Powder manufacturing method, powder, and powder dispersion

Abstract

[Problem] To provide a powder that has high dispersion stability in a powder dispersion liquid, a method for producing the powder, and a powder dispersion liquid having high dispersion stability. [Solution] A method for producing a powder according to the present invention, said method comprising thermally treating, under fluidization, aggregates of a hot melt tetrafluoroethylene-based polymer at a temperature not higher than the melting point of the tetrafluoroethylene-based polymer and then grinding the same to give a powder of the tetrafluoroethylene-based polymer. The powder shows a unimodal grain size distribution and has an average particle size of more than 1 μm and not more than 10 μm and a specific surface area of 1 m2 / g or more and less than 8 m2 / g.

Description

[Technical Field]

[0001] The present invention relates to a tetrafluoroethylene-based polymer powder having a predetermined particle size distribution and specific surface area, a method for producing the same, and a powder dispersion containing the powder. [Background technology]

[0002] Heat-meltable tetrafluoroethylene polymers are excellent in various physical properties (electrical insulation, water and oil repellency, chemical resistance, etc.) and melt processability. For this reason, tetrafluoroethylene polymer powders are used as powder coatings themselves (see Patent Document 1) or as powder dispersions containing them (see Patent Document 2).

[0003] Patent Document 1 describes a tetrafluoroethylene polymer aggregate having a specific surface area of ​​8 to 25 m, which is obtained by heat-treating an aggregate recovered from a polymerization solution of a tetrafluoroethylene polymer and then pulverizing it. 2 / g and an average particle size of 1 to 100 μm. However, in Patent Document 1, the agglomerates are heat-treated in a stationary state, which tends to result in unevenness in the degree of heat treatment. Therefore, the powder obtained by the subsequent pulverization is unlikely to have a small specific surface area and tends to have a multi-modal particle size distribution.

[0004] On the other hand, Patent Document 2 discloses a tetrafluoroethylene polymer powder having a specific surface area of ​​15 m2 to be dispersed in a powder dispersion liquid. 2 / g or less and an average particle size of 1 μm or less. Patent Document 2 states that if the average particle size of the powder exceeds 1 μm, the dispersion stability of the powder in the powder dispersion liquid decreases. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-169339 [Patent Document 2] Japanese Patent Application Publication No. 2017-088861 Summary of the Invention [Problem to be solved by the invention]

[0006] The present inventors have intensively investigated methods for improving the dispersion stability of the above powder in a powder dispersion, and as a result have found that the dispersion stability of the powder in a powder dispersion can be improved by adjusting the average particle size within a predetermined range, making the particle size distribution unimodal, and reducing the specific surface area. An object of the present invention is to provide a powder having excellent dispersion stability in a powder dispersion, a method for producing the powder, and a powder dispersion having excellent dispersion stability. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] Agglomerates of a heat-fusible tetrafluoroethylene-based polymer are fluidized and heat-treated at a temperature equal to or lower than the melting point of the tetrafluoroethylene-based polymer, and then pulverized to produce particles having an average particle size of more than 1 μm and 10 μm or less and a specific surface area of ​​1 m 2 / g or more 8m 2 The method for producing a powder of the tetrafluoroethylene-based polymer is characterized in that the powder has a unimodal particle size distribution of less than 1 / g. [2] The production method according to [1], wherein the tetrafluoroethylene-based polymer is a tetrafluoroethylene-based polymer containing units based on perfluoro(alkyl vinyl ether) or units based on hexafluoropropylene. [3] The method according to [1] or [2], wherein the tetrafluoroethylene-based polymer is a tetrafluoroethylene-based polymer having a polar functional group. [4] The method according to any one of [1] to [3], wherein the aggregates have an average particle size of 100 μm to 5 mm. [5] The method according to any one of [1] to [4], wherein the temperature during the heat treatment of the aggregate is higher than the melting point of the tetrafluoroethylene-based polymer minus 100°C. [6] The production method according to any one of [1] to [5], wherein the aggregate is heat-treated while being tumbled in a rotary furnace that rotates about a central axis. [7] The production method according to [6], wherein the inclination angle of the rotary furnace with respect to the horizontal direction is 0.01 to 5°. [8] The production method according to [6] or [7], wherein the rotational speed of the rotary furnace is 1 to 20 rpm. [9] The production method according to any one of [6] to [8], wherein the residence time of the aggregate in the rotary furnace is 1 to 60 minutes.

[10] The production method according to any one of [1] to [9], wherein the hardness of the aggregate before pulverization is 0.2 N / mm or more.

[11] The production method according to any one of [1] to

[10] , wherein the aggregate is an aggregate of particles of the tetrafluoroethylene-based polymer formed by polymerization of a raw material monomer of the tetrafluoroethylene-based polymer.

[12] A powder of a heat-meltable tetrafluoroethylene-based polymer, having an average particle diameter exceeding 1 μm and not exceeding 10 μm, and a specific surface area of 1 m 2 / g or more and less than 8 m 2 / g, and having a unimodal particle size distribution.

[13] The powder according to

[12] , wherein the full width at half maximum of the particle size distribution is 0.5 to 3.5 μm.

[14] A powder dispersion liquid containing the powder according to

[12] or

[13] and a liquid dispersion medium.

[15] The powder dispersion liquid according to

[14] , wherein the viscosity of the powder dispersion liquid is 1000 mPa·s or less. [Advantages of the Invention]

[0008] According to the present invention, a powder of a tetrafluoroethylene-based polymer having a predetermined average particle diameter, particle size distribution, and specific surface area can be easily produced. Further, a powder dispersion liquid excellent in such powder and dispersion stability can be obtained. [Embodiments for Carrying Out the Invention]

[0009] The following terms have the following meanings. "Average particle size (D50)" is the volume-based cumulative 50% diameter of a target object (powder or aggregate) determined by laser diffraction / scattering. In other words, the particle size distribution of the target object is measured by laser diffraction / scattering, and a cumulative curve is calculated with the total volume of the target particle group as 100%. The average particle size (D50) is the particle size at the point on the cumulative curve where the cumulative volume is 50%. "D90" is the volume-based cumulative 90% diameter of the object, measured in the same manner. The "full width at half maximum" is the width of the peak at half the height of the peak (maximum value) in the particle size distribution curve of the object. The "melting temperature (melting point) of a polymer" is the temperature corresponding to the maximum value of the melting peak measured by differential scanning calorimetry (DSC). The "glass transition temperature (Tg) of a polymer" is a value measured by analyzing a polymer using dynamic mechanical analysis (DMA). The "viscosity of the dispersion" is the viscosity of the liquid measured using a Brookfield viscometer at room temperature (25°C) and a rotation speed of 60 rpm. The "thixotropy ratio of the dispersion" is a value calculated by dividing the viscosity of the dispersion measured at a rotation speed of 30 rpm by the viscosity of the dispersion measured at a rotation speed of 60 rpm. The "unit" in a polymer may be an atomic group formed directly from a monomer, or may be an atomic group in which a part of the structure is converted by treating the obtained polymer in a predetermined manner. A unit based on monomer A contained in a polymer is also simply referred to as a "monomer A unit."

[0010] The method for producing a powder of the present invention (hereinafter also referred to as "this method") involves fluidizing an aggregate of a heat-fusible tetrafluoroethylene-based polymer (hereinafter also referred to as "F polymer"), heat-treating it at a temperature equal to or lower than the melting point of the F polymer, and then pulverizing it to produce a powder having a D50 of more than 1 μm and 10 μm or less and a specific surface area of ​​1 m. 2 / g or more 8m 2 The method for obtaining a powder of F polymer having a particle size distribution of less than 1 / g is described below. The powder of F polymer obtained by this method has a monomodal particle size distribution.

[0011] According to this method, the F polymer aggregates are heat-treated not in a static state but in a fluidized state. This facilitates uniform heat transfer to the aggregates during heating, resulting in a highly heat-treated surface and a softened interior. Furthermore, the fluidized aggregates are subjected to moderate impact (vibration), which is believed to result in the formation of aggregates that are dense overall, have a small specific surface area, and are highly hard (particularly surface hardness). The resulting powder is believed to have a small D50 and to be less prone to the generation of irregularly shaped particles. In other words, the resulting powder is believed to have a sufficiently small D50, a unimodal particle size distribution, and a small specific surface area. It is believed that by using such powder, a powder dispersion liquid with excellent dispersion stability can be easily obtained.

[0012] The F polymer in the present invention is a heat-meltable polymer containing units based on tetrafluoroethylene (TFE) (TFE units). The melting temperature of the F polymer is preferably 260 to 320°C, more preferably 285 to 320°C. The glass transition point of the F polymer is preferably from 75 to 125°C, more preferably from 80 to 100°C.

[0013] The F polymer is preferably a polymer containing TFE units and units based on perfluoro(alkyl vinyl ether) (PAVE) (PAVE units) or units based on hexafluoropropylene (HFP) (HFP units). In this case, the F polymer forms small spherulites, which increases the surface smoothness of the aggregates and tends to reduce the specific surface area of ​​the resulting powder. In addition, it is easier to more effectively suppress the denaturation of the F polymer during heat treatment. The F polymer may contain both PAVE units and HFP units, or may contain only one of them. The PAVE is preferably CF2=CFOCF3, CF2=CFOCF2CF3, CF2=CFOCF2CF2CF3 (PPVE) or CF2=CFOCF(CF3)CF2OCF2CF2CF3, more preferably PPVE.

[0014] The F polymer preferably has a polar functional group, which makes it easier to produce a powder with excellent dispersion stability while more highly suppressing the denaturation of the F polymer during heat treatment. The polar functional group may be contained in a unit in the F polymer or in a terminal group of the main chain of the polymer. Examples of the latter include an F polymer having a polar functional group as a terminal group derived from a polymerization initiator, a chain transfer agent, etc., and an F polymer having a polar functional group obtained by subjecting an F polymer to plasma treatment or ionizing radiation treatment.

[0015] The polar functional group is preferably a hydroxyl group-containing group or a carbonyl group-containing group, and from the viewpoint of dispersion stability of the powder, a carbonyl group-containing group is more preferred. The hydroxyl group-containing group is preferably a group containing an alcoholic hydroxyl group, and is preferably -CF2CH2OH or -C(CF3)2OH. The carbonyl group-containing group is a group containing a carbonyl group (>C(O)), and is preferably a carbonyl group, a carboxyl group, an alkoxycarbonyl group, an amide group, an isocyanate group, a carbamate group (-OC(O)NH), an acid anhydride residue (-C(O)OC(O)-), an imide residue (-C(O)NHC(O)-, etc.), or a carbonate group (-OC(O)O-). When the F polymer has a carbonyl group-containing group, the number of carbonyl group-containing groups in the F polymer is 1×10 6 The number per unit is preferably 10 to 5000, more preferably 100 to 3000, and further preferably 800 to 1500. The number of carbonyl group-containing groups in the F polymer can be quantified by the composition of the polymer or the method described in WO 2020 / 145133.

[0016] The F polymer is preferably a polymer (1) having a polar functional group containing TFE units, PAVE units, and units based on a monomer having a polar functional group, or a polymer (2) having no polar functional group but containing TFE units and PAVE units and 2.0 to 5.0 mol % of PAVE units based on all units. These F polymers not only have excellent dispersion stability, ease of handling, and blendability in their powder form, but also tend to form microspherulites in molded articles (such as the F layer described below) formed from them, thereby improving their adhesion.

[0017] The polymer (1) preferably contains, based on all units, 90 to 99 mol % of TFE units, 0.5 to 9.97 mol % of PAVE units, and 0.01 to 3 mol % of units derived from a monomer having a polar functional group. Furthermore, the monomer having a polar functional group is preferably maleic anhydride, itaconic anhydride, citraconic anhydride, or 5-norbornene-2,3-dicarboxylic anhydride (also known as himic anhydride; hereinafter also referred to as "NAH"). Specific examples of polymer (1) include the polymers described in WO 2018 / 16644.

[0018] The polymer (2) is composed only of TFE units and PAVE units, and preferably contains 95.0 to 98.0 mol % of TFE units and 2.0 to 5.0 mol % of PAVE units based on all units. The content of PAVE units in the polymer (2) is preferably 2.1 mol % or more, more preferably 2.2 mol % or more, based on all units. The term "polymer (2) has no polar functional group" means that the number of carbon atoms constituting the polymer main chain is 1 × 10 6 This means that the number of polar functional groups possessed by the polymer per unit area is less than 500. The number of polar functional groups is preferably 100 or less, and more preferably less than 50. The lower limit of the number of polar functional groups is usually 0.

[0019] Polymer (2) may be produced using a polymerization initiator or chain transfer agent that does not generate a polar functional group as the terminal group of the polymer chain, or may be produced by fluorinating an F polymer having a polar functional group (such as an F polymer having a polar functional group derived from a polymerization initiator at the terminal group of the polymer main chain). Fluorination methods include methods using fluorine gas (see, for example, JP 2019-194314 A).

[0020] The F polymer agglomerates are preferably agglomerates of F polymer particles formed by polymerization of raw material monomers for the F polymer, and more preferably agglomerates of primary particles of F polymer formed by polymerization of raw material monomers for the F polymer. The average particle size of the F polymer particles is preferably less than 1 μm. Furthermore, the average particle size of the F polymer particles is preferably 0.01 μm or more. The powder obtained by heat-treating and then pulverizing such agglomerates tends to have a smaller D50. Furthermore, the particle size distribution of the powder tends to be sharp and unimodal. Examples of methods for aggregating F polymer particles include a method of aggregating F polymer particles by mixing a liquid composition containing F polymer particles with an aggregating agent, a method of aggregating F polymer particles by shear stirring the liquid composition, etc. The liquid composition is preferably a liquid composition containing primary particles of F polymer and a polymerization solvent, which is formed by polymerizing raw material monomers of the F polymer in a polymerization solvent. The liquid composition containing the F polymer agglomerates formed by these methods can be subjected to solid-liquid separation to recover the agglomerates, which can then be dried to obtain dried agglomerates. The D50 of the agglomerates is preferably 100 μm to 5 mm, more preferably 1 to 3 mm, in which case the D50 of the resulting powder can be made sufficiently small while ensuring the handleability of the agglomerates.

[0021] In the present invention, the agglomerates are heat-treated while being fluidized. Examples of such heat-treatment methods include Method I, which uses a rotary kiln, and Method II, which uses a heat treatment device that generates swirling hot air (hot air fluidizer). In the former method I, the agglomerates are heat-treated by passing them through a cylindrical rotary furnace while rolling inside the rotary furnace, which rotates around its central axis. According to method I, the agglomerates roll while coming into contact with the inner surface of the heated rotary furnace, which makes it particularly easy for heat to be transferred uniformly to the agglomerates.

[0022] The temperature during heat treatment is below the melting temperature of the F polymer, preferably above the melting temperature of the F polymer minus 100°C but below the melting temperature of the F polymer, and more preferably above 200°C but below 300°C. Heat-treating the agglomerates at such temperatures increases the density of the agglomerates and their hardness. As a result, the powder obtained by pulverization tends to have a smaller D50 and a sharper, unimodal particle size distribution. The specific hardness value of the agglomerate before pulverization is preferably 0.2 N / mm or more, more preferably 0.3 N / mm or more. The hardness value is preferably 0.8 N / mm or less. In this case, the impact force applied to the agglomerate during pulverization increases, making it easier to pulverize the agglomerate into more uniform particles.

[0023] The tilt angle of the rotary kiln relative to the horizontal direction (the rotary kiln's ground surface) is preferably 0.01 to 5°, more preferably 0.1 to 3°. By tilting the rotary kiln at such an angle, the passage time of the agglomerates through the rotary kiln (the residence time of the agglomerates in the rotary kiln) can be ensured to be sufficiently long. As a result, the agglomerates can be heated more uniformly and sufficiently. The residence time of the agglomerates in the rotary furnace is preferably 1 to 60 minutes, more preferably 5 to 40 minutes, which ensures sufficient heating time for the agglomerates. The rotation speed of the rotary furnace is preferably 1 to 20 rpm, and more preferably 3 to 10 rpm. In this case, excessive impact force is unlikely to be applied to the agglomerates, and therefore unintended crushing of the agglomerates during rolling (flowing) can be prevented.

[0024] The rate at which the agglomerates are introduced into the rotary furnace is preferably such that the packing ratio of the agglomerates in the rotary furnace is 0.1 to 40%, more preferably 1 to 20%, which allows the agglomerates to be heat-treated sufficiently and efficiently. As such a rotary kiln, for example, a device manufactured by Noritake Company Limited or a device manufactured by Sanai Kanetsuko Co., Ltd. can be used.

[0025] In the latter method II, hot air is swirled and supplied into a cylindrical treatment space, and the aggregates are supplied into the treatment space tangentially with a carrier gas, and the aggregates are heat-treated with the hot air. This method II can prevent turbulence at the moment the carrier gas containing the aggregates is introduced into the treatment space, thereby suppressing the generation of coalesced particles (coarse particles). Furthermore, it is preferable to configure the system so that the carrier gas containing the aggregates is supplied into the processing space from multiple aggregate supply ports arranged along the circumferential direction of the processing space and on the same plane. The greater the number of aggregate supply ports, the more the amount of aggregates supplied from each aggregate supply port can be reduced. Therefore, the temperature of the hot air required for heat treatment of the aggregates can be reduced depending on the number of aggregate supply ports. Furthermore, the generation of coalesced particles can be suppressed. The agglomerates are transported in the treatment space while flowing in the swirling current of hot air, which facilitates the heat transfer to the agglomerates in a particularly uniform manner, facilitating the densification and spheroidization of the agglomerates.

[0026] The heat-treated agglomerate is cooled by cold air supplied from a cold air supply port installed downstream (vertically below) of the agglomerate supply port. Like the aggregate supply port, the cold air supply port also supplies cold air from a tangential direction of the processing space. It is also preferable to supply cold air into the processing space from multiple cold air supply ports arranged on the same plane along the circumferential direction of the processing space. This configuration can prevent turbulence at the moment the cold air is introduced into the processing space, thereby suppressing the generation of coalesced particles (coarse particles). The more cold air supply ports are provided, the more efficiently the heat-treated agglomerates can be cooled, which reduces unevenness in cooling of the agglomerates and makes it easier to obtain agglomerates with uniform circularity. The arrangement of the cold air supply port makes it possible to maintain a swirling flow within the treatment space, and therefore to prevent the adhesion of aggregates due to retention at the bottom of the heat treatment device that defines the treatment space.

[0027] The aggregates that have passed through the treatment space are sucked by a suction device and discharged (recovered) from an aggregate discharge port installed downstream (vertically below) of the cold air supply port. In such a heat treatment apparatus, the total flow rate (total supply amount) Q of the compressed air, hot air, and cold air supplied into the treatment space is IN and the flow rate (total discharge) Q discharged from the treatment space by the suction device OUT The relationship between is preferably Q IN ≦Q OUT In this case, the pressure in the processing space becomes negative, which makes it easier to discharge the aggregates in the processing space, thereby preventing excessive heat from being applied to the aggregates. This also reduces the generation of coalesced particles and the amount of aggregates adhering to the inside of the heat treatment device. The hardness of the aggregates discharged from the treatment space tends to be in the range described above. The temperature of the hot air (temperature during the heat treatment) is also the same as above. As such a heat treatment device, for example, a flash jet dryer (manufactured by Seishin Enterprise Co., Ltd.) can be used.

[0028] The heat-treated agglomerates are pulverized (broken down) to obtain a powder of the F polymer, which is preferably pulverized by mechanical pulverization. Mechanical grinding is carried out using equipment capable of exerting sufficient shear and / or crushing forces to break down agglomerates into smaller particles (powder). Such equipment includes a hammer mill including a disintegrator, a pin mill, a disc mill, a rotary mill, a jet mill, a fluidized bed air jet mill, a jaw crusher, a gyratory crusher, a cage mill, a pan crusher, a ball mill, a pebble mill, a rod mill, a tube mill, a disc attrition mill, an attritor, and a disc refiner.

[0029] Among these, the mechanical pulverization treatment is preferably carried out using a hammer mill, pin mill, disk mill, rotary mill, or jet mill. Using these devices, the D50 of the resulting powder is likely to be small, and a sharper, unimodal particle size distribution is likely to be obtained. In addition, the specific surface area of ​​the powder is likely to be small. The temperature during pulverization is preferably −40° C. or lower, more preferably −100° C. or lower, and even more preferably −160° C. or lower. In this case, due to the low-temperature brittleness of the F polymer, it is easier to reduce the D50 of the powder compared to when the aggregates are not cooled. The cooling is preferably carried out using solidified carbon dioxide or liquid nitrogen. Specific examples of the above-mentioned device include a jet mill ("Counter Jet Mill" manufactured by Hosokawa Micron Corporation) and a planetary ball mill ("Planetary Ball Mill PM100" manufactured by Retsch).

[0030] The F polymer powder in the present invention is a powder containing an F polymer, and is preferably a powder consisting of an F polymer. Other components that can be contained in the F polymer powder include aromatic polyester, polyamideimide, thermoplastic polyimide, polyphenylene ether, and polyphenylene oxide. The D50 of the F polymer powder is greater than 1 μm and equal to or less than 10 μm, preferably greater than 1 μm and equal to or less than 8 μm, and more preferably greater than 1 μm and equal to or less than 6 μm. In this case, the F polymer powder has high fluidity, and the powder dispersion containing it has improved handleability and excellent dispersion stability.

[0031] Such powder has a unimodal particle size distribution, with a full width at half maximum of preferably 0.5 to 3.5 μm, more preferably 1 to 2.5 μm, which means that the powder does not contain irregularly shaped particles (coarse particles), and when preparing a powder dispersion, aggregation of the powder originating from the irregularly shaped particles is unlikely to occur. The specific surface area of ​​the powder is 1m 2 / g or more 8m 2 / g or less, 1-5m 2 / g, and 1 to 3 m 2 / g, which is more preferable. In this case, the specific surface area is smaller, and therefore the dispersion stability of the powder dispersion is improved.

[0032] The powder of the present invention (hereinafter also referred to as "the present powder") is a powder of F polymer, and has a unimodal particle size distribution with D50 of more than 1 μm and 10 μm or less, and a specific surface area of ​​1 m 2 / g or more 8m 2 / g. The definitions and scope of the present powder, including preferred embodiments thereof, are the same as those in the present method. The definitions and scope of the F polymer in the present powder, including preferred embodiments thereof, are the same as those in the present method. The present powder is preferably a powder produced by the present method.

[0033] The powder dispersion of the present invention (hereinafter also referred to as "the present dispersion") contains the present powder and a liquid dispersion medium. Because the present powder has the above-mentioned characteristics, the present dispersion has excellent dispersion stability. As described above, the present powder does not contain irregular-shaped particles, or even if it does contain irregular-shaped particles, the content of such particles is extremely small. Therefore, when the present powder is mixed with a liquid dispersion medium by stirring or the like to prepare the present dispersion, denaturation of the present powder is highly suppressed, and the present dispersion is less likely to thicken. The degree of resistance to thickening of the present dispersion can be evaluated based on the rate of thickening of the present dispersion before and after stirring.

[0034] Specifically, when the dispersion is stirred at 8000 rpm for 30 minutes, the viscosity increase rate calculated according to the following formula based on the viscosity measurements before and after stirring is preferably less than 60%, more preferably less than 40%. The lower limit of the viscosity increase rate of the dispersion is 0%. Formula: Viscosity increase rate = {(viscosity after stirring / viscosity before stirring)-1} x 100 The liquid dispersion medium is preferably a liquid compound that functions as a dispersion medium for the present powder and is inactive at 25° C. The liquid dispersion medium may be used alone or in combination of two or more. The boiling point of the liquid dispersion medium is preferably 125 to 250° C. In this case, when a liquid coating made from the dispersion is dried to form a dry coating, the fluidity of the powder proceeds effectively as the liquid dispersion medium volatilizes, facilitating dense packing of the powder.

[0035] Specific examples of liquid dispersion media include water, N,N-dimethylformamide, N,N-dimethylacetamide, 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, N-methyl-2-pyrrolidone, γ-butyrolactone, cyclohexanone, cyclopentanone, butyl acetate, methyl isopropyl ketone, cyclopentanone, and cyclohexanone. The liquid dispersion medium is preferably an amide, ketone or ester, and more preferably N,N-dimethylacetamide, 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, N-methyl-2-pyrrolidone or γ-butyrolactone.

[0036] The content of the F polymer in the present dispersion is preferably from 5 to 70% by mass, more preferably from 10 to 60% by mass. The content of the liquid dispersion medium in the present dispersion is preferably from 30 to 95% by mass, more preferably from 40 to 90% by mass. The dispersion may contain components other than the F polymer and the liquid dispersion medium. The present dispersion preferably further contains a surfactant, which facilitates dispersion of the F polymer powder and improves the dispersion stability of the present dispersion. In this case, the content of the surfactant in the dispersion is preferably 1 to 15% by mass. The surfactant is preferably nonionic.

[0037] The hydrophilic portion of the surfactant preferably has an oxyalkylene group or an alcoholic hydroxyl group. The oxyalkylene group may be composed of one type or two or more types, and in the latter case, the different types of oxyalkylene groups may be arranged randomly or in blocks. The oxyalkylene group is preferably an oxyethylene group or an oxypropylene group, more preferably an oxyethylene group. The hydrophobic portion of the surfactant preferably has an acetylene group, a polysiloxane group, a perfluoroalkyl group, or a perfluoroalkenyl group. The surfactant is preferably a glycol surfactant, an acetylene surfactant, a silicone surfactant, or a fluorine surfactant, and more preferably a silicone surfactant. One or more types of nonionic surfactants may be used. When two types of nonionic surfactants are used, the nonionic surfactants are preferably a silicone surfactant and a glycol surfactant.

[0038] The fluorosurfactant is preferably a fluorosurfactant having a hydroxyl group (especially an alcoholic hydroxyl group) or an oxyalkylene group and a perfluoroalkyl group or a perfluoroalkenyl group, wherein an etheric oxygen atom may be inserted between carbon atoms in the perfluoroalkyl group. The weight average molecular weight of the surfactant is preferably 5,000 to 300,000. The fluorine content of the surfactant is more preferably 15 to 90% by mass. The content of the oxyalkylene group in the surfactant is preferably 10 to 60% by mass. The hydroxyl value of the surfactant is preferably 10 to 100 mgKOH / g.

[0039] The fluorine-based surfactant is preferably a copolymer of fluoro(meth)acrylate and hydrophilic (meth)acrylate. Specific examples of fluoro(meth)acrylates include CH2=C(CH3)C(O)OCH2CH2(CF2)4F, CH2=C(CH3)C(O)OCH2CH2(CF2)6F, CH2=C(CH3)C(O)OCH2CH2CH2CH2OCF(CF3)C(=C(CF3)2)(CF(CF3)2), and CH2=C(CH3)C(O)OCH(CH3)OCH2(CF2)6F.

[0040] Specific examples of hydrophilic (meth)acrylates include CH2=C(CH3)C(O)(OCH2CH2)4OH, CH2=C(CH3)C(O)(OCH2CH2)9OH, and CH2=C(CH3)C(O)(OCH2CH2) 23 OH is an example. Specific examples of such surfactants include the "Ftergent" series (manufactured by Neos Corporation), the "Surflon" series (manufactured by AGC Seimi Chemical Co., Ltd.), the "Megafac" series (manufactured by DIC Corporation), the "Unidyne" series (manufactured by Daikin Industries, Ltd.), "BYK-347", "BYK-349", "BYK-378", "BYK-3450", "BYK-3451", "BYK-3455", and "BYK-3456" (manufactured by BYK Japan KK), "KF-6011", and "KF-6043" (manufactured by Shin-Etsu Chemical Co., Ltd.), and the "Tergitol" series (manufactured by The Dow Chemical Company, such as "Tergitol TMN-100X"). When the present dispersion contains a nonionic surfactant, the content of the nonionic surfactant in the present dispersion is preferably 1 to 15% by mass.

[0041] The dispersion may further comprise a polymer different from the F polymer, such as an aromatic polyester, polyamideimide, polyimide, polyphenylene ether, polyphenylene oxide, or maleimide, preferably a thermoplastic aromatic polyimide. The dispersion may further contain an inorganic filler, a thixotropic agent, an antifoaming agent, a silane coupling agent, a dehydrating agent, a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a brightener, a colorant, a conductive agent, a release agent, a surface treatment agent, a viscosity adjuster, or a flame retardant. The viscosity of the present dispersion is preferably 1000 mPa·s or less, more preferably 50 to 750 mPa·s. Because the present powder has the above characteristics, the present dispersion tends to have a low viscosity. The thixotropy ratio of the present dispersion is preferably 1.0 to 3.0, more preferably 1.0 to 2.0. When the viscosity and thixotropy ratio of the dispersion are within the above ranges, it is easy to obtain a dispersion in which the powder is highly dispersed at a high content. Furthermore, the dispersion has excellent handling properties such as coatability and blendability with other components.

[0042] The dispersion is useful as a coating material for imparting insulating properties, heat resistance, corrosion resistance, chemical resistance, water resistance, impact resistance, and thermal conductivity. Specifically, this dispersion can be used in printed wiring boards, thermal interface materials, power module substrates, coils used in power devices such as motors, automotive engines, heat exchangers, vials, syringes, ampoules, medical wires, secondary batteries such as lithium ion batteries, primary batteries such as lithium batteries, radical batteries, solar cells, fuel cells, lithium ion capacitors, hybrid capacitors, capacitors (aluminum electrolytic capacitors, tantalum electrolytic capacitors, etc.), electrochromic elements, electrochemical switching elements, electrode binders, electrode separators, and electrodes (positive and negative electrodes).

[0043] The dispersion is also useful as an adhesive for bonding parts. Specifically, the dispersion can be used for bonding ceramic parts, metal parts, electronic parts such as IC chips, resistors, and capacitors on substrates for semiconductor elements and module parts, bonding circuit boards and heat sinks, and bonding LED chips to substrates. Furthermore, the dispersion containing a conductive filler can be used in applications requiring electrical conductivity, such as in the field of printed electronics, for example, for the manufacture of conductive elements in printed circuit boards, sensor electrodes, etc.

[0044] This dispersion is applied to the surface of a substrate layer to form a liquid coating, which is then heated to volatilize the liquid dispersion medium to obtain a dry coating, and further heated to bake the F polymer, resulting in a laminate having a substrate layer and a polymer layer containing the F polymer (hereinafter also referred to as the "F layer"). As described above, the present dispersion is less likely to thicken during preparation because the denaturation of the present powder is highly suppressed. Therefore, the present dispersion has excellent leveling properties. Furthermore, the present powder is prone to dense packing. Therefore, the F layer formed from the present dispersion has high surface smoothness and density. The arithmetic mean roughness (Ra) of the surface of the F layer (the surface opposite to the substrate layer) is preferably less than 0.6 μm, more preferably less than 0.4 μm, and is usually 0.05 μm or more.

[0045] The temperature in the former heating is preferably 120 to 200° C. The temperature in the latter heating is preferably a temperature equal to or higher than the melting temperature of the F polymer, more specifically, more preferably 300 to 380° C. In this case, the F layer tends to have excellent surface smoothness, flexibility, and electrical properties. Examples of the heating method include a method using an oven, a method using a ventilation drying furnace, and a method using heat rays such as infrared rays.

[0046] Examples of substrates that constitute the substrate layer include metal substrates (substrates of copper, nickel, aluminum, titanium, alloys thereof, etc.), resin films (films of polyimide, polyarylate, polysulfone, polyarylsulfone, polyamide, polyetheramide, polyphenylene sulfide, polyaryletherketone, polyamideimide, liquid crystalline polyester, liquid crystalline polyesteramide, etc.), and prepregs (precursors of fiber-reinforced resin substrates). Examples of methods for applying the dispersion include spraying, roll coating, spin coating, gravure coating, microgravure coating, gravure offset, knife coating, kiss coating, bar coating, die coating, fountain-meyer bar coating, and slot die coating.

[0047] The thickness of the F layer is preferably 0.1 to 100 μm, more preferably 0.5 to 50 μm. The present dispersion has excellent handleability, and therefore an F layer of any thickness can be easily formed that is also excellent in physical properties such as surface smoothness. The dispersion may be applied to only one surface of the substrate layer, or may be applied to both surfaces of the substrate layer. In the former case, a laminate having a substrate layer and an F layer on one surface of the substrate layer is obtained, while in the latter case, a laminate having a substrate layer and an F layer on both surfaces of the substrate layer is obtained. The latter laminate is less likely to warp, and therefore has excellent handling properties during processing.

[0048] Specific examples of such laminates include a metal clad laminate having a metal foil and an F layer on at least one surface of the metal foil, and a multilayer film having a polyimide film and an F layer on both surfaces of the polyimide film. These laminates have excellent physical properties such as electrical properties, heat resistance including solder reflow resistance, chemical resistance, and surface smoothness, and are suitable as materials for printed circuit boards, etc. Specifically, such laminates can be used to produce flexible printed circuit boards and rigid printed circuit boards.

[0049] The metal foil may be a carrier-attached metal foil containing two or more layers of metal foil. Examples of carrier-attached metal foils include carrier-attached copper foils consisting of a carrier copper foil (thickness: 10 to 35 μm) and an ultrathin copper foil (thickness: 2 to 5 μm) laminated on the carrier copper foil via a release layer. The use of such carrier-attached copper foils makes it possible to form fine patterns using the MSAP (modified semi-additive) process. The release layer is preferably a metal layer containing nickel or chromium, or a multilayer metal layer in which such metal layers are laminated. A specific example of the metal foil with a carrier is "FUTF-5DAF-2" manufactured by Fukuda Metal Foil and Powder Co., Ltd. The ten-point average roughness of the surface of the substrate is preferably 0.01 to 0.05 μm. Because the powder is easily packed densely, it is possible to form a laminate with excellent peel strength even on a substrate with a smooth surface.

[0050] The outermost surface of the laminate (the surface of the polymer layer opposite the substrate layer) may be further surface-treated to further improve its low linear expansion and adhesiveness. Examples of the surface treatment method include annealing treatment, corona treatment, plasma treatment, ozone treatment, excimer treatment, and silane coupling treatment. The annealing conditions are preferably a temperature of 120 to 180° C., a pressure of 0.005 to 0.015 MPa, and a time of 30 to 120 minutes. Gases used in the plasma treatment include oxygen gas, nitrogen gas, rare gas (argon, etc.), hydrogen gas, ammonia gas, and vinyl acetate. These gases may be used alone or in combination of two or more.

[0051] Another substrate may be further laminated on the outermost surface of the laminate. Other substrates include heat-resistant resin films, prepregs which are precursors of fiber-reinforced resin plates, laminates having a heat-resistant resin film layer, and laminates having a prepreg layer. The prepreg is a sheet-like substrate in which a base material (tow, woven fabric, etc.) of reinforcing fibers (glass fiber, carbon fiber, etc.) is impregnated with a thermosetting resin or a thermoplastic resin. The heat-resistant resin film is a film containing one or more heat-resistant resins, such as polyimide, polyarylate, polysulfone, polyarylsulfone, aromatic polyamide, aromatic polyetheramide, polyphenylene sulfide, polyaryletherketone, polyamideimide, liquid crystalline polyester, and liquid crystalline polyesteramide, and polyimide (particularly, aromatic polyimide) is preferred.

[0052] As a lamination method, a method of hot pressing the laminate and another substrate can be mentioned. When the other substrate is a prepreg, the conditions for the hot pressing are preferably a temperature of 120 to 400° C., a vacuum atmosphere of 20 kPa or less, and a pressing pressure of 0.2 to 10 MPa. The laminate has a polymer layer with excellent electrical properties, and is therefore suitable as a material for printed circuit boards. Specifically, the laminate produced by this method can be used as a flexible metal-clad laminate or a rigid metal-clad laminate for producing printed circuit boards, and is particularly suitable for use as a flexible metal-clad laminate for producing flexible printed circuit boards.

[0053] A printed circuit board can be obtained by etching the metal foil of a laminate (polymer-layered metal foil) whose base layer is a metal foil to form a transmission circuit. Specifically, a printed circuit board can be manufactured by etching the metal foil to process it into a predetermined transmission circuit, or by processing the metal foil into a predetermined transmission circuit by electrolytic plating (semi-additive method (SAP method), MSAP method, etc.). A printed circuit board manufactured from a polymer-layered metal foil has a transmission circuit formed from the metal foil and a polymer layer in this order. Specific examples of the configuration of a printed circuit board include transmission circuit / polymer layer / prepreg layer and transmission circuit / polymer layer / prepreg layer / polymer layer / transmission circuit. In the manufacture of such a printed circuit board, an interlayer insulating film may be formed on a transmission circuit, a solder resist may be laminated on the transmission circuit, or a coverlay film may be laminated on the transmission circuit. These interlayer insulating films, solder resists, and coverlay films may be formed using the present dispersion.

[0054] Although the method for producing powder, the powder, and the powder dispersion of the present invention have been described above, the present invention is not limited to the configurations of the above-described embodiments. For example, the powder and powder dispersion of the present invention may each have any other components added to the configurations of the above embodiments, or may be substituted with any other components that exhibit similar functions. Furthermore, the method for producing powder of the present invention may have any other optional step added to the configuration of the above embodiment, or may be replaced with any other optional step that produces a similar effect. [Example]

[0055] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following description. 1. Aggregate Preparation [Preparation of Aggregate 1] First, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (boiling point: 56°C; manufactured by AGC, "AE-3000"), methanol, and PPVE were charged into a evacuated stainless steel polymerization tank (internal volume: 1.3 L), and while stirring the contents inside the polymerization tank, TFE gas was charged and the internal temperature of the polymerization tank was maintained at 50°C. Next, a 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether solution (initiator solution) containing 0.05% by mass of bis(perfluorobutyryl) peroxide was pressure-fed into the polymerization tank to initiate polymerization, and TFE gas was pressure-fed so that the internal pressure of the polymerization tank was kept constant at 1.0 MPa to continue the polymerization.

[0056] The initiator solution was intermittently injected into the polymerization vessel so that the TFE gas consumption rate was 0.5 g / min. In addition, 1 mL of a 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether solution containing 1% by mass of NAH was injected into the polymerization vessel every time 5 g of TFE gas was consumed. After 290 minutes from the start of polymerization, the polymerization vessel was cooled to terminate the polymerization. Thereafter, the residual monomer gas was purged from the polymerization vessel to atmospheric pressure to obtain a crude polymerization liquid 1. The polymerization crude liquid 1 contained 98.0 mol %, 1.9 mol %, and 0.1 mol % of TFE units, PPVE units, and NAH units, in that order, and contained 13 mass % of polymer 1 (melting temperature: 300° C.) having a polar functional group. In addition, in the polymerization crude liquid 1, primary particles of polymer 1 (D50: 0.5 μm) were dispersed in 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. Water was added to this polymerization crude liquid 1 and stirred to aggregate the particles, and then the aggregate was recovered by solid-liquid separation. The recovered aggregate was dried at 150°C for 15 hours to obtain aggregate 1 (D50: 1.5 mm) of primary particles of polymer 1.

[0057] [Preparation of Aggregate 2] Polymerization crude liquid 2 was obtained in the same manner as for polymerization crude liquid 1, except that the use of NAH was omitted. The polymerization crude liquid 2 contained 98.7 mol % of TFE units and 1.3 mol % of PPVE units, in that order, and 13 mass % of polymer 2 (melting temperature: 305° C.) having no polar functional groups. In the polymerization crude liquid 2, primary particles of polymer 2 (D50: 0.4 μm) were dispersed in 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether. In the same manner as for Coagulate 1, Coagulate 2 (D50: 2.4 mm) was recovered from this polymerization crude liquid 2.

[0058] 2. Powder production [Example 1] First, agglomerate 1 was supplied to and passed through a rotary furnace of a rotary kiln (manufactured by Sanai Kanetsyo Co., Ltd.) for heat treatment. The rotary furnace was heated so that the heating temperature of agglomerate 1 was 250°C. The rotary furnace was rotated at a rotation speed of 7 rpm so that the residence time (heating time) of the agglomerate in the rotary furnace was 30 minutes. The agglomerate was introduced at a rate such that the filling rate of the agglomerate in the rotary furnace was 10%, and the inclination angle of the rotary furnace with respect to the horizontal direction was 0.2°. The hardness of the agglomerate after passing through the rotary furnace was 0.41 N / mm.

[0059] Next, the obtained aggregate was pulverized in a counter jet mill (manufactured by Hosokawa Micron Corporation) at a grind pressure and a push pressure of 0.65 MPa to obtain Powder 1. The D50 of the obtained powder 1 was 2 μm and the specific surface area was 3 m 2 / g. The particle size distribution of Powder 1 was unimodal, with a full width at half maximum of 2.1 μm.

[0060] [Example 2] Powder 2 was obtained in the same manner as in Example 1, except that Agglomerate 2 was used. The D50 of the obtained powder 2 was 4 μm and the specific surface area was 5 m 2 / g. The particle size distribution of Powder 2 was unimodal, with a full width at half maximum of 2.5 μm. [Example 3] Powder 3 was obtained in the same manner as in Example 1, except that the rotary furnace was heated so that the heating temperature of Aggregate 1 became 200°C. The D50 of the obtained powder 3 was 2 μm and the specific surface area was 6 m 2 / g. The particle size distribution of Powder 3 was unimodal, with a full width at half maximum of 2.8 μm.

[0061] [Example 4 (Comparative Example)] Powder 4 was obtained in the same manner as in Example 1, except that the rotary furnace was not heated. The D50 of the obtained powder 4 was 2 μm and the specific surface area was 12 m 2 / g. The particle size distribution of Powder 4 was broad. [Example 5 (Comparative Example)] Powder 5 was obtained in the same manner as in Example 1, except that the rotary furnace was operated so that the heating temperature of Aggregate 1 was 330° C. and the residence time was 1 minute. The D50 of the obtained powder 5 is 5 μm and the specific surface area is 6 m 2 / g. The particle size distribution of Powder 5 was bimodal. [Example 6 (Comparative Example)] Powder 6 was obtained in the same manner as in Example 1, except that Aggregate 1 was heated in an oven to a heating temperature of 250°C. The D50 of the obtained powder 6 was 24 μm and the specific surface area was 6 m 2 / g. The particle size distribution of Powder 6 was bimodal.

[0062] 3. Preparation of Powder Dispersion 55 parts by mass of each of the powders 1 to 6, 2 parts by mass of a surfactant (Neos Corporation, "Ftergent 250"), and 43 parts by mass of N-methyl-2-pyrrolidone (NMP) were placed in a ball mill, ceramic balls were added, and the mixture was mixed for 1 hour to prepare powder dispersions 1 to 6.

[0063] 4. Measurement and Evaluation 4-1. Hardness measurement of aggregates The aggregates were compressed at 0.5 mm / min using a Strograph (manufactured by Toyo Seiki Co., Ltd.), and the maximum load was taken as the hardness of the aggregates.

[0064] 4-2.Measurement of powder specific surface area The specific surface area of ​​each of Powders 1 to 6 was measured by the gas adsorption (constant volume method) BET multipoint method using a NOVA4200e (manufactured by Quantachrome Instruments) as the measuring device. As a pretreatment, each of Powders 1 to 6 was vacuum degassed at 200°C for 30 minutes. Then, nitrogen gas was introduced while maintaining a constant temperature with liquid nitrogen to create an adsorption isotherm. The specific surface area of ​​each of Powders 1 to 6 was determined at a point of relative pressure with good linearity.

[0065] 4-3. Measuring D50 of powder The particle size distribution of each of Powders 1 to 6 was measured using a laser interferometer (manufactured by Horiba, Ltd., "LA-960V2"), and D50 was calculated. The particle size distribution was defined as unimodal when it had one maximum value, and as bimodal when it had two maximum values. In general, bimodality is a phenomenon observed when such a particle size distribution does not actually exist and the powder contains irregularly shaped particles.

[0066] 4-4. Evaluation of the cohesion of powder dispersions Each of the powder dispersions 1 to 6 was passed through a metal mesh with a mesh size of 50 μm. The solid matter captured by the metal mesh was then dried and its mass was measured. The capture rate was calculated from the mass of the powder contained in the original powder dispersion according to the following formula, and the cohesion was evaluated according to the following criteria. Formula: Capture rate = mass of solid matter captured by the metal mesh / mass of powder contained in the original powder dispersion [Evaluation criteria] 〇: Capture rate is less than 5% △: Capture rate is between 5% and 10% ×: Capture rate is 10% or more

[0067] 4-5. Viscosity measurement of powder dispersion liquid The viscosity of each of the Powder Dispersions 1 to 6 was measured using a B-type viscometer (manufactured by Eiko Seiki Co., Ltd., "DV2T") with an RV-2 spindle rotating at a rotation speed of 60 rpm.

[0068] 4-6. Evaluation of thickening properties of powder dispersion Each of the powder dispersions 1 to 6 was stirred at 8000 rpm for 30 minutes using a homomixer, and the viscosity was measured in the same manner as in "4-5." The viscosity increase rate of the dispersion before and after stirring was calculated using the following formula, and evaluated according to the following criteria. Formula: Viscosity increase rate = {(viscosity after stirring / viscosity before stirring)-1} x 100 [Evaluation criteria] 〇: Viscosity increase rate is less than 40% △: Viscosity increase rate is 40% or more but less than 60% ×: Viscosity increase rate is 60% or more

[0069] 4-7. Evaluation of surface smoothness of laminate Powder Dispersion 1 was applied to the surface of a stainless steel plate using an applicator to form a wet film. The stainless steel plate on which the wet film was formed was then passed through a drying oven at 100°C for 5 minutes and dried by heating to obtain a dry film. The dry film was then heated at 350°C for 5 minutes in a nitrogen oven. This resulted in Laminate 1, which had a stainless steel plate and a polymer layer as a molded product containing a molten and baked product of Powder 1 on its surface. Laminates 2 to 6 were obtained in the same manner as for laminate 1, except that powder dispersion 1 was changed to powder dispersions 2 to 6.

[0070] For each of the laminates 1 to 6, the arithmetic mean roughness (Ra) of the polymer layer surface was measured using a surface roughness meter (Surfcom NEX100, manufactured by Tokyo Seimitsu Co., Ltd.) in accordance with JIS B0601:2013 (ISO4287:1997, Amd.1:2009). The reference length lr (cutoff value λc) for the roughness curve when determining Ra was set to 0.8 mm. [Evaluation criteria] ○: Ra is less than 0.4 μm △: Ra is 0.4 μm or more and less than 0.6 μm ×: Ra is 0.6 μm or more These results are shown in Table 1 below.

[0071] [Table 1] [Industrial Applicability]

[0072] The powder and powder dispersion of the present invention can be used to produce molded articles such as films, impregnated materials (prepregs, etc.), and laminates (metal laminates such as copper-clad laminates), and can be used to produce molded articles for applications requiring mold releasability, electrical properties, water and oil repellency, chemical resistance, weather resistance, heat resistance, slip properties, abrasion resistance, etc. Molded articles formed from the dispersion of the present invention are useful as antenna parts, printed circuit boards, aircraft parts, automobile parts, sports equipment, food industry supplies, paints, cosmetics, etc., and specifically useful as electric wire coating materials, electrical insulating tape, insulating tape for oil drilling, materials for printed circuit boards, separators, battery materials (electrode binder materials for lithium secondary batteries, fuel cells, etc.), copy rolls, furniture, automobile dashboards, covers for home appliances, sliding parts (load bearings, sliding shafts, valves, bearings, gears, cams, belt conveyors, food transport belts, etc.), tools (shovels, files, saws, saws, etc.), boilers, hoppers, pipes, ovens, baking molds, chutes, dies, toilets, and container coating materials.

Claims

1. The aggregates of the heat-fusible tetrafluoroethylene-based polymer are fluidized and heat-treated at a temperature equal to or lower than the melting temperature of the tetrafluoroethylene-based polymer, and then pulverized to produce aggregates having an average particle size of more than 1 μm and 10 μm or less and a specific surface area of ​​1 m. ２ / g or more 8m ２ The method for producing a powder of the tetrafluoroethylene-based polymer is characterized in that the powder has a unimodal particle size distribution of less than 1 / g.

2. The method according to claim 1 , wherein the tetrafluoroethylene-based polymer is a tetrafluoroethylene-based polymer containing units based on perfluoro(alkyl vinyl ether) or units based on hexafluoropropylene.

3. The method according to claim 1 or 2, wherein the tetrafluoroethylene-based polymer is a tetrafluoroethylene-based polymer having a polar functional group.

4. The method according to any one of claims 1 to 3, wherein the average particle size of the aggregates is 100 µm to 5 mm.

5. The method according to any one of claims 1 to 4, wherein the temperature during the heat treatment of the aggregate is higher than the melting point of the tetrafluoroethylene-based polymer minus 100°C.

6. The method according to any one of claims 1 to 5, wherein the agglomerate is heat-treated while being tumbled in a rotary furnace that rotates about a central axis.

7. The method according to claim 6, wherein the rotary furnace has an inclination angle of 0.01 to 5° with respect to the horizontal direction.

8. The method according to claim 6 or 7, wherein the rotation speed of the rotary furnace is 1 to 20 rpm.

9. The method according to any one of claims 6 to 8, wherein the residence time of the agglomerate in the rotary furnace is 1 to 60 minutes.

10. The method according to any one of claims 1 to 9, wherein the agglomerate has a hardness of 0.2 N / mm or more before being pulverized.

11. The method according to any one of claims 1 to 10, wherein the aggregate is an aggregate of particles of the tetrafluoroethylene-based polymer formed by polymerization of raw material monomers of the tetrafluoroethylene-based polymer.

12. A powder of a heat-fusible tetrafluoroethylene polymer having an average particle size of more than 1 μm and not more than 10 μm and a specific surface area of ​​1 m ２ / g or more 8m ２ / g, and the full width at half maximum of the particle size distribution is 0.5 to 3.5 μm.

13. A powder dispersion comprising the powder according to claim 12 and a liquid dispersion medium.

14. 14. The powder dispersion according to claim 13, wherein the viscosity of the powder dispersion is 1000 mPa·s or less.

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