Polytetrafluoroethylene powder production method, polytetrafluoroethylene powder, and molded article

Abstract

The purpose of the present disclosure is to provide: polytetrafluoroethylene powder production method that makes it possible to remove irregularly shaped particles and coarse particles while suppressing clogging of a device and adhesion of powder to the device during classification; a polytetrafluoroethylene powder; and a molded article obtained by using the same. The present disclosure is a polytetrafluoroethylene powder production method comprising an air sieve classification step for removing, via air classification using a sieve mesh, irregularly shaped particles and coarse particles from a polytetrafluoroethylene powder to be processed.

Description

Method for producing polytetrafluoroethylene powder, polytetrafluoroethylene powder, and molded body

[0001] The present disclosure relates to a method for producing polytetrafluoroethylene powder, the polytetrafluoroethylene powder, and a molded article.

[0002] Polytetrafluoroethylene (PTFE) molding powder is a PTFE powder obtained by suspension polymerization of tetrafluoroethylene (TFE), and is used as a molding material for various molded products.

[0003] Patent Document 1 describes that PTFE molding powder is obtained by crushing granular PTFE with a cutter mill and then crushing it with a jet mill.

[0004] International Publication No. 2014 / 123075

[0005] The present disclosure aims to provide a method for producing PTFE powder that can remove irregularly shaped particles and coarse particles while suppressing adhesion of powder to the device and clogging of the device during classification, as well as a PTFE powder and a molded body obtained using the same.

[0006] The present disclosure (1) is a method for producing polytetrafluoroethylene powder, including an air flow sieve classification step of removing irregularly shaped particles and coarse particles from a treated polytetrafluoroethylene powder by air flow classification using a sieve.

[0007] The present disclosure (2) is the manufacturing method according to the present disclosure (1), wherein the mesh size of the sieve in the airflow sieve classification step is 100 to 2000 μm.

[0008] The present disclosure (3) is directed to a method in which the air flow rate in the air flow sieve classification step is 20 to 200 Nm ３ / min・m ２ This is the manufacturing method described in the present disclosure (1) or (2).

[0009] The present disclosure (4) is a method for producing an arbitrary combination with any of the present disclosures (1) to (3), in which the standard specific gravity of the polytetrafluoroethylene is 2.130 to 2.280.

[0010] The present disclosure (5) is a manufacturing method of any combination with any of the present disclosures (1) to (4), in which the average particle size of the powder to be treated is 1 to 200 μm.

[0011] The present disclosure (6) is a method for producing a sieve having an opening of 300 to 500 μm in the airflow sieve classification step, and an air flow rate of 50 to 100 Nm ３ / min・m ２ and a manufacturing method for optionally combining with any one of the present disclosures (1) to (5), wherein the average particle diameter of the powder to be treated is 25 to 60 μm.

[0012] The present disclosure (7) provides a granular material having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g / ml, and a specific surface area of ​​1.0 to 5.0 m ２ / g and an average circularity of 0.60 or more.

[0013] The present disclosure (8) is directed to a method for producing a granular ... ２ / g.

[0014] The present disclosure (9) provides a granular material having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g / ml, and a specific surface area of ​​1.0 to 5.0 m ２ / g and is substantially free of irregular particles having an area-equivalent diameter of 200 μm or more and an irregularity of 2.5 or more, and coarse particles having an area-equivalent diameter of 300 μm or more and an irregularity of less than 2.5.

[0015] The present disclosure (10) is the polytetrafluoroethylene powder according to the present disclosure (9), in which the amount of the irregular-shaped particles is 0.001% by mass or less relative to the polytetrafluoroethylene powder.

[0016] The present disclosure (11) is the polytetrafluoroethylene powder according to the present disclosure (9) or (10), in which the amount of the coarse particles is 0.001% by mass or less relative to the polytetrafluoroethylene powder.

[0017] The present disclosure (12) is directed to a method for producing a granular ... ２ / g and any combination of any of the polytetrafluoroethylene powders of the present disclosure (9) to (11).

[0018] The present disclosure (13) is a molded article obtained by molding a polytetrafluoroethylene powder in any combination with any of the present disclosures (7) to (12).

[0019] The present disclosure (14) is the molded body according to the present disclosure (13), which is a sheet.

[0020] According to the present disclosure, it is possible to provide a method for producing PTFE powder that can remove irregularly shaped particles and coarse particles while suppressing adhesion of powder to the device and clogging of the device during classification, as well as a PTFE powder and a molded body obtained using the same.

[0021] 1A and 1B are schematic diagrams showing examples of irregular-shaped particles.

[0022] PTFE powder contains irregularly shaped particles and coarse particles that cannot be completely removed by conventional classification methods, and when processed into molded products such as sheets, these particles cause defects such as cracks and reduce strength.After extensive research, it was discovered that the above problems could be solved by implementing a specific classification process.

[0023] The present disclosure will be specifically described below.

[0024] The present disclosure relates to a method for producing PTFE powder, which includes an air flow sieve classification step in which irregularly shaped particles and coarse particles are removed from a treated PTFE powder by air flow classification using a sieve mesh.

[0025] PTFE powder has high cohesiveness and adhesiveness, and in conventional classification methods using a sieve, irregularly shaped particles and coarse particles adhere to the sieve or become clogged, making it difficult to remove them. In contrast, the manufacturing method of the present disclosure includes the airflow sieve classification step, so it is possible to manufacture PTFE powder from which irregularly shaped particles and coarse particles that cause defects and reduced strength in molded products have been removed, without causing problems such as clogging.

[0026] In the airflow sieve classification process, irregularly shaped particles and coarse particles are removed from the treated PTFE powder by airflow classification using a sieve. The treated powder may contain the irregularly shaped particles and coarse particles. PTFE has high cohesiveness and adhesiveness, making classification using conventional sieves difficult. In the manufacturing method disclosed herein, the airflow sieve classification process can remove irregularly shaped particles and coarse particles while suppressing adhesion and clogging.

[0027] The powder to be treated that is subjected to the air sieve classification step may contain more than 0.005% by mass of the irregular-shaped particles relative to the powder to be treated. Also, the powder to be treated that is subjected to the air sieve classification step may contain more than 0.005% by mass of the coarse particles relative to the powder to be treated.

[0028] In the air sieve classification step, at least a portion of the irregular-shaped particles is removed, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, still more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass of the irregular-shaped particles contained in the powder to be treated in the air sieve classification step is removed.

[0029] In the air sieve classification step, at least a portion of the coarse particles is removed, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, still more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass of the coarse particles contained in the powder to be treated that is subjected to the air sieve classification step is removed.

[0030] The irregular particles are particles having an area-equivalent diameter of 200 μm or more and an irregularity of 2.5 or more. The coarse particles are particles having an area-equivalent diameter of 300 μm or more and an irregularity of less than 2.5. The area-equivalent diameter is a value obtained by photographing particles at 20 to 200 magnifications using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), determining the projected area of ​​the particle through image analysis, and calculating the diameter of a perfect circle having an area equivalent to this area value. The irregularity is a value obtained by photographing particles at 20 to 200 magnifications using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000) and analyzing the images to determine the diameter of the perfect circle circumscribing the particle (circumscribing circle diameter) and the diameter of the perfect circle inscribing the particle (inscribing circle diameter), and then dividing the circumscribing circle diameter by the inscribing circle diameter.

[0031] The shape of the irregular-shaped particles may be film-like, fibrous, string-like, strip-like, compressed piece-like, rod-like, etc. Examples of the irregular-shaped particles are shown in Figure 1. In Figure 1(a), irregular-shaped particle 1 is film-like. In irregular-shaped particle 1, the circumscribing circle is indicated by 11, and the inscribing circle is indicated by 12. In Figure 1(b), irregular-shaped particle 2 is string-like. In irregular-shaped particle 2, the circumscribing circle is indicated by 13, and the inscribing circle is indicated by 14.

[0032] The irregularly shaped particles may be generated, for example, during the manufacturing or transport process of PTFE particles. The irregularly shaped particles have a large air resistance and cannot be sufficiently removed by ordinary air classification.

[0033] In the airflow classification using a sieve screen (airflow sieve classification), particles dispersed in an airflow are classified by passing them through a sieve screen. The airflow sieve classification can be performed, for example, using an airflow classifier equipped with a sieve screen. As the airflow classifier equipped with a sieve screen, a known classifier such as a blow-through classifier can be used.

[0034] The sieve may be made of metal such as stainless steel or resin such as nylon. The sieve may be in the form of a woven mesh, a microsieve, or the like. The sieve may be flat.

[0035] The mesh size of the sieve can be selected depending on the particle size and particle size distribution of the target PTFE particles, adhesion to the sieve, etc., and in one embodiment of the present disclosure, may be 100 to 2000 μm. The mesh size may also be 200 μm or more, 300 μm or more, or 1500 μm or less, 1000 μm or less, 750 μm or less, 500 μm or less, etc.

[0036] The air flow rate in the airflow sieve classification step can be selected depending on the scale of the equipment, etc., but in one embodiment of the present disclosure, it is 20 to 200 Nm ３ / min・m ２ The air flow rate may also be 40 Nm ３ / min・m ２ Above, 50Nm ３ / min・m ２ or more, 150 Nm ３ / min・m ２ Below, 100Nm ３ / min・m ２ The following may also be used:

[0037] The temperature of the air in the air flow sieve classification step is preferably −200° C. or higher, more preferably −100° C. or higher, and even more preferably 5° C. or higher, from the viewpoint of energy efficiency, and is preferably 40° C. or lower, more preferably 20° C. or lower, and even more preferably 10° C. or lower, from the viewpoint of easily suppressing adhesion and aggregation of particles.

[0038] The temperature of the powder to be treated supplied in the air sieve classification step is preferably −100° C. or higher, and even more preferably 5° C. or higher, from the viewpoint of efficiently removing irregularly shaped particles and coarse particles, and is preferably 40° C. or lower, more preferably 20° C. or lower, and even more preferably 10° C. or lower.

[0039] The processing speed of the powder to be processed in the air sieve classification step can be selected depending on the scale of the equipment, etc., but in one embodiment of the present disclosure, it is 250 to 5000 kg / h m ２ It may be.

[0040] The average particle size of the powder to be treated that is subjected to the air sieve classification step is preferably 1 to 200 μm. When attempting to classify such small PTFE particles using a sieve, the particles aggregate and adhere to the sieve, causing clogging and making classification difficult. By performing the air sieve classification step, adhesion and clogging are less likely to occur even with small particle sizes. The average particle size can be selected according to the molding method, the use of the molded body, and other objectives, and may be, for example, 10 μm or more, 15 μm or more, 20 μm or more, 25 μm or more, or 170 μm or less, 150 μm or less, 100 μm or less, 70 μm or less, 60 μm or less, etc. The average particle size is measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and is defined as being equal to the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

[0041] In one aspect of the present disclosure, the PTFE is preferably a high molecular weight PTFE. High molecular weight PTFE usually has fibrillating properties and has particularly high cohesiveness and adhesiveness. In the manufacturing method of the present disclosure, even if a high molecular weight PTFE with high cohesiveness and adhesiveness is used, adhesion to the device during classification and clogging of the device are unlikely to occur.

[0042] The high molecular weight PTFE may have a standard specific gravity (SSG) of 2.130 to 2.280. The standard specific gravity may also be 2.140 or more, 2.150 or more, or 2.230 or less, 2.200 or less, 2.180 or less, etc. The standard specific gravity is measured by the water displacement method in accordance with ASTM D792 using a sample molded in accordance with ASTM D4894. The term "high molecular weight" for PTFE means that the standard specific gravity is within the above range.

[0043] The high-molecular-weight PTFE generally has non-melt-fabricability, which means that the melt flow rate cannot be measured at a temperature higher than the melting point in accordance with ASTM D-1238 and D-2116, in other words, the PTFE does not flow easily even in the melting temperature range.

[0044] The high molecular weight PTFE may be a homopolymer of TFE, or may be a modified PTFE that comprises a polymerized unit (TFE unit) based on TFE and a polymerized unit (modified monomer unit) based on modified monomer.The modified PTFE may comprise 99.0 mass% or more of TFE unit and 1.0 mass% or less of modified monomer unit.Also, the modified PTFE may be composed only of TFE unit and modified monomer unit.

[0045] The content of the modified monomer unit in the modified PTFE is preferably in the range of 0.00001 to 1.0% by mass relative to the total polymerized units. The lower limit of the content of the modified monomer unit is more preferably 0.0001% by mass, even more preferably 0.001% by mass, even more preferably 0.005% by mass, and particularly preferably 0.010% by mass. The upper limit of the content of the modified monomer unit is preferably 0.90% by mass, more preferably 0.50% by mass, even more preferably 0.40% by mass, even more preferably 0.30% by mass, even more preferably 0.20% by mass, even more preferably 0.15% by mass, and particularly preferably 0.10% by mass. In this specification, the modified monomer unit refers to a part of the molecular structure of PTFE that is derived from the modified monomer.

[0046] The content of each of the above-mentioned polymerized units can be calculated by appropriately combining NMR, FT-IR, elemental analysis, and X-ray fluorescence analysis depending on the type of monomer.

[0047] The above-mentioned modified monomer is not particularly limited as long as it can be copolymerized with TFE, and for example, can be listed perfluoroolefin such as hexafluoropropylene (HFP); hydrogen-containing fluoroolefin such as trifluoroethylene, vinylidene fluoride (VDF); perhaloolefin such as chlorotrifluoroethylene; perfluorovinyl ether; perfluoroallyl ether; (perfluoroalkyl) ethylene, ethylene etc. In addition, the modified monomer used can be one kind or multiple kinds.

[0048] The perfluorovinyl ether is not particularly limited, and examples thereof include perfluorovinyl ethers represented by the following general formula (A): CF ２═CF-ORf (A) (wherein Rf represents a perfluoroorganic group). In this specification, the term "perfluoroorganic group" refers to an organic group in which all hydrogen atoms bonded to carbon atoms are substituted with fluorine atoms. The perfluoroorganic group may have an ether oxygen.

[0049] An example of the perfluorovinyl ether is perfluoro(alkyl vinyl ether) [PAVE], where Rf in the general formula (A) is a perfluoroalkyl group having 1 to 10 carbon atoms. The number of carbon atoms in the perfluoroalkyl group is preferably 1 to 5.

[0050] Examples of the perfluoroalkyl group in the PAVE include a perfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, a perfluorobutyl group, a perfluoropentyl group, and a perfluorohexyl group.

[0051] The perfluorovinyl ether further includes those in which Rf in the general formula (A) is a perfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, and those in which Rf ... the following formula:

[0052]

[0053] (wherein m represents 0 or an integer of 1 to 4), and Rf is a group represented by the following formula:

[0054]

[0055] (wherein n represents an integer of 1 to 4).

[0056] The (perfluoroalkyl)ethylene (PFAE) is not particularly limited, and examples thereof include (perfluorobutyl)ethylene (PFBE) and (perfluorohexyl)ethylene.

[0057] Examples of perfluoroallyl ethers include those represented by the general formula (B): CF ２ =CF-CF ２ -ORf １ (B) (wherein, Rf １represents a perfluoroorganic group.

[0058] The above Rf １ is preferably a perfluoroalkyl group having 1 to 10 carbon atoms or a perfluoroalkoxyalkyl group having 1 to 10 carbon atoms. ２ =CF-CF ２ -O-CF ３ , C.F. ２ =CF-CF ２ -O-C ２ F ５ , C.F. ２ =CF-CF ２ -O-C ３ F ７ , and CF ２ =CF-CF ２ -O-C ４ F ９ At least one selected from the group consisting of CF ２ =CF-CF ２ -O-C ２ F ５ , C.F. ２ =CF-CF ２ -O-C ３ F ７ , and CF ２ =CF-CF ２ -O-C ４ F ９ More preferably, at least one selected from the group consisting of CF ２ =CF-CF ２ -O-CF ２ CF ２ CF ３ is more preferred.

[0059] The high-molecular-weight PTFE can be produced by suspension polymerization or emulsion polymerization. The high-molecular-weight PTFE is preferably obtained by suspension polymerization, and the PTFE powder obtained by the production method of the present disclosure is preferably a PTFE molding powder.

[0060] The suspension polymerization can be carried out by a known method. For example, by dispersing a polymerization initiator in an aqueous medium without using an anionic fluorine-containing surfactant or using a limited amount of anionic fluorine-containing surfactant, and polymerizing the monomers necessary to constitute the PTFE, the suspension-polymerized particles of PTFE can be directly isolated.

[0061] The PTFE is preferably low-molecular-weight PTFE. Since the low-molecular-weight PTFE powder is a fine particle (for example, a particle size of 10 μm or less), the weight of the particles relative to the contact point is small and they are unlikely to fall through the sieve. In the manufacturing method of the present disclosure, even if low-molecular-weight PTFE is used, adhesion to the device during classification and clogging of the device are unlikely to occur.

[0062] The low-molecular-weight PTFE has a melt viscosity of 1.0×10 at 380° C. ２ ~7.0 x 10 ５ In the present disclosure, "low molecular weight" means that the melt viscosity is within the above range. The melt viscosity is preferably 1.5 × 10 ３ It is preferable that the viscosity is 3.0×10 Pa s or more. ５ It is preferable that the viscosity is 1.0×10 Pa·s or less. ５ It is more preferable that the viscosity is Pa·s or less.

[0063] The melt viscosity is a value measured in accordance with ASTM D 1238 using a flow tester (manufactured by Shimadzu Corporation) and a 2φ-8L die, with a 2 g sample preheated at 380°C for 5 minutes and maintained at the above temperature under a load of 0.7 MPa.

[0064] The low-molecular-weight PTFE preferably has a melting point of 320 to 340°C, more preferably 324°C or higher, and more preferably 336°C or lower.

[0065] The melting point is measured using a differential scanning calorimeter (DSC) by calibrating the temperature in advance using indium and lead as standard samples, placing about 3 mg of low-molecular-weight PTFE in an aluminum pan (crimp container), and raising the temperature at 10°C / min in a temperature range of 250 to 380°C under an air flow of 200 ml / min, and the minimum point of the heat of fusion in the above range is taken as the melting point.

[0066] Above-mentioned low molecular weight PTFE can be TFE homopolymer, or can be the modified PTFE that comprises the polymerization unit (TFE unit) based on TFE and the polymerization unit (modified monomer unit) based on modified monomer.The type of modified monomer and the content of modified monomer unit can be the same as those of above-mentioned high molecular weight PTFE.

[0067] The low-molecular-weight PTFE can be produced by direct polymerization using methods such as emulsion polymerization or suspension polymerization, or by thermal decomposition or radiation decomposition of high-molecular-weight PTFE. The low-molecular-weight PTFE is preferably produced by direct polymerization, more preferably by emulsion polymerization.

[0068] The manufacturing method of the present disclosure may include a pulverization step of pulverizing PTFE. When the PTFE is a high-molecular-weight PTFE obtained by suspension polymerization, it is particularly preferable to perform the pulverization step. The pulverization step is preferably performed before the air sieve classification step. For example, the PTFE obtained in the polymerization step, washing step, or drying step can be used as the PTFE to be subjected to the pulverization step, and the PTFE obtained in the drying step can be preferably used. In one preferred embodiment, the manufacturing method of the present disclosure includes the polymerization step, washing step, drying step, and pulverization step, and the dried PTFE is used as the PTFE in the pulverization step.

[0069] The PTFE pulverized in the pulverization step is preferably particles with an average particle size of 100 to 800 μm. The average particle size is more preferably 750 μm or less, and even more preferably 700 μm or less. The average particle size is measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and is defined as the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

[0070] The pulverization in the pulverization step may be dry pulverization, and can be carried out, for example, by a pulverizer such as a hammer mill, pin mill, jet mill, or cutter mill.

[0071] The average particle size of the pulverized particles obtained in the pulverization step can be determined depending on the application, but in one embodiment of the present disclosure, it may be 1 to 200 μm. The average particle size is measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and is defined as being equal to the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

[0072] The manufacturing method of the present disclosure may include an air classification step in which coarse particles are removed from the treated PTFE powder by air classification. When the PTFE is a high-molecular-weight PTFE obtained by suspension polymerization, it is particularly preferable to perform the air classification step. The air classification step is preferably performed before the air sieve classification step. When the manufacturing method of the present disclosure includes the pulverization step, the air classification step may be performed after the pulverization step, or may be performed simultaneously with the pulverization step from the viewpoint of improving production efficiency and suppressing adhesion of PTFE particles. Since coarse particles can be removed in the air sieve classification step, the air classification step is not essential. However, by removing coarse particles to a certain extent in the air classification step and then removing the coarse particles that were not completely removed in the air classification step in the air sieve classification step, coarse particles can be removed more efficiently.

[0073] In the air classification step, at least a portion of the coarse particles is removed, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 97% by mass or more, still more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass of the coarse particles contained in the powder to be treated that is subjected to the air classification step is removed.

[0074] The removed coarse particles may be subjected to the pulverization step again.

[0075] The air classification can be carried out using, for example, an air classifier. The air classification may not use the sieve mesh described above. In an air classifier, particles are sent by an air current to a classification chamber (e.g., a cylindrical classification chamber), where they are dispersed by the swirling air current within the chamber and classified by centrifugal force. The particles are collected from the center into a cyclone and a bag filter. A rotating body such as a cone or rotor is installed in the classification chamber to ensure uniform swirling motion of the particles and air.

[0076] When using classification cones, the classification point is adjusted by adjusting the volume of air introduced into the classification chamber and the gap between the classification cones. When using rotors, the classification point is adjusted by the rotor rotation speed and the volume of air introduced.

[0077] The air classifier preferably includes a rotor (classification rotor) for classifying particles. The classification rotor preferably has openings such as slits that allow particles of a predetermined size to pass through. The size of the particles that pass through can be adjusted by the size of the openings of the classification rotor, the rotation speed, and the volume of air passing through the rotor. The coarse particles do not pass through the classification rotor.

[0078] The opening size of the classifying rotor can be determined depending on the target classification point, and may be, for example, 5 mm or more or 50 mm or less. The opening size of the classifying rotor may be larger than the opening size of the sieve mesh used in the air sieve classification step described above.

[0079] The rotation speed of the classification rotor can be determined depending on the target classification point, and may be, for example, 500 rpm or more and 10,000 rpm or less.

[0080] The air flow rate in the air classification step can be determined depending on the target classification point, for example, 1 Nm ３ / min・m ２ It may be 100 Nm or more. ３ / min・m ２ It may be the following:

[0081] The air classifier is preferably one having both a pulverizing function and an air classifying function, since the pulverizing step and the air classifying step can be carried out simultaneously. As the air classifier having both a pulverizing function and an air classifying function, a known one can be used.

[0082] In the air classification step, the coarse particles are removed to obtain classified particles, which are the fine particles (e.g., particles that have passed through the classifying rotor) among the particles separated into the coarse particles and the fine particles by the air classification.

[0083] In one embodiment of the present disclosure, the average particle size of the classified particles may be 1 to 200 μm. The average particle size is measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and is defined as being equal to the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

[0084] The manufacturing method of the present disclosure may include a sieve classification step using a sieve mesh with openings that do not clog, as necessary.

[0085] The manufacturing method of the present disclosure provides a PTFE powder that is substantially free of the above-mentioned irregularly shaped particles and coarse particles. "Substantially free of irregularly shaped particles" means that the amount of the irregularly shaped particles is 0.005% by mass or less relative to the PTFE powder. The amount of the irregularly shaped particles is preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, relative to the PTFE powder. "Substantially free of coarse particles" means that the amount of the coarse particles is 0.005% by mass or less relative to the PTFE powder. The amount of the coarse particles is preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, relative to the PTFE powder.

[0086] According to the manufacturing method of the present disclosure, the following PTFE powders can also be manufactured. The present disclosure also relates to these PTFE powders: (1) an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g / ml, and a specific surface area of ​​1.0 to 5.0 m ２(2) A PTFE powder having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g / ml, and a specific surface area of ​​1.0 to 5.0 m ２ / g and substantially free of irregular particles having an area-equivalent diameter of 200 μm or more and an irregularity of 2.5 or more, and coarse particles having an area-equivalent diameter of 300 μm or more and an irregularity of less than 2.5 (hereinafter also referred to as the second PTFE powder).

[0087] The first and second PTFE powders make it possible to obtain a molded article with few defects such as cracks and excellent strength.

[0088] In one embodiment of the present disclosure, the first and second PTFE powders are preferably high-molecular-weight PTFE powders, and more preferably high-molecular-weight PTFE powders (PTFE molding powders) obtained by suspension polymerization. Also, in one embodiment of the present disclosure, the first and second PTFE powders are preferably low-molecular-weight PTFE. The high-molecular-weight PTFE and low-molecular-weight PTFE are as described above.

[0089] The first and second PTFE powders have an average particle size of 1 to 200 μm. This results in excellent compactness of the molded body and ease of handling as a powder. The average particle size may be determined depending on the application, and may be, for example, 10 μm or more, 15 μm or more, 20 μm or more, 25 μm or more, or 170 μm or less, 150 μm or less, 100 μm or less, 70 μm or less, 60 μm or less, etc. The average particle size is measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and is defined as the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

[0090] The first and second PTFE powders have an apparent density of 0.20 to 0.55 g / ml. This results in excellent compactness of the molded body and ease of handling as a powder. The apparent density may be 0.20 g / ml or more, 0.25 g / ml or more, or 0.50 g / ml or less, 0.45 g / ml or less, etc. The apparent density is measured in accordance with JIS K 6891.

[0091] The first and second PTFE powders have a specific surface area of ​​1.0 to 5.0 m ２ This results in excellent fusion properties during firing and excellent removal of impurities. ２ / g or more, 1.0m ２ / g or more, 1.5m ２ / g or more, and 4.0m ２ / g or less, 3.0m ２ The specific surface area is measured by the BET method using a surface analyzer (product name: BELSORP-mini II, manufactured by Microtrac-Bell Corporation) and a mixed gas of 30% nitrogen and 70% helium as a carrier gas, with liquid nitrogen for cooling.

[0092] The first PTFE powder has an average circularity of 0.60 or more. This means that the PTFE powder is substantially free of the irregularly shaped particles and coarse particles described above. The average circularity is preferably 0.65 or more, more preferably 0.70 or more, and even more preferably 0.75 or more. The average circularity is usually less than 1.0, and may be 0.9 or less, or even 0.8 or less. The average circularity is the average circularity measured by dispersing a target powder sample on a flat plate and measuring 100 randomly selected particles using the following procedure. The circularity is defined as B / A, where A is the perimeter of the projected image of the particle, and B is the diameter of a perfect circle having an area equivalent to the projected area. This value gradually approaches 1.0 as the particle approaches a perfect circle.

[0093] The second PTFE powder is substantially free of irregular-shaped particles having an area-equivalent diameter of 200 μm or more and an irregularity of 2.5 or more. The area-equivalent diameter and irregularity are as described above. "Substantially free of irregular-shaped particles" means that the amount of the irregular-shaped particles is 0.005% by mass or less relative to the PTFE powder. The amount of the irregular-shaped particles is preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, relative to the PTFE powder.

[0094] The second PTFE powder is substantially free of coarse particles having an area-equivalent diameter of 300 μm or more and an irregularity degree of less than 2.5. "Substantially free of coarse particles" means that the amount of coarse particles is 0.005% by mass or less relative to the PTFE powder. The amount of coarse particles is preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, relative to the PTFE powder.

[0095] In one embodiment of the present disclosure, the first and second PTFE powders may have a particle size X10 of 0.1 to 100 μm. The X10 is equal to the particle size corresponding to 10% of the integrated particle size distribution (volume basis) measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar.

[0096] In one embodiment of the present disclosure, the first and second PTFE powders may have a particle diameter X90 of 5 to 800 μm. The X90 is equal to the particle diameter corresponding to 90% of the integrated particle size distribution (volume basis) measured using a laser diffraction particle size distribution analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar.

[0097] The PTFE powder obtained by the manufacturing method of the present disclosure, and the first and second PTFE powders of the present disclosure, can be suitably used as molding materials. The present disclosure also relates to a molded article obtained by molding the first and second PTFE powders. The molded article of the present disclosure has few defects such as cracks and is excellent in strength.

[0098] The method for molding the PTFE powder is not particularly limited, but examples include compression molding, ram extrusion molding, isostatic molding, etc. Of these, compression molding is preferred.

[0099] The compression molding can be carried out, for example, by maintaining a pressure of 10 to 50 MPa for 1 minute to 30 hours.

[0100] After the molding, the product may be calcined, for example, by heating at a temperature of 350 to 380° C. for 0.5 to 50 hours.

[0101] After the firing, the product may be cut and processed into a shape such as a sheet.

[0102] The shape of the molded article of the present disclosure is not particularly limited, and examples thereof include a sheet shape, a film shape, a rod shape, a pipe shape, and a fiber shape.

[0103] The molded article of the present disclosure preferably has a tensile breaking strength of 15 MPa or more, more preferably 20 MPa or more, and even more preferably 25 MPa or more. The upper limit is not particularly limited, but may be, for example, 75 MPa, 70 MPa, or 65 MPa. The tensile breaking strength is measured in accordance with ASTM D1708.

[0104] The molded article of the present disclosure can be suitably used as a lining sheet, an insulating film, a valve, a gasket, a bellows, a semiconductor chemical tank, and the like.

[0105] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims.

[0106] The present disclosure will now be described in more detail with reference to examples, but the present disclosure is not limited to these examples.

[0107] Various physical properties were measured by the following methods.

[0108] <Standard Specific Gravity (SSG)> Using a sample molded in accordance with ASTM D4894, the standard specific gravity was measured by the water displacement method in accordance with ASTM D792.

[0109] <Particle size> Measurement was performed using a laser diffraction particle size analyzer (HELOS & RODOS) manufactured by JEOL Ltd. without using a cascade and at a dispersion pressure of 2.0 bar, and the particle size was calculated based on the obtained particle size distribution integration (volume basis). The particle size corresponding to 50% of the particle size distribution integration was defined as the average particle size (X50), the particle size corresponding to 10% was defined as X10, and the particle size corresponding to 90% was defined as X90.

[0110] <Equivalent Area Diameter> Using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), images of particles were taken at 20x to 200x magnification, and the projected area of ​​the particles was determined by image analysis. The diameter of a perfect circle having an area equivalent to this area was calculated, and this value was taken as the equivalent area diameter.

[0111] <Irregularity> Particles were photographed at 20 to 200 magnifications using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000). The images were analyzed to determine the diameter of a circle circumscribing the particle (circumscribed circle diameter) and the diameter of a circle inscribing the particle (inscribed circle diameter). The circumscribed circle diameter was divided by the inscribed circle diameter to obtain the irregularity.

[0112] <Apparent Density> Measured in accordance with JIS K 6891.

[0113] <Average Circularity> The average circularity of 100 randomly selected particles measured by the following procedure was taken as the average circularity. Using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), particles were photographed at 20x to 200x magnification, and the images were analyzed to determine the perimeter of the projected image of the particle, designated as A, and the diameter of a perfect circle having an area equivalent to the projected area, designated as B, where B is the calculated value and the average circularity is B / A. This value gradually approaches 1.0 as the particles approach a perfect circle.

[0114] <Specific Surface Area> Measurement was performed by the BET method using a surface analyzer (product name: BELSORP-mini II, manufactured by Microtrac-Bell Corporation) and a mixed gas of 30% nitrogen and 70% helium as the carrier gas, with liquid nitrogen for cooling.

[0115] Production example 1 According to the method described in production example 3 of International Publication No. 2003 / 035724, carry out polymerization of tetrafluoroethylene.The PTFE suspension polymerization particles obtained are taken out, washed with pure water, and are pulverized to an average particle diameter of 650 μ m by using line mixer pulverization, so as to obtain PTFE particles.The PTFE particles obtained are dehydrated and dried, and then pulverized and particle size adjusted, so as to obtain the PTFE dry particles A of average particle diameter 25.6 μ m and the PTFE dry particles B of average particle diameter 52.8 μ m.

[0116] Example 1 A SUS sieve with a mesh size of 300 μm was set in a Toyo Hightec Co., Ltd. Hibolter NR-300 model, and a sieve area of ​​72 Nm ３ / min・m ２ Entrained air with a flow rate of 1000 rpm and 5 kg of PTFE dried particles A were continuously supplied. The particles sent from the primary side of the sieve to the coarse powder recovery line were recovered, and the agglomerates were broken down using a standard sieve with a mesh size of 300 μm. After separating the particles that passed through the standard sieve, the shape, irregularity, and area-equivalent diameter of the particles remaining on the sieve were confirmed. The results are shown in Table 2. Meanwhile, various powder properties were measured for the fine powder recovered on the secondary side that had passed through the sieve. The results are shown in Table 1. The shape, irregularity, and area-equivalent diameter of the particles that had passed through the sieve were also confirmed. The results are shown in Table 2.

[0117] Example 2 A SUS sieve with a mesh size of 300 μm was set in a Toyo Hightec Co., Ltd. Hi-Bolter NR-300 model, and a sieve area of ​​72 Nm ３ / min・m ２ Entrained air with a flow rate of 1000 rpm and 5 kg of PTFE dried particles B were continuously supplied. The particles sent from the primary side of the sieve to the coarse powder recovery line were recovered, and the agglomerates were broken down using a standard sieve with a mesh size of 300 μm. After separating the particles that passed through the standard sieve, the shape, irregularity, and area-equivalent diameter of the particles remaining on the sieve were confirmed. The results are shown in Table 2. Meanwhile, various powder properties were measured for the fine powder recovered on the secondary side that had passed through the sieve. The results are shown in Table 1. The shape, irregularity, and area-equivalent diameter of the particles that had passed through the sieve were also confirmed. The results are shown in Table 2.

[0118]

[0119]

[0120] 1, 2: irregular particle 11, 13: circumscribed circle 12, 14: inscribed circle

Claims

1. A method for producing polytetrafluoroethylene powder, comprising an airflow sieve classification step of removing irregularly shaped particles and coarse particles from the treated polytetrafluoroethylene powder by airflow classification using a sieve.

2. The manufacturing method according to claim 1, wherein the mesh size of the sieve used in the airflow sieve classification step is 100 to 2000 μm.

3. The air flow rate in the air flow sieve classification process is 20 to 200 Nm ３ / min・m ２ The method according to claim 1 or 2, wherein 4. The method according to any one of claims 1 to 3, wherein the standard specific gravity of the polytetrafluoroethylene is 2.130 to 2.

280.

5. The manufacturing method according to any one of claims 1 to 4, wherein the average particle size of the powder to be treated is 1 to 200 µm.

6. The mesh size of the sieve in the airflow sieve classification process is 300 to 500 μm, and the air flow rate in the airflow sieve classification process is 50 to 100 Nm ３ / min・m ２ The method according to any one of claims 1 to 5, wherein the average particle size of the powder to be treated is 25 to 60 µm.

7. Average particle size: 1-200 μm, apparent density: 0.20-0.55 g / ml, specific surface area: 1.0-5.0 m ２ / g, and a polytetrafluoroethylene powder having an average circularity of 0.60 or more.

8. The average particle size is 25 to 60 μm, the apparent density is 0.25 to 0.45 g / ml, and the specific surface area is 1.5 to 3.0 m ２ The polytetrafluoroethylene powder according to claim 7, wherein the molecular weight is 1 / g.

9. Average particle size: 1-200 μm, apparent density: 0.20-0.55 g / ml, specific surface area: 1.0-5.0 m ２ / g, and is substantially free of irregular particles having an area-equivalent diameter of 200 μm or more and an irregularity of 2.5 or more, and coarse particles having an area-equivalent diameter of 300 μm or more and an irregularity of less than 2.

5.

10. The polytetrafluoroethylene powder according to claim 9, wherein the amount of the irregularly shaped particles is 0.001% by mass or less relative to the polytetrafluoroethylene powder.

11. The polytetrafluoroethylene powder according to claim 9 or 10, wherein the amount of the coarse particles is 0.001% by mass or less based on the polytetrafluoroethylene powder.

12. The average particle size is 25 to 60 μm, the apparent density is 0.25 to 0.45 g / ml, and the specific surface area is 1.5 to 3.0 m ２ The polytetrafluoroethylene powder according to any one of claims 9 to 11, wherein the molecular weight is 1 / g.

13. A molded article obtained by molding the polytetrafluoroethylene powder according to any one of claims 7 to 12.

14. The molded article according to claim 13, which is a sheet.

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