WO2025178109A1 - Polytetrafluoroethylene powder production method, polytetrafluoroethylene powder, and molded article - Google Patents

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

This disclosure relates to a method for producing polytetrafluoroethylene powder, polytetrafluoroethylene powder, and a molded body.

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.

Patent Document 1 describes how granular PTFE is crushed in a cutter mill and then crushed in a jet mill to obtain PTFE molding powder.

International Publication No. 2014/123075

The purpose of this disclosure is to provide a method for producing PTFE powder that can remove irregularly shaped particles and coarse particles while suppressing adhesion of powder to the classification equipment and clogging of the equipment, as well as to provide PTFE powder and molded articles obtained using the same.

The present disclosure (1) includes an air flow sieve classification step of removing irregular particles and coarse particles from a treated powder of polytetrafluoroethylene by air flow classification using a sieve mesh,
A method for producing polytetrafluoroethylene powder.

Disclosure (2) is a manufacturing method described in Disclosure (1), in which the mesh size of the sieve used in the airflow sieve classification process is 100 to 2000 μm.

The present disclosure (3) is the manufacturing method according to the present disclosure (1) or (2), wherein the air flow rate in the airflow sieve classification step is 20 to 200 Nm ³ /min·m ² .

The present disclosure (4) is a manufacturing method for any combination of the present disclosures (1) to (3) in which the standard specific gravity of the polytetrafluoroethylene is 2.130 to 2.280.

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

The present disclosure (6) is characterized in that the mesh size of the sieve in the airflow sieve classification step is 300 to 500 μm,
The air flow rate in the air flow sieve classification step is 50 to 100 Nm ³ /min·m ² ,
This is a manufacturing method in any combination with any of the present disclosures (1) to (5), wherein the average particle size of the powder to be treated is 25 to 60 μm.

The present disclosure (7) is a polytetrafluoroethylene powder having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, a specific surface area of 1.0 to 5.0 ^{m 2} /g, and an average circularity of 0.60 or more.

The present disclosure (8) is the polytetrafluoroethylene powder according to the present disclosure (7), wherein 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 /g.

The present disclosure (9) is a polytetrafluoroethylene powder having an average particle diameter of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, 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.

The present disclosure (10) is the polytetrafluoroethylene powder described in 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.

The present disclosure (11) is the polytetrafluoroethylene powder described in 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.

The present disclosure (12) is a polytetrafluoroethylene powder in any combination with any of the present disclosures (9) to (11), wherein 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} /g.

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

The present disclosure (14) is a molded body described in the present disclosure (13) that is a sheet.

This disclosure provides a method for producing PTFE powder that can remove irregularly shaped particles and coarse particles while suppressing adhesion of powder to the classification equipment and clogging of the equipment, as well as PTFE powder and molded articles obtained using the same.

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

PTFE powder contains irregularly shaped particles and coarse particles that cannot be completely removed by conventional classification methods, and these particles cause defects such as cracks and reduced strength when processed into molded products such as sheets.
As a result of extensive investigation, it was found that the above problems could be solved by carrying out a specific classification process.

This disclosure is explained in detail below.

This disclosure relates to a method for producing PTFE powder, which includes an air sieve classification process in which irregularly shaped particles and coarse particles are removed from the treated PTFE powder by air classification using a sieve.

PTFE powder has high cohesive and adhesive properties, and conventional classification methods using a sieve make it difficult to remove irregularly shaped and coarse particles because the particles adhere to the sieve or clog the screen. In contrast, the manufacturing method disclosed herein includes the airflow sieve classification step, making it possible to produce PTFE powder from which irregularly shaped and coarse particles that cause defects and reduced strength in molded products have been removed, without causing problems such as clogging.

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 mesh. The treated powder may contain the irregularly shaped particles and coarse particles. PTFE has high cohesiveness and adhesive properties, making it difficult to classify using conventional sieve meshes. In the manufacturing method disclosed herein, the airflow sieve classification process makes it possible to remove irregularly shaped particles and coarse particles while suppressing adhesion and clogging.

The powder to be treated that is subjected to the air flow sieve classification step may contain more than 0.005 mass % of the irregular-shaped particles relative to the powder to be treated.
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.

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

In the air sieve classification process, at least a portion of the coarse particles are removed. Preferably, 90% by mass or more of the coarse particles contained in the powder to be treated and subjected to the air sieve classification process are removed, more preferably 95% by mass or more, even more preferably 97% by mass or more, even more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass.

The irregularly shaped particles refer to particles having an area-equivalent diameter of 200 μm or more and an irregularity degree of 2.5 or more.
The coarse particles refer to particles having an area-equivalent diameter of 300 μm or more and an irregularity degree of less than 2.5.
The above-mentioned area-equivalent diameter is a value obtained by taking an image of the particle at 20 to 200 magnifications using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), determining the projected area of the particle by 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), analyzing the images to determine the diameter of a perfect circle circumscribing the particle (circumscribing circle diameter) and the diameter of a perfect circle inscribing the particle (inscribing circle diameter), and then dividing the circumscribing circle diameter by the inscribing circle diameter.

The irregular shaped particles may have a shape such as a film, fiber, string, strip, compressed piece, rod, etc. Examples of the irregular shaped particles are shown in FIG.
1(a), the irregular-shaped particle 1 is in the form of a film. In the irregular-shaped particle 1, the circumscribing circle is indicated by 11 and the inscribing circle is indicated by 12.
1(b), the irregular-shaped particle 2 is string-shaped. The irregular-shaped particle 2 has a circumscribing circle 13 and an inscribing circle 14.

These irregularly shaped particles can be generated, for example, during the manufacturing and transportation processes of PTFE particles. These irregularly shaped particles have high air resistance and cannot be adequately removed by conventional air classification.

In air classification using the above-mentioned sieve screen (air sieve classification), particles dispersed in an airflow are classified by passing them through the sieve screen. The above-mentioned air sieve classification can be carried out, for example, using an air classifier equipped with a sieve screen. Known air classifiers equipped with the above-mentioned sieve screen can be used, such as blow-through classifiers.

The sieve may be made of metal such as stainless steel or resin such as nylon, and may be in the form of a woven mesh, a microsieve, or the like.
The sieve screen may have a flat shape.

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., but in one embodiment of the present disclosure, it may be 100 to 2000 μm. The mesh size may also be 200 μm or more, 300 μm or more, etc., or 1500 μm or less, 1000 μm or less, 750 μm or less, 500 μm or less, etc.

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 may be 20 to 200 Nm ³ /min·m ^2. The air flow rate may also be 40 Nm ³ /min·m ² or more, 50 Nm ³ /min·m ² or more, etc., or 150 Nm ³ /min·m ² or less, 100 Nm ³ /min·m ² or less, etc.

The air temperature in the above-mentioned air flow sieve classification process is preferably -200°C or higher from the standpoint of energy efficiency, more preferably -100°C or higher, and even more preferably 5°C or higher. Furthermore, in terms of easily suppressing particle adhesion and aggregation, it is preferably 40°C or lower, more preferably 20°C or lower, and even more preferably 10°C or lower.

The temperature of the powder to be treated supplied in the air sieve classification process is preferably -100°C or higher, and even more preferably 5°C or higher, in order to efficiently remove 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.

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 may be 250 to 5000 kg/h·m ² .

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. If such small PTFE particles are to be classified using a sieve, the particles will 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 depending on the purpose, such as the molding method and the use of the molded body, 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).

In one aspect of the present disclosure, the PTFE is preferably high-molecular-weight PTFE. High-molecular-weight PTFE typically has fibrillating properties and is particularly cohesive and adhesive. In the manufacturing method of the present disclosure, even if high-molecular-weight PTFE with high cohesive and adhesive properties is used, adhesion to the classification device and clogging of the device are unlikely to occur.

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 according to ASTM D792 using a sample molded according to ASTM D4894.
"High molecular weight" with respect to PTFE means that the standard specific gravity is within the ranges indicated above.

The above-mentioned high molecular weight PTFE is typically non-melt-processable. "Non-melt-processable" means that the melt flow rate cannot be measured at temperatures higher than the melting point in accordance with ASTM D-1238 and D-2116; in other words, it does not flow easily even in the melting temperature range.

The high-molecular-weight PTFE may be a homopolymer of TFE, or may be modified PTFE containing polymerized units based on TFE (TFE units) and polymerized units based on a modified monomer (modified monomer units). The modified PTFE may contain 99.0% by mass or more of TFE units and 1.0% by mass or less of modified monomer units. Alternatively, the modified PTFE may consist solely of TFE units and modified monomer units.

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

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.

The modifying monomer is not particularly limited as long as it is copolymerizable with TFE, and examples include perfluoroolefins such as hexafluoropropylene (HFP); hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride (VDF); perhaloolefins such as chlorotrifluoroethylene; perfluorovinyl ether; perfluoroallyl ether; (perfluoroalkyl)ethylene; ethylene; and the like. The modifying monomer used may be one type or multiple types.

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.

An example of the perfluorovinyl ether is perfluoro(alkyl vinyl ether) [PAVE], where Rf in the above 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.

Examples of the perfluoroalkyl group in the PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, and perfluorohexyl groups.

The above-mentioned perfluorovinyl ether further includes those represented by the above general formula (A), in which Rf is a perfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, and those represented by the following formula:

(wherein m represents 0 or an integer from 1 to 4), and Rf represents the following formula:

(wherein n represents an integer from 1 to 4)

(Perfluoroalkyl)ethylenes (PFAEs) are not particularly limited, and examples include (perfluorobutyl)ethylene (PFBE) and (perfluorohexyl)ethylene.

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

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. The perfluoroallyl ether is preferably at least one selected from the group consisting of CF ₂ ═CF-CF ₂ -O-CF ₃ , CF ₂ ═CF-CF ₂ -O-C ₂ F ₅ , CF 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-C ₂ F ₅ , CF ₂ ═CF-CF ₂ -O-C ₃ F ₇ , and CF ₂ ═CF-CF ₂ -O-C ₄ F ₉ , and even more preferably CF ₂ ═CF-CF ₂ -O-CF ₂ CF ₂ CF ₃ .

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 disclosed herein is preferably PTFE molding powder.

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

It is also preferable that the PTFE be low-molecular-weight PTFE. Because low-molecular-weight PTFE powder is a fine particle (e.g., particle size of 10 μm or less), the weight of the particles relative to the contact point is small and they are less likely to fall through the sieve. In the manufacturing method disclosed herein, even when low-molecular-weight PTFE is used, adhesion to the classification equipment and clogging of the equipment are less likely to occur.

The low-molecular-weight PTFE preferably has a melt viscosity of 1.0×10 ² to 7.0×10 ⁵ Pa·s at 380° C. 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 Pa·s or more, and is preferably 3.0×10 ⁵ Pa·s or less, and more preferably 1.0×10 ⁵ Pa·s or less.

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

The melting point of the above-mentioned low-molecular-weight PTFE is preferably 320 to 340°C, more preferably 324°C or higher, and even more preferably 336°C or lower.

The melting point was measured using a differential scanning calorimeter (DSC). After temperature calibration was performed in advance using indium and lead as standard samples, approximately 3 mg of low molecular weight PTFE was placed in an aluminum pan (crimp container), and the temperature was raised at a rate of 10°C/min over the temperature range of 250-380°C in an air flow of 200 ml/min. The melting point was determined to be the minimum point in the heat of fusion within the above range.

The low-molecular-weight PTFE may be a homopolymer of TFE, or may be modified PTFE containing polymerized units based on TFE (TFE units) and polymerized units based on a modified monomer (modified monomer units). The type of modified monomer and the content of the modified monomer units may be the same as those for the high-molecular-weight PTFE.

The above-mentioned low-molecular-weight PTFE can be produced by direct polymerization using methods such as emulsion polymerization or suspension polymerization, or by thermal decomposition or radiolysis of high-molecular-weight PTFE. The above-mentioned low-molecular-weight PTFE is preferably produced by direct polymerization, and more preferably by emulsion polymerization.

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 carried out before the air sieve classification step.
The PTFE to be subjected to the pulverization step can be, for example, the PTFE obtained in the polymerization step, washing step, or drying step, and among these, the PTFE obtained in the drying step can be preferably used.
In one preferred embodiment, the production method of the present disclosure comprises the polymerization step, washing step, drying step, and pulverization step, and the dried PTFE is used as the PTFE in the pulverization step.

The PTFE pulverized in the pulverization step preferably has an average particle size of 100 to 800 μm, 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 being equal to the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

The grinding in the grinding step may be dry grinding, and can be carried out, for example, using a grinder. Examples of grinders include a hammer mill, pin mill, jet mill, and cutter mill.

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).

The manufacturing method of the present disclosure may include an air classification step of removing coarse particles 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 carry out the air classification step.
The air classification step is preferably carried out before the air sieve classification step. When the production method of the present disclosure includes the pulverization step, the air classification step may be carried out after the pulverization step, or may be carried out simultaneously with the pulverization step from the viewpoints of improving production efficiency and suppressing adhesion of PTFE particles.
Since coarse particles can be removed in the air current classification step, the air current classification step is not essential. However, by removing some of the coarse particles in the air current classification step and then removing the coarse particles that were not completely removed in the air current classification step in the air current classification step, the coarse particles can be removed more efficiently.

In the air classification process, at least a portion of the coarse particles are removed. Preferably, 90% by mass or more of the coarse particles contained in the powder to be treated and subjected to the air classification process are removed, more preferably 95% by mass or more, even more preferably 97% by mass or more, even more preferably 99% by mass or more, particularly preferably 99.5% by mass or more, and most preferably 100% by mass.

The removed coarse particles may be subjected to the grinding process again.

The air classification can be carried out using, for example, an air classifier. The air classification may be carried out without using the sieve screen described above.
In an air classifier, particles are sent by air currents to a classification chamber (for example, a cylindrical classification chamber), where they are dispersed by the swirling air currents within the chamber and classified by centrifugal force. The particles are collected from the center into a cyclone and a bag filter. Rotating bodies such as cones and rotors are installed within the classification chamber to ensure uniform swirling motion of the particles and air.

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.

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 certain size to pass through. The size of the particles that pass through can be adjusted by the size of the openings in 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.

The opening size of the openings of the classification 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 openings of the classification rotor may have a larger opening than the openings of the sieve mesh used in the airflow sieve classification step described above.

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

The air flow rate in the air classification step can be determined depending on the target classification point, and may be, for example, 1 Nm ³ /min·m ² or more, and 100 Nm ³ /min·m ² or less.

The air classifier is preferably one that has both a pulverizing function and an air classifying function, as this allows the pulverizing and air classifying processes to be carried out simultaneously. Known air classifiers that have both a pulverizing function and an air classifying function can be used.

In the air classification process, the coarse particles are removed to obtain classified particles. The classified particles are the fine particles (e.g., particles that have passed through the classification rotor) among the particles separated into coarse and fine particles by air classification.

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).

The manufacturing method of the present disclosure may, if necessary, include a sieve classification step using a sieve with openings large enough to prevent clogging.

The manufacturing method of the present disclosure can provide a PTFE powder that is substantially free of the irregularly shaped particles and coarse particles described above.
The term "substantially free of irregular-shaped particles" means that the amount of irregular-shaped particles is 0.005% by mass or less, preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, relative to the PTFE powder.
The term "substantially free of coarse particles" means that the amount of coarse particles is 0.005% by mass or less, preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, based on the PTFE powder.

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) A PTFE powder having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, a specific surface area of 1.0 to 5.0 m ² /g, and an average circularity of 0.60 or more (hereinafter also referred to as a first PTFE powder).
(2) A PTFE powder (hereinafter also referred to as the second PTFE powder) having an average particle diameter of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, 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.

The first and second PTFE powders produce molded articles with few defects such as cracks and excellent strength.

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

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 being equal to the particle size corresponding to 50% of the integrated particle size distribution (volume basis).

The first and second PTFE powders have an apparent density of 0.20 to 0.55 g/ml. This ensures 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.

The first and second PTFE powders have a specific surface area of 1.0 to 5.0 ^{m 2} /g, which provides excellent fusion properties during firing and excellent removal of impurities. The specific surface area may be 0.1 ^{m 2} /g or more, 1.0 m ² /g or more, 1.5 m ² /g or more, or 4.0 m ² /g or less, 3.0 m ² /g or less, etc.
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 used for cooling.

The first PTFE powder has an average circularity of 0.60 or more, which 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 also usually less than 1.0, and may be 0.9 or less, or 0.8 or less.
The average circularity is the average value of the circularity measured for 100 particles randomly selected from a powder sample dispersed on a flat plate, using the following procedure. Images of the particles are taken at 20x to 200x magnification using a microscope (Keyence Corporation, Model No. VHX-6000), and the image is analyzed to determine the perimeter of the projected image of the particle, where A is the perimeter 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 particles approach a perfect circle.

The second PTFE powder is substantially free of irregular-shaped particles having an area-equivalent diameter of 200 μm or more and an irregularity degree of 2.5 or more. The area-equivalent diameter and the irregularity degree are as described above.
The phrase "substantially free of irregular-shaped particles" means that the amount of 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, based on the PTFE powder.

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.
The phrase "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 the coarse particles is preferably 0.001% by mass or less, and more preferably 0.0001% by mass or less, based on the PTFE powder.

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 above X10 is equal to the particle diameter corresponding to 10% of the integrated particle size distribution (volume basis) when measurement is performed 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.

In one embodiment of the present disclosure, the first and second PTFE powders may have a particle size 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) when 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.

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.

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

The above compression molding can be carried out, for example, by holding the mixture at a pressure of 10 to 50 MPa for 1 minute to 30 hours.

After the molding, the material may be fired. The firing can be carried out, for example, by heating at a temperature of 350 to 380°C for 0.5 to 50 hours.

Furthermore, after the firing, the material may be cut and processed into a sheet or other shape.

The shape of the molded product of the present disclosure is not particularly limited, and examples include sheet, film, rod, pipe, and fiber shapes.

The molded article of the present disclosure preferably has a tensile 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 strength at break is measured in accordance with ASTM D1708.

The molded articles disclosed herein can be suitably used as lining sheets, insulating films, valves, gaskets, bellows, semiconductor chemical tanks, etc.

Although the embodiments have been described above, it will be understood that various changes in form and details are possible without departing from the spirit and scope of the claims.

The following examples will further illustrate the present disclosure, but the present disclosure is not limited to these examples.

Various physical properties were measured using the following methods.

<Standard specific gravity (SSG)>
Using a sample molded in accordance with ASTM D4894, the measurement was carried out by the water displacement method in accordance with ASTM D792.

<Particle size>
Measurement was performed 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 the particle size distribution was calculated based on the obtained integrated particle size distribution (volume basis). The particle size corresponding to 50% of the integrated particle size distribution 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.

<Area equivalent diameter>
Using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), images of the particles were taken at 20 to 200 magnifications, 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 area-equivalent diameter.

<Degree of abnormality>
The 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 value obtained by dividing the circumscribing circle diameter by the inscribed circle diameter was defined as the irregularity.

<Apparent density>
Measurement was carried out in accordance with JIS K 6891.

<Average circularity>
The average value of the circularity values measured for 100 randomly selected particles using the following procedure was taken as the average circularity. Using a microscope (manufactured by KEYENCE Corporation, model number: VHX-6000), images of the particles were taken 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.

<Specific surface area>
Measurements were carried out by the BET method using a surface analyzer (trade name: BELSORP-mini II, manufactured by Microtrac-Bell Co., Ltd.) and a mixed gas of 30% nitrogen and 70% helium as the carrier gas, with liquid nitrogen used for cooling.

Production Example 1
Polymerization of tetrafluoroethylene was carried out by the method described in Production Example 3 of WO 2003/035724.
The resulting PTFE suspension-polymerized particles were taken out, washed with pure water, and pulverized in a line mixer to an average particle size of 650 μm to obtain PTFE particles.
The obtained PTFE particles were dehydrated and dried, then pulverized and the particle size was adjusted to obtain PTFE dry particles A having an average particle size of 25.6 μm and PTFE dry particles B having an average particle size of 52.8 μm.

Example 1
A SUS sieve with 300 μm openings was set in a Toyo Hitec Hibolter NR-300, and entrained air at a rate of 72 Nm ³ /min·m ² relative to the sieve area and 5 kg of PTFE dry 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 aggregates 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.
On the other hand, the fine powder recovered on the secondary side after passing through the sieve was subjected to measurements of various powder properties. The results are shown in Table 1. In addition, the shape, irregularity, and area-equivalent diameter of the particles that passed through the sieve were confirmed. The results are shown in Table 2.

Example 2
A SUS sieve with 300 μm openings was set in a Toyo Hitec Hibolter NR-300, and entrained air at a rate of 72 Nm ³ /min·m ² relative to the sieve area and 5 kg of PTFE dry 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 aggregates 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.
On the other hand, the fine powder recovered on the secondary side after passing through the sieve was subjected to measurements of various powder properties. The results are shown in Table 1. In addition, the shape, irregularity, and area-equivalent diameter of the particles that passed through the sieve were confirmed. The results are shown in Table 2.

1, 2: irregular shaped particles 11, 13: circumscribed circles 12, 14: inscribed circles

Claims (14)

The method includes an airflow sieve classification step of removing irregular particles and coarse particles from the polytetrafluoroethylene powder to be treated by airflow classification using a sieve mesh,
Method for producing polytetrafluoroethylene powder. The manufacturing method described in claim 1, wherein the mesh size of the sieve used in the air sieve classification process is 100 to 2000 μm. 3. The method according to claim 1, wherein the air flow rate in the air flow sieve classification step is 20 to 200 Nm ³ /min·m ² . The manufacturing method described in any one of claims 1 to 3, wherein the standard specific gravity of the polytetrafluoroethylene is 2.130 to 2.280. The manufacturing method described in any one of claims 1 to 4, wherein the average particle size of the powder to be treated is 1 to 200 μm. The mesh size of the sieve in the airflow sieve classification step is 300 to 500 μm,
The air flow rate in the air flow sieve classification step is 50 to 100 Nm ³ /min·m ² ,
The method according to any one of claims 1 to 5, wherein the powder to be treated has an average particle size of 25 to 60 µm. A polytetrafluoroethylene powder having an average particle size of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, a specific surface area of 1.0 to 5.0 m ² /g, and an average circularity of 0.60 or more. 8. The polytetrafluoroethylene powder according to claim 7, wherein 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 /g. A polytetrafluoroethylene powder having an average particle diameter of 1 to 200 μm, an apparent density of 0.20 to 0.55 g/ml, 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. 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. The polytetrafluoroethylene powder according to claim 9 or 10, wherein the amount of the coarse particles is 0.001% by mass or less relative to the polytetrafluoroethylene powder. The polytetrafluoroethylene powder according to any one of claims 9 to 11, wherein 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 /g. A molded article obtained by molding the polytetrafluoroethylene powder described in any one of claims 7 to 12. The molded article according to claim 13, which is a sheet.