Method for producing low molecular weight polytetrafluoroethylene

Abstract

The present invention provides a method for producing a low-molecular-weight polytetrafluoroethylene which has a small variation in the molecular weight. The present invention provides a method for producing a low-molecular-weight polytetrafluoroethylene, the method comprising a step (1) in which a low-molecular-weight polytetrafluoroethylene that has a melt viscosity of 1.0 × 102 Pa∙s to 7.0 × 105 Pa∙s at 380°C is obtained by irradiating a high-molecular-weight polytetrafluoroethylene with radiation such that the ratio (maximum dose) / (minimum dose) of the maximum dose to the minimum dose is 1.55 or less.

Description

Technical Field

[0001] The present disclosure relates to a method for producing low molecular weight polytetrafluoroethylene.

Background Art

[0002] Low molecular weight polytetrafluoroethylene (also called "polytetrafluoroethylene wax" or "polytetrafluoroethylene micropowder") with a molecular weight of several thousand to several hundred thousand has excellent chemical stability, extremely low surface energy, and is difficult to fibrillate. Therefore, as an additive for improving slipperiness and the texture of the coating film surface, it is used in the production of plastics, inks, cosmetics, paints, greases, etc. (for example, see Patent Document 1).

[0003] As methods for producing low molecular weight polytetrafluoroethylene, a polymerization method, a radiation decomposition method, a thermal decomposition method, etc. are known. Patent Document 2 describes a method for producing low molecular weight polytetrafluoroethylene by a radiation decomposition method.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present disclosure is to provide a method for producing low molecular weight polytetrafluoroethylene (PTFE) with little variation in molecular weight.

Means for Solving the Problems

[0006] (1) This disclosure describes irradiating high molecular weight polytetrafluoroethylene with radiation such that the ratio of the maximum dose rate to the minimum dose rate (maximum dose rate / minimum dose rate) is 1.55 or less, and the melt viscosity at 380°C is 1.0 × 10⁻⁶. 2 Pa·s or more, 7.0×10 5 This is a method for producing low molecular weight polytetrafluoroethylene, comprising step (1) of obtaining low molecular weight polytetrafluoroethylene having a Pa·s or less.

[0007] Disclosure (2) is a method for manufacturing the above-mentioned radiation, wherein the minimum absorbed dose of the radiation is 200 kGy or more.

[0008] Disclosure (3) is a method for manufacturing the present invention (1) or (2) in which the irradiation is carried out substantially in the absence of oxygen.

[0009] Disclosure (4) is a method for manufacturing any combination of the above radiation being an electron beam, gamma ray, or X-ray, as described in Disclosures (1) to (3).

[0010] Disclosure (5) is a method of manufacturing any combination of any of Disclosures (1) to (4) wherein the irradiation is performed with the distance from the radiation source to the furthest part of the high molecular weight polytetrafluoroethylene being 10 m or less.

[0011] Disclosure (6) is a method of manufacturing any combination of any of Disclosures (1) to (5) wherein the irradiation is performed with a distance of 5 cm or more from the radiation source to the nearest part of the high molecular weight polytetrafluoroethylene.

[0012] Disclosure (7) is a method for manufacturing any combination of Disclosure (1) to (6), wherein the high molecular weight polytetrafluoroethylene is placed in a position facing the effective region of the radiation source, and the irradiation is performed, and the effective region is a region in which the distance from the center of the radiation source is 95% or less of the distance from the center to the end of the radiation source.

[0013] Disclosure (8) is a method of producing the above-mentioned high molecular weight polytetrafluoroethylene, which is filled into an irradiation vessel made of at least one material selected from the group consisting of metals, glass, ceramics, and organic materials, in any combination of any of Disclosures (1) to (7).

[0014] This disclosure (9) is a method for manufacturing the irradiation container according to this disclosure (8), wherein the irradiation container is cylindrical or prismatic in shape.

[0015] Disclosure (10) is a method for manufacturing the irradiation container according to Disclosure (8) or (9), wherein the irradiation container has at least one surface selected from the group consisting of a plate-like surface, a mesh-like surface, and a surface having a slit-like opening.

[0016] Disclosure (11) is a method of manufacturing by arranging a plurality of irradiation containers filled with the above-mentioned high molecular weight polytetrafluoroethylene in a row and performing the above-mentioned irradiation in any combination with any of Disclosures (1) to (10).

[0017] Disclosure (12) is a method for producing any combination of the above high molecular weight polytetrafluoroethylene with any of Disclosures (1) to (11), wherein the standard specific gravity of the high molecular weight polytetrafluoroethylene is 2.130 or higher and 2.230 or lower.

[0018] Disclosure (13) is a method for producing any combination of the above high molecular weight polytetrafluoroethylene and the above low molecular weight polytetrafluoroethylene in any of Disclosures (1) to (12), wherein both are in powder form.

[0019] This disclosure (14) further includes step (2) of heating the high molecular weight polytetrafluoroethylene above its primary melting point before step (1) to obtain a molded article, wherein the molded article has a specific gravity of 1.0 g / cm³. 3 The above is a method of manufacturing any combination of any of the above (1) to (13) of the present disclosure. [Effects of the Invention]

[0020] According to this disclosure, it is possible to provide a method for producing low molecular weight PTFE with small molecular weight variation. [Brief explanation of the drawing]

[0021] [Figure 1] This figure shows an example of a radiation irradiation method. [Figure 2] This is a cross-sectional view showing an example of an irradiation container filled with high molecular weight PTFE. [Figure 3] This is a cross-sectional view showing another example of an irradiation vessel filled with high molecular weight PTFE. [Figure 4] This figure shows an example of the arrangement of the irradiation container. [Figure 5] This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, in Example 1 and Comparative Example 1. [Figure 6] This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, in Examples 2, 4-7 and Comparative Examples 2-6. [Figure 7] This diagram shows the positional relationship of the main drum in Example 3. [Figure 8] This figure shows the sampling locations in Example 3. [Figure 9] This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations in Example 8. [Figure 10] This figure shows the state in Example 8 where five 18-liter cans are stacked. [Modes for carrying out the invention]

[0022] Conventionally, when high molecular weight PTFE is irradiated with radiation, there is a problem in that the absorbed radiation dose differs greatly depending on the position within the irradiation chamber, resulting in large variations in the molecular weight of the resulting low molecular weight PTFE. The inventors have discovered that by irradiating the material such that the ratio of the maximum dose rate to the minimum dose rate of radiation falls within a specific range, the variation in the molecular weight of the resulting low molecular weight PTFE can be suppressed, and have thus completed the manufacturing method disclosed herein. The following provides a detailed explanation of this disclosure.

[0023] In this specification, there are instances where "~" is used to indicate a numerical range from a lower limit to an upper limit. In these instances, the numerical range refers to a range of values ​​that are greater than or equal to the lower limit and less than or equal to the upper limit, including the lower limit itself and the upper limit itself.

[0024] This disclosure describes how high molecular weight PTFE is irradiated with radiation such that the ratio of the maximum dose rate to the minimum dose rate (maximum dose rate / minimum dose rate) is 1.55 or less, and the melt viscosity at 380°C is 1.0 × 10⁻⁶. 2 Pa·s or more, 7.0×10 5 The present invention relates to a method for producing low molecular weight PTFE, which includes step (1) of obtaining low molecular weight PTFE with a Pa·s or less.

[0025] In step (1), high molecular weight PTFE is irradiated with radiation such that the ratio of the maximum dose rate to the minimum dose rate (hereinafter also referred to as the dose rate ratio) is 1.55 or less. By keeping the dose rate ratio within the above range, low molecular weight PTFE with small molecular weight variation is obtained. The above dose rate ratio is preferably 1.50 or less, more preferably 1.45 or less, even more preferably 1.40 or less, even more preferably 1.35 or less, and particularly preferably 1.30 or less. The above dose rate ratio may also be 1.00 or higher, or 1.10 or higher.

[0026] The above dose rate ratio is determined as the ratio of the maximum and minimum absorbed dose rates obtained at multiple locations within the irradiation chamber containing the sample. The above absorbed dose rates are determined at least at the location where the lowest dose value is expected (e.g., the center of the irradiation chamber or the inner surface of the irradiation chamber furthest from the radiation source) and at the location where the highest dose is expected (e.g., the inner surface of the irradiation chamber closest to the radiation source). The absorbed dose rate can be measured by two methods: direct measurement using a chemical dosimeter and computational simulation using the Monte Carlo simulation code "PHITS" (Journal of Nuclear Science and Technology, 2018, Vol. 55, No. 6, pp. 684-690). Alanine dosimeters and PMMA dosimeters can be used as chemical dosimeters. Three alanine dosimeters will be installed at locations where the lowest dose values ​​are expected. In addition, five PMMA dosimeters, which are used for radiation process control, will be installed per surface at locations where the highest dose values ​​are expected. When using an alanine dosimeter, the dose is calculated using a calibration curve created from alanine dosimeter standard samples, based on the ESR spectrum obtained by integrating measurements specifically for alanine-derived radicals in the crystal using electron spin resonance (microwave frequency set to 9.8 GHz, resonance magnetic field set to 350 mT), in accordance with ISO / ASTM 51607 and JIS standards (JIS Z4571). When using a PMMA dosimeter, a RadixW (manufactured by Radie Industries Co., Ltd.) is used as the dosimeter. After irradiation in accordance with ISO / ASTM 51276 and JIS standards (JIS Z4572), the absorbance at 320 nm or 280 nm is measured using a spectrophotometer, and the dose is calculated from the calibration curve. In the PHITS computational simulation, the actual size, quantity, and irradiation time of the cobalt source, the size, material, and shape of the irradiation vessel, and the size, material, and actual arrangement of the walls and floor inside the irradiation chamber that affect radiation transmission and scattering are simulated, and Monte Carlo simulations are performed in accordance with ISO / ASTM 52303 and JIS standards (JIS Z4574). The computational simulation allows for a more detailed evaluation of the dose distribution inside the irradiation vessel. By comparing the dose rate evaluation results at the same locations as the actual measurement points using a chemical dosimeter, among the multiple evaluation points obtained from this PHITS calculation simulation, the validity of the calculation simulation results was confirmed.

[0027] When irradiating a sample by placing it in multiple irradiation containers, it is preferable that the dose rate ratio between the containers also be within the range described above. In this case, the dose rate ratio obtained from the maximum and minimum absorbed dose rates of the sample as a whole, contained in the multiple containers, may be adjusted to fall within the above range, or the dose ratio obtained from the minimum absorbed dose of the sample as a whole and the maximum absorbed dose in each container may be adjusted to fall within the above range.

[0028] The above-mentioned radiation is not particularly limited as long as it is ionizing radiation, and examples include electron beams, gamma rays, X-rays, neutron beams, and high-energy ions. However, electron beams, gamma rays, or X-rays are preferred for industrial use, and electron beams or gamma rays are more preferred. Electron beams can be generated, for example, from an electron accelerator. Gamma rays can be generated, for example, from radioisotopes. X-rays can be generated, for example, by irradiating a target such as a metal with a particle beam from a particle accelerator. Alternatively, quasi-monochromatic X-rays can be generated by colliding a laser beam with a high-energy electron beam, causing inverse Compton scattering (laser-Compton scattering). Furthermore, X-rays can be generated by synchrotron radiation, or by installing undulators or wigglers in the lower stage of a particle accelerator.

[0029] As a gamma ray source, a cobalt-60 source is preferred because it is widely used in industry. Examples of such sources include rod-shaped or plate-shaped sources, with plate-shaped sources being preferred. An example of a plate-shaped source is a rod-shaped source pencil arranged on a flat plate.

[0030] The average absorbed dose of the above radiation may be, for example, 10 kGy or more, preferably 100 kGy or more, more preferably 150 kGy or more, even more preferably 200 kGy or more, even more preferably 250 kGy or more, particularly preferably 300 kGy or more, and most preferably 350 kGy or more. Furthermore, the average absorbed dose may be 1000 kGy or less, preferably 750 kGy or less, more preferably 600 kGy or less, and even more preferably 500 kGy or less. When the average absorbed dose is high, the difference in dose due to location tends to have a greater impact on the variation in molecular weight of low molecular weight PTFE. In the manufacturing method disclosed herein, by setting the dose rate ratio to 1.55 or less, it is possible to suppress variations in the molecular weight of low molecular weight PTFE even when the average absorbed dose is high. The average absorbed dose mentioned above is calculated as the average of the absorbed doses measured by dosimeters placed at multiple locations on the source-side surface inside the irradiation container. The dosimeters (chemical dosimeters) are as described above.

[0031] Radiation irradiation may be carried out continuously until the desired absorbed dose is reached, or it may be carried out intermittently and repeatedly until the desired absorbed dose is reached cumulatively.

[0032] The average absorbed dose rate during irradiation with the above-mentioned radiation is not particularly limited, but for example, in the case of gamma rays emitted from cobalt-60, it is preferably 0.1 kGy / h or more, more preferably 1 kGy / h or more, and even more preferably 2 kGy / h or more. For electron beams from an electron accelerator, a pass rate of 0.1 kGy / s or higher is preferred, 1 kGy / s or higher is more preferred, and 10 kGy / s or higher is even more preferred. When irradiating while moving the sample on a conveyor or the like, a pass rate of 0.1 kGy / pass or higher is preferred, 1 kGy / pass or higher is more preferred, and 10 kGy / pass or higher is even more preferred. For X-rays generated by irradiating an X-ray generation target with a particle beam from a particle accelerator, particularly an electron beam from an electron accelerator, a pass rate of 5 Gy / s or higher is preferred, more preferably 50 Gy / s or higher, even more preferably 0.1 kGy / s or higher, even more preferably 0.5 kGy / s or higher, particularly preferred 1 kGy / s or higher, and most preferably 10 kGy / s or higher. When irradiating while moving the sample on a conveyor or the like, a pass rate of 0.1 kGy / pass or higher is preferred, more preferably 1 kGy / pass or higher, and even more preferably 10 kGy / pass or higher. The average absorbed dose rate mentioned above is determined as the average of the absorbed dose rates measured by dosimeters placed at multiple locations on the surface inside the irradiation vessel. The dosimeters (chemical dosimeters) are as described above. It can also be determined by the PHITS calculation simulation described above. Dosimeters and calculation simulations may be used in combination.

[0033] The minimum absorbed dose of the above-mentioned radiation may be, for example, 10 kGy or more, preferably 100 kGy or more, more preferably 150 kGy or more, even more preferably 200 kGy or more, even more preferably 250 kGy or more, particularly preferably 300 kGy or more, and most preferably 350 kGy or more. Furthermore, the minimum absorbed dose may be 1000 kGy or less, preferably 750 kGy or less, more preferably 600 kGy or less, and even more preferably 500 kGy or less. The minimum absorbed dose mentioned above is determined as the absorbed dose value measured by a dosimeter placed at the location where the lowest dose value is expected (for example, the center of the irradiation vessel or the inner surface of the irradiation vessel furthest from the radiation source). The dosimeter (chemical dosimeter) is as described above.

[0034] The irradiation temperature of the above-mentioned radiation is not particularly limited as long as it is above the γ dispersion temperature near -80°C and below the melting point of high molecular weight PTFE. It is known that the molecular chains of high molecular weight PTFE crosslink near the melting point, and in order to suppress crosslinking, a temperature of 320°C or lower is preferred, more preferably 310°C or lower, and even more preferably 300°C or lower. Economically, irradiation is preferred in a temperature range from room temperature to about 50°C, but the temperature may be raised for irradiation in order to increase the efficiency of radiation decomposition. The irradiation temperature mentioned above is the temperature of the sample (high molecular weight PTFE) during irradiation. Furthermore, the sample temperature may change between -80°C and 320°C during continuous radiation irradiation.

[0035] The above irradiation may be carried out in any atmosphere, for example, in air, inert gas, or in a vacuum. It is also preferable to carry out the above irradiation in the substantially absence of oxygen. When irradiation is performed in the virtually oxygen-free environment, a high absorbed dose may be required to obtain low molecular weight PTFE. As mentioned above, the higher the absorbed dose, the greater the influence of location-dependent dose differences on the molecular weight variation of low molecular weight PTFE. In the manufacturing method disclosed herein, by setting the dose rate ratio to 1.55 or less, it is possible to suppress variations in the molecular weight of low molecular weight PTFE even when irradiation is performed in the substantially absence of oxygen.

[0036] In this specification, "substantially oxygen-free" means that the oxygen concentration in the atmosphere in which the process is carried out is less than 2.0 volume%. The oxygen concentration may be 1.0 volume% or less, less than 1.0 volume%, 0.5 volume% or less, 0.1 volume% or less, or 0.01 volume% or less. The lower limit of the oxygen concentration may be below the detection limit. The main component gas in this case may be an inert gas. Examples of inert gases include nitrogen gas, argon gas, helium gas, and mixtures thereof. For industrial use, nitrogen gas is preferred. The above oxygen concentration can be easily measured by methods such as analyzing the gas phase portion of the container in which the process is carried out using gas chromatography, using an oxygen concentration measuring instrument, or by examining the color of an oxygen indicator placed inside the container.

[0037] Furthermore, the environment in which step (1) is carried out may be under pressure, atmospheric pressure, or a reduced pressure environment. Here, a reduced pressure environment means an environment in which the air has been removed to a vacuum of 100 Pa or less using a vacuum pump such as a diaphragm pump, oil rotary pump, or scroll pump. The vacuum is preferably 50 Pa or less, more preferably 10 Pa or less, and even more preferably 1 Pa or less. As a method for maintaining the reduced pressure environment during the above process (1), a sealed container for reduced pressure may be used, or the reduced pressure environment may be maintained by continuously evacuating the container with a vacuum pump, or the reduced pressure environment inside the container may be maintained by repeatedly turning the vacuum pump on and off intermittently. To remove oxygen present in the environment and create a virtually oxygen-free environment, an oxygen adsorbent may be used. Oxygen absorbents are also called deoxidizing agents and are synonymous. Of course, oxygen adsorbents may be used in combination with the above method. Methods of combination include placing the oxygen adsorbent together with the high molecular weight PTFE in the above-mentioned sealed container, or uniformly or unevenly coating the inside of the sealed container with the oxygen adsorbent.

[0038] One method for carrying out step (1) in the substantially absence of oxygen is to carry out step (1) in a space in which substantially no oxygen is present.

[0039] The above-mentioned space in which virtually no oxygen exists refers to a space in which the oxygen concentration can be locally adjusted during the execution of process (1). For example, a sealed container (hereinafter referred to as a sealed container) can be used to adjust the oxygen concentration in the internal space. Alternatively, the space in which process (1) is carried out may be locally made into a space where oxygen is substantially absent by a gas shower with an inert gas or differential exhaust using a vacuum pump system. Furthermore, as a method for maintaining the substantially oxygen-free state in step (1) above using an inert gas, a sealed container may be used, the state may be maintained by circulating the inert gas, or the state may be maintained by repeatedly switching the circulation of the inert gas on and off intermittently.

[0040] The above-mentioned sealed container may be connected to piping for intake and exhaust of inert gases, etc., as described later, or for exhausting gases inside the sealed container, and may also be connected to other piping, lids, valves, flanges, etc. Furthermore, the shape of the above-mentioned sealed container is not particularly limited and may be cylindrical, prismatic, spherical, etc., or may be a bag with a variable internal volume. Furthermore, the material is not particularly limited and may be metal, glass, paper, polymer, composite material made by laminating these, etc. The above-mentioned sealed container is preferably made of a material and structure that allows radiation to pass through and does not deteriorate by radiation irradiation, but is not limited thereto. Furthermore, the above-mentioned sealed container is not limited to a pressure-resistant container.

[0041] In the above-mentioned sealed container, as the material for the bag with a variable internal volume, rubber materials that can be sealed by physical stress, such as ethylene-propylene rubber, tetrafluoroethylene-propylene rubber, chloroprene rubber, and polyester elastomer, as well as materials that can be sealed by heat fusion or epoxy adhesives, are preferred. Among these, thermoplastic organic materials that can be sealed by heat fusion are particularly preferred. In terms of being able to withstand the radiation irradiation in step (1), among the above-mentioned thermoplastic organic materials, polyesters such as polyethylene terephthalate (PET), polyamide (PA), polyethylene (PE), polyamide-imide (PAI), thermoplastic polyimide (TPI), polyphenylene sulfide (PPS), polyetherimide (PEI), cyclic polyolefin (COP), polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), hexafluoropropylene-tetrafluoroethylene copolymer (FEP), and perfluoroalkoxyalkane (PFA) are preferred. Furthermore, these materials may be multilayer materials with two or three layers, or even composite multilayer materials of organic and inorganic materials combined with aluminum or the like. Methods for compounding include vapor deposition, sputtering, and simply bonding the materials of each layer.

[0042] The state in which substantially no oxygen is present in the sealed container can be achieved, for example, by creating a substantially vacuum inside the sealed container or by filling it with an inert gas. Here, substantially vacuum means that the pressure inside the container is 100 Pa or less, preferably 50 Pa or less, more preferably 10 Pa or less, and even more preferably 1 Pa or less.

[0043] The inert gas mentioned above must be inert to the reaction of reducing the molecular weight of high molecular weight PTFE by radiation irradiation. Examples of such inert gases include nitrogen, helium, and argon. Among these, nitrogen is preferred.

[0044] The above inert gas preferably has an oxygen content of less than 2.0 volume%, more preferably 1.0 volume% or less, even more preferably less than 1.0 volume%, even more preferably 0.5 volume% or less, even more preferably 0.1 volume% or less, and particularly preferably 0.01 volume% or less. The lower limit is not particularly limited and may be an amount below the detection limit. The oxygen content can be determined by gas chromatography analysis, as well as by galvanic cell oxygen meter, zirconia oxygen meter, oxygen detection paper, etc.

[0045] The oxygen adsorbent described above is not particularly limited as long as it has the function of adsorbing oxygen, and known adsorbents that exhibit oxygen adsorption effects can be used, such as inorganic oxygen adsorbents such as iron-based, zinc-based, and hydrosulfite-based, and organic oxygen adsorbents such as ascorbic acid-based, polyhydric alcohol-based, and activated carbon-based. The oxygen adsorbent described above may be a water-dependent type that requires water when reacting with oxygen, or a self-reacting type that does not require water, but a self-reacting type is preferred. As the oxygen adsorbent described above, iron-based self-reacting oxygen adsorbents and quicklime are preferred, and among these, iron-based self-reacting oxygen adsorbents are preferred.

[0046] When step (1) is carried out in substantially the absence of oxygen, the manufacturing method of the present disclosure preferably includes a step of placing the high molecular weight PTFE into a sealed container in substantially the absence of oxygen before step (1). Methods for introducing the above-mentioned high molecular weight PTFE into a sealed container in substantially the absence of oxygen include, for example, a method in which the high molecular weight PTFE is placed in the sealed container, and then an oxygen adsorbent is added to the sealed container as needed, followed by vacuum degassing of the sealed container; a method in which the high molecular weight PTFE and at least one selected from the group consisting of an inert gas and an oxygen adsorbent are introduced into the sealed container; and a combination of these methods.

[0047] More specifically, examples include a vacuum degassing method in which the high molecular weight PTFE is placed in the sealed container, the container is degassed into a reduced-pressure environment using a vacuum pump, and the sealed container is then sealed; a gas replacement method in which the high molecular weight PTFE is placed in the sealed container, the container is evacuated as needed, and then the container is filled with the inert gas; a method in which the vacuum degassing method and the gas replacement method are repeated to create a substantially oxygen-free environment; and a gas flow replacement method in which the high molecular weight PTFE is placed in the sealed container, and the inert gas is continuously circulated through the container to gradually reduce the oxygen concentration and create a substantially oxygen-free environment as desired. Furthermore, when using the above-mentioned oxygen adsorbent, possible methods include: placing the above-mentioned high molecular weight PTFE and the above-mentioned oxygen adsorbent in the above-mentioned sealed container in air, and then sealing the sealed container; placing the above-mentioned high molecular weight PTFE and the above-mentioned oxygen adsorbent in the above-mentioned sealed container, then vacuum-degassing the inside of the sealed container, and then sealing the sealed container; or placing the above-mentioned high molecular weight PTFE and the above-mentioned oxygen adsorbent in the above-mentioned sealed container, vacuum-degassing the inside of the sealed container as necessary, and then filling the inside of the sealed container with the above-mentioned inert gas.

[0048] The irradiation in step (1) may be carried out by fixing the high molecular weight PTFE in front of the radiation source, or by moving the high molecular weight PTFE using a conveyor or the like so that it passes in front of the radiation source. It is preferable to irradiate the high molecular weight PTFE while moving it, as this makes it easy to obtain low molecular weight PTFE with small molecular weight variations.

[0049] Figure 1 shows an example of a radiation irradiation method. In the embodiment shown in Figure 1, an irradiation container 11 filled with high molecular weight PTFE powder is placed on an irradiation table 12, and radiation is irradiated onto the high molecular weight PTFE from a radiation source 10. If the irradiation platform is a transport device such as a conveyor, the irradiation is performed while moving the irradiation container so that it passes in front of the radiation source 10 (in the x-axis direction in the figure). In each figure, the x-axis represents the width direction of the radiation source 10, the y-axis represents the direction perpendicular to the radiation emission surface, and the z-axis represents the height direction of the radiation source 10.

[0050] In the above irradiation, the distance from the radiation source to the nearest part of the high molecular weight PTFE is not particularly limited, but for safety reasons, it is preferable to set it to 5 cm or more. Furthermore, the distance from the radiation source to the furthest part of the high molecular weight PTFE may be 10 m or less, 5 m or less, or 2 m or less. This makes it easy to keep the dose rate ratio within the aforementioned range. Note that the distance from the radiation source refers to the distance from the radiation emission surface.

[0051] Figure 2 is a cross-sectional view showing an example of an irradiation vessel filled with high molecular weight PTFE. In Figure 2, the distance from the radiation source 10 to the furthest part of the high molecular weight PTFE powder 100 filled in the irradiation container 11 is a, and the distance from the radiation source 10 to the closest part of the high molecular weight PTFE powder 100 is b.

[0052] In the above irradiation, it is preferable that the high molecular weight PTFE is positioned facing the effective region of the radiation source described below. The effective region may be the region on the radiation emission surface. (Effective area) A region in which the distance from the center of the radiation source is 95% or less, preferably 90% or less, of the distance from the center to the end of the radiation source. When the radiation source is rod-shaped or plate-shaped, a sufficient absorbed dose may not be obtained from its ends, such as near the top and bottom ends. By placing high molecular weight PTFE in a position facing the effective region of the radiation source as described above, a sufficient absorbed dose can be obtained from the entire high molecular weight PTFE, and the dose rate ratio can be easily kept within the range mentioned above. When irradiating the high molecular weight PTFE while moving it, it is preferable to move it so that each part of the high molecular weight PTFE passes through the above range at least once. When irradiating while moving the high molecular weight PTFE, the ends of the radiation source in the definition of the effective region may be the ends perpendicular to the direction of movement of the high molecular weight PTFE. For example, when moving the high molecular weight PTFE in the x-axis direction in Figure 1, the ends of the radiation source in the definition of the effective region may be the ends (upper and lower ends) in the z-axis direction.

[0053] Figure 3 is a cross-sectional view showing another example of an irradiation vessel filled with high molecular weight PTFE. In Figure 3, the high molecular weight PTFE powder 100 is positioned so as to face the effective region of the radiation source 10, at a distance c below the position corresponding to the upper end of the radiation source 10 and at a distance d above the position corresponding to the lower end of the radiation source 10.

[0054] The above irradiation is typically performed on the high molecular weight PTFE packed in the irradiation container. The above-mentioned irradiation container is preferably made of at least one material selected from the group consisting of metals, glass, ceramics, and organic materials, and is more preferably made of metal because it is less susceptible to degradation by radiation. Examples of the above-mentioned metals include stainless steel, copper, iron, titanium, and aluminum, with stainless steel being preferred for its versatility. More preferably, highly corrosion-resistant stainless steel is used. Examples of the above-mentioned organic materials include paper, rubber, FRP (fiber-reinforced plastic), thermoplastic organic materials, and the like. As for the irradiation container, a paper irradiation container is preferred because it can be used as a disposable item, and an FRP or aluminum irradiation container is preferred because it is lightweight. More preferably, a fiber drum is used, which is a combination of these.

[0055] The above-mentioned irradiation container is preferably a sealed container.

[0056] The above irradiation container has a density of 0.3 g / cm³. 3 Preferably, it is 1.0 g / cm³ or more. 3 It is more preferable that the amount be greater than or equal to 3.0 g / cm³. 3 It is even more preferable that the amount be greater than or equal to 19.3 g / cm³.3 It is preferably the following, 8.9 g / cm 3 More preferably, it is the following, 8.0 g / cm 3 Even more preferably, it is the following. By setting the density of the irradiation container within the above range, the dose rate ratio can be easily set within the above-mentioned range. The above density is measured by a normal density measurement method as an average density.

[0057] The shape of the irradiation container is not particularly limited and may be, for example, cylindrical, prismatic, etc. From the viewpoint of uniform irradiation, it is preferably cylindrical. Also, from the viewpoints of reducing the dead space at the installation location, facilitating the installation of the dosimeter, and facilitating container replacement and transportation, it is preferably prismatic, and more preferably quadrangular prismatic. The irradiation container may be a container with a variable internal volume, such as a bag. A container with a variable internal volume and a container with a non-variable internal volume may be used in combination.

[0058] The surface constituting the irradiation container may be plate-shaped, but from the viewpoint of practical weight reduction, for example, it may be net-shaped or may have slit-shaped openings. The irradiation container preferably has at least one surface selected from the group consisting of a plate-shaped surface, a net-shaped surface, and a surface having slit-shaped openings.

[0059] The size of the irradiation container is preferably such that the distance between the radiation source and the high molecular weight PTFE can be within the above-mentioned range. For example, when the irradiation container is cylindrical, the diameter is preferably 5 to 110 cm, more preferably 10 cm or more, and more preferably 55 cm or less. When the irradiation container is prismatic, the length of at least one side of the bottom surface is preferably 5 to 110 cm, more preferably 10 cm or more, and more preferably 55 cm or less. The height (depth) of the irradiation vessel can be determined, for example, depending on the amount of high molecular weight PTFE to be packed and the size of the radiation source.

[0060] The thickness of the irradiation container, preferably the thickness of the side surface of the irradiation container, is preferably 0.1 to 10 mm, more preferably 1 mm or more, and even more preferably 3 mm or less. By setting the thickness of the irradiation container within the above range, the dose rate ratio can be easily set within the aforementioned range.

[0061] The above irradiation is preferably carried out by arranging multiple irradiation containers filled with the above-mentioned high molecular weight PTFE in a row. Radiation can be scattered by the irradiation stand or other equipment on which the irradiation containers are placed, and scattered radiation can wrap around to the sides of the irradiation containers, which can result in a large difference in dose between the center and the edges of the containers. By arranging multiple irradiation containers in a row, the scattering of radiation is shielded by adjacent irradiation containers, enabling more uniform irradiation. The above embodiment is particularly suitable when the radiation source is plate-shaped. The irradiation containers are preferably arranged in a line in the width direction of the radiation source. The multiple irradiation containers may or may not be in contact with each other, but to enhance the shielding effect of scattered radiation, it is preferable to arrange them as close together as possible.

[0062] Figure 4 shows an example of the arrangement of the irradiation container. In Figure 4, multiple irradiation containers 11 are arranged in a line along the width direction (x-axis direction in the figure) of the radiation source 10.

[0063] The above irradiation may be carried out by stacking multiple irradiation containers filled with the high molecular weight PTFE in the height direction. In this embodiment, multiple irradiation containers may also be arranged in the width direction of the radiation source. This makes it possible to reduce the variation in molecular weight even in the upper section, where the variation in molecular weight tends to be large under normal circumstances.

[0064] The above irradiation is preferably performed on the high molecular weight PTFE from multiple different directions. Performing the irradiation in this manner allows for more uniform irradiation. Irradiation from multiple different directions does not necessarily need to be done simultaneously; it can be done sequentially by changing the orientation of the high molecular weight PTFE (or irradiation container) as described above. In the above embodiment, irradiation may be performed from two or more directions, but the more directions from which irradiation is performed, the more uniform the irradiation becomes. It is preferable to irradiate from at least one direction and the opposite direction.

[0065] The manufacturing method of the present disclosure may also include, prior to step (1), step (2) of heating the high molecular weight PTFE to above its primary melting point to obtain a molded article. In this case, the molded article obtained in step (2), the cutting chips generated when the molded article is machined, or the crushed molded article (coarse particles, powder, etc.) can be used as the high molecular weight PTFE in step (1). The primary melting point is preferably 300°C or higher, more preferably 310°C or higher, and even more preferably 320°C or higher. The primary melting point mentioned above refers to the maximum peak temperature of the endothermic curve that appears on the crystal melting curve when uncalcined high molecular weight PTFE is measured using a differential scanning calorimeter. The endothermic curve mentioned above was obtained by heating the material using a differential scanning calorimeter at a heating rate of 10°C / min.

[0066] The molded product in step (2) has a specific gravity of 1.0 g / cm³. 3 Preferably, it is 1.5 g / cm³ or more. 3 It is more preferable that the amount be greater than or equal to 2.5 g / cm³. 3 The following is preferable: When the specific gravity of the molded product is within the above range, the pores and irregularities on the surface become smaller, and as a result, a low molecular weight PTFE with a small specific surface area can be obtained. The above specific gravity can be measured by the submersion method. The manufacturing method of the present disclosure may further include a step of grinding the molded article after step (2). The molded article may be coarsely ground and then further ground into a finer consistency.

[0067] The manufacturing method of the present disclosure may also include, after step (1), a step of further grinding the low molecular weight PTFE to obtain a low molecular weight PTFE powder.

[0068] The above-mentioned crushing method is not particularly limited, but one example is crushing using a crushing machine. Examples of such crushing machines include impact-type machines such as planetary mills, hammer mills, pin mills, and jet mills, and grinding-type machines such as cutter mills, which crush using shear force due to the uneven surface of the rotating blades and outer stator.

[0069] The grinding temperature is preferably between -200°C and below 50°C. While cryogenic grinding is typically performed at -200 to -100°C, grinding may also be performed at temperatures near room temperature (10 to 30°C). Cryogenic grinding generally uses liquid nitrogen, but this requires extensive equipment and increases grinding costs. Grinding at 10°C and below 50°C is more preferable, grinding at 10 to 40°C is even more preferable, and grinding at 10 to 30°C is particularly preferable, as it simplifies the process and reduces grinding costs.

[0070] After the grinding described above, fine particles and fibrous particles may be removed by airflow classification, and then coarse particles may be removed by further classification.

[0071] In airflow classification, crushed particles are sent to a cylindrical classification chamber by reduced-pressure air, dispersed by a swirling airflow within the chamber, and then classified by centrifugal force. The fine particles are collected from the center into a cyclone and bag filter. Rotating bodies such as conical cones and rotors are installed inside the classification chamber to ensure that the crushed particles and air move uniformly in a swirling motion.

[0072] When using classification cones, the classification point is adjusted by adjusting the airflow of the secondary air and the gap between the classification cones. When using a rotor, the airflow inside the classification chamber is adjusted by the rotation speed of the rotor.

[0073] Methods for removing coarse particles include airflow classification using mesh, vibrating screens, and ultrasonic screens, but airflow classification is preferred.

[0074] Next, we will describe the high molecular weight PTFE irradiated with radiation in step (1) of the manufacturing method of this disclosure, and the low molecular weight PTFE obtained after carrying out step (1).

[0075] The low molecular weight PTFE obtained after carrying out step (1) has a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 Pa·s or more, 7.0×10 5 It is Pa·s or less. In this disclosure, "low molecular weight" means that the melt viscosity is within the above range. The above melt viscosity is 1.0 × 10⁻⁶ 3 It is preferable that it be Pa·s or higher, and 1.5 × 10 3 It is more preferable that it be Pa·s or higher, and 1.0 × 10 4 It is even more preferable that it be Pa·s or higher, and also 3.0 × 10 5 It is preferable that it be less than or equal to Pa·s, and 1.0 × 10 5 It is more preferable that it be less than or equal to Pa·s.

[0076] The above melt viscosity was measured in accordance with ASTM D 1238, using a flow tester (manufactured by Shimadzu Corporation) and a 2φ-8L die. A 2g sample, preheated to 380°C for 5 minutes, was maintained at the above temperature under a load of 0.7MPa.

[0077] The high molecular weight PTFE irradiated with the above radiation preferably has a standard specific gravity (SSG) of 2.130 to 2.230. The above standard specific gravity (SSG) is a value measured in accordance with ASTM D 4895.

[0078] The high molecular weight PTFE described above has an extremely high melt viscosity compared to the low molecular weight PTFE described above, making it difficult to accurately measure its melt viscosity. On the other hand, while the melt viscosity of low molecular weight PTFE can be measured, it is difficult to obtain molded articles from low molecular weight PTFE that can be used to measure standard specific gravity, making it difficult to accurately measure its standard specific gravity. Therefore, in this disclosure, standard specific gravity is used as an indicator of the molecular weight of the high molecular weight PTFE, and melt viscosity is used as an indicator of the molecular weight of the low molecular weight PTFE. It should be noted that no measurement method is known that can directly determine the molecular weight of either the high molecular weight PTFE or the low molecular weight PTFE.

[0079] The above low molecular weight PTFE preferably has a melting point of 320 to 340°C, and more preferably 324 to 336°C.

[0080] The melting point described above was determined using a differential scanning calorimeter (DSC). After pre-calibrating the temperature using standard samples of indium and lead, approximately 3 mg of low molecular weight PTFE was placed in an aluminum pan (crimped container) and heated in the temperature range of 250-380°C at a rate of 10°C / min under an air flow of 200 ml / min. The point at which the heat of fusion was minimized in the above range was defined as the melting point.

[0081] In the manufacturing method of the present disclosure, the shape of the high molecular weight PTFE is not particularly limited and may be a powder (fine powder, molding powder, etc.), a molded article of the high molecular weight PTFE, cutting chips generated when the molded article of the high molecular weight PTFE is machined, or a pulverized molded article of the high molecular weight PTFE (coarse particles, powder, etc.). If the high molecular weight PTFE is a powder, the low molecular weight PTFE powder can be easily obtained. Furthermore, the above-mentioned high molecular weight PTFE may be crosslinked.

[0082] Furthermore, the form of the low molecular weight PTFE obtained by the manufacturing method of this disclosure is not particularly limited, but it is preferably in powder form.

[0083] When the low molecular weight PTFE obtained by the manufacturing method of this disclosure is in powder form, the specific surface area is 0.5 to 25 m². 2 It is preferable that it be / g. As a low molecular weight PTFE powder, it has a specific surface area of ​​0.5 m². 2 / g or more, 7.0m 2 Types with a low specific surface area of ​​less than / g, and types with a specific surface area of ​​7.0m² 2 / g or more, 25m 2 Types with a high specific surface area of ​​less than / g are being sought. Low molecular weight PTFE powders with a low specific surface area have the advantage of being easily dispersed in matrix materials such as paints, but their dispersion particle size in the matrix material is large, resulting in poor fine dispersion. The specific surface area of ​​low molecular weight PTFE powder of the type with a low specific surface area is 1.0 m². 2 Preferably 5.0 m 2 Preferably less than / g, and 3.0m 2 A value of less than / g is more preferable. In addition to plastics and inks, paints and the like can also be suitably used as matrix materials. Low molecular weight PTFE powders with a high specific surface area, when dispersed in a matrix material such as paint, have a small particle size distribution within the matrix material, resulting in a high surface modification effect, such as improving the texture of the coating surface, and also increasing oil absorption. However, they may not disperse easily, requiring a long time to disperse into the matrix material, and may also increase the viscosity of the paint. The specific surface area of ​​a low molecular weight PTFE powder with a high specific surface area is 8.0 m². 2 Preferably 20m / g or more. 2 A value of less than / g is preferred. In addition to oils, greases, and paints, plastics and the like are also suitably used as matrix materials.

[0084] The specific surface area described above is measured using a surface analyzer (product name: BELSORP-miniII, manufactured by Microtrac-Bel Co., Ltd.) with a mixed gas of 30% nitrogen and 70% helium as the carrier gas, and liquid nitrogen for cooling, using the BET method.

[0085] When the low molecular weight PTFE obtained by the manufacturing method of this disclosure is in powder form, the average particle size is preferably 0.5 to 200 μm, more preferably 50 μm or less, even more preferably 25 μm or less, and particularly preferably 10 μm or less. Having a relatively small average particle size allows for the formation of coating films with superior surface smoothness, for example, when used as an additive in paints.

[0086] The above average particle diameter was measured using a laser diffraction particle size distribution analyzer (HELOS&RODOS) manufactured by JEOL Ltd., without using a cascade, at a dispersion pressure of 3.0 bar, and was defined as being equal to the particle diameter corresponding to 50% of the integrated particle size distribution.

[0087] The above low molecular weight PTFE may have carboxyl groups at the end of its molecular chain. The number of carboxyl groups mentioned above corresponds to 10 carbon atoms in the main chain. 6 Each unit may contain 5 or fewer items, more than 5 items, or 30 or more items. When the above irradiation is carried out in the substantially absence of oxygen, the number of carboxyl groups may be five or less. The number of carboxyl groups mentioned above was measured using the method described below. The detection limit for this measurement method is 0.5 groups. (Measurement method) The following measurements are performed in accordance with the method for analyzing terminal groups described in Japanese Patent Publication No. 4-20507. Low molecular weight PTFE powder is pre-formed by hand pressing to create films with a thickness of 0.1 to 1.0 mm. The prepared films are subjected to infrared absorption spectroscopy. Infrared absorption spectroscopy is also performed on PTFE with completely fluorinated ends, which is prepared by contacting PTFE with fluorine gas. The number of terminal carboxyl groups is calculated from the difference spectra of the two films using the following formula. Number of terminal carboxyl groups (10 carbon atoms) 6 (per item) = (l × K) / t l: Absorbance K: Correction coefficient t: Film thickness (mm) The absorption frequency of the carboxyl group is 3560 cm⁻¹.-1 The correction factor for both the front and rear is set to 440.

[0088] The molecular chain ends of the low molecular weight PTFE may have unstable end groups derived from the chemical structure of the polymerization initiator or chain transfer agent used in the polymerization reaction of the high molecular weight PTFE. The unstable end groups are not particularly limited and include, for example, -CH2OH, -COOH, -COOCH3, etc.

[0089] The low molecular weight PTFE described above may have its unstable end groups stabilized. The method for stabilizing the unstable end groups is not particularly limited and includes, for example, a method of changing the end to a trifluoromethyl group [-CF3] by exposure to a fluorine-containing gas.

[0090] The low molecular weight PTFE described above may also be subject to terminal amidation. The method of terminal amidation is not particularly limited, and examples include contacting ammonia gas with a fluorocarbonyl group [-COF] obtained by exposure to a fluorine-containing gas, as disclosed in Japanese Patent Application Publication No. 4-20507.

[0091] When the above-mentioned low molecular weight PTFE undergoes stabilization of the unstable end groups or terminal amidation, it can be used as an additive to mating materials such as paints, greases, cosmetics, plating solutions, toners, and plastics, as it blends well with the mating material and improves dispersibility.

[0092] The high molecular weight PTFE irradiated with radiation may be homo-PTFE consisting only of tetrafluoroethylene (TFE) units, or it may be modified PTFE containing TFE units and modified monomer units based on modified monomers copolymerizable with TFE. In the manufacturing method of this disclosure, the polymer composition does not change, so the low molecular weight PTFE has the same composition as the high molecular weight PTFE irradiated with radiation.

[0093] In the above-mentioned modified PTFE, the content of the modified monomer units is preferably 0.001 to 1% by mass of the total monomer units, more preferably 0.01% by mass or more, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less. In this specification, the above-mentioned modified monomer units mean a part of the molecular structure of modified PTFE that is derived from the modified monomer, and the total monomer units mean a part derived from all monomers in the molecular structure of modified PTFE. The content of the above-mentioned modified monomer units can be determined by known methods such as Fourier transform infrared spectroscopy (FT-IR).

[0094] The above-mentioned modified monomers are not particularly limited as long as they can copolymerize with TFE, and examples include perfluoroolefins such as hexafluoropropylene [HFP]; chlorofluoroolefins such as chlorotrifluoroethylene [CTFE]; hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride [VDF]; perfluorovinyl ethers; perfluoroalkyl ethylenes; and ethylene. Furthermore, one or more modified monomers may be used.

[0095] The above perfluorovinyl ether is not particularly limited, for example, the following general formula (1) CF2 = CF - ORf (1) Examples include perfluorounsaturated compounds represented by the formula (wherein Rf represents a perfluoroorganic group). In this specification, the term "perfluoroorganic group" means an organic group in which all hydrogen atoms bonded to a carbon atom are replaced with fluorine atoms. The perfluoroorganic group may have an ether oxygen.

[0096] Examples of the perfluorovinyl ethers mentioned above include perfluoro(alkyl vinyl ether) [PAVE] in which Rf in the general formula (1) above represents a perfluoroalkyl group having 1 to 10 carbon atoms. The number of carbon atoms in the perfluoroalkyl group is preferably 1 to 5.

[0097] Examples of perfluoroalkyl groups in the above-mentioned PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, and perfluorohexyl groups, but perfluoro(propyl vinyl ether) [PPVE] in which the perfluoroalkyl group is a perfluoropropyl group is preferred.

[0098] The above perfluorovinyl ethers are further defined as those in the above general formula (1) where Rf is a perfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, and where Rf is in the following formula:

[0099] [ka]

[0100] (In the formula, m represents an integer from 0 to 4.) The base is represented by the following formula, where Rf is:

[0101] [ka]

[0102] Examples include the base represented by (wherein n represents an integer from 1 to 4).

[0103] The perfluoroalkylethylene is not particularly limited and examples include (perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene, and (perfluorooctyl)ethylene.

[0104] The modified monomer in the above-mentioned modified PTFE is preferably at least one selected from the group consisting of HFP, CTFE, VDF, PPVE, PFBE, and ethylene. More preferably, it is at least one selected from the group consisting of HFP and CTFE.

[0105] The above-mentioned low molecular weight PTFE can be suitably used as a molding material, ink, cosmetic, paint, grease, component for office automation equipment, additive for modifying toner, organic photoreceptor material for photocopiers, additive for plating solutions, etc. Examples of the molding material include engineering plastics such as polyoxybenzoyl polyester, polyimide, polyamide, polyamide-imide, polyacetal, polycarbonate, and polyphenylene sulfide. The above-mentioned low molecular weight PTFE is particularly suitable as a viscosity modifier for greases.

[0106] The above-mentioned low molecular weight PTFE can be suitably used as an additive to molding materials, for example, to improve the non-stick and sliding properties of copy rolls, to improve the texture of engineering plastic molded products such as surface sheets for furniture, car dashboards, and covers for home appliances, to improve the lubricity and wear resistance of mechanical parts that generate mechanical friction, such as light load bearings, gears, cams, push-button buttons, projectors, camera parts, and sliding materials, and as a processing aid for engineering plastics.

[0107] The above-mentioned low molecular weight PTFE can be used as an additive in paints to improve the slipperiness of varnishes and paints. The above-mentioned low molecular weight PTFE can be used as an additive in cosmetics to improve the slipperiness of cosmetics such as foundations.

[0108] The low molecular weight PTFE described above is also suitable for applications that improve the oil-repellent or water-repellent properties of waxes, etc., and for applications that improve the lubricity of greases and toners.

[0109] The above-mentioned low molecular weight PTFE can also be used as an electrode binder for secondary batteries and fuel cells, a hardness modifier for electrode binders, and a water-repellent treatment agent for electrode surfaces.

[0110] Grease can also be prepared using the above-mentioned low molecular weight PTFE and lubricating oil. Since the above-mentioned grease contains the above-mentioned low molecular weight PTFE and lubricating oil, the low molecular weight PTFE is uniformly and stably dispersed in the lubricating oil, and it has excellent properties such as load-bearing capacity, electrical insulation, and low moisture absorption.

[0111] The above-mentioned lubricating oil (base oil) may be a mineral oil or a synthetic oil. Examples of the above-mentioned lubricating oil (base oil) include paraffinic or naphthenic mineral oils, synthetic hydrocarbon oils, ester oils, fluorine oils, and silicone oils. From the viewpoint of heat resistance, fluorine oil is preferred, and examples of the above-mentioned fluorine oil include perfluoropolyether oil and low polymers of trifluoroethylene chloride. The low polymers of trifluoroethylene chloride may have a weight-average molecular weight of 500 to 1200.

[0112] The above grease may further contain a consistency thickener. Examples of the above consistency thickener include metal soaps, complex metal soaps, bentonite, phthalocyanine, silica gel, urea compounds, urea-urethane compounds, urethane compounds, and imide compounds. Examples of the above metal soaps include sodium soap, calcium soap, aluminum soap, and lithium soap. Examples of the above urea compounds, urea-urethane compounds, and urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, urea-urethane compounds, diurethane compounds, or mixtures thereof.

[0113] The above grease preferably contains 0.1 to 60% by mass of the above low molecular weight PTFE, more preferably 0.5% by mass or more, even more preferably 5% by mass or more, and most preferably 50% by mass or less. If the amount of the above low molecular weight PTFE is too high, the grease may become too hard and may not be able to exhibit sufficient lubrication, and if the amount of the above low molecular weight PTFE is too low, it may not be able to exhibit sealing properties.

[0114] The above-mentioned grease may also contain solid lubricants, extreme pressure agents, antioxidants, oiliness agents, rust inhibitors, viscosity index improvers, cleaning dispersants, etc. [Examples]

[0115] The present disclosure will now be further described with reference to examples, but the present disclosure is not limited to these examples.

[0116] The "Overall Assessment" column in the table was determined according to the following criteria. ○: PTFE melt viscosity of 1.0 × 10 at all sampling locations 4 Pa·s or more, 1.0×10 5 It falls within the range of Pa·s or less. ×: Other than the above

[0117] Example 1 8.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed on a balance and placed in a 30L aluminum airtight bag (polyethylene inner bag with stopcock). Next, an amount equivalent to the volume of space was added and the bag was sealed. After evacuating the bag using an oil rotary pump through the stopcock attached to the airtight bag, the inside of the bag was reduced to a reduced pressure state (20 kPa). The pressure was returned to atmospheric pressure with nitrogen gas and then reduced again. After repeating this three times, the base of the stopcock was heat-sealed to completely seal the bag. This airtight bag was placed in a 238mmW×238mmD×350mmH SPTFS, 0.3mm thick 18-liter can, and the lid was secured with aluminum tape to prevent it from coming off. Three of these cans were prepared. The three 18-liter cans were arranged parallel to the gamma ray source. The irradiation temperature was 30°C. After 36.5 hours of irradiation, the irradiation was stopped and the 18-liter can was rotated 180°. Irradiation was restarted and terminated after another 36.5 hours. The airtight bag was removed from the can and heat-treated in an electric furnace at 80°C for 24 hours to obtain low molecular weight PTFE powder. The airtight bag was opened and samples were taken from five locations using a pencil-type powder sampler (manufactured by Tsutsui Rikagakukikai Co., Ltd.). Figure 5 shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, as viewed from above. The melt viscosity of the obtained low molecular weight PTFE was measured. Based on the calculation formula below, which was obtained from the experiment described later, the absorbed dose at that location was calculated from the measured melt viscosity. Absorbed dose (kGy) = (LN(Melting viscosity (10) 3 Pa·s))-7.32) / (-0.0093) The dose rate was calculated from the obtained absorbed dose and irradiation time, and the ratio of the maximum to minimum value was determined. The results are shown in Table 1. Furthermore, the results of the calculation simulation are shown in Table 1. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0118] Experiment to determine the relationship between the melt viscosity and absorbed dose of low molecular weight PTFE The relationship between the melt viscosity and absorbed dose of low molecular weight PTFE was determined by the following experiment. 20 g of high molecular weight PTFE fine powder (standard specific gravity measured according to ASTM D 4895: 2.171, homogeneous) was weighed using a balance and placed in a barrier nylon bag. One oxygen absorber (A-500HS, manufactured by AS ONE Corporation) was then added. Next, the inside of the bag was reduced to a reduced pressure (20 torr) using a vacuum sealer (V-300-10W, manufactured by Fuji Impulse Co., Ltd.), and then sealed by heat sealing. After confirming the absence of oxygen inside the bag using an oxygen indicator that had been pre-installed inside the bag, the barrier nylon bag was irradiated with 400 kGy (calculated value from simulation) of gamma rays. The irradiation conditions at this time were an average dose rate of 6.25 kGy / h at the center of the sample (measured value by chemical dosimeter), and the room temperature in the irradiation facility was 25°C. Next, the bag was stored at room temperature (20-28°C) for 15 days without opening (natural deactivation process). Then, without opening the bag, it was heat-treated at 80°C for 18 hours using a FORCED CONVECTION OVEN (DRX620DA, Advantech Co., Ltd.) to obtain low molecular weight PTFE powder. Using a similar method, low molecular weight PTFE was obtained by irradiating with gamma rays at absorbed doses of 300 kGy, 350 kGy, 450 kGy, and 500 kGy. The melt viscosity of these powders was measured, and a relationship between the logarithm of the melt viscosity and the absorbed dose was determined.

[0119] Comparative Example 1 For comparison, the procedure was the same as in Example 1, except that the 18-liter can was not inverted during gamma ray irradiation. The results are shown in Table 1. At sampling points C to E, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, sampling locations A and B deviate from this range, indicating a large variation in the molecular weight of low molecular weight PTFE.

[0120] [Table 1]

[0121] Example 2 11.0 kg of high molecular weight PTFE fine powder (2) (standard specific gravity 2.175, modified form, measured according to ASTM D 4895) was weighed on a balance and placed in a 30L aluminum airtight bag (polyethylene inner bag with stopcock). The rest of the procedure was the same as in Example 1. One of the 18L cans was placed parallel to the gamma ray source. The irradiation temperature was 30°C. After 64 hours of irradiation, the irradiation was stopped and the 18L can was inverted 180°C. Irradiation was restarted and ended after 70 hours. The airtight bag was removed from the 18L can and heat-treated in an electric furnace at 80°C for 18 hours to obtain low molecular weight PTFE powder. The airtight bag was opened and samples were taken from three locations using a pencil-type powder sampler (manufactured by Tsutsui Rikagakukikai Co., Ltd.). Figure 6 shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, as viewed from above. The melt viscosity of the obtained low molecular weight PTFE was measured. Based on the calculation formula below, which was obtained from the experiment described later, the absorbed dose at that location was calculated from the measured melt viscosity. Absorbed dose (kGy) = (LN(Melting viscosity (10) 3 Pa·s))-7.32) / (-0.0098). The dose rate was calculated from the obtained absorbed dose and irradiation time, and the ratio of the maximum value to the minimum value was determined. The results are shown in Table 2. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0122] Experiment to determine the relationship between the melt viscosity and absorbed dose of low molecular weight PTFE The relationship between the melt viscosity and absorbed dose of low molecular weight PTFE was determined by the following experiment. 20 g of high molecular weight PTFE fine powder (standard specific gravity measured according to ASTM D 4895: 2.175, modified form) was weighed using a balance and placed in a barrier nylon bag. One oxygen absorber (A-500HS, manufactured by AS ONE Corporation) was then added. Next, the inside of the bag was reduced to a reduced pressure (20 torr) using a vacuum sealer (Fuji Impulse Co., Ltd. V-300-10W), and then sealed by heat sealing. After confirming the absence of oxygen inside the bag using an oxygen indicator that had been pre-installed inside the bag, the barrier nylon bag was irradiated with 400 kGy (calculated value by simulation) of gamma rays. The irradiation conditions at this time were an average dose rate of 6.25 kGy / h at the center of the sample (measured value by chemical dosimeter), and the room temperature in the irradiation facility was 25°C. Next, the bag was stored at room temperature (20-28°C) for 15 days without opening (natural deactivation process). Then, without opening the bag, it was heat-treated at 80°C for 18 hours using a FORCED CONVECTION OVEN (DRX620DA, Advantech Co., Ltd.) to obtain low molecular weight PTFE powder. Using a similar method, low molecular weight PTFE was obtained by irradiating with gamma rays at absorbed doses of 300 kGy, 350 kGy, 450 kGy, and 500 kGy. The melt viscosity of these powders was measured, and a relationship between the logarithm of the melt viscosity and the absorbed dose was determined.

[0123] Comparative Example 2 For comparison, the procedure was the same as in Example 2, except that the 18-liter can was not inverted during gamma ray irradiation. The results are shown in Table 2. At sampling point C, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, sampling locations A and B deviate from this range, indicating a large variation in the molecular weight of low molecular weight PTFE.

[0124] [Table 2]

[0125] Example 3 10.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed on a balance and placed in a PBT-composed bag (600 mm x 1000 mm). Next, a self-acting oxygen absorber was added in an amount equivalent to the empty space. For vacuum packaging, a chamber-type vacuum packaging machine (MULTIVAC Japan, model C550, manufactured by Tokyo Food Machinery Co., Ltd.) was used. After reducing the pressure inside the chamber (5 mbar), the pressure was returned to 650 mbar with 99.998% pure nitrogen gas, and this process was repeated three times. Finally, the bag was sealed completely by welding with a sealing bar. Two of these packages were placed in a fiber drum (φ525 mm x 320 mm H) and the lid was closed. A total of four similar fiber drums containing the raw material were prepared (main drums). In addition to the above, 20.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed using a balance, placed in a PBT-structured bag (600 mm x 1000 mm), sealed with a cable tie, and placed in the same fiber drum as above to create eight dummy drums. On the first layer of an aluminum pallet (110mm x 110mm), four dummy drums were placed, and four main drums were placed on top of them. Four more dummy drums were placed on the third layer, and four empty fiber drums were placed on the fourth layer. In the same facility, a pallet containing a drum was placed on a conveyor belt, and irradiation was performed 360° around the pallet. After 51 passes of irradiation, the pallet was removed from the facility. After rotating the fiber drum inside the pallet, the pallet was brought back into the facility and irradiated for another 51 passes. At a rate of 1.02 hours per pass, the total irradiation time was 103 hours. After irradiation, the airtight bag in one of the main drums was opened, and samples were taken from 10 locations using a pencil-type powder sample collector (manufactured by Tsutsui Rikagakukikai Co., Ltd.). Figure 7 shows the relative positions of the main drum as seen from above, and Figure 8 shows the sampling locations. The melt viscosity of the obtained low molecular weight PTFE was measured. Based on the calculation formula obtained in Example 1, the absorbed dose at that location was calculated from the measured melt viscosity. Absorbed dose (kGy) = (LN(Melting viscosity (10) 3 Pa·s))-7.32) / (-0.0093) The dose rate was calculated from the obtained absorbed dose and irradiation time, and the ratio of the maximum to minimum value was determined. The results are shown in Table 3. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0126] [Table 3]

[0127] Example 4 20.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed on a balance and placed in a 50L aluminum airtight bag (polyethylene inner bag with stopcock). Next, an amount equivalent to the volume of space was added and the bag was sealed. After evacuating the bag using an oil rotary pump through the stopcock attached to the airtight bag, the inside of the bag was reduced to a reduced pressure state (20 kPa). The pressure was returned to atmospheric pressure with nitrogen gas and then reduced again. After repeating this three times, the base of the stopcock was heat-sealed to completely seal the bag. The airtight bag was placed in a fiber drum (material: cardboard) measuring Φ525 mm × 350 mm H and the lid was closed. The fiber drum was placed parallel to the gamma ray source. The irradiation temperature was 30°C. After 15 hours of irradiation, the irradiation was stopped and the fiber drum was rotated 90°. Irradiation was restarted and stopped again after another 15 hours. The fiber drum was then rotated another 90°. This process was repeated to irradiate each of the four sides for 15 hours, for a total of 60 hours. The airtight bag was removed from the fiber drum and heated in an electric furnace at 80°C for 24 hours to obtain low molecular weight PTFE powder. The airtight bag was opened, and samples were taken from the center and wall of the fiber drum using a pencil-type powder sampler (manufactured by Tsutsui Rikagakukikai Co., Ltd.). The melt viscosity of the obtained low molecular weight PTFE was measured. Similar to Example 1, the absorbed dose at each location was calculated from the measured melt viscosity, the dose rate was calculated from the obtained absorbed dose and irradiation time, and the ratio of the maximum and minimum values ​​was determined. The maximum dose rate was observed at locations A and B in Figure 6, and the minimum dose rate was observed at location C in Figure 6. The results are shown in Table 4. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0128] Comparative Example 3 For comparison, irradiation was stopped after 30 hours, and the fiber drum was rotated 180°. Irradiation was restarted and stopped after another 30 hours, for a total of 60 hours of irradiation. Otherwise, the same procedure as in Example 4 was followed. The maximum dose rate was observed at locations A and B in Figure 6, and the minimum dose rate was observed at location C in Figure 6. The results are shown in Table 4. At sampling locations A and B, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, sampling point C deviates from the above range, indicating that the molecular weight variation of low molecular weight PTFE is large.

[0129] Example 5 8.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed on a balance and placed in a 30L aluminum airtight bag (polyethylene inner bag with stopcock). This airtight bag was placed in a fiber drum (material: cardboard) measuring Φ305 mm × 345 mm H and the lid was closed. Otherwise, the procedure was the same as in Example 4. The dose rate was highest at locations A and B in Figure 6, and lowest at location C in Figure 6. The results are shown in Table 4. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0130] Comparative Example 4 8.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) was weighed on a balance and placed in a 30L aluminum airtight bag (polyethylene inner bag, with stopcock). This airtight bag was placed in a fiber drum (material: cardboard) measuring Φ305 mm × 345 mm H and the lid was closed. Otherwise, the same procedure as in Comparative Example 3 was performed. The dose rate was highest at locations A and B in Figure 6, and lowest at location C in Figure 6. The results are shown in Table 4. At sampling locations A and B, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, sampling point C deviates from the above range, indicating that the molecular weight variation of low molecular weight PTFE is large.

[0131] [Table 4]

[0132] Example 6 The procedure was the same as in Example 4, except that a container with dimensions of Φ500mm × 400mmH and made of SUS (stainless steel) was used. The dose rate was highest at locations A and B in Figure 6, and lowest at location C in Figure 6. The results are shown in Table 5. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0133] Comparative Example 5 The procedure was the same as in Comparative Example 3, except that the container was made of SUS with dimensions of Φ500mm × 400mmH. The results are shown in Table 5. At sampling locations A and B, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5Although the values ​​fall within the range of Pa·s or less, sampling point C deviates from the above range, indicating that the molecular weight variation of low molecular weight PTFE is large.

[0134] Example 7 The procedure was the same as in Example 4, except that a container with dimensions of Φ300mm × 400mmH and made of SUS (stainless steel) was used. The dose rate was highest at locations A and B in Figure 6, and lowest at location C in Figure 6. The results are shown in Table 5. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0135] Comparative Example 6 The procedure was the same as in Comparative Example 3, except that the container was made of stainless steel with dimensions of Φ300mm × 400mmH. The results are shown in Table 5. At sampling locations A and B, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, sampling point C deviates from the above range, indicating that the molecular weight variation of low molecular weight PTFE is large.

[0136] [Table 5]

[0137] Example 8 The dose distribution was simulated for three 18-liter cans (238mmW×238mmD×350mmH, made of SPTFS, 0.3mm thick) containing 8.0 kg of high molecular weight PTFE fine powder (1) (standard specific gravity 2.171, homogeneous, measured according to ASTM D 4895) placed in these cans and secured with aluminum tape to prevent the lids from coming off. These cans were arranged in a plane parallel to the radiation source. Figure 9 shows the positional relationship between the radiation source and the irradiated object as viewed from above, as well as the sampling locations. In the plane parallel to the source (X-axis direction), the dose distribution at a point 10 cm from the source (Y-axis direction) was calculated for five points at distances of -10 cm, -5 cm, 0 cm, 5 cm, and 10 cm from the center of the can. Furthermore, the dose distribution in the vertical direction (Z-axis direction) when the cans were stacked five high was calculated. Figure 10 shows the stacked cans five high. As a result, the dose rate tended to increase as the layers were stacked from the first to the fifth layer. When the dose rate ratio was calculated between the value at the first layer (where the dose rate is minimum) and the maximum value at each layer, all values ​​were 1.55 or less. At all sampling locations, the melt viscosity was 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 The values ​​fall within the range of Pa·s or less, indicating that the molecular weight variation of low molecular weight PTFE has been reduced.

[0138] [Table 6]

[0139] Example 9 The procedure was the same as in Example 8, except that instead of arranging three 18-liter cans side by side, only one was placed. In stages 1-3, the melt viscosity is 1.0 × 10⁻⁶. 4 Pa·s or more, 1.0×10 5 Although the values ​​fall within the range of Pa·s or less, the 4th and 5th stages deviate from the above range, and when the evaluation includes the 4th and 5th stages, the variation in molecular weight of low molecular weight PTFE becomes somewhat larger.

[0140] [Table 7] [Explanation of Symbols]

[0141] 10: Source 11: Irradiation container 12: Irradiation table 100: High molecular weight PTFE powder

Claims

1. A high molecular weight polytetrafluoroethylene having a standard specific gravity of 2.130 or more and 2.230 or less is irradiated with radiation such that the ratio of the maximum dose rate to the minimum dose rate (maximum dose rate / minimum dose rate) is 1.55 or less, and the melt viscosity at 380°C is 1.0 × 10 ２ Pa・s or more, 7.0×10 ５ The process includes (1) obtaining low molecular weight polytetrafluoroethylene having a density of Pa·s or less, Multiple irradiation containers filled with the aforementioned high molecular weight polytetrafluoroethylene are arranged in a row, and the irradiation is performed. A method for producing low molecular weight polytetrafluoroethylene.

2. The manufacturing method according to claim 1, wherein the minimum absorbed dose of the radiation is 200 kGy or more.

3. The manufacturing method according to claim 1 or 2, wherein the irradiation is carried out substantially in the absence of oxygen.

4. The manufacturing method according to claim 1 or 2, wherein the radiation is an electron beam, a gamma ray, or an X-ray.

5. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed with the distance from the radiation source to the furthest part of the high molecular weight polytetrafluoroethylene being 10 m or less.

6. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed with a distance of 5 cm or more from the radiation source to the nearest portion of the high molecular weight polytetrafluoroethylene.

7. The manufacturing method according to claim 1 or 2, wherein the high molecular weight polytetrafluoroethylene is placed at a position facing the effective region of the radiation source, and the irradiation is performed, wherein the effective region is a region where the distance from the center of the radiation source is 95% or less of the distance from the center to the end of the radiation source.

8. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed on the high molecular weight polytetrafluoroethylene filled in an irradiation container made of at least one material selected from the group consisting of metals, glass, ceramics, and organic materials.

9. The manufacturing method according to claim 8, wherein the irradiation container is cylindrical and prismatic.

10. The manufacturing method according to claim 8, wherein the irradiation container has at least one surface selected from the group consisting of a plate-shaped surface, a mesh-like surface, and a surface having a slit-shaped opening.

11. The manufacturing method according to claim 1 or 2, wherein both the high molecular weight polytetrafluoroethylene and the low molecular weight polytetrafluoroethylene are in powder form.

12. The process includes, prior to step (1), step (2) of heating the high molecular weight polytetrafluoroethylene to above its primary melting point to obtain a molded article, wherein the molded article has a specific gravity of 1.0 g / cm³. ３ The manufacturing method according to claim 1 or 2, wherein the above is true.

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