Process for the production of low molecular weight polytetrafluoroethylene and powder

Abstract

This invention provides a method for manufacturing low molecular weight polytetrafluoroethylene (PTFE) with reduced content of perfluorocarboxylic acids and their salts containing 4 to 16 carbon atoms. The invention relates to a method for manufacturing low molecular weight PTFE, comprising the step (1) irradiating PTFE containing perfluorocarboxylic acids or their salts containing 4 to 16 carbon atoms, and whose heat of fusion reduction rate from the first heating to the second heating in differential scanning calorimetry is less than 40%, with 5 to 1000 kGy of radiation at a temperature less than 100°C under substantially oxygen-free conditions, thereby obtaining PTFE with reduced content of the aforementioned perfluorocarboxylic acids and their salts and a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 ~7.0×10 5 Low molecular weight polytetrafluoroethylene (PTFE) with a strength of Pa·s.

Description

Technical Field

[0001] This invention relates to a method for manufacturing low molecular weight polytetrafluoroethylene and to powder thereof. Background Technology

[0002] Low molecular weight polytetrafluoroethylene (also known as "PTFE wax" or "PTFE micro powder") with a molecular weight of several thousand to hundreds of thousands has excellent chemical stability, very low surface energy, and is not prone to fibrillation. Therefore, it is used as an additive to improve lubricity and coating surface texture in the manufacture of plastics, inks, cosmetics, coatings, greases, etc. (see, for example, Patent Document 1).

[0003] Methods for manufacturing low molecular weight polytetrafluoroethylene (PTFE) include polymerization, radiolysis, and thermal decomposition. In radiolysis, high molecular weight PTFE is typically irradiated with radiation in an air atmosphere to obtain low molecular weight PTFE.

[0004] In addition, methods for reducing the perfluorocarboxylic acids and their salts that can be produced as byproducts by radioactive decomposition have been studied (see, for example, Patent Document 2).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 10-147617

[0008] Patent Document 2: Japanese Patent Application Publication No. 2018-24868 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] The purpose of this invention is to provide a method for manufacturing low molecular weight polytetrafluoroethylene with reduced content of perfluorocarboxylic acids and their salts having carbon atoms of 4 to 16.

[0011] In addition, the present invention aims to provide a new low molecular weight polytetrafluoroethylene powder with low content of perfluorooctanoic acid and its salts.

[0012] Methods for solving problems

[0013] This invention relates to a method for manufacturing low molecular weight polytetrafluoroethylene (PTFE), comprising the step (1): irradiating PTFE containing perfluorocarboxylic acids or their salts having 4 to 16 carbon atoms, and exhibiting a reduction in heat of fusion of less than 40% from the first heating to the second heating in differential scanning calorimetry, with 5 to 1000 kGy of radiation at a temperature below 100°C under substantially oxygen-free conditions, thereby obtaining PTFE with reduced content of the aforementioned perfluorocarboxylic acids and their salts and a melt viscosity of 1.0 × 10⁻⁶ at 380°C.2 ~7.0×10 5 Low molecular weight polytetrafluoroethylene (PTFE) with a strength of Pa·s.

[0014] Both the aforementioned polytetrafluoroethylene and the aforementioned low molecular weight polytetrafluoroethylene are preferably in powder form.

[0015] This invention also relates to a powder comprising low molecular weight polytetrafluoroethylene, wherein the content of perfluorooctanoic acid and its salts is below 1500 ppb by mass, and the low molecular weight polytetrafluoroethylene has a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 ~7.0×10 5 In the first differential spectral line obtained by electron spin resonance method, the difference (P1-P2) between the signal intensity P1 with a g value of 2.020 and the signal intensity P2 with a g value of 2.023 is greater than -0.07.

[0016] The specific surface area of ​​the above-mentioned powder is preferably 0.5 to 20 m². 2 / g.

[0017] The effects of the invention

[0018] According to the present invention, a method for manufacturing low molecular weight polytetrafluoroethylene with reduced content of perfluorocarboxylic acids and their salts having carbon numbers of 4 to 16 is provided.

[0019] In addition, according to the present invention, it is also possible to provide a new low molecular weight polytetrafluoroethylene powder with low content of perfluorooctanoic acid and its salts. Attached Figure Description

[0020] Figure 1 This is a diagram showing an example of P1 and P2 in an electron spin resonance (ESR) spectrum. Detailed Implementation

[0021] The present invention will now be described in detail.

[0022] This invention relates to a method for manufacturing low molecular weight polytetrafluoroethylene (PTFE), comprising the step (1): irradiating polytetrafluoroethylene (PTFE) containing perfluorocarboxylic acids or their salts having 4 to 16 carbon atoms, and exhibiting a reduction in heat of fusion of less than 40% from the first heating to the second heating in differential scanning calorimetry, with 5 to 1000 kGy of radiation at a temperature below 100°C under substantially oxygen-free conditions, thereby obtaining a PTFE with a melt viscosity of 1.0 × 10⁻⁶ at 380°C, having reduced content of the aforementioned perfluorocarboxylic acids and their salts. 2 ~7.0×10 5 Low molecular weight polytetrafluoroethylene (low molecular weight PTFE) with a strength of Pa·s.

[0023] According to the manufacturing method of the present invention, low molecular weight PTFE with reduced content of the aforementioned perfluorocarboxylic acids and their salts can be obtained from PTFE containing perfluorocarboxylic acids or their salts having 4 to 16 carbon atoms.

[0024] It can be assumed that by irradiating the aforementioned specific dose of radiation, the perfluorocarboxylic acids and their salts contained in the PTFE, which is the irradiated object, decompose. Furthermore, it can be assumed that because the irradiation is carried out under conditions of substantially the absence of oxygen, even if a portion of the PTFE decomposes upon irradiation, the regeneration of the aforementioned perfluorocarboxylic acids or their salts is unlikely. The result is considered to be a low molecular weight PTFE with a reduced content of the aforementioned perfluorocarboxylic acids and their salts.

[0025] Furthermore, since the manufacturing method of the present invention does not require heating at high temperatures, heating equipment is unnecessary. Additionally, there is no need for a step of moving the obtained low molecular weight PTFE from the irradiation equipment to the heating equipment, etc. Therefore, productivity can be improved and manufacturing costs can be reduced.

[0026] In step (1), the PTFE irradiated with radiation exhibits a heat of fusion reduction rate of less than 40% during the first and second heating cycles in differential scanning calorimetry. This heat of fusion reduction rate is an indicator of the molecular weight of PTFE; the greater the reduction rate, the higher the molecular weight tends to be.

[0027] The reduction rate of the heat of fusion is preferably 35% or less, more preferably 30% or less, and even more preferably 27% or less. Alternatively, it can be -30% or more or -20% or more.

[0028] The reduction rate of the heat of fusion was obtained by measuring using a differential scanning calorimeter (DSC, trade name: DSC7020, manufactured by Hitachi High-Tech Science Co., Ltd.). Specifically, approximately 10 mg of PTFE was accurately weighed and placed in a dedicated aluminum dish. The dish was heated to 200°C under a nitrogen atmosphere and held for 5 minutes. The temperature was then increased to 390°C at a rate of 10°C / min (first heating) to allow complete melting of the crystals, and the heat of fusion was measured. The temperature was then decreased from 390°C to 200°C at a rate of 10°C / min, and then increased to 390°C at a rate of 10°C / min (second heating), and the heat of fusion was measured again. Using the measured values ​​of the heat of fusion during the first and second heatings, the reduction rate of the heat of fusion was calculated using the following formula.

[0029] Reduction rate of heat of fusion (%) = (Heat of fusion during the first heating - Heat of fusion during the second heating) / (Heat of fusion during the first heating) × 100

[0030] PTFE exhibiting the aforementioned reduction in heat of fusion can be manufactured, for example, by subjecting high molecular weight PTFE to radioactive or thermal decomposition. This radioactive or thermal decomposition can be carried out in any atmosphere, such as air, an inert gas, or a vacuum. From the perspective of low-cost implementation, it is preferable to carry out the decomposition in air; from the perspective of minimizing the formation of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms, it is preferable to carry out the decomposition under substantially oxygen-free conditions. However, since the manufacturing method of the present invention includes the aforementioned step (1), it is not necessary to carry out the radioactive or thermal decomposition under substantially oxygen-free conditions. It can also be carried out in the presence of oxygen (e.g., air).

[0031] The preferred standard specific gravity (SSG) of the aforementioned high molecular weight PTFE is 2.130 to 2.230. The aforementioned standard specific gravity (SSG) is a value obtained according to ASTM D 4895.

[0032] Alternatively, the aforementioned PTFE can also be directly manufactured through the polymerization of tetrafluoroethylene (TFE). In this case, the reduction rate of heat of fusion can be within the aforementioned range by adjusting the choice of initiator, polymerization temperature, or polymerization time.

[0033] The PTFE mentioned above contains perfluorocarboxylic acids or their salts with 4 to 16 carbon atoms.

[0034] The aforementioned perfluorocarboxylic acids or their salts are sometimes used in the polymerization of PTFE. Furthermore, the aforementioned perfluorocarboxylic acids or their salts are typically generated during the radioactive decomposition of high molecular weight PTFE in an air atmosphere. According to the manufacturing method of the present invention, even when using PTFE containing the aforementioned perfluorocarboxylic acids or their salts for some reason as a raw material, it is possible to obtain low molecular weight PTFE with a reduced content of the aforementioned perfluorocarboxylic acids and their salts.

[0035] In the aforementioned PTFE, the content (total) of the aforementioned perfluorocarboxylic acid and its salt can be 5 ppb or more, greater than 10 ppb, greater than 15 ppb, greater than 20 ppb, greater than 25 ppb, greater than 50 ppb, greater than 100 ppb, greater than 500 ppb, greater than 1000 ppb, greater than 2000 ppb, greater than 3000 ppb, or greater than 10000 ppb.

[0036] The amounts of the aforementioned perfluorocarboxylic acids and their salts can be determined by liquid chromatography.

[0037] The PTFE described above may contain perfluorooctanoic acid (PFOA) or its salts. The content of PFOA and its salts in the PTFE may be, for example, 5 ppb or more by mass, greater than 10 ppb by mass, greater than 15 ppb by mass, greater than 20 ppb by mass, greater than 25 ppb by mass, greater than 50 ppb by mass, greater than 100 ppb by mass, greater than 300 ppb by mass, greater than 500 ppb by mass, or greater than 1500 ppb by mass.

[0038] The amounts of perfluorooctanoic acid and its salts mentioned above can be determined by liquid chromatography.

[0039] The PTFE described above may contain perfluorosulfonic acid or its salts having 4 to 16 carbon atoms. The total amount of the perfluorosulfonic acid and its salts in the PTFE may be 5 ppb or more by mass, greater than 10 ppb by mass, greater than 15 ppb by mass, greater than 20 ppb by mass, or greater than 25 ppb by mass.

[0040] The amounts of the above-mentioned perfluorosulfonic acid and its salts can be determined by liquid chromatography.

[0041] The shape of the PTFE is not particularly limited, but powder is preferred.

[0042] The radiation dose in step (1) is 5 to 1000 kGy. From the perspective of obtaining low molecular weight PTFE with a smaller amount of the aforementioned perfluorocarboxylic acid and its salts, the aforementioned dose is preferably 10 kGy or more, more preferably 20 kGy or more, further preferably 30 kGy or more, and particularly preferably 50 kGy or more. Furthermore, from the perspective of obtaining low molecular weight PTFE with a moderately high molecular weight, the aforementioned dose is preferably 500 kGy or less, more preferably 300 kGy or less, further preferably less than 250 kGy, and particularly preferably 200 kGy or less.

[0043] The dosage mentioned above refers to the absorbed dosage.

[0044] As for the aforementioned radiation, there are no particular limitations as long as it is ionizing radiation. Examples include electron beams, gamma rays, X-rays, neutron rays, and high-energy ions, with electron beams or gamma rays being preferred.

[0045] The irradiation temperature of the aforementioned radiation is less than 100°C. Preferably, the irradiation temperature is less than 70°C, more preferably less than 50°C, and most preferably 5°C or higher.

[0046] From an economic perspective, irradiation at room temperature is preferred. Room temperature, including the heat released by irradiation, can be in the range of 5 to 60°C, preferably 10 to 50°C, more preferably 10 to less than 50°C, and even more preferably 15 to 45°C.

[0047] According to the manufacturing method of the present invention, the above-mentioned perfluorocarboxylic acids and their salts can be sufficiently reduced by such treatment at a relatively low temperature.

[0048] The irradiation in step (1) is carried out under conditions where oxygen is substantially absent. Here, conditions where oxygen is substantially absent mean that the oxygen concentration in the atmosphere in which the step is carried out is 0.5 vol% or less. From the viewpoint of obtaining low molecular weight PTFE with less of the above-mentioned perfluorocarboxylic acid and its salt, the above-mentioned oxygen concentration is preferably 0.25 vol% or less, more preferably 0.1 vol% or less, further preferably 0.01 vol% or less, and particularly preferably 0.001 vol% or less. The lower limit is not particularly limited and can be a concentration below the detection limit.

[0049] The oxygen concentration mentioned above can be determined by analyzing the gas phase in the space where the process is carried out using gas chromatography, by using an oxygen concentration meter, or by investigating the hue of the oxygen detector set in the space.

[0050] The irradiation in step (1) is preferably carried out in a closed container. The closed container refers to a container that can be sealed in a way that allows adjustment of the oxygen concentration within the container. Therefore, it can be connected to piping for drawing in and discharging inert gases or for discharging gases from the closed container, and can also be connected to piping, caps, valves, flanges, etc., that are not open during radiation irradiation. Furthermore, its shape is not particularly limited; it can be cylindrical, prismatic, spherical, etc., or it can be a bag with variable internal volume. Furthermore, its material is not particularly limited; it can be metal, glass, polymer, etc. The closed container needs to be made of a material / structure that is permeable to radiation and will not deteriorate due to radiation irradiation, but it does not need to be a pressure-resistant container.

[0051] The manufacturing method of the present invention may include, prior to step (1), a step of introducing the aforementioned PTFE into a sealed container under substantially oxygen-free conditions. Introducing the PTFE into the sealed container under substantially oxygen-free conditions means that the oxygen concentration in the atmosphere within the sealed container after introduction is within the aforementioned range.

[0052] As a method for introducing the PTFE into a sealed container under substantially oxygen-free conditions, for example, a method can be given by introducing the PTFE into a sealed container along with at least one selected from the group consisting of an inert gas and an oxygen adsorbent.

[0053] As a method for placing the aforementioned substances into the sealed container, examples include placing the PTFE inside the sealed container and then filling or evacuating the sealed container with the inert gas. Furthermore, when using the oxygen adsorbent, examples include: placing the PTFE and the oxygen adsorbent in air inside the sealed container and then sealing the sealed container; placing the PTFE and the oxygen adsorbent inside the sealed container and then filling the sealed container with the inert gas; placing the PTFE and the oxygen adsorbent inside the sealed container and then evacuating the sealed container, etc.

[0054] The aforementioned inert gas needs to be inert to the decomposition reaction of the aforementioned perfluorocarboxylic acids and their salts (and the aforementioned PTFE) caused by radiation irradiation. Examples of such inert gases include nitrogen, helium, and argon. Nitrogen is preferred.

[0055] The oxygen content of the aforementioned inert gas is preferably 0.5 vol% or less, more preferably 0.25 vol% or less, even more preferably 0.1 vol% or less, further preferably 0.01 vol% or less, and particularly preferably 0.001 vol% or less. The lower limit is not particularly limited and can be an amount below the detection limit. When the oxygen content in the aforementioned inert gas is within the above range, a low molecular weight PTFE with a smaller amount of the aforementioned perfluorocarboxylic acid and its salts can be obtained.

[0056] In addition to gas chromatography, the oxygen content can also be confirmed using an oxygen concentration meter and oxygen test strips.

[0057] The aforementioned oxygen adsorbents are not particularly limited as long as they possess the function of adsorbing oxygen. Known oxygen adsorbents such as inorganic oxygen adsorbents (iron-based, zinc-based, bisulfite-based, etc.) and organic oxygen adsorbents (ascorbic acid-based, polyol-based, activated carbon-based, etc.) can be used. These oxygen adsorbents can be water-dependent (requiring moisture when reacting with oxygen) or self-reactive (not requiring moisture), with self-reactive types being preferred. As for the aforementioned oxygen adsorbents, iron-based self-reactive oxygen adsorbents and quicklime are preferred, with iron-based self-reactive oxygen adsorbents being the most preferred.

[0058] The amount of oxygen adsorbent added is preferably an amount that can keep the oxygen concentration in the sealed container within the range described above.

[0059] The irradiation in step (1) is preferably carried out under conditions where there are no halogenated polymers having halogen atoms other than fluorine atoms.

[0060] The aforementioned halogenated polymers also include polymers that have halogen atoms other than fluorine atoms and have fluorine atoms.

[0061] Examples of the aforementioned halogenated polymers include polyvinyl chloride (PVC), polyvinylidene chloride (PVdC), and polychlorotrifluoroethylene (PCTFE), which contain chlorine atoms.

[0062] The absence of the aforementioned halogenated polymers means that the amount of the aforementioned halogenated polymers present is less than 0.001% by mass relative to the aforementioned PTFE. The aforementioned amount is preferably less than 0.0001% by mass. There is no particular limitation on the lower limit; it can be an amount below the detection limit.

[0063] The irradiation in step (1) is preferably carried out under conditions in which hydrocarbons, chlorinated hydrocarbons, alcohols and carboxylic acids are substantially absent.

[0064] Examples of such hydrocarbons include saturated hydrocarbons with 1 to 20 carbon atoms.

[0065] Examples of chlorinated hydrocarbons include, for instance, chlorides of saturated hydrocarbons having 1 to 18 carbon atoms.

[0066] Examples of the aforementioned alcohols include monohydric saturated alcohols with 1 to 12 carbon atoms.

[0067] Examples of the aforementioned carboxylic acids include saturated monocarboxylic acids having 1 to 13 carbon atoms.

[0068] The absence of the aforementioned compounds is defined as the presence (total amount) of the aforementioned compounds being less than 0.001% by mass relative to the PTFE. The presence amount is preferably less than 0.0001% by mass. There is no particular limitation on the lower limit; it can be an amount below the detection limit.

[0069] In step (1), a low molecular weight PTFE is obtained with a reduced content of perfluorocarboxylic acids and their salts having 4 to 16 carbon atoms. In other words, the content of the perfluorocarboxylic acids and their salts in the low molecular weight PTFE is less than the content in the PTFE before radiation irradiation in step (1).

[0070] In step (1), the reduction rate (PFC reduction rate) of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms is preferably 10% by mass or more, more preferably 20% by mass or more, further preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 50% by mass or more. The higher the PFC reduction rate, the better, and there is no particular upper limit, which can be 100% by mass.

[0071] Regarding the PFC reduction rate, the content of PFC in the PTFE before radiation irradiation in step (1) and the low molecular weight PTFE obtained in step (1) can be determined by liquid chromatography, and the reduction rate can be calculated using the following formula using these measured values.

[0072] PFC reduction rate (mass%) = (PFC content in PTFE before irradiation - PFC content in the resulting low molecular weight PTFE) / (PFC content in PTFE before irradiation) × 100

[0073] In process (1), low molecular weight PTFE with reduced perfluorooctanoic acid and its salts content can also be obtained. In other words, the content of perfluorooctanoic acid and its salts in the low molecular weight PTFE can be reduced compared to the content in the PTFE before radiation irradiation in process (1).

[0074] In step (1), the reduction rate of perfluorooctanoic acid and its salts (PFOA) is preferably 10% by mass or more, more preferably 20% by mass or more, further preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 50% by mass or more. The higher the PFOA reduction rate, the better, and there is no particular upper limit, which can be 100% by mass.

[0075] Regarding the PFOA reduction rate, the PFOA content in the PTFE before radiation irradiation in step (1) and the low molecular weight PTFE obtained in step (1) can be determined by liquid chromatography, and the reduction rate can be calculated using the following formula using these measured values.

[0076] PFOA reduction rate (mass%) = (PFOA content in PTFE before irradiation - PFOA content in the resulting low molecular weight PTFE) / (PFOA content in PTFE before irradiation) × 100

[0077] In the low molecular weight PTFE obtained in step (1), the content (total amount) of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms can be less than 10,000 ppb by mass, less than 3,000 ppb by mass, less than 2,000 ppb by mass, less than 1,000 ppb by mass, less than 500 ppb by mass, less than 100 ppb by mass, less than 50 ppb by mass, less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. There is no specific lower limit; it can be below the detection limit.

[0078] The amounts of the aforementioned perfluorocarboxylic acids and their salts can be determined by liquid chromatography.

[0079] In addition, the content of perfluorooctanoic acid and its salts in the aforementioned low molecular weight PTFE can be less than 1500 ppb by mass, less than 500 ppb by mass, less than 300 ppb by mass, less than 100 ppb by mass, less than 50 ppb by mass, less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. There is no specific lower limit; it can be an amount below the detection limit.

[0080] The amounts of perfluorooctanoic acid and its salts mentioned above can be determined by liquid chromatography.

[0081] In addition, in the aforementioned low molecular weight PTFE, the amount of perfluorosulfonic acid and its salts with 4 to 16 carbon atoms can be less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. There is no specific lower limit; it can be an amount below the detection limit.

[0082] The amounts of the above-mentioned perfluorosulfonic acid and its salts can be determined by liquid chromatography.

[0083] The content of the aforementioned perfluorosulfonic acid and its salt in the aforementioned low molecular weight PTFE can be less than that in the aforementioned PTFE that is the irradiation target.

[0084] The melt viscosity of the aforementioned low molecular weight PTFE at 380℃ is 1.0 × 10⁻⁶. 2 ~7.0×10 5 Pa·s. The preferred melt viscosity is 1.5 × 10⁻⁶ Pa·s. 3 Pa·s or higher, more preferably 7.0 × 10⁻⁶ Pa·s or higher. 3 Pa·s or higher, and preferably 3.0 × 10⁻⁶ Pa·s or higher. 5 Pa·s or less, more preferably 1.0 × 10 Pa·s 5 Pa·s or less, more preferably 9.0 × 10⁻⁶ Pa·s or less. 4 Pa·s and below.

[0085] The melt viscosity values ​​mentioned above are the values ​​measured as follows: according to ASTM D 1238, using a flow tester (manufactured by Shimadzu Corporation) and The die head was used to hold a 2g sample that had been preheated at 380°C for 5 minutes under a load of 0.7MPa at the above temperature, and the resulting value was the melt viscosity.

[0086] The melting point of the aforementioned low molecular weight PTFE is preferably 320–340°C, more preferably 324–336°C.

[0087] Regarding the melting point mentioned above, a differential scanning calorimeter (DSC) was used as a standard sample. The temperature was calibrated beforehand using indium and lead. Then, about 3 mg of low molecular weight PTFE was placed in an aluminum pan (capped container) and heated at 10 °C / min in the temperature range of 250–380 °C under an air flow of 200 ml / min. The point at which the heat of fusion was minimal in the above range was taken as the melting point.

[0088] The aforementioned low molecular weight PTFE can be a homopolymer PTFE containing only tetrafluoroethylene (TFE) units, or a modified PTFE containing TFE units and modified monomer units based on modified monomers capable of copolymerizing with TFE. In the manufacturing method of the present invention, since the composition of the polymer remains unchanged, the aforementioned low molecular weight PTFE still possesses the composition of the aforementioned PTFE.

[0089] In the modified PTFE described above, the content of the modified monomer unit is preferably 0.001 to 1% by mass of all monomer units, more preferably 0.01% by mass or more, and even more preferably 0.5% by mass or less, and further preferably 0.1% by mass or less. In this specification, the modified monomer unit refers to a portion of the molecular structure of the modified PTFE, that is, the portion derived from the modified monomer; all monomer units refer to the portion of the molecular structure of the modified PTFE derived from all monomers. The content of the modified monomer unit can be determined by known methods such as Fourier transform infrared spectroscopy (FT-IR).

[0090] As for the aforementioned modified monomers, there are no particular limitations as long as they are monomers capable of copolymerizing with TFE. Examples include perfluoroolefins such as hexafluoropropylene [HFP]; chlorofluoroolefins such as trifluorochloroethylene [CTFE]; hydrofluoroolefins such as trifluoroethylene and vinylidene fluoride [VDF]; perfluorovinyl ethers; perfluoroalkyl ethylene; and ethylene. In addition, one or more modified monomers may be used.

[0091] There are no particular limitations on the above-mentioned perfluorovinyl ethers; for example, the following general formula (1) can be cited.

[0092] CF2 = CF - ORf (1)

[0093] (In the formula, Rf represents a perfluorinated organic group) refers to perfluorinated unsaturated compounds, etc. In this specification, the term "perfluorinated organic group" refers to an organic group in which all hydrogen atoms bonded to carbon atoms are replaced by fluorine atoms. The aforementioned perfluorinated organic groups may also have ether oxygen.

[0094] Examples of perfluorovinyl ethers include perfluoro(alkylvinyl ether) [PAVE], where Rf in the above general formula (1) represents a perfluoroalkyl group having 1 to 10 carbon atoms. The number of carbon atoms in the perfluoroalkyl group is preferably 1 to 5.

[0095] Examples of perfluoroalkyl groups in the above-mentioned PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, and perfluorohexyl, with perfluoroalkyl groups preferably being perfluoropropyl perfluoro(propyl vinyl ether) [PPVE].

[0096] As for the aforementioned perfluorovinyl ethers, examples can be given of substances in which Rf in the above general formula (1) is a perfluoro(alkoxyalkyl) compound with 4 to 9 carbon atoms, and Rf is the following formula:

[0097] [Chemistry 1]

[0098]

[0099] (In the formula, m represents 0 or an integer from 1 to 4) The substance of the group represented by the following formula, Rf:

[0100] [Chemistry 2]

[0101]

[0102] (where n represents an integer from 1 to 4) refers to substances containing groups, etc.

[0103] There is no particular limitation on perfluoroalkyl ethylene, for example, (perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene, (perfluorooctyl)ethylene, etc.

[0104] The modifying 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 shape of the aforementioned low molecular weight PTFE is not particularly limited, but powder is preferred. It is also preferred that both the aforementioned PTFE and the aforementioned low molecular weight PTFE are powders.

[0106] The manufacturing method of the present invention may further include a step of exposing the low molecular weight PTFE obtained in step (1) to air.

[0107] The time from the end of radiation irradiation in process (1) to the exposure of the low molecular weight PTFE to air is, for example, less than 1 day, less than 10 hours, less than 1 hour, less than 10 minutes, or less than 5 minutes.

[0108] In the manufacturing method of the present invention, the PTFE and the low molecular weight PTFE are preferably not placed at a temperature above 100°C, more preferably not placed at a temperature above 70°C, and even more preferably not placed at a temperature above 50°C.

[0109] In addition, the PTFE and the low molecular weight PTFE mentioned above are preferably not placed at the above temperature for more than 30 minutes, more preferably not for more than 10 minutes, and more preferably not for more than 10 seconds.

[0110] The manufacturing method of the present invention may further include a step of pulverizing the low molecular weight PTFE obtained in step (1). The pulverization method is not particularly limited and can be any known method.

[0111] The low molecular weight PTFE obtained by the manufacturing method of the present invention will be further described below.

[0112] When the aforementioned low molecular weight PTFE is in powder form, the average particle size is preferably 0.5 to 200 μm, more preferably 100 μm or less, even more preferably 50 μm or less, and even more preferably 25 μm or less, and particularly preferably 10 μm or less. In this way, by using a powder with a relatively small average particle size, for example when used as an additive in coatings, a coating film with superior surface smoothness can be formed.

[0113] Regarding the aforementioned average particle size, it was measured using a laser diffraction particle size distribution measuring device (HELOS & RODOS) manufactured by Nippon Electronics Co., Ltd., without using cascades, at a dispersion pressure of 3.0 bar. This average particle size is equal to the particle size corresponding to 50% of the cumulative particle size distribution.

[0114] When the aforementioned low molecular weight PTFE is in powder form, the preferred specific surface area is 0.5–20 m². 2 / g.

[0115] As a low molecular weight PTFE powder, a specific surface area of ​​0.5 m² is required. 2 / g or more and less than 7.0m 2 Low specific surface area type, with a specific surface area of ​​7.0 m² / g. 2 / g or more 20m 2 High specific surface area type with a surface area of ​​less than / g.

[0116] Low molecular weight PTFE powder with low specific surface area has the advantage of being easy to disperse in matrix materials such as coatings. On the other hand, its particle size in matrix materials is large and its micro-dispersion is poor.

[0117] The preferred specific surface area of ​​low molecular weight PTFE powder is 1.0 m². 2 / g or more, preferably 5.0m 2 / g or less, more preferably 3.0m 2 Below / g. As a base material, it is suitable for use in coatings, in addition to plastics and inks.

[0118] High specific surface area type low molecular weight PTFE powder, for example, when dispersed in matrix materials such as coatings, has a small dispersed particle size in the matrix material, which can improve the texture of the coating surface, and has a high effect on surface modification and oil absorption. However, it requires a long dispersion time in the matrix material, which may make it difficult to disperse, and the viscosity of the coating may also increase.

[0119] The preferred specific surface area of ​​high specific surface area low molecular weight PTFE powder is 8.0 m². 2 / g or more, preferably 25m 2 / g or less, more preferably 20m 2 Below / g. As a base material, it is suitable for use with plastics, in addition to oils, greases, and coatings.

[0120] Regarding the specific surface area mentioned above, a surface analyzer (trade name: BELSORP-miniII, manufactured by Microtrac BEL Co., Ltd.) was used, with a mixture of 30% nitrogen and 70% helium as the carrier gas, and liquid nitrogen was used for cooling. The measurement was performed by the BET method.

[0121] The aforementioned low molecular weight PTFE may have carboxyl groups at the ends of the molecular chain. The number of carboxyl groups is not particularly limited, for example, relative to every 10... 6 The number of carbon atoms in a main chain can range from 0 to 500.

[0122] The number of carboxyl groups mentioned above can be determined by the following method. The detection limit of this method is 0.5.

[0123] (Determination Method)

[0124] The following determinations were performed according to the analytical method for the terminal groups described in Japanese Patent Application Publication No. 4-20507.

[0125] Low molecular weight PTFE powder was pre-formed using a hand press to produce a film approximately 0.1 mm thick. Infrared absorption spectroscopy analysis was performed on the fabricated film. Infrared absorption spectroscopy analysis was also performed on PTFE with fully fluorinated terminals produced by contacting fluorine gas with PTFE. The number of terminal carboxyl groups was calculated using the following formula based on the differential spectra of the two methods.

[0126] The number of terminal carboxyl groups (relative to every 10) 6 Number of carbon atoms) = (l × K) / t

[0127] l: Absorbance

[0128] K: Correction coefficient

[0129] t: Membrane thickness (mm)

[0130] The absorption frequency of the carboxyl group is 3560 cm⁻¹. -1 The correction factor is 440.

[0131] Unstable terminal groups derived from the chemical structure of the polymerization initiator or chain transfer agent used in the polymerization reaction of the aforementioned low molecular weight PTFE can be generated at the molecular chain ends of the PTFE. There are no particular limitations on the unstable terminal groups; examples include -CH2OH, -COOH, and -COOCH3.

[0132] The aforementioned low molecular weight PTFE can be a substance that has undergone stabilization with unstable terminal groups. There are no particular limitations on the method for stabilizing these unstable terminal groups; for example, methods such as exposing the terminal group to a fluorine-containing gas to change it to trifluoromethyl[-CF3] can be cited.

[0133] The aforementioned low molecular weight PTFE can also be a substance that has undergone terminal amidation. There are no particular limitations on the method of terminal amidation; for example, as disclosed in Japanese Patent Application Publication No. 4-20507, a method can be used to contact a fluorocarbonyl group [-COF] obtained by exposure to a fluorine-containing gas with ammonia gas.

[0134] When the aforementioned low molecular weight PTFE is a substance that has undergone stabilization or terminal amidation of the aforementioned unstable terminal groups, it is easy to integrate with the target material when used as an additive for materials such as coatings, greases, cosmetics, electroplating solutions, colorants, and plastics, thereby improving dispersibility.

[0135] The manufacturing method of the present invention may further include a step (M1) before step (1), in which a molded article is obtained by further heating high molecular weight polytetrafluoroethylene to above its primary melting point.

[0136] The primary melting point is preferably 300°C or higher, more preferably 310°C or higher, and even more preferably 320°C or higher.

[0137] The aforementioned primary melting point refers to the maximum peak temperature of the endothermic curve appearing on the crystallization melting curve when measured using a differential scanning calorimeter (DSC) on unfired high molecular weight PTFE. This endothermic curve was obtained by heating the material using a DSC at a rate of 10°C / min.

[0138] The preferred specific gravity of the molded article in process (M1) is 1.0 g / cm³. 3 The above, and more preferably 1.5 g / cm 3 The above, and preferably 2.5 g / cm³ 3The following applies. When the specific gravity of the molded article is within the above range, the pores and unevenness of the surface are reduced, resulting in the production of low molecular weight PTFE with a small specific surface area.

[0139] The above specific gravity can be determined by the water displacement method.

[0140] Preferably, a further step (M2) is performed after step (M1): by subjecting the molded article obtained in step (M1) to radioactive decomposition or thermal decomposition, PTFE with a reduction rate of less than 40% in the heat of fusion from the first heating to the second heating in differential scanning calorimetry is obtained.

[0141] In this case, the PTFE obtained in process (M2) can be used as the PTFE mentioned above in process (1).

[0142] The aforementioned radioactive decomposition or thermal decomposition can be carried out in any atmosphere, such as in air, in an inert gas, or in a vacuum. From the perspective of low-cost implementation, it is preferred to carry out the decomposition in air, and from the perspective of minimizing the formation of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms, it is preferred to carry out the decomposition under substantially oxygen-free conditions. However, since the manufacturing method of the present invention includes the aforementioned step (1), it is not necessary to carry out the aforementioned radioactive decomposition or thermal decomposition under substantially oxygen-free conditions. It can also be carried out in the presence of oxygen (e.g., in air).

[0143] It should be noted that, if the above-mentioned process of adding the PTFE into a closed container under substantially oxygen-free conditions is also carried out, it is preferable to perform processes (M1) and (M2) before the above-mentioned addition process.

[0144] The manufacturing method of the present invention may further include a step of pulverizing the above-mentioned molded article to obtain powder after step (M1) and before step (M2). After coarsely pulverizing the above-mentioned molded article, it can be further finely pulverized.

[0145] This invention also relates to a powder comprising low molecular weight polytetrafluoroethylene, wherein the content of perfluorooctanoic acid and its salts is below 1500 ppb by mass, and the low molecular weight polytetrafluoroethylene has a melt viscosity of 1×10⁻⁶ at 380°C. 2 ~7×10 5 In the first differential spectral line obtained by electron spin resonance (ESR), the difference (P1-P2) between the signal intensity P1 with a g value of 2.020 and the signal intensity P2 with a g value of 2.023 is greater than -0.07.

[0146] The low molecular weight PTFE in the powder of the present invention exhibits the specific signal described above in the first differential spectrum (hereinafter also referred to as ESR spectrum) obtained by electron spin resonance (ESR).

[0147] Low molecular weight PTFE obtained by conventional methods, such as radiation irradiation in the presence of oxygen, does not exhibit the specific signals described above. It is presumed that the structure and proportion of free radicals contained in the low molecular weight PTFE obtained by conventional methods differ from those in the powder of this invention.

[0148] First, the measurement conditions for ESR and the definitions of related terms will be explained.

[0149] The measurement conditions are as follows.

[0150] Device: JEOL Ltd., JES-FR30EX

[0151] Measurement temperature: 23±3℃

[0152] Microwave frequency: 9.42GHz

[0153] Microwave output: 0.4mW

[0154] Central magnetic field: 347.548 mT

[0155] Scan width: ±25mT

[0156] Scan time: 60s

[0157] Time constant: 0.03s

[0158] Magnetic field modulation width: 0.32mT

[0159] Number of scans: 1

[0160] Modulation frequency: 100kHz

[0161] Marker: Mn 2+

[0162] In the above ESR spectrum, the vertical axis represents the correction signal intensity, and the horizontal axis represents the g value.

[0163] The strength of the above-mentioned correction signal is defined by the following formula:

[0164] Corrected signal strength (mg) -1 = Int.[PTFE] / Int.[Mn 2+ Sample mass (mg)

[0165] (In the formula, Int.[PTFE] is the signal intensity before sample calibration, and Int.[Mn 2+ [The signal strength of the marker].

[0166] In this specification, unless otherwise stated, when the signal of the ESR spectrum of PTFE is referred to as intensity, it refers to the intensity of the above-mentioned correction signal.

[0167] The g value of the ESR signal is defined by the following formula:

[0168] g=hν / βH

[0169] (In the formula, h is Planck's constant, ν is the frequency of the electromagnetic wave being measured, β is the Bohr magneton, and H is the magnetic field strength of the signal being obtained).

[0170] As the aforementioned g value, Mn, used as a marker, is employed. 2+ The values ​​were corrected based on the known g values ​​of 2.034 and 1.981 corresponding to the third and fourth peaks from the low magnetic field side out of the six peaks.

[0171] In the case of baseline shift in the above ESR spectrum, baseline correction is performed with the signal intensity approximately 0 around g values ​​of 2.05 and 1.98.

[0172] In the above ESR spectrum, a positive signal refers to the signal that appears in the positive region of the spectrum (above the baseline), and a negative signal refers to the signal that appears in the negative region of the spectrum (below the baseline).

[0173] In the ESR spectrum above, the difference (P1-P2) between the signal intensity P1 with a g value of 2.020 and the signal intensity P2 with a g value of 2.023 in the low molecular weight PTFE of the powder of the present invention is greater than -0.07.

[0174] The signals with a g value of 2.020 and 2.023 can be considered positive signals. Furthermore, P1 and P2 can be the absolute values ​​of the signal strengths mentioned above.

[0175] It is understood that a signal (peak) with a g value of 2.020 is based on

[0176] The following formula:

[0177] [Chemistry 3]

[0178]

[0179] (The wavy lines in the formula represent the polymer chains of PTFE. The same applies below.) The signal of free radical 1 represented by the formula, and the signal (peak) with a g value of 2.023, are based on the following formula:

[0180] [Chemistry 4]

[0181]

[0182] The signal represented by free radical 2.

[0183] The aforementioned low molecular weight PTFE with a reduction rate of less than 40% in heat of fusion contains free radical 1 and free radical 2.

[0184] Subsequently, when irradiation is carried out under conditions where oxygen is not actually present (step (1)), a portion of the free radical 2 is decomposed and transformed into free radical 1 by irradiation, and thus the proportion of free radical 1 tends to increase.

[0185] On the other hand, when the above irradiation is carried out in the air, free radical 1 and free radical 2 are generated, so the proportion of free radical 1 will not increase.

[0186] As described in the embodiments below, by means of the manufacturing method of the present invention, the difference P1-P2, which was less than -0.07 before irradiation (Comparative Example 2), becomes more than -0.07 after irradiation (Example 1, etc.). This means that by irradiation in step (1), free radical 2 decreases and free radical 1 increases.

[0187] The difference P1-P2 being -0.07 or higher means that the difference in the amount of free radical 1 and free radical 2 in low molecular weight PTFE is above a certain level. In other words, it means that the amount of free radical 1 is greater than that of free radical 2.

[0188] Low molecular weight PTFE with a difference P1-P2 of -0.07 or higher can be manufactured, for example, by the manufacturing method of the present invention described above. Therefore, compared with existing known low molecular weight PTFE powders, the powder of the present invention can be manufactured at a lower cost. When the difference P1-P2 is less than -0.07, the manufacturing cost may increase.

[0189] The difference P1-P2 is preferably greater than 0.00, more preferably 0.01 or more, and even more preferably 0.02 or more. Alternatively, the difference P1-P2 can be 0.10 or less, more preferably 0.05 or less.

[0190] By implementing the above process (1), the difference P1-P2 can be made to fall within the above range.

[0191] Figure 1 An example of P1 and P2 in the ESR spectrum is shown.

[0192] The low molecular weight PTFE in the powder of this invention has a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 ~7.0×10 5 Pa·s. The preferred melt viscosity is 1.5 × 10⁻⁶ Pa·s. 3 Pa·s or higher, more preferably 7.0 × 10⁻⁶ Pa·s or higher. 3 Pa·s or higher, and preferably 3.0 × 10⁻⁶ Pa·s or higher. 5 Pa·s or less, more preferably 1.0 × 10 Pa·s 5Pa·s or less, more preferably 9.0 × 10⁻⁶ Pa·s or less. 4 Pa·s and below.

[0193] Regarding the composition, melting point, and molecular chain ends (number of carboxyl groups, unstable terminal groups, and their stabilization or amidation) of the low molecular weight PTFE in the powder of the present invention, the same situation as described in the low molecular weight PTFE obtained by the manufacturing method of the present invention can be adopted.

[0194] The powder of the present invention may substantially contain only the aforementioned low molecular weight PTFE. The amount of the aforementioned low molecular weight PTFE relative to the aforementioned powder may be 95.0% by mass or more, preferably 99.0% by mass or more, and more preferably 99.5% by mass or more.

[0195] In the powder of this invention, the content of perfluorooctanoic acid (PFOA) and its salts is less than 1500 ppb by mass. The amount of PFOA and its salts relative to the powder can be less than 500 ppb by mass, less than 300 ppb by mass, less than 100 ppb by mass, less than 50 ppb by mass, less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. The lower limit is not particularly limited and can be an amount below the detection limit.

[0196] In the powder of the present invention, the content (total) of perfluorocarboxylic acids and their salts having 4 to 16 carbon atoms can be less than 10,000 ppb by mass, less than 3,000 ppb by mass, less than 2,000 ppb by mass, less than 1,000 ppb by mass, less than 500 ppb by mass, less than 100 ppb by mass, less than 50 ppb by mass, less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. The lower limit is not particularly limited and can be an amount below the detection limit.

[0197] In the powder of the present invention, the amount of perfluorosulfonic acid and its salts having 4 to 16 carbon atoms can be less than 25 ppb by mass, less than 20 ppb by mass, less than 15 ppb by mass, less than 10 ppb by mass, or less than 5 ppb by mass. The lower limit is not particularly limited and can be an amount below the detection limit.

[0198] The specific surface area of ​​the powder of the present invention is preferably 0.5 to 20 m². 2 / g.

[0199] The average particle size of the powder of the present invention is preferably 0.5 to 200 μm, more preferably 100 μm or less, even more preferably 50 μm or less, and even more preferably 25 μm or less, and particularly preferably 10 μm or less. In this way, by using a powder with a relatively small average particle size, for example when used as an additive in coatings, a coating film with superior surface smoothness can be formed.

[0200] The powder of the present invention can be obtained, for example, by manufacturing low molecular weight PTFE in powder form using the manufacturing method of the present invention described above. The powder of the present invention has excellent physical properties that are in no way inferior to those of conventionally known low molecular weight PTFE powders, and can be used in the same way as conventionally known low molecular weight PTFE powders for the same purposes.

[0201] The low molecular weight PTFE obtained by the manufacturing method of the present invention and the powder of the present invention can be suitably used as molding materials, inks, cosmetics, coatings, greases, components for office automation equipment, additives for modifying colorants, organic photosensitive materials for copiers, additives for plating solutions, etc. Examples of the molding materials mentioned above include engineering plastics such as polyoxybenzoyl polyester, polyimide, polyamide, polyamide-imide, polyacetal, polycarbonate, and polyphenylene sulfide. The low molecular weight PTFE and the powder mentioned above are particularly suitable as thickeners for greases.

[0202] The aforementioned low molecular weight PTFE and the aforementioned powder, as additives for molding materials, can be appropriately used for, for example, improving the non-adhesive / slip properties of copier rollers, improving the texture of engineering plastic molded products such as furniture surface sheets, automotive dashboards, and appliance housings, improving the sliding or wear resistance of mechanical parts that generate mechanical friction, such as light-load bearings, gears, cams, button telephone buttons, projector and camera parts, and sliding materials, and as processing aids for engineering plastics.

[0203] The aforementioned low molecular weight PTFE and powder can be used as additives in coatings to improve the slip properties of varnishes or paints. They can also be used as additives in cosmetics to improve the slip properties of foundations and other cosmetic products.

[0204] The aforementioned low molecular weight PTFE and the aforementioned powder are also suitable for applications that improve the oil resistance or water resistance of waxes, etc., and for applications that improve the lubrication of greases or colorants.

[0205] The aforementioned low molecular weight PTFE and powder can also be used as electrode binders for secondary batteries or fuel cells, hardness modifiers for electrode binders, and waterproofing agents for electrode surfaces.

[0206] The aforementioned low molecular weight PTFE or powder, along with lubricating oil, can also be used to prepare grease. The grease contains the aforementioned low molecular weight PTFE or powder and lubricating oil, thus the aforementioned low molecular weight PTFE or powder is uniformly and stably dispersed in the lubricating oil, exhibiting excellent load-bearing capacity, electrical insulation, and low moisture absorption.

[0207] The aforementioned lubricating oil (base oil) can be either mineral oil or synthetic oil. Examples of such lubricating oils (base oils) include alkane-based or cycloalkane-based mineral oils, synthetic hydrocarbon oils, ester oils, fluorinated oils, and silicone oils. From a heat resistance perspective, fluorinated oils are preferred; examples of such fluorinated oils include perfluoropolyether oils and oligomers of trifluorochloroethylene. The weight-average molecular weight of trifluorochloroethylene oligomers can be 500–1200.

[0208] The aforementioned grease may further contain a thickener. Examples of such thickeners include metal soaps, complex metal soaps, bentonite, phthalocyanine, silica gel, urea compounds, urea-carbamate compounds, carbamate compounds, and imide compounds. Examples of such metal soaps include sodium soaps, calcium soaps, aluminum soaps, and lithium soaps. Examples of such urea compounds, urea-carbamate compounds, and carbamate compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, urea-carbamate compounds, dicarbamate compounds, or mixtures thereof.

[0209] In the aforementioned grease, the aforementioned low molecular weight PTFE or the aforementioned powder preferably contains 0.1 to 60% by mass, more preferably 0.5% by mass or more, further preferably 5% by mass or more, and more preferably less than 50% by mass. If the amount of the aforementioned low molecular weight PTFE or the aforementioned powder is too large, the grease will be too hard and may not be able to exert sufficient lubricity; if the amount of the aforementioned low molecular weight PTFE or the aforementioned powder is too small, it may not be able to exert sealing performance.

[0210] The aforementioned greases may also contain solid lubricants, extreme pressure agents, antioxidants, oiliness agents, rust inhibitors, viscosity index improvers, cleaning and dispersing agents, etc.

[0211] Example

[0212] The invention will now be described in more detail by way of examples, but the invention is not limited to these examples.

[0213] The values ​​in the examples were determined by the following method.

[0214] Reduction rate of heat of fusion

[0215] The measurements were performed using a differential scanning calorimeter (DSC, trade name: DSC7020, manufactured by Hitachi High-TechScience Co., Ltd.). Specifically, approximately 10 mg of PTFE was accurately weighed and placed in a dedicated aluminum dish. The dish was heated to 200°C under a nitrogen atmosphere and held for 5 minutes. The temperature was then increased to 390°C at a rate of 10°C / min (first heating) to allow complete melting of the crystals, and the heat of fusion was measured. The temperature was then decreased from 390°C to 200°C at a rate of 10°C / min, followed by a second heating from 200°C to 390°C at a rate of 10°C / min, and the heat of fusion was measured again. The reduction rate of heat of fusion was calculated using the measured values ​​of the heat of fusion during the first and second heating cycles, according to the following formula.

[0216] Reduction rate of heat of fusion (%) = (Heat of fusion during the first heating - Heat of fusion during the second heating) / (Heat of fusion during the first heating) × 100

[0217] Melt viscosity (MV)

[0218] According to ASTM D 1238, a flow testing apparatus (manufactured by Shimadzu Corporation) was used. The die head was used to hold a 2g sample, which had been preheated at 380°C for 5 minutes, at the above temperature under a load of 0.7MPa for measurement.

[0219] Electron spin resonance (ESR) based measurement

[0220] Device: JEOL Ltd., JES-FR30EX

[0221] Measurement temperature: 23±3℃

[0222] Microwave frequency: 9.42GHz

[0223] Microwave output: 0.4mW

[0224] Central magnetic field: 347.548 mT

[0225] Scan width: ±25mT

[0226] Scan time: 60s

[0227] Time constant: 0.03s

[0228] Magnetic field modulation width: 0.32mT

[0229] Number of scans: 1

[0230] Modulation frequency: 100kHz

[0231] Marker: Mn 2+

[0232] Content of perfluorooctanoic acid and its salts (PFOA)

[0233] Perfluorooctanoic acid (PFOA) and its salts were determined using a liquid chromatography-mass spectrometry (LC-MS ACQUITY UPLC / TQD) system. 5 ml of acetonitrile was added to 1 g of the test powder, and the mixture was sonicated for 60 minutes to extract PFOA. The resulting liquid phase was analyzed using MRM (Multiple Reaction Monitoring). Acetonitrile (A) and an aqueous solution of ammonium acetate (20 mmol / L) (B) were injected as the mobile phase at a concentration gradient (A / B = 40 / 60 - 2 min - 80 / 20 - 1 min). A separation column (ACQUITY UPLC BEH C18 1.7 μm) was used, with a column temperature of 40 °C and an injection volume of 5 μL. Ionization was performed using ESI (electrospray ionization) in negative ion mode, with the cone voltage set to 25 V. The precursor ion molecular weight / product ion molecular weight was determined to be 413 / 369. The content of perfluorooctanoic acid (PFOA) and its salts was calculated using an external standard method. The detection limit for this assay was 5 ppb.

[0234] Content of perfluorocarboxylic acids and their salts (PFCs) with 4 to 16 carbon atoms

[0235] Perfluorocarboxylic acids and their salts with carbon numbers of 4–16 were determined using a Waters LC-MS ACQUITY UPLC / TQD system. The solutions used were the liquid phase extracted for the determination of perfluorooctanoic acid (PFOA), and the MRM method was employed. Regarding the determination conditions, the concentration gradient was changed from that used for PFOA (A / B = 10 / 90 - 1.5 min - 90 / 10 - 3.5 min). The molecular weight ratios of precursor ions to product ions were as follows: perfluorobutyric acid (4 carbons) 213 / 169, perfluoropentanoic acid (5 carbons) 263 / 219, perfluorohexanoic acid (6 carbons) 313 / 269, perfluoroheptanoic acid (7 carbons) 363 / 319, perfluorooctanoic acid (8 carbons) 413 / 369, and perfluorononanoic acid... The following acid (9 carbon atoms) was measured to be 463 / 419, perfluorodecanoic acid (10 carbon atoms) was measured to be 513 / 469, perfluoroundecanoic acid (11 carbon atoms) was measured to be 563 / 519, perfluorododecanic acid (12 carbon atoms) was measured to be 613 / 569, perfluorotridecanoic acid (13 carbon atoms) was measured to be 663 / 619, perfluorotetradecanoic acid (14 carbon atoms) was measured to be 713 / 669, perfluoropentadecanic acid (15 carbon atoms) was measured to be 763 / 719, and perfluorohexadecanoic acid (16 carbon atoms) was measured to be 813 / 769.

[0236] The total amount of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms is calculated using the following formula based on the perfluorooctanoic acid content (X) obtained from the above determination. The detection limit for this determination is 5 ppb.

[0237] (A C4 +A C5 +A C6 +A C7 +A C8 +A C9 +A C10 +A C11 +A C12 +A C13 +A C14 +A C15 +A C16 ) / A C8 ×X

[0238] A C4 Peak area of ​​perfluorobutyric acid

[0239] A C5 Peak area of ​​perfluorovalerate

[0240] A C6 Peak area of ​​perfluorohexanoic acid

[0241] A C7 Peak area of ​​perfluoroheptanoic acid

[0242] A C8 Peak area of ​​perfluorooctanoic acid

[0243] A C9 Peak area of ​​perfluorononanoic acid

[0244] A C10 Peak area of ​​perfluorodecanoic acid

[0245] A C11 Peak area of ​​perfluoroundecanoic acid

[0246] A C12 Peak area of ​​perfluorododecanoic acid

[0247] A C13 Peak area of ​​perfluorotridecanoic acid

[0248] A C14 Peak area of ​​perfluorotetradecanoic acid

[0249] A C15 Peak area of ​​perfluoropentadecanic acid

[0250] A C16 Peak area of ​​perfluorohexadecanoic acid

[0251] X: Perfluorooctanoic acid (PFOA) content calculated using the external standard method based on the results of the MRM method.

[0252] Reduction rate of perfluorocarboxylic acids and their salts with 4 to 16 carbon atoms (PFC reduction rate)

[0253] The PFC content in PTFE before radiation irradiation and in the obtained low molecular weight PTFE was determined using the above method, and the reduction rate was calculated using the following formula using these measured values.

[0254] PFC reduction rate (mass%) = (PFC content in PTFE before irradiation - PFC content in the resulting low molecular weight PTFE) / (PFC content in PTFE before irradiation) × 100

[0255] Oxygen concentration in a sealed container

[0256] The oxygen concentration was determined by analyzing the gas layer within the sealed container using gas chromatography. Furthermore, the oxygen concentration was confirmed to be below 0.1% by volume (oxygen was absent) by observing a change in hue from blue to pink on an oxygen test strip also sealed in the sealed container.

[0257] Example 1

[0258] Weigh 50g of PTFE micropowder (PTFE-MP) (1) (reduction rate of heat of fusion: 10.7%, PFOA content: 155 ppb by mass, PFC content: 1008 ppb by mass) in a bag made of barrier nylon.

[0259] Further, an iron-based self-reactive oxygen adsorbent (AGELESS ZP-100 manufactured by Mitsubishi Gas Chemical Co., Ltd.) was added as an oxygen adsorbent, and the nylon-based bag was sealed using heat sealing. After confirming the absence of oxygen using oxygen-measuring paper pre-placed inside the bag, the PTFE micropowder inside the bag was irradiated with 10 kGy of cobalt-60 gamma rays at an atmosphere temperature of 20–45°C to obtain low molecular weight PTFE powder. It should be noted that the atmosphere temperature during irradiation includes the temperature of the irradiation exothermic reaction (the same applies to the following examples and comparative examples).

[0260] Various properties of the obtained low molecular weight PTFE powder were determined. For each property determination, the bag was opened immediately after irradiation, and the powder was left in the air for approximately 30 minutes before the determination was performed. The results are shown in Table 1.

[0261] Example 2

[0262] In addition to irradiation with 100 kGy of cobalt-60 γ rays, low molecular weight PTFE powder was obtained in the same manner as in Example 1.

[0263] The various properties of the obtained low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 1.

[0264] Example 3

[0265] In addition to irradiation with 400 kGy of cobalt-60 γ rays, low molecular weight PTFE powder was obtained in the same manner as in Example 1.

[0266] The various properties of the obtained low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 1.

[0267] Examples 4-9

[0268] Except that PTFE micropowders (2) to (7) with the reduction rate of heat of fusion, PFOA content, and PFC content shown in Table 1 were used instead of PTFE micropowder (1), low molecular weight PTFE powder was obtained in the same manner as in Example 2. Various physical properties of the obtained low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 1.

[0269] Comparative Example 1

[0270] 50 g of PTFE micro powder (1) was weighed into a bag made of barrier nylon. The bag was then sealed by heat sealing. The PTFE micro powder in the bag was irradiated with 100 kGy cobalt-60 γ rays at an atmosphere temperature of 20–45 °C to obtain low molecular weight PTFE powder.

[0271] The various properties of the obtained low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 1.

[0272] Comparative Example 2

[0273] For PTFE micropowder (1), without radiation exposure, various physical properties were measured in the same manner as in Example 1. The results are shown in

[0274] Table 1.

[0275]

[0276] Comparative Example 3

[0277] 50 g of PTFE micropowder (1) was weighed into an aluminum cup and heat-treated at 100 °C for 3 hours using a hot air circulating electric furnace (Espec STPH-202M high-temperature thermostat). The various physical properties of the resulting low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 2.

[0278] Comparative Example 4

[0279] Except that the heat treatment conditions were set at 100°C for 0.5 hours, low molecular weight PTFE powder was obtained in the same manner as in Comparative Example 3. Various physical properties of the obtained low molecular weight PTFE powder were measured in the same manner as in Example 1. The results are shown in Table 2.

[0280]

[0281] The methods in Comparative Examples 3 and 4 require heating equipment, thus incurring significant manufacturing costs. On the other hand, the methods in Examples 1-9 do not require heating equipment. Furthermore, there is no need for the step of moving the obtained low molecular weight PTFE from the irradiation equipment to the heating equipment, etc. Therefore, the methods in Examples 1-9 can improve productivity and reduce manufacturing costs.

Claims

1. A method for manufacturing low molecular weight polytetrafluoroethylene, comprising the step (1): irradiating polytetrafluoroethylene containing perfluorocarboxylic acids or their salts having 4 to 16 carbon atoms, and having a heat of fusion reduction rate of less than 40% from the first heating to the second heating in differential scanning calorimetry, with 5 kGy to 1000 kGy of radiation at a temperature less than 100°C under substantially oxygen-free conditions, thereby obtaining polytetrafluoroethylene with reduced content of said perfluorocarboxylic acids and their salts and a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 Pa·s ~ 7.0 × 10 5 The irradiation in step (1) is carried out on low molecular weight polytetrafluoroethylene of Pa·s under the condition that there are no halogenated polymers having halogen atoms other than fluorine atoms.

2. The manufacturing method as described in claim 1, wherein, Both the polytetrafluoroethylene and the low molecular weight polytetrafluoroethylene are powders.

3. A powder comprising low molecular weight polytetrafluoroethylene, wherein, The powder contains less than 1500 ppb by mass of perfluorooctanoic acid and its salts. The low molecular weight polytetrafluoroethylene has a melt viscosity of 1.0 × 10⁻⁶ at 380°C. 2 Pa·s ~ 7.0 × 10 5 In the first differential spectral line obtained by electron spin resonance method, the difference between the signal intensity P1 with a g value of 2.020 and the signal intensity P2 with a g value of 2.023, P1-P2, is greater than -0.07 and less than 0.

10.

4. The powder as described in claim 3, wherein, The specific surface area of ​​the powder is 0.5 m². 2 / g~20m 2 / g.

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