Patent Description:
Since low molecular weight polytetrafluoroethylene (also referred to as "low molecular weight PTFE" hereinafter) not only has excellent chemical stability and extremely low surface energy but also is unlikely to suffer fibrillation, it has been used for producing e.g. plastics, inks, cosmetics, coating materials and greases, as an additive for enhancing lubricity or texture of a coating surface (e.g., <CIT>). One of the methods for producing a composition containing such low molecular weight PTFE is a method of irradiating high molecular weight polytetrafluoroethylene (sometimes referred to as "PTFE" hereinafter) with ionizing radiation (e.g., <CIT>).

<CIT> discloses a method to control the melting point and particle size of pulverized degradable polymers, such as PTFE, by irradiating the polymer with radiation and regulating the oxygen concentration in the irradiation atmosphere surrounding the polymer.

<CIT> describes a method for crosslinking treatment of a fluororesin, capable of controlling the molecular chain length between the crosslinking points of the fluororesin, by irradiating an ionizing radiation to break molecular chains, and then crosslinking by irradiating ionizing radiation to the fluororesin at a temperature of the melting point of the fluororesin or above under vacuum or in an atmosphere wherein oxidation by oxygen is suppressed by introducing an inert gas.

<NPL>) is a review article relating to the possibilities for the modification of perfluorinated polymers using high energy radiation. The reaction mechanisms, the decomposition of macromolecules to low-molecular compounds and the formation of small molecules and/or functional groups by reaction with oxygen are discussed.

In such a composition containing low molecular weight PTFE as in <CIT>, a low molecular weight fluorine-containing compound (sometimes referred to as "PFC" hereinafter), such as perfluorooctanoic acid (sometimes referred to as "PFOA" hereinafter) or its salt, can be contained. As a result of earnest studies by the present inventor, it has been found that PFC typified by such PFOA as above can be produced by irradiation with ionizing radiation.

It is an object of the present invention to obtain a composition containing low molecular weight PTFE, which has been reduced in a content of PFC.

According to the present invention, provided is a method as set out in the appended claims (also referred to as "the present method" hereinafter) for producing a composition containing low molecular weight PTFE, comprising:.

According to the present invention, a method suitable for producing a composition containing low molecular weight PTFE, which has been reduced in a content of PFC, can be obtained.

The PTFE can be one obtained by a polymerization method that can be usually carried out, such as emulsion polymerization or suspension polymerization.

The PTFE may be one having, as a constituent unit derived from a monomer in a molecular structure, only a constituent unit derived from tetrafluoroethylene (TFE), or a constituent unit derived from a monomer having a structure other than TFE and a constituent unit derived from TFE (sometimes referred to as "modified PTFE" hereinafter).

The monomer having a structure other than TFE is not limited as long as it is copolymerizable with TFE, and a monomer usually used (e.g., perfluoroolefin such as hexafluoropropylene [HFP]; chlorofluoroolefin such as chlorotrifluoroethylene [CTFE]; hydrogen atom-containing fluoroolefin such as trifluoroethylene or vinylidene fluoride [VDF]; perfluorovinyl ether; perfluoroalkylethylene; ethylene) can be used. Only one of the monomers having a structure other than TFE may be used, or a plurality of them may be used.

Examples of the perfluorovinyl ether include an unsaturated perfluoro compound of formula (<NUM>):.

wherein Rf is a perfluoro organic group. In the present specification, the "perfluoro organic group" means an organic group in which hydrogen atoms bonded to carbon atoms are all substituted by fluorine atoms. Examples of the perfluoro organic group include perfluoroalkyl and perfluoro(alkoxyalkyl). The perfluoro organic group may have an oxygen atom that forms an ether linkage.

In one embodiment, the perfluorovinyl ether is, for example, a perfluoroalkyl vinyl ether (PAVE), wherein Rf is C<NUM>-<NUM>-perfluoroalkyl (preferably C<NUM>-<NUM>-perfluoroalkyl) in the above formula (<NUM>).

Examples of the perfluoroalkyl group in the PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl and perfluorohexyl. A preferred perfluoroalkyl group is, for example, perfluoropropyl.

Preferred PAVE is perfluoropropyl vinyl ether (PPVE) in which the perfluoroalkyl group is perfluoropropyl.

In another embodiment, the perfluorovinyl ether is, for example, in formula (<NUM>), one in which Rf is C<NUM>-<NUM>-perfluoro(alkoxyalkyl) ;.

Examples of the perfluoroalkylethylene include (perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene and (perfluorooctyl)ethylene.

The monomer having a structure other than TFE is preferably at least one of HFP, CTFE, VDF, PPVE, PFBE and ethylene, and is more preferably at least one of HFP and CTFE.

In the modified PTFE, the constituent unit derived from the monomer having a structure other than TFE is preferably contained in an amount of <NUM>-<NUM> mass%, more preferably ≥ <NUM> mass% and ≤ <NUM> mass%, still more preferably ≤ <NUM> mass%. The content of the constituent unit derived from the monomer having a structure other than TFE can be determined by a known method such as Fourier transform infrared spectroscopy (FT-IR).

The standard specific gravity (SSG) of the PTFE is in the range of <NUM>-<NUM>. The SSG is a value measured in accordance with ASTM D <NUM> when the PTFE is obtained by suspension polymerization, and is a value measured in accordance with ASTM D <NUM> when the PTFE is obtained by emulsion polymerization. SSG or a melt viscosity can be usually used as an indication of a molecular weight of PTFE. PTFE having such SSG as above has an extremely high melt viscosity, and it is difficult to accurately measure the melt viscosity. Accordingly, in PTFE having such SSG as above, the SSG is usually used as an indication of the molecular weight.

The melting point of the PTFE is preferably in the range of <NUM>-<NUM>. The melting point is a value measured by using, for example, a differential scanning calorimeter (DSC). Specifically, the melting point can be a minimum point of heat of fusion, which is obtained by using a DSC having been subjected to temperature calibration in advance using indium and lead as standard samples and increasing the temperature at <NUM>/min in the temperature region of <NUM>-<NUM> in an air stream at <NUM>/min.

The composition containing PTFE is preferably substantially composed of PTFE. The expression "substantially composed of PTFE" means that PTFE can be contained in an amount of ≥ <NUM> mass parts (pbm), specifically ≥ <NUM> pbm, based on <NUM> pbm of the composition. The upper limit of the content of PTFE based on <NUM> pbm of the composition is not limited, but it can be, for example, ≤ <NUM> pbm, specifically ≤ <NUM> pbm.

The composition containing PTFE may be one containing PFOA or its salt or may be one substantially free from PFOA or its salt. Here, the expression "substantially free from" means that the amount of PFOA, in terms of mass, based on PTFE is < <NUM> ppb, preferably ≤ <NUM> ppb, more preferably ≤ <NUM> ppb, particularly preferably less than the detection limit. The expression "less than the detection limit" means, for example, < <NUM> ppb. In the present specification, the above content indicates a total amount of PFOA and its salt, unless otherwise noted.

The composition containing PTFE is preferably substantially free from PFOA or its salt. Here, the expression "substantially free from" has the same meaning as above. Even when the composition containing PTFE and substantially free from PFOA or its salt is used as in the present embodiment, PFOA or its salt can be formed by irradiating the PTFE with ionizing radiation in step (I).

Examples of the salt of PFOA include an ammonium salt, a sodium salt and a potassium salt of PFOA.

The content of the PFOA and its salt can be measured by using liquid chromatography. More specifically, the PFOA and its salt are extracted from the composition containing PTFE using a solvent, and the amount of the PFOA and its salt contained in the solvent after extraction can be measured by using liquid chromatography.

As the composition containing PTFE, one containing PFC may be used, or one substantially free from PFC may be used. Here, the expression "substantially free from" means that the amount of PFC, in terms of mass, based on PTFE is < <NUM> ppb, preferably ≤ <NUM> ppb, more preferably ≤ <NUM> ppb, particularly preferably less than the detection limit. The expression "less than the detection limit" means, for example, < <NUM> ppb.

The composition containing PTFE is preferably substantially free from PFC. Even when the composition containing PTFE and substantially free from PFC is used as in the present embodiment, PFC can be formed by irradiating the PTFE with ionizing radiation in the step (I). Here, the expression "substantially free from" has the same meaning as above. In the present specification, when PFCs of a plurality of structures is present, the above content indicates a total amount of PFCs of a plurality of structures, unless otherwise noted.

The content of the PFC can be measured by using, for example, liquid chromatography. More specifically, from the composition containing PTFE, the PFC is extracted using a solvent, and the amount of the PFC contained in the solvent after extraction can be measured by using liquid chromatography.

In the present specification, "PFC" means a low molecular weight fluorine-containing compound such as PFOA or its salt, specifically means a low molecular weight fluorine-containing compound having a functional group or its derivative, and more specifically means an acid having a fluorine-containing carbon chain or its derivative. The fluorine-containing carbon chain indicates a carbon chain in which one or more of hydrogen atoms bonded to carbon atoms are substituted by fluorine atoms. Examples of the acid include carboxylic acid and sulfonic acid. Examples of the derivative include salts. Examples of the salt include an ammonium salt, a sodium salt and a potassium salt.

Examples of the PFC include an acid having a fluorine-containing C<NUM>-<NUM>-carbon chain and its derivative.

Examples of the shape of the composition containing PTFE include, but not limited to, a powder, a molded body, a scrap, a fragment that can be produced during formation of a molded body, a cutting chip produced during cutting of a molded body, and a preform.

In one embodiment, the shape of the composition containing PTFE is powder. The present embodiment is advantageous from the viewpoint that the resin component containing PTFE can be uniformly irradiated with ionizing radiation. According to the present embodiment, a powdery composition containing low molecular weight PTFE can be easily obtained.

In one embodiment, the composition containing PTFE is a molded body. In the present embodiment, the composition containing PTFE can be one having been subjected to a molding step of heating the composition containing PTFE to a primary melting point or higher. That is to say, the present method can further include, prior to the step (I), a step of heating the composition containing PTFE to a primary melting point of PTFE or higher to form a molded body of the composition containing PTFE.

The primary melting point can be measured by using DSC. Specifically, the primary melting point is a maximum peak temperature of an endothermic curve appearing on a crystal melting curve, and the endothermic curve is obtained by heating uncalcined PTFE under the conditions of a temperature-increasing rate of <NUM>/min. The primary melting point can be usually a temperature of ≥ <NUM>. The uncalcined PTFE indicates PTFE free from heat history of being heated to a temperature of the primary melting point or higher.

The shape of the molded body is not limited. Examples of the shape of the molded body include tape-like, sheet-like, rod-like, tubular, and fibrous shapes.

The specific gravity of the molded body is preferably ≥ <NUM>/cm<NUM>, more preferably ≥ <NUM>/cm<NUM>, and is preferably ≤ <NUM>/cm<NUM>. The specific gravity can be measured by an underwater replacement method.

In one embodiment, the composition containing PTFE is a scrap, a fragment that can be produced during formation of a molded body, or a cutting chip produced during cutting of a molded body.

In one embodiment, the composition containing PTFE is a preform. Here, the preform refers to a molded body that is obtained by compressing powdery PTFE and has not been subjected to a calcining step (has not been heated to a temperature of the primary melting point or higher).

Irradiation with ionizing radiation can be carried out by a conventionally known method and under the conventionally known conditions. By the irradiation with ionizing radiation, the PTFE contained in the composition containing PTFE is decreased in molecular weight and becomes low molecular weight PTFE. By the irradiation with ionizing radiation, PFC such as PFOA or its salt can be produced.

Examples of the ionizing radiation include electron ray, γ-ray, X-ray, neutron beam and high energy ion. The ionizing radiation is preferably electron ray or γ-ray.

The irradiation with ionizing radiation is not limited, but can be carried out in, for example, air, a hydrocarbon-based gas, water, or a solvent.

In one embodiment, the irradiation with ionizing radiation can be carried out in air. The present embodiment is preferable from the viewpoint of reduction of cost.

The irradiation dose of the ionizing radiation is preferably in the range of <NUM>-<NUM>,<NUM> kGy, more preferably ≤ <NUM>,<NUM> kGy and ≥ <NUM> kGy , still more preferably ≤ <NUM> kGy, particularly preferably ≤ <NUM> kGy and ≥ <NUM> kGy.

The irradiation temperature of the ionizing radiation is, for example, ≥ <NUM>. The upper limit of the irradiation temperature of the ionizing radiation is not limited as long as it is a melting point of PTFE or lower, and from the viewpoint of suppression of crosslinking of PTFE molecular chain, it is preferably ≤ <NUM>, more preferably ≤ <NUM>, still more preferably ≤ <NUM>. From the viewpoint of reduction of production cost, irradiation with ionizing radiation can be carried out at <NUM> (e.g., <NUM>) to <NUM>.

The resin component contained in the composition obtained in the step (I) is preferably substantially composed of low molecular weight PTFE. The expression "substantially composed of low molecular weight PTFE" means that the low molecular weight PTFE can be contained in an amount of, for example, ≥ <NUM> pbm, specifically ≥ <NUM> pbm, based on <NUM> pbm of the composition. The upper limit of the low molecular weight PTFE based on <NUM> pbm of the composition is not limited, but it can be, for example, ≤ <NUM> pbm, specifically ≤ <NUM> pbm.

The shape of the composition obtained in the step (I) is not limited.

In one embodiment, the shape of the composition obtained in the step (I) is powder. This composition is advantageous from the viewpoint of ease of treatment in or after the step (II).

In the above embodiment, the average particle size of the low molecular weight PTFE in the composition obtained in the step (I) is preferably ≤ <NUM>,<NUM>, more preferably ≤ <NUM>, still more preferably ≤ <NUM>. The lower limit of the average particle size of the low molecular weight PTFE is not limited, but it is, for example, > <NUM>. Since the low molecular weight PTFE has such an average particle size, the diameters of particles contained in the composition can become relatively small.

The average particle size of the low molecular weight PTFE can be measured by using a laser diffraction particle size distribution measurement apparatus. Specifically, the average particle size is taken to be equal to a particle size corresponding to <NUM>% (based on volume) of particle size distribution integration obtained by carrying out measurement at a dispersion pressure of <NUM> bar without using a cascade. The laser diffraction particle size distribution measurement apparatus may be, for example, laser diffraction particle size distribution measurement apparatus manufactured by JEOL Ltd. (trade name: HELOS & RODOS).

The melt viscosity of the low molecular weight PTFE at <NUM> is in the range of <NUM>×<NUM><NUM> to <NUM>×<NUM><NUM> Pa·s. A more preferred lower limit of the melt viscosity is ≥ <NUM>×<NUM><NUM> Pa·s. A more preferred upper limit of the melt viscosity is ≤ <NUM>×<NUM><NUM> Pa·s. In the present invention, the "low molecular weight PTFE" means PTFE having a melt viscosity in the above range. In the case of low molecular weight PTFE having such a melt viscosity as above, it is difficult to form a molded body for use in the measurement of the aforesaid standard specific gravity (SSG), and it is difficult to measure accurate SSG of the low molecular weight PTFE. Accordingly, a melt viscosity is usually used as an indication of a molecular weight of low molecular weight PTFE.

In accordance with ASTM D <NUM>, the melt viscosity can be measured using <NUM> of a sample having been heated in advance at the measuring temperature (<NUM>) for <NUM> minutes and using a flow tester (manufactured by Shimadzu Corporation) and a die of 2φ-<NUM> while maintaining the sample at the above temperature under a load of <NUM> MPa.

The low molecular weight PTFE can be non-fibrillatable.

The melting point of the low molecular weight PTFE is preferably in the range of <NUM>-<NUM>. This melting point can be measured using a method described above as the method for measuring a melting point of PTFE.

Step (II) will be described hereinafter.

Step (II) is a step in which PFC contained in the composition containing low molecular weight PTFE obtained in the step (I) is decomposed. By the present step, a composition containing low molecular weight PTFE and having a low content of PFC can be obtained.

Although the content of PFC contained in the composition containing low molecular weight PTFE obtained in the step (I) is not limited, it is, for example, ≥ <NUM> mass ppb based on the low molecular weight PTFE.

A content of the PFOA or its salt contained in the composition containing low molecular weight PTFE obtained in the step (I) is not limited, but it is, for example, ≥ <NUM> mass ppb based on the low molecular weight PTFE.

The decomposition of the PFC is carried out by irradiation with non-ionizing radiation.

The irradiation with non-ionizing radiation can be carried out by using a usual method.

The non-ionizing radiation is ultraviolet light of <NUM>-<NUM>, for example, near ultraviolet light of <NUM>-<NUM>, vacuum ultraviolet light of <NUM>-<NUM>, and extreme ultraviolet light of <NUM>-<NUM>.

The irradiation with ultraviolet light is preferably by irradiation with ultraviolet light of <NUM>-<NUM>, and may be carried out by combining ultraviolet lights having different wavelengths.

The irradiation with ultraviolet light can be carried out for, for example, <NUM>-<NUM> minutes, more specifically <NUM>-<NUM> minutes.

The decomposition of PFC may be carried out once, or may be carried out a plurality of times. When the decomposition of PFC is carried out a plurality of times, the same method may be carried out a plurality of times, or different methods may be carried out in combination.

Step (II) can further include grinding the composition obtained in step (I). Specifically, step (II) can include decomposing PFC contained in the composition obtained in step (I) and grinding the composition.

In one embodiment, in step (II), decomposition of PFC contained in the composition obtained in step (I) is carried out, and thereafter, the composition is ground.

In one embodiment, in step (II), the composition obtained in step (I) is ground, and thereafter, PFC contained in the ground composition is decomposed.

In a preferred embodiment, in step (II), the composition obtained in step (I) is ground, and thereafter, PFC contained in the ground composition is decomposed. According to the present embodiment, decomposition of PFC can be more effective.

A method for the grinding treatment is not limited, but it is, for example, a method of grinding using a grinder. Examples of the grinder include impact type grinders, such as a hammer mill, a pin mill and a jet mill, and mill type grinders, such as a cutter mill that performs grinding by means of shear force produced by a rotary blade and an outer peripheral stator (stationary blade).

The temperature for carrying out the grinding treatment is preferably -<NUM> to < <NUM>.

In one embodiment, the grinding is freeze grinding and can be usually carried out at a temperature in the range of - <NUM> to -<NUM>. In the present embodiment, the temperature can be generally adjusted using liquid nitrogen.

In one embodiment, the grinding is carried out at a temperature in the vicinity of room temperature (e.g., <NUM>-<NUM>).

In one embodiment, the grinding can be carried out at <NUM> to < <NUM>, preferably <NUM>-<NUM>, and more preferably <NUM>-<NUM>. The present embodiment is preferable from the viewpoints of simplification of the step and reduction of cost required for the grinding.

In the present method, step (II) can further include carrying out at least one of washing, steam treatment and decompression treatment. That is to say, step (II) can include decomposing PFC contained in the composition obtained in step (I) and carrying out at least one of washing, steam treatment and decompression treatment on the composition.

Preferably, step (II) includes decomposing PFC contained in the composition obtained in step (I) and thereafter carrying out at least one washing, steam treatment and decompression treatment on the resulting composition.

In one embodiment, step (II) can be a step in which the composition obtained in step (I) is ground, PFC contained in the composition is decomposed, and thereafter, at least one of washing, steam treatment and decompression treatment is carried out on the resulting composition. According to the present embodiment, removal of decomposed PFC can become easy.

In one embodiment, step (II) can be a step in which the composition obtained in step (I) is ground, at least one of washing, steam treatment and decompression treatment is carried out on the resulting composition, and subsequently, PFC contained in the composition is decomposed. According to the present embodiment, removal of decomposed PFC can become easy.

The at least one treatment selected from washing, steam treatment and decompression treatment may be carried out only once, or may be carried out a plurality of times. When these are carried out a plurality of times, the same method may be repeatedly carried out, or different methods may be carried out in combination.

The washing can be carried out using a solution containing at least one of water and an organic solvent (sometimes referred to as a "washing solution" hereinafter).

It is preferable to use a polar solvent as the organic solvent. Examples of the polar solvent include alcohols, such as methanol, ethanol, and isopropyl alcohol (IPA); ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone (MIBK), and diisobutyl ketone (DISK); nitriles, such as acetonitrile; N,N-dimethylformamide, and dimethyl sulfoxide.

It is preferable to use an alkaline solution as the washing solution, and it is more preferable to use a solution of e. g ammonia or sodium hydroxide. From the viewpoint of obtaining low molecular weight PTFE having a low content of a metallic atom, a solution of ammonia is preferably used.

The washing may be carried out only once or may be carried out a plurality of times. When the washing is carried out a plurality of times, the washing solutions may be the same as one another, or may be different from one another. The washing solutions may be those of the same concentration or may be those of different concentrations.

The washing can be carried out by applying at least one of stirring, heating and ultrasonic waves to the composition immersed in the washing solution.

A method for applying stirring is not limited, and a method that can be usually used, such as a method of using a stirrer or a stirring blade, can be used.

A method for applying ultrasonic waves is not limited, and a method that can be usually used can be used.

The washing can be carried out under pressure, at normal pressure or under reduced pressure. From the viewpoint of suppression of production cost, the washing is preferably carried out at normal pressure.

In a preferred embodiment, the washing is carried out by using water, an alkaline aqueous solution or a polar solvent as the washing solution and applying at least one of stirring, heating and ultrasonic waves to the composition immersed in the washing solution.

In a preferred embodiment, water washing is further carried out after the washing. The water washing is preferably carried out until pH of wash water becomes <NUM>-<NUM>.

In a more preferred embodiment, drying is carried out after the washing.

A method for carrying out the drying is not limited, but the drying can be carried out using, for example, a box type dryer, a hot air circulation type dryer, a vacuum dryer, a spray dryer, a vibration dryer, a fluidized bed dryer, or a fixed bed dryer. The drying can be specifically carried out using e.g. a box type dryer or a hot air circulation type dryer.

The drying can be carried out in the temperature range of, for example, <NUM>-<NUM> and, from the viewpoints of productivity and safety, preferably <NUM>-<NUM>.

The drying can be carried out to remove e.g. water or a solvent used for the washing. The time for carrying out the drying can be set to, for example, <NUM>-<NUM> hours.

By carrying out the drying, the content of PFC contained in the composition containing low molecular weight PTFE obtained by the present method can be further reduced.

By carrying out the drying, the content of PFOA or its salt contained in the composition containing low molecular weight PTFE obtained by the present method can be further reduced.

The steam treatment can be carried out by bringing a treatment solution in the form of steam and the composition provided to step (II) into contact with each other.

The treatment solution may be water, an alkaline aqueous solution, or a polar solvent.

In a preferred embodiment, the steam treatment can be carried out at a temperature of <NUM>-<NUM> though the temperature varies depending on the treatment solution.

The steam treatment is preferably carried out using an alkaline solution having been heated to <NUM>-<NUM>, more preferably heated to <NUM>-<NUM>.

The steam treatment is preferably carried out under pressure, more preferably at a pressure of <NUM>-<NUM> MPa, and still more preferably <NUM>-<NUM> MPa.

The steam treatment can be carried out by bringing the treatment solution in the form of steam and the composition provided to step (II) into contact with each other for, for example, <NUM> minutes to <NUM> hour.

In a more preferred embodiment, drying is carried out after the steam treatment. The drying can be carried out in the same manner as described above.

The decompression treatment can be carried out at a gauge pressure of -<NUM> to -<NUM> MPa, specifically -<NUM> to - <NUM> MPa.

The time for carrying out the decompression treatment is not limited, but it is in the range of, for example, several minutes to <NUM> days.

A method for carrying out the decompression treatment is not limited, but the decomposition treatment can be carried out by using, for example, a vacuum dryer, an accumulator, or a double cone vacuum stirring machine.

The present method may further include a classification step.

In the classification step, for example, airflow classification can be used. Specifically, the classification step may be one in which fine particles or fibrous particles are removed by airflow classification and then coarse particles are further removed by classification.

In the airflow classification, low molecular weight PTFE particles (e.g., ground particles) are fed to a column-like classification room by decompressed air and dispersed by swirl airflow in the room, and the fine particles are classified by centrifugal force. The fine particles are recovered with a cyclone and a bug filter from the center. In the classification room, a cone in a conical shape or a rotating body such as a rotor is installed in order to allow the ground particles and air to uniformly perform swirling motion.

When classification cones are used, adjustment of classification points is carried out by adjusting a flow rate of secondary air and a gap between the classification cones. When a rotor is used, a flow rate in the classification room is adjusted by the number of revolutions of the rotor.

Examples of the method for removing coarse particles include airflow classification using a mesh, a vibrating screen and an ultrasonic sieve, and airflow classification is preferable.

In one embodiment, the classification treatment may be carried out prior to step (I).

In one embodiment, the classification treatment is carried out on the composition having been subjected to the grinding treatment.

In a preferred embodiment, step (II) is a step in which grinding treatment is carried out on the composition obtained in step (I), then classification treatment is carried out, and thereafter, PFC is decomposed.

In another preferred embodiment, step (II) is a step in which grinding treatment is carried out on the composition obtained in step (I), then PFC is decomposed, and thereafter, classification treatment is carried out to obtain a composition containing low molecular weight PTFE.

According to the present invention, PFC typified by e.g. PFOA is decomposed as described above, and a composition containing low molecular weight PTFE and having a low content of PFC can be obtained.

The content of PFOA and its salt contained in the composition containing low molecular weight PTFE obtained in the present method is preferably < <NUM> ppb, more preferably ≤ <NUM> ppb, still more preferably < <NUM> ppb, in terms of mass, based on the low molecular weight PTFE. The lower limit of the content of PFOA and its salt is not limited, but it is, for example, less than the detection limit, more specifically <NUM> ppb. According to the present invention, a composition containing low molecular weight PTFE and having a low content of PFOA and its salt as described above can be obtained. The above content indicates a total amount of PFOA and its salt. The salt of PFOA is as previously described.

The content of the PFOA and its salt can be measured by using liquid chromatography. More specifically, the PFOA and its salt are extracted from the composition containing low molecular weight PTFE using a solvent, and the amount of the PFOA and its salt contained in the solvent after extraction can be measured by using liquid chromatography.

The content of PFC contained in the composition containing low molecular weight PTFE obtained in the present method is preferably ≤ <NUM> ppb, more preferably < <NUM> ppb, still more preferably ≤ <NUM> ppb, particularly preferably ≤ <NUM> ppb, more preferably < <NUM> ppb, in terms of mass, based on the low molecular weight PTFE. The lower limit of the content of the PFC is not limited, but it is, for example, less than the detection limit, more specifically <NUM> ppb. According to the present invention, a composition containing low molecular weight PTFE and having a low content of PFC as described above can be obtained. When PFCs of a plurality of structures are present, the above content indicates a total amount of PFCs of a plurality of structures, unless otherwise noted. The PFC is as previously described.

The content of the PFC can be measured by using, for example, liquid chromatography. More specifically, the PFC is extracted from the composition containing low molecular weight PTFE using a solvent, and the amount of the PFC contained in the solvent after extraction can be measured by using liquid chromatography.

In one embodiment, the shape of the composition containing low molecular weight PTFE obtained by the present method is powder. The specific surface area of the powder is preferably <NUM>-<NUM><NUM>/g. The specific surface area can be obtained by using a surface analyzer. Specifically, the specific surface area is measured by BET method using a mixed gas consisting of <NUM>% of nitrogen and <NUM>% of helium as a carrier gas and using liquid nitrogen as a refrigerant. The surface analyzer may be, for example, BELSORP-mini II (trade name, manufactured by MicrotracBEL Corp. The specific surface area is more preferably ≥ <NUM><NUM>/g.

In one embodiment, the specific surface area of the powder is <NUM> to < <NUM><NUM>/g. A method for measuring the specific surface area is as described above. Such low molecular weight PTFE can be easily dispersed in a matrix material. The matrix material may be e.g. a plastic, an ink or a coating material.

The lower limit of the specific surface area of the powder (specifically, low molecular weight PTFE) of the present embodiment is preferably ≥ <NUM><NUM>/g, and the upper limit is preferably ≤ <NUM><NUM>/g, more preferably ≤ <NUM><NUM>/g.

In one embodiment, the specific surface area of the powder is <NUM>-<NUM><NUM>/g. A method for measuring the specific surface area is as described above. In the case of such low molecular weight PTFE, the sizes of particles dispersed in a matrix material are small, so that an effect of surface modification, such as an effect of enhancing texture of a coating surface, is high, and the oil absorption can be high. The matrix material may be e.g. an oil, a grease, a coating material or a plastic.

The lower limit of the specific surface area of the powder (specifically, low molecular weight PTFE) of the above embodiment is preferably ≥ <NUM><NUM>/g and the upper limit is preferably ≤ <NUM><NUM>/g, more preferably ≤ <NUM><NUM>/g.

In one embodiment, the average particle size of the powder is preferably <NUM>-<NUM>, more preferably ≤ <NUM>, still more preferably ≤ <NUM>. The average particle size is, for example, <NUM> or more. The low molecular weight PTFE obtained by the present method has such an average particle size as above, and therefore, when it is added to a coating material as, for example, an additive, it can contribute to formation of a coating film having more excellent surface smoothness.

A method for measuring the average particle size of the composition containing low molecular weight PTFE obtained by the present method is as described for measuring the average particle size of the low molecular weight PTFE contained in the composition obtained in step (I).

In a preferred embodiment, the composition containing low molecular weight PTFE obtained by the present method is in powder form.

The composition containing low molecular weight PTFE obtained by the present method can be used after it is molded, when needed.

In one embodiment, the present method is a method for purifying a composition containing low molecular weight PTFE.

The composition containing low molecular weight PTFE obtained by the present method is preferably a powder, and can be preferably used as an additive for modifying a molding material, an ink, a cosmetic, a coating material, a grease, a member for office automation equipment, a toner or an additive for a plating solution. Examples of the molding material include engineering plastics, such as polyoxybenzoyl polyester, polyimide, polyamide, polyamideimide, polyacetal, polycarbonate and polyphenylene sulfide. The composition containing low molecular weight PTFE can be preferably used particularly as a thickening agent for grease.

The composition containing low molecular weight PTFE obtained by the present invention is preferably a powder and can be used as an additive for a molding material. The composition can be preferably used, for example, for the purpose of enhancing non-stickiness and sliding property of a copy roll or a photoreceptor; for the purpose of enhancing texture of engineering plastic molded products, such as a surface layer sheet of furniture, a dashboard of an automobile, and a cover of an appliance; for the purpose of enhancing lubricity or abrasion resistance of machine parts that can cause mechanical friction, such as a light-load bearing, a gear, a cum, buttons of push-button telephone, a projector, camera parts, and a sliding material; or as a processing aid for engineering plastics.

The composition containing low molecular weight PTFE obtained by the present method is preferably a powder, and can be used for the purpose of enhancing lubricity of a varnish or a paint, as an additive for a coating material. The composition containing low molecular weight PTFE can be used for the purpose of enhancing lubricity of cosmetics such as foundation, as an additive for cosmetics.

The composition containing low molecular weight PTFE obtained by the present invention is preferably a powder, and can be preferably used also for the purpose of enhancing oil-repellency or water-repellency of e.g. a wax and for the purpose of enhancing lubricity of a thickening agent for grease, a grease or a toner.

The composition containing low molecular weight PTFE obtained by the present invention is preferably a powder, and can be used also as e.g. an electrode binder for a secondary battery or a fuel battery, a hardness adjuster for an electrode binder or a water-repellent for an electrode surface.

The composition containing low molecular weight PTFE obtained by the present method is preferably a powder, and can be used together with a lubricating oil to prepare a grease. Since the grease is characterized by containing the powdery composition containing low molecular weight PTFE and a lubricating oil, the composition containing low molecular weight PTFE can be uniformly and stably dispersed in the lubricating oil. The grease can be excellent in properties such as heat resistance, load-bearing property, electrical insulation property and low hygroscopicity.

The lubricating oil (base oil) may be a mineral oil, or may be a synthetic oil. Examples of the lubricating oil (base oil) include paraffinic or naphthenic mineral oils, and synthetic oils, such as synthetic hydrocarbon oil, ester oil, fluorine oil and silicone oil. From the viewpoint of heat resistance, fluorine oil is preferable, and examples of the fluorine oil include perfluoropolyether oil and a low polymer of ethylene chloride trifluoride. The low polymer of ethylene chloride trifluoride may have a weight-average molecular weight of <NUM>-<NUM>,<NUM>.

In the grease, other thickening agents may be further used in combination. Examples of the thickening agent include metallic soap, composite metallic soap, bentonite, phthalocyanine, silica gel, a urea compound, a urea-urethane compound, a urethane compound, and an imide compound. Examples of the metallic soap include sodium soap, calcium soap, aluminum soap and lithium soap. Examples of the urea compound, the urea-urethane compound and the urethane compound include a diurea compound, a triurea compound, a tetraurea compound, other polyurea compounds, urea-urethane compounds and diurethane compounds, and mixtures thereof.

The grease preferably contains the powdery composition containing low molecular weight PTFE in an amount of <NUM>-<NUM> mass%, more preferably ≥ <NUM> mass% and ≤ <NUM> mass%. Since the powdery composition containing low molecular weight PTFE is contained in the above amount, a grease having an appropriate hardness can be obtained. The grease can exhibit sufficient lubricity and can exhibit appropriate sealing property.

The grease can further contain, for example, a solid lubricant, an extreme pressure agent, an antioxidant, an oiliness agent, a rust preventive, a viscosity index improver, a detergent dispersant.

The present invention will be more specifically described with reference to the following Examples.

In a barrier nylon bag, <NUM> of POLYFLON (R) PTFE F-<NUM> (manufactured by DAIKIN INDUSTRIES, LTD. , concentration of PFC and concentration of PFOA were each the detection limit or less) was weighed, and the bag was sealed by means of heat sealing. Subsequently, the PTFE F-<NUM> in the bag was irradiated with cobalt-<NUM>γ-ray at <NUM> kGy at room temperature, thereby obtaining a low molecular weight PTFE powder. The resulting low molecular weight PTFE powder was used as a sample for measuring contents of PFOA and a C<NUM>-<NUM>-perfluorocarboxylic acid and its derivative. Further, a melt viscosity of the resulting low molecular weight PTFE powder was measured. The results are set forth in Table <NUM>.

The low molecular weight PTFE obtained in Comparative Example <NUM> was irradiated with ultraviolet lights (UV) of <NUM> and <NUM> at the same time, thereby obtaining treated low molecular weight PTFE.

The low molecular weight PTFE obtained in Comparative Example <NUM> was subjected to ultraviolet light irradiation under the conditions described in Table <NUM>, thereby obtaining treated low molecular weight PTFE.

Melt viscosity of the low molecular weight PTFE obtained in each of Examples and Comparative Example was measured. In accordance with ASTM D <NUM>, the melt viscosity was measured using <NUM> of a sample having been heated in advance at the measuring temperature (<NUM>) for <NUM> minutes and using a flow tester (manufactured by Shimadzu Corporation) and a die of 2φ-<NUM> while maintaining the sample at the above temperature under a load of <NUM> MPa.

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITY UPLC/TQD), a content of PFOA in the low molecular weight PTFE obtained in each of Examples and Comparative Example, based on <NUM> pbm of the low molecular weight PTFE, was measured.

Specifically, <NUM> of acetonitrile was added to <NUM> of a powder for measurement, and the mixture was subjected to ultrasonic treatment for <NUM> minutes to extract PFOA. A content of PFOA in the resulting liquid phase was measured using MRM (Multiple Reaction Monitoring) method. Acetonitrile (A) and an ammonium acetate aqueous solution (<NUM> mmol/L) (B) were used as mobile phases, and they were fed at a concentration gradient (A/B = <NUM>/<NUM>-<NUM>-<NUM>/<NUM>-<NUM>). ACQUITY UPLC BEH C18 <NUM> was used as a separatory column, the column temperature was set at <NUM>, and the injection quantity was set to <NUM>µL. In the ionization method, ESI (Electrospray ionization) Negative was used, a cone voltage was set at <NUM> V, and measurement of precursor ion molecular weight/product ion molecular weight resulted in <NUM>/<NUM>. The content of PFOA was calculated using an external standard method. The detection limit in this measurement was <NUM> ppb.

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITY UPLC/TQD), a content of a C<NUM>-<NUM>-perfluorocarboxylic acid in the low molecular weight PTFE obtained in each of Examples and Comparative Example, based on <NUM> pbm of the low molecular weight PTFE, was measured.

Specifically, the above content was measured by the use of the MRM method using, as a solution, a liquid phase extracted in the measurement of PFOA. Under the measurement conditions changed in concentration gradient (A/B = <NUM>/<NUM>-<NUM>-<NUM>/<NUM>-<NUM>) from the measurement conditions for PFOA, the precursor ion molecular weight/product ion molecular weight described in Table <NUM> were measured. The total amount of C<NUM>-<NUM>-perfluorocarboxylic acids was calculated from the following formula using the content (X) of perfluorooctanoic acid obtained in the above measurement. The detection limit in this measurement was <NUM> ppb. <MAT> wherein Ac6 to Ac14 represent peak areas of C<NUM>-<NUM>-carboxylic acids, respectively; and.

X represents the content of perfluorooctanoic acid (<NUM> carbon atoms) calculated from the result of the measurement by the MFM method, using an external standard method.

The results are shown in the following table. In the following table, the "content of C6 to <NUM>" indicates a content of C<NUM>-<NUM>-perfluorocarboxylic acids and their derivatives.

Claim 1:
A method for producing a composition containing low molecular weight polytetrafluoroethylene (PTFE), comprising:
(I) irradiating a composition containing high molecular weight PTFE having standard specific gravity (SSG), measured according to ASTM D <NUM> when obtained by suspension polymerization, and measured according to ASTM D <NUM> when obtained by emulsion polymerization, of <NUM>-<NUM> with ionizing radiation to obtain a composition containing low molecular weight PTFE having a melt viscosity at <NUM> in the range of <NUM>×<NUM><NUM> to <NUM>×<NUM><NUM> Pa·s, measured in accordance with ASTM D <NUM> using <NUM> of a sample heated in advance at <NUM> for <NUM>. and using a flow tester (Shimadzu Corporation) and a die of 2φ-<NUM> while maintaining the sample at <NUM> under a load of <NUM> MPa; and
(II) decomposing a low molecular weight fluorine-containing compound (PFC) contained in the composition obtained in step (I) using non-ionizing radiation which is UV-light of <NUM>-<NUM> wavelength.