Solid composition for producing stretched film, and stretched film using the same

Abstract

Provided is a solid composition for stretched film production which comprises a first polymer that is either a crystalline fluoropolymer having a melting point lower than 20°C or a noncrystalline fluoropolymer having a glass transition temperature lower than 20°C and a second polymer that includes units based on tetrafluoroethylene and differs from the first polymer, wherein the content of the first polymer is 0.01-4.0 mass% with respect to the total mass of the first polymer and the second polymer and the sum of the content of a compound represented by formula (S1) and the content of a compound represented by formula (S2) is 100 mass ppb or less.

Description

[Technical Field]

[0001] The present invention relates to a solid composition for producing a stretched membrane, and a stretched membrane using the solid composition. This application claims priority based on Japanese Patent Application No. 2024-017050, filed February 7, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] Fluorine-containing polymers such as tetrafluoroethylene copolymers are used in various industrial fields because of their excellent heat resistance, chemical resistance, flame retardancy, weather resistance, and the like. Patent Document 1 describes a method in which modified tetrafluoroethylene polymerized using a specific fluorine-containing surfactant is powdered, paste-extruded, and then stretched to obtain a stretched membrane. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017-030094 Summary of the Invention [Problem to be solved by the invention]

[0004] However, stretched membranes produced using fluoropolymers produced by conventional methods do not necessarily have sufficient uniformity. An object of the present invention is to provide a solid composition for producing a stretched membrane, which can improve the uniformity of the stretched membrane, and a stretched membrane using the same. [Means for solving the problem]

[0005] The present invention has the following aspects. [1] A composition comprising a first polymer which is a crystalline fluorine-containing polymer having a melting point below 20°C or an amorphous fluorine-containing polymer having a glass transition temperature below 20°C, and a second polymer which contains units based on tetrafluoroethylene and is different from the first polymer, the content of the first polymer is 0.01 to 4.0% by mass with respect to the total mass of the first polymer and the second polymer, A solid composition for producing a stretched membrane, wherein the total content of a compound represented by the following formula (S1) and a compound represented by the following formula (S2) is 100 mass ppb or less. Formula (S1):H-(CF2) n1 -COOM Formula (S2):H-(CF2) n2 -SO3M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH4; n1 represents an integer of 3 to 13, or 15 or 17; and n2 represents an integer of 4 to 10, or 12. [2] The solid composition according to [1], comprising primary particles containing the first polymer and the second polymer. [3] The solid composition according to [2], wherein the aspect ratio of the primary particles is 1.5 or less. [4] The first polymer contains units based on perfluoro(alkyl vinyl ether), The solid composition according to any one of [1] to [3], wherein the content of the units based on perfluoro(alkyl vinyl ether) is 0.1 to 3.0 mol % relative to the total of all units of the first polymer and all units of the second polymer. [5] The solid composition according to any one of [1] to [4], wherein the first polymer is water-insoluble. [6] A stretched film formed from the solid composition according to any one of the above [1] to [5]. [Effects of the Invention]

[0006] According to the present invention, a stretched membrane with excellent uniformity can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0007] The following definitions of terms apply throughout the specification and claims. The term "unit" refers collectively to an atomic group derived from one molecule of the monomer formed directly by polymerization of the monomer, and an atomic group obtained by chemically converting a part of the atomic group. Hereinafter, a "unit based on a monomer" will also be simply referred to as a "unit." A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the Examples. The "average particle size of primary particles in an aqueous dispersion" is the average particle size (hydrodynamic diameter) determined by cumulant analysis from the autocorrelation function obtained by dynamic light scattering. The "average primary particle diameter of aggregated particles" is the arithmetic mean value of the approximate circular radii obtained by image analysis of the observed image of a scanning electron microscope (SEM image). The "aspect ratio of a particle" is the ratio (a / b) of the particle's major axis a to the longest diameter b among the diameters perpendicular to the major axis a. "Non-melt-formable" means that it does not exhibit melt flowability. "Exhibiting melt fluidity" means that there exists a temperature at which the melt flow rate is 0.1 to 1000 g / 10 min under a load of 49 N at a temperature at least 20° C. higher than the melting point of the resin. "Melt flow rate" means the melt mass flow rate (MFR) defined in JIS K 7210:1999 (ISO 1133:1997). Standard specific gravity (SSG) is an index of average molecular weight, with a higher value indicating a lower molecular weight. It can be measured in accordance with ASTM D4895-04. The term "amorphous fluorine-containing polymer" refers to a polymer containing fluorine atoms, in which no significant crystalline peak is detected within a diffraction angle (2θ) range of 10 to 50° in an X-ray diffraction (XRD) pattern obtained using CuKα rays (wavelength: 1.5418 Å) as X-rays. Amorphous fluorine-containing polymers do not have a melting point. The term "crystalline fluorine-containing polymer" refers to a polymer containing fluorine atoms, in which a significant crystalline peak is detected within a diffraction angle (2θ) range of 10 to 50° in an X-ray diffraction (XRD) pattern obtained using CuKα rays (wavelength: 1.5418 Å) as X-rays. The content (mass % or mol %) of each unit relative to all units contained in the polymer is determined by analyzing the polymer by solid-state nuclear magnetic resonance spectroscopy (NMR), and usually, the content of each unit calculated from the amount of each monomer charged substantially coincides with the actual content of each unit. In this specification, each component may be a single substance corresponding to the component, or two or more substances may be used in combination. When two or more substances are used in combination for each component, the content of the component refers to the total content of the substances used in combination, unless otherwise specified. As used herein, a combination of two or more preferred embodiments is a more preferred embodiment.

[0008] The solid composition for producing a stretched film of this embodiment (hereinafter also simply referred to as "solid composition") contains a first polymer and a second polymer different from the first polymer. The content of the first polymer is 0.01 to 4.0 mass% relative to the total mass of the first polymer and the second polymer. The solid composition is preferably a powder composition.

[0009] <First polymer> The first polymer is a crystalline fluorine-containing polymer having a melting point below 20° C. or an amorphous fluorine-containing polymer having a glass transition temperature (hereinafter also referred to as “Tg”) below 20° C. When the first polymer does not have a Tg or melting point above 20° C., the polymerization reaction of the second polymer is more likely to proceed. The Tg of the first polymer is preferably lower than the polymerization temperature of the second polymer.

[0010] The Tg of the first polymer is preferably 10° C. or less, more preferably 5° C. or less, even more preferably 3° C. or less, and particularly preferably 0° C. or less. When the Tg of the first polymer is the above upper limit or less, primary particles containing the first polymer and the second polymer are easily formed. The Tg of the first polymer is preferably not less than −50° C., more preferably not less than −45° C., and even more preferably not less than −40° C. When the Tg of the first polymer is not less than the lower limit, the thermal stability of the stretched film is improved. The Tg of the first polymer can be adjusted to fall within the above range, for example, by adjusting the type and amount of the monomer used in producing the first polymer.

[0011] The first polymer has units based on a fluorine-containing monomer that contains a fluorine atom. Examples of the fluorine-containing monomer that constitutes the first polymer include tetrafluoroethylene (hereinafter also referred to as "TFE"), perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE"), hexafluoropropylene (hereinafter also referred to as "HFP"), vinylidene fluoride (hereinafter also referred to as "VdF"), CH2=CF-CF2-O-Rf-COOH, CH2=CF-CF2-O-Rf-SOH, CF2=CF-CF2-O-Rf-COOH, CF2=CF-CF2-O-Rf-SOH, CH2=CF-O-Rf-COOH, CH2=CF-O-Rf-SOH, CF2=CF-O-Rf-COOH, and CF2=CF-O-Rf-SOH (Rf represents a perfluoroalkyl group having 1 to 10 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom between carbon atoms). The fluorine-containing monomer may have, as a substituent, a functional group that contributes to water solubility. Specific examples of the functional group that contributes to water solubility include a hydroxyl group and an ionic functional group. The ionic functional group may be either a cationic functional group or an anionic functional group. Specific examples of the ionic functional group include a carboxylic acid group (-COO - ), sulfonic acid group (-SO3 -), sulfate group (-SO4 2- ), phosphonic acid group (-PO3 2- ), sulfonimide group (-N - (SO2)), and phosphate groups (-PO4 3- The functional group that contributes to water solubility is preferably a monovalent group. The first polymer may have one or more functional groups that contribute to water solubility. The first polymer may also have functional groups that contribute to water solubility on side chains or at terminals.

[0012] The first polymer preferably contains units based on TFE (hereinafter also referred to as "TFE units"). When the first polymer contains TFE units, the content of the TFE units relative to the total units of the first polymer is preferably 30 mol% or more, more preferably 40 mol% or more, and even more preferably 50 mol% or more, since a higher content of TFE units contributes to improved heat resistance. The upper limit of the content of the TFE units can be adjusted depending on the type of units other than the TFE units so as to obtain a desired Tg. For example, it is preferably 90 mol% or less, more preferably 80 mol% or less, and even more preferably 70 mol% or less.

[0013] The first polymer is preferably water-insoluble in view of the ease of removing the first polymer from water. In this specification, "water-insoluble" means that the solubility in 1000 g of water at 25°C is less than 100 mg. When the first polymer contains the functional group that contributes to water solubility, the content thereof is preferably in a range that makes the first polymer water-insoluble. For example, the content of the functional group that contributes to water solubility is preferably 0 to 20 mol %, more preferably 0.000001 to 15 mol %, and even more preferably 0.0001 to 10 mol %, based on all units constituting the first polymer.

[0014] The first polymer preferably contains TFE units and units based on PAVE (hereinafter also referred to as "PAVE units"), since this makes it easier to adjust the Tg to the above range and provides better effects of the present invention.

[0015] The PAVE is preferably a monomer represented by formula (1) from the viewpoints of excellent polymerization reactivity in producing the first polymer and of enabling more efficient production of the second polymer. CF2=CF-OR f1 (1) In formula (1), R f1 represents a perfluoroalkyl group having 1 to 10 carbon atoms. f1 The number of carbon atoms is preferably 1 to 8, more preferably 1 to 6, further preferably 1 to 5, and particularly preferably 1 to 3, in terms of better polymerization reactivity. The perfluoroalkyl group may be straight-chain or branched-chain.

[0016] Specific examples of PAVE include perfluoro(methyl vinyl ether) (hereinafter also referred to as "PMVE"), perfluoro(ethyl vinyl ether) (hereinafter also referred to as "PEVE"), and perfluoro(propyl vinyl ether) (hereinafter also referred to as "PPVE"). Among these, PMVE and PPVE are preferred, with PMVE being more preferred, as they allow the second polymer to be produced more efficiently.

[0017] When the first polymer contains TFE units and PAVE units, the content of PAVE units relative to the total of TFE units and PAVE units is preferably 20 to 60 mol%, more preferably 25 to 60 mol%, and even more preferably 30 to 55 mol%, from the viewpoints of making it easier to adjust the Tg within the above range and of enabling more efficient production of the second polymer.

[0018] When the first polymer contains TFE units and PAVE units, the first polymer may contain units based on monomers other than TFE and PAVE, but it is preferable that the first polymer is substantially free of units based on other monomers in order to more efficiently produce the second polymer. Substantially free of units derived from other monomers means that the content of units derived from other monomers is 0.01 mol % or less, more preferably 0 mol %, based on the total units of the first polymer. When units based on other monomers are contained, the other monomers are preferably HFP and propylene. When HFP is contained as the other monomer, the content of units based on HFP relative to all units of the first polymer is preferably 0 to 10 mol%, more preferably 0 to 5 mol%, and even more preferably 0 to 1 mol%.

[0019] <Second polymer> The second polymer is a fluoropolymer containing TFE units. The second polymer does not contain the first polymer. The second polymer is preferably a non-melt-processable fluoropolymer. The monomers constituting the second polymer include at least TFE, and may further include a monomer other than TFE. The second polymer contains at least TFE units and may further contain monomer units other than TFE units.

[0020] The monomer other than TFE may be a fluorine-containing monomer or a fluorine-free monomer that does not contain a fluorine atom. Examples of fluorine-containing monomers include chlorotrifluoroethylene (hereinafter also referred to as "CTFE"), vinylidene fluoride (hereinafter also referred to as "VdF"), FAE, PAVE, hexafluoropropylene, perfluoro(2,2-dimethyl-1,3-dioxole), and perfluoro(4-methoxy-1,3-dioxole). Among these, CTFE, VdF, PAVE, perfluoroalkylethylene, and hexafluoropropylene are preferred, CTFE, VdF, perfluorobutylethylene, perfluoro(methyl vinyl ether), perfluoro(propyl butyl ether), and hexafluoropropylene are more preferred, and perfluorobutylethylene, perfluoro(methyl vinyl ether), and perfluoro(propyl butyl ether) are even more preferred. The non-fluorine monomers include ethylene, propylene, vinyl chloride, and vinylidene chloride.

[0021] The content of TFE units contained in the second polymer is preferably 99.0 to 100.0 mol %, more preferably 99.5 to 100.0 mol %, and even more preferably 99.9 to 100.0 mol %, based on all units of the second polymer.

[0022] <Solid composition> The solid composition includes a first polymer and a second polymer. The content of the first polymer is 0.01 to 4.0% by mass, preferably 0.1 to 3.5% by mass, and more preferably 0.3 to 3.0% by mass, based on the total mass of the first polymer and the second polymer. When the content of the first polymer is at least the lower limit of the above range, the emulsion stability of the second polymer is improved, and when it is at most the upper limit, the heat resistance of the second polymer can be maintained.

[0023] The content of the first polymer relative to the total mass of the solid composition is preferably from 0.1 to 4 mass%, more preferably from 0.2 to 4 mass%, and even more preferably from 0.3 to 3 mass%. The content of the second polymer relative to the total mass of the solid composition is preferably 96 to 99.9 mass %, more preferably 96 to 99.8 mass %, and even more preferably 97 to 99.7 mass %. The total content of the first polymer and the second polymer relative to the total mass of the solid composition is preferably 99.0 to 100 mass%, more preferably 99.5 to 100 mass%, and even more preferably 99.8 to 100 mass%.

[0024] When the first polymer contains PAVE units, the content of PAVE units is preferably 0.1 to 3.0 mol %, more preferably 0.2 to 3.0 mol %, and even more preferably 0.3 to 2.0 mol %, based on the total of all units of the first polymer and all units of the second polymer. When the content of PAVE units is at least the lower limit of the above range, the stability of the aqueous dispersion during production of the second polymer is improved, and when it is at most the upper limit, the heat resistance is improved. The content of TFE units relative to the total of all units of the first polymer and all units of the second polymer is preferably 90 to 99.8 mol%, more preferably 93 to 99.5 mol%, and even more preferably 95 to 99.0 mol%. When the content of TFE units is at least the lower limit of the above range, heat resistance is improved, and when it is at most the upper limit, stability of the aqueous dispersion during production is improved.

[0025] The solid composition preferably comprises primary particles containing a first polymer and a second polymer, and in the primary particles, the first polymer and the second polymer may be partially copolymerized. The primary particles containing the first polymer and the second polymer may contain impurities unavoidable in the production process in addition to the first polymer and the second polymer. The content of the impurities relative to the total mass of the first polymer and the second polymer is preferably less than 250 ppb by mass, more preferably less than 25 ppb by mass, even more preferably less than 1 ppb by mass, and particularly preferably less than 0.1 ppb by mass. The content of the impurities may be 0 ppb by mass.

[0026] In the solid composition, the content of the compound represented by the following formula (S1) and the content of the compound represented by the following formula (S2) are each preferably 100 mass ppb or less, more preferably 50 mass ppb or less, even more preferably 25 mass ppb or less, and particularly preferably 0 mass ppb, relative to the total mass of the first polymer and the second polymer.

[0027] Formula (S1):H-(CF2) n1 -COOM Formula (S2):H-(CF2) n2 -SO3M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH4; n1 represents an integer of 3 to 13, 15, or 17; and n2 represents an integer of 4 to 10, or 12.

[0028] The compound represented by formula (S1) and the compound represented by formula (S2) are components that can be generated when a fluorine-containing monomer such as TFE is polymerized in the presence of a polymerization initiator, a chain transfer agent, and an emulsifier (particularly, a hydrocarbon-based emulsifier). When no emulsifier is used in the polymerization step of the second polymer in the method for producing the solid composition of this embodiment described below, the amounts of the compound represented by formula (S1) and the compound represented by formula (S2) generated can be suppressed, making it easy to keep the contents of these compounds within the above ranges.

[0029] The average particle size of the primary particles is preferably 100 to 400 nm, more preferably 150 to 300 nm, and even more preferably 170 to 270 nm. When the average particle size is at least the lower limit of the above range, productivity and mechanical properties are improved, and when it is at most the upper limit, emulsion stability can be maintained during production of the second polymer. The aspect ratio of the primary particles is preferably 1.5 or less, more preferably 1.0 to 1.5, even more preferably 1.1 to 1.5, and particularly preferably 1.2 to 1.5. When the aspect ratio is equal to or less than the upper limit of the above range, emulsion stability can be imparted.

[0030] The solid composition may be a powder composition containing secondary particles formed by aggregation of primary particles containing the first polymer and the second polymer. When the solid composition is a powder composition, the average primary particle size of the solid composition is preferably 100 to 400 nm, more preferably 150 to 300 nm, and even more preferably 170 to 270 nm. The powder composition containing the secondary particles may contain impurities unavoidable in the production process in addition to the first polymer and the second polymer. The content of the impurities in the powder composition is preferably less than 250 ppb by mass, more preferably less than 100 ppb by mass, and even more preferably less than 25 ppb by mass.

[0031] The standard specific gravity (SSG) of the solid composition is preferably 2.13 to 2.23, more preferably 2.14 to 2.20, and even more preferably 2.15 to 2.17. When the SSG is equal to or less than the upper limit of the above range, the molecular weight is large and the mechanical strength during processing is improved.

[0032] The thermal instability index (hereinafter also referred to as "TII") of the solid composition is preferably in the range of 0 to 10, more preferably 0 to 8. TII is an index of heat resistance, and a smaller TII indicates better heat resistance. When the TII is equal to or greater than the lower limit of the above range, the amount of heat volatile matter is small, and when it is equal to or less than the upper limit, the heat resistance is excellent. The TII of the TFE copolymer is measured in accordance with ASTM D4895-04. The detailed measurement method is as described in the Examples section below.

[0033] The extrusion pressure of the solid composition in the extrusion test described below is preferably 10 to 30 MPa, more preferably 15 to 25 MPa, and even more preferably 17 to 23 MPa. When the extrusion pressure is at least the lower limit of the above range, the tensile strength during stretching is high, and when it is at most the upper limit, the processability is excellent.

[0034] <Method for producing solid composition> The method for producing a solid composition preferably includes a step of polymerizing a monomer (hereinafter also referred to as "second monomer") constituting a second polymer in an aqueous dispersion C containing a first polymer and an aqueous medium to produce an aqueous dispersion D containing the second polymer and the first polymer. It also preferably includes a step of aggregating the first polymer and the second polymer in the aqueous dispersion D. The polymerization step of the first polymer and the polymerization step of the second polymer will be described below. Furthermore, the aggregation step of the first polymer and the second polymer will be described. Hereinafter, reaction liquid A refers to an aqueous dispersion containing the first polymer and an aqueous medium, and refers to the aqueous dispersion immediately after the production of the first polymer. Aqueous dispersion B refers to an aqueous dispersion obtained by subjecting reaction solution A to an ion exchange treatment. Aqueous Dispersion C refers to an aqueous dispersion containing the First Polymer and an aqueous medium, which is used as a polymerization reaction liquid for the Second Polymer, and may be a diluted version of Aqueous Dispersion B. The total mass of Aqueous Dispersion C is the sum of the First Polymer, the aqueous medium, and optional additives, and does not include the masses of the Second Monomer, the polymerization initiator, and the chain transfer agent. Aqueous dispersion D is an aqueous dispersion containing primary particles containing a first polymer and a second polymer, and is obtained by the polymerization step of the second polymer.

[0035] <Polymerization step of first polymer> Two embodiments of the polymerization step of the first polymer will be described below. The first embodiment will also be referred to as polymerization step (1) of the first polymer, and the second embodiment will also be referred to as polymerization step (2) of the first polymer. The polymerization step (1) of the first polymer is preferably a method of polymerizing a monomer (hereinafter also referred to as "first monomer") constituting the first polymer in an aqueous medium in the presence of a polymerization initiator. This method produces a reaction solution A containing the first polymer in an aqueous medium. Preferred aspects such as the type and ratio of the first monomer are determined based on the composition of the first polymer described above. The reaction liquid A is preferably a dispersion liquid in which primary particles of the first polymer are dispersed in an aqueous medium.

[0036] The polymerization initiator used in producing the first polymer is preferably a water-soluble polymerization initiator, more preferably a persulfate such as ammonium persulfate, sodium persulfate, or potassium persulfate, or an organic polymerization initiator such as disuccinic acid peroxide or azobisisobutylamidine dihydrochloride, still more preferably a persulfate, and particularly preferably ammonium persulfate. Water-soluble oxidation-reduction catalysts, which will be described later, are also preferred.

[0037] Examples of the aqueous medium used in producing the first polymer include water and a mixed solvent of water and a water-soluble organic solvent. Examples of the water-soluble organic solvent include tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol.

[0038] The first monomer is supplied to the reaction system (i.e., polymerization reaction vessel) by a conventional method. For example, the first monomer may be supplied to the reaction system continuously or intermittently so that the polymerization pressure reaches a predetermined pressure. Alternatively, the first monomer may be dissolved in an aqueous medium, and the resulting solution may be supplied to the reaction system continuously or intermittently. The polymerization initiator may be supplied to the reaction system all at once or in portions.

[0039] The polymerization temperature is preferably from 20 to 150°C, more preferably from 50 to 100°C. The polymerization pressure is preferably 0 to 3 MPa, more preferably 1.0 to 2.0 MPa. In the case of batch processing, the polymerization time is preferably 1 to 120 minutes, more preferably 10 to 60 minutes.

[0040] The obtained reaction liquid A may be directly subjected to the polymerization step of the second polymer, or may be subjected to one or more treatments selected from the group consisting of solvent adjustment treatment, heat treatment, and ion exchange treatment before being subjected to the polymerization step of the second polymer.

[0041] Examples of the solvent adjustment treatment include adding another aqueous medium to the reaction liquid A, or replacing the solvent in the reaction liquid A and dispersing the first polymer in another aqueous medium.

[0042] Heat treatment deactivates the polymerization initiator contained in reaction solution A, making the polymerization step of the second polymer less susceptible to the influence of the polymerization initiator used in producing the first polymer, and as a result, it is easier to obtain a second polymer with a high molecular weight. The heating temperature is preferably 70 to 100° C., more preferably 80 to 98° C., and even more preferably 85 to 95° C. When the heating temperature is within the above range, the deactivation of the polymerization initiator in the aqueous medium can be further promoted.

[0043] An example of an ion exchange treatment is to contact reaction liquid A with either or both of an anion exchange resin and a cation exchange resin to remove either or both of anions and cations, which are impurities contained in reaction liquid A.

[0044] An example of an anion impurity is sulfate ion. Sulfate ion originates from the polymerization initiator (particularly ammonium persulfate) and may be contained in reaction liquid A. By subjecting reaction liquid A to ion exchange treatment using an anion exchange resin to obtain aqueous dispersion B, the sulfate ion content in aqueous dispersion C after dilution can be reduced. The content of sulfate ion relative to the total mass of aqueous dispersion C is preferably 10 mass ppm or less, more preferably 5 mass ppm or less. The lower limit is not particularly limited, and may be 0 mass ppm. When the content of sulfate ion is equal to or less than the above upper limit, it is believed that the formation of terminal groups with low heat resistance in the second polymer can be suppressed. As a result, it is presumed that coloration of the second polymer (primary particles) is suppressed.

[0045] An example of an anion impurity is fluoride ion. Fluoride ion may be generated by the reaction between a polymerization initiator (e.g., ammonium persulfate) and a monomer used in producing the first polymer, and may be contained in aqueous dispersion B. By subjecting reaction solution A to ion exchange treatment using an anion exchange resin to obtain aqueous dispersion B, the fluoride ion content in aqueous dispersion C after dilution can be reduced. The content of fluoride ion relative to the total mass of aqueous dispersion C is preferably 100 mass ppm or less, more preferably 50 mass ppm or less. The lower limit is not particularly limited and may be 0 mass ppm. When the content of fluoride ion is equal to or less than the upper limit, polymerization is likely to be stable in the polymerization step of the second polymer.

[0046] An example of a cation impurity is ammonium ion. Ammonium ion may be derived from the polymerization initiator (particularly ammonium persulfate) and be contained in reaction solution A. By subjecting reaction solution A to ion exchange treatment using a cation exchange resin to obtain aqueous dispersion B, the ammonium ion content in aqueous dispersion C after dilution can be reduced. The content of ammonium ion relative to the total mass of aqueous dispersion C is preferably 20 ppm by mass or less, more preferably 10 ppm by mass or less. The lower limit is not particularly limited, and may be 0 ppm by mass. When the content of ammonium ion is equal to or less than the upper limit, the ionic strength in the aqueous medium is reduced, which is thought to suppress aggregation of the second polymer. As a result, it is expected that the production efficiency of the second polymer will be improved.

[0047] Next, the polymerization step (2) of the first polymer will be described. In the polymerization step (2) of the first polymer, a first monomer is polymerized in a first aqueous medium in the presence of compound (1) to produce a first polymer. This polymerization is preferably carried out in the presence of compound (1) and a polymerization initiator, and the same polymerization initiator as in the polymerization step (1) of the first polymer can be used as the polymerization initiator. As a result of the polymerization step (2) of the first polymer, a first post-polymerization dispersion is obtained, which is a dispersion containing the first polymer and the first aqueous medium.

[0048] <Compound (1)> The compound (1) is a compound represented by the following formula (2). CX 1 X 2 =CX 3 -LZ …(2) In formula (2), X 1 and X 2 are each independently a hydrogen atom or an alkyl group, X 3 is a hydrogen atom, a fluorine atom, or an alkyl group, L is a single bond or a divalent linking group, Z is -SO3M 1 , -OSO3M 1 , -P(=O)(OM 1 )2, -OP(=O)(OM1 )2, or -COOM 1 and M 1 is a hydrogen atom, a metal atom, N(R M11 )4 or P(R M12 )4 and M 1 If there are multiple M 1 may be the same or different from each other, R M11 and R M12 are each independently a hydrogen atom or a substituent, and R M11 Any two of R may be bonded to each other to form a ring, and multiple R M11 may be the same or different from each other, R M12 Any two of R may be bonded to each other to form a ring, and multiple R M12 may be the same or different from each other.

[0049] In formula (2), X 1 and X 2 are each independently a hydrogen atom or an alkyl group. The alkyl group may be linear, branched, or cyclic. The alkyl group preferably has 1 to 10 carbon atoms, more preferably 1 to 3 carbon atoms, and even more preferably 1 carbon atom. X 1 and X 2 are preferably all hydrogen atoms, from the viewpoint of increasing the number of particles of the first polymer. In formula (2), X 3 is a hydrogen atom, a fluorine atom, or an alkyl group. Specific examples and preferred embodiments of the alkyl group are: X 1 and X 2 The specific examples and preferred embodiments of the alkyl group are the same as those in the above. X 3 is preferably a fluorine atom or a hydrogen atom, more preferably a hydrogen atom, from the viewpoint of increasing the number of particles of the first polymer.

[0050] In formula (2), L is a single bond or a divalent linking group. Examples of the divalent linking group include an alkylene group, a carbonyl group, an ether bond, a thioether bond, a sulfonyl group, -NH-, -SiH2-, a phenylene group, -CF2-, and a group combining two or more of these. Examples of the group combining two or more of these include an ester bond, a thioester bond, an amide bond, a sulfonamide bond, a combination of an alkylene group and an ether bond, a combination of an alkylene group and an ester bond, and a combination of an alkylene group and an amide bond. The alkylene group may be linear, branched, or cyclic, preferably linear or branched, and more preferably branched. The alkylene group may have 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms.

[0051] Specific examples of L include a single bond, an alkylene group, an ether bond, an ester bond, and C -CO-NH-R-* Z and the like, and examples thereof include a single bond, an alkylene group having 1 to 6 carbon atoms, and * C -CO-NH-R-* Z are preferred, and particularly preferred are a single bond, an alkylene group having 1 to 2 carbon atoms, and * C -CO-NH-R-* Z is more preferred, where * C is the bonding site to the carbon atom in formula (2), and * Z is the bonding site to Z in formula (2), and R is an alkylene group having 1 to 6 carbon atoms.

[0052] In formula (2), Z is -SO3M 1 , -OSO3M 1 , -P(=O)(OM 1 )2, -OP(=O)(OM 1 )2 or -COOM 1 is. As Z, from the viewpoint of stabilizing the dispersion and increasing the number of particles of the first polymer, -SO3M 1 and -COOM 1 is preferred, -SO3Na and -COONa are more preferred, and -SO3Na is even more preferred.

[0053] M1 is a hydrogen atom, a metal atom, N(R M11 )4 or P(R M12 )4 and R M11 and R M12 are each independently a hydrogen atom or a substituent. M 1 The metal atom represented by the formula (I) is preferably a metal atom of Group 1, and more preferably Li, Na, or K. R M11 and R M12 The substituent represented by the formula (I) is preferably a monovalent organic group, more preferably a monovalent hydrocarbon group, and further preferably an alkyl group or an aromatic hydrocarbon group. The substituent preferably has 1 to 10 carbon atoms. The alkyl group may be linear, branched, or cyclic. The aromatic hydrocarbon group may be either monocyclic or polycyclic, and is preferably a phenyl group.

[0054] The molecular weight of the compound (1) is, for example, 70 to 500, and from the viewpoint of dispersion stability, it is preferably 70 to 450, and more preferably 100 to 300.

[0055] Specific examples of compound (1) include vinyl sulfonic acid, vinyl phosphonic acid, (meth)acrylic acid, allyl sulfonic acid, allyl phosphonic acid, butenoic acid, crotonic acid, vinyl acetic acid, 2-sulfoethyl methacrylic acid, 4-vinyl benzene sulfonic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid, N-tigloylglycine, 6-acrylamidohexanoic acid, 1,1-difluoro-2-methyl-2-[(1-oxo-2-propen-1-yl)amino]-1-propanesulfonic acid, 3-methyl-3-[(2-methyl-1-oxo-2-propen-1-yl)amino]-2-butanesulfonic acid, 2-methacrylamido-2-methylpropanesulfonic acid, 2,3-dimethyl-3-[(1-oxo-2-propen-1-yl)amino]-2-butanesulfonic acid, and metal salts thereof. The metal salts include M 1 Examples of the metal salt include a metal salt of a metal atom represented by the following formula:

[0056] As the compound (1), vinyl compounds having a sulfonic acid group, a phosphonic acid group, or a carboxy group, allyl compounds having a sulfonic acid group, a phosphonic acid group, or a carboxy group, (meth)acrylic acid, (meth)acrylamides having a sulfonic acid group, a phosphonic acid group, or a carboxy group, and metal salts thereof are preferred, and vinyl sulfonic acid, sodium vinyl sulfonate, allyl sulfonic acid, sodium allyl sulfonate, 2-acrylamido-2-methyl-1-propanesulfonic acid, sodium 2-acrylamido-2-methyl-1-propanesulfonate, 2-methacrylamido-2-methyl-1-propanesulfonic acid, or sodium 2-methacrylamido-2-methyl-1-propanesulfonate are preferred. Note that the term "(meth)acrylic acid" encompasses both acrylic acid and methacrylic acid, and the term "(meth)acrylamide" encompasses both acrylamide and methacrylamide.

[0057] Before starting polymerization of the first monomer, the content of compound (1) is preferably 1.0 to 1000 ppm by mass relative to the entire first aqueous dispersion, and in terms of better effects of the present invention, is more preferably 1.0 to 800 ppm by mass, even more preferably 3.0 to 500 ppm by mass, and particularly preferably 5.0 to 300 ppm by mass. The concept of the "first aqueous dispersion" before the start of polymerization of the first monomer includes compound (1) and an aqueous medium, but does not include the first monomer and polymerization initiator used in the polymerization of the first polymer. For example, even if a first aqueous dispersion containing compound (1) and an aqueous medium is mixed with the first monomer and the polymerization initiator before the start of polymerization of the first monomer, the "first aqueous dispersion" refers to a mixture of components excluding the first monomer and the polymerization initiator.

[0058] <First aqueous medium> Specific examples of the first aqueous medium include water and a mixed solvent of water and a water-soluble organic solvent, such as tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol.

[0059] Before starting the polymerization of the first monomer, the content of the first aqueous medium is preferably 20 to 90% by volume, more preferably 40 to 80% by volume, based on the volume of the reactor.

[0060] <First monomer> The first monomer used in the present production method includes at least one selected from the group consisting of tetrafluoroethylene (hereinafter also referred to as "TFE") and hexafluoropropylene (hereinafter also referred to as "HFP"). The first monomer may contain a monomer other than TFE and HFP, and preferably contains a monomer other than TFE and HFP. Examples of the monomer other than TFE and HFP include perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE"), propylene, vinylidene fluoride (hereinafter also referred to as "VdF"), CH2=CF-CF2-O-Rf-COOH, CH2=CF-CF2-O-Rf-S03H, CF2=CF-CF2-O-Rf-COOH, CF2=CF-CF2-O-Rf-S03H, CH2=CF-O-Rf-COOH, CH2=CF-O-Rf-S03H, CF2=CF-O-Rf-COOH, and CF2=CF-O-Rf-S03H (Rf represents a perfluoroalkyl group having 1 to 10 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom between carbon atoms). When the first monomer contains TFE, the amount of TFE used is preferably 10 to 90 mol %, more preferably 30 to 85 mol %, and even more preferably 40 to 80 mol %, based on the amount of the first monomer used. When the first monomer contains HFP, the amount of HFP used is preferably 30 to 95 mol %, more preferably 40 to 90 mol %, and even more preferably 50 to 85 mol %, based on the amount of the first monomer used.

[0061] In terms of excellent polymerization reactivity of the first polymer and better effects of the present invention, the first monomer preferably contains PAVE. The PAVE is preferably a monomer represented by formula (11) from the viewpoints of excellent polymerization reactivity in producing the first polymer and of enabling more efficient production of the second polymer.

[0062] CF2=CF-OR f1 (11) In formula (11), R f1 is a perfluoroalkyl group having 1 to 10 carbon atoms. R f1 The number of carbon atoms in the alkyl group is preferably 1 to 8, more preferably 1 to 6, still more preferably 1 to 5, and particularly preferably 1 to 3, in terms of better polymerization reactivity. The perfluoroalkyl group may be linear or branched.

[0063] Specific examples of PAVE include perfluoro(methyl vinyl ether) (hereinafter also referred to as "PMVE"), perfluoro(ethyl vinyl ether) (hereinafter also referred to as "PEVE"), and perfluoro(propyl vinyl ether) (hereinafter also referred to as "PPVE"), and PMVE or PPVE are preferred, with PMVE being more preferred, from the viewpoint of enabling more efficient production of the second polymer.

[0064] When the first monomer contains PAVE, the amount of PAVE used is preferably 20 to 95 mol %, more preferably 25 to 80 mol %, and even more preferably 30 to 60 mol %, based on the amount of the first monomer used. The suitable amount is the same whether the PAVE is PMVE, PEVE, or PPVE, or a mixture of two or more of these.

[0065] In terms of excellent polymerization reactivity of the first polymer and better effects of the present invention, it is also preferable that the first monomer contains at least one selected from the group consisting of propylene and VdF.

[0066] When the first monomer contains propylene, the amount of propylene used is preferably 5 to 90 mol %, more preferably 8 to 70 mol %, and even more preferably 10 to 60 mol %, based on the amount of the first monomer used. When the first monomer contains VdF, the amount of VdF used is preferably 5 to 90 mol %, more preferably 8 to 80 mol %, and even more preferably 10 to 70 mol %, based on the amount of the first monomer used.

[0067] The first monomer preferably includes any combination of TFE and PAVE, TFE and propylene, and HFP and VdF. When the first monomer contains TFE and PAVE, the amount of PAVE used is preferably 20 to 95 mol %, more preferably 25 to 80 mol %, and even more preferably 30 to 60 mol %, based on the total amount of TFE and PAVE used. When the first monomer contains TFE and PAVE, the total amount of TFE and PAVE used is preferably 99.0 to 100.0 mol %, more preferably 99.5 to 100.0 mol %, and even more preferably 99.9 to 100.0 mol %, based on the amount of the first monomer used. When the first monomer contains TFE and propylene, the amount of propylene used is preferably 5 to 90 mol %, more preferably 8 to 70 mol %, and even more preferably 10 to 60 mol %, based on the total amount of TFE and propylene used. When the first monomer contains TFE and propylene, the total amount of TFE and propylene used is preferably 99.0 to 100.0 mol %, more preferably 99.5 to 100.0 mol %, and even more preferably 99.9 to 100.0 mol %, based on the amount of the first monomer used. When the first monomer contains HFP and VdF, the amount of VdF used is preferably 5 to 90 mol %, more preferably 8 to 80 mol %, and even more preferably 10 to 70 mol %, based on the total amount of HFP and VdF used. When the first monomer contains HFP and VdF, the total amount of HFP and VdF used is preferably 99.0 to 100.0 mol %, more preferably 99.5 to 100.0 mol %, and even more preferably 99.9 to 100.0 mol %, based on the amount of the first monomer used.

[0068] The first monomer may contain other monomers in addition to those mentioned above, but may be substantially free of other monomers in order to more efficiently produce the second polymer. "Substantially free of other monomers" means that the amount of other monomers used is 0.01 mol % or less, preferably 0 mol %, relative to the amount of the first monomer used.

[0069] In the first aspect, the polymerization steps (1) and (2) of the first polymer are preferably carried out under conditions in which an emulsifier having a fluorine atom and an emulsifier not having a fluorine atom are substantially absent, from the viewpoint of suppressing a decrease in the molecular weight of the resulting first polymer. In other words, it is preferable that the aqueous dispersion is substantially free of an emulsifier having a fluorine atom and an emulsifier not having a fluorine atom. "Substantially free of emulsifiers having fluorine atoms and emulsifiers not having fluorine atoms (hereinafter also collectively referred to as "emulsifiers")" means that the content of emulsifiers is 10 mass ppm or less, preferably 150 mass ppb or less, and more preferably 50 mass ppb or less, relative to the total mass of the first aqueous dispersion. The lower limit is 0 mass ppb. The content of various emulsifiers can be measured using a liquid chromatograph mass spectrometer. Specifically, the measurement method described in paragraphs

[0721] to

[0732] of WO 2018 / 181904 can be used.

[0070] Examples of emulsifiers having fluorine atoms and emulsifiers not having fluorine atoms include water-soluble emulsifiers. A water-soluble emulsifier means an emulsifier having a solubility of 100 mg or more in 1000 g of water at 25°C, and a water-insoluble emulsifier means an emulsifier other than the above-mentioned water-soluble emulsifiers. The water-soluble emulsifier may be either ionic or nonionic. The emulsifiers having fluorine atoms and the emulsifiers not having fluorine atoms include those not having a carbon-carbon double bond. Furthermore, none of the compound (1), the first polymer described below, and the second fluorine-containing polymer described below corresponds to an emulsifier.

[0071] Examples of the emulsifier having a fluorine atom include anionic fluorine-containing emulsifiers. Examples of the anionic fluorine-containing emulsifier include emulsifiers containing fluorine atoms whose total carbon number excluding the anionic group is 20 or less, and emulsifiers containing fluorine atoms whose anionic moiety has a molecular weight of 800 or less. The "anionic moiety" means the moiety excluding the cation of the fluorine-containing emulsifier.

[0072] The fluorine-free emulsifier is an emulsifier that does not contain fluorine atoms and has a hydrocarbon group such as an alkyl group as a hydrophobic moiety. It is also possible to substitute a hydrogen atom of the hydrocarbon group of the fluorine-free emulsifier with a halogen atom other than a fluorine atom.

[0073] The emulsifiers having no fluorine atoms include anionic hydrocarbon emulsifiers and nonionic hydrocarbon emulsifiers.

[0074] Anionic hydrocarbon emulsifiers refer to emulsifiers having a negatively charged hydrophilic portion such as a carboxylic acid group, a sulfonic acid group, a sulfate group, a phosphonic acid group, or a phosphate group, and a hydrocarbon group such as an alkyl group as a hydrophobic portion. Specific examples of anionic hydrocarbon emulsifiers include sodium dodecyl sulfate, a highly branched C10 tertiary carboxylic acid supplied by Resolution Performance Products as Versatic® 10, sodium linear alkyl polyether sulfonate supplied by BASF as the Avanel® S series, and sodium methyl ... An example of an available sulfosuccinate emulsifier is Lankropol® K8300.

[0075] A nonionic hydrocarbon emulsifier is an emulsifier that exhibits surface activity in water without dissociating into ions and has a hydrocarbon group such as an alkyl group as the hydrophobic portion. The hydrophilic portion of the nonionic hydrocarbon emulsifier includes a water-soluble functional group such as a polyethylene oxide chain obtained from the polymerization of ethylene oxide. Nonionic hydrocarbon emulsifiers include polyalkylene oxide block copolymers, such as block copolymers having polyethylene oxide and polypropylene oxide.

[0076] Further, other nonionic hydrocarbon emulsifiers include those described in paragraphs

[0043] to

[0052] of JP-A No. 2016-537499.

[0077] The emulsifier having a fluorine atom and the emulsifier not having a fluorine atom may contain a silicon atom. Examples of the emulsifier containing a silicon atom include a siloxane emulsifier. The siloxane emulsifier is a hydrocarbon-containing emulsifier having a siloxane skeleton. Siloxane emulsifiers include those described in US Pat. Nos. 6,841,616 (Wille et al.) and 7,977,438 (Brothers et al.).

[0078] The emulsifier having a fluorine atom and the emulsifier not having a fluorine atom may be a polymer emulsifier. Examples of the polymer emulsifier include a polymer having a hydrophilic group in a side chain. Examples of such a polymer emulsifier include a polymer containing a unit based on a compound having a site capable of polymerization reaction and a hydrophilic group. Further, even if the polymer does not originally have a hydrophilic group, a polymer containing a unit based on a compound having a group that can become a hydrophilic group and subjected to post-treatment such as hydrolysis may also be used.

[0079] When the first monomer is polymerized in the presence of an emulsifier having no fluorine atoms, typically 0.1 to 15 parts by mass of the emulsifier having no fluorine atoms is used per 100 parts by mass of the aqueous medium.

[0080] <Polymerization step of second polymer> The polymerization step of the second polymer is preferably a method of polymerizing the second monomer in an aqueous dispersion C containing the first polymer and an aqueous medium. If necessary, a polymerization initiator and a chain transfer agent may be supplied. This method results in an aqueous dispersion D containing the first polymer and the second polymer in an aqueous medium. Preferred aspects such as the type and ratio of the second monomer are determined based on the composition of the second polymer described above.

[0081] The content of the first polymer relative to the total mass of the aqueous dispersion C is preferably 0.01 to 4.0 mass%, more preferably 0.01 to 0.6 mass%, and even more preferably 0.01 to 0.5 mass%. When the content of the first polymer is within the above range, the second polymer can be produced more efficiently. The content of the first polymer relative to the total mass of the aqueous dispersion C can be adjusted by the solvent adjustment treatment described above.

[0082] It is preferred that primary particles of the first polymer are dispersed in the aqueous medium of aqueous dispersion C. The average particle size of the primary particles of the first polymer is preferably 1 to 150 nm, more preferably 10 to 120 nm, and even more preferably 50 to 120 nm. When the average particle size of the first polymer is within the above range, the second polymer can be produced more efficiently. The average particle size of the primary particles of the first polymer is the average particle size (hydrodynamic diameter) obtained by measuring using a dynamic scattering method and analyzing using the cumulant method, and detailed measurement conditions are as described in the Examples section.

[0083] Examples of the aqueous medium contained in the aqueous dispersion C include the same aqueous media as those described in the production process of the reaction liquid A. The content of the aqueous medium relative to the total mass of the aqueous dispersion C is preferably from 60 to 99.9 mass %, more preferably from 80 to 99.9 mass %, and even more preferably from 90 to 99.9 mass %.

[0084] Aqueous Dispersion C may contain other components in addition to the first polymer and the aqueous medium, such as a pH adjuster, a wax, an emulsifier, and a reducing agent.

[0085] Examples of pH adjusters include inorganic salts. Examples of inorganic salts include phosphates such as disodium hydrogen phosphate and sodium dihydrogen phosphate, and carbonates such as sodium hydrogen carbonate and sodium carbonate. Preferred phosphates include disodium hydrogen phosphate dihydrate and disodium hydrogen phosphate dodecahydrate.

[0086] The wax is more preferably paraffin wax. Paraffin wax may be liquid, semi-solid, or solid at room temperature. Among them, saturated hydrocarbons having 12 or more carbon atoms are preferred. The melting point of the paraffin wax is preferably 40 to 65°C, more preferably 50 to 65°C.

[0087] Examples of emulsifiers include fluorine-based emulsifiers and non-fluorine-based emulsifiers, but in this embodiment, it is preferable that the aqueous dispersion C does not substantially contain a fluorine-based emulsifier. That is, it is preferable that a fluorine-based emulsifier is not used in the polymerization step of the second polymer. The fluorine-based emulsifier refers to an emulsifier in which the hydrophobic moiety contains a fluorine atom in the hydrophilic moiety and the hydrophobic moiety of the emulsifier. Specific examples of the fluorine-based emulsifier include fluorine-containing alkanoates and fluorine-containing ether carboxylic acid compounds. The non-fluorine-based emulsifier refers to an emulsifier other than a fluorine-based emulsifier. Examples of non-fluorine-based emulsifiers include sodium lauryl sulfate, Perex SS-H manufactured by Kao Chemical Corporation, and Newcol 1305-SN manufactured by Nippon Nyukazai Co., Ltd.

[0088] "Substantially free of fluorine-based emulsifier" means that the content of fluorine-based emulsifier relative to the total mass of the first polymer in aqueous dispersion C is 100 ppm by mass or less, preferably 50 ppm by mass or less, more preferably 25 ppm by mass or less, and even more preferably 5 ppm by mass or less. The lower limit is not particularly limited, and may be 0 ppm by mass.

[0089] When aqueous dispersion C contains an emulsifier other than a fluorine-based emulsifier, the content of the emulsifier other than a fluorine-based emulsifier is preferably 0.01 to 5% by mass relative to the aqueous medium. In this embodiment, aqueous dispersion C preferably does not substantially contain an emulsifier. That is, it is preferable that no emulsifier is used in the polymerization step of the second polymer. "Substantially does not contain an emulsifier" means that the content of the emulsifier is 100 ppm by mass or less, preferably 10 ppm by mass or less, more preferably 100 ppb by mass or less, and even more preferably 1 ppb by mass or less, relative to the total mass of the first polymer and the second polymer. It may be 0.1 ppb by mass or less, or even 0 ppb by mass. When an emulsifier is not used, the generation amounts of the compound represented by formula (S1) and the compound represented by formula (S2) can be suppressed, making it easier to keep the contents of these compounds within the above-mentioned ranges.

[0090] The method for producing a solid composition according to this embodiment allows efficient production of a second polymer without the use of an emulsifier. It is believed that during polymerization of the second monomer, the first polymer adsorbs the second monomer at its hydrophobic portion and incorporates it into the first polymer, thereby solubilizing the second monomer. As a result, it is believed that the second monomer is polymerized within or near the first polymer. Furthermore, it is believed that the first polymer contributes to dispersion stabilization in an aqueous medium. When primary particles of the first polymer are present in aqueous dispersion C, it is presumed that the second monomer polymerizes within or near the primary particles of the first polymer, thereby producing primary particles containing the first polymer and the second polymer.

[0091] When the aqueous dispersion C contains a pH adjuster, the content of the pH adjuster is preferably 0.01 to 3.0 parts by mass relative to 100 parts by mass of the aqueous medium. When aqueous dispersion C contains a wax, the content of the wax is preferably 1 to 10 parts by mass relative to 100 parts by mass of the aqueous medium.

[0092] When the aqueous dispersion C contains a reducing agent, the amount of the reducing agent used is preferably 1 to 2000 ppm relative to 100 parts by mass of the second monomer supplied.

[0093] The reaction is initiated by supplying a second monomer to the aqueous dispersion C, and optionally supplying a polymerization initiator. During the reaction, a chain transfer agent may be supplied.

[0094] The second monomer is supplied to the reaction system (i.e., polymerization reaction vessel) by a conventional method. For example, the second monomer may be supplied to the reaction system continuously or intermittently so that the polymerization pressure reaches a predetermined pressure. Alternatively, the second monomer may be dissolved in an aqueous medium, and the resulting solution may be supplied to the reaction system continuously or intermittently. When a polymerization initiator is used, the polymerization initiator may be supplied to the reaction system all at once or in portions.

[0095] The amount of the second monomer supplied is preferably 1 to 50 parts by mass, more preferably 1 to 40 parts by mass, and even more preferably 1 to 30 parts by mass, relative to 100 parts by mass of the aqueous medium contained in the aqueous dispersion C.

[0096] Examples of the polymerization initiator include an oil-soluble radical initiator, a water-soluble radical initiator, and a water-soluble oxidation-reduction catalyst. Examples of oil-soluble radical initiators include oil-soluble organic peroxides such as tert-butyl peroxypivalate (hereinafter also referred to as "PBPV") and diisopropyl peroxydicarbonate (hereinafter also referred to as "IPP"). Examples of the water-soluble radical initiator include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble organic peroxides such as disuccinic acid peroxide, bisglutaric acid peroxide, and tert-butyl hydroperoxide. Examples of water-soluble redox catalysts include combinations of oxidizing agents such as bromic acid or its salts, chloric acid or its salts, persulfuric acid or its salts, permanganic acid or its salts, and hydrogen peroxide with reducing agents such as sulfurous acid or its salts, hydrogen sulfite or its salts, thiosulfuric acid or its salts, organic acids, and inorganic salts. Potassium persulfate and ammonium persulfate are preferred as persulfates. Sodium sulfite is preferred as sulfites. Inorganic salts include combinations of sulfate anions, sulfite anions, and chloride anions with metal ions. Transition metal ions are preferred, including manganese, iron, cobalt, nickel, copper, zinc, cerium, and silver ions, with iron ions being preferred. Iron (II) sulfate is preferred as an inorganic salt. The polymerization initiator is preferably an oil-soluble radical initiator or a water-soluble radical initiator. From the viewpoint of more efficient production of fluoropolymers, oil-soluble radical initiators are more preferred, and oil-soluble organic peroxides are even more preferred. Two or more types of polymerization initiators may be used in combination.

[0097] The amount of the polymerization initiator to be fed is preferably 1 to 1000 ppm, more preferably 5 to 750 ppm, and even more preferably 10 to 500 ppm, relative to 100 parts by mass of the amount of the second monomer to be fed.

[0098] Examples of chain transfer agents include ethyl acetate, methanol, ethanol, t-butyl methyl ether, diethyl ether, n-pentane, cyclohexane, methane, and propane.

[0099] The amount of the chain transfer agent supplied is preferably 0.1 to 5 parts by mass relative to 100 parts by mass of the aqueous medium, and more preferably 0.1 to 20 parts by mass, more preferably 0.1 to 15 parts by mass, and even more preferably 0.1 to 10 parts by mass relative to 100 parts by mass of the second monomer supplied.

[0100] The polymerization temperature is preferably from 10 to 95°C, more preferably from 15 to 90°C. The polymerization pressure is preferably from 0.5 to 4.0 MPa, more preferably from 0.6 to 3.5 MPa. In the case of batch processing, the polymerization time is preferably from 90 to 1000 minutes, more preferably from 90 to 700 minutes.

[0101] (Aqueous dispersion D) Aqueous dispersion D is an aqueous dispersion containing a first polymer and a second polymer, and is obtained by a polymerization step of the second polymer. Aqueous dispersion D preferably contains primary particles containing the first polymer and the second polymer.

[0102] It is preferable that the aqueous dispersion D is substantially free of an emulsifier. Examples of the emulsifier include the above-mentioned fluorine-based emulsifiers and non-fluorine-based emulsifiers. The phrase "aqueous dispersion D is substantially free of emulsifier" means that the content of emulsifier is 0.03 mass ppm or less, preferably 0.02 mass ppm or less, and more preferably 0 mass ppm, relative to the total mass of aqueous dispersion D.

[0103] When no emulsifier is used in the polymerization step of the second polymer, aqueous dispersion D can easily be made into a dispersion in an organic solvent such as N-methylpyrrolidone or acetone by solvent substitution. For example, aqueous dispersion D can be mixed with an organic solvent and dehydrated using evaporation or anhydrous sodium sulfate or the like to form a dispersion in an organic solvent.

[0104] The content of the first polymer relative to the total mass of the aqueous dispersion D is preferably from 0.10 to 2.00 mass %, more preferably from 0.15 to 1.50 mass %, and even more preferably from 0.20 to 0.80 mass %. The content of the second polymer relative to the total mass of the aqueous dispersion D is preferably from 10 to 40 mass %, more preferably from 12 to 35 mass %, and even more preferably from 15 to 30 mass %. The total content of the first polymer and the second polymer relative to the total mass of the aqueous dispersion D is preferably from 10 to 40 mass %, more preferably from 12 to 35 mass %, and even more preferably from 15 to 35 mass %.

[0105] The content of the aqueous medium relative to the total mass of the aqueous dispersion D is preferably from 50 to 99 mass %, more preferably from 60 to 99 mass %, and even more preferably from 70 to 99 mass %.

[0106] When the first polymer contains PAVE units, the content of PAVE units relative to the sum of all units of the first polymer and all units of the second polymer is preferably 0.1 to 5.0 mol%, more preferably 0.2 to 3.0 mol%, and even more preferably 0.3 to 2.0 mol%. When the content of PAVE units is at least the lower limit of the above range, excellent dispersibility is achieved, and when it is at most the upper limit, excellent heat resistance is achieved.

[0107] The content of TFE units relative to the total of all units of the first polymer and all units of the second polymer is preferably from 90 to 99.8 mol %, more preferably from 93 to 99.5 mol %, and even more preferably from 95 to 99.0 mol %.

[0108] In aqueous dispersion D, the content of the compound represented by formula (S1) and the content of the compound represented by formula (S2) are each preferably 100 mass ppb or less, more preferably 50 mass ppb or less, even more preferably 25 mass ppb or less, and particularly preferably 0 mass ppb, relative to the total mass of the first polymer and the second polymer.

[0109] When aqueous dispersion D contains primary particles, the average particle size of the primary particles is preferably 100 to 400 nm, more preferably 150 to 300 nm, and even more preferably 170 to 270 nm. When the average particle size is at least the lower limit of the above range, productivity and mechanical properties are improved, and when it is at most the upper limit, emulsion stability can be maintained during production of the second polymer. The aspect ratio of the primary particles is preferably 1.5 or less, more preferably 1.0 to 1.5, even more preferably 1.1 to 1.5, and particularly preferably 1.2 to 1.5. When the aspect ratio is equal to or less than the upper limit of the above range, emulsion stability can be imparted.

[0110] <Step of Aggregating First Polymer and Second Polymer> The solid in the aqueous dispersion D is agglomerated, and the dispersion medium such as the aqueous medium is removed to obtain a wet powder, which is then dried to obtain a solid composition in the form of a dry powder. When aqueous dispersion D contains primary particles, the aspect ratio of the primary particles measured after drying aqueous dispersion D on a substrate can be considered to be the same as the aspect ratio of the primary particles in the dry powder. Also, the average primary particle diameter measured after drying aqueous dispersion D on a substrate can be considered to be the same as the average primary particle diameter in the dry powder. The aggregating step, the step of removing the dispersion medium to obtain a wet powder, and the step of drying the wet powder to obtain a dry powder can be carried out using known techniques.

[0111] The flocculation method includes, but is not limited to, freeze flocculation, acid flocculation, base flocculation, flocculation using a coagulant, and mechanical flocculation. In the case of freeze aggregation, the aggregation temperature is preferably −20 to 0° C. The aggregation time is preferably 1 hour or more, more preferably 2 hours or more. In the case of acid coagulation, a method in which an acid-containing solution is added to aqueous dispersion D is preferred. Examples of the acid to be added include hydrochloric acid, nitric acid, sulfuric acid, oxalic acid, and hydrofluoric acid, with hydrochloric acid being preferred. The concentration of the acid in the acid-containing solution is preferably 0.1 to 50 mass%, more preferably 1 to 30 mass%, and even more preferably 1 to 10 mass%. A preferred method for base coagulation is to add a solution containing a base to aqueous dispersion D. Examples of the base to be added include sodium hydroxide, potassium hydroxide, and ammonium carbonate, with sodium hydroxide being preferred. The concentration of the base in the solution containing the base is preferably 0.1 to 50% by mass, more preferably 1 to 30% by mass, and even more preferably 1 to 10% by mass. For aggregation using a coagulant, known coagulants can be used. Known coagulants include aluminum salts, calcium salts, and magnesium salts. Specific examples include aluminum sulfate, alum represented by the general formula M'Al(SO4)2·12H2O (where M' is a monovalent cation other than lithium), calcium nitrate, and magnesium sulfate. Alum is preferred, and potassium alum, where M' is potassium, is more preferred. Examples of mechanical coagulation include known methods such as those described in

[0032] of WO 2023 / 115278. As the aggregation method, mechanical aggregation is preferred because it can be carried out easily.

[0112] <Stretched film> The stretched membrane of this embodiment is formed from a solid composition. A known method can be used to form a stretched film from the solid composition. For example, first, the solid composition is paste-extruded to obtain an extrusion bead (paste extrusion step). Next, if necessary, the extrusion bead is calendered (rolled) to obtain a sheet-like or tape-like rolled body (rolling step). Next, the rolled body is biaxially stretched to form a membrane, producing a stretched membrane (stretching step). A stretched membrane may be produced by biaxially stretching an extrusion bead without providing a rolling step.

[0113] The paste extrusion step can be carried out by a known method, for example, a method in which a lubricant is mixed with a powdery solid composition to form a composition for stretching having flowability, and the composition for stretching is paste-extruded into a desired shape. As the lubricant, naphtha, a petroleum hydrocarbon having a dry point of at least 100, is preferred. After the paste extrusion step, the lubricant can be removed by heating and drying. The amount of the lubricant added is preferably 15 to 30 parts by mass, more preferably 20 to 25 parts by mass, per 100 parts by mass of the total mass of the first polymer and the second polymer. The composition for stretching may further contain any additives in addition to the solid composition and the lubricant. Examples of the additives include pigments for coloring and various fillers for imparting strength, conductivity, etc.

[0114] The rolling step can be carried out using a known method. For example, a string-like extrusion bead is sandwiched between a pair of rotating rollers and rolled to obtain a rolled body. The shape of the rolled body can be appropriately designed so that a stretched membrane of the desired shape can be obtained when stretched in the subsequent stretching step. The stretching step can be carried out using a known biaxial stretching method.

[0115] As will be shown in the examples below, the solid composition for producing a stretched membrane of this embodiment has a highly uniform stretched state when stretched, and can improve the uniformity of the stretched membrane. The reason why the uniformity of the stretched membrane is improved is not clear, but it is thought that the first polymer fills the grain boundaries, thereby making the membrane uniform as a whole.

[0116] As will be shown in the examples described later, a solid composition containing primary particles including the first polymer and the second polymer can be obtained by polymerizing a second monomer in an aqueous dispersion containing a predetermined amount of a particulate first polymer, without using an emulsifier. The reason for this is believed to be that the second monomer is solubilized by adsorbing and incorporating it into the hydrophobic portion of the first polymer, and then, by adding a polymerization initiator, the second monomer is polymerized while being integrated with the first polymer particles. It is also believed that the first polymer contributes to dispersion stabilization in the aqueous medium. [Example]

[0117] The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[0118] <Measurement and evaluation methods> [Glass transition temperature (Tg)] Tg was measured using a NEXTA DSC600 manufactured by Hitachi High-Technologies Corporation. Specifically, 5 mg of the sample for measurement was weighed into an aluminum sample pan, and the sample was heated to 100°C at a heating rate of 10°C / min under a nitrogen atmosphere. It was then cooled to -60°C at a rate of 10°C / min. Once the specified temperature was reached, the temperature was again raised to 100°C at 10°C / min. Tg was estimated from the inflection point observed during this second heating operation.

[0119] [Proportion of each unit in the polymer] The ratio of each unit in the polymer is 19 The values ​​were determined by F-NMR analysis and infrared absorption spectroscopy.

[0120] [Solid content of aqueous dispersion] The solid content concentration was determined by measuring the heating residue. Specifically, 7 to 8 g of the sample to be measured was weighed out onto an aluminum dish whose mass had been measured in advance, and heated at 120°C for 2 hours to evaporate the water. Next, the mass of the aluminum dish, including the solid content remaining on the aluminum dish, was measured. The solid content concentration (unit: mass%) was calculated by dividing the mass of the solid content by the mass of the sample used in the measurement.

[0121] [Moisture content of wet powder] 50 g of the wet powder to be measured was weighed into a pre-measured aluminum dish and heated at 120°C for 12 hours or more to evaporate the water. The mass of the aluminum dish, including the solid content remaining on the dish, was then measured. The solid content W (unit: mass%) was calculated by dividing the mass of the solid content by the mass of the wet powder used in the measurement. The moisture content (unit: mass%) was calculated using the following formula: Moisture content=100-W

[0122] [Average particle size of primary particles in aqueous dispersion] Measurements were made using dynamic light scattering. The solids concentration of the aqueous dispersion to be measured was adjusted to 5.0% by mass to prepare the sample solution. However, if the solids concentration of the aqueous dispersion to be measured (stock solution) was less than 5%, the stock solution was used as the sample solution. Measurements were made at 23°C with a total of 125 measurements using a particle size measurement system (Otsuka Electronics Co., Ltd. product name "ELSZ-neo"), and the average particle size determined by the cumulant method was used as the average particle size (unit: nm) of primary particles. The refractive index of the solvent (water) was 1.333, and the viscosity of the solvent (water) was 0.93 mPa·s.

[0123] [Number of particles in aqueous dispersion] Using the relationship: solids concentration x of aqueous dispersion = number of primary particles N × primary particle volume V × primary particle specific gravity ρ1 / specific gravity of aqueous dispersion ρ2, the number of primary particles per mL of aqueous dispersion was calculated using the following formula. Primary particles were considered to be spherical. N=x·ρ2 / V·ρ1 N (unit: particles / mL): Number of primary particles per mL. x (unit: mass%): solids concentration of the aqueous dispersion. V (unit: mL / particle): volume of primary particles, V = 4 / 3 π (r / 2 × 10 -7 ) 3 r (unit: nm): primary particle diameter. ρ1 (unit: g / mL): density of primary particles. If it can be measured as polytetrafluoroethylene (PTFE), the value of SSG was used as ρ1. If it cannot be measured as PTFE, ρ1 = 2.2. ρ2 (unit: g / mL): specific gravity of aqueous dispersion, empirically obtained value ρ2 = 0.492 x 2 +0.5319x+0.09992 was used, where x is the solid concentration.

[0124] Aspect Ratio Aqueous dispersion D was diluted to a solids concentration of 0.2% by mass to prepare a sample dispersion. The sample dispersion was dropped onto a substrate and dried. The resulting sample was then platinum-deposited and observed using a scanning electron microscope (SEM, JEOL JSM-IT700HR InTouchScope) at a magnification of 20,000x. Fields of view were randomly selected so that particles did not overlap, and at least four observation images were saved. At least 800 ellipsoidal particles were selected from the observation images and analyzed for their long-to-short side ratio (aspect ratio) using the image analysis software "MultiImage Tool." The brightness was adjusted and the particles and substrate were binarized for image processing. The average aspect ratio of each particle was taken as the aspect ratio of the primary particles.

[0125] [Total Content of Compound (S1) Represented by Formula (S1) and Compound (S2) Represented by Formula (S2)] The content of the compounds represented by the above formula (S1) and formula (S2) relative to the total mass of the fluoropolymer in the dry powder obtained in each example described below (hereinafter also referred to as "content M") was measured using a liquid chromatograph mass spectrometer as follows. 5 mL of methanol was added to 2.5 g of the dry powder obtained in each example described below, and the mixture was subjected to ultrasonic treatment at 50°C for 2 hours and centrifuged (5000 rpm, 5 minutes) to precipitate the fluoropolymer, and the supernatant was extracted as an extract. The extract was diluted with water or methanol as necessary and used for measurement. In this way, extract M to be used for measuring the content M was obtained.

[0126] (Measurement procedure) For extract M, the peak areas of the compounds represented by formula (S1) and formula (S2) with each carbon number were determined using the MRM (Multiple Reaction Monitoring) method. Compounds in formula (S1) where n1 is 3 to 13, 15, and 17 were calculated by converting them into perfluorocarboxylic acids with the same number of carbon atoms. Compounds in formula (S2) where n2 is 4 to 10, and 12 were calculated by converting them into perfluorosulfonic acids with the same number of carbon atoms.

[0127] The measurement equipment and measurement conditions are shown in Table 1 below. The MRM measurement parameters are shown in Tables 2 to 5 below. First, five levels of methanol standard solutions of perfluorocarboxylic acid and perfluorosulfonic acid with known concentrations of 1 to 180 ng / g were prepared, and a was calculated using the following formula (A1) by using a first-order approximation from each sample concentration and peak integral value. A=a×X (A1) A: peak area of ​​perfluorocarboxylic acid and perfluorosulfonic acid, X: concentration of perfluorocarboxylic acid and perfluorosulfonic acid (ng / g)

[0128] [Table 1]

[0129] [Table 2]

[0130] [Table 3]

[0131] [Table 4]

[0132] [Table 5]

[0133] Next, the content of compounds with carbon number (n1+1) in extract M was calculated using the following formula (A2): In formula (A2), a means a calculated by the above formula (A1). XCm=ACm / a×ρ1 / ρ2 (A2) XCm: Content (ng / g) of compounds with carbon number (n1+1) in the extract ACm: Peak area of ​​compounds with carbon number (n1+1) in the extract ρ1: density of methanol ρ2: Density of the extract The limit of quantitation for this assay is 1 ng / g.

[0134] Furthermore, the content (ZCm) of each compound relative to the content of the fluoropolymer in the solid composition was calculated from the value of XCm of each compound obtained using extract M by the following formula (A3). ZCm = XCm × dilution ratio × W2a / W2b (A3) ZCm: Content of (n1+1) compounds in the powder (relative to fluorine-containing polymer) W2a: Mass of extract M (g) W2b: Mass (g) of dry powder used to prepare the sample of extract M The dilution ratio indicates the mass ratio at which the extract is diluted with water or methanol so that XCm becomes 180 ng / g or less. The ZCm values ​​of each compound were summed up to obtain the content M.

[0135] [Content of emulsifier in aqueous dispersion] The content of the emulsifier in the aqueous dispersion was calculated from the amount charged.

[0136] [Content of first polymer in dry powder] In the dry powder, the content of the first polymer relative to the total mass of the first polymer and the second polymer was calculated from the charged amounts, and the obtained value was taken as the content of the first polymer in the dry powder.

[0137] [Average primary particle size of dry powder] Aqueous dispersion D was diluted to a solids concentration of 0.2% by mass, and the sample dispersion was dropped onto a substrate and dried. Pt was then deposited on the sample, and a scanning electron microscope (SEM, JEOL JSM-IT700HR InTouchScope) was used to randomly select fields of view at 20,000x magnification so that particles did not overlap, and four or more images were saved. Using the image analysis software "MultiImage Tool," brightness was adjusted and the particles and substrate were binarized. The approximate circular radius of each particle was measured and used as the particle diameter. The arithmetic mean of these particle diameters was considered to be the average primary particle diameter of the dry powder.

[0138] [Standard specific gravity (SSG)] Standard specific gravity (SSG) was measured in accordance with ASTM D4895-04. Specifically, 12.0 g of sample was weighed and compression-molded in a cylindrical mold with an inner diameter of 28.6 mm to obtain a pellet sample. This was placed in an oven at 290°C and heated at a rate of 120°C / hour. After holding at 380°C for 30 minutes, the temperature was decreased at a rate of 60°C / hour and held at 294°C for 24 minutes. The sample was then kept in a desiccator at 23°C for 12 hours, after which the specific gravity of the sample relative to water at 23°C was measured and used as the standard specific gravity. The smaller the SSG value, the higher the molecular weight.

[0139] "Thermal Instability Index (TII)" The measurement was performed in accordance with ASTM D4895-04. The expanded specific gravity (ESG) sample was prepared in the same manner as for the SSG sample, except that the holding time at 380°C was changed from 30 minutes to 360 minutes. Calculated as TII = (ESG-SSG) x 1000. The smaller the TII value, the less the molecular weight changes even when the holding time at 380°C is extended, which indicates excellent heat resistance.

[0140] [Extrusion pressure (hereinafter referred to as EP)] After leaving the dried powder at room temperature for at least 2 hours, 100 g was placed in a 500 mL glass bottle, and 21.7 g of lubricating oil (Isopar H (registered trademark), manufactured by Exxon) was added and mixed for 3 minutes to obtain a mixture. The resulting mixture was then left in a 25°C thermostatic chamber for 2 hours. The paste was then extruded at 25°C through an orifice with a diameter of 2.5 cm, a land length of 1.1 cm, and an entrance angle of 30° under the conditions of a reduction ratio (ratio of the cross-sectional area of ​​the die entrance to the cross-sectional area of ​​the exit) of 100 and an extrusion rate of 51 cm / min to obtain an extrusion bead (string-like material). The pressure required for extrusion at this time was measured and recorded as the extrusion pressure (unit: MPa).

[0141] [Breaking strength] An extrusion bead was obtained in the same manner as in the measurement of extrusion pressure, and this was dried at 230°C for 30 minutes to remove the lubricant. Next, the extrusion bead was cut to an appropriate length, both ends were fixed so that the clamp distance was 3.8 cm, and the extrusion bead was heated to 300°C in an air circulating oven. Subsequently, it was stretched at a stretching speed of 1000% / sec and a stretch ratio of 2400% to obtain stretched porous body B (hereinafter also referred to as "stretched bead B"). The tensile breaking load was measured for each of three samples: one cut from each end of one stretched bead B (excluding any neckdown within the clamping range) and one cut from the center of stretched bead B, using a tensile tester (manufactured by A&D Co., Ltd.), and the maximum stress generated until breakage was taken as the breaking strength (unit: N). Two stretched beads were measured, and the breaking strength was measured for a total of six samples (samples (1) to (6)). In the measurement using the tensile tester, the sample was clamped and fixed between movable jaws with a gauge length of 5.0 cm, and tensile stress was applied by driving the movable jaws at a speed of 300 mm / min at room temperature (24°C).

[0142] "Evaluation of stretchability during biaxial stretching" The average value and standard deviation of the breaking strength measured for the above samples (1) to (6) were calculated. Based on the standard deviation, the stretchability when forming a stretched film by biaxial stretching was evaluated. In other words, the smaller the standard deviation of the measured breaking strength at various points on the stretching bead, the more uniform the stretched state is, and the more uniform the membrane obtained by biaxial stretching is, i.e., the better the stretchability during biaxial stretching. The stretchability was evaluated according to the following criteria. Very good: standard deviation less than 0.8. Good: Standard deviation is greater than or equal to 0.8 and less than 1.5. Poor: Standard deviations greater than 1.5.

[0143] [Stress relaxation time] Both ends of the stretched bead B were connected to fixtures to create a taut bead sample with a total length of 8 inches (20 cm). The oven was kept at 390°C, and the fixture and bead sample were inserted into the oven through a (covered) slit in the side of the oven. The time required from the time of insertion into the oven until the bead sample broke was measured as the stress relaxation time (unit: seconds). A longer stress relaxation time indicates better heat resistance, and higher molecular weight and crystallinity after stretching.

[0144] [Production Example 1: Production of First Polymer P1-1] A 60L stainless steel autoclave equipped with a baffle and a stirrer was charged with 33 kg of deionized water, and the autoclave was then purged with nitrogen. The autoclave was then depressurized and 2450 g of PMVE was added. The temperature was raised to 90°C with stirring, and 324 g of TFE was added, pressurizing the autoclave to 1.7 MPa. 150 g of ultrapure water containing 8.35 g of APS (ammonium persulfate) was then added under pressure to initiate polymerization. The polymerization was continued while TFE was added to maintain the internal pressure of the autoclave at 1.62 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 60 g, and the autoclave was cooled and then the TFE in the autoclave was released into the atmosphere. The polymerization time was 24 minutes. Nitrogen was pressurized to 0.2 MPa, and the temperature was raised to 90°C. After heating the autoclave for 3 hours, it was cooled and reaction liquid A1 was extracted. The first polymer P1-1 in the reaction liquid A1 is water-insoluble, and the reaction liquid A1 is an aqueous dispersion in which particles of the first polymer P1-1 are dispersed in an aqueous medium. The reaction solution A1 was freeze-aggregated and then filtered. The resulting first polymer P1-1 was analyzed by NMR, and found to have a TFE unit / PMVE unit ratio of 62 / 38 (molar ratio). The Tg was -6°C. The polymer had no melting point. This production was carried out twice.

[0145] Two ion exchange resin packed columns filled with Dowex Monosphere 650C (manufactured by DuPont, cation exchange resin, 843 mL) and Purolite A300 (manufactured by Purolite, anion exchange resin, 843 mL) were prepared, and the reaction liquid A1 was passed through them to remove impurities such as ionic species derived from the initiator and by-product ionic species, thereby obtaining an aqueous dispersion B1 of the first polymer P1-1. The solids concentration (content of the first polymer) of aqueous dispersion B1, the average particle size of primary particles in aqueous dispersion B1, and the number of particles in aqueous dispersion B1 were determined using the methods described above. The results are shown in Table 6 (the same applies hereinafter).

[0146] Example 1: Preparation of Second Polymer P2-1 A 100L stainless steel autoclave equipped with a baffle and a stirrer was charged with 1500g of paraffin wax, 47.6kg of aqueous dispersion B1, and 11.4L of deionized water. The autoclave was purged with nitrogen, then reduced in pressure and heated to 70°C. Stirring was initiated, and the autoclave was pressurized to 1.86MPa with TFE. The polymerization reaction was initiated by adding 1L of deionized water containing 3.36g of DSAP (disuccinic acid peroxide). The polymerization was continued while adding TFE to maintain the internal pressure of the autoclave at 1.86MPa, synthesizing the second polymer P2-1 (PTFE). PTFE, a homopolymer of TFE, is non-melt moldable. That is, TFE, which is a second monomer, was polymerized in aqueous dispersion C1, which was prepared by adding deionized water to aqueous dispersion B1. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 13.1 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 150 minutes. The resulting reaction solution was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D1. The obtained aqueous dispersion D1 was diluted with deionized water to a solids concentration of 10% by mass, the temperature was adjusted to 16°C, and the mixture was stirred to cause aggregation, followed by filtration to obtain a wet powder. The moisture content of the wet powder was 38.1% by mass. The moisture content of the wet powder is shown in Table 8 (the same applies below). The obtained wet powder was dried to a solids concentration of 99% by mass or more to obtain a dry powder. Specifically, the wet powder was dried at 195°C for 7.1 hours. In the obtained dry powder, the content of the first polymer P1-1 relative to the total mass of the first polymer and the second polymer (represented as "P1 / (P1+P2)" in the table) was 1.89% by mass. The results are shown in Table 8 (the same applies hereinafter). The composition of the dry powder was calculated using NMR, and the PAVE unit content relative to the total of TFE units and PAVE units (PMVE units in this example) was 0.6 (mol%). The results are shown in Table 8 (the same applies below).

[0147] The production conditions in the step of polymerizing the second monomer (TFE) in the aqueous dispersion C1 are shown in Table 7 (the same applies hereinafter). Specifically, the contents of the aqueous medium and the first polymer in the aqueous dispersion C before the start of polymerization of the second monomer are shown in the table. The content of paraffin wax and the amount of the second monomer (TFE) used per 100 parts by mass of the aqueous medium in aqueous dispersion C are shown in the table above. The amount of polymerization initiator used per 100 parts by mass of the second monomer used is shown in the table above. The aqueous dispersion C1 does not contain any of an emulsifier, fluoride ions, sulfate ions, and ammonium ions.

[0148] The solids concentration of aqueous dispersion D1, the average particle size of primary particles in aqueous dispersion D1, and the number of particles in aqueous dispersion D1 were determined using the methods described above. The results are shown in Table 8 (the same applies hereinafter). As shown in Tables 6 and 8, the particle numbers of aqueous dispersions B1 and D1 are almost the same, and the average particle size of the primary particles of aqueous dispersion D1 is larger. This suggests that the second polymer P2-1 was produced while integrating with particles of the first polymer P1-1, and primary particles composed of the first polymer and the second polymer were formed. The particle size of the primary particles in aqueous dispersion B and the particle size of the primary particles in aqueous dispersion C are the same. Table 8 shows the measurement results of the aspect ratio of the primary particles in aqueous dispersion D1. As shown in Table 8, in aqueous dispersion D1, the contents of compound (S1) and compound (S2) relative to the total mass of primary particles were both below the detection limit, and the total content was in the range of 100 mass ppb or less.

[0149] The standard specific gravity (SSG) and thermal instability index (TII) of the dry powder obtained in this example were measured by the methods described above. The mixture (composition for stretching) obtained in this example by adding a lubricating oil to the dry powder was subjected to measurement of extrusion pressure (EP) by the method described above. Also, by the method described above, stretch beads were formed from the mixture (composition for stretching), and the breaking strength was measured to evaluate the stretchability during biaxial stretching. These results are shown in Table 8 (the same applies below).

[0150] [Production Example 2: Production of First Polymer P1-2] In this example, a first polymer having a molar ratio of TFE units / PMVE units and a Tg different from those in Production Example 1 was produced. A 60L stainless steel autoclave equipped with a baffle and a stirrer was charged with 33 kg of deionized water, and the autoclave was then purged with nitrogen, reduced pressure, and charged with 2441 g of PMVE. The temperature was raised to 90°C with stirring, and 210 g of TFE was charged, pressurizing the autoclave to 1.40 MPa. 150 g of ultrapure water containing 8.35 g of APS was then added under pressure to initiate polymerization. The polymerization was continued while adding TFE to maintain the internal pressure of the autoclave at 1.40 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 120 g, and the autoclave was cooled and then the TFE in the autoclave was released into the atmosphere. The polymerization time was 84 minutes. Nitrogen was pressurized to 0.2 MPa, and the temperature was raised to 90°C. After heating the autoclave for 3 hours, it was cooled and reaction liquid A2 was extracted. The first polymer P1-2 in reaction liquid A2 was water-insoluble. The reaction solution A2 was freeze-aggregated and then filtered to obtain a first polymer P1-2. The first polymer P1-2 had a TFE unit / PMVE unit ratio of 50 / 50 (molar ratio). The Tg was -5°C. It had no melting point. This production was carried out twice.

[0151] In the same manner as in Production Example 1, the reaction liquid A2 was passed through an ion exchange resin to remove impurities, thereby obtaining an aqueous dispersion B2 of the first polymer P1-2.

[0152] Example 2: Preparation of second polymer P2-2 In this example, the second monomer, TFE, was polymerized in aqueous dispersion C2, which was prepared by adding deionized water to aqueous dispersion B2. A 100 L stainless steel autoclave equipped with a baffle and a stirrer was charged with 1500 g of paraffin wax, 47.6 kg of aqueous dispersion B2, and 9.9 L of deionized water. The autoclave was purged with nitrogen, then reduced in pressure and heated to 70 °C. Stirring was started, and the autoclave was pressurized to 1.76 MPa with TFE. 1 L of deionized water containing 2.80 g of DSAP was added to initiate the polymerization reaction. The polymerization was continued while adding TFE to maintain the internal pressure of the autoclave at 1.76 MPa, synthesizing the second polymer P2-2 (PTFE). When the amount of TFE added after the start of polymerization reached 7.88 kg, 1 L of deionized water containing 1.07 g of ammonium sulfite was added over 6 minutes, and the temperature inside the autoclave was then increased to 90°C at a rate of 15°C per hour. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 14.8 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 193 minutes. The resulting reaction solution was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D2. In the same manner as in Example 1, a wet powder was obtained from the obtained aqueous dispersion D2, and this was dried to obtain a dry powder.

[0153] [Production Example 3: Production of First Polymer P1-3] In this example, PPVE was used as PAVE, and a first polymer having a molar ratio of TFE units / PAVE units and a Tg different from those in Production Examples 1 and 2 was produced. A 100L stainless steel autoclave equipped with a baffle and a stirrer was charged with 61 kg of deionized water, and the autoclave was then purged with nitrogen, reduced pressure, and charged with 2470 g of PPVE (perfluoropropyl vinyl ether). The temperature was raised to 90°C with stirring, and the pressure reached 0.48 MPa. 900 g of TFE was charged, and the pressure was increased to 0.82 MPa. 1500 g of ultrapure water containing 19.1 g of APS was then added under pressure to initiate polymerization. The polymerization reaction was terminated when the internal pressure of the autoclave after the start of polymerization dropped by 0.1 MPa after the addition of the APS aqueous solution, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 19 minutes. Nitrogen was injected up to 0.5 MPa, and purging was repeated five times. After cooling the reaction solution, reaction solution A3 was extracted. The first polymer P1-3 in reaction solution A3 was water-insoluble. The reaction solution A3 was freeze-aggregated and then filtered to obtain a first polymer P1-3. The first polymer P1-3 had a TFE unit / PPVE unit ratio of 56 / 44 (molar ratio). The Tg was -5.5°C. The polymer had no melting point.

[0154] In the same manner as in Production Example 1, the reaction liquid A3 was passed through an ion exchange resin to remove impurities, thereby obtaining an aqueous dispersion B3 of a first polymer P1-3.

[0155] Example 3: Preparation of second polymer P2-3 In this example, the second monomer, TFE, was polymerized in aqueous dispersion C3, which was prepared by adding deionized water to aqueous dispersion B3. A 100 L stainless steel autoclave equipped with a baffle and a stirrer was charged with 1500 g of paraffin wax, 47.6 kg of aqueous dispersion B3, and 11.4 L of deionized water. The autoclave was purged with nitrogen, then reduced in pressure and heated to 70°C. Stirring was started, and the autoclave was pressurized to 1.86 MPa with TFE. 1 L of deionized water containing 3.36 g of DSAP was added to initiate the polymerization reaction. The polymerization was allowed to proceed while TFE was added to maintain the internal pressure of the autoclave at 1.86 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 13.1 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 188 minutes. The resulting reaction solution was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D3. In the same manner as in Example 1, a wet powder was obtained from the obtained aqueous dispersion D3, and this was dried to obtain a dry powder.

[0156] [Example 4: PTFE production] This example is a comparative example in which PTFE was synthesized using an emulsifier that was a water-soluble fluorine-containing polymer, without using the first polymer. A 100 L stainless steel autoclave equipped with a baffle and a stirrer was charged with 9 g of polymer P4 (polymerization unit -CFCF(OCFCFCOONH)-) (molecular weight Mn 4500, water-soluble, powder that did not melt even when heated to 220°C, had no melting point or Tg, and thermally decomposed when heated above 220°C), obtained in Example 1 of JP-A-11-181009

[0025] to

[0027] , 1500 g of paraffin wax, and 60 L of deionized water. The autoclave was then purged with nitrogen, reduced pressure, and heated to 75°C. After stirring, the autoclave was pressurized to 1.86 MPa with TFE. 5.60 g of DSAP dissolved in 1 L of deionized water was added to initiate the polymerization reaction. The polymerization was allowed to proceed while adding TFE to maintain the internal pressure of the autoclave at 1.86 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 8.8 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 102 minutes. The resulting reaction solution was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D4. In the same manner as in Example 1, a wet powder was obtained from the obtained aqueous dispersion D4, and this was dried to obtain a dry powder. In this example, the polymer P4 used as an emulsifier was designated as "P1," and the PTFE produced by polymerizing TFE in the presence of the polymer P4 was designated as "P2," and "P1 / (P1+P2)" was calculated.

[0157] [Production Example 5: Production of First Polymer P1-5] In this example, a first polymer having a molar ratio of TFE units / PMVE units and a Tg different from those in Production Example 1 was produced. A 60L stainless steel autoclave equipped with a baffle and a stirrer was charged with 33 kg of deionized water, and the autoclave was then purged with nitrogen, reduced pressure, and charged with 2450 g of PMVE (perfluoromethyl vinyl ether). The temperature was raised to 90°C with stirring, and 324 g of TFE was charged, pressurizing the autoclave to 1.7 MPa. 150 g of ultrapure water containing 8.35 g of APS was then added under pressure to initiate polymerization. The polymerization was allowed to proceed while TFE was added to maintain the internal pressure of the autoclave at 1.62 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 150 g, and the autoclave was cooled and then the TFE in the autoclave was released into the atmosphere. The polymerization time was 60 minutes. Nitrogen was pressurized to 0.2 MPa, and the temperature was raised to 90°C. After heating the autoclave for 3 hours, it was cooled and reaction liquid A5 was extracted. The first polymer P1-5 in reaction liquid A5 was water-insoluble. The reaction solution A5 was freeze-aggregated and then filtered to obtain a first polymer P1-5. The first polymer P1-5 had a TFE unit / PMVE unit ratio of 55 / 45 (molar ratio). The Tg was -3°C. It had no melting point. This production was carried out twice.

[0158] In the same manner as in Production Example 1, the reaction liquid A5 was passed through an ion exchange resin to remove impurities, thereby obtaining an aqueous dispersion B5 of the first polymer P1-5.

[0159] Example 5: Preparation of second polymer P2-5 This example is a comparative example in which the amount of the first polymer used is large. A 100 L stainless steel autoclave equipped with a baffle and a stirrer was charged with 1500 g of paraffin wax, 47.6 kg of aqueous dispersion B5, and 11.4 L of deionized water. The autoclave was purged with nitrogen, then reduced in pressure and heated to 70 °C. Stirring was started, and the autoclave was pressurized to 1.86 MPa with TFE. 1 L of deionized water containing 3.36 g of DSAP was added to initiate the polymerization reaction. The polymerization was continued while adding TFE to maintain the internal pressure of the autoclave at 1.86 MPa, synthesizing the second polymer P2-5 (PTFE). The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 13.1 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 150 minutes. The resulting reaction solution was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D5. In the same manner as in Example 1, a wet powder was obtained from the obtained aqueous dispersion D5, and this was dried to obtain a dry powder.

[0160] [Production Example 6: Production of First Polymer P1-6] A 60 L stainless steel autoclave equipped with a baffle and a stirrer was charged with 33 kg of deionized water and 1.66 g of a 50% by weight aqueous solution of sodium 2-acrylamido-2-methyl-1-propanesulfonate (hereinafter also referred to as "NaAAMPS"). The autoclave was then purged with nitrogen, reduced pressure, and 2443 g of PMVE was added. The temperature was raised to 90 °C with stirring, and 243 g of TFE was added and the pressure was increased to 1.5 MPa. 400 g of ultrapure water containing 84.3 g of APS (ammonium persulfate) was added under pressure to initiate polymerization. The polymerization was allowed to proceed while TFE was added to maintain the internal pressure of the autoclave at 1.5 MPa. The polymerization reaction was terminated when the amount of TFE added after the start of polymerization reached 1,200 g, and the autoclave was cooled. The TFE in the autoclave was then released into the atmosphere. Nitrogen was introduced under pressure up to 0.2 MPa, and the temperature was raised to 90°C. The autoclave was heated for 3 hours, cooled, and reaction liquid A6 was extracted. The first polymer P1-6 in the reaction liquid A6 is water-insoluble, and the reaction liquid A6 is an aqueous dispersion in which particles of the first polymer P1-6 are dispersed in an aqueous medium. The reaction solution A6 was freeze-aggregated and then filtered. The resulting first polymer P1-6 was analyzed by NMR, and found to have a TFE unit / PMVE unit ratio of 63 / 37 (molar ratio). The Tg was -6°C. The polymer had no melting point. Except for diluting reaction solution A6 five times, the same procedure as in Production Example 1 was repeated, except that reaction solution A6 was diluted five times, and impurities were removed by passing reaction solution A6 through an ion exchange resin to obtain an aqueous dispersion B6 of the first polymer P1-6.

[0161] Example 6: Preparation of second polymer P2-6 A 100L stainless steel autoclave equipped with a baffle and a stirrer was charged with 1500g of paraffin wax, 33.7kg of aqueous dispersion B6 containing 1.1% by mass of the first polymer P1-6, and 24.9L of deionized water. The autoclave was purged with nitrogen, then evacuated and heated to 65°C. Stirring was initiated, and the autoclave was pressurized to 1.76MPa with TFE. The polymerization reaction was initiated by adding 1L of deionized water containing 2.8g of DSAP (disuccinic acid peroxide). The polymerization was continued while adding TFE to maintain the internal pressure of the autoclave at 1.76MPa, synthesizing the second polymer P2-6 (PTFE). PTFE, a homopolymer of TFE, is non-melt moldable. When the amount of TFE added after the start of polymerization reached 15.8 kg, the polymerization reaction was terminated, and the TFE in the autoclave was released into the atmosphere. The resulting reaction liquid was cooled, and the supernatant paraffin wax was removed to obtain aqueous dispersion D6. In the same manner as in Example 1, a wet powder was obtained from the obtained aqueous dispersion D6, and this was dried to obtain a dry powder.

[0162] [Table 6]

[0163] [Table 7]

[0164] [Table 8]

[0165] As shown in the results of Tables 6 to 8, the dry powders obtained in Examples 1 to 3 and 6 have good stretchability when biaxially stretched, and stretched films with excellent uniformity can be obtained. The dry powder of Example 4, in which the second monomer was polymerized without using the first polymer, had poor stretchability. The dry powder of Example 5, in which the content of the first polymer relative to the total mass of the first polymer and the second polymer exceeded 4.0 mass %, broke during stretching.

Claims

1. a first polymer which is a crystalline fluorine-containing polymer having a melting point below 20°C or an amorphous fluorine-containing polymer having a glass transition temperature below 20°C, and a second polymer which contains units based on tetrafluoroethylene and is different from the first polymer, the first polymer contains units based on tetrafluoroethylene and units based on perfluoro(alkyl vinyl ether), and the content of the units based on perfluoro(alkyl vinyl ether) is 20 to 60 mol % relative to the total of the units based on tetrafluoroethylene and the units based on perfluoro(alkyl vinyl ether); the content of units based on tetrafluoroethylene contained in the second polymer is 99.0 to 100.0 mol %, the content of the first polymer is 0.01 to 4.0% by mass with respect to the total mass of the first polymer and the second polymer, A solid composition for producing a stretched membrane, wherein the total content of a compound represented by the following formula (S1) and a compound represented by the following formula (S2) is 100 mass ppb or less: A solid composition, wherein the total content of the first polymer and the second polymer is 99.0 to 100% by mass relative to the total mass of the solid composition. Formula (S1): H-(CF) ２ ) ｎ１ -COM Formula (S2): H-(CF ２ ) ｎ２ -SO ３ M In formula (S1) and formula (S2), M each independently represents a hydrogen atom, Na, K, or NH ４ wherein n1 represents an integer of 3 to 13, 15 or 17, and n2 represents an integer of 4 to 10, or 12.

2. 10. The solid composition of claim 1, comprising primary particles comprising said first polymer and said second polymer.

3. 3. The solid composition of claim 2, wherein the primary particles have an aspect ratio of 1.5 or less.

4. 2. The solid composition according to claim 1, wherein the content of units based on perfluoro(alkyl vinyl ether) contained in the first polymer is 0.1 to 3.0 mol % relative to the sum of all units of the first polymer and all units of the second polymer.

5. The solid composition of claim 1 , wherein the first polymer is water-insoluble.

6. A stretched film formed from the solid composition according to any one of claims 1 to 5.

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