Method for producing aqueous polytetrafluoroethylene dispersion

Abstract

Provided is a method for producing a polytetrafluoroethylene aqueous dispersion, with which it is possible to obtain, in a simple manner, a high-concentration polytetrafluoroethylene aqueous dispersion wherein the amount of a fluorine-based oligomer produced as a by-product is small, and a coating film formed from said dispersion has a large CFT.　This method for producing a polytetrafluoroethylene aqueous dispersion includes: a step A1 for polymerizing a non-fluorine-based monomer in an aqueous medium to obtain a solution 1 containing a polymer that contains a unit based on a non-fluorine-based monomer; a step A2 for polymerizing tetrafluoroethylene in the solution 1, without substantially adding a surfactant to the solution 1, to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and a step A3 for adding a nonionic surfactant to the aqueous emulsion, in an amount of 10-150 mass% with respect to the mass of PTFE contained in the aqueous emulsion, and subsequently concentrating the aqueous emulsion to obtain a polytetrafluoroethylene aqueous dispersion, wherein the amount of the non-fluorine-based monomer used is at most 200 mass ppm with respect to the amount of tetrafluoroethylene supplied to a polymerization system.

Description

[Technical Field]

[0001] The present invention relates to a method for producing an aqueous polytetrafluoroethylene dispersion. [Background technology]

[0002] Polytetrafluoroethylene is used in a variety of applications due to its excellent properties. Conventionally, fluorosurfactants such as perfluorooctanoates have been used in the production of polytetrafluoroethylene, but from an environmental perspective, it has been desired to reduce the use of fluorosurfactants. Therefore, as one of the new methods for producing polytetrafluoroethylene, a method has been proposed in which a hydrocarbon-containing surfactant is used when polymerizing tetrafluoroethylene (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2016-537499 Summary of the Invention [Problem to be solved by the invention]

[0004] On the other hand, when a hydrocarbon-containing surfactant is used, a chain transfer reaction occurs with the hydrocarbon-containing surfactant during polymerization of tetrafluoroethylene. Therefore, in the method using this hydrocarbon-containing surfactant, the presence of fluorine-containing oligomers with various chain lengths (especially fluorine-containing oligomers having hydrophilic functional groups) was confirmed in the product, which was not observed in conventional methods using fluorine-containing surfactants. As mentioned above, such fluorine-containing oligomers are by-products generated when the hydrocarbon-containing surfactant is the starting point for chain transfer. The presence of such by-products is undesirable from an environmental perspective.

[0005] Furthermore, it is desirable that the coating film formed from an aqueous dispersion containing polytetrafluoroethylene particles have a large critical film thickness (hereinafter also referred to as CFT). The CFT corresponds to the thickness at which cracks begin to occur when an aqueous dispersion containing polytetrafluoroethylene particles is applied to form a coating film of a certain thickness or more and then baked.

[0006] An object of the present invention is to provide a method for producing an aqueous polytetrafluoroethylene dispersion, which produces little fluorine-based oligomer by-product, forms a coating film with a large CFT, and easily obtains a highly concentrated aqueous polytetrafluoroethylene dispersion. [Means for solving the problem]

[0007] As a result of extensive investigation, the present inventors have found that the above-mentioned problems can be solved by the following configuration.

[0008] (1) Step A1 of polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a polymer containing units based on the non-fluorinated monomer; a step A2 of polymerizing tetrafluoroethylene in the solution 1 without adding a surfactant to the solution 1 to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and a step A3 of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150 mass % relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, A method for producing an aqueous polytetrafluoroethylene dispersion, wherein the amount of the non-fluorinated monomer used is 200 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system. (2) The method for producing an aqueous polytetrafluoroethylene dispersion according to (1), wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion between the step A2 and the step A3, or during the step A3. (3) The method for producing an aqueous polytetrafluoroethylene dispersion according to (1) or (2), wherein the non-fluorinated monomer is a monomer represented by the formula (1) described below. (4) The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of (1) to (3), wherein the content of the polytetrafluoroethylene particles in the aqueous polytetrafluoroethylene dispersion is 50 to 70 mass % based on the total amount of the aqueous polytetrafluoroethylene dispersion. (5) The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of (1) to (4), wherein in step A3, a nonionic surfactant is added to the aqueous emulsion in an amount of 10 to 150% by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for an additional 1 to 10 hours to cause phase separation of the aqueous emulsion, and the lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion. (6) Step B1 of mixing at least one nucleating additive selected from the group consisting of polyalkylene oxide compounds and hydrocarbon-containing surfactants with an oxidizing agent in an aqueous medium to obtain a solution 2; Step B2 of polymerizing tetrafluoroethylene in the solution 2 without adding a surfactant to the solution 2 to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and a step B3 of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150 mass % relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, A method for producing an aqueous polytetrafluoroethylene dispersion, wherein the amount of the nucleation additive used is 100 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system. (7) The method for producing an aqueous polytetrafluoroethylene dispersion according to (6), wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion between the step B2 and the step B3, or during the step B3. (8) The method for producing an aqueous polytetrafluoroethylene dispersion according to (6) or (7), wherein the content of the polytetrafluoroethylene particles in the aqueous polytetrafluoroethylene dispersion is 50 to 70 mass % based on the total amount of the aqueous polytetrafluoroethylene dispersion. (9) The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of (6) to (8), wherein in step B3, a nonionic surfactant is added to the aqueous emulsion in an amount of 10 to 150% by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for an additional 1 to 10 hours to cause phase separation of the aqueous emulsion, and the lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion. (10) The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of (6) to (9), wherein the nucleation additive is a polyalkylene oxide compound. (11) The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of (6) to (10), wherein the amount of the oxidizing agent used is 0.5 to 100 ppm by mass based on the total mass of the aqueous medium. (12) A step C1 of polymerizing tetrafluoroethylene in an aqueous medium in the presence of a polyalkylene oxide compound without substantially using a surfactant to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and step C2 of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150 mass % relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, A method for producing an aqueous polytetrafluoroethylene dispersion, wherein the amount of the polyalkylene oxide compound used is 100 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system. (13) The method for producing an aqueous polytetrafluoroethylene dispersion according to (12), wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion between the step C1 and the step C2, or during the step C2. (14) The method for producing an aqueous polytetrafluoroethylene dispersion according to (12) or (13), wherein in step C2, a nonionic surfactant is added to the aqueous emulsion in an amount of 10 to 150% by mass relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for an additional 1 to 10 hours to cause phase separation of the aqueous emulsion, and the lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for producing an aqueous polytetrafluoroethylene dispersion, which produces little fluorine-based oligomer by-product, forms a coating film with a large CFT, and easily obtains a highly concentrated aqueous polytetrafluoroethylene dispersion. DETAILED DESCRIPTION OF THE INVENTION

[0010] The terms used in the present invention have the following meanings. A "unit" is a general term for an atomic group derived from one monomer molecule, formed directly by the polymerization of a monomer. A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0011] The first and second embodiments of the method for producing an aqueous polytetrafluoroethylene (hereinafter also referred to as "PTFE") dispersion of the present invention are characterized in that they use solution 1 containing a polymer containing units based on a non-fluorinated monomer (hereinafter also referred to as "specific polymer") and an aqueous medium, or use solution 2 obtained by mixing at least one nucleation additive selected from the group consisting of polyalkylene oxide compounds and hydrocarbon-containing surfactants with an oxidizing agent in an aqueous medium; they do not substantially use a surfactant when polymerizing tetrafluoroethylene (hereinafter also referred to as "TFE"); they adjust the amounts of the non-fluorinated monomer and the nucleation additive used; and they use a predetermined amount of a nonionic surfactant when concentrating the aqueous emulsion after polymerization of TFE. Solutions 1 and 2 provide a hydrophobic environment in which TFE polymerization can proceed smoothly. Furthermore, the substantial absence of surfactants allows stable TFE polymerization while suppressing chain transfer. As a result, the formation of fluorine-containing oligomers as by-products can be suppressed. Furthermore, it has been found that the CFT of the resulting coating film can be increased by adjusting the amounts of non-fluorine-containing monomers and nucleation additives used. The fluorine-based oligomer mainly includes oligomers having about 6 to 34 carbon atoms and formed by linking CF2 groups. It has also been found that a highly concentrated aqueous PTFE dispersion can be easily obtained by using a predetermined amount of a nonionic surfactant.

[0012] The third embodiment of the method for producing an aqueous PTFE dispersion of the present invention is characterized in that TFE is polymerized in an aqueous medium in the presence of a predetermined amount of a polyalkylene oxide compound without substantially adding a surfactant, and that a predetermined amount of a nonionic surfactant is used when concentrating the aqueous emulsion after TFE polymerization.

[0013] <<First embodiment>> A first embodiment of the method for producing an aqueous PTFE dispersion includes the following three steps. Step A1: A step of polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a specific polymer Step A2: A step of polymerizing TFE in Solution 1 without adding a surfactant to Solution 1 to obtain an aqueous emulsion containing PTFE particles. Step A3: A step of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150 mass % based on the mass of polytetrafluoroethylene contained in the aqueous emulsion (hereinafter also referred to as "PTFE mass"), and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. In the first embodiment, the amount of the non-fluorinated monomer used is 200 mass ppm or less relative to the amount of TFE supplied to the polymerization system (hereinafter also referred to as "TFE supply amount"). The present invention will be described in detail below, taking the above preferred embodiment as an example.

[0014] <Process A1> Step A1 is a step of polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a specific polymer. In the following, first, the materials used in step A1 will be described in detail, and then the procedure of step A1 will be described in detail.

[0015] (non-fluorinated monomer) The non-fluorine-based monomer is a monomer that does not contain a fluorine atom. The non-fluorine-containing monomer usually has a polymerizable group, and the number of polymerizable groups is preferably 1 to 3, and more preferably 1. The polymerizable group is preferably an ethylenically unsaturated group, more specifically, an acryloyl group, a methacryloyl group, a vinyl ether group, a vinyl ester group, a vinyl group, or an allyl group, with an acryloyl group, a methacryloyl group, a vinyl ester group, or a vinyl ether group being preferred.

[0016] The non-fluorine-containing monomer is preferably a monomer represented by formula (1). Formula (1) CH2=CR 11 -L 1 -R 12 R 11 represents a hydrogen atom or an alkyl group. The alkyl group preferably has 1 to 3 carbon atoms, and more preferably 1 carbon atom. L 1 represents a single bond, -CO-O-*, -O-CO-* or -O-. * represents R 12 For example, L 1 is -CO-O-*, then equation (1) is CH2=CR 11 -CO-OR 12 Represents. R 12 represents a hydrogen atom, an alkyl group, an alkenyl group, or a nitrile group. 1 If is a single bond, R 12 is a nitrile group. The alkyl group and alkenyl group preferably have 1 to 10 carbon atoms, more preferably 1 to 6 carbon atoms, and even more preferably 1 to 4 carbon atoms. The alkyl group may be linear or cyclic. When the alkyl group is cyclic, it corresponds to a cycloalkyl group. The alkenyl group may be linear or cyclic.

[0017] The monomer represented by formula (1) is preferably a monomer selected from the group consisting of a monomer represented by formula (1-1), a monomer represented by formula (1-2), a monomer represented by formula (1-3), and a monomer represented by formula (1-4). Formula (1-1) CH2=CR 11 -CO-OR 13 Formula (1-2) CH2=CR 11 -O-CO-R 14 Formula (1-3) CH2=CR 11 -OR 15 Formula (1-4) CH2=CR 11 -R 16 R 11 The definition of is as described above. R13 represents a hydrogen atom, an alkyl group, or an alkenyl group, and is preferably an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 1 to 6 carbon atoms. R 14 represents an alkyl group, preferably an alkyl group having 1 to 3 carbon atoms, more preferably a methyl group. R 15 represents an alkyl group, preferably a linear alkyl group or a cyclic alkyl group. R 16 represents a nitrile group.

[0018] Examples of non-fluorine-containing monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate, vinyl methacrylate, vinyl acetate, acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, ethyl vinyl ether, and cyclohexyl vinyl ether. The non-fluorine-containing monomers may be used alone or in combination of two or more. The non-fluorine-containing monomer is preferably a monomer represented by formula (1-1) or a monomer represented by formula (1-2), and R 13 is an alkyl group. The monomers represented by formula (1-1) and formula (1-2) have a hydrophilic group such as an ester group or a carboxy group, and therefore the monomers and their polymers have hydrophilicity. Therefore, it is believed that the monomers and their polymers can be stably dispersed in an aqueous medium without the need for a surfactant, particularly at low concentrations.

[0019] (Specific polymer) The specific polymer is a polymer containing units based on a non-fluorine-containing monomer. The specific polymer usually contains only units based on a non-fluorine-containing monomer, but may contain units based on a fluorine-containing monomer within a range that does not impair the effects of the present invention. That is, in addition to the non-fluorine-containing monomer, a fluorine-containing monomer may be used in step A1. The fluorine-containing monomer is a monomer having a fluorine atom, and an example of the fluorine-containing monomer is TFE. The content of units based on a non-fluorine-containing monomer in the specific polymer is preferably 90% by mass or more, more preferably 95% by mass or more, relative to all units of the specific polymer, and the upper limit may be 100% by mass.

[0020] (aqueous medium) Examples of aqueous media include water and mixtures of water and water-soluble organic solvents. Examples of the water-soluble organic solvent include tert-butanol, propylene glycol, and dipropylene glycol. In the case of a mixture of water and a water-soluble organic solvent, the concentration of the water-soluble organic solvent is preferably 10% by mass or less. The aqueous medium is preferably water alone.

[0021] (Polymerization initiator) In step A1, a polymerization initiator may be used, that is, a polymerization initiator may be used when polymerizing the non-fluorine-based monomer. As the polymerization initiator, a water-soluble radical initiator or a water-soluble oxidation-reduction catalyst is preferred. The water-soluble radical initiator is preferably a persulfate such as ammonium persulfate or potassium persulfate, or a water-soluble organic peroxide such as disuccinic acid peroxide, bisglutaric acid peroxide, or tert-butyl hydroperoxide. As the water-soluble oxidation-reduction catalyst, a combination of an oxidizing agent such as bromic acid or a salt thereof, chloric acid or a salt thereof, persulfuric acid or a salt thereof, permanganic acid or a salt thereof, or hydrogen peroxide, and a reducing agent such as sulfurous acid or a salt thereof, hydrogen sulfite or a salt thereof, thiosulfuric acid or a salt thereof, or an organic acid, is preferred. Among these, a combination of bromic acid or a salt thereof and sulfurous acid or a salt thereof (e.g., ammonium sulfite), and a combination of permanganic acid or a salt thereof (e.g., potassium permanganate) and oxalic acid are more preferred. The polymerization initiator is preferably ammonium persulfate alone or a mixture of a persulfate and disuccinic acid peroxide, more preferably ammonium persulfate alone or a mixture of ammonium persulfate and disuccinic acid peroxide. The polymerization initiator may be used alone or in combination of two or more. The polymerization initiator may be charged in its entirety into the polymerization system before the start of the polymerization reaction, or may be added to the polymerization system continuously or intermittently.

[0022] (Step A1 procedure) In step A1, a non-fluorine-containing monomer is polymerized in an aqueous medium. Specifically, it is preferable to mix the non-fluorine-containing monomer with an aqueous medium and polymerize the non-fluorine-containing monomer in the resulting mixed liquid. As described above, a fluorine-containing monomer may be used in combination, if necessary.

[0023] The amount of the non-fluorinated monomer used is 200 mass ppm or less, preferably 1 to 150 mass ppm or less, more preferably 5 to 100 mass ppm, and even more preferably 5 to 50 mass ppm, relative to the amount of TFE supplied (amount of TFE used) used in step A2 described below. The non-fluorine-containing monomer is preferably added all at once in the initial stage, that is, the entire amount is added to the polymerization system before the polymerization reaction is started.

[0024] The content of the non-fluorine-based monomer in the dispersion obtained by mixing the non-fluorine-based monomer with the aqueous medium is preferably 0.000015 to 0.0030 mass %, more preferably 0.000075 to 0.0023 mass %, relative to the total mass of the dispersion. Since the entire amount of the non-fluorinated monomer is usually polymerized to form the specific polymer, the concentration of the specific polymer in the obtained solution 1 falls within the above-mentioned numerical range. The non-fluorinated monomer concentration and the specific polymer concentration are those when the obtained solution 1 is used in step A2 without diluting it with an aqueous medium. When the obtained solution 1 is diluted with an aqueous medium to have the above-mentioned specific polymer concentration and the diluted solution is used in step A2, a high-concentration solution is produced in step A1 according to the dilution ratio. The dilution ratio is not particularly limited, but is preferably 10 times or less.

[0025] The amount of the polymerization initiator used is preferably from 0.2 to 1000 mass %, more preferably from 0.2 to 500 mass %, based on the total amount of non-fluorinated monomers.

[0026] The amount of the polymerization initiator used is preferably 0.1 to 1000 mol %, more preferably 0.1 to 300 mol %, based on the total amount of non-fluorinated monomers.

[0027] The polymerization temperature for the non-fluorine-based monomer is preferably 10 to 95° C., more preferably 50 to 90° C. The polymerization time is preferably 5 to 400 minutes, more preferably 5 to 300 minutes, and even more preferably 5 to 200 minutes. The pressure conditions during polymerization are preferably reduced pressure conditions or normal pressure conditions, and among these, 0 to 2.0 MPa is preferred, 0 to 1.0 MPa is more preferred, and 0 to 0.5 MPa is even more preferred. Alternatively, the polymerization may be carried out in a TFE atmosphere. Note that the polymerization of the non-fluorinated monomer in an aqueous medium usually proceeds preferentially over the polymerization of TFE.

[0028] By the above step A1, a solution 1 containing a specific polymer is obtained. The specific polymer may be dissolved in solution 1 or may be dispersed in the aqueous medium in the form of particles. During the polymerization of TFE in step A2 described below, the specific polymer is not an emulsifier, but it is presumed that the balance of interfacial tensions with both the aqueous medium and the PTFE particles causes the specific polymer to exist at the boundary between the two, thereby contributing to the stabilization of the dispersion of the PTFE particles in the aqueous medium. The particle size of the particles of the specific polymer is preferably 0.1 to 100 nm, more preferably 0.1 to 50 nm.

[0029] Solution 1 obtained in step A1 may contain unreacted non-fluorinated monomers. Furthermore, the polymerization atmosphere in step A1 may be set to a TFE-containing atmosphere in consideration of step A2. In such cases, it is considered that a part of the specific polymer in step A2 may become a polymer containing TFE units. From another perspective, the PTFE particles obtained in step A2 are not limited to particles consisting of a physical mixture of a specific polymer and PTFE, but can also be considered to be particles containing a TFE copolymer having units based on a non-fluorinated monomer.

[0030] <Process A2> Step A2 is a step of polymerizing TFE in the solution 1 obtained in step A1 without adding substantially any surfactant to the solution 1, thereby obtaining an aqueous emulsion containing PTFE particles. In the following, first, the materials used in step A2 will be described in detail, and then the procedure of step A2 will be described in detail.

[0031] (TFE) In step A2, TFE is used.

[0032] (Other monomers) In step A2, a monomer other than TFE may be further used within the range that does not impair the effects of the present invention. The other monomers include monomers having polar groups (hereinafter simply referred to as "specific monomers"). The polar groups in the specific monomers interact with the aqueous medium, and are presumed to function as surfactants by being positioned between the TFE and the aqueous medium during TFE polymerization. As a result, TFE polymerization proceeds smoothly and chain transfer is suppressed.

[0033] Examples of the polar group contained in the specific monomer include a sulfonic acid group, a sulfonate group, a carboxylic acid group, a carboxylate group, a phosphonic acid group, and a phosphonate group. Among them, the group represented by formula (A) or the group represented by formula (B) is preferred, and the group represented by formula (A) is more preferred, in terms of further suppressing the formation of fluorine-based oligomers. Formula (A) -SO3M Formula (B) -COOM In formula (A) and formula (B), M represents a hydrogen atom, NH4, or an alkali metal atom. Examples of alkali metal atoms include a lithium atom, a sodium atom, and a potassium atom.

[0034] The specific monomer usually has a polymerizable group, and the number of polymerizable groups is preferably 1 to 3, and more preferably 1. The polymerizable group is preferably an ethylenically unsaturated group, more specifically, an acryloyl group, a methacryloyl group, a vinyl ether group, a vinyl ester group, a vinyl group, or an allyl group, with an acryloyl group, a methacryloyl group, a vinyl ester group, or a vinyl ether group being preferred.

[0035] The specific monomer is preferably a monomer represented by formula (3) in that the formation of fluorine-based oligomers is further suppressed. Formula (3) CR 31 R 32 =CR 33 -L 3 -R 34 In formula (3), R 31 and R 32 each independently represents a hydrogen atom or a fluorine atom.

[0036] R 33 represents a hydrogen atom, a fluorine atom, or an alkyl group optionally substituted with a fluorine atom. Among these, a hydrogen atom or a fluorine atom is preferred in terms of better copolymerizability with TFE. The term "alkyl group which may be substituted with a fluorine atom" refers to an alkyl group in which at least one hydrogen atom may be substituted with a fluorine atom. The alkyl group which may be substituted with a fluorine atom preferably has 1 to 3 carbon atoms, and more preferably 1 carbon atom.

[0037] L 3represents a single bond or a divalent linking group. Among these, a single bond is preferred in that it has better copolymerizability with TFE. Examples of the divalent linking group include a divalent hydrocarbon group (which may be a divalent saturated hydrocarbon group, a divalent aromatic hydrocarbon group, an alkenylene group, or an alkynylene group. The divalent saturated hydrocarbon group may be linear, branched, or cyclic, and an example thereof is an alkylene group. The carbon number is preferably 1 to 20. The divalent aromatic hydrocarbon group preferably has 5 to 20 carbon atoms, and an example thereof is a phenylene group. In addition, the divalent aromatic hydrocarbon group may be an alkenylene group having 2 to 20 carbon atoms or an alkynylene group having 2 to 20 carbon atoms), a divalent heterocyclic group, -O-, -S-, -SO2-, -C(O)-, -Si(R a )2-, -N(R b )-, and groups formed by combining two or more of these. a represents an alkyl group (preferably having 1 to 10 carbon atoms) or a phenyl group. b represents a hydrogen atom or an alkyl group (preferably having 1 to 10 carbon atoms). Examples of groups that combine two or more of these include -OC(O)-, -C(O)N(R b )-, alkylene group-O-alkylene group, alkylene group-OC(O)-alkylene group, alkylene group-Si(R a )2-phenylene group -Si(R a )2 are listed. The divalent hydrocarbon group may have a substituent. Examples of the substituent include a halogen atom (e.g., a fluorine atom, a chlorine atom). In other words, the hydrogen atom in the divalent hydrocarbon group may be substituted with a halogen atom.

[0038] R 34 represents a group represented by the above formula (A) or a group represented by the above formula (B).

[0039] The monomer represented by formula (3) is preferably a monomer selected from the group consisting of a monomer represented by formula (3-1), a monomer represented by formula (3-2), a monomer represented by formula (3-3), a monomer represented by formula (3-4), a monomer represented by formula (3-5), and a monomer represented by formula (3-6), and more preferably a monomer represented by formula (3-1). Formula (3-1) CR 31 R 32 =CR 33 -R 34 Formula (3-2) CR 31 R 32 =CR 33 -(CF2) m1 -R 34 Formula (3-3) CR 31 R 32 =CR 33 -(CF2C(CF3)F) m2 -R 34 Formula (3-4) CR 31 R 32 =CR 33 -O-(CFR 35 ) m3 -R 34 Formula (3-5) CR 31 R 32 =CR 33 -O-(CF2CFR 35 O) m4 -CF2CF2-R 34 Formula (3-6) CR 31 R 32 =CR 33 -CF2-O-(CF(CF3)CF2O) m5 -CF(CF3)-R 34

[0040] In formulas (3-1) to (3-6), R 31 ~R 34 The definition of is as described above. In formula (3-2), m1 represents an integer of 1 to 10. In the formula (3-3), m2 represents an integer of 1 to 5. In formula (3-4), m3 represents an integer of 1 to 10.35 represents a fluorine atom or CF3. In formula (3-5), m4 represents an integer of 1 to 10. 35 The definition of is as described above. In formula (3-6), m5 represents 0 or an integer of 1 to 10.

[0041] A specific example of the specific monomer is ammonium vinyl sulfonate. The specific monomer may be used alone or in combination of two or more.

[0042] (Polymerization initiator) In step A2, a polymerization initiator may be used, that is, a polymerization initiator may be used during polymerization of TFE. The polymerization initiator used may be the polymerization initiator described in step A1. As the polymerization initiator, a mixture of persulfate and disuccinic acid peroxide is preferred, and a mixture of ammonium persulfate and disuccinic acid peroxide is more preferred. The amount of the polymerization initiator used is preferably 0.10% by mass or more, more preferably 0.10 to 1.5% by mass, and even more preferably 0.20 to 1.0% by mass, based on the total amount of TFE supplied to the polymerization system.

[0043] (Stabilizing agent) In step A2, a stabilizing agent may be used. As the stabilizing aid, paraffin wax, fluorine-based solvents, and silicone oils are preferred, with paraffin wax being more preferred. Paraffin wax may be liquid, semi-solid, or solid at room temperature. Among these, 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. The stabilizing aids may be used alone or in combination of two or more.

[0044] (others) In step A2, a monomer other than TFE and the specific monomer may be used within a range that does not impair the effects of the present invention. However, in terms of the various properties of PTFE being more excellent, the amount of TFE used is preferably 99.5 mass% or more, and more preferably 99.8 mass% or more, based on the total amount of the monomers used in step A2.

[0045] (Step A2 procedure) In step A2, substantially no surfactant is added to solution 1. That is, in step A2, polymerization of TFE is carried out in solution 1 without substantially adding a new surfactant to solution 1. A surfactant is a compound having a hydrophilic group (e.g., a polar group) and a hydrophobic group (e.g., a hydrocarbon group). The definition of the polar group is the same as the definition of the polar group contained in the specific monomer. The surfactant may be a known surfactant, such as a nonionic surfactant or an ionic surfactant, or more specifically, a hydrocarbon-containing surfactant or a fluorine-containing surfactant. The hydrocarbon-containing surfactant is defined below. In step A2, it is preferable that at least one surfactant selected from the group consisting of hydrocarbon-containing surfactants and fluorine-containing surfactants is not substantially added to solution 1. The above phrase "substantially not added" means that no surfactant is added, or if a surfactant is added, the amount of surfactant added is 200 mass ppm or less relative to the total mass of Solution 1. There is no particular lower limit, but 0 mass ppm is preferred. In other words, it is preferred that no surfactant is added to Solution 1 in Step A2.

[0046] TFE is added to the polymerization system (i.e., polymerization reaction vessel) by a conventional method. For example, TFE is added to the polymerization system continuously or intermittently so that the polymerization pressure becomes a predetermined pressure. When a polymerization initiator is used, the polymerization initiator may be added to the polymerization system all at once or in portions.

[0047] When a specific monomer is used, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.150% by mass or less, that is, the amount of the specific monomer charged relative to the total amount of TFE charged is preferably 0.150% by mass or less. From the viewpoint of emulsion stability during polymerization, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.100% by mass or less, more preferably 0.090% by mass or less, and from the viewpoint of improving molecular weight, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.005% by mass or more, more preferably 0.010% by mass or more. When two or more specific monomers are used, the total amount of the specific monomers used may be within the above range.

[0048] When a specific monomer is used, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.150 mol % or less, that is, the amount of the specific monomer charged relative to the total amount of TFE charged is preferably 0.150 mol % or less. From the viewpoint of emulsion stability during polymerization, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.100 mol% or less, more preferably 0.090 mol% or less, and from the viewpoint of improving molecular weight, the amount of the specific monomer used relative to the total amount of TFE is preferably 0.001 mol% or more, more preferably 0.005 mol% or more. When two or more specific monomers are used, the total amount of the specific monomers used may be within the above range.

[0049] The polymerization temperature is preferably 10 to 95° C., more preferably 15 to 90° C. The polymerization pressure is preferably 0.5 to 4.0 MPa, more preferably 0.6 to 3.5 MPa. The polymerization time is preferably 50 to 520 minutes, more preferably 50 to 450 minutes, and even more preferably 50 to 300 minutes.

[0050] Step A1 and step A2 may be carried out consecutively in the same polymerization reaction vessel. In the production method of the present invention, it is sufficient that the specific polymer is formed in step A1, and step A2 may be carried out before the non-fluorinated monomer is completely consumed in step A1.

[0051] The above procedure results in an aqueous emulsion in which PTFE is dispersed in a particulate form (aqueous emulsion containing PTFE particles). The concentration of the PTFE particles in the aqueous emulsion is preferably 10 to 45% by mass, more preferably 10 to 30% by mass, and even more preferably 10 to 25% by mass, based on the total amount of the aqueous emulsion. Within the above range, the PTFE particles in the aqueous emulsion can be more easily coagulated, and clouding of the coagulated liquid can be suppressed. The average primary particle size of the PTFE particles is preferably 100 to 500 nm, more preferably 150 to 300 nm. The average primary particle size of the PTFE particles corresponds to D50 measured by a laser scattering particle size distribution analyzer.

[0052] The PTFE in the PTFE particles obtained by the above procedure usually contains TFE units as the main component. The main component means that the content of TFE units relative to the total PTFE units is 99.700 mass% or more, preferably 99.900 mass% or more. The upper limit can be 100 mass%. When PTFE contains units based on a specific monomer, the content of the units based on the specific monomer is preferably 0.005 to 0.150 mass %, more preferably 0.010 to 0.100 mass %, based on the total units of PTFE. When two or more specific monomers are used, the total content of the units based on the specific monomers may be within the above range. When PTFE contains units based on a non-fluorine-based monomer, the content of the units based on the non-fluorine-based monomer is preferably 200 ppm by mass or less, more preferably 1 to 150 ppm by mass, still more preferably 5 to 100 ppm by mass, and particularly preferably 5 to 50 ppm by mass, based on the total units of PTFE. When two or more types of non-fluorine-containing monomers are used, the total content of the units based on the respective non-fluorine-containing monomers may be within the above range.

[0053] <Process A3> Step A3 is a step of adding a nonionic surfactant to the aqueous emulsion obtained in Step A2 in an amount of 10 to 150 mass % relative to the mass of PTFE contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. That is, it is a step of adding a nonionic surfactant to the low-concentration aqueous PTFE dispersion obtained in Step A2 (corresponding to the aqueous emulsion described above), and then concentrating the low-concentration aqueous PTFE dispersion to obtain a high-concentration aqueous PTFE dispersion (corresponding to the aqueous PTFE dispersion described above). By performing Step A3, an aqueous PTFE dispersion having a PTFE particle concentration higher than the PTFE particle concentration of the aqueous dispersion can be obtained. The entire amount of the aqueous emulsion obtained in step A2 may be supplied to step A3, or a portion of the aqueous emulsion obtained in step A2 may be supplied to step A3. In either case, the nonionic surfactant is added in an amount of 10 to 150 mass % based on the mass of PTFE contained in the aqueous emulsion supplied to step A3.

[0054] If the amount of nonionic surfactant added is less than 10% by mass relative to the mass of PTFE contained in the aqueous emulsion supplied to step A3, the low-concentration aqueous PTFE dispersion cannot be concentrated, and a high-concentration aqueous PTFE dispersion cannot be obtained. The amount of nonionic surfactant added is preferably 20% by mass or more, more preferably 21% by mass or more, even more preferably 22% by mass or more, still more preferably 30% by mass or more, and particularly preferably 50% by mass or more, relative to the mass of PTFE contained in the aqueous emulsion supplied to step A3. If the amount of nonionic surfactant added is more than 150% by mass relative to the mass of PTFE contained in the aqueous emulsion used in step A3, the aqueous emulsion will condense, making it impossible to concentrate the low-concentration aqueous PTFE dispersion, or the viscosity of the aqueous PTFE dispersion will increase, resulting in a decrease in the CFT of the coating film that is formed. The amount of nonionic surfactant added is preferably 120% by mass or less, and more preferably 130% by mass or less, relative to the mass of PTFE contained in the aqueous emulsion used in step A3.

[0055] In the following, first, the materials used in step A3 will be described in detail, and then the procedure of step A3 will be described in detail.

[0056] (nonionic surfactants) The nonionic surfactant may be a nonionic surfactant. As the nonionic surfactant, a nonionic surfactant represented by formula (4) or a nonionic surfactant represented by formula (5) is preferred. Formula (4):R 41 -OAH Formula (5): R 51 -C6H4-OBH In the formula, R 41 represents an alkyl group having 8 to 18 carbon atoms. A represents a polyoxyalkylene chain composed of 5 to 20 oxyethylene groups and 0 to 2 oxypropylene groups. In the formula, R 51 represents an alkyl group having 4 to 12 carbon atoms. B represents a polyoxyethylene chain composed of 5 to 20 oxyethylene groups. Furthermore, as the nonionic surfactant, a nonionic surfactant represented by formula (6) is also preferred. Formula (6):R 61 -ODH In the formula, R 61 represents an alkyl group having 8 to 18 carbon atoms. D represents a polyoxyalkylene chain composed of 5 to 20 oxyethylene groups and 0.1 to 3 oxybutylene groups.

[0057] In equation (4), R 41 The number of carbon atoms in the alkyl group represented by the formula (I) is 8 to 18, preferably 10 to 16, and more preferably 12 to 16. When the number of carbon atoms is 18 or less, the PTFE particles are less likely to settle even when the aqueous PTFE dispersion is left standing for a long period of time, and the storage stability is excellent. Furthermore, when the number of carbon atoms is 8 or more, the surface tension of the aqueous PTFE dispersion is low, and the permeability and wettability are excellent. In formula (4), the hydrophilic group A is preferably a polyoxyalkylene chain having 7 to 12 oxyethylene groups and 0 to 2 oxypropylene groups. In particular, when the number of oxypropylene groups in A is 0.5 to 1.5, good defoaming properties are obtained, which is preferred.

[0058] In equation (5), R 51 The alkyl group represented by the formula (I) has 4 to 12 carbon atoms, preferably 6 to 10, and more preferably 8 to 9. When the alkyl group has 4 or more carbon atoms, the surface tension of the aqueous PTFE dispersion is low, and the permeability and wettability are excellent. When the alkyl group has 12 or less carbon atoms, the PTFE particles are less likely to settle even when the aqueous PTFE dispersion is left standing for a long period of time, and the storage stability is excellent. In the formula (5), the number of oxyethylene groups in B, which is a hydrophilic group, is preferably 6 to 16, and more preferably 7 to 12.

[0059] In equation (6), R 61 The number of carbon atoms in the alkyl group represented by the formula (I) is 8 to 18, preferably 10 to 16, and more preferably 12 to 16. When the number of carbon atoms is 18 or less, the PTFE particles are less likely to settle even when the aqueous PTFE dispersion is left standing for a long period of time, and the storage stability is excellent. Furthermore, when the number of carbon atoms is 8 or more, the surface tension of the aqueous PTFE dispersion is low, and the permeability and wettability are excellent. In formula (6), the hydrophilic group D is preferably a polyoxyalkylene chain having 7 to 12 oxyethylene groups and 0.1 to 3 oxybutylene groups. In particular, when the number of oxybutylene groups in D is 0.5 to 2, good defoaming properties are obtained, which is preferred. Furthermore, the number of oxybutylene groups is more preferably 0.7 to 1.7, and even more preferably 0.9 to 1.5. The number of oxyethylene groups is preferably 6 to 15, and more preferably 7 to 12.

[0060] The average molecular weight of the nonionic surfactant represented by formula (4), the average molecular weight of the nonionic surfactant represented by formula (5), and the average molecular weight of the nonionic surfactant represented by formula (6) are each preferably 450 to 800, more preferably 500 to 750, and even more preferably 550 to 700.

[0061] Examples of the nonionic surfactant represented by formula (4) include C 13 H 27 -(OC2H4) 10 -OH, C 12 H 25 -(OC2H4)10 -OH, C 10 H 21 CH(CH3)CH2-(OC2H4)9-OH, C 13 H 27 -(OC2H4)9-OCH(CH3)CH2-OH, C 16 H 33 -(OC2H4) 10 -OH, HC(CH 11 )(C7H 15 )-(OC2H4)9-OH. Commercially available products include the Tergitol (registered trademark) 15S series and Tergitol (registered trademark) TMN series manufactured by Dow Chemical Company, the Genapol (registered trademark) X series manufactured by Clariant, the Leocol (registered trademark) TD series and Lionol (registered trademark) TDL series manufactured by Lion Corporation, and the Newcol (registered trademark) series manufactured by Nippon Nyukazai Co., Ltd. Examples of the nonionic surfactant represented by formula (5) include C8H 17 -C6H4-(OC2H4) 10 -OH, CH 19 -C6H4-(OC2H4) 10 Commercially available products include the Triton (registered trademark) X series manufactured by Dow and the Nikkor (registered trademark) OP series or NP series manufactured by Nikko Chemical Co., Ltd. Examples of the nonionic surfactant represented by formula (6) include C 13 H 27 OCH2CH(C2H5)O(C2H4O)8H, C 10 H 21 CH(CH3)CH2OCH2CH(C2H5)O(C2H4O)8H,C 12 H 25 OCH2CH(C2H5)O(C2H4O)8H, C8H 17 OCH2CH(C2H5)O(C2H4O) 10 H, C 13 H 27 OCH2CH2OCH2CH(C2H5)O(C2H4O)8H,C 10 H 21 CH(CH3)CH2O(C2H4O)9CH2CH(C2H5)OH,C 16 H 33OC2H4OCH(C2H5)CH2O(C2H4O)9H,C 12 H 25 OCH2CH(C2H5)O(C2H4O)8CH2CH(C2H5)OH,C 13 H 27 OCH(CH3)CH(CH3)O(C2H4O)8H, C 12 H 25 OCH(CH3)CH(CH3)O(C2H4O)8H, C 13 H 27 O(CH2)4O(C2H4O)8H, C 12 H 25 Examples include O(CH2)2CH(CH3)O(C2H4O)8H.

[0062] The nonionic surfactant represented by formula (4) and / or the nonionic surfactant represented by formula (5) may be used singly or in combination of two or more. The nonionic surfactants represented by formula (6) may be used singly or in combination of two or more. Furthermore, the nonionic surfactant represented by formula (6) can be used in combination with the nonionic surfactant represented by formula (4) or the nonionic surfactant represented by formula (5). Note that a nonionic surfactant is a mixture of multiple substances with different molecular structures, and the number of carbon atoms in the alkyl group in the nonionic surfactant, and the number of oxyethylene groups, oxypropylene groups, and oxybutylene groups in the polyoxyalkylene chain are treated as average values, and each value is not limited to an integer.

[0063] (Step A3) The aqueous emulsion is preferably concentrated by a phase separation method. Polymerization using fluorosurfactants or hydrocarbon-based surfactants leaves undesirable water-soluble fluorine compounds behind. For example, when using hydrocarbon-containing surfactants, the by-products are water-soluble fluorine oligomers with structures similar to perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), substances of environmental concern. Therefore, it is desirable to remove these by-products to the detection limit. Therefore, a removal process using ion exchange resins is required. To remove these by-products to the detection limit, the solution must be concentrated, then diluted, and then reconcentrated, followed by discharging the undesirable water-soluble fluorine compounds into the supernatant. This is a very complicated process, requiring new capital investment and complex operational management. Furthermore, aqueous emulsions obtained by polymerization without fluorosurfactants or hydrocarbon-based surfactants do not contain undesirable water-soluble fluorine compounds. However, when concentrated by electrophoresis, the ionic species from the remaining initiator residue increase the current and easily generate heat, making operational management complicated and limiting the concentration yield. By using the phase separation method, new capital investment, complicated processes, and complex operation management can be avoided.

[0064] The phase separation method is a method in which the aqueous emulsion is heated and then allowed to stand for a certain period of time to precipitate the PTFE particles. Specifically, this method involves adding 10 to 150% by mass (preferably 21 to 150% by mass, more preferably 22 to 150% by mass) of a nonionic surfactant to the aqueous emulsion relative to the mass of PTFE contained in the aqueous emulsion used in step A3, heating the resulting aqueous emulsion to 50 to 100°C (preferably 60 to 90°C), and allowing it to stand for 1 to 10 hours (preferably 1 to 5 hours) to cause phase separation of the aqueous emulsion, and recovering the lower phase in which the PTFE particles are concentrated as a PTFE aqueous dispersion.

[0065] It is preferable that the aqueous emulsion before concentration is not subjected to a step using an ion exchange resin, an adsorbent, or the like.

[0066] Before concentration, the aqueous emulsion is preferably adjusted to a pH of 9 to 12, more preferably 10 to 12, with an alkaline ion species such as ammonia.

[0067] Between step A2 and step A3, or during step A3, it is preferable to add an anionic surfactant (e.g., ammonium laurate, triethanolamine laurate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, etc.) to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of PTFE contained in the aqueous emulsion to be subjected to step A3. When the amount added is 1000 ppm by mass or more, the amount of aggregates during concentration can be further reduced. Also, when the amount added is 5000 ppm by mass or less, coloration of a coating film obtained using the PTFE aqueous dispersion after processing and baking is suppressed. The amount of the anionic surfactant added to the aqueous emulsion is more preferably 1000 to 3000 ppm by mass, even more preferably 1000 to 2500 ppm by mass, and particularly preferably 1500 to 2500 ppm by mass.

[0068] The concentration of PTFE particles in the aqueous PTFE dispersion obtained by the above procedure is preferably 15 to 75 mass% (specific gravity: 1.09 to 1.74), more preferably 20 to 75 mass% (specific gravity: 1.13 to 1.74), even more preferably 30 to 75 mass% (specific gravity: 1.20 to 1.74), and particularly preferably 40 to 75 mass% (specific gravity: 1.29 to 1.74), relative to the total mass of the aqueous PTFE dispersion. Aqueous PTFE dispersions with a PTFE particle concentration of 15 to 70 mass% are preferably used for impregnating cloth or string made from woven fibers such as glass fiber with the aqueous PTFE dispersion, for mixing with inorganic powders or plastic powders, and for adding small amounts to paints. In particular, when the PTFE aqueous dispersion is used for coating or for processing into PTFE fibers, the concentration of the PTFE particles is preferably 50 to 75 mass% (specific gravity: 1.39 to 1.74), more preferably 52 to 75 mass% (specific gravity: 1.41 to 1.74), even more preferably 55 to 75 mass% (specific gravity: 1.45 to 1.74), and particularly preferably 55 to 70 mass% (specific gravity: 1.45 to 1.66). The pH of the aqueous PTFE dispersion is preferably 2-13, more preferably 3-11.

[0069] The PTFE constituting the PTFE particles in the aqueous PTFE dispersion includes not only TFE homopolymers but also so-called modified PTFEs containing polymerization units based on copolymerization components copolymerizable with TFE, such as halogenated ethylenes such as chlorotrifluoroethylene, halogenated propylenes such as hexafluoropropylene, and fluorovinyl ethers such as perfluoro(alkyl vinyl ether), in amounts so small that melt processing is substantially impossible.

[0070] The surface tension of the aqueous PTFE dispersion is preferably 24 to 35 mN / m, more preferably 25 to 32 mN / m. When the surface tension is 24 mN / m or more, the defoaming property is excellent, and when it is 35 mN / m or less, repellency is unlikely to occur.

[0071] The PTFE aqueous dispersion may contain one or more of a fluorine-free emulsifier, various leveling agents, preservatives, colorants, fillers, organic solvents, aqueous ammonia, and other known components. Furthermore, when the aqueous PTFE dispersion contains a viscosity modifier such as polyethylene oxide or polyurethane, the aqueous PTFE dispersion has excellent mechanical stability.

[0072] From the viewpoint of ease of application, the viscosity of the aqueous PTFE dispersion is preferably 300 mPa·s or less at 23° C., more preferably 3 to 100 mPa·s, and even more preferably 5 to 50 mPa·s. The thickening temperature of the aqueous PTFE dispersion is preferably 30 to 60° C., more preferably 35 to 55° C., and even more preferably 40 to 50° C. When the thickening temperature is within the above range, viscosity changes due to variations in application temperature are unlikely to occur, and repellency is unlikely to occur.

[0073] <<Second embodiment>> A second embodiment of the method for producing an aqueous PTFE dispersion includes the following three steps. Step B1: A step of mixing at least one nucleating additive selected from the group consisting of polyalkylene oxide compounds and hydrocarbon-containing surfactants with an oxidizing agent in an aqueous medium to obtain a solution 2. Step B2: A step of polymerizing TFE in the solution 2 obtained in step B1 without adding a surfactant to the solution 2 to obtain an aqueous emulsion containing PTFE particles. Step B3: A step of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150 mass % relative to the mass of PTFE contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. In the second embodiment, the amount of the nucleation additive used is 100 mass ppm or less relative to the amount of TFE supplied. The present invention will be described in detail below, taking the above preferred embodiment as an example.

[0074] <Process B1> Step B1 is a step of mixing at least one nucleating additive selected from the group consisting of polyalkylene oxide compounds and hydrocarbon-containing surfactants with an oxidizing agent in an aqueous medium to obtain solution 2. In the following, first, the materials used in step B1 will be described in detail, and then the procedure of step B1 will be described in detail.

[0075] (Polyalkylene oxide compound) The polyalkylene oxide compound is a compound that forms nuclei (seeds) during the polymerization of TFE, that is, it corresponds to a nucleation additive. The polyalkylene oxide compound is a compound containing a polyalkylene oxide chain, and examples of the polyalkylene oxide chain include a polymethylene oxide chain, a polyethylene oxide chain, a polypropylene oxide chain, and a polytetramethylene oxide chain.

[0076] The polyalkylene oxide compound preferably has a surface tension in water at a concentration of 1000 ppm greater than about 40 dynes / cm, more preferably greater than about 42 dynes / cm, and even more preferably greater than about 45 dynes / cm, and preferably less than or equal to about 73 dynes / cm.

[0077] The number average molecular weight of the polyalkylene oxide compound is preferably 50 to 2,000, more preferably 100 to 1,500, and even more preferably 150 to 1,300.

[0078] The polyalkylene oxide compound is preferably a compound represented by formula (2) in that the formation of fluorine-based oligomers is further suppressed. Formula (2) R 21 -(OL 2 ) n -OR 22 In formula (2), R 21 and R 22 each independently represents a hydrogen atom, an alkyl group, an acryloyl group, or a methacryloyl group. L 2 represents an alkylene group having 1 to 4 carbon atoms, which may be linear or branched. n represents 1 to 50.

[0079] Examples of polyalkylene oxide compounds include polyethylene glycol, polyethylene glycol acrylate, polyethylene glycol methacrylate, polyethylene glycol methyl ether, polyethylene glycol dimethyl ether, polyethylene glycol butyl ether, polypropylene glycol, polypropylene glycol acrylate, polypropylene glycol methacrylate, polypropylene glycol dimethacrylate, polypropylene glycol methyl ether, polypropylene glycol dimethyl ether, polypropylene glycol butyl ether, polypropylene glycol dimethacrylate, and polytetramethylene glycol.

[0080] The polyalkylene oxide compounds may be used singly or in combination of two or more.

[0081] (hydrocarbon-containing surfactants) Hydrocarbon-containing surfactant is a surfactant that contains hydrocarbon.More specifically, at least some of the monovalent substituents on carbon atom are hydrogen atoms, and can also be substituted with halogen atoms such as fluorine atoms and chlorine atoms.In preferred hydrocarbon-containing surfactants, at least 75%, preferably at least 85%, more preferably at least 95% of the monovalent substituents on carbon atom are hydrogen atoms.

[0082] Hydrocarbon-containing surfactants include, for example, hydrocarbon surfactants and siloxane surfactants. A hydrocarbon surfactant refers to a surfactant that does not contain silicon atoms and in which 100% of the monovalent substituents substituted on carbon atoms are hydrogen atoms, and therefore does not contain halogen atoms such as chlorine atoms and fluorine atoms. By siloxane surfactant is meant a hydrocarbon-containing surfactant having a hydrophobic group that includes a siloxane backbone containing multiple siloxane units.

[0083] The hydrocarbon surfactant is preferably an anionic hydrocarbon surfactant, which means a hydrocarbon surfactant 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 portion such as an alkyl group as a hydrophobic portion. An example of an anionic hydrocarbon surfactant is a highly branched C10 tertiary carboxylic acid supplied as Versatic® 10 by Resolution Performance Products. Other examples of anionic hydrocarbon surfactants include the sodium linear alkyl polyether sulfonates supplied by BASF as their Avanel® S series.

[0084] The anionic hydrocarbon surfactant is also preferably an anionic hydrocarbon surfactant represented by formula (7). Equation (7) R 71 -L 7 -M R 71 represents an alkyl group. The alkyl group may be linear, branched, or cyclic, and is preferably linear. The alkyl group may have 6 to 20 carbon atoms, for example. L 7 is -ArSO3 - , -SO3 - , -SO4 - , -PO3 - , -PO4 - , or -COO - Here, Ar represents an arylene group. M represents a monovalent cation. Examples of the monovalent cation include H + , Na + , K. + , NH4 + Examples include: An example of the anionic hydrocarbon surfactant represented by formula (7) is sodium dodecyl sulfate.

[0085] Another example of an anionic hydrocarbon surfactant is the sulfosuccinate surfactant Lankropol® K8300, available from Akzo Nobel Surface Chemistry LLC.

[0086] Nonionic hydrocarbon surfactants are also preferred as hydrocarbon surfactants. Nonionic hydrocarbon surfactants do not have charged groups, but have a hydrophobic portion, which is often a long-chain hydrocarbon. The hydrophilic portion of nonionic hydrocarbon surfactants includes water-soluble functional groups such as polyethylene oxide chains obtained from the polymerization of ethylene oxide. Nonionic hydrocarbon surfactants include block copolymers having various types of polyalkylene oxide blocks, such as polyethylene oxide and polypropylene oxide.

[0087] Suitable nonionic hydrocarbon surfactants include those described in paragraphs 0043 to 0052 of JP-A No. 2016-537499.

[0088] Suitable siloxane surfactants include those described in US Pat. Nos. 6,841,616 (Wille et al.) and 7,977,438 (Brothers et al.).

[0089] (oxidizing agent) The oxidizing agent may, for example, be hydrogen peroxide or a polymerization initiator. Examples of the polymerization initiator include the compounds exemplified as the polymerization initiator described in the above step A1. The polymerization initiator is preferably a persulfate, more preferably ammonium persulfate or potassium persulfate.

[0090] The aqueous medium may be the aqueous medium used in step A1.

[0091] (Step B1 procedure) In step B1, a nucleating additive and an oxidizing agent are mixed in an aqueous medium to obtain solution 2. In other words, in this step, the nucleating additive is exposed to an oxidizing agent in an aqueous medium. When a nucleating additive and an oxidizing agent are mixed in an aqueous medium, a solution in which lipophilic nucleation sites are dispersed in the aqueous medium is obtained. More specifically, when a nucleating additive such as a polyalkylene oxide compound and a hydrocarbon-containing surfactant is mixed with an oxidizing agent, the hydrophilic portions of the nucleating additive are decomposed, and the hydrophobic portions of the nucleating additive become lipophilic nucleation sites. The lipophilic nucleation sites are dispersed in the aqueous medium, and the fluoropolymer can be finely dispersed at these sites. Because lipophilic nucleation sites have excellent affinity for TFE, polymerization of TFE is likely to proceed in Solution 2 containing lipophilic nucleation sites. In other words, lipophilic nucleation sites can provide a hydrophobic environment for TFE polymerization.

[0092] The amount of the nucleation additive used is 100 ppm by mass or less, preferably 1 to 50 ppm by mass or less, and more preferably 1 to 25 ppm by mass, relative to the amount of TFE supplied (amount of TFE used) used in step B2 described below. The amount of the oxidizing agent used is preferably from 0.5 to 100 ppm by mass, more preferably from 0.5 to 50 ppm by mass, based on the total mass of the aqueous medium. The temperature when mixing the nucleation additive and the oxidizing agent is preferably 20 to 120°C, more preferably 40 to 120°C. The mixing time when mixing the nucleation additive and the oxidizing agent is preferably 0.05 to 1.0 hour.

[0093] Preferably, a water-soluble inorganic salt is added to the aqueous medium before or during mixing of the nucleating additive and the oxidizing agent, which is useful for increasing the number of fluoropolymer particles formed during nucleation. The amount of the water-soluble inorganic salt used is preferably from 0.01 to 80 ppm by mass, more preferably from 1 to 50 ppm by mass, based on the total mass of the aqueous medium. Examples of water-soluble inorganic salts include sodium sulfite, sodium hydrogen sulfite, sodium chloride, potassium sulfite, potassium hydrogen sulfite, potassium carbonate, ammonium oxalate, sodium tetraborate, sodium acetate, ammonium carbonate, ammonium dihydrogen phosphate, and diammonium phosphate. Sulfites are preferred, and sodium sulfite and ammonium sulfite are more preferred.

[0094] <Process B2> Step B2 is a step of polymerizing TFE in solution 2 obtained in step B1 without adding substantially any surfactant to solution 2, to obtain an aqueous emulsion containing PTFE particles. This step is carried out in the same manner as in Step A2 above, except that Solution 2 is used instead of Solution 1, and therefore a description thereof will be omitted. The various properties of the PTFE obtained in step B2 are as explained in the various properties of the PTFE obtained in step A2. The above phrase "substantially not added" means that no surfactant is added, or if added, the amount of surfactant added is 200 mass ppm or less relative to the total mass of Solution 2. There is no particular lower limit, but 0 mass ppm is preferred. In other words, it is preferred that no surfactant is added to Solution 2 in Step B2.

[0095] <Process B3> Step 3 is a step of adding a nonionic surfactant to the aqueous emulsion obtained in step B2 in an amount of 10 to 150 mass % relative to the mass of PTFE contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. That is, this is a step of adding a nonionic surfactant to the low-concentration aqueous PTFE dispersion obtained in step B2 (corresponding to the aqueous emulsion described above), and then concentrating the low-concentration aqueous PTFE dispersion to obtain a high-concentration aqueous PTFE dispersion (corresponding to the aqueous PTFE dispersion described above). By carrying out step B3, an aqueous PTFE dispersion having a PTFE particle concentration higher than that of the aqueous dispersion can be obtained. The entire amount of the aqueous emulsion obtained in step B2 may be supplied to step B3, or a portion of the aqueous emulsion obtained in step B2 may be supplied to step B3. In either case, the nonionic surfactant is added in an amount of 10 to 150 mass % based on the mass of PTFE contained in the aqueous emulsion supplied to step B3. This step is carried out in the same manner as in step A3 above, except that the aqueous emulsion obtained in step B2 is used instead of the aqueous emulsion obtained in step A2, and therefore a description thereof will be omitted. In step B3 as well, the aqueous emulsion is preferably concentrated using a phase separation method. The various properties of the aqueous PTFE dispersion obtained in step B3 are as described above for the various properties of the aqueous PTFE dispersion obtained in step A3.

[0096] Between step B2 and step B3, it is preferable to add an anionic surfactant (e.g., ammonium laurate, triethanolamine laurate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, etc.) to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of PTFE contained in the aqueous emulsion to be subjected to step B3. When the amount added is 1000 ppm by mass or more, the amount of aggregates during concentration can be further reduced. Also, when the amount added is 5000 ppm by mass or less, coloration of a coating film obtained using the PTFE aqueous dispersion after processing and baking is suppressed. The amount of the anionic surfactant added to the aqueous emulsion is more preferably 1000 to 3000 ppm by mass, even more preferably 1000 to 2500 ppm by mass, and particularly preferably 1500 to 2500 ppm by mass.

[0097] <<Third embodiment>> A third embodiment of the method for producing an aqueous PTFE dispersion includes the following two steps. Step C1: A step of polymerizing TEF in an aqueous medium in the presence of a polyalkylene oxide compound without adding a surfactant to obtain an aqueous emulsion containing PTFE particles. Step C2: A step of adding a nonionic surfactant to the aqueous emulsion in an amount of 10 to 150% by mass relative to the mass of PTFE contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. In the third embodiment, the amount of the polyalkylene oxide compound used is 100 mass ppm or less relative to the amount of TFE supplied.

[0098] <Process C1> Step C1 is a step of polymerizing TEF in an aqueous medium in the presence of a polyalkylene oxide compound without adding a surfactant to obtain an aqueous emulsion containing PTFE particles. First, the materials used in step C1 will be described in detail below.

[0099] (Polyalkylene oxide compound) The polyalkylene oxide compound is a compound containing a polyalkylene oxide chain, and examples of the polyalkylene oxide chain include a polymethylene oxide chain, a polyethylene oxide chain, a polypropylene oxide chain, and a polytetramethylene oxide chain.

[0100] The polyalkylene oxide compound preferably has a surface tension in water at a concentration of 1000 ppm by weight greater than about 40 dynes / cm, more preferably greater than about 42 dynes / cm, and even more preferably greater than about 45 dynes / cm, and preferably less than or equal to about 73 dynes / cm.

[0101] The number average molecular weight of the polyalkylene oxide compound is preferably 300 to 10,000,000, more preferably 400 to 10,000,000, even more preferably 400 to 5,000,000, still more preferably 3,000 to 5,000,000, and particularly preferably 200,000 to 5,000,000. When the number average molecular weight of the polyalkylene oxide compound is within the above range, the polymerization rate is faster and the emulsion stability is also superior.

[0102] Examples of polyalkylene oxide compounds include polyethylene glycol, polyethylene glycol acrylate, polyethylene glycol methacrylate, polyethylene glycol methyl ether, polyethylene glycol dimethyl ether, polyethylene glycol butyl ether, polypropylene glycol, polypropylene glycol acrylate, polypropylene glycol methacrylate, polypropylene glycol dimethacrylate, polypropylene glycol methyl ether, polypropylene glycol dimethyl ether, polypropylene glycol butyl ether, polypropylene glycol dimethacrylate, and polytetramethylene glycol.

[0103] The polyalkylene oxide compounds may be used singly or in combination of two or more.

[0104] The amount of the polyalkylene oxide compound used is 100 ppm by mass or less, preferably 1 to 80 ppm by mass, and more preferably 1 to 50 ppm by mass, relative to the amount of TFE supplied (amount of TFE used). When the number average molecular weight of the polyalkylene oxide compound is high, for example, 200,000 to 5,000,000, the amount of the polyalkylene oxide compound used is preferably 1 to 50 ppm by mass, more preferably 1 to 25 ppm by mass, relative to the amount of TFE supplied (amount of TFE used).

[0105] The aqueous medium may be the aqueous medium used in step A1.

[0106] The polymerization of TFE in this step is carried out in the same manner as in the above-mentioned step A2, except that a solution in which a polyalkylene oxide compound is mixed in an aqueous medium is used instead of solution 1, and therefore a description thereof will be omitted. The ether oxygen of polyalkylene oxide interacts with the aqueous medium, and is thought to function as a surfactant between the TFE and the aqueous medium during TFE polymerization. As a result, TFE polymerization proceeds smoothly and chain transfer is suppressed. The various properties of the PTFE obtained in step C1 are as explained in the various properties of the PTFE obtained in step A2.

[0107] <Process C2> Step C2 is a step of adding a nonionic surfactant to the aqueous emulsion obtained in Step C1 in an amount of 10 to 150 mass % relative to the mass of PTFE contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous PTFE dispersion. That is, this is a step of adding a nonionic surfactant to the low-concentration aqueous PTFE dispersion obtained in Step C1 (corresponding to the aqueous emulsion described above), and then concentrating the low-concentration aqueous PTFE dispersion to obtain a high-concentration aqueous PTFE dispersion (corresponding to the aqueous PTFE dispersion described above). By performing Step C2, an aqueous PTFE dispersion exhibiting a PTFE particle concentration higher than the PTFE particle concentration of the aqueous dispersion can be obtained. The entire amount of the aqueous emulsion obtained in step C1 may be supplied to step C2, or a portion of the aqueous emulsion obtained in step C1 may be supplied to step C2. In either case, the nonionic surfactant is added in an amount of 10 to 150 mass % based on the mass of PTFE contained in the aqueous emulsion supplied to step C2. This step is carried out in the same manner as in step A3 above, except that the aqueous emulsion obtained in step C1 is used instead of the aqueous emulsion obtained in step A2, and therefore a description thereof will be omitted. In step C2 as well, the aqueous emulsion is preferably concentrated using a phase separation method. The various properties of the aqueous PTFE dispersion obtained in step C2 are as described above for the various properties of the aqueous PTFE dispersion obtained in step A3.

[0108] Between step C2 and step C3, or during step C3, it is preferable to add an anionic surfactant (e.g., ammonium laurate, triethanolamine laurate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, etc.) to the aqueous emulsion in an amount of 1,000 to 5,000 ppm by mass relative to the mass of PTFE contained in the aqueous emulsion. When the addition amount is 1000 mass ppm or more, aggregates during concentration can be further reduced. Also, when the addition amount is 5000 mass ppm or less, coloring after processing and firing of the coating film obtained using the PTFE aqueous dispersion is suppressed. The addition amount of the anionic surfactant to the aqueous emulsion is more preferably 1000 to 3000 mass ppm, still more preferably 1000 to 2500 mass ppm, and particularly preferably 1500 to 2500 mass ppm.

[0109] <<PTFE Products>> PTFE products mean films, sheets, fibers mainly composed of PTFE, heat-resistant articles having a PTFE coating film, and articles containing PTFE as a sub-component, which are obtained using a PTFE aqueous dispersion.

[0110] PTFE products include, for example, packings made by impregnating a PTFE aqueous dispersion into a substrate made of woven or braided cloth of glass fiber, aramid fiber, carbon fiber, or other synthetic or natural fibers, and then drying the resulting material; heat-resistant conveyor belts, architectural membrane structure sheets, packing, and printed circuit board materials made by impregnating a substrate made of woven or braided cloth of heat-resistant fibers such as glass fiber, aramid fiber, or carbon fiber with a PTFE aqueous dispersion and then baking the resulting material at a temperature above the melting point of PTFE; kitchen equipment such as frying pans and electric kettles made by coating a metal plate such as aluminum or stainless steel with a PTFE aqueous dispersion containing a pigment or heat-resistant resin, and then baking the resulting material; binders made by kneading a PTFE aqueous dispersion with powdered active materials for batteries such as carbon, manganese dioxide, or nickel hydroxide; and molding materials and components made by mixing a PTFE aqueous dispersion to prevent dripping during combustion of plastic moldings such as polycarbonate and ABS resin. Forms (anti-dripping agents); powders with reduced dust generation obtained by mixing PTFE aqueous dispersions with chemical fertilizers, lime, incineration ash, etc.; sliding materials such as oil-free bearings, in which a paste made by mixing fillers such as lead, zinc, or carbon powder with PTFE aqueous dispersions is coated on a porous material; PTFE fibers, in which a thickener such as viscose is added to PTFE aqueous dispersions and pressure-spun into a coagulation bath followed by calcination; ultra-thin PTFE sheets, obtained by coating a heat-resistant sheet substrate such as aluminum or stainless steel with PTFE aqueous dispersions, calcining the PTFE layer, and then peeling off the PTFE layer; thin films obtained by casting PTFE aqueous dispersions and having heat resistance, high insulation properties, and low dielectric loss tangent, such as coil insulation, interlayer insulation, and electrical insulation materials used in motors, transformers, relays, and switches; paints, resins, and rubber materials to which PTFE aqueous dispersions have been added to improve their lubricity and stain resistance.

[0111] The PTFE product is obtained by coating or mixing the PTFE aqueous dispersion, followed by drying or heat treatment at a temperature of room temperature to 420° C. The temperature for the drying or heat treatment is preferably 50 to 400° C., more preferably 100 to 395° C. The PTFE content in the PTFE product varies depending on the application, but is preferably 0.01 to 100 mass %, more preferably 0.1 to 100 mass %, and even more preferably 1 to 100 mass %. [Example]

[0112] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto. Examples 1 to 10 all fall under the category of examples.

[0113] The various measurement and evaluation methods are as follows. (A) Average primary particle size of PTFE particles (hereinafter also referred to as "PPS") The PTFE aqueous dispersion was used as a sample and was measured using a laser scattering particle size distribution analyzer (manufactured by Horiba Ltd., trade name "LA-920"). (B) Standard Specific Gravity (hereinafter referred to as "SSG") Measurements were performed in accordance with ASTM D4895-04. A 12.0 g sample (PTFE powder) was weighed and placed in a cylindrical mold with an inner diameter of 28.6 mm at 34.5 MPa for 2 minutes. It was then placed in a 290°C oven and heated at 120°C / hr. After being held at 380°C for 30 minutes, the temperature was lowered at 60°C / hr and held at 294°C for 24 minutes. The sample was then placed 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. A smaller SSG value indicates a higher molecular weight. (C) PTFE concentration and surfactant concentration Approximately 7 g of the PTFE aqueous dispersion was placed in an aluminum dish (mass W0) and weighed (mass W1). The PTFE concentration and surfactant concentration (ratio of surfactant to TFE supply amount) were calculated using the following formula from the mass after drying at 120°C for 1 hour (mass W2) and the mass after drying at 380°C for 35 minutes (mass W3). PTFE concentration (mass%)=[(W3-W0) / (W1-W0)]×100 Surfactant concentration (mass% / PTFE) = [(W2-W3) / (W3-W0)] x 100 (D) Viscosity The viscosity of the PTFE aqueous dispersion was measured using a Brookfield viscometer with a #1 spindle at a liquid temperature of 23°C and 60 rpm. (E) pH The pH of the PTFE aqueous dispersion was measured by the glass electrode method. (F)Surface tension The surface tension of the PTFE aqueous dispersion was measured by the ring method using a platinum wire ring. (G) CFT (Crack Thickness Limit) Using an applicator with a continuously variable coating thickness up to 200 μm, the PTFE aqueous dispersion was applied to a 0.5 mm thick aluminum plate, dried at 120°C for 10 minutes, and then baked at 380°C for 10 minutes. The PTFE coating film was observed, and the thickness of the tip of the crack that had occurred was measured at five points using a permscope, and the average value (μm) was calculated and used as the CFT (crack critical film thickness). (H) Measurement of fluorine-based oligomers The sample (PTFE powder) was subjected to Soxhlet extraction with ethanol for 5 hours, and the ethanol extract was then analyzed by LC / MS. Using perfluorooctylsulfonic acid and perfluorooctanoic acid as standards, oligomers with CF2 chains and carbon numbers of 6 to 34 were mainly quantified. Cases where the presence of oligomers was confirmed were recorded as "present," and cases where they were not confirmed were recorded as "absent." For LC / MS analysis, an Agilent 1260 series HPLC / 6460MS was used, and the column used was an Imtakt Cadenza CD-C18 2mmφ×100mm 3μm particle size. A gradient of ammonium acetate aqueous solution and methanol was applied during the measurement.

[0114] (Example 1) [Process A1] A 100 L stainless steel autoclave was charged with paraffin wax (1500 g) and deionized water (60 L). After replacing the autoclave with nitrogen, the pressure was reduced and i-butyl methacrylate (i-BMA) (0.25 g) and deionized water (0.5 L) were poured into the autoclave. Next, the pressure inside the autoclave was reduced to below atmospheric pressure, and the solution inside the autoclave was heated to 75°C while being stirred. After that, a solution of ammonium persulfate (0.055 g) as a polymerization initiator dissolved in deionized water (1 L) was injected into the autoclave, and i-butyl methacrylate was polymerized.

[0115] [Process A2] After 20 minutes, the pressure was increased to 1.96 MPa with TFE, and a solution of ammonium persulfate (0.54 g) and disuccinic acid peroxide (concentration 80% by mass, remainder water) (53 g) dissolved in warm water (1 L) at approximately 70°C was injected into the autoclave. After the internal pressure in the autoclave had dropped to 1.89 MPa, TFE was added to maintain the pressure at 1.96 MPa, and polymerization of TFE was allowed to proceed. When the amount of TFE added reached 9 kg, the reaction was terminated and the TFE in the autoclave was released into the atmosphere. The polymerization time was 88 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 12 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.206 μm (206 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the resulting PTFE powder was 2.202. Furthermore, no by-product fluorine-based oligomers were found in the obtained PTFE powder.

[0116] [Process A3] Approximately 5 kg of the aqueous emulsion obtained in step A2 was dissolved in a nonionic surfactant (a) (Newcol 1308FA, manufactured by Nippon Nyukazai Co., Ltd.) at a ratio of 100% by mass relative to the PTFE mass (630 g) contained in the aqueous emulsion, ammonium laurate at a ratio of 0.17% by mass relative to the PTFE mass contained in the aqueous emulsion, and triethanolamine lauryl sulfate at a ratio of 0.06% by mass relative to the PTFE mass contained in the aqueous emulsion. The pH was adjusted to 10.4 with aqueous ammonia, the solution was transferred to a container equipped with a bottom tap, and kept in an oven at 80°C for 150 minutes. The solution was then cooled to room temperature, and after 2 hours, the concentrated lower phase was phase-separated and discharged when the container surface temperature reached 30°C. An aqueous PTFE dispersion was obtained, with a PTFE concentration of 67.5% by mass and a nonionic surfactant (a) concentration of 2.6% by mass relative to the PTFE mass contained in the aqueous emulsion. To this PTFE aqueous dispersion, 0.05% by mass of PEO relative to the mass of PTFE contained in the aqueous emulsion, water, and aqueous ammonia were added, and nonionic surfactant (a) was added so that the content of nonionic surfactant (a) relative to the mass of PTFE contained in the aqueous emulsion was 4.8% by mass, resulting in a PTFE aqueous dispersion with a PTFE concentration of 60.9% by mass. The viscosity of the resulting PTFE aqueous dispersion at 23°C was 24 mPa·s, pH = 10.5, and surface tension 30 (mN / m). The average primary particle size of the PTFE particles in the PTFE aqueous dispersion was the same as the average primary particle size of the PTFE particles measured in the aqueous emulsion.

[0117] This aqueous PTFE dispersion was used to determine the crack limit film thickness. The evaluation results of the resulting aqueous PTFE dispersion are shown in Table 1. The recovery rate was determined by the following procedure. The mass of PTFE contained in the aqueous emulsion recovered from the concentrated lower phase after phase separation was measured and divided by the mass of PTFE contained in the aqueous emulsion charged before the start of concentration. Similarly, 5 kg of the aqueous emulsion obtained in step A2 was concentrated with the nonionic surfactant (a) at a ratio of 22 mass % relative to the mass of PTFE contained in the aqueous emulsion charged before the start of concentration, and the same preparation was carried out, and the results are shown in Example 2. Furthermore, the nonionic surfactant (a) was concentrated at a ratio of 150 mass % relative to the mass of PTFE contained in the aqueous emulsion charged before the start of concentration, and the results are shown in Example 3.

[0118] [Process A2'] The same procedure as in [Step A2] was carried out except that 1 mg of hydroquinone was added before the introduction of TFE, the mixture was stirred for 5 minutes, and then the pressure was increased to 1.96 MPa with TFE. The polymerization time was 91 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 12 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.204 μm (204 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the resulting PTFE powder was 2.204. Furthermore, no by-product fluorine-based oligomers were detected in the resulting PTFE powder.

[0119] [Process A3'] The same procedure as in [Step A3] was carried out for approximately 5 kg of the aqueous emulsion obtained in Step A2', except that nonionic surfactant (b) (Tergitol TMN-100X, manufactured by The Dow Chemical Company) was used in place of nonionic surfactant (a) in a proportion of 40 mass% relative to the mass of PTFE (597 g) contained in the aqueous emulsion. An aqueous PTFE dispersion was obtained with a PTFE concentration of 69.2% by mass and a nonionic surfactant (b) concentration of 3.2% by mass relative to the mass of PTFE contained in the aqueous emulsion. To this aqueous PTFE dispersion, 0.05% by mass of PEO relative to the mass of PTFE contained in the aqueous emulsion, water, and aqueous ammonia were added, and nonionic surfactant (b) was added so that the content of nonionic surfactant (b) relative to the mass of PTFE contained in the aqueous emulsion was 4.8% by mass, thereby preparing an aqueous PTFE dispersion with a PTFE concentration of 60.7% by mass. The viscosity of the resulting aqueous PTFE dispersion at 23°C was 22 mPa·s, pH = 10.2, and surface tension 29 (mN / m). The average primary particle size of PTFE particles in the aqueous PTFE dispersion was the same as the average primary particle size of PTFE particles measured in the aqueous emulsion.

[0120] The same procedure as in [Step A3] was carried out, and the crack limit film thickness and recovery rate were determined, as shown in Example 9 in Table 1.

[0121] (Example 4) [Process B1] Paraffin wax (1500 g) and deionized water (60 L) were charged into a 100 L stainless steel autoclave. After replacing the autoclave with nitrogen, the pressure was reduced and PEG1000 (number average molecular weight: 900 to 1100, polyethylene glycol) (0.098 g) and deionized water (1 L) were poured into the autoclave. Next, the pressure inside the autoclave was reduced to below atmospheric pressure, and the solution inside the autoclave was heated to 75°C while being stirred. After that, a solution of ammonium persulfate (0.11 g) as an oxidizing agent dissolved in deionized water (1 L) was poured into the autoclave.

[0122] [Process B2] After 10 minutes, the pressure was increased to 1.96 MPa with TFE, and a solution of ammonium persulfate (0.54 g) and disuccinic acid peroxide (concentration 80% by mass, remainder water) (53 g) dissolved in warm water (1 L) at approximately 70°C was injected into the autoclave. After the internal pressure in the autoclave had dropped to 1.89 MPa, TFE was added to maintain the pressure at 1.96 MPa, and polymerization of TFE was allowed to proceed. When the amount of TFE added reached 9 kg, the reaction was terminated and the TFE in the autoclave was released into the atmosphere. The polymerization time was 89 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 12 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.192 μm (192 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the resulting PTFE powder was 2.209. Furthermore, no by-product fluorine-based oligomers were found in the obtained PTFE powder.

[0123] [Process B3] The aqueous emulsion obtained in step B2 was concentrated in the same manner as in Example 1 to prepare an aqueous PTFE dispersion having a PTFE concentration of 60.5% by mass. The viscosity of the obtained aqueous PTFE dispersion at 23°C was 23 mPa·s, pH = 10.2, and surface tension was 31 (mN / m). The average primary particle size of the PTFE particles in the aqueous PTFE dispersion was the same as the average primary particle size of the PTFE particles measured in the aqueous emulsion. The evaluation results are shown in Example 4 in Table 1.

[0124] (Example 5) [Process C1] Paraffin wax (1500 g) and deionized water (60 L) were charged into a 100 L stainless steel autoclave. After replacing the autoclave with nitrogen, the pressure was reduced and polyethylene glycol 4,000,000 (average molecular weight: 3.6 million to 4 million) (0.087 g) and deionized water (1 L) were poured into the autoclave. Next, the pressure inside the autoclave was reduced to below atmospheric pressure, and the solution inside the autoclave was heated to 75°C while being stirred. The autoclave was pressurized to 1.96 MPa with TFE, and a solution prepared by dissolving ammonium persulfate (0.54 g) and disuccinic acid peroxide (concentration 80% by mass, remainder water) (53 g) in warm water (1 L) at approximately 70°C was injected into the autoclave. After the internal pressure in the autoclave had dropped to 1.89 MPa, TFE was added to maintain the pressure at 1.96 MPa, and polymerization of TFE was allowed to proceed. When the amount of TFE added reached 9 kg, the reaction was terminated and the TFE in the autoclave was released into the atmosphere. The polymerization time was 89 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 13 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.200 μm (200 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The resulting PTFE powder had an SSG of 2.199. Furthermore, no by-product fluorine-based oligomers were found in the obtained PTFE powder.

[0125] [Process C2] The aqueous emulsion obtained in step C1 was concentrated in the same manner as in Example 1 to prepare an aqueous PTFE dispersion with a PTFE concentration of 60.8 mass%. The viscosity of the obtained aqueous PTFE dispersion at 23°C was 23 mPa·s, pH = 10.5, and surface tension was 30 (mN / m). The average primary particle size of the PTFE particles in the aqueous PTFE dispersion was the same as the average primary particle size of the PTFE particles measured in the aqueous emulsion. The evaluation results are shown in Example 5 in Table 1. Similarly, the nonionic surfactant (a) was concentrated at a ratio of 22 mass % relative to the mass of PTFE contained in the aqueous emulsion charged before the concentration started, and similar compounding was carried out, and the results are shown in Example 6. Furthermore, the nonionic surfactant (a) was concentrated at a ratio of 150 mass % relative to the mass of PTFE contained in the aqueous emulsion charged before the concentration started, and similar compounding was carried out, and the results are shown in Example 7.

[0126] (Example 8) [Process C1] Polymerization was carried out by replacing the polyethylene glycol 4,000,000 used in step C1 of Example 5 with polyethylene glycol 400 (average molecular weight: 360 to 440) (0.45 g). The polymerization time was 95 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 12 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.188 μm (188 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the resulting PTFE powder was 2.187. Furthermore, no by-product fluorine-based oligomers were found in the obtained PTFE powder.

[0127] [Process C2] The aqueous emulsion obtained in step C1 was concentrated in the same manner as in Example 1 to prepare an aqueous PTFE dispersion having a PTFE concentration of 60.5% by mass. The viscosity of the obtained aqueous PTFE dispersion at 23°C was 24 mPa·s, pH = 10.2, and surface tension was 31 (mN / m). The average primary particle size of the PTFE particles in the aqueous PTFE dispersion was the same as the average primary particle size of the PTFE particles measured in the aqueous emulsion. The evaluation results are shown in Example 8 in Table 1.

[0128] (Example 10) [Process C1] Polymerization was carried out by replacing the polyethylene glycol 4,000,000 used in step C1 of Example 5 with PEO-2 (Sumitomo Seika Chemicals Co., Ltd., average molecular weight: 400,000 to 600,000) (0.087 g). The polymerization time was 85 minutes. The solids concentration (PTFE particle concentration) of the aqueous emulsion was approximately 12 mass %, and the average primary particle size of the PTFE particles in the aqueous emulsion was 0.199 μm (199 nm). A portion of the resulting aqueous emulsion was adjusted to 20°C and stirred to aggregate the PTFE particles, yielding a PTFE powder. This PTFE powder was then dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the resulting PTFE powder was 2.207. Furthermore, no by-product fluorine-based oligomers were found in the obtained PTFE powder.

[0129] [Process C2'] The same procedure as in [Step C2] was carried out for approximately 5 kg of the aqueous emulsion obtained in Step C1, except that nonionic surfactant (b) (Tergitol TMN-100X, manufactured by The Dow Chemical Company) was used in place of nonionic surfactant (a) in a proportion of 40 mass% relative to the mass of PTFE (617 g) contained in the aqueous emulsion. An aqueous PTFE dispersion was obtained with a PTFE concentration of 68.9% by mass and a nonionic surfactant (b) concentration of 3.3% by mass relative to the mass of PTFE contained in the aqueous emulsion. To this aqueous PTFE dispersion, 0.05% by mass of PEO relative to the mass of PTFE contained in the aqueous emulsion, water, and aqueous ammonia were added, and nonionic surfactant (b) was added so that the content of nonionic surfactant (b) relative to the mass of PTFE contained in the aqueous emulsion was 4.8% by mass, thereby preparing an aqueous PTFE dispersion with a PTFE concentration of 60.8% by mass. The viscosity of the resulting aqueous PTFE dispersion at 23°C was 21 mPa·s, pH = 10.0, and surface tension 29 (mN / m). The average primary particle size of PTFE particles in the aqueous PTFE dispersion was the same as the average primary particle size of PTFE particles measured in the aqueous emulsion.

[0130] The same procedure as in [Step A3] was carried out to determine the crack limit film thickness and recovery rate, which are shown in Example 10 of Table 1.

[0131] In Table 1, the column "Amount of non-fluorinated monomer used" indicates the amount of non-fluorinated monomer used relative to the amount of TFE supplied. The column "Amount of nucleating additive used" indicates the amount of nucleating additive used relative to the amount of TFE fed. The column "Amount of polyalkylene oxide used" indicates the amount of polyalkylene oxide used relative to the amount of TFE supplied. The column "Amount of nonionic surfactant used" indicates the amount of nonionic surfactant used relative to the mass of PTFE contained in the aqueous emulsion before the start of concentration.

[0132] [Table 1]

[0133] As shown in Table 1, the production method of the present invention makes it possible to produce an aqueous PTFE dispersion that exhibits the desired effects. The entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2021-027626, filed on February 24, 2021, are hereby incorporated by reference as part of the disclosure of the specification of the present invention.

Claims

1. A step A1 of polymerizing a non-fluorine-based monomer in an aqueous medium to obtain a solution 1 containing a polymer containing units based on the non-fluorine-based monomer; a step A2 of polymerizing tetrafluoroethylene in the solution 1 without adding a surfactant to the solution 1 to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and a step A3 of adding a nonionic surfactant to the aqueous emulsion in an amount of 50 to 150 mass % based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, a non-fluorinated monomer used in an amount of 200 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system;

2. 2. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 1, wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1000 to 5000 ppm by mass, based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, between step A2 and step A3, or during step A3.

3. 3. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 1, wherein the non-fluorinated monomer is a monomer represented by formula (1): Equation (1) CH 2 =CR 11 -L 1 -R 12 In formula (1), R 11 represents a hydrogen atom or an alkyl group. 1 represents a single bond, -CO-O-*, -O-CO-* or -O-. * represents R 12 represents the bonding position with 12 represents a hydrogen atom, an alkyl group, an alkenyl group, or a nitrile group. １ When is a single bond, R 12 is a nitrile group.

4. The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of claims 1 to 3, wherein the content of the polytetrafluoroethylene particles in the aqueous polytetrafluoroethylene dispersion is 50 to 70 mass% with respect to the total amount of the aqueous polytetrafluoroethylene dispersion.

5. 5. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 1, wherein in step A3, a nonionic surfactant is added to the aqueous emulsion in an amount of 50 to 150 mass % relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for a further 1 to 10 hours to cause phase separation of the aqueous emulsion, and a lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion.

6. Step B1 of mixing at least one nucleating additive selected from the group consisting of polyalkylene oxide compounds and hydrocarbon-containing surfactants with an oxidizing agent in an aqueous medium to obtain a solution 2; a step B2 of polymerizing tetrafluoroethylene in the solution 2 without adding a surfactant to the solution 2 to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and a step B3 of adding a nonionic surfactant to the aqueous emulsion in an amount of 50 to 150 mass % based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, a method for producing an aqueous polytetrafluoroethylene dispersion, wherein the amount of the nucleation additive used is 100 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system.

7. 7. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 6, wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1000 to 5000 ppm by mass, based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, between step B2 and step B3, or during step B3.

8. 8. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 6, wherein the content of the polytetrafluoroethylene particles in the aqueous polytetrafluoroethylene dispersion is 50 to 70 mass% based on the total amount of the aqueous polytetrafluoroethylene dispersion.

9. 9. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 6, wherein in step B3, a nonionic surfactant is added to the aqueous emulsion in an amount of 50 to 150 mass % relative to the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for a further 1 to 10 hours to cause phase separation of the aqueous emulsion, and a lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion.

10. The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of claims 6 to 9, wherein the nucleating additive is a polyalkylene oxide compound.

11. The method for producing an aqueous polytetrafluoroethylene dispersion according to any one of claims 6 to 10, wherein the amount of the oxidizing agent used is 0.5 to 100 ppm by mass, based on the total mass of the aqueous medium.

12. a step C1 of polymerizing tetrafluoroethylene in an aqueous medium in the presence of a polyalkylene oxide compound without substantially using a surfactant to obtain an aqueous emulsion containing polytetrafluoroethylene particles; and step C2 of adding a nonionic surfactant to the aqueous emulsion in an amount of 50 to 150 mass % based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion, a method for producing an aqueous polytetrafluoroethylene dispersion, wherein the amount of the polyalkylene oxide compound used is 100 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to a polymerization system.

13. 13. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 12, wherein an anionic surfactant is added to the aqueous emulsion in an amount of 1000 to 5000 ppm by mass, based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, between the step C1 and the step C2, or during the step C2.

14. 14. The method for producing an aqueous polytetrafluoroethylene dispersion according to claim 12 or 13, wherein in step C2, a nonionic surfactant is added to the aqueous emulsion in an amount of 50 to 150 mass % based on the mass of polytetrafluoroethylene contained in the aqueous emulsion, and then the aqueous emulsion is heated at 50 to 100°C and allowed to stand for a further 1 to 10 hours to cause phase separation of the aqueous emulsion, and the lower phase in which the polytetrafluoroethylene particles are concentrated is recovered as the aqueous polytetrafluoroethylene dispersion.

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