Method for producing a polytetrafluoroethylene mixed aqueous dispersion

Abstract

To provide a polytetrafluoroethylene mixed aqueous dispersion that can form a coating layer with a good appearance and resistance to crack or the like, and a method for producing the same.SOLUTION: A method for producing a polytetrafluoroethylene mixed aqueous dispersion includes mixing a predetermined polytetrafluoroethylene aqueous dispersion 1 with a predetermined polytetrafluoroethylene aqueous dispersion 2 so that the solid content mass ratio of the polytetrafluoroethylene aqueous dispersion 1 to the polytetrafluoroethylene aqueous dispersion 2 is 90 / 10-10 / 90.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Fluorine-containing polymers are widely used to impart properties such as stain resistance and weather resistance. For example, a cloth sheet (coated woven fabric) made by coating or impregnating a glass fiber woven fabric with a fluorine-containing polymer and then firing it to form a film is used as roofing material for membrane structures, conveyor belts in food manufacturing lines, etc. (see, for example, Patent Document 1). Polytetrafluoroethylene, in particular, tends to have superior heat resistance compared to other fluorine-containing polymers, and is therefore preferred for applications requiring heat resistance. A polytetrafluoroethylene coating is formed, for example, using an aqueous dispersion of polytetrafluoroethylene.

[0003] However, polytetrafluoroethylene coatings have low flexibility and poor bendability, which makes them prone to peeling and cracking in areas subjected to large strains. Cross sheets using polytetrafluoroethylene as a fluorine-containing polymer also had similar problems. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 3118468 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The present invention aims to provide a polytetrafluoroethylene mixed aqueous dispersion that can form a coating with excellent appearance and is resistant to cracking, and a method for producing the same. [Means for solving the problem]

[0006] As a result of diligent research, the inventors have found that the above problem can be solved by the following configuration.

[0007] (1) A method for producing a polytetrafluoroethylene mixed aqueous dispersion, comprising mixing the following polytetrafluoroethylene aqueous dispersion 1 and the following polytetrafluoroethylene aqueous dispersion 2 such that the solid content mass ratio is polytetrafluoroethylene aqueous dispersion 1 / polytetrafluoroethylene aqueous dispersion 2 = 90 / 10 to 10 / 90. [Polytetrafluoroethylene aqueous dispersion 1] Step A1 involves polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a polymer that includes units based on the non-fluorinated monomer, Step A2 involves polymerizing tetrafluoroethylene in the above solution 1 without substantially adding a surfactant to the above solution 1 to obtain an aqueous emulsion containing polytetrafluoroethylene particles, The method includes step A3, which involves adding a nonionic surfactant to the above aqueous emulsion, and then concentrating the above aqueous emulsion to obtain a polytetrafluoroethylene aqueous dispersion. A polytetrafluoroethylene aqueous dispersion in which the amount of the non-fluorinated monomer used is 200 ppm by mass or less relative to the amount of the tetrafluoroethylene supplied to the polymerization system. [Polytetrafluoroethylene aqueous dispersion 2] An aqueous dispersion containing polytetrafluoroethylene particles and an aqueous medium, which is different from polytetrafluoroethylene aqueous dispersion 1. (2) The method for producing a polytetrafluoroethylene mixed aqueous dispersion according to (1), wherein the polytetrafluoroethylene aqueous dispersion 2 is obtained by a method comprising step B1 of emulsion polymerization of tetrafluoroethylene in an aqueous medium in the presence of a surfactant to obtain an aqueous emulsion, and step B2 of adding a nonionic surfactant to the aqueous emulsion and then concentrating the aqueous emulsion. (3) A mixed aqueous dispersion of polytetrafluoroethylenes containing the following polytetrafluoroethylene 1 and the following polytetrafluoroethylene 2 in a mass ratio of polytetrafluoroethylene 1 / polytetrafluoroethylene 2 = 90 / 10 to 10 / 90. [Polytetrafluoroethylene 1] Polytetrafluoroethylene particles with an average primary particle size of 0.1 to 0.5 μm and no detection of CF2 chain oligomers with 6 to 34 carbon atoms. [Polytetrafluoroethylene 2] Polytetrafluoroethylene particles with an average primary particle diameter of 0.1 to 0.5 μm, distinct from polytetrafluoroethylene 1. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a polytetrafluoroethylene mixed aqueous dispersion that can form a coating with excellent appearance and is resistant to cracking, and a method for producing the same. [Modes for carrying out the invention]

[0009] The meanings of the terms used in this invention are as follows: A "unit" is a general term for an atomic group derived from a single monomer molecule, which is directly formed by the polymerization of monomers. A numerical range represented using "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0010] The present invention provides a method for producing a polytetrafluoroethylene (hereinafter also referred to as "PTFE") mixed aqueous dispersion, which involves mixing PTFE aqueous dispersion 1 and PTFE aqueous dispersion 2, as described later, so that the solid content mass ratio is PTFE aqueous dispersion 1 / PTFE aqueous dispersion 2 = 90 / 10 to 10 / 90.

[0011] PTFE aqueous dispersion 1 is prepared by step A1, which involves polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a polymer that includes units based on the non-fluorinated monomer, Step A2 involves polymerizing tetrafluoroethylene in solution 1 without substantially adding a surfactant to solution 1 to obtain an aqueous emulsion containing polytetrafluoroethylene particles, The method includes step A3, which involves adding a nonionic surfactant to an aqueous emulsion, and then concentrating the aqueous emulsion to obtain an aqueous polytetrafluoroethylene dispersion. This is an aqueous polytetrafluoroethylene dispersion in which the amount of non-fluorinated monomer used relative to the amount of tetrafluoroethylene supplied to the polymerization system is 200 ppm by mass or less.

[0012] <Process A1> Step A1 is a step in which a non-fluorinated monomer is polymerized in an aqueous medium to obtain a solution 1 containing a specific polymer. Below, we will first describe in detail the materials used in process A1, and then describe in detail the procedure for process A1.

[0013] (Non-fluorinated monomers) A non-fluorinated monomer is a monomer that does not contain fluorine atoms. Non-fluorinated monomers typically have polymerizable groups, and the number of polymerizable groups is preferably 1 to 3, and more preferably 1. As polymerizable groups, ethylenically unsaturated groups are preferred. More specifically, acryloyl groups, methacryloyl groups, vinyl ether groups, vinyl ester groups, vinyl groups, and allyl groups are examples, with acryloyl groups, methacryloyl groups, vinyl ester groups, and vinyl ether groups being preferred.

[0014] As a non-fluorinated monomer, the monomer represented by formula (1) is preferred. Formula (1) CH2=CR 11 -L 1 -R 12 R 11 represents a hydrogen atom or an alkyl group. The number of carbon atoms in the alkyl group is preferably 1 to 3, and more preferably 1. L 1 The asterisk (*) represents a single bond, -CO-O-*, -O-CO-*, or -O-. 12represents the bonding position with. For example, when L 1 is -CO-O-*, formula (1) is CH2=CR 11 -CO-O-R 12 represents. R 12 represents a hydrogen atom, an alkyl group, an alkenyl group or a nitrile group. However, when L 1 is a single bond, R 12 is a nitrile group. The number of carbon atoms of the alkyl group and the alkenyl group is preferably 1 to 10, more preferably 1 to 6, and even more preferably 1 to 4. 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.

[0015] As the monomer represented by formula (1), a monomer selected from the group consisting of the monomer represented by formula (1-1), the monomer represented by formula (1-2), the monomer represented by formula (1-3), and the monomer represented by formula (1-4) is preferred. Formula (1-1) CH2=CR 11 -CO-O-R 13 Formula (1-2) CH2=CR 11 -O-CO-R 14 Formula (1-3) CH2=CR 11 -O-R 15 Formula (1-4) CH2=CR 11 -R 16 R 11 is as defined above. R 13 represents a hydrogen atom, an alkyl group or an alkenyl group, and an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 1 to 6 carbon atoms is preferred. R 14 represents an alkyl group, and an alkyl group having 1 to 3 carbon atoms is preferred, and a methyl group is more preferred. R 15 represents an alkyl group, and a linear alkyl group or a cyclic alkyl group is preferred. R 16 This represents a nitrile group.

[0016] Examples of non-fluorinated 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. Non-fluorinated monomers may be used individually or in combination of two or more. As the non-fluorinated monomer, the monomer represented by formula (1-1) or the monomer represented by formula (1-2) is preferred, R 13 A monomer represented by formula (1-1), in which the alkyl group is more preferred. Since the monomers represented by formula (1-1) and formula (1-2) have ester groups or carboxyl groups, which are water-affinity groups, the monomers and their polymers are water-affinity. Therefore, especially at low concentrations, the monomers and their polymers are considered to disperse stably in aqueous media without the need for surfactants.

[0017] (Specific polymers) A specific polymer is a polymer that contains units based on non-fluorinated monomers. The specific polymer usually contains only units based on non-fluorinated monomers, but it may also contain units based on fluorinated monomers to the extent that it does not impair the effects of the present invention. In other words, in addition to non-fluorinated monomers, fluorinated monomers may be used in step A1. Fluorinated monomers are monomers having fluorine atoms, and TFE is an example of such a monomer. The content of units based on non-fluorinated monomers in the specific polymer is preferably 90% by mass or more, and more preferably 95% by mass or more, relative to the total units of the specific polymer. The upper limit is 100% by mass.

[0018] (aqueous medium) Examples of aqueous media include water and mixtures of water and water-soluble organic solvents. Examples of water-soluble organic solvents 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. As the aqueous medium, water alone is preferable.

[0019] (Polymerization initiator) In step A1, a polymerization initiator may be used. In other words, a polymerization initiator may be used during the polymerization of non-fluorinated monomers. Preferred polymerization initiators are water-soluble radical initiators and water-soluble redox catalysts. Preferred water-soluble radical initiators include persulfates such as ammonium persulfate and potassium persulfate, and water-soluble organic peroxides such as disuccinic acid peroxide, bisglutaric acid peroxide, and tert-butyl hydroperoxide. As a water-soluble redox catalyst, a combination of an oxidizing agent such as bromate or its salt, chloric acid or its salt, persulfate or its salt, permanganate or its salt, or hydrogen peroxide, and a reducing agent such as sulfurous acid or its salt, bisulfite or its salt, thiosulfate or its salt, or organic acid is preferred. Among these, a combination of bromate or its salt and sulfurous acid or its salt (e.g., ammonium sulfite), and a combination of permanganate or its salt (e.g., potassium permanganate) and oxalic acid are more preferred. As polymerization initiators, ammonium persulfate alone or a mixture of persulfate and disuccinate peroxide is preferred, and ammonium persulfate alone or a mixture of ammonium persulfate and disuccinate peroxide is more preferred. Polymerization initiators may be used individually or in combination of two or more. Regarding the method of adding the polymerization initiator, the entire amount may be added to the polymerization system before starting the polymerization reaction, or it may be added to the polymerization system continuously or intermittently.

[0020] (Procedure for Step A1) In step A1, polymerization of the non-fluorinated monomer is carried out in an aqueous medium. Specifically, it is preferable to mix the non-fluorinated monomer with the aqueous medium and carry out polymerization of the non-fluorinated monomer in the resulting mixture. As mentioned above, fluorine-based monomers may be used in combination as needed.

[0021] The amount of non-fluorinated monomer used is 200 ppm by mass or less relative to the amount supplied to the TFE polymerization system (amount of TFE used) in step A2 described later, preferably 1 to 150 ppm by mass or less, more preferably 5 to 100 ppm by mass, and even more preferably 5 to 50 ppm by mass. Furthermore, as a method for adding non-fluorinated monomers, it is preferable to add the entire amount to the polymerization system at the beginning of the polymerization reaction.

[0022] The content of the non-fluorinated monomer in the dispersion obtained by mixing the non-fluorinated monomer with an aqueous medium is preferably 0.000015 to 0.0030% by mass, and more preferably 0.000075 to 0.0023% by mass, based on the total mass of the dispersion. Since non-fluorinated monomers usually polymerize entirely to form a specific polymer, the concentration of the specific polymer in the resulting solution 1 will be within the above numerical range. The above non-fluorinated monomer concentrations and specific polymer concentrations are those obtained when the resulting solution 1 is used in step A2 without dilution with an aqueous medium. When the obtained solution 1 is diluted with an aqueous medium to obtain the above specific polymer concentration and the diluted solution is used in step A2, a high-concentration solution corresponding to the dilution ratio is produced in step A1. The dilution ratio is not particularly limited, but 10 times or less is preferred.

[0023] The amount of polymerization initiator used is preferably 0.2 to 1000% by mass, and more preferably 0.2 to 500% by mass, relative to the total amount of non-fluorinated monomers.

[0024] The amount of polymerization initiator used is preferably 0.1 to 1000 mol%, and more preferably 0.1 to 300 mol%, relative to the total amount of non-fluorinated monomers.

[0025] The polymerization temperature of the non-fluorinated 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 or atmospheric pressure. Of 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, polymerization may be carried out in a TFE atmosphere. Generally, polymerization of non-fluorinated monomers in an aqueous medium proceeds preferentially over TFE polymerization.

[0026] Step A1 described above yields solution 1 containing the specific polymer. The specific polymer may be dissolved in solution 1 or dispersed in particulate matter in an aqueous medium. During the polymerization of TFE in step A2 described later, although the specific polymer is not an emulsifier, it is presumed that the specific polymer exists at the boundary between the aqueous medium and the PTFE particles due to the balance of interfacial tension between the two, thereby contributing to the stabilization of the dispersion of PTFE particles in the aqueous medium. The particle size of the specific polymer particles is preferably 0.1 to 100 nm, and more preferably 0.1 to 50 nm.

[0027] Furthermore, the solution 1 obtained in step A1 may contain unreacted non-fluorinated monomers. Also, the polymerization atmosphere in step A1 may be carried out under a TFE-containing atmosphere, taking step A2 into consideration. In such a case, it is conceivable that a portion of the specific polymer in step A2 may become a polymer containing TFE units. Alternatively, 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 non-fluorinated monomers.

[0028] <Process A2> Step A2 is a step in which TFE is polymerized in solution 1 obtained in step A1 without substantially adding a surfactant to solution 1, thereby obtaining an aqueous emulsion containing PTFE particles. Below, we will first describe in detail the materials used in process A2, and then describe in detail the procedure for process A2.

[0029] (TFE) In process A2, TFE is used.

[0030] (Other monomers) In step A2, other monomers other than TFE may be used, as long as they do not impair the effects of the present invention. Other monomers include monomers with polar groups (hereinafter also simply referred to as "specific monomers"). Since the polar groups in specific monomers interact with aqueous media, it is presumed that they are positioned between TFE and the aqueous media during TFE polymerization and exhibit surfactant-like functions. As a result, TFE polymerization proceeds smoothly, and the occurrence of chain transfer is suppressed.

[0031] Examples of polar groups contained in specific monomers include sulfonic acid groups, sulfonic acid bases, carboxylic acid groups, carboxylic acid bases, phosphonic acid groups, and phosphonic acid bases. Among these, the group represented by formula (A) or the group represented by formula (B) is preferred, with the group represented by formula (A) being more preferred, as it further suppresses the formation of fluorine-based oligomers. Formula (A) -SO3M Formula (B) -COOM In formulas (A) and (B), M represents a hydrogen atom, NH4, or an alkali metal atom. Examples of alkali metal atoms include lithium, sodium, and potassium atoms.

[0032] The specific monomer usually has polymerizable groups, and the number of polymerizable groups is preferably 1 to 3, and more preferably 1. As polymerizable groups, ethylenically unsaturated groups are preferred. More specifically, acryloyl groups, methacryloyl groups, vinyl ether groups, vinyl ester groups, vinyl groups, and allyl groups are examples, with acryloyl groups, methacryloyl groups, vinyl ester groups, and vinyl ether groups being preferred.

[0033] In terms of further suppressing the formation of fluorine-based oligomers, the monomer represented by formula (3) is preferred as the specific monomer. Formula (3) CR 31 R 32 =CR 33 -L 3 -R 34 In formula (3), R 31 and R 32 Each of these independently represents either a hydrogen atom or a fluorine atom.

[0034] R 33 This represents a hydrogen atom, a fluorine atom, or an alkyl group which may be substituted with a fluorine atom. Among these, a hydrogen atom or a fluorine atom is preferred because it exhibits better copolymerization with TFE. Furthermore, "alkyl groups that may be substituted with fluorine atoms" means alkyl groups in which at least one hydrogen atom may be substituted with a fluorine atom. The number of carbon atoms in the alkyl group, which may be substituted with a fluorine atom, is preferably 1 to 3, and more preferably 1.

[0035] L 3 This represents a single bond or a divalent linking group. Of these, a single bond is preferred because it exhibits better copolymerization with TFE. Examples of divalent linking groups include divalent hydrocarbon groups (which may be divalent saturated hydrocarbon groups, divalent aromatic hydrocarbon groups, alkenylene groups, or alkylylene groups. Divalent saturated hydrocarbon groups may be linear, branched, or cyclic, for example, alkylene groups. The number of carbon atoms is preferably 1 to 20. Divalent aromatic hydrocarbon groups are preferably 5 to 20 carbon atoms, for example, phenylene groups. Other options include alkenylene groups with 2 to 20 carbon atoms and alkylylene groups with 2 to 20 carbon atoms), divalent heterocyclic groups, -O-, -S-, -SO2-, -C(O)-, and -Si(R a )2-, -N(R b )-, and groups formed by combining two or more of these. Here, R aR represents an alkyl group (preferably having 1 to 10 carbon atoms) or a phenyl group. b This represents a hydrogen atom or an alkyl group (preferably with 1 to 10 carbon atoms). Examples of groups formed by combining two or more of the above include -OC(O)- and -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 is one example. The above-mentioned divalent hydrocarbon group may have substituents. Examples of substituents include halogen atoms (e.g., fluorine atoms, chlorine atoms). In other words, the hydrogen atoms in the above-mentioned divalent hydrocarbon group may be substituted with halogen atoms.

[0036] R 34 This represents a group represented by formula (A) or a group represented by formula (B) above.

[0037] The monomer represented by formula (3) is preferably selected from the group consisting of the monomer represented by formula (3-1), the monomer represented by formula (3-2), the monomer represented by formula (3-3), the monomer represented by formula (3-4), the monomer represented by formula (3-5), and the monomer represented by formula (3-6), with the monomer represented by formula (3-1) being more preferred. 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

[0038] In formulas (3-1) to (3-6), R 31 ~R 34 The definition is as stated above. In equation (3-2), m1 represents an integer between 1 and 10. In equation (3-3), m² represents an integer between 1 and 5. In equation (3-4), m3 represents an integer from 1 to 10. 35 This represents a fluorine atom or CF3. In equation (3-5), m4 represents an integer from 1 to 10. 35 The definition is as stated above. In equation (3-6), m5 represents 0 or an integer from 1 to 10.

[0039] A specific example of a particular monomer is ammonium vinylsulfonate. A specific monomer may be used alone or in combination of two or more types.

[0040] (Polymerization initiator) In step A2, a polymerization initiator may be used. In other words, a polymerization initiator may be used during the polymerization of TFE. Examples of polymerization initiators used include those described in step A1. As a 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 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, relative to the total amount of TFE supplied to the polymerization system.

[0041] (Stabilizing agent) In step A2, a stabilizing agent may be used. Preferred stabilizing agents include paraffin wax, fluorinated solvents, and silicone oil, with paraffin wax being more preferred. The paraffin wax may be liquid, semi-solid, or solid at room temperature. Among these, saturated hydrocarbons with 12 or more carbon atoms are preferred. The melting point of the paraffin wax is preferably 40-65°C, and more preferably 50-65°C. Stabilizing agents may be used individually or in combination of two or more.

[0042] (others) Furthermore, in step A2, monomers other than TFE and specific monomers may be used as long as they do not impair the effects of the present invention. However, since PTFE exhibits superior various properties, the amount of TFE used is preferably 99.5% by mass or more, and more preferably 99.8% by mass or more, relative to the total amount of monomers used in step A2.

[0043] (Step A2 procedure) In step A2, virtually no surfactant is added to solution 1. In other words, in step A2, TFE polymerization is carried out in solution 1 without substantially adding any new surfactant to solution 1. A surfactant is a compound that has both a hydrophilic group (e.g., a polar group) and a hydrophobic group (e.g., a hydrocarbon group). The definition of a polar group is the same as the definition of a polar group contained in a specific monomer. Examples of surfactants include well-known surfactants, such as nonionic surfactants and ionic surfactants, and more specifically, hydrocarbon-containing surfactants and fluorinated surfactants. The definition of hydrocarbon-containing surfactants is as described below. In step A2, it is preferable that at least one surfactant selected from the group consisting of hydrocarbon-containing surfactants and fluorine-based surfactants is not substantially added to solution 1. The phrase "substantially not added" above means that no surfactant is added, or if one is added, the amount of surfactant added is 200 ppm by mass or less relative to the total mass of solution 1. There is no particular lower limit, but 0 ppm by mass is preferred. In other words, it is preferable not to add a surfactant to solution 1 in step A2.

[0044] TFE is introduced into the polymerization system (i.e., the polymerization reaction vessel) by conventional methods. For example, TFE is introduced into the polymerization system continuously or intermittently so that the polymerization pressure reaches a predetermined pressure. When a polymerization initiator is used, the polymerization initiator may be added to the polymerization system all at once or in separate additions.

[0045] When using specific monomers, the amount of specific monomers used relative to the total amount of TFE is preferably 0.150% by mass or less. In other words, the amount of specific monomers added relative to the total amount of TFE is preferably 0.150% by mass or less. From the viewpoint of the stability of the emulsion during polymerization, the amount of specific monomer used relative to the total amount of TFE is preferably 0.100% by mass or less, and more preferably 0.090% by mass or less. Furthermore, from the viewpoint of improving molecular weight, the amount of specific monomer used relative to the total amount of TFE is preferably 0.005% by mass or more, and more preferably 0.010% by mass or more. Furthermore, when using two or more specific monomers, the total amount of specific monomers used must be within the above range.

[0046] When using a specific monomer, the amount of the specific monomer relative to the total amount of TFE is preferably 0.150 mol% or less. In other words, the amount of the specific monomer relative to the total amount of TFE charged is preferably 0.150 mol% or less. From the viewpoint of the stability of the emulsion during polymerization, the amount of specific monomer used relative to the total amount of TFE is preferably 0.100 mol% or less, and more preferably 0.090 mol% or less. Furthermore, from the viewpoint of improving molecular weight, the amount of specific monomer used relative to the total amount of TFE is preferably 0.001 mol% or more, and more preferably 0.005 mol% or more. Furthermore, when using two or more specific monomers, the total amount of specific monomers used must be within the above range.

[0047] The polymerization temperature is preferably 10 to 95°C, and more preferably 15 to 90°C. The polymerization pressure is preferably 0.5 to 4.0 MPa, and 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.

[0048] Steps A1 and A2 may be carried out continuously in the same polymerization reaction vessel. Furthermore, in the manufacturing method of the present invention, it is sufficient that a 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.

[0049] The above procedure yields an aqueous emulsion in which PTFE is dispersed in particulate form (an aqueous emulsion containing PTFE particles). The concentration of 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 volume of the aqueous emulsion. Within this range, the PTFE particles in the aqueous emulsion can be coagulated more easily, and the turbidity of the coagulated liquid can be suppressed. The average primary particle diameter of the PTFE particles is preferably 100 to 500 nm, and more preferably 150 to 300 nm. The average primary particle diameter of PTFE particles corresponds to D50, as measured by a laser scattering particle size distribution analyzer.

[0050] The PTFE particles obtained by the above procedure typically contain TFE units as the main component. The main component refers to a TFE unit content of 99.700% by mass or more, preferably 99.900% by mass or more, relative to the total units of PTFE. An upper limit of 100% by mass is given. When PTFE contains units based on specific monomers, the content of units based on specific monomers is preferably 0.005 to 0.150% by mass, and more preferably 0.010 to 0.100% by mass, relative to the total units of PTFE. Furthermore, when using two or more specific monomers, the total content of each specific monomer must be within the above range. When PTFE contains units based on non-fluorinated monomers, the content of units based on non-fluorinated monomers is preferably 200 ppm by mass or less, more preferably 1 to 150 ppm by mass, even more preferably 5 to 100 ppm by mass, and particularly preferably 5 to 50 ppm by mass, relative to the total units of PTFE. Furthermore, when using two or more non-fluorinated monomers, the total content of each non-fluorinated monomer must be within the above range.

[0051] <Process A3> Step A3 is a process in which a nonionic surfactant is added to the aqueous emulsion obtained in Step A2, and then the aqueous emulsion is concentrated to obtain an aqueous PTFE dispersion. In other words, it is a process in which a nonionic surfactant is added to the low-concentration aqueous PTFE dispersion obtained in Step A2 (corresponding to the aqueous emulsion mentioned above), and then the low-concentration aqueous PTFE dispersion is concentrated to obtain a high-concentration aqueous PTFE dispersion (corresponding to the aqueous PTFE dispersion mentioned above). By carrying out Step A3, an aqueous PTFE dispersion exhibiting a higher PTFE particle concentration than that of the aqueous dispersion is obtained. Below, we will first describe in detail the materials used in process A3, and then describe in detail the procedure for process A3.

[0052] (Nonionic surfactant) Nonionic surfactants include nonionic surfactants. Furthermore, as nonionic surfactants, nonionic surfactants represented by formula (4) and nonionic surfactants represented by formula (5) are preferred. Formula (4):R 41 -OAH Formula (5):R 51 -C6H4-OBH In the formula, R 41 represents an alkyl group with 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 with 4 to 12 carbon atoms. B represents a polyoxyethylene chain composed of 5 to 20 oxyethylene groups. Furthermore, nonionic surfactants represented by formula (6) are also preferred as nonionic surfactants. Formula (6):R 61 -ODH In the formula, R 61 represents an alkyl group with 8 to 18 carbon atoms. D represents a polyoxyalkylene chain composed of 5 to 20 oxyethylene groups and 0.1 to 3 oxybutylene groups.

[0053] In equation (4), R 41 The alkyl group represented by has 8 to 18 carbon atoms, 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 PTFE aqueous dispersion is left standing for a long period of time, resulting in excellent storage stability. When the number of carbon atoms is 8 or more, the surface tension of the PTFE aqueous dispersion is lower, resulting in excellent permeability and wettability. In formula (4), the hydrophilic group A is preferably a polyoxyalkylene chain composed of 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, it is preferable because it has good anti-foaming properties.

[0054] In equation (5), R 51The number of carbon atoms of the alkyl group represented by [alkyl group] is 4 to 12, preferably 6 to 10, and more preferably 8 to 9. When the number of carbon atoms of the alkyl group is 4 or more, the surface tension of the PTFE aqueous dispersion becomes low, and the permeability and wettability are excellent. When the number of carbon atoms is 12 or less, even when the PTFE aqueous dispersion is left for a long time, the PTFE particles are hardly sedimented, and the storage stability is excellent. In 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.

[0055] In formula (6), R 61 The number of carbon atoms of the alkyl group represented by [alkyl group] is 8 to 18, preferably 10 to 16, and more preferably 12 to 16. When the number of carbon atoms is 18 or less, even when the PTFE aqueous dispersion is left for a long time, the PTFE particles are hardly sedimented, and the storage stability is excellent. Further, when the number of carbon atoms is 8 or more, the surface tension of the PTFE aqueous dispersion becomes low, and the permeability and wettability are excellent. In formula (6), as the hydrophilic group D, a polyoxyalkylene chain having 7 to 12 oxyethylene groups and 0.1 to 3 oxybutylene groups is preferable. Among them, when the number of oxybutylene groups in D is 0.5 to 2, the foam disappearance property is good, which is preferable. Further, 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.

[0056] 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.

[0057] Examples of the nonionic surfactant represented by formula (4) include, for example, C 13 H 27 -(OC2H4) 10 -OH, C 12 H 25 -(OC2H4) 10 -OH, C 10 H 21CH(CH3)CH2-(OC2H4)9-OH, C 13 H 27 -(OC2H4)9-OCH(CH3)CH2-OH, C 16 H 33 -(OC2H4) 10 -OH, HC(C5H 11 )(C7H 15 )-(OC2H4)9-OH can be mentioned. As commercially available products, there are Dow's Triton (registered trademark) 15S series and Lion's Lionol (registered trademark) TD series. As the nonionic surfactant represented by formula (5), for example, C8H 17 -C6H4-(OC2H4) 10 -OH, C9H 19 -C6H4-(OC2H4) 10 -OH can be mentioned. As commercially available products, there are Dow's Triton (registered trademark) X series, Nikko Chemicals' Nikkol (registered trademark) OP series or NP series. As the nonionic surfactant represented by formula (6), for example, 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 33 OC2H4OCH(C2H5)CH2O(C2H4O)9H, C 12 H 25 OCH2CH(C2H5)O(C2H4O)8CH2CH(C2H5)OH, C 13 H 27OCH(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 O(CH2)2CH(CH3)O(C2H4O)8H is one example.

[0058] The nonionic surfactant represented by formula (4) and / or the nonionic surfactant represented by formula (5) may be used individually or in combination of two or more. Furthermore, the nonionic surfactant represented by formula (6) may be used alone or in combination of two or more types. Furthermore, the nonionic surfactant represented by formula (6) can be mixed with the nonionic surfactant represented by formula (4) or the nonionic surfactant represented by formula (5). Furthermore, nonionic surfactants are mixtures of multiple substances with different molecular structures, and the number of carbon atoms in the alkyl group and the number of oxyethylene, oxypropylene, and oxybutylene groups in the polyoxyalkylene chain will be treated as average values. These values ​​are not limited to integers.

[0059] (Step A3 procedure) The method for concentrating the aqueous emulsion is not particularly limited, and conventionally known methods can be used. Examples of concentration methods include centrifugal sedimentation, electrophoresis, and phase separation, as described on page 32 of the Fluororesin Handbook (edited by Takaomi Satokawa, published by Nikkan Kogyo Shimbun).

[0060] Electrophoresis is a method that utilizes the fact that PTFE particles are negatively charged. Specifically, 1 to 10% by mass (preferably 2 to 8% by mass) of a nonionic surfactant relative to the mass of PTFE is dissolved in an aqueous emulsion. Next, a voltage of 50 to 500 V / m (preferably 100 to 300 V / m) is applied to the resulting aqueous emulsion in a container having a semipermeable membrane such as a cellulose membrane, causing the PTFE particles to undergo electrophoresis. The aqueous PTFE dispersion that accumulates on the surface of the semipermeable membrane and then settles at the bottom due to the difference in specific gravity is then collected. The pH of the aqueous emulsion before concentration is preferably 2 to 10, and more preferably 3 to 9.

[0061] The phase separation method involves heating and letting it stand for a certain period of time to allow the PTFE particles to settle. Specifically, 8 to 20% by mass (preferably 12 to 18% by mass) of a nonionic surfactant relative to the mass of PTFE is dissolved in an aqueous emulsion. Next, the resulting aqueous emulsion is heated at 50 to 100°C (preferably 60 to 90°C) and left for 1 to 100 hours (preferably 5 to 20 hours), and the aqueous PTFE dispersion that has accumulated at the bottom due to the difference in specific gravity is recovered.

[0062] Furthermore, during concentration, anionic surfactants (e.g., ammonium laurate, triethanolamine laurate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, etc.) may be added to the PTFE aqueous emulsion at a concentration of 0.20% by mass or less relative to the mass of PTFE to accelerate the concentration process. These additives not only accelerate the concentration of the PTFE aqueous emulsion but also improve its viscosity and dispersion stability.

[0063] Furthermore, after adding a nonionic surfactant to the aqueous emulsion obtained in step A2, polymerization initiator residues and the like may be adsorbed and removed by an ion exchange resin or the like before concentrating the aqueous emulsion. This removes excess water-soluble ionic compounds, improving the coating performance of the PTFE aqueous dispersion and facilitating its application to insulating materials.

[0064] The concentration of PTFE particles in the aqueous PTFE dispersion obtained by the above procedure is preferably 15 to 70% by mass, and more preferably 20 to 70% by mass, based on the total mass of the aqueous PTFE dispersion. An aqueous PTFE dispersion with a concentration of 15 to 70% by mass is preferably used for impregnating cloths or strings woven with fibers such as glass fibers, mixing with inorganic powders or plastic powders, and adding small amounts to paints. Furthermore, in particular for applications involving coating with PTFE aqueous dispersions or processing into PTFE fibers, the concentration of PTFE particles is preferably 50-70% by mass, and more preferably 52-68% by mass. The pH of the PTFE aqueous dispersion is preferably 2 to 13, and more preferably 3 to 11.

[0065] The PTFE particles in the aqueous PTFE dispersion include not only homopolymers of TFE, but also so-called modified PTFE that contains polymerization units based on copolymer components that can copolymerize with TFE, such as halogenated ethylenes like chlorotrifluoroethylene, halogenated propylenes like hexafluoropropylene, and fluorovinyl ethers like perfluoro(alkyl vinyl ether), in amounts that are substantially incapable of melt processing.

[0066] The content of nonionic surfactant in the aqueous PTFE dispersion is preferably 1 to 20% by mass, more preferably 1.5 to 15% by mass, and even more preferably 2 to 10% by mass, relative to the mass of PTFE. When the above content is 1% by mass or more, the PTFE aqueous dispersion exhibits excellent mechanical stability and wettability. Furthermore, when the above content is 20% by mass or less, cracks are less likely to occur in the coated film, resulting in superior durability of the PTFE product. In particular, to improve wettability during coating and reduce the likelihood of cracking, the content of nonionic surfactant in the PTFE aqueous dispersion is preferably 3 to 10% by mass.

[0067] The surface tension of the PTFE aqueous dispersion is preferably 24 to 35 mN / m, and more preferably 25 to 32 mN / m. When the surface tension is 24 mN / m or higher, it exhibits excellent defoaming properties, and when it is 35 mN / m or lower, it is less prone to repelling.

[0068] The PTFE aqueous dispersion may contain one or more of the following known components: a fluorine-free emulsifier, various leveling agents, preservatives, colorants, fillers, organic solvents, ammonia water, and other known components. Furthermore, when the PTFE aqueous dispersion contains polyethylene oxide or polyurethane-based viscosity modifiers, the mechanical stability of the PTFE aqueous dispersion is superior.

[0069] The viscosity of the PTFE aqueous 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, from the viewpoint of ease of application. The viscosity-enhancing 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 viscosity-enhancing temperature is within the above range, viscosity changes due to fluctuations in application temperature are less likely to occur, and repulsion is less likely to occur.

[0070] PTFE aqueous dispersion 2 comprises PTFE particles and an aqueous medium, and may further contain surfactants and other components as needed. However, PTFE aqueous dispersion 2 is a different aqueous dispersion from PTFE aqueous dispersion 1. Although the PTFE aqueous dispersion 2 can be produced by known methods, it is preferable to obtain it by a method comprising step B1, in which tetrafluoroethylene is emulsion polymerized in an aqueous medium in the presence of a surfactant to obtain an aqueous emulsion, and step B2, in which a nonionic surfactant is added to the aqueous emulsion and the aqueous emulsion is then concentrated.

[0071] <Process B1> Step B1 is a step in which tetrafluoroethylene is emulsion polymerized in an aqueous medium in the presence of a surfactant to obtain an aqueous emulsion. The aqueous emulsion in step B1 contains PTFE particles. PTFE particles are particles of a non-melt-molten TFE polymer, and the term includes both TFE homopolymer particles and modified PTFE particles.

[0072] Examples of comonomers used in the production of modified PTFE include hexafluoropropylene, perfluoro(alkyl vinyl ether), chlorotrifluoroethylene, (perfluoroalkyl)ethylene, vinylidene fluoride, perfluoro(alkenyl vinyl ether), perfluoro(2,2-dimethyl-1,3-dioxol), and perfluoro(4-alkyl-1,3-dioxol). Comonomers may be used individually or in combination of two or more. As the comonomer, (perfluoroalkyl)ethylene is preferred, and (perfluoroalkyl)ethylene selected from the group consisting of (perfluoroethyl)ethylene, (perfluorobutyl)ethylene, and (perfluorohexyl)ethylene is more preferred. The comonomer unit content in modified PTFE is preferably 0.5% by mass or less, and more preferably 0.4% by mass or less, relative to the total units. In the production of modified PTFE, the total amount of TFE and comonomer consumed in the copolymerization reaction between TFE and comonomer is approximately equal to the amount of modified PTFE produced.

[0073] Examples of surfactants include fluorine-containing anionic surfactants. Examples of fluorine-containing anionic surfactants include fluorine-containing alkyl carboxylic acids and their salts, and fluorine-containing alkyl carboxylic acids and their salts having an etheric oxygen atom. Examples of fluorinated alkyl carboxylic acids or their salts include ammonium perfluorooctanoate and ammonium perfluorohexanoate. Examples of fluorine-containing alkyl carboxylic acids or salts thereof having an etheric oxygen atom include compounds represented by the following formula 1. F(CF2) p O(CF(X)CF2O) qCF(X)COOA...Equation 1. However, X is a fluorine atom or a perfluoroalkyl group having 1 to 3 carbon atoms, A is a hydrogen atom, an alkali metal atom, or NH4, p is an integer from 1 to 10, and q is an integer from 0 to 3.

[0074] Specific examples of compounds represented by Equation 1 are shown below. F(CF2)2OCF2CF2OCF2COONH4 (hereinafter also referred to as EEA), F(CF2)2O(CF2CF2O)2CF2COONH4, F(CF2)3O(CF(CF3)CF2O)2CF(CF3)COONH4, F(CF2)3OCF2CF2OCF2COONH4, F(CF2)3O(CF2CF2O)2CF2COONH4, F(CF2)4OCF2CF2OCF2COONH4, F(CF2)4O(CF2CF2O)2CF2COONH4, F(CF2)2OCF(CF3)CF2OCF(CF3)COONH4, F(CF2)2OCF2CF2OCF2COONa, F(CF2)2O(CF2CF2O)2CF2COONa, F(CF2)3OCF2CF2OCF2COONa, F(CF2)3O(CF2CF2O)2CF2COONa, F(CF2)4OCF2CF2OCF2COONa, F(CF2)4O(CF2CF2O)2CF2COONa etc.

[0075] Examples of emulsion polymerization include known methods such as those described in Japanese Patent Publication No. 5141256.

[0076] The number-average molecular weight of PTFE is preferably between 300,000 and 30,000,000, and more preferably between 500,000 and 25,000,000. If the number-average molecular weight of PTFE is above the lower limit, the mechanical properties of the coating are superior, and if it is below the upper limit, industrial manufacturing is easier. The number-average molecular weight of PTFE was determined using the method described in Suwa et al., Journal of Applied Polymer Science, 17, 3253 (1973), utilizing the heat of crystallization.

[0077] The standard specific gravity (SSG) of PTFE is preferably 2.14 or higher and less than 2.22, and more preferably between 2.15 and 2.21. When the SSG is within the above range, the mechanical properties of the coating are superior.

[0078] The average primary particle diameter of the PTFE particles is preferably 0.1 to 0.5 μm, more preferably 0.18 to 0.45 μm, and particularly preferably 0.20 to 0.35 μm. If the average primary particle diameter of the PTFE particles is above the lower limit, cracks are less likely to occur in the coating, and if it is below the upper limit, the PTFE particles are less likely to settle in the aqueous dispersion, resulting in excellent storage stability.

[0079] [Aqueous medium] Examples of aqueous media include water, or a mixture of water and a water-soluble organic solvent. As a water-soluble organic solvent, any known solvent can be used as appropriate, such as alcohols including tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol. When using a water-soluble organic solvent, it is preferable to have a low content of the water-soluble organic solvent in the aqueous medium. Specifically, the amount of water-soluble organic solvent per 100 parts by mass of water is preferably less than 1 part by mass, more preferably 0.5 parts by mass or less, and particularly preferably 0.1 parts by mass or less. The aqueous medium is most preferably free of water-soluble organic solvents.

[0080] <Process B2> Step B2 is a step in which a nonionic surfactant is added to the aqueous emulsion, and then the aqueous emulsion is concentrated. In step B2, it is preferable to use the same nonionic surfactant and concentration method as in step A3 of the PTFE aqueous dispersion 1 described above. Commercially available PTFE aqueous dispersions can also be used.

[0081] The present invention provides a method for producing a PTFE aqueous dispersion, which involves mixing PTFE aqueous dispersion 1 and PTFE aqueous dispersion 2 such that the solid content mass ratio is PTFE aqueous dispersion 1 / PTFE aqueous dispersion 2 = 90 / 10 to 10 / 90. Preferably, the solid content mass ratio of PTFE aqueous dispersion 1 / PTFE aqueous dispersion 2 is 50 / 50 to 10 / 90, and more preferably 20 / 80 to 10 / 90. Within this range, discoloration of the coating is suppressed, resulting in a superior appearance, and cracks are less likely to occur in the coating. There are no particular restrictions on the method of mixing each aqueous dispersion; each aqueous dispersion can be manufactured by conventional methods such as weighing and mixing / stirring each dispersion so that it matches the above-mentioned solid content mass ratio.

[0082] The PTFE mixed aqueous dispersion of the present invention contains PTFE1 and PTFE2 in a mass ratio of PTFE1 / PTFE2 = 90 / 10 to 10 / 90. [PTFE1] PTFE particles with an average primary particle size of 0.1-0.5 μm and in which oligomer groups of CF2 chains with 6-34 carbon atoms are not detected. [PTFE2] These PTFE particles have an average primary particle diameter of 0.1 to 0.5 μm and are different from PTFE1.

[0083] The values ​​for the oligomer group were measured using the following measurement method. <Measurement method> The sample (PTFE particles) was subjected to Soxhlet extraction with ethanol for 5 hours. LC / MS analysis was performed on the ethanol extract, and the oligomer group mainly consisting of CF2 chains with 6 to 34 carbon atoms was quantified using perfluorooctylsulfonic acid and perfluorooctanoic acid as standards. For LC / MS analysis, an Agilent 1260 series HPLC / 6460MS was used, and an Imtakt Cadenza CD-C18 column (2mmφ×100mm, 3μm particle size) was employed. A gradient of ammonium acetate aqueous solution and methanol was applied during the measurement.

[0084] The PTFE mixed aqueous dispersion of the present invention is preferably manufactured by the method for producing the PTFE mixed aqueous dispersion of the present invention described above.

[0085] The PTFE aqueous dispersion of the present invention can be suitably used in the manufacture of various PTFE products, such as films, sheets, and fibers mainly composed of PTFE, heat-resistant articles having a PTFE coating, and articles containing PTFE as a secondary component.

[0086] Examples of PTFE products include: packing made by impregnating a PTFE aqueous dispersion into a base material consisting of woven or braided fabric or cord made of glass fiber, aramid fiber, carbon fiber, or other various synthetic or natural fibers, and then drying it; heat-resistant conveying belts, building membrane structural sheets, packing, and printed circuit board materials made by impregnating a PTFE aqueous dispersion into a base material consisting of woven or braided fabric or cord made of heat-resistant fibers such as glass fiber, aramid fiber, and carbon fiber, and then firing it at a temperature above the melting point of PTFE; kitchen equipment such as frying pans and electric rice cookers made by coating metal plates such as aluminum and stainless steel with a PTFE aqueous dispersion containing pigments and heat-resistant resins, and then firing them; binders made by kneading active material powders for batteries such as carbon, manganese dioxide, and nickel hydroxide with a PTFE aqueous dispersion; and molding raw materials mixed with a PTFE aqueous dispersion to prevent dripping during combustion of plastic molded products such as polycarbonate and ABS resin. Examples of PTFE materials include: molded bodies (anti-dripping agents); powders obtained by mixing PTFE aqueous dispersion with chemical fertilizers, lime, incinerator ash, etc. to reduce dust generation; sliding materials such as oil-free bearings coated on porous materials by mixing fillers such as lead, zinc, and carbon powder with PTFE aqueous dispersion to form a paste; PTFE fibers obtained by adding a thickener such as viscose to PTFE aqueous dispersion, pressurizing and spinning it in a solidification bath, and then firing it; ultra-thin PTFE sheets obtained by coating a heat-resistant sheet substrate such as an aluminum plate or stainless steel plate with PTFE aqueous dispersion, firing it, and then peeling off the PTFE layer; thin films obtained by casting from PTFE aqueous dispersion, etc., which have heat resistance, high insulation properties, and low dielectric loss tangent capacity, and are used as coil insulation, interlayer insulating films, and electrical insulating materials for motors, transformers, relays, switches, etc.; and paints, resins, and rubber materials with improved lubricity and antifouling properties by adding PTFE aqueous dispersion.

[0087] PTFE products are obtained by coating or mixing an aqueous PTFE dispersion with a material, followed by drying or heat treatment at a temperature of room temperature to 420°C. The drying or heat treatment temperature is preferably 50 to 400°C, and more preferably 100 to 395°C. The PTFE content in a PTFE product varies depending on the application, but is preferably 0.01 to 100% by mass, more preferably 0.1 to 100% by mass, and even more preferably 1 to 100% by mass. [Examples]

[0088] 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.

[0089] [Coating test] Using an RDS #14 bar, a mixed aqueous dispersion obtained by the procedure described later was applied to an aluminum plate measuring 20 cm in length, 15 cm in width, and 0.2 mm in thickness. The plate was dried in a 120°C oven for 10 minutes, baked in a 380°C oven for 10 minutes, and then allowed to cool naturally. The formed PTFE layer was then coated, dried, and fired, and this process was repeated twice to form a total of three layers of PTFE. After cooling, only the film layer was peeled off to produce a PTFE film.

[0090] [exterior] The appearance of the PTFE film prepared in the coating test was visually inspected.

[0091] [crack] The PTFE film prepared in the coating test was dried at 120°C for 10 minutes, then baked at 380°C for 10 minutes, and the presence or absence of cracks was visually checked.

[0092] [Mechanical stability (abrasion stability)] A Tygon tube with an outer diameter of 7.9 mm and an inner diameter of 4.8 mm was attached to a Cohlpalmer tube pump. Both ends of the tube were placed in a 200 cc beaker containing 100 cc of PTFE aqueous dispersion, and the openings were covered with aluminum foil to prevent the liquid from drying out. Using this apparatus, the mixed aqueous dispersion obtained by the procedure described later was circulated for 1 hour at room temperature (23°C) at a flow rate of 200 cc per minute. After the circulation, the dispersion was filtered through a 200-mesh nylon filter to collect aggregates, and the weight was measured after drying at 120°C for 1 hour.

[0093] (Manufacturing example 1): 1500g of paraffin wax and 60L of deionized water were placed in a 100L stainless steel autoclave. After purging the autoclave with nitrogen and reducing the pressure, i-butyl methacrylate (i-BMA) (0.1g) and deionized water (0.5L) were added to the autoclave by pouring them in. Next, the autoclave was heated to 75°C while the solution inside was stirred, with the pressure below atmospheric pressure. Then, a solution of ammonium persulfate (0.055 g), which is a polymerization initiator, dissolved in deionized water (1 L) was poured into the autoclave to polymerize i-butyl methacrylate. After 20 minutes, the autoclave was pressurized to 1.96 MPa using a TFE, and a solution of ammonium persulfate (0.54 g) and disuccinate peroxide (80% by mass, remainder water) (53 g) dissolved in approximately 70°C warm water (1 L) was injected into the autoclave. After the internal pressure inside the autoclave dropped to 1.89 MPa, TFE was added to maintain the pressure at 1.96 MPa, allowing the polymerization of TFE to proceed. The reaction was terminated when the amount of TFE added reached 9 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 94 minutes. The solid content concentration (PTFE particle concentration) of the aqueous emulsion was approximately 11% by mass. The average primary particle size of the PTFE particles in the aqueous emulsion was 0.24 μm (240 nm). A portion of the obtained aqueous emulsion was adjusted to 20°C and stirred to agglomerate the PTFE particles, thereby obtaining PTFE powder. Next, this PTFE powder was dried at 275°C with an aqueous ammonium carbonate solution. The SSG of the obtained PTFE powder was 2.201. Furthermore, no by-product fluorine-based oligomers were detected in the obtained PTFE powder. To the resulting aqueous emulsion, add a nonionic surfactant (a) (Newcol 1308FA, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27-(OC2H4)8-OCH(CH3)CH2-OH) was dissolved in 2.7% by mass relative to the mass of PTFE, ammonium laurate in 0.06% by mass relative to the mass of PTFE, and triethanolamine lauryl sulfate in 0.02% by mass relative to the mass of PTFE, and the mixture was concentrated by electrophoresis. The supernatant was removed to obtain an aqueous PTFE dispersion with a PTFE concentration of 65.8% by mass and a nonionic surfactant (a) concentration of 2.1% by mass relative to the mass of PTFE. This PTFE aqueous dispersion contains a nonionic surfactant (b) (Newcol G1301H, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27 -OCH2CH(C2H5)-(OC2H4)8-OH) at 1.2% by mass relative to the mass of PTFE, nonionic surfactant (c) (Newcol FAA09801, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27 -OCH2CH(C2H5)-(OC2H4) 11 -OH) was added at a concentration of 1.2% by mass relative to the mass of PTFE, PEO at a concentration of 0.1% by mass relative to the mass of PTFE, water and aqueous ammonia were added, and nonionic surfactant (a) was added so that its content relative to the mass of PTFE was 2.4% by mass, to obtain PTFE aqueous dispersion 1-1 with a PTFE concentration of 60.5% by mass and a pH of 10.2. The viscosity of the obtained PTFE aqueous dispersion 1-1 at 23°C was 25.8 mPa·s, pH = 10.2, and surface tension was 30 (mN / m).

[0094] (Manufacturing example 2): 36g of F(CF2)2OCF2CF2OCF2COONH4, 555g of paraffin wax (melting point 55°C), and 61.3 liters of deionized water were placed in a 100L stainless steel autoclave. After purging the inside of the autoclave with nitrogen and reducing the pressure, TFE monomer was introduced and the temperature was raised to 62°C while stirring. Further TFE monomer was injected under pressure until the internal pressure reached 1.765 MPa [gauge], and 26.3g of disuccinate peroxide (concentration 80% by mass, the remainder being water) was dissolved in 1 liter of warm water at approximately 70°C and injected. After approximately 3 minutes, the autoclave pressure dropped to 1.716 MPa [gauge], so TFE monomer was injected under pressure to maintain the internal pressure at 1.765 MPa [gauge] and polymerization proceeded. During polymerization, EEA was dissolved in warm water, and a total of 53 g of EEA was injected in two separate injections. The autoclave temperature was gradually raised to 72°C, and the reaction was terminated when the amount of TFE monomer injected reached 22 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 105 minutes. After cooling, the solidified paraffin wax on top was removed, and a PTFE aqueous emulsion was obtained. The PTFE concentration in the PTFE aqueous emulsion was approximately 25.0% by mass, and the EEA concentration was 0.40% by mass relative to the mass of PTFE. The average particle size of PTFE particles in the aqueous emulsion was 0.26 μm. The average molecular weight of PTFE was 760,000, and the standard specific gravity of PTFE was 2.21. To the resulting aqueous emulsion, add a nonionic surfactant (a) (Newcol 1308FA, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27 -(OC2H4)8-OCH(CH3)CH2-OH) was dissolved at a ratio of 2.7% by mass relative to the mass of PTFE, ammonium laurate at 0.02% by mass relative to the mass of PTFE, and triethanolamine lauryl sulfate at 0.01% by mass relative to the mass of PTFE. The mixture was then concentrated by electrophoresis. The supernatant was removed to obtain an aqueous PTFE dispersion with a PTFE concentration of 60.5% by mass and a nonionic surfactant (a) concentration of 2.2% by mass relative to the mass of PTFE. This PTFE aqueous dispersion contains a nonionic surfactant (b) (Newcol G1301H, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27 -OCH2CH(C2H5)-(OC2H4)8-OH) at 1.3% by mass relative to the mass of PTFE, nonionic surfactant (c) (Newcol FAA09801, manufactured by Nippon Emulsifier Co., Ltd., C 13 H 27 -OCH2CH(C2H5)-(OC2H4) 11-OH) was added at a concentration of 1.3% by mass relative to the mass of PTFE, PEO at a concentration of 0.1% by mass relative to the mass of PTFE, 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 was 2.2% by mass to obtain PTFE aqueous dispersion 2.

[0095] (Manufacturing example 3): Instead of nonionic surfactant (a), use nonionic surfactant (d) (Tergitol TMN100X, C, manufactured by DOW). 12 H 25 -(OC2H4) 10 PTFE aqueous dispersions 1-2 were obtained by following the same procedure as in Example 1, except that -OH) was used.

[0096] (Manufacturing example 4) 1500g of paraffin wax and 60L of deionized water were placed in a 100L stainless steel autoclave. After purging the autoclave with nitrogen and reducing the pressure, i-butyl methacrylate (i-BMA) (0.1g) and deionized water (0.5L) were added to the autoclave by pouring them in. Next, the autoclave was heated to 75°C while the solution inside was stirred, with the pressure below atmospheric pressure. Then, a solution of ammonium persulfate (0.055 g), which is a polymerization initiator, dissolved in deionized water (1 L) was poured into the autoclave to polymerize i-butyl methacrylate. After 20 minutes, the autoclave was pressurized to 1.96 MPa using TFE, and a solution of ammonium persulfate (0.54 g) and disuccinate peroxide (80% by mass, remainder water) (53 g) dissolved in approximately 70°C warm water (1 L) was injected into the autoclave. Next, TFE was added to maintain the internal pressure of the autoclave at 1.96 MPa, and the polymerization of TFE proceeded. After adding 1 kg of TFE, a solution of ammonium vinylsulfonate (5.0 g) dissolved in deionized water (1.5 L) was added, while monitoring the amount of TFE supplied with a flow meter, so that the amount of ammonium vinylsulfonate was 0.15 g for every 1 kg of TFE supplied. The reaction was terminated when the amount of TFE added reached 9 kg, and the TFE in the autoclave was released into the atmosphere. The polymerization time was 97 minutes. The solid content concentration (concentration of modified PTFE) of the aqueous emulsion was approximately 12% by mass. The average primary particle size of the modified PTFE particles in the aqueous emulsion was 0.21 μm. A portion of the obtained aqueous emulsion was adjusted to 20°C and stirred to agglomerate the modified PTFE particles, thereby obtaining modified PTFE powder. Next, this modified PTFE powder was dried with an aqueous ammonium carbonate solution at 275°C. The SSG of the obtained modified PTFE powder was 2.193. Furthermore, no by-product fluorine-based oligomers were detected in the obtained modified PTFE powder. The aqueous emulsion obtained in step A2 was concentrated in the same manner as in production example 1 to obtain aqueous PTFE dispersions 1-3. The average primary particle size of the PTFE particles in aqueous PTFE dispersions 1-3 was the same as the average primary particle size of the PTFE particles measured in the aqueous emulsion.

[0097] (Manufacturing example 5) PTFE aqueous dispersions 1-4 were obtained by following the same procedure as in Production Example 3, except that nonionic surfactant (d) was used instead of nonionic surfactant (a).

[0098] (Example 1) The PTFE aqueous dispersion 1-1 obtained in Production Example 1 and the PTFE aqueous dispersion 2 obtained in Production Example 2 were mixed in a ratio of 20 / 80 (solids by mass) to obtain a mixed aqueous dispersion. (Examples 2-12) A mixed aqueous dispersion was obtained in the same manner as in Example 1, except that the solid content mass ratio was changed to the value shown in Table 1.

[0099] [Table 1]

Claims

1. A method for producing a polytetrafluoroethylene mixed aqueous dispersion, comprising mixing the following polytetrafluoroethylene aqueous dispersion 1 and the following polytetrafluoroethylene aqueous dispersion 2 such that the solid content mass ratio is polytetrafluoroethylene aqueous dispersion 1 / polytetrafluoroethylene aqueous dispersion 2 = 90 / 10 to 10 / 90. [Polytetrafluoroethylene aqueous dispersion 1] Step A1 involves polymerizing a non-fluorinated monomer in an aqueous medium to obtain a solution 1 containing a polymer that includes units based on the non-fluorinated monomer, Step A2 involves polymerizing tetrafluoroethylene in the solution 1 without substantially adding a surfactant to the solution 1 to obtain an aqueous emulsion containing polytetrafluoroethylene particles. The method includes step A3, which involves adding a nonionic surfactant to the aqueous emulsion, and then concentrating the aqueous emulsion to obtain a polytetrafluoroethylene aqueous dispersion. A polytetrafluoroethylene aqueous dispersion in which the amount of the non-fluorinated monomer used is 200 ppm by mass or less relative to the amount of tetrafluoroethylene supplied to the polymerization system. [Polytetrafluoroethylene aqueous dispersion 2] An aqueous dispersion containing polytetrafluoroethylene particles and an aqueous medium, which is different from the polytetrafluoroethylene aqueous dispersion 1.

2. The method for producing a polytetrafluoroethylene mixed aqueous dispersion according to claim 1, wherein the polytetrafluoroethylene aqueous dispersion 2 is obtained by a method comprising step B1 of emulsion polymerization of tetrafluoroethylene in an aqueous medium in the presence of a surfactant to obtain an aqueous emulsion, and step B2 of adding a nonionic surfactant to the aqueous emulsion and then concentrating the aqueous emulsion.

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