Method for producing an aqueous dispersion of fluoropolymer, method for producing a fluoropolymer, and method for reducing the concentration of a fluorine-containing emulsifier in an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier.

Abstract

To propose a method for effectively reducing the amount of fluorine-containing emulsifiers that may remain in aqueous emulsion polymerization solutions of fluoropolymers, including fluoroelastomers and perfluoroelastomers. [Solution] The following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step to obtain an aqueous fluoropolymer dispersion by passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. A method for producing an aqueous dispersion of a fluoropolymer containing at least [a specific substance] is provided. Furthermore, a method for producing a fluoropolymer is provided for obtaining a fluoropolymer from the obtained aqueous dispersion of a fluoropolymer.

Description

Technical Field

[0001] The present invention relates to a method for producing an aqueous dispersion of a fluoropolymer. Further, the present invention relates to a method for producing a fluoropolymer. Further, the present invention relates to a method for reducing the concentration of a fluorinated emulsifier in an aqueous dispersion of a fluoropolymer containing the fluorinated emulsifier.

Background Art

[0002] Fluoroelastomers (FKM) and perfluoroelastomers (FFKM) are known to have extremely excellent heat resistance, chemical resistance, weather resistance, low friction, non-stickiness, and plasma resistance at the same level as fluoropolymers. Fluoropolymers (fluorinated polymers) including FKM and FFKM are usually produced by emulsion polymerization of fluoromonomers using a fluorinated compound as an emulsifier (fluorinated emulsifier). However, in the chemical substance regulation by the European Chemicals Agency (ECHA) or the comprehensive registration, evaluation, authorization, and restriction system of European chemical substances known as the REACH regulation, it is required to reduce the fluorinated emulsifier contained in the fluoropolymer.

[0003] As a method for reducing the amount of the fluorinated emulsifier contained in an aqueous dispersion of a fluoropolymer, a method has been proposed in which an aqueous dispersion of a fluoropolymer is brought into contact with a basic anion exchange resin to adsorb the fluorinated emulsifier to the basic anion exchange resin. For example, it has been proposed to add a nonionic surfactant to an aqueous emulsion polymerization liquid of a fluoropolymer to produce an aqueous dispersion of a fluoropolymer, and then to bring the aqueous dispersion of the fluoropolymer into contact with a basic anion exchange resin to produce an aqueous dispersion of a fluoropolymer having a low content of the fluorinated emulsifier (Patent Document 1, Patent Document 2).

[0004] Patent Document 1 discloses the use of Triton X100, TergitolTMN100X, Antarox863, Rodasurf870, and GenapolX080 as nonionic surfactants, and Patent Document 2 discloses the use of TergitolTMN-6, TergitolTMN-10, and Tergitol100X as nonionic surfactants. However, when these nonionic surfactants are added to an aqueous emulsion polymerization solution of a fluoropolymer, the fluoropolymer is stabilized in the aqueous dispersion, making it difficult to coagulate using conventional methods such as salts or acids, thus making it difficult to obtain a fluoropolymer as a product.

[0005] Patent Document 3 is the only reported example in which a fluoropolymer was obtained by adding a nonionic surfactant to an aqueous emulsion polymerization solution of a fluoropolymer, then contacting it with a basic anion exchange resin, and finally coagulating it. However, this example proposes the use of an organic solvent and a forced stirrer during coagulation, which makes it impractical from the standpoint of commercializing fluoropolymers.

[0006] On the other hand, fluoroelastomers are known that have nitrile groups in the polymer molecular chain, which are substituents for crosslinking in subsequent processes. However, when an aqueous emulsion polymerization solution of such a fluoroelastomer is brought into contact with an anion exchange resin preconditioned with potassium hydroxide (strongly basic), the nitrile moieties in the fluoroelastomer molecular chain decompose because the inside of the anion exchange resin column becomes basic. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 4494932 [Patent Document 2] Patent No. 5364576 [Patent Document 3] International Publication No. 2008 / 134138 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention aims to propose a method for effectively reducing the amount of fluorine-containing emulsifier that may remain in aqueous emulsion polymerization solutions of fluoropolymers containing FKM and FFKM. [Means for solving the problem]

[0009] One aspect of the present invention involves the following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step to obtain an aqueous fluoropolymer dispersion by passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. This is a method for producing an aqueous dispersion of a fluoropolymer containing at least [a specific substance].

[0010] A second aspect of the present invention involves the following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step of obtaining an aqueous fluoropolymer dispersion by passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed, and The process involves adding an acid to the aqueous dispersion of the fluoropolymer to cause coagulation and obtain a fluoropolymer. This is a method for producing fluoropolymers, which include [the specified element].

[0011] A third aspect of the present invention is the following step: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step of passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. This is a method for reducing the concentration of a fluorine-containing emulsifier in an aqueous dispersion of a fluoropolymer containing a fluorine-containing emulsifier, which contains at least [specific element]. [Effects of the Invention]

[0012] The method of the present invention removes fluorine-containing emulsifiers contained in aqueous fluoropolymer dispersions without hindering the coagulation of fluoropolymers in the aqueous fluoropolymer dispersions, thus enabling the provision of high-quality fluoropolymers without impeding the commercialization of fluoropolymers. When removing fluorine-containing emulsifiers from aqueous dispersions of fluoroelastomers containing polymerizable substituents such as nitrile groups or substituents that act as crosslinking points in the molecular chains using the method of the present invention, the polymerizable substituents or substituents that act as crosslinking points in the fluoroelastomer molecular chains do not decompose, thus enabling the efficient provision of high-quality fluoroelastomers with the intended structure. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described in more detail below, but the present invention is not limited to the following embodiments.

[0014] One embodiment involves the following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step of passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorinated emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed to obtain a fluoropolymer aqueous dispersion, A method for producing a fluoropolymer aqueous dispersion, which at least includes this.

[0015] In one embodiment, a fluoropolymer, also called a fluorinated polymer etc., has been conventionally used in a wide range of applications such as the chemical industry, the electric and electronic fields, the automotive and aerospace industries, household goods, the medical and bio fields, etc. Examples of common fluoropolymers include thermoplastic or thermosetting fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene - hexafluoropropene copolymer (FEP), tetrafluoroethylene - ethylene copolymer (ETFE), etc., and fluorine elastomers (FKM) and perfluoroelastomers (FFKM) etc. having a polymerizable substituent or a substituent serving as a crosslinking point in the polymer molecular chain. In one embodiment, the term fluoropolymer shall include all polymers containing fluorine atoms in the polymer molecular chain.

[0016] A fluoropolymer is obtained by polymerizing a fluoromonomer. Here, a fluoromonomer is a compound having at least one polymerizable substituent in the molecule, and refers to a partially fluorinated or perfluorinated compound. In this specification, a fluoromonomer may be referred to as a fluorinated monomer, and these may be considered synonymous. Examples of fluoromonomers include tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), perfluoro(2 - phenoxypropyl) vinyl ether (PVE), vinylidene fluoride (VDF), trifluoroethylene (TrFE), perfluoroalkyl vinyl ethers such as perfluoromethyl vinyl ether, and perfluoroalkoxyalkyl vinyl ethers (PAAVE) such as perfluoromethyl - oligo(isopropoxy) vinyl ether, etc.

[0017] The fluoropolymer aqueous dispersion produced according to one embodiment is an aqueous liquid in which the above fluoropolymer is emulsified or dispersed. When referring to a fluoropolymer aqueous dispersion in this specification, it shall also include an aqueous emulsion of a fluoropolymer. That is, when referring to a fluoropolymer aqueous dispersion in this specification, it shall also include an aqueous emulsion polymerization liquid of a fluoropolymer obtained by aqueous emulsion polymerization of a fluoromonomer.

[0018] One embodiment is a method for producing a fluoropolymer aqueous dispersion by removing a fluorinated emulsifier from a fluoropolymer aqueous dispersion containing a fluorinated emulsifier. Here, the fluorinated emulsifier refers to all emulsifiers having a fluorine atom in the molecule. When obtaining a fluoropolymer aqueous dispersion by aqueous emulsion polymerization of a fluoromonomer, first, it is necessary to emulsify the fluoromonomer in water, so the addition of an emulsifier is required. Conventionally, fluorinated emulsifiers have been used here. Examples of fluorinated emulsifiers include perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), fluorotelomer alcohol (FTOHs), and PFOA alternative compounds such as hexafluoropropylene oxide dimer carboxylic acid (HFPO-DA), hexafluoropropylene oxide trimer carboxylic acid (HFPO-TA), or 3H-perfluoro-3-(3-methoxypropyl)-propanoic acid (ADONA).

[0019] As described above, generally, the polymerization of a fluoromonomer is carried out by aqueous emulsion polymerization of the fluoromonomer in an aqueous dispersion, so the obtained fluoropolymer aqueous dispersion contains a fluorinated emulsifier. However, in recent years, many countries have imposed regulations on the use of fluorinated compounds such as PFOA and PFOS, and it is required to reduce these compounds contained in products as much as possible. For this reason, one embodiment attempts to adsorb a fluorinated emulsifier onto a strongly basic anion exchange resin to remove the fluorinated emulsifier from the fluoropolymer aqueous dispersion. In one embodiment, first, a step of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin is performed.

[0020] Anion exchange resins are ion exchange resins that can exchange anions in a substance. Anion exchange resins contain a fixed ion with a positive charge bonded to a negatively charged ion, and these negative ions exchange with other anions. Anion exchange resins include strongly basic anion exchange resins, consisting of strongly basic anion exchange resin type I or strongly basic anion exchange resin type II, and weakly basic anion exchange resins. In one embodiment, a hydroxide ion type strongly basic anion exchange resin is preferably used. A hydroxide ion type strongly basic anion exchange resin is a hydroxide ion (OH) - It is a strongly basic anion exchange resin having ) as an exchange group. As the strongly basic anion exchange resin, resins such as styrene-divinylbenzene copolymer, acrylic, and polyvinylbenzylamine can be used. As strongly basic anion exchange resins, commercially available strongly basic anion exchange resins such as Mitsubishi Chemical Corporation's DIANON® series, DuPont's AMBERLITE® IRA400JCl, IRA402BL CL, IRA900J CL, IRA900J CL, etc., Dow Chemical's DOWEX 1X8, and Lanxess's LEWATIT MP 500 can be obtained.

[0021] In one embodiment, a step is performed in which a weakly acidic aqueous solution is passed through a hydroxide ion type strongly basic anion exchange resin. This step is particularly necessary when the fluoropolymer contained in the aqueous dispersion of fluoropolymer that is brought into contact with the strongly basic anion exchange resin is a fluoroelastomer or perfluoroelastomer containing polymerizable substituents such as nitrile groups or substituents that act as crosslinking points. By passing a weakly basic aqueous solution through a hydroxide ion type strongly basic anion exchange resin, the pH inside the strongly basic anion exchange resin column can be slightly lowered. As the weakly acidic aqueous solution to be passed through the hydroxide ion type strongly basic anion exchange resin, aqueous solutions of acetic acid, citric acid, tartaric acid, oxalic acid, lactic acid, pyruvic acid, malic acid, butyric acid, and succinic acid can be used, as well as carbonated water.

[0022] In one embodiment, preconditioning (pretreatment) of the strongly basic anion exchange resin may be performed prior to the step of passing a weakly acidic aqueous solution through the hydroxide ion type strongly basic anion exchange resin. Specifically, preconditioning includes initial adjustment and activation treatment performed before using the strongly basic anion exchange resin. In one embodiment, first, a step of preconditioning the strongly basic anion exchange resin using a strongly basic aqueous solution is performed. This step includes washing with water to remove any fine particles, impurities, suspended matter, etc. that may be contained in the strongly basic anion exchange resin, and if necessary, the strongly basic anion exchange resin after washing is brought into contact with a strongly basic aqueous solution to activate the strongly basic anion exchange resin. If necessary, the strongly basic anion exchange resin may be washed again with water to stabilize the pH of the activated strongly basic anion exchange resin, and this subsequent washing is also included in the preconditioning step. The strongly basic aqueous solution used to activate the strongly basic anion exchange resin is an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, etc. By preconditioning a strongly basic anion exchange resin, a hydroxide ion type strongly basic anion exchange resin can be obtained.

[0023] Next, a fluoropolymer aqueous dispersion containing a fluorine-containing emulsifier is brought into contact with a strongly basic anion exchange resin. As described above, fluoropolymers are produced by emulsion polymerization in an aqueous dispersion of fluoromonomers, but fluorine-containing emulsifiers such as PFOA and PFOS are used during the emulsification of fluoromonomers in water. Therefore, fluorine-containing emulsifiers remain in the fluoropolymer aqueous dispersion obtained by emulsion polymerization of the aqueous dispersion of fluoromonomers. In order to commercialize the fluoropolymer aqueous dispersion and fluoropolymer, it is necessary to remove the fluorine-containing emulsifier as much as possible. Therefore, the fluoropolymer aqueous dispersion containing a fluorine-containing emulsifier is brought into contact with a strongly basic anion exchange resin to adsorb the fluorine-containing emulsifier onto the ion exchange resin and remove it from the fluoropolymer aqueous dispersion.

[0024] Prior to contacting the aqueous fluoropolymer dispersion containing a fluorine-containing emulsifier with the strongly basic anion exchange resin, the first step is to add polyoxyethylene(20) sorbitan monolaurate to the aqueous fluoropolymer dispersion containing the fluorine-containing emulsifier. This step is independent of the preconditioning of the strongly basic anion exchange resin described above. The polyoxyethylene(20) sorbitan monolaurate used here is a type of so-called nonionic surfactant, and commercially available products such as Tween 20 (Aldrich, Kishida Chemical Co., Ltd., etc.) and Polysorbate 20 (Fujifilm Wako Pure Chemical Industries, Ltd., Nikko Chemicals Co., Ltd., etc.) are known. When polyoxyethylene(20) sorbitan monolaurate is added to the aqueous fluoropolymer dispersion containing the fluorine-containing emulsifier, the aqueous fluoropolymer dispersion containing the fluorine-containing emulsifier is stabilized. However, conventionally used nonionic surfactants such as Triton X100 and Tergitol TMN-6 stabilize the aqueous fluoropolymer dispersion to such an extent that coagulation of the fluoropolymer becomes difficult in the final step of the second embodiment described later, which involves coagulation using acid. On the other hand, if polyoxyethylene (20) sorbitan monolaurate is added to the aqueous fluoropolymer dispersion, the ester moiety of the polyoxyethylene (20) sorbitan monolaurate hydrolyzes in the final step of the second embodiment described later, which involves coagulation using acid, destabilizing the aqueous fluoropolymer dispersion system. For this reason, in this embodiment, it is preferable to add polyoxyethylene (20) sorbitan monolaurate to the aqueous fluoropolymer dispersion containing a fluorine-containing emulsifier at this stage.

[0025] Next, a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier is passed through a strongly basic anion exchange resin through which a weakly acidic aqueous solution has been passed to obtain a fluoropolymer aqueous dispersion. In this step, the fluorine-containing emulsifier is adsorbed onto the strongly basic anion exchange resin, so a fluoropolymer aqueous dispersion with a reduced fluorine-containing emulsifier content, preferably one that contains almost no fluorine-containing emulsifier, can be obtained. As explained above, since a weakly acidic aqueous solution has been passed through the strongly basic anion exchange resin in advance to slightly lower the pH inside the strongly basic anion exchange resin column, even if the fluoropolymer contained in the fluoropolymer aqueous dispersion contains polymerizable substituents such as FKM or FFKM or substituents that act as crosslinking points (e.g., nitrile groups), such substituents will not decompose. Therefore, it is possible to effectively remove the fluorine-containing emulsifier from the fluoropolymer aqueous dispersion containing the fluorine-containing emulsifier and obtain an aqueous dispersion of the fluoropolymer while maintaining the intended structure.

[0026] A second embodiment of the present invention involves the following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. A step of adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of fluoropolymer containing a fluorine-containing emulsifier, followed by A step of obtaining an aqueous fluoropolymer dispersion by passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed, and The process involves adding acid to the aqueous dispersion of the fluoropolymer to cause coagulation and obtain a fluoropolymer. This is a method for producing fluoropolymers, which include [the specified element].

[0027] The second embodiment is a method for obtaining a fluoropolymer by removing the fluorine-containing emulsifier from an aqueous fluoropolymer dispersion containing the fluorine-containing emulsifier according to the first embodiment of this specification to obtain an aqueous fluoropolymer dispersion that is substantially free of the fluorine-containing emulsifier, and then adding an acid to cause coagulation and obtain a fluoropolymer. As the acid, nitric acid, sulfuric acid, hydrochloric acid, etc. can be used, but it is preferable to use nitric acid, which promotes the decomposition of polyoxyethylene (20) sorbitan monolaurate.

[0028] The third embodiment involves the following steps: A step of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin, followed by A step of passing a fluoropolymer aqueous dispersion containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. This is a method for reducing the concentration of a fluorine-containing emulsifier in an aqueous dispersion of a fluoropolymer containing a fluorine-containing emulsifier, which contains at least [specific element].

[0029] The third embodiment is a method for reducing the concentration of a fluorine-containing emulsifier in an aqueous dispersion of a fluoropolymer containing a fluorine-containing emulsifier by performing the same steps as in the first embodiment of this specification. In this specification, "reducing the concentration of the fluorine-containing emulsifier" means making the concentration of the fluorine-containing emulsifier smaller, and also includes making the concentration of the fluorine-containing emulsifier 0% by mass, that is, completely removing the fluorine-containing emulsifier. Furthermore, "removing the fluorine-containing emulsifier" generally means making the concentration of the fluorine-containing emulsifier 0% by mass, but also includes making the concentration of the fluorine-containing emulsifier approximately 0% by mass, that is, not completely 0% by mass, but at a concentration that is almost undetectable.

[0030] According to the three embodiments, the concentration of the fluorine-containing emulsifier in an aqueous dispersion of fluoropolymer containing the fluorine-containing emulsifier can be reduced, and from the aqueous dispersion of fluoropolymer with reduced fluorine-containing emulsifier concentration, a fluoropolymer can be obtained that has reduced fluorine-containing emulsifier concentration while maintaining the originally intended structure. [Examples]

[0031] The embodiments of the present invention will be described in detail below. The present invention is not limited to the following embodiments.

[0032] [Example 1] An aqueous perfluoroelastomer dispersion was obtained by emulsion polymerization. In a 500-liter autoclave, 6.1 kg of tetrafluoroethylene (TFE), 5.7 kg of perfluoromethyl vinyl ether (FMVE), 431.6 kg of perfluorocyanopentyl vinyl ether (CPeVE), 3870 g of hexafluoropropylene oxide trimer acid (HFPO-TA) (pre-neutralized with 9700 g of 1.4% aqueous ammonia), 3480 g of potassium dihydrogen phosphate, and 243.8 kg of deionized water were added, and the temperature was raised to 60°C. 623 g of ammonium peroxodisulfate, a polymerization initiator, was added, and polymerization was carried out under a pressure of 0.80 MPa. 39.8 kg of TFE, 39.8 kg of FMVE, 41.0 kg of FMVE, and 2954.6 g of CPeVE were added in portions while maintaining a pressure of 0.80 MPa inside the autoclave. After polymerization, the mixture was aged for 1 hour to obtain an aqueous dispersion of the perfluoroelastomer. To 1750 g of the obtained aqueous dispersion, 39.5 g of the nonionic surfactant Tween20 (ALDRICH) and 750 g of water were added to obtain an aqueous dispersion of the perfluoroelastomer containing the nonionic surfactant.

[0033] The strongly basic anion exchange resin AmberliteIRA402CL (DuPont) was preconditioned with a 1N potassium hydroxide aqueous solution at a flow rate (BV) of 6 and a space velocity (SV) of 5. It was then extruded with water at a BV of 1.5 and an SV of 4, and further washed with water at a BV of 10 and an SV of 20. Subsequently, a 1N acetic acid aqueous solution was passed through this strongly basic anion exchange resin at a BV of 6 and an SV of 5, followed by extrusion with water at a BV of 1.5 and an SV of 4, and further washed with water at a BV of 10 and an SV of 20.

[0034] The aqueous dispersion of the perfluoroelastomer containing the previously obtained nonionic surfactant was passed through the strongly basic anion exchange resin prepared in this manner using BV2.7 and SV1. NMR measurements were performed on the liquids before and after the passage, using sodium hexafluorophosphate as the internal standard. The concentrations of HFPO-TA were 7181 ppm and 691 ppm, respectively. Furthermore, the pH of the liquids before and after passage remained below 7 in both cases. Next, 8% by weight nitric acid was added to the aqueous dispersion of the perfluoroelastomer obtained after liquid flow, and the mixture was heated at 80°C for 1 hour, yielding a white solid (polymer). The obtained polymer was washed 22 times with tap water at 90°C. This was dried in a dryer to obtain the perfluoroelastomer. The yield of the perfluoroelastomer was 89.7%, and the amount of HFPO-TA remaining in the perfluoroelastomer was calculated to be 235 ppm by gas chromatography-mass spectrometry (GCMS). Furthermore, infrared absorption spectroscopy (IR) of the obtained perfluoroelastomer showed that the peak intensity normalized by the peak intensity derived from CF bonds to the peak intensity derived from nitrile groups was 0.0189, indicating that no decomposition of nitrile groups in the perfluoroelastomer molecular chain was observed.

[0035] [Comparative Example 1] To 80 g of an aqueous dispersion of perfluoroelastomer obtained in the same manner as in Example 1, 20 g of water and 2 g of the nonionic surfactant Genapol-X080 (isotridecyl alcohol polyglycol ether (8EO), Sigma-Aldrich) were added. The strongly basic anion exchange resin AmberliteIRA402CL was preconditioned in the same manner as in Example 1.

[0036] The aqueous dispersion of the perfluoroelastomer containing the previously obtained nonionic surfactant was passed through the strongly basic anion exchange resin prepared in this manner using BV2.7 and SV1. NMR measurements were performed on the liquids before and after passage, using sodium hexafluorophosphate as the internal standard. The concentrations of HFPO-TA were found to be 7181 ppm and 321 ppm, respectively.

[0037] Next, 2 g of 8% by weight nitric acid was added to 20 g of the aqueous dispersion of the perfluoroelastomer obtained after liquid flow, and the mixture was heated at 90°C for 1 hour, but the polymer did not coagulate.

[0038] [Comparative Example 2] In the same manner as in Example 1, Tween20, a nonionic surfactant, and water were added to the aqueous dispersion of the perfluoroelastomer to obtain an aqueous dispersion of the perfluoroelastomer containing the nonionic surfactant.

[0039] The strongly basic anion exchange resin AmberliteIRA402CL was preconditioned with a 1N potassium hydroxide aqueous solution at a water flow rate (BV) of 6 and a space velocity (SV) of 5, as in Example 1. Then, it was extruded with water at a BV of 1.5 and an SV of 4, and further washed with water at a BV of 10 and an SV of 20.

[0040] The aqueous dispersion of the perfluoroelastomer containing the previously obtained nonionic surfactant was passed through the strongly basic anion exchange resin prepared in this manner using BV2.7 and SV1. Similar to Example 1, 8% by weight nitric acid was added to the aqueous dispersion of perfluoroelastomer obtained after passing through the resin, and the mixture was heated at 80°C for 1 hour to obtain a white solid (polymer). The obtained polymer was washed 22 times with tap water at 90°C. This was dried in a dryer to obtain a perfluoroelastomer. When the obtained perfluoroelastomer was measured by infrared absorption spectroscopy (IR), the value of the peak intensity derived from the nitrile group normalized by the peak intensity derived from the CF bond was 0.0057, indicating decomposition of the nitrile group.

[0041] The method of the present invention made it possible to reduce the amount of fluorine-containing emulsifier in an aqueous dispersion of a fluoropolymer containing a fluorine-containing emulsifier. Furthermore, the method of the present invention made it possible to easily coagulate the fluoropolymer and obtain a fluoropolymer that contains almost no fluorine-containing emulsifier. No decomposition of substituents that form crosslinking points in the molecular chain of the obtained fluoropolymer was observed (Example 1).

[0042] On the other hand, with conventional methods, the fluoropolymer did not coagulate easily (Comparative Example 1). Furthermore, when an aqueous dispersion of a fluoropolymer containing a fluorine-containing emulsifier was passed through a strongly basic anion exchange resin without passing an aqueous acetic acid solution through it, decomposition of substituents that form crosslinking points in the fluoropolymer molecular chain was observed (Comparative Example 2).

Claims

1. The following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. The process involves adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of a fluoropolymer obtained by polymerizing a fluoromonomer mixture containing a fluorine-containing emulsifier and one or more fluoromonomers selected from the group consisting of perfluoro(2-phenoxypropyl) vinyl ether, perfluoroalkyl vinyl ether, perfluorocyanopentyl vinyl ether, and perfluoroalkoxyalkyl vinyl ether, and then... A step to obtain an aqueous fluoropolymer dispersion by passing an aqueous dispersion of a fluoropolymer containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. A method for producing an aqueous dispersion of a fluoropolymer containing at least [a certain substance].

2. The following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. The process involves adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of a fluoropolymer obtained by polymerizing a fluoromonomer mixture containing a fluorine-containing emulsifier and one or more fluoromonomers selected from the group consisting of perfluoro(2-phenoxypropyl) vinyl ether, perfluoroalkyl vinyl ether, perfluorocyanopentyl vinyl ether, and perfluoroalkoxyalkyl vinyl ether, and then... A step of obtaining an aqueous fluoropolymer dispersion by passing an aqueous dispersion of a fluoropolymer containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed, and The process involves adding acid to the aqueous dispersion of the fluoropolymer to cause coagulation and obtain a fluoropolymer. A method for producing fluoropolymers, including the following:

3. The following steps: A process of passing a weakly acidic aqueous solution through a hydroxide ion type strongly basic anion exchange resin. The process involves adding polyoxyethylene (20) sorbitan monolaurate to an aqueous dispersion of a fluoropolymer obtained by polymerizing a fluoromonomer mixture containing a fluorine-containing emulsifier and one or more fluoromonomers selected from the group consisting of perfluoro(2-phenoxypropyl) vinyl ether, perfluoroalkyl vinyl ether, perfluorocyanopentyl vinyl ether, and perfluoroalkoxyalkyl vinyl ether, and then... A step of passing an aqueous dispersion of a fluoropolymer containing polyoxyethylene (20) sorbitan monolaurate and a fluorine-containing emulsifier through a strongly basic anion exchange resin through which the weakly acidic aqueous solution has been passed. A method for reducing the concentration of a fluorine-containing emulsifier in an aqueous dispersion of a fluoropolymer containing at least the fluorine-containing emulsifier.