JP7715223B2 - Fluorine resin manufacturing method - Google Patents

Description

The present invention relates to a fluororesin with few impurities and excellent optical properties, and a method for producing the same.

Fluorine-based resins have traditionally been used in protective films for semiconductors and other electronic components, water-repellent films for inkjet printing, waterproof and oil-repellent coatings for filters, and optical components due to their excellent electrical and optical properties, chemical resistance, waterproofness, and oil-repellent properties.

Of these, fluororesins containing oxolane rings have a bulky ring structure, making them amorphous and highly transparent and heat-resistant. Furthermore, because they are composed only of carbon, fluorine, and oxygen, they have excellent optical and electrical properties, chemical resistance, waterproofing, and liquid and oil repellency. Furthermore, because they are amorphous, they can be melt-molded.

Non-Patent Document 1 describes a fluororesin containing an oxolane ring, noting that the transparency of the resin decreases when stored in the air for more than two weeks, and that it turns yellow when heated at 260-290°C. It also describes how re-precipitating and purifying this resin removes impurities with carboxylic acid groups that are generated in side reactions during polymerization, preventing a decrease in transparency even when stored in the air; however, it does not describe discoloration upon heating. According to the inventors, when the fluororesin is re-precipitated, the metal components contained in the resin cannot be sufficiently removed, resulting in the problem of discoloration upon heating.

Patent Document 1 also describes the possibility of obtaining fluororesin particles by means of suspension polymerization, emulsion polymerization, or other methods. However, the dispersants and emulsifiers used as polymerization aids remain inside the resin particles and cause discoloration when heated, impairing the transparency and heat resistance that are characteristic of this resin. According to the inventors, there was also the issue of discoloration occurring when heated due to the fluororesin containing certain metals.

Since overheating is required during molding processing, there is a risk that the above-mentioned fluororesin processed products may become discolored. Therefore, with the aim of using them in optical applications, there has been a demand for fluororesins that reduce discoloration when heated.

WO2014/156996 Public Relations Macromolecules, 2005, 38, 4237-4245

The present invention was made in consideration of the above-mentioned problems, and provides a fluororesin that exhibits reduced discoloration when heated, and a method for producing the same.

The inventors discovered that a fluororesin containing a residue unit represented by the following general formula (1) and having a total content of chromium, iron, and nickel of 500 ppb by mass or less exhibits little discoloration when heated, leading to the completion of the present invention.

(In formula (1), Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ each independently represent a group selected from the group consisting of a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms, which may have an etheric oxygen atom. Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked together to form a ring having 4 to 8 carbon atoms.)

The invention is described in detail below.

The present invention relates to a resin that contains a residue unit represented by general formula (1) and has a total content of chromium, iron, and nickel metals of 500 ppb by mass or less.

The fluororesin of the present invention is amorphous and has high transparency and heat resistance due to the bulky ring structure contained in general formula (1). Furthermore, because it is composed only of carbon, fluorine, and oxygen, it has excellent electrical properties, chemical resistance, waterproofness, and liquid and oil repellency.

In the present invention, the Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ groups in the residue unit represented by general formula (1) each independently represent one type of a group consisting of a linear, branched or cyclic perfluoroalkyl group which may have a fluorine atom or an etheric oxygen atom having 1 to 7 carbon atoms. Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked to each other to form a ring having 4 to 8 carbon atoms. Examples of linear perfluoroalkyl groups having 1 to 7 carbon atoms include a trifluoromethyl group, a pentafluoroethyl group, a heptafluoropropyl group, a nonafluorobutyl group, an undecafluoropentyl group, a tridecafluorohexyl group, and a pentadecafluoroheptyl group. Examples of branched perfluoroalkyl groups having 3 to 7 carbon atoms include a heptafluoroisopropyl group, a nonafluoroisobutyl group, a nonafluorosec-butyl group, and a nonafluorotert-butyl group. Examples of cyclic perfluoroalkyl groups having 3 to 7 carbon atoms include a heptafluorocyclopropyl group, a nonafluorocyclobutyl group, and a tridecafluorocyclohexyl group. Examples of linear perfluoroalkyl groups having 1 to 7 carbon atoms which may have an etheric oxygen atom include -CF ₂ OCF ₃ groups, -(CF ₂ ) ₂ OCF ₃ groups, and -(CF ₂ ) ₂ OCF ₂ CF ₃ groups. Examples of cyclic perfluoroalkyl groups having 3 to 7 carbon atoms which may have an etheric oxygen atom include 2-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl groups, 4-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl groups, and 2-(2,3,3,4,4,5,5-heptafluoro)-furanyl groups.

In order to obtain excellent heat resistance, it is preferable that at least one of Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ is a group selected from the group consisting of linear, branched or cyclic perfluoroalkyl groups having 1 to 7 carbon atoms.

Specific examples of residue units represented by general formula (1) include the following residue units:

Among these, resins containing the following residue units are preferred due to their excellent heat resistance and moldability, and resins containing perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue units represented by general formula (3) are more preferred.

The resin of the present invention has a total content of chromium, iron, and nickel metals of 500 ppb by mass or less. This results in a fluororesin with reduced discoloration when heated. The content is preferably 300 ppb by mass or less, and more preferably 120 ppb by mass or less. This further reduces discoloration when heated. Typically, the content is expressed as 5 ppb by mass or more, but this is due to the lower analytical limit, and if measurable, it may be below 5 ppb by mass.

Here, the metal contents of chromium, iron, and nickel can each be measured using standard composition analysis, such as IPC-MS.

Furthermore, since this results in a fluororesin with reduced coloration, it is more preferable that the chromium and nickel metal contents are each 100 ppb by mass or less.

The resin of the present invention preferably has a sodium content of 1,000 ppb by mass or less.

Because the resin of the present invention has excellent fluidity and moldability, it is preferably in a particulate form, and more preferably has a volume average particle diameter of 5 μm or more and 500 μm or less. This is preferable because it allows for continuous supply to molding machines and the like when processing the resin. A volume average particle diameter of 5 μm or more makes the resin less likely to be blown away by air currents, improving handleability. Furthermore, a volume average particle diameter of 500 μm or less provides high fluidity, allowing for continuous supply to molding machines and the like, improving handleability.

When the resin of the present invention has a particulate shape, the volume average particle diameter can be evaluated by particle size distribution measurement (volume distribution) using laser diffraction scattering. Particle size distribution using laser diffraction scattering can be quantified with good reproducibility by dispersing resin particles in water and measuring after subjecting them to a process using an ultrasonic homogenizer to homogenize the dispersion state of the crystalline particles. An example of a laser scattering meter is the MT3000 manufactured by Microtrac.

When the resin of the present invention has a particulate shape, it is preferably a precipitation polymer, as it does not contain an emulsifier or dispersant and has excellent transparency and heat resistance.

When the resin of the present invention has a particulate shape, the bulk density is preferably 0.2 g/cm ³ or more and 1.5 g/cm ³ or less from the viewpoint of packing properties.

The resin of the present invention may contain other monomer residue units, such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), trifluoroethylene, hexafluoroisobutylene, perfluoroalkylethylene, fluorovinyl ether, vinyl fluoride (VF), vinylidene fluoride (VDF), perfluoro-2,2-dimethyl-1,3-dioxole (PDD), perfluoro(allyl vinyl ether), and perfluoro(butenyl vinyl ether).

In the present invention, there are no limitations on the molecular weight of the resin, and examples include a weight-average molecular weight of 2,500 to 2,000,000 in terms of PMMA as measured by gel permeation chromatography (GPC). From the standpoint of the resin's melt viscosity and mechanical strength, a molecular weight of 10,000 to 1,000,000 (g/mol) is preferred.

The yellowness index of the fluororesin of the present invention is preferably 3.0 or less, and more preferably 1.0 or less. This allows the molded article to be more suitably used for optical applications. The yellowness index can be measured, for example, by press-molding the fluororesin into the desired shape.

Here, yellowness can be measured using a common spectrophotometer.

Next, we will explain the method for producing the resin of the present invention.

A first aspect of the method for producing a resin of the present invention is
Examples of the method for producing a fluororesin include a precipitation polymerization step in which precipitation polymerization is carried out in the presence of a monomer represented by the following general formula (4), a radical polymerization initiator, and an organic solvent, with the water content in the reaction system being 1,000 ppm by mass or less.

Furthermore, since a particulate resin with excellent flowability and moldability can be obtained, it is preferable that the organic solvent dissolves the monomer represented by the following general formula (4) and precipitates a resin containing a residue unit represented by the following general formula (5).

(In formula (4), Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ each independently represent a group selected from the group consisting of a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms, which may have an etheric oxygen atom. Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ may be linked together to form a ring having 4 to 8 carbon atoms.)

(In formula (5), Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ each independently represent a group selected from the group consisting of a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms, which may have an etheric oxygen atom. Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ may be linked together to form a ring having 4 to 8 carbon atoms.)

In the resin production method of the present invention, an organic solvent (hereinafter referred to as the "precipitation polymerization solvent") is used as the polymerization solvent to dissolve the monomer represented by general formula (4) and precipitate a resin containing a residue unit represented by general formula (5). This allows the resin produced by the polymerization reaction to be precipitated as particles having a specific volume average particle size, resulting in the production of a resin with a particle shape that exhibits excellent moldability and packing properties. Furthermore, because no polymerization aids such as emulsifiers or dispersants are used, resin particles can be produced that do not contain emulsifiers or dispersants, which can impair transparency and heat resistance.

Whether a certain organic solvent will precipitate a certain resin can be determined by whether the polarity of the organic solvent is within a certain range. In the present invention, it is preferable to select an organic solvent with a polarity within a certain range based on Hansen solubility parameters as the precipitation polymerization solvent.

The Hansen solubility parameter is a solubility parameter introduced by Hildebrand, which Hansen divided into three components - the dispersion term δD, the polar term δP, and the hydrogen bonding term δH - and displayed in three-dimensional space. The dispersion term δD indicates the effect of dispersion forces, the polar term δP indicates the effect of dipole-dipole forces, and the hydrogen bonding term δH indicates the effect of hydrogen bonding forces. In three-dimensional space, the greater the distance between the coordinates of a resin and the coordinates of a certain organic solvent, the more likely the resin will precipitate in that organic solvent.

The definition and calculation method of the Hansen solubility parameter are described in the following literature: Charles M. Hansen, "Hansen Solubility Parameters: A Users Handbook," CRC Press, 2007. Additionally, for organic solvents for which literature values are not known, the Hansen solubility parameter can be easily estimated from their chemical structure using computer software (Hansen Solubility Parameters in Practice (HSPiP)).

In this invention, HSPiP 5th Edition is used, and values registered in the database are used for organic solvents, while estimated values are used for organic solvents not registered.

The Hansen solubility parameter of a resin is usually determined by conducting a solubility test in which the resin is dissolved in a number of different organic solvents for which the Hansen solubility parameter has been established, and the solubility is measured. Specifically, when the coordinates of the Hansen solubility parameters of all the organic solvents used in the solubility test are plotted in three-dimensional space, a sphere (solubility sphere) is found such that the coordinates of the organic solvents that dissolved resin A are all contained within the sphere, and the coordinates of the organic solvents used to precipitate the resin are on the outside, and the central coordinates of the solubility sphere are taken to be the Hansen solubility parameter of the resin.

If the coordinates of the Hansen solubility parameters of an organic solvent not used in the solubility test are (δD, δP, δH), and these coordinates are contained within the solubility sphere, then the organic solvent is considered to dissolve the resin. On the other hand, if these coordinates are outside the solubility sphere, then the organic solvent is considered to precipitate the resin.

In the present invention, the Hansen solubility parameter of the resin is the value calculated using HSPiP for the compound represented by the following general formula (6) (pentamer of the compound represented by general formula (5)). Using this method, for example, the Hansen solubility parameters δD, δP, and δH of resin particles containing perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue units represented by general formula (3) are 11.6, 3.5, and 1.4 (MPa ^1/2 ), respectively.

(In formula (6), Rf ₉ , Rf ₁₀ , Rf ₁₁ and Rf ₁₂ each independently represent a group selected from the group consisting of a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms, which may have an etheric oxygen atom. Rf ₉ , Rf ₁₀ , Rf ₁₁ and Rf ₁₂ may be linked to each other to form a ring having 4 to 8 carbon atoms.)

As the precipitation polymerization solvent in the present invention, it is preferable to select an organic solvent having a resin solubility index R of 4 or more, calculated from the Hansen solubility parameter according to formula (7).
R=4×{(δD ₁ - δD ₂ ) ² + (δP ₁ - δP ₂ ) ² + (δH ₁ - δH ₂ ) ² } ^0.5 ...(7)
Here, δD ₁ , δP ₁ and δH ₁ are the dispersion term, polar term and hydrogen term of the Hansen solubility parameter of the resin particles, respectively, and δD ₂ , δP ₂ and δH ₂ are the dispersion term, polar term and hydrogen term of the Hansen solubility parameter of the organic solvent, respectively.

For example, examples of organic solvents that have an affinity Ra of 4 or higher with resins containing perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue units include the organic solvents listed in Table 1 below.

Furthermore, organic solvents containing fluorine atoms and hydrogen atoms in the molecule are preferred as precipitation polymerization solvents, as they are less likely to cause chain transfer reactions in radical polymerization, have excellent polymerization yields, and are more likely to produce high-molecular-weight polymers. Specific examples of precipitation polymerization solvents containing fluorine atoms and hydrogen atoms in the molecule include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoroisopropanol, and 1,2,2,3,3,4,4-heptafluorocyclopentane.

Examples of radical polymerization initiators for radical polymerization include organic peroxides such as benzoyl peroxide, lauryl peroxide, octanoyl peroxide, acetyl peroxide, di-tetributyl peroxide, tetributylcumyl peroxide, dicumyl peroxide, tetributyl peroxyacetate, perfluoro(di-tetributyl peroxide), bis(2,3,4,5,6-pentafluorobenzoyl) peroxide, tetributyl peroxybenzoate, and tetributyl perpivalate; and azo initiators such as 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-butyronitrile), 2,2'-azobisisobutyronitrile, dimethyl-2,2'-azobisisobutyrate, and 1,1'-azobis(cyclohexane-1-carbonitrile).

In the production method of the present invention, it is preferable that the monomer represented by the general formula (4) is perfluoro(4-methyl-2-methylene-1,3-dioxolane) represented by the following general formula (8), and the residue unit represented by the general formula (5) is a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit represented by the following general formula (9).

As a second aspect of the method for producing a resin of the present invention,
The fluororesin can be produced by a method for producing the fluororesin, which comprises a solution polymerization step of carrying out solution polymerization in the presence of a monomer represented by the following general formula (4), a radical polymerization initiator, and an organic solvent at a water content of 1,000 ppm by mass or less in the reaction system, and a precipitation step of contacting a solution containing the fluororesin obtained by the solution polymerization with a poor solvent to precipitate the fluororesin while maintaining the water content in the system at 100 ppm or less.

Here, the poor solvent is preferably the same solvent as the organic solvent in the first embodiment of the resin production method of the present invention.

The organic solvent in the second embodiment may be either a good or poor solvent for the fluororesin of the present invention, but is preferably a good solvent.

In both the first and second aspects of the production method of the present invention, it is preferable to purify at least one of the monomer represented by general formula (4), the initiator, and the organic solvent using at least one of a filtration filter, an ion exchange resin, a metal ion removal filter, and a metal ion remover before carrying out polymerization.

The present invention provides a fluororesin with reduced coloration and a method for producing the fluororesin.

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

<Volume average particle size>
The volume average particle size (unit: μm) was measured using an MT3000 manufactured by Microtrac and methanol as a dispersion medium.
<Metal component content>
The fluororesin was dissolved in hexafluorobenzene and then extracted with a dilute acid aqueous solution, and the metal components (unit: ppb) were measured using ELAN DRC II manufactured by Perkin Elmer.
<Yellowness (YI)>
The fluororesin was hot-pressed at 230° C. for 50 minutes in a nitrogen atmosphere to prepare a pressed sheet having a thickness of 200 nm, and the yellowness index (YI) was measured using SD5000 manufactured by Nippon Denshoku Industries Co., Ltd.

Example 1 Production of Perfluoro(4-methyl-2-methylene-1,3-dioxolane) Resin Particles The interior of a 1 L SUS316 autoclave equipped with a SUS316 paddle-type stirring blade, a nitrogen inlet tube, and a thermometer was purged with nitrogen. 1.215 g (0.00288 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 140.0 g (0.574 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 1250 g of Asahiklin AE-3000 (manufactured by Asahi Glass Co., Ltd., 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether) as a precipitation polymerization solvent were added, and the mixture was maintained at 55°C with stirring for 24 hours to carry out precipitation polymerization. After cooling to room temperature, the liquid containing the purified resin particles was filtered and washed with acetone to obtain perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles (Resin A) (yield: 63%). The volume average particle size, metal content, and yellowness index of the obtained resin particles are shown in Table 2. The obtained resin particles had excellent fluidity and filling properties, low metal content, low yellowness, and good transparency.

Comparative Example 1: Preparation of Perfluoro(4-methyl-2-methylene-1,3-dioxolane) Resin. A fluororesin was prepared according to Non-Patent Document 1. The interior of a 1-L SUS316 autoclave equipped with a SUS316 paddle-type stirring blade, a nitrogen inlet tube, and a thermometer was purged with nitrogen. 0.476 g (0.00113 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as the initiator, 140.0 g (0.574 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as the monomer, and 230 g of hexafluorobenzene as the polymerization solvent were charged and stirred at 55°C for 24 hours to carry out radical solution polymerization, yielding a viscous liquid in which the resin was dissolved. The resin solution was cooled to room temperature, and diluted with 1,000 g of hexafluorobenzene to adjust the viscosity, to prepare a diluted resin solution. 3 L of hexane was added to the vat, and the diluted resin solution was extruded into the hexane with a syringe to precipitate the resin, yielding perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 68%).

Because the resulting resin was amorphous, the volume average particle size could not be measured. The metal content and yellowness of the resulting resin are shown in Table 2. The resulting resin particles had poor fluidity and filling properties, and due to the high metal content and high yellowness, there were also issues with transparency. Furthermore, water droplets that had aggregated due to the heat of vaporization of the poor solvent were found in the poor solvent used in the precipitation process, and the water content in the system was over 100 ppm.

Comparative Example 2: Preparation of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles. The interior of a 1 L SUS316 autoclave equipped with a SUS316 paddle-type stirring blade, a nitrogen inlet tube, and a thermometer was purged with nitrogen. 0.775 g (0.00184 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 5.289 g of Newcol 714SN (Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether sulfate) as a dispersion stabilizer, 16.300 g of methanol as a chain transfer agent, 140.0 g (0.574 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 190.0 g of ion-exchanged water as a suspension polymerization solvent were added, and the mixture was stirred at 55°C for 24 hours to carry out suspension polymerization. After cooling to room temperature, the liquid containing the purified resin particles was filtered and washed with acetone to obtain perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles (yield: 85%). The volume average particle size, metal content, and yellowness index of the obtained resin particles are shown in Table 2. The obtained resin particles had excellent fluidity and packing properties, but had a problem with transparency due to the high metal content and high yellowness index.

Example 2: Production of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles. The interior of a 1-liter glass autoclave equipped with a SUS316 paddle-type stirring blade, a nitrogen inlet tube, and a thermometer was purged with nitrogen. 1.190 g (0.00282 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 140.0 g (0.574 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 1250 g of Asahiklin AE-3000 (manufactured by Asahi Glass Co., Ltd., 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether) as a precipitation polymerization solvent were added, and the mixture was stirred at 55°C for 24 hours to carry out precipitation polymerization. After cooling to room temperature, the liquid containing the purified resin particles was filtered and washed with acetone to obtain perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles (Resin A) (yield: 66%). The volume average particle size, metal content, and yellowness index of the obtained resin particles are shown in Table 2. The obtained resin particles had a low metal content and were excellent in yellowness, fluidity, and packing ability.

Comparative Example 3: Preparation of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles. The interior of a 1-liter glass autoclave equipped with a SUS316 paddle-type stirring blade, a nitrogen inlet tube, and a thermometer was purged with nitrogen. 0.726 g (0.00172 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 5.314 g of Newcol 714SN (Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether sulfate) as a dispersion stabilizer, 16.280 g of methanol as a chain transfer agent, 140.0 g (0.573 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 186.7 g of ion-exchanged water as a suspension polymerization solvent were added, and the mixture was stirred at 55°C for 24 hours to carry out suspension polymerization. After cooling to room temperature, the liquid containing the purified resin particles was filtered and washed with acetone to obtain perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin particles (yield: 85%). The volume average particle size, metal content, and yellowness index of the obtained resin particles are shown in Table 2. The obtained resin particles had excellent fluidity and packing properties, but the high metal content and high yellowness posed a problem in terms of transparency.

The present invention can provide a fluororesin that reduces discoloration when heated and a method for producing the fluororesin.

Claims (2)

A method for producing a fluororesin comprising a residue unit represented by the following general formula (1), and having a total content of chromium, iron and nickel of 500 ppb by mass or less, the method comprising: a solution polymerization step of carrying out solution polymerization in the presence of a monomer represented by the following general formula (2), a radical polymerization initiator, and an organic solvent, with the water content in the reaction system being 1000 ppm by mass or less; and a precipitation step of contacting a solution containing the fluororesin obtained by the solution polymerization with a poor solvent to precipitate the fluororesin while maintaining the water content in the system at 100 ppm by mass or less, wherein the poor solvent is an organic solvent containing a fluorine atom and a hydrogen atom in the molecule:
(In formula (1), Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ each independently represent a group selected from the group consisting of a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms which may have an etheric oxygen atom. Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked together to form a perfluoroaliphatic ring having from 4 to 8 carbon atoms which may have an etheric oxygen atom.)
(In formula (2), Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ each independently represent a fluorine atom or a linear, branched or cyclic perfluoroalkyl group having 1 to 7 carbon atoms which may have an etheric oxygen atom. Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ may be linked together to form a perfluoroaliphatic ring having 4 to 8 carbon atoms which may have an etheric oxygen atom.) 2. The method for producing a fluororesin according to claim 1, wherein at least one of the monomer represented by the general formula ( 2 ), the initiator, and the organic solvent used in the solution polymerization step is purified with at least one of a filtration filter, an ion exchange resin, a metal ion removal filter, and a metal ion remover before polymerization.