JP7583521B2 - Manufacturing method of fluororesin - Google Patents

Description

The present invention relates to a fluororesin and its manufacturing method.

Amorphous fluororesins are used in a variety of applications in the optical and electronic fields due to their excellent transparency, liquid repellency, durability, and electrical properties. In the optical field, amorphous fluoropolymers are used as optical components such as optical waveguides and pellicles, which are dust-proof films for semiconductor photomasks.

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

Non-Patent Document 1 describes the synthesis and properties of a polymer (polyPFMMD) of perfluoro-2-methylene-4-methyl-1,3-dioxolane (PFMMD), a type of fluororesin containing an oxolane ring. PolyPFMMD has excellent heat resistance.

Macromolecules 2005, 38, 4237-4245

According to the inventors' investigations, the resin produced by the method described in Non-Patent Document 1 had the problem that the haze value of the melt-molded product was high.

The present invention aims to solve the problems with fluororesins containing oxolane rings, specifically to provide fluororesins containing oxolane rings that produce melt-molded products with a small haze value, and a method for producing the same.

The present invention is as follows.
[1]
A fluororesin containing a residue unit represented by the following general formula (1), the haze value of a hot press molded product (thickness 1 mm) being 2% or less:
(In formula (1), Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ each independently represent one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be bonded to each other to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)
[2]
The fluororesin according to [1], wherein the amount of insoluble matter when the fluororesin is dissolved in 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane is 0.2% by weight or less based on the fluororesin.
[3]
The fluororesin according to [1] or [2], wherein the bulk density of the fluororesin is 0.1 to 1.5 g/cm ³ .
[4]
The fluororesin according to [1] or [2], wherein the bulk density of the fluororesin is 0.12 to 0.25 g/cm ^3.
[5]
The fluororesin according to any one of [1] to [4], wherein a melt-molded product (thickness: 3 mm) of the fluororesin after heating at 280° C. for 24 hours has a yellowness index of 4 or less.
[6]
The fluororesin according to any one of [1] to [5], wherein the weight average molecular weight of the fluororesin is 5×10 ⁴ to 3×10 ⁵ .
[7]
A polymerization step (1) of polymerizing a monomer represented by the following general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by the following general formula (5):
an insoluble matter removal step (2) of removing insoluble matter from a mixture containing the fluororesin A containing the residue unit represented by general formula (5) obtained in the polymerization step and the solvent S2 to obtain a fluororesin A solution;
The method includes a precipitation step (3) of precipitating fluororesin A from the fluororesin A solution obtained in the insoluble matter removal step,
A method for producing a fluororesin having a haze value of 2% or less in a hot press molded product (thickness 1 mm).
(In formula (4) and formula (5), Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ each independently represent one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ may be bonded to each other to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)
[8]
The method according to [7], wherein the polymerization step (1) is any one of the following steps (1a), (1b) or (1c):
(1a) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A and the good solvent b1;
(1b) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for fluororesin A to precipitate fluororesin A, recovering the precipitated fluororesin A, and mixing the recovered fluororesin A with a good solvent b1 for fluororesin A to obtain a mixture containing fluororesin A and the good solvent b1.
(1c) A step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for the fluororesin A to precipitate the fluororesin A, and mixing with a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A, the good solvent b1, and the poor solvent c1.
[9]
The production method according to [8], wherein the step (1a) is carried out in the presence of a radical polymerization initiator, a good solvent b1 for the fluororesin A, and a poor solvent c1 for the fluororesin A.
[10]
The method according to any one of [7] to [9], wherein the insoluble matter removal step (2) is either the following step (2a) or (2b):
(2a) a step of filtering a mixture containing the fluororesin A and the solvent S2 through a filter to remove insoluble matters;
(2b) A step of subjecting the mixture containing the fluororesin A and the solvent S2 to centrifugation to remove insoluble matters.
[11]
The method according to [10], wherein the solvent S2 is a good solvent b2 for the fluororesin A or a mixed solvent of the good solvent b2 and a poor solvent c2 for the fluororesin A.
[12]
The method according to [10] or [11], wherein the insoluble matter removing step (2) is (2a).
[13]
The method according to any one of [10] to [12], wherein the filter is a filter having a 99% capture particle size of 10 μm or less or a screen filter having a pore size of 10 μm or less.
[14]
The method according to any one of [7] to [13], wherein the precipitation step (3) is any one of the following steps (3a), (3b), (3c), or (3d).
(3a) a step of lowering the temperature of the fluororesin A solution to precipitate the fluororesin A;
(3b) adding the fluororesin A solution to a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3c) a step of precipitating fluororesin A by adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution; and (3d) a step of precipitating fluororesin A by volatilizing the solvent from the fluororesin A solution.
[15]
The method according to [14], wherein the solvent for the fluororesin A solution in the precipitation step (3a) is a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A.
[16]
The method according to [14] or [15], wherein, in the precipitation step (3a), when the solution temperature T1 before the temperature is reduced is 30° C. or higher and the solution temperature after the temperature is reduced is T2, T1-T2 is 5° C. or higher.
[17]
The production method according to any one of [7] to [16], further comprising a separation step (5) of separating the fluororesin A from the solution in which the fluororesin A has been precipitated obtained in the precipitation step (3) or from the solution to which the poor solvent c4 has been added in the poor solvent adding step (4), and a washing step (6) of washing the separated fluororesin A with the poor solvent c5.
[18]
The method according to any one of [14] to [17], wherein the polymerization step (1) is step (1b), and the precipitation step (3) is step (3a), (3b), (3c), or (3d).
[19]
The method according to any one of [14] to [18], wherein the precipitation step (3) is step (3a) or (3c).
[20]
The method according to any one of claims 14 to 17, wherein the precipitating step (3) is any one of steps (3a), (3b) and (3c), and the solvent S2 is an aliphatic fluorine-containing solvent.
[21]
The method according to any one of claims 7 to 20, wherein the insoluble matter removed in the insoluble matter removal step (2) comprises at least a fluororesin containing a residue unit represented by general formula (1).

The present invention provides a fluororesin that contains a residue unit represented by general formula (1) and has a haze value of 2% or less when molded by hot press (thickness 1 mm).

<Fluoropolymer>
The present invention relates to a fluororesin which contains a residue unit represented by the following general formula (1) and has a haze value of 2% or less when molded by hot press (thickness: 1 mm).
(In formula (1), Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ each independently represent one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be bonded to each other to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)
The invention will be described in detail below.

The present invention is a fluororesin containing a residue unit represented by a specific general formula (1). Since the fluororesin of the present invention has a bulky ring structure contained in the specific general formula (1), it is amorphous and has high transparency and high heat resistance. Furthermore, since it is composed only of carbon, fluorine, and oxygen, it has high electrical properties, chemical resistance, waterproofness, and liquid and oil repellency.

In the present invention, Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ groups in the residue unit represented by general formula (1) each independently represent one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms. The perfluoroalkyl group may have an etheric oxygen atom. Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ may be bonded to each other to form a ring having 4 to 8 carbon atoms, and the ring may contain an etheric oxygen atom. _Rf1 , _Rf2 , _Rf3 , and _Rf4 in general formula (1) are respectively defined as _Rf5 , _Rf6 , _Rf7 , and _Rf8 in general formulas (4) and (5) described below, and specific examples of _Rf1 , _Rf2 , _Rf3 , and _Rf4 described below are also specific examples of _Rf5 , _Rf6 , _Rf7 , and _Rf8 .

Examples of linear perfluoroalkyl groups having 1 to 7 carbon atoms include trifluoromethyl, pentafluoroethyl, heptafluoropropyl, nonafluorobutyl, undecafluoropentyl, tridecafluorohexyl, and pentadecafluoroheptyl groups. Examples of branched perfluoroalkyl groups having 3 to 7 carbon atoms include heptafluoroisopropyl, nonafluoroisobutyl, nonafluorosec-butyl, and nonafluorotert-butyl groups. Examples of cyclic perfluoroalkyl groups having 3 to 7 carbon atoms include heptafluorocyclopropyl, nonafluorocyclobutyl, and tridecafluorocyclohexyl groups. Examples of linear perfluoroalkyl groups having 1 to 7 carbon atoms that may have an etheric oxygen atom include _-CF2OCF3 , - _{( CF2 ) 2OCF3} , and - _{( CF2} ) _{2OCF2CF3 groups} . Examples of cyclic perfluoroalkyl groups having 3 to 7 carbon atoms which may have an etheric oxygen atom include a 2-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl group, a 4-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl group, and a 2-(2,3,3,4,4,5,5-heptafluoro)-furanyl group.

From the viewpoint of exhibiting excellent heat resistance, a fluororesin in which at least one of Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ is a member of the group consisting of a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms is preferred.

Specific examples of the residue unit represented by general formula (1) include various residue units represented by the following formula (2):

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

The fluororesin of the present invention has a haze value of 2% or less in a melt molded product (thickness 1 mm). Having a haze value of 2% or less in a hot press molded product (thickness 1 mm) has the advantage that when used as an optical component, an optical component with excellent transparency and performance can be obtained. A method for producing the fluororesin of the present invention having a haze value of 2% or less in a hot press molded product (thickness 1 mm) will be described later. The haze value is measured by the following method. A mold with a 1 mm thick plate with the center hollowed out was placed on a smooth metal plate with a polyimide film placed on it, and the fluororesin was placed on the hollowed out area, and then the polyimide film and metal plate were placed on top of it, sandwiched, placed on a press, heated at 280°C for 10 minutes without pressure, then hot-pressed at 280°C for 10 minutes with a press at a pressure of 10 MPa and 10 MPa, and then depressurized and hot-pressed at a pressure of 10 MPa was repeated for 5 minutes, and then hot-pressed at 280°C and 10 MPa with a press at 10 minutes, and then depressurized, and the molded product sandwiched between the metal plates was further sandwiched between cooling metal plates and cooled to obtain a hot-press molded product (thickness 1 mm). The obtained hot-press molded product (thickness 1 mm) was measured according to JIS K7136 using a haze meter NDH5000 (light source: white LED) manufactured by Nippon Denshoku Kogyo Co., Ltd. to determine the haze (%).

The fluororesin of the present invention has a haze value of 2% or less, preferably 1% or less, more preferably 0.8% or less, and more preferably 0.5% or less, in a hot press molded product (thickness 1 mm). There is no lower limit for the haze value, and the lower the better, but an example of the haze value is 0.01% or more.

The fluororesin of the present invention preferably has a yellowness index (hereinafter also referred to as "YI") of 6 or less for a melt-molded product (thickness 3 mm) that is heated at 280°C for 24 hours. When the melt-molded product (thickness 3 mm) has a yellowness index of 6 or less, an optical component that is excellent in transparency and performance when used as an optical component can be obtained. YI is preferably 4 or less, more preferably 3 or less, more preferably 2 or less, and more preferably 1 or less. There is no lower limit for YI, and the lower the better, but for example, 0.01 or more can be mentioned. YI is measured by the following method. First, the transmittance of a fluororesin melt-molded product having a thickness of 3 mm is measured at wavelengths of 200 nm to 1500 nm using a spectrophotometer. Data for wavelengths of 380 nm to 780 nm is extracted from the measured transmittance data. From the transmittance data, calculate the tristimulus values X, Y, and Z of the XYZ color system in accordance with JIS Z8701, and calculate YI under the C light source in accordance with JIS K7373.

The fluororesin of the present invention preferably has a bulk density of 0.1 to 1.5 g/cm ³ in consideration of, for example, handleability and moldability. The bulk density is more preferably 0.25 to 1.5 g/cm ³ , and even more preferably 0.25 to 1.0 g/cm ^3. The inventors' studies have revealed that when the bulk density is in a specific range, the YI exhibited by the fluororesin of the present invention is in a good range (see Examples 2 and 4 to 6). From this viewpoint, the bulk density is preferably 0.12 to 0.25 g/cm ³ , and even more preferably 0.14 to 0.22 g/cm ^3. The measurement of the bulk density is carried out as follows. Fluororesin A is weighed out and poured into a 13.5 mL glass sample tube (the liquid level is 2.8 cm when 10 mL of water is poured) whose height per unit volume has been measured in advance, without applying vibration, and the bulk density can be calculated from the powder height and weight at that time according to the following formula. The bulk density at this time is called loose bulk density.
Bulk density = (weight of powder (g)) / ((height of powder (cm)) / 0.28 (cm/mL))

There is no limitation on the weight average molecular weight Mw, but examples thereof include 1×10 ³ to 5×10 ^7. Since the haze value of the hot press molded product is excellent, the weight average molecular weight Mw is preferably in the range of 5×10 ⁴ to 5×10 ^5. Furthermore, since the haze value of the hot press molded product is excellent, the weight average molecular weight Mw is preferably in the range of 5×10 ⁴ to 3×10 ^5. By having the weight average molecular weight Mw in this range, the haze value of the hot press molded product is excellent, and the melt viscosity at a shear rate of 10 ^-2 s and 250° C. can be 1×10 ² to 3×10 ⁵ Pa·s, resulting in excellent melt molding processability. Furthermore, the defoaming property during melting is also excellent. Furthermore, by having the weight average molecular weight Mw in this range, cracks during heating and cooling are reduced. From the viewpoints of excellent haze value, excellent melt molding processability, and excellent defoaming property when melted, the fluororesin of the present invention preferably has a weight average molecular weight Mw in the range of 5×10 ⁴ to 2×10 ^5. With the weight average molecular weight Mw in this range, the melt viscosity at a shear rate of 10 ^-2 s and 250° C. can be 1×10 ² to 2×10 ⁴ Pa·s, and as a result, the fluororesin has excellent melt molding processability and defoaming property, which is preferable. From the viewpoints of excellent haze value, excellent melt molding processability, and excellent defoaming property when melted, the weight average molecular weight Mw is more preferably in the range of 5×10 ⁴ to 1.5×10 ⁵ , and from the viewpoint of reducing the occurrence of cracks when heated and cooled, the weight average molecular weight Mw is even more preferably in the range of 6×10 ⁴ to 1.5×10 ⁵ .

The weight average molecular weight Mw of the fluororesin of the present invention can be calculated using gel permeation chromatography (GPC), for example, by using standard polymethylmethacrylate with a known molecular weight as the standard sample and a solvent capable of dissolving both the standard sample and the fluororesin as the eluent, from the elution times of the sample and standard sample, and the molecular weight of the standard sample. An example of the solution is Asahiklin AK-225 (manufactured by Asahi Glass Co., Ltd.) to which 10 wt % of 1,1,1,3,3,3-hexafluoro-2-propanol (manufactured by Wako Pure Chemical Industries, Ltd.) has been added relative to the AK-225.

The molecular weight distribution Mw/Mn, which is the ratio of the weight average molecular weight Mw to the number average molecular weight Mn of the fluororesin of the present invention, is not particularly limited, but from the viewpoint of achieving excellent haze value, suppressed yellowing after heating and melting, excellent melt molding processability, excellent defoaming properties during melting, and less cracking during heating and cooling, the molecular weight distribution Mw/Mn is preferably 1.2 to 8, more preferably 1.2 to 5, even more preferably 1.5 to 3, and even more preferably 2.0 to 3. The number average molecular weight Mn can be measured by the same method as the method for measuring the weight average molecular weight Mw described above, and the molecular weight distribution Mw/Mn can be calculated by dividing the weight average molecular weight Mw by the number average molecular weight Mn.

The particle size of the fluororesin of the present invention is not particularly limited, but in order to provide excellent handleability during molding processing, the volume average particle size is preferably 1 to 10,000 μm, more preferably 1 to 2,000 μm, more preferably 1 to 1,000 μm, and even more preferably 10 to 1,000 μm.
The volume average particle size of the fluororesin of the present invention can be evaluated by particle size distribution measurement (volume distribution) by laser diffraction scattering method. The particle size distribution by laser diffraction scattering method can be measured by dispersing the resin particles in water or an organic solvent such as methanol and measuring it. As an example of a laser scattering meter, Microtrack manufactured by Microtrack Bell Co., Ltd. can be mentioned.
The volume average particle diameter is also called Mean Volume Diameter, and is the average particle diameter expressed on a volume basis. When the particle diameter distribution is divided into each particle diameter channel, the representative particle diameter value of each particle diameter channel is d, and the volume-based percentage of each particle diameter channel is v, it is expressed as Σ(vd)/Σ(v).

Since the fluororesin of the present invention has a small haze value, the amount of _insoluble matter when dissolved in 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-( _{trifluoromethyl} )pentane ( _C2F5CF ( _OCH3 ) _C3F7 , manufactured by 3M Japan, Novec7300) is preferably 0.2% by weight or less, more preferably 0.1% by weight or less, even more preferably 0.05% by weight or less, and even more preferably 0.01% by weight or less, based on the fluororesin. The method for measuring the amount of insoluble matter is as follows. 1,1,1,2,3,4,4,5,5,5-Decafluoro-3-methoxy-2-( _{trifluoromethyl} )pentane ( _C2F5CF ( _OCH3 ) _C3F7 , Novec7300, manufactured by 3M Japan) is added to the _fluororesin to adjust the solid concentration to 10% by weight. Dissolve at 50°C for 5 hours and shake and stir to prepare a fluororesin solution. The solution was pressure-filtered using a pressure filter equipped with a PTFE membrane filter having a pore size of 0.1 μm, the weight of which had been recorded in advance, and then pressure-filtered using Novec 7300 from which foreign matter had been removed using a filter with a pore size of 0.1 μm. The Novec 7300 from which foreign matter had been removed was placed in the pressure filter and pressure-filtered repeatedly to wash away the remaining fluororesin. The filter was then removed and vacuum-dried, and the weight of the resulting filter was subtracted by the weight of the filter before filtration to determine the amount of matter remaining on the filter, and the amount of matter remaining on the filter was divided by the weight of the resin used to determine the percentage, thereby determining the amount of insoluble matter (wt %).

<Method of manufacturing fluororesin>
The method for producing a fluororesin of the present invention is a method for producing a fluororesin having a haze value of 10% or less in a melt-molded product (thickness: 1 mm),
A polymerization step (1) of polymerizing a monomer represented by the following general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by the following general formula (5):
The method includes an insoluble matter removal step (2) of obtaining a fluororesin A solution by removing insoluble matter from a mixture of a fluororesin A containing a residue unit represented by general formula (5) obtained in the polymerization step and a solvent S2, and a precipitation step (3) of precipitating fluororesin A from the fluororesin A solution obtained in the insoluble matter removal step.

In formulas (4) and (5), Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ each independently represent a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₅ , Rf 6 , Rf ₇ , and Rf ₈ may be linked together to form a ring having 4 to 8 carbon atoms, which may contain an etheric oxygen atom. Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ in formulas ( ₄ ) and (5) are synonymous with Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ in formula (1), respectively.

Polymerization step (1)
The polymerization step (1) is a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by general formula (5). There are no limitations on the polymerization method in the polymerization step (1), and examples of the polymerization method include solution polymerization, precipitation polymerization, bulk polymerization, emulsion polymerization, and suspension polymerization.

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

Examples of radical polymerization initiators for carrying _out radical polymerization include bis( _{perfluorobenzoyl} ) _peroxide (PFBPO), ( _CF3COO ) ₂ , ( _CF3CF2COO ) ₂ , ( _{C3F7COO ) 2} , (C4F9COO) _{2 , ( C5F11COO} ) _{2 ,} ( _C6F13COO ) ₂ , ( _C7F15COO ) ₂ , and ( _C8F17COO ) _{. 2} and the like; organic peroxides such as benzoyl peroxide, lauryl peroxide, octanoyl peroxide, acetyl peroxide, di-tert-butyl peroxide, tert-butylcumyl peroxide, dicumyl peroxide, tert-butyl peroxyacetate, perfluoro(di-trt-butyl peroxide), bis(2,3,4,5,6-pentafluorobenzoyl) peroxide, tert-butyl peroxybenzoate, and tert-butyl 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).

As the radical polymerization initiator, from the viewpoints of low haze value, suppression of yellowing after heating and melting, excellent melt molding processability, excellent defoaming properties when melted, and little cracking during heating and cooling, perfluoro organic peroxides are preferred, and bis(perfluorobenzoyl) peroxide (PFBPO) is more preferred. Here, perfluoro organic peroxide refers to a compound in which the hydrogen atoms of an organic peroxide are replaced with fluorine atoms.

The polymerization step (1) is suitably carried out in the presence of a solvent, and depending on the type of solvent, it can be, for example, either the following step (1a) or (1b).
(1a) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A and the solvent b1;
(1b) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for the fluororesin A to precipitate the fluororesin A, recovering the precipitated fluororesin A, and mixing the recovered fluororesin A with a good solvent b1 to obtain a mixture containing the fluororesin A and the good solvent b1.
(1c) A step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for the fluororesin A to precipitate the fluororesin A, and mixing with a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A, the good solvent b1, and the poor solvent c1.

In this specification, a good solvent for fluororesin A means an organic solvent capable of dissolving fluororesin A at 50° C. Dissolvable means that at least a portion of fluororesin A having a weight average molecular weight Mw of 5 to 15×10 ⁴ is soluble in the organic solvent, and for example, when a sample of fluororesin A is immersed in a 20-fold amount (w/w) of an organic solvent at 50° C. for 5 hours or more, if 80% by weight or more of the sample of fluororesin A is dissolved in the solvent, this solvent can be considered as a good solvent. Here, fluororesin A can be a fluororesin containing a residue unit represented by the general formula (3) above.

A poor solvent for fluororesin A means a solvent that does not easily dissolve fluororesin A. For example, when a sample of fluororesin A having a weight average molecular weight Mw of 5 to 15×10 ⁴ is immersed in 20 times (w/w) the amount of the solvent at 50° C. for 5 hours or more and cooled to 25° C., a solvent in which the amount of the fluororesin A sample dissolved in the solvent is less than 20% by weight, preferably less than 10% by weight, can be defined as a poor solvent. Furthermore, in the present invention, a poor solvent for fluororesin A is also a solvent that can precipitate fluororesin A from a solution of fluororesin A obtained by dissolving a fluororesin in a good solvent. The poor solvent is preferably a solvent in which fluororesin A precipitates when a solution of fluororesin A dissolved in a certain good solvent is dropped into a solvent 10 times the amount of the good solvent at 25° C. Here, fluororesin A can be a fluororesin containing a residue unit represented by the general formula (3).

In this specification, solvents are designated by the symbol S, and the solvent used in step (1) is designated by S1, the solvent used in step (2) by S2, the solvent used in step (3) by S3, the solvent used in step (4) by S4, and the solvent used in step (n) by Sn (n is an integer). Good solvents are designated by the symbol b, and the good solvent used in step (1) is designated by b1, the good solvent used in step (2) by b2, the good solvent used in step (3) by b3, the good solvent used in step (4) by b4, and the good solvent used in step (n) by bn (n is an integer). Poor solvents are designated by the symbol c, and the poor solvent used in step (1) is designated by c1, the poor solvent used in step (2) by c2, the poor solvent used in step (3) by c3, the poor solvent used in step (4) by c4, and the poor solvent used in step (n) by cn (n is an integer).

The solvent that can be a good solvent is preferably at least one selected from the group consisting of aliphatic fluorine-containing solvents such as perfluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, hydrofluoroethers, and hydrofluoroolefins, or aromatic fluorine compounds. Since a fluororesin that exhibits good coloring when heated can be obtained, an aliphatic fluorine-containing solvent is more preferable, and further preferable are perfluorohexane, perfluoro- _N -methylmorpholine, perfluoro- _{N -} propylmorpholine, perfluorotriethylamine _{, perfluoromethyldibutylamine , perfluorotributylamine , CF3CF2CHCl2 , CF3CHFCHFCF2CF3 , CF3CF2CF2CF2CF2CF2CF2CF2H , CF3 ( CF2} ) _{5CH2CH3 , C4F9OCH3 , and C4F9OC2H5 .} , 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane (C ₂ F ₅ CF(OCH ₃ )C ₃ F ₇ ), and hexafluorobenzene.

For example, perfluorocarbons such as Fluorinert FC-5052, FC-72, FC-770, FC-3283, FC-40, and FC-43 (all manufactured by 3M Japan Co., Ltd.); hydrochlorofluorocarbons such as Asahiklin AK-225 (manufactured by Asahi Glass Co., Ltd.); hydrofluorocarbons such as Vertrel XF (manufactured by Mitsui-Chemours Co., Ltd.), Asahiklin AC-2000, and AC-6000 (both manufactured by Asahi Glass Co., Ltd.); hydrofluoroethers such as Novec 7100, Novec 7200, and Novec 7300 (manufactured by 3M Japan Co., Ltd.); hydrofluoroolefins such as Opteon SF10 (manufactured by Mitsui-Chemours Co., Ltd.); and aromatic fluorine-containing solvents such as hexafluorobenzene. A preferred example of the good solvent is 1,1,1,2,3,4,4,5,5,5-decafluoro-3-methoxy-2-(trifluoromethyl)pentane (C ₂ F ₅ CF(OCH ₃ )C ₃ F ₇ , Novec 7300, manufactured by 3M Japan).

Since particles having a large bulk density and excellent handling properties as a powder can be obtained, the good solvent is preferably a fluorine-containing solvent, more preferably an aliphatic fluorine-containing solvent having a hydrogen atom in the molecule such as hydrofluorocarbon, hydrofluoroether, hydrochlorofluorocarbon, or hydrofluoroolefin; or an aromatic fluorine-containing solvent, even more preferably an aliphatic fluorine-containing solvent having a hydrogen atom in the molecule such as hydrofluorocarbon, hydrofluoroether, hydrochlorofluorocarbon, or hydrofluoroolefin, even more preferably a hydrofluorocarbon or hydrofluoroether, and particularly preferably a hydrofluoroether. Here, the aliphatic fluorine-containing solvent having a hydrogen atom may be saturated or unsaturated, and may be linear or cyclic.

Examples of solvents that can be poor solvents include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoroisopropanol, 1,2,2,3,3,4,4-heptafluorocyclopentane, 1H,1H-pentafluoropropanol, 1H,1H-heptafluorobutanol, 2-perfluorobutylethanol, 4,4 ,4-trifluorobutanol, 1H,1H,3H-tetrafluoropropanol, 1H,1H,5H-octafluoropropanol, 1H,1H,7H-dodecafluoroheptanol, 1H,1H,3H-hexafluorobutanol, 2,2,3,3,3-pentafluoropropyl difluoromethyl ether, 2,2,3,3,3-pentafluoropropyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2 - At least one of the following fluorine-containing solvents having hydrogen atoms in the molecule, such as tetrafluoroethyl ethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, hexafluoroisopropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, and 2,2,3,4,4,4-hexafluorobutyl difluoromethyl ether; and fluorine-free organic solvents, such as hexane, heptane, toluene, acetone, methanol, ethanol, isopropanol, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, chloroform, dichloromethane, dichloroethane, and trichloroethane.

Since a fluororesin having excellent productivity, large bulk density, and excellent handling properties as a powder can be obtained, the poor solvent is preferably a fluorine-containing solvent, more preferably a fluorine-containing solvent having hydrogen atoms in the molecule, and more preferably at least one of the group consisting of 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. From the viewpoint of economy, the poor solvent is preferably a fluorine-free organic solvent such as hexane, heptane, toluene, acetone, methanol, ethanol, isopropanol, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, tetrahydrofuran, chloroform, dichloromethane, dichloroethane, or trichloroethane. In addition, non-chlorine-based solvents are more preferable because they have excellent yellowness, and examples of such solvents include hexane, heptane, toluene, acetone, methanol, ethanol, isopropanol, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, and tetrahydrofuran.

The polymerization step (1a) is a step in which polymerization is carried out in the presence of a good solvent b1 for the fluororesin A, and is preferably a step of solution polymerization in which the fluororesin A is dissolved in a solvent containing the good solvent b1.

In the polymerization step (1a), in addition to the radical polymerization initiator and the good solvent b1 for the fluororesin A, the polymerization can also be carried out in the presence of a poor solvent c1 for the fluororesin A. The poor solvent c1 will be described later. By carrying out the polymerization in the presence of the poor solvent c1, it is possible to reduce the amount of poor solvent used in the precipitation step described later. The content of the poor solvent c1 is preferably set to an amount such that the fluororesin produced by polymerization in the polymerization step does not precipitate, and the ratio of the good solvent b1 to the poor solvent c1 can be, for example, in the range of 1 to 50% by weight of the poor solvent c1 relative to the total of the good solvent b1 and the poor solvent c1.

The polymerization step (1b) is a step in which polymerization is carried out in the presence of a poor solvent c1 for the fluororesin A to precipitate the fluororesin A. The poor solvent c1 in the polymerization step (1b) can be water. When the poor solvent c1 is water, it is generally called suspension polymerization when no emulsifier is present, and emulsion polymerization when an emulsifier is present. In particular, it is preferable that the poor solvent c1 dissolves the monomer represented by general formula (4), and precipitation polymerization is more preferable, since this reduces the haze of the hot press molded product. Here, precipitation polymerization refers to polymerization carried out in a solvent that dissolves the monomer and precipitates the polymer.

The poor solvent c1 is preferably a solvent that precipitates the fluororesin A dissolved in the good solvent b1 at the polymerization temperature (e.g., 30 to 70°C). The poor solvent c1 is preferably one in which the solubility of the fluororesin A in 20 times the amount of the solvent is less than 20% by weight, and more preferably less than 10% by weight.

The conditions in the polymerization step (1), such as the polymerization temperature, polymerization time, concentration of the radical polymerization initiator, concentration of the monomer, the ratio of the initiator to the monomer, and the amount of the solvent used, can be appropriately determined in consideration of the types of the monomer, radical polymerization initiator, solvent, etc. used. Examples are as follows.
The polymerization temperature is, for example, in the range of 30 to 70° C., and the polymerization time is, for example, in the range of 5 to 96 hours.
The concentration of the radical polymerization initiator is, for example, in the range of 0.1 to 5 mol% based on the monomer.
The monomer concentration may be, for example, in the range of 5 to 40% by weight based on the total weight of the monomer and the solvent. However, these numerical ranges are merely examples and are not intended to be limiting. In particular, the monomer concentration is appropriately determined depending on the type of monomer and the type of solvent, and taking into consideration the solubility of the resulting polymer in the solvent.

In addition to the monomer and the radical polymerization initiator, it is preferable to use a chain transfer agent in the polymerization from the viewpoint of reducing the haze value of the hot press molded product. There are no particular limitations on the chain transfer agent, but for example, an organic compound having 1 to 20 carbon atoms and containing at least one atom selected from the group consisting of hydrogen atoms and chlorine atoms can be used. Here, the chain transfer agent refers to a substance that has the effect of lowering the molecular weight by being present in the system during the radical polymerization of the fluororesin. Specific examples of the chain transfer agent include organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms, such as toluene, acetone, ethyl acetate, tetrahydrofuran, methyl ethyl ketone, methanol, ethanol, and isopropanol; organic compounds having 1 to 20 carbon atoms and containing chlorine atoms, such as chloroform, dichloromethane, tetrachloromethane, chloromethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane, hexachloroethane, benzyl chloride, pentafluorobenzyl chloride, and pentafluorobenzoyl chloride. Among these, from the viewpoints of suppressing the haze value of the hot press molded product, suppressing yellowing after heating and melting, controlling the molecular weight of the fluororesin, having excellent melt molding processability, excellent degassing properties when melted, causing less cracking when heated and cooled, and having an excellent yield, it is preferable to use an organic compound having 1 to 20 carbon atoms containing a chlorine atom, and more preferably one represented by general formula (A).

(In formula (A), m is an integer of 0 to 3, n is an integer of 1 to 3, p is an integer of 0 to 1, q is an integer of 0 to 1, and m+n+p+q is 4. ^{R 1} and R ² are each independently a hydrocarbon group having 1 to 19 carbon atoms or an oxygen atom, and the oxygen atom may form a double bond with an adjacent carbon atom. The total number of carbon atoms in ^{R 1} and R ² is 1 to 19, and the hydrocarbon group may have one or more atoms selected from an oxygen atom, a fluorine atom, and a chlorine atom, and may not have a hydrogen atom. The hydrocarbon group may be linear, branched, alicyclic, or aromatic, and R ¹ and R ² may be bonded to each other to form a ring having 3 to 19 carbon atoms.)

Among them, organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms are more preferable from the viewpoints of suppressing the haze value of hot press molded products, suppressing yellowing after heating and melting, controlling the molecular weight of the fluororesin, providing excellent melt molding processability, excellent defoaming properties when melted, causing less cracking when heated and cooled, and providing excellent yields. Examples of organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms include chloroform, dichloromethane, chloromethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane, benzyl chloride, and pentafluorobenzyl chloride. In addition, in an organic compound having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms, from the viewpoints of suppressing the haze value of a hot press molded product, suppressing yellowing after heating and melting, controlling the molecular weight of the fluororesin, having excellent melt molding processability, excellent defoaming properties when melted, little cracking during heating and cooling, and excellent yield, the ratio of hydrogen atoms to chlorine atoms is preferably in the range of 1:9 to 9:1, and more preferably in the range of 1:9 to 5:5. In addition, from the viewpoints of suppressing the haze value of a hot press molded product, suppressing yellowing after heating and melting, controlling the molecular weight of the fluororesin, having excellent melt molding processability, excellent defoaming properties when melted, little cracking during heating and cooling, and excellent yield, the organic compound having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms is preferably represented by the following general formula (B) or (C), and more preferably represented by general formula (B).

(In formula (B), m and n are each independently an integer of 1 to 3, p is an integer of 0 to 1, q is an integer of 0 to 1, and m+n+p+q is 4. ^{R 1} and R ² are each independently a hydrocarbon group having 1 to 19 carbon atoms, the total number of carbon atoms in R ¹ _p and R ² _q is 0 to 19, and the hydrocarbon group may have one or more atoms selected from oxygen atoms, fluorine atoms, and chlorine atoms, or may have no hydrogen atoms. In addition, the hydrocarbon group may be linear, branched, alicyclic, or aromatic, and R ¹ and R ² may be bonded to each other to form a ring having 3 to 19 carbon atoms.)

(In formula (C), m, n, u, and v are each independently an integer of 0 to 3, m+u is 1 to 5, n+v is 1 to 5, p, q, r, s, and t are each independently an integer of 0 to 1, m+n+p+q is 3, and r+s+u+v is 3; R ¹ , R ² , R ³ , R ⁴ , and R ⁵ are each independently a hydrocarbon group having 1 to 18 carbon atoms, the total number of carbon atoms in R ¹ , R ² , R ³ , R ⁴ , and R ⁵ is 0 to 18, and the hydrocarbon group may have one or more atoms selected from an oxygen atom, a fluorine atom, and a chlorine atom, and may not have a hydrogen atom. In addition, the hydrocarbon group may be linear, branched, alicyclic, or aromatic, and R ¹ , R ² , R ³ , R ⁴ , and R Two or more groups selected from ⁵ may be linked together to form a ring having 3 to 19 carbon atoms, and there may be multiple rings.

Examples of organic compounds having 1 to 20 carbon atoms and containing chlorine atoms and represented by general formula (A) include chloroform, dichloromethane, tetrachloromethane, chloromethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane, hexachloroethane, benzyl chloride, pentafluorobenzyl chloride, pentafluorobenzoyl chloride, etc. Examples of organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms and represented by general formula (B) include chloroform, dichloromethane, chloromethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane, benzyl chloride, pentafluorobenzyl chloride, etc. Examples of organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms and represented by general formula (C) include 1,1,1-trichloroethane, etc.

Furthermore, since a fluororesin that suppresses the haze value of the hot press molded product, suppresses yellowing after heating and melting, balances defoaming properties and cracking during melting, and has excellent defoaming properties and heat resistance during melting, low melt viscosity, and little cracking, and also has an excellent yield, the amount of the chain transfer agent is preferably 0.01 to 95% by weight, more preferably 1 to 50% by weight, and even more preferably 3 to 50% by weight, based on the total amount of the monomer and the chain transfer agent.

Insoluble matter removal step (2)
The insoluble matter removal step (2) is a step of removing insoluble matter from a mixture containing the fluororesin A containing the residue unit represented by the general formula (5) obtained in the polymerization step (1) and the solvent S2. This is a process for obtaining a fluororesin A solution. By providing the insoluble matter removal process, the haze of the resulting fluororesin heat-melt molded product (1 mm thick) can be reduced to 2% or less. The presence of insoluble matter in a mixture containing the above-mentioned and a solution of fluororesin A can be confirmed by, for example, visually observing the mixture or the solution, or by sieving the mixture or the solution through a 0.1 μm pore size filter having a weight recorded in advance. The mixture is filtered under pressure using a PTFE membrane filter of 0.1 μm in diameter, and then filtered under pressure using a good solvent such as Novec 7300, from which foreign matter has been removed using a filter of 0.1 μm in diameter. The remaining resin is then filtered under pressure using the good solvent. After washing, remove the filter, vacuum dry it, subtract the weight of the filter from the weight of the filter before filtration, and calculate the amount of residue on the filter, or observe the residue on the filter. The removal of at least a part of the insoluble matter can be evaluated by, for example, a method of visually observing the mixture or solution, or a method of inserting the fluororesin solution A into a pore with a diameter of 0.1 μm whose weight has been recorded in advance. The mixture is filtered under pressure using a PTFE membrane filter, and then filtered under pressure using a good solvent such as Novec 7300, from which foreign matter has been removed using a filter with a pore size of 0.1 μm. The remaining resin is washed by repeatedly adding the good solvent and filtering under pressure. After that, the filter is taken out and dried in vacuum, and the weight of the filter is subtracted by the weight of the filter before filtration to calculate the amount of residue on the filter, or the amount of residue on the filter can be evaluated by observing the amount of residue on the filter.

The above fluororesin A solution is obtained by removing at least a part of the insoluble matter in the insoluble matter removal step (2), and it is preferable that at least a part of the removed insoluble matter is a fluororesin containing a residue unit represented by general formula (1) from the viewpoint that the fluororesin A of the present invention finally obtained has a reduced haze value. In this case, the structure of the insoluble matter, i.e., the inclusion of the residue unit represented by general formula (1), can be confirmed by microscopic FT-IR or the like, and can be evaluated, for example, by the following method. After the operation of passing 50 g of Novec 7300 through a 0.1 μm PTFE filter used for filtering the diluted resin solution and washing it is repeated five times, and after drying, the foreign matter on the filter is picked up, and the microscopic IR is measured and compared with the IR chart of the fluororesin containing the residue unit represented by general formula (1) to make a judgment. As shown in the examples, it was confirmed that the insoluble matter removed in the insoluble matter removal step (2) was a resin containing a residue unit represented by general formula (1), and the fluororesin A of the present invention from which at least a part of the insoluble matter was removed had a reduced haze value.

The fluororesin A containing the residue unit represented by the general formula (5) obtained in the polymerization step (1) is obtained as a mixture with different solvents depending on the type of polymerization step. In the case of the polymerization step (1a), the fluororesin A is, for example, a mixture with a good solvent b1 or a mixed solvent of a good solvent b1 and a poor solvent c1. In this case, in the insoluble matter removal step (2), these solvents can be used as they are as solvent S2. The mixture of the good solvent b1 or the mixed solvent of the good solvent b1 and the poor solvent c1 can be used as it is as good solvent b2 or a mixed solvent of the good solvent b2 and the poor solvent c2. Alternatively, it can be further mixed with other solvents to obtain solvent S2.

In the case of the polymerization step (1b), the fluororesin A is obtained as a precipitate. Solvent S2 can be obtained by recovering the precipitate of fluororesin A obtained in the polymerization step (1b) by solid-liquid separation or the like, and after washing and/or drying as necessary, forming a mixture containing good solvent b2 or a mixed solvent of good solvent b2 and poor solvent c2.

When solvent S2 is a mixed solvent, the content of poor solvent c2 is preferably set to a level that allows insoluble matter to coexist but does not cause precipitation of the fluororesin, taking into consideration the concentration of fluororesin A, and the ratio of good solvent b2 to poor solvent c2 can be, for example, in the range of 1 to 50% by weight of poor solvent c2 relative to the total of good solvent b2 and poor solvent c2.

In either case, the concentration of fluororesin A in the mixture of fluororesin A and solvent S2 to be subjected to the insoluble matter removal step is preferably 1 to 40% by weight, more preferably 5 to 30% by weight, from the viewpoint of effectively reducing haze in the heat-melt molded product of the fluororesin.

The insoluble matter removal step (2) can be, for example, either the following step (2a) or (2b).
(2a) a step of filtering a mixture containing the fluororesin A and the solvent S2 through a filter to remove insoluble matters;
(2b) A step of subjecting the mixture containing the fluororesin A and the solvent S2 to centrifugation to remove insoluble matters.

In the insoluble matter removal step (2a), the mixture containing the fluororesin A and the solvent S2 is filtered through a filter to remove the insoluble matter. There is no particular limitation on the filtration method, but examples include pressure filtration, reduced pressure filtration, and centrifugal filtration. There is no limitation on the particulate matter removal performance of the filter used, but since the haze in the heat-melt molded product of the fluororesin A is effectively reduced, the 99% capture particle size of the filter is preferably 10 μm or less, more preferably 5 μm or less, more preferably 1 μm or less, more preferably 0.5 μm or less, even more preferably 0.2 μm or less, and even more preferably 0.1 μm or less. Here, the 99% capture particle size represents the particle size of particles that the filter can capture 99% or more of, and is described in the catalog or technical documentation of the filter, and can also be known by investigating the capture rate of standard particles with known particle sizes.

The filter material used may be, for example, resins such as polypropylene, polyethylene, polyethylene terephthalate, nylon, PTFE (polytetrafluoroethylene), PES (polyethersulfone), mixed cellulose esters, cellulose acetate, polycarbonate, cellulose, nylon, polyamide, etc.; ceramics such as silica fiber and glass fiber; metals such as stainless steel and Hastelloy, etc., and among these, PTFE is preferred because it can effectively reduce haze in the heat-melt molded product of fluororesin A. In addition, the filter may be either hydrophobic or hydrophilic.

Examples of the types of filters used include depth filters and screen filters. Examples of screen filters include mesh filters and membrane filters. Among them, it is preferable to use a screen filter because it can effectively reduce haze in a heat-melt molded product of fluororesin A, and it is even more preferable to use a membrane filter, and it is even more preferable to use a membrane filter made of PTFE. A depth filter is a filter that captures particles inside the filter, and a screen filter is a filter that captures particles on the surface of the filter. A membrane filter is a type of screen filter. In addition, since excellent filtering properties are obtained, multiple types of filters may be used in combination. For example, a combination of a depth filter and a screen filter, and a combination of screen filters with different captured particle sizes can be used. When a screen filter is combined with another filter such as a depth filter or a screen filter with a different captured particle size, it is preferable that the 99% captured particle size of the filter combined with the screen filter is 1 to 10 μm because excellent filtering properties are obtained.

Since this effectively reduces haze in hot-melt molded products of fluororesin A, it is preferable to use a screen filter with a pore size of 10 μm or less, more preferably a screen filter with a pore size of 5 μm or less, more preferably a screen filter with a pore size of 1 μm or less, even more preferably a screen filter with a pore size of 0.5 μm or less, and even more preferably a screen filter with a pore size of 0.2 μm or less. In general, when the pore size of a membrane filter is C μm, the 99% captured particle size is below C μm, and depending on the product, 99.99% or more of C μm particles are captured.

In the insoluble matter removal step (2b), the mixture containing fluororesin A and solvent S2 is centrifuged to remove the insoluble matter. There are no particular limitations on the centrifugation method, but examples include a method in which the mixture containing fluororesin A and solvent S2 is placed in a container, and centrifugal force is applied to the container to cause the insoluble matter to settle and separate the solution, thereby removing the insoluble matter. The centrifugation method may be a batch type, a continuous type, or an intermediate type between the batch type and the continuous type.

Precipitation step (3)
In the precipitation step (3), fluororesin A is precipitated from the fluororesin A solution obtained in the insoluble matter removal step. As the solvent S3 for the fluororesin A solution, the solvent S2 used for removing the insoluble matter in the insoluble matter removal step (2) may be used as it is, or a solvent of a different type or composition may be used depending on the method.

The method for precipitating a polymer from the fluororesin A solution is not particularly limited, and the precipitation step (3) can be, for example, any one of the following steps (3a), (3b), (3c) or (3d).
(3a) a step of lowering the temperature of the fluororesin A solution to precipitate the fluororesin A;
(3b) adding the fluororesin A solution to a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3c) a step of precipitating fluororesin A by adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution; and (3d) a step of precipitating fluororesin A by volatilizing the solvent from the fluororesin A solution.

The precipitation step (3a) is a step in which the temperature of the fluororesin A solution is lowered to precipitate the fluororesin A. The solvent S3 of the fluororesin A solution may be the solvent S2 used in the insoluble matter removal step (2) or a solvent of a different type or composition, depending on the method. The good solvent b2 or the mixed solvent of the good solvent b2 and the poor solvent c2 used in the insoluble matter removal step (2) may be used as the solvent in the precipitation step (3a). That is, the good solvent b2 or the mixed solvent of the good solvent b2 and the poor solvent c2 may be the good solvent b3 for the fluororesin A or the mixed solvent of the good solvent b3 for the fluororesin A and the poor solvent c3 for the fluororesin A, respectively. The concentration of the fluororesin A in the fluororesin A solution is preferably 1 to 40% by weight, more preferably 1 to 30% by weight, and even more preferably 2 to 20% by weight, from the viewpoint of obtaining particles having excellent productivity and excellent handling properties as a powder. From the viewpoint of obtaining particles having excellent productivity and excellent handling properties as a powder, it is preferable that the solvent S3 used in the precipitation step is a mixed solvent of the good solvent b3 and the poor solvent c3. When the solvent S3 used in the precipitation step is a mixed solvent of the good solvent b3 and the poor solvent c3, the ratio of the good solvent b3 to the poor solvent c3 is preferably 10:90 to 99:1 by weight, more preferably 20:80 to 95:5, even more preferably 30:70 to 95:5, even more preferably 30:70 to 90:10, and even more preferably 30:70 to 80:20, since it is possible to obtain particles having excellent productivity and excellent handling properties as a powder, and the coloring of the heat-melted product is reduced.

In the precipitation step (3a), the solution temperature T1 before the temperature is lowered is, for example, preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 50°C or higher. If the solution temperature after the temperature is lowered is T2, T1-T2 is preferably 5°C or higher, more preferably 10°C or higher, more preferably 15°C or higher, and even more preferably 20°C or higher. This allows the precipitation of fluororesin A to be carried out sufficiently.

In the precipitation step (3a), it is preferable to lower the temperature over a period of 1 to 600 minutes, and more preferably over a period of 5 to 300 minutes, because this provides excellent productivity, excellent powder handling, and reduces coloring of the heat-melted product.

In the precipitation step (3a), it is preferable to lower the temperature at a rate of 0.05 to 20°C per minute, and it is particularly preferable to lower the temperature at a rate of 0.1 to 5°C per minute, since this provides particles with excellent productivity and excellent handling properties as a powder.

The precipitation step (3b) is a step of precipitating fluororesin A by adding a poor solvent c3 for fluororesin A to the fluororesin A solution, and the precipitation step (3c) is a step of precipitating fluororesin A by adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution. The solvent S3 of the fluororesin A solution in the precipitation steps (3b) and (3c) may be the solvent S2 used for removing insoluble matter in the insoluble matter removal step (2). However, from the viewpoint that the precipitation of fluororesin A is easy by mixing with the poor solvent c3, it is preferable that the solvent s2 used for removing insoluble matter in the insoluble matter removal step (2) is a mixed solvent of the good solvent b2 and the poor solvent c2. From the viewpoint that the precipitation of fluororesin A is easy and the coloring of the heat-melted product is small, the precipitation step (3b) in which fluororesin A is precipitated by adding a fluororesin A solution to the poor solvent c3 for fluororesin A is preferable. On the other hand, from the viewpoint of excellent powder handling properties, a precipitation step (3c) in which fluororesin A is precipitated by adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution is preferred.

In any process, the productivity is excellent, particles are prevented from adhering to each other, and particles that are easy to handle as a powder are obtained. Therefore, the weight ratio of the good solvent:poor solvent after mixing with the poor solvent c3 is preferably in the range of 10:90 to 90:10, more preferably 20:80 to 80:20, even more preferably 30:70 to 70:30, and even more preferably 30:70 to 60:40.

In the precipitation step (3d), the solvent S3 is evaporated from the fluororesin A solution to precipitate the fluororesin A. From the viewpoint of removing the solvent S3 by evaporation, the solvent S3 can be a solvent with a relatively low boiling point. The volatilization operation of the solvent S3 can be carried out by a known method, for example, a method of volatilizing the solvent using a thin-film evaporator such as Exeva, a method of passing the solvent through a heated flash tank to volatilize the solvent, a method of heating the solution in an extruder using a volatilization extrusion device to volatilize the solvent, a method of dispersing the fluororesin A solution in a solvent that is not miscible with the fluororesin A solution such as water, and volatilizing the solvent by heating or introducing steam (when steam is introduced, this is generally called steam stripping), a method of heating a fluororesin A solution containing a low-boiling good solvent and a high-boiling poor solvent and volatilizing the low-boiling good solvent to precipitate the fluororesin A, and the like, and a combination of multiple methods may be used. In addition, after removing the solvent by these methods, the fluororesin A may be processed into pellets by a pelletizer or the like.

The precipitation steps (3a) to (3d) can also be used in appropriate combination. For example, the fluororesin A solution obtained in the insoluble matter removal step can be subjected to the precipitation step (3b) or (3c), and the remaining fluororesin A solution can be further subjected to the precipitation step (3a) or (3d) to further recover the remaining fluororesin A.

In the precipitation step, it is preferable to stir the fluororesin A solution, since this provides particles that are highly productive and easy to handle as a powder. For example, stirring with a stirring blade or stirring by vibration is one example. In all of the precipitation steps (3a) to (3d), it is preferable to stir the solution when precipitating the resin.

In the precipitation step, since particles having excellent productivity and excellent powder handleability can be obtained, it is preferable to precipitate a particulate solid by lowering the temperature while stirring so that the Pv value, which is the value of the agitator motor power per unit stirring volume, is 0.05 to 50 kW/m ³ , with the Pv value being more preferably 0.2 to 50 kW/m ³ , even more preferably 0.5 to 30 kW/m ³ , and particularly preferably 0.5 to 10 kW/m ^3. Here, the Pv value (kW/m ³ ) can be calculated by the following formula (10).

(Here, Np is the power number, ρ is the density of the solution (kg/m ³ ), n is the rotation speed of the impeller (rpm), d is the diameter of the impeller (mm), and V is the amount of the solution (L).)

In formula (10), Np is a dimensionless number called the power number, which varies depending on the shape of the impeller. This Np can be obtained from known literature, such as "Chemical Equipment, August 1995 issue, pp. 71-79" and "Shinko Pfaudler Technical Report, vol. 28, No. 8 (October 1984), pp. 13-16." In this case, if the ratio b/d of the blade width b to the impeller diameter d differs from the impellers described in the literature, it can be calculated using the following formula (11).

Actual Np = literature Np × (actual b/d) / (literature b/d) (11)
(Here, Np: power number, b: impeller width (mm), d: impeller diameter (mm).)

In the present invention, there are no particular limitations on the combination of the polymerization step (1), the insoluble matter removal step (2), and the precipitation step (3). However, from the viewpoint that including a step in which the fluororesin is precipitated as particles results in a fluororesin with fewer impurities and reduces discoloration of the heat-melted product, it is preferable that the polymerization step (1) is step (1a) or (1c) and the precipitation step (3) is step (3a), (3b), or (3c), for example, and it is also preferable that the polymerization step (1) is step (1b) and the precipitation step (3) is step (3a), (3b), (3c), or (3d). More preferably, the polymerization step (1) is step (1a), the insoluble matter removal step (2) is step (2a), and the precipitation step (3) is step (3a) or (3b) or (3c). Also, it is preferable that the polymerization step (1) is step (1b), the insoluble matter removal step (2) is step (2a), and the precipitation step (3) is step (3a), (3b), (3c) or (3d). When the polymerization step (1) and the precipitation step (3) are combined as above, a particulate fluororesin A is obtained in either step, and further, a step such as washing the particulate fluororesin A can be performed, so that a fluororesin with little coloring in the heat-melted product is easily obtained, which is preferable. In addition, the precipitation step is preferably step (3a) or (3c) because particles with high bulk density and excellent handling properties as powder are obtained. In addition, from the viewpoint of being less susceptible to torque increases and excellent productivity during the particle precipitation process, it is preferable that the precipitation process be step (3a).

In the fluororesin A solution from which the resin is precipitated obtained in the precipitation step (3), it is preferable to perform a poor solvent addition step (4) in which poor solvent c4 is added, since this prevents the resulting resin from adhering to itself and results in a resin that is easy to handle as a powder. The amount of poor solvent c4 added in the poor solvent addition step (4) is preferably 0.1 times or more the weight of the fluororesin A-containing solution obtained in the precipitation step, and more preferably 0.5 times or more and 1 times or more the weight of the poor solvent c4 is added, since this provides excellent productivity, prevents particles from adhering to each other, and results in a resin that is easy to handle as a powder.

In the present invention, any other process may be added, and a separation process (5) of extracting a solid by solid-liquid separation may be included after the precipitation process (3) or the poor solvent addition process (4). The solid-liquid separation method is not particularly limited, and examples thereof include pressure filtration, reduced pressure filtration, centrifugation, and centrifugal filtration. The size of the filter used is not limited, and examples thereof include filters with a capture particle size of 30 μm or less. The material of the filter used is not limited, and examples thereof include polypropylene, polyethylene, polyethylene terephthalate, nylon, PTFE, PES, and the like.

In the present invention, any other process may be added, and may include a washing process for washing the particles of fluororesin A and/or a drying process for drying the particles. In the washing process (6), it is preferable to use a poor solvent c6, which is an organic solvent that precipitates fluororesin A, preferably at 25°C. There are no particular limitations on the drying method, but examples include vacuum drying, reduced pressure drying, normal pressure drying, air drying, shaking drying, hot air drying, and heat drying.

In the present invention, it is preferable to further include a separation step (5) of separating fluororesin A from the solution in which fluororesin A has precipitated obtained in the precipitation step (3) or from the solution to which poor solvent c4 has been added in the poor solvent addition step (4), and a washing step (6) of washing the separated fluororesin A with poor solvent c6. This allows for the production of particles with a superior yellowness.

In addition, it is preferable to use a solvent filtered through a filter having a 99% capture particle size of 5 μm or less or a screen filter with a pore size of 5 μm or less as the poor solvent c6, because this can effectively reduce haze in the hot-melt molded product.

The present invention will be described in more detail below with reference to examples. However, the examples are merely illustrative of the present invention, and the present invention is not intended to be limited to the examples.

<Physical property measurement method>
(1) Weight average molecular weight Mw
Measurements were performed using gel permeation chromatography equipped with a column TSKgel SuperHZM-M manufactured by Tosoh Corporation and an RI detector. As an eluent, Asahiklin AK-225 (manufactured by Asahi Glass Co., Ltd.) was used to which 10 wt% of 1,1,1,3,3,3-hexafluoro-2-propanol (manufactured by Wako Pure Chemical Industries, Ltd.) was added relative to AK-225. Standard polymethyl methacrylate manufactured by Agilent was used as a standard sample, and the weight average molecular weight Mw in terms of polymethyl methacrylate was calculated from the elution times of the sample and the standard sample.

(2) Measurement of Volume Average Particle Diameter The volume average particle diameter (unit: μm) was measured using a Microtrac MT3000 manufactured by Microtrac Bell Inc. and methanol as a dispersion medium.

(3) Calculation of Pv Value The Pv value, which is the value of the agitator motor power per unit agitation volume, was calculated from the following formula: Np was 4.2 when using four diagonal paddle agitating blades (blade diameter 50 mm, angle 45°).
(Here, Np is the power number, ρ is the density of the solution (kg/m ³ ), n is the rotation speed of the impeller (rpm), d is the diameter of the impeller (mm), and V is the amount of the solution (L).)

(4) Haze measurement A mold with a 1 mm thick plate with the center hollowed out was placed on a smooth metal plate with a polyimide film placed on it, placed on the hollowed out part of the fluororesin, placed a polyimide film and a metal plate on top of it, sandwiched, placed on a press, heated at 280 ° C for 10 minutes without applying pressure, then heated and pressed at a pressure of 10 MPa and 280 ° C for 10 minutes in a press, and then depressurized and heated and pressed at a pressure of 10 MPa for 5 minutes, and then heated and pressed at 280 ° C and a pressure of 10 MPa in a press for 10 minutes, and then depressurized, and the molded product sandwiched between the metal plates was further sandwiched between metal plates for cooling and cooled to obtain a hot press molded product (thickness 1 mm). The obtained hot press molded product (thickness 1 mm) was measured according to JIS K7136 using a haze meter NDH5000 (light source: white LED) manufactured by Nippon Denshoku Kogyo Co., Ltd. to obtain a haze (%).

(5) Measurement of insoluble content 1,1,1,2,3,4,4,5,5,5-Decafluoro-3-methoxy-2-( _{trifluoromethyl} )pentane ( _C2F5CF ( _OCH3 ) _C3F7 , Novec7300, manufactured by 3M Japan) is added to the _fluororesin to prepare a solids concentration of 10% by weight. The mixture is dissolved at 50°C for 5 hours and shaken to prepare a fluororesin solution. The solution was pressure-filtered using a pressure filter equipped with a PTFE membrane filter having a pore size of 0.1 μm, the weight of which had been recorded in advance, and then pressure-filtered using Novec 7300 from which foreign matter had been removed using a filter with a pore size of 0.1 μm. The Novec 7300 from which foreign matter had been removed was placed in the pressure filter and pressure-filtered repeatedly to wash away the remaining fluororesin. The filter was then removed and vacuum-dried, and the weight of the resulting filter was subtracted by the weight of the filter before filtration to determine the amount of matter remaining on the filter, and the amount of matter remaining on the filter was divided by the weight of the resin used to determine the percentage, thereby determining the amount of insoluble matter (wt %).

(6) Measurement of bulk density Fluororesin A is weighed out and placed into a 13.5 mL glass sample tube (the liquid level when 10 mL of water is placed is 2.8 cm) whose height per unit volume has been measured in advance, without applying vibration, and the bulk density can be calculated from the powder height and weight at that time according to the following formula. The bulk density at this time is called loose bulk density.
Bulk density = (weight of powder (g)) / ((height of powder (cm)) / 0.28 (cm/mL))

(7) Measurement of Yellowness (YI) 2.0 g of fluororesin was weighed into a petri dish with an inner diameter of 26.4 mm (a set of a flat petri dish lid and a receiver made by Flat Co., Ltd., only the receiver, the glass thickness of the bottom of the receiver was 1 mm), and placed in an inert oven (DN411I made by Yamato Scientific Co., Ltd.), and left to stand at room temperature for 30 minutes under air flow (20 L/min), then heated to 280 ° C. over 30 minutes, and heated at 280 ° C. for 24 hours. Thereafter, while maintaining the air flow (20 L/min), the oven door was kept closed, the inert oven was turned off, and the sample was left to cool for 12 hours, and then the sample was taken out to obtain a fluororesin heat-melt molded product with a thickness of 3 mm and a diameter of 26.4 mm on the petri dish. At this time, air compressed by a compressor and passed through a dehumidifier (dew point temperature -20 ° C. or less) was used as the air. The obtained fluororesin heat melt molded product was measured for transmittance at each wavelength at 1 nm intervals at wavelengths of 200 nm to 1500 nm using a spectrophotometer (U-4100 manufactured by Hitachi High-Tech Science Corporation). Data at 5 nm intervals at wavelengths of 380 nm to 780 nm were extracted from the measured transmittance data, and the tristimulus values X, Y, and Z of the XYZ color system were calculated according to the method of JIS Z8701. YI was calculated for a C light source (auxiliary illuminant C) according to the method of JIS K7373, and the YI of the fluororesin heat melt molded product including the petri dish was obtained. The YI of the petri dish alone (receiver only) was measured, and the YI of the fluororesin heat melt molded product with a thickness of 3 mm was obtained by subtracting the YI of the petri dish alone (receiver only) from the YI of the fluororesin molded product including the petri dish. The YI of the petri dish alone (receiver only) was 0.21.

Example 1
A 75 mL glass ampoule was charged with 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 80.00 g of Novec 7300 (3M Japan, _C2F5CF ( _OCH3 ) _C3F7 ) as a polymerization solvent, and 2.22 g (0.0186 _mol ) of chloroform (Wako Pure Chemical Industries, Ltd.) as a chain transfer agent, and the _ampoule was subjected to repeated nitrogen replacement by freeze degassing and depressurization, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic bath at 55°C and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin was dissolved. After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 100g of Novec7300 to adjust the viscosity, producing a diluted resin solution (solid content concentration 10% by weight). The diluted resin solution was placed in a pressure filtration device (manufactured by ADVANTEC) equipped with a PTFE membrane filter (manufactured by ADVANTEC T010A) with a pore size of 0.1 μm, and pressure filtration was performed to remove components insoluble in the solvent.

The solution was transferred to a 1000 mL separable flask equipped with a four-blade diagonal paddle impeller (blade diameter 50 mm, blade width 12 mm, 45° inclination) heated to 50°C, a three-one motor, and a water bath, and heated to 50°C while stirring at 200 rpm and held for 5 minutes, after which 270 g of Zeororora H (manufactured by Zeon Corporation, 1,2,2,3,3,4,4-heptafluorocyclopentane) was added and held at 50°C for 5 minutes while stirring at 200 rpm (Zeorolla H/Novec7300=60/40 (wt/wt)). The water bath was removed while stirring at 600 rpm (Pv value: 8.1 kW/m ³ ), and the mixture was allowed to cool in the air and cooled to 30°C in about 30 minutes to obtain a particulate solid. Thereafter, 150 g of Zeororah H was further added while stirring at 600 rpm (Zeorollah H/Novec7300=70/30 (wt/wt)). Suction filtration was performed, washing with acetone was performed twice, and vacuum drying was performed under heating to obtain particles of fluororesin A. The obtained resin had a weight average molecular weight of 7.2×10 ⁴ , was a fine particle with a volume average particle size of 88 μm, and was almost free of coarse particles. At this time, the acetone used was previously filtered through a 0.1 μm PTFE filter. The evaluation results of the fluororesin are shown in Table 1. Meanwhile, the PTFE filter used for filtering the diluted resin solution was washed by passing 50 g of Novec7300 through it five times, and then dried. When the insoluble matter on the obtained filter was confirmed by microscopic IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unwanted matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 2
A solution of 0.0865 g (0.000205 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator dissolved in 0.260 g of hexafluorobenzene was placed in a glass ampoule having a diameter of 30 mm and equipped with a magnetic stirrer, 10.0 g (0.0205 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 39.74 g of Zeorola-H (manufactured by Nippon Zeon Co., Ltd., 1,2,2,3,3,4,4-heptafluorocyclopentane) as a polymerization solvent, and 1.111 g (0.00931 mol) of chloroform (manufactured by Wako Pure Chemical Industries, Ltd.) as a chain transfer agent were placed in the ampoule, which was equipped with a magnetic stirrer and had a diameter of 30 mm. The ampoule was then repeatedly subjected to nitrogen replacement by freeze degassing and depressurization, and then sealed under reduced pressure (amount of chain transfer agent: 10% by weight based on the total weight of the monomer and chain transfer agent). The ampoule was held upright and stirred with a magnetic stirrer at 55°C for 24 hours to carry out precipitation polymerization. The mixture became cloudy, and a slurry was obtained in which the resin precipitated in the polymerization solvent. After cooling to room temperature, the ampoule was opened, and the liquid containing the generated resin particles was filtered, washed with acetone, and vacuum dried to obtain a particulate perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin having a volume average particle size of 95 μm. 90 g of Novec7300 (manufactured by 3M Japan, C ₂ F ₅ CF(OCH ₃ )C ₃ F ₇ ) was added to 10.0 g of the obtained fluororesin, and the resin was dissolved by heating at 50°C for 4 hours to prepare a diluted resin solution (solid concentration 10% by weight). The resin dilution solution was placed in a pressure filter (manufactured by ADVANTEC) equipped with a 0.1 μm PTFE membrane filter (T010A manufactured by ADVANTEC), and components insoluble in the solvent were removed by pressure filtration. 2 L of acetone was placed in a plastic cup equipped with an anchor blade, and the resin dilution solution filtered under pressure was added to a beaker under stirring to precipitate the resin. The precipitated resin was recovered by filtration, washed once with acetone, and vacuum dried to obtain a powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The weight average molecular weight of the obtained fluororesin was 9.7 × 10 ^4. The evaluation results of the fluororesin are shown in Table 1. At this time, the acetone used was previously filtered through a 0.1 μm PTFE filter. Meanwhile, the PTFE filter used for filtering the resin dilution solution was washed by passing 50 g of Novec7300 through it five times, and then dried. The insoluble matter on the filter was confirmed to contain a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit by microscopic IR. By removing this resin component (unwanted matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 3
A 75 mL glass ampoule was charged with 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 80.00 g of Novec 7300 (3M Japan, _C2F5CF ( _OCH3 ) _C3F7 ) as a polymerization solvent, and 2.22 g (0.0186 _mol ) of chloroform (Wako Pure Chemical Industries, Ltd.) as a chain transfer agent, and the _ampoule was subjected to repeated nitrogen replacement by freeze degassing and depressurization, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic bath at 55°C and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin was dissolved. After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 100g of Novec7300 to adjust the viscosity, producing a diluted resin solution (solid content concentration 10% by weight). The diluted resin solution was placed in a pressure filtration device (manufactured by ADVANTEC) equipped with a PTFE membrane filter (manufactured by ADVANTEC T010A) with a pore size of 0.1 μm, and pressure filtration was performed to remove components insoluble in the solvent.

The solution was transferred to a 1000 mL separable flask equipped with four diagonal paddle impellers (blade diameter 50 mm, blade width 12 mm, 45° inclination) heated to 50° C., a three-one motor, and a water bath, and 420 g of Zeororah H (1,2,2,3,3,4,4-heptafluorocyclopentane, manufactured by Zeon Corporation) was slowly added while stirring at 600 rpm (Pv value: 20.6 kW/m ³ ), to obtain a particulate solid (Zeorollah H/Novec7300=70/30 (wt/wt), Pv value after addition: 6.1 kW/m ³ ). The solution was filtered by suction, washed twice with acetone, and vacuum dried under heating to obtain particles of fluororesin A. The obtained resin had a weight average molecular weight of 7.9×10 ⁴ , was fine particles with a volume average particle size of 87 μm, and was almost free of coarse particles. In this case, Zeorora H, Novec7300, and acetone used in the precipitation process and thereafter were previously filtered through a 0.1 μm PTFE filter. The evaluation results of the fluororesin are shown in Table 1. Meanwhile, the PTFE filter used to filter the diluted resin solution was washed by passing 50 g of Novec7300 through it five times, and then dried. When the insoluble matter on the obtained filter was confirmed by micro-IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unnecessary matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 4
A 75 mL glass ampoule was charged with 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 80.00 g of Novec 7300 (3M Japan, _C2F5CF ( _OCH3 ) _C3F7 ) as a polymerization solvent, and 2.22 g (0.0186 _mol ) of chloroform (Wako Pure Chemical Industries, Ltd.) as a chain transfer agent, and the _ampoule was subjected to repeated nitrogen replacement by freeze degassing and depressurization, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic bath at 55 ° C. and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin was dissolved. After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 100 g of Novec 7300 to adjust the viscosity to prepare a diluted resin solution (solid content concentration 10 wt%). The diluted resin solution was placed in a pressure filtration device (manufactured by ADVANTEC Co., Ltd.) equipped with a PTFE membrane filter (manufactured by ADVANTEC Co., Ltd. T010A) having a pore size of 0.1 μm, and pressure filtration was performed to remove components insoluble in the solvent. 2 L of acetone was placed in a plastic cup equipped with an anchor blade, and the pressure-filtered diluted resin solution was added to a beaker under stirring to precipitate the resin, and the precipitated resin was collected by filtration, washed twice with acetone, and vacuum dried to obtain a powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The weight average molecular weight of the obtained fluororesin was 5.7 x ^104. At this time, the acetone used was previously filtered through a 0.1 μm PTFE filter. The evaluation results of the fluororesin are shown in Table 1. On the other hand, the PTFE filter used for filtering the diluted resin solution was washed five times with 50 g of Novec7300, and then dried. When the insoluble matter on the obtained filter was confirmed by microscopic IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unnecessary matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 5
A 75 mL glass ampoule was charged with 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 80.00 g of Novec 7300 (3M Japan, _C2F5CF ( _OCH3 ) _C3F7 ) as a polymerization solvent, and 2.22 g (0.0186 _mol ) of chloroform (Wako Pure Chemical Industries, Ltd.) as a chain transfer agent, and the _ampoule was subjected to repeated nitrogen replacement by freeze degassing and depressurization, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic bath at 55°C and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin was dissolved. The ampoule was opened after cooling to room temperature (solid content concentration 20% by weight). The resin solution was placed in a pressure filtration device (manufactured by ADVANTEC) equipped with a PTFE membrane filter (manufactured by ADVANTEC T500A) with a pore size of 5 μm, and pressure filtration was performed to remove components that were insoluble in the solvent. To adjust the viscosity, the resin solution was diluted with 100 g of Novec 7300 that had been previously filtered through a 0.1 μm PTFE filter to prepare a diluted resin solution (solid content concentration 10% by weight). 2 L of acetone was placed in a plastic cup equipped with an anchor blade, and the resin diluted solution obtained by pressure filtration was added to a beaker under stirring to precipitate the resin. The precipitated resin was recovered by filtration, washed twice with acetone, and vacuum dried to obtain a powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The weight-average molecular weight of the obtained fluororesin was 5.5 x 10 ^4. At this time, the acetone used had been previously filtered through a 0.1 μm PTFE filter. The evaluation results of the fluororesin are shown in Table 1. Meanwhile, the PTFE filter used to filter the resin diluted solution was washed five times with 50 g of Novec 7300, and then dried. When the insoluble matter on the obtained filter was confirmed by microscopic IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unnecessary matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 6
In a 75 mL glass ampoule, 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide was added as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane), 80.00 g of FC-72 (3M Japan, perfluorohexane) was added as a polymerization solvent, and 2.22 g (0.0186 mol) of chloroform (Wako Pure Chemical Industries, Ltd.) was added as a chain transfer agent. The mixture was repeatedly subjected to nitrogen replacement and depressurization by freeze degassing, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic chamber at 55° C. and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin had dissolved. The ampoule was opened after cooling to room temperature (solid concentration 20% by weight). After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 100 g of FC-72 to adjust the viscosity, to prepare a resin dilution solution (solid concentration 10 wt%). The resin dilution solution was placed in a pressure filter (manufactured by ADVANTEC) equipped with a 0.1 μm pore size PTFE membrane filter (manufactured by ADVANTEC T010A), and components insoluble in the solvent were removed by pressure filtration. 2 L of hexane was placed in a plastic cup equipped with an anchor blade, and the resin dilution solution filtered under pressure was added to a beaker under stirring to precipitate the resin. The precipitated resin was recovered by filtration, washed twice with acetone, and vacuum dried to obtain a powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The weight average molecular weight of the obtained fluororesin was 7.2 × 10 ^4. At this time, hexane and acetone were used that had been previously filtered through a 0.1 μm PTFE filter. The evaluation results of the fluororesin are shown in Table 1. On the other hand, the PTFE filter used for filtering the diluted resin solution was washed five times with 50 g of Novec7300, and then dried. When the insoluble matter on the obtained filter was confirmed by microscopic IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unwanted matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Example 7
In a 75 mL glass ampoule, 0.173 g (0.000410 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide was added as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) was added as a monomer, 80.00 g of hexafluorobenzene (Tokyo Chemical Industry Co., Ltd.) was added as a polymerization solvent, and 2.22 g (0.0186 mol) of chloroform (Wako Pure Chemical Industries, Ltd.) was added as a chain transfer agent. The mixture was repeatedly subjected to nitrogen replacement and depressurization by freeze degassing, and then sealed under reduced pressure (monomer/solvent=20/80 (wt/wt)). The ampoule was placed in a thermostatic chamber at 55° C. and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin had dissolved. The ampoule was opened after cooling to room temperature (solid concentration 20% by weight). After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 100 g of hexafluorobenzene to adjust the viscosity, to prepare a resin dilution solution (solid concentration 10 wt%). The resin dilution solution was placed in a pressure filter (manufactured by ADVANTEC) equipped with a PTFE membrane filter (manufactured by ADVANTEC T010A) with a pore size of 0.1 μm, and components insoluble in the solvent were removed by pressure filtration. 2 L of chloroform was placed in a plastic cup equipped with an anchor blade, and the resin dilution solution filtered under pressure was added to a beaker under stirring to precipitate the resin. The precipitated resin was recovered by filtration and then vacuum dried to obtain a powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The weight average molecular weight of the obtained fluororesin was 6.5 × 10 ^4. The evaluation results of the fluororesin are shown in Table 1. On the other hand, the PTFE filter used for filtering the diluted resin solution was washed five times with 50 g of Novec7300, and then dried. When the insoluble matter on the obtained filter was confirmed by microscopic IR, it was confirmed that it contained a fluororesin component containing a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit. By removing this resin component (unwanted matter), the haze of the fluororesin was clearly reduced compared to Comparative Example 1.

Comparative Example 1
The procedure was carried out according to the description of Sample 93 in Table 2 of Non-Patent Document 1. However, since there was no description of the polymer concentration during reprecipitation purification, the polymer was diluted to 10 wt%. A 75 mL glass ampoule was charged with 0.0880 g (0.000209 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 20.0 g (0.0820 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 32.63 g of hexafluorobenzene as a polymerization solvent, and the mixture was repeatedly subjected to nitrogen replacement and depressurization by freeze degassing, and then sealed under reduced pressure (monomer/solvent=38/62 (wt/wt)). The ampoule was placed in a thermostatic bath at 60° C. and held for 24 hours to carry out radical solution polymerization, resulting in a viscous liquid in which the resin had dissolved. After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 147 g of hexafluorobenzene to adjust the viscosity, to prepare a diluted resin solution. 1 L of chloroform was placed in a beaker equipped with an anchor blade, and the diluted resin solution was added to the beaker under stirring to precipitate the resin. The precipitated resin was collected by filtration and then vacuum dried to obtain an amorphous perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin. The molded product of the obtained fluororesin after heating at 280°C for 24 hours had many bubbles, but the coloring was more intense than in Example 1 when observed visually, and was equal to or slightly more intense than in Example 7. Furthermore, when the size of the average size of the obtained fluororesin was measured with a ruler, the average size was about 10 mm. The weight average molecular weight of the obtained fluororesin was 3.7 x 10 ^5. The evaluation results of the fluororesin are shown in Table 1.

Reference Example 1
The fluororesin A (weight average molecular weight Mw = 7.2 × ¹⁰ ) produced in Example 1 was immersed in various organic solvents in an amount 20 times (w/w) the amount of fluororesin A at 50°C for 5 hours or more, and the dissolution was confirmed visually, with the following results.
Soluble: FC-72, FC-770, Novec 7200, Novec 7300, hexafluorobenzene. When the solutions dissolved in these solvents were cooled to 25°C, all of them maintained a dissolved state. In all of them, there was almost no residual residue, and the solubility was 90 wt% or more.
Insoluble: Zeorora H, AE-3000, trifluoroethanol, ethyl acetate, chloroform, acetone, hexane. In all cases, after cooling to 25°C, filtration and drying, the recovery rate of fluororesin A exceeded 80%, and the solubility was less than 20 wt%.

Reference Example 2
A solution of fluororesin A (weight average molecular weight Mw = 7.2 × ¹⁰ ) dissolved in Novec 7300 to a solid content concentration of 10 wt % was added dropwise at 25°C to the following organic solvent in an amount 10 times the amount of the fluororesin A solution. The resulting precipitate was visually confirmed as follows:
No solid precipitated: FC-72, FC-770, Novec 7200, Novec 7300, hexafluorobenzene. All of them had no precipitate and a solubility of 90 wt % or more.
A solid precipitated: Zeorora H, AE-3000, trifluoroethanol, ethyl acetate, chloroform, acetone, and hexane. In all cases, the recovery rate of fluororesin A after filtration and drying exceeded 80%, and the solubility was less than 20 wt %.

The present invention is useful in fields related to fluororesins.

Claims (15)

A polymerization step (1) of polymerizing a monomer represented by the following general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by the following general formula (5):
an insoluble matter removal step (2) of removing insoluble matter from a mixture containing the fluororesin A containing the residue unit represented by general formula (5) obtained in the polymerization step and the solvent S2 to obtain a fluororesin A solution;
The method includes a precipitation step (3) of precipitating fluororesin A from the fluororesin A solution obtained in the insoluble matter removal step,
Solvent S2 is an aliphatic fluorine-containing solvent,
The precipitation step (3) is any one of the following steps (3a), (3b) or (3c),
(3a) a step of lowering the temperature of a fluororesin A solution containing a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3b) adding the fluororesin A solution to a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3c) adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution to precipitate the fluororesin A;
The poor solvent c3 includes a non-chlorinated solvent;
A method for producing a fluororesin, in which a hot press molded product (thickness 1 mm) has a haze value of 2% or less, and a melt molded product (thickness 3 mm) heated at 280° C. for 24 hours has a yellowness index of 3 or less.
(In formula (4) and formula (5), Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ each independently represent one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₅ , Rf ₆ , Rf ₇ , and Rf ₈ may be bonded to each other to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.) The method according to claim 1 , wherein the polymerization step (1) is any one of the following steps (1a), (1b) and (1c):
(1a) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A and the good solvent b1;
(1b) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for fluororesin A to precipitate fluororesin A, recovering the precipitated fluororesin A, and mixing the recovered fluororesin A with a good solvent b1 for fluororesin A to obtain a mixture containing fluororesin A and the good solvent b1.
(1c) A step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for the fluororesin A to precipitate the fluororesin A, and mixing with a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A, the good solvent b1, and the poor solvent c1. The method according to claim 2 , wherein the step (1a) is carried out in the presence of a poor solvent c1 for the fluororesin A in addition to the radical polymerization initiator and the good solvent b1 for the fluororesin A. The method according to any one of claims 1 to 3 , wherein the insoluble matter removing step (2) is either the following step (2a) or (2b):
(2a) a step of filtering a mixture containing the fluororesin A and the solvent S2 through a filter to remove insoluble matters;
(2b) A step of subjecting the mixture containing the fluororesin A and the solvent S2 to centrifugation to remove insoluble matters. The method according to claim 4 , wherein the solvent S2 is a good solvent b2 for the fluororesin A or a mixed solvent of the good solvent b2 and a poor solvent c2 for the fluororesin A. The method according to claim 4 or 5 , wherein the insoluble matter removing step (2) is (2a). The method according to any one of claims 4 to 6 , wherein the filter is a filter having a 99% capture particle size of 10 µm or less or a screen filter having a pore size of 10 µm or less. The method according to any one of claims 1 to 7 , wherein the solvent of the fluororesin A solution in the precipitation step (3a) is a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A. The method according to any one of claims 1 to 8, wherein in the precipitation step (3a), when the solution temperature T1 before the temperature is reduced is 30°C or more and the solution temperature after the temperature is reduced is T2, T1 - T2 is 5°C or more. The production method according to any one of claims 1 to 9, further comprising a separation step (5) of separating the fluororesin A from the solution in which the fluororesin A has been precipitated obtained in the precipitation step (3) or from the solution to which the poor solvent c4 has been added in the poor solvent adding step (4), and a washing step (6) of washing the separated fluororesin A with the poor solvent c5 . The production method according to any one of claims 1 to 10, wherein the polymerization step (1) is (1b) a step of polymerizing a monomer represented by general formula (4) in the presence of a radical polymerization initiator and a poor solvent c1 for the fluororesin A to precipitate the fluororesin A, recovering the precipitated fluororesin A, and mixing the recovered fluororesin A with a good solvent b1 for the fluororesin A to obtain a mixture containing the fluororesin A and the good solvent b1 , and the precipitation step (3) is step (3a), (3b) or (3c). The method according to any one of claims 1 to 11 , wherein the precipitation step (3) is step (3a) or (3c). The method according to any one of claims 1 to 12 , wherein the insoluble matter removed in the insoluble matter removal step (2) comprises at least a fluororesin containing a residue unit represented by general formula (1). A polymerization step (1) of polymerizing a monomer represented by the following general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by the following general formula (5):
an insoluble matter removal step (2) of removing insoluble matter from a mixture containing the fluororesin A containing the residue unit represented by general formula (5) obtained in the polymerization step and the solvent S2 to obtain a fluororesin A solution;
The method includes a precipitation step (3) of precipitating fluororesin A from the fluororesin A solution obtained in the insoluble matter removal step,
Solvent S2 is an aliphatic fluorine-containing solvent,
The precipitation step (3) is any one of the following steps (3a), (3b) or (3c),
(3a) a step of lowering the temperature of a fluororesin A solution containing a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3b) adding the fluororesin A solution to a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3c) adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution to precipitate the fluororesin A;
The solvent of the fluororesin A solution in the precipitation step (3a) is a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A.
A method for producing a fluororesin, in which a hot press molded product (thickness 1 mm) has a haze value of 2% or less, and a melt molded product (thickness 3 mm) heated at 280° C. for 24 hours has a yellowness index of 3 or less.
(In formula (4) and formula (5), Rf ₅ , Rf ₆ , Rf ₇ , Rf ₈ each independently represents one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an ether oxygen atom, and Rf ₅ , Rf ₆ , Rf ₇ , Rf ₈ may be linked together to form a ring having 4 to 8 carbon atoms, and the ring may contain an ether oxygen atom. A polymerization step (1) of polymerizing a monomer represented by the following general formula (4) in the presence of a radical polymerization initiator to obtain a fluororesin A containing a residue unit represented by the following general formula (5):
an insoluble matter removal step (2) of removing insoluble matter from a mixture containing the fluororesin A containing the residue unit represented by general formula (5) obtained in the polymerization step and the solvent S2 to obtain a fluororesin A solution;
The method includes a precipitation step (3) of precipitating fluororesin A from the fluororesin A solution obtained in the insoluble matter removal step,
Solvent S2 is an aliphatic fluorine-containing solvent,
The precipitation step (3) is any one of the following steps (3a), (3b) or (3c),
(3a) a step of lowering the temperature of a fluororesin A solution containing a mixed solvent of a good solvent b3 for the fluororesin A and a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3b) adding the fluororesin A solution to a poor solvent c3 for the fluororesin A to precipitate the fluororesin A;
(3c) adding a poor solvent c3 for the fluororesin A solution to the fluororesin A solution to precipitate the fluororesin A;
In the precipitation step (3a), when the solution temperature T1 before the temperature is lowered is 30° C. or higher and the solution temperature after the temperature is lowered is T2, T1-T2 is 5° C. or higher.
A method for producing a fluororesin, in which a hot press molded product (thickness 1 mm) has a haze value of 2% or less, and a melt molded product (thickness 3 mm) heated at 280° C. for 24 hours has a yellowness index of 3 or less.
(In formula (4) and formula (5), Rf ₅ , Rf ₆ , Rf ₇ , Rf ₈ each independently represents one of the group consisting of a fluorine atom, a linear perfluoroalkyl group having 1 to 7 carbon atoms, a branched perfluoroalkyl group having 3 to 7 carbon atoms, or a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, the perfluoroalkyl group may have an ether oxygen atom, and Rf ₅ , Rf ₆ , Rf ₇ , Rf ₈ may be linked together to form a ring having 4 to 8 carbon atoms, and the ring may contain an ether oxygen atom.