JP7704187B2 - Fluorine resin - Google Patents

Description

The present invention relates to a fluororesin and a method for producing the same.

Fluoropolymers have excellent heat resistance, electrical properties, chemical resistance, waterproofness, liquid and oil repellency, and optical properties, so they are used in protective films for semiconductors and other electronic components, water-repellent films for inkjet printer heads, waterproof and oil-resistant coatings for filters, and optical components.

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 characteristics of a polymer (polyPFMMD) obtained by polymerizing perfluoro-2-methylene-4-methyl-1,3-dioxolane (PFMMD), a type of fluororesin containing an oxolane ring, using perfluorobenzoyl peroxide as a radical polymerization initiator. PolyPFMMD has excellent heat resistance. According to the inventors' investigations, the polyPFMMD described in Non-Patent Document 1 yellows significantly after heat melt molding, and the yellowing is particularly significant in thick molded products. Therefore, there is a strong demand for technology that can achieve high transparency with less coloring, even in thick molded products of polyPFMMD.

As a method for reducing the coloring of fluororesin during heating, a method for obtaining a fluororesin having an aliphatic perfluoroalkyl group terminal using an aliphatic perfluorinated initiator is known. For example, Non-Patent Document 2 reports an example of cyclopolymerization of perfluoro(butenyl vinyl ether) by bulk polymerization using (CF ₃ CF ₂ CF ₂ COO) _2. However, according to the study by the present inventors, even if the technology described in Non-Patent Document 2 is used for the synthesis of polyPFMMD, it cannot be said that the yellowing of polyPFMMD after heat melt molding is sufficiently reduced. In addition, the yield is extremely poor, the productivity is poor, and the molecular weight distribution Mw/Mn is large.

Macromolecules 2005, 38, 4237-4245 Journal of the Chemical Society of Japan 2001, No. 12, 659-668

The present invention has been made with the aim of solving problems with fluororesins containing oxolane rings. Specifically, the aim is to provide a fluororesin containing oxolane rings that is inhibited from yellowing after heating and melting, and that reduces coloring even when molding thick-walled products, and a method for producing the same.

As a result of intensive research, the inventors discovered that a fluororesin containing an oxolane ring with a terminal group of a specific structure can solve the above problems, and thus completed the present invention.

That is, the present invention is a fluororesin containing a residue unit represented by the following formula (1) and a terminal group represented by the following formula (2).

(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 and 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.)

(In formula (2), i is an integer between 3 and 20)

The present invention provides a fluororesin containing an oxolane ring that suppresses yellowing during melt molding of a thick molded product, and a method for producing the same.

The fluororesin, which is one aspect of the present invention, is described in detail below.

The fluororesin of the present invention contains a residue unit represented by the following formula (1) and a terminal group represented by the following formula (2).

(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.)

(In formula (2), i is an integer from 3 to 20.)
The fluororesin of the present invention is amorphous and has high transparency and high heat resistance because it has a bulky ring structure contained in the specific formula (1). In addition, since it is composed only of carbon atoms, fluorine atoms, and oxygen atoms, it has high electrical properties, chemical resistance, waterproofness, and liquid and oil repellency.

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. 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.

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 the linear perfluoroalkyl group having 1 to 7 carbon atoms which may have an etheric oxygen atom include a _-CF2OCF3 group, a -( _CF2 ) _{2OCF3 group} , a -( _{CF2 ) 2OCF2CF3 group ,} and the like.

Examples of cyclic perfluoroalkyl groups having 3 to 7 carbon atoms and optionally having an etheric oxygen atom include 2-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl group, 4-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl group, and 2-(2,3,3,4,4,5,5-heptafluoro)-furanyl group.

At least one of Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ is preferably a fluororesin which is one kind of the group consisting of linear perfluoroalkyl groups having 1 to 7 carbon atoms, branched perfluoroalkyl groups having 3 to 7 carbon atoms and cyclic perfluoroalkyl groups having 3 to 7 carbon atoms, thereby allowing the fluororesin of the present invention to exhibit superior heat resistance.

A specific example of the residue unit represented by formula (1) is the residue unit represented by the following formula (3):

Among these, fluororesins containing a residue unit represented by the following formula (4) 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 contains an end group represented by the following formula (2). This suppresses yellowing during melt molding of a thick molded product.

Here, "terminal group" refers to a group present at the end of the polymer main chain.

The terminal group represented by formula (2) preferably has a structure represented by the following formula (5):

The fact that the fluororesin of the present invention contains the terminal group unit represented by formula (5) can be confirmed, for example, by the following method.

That is, when it is confirmed that the fluororesin exhibits peaks with maxima in the ranges of from −140 ppm to 142 ppm and from −143 ppm to 145 ppm in solid-state ^19F -NMR spectrum analysis, it can be determined that the fluororesin of the present invention has an end group represented by formula (5). The peaks with maxima in the ranges of from −140 ppm to 142 ppm and from −143 ppm to 145 ppm are assigned to the —CF ₂ — groups at the 3rd, 4th and 5th positions of formula (5).

The solid-state ¹⁹ F-NMR measurement of the fluororesin may be performed using a general ¹⁹ F-NMR measurement device. For example, a method of performing solid-state 19 F-NMR measurement using a Varian VNMRS-400 with a magnetic field strength of 376.18 MHz (19F) and a 1.6 mm FAST MAS probe, using the Hahn-echo method, pulse width 1.3 μs, spectrum width 250 kHz (664.6 ppm), spectrum center: −120 ppm, waiting time: 10 seconds, MAS rotation speed: 39 kHz, and cumulative number of 2048 times, using PTFE (−122.0 ppm) as a standard, and using about ¹⁰ mg of the fluororesin, can be exemplified.

In the solid-state ^19F -NMR spectrum analysis of the fluororesin of the present invention, the integral value of the area of the terminal peak (6F) due to the terminal group showing a maximum in the range of -140 ppm to 142 ppm and -143 ppm to 145 ppm is preferably 0.001 to 10 in total, more preferably 0.01 to 5, and even more preferably 0.05 to 5, relative to the integral value 500 of the area of the main chain peak (5F) showing a maximum at a position of -81 ppm. This further suppresses yellowing of the fluororesin. Here, the main chain peak (5F) showing a maximum at a position of -81 ppm is assigned to a _-CF2O- group and a _-CF3 group.

The fact that the fluororesin of the present invention contains a residue unit represented by formula (2) or formula (5) can also be confirmed by, for example, subjecting an oligomer of the fluororesin to solid-state ^13C -NMR measurement. For example, an oligomer dissolved in a solvent such as Zeorola H during the production process is isolated, and the resulting oligomer is subjected to solid-state ^13C -NMR measurement. In this case, if a peak showing a maximum in the range of 90 to 92 ppm, which is derived from the F atom at the 1-position of the perfluorocycloalkyl group directly bonded to the residue unit represented by formula (1), is observed, it can be confirmed that the fluororesin contains an end group represented by formula (2) or formula (5).

It is preferable that the terminal group represented by formula (2) or formula (5) is directly bonded to the residue unit represented by general formula (1) without going through another functional group. In addition, it is preferable that the fluororesin of the present invention does not contain a carbonyl group.

The fluororesin of the present invention preferably has a weight average molecular weight Mw in the range of 5×10 ⁴ to 3×10 ^5. With the weight average molecular weight Mw in this range, the melt molding processability and defoaming property when melted are excellent. Furthermore, with the weight average molecular weight Mw in this range, the occurrence of cracks during heating and cooling is reduced. From the viewpoint of excellent melt molding processability and excellent defoaming property when melted, the fluororesin of the present invention more preferably has a weight average molecular weight Mw in the range of 5×10 ⁴ to 2×10 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 suppressing yellowing after heating and melting, providing excellent melt molding processability, excellent defoaming properties during melting, and reducing the occurrence of cracks 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 fluororesin of the present invention preferably has a yellowness index (YI) of 10 or less, more preferably 4 or less, and even more preferably 3 or less, measured in the diameter direction of a thick melt-molded product (a cylindrical molded product having a diameter of 10 mm and a height of about 17 mm, which is heated and melted at 280° C. for 24 hours in a test tube) according to the present inventors. Compared to heating in an open environment, melt molding of a thick molded product in a closed environment causes significant yellowing of the fluororesin after heat melt molding. As a method for evaluating the yellowness and coloration of a thick melt-molded product, for example, a method can be mentioned in which 3 g of the fluororesin of the present invention is heated in a test tube having an outer diameter of 13 mm at 280° C. for 24 hours, melted and molded, and the yellowness index (YI) measured in the diameter direction of a cylindrical body (diameter 10 mm and height about 17 mm). Here, the diameter direction is the up-down direction when the test tube is laid down on a desk or the like. The yellowness index was measured by placing the obtained resin molded product in a test tube, laying the test tube on its side on a sheet of white paper, taking a digital photo from above, reading the RGB values of the molded product from the obtained image using software, and calculating the RGB values according to the following formula: X = 0.4124R + 0.3576G + 0.1805B
Y=0.2126R+0.7152G+0.0722B
Z=0.0193R+0.1192G+0.9505B
The tristimulus values X, Y, and Z of the XYZ color system are calculated by the above equation, and the yellowness index (YI) under C light source (auxiliary illuminant C) is calculated from X, Y, and Z in accordance with the method of JIS K7373.

The fluororesin of the present invention preferably has a yellowness index (YI) of 1 or less when it is melt-molded into a thin piece (3 mm thick, molded by heating and melting in a petri dish at 280°C for 24 hours). The molding method and evaluation method can be the method described in the examples.

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

There is no particular limit to the particle size of the fluororesin of the present invention, but the resin powder has high fluidity, allowing for continuous supply to molding machines and the like, suppressing the residue of solvent in the resin, and providing high bulk density, increased packing properties, and excellent handling during molding. Therefore, the volume average particle size is preferably 1 to 1000 μm, more preferably 1 to 500 μm, and even more preferably 1 to 300 μm.

The volume average particle size of the fluororesin of the present invention can be evaluated by particle size distribution measurement (volume distribution) using a laser diffraction scattering method. The particle size distribution using the laser diffraction scattering method can be measured by dispersing the resin particles in water or an organic solvent such as methanol and measuring the particle size distribution. An example of a laser scattering meter is Microtrack manufactured by Microtrack Bell Co., Ltd.

The volume average particle diameter, also known as the Mean Volume Diameter, is the average particle diameter expressed on a volume basis. When the particle size distribution is divided into each particle size channel, the representative particle size value of each particle size channel is d, and the volume-based percentage of each particle size channel is v, it is expressed as Σ(vd)/Σ(v).

The fluororesin of the present invention is in powder form, and preferably has a volume average particle size of 1 to 1000 μm.

The method for producing fluororesin, which is one aspect of the present invention, is described in detail below.

The fluororesin of the present invention can be produced by polymerizing a mixture containing a radical polymerization initiator represented by the following formula (6) and a monomer represented by the following formula (7).

(In formula (6), j is an integer between 3 and 20.)

(In formula (7), 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.)
Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ in formula (7) have the same meanings as Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ in formula (1), respectively.

In the method for producing a fluororesin of the present invention, by using the radical polymerization initiator represented by formula (6), a fluororesin that suppresses yellowing during melt molding of a thick molded product can be obtained. Furthermore, a fluororesin with a narrow molecular weight distribution Mw/Mn can be obtained. The narrow molecular weight distribution Mw/Mn improves the heat melt moldability. Furthermore, the fluororesin can be obtained with excellent yield and productivity. In addition, by using the radical polymerization initiator represented by formula (6), the radical polymerization initiator is decarboxylated and then added to the polymer, so that the resulting polymer contains almost no carbonyl groups, and the end group represented by formula (2) is directly added to the polymer, which is advantageous in obtaining a fluororesin that suppresses yellowing during melt molding of a thick molded product.

In the method for producing a fluororesin of the present invention, it is more preferable to use a radical polymerization initiator represented by the following formula (8). By using a radical polymerization initiator represented by the following formula (8), a fluororesin can be obtained in which yellowing during melt molding of a thick molded product is further suppressed. Furthermore, a fluororesin with a narrow molecular weight distribution Mw/Mn can be obtained. The narrow molecular weight distribution Mw/Mn improves the heat melt moldability. Furthermore, the fluororesin can be obtained with excellent yield and productivity. In this specification, the radical polymerization initiator represented by the following formula (8) is sometimes called bis(perfluorocyclohexylcarbonyl) peroxide.

Bis(perfluorocyclohexylcarbonyl) peroxide can be obtained by the method described in JP-A-11-49749 or J. App. Polym. Sci, 1999, 72, 1101-1108. In this case, perfluorohexane (FC-72, 3M Japan) or the like can be used as a solvent instead of AK-225. The bis(perfluorocyclohexylcarbonyl) peroxide of the present invention may be synthesized by a method other than the method described in the above-mentioned literature. For example, a method for synthesizing a fluorinated peroxide is described in Chem. Rev, 1996, 96, 1779-1808. Examples of the method include a method of reacting an acid fluoride with hydrogen peroxide, a method of reacting an acid chloride with hydrogen peroxide, and a method of reacting an acid anhydride with hydrogen peroxide. In this case, the reaction proceeds when a base such as sodium hydroxide is present in the system.

In the method for producing the fluororesin of the present invention, a chain transfer agent may be used for the purpose of adjusting the molecular weight. Examples of the chain transfer agent include organic compounds having 1 to 20 carbon atoms and containing at least one atom selected from the group consisting of hydrogen atoms and chlorine atoms. 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, organic compounds having 1 to 20 carbon atoms and containing chlorine atoms are preferred from the viewpoint of suppressing yellowing after heating and melting, and organic compounds having 1 to 20 carbon atoms and containing hydrogen atoms and chlorine atoms are more preferred. The amount of chain transfer agent is, for example, 0.01 to 50% by weight based on the total amount of the monomer and chain transfer agent.

In the method for producing the resin of the present invention, there are no limitations on the polymerization method, but examples include solution polymerization, precipitation polymerization, bulk polymerization, emulsion polymerization, and suspension polymerization.

In the manufacturing method of the present invention, it is preferable to further mix an organic solvent into the mixture in the polymerization step.

In the method for producing a resin of the present invention, the resin powder has high fluidity, can be continuously supplied to a molding machine, etc., can suppress the residual solvent in the resin, has a large bulk density, increases packing properties, and is easy to handle during molding. Therefore, it is preferable to use an organic solvent in which at least the monomer represented by formula (7) dissolves, and at least a part of the resin produced by polymerization does not dissolve, causing precipitation of the resin, and the resin produced by polymerization precipitates in the organic solvent as particles. In the method for producing a resin of the present invention, the organic solvent may be referred to as a "precipitation polymerization solvent". By using the precipitation polymerization solvent, the resin produced by the polymerization reaction can be precipitated as particles having a specific volume average particle size, and as a result, resin particles with excellent moldability and packing properties can be produced. In addition, since no polymerization aids such as emulsifiers and dispersants are used, resin particles that do not contain emulsifiers and dispersants that cause deterioration of transparency and heat resistance can be produced.

Here, the precipitation polymerization solvent means a solvent in which resin particles containing a residue unit represented by formula (1) remain after immersion in the organic solvent for a long period of time. Specifically, when resin particles containing a residue unit represented by general formula (1) and having a weight average molecular weight Mw of 5×10 ⁴ to 70×10 ⁴ are immersed in an organic solvent in an amount (w/w) 10 times the amount of the resin particles at 50° C. for 5 hours or more, and the remaining resin particles can be confirmed with the naked eye in the organic solvent, the organic solvent can be regarded as the precipitation polymerization solvent A. The precipitation polymerization solvent A is preferably an organic solvent in which the weight loss rate of the resin sample remaining in a solid state is recovered after cooling the solution to 25° C. after immersion for 5 hours or more at 50° C. is less than 20% by weight. The weight loss rate of the resin sample is more preferably less than 12% by weight, and even more preferably less than 10% by weight.

The resin weight loss rate can be measured by the following method. After filtering the cooled solution, the solids on the filter are rinsed with the solvent, washed multiple times with acetone, and then dried to recover the resin sample on the filter. The weight of the recovered resin is measured, and the resin weight is subtracted from the amount of resin immersed in the organic solvent, and the result is divided by the amount of resin immersed in the organic solvent to obtain the resin weight loss rate.

Precipitation polymerization solvents include non-halogenated organic solvents such as acetone, methyl ethyl ketone, hexane, and butyl acetate, chlorine-based organic solvents such as dichloromethane and chloroform, and organic solvents that contain fluorine atoms in the molecule.

Furthermore, as the precipitation polymerization solvent, an organic solvent containing fluorine atoms and hydrogen atoms in the molecule is preferable because it is difficult for a chain transfer reaction to occur in radical polymerization, has a high polymerization yield, and is easy to obtain a high molecular weight substance. Specific examples of precipitation polymerization solvents containing fluorine atoms and hydrogen atoms in the molecule include 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoroisopropanol, 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-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, 2,2,3,4,4,4-hexafluorobutyl difluoromethyl ether, and the like.

Among them, 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 are preferred, and 1,2,2,3,3,4,4-heptafluorocyclopentane is preferred because it has excellent polymerization yield and is easy to obtain high molecular weight products. The ratio of fluorine atoms and hydrogen atoms in the molecules of the precipitation polymerization solvent is preferably fluorine atoms:hydrogen atoms=1:9 to 9:1 in terms of the number of atoms, since it has excellent polymerization yield, more preferably 1:9 to 7:3, and even more preferably 4:6 to 7:3. The precipitation polymerization solvent contains fluorine atoms and hydrogen atoms in the molecules, since it has excellent polymerization yield, and the content of hydrogen atoms in the solvent is preferably 1% by weight or more, more preferably 1.5% by weight or more, based on the weight of the solvent molecule. In addition, since it is easy to obtain a high molecular weight substance with a high polymerization yield, it is preferably 1% by weight or more and 5% by weight or less, and more preferably 1.5% by weight or more and 4% by weight or less. In addition, as a precipitation polymerization solvent, it is preferable to use one that does not contain a chlorine atom in the molecule, since it is easy to obtain a high molecular weight substance with a high polymerization yield.

The ratio of the monomer represented by formula (7) to the precipitation polymerization solvent is preferably 1:99 to 50:50 by weight, more preferably 5:95 to 40:60, and even more preferably 5:95 to 30:70, since this provides excellent productivity and particles with excellent flow properties.

In the manufacturing method of the present invention, it is preferable that the monomer represented by formula (7) is perfluoro(4-methyl-2-methylene-1,3-dioxolane) represented by the following formula (9), and the residue unit represented by formula (1) is a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit represented by formula (4).

By producing using the method of the present invention, it is possible to obtain a fluororesin that suppresses yellowing during melt molding of a thick molded product. Furthermore, it is possible to obtain a fluororesin that has a narrow molecular weight distribution Mw/Mn while exhibiting the above characteristics. Furthermore, it is possible to obtain a fluororesin with excellent yield and productivity while exhibiting the above characteristics.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these.

[Measurement of weight average molecular weight Mw and molecular weight distribution Mw/Mn]
Measurements were performed using gel permeation chromatography equipped with a column TSKgel SuperHZM-M manufactured by Tosoh Corporation and an RI detector. The eluent used was 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.) was added relative to AK-225. Standard polymethyl methacrylate manufactured by Agilent was used as the standard sample, and the weight average molecular weight Mw and molecular weight distribution Mw/Mn calculated in terms of polymethyl methacrylate were calculated from the elution times of the sample and the standard sample.

[Solid ^19F -NMR Measurement]
Using a Varian VNMRS-400, a magnetic field strength of 376.18 MHz (19F), a 1.6 mm FAST MAS probe, and the hahn-echo method, a pulse width of 1.3 μs, a spectrum width of 250 kHz (664.6 ppm), a spectrum center: −120 ppm, a waiting time: 10 seconds, a MAS rotation speed: 39 kHz, and an accumulation count of 2048 were used to carry out solid-state ¹⁹ F-NMR measurement using about 10 mg of the fluororesin as a standard, with PTFE (−122.0 ppm).

[Solid ^13C -NMR Measurement]
Using a Varian VNMRS-400, a magnetic field strength of 100.55 MHz (13C), a 4.0 mm MAS probe, and a CP/MAS method, the solid ^13C -NMR measurement was carried out using about 50 mg of the fluororesin with TMS (0 ppm) as the standard, with a spectrum width of 30.5 kHz, a spectrum center of 77.5 ppm, a waiting time of 3 seconds, a MAS rotation speed of 10 kHz, and an accumulation number of 4096.

[Measurement of yellowness index (YI) of thick melt-molded product (φ10 mm × H17 mm, 280°C in test tube for 24 h)]
3.0 g of fluororesin was placed in a glass test tube (ST-13M, manufactured by Nichiden Rika Glass Co., Ltd.) with an outer diameter of φ13 mm and a total length of 100 mm, the mouth of the test tube was covered with aluminum foil and an aluminum cap (M-1, manufactured by Maruemu), the tube was placed in a test tube stand, and the tube was placed in an oven and heated at 280°C for 24 hours, and then allowed to cool to obtain a cylindrical resin molded product in the test tube (diameter: 10 mm, height: about 17 mm). With the resin molded product still in the test tube, the test tube was placed on its side on a piece of white paper, and a digital photograph was taken from above under white fluorescent light using PowerShot SX620HS (manufactured by Canon Inc.), and the RGB values of the molded product were read from the obtained image using Paint (image processing software manufactured by Microsoft Corporation). The read RGB values were calculated using the following formula: X = 0.4124R + 0.3576G + 0.1805B
Y=0.2126R+0.7152G+0.0722B
Z=0.0193R+0.1192G+0.9505B
The tristimulus values X, Y, and Z of the XYZ color system were obtained by the above formula. The yellowness index (YI) under light source C (auxiliary illuminant C) was calculated from X, Y, and Z according to the method of JIS K7373, and the yellowness index (YI) of a thick melt-molded product (φ10 mm×H17 mm, 280° C. in a test tube for 24 h) was obtained.

[Yellowness index (YI) of thin melt-molded product (3 mm thick, in a petri dish at 280°C for 24 h)]
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 an air flow (20 L/min), and then heated to 280 ° C. over 30 minutes, and then 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-melted 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 wavelengths of 200 nm to 1500 nm at 1 nm intervals 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. Furthermore, according to the method of JIS K7373, the yellowness index (YI) in the C light source (auxiliary illuminant C) was calculated, and the yellowness index (YI) of the fluororesin heat melt molded product including the petri dish was obtained. The yellowness index (YI) of the petri dish alone (receiver only) was measured, and the yellowness index (YI) of the fluororesin heat melt molded product with a thickness of 3 mm was obtained by subtracting the yellowness index (YI) of the petri dish alone (receiver only) from the yellowness index (YI) of the fluororesin molded product including the petri dish. The yellowness index (YI) of the petri dish alone (receiver only) was 0.21.

[Measurement of volume average particle size]
The volume average particle size (unit: μm) was measured using a Microtrac MT3000 manufactured by Microtrac Bell Co., Ltd. and methanol as a dispersion medium.

[Example 1]
The inside of a 3L SUS316 autoclave equipped with a paddle-type stirring blade, a nitrogen inlet tube and a thermometer was replaced with nitrogen. A solution of 2.33 g (0.0036 mol) of bis(perfluorocyclohexylcarbonyl)peroxide as an initiator dissolved in 230 g of FC-72 (manufactured by 3M Japan, perfluorohexane), 175 g (0.72 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 470 g of FC-72 as a polymerization solvent were placed in an autoclave after removing dissolved oxygen, and solution polymerization was performed by holding the mixture at 55°C for 24 hours under stirring to obtain a viscous liquid in which the resin was dissolved. After cooling to room temperature, the ampoule was opened, and the obtained resin solution was poured into 875 g of FC-72 for viscosity adjustment, and diluted to prepare a resin diluted solution. In a plastic cup equipped with a stirring blade, 5250 g of Zeorora H (manufactured by Nippon Zeon Co., Ltd., 1,2,2,3,3,4,4-heptafluorocyclopentane, hydrogen atom content in the solvent molecule: 1.55% by weight, fluorine atoms: hydrogen atoms in the solvent molecule = 7:3 (number ratio)) was placed, and the resin dilution solution was added to the plastic cup 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 (yield: 93%).

The weight average molecular weight of the obtained fluororesin was 9.5×10 ⁴ , and the molecular weight distribution Mw/Mn was 3.74. The evaluation results of the fluororesin are shown in Table 1.

As a result of performing solid-state ^19F -NMR measurement on the obtained fluororesin, peaks derived from the terminal perfluorocyclohexyl groups were confirmed at -140.7 ppm and -143.9 ppm. The integral value of the main chain peak (5F) at -81.3 ppm was 500, while the integral value of the peak (6F) derived from the terminal groups was a total of 1.46.

During the above operation, the filtrate obtained by precipitating the resin using Zeorora H was dried, and the solid was washed with acetone and vacuum dried to obtain an oligomer.

The solid-state ^13C -NMR measurement of the obtained oligomer confirmed a peak at 91 ppm attributable to the F atom at position 1 of the perfluorocyclohexyl group directly bonded to the polymer end, while no peak was observed in the carbonyl group region at 130 to 150 ppm.

[Example 2]
The inside of a 3 L SUS316 autoclave equipped with a paddle-type stirring blade, a nitrogen inlet tube and a thermometer was replaced with nitrogen.

A solution of 0.80 g (0.0012 mol) of bis(perfluorocyclohexylcarbonyl) peroxide as an initiator dissolved in 80 g of FC-72, 300 g (1.23 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 1,120 g of Zeorola-H as a polymerization solvent, and 33.33 g (0.279 mol) of chloroform (manufactured by Wako Pure Chemical Industries, Ltd., amount of chain transfer agent: 10% by weight based on the total of monomer and chain transfer agent) were placed in an autoclave after removing dissolved oxygen, and held at 40°C for 24 hours with stirring to carry out precipitation polymerization, resulting in a milky white slurry in which resin had precipitated in the polymerization solvent. The slurry was cooled to room temperature, and the resulting resin particles were collected by filtration, washed with acetone, and vacuum dried to obtain powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 90%).

The weight average molecular weight of the obtained fluororesin was 8.8 × 10 ⁴ , and the molecular weight distribution Mw/Mn was 2.48. The volume average particle size of the obtained fluororesin was 26 μm, and the powder had excellent fluidity, which was superior to that of Example 1.

[Example 3]
As a result of performing solid-state ^19F -NMR measurement of the fluororesin obtained in Example 2, peaks derived from the terminal perfluorocyclohexyl groups were confirmed at -140.2 ppm and -144.4 ppm. The integral value of the main chain peak (5F) at -81.3 ppm was 500, while the integral value of the peak (6F) derived from the terminal groups was a total of 0.23.

[Comparative Example 1]
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, 10.0 g (0.0410 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) was added as a monomer, and 40.0 g of FC-72 was added as a polymerization solvent. 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 was dissolved. After cooling to room temperature, the ampoule was opened, and the resulting resin solution was poured into 50 g of FC-72 for viscosity adjustment, and diluted to prepare a resin dilution solution. 240 g of Zeorora H was placed in a beaker equipped with a stirrer, and the diluted resin solution was added to the beaker under stirring to precipitate the resin. The precipitated resin was collected by filtration, washed twice with acetone, and dried in vacuum to obtain a lump of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield 94%).

The weight average molecular weight of the obtained fluororesin was 21×10 ⁴ , and the molecular weight distribution Mw/Mn was 2.8. The obtained fluororesin was in a lump form and had poor powder fluidity. The evaluation results of the fluororesin are shown in Table 1.

When the fluororesin was subjected to solid-state ^19F -NMR measurement, no peaks showing maxima were detected at -140 to 142 ppm or -143 to 145 ppm. The filtrate from which the resin was precipitated on Zeorora H by the above procedure was dried, the solid was washed with acetone, and vacuum dried. The resulting oligomer was subjected to solid-state ^13C -NMR measurement, and no peaks were confirmed in the region of 80 to 100 ppm. Furthermore, carbonyl group peaks were detected at 139 ppm and 143 ppm.

[Comparative Example 2]
A 30 mm diameter glass ampoule equipped with a magnetic stirrer was charged with 0.0865 g (0.000205 mol) of bis(2,3,4,5,6-pentafluorobenzoyl) peroxide as an initiator, 10.0 g (0.0205 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 40.0 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, and the ampoule was subjected to repeated 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). Precipitation polymerization was carried out by holding the ampoule upright at 55°C for 24 hours while stirring with a magnetic stirrer, resulting in a cloudy slurry 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 particulate perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 82%).

The weight average molecular weight of the resulting fluororesin was 9.6×10 ⁴ , and the molecular weight distribution Mw/Mn was 2.6.

[Comparative Example 3]
A solution of 0.52 g (0.0012 mol) of (CF ₃ CF ₂ CF ₂ COO) ₂ as an initiator diluted with 52 g of FC-72 and 30.0 g (0.12 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer were placed in a 75 mL glass ampoule, and the mixture was repeatedly subjected to nitrogen replacement and depressurization by freeze degassing, and then sealed under reduced pressure. The ampoule was kept at 25° C. for 24 hours to carry out radical polymerization. The ampoule was opened, and the contents were placed in a beaker containing 600 g of hexane under stirring, and the solid was collected by filtration, washed twice with acetone, and dried in vacuum to obtain a lump of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield 18%). The evaluation results of the fluororesin are shown in Table 1. The weight average molecular weight of the obtained fluororesin was 86×10 ⁴ , and the molecular weight distribution Mw/Mn was 26.2. The yield was very low, and the molecular weight distribution was very large.

[Comparative Example 4]
A polymerization glass tube equipped with a stir bar was charged with 4.8 g (0.020 mole) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 3 mL of dichloropentafluoropropane (AK-225, manufactured by AGC) as a solvent, 0.21 g of ammonium perfluorooctanoate as an emulsifier, 0.24 g of Na ₂ HPO ₄ ·7H ₂ O as a pH adjuster, 0.15 g of (NH ₄ ) ₂ S ₂ O ₈ as an initiator, and 50 mL of distilled water degassed with N ₂ as a solvent. The headspace above the solution was purged with N ₂ and then slightly pressurized with N ₂ . The contents of the tube were then heated at 75°C for 5 hours while stirring with a stir bar. The resulting reaction mixture was treated with 80 mL of aqueous HCl (6.3 M) to precipitate the polymer. The polymer was washed three times with 200 mL of distilled water, and then washed three times with 200 mL of acetone. Next, the polymer was placed in a vacuum oven and dried at 150° C. under vacuum (150 mmHg) for 24 hours to obtain a white powdery polymer (yield: 3%). The above-mentioned operations from polymerization to drying were carried out two more times, and the obtained polymers were mixed to obtain a polymer for evaluation. The evaluation results of the obtained polymer are shown in Table 2. The weight average molecular weight of the obtained fluororesin was 34×10 ⁴ , and the molecular weight distribution Mw/Mn was 25. The yield was very low, and the molecular weight distribution was very large. When the solid ¹⁹ F-NMR measurement of the fluororesin was performed, no peaks showing maximum values at −140 to 142 ppm or −143 to 145 ppm were detected.

[Comparative Example 5]
A 75 mL glass ampoule was charged with 10.0 g (0.041 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 35 g of AK-225 as a solvent, and 0.02 g of 4,4-bis(t-butylcyclohexyl) peroxydicarbonate (Perloyl TCP, manufactured by NOF Corp.) as an initiator. After repeated nitrogen replacement and depressurization by freeze degassing, the ampoule was sealed under reduced pressure. The ampoule was heated at 60° C. for 3 hours while shaking with a thermostatic shaker. The polymer removed from the ampoule was dried at 100° C. for 24 hours under vacuum (150 mmHg) to obtain a polymer (yield: 76%). The above-mentioned operations from polymerization to drying were carried out twice more, and the obtained polymers were mixed to obtain a polymer for evaluation. The evaluation results of the obtained polymer are shown in Table 2. The weight average molecular weight of the obtained fluororesin was 12×10 ⁴ , and the molecular weight distribution Mw/Mn was 1.8. When the fluororesin was subjected to solid-state ¹⁹ F-NMR measurement, no peaks showing maximum values were detected at −140 to 142 ppm or −143 to 145 ppm.

[Comparative Example 6]
When the fluororesin obtained in Comparative Example 2 was subjected to solid-state ¹⁹ F-NMR measurement, no peaks showing maxima were detected at −140 to 142 ppm or −143 to 145 ppm.

[Comparative Example 7]
When the fluororesin obtained in Comparative Example 3 was subjected to solid-state ¹⁹ F-NMR measurement, no peaks showing maxima were detected at −140 to 142 ppm or −143 to 145 ppm.

[Reference example 1]
The resin particles obtained in Example 2 were immersed in 10 times the amount of each of the solvents at 50° C. for 5 hours, and the presence or absence of remaining resin particles was observed with the naked eye.

Residual resin particles were confirmed with the naked eye in the following organic solvents:
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, chloroform.

Then, the mixture was cooled to 25°C, filtered, and the resin particles were extracted by rinsing with the solvent. The resin particles were then washed twice with 10 times the amount of acetone and vacuum dried. The recovery rate was calculated from the dry weight, and in both cases the recovery rate was 90% or more. The filtrate obtained above was also distilled off, and the solid content in the filtrate was calculated. The solid content in the filtrate was less than 10% of the resin particles used. From the above results, it was confirmed that the weight loss rate of the resin weight was less than 10% by weight.

As shown in Examples 1 and 2, the fluororesin of the present invention has a smaller yellowness in thick melt-molded products (φ10 mm x H17 mm, 280°C in a test tube for 24 h) than Comparative Examples 1 to 3, and yellowing is suppressed even in thick molded products.

The method for producing fluororesin of the present invention has a higher yield than the method of Comparative Example 3. As shown in Examples 1 and 2, fluororesin can be produced with a yield of 80% or more, and depending on the conditions, fluororesin can be produced with a yield of 85% or more, or even 90% or more.

The fluororesin obtained by the method for producing fluororesin of the present invention has improved yellowness when heated in a test tube at 280°C for 24 hours, and has a narrower molecular weight distribution than the method of Comparative Example 3. It is possible to produce a fluororesin with a molecular weight distribution Mw/Mn of 5 or less, and under some conditions, 4 or less, or even 3 or less.

The fluororesin of the present invention is useful in fields related to fluororesins.

Claims (3)

A fluororesin comprising a residue unit represented by the following formula (1) and a terminal group represented by the following formula ( 3 ):

(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 and 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 claim 1 , which exhibits maximum peaks in the ranges of -142 ppm to -140 ppm and -145 ppm to -143 ppm in solid-state ^19F -NMR spectrum analysis. 3. The fluororesin according to claim 1 , wherein the fluororesin is in a powder form and has a volume average particle size of 1 to 1000 μm.