JP7740319B2 - 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, waterproofing, 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.

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

Non-Patent Document 1 describes the synthesis and properties 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. However, according to the inventors' investigations, the polyPFMMD described in Non-Patent Document 1 exhibits significant yellowing after heat-melt molding, particularly in thick molded articles. Therefore, there is a strong demand for technology that can achieve low coloration and high transparency even in thick molded polyPFMMD articles.

Furthermore, a method for producing fluororesin containing oxolane rings that suppresses yellowing after heat-melt molding, has excellent yield and productivity, and has a narrow molecular weight distribution Mw/Mn was not yet clear.

Macromolecules 2005, 38, 4237-4245

The present invention aims to solve the problems associated with fluororesins containing oxolane rings. Specifically, the objective is to provide a fluororesin containing oxolane rings that is inhibited from yellowing after heat melting, and that exhibits reduced discoloration, particularly when molded into thick-walled products, and a method for producing the same.

After extensive research, the inventors discovered that a fluororesin with low light absorption and high transmittance at a wavelength of 275 nm could solve the above-mentioned problems, leading to the completion of the present invention.

That is, the present invention relates to a fluororesin that contains a residue unit represented by the following formula (1), and that, when dissolved in perfluorohexane to form a 10 wt % perfluorohexane solution, has a transmittance of 50% or more as measured at an optical path length of 10 mm and a wavelength of 275 nm.

(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, wherein the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked together to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)

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

1 shows the transmission spectra of a 10 wt % fluororesin-perfluorohexane solution at an optical path length of 10 mm in Examples 1 and 2 and Comparative Example 1.

The fluororesin, 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 when dissolved in a perfluorohexane solution to form a 10 wt % perfluorohexane solution, has a transmittance of 50% or more measured at an optical path length of 10 mm and a wavelength of 275 nm.

(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, wherein the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked together to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)
The fluororesin of the present invention is amorphous and has high transparency and heat resistance due to the bulky ring structure contained in the specific formula (1). Furthermore, because it is composed only of carbon, fluorine, and oxygen, it has excellent electrical properties, chemical resistance, waterproofness, and liquid and oil repellency.

In 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. Furthermore, Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ may be linked together 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 groups, nonafluorocyclobutyl groups, and tridecafluorocyclohexyl groups.

Examples of the linear perfluoroalkyl group having 1 to 7 carbon atoms which may have an etheric oxygen atom include a -CF ₂ OCF ₃ group, a -(CF ₂ ) ₂ OCF ₃ group, and a -(CF ₂ ) ₂ OCF ₂ CF ₃ group.

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 groups, 4-(2,3,3,4,4,5,5,6,6-decafluoro)-pyrinyl groups, and 2-(2,3,3,4,4,5,5-heptafluoro)-furanyl groups.

At least one of Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ is preferably a fluororesin that is one type 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 and a cyclic perfluoroalkyl group having 3 to 7 carbon atoms, thereby allowing the fluororesin of the present invention to exhibit excellent heat resistance.

Specific examples of residue units represented by formula (1) include residue units represented by formula (2) below.

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

The fluororesin of the present invention contains a residue unit represented by formula (1) and, when dissolved in perfluorohexane to form a 10 wt % perfluorohexane solution, has a transmittance of 50% or more as measured at an optical path length of 10 mm and a wavelength of 275 nm.

When the fluororesin of the present invention is dissolved in a perfluorohexane solution to prepare a 10 wt% perfluorohexane solution, the transmittance measured at 275 nm with a 10 mm optical path length is 50% or greater. Herein, the transmittance at a 10 mm optical path length may refer to either the value obtained when light actually penetrates the solution for 10 mm, or the value calculated for 10 mm assuming light penetrates the solution for 10 mm. In the latter case, it can also be referred to as the transmittance when the optical path length is converted to 10 mm. Hereinafter, a 10 wt% fluororesin-perfluorohexane solution of the present invention will be referred to simply as the "measurement solution." A measurement solution exhibiting a transmittance of 50% or greater suppresses yellowing during melt molding of a thick molded article using the fluororesin of the present invention. According to the inventors, when the measurement solution exhibits high optical absorption and low transmittance at the 275 nm wavelength in the ultraviolet region, significant yellowing occurs during melt molding of a thick molded article using the fluororesin of the present invention. On the other hand, it has been found that when the measurement solution has low light absorption and high transmittance at a wavelength of 275 nm, yellowing is suppressed when the fluororesin of the present invention is used to melt-mold a thick molded product.

The transmittance of the measurement solution at a wavelength of 275 nm is preferably 70% or more, more preferably 80% or more, even more preferably 85% or more, even more preferably 90% or more, and even more preferably 95% or more. This further suppresses yellowing after heating and melting.

It is also preferable that the measurement solution does not exhibit a minimum peak in the transmission spectrum at an optical path length of 10 mm and wavelengths of 250 to 300 nm.

The fluororesin of the present invention preferably does not have a peak in the range of −150 to 170 ppm in solid-state ¹⁹ F-NMR, since yellowing during melt molding of a thick molded article is suppressed.

The fluororesin of the present invention preferably has a weight average molecular weight Mw in the range of 5 x 10 ⁴ to 3 x 10 ^5. Having a weight average molecular weight Mw in this range results in excellent melt molding processability and defoaming ability when melted. Furthermore, having a weight average molecular weight Mw in this range results in less cracking during heating and cooling. From the viewpoint of excellent melt molding processability and excellent defoaming ability when melted, the fluororesin of the present invention more preferably has a weight average molecular weight Mw in the range of 5 x 10 ⁴ to 2 x 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, using a standard polymethyl methacrylate of 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 such a 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.

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 standpoint of suppressing yellowing after heating and melting, providing excellent melt-molding processability, excellent degassing properties during melting, and minimizing 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, and even more preferably 1.5 to 3. The number average molecular weight Mn can be measured using the same method as 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 diametrically on a thick-walled melt-molded article (a cylindrical article 10 mm in diameter x approximately 17 mm in height, melt-molded in a test tube at 280°C for 24 hours). According to the present inventors, melt-molding a thick article in a closed environment results in more pronounced yellowing of the fluororesin after melt-molding, compared to heating in an open environment. One method for evaluating the yellowness and coloration of a thick melt-molded article is to heat 3 g of the fluororesin of the present invention in a test tube with an outer diameter of 13 mm at 280°C for 24 hours, melt it, and mold it into a cylindrical article (10 mm in diameter x approximately 17 mm in height), and evaluate the yellowness index (YI) measured diametrically. Here, the diametric direction refers to the vertical direction when the test tube is placed on its side on a desk or the like. The yellowness index was measured by placing the obtained resin molded product in a test tube on its side on a piece of white paper, taking a digital photograph from above, reading the RGB values of the molded product from the obtained image using software, and then calculating the read RGB values 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 are calculated by the above equation, and the yellowness index (YI) under Illuminant C (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 formed into a thin melt-molded product (3 mm thick, molded by heating and melting in a petri dish at 280°C for 24 hours). Examples of molding and evaluation methods include those 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 are no particular limitations on the particle size of the fluororesin of the present invention, but 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, because this provides high fluidity to the resin powder, enabling continuous supply to molding machines, suppressing residual solvent in the resin, and increasing bulk density and packing properties, resulting in excellent handleability during molding processing.

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. Particle size distribution using the laser diffraction scattering method can be measured by dispersing resin particles in water or an organic solvent such as methanol and then measuring. An example of a laser scattering meter is the Microtrac, manufactured by Microtrac 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. It is expressed as Σ(vd)/Σ(v) when the particle size distribution is divided into particle size channels, d is the representative particle size value for each particle size channel, and v is the volume-based percentage for each particle size channel.

The fluororesin of the present invention is in powder form, and preferably has a volume average particle size of 1 to 1,000 μm.

The method for producing fluororesin, 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 (4) and a monomer represented by the following formula (5).

(In formula (4), Rf ⁹ , Rf ¹⁰ , Rf ¹¹ , and Rf ¹² each independently represent a perfluoroalkyl group having 1 to 20 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom; Rf ⁹ and Rf ¹⁰ may be bonded to each other to form a ring having 4 to 8 carbon atoms; Rf ¹¹ and Rf ¹² may be bonded to each other to form a ring having 4 to 8 carbon atoms, and the ring may be a ring containing an etheric oxygen atom.)

(In 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, and 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, and the ring may contain an etheric oxygen atom.)
Rf ₅ , Rf ₆ , Rf ₇ and Rf ₈ in formula (5) 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, the use of a radical polymerization initiator represented by formula (4) enables the production of a fluororesin that exhibits reduced yellowing during melt molding of thick molded articles. Furthermore, the use of a radical polymerization initiator represented by formula (4) enables the production of a fluororesin with superior yield and productivity compared to the use of a linear perfluorodiacyl peroxide. Furthermore, a fluororesin with a narrow molecular weight distribution Mw/Mn can be obtained. A narrow molecular weight distribution Mw/Mn improves thermal melt moldability. Furthermore, the use of a radical polymerization initiator represented by formula (4) decarboxylates the radical polymerization initiator before adding it to the polymer, resulting in a polymer that contains almost no carbonyl groups. It is believed that this results in a structure in which the terminal represented by formula (6) below is directly attached to the polymer, which is advantageous for producing a fluororesin that exhibits reduced yellowing during melt molding of thick molded articles.

(In formula (6), Rf ¹⁵ and Rf ¹⁶ each independently represent a perfluoroalkyl group having 1 to 20 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom. Furthermore, Rf ¹⁵ and Rf ¹⁶ 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.)
In formula (4), Rf ⁹ , Rf ¹⁰ , Rf ¹¹ , and Rf ¹² each independently represent a perfluoroalkyl group having 1 to 20 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom. Rf ⁹ and Rf ¹⁰ may be bonded to each other to form a ring having 4 to 8 carbon atoms, and Rf ¹¹ and Rf ¹² may be bonded to each other to form a ring having 4 to 8 carbon atoms, and the ring may be a ring containing an etheric oxygen atom.

Examples of the perfluoroalkyl group having 1 to 20 carbon atoms in formula (4) include linear perfluoroalkyl groups having 1 to 20 carbon atoms, such as trifluoromethyl, pentafluoroethyl, heptafluoropropyl, nonafluorobutyl, undecafluoropentyl, tridecafluorohexyl, pentadecafluoroheptyl, and perfluorohexadecyl; branched perfluoroalkyl groups having 3 to 7 carbon atoms, such as heptafluoroisopropyl, _{nonafluoroisobutyl} , nonafluorosec-butyl, and nonafluorotert-butyl; cyclic perfluoroalkyl groups, such as nonafluorocyclobutyl and tridecafluorocyclohexyl; and perfluoroalkyl groups having an etheric oxygen atom, such as _-OCF3 , _-OCF2CF3 , and _{-OC3F7 .} Furthermore, when these groups are linked together to form a ring having 4 to 20 carbon atoms, examples of the ring include a perfluorocyclobutyl group, a perfluorocyclopentyl group, a perfluorocyclohexyl group, a perfluoropentyl group, a perfluorooctyl group, a perfluorooxolane group, and a perfluorodioxolane group.

An example of the radical polymerization initiator represented by formula (4) is the radical polymerization initiator represented by formula (7) below.

In the method for producing a fluororesin of the present invention, it is further preferable to use a radical polymerization initiator represented by the following formula (8). By using a radical polymerization initiator represented by formula (8), it is possible to obtain a fluororesin that is further inhibited from yellowing when melt-molding a thick molded product. Furthermore, it is possible to obtain a fluororesin with a narrow molecular weight distribution Mw/Mn. A narrow molecular weight distribution Mw/Mn improves hot melt moldability. Furthermore, it is possible to obtain a fluororesin with excellent yield and productivity.

(In formula (8), Rf ¹³ and Rf ¹⁴ each independently represent a perfluoroalkyl group having 3 to 20 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom.)
Examples of the perfluoroalkyl group having 3 to 20 carbon atoms in formula (8) include a -CF ₂ CF ₂ CF ₂ - group, a -CF ₂ CF ₂ CF ₂ CF ₂ - group, a -CF ₂ CF ₂ CF _{2 CF 2 CF 2} - group, a -CF ₂ CF ₂ CF ₂ CF ₂ CF ₂ CF ₂ - group, a -CF ₂ CF ₂ CF ₂ CF ₂ CF ₂ CF _{2 -} group, a -CF ₂ CF ₂ CF(CF ₃ )CF ₂ CF ₂ - group, a -CF ₂ CF ₂ OCF ₂ CF ₂ - group, and a -OCF ₂ CF ₂ CF ₂ O- group.

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 (9) or (10). By using a radical polymerization initiator represented by formula (9) or (10), yellowing after heat melting is further suppressed, and a fluororesin can be obtained that exhibits even less yellowing when melt-molded into a thick molded product in a closed environment, such as a test tube. Furthermore, a fluororesin with a narrow molecular weight distribution Mw/Mn can be obtained. A narrow molecular weight distribution Mw/Mn improves heat melt moldability. Furthermore, the fluororesin can be obtained with excellent yield and productivity. In this specification, the radical polymerization initiator represented by formula (10) is sometimes referred to as bis(perfluorocyclohexylcarbonyl) peroxide.

(In formula (9), j is an integer between 3 and 20.)

Bis(perfluorocyclohexylcarbonyl) peroxide can be obtained by methods such as those described in Japanese Patent Application Laid-Open No. 11-49749 and J. App. Polym. Sci., 1999, 72, 1101-1108. In this case, perfluorohexane (FC-72, manufactured by 3M Japan) or the like can be used as a solvent instead of AK-225. The bis(perfluorocyclohexylcarbonyl) peroxide of the present invention may also be synthesized by methods other than those described in the above-mentioned literature. For example, Chem. Rev., 1996, 96, 1779-1808 describes a method for synthesizing fluorinated peroxides. Examples of such methods include reacting an acid fluoride with hydrogen peroxide, reacting an acid chloride with hydrogen peroxide, and reacting an acid anhydride with hydrogen peroxide. In this case, the presence of a base such as sodium hydroxide in the system promotes the reaction.

In the manufacturing method of the fluororesin of the present invention, a chain transfer agent may be used to adjust the molecular weight. Examples of chain transfer agents include organic compounds having 1 to 20 carbon atoms and containing at least one atom selected from the group consisting of hydrogen and chlorine atoms. Specific examples of chain transfer agents 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; and 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 standpoint of suppressing yellowing after heat melting, and organic compounds having 1 to 20 carbon atoms and containing hydrogen and chlorine atoms are even more preferred. The amount of chain transfer agent is, for example, 0.01 to 50% by weight of the total 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 during the polymerization step. In the resin manufacturing method of the present invention, the resin powder has high fluidity, allowing continuous supply to molding machines and the like, and the residual solvent in the resin can be suppressed. Therefore, a powder with high bulk density, increased packing properties, and excellent handleability during molding can be obtained. Therefore, it is preferable to use an organic solvent that dissolves at least the monomer represented by formula (5) but does not dissolve at least a portion of the resin containing the residue unit represented by formula (1) produced by polymerization, resulting in precipitation of the resin, and in which the resin produced by polymerization precipitates as particles in the organic solvent. In the resin manufacturing method of the present invention, the organic solvent is sometimes 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, resulting in the production of resin particles with excellent moldability and packing properties. Furthermore, because no polymerization aids such as emulsifiers and dispersants are used, resin particles can be produced that do not contain emulsifiers or dispersants, which can impair transparency and heat resistance.

Here, the precipitation polymerization solvent refers to a solvent in which resin particles containing a residue unit represented by formula (1) remain after being immersed in the organic solvent for a long period of time. Specifically, when resin particles containing a residue unit represented by 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 10 times (w/w) 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 considered to be precipitation polymerization solvent A. Precipitation polymerization solvent A is preferably an organic solvent in which the weight loss of the resin sample is less than 20 wt % when the solution is cooled to 25°C after immersion for 5 hours or more at 50°C and the remaining resin sample in a solid state is recovered. The weight loss of the resin sample is more preferably less than 12 wt %, and even more preferably less than 10 wt %.

The resin weight loss rate can be measured using the following method. After filtering the cooled solution, the solid on the filter is 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 weight of the recovered resin is subtracted from the amount of resin immersed in the organic solvent. The percentage of this value, calculated by dividing the weight by the amount of resin immersed in the organic solvent, is used as the resin weight loss rate.

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

Furthermore, organic solvents containing fluorine atoms and hydrogen atoms in the molecule are preferred as precipitation polymerization solvents because they are less likely to cause chain transfer reactions in radical polymerization, have excellent polymerization yields, and are more likely to produce high molecular weight products. Specific 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, and 2,4,4-trifluorobutanol. , 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 these, 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, with 1,2,2,3,3,4,4-heptafluorocyclopentane being preferred due to its excellent polymerization yield and ease of obtaining high molecular weight polymers. The ratio of fluorine atoms to hydrogen atoms in the precipitation polymerization solvent molecules is preferably fluorine atoms:hydrogen atoms = 1:9 to 9:1, more preferably 1:9 to 7:3, and even more preferably 4:6 to 7:3, due to its excellent polymerization yield. The precipitation polymerization solvent contains fluorine atoms and hydrogen atoms in its molecules, and the hydrogen atom content 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 molecules, due to its excellent polymerization yield. Furthermore, in order to achieve a high polymerization yield and facilitate the production of high molecular weight products, the concentration 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. Furthermore, as a precipitation polymerization solvent, one that does not contain chlorine atoms in the molecule is preferred, in order to achieve a high polymerization yield and facilitate the production of high molecular weight products.

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

In the production method of the present invention, it is preferable that the monomer represented by formula (5) is perfluoro(4-methyl-2-methylene-1,3-dioxolane) represented by formula (11), and the residue unit represented by formula (1) is a perfluoro(4-methyl-2-methylene-1,3-dioxolane) residue unit represented by formula (3).

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

The present invention will be described in more detail below based on examples. However, these examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention.

[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 Super HZM-M manufactured by Tosoh Corporation and an RI detector. Asahiklin AK-225 (manufactured by Asahi Glass Co., Ltd.) was used as the eluent, to which 10 wt % of 1,1,1,3,3,3-hexafluoro-2-propanol (manufactured by Wako Pure Chemical Industries, Ltd.) was added. 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-state ^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, solid-state 19F-NMR measurements were carried out using approximately 10 mg of fluororesin, with PTFE (-122.0 ppm) as the standard, under the following conditions: 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 number of accumulations: 2048.

[Measurement of transmittance of polymer solution]
A fluororesin-perfluorohexane solution with a concentration of 10 wt% was prepared by dissolving the fluororesin in perfluorohexane (FC-72, manufactured by 3M Japan). Using a quartz cell with a 10 mm optical path length and a UV-visible spectrophotometer UH5300 (manufactured by Hitachi, Ltd.), the quartz cell containing the obtained polymer solution was set on the sample side of the spectrophotometer, and a quartz cell containing perfluorohexane (FC-72) was set on the reference side. The transmittance of the polymer solution was measured over a wavelength range of 190 to 700 nm, at a scan speed of 400 nm/min and intervals of 1 nm. The transmittance value obtained here at a wavelength of 275 nm was taken as the transmittance of a 10 wt% perfluorohexane solution of the fluororesin at a wavelength of 275 nm with an optical path length of 10 mm. Furthermore, a graph of the relationship between wavelength and transmittance was used to determine whether there was a peak showing a minimum at wavelengths of 250 to 300 nm.

[Measurement of yellowness index (YI) of thick melt-molded product (φ10 mm × H17 mm, in a test tube at 280°C for 24 hours)]
3.0 g of fluororesin was placed in a glass test tube (Nichiden Rika Glass, ST-13M) with an outer diameter of 13 mm and a total length of 100 mm, and the mouth of the test tube was covered with aluminum foil and an aluminum cap (Maruem, M-1). The tube was then placed in a test tube stand, placed in an oven, and heated at 280 ° C. for 24 hours. After that, the tube was allowed to cool to obtain a cylindrical resin molded product in the test tube (diameter: 10 mm, height: approximately 17 mm). The obtained resin molded product was placed in the test tube on its side on white paper, and a digital photograph was taken from above under white fluorescent light using a PowerShot SX620HS (Canon Inc.). The RGB values of the molded product were read from the obtained image using Paint (image processing software manufactured by Microsoft). 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 determined by the above formula. The yellowness index (YI) under Illuminant 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-walled melt-molded product (φ10 mm×H17 mm, in a test tube at 280°C for 24 hours) was determined.

[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 26.4 mm inner diameter dish (a flat dish lid and receiver set manufactured by Flat Corporation, with the receiver only, and the glass thickness of the bottom of the receiver 1 mm), placed in an inert oven (DN411I manufactured by Yamato Scientific), and left to stand at room temperature for 30 minutes under an air flow (20 L/min). After that, the temperature was raised 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 closed, the inert oven was turned off, and the sample was allowed to cool for 12 hours. After that, a 3 mm thick, 26.4 mm diameter fluororesin hot melt molded product was obtained on the dish. At this time, air compressed by a compressor and passed through a dehumidifier (dew point temperature -20 ° C. or less) was used. The transmittance of the obtained fluororesin heat-melt molded article, together with the Petri dish, was measured 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) under Illuminant C (auxiliary illuminant C) was calculated to determine the yellowness index (YI) of the fluororesin heat-melt molded article including the Petri dish. The yellowness index (YI) of the Petri dish alone (receiver only) was measured, and the yellowness index (YI) of the fluororesin molded article with a thickness of 3 mm was calculated by subtracting the yellowness index (YI) of the Petri dish alone (receiver only) from the yellowness index (YI) of the fluororesin molded article including the Petri dish. The yellowness index (YI) of the petri dish alone (receiver only) was 0.21.

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

[Example 1]
A 75 mL glass ampoule was charged with 0.133 g (0.000205 mol) of bis(perfluorocyclohexylcarbonyl) peroxide as an initiator, 10.0 g (0.0410 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, and 40.0 g of FC-72 as a polymerization solvent. The ampoule was repeatedly subjected to nitrogen substitution and depressurization by freeze degassing, and then sealed under reduced pressure (monomer/solvent = 20/80 (wt/wt)). The ampoule was placed in a constant temperature bath at 55 ° C. and maintained for 24 hours to carry out radical solution polymerization, resulting in a viscous solution containing the dissolved resin. After cooling to room temperature, the ampoule was opened, and the resin solution was diluted with 50 g of FC-72 to adjust the viscosity. A diluted resin solution was prepared. 240 g of Zeorora H (1,2,2,3,3,4,4-heptafluorocyclopentane, manufactured by Nippon Zeon Co., Ltd.) was placed in a beaker equipped with a stirrer, and the diluted resin solution was added to the beaker while stirring to precipitate a resin. The precipitated resin was recovered by filtration, washed twice with acetone, and vacuum dried to obtain powdered perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 93%). The weight-average molecular weight of the resulting fluororesin was 8.6 × 10 ⁴ , and the molecular weight distribution Mw/Mn was 3.5. Solid-state ^19F -NMR showed no peak between -150 and 170 ppm, and no fluorinated aromatic rings such as pentafluorophenyl groups were detected. The evaluation results of the resulting fluororesin are shown in Table 1 and Figure 1.

[Example 2]
A solution of 0.0266 g (0.000041 mol) of bis(perfluorocyclohexylcarbonyl) peroxide as an initiator dissolved in 2.7 g of FC-72 was placed in a glass ampoule having a diameter of 30 mm and equipped with a magnetic stirrer, and 10.0 g (0.0410 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as a monomer, 37.3 g of Zeorora-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 then placed in the ampoule. The ampoule was repeatedly subjected to nitrogen substitution 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 40°C for 24 hours while stirring with a magnetic stirrer. The resulting solution became cloudy, and a slurry of resin precipitated in the polymerization solvent was obtained. After cooling to room temperature, the ampoule was opened, and the liquid containing the resulting resin particles was filtered, washed with acetone, and vacuum dried to obtain a powder of perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 90%). The resulting fluororesin had a weight-average molecular weight of 9.2 x 10 ⁴ and a molecular weight distribution Mw/Mn of 2.7. The resulting fluororesin had a volume-average particle diameter of 18 μm and excellent powder fluidity, even superior to that of Example 1. Solid-state ^19F -NMR revealed no peaks between -150 and 170 ppm, and no fluorinated aromatic rings such as pentafluorophenyl groups were detected. The evaluation results of the resulting fluororesin are shown in Table 1 and Figure 1.

[Comparative Example 1]
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 Zeorora-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 substitution by freeze degassing and depressurization, followed by sealing 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. The resulting solution became cloudy, and a slurry of resin precipitated in the polymerization solvent was obtained. After cooling to room temperature, the ampoule was opened, and the liquid containing the resulting resin particles was filtered, washed with acetone, and vacuum dried to obtain particulate perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield: 81%). The weight-average molecular weight of the resulting fluororesin was 9.8 x 10 ⁴ , and the molecular weight distribution Mw/Mn was 2.6. Solid-state ^19F -NMR confirmed a peak at -163 ppm attributable to a pentafluorophenyl group (fluorine-containing aromatic ring). Evaluation results for the resulting fluororesin are shown in Table 1 and FIG. 1.

[Comparative Example 2]
A polymerization tube equipped with a stir bar was charged with 4.8 g (0.020 mol) of perfluoro(4-methyl-2-methylene-1,3-dioxolane) as the monomer, 3 mL of dichloropentafluoropropane (AGC, AK-225) as the solvent, 0.21 g of ammonium perfluorooctanoate as the emulsifier, 0.24 g _{of NaHPO4} · _7H2O as the pH adjuster, 0.15 _{g of (NH4)2S2O8 as the} initiator, and 50 mL of distilled water degassed with _N2 as the solvent. The headspace above the solution was purged with _N2 and then slightly overpressurized with _N2 . 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. This polymer was washed three times with 200 mL of distilled water and then three times with 200 mL of acetone. The polymer was then placed in a vacuum oven and dried at 150°C under vacuum (150 mmHg) for 24 hours, yielding a white powdery polymer (yield: 3%). The above procedures from polymerization to drying were repeated two more times, and the resulting polymers were mixed to obtain a polymer for evaluation. The evaluation results of the resulting polymer are shown in Table 2. The weight-average molecular weight of the resulting fluororesin was 34 x 10 ⁴ , and the molecular weight distribution Mw/Mn was 25. The yield was very low, and the molecular weight distribution was very large.

[Comparative Example 3]
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 substitution and depressurization by freeze degassing, the ampoule was sealed under reduced pressure. The ampoule was heated at 60°C for 3 hours while shaking in a thermostatic shaker. The polymer removed from the ampoule was dried at 100°C under vacuum (150 mmHg) for 24 hours to obtain a polymer (yield: 76%). The above procedures from polymerization to drying were repeated two more times, and the resulting polymers were mixed to obtain a polymer for evaluation. The evaluation results of the resulting polymer are shown in Table 2. The weight average molecular weight of the resulting fluororesin was 12×10 ⁴ , and the molecular weight distribution Mw/Mn was 1.8.

[Reference example 1]
A 75 mL glass ampoule was _charged with a solution of 0.52 g (0.0012 mol) _of ( _CF3CF2CF2COO ) 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. The mixture was repeatedly subjected to nitrogen substitution and depressurization by freeze degassing, and then sealed under reduced pressure. The ampoule was maintained at 25°C for 24 hours to allow radical polymerization. The ampoule was opened, and the contents were placed in a beaker containing 600 g of hexane under stirring. The solid was recovered by filtration, washed twice with acetone, and vacuum dried to obtain a bulk perfluoro(4-methyl-2-methylene-1,3-dioxolane) resin (yield 17%). The weight-average molecular weight of the resulting fluororesin was 83 x ¹⁰⁴ , and the molecular weight distribution Mw/Mn was 25. The yield was very low and the molecular weight distribution was very wide.

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

The organic solvents in which residual resin particles were confirmed with the naked eye are as follows:
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.

The mixture was then cooled to 25°C, filtered, and rinsed with the solvent to remove the resin particles. 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 all cases the recovery rate was over 90%. The filtrate obtained above was also distilled off, and the solid content of the filtrate was determined. It was found that the solid content of the filtrate was less than 10% of the resin particles used. These results confirmed that the weight loss rate of the resin was less than 10% by weight.

As shown in Examples 1 and 2, the fluororesin of the present invention has a lower yellowness index in thick-walled melt-molded products (φ10 mm x H17 mm, 280°C in a test tube for 24 hours) than Comparative Example 1, and yellowing during melt molding of thick molded products is suppressed.

The method for producing fluororesin of the present invention has a higher yield than the method of Reference Example 1. 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 fluororesin manufacturing method 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 Reference Example 1, with a molecular weight distribution Mw/Mn of 5 or less, and under certain 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 containing a residue unit represented by the following formula (1), having a molecular weight distribution Mw/Mn of 1.2 to 8, and having an end represented by the following formula (6) directly attached to the polymer, which, when dissolved in perfluorohexane to form a 10 wt % perfluorohexane solution, has a transmittance of 50% or more as measured at an optical path length of 10 mm and a wavelength of 275 nm :
A thick-walled melt-molded product of the fluororesin (a cylindrical molded body having a diameter of 10 mm and a height of approximately 17 mm, molded by heating and melting in a test tube at 280°C for 24 hours) has a yellowness index of 4 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, wherein the perfluoroalkyl group may have an etheric oxygen atom, and Rf ₁ , Rf ₂ , Rf ₃ and Rf ₄ may be linked together to form a ring having 4 to 8 carbon atoms, which ring may contain an etheric oxygen atom.)
(In formula (6), Rf ¹⁵ and Rf ¹⁶ each independently represent a perfluoroalkyl group having 1 to 20 carbon atoms, and the perfluoroalkyl group may have an etheric oxygen atom. Furthermore, Rf ¹⁵ and Rf ¹⁶ 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.) The fluororesin according to claim 1, which, when dissolved in perfluorohexane to form a 10 wt % perfluorohexane solution, does not exhibit a minimum peak in the transmission spectrum at an optical path length of 10 mm and wavelengths of 250 to 300 nm. The fluororesin according to claim 1 or 2, wherein the fluororesin is in powder form and has a volume average particle size of 1 to 1000 μm.