ELECTRICAL MATERIAL AND INDUCTION CONVERSION ELEMENT

Description

Technical Field

[0001] The present disclosure relates to an electret material, an electrostatic induction conversion element comprising the same, and the use of a coating agent for forming an electret material.Background Art

[0002] The use of fluoropolymers as electret materials for electrostatic induction elements is being considered (WO2009 / 104699, JP-A-2006-180450 and JP-A-2007-312551). Electret materials are required to play the role of storing (charging) electric potential. Since the amount of charge generally increases with the increase in the thickness of electret materials, there has been a demand for thicker films (JP-A-2006-180450). In addition, electret materials are further required to prevent the occurrence of peeling from base materials and their own fracture caused by, for example, vibration for power generation and stress in harsh environments due to outdoor use.

[0003] US-A-2011 / 0105686 discloses an electret containing a fluoropolymer which comprises at least one repeating unit selected from repeating units of specified formulae (a), (b) and (c) in a total amount of ≥ 80 mol % based on all repeating units, the units (c) being units derived from fluorinated 2-methylene-1,3-dioxolane derivates, and which has a glass transition temperature of 110-350°C.

[0004] US-A-2014 / 0132111 describes an electret having an electric charge injected to a laminate having resin layers (A) and (B) laminated directly to each other, wherein the layer (A), but not the layer (B), contains a reaction product of a fluorinated polymer with a silane coupling agent, and the layer (B) contains a fluorinated polymer, the layer (B) being disposed as the outermost layer in contact with air, and the total thickness of the layer (B) is 5-55% of that of the layer (A).Summary of InventionTechnical Problem

[0005] An object of the present disclosure is to provide an electret material comprising a fluoropolymer. Another object of the present disclosure is to provide an electrostatic induction conversion element comprising an electret material comprising a fluoropolymer. Another object of the present disclosure is to provide the use of a specified coating agent for forming an electret material comprising a fluoropolymer.Solution to Problem

[0006] The present invention provides an electret material (also referred to as "the present electret material" hereinafter) comprising a fluoropolymer which comprises ≥ 50 mol%, based on the total monomer units in the polymer, of monomer units of formula (1): wherein R 1< -R 4< each independently are F, fluoroalkyl or fluoroalkoxy; and has a mass average molecular weight, determined by GPC analysis according to the method defined in the description, of 1-5 million, or 5,000 to < 20,000.

[0007] Also, the present invention provides (i) an electrostatic induction conversion element comprising the present electret material and (ii) the use of a coating agent comprising the fluoropolymer as defined in the present electret material above, and an aprotic solvent, for forming the present electret material.

[0008] Preferred embodiments of the invention are as defined in the appended dependent claims and / or in the following detailed description.Advantageous Effects of Invention

[0009] It was found that when a fluoropolymer comprising ≥ 50 mol%, based on the total monomer units in the polymer, of the monomer units of formula (1) is used as an electret material, the molecular weight thereof is set to 1-5 million , or 5000 to < 20000, whereby peeling from the base material is prevented, and thick film formation is possible.

[0010] The present invention is capable of providing a fluoropolymer electret material that is prevented from being peeled off from a base material; a highly rigid fluoropolymer electret material by using the fluoropolymer having a mass average molecular weight of 1-5 million; a fluoropolymer electret material having a large end content by using the fluoropolymer having a mass average molecular weight of 5000 to < 20000,. a fluoropolymer electret material having a large initial surface potential; a fluoropolymer electret material having excellent potential retention; a fluoropolymer electret material with a large thickness; and the use of a coating agent for forming an electret material in which a fluoropolymer is dissolved at a high concentration, and that is advantageous for the production of thick films. Brief Description of Drawing

[0011] Fig. 1 is a schematic diagram of a device used in corona discharge treatment for electret formation in the Examples and Comparative Example.Description of EmbodimentsTerms

[0012] Unless otherwise specified, the symbols and abbreviations in the present specification can be understood in the sense commonly used in the technical field to which the present disclosure pertains, according to the context of the present specification. Also, unless otherwise specified in the present specification the following applies.

[0013] The terms "comprise" and "contain" are used with the intention of including the terms "consisting essentially of" and "consisting of."

[0014] The steps, treatments, or operations described herein can be performed at room temperature. Room temperature can mean a temperature of 10-40°C.

[0015] The phrase "C n-m " (n and m are each a number) indicates that the number of carbon atoms is n to m, as would be commonly understood by a person skilled in the art.

[0016] The expression "thickness" or simply "film thickness" with respect to a film means an average film thickness. The average film thickness is determined as follows.Average Film Thickness

[0017] The average film thickness is the average value of a thickness measured 5 times with a micrometer. When measuring the thickness of a film itself is difficult, such as when a film formed on a base material of a substrate cannot be peeled off, the average film thickness is calculated by measuring the thickness of the base material before film formation and after film formation 5 times each with a micrometer, and subtracting the average value of the thickness before film formation from the average value of the thickness after film formation.

[0018] When the measurement cannot be performed with a micrometer, the film thickness obtained by measuring the line profile of the cut surface of a film to be measured with an atomic force microscope (AFM) is defined as the average film thickness.

[0019] Specifically, the average film thickness is a value determined by the method described in a specific example herein.

[0020] The simple expression "molecular weight" means mass average molecular weight. The mass average molecular weight is determined as follows.Mass Average Molecular Weight

[0021] The mass average molecular weight is measured by using the following GPC analysis method. Specifically, the mass average molecular weight is a value determined by the method described in a specific example herein.GPC Analysis MethodSample Preparation Method

[0022] A polymer is dissolved in perfluorobenzene to prepare a 2 mass% polymer solution, and the polymer solution is passed through a membrane filter (0.22 µm) to obtain a sample solution.Measurement Method

[0023] Standard sample for measurement of molecular weight: polymethyl methacrylate Detection method: RI (differential refractometer)

[0024] The "indentation hardness" and "indentation elastic modulus" are determined as follows.Indentation Hardness and Indentation Elastic Modulus

[0025] The indentation hardness (H IT ; indentation hardness) of a sample is measured using an ENT-2100 ultra-fine hardness tester produced by Nanotec Corporation. The indentation elastic modulus is also measured at the same time. The test is performed by adjusting the indentation depth to be ≤ 1 / 10 of the thickness. Specifically, the indentation hardness and indentation elastic modulus are values determined by the method described in a specific example herein.

[0026] The "surface potential" is determined as follows.Surface Potential

[0027] A fluoropolymer formed into an electret is measured with a surface electrometer (Model 347, produced by Trek Japan). Specifically, the surface potential is a value determined by the method described in a specific example of the present disclosure.

[0028] The "end content" is determined as follows.End Content

[0029] The end content is determined as a relative value of the number of carbonate end groups per 1 x 10 6< carbon atoms in the monomer unit forming the fluoropolymer backbone. Specifically, this is expressed as a value relative to the number of carbonate end groups in the fluoropolymer of Reference Preparation Example 1, described later, which is taken as 100. The number of carbonate end groups is determined as follows.

[0030] A film with a thickness of 0.05 mm or more and 0.2 mm or less is formed from a cut piece of a cast film or press-molded film of the fluoropolymer. The absorbance of the peak at 1809 cm -1< attributed to the carbonyl group of the carbonate group (-OC(=O)O-) is measured by infrared absorption spectrum analysis of the film. The number (N) of carbonate groups per 10 6< backbone carbon atoms is calculated according to the following equation. N = 500 AW / ε df A: Absorbance of the peak at 1809 cm -1< derived from the carbonate group (-OC(=O)O-) ε: Molar absorbance coefficient of the peak at 1809 cm -1< derived from the carbonate group (-OC(=O)O-) (1·cm -1< ·mol -1< ). From the model compound, ε = 170. W: Composition average monomer molecular weight calculated from monomer composition d: Film density (g / cm 3< ) f: Film thickness (mm)

[0031] The infrared absorption spectrum analysis is performed using a Perkin-Elmer 1760x FTIR spectrometer (produced by Perkin-Elmer) with 40 scans. The baseline of the IR spectrum obtained is automatically determined by using Perkin-Elmer Spectrum for Windows, Ver. 1.4C, and the absorbance A of the peak at 1809 cm -1< is measured. Further, the thickness of the film is measured with a micrometer.

[0032] Specifically, the end content is a value determined by the method described in a specific example herein.

[0033] The "glass transition temperature" is determined as follows.Glass Transition Temperature (Tg)

[0034] The temperature is increased (first run), decreased, and then increased (second run) at 10°C / minute in the temperature range of 30-200°C using a DSC (differential scanning calorimeter; Hitachi High-Tech Science Corporation, DSC7000); the midpoint of the endothermic curve in the second run is determined to be the glass transition temperature (°C).

[0035] Examples of "alkyl" include linear or branched C 1 - 10 -alkyl, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl, nonyl, and decyl.

[0036] "Fluoroalkyl" is alkyl in which at least one H is replaced by F. "Fluoroalkyl" can be linear or branched fluoroalkyl.

[0037] The number of carbon atoms in "fluoroalkyl" can be, for example, 1-12, 1-6, 1-5, 1-4, 1-3, 6, 5, 4, 3, 2, or 1.

[0038] The number of fluorine atoms in "fluoroalkyl" can be 1 or more (e.g., 1-3, 1-5, 1-9, 1-11, or 1 to the maximum substitutable number).

[0039] "Fluoroalkyl" includes perfluoroalkyl.

[0040] "Perfluoroalkyl" is alkyl in which all H are replaced by F.

[0041] Examples of perfluoroalkyl include trifluoromethyl (CF 3 -), pentafluoroethyl (C 2 F 5 -), heptafluoropropyl (CF 3 CF 2 CF 2 -), and heptafluoroisopropyl ((CF 3 ) 2 CF-).

[0042] Specific examples of "fluoroalkyl" include monofluoromethyl, difluoromethyl, trifluoromethyl (CF 3 -), 2,2,2-trifluoroethyl (CF 3 CH 2 -), perfluoroethyl (C 2 F 5 -), tetrafluoropropyl (e.g., HCF 2 CF 2 CH 2 -), hexafluoropropyl (e.g., (CF 3 ) 2 CH-), perfluorobutyl (e.g., CF 3 CF 2 CF 2 CF 2 -), octafluoropentyl (e.g., HCF 2 CF 2 CF 2 CF 2 CH 2 -), perfluoropentyl (e.g., CF 3 CF 2 CF 2 CF 2 CF 2 -), and perfluorohexyl (e.g., CF 3 CF 2 CF 2 CF 2 CF 2 CF 2 -).

[0043] "Alkoxy" can be a group represented by RO-, wherein R is alkyl (e.g., C 1-10 -alkyl).

[0044] Examples of "alkoxy" include linear or branched C 1 -C 10 alkoxy, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy, isopentyloxy, neopentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy, and decyloxy.

[0045] "Fluoroalkoxy" is alkoxy in which at least one H is replaced by F. "Fluoroalkoxy" can be linear or branched fluoroalkoxy.

[0046] The number of carbon atoms in "fluoroalkoxy" can be, for example, 1-12, 1-6, 1-5, 1-4, 1-3, 6, 5, 4, 3, 2, or 1.

[0047] The number of fluorine atoms in "fluoroalkoxy" can be 1 or more (e.g., 1-3, 1-5, 1-9, 1-11, or 1 to the maximum substitutable number).

[0048] "Fluoroalkoxy" includes perfluoroalkoxy.

[0049] "Perfluoroalkoxy" is alkoxy in which all H are replaced by F.

[0050] Examples of "perfluoroalkoxy" include trifluoromethoxy (CF 3 O-), pentafluoroethoxy (C 2 F 5 O-), heptafluoropropoxy (CF 3 CF 2 CF 2 O-), and heptafluoroisopropoxy ((CF 3 ) 2 CFO-).

[0051] Specific examples of "fluoroalkoxy" include monofluoromethoxy, difluoromethoxy, trifluoromethoxy, 2,2,2-trifluoroethoxy (CF 3 CH 2 O-), perfluoroethoxy (C 2 F 5 O-), tetrafluoropropyloxy (e.g. HCF 2 CF 2 CH 2 O-), hexafluoropropyloxy (e.g., (CF 3 ) 2 CHO-), perfluorobutyloxy (e.g., CF 3 CF 2 CF 2 CF 2 O-), octafluoropentyloxy (e.g., HCF 2 CF 2 CF 2 CF 2 CH 2 O-), perfluoropentyloxy (e.g., CF 3 CF 2 CF 2 CF 2 CF 2 O-), and perfluorohexyloxy (e.g., CF 3 CF 2 CF 2 CF 2 CF 2 CF 2 O-).Electret Material

[0052] The electret material comprises: a fluoropolymer comprising ≥ 50 mol%, based on the total monomer units in the polymer, of monomer units of formula (1) : wherein R 1< -R 4< are each independently F, fluoroalkyl or fluoroalkoxy (which may be referred to as "unit (1)" in the present specification); and having a mass average molecular weight of 1-5 million (which may be referred to as "high-molecular-weight fluoropolymer" in the present specification); or a fluoropolymer comprising ≥ 50 mol%, based on the total monomer units in the polymer, of unit (1) and having a mass average molecular weight of 5000 to < 20000 (which may be referred to as "low-molecular-weight fluoropolymer" in the present specification).

[0053] Herein, phrases such as "comprising a monomer unit as a main component" mean that the percentage of the specific monomer unit is ≥ 50 mol% based on the total monomer units in a polymer.

[0054] Unit (1) as a monomer unit constituting the fluoropolymer may be used singly or in a combination of two or more.

[0055] The percentage of unit (1) may be, for example, ≥ 70 mol%, preferably ≥ 80 mol%, more preferably ≥ 90 mol%, and particularly preferably 100 mol%, based on the total monomer units of the fluoropolymer.Fluoropolymer Comprising Unit (1) as Main Component

[0056] Fluoropolymers comprising unit (1) as a main component are roughly divided into a high-molecular-weight fluoropolymer, a low-molecular-weight fluoropolymer, and a medium-molecular-weight fluoropolymer (a polymer having a mass average molecular weight of 20000 to < 1 million). The present electret material comprises a high-molecular-weight fluoropolymer or a low-molecular-weight fluoropolymer.High-Molecular-Weight Fluoropolymer

[0057] The electret material comprising a high-molecular-weight fluoropolymer has high hardness and rigidity, is prevented from being peeled off from the base material, has a large initial surface potential, and has excellent potential retention.

[0058] General high-molecular-weight fluoropolymers had low solubility in solvents, it was difficult to obtain high-concentration liquids, and even if they were dissolved, the viscosity tended to increase. It was thus considered difficult to use them for film formation by coating. However, by polymerizing a monomer corresponding to unit (1) in an aprotic solvent (solution polymerization) to produce a fluoropolymer comprising unit (1) as a main component, it was possible to obtain a polymerization reaction mixture in which a high-molecular-weight fluoropolymer was dissolved at a high concentration and whose viscosity was not high. In the present disclosure, the use of this polymerization reaction mixture as a coating agent facilitated the formation of high-molecular-weight fluoropolymer films and reduced peeling of the films from base materials. Therefore, an electret material comprising a high-molecular-weight fluoropolymer could be provided.

[0059] The mass average molecular weight of the high-molecular-weight fluoropolymer can be, for example, 1-5 million, or > 1 to 5 million, preferably 1.1-3 million, more preferably 1.1-2 million. When the molecular weight is ≥ 1 million, the rigidity of the electret material increases. When the molecular weight is ≤ 5 million, it is easy to set the polymerization conditions during polymer production, and it is easy to mold the polymer.Low-Molecular-Weight Fluoropolymer

[0060] The electret material comprising a low-molecular-weight fluoropolymer is prevented from being peeled off from the base material, has a large initial surface potential, has excellent potential retention, and has a high end content.Fluoropolymer Comprising Unit (1) as Main Component

[0061] General low-molecular-weight fluoropolymers are known to have low polymer strength. Therefore, it was difficult to use low-molecular-weight fluoropolymers as electret materials exposed to vibration etc. because there was concern about the occurrence of peeling from base materials and the fracture of the polymers. However, it was found that low-molecular-weight fluoropolymers had better adhesion to base materials than that of medium-molecular-weight fluoropolymers, and could withstand use as electret materials (e.g., outdoor use). It is considered that due to the low molecular weight, the amount of polymer ends (number of ends) per unit mass of the fluoropolymer increases, which enhances the adhesion to the base material. In WO2009 / 104699, the adhesion to base materials is ensured by introducing a carboxylic acid group at the end of a fluoropolymer comprising unit (1) as a main component. In contrast, fluoropolymer films formed using a coating agent containing a polymerization reaction mixture obtained by solution polymerization are prevented from being peeled off from base materials, even without introducing a carboxylic acid group at the fluoropolymer end.

[0062] The end content can be, for example, ≥ 110, ≥ 120, ≥ 130, ≥ 140, ≥ 150, ≥ 200 or more, 110-1000, 120-1000, 130-1000, 140-1000, 150-1000, 200-1000, 110-600, 120-600, 130-600, 140-600, 150-600, or 200-600.

[0063] The mass average molecular weight of the low-molecular-weight fluoropolymer can be, for example, 5000 to < 20000, preferably 8000 to < 20000, and more preferably 10000 to < 20000. When the molecular weight is ≥ 5000, the heat resistance is enhanced. When the molecular weight is < 20000, the end content increases, and the adhesion is enhanced.

[0064] In each of R 1< -R 4< , fluoroalkyl can be, for example, C 1-5 -fluoroalkyl, C 1-4 -fluoroalkyl, C 1-3 -fluoroalkyl, , each linear or branched, or C 1-2 -fluoroalkyl. These groups each independently are preferably perfluoroalkyl groups.

[0065] In each of R 1< -R 4< , fluoroalkoxy can be, for example, C 1-5 -fluoroalkoxy, C 1-4 -fluoroalkoxy, C 1-3 -fluoroalkoxy, each linear or branched, or C 1-2 -fluoroalkoxy. These groups each independently are preferably perfluoroalkoxy groups.

[0066] R 1< -R 4< can be each independently F, or C 1-5 -fluoroalkyl or C 1-5 -fluoroalkoxy each linear or branched. R 1< -R 4< can be each independently F, or C 1-5 -perfluoroalkyl or C 1-5 -perfluoroalkoxy each linear or branched.

[0067] R 1< -R 4< can be each independently F, or C 1-4 -fluoroalkyl or C 1-4 -fluoroalkoxy each linear or branched.

[0068] R 1< -R 4< can be each independently F, or C 1-4 -perfluoroalkyl or C 1-4 -perfluoroalkoxy each linear or branched.

[0069] R 1< -R 4< can be each independently F, or C 1-3 -fluoroalkyl or C 1-3 -fluoroalkoxy each linear or branched.

[0070] R 1< -R 4< can be each independently F, or C 1-3 -perfluoroalkyl or C 1-3 -perfluoroalkoxy each linear or branched.

[0071] R 1< -R 4< can be each independently F, C 1-2 -fluoroalkyl or C 1-2 -fluoroalkoxy.

[0072] R 1< -R 4< can be each independently F, C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy.

[0073] R 1< -R 4< can be each independently F, trifluoromethyl, pentafluoroethyl, or trifluoromethoxy.

[0074] At least one of R 1< -R 4< can be F, and the other groups in R 1< -R 4< can be independently C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy when two or more such other groups are present.

[0075] At least two of R 1< -R 4< can be F, and the other groups in R 1< -R 4< can be independently C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy when two or more such other groups are present.

[0076] At least three of R 1< -R 4< can be F, and the other group in R 1< -R 4< can be C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy.

[0077] At least three of R 1< -R 4< can be F, and the other group in R 1< -R 4< can be C 1-2 -perfluoroalkyl.

[0078] R 1< -R 4< can be all F.

[0079] Unit (1) can be a monomer unit of formula (1-1) (this unit may be referred to as "unit (1-1)" herein). In the formula, R 1< is F, , fluoroalkyl or fluoroalkoxy.

[0080] Unit (1-1) as a monomer unit constituting the fluoropolymer may be used singly or in a combination of two or more.

[0081] In unit (1-1), R 1< can be F, or C 1-5 -fluoroalkyl or C 1-5 -fluoroalkoxy each linear or branched. R 1< -R 4< can be each independently F, or C 1-5 -perfluoroalkyl or C 1-5 -perfluoroalkoxy each linear or branched.

[0082] In unit (1-1), R 1< can be F, or C 1-4 -fluoroalkyl or C 1-4 -fluoroalkoxy each linear or branched.

[0083] In unit (1-1), R 1< can be F, or C 1-4 -perfluoroalkyl or C 1-4 -perfluoroalkoxy each linear or branched.

[0084] In unit (1-1), R 1< can be F, or C 1-3 -fluoroalkyl or C 1-3 -fluoroalkoxy each linear or branched.

[0085] In unit (1-1), R 1< can be F, or C 1-3 -perfluoroalkyl or C 1-3 -perfluoroalkoxy each linear or branched.

[0086] In unit (1-1), R 1< can be F, C 1-2 -fluoroalkyl or C 1-2 -fluoroalkoxy.

[0087] In unit (1-1), R 1< can be F, C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy.

[0088] In unit (1-1), R 1< can be F, trifluoromethyl, pentafluoroethyl, or trifluoromethoxy.

[0089] In unit (1-1), R 1< can be C 1-2 -perfluoroalkyl or C 1-2 -perfluoroalkoxy.

[0090] In unit (1-1), R 1< can be C 1 -C 2 C 1-2 -perfluoroalkyl.

[0091] Preferred examples of unit (1-1) include a monomer unit of formula (1-11) (this monomer unit may be referred to as "unit (1-11)" herein).

[0092] The fluoropolymer may comprise a fluoroolefin unit in addition to unit (1).

[0093] The fluoroolefin unit may be used singly or in a combination of two or more.

[0094] The percentage of the fluoroolefin unit can be ≤ 50 mol%, preferably ≤ 30 mol%, more preferably ≤ 20 mol%, even more preferably ≤ 10 mol%, and particularly preferably 0 mol%, based on the total monomer units.

[0095] The fluoroolefin unit is a monomer unit that is formed after polymerization of a monomer containing fluorine and a carbon-carbon double bond.

[0096] The atoms constituting the fluoroolefin unit may be only F, halogen other than F, C, H and O.

[0097] The atoms constituting the fluoroolefin unit may be only F, halogen other than F, C and H.

[0098] The atoms constituting the fluoroolefin unit may be only F, C and H.

[0099] The atoms constituting the fluoroolefin unit may be only F and C.

[0100] The fluoroolefin unit includes at least one unit selected from, for example, a fluorine-containing perhaloolefin unit, a vinylidene fluoride unit (-CH 2 -CF 2 -), a trifluoroethylene unit (-CFH-CF 2 -), a pentafluoropropylene unit (-CFH-CF(CF 3 )-, - CF 2 -CF(CHF 2 )-), and a 1,1,1,2-tetrafluoro-2-propylene unit (-CH 2 -CF(CF 3 ) -< ).

[0101] The fluorine-containing perhaloolefin unit is a monomer unit that is formed after polymerization of a monomer containing fluorine and a carbon-carbon double bond, and optionally halogen other than fluorine.

[0102] The fluorine-containing perhaloolefin unit includes at least one member selected from a chlorotrifluoroethylene unit (-CFCl-CF 2 -), a tetrafluoroethylene unit (-CF 2 -CF 2 -), a hexafluoropropylene unit (-CF 2 -CF(CF 3 )-), a perfluoro(methyl vinyl ether) unit (-CF 2 -CF(OCF 3 )-), a perfluoro(ethyl vinyl ether) unit (-CF 2 -CF(OC 2 F 5 )-), perfluoro (propyl vinyl ether) unit (-CF 2 -CF(OCF 2 C 2 F 5 )-), perfluoro (butyl vinyl ether) unit (-CF 2 -CF(O(CF 2 ) 2 C 2 F 5 )-), and a perfluoro(2,2-dimethyl-1,3-dioxol) unit (-CF-CAF- (wherein A represents a perfluorodioxolane ring formed with the adjacent carbon atom shown in the formula, with two trifluoromethyl bonded to the carbon atom at position 2 of the dioxolane ring).

[0103] The fluoroolefin unit includes at least one member selected from a chlorotrifluoroethylene unit, a tetrafluoroethylene unit, a hexafluoropropylene unit, a perfluoro(methyl vinyl ether) unit, and a perfluoro(propyl vinyl ether) unit.

[0104] The fluoropolymer may further contain one or more other monomer units in addition to unit (1) and the fluoroolefin unit. However, it is preferable to contain no other monomer units.

[0105] Examples of the other monomer units include CH 2 =CHRf (wherein Rf is C 1-10 -fluoroalkyl), alkyl vinyl ether (e.g., a cyclohexyl vinyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl vinyl ether), alkenyl vinyl ether (e.g., a polyoxyethylene allyl ether and ethyl allyl ether), organosilicon compounds having a reactive α,β-unsaturated group (e.g., a vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltris(methoxyethoxy)silane), acrylic esters (e.g., a methyl acrylate and ethyl acrylate), methacrylic esters (e.g., a methyl methacrylate and ethyl methacrylate), vinyl esters (e.g., vinyl acetate, vinyl benzoate, and a VeoVA (vinyl ester produced by Shell plc)).

[0106] The percentage of the other monomer units may be, for example, 0-20 mol%, or 0-10 mol%, based on the total monomer units.

[0107] The fluoropolymer preferably has a glass transition temperature (Tg) of ≥ 110°C, more preferably 110-300°C, even more preferably 120-300°C, and particularly preferably 125-200°C. A glass transition temperature within these ranges is advantageous in terms of high bending durability of the electret material.

[0108] A fluoropolymer can be produced, for example, by polymerizing one or more monomers corresponding to one or more monomer units constituting the fluoropolymer by an appropriate polymerization method. For example, a fluoropolymer can be produced by polymerizing one or more monomers corresponding to unit (1).

[0109] A fluoropolymer can also be produced by polymerizing one or more monomers corresponding to unit (1), optionally with at least one monomer selected from the group consisting of fluoroolefins and other monomers.

[0110] A person skilled in the art would be able to understand monomers corresponding to the monomer units constituting a fluoropolymer. For example, a monomer corresponding to unit (1) is a compound of formula (M1): wherein R 1< -R 4< are as defined above (this compound may be referred to as "monomer (M1)" herein).

[0111] For example, a monomer corresponding to unit (1-1) is a compound of formula (M1-1): wherein R 1< is F, fluoroalkyl or fluoroalkoxy (this compound may be referred to as "monomer (M1-1)" herein).

[0112] For example, the monomer corresponding to unit (1-11) is a compound of formula (M1-11): (this compound may be referred to as "monomer (M1-11)" herein).

[0113] The fluoroolefins for use may be monomers corresponding to the fluoroolefin units mentioned above. For example, the monomers corresponding to the tetrafluoroethylene unit, hexafluoropropylene unit, and vinylidene fluoride unit are tetrafluoroethylene (CF 2 =CF 2 ), hexafluoropropylene (CF 3 CF=CF 2 ), and vinylidene fluoride (CH 2 =CF 2 ), respectively. Thus, the details regarding fluoroolefins would be able to be understood by a person skilled in the art from the description of the corresponding fluoroolefin units.

[0114] For example, the fluoroolefin may be at least one member selected from fluorine-containing perhaloolefins, vinylidene fluoride, trifluoroethylene, pentafluoropropylene, and 1,1,1,2-tetrafluoro-2-propylene. Preferably, the fluoroolefin may be at least one member selected from chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, perfluoro(methyl vinyl ether), and perfluoro(propyl vinyl ether).

[0115] The fluorine-containing perhaloolefin may be at least one member selected from chlorotrifluoroethylene, tetrafluoroethylene, hexafluoropropylene, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro(butyl vinyl ether), and perfluoro(2,2-dimethyl-1,3-dioxol).

[0116] The other monomers for use may be monomers corresponding to the other monomer units mentioned above. Thus, the details regarding the other monomers would be able to be understood by a person skilled in the art from the description of the corresponding other monomer units.

[0117] The polymerization method includes, for example, a method of using appropriate amounts of monomers corresponding to the monomer units that constitute the fluoropolymer, with the monomers being optionally dissolved or dispersed in a solvent (e.g., an aprotic solvent) and a polymerization initiator being optionally added, and performing polymerization (e.g., radical polymerization, bulk polymerization, solution polymerization, suspension polymerization, dispersion polymerization, or emulsion polymerization).

[0118] The polymerization method is preferably solution polymerization because solution polymerization can produce a high-concentration solution of the fluoropolymer and thereby achieve a high manufacturing yield, and is advantageous for the formation of a thick film and purification. The polymerization method is also preferably bulk polymerization that tends to increase the molecular weight. The fluoropolymer is preferably produced by solution polymerization. The fluoropolymer is more preferably produced by solution polymerization in which one or more monomers are polymerized in the presence of an aprotic solvent.

[0119] The solvent for use in solution polymerization of the fluoropolymer is preferably an aprotic solvent. The amount of aprotic solvent for use in the production of the fluoropolymer is, for example, ≤ 80 mass%, < 80 mass%, ≤ 75 mass%, ≤ 70 mass%, 35-95 mass%, 35-90 mass%, 35-80 mass%, 35-70 mass%, 35 to < 70 mass%, or 60-80 mass%, based on the sum of the mass of the monomers and the mass of the solvent. The amount can be preferably 35 to < 80 mass%, more preferably 40-75 mass%, and particularly preferably 50-70 mass%.

[0120] The aprotic solvent for use in the polymerization of fluoropolymers can be, for example, at least one member selected from perfluoroaromatic compounds, perfluorotrialkylamines, perfluoroalkanes, hydrofluorocarbons, perfluorocyclic ethers, hydrofluoroethers, and olefin compounds containing at least one chlorine atom.

[0121] The perfluoroaromatic compound is, for example, a perfluoroaromatic compound optionally having one or more perfluoroalkyl groups. The aromatic ring of the perfluoroaromatic compound can be at least one ring selected from a benzene ring, a naphthalene ring, and an anthracene ring. The perfluoroaromatic compound can have one or more (e.g., one, two, or three) aromatic rings.

[0122] The perfluoroalkyl group as a substituent is, for example, linear or branched, C 1-6 -, C 1-5 - or C 1-4 -perfluoroalkyl, and preferably linear or branched C 1-3 -perfluoroalkyl.

[0123] The number of substituents is, for example, one to four, preferably one to three, and more preferably one or two. When a plurality of substituents are present, the substituents may be the same or different.

[0124] Examples of perfluoroaromatic compounds include perfluorobenzene, perfluorotoluene, perfluoroxylene, and perfluoronaphthalene.

[0125] Preferred examples of perfluoroaromatic compounds include perfluorobenzene and perfluorotoluene.

[0126] The perfluorotrialkylamine is, for example, an amine substituted with three linear or branched perfluoroalkyl groups. The number of carbon atoms of each perfluoroalkyl group is, for example, 1-10, preferably 1-5, and more preferably 1-4. The perfluoroalkyl groups can be the same or different, and are preferably the same.

[0127] Examples of perfluorotrialkylamines include perfluorotrimethylamine, perfluorotriethylamine, perfluorotripropylamine, perfluorotriisopropylamine, perfluorotributylamine, perfluorotri-sec-butylamine, perfluorotri-tert-butylamine, perfluorotripentylamine, perfluorotriisopentylamine, and perfluorotrineopentylamine.

[0128] Preferred examples of perfluorotrialkylamines include perfluorotripropylamine and perfluorotributylamine.

[0129] The perfluoroalkane is, for example, a linear, branched, or cyclic C 3-12 - (preferably C 3-10 -, more preferably C 3-12 -) perfluoroalkane.

[0130] Examples of perfluoroalkanes include perfluoropentane, perfluoro-2-methylpentane, perfluorohexane, perfluoro-2-methylhexane, perfluoroheptane, perfluorooctane, perfluorononane, perfluorodecane, perfluorocyclohexane, perfluoro(methylcyclohexane), perfluoro(dimethylcyclohexane) (e.g., perfluoro(1,3-dimethylcyclohexane)), and perfluorodecalin.

[0131] Preferred examples of perfluoroalkanes include perfluoropentane, perfluorohexane, perfluoroheptane, and perfluorooctane.

[0132] The hydrofluorocarbon is, for example, a C 3-8 -hydrofluorocarbon. Examples of hydrofluorocarbons include CF 3 CH 2 CF 2 H, CF 3 CH 2 CF 2 CH 3 , CF 3C HFCHFC 2 F 5 , 1,1,2,2,3,3,4-heptafluorocyclopentane, CF 3 CF 2 CF 2 CF 2 CH 2 CH 3 , CF 3 CF 2 CF 2 CF 2 CF 2 CHF 2 , and CF 3 CF 2 CF 2 CF 2 CF 2 CF 2 CH 2 CH 3 .

[0133] Preferred examples of hydrofluorocarbons include CF 3 CH 2 CF 2 H and CF 3 CH 2 CF 2 CH 3 .

[0134] The perfluorocyclic ether is, for example, a perfluorocyclic ether optionally having one or more perfluoroalkyl groups. The ring of the perfluorocyclic ether may be a 3- to 6-membered ring. The ring of the perfluorocyclic ether may have one or more oxygen atoms as a ring-constituting atom. The ring preferably has one or two oxygen atoms, and more preferably one oxygen atom.

[0135] The perfluoroalkyl group as a substituent is, for example, linear or branched C 1-6 -, C 1-5 - or C 1-4 -perfluoroalkyl. The perfluoroalkyl group is preferably linear or branched C 1-3 -perfluoroalkyl.

[0136] The number of substituents is, for example, one to four, preferably one to three, and more preferably one or two. When a plurality of substituents are present, they may be the same or different.

[0137] Examples of perfluorocyclic ethers include perfluorotetrahydrofuran, perfluoro-5-methyltetrahydrofuran, perfluoro-5-ethyltetrahydrofuran, perfluoro-5-propyltetrahydrofuran, perfluoro-5-butyltetrahydrofuran, and perfluorotetrahydropyran.

[0138] Preferred examples of perfluorocyclic ethers include perfluoro-5-ethyltetrahydrofuran and perfluoro-5-butyltetrahydrofuran.

[0139] The hydrofluoroether is, for example, a fluorine-containing ether.

[0140] The hydrofluoroether preferably has a global warming potential (GWP) of ≤ 400, and more preferably ≤ 300.

[0141] Examples of hydrofluoroethers include CF 3 CF 2 CF 2 CF 2 OCH 3 , CF 3 CF 2 CF(CF 3 )OCH 3 , CF 3 CF(CF 3 )CF 2 OCH 3 , CF 3 CF 2 CF 2 CF 2 OC 2 H 5 , CF 3 CH 2 OCF 2 CHF 2 , C 2 F 5 CF(OCH 3 )C 3 F 7 , (CF 3 ) 2 CHOCH 3 , (CF 3 ) 2 CFOCH 3 , CHF 2 CF 2 OCH 2 CF 3 , CHF 2 CF 2 CH 2 OCF 2 CHF 2 , CF 3 CHFCF 2 OCH 3 , CF 3 CHFCF 2 OCF 3 , trifluoromethyl 1,2,2,2-tetrafluoroethyl ether (HFE-227me), difluoromethyl 1,1,2,2,2-pentafluoroethyl ether (HFE-227mc), trifluoromethyl 1,1,2,2-tetrafluoroethyl ether (HFE-227pc), difluoromethyl 2,2,2-trifluoroethyl ether (HFE-245mf), 2,2-difluoroethyltrifluoromethyl ether (HFE-245pf), 1,1,2,3,3,3-hexafluoropropylmethyl ether (CF 3 CHFCF 2 OCH 3 ), 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (CHF 2 CF 2 OCH 2 CF 3 ), and 1,1,1,3,3,3-hexafluoro-2-methoxypropane (CF 3 ) 2 CHOCH 3 ).

[0142] Preferred examples of hydrofluoroethers include CF 3 CF 2 CF 2 CF 2 OCH 3 , CF 3 CF 2 CF 2 CF 2 OC 2 H 5 , CF 3 CH 2 OCF 2 CHF 2 , and C 2 F 5 CF(OCH 3 )C 3 F 7 .

[0143] The hydrofluoroether is preferably a compound of formula (B1):         R 21< -O-R 22<      (B1), wherein R 21< is linear or branched perfluorobutyl and R 22< is methyl or ethyl.

[0144] The olefin compound containing at least one chlorine atom is a C 2-4 - (preferably C 2-3 -) olefin compound containing at least one chlorine atom in its structure. The olefin compound containing at least one chlorine atom is a compound in which at least one of the hydrogen atoms bonded to the carbon atoms in a C 2 -C 4 hydrocarbon having one or two (preferably one) carbon-carbon double bonds (C=C) is replaced with chlorine. A compound in which at least one of the hydrogen atoms bonded to two carbon atoms constituting the carbon-carbon double bond in a C 2 -C 4 hydrocarbon is replaced with chlorine is preferred.

[0145] The number of chlorine atoms is one to the maximum substitutable number. The number of chlorine atoms may be, for example, one, two, three, four, or five.

[0146] The olefin compound containing at least one chlorine atom may contain at least one (e.g., one, two, three, four, or five) fluorine atom.

[0147] Examples of olefin compounds containing at least one chlorine atom include CH 2 =CHCl, CHCl=CHCl, CCl 2 =CHCl, CCl 2 =CCl 2 , CF 3 CH=CHCl, CHF 2 CF=CHCl, CFH 2 CF=CHCl, CF 3 CCl=CFCl, CF 2 HCl=CFCl, and CFH 2 Cl=CFCl.

[0148] Preferred examples of olefin compounds containing at least one chlorine atom include CHCl=CHCl, CHF 2 CF=CHCl, CF 3 CH=CHCl, and CF 3 CCl=CFCl.

[0149] As the aprotic solvent, a hydrofluoroether is preferable because it has less environmental impact during use and polymers can be dissolved at high concentrations in it.

[0150] Preferred examples of polymerization initiators used in production of the fluoropolymer include di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, diisobutyryl peroxide, di(ω-hydro-dodecafluoroheptanoyl)peroxide, di(ω-hydro-hexadecafluorononanoyl)peroxide, ω-hydro-dodecafluoroheptanoyl-ω-hydro-hexadecafluorononanoyl-peroxide, benzoyl peroxide, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, ammonium persulfate, sodium persulfate, and potassium persulfate.

[0151] Particularly preferred examples of polymerization initiators include di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, diisobutyryl peroxide, di(ω-hydro-dodecafluoroheptanoyl)peroxide, benzoyl peroxide, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, and ammonium persulfate.

[0152] The amount of the polymerization initiator for use in the polymerization reaction can be, for example, 0.0001-0.05 g, preferably 0.0001-0.01 g, and more preferably 0.0005-0.008 g, per gram of all of the monomers subjected to the reaction.

[0153] The temperature of the polymerization reaction can be, for example, -10°C to 160°C, preferably 0-160°C, and more preferably 0-100°C.

[0154] The reaction time for the polymerization reaction is preferably 0.5-72 hours, more preferably 1-48 hours, and even more preferably 3-30 hours.

[0155] The polymerization reaction can be performed in the presence or absence of an inert gas (e.g., nitrogen gas), and preferably in the presence of an inert gas.

[0156] The polymerization reaction can be performed under reduced pressure, atmospheric pressure, or increased pressure.

[0157] The polymerization reaction can be performed by adding one or more monomers to an aprotic solvent containing a polymerization initiator and subjecting it to polymerization conditions. The polymerization reaction can also be performed by adding a polymerization initiator to an aprotic solvent containing one or more monomers and subjecting it to polymerization conditions.

[0158] The fluoropolymer produced by the polymerization reaction may be purified, if desired, by a conventional method, such as extraction, dissolution, concentration, filtration, precipitation, reprecipitation, dehydration, adsorption, or chromatography, or a combination of these methods. Alternatively, a solution of the fluoropolymer produced by the polymerization reaction, a dilute solution thereof, or a mixture of the solution with other optional components may be dried or heated (e.g., 30-150°C) to form an electret material containing the fluoropolymer.

[0159] The content of the fluoropolymer in the electret material can be, for example, 50-100 mass%, preferably 60-100 mass%, more preferably 80-100 mass%, and particularly preferably 90-100 mass%, based on the total mass of the electret material.

[0160] The electret material may comprise other components in addition to the fluoropolymer. Such other components may be components generally used in electret materials. Examples include wetting agents, levelling agents, colorants, light-diffusing agents, fillers, plasticizers, viscosity modifiers, flexibilizers, light-resistant stabilizers, reaction inhibitors, and adhesion promoters.

[0161] The electret material may comprise other components in appropriate amounts as long as the effects of the present disclosure are obtained. The content of the other components can be, for example, 0-50 mass%, preferably 0-40 mass%, more preferably 0-20 mass%, and particularly preferably 0-10 mass%, based on the total mass of the electret material.

[0162] The electret material can be produced, for example, by drying or heating a solution obtained by dissolving or dispersing the fluoropolymer in a solvent to thus remove the solvent from the solution. Preferably, the electret material can be produced by removing the solvent from the coating agent described below.

[0163] The thickness of the electret material can be appropriately selected according to the functions etc. required for the film, and can be, for example, ≥ 10 nm, 10 nm to 1000 µm, 30 nm to 500 µm, 50 nm to 500 µm, or ≥ 0.1 µm. The thickness can be preferably 100 nm to 500 µm, more preferably 500 nm to 300 µm, even more preferably 800 nm to 200 µm, and particularly preferably 10-200 µm. When the average film thickness is within these ranges, it is advantageous in terms of preventing peeling from the base material.

[0164] The electret material has a high indentation hardness and indentation elastic modulus by comprising a fluoropolymer. Therefore, the electret material is advantageous as an electret material for electrostatic induction conversion elements that are used in harsh conditions such as outdoor environments.

[0165] The indentation hardness of the electret material can be, for example, 450-1000 N / mm 2< , preferably 450-800 N / mm 2< , and more preferably 450-600 N / mm 2< .

[0166] The indentation elastic modulus of the electret material can be, for example, 3.5-10 GPa, preferably 3.5-8 GPa, and more preferably 4.0-6 GPa.Electrostatic Induction Conversion Element

[0167] One embodiment of the present invention is an electrostatic induction conversion element comprising the electret material. The type of electrostatic induction conversion element is not limited as long as the electret material can be applied. The electrostatic induction conversion element may be incorporated into instruments such as vibration generators, actuators, and sensors, and is useful in terms of e.g. durability particularly when these instruments are used outdoor.

[0168] For the details of the electret material in the electrostatic induction conversion element, the description in the above explanation can be applied.Coating Agent

[0169] One embodiment of the present invention is the use of a specified coating agent for forming an electret material comprising a specific fluoropolymer.

[0170] The coating agent comprises a fluoropolymer and an aprotic solvent.

[0171] The fluoropolymer in the coating agent is the fluoropolymer described above for the electret material. Therefore, the details of the fluoropolymer for the electret material are applicable to the details of the fluoropolymer for the coating agent.

[0172] The content of the fluoropolymer in the coating agent can be, for example, 5-65 mass%, 10-65 mass%, 20-65 mass%, 30-65 mass%, > 30 to 65 mass%, or 20-40 mass%, based on the total mass of the coating agent. The content is preferably > 20 to 65 mass%, more preferably 25-60 mass%, and particularly preferably 30-50 mass%.

[0173] The aprotic solvent in the coating agent may be the aprotic solvent described above for the electret material. Therefore, the details of the aprotic solvent for the electret material are applicable to the details of the aprotic solvent for the coating agent.

[0174] The content of the aprotic solvent in the coating agent can be, for example, 35-95 mass%, 35-90 mass%, 35-80 mass%, 35-70 mass%, 35 to < 70 mass%, or 60-80 mass%, based on the total mass of the coating agent. The content is preferably 35 to < 80 mass%, more preferably 40-75 mass%, and particularly preferably 50-70 mass%.

[0175] The coating agent may comprise a polymerization initiator. The polymerization initiator for the coating agent may be the polymerization initiator described above for the electret material. Therefore, the details of the polymerization initiator for the electret material are applicable to the details of the polymerization initiator for the coating agent.

[0176] The content of the polymerization initiator in the coating agent is, for example, 0.00001-10 mass%, preferably 0.00005-10 mass%, and more preferably 0.0001-10 mass%, based on the total mass of the coating agent.

[0177] The coating agent may comprise the fluoropolymer and an aprotic solvent, and optionally a polymerization initiator and optionally other components, in appropriate amounts. Examples of other components can include colorants, light-diffusing agents, fillers, plasticizers, viscosity modifiers, flexibilizers, light-resistant stabilizers, reaction inhibitors, and adhesion promoters. The content of the other components can be, for example, 0.01-50 mass%, preferably 0.01-30 mass%, more preferably 0.01-20 mass%, based on the total mass of the coating agent.

[0178] The coating agent can be produced by mixing the fluoropolymer and an aprotic solvent, optionally with a polymerization initiator and optionally with other components.

[0179] The coating agent can be produced by mixing a polymerization reaction mixture obtained by the solution polymerization of fluoropolymer described above (this reaction mixture contains at least a fluoropolymer and an aprotic solvent) optionally with an aprotic solvent and / or other components.

[0180] When solution polymerization is performed, the fluoropolymer concentration or the amount of fluoropolymer dissolved in the polymerization reaction mixture can be increased, and the step of isolating the fluoropolymer from the polymerization reaction mixture can be omitted. For this reason, the coating agent preferably contains a polymerization reaction mixture obtained by solution polymerization.

[0181] In the coating agent, the content of the polymerization reaction mixture of solution polymerization can be appropriately selected according to the concentration of the fluoropolymer in the polymerization reaction mixture and the functions, thickness, etc. of the film to be produced. The content of the polymerization reaction mixture of solution polymerization in the coating agent can be, for example, 5-100 mass%, preferably 20-100 mass%, and more preferably 30-100 mass%, based on the total mass of the coating agent.

[0182] The coating agent comprising an aprotic solvent in which the fluoropolymer is dissolved or dispersed can form a film, for example, by applying the agent to a portion in which the formation of protective film is required by an appropriate method (e.g., spray coating, dip coating, bar coating, gravure coating, roll coating, ink jet, spin coating), and then removing the solvent by e.g. drying or heating. After application of the coating agent, heating is preferably performed. The drying or heating temperature is, for example, 30-150°C, and preferably 30-80°C.

[0183] For example, a film can be formed by applying the coating agent of the present disclosure to a base material, followed by drying in a dryer at 80°C.Examples

[0184] An embodiment of the present disclosure is described in more detail below with Examples.

[0185] The symbols and abbreviations in the Examples are used with the following meanings. Initiator solution (1): a methanol solution containing 50 mass% di-n-propyl peroxydicarbonate (10-hour half-life temperature: 40°C) Fluoropolymer (1-11): a polymer composed of unit (1-11) Mw: mass average molecular weight Film thickness: average film thickness GPC Analysis Method (Measurement of Mass Average Molecular Weight of Fluoropolymer)Sample Preparation Method

[0186] A polymer was dissolved in perfluorobenzene to prepare a 2 mass% polymer solution, and the polymer solution was passed through a membrane filter (0.22 µm) to obtain a sample solution.Measurement Method

[0187] Standard sample for measurement of molecular weight: polymethyl methacrylate Detection method: RI (differential refractometer) Confirmation of Polymer Solubility

[0188] Whether the polymer was dissolved in the liquid was determined as follows.

[0189] Each of the prepared liquids was visually observed, and when no undissolved polymer was observed and the entire liquid flowed uniformly at room temperature, it was determined that the polymer was dissolved.Average Film Thickness

[0190] The average film thickness was defined as the average value of a thickness measured 5 times with a micrometer. When measuring the thickness of a film itself was difficult, such as when a film formed on a base material of a substrate could not be peeled off, the average film thickness was calculated by measuring the thickness of the base material before film formation and the thickness of the base material after film formation (the sum of the film thickness and the base material thickness) 5 times each with a micrometer, and subtracting the average value of the thickness before film formation from the average value of the thickness after film formation.Indentation Hardness and Indentation Elastic Modulus

[0191] The indentation hardness (H IT ; indentation hardness) of the sample was measured using an ENT-2100 ultra-fine hardness tester produced by Nanotec Corporation. The indentation elastic modulus was also measured at the same time. The test was performed by adjusting the indentation depth to be ≤ 1 / 10 of the thickness.Corona Discharge Treatment (Charge Treatment)

[0192] Using the device shown in Fig. 1, potential was injected into an electret material by arc discharge. A copper plate 4 comprising a fluoropolymer 1 produced in the Examples or Comparative Example was connected to the ground to form an earth electrode, and a voltage was applied to a needle-shaped electrode 3 with a high-voltage DC power supply device 2 to inject potential into the fluoropolymer 1. Specifically, a corona discharge was generated under the conditions of a voltage of -10 kV, a discharge distance of 30 mm, and a voltage application time of 10 seconds, air molecules were ionized, and the generated air ions were repelled by the polarity of the needle-shaped electrode 3 to apply a DC electric field to the fluoropolymer 1 to inject potential, thereby charging the fluoropolymer as a whole. During the charge treatment, the entire system was placed in an electric furnace, and the surface temperature of the fluoropolymer 1 was maintained at 130°C or higher.Surface Potential Measurement

[0193] The surface potential of the fluoropolymer was measured with a surface electrometer (Model 347, produced by Trek Japan).Bending Test

[0194] A film (long side: 50 mm, short side: 15 mm, thickness: 0.1 mm) of the fluoropolymer was attached along the outer circumference of a disk (made of SUS) having a diameter of 120 mm and a thickness of 20 mm, and held for 60 seconds. Then, the appearance was visually evaluated. Polymers with no fractures or cracks were evaluated as A, polymers with cracks were evaluated as B, and polymers with fractures were evaluated as C. Fractures indicate that the strength of the polymer is low, and cracks indicate that the strength of the polymer is not so low as to break, but is not high enough.Adhesion Test (Cross-Cut Test)

[0195] Evaluation was performed according to the cross-cut cellophane tape peeling test of JIS K5600. Cross-cut notches at 1-mm intervals were made with a utility knife from the upper part of the produced laminate on the fluoropolymer layer side, and cellophane tape was attached and then peeled off. No peeling was given 10 points, 8 points for 0 to < 5%, 6 points for 5 to < 15%, 4 points for 15 to < 35%, 2 points for 35 to < 65%, and 0 points for ≥ 65%.End Content

[0196] The end content was determined as a relative value of the number of carbonate end groups per 1 x 10 6< carbon atoms in the monomer unit forming the fluoropolymer backbone. Specifically, this was expressed as a value relative to the number of carbonate end groups in the fluoropolymer of Reference Preparation Example 1, described later, which was taken as 100. The number of carbonate end groups was determined as follows.

[0197] A film with a thickness of 0.05-0.2 mm is formed from a cut piece of a cast film or press-molded film of the fluoropolymer. The absorbance of the peak at 1809 cm -1< attributed to the carbonyl group of the carbonate group (-OC(=O)O-) is measured by infrared absorption spectrum analysis of the film. The number (N) of carbonate groups per 10 6< backbone carbon atoms is calculated according to the following equation. N = 500 AW / ε df A: Absorbance of the peak at 1809 cm -1< derived from the carbonate group (-OC(=O)O-) ε: Molar absorbance coefficient of the peak at 1809 cm -1< derived from the carbonate group (-OC(=O)O-) (1·cm -1< ·mol -1< ). From the model compound, ε = 170. W: Composition average monomer molecular weight calculated from monomer composition d: Film density (g / cm 3< ) f: Film thickness (mm)

[0198] The infrared absorption spectrum analysis was performed using a Perkin-Elmer 1760x FTIR spectrometer (produced by Perkin-Elmer) with 40 scans. The baseline of the IR spectrum obtained was automatically determined by using Perkin-Elmer Spectrum for Windows, Ver. 1.4C, and the absorbance A of the peak at 1809 cm -1< was measured. Further, the thickness of the film was measured with a micrometer.

[0199] The strength value of the carbonate end of the polymer of Reference Preparation Example 1 obtained in this manner was taken as 100, and the strength value of the carbonate end of the other polymers relative to this value was used as the end content of each polymer.Preparation Example 1: Polymerization of High-Molecular-Weight Fluoropolymer Comprising Unit (1-11) as Main Component and Production of Polymer Solution (Polymerization Reaction Mixture)

[0200] The monomer (M1-11) (10 g), perfluorotripropylamine (15 g) as a solvent, and the initiator solution (1) (4.87 mg) were placed in a 50-mL glass container, and a polymerization reaction was then performed for 20 hours while the mixture was heated such that the internal temperature was 40°C, thereby producing a fluoropolymer (1-11) (6.9 g; Mw: 1125000). The fluoropolymer in the polymerization reaction mixture was dissolved, and the concentration was 31 mass%.

[0201] The weight of the polymer in the composition was measured by distilling off the unreacted starting material, the solvent, the initiator residue, and the impurities contained in a trace amount in the monomer by vacuum drying at 120°C after the completion of the polymerization reaction.Preparation Example 2

[0202] Perfluorotripropylamine was further added to the polymerization reaction mixture obtained in Preparation Example 1 to prepare a solution having a fluoropolymer concentration of 20 mass%.Preparation Example 3: Polymerization of Low-Molecular-Weight Fluoropolymer Comprising Unit (1-11) as Main Component and Production of Polymer Solution (Polymerization Reaction Mixture)

[0203] The monomer (M1-11) (3 g), methyl nonafluorobutyl ether (6 g) as a solvent, and the initiator solution (1) (0.03 g) were placed in a 50-mL glass container, and a polymerization reaction was then performed for 20 hours while the mixture was heated such that the internal temperature was maintained at 40°C, thereby producing a fluoropolymer (1-11) (2.6 g; Mw: 19500). The fluoropolymer in the polymerization reaction mixture was dissolved, and the concentration was 46 mass%.

[0204] The weight of the polymer in the composition was measured by distilling off the unreacted starting material, the solvent, the initiator residue, and the impurities contained in a trace amount in the monomer by vacuum drying at 120°C after the completion of the polymerization reaction.Preparation Example 4

[0205] Methyl nonafluorobutyl ether was further added to the polymerization reaction mixture obtained in Preparation Example 2 to prepare a solution having a fluoropolymer concentration of 30 mass%.Reference Preparation Example 1: Polymerization of Fluoropolymer Having Molecular Weight of about 98000 (Medium Molecular Weight) Comprising Unit (1-11) as Main Component and Production of Polymer Solution (Polymerization Reaction Mixture)

[0206] The monomer (M1-11) (10 g), ethyl nonafluorobutyl ether (20 g) as a solvent, and the initiator solution (1) (0.041 g) were placed in a 50-mL glass container, and a polymerization reaction was then performed for 20 hours while the mixture was heated such that the internal temperature was maintained at 40°C, thereby producing a fluoropolymer (1-11) (9.0 g; Mw: 97533). The fluoropolymer in the polymerization reaction mixture was dissolved, and the concentration was 31 mass%.

[0207] The weight of the polymer in the composition was measured by distilling off the unreacted starting material, the solvent, the initiator residue, and the impurities contained in a trace amount in the monomer by vacuum drying at 120°C after the completion of the polymerization reaction.Comparative Preparation Example 1

[0208] The polymerization reaction mixture obtained in Preparation Example 1 was added dropwise to toluene to precipitate the polymer, followed by heat treatment at 120°C for 12 hours, thereby obtaining a medium-molecular-weight fluoropolymer (1-11) (1 g). The obtained fluoropolymer (1-11) was pressed with a heat press under heating conditions of 270°C to obtain a film having an average film thickness of 50 µm.Example 1: Electret Material of High-Molecular-Weight Fluoropolymer

[0209] The polymerization reaction mixture prepared in Preparation Example 2 was directly used as a coating agent, and an electret material was produced in the following manner.

[0210] The coating agent was applied to a copper plate (thickness: 0.5 mm x length: 30 mm x width: 15 mm) by bar coating such that the thickness after drying was 20 µm. The copper plate used was treated beforehand with γ-aminopropyltriethoxysilane. After coating, air drying was performed, followed by drying with a blower dryer at 80°C for 4 hours, thereby obtaining a laminate in which a fluoropolymer layer (electret material; length: 30 mm x width: 15 mm) was formed on the copper plate. The fluoropolymer layer had a thickness of 21 µm, and was a uniform coating film with no unevenness in thickness.

[0211] The indentation hardness and indentation elastic modulus of the surface of the laminate on the fluoropolymer side were measured with a nanoindenter.

[0212] Next, the laminate was set in the device shown in Fig. 1, and corona discharge treatment was performed to inject potential into the fluoropolymer layer to charge it, thereby producing an electret. The surface potential of the produced electret was measured.

[0213] Then, the laminate was subjected to heat treatment. The heat treatment was performed by holding the laminate in a blower dryer at 100°C for 1000 hours. After the laminate was taken out from the dryer and cooled to room temperature, the surface potential of the electret was measured again. The measured surface potential value (initial surface potential) of the electret before heat treatment was taken as 100, and the measured surface potential value after heat treatment (surface potential after heat treatment) relative to the initial surface potential was calculated as the potential retention rate (%). No peeling of the film from the copper plate was visually confirmed.

[0214] Separately, a glass substrate was coated with the same coating agent such that the thickness after drying was 100 µm, and dried at 80°C for 4 hours to form a transparent film. Then, the film was removed from the glass plate to obtain a film of the fluoropolymer (1-11) having an average film thickness of 100 µm. This film was used to perform a bending test.

[0215] The above results are shown in Table 1. The electret material produced using the coating agent of Preparation Example 2 had a high molecular weight, high hardness (indentation hardness and indentation elastic modulus), no fractures or cracks in the bending test, and a high potential retention rate. These results confirmed that this electret material had high hardness, a high potential retention rate, low peel properties due to no peeling from the base material, and high rigidity due to no fractures or cracks.Example 2: Electret Material of Low-Molecular-Weight Fluoropolymer

[0216] The production of an electret material, various measurements, and the like were performed in the same manner as in Example 1, except that the polymerization reaction mixture of Preparation Example 4 was directly used as a coating agent in place of the polymerization reaction mixture of Preparation Example 2, the film thickness was 30 µm, the end content was measured in place of the measurements of indentation hardness and indentation elastic modulus, and a cross-cut test was performed in place of the bending test. The results are shown in Table 1. No peeling of the film from the copper plate was visually confirmed.

[0217] The electret material produced using the coating agent of Preparation Example 4 had a low molecular weight, a high potential retention rate, a high end content, and a high adhesion in the cross-cut test. These results confirmed that this electret material had a high potential retention rate and excellent adhesion to the base material due to no peeling from the base material. Further, the excellent adhesion of the electret material was considered to also contribute to the effect of improving the adhesion to the base material due to the end.Reference Example 1: Electret Material of Fluoropolymer Having Molecular Weight of about 98000

[0218] The production of an electret material and various measurements were performed in the same manner as in Example 1, except that the polymerization reaction mixture of Reference Preparation Example 1 was directly used as a coating agent in place of the polymerization reaction mixture of Preparation Example 2, the film thickness was 25 µm, the end content was measured, and a cross-cut test was performed. The results are shown in Table 1.

[0219] In the electret material produced using the coating agent of Reference Preparation Example 1, fractures and cracks were both observed in the bending test, the potential retention rate was slightly lower than that of the electret materials of Examples 1 and 2, the indentation hardness and indentation elastic modulus were lower than those of the electret of Example 1, and the adhesion was low in the cross-cut test. It was considered that because the molecular weight of the fluoropolymer forming the electret material was lower than the molecular weight of the fluoropolymer forming the electret material of Example 1, the electret material had low rigidity, low hardness, and a low elastic modulus, and showed fractures and cracks. It was considered that because the molecular weight of the fluoropolymer forming the electret material was higher than the molecular weight of the fluoropolymer forming the electret material of Example 2, the adhesion to the base material was insufficient, which caused peeling.

[0220] It was confirmed from the above results that fluoropolymers having the same monomer unit did not have the same suitability as electret materials depending on their molecular weight. It was confirmed that the more appropriate molecular weight of the fluoropolymer (1) used in the electret material was < 20000 or ≥ 1 million.Comparative Example 1: Electret Material of Heat Press-Molded Fluoropolymer

[0221] The polymer film produced in Comparative Preparation Example 1 was placed on the same copper plate as used in Example 1, and bonded to the copper plate with a heat press under heating conditions of 200°C. Then, corona discharge treatment was performed in the same manner as in Example 1; however, the film was peeled off from the copper plate during the treatment. Accordingly, the treatment was stopped, and no potential could be injected into the fluoropolymer film.

[0222] The fluoropolymer film produced from a solution in which a solid fluoropolymer was dissolved was difficult to use as an electret material. Table 1Example 1Example 2Comparative Example 1Coating agentPreparation Example 2Preparation Example 4Reference Preparation Example 1Polymer molecular weight11250001950097533Film thickness (µm)213025Indentation hardness (N / mm 2< )480-380Indentation elastic modulus (GPa)4.1-2.6Initial surface potential (V)10241002971Potential retention rate (%)908885Bending testA-CCross-cut test-106End content-475100 Reference Signs List

[0223] 1. Electret material (fluoropolymer) 2. High-voltage DC power supply 3. Needle-shaped electrode 4. Earth electrode (copper plate)

Claims

1. An electret material comprising a fluoropolymer which - comprises ≥ 50 mol%, based on the total monomer units in the polymer, of monomer units of formula (1): wherein R1-R4 each independently are F, fluoroalkyl or fluoroalkoxy; and - has a mass average molecular weight, determined by GPC analysis according to the method defined in the description, of 1-5 million, or 5,000 to < 20,000.

2. The electret material of claim 1, wherein the fluoropolymer further comprises a fluoroolefin unit.

3. The electret material of claim 2, wherein the fluoroolefin unit is at least one of a fluorine-containing perhaloolefin unit, a vinylidene fluoride unit, a trifluoroethylene unit, a pentafluoropropylene unit, and a 1,1,1,2-tetrafluoro-2-propylene unit.

4. The electret material of claim 3, wherein the fluorine-containing perhaloolefin unit is at least one of a chlorotrifluoroethylene unit, a tetrafluoroethylene unit, a hexafluoropropylene unit, a perfluoro(methyl vinyl ether) unit, a perfluoro(ethyl vinyl ether) unit, a perfluoro(propyl vinyl ether) unit, a perfluoro(butyl vinyl ether) unit, and a perfluoro(2,2-dimethyl-1,3-dioxol) unit.

5. The electret material of claim 2, wherein the fluoroolefin unit is at least one of a chlorotrifluoroethylene unit, a tetrafluoroethylene unit, a hexafluoropropylene unit, a perfluoro(methyl vinyl ether) unit, and a perfluoro(propyl vinyl ether) unit.

6. The electret material of any of claims 1-5, which has an average thickness of ≥ 0.1 µm.

7. The electret material of any of claims 1-6, wherein the fluoropolymer has a mass average molecular weight of 1-5 million.

8. The electret material of any of claims 1-7, which has an indentation hardness, measured using an ENT-2100 ultra-fine hardness tester of Nanotec Corporation, of 450-1000 N / mm2.

9. The electret material of any of claims 1-8, which has an indentation elastic modulus, measured using an ENT-2100 ultra-fine hardness tester of Nanotec Corporation, of 3.5-10 GPa.

10. The electret material of any of claims 1-6, wherein the fluoropolymer has a mass average molecular weight of 5,000 to < 20,000.

11. The electret material of any of claims 1-10, wherein the fluoropolymer has a carbonate end group content, determined by the method defined in the description, of ≥ 150 per 106 carbon atoms of the monomer unit forming the fluoropolymer backbone.

12. An electrostatic induction conversion element comprising the electret material of any of claims 1-11, and preferably the electret material of any of claims 7-9 or of claim 10 or 11.

13. The use of a coating agent comprising a fluoropolymer and an aprotic solvent, wherein the fluoropolymer - comprises ≥ 50 mol%, based on the total monomer units in the polymer, of monomer units of formula (1): wherein R1-R4 each independently are F, fluoroalkyl or fluoroalkoxy, and - has a mass average molecular weight, determined by GPC analysis according to the method defined in the description, of 1-5 million, or 5,000 to < 20,000, for forming the electret material of any of claims 1-11.

14. The use of claim 13, wherein the content of the fluoropolymer in the coating agent is 20-65 mass% based on the total mass of the coating agent.

15. The use of claim 13 or 14, wherein the aprotic solvent in the coating agent is at least one solvent selected from perfluoroaromatic compounds, perfluorotrialkylamines, perfluoroalkanes, hydrofluorocarbons, perfluorocyclic ethers, hydrofluoroethers, and olefin compounds containing at least one chlorine atom, and preferably is at least one hydrofluoroether.