Fluororesin piezoelectric film and method for producing same

Abstract

To provide a fluororesin piezoelectric film having high transparency and a high compressive elastic modulus in the thickness direction.SOLUTION: The fluororesin film has a compression modulus Y33 in the thickness direction of 3. 0GPa or more and 4. 5GPa or less, a retardation of 100nm or more and 3000nm or less, an internal haze of less than 1.2%, and a piezoelectric coefficient d33 of 5. 0pC / N or more and 40. 0pC / N or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a fluorine-based resin piezoelectric film and a method for producing the same. [Background technology]

[0002] Display and input devices with a touch panel installed on the front of a display device such as a liquid crystal display are widely used, and are used in mobile devices such as mobile phones and operation panels for home appliances. Display and input devices using a touch panel allow users to operate the device by pressing the display on the screen. Therefore, the piezoelectric film used in touch panels must have high piezoelectricity to increase detection sensitivity and high transparency to allow accurate viewing of the display image.

[0003] As described in Patent Documents 1 and 2, piezoelectricity can be imparted to a fluorine-based resin film by polarization treatment.

[0004] Known methods for producing fluororesin films include a solution casting method using a solution in which a fluororesin is dissolved (Patent Document 1, etc.), and an extrusion molding method in which a fluororesin is thermally melted (Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2015 / 064324 [Patent Document 2] Japanese Patent Application Publication No. 05-102548 [Patent Document 3] International Publication No. 2022 / 091828 Summary of the Invention [Problem to be solved by the invention]

[0006] Piezoelectric films are produced from fluororesin films formed by solution casting by poling them without stretching them (Patent Document 1).On the other hand, piezoelectric films are produced from fluororesin films formed by hot melt extrusion by stretching the films and then poling them.

[0007] The solution casting method described in Patent Document 1 uses a large amount of polar organic solvent to dissolve the fluororesin, which requires solvent recovery, resulting in higher production costs and the need to consider the impact of the organic solvent on the working environment and the natural environment. Furthermore, if the polar organic solvent used to dissolve the fluororesin remains in the film, it can interfere with polarization of the piezoelectric film and impair the smoothness of the film surface as the solvent evaporates.

[0008] In contrast, the method of extrusion molding a fluororesin by thermal melting, as described in Patent Document 2, allows for the production of a fluororesin film without using an organic solvent, and is therefore less likely to cause deterioration of the working environment or the natural environment, or insufficient polarization or a decrease in the smoothness of the film surface due to a polar organic solvent remaining in the film.

[0009] To increase the detection sensitivity of a touch panel, the piezoelectric film must efficiently convert applied energy into electrical energy, which requires increasing the compressive modulus of the piezoelectric film in the thickness direction. To increase the compressive modulus, the film must be stretched to increase the degree of molecular chain orientation. However, the piezoelectric films in Patent Documents 1 and 2 are not stretched, and therefore a high compressive modulus cannot be expected.

[0010] Patent Document 3 discloses a fluororesin piezoelectric film that has been subjected to a stretching process and then a polarization process. However, the piezoelectric film disclosed in Patent Document 3 has high haze and does not disclose the compressive modulus of elasticity in the thickness direction.

[0011] According to the studies of the present inventors, fluororesin piezoelectric films produced at high stretch ratios tend to have improved compressive modulus in the thickness direction and higher conversion efficiency between electrical energy and mechanical energy. On the other hand, increasing the stretch ratio can lead to the risk of film breakage, and further, as the film crystallizes, the difference in refractive index between the crystalline and amorphous portions increases light scattering, which tends to reduce transparency. As such, it is difficult to achieve both a high compressive modulus and high transparency in a piezoelectric film. Therefore, when such a film is used in a touch panel, one of the transparency and high touch detection sensitivity is reduced.

[0012] The present invention has been made in view of the above problems, and an object of the present invention is to provide a fluorine-based resin piezoelectric film that is highly transparent and has a high compressive modulus of elasticity in the thickness direction, and a method for producing the same. [Means for solving the problem]

[0013] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [1] to [6]. [1] Compressive modulus Y in the thickness direction 33 is 3.0 GPa or more and 4.5 GPa or less, the retardation is 100 nm or more and 3000 nm or less, the internal haze is less than 1.2%, and the piezoelectric constant d 33 Fluorine-based resin piezoelectric film having a dielectric constant of 5.0 pC / N or more and 40.0 pC / N or less. [2] Electromechanical coupling coefficient k 33 The fluorine-based resin piezoelectric film according to [1], wherein the value of the ρ is 0.10 or more and 0.20 or less. [3] Measurement temperature: 260°C, shear rate: 50 s -1 The fluorine-based resin piezoelectric film according to [1] or [2], which is a fluorine-based resin piezoelectric film having a melt viscosity η measured by a method of 600 Pa·s or more and 4000 Pa·s or less. [4] The fluorine-based resin piezoelectric film according to any one of [1] to [3], which contains vinylidene fluoride resin as a main component. [5] The number of foreign particles whose size, which is the arithmetic mean value of the maximum and minimum widths when the film is viewed in a plane, is 100 μm or more is 7 / 0.25 m 2 The fluorine-based resin piezoelectric film according to any one of [1] to [4] below: [6] The fluorine-based resin piezoelectric film according to any one of [1] to [5], wherein the surface roughness in surface height Rz of the surface on the side where the Rz is smaller as measured in accordance with JIS B 0601:2001 is 0.50 μm or less.

[0014] One embodiment of the present invention for solving the above problems relates to the following methods for producing a piezoelectric film [7] to

[10] . [7] A method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [6], comprising the steps of: heating and melting a resin composition containing a fluorine-based resin; extruding the melted resin composition to form a film; stretching the formed film at a stretching temperature of more than 125°C and not more than 165°C at a stretching ratio of 4.1 times to 8.0 times; and polarizing the formed film at an applied voltage of 7.0 kV to 50.0 kV. [8] Measurement temperature: 260°C, shear rate: 50 s -1 The method for producing a fluorine-based resin piezoelectric film according to [7], wherein the melt viscosity η measured by the method is 600 Pa·s or more and 4000 Pa·s or less. [9] The method for producing a fluorine-based resin piezoelectric film according to [7] or [8], further comprising a step of filtering the resin composition melted in the melting step through a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

[10] The method for producing a fluorine-based resin piezoelectric film according to any one of [7] to [9], wherein in the film-forming step, the extruded resin composition is cooled by contacting it with a cooling roll having a surface temperature of 125°C or less. [Effects of the Invention]

[0015] According to the present invention, a fluororesin piezoelectric film having high transparency and a high compressive modulus of elasticity in the thickness direction, and a method for producing the same are provided. DETAILED DESCRIPTION OF THE INVENTION

[0016] [Fluoroplastic piezoelectric film] An embodiment of the present invention relates to a fluorine-based resin piezoelectric film.

[0017] The fluororesin piezoelectric film contains a fluororesin.

[0018] Fluorine-based resins can be homopolymers or copolymers obtained by polymerizing tetrafluoroethylene (TFE), vinylidene fluoride (VDF), etc. Examples of fluorine-based resins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, hexafluoropropylene (HFP), etc. Examples of fluorine-based resins obtained by polymerizing VDF include VDF homopolymers and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), TFE, tetrafluoropropene, HFP, perfluoroalkyl vinyl ether, etc.

[0019] Among these, from the viewpoint of facilitating polarization of the fluororesin film, VDF resin obtained by polymerizing VDF is preferred, and a VDF homopolymer, a copolymer of VDF and HFP, a copolymer of VDF and TrFE, a copolymer of VDF and TFE, and a copolymer of VDF, TrFE and TFE are more preferred, with a VDF homopolymer being even more preferred. These fluororesins may be used alone or in combination.

[0020] When the VDF resin is a copolymer, it is preferably a resin containing VDF as the main component, specifically a resin in which the proportion of VDF-derived structural units is 50% by mass or more. The proportion of VDF-derived structural units to the total mass of the VDF resin is more preferably 50% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and particularly preferably 90% by mass or more and 100% by mass or less.

[0021] The fluororesin piezoelectric film preferably contains a VDF resin, preferably a VDF homopolymer, as a main component. "Containing such a resin as a main component" means that the proportion of the resin containing VDF as a constituent unit relative to the total mass of the fluororesin piezoelectric film is 50% by mass or more. The content of such a resin relative to the total mass of the fluororesin piezoelectric film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0022] The content of resin whose constituent unit is VDF contained in the fluororesin film and the fluororesin piezoelectric film is as follows: 19 It can be measured by quantitative analysis using an internal standard using F-NMR.

[0023] The fluorine resin piezoelectric film was measured at a temperature of 260°C and a shear rate of 50 s -1 (Hereinafter, unless otherwise specified, the measurement temperature is 260°C, and the shear rate during measurement is 50 s -1The melt viscosity (measured by the method described above) is preferably 600 Pa·s or more and 4000 Pa·s or less. Because the piezoelectric film is produced by forming a film from a molten resin composition, the melt viscosity of the fluororesin piezoelectric film serves as an indicator of the melt viscosity of the resin composition containing the fluororesin (hereinafter, the resin composition containing the fluororesin is referred to as the "resin composition"). Resin compositions that produce piezoelectric films with a melt viscosity of 4000 Pa·s or less have a low melting temperature, resulting in minimal resin degradation due to heating and melting. Furthermore, the low melt viscosity of the resin allows for filtration, resulting in minimal speckled irregularities, resin decomposition products, and foreign matter on the film surface of the resulting piezoelectric film. Furthermore, the piezoelectric film can be stretched at a high ratio, resulting in a high compressive modulus. On the other hand, piezoelectric films with a melt viscosity of 600 Pa·s or more have long molecular chains that suppress crystal growth, thereby suppressing light scattering due to the difference in refractive index between the crystalline and amorphous regions caused by the enlargement of crystallites, resulting in low internal haze. The melt viscosity of the piezoelectric film is more preferably 600 Pa·s or more and 3500 Pa·s or less, further preferably 600 Pa·s or more and 2400 Pa·s or less, particularly preferably 600 Pa·s or more and 2000 Pa·s or less, and most preferably 600 Pa·s or more and 1500 Pa·s or less.

[0024] The melt viscosity is measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (Capillograph 1D, manufactured by Toyo Seiki Seisakusho Co., Ltd.) is used, and the melt viscosity is measured at a measurement temperature of 260°C and a shear rate of 50 s using a capillary die with an inner diameter of φ1 mm and a tube length of 10 mm. -1 The viscosity measured is taken as the viscosity.

[0025] The fluorine-based resin piezoelectric film has a compressive elastic modulus Y 33is 3.0 GPa or more and 4.5 GPa or less, preferably 3.2 GPa or more and 4.5 GPa or less, more preferably 3.4 GPa or more and 4.5 GPa or less, even more preferably 3.6 GPa or more and 4.5 GPa or less, particularly preferably 3.6 GPa or more and 4.3 GPa or less, very preferably 3.6 GPa or more and 4.2 GPa or less, and most preferably 3.8 GPa or more and 4.2 GPa or less. As will be described later, the compressive modulus Y in the thickness direction 33 The larger the electromechanical coupling coefficient k 33 Therefore, the compressive elastic modulus Y 33 The larger the value, the higher the efficiency of conversion between electrical and mechanical energy.

[0026] Compressive elastic modulus Y of fluororesin piezoelectric film in the thickness direction 33 is measured by the nanoindentation method in accordance with ISO 14577:2015 within the area including the intersection of the diagonal lines of the piezoelectric film, and the measured value is taken as the compressive modulus Y of the fluororesin piezoelectric film. 33 The representative value of the hardness is used. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set, and the area including the intersection of the diagonal lines of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the above method. Specifically, a dynamic ultra-micro hardness tester "DUH-211S" manufactured by Shimadzu Corporation is used. A drop of "Pen Correction Fluid Extra Fine" manufactured by Pentel Co., Ltd. is applied to the side opposite the polarized surface of the film, and the sample is fixed to a glass plate via the correction fluid. The polarized surface of the film is used as the measurement surface, and the compressive elastic modulus Y in the thickness direction is measured under the following conditions. 33 Measure. Indenter: Berkovich type, inter-edge angle: 115° Test mode: Load-unload test Test force: 20mN Minimum test force: 0.2 mN Load speed: 6.6620mN / s Load holding time: 2 seconds Unloading holding time: 1 second

[0027] The fluororesin piezoelectric film has a retardation of 100 nm to 3000 nm, preferably 500 nm to 2500 nm, and more preferably 700 nm to 2000 nm. The larger the retardation, the higher the degree of molecular orientation of the fluororesin film and the more sufficiently the proportion of the β phase, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through the polarization step.

[0028] The retardation is measured using a light source with a wavelength of 587.8 nm by the parallel Nicol rotation method on a 20 mm x 20 mm piece of film cut from an area including the intersection of the diagonals of a fluororesin piezoelectric film, and this value is used as the representative retardation value of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, an arbitrary rectangle with a side of 300 mm is assumed, and the retardation of a 20 mm x 20 mm piece of film cut from an area including the intersection of the diagonals of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, an arbitrary square with a side of 300 mm is cut on the piezoelectric film, and the measurement range is set according to the method described above.

[0029] The fluororesin piezoelectric film has an internal haze of less than 1.2%, preferably 0.1% to 1.1%, more preferably 0.1% to 1.0%, particularly preferably 0.1% to 0.8%, and most preferably 0.1% to 0.6%. The lower the internal haze, the more transparent the fluororesin piezoelectric film.

[0030] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 5.0%, and even more preferably 0.0% to 3.0%. The lower the haze, the more transparent the fluororesin piezoelectric film.

[0031] The internal haze of a fluororesin piezoelectric film is determined by forming coating layers on both sides of the film, removing external haze due to scratches, etc., and measuring the haze of the film in accordance with ISO 14782:2021. Specifically, a 50 mm x 50 mm rectangular film is cut out of the fluororesin piezoelectric film so that it includes the intersection of the diagonals. If the length of one side of the piezoelectric film is 300 mm or longer, a rectangle with a side of 300 mm is arbitrarily assumed, and the rectangular film is cut out from the area including the intersection of the diagonals of the rectangular film. If both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above, and the rectangular film is cut out. Next, a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) is applied to one surface (side A) of the rectangular film using a bar coater and dried at 80°C for 30 minutes. Then, an ultraviolet (UV) irradiation device (GS NIPPON DENCHI, CSOT040) was used to achieve a target cumulative light dose of 400 mJ / cm 2 The film is irradiated with UV light so that the thickness becomes 2 μm, forming a coating layer. A coating layer similar to that on side A is also formed on the other surface (side B) of the rectangular film with side A coated, to prepare a film for internal haze measurement in which external haze due to scratches on the film surface, etc., has been removed. Haze is measured in accordance with ISO 14782:2021 for an area including the intersection of the diagonal lines of the obtained measurement film using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.), and the value obtained is used as the representative value of the internal haze of the fluororesin piezoelectric film.

[0032] Fluorine resin piezoelectric film has a piezoelectric constant d 33 is 5.0 pC / N or more and 40.0 pC / N or less, preferably 8.0 pC / N or more and 40.0 pC / N or less, more preferably 10.0 pC / N or more and 35.0 pC / N or less, even more preferably 15.0 pC / N or more and 35.0 pC / N or less, and particularly preferably 20.0 pC / N or more and 30.0 pC / N or less.

[0033] The piezoelectric constant of the fluororesin piezoelectric film was measured by the direct quasi-static method (d 33Piezoelectric constant d by Mehta method, Berlincourt method 33 Measurements are made in accordance with ISO 19622:2018, the test method for piezoelectric constant measurement. Specifically, a piezoelectric constant measurement device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold the piezoelectric film at a measurement temperature of 25°C with a holding force of 1.0 N, and measure the charge generated when an alternating force of 0.15 N and a frequency of 110 Hz is applied. The piezoelectric constant d of the fluororesin piezoelectric film is 33 The measurement is carried out on the polarization surface of the fluorine-based resin piezoelectric film. The electric charge in the area including the intersection of the diagonal lines of the piezoelectric film is measured by the above method, and the piezoelectric constant d is calculated from the measured value. 33 The absolute value of the piezoelectric constant d 33 The representative value of the measurement range is set as follows. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set and the area including the intersection of the diagonal lines of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film and the measurement range is set according to the method described above.

[0034] The fluorine-based resin piezoelectric film has a dielectric constant ε of 0.90×10 -10 F / m or more 1.42×10 -10 F / m or less is preferable, and 0.90×10 -10 F / m or more 1.30×10 -10 F / m or less is more preferable, and 0.90×10 -10 F / m or more 1.26×10 -10 F / m or less is more preferable, and 0.90×10 -10 F / m or more 1.22×10 -10 F / m or less is particularly preferable.

[0035] The dielectric constant ε is measured in accordance with JIS C 2151:2019 using an LCR meter "IM3533" manufactured by Hioki E.E. Corporation. Electrode area: 3.0 cm 2The measurement is taken at a measurement voltage of 1 V and a measurement frequency of 1 kHz. The dielectric constant ε in the area including the intersection of the diagonal lines of the piezoelectric film is measured using the above method, and this is taken as the representative value of the dielectric constant of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set, and the area including the intersection of the diagonal lines of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the above method.

[0036] In addition, the electromechanical coupling coefficient k of the fluororesin piezoelectric film 33 is preferably 0.10 or more and 0.20 or less, more preferably 0.11 or more and 0.20 or less, further preferably 0.12 or more and 0.20 or less, and particularly preferably 0.13 or more and 0.20 or less. 33 The higher the value, the higher the efficiency of conversion between electrical and mechanical energy.

[0037] Electromechanical coupling coefficient k 33 is the piezoelectric constant d 33 , compressive modulus Y 33 and the dielectric constant ε, it can be calculated by the following formula:

[0038]

number

[0039] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles 100 μm or larger is 0 per 0.25 m 2 More than 7 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 5 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 3 pieces / 0.25m 2 The following is even more preferable: 0 pieces / 0.25m 2 More than 1 piece / 0.25m 2 The following are particularly preferred:

[0040] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles larger than 200 μm is 0 per 0.25 m 2 More than 3 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 2 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 1 piece / 0.25m 2 The following is even more preferred:

[0041] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles less than 100 μm in size is 0 / 0.25 m 2 More than 50 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 25 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 16 pieces / 0.25m 2 The following is even more preferred:

[0042] The fewer these foreign matters there are, the more transparent the fluororesin piezoelectric film will be, and the more uniform the stretching and polarization will be when the fluororesin piezoelectric film is stretched or polarized.

[0043] The number of these foreign particles is measured by cutting four rectangular films (observation pieces) adjacent to each other from the fluororesin piezoelectric film, and then calculating the sum of the number of foreign particles measured from each observation piece. Specifically, four rectangular films (observation pieces) adjacent to each other from the fluororesin film are cut out at 0.010 m 2 Four observation pieces each measuring 100 mm x 100 mm are cut out. The sum of the number of foreign particles measured from each observation piece is calculated, and the sum is multiplied by 25 / 4. The result is rounded to the nearest tenth and equals 0.25 m. 2 The number of foreign particles per unit area is measured. The foreign particles are observed using transmitted light, marked, and the marked areas are observed under a microscope to determine the size of the foreign particles. The size of the foreign particles is the arithmetic mean value of the maximum and minimum widths of the foreign particles.

[0044] The surface roughness Rz of the fluororesin piezoelectric film is preferably 0.50 μm or less, more preferably 0.05 μm or more and 0.50 μm or less, even more preferably 0.05 μm or more and 0.40 μm or less, and particularly preferably 0.05 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin piezoelectric film, the less likely it is that haze will occur on the surface of the film and the less likely the film will wrinkle.

[0045] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter conforming to JIS B 0601:2001 (Keyence Corporation, shape analysis laser microscope VK-X260) is used. Measurements are then made over an area including the intersection of the diagonals of the fluororesin piezoelectric film, and the resulting value is used as the representative value of the surface height roughness Rz of the fluororesin piezoelectric film. When the length of one side of the piezoelectric film is 300 mm or longer, a rectangle with a side of 300 mm is arbitrarily set, and measurements are made over an area including the intersection of the diagonals of the rectangular film. When both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above. The surface of the fluororesin piezoelectric film that comes into contact with the chill roll tends to have a smaller surface height roughness Rz than the surface that does not come into contact with the chill roll. This is because the fluororesin piezoelectric film becomes less uneven on the surface when pressed against the cooling roll. This tendency remains the same even after stretching and polarization. Here, the measurement results for the surface with the smallest surface height roughness Rz (the surface in contact with the cooling roll) are taken as the surface height roughness Rz.

[0046] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm to 200 μm, more preferably 20 μm to 80 μm, and even more preferably 30 μm to 80 μm. A thicker film is more advantageous in terms of electrical properties such as insulation and piezoelectric properties. A thinner film is more advantageous in terms of optical properties such as transparency and cost.

[0047] The thickness of a fluororesin piezoelectric film is generally measured using a micrometer (JIS C 2151:2019), but measurements can also be taken of the area including the intersection of the diagonals of the fluororesin piezoelectric film using known methods such as a laser displacement meter, a capacitance displacement meter, or an infrared method, and the resulting value is used as the representative thickness of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or longer, a rectangle with a side of 300 mm is arbitrarily set and the area including the intersection of the diagonals of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm is arbitrarily set on the piezoelectric film and the measurement area is determined according to the method described above.

[0048] The fluororesin piezoelectric film may contain resins other than fluororesin or other additives as long as it satisfies the above physical properties.

[0049] [Method of manufacturing fluororesin piezoelectric film] The method for producing the above-mentioned fluororesin piezoelectric film is not particularly limited, but preferably includes the steps of: a step of heating and melting a resin composition containing a fluororesin (hereinafter, a resin composition containing a fluororesin may be referred to as a "resin composition") (melting step); a step of forming the heat-melted resin composition into a film by extrusion molding to produce a fluororesin film (film formation step); a step of stretching the formed fluororesin film (stretching step); and a step of polarizing the formed fluororesin film (polarization step).

[0050] At this time, a step of filtering the molten resin composition (filtration step) may be carried out as needed.

[0051] (Melting process) In the melting step, the resin composition containing the fluororesin is heated and melted. This step can be carried out, for example, by melt-kneading the resin composition using an extruder.

[0052] The resin composition melted in the melting step may be any resin composition containing the above-mentioned fluororesin. If the resin composition contains a solvent component, the remaining solvent component may interfere with polarization in a subsequent step. Therefore, the content of the solvent component in the resin composition is preferably low, and the content of the solvent component relative to the total mass of the resin composition is preferably 1% by mass or less, more preferably 0.1% by mass or less. In particular, the content of the polar solvent component relative to the total mass of the resin composition is preferably 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less.

[0053] Furthermore, the resin composition melted during the melting process preferably has a melt viscosity of 600 Pa·s or more and 4000 Pa·s or less. A resin composition with a melt viscosity of 4000 Pa·s or less facilitates filtration of the molten resin composition and reduces the dissolution temperature of the resin composition for film formation, thereby suppressing resin degradation due to melt heating. Furthermore, the resin's molecular chains are less entangled, allowing the film to be stretched at a high ratio without breaking, thereby increasing the film's compressive modulus. On the other hand, resin compositions with low melt viscosity have a low degree of polymerization, which facilitates free movement of molecular chains and facilitates selective alignment of molecular chains, making them more prone to crystallization during the production of fluororesin films. As crystallization progresses, the crystallites become larger, and the difference in refractive index between the crystalline and amorphous portions causes light scattering, which tends to reduce the transparency of the film (increasing internal haze). Using a resin composition with a melt viscosity of 600 Pa·s or more for the piezoelectric film reduces internal haze due to the progression of crystallization. The melt viscosity of the piezoelectric film is more preferably 600 Pa·s to 3500 Pa·s, even more preferably 600 Pa·s to 2400 Pa·s, particularly preferably 600 Pa·s to 2000 Pa·s, and most preferably 600 Pa·s to 1500 Pa·s.

[0054] The melting temperature of the resin composition is preferably 75°C or more higher than the melting point of the resin composition but not higher than 105°C, more preferably 75°C or more higher but not higher than 100°C, even more preferably 80°C or more higher but not higher than 100°C, and particularly preferably 85°C or more higher but not higher than 95°C. By setting the melting temperature at 75°C or more higher than the melting point of the resin composition, the viscosity of the resin composition can be reduced to a level that allows filtration in the subsequent process. By setting the melting temperature at 105°C or less higher than the melting point of the resin composition, decomposition and condensation of the resin composition due to heating can be suppressed, and the resulting generation of decomposition products can be suppressed. Suppressing the generation of decomposition products can reduce the amount of foreign matter in the fluororesin piezoelectric film and improve the transparency and smoothness of the fluororesin piezoelectric film. Furthermore, suppressing the generation of decomposition products can suppress filter clogging and improve the filtration efficiency of the resin composition. The melting point of the resin composition is a value measured in accordance with JIS K 7121: 1987. Specifically, 5 mg of a measurement sample is sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation), and the temperature is raised from room temperature to 230°C at a heating rate of 10°C / min in a nitrogen atmosphere to measure a DSC curve, and the maximum melting peak temperature is taken as the melting point of the resin composition.

[0055] According to the findings of the present inventors, high melt viscosity resin compositions must be heated to a high temperature to achieve a filterable viscosity. However, heating to high temperatures can easily cause decomposition, condensation, and other reactions, resulting in filter clogging. Therefore, it has been difficult to melt and filter fluororesin-containing resin compositions. In response, the present inventors discovered that controlling the melting temperature of a resin composition having an appropriate melt viscosity within the above range allows the resin composition to be filtered with high filtration efficiency. By adjusting the melting temperature of the resin composition within the above range and filtering the resin composition with a reduced viscosity through melting, the resin composition can be filtered without using a polar solvent. Furthermore, the absence of a solvent reduces the likelihood of polarization inhibition due to residual polar solvent in the fluororesin film during polarization treatment. Furthermore, since polar solvents are not generally used in the film formation process, it is possible to reduce the burden on the working environment and the natural environment, as well as the manufacturing costs associated with recovering the polar solvent.

[0056] (filtration process) In the filtration step, the resin composition that has been melted and reduced in viscosity in the melting step is filtered. The filtration method is not particularly limited, and the molten resin composition may be passed through a filter, and known filter types such as pleated filters and leaf disc filters may be used.

[0057] In the filtration step, the resin composition is preferably filtered through a filter with a filtration accuracy of 10 μm or more and 40 μm or less. Using a filter with a filtration accuracy of 10 μm or more facilitates filtration of the resin composition that has been heated to the above temperature and melted, and also prevents the filtration pressure from becoming too high, allowing for a shorter filtration time. Using a filter with a filtration accuracy of 40 μm or less allows for sufficient removal of foreign matter from the resin composition, resulting in a fluororesin film with little foreign matter. The filtration accuracy of the filter is preferably 10 μm or more and 30 μm or less, and more preferably 15 μm or more and 30 μm or less.

[0058] The resin composition is filtered using a multilayer filter consisting of multiple layers with different shapes, mesh sizes, etc. The filtration accuracy of the filter used to filter the resin composition refers to the filtration efficiency of the filter, i.e., the filter's ability to filter out particles of a certain size with a predetermined filtration efficiency. For example, in this specification, a filtration accuracy of 10 μm means that the filter can filter out particles of 10 μm or larger with a filtration efficiency of 95% or higher.

[0059] In this step, the resin composition may be passed through multiple filters. For example, a filter with low filtration accuracy (large filtration accuracy value) may be used in the first stage to remove coarse foreign matter, and then a filter with high filtration accuracy (small filtration accuracy value) may be used in the second stage to remove finer foreign matter. In this case, the filtration accuracy is the value of the filter with the highest filtration accuracy.

[0060] The filter may be disposed between the extruder used in the film-forming step and the die. Alternatively, the filter may be disposed in an extruder or melt-kneading apparatus different from the extruder used in the film-forming step, and the resin composition filtered through the filter may be fed into the extruder used in the melting step to form a film.

[0061] (Film forming process) In the film-forming step, the resin composition filtered in the filtration step is formed into a film.

[0062] The film-forming method is not particularly limited, and a known method can be used, such as extruding a molten and filtered resin composition through a T-die and cooling it by contacting it with a cooling roll.

[0063] Furthermore, as crystallization progresses during the production of a fluororesin film, the crystallites become larger, and the difference in refractive index between the crystalline and amorphous parts causes light scattering, which tends to deteriorate the transparency of the film. In contrast, by forming a molten resin into a film and then rapidly cooling it before crystallization progresses, the crystallization of the fluororesin is suppressed, reducing the scattering of light that occurs between the crystalline and amorphous parts, and thereby improving the transparency of the resulting fluororesin film.

[0064] The surface temperature of the cooling roll is preferably 125°C or lower. Rapid cooling of the resin composition film extruded using a cooling roll with a low surface temperature can suppress crystal growth and increase the transparency of the resulting fluororesin film. The surface temperature of the cooling roll is preferably 5°C to 115°C, more preferably 10°C to 100°C, even more preferably 20°C to 90°C, particularly preferably 20°C to 80°C, very preferably 30°C to 70°C, and most preferably 30°C to 60°C.

[0065] The fluorine-based resin film thus obtained may be stored after being wound up, or may be transported to a subsequent process such as a stretching process or a polarization process.

[0066] (Stretching process) In the stretching step, the formed fluororesin film is stretched. In the stretching step, the fluororesin film produced in the film formation step may be stretched as is, or the fluororesin film may be stretched after being wound up and stored. In addition, the fluororesin film may be heated and stretched as necessary.

[0067] The fluororesin film can be stretched in the machine direction (MD) and, if necessary, in the direction perpendicular to the machine direction (TD) while being transported by multiple rolls. In this embodiment, the stretching ratio (ratio in the MD direction) is 4.1 to 8.0 times. The stretching ratio is preferably 4.3 to 8.0 times, more preferably 4.3 to 7.7 times, even more preferably 4.5 to 7.7 times, particularly preferably 5.0 to 7.7 times, and most preferably 5.5 to 7.5 times.

[0068] The higher the stretching ratio, the higher the proportion of the β phase, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through the polarization process. By setting the stretching ratio to 8.0 times or less, the fluororesin film is less likely to break during the stretching process.

[0069] In addition, by stretching the formed film at a high magnification, the lamellae that make up the crystals can be elongated and at the same time the orientation of the molecular chains that were insufficiently oriented can be promoted. Therefore, by stretching the film at a high magnification, the film can be made rigid, and the compressive elastic modulus Y in the thickness direction of the fluorine-based resin piezoelectric film can be increased. 33 can be made larger.

[0070] Furthermore, the stretching temperature of the fluororesin film can be controlled by controlling the surface temperature of the stretching roll during the stretching process. Here, the surface temperature of the stretching roll is referred to as the stretching temperature. The surface temperature of the stretching roll is preferably 125°C or higher and 165°C or lower, more preferably 125°C or higher and 160°C or lower, even more preferably 125°C or higher and 150°C or lower, and particularly preferably 130°C or higher and 150°C or lower. By maintaining the surface temperature of the roll at 125°C or higher during stretching, the temperature inside the film is increased during stretching, facilitating deformation in the stretching direction, thereby preventing film breakage and enabling stretching at a high magnification. Furthermore, by stretching while heating, the film is uniformly stretched, suppressing localized crystallization, and uniformly aligning molecular chains throughout the film, thereby suppressing light scattering due to differences in crystallinity within the film and reducing internal haze. By keeping the surface temperature of the rolls below 165°C during stretching, the efficiency of molecular chain alignment due to stretching of the film is improved, the proportion of the beta phase structure that contributes to the expression of piezoelectricity is increased, and deformation of the film due to melting of the fluororesin can be suppressed.

[0071] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. Fluororesins containing vinylidene fluoride homopolymers or copolymers undergo a transition from α phase to β phase during the stretching process, increasing the proportion of the β phase. By applying a DC voltage to a fluororesin film with an increased proportion of the polar β phase, a fluororesin piezoelectric film with high piezoelectricity can be obtained.

[0072] The applied DC voltage is preferably 7.0 kV or more and 50.0 kV or less, more preferably 7.5 kV or more and 30.0 kV or less, even more preferably 8.0 kV or more and 30.0 kV or less, and particularly preferably 8.5 kV or more and 30.0 kV or less.

[0073] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0074] The fluororesin film after the film-forming step or the fluororesin piezoelectric film after the polarization step can be wound into a roll for storage, transportation, and the like.

[0075] [Application] The above-mentioned fluorine-based resin piezoelectric film can be used in various applications such as touch sensors and touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0076] [Other embodiments] It should be noted that the above-described embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept. [Example]

[0077] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0078] 1. Preparation of Fluoropolymer Piezoelectric Film Films 1 to 12 and Film 15, all of which were fluorine-based resin piezoelectric films, were produced by the following procedure.

[0079] The melt viscosity of the fluororesin used as the material was measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (Capillograph 1D, manufactured by Toyo Seiki Seisakusho Co., Ltd.) was used, and the melt viscosity was measured at a temperature of 260 °C and a shear rate of 50 s using a capillary die with an inner diameter of φ1 mm and a tube length of 10 mm. -1 The viscosity was measured at 1000 kJ / min.

[0080] The melting point of the resin composition was measured in accordance with JIS K 7121: 1987. Specifically, 5 mg of a measurement sample was sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation), and the temperature was raised from room temperature to 230°C at a heating rate of 10°C / min in a nitrogen atmosphere to measure a DSC curve. The maximum melting peak temperature in the DSC curve was determined as the melting point of the resin composition.

[0081] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0082] 1-1.Film 1 Vinylidene fluoride homopolymer (PVDF) with a melt viscosity of 800 Pa·s and a melting point of 173°C was melted at a melting temperature of 260°C in a 50mm diameter single-screw extruder and then filtered through a sintered metal nonwoven filter element (Fuji Metal Fiber, manufactured by Fuji Filter Industrial Co., Ltd.) with a filtration accuracy of 20μm. The filtered resin was extruded into a film from a T-die and cooled by contacting it with the surface of a cooling roll heated to 50°C, yielding an unstretched film with a thickness of 160μm.

[0083] The unstretched film was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and stretched 5.6 times in the machine direction (MD) at 130°C by adjusting the surface temperature and rotation speed ratio of each roll. Furthermore, a voltage of 9.1 kV was applied from the surface of the film to the thickness direction to obtain Film 1.

[0084] 1-2.Film 2 Film 2 was obtained in the same manner as Film 1, except that the surface temperature of the cooling roll was set to 70° C. and the stretching ratio was set to 5.9 times.

[0085] 1-3.Film 3 Film 3 was obtained in the same manner as Film 1, except that a 190 μm thick unstretched film was stretched at a stretching temperature of 140° C. and a stretching ratio of 7.5, and then polarized at an applied voltage of 12.8 kV.

[0086] 1-4.Film 4 Film 4 was obtained in the same manner as film 2, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used, the stretching ratio was 5.3 times, and the applied voltage was 7.4 kV.

[0087] 1-5.Film 5 Film 5 was obtained in the same manner as in the production of Film 4, except that the stretching temperature was 150° C., the stretching ratio was 4.9 times, and the applied voltage was 9.8 kV.

[0088] 1-6.Film 6 Film 6 was obtained in the same manner as Film 4, except that the surface temperature of the cooling roll was 110° C., the draw ratio was 5.1 times, and the applied voltage was 7.8 kV.

[0089] 1-7.Film 7 Film 7 was obtained in the same manner as in the production of Film 6, except that the stretching ratio was 4.3 times and the applied voltage was 7.2 kV.

[0090] 1-8.Film 8 Film 8 was obtained in the same manner as film 4, except that the filtration step was not performed, the surface temperature of the cooling roll was 130°C, the stretching temperature was 110°C, the stretching ratio was 4.2 times, and the applied voltage was 7.5 kV.

[0091] 1-9.Film 9 Film 9 was obtained in the same manner as Film 1, except that the surface temperature of the cooling roll was set to 110° C. and the stretching ratio was set to 3.5 times.

[0092] 1-10.Film 10 Film 10 was obtained in the same manner as in the production of Film 6, except that the stretching ratio was 3.5 times and the applied voltage was 9.0 kV.

[0093] 1-11.Film 11 Film 11 was obtained in the same manner as Film 6, except that the thickness of the unstretched film was 190 μm, the stretching temperature was 120° C., the stretching ratio was 4.8 times, and the applied voltage was 7.4 kV.

[0094] 1-12.Film 12 Film 12 was obtained in the same manner as Film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used, the melting temperature was 280°C, no filtration process was performed, the surface temperature of the cooling roll was 130°C, the draw ratio was 4.0 times, and the applied voltage was 7.5 kV.

[0095] 1-13. Film 13 (unable to be produced) An attempt was made to produce Film 13 in the same manner as Film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used; however, the resin clogged the filter, and no film could be obtained.

[0096] 1-14. Film 14 (unable to produce) An attempt was made to produce Film 14 in the same manner as Film 4, except that a polymer filter with a filtration accuracy of 5 μm was used, but the filter became clogged with resin, and no film could be obtained.

[0097] 1-15.Film 15 100 g of PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was weighed and added to 900 ml of n-methylpyrrolidone (NMP). The mixture was heated to 60°C while stirring with a stirrer and continued stirring for 6 hours to produce a resin solution. This resin solution was filtered through a sintered metal nonwoven filter element (Fuji Metal Fiber, manufactured by Fuji Filter Industrial Co., Ltd.) with a filtration accuracy of 40 μm. The filtered resin solution was then placed in an automatic coating machine to produce a 600 μm thick coating film. The resulting film was dried at 120°C for 1 hour, and a voltage of 9.1 kV was applied across the thickness of the film to produce Film 15.

[0098] 2. Evaluation of fluororesin piezoelectric film Unless otherwise specified, the obtained films 1 to 12 and 15 were each cut into a square film with a side of 300 mm, and the compressive elastic modulus Y in the thickness direction was measured using the following method for the area including the intersection of the diagonal lines of the square film. 33 , dielectric constant ε, piezoelectric constant d 33 , electromechanical coupling coefficient k 33 The retardation, internal haze, surface height roughness Rz, number of foreign particles, and thickness were measured and used as representative values.

[0099] 2-1. Compressive elastic modulus Y in the thickness direction 33 The compressive elastic modulus Y of the film in the thickness direction 33 was measured by the nanoindentation method in accordance with ISO 14577:2015. Specifically, a dynamic ultra-micro hardness tester "DUH-211S" manufactured by Shimadzu Corporation was used. A drop of "Pen Correction Fluid Extra Fine" manufactured by Pentel Co., Ltd. was applied to the side opposite the polarized surface of the measurement film, and the side opposite the polarized surface of the square film was fixed to a glass plate via the correction fluid. The polarized surface was used as the measurement surface, and the compressive elastic modulus Y in the thickness direction was measured under the following conditions. 33 was measured. Indenter: Berkovich type, inter-edge angle: 115° Test mode: Load-unload test Test force: 20mN Minimum test force: 0.2 mN Load speed: 6.6620mN / sec Load holding time: 2 seconds Unloading holding time: 1 second

[0100] 2-2.Dielectric constant ε The dielectric constant ε of each square film was measured in accordance with JIS C 2151:2019 using an LCR meter "IM3533" manufactured by Hioki E.E. Corporation. Electrode area: 3.0 cm 2 The measurements were performed at an applied voltage of 1 V and a measurement frequency of 1 kHz.

[0101] 2-3.Piezoelectric constant d 33 Piezoelectric constant d 33 is the direct quasi-static method (d 33 Piezoelectric constant d of piezoelectric ceramics measured by Meter method and Berlincourt method 33 The measurement was performed in accordance with the test method ISO 19622:2018. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) was used to hold the test film as a test piece at a measurement temperature of 25°C with a holding force of 1.0 N, and an alternating force of 0.15 N and a frequency of 110 Hz was applied, and the generated charge was measured on the polarization surface, and the piezoelectric constant d was calculated from the value. 33 The absolute value of the piezoelectric constant d 33 was used as a representative value.

[0102] 2-4. Electromechanical coupling coefficient k 33 Piezoelectric constant d 33 , compressive modulus Y 33 , and the dielectric constant ε, the electromechanical coupling coefficient k of the fluorine-based resin piezoelectric film is calculated using the following formula: 33 was calculated.

[0103]

number

[0104] 2-5.Retardation The retardation of a 20mm x 20mm film cut out from the area including the intersection of the diagonal lines of the square film was measured using a KOBRA-HB made by Oji Scientific Instruments by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8nm was taken as the retardation of the film.

[0105] 2-6. Internal haze A rectangular film was cut to 50 mm x 50 mm so as to include the intersection of the diagonals of the square film. A hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) was applied to one surface (side A) of the film using a bar coater and dried at 80°C for 30 minutes. Then, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to apply a target cumulative light dose of 400 mJ / cm. 2 The film was irradiated with UV light so that the thickness was 2 μm, forming a coating layer. A coating layer similar to that on side A was also formed on the other surface (side B) of the rectangular film with side A coated. Coating layers were formed on both surfaces of the film using the method described above, and external haze due to scratches on the film surface, etc. was removed. The internal haze of the fluororesin piezoelectric film was measured in accordance with ISO 14782:2021 using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) for a range including the intersection of the diagonals of the film for internal haze measurement, and the result was used as a representative value.

[0106] 2-7. Hayes The haze was measured using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) In accordance with ISO 14782:2021, the haze was measured in an area including the intersection of the diagonal lines of the rectangular film before the coating layer was formed, and the measured value was used as the representative haze value of the fluororesin piezoelectric film.

[0107] 2-8. Number of foreign objects 0.010m so that each film is cut from adjacent positions in succession 2Four rectangular films (observation pieces) measuring 100 mm x 100 mm were cut out from the fluorine-based resin piezoelectric film. The sum of the number of foreign particles measured by observing each observation piece was multiplied by 25 / 4, and the result was rounded to the first decimal place to obtain a value of 0.25 m. 2 The number of foreign particles per unit area was calculated. The foreign particles were observed using transmitted light, marked, and the marked areas were then observed under a microscope to determine the size of the foreign particles. The size of the foreign particles was calculated as the arithmetic mean value of the maximum and minimum widths of the foreign particles. In this way, the number of foreign particles larger than 200 μm, foreign particles between 100 μm and 200 μm, and foreign particles smaller than 100 μm were calculated.

[0108] 2-9. Surface height roughness Rz The surface height roughness Rz of the measurement film was measured using a surface roughness meter (Keyence Corporation, shape analysis laser microscope VK-X260) conforming to JIS B 0601:2001. The surface height roughness Rz was measured over an area including the intersection of the diagonal lines of the fluororesin piezoelectric film, and this value was used as the representative value of the surface height roughness Rz of the film. The Rz measurement was performed on the surface of the fluororesin piezoelectric film that had come into contact with the cooling roll.

[0109] 2-10.Thickness A digital linear gauge (DG525H, manufactured by Ono Sokki Co., Ltd.) and a gauge stand (SH-022, manufactured by Ono Sokki Co., Ltd.) were used. The thickness was measured in the area including the intersection of the diagonal lines of the fluororesin piezoelectric film, and this value was used as the representative value of the film thickness.

[0110] 3.Results The production conditions and evaluation results for each film are shown in Tables 1 to 3. Film 15 was very opaque due to the haze, and it was not possible to measure the number of foreign particles.

[0111] [Table 1]

[0112] [Table 2]

[0113] [Table 3] [Industrial Applicability]

[0114] The fluororesin piezoelectric film according to the present invention is useful as a piezoelectric film having high transparency and high sensitivity.

Claims

1. Compressive elastic modulus Y in the thickness direction ３３ is 3.0 GPa or more and 4.5 GPa or less, The retardation is 100 nm or more and 3000 nm or less, The internal haze is less than 1.2%; Piezoelectric constant d ３３ is 5.0 pC / N or more and 40.0 pC / N or less, Fluorine-based resin piezoelectric film.

2. Electromechanical coupling coefficient k ３３ is 0.10 or more and 0.20 or less, The fluorine-based resin piezoelectric film according to claim 1 .

3. Measurement temperature: 260°C, shear rate: 50 s －１ The melt viscosity η measured by is 600 Pa s or more and 4000 Pa s or less, Fluorine-based resin piezoelectric film according to claim 1.

4. The main component is vinylidene fluoride resin. The fluorine-based resin piezoelectric film according to claim 1 .

5. The number of foreign particles having a size of 100 μm or more, which is the arithmetic mean value of the maximum and minimum widths when the film is viewed in plan, is 7 / 0.25 m ２ Below is the The fluorine-based resin piezoelectric film according to claim 1 .

6. The surface height roughness Rz of the surface having a smaller Rz measured in accordance with JIS B 0601:2001 is 0.50 μm or less. The fluorine-based resin piezoelectric film according to claim 1 .

7. a step of heating and melting a resin composition containing a fluorine-based resin; a step of extruding the molten resin composition to form a film; stretching the formed film at a stretching temperature of 125°C or higher and 165°C or lower and a stretching ratio of 4.1 times or higher and 8.0 times or lower; Polarizing the deposited film; The method for producing a fluorine-based resin piezoelectric film according to any one of claims 1 to 6, comprising:

8. The resin composition was measured at a temperature of 260°C and a shear rate of 50 s －１ 8. The method for producing a fluorine-based resin piezoelectric film according to claim 7, wherein the melt viscosity η measured by HPLC is 600 Pa·s or more and 4000 Pa·s or less.

9. The resin composition melted in the melting step is filtered through a filter having a filtration accuracy of 10 μm or more and 40 μm or less. The method for producing the fluorine-based resin piezoelectric film according to claim 7 .

10. In the film-forming step, the extruded resin composition is cooled by contacting it with a cooling roll having a surface temperature of 125°C or less. The method for producing the fluorine-based resin piezoelectric film according to claim 7 .

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