Fluororesin piezoelectric film and method for producing same

Abstract

Provided is a fluororesin piezoelectric film having high transparency and a high compressive modulus in the direction of thickness. This fluororesin film has: a compressive modulus Y33 in the direction of thickness of 3.0 to 4.5 GPa; a retardation of 100 to 3,000 nm; an internal haze of less than 1.2%; and a piezoelectric constant d33 of 5.0 to 40.0 pC / N.

Description

Fluorine-based resin piezoelectric film and its manufacturing method

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

[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 fluororesin film by polarization treatment. Known methods for producing a fluororesin film 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).

[0004] International Publication No. 2015 / 064324 Japanese Patent Application Laid-Open No. 05-102548 International Publication No. 2022 / 091828

[0005] Piezoelectric films are produced from fluororesin films formed by solution casting by poling without stretching, whereas piezoelectric films are produced from fluororesin films formed by hot melt extrusion by stretching the film and then poling it.

[0006] In the solution casting method described in Patent Document 1, a large amount of polar organic solvent is used to dissolve the fluororesin, which requires the recovery of the solvent, which not only increases production costs but also requires consideration of 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 hinder polarization of the piezoelectric film and impair the smoothness of the film surface as the solvent evaporates.

[0007] 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 therefore is 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.

[0008] 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.

[0009] Patent Document 3 discloses a fluororesin piezoelectric film that has been subjected to a stretching treatment and then a polarization treatment. However, the piezoelectric film disclosed in Patent Document 3 has high haze and does not disclose the compressive modulus in the thickness direction.

[0010] 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.

[0011] 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.

[0012] 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 ３３is 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 [2] A fluorine-based resin piezoelectric film having an electromechanical coupling coefficient k of 5.0 pC / N or more and 40.0 pC / N or less. ３３ [3] The fluorine-based resin piezoelectric film according to [1], wherein the value of the shear rate during measurement is 0.10 or more and 0.20 or less. －１ [4] The fluororesin 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 having a size of 100 μm or more, which is the arithmetic mean of the maximum and minimum widths when viewed from above, of the film is 7 / 0.25 m. ２ [6] The fluororesin 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.

[0013] One embodiment of the present invention for solving the above problems relates to a method for producing a piezoelectric film according to the following items [7] to

[13] . [7] A method for producing a fluororesin piezoelectric film, comprising the steps of: heating and melting a resin composition containing a fluororesin; extruding the melted resin composition to form a film; stretching the resin composition at a stretching temperature of 125°C to 165°C and at a stretching ratio of 4.1 to 8.0; and polarizing the film at an applied voltage of 7.0 kV to 50.0 kV. [8] A method for producing a fluororesin piezoelectric film, comprising the steps of: heating and melting a resin composition containing a fluororesin; extruding the resin composition to form a film; stretching the resin composition at a stretching temperature of 125°C to 165°C and at a stretching ratio of 4.1 to 8.0; and polarizing the film at an applied voltage of 7.0 kV to 50.0 kV. －１[9] The method for producing a fluororesin piezoelectric film according to [7], wherein the resin composition melted in the melting step has a melt viscosity η of 600 Pa·s or more and 4000 Pa·s or less, as measured by a method using a filter.

[10] The method for producing a fluororesin piezoelectric film according to [7] or [8], wherein the resin composition melted in the melting step is filtered through a filter with a filtration accuracy of 10 μm or more and 40 μm or less.

[11] The method for producing a fluororesin piezoelectric film according to any one of [7] to

[10] , wherein the resin composition is mainly composed of vinylidene fluoride resin.

[12] The method for producing a fluororesin piezoelectric film according to any one of [7] to

[10] , wherein the resin composition has a 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%, and the piezoelectric constant d ３３

[13] The method for producing a fluorine-based resin piezoelectric film according to any one of [7] to

[11] , wherein the film has a surface roughness of 7.0 pC / N or more and 40.0 pC / N or less. ... ２ The method for producing a fluorine-based resin piezoelectric film according to any one of [7] to

[12] below.

[0014] 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.

[0015] [Fluorocarbon Resin Piezoelectric Film] One embodiment of the present invention relates to a fluorocarbon resin piezoelectric film.

[0016] The fluororesin piezoelectric film contains a fluororesin.

[0017] The fluororesin can be a homopolymer or copolymer obtained by polymerizing tetrafluoroethylene (TFE), vinylidene fluoride (VDF), or the like. Examples of fluororesins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, hexafluoropropylene (HFP), or the like. Examples of fluororesins obtained by polymerizing VDF include homopolymers of VDF, and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), TFE, tetrafluoropropene, HFP, perfluoroalkyl vinyl ether, or the like.

[0018] 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.

[0019] When the VDF resin is a copolymer, it is preferably a resin containing VDF as the main component, specifically a resin containing 50% by mass or more of VDF-derived structural units. The proportion of the VDF-derived structural units relative to the total mass of the VDF resin 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 particularly preferably 90% by mass or more and 100% by mass or less.

[0020] 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 content of the resin having 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.

[0021] The content of the resin having VDF as a constituent unit contained in the fluororesin film and the fluororesin piezoelectric film is １９ It can be measured by quantitative analysis using an internal standard using F-NMR.

[0022] The fluorine-based 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 piezoelectric film serves as an indicator of the melt viscosity of the resin composition containing a fluororesin (a resin composition containing a fluororesin is referred to as a "resin composition"). Resin compositions that produce piezoelectric films with a melt viscosity of 4000 Pa·s or less have low melting temperatures, resulting in little resin deterioration. Furthermore, because melt filtration is possible, the resulting film has fewer speckled irregularities, resin decomposition products, and foreign matter on its surface. Furthermore, piezoelectric films produced from such resin compositions can be stretched at a high ratio, resulting in a high compressive modulus. Meanwhile, resin compositions with a melt viscosity of 600 Pa·s or more can suppress crystal growth during film formation, thereby reducing internal haze due to light scattering caused by the difference in refractive index between the crystalline and amorphous regions. The melt viscosity of the piezoelectric film is more preferably 600 Pa·s or more and 4000 Pa·s or less, even more preferably 600 Pa·s or more and 3500 Pa·s or less, still more preferably 600 Pa·s or more and 2400 Pa·s or less, still more preferably 600 Pa·s or more and 2000 Pa·s or less, and particularly preferably 600 Pa·s or more and 1500 Pa·s or less.

[0023] 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 －１ The viscosity measured is taken as the viscosity.

[0024] The fluorine-based resin piezoelectric film has a compressive elastic modulus Y in the thickness direction. ３３ is 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, 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 elastic modulus Y in the thickness direction ３３The larger the electromechanical coupling coefficient k ３３ Therefore, the compressive elastic modulus Y in the thickness direction ３３ The larger the value, the higher the efficiency of conversion between electrical and mechanical energy.

[0025] Compressive elastic modulus Y of fluorine-based resin piezoelectric film in the thickness direction ３３ is measured in an area including the intersection of the diagonal lines of the piezoelectric film by a nanoindentation method in accordance with ISO 14577:2015, 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. When 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 range including the intersection of the diagonal lines of the rectangular film is measured. When 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 to 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 Indenter: Berkovich type, edge angle: 115° Test mode: load-unload test Test force: 20 mN Minimum test force: 0.2 mN Load rate: 6.6620 mN / s Load holding time: 2 seconds Unload holding time: 1 second

[0026] 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 by the polarization step.

[0027] The retardation is measured by the parallel Nicol rotation method using a light source with a wavelength of 587.8 nm on a 20 mm x 20 mm piece of film cut from a region including the intersection of the diagonals of the fluororesin piezoelectric film, and this value is used as the representative value of the retardation of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, an arbitrary rectangle with sides of 300 mm is assumed, and the retardation of a 20 mm x 20 mm piece of film cut from a region 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 sides of 300 mm is cut on the piezoelectric film, and the measurement region is set according to the method described above.

[0028] 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%, even more preferably 0.1% to 0.8%, particularly preferably 0.1% to 0.6%, and very preferably 0.1% to 0.4%. The lower the internal haze, the more improved the transparency of the fluororesin piezoelectric film.

[0029] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 5.0%, even more preferably 0.0% to 3.0%, and particularly preferably 0.0% to 2.0%. The lower the haze, the more improved the transparency of the fluororesin piezoelectric film.

[0030] 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 as to include the intersection of the diagonals. 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 rectangular film is cut out from an area including the intersection of the diagonals of the rectangular film. If both sides of the piezoelectric film are 300 mm or more, an arbitrary square with a side of 300 mm is set on the piezoelectric film, and the measurement range is set according to the above method, 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. Thereafter, an ultraviolet (UV) irradiation device (GS NIPPON DENCHI, CSOT040) was used to irradiate the sample with a target cumulative light dose of 400 mJ / cm. ２ The film is irradiated with UV light so that a coating layer with a thickness of 2 μm is formed. 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, and an internal haze measurement film is prepared in which external haze due to scratches on the film surface has been removed. The haze is measured in accordance with ISO 14782:2021 using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) for an area including the intersection of the diagonals of the obtained measurement film, and the value obtained is used as a representative value of the internal haze of the fluororesin piezoelectric film.

[0031] The haze is measured in the same manner as in the internal haze measurement method, except that no surface coating layer is formed on the rectangular film, and the measured value is used as a representative value.

[0032] The fluorine-based resin piezoelectric film has a piezoelectric constant d ３３is 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] Piezoelectric constant d of fluororesin piezoelectric film ３３ is the direct quasi-static method (d ３３ Piezoelectric constant d by Mehta method, Berlincourt method ３３ The piezoelectric constant d of the fluororesin piezoelectric film is measured in accordance with ISO 19622:2018, which is a test method for determining the piezoelectric constant of the fluororesin piezoelectric film. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold the piezoelectric film as a test piece at a measurement temperature of 25°C with a holding force of 1.0 N, and measure the electric charge generated when an alternating force of 0.15 N and a frequency of 110 Hz is applied. ３３ The measurement is carried out on the polarization surface of the 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 above method.

[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 preferred.

[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 ２ The measured values ​​are measured at a measurement voltage of 1 V and a measurement frequency of 1 kHz. The dielectric constant ε is measured by the above method in a range including the intersection of the diagonals of the fluororesin piezoelectric film, and is used as a representative value of the dielectric constant of the fluororesin piezoelectric film. When 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 range including the intersection of the diagonals of the rectangular film is measured. When 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 fluorine-based resin 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 elastic modulus Y ３３ and the dielectric constant ε, it can be calculated by the following formula:

[0038]

[0039] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles having a size of 100 μm or more is 0 / 0.25 m. ２ More than 7 pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 5 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２The following are particularly preferred:

[0040] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles larger than 200 μm is 0 / 0.25 m ２ 3 or more pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 2 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following is even more preferred:

[0041] The fluorine-based resin piezoelectric film has 0 foreign particles per 0.25 m when viewed in plan. ２ More than 50 pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ More than 25 pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m 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 fluorine-based resin piezoelectric film, and then calculating the sum of the number of foreign particles measured from each observation piece. ２ Four observation pieces each measuring 100 mm x 100 mm were cut out. The sum of the number of foreign particles measured from each observation piece was calculated, and the sum was multiplied by 25 / 4, and the result was rounded to the nearest tenth to obtain a value of 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 height 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. Measurement is then performed 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 more, 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. When 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. 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 surface irregularities are reduced by pressing the fluororesin piezoelectric film against the cooling roll. This tendency is maintained even after stretching and polarization. Here, the measurement result for the surface with the smallest surface height roughness Rz (the surface in contact with the cooling roll) is 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 made using known methods such as a laser displacement meter, a capacitance displacement meter, or an infrared method over an area including the intersection of the diagonals of the fluororesin piezoelectric film, and the resulting value is used as the representative thickness of the fluororesin piezoelectric film. When 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 diagonals of the rectangular film is measured. When 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 area is set according to the above method.

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

[0049] [Method of manufacturing fluororesin piezoelectric film]

[0050] 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 will 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).

[0051] At this time, a step of filtering the molten resin composition (filtration step) may be carried out as needed.

[0052] (Melting Step) 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.

[0053] 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 solvent component remaining without volatilization may interfere with polarization in a subsequent step. Therefore, the content of the solvent component in the resin composition is preferably low, preferably 1% by mass or less, more preferably 0.1% by mass or less, based on the total mass of the resin composition. In particular, the content of the polar solvent is preferably 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less, based on the total mass of the resin composition.

[0054] Furthermore, the resin composition melted in the melting step preferably has a melt viscosity of 600 Pa·s or more and 4000 Pa·s or less. When the melt viscosity of the resin composition is 4000 Pa·s or less, the molten resin composition can be easily filtered and the dissolution temperature of the resin composition for film formation can be lowered, thereby suppressing resin deterioration due to melt heating. Furthermore, since the molecular chains of the resin are less entangled, the film can be stretched at a high ratio without breaking, thereby increasing the compressive modulus of the film. On the other hand, a resin composition with a low melt viscosity has a low degree of polymerization, which facilitates free movement of molecular chains and facilitates selective alignment of molecular chains, making it more prone to crystal growth during film production. As crystal growth progresses, the crystallites become larger, which increases light scattering due to the difference in refractive index between the crystalline and amorphous portions, tending to deteriorate the transparency of the film (increasing internal haze). Using a resin composition with a melt viscosity of 600 Pa·s or more can reduce internal haze due to the progression of crystallization. The melt viscosity of the resin composition is more preferably 600 Pa·s or more and 3500 Pa·s or less, even more 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 very preferably 600 Pa·s or more and 1500 Pa·s or less.

[0055] The melting temperature of the resin composition is preferably 75°C or more higher than the melting point of the resin composition but not exceeding 105°C, more preferably 75°C or more higher but not exceeding 100°C, even more preferably 80°C or more higher but not exceeding 100°C, and particularly preferably 85°C or more higher but not exceeding 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. By suppressing the generation of the decomposition products, the amount of foreign matter in the fluororesin piezoelectric film can be reduced, and the transparency and smoothness of the fluororesin piezoelectric film can be improved. Furthermore, by suppressing the generation of the decomposition products, the clogging of the filter due to these products can be prevented, and the filtration efficiency of the resin composition can be improved. 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.

[0056] According to the findings of the present inventors, fluororesins have high melt viscosities, and high temperatures are required to reduce the viscosity to a level suitable for filtration. However, heating to high temperatures is prone to decomposition, condensation, and other reactions, resulting in filter clogging. Therefore, it has been difficult to melt and filter a resin composition containing a fluororesin. In response, the present inventors discovered that a resin composition containing a fluororesin can be efficiently filtered by selecting a resin composition with an appropriate melt viscosity and adjusting the melting temperature within the above range. By adjusting the melting temperature of a resin composition containing a fluororesin within the above range and filtering the resin composition with a reduced viscosity due to 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, this reduces the burden on the working environment and the natural environment, and also reduces the production costs associated with recovering the polar solvent.

[0057] In the filtration step, the resin composition 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 any known filter type such as a pleated filter or a leaf disc filter may be used.

[0058] 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 makes it easier to filter the resin composition that has been heated to the above temperature and melted, and also prevents the filtration pressure from becoming too high, allowing the filtration time to be shortened. Using a filter with a filtration accuracy of 40 μm or less allows foreign matter in the resin composition to be sufficiently removed, 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.

[0059] 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 given size with a given 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.

[0060] 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.

[0061] The filter may be disposed between the extruder that performs the film-forming step and the die. Alternatively, the filter may be disposed in an extruder or melt-kneading apparatus that is different from the extruder that performs the film-forming step, and the resin composition that has been filtered through the filter may be fed into the extruder that is used in the film-forming step to form a film.

[0062] (Film Forming Step) In the film forming step, the resin composition filtered in the filtration step is formed into a film.

[0063] 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.

[0064] 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 response to this, by forming a molten resin into a film and then rapidly cooling it before crystallization progresses, the crystallization of the fluororesin can be suppressed, reducing the scattering of light that occurs between the crystalline and amorphous parts, and improving the transparency of the resulting fluororesin film.

[0065] The surface temperature of the chill roll is preferably 125°C or lower. Rapid cooling of the resin composition film extruded by the chill roll with a low surface temperature can suppress crystal growth and increase the transparency of the resulting fluororesin film. The surface temperature of the chill roll is preferably 5°C or higher and 115°C or lower, more preferably 10°C or higher and 100°C or lower, even more preferably 20°C or higher and 90°C or lower, particularly preferably 20°C or higher and 80°C or lower, very preferably 30°C or higher and 70°C or lower, and most preferably 30°C or higher and 60°C or lower.

[0066] 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.

[0067] (Stretching step) In the stretching step, the formed fluororesin film is stretched. In the stretching step, the fluororesin film formed in the film formation step may be stretched as is, or the fluororesin film that has been wound up and stored may be stretched. Furthermore, the fluororesin film may be heated and stretched as necessary.

[0068] The stretching can be performed in the machine direction (MD direction) and, if necessary, in the direction perpendicular to the machine direction (TD direction) while the fluororesin film is transported by a plurality of rolls. In this embodiment, the stretching ratio (ratio in the MD direction) is set to 4.1 times or more and 8.0 times or less.

[0069] 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 very preferably 5.5 to 7.5 times. 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 step. By setting the stretching ratio to 8.0 times or less, the fluororesin film is less likely to break during the stretching step.

[0070] In addition, by stretching the formed film at a high ratio, the lamellae constituting 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 ratio, 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.

[0071] 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 roll during stretching is 125°C or higher and 165°C or lower, preferably higher than 120°C and lower than 160°C, 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 enabling the film to be stretched at a high ratio while preventing breakage. Furthermore, stretching while heating allows the film to be stretched uniformly, suppressing localized crystallization and aligning molecular chains uniformly 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 at 165°C or less during stretching, the efficiency of molecular chain alignment due to stretching of the film can be improved, the proportion of the β-phase structure that contributes to the expression of piezoelectricity can be increased, and deformation of the film due to melting of the fluororesin can be suppressed.

[0072] (Polarization Step) In the polarization step, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. Fluororesin containing a homopolymer or copolymer of vinylidene fluoride undergoes a transition from α phase to β phase during the stretching step, 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.

[0073] 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.

[0074] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0075] 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.

[0076] [Uses] The above-described 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.

[0077] [Other Embodiments] It goes without saying that the above-described embodiments are exemplary embodiments of the present invention, and the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept.

[0078] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0079] The melt viscosity of the fluororesin 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 Seisaku-sho, Ltd.) was used, and the melt viscosity was measured at a temperature of 260°C and a shear rate of 50 s−1 using a capillary die with an inner diameter of 1 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 polymer filter in the filtration device or in the conduit from the extruder to the filtration device.

[0082] 1. Preparation of Fluorine-Based Resin Piezoelectric Films Films 1 to 12 and 15, all of which were fluorine-based resin piezoelectric films, were prepared by the following procedure.

[0083] 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 single-screw extruder with a bore of φ50 mm, 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 20 μm. The filtered resin was extruded into a film from a T-die and cooled by contacting it with the surface of a cooling roll whose surface was heated to 50°C, yielding an unstretched film with a thickness of 160 μm.

[0084] The resulting unstretched film was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and the surface temperature and rotation speed ratio of each roll was adjusted to stretch it 5.6 times in the machine direction (MD direction) at 130° C. Furthermore, a voltage of 9.1 kV was applied from the surface of the film in the thickness direction to obtain Film 1.

[0085] 1-2. Film 2 An unstretched film having a thickness of 190 μm was stretched 7.5 times in the machine direction (MD direction) at 140° C. Film 2 was obtained in the same manner as in Film 1, except that a voltage of 12.8 kV was applied from the surface of the film in the thickness direction.

[0086] 1-3. Film 3 Film 3 was obtained in the same manner as in the production of 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.

[0087] 1-4. Film 4 Film 4 was obtained in the same manner as Film 3, except that PVDF having 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.

[0088] 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.

[0089] 1-6. Film 6 Film 6 was obtained in the same manner as in the production of 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.

[0090] 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.

[0091] 1-8. Film 8 Film 8 was obtained in the same manner as in the production of 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] 1-9. Film 9 Film 9 was obtained in the same manner as in Film 4, except that, without filtering the resin, an unstretched film having a thickness of 160 μm produced at a chill roll temperature of 130° C. was stretched at a temperature of 110° C., a stretching ratio of 4.2 times, and an applied voltage of 7.5 kV.

[0093] 1-10. Film 10 Film 10 was obtained in the same manner as in Film 6, except that an unstretched film having a thickness of 190 μm was used, the stretching temperature was 120° C., the stretching ratio was 4.8 times, and the applied voltage was 7.4 kV.

[0094] 1-11. Film 11 Film 11 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.

[0095] 1-12. Film 12 Film 12 was obtained in the same manner as Film 1, except that PVDF having a melt viscosity of 4,500 Pa s and a melting point of 173°C was used, no polymer filter was used, the surface temperature of the chill roll was 130°C, the draw ratio was 4.0 times, and the applied voltage was 7.5 kV.

[0096] 1-13. Film 13 (unable to be produced) An attempt was made to produce Film 10 in the same manner as Film 1, except that PVDF with a melt viscosity of 4,500 Pa s and a melting point of 173°C was used. However, the filter was clogged with resin, and a film could not be obtained.

[0097] 1-14. Film 14 (unable to be produced) An attempt was made to produce Film 14 in the same manner as Film 3, except that a polymer filter with a filtration accuracy of 5 μm was used, but the filter became clogged with resin, and a film could not be obtained.

[0098] 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 temperature was raised to 60°C while stirring with a stirrer, and stirring was continued for 6 hours to prepare 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 placed in an automatic coater to prepare a coating film with a liquid thickness of 600 μm. The film was dried at 120°C for 1 hour, and a voltage of 9.1 kV was applied from the surface to the thickness direction to obtain Film 15.

[0099] 2. Evaluation of Fluorine-Based Resin Piezoelectric Films Unless otherwise specified, the obtained Films 1 to 12 and Film 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 , retardation, internal haze, surface height roughness Rz, number of foreign particles, and thickness were measured.

[0100] 2-1. Compressive elastic modulus Y in the thickness direction ３３ The compressive elastic modulus Y in the thickness direction of the film 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 to the polarized surface of the measurement film, and the side opposite to the polarized surface of the measurement 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 Indenter: Berkovich type, edge angle: 115° Test mode: load-unload test Test force: 20 mN Minimum test force: 0.2 mN Load rate: 6.6620 mN / sec Load holding time: 2 seconds Unload holding time: 1 second

[0101] 2-2. Dielectric constant ε The dielectric constant ε of each measurement 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 measurement was performed at an applied voltage of 1 V and a measurement frequency of 1 kHz.

[0102] 2-3. Piezoelectric constant d ３３ Piezoelectric constant d ３３ is the direct quasi-static method (d ３３ The piezoelectric constant d of the piezoelectric ceramic measured by the Meter method and the Berlincoat method ３３ The measurement was carried out 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 apply an alternating force of 0.15 N and a frequency of 110 Hz to measure the electric charge generated. The piezoelectric constant d ３３ The absolute value of the piezoelectric constant d ３３ was used as a representative value.

[0103] 2-4. Electromechanical coupling coefficient k ３３ Piezoelectric constant d ３３ , compressive elastic 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.

[0104]

[0105] 2-5. Retardation Using a KOBRA-HB manufactured by Oji Scientific Instruments, the retardation of a film cut into a 20 mm x 20 mm area from the area including the intersection of the diagonal lines of the measurement film was measured by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8 nm was taken as the retardation of the film.

[0106] 2-6. Internal haze A rectangular film was cut into a size of 50 mm x 50 mm so as to include the intersection of the diagonals of the square film. One surface (side A) of the rectangular film was coated with a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) 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 hard coating agent to the surface of the rectangular film (side A) until the target integrated light intensity reached 400 mJ / cm. ２ The film was irradiated with UV light so that the thickness of the coating layer was 2 μm. A coating layer similar to that on the A side was also formed on the other surface (side B) of the rectangular film with the A side coated. Coating layers were formed on both sides of the film by the above method, 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.

[0107] 2-7. Haze Measurement was performed 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.

[0108] 2-8. Number of foreign objects: 0.010 m so that each film is cut out from adjacent positions continuously.２ Four rectangular films (observation pieces) each 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 calculated, and the sum was multiplied by 25 / 4. The result was rounded to the nearest tenth to obtain a value of 0.25 m. ２ The number of foreign particles per particle was determined. At this time, the foreign particles were observed using transmitted light, marked, and the marked areas were observed under a microscope to determine the size of the foreign particles. The size of the foreign particles was determined as the arithmetic mean value of the maximum and minimum widths of the foreign particles. In this way, the numbers of foreign particles larger than 200 μm in size, foreign particles with a size of 100 μm to 200 μm in size, and foreign particles with a size of less than 100 μm were determined.

[0109] 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 been in contact with the cooling roll.

[0110] 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 thickness of the film.

[0111] 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 its haze, making it impossible to measure the number of foreign particles.

[0112]

[0113]

[0114]

[0115] This application claims priority from Japanese Patent Application No. 2024-124967 filed on July 31, 2024, and Japanese Patent Application No. 2024-124972 filed on July 31, 2024. The matters described in the specification, claims, and drawings of those applications as originally filed are incorporated herein by reference.

[0116] The fluororesin piezoelectric film according to the present invention is useful as a highly sensitive piezoelectric film.

Claims

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 A fluorine-based resin piezoelectric film having a piezoelectric 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 claim 1 , wherein the ρ is 0.10 or more and 0.20 or less.

3. Measurement temperature: 260°C, shear rate: 50 s －１ 2. The fluorine-containing resin piezoelectric film according to claim 1, wherein the film has a melt viscosity η measured by a method of 600 Pa·s or more and 4000 Pa·s or less.

4. The fluororesin piezoelectric film according to claim 1, which is primarily composed of vinylidene fluoride resin.

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 claim 1 , wherein:

6. The fluororesin piezoelectric film according to claim 1, wherein the surface roughness in surface height Rz measured in accordance with JIS B 0601:2001 on the surface having a smaller Rz is 0.50 μm or less.

7. A method for producing a fluorine-based resin piezoelectric film, comprising the steps of: heating and melting a resin composition containing a fluorine-based resin; extruding the molten resin composition to form a film; stretching the formed film at a stretching temperature of 125°C to 165°C and at a stretching ratio of 4.1 to 8.0; and polarizing the formed film at an applied voltage of 7.0 kV to 50.0 kV.

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

11. The method for producing a fluorine-based resin piezoelectric film according to claim 7, wherein the resin composition contains vinylidene fluoride resin as a main component.

12. The film has a 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 The method for producing a fluorine-based resin piezoelectric film according to claim 7 , wherein the piezoelectric constant is 7.0 pC / N or more and 40.0 pC / N or less.

13. 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 method for producing a fluorine-based resin piezoelectric film according to claim 7 , wherein:

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