Fluorine-based resin piezoelectric film and manufacturing method therefor

Abstract

This fluorine-based resin piezoelectric film has a retardation of 100-2,000 nm, an internal haze of less than 1.7%, an average value of piezoelectric constants d33 of 6.0-40.0 pC / N, as measured at 21 measurement points determined in a fast-axis direction, and a standard deviation of the piezoelectric constants d33 of at most 2.0 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] It is known that a fluorine-based resin film can be transformed into a piezoelectric film having high piezoelectricity by polarization treatment (see, for example, Patent Document 1). Piezoelectric bodies made of fluorine-based resins are used in transparent touch panels and the like because of their high transparency.

[0003] A known method for polarizing a fluororesin film is to apply a DC voltage to the fluororesin film while it is passing between a needle-like electrode and a metal roll (sometimes referred to as a "polarizing roll" to distinguish it from a conveying roll) (see, for example, Patent Document 2). Since the entire surface of the fluororesin film needs to be polarized, a method has been disclosed in which multiple needle-like electrodes are arranged in a row perpendicular to the film surface in a direction (TD) perpendicular to the conveying direction (MD) of the fluororesin film, and DC current is discharged from these needle-like electrodes to the polarization roll, thereby polarizing the fluororesin film passing between the polarization roll and the needle-like electrode (see, for example, Patent Document 3).

[0004] Piezoelectric films used in touch panels and the like are mass-produced as large-area piezoelectric films, but piezoelectric films used in sensors and the like are often small in area, and large-area piezoelectric films are often cut into small pieces for use. Therefore, to ensure the quality of the piezoelectric film, it is required that the variation in the piezoelectric constant of the entire film is small. To address these issues, Patent Document 3 discloses a method for controlling the piezoelectric constant d of the fluororesin film by keeping the distance between each of a plurality of needle-like electrodes and the fluororesin film constant during polarization treatment. ３３ We are trying to reduce the fluctuations in

[0005] JP 60-047034 A JP 2019-067908 A JP 2016-219804 A

[0006] The inventors of the present invention believed that densely arranging needle-shaped electrodes would be desirable for uniformly poling a fluororesin film and minimizing in-plane variations in the piezoelectric constant. Therefore, the inventors attempted to perform polarization by reducing the distance between the needle-shaped electrodes. However, they found that reducing the distance between the electrodes caused interference between the electric fields generated by discharge from the needle-shaped electrodes, resulting in uneven field strength, preventing uniform polarization and, instead, increasing variations in the piezoelectric constant within the film plane.

[0007] Patent Document 3 describes a fluororesin film with reduced variation in the piezoelectric constant in the in-plane direction. However, the fluororesin film described in Patent Document 3 has low transparency and a low piezoelectric constant. As such, a fluororesin piezoelectric film that satisfies the requirements of high transparency, a high piezoelectric constant, and reduced variation in the piezoelectric constant has not yet been obtained.

[0008] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a fluororesin piezoelectric film that has high transparency, a high piezoelectric constant, and small variation in the piezoelectric constant in the in-plane direction, and a method for producing the same.

[0009] One embodiment of the present invention for solving the above problems relates to the following fluororesin piezoelectric films [1] to [5]: [1] A fluororesin piezoelectric film having a retardation of 100 nm or more and 2000 nm or less, an internal haze of less than 1.7%, and a piezoelectric constant d ３３ The average value of the piezoelectric constant d ３３ [2] The fluororesin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less. [3] The fluororesin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less. [4] The fluororesin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less. [5] The fluororesin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less. [6] The fluororesin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less. [7] The fluororesin piezoelectric film －１[4] The fluorine-based resin piezoelectric film according to [1] or [2], wherein the melt viscosity η measured by a method of measuring 600 Pa·s or more and 4000 Pa·s or less. [4] The number of foreign particles having a size of 100 μm or more, which is the arithmetic mean value of the maximum width and the minimum width when viewed in plan, of the film is 7 / 0.25 m ２ The fluororesin piezoelectric film according to any one of [1] to [3], which is the following: [5] The fluororesin piezoelectric film according to any one of [1] to [4], which contains a homopolymer of vinylidene fluoride.

[0010] One embodiment of the present invention for solving the above problems relates to a method for producing a fluororesin piezoelectric film according to the following [6] to

[14] . [6] A method for producing a fluororesin piezoelectric film according to any one of [1] to [5], comprising the steps of heating and melting a fluororesin, forming the molten fluororesin into a film, uniaxially stretching the formed film by 2.5 to 6.0 times, and polarizing the formed film. [7] A method for producing a fluororesin piezoelectric film according to [6], comprising the step of filtering the molten fluororesin through a filter having a filtration accuracy of 10 μm to 40 μm. [8] A method for producing a fluororesin piezoelectric film according to [6] or [7], comprising the step of cooling the formed film by contacting it with a cooling roll having a surface temperature of 125°C or less. [9] A method for producing a fluorine-based resin piezoelectric film, comprising the step of polarizing a film passing between a polarization roll and a plurality of needle-like electrodes by DC discharge between the polarization roll and each of the plurality of needle-like electrodes, wherein the plurality of needle-like electrodes are all positioned so that the distance between the tip of the needle-like electrode and the surface of the polarization roll is 5 mm or more and 30 mm or less, and the distance between the nearest adjacent needle-like electrodes is more than 15 mm and 100 mm or less.

[10] The method for producing a fluorine-based resin piezoelectric film according to [9], wherein the plurality of needle-like electrodes are arranged in a row in a direction (TD) perpendicular to the direction in which the film passes (MD), thereby forming an electrode array.

[11] The method for producing a fluorine-based resin piezoelectric film according to

[10] , wherein the needle-like electrodes arranged in a row are spaced equally apart.

[12] The method for producing a fluorine-based resin piezoelectric film according to any of [9] to

[11] , wherein a plurality of electrode arrays are arranged in the MD direction.

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

[12] , wherein the needle-like electrodes constituting a certain electrode row and the needle-like electrodes constituting an electrode row adjacent to the said electrode row are arranged so as not to overlap in the MD direction, and the needle-like electrodes constituting the adjacent electrode row are arranged at intersections of a perpendicular line to the electrode row and the electrode row adjacent to the said electrode row, passing through points dividing the space between adjacent needle-like electrodes in the electrode row at equal intervals.

[14] The method for producing a fluorine-based resin piezoelectric film according to

[13] , wherein the number of electrode rows is an integer multiple of the number of divisions obtained by dividing adjacent needle-like electrodes in the electrode rows.

[15] The method for producing a fluorine-based resin piezoelectric film according to [9], comprising the steps of: heating and melting a fluorine-based resin; forming the molten fluorine-based resin into a film; and uniaxially stretching the formed film by 2.5 times or more and 6.0 times or less, wherein the formed film is polarized in the polarization treatment step.

[0011] According to the present invention, there are provided a fluororesin piezoelectric film that is highly transparent, has a high piezoelectric constant, and exhibits little variation in the piezoelectric constant in the in-plane direction, and a method for producing the same.

[0012] Fig. 1 is a schematic diagram showing the process of stretching and poling a fluororesin film. Fig. 2 is a schematic diagram showing an example of the arrangement of needle electrodes in a needle electrode array relative to the surface of a polarized roll. Fig. 3 is a schematic diagram showing another example of the arrangement of needle electrodes in a plurality of needle electrode arrays relative to the surface of a polarized roll. Fig. 4 is a schematic diagram showing in more detail the positions of the needle electrodes in a plurality of needle electrode arrays in the example shown in Fig. 3.

[0013] [Fluorocarbon Resin Piezoelectric Film] One embodiment of the present invention relates to a fluorocarbon resin piezoelectric film.

[0014] The fluororesin piezoelectric film contains a fluororesin.

[0015] 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 ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP) with TFE. 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, CTFE, TFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether.

[0016] Of these, from the viewpoint of facilitating polarization of the fluororesin film, fluororesins obtained by polymerizing VDF are preferred, with VDF homopolymers, copolymers of VDF and HFP, copolymers of VDF and trifluoroethylene, copolymers of VDF and TFE, copolymers of VDF, trifluoroethylene and TFE, copolymers of VDF, trifluoroethylene, TFE and CTFE, and copolymers of VDF, trifluoroethylene, TFE and 1-chloro-1-fluoroethylene being more preferred, and VDF homopolymers being even more preferred. These fluororesins may be used alone or in combination of two or more.

[0017] The fluororesin piezoelectric film preferably contains, as a main component, a fluororesin obtained by polymerizing VDF, preferably a VDF homopolymer. "Containing such a resin as a main component" means that the content of structural units derived from VDF 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.

[0018] The fluorine-based resin piezoelectric film is not particularly limited and can be used, but the measurement temperature is 260°C, and the shear rate during measurement is 50 s －１ The melt viscosity measured by is preferably 600 Pa·s or more and 4000 Pa·s or less, more preferably 600 Pa·s or more and 3500 Pa·s or less, and even more preferably 600 Pa·s or more and 2400 Pa·s or less. The lower the melt viscosity, the less likely the filter is to clog and the easier the filtration. Therefore, it is not necessary to increase the melting temperature to enable filtration, and it is possible to prevent the occurrence of spot-like irregularities on the film surface due to the resin denaturing due to heat and the generation of foreign matter. In addition, the higher the melt viscosity, the larger the molecular weight of the resin, making it difficult to selectively align molecular chains and less likely to crystallize, resulting in excellent transparency and stabilizing various physical properties of the film. In addition, the higher the melt viscosity, the less likely the resin constituting the film is to crystallize, so light scattering due to the difference in refractive index between the crystalline and amorphous portions is less likely to occur, and transparency is less likely to deteriorate.

[0019] 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.

[0020] The fluororesin piezoelectric film has a retardation of 100 nm to 2000 nm, preferably 500 nm to 1800 nm, more preferably 700 nm to 1600 nm, even more preferably 910 nm to 1600 nm, and particularly preferably 1050 nm to 1600 nm. The larger the retardation, the higher the degree of molecular orientation of the fluororesin film, and the more sufficiently the proportion of β crystals, making it easier to obtain a fluororesin piezoelectric film with a high piezoelectric constant by the polarization step.

[0021] The retardation is measured by the parallel Nicol rotation method (KOBRA-HB, manufactured by Oji Scientific Instruments Co., Ltd.) at a measurement wavelength of 587.8 nm. At this time, the fast axis and slow axis are determined from the in-plane birefringence of the film. The slow axis direction coincides with the average direction of molecular chains oriented by stretching or extrusion. Therefore, if the film is stretched in the flow direction (machine direction), the MD direction and the slow axis direction coincide.

[0022] The fluororesin piezoelectric film has an internal haze of less than 1.7%, preferably 0.0% or more and less than 1.7%, more preferably 0.0% or more and less than 1.5%, even more preferably 0.0% or more and less than 1.2%, particularly preferably 0.0% or more and less than 0.8%, and most preferably 0.0% or more and less than 0.7%. The lower the internal haze, the more improved the transparency of the fluororesin piezoelectric film. Furthermore, according to the findings of the present inventors, the smaller the internal haze, the lower the piezoelectric constant d ３３ It is easier to make it higher.

[0023] The internal haze of the fluorine-based resin piezoelectric film was measured by applying a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) to one surface of the film using a bar coater and drying it at 80°C for 30 minutes. After that, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to irradiate the film with a target integrated light dose of 400 mJ / cm. ２ The film was irradiated with UV light so that a coating layer with a thickness of 2 μm was formed. A similar coating layer was formed on the other surface. This coating layer allowed the preparation of a film for measuring internal haze, in which external haze due to scratches on the film surface had been removed.

[0024] The direction of the fast axis determined by measuring the birefringence of the film to be measured is defined as the width direction, and the length obtained by subtracting 20 mm from the overall width L mm of the film is defined as the internal haze measurement width (L-20). When an arbitrary point on the midpoint of the line segment connecting both ends of the film to be measured is defined as point A, 21 measurement points are set on the line segment in the width direction passing through point A, with an interval of (L-20) / 20 from each point from point A toward both ends. The average value of haze measured at these 21 measurement points is defined as the representative value of the internal haze of the film. The haze is measured using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K 7136:2000.

[0025] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 6.0%, even more preferably 0.0% to 5.0%, particularly preferably 0.0% to 3.0%, and most preferably 0.0% to 2.0%. The lower the haze, the more improved the transparency of the fluororesin piezoelectric film.

[0026] The haze of a fluororesin piezoelectric film is measured using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K 7136:2000 at 21 measurement points set in the same manner as the measurement points for the internal haze, for a piezoelectric film that does not have a coating layer to remove external haze due to scratches on the film surface, etc., and the average value of the haze is used as the representative value of the haze of the film.

[0027] The fluorine-based resin piezoelectric film has a piezoelectric constant d ３３ The average value is 6.0 pC / N or more and 40.0 pC / N or less, preferably 11.0 pC / N or more and 40.0 pC / N or less, more preferably 13.0 pC / N or more and 35.0 pC / N or less, even more preferably 19.0 pC / N or more and 35.0 pC / N or less, particularly preferably 21.0 pC / N or more and 30.0 pC / N or less, and most preferably 22.5 pC / N or more and 30.0 pC / N or less.

[0028] In addition, the fluorine-based resin piezoelectric film has a piezoelectric constant d ３３The standard deviation σ of the chromatic aberration is 2.0 pC / N or less, preferably 0.1 pC / N or more and 2.0 pC / N or less, more preferably 0.1 pC / N or more and 1.8 pC / N or less, even more preferably 0.1 pC / N or more and 1.6 pC / N or less, and particularly preferably 0.1 pC / N or more and 1.4 pC / N or less.

[0029] Furthermore, the fluorine-based resin piezoelectric film has a piezoelectric constant d ３３ The smaller the coefficient of variation CV, the smaller the variation in the piezoelectric constant within the film and the more uniform it is. Therefore, the coefficient of variation CV is preferably 0.00 or more and 0.50 or less, more preferably 0.00 or more and 0.30 or less, still more preferably 0.00 or more and less than 0.12, particularly preferably 0.00 or more and less than 0.10, and most preferably 0.00 or more and 0.06 or less.

[0030] 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 measurement is performed in accordance with ISO 19622:2018, a test method for determining the piezoelectric constant of a fluororesin piezoelectric film. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold a test piece of fluororesin piezoelectric film 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 absolute value of the measured value is then used to calculate the piezoelectric constant. At this time, the fast axis direction determined by measuring the birefringence of the film is defined as the width direction, and the length obtained by subtracting 20 mm from the total width length L mm is defined as the thickness measurement width (L-20). An arbitrary point on the midpoint line of the line segment connecting both ends of the film is defined as point A, and the average value of the piezoelectric constants measured at 21 measurement points, including point A and 20 points set on the line segment in the width direction passing through point A at intervals of (L-20) / 20 from each point toward both ends, is defined as the piezoelectric constant d of the fluororesin piezoelectric film. ３３ The average value of the piezoelectric constant d ３３ The standard deviation of the piezoelectric constant of the film is defined as the standard deviation σ of the piezoelectric constant of the film. Furthermore, the value obtained by dividing the standard deviation σ by the average value is defined as the coefficient of variation CV.

[0031] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm to 300 μm, more preferably 15 μm to 200 μm, even more preferably 20 μm to 150 μm, particularly preferably 25 μm to 120 μm, very preferably 30 μm to 80 μm, and most preferably 30 μm to 60 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.

[0032] The thickness of a fluororesin piezoelectric film is generally measured using a micrometer (JIS C 2151:2019), but can also be measured by other known methods, such as a laser displacement meter, a capacitance displacement meter, or an infrared ray. The thickness is measured at measurement points set by the following method. The fast axis direction determined by measuring the birefringence of the film is defined as the width direction, and the thickness measurement width (L-20) is the length obtained by subtracting 20 mm from the overall width L mm. When an arbitrary point on the midpoint of the line segment connecting both ends of the film is designated as point A, the thickness measurement points are 11 points: point A and 10 points set on the line segment in the width direction passing through point A, spaced apart by (L-20) / 10 from each point toward both ends. The average of the thicknesses measured at these measurement points is used as the representative value of the film's thickness.

[0033] The width of the fluororesin piezoelectric film is not particularly limited, but the film width is preferably 250 mm or more and 2000 mm or less, more preferably 250 mm or more and 1750 mm or less, even more preferably 250 mm or more and 1500 mm or less, and particularly preferably 300 mm or more and 1300 mm or less. The wider the film width, the higher the productivity tends to be. On the other hand, the narrower the film width, the easier it is to stretch uniformly.

[0034] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles with a size of 100 μm or more is 7 per 0.25 m ２ Preferably, 0 pieces / 0.25m or less ２ 5 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２3 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following are particularly preferred:

[0035] 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:

[0036] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles less than 100 μm in size is 0 / 0.25 m ２ More than 50 pieces / 0.25m ２ Preferably, the number is 0 / 0.25m or less. ２ More than 25 pieces / 0.25m ２ More preferably, the number is 0 / 0.25m or less. ２ More than 16 pieces / 0.25m ２ It is even more preferable that:

[0037] The fewer these foreign matters there are, the more the transparency of the fluororesin piezoelectric film can be improved, and the more easily the fluororesin film can be stretched or polarized uniformly when stretched or polarized.

[0038] The number of these foreign matters is measured by cutting out 25 rectangular films (observation pieces) from a fluororesin film having a thickness of 40 μm or less so that they are adjacent to each other, and then calculating the sum of the number of foreign matters measured from each of the observation pieces. For a fluororesin film having a thickness of more than 40 μm, the film is stretched until the thickness is reduced from 35 μm to 40 μm or less, and then the number of foreign matters is measured by the method described below. Specifically, 0.010 m of the film is cut out so that the film is adjacent to the fluororesin film. ２Twenty-five pieces of the above specimen, each measuring 100 mm x 100 mm, are cut out. The sum of the number of foreign particles measured from each specimen is then calculated. At this time, foreign particles are marked using transmitted light, and the marked areas are observed under a microscope to determine the size of the foreign particles. The size of the foreign particles is defined as the arithmetic mean value of the maximum and minimum widths of the foreign particles.

[0039] The fluororesin piezoelectric film may contain resins other than fluororesin or other additives as long as the above physical properties can be satisfied.

[0040] Examples of resins other than the above-mentioned fluorine-based resins include polyesters such as polycarbonate, polyethylene terephthalate, and polyethylene naphthalate, silicone resins, polyethers, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, which can be added to enhance flexibility; acrylic resins, epoxy resins, polyethylene oxide, polypropylene oxide, polyphenylene oxide, polyphenylene sulfide, polyamide, polyimide, polyamideimide, polystyrene, and polybenzimidazole, which can be added to enhance strength; and odd-numbered polyamides, cyanopullulan, and copper phthalocyanine-based polymers, which can be added to further enhance dielectric properties.

[0041] [Method for Manufacturing Fluorine-Based Resin Piezoelectric Film] The method for manufacturing the above-mentioned fluororesin piezoelectric film is not particularly limited, but preferably includes the steps of producing a fluororesin film by extrusion molding a heated and melted resin, stretching the obtained fluororesin film, and performing a polarization treatment.

[0042] For example, a fluororesin film can be produced by carrying out the following steps: heating and melting the fluororesin (melting step), filtering the molten fluororesin (filtration step), forming the filtered fluororesin into a film (film formation step), stretching the formed film (stretching step), and polarizing the formed film (polarization step). By adjusting the arrangement of the needle-like electrodes in the polarization treatment, a piezoelectric film with a high piezoelectric constant and small in-plane variation in the piezoelectric constant can be obtained.

[0043] (Melting Step) In the melting step, the fluororesin is melted. This step can be carried out by melt-kneading the fluororesin using an extruder, for example.

[0044] If the fluororesin melted in the melting step contains a solvent component, the solvent component remaining without volatilization may interfere with polarization in a subsequent step, and therefore the content of the solvent component in the fluororesin is preferably small, preferably 1 mass % or less, more preferably 0.1 mass % or less, based on the total mass of the fluororesin. 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 fluororesin.

[0045] The melting temperature of the fluororesin is preferably at least 75°C higher than the melting point of the fluororesin but not more than 105°C higher, more preferably at least 75°C higher but not more than 100°C higher, even more preferably at least 80°C higher but not more than 100°C higher, and particularly preferably at least 85°C higher but not more than 95°C higher. By setting the melting temperature at least 75°C higher than the melting point of the fluororesin, the viscosity of the fluororesin can be reduced to a level that allows filtration in the subsequent process. By setting the melting temperature at most 105°C higher than the melting point of the fluororesin, decomposition and condensation of the fluororesin due to heating can be suppressed, and the generation of decomposition products and the like due to these can be suppressed. By suppressing the generation of the decomposition products and the like, 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 and the like, filter clogging due to these products can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0046] According to the findings of the present inventors, fluororesins with high melt viscosity require high temperatures to achieve a filterable viscosity. 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 fluororesins. In response, the present inventors discovered that fluororesins with appropriate melt viscosity can be efficiently filtered by adjusting the melting temperature within the above range. By adjusting the melting temperature of the fluororesin within the above range and filtering the fluororesin with reduced viscosity through melting, the fluororesin 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.

[0047] (Filtration step) In the filtration step, the fluororesin that has been melted and reduced in viscosity in the melting step is filtered. The filtration method is not particularly limited, and the molten fluororesin may be passed through a filter, and known filter types such as pleated filters and leaf disc filters may be used.

[0048] In the filtration step, the fluororesin 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 fluororesin melted by heating to the above temperature, 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 fluororesin, resulting in a fluororesin film with little foreign matter. The filtration accuracy of the filter is more preferably 10 μm or more and 30 μm or less, and even more preferably 15 μm or more and 30 μm or less.

[0049] Fluorine-based resins are filtered using a multilayer filter consisting of multiple layers with different shapes, mesh sizes, etc. The filtration accuracy of a filter used to filter fluorine-based resins 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.

[0050] In this step, the fluororesin may be filtered multiple times. For example, coarse foreign matter may be removed by a filter with low filtration accuracy (large filtration accuracy value) placed in the front stage, and then finer foreign matter may be removed by a filter with high filtration accuracy (small filtration accuracy value) placed in the rear stage. In this case, the filtration accuracy is the value of the filter with the highest filtration accuracy.

[0051] 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 fluororesin that has been filtered through the filter may be fed into the extruder that performs the film-forming step to form a film.

[0052] (Film Forming Step) In the film forming step, the fluorine-based resin filtered in the filtration step is formed into a film.

[0053] The film formation method is not particularly limited, and a known method can be used, such as extruding a molten and filtered fluorine-based resin through a T-die and cooling it by contacting it with a cooling roll.

[0054] The cooling roll may be a metal mirror-finish roll, etc. The surface temperature of the cooling roll is preferably 115° C. or less, more preferably 30° C. or more and 115° C. or less. By bringing the fluororesin into contact with the cooling roll set at a relatively low temperature and quenching it, it is possible to appropriately suppress crystallization of the fluororesin, appropriately control the crystalline structure of the fluororesin, and reduce the internal haze of the film.

[0055] The film formation process may be carried out in a batch system, but from the viewpoint of mass productivity, a system in which the formed film is transported and continuously wound up is preferred. In the film formation process and the subsequent stretching process, the length of the film in the direction perpendicular to the transport direction (film width) is preferably as large as possible from the viewpoint of mass productivity. On the other hand, the smaller the film width, the easier it is to control the film formation conditions, and therefore the film formation accuracy tends to be higher. The film width is preferably 250 mm or more and 1500 mm or less, more preferably 300 mm or more and 1100 mm or less, even more preferably 400 mm or more and 850 mm or less, and particularly preferably 450 mm or more and 600 mm or less.

[0056] The fluorine-based resin film thus obtained may be stored after being wound up, or may be transported directly to the stretching step and polarization step.

[0057] In this embodiment, a direct current voltage is applied to the fluorine-based resin film to impart piezoelectricity to the fluorine-based resin film.

[0058] The fluororesin film may be stretched before or simultaneously with the polarization step (stretching step). The stretching direction in the stretching step is not particularly limited as long as it is uniaxial stretching. When mass-producing stretched films, the film can be continuously stretched by passing it between rollers with different feed speeds. Therefore, the stretching direction of the film is preferably the film transport direction. In fluororesins containing vinylidene fluoride homopolymers or copolymers, the stretching step causes a transition from α crystals to β crystals, increasing the proportion of β crystals. By applying a DC voltage to a fluororesin film with an increased proportion of polar β crystals, a fluororesin piezoelectric film with a high piezoelectric constant can be obtained.

[0059] 1 is a schematic diagram showing the process of successively stretching and polarizing a fluororesin film. The fluororesin film 110 is stretched by a stretching roll 122 and a stretching roll 124, and then polarized on a polarization roll 130 by direct current discharge from a plurality of needle-like electrodes 144.

[0060] The stretching can be performed in the transport direction and, if necessary, in a direction perpendicular to the transport direction, while the fluororesin film is transported between multiple rolls (stretching roll 122 and stretching roll 124 in FIG. 1 ). Fluororesin containing a vinylidene fluoride homopolymer or copolymer undergoes a transition from α crystal to β crystal during the stretching process, increasing the proportion of β crystal. A fluororesin piezoelectric film with high piezoelectricity can be obtained by applying a DC voltage to a fluororesin film with an increased proportion of polar β crystal.

[0061] The stretching ratio is preferably 2.5 to 6.0 times, more preferably 3.5 to 5.0 times. By setting the stretching ratio to 2.5 times or more, the proportion of β crystals in the fluororesin is sufficiently increased, making it easier to obtain a fluororesin piezoelectric film with a high piezoelectric constant through the polarization step. By setting the stretching ratio to 6.0 times or less, the fluororesin film is less likely to break during the stretching step.

[0062] (Polarization Step) In the polarization step, a DC discharge is applied to the grounded polarization roll 130 from each of the multiple needle-shaped electrodes 144, each connected to a DC high-voltage power supply 146. Then, the fluorine-based resin film 110, which is in close contact with the surface of the polarization roll 130, is passed between the DC-discharged needle-shaped electrodes 144 and the polarization roll 130.

[0063] The multiple needle electrodes 144 are preferably arranged at positions where the distance between the tip of each needle electrode 144 and the surface of the polarization roll 130 is 5 mm or more and 30 mm or less. If the distance is 5 mm or more, a short circuit is less likely to occur between the needle electrodes 144 and the polarization roll 130 during polarization, and non-uniform polarization due to the short circuit is less likely to occur. If the distance is 30 mm or less, the piezoelectric constant of the manufactured fluororesin piezoelectric film can be increased and fluctuations in the piezoelectric constant can be reduced. The distance is more preferably 10 mm or more and 25 mm or less. It is preferable that the distance between the tip of each needle electrode 144 and the surface of the polarization roll 130 is the same for each of the multiple needle electrodes 144.

[0064] Incidentally, in order to uniformly polarize the fluorine-based resin film and reduce variations in the piezoelectric constant in the in-plane direction, it is considered desirable to arrange the needle-shaped electrodes 144 closely. However, according to the findings of the present inventors, it has become clear that if the distance between the needle-shaped electrodes 144 is reduced, the electric fields between the needle-shaped electrodes 144 interfere with each other, causing unevenness in the electric field strength, and thus making it difficult to achieve uniform polarization. Therefore, in this embodiment, it is preferable to arrange the multiple needle-shaped electrodes 144 so that the distance between the nearest needle-shaped electrodes 144 is greater than 15 mm and less than or equal to 100 mm. If the distance between the nearest needle-shaped electrodes 144 is greater than 15 mm, unevenness in the electric field strength due to interference between the electric fields generated by the needle-shaped electrodes 144 does not occur, and therefore uniform polarization can be achieved. When the distance between the nearest needle-like electrodes 144 is 100 mm or less, no gaps are generated in the electric field generated by adjacent needle-like electrodes, and the electric field can be sufficiently high even in areas distant from the needle-like electrodes, allowing the electric field to be applied uniformly within the plane of the fluorine-based resin film 110, thereby enabling uniform polarization treatment of the entire fluorine-based resin film 110. The distance between the nearest needle-like electrodes 144 is more preferably 16 mm to 50 mm, and even more preferably 20 mm to 50 mm.

[0065] As long as the distance between the closest needle-shaped electrodes is within the above range, the arrangement of the needle-shaped electrodes is not particularly limited and may be, for example, randomly arranged. However, in order to polarize the entire width direction of the fluororesin film 110 densely and more uniformly, it is preferable that the multiple needle-shaped electrodes be arranged in a row in the TD direction to form an electrode array. In this case, it is preferable that the distance between the multiple needle-shaped electrodes arranged in a row is equal. There may be only one electrode array, or multiple electrode arrays may be arranged at intervals in the transport direction of the fluororesin film. In this case, it is preferable that the distance between the multiple electrode arrays is equal. In the example shown in FIG. 1, three electrode arrays are arranged at equal intervals. In order to uniformly discharge the needle-shaped electrodes, it is preferable that the needle-shaped electrodes 144 are arranged perpendicular to the polarization roll surface.

[0066] 2 is a schematic diagram showing an example of the arrangement positions of the needle electrodes in the needle electrode array 144 relative to the surface of the polarization roll 130. In FIG. 2, the polarization roll 130 rotates so that the surface moves from top to bottom in the figure, thereby transporting the fluororesin film 110 that is in close contact with the surface of the polarization roll 130 from top to bottom (in the MD direction) in FIG. 2. In this specification, the transport direction of the fluororesin film 110 (the direction of the arrow in each figure) is also referred to as the MD direction, and the direction along the surface of the fluororesin film 110 that is perpendicular to the MD direction is also referred to as the TD direction.

[0067] In FIG. 2 , one electrode array 240 is arranged in the extension direction of the polarization roll 130 (toward the viewer in the drawing). The electrode array 240 includes an aluminum rod 242 arranged to extend in the TD direction, and multiple needle-like electrodes 244-1 to 244-4 arranged on the aluminum rod 242 at equal intervals with the needle tips facing the polarization roll 130. In FIG. 2 , the distance between adjacent needle-like electrodes (L in FIG. 2 ; in FIG. 2 , L is also the distance between the closest needle-like electrodes) may be greater than 15 mm and less than 100 mm, preferably greater than 15 mm and less than 30 mm, more preferably 16 mm to 25 mm, and even more preferably 20 mm to 25 mm. Note that, although the multiple needle-like electrodes 244-1 to 244-4 are arranged at equal intervals in FIG. 2 , they may be arranged non-uniformly as long as the distance between each of the needle-like electrodes is within the above-mentioned range.

[0068] Figure 3 is a schematic diagram showing another example of the position of the needle electrode 144 relative to the surface of the polarization roll 130. In Figure 3, the polarization roll 130 also rotates so that the surface moves from top to bottom (in the MD direction) in the figure, thereby moving the fluororesin film 110 that is in close contact with the surface of the polarization roll 130 from top to bottom (in the MD direction) in Figure 3.

[0069] 3, multiple electrode rows 340a to 340e are arranged in the extension direction relative to the surface of polarization roll 130. In the example shown in Fig. 3, electrode rows 340a to 340e include multiple aluminum rods 342a to 342e arranged to extend in the TD direction, and multiple needle-like electrodes 344a-1 to 344e-4 arranged at equal intervals on each aluminum rod 342a to 342e with the needle tips facing toward polarization roll 130.

[0070] In Fig. 3, the distance between adjacent needle-shaped electrodes in each of electrode rows 340a to 340e (L in Fig. 3) is preferably greater than 15 mm and less than 30 mm, more preferably greater than 16 mm and less than 25 mm, and even more preferably greater than 20 mm and less than 25 mm. The spacing between adjacent electrode rows (I in Fig. 3) is preferably greater than 15 mm and less than 30 mm, more preferably greater than 15 mm and less than 25 mm, and even more preferably greater than 15 mm and less than 20 mm. The distance between the closest needle-shaped electrodes (D in Fig. 3) may be greater than 15 mm and less than 100 mm, but is preferably greater than 15 mm and less than 30 mm, more preferably greater than 15 mm and less than 25 mm, and even more preferably greater than 15 mm and less than 20 mm.

[0071] The interval between adjacent electrode rows (I in FIG. 3) is preferably smaller than the distance between adjacent needle electrodes (L in FIG. 3).The needle electrode closest to the needle electrode of a certain electrode row is preferably the needle electrode of the adjacent electrode row (D in FIG. 3 is set between the needle electrodes of the adjacent electrode rows).

[0072] In the example shown in FIG. 3, the plurality of electrode rows 340a to 340e are arranged so that the positions of the needle electrodes in the TD direction do not overlap between adjacent electrode rows.

[0073] 4 is a schematic diagram showing in more detail the positions of the needle electrodes in the multiple needle electrode arrays in the example shown in FIG. 3. The position (coordinates) of one needle electrode 344a-1 in the electrode array 340a shown in FIG. 4 is expressed as (TD axis, MD axis) = (T １ , M １) In addition, the coordinates at which the needle electrode 344a-2 adjacent to the needle electrode 344a-1 in the electrode array 340a is arranged are defined as (TD axis, MD axis) = (T ２ , M １ ) on the electrode array 340a in this coordinate space. １ and T ２ The positions of the four points between and in the TD direction are T １－１ , T １－２ , and T １－３ Let's say.

[0074] In this embodiment, the needle-shaped electrode 340b-1 of the electrode row 340b adjacent to the electrode row 340a is T １ and T ２ The position in the TD direction is shifted by one of the four equal division points between １－１ ) and the intersection of the perpendicular line of the electrode row passing through the adjacent electrode row 340b, (TD axis, MD axis) = (T １－１ , M ２ ) and the needle-like electrode 340c-1 of the adjacent electrode row 340c is arranged at T １ and T ２ The position in the TD direction is shifted by two of the four equal divisions between and , and the other division points (here, T １－２ ) and the intersection of the perpendicular line of the electrode row passing through the adjacent electrode row 340c, (TD axis, MD axis) = (T １－２ , M ３ Similarly, the needle-shaped electrode 340c-1 of the electrode array 340d is arranged at T １ and T ２ The position in the TD direction is shifted by three of the four equal divisions between and , and other division points (here, T １－３ ) and the intersection of the perpendicular line of the electrode row passing through the adjacent electrode row 340d, (TD direction, MD direction) = (T １－３ , M ４ The other needle electrodes are arranged in the same way.

[0075] In this way, by displacing the positions of the needle-like electrodes in the TD direction between adjacent electrode rows, it is possible to suppress non-uniformity in the TD direction of the polarization treatment of the fluorine-based resin film 110. Furthermore, by dividing the distance between adjacent needle-like electrodes (needle-like electrodes 344a-1 and 344a-2) in a certain electrode row (electrode row 340a in the above example) at equally spaced points (T １－１ , T １－２ , T １－３ By arranging the needle-like electrodes (needle electrode 344b-1, needle electrode 344c-1, needle electrode 344d-1, etc.) of adjacent electrode rows (electrode row 340b, electrode row 340c, electrode row 340d, etc.) at the intersection of a perpendicular line of the electrode row passing through the electrode row and the adjacent electrode row, it is possible to perform a dense and more uniform polarization treatment on the fluorine-based resin film 110 while maintaining a distance such that the nearest needle-like electrodes do not interfere with each other.

[0076] The number of divisions when dividing the space between adjacent needle-shaped electrodes into equal intervals is not particularly limited, and a larger number of divisions is preferable. The larger the number of divisions, the shorter the inter-electrode distance in the TD direction of the needle-shaped electrodes can be while maintaining the inter-electrode distance of the needle-shaped electrodes, allowing for denser and more uniform polarization treatment. Two or more divisions are preferable, three or more divisions are more preferable, four or more divisions are even more preferable, and five or more divisions are particularly preferable. However, it is preferable to set the number of divisions so that the distance between each division point (L' in FIG. 4 ) is smaller than the distance between adjacent electrode rows (I in FIG. 4 ).

[0077] In this case, the number of electrode rows used to polarize the fluorine-based resin film 110 is preferably an integer multiple of the division number. This allows each divided position to be polarized evenly (the same number of times), thereby reducing variation in the piezoelectric constant of the manufactured fluorine-based resin piezoelectric film. Furthermore, increasing the number of electrode rows increases the polarization range in the MD direction, which is preferable because it increases the polarization rate of the film. The number of electrode rows may be 1 time, 2 times or more, or 3 times or more the division number. In order to increase the number of electrode rows while maintaining the distance between the needle-shaped electrodes, the diameter of the polarization roll must be increased. While it is theoretically possible to increase the number of electrode rows by increasing the polarization roll diameter, there is a limit to how much the polarization roll diameter can be increased, so in reality, a value of 10 times or less is preferable.

[0078] The DC voltage applied from each needle electrode in the polarization step is preferably 7.0 kV or more and 50.0 kV or less, and more preferably 7.5 kV or more and 30.0 kV or less.

[0079] After the polarization step, the fluororesin piezoelectric film can be wound into a roll for storage, transportation, and the like.

[0080] [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.

[0081] [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.

[0082] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0083] 1. Preparation of Piezoelectric Films 1-1. Piezoelectric Film 1 Vinylidene fluoride resin (KF Polymer manufactured by Kureha Corporation) with a melt viscosity of 2500 Pa s and a melting point of 173°C was supplied to a single-screw extruder with a bore of φ50 mm and thermally melted in the extruder and attached conduit set to 260°C. The resin was then passed through a polymer filter with a filtration accuracy of 20 μm, also set to 260°C, to remove foreign matter. The molten resin was then extruded into a film form from a T-die and cooled by contacting it with the surface of a cooling roll whose surface was set to 110°C, producing a vinylidene fluoride resin film (PVDF film) with a thickness of approximately 160 μm.

[0084] Next, using a uniaxial stretching device, the rotation speed ratio of the rolls was adjusted to stretch the PVDF film 4.2 times in the machine direction (MD direction) to produce a stretched film.

[0085] An electrode array was installed, with multiple needle-shaped electrodes arranged in a row at 15.3 mm intervals in the TD direction. Three electrode arrays were arranged parallel to this electrode array, with 15.0 mm intervals in the MD direction. The needle-shaped electrodes of an adjacent electrode array were offset by 5.1 mm in the TD direction (dividing 15.3 mm into thirds) from the positions of the needle-shaped electrodes of one electrode array. The multiple electrode arrays were then installed in the extension direction of the grounded polarization roll, with the distance between the tip of each needle-shaped electrode and the surface of the polarization roll being 10 mm.

[0086] In this state, the stretched PVDF film was passed between the polarizing roll and the multiple needle-shaped electrodes, and at the same time, a voltage of 8.0 kV or more was applied from the surface of the film in the thickness direction by each electrode, thereby obtaining piezoelectric film 1.

[0087] 1-2. Piezoelectric Films 2 to 5 and Piezoelectric Films 9 to 11 Piezoelectric films 2 to 5 and 9 to 11 were obtained in the same manner as piezoelectric film 1, except that the arrangement of the needle electrodes and the distance between the tip of the needle electrode and the surface of the polarizing roll were changed as shown in Tables 1 and 3. Films 9 and 10 had only one row of electrodes.

[0088] 1-3. Piezoelectric Film 6 Piezoelectric film 6 was obtained in the same manner as piezoelectric film 1, except that a PVDF film was produced from PVDF with a melt viscosity of 4500 Pa s without filtering it through a polymer filter, and the arrangement of the needle electrodes was changed as shown in Table 2.

[0089] 1-4. Piezoelectric Film 7 Piezoelectric film 7 was obtained in the same manner as piezoelectric film 1, except that PVDF having a melt viscosity of 800 Pa·s was used and the cooling roll temperature in the production of the PVDF film was set to 50°C.

[0090] 1-5. Piezoelectric Film 8 Piezoelectric film 8 was obtained in the same manner as piezoelectric film 1, except that a PVDF film with a melt viscosity of 3400 Pa·s was used.

[0091] 1-6. Piezoelectric Film 12 Piezoelectric film 12 was obtained in the same manner as for piezoelectric film 1, except that the cooling roll temperature in the preparation of the PVDF film was changed to 140° C. and the arrangement of the needle electrodes was changed as shown in Table 3.

[0092] 100 g of PVDF with a melt viscosity of 2500 Pa s and a melting point of 173°C was 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 filter with a filtration accuracy of 40 μm. The filtered resin solution was added to an automatic coater to prepare a coating film with a liquid thickness of 600 μm, which was then dried at 120°C for 1 hour to obtain a PVDF film.

[0093] The obtained PVDF film was not stretched but was subjected to the same polarization treatment as in Piezoelectric Film 1 to obtain Piezoelectric Film 13. Piezoelectric Film 14 was obtained in the same manner as Piezoelectric Film 1, except that it was stretched 2.0 times in the MD direction and needle-shaped electrodes were arranged as shown in Table 4.

[0094] 2. Evaluation of Piezoelectric Film 2-1. 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 total width L in the fast axis direction (width direction) determined by measuring the birefringence of the film was 520 mm, and the measurement width (L-20), which was the length obtained by subtracting 20 mm from L, was 500 mm. An arbitrary point on the midpoint of the line segment connecting both ends of the film was designated as point A. Ten points were set on the line segment in the width direction passing through point A, spaced 50 mm apart from each point toward both ends, equivalent to the value of (L-20) / 10, for a total of 11 measurement points. The average value of the thicknesses measured at these 11 measurement points was used as the representative value of the film thickness.

[0095] 2-2. Retardation The retardation of a film cut to a size of 20 mm x 20 mm was measured using a KOBRA-HB manufactured by Oji Scientific Instruments by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8 nm was taken as the retardation of the film. At this time, the fast axis and slow axis were determined from the in-plane birefringence of the film. The slow axis direction coincides with the average direction of the molecular chains migrated by stretching or extrusion. The film used here was stretched in the flow direction (machine direction) of the film, so the MD direction and the slow axis direction coincide.

[0096] 2-3. Piezoelectric constant d ３３ Piezoelectric constant d of fluororesin piezoelectric film ３３ is the direct quasi-static method (d ３３ Piezoelectric constant d by Mehta method, Berlincourt method ３３The measurement was performed in accordance with ISO 19622:2018, a test method for the piezoelectric constant measurement. Specifically, a piezoelectric constant measurement device (Piezometer System PM300, manufactured by PIEZOTEST) was used to hold a test piece, a film, 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 was applied. The charge measurement was performed on the polarization surface of the film, and the absolute value of the measured value was used to calculate the piezoelectric constant. An arbitrary point on the midpoint of the line segment connecting both ends in the fast axis direction obtained by measuring the birefringence of the film was designated as point A, and the piezoelectric constant d was measured at 21 measurement points, including point A and 20 points set on a line segment in the width direction passing through point A at intervals of 25 mm, which is a value equivalent to (L-20) / 20 from each point toward both ends. ３３ The average value of the piezoelectric constant d ３３ The standard deviation of the piezoelectric constant of the film was taken as the standard deviation σ. Furthermore, the standard deviation σ was divided by the average value to obtain the coefficient of variation CV.

[0097] 2-4. Internal Haze The internal haze of the fluorine-based resin piezoelectric film was measured by applying a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) to one surface of the film using a bar coater and drying at 80°C for 30 minutes. After that, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to irradiate the film with a target integrated light dose of 400 mJ / cm. ２ The film was irradiated with UV light so that the film was exposed to UV light to form a 2 μm thick coating layer. This coating layer removed external haze due to scratches on the film surface, and a film for internal haze measurement was prepared. Point A was an arbitrary point on the midpoint of the line segment connecting both ends in the fast axis direction, determined by measuring the birefringence of the film. Point A and 20 other measurement points were set on a line segment in the width direction passing through point A, spaced 25 mm apart from each other, corresponding to (L-20) / 20, toward both ends. These 21 measurement points were measured using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K 7136:2000. The average value of the haze measured at these 21 measurement points was used as a representative value for the internal haze of the film.

[0098] 2-5. Number of foreign objects Each film is cut out from adjacent positions in succession, with a spacing of 0.010 m. ２ Twenty-five rectangular films (observation pieces) each measuring 100 mm x 100 mm were cut out from the film. The sum of the number of foreign particles measured by observing each observation piece was 0.250 m ２ The number of foreign particles per unit area was calculated. At this time, 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 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 with a size of 100 μm to 200 μm, and foreign particles with a size of less than 100 μm was calculated.

[0099] 3. Results The production conditions and evaluation results of each piezoelectric film are shown in Tables 1 to 4. Film 13 was opaque with a large haze, and it was not possible to measure the number of foreign particles.

[0100]

[0101]

[0102]

[0103]

[0104] This application claims priority to Japanese Patent Application No. 2024-029029, filed February 28, 2024, and Japanese Patent Application No. 2024-029030, filed February 28, 2024. The entire contents of the specification, claims, and drawings of those applications as originally filed are incorporated herein by reference.

[0105] The fluororesin piezoelectric film according to the present invention has high transparency, a high piezoelectric constant, and small in-plane variation in the piezoelectric constant.

Claims

1. A fluorine-based resin piezoelectric film having a retardation of 100 nm or more and 2000 nm or less, an internal haze of less than 1.7%, and a piezoelectric constant d measured at 21 measurement points determined along the fast axis. ３３ The average value of the piezoelectric constant d ３３ A fluorine-based resin piezoelectric film, wherein the standard deviation of the piezoelectric constant is 2.0 pC / N or less.

2. The fluororesin piezoelectric film according to claim 1, wherein the average thickness measured at 11 measurement points along the fast axis is 10 μm or more and 300 μm or less.

3. Measurement temperature: 260°C, shear rate: 50 s －１ 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 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:

5. The fluororesin piezoelectric film according to claim 1, which contains a homopolymer of vinylidene fluoride.

6. A method for producing a fluororesin piezoelectric film according to any one of claims 1 to 5, comprising the steps of: heating and melting a fluororesin; forming the molten fluororesin into a film; uniaxially stretching the formed film by a factor of 2.5 to 6.0; and polarizing the formed film.

7. The method for producing a fluorine-based resin piezoelectric film according to claim 6, further comprising a step of filtering the molten fluorine-based resin through a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

8. The method for producing a fluorine-based resin piezoelectric film according to claim 6, further comprising a step of cooling the formed film by contacting it with a cooling roll having a surface temperature of 125°C or less.

9. A method for producing a fluorine-based resin piezoelectric film, comprising a step of polarizing a film passing between a polarization roll and a plurality of needle-like electrodes by direct current discharge between each of the plurality of needle-like electrodes and the polarization roll, wherein the plurality of needle-like electrodes are all positioned such that the distance between the tip of the needle-like electrode and the surface of the polarization roll is 5 mm or more and 30 mm or less, and the distance between the nearest adjacent needle-like electrodes is more than 15 mm and 100 mm or less.

10. The method for producing a fluorine-based resin piezoelectric film according to claim 9, wherein the plurality of needle-like electrodes are arranged in a row in a direction (TD direction) perpendicular to the direction in which the film passes (MD direction) to form an electrode row.

11. The method for producing a fluorine-based resin piezoelectric film according to claim 10, wherein the needle-like electrodes arranged in a row are spaced at equal intervals.

12. The method for producing a fluorine-based resin piezoelectric film according to claim 10 or 11, wherein a plurality of the electrode rows are arranged in the MD direction.

13. A method for producing a fluorine-based resin piezoelectric film according to claim 12, wherein the needle-like electrodes constituting a certain electrode row and the needle-like electrodes constituting an electrode row adjacent to said electrode row are arranged so as not to overlap in the MD direction, and the needle-like electrodes constituting the adjacent electrode row are arranged at the intersection of a perpendicular to said electrode row and said electrode row and the adjacent electrode row, passing through a point dividing the space between adjacent needle-like electrodes in said electrode row at equal intervals.

14. The method for producing a fluorine-based resin piezoelectric film according to claim 13, wherein the number of electrode rows is an integer multiple of the number of divisions obtained by dividing the space between adjacent needle-like electrodes in the electrode rows.

15. A method for producing a fluororesin piezoelectric film according to claim 9, comprising: a step of heating and melting a fluororesin; a step of forming the molten fluororesin into a film; and a step of uniaxially stretching the formed film by 2.5 times or more and 6.0 times or less, wherein the formed film is polarized in the polarization treatment step.

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