Fluororesin piezoelectric film and method for producing the same

Abstract

To provide a fluororesin piezoelectric film exhibiting superior transparency, a high piezoelectric constant, and reduced fluctuation of the piezoelectric constant in the in-plane direction.SOLUTION: 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%, an average value of a piezoelectric constant d33 measured at 21 measurement points defined in a fast axis direction being 6.0 pC / N or more and 40.0 pC / N or less, and a standard deviation of the piezoelectric constant d33 being 2.0 pC / N or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] It is known that fluorine-based resin films can be transformed into piezoelectric films with high piezoelectricity by polarization treatment (see, for example, Patent Document 1). Piezoelectric bodies made from 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 while the fluororesin film is passing between a needle-like electrode and a metal roll (sometimes called a "polarizing roll" to distinguish it from a transport roll) (see, for example, Patent Document 2). Since it is necessary to polarize the entire surface of the fluororesin film, 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 transport 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 those 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 [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 60-047034 [Patent Document 2] Japanese Patent Application Publication No. 2019-067908 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-219804 Summary of the Invention [Problem to be solved by the invention]

[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 in-plane variation in piezoelectric constant. 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. [Means for solving the problem]

[0009] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [1] to [5]. [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 is 6.0 pC / N or more and 40.0 pC / N or less, and the piezoelectric constant d 33 A fluorine-based resin piezoelectric film having a standard deviation of 2.0 pC / N or less. [2] The fluorine-based resin piezoelectric film according to [1], wherein the average thickness measured at 11 measurement points determined in the fast axis direction 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 [1] or [2], having 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 any one of [1] to [3] below: [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 the following methods for producing a fluorine-based resin piezoelectric film [6] to

[14] . [6] A method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [5], comprising the steps of: heating and melting a fluorine-based resin; forming the molten fluorine-based resin into a film; uniaxially stretching the formed film by 2.5 times or more and 6.0 times or less; and polarizing the formed film. [7] The method for producing a fluorine-based resin piezoelectric film according to [6], further comprising the 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 [6] or [7], 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. [6] A method for producing a fluorine-based resin piezoelectric film according to any one of [6] to [8].

[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 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

[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 any one of [9] to

[11] , wherein a plurality of the electrode rows 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 the 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 between adjacent needle-like electrodes constituting the electrode rows.

[15] A 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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing how a fluorine-based resin film is stretched and polarized. [Figure 2] FIG. 2 is a schematic diagram showing an example of the arrangement positions of the needle electrodes in the needle electrode array relative to the surface of the polarizing roll. [Figure 3] FIG. 3 is a schematic diagram showing another example of the positions of the needle electrodes in a plurality of needle electrode rows relative to the surface of the polarizing roll. [Figure 4] FIG. 4 is a schematic diagram showing in more detail the positions of the needle electrodes in the arrays of needle electrodes in the example shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Fluoroplastic piezoelectric film] An embodiment of the present invention relates to a fluorine-based 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 TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP). 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] Among 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.

[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 constituent 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] There are no particular limitations on the fluorine-based resin piezoelectric film that can be used, but the measurement temperature should be 260°C and the shear rate should be 50 s -1 The melt viscosity measured by HPLC 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. This eliminates the need for a high melt temperature to enable filtration, and reduces the risk of the film surface becoming uneven due to heat-induced resin denaturation and the generation of foreign matter. Furthermore, the higher the melt viscosity, the larger the resin molecular weight, making selective alignment of molecular chains difficult and less likely to crystallize, resulting in excellent transparency and stable film properties. Furthermore, the higher the melt viscosity, the less likely the resin constituting the film is to crystallize, making it less likely to experience light scattering due to differences in refractive index between the crystalline and amorphous portions, and therefore less likely to deteriorate transparency.

[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 using a capillary die with an inner diameter of 1 mm and a tube length of 10 mm. -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 process.

[0021] The retardation is measured using the parallel Nicol rotation method (KOBRA-HB, manufactured by Oji Scientific Instruments Co., Ltd.) at a 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 the 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 will 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 fluororesin 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 cumulative light dose of 400 mJ / cm. 2 The film was irradiated with UV light so that the film was coated with a 2 μm thick coating layer. A similar coating layer was formed on the other surface. This coating layer removed external haze caused by scratches on the film surface, allowing the preparation of a film for measuring internal haze.

[0024] The direction of the fast axis determined by measuring the birefringence of the film 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 is defined as point A, 21 measurement points are set along 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 the haze measured at these 21 measurement points is defined as the representative value of the internal haze of the film 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% or more and 10.0% or less, more preferably 0.0% or more and 6.0% or less, even more preferably 0.0% or more and 5.0% or less, particularly preferably 0.0% or more and 3.0% or less, and most preferably 0.0% or more and 2.0% or less. 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 in which the coating layer does not remove external haze caused by scratches on the film surface, etc. The average value of the haze measured at the 21 measurement points set in accordance with JIS K 7136:2000 is used as the representative value of the haze of the film.

[0027] Fluorine 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 d33 The standard deviation σ of 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, even 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 Measurements are performed in accordance with ISO 19622:2018, a test method for piezoelectric constant measurement. Specifically, a piezoelectric constant measurement device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold a fluororesin piezoelectric film specimen 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. 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 total width length L mm. Point A is defined as an arbitrary point on the midpoint of the line segment connecting both ends of the film. The average value of the piezoelectric constants measured at 21 measurement points, including point A and 20 points set on the line segment passing through point A in the width direction from point A toward both ends, at intervals of (L-20) / 20 from each point, is used to determine 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. 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 piezoelectricity. A thinner film is more advantageous in terms of optical properties such as transparency and cost.

[0032] The thickness of fluororesin piezoelectric films is typically measured using a micrometer (JIS C 2151:2019), but can also be measured using other well-known methods, such as a laser displacement meter, a capacitance displacement meter, or an infrared ray. Thickness measurements are performed at measurement points set using 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 total width (L mm minus 20 mm). Point A is an arbitrary point on the midpoint of the line segment connecting both ends of the film. Point A and 10 other points along the widthwise line segment passing through point A, spaced apart by (L-20) / 10 from each point toward both ends, form a total of 11 thickness measurement points. 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 to 2000 mm, more preferably 250 mm to 1750 mm, even more preferably 250 mm to 1500 mm, and particularly preferably 300 mm to 1300 mm. 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 a plane, the number of foreign particles with a size of 100 μm or more is 7 per 0.25 m 2 The following is preferred: 0 pieces / 0.25m2 More than 5 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 3 pieces / 0.25m 2 The following is even more preferable: 0 pieces / 0.25m 2 More than 1 piece / 0.25m 2 The following are particularly preferred:

[0035] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles larger than 200 μm is 0 per 0.25 m 2 More than 3 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 2 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 1 piece / 0.25m 2 The following is even more preferred:

[0036] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles less than 100 μm in size is 0 / 0.25 m 2 More than 50 pieces / 0.25m 2 Preferably, 0 pieces / 0.25m or less 2 More than 25 pieces / 0.25m 2 It is more preferable that the number is less than 0 / 0.25m. 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 particles is determined by cutting 25 rectangular films (observation pieces) adjacent to each other from a fluororesin film having a thickness of 40 μm or less, and then calculating the sum of the number of foreign particles 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 particles is measured by the method described below. Specifically, 0.010 m thick films are cut out so that they are adjacent to each other from the fluororesin film. 2 Twenty-five pieces of the above specimen, each measuring 100 mm x 100 mm, are cut out. The total 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 it can satisfy the above physical properties.

[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 of manufacturing fluororesin piezoelectric film] The method for producing the above-mentioned fluororesin piezoelectric film is not particularly limited, but preferably includes the steps of: producing a fluororesin film by extrusion molding a heat-molten 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: a step of heating and melting the above-mentioned fluororesin (melting step), a step of filtering the molten fluororesin (filtration step), a step of forming the filtered fluororesin into a film (film formation step), a step of stretching the formed film (stretching step), and a step of 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 process) 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 remaining solvent component without volatilization may interfere with polarization in a subsequent step, so 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 75°C or more higher than the melting point of the fluororesin but not more than 105°C higher, more preferably 75°C or more higher but not more than 100°C higher, even more preferably 80°C or more higher but not more than 100°C higher, and particularly preferably 85°C or more higher but not more than 95°C higher. Setting the melting temperature at 75°C or more higher than the melting point of the fluororesin can reduce the viscosity of the fluororesin to a level that allows it to be filtered in the subsequent process. Setting the melting temperature at 105°C or less higher than the melting point of the fluororesin can suppress decomposition and condensation of the fluororesin due to heating, thereby suppressing the generation of decomposition products and the like. Suppressing the generation of decomposition products and the like reduces the amount of foreign matter in the fluororesin piezoelectric film and improves the transparency and smoothness of the fluororesin piezoelectric film. Suppressing the generation of decomposition products and the like also suppresses filter clogging and improves the filtration efficiency of the fluororesin.

[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 process) In the filtration step, it is preferable to filter the fluororesin that has been melted and reduced in viscosity in the melting step. 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 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 fluororesin that has been heated to the above temperature and melted, and also prevents the filtration pressure from becoming too high, allowing for a shorter filtration time. Using a filter with a filtration accuracy of 40 μm or less allows for sufficient removal of foreign matter from the fluororesin, 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.

[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 fluororesins 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 using a filter with low filtration accuracy (large filtration accuracy value) placed in the front stage, and then finer foreign matter may be removed using 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 process) 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 rapidly cooling 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 production, a method in which the formed film is transported and continuously wound up is preferred. From the viewpoint of mass production, the longer the length of the film in the direction perpendicular to the transport direction (film width) in the film formation process and the subsequent stretching process, the better. On the other hand, the smaller the film width, the easier it is to control the film formation conditions, and therefore the higher the film formation accuracy tends to be. 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 (stretching process) before or simultaneously with the polarization process. The stretching direction in the stretching process 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 process 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] (Stretching process) 1 is a schematic diagram showing the process of successively stretching and poling a fluororesin film. Fluororesin film 110 is stretched by stretching roll 122 and stretching roll 124, and then polarized on polarization roll 130 by direct current discharge from multiple needle-like electrodes 144.

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

[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 process. By setting the stretching ratio to 6.0 times or less, the fluororesin film is less likely to break during the stretching process.

[0062] (Polarization process) In the polarization process, a DC discharge is applied to the grounded polarization roll 130 from each of the multiple needle electrodes 144, all of which are connected to a DC high-voltage power supply 146. Then, the fluororesin film 110, which is in close contact with the surface of the polarization roll 130, is passed between the DC-discharged needle 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. More preferably, the distance is 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 all 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-like electrodes 144 closely. However, according to the findings of the present inventors, it has become clear that if the distance between the needle-like electrodes 144 is reduced, the electric fields between the needle-like 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-like electrodes 144 so that the distance between the nearest needle-like electrodes 144 is more than 15 mm and 100 mm or less. If the distance between the nearest needle-like electrodes 144 is more than 15 mm, unevenness in the electric field strength due to interference between the electric fields generated by the needle-like 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 created 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 nearest adjacent needle-like electrodes is within the above range, the arrangement of the needle-like electrodes is not particularly limited and may be, for example, randomly arranged. However, in order to polarize the entire width direction of the fluorine-based resin film 110 densely and more uniformly, it is preferable that the multiple needle-like electrodes are arranged in a row in the TD direction to form an electrode array. In this case, the distance between the multiple needle-like electrodes arranged in a row is preferably equal. There may be only one electrode array, or multiple electrode arrays may be arranged at intervals in the transport direction of the fluorine-based resin film. In this case, the distance between the multiple electrode arrays is preferably equal. In the example shown in FIG. 1, three electrode arrays are arranged at equal intervals. In order to uniformly discharge the needle-like electrodes, it is preferable that the needle-like electrodes 144 are arranged perpendicular to the polarization roll surface.

[0066] 2 is a schematic diagram showing an example of the arrangement position of needle electrodes in needle electrode array 144 relative to the surface of polarization roll 130. In FIG. 2, polarization roll 130 rotates so that its surface moves from top to bottom in the figure, thereby transporting fluororesin film 110, which is in close contact with the surface of polarization roll 130, from top to bottom (MD direction) in FIG. 2. In this specification, the transport direction of fluororesin film 110 (the direction of the arrow in each figure) is also referred to as MD direction, and the direction along the surface of fluororesin film 110 that is perpendicular to the MD direction is also referred to as 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 or equal to 100 mm, preferably greater than 15 mm and less than or equal to 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 needle electrodes 144 relative to the surface of polarization roll 130. In Figure 3, polarization roll 130 also rotates so that its surface moves from top to bottom (MD direction) in the figure, thereby moving fluorine-based resin film 110, which is in close contact with the surface of polarization roll 130, from top to bottom (MD direction) in Figure 3.

[0069] 3, multiple electrode rows 340a to 340e are arranged in the extension direction on 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 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 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). It is also preferable that the needle electrode closest to the needle electrode of a certain electrode row is 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 defined as (TD axis, MD axis) = (T1, M1). The position (coordinates) of the needle electrode 344a-2 adjacent to the needle electrode 344a-1 in the electrode array 340a is defined as (TD axis, MD axis) = (T2, M1). The positions in the TD direction of each of the points dividing the space between T1 and T2 on the electrode array 340a in this coordinate space into four equal parts are defined as T 1-1 , T 1-2 , and T 1-3 Let's say.

[0074] In this embodiment, the needle-like electrode 340b-1 of the electrode row 340b adjacent to the electrode row 340a is shifted in position in the TD direction by one of the four equal divisions between T1 and T2, and is positioned at the other division point (here, T 1-1 ) and the adjacent electrode row 340b, (TD axis, MD axis)=(T 1-1 , M2). The needle-like electrode 340c-1 of the adjacent electrode array 340c is shifted in position in the TD direction by two of the four equal divisions between T1 and T2, and is positioned at another division point (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 Similarly, the needle-like electrode 340c-1 of the electrode array 340d is shifted in position in the TD direction by three of the four divisions between T1 and T2, and is arranged at 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 , M4). 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 the perpendicular line of the electrode row passing through (340a), the fluorine-based resin film 110 can be polarized more densely and more uniformly while maintaining a distance that prevents the nearest needle-like electrodes from interfering with each other.

[0076] The number of divisions when dividing the space between adjacent needle-shaped electrodes into equal intervals is not particularly limited, but the greater the number of divisions, the better. The greater 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 preferred, three or more divisions are more preferred, four or more divisions are even more preferred, and five or more divisions are particularly preferred. 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, as the number of electrode rows increases, the polarization range in the MD direction increases, 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-like electrodes, it is necessary to increase the diameter of the polarization roll. 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 to 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] [Application] The above-mentioned fluorine-based resin piezoelectric film can be used in various applications such as touch sensors and touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0081] [Other embodiments] It should be noted that the above-described embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept. [Example]

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

[0083] 1. Fabrication of Piezoelectric Film 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 fed into a 50mm diameter single-screw extruder 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 20μm, 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, the surface of which was set to 110°C, to produce a polyvinylidene fluoride resin film (PVDF film) with a thickness of approximately 160μm.

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

[0085] An electrode row was installed, with multiple needle-shaped electrodes arranged in a row at 15.3 mm intervals in the TD direction. Three electrode rows were arranged parallel to this electrode row, with 15.0 mm intervals in the MD direction. The needle-shaped electrodes of an adjacent electrode row were offset by 5.1 mm in the TD direction (dividing 15.3 mm into thirds) from the positions of the needle-shaped electrodes in one electrode row. The multiple electrode rows 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 above-mentioned 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 9 to 11 Piezoelectric films 2 to 5 and 9 to 12 were obtained in the same manner as piezoelectric film 1, except that the arrangement of the needle electrodes and the distance between the tips of the needle electrodes and the surface of the polarizing roll were changed as shown in Tables 1 and 3. Films 9 and 10 had only one electrode row.

[0088] 1-3.Piezoelectric film 6 Piezoelectric film 6 was obtained in the same manner as piezoelectric film 1, except that a PVDF film with a melt viscosity of 4500 Pa·s was produced without filtering through a polymer filter, and the needle electrode arrangement 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 with 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 piezoelectric film 1, except that the cooling roll temperature in the production of the PVDF film was changed to 140° C. and the arrangement of the needle electrodes was changed as shown in Table 3.

[0092] 100g of PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was added to 900ml of n-methylpyrrolidone (NMP). The mixture was heated to 60°C while stirring with a stirrer, and stirring was continued for 6 hours to create a resin solution. This resin solution was filtered through a filter with a filtration accuracy of 40μm. The filtered resin solution was added to an automatic coating machine to create a coating film with a liquid thickness of 600μm, 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-like 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. Point A and 10 points were set on the line segment in the width direction passing through point A, spaced 50 mm apart from each other, corresponding to the value of (L-20) / 10, from each point toward both ends, for a total of 11 measurement points. The average value of the thickness 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 20mm x 20mm was measured using the parallel Nicol rotation method using a KOBRA-HB made by Oji Scientific Instruments. The value at a measurement wavelength of 587.8nm 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 that have migrated due to 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 33Measurements were performed in accordance with ISO 19622:2018, a test method for piezoelectric constant measurement. Specifically, a piezoelectric constant measurement device (Piezometer System PM300, manufactured by PIEZOTEST) was used to hold the film specimen 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. Point A was an arbitrary point on the midpoint of the line segment connecting both ends in the fast axis direction, which was determined by measuring the birefringence of the film. Point A was then set on the line segment in the width direction passing through point A, and 20 points were set at intervals of 25 mm from point A toward both ends, which corresponds to (L-20) / 20. The piezoelectric constant d was measured at 21 measurement points. 33 The average value of the piezoelectric constant d 33 The standard deviation of the piezoelectric constants 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 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 cumulative light dose of 400 mJ / cm. 2The film was irradiated with UV light so that a coating layer with a thickness of 2 μm was formed. 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 the representative value of the internal haze of the film.

[0098] 2-5. Number of foreign objects 0.010m so that each film is cut from adjacent positions in succession 2 Twenty-five rectangular films (observation pieces) 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. The foreign particles were observed using transmitted light, marked, and the marked areas were then observed under a microscope to determine the size of the foreign particles. The size of the foreign particles was calculated as the arithmetic mean value of the maximum and minimum widths of the foreign particles. In this way, the number of foreign particles larger than 200 μm, foreign particles between 100 μm and 200 μm, and foreign particles smaller than 100 μm were calculated.

[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] [Table 1]

[0101] [Table 2]

[0102] [Table 3]

[0103] [Table 4] [Industrial Applicability]

[0104] 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, The retardation is 100 nm or more and 2000 nm or less, The internal haze is less than 1.7%; The piezoelectric constant d measured at 21 measurement points determined along the fast axis ３３ The average value of the piezoelectric constant d ３３ The standard deviation is 2.0 pC / N or less. Fluorine-based resin piezoelectric film.

2. the average thickness measured at 11 measurement points determined in the fast axis direction is 10 μm or more and 300 μm or less; The fluorine-based resin piezoelectric film according to claim 1 .

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

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

5. including homopolymers of vinylidene fluoride, The fluorine-based resin piezoelectric film according to claim 1 .

6. a step of heating and melting a fluororesin; forming the molten fluorine-based resin into a film; uniaxially stretching the formed film by 2.5 times or more and 6.0 times or less; Polarizing the deposited film; The method for producing a fluorine-based resin piezoelectric film according to any one of claims 1 to 5, comprising:

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

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

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, each of the plurality of needle-like electrodes is arranged at a position where 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; A method for manufacturing a fluorine-based resin piezoelectric film.

10. 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; The method for producing the fluorine-based resin piezoelectric film according to claim 9 .

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

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

13. 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, a needle-like electrode constituting the adjacent electrode row is disposed at an intersection of a perpendicular line to the electrode row and the adjacent electrode row, the perpendicular line passing through a point dividing the space between adjacent needle-like electrodes in the electrode row at equal intervals; The method for producing the fluorine-based resin piezoelectric film according to claim 12 .

14. the number of the 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; The method for producing the fluorine-based resin piezoelectric film according to claim 13 .

15. a step of heating and melting a fluororesin; forming the molten fluorine-based resin into a film; and uniaxially stretching the formed film by a factor of 2.5 to 6.

0. In the polarization treatment step, the deposited film is polarized. The method for producing a fluorine-based resin piezoelectric film according to claim 9 , comprising:

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