Fluorine-based resin film, method for producing same, method for producing piezoelectric film, and multilayer piezoelectric body

Abstract

Disclosed is a fluorine-based resin film which has a melt viscosity η of 600 Pa∙s to 4,000 Pa∙s inclusive as measured at a measurement temperature of 260°C and a measurement shear rate of 50 s-1. The fluorine-based resin film has a surface roughness height Rz of 0.50 μm or less as measured in accordance with JIS B 0601 (2001), and the number of foreign substances having a size of 100 μm or more is 7 per 0.25 m2 or less, the size being the arithmetic average value of the maximum width and the minimum width when the film is viewed in plan.

Description

Fluorine-based resin film and its manufacturing method, piezoelectric film manufacturing method, and laminated piezoelectric body

[0001] The present invention relates to a fluororesin film and a method for producing the same, a method for producing a piezoelectric film, and a laminated piezoelectric element.

[0002] Fluorine-based resin films have excellent properties such as weather resistance, heat resistance, contamination resistance, chemical resistance, solvent resistance, mechanical properties, and ease of secondary processability, and are therefore used in a variety of applications such as interior and exterior building components, surface materials for molded products, surface or back surface protective materials for solar cell modules, fuel cell components, etc. In particular, because of their high weather resistance, contamination resistance, and chemical resistance, they are expected to be used in protective films and optical components, and there is a great demand for fluorine-based films that are highly transparent and contain little foreign matter.

[0003] It is also known that some fluorine-based resin films exhibit piezoelectricity through polarization treatment. Piezoelectric bodies made of fluorine-based resin films are expected to have high transparency and flexibility, and therefore are being considered for use in transparent touch panels and the like. In this specification, the term "fluorine-based resin piezoelectric film" or "piezoelectric film" refers to a film having a piezoelectric constant d measured by the method described below. ３３ The term "fluorine-based resin film" means a fluorine-based resin film having a modulus of 1.0 pC / N or more.

[0004] During the production of fluororesin, foreign matter such as polymerized lumps may be generated. Such foreign matter may reduce the transparency of the fluororesin film or cause non-uniform polarization. Therefore, foreign matter may be removed from the fluororesin before the film is produced. For example, in Patent Document 1, a solution in which a fluororesin is dissolved in methyl ethyl ketone is filtered through a filter to remove foreign matter. In addition, in Patent Document 2, a solution in which a fluororesin is dissolved in a fluorinated aliphatic solvent is filtered through a filter to remove foreign matter.

[0005] International Publication No. 2015 / 064324 Japanese Patent Application Laid-Open No. 2020-164781

[0006] As described in Patent Documents 1 and 2, methods are known in which a fluororesin is dissolved in a solvent and then filtered. When filtration is performed using these methods, the solvent evaporates during subsequent film formation, which tends to cause unevenness on the film surface. Furthermore, since a large amount of polar organic solvent is used to dissolve the fluororesin, consideration must be given to the working environment and the natural environment, and production costs are relatively high due to the need to recover the solvent, etc.

[0007] Furthermore, in the solution casting method, a large amount of polar organic solvent is used to dissolve the fluororesin, which not only requires solvent recovery, resulting in higher production costs, but also requires consideration of the impact of the organic solvent on the working environment and the natural environment. Furthermore, because a polar organic solvent is used to dissolve the fluororesin, if the solvent remains in the fluororesin film, there is a risk that polarization in the manufacturing process of the piezoelectric film will be hindered. Therefore, it is preferable to form a film by extrusion molding a thermally molten fluororesin without using an organic solvent.

[0008] On the other hand, in the method of melting a fluororesin and extruding it, there is a risk of problems such as the resin being denatured when heated to a high temperature, resulting in the generation of decomposed substances (foreign matter).

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a fluororesin film having little foreign matter and little surface irregularity, a method for manufacturing the same, a method for manufacturing a piezoelectric film, and a laminated piezoelectric element.

[0010] One embodiment of the present invention for solving the above problems relates to the following fluororesin films [1] to [4]. [1] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A fluorine-based resin film having a melt viscosity η of 600 Pa·s or more and 4000 Pa·s or less, measured in accordance with JIS B 0601:2001, in which the surface roughness Rz on the surface on the smaller side is 0.50 μm or less, and 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[2] The fluororesin film according to [1], which contains a structural unit derived from vinylidene fluoride as a main component. [3] The fluororesin film according to [1] or [2], which is a film for a piezoelectric film and has a thickness of 80 μm or more and 1000 μm or less. [4] A piezoelectric constant d measured in accordance with ISO 19622:2018 ３３ The fluorine-based resin film according to any one of [1] to [3], wherein the viscosity is 5.0 pC / N or more and 40.0 pC / N or less.

[0011] One embodiment of the present invention for solving the above problems relates to the following methods for producing a fluorine-based resin film [5] to [6]. [5] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ [6] A method for producing a fluorine-containing resin film according to any one of [1] to [3], comprising the steps of: heating and melting a fluorine-containing resin having a melt viscosity η of 600 Pa s to 4000 Pa s as measured by a method using a filter having a filtration accuracy of 10 μm to 40 μm; and filtering the fluorine-containing resin melted at said temperature through a filter having a filtration accuracy of 10 μm to 40 μm; and forming the filtered fluorine-containing resin into a film. [7] A method for producing a piezoelectric film according to [4], comprising the steps of: stretching the fluorine-containing resin film according to any one of [1] to [3] at a draw ratio of 2.5 to 6.0; and polarizing the stretched film by applying a DC voltage of 7.0 kV to 50.0 kV.

[0012] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [1] to [3]. [1] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A fluorine-based resin piezoelectric film having a melt viscosity η of 600 Pa·s or more and 4000 Pa·s or less, as measured by a method of measuring a thickness of 100 μm or more, wherein 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 the film is viewed in plan, is 7 / 0.25 m ２ The retardation is 100 nm or more and 2000 nm or less, and the piezoelectric constant d ３３[2] The fluororesin piezoelectric film according to [1], wherein the surface roughness in height Rz of the surface on the smaller side measured in accordance with JIS B 0601:2001 is 0.50 μm or less. [3] The fluororesin piezoelectric film according to [1] or [2], wherein the fluororesin piezoelectric film contains, as a main component, a structural unit derived from vinylidene fluoride.

[0013] One embodiment of the present invention for solving the above problem relates to the following method for producing a fluorine-based resin piezoelectric film [4]. [4] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ a step of melting a fluororesin having a melt viscosity η of 600 Pa s or more and 4000 Pa s or less, as measured by a method of measuring the melt viscosity η, at a temperature that is 75°C or more and 105°C or less higher than the melting point of the fluororesin; a step of filtering the molten fluororesin through a filter having a filtration accuracy of 10 μm or more and 40 μm or less; a step of molding the filtered fluororesin into a film; a step of stretching the molded film; and a step of polarizing the molded film.

[0014] One embodiment of the present invention for solving the above problems relates to a laminated piezoelectric element according to the following [1] to [6]. [1] A laminated piezoelectric element comprising the fluororesin piezoelectric film according to any one of [1] to [3], wherein the laminated piezoelectric element has a total light transmittance of 80% or more. [2] A laminated piezoelectric element having a surface resistivity of 1.0 × 10, disposed on at least one surface of the fluororesin piezoelectric film. －１ Ω / sq. More than 1.0×10 ４Ω / sq. or less. [3] The laminated piezoelectric body according to [2], wherein the electrode layer comprises at least one material selected from the group consisting of a metal film, a metal oxide film, a metal nanowire, a metal mesh, a conductive polymer compound, a carbon nanotube, and graphene. [4] The laminated piezoelectric body according to any one of [1] to [3], further comprising a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film. [5] The laminated piezoelectric body according to any one of [1] to [3], further comprising a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film, wherein the hard coat layer has a surface resistivity of 1.0×10 ４ Ω / sq. More than 1.0×10 ９ [6] The laminated piezoelectric element according to any one of [1] to [4], further comprising an antistatic layer having a surface resistivity of 1.0×10 Ω / sq. or less. [6] An antistatic layer and a hard coat layer are disposed in this order on at least one surface of the fluorine-based resin piezoelectric film, and the surface resistivity measured on the hard coat layer is 1.0×10 Ω / sq. or less. ６ Ω / sq. More than 1.0×10 １２ The laminated piezoelectric element according to any one of [1] to [5], wherein the dielectric constant is Ω / sq. or less.

[0015] According to the present invention, there are provided a fluororesin film having little foreign matter and little surface irregularity, a method for producing the same, a piezoelectric film, and a method for producing a laminated piezoelectric element.

[0016] FIG. 1 is a schematic diagram showing a laminated piezoelectric element according to one embodiment of the present invention.

[0017] [Fluorine-based resin film] One embodiment of the present invention relates to a fluorine-based resin film. The fluorine-based resin film may be an unstretched film or a stretched film.

[0018] The fluororesin film may be any film containing a fluororesin as a main component. A fluororesin is a resin obtained by polymerizing a monomer made of a fluorine-containing olefin, and containing a fluororesin as a main component means that the content of structural units derived from the fluororesin monomer relative to the total mass of the fluororesin film is 50% by mass or more. The content of the fluororesin relative to the total mass of the 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.

[0019] 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), hexafluoropropylene (HFP), or the like. Examples of fluororesins obtained by polymerizing VDF include homopolymers of VDF, and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene, TFE, CTFE, tetrafluoropropene, HFP, perfluoroalkyl vinyl ether, or the like.

[0020] Of these, from the viewpoint of facilitating polarization of the fluororesin film, fluororesins (VDF resins) obtained by polymerizing monomers containing 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, TFE and CTFE, and copolymers of vinylidene fluoride 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.

[0021] The fluororesin film preferably contains a structural unit derived from VDF as a main component, and a VDF homopolymer is most preferred. "Containing a structural unit derived from VDF as a main component" means that the content of the structural unit derived from VDF relative to the total mass of the fluororesin film is 50% by mass or more. The content of the structural unit derived from VDF relative to the total mass of the fluororesin film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0022] Among these, since fluororesin films and fluororesin piezoelectric films are expected to have high piezoelectricity, the higher the content of vinylidene fluoride homopolymer, the better. The content of the homopolymer relative to the total mass of the fluororesin 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.

[0023] The content of the resin having VDF as a constituent unit contained in the fluororesin film and the fluororesin piezoelectric film is １９ It can be measured by quantitative analysis using an internal standard using F-NMR.

[0024] The fluorine-based resin film was measured at a temperature of 260°C and a shear rate of 50 s －１ The melt viscosity measured by is 600 Pa·s or more and 4000 Pa·s or less, preferably 600 Pa·s or more and 3500 Pa·s or less, and 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, there is no need to increase the melting temperature to enable filtration, and the occurrence of spot-like irregularities on the film surface due to heat-induced resin denaturation and the generation of foreign matter can be suppressed. Furthermore, the higher the melt viscosity, the larger the molecular weight of the resin, making selective alignment of molecular chains more difficult and less likely to crystallize. Therefore, light scattering due to the difference in refractive index between the crystalline and amorphous portions is less likely to occur, increasing the transparency of the film and stabilizing various physical properties of the film.

[0025] 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.

[0026] The thinner the film, the more likely it is that foreign matter in the resin will be present near the surface, and so foreign matter tends to be easily detected by a planar view of the film. Therefore, to quantify the amount of foreign matter in a film, the film thickness used for foreign matter measurement is set to 40 μm or less. When a fluororesin film with a film thickness of 40 μm or less is viewed in plan, the number of foreign matter particles with a size of 100 μm or more is 7 per 0.25 m. ２ or less, 0 pieces / 0.25m ２ 5 or more pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 3 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:

[0027] When a fluororesin film with a thickness of 40 μm or less 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 More preferably, 0 pieces / 0.25 m ２ is particularly preferred.

[0028] When a fluororesin film with a thickness of 40 μm or less is viewed in plan, the number of foreign particles with a size of less than 100 μm 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 ２More preferably, 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following is even more preferred:

[0029] The less these foreign matters there are, the more the transparency of the fluororesin film is improved, and when the fluororesin film is stretched or polarized, the more uniform the stretching or polarization can be achieved.

[0030] 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. ２ Twenty-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.

[0031] The fluororesin film may further satisfy one or more of the following properties:

[0032] The fluororesin film preferably has a surface height roughness Rz of 0.50 μm or less, more preferably 0.01 μm or more and 0.50 μm or less, even more preferably 0.01 μm or more and 0.40 μm or less, and particularly preferably 0.01 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin film, the less likely it is that haze will occur on the film surface, resulting in excellent transparency. Furthermore, a small surface height roughness Rz indicates fewer irregularities such as wrinkles on the film.

[0033] The surface height roughness Rz of a fluororesin film is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter conforming to JIS B 0601:2001 (Keyence Corporation, Shape Analysis Laser Microscope VK-X260) is used. The intersection of the diagonals of a rectangular fluororesin film is defined as center point A, and the long side direction is defined as the width direction. The surface height roughness Rz is measured at a total of three measurement points: center point A (measurement point) and two measurement points set 30 mm away from center point A toward both ends on a line segment passing through midpoint A parallel to the long side. The average of these measurements is taken as the surface height roughness of the fluororesin film. When the fluororesin film has a circular or polygonal shape, the film is cut into a rectangle with the largest area, and the surface height roughness is measured using the method described above. The surface of the fluororesin film that comes into contact with the chill roll tends to have a smaller surface roughness Rz than the surface that does not come into contact with the roll. This is because the fluororesin film is pressed against the chill roll, reducing surface irregularities. Here, the measurement result for the surface with the smaller surface roughness Rz (the surface that comes into contact with the chill roll) is referred to as the surface height roughness Rz.

[0034] The thickness of the fluororesin film is not particularly limited, but for a film for a piezoelectric film before stretching and polarization treatment, it is preferably 80 μm to 1000 μm, more preferably 100 μm to 500 μm, even more preferably 100 μm to 300 μm, still more preferably 120 μm to 200 μm, and even more preferably 120 μm to 180 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.

[0035] The thickness of a fluororesin film is generally measured using a micrometer (JIS C 2151:2019), but can also be measured by other known methods, such as a method using a laser displacement meter, a method using a capacitance displacement meter, or a method using infrared rays. When the intersection of two diagonals of the fluororesin film is defined as center point A and the long side direction is defined as the width direction, the thickness is measured at a total of three measurement points: center point A (measurement point) and two measurement points set 30 mm away from center point A toward both ends on a line segment passing through midpoint A parallel to the long side, and the average of these measurement points is taken as the thickness of the fluororesin film. When the fluororesin film has a circular or polygonal shape, the film is cut into a rectangle with the largest area, and the thickness is measured by the above method.

[0036] The fluorine-based resin film may be polarized to impart piezoelectricity. ３３ is preferably 5.0 pC / N or more and 40.0 pC / N or less, more preferably 8.0 pC / N or more and 40.0 pC / N or less, and even more preferably 10.0 pC / N or more and 30.0 pC / N or less.

[0037] The piezoelectric constant of the fluororesin piezoelectric film was measured by 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 piezoelectric constant measurement. Specifically, a piezoelectric constant measurement device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold a fluororesin film test piece 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 charge measurement is performed on the polarization surface of the fluororesin film, and the absolute value of the measured value is used to calculate the piezoelectric constant. At this time, the fast axis direction determined by measuring the birefringence of the fluororesin piezoelectric film is defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends of the film in the width direction is defined as point A. The piezoelectric constant d is measured at a total of three measurement points: point A (measurement point), and two measurement points set on the width direction line segment passing through point A at positions 30 mm away from point A toward both ends. ３３The average value of the piezoelectric constants obtained from these measurements was taken as the piezoelectric constant d ３３ Let's say.

[0038] The fluororesin piezoelectric film has a retardation of 100 nm to 2000 nm, preferably 500 nm to 1800 nm, and more preferably 700 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.

[0039] The retardation is a value measured by the parallel Nicol rotation method using a light source with 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 MD direction (machine direction), the MD direction and the slow axis direction will coincide.

[0040] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm to 200 μm, more preferably 15 μm to 80 μm, even more preferably 20 μm to 80 μm, particularly preferably 30.0 μm to 80.0 μm, very preferably 35.0 μm to 70.0 μm, and most preferably 35.0 μm to 50.0 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.

[0041] The thickness of the fluororesin piezoelectric film is also measured in the same manner as above, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the piezoelectric film is defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction is defined as point A. The thickness is measured at a total of three measurement points: point A (measurement point) and two measurement points set at 30 mm intervals from point A on a line segment in the width direction passing through point A, and the average value of these measurement points is defined as the thickness of the fluororesin piezoelectric film.

[0042] The surface roughness in height Rz of the fluororesin piezoelectric film is preferably 0.50 μm or less, more preferably 0.05 μm or more and 0.50 μm or less, even more preferably 0.05 μm or more and 0.40 μm or less, and particularly preferably 0.05 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin piezoelectric film, the less likely haze will occur on the film surface, resulting in excellent transparency. Furthermore, a small surface roughness in height Rz indicates fewer wrinkles and other irregularities in the film.

[0043] The surface height roughness Rz of the fluororesin piezoelectric film is also measured in the same manner as described above, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the piezoelectric film is defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction is defined as point A. The surface height roughness Rz is measured at a total of three measurement points: point A (measurement point) and two measurement points set at 30 mm intervals from point A toward both ends on the line segment in the width direction passing through point A, and the average value of these measurement points is defined as the surface height roughness Rz of the fluororesin piezoelectric film.

[0044] The haze per unit thickness of the fluororesin film is preferably 0.00% / μm or more and 0.35% / μm or less. 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 5.0% or less, and even more preferably 0.0% or more and 3.0% or less. The lower the haze, the more improved the transparency of the fluororesin film.

[0045] The haze of a fluororesin film is measured in accordance with JIS K 7136:2000. Specifically, a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) is used. The intersection of the diagonals of a rectangular fluororesin film is defined as center point A, and the long side direction is defined as the width direction. The haze is measured at three measurement points: center point A (measurement point) and two measurement points set 30 mm away from center point A toward both ends on a line segment passing through midpoint A parallel to the long side. The average of these measurement points is defined as the haze of the fluororesin film. When the fluororesin film has a circular or polygonal shape, the film is cut into a rectangle with the maximum area, and the haze is measured by the above method. The haze of the fluororesin film is divided by the thickness of the fluororesin film to determine the haze per unit thickness of the fluororesin film.

[0046] The haze of a fluororesin piezoelectric film is also measured in the same manner as described above, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the fluororesin piezoelectric film is defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction is defined as point A. The haze is measured at a total of three measurement points: point A (measurement point) and two measurement points set at 30 mm intervals from point A on a line segment in the width direction passing through point A, and the average value of these measurement points is defined as the haze of the fluororesin piezoelectric film.

[0047] The fluororesin film may contain resins other than fluororesins or other additives, as long as the above physical properties can be satisfied.

[0048] Examples of resins other than the above-mentioned fluorine-based resins include polycarbonate, polyesters such as 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.

[0049] [Method for Producing Fluorine-Based Resin Film] The method for producing the above-mentioned fluorine-based resin film is not particularly limited, but preferably includes a step of forming a film by extrusion molding of a heat-molten resin.

[0050] For example, the fluororesin film can be produced by a production method including a step of heating and melting the above-mentioned fluororesin (melting step), a step of filtering the molten fluororesin (filtration step), and a step of forming the filtered fluororesin into a film (film formation step).

[0051] The obtained fluororesin film may be subjected to a stretching step (stretching step) as needed. Furthermore, the stretched fluororesin film may be subjected to a polarization step (polarization step) as needed to impart piezoelectricity. The fluororesin film to which piezoelectricity has been imparted can be used as a piezoelectric film.

[0052] (Melting Step) In the melting step, the fluororesin is heated and melted. This step can be carried out, for example, by melt-kneading the fluororesin using an extruder.

[0053] 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.

[0054] The heating temperature of the fluororesin in the melting step is preferably 75°C or more higher than the melting point of the fluororesin and not higher than 105°C higher than the melting point, more preferably 75°C or more higher but not higher than 100°C, even more preferably 80°C or more higher but not higher than 100°C, and particularly preferably 85°C or more higher but not higher than 95°C. By setting the melting temperature to a temperature 75°C or more 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 step. By setting the melting temperature to a temperature not higher than 105°C higher than the melting point of the fluororesin, decomposition and condensation of the fluororesin due to heating can be suppressed, and the resulting generation of decomposition products can be suppressed. By suppressing the generation of the decomposition products, the amount of foreign matter in the fluororesin film can be reduced, and the transparency and smoothness of the fluororesin film can be improved. Furthermore, by suppressing the generation of the decomposition products, the clogging of the filter caused by these products can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0055] According to the findings of the present inventors, fluororesins with high melt viscosity require high temperatures to achieve a viscosity suitable for filtration. However, heating to high temperatures is prone to decomposition, condensation, and other reactions, resulting in filter clogging. Therefore, it has been difficult to melt and filter 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 a reduced viscosity through melting, the fluororesin can be filtered without using a polar solvent. Furthermore, the absence of a solvent makes it less likely that residual polar solvent in the fluororesin film will inhibit polarization 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 reduces the production costs associated with recovering polar solvents.

[0056] (Filtration step) In the filtration step, the fluororesin is thermally melted in a melting step and then 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.

[0057] 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 facilitates filtration of the molten fluororesin, and also shortens the filtration time without increasing the filtration pressure. Using a filter with a filtration accuracy of 10 μm or more makes it easier for the fluororesin melted by heating to the above temperature to pass through the filter. Furthermore, using a filter with a filtration accuracy of 10 μm or more prevents the filtration pressure from increasing too much and shortens the filtration time. Using a filter with a filtration accuracy of 40 μm or less sufficiently removes 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 35 μm or less, and more preferably 15 μm or more and 30 μm or less.

[0058] 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.

[0059] 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.

[0060] 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 filtered through the filter may be fed into the extruder that performs the film-forming step to form a film.

[0061] (Film Forming Step) In the film forming step, the fluorine-based resin filtered in the filtration step is formed into a film.

[0062] The film formation method is not particularly limited, and known methods such as extruding a molten and filtered fluororesin through a T-die and winding it up on a roll can be used. The film extruded through the T-die and maintained at a temperature of 150°C or higher can be rapidly cooled by contacting it with a cooling roll, thereby suppressing the progress of crystallization of the fluororesin. As the crystallization of the resin progresses, the refractive index of the crystalline and amorphous portions differs, and the haze of films containing the resin tends to increase due to light scattering. Suppressing the progress of crystallization is effective in increasing the transparency of the film. To suppress the crystallization, the surface temperature of the cooling roll is preferably 125°C or less, more preferably 110°C or less, even more preferably 80°C or less, and particularly preferably 50°C or less.

[0063] In this manner, the fluororesin film according to this embodiment can be obtained. The produced fluororesin film may be stored as is, for example, by being wound up, or may be transported to a subsequent process such as a stretching process or a polarization process.

[0064] In the stretching step, the formed fluororesin film is uniaxially stretched. In the stretching step, the fluororesin film formed in the film formation step may be stretched as is, or the fluororesin film that has been wound up and stored may be stretched, if necessary, by heating or the like.

[0065] The stretching direction is not limited as long as it is uniaxial stretching. In the stretching process for mass production, it is preferable to continuously uniaxially stretch the fluororesin film. Specifically, the fluororesin film can be uniaxially stretched in the conveying direction (MD direction) while being conveyed by multiple rolls. The stretching ratio (ratio in the MD direction) is preferably 2.5 times or more and 6.0 times or less, and more preferably 3.5 times or more and 5.0 times or less. By setting the stretching ratio to 2.5 times or more, the proportion of β crystals described below is sufficiently increased, making it easier to obtain a fluororesin film with a high piezoelectric constant in 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. In a film uniaxially stretched in the MD direction, the MD direction is the slow axis, and the direction perpendicular to the stretching direction (TD direction) is the fast axis.

[0066] (Polarization Step) In the polarization step, a DC voltage is applied to the fluororesin film stretched in the stretching step to impart piezoelectricity to the fluororesin film. Fluororesins containing a VDF homopolymer or copolymer undergo a transition from α crystal to β crystal during the stretching step, 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 film with a high piezoelectric constant can be obtained.

[0067] The applied DC voltage 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.

[0068] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0069] After the film-forming step or the polarization step, the fluororesin film can be wound into a roll for storage, transportation, and the like.

[0070] [Uses] The above-mentioned fluorine-based resin film can be used in various applications such as touch sensors and touch panels, piezoelectric films for actuators, interior and exterior materials for automobiles, furniture, and building materials, protective films, and retardation films.

[0071] [Laminated Piezoelectric Body] The fluororesin piezoelectric film (hereinafter also referred to simply as "piezoelectric film") can be laminated with other functional layers or films to form a laminated piezoelectric body. The laminated piezoelectric body may optionally include an antistatic layer to prevent static electricity from being generated during handling, and may also include a hard coat layer or an optical adjustment layer to prevent scratches on the antistatic layer or adjust the color. Furthermore, the laminated piezoelectric body may optionally include an electrode layer for detecting an electrical signal generated by the piezoelectricity of the piezoelectric film. Furthermore, a moisture-proof layer may be included to further prevent discoloration and loss of transparency of the fluororesin piezoelectric film over time when moisture penetrates from the outside under high temperature and high humidity conditions. When the electrode layer is a film with excellent moisture resistance, such as a metal film or a metal oxide film, the film can also be used as a moisture-proof layer. On the other hand, when the electrode layer includes metal nanowires or a metal mesh, the metal may be oxidized, potentially reducing conductivity. Therefore, the laminated piezoelectric body may further include an overcoat layer on the non-adhesive side of the electrode layer (the side opposite the piezoelectric film).

[0072] That is, the laminated piezoelectric body includes at least the piezoelectric film. The laminated piezoelectric body may further include one or more of an antistatic layer, a hard coat layer, an optical adjustment layer, a moisture-proof layer, an electrode layer, and an overcoat layer on at least one surface of the piezoelectric film. These layers may be combined in any manner. For example, the laminated piezoelectric body may include an electrode layer on at least one surface of the piezoelectric film, and may further include an antistatic layer and an overcoat layer.

[0073] A laminated piezoelectric element and a method for manufacturing the same according to one embodiment of the present invention will be specifically described below, although the laminated piezoelectric element is not limited to this embodiment.

[0074] Fig. 1 is a schematic cross-sectional view showing a laminated piezoelectric body of this embodiment. As shown in Fig. 1, a laminated piezoelectric body 10 can have, in this order, a piezoelectric film 11, an antistatic layer 12, a hard coat layer 13, an optical adjustment layer 14, and an electrode layer 15. Each layer will be described below.

[0075] (Piezoelectric Film) As the piezoelectric film, the above-mentioned fluorine-based resin piezoelectric film can be used. The piezoelectric constant d of the piezoelectric film ３３ and the thickness of the piezoelectric film is determined by the piezoelectric constant d ３３ and thickness, respectively.

[0076] (Antistatic Layer) The antistatic layer can be disposed on at least one surface of the piezoelectric film. In this embodiment, the antistatic layer is preferably in contact with the piezoelectric film. The antistatic layer can make it difficult for static electricity to be generated in the laminated piezoelectric element.

[0077] The antistatic layer may contain a cured product of a curable composition containing a conductive material. The curable composition containing a conductive material may contain a conductive material, a curable resin, and optionally a crosslinking agent, or may contain a conductive material and a crosslinking agent.

[0078] The conductive material may be an ion-conductive conductive material or an electron-conductive conductive material.

[0079] Examples of the ion-conductive conductive material include: (a) cationic antistatic agents having a cationic group such as a quaternary ammonium salt, a pyridinium salt, or a primary, secondary, or tertiary amino group; (b) anionic antistatic agents having an anionic group such as a sulfonate group, a sulfate ester group, a phosphate ester group, or a phosphonate group; (c) amphoteric antistatic agents such as amino acid-based or amino sulfate-based; and (d) nonionic antistatic agents such as amino alcohol-based, glycerin-based, or polyethylene glycol-based.

[0080] Examples of electron-conductive conductive materials include conductive polymers and other conductive materials. Examples of conductive polymers include polyacetylene or its derivatives, polythiophene or its derivatives, polypyrrole or its derivatives, and polyaniline or its derivatives. Among them, polythiophene or its derivatives are preferred from the viewpoint of high transparency and high conductivity. These conductive polymers may have functional groups such as sulfonic acid groups, amino groups, amide groups, hydroxyl groups, mercapto groups, and carboxyl groups. Examples of conductive materials other than conductive polymers include carbon nanotubes and graphene.

[0081] Among these, it is preferable to include an electron-conductive conductive material, and carbon nanotubes are more preferable, from the viewpoint of easily reducing the surface resistivity of the laminated piezoelectric body and making bleed-out less likely to occur.

[0082] Examples of the curable resin include acrylic resins, epoxy resins, polyurethane resins, polyimide resins, melamine resins, polyester resins, polyvinyl alcohol resins, polystyrene resins, polyvinyl acetate resins, silicone compounds, polyethylene glycols, and the like, which have functional groups such as hydroxyl groups, methylol groups, carboxyl groups, sulfonyl groups, epoxy groups, and amino groups.

[0083] The crosslinking agent may be any agent that reacts with functional groups of the curable resin or conductive polymer to form crosslinks, and examples of the crosslinking agent include melamine-based crosslinking agents, polycarbodiimide-based crosslinking agents, polyoxazolidine-based crosslinking agents, polyepoxy-based crosslinking agents, and polyisocyanate-based crosslinking agents.

[0084] As described above, curable resins and crosslinking agents having amino groups, such as melamine resins (also referred to as "amine-based materials"), are sometimes used as curable resins and crosslinking agents for obtaining antistatic layers. Among these, melamine resins are sometimes preferred because of their low-temperature curing properties. However, such amine-based materials can generate amines (bases) that cause fluorine-based resins to undergo a dehydrofluorination reaction under high-temperature and high-humidity conditions, which can cause discoloration of the piezoelectric film. Even in such cases, providing the electrode layer or moisture-proof layer described above can make it difficult for amines to be generated, thereby further suppressing discoloration of the piezoelectric film in the laminated piezoelectric element.

[0085] The surface resistivity of the antistatic layer is, for example, 1.0×10 ４ Ω / sq. More than 1.0×10 ９ Ω / sq. or less, preferably 1.0×10 ５ Ω / sq. More than 1.0×10 ８ It is more preferable that the surface resistivity of the antistatic layer is 1.0×10 / sq. or less. ９ When the surface resistivity of the laminated piezoelectric body is 1.0×10 Ω / sq. or less, the surface resistivity of the laminated piezoelectric body can be further reduced, and sufficient antistatic properties can be imparted. In addition, in order to prevent damage to the laminated piezoelectric body, the laminated piezoelectric body is sometimes handled with a hard coat layer formed on the antistatic layer, and the surface resistivity measured from above the hard coat layer is 1.0×10 Ω / sq. or less. ６ Ω / sq. More than 1.0×10 １２ When the surface roughness is 1.0×10 Ω / sq. or less, sufficient antistatic properties are likely to be obtained. ８ Ω / sq. More than 5.0×10 １１ The surface resistivity of the antistatic layer can be measured in accordance with JIS C 2139-3-2:2018 using, for example, a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800, URS probe)).

[0086] The thickness of the antistatic layer is not particularly limited, but is preferably, for example, 0.010 μm or more and 0.40 μm or less. When the thickness of the antistatic layer is 0.010 μm or more, the generation of static electricity in the laminated piezoelectric body can be further suppressed. When the thickness of the antistatic layer is 0.40 μm or less, discoloration of the piezoelectric film in the laminated piezoelectric body can be further prevented. From the same viewpoint, the thickness of the antistatic layer is more preferably 0.030 μm or more and 0.40 μm or less, and even more preferably 0.045 μm or more and 0.30 μm or less.

[0087] The thickness of the antistatic layer can be measured using a spectral interference film thickness meter (for example, Optical NanoGauge C13027-11 manufactured by Hamamatsu Photonics K.K.). The thickness of each layer is measured at three points in an area including the center of the surface of the laminated piezoelectric body, and the thickness can be calculated as the arithmetic mean value. The thickness of each of the following layers can also be measured in a similar manner.

[0088] (Hard Coat Layer) The hard coat layer can be disposed between the piezoelectric film and the electrode layer. In this embodiment, the hard coat layer is disposed adjacent to the piezoelectric film (see FIG. 1). The hard coat layer fills in scratches on the surface of the piezoelectric film to smooth it, and also makes the surface of the piezoelectric film less susceptible to scratches during the manufacturing process of the laminated piezoelectric body. This can further reduce the haze of the laminated piezoelectric body.

[0089] The thickness of the hard coat layer is not particularly limited, but is 0.05 μm or more, preferably 0.3 μm or more and 3.0 μm or less, more preferably 0.5 μm or more and 2.0 μm or less, and even more preferably 0.5 μm or more and 1.5 μm or less.

[0090] As described above, the refractive index of the hard coat layer is preferably higher than that of the piezoelectric film and lower than that of the electrode layer. Specifically, the refractive index of the hard coat layer is preferably 1.40 or higher but lower than 1.60, more preferably 1.47 or higher but lower than 1.57, and even more preferably 1.49 or higher but lower than 1.55. When the refractive index of the hard coat layer is within the above range, the transmittance can be further increased while interference fringes can be further suppressed.

[0091] The difference in refractive index between the hard coat layer and the piezoelectric film is preferably 0.01 or more and 0.20 or less. When the difference in refractive index is 0.01 or more, the transmittance can be further increased, and when the difference is 0.20 or less, interference fringes can be further suppressed.

[0092] The hard coat layer may be made of any material that satisfies the refractive index described above, and is usually a resin layer. The resin layer may be formed by applying a coating liquid containing a resin and then drying it, or by applying a curable composition containing a polymerizable compound and then drying and curing it.

[0093] The polymerizable compound may be any one of a monomer, an oligomer, or a polymer. The polymerizable compound may be a thermosetting compound or an ionizing radiation compound, but is preferably an ionizing radiation compound. The ionizing radiation may typically be ultraviolet (UV) or an electron beam (EB).

[0094] The ionizing radiation-curable compound is a compound having an ionizing radiation-curable functional group. Examples of the ionizing radiation-curable functional group include ethylenically unsaturated bond groups such as (meth)acryloyl groups, vinyl groups, and allyl groups, and ring-opening polymerizable groups such as epoxy groups and oxetanyl groups. Among these, compounds having an ethylenically unsaturated bond group are preferred, compounds having two or more ethylenically unsaturated bond groups are more preferred, and polyfunctional (meth)acrylate compounds are even more preferred. The term "(meth)acrylate" refers to either or both of acrylate and methacrylate.

[0095] Among the polyfunctional (meth)acrylate compounds, examples of bifunctional (meth)acrylate monomers include ethylene glycol di(meth)acrylate, bisphenol A tetraethoxydiacrylate, bisphenol A tetrapropoxydiacrylate, 1,6-hexanediol diacrylate, etc. Examples of trifunctional or higher functional (meth)acrylate monomers include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol tetra(meth)acrylate, isocyanuric acid-modified tri(meth)acrylate, etc. Furthermore, the above-mentioned (meth)acrylate monomers may be partially modified in the molecular skeleton, and those modified with ethylene oxide, propylene oxide, caprolactone, isocyanuric acid, alkyl, cyclic alkyl, aromatic, bisphenol, etc. can also be used.

[0096] Examples of polyfunctional (meth)acrylate oligomers include acrylate polymers such as urethane (meth)acrylate, epoxy (meth)acrylate, polyester (meth)acrylate, polyether (meth)acrylate, etc. Urethane (meth)acrylate can be obtained, for example, by reacting a polyhydric alcohol and an organic diisocyanate with a hydroxy (meth)acrylate.

[0097] When the ionizing radiation-curable compound is an ultraviolet-curable compound, the curable composition preferably contains a photopolymerization initiator, such as one or more selected from acetophenone, benzophenone, α-hydroxyalkylphenone, Michler's ketone, benzoin, benzyl methyl ketal, benzoyl benzoate, α-acyloxime ester, and thioxanthones.

[0098] The curable composition may further contain other components as needed. For example, from the viewpoint of suppressing blocking of the hard coat layer during the production process or adjusting the refractive index, the curable composition may further contain particles. The particles may be inorganic particles or organic particles.

[0099] Examples of inorganic particles include particles of silica (silicon oxide), titanium oxide, zirconium oxide, aluminum oxide, zinc oxide, tin oxide, etc., diamond powder, sapphire particles, boron carbide particles, silicon carbide particles, antimony pentoxide particles, etc. Examples of organic particles include resin particles of acrylic resin, acrylic-styrene copolymer, silicone resin, etc. Among these, inorganic particles are preferred, and silica particles are more preferred, as they are less likely to impair the transparency of the hard coat layer. The surfaces of the inorganic particles may be treated with a surface modifier such as a silane coupling agent.

[0100] As described below, the laminated piezoelectric body may further include an optical adjustment layer between the antistatic layer and the electrode layer. The total thickness of the hard coat layer and the optical adjustment layer is preferably 0.30 μm or more and 4.0 μm or less. When the total thickness is 4.0 μm or less, moisture penetration from the outside through these layers can be further suppressed, and the effect of providing a moisture-proof layer to suppress discoloration of the piezoelectric film can be more easily maintained. Furthermore, when the total thickness is 0.30 μm or less, the color of the laminated piezoelectric body can be suppressed and transparency can be increased. From the same perspective, the total thickness is more preferably 0.50 μm or more and 3.0 μm or less, and even more preferably 0.60 μm or more and 2.0 μm or less.

[0101] (Optical Adjustment Layer) The optical adjustment layer can be disposed between the hard coat layer and the transparent conductive layer. The refractive index and thickness of the optical adjustment layer are appropriately adjusted, thereby suppressing the color of the laminated piezoelectric body.

[0102] Specifically, as described above, the refractive index of the optical adjustment layer is preferably higher than that of the hard coat layer and lower than that of the transparent conductive layer. A laminated piezoelectric body including such an optical adjustment layer can suppress color due to interference between light that is incident on and reflected from the hard coat layer and light that is reflected at the interface between the optical adjustment layer and the hard coat layer.

[0103] From the above viewpoint, the refractive index of the optical adjustment layer is preferably 1.60 or more and less than 1.80, more preferably 1.63 or more and less than 1.78, and even more preferably 1.65 or more and 1.75 or less.

[0104] The difference in refractive index between the optical adjustment layer and the hard coat layer is preferably 0.05 or more. When the difference in refractive index is 0.05 or more, the transmittance can be further increased.

[0105] The thickness of the optical adjustment layer is not particularly limited, but may be 0.05 μm or more. For example, the thickness of the optical adjustment layer may be 0.05 μm or more and 0.5 μm or less, or 0.09 μm or more and 0.18 μm or less. When the thickness of the optical adjustment layer is within the above range, the color of the laminated piezoelectric body can be suppressed.

[0106] The material of the optical adjustment layer may be any material that satisfies such a refractive index. For example, the refractive index may be adjusted by adding metal oxide particles to the curable composition exemplified as the material of the hard coat layer.

[0107] The metal oxide particles are preferably made of a material having a refractive index of 1.50 or more, such as aluminum oxide, titanium oxide, zirconium oxide, zinc oxide, and tin oxide, with titanium oxide and zirconium oxide being preferred.

[0108] (Transparent Electrode) The transparent electrode (electrode layer) may be formed by sputtering a conductive material onto the piezoelectric film on which each layer is formed, or by mixing a conductive material with a resin or the like and applying the mixture.

[0109] The electrode layer may be disposed on at least one side of the piezoelectric film. The form of the electrode layer is not limited, and may be a nanowire, a mesh, or a thin film. The thin film may be a single layer or a laminate of multiple layers.

[0110] The conductive material constituting the electrode layer is not limited, and at least one metal or metal oxide selected from the group consisting of In, Sn, Zn, Ga, Sb, Ti, Si, Zr, Mg, Al, Au, Ag, Cu, Pd, and W is preferably used. The metal oxide may further contain a metal atom listed in the above group, if necessary. Preferred examples of the metal oxide include indium-tin composite oxide (ITO) and antimony-tin composite oxide (ATO), with ITO being particularly preferred. Other representative examples of conductive materials for the electrode layer include at least one selected from the group consisting of metal nanowires, metal meshes, conductive polymer compounds, carbon nanotubes, and graphene. Among these, metal nanowires and metal meshes include silver nanowires, silver meshes, copper nanowires, and copper meshes. Examples of conductive polymer compounds include polyacetylene and its derivatives, polythiophene and its derivatives, polypyrrole and its derivatives, polyaniline and its derivatives, and the like. In this embodiment, the conductive material is preferably a metal oxide, and more preferably ITO.

[0111] As devices become larger and more functional, silver nanowires have attracted attention as a conductive material suitable for achieving lighter weight, lower resistance, higher transparency, and greater flexibility in transparent conductive materials. Silver nanowires, in particular, can be used to form flexible electrode layers using wet processes, making them suitable for a variety of applications. However, because silver nanowires are made of metallic silver, their conductivity tends to decrease under high humidity and temperature conditions, both under sunlight and artificial light. Therefore, it is preferable to apply, dry, and cure an overcoat agent, such as a UV-curable resin, to the silver nanowire coating to form a protective layer (overcoat layer). This allows for the formation of an electrode layer on the substrate that is scratch-resistant and durable, achieving long-term stability of surface electrical resistance under sunlight.

[0112] The lower the surface resistivity of the electrode layer, the easier it is to detect minute signals from the piezoelectric film. On the other hand, to lower the surface resistivity of the electrode layer, it is necessary to make the electrode layer thicker or increase the amount of conductive material added to form the electrode layer, which tends to reduce transparency. Therefore, the surface resistivity of the electrode layer is set to 1.0 × 10 －１ Ω / sq. More than 1.0×10 ４ is preferably 1.0×10 －１ Ω / sq. More than 1.0×10 ３ is preferably 1.0×10 or less. －１ Ω / sq. More than 1.0×10 ２ and particularly preferably 1.0 Ω / sq. or more and 1.0×10 ２ The following is the result.

[0113] The surface resistivity (Ω / sq.) of the electrode layer is measured using a resistivity meter ("LorestaGP MCP-T610", manufactured by Mitsubishi Chemical Analytech Co., Ltd.) by the DC four-probe method in accordance with JIS K 7194-1994. When an overcoat layer is formed on the electrode layer, the surface resistivity detected from above the overcoat layer is taken as the surface resistivity of the electrode layer. Since volume resistivity is the product of surface resistivity and thickness, surface resistivity is measured in accordance with the volume resistivity measurement method. The surface resistivity is measured in the range including the intersection of the diagonals of the laminated piezoelectric body.

[0114] The thickness of the electrode layer is not limited, but from the viewpoint of good conductivity, a thicker electrode layer is preferable. On the other hand, if the electrode layer is too thick, transparency may decrease, and if it is too thin, electrical resistance may increase and discontinuous conductive portions may be formed in the film structure. The thickness is preferably 10 nm to 55 nm, more preferably 15 nm to 55 nm, even more preferably 20 nm to 55 nm, even more preferably 20 nm to 45 nm, and particularly preferably 20 nm or more. The thickness of the electrode layer can be determined by a known method of observing the cross section of such a laminate. The thickness of the electrode layer is determined by observing the cross section of the laminated piezoelectric body using a scanning electron microscope ("SU3800" manufactured by Hitachi High-Technologies Corporation) under conditions of an acceleration voltage of 3.0 kV and a magnification of 50,000 times, measuring the thickness of the electrode layer in the area including the intersection of the diagonal lines of the laminated piezoelectric body, and using this as a representative value of the thickness of the electrode layer of the laminated piezoelectric body.

[0115] In the above embodiment, the electrode layer is formed on the optical adjustment layer, but this is not limited to this. For example, as described above, if the electrode layer contains metal nanowires or a metal mesh as a conductive material, the optical adjustment layer may be omitted. In this case, an overcoat layer may be further disposed on the electrode layer to suppress oxidation of the metal nanowires or the metal mesh. When an overcoat layer is disposed on the electrode layer, the surface resistivity of the electrode layer refers to the surface resistivity measured on the overcoat layer.

[0116] In addition, in the above embodiment, an electrode layer is formed on the optical adjustment layer, but a transparent electrode film having an electrode layer formed on the surface of a substrate such as glass or a polymer material may also be formed by adhering it to the optical adjustment layer.

[0117] (Transparent Electrode Film) The transparent electrode may be used in combination with an existing substrate film. The existing electrode layer may be, for example, a substrate containing one or more polymer materials such as polyethylene terephthalate (PET), cycloolefin polymer (COP), and polycarbonate (PC), or a substrate containing an inorganic material such as glass, and the electrode layer may be laminated on the substrate.

[0118] The substrate is a transparent resin layer for supporting the electrode layer. The transparent resin contained in the substrate is preferably a material having heat resistance that can withstand the heat required to crystallize ITO or the like. Examples of such transparent resins include polyesters such as polyethylene terephthalate (PET). Of these, PET is preferred.

[0119] The thickness of the substrate is not particularly limited as long as it can support the electrode layer, but is preferably 2 μm or more and 300 μm or less, more preferably 10 μm or more and 200 μm or less, even more preferably 20 μm or more and 150 μm or less, and particularly preferably 30 μm or more and 130 μm or less.

[0120] The electrode layer is disposed on the surface of the substrate on the piezoelectric film side (the optical adjustment layer side in FIG. 1 ). The electrode layer is preferably an inorganic electrode such as ITO (indium tin oxide complex oxide) or tin oxide, and more preferably ITO.

[0121] (Moisture-proof layer) In the above embodiment, the laminated piezoelectric element has a piezoelectric film, but may include a moisture-proof layer to further prevent discoloration and loss of transparency of the piezoelectric film over time when moisture penetrates from the outside under high temperature and high humidity conditions. The material of the moisture-proof layer is not particularly limited as long as it is transparent and can prevent moisture from penetrating, but it is preferably made of an inorganic oxide, and more preferably a thin film of an inorganic oxide (such as a vapor-deposited film).

[0122] The inorganic oxide is a metal oxide, a nonmetal oxide, or a submetal oxide. Examples of the inorganic oxide include aluminum oxide, zinc oxide, antimony oxide, indium oxide, indium tin oxide, calcium oxide, cadmium oxide, silver oxide, gold oxide, chromium oxide, silicon oxide, cobalt oxide, zirconium oxide, tin oxide, titanium oxide, iron oxide, copper oxide, nickel oxide, platinum oxide, palladium oxide, bismuth oxide, magnesium oxide, manganese oxide, molybdenum oxide, vanadium oxide, and barium oxide, and indium tin oxide and silicon oxide are particularly preferred.

[0123] The thickness of the moisture-proof layer is not particularly limited, but from the viewpoint of achieving both moisture resistance and transparency at a higher level, the thickness is preferably from 0.005 μm to 0.100 μm, more preferably from 0.010 μm to 0.060 μm, and even more preferably from 0.015 μm to 0.050 μm.

[0124] [Physical properties of laminated piezoelectric material] (piezoelectric constant d ３３ ) Piezoelectric constant d of the laminated piezoelectric material ３３ The piezoelectric constant d of the laminated piezoelectric body is preferably 7.0 pC / N or more and 40.0 pC / N or less. ３３ When the piezoelectric constant d of the laminated piezoelectric material is 7.0 pC / N or more, higher pressure sensitivity is likely to be obtained. ３３ From the same viewpoint, when the piezoelectric constant d of the laminated piezoelectric body is 40.0 pC / N or less, the above-mentioned appearance defects can be further reduced. ３３ is more preferably 10.0 pC / N or more and 40.0 pC / N or less, further preferably 13.0 pC / N or more and 35.0 pC / N or less, and particularly preferably 15.0 pC / N or more and 30.0 pC / N or less. ３３ can be measured in the same manner as above, except that the measurement point is one intersection of the diagonal lines of the laminated piezoelectric body.

[0125] Piezoelectric constant d of the laminated piezoelectric material ３３ is the piezoelectric constant d of a piezoelectric film, which is one of the components of the laminated piezoelectric body. ３３ The piezoelectric constant d of the piezoelectric film can be adjusted by ３３ When the piezoelectric constant d ３３ It is also likely to be expensive.

[0126] (Total Light Transmittance) The laminated piezoelectric body preferably has high transparency from the viewpoint of application to, for example, a touch panel. Specifically, the total light transmittance of the laminated piezoelectric body is preferably 80% or more, and more preferably 85% or more.

[0127] The total light transmittance of the laminated piezoelectric body can be measured using a haze meter (for example, NDH7000SP II manufactured by Nippon Denshoku Industries Co., Ltd.) based on the method described in JIS K 7361-1. The total light transmittance is measured at a position including the intersection of the diagonal lines of the laminated piezoelectric body.

[0128] The total light transmittance of the laminated piezoelectric body can be adjusted by the layer structure and the refractive index and thickness of each layer. For example, if the laminated piezoelectric body includes a hard coat layer, the haze of the laminated piezoelectric body can be further reduced, and therefore the total light transmittance tends to be higher.

[0129] (Hue b ＊ value, Δb ＊ ) Color b of the laminated piezoelectric body ＊ The value is preferably, for example, between −5.0 and 5.0. ＊ When the laminated piezoelectric body is stored in an environment of 85°C and 85% RH for 500 hours, the hue difference Δb ＊ is preferably 4.0 or less, and more preferably 3.5 or less. ＊ When the value is 4.0 or less, the laminated piezoelectric body undergoes little change in hue under high temperature and high humidity conditions, and therefore transparency and visibility can be maintained at a good level.

[0130] b of the laminated piezoelectric body ＊ The hue value can be measured using a spectrophotometer (for example, SD7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z 8722. ＊ The value is measured at a position including the intersection of the diagonal lines of the laminated piezoelectric body. ＊ The value is the thickness of the laminated piezoelectric body 25 cm ２ The laminated piezoelectric body was cut into a square shape, and the four corners of the laminated piezoelectric body were fixed to a SUS plate with tape. The laminated piezoelectric body was then placed in a thermo-hygrostat chamber set at a temperature of 85°C and a humidity of 85% RH for 500 hours. After the laminated piezoelectric body was taken out of the thermo-hygrostat chamber, the thickness of the laminated piezoelectric body was measured. ＊ The value may be measured in the same manner as above.

[0131] b of the laminated piezoelectric body ＊ value and Δb ＊ can be adjusted by the thickness of the antistatic layer and the composition and thickness of the moisture-proof layer. For example, if the thickness of the antistatic layer is reduced, b ＊ value and Δb ＊In addition, by increasing the thickness of the moisture-proof layer, discoloration of the piezoelectric film due to moisture penetration can be reduced. ＊ value and Δb ＊ can be made smaller.

[0132] [Method for Manufacturing a Laminated Piezoelectric Body] The laminated piezoelectric body can be manufactured by any method. For example, the laminated piezoelectric body shown in FIG. 1 can be manufactured through the following steps: (1) preparing a fluorine-based resin piezoelectric film; (2) forming an antistatic layer on the piezoelectric film; (3) forming a hard coat layer on the antistatic layer; (4) forming an optical adjustment layer on the hard coat layer; and (5) forming an electrode layer on the optical adjustment layer. If the laminated piezoelectric body does not include a hard coat layer, step (3) can be omitted, and steps (4) and (5) can also be omitted as needed. If the electrode layer includes metal nanowires or a metal mesh as a conductive material, step (4) may be omitted. To further suppress oxidation of the metal nanowires or the metal mesh, steps (6) forming an overcoat layer on the electrode layer and (7) forming a moisture-proof layer may be further performed. In addition, instead of step (5), after step (2) or step (3), a step (6) of forming a transparent adhesive (OCA) layer on the hard coat layer or the optical adjustment layer, and a step (8) of laminating an electrode film on the OCA layer can be performed.

[0133] (Step of Preparing Piezoelectric Film) As the piezoelectric film containing a fluorine-based resin, the above-mentioned fluorine-based resin piezoelectric film can be used.

[0134] (Step of forming an antistatic layer) The above-mentioned curable composition for the antistatic layer is applied to the obtained piezoelectric film, and then dried and cured to form an antistatic layer. From the viewpoint of improving low-temperature curability, the curable composition for the antistatic layer is preferably a curable composition containing a conductive material and an amine-based material, and more preferably a curable composition containing a conductive material and a melamine resin.

[0135] The curable composition may further contain water or a solvent, examples of which include alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol.

[0136] The method for applying the curable composition is not particularly limited, and may be any of spin coating, gravure coating, die coating, bar coating, dip coating, and the like.

[0137] The curable composition can be dried by heating the applied curable composition. The heating temperature is preferably a temperature at which the solvent can be removed or higher and a temperature at which the fluorine-based resin constituting the piezoelectric film is not higher than the heat distortion temperature, and can be, for example, 100° C. or higher and 150° C. or lower. The heat distortion temperature can be measured, for example, in accordance with JIS K 7191-2:2015.

[0138] (Step of Forming Hard Coat Layer) In this embodiment, the above-described curable composition for the hard coat layer is applied onto the piezoelectric film or the antistatic layer, and then dried and cured to form a hard coat layer.

[0139] The curable composition may further contain a dilution solvent. The dilution solvent preferably has a polarity similar to that of the particles. Examples of the dilution solvent include organic solvents such as alcohol-based solvents, ketone-based solvents, ester-based solvents, carbonate-based solvents, and aromatic solvents.

[0140] The curable composition can be applied by a known wet process. Typical wet process methods include dip coating, spray coating, spin coating, gravure coating, die coating, roll coating, flow coating, and curtain coating. Among these, methods that can form layers continuously, such as roll coating and gravure coating, are preferred from the viewpoint of productivity.

[0141] The curable composition may be applied and dried in the same manner as described above. The heating temperature may be set to a temperature within a range that allows the solvent to be volatilized and is equal to or lower than the heat distortion temperature of the fluororesin constituting the piezoelectric film, for example, 60°C or higher and 100°C or lower.

[0142] The curable composition may be cured by heat or by ionizing radiation. Curing by ionizing radiation can be carried out by irradiation with ultraviolet rays or electron beams. Alternatively, curing by heat and curing by ionizing radiation may be used in combination.

[0143] (Step of forming an optical adjustment layer) As in the step of forming a hard coat layer, the curable composition is applied to a piezoelectric film, an antistatic layer, or a hard coat layer, and then dried and cured to form an optical adjustment layer. However, when the electrode layer uses metal nanowires or metal mesh as a conductive material, the conductive layer has high transparency, so the optical adjustment layer can be omitted.

[0144] (Step of forming electrode layer) The electrode layer can be formed on the optical adjustment layer by sputtering a transparent conductive material or by applying a solution containing a transparent conductive material and a resin. When the electrode layer uses metal nanowires or metal mesh as the conductive material, there is a risk that the conductive material will be oxidized and deteriorated, resulting in an increase in electrical resistance. Therefore, an overcoat layer can be formed on the electrode layer.

[0145] [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.

[0146] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0147] 1. Preparation of Fluorine-Based Resin Films Films 1 to 10 and 13, all of which were fluorine-based resin films, were prepared by the following procedure.

[0148] The melt viscosity of the fluororesin material was measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (Capillograph 1D, manufactured by Toyo Seiki Seisaku-sho, Ltd.) was used, and the melt viscosity was measured at a temperature of 260°C and a shear rate of 50 s−1 using a capillary die with an inner diameter of 1 mm and a tube length of 10 mm.－１ The viscosity was measured at 1000 kJ / min.

[0149] The melting point of the fluororesin was determined as the maximum peak temperature of the endothermic curve in a DSC curve obtained by sealing 5 mg of a measurement sample in an aluminum pan, placing the pan in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation), and heating the sample from room temperature to 230°C at a heating rate of 10°C / min under a nitrogen atmosphere.

[0150] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0151] 1-1. Film 1 Vinylidene fluoride homopolymer (PVDF) with a melt viscosity of 2500 Pa s and a melting point of 173°C was melted using a single-screw extruder with a diameter of 50 mm. The molten resin was filtered through a pleated polymer filter with a filtration accuracy of 20 μm, extruded from a T-die, and cooled by contact with a cooling roll to obtain an unstretched film with a thickness of 160 μm. The melting temperature and filtration temperature were 260°C.

[0152] The unstretched film was introduced into a uniaxial stretching apparatus equipped with multiple metal rolls and pinch rolls, and stretched 4.0 times in the machine direction by adjusting the rotation speed ratio of each roll. A voltage of 11.6 kV was then applied from the surface of the film to the thickness direction to obtain Film 1 having a thickness of 39 μm.

[0153] 1-2. Film 2 Film 2 was obtained in the same manner as in the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 40 μm was used.

[0154] 1-3. Film 3 Film 3 was obtained in the same manner as in the production of Film 1, except that PVDF having a melt viscosity of 800 Pa·s and a melting point of 173°C was used.

[0155] 1-4. Film 4 Film 4 was obtained in the same manner as in the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was 275°C.

[0156] 1-5. Film 5 Film 5 was obtained in the same manner as in the production of Film 1, except that PVDF having a melt viscosity of 3,400 Pa·s and a melting point of 173°C was used.

[0157] 1-6. Film 6 Film 6 was obtained in the same manner as in the production of Film 1, except that PVDF having a melt viscosity of 800 Pa·s and a melting point of 173°C was used and the melting temperature was set to 250°C.

[0158] 1-7. Film 7 Film 7 was obtained in the same manner as in the production of Film 1, except that no filter was attached to the single-screw extruder.

[0159] 1-8. Film 8 Film 8 was obtained in the same manner as in the production of Film 1, except that PVDF having a melt viscosity of 4500 Pa·s and a melting point of 173° C. was used and no filter was attached to the single-screw extruder.

[0160] 1-9. Film 9 Film 9 was obtained in the same manner as in the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 60 μm was used.

[0161] 1-10. Film 10 Film 10 was obtained in the same manner as in Film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was 280° C. The surface height roughness Rz of Film 10 was large, making it difficult to accurately detect the amount of foreign matter.

[0162] 1-11. Film 11 (unable to be produced) An attempt was made to produce Film 11 in the same manner as Film 1, except that a pleated polymer filter with a filtration accuracy of 5 μm was used, but the filter became clogged with resin, and a film could not be obtained.

[0163] 1-12. Film 12 (unable to be produced) An attempt was made to produce Film 12 in the same manner as Film 1, except that PVDF with a melt viscosity of 4,500 Pa s and a melting point of 173°C was used. However, the filter became clogged with resin, and a film could not be obtained.

[0164] 1-13. Film 13 100 g of PVDF with a melt viscosity of 2500 Pa s and a melting point of 173°C was weighed and added to 900 ml of N-methylpyrrolidone (NMP). The temperature was raised to 60°C while stirring with a stirrer, and stirring was continued for 6 hours to produce 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 produce a coating film with a liquid thickness of 600 μm, which was then dried at 120°C for 1 hour to obtain Film 13. Film 13 had a large surface height roughness Rz, making it difficult to accurately detect the amount of foreign matter.

[0165] 2. Evaluation of Fluorine-Based Resin Films The obtained Films 1 to 10 and Film 13 were evaluated for surface roughness Rz, number of foreign particles, thickness, and piezoelectric constant d by the following methods. ３３ , and haze were measured.

[0166] 2-1. Surface Height Roughness Rz 2-1-1. Fluorocarbon Resin Film (Other Than Piezoelectric Film) A surface roughness meter conforming to JIS B 0601:2001 (Keyence Corporation, Shape Analysis Laser Microscope VK-X260) was used. The surface roughness of the contact surface (the surface with smaller Rz) of the fluorocarbon resin film with the cooling roll was measured. Specifically, when the intersection of the diagonals of the rectangular fluorocarbon resin film is defined as center point A and the long side direction is defined as the width direction, the surface height roughness Rz was measured at a total of three measurement points: center point A (measurement point) and two measurement points set 30 mm away from center point A toward both ends on a line segment passing through midpoint A parallel to the long side, and the average value of these was taken as the surface height roughness of the fluorocarbon resin film.

[0167] 2-1-2. Fluorine-based resin piezoelectric film The surface height roughness Rz of a fluororesin piezoelectric film was also measured in the same manner as in 2-1-1, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the piezoelectric film was defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction was designated as point A. The surface height roughness Rz was measured at a total of three measurement points: point A (measurement point) and two measurement points set at 30 mm intervals from point A on a line segment in the width direction passing through point A toward both ends, and the average of these measurement points was defined as the surface height roughness Rz of the fluororesin film.

[0168] 2-2. Number of foreign particles After uniaxially stretching the fluororesin film to a thickness of 35 to 40 μm, the film was cut out from adjacent positions in a continuous manner by 0.010 mm ２ Twenty-five rectangular films (observation pieces) each measuring 100 mm x 100 mm were cut out from the fluorine-based resin 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.

[0169] 2-3. Thickness 2-3-1. Fluoropolymer Films (Other Than Piezoelectric Films) 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. Specifically, when the intersection of the diagonals of a rectangular fluoropolymer film is defined as center point A and the long side direction is defined as the width direction, thickness was measured at a total of three measurement points: center point A (measurement point) and two measurement points set on a line segment passing through midpoint A parallel to the long side, at positions 30 mm away from center point A towards both ends, and the average of these measurements was taken as the thickness of the fluoropolymer film.

[0170] 2-3-2. Fluoroplastic Resin Piezoelectric Film The thickness of the fluoroplastic resin piezoelectric film was also measured in the same manner as in 2-3-1, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the piezoelectric film was defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction was designated as point A. The thickness was measured at a total of three measurement points: point A (measurement point) and two measurement points set on the width direction line segment passing through point A, each 30 mm away from point A toward both ends, and the average of these measurement points was defined as the thickness of the fluoroplastic resin piezoelectric film.

[0171] 2-4. Haze 2-4-1. Fluorine-Based Resin Films (Other Than Piezoelectric Films) A haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) was used. Haze was measured in accordance with ISO 14782:2021. Specifically, when the intersection of the diagonals of a rectangular fluororesin film is defined as center point A and the long side direction is defined as the width direction, haze was measured at a total of three measurement points: center point A (measurement point) and two measurement points set 30 mm away from center point A toward both ends on a line segment passing through midpoint A parallel to the long side, and the average value of these was used as the haze of the fluororesin film. The haze was divided by the film thickness measured in 2-3-1 to obtain the haze per unit thickness of the fluororesin film.

[0172] 2-4-2. Fluorocarbon Resin Piezoelectric Film The haze of a fluorocarbon resin piezoelectric film was also measured in the same manner as in 2-5-1, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the piezoelectric film was defined as the width direction, and an arbitrary point on the midpoint of a line segment connecting both ends of the film in the width direction was defined as point A. Haze measurements were performed at a total of three measurement points: point A (measurement point) and two measurement points set 30 mm apart from point A on a line segment in the width direction passing through point A, each of which was located at an interval of 30 mm from point A toward both ends, and the average of these measurement points was defined as the haze of the fluorocarbon resin piezoelectric film.

[0173] 2-5. Piezoelectric constant d ３３ Piezoelectric constant d according to ISO 19622:2018 ３３was measured. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) was used to hold a fluororesin film test piece with a holding force of 1.0 N, and the charge generated when an alternating force of 0.15 N and a frequency of 110 Hz was applied was measured. The charge measurement was performed on the polarization surface of the fluororesin film, and the absolute value of the measured value was used to calculate the piezoelectric constant. The fast axis direction determined by measuring the birefringence of the fluororesin film was defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends of the film in the width direction was designated as point A. The piezoelectric constant d was measured at a total of three measurement points: point A (measurement point), and two measurement points on the line segment in the width direction passing through point A, set at 30 mm intervals from point A toward both ends. ３３ The average value of the piezoelectric constants obtained from these measurements was taken as the piezoelectric constant d ３３ It was decided.

[0174] 2-6. 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 oriented 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.

[0175] 3. Results The preparation conditions and evaluation results for each film are shown in Tables 1 and 2. Film 10 had very large irregularities on the leaf surface, making it impossible to count the number of foreign particles. Film 13 was opaque with a large haze, making it impossible to count the number of foreign particles.

[0176]

[0177]

[0178] 4. Preparation of Laminated Piezoelectric Body 4-1. Antistatic Layer 4-1-1. Antistatic Layer 1 A solution obtained by mixing paint P-400MP-A (manufactured by Nagase ChemteX Corporation) containing PEDOT:PSS as a conductive polymer and paint P-400MP-B (manufactured by Nagase ChemteX Corporation) containing a crosslinking agent and a conductivity improver in a ratio of 4:1 was applied using a gravure coater (multi-coater manufactured by Hirano Tecseed Co., Ltd.), and the mixture was heat-treated at 130°C for 0.67 minutes to form an antistatic layer with a thickness of 80 nm. The surface resistivity of the antistatic layer was 1.2 x 10 ８ Ω / sq.

[0179] 4-1-2. Antistatic Layer 2 A solution (C-169PF) obtained by mixing a paint C-169PF-A (manufactured by Nagase ChemteX Corporation) containing single-walled carbon nanotubes and a paint C-169PF-B (manufactured by Nagase ChemteX Corporation) containing a crosslinking agent in a ratio of 3:2 was applied using a multi-coater (manufactured by Hirano Tecseed Co., Ltd.) and heat-treated at 130°C for 1 minute to form an antistatic layer 2 having a thickness of 100 nm. The surface resistivity of the antistatic layer was 7.1 x 10 ６ Ω / sq.

[0180] 4-2. Formation of hard coat layer A hard coat agent (BS-CH271 manufactured by Arakawa Chemical Industries, Ltd., amorphous silica average particle size 60 nm) was applied using a multi-coater, and then heat-treated at 80°C for 2 minutes, and then subjected to irradiation with an integrated light dose of 200 mJ / cm ２ The coating was photocured by irradiating it with UV rays of 1.50 to form a hard coat layer having a thickness of 700 nm and a refractive index of 1.50.

[0181] 4-3. Optical Adjustment Layer An ultraviolet-curable composition containing zirconium oxide particles (Opstar RA004 manufactured by Arakawa Chemical Industries, Ltd.) was applied and dried at 40°C for 30 seconds, and then irradiated with 250 mJ / cm ２ An optical adjustment layer (thickness: 102 nm, refractive index: 1.65) was formed by irradiating ultraviolet light with an integrated light amount of 102 nm.

[0182] 4-4. Formation of Transparent Electrode 4-4-1. Transparent Electrode Layer 1 An indium tin metal target (the tin content relative to the sum of the indium content and the tin content was 3% by mass) was set as a target material in a magnetron sputtering device, and a piezoelectric film on which a predetermined layer was formed was set as a substrate. Then, while winding up the piezoelectric film on which the predetermined layer was formed, dehydration and degassing were performed until the degree of vacuum reached 7×10 －５ The chamber was then evacuated until the pressure reached 0.4 Pa. Subsequently, the substrate temperature was returned to room temperature (25° C.), and a mixed gas of 99.2 mol % argon gas and 0.8 mol % oxygen gas was introduced into the chamber. While the gas in the chamber was being evacuated so that the pressure in the chamber reached 0.4 Pa, a transparent electrode layer of 30 nm thickness made essentially of indium tin composite oxide was formed by reactive sputtering, thereby obtaining a laminated piezoelectric body.

[0183] 4-4-2. Transparent electrode layer 2 A solution of T-AG230 (manufactured by Seiko PMC Corporation) containing silver nanowires diluted appropriately with alcohol was applied using a coater and dried with hot air at 40°C for 30 seconds to form a transparent electrode layer with a thickness of 80 nm. Next, a solution of T-YP562 (manufactured by Seiko PMC Corporation) mixed with an additive, T-YP462 (manufactured by Seiko PMC Corporation), was applied using a coater as a transparent electrode protective layer on the transparent electrode layer, and after drying with hot air at 40°C for 60 seconds, a 330 mJ / cm ２ The transparent electrode layer and the protective layer were collectively referred to as the transparent electrode layer 2.

[0184] 4-4-3. Transparent Electrode Film A transparent electrode precursor film (Tetlite TCF KH100NMH3-100-U8, manufactured by Oike Kogyo Co., Ltd.) prepared by sputtering indium tin oxide (ITO) onto a polyethylene terephthalate (PET) film was crystallized at 150°C for 90 minutes to produce a transparent electrode film. The surface resistivity of the transparent electrode film 1 was 100 Ω / sq. Next, an optically clear adhesive (OCA) sheet (Nitto Denko Corporation, CS9862UA, thickness 50 μm) was bonded to the polymeric piezoelectric film on which the predetermined layers had been formed. The release film was then peeled off from the OCA sheet, and the transparent electrode film was bonded to the OCA sheet surface with the conductive layer facing the OCA sheet surface.

[0185] 4-5. Laminated Piezoelectric Body 4-5-1. Laminated Piezoelectric Body 1 The antistatic layer 1 was formed on surface A of piezoelectric film 1, and then a hard coat layer was formed on antistatic layer 1 to prepare laminated piezoelectric body 1.

[0186] 4-5-2. Laminated Piezoelectric Body 2 A transparent electrode layer 1 was formed on the hard coat layer of the laminated piezoelectric body 1 to prepare a laminated piezoelectric body 2.

[0187] 4-5-3. Laminated Piezoelectric Body 3 A transparent electrode film was attached onto the hard coat layer of the laminated piezoelectric body 1 to prepare the laminated piezoelectric body 3.

[0188] 4-5-4. Laminated Piezoelectric Body 4 Laminated piezoelectric body 4 was fabricated in the same manner as laminated piezoelectric body 2, except that transparent electrode layer 2 was used instead of transparent electrode layer 1.

[0189] 4-5-5. Laminated Piezoelectric Body 5 Laminated piezoelectric body 5 was fabricated in the same manner as laminated piezoelectric body 4, except that antistatic layer 1 was changed to antistatic layer 2.

[0190] 4-5-6. Laminated Piezoelectric Body 6 Laminated piezoelectric body 6 was produced in the same manner as laminated piezoelectric body 2, except that an optical adjustment layer was formed between the hard coat layer and the transparent electrode layer 1.

[0191] 4-5-7. Laminated Piezoelectric Body 7 The antistatic layer 1 was formed on surface A of the piezoelectric film 1, and a transparent electrode film 1 was attached onto the antistatic layer 1 to prepare a laminated piezoelectric body 7.

[0192] 4-5-8. Laminated Piezoelectric Body 8 The antistatic layer 1 was formed on the surface A of the piezoelectric film 1, and then a transparent electrode layer 1 was formed on the antistatic layer 1, to prepare a laminated piezoelectric body 8.

[0193] 4-5-9. Laminated Piezoelectric Body 9 Laminated piezoelectric body 9 was fabricated in the same manner as laminated piezoelectric body 8, except that transparent electrode layer 1 was changed to transparent electrode layer 2.

[0194] 4-5-10. Laminated Piezoelectric Body 10 The antistatic layer 1 was formed on surface A of the piezoelectric film 1, the optical adjustment layer was then formed on the antistatic layer 1, and the transparent electrode layer 1 was formed on the optical adjustment layer to produce the laminated piezoelectric body 10.

[0195] 4-5-11. Laminated Piezoelectric Body 11 Laminated piezoelectric body 11 was fabricated in the same manner as laminated piezoelectric body 8, except that a hard coat layer was formed on side A of piezoelectric film 1 instead of antistatic layer 1.

[0196] 4-5-12. Laminated Piezoelectric Body 12 Laminated piezoelectric body 12 was fabricated in the same manner as laminated piezoelectric body 11, except that transparent electrode layer 1 was replaced with transparent electrode layer 2.

[0197] The hard coat layer was formed on surface A of the piezoelectric film 1, the optical adjustment layer 1 was then formed on the hard coat layer, and the transparent electrode layer 1 was then formed on the optical adjustment layer 1 to produce the laminated piezoelectric body 13.

[0198] 4-5-14. Laminated Piezoelectric Body 14 A laminated piezoelectric body 14 was produced in the same manner as the laminated piezoelectric body 11, except that the hard coat layer was replaced with an optical adjustment layer.

[0199] 4-5-15. Laminated Piezoelectric Body 15 An antistatic layer 1 was formed on surface A of a piezoelectric film 1, a hard coat layer was formed on surface B of the piezoelectric film 1, the separator film protecting the adhesive of a PET protective film (SAT TM30125T manufactured by San-A Kaken Co., Ltd.) was peeled off, and the PET protective film was laminated on top of the hard coat layer on surface B. Next, a hard coat layer and an optical adjustment layer were formed on the antistatic layer 1 on surface A, and further a transparent electrode layer 1 was formed on the optical adjustment layer to produce a laminated piezoelectric body 15.

[0200] 4-5-16. Laminated piezoelectric element 16 The antistatic layer 1 was formed on surface A of piezoelectric film 1, a hard coat layer was formed on surface B of piezoelectric film 1, the separator film protecting the adhesive of a PET protective film (SAT TM30125T manufactured by San-A Kaken Co., Ltd.) was peeled off, and the PET protective film was laminated on top of the hard coat layer on surface B. Next, a hard coat layer and a transparent electrode layer 2 were formed on the antistatic layer 1 on surface A, thereby producing laminated piezoelectric element 16.

[0201] 5. Evaluation of the laminated piezoelectric bodies The piezoelectric constant d of the obtained laminated piezoelectric bodies 1 to 16 was measured by the following method. ３３ , total light transmittance, surface resistivity, haze, and b ＊ was measured.

[0202] 5-1. Piezoelectric constant The value measured in the range including the intersection of the diagonal lines of the laminated piezoelectric material is taken as the piezoelectric constant d ３３ The measurement was carried out in the same manner as in the measurement method (2-5) above, except that the representative value was used.

[0203] The total light transmittance of the laminated piezoelectric body was measured in accordance with JIS K 7361-1 using a haze meter (NDH7000SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) in a range including the intersection of the diagonal lines of the laminated piezoelectric body, and the measured value was used as a representative value.

[0204] 5-3. Surface Resistivity 5-3-1. Surface Resistivity of Antistatic Layer The surface resistivity of the antistatic layer of the laminated piezoelectric body was measured in accordance with JIS C 2139-3-2:2018 using, for example, a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800, URS probe)). The measurement location was an area including the intersection of the diagonal lines of the rectangular laminated piezoelectric body, and the surface resistivity of the antistatic layer measured was used as a representative value.

[0205] 5-3-2. Surface Resistivity of Transparent Electrode The surface resistivity of the transparent electrode of the laminated piezoelectric body was measured using a resistivity meter ("LorestaGP MCP-T610", manufactured by Nitto Seiko Analytech Co., Ltd.) using a DC four-probe method in accordance with JIS K 7194. Since volume resistivity is the product of surface resistivity and thickness, the surface resistivity was measured in accordance with the volume resistivity measurement method. The measurement location was the surface resistivity of the transparent electrode measured in an area including the intersection of the diagonals of the rectangular laminated piezoelectric body, and the representative value was used. When a protective layer was formed on the electrode, the surface resistivity was measured from above the protective layer.

[0206] The haze value of the laminated piezoelectric body was measured using a haze meter ("NDH7000SP II", manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K7136, and the haze measurement result in the range including the intersection of the diagonal lines of the laminated piezoelectric body was used as a representative value.

[0207] 5-5. Hue difference Δb ＊ The measurement results of the area including the intersection of the diagonal lines of the laminated piezoelectric body before storage in a humid and heated environment using a spectrophotometer (SD7000, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z 8722 were recorded as hue b ＊ The values ​​were taken as representative values.

[0208] 5-6. Thickness The thickness of the antistatic layer was measured using a spectral interference film thickness meter (e.g., Optical NanoGauge C13027-11, manufactured by Hamamatsu Photonics). The thickness of each layer was measured in an area including the intersection of the diagonal lines of the laminated piezoelectric body. The thickness of each layer other than the transparent electrode and piezoelectric film was also measured in a similar manner. The thickness of the transparent electrode was measured by observing the cross section of the laminated piezoelectric body using a scanning electron microscope ("SU3800", manufactured by Hitachi High-Technologies Corporation) under conditions of an acceleration voltage of 3.0 kV and a magnification of 50,000 times, and measuring the thickness of the transparent electrode in an area including the intersection of the diagonal lines of the laminated piezoelectric body. The obtained measurement value was used as the representative value of the thickness of the transparent electrode.

[0209] 6. Results Tables 3 and 4 show the preparation conditions and evaluation results of each laminated piezoelectric body.

[0210]

[0211]

[0212] This application claims priority to Japanese Patent Application No. 2024-006086 filed on January 18, 2024, and Japanese Patent Application No. 2024-006105 filed on January 18, 2024. The entire contents of the specification, claims, and drawings of those applications as originally filed are incorporated herein by reference.

[0213] The fluorine-containing resin film according to the present invention contains little foreign matter and has little surface irregularities.

[0214] REFERENCE SIGNS LIST 10 laminated piezoelectric body 11 piezoelectric film 12 antistatic layer 13 hard coat layer 14 optical adjustment layer 15 electrode layer

Claims

1. A fluororesin film having a melt viscosity η measured at a measurement temperature of 260°C and a shear rate of 50 s －１ is 600 Pa·s or more and 4000 Pa·s or less, and the Rz of the surface on the side with a small surface height roughness measured according to JIS B 0601:2001 is 0.50 μm or less, and the number of foreign substances having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in a plan view is 7 pieces / 0.25 m ２ or less. A fluororesin film.

2. The fluororesin film according to claim 1, comprising a structural unit derived from vinylidene fluoride as a main component.

3. The fluororesin film according to claim 1, which is a film for a piezoelectric film and has a thickness of 80 μm or more and 1000 μm or less.

4. Piezoelectric constant d measured in accordance with ISO 19622:2018 ３３ The fluororesin film according to claim 1, wherein ３３ is 5.0 pC / N or more and 40.0 pC / N or less.

5. The fluororesin having a melt viscosity η measured at a measurement temperature of 260°C and a shear rate of 50 s －１ is heated and melted at a temperature 75°C or higher and 105°C or lower higher than the melting point of the fluororesin, and the melted fluororesin at the temperature is filtered through a filter having a filtration accuracy of 10 μm or more and 40 μm or less, and the filtered fluororesin is formed into a film, and the method for producing a fluororesin film according to any one of claims 1 to 3.

6. A step of stretching the fluororesin film according to any one of claims 1 to 3 at a stretching ratio of 2.5 times or more and 6.0 times or less, and a step of polarizing the stretched film by applying a DC voltage of 7.0 kV or more and 50.0 kV or less. A method for producing a fluororesin film according to claim 4, comprising:

7. Measured temperature: 260 °C, shear rate during measurement: 50 s －１ A fluororesin piezoelectric film having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less, wherein the number of foreign substances having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view is 7 pieces / 0.25 m ２ or less, retardation is 100 nm or more and 2000 nm or less, and piezoelectric constant d ３３ is 5.0 pC / N or more and 40.0 pC / N or less. A fluororesin piezoelectric film.

8. The fluororesin piezoelectric film according to claim 7, wherein the Rz of the surface on the side with a small surface height roughness Rz measured according to JIS B 0601: 2001 is 0.50 μm or less.

9. The fluororesin piezoelectric film according to claim 7, comprising a structural unit derived from vinylidene fluoride as a main component.

10. The measurement temperature is 260°C, and the shear rate during measurement is 50 s －１ A step of heating and melting a fluororesin having a melt viscosity η measured at a shear rate of 50 s at a measurement temperature of 260°C of 600 Pa·s or more and 4000 Pa·s or less at a temperature 75°C or more higher than the melting point of the fluororesin and at a temperature not higher than 105°C higher; a step of filtering the molten fluororesin through a filter having a filtration accuracy of 10 μm or more and 40 μm or less; a step of forming the filtered fluororesin into a film; a step of stretching the formed film; and a step of polarizing the formed film. The method for producing a fluororesin piezoelectric film according to any one of claims 7 to 9, comprising:

11. A laminated piezoelectric body comprising the fluororesin piezoelectric film according to claim 7, wherein the total light transmittance is 80% or more.

12. An electrode layer having a surface resistivity of 1.0×10 －１ Ω / sq. or more and 1.0×10 ４ Ω / sq. or less, disposed on at least one surface of the fluororesin piezoelectric film; the laminated piezoelectric body according to claim 11.

13. The laminated piezoelectric body according to claim 12, wherein the electrode layer contains at least one selected from the group consisting of a metal film, a metal oxide film, a metal nanowire, a metal mesh, a conductive polymer compound, a carbon nanotube, and graphene.

14. The laminated piezoelectric body according to claim 11 or 12, further comprising a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film.

15. A charge prevention layer having a surface resistivity of 1.0×10 ４ Ω / sq. or more and 1.0×10 ９ Ω / sq. or less, which is disposed on at least one surface of the fluororesin piezoelectric film, and the laminated piezoelectric body according to claim 11 or 12.

16. An antistatic layer and a hard coat layer are arranged in this order on at least one surface of the fluororesin piezoelectric film, and the surface resistivity measured on the hard coat layer is 1.0×10 ６ Ω / sq. or more and 1.0×10 １２ Ω / sq. or less. The laminated piezoelectric body according to claim 11 or 12.

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