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

Abstract

This fluorine-based resin film has a melt viscosity η of 600-4,000 Pa·s as measured at a measurement temperature of 260°C and a shear rate of 50 s-1 during measurement, a thickness of 80-1,000 μm, and a haze per unit thickness of 0.35% / μm or less.

Description

Fluorine-based resin film, fluorine-based resin piezoelectric film and method for producing the same, and laminated piezoelectric body

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

[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 high demand for highly transparent fluorine-based films.

[0003] It is also known that a fluororesin film can be transformed into a piezoelectric film by piezoelectric treatment. The piezoelectric film is required to be transparent since it is used in touch panels and the like. The transparency of the piezoelectric film is largely determined by the transparency of the fluororesin film before the piezoelectric treatment, so there is a demand for the development of a highly transparent fluororesin film. In this specification, the term "fluororesin 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] Known methods for producing a fluororesin film include a solution casting method using a solution in which a fluororesin is dissolved (Patent Document 1, etc.), and an extrusion molding method in which a fluororesin is thermally melted (Patent Document 2).

[0005] International Publication No. 2015 / 064324 JP 05-102548

[0006] In the solution casting method, a large amount of polar organic solvent is used to dissolve the fluororesin, which requires the recovery of the solvent, which not only increases the production cost 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, it may hinder polarization in the manufacturing process of the piezoelectric film.

[0007] 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).

[0008] In order to perform extrusion molding by the hot melting method while suppressing degradation such as decomposition of the resin, it is preferable to use a fluororesin having a low melt viscosity. However, a fluororesin film formed by extrusion molding the hot melted resin has a problem of low transparency.

[0009] High transparency and a high piezoelectric constant are particularly desirable for piezoelectric films used in touch panels, etc. To produce a highly transparent fluororesin piezoelectric film, the fluororesin film used as the material is desirably highly transparent.

[0010] 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 that is highly transparent and exhibits high piezoelectricity through stretching and polarization treatment, a fluororesin piezoelectric film that has high transparency and piezoelectricity, a method for producing the same, and a laminated piezoelectric body.

[0011] One embodiment of the present invention for solving the above problems relates to the following fluororesin films [1] to [5]. [1] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ [2] A fluororesin film having a melt viscosity η of 600 Pa·s or more and 4000 Pa·s or less, measured by a method of 1000 rpm, a thickness of 80 μm or more and 1000 μm or less, and a haze per unit thickness of 0.35% / μm or less. [3] 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[3] The fluororesin film according to [1], wherein the surface roughness in height Rz of the surface with a smaller roughness, measured in accordance with JIS B 0601:2001, is 0.50 μm or less. [4] The fluororesin film according to any one of [1] to [3], which contains, as a main component, a structural unit derived from vinylidene fluoride. [5] The fluororesin film according to any one of [1] to [4], which is a film for use in a piezoelectric film.

[0012] One embodiment of the present invention for solving the above problems relates to the following methods for producing a fluorine-based resin film [6] to [7]. [6] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ [7] The method for producing a fluororesin film according to any one of [1] to [5], comprising the steps of: heat-melting a fluororesin having a melt viscosity η measured by HPLC of 600 Pa s to 4000 Pa s; extruding the heat-melted fluororesin to form a film; and cooling the formed film by contacting it with a cooling roll having a surface temperature of 125° C. or less. [7] The method for producing a fluororesin film according to [6], further comprising the steps of: in the heat-melting step, heat-melting the fluororesin at a temperature that is 75° C. or more and 105° C. or less higher than the melting point of the fluororesin; and filtering the molten fluororesin at said temperature through a filter having a filtration accuracy of 10 μm to 40 μm.

[0013] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [1] to [4]. [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 ３３[2] A fluororesin piezoelectric film according to [1], wherein the surface roughness in terms of surface height Rz of the surface on the side where the surface roughness Rz is smaller as measured in accordance with JIS B 0601:2001 is 0.50 μm or less. [3] 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 fluororesin piezoelectric film according to [1] or [2], which is: [4] The fluororesin piezoelectric film according to any one of [1] to [3], which contains a homopolymer of vinylidene fluoride as a main component.

[0014] One embodiment of the present invention for solving the above problems relates to a method for manufacturing a piezoelectric film according to the following [5] to [7]. [5] Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A method for producing a fluororesin piezoelectric film according to any one of [1] to [4], comprising the steps of: heating and melting a fluororesin having a melt viscosity η measured by FTIR of 600 Pa s to 4000 Pa s; extruding the molten fluororesin to form a film; cooling the formed film by contacting it with a cooling roll having a surface temperature of 125°C or less; stretching the cooled film; and polarizing the cooled film. [6] A method for producing a fluororesin piezoelectric film according to [5], wherein in the melting step, the fluororesin is melted at a temperature that is 75°C or more and 105°C or less higher than the melting point of the fluororesin. [7] A method for producing a fluororesin piezoelectric film according to [5] or [6], comprising the step of filtering the fluororesin melted in the melting step through a filter having a filtration accuracy of 10 μm to 40 μm.

[0015] 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 [4], 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 ４ Ω / 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.

[0016] According to the present invention, there are provided a highly transparent fluororesin film, a highly transparent fluororesin piezoelectric film, a method for producing the same, and a laminated piezoelectric body.

[0017] Figure 1 shows the relationship between the haze per unit thickness and the piezoelectric constant d ３３ 2 is a graph showing the relationship between the piezoelectric constant and the surface roughness of the laminated piezoelectric element according to one embodiment of the present invention.

[0018] One embodiment of the present invention relates to a fluororesin film and a fluororesin piezoelectric film produced from the same.

[0019] [Materials for Fluorine-Based Resin Film and Fluorine-Based Resin Piezoelectric Film] The fluororesin film and fluororesin piezoelectric 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. "Containing a fluororesin as a main component" means that the content of structural units derived from a monomer made of a fluorine-containing olefin relative to the total mass of the fluororesin film is 50% by mass or more and 100% by mass or less, preferably 70% by mass or more and 100% by mass or less, and more preferably 90% by mass or more and 100% by mass or less.

[0020] The fluororesin can be a homopolymer or copolymer obtained by polymerizing tetrafluoroethylene (TFE) or vinylidene fluoride (VDF). Examples of fluororesins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP). Examples of fluororesins obtained by polymerizing VDF include homopolymers of VDF, and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene, TFE, CTFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ethers.

[0021] Of these, from the viewpoint of facilitating polarization of the fluororesin film, fluororesins obtained by polymerizing monomers including 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 of two or more types.

[0022] The fluororesin film and fluororesin piezoelectric film preferably contain a structural unit derived from VDF as a main component, with a VDF homopolymer being 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 and fluororesin piezoelectric 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 and fluororesin piezoelectric film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0023] 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.

[0024] 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.

[0025] The fluororesin film and the fluororesin piezoelectric film were measured at a temperature of 260°C and a shear rate of 50 s －１The 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 of a fluororesin, the easier it is to form a film by extrusion at a low melting temperature. However, since resins with low melt viscosity facilitate molecular chain movement, selective alignment of molecular chains progresses during film formation, and crystals tend to grow. As the resin crystals grow, the refractive index of light differs between the crystalline and amorphous portions, increasing light scattering at the interface between the crystalline and amorphous portions, resulting in increased film haze and reduced transparency. In contrast, even with a fluororesin with a low melt viscosity, rapid cooling after film formation before crystal growth suppresses crystal growth in the film, reducing light scattering and resulting in a film with low haze and high transparency. Furthermore, the lower the melt viscosity, the easier it is to filter the film and the lower the melting temperature can be kept, so that speckled irregularities on the film surface due to resin modification are less likely to occur.

[0026] 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.

[0027] [Characteristics of Fluorine-Based Resin Film and Fluorine-Based Resin Piezoelectric Film] The characteristics of the fluorine-based resin film and fluorine-based resin piezoelectric film according to this embodiment will be described below. Note that the respective physical properties described below are values ​​measured by the following methods.

[0028] (Method of Measuring Physical Properties) The thickness of a non-piezoelectric fluororesin film is generally measured using a micrometer (JIS C 2151:2019), but can also be measured by 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 the 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.

[0029] On the other hand, the thickness of a fluororesin film having piezoelectricity (fluororesin piezoelectric film) is measured in the same manner as the method for measuring the thickness of a film without piezoelectricity, except for the setting of 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 line of the 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 the 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.

[0030] The haze of a non-piezoelectric 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 using 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.

[0031] 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.

[0032] At this time, a transparent coating layer is formed on the surface of the film to be measured, and the haze measured under the assumption that external haze due to scratches on the film surface has been removed is taken as the internal haze of the film. 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 the line segment connecting both ends of the film in the width direction is defined as point A. A measurement sample measuring 30 mm x 30 mm is cut out so that point A is at the center of the diagonal of the measurement sample, and two measurement samples are cut out adjacent to the measurement sample at both ends of the width, for a total of three measurement samples. For each of the three cut measurement samples, a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) is applied to one surface of the measurement sample using a bar coater and dried at 80 ° C. for 30 minutes. Thereafter, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) is used to apply a target integrated light dose of 400 mJ / cm. ２ The film is irradiated with UV light so that a coating layer with a thickness of 2 μm is formed. It is assumed that the coating layer removes external haze caused by scratches on the film surface, and the measured haze value is taken as the internal haze. The average value of three samples measured in the same manner is taken as the representative value of the fluororesin piezoelectric film.

[0033] The thinner the film, the greater the proportion of foreign particles present near the surface, making them more easily detectable by planar viewing of the film. Therefore, to quantify the amount of foreign particles in a film, the film thickness used for foreign particle measurement is set to 40 μm or less. Twenty-five rectangular films (observation pieces) are cut out adjacent to each other from a fluororesin or fluororesin piezoelectric film having a film thickness of 40 μm or less, and the sum of the number of foreign particles measured from each of the observation pieces is calculated. For fluororesin films with a thickness of more than 40 μm, the film is stretched until the thickness is reduced from 35 μm to 40 μm or less, and then the number of foreign particles is measured using the method described below. Specifically, a 0.010 m thick film is cut out adjacent to the film to be measured. ２Twenty-five pieces of the above specimen, each measuring 100 mm x 100 mm, are cut out. The sum of the number of foreign particles measured from each specimen is then calculated. At this time, foreign particles are marked using transmitted light, and the marked areas are observed under a microscope to determine the size of the foreign particles. The size of the foreign particles is defined as the arithmetic mean value of the maximum and minimum widths of the foreign particles.

[0034] The surface height roughness Rz of a non-piezoelectric 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. Then, with the intersection of the diagonals of a rectangular fluororesin film defined as center point A and the long side direction 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, and the average of these measurement points 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 above method. 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. This tendency is also maintained in the fluororesin piezoelectric film after stretching and polarization. Here, the measurement results for the surface with the smaller surface roughness Rz (the surface that comes into contact with the chill roll) are referred to as the surface height roughness Rz.

[0035] On the other hand, the surface height roughness Rz of a fluororesin piezoelectric film is measured in the same manner as the above-mentioned method for measuring Rz of a film without piezoelectricity, 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 on the line segment in the width direction passing through point A toward both ends, and the average of these measurement points is defined as the surface height roughness Rz of the fluororesin piezoelectric film.

[0036] The retardation of the fluororesin piezoelectric film is measured by the parallel Nicol rotation method using a light source with a wavelength of 587.8 nm. The fast and slow axes 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 (machine direction), the MD direction and the slow axis direction will coincide.

[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 piezoelectric 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 piezoelectric 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] (Characteristics of Fluorine-Based Resin Films Having No Piezoelectricity) The thickness of the fluorine-based resin film is 80 μm or more and 1000 μm or less, preferably 100 μm or more and 500 μm or less, more preferably 100 μm or more and 300 μm or less, even more preferably 120 μm or more and 200 μm or less, and particularly preferably 120 μm or more and 180 μm or less. The thicker the film, the more advantageous it is in terms of mechanical properties such as abrasion resistance, durability, and insulation. On the other hand, the thinner the film, the more advantageous it is in terms of optical properties such as transparency and cost.

[0039] The haze per unit thickness of the fluororesin film is less than 0.35% / μm, preferably 0% / μm or more and 0.30% / μm or less, and more preferably 0% / μm or more and 0.25% / μm or less. The lower the haze, the more improved the transparency of the fluororesin film. Furthermore, according to the findings of the present inventors, the smaller the haze per unit thickness, the lower the piezoelectric constant d ３３ This suggests that the smaller the haze per unit thickness, the smaller the crystal size in the film, and this moderate crystal size increases the amount of orientation polarization in the polarization process. ３３ This is thought to be because it allows for higher

[0040] The thinner the film, the more likely foreign matter is to 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. For fluororesin films with a film thickness of 40 μm or less, the number of foreign matter particles with a size of 100 μm or more is set to 7 per 0.25 m when the film is viewed in plan. ２ Preferably, 0 pieces / 0.25m or less ２ 5 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２1 piece or more / 0.25m ２ The following are particularly preferred:

[0041] For fluorine-based resin films with a thickness of 40 μm or less, the number of foreign particles larger than 200 μm in size is 3 per 0.25 m when the film is viewed in plan view. 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.

[0042] For fluorine-based resin films with a thickness of 40 μm or less, the number of foreign particles with a size of less than 100 μm is 50 / 0.25 m when the film is viewed in plan view. ２ Preferably, 0 pieces / 0.25m or less ２ More than 25 pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following is even more preferred:

[0043] 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.

[0044] 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 0.45 μm or less, and particularly preferably 0.01 μm or more and 0.40 μm or less. The smoother the surface of the fluororesin film, the less likely haze is to 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.

[0045] (Characteristics of Fluorine-Based Resin Piezoelectric Film) The fluorine-based resin piezoelectric film has an internal haze of less than 1.2%, preferably 0.0% to 1.1%, more preferably 0.0% to 0.7%, particularly preferably 0.0% to 0.6%, and most preferably 0.0% to 0.5%. The lower the internal haze, the more improved the transparency of the fluorine-based resin piezoelectric film. Furthermore, according to the findings of the present inventors, the smaller the internal haze, the lower the piezoelectric constant d ３３ It is easier to make it higher.

[0046] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 5.0%, even more preferably 0.0% to 3.0%, particularly preferably 0.0% to 2.5%, very preferably 0.0% to 2.0%, and most preferably 0.0% to 1.5%. The lower the haze, the more improved the transparency of the fluororesin piezoelectric film.

[0047] 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.

[0048] The fluorine-based resin piezoelectric film has a piezoelectric constant d ３３ is from 5.0 pC / N to 40.0 pC / N, preferably from 8.0 pC / N to 40.0 pC / N, more preferably from 10.0 pC / N to 30.0 pC / N, even more preferably from 15.0 pC / N to 30.0 pC / N, particularly preferably from 18.0 pC / N to 30.0 pC / N, and very preferably from 20.0 pC / N to 30.0 pC / N.

[0049] 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.

[0050] When a fluorine-based resin piezoelectric film having a thickness of 40 μm or less is viewed in plan, the number of foreign particles having a size of 100 μm or more is 0 / 0.25 m. ２ More than 7 pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 5 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following are particularly preferred:

[0051] When a fluorine-based resin piezoelectric film having 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 The following is even more preferred:

[0052] When a fluorine-based resin piezoelectric film having a thickness of 40 μm or less is viewed in plan, the number of foreign particles having 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 ２ More preferably, 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following is even more preferred:

[0053] The fewer these foreign matters there are, the more the transparency of the fluorine-based resin piezoelectric film can be improved and the more uniform the stretching and polarization can be.

[0054] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10.0 μm to 200.0 μm, more preferably 15.0 μm to 80.0 μm, even more preferably 20.0 μm to 80.0 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 advantageous in terms of electrical properties such as insulation and piezoelectric properties. A thinner film is advantageous in terms of optical properties such as transparency and cost.

[0055] [Others] The fluororesin film may contain resins other than fluororesins or other additives, as long as the above physical properties can be satisfied.

[0056] 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.

[0057] [Method for producing fluororesin film and fluororesin piezoelectric film] The method for producing the above-mentioned fluororesin film and fluororesin piezoelectric film is not particularly limited, but preferably includes a step of forming a film by extrusion molding of a heat-molten resin.

[0058] For example, the fluororesin film can be produced by carrying out a step of heating and melting the above-mentioned fluororesin (melting step) and a step of forming the filtered fluororesin into a film (film formation step). In this case, a step of filtering the fluororesin melted in the melting step (filtration step) may be further carried out.

[0059] Furthermore, a fluorine-based resin piezoelectric film can be produced by subjecting a fluorine-based resin film to a stretching step (stretching step) and a polarization step (polarization step) to impart piezoelectricity.

[0060] (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.

[0061] 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.

[0062] The melting temperature of the fluororesin is preferably 75°C or more higher than the melting point of the fluororesin but not more than 105°C higher, more preferably 75°C or more higher but not more than 100°C higher, even more preferably 80°C or more higher but not more than 100°C higher, and particularly preferably 85°C or more higher but not more than 95°C higher. By setting the melting temperature to 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 105°C or less higher than the melting point of the fluororesin, decomposition and condensation of the fluororesin due to heating can be suppressed, and the generation of decomposition products and the like due to these can be suppressed. By suppressing the generation of the decomposition products and the like, the amount of foreign matter in the fluororesin 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 and the like, clogging of the filter due to these when filtering the fluororesin can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0063] According to the findings of the present inventors, fluororesins with high melt viscosity require high temperatures to achieve a filterable viscosity. Heating to high temperatures is prone to decomposition, condensation, and other reactions, resulting in filter clogging. Therefore, it has been difficult to melt and filter fluororesins. In response, the present inventors discovered that fluororesins with appropriate melt viscosity can be efficiently filtered by adjusting the melting temperature within the above range. By adjusting the melting temperature of the fluororesin within the above range and filtering the fluororesin with reduced viscosity through melting, the fluororesin can be filtered without using a polar solvent. Furthermore, the absence of a solvent reduces the likelihood of polarization inhibition due to residual polar solvent in the fluororesin film during polarization treatment. Furthermore, since polar solvents are not generally used in the film formation process, this reduces the burden on the working environment and the natural environment, and also reduces the production costs associated with recovering the polar solvent.

[0064] (Filtration step) In the filtration step, the fluororesin that has been melted and reduced in viscosity in the melting step is filtered. The filtration method is not particularly limited, and the molten fluororesin may be passed through a filter, and known filter types such as pleated filters and leaf disc filters may be used.

[0065] The filtration accuracy of the filter used in the filtration step is preferably 10 μm or more and 40 μm or less, more preferably 10 μm or more and 30 μm or less, and even more preferably 15 μm or more and 30 μm or less. Using a filter with a filtration accuracy of 10 μm or more makes it easy to filter the molten fluororesin, and also makes it possible to shorten the filtration time without increasing the filtration pressure too much. Furthermore, using a filter with a filtration accuracy of 10 μm or more makes it possible to shorten the filtration time without increasing the filtration pressure too much. Using a filter with a filtration accuracy of 40 μm or less makes it possible to sufficiently remove foreign matter from the fluororesin, thereby obtaining a fluororesin film with little foreign matter.

[0066] 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.

[0067] 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.

[0068] The filter may be disposed between the extruder performing the film-forming step and the die. Alternatively, the filter may be disposed in an extruder or melt-kneading apparatus different from the extruder performing the film-forming step, and the fluororesin filtered through the filter may be fed into the extruder performing the film-forming step to form a film.

[0069] (Film Forming Step) In the film forming step, the fluorine-based resin filtered in the filtration step is formed into a film.

[0070] The film formation method is not particularly limited, and known methods can be used, such as extruding a molten and filtered fluororesin through a T-die and contacting it with a chill roll to cool it. In this embodiment, the surface temperature of the chill roll is set to 125°C or lower. Rapidly cooling the extruded fluororesin film using a chill roll with a low surface temperature allows the formation of numerous fine crystals, thereby reducing the haze per unit thickness of the resulting fluororesin film. According to the findings of the inventors, the fluororesin used in this embodiment with a low melt viscosity has a different crystallization rate from that of fluororesins with a higher melt viscosity, and therefore the haze of the resulting fluororesin film is likely to change significantly depending on the temperature of the chill roll. Therefore, in this embodiment, it is considered important to adjust the temperature of the chill roll. The surface temperature of the chill roll is preferably 5°C or higher and 110°C or lower, more preferably 20°C or higher and 80°C or lower, and even more preferably 30°C or higher and 60°C or lower.

[0071] The fluorine-based resin film thus obtained may be stored after being wound up, or may be transported to a stretching step as it is.

[0072] In the stretching step, the fluororesin film is uniaxially stretched. In the stretching step, the fluororesin film formed in the film-forming 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.

[0073] 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) by passing it between multiple rolls with different rotation speeds while being transported 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.

[0074] (Polarization Step) In the polarization step, a DC voltage is applied to the fluororesin film 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.

[0075] 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 kV or less.

[0076] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0077] After the polarization step, the fluororesin piezoelectric film can be wound into a roll for storage, transportation, and the like.

[0078] In this manner, the fluorine-based resin film and the fluorine-based resin piezoelectric film according to this embodiment can be obtained.

[0079] [Applications] The above-mentioned fluororesin film can be used for producing fluororesin piezoelectric films, as well as for various applications such as interior and exterior materials for automobiles, furniture, and building materials, protective films, and retardation films. The above-mentioned fluororesin piezoelectric films can also be used for 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.

[0080] An embodiment of the present invention relates to a laminated piezoelectric element.

[0081] [Laminated Piezoelectric Body] The fluororesin film described above can be used as a piezoelectric film. The fluororesin piezoelectric film (hereinafter simply referred to 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, resulting in a decrease in 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).

[0082] 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.

[0083] 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.

[0084] 2 is a schematic cross-sectional view showing the laminated piezoelectric body of this embodiment. As shown in FIG. 2, the 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.

[0085] (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.

[0086] (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.

[0087] 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.

[0088] The conductive material may be an ion-conductive conductive material or an electron-conductive conductive material.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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 less than 1.0×10 Ω / sq., the surface resistivity of the laminated piezoelectric body can be further reduced, and sufficient antistatic properties can be imparted. In order to prevent deterioration of 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 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 K 6911 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)).

[0096] 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.

[0097] 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.

[0098] (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. 2). 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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).

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] The thickness of the hard coat layer is not particularly limited, but is preferably 0.05 μm or more, more preferably 0.3 μm to 3.0 μm, even more preferably 0.5 μm to 2.0 μm, and even more preferably 0.5 μm to 1.5 μm.

[0111] 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.

[0112] (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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] (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.

[0120] 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.

[0121] 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 oxide (ITO) and antimony-tin 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.

[0122] 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.

[0123] 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.

[0124] 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, the 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 element, and is taken as a representative value.

[0125] 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 including the intersection of the diagonal lines 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 electrode layer.

[0126] 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.

[0127] 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.

[0128] (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.

[0129] 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.

[0130] 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.

[0131] The electrode layer is disposed on the surface of the substrate on the piezoelectric film side (the optical adjustment layer side in FIG. 2). As the electrode layer, an inorganic electrode such as ITO (indium tin oxide complex oxide) or tin oxide is preferable, and ITO is more preferable.

[0132] (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).

[0133] 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.

[0134] 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.

[0135] [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 material 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 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 position is set to one point at the intersection of the diagonal lines of the laminated piezoelectric element.

[0136] 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.

[0137] (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.

[0138] 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.

[0139] 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.

[0140] (Hue b ＊ value, Δb ＊ ) Color b of laminated piezoelectric body ＊ The value is preferably −5.0 or more and 5.0 or less. ＊ By setting the value to be −5.0 or more and 5.0 or less, it is possible to achieve good transparency and visibility. ＊ The value is preferably, for example, −5.0 or more and 5.0 or less. When the laminated piezoelectric element 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.

[0141] b of the laminated piezoelectric body ＊ The 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.

[0142] b of the laminated piezoelectric body ＊ Value and △b ＊ 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 △b ＊ 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 △b ＊ can be made smaller.

[0143] [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. 2 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.

[0144] (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.

[0145] (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.

[0146] The curable composition may further contain water or a solvent, examples of which include alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol.

[0147] 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.

[0148] 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.

[0149] (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.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] (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.

[0155] (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.

[0156] [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.

[0157] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0158] 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.

[0159] 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.

[0160] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0161] [Experiment 1] Preparation and Evaluation of Fluorine-Based Resin Film In Experiment 1, an unstretched, non-polarized fluorine-based resin film was prepared and evaluated.

[0162] 1. Fluorine-Based Resin Film 1-1 Preparation of Film Films 1-1 to 1-7 and 10, all of which were fluorine-based resin films, were prepared by the following procedure.

[0163] 1-1-1. Film 1-1 A vinylidene fluoride homopolymer (PVDF) having a melt viscosity of 800 Pa s and a melting point of 173°C was melted in a single-screw extruder with a diameter of φ50 mm, filtered through a pleated polymer filter with a filtration accuracy of 20 μm, and extruded into a film from a T-die. The melt temperature and filtration temperature at this time were 260°C. The extruded film was brought into contact with a cooling roll with a surface temperature of 50°C while maintaining a temperature of 150°C or higher, to obtain Film 1-1, an unstretched fluororesin film.

[0164] 1-1-2. Film 1-2 Film 1-2 was obtained in the same manner as in the production of Film 1-1, except that no filter was attached to the single-screw extruder and the surface temperature of the cooling roll was set to 70°C.

[0165] Film 1-3 was obtained in the same manner as in Film 1-1, except that PVDF having a melt viscosity of 2500 Pa s and a melting point of 173°C was used, a pleated polymer filter having a filtration accuracy of 15 μm was used, and the surface temperature of the cooling roll was set to 110°C.

[0166] Film 1-4 Film 1-4 was obtained in the same manner as Film 1-1, except that a vinylidene fluoride homopolymer (PVDF) having a melt viscosity of 3400 Pa s and a melting point of 173°C was used, which was prepared by mixing PVDF having a melt viscosity of 2500 Pa s and a melting point of 173°C with PVDF having a melt viscosity of 4500 Pa s and a melting point of 173°C in a mass ratio of 1:1, and the surface temperature of the cooling roll was set to 110°C.

[0167] 1-1-5. Film 1-5 Film 1-5 was obtained in the same manner as in the production of Film 1-1, except that the surface temperature of the cooling roll was set to 130°C.

[0168] 1-1-6. Film 1-6 Film 1-6 was obtained in the same manner as in the production of Film 1-1, except that PVDF having a melt viscosity of 2500 Pa·s and a melting point of 173°C was used and the surface temperature of the cooling roll was set to 130°C.

[0169] 1-1-7. Film 1-7 Film 1-7 was obtained in the same manner as in Film 1-1, except that PVDF with a melt viscosity of 4,500 Pa s and a melting point of 173°C was used, the melt temperature was set to 280°C, a filter was not attached to the single-screw extruder, and the surface temperature of the cooling roll was set to 140°C.

[0170] 1-1-8. Film 1-8 (unable to be produced) An attempt was made to produce Film 1-8 in the same manner as Film 1-1, except that PVDF with a melt viscosity of 4,500 Pa s and a melting point of 173°C was used, but the resin clogged the filter, and a film could not be obtained.

[0171] 1-1-9. Film 1-9 (unable to be produced) An attempt was made to produce Film 1-9 in the same manner as Film 1-1, 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.

[0172] 1-1-10. Film 1-10 100 g of PVDF with a melt viscosity of 2500 Pa s and a melting point of 173°C was weighed and added to 900 ml of n-methylpyrrolidone (NMP). The temperature was raised to 60°C while stirring with a stirrer, and stirring was continued for 6 hours to prepare a resin solution. This resin solution was filtered through a filter with a filtration accuracy of 40 μm. This resin solution was added to an automatic coater to prepare a coating film with a liquid thickness of 600 μm, which was dried at 120°C for 1 hour to obtain Film 1-10. Film 1-10 had a large surface height roughness Rz, making it difficult to accurately detect the amount of foreign matter.

[0173] 1-2. Evaluation of Fluorine-Based Resin Films The thickness, haze, number of foreign matters, and surface roughness Rz of the obtained Films 1-1 to 1-7 and 1-10 were measured by the following methods.

[0174] 1-2-1. Thickness A digital linear gauge (DG525H, manufactured by Ono Sokki Co., Ltd.) and a gauge stand (SH-022, manufactured by Ono Sokki Co., Ltd.) were used. 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, 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 fluororesin film.

[0175] 1-2-2. Haze 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 the center point A and the long side direction is the width direction, haze measurements were performed at a total of three measurement points: the center point A (measurement point) and two measurement points set 30 mm away from the center point A toward both ends on a line segment passing through the 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) to obtain the haze per unit thickness of the fluororesin film.

[0176] 1-2-3. Number of foreign particles The fluororesin film was uniaxially stretched to a thickness of 35 μm to 40 μm, and each film was cut out continuously from adjacent positions. 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.

[0177] 1-2-4. Surface Height Roughness Rz A surface roughness meter (Keyence Corporation, shape analysis laser microscope VK-X260) conforming to JIS B 0601:2001 was used. Specifically, when the intersection of the diagonals of a rectangular fluororesin film is defined as center point A and the direction of the long side is defined as the width direction, 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 on a line segment in the width direction passing through point A toward both ends from point A, and the average value of these measurements was defined as the surface height roughness Rz of the fluororesin film.

[0178] 1-3. Evaluation Results of Fluorine-Based Resin Films 1-3-1. Physical Properties of Fluorine-Based Resin Films The production conditions and evaluation results for each film are shown in Tables 1 and 2. Film 1-10 was opaque with a large haze, and it was not possible to measure the number of foreign matters.

[0179]

[0180]

[0181] 1-3-2. Relationship between Haze and Piezoelectric Constant Films 1-1 and 1-5 were introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and stretched 4.6 times in the machine direction by adjusting the rotation speed ratio of each roll. Furthermore, a voltage of 7.0 kV to 10.0 kV was applied from the surface of the film to the thickness direction to polarize the film.

[0182] The piezoelectric constant d of the poled film according to ISO 19622:2018 ３３ was measured. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) was used to hold a fluorine-based resin 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 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 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 width direction line segment passing through point A, each set 30 mm away from point A toward both ends. ３３ The average value of the piezoelectric constants obtained from these measurements was used as the piezoelectric constant d ３３ It was decided.

[0183] The applied voltage and the piezoelectric constant d of Film 1-1 (haze per unit thickness: 0.20% / μm) and Film 1-5 (haze per unit thickness: 0.51% / μm) ３３ The relationship between these is shown in Figure 1.

[0184] As shown in FIG. 1, the film 1-1, which has a small haze per unit thickness, has a piezoelectric constant d ３３ was getting higher.

[0185] [Experiment 2] Preparation and Evaluation of Fluorine-Based Resin Piezoelectric Film In Experiment 2, a fluorine-based resin film was stretched and polarized to prepare a fluorine-based resin piezoelectric film, which was then evaluated.

[0186] 2. Fluorine-Based Resin Piezoelectric Films 2-1. Fabrication of Fluorine-Based Resin Piezoelectric Films Piezoelectric films 2-1 to 2-8, all of which were fluorine-based resin piezoelectric films, were fabricated by the following procedure.

[0187] 2-1-1. Piezoelectric Film 2-1 Film 1-1 obtained in Experiment 1 was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and the rotational speed ratio of each roll was adjusted to stretch it 4.6 times in the machine direction. Furthermore, a voltage of 8.0 kV was applied from the surface of the film in the thickness direction to obtain piezoelectric film 2-1.

[0188] 2-1-2. Piezoelectric Film 2-2 A fluororesin film obtained in the same manner as in the production of the film in Experiment 1, except that a filter was not attached to the single-screw extruder, was used as a material, and a piezoelectric film 2-2 was obtained in the same manner as in the production of piezoelectric film 2-1, except that the voltage applied after stretching was 8.8 kV.

[0189] 2-1-3. Piezoelectric Film 2-3 A fluororesin film obtained in the same manner as in the film preparation in Experiment 1, except that the surface temperature of the cooling roll was set to 110°C, was used as the material, and a piezoelectric film 2-3 was obtained in the same manner as in the preparation of piezoelectric film 2-1, except that the voltage applied after stretching was set to 8.8 kV.

[0190] 2-1-4. Piezoelectric Film 2-4 A fluororesin film obtained in the same manner as in the production of the film in Experiment 1 was used as the material, except that PVDF with a melt viscosity of 2500 Pa s and a melting point of 173°C was used and the surface temperature of the cooling roll was set to 80°C. The other manufacturing conditions were the same as in the production of Piezoelectric Film 2-1, and Piezoelectric Film 2-4 was obtained.

[0191] Piezoelectric film 2-5 was obtained in the same manner as in the production of piezoelectric film 2-1, except that PVDF having a melt viscosity of 2500 Pa s and a melting point of 173°C was used, the surface temperature of the cooling roll was set to 110°C, and the voltage applied after stretching was set to 8.8 kV.

[0192] 2-1-6. Piezoelectric Film 2-6 Piezoelectric film 2-6 was obtained in the same manner as for the piezoelectric film 2-1, except that film 1-4 obtained in Experiment 1 was used as the material and the voltage applied after stretching was 8.8 kV.

[0193] 2-1-7. Piezoelectric Film 2-7 Piezoelectric film 2-7 was obtained in the same manner as in the production of piezoelectric film 2-1, except that film 1-5 obtained in Experiment 1 was used as the material and the voltage was applied after stretching.

[0194] 2-1-8. Piezoelectric Film 2-8 Piezoelectric film 2-8 was obtained in the same manner as for producing piezoelectric film 2-1, except that film 1-8 obtained in Experiment 1 was used as the material and the voltage applied after stretching was 9.5 kV.

[0195] 2-1-9. Piezoelectric Film 2-9 A voltage of 8.0 kV was applied from the surface of the film to the thickness direction of the film 1-10 obtained in Experiment 1 to obtain a piezoelectric film 2-9. Although the piezoelectric film 2-9 does not have piezoelectricity, it will be referred to as a piezoelectric film here.

[0196] 2-2. Evaluation of Fluorine-Based Resin Piezoelectric Films The obtained piezoelectric films 2-1 to 2-9 were evaluated for haze, internal haze, retardation, and piezoelectric constant d ３３ , surface height roughness Rz, number of foreign particles, and thickness were measured.

[0197] 2-2-1. Haze of Film The haze of the fluororesin piezoelectric film was measured in the same manner as in the measurement method (1-2-3) of Experiment 1, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the fluororesin 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. Haze measurements were performed at a total of three measurement points: point A (measurement point) and two measurement points set on the line segment in the width direction passing through point A, each 30 mm away from point A toward both ends. The average value of the haze values ​​determined from these measurements was defined as the haze of the fluororesin piezoelectric film.

[0198] 2-2-2. Internal Haze The fast axis direction determined by measuring the birefringence of the fluorine-based resin piezoelectric film was defined as the width direction, and an arbitrary point on the midpoint of the line segment connecting both ends of the film in the width direction was designated as point A. A measurement sample measuring 30 mm x 30 mm was cut out so that point A was the center of the diagonal of the measurement sample. Two measurement samples were cut out adjacent to the measurement sample in both width directions, for a total of three measurement samples. For each of the three cut measurement samples, a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) was applied to one surface of the measurement sample using a bar coater and dried at 80 °C for 30 minutes. Thereafter, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to apply a target integrated light dose of 400 mJ / cm. ２ The film was irradiated with UV light so that a coating layer with a thickness of 2 μm was formed. It was assumed that the coating layer removed external haze caused by scratches on the film surface, and the measured haze value was taken as the internal haze. The average value of three samples measured in the same manner was taken as the representative value of the fluororesin piezoelectric film.

[0199] 2-2-3. Retardation The retardation of a film cut to a size of 20 mm x 20 mm was measured by the parallel Nicol rotation method using a KOBRA-HB manufactured by Oji Scientific Instruments as a light source with a measurement wavelength of 587.8 nm.

[0200] 2-2-4. Piezoelectric constant d ３３ The piezoelectric constant of the fluororesin piezoelectric film was measured in the same manner as in the measurement method (1-3-2) of Experiment 1, except for the setting of the measurement points. Specifically, the fast axis direction determined by measuring the birefringence of the fluororesin piezoelectric film was defined as the width direction, and an arbitrary point on the midpoint of the line segment connecting both ends of the film in the width direction was defined as point A. The piezoelectric constant d ３３ The average value of the piezoelectric constants obtained from these measurements was taken as the piezoelectric constant d ３３ It was decided.

[0201] 2-2-5. Surface Height Roughness Rz The surface height roughness Rz of the fluororesin piezoelectric film was measured in the same manner as in the measurement method (1-2-4) of Experiment 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 piezoelectric film.

[0202] 2-2-6. Number of foreign matters Since the thickness of the fluororesin piezoelectric film was 40 μm or less, measurements were carried out in the same manner as in Experiment 1 (1-2-3), except that uniaxial stretching to adjust the film thickness was not performed.

[0203] 2-2-7. Thickness The thickness of the fluororesin piezoelectric film was measured in the same manner as in Experiment 1 (1-2-1), 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. The thickness 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 that passes through Point A toward both ends, and the average value of these measurement points was defined as the thickness of the fluororesin piezoelectric film.

[0204] 2-3. Results Tables 3 and 4 show the preparation conditions and evaluation results of each film.

[0205]

[0206]

[0207] [Experiment 3] Fabrication and Evaluation of Laminated Piezoelectric Body In Experiment 3, a laminated piezoelectric body was fabricated using a fluorine-based resin piezoelectric film and evaluated.

[0208] 3. Laminate 3-1. Method for Producing Laminated Piezoelectric Body 3-1-1. Antistatic Layer (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 resulting 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.

[0209] (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.

[0210] 3-1-2 Formation of Hard Coat Layer After applying a hard coat agent (BS-CH271 manufactured by Arakawa Chemical Industries, Ltd., amorphous silica average particle size 60 nm) using a multi-coater, the layer was heat-treated at 80°C for 2 minutes and then exposed to 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.

[0211] 3-1-3. 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.

[0212] 3-1-4. Transparent electrode (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.

[0213] (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.

[0214] (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 polymer 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.

[0215] 3-2. Layer Structure of Laminated Piezoelectric Body 3-2-1. Laminated Piezoelectric Body 1 The antistatic layer 1 was formed on surface A of piezoelectric film 2-1, and then a hard coat layer was formed on antistatic layer 1 to prepare laminated piezoelectric body 1.

[0216] 3-2-2. 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.

[0217] 3-2-3. 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.

[0218] 3-2-4. 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.

[0219] 3-2-5. 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 replaced with antistatic layer 2.

[0220] 3-2-6. 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.

[0221] 3-2-7. 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.

[0222] 3-2-8. 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.

[0223] 3-2-9. 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.

[0224] 3-2-10. 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.

[0225] 3-2-11. 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.

[0226] 3-2-12. 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.

[0227] 3-2-13. Laminated Piezoelectric Body 13 The hard coat layer was formed on surface A of piezoelectric film 1, then the optical adjustment layer 1 was formed on the hard coat layer, and the transparent electrode layer 1 was formed on the optical adjustment layer 1 to produce laminated piezoelectric body 13.

[0228] 3-2-14. Laminated Piezoelectric Body 14 Laminated piezoelectric body 14 was produced in the same manner as laminated piezoelectric body 11, except that the hard coat layer was replaced with an optical adjustment layer.

[0229] 3-2-15. 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.

[0230] 3-2-16. 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.

[0231] 3-3. 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.

[0232] 3-3-1. 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 performed in the same manner as in Experiment 2 (2-2-4), except that the representative value was used.

[0233] The total light transmittance of the laminated piezoelectric body was measured 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 in accordance with the method described in JIS K 7361-1, and the measured value was used as a representative value.

[0234] 3-3-3. Surface Resistivity (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 surface resistivity of the antistatic layer measured in an area including the intersection of the diagonals of the rectangular laminated piezoelectric body was used as a representative value.

[0235] (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 the surface resistivity. When a protective layer was formed on the electrode, the surface resistivity was measured from above the protective layer.

[0236] 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.

[0237] 3-3-5. Hue difference Δb ＊ The hue b of the laminated piezoelectric body before storage in a humid and heated environment was measured using a spectrocolorimeter (SD7000, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z 8722. ＊ The values ​​were measured.

[0238] 3-3-6. 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 the 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.

[0239] 3-4. Results Tables 5 and 6 show the preparation conditions and evaluation results of each laminated piezoelectric body.

[0240]

[0241]

[0242] This application claims priority to Japanese Patent Application No. 2024-006093 filed on January 18, 2024, and Japanese Patent Application No. 2024-006094 filed on January 18, 2024. The matters described in the specification, claims, and drawings of those applications as originally filed are incorporated herein by reference.

[0243] The fluororesin film and fluororesin piezoelectric film according to the present invention have high transparency.

[0244] 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, having a thickness of 80 μm or more and 1000 μm or less, and having a haze per unit thickness of 0.35% / μm or less. Fluororesin film.

2. The number of foreign matters with a size, which is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in a plan view, of 100 µm or more is 7 pieces / 0.25 m ２ The fluororesin film according to claim 1, wherein the number is 7 pieces / 0.25 m or less.

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

4. The fluororesin film according to claim 1, which contains a structural unit derived from vinylidene fluoride as a main component.

5. The fluororesin film according to claim 1, which is a film for a piezoelectric film.

6. Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A step of heating and melting a fluororesin having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less at a measurement temperature of 260°C and a shear rate of 50 s during measurement; a step of extruding the heated and melted fluororesin to form a film; and a step of cooling the formed film by bringing it into contact with a cooling roll having a surface temperature of 125°C or less. The method for producing a fluororesin film according to any one of claims 1 to 5.

7. In the step of heating and melting, the fluororesin 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 method for producing a fluororesin film according to claim 6 further includes a step of filtering the fluororesin melted at the temperature with a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

8. Measured temperature: 260°C, shear rate during measurement: 50 s －１ A fluororesin piezoelectric film 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, an internal haze of less than 1.2%, a retardation of 100 nm or more and 2000 nm or less, and a piezoelectric constant d ３３ of 5.0 pC / N or more and 40.0 pC / N or less, the fluororesin piezoelectric film.

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

10. The number of foreign matters having a size, which is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view, of 100 µm or more is 7 pieces / 0.25 m ２ The fluororesin piezoelectric film according to claim 8, which is as follows.

11. The fluororesin piezoelectric film according to claim 8, which contains a homopolymer of vinylidene fluoride as a main component.

12. A step of heating and melting a fluororesin having a melt viscosity η measured at a measurement temperature of 260° C. and a shear rate of 50 s−1 of 600 Pa·s or more and 4000 Pa·s or less; a step of extruding the melted fluororesin to form a film; a step of cooling the formed film by bringing it into contact with a cooling roll having a surface temperature of 125° C. or less; a step of stretching the cooled film; and a step of polarizing the cooled film. The method for producing a fluororesin piezoelectric film according to any one of claims 1 to 4, comprising: －１ a step of heating and melting a fluororesin having a melt viscosity η measured at a measurement temperature of 260° C. and a shear rate of 50 s−1 of 600 Pa·s or more and 4000 Pa·s or less; a step of extruding the melted fluororesin to form a film; a step of cooling the formed film by bringing it into contact with a cooling roll having a surface temperature of 125° C. or less; a step of stretching the cooled film; and a step of polarizing the cooled film. The method for producing a fluororesin piezoelectric film according to any one of claims 1 to 4, comprising:

13. In the melting step, the fluororesin is melted at a temperature 75°C or higher and 105°C or lower higher than the melting point of the fluororesin, and the method for producing a fluororesin piezoelectric film according to claim 12.

14. The method for producing a fluororesin piezoelectric film according to claim 12 includes a step of filtering the fluororesin melted in the melting step with a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

15. A laminated piezoelectric body including the fluororesin piezoelectric film according to claim 8, wherein the total light transmittance is 80% or more.

16. 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 15.

17. The laminated piezoelectric body according to claim 16, 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.

18. The laminated piezoelectric body according to claim 15 or 16, further including a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film.

19. 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, disposed on at least one surface of the fluororesin piezoelectric film, the laminated piezoelectric body according to claim 15 or 16.

20. 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 15 or 16.

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