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

Abstract

To provide a fluorine-based resin piezoelectric film having high transparency, and a method for producing the same.SOLUTION: A fluorine-based piezoelectric resin film has melt viscosity η measured at a measurement temperature of 260°C and a shear rate 50 s-1 at the time of measurement of 600 Pa s or more and 4000 Pa s or less. The fluorine-based resin film has a lamellar long period determined by a small angle X-ray scattering method of 11.5 nm or less, retardation of 100 nm or more and 2000 nm or less, and a piezoelectric constant d33 of 5.0 pC / N or more and 40.0 pC / N or less.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Display and input devices with a touch panel installed on the front surface of a display device such as a liquid crystal display are widely used and are used for operation panels of portable devices such as mobile phones and home electric appliances. In a display and input device using a touch panel, since a user operates the device by pressing a display on the screen, the piezoelectric film used for the touch panel is required to have high piezoelectricity to enhance detection sensitivity and high transparency to accurately view an image of the display.

[0003] As described in Patent Document 1 and Patent Document 2, piezoelectricity can be imparted to a fluororesin film by polarization treatment. As methods for producing a fluororesin film, a method of producing by a solution casting method using a solution in which a fluororesin is dissolved (Patent Document 1, etc.) and a method of thermally melting a fluororesin and performing extrusion molding (Patent Document 2) are known. For a piezoelectric film used for a touch panel or the like, high transparency and piezoelectricity are desired. In this specification, the fluororesin piezoelectric film or piezoelectric film means a fluororesin film having a piezoelectric constant d 33 measured by the method described below of 1.0 pC / N or more.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

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

[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, resulting in higher production costs and the need to consider 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, or the smoothness of the film surface may be impaired as the solvent evaporates.

[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 a 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 low transparency, and a fluororesin piezoelectric film produced from this fluororesin film also has low transparency, which is a problem.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a highly transparent fluorine-based resin piezoelectric film, a method for producing the same, and a laminated piezoelectric element. [Means for solving the problem]

[0010] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [1] to [5]. [1] Measurement temperature: 260°C, shear rate: 50 s -1A fluororesin piezoelectric film having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less, wherein the lamellar long period determined by small-angle X-ray scattering is 11.5 nm or less, the retardation is 100 nm or more and 2000 nm or less, and the piezoelectric constant d 33 is 5.0 pC / N or more and 40.0 pC / N or less, a fluororesin piezoelectric film. [2] having an internal haze of less than 1.2%, the fluororesin piezoelectric film according to [1]. [3] The Rz of the surface on the smaller side of the surface height roughness Rz measured according to JIS B 0601:2001 is 0.50 μm or less, the fluororesin piezoelectric film according to [1] or [2]. [4] The number of foreign matters having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view and is 100 μm or more is 7 pieces / 0.25 m 2 or less, the fluororesin piezoelectric film according to any one of [1] to [3]. [5] containing a structural unit derived from vinylidene fluoride as a main component, the fluororesin piezoelectric film according to any one of [1] to [4].

[0011] One embodiment of the present invention for solving the above problems relates to a method for manufacturing a piezoelectric film according to the following [6] to [8]. [6] 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 at 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, a step of polarizing the cooled film, The manufacturing method of the fluororesin piezoelectric film according to any one of [1] to [5], which has [7] In the melting step, the fluororesin is melted at a temperature 75°C or more higher than the melting point of the fluororesin and at a temperature not higher than 105°C higher. [6] The manufacturing method of the fluororesin piezoelectric film according to the above. [8] The method 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. [6] Or the manufacturing method of the fluororesin piezoelectric film according to [7].

[0012] One embodiment of the present invention for solving the above problems relates to the following laminated piezoelectric body of [9] to

[14] . [9] A laminated piezoelectric body including the fluororesin piezoelectric film according to any one of [1] to [4], The total light transmittance of which is 80% or more, Laminated piezoelectric body.

[10] An electrode layer disposed on at least one surface of the fluororesin piezoelectric film, having a surface resistivity of 1.0×10 -1 Ω / sq. or more and 1.0×10 4 Ω / sq. or less, further comprising The laminated piezoelectric body according to [9].

[11] The electrode layer includes 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. The laminated piezoelectric body according to

[10] .

[12] Further comprising a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film. The laminated piezoelectric body according to any one of [9] to

[11] . [I3] Further comprising an antistatic layer disposed on at least one surface of the fluororesin piezoelectric film, having a surface resistivity of 1.0×10 4 Ω / sq. or more and 1.0×10 9 Ω / sq. or less. The laminated piezoelectric body according to any one of [9] to

[12] .

[14] 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; The surface resistivity measured on the hard coat layer is 1.0×10 6 Ω / sq. or more 1.0×10 12 Ω / sq. or less, The laminated piezoelectric element according to any one of [9] to

[13] . [Effects of the Invention]

[0013] According to the present invention, a fluororesin piezoelectric film having high transparency and excellent surface smoothness, a method for producing the same, and a laminated piezoelectric element are provided. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing a laminated piezoelectric element according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] [Fluoroplastic piezoelectric film] An embodiment of the present invention relates to a fluorine-based resin piezoelectric film.

[0016] The fluororesin piezoelectric film may be any film containing fluororesin as its main component. Fluororesin is a resin obtained by polymerizing a monomer made of fluorine-containing olefin, and containing fluororesin as its main component means that the content of structural units derived from fluororesin monomers relative to the total mass of the fluororesin film is 50% by mass or more. The content of fluororesin relative to the total mass of the piezoelectric film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

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

[0018] Among 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 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.

[0019] The fluororesin piezoelectric film preferably contains 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 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 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.

[0020] In particular, since fluororesin piezoelectric films are expected to have high piezoelectric properties, 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.

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

[0022] The fluorine resin piezoelectric film was measured at a temperature of 260°C and a shear rate of 50 s -1 The melt viscosity measured by NMR 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. Resins with lower melt viscosity can be extruded at lower temperatures. However, resins with low melt viscosity facilitate molecular chain movement, which promotes selective alignment of molecular chains during film formation and facilitates crystal growth. As resin crystals grow, the refractive index of the crystalline and amorphous regions differs, increasing light scattering at the interface between the crystalline and amorphous regions and tending to reduce film transparency. In contrast, even for fluororesins with low melt viscosity, rapid cooling of the film after film formation suppresses crystal growth, reducing light scattering and resulting in highly transparent films. The lamellar periodicity determined by X-ray diffraction is a good indicator of the crystallite size of the film; the smaller the crystallite size, the smaller the lamellar long period. Furthermore, the lower the melt viscosity, the easier it is to filter the resin and the lower the melting temperature required for film formation, making it less likely that the film surface will develop spot-like irregularities due to resin denaturation.

[0023] The melt viscosity is measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, using a capillary rheometer (manufactured by Toyo Seiki Seisaku-sho, Ltd., Capillograph 1D), the viscosity measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 shall be the viscosity measured with a capillary die having an inner diameter of φ1 mm and a tube length of 10 mm.

[0024] The fluororesin piezoelectric film has a lamellar long period of 11.5 nm or less as measured by the small-angle X-ray scattering method, preferably 8.0 nm or more and 11.0 nm or less, more preferably 8.5 nm or more and 11.0 nm or less. The shorter the lamellar long period, the smaller the crystallite size of the resin constituting the fluororesin piezoelectric film. By reducing the crystal size, the transparency of the fluororesin piezoelectric film can be increased. Further, according to the findings of the present inventors, when the lamellar long period is 11.5 nm or less, the piezoelectric constant d 33 is more likely to be higher when the fluororesin film is poled. This is considered to be because by shortening the lamellar long period, that is, appropriately reducing the crystallite size, the amount of orientation polarization in the poling process can be increased, thereby increasing the piezoelectric constant d 33 to be high.

[0025] The lamellar long period can be calculated by measuring the X-ray diffraction pattern of the fluororesin piezoelectric film with a small-angle X-ray scattering measurement device and using the black reflection method. Specifically, the fluororesin piezoelectric film is installed in the small-angle X-ray scattering measurement device, irradiated with CuKα rays (wavelength λ = 0.15418 nm) monochromatized with a Ni filter as the X-ray source, and a two-dimensional diffraction image is measured using a two-dimensional detector. Then, subtracting the X-ray scattering intensity (B(2θ)) by air from the X-ray scattering (diffraction) intensity (A(2θ)) measured by the above method, the X-ray scattering (diffraction) intensity (I(2θ)) with respect to the diffraction angle 2θ in the range of the diffraction angle 2θ of 0.200° to 2.000° is obtained.

[0026] The background of I(2θ) is calculated by linear approximation from the diffraction angle 2θ that shows the lowest X-ray intensity in the range of 2θ = 0.200° to 0.450°, its X-ray intensity, and the X-ray intensity at 2θ = 2.000°. Next, the background is subtracted from I(2θ), the diffraction angle 2θ at which the X-ray intensity reaches its maximum (peak top) is determined, and the lamellar long period L is calculated using Bragg's reflection formula shown in Equation 1.

[0027]

number

[0028] 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 through the polarization process.

[0029] Retardation is measured using a light source with a wavelength of 587.8 nm using the parallel Nicol rotation method. At this time, the fast axis and slow axis are determined from the in-plane birefringence of the film. The slow axis direction coincides with the average direction of the molecular chains oriented by stretching or extrusion. Therefore, if the film is stretched in the MD (machine direction), the MD and slow axis directions will coincide.

[0030] Fluorine resin piezoelectric film has a piezoelectric constant d 33 is 5.0 pC / N or more and 40.0 pC / N or less, preferably 8.0 pC / N or more and 40.0 pC / N or less, and more preferably 10.0 pC / N or more and 30.0 pC / N or less.

[0031] The piezoelectric constant of the fluororesin piezoelectric film was measured by the direct quasi-static method (d 33Measurements were performed in accordance with ISO 19622:2018, a test method for the piezoelectric constant d33 by the Berlin-Corte method (Meter method). Specifically, a piezoelectric constant measurement device (PIEZOTEST, Piezometer System PM300) was used to hold the fluororesin piezoelectric film test specimen with a holding force of 1.0 N, and measure the charge generated when an alternating force of 0.15 N and a frequency of 110 Hz was applied. The charge measurement was performed on the polarization surface of the fluororesin piezoelectric film, and the absolute value of the measured value was used to calculate the piezoelectric constant. Here, 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 designated as Point A. The piezoelectric constant d33 was measured at a total of three measurement points: Point A (measurement point), and two measurement points set on the width direction line segment passing through Point A, each 30 mm away from Point A toward both ends. 33 The average value of the piezoelectric constants obtained from these measurements was used as the piezoelectric constant d 33 Let's say.

[0032] The haze of the fluororesin piezoelectric film is preferably 0.0% or more and 10.0% or less, more preferably 0.0% or more and 5.0% or less, and even more preferably 0.0% or more and 3.0% or less. The lower the haze, the more improved the transparency of the fluororesin piezoelectric film.

[0033] The fluororesin piezoelectric film preferably has an internal haze of less than 1.2%, more preferably 0.3% to 1.1%, and even more preferably 0.1% to 1.0%. The lower the internal haze, the more transparent the fluororesin piezoelectric film.

[0034] The haze of the fluororesin piezoelectric film is measured in accordance with JIS K 7136:2000. Specifically, a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II) is used. Then, the advancing axis direction obtained 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 in the width direction of the film is defined as point A. Haze measurement is performed at a total of three measurement points, namely point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A. The average value of these is taken as the haze of the fluororesin piezoelectric film.

[0035] The internal haze is the haze measured assuming that an external haze due to scratches on the surface of the fluororesin piezoelectric film is removed by forming a transparent coating layer on the surface of the film. The advancing axis direction obtained 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 in the width direction of the film is defined as point A. A measurement sample with a size of 30 mm × 30 mm is cut out so that point A is at the center of the diagonal of the measurement sample. Two measurement samples are cut out in both end directions of the width so as to be adjacent to the measurement sample, and a total of three measurement samples are collected. For each of the three cut-out measurement samples, a hard coat agent (manufactured by Arakawa Chemical Industries, Ltd., BS CH271) is applied to one surface of the measurement sample with a bar coater and dried at 80°C for 30 minutes. Then, using an ultraviolet (UV) irradiation device (manufactured by GS NIPPON DENCHI, CSOT040), UV is irradiated so that the target integrated light amount becomes 400 mJ / cm 2 and a coating layer with a thickness of 2 μm is formed. Assuming that the external haze due to scratches on the film surface is removed by these coating layers, the measured haze value is taken as the internal haze. The average value of the three measurement samples measured in the same way is taken as the representative value of the fluororesin piezoelectric film.

[0036] The fluororesin piezoelectric film preferably has a surface height roughness Rz of 0.50 μm or less, more preferably 0.05 μm or more and 0.50 μm or less, still 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 is, the less likely it is to cause haze on the film surface and the better the transparency obtained. Also, a small surface height roughness Rz indicates that there are few irregularities such as wrinkles on the film.

[0037] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter (VK-X260, a shape analysis laser microscope manufactured by KEYENCE CORPORATION) compliant with JIS B 0601:2001 is used. Then, the advancing axis direction obtained 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 in the width direction of the film is defined as point A. For a total of three measurement points including point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the surface height roughness Rz is measured, and the average value of these is taken as the surface height roughness Rz of the fluororesin piezoelectric film. Note that the contact surface of the fluororesin piezoelectric film with the cooling roll tends to have a smaller surface roughness Rz compared to the non-contact surface with the roll. This is because the irregularities on the surface become smaller due to the fluororesin piezoelectric film being pressed against the cooling roll. This tendency is maintained even after stretching and polarization. Here, the measurement result of the surface with a small surface roughness Rz (the contact surface with the cooling roll) is taken as the above surface height roughness Rz.

[0038] Since the proportion of resin foreign matter present near the surface increases as the film becomes thinner, foreign matter is more likely to be detected in a plan view of the film. Therefore, for quantifying the amount of foreign matter in the film, the film thickness used for foreign matter measurement is set to 40 μm or less. When a fluororesin piezoelectric film with a film thickness of 40 μm or less is viewed in plan view, the number of foreign matters with a size of 100 μm or more is 0 pieces / 0.25 m 2 to 7 pieces / 0.25 m 2The following is preferred: 0 pieces / 0.25m 2 More than 5 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 3 pieces / 0.25m 2 The following is even more preferable: 0 pieces / 0.25m 2 More than 1 piece / 0.25m 2 The following are particularly preferred:

[0039] For fluorine-based resin piezoelectric films with a film thickness of 40 μm or less, the number of foreign particles larger than 200 μm in size is 0 / 0.25 m when the film is viewed in plan view. 2 More than 3 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 2 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 1 piece / 0.25m 2 The following is even more preferred:

[0040] When a fluorine-based resin piezoelectric film with a film thickness of 40 μm or less is viewed in plan, the number of foreign particles less than 100 μm in size is 0 / 0.25 m 2 More than 50 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 25 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 16 pieces / 0.25m 2 The following is even more preferred:

[0041] The fewer these foreign matters there are, the more the transparency of the fluororesin piezoelectric film can be improved, and when the fluororesin piezoelectric film is stretched or polarized, the more uniform the stretching or polarization can be.

[0042] The number of these foreign substances is determined by continuously cutting out 25 rectangular films (observation pieces) from a fluororesin piezoelectric film with a film thickness of 40 μm or less so that they are adjacent to each other, and obtaining the sum of the numbers of foreign substances measured from each of the said observation pieces. For a fluororesin piezoelectric film with a thickness exceeding 40 μm, after stretching until the film thickness becomes 35 μm or more and 40 μm or less, the number of foreign substances is measured by the method described below. Specifically, 0.010 m 2 (100 mm × 100 mm) of 25 said observation pieces having the size are cut out. Then, the sum of the numbers of foreign substances measured from each observation piece is obtained. At this time, the foreign substances are marked by observing with transmitted light, and the size of the foreign substances is obtained by observing the marked portions with a microscope. The size of the foreign substances is the arithmetic mean value of the maximum width and the minimum width of the foreign substances.

[0043] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm or more and 200 μm or less, more preferably 15 μm or more and 80 μm or less, still more preferably 20 μm or more and 80 μm or less, particularly preferably 30.0 μm or more and 80.0 μm or less, very preferably 35.0 μm or more and 70.0 μm or less, and most preferably 35.0 μm or more and 50.0 μm or less. The thicker the film, the more advantageous it is for electrical properties such as insulation and piezoelectric properties. The thinner the film, the more advantageous it is for optical properties such as transparency and cost.

[0044] The thickness of the fluororesin piezoelectric film is generally measured by a method using a micrometer (JIS C 2151:2019), but can 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. At this time, the advancing axis direction obtained by measuring the birefringence is defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the said film is defined as point A. For a total of three measurement points including point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the thickness is measured, and the average value of these is taken as the thickness of the fluororesin piezoelectric film.

[0045] In addition, as long as the fluororesin piezoelectric film satisfies the above physical properties, it may contain resins other than the fluororesin or other additives.

[0046] Examples of the resins other than the above fluororesin include polyesters such as polycarbonate, polyethylene terephthalate, and polyethylene naphthalate that can be added to enhance flexibility, silicone resin, polyether, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, acrylic resin, epoxy resin, polyethylene oxide, polypropylene oxide, polyphenylene oxide, polyphenylene sulfide, polyamide, polyimide, polyamideimide, polystyrene, and polybenzimidazole that can be added to enhance strength, and odd polyamides, cyanopurans, and copper phthalocyanine-based polymers that can be added to further enhance dielectric properties.

[0047] [Manufacturing method of fluororesin piezoelectric film] The manufacturing method of the above-mentioned fluororesin film is not particularly limited, but preferably includes a step of forming a fluororesin film by extruding a heat-melted resin, a step of stretching the obtained fluororesin film, and a step of polarization treatment.

[0048] For example, the fluororesin film can be produced by performing a step of heating and melting the above-mentioned fluororesin (melting step), a step of forming the filtered fluororesin into a film (film forming step), a step of stretching the formed film (stretching step), and a step of polarizing the formed film (polarization step).

[0049] At this time, a step of filtering the molten fluororesin (filtering step) may be performed as necessary.

[0050] (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.

[0051] If the fluororesin melted in the melting step contains a solvent component, the remaining solvent component without volatilization may hinder polarization in a subsequent step, so the content of the solvent component in the fluororesin is preferably small, preferably 1 mass % or less, more preferably 0.1 mass % or less, based on the total mass of the fluororesin. In particular, the content of the polar solvent is preferably 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less, based on the total mass of the fluororesin.

[0052] The melting temperature of the fluororesin is preferably 75°C or more higher than the melting point of the fluororesin but not exceeding 105°C, more preferably 75°C or more higher but not exceeding 100°C, even more preferably 80°C or more higher but not exceeding 100°C, and particularly preferably 85°C or more higher but not exceeding 95°C. Setting the melting temperature at 75°C or more higher than the melting point of the fluororesin can reduce the viscosity of the fluororesin to a level that allows it to be filtered in the subsequent process. Setting the melting temperature at 105°C or less higher than the melting point of the fluororesin can suppress decomposition and condensation of the fluororesin due to heating, thereby suppressing the generation of decomposition products and the like. Suppressing the generation of such decomposition products and the like reduces the amount of foreign matter in the fluororesin piezoelectric film and improves the transparency and smoothness of the fluororesin piezoelectric film. Furthermore, suppressing the generation of such decomposition products and the like can suppress filter clogging caused by these products when filtering the fluororesin, thereby improving the filtration efficiency of the fluororesin.

[0053] According to the findings of the present inventors, in order to make a resin with a high melt viscosity into a viscosity that can be filtered, it is necessary to heat it to a high temperature. When heated to a high temperature, decomposition, condensation, etc. are likely to occur, and clogging of the filter is likely to occur. Therefore, it has been difficult to melt and filter fluororesins. In contrast, the present inventors have found that by setting the melting temperature of a fluororesin having an appropriate melt viscosity within the above range, the fluororesin can be efficiently filtered. By setting the melting temperature of the fluororesin within the above range and filtering the fluororesin whose viscosity has been lowered by melting, the fluororesin can be filtered without using a polar solvent. Further, by not using a solvent, it is less likely to cause inhibition of polarization by the solvent during the polarization treatment due to the polar solvent remaining in the fluororesin film. Furthermore, since a polar solvent is basically not used in the film-forming step, it is possible to reduce the burden on the working environment and the natural environment, and to reduce the manufacturing cost for recovering the polar solvent.

[0054] (Filtering step) In the filtering step, the fluororesin melted and reduced in viscosity by the melting step is filtered. The filtering method is not particularly limited, and the melted fluororesin may be passed through a filter, and known filter types such as a pleated type and a leaf disk type filter can be used.

[0055] The filtration accuracy of the filter used in the filtering step is preferably 10 μm or more and 40 μm or less, more preferably 10 μm or more and 35 μm or less, and even more preferably 15 μm or more and 30 μm or less. By using a filter with a filtration accuracy of 10 μm or more, it is possible to easily pass the filter through the fluororesin melted by heating to the above temperature. Further, by using a filter with a filtration accuracy of 10 μm or more, the filtration pressure does not become too high, and the filtration time can be shortened. By using a filter with a filtration accuracy of 40 μm or less, foreign matters in the fluororesin can be sufficiently removed, and a fluororesin film with few foreign matters can be obtained.

[0056] Fluorine-based resins are filtered using a multilayer filter consisting of multiple layers with different shapes, mesh sizes, etc. The filtration accuracy of a filter used to filter fluororesins refers to the filtration efficiency of the filter, i.e., the filter's ability to filter out particles of a certain size with a predetermined filtration efficiency. For example, in this specification, a filtration accuracy of 10 μm means that the filter can filter out particles of 10 μm or larger with a filtration efficiency of 95% or higher.

[0057] In this step, the fluororesin may be filtered multiple times. For example, coarse foreign matter may be removed using a filter with low filtration accuracy (large filtration accuracy value) placed in the front stage, and then finer foreign matter may be removed using a filter with high filtration accuracy (small filtration accuracy value) placed in the rear stage. In this case, the filtration accuracy is the value of the filter with the highest filtration accuracy.

[0058] The filter may be disposed between the extruder that performs the film-forming step and the die. Alternatively, the filter may be disposed in an extruder or melt-kneading apparatus that is different from the extruder that performs the film-forming step, and the fluororesin that has been filtered through the filter may be fed into the extruder that performs the film-forming step to form a film.

[0059] (Film forming process) In the film-forming step, the fluorine-based resin filtered in the filtration step is formed into a film.

[0060] 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 cooling it by contacting it with a cooling roll. In this embodiment, the surface temperature of the cooling roll at this time is set to 125°C or lower. By rapidly cooling the extruded fluororesin film using a cooling roll with a low surface temperature, a large number of fine crystals can be formed, shortening the lamellar long period of the resulting fluororesin film. The surface temperature of the cooling 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.

[0061] The fluororesin film thus obtained may be wound up once and stored, or may be conveyed to subsequent processes such as a stretching process and a polarization process.

[0062] (Stretching process) In the stretching process, the formed fluororesin film is uniaxially stretched. In the stretching process, the fluororesin film formed in the film-forming process may be stretched as it is, or the fluororesin film wound up and stored once may be stretched after being heated as necessary.

[0063] In the case of uniaxial stretching, the stretching direction is not limited. In the stretching process for mass production, it is preferable to continuously uniaxially stretch the fluororesin film. Specifically, while conveying the fluororesin film by a plurality of rolls, it can be uniaxially stretched in the conveying direction (MD direction) by passing between a plurality of rolls with different rotational speeds. The stretching ratio (the 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 ratio of β-crystals described later is sufficiently increased, and it becomes easier to obtain a fluororesin piezoelectric film with a high piezoelectric constant by the polarization process. By setting the stretching ratio to 6.0 times or less, breakage of the fluororesin film in the stretching process is less likely to occur. In the film uniaxially stretched in the MD direction, the MD direction is the slow axis direction, and the direction perpendicular to the stretching direction (TD direction) is the fast axis direction.

[0064] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. In a fluororesin containing a homopolymer or copolymer of VDF, a transition from α-crystals to β-crystals occurs in the stretching process, and the ratio of β-crystals increases. By applying a DC voltage to a fluororesin film with an increased ratio of polar β-crystals, a fluororesin piezoelectric film with a high piezoelectric constant can be obtained.

[0065] The DC voltage to be applied 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.

[0066] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0067] The fluororesin film after the film-forming step or the fluororesin piezoelectric film after the polarization step can be wound into a roll for storage, transportation, and the like.

[0068] [Application] The above-mentioned fluorine-based resin piezoelectric film can be used in various applications such as touch sensors and touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0069] [Laminated piezoelectric element] The fluorine-based resin piezoelectric film (hereinafter also referred to simply as "piezoelectric film") can be laminated with other functional layers or films to form a laminated piezoelectric body. The laminated piezoelectric body may optionally include an antistatic layer to prevent static electricity 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 fluorine-based resin 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, which may reduce conductivity. Therefore, the laminated piezoelectric body may further include an overcoat layer on the non-adhesive surface of the electrode layer (the surface opposite the piezoelectric film).

[0070] That is, the laminated piezoelectric body includes at least the piezoelectric film. The laminated piezoelectric body can 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 can be arbitrarily combined. For example, the laminated piezoelectric body can include an electrode layer on at least one surface of the piezoelectric film, and may further include an antistatic layer or an overcoat layer.

[0071] Hereinafter, the laminated piezoelectric body and its manufacturing method according to an embodiment of the present invention will be specifically described. However, the laminated piezoelectric body is not limited to this embodiment.

[0072] FIG. 1 is a schematic cross-sectional view showing the laminated piezoelectric body of this embodiment. As shown in FIG. 1, the laminated piezoelectric body 10 can have a piezoelectric film 11, an antistatic layer 12, a hard coat layer 13, an optical adjustment layer 14, and an electrode layer 15 in this order. Hereinafter, each layer will be described.

[0073] (Piezoelectric Film) As the piezoelectric film, the fluororesin piezoelectric film can be used. The piezoelectric constant d of the piezoelectric film 33 and the thickness of the piezoelectric film can be the same as the piezoelectric constant d 33 and the thickness of the above-mentioned fluororesin piezoelectric film, respectively.

[0074] (Antistatic Layer) The antistatic layer can be disposed on at least one surface of the piezoelectric film. In this embodiment, it is preferable that the antistatic layer is in contact with the piezoelectric film. The antistatic layer can make it difficult for the laminated piezoelectric body to generate static electricity.

[0075] The antistatic layer can include a cured product of a curable composition containing a conductive material. The curable composition containing a conductive material may include a conductive material, a curable resin, and a cross-linking agent as necessary, or may include a conductive material and a cross-linking agent.

[0076] The conductive material may be an ion-conductive conductive material or an electron-conductive conductive material.

[0077] Examples of the ion-conductive conductive material include (a) cationic antistatic agents having cationic groups such as quaternary ammonium salts, pyridinium salts, and primary to tertiary amino groups, (b) anionic antistatic agents having anionic groups such as sulfonate groups, sulfate ester groups, phosphate ester groups, and phosphonate groups, (c) amphoteric antistatic agents such as amino acid-based and amino sulfate ester-based antistatic agents, and (d) nonionic antistatic agents such as amino alcohol-based, glycerin-based, and polyethylene glycol-based antistatic agents.

[0078] Examples of the electron-conductive conductive material include conductive polymers and other conductive materials. Examples of the conductive polymer include polyacetylene or its derivatives, polythiophene or its derivatives, polypyrrole or its derivatives, polyaniline or its derivatives, etc. Among them, polythiophene or its derivatives are preferable from the viewpoints of high transparency and high conductivity. These conductive polymers may have functional groups such as sulfone groups, amino groups, amide groups, hydroxyl groups, mercapto groups, and carboxyl groups. Examples of the conductive material other than the conductive polymer include carbon nanotubes, graphene, etc.

[0079] Among these, from the viewpoints of easily lowering the surface resistivity of the laminated piezoelectric body and hardly causing bleed-out, it is preferable to include an electron-conductive conductive material, and carbon nanotubes are more preferable.

[0080] 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 glycol, etc., which have functional groups such as hydroxyl groups, methylol groups, carboxyl groups, sulfonyl groups, epoxy groups, and amino groups.

[0081] The crosslinking agent may be any substance that reacts with the functional groups of the curable resin or conductive polymer to cause crosslinking. Examples of crosslinking agents include melamine-based crosslinking agents, polycarbodiimide-based crosslinking agents, polyoxazolidine-based crosslinking agents, polyepoxy-based crosslinking agents, and polyisocyanate-based crosslinking agents.

[0082] As described above, as the curable resin and crosslinking agent for obtaining the antistatic layer, for example, a curable resin and crosslinking agent having an amino group such as melamine resin (these are also referred to as "amine-based materials") may be used. Among them, melamine resin may be preferably used from the viewpoint of having low-temperature curability. On the other hand, such amine-based materials may generate an amine (base) that causes a defluorination reaction of the fluororesin under high temperature and high humidity, which may cause discoloration of the piezoelectric film. Even in such a case, by providing the above-described electrode layer or moisture-proof layer, it is possible to make it difficult to generate amine and further suppress the discoloration of the piezoelectric film in the laminated piezoelectric body.

[0083] The surface resistivity of the antistatic layer is, for example, 1.0×10 4 Ω / sq. or more and 1.0×10 9 Ω / sq. or less is preferable, and 1.0×10 5 Ω / sq. or more and 1.0×10 8 / sq. or less is more preferable. When the surface resistivity of the antistatic layer is 1.0×10 9 Ω / sq. or less, the surface resistivity of the laminated piezoelectric body can be made lower, and sufficient antistatic properties can be imparted. In order to prevent deterioration of the laminated piezoelectric body, it may be handled in a state where a hard coat layer is formed on the antistatic layer, and the surface resistivity measured from above the hard coat layer is 1.0×10 6 Ω / sq. or more and 1.0×10 12 Ω / sq. or less, sufficient antistatic properties are easily obtained. It is more preferable that it is 1.0×10 8 Ω / sq. or more and 5.0×10 11 Ω / sq. or less. The surface resistivity of the antistatic layer can be measured in accordance with JIS C 2139-3-2:2018 using, for example, a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800, URS probe)).

[0084] 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 suppressed. 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.

[0085] The thickness of the antistatic layer can be measured using a spectroscopic interference film thickness meter (e.g., Optical NanoGauge C13027-11, manufactured by Hamamatsu Photonics). 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.

[0086] (Hard coat layer) The hard coat layer can be disposed between the piezoelectric film and the electrode layer. In this embodiment, the hard coat layer is disposed adjacent to the piezoelectric film (see FIG. 1). The hard coat layer fills in scratches on the surface of the piezoelectric film to smooth it out, 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.

[0087] The thickness of the hard coat layer is not particularly limited, but is 0.05 μm or more, preferably 0.3 μm to 3.0 μm, more preferably 0.5 μm to 2.0 μm, and even more preferably 0.5 μm to 1.5 μm.

[0088] As described above, it is preferable that the refractive index of the hard coat layer is 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 more and less than 1.60, more preferably 1.47 or more and 1.57 or less, and even more preferably 1.49 or more and 1.55 or less. When the refractive index of the hard coat layer is within the above range, it is possible to further increase the transmittance and further suppress interference fringes.

[0089] 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 it is 0.20 or less, interference fringes can be further suppressed.

[0090] The hard coat layer only needs to be composed of a material that satisfies the above refractive index, and is usually a resin layer. The resin layer may be obtained 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.

[0091] The polymerizable compound may be any of a monomer, an oligomer, or a polymer. The polymerizable compound may be a thermosetting compound or an ionizing radiation curable compound, but preferably an ionizing radiation curable compound. The ionizing radiation may usually be ultraviolet rays (UV) or electron beams (EB).

[0092] 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 group, vinyl group, and allyl group, and ring-opening polymerizable groups such as epoxy group and oxetanyl group. Among them, a compound having an ethylenically unsaturated bond group is preferable, a compound having two or more ethylenically unsaturated bond groups is more preferable, and a polyfunctional (meth)acrylate-based compound is even more preferable. (Meth)acrylate means one or both of acrylate and methacrylate.

[0093] 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 (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. may also be used.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] Examples of inorganic particles include particles of silica (silicon oxide), titanium oxide, zirconium oxide, aluminum oxide, zinc oxide, tin oxide, 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 them, inorganic particles are preferred, and silica particles are more preferred, because 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.

[0098] As described below, the laminated piezoelectric element 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 in suppressing 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 element can be suppressed and its transparency can be increased. From the same viewpoint, 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.

[0099] (Optical adjustment layer) The optical adjustment layer can be disposed between the hard coat layer and the transparent conductive layer. The optical adjustment layer can suppress the color of the laminated piezoelectric body by appropriately adjusting the refractive index and thickness.

[0100] Specifically, 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 as described above. The laminated piezoelectric body including such an optical adjustment layer can suppress the color tone due to the interference effect between the light incident on the hard coat layer and reflected, and the light reflected at the interface between the optical adjustment layer and the hard coat layer, etc.

[0101] From the above viewpoints, 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 still more preferably 1.65 or more and 1.75 or less.

[0102] 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.

[0103] The thickness of the optical adjustment layer is not particularly limited, but it 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 may be 0.09 μm or more and 0.18 μm or less. By the thickness of the optical adjustment layer being within the above range, the color tone of the laminated piezoelectric body can be suppressed.

[0104] The material of the optical adjustment layer may be any material that satisfies such a refractive index. For example, metal oxide particles may be added to the curable composition exemplified as the material of the hard coat layer to adjust the refractive index.

[0105] The metal oxide particles are preferably refractive index materials having a refractive index of 1.50 or more. Examples of such metal oxide particles include aluminum oxide, titanium oxide, zirconium oxide, zinc oxide, and tin oxide. Among them, titanium oxide and zirconium oxide are preferable.

[0106] (Transparent electrode) The transparent electrode (electrode layer) may be formed by sputtering a conductive material on the piezoelectric film on which each layer is formed, or by mixing a conductive material with a resin or the like and applying it.

[0107] 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.

[0108] 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 conductive materials for the electrode layer include at least one selected from the group consisting of metal nanowires, metal meshes, conductive polymers, carbon nanotubes, and graphene. Examples of metal nanowires and metal meshes include silver nanowires, silver meshes, copper nanowires, and copper meshes. Examples of conductive polymers include polyacetylene and its derivatives, polythiophene and its derivatives, polypyrrole and its derivatives, and polyaniline and its derivatives. In this embodiment, the conductive material is preferably a metal oxide, and more preferably ITO.

[0109] Note that silver nanowires have attracted attention as conductive materials suitable for weight reduction, low resistance, high transparency, and high flexibility of transparent conductive materials, along with the increase in the area and functionality of devices. In particular, silver nanowires can form a flexible electrode layer by a wet process and are expected to be used in various applications. However, since silver nanowires are made of metallic silver, their conductivity tends to decrease under high humidity and high temperature conditions, whether under sunlight or artificial light. Therefore, it is preferable to apply, dry, and cure an overcoat agent such as a UV-curable resin on the silver nanowire coating film to form a protective layer (overcoat layer). Thereby, an electrode layer with scratch resistance and durability can be formed on the substrate, and long-term stability of the surface electrical resistance value under sunlight can be realized.

[0110] The lower the surface resistivity of the electrode layer, the easier it is to detect the minute signals of the piezoelectric film. On the other hand, to lower the surface resistivity of the electrode layer, it is necessary to increase the thickness of the electrode layer or the addition amount of the conductive material forming the electrode layer, which tends to reduce transparency. Therefore, the surface resistivity of the electrode layer is 1.0×10 -1 Ω / sq. or more and 1.0×10 4 Ω / sq. or less, preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 3 Ω / sq. or less, more preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 2 Ω / sq. or less, and particularly preferably 1.0 Ω / sq. or more and 1.0×10 2 Ω / sq. or less.

[0111] The surface resistivity (Ω / sq.) of the electrode layer is measured by the four-probe DC method in accordance with JIS K 7194-1994 using a resistivity meter (「LorestaGP MCP-T610」, manufactured by Mitsubishi Chemical Analytech Co., Ltd.). 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 the volume resistivity is the product of the surface resistivity and the thickness, the surface resistivity is measured in accordance with the measurement method of the volume resistivity. The surface resistivity is measured in the range including the intersection of the diagonals of the laminated piezoelectric body.

[0112] The thickness of the electrode layer is not limited. From the perspective of having good conductivity, the thicker the electrode layer, the better. On the other hand, if the electrode layer is too thick, the transparency may decrease; if it is too thin, the electrical resistance may increase, and non - continuous conductive portions may be formed in the film structure. The thickness is preferably 10 nm or more and 55 nm or less, more preferably 15 nm or more and 55 nm or less, still more preferably 20 nm or more and 55 nm or less, still more preferably 20 nm or more and 45 nm or less, and particularly preferably 20 nm or more. The thickness of the electrode layer can be determined by a known method obtained from the observation of the cross - section of such a laminate. The thickness of the electrode layer is determined by observing the cross - section of the laminated piezoelectric body using a scanning electron microscope (「SU3800」, manufactured by Hitachi High - Technologies Corporation) under the conditions of an acceleration voltage of 3.0 kV and a magnification of 50,000 times, measuring the thickness of the electrode layer, and taking it as the thickness of the electrode layer of the laminated piezoelectric body.

[0113] In the above - described embodiment, the electrode layer is formed on the optical adjustment layer, but it is not limited thereto. For example, as described above, when the electrode layer contains metal nanowires or a metal mesh as a conductive material, the optical adjustment layer may be omitted. Also, in that case, from the perspective of suppressing the oxidation of the metal nanowires or the metal mesh, an overcoat layer may be further disposed on the electrode layer. When an overcoat layer is disposed on the electrode layer, the surface resistivity of the electrode layer is taken as the surface resistivity measured on the overcoat layer.

[0114] Also, in the above - described embodiment, the electrode layer is formed on the optical adjustment layer, but a transparent electrode film formed by forming the electrode layer on the surface of a substrate such as glass or a polymer material may be adhered to the optical adjustment layer.

[0115] (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 polymeric materials such as polyethylene terephthalate (PET), cycloolefin polymer (COP), and polycarbonate (PC), or a substrate containing an inorganic material such as glass, on which the above-mentioned electrode layer is laminated.

[0116] 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 for crystallizing ITO or the like. Examples of such transparent resins include polyesters such as polyethylene terephthalate (PET). Of these, PET is preferred.

[0117] 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.

[0118] The electrode layer is disposed on the surface of the substrate on the piezoelectric film side (on the optical adjustment layer side in FIG. 1). The electrode layer is preferably an inorganic electrode such as ITO (indium tin oxide complex oxide) or tin oxide, and more preferably ITO.

[0119] (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 penetration, 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).

[0120] 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.

[0121] The thickness of the moisture-proof layer is not particularly limited, but from the viewpoint of achieving a higher degree of both moisture resistance and transparency, it is preferably 0.005 μm or more and 0.100 μm or less, more preferably 0.010 μm or more and 0.060 μm or less, and even more preferably 0.015 μm or more and 0.050 μm or less.

[0122] [Physical properties of laminated piezoelectric materials] (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 If the piezoelectric constant d of the laminated piezoelectric material is 40.0 pC / N or less, the above-mentioned appearance defects can be further reduced. 33 is more preferably 10.0 pC / N or more and 40.0 pC / N or less, further preferably 13.0 pC / N or more and 35.0 pC / N or less, and particularly preferably 15.0 pC / N or more and 30.0 pC / N or less. 33 can be measured in the same manner as above, except that the measurement position is set to one point at the intersection of the diagonal lines of the laminated piezoelectric element.

[0123] Piezoelectric constant d of the laminated piezoelectric material 33 is the piezoelectric constant d of the piezoelectric film, which is one of the components of the laminated piezoelectric element. 33The piezoelectric constant d of the piezoelectric film can be adjusted by 33 When the value is high, the piezoelectric constant d 33 It is also likely to be expensive.

[0124] (Total light transmittance) The laminated piezoelectric body preferably has high transparency from the viewpoint of application to touch panels, etc. Specifically, the total light transmittance of the laminated piezoelectric body is preferably 80% or more, and more preferably 85% or more.

[0125] The total light transmittance of the laminated piezoelectric body can be measured using a haze meter (e.g., NDH7000SP II manufactured by Nippon Denshoku Industries Co., Ltd.) according to 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.

[0126] 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.

[0127] (hue b * value, Δb * ) Color b of laminated piezoelectric material * The value is preferably between -5.0 and 5.0. * By setting the value to be between -5.0 and 5.0, it is possible to achieve good transparency and visibility. In addition, 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.

[0128] b of the laminated piezoelectric body *The b 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 element. b of the laminated piezoelectric body after storage * The value is the thickness of the laminated piezoelectric material 25cm 2 The laminated piezoelectric element was cut into a square shape, and the four corners of the laminated piezoelectric element were fixed to a SUS plate with tape. The laminated piezoelectric element was then placed in a constant temperature and humidity chamber set at a temperature of 85°C and a humidity of 85% RH. After maintaining the laminated piezoelectric element under the above conditions for 500 hours, the laminated piezoelectric element was taken out of the constant temperature and humidity chamber. * The value may be measured in the same manner as above.

[0129] b of the laminated piezoelectric body * value and Δb * can be adjusted by the thickness of the antistatic layer and the composition and thickness of the moisture-proof layer. For example, if the thickness of the antistatic layer is reduced, b * value and Δb * In addition, by increasing the thickness of the moisture-proof layer, discoloration of the piezoelectric film due to moisture penetration can be reduced, and b * value and Δb * can be made smaller.

[0130] [Method of manufacturing laminated piezoelectric body] The laminated piezoelectric element can be manufactured by any method. For example, the laminated piezoelectric element shown in Fig. 1 can be manufactured through the steps of (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. When the laminated piezoelectric body does not include a hard coat layer, the step (3) can be omitted, and the steps (4) and (5) can also be omitted as needed. In addition, when the electrode layer contains metal nanowires or a metal mesh as the conductive material, the step (4) may be omitted. Further, from the viewpoint of further suppressing the oxidation of the metal nanowires or the metal mesh, a step of forming an overcoat layer on the electrode layer (6) and a step of forming a moisture-proof layer (7) may be further performed. Alternatively, instead of the step (5), after the step (2) or the step (3), a step of forming a transparent adhesive (OCA) layer on the hard coat layer or the optical adjustment layer (6) and an electrode film can be bonded onto the OCA layer (8).

[0131] (Step of preparing a piezoelectric film) As the piezoelectric film containing a fluororesin, the above-described fluororesin piezoelectric film can be used.

[0132] (Step of forming an antistatic layer) After applying the above-described curable composition for an antistatic layer onto the obtained piezoelectric film, it is dried and cured to form an antistatic layer. The curable composition for an 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, from the viewpoint of enhancing low-temperature curability.

[0133] The above curable composition may further contain water or a solvent. Examples of the solvent include alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol.

[0134] The coating method of the above curable composition is not particularly limited, and any of a spin coating method, a gravure coating method, a die coating method, a bar coating method, a dip coating method, etc. may be used.

[0135] The drying of the above curable composition can be performed by heating the applied curable composition. The heating temperature is preferably a temperature equal to or higher than the temperature at which the solvent can be removed and equal to or lower than the heat distortion temperature of the fluororesin constituting the piezoelectric film, 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.

[0136] (Step of forming a hard coat layer) In this embodiment, after applying the above-described curable composition for a hard coat layer on a piezoelectric film or an antistatic layer, it is dried and cured to form a hard coat layer.

[0137] The curable composition may further contain a diluent solvent. As the diluent solvent, those having a polarity close to that of the particles are preferable. Examples of the diluent solvent include organic solvents such as alcohol-based solvents, ketone-based solvents, ester-based solvents, carbonate-based solvents, and aromatic-based solvents.

[0138] The application of the curable composition can be performed by a known wet process method. As the wet process method, for example, dip coating, spray coating, spin coating, gravure coating, die coating, roll coating, flow coating, curtain coating, etc. are mentioned as typical methods. Among them, methods capable of continuously forming a layer, such as roll coating method and gravure coating method, are preferable from the viewpoint of productivity.

[0139] The application and drying methods of the curable composition can be the same as the above-described application and drying methods. The heating temperature may be a temperature equal to or lower than the heat distortion temperature of the fluororesin constituting the piezoelectric film within a range where the solvent can be volatilized and removed, and can be, for example, 60°C or higher and 100°C or lower.

[0140] The curing of the curable composition may be by heat curing or by ionization radiation curing. Curing by ionization radiation can be performed by irradiating ultraviolet rays or electron beams. Further, heat curing and curing by ionization radiation may be used in combination.

[0141] (Step of forming an optical adjustment layer) Similar to the step of forming the hard coat layer, after applying the curable composition onto the piezoelectric film, the antistatic layer, or the hard coat layer, it is dried and cured to form an optical adjustment layer. However, when the electrode layer uses metal nanowires or a metal mesh as the conductive material, since the transparency of the conductive layer is high, the optical adjustment layer can be omitted.

[0142] (Step of forming the electrode layer) The electrode layer can be formed by sputtering a transparent conductive material or applying a solution containing a transparent conductive material and a resin onto the optical adjustment layer. When the electrode layer uses metal nanowires or a metal mesh as the conductive material, since the conductive material may be oxidized and deteriorated, increasing the electrical resistance, an overcoat layer can be formed on the electrode layer.

[0143] [Other Embodiments] It should be noted that the above-described embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-described embodiments within the scope of the core technical idea.

Examples

[0144] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0145] 1. Production of Fluorine-based Resin Piezoelectric Film Films 1 to 8 and Film 11, all of which are fluorine-based resin piezoelectric films, were produced according to the following procedure.

[0146] The melt viscosity of the fluorine-based resin used as the material was measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (manufactured by Toyo Seiki Seisakusho, Ltd., Capillograph 1D) was used, and the viscosity was measured at a measurement temperature of 260°C and a shear rate of 50 s -1 using a capillary die with an inner diameter of φ1 mm × tube length of 10 mm.

[0147] Also, the melting point of the fluororesin was determined as the maximum peak temperature of the endothermic curve among the DSC curves obtained by enclosing 5 mg of the measurement sample in an aluminum pan, installing it in a differential scanning calorimeter (manufactured by Shimadzu Corporation, DSC-60A), and raising the temperature from room temperature to 230°C at a heating rate of 10°C / min under a nitrogen atmosphere.

[0148] Also, the melting temperature was defined as the maximum temperature in the conduit from the extruder to the filtration device.

[0149] 1-1. Piezoelectric Film 1 A polyvinylidene fluoride homopolymer (PVDF) with 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, then filtered through a pleated polymer filter with a filtration accuracy of 20 μm, and extruded into a film shape from a T-die. The extruded film was brought into contact with a cooling roll whose surface temperature was set to 50°C while maintaining a temperature of 150°C or higher, to obtain an unstretched film with a thickness of 160 μm. At this time, both the melting temperature and the filtration temperature were 260°C.

[0150] The obtained unstretched film was guided to a uniaxial stretching device equipped with a plurality of metal rolls and pinch rolls, and stretched 4.6 times in the MD direction by adjusting the rotation speed ratio of each roll. Furthermore, a voltage of 8.0 kV was applied from the surface of the film in the thickness direction to obtain Piezoelectric Film 1.

[0151] 1-2. Piezoelectric Film 2 Piezoelectric Film 2 was obtained in the same manner as the production of Piezoelectric Film 1, except that a filter was not attached to the single-screw extruder and the voltage applied after stretching was 8.8 kV.

[0152] 1-3. Piezoelectric Film 3 Piezoelectric Film 3 was obtained in the same manner as the production of Piezoelectric Film 1, except that the surface temperature of the cooling roll was set to 110°C and the voltage applied after stretching was 8.8 kV.

[0153] 1-4. Piezoelectric Film 4 A piezoelectric film 4 was obtained in the same manner as the production of the piezoelectric film 1, 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.

[0154] 1-5. Piezoelectric film 5 A piezoelectric film 5 was obtained in the same manner as the production of the piezoelectric film 1, except that PVDF with 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.

[0155] 1-6. Piezoelectric film 6 A piezoelectric film 6 was obtained in the same manner as the production of the piezoelectric film 1, except that a polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 3400 Pa·s and a melting point of 173°C, which was prepared by mixing PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C and PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C at a mass ratio of 1:1, 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.

[0156] 1-7. Piezoelectric film 7 A piezoelectric film 7 was obtained in the same manner as the production of the film 1, except that the surface temperature of the cooling roll was set to 130°C and the voltage applied after stretching was set to 9.5 kV.

[0157] 1-8. Piezoelectric film 8 A piezoelectric film 8 was obtained in the same manner as the production of the piezoelectric film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used, no filter was attached to the single-screw extruder, the melting temperature was set to 280°C, the surface temperature of the cooling roll was set to 140°C, and the voltage applied after stretching was set to 9.5 kV.

[0158] 1-9. Piezoelectric film 9 (Could not be produced) An attempt was made to produce a piezoelectric film 9 in the same manner as the production of the piezoelectric film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used. However, the filter was clogged with resin and a piezoelectric film could not be obtained.

[0159] 1 - 10. Piezoelectric film 10 (could not be produced) An attempt was made to produce film 10 in the same manner as the production of piezoelectric film 1, except that a pleated - type polymer filter with a filtration accuracy of 5 μm was used. However, the filter became clogged with resin, and a piezoelectric film could not be obtained.

[0160] 1 - 11. Piezoelectric film 11 100 g of PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was weighed and put into 900 ml of n - methylpyrrolidone (NMP). While stirring with a stirrer, the temperature was raised to 60°C, and stirring was continued for 6 hours as it was to prepare a resin solution. This resin solution was filtered through a filter with a filtration accuracy of 40 μm. The filtered resin solution was put into an automatic coater to prepare a coating film with a liquid thickness of 600 μm, and it was dried at 120°C for 1 hour to obtain piezoelectric film 11.

[0161] 2. Evaluation of fluorine - based resin piezoelectric films Regarding the obtained piezoelectric films 1 to piezoelectric film 8 and piezoelectric film 11, the lamellar long period, retardation, piezoelectric constant d 33 , haze, internal haze, surface height roughness Rz, the number of foreign matters, and thickness were measured by the following methods.

[0162] 2 - 1. Lamellar long period The X - ray diffraction pattern of each film was measured by a small - angle X - ray scattering measurement device, and the lamellar long period was calculated by the black reflection method.

[0163] Specifically, each film was placed in the small-angle X-ray scattering measurement device (Rigaku Corporation, NANO-Viewer) equipped with a two-dimensional detector, and CuKα radiation (wavelength λ = 0.15418 nm) monochromated through a Ni filter was irradiated as the X-ray source to measure the two-dimensional diffraction pattern of the film. The X-ray diffraction intensity (A(2θ)) relative to the diffraction angle 2θ in the range of 0.200° to 2.000° was then calculated from the intensity distribution of the X-ray diffraction (scattering). Because A(2θ) includes the X-ray scattering intensity of air (B(2θ)), B(2θ) was measured without the film, and the X-ray diffraction intensity (I(2θ)) of the film was obtained by subtracting B(2θ) from A(2θ).

[0164] The background of I(2θ) was calculated by linear approximation from the diffraction angle 2θ that showed the lowest X-ray intensity in the range of 2θ = 0.200° to 0.450°, its X-ray intensity, and the X-ray intensity at 2θ = 2.000°. Next, the background was subtracted from I(2θ) to determine the diffraction angle 2θ at which the X-ray intensity was maximized, and the lamellar periodic length was calculated using Bragg's reflection equation (Equation 1).

[0165]

number

[0166] (Method for measuring small-angle X-ray scattering (diffraction)) Measurement device: NANO-Viewer X-ray source: CuKα (monochromatization: Ni filter) Applied current: 20mA, Applied voltage: 40kV 1st slit: φ0.7 mm, 2nd slit: φ0.6 mm, 3rd slit: φ0.8 mm Camera length: 830mm Detector: Imaging plate Measurement time: 2 hours Diffraction angle correction: Si(111) surface · Shape and size of the sample: With the stretching direction (MD) as the length direction of the sample, the film was cut into samples with a length of 30 mm and a width of 10 mm. The samples were set so that the longitudinal direction of the measurement folder was the same as the sample length (the stretching direction of the film). At this time, one or more samples were stacked and adjusted so that the total thickness was 200 ± 20 μm. · When converting from a two-dimensional diffraction image obtained by an X-ray two-dimensional detector to an X-ray scattering (diffraction) intensity profile with respect to the diffraction angle 2θ, the sampling width was set to 0.001°.

[0167] 2-2. Retardation Using a KOBRA-HB manufactured by Oji Scientific Instruments, the retardation of a film cut into 20 mm × 20 mm was measured by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8 nm was taken as the retardation of the film. At this time, the fast axis and the slow axis were determined from the in-plane birefringence of the film. The slow axis direction coincides with the average direction of the molecular chains oriented by stretching or extrusion. Since the film used this time was stretched in the flow direction (machine direction) of the film, the MD direction and the slow axis direction coincide.

[0168] 2-3. Piezoelectric constant d 33 The piezoelectric constant d was measured in accordance with ISO 19622:2018. 33 Specifically, using a piezoelectric constant measuring device (PiezoTest, Piezometer System PM300), the film as a test piece was held with a holding force of 1.0 N, and the charge generated when an alternating force with a vibration force of 0.15 N and a frequency of 110 Hz was applied was measured. The charge was measured on the polarization plane of the film, and the piezoelectric constant was calculated using the absolute value of the measured value. The fast axis direction obtained 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 in the width direction of the film was defined as point A. For a total of three measurement points, namely point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the piezoelectric constant d 33The average value of the piezoelectric constants obtained from these measurements was used as the piezoelectric constant d 33 It was decided.

[0169] 2-4.Film haze A haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) was used. In accordance with ISO 14782:2021, the fast axis direction determined by measuring the birefringence 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. 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, and the average value of these measurement points was defined as the haze of the film.

[0170] 2-5. Internal haze The fast axis direction determined by measuring the birefringence of the fluorine-based resin piezoelectric film was defined as the width direction. 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 30 mm x 30 mm measurement sample 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. Then, 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 for the fluororesin piezoelectric film.

[0171] 2-6. Surface height roughness Rz A surface roughness meter (Keyence Corporation's shape analysis laser microscope VK-X260) compliant with JIS B 0601:2001 was used. Then, the surface roughness of the contact surface (the surface with a smaller Rz) of the fluororesin film with the cooling roll was measured. The advancing axis direction obtained by measuring the birefringence was defined as the width direction. Taking an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film as point A, surface height roughness Rz was measured at a total of three measurement points: point A (the measurement point), and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A. The average value of these was taken as the surface height roughness Rz of the film.

[0172] 2-7. Number of foreign matters Since the thicknesses of the fabricated piezoelectric films were all 40 μm or less, no processing such as stretching was performed on the piezoelectric films. Instead, 25 rectangular films (observation pieces) with a size of 0.010 m 2 (100 mm × 100 mm) were cut out from the film so that each film was cut out from continuously adjacent positions. The sum of the number of foreign matters measured by observing each observation piece was determined as the number of foreign matters per 0.250 m 2 At this time, the foreign matters were marked by observing with transmitted light, and the size of the foreign matters was determined by observing the marked locations with a microscope. The size of the foreign matters was taken as the arithmetic mean value of the maximum width and the minimum width of the foreign matters. In this way, the number of foreign matters larger than 200 μm, the number of foreign matters with a size of 100 μm or more and 200 μm or less, and the number of foreign matters with a size of less than 100 μm were determined respectively.

[0173] 2-8. Thickness A digital linear gauge (manufactured by Ono Sokki Co., Ltd., DG525H) and a gauge stand (manufactured by Ono Sokki Co., Ltd., SH-022) were used. At this time, the advancing axis direction obtained 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 in the width direction of the film was defined as point A. Thickness measurements were taken at a total of three measurement points: point A and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A. The average value of these measurements was taken as the thickness of this film.

[0174] 3. Results The production conditions and evaluation results of each piezoelectric film are shown in Tables 1 and 2. Note that for Film 11, the haze was large and it was opaque, so the number of foreign objects could not be measured.

[0175]

Table 1

[0176]

Table 2

[0177] 4. Fabrication of the laminated piezoelectric body 4-1. Antistatic layer 4-1-1. Antistatic layer 1 Using a gravure coater (Multi Coater manufactured by Hirano Texseed Co., Ltd.), a solution in which a paint P-400MP-A (manufactured by Nagase ChemteX Corporation) containing PEDOT:PSS as a conductive polymer and a paint P-400MP-B (manufactured by Nagase ChemteX Corporation) containing a crosslinking agent and a conductivity improver were mixed at a ratio of 4:1 was applied with the Multi Coater (manufactured by Hirano Texseed Co., Ltd.), and 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 1.2×10 8 Ω / sq.

[0178] 4-1-2. Antistatic layer 2 A solution (C-169PF) made by mixing paint C-169PF-A (manufactured by Nagase ChemteX Corporation) containing single-walled carbon nanotubes and 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 with a thickness of 100 nm. The surface resistivity of the antistatic layer was 7.1 x 10 6 It was Ω / sq.

[0179] 4-2. Formation of hard coat layer After applying a hard coating agent (BS-CH271 manufactured by Arakawa Chemical Industries, Ltd., amorphous silica average particle size 60 nm) using a multi-coater, the sample 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.

[0180] 4-3. Optical adjustment layer An ultraviolet-curable composition containing zirconium oxide particles (Opstar RA004 manufactured by Arakawa Chemical Industries, Ltd.) was applied and dried at 40°C for 30 seconds, and then 250 mJ / cm 2 An optical adjustment layer (thickness: 102 nm, refractive index: 1.65) was formed by irradiating the film with ultraviolet light at an integrated light amount of 102 nm.

[0181] 4-4. Formation of transparent electrodes 4-4-1.Transparent electrode layer 1 An indium tin metal target (the tin content relative to the sum of the indium content and the tin content was 3% by mass) was set as the target material in a magnetron sputtering device, and a piezoelectric film on which a predetermined layer was formed was set as the substrate. Then, while the piezoelectric film on which the predetermined layer was formed was wound up, dehydration and degassing were performed until the vacuum degree reached 7×10 -5The 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 evacuated until the pressure in the chamber reached 0.4 Pa, a 30 nm thick transparent electrode layer consisting essentially of indium tin composite oxide was formed by reactive sputtering, thereby obtaining a laminated piezoelectric body.

[0182] 4-4-2.Transparent electrode layer 2 A solution of T-AG230 (Seiko PMC Corporation) paint 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 (Seiko PMC Corporation) paint mixed with T-YP462 (Seiko PMC Corporation) as an additive 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 A protective layer having a thickness of 80 nm was formed by irradiating ultraviolet light with an integrated light amount of 1000 nm. Here, the transparent electrode layer and the protective layer are collectively referred to as the transparent electrode layer 2.

[0183] 4-4-3.Transparent electrode film A transparent electrode precursor film (Tetrite 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 optical clear adhesive (OCA) sheet (Nitto Denko Corporation, CS9862UA, thickness 50 μm) was laminated 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 laminated with the conductive layer facing the OCA sheet surface.

[0184] 4-5.Laminated piezoelectric element 4-5-1.Laminated piezoelectric element 1 The antistatic layer 1 was formed on the surface A of the piezoelectric film 1, and then a hard coat layer was formed on the antistatic layer 1, thereby producing a laminated piezoelectric element 1.

[0185] 4-5-2.Laminated piezoelectric element 2 A transparent electrode layer 1 was formed on the hard coat layer of the laminated piezoelectric element 1 to prepare a laminated piezoelectric element 2.

[0186] 4-5-3.Laminated piezoelectric element 3 A transparent electrode film was attached onto the hard coat layer of the laminated piezoelectric element 1 to prepare a laminated piezoelectric element 3.

[0187] 4-5-4.Laminated piezoelectric element 4 A laminated piezoelectric body 4 was produced in the same manner as the laminated piezoelectric body 2, except that the transparent electrode layer 1 was replaced with the transparent electrode layer 2.

[0188] 4-5-5.Laminated piezoelectric element 5 A laminated piezoelectric body 5 was prepared in the same manner as the laminated piezoelectric body 4, except that the antistatic layer 1 was changed to the antistatic layer 2.

[0189] 4-5-6.Laminated piezoelectric element 6 A laminated piezoelectric body 6 was produced in the same manner as the laminated piezoelectric body 2, except that an optical adjustment layer was formed between the hard coat layer and the transparent electrode layer 1.

[0190] 4-5-7.Laminated piezoelectric element 7 The antistatic layer 1 was formed on the 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 element .

[0191] 4-5-8.Laminated piezoelectric element 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, thereby producing a laminated piezoelectric element 8.

[0192] 4-5-9.Laminated piezoelectric element 9 A laminated piezoelectric body 9 was produced in the same manner as the laminated piezoelectric body 8, except that the transparent electrode layer 1 was changed to the transparent electrode layer 2.

[0193] 4-5-10. Multilayer piezoelectric body 10 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, then the optical adjustment layer was formed on the antistatic layer 1, the transparent electrode layer 1 was formed on the optical adjustment layer, and the multilayer piezoelectric body 10 was fabricated.

[0194] 4-5-11. Multilayer piezoelectric body 11 The multilayer piezoelectric body 11 was fabricated in the same manner as the multilayer piezoelectric body 8, except that a hard coat layer was formed on the A surface of the piezoelectric film 1 instead of the antistatic layer 1.

[0195] 4-5-12. Multilayer piezoelectric body 12 The multilayer piezoelectric body 12 was fabricated in the same manner as the multilayer piezoelectric body 11, except that the transparent electrode layer 1 was replaced with the transparent electrode layer 2.

[0196] 4-5-13. Multilayer piezoelectric body 13 The hard coat layer was formed on the A surface of the piezoelectric film 1, then the optical adjustment layer 1 was formed on the hard coat layer, the transparent electrode layer 1 was formed on the optical adjustment layer 1, and the multilayer piezoelectric body 13 was fabricated.

[0197] 4-5-14. Multilayer piezoelectric body 14 The multilayer piezoelectric body 14 was fabricated in the same manner as the multilayer piezoelectric body 11, except that the hard coat layer was replaced with the optical adjustment layer.

[0198] 4-5-15. Multilayer piezoelectric body 15 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, the hard coat layer was formed on the B surface of the piezoelectric film 1, the separator film protecting the adhesive of the PET protective film (SAT TM30125T manufactured by Sun Ace Chemical Co., Ltd.) was peeled off, and the PET protective film was bonded onto the hard coat layer on the B surface. Next, the hard coat layer and the optical adjustment layer were formed on the antistatic layer 1 on the A surface, and further the transparent electrode layer 1 was formed on the optical adjustment layer, and the multilayer piezoelectric body 15 was fabricated.

[0199] 4-5-16. Multilayer piezoelectric body 16 The antistatic layer 1 was formed on surface A of the 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 Chemical Co., Ltd.) was peeled off, and the PET protective film was attached to 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, and a laminated piezoelectric element 16 was produced.

[0200] 5. Evaluation of the multilayer piezoelectric material The piezoelectric constants d of the obtained laminated piezoelectric bodies 1 to 16 were measured by the following method. 33 , total light transmittance, surface resistivity, haze, and b * was measured.

[0201] 5-1.Piezoelectric constant The value measured in the area including the intersection of the diagonal lines of the laminated piezoelectric material is the piezoelectric constant d 33 The measurement was carried out in the same manner as in the measurement method (2-3) above, except that the representative value was used.

[0202] 5-2.Total light transmittance The total light transmittance of the laminated piezoelectric body was measured in accordance with JIS K 7361-1 using a haze meter (NDH7000SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) in a range including the intersection of the diagonal lines of the laminated piezoelectric body, and this value was used as a representative value.

[0203] 5-3.Surface resistivity 5-3-1. Surface resistivity of antistatic layer The surface resistivity of the antistatic layer of the laminated piezoelectric body was measured in accordance with JIS C 2139-3-2:2018 using, for example, a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800, URS probe)). The measurement location was an area including the intersection of the diagonal lines of the rectangular laminated piezoelectric body, and the surface resistivity of the antistatic layer measured was used as a representative value.

[0204] 5-3-2. Surface resistivity of transparent electrodes 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 Analytic Co., Ltd.) in accordance with JIS K 7194 using the DC four-probe method. Since the volume resistivity is the product of the surface resistivity and the thickness, the surface resistivity was measured in accordance with the method for measuring the volume resistivity. The measurement location was the surface resistivity of the transparent electrode measured in the range including the intersection of the diagonals of the rectangular laminated piezoelectric body as the representative value. When a protective layer was formed on the electrode, the surface resistivity was measured from above the protective layer.

[0205] 5-4. Haze 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 measurement result of the haze in the range including the intersection of the diagonals of the laminated piezoelectric body was taken as the representative value.

[0206] 5-5. Chromatic aberration Δb * Value In accordance with the method conforming to JIS Z 8722, using a spectroscopic colorimeter (manufactured by Nippon Denshoku Industries Co., Ltd., SD7000), the measurement result of the range including the intersection of the diagonals of the laminated piezoelectric body before damp heat storage was taken as the representative value of the hue b * Value.

[0207] 5-6. Thickness The thickness of the antistatic layer was measured using a spectroscopic interference type film thickness meter (for example, manufactured by Hamamatsu Photonics, Optical NanoGauge C13027-11). The thickness of each layer in the range including the intersection of the diagonals of the laminated piezoelectric body was measured. The thickness of each layer other than the transparent electrode and the piezoelectric film was also measured by the same method. 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-Tech Corporation) under the conditions of an acceleration voltage of 3.0 kV and a magnification of 50,000 times, and the thickness of the transparent electrode in the range including the intersection of the diagonals of the laminated piezoelectric body was measured. The obtained measurement value was taken as the representative value of the thickness of the transparent electrode.

[0208] 6. Results The production conditions and evaluation results of each laminated piezoelectric body are shown in Table 3 and Table 4.

[0209]

Table 3

[0210]

Table 4

Industrial Applicability

[0211] The fluororesin piezoelectric film according to the present invention is useful as a piezoelectric film with high transparency.

Explanation of Signs

[0212] 10 Multilayer piezoelectric body 11 Piezoelectric film 12 Antistatic layer 13 Hard coat layer 14 Optical adjustment layer 15 Electrode layer

Claims

1. Measurement temperature: 260 °C, shear rate during measurement: 50 s －１ a fluororesin piezoelectric film having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less The lamellar long period determined by the small-angle X-ray scattering method is 11.5 nm or less, the retardation is 100 nm or more and 2000 nm or less, and the piezoelectric constant d ３３ is 5.0 pC / N or more and 40.0 pC / N or less, A fluororesin piezoelectric film.

2. The fluororesin piezoelectric film according to Claim 1, having an internal haze of less than 1.2%.

3. The fluororesin piezoelectric film according to Claim 1, having an Rz of the surface on the smaller side of the surface height roughness Rz measured according to JIS B 0601:2001 of 0.50 μm or less.

4. The fluororesin piezoelectric film according to Claim 1.

5. 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 a plan view, of 100 µm or more is 7 pieces / 0.25 m ２ is as follows The fluororesin piezoelectric film according to Claim 1, containing a structural unit derived from vinylidene fluoride as a main component.

6. A method for manufacturing a fluororesin piezoelectric film according to any one of Claims 1 to 5, comprising: a step of extruding the molten 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 lower; 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 －１ a step of stretching the cooled film; a step of polarizing the cooled film.

7. 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. The method for manufacturing a fluororesin piezoelectric film according to Claim 6.

8. The method for manufacturing a fluororesin piezoelectric film according to Claim 7, comprising 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.

9. A laminated piezoelectric body including the fluororesin piezoelectric film according to Claim 1, having a total light transmittance of 80% or more.

10. The laminated piezoelectric body according to Claim 9.

11. The laminated piezoelectric body according to Claim 10, wherein the electrode layer includes 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.

12. The laminated piezoelectric body according to Claim 9 or 10, further including a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film. 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, which is disposed on at least one surface of the fluororesin piezoelectric film, is further included.

13. The laminated piezoelectric body according to Claim 9 or 10.

14. The laminated piezoelectric body according to Claim 9 or 10, wherein an antistatic layer and a hard coat layer are disposed in this order on at least one surface of the fluororesin piezoelectric film. ​ ​ ​ ​ Disposed on at least one surface of the fluororesin piezoelectric film, further comprising an antistatic layer having a surface resistivity of 1.0×10 ４ Ω / sq. or more and 1.0×10 ９ Ω / sq. or less ​ ​ ​ 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. ​

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