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 resin piezoelectric 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 4,000 Pa s or less. The fluorine-based resin piezoelectric film has an internal haze of less than 1.2%, retardation of 100 nm or more and 2,000 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 manufacturing the same, and a laminated piezoelectric body.

Background Art

[0002] Display and input devices with a touch panel installed on the front 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 household electrical 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 manufacturing a fluororesin film, a method of manufacturing 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 extruding it (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 of 1.0 pC / N or more measured by the method described later.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] From a fluororesin film formed by the solution casting method, a piezoelectric film is manufactured by polarization treatment without stretching. However, from a fluororesin film formed by the hot melt extrusion method, a piezoelectric film is manufactured by stretching the film and then performing polarization treatment.

[0006] In the solution casting method, since a large amount of polar organic solvent is used for dissolving the fluororesin, it is necessary to recover the solvent, which not only increases the manufacturing cost but also requires consideration of the impact on the working environment and natural environment due to the organic solvent. Further, since a polar organic solvent is used for dissolving the fluororesin, if the solvent remains in the fluororesin film, there is a risk that polarization in the manufacturing process of the piezoelectric film will be hindered. Therefore, it is preferable to form a film by extruding a hot-melted fluororesin without using an organic solvent.

[0007] On the other hand, in the method of melting and extruding a fluororesin, when heated to a high temperature, there is a risk that problems such as resin denaturation and generation of decomposition products (foreign substances) will occur.

[0008] In order to perform extrusion molding by the hot melt method while suppressing denaturation such as decomposition of the resin, it is preferable to use a fluororesin having a low melt viscosity. However, a fluororesin film formed by extruding the melted resin has low transparency, and there is a problem that a fluororesin piezoelectric film produced from this fluororesin film also has low transparency.

[0009] The present invention has been made in view of the above problems, and an object thereof is to provide a fluororesin piezoelectric film having high transparency, a manufacturing method thereof, and a laminated piezoelectric body.

Means for Solving the Problems

[0010] One embodiment of the present invention for solving the above problems relates to a fluororesin piezoelectric film of the following [1] to [4]. [1] Measurement temperature 260 ° C, shear rate at the time of measurement 50 s -1A fluororesin piezoelectric film having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less, having an internal haze of less than 1.2%, a retardation of 100 nm or more and 2000 nm or less, and a piezoelectric constant d 33 of 5.0 pC / N or more and 40.0 pC / N or less, a fluororesin piezoelectric film. [2] The Rz of the surface on the side with a smaller surface height roughness Rz measured in accordance with JIS B 0601:2001 is 0.50 μm or less, [1] The fluororesin piezoelectric film according to [1]. [3] 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 / 0w.25 m 2 or less, [1] or [2] The fluororesin piezoelectric film according to [1]. [4] Containing a structural unit derived from vinylidene fluoride as a main component, [1] to [3] The fluororesin piezoelectric film according to any one of [1] to [3].

[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 [5] to [7]. [5] 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, A method for manufacturing a fluororesin piezoelectric film according to any one of [1] to [4], which has [6] In the melting step, the fluororesin is melted at a temperature 75° C. or more and 105° C. or less higher than the melting point of the fluororesin. The manufacturing method of the fluororesin piezoelectric film described in [5]. [7] 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. The manufacturing method of the fluororesin piezoelectric film described in [5] or [6].

[0012] One embodiment of the present invention for solving the above problems relates to the laminated piezoelectric body of the following [8] to

[13] . [8] A laminated piezoelectric body including the fluororesin piezoelectric film described in any one of [1] to [4], having a total light transmittance of 80% or more, Laminated piezoelectric body. [9] 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 described in [8].

[10] 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 described in [9].

[11] Further comprising a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film, The laminated piezoelectric body described in any one of [8] to

[10] .

[12] 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, further comprising The laminated piezoelectric body described in any one of [8] to

[11] .

[13] An antistatic layer and a hard coat layer are disposed in this order on at least one surface of the fluororesin piezoelectric film, The surface resistivity measured on the hard coat layer is 1.0×10 61.0×10 or more Ω / sq 12 and 1.0×10 or less Ω / sq The laminated piezoelectric body according to any one of [8] to

[12] .

Advantages of the Invention

[0013] According to the present invention, there are provided a fluororesin piezoelectric film having high transparency, a method for producing the same, and a laminated piezoelectric body.

Brief Description of the Drawings

[0014]

Figure 1

Mode for Carrying Out the Invention

[0015] [Fluororesin Piezoelectric Film] One embodiment of the present invention relates to a fluororesin piezoelectric film.

[0016] The fluororesin piezoelectric film may be a film mainly composed of a fluororesin. The fluororesin is a resin obtained by polymerizing a monomer composed of an olefin containing fluorine, and containing the fluororesin as a main component means that the content ratio of the structural unit derived from the monomer of the fluororesin to the total mass of the fluororesin film is 50% by mass or more. The ratio of the content of the fluororesin 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 ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP) with TFE. Examples of fluororesins obtained by polymerizing VDF include homopolymers of VDF, as well as copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene, TFE, CTFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether.

[0018] Among these, from the viewpoint of facilitating the polarization of the fluororesin film, fluororesins obtained by polymerizing monomers containing VDF are preferred. More preferred are homopolymers of VDF, 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, with homopolymers of VDF being even more preferred. These fluororesins may be used alone or in combination of multiple types.

[0019] The fluororesin piezoelectric film preferably contains a structural unit derived from VDF as a main component, with homopolymers of VDF 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 these resins 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] Among them, since a fluororesin piezoelectric film is expected to have high piezoelectricity, the higher the content rate of the homopolymer of vinylidene fluoride, the more preferable. The content rate of the homopolymer with respect 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 rate of the resin having VDF as a constitutional unit contained in the fluororesin film and the fluororesin piezoelectric film 19 can be measured by quantitative analysis using F-NMR with an internal standard.

[0022] The fluororesin piezoelectric film has a melt viscosity measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 in the range of 600 Pa·s or more and 4000 Pa·s or less, preferably 600 Pa·s or more and 3500 Pa·s or less, and more preferably 600 Pa·s or more and 2400 Pa·s or less. The lower the melt viscosity of the fluororesin, the easier it is to form a film by extrusion at a lower melt temperature. However, since a resin with a lower melt viscosity has easy movement of molecular chains, during film formation, the selective arrangement of molecular chains progresses and crystals are likely to grow. When the crystals of the resin grow, since the refractive indices of light in the crystalline part and the amorphous part are different, light scattering at the interface between the crystalline part and the amorphous part increases, and the haze of the film tends to increase and the transparency decreases. On the other hand, even for a fluororesin with a low melt viscosity, after film formation, by rapidly cooling before crystallization progresses, crystal growth in the film can be suppressed, light scattering can be reduced, and a film with low haze and high transparency can be obtained. Also, the lower the melt viscosity, the easier it is to filter by a filter and the melt temperature can be kept low, so that spot-like irregularities on the film surface due to resin modification are less likely to occur.

[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 Seisakusho, Ltd., Capillograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, the viscosity is measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 shall be the viscosity measured under such conditions.

[0024] The fluororesin piezoelectric film has an internal haze of less than 1.2%, preferably 0% or more and 1.1% or less, more preferably 0% or more and 1.0% or less. The lower the internal haze, the higher the transparency of the fluororesin piezoelectric film. Also, according to the findings of the present inventors, the smaller the internal haze, the easier it is to increase the piezoelectric constant d 33 when the fluororesin piezoelectric film is poled.

[0025] The fluororesin piezoelectric film preferably has a film haze of 0.0% or more and 10.0% or less, more preferably 0.0% or more and 5.0% or less, still more preferably 0.0% or more and 3.0% or less. The lower the haze, the higher the transparency of the fluororesin piezoelectric film.

[0026] The fluororesin piezoelectric film preferably has an internal haze of less than 1.2%, more preferably 0.3% or more and 1.1% or less, still more preferably 0.1% or more and 1.0% or less. The lower the internal haze, the higher the transparency of the fluororesin piezoelectric film.

[0027] 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. At this time, 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. Regarding 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 haze is measured, and the average value of these is taken as the haze of the fluororesin piezoelectric film.

[0028] 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 fluororesin piezoelectric film, and is defined as the internal haze of the fluororesin piezoelectric film. Specifically, 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 to form a coating layer with a thickness of 2 μm. 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 measured samples measured in the same manner is taken as the representative value of the fluororesin piezoelectric film.

[0029] The fluororesin piezoelectric film has a retardation of 100 nm or more and 2000 nm or less, preferably 500 nm or more and 1800 nm or less, and more preferably 700 nm or more and 1600 nm or less. The larger the retardation, the higher the molecular orientation degree of the fluororesin film, the sufficiently increased ratio of β-crystals, and the easier it is to obtain a fluororesin piezoelectric film with a high piezoelectric constant by the polarization process.

[0030] The retardation is a value measured by the parallel Nicol rotation method using a light source having a wavelength of 587.8 nm. At this time, the fast axis and the 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 a stretching treatment is performed in the MD direction (machine direction) of the film, the MD direction and the slow axis direction will coincide.

[0031] The fluororesin piezoelectric film has a piezoelectric constant d 33 of 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.

[0032] The piezoelectric constant of the fluororesin piezoelectric film is the piezoelectric constant d 33 measured by the direct quasi-static method (d 33Measure in accordance with ISO 19622:2018, which is a test method. Specifically, use a piezoelectric constant measuring device (manufactured by PIEZOTEST, piezometer system PM300), hold the fluororesin piezoelectric film, which is the test piece, with a holding force of 1.0 N, and measure the charge generated when an alternating force with a vibration force of 0.15 N and a frequency of 110 Hz is applied. The charge is measured on the polarization surface of the fluororesin piezoelectric film, and the piezoelectric constant is calculated using the absolute value of the measured value. At this time, the advancing axis direction obtained by measuring the birefringence of the fluororesin piezoelectric film is 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, 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, measure the piezoelectric constant d 33 and use the average value of the piezoelectric constants obtained from these to be the piezoelectric constant d 33 of the fluororesin piezoelectric film.

[0033] The surface height roughness Rz of the fluororesin piezoelectric film is preferably 0.50 μm or less, more preferably 0.05 μm or more and 0.50 μm or less, even more preferably 0.05 μm or more and 0.40 μm or less, and particularly preferably 0.05 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin piezoelectric film, the less likely it is to generate haze on the film surface and the better transparency can be obtained. Also, a small surface height roughness Rz indicates that there are few irregularities such as wrinkles on the film.

[0034] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter (Keyence Corporation, Shape Analysis Laser Microscope VK-X260) compliant with JIS B 0601:2001 is used. At this time, the advancing axis direction obtained by measuring the birefringence of the fluororesin piezoelectric film is defined as the width direction. 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, namely 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 surface height roughness Rz is measured, and the average value thereof 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 unevenness of the surface becomes smaller when the fluororesin film is 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-mentioned surface height roughness Rz.

[0035] Foreign substances in the resin tend to have an increased ratio of existing near the surface as the film becomes thinner, so foreign substances are likely to be detected by viewing the film in plan view. Therefore, for the quantification of the amount of foreign substances in the film, the film thickness used for foreign substance 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 substances with a size of 100 μm or more is 2 7 or less per 0.25 m 2 preferably 0 or less per 0.25 m 2 5 or less per 0.25 m 2 more preferably 0 or less per 0.25 m 2 3 or less per 0.25 m 2 even more preferably 0 or less per 0.25 m 2 1 or less per 0.25 m 2 is particularly preferably 0 or less per 0.25 m.

[0036] When the fluorine-based resin piezoelectric film with a film thickness of 40 μm or less is viewed in a plan view, the number of foreign matters with a size larger than 200 μm is 0 pieces / 0.25 m 2 3 pieces or more / 0.25 m 2 Less is preferable, 0 pieces / 0.25 m 2 2 pieces or more / 0.25 m 2 Less is more preferable, 0 pieces / 0.25 m 2 1 piece or more / 0.25 m 2 Less is even more preferable.

[0037] When the fluorine-based resin piezoelectric film with a film thickness of 40 μm or less is viewed in a plan view, the number of foreign matters with a size less than 100 μm is 0 pieces / 0.25 m 2 50 pieces or more / 0.25 m 2 Less is preferable, 0 pieces / 0.25 m 2 25 pieces or more / 0.25 m 2 Less is more preferable, 0 pieces / 0.25 m 2 16 pieces or more / 0.25 m 2 Less is even more preferable.

[0038] The fewer these foreign matters are, the more the transparency of the fluorine-based resin piezoelectric film is improved, and when stretching treatment or polarization treatment is performed on the fluorine-based resin piezoelectric film, stretching and polarization can be made uniform.

[0039] The number of these foreign matters is obtained by continuously cutting out 25 rectangular films (observation pieces) from the fluorine-based resin film with a film thickness of 40 μm or less so as to be adjacent to each other, and obtaining the sum of the number of foreign matters measured from each of the observation pieces. For a fluorine-based resin film with a thickness exceeding 40 μm, after stretching until the film thickness becomes 35 μm to 40 μm or less, the number of foreign matters is measured by the method described below. Specifically, 0.010 m so as to be in adjacent positions continuously from the fluorine-based resin film 2Cut out 25 pieces of the observation pieces having a size of (100 mm × 100 mm). Then, obtain the sum of the number of foreign matters measured from each observation piece. At this time, observe with transmitted light, mark the foreign matters, and observe the marked locations with a microscope to obtain the size of the foreign matters. The size of the foreign matters shall be the arithmetic mean value of the maximum width and the minimum width of the foreign matters.

[0040] 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 properties and piezoelectric properties. The thinner the film, the more advantageous it is for optical properties such as transparency and cost.

[0041] The thickness of the fluororesin piezoelectric film is generally measured by a method using a micrometer (JIS C 2151:2019), but is measured by known methods such as a method using a laser displacement meter, a method using a capacitance displacement meter, and a method using infrared rays. At this time, define the advance axis direction obtained by measuring the birefringence as the width direction, and set an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film as point A. Measure the thickness at a total of three measurement points: point A (measurement point) and two measurement points set at intervals of 30 mm from point A in both end directions on the line segment in the width direction passing through point A, and take the average value of these as the thickness of the fluororesin piezoelectric film.

[0042] Note that the fluororesin piezoelectric film may contain a resin other than the fluororesin or other additives as long as it can satisfy the above physical properties.

[0043] Examples of resins other than the above fluororesins include polycarbonate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, silicone resins, polyethers, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, which can be added to enhance flexibility; acrylic resins, epoxy resins, polyethylene oxide, polypropylene oxide, polyphenylene oxide, polyphenylene sulfide, polyamide, polyimide, polyamideimide, polystyrene, and polybenzimidazole, etc., which can be added to enhance strength; and odd polyamides, cyanopullulan, and copper phthalocyanine-based polymers, etc., which can be added to further enhance dielectric properties.

[0044] [Method for manufacturing fluororesin piezoelectric film] The method for manufacturing the above-mentioned fluororesin piezoelectric 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.

[0045] For example, the fluororesin film can be produced by a manufacturing method including a step of heating and melting the above-mentioned fluororesin (melting step), a step of forming the melted 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). At this time, a step of filtering the fluororesin melted in the melting step (filtering step) may be further performed.

[0046] (Melting step) In the melting step, the fluororesin is heated and melted. This step can be performed, for example, by melt-kneading the fluororesin with an extruder.

[0047] When the fluororesin melted in the melting process contains a solvent component, there is a risk that the polarization in the subsequent process may be hindered by the solvent component remaining without volatilization. Therefore, the content of the solvent component in the fluororesin is preferably low, preferably 1% by mass or less, more preferably 0.1% by 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.

[0048] The melting temperature of the fluororesin is preferably 75 °C or more higher than the melting point of the fluororesin and 105 °C or lower higher than the melting point, more preferably 75 °C or more higher than the melting point and 100 °C or lower higher than the melting point, even more preferably 80 °C or more higher than the melting point and 100 °C or lower higher than the melting point, and particularly preferably 85 °C or more higher than the melting point and 95 °C or lower higher than the melting point. By setting the melting temperature 75 °C or more higher than the melting point of the fluororesin, the viscosity of the fluororesin can be reduced to such an extent that it can be filtered in the next process. By setting the melting temperature 105 °C or lower higher than the melting point of the fluororesin, decomposition, condensation, etc. of the fluororesin due to heating can be suppressed, and the generation of decomposition products, etc. can be suppressed. By suppressing the generation of the above decomposition products, etc., the amount of foreign matter in the fluororesin piezoelectric film can be reduced, and the transparency and smoothness of the fluororesin piezoelectric film can be improved. Further, by suppressing the generation of the above decomposition products, etc., when filtering the fluororesin, clogging of the filter due to these can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0049] 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. On the other hand, 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 reduced 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 process, 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.

[0050] (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 pleated type and leaf disk type filters can be used.

[0051] 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.

[0052] In addition, the filtration of the fluororesin is performed by a filter having a multilayer structure composed of a plurality of layers with different shapes, mesh sizes, etc. The filtration accuracy of the filter for filtering the fluororesin indicates the filtration efficiency of the filter, that is, the filter can separate particles of a given size with a predetermined filtration efficiency. For example, in this specification, a filtration accuracy of 10 μm means that the filter can separate particles with a size of 10 μm or more with a filtration efficiency of 95% or more.

[0053] In this step, the filtration of the fluororesin may be performed multiple times. For example, coarse foreign matter may be removed with a filter having a low filtration accuracy (a large numerical value of filtration accuracy) arranged in the previous stage, and then finer foreign matter may be removed with a filter having a high filtration accuracy (a small numerical value of filtration accuracy) arranged in the subsequent stage. The filtration accuracy at this time shall be the value of the filter with the highest filtration accuracy.

[0054] The filter may be arranged between the extruder and the die for performing the film-forming process. Alternatively, the filter may be arranged in an extruder different from the extruder for performing the film-forming process or a melt-kneading device, and the fluororesin filtered by the filter may be put into the extruder for performing the film-forming process to form a film.

[0055] (Film-forming process) In the film-forming process, the fluororesin filtered in the filtration process is formed into a film.

[0056] The film-forming method is not particularly limited, and known methods such as extruding the melted and filtered fluororesin from a T-die and contacting it with a cooling roll for cooling can be used. In this embodiment, the surface temperature of the cooling roll at this time is set to 125°C or lower. By rapidly cooling the film of the fluororesin extruded by the cooling roll with a low surface temperature, a large number of minute crystals can be formed, and the internal haze of the obtained fluororesin film can be reduced. According to the findings of the present inventors, the fluororesin having a low melt viscosity used in this embodiment has a different crystallization rate from the fluororesin having a higher melt viscosity, so the haze of the obtained fluororesin film is likely to change greatly depending on the temperature of the cooling roll. Therefore, in this embodiment, it is considered important to adjust the temperature of the cooling roll. 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.

[0057] The fluororesin film thus obtained may be wound up once and stored, or may be conveyed to a stretching step or a polarization step, etc., which are subsequent steps.

[0058] (Stretching step) In the stretching step, the fluororesin film formed in the above film-forming step is uniaxially stretched. In the stretching step, the fluororesin film formed in the film-forming step may be stretched as it is, or the fluororesin film wound up and stored once may be heated as necessary and then stretched.

[0059] In the case of uniaxial stretching, the stretching direction is not limited. In the stretching process during mass production, it is preferable to continuously uniaxially stretch the fluororesin film. Specifically, while transporting the fluororesin film by a plurality of rolls, it can be uniaxially stretched in the transport 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, more preferably 3.5 times or more and 5.0 times or less. By setting the stretching ratio to 2.5 times or more, the proportion of β-crystals described 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, and the direction perpendicular to the stretching direction (TD direction) is the fast axis direction.

[0060] (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 due to the stretching process, and the proportion of β-crystals increases. By applying a DC voltage to a fluororesin film with an increased proportion of polar β-crystals, a fluororesin piezoelectric film with a high piezoelectric constant can be obtained.

[0061] The DC voltage to be applied is preferably 7.0 kV or more and 50.0 kV or less, more preferably 7.5 kV or more and 30.0 kV or less.

[0062] The stretching process and the polarization process may be performed simultaneously, or the polarization process may be performed after the stretching process.

[0063] The fluororesin film after the film formation process or the fluororesin piezoelectric film after the polarization process can be wound into a roll for storage and transfer, etc.

[0064] [Applications] The above-mentioned fluororesin piezoelectric film can be used for various applications such as touch sensors, touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0065] [Multilayer piezoelectric body] By laminating the above-mentioned fluororesin piezoelectric film (hereinafter, also simply referred to as "piezoelectric film") with other functional layers or films, a multilayer piezoelectric body can be obtained. The multilayer piezoelectric body may include an antistatic layer for the purpose of preventing static electricity generated during handling, if necessary. From the viewpoints of preventing damage to the antistatic layer and adjusting the hue, it may also include a hard coat layer or an optical adjustment layer. Furthermore, the multilayer piezoelectric body may include an electrode layer, if necessary, for detecting an electrical signal generated by the piezoelectricity of the piezoelectric film. In addition, in order to further prevent discoloration and a decrease in transparency of the fluororesin piezoelectric film over time when moisture enters from the outside under high temperature and high humidity conditions, a moisture-proof layer can be included. When the electrode layer is a film excellent in moisture-proof properties such as a metal film or a metal oxide film, the film can also be shared as the moisture-proof layer. On the other hand, when the electrode layer includes metal nanowires or a metal mesh, etc., since there is a risk that the metal is oxidized and the conductivity decreases, the multilayer piezoelectric body can further include an overcoat layer on the non-adhesive surface (the surface opposite to the piezoelectric film) of the electrode layer.

[0066] That is, the multilayer piezoelectric body includes at least the above-mentioned piezoelectric film. The multilayer 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 multilayer piezoelectric body may include an electrode layer on at least one surface of the piezoelectric film, and may further include an antistatic layer or an overcoat layer.

[0067] Hereinafter, a multilayer piezoelectric body according to an embodiment of the present invention and a method for manufacturing the same will be specifically described. However, the multilayer piezoelectric body is not limited to this embodiment.

[0068] FIG. 1 is a schematic cross-sectional view showing the laminated piezoelectric body of the present 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.

[0069] (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.

[0070] (Antistatic layer) [[ID=I8]]The antistatic layer can be disposed on at least one surface of the piezoelectric film. In the present 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.

[0071] 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 crosslinking agent as necessary, or may include a conductive material and a crosslinking agent.

[0072] The conductive material may be an ion conductive type conductive material or an electron conductive type conductive material.

[0073] Examples of the ion conductive type conductive material include (a) quaternary ammonium salts, pyridinium salts, cationic antistatic agents having a cationic group such as primary to tertiary amino groups, (b) anionic antistatic agents having an anionic group such as a sulfonate group, a sulfate ester group, a phosphate ester group, and a phosphonate group, (c) amphoteric antistatic agents such as amino acid-based and amino sulfate ester-based, and (d) nonionic antistatic agents such as amino alcohol-based, glycerin-based, and polyethylene glycol-based.

[0074] 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, and the like. 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 a sulfone group, an amino group, an amide group, a hydroxyl group, a mercapto group, and a carboxyl group. Examples of the conductive material other than the conductive polymer include carbon nanotubes, graphene, and the like.

[0075] 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.

[0076] 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, and the like, which have functional groups such as a hydroxy group, a methylol group, a carboxyl group, a sulfonyl group, an epoxy group, and an amino group.

[0077] The crosslinking agent may be any one that reacts with the functional groups possessed by the curable resin or the conductive polymer to effect crosslinking. Examples of the crosslinking agent include melamine-based crosslinking agents, polycarbodiimide-based crosslinking agents, polyoxazolidine-based crosslinking agents, polyepoxy-based crosslinking agents, and polyisocyanate-based crosslinking agents.

[0078] 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 a 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.

[0079] 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 property 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 property is likely to be 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, for example, in accordance with JIS C 2139-3-2:2018 using a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytic Co., Ltd., High Resister UX, model number: MCP-HT800, URS probe)).

[0080] The thickness of the antistatic layer is not particularly limited, but for example, it is preferably 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 made less likely to occur. 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.

[0081] The thickness of the antistatic layer can be measured using a spectroscopic interference film thickness meter (for example, Optical NanoGauge C13027-11 manufactured by Hamamatsu Photonics K.K.). The thickness of each layer in the range including the center of the surface of the laminated piezoelectric body can be measured at three points, and the measured value can be the arithmetic mean value. The thicknesses of the following each layer can also be measured in the same manner.

[0082] (Hard coat layer) The hard coat layer can be disposed between the piezoelectric film and the electrode layer. In the present embodiment, the hard coat layer is disposed adjacent to the piezoelectric film (see FIG. 1). The hard coat layer fills the scratches on the surface of the piezoelectric film to make it smooth, and makes it difficult for the surface of the piezoelectric film to be scratched in the manufacturing process of the laminated piezoelectric body. Thereby, the haze of the laminated piezoelectric body can be further reduced.

[0083] The thickness of the hard coat layer is not particularly limited, but it is 0.05 μm or more, preferably 0.3 μm or more and 3.0 μm or less, more preferably 0.5 μm or more and 2.0 μm or less, and even more preferably 0.5 μm or more and 1.5 μm or less.

[0084] As described above, the refractive index of the hard coat layer is preferably higher than that of the piezoelectric film and lower than that of the electrode layer. Specifically, the refractive index of the hard coat layer is preferably 1.40 or 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 while suppressing interference fringes more effectively.

[0085] 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 suppressed more effectively.

[0086] 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 solution containing a resin and then drying it, or by applying a curable composition containing a polymerizable compound and then drying and curing it.

[0087] 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 light (UV) or electron beam (EB).

[0088] 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.

[0089] Among 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, and the like. Examples of trifunctional or higher (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, and the like. Further, the above (meth)acrylate monomers may be those in which a part of the molecular skeleton is modified, and those modified with ethylene oxide, propylene oxide, caprolactone, isocyanuric acid, alkyl, cycloalkyl, aromatic, bisphenol, etc. can also be used.

[0090] Examples of polyfunctional (meth)acrylate oligomers include acrylate - based polymers such as urethane (meth)acrylate, epoxy (meth)acrylate, polyester (meth)acrylate, polyether (meth)acrylate, etc. Urethane (meth)acrylate is obtained, for example, by the reaction of a polyhydric alcohol and an organic diisocyanate with hydroxy (meth)acrylate.

[0091] When the radiation - curable compound is an ultraviolet - curable compound, the curable composition preferably contains a photoinitiator. Examples of the photoinitiator include one or more selected from acetophenone, benzophenone, α - hydroxyalkylphenone, Michler's ketone, benzoin, benzyl methyl ketal, benzoyl benzoate, α - acyloxime ester, thioxanthones, and the like.

[0092] The curable composition may further contain other components other than the above, if necessary. For example, from the viewpoints of suppressing blocking of the hard coat layer in the manufacturing process and adjusting the refractive index, the curable composition may further contain particles. The particles may be inorganic particles or organic particles.

[0093] Examples of the inorganic particles include particles such as silica (silicon oxide), titanium oxide, zirconium oxide, aluminum oxide, zinc oxide, and tin oxide, diamond powder, sapphire particles, boron carbide particles, silicon carbide particles, and antimony pentoxide particles. Examples of the organic particles include resin particles such as acrylic resin, acrylic-styrene copolymer, and silicone resin. Among them, inorganic particles are preferable, and silica particles are more preferable, from the viewpoint that the transparency of the hard coat layer is less likely to be impaired. The surface of the inorganic particles may be treated with a surface modifier such as a silane coupling agent.

[0094] Note that, as will be described later, the laminated piezoelectric body may further contain 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, it is possible to further suppress the intrusion of moisture from the outside through these layers, and the discoloration suppression effect of the piezoelectric film by providing the moisture-proof layer is more easily maintained. Further, when the total thickness is 0.30 μm or less, the color tone of the laminated piezoelectric body can be suppressed and the transparency can be enhanced. From the same viewpoint, the total thickness is more preferably 0.50 μm or more and 3.0 μm or less, and further preferably 0.60 μm or more and 2.0 μm or less.

[0095] (Optical adjustment layer) The optical adjustment layer can be disposed between the hard coat layer and the transparent conductive layer. By appropriately adjusting the refractive index and thickness of the optical adjustment layer, the color tone of the laminated piezoelectric body can be suppressed.

[0096] 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. A laminated piezoelectric body including such an optical adjustment layer can suppress coloration due to, for example, the interference between the light incident on the hard coat layer and reflected therefrom and the light reflected at the interface between the optical adjustment layer and the hard coat layer.

[0097] 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 even more preferably 1.65 or more and 1.75 or less.

[0098] 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.

[0099] The thickness of the optical adjustment layer is not particularly limited, but may be 0.05 μm or more. For example, the thickness of the optical adjustment layer may be 0.05 μm or more and 0.5 μm or less, or may be 0.09 μm or more and 0.18 μm or less. When the thickness of the optical adjustment layer is within the above range, the coloration of the laminated piezoelectric body can be suppressed.

[0100] 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.

[0101] 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.

[0102] (Transparent electrode) The transparent electrode (electrode layer) may be formed by sputtering a conductive material onto the piezoelectric film on which each layer is formed, or by mixing a conductive material with a resin or the like and applying the mixture.

[0103] The electrode layer only needs to 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.

[0104] The conductive material constituting the electrode layer is not limited, and at least one metal and metal oxide selected from the group consisting of In, Sn, Zn, Ga, Sb, Ti, Si, Zr, Mg, Al, Au, Ag, Cu, Pd, and W are preferably used. The metal oxide may further contain the metal atoms shown in the above group as necessary. As the metal oxide, indium tin oxide composite oxide (ITO), antimony tin composite oxide (ATO), etc. are preferably used, and ITO is particularly preferably used. Examples of other typical conductive materials for the electrode layer include at least one selected from the group consisting of metal nanowires, metal meshes, conductive polymer compounds, carbon nanotubes, and graphene. Among these, as the metal nanowires and metal meshes, silver nanowires, silver meshes, copper nanowires, and copper meshes are included. Examples of the conductive polymer compound include polyacetylene and its derivatives, polythiophene and its derivatives, polypyrrole and its derivatives, polyaniline and its derivatives, etc. In this embodiment, the conductive material is preferably a metal oxide, and more preferably ITO.

[0105] Note that silver nanowires have attracted attention as a conductive material 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.

[0106] 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 or less, preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 3 or less, more preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 2 or less, and particularly preferably 1.0 Ω / sq. or more and 1.0×10 2 or less.

[0107] 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.

[0108] The thickness of the electrode layer is not limited. However, from the viewpoint of having good conductivity, the thicker the electrode layer, the more preferable. On the other hand, if the electrode layer is too thick, the transparency may decrease, and if it is too thin, the electrical resistance may increase, and a non - continuous conductive portion 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 - Tech 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 in the range including the intersection of the diagonal lines of the laminated piezoelectric body, and taking it as the thickness of the electrode layer.

[0109] 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 viewpoint 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 the surface resistivity measured on the overcoat layer.

[0110] Also, in the above - described embodiment, the electrode layer is formed on the optical adjustment layer, but a transparent electrode film having an electrode layer formed on the surface of a substrate such as glass or a polymer material may be adhered to the optical adjustment layer to form it.

[0111] (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 any one or a plurality of polymer materials such as polyethylene terephthalate (PET), cycloolefin polymer (COP), and polycarbonate (PC), or a substrate containing an inorganic material such as glass, with the above-described electrode layer laminated thereon.

[0112] 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 to withstand heat for crystallizing ITO or the like. Examples of such transparent resins include polyesters such as polyethylene terephthalate (PET). Among them, PET is preferred.

[0113] The thickness of the substrate is not particularly limited as long as it can support the electrode layer, but it is preferably 2 μm or more and 300 μm or less, more preferably 10 μm or more and 200 μm or less, still more preferably 20 μm or more and 150 μm or less, and particularly preferably 30 μm or more and 130 μm or less.

[0114] The electrode layer is disposed on the surface of the substrate on the piezoelectric film side (the optical adjustment layer side in FIG. 1). As the electrode layer, inorganic electrodes such as ITO (indium tin composite oxide) and tin oxide are preferred, and ITO is more preferred.

[0115] (Moisture-proof layer) Further, in the above embodiment, the laminated piezoelectric body has a piezoelectric film, but in order to further prevent discoloration and reduction in transparency of the piezoelectric film over time when moisture enters from the outside under high temperature and high humidity, it can include a moisture-proof layer. The material of the moisture-proof layer is not particularly limited as long as it has transparency and can suppress the intrusion of moisture, but it is preferably composed of an inorganic oxide, and more preferably a thin film (such as a vapor deposition film) of an inorganic oxide.

[0116] An inorganic oxide is an oxide of a metal, a non-metal, or a sub-metal. Examples of inorganic oxides 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, barium oxide, etc. Among them, indium tin oxide and silicon oxide are particularly preferred.

[0117] The thickness of the moisture-proof layer is not particularly limited. From the perspective of achieving a higher degree of compatibility between moisture-proof property 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.

[0118] [Physical Properties of the Multilayer Piezoelectric Body] (Piezoelectric Constant d 33 ) The piezoelectric constant d 33 of the multilayer piezoelectric body is preferably, for example, 7.0 pC / N or more and 40.0 pC / N or less. When the piezoelectric constant d 33 of the multilayer piezoelectric body is 7.0 pC / N or more, higher pressure sensitivity is more easily obtained. When the piezoelectric constant d 33 of the multilayer piezoelectric body is 40.0 pC / N or less, the appearance defects as described above can be further reduced. From the same perspective, the piezoelectric constant d 33 of the multilayer piezoelectric body is more preferably 10.0 pC / N or more and 40.0 pC / N or less, even more 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. The piezoelectric constant d 33 of the multilayer piezoelectric body can be measured in the same manner as above, except that the measurement position is one point at the intersection of the diagonal lines of the multilayer piezoelectric body.

[0119] The piezoelectric constant d 33 of the multilayer piezoelectric body is, for example, the piezoelectric constant d 33can be adjusted. The piezoelectric constant d of the piezoelectric film 33 is high, and the piezoelectric constant d of the laminated piezoelectric body 33 also tends to be high.

[0120] (Total light transmittance) From the viewpoint of application to, for example, a touch panel, the laminated piezoelectric body preferably has high transparency. Specifically, the total light transmittance of the laminated piezoelectric body is preferably 80% or more, more preferably 85% or more.

[0121] The total light transmittance of the laminated piezoelectric body can be measured using a haze meter (for example, NDH7000SP II manufactured by Nippon Denshoku Industries Co., Ltd.) based on the method described in JIS K 7361-1. The total light transmittance shall be measured at a position including the intersection of the diagonals of the laminated piezoelectric body.

[0122] The total light transmittance of the laminated piezoelectric body can be adjusted by the layer structure, the refractive index and thickness of each layer. For example, when the laminated piezoelectric body includes a hard coat layer, the haze of the laminated piezoelectric body can be further reduced, so the total light transmittance tends to be higher.

[0123] (Hue b * value, Δb * ) The hue b * value of the laminated piezoelectric body is preferably -5.0 or more and 5.0 or less. By setting the hue b * value to -5.0 or more and 5.0 or less, good transparency and visibility can be achieved. Also, when the laminated piezoelectric body is stored in an environment of 85°C and 85% RH for 500 hours, the color difference Δb * before and after storage is preferably 4.0 or less, more preferably 3.5 or less. When the Δb * before and after storage of the laminated piezoelectric body is 4.0 or less, the change in the hue of the laminated piezoelectric body under high temperature and high humidity is small, so good transparency and visibility can be maintained.

[0124] The b *The value can be measured using a spectrocolorimeter (e.g., SD7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in a method compliant with JIS Z 8722. For the b value of the laminated piezoelectric body * measurement shall be made at a position including the intersection point of the diagonal lines of the laminated piezoelectric body. For the b value of the laminated piezoelectric body after storage * measurement shall be made after cutting the laminated piezoelectric body into a 25 cm 2 square, fixing the four corners of the laminated piezoelectric body to a SUS plate with tape, placing it in a thermo-hygrostat set at a temperature of 85 °C and a humidity of 85% RH, holding it for 500 hours under the above conditions, and then measuring the b value of the laminated piezoelectric body taken out from the thermo-hygrostat in the same manner as the above method. * The value may be measured in the same manner as the above method.

[0125] For the b value of the laminated piezoelectric body * and Δb * they can be adjusted according to the thickness of the antistatic layer, the composition and thickness of the moisture-proof layer. For example, when the thickness of the antistatic layer is reduced, the b value * and Δb * can be reduced. Also, when the thickness of the moisture-proof layer is increased, discoloration of the piezoelectric film due to moisture intrusion can be reduced, and the b value * and Δb * can be reduced.

[0126] [Method for manufacturing a laminated piezoelectric body] The laminated piezoelectric body can be manufactured by any method. For example, the laminated piezoelectric body in FIG. 1 can be manufactured through the steps of: (1) preparing a fluororesin 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. In the case where 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 necessary. 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) or 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).

[0127] (Step of preparing a piezoelectric film) As the piezoelectric film containing a fluororesin, the above-described fluororesin piezoelectric film can be used.

[0128] (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 the low-temperature curability.

[0129] 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.

[0130] 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.

[0131] 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 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.

[0132] (Step of forming a hard coat layer) In this embodiment, after applying the curable composition for the hard coat layer onto the piezoelectric film or the antistatic layer, it is dried and cured to form a hard coat layer.

[0133] The curable composition may further contain a diluent solvent. As the diluent solvent, those having a polarity close to that of the particles are preferred. 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.

[0134] The application of the curable composition can be carried out by a known wet process method. Representative wet process methods include, for example, dip coating, spray coating, spin coating, gravure coating, die coating, roll coating, flow coating, curtain coating, etc. Among them, methods capable of continuously forming a layer, such as the roll coating method and the gravure coating method, are preferred from the viewpoint of productivity.

[0135] 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 within a range where the solvent can be volatilized and removed and below the heat distortion temperature of the fluororesin constituting the piezoelectric film, and can be, for example, 60°C or higher and 100°C or lower.

[0136] The curing of the curable composition may be by heat curing or by radiation curing. Radiation curing can be carried out by irradiating ultraviolet rays or electron beams. Also, heat curing and radiation curing may be used in combination.

[0137] (Step of forming an optical adjustment layer) In the same manner as 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 a metal nanowire or a metal mesh as the conductive material, since the transparency of the conductive layer is high, the optical adjustment layer can be omitted.

[0138] (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 a metal nanowire 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.

[0139] [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

[0140] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0141] 1. Fabrication of Fluorine-based Resin Film Piezoelectric Films 1 to 8 and Film 11, all of which are fluorine-based resin piezoelectric films, were fabricated according to the following procedure.

[0142] 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, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho, Ltd., Capilograph 1D), with a capillary die having an inner diameter of φ1 mm × a tube length of 10 mm, the viscosity was measured at a measurement temperature of 260°C and a shear rate of 50 s -1 as the measured viscosity.

[0143] 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.

[0144] Also, the melting temperature was defined as the maximum temperature in the conduit from the extruder to the filtration device.

[0145] 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, the melting temperature and the filtration temperature were 260 °C.

[0146] The obtained unstretched film was guided to a uniaxial stretching device equipped with a plurality of metal rolls and pinch rolls, and the rotation speed ratio of each roll was adjusted to stretch it 4.6 times in the flow direction. Furthermore, a voltage of 8.0 kV was applied from the surface of the film in the thickness direction to obtain Piezoelectric Film 1.

[0147] 1-2. Piezoelectric Film 2 Piezoelectric Film 2 was obtained in the same manner as the production of Piezoelectric Film 1, except that no filter was attached to the single-screw extruder and the voltage applied after stretching was 8.8 kV.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152] 1-7. Piezoelectric film 7 A piezoelectric film 7 was obtained in the same manner as the production of the piezoelectric 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.

[0153] 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 a polyvinylidene fluoride homopolymer (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.

[0154] 1-9. Piezoelectric film 9 (could not be produced) A piezoelectric film 9 was attempted to be fabricated in the same manner as the fabrication 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 became clogged with resin, and a piezoelectric film could not be obtained.

[0155] 1-10. Piezoelectric film 10 (not fabricated) A piezoelectric film 10 was attempted to be fabricated in the same manner as the fabrication of the piezoelectric film 4, except that a pleated 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.

[0156] 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). The temperature was raised to 60°C while stirring with a stirrer, 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 dried at 120°C for 1 hour to obtain a piezoelectric film 11. The piezoelectric film 11 had a large surface height Rz, and it was difficult to accurately detect the amount of foreign matter.

[0157] 2. Evaluation of fluororesin piezoelectric films Regarding the obtained piezoelectric films 1 to 8 and piezoelectric film 11, haze, internal haze, retardation, piezoelectric constant d 33 , surface height roughness Rz, the number of foreign matters, and thickness were measured by the following methods.

[0158] 2-1. Haze of the film A haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., model NDH7700SP II) was used. In accordance with ISO 14782:2021, the advancing axis direction determined by measuring the birefringence was defined as the width direction. An arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film was designated as point A. Haze measurements were taken 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 measurements was taken as the haze of the film.

[0159] 2-2. Internal Haze The advancing axis direction determined by measuring the birefringence of the fluororesin piezoelectric film was defined as the width direction. An arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film was designated as point A. A measurement sample with a size of 30 mm × 30 mm was cut out so that point A was at the center of the diagonal of the measurement sample. Two measurement samples were cut out in both width end directions adjacent to the measurement sample, and a total of three measurement samples were collected. For each of the three cut-out measurement samples, a hard coat agent (manufactured by Arakawa Chemical Industries, Ltd., BS CH271) was 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, model CSOT040), UV was irradiated so that the target integrated light amount was 400 mJ / cm 2 to form a coating layer with a thickness of 2 μm. Assuming that the external haze due to scratches on the film surface was removed by these coating layers, the measured haze value was taken as the internal haze. Similarly, the average value of the three measured samples was taken as the representative value of the fluororesin piezoelectric film.

[0160] 2-3. Retardation The manufacturer of the prince measuring instrument measured the retardation of a film cut into 20 mm × 20 mm by the parallel Nicol rotation method using KOBRA-HB. 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 direction of the slow axis 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 coincides with the direction of the slow axis.

[0161] 2-4. Piezoelectric constant d 33 The piezoelectric constant d was measured in accordance with ISO 19622:2018. 33 Specifically, a piezoelectric constant measuring device (PiezoTest, piezometer system PM300) was used to hold the film, which is the test piece, 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 surface of the film, and the piezoelectric constant was calculated using the absolute value of the measured value. At this time, 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 set as point A. The piezoelectric constant d was measured at a total of three measurement points: 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 the piezoelectric constants obtained from these was taken as the piezoelectric constant d of the film. 33 and was used as the piezoelectric constant d of the film. 33

[0162] 2-5. 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.

[0163] 2-6. Number of foreign matters Since the thicknesses of the fabricated piezoelectric films were all 40 μm or less, no processes such as stretching were performed on the piezoelectric films, and 0.010 m 2 Twenty-five rectangular films (observation pieces) with a size of (100 mm × 100 mm) were cut out from the film. 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, 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 smaller than 100 μm were determined respectively.

[0164] 2-7. 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. For a total of three measurement points, namely 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 thickness was measured, and the average value of these was taken as the thickness of the film.

[0165] 3. Results The production conditions and evaluation results of each film are shown in Table 1 and Table 2. Note that Film 11 had a large haze and was opaque, and the number of foreign substances could not be measured.

[0166] [Table 1]

[0167] [Table 2]

[0168] 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.), heat-treated at 130 °C for 0.67 minutes, and an antistatic layer with a thickness of 80 nm was formed. The surface resistivity of the antistatic layer was 1.2×10 8 Ω / sq.

[0169] 4-1-2. Antistatic layer 2 A solution (C-169PF) obtained 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 at a ratio of 3:2 was applied with a multi-coater (manufactured by Hirano Techno Seed 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 7.1×10 6 Ω / sq.

[0170] 4-2. Formation of hard coat layer A hard coat agent (BS-CH271 manufactured by Arakawa Chemical Industries, Ltd., average particle diameter of amorphous silica is 60 nm) was applied with a multi-coater, heat-treated at 80 °C for 2 minutes, and further irradiated with UV with an integrated light amount of 200 mJ / cm 2 to be photocured to form a hard coat layer with a thickness of 700 nm and a refractive index of 1.50.

[0171] 4-3. Optical adjustment layer An ultraviolet curable composition containing zirconium oxide particles (Opsstar RA004 manufactured by Arakawa Chemical Industries, Ltd.) was applied, dried at 40 °C for 30 seconds, and then irradiated with ultraviolet rays with an integrated light amount of 250 mJ / cm 2 to form an optical adjustment layer (thickness 102 nm, refractive index 1.65).

[0172] 4-4. Formation of transparent electrode 4-4-1. Transparent electrode layer 1 An indium tin metal target (tin content ratio to the sum of indium content and tin content is 3% by mass) was mounted on a magnetron sputtering apparatus as a target material, and a piezoelectric film on which a predetermined layer was formed was mounted as a substrate. Then, while winding up the piezoelectric film on which the predetermined layer was formed, dehydration and degassing were performed, and the degree of vacuum was 7×10 -5Exhaustion was carried out until it reached Pa. Subsequently, the substrate temperature was set to room temperature (25 °C), and a mixed gas with a composition of 99.2 mol% argon gas and 0.8 mol% oxygen gas was introduced into the chamber. While discharging the gas in the chamber so that the pressure in the chamber became 0.4 Pa, a transparent electrode layer with a thickness of 30 nm substantially composed of indium tin composite oxide was formed by reactive sputtering method, and a laminated piezoelectric body was obtained.

[0173] 4-4-2. Transparent Electrode Layer 2 A solution obtained by appropriately diluting a paint T-AG230 (manufactured by Seiko PMC Co., Ltd.) containing silver nanowires with alcohol was applied with 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 in which T-YP462 (manufactured by Seiko PMC Co., Ltd.), an additive, was mixed with paint T-YP562 (manufactured by Seiko PMC Co., Ltd.) was applied with 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, it was irradiated with ultraviolet rays with an integrated light amount of 330 mJ / cm 2 to form a protective layer with a thickness of 80 nm. Here, the transparent electrode layer and the protective layer together are referred to as the transparent electrode layer 2.

[0174] 4-4-3. Transparent Electrode Film A transparent electrode precursor film (manufactured by Oike Kogyo Co., Ltd., Tetrite TCF KH100NMH3-100-U8) obtained by sputtering indium tin oxide (ITO) on 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 bonded to the polymer piezoelectric film on which the above-mentioned predetermined layer was formed, the release film was peeled off from the OCA sheet, and the transparent electrode film with the conductive layer surface facing the OCA sheet surface was bonded.

[0175] 4-5. Laminated Piezoelectric Body 4-5-1. Laminated Piezoelectric Body 1 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, and then a hard coat layer was formed on the antistatic layer 1 to fabricate the laminated piezoelectric body 1.

[0176] 4-5-2. Laminated piezoelectric body 2 A transparent electrode layer 1 was formed on the hard coat layer of the laminated piezoelectric body 1 to fabricate the laminated piezoelectric body 2.

[0177] 4-5-3. Laminated piezoelectric body 3 A transparent electrode film was bonded onto the hard coat layer of the laminated piezoelectric body 1 to fabricate the laminated piezoelectric body 3.

[0178] 4-5-4. Laminated piezoelectric body 4 The laminated piezoelectric body 4 was fabricated in the same manner as the laminated piezoelectric body 2, except that the transparent electrode layer 2 was used instead of the transparent electrode layer 1.

[0179] 4-5-5. Laminated piezoelectric body 5 The laminated piezoelectric body 5 was fabricated in the same manner as the laminated piezoelectric body 4, except that the antistatic layer 1 was changed to the antistatic layer 2.

[0180] 4-5-6. Laminated piezoelectric body 6 The laminated piezoelectric body 6 was fabricated 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.

[0181] 4-5-7. Laminated piezoelectric body 7 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, and a transparent electrode film 1 was bonded onto the antistatic layer 1 to fabricate the laminated piezoelectric body 7.

[0182] 4-5-8. Laminated piezoelectric body 8 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, and then a transparent electrode layer 1 was formed on the antistatic layer 1 to fabricate the laminated piezoelectric body 8.

[0183] 4-5-9. Laminated piezoelectric body 9 The laminated piezoelectric body 9 was fabricated 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.

[0184] 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.

[0185] 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.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] 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.

[0190] 4-5-16. Multilayer piezoelectric body 16 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, a hard coat layer and a transparent electrode layer 2 were formed on the antistatic layer 1 on the A surface to fabricate the laminated piezoelectric body 16.

[0191] 5. Evaluation of the laminated piezoelectric body For the obtained laminated piezoelectric bodies 1 to 16, the piezoelectric constant d 33 , total light transmittance, surface resistivity, haze, and b * were measured.

[0192] 5-1. Piezoelectric constant The value measured in the range including the intersection of the diagonals of the laminated piezoelectric body was taken as the representative value of the piezoelectric constant d 33 of the laminated piezoelectric body, and the measurement was performed in the same manner as the above measurement method (2-4).

[0193] 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.), and the range including the intersection of the diagonals of the laminated piezoelectric body was measured, and the value was taken as the representative value.

[0194] 5-3. Surface resistivity 5-3-1. Surface resistivity of the 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 a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytic Co., Ltd., High Resister UX, model number: MCP-HT800, URS probe)). The measurement location was the surface resistivity of the antistatic layer measured in the range including the intersection of the diagonals of the rectangular laminated piezoelectric body, which was taken as the representative value.

[0195] 5-3-2. Surface resistivity of the transparent electrode The surface resistivity of the transparent electrode of the laminated piezoelectric body was measured using a resistivity meter ("LorestaGP MCP-T610", manufactured by Nitto Seiko 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.

[0196] 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.

[0197] 5-5. Chromatic aberration Δb * Value In accordance with the method conforming to JIS Z 8722, using a spectrocolorimeter (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.

[0198] 5-6. Thickness The thickness of the antistatic layer was measured using a spectroscopic interference film thickness meter (for example, manufactured by Hamamatsu Photonics K.K., 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.

[0199] The thickness of the transparent electrode was measured by observing the cross-section of the laminated piezoelectric body using a scanning electron microscope ("SU3800", manufactured by Hitachi High-Technologies Corporation) under 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.

[0200] 6. Results The production conditions and evaluation results of each laminated piezoelectric body are shown in Table 3 and Table 4.

[0201]

Table 3

[0202]

Table 4

Industrial Applicability

[0203] The fluororesin film according to the present invention is useful as a piezoelectric film with high transparency.

Explanation of Signs

[0204] 10 Laminated 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 internal haze is less than 1.2%, 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 Rz of the surface on the side with a smaller surface height roughness Rz measured in accordance with JIS B 0601:2001 is 0.50 μm or less, The fluororesin piezoelectric film according to Claim 1.

3. The number of foreign substances having a size, which is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view, of 100 µm or more is 7 pieces / 0.25 m ２ is as follows The fluororesin piezoelectric film according to Claim 1.

4. Containing a structural unit derived from vinylidene fluoride as a main component, The fluororesin piezoelectric film according to Claim 1.

5. Measuring temperature: 260°C, shear rate during measurement: 50 s －１ A step of heating and melting a fluororesin in which the melt viscosity η measured at －１ is 600 Pa·s or more and 4000 Pa·s or less 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 less, A step of stretching the cooled film, A step of polarizing the cooled film, The method for producing a fluororesin piezoelectric film according to any one of Claims 1 to 4, which has these steps.

6. In the melting step, the fluororesin is melted at a temperature 75°C or more higher than the melting point of the fluororesin and 105°C or lower, The method for producing a fluororesin piezoelectric film according to Claim 5.

7. The method for producing a fluororesin piezoelectric film according to Claim 5, which has 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. The method for producing a fluororesin piezoelectric film according to Claim 5.

8. A laminated piezoelectric body including the fluororesin piezoelectric film according to Claim 1, The total light transmittance is 80% or more, Laminated piezoelectric body. 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. [[ID= ​ ​ ​ ​ ​ ​ ​ 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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