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 the small number of foreign matters and a high piezoelectric constant.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 film has 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, in which the number of foreign matters having an arithmetic average value of a maximum width and a minimum width of 100 μm or more when the film is viewed in plan view is 7 pieces / 0.25 m2 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 surface of a display device such as a liquid crystal display are widely used and are used for operation panels of portable devices such as mobile phones and 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 for accurately viewing an image of the display.

[0003] It is known that a fluororesin film exhibits high piezoelectricity by polarization treatment. A piezoelectric film made of a fluororesin piezoelectric film is being used for transparent touch panels and the like because it can have high transparency and flexibility. The piezoelectric film used for a touch panel or the like is required to have high transparency to enhance the visibility of an image of the display. Since the transparency of the piezoelectric film is largely dominated by the transparency of the fluororesin-based film before piezoelectric treatment, the development of a fluororesin piezoelectric film with high transparency is desired. In this specification, the fluororesin piezoelectric film or piezoelectric film means a fluororesin film having a piezoelectric constant d 33 measured by the method described below of 1.0 pC / N or more.

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

[0005] [Patent Document 1] International Publication No. 2015 / 064324 [Patent Document 2] Japanese Patent Publication No. 2020-164781 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Documents 1 and 2, methods of dissolving a fluororesin in a solvent and filtering the resulting solution are known. However, when filtering is performed using these methods, polarization is likely to be hindered by the polar solvent remaining in the fluororesin film. Furthermore, since a large amount of polar organic solvent is used to dissolve the fluororesin, consideration must be given to the working environment and the natural environment, and production costs, such as solvent recovery, are relatively high.

[0007] Furthermore, in the solution casting method, a large amount of polar organic solvent is used to dissolve the fluororesin, which requires the recovery of the solvent, which not only increases the production cost but also requires consideration of the impact of the organic solvent on the working environment and the natural environment.Furthermore, because a polar organic solvent is used to dissolve the fluororesin, if the solvent remains in the fluororesin film, there is a risk that it will interfere with polarization in the manufacturing process of the piezoelectric film.

[0008] On the other hand, in the method of hot-melting and extrusion-molding a fluororesin, there is a risk of problems such as the resin being denatured when heated to a high temperature, generating decomposition products (foreign substances).

[0009] The present invention has been made in view of the above problems, and an object thereof is to provide a fluororesin piezoelectric film that is manufactured from a fluororesin with a small number of foreign substances, and that has a high piezoelectric constant, a method for manufacturing the same, 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 [3]. [1] A fluororesin piezoelectric film having a melt viscosity η measured at a measurement temperature of 260°C and a shear rate of 50 s -1 during measurement of 600 Pa·s or more and 4000 Pa·s or less, wherein the number of foreign substances having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view of 100 μm or more is 7 pieces / 0.25 m 2 or less, 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. Fluororesin piezoelectric film. [2] The Rz of the surface on the smaller side of the surface height roughness Rz measured in accordance with JIS B 0601:2001 is 0.50 μm or less. The fluororesin piezoelectric film according to [1]. [3] Containing a structural unit derived from vinylidene fluoride as a main component. The fluororesin piezoelectric film according to [1] or [2].

[0011] One embodiment of the present invention for solving the above problems relates to a method for manufacturing a fluororesin piezoelectric film of the following [4]. [4] At a measurement temperature of 260°C and a shear rate of 50 s during measurement -1A step of heating and melting a fluororesin having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less at a temperature 75°C or more higher than the melting point of the fluororesin and at a temperature not higher than 105°C higher; A step of filtering the molten fluororesin with a filter having a filtration accuracy of 10 μm or more and 40 μm or less; A step of forming the filtered fluororesin into a film; A step of stretching the formed film; A step of polarizing the formed film; A method for manufacturing a fluororesin piezoelectric film according to any one of [1] to [3], comprising the above steps.

[0012] One embodiment of the present invention for solving the above problems relates to a laminated piezoelectric body of the following [5] to

[10] . [5] A laminated piezoelectric body including a fluororesin piezoelectric film according to any one of [1] to [3], having a total light transmittance of 80% or more, laminated piezoelectric body. [6] Further including 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, The laminated piezoelectric body according to [5]. [7] The electrode layer includes at least one selected from the group consisting of a metal film, a metal oxide film, a metal nanowire, a metal mesh, a conductive polymer compound, a carbon nanotube, and graphene, The laminated piezoelectric body according to [6]. [8] Further including a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film, The laminated piezoelectric body according to any one of [5] to [7]. [9] Further including an antistatic layer disposed on at least one surface of the fluororesin piezoelectric film, having a surface resistivity of 1.0×10 4 Ω / sq. or more and 1.0×10 9 Ω / sq. or less, The laminated piezoelectric body according to any one of [5] to [8].

[10] An antistatic layer and a hard coat layer are arranged 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 6 Ω / sq. or more and 1.0×10 12 Ω / sq. or less. The laminated piezoelectric body according to any one of [5] to [9]. [Advantages of the Invention]

[0013] According to the present invention, there are provided a fluororesin piezoelectric film having a small number of foreign matters and a high piezoelectric constant, a method for producing the same, and a laminated piezoelectric body. [Brief Description of the Drawings]

[0014]

Figure 1

[0015] [Fluororesin Piezoelectric Film] An 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-based film is 50% by mass or more. The proportion of the content of the fluororesin-based resin in 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), etc. Examples of fluororesins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoro-ethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP), etc. Examples of fluororesins obtained by polymerizing VDF include the homopolymer of VDF, and copolymers of VDF with 1-chloro-1-fluoro-ethylene, 1-chloro-2-fluoro-ethylene, trifluoroethylene, TFE, CTFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether, etc.

[0018] Among these, from the viewpoint of facilitating the polarization of the fluororesin film, fluororesins obtained by polymerizing monomers containing VDF are preferred. The homopolymer of VDF, the copolymer of VDF and trifluoroethylene, the copolymer of VDF and HFP, the copolymer of VDF and TFE, the copolymer of VDF, trifluoroethylene, TFE, and CTFE, and the copolymer of VDF, trifluoroethylene, TFE, and 1-chloro-1-fluoro-ethylene are more preferred, and the homopolymer of VDF is 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, and the homopolymer of VDF is most preferred. Containing a structural unit derived from VDF as a main component means that the content rate of the structural unit derived from VDF with respect to the total mass of the fluororesin piezoelectric film is 50% by mass or more. The content rate of these resins with respect 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 it is. 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 structural unit contained in the fluororesin film and the fluororesin piezoelectric film is 19 It 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 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, the less likely the filter is to be clogged and the easier the filtration becomes. Therefore, it is not necessary to increase the melting temperature to enable filtration, and it is possible to suppress the generation of spot-like irregularities on the film surface due to the resin being denatured by heat and foreign substances being generated. Also, the higher the melt viscosity, the larger the molecular weight of the resin, so it becomes difficult to selectively arrange the molecular chains and crystallization is difficult. Therefore, light scattering due to the difference in refractive index between the crystalline part and the non-crystalline part is less likely to occur, the transparency of the film is increased, and various physical properties of the film are likely to be stable.

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

[0024] Foreign substances in the resin tend to be more likely to be detected by the plan view of the film because the proportion of foreign substances present near the surface increases as the film becomes thinner. 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 7 pieces / 0.25 m 2 or less, and 0 pieces / 0.25 m 2 or more and 5 pieces / 0.25 m 2 or less is preferable, and 0 pieces / 0.25 m 2 or more and 3 pieces / 0.25 m 2 or less is more preferable, and 0 pieces / 0.25 m 2 or more and 1 piece / 0.25 m 2 or less is even more preferable.

[0025] 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 larger than 200 μ is 0 pieces / 0.25 m 2 or more and 3 pieces / 0.25 m 2 or less is preferable, and 0 pieces / 0.25 m 2 or more and 2 pieces / 0.25 m 2 or less is more preferable, and 0 pieces / 0.25 m 2 or more and 1 piece / 0.25 m 2 or less is even more preferable.

[0026] 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 less than 100 μm is 0 pieces / 0.25 m 2 or more and 50 pieces / 0.25 m 2 or less is preferable, and 0 pieces / 0.25 m 2 or more and 25 pieces / 0.25 m 2 or less is more preferable, and 0 pieces / 0.25 m 2 or more and 16 pieces / 0.25 m 2 or less is even more preferable.

[0027] The fewer these foreign substances are, the more the transparency of the fluororesin piezoelectric film is improved, and when stretching or polarizing the fluororesin film, it becomes easier to make the stretching or polarization uniform.

[0028] The number of these foreign matters is obtained by continuously cutting out 25 rectangular films (observation pieces) from a fluororesin piezoelectric film with a film thickness of 40 μm or less so that they are adjacent to each other, and finding the sum of the numbers of foreign matters measured from each of the said observation pieces. Specifically, 0.010 m 2 (100 mm × 100 mm) of 25 pieces of the said observation pieces are cut out. Then, the sum of the numbers of foreign matters measured from each observation piece is obtained. In the case of a fluororesin piezoelectric 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. At this time, foreign matters are marked by observing with transmitted light, and the size of the foreign matter is obtained by observing the marked portion with a microscope. The size of the foreign matter is taken as the arithmetic mean value of the maximum width and the minimum width of the foreign matter.

[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 higher the ratio of the β crystal, 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 process 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 measured by the direct quasi-static method (d33 The piezoelectric constant d is measured in accordance with ISO 19622:2018, which is a test method (meter method, Berlin court method). Specifically, a piezoelectric constant measuring device (manufactured by PIEZOTEST, piezometer system PM300) is used to hold the fluororesin piezoelectric 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 is applied is measured. 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 film is defined as the width direction, and any point on the midpoint line of the line segment connecting both ends in the width direction of the film is defined as point A. The piezoelectric constant is 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 is taken as the piezoelectric constant d of the fluororesin piezoelectric film. 33 33 Let it be so.

[0033] The fluororesin piezoelectric film has a surface height roughness Rz of 0.50 μm or less, preferably 0.05 μm or more and 0.50 μm or less, more preferably 0.05 μm or more and 0.40 μm or less, and even more 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 surface of the film, and excellent 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. Then, 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, the surface height roughness Rz is measured for 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 these is taken as the surface height roughness Rz of the fluororesin piezoelectric film. Note that the contact surface of the fluororesin piezoelectric film with the cooling roll tends to have a smaller surface roughness Rz compared to the non-contact surface with the roll. This is because the 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 for the surface with a smaller surface roughness Rz (the contact surface with the cooling roll) is taken as the above-mentioned surface height roughness Rz.

[0035] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm or more and 200 μm or less, more preferably 15 μm or more and 80 μm or less, still more preferably 20 μm or more and 80 μm or less, particularly preferably 30.0 μm or more and 80.0 μm or less, very preferably 35.0 μm or more and 70.0 μm or less, and most preferably 35.0 μm or more and 50.0 μm or less. The thicker the film, the more advantageous it is for electrical properties such as insulation and piezoelectric properties. The thinner the film, the more advantageous it is for optical properties such as transparency and in terms of cost.

[0036] The thickness of the fluororesin piezoelectric film is generally measured by a method using a micrometer (JIS C 2151:2019), but it can also be 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, the advancing axis direction obtained by measuring the birefringence is defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film is defined as point A. For a total of three measurement points, which are point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the thickness is measured, and the average value of these is taken as the thickness of the fluororesin piezoelectric film.

[0037] The haze of the fluororesin piezoelectric film is preferably 0.0% or more and 10.0% or less, more preferably 0.0% or more and 5.0% or less, and even more preferably 0.0% or more and 3.0% or less. The lower the haze, the higher the transparency of the fluororesin piezoelectric film.

[0038] The haze of the fluororesin piezoelectric film is measured in accordance with JIS K 7136:2000. Specifically, a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II) is used. Then, the advancing axis direction obtained by measuring the birefringence of the fluororesin piezoelectric film is defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film is defined as point A (measurement point). For a total of three measurement points, which are 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 measurement is performed, and the average value of these is taken as the haze of the fluororesin piezoelectric film.

[0039] 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.

[0040] Examples of resins other than the above fluororesin include polycarbonate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, silicone resin, polyether, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, which can be added to enhance flexibility; acrylic resin, epoxy resin, 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.

[0041] [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.

[0042] For example, the fluororesin film can be produced by performing a step of heating and melting the above-mentioned fluororesin (melting step), a step of filtering the melted fluororesin (filtering step), a step of forming the filtered fluororesin into a film (film-forming step), a step of stretching the formed film (stretching step), and a step of polarizing the formed film (polarization step).

[0043] (Melting step) In the melting step, it is preferable to heat-melt the fluororesin. This step can be carried out, for example, by melt-kneading the fluororesin with an extruder.

[0044] 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.

[0045] 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 temperature, more preferably 75 °C or more higher than the melting point of the fluororesin and 100 °C or lower higher temperature, still more preferably 80 °C or more higher than the melting point of the fluororesin and 100 °C or lower higher temperature, and particularly preferably 85 °C or more higher than the melting point of the fluororesin and 95 °C or lower higher temperature. By setting the melting temperature to 75 °C or more higher than the melting point of the fluororesin, the viscosity of the fluororesin can be reduced to such an extent that it can be filtered in the next step. By setting the melting temperature to 105 °C or lower higher temperature 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 and the like can be suppressed. By suppressing the generation of the decomposition products and the like, 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 decomposition products and the like, clogging of the filter due to these can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0046] 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 the resin to a high temperature. When the resin is 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 step, it is possible to reduce the burden on the working environment and the natural environment, and to reduce the manufacturing cost for recovering the polar solvent.

[0047] (Filtering step) In the filtering step, the fluororesin melted and made to have a low viscosity in the melting step is filtered. The filtering method is not particularly limited, and it is sufficient to pass the melted fluororesin through a filter, and known filter types such as a pleated type and a leaf disk type filter can be used.

[0048] In the filtering step, it is preferable to filter the fluororesin with a filter having a filtration accuracy of 10 μm or more and 40 μm or less. By using a filter having a filtration accuracy of 10 μm or more, the filtration of the fluororesin melted by heating to the above temperature becomes easy, and the filtration pressure does not become too high, and the filtration time can be shortened. By using a filter having 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. The filtration accuracy of the filter is more preferably 10 μm or more and 30 μm or less, and even more preferably 15 μm or more and 30 μm or less.

[0049] Note that 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 ability of the filter to 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.

[0050] In this step, the filtration of the fluororesin may be performed multiple times. For example, coarse foreign matter may be removed using a filter with a low filtration accuracy (a large numerical value of filtration accuracy) arranged in the previous stage, and then finer foreign matter may be removed using a filter with 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.

[0051] The filter may be arranged between the extruder and the die for performing the film-forming step. Alternatively, the filter may be arranged in an extruder or a melt-kneading device different from the extruder for performing the film-forming step, and the fluororesin filtered by the filter may be introduced into the extruder used in the melting step and formed into a film.

[0052] (Film-forming step) In the film-forming step, the fluororesin filtered in the filtration step is formed into a film.

[0053] The film-forming method is not particularly limited, and known methods such as extruding the melted and filtered fluororesin from a T-die and cooling it by contacting a cooling roll can be used.

[0054] The fluororesin film thus obtained may be wound up once and stored, or may be directly conveyed to the stretching step.

[0055] (Stretching step) In the stretching process, the above-mentioned fluororesin film formed is uniaxially stretched. In the stretching process, the fluororesin film formed in the film-forming process may be stretched as it is, or the fluororesin film once wound up and stored may be heated or the like as necessary and then stretched.

[0056] For stretching, as long as it is uniaxial stretching, the stretching direction is not limited. In the stretching process for mass production, it is preferable to continuously uniaxially stretch the fluororesin film. Specifically, while 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, and more preferably 3.5 times or more and 5.0 times or less. By setting the stretching ratio to 2.5 times or more, the ratio of β-crystals described later is sufficiently increased, and it becomes easier to obtain a fluororesin piezoelectric film with a high piezoelectric constant by the poling process. By setting the stretching ratio to 6.0 times or less, breakage of the fluororesin film in the stretching process is less likely to occur. In the film uniaxially stretched in the MD direction, the MD direction is the slow axis direction, and the direction perpendicular to the stretching direction (TD direction) is the fast axis direction.

[0057] (Poling process) In the poling 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 ratio of β-crystals increases. By applying a DC voltage to a fluororesin film with an increased ratio of polar β-crystals, a fluororesin piezoelectric film having a high piezoelectric constant can be obtained.

[0058] The DC voltage to be applied is preferably 7.0 kV or more and 50.0 kV or less, and more preferably 7.5 kV or more and 30.0 kV or less.

[0059] The stretching process and the poling process may be performed simultaneously, or the poling process may be performed after the stretching process.

[0060] The fluororesin piezoelectric film after the polarization process can be wound into a roll for storage, transportation, etc.

[0061] In this way, the fluororesin piezoelectric film according to this embodiment can be obtained.

[0062] [Applications] The above-mentioned fluororesin piezoelectric film can be used in various applications such as touch sensors, touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0063] [Multilayer Piezoelectric Body] By laminating the fluororesin piezoelectric film (hereinafter also simply referred to as "piezoelectric film") with other functional layers or films, a multilayer piezoelectric body can be formed. 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 the electrical signal generated by the piezoelectricity of the piezoelectric film. In addition, a moisture-proof layer can be included 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. When the electrode layer is a film with excellent 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., there is a risk that the metal will be oxidized and the conductivity will decrease. Therefore, 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.

[0064] That is, the laminated piezoelectric body includes at least the piezoelectric film. The laminated piezoelectric body can further include one or more of an antistatic layer, a hard coat layer, an optical adjustment layer, a moisture-proof layer, an electrode layer, and an overcoat layer on at least one surface of the piezoelectric film. These layers can be arbitrarily combined. For example, the laminated piezoelectric body can include an electrode layer on at least one surface of the piezoelectric film, and may further include an antistatic layer or an overcoat layer.

[0065] Hereinafter, the laminated piezoelectric body and its manufacturing method according to an embodiment of the present invention will be specifically described. However, the laminated piezoelectric body is not limited to this embodiment.

[0066] FIG. 1 is a schematic cross-sectional view showing the laminated piezoelectric body of this embodiment. As shown in FIG. 1, the laminated piezoelectric body 10 can have a piezoelectric film 11, an antistatic layer 12, a hard coat layer 13, an optical adjustment layer 14, and an electrode layer 15 in this order. Hereinafter, each layer will be described.

[0067] (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.

[0068] (Antistatic layer) The antistatic layer can be disposed on at least one surface of the piezoelectric film. In this embodiment, the antistatic layer is preferably in contact with the piezoelectric film. The antistatic layer can make it difficult for the laminated piezoelectric body to generate static electricity.

[0069] 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.

[0070] The conductive material may be an ion-conductive conductive material or an electron-conductive conductive material.

[0071] Examples of the ion-conductive conductive material include (a) cationic antistatic agents having a cationic group such as a quaternary ammonium salt, a pyridinium salt, and a primary to tertiary amino group, (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 antistatic agents, and (d) nonionic antistatic agents such as amino alcohol-based, glycerin-based, and polyethylene glycol-based antistatic agents.

[0072] Examples of the electron-conductive conductive material include conductive polymers and other conductive materials. Examples of the conductive polymer include polyacetylene or its derivatives, polythiophene or its derivatives, polypyrrole or its derivatives, polyaniline or its derivatives, etc. Among them, polythiophene or its derivatives are preferable from the viewpoint 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, etc.

[0073] Among these, from the viewpoint 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.

[0074] Examples of the curable resin include acrylic resins, epoxy resins, polyurethane resins, polyimide resins, melamine resins, polyester resins, polyvinyl alcohol resins, polystyrene resins, polyvinyl acetate resins, silicone compounds, polyethylene glycol, etc., which have functional groups such as a hydroxyl group, a methylol group, a carboxyl group, a sulfonyl group, an epoxy group, and an amino group.

[0075] The crosslinking agent may be any one that reacts with the functional groups of the curable resin or the conductive polymer to cause 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.

[0076] As described above, as the curable resin and the crosslinking agent for obtaining the antistatic layer, for example, a curable resin and a 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, the 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 an amine and further suppress the discoloration of the piezoelectric film in the laminated piezoelectric body.

[0077] The surface resistivity of the antistatic layer is, for example, 1.0×10 4 Ω / sq. or more and 1.0×10 9 Ω / sq. or less is preferable, and 1.0×10 5 Ω / sq. or more and 1.0×10 8 / sq. or less is more preferable. When the surface resistivity of the antistatic layer is 1.0×10 9 Ω / sq. or less, the surface resistivity of the laminated piezoelectric body can be made lower, and sufficient antistatic properties can be imparted. In order to prevent damage to the laminated piezoelectric body, it may be handled with a hard coat layer formed on the antistatic layer, and the surface resistivity measured from above the hard coat layer is 1.0×10 6 Ω / sq. or more and 1.0×10 12 Ω / sq. or less, sufficient antistatic properties are easily obtained. It is more preferable that it is 1.0×10 8 Ω / sq. or more and 5.0×10 11 Ω / sq. or less. The surface resistivity of the antistatic layer can be measured, 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)).

[0078] The thickness of the antistatic layer is not particularly limited, but is preferably, for example, 0.010 μm or more and 0.40 μm or less. When the thickness of the antistatic layer is 0.010 μm or more, the generation of static electricity in the laminated piezoelectric body can be further suppressed. When the thickness of the antistatic layer is 0.40 μm or less, discoloration of the piezoelectric film in the laminated piezoelectric body can be 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.

[0079] 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 arithmetic mean value thereof can be used as the measurement result. The thicknesses of the following layers can also be measured in the same manner.

[0080] (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.

[0081] The thickness of the hard coat layer is not particularly limited, but is 0.05 μm or more, preferably 0.3 μm or more and 3.0 μm or less, more preferably 0.5 μm or more and 2.0 μm or less, and even more preferably 0.5 μm or more and 1.5 μm or less.

[0082] 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 further suppressing interference fringes.

[0083] The difference in refractive index between the hard coat layer and the piezoelectric film is preferably 0.01 or more and 0.20 or less. When the difference in refractive index is 0.01 or more, the transmittance can be further increased, and when it is 0.20 or less, interference fringes can be further suppressed.

[0084] The hard coat layer only needs to be composed of a material that satisfies the above refractive index, and is usually a resin layer. The resin layer may be obtained by applying a coating liquid containing a resin and then drying it, or by applying a curable composition containing a polymerizable compound and then drying and curing it.

[0085] The polymerizable compound may be any of a monomer, an oligomer, or a polymer. The polymerizable compound may be a thermosetting compound or an ionizing radiation curable compound, but preferably an ionizing radiation curable compound. The ionizing radiation may usually be ultraviolet rays (UV) or electron beams (EB).

[0086] 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.

[0087] 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 (meth)acrylate monomers having three or more functional groups 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.

[0088] 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 can be obtained, for example, by the reaction of a polyhydric alcohol, an organic diisocyanate, and hydroxy(meth)acrylate.

[0089] 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.

[0090] The curable composition may further contain other components other than those described 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.

[0091] 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.

[0092] Note that, as described later, the laminated piezoelectric body may further include an optical adjustment layer between the antistatic layer and the electrode layer. The total thickness of the hard coat layer and the optical adjustment layer is preferably 0.30 μm or more and 4.0 μm or less. When the total thickness is 4.0 μm or less, 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.

[0093] (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.

[0094] 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 light incident on the hard coat layer and reflected therefrom and light reflected at the interface between the optical adjustment layer and the hard coat layer.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] (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.

[0101] The electrode layer may be disposed on at least one side of the piezoelectric film. The form of the electrode layer is not limited and may be a nanowire, a mesh, or a thin film. The thin film may be a single layer or a laminate of multiple layers.

[0102] 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 (ITO), antimony tin 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.

[0103] Note that silver nanowires have attracted attention as conductive materials suitable for weight reduction, low resistance, high transparency, and high flexibility of transparent conductive materials, along with the increase in the area and functionality of devices. In particular, silver nanowires can form a flexible electrode layer by a wet process and are expected to be used in various applications. However, since silver nanowires are made of metallic silver, their conductivity tends to decrease under high humidity and high temperature conditions, whether under sunlight or artificial light. Therefore, it is preferable to apply, dry, and cure an overcoat agent such as a UV curable resin on the silver nanowire coating film to form a protective layer (overcoat layer). Thereby, an electrode layer with scratch resistance and durability can be formed on the substrate, and long-term stability of the surface electrical resistance value under sunlight can be realized.

[0104] The lower the surface resistivity of the electrode layer, the easier it is to detect the minute signals of the piezoelectric film. On the other hand, to lower the surface resistivity of the electrode layer, it is necessary to increase the thickness of the electrode layer or the addition amount of the conductive material forming the electrode layer, which tends to reduce transparency. Therefore, the surface resistivity of the electrode layer is 1.0×10 -1 Ω / sq. or more and 1.0×10 4 Ω / sq. or less, preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 3 Ω / sq. or less, more preferably 1.0×10 -1 Ω / sq. or more and 1.0×10 2 Ω / sq. or less, and particularly preferably 1.0 Ω / sq. or more and 1.0×10 2 Ω / sq. or less.

[0105] 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.

[0106] The thickness of the electrode layer is not limited, but 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, particularly preferably 20 nm or more and 45 nm or less, and very preferably 20 nm or more. The thickness of the electrode layer can be determined by a known method obtained from the observation of the cross - section of such a laminate. The thickness of the electrode layer is determined by observing the cross - section of the laminated piezoelectric body using a scanning electron microscope (「SU3800」, manufactured by Hitachi High - Technologies Corporation) under the conditions of an acceleration voltage of 3.0 kV and a magnification of 50,000 times, measuring the thickness of the electrode layer 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 of the laminated piezoelectric body.

[0107] In addition, 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. Further, 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 refers to the surface resistivity measured on the overcoat layer.

[0108] Also, in the above - described embodiment, the electrode layer is formed on the optical adjustment layer, but a transparent electrode film formed by forming an electrode layer on the surface of a substrate such as glass or a polymer material may be adhered to the optical adjustment layer.

[0109] (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 of polyethylene terephthalate (PET), cycloolefin polymer (COP), polycarbonate (PC), etc., or a plurality of polymer materials, or a substrate containing an inorganic material such as glass, with the above-described electrode layer laminated thereon.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] (Moisture-proof layer) Also, 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, a moisture-proof layer can be included. 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.

[0114] 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.

[0115] The thickness of the moisture-proof layer is not particularly limited. From the perspective of achieving a higher level of both 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.

[0116] [Physical Properties of the Multilayer Piezoelectric Body] (Piezoelectric Constant d 33 ) The piezoelectric constant d of the multilayer piezoelectric body 33 is preferably, for example, 7.0 pC / N or more and 40.0 pC / N or less. When the piezoelectric constant d of the multilayer piezoelectric body 33 is 7.0 pC / N or more, higher pressure sensitivity is more easily obtained. When the piezoelectric constant d of the multilayer piezoelectric body 33 is 40.0 pC / N or less, the appearance defects as described above can be more reduced. From the same perspective, the piezoelectric constant d of the multilayer piezoelectric body 33 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 of the multilayer piezoelectric body 33 can be measured in the same manner as described above, except that the measurement position is one point at the intersection of the diagonal lines of the multilayer piezoelectric body.

[0117] The piezoelectric constant d of the multilayer piezoelectric body 33 can be adjusted, for example, by the piezoelectric constant d 33 of the piezoelectric film, which is one of the components of the multilayer piezoelectric body. The piezoelectric constant d of the piezoelectric film33 is high, the piezoelectric constant d of the laminated piezoelectric body 33 also tends to be high.

[0118] (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.

[0119] 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.

[0120] 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.

[0121] (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.

[0122] The b * value of the laminated piezoelectric body can be measured using a spectrophotometer (for example, SD7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with the method conforming to JIS Z 8722. The b* The value shall be measured at a position including the intersection of the diagonal lines of the laminated piezoelectric body. The b of the laminated piezoelectric body after storage * The value shall be obtained by 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 of the laminated piezoelectric body taken out from the thermo-hygrostat * value in the same manner as the above method.

[0123] The b of the laminated piezoelectric body * value and Δb * can be adjusted by 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.

[0124] [Manufacturing method of 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. Also, when the electrode layer includes metal nanowires or a metal mesh as a 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 (6) on the electrode layer and a step of forming a moisture-proof layer (7) may be further performed. Alternatively, instead of the step (5), after the step (2) or the step (3), a step of forming a transparent adhesive (OCA) layer on the (6) hard coat layer or on the optical adjustment layer, and (8) an electrode film can be bonded on the OCA layer.

[0125] (Step of preparing a piezoelectric film) As the piezoelectric film containing a fluororesin, the above-mentioned fluororesin piezoelectric film can be used.

[0126] (Step of forming an antistatic layer) After applying the above-mentioned curable composition for the antistatic layer on the obtained piezoelectric film, it is dried and cured to form an antistatic layer. From the viewpoint of enhancing low-temperature curability, the curable composition for the antistatic layer is preferably a curable composition containing a conductive material and an amine-based material, and more preferably a curable composition containing a conductive material and a melamine resin.

[0127] The above-mentioned curable composition may further contain water or a solvent. Examples of the solvent include alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol.

[0128] The coating method of the above-mentioned 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.

[0129] The drying of the above-mentioned 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.

[0130] (Step of forming a hard coat layer) In this embodiment, after applying the above-mentioned curable composition for the hard coat layer on the piezoelectric film or the antistatic layer, it is dried and cured to form a hard coat layer.

[0131] The above 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 solvents, ketone solvents, ester solvents, carbonate solvents, and aromatic solvents.

[0132] The application of the above curable composition can be carried out by a known wet process method. As the wet process method, for example, dip coating, spray coating, spin coating, gravure coating, die coating, roll coating, flow coating, curtain coating, etc. are cited as typical methods. Among them, methods capable of continuously forming a layer, such as the roll coating method and the gravure coating method, are more preferable from the viewpoint of productivity.

[0133] The application and drying method of the above 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.

[0134] The curing of the above curable composition may be by heat curing or by curing with ionizing radiation. Curing with ionizing radiation can be carried out by irradiating ultraviolet rays or electron beams. Also, heat curing and curing with ionizing radiation may be used in combination.

[0135] (Step of forming an optical adjustment layer) Similar to the step of forming the hard coat layer, after applying the above curable composition on 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.

[0136] (Step of forming an 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.

[0137] [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

[0138] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0139] 1. Preparation of Fluorine-based Resin Piezoelectric Films Films 1 to 10 and Film 13, all of which are fluorine-based resin piezoelectric films, were prepared according to the following procedure.

[0140] The melt viscosity of the fluorine-based resin used as the material was measured according to ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho Co., Ltd., Capilograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, the measurement temperature was 260 °C, and the shear rate was 50 s -1 The measured viscosity was used.

[0141] The melting point of the fluorine-based resin was obtained by enclosing 5 mg of the measurement sample in an aluminum pan and placing it in a differential scanning calorimeter (manufactured by Shimadzu Corporation, DSC-60A). Under a nitrogen atmosphere, the temperature was raised from room temperature to 230 °C at a heating rate of 10 °C / min, and the maximum peak temperature of the endothermic curve in the obtained DSC curve was used.

[0142] The melting temperature was set as the maximum temperature in the conduit from the extruder to the filtration device.

[0143] 1-1. Piezoelectric Film 1 A polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 2500 Pa·s and a melting point of 173°C was melted using a single-screw extruder with a diameter of φ50 mm. The molten resin was filtered through a pleated polymer filter with a filtration accuracy of 20 μm, extruded from a T-die, and brought into contact with a cooling roll for cooling to obtain an unstretched film with a thickness of 160 μm. At this time, both the melting temperature and the filtration temperature were 260°C.

[0144] 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 4.0 times in the MD direction. Further, a voltage of 11.6 kV was applied from the surface of the film in the thickness direction to obtain a piezoelectric film 1 with a thickness of 39 μm.

[0145] 1-2. Piezoelectric Film 2 A piezoelectric film 2 was obtained in the same manner as the production of film 1, except that a pleated polymer filter with a filtration accuracy of 40 μm was used.

[0146] 1-3. Piezoelectric Film 3 A piezoelectric film 3 was obtained in the same manner as the production of film 1, except that PVDF with a melt viscosity of 800 Pa·s and a melting point of 173°C was used.

[0147] 1-4. Piezoelectric Film 4 A piezoelectric film 4 was obtained in the same manner as the production of piezoelectric film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was set to 275°C.

[0148] 1-5. Piezoelectric Film 5 A piezoelectric film 5 was obtained in the same manner as the production of piezoelectric film 1, except that PVDF with a melt viscosity of 3400 Pa·s and a melting point of 173°C was used.

[0149] 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 PVDF with a melt viscosity of 800 Pa·s and a melting point of 173°C was used and the melting temperature was set to 250°C.

[0150] 1-7. Piezoelectric film 7 A film 7 was obtained in the same manner as the production of the piezoelectric film 1, except that a filter was not attached to the single-screw extruder.

[0151] 1-8. Piezoelectric film 8 A piezoelectric film 8 was obtained in the same manner as the production of the piezoelectric film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used and a filter was not attached to the single-screw extruder.

[0152] 1-9. Piezoelectric film 9 A piezoelectric film 9 was obtained in the same manner as the production of the piezoelectric film 1, except that a pleated polymer filter with a filtration accuracy of 60 μm was used.

[0153] 1-10. Piezoelectric film 10 A piezoelectric film 10 was obtained in the same manner as the production of the film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was set to 280°C. The piezoelectric film 10 had a large surface height roughness Rz, and it was difficult to accurately detect the amount of foreign matter.

[0154] 1-11. Piezoelectric film 11 (could not be produced) An attempt was made to produce a piezoelectric film 11 in the same manner as the production of the film 1, 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.

[0155] 1-12. Piezoelectric film 12 (could not be produced) A piezoelectric film 12 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.

[0156] 1 - 13. Piezoelectric film 13 100 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 it was dried at 120°C for 1 hour to obtain a piezoelectric film 13. The piezoelectric film 13 had a large surface height roughness Rz, and it was difficult to accurately detect the amount of foreign matter.

[0157] 2. Evaluation of fluororesin piezoelectric films Regarding the obtained films 1 to 10 and film 13, the number of foreign matters, retardation, piezoelectric constant d 33 , surface height roughness Rz, haze, and thickness were measured by the following methods.

[0158] 2 - 1. Number of foreign matters Since the thicknesses of the prototype piezoelectric films were all 40 μm or less, without performing processes such as stretching on the piezoelectric films, 25 rectangular films (observation pieces) with a size of 0.010 m 2 (100 mm × 100 mm) were cut out from the films so that each film was continuously cut out from adjacent positions. The sum of the number of foreign matters measured by observing each observation piece was determined as the number of foreign matters per 0.250 m 2 . The size of the foreign matter was taken as the arithmetic mean value of the maximum width and the minimum width of the foreign matter. 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 less than 200 μm, and the number of foreign matters with a size of less than 100 μm were determined respectively.

[0159] 2 - 2. Retardation Using the KOBRA-HB manufactured by Oji Measuring Instruments, the retardation of a film cut into 20 mm × 20 mm was measured by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8 nm was taken as the retardation of the film. At this time, the fast axis and the slow axis were determined from the in-plane birefringence of the film. The direction of the slow axis coincides with the average direction of the molecular chains oriented by stretching or extrusion. Since the piezoelectric film used this time has been stretched in the MD direction (machine direction) of the film, the MD direction and the slow axis direction coincide.

[0160] 2-3. 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 was 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. The fast axis direction obtained by measuring the birefringence of the film was defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film was defined as point A. For a total of three measurement points, namely point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the piezoelectric constant d 33 was measured, and the average value of the piezoelectric constants obtained therefrom was taken as the piezoelectric constant d 33 of the film.

[0161] 2-4. Surface height roughness Rz A surface roughness meter (Keyence Corporation, shape analysis laser microscope VK-X260) compliant with JIS B 0601:2001 was used. The direction of the fast axis obtained 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 defined 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 each 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.

[0162] 2-5. Haze A haze meter (Nippon Denshoku Industries Co., Ltd., NDH7700SP II) was used. In accordance with ISO 14782:2021, the direction of the fast axis obtained 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 defined as point A. Haze 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 haze of the film.

[0163] 2-6. Thickness of the piezoelectric film A digital linear gauge (Ohno Sokki Co., Ltd., DG525H) and a gauge stand (Ohno Sokki Co., Ltd., SH-022) were used. At this time, the direction of the fast axis obtained 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 defined as point A. Thickness 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 thickness of the film.

[0164] 3. Results The production conditions and evaluation results of each piezoelectric film are shown in Tables 1 and 2. Note that for Film 10, the unevenness on both sides was very large, and the number of foreign objects could not be measured. Also, for Film 13, the haze was large and it was opaque, and the number of foreign objects could not be measured.

[0165]

Table 1

[0166]

Table 2

[0167] 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 paint P-400MP-A (manufactured by Nagase ChemteX Corporation) containing PEDOT:PSS as a conductive polymer and paint P-400MP-B (manufactured by Nagase ChemteX Corporation) containing a crosslinking agent and a conductivity improver were mixed at a ratio of 4:1 was applied with the multi coater (manufactured by Hirano Texseed Co., Ltd.), and heat-treated at 130 °C for 0.67 minutes to form an antistatic layer with a thickness of 80 nm. The surface resistivity of the antistatic layer was 1.2×10 8 Ω / sq.

[0168] 4-1-2. Antistatic layer 2 A solution (C-169PF) in which 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 were mixed at a ratio of 3:2 was applied with the multi coater (manufactured by Hirano Texseed 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.

[0169] 4-2. Formation of the hard coat layer A hard coat agent (BS-CH271 manufactured by Arakawa Chemical Industries, Ltd., with an average particle diameter of amorphous silica of 60 nm) was applied with a multi-coater, heat-treated at 80 °C for 2 minutes, and further irradiated with UV having an integrated light amount of 200 mJ / cm 2 to form a hard coat layer with a thickness of 700 nm and a refractive index of 1.50 by photocuring.

[0170] 4-3. Optical adjustment layer An ultraviolet curable composition containing zirconium oxide particles (Opster 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).

[0171] 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 the piezoelectric film on which the predetermined layer was formed, dehydration and degassing were performed, and the exhaust was carried out until the degree of vacuum reached 7×10 -5 Pa. Subsequently, the substrate temperature was set to room temperature (25 °C), a mixed gas with a composition of 99.2 mol% of argon gas and 0.8 mol% of oxygen gas was introduced into the chamber, and 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 to obtain a laminated piezoelectric body.

[0172] 4-4-2. Transparent electrode layer 2 A solution obtained by appropriately diluting 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 on the transparent electrode layer, dried with hot air at 40°C for 60 seconds, and then irradiated with ultraviolet rays at an integrated light quantity 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.

[0173] 4-4-3. Transparent Electrode Film A transparent electrode precursor film (manufactured by Oike Kogyo Co., Ltd., Tetratec 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 (manufactured by 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.

[0174] 4-5. Multilayer Piezoelectric Body 4-5-1. Multilayer 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 produce the multilayer piezoelectric body 1.

[0175] 4-5-2. Multilayer Piezoelectric Body 2 A transparent electrode layer 1 was formed on the hard coat layer of the multilayer piezoelectric body 1 to produce the multilayer piezoelectric body 2.

[0176] 4-5-3. Multilayer Piezoelectric Body 3 A transparent electrode film was laminated on the hard coat layer of the multilayer piezoelectric body 1 to fabricate a multilayer piezoelectric body 3.

[0177] 4-5-4. Multilayer piezoelectric body 4 A multilayer piezoelectric body 4 was fabricated in the same manner as the multilayer piezoelectric body 2, except that the transparent electrode layer 2 was used instead of the transparent electrode layer 1.

[0178] 4-5-5. Multilayer piezoelectric body 5 A multilayer piezoelectric body 5 was fabricated in the same manner as the multilayer piezoelectric body 4, except that the antistatic layer 1 was changed to the antistatic layer 2.

[0179] 4-5-6. Multilayer piezoelectric body 6 A multilayer piezoelectric body 6 was fabricated in the same manner as the multilayer piezoelectric body 2, except that an optical adjustment layer was formed between the hard coat layer and the transparent electrode layer 1.

[0180] 4-5-7. Multilayer 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 laminated on the antistatic layer 1 to fabricate a multilayer piezoelectric body 7.

[0181] 4-5-8. Multilayer piezoelectric body 8 The antistatic layer 1 was formed on the A surface of the piezoelectric film 1, and then the transparent electrode layer 1 was formed on the antistatic layer 1 to fabricate a multilayer piezoelectric body 8.

[0182] 4-5-9. Multilayer piezoelectric body 9 A multilayer piezoelectric body 9 was fabricated in the same manner as the multilayer piezoelectric body 8, except that the transparent electrode layer 1 was changed to the transparent electrode layer 2.

[0183] 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, and the transparent electrode layer 1 was formed on the optical adjustment layer to fabricate a multilayer piezoelectric body 10.

[0184] 4-5-11. Multilayer piezoelectric body 11 A laminated piezoelectric body 11 was fabricated in the same manner as the laminated 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.

[0185] 4-5-12. Laminated piezoelectric body 12 A laminated piezoelectric body 12 was fabricated in the same manner as the laminated piezoelectric body 11, except that the transparent electrode layer 1 was replaced with the transparent electrode layer 2.

[0186] 4-5-13. Laminated 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, and the transparent electrode layer 1 was formed on the optical adjustment layer 1 to fabricate the laminated piezoelectric body 13.

[0187] 4-5-14. Laminated piezoelectric body 14 A laminated piezoelectric body 14 was fabricated in the same manner as the laminated piezoelectric body 11, except that the hard coat layer was replaced with the optical adjustment layer.

[0188] 4-5-15. Laminated 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 developed by Sun Ace Kaken 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 to fabricate the laminated piezoelectric body 15.

[0189] 4-5-16. Laminated 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 developed by Sun Ace Kaken 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 transparent electrode layer 2 were formed on the antistatic layer 1 on the A surface to fabricate the laminated piezoelectric body 16.

[0190] 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 by the following method.

[0191] 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 (2-3).

[0192] 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 (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7000SP II), 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.

[0193] 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.

[0194] 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. Also, when a protective layer was formed on the electrode, the surface resistivity was measured from above the protective layer.

[0195] 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 used as the representative value.

[0196] 5-5. Color difference 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 used as the representative value of the hue b * Value.

[0197] 5-6. Thickness The thickness of the antistatic layer was measured using a spectroscopic ellipsometry 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. 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 used as the representative value of the thickness of the transparent electrode.

[0198] 6. Results The production conditions and evaluation results of each laminated piezoelectric body are shown in Tables 3 and 4.

[0199]

Table 3

[0200]

Table 4

Industrial Applicability

[0201] The fluororesin piezoelectric film according to the present invention has a small number of foreign substances and a high piezoelectric constant.

Explanation of Signs

[0202] 10 Laminated piezoelectric body 11 Piezoelectric film 12 Antistatic layer 13 Hard coat layer 14 Optical adjustment layer 15 Electrode layer

Claims

1. Measuring 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 number of foreign matters having a size, which is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view, of 100 μm or more is 7 pieces / 0.25 m ２ or less, the retardation is 100 nm or more and 2000 nm or less, and the piezoelectric constant d ３３ is 5.0 pC / N or more and 40.0 pC / N or less A fluororesin piezoelectric film.

2. The Rz of the surface on the side where the surface height roughness Rz measured in accordance with JIS B 0601:2001 is small is 0.50 μm or less, The fluororesin piezoelectric film according to Claim 1.

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

4. Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A step of heating and melting a fluororesin having a melt viscosity η measured at －１ of 600 Pa·s or more and 4000 Pa·s or less at a temperature 75°C or more higher than the melting point of the fluororesin and at a temperature not higher than 105°C higher; A step of filtering the molten fluororesin with a filter having a filtration accuracy of 10 μm or more and 40 μm or less; A step of forming the filtered fluororesin into a film; A step of stretching the formed film; A step of polarizing the formed film; The method for producing a fluororesin piezoelectric film according to any one of Claims 1 to 3, which has these steps.

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

6. 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. The laminated piezoelectric body according to Claim 5.

7. The electrode layer includes at least one selected from the group consisting of a metal film, a metal oxide film, a metal nanowire, a metal mesh, a conductive polymer compound, a carbon nanotube, and graphene, The laminated piezoelectric body according to Claim 6.

8. Further including a hard coat layer disposed on at least one surface of the fluororesin piezoelectric film, The laminated piezoelectric body according to Claim 5 or 6.

9. 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 laminated piezoelectric body according to Claim 5 or 6.

10. 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 ６ Ω / sq. or more and 1.0×10 １２ Ω / sq. or less, The laminated piezoelectric body according to Claim 5 or 6.

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