Laminated piezoelectric film

Abstract

Provided is a laminated piezoelectric film having high rigidity and excellent thermal stability and smoothness. A laminated piezoelectric film provided with a piezoelectric film and a protective film laminated on one surface of the piezoelectric film, the rigidity of the protective film being 1.0-20 times the rigidity of the piezoelectric film, and the thickness of the protective film being 50-200 [mu] m.

Description

Technical Field

[0001] The present invention relates to a stacked piezoelectric film. Background Art

[0002] In recent years, touch sensors have been introduced into electronic devices such as smartphones and tablets as human-machine interfaces that enable intuitive operation. Touch sensors operate electronic devices by detecting a two-dimensional position touched by a finger or pen (for example, see Patent Document 1).

[0003] In addition, in recent years, a touch sensor that senses pressing force has been developed for the purpose of increasing input information and improving operability. For example, there are methods of detecting pressing force by changes in electrostatic capacitance when the shell is deformed, changes in resistance value using pressure-sensitive rubber, and methods of detecting changes in charge of piezoelectric materials. As a piezoelectric film of a touch panel that can also detect such pressing force (Z coordinate), a fluorine-based resin piezoelectric body with polyvinylidene fluoride and polyvinylidene fluoride-tetrafluoroethylene copolymer as the main component is known.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 5-324203 Summary of the invention

[0007] Problem that the invention aims to solve

[0008] However, the inventors have conducted research and found that in order to improve the piezoelectric properties of the fluororesin film, the film is sometimes stretched or thermally polarized. Therefore, the piezoelectric film subjected to the above treatment has a tendency to have reduced smoothness due to large undulations on the surface compared to non-piezoelectric films such as PET in its manufacturing method. In the manufacturing process of a stacked body using the piezoelectric film, there is a problem that defects may occur during the stacking process and the piezoelectric sensitivity of the stacked body may be reduced.

[0009] The present inventors have conducted intensive studies on the above-mentioned problems and have found that smoothness can be improved by stacking a protective film with high rigidity (hereinafter, the product of the tensile elastic modulus of the film and the thickness of the film is referred to as "rigidity") on the surface of the piezoelectric film.

[0010] In addition, in the stacked film formed by stacking the protective film on the piezoelectric film, it is required that: the surface of the piezoelectric film is smooth (smoothness); in the process of further stacking the conductive layer on the stacked film, heat treatment is sometimes performed, but the stacked piezoelectric film does not curl during these heat treatments (high thermal stability); and wrinkles are not generated due to stress during transportation, etc. (high rigidity).

[0011] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a laminated piezoelectric film having high rigidity and excellent thermal stability and smoothness.

[0012] Technical Solution

[0013] The inventors have found that the above-mentioned problems can be solved by forming a stacked piezoelectric film with a protective film, wherein the rigidity B of the protective film and the rigidity A of the piezoelectric film satisfy a specific range, and the thickness of the protective film satisfies a specific range, thereby completing the present invention. Specifically, the present invention relates to the following.

[0014] The present invention is a stacked piezoelectric film, which comprises a piezoelectric film and a protective film stacked on one surface of the piezoelectric film, wherein the rigidity B of the protective film is greater than 1.0 times and less than 20 times the rigidity A of the piezoelectric film, and the thickness of the protective film is greater than 50 μm and less than 200 μm.

[0015] Preferably, the thickness of the protective film is 1.5 times or more the thickness of the piezoelectric film.

[0016] Preferably, the rigidity B of the protective film is 3.0 times or more and 20 times or less of the rigidity A of the piezoelectric film.

[0017] Preferably, the surface roughness of the piezoelectric film on the side opposite to the side on which the protective film is stacked is 80 μm or less.

[0018] Preferably, the piezoelectric film contains polyvinylidene fluoride as a main component.

[0019] Effects of the Invention

[0020] According to the present invention, it is possible to provide a laminated piezoelectric film having high rigidity and excellent thermal stability and smoothness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 1 is a cross-sectional view schematically showing a stacked piezoelectric film 1 as one embodiment of the stacked piezoelectric film of the present invention.

[0022] Figure 2 FIG. 1 is a cross-sectional view schematically showing a stacked piezoelectric film 2 as another embodiment of the stacked piezoelectric film of the present invention. DETAILED DESCRIPTION

[0023] Hereinafter, an embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail with reference to the drawings, but the present invention is not limited thereto and various modifications can be made without departing from the gist of the invention.

[0024] In this specification, "laminated" means that each layer may be laminated in order, and other layers may be laminated between each layer.

[0025] 《Stacked piezoelectric film》

[0026] The stacked piezoelectric film of the present invention is a stacked piezoelectric film comprising a piezoelectric film and a protective film stacked on one surface of the piezoelectric film, wherein the rigidity B of the protective film is greater than 1.0 times and less than 20 times the rigidity A of the piezoelectric film, and the thickness of the protective film is greater than 50 μm and less than 200 μm.

[0027] In particular, by making the rigidity B of the protective film 1.0 times or more and 20 times or less of the rigidity A of the piezoelectric film, it is easy to obtain a laminated piezoelectric film with excellent thermal stability and smoothness. It is speculated that the rigidity of the film indicates the degree of resistance to deformation, and by making the rigidity B of the protective film and the rigidity A of the piezoelectric film within a specific range, the protective film smoothes the undulations on the surface of the piezoelectric film, thereby reducing the unevenness of the surface of the piezoelectric film, and further suppresses the curling of the laminated piezoelectric film caused by processing heat.

[0028] <Laminated piezoelectric film, protective film, and characteristics of piezoelectric film>

[0029] In the laminated piezoelectric film, a rigidity B of the protective film is 1.0 times or more and 20 times or less of a rigidity A of the piezoelectric film, and a thickness of the protective film is 50 μm or more and 200 μm or less.

[0030] If the rigidity of the protective film is less than 1.0 times the rigidity of the piezoelectric film, it is difficult to suppress the undulations on the surface of the piezoelectric film by the rigidity of the protective film, and the concavity and convexity of the surface of the piezoelectric film becomes high, so it is not preferred. In addition, from the viewpoint of production processes such as conveying / winding, the rigidity of the protective film can also be less than 10 times the rigidity of the piezoelectric film. The rigidity of the protective film is preferably more than 1.5 times and less than 20 times the rigidity of the piezoelectric film, more preferably more than 2.5 times and less than 20 times, further preferably more than 3.0 times and less than 20 times, particularly preferably more than 5.0 times and less than 15 times, and most preferably more than 5.0 times and less than 10 times. If it is more than 1.5 times, it is easy to obtain excellent thermal stability and smoothness.

[0031] The tensile modulus of the film sometimes varies depending on the measurement direction, but in this specification, the tensile modulus of the protective film, the piezoelectric film, and the stacked piezoelectric film refers to the minimum value thereof. In addition, the tensile modulus is a value measured in accordance with JIS K7127 and calculated based on 10.3 of JIS K 7161-1, and can be specifically measured by the method described in the examples described later.

[0032] The curling height of the stacked piezoelectric film caused by heating becomes a measure of the thermal stability of the film. The highest height of the film reached by bending the film from the film grounding position is called the curling height, and the smaller the curling height, the higher the thermal stability. The curling height is preferably less than 20 mm, more preferably less than 15 mm, and further preferably less than 10 mm.

[0033] In this specification, the curl height of the laminated piezoelectric film can be measured by the method described in the examples described later. It should be noted that a large curling degree results in the test piece becoming cylindrical, and in this case, the curl height is considered to exceed 20 mm.

[0034] The curl height of the stacked piezoelectric film can be appropriately adjusted by, for example, considering the thickness and tensile elastic modulus of the protective film, the thickness and tensile elastic modulus of the piezoelectric film, the difference in linear expansion coefficient between the protective film and the piezoelectric film, and the like.

[0035] The thickness of the laminated piezoelectric film is preferably 50 μm or more and 300 μm or less, more preferably 100 μm or more and 300 μm or less, and further preferably 150 μm or more and 250 μm or less. If the thickness of the film is 50 μm or more, the rigidity is likely to be sufficient. In addition, from the perspective of production processes such as conveying / winding, the thickness of the film is preferably 300 μm or less.

[0036] The thickness of the protective film is preferably 50 μm to 200 μm, more preferably 70 μm to 200 μm, further preferably 90 μm to 150 μm, and particularly preferably 110 μm to 150 μm. If the thickness of the protective film is 50 μm or more, it is easier to achieve high rigidity and excellent thermal stability and smoothness.

[0037] The thickness of the piezoelectric film is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 200 μm or less, further preferably 30 μm or more and 120 μm or less, and particularly preferably 30 μm or more and 80 μm or less. If the thickness of the film is 10 μm or more, the strength is likely to be sufficient. In addition, if the thickness of the film is 200 μm or less, the transparency is likely to be sufficient and it is easy to be used for optical purposes.

[0038] The thickness of the protective film is preferably 1.5 times or more and 10 times or less of the thickness of the piezoelectric film, more preferably 2.0 times or more and 10 times or less. If it is 1.5 times or more, the rigidity of the protective film dominates the rigidity of the stacked piezoelectric film. The protective film is not subjected to the treatment for improving piezoelectricity as performed in the piezoelectric film. Therefore, the thermal stability and smoothness of the protective film are excellent, so the rigidity of the protective film dominates the rigidity of the stacked piezoelectric film, and thus the stacked piezoelectric film is easy to obtain excellent thermal stability and smoothness.

[0039] The tensile modulus of the protective film is preferably 1.0 GPa or more and 5.0 GPa or less, more preferably 2.0 GPa or more and 5.0 GPa or less, and further preferably 3.0 GPa or more and 5.0 GPa or less. If it is 1.0 GPa or more, the rigidity tends to be high, and excellent thermal stability and smoothness are easily obtained. In addition, from the viewpoint of production processes such as conveying / winding, the tensile modulus of the protective film may also be 5.0 GPa or less.

[0040] The tensile elastic modulus of the piezoelectric film is preferably 0.5 GPa to 3.0 GPa, more preferably 1.0 GPa to 3.0 GPa, further preferably 1.5 GPa to 3.0 GPa, and particularly preferably 1.5 GPa to 2.0 GPa. Within the above numerical range, piezoelectricity is likely to be sufficient.

[0041] The rigidity of the stacked piezoelectric film is preferably 100 N / mm or more and 1000 N / mm or less, more preferably 200 N / mm or more and 1000 N / mm or less, further preferably 300 N / mm or more and 1000 N / mm or less, and particularly preferably 400 N / mm or more and 1000 N / mm or less. If the rigidity of the stacked piezoelectric film is 100 N / mm or more, the film is likely to have excellent thermal stability and smoothness.

[0042] The rigidity of the protective film is preferably 100 N / mm or more and 1000 N / mm or less, more preferably 150 N / mm or more and 1000 N / mm or less, further preferably 300 N / mm or more and 1000 N / mm or less, and particularly preferably 400 N / mm or more and 1000 N / mm or less. If the rigidity of the protective film is 100 N / mm or more, the laminated piezoelectric film using the film is easy to obtain excellent thermal stability and smoothness.

[0043] The rigidity of the piezoelectric film is preferably 10 N / mm to 200 N / mm, more preferably 50 N / mm to 200 N / mm, and further preferably 50 N / mm to 150 N / mm. If the rigidity of the piezoelectric film is 10 N / mm or more, a laminated piezoelectric film using the film can easily obtain high piezoelectricity.

[0044] The surface roughness of the piezoelectric film on the side opposite to the side on which the protective film is laminated is preferably 80 μm or less, more preferably 60 μm or less, further preferably 40 μm or less, and particularly preferably 30 μm or less. If the surface roughness of the film surface is 100 μm or less, it is easy to obtain excellent smoothness, reduce the undesirable conditions in the lamination process using the film, or improve the piezoelectric sensitivity of the laminated body obtained by the process.

[0045] In this specification, the surface roughness can be measured by the method described in the examples described later. The surface roughness in this specification is not a value based on fine surface roughness, but a value based on surface undulations and wrinkles.

[0046] The surface roughness can be appropriately adjusted by, for example, adjusting the thickness and tensile elastic modulus of the protective film, the thickness and tensile elastic modulus of the piezoelectric film, and the like.

[0047] Next, each layer of the stacked piezoelectric film will be described with reference to the drawings.

[0048] Figure 1 1 is a cross-sectional view schematically showing a laminated piezoelectric film 1 as one embodiment of the laminated piezoelectric film. In the laminated piezoelectric film 1 , a protective film 31 is laminated on one surface of a piezoelectric film 11 .

[0049] Figure 2 1 is a cross-sectional view schematically showing a laminated piezoelectric film 2 as another embodiment of the laminated piezoelectric film. The laminated piezoelectric film 2 is different from the laminated piezoelectric film 1 in that an adhesive layer 21 is provided between the piezoelectric film 11 and the protective film 31 .

[0050] <Piezoelectric film>

[0051] The piezoelectric film 11 is a film (thin film) having piezoelectricity (a property of converting an applied force into a voltage or a property of converting an applied voltage into a force).

[0052] As the piezoelectric film 11, there can be listed polar polymer compounds that can exhibit piezoelectricity by orienting molecular dipoles by a polarization treatment generally referred to as a thermal polarization treatment, and stretched chiral polymer compounds that can exhibit piezoelectricity by applying a stretching treatment to a chiral polymer compound. As polar polymer compounds, there can be listed: fluorine-based resins; vinyl cyanide polymers; vinyl acetate polymers; odd-numbered nylons such as nylon 9 and nylon 11; polyureas, etc. As stretched chiral polymer compounds, for example, there can be listed: helical chiral polymer compounds such as polylactic acid; polyhydroxycarboxylates such as polyhydroxybutyrate; cellulose derivatives, etc. They can be used alone or in combination of two or more. Among them, fluorine-based resins are preferred from the viewpoint that the smoothness of the surface of the piezoelectric film is easily reduced and the smoothness improvement effect of the present invention is easily exerted.

[0053] If the piezoelectric film is a uniaxially stretched film, the surface smoothness is easily reduced, and the smoothness improvement effect of the present invention is easily exerted. In addition, if the piezoelectric film is a fluorine-based resin, if the polarization degree is large, the surface smoothness is easily reduced, and the smoothness improvement effect of the present invention is easily exerted.

[0054] As fluorine-based resins, there can be listed: polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers (for example, vinylidene fluoride / trifluoroethylene copolymers, vinylidene fluoride / trifluoroethylene / trifluorochloroethylene copolymers, hexafluoropropylene / vinylidene fluoride copolymers, perfluorovinyl ether / vinylidene fluoride copolymers, tetrafluoroethylene / vinylidene fluoride copolymers, hexafluoropropylene oxide / vinylidene fluoride copolymers, hexafluoropropylene oxide / tetrafluoroethylene / vinylidene fluoride copolymers, hexafluoropropylene / tetrafluoroethylene / vinylidene fluoride copolymers); tetrafluoroethylene polymers; trifluorochloroethylene polymers, etc. They can be used alone or in combination of two or more. Among them, from the perspective of the obtained piezoelectricity, weather resistance, heat resistance, etc., it is preferred to use polyvinylidene fluoride or vinylidene fluoride copolymers as the main component, and more preferably polyvinylidene fluoride as the main component. In this specification, when the mass of the components constituting a certain polymer compound is 50% by mass or more relative to the total mass of the polymer compound (resin) constituting the piezoelectric film, the polymer compound is referred to as the main component.

[0055] The piezoelectric film 11 may further contain commonly used additives (fillers, surfactants, etc.).

[0056] <Protective film>

[0057] As the protective film 31, as long as it has the above-mentioned characteristics, it is not particularly limited, and examples thereof include: polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); polyolefin resins such as polypropylene (PP) and polyethylene (PE); halogen-containing polymers such as polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF); acrylic polymers such as polymethyl methacrylate; styrene polymers such as polystyrene and styrene-methyl methacrylate copolymers, etc. Among them, from the viewpoint of obtaining the effect of the present invention more well, PET or PP is preferred, and PET is more preferred. In addition, these films are preferably biaxially stretched films.

[0058] <Adhesive layer>

[0059] The laminated piezoelectric film of the present invention may include an adhesive layer.

[0060] The laminated piezoelectric film 2 includes an adhesive layer 21 between the piezoelectric film 11 and the protective film 31. That is, the piezoelectric film 11 and the protective film 31 may be bonded together via the adhesive layer 21.

[0061] The adhesive layer is not particularly limited as long as it is easily peeled from the piezoelectric film together with the protective film, and for example, acrylic resins, natural rubber, synthetic rubber and other rubber-based resins can be used.

[0062] <Applications of laminated piezoelectric films>

[0063] The laminated piezoelectric film of the present invention is suitable for devices such as piezoelectric panels including electrostatic capacitance type, resistance film type touch panels, pressure sensors, actuators for tactile sensing devices, piezoelectric vibration power generation devices, and flat speakers after peeling off the protective film.

[0064] The device may further include a common display panel unit such as a liquid crystal display (LCD: Liquid Crystal Display) under the piezoelectric film.

[0065] The above-mentioned device is preferably used in smart phones, mobile information terminals, tablet PCs, notebook computers, medical equipment, car navigation systems, etc.

[0066] <Method for Manufacturing Laminated Piezoelectric Film>

[0067] The laminated piezoelectric film of the present embodiment can be produced by a method including (1) a step of producing a piezoelectric film and (2) a step of laminating a protective film.

[0068] (1) Manufacturing process of piezoelectric film

[0069] The method for producing the piezoelectric film is not particularly limited, and the piezoelectric film can be produced, for example, by the following method.

[0070] When manufacturing a piezoelectric film containing a fluorine resin, the piezoelectric film can be obtained by subjecting the film containing the fluorine resin to a polarization process. The film containing the fluorine resin can be a stretched film or an unstretched film. In this embodiment, from the viewpoint of exhibiting a high piezoelectric effect, it is preferred to stretch the film containing the fluorine resin and then polarize it.

[0071] The film containing fluorine resin can be manufactured by any method such as melt extrusion method and solution casting method. Among them, from the viewpoint of easily obtaining a piezoelectric film with a thickness above the prescribed value, the film containing fluorine resin is preferably manufactured by melt extrusion method. In the melt extrusion method, the fluorine resin and any additives can be heated and melted in the barrel of the extruder, and then extruded from the die head to obtain a film.

[0072] The obtained film has a structure in which α-type crystals (the main chain is a helical structure) and β-type crystals (the main chain is a planar zigzag structure) are mixed. β-type crystals have a large polarization structure. By stretching the film, α-type crystals can be made into β-type crystals. In order to make the fluorine-based resin into β-type crystals, the stretching process is preferably stretched as needed. The stretching direction can be the TD direction, or the MD direction, and more preferably the MD direction.

[0073] The stretching method is not particularly limited, and a known stretching method such as a tenter method and a roll method may be used.

[0074] The stretching ratio can be, for example, 3.0 times or more and 6.0 times or less. If the stretching ratio is 3.0 times or more, it is easy to adjust the thickness and polarizability of the film to a more appropriate range. If the stretching ratio is 3.0 times or more, the dislocation of the β-type crystal becomes more sufficient, not only is it easy to show higher piezoelectricity, but also the transparency can be further improved. If the stretching ratio is 6.0 times or less, the fracture caused by stretching can be further suppressed.

[0075] The obtained stretched film is subjected to polarization treatment. The polarization treatment can be performed, for example, by applying a DC voltage between a ground electrode and a needle-shaped electrode. The voltage can be adjusted according to the thickness of the stretched film, and can be, for example, 1 kV or more and 50 kV or less.

[0076] As described above, in this embodiment, a piezoelectric film can be obtained by subjecting a stretched film to a polarization treatment.

[0077] (2) Process of attaching protective film

[0078] The protective film may be a commercial product or a manufactured product. From the viewpoint of improving the smoothness of the surface of the piezoelectric film, the protective film is preferably a film with a high tensile elastic modulus. Specifically, protective films of the above-mentioned tensile elastic modulus and types can be used.

[0079] The method of laminating the protective film is not particularly limited, and for example, a method of laminating the protective film and the piezoelectric film via an adhesive layer using a laminator or the like can be cited. In the case where the protective film has self-adhesive properties, the protective film and the piezoelectric film can be laminated without an adhesive layer. In the case of an adhesive layer, the protective film formed with an adhesive layer can be laminated to the piezoelectric film, or the protective film can be laminated after an adhesive layer is formed on the piezoelectric film.

[0080] Example

[0081] The present invention will be further described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[0082] The properties of the laminated piezoelectric film of the present invention were measured by the following method. The results are shown in Table 1.

[0083] (Tensile elastic modulus)

[0084] The tensile modulus of each piezoelectric film, protective film, and laminated piezoelectric film was measured according to JIS K 7127. A test piece cut into 10 mm × 100 mm with the long side parallel to the longitudinal direction (MD) was used and measured at a tensile speed of 50 mm / min and a chuck spacing of 50 mm. The tensile modulus at this time was calculated based on 10.3 of JIS K 7161-1.

[0085] The tensile modulus was similarly calculated using a test piece cut so that the long side was parallel to another direction such as the transverse direction (TD). However, in any of the Examples and Comparative Examples, the tensile modulus in the longitudinal direction was the smallest.

[0086] (Curl Height)

[0087] A test piece obtained by cutting the laminated piezoelectric film into a 10 cm square was placed in an oven set at 50° C. for 1 minute for heat treatment. The curling amount of the four corners in the vertical direction of the heat-treated test piece was measured, and the maximum height was defined as the curling height (mm).

[0088] (Surface roughness)

[0089] The surface roughness (μm) of the laminated piezoelectric film was measured using a 3D shape measuring machine VR-5000 (manufactured by Keyence Corporation).

[0090] A test piece cut to the same size was placed on a 20 cm × 30 cm stainless steel plate (1 mm thick) with the surface of the piezoelectric film as the front surface. The center of the long side of the test piece was pressed with a magnet without applying tension, and the four corners were fixed with tape.

[0091] As pre-processing, a reference surface was set using a designated area (75 mm x 140 mm), and noise reduction was set to medium.

[0092] The specified area was photographed with a low-magnification camera for measurement. The measurement was carried out under the conditions of multi-line roughness mode, 11 roots, 100 skipped roots, horizontal and vertical areas, and no truncation, and the arithmetic average height Ra of a total of 22 roots was calculated.

[0093] The test piece was reset each time, and the measurement was performed three times in total. The arithmetic mean of the three times was calculated as the surface roughness.

[0094] [Examples 1 to 2 and Comparative Examples 1 to 4]

[0095] A resin film (thickness 120 μm) formed of polyvinylidene fluoride (manufactured by KUREHA Co., Ltd.) having a logarithmic viscosity of 1.1 dl / g was uniaxially stretched at a stretching ratio of 4.2 times. After stretching, a DC voltage was applied between a ground electrode and a needle electrode while increasing from 0 kV to 12.0 kV, thereby performing a polarization treatment to obtain a piezoelectric film. The polarized film was further heat-treated at 130°C for 1 minute to obtain a transparent piezoelectric film with a thickness of 40 μm. The tensile modulus of the piezoelectric film was 1812 MPa, and the product of the thickness and the tensile modulus was 72.5 N / mm.

[0096] Next, the laminated film having an adhesive layer formed on a protective film as described in Table 1 was bonded to the piezoelectric film using a laminator and then rolled into a roll to obtain a laminated piezoelectric film. The laminator was set to a line speed of 5 m / min and a laminating roll contact pressure of about 0.3 N.

[0097] [Table 1]

[0098]

[0099]

[0100] (The product of thickness and tensile modulus is called rigidity)

[0101] As shown in Table 1, in the examples, the product of the thickness and the tensile elastic modulus of the laminated piezoelectric film was high, the curl height was low, and the surface roughness was low. Therefore, it was confirmed that the present invention can obtain a laminated piezoelectric film having high rigidity and excellent thermal stability and smoothness.

[0102] Description of Reference Numerals

[0103] 1, 2: stacked piezoelectric films; 11: piezoelectric film; 21: adhesive layer; 31: protective film.

Claims

1. A laminated piezoelectric film comprising a piezoelectric film and a protective film laminated on one surface of the piezoelectric film, The rigidity of the protective film is 1.0 times or more and 20 times or less than the rigidity of the piezoelectric film. The protective film has a thickness of 50 μm or more and 200 μm or less.

2. The stacked piezoelectric film according to claim 1, wherein: The thickness of the protective film is 1.5 times or more of the thickness of the piezoelectric film.

3. The laminated piezoelectric film according to claim 1 or 2, wherein: The rigidity of the protective film is 3.0 times or more and 20 times or less of the rigidity of the piezoelectric film.

4. The laminated piezoelectric film according to claim 1 or 2, wherein: The surface roughness of the piezoelectric film on the side opposite to the side on which the protective film is stacked is 80 μm or less.

5. The laminated piezoelectric film according to claim 1 or 2, wherein: The piezoelectric film has polyvinylidene fluoride as a main component.

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