Multilayer piezoelectric sheet

Abstract

To provide a laminated piezoelectric sheet with small variations in an output voltage without impairing the piezoelectric properties of an electret film, and a piezoelectric sheet that can be manufactured continuously while improving the piezoelectric properties of the electret film.SOLUTION: In a laminated piezoelectric sheet, an electrode is laminated only on one side of an electret film, the adhesion strength between the electret film and the electrode is 1 N / cm or more, and the variation in output voltage is less than 0.36 V. Moreover, the electret film is a porous electret film.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a laminated piezoelectric sheet. [Background technology]

[0002] An electret is a material made of a polymeric material or an inorganic material that does not easily conduct electricity, which is subjected to thermal or electrical treatment to semi-permanently polarize part of the material. For example, porous electrets using porous resin films are known to exhibit excellent piezoelectric effects and are widely used in vibration power generation, sensor devices, and the like. In order to extract the charges generated by utilizing the charges held in the electret film to the outside, it is necessary to form electrode layers on both sides of the electret film. However, forming electrodes on both sides of the electret film causes the charges held on the surface of the electret film to disappear, resulting in a problem of a significant decrease in piezoelectric properties. To address this issue, it has been reported that high piezoelectric properties can be achieved by sealing the porous electret film and the electrodes without using an adhesive (Patent Document 1). Also reported is a method of improving adhesion by forming an adhesive layer on only a portion of one surface of an electret film and then forming an electrode layer on the adhesive layer (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-097107 [Patent Document 2] International Publication No. 2019 / 208580 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the laminated piezoelectric sheet described in Patent Document 1 does not have sufficient adhesion between the porous electret film and the electrodes, so in large sensor devices, the signal strength may vary greatly depending on the position. Also, the electret laminate described in Patent Document 2 may not be able to obtain sufficient piezoelectric properties because part of the electret film is deactivated by the adhesive layer.

[0005] Furthermore, mass production of electret films requires continuous production. However, as mentioned above, if the film is charged and then electrodes are formed on both sides, the charge retained on the surface of the electret film disappears, resulting in a significant decline in piezoelectric properties. On the other hand, if electrodes are formed on both sides of the film before charging, continuous methods such as roll-to-roll production can result in electric shock from the unwinding device to the winding device, and the electret film may not be able to retain a sufficient charge due to shielding by the electrodes.

[0006] Therefore, a first problem to be solved by the present invention is to provide a laminated piezoelectric sheet that has small variations in output voltage without impairing the piezoelectric properties of the electret film. A second object of the present invention is to provide a piezoelectric sheet that can be continuously produced while improving the piezoelectric properties of the electret film. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the inventors discovered that the above object can be achieved by a laminated piezoelectric sheet in which an electrode is laminated on only one side of an electret film and which has a certain level of adhesive strength with the electrode, and thus completed the present invention.

[0008] [1] A laminated piezoelectric sheet in which an electrode is laminated on only one side of an electret film, and the adhesive strength between the electret film and the electrode is 1 N / cm or more. [2] The laminated piezoelectric sheet according to [1] above, wherein the variation in output voltage measured by the following method is less than 0.36 V. <Output voltage variation> The laminated piezoelectric sheet was placed with the electrode facing down, and a 100μm thick polyester film with aluminum vapor deposition was placed on top of it with the vapor deposition side facing down. Both were sandwiched between 100μm thick polyester film clear files, and the laminated piezoelectric sheet and the vapor deposition side were each connected to an oscilloscope. A 40mm diameter, 2.7g ping-pong ball was dropped onto the laminate from a height of 40mm, and the maximum voltage of the pulse generated was measured with the oscilloscope. This procedure was repeated five times, and the standard deviation of the measurements was taken as the "output voltage variation." [3] The laminated piezoelectric sheet according to the above [1] or [2], wherein the electret film is a porous electret film. [4] The laminated piezoelectric sheet according to the above [3], wherein the porous electret film is composed mainly of a polyolefin resin. [5] The laminated piezoelectric sheet according to the above [4], wherein the polyolefin resin is a polypropylene resin having a β-crystal generating ability of 80% or more. [6] The laminated piezoelectric sheet according to any one of the above [3] to [5], wherein the porous electret film has a porosity of 5% or more and 60% or less. [7] The laminated piezoelectric sheet according to any one of the above [1] to [6], wherein the thickness of the electret film is 10 μm or more and 200 μm or less. [8] A wound body in which an electrode is laminated on only one side of the electret film, and the adhesive strength between the electret film and the electrode is 1 N / cm or more. [9] A piezoelectric sheet in which a first electrode, a first electret film, a second electret film, and a second electrode are laminated in this order, and the adhesive strength between the electret film and the electrodes is 1 N / cm or more.

[10] A method for producing a piezoelectric sheet, comprising: (A) a step of obtaining two laminated piezoelectric sheets each having an electrode laminated on one side of an electret film; (B) a step of positively charging the surface of one of the laminated piezoelectric sheets facing the electret film to obtain a first laminated piezoelectric sheet, and negatively charging the surface of the other laminated piezoelectric sheet facing the electret film to obtain a second laminated piezoelectric sheet; and (C) a step of stacking the first laminated piezoelectric sheet and the second laminated piezoelectric sheet so that the surfaces facing the electret films are in contact with each other.

[11] The method for producing a piezoelectric sheet according to the above

[10] , wherein in the step (A), the electret film and the electrode are laminated via an adhesive.

[12] The method for producing a piezoelectric sheet according to the above

[10] or

[11] , wherein in the step (C), the surfaces of the laminated piezoelectric sheets on the electret film side are laminated together via an adhesive. [Effects of the Invention]

[0009] The laminated piezoelectric sheet proposed by the present invention has good piezoelectric properties and good adhesion between the electret film and the electrodes, and therefore, the output voltage variation is small. Furthermore, in the laminated piezoelectric sheet proposed by the present invention, an electrode is formed on only one surface of the electret film, so that charging can be performed by a continuous method such as roll-to-roll. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following describes in detail the embodiments of the present invention, but the present invention is not limited to the embodiments described below.

[0011] [Laminated piezoelectric sheet] The laminated piezoelectric sheet of the present invention is a piezoelectric sheet formed by laminating at least two materials, an electret film and an electrode, and is characterized in that the adhesion strength between the electret film and the electrode is 1 N / cm or more. With an adhesion strength between the electret film and the electrode of 1 N / cm or more, for example, even in a large-sized sensor device, the signal strength does not vary depending on the position. From the above perspective, the adhesion strength between the electret film and the electrode is more preferably higher than 6 N / cm, and even more preferably 8 N / cm or more. On the other hand, there is no particular upper limit, but from the viewpoint of device disassembly, sorting, and recycling, it is preferably 80 N / cm or less. The characteristics of this laminated piezoelectric sheet will be described below.

[0012] (1) Output voltage The output voltage of the laminated piezoelectric sheet of the present invention is preferably 1 V or more and 100 V or less, more preferably 2 V or more and 50 V or less, and even more preferably 3 V or more and 10 V or less. When the output voltage is equal to or greater than the above-mentioned lower limit, sufficient sensitivity can be obtained when used as a sensor. On the other hand, when the output voltage is equal to or less than the above-mentioned upper limit, the risk of spark discharge when incorporated into a sensor or actuator can be reduced. The output voltage of the laminated piezoelectric sheet of the present invention is measured by the following method. <How to measure output voltage> The laminated piezoelectric sheet is placed with the electrode facing downwards, and a 100μm thick polyester film with aluminum vapor-deposited on top is placed with the vapor-deposited side facing downwards. Both are sandwiched between 100μm thick polyester film clear files, and both the laminated piezoelectric sheet and the vapor-deposited side are connected to an oscilloscope. A 40mm diameter, 2.7g ping-pong ball is dropped onto the laminate from a height of 40mm, and the maximum voltage of the generated pulse is measured with the oscilloscope. This procedure is repeated five times and the average value is calculated.

[0013] (2) Output voltage variation The laminated piezoelectric sheet of the present invention preferably has an output voltage variation of less than 0.36V, and more preferably 0V or more and 0.34V or less. The variation in output voltage is the standard deviation of five measurements obtained in the above <Method for measuring output voltage>.

[0014] (3) Thickness The thickness of the laminated piezoelectric sheet of the present invention is preferably 50 μm or more and 500 μm or less. The lower limit is more preferably 70 μm or more, and even more preferably 100 μm or more. On the other hand, the upper limit is more preferably 400 μm or less, and even more preferably 300 μm or less. If the thickness is 50 μm or more, a responsive laminated piezoelectric sheet can be obtained. Furthermore, if the thickness is 500 μm or less, it can be transported and wound from roll to roll, making subsequent processing easy. The thickness can be measured using a 1 / 1000 mm dial gauge.

[0015] The configuration of the laminated piezoelectric sheet of the present invention will be described below.

[0016] 1. Electret film The laminated piezoelectric sheet of the present invention comprises at least one electret film. The type of electret film is not particularly limited as long as it has piezoelectric properties, but a porous electret film is preferred in order to further enhance the piezoelectric properties. Furthermore, it is more preferred to use an electret film that is a charged porous film. When a porous electret film is used, the method for making the film porous is not particularly limited, but examples thereof include chemical or physical foaming and stretching. Among these, stretching is preferred because it provides a dense porous structure and the shape of the pores can be easily controlled. Materials for the electret film include polyolefin resins, fluororesins, vinyl chloride resins, polystyrene resins, butadiene resins, polyester resins, and acrylic resins, among which polyolefin resins are preferably used because they have a small environmental impact and are easy to charge.

[0017] The electret film used in the laminated piezoelectric sheet of the present invention may have a single layer structure, or may be a laminate of two or more layers with different porous structures or materials. In this case, the films can be bonded together with an adhesive or the like, but using a multilayer die makes it possible to obtain a laminated structure all at once, which is preferable in terms of productivity.

[0018] (1) Polyolefin resin The electret film constituting the laminated piezoelectric sheet of the present invention preferably contains a polyolefin resin as the main component, and more preferably contains a polypropylene resin as the main component. Examples of polypropylene resins include homopolypropylene (propylene homopolymer), and random or block copolymers of propylene with α-olefins such as ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene. Among these, homopolypropylene is more preferably used from the viewpoint of mechanical strength.

[0019] Furthermore, the isotactic pentad fraction of polypropylene resins, which indicates stereoregularity, is preferably 80 to 99%, more preferably 83 to 98%, and even more preferably 85 to 97%. An isotactic pentad fraction of 80% or higher provides good mechanical strength. While the upper limit of the isotactic pentad fraction is currently defined as the upper limit industrially achievable, this limit may not apply in the future if resins with higher stereoregularity are developed at the industrial level. The isotactic pentad fraction refers to a stereostructure in which all five methyl groups in the side chains are positioned in the same direction relative to the main chain formed by carbon-carbon bonds composed of any five consecutive propylene units, or the proportion thereof. The signal assignments for the methyl group region conform to A. Zambelli et al. (Macromol. 8, 687 (1975)).

[0020] Furthermore, the polypropylene resin preferably has a Mw / Mn, a parameter indicating molecular weight distribution, of 1.5 to 10.0. It is more preferably 2.0 to 8.0, and even more preferably 2.0 to 6.0. A smaller Mw / Mn means a narrower molecular weight distribution, but by making the Mw / Mn 1.5 or more, sufficient extrusion moldability can be obtained, making industrial mass production possible. On the other hand, by making the Mw / Mn 10.0 or less, sufficient mechanical strength can be ensured. Mw / Mn is measured by GPC (gel permeation chromatography) method.

[0021] The melt flow rate (MFR) of the polypropylene resin is not particularly limited, but is preferably 0.5 to 15 g / 10 min, and more preferably 1.0 to 10 g / 10 min. By setting the MFR to 0.5 g / 10 min or more, sufficient melt viscosity can be obtained during molding, ensuring high productivity. On the other hand, by setting the MFR to 15 g / 10 min or less, sufficient strength can be ensured. The MFR is measured in accordance with JIS K7210-1 (2014) at a temperature of 230°C and a load of 2.16 kg.

[0022] The method for producing the polypropylene resin is not particularly limited, and examples thereof include known polymerization methods using known polymerization catalysts, such as polymerization methods using multi-site catalysts represented by Ziegler-Natta catalysts or single-site catalysts represented by metallocene catalysts.

[0023] Examples of polypropylene-based resins that can be suitably used in the present invention include commercially available products such as "Novatec PP," "WINTEC," and "WAYMAX" (manufactured by Japan Polypropylene Corporation), "Versify," "Notio," and "Tafmer XR" (manufactured by Mitsui Chemicals, Inc.), "Zelas" and "Thermorun" (manufactured by Mitsubishi Chemical Corporation), "Sumitomo Noblen" and "Tafthren" (manufactured by Sumitomo Chemical Co., Ltd.), "Prime Polypro" and "Prime TPO" (manufactured by Prime Polymer Co., Ltd.), "Adflex," "Adsyl," and "HMS-PP (PF814)" (manufactured by SunAllomer Co., Ltd.), and "Inspire" (manufactured by The Dow Chemical Company).

[0024] The porous electret film used in the laminated piezoelectric sheet of the present invention is preferably made of a resin composition mainly composed of a polypropylene-based resin containing a large amount of β-crystals, which is one of the crystalline forms. A non-porous film made of a resin composition mainly composed of a polypropylene-based resin containing a large amount of β-crystals exhibits excellent piezoelectricity after charging treatment, but by stretching it to form a porous structure, even better piezoelectricity can be obtained. The formation of a porous structure using β-crystals is advantageous for preparing a porous structure because pores are formed during the stretching process, when the β-crystals in the polypropylene-based resin transform into α-crystals. This dense porous structure is independent of particle size or dispersion diameter, compared to conventional pore formation methods using inorganic fillers or incompatible organic substances.

[0025] The β-crystal activity of the porous electret film used in the laminated piezoelectric sheet of the present invention can be regarded as an index showing that the polypropylene resin in the non-porous film-like material before stretching has generated β-crystals. If the polypropylene resin in the non-porous film-like material before stretching has generated β-crystals, many fine and uniform pores are formed by subsequent stretching, resulting in excellent mechanical properties and excellent voltage resistance due to the formation of fine and uniform pores.

[0026] The presence or absence of β-crystal activity of the porous electret film used in the laminated piezoelectric sheet of the present invention is determined by using a differential scanning calorimeter to perform differential thermal analysis of the polypropylene resin, and determining whether or not the crystalline melting peak temperature derived from the β-crystal of the polypropylene resin is detected. Specifically, the laminated porous film is heated from 25 ° C to 240 ° C at a heating rate of 10 ° C / min and then held for 1 minute, then cooled from 240 ° C to 25 ° C at a cooling rate of 10 ° C / min and held for 1 minute, and then heated again from 25 ° C to 240 ° C at a heating rate of 10 ° C / min. When the crystalline melting peak temperature (Tmβ) derived from the β-crystal of the polypropylene resin is detected during re-heating, it is determined to have β-crystal activity.

[0027] The presence or absence of β-crystal activity can also be determined from the diffraction profile obtained by X-ray diffraction measurement of a porous electret film that has been subjected to a specific heat treatment. Specifically, the porous electret film is heat-treated at 170 to 190°C, which is a temperature exceeding the crystalline melting peak temperature of the polypropylene-based resin, and then slowly cooled to generate and grow β-crystals. When X-ray diffraction measurement is performed on the porous electret film, if a diffraction peak derived from the (300) plane of the β-crystals of the propylene-based resin is detected in the range of 2θ = 16.0° to 16.5°, it is determined that the porous electret film has β-crystal activity. For details regarding the β-crystal structure of polypropylene resins and X-ray diffraction measurements, reference can be made to Macromol. Chem. 187, 643-652 (1986), Prog. Polym. Sci. Vol. 16, 361-404 (1991), Macromol. Symp. 89, 499-511 (1995), Macromol. Chem. 75, 134 (1964), and references cited therein.

[0028] Methods for obtaining the β-crystal activity of the polypropylene resin described above include a method of not adding a substance that promotes the formation of α-crystals in the polypropylene resin, a method of adding a polypropylene resin that has been treated to generate peroxide radicals as described in Japanese Patent No. 3739481, and a method of adding a β-crystal nucleating agent. In the present invention, however, it is particularly preferable to obtain β-crystal activity by adding a β-crystal nucleating agent. By adding a β-crystal nucleating agent, it is possible to more uniformly and efficiently promote the formation of β-crystals in the polypropylene resin, and a porous electret film having β-crystal activity can be obtained.

[0029] The degree of β-crystal activity can be quantified by measuring the β-crystal generation ability. The β-crystal generation ability is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. When the β-crystal generation ability is 80% or more, the laminated piezoelectric sheet can exhibit suitable piezoelectricity. There is no particular upper limit, but it is preferably 100% or less. The β-crystal forming ability can be measured as follows.

[0030] The β-crystal formation ability is calculated by the following formula using the heat of fusion of α-crystal-derived crystals (ΔHmα) and β-crystal-derived crystals (ΔHmβ) of the polypropylene resin detected by differential thermal analysis of the porous electret film using a differential scanning calorimeter (DSC). β crystal formation ability (%) = [ΔHmβ / (ΔHmβ+ΔHmα)]×100 For example, when the polypropylene resin is a homopolypropylene, it can be calculated from the heat of crystalline fusion (ΔHmβ) derived from β crystals, which is mainly detected in the range of 145°C or higher but lower than 160°C, and the heat of crystalline fusion (ΔHmα) derived from α crystals, which is mainly detected in the range of 160°C or higher but lower than 170°C. Also, when the polypropylene resin is a random polypropylene in which 1 to 4 mol% ethylene is copolymerized, it can be calculated from the heat of crystalline fusion (ΔHmβ) derived from β crystals, which is mainly detected in the range of 120°C or higher but lower than 140°C, and the heat of crystalline fusion (ΔHmα) derived from α crystals, which is mainly detected in the range of 140°C or higher but lower than 165°C.

[0031] The porous electret film used in the laminated piezoelectric sheet of the present invention contains a polypropylene resin as its main component, and specifically, the content thereof is 50% by mass or more, preferably 70 to 99.9999% by mass, more preferably 80 to 99.999% by mass, and even more preferably 90 to 99.99% by mass.

[0032] (2) β-crystal nucleating agent The porous electret film used in the laminated piezoelectric sheet of the present invention preferably contains a β-crystal nucleating agent in order to obtain excellent piezoelectricity. The inclusion of a β-crystal nucleating agent allows the film to have β-crystal activity. Examples of the β-crystal nucleating agent used in the present invention include the following. If necessary, two or more types of β-crystal nucleating agents may be mixed and used.

[0033] Examples of β-crystal nucleating agents include amide compounds; tetraoxaspiro compounds; quinacridones; iron oxides having nanoscale sizes; alkali or alkaline earth metal salts of carboxylic acids, such as potassium 1,2-hydroxystearate, magnesium benzoate or magnesium succinate, and magnesium phthalate; aromatic sulfonic acid compounds, such as sodium benzenesulfonate or sodium naphthalenesulfonate; di- or triesters of di- or tribasic carboxylic acids; phthalocyanine pigments, such as phthalocyanine blue; two-component compounds consisting of component A, which is an organic dibasic acid, and component B, which is an oxide, hydroxide, or salt of a metal in Group 2 of the periodic table; and compositions consisting of a cyclic phosphorus compound and a magnesium compound.

[0034] Among these β-crystal nucleating agents, amide compounds are preferred in terms of the piezoelectricity of the resulting laminated piezoelectric sheet. Examples of amide compounds include N,N'-dicyclohexyl-2,6-naphthalenedicarboxyamide, N,N'-dicyclohexylterephthalamide, and N,N'-diphenylhexanediamide, with N,N'-dicyclohexyl-2,6-naphthalenedicarboxyamide being preferred. Because amide compounds have highly polar amide groups, they are capable of localizing charge in the crystal structure, which is thought to result in high piezoelectric properties. On the other hand, highly polar compounds such as amide compounds have the problem of poor dispersibility and tendency to aggregate in low-polarity polypropylene-based resins due to electrostatic interactions. However, typical β-crystal nucleating agents have the property of dissolving in polypropylene-based resins within a certain temperature range. This property allows the β-crystal nucleating agent to be uniformly dispersed in the polypropylene-based resin, making it easier for crystals derived from the β-crystal nucleating agent to precipitate uniformly. Therefore, it is believed that crystals of highly polar amide compounds are uniformly dispersed in low-polarity polypropylene-based resins, resulting in high piezoelectric properties.

[0035] Specific examples of commercially available β-crystal nucleating agents include the β-crystal nucleating agent "Njestar NU-100" manufactured by New Japan Chemical Co., Ltd., and specific examples of propylene-based resins to which a β-crystal nucleating agent has been added include the polypropylene "Bepol B-022SP" manufactured by Aristech, the polypropylene "Beta(β)-PP BE60-7032" manufactured by Borealis, and the polypropylene "BNX BETAPP-LN" manufactured by Mayzo.

[0036] The content of the β-crystal nucleating agent in the porous electret film used in the laminated piezoelectric sheet of the present invention can be adjusted appropriately depending on the type of β-crystal nucleating agent or the composition of the polypropylene-based resin, but is preferably 0.0001 to 5.0 parts by mass, more preferably 0.001 to 3.0 parts by mass, and even more preferably 0.01 to 1.0 parts by mass per 100 parts by mass of the polypropylene-based resin. If the content is 0.0001 parts by mass or more, sufficient β-crystals of the polypropylene-based resin can be generated and grown during production, ensuring sufficient β-crystal activity. Therefore, sufficient β-crystal activity can be ensured even when the porous film is made, and a porous electret film with the desired piezoelectricity can be obtained by charging. On the other hand, adding 5.0 parts by mass or less is preferable because it is economically advantageous and prevents bleeding of the β-crystal nucleating agent onto the film surface.

[0037] (Thickness) The thickness of the electret film used in the laminated piezoelectric sheet of the present invention is preferably 10 μm or more and 200 μm or less. By setting the thickness within this range, the piezoelectric properties can be improved without making the laminated piezoelectric sheet thicker than necessary. From this perspective, the thickness of the electret film is more preferably 15 μm or more and 150 μm or less, and even more preferably 20 μm or more and 120 μm or less.

[0038] (porosity) When the laminated piezoelectric sheet of the present invention has a porous electret film, the porosity of the porous electret film is preferably 5% or more and 60% or less. By setting the porosity within the above range, it is possible to suppress deterioration of the piezoelectric properties due to plastic deformation caused by external pressure. The porosity is preferably 7% or more and 55% or less, and more preferably 9% or more and 50% or less. The porosity was measured as follows. The actual mass W1 of the measurement sample is measured, and the mass W0 when the porosity is 0% is calculated based on the density of the resin composition that is the raw material of the porous electret film. The porosity is calculated from these values ​​based on the following formula. Porosity (%)={(W0-W1) / W0}×100

[0039] (additives) The electret film used in the laminated piezoelectric sheet of the present invention may contain various additives, such as heat stabilizers, antioxidants, ultraviolet absorbers, light stabilizers, crystal nucleating agents, colorants, antistatic agents, hydrolysis inhibitors, lubricants, flame retardants, conductive agents, elastomers, etc., to an extent that does not impair its properties. In addition, other resin compositions may be contained to an extent that does not impair its properties.

[0040] 2.Electrode The laminated piezoelectric sheet of the present invention is characterized by having an electrode layer on only one side of the electret film. The electrode layer may be conductive, and suitable materials include metal foils such as aluminum foil, copper foil, silver foil, gold foil, nickel foil, and tin foil, carbon sheets, conductive paints, composites with resins such as conductive rubber sheets, and inorganic vapor-deposited films such as aluminum, gold, and ITO. The thickness of the electrode is preferably 1 nm to 100 μm, more preferably 2 nm to 50 μm, and even more preferably 5 nm to 30 μm. A thickness of 1 nm or more ensures stable conductivity as an electrode. On the other hand, a thickness of 100 μm or less ensures flexibility when made into a laminated piezoelectric sheet.

[0041] 3.Adhesive layer In the laminated piezoelectric sheet of the present invention, an adhesive layer may be provided between the electret film and the electrode, which is preferable in that it can prevent the electrodes from being displaced during the production of the laminated piezoelectric sheet. In contrast, for example, in the laminated piezoelectric sheet disclosed in Patent Document 1, no adhesive layer is interposed between the porous electret film and the electrode. This is to prevent the adhesive layer from reducing the electromotive force of the multilayer piezoelectric sheet and impairing the piezoelectric properties. This is thought to be due to the adhesive constituting the adhesive layer penetrating into the pores of the porous electret film. However, in the present invention, by performing the charging treatment after forming the adhesive layer, the adhesive layer does not reduce the electromotive force of the multilayer piezoelectric sheet or impair the piezoelectric properties, which is preferable.

[0042] The adhesive layer that interposes the electret film and the electrode may or may not be conductive. Having conductivity has the effect of stabilizing the conduction with the electrode tab. The adhesive layer may be a pressure-sensitive adhesive layer or may not be a pressure-sensitive adhesive layer, but is preferably a pressure-sensitive adhesive layer. By using the adhesive layer, the electrode and the protective film can be bonded together simply by laminating them with the adhesive layer interposed therebetween and applying pressure.

[0043] The adhesive layer is not particularly limited as long as it is made of an adhesive, but when the adhesive layer is a pressure-sensitive adhesive layer, it is preferably made of a pressure-sensitive adhesive. The pressure-sensitive adhesive is not particularly limited, but examples thereof include acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, synthetic rubber pressure-sensitive adhesives, natural rubber pressure-sensitive adhesives, and silicone pressure-sensitive adhesives, and among these, acrylic pressure-sensitive adhesives are preferred. The adhesive contains a main polymer such as an acrylic resin, a urethane resin, a synthetic rubber, a natural rubber, or a silicone resin, and the main polymer may be blended with at least one additive selected from a crosslinking agent, a tackifier, a plasticizer, a softener, a metal deactivator, an antioxidant, a pigment, a dye, etc.

[0044] In addition, when the adhesive layer is a conductive adhesive layer having electrical conductivity, a conductive adhesive may be used as the adhesive. The conductive adhesive is not particularly limited as long as it has electrical conductivity, but it is preferable that conductive particles are blended therein. Examples of conductive particles include metal powder particles such as gold, silver, copper, nickel, and aluminum, conductive carbon particles such as carbon and graphite, and particles having a metal coating on the surface of a core material such as resin, solid glass beads, and hollow glass beads. Among these, metal powder particles such as nickel powder particles, copper powder particles, and silver powder particles are preferred because of their excellent conductivity, adhesiveness, and productivity. Furthermore, the shape of the conductive particles is not particularly limited, and may be spherical, have a needle-like surface, or may have a shape in which multiple conductive particles are connected together by forming bonds between them.

[0045] The conductive particles may be used alone or in combination of two or more types. The content of the conductive particles in the conductive adhesive may be adjusted as appropriate to impart the desired conductivity, but is preferably 1% by mass or more and 50% by mass or less, more preferably 5% by mass or more and 25% by mass or less, and even more preferably 8% by mass or more and 20% by mass or less.

[0046] Furthermore, when the adhesive layer does not have pressure-sensitive adhesive properties, the adhesive constituting the adhesive layer may be a thermosetting adhesive, a photocurable adhesive, a hot-melt adhesive, a moisture-curable adhesive, or the like. These may or may not be conductive, but if they are conductive, they may be a conductive adhesive containing conductive particles. The details of the conductive particles, their content, etc. are as described above.

[0047] The thickness of the adhesive layer is not particularly limited, but from the viewpoint of ensuring adhesion between the electret film and the electrode without making the laminated piezoelectric sheet thicker than necessary, it is preferably 1 μm or more and 100 μm or less, more preferably 2 μm or more and 50 μm or less, and even more preferably 4 μm or more and 35 μm or less. Alternatively, an adhesive tape having an adhesive layer pre-laminated on one side of the electrode may be used, such as an adhesive tape having an adhesive layer laminated on one side of a metal foil such as copper foil or aluminum foil. Such adhesive tapes may be commercially available products, such as "E-2300ND," "E20CU," "E30CU," "E40CU," "E50CU," "E65CU," and "52050AD" manufactured by DIC Corporation.

[0048] 4.Other In addition to the constituent members listed above, the laminated piezoelectric sheet of the present invention may be given functions such as a protective layer, a tab electrode, a shielding layer, and a buffer layer in order to improve the handling properties and electrical properties when made into a device. In addition to the components described above, the present invention may also appropriately contain commonly used additives within the range that does not significantly impair the effects of the present invention. Examples of such additives include recycled resins resulting from trimming losses such as selvage, inorganic particles such as silica, talc, kaolin, and calcium carbonate, pigments such as titanium oxide and carbon black, flame retardants, weathering stabilizers, heat stabilizers, antistatic agents, melt viscosity modifiers, crosslinking agents, lubricants, nucleating agents, plasticizers, antioxidants, antioxidants, light stabilizers, UV absorbers, neutralizers, antifogging agents, antiblocking agents, slip agents, and colorants, which are added for the purpose of improving or adjusting moldability, productivity, and various physical properties of the laminated piezoelectric sheet.

[0049] 5. Manufacturing method of laminated piezoelectric sheet A method for manufacturing a laminated piezoelectric sheet of the present invention will be described. However, the following description is an example of a method for manufacturing a laminated piezoelectric sheet of the present invention, and the laminated piezoelectric sheet of the present invention is not limited to laminated piezoelectric sheets manufactured by such a manufacturing method.

[0050] The method for producing a laminated piezoelectric sheet of the present invention may include the following steps, and may further include other steps or treatments.

[0051] The film-forming step, the stretching step, the electrode-forming step, and the charging treatment will be described below in order. Note that the stretching step is not essential and may be omitted as appropriate.

[0052] (1) Film forming process In the film-forming process, a film made of a material constituting the electret film is formed. In the film-forming process, the material constituting the electret film is not particularly limited as long as it is formed into a film by a known method, but for example, the resin (material resin) constituting the electret film may be heated and melted to form a film, specifically, the film may be formed by a T-die method, an inflation method, or the like, and among them, the T-die method is preferably used. In addition, practically, it is preferable to melt-extrude the material resin through a T-die and cast it using a cast roll (chill roll, cast drum, etc.). The material resin may be appropriately blended with additives, or two or more resin components may be mixed and formed into a film as a resin composition containing two or more components.

[0053] The materials constituting the electret film may be kneaded in a kneading device and then formed into a film. The kneading device used when kneading is not particularly limited. For example, known extruders such as single-screw extruders, twin-screw extruders, and multi-screw extruders can be used. In addition, depending on the equipment structure and needs, a pressure reducer may be connected to the vent port of the extruder to remove moisture and low-molecular-weight substances from the materials constituting the electret film.

[0054] When using a casting roll as described above, the sheet-shaped molten resin (resin composition) extruded from a T-die is extruded onto the casting roll and then taken up while being tightly attached to the rotating casting roll to form a film-like material. In addition, a touch roll, an air knife, an electric contact device, or the like may be attached to the casting roll to tightly attach the film-like material to the casting roll.

[0055] Furthermore, when the molten resin (resin composition) is cooled and molded into a film, the temperature of the casting roll is preferably 100°C or higher, more preferably 110°C or higher, and even more preferably 120°C or higher. In the present invention, the porosity can also be increased by opening holes in the crystalline and amorphous parts of the polypropylene resin during the stretching process, so it is preferable to set the casting roll temperature to 100°C or higher to obtain a film with a high degree of crystallinity. Furthermore, the casting roll temperature is preferably 140°C or lower, more preferably 135°C or lower, and even more preferably 130°C or lower. Setting the casting roll temperature to 140°C or lower makes it easy to peel the film from the casting roll during film formation.

[0056] The thickness of the effective portion of the obtained film, excluding both ends, is preferably 30 μm or more and 500 μm or less, more preferably 40 μm or more, even more preferably 50 μm or more, and more preferably 300 μm or less, and even more preferably 200 μm or less. If the film thickness is 30 μm or more, breakage during stretching can be prevented, and if the film thickness is 500 μm or less, the film can be easily stretched. The layer structure of the electret film is not limited to the single layer structure described above, and may be a structure in which other layers are combined.

[0057] (2) Stretching process The film obtained in the film-forming process may be stretched. Stretching the film can easily produce a porous film. In the stretching process, the film may be uniaxially or biaxially stretched. The uniaxial stretching may be longitudinal uniaxial stretching or transverse uniaxial stretching. The biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching. Among these, sequential biaxial stretching makes it relatively easy to control the porous structure and easily balances other physical properties such as mechanical strength and shrinkage rate. Sequential biaxial stretching is not particularly limited, but may be performed, for example, by longitudinal stretching followed by transverse stretching. Stretching in the machine direction (MD) of the film is called "longitudinal stretching," and stretching in the direction perpendicular to the machine direction (TD) is called "transverse stretching."

[0058] The stretching temperature must be appropriately selected depending on the composition, crystalline melting peak temperature, crystallinity, etc. of the resin composition used, but the longitudinal stretching temperature is preferably 60°C or higher and 140°C or lower, more preferably 80°C or higher and 120°C or lower. A longitudinal stretching temperature of 140°C or lower is preferred because stretching without breakage is possible below the melting point of the polypropylene resin, which is the main component. On the other hand, a temperature of 60°C or higher is preferred because breakage during stretching can be suppressed. The transverse stretching temperature is preferably 90°C or higher and 160°C or lower, more preferably 100°C or higher and 150°C or lower. When the transverse stretching temperature is within the specified range, pores are sufficiently formed, the porosity can be increased, and sufficient piezoelectric properties can be obtained. In addition, in the case of sequential biaxial stretching, for example, pores generated during longitudinal stretching can be enlarged, thereby increasing the porosity of the porous layer. The temperatures described above are those for uniaxial stretching or sequential biaxial stretching, but the stretching temperature for simultaneous biaxial stretching may be adjusted, from the above viewpoint, within a range of preferably 90°C or higher and 140°C or lower, more preferably 100°C or higher and 120°C or lower.

[0059] The stretching ratio may be selected arbitrarily according to the desired porosity, but the stretching ratio per uniaxial stretching is preferably 1.1 to 20 times, more preferably 1.5 to 18 times, and even more preferably 2 to 16 times. By setting the stretching ratio per uniaxial stretching to 1.1 times or more, whitening progresses, and minute crazes are elongated, resulting in sufficient porosity due to stretching. Furthermore, by setting the stretching ratio to 20 times or less, film breakage during production can be suppressed. Furthermore, in the case of sequential biaxial stretching, stretching at the stretching ratio specified above for each axis prevents pores generated during the previous stretching from being deformed during the subsequent stretching.

[0060] (3) Electrode formation process Next, an electrode can be formed on one side of the film by a method appropriate for the electrode material. If the electrode is a metal foil, it is preferable to bond it using an adhesive. If the electrode is a composite with a resin, such as a conductive paint, it can be obtained by dissolving it in a solvent, applying it, and drying it, or by laminating it using heat. If the electrode is a vapor-deposited film, it can be obtained by placing the film in a vapor deposition oven and vapor-depositing it on the film surface. Among these methods, the method of bonding a metal foil using an adhesive and the method of applying a conductive paint using a solvent and drying it are preferred because of their high productivity.

[0061] (4) Charging treatment Next, the porous film with the electrodes formed thereon is subjected to an electrification treatment to obtain the laminated piezoelectric sheet of the present invention. Since the laminated piezoelectric sheet of the present invention has an electrode on only one side, it can be electrified while being grounded. The electrification treatment may be continuous or batchwise. In particular, since the laminated piezoelectric sheet can be produced continuously by roll-to-roll processing, high production efficiency can be achieved. When electrifying, a method in which the film is passed between electrodes such as needle electrodes, wire electrodes, roll electrodes, or plate electrodes on the front and back of the film and an electric field is applied between the electrodes is preferred from the standpoint of productivity. The applied electric field is preferably 0.1 MV / m or more and 10 MV / m or less, more preferably 0.2 MV / m or more and 8 MV / m or less, and even more preferably 0.3 MV / m or more and 6 MV / m or less. When the electric field is 0.1 MV / m or more, the porous electret film can have excellent piezoelectric properties. When the electric field is 10 MV / m or less, there is an effect of preventing dielectric breakdown during charging processing.

[0062] (5) Other Next, a protective layer or a shielding layer may be formed on the laminated piezoelectric sheet obtained above for the purpose of improving insulation properties and reliability. Suitable methods for forming such layers include bonding using an adhesive or thermal lamination.

[0063] <Wound body> The wound body of the present invention is characterized in that an electrode is laminated on one side of the electret film, and the adhesive strength between the electret film and the electrode is 1 N / cm or more. In an embodiment in which an electrode is laminated on one side of the electret film, the electret film can be charged while being grounded, and the wound body can be easily produced by transporting the electret film from roll to roll and winding it. Furthermore, since the electret film and the electrode can be charged after being bonded together, it is possible to suppress a reduction in the piezoelectric effect caused by the adhesive. In other words, since an adhesive can be used to bond the electret film and the electrode, high adhesive strength between the electret film and the electrode can be obtained. Furthermore, forming the wound body has the advantage of easy subsequent processing.

[0064] <Piezoelectric Sheet and Method for Manufacturing the Piezoelectric Sheet> The piezoelectric sheet of the present invention comprises a first electrode, a first electret film, a second electret film, and a second electrode laminated in this order, and the adhesive strength between the electret film and the electrodes is 1 N / cm or more. The piezoelectric sheet of the present invention is suitable for use as a sensor device.

[0065] The piezoelectric sheet can be produced by combining two of the above-mentioned laminated piezoelectric sheets, and more specifically, can be produced by the following steps (A) to (C). (A) A step of obtaining two laminated piezoelectric sheets each having an electrode laminated on one side of a porous electret film. (B) A step of positively charging the surface of one of the piezoelectric laminate sheets facing the electret film to obtain a first piezoelectric laminate sheet, and negatively charging the surface of the other piezoelectric laminate sheet facing the electret film to obtain a second piezoelectric laminate sheet. (C) A step of laminating the first laminated piezoelectric sheet and the second laminated piezoelectric sheet so that the electret film side surfaces are in contact with each other.

[0066] In the piezoelectric sheet, the porous electret film and the electrode may be laminated via an adhesive in the step (A). Even if an adhesive is used, the adhesive does not reduce the adhesive strength between the porous electret film and the electrode because the charging treatment is carried out in the step (B) after bonding with the adhesive. Furthermore, in this piezoelectric sheet, since the first and second piezoelectric sheets have different electric charges, sufficient adhesion can be achieved through electrostatic adhesion. Furthermore, in step (C), the surfaces of the laminated piezoelectric sheets may be laminated together via an adhesive. Note that the laminated piezoelectric sheets only need to be charged near the electrodes, and the adhesive is generally not considered to have any effect on the adhesion between the porous electret films.

[0067] <Sensor device> Since the laminated piezoelectric sheet of the present invention has one electrode, a sensor device can be created by combining two laminated piezoelectric sheets with different polarities. In other words, a sensor device can be constructed using the above-mentioned piezoelectric sheet. Furthermore, by providing lead wires and circuit implementation, a sophisticated device can be created. As described above, in the piezoelectric sheet of the present invention, the electret film and the electrode are bonded together via an adhesive, and the adhesive strength is 1 N / cm or more, providing strong adhesion. Furthermore, the electret films are also firmly bonded together, preferably via an adhesive, in addition to electrostatic adhesion, so that a sensor device can be obtained in which the electret and the electrode are completely bonded together.

[0068] The piezoelectric sheet of the present invention is useful as a sensor device and can be suitably used as a mat sensor, a sensor for a robot hand, etc. In addition to sensors, the sheet can also be used as a vibration power generator, a water level gauge, and an acoustic detector. [Example]

[0069] The laminated piezoelectric sheet of the present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited in any way.

[0070] The materials used in the present invention are described below. (Polypropylene resin) A-1: Homopolypropylene (Novatec PP FY6HA, MFR: 2.4 g / 10 min [230°C, 2.16 kg load], Mw / Mn = 3.2, manufactured by Japan Polypropylene Corporation) (β-crystal nucleating agent) B-1: N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide (New Japan Chemical Co., Ltd., NU-100) (antioxidant) C-1: A 1:1 mixture of tris(2,4-di-t-butylphenyl)phosphite and tetrakis[3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionic acid]pentaerythritol (IRGANOX-B225, manufactured by BASF)

[0071] Production Example 1 (Production of porous film) A resin composition was obtained by mixing 100 parts by mass of polypropylene resin (A-1), 0.2 parts by mass of β-crystal nucleating agent (B-1), and 0.1 parts by mass of antioxidant (C-1) and melt-extruding the mixture in a twin-screw extruder at 280° C. The resin composition was poured into an extruder connected to a T-die with a lip opening of 1 mm and molded, and then guided through a casting roll to obtain a non-porous membrane with a thickness of 300 μm. Thereafter, the film was stretched 7 times in the transverse direction at a stretching temperature of 100°C using a film tenter facility (manufactured by Kyoto Kikai Co., Ltd.) to obtain a porous film. The obtained porous film had β-crystal activity, and the β-crystal generation ability of the polypropylene resin contained therein was 92%, and the porosity was 20%.

[0072] (Manufacturing of laminated piezoelectric sheets) Example 1 The porous film obtained in Production Example 1 was cut into 10 cm squares, and conductive copper foil adhesive tape "E20CU" (manufactured by DIC Corporation) with an adhesive layer on one side was cut into 9 cm squares as electrodes. These were pasted together so that the porous film extended 0.5 cm from both ends of the copper foil, and the film was placed on an earth plate so that the copper foil side was in contact with the earth roll. An electric charge of -15 kV was applied for 60 seconds at room temperature from needle electrodes with a distance of 20 mm between the electrodes, thereby carrying out an electrification treatment.

[0073] Example 2 A laminated piezoelectric sheet was obtained in the same manner as in Example 1, except that the applied voltage was changed from −15 kV to +15 kV.

[0074] (Comparative Example 1) The porous film obtained in Production Example 1 was cut into a 10 cm square, and two 9 cm square pieces of conductive copper foil adhesive tape "E20CU" (manufactured by DIC Corporation) with an adhesive layer on one side were attached to both sides as electrodes, placed on an earth plate, and charged by applying a charge of -15 kV for 60 seconds at room temperature from needle electrodes with a distance of 20 mm between the electrodes.

[0075] (Comparative Example 2) The porous film obtained in Production Example 1 was cut into a 10 cm square, and a 9 cm square piece of aluminum foil (Mitsubishi Aluminum, FOIL, thickness 11 μm) was placed on one side as an electrode, and placed on an earth plate. A charge of -15 kV was applied for 60 seconds at room temperature from needle electrodes with a distance of 20 mm between the electrodes, thereby carrying out an electrification treatment. The adhesion method in this example is referred to as electrostatic adhesion in the tables.

[0076] (Comparative Example 3) A laminated piezoelectric sheet was obtained in the same manner as in Comparative Example 2, except that the applied voltage was changed from −15 kV to +15 kV.

[0077] (Sensor device manufacturing) Example 3 The laminated piezoelectric sheets obtained in Examples 1 and 2 were stacked together with the electrodes facing outward, electrode tabs were attached, and the four sides were sealed with an 11 cm square laminate film to obtain a sensor device.

[0078] Comparative Example 4 Electrode tabs were attached to the laminated piezoelectric sheet obtained in Comparative Example 1, and the four sides were sealed with an 11 cm square laminate film to obtain a sensor device.

[0079] The laminated piezoelectric sheets and sensor devices obtained in the examples and comparative examples were measured for output voltage, output variation, electrode adhesion strength, and signal strength by the following methods.

[0080] (1) Output voltage The laminated piezoelectric sheet was placed with the electrode facing down, and a 100μm thick polyester film with aluminum vapor deposition was placed on top of it with the vapor-deposited side facing down. Both were sandwiched between 100μm thick polyester film clear files, and the laminated piezoelectric sheet and the vapor-deposited side were each connected to an oscilloscope. A 40mm diameter, 2.7g ping-pong ball was dropped onto the laminate from a height of 40mm, and the maximum voltage of the pulse generated was measured with the oscilloscope. This procedure was repeated five times, and the average value was taken as the output voltage. ○: Output voltage is 1V or more ×: Output voltage is less than 1V

[0081] (2) Output Variation The standard deviation of the five measurements of the output voltage was taken as the signal variation. ○: Standard deviation is less than 0.36V ×: Standard deviation is 0.36V or more

[0082] (3) Adhesion strength of electrodes The peel strength between the porous electret film and the electrode of the laminated piezoelectric sheet was measured in accordance with JIS Z0237. First, a sample of 10 mm wide x 100 mm long was cut from the laminated piezoelectric sheet, and the electrode and porous electret film were peeled off by 25 mm. Next, one end of the peeled sample was fixed to the lower chuck of a tensile tester (Intesco IM-20ST, manufactured by Intesco Co., Ltd.) and the other end was fixed to the upper chuck, and the peel strength was measured at a test speed of 300 mm / min. After the measurement, the measurement value for the first 25 mm length was ignored, and the peel strength measurements for the 50 mm length peeled from the test piece were averaged to determine the peel strength. If the electrode did not peel off from the porous electret film, tape was applied to the electrode and the peel strength of the tape was measured. The peel strength of this laminated piezoelectric sheet was deemed to be equal to or greater than the peel strength of the tape alone.

[0083] (4) Signal strength The sensor device of the present invention was connected to an oscilloscope, and a ping-pong ball with a diameter of 40 mm and a weight of 2.7 g was dropped onto any location on the surface from a height of 40 mm. The maximum voltage of the generated pulse was measured with the oscilloscope. This procedure was repeated five times, and the average value was calculated. ○: Signal strength is 4V or more ×: Signal strength is less than 4V

[0084] Tables 1 and 2 show the evaluation results for the examples and comparative examples.

[0085] [Table 1]

[0086] [Table 2]

[0087] Manufacturing Example 2 (Roll-to-roll manufacturing of laminated piezoelectric sheets) 100 parts by mass of polypropylene resin (A-1), 0.2 parts by mass of β-crystal nucleating agent (B-1), and 0.1 parts by mass of antioxidant (C-1) were mixed and melt-extruded at 280°C using a twin-screw extruder to obtain resin composition 1. Resin composition 1 was introduced into the extruder connected to a T-die with a lip opening of 1 mm and molded, and then guided to a casting roll at a temperature of 127°C to obtain non-porous membrane material 1 with a thickness (thickness of the effective portion excluding both ends) of 100 μm. Thereafter, the film was stretched 7 times in the transverse direction at a stretching temperature of 100°C using a film tenter facility (manufactured by Kyoto Machinery Co., Ltd.) to obtain a porous film 1 with a thickness of 20 μm. The porous film 1 was then bonded to conductive copper foil adhesive tape "E20CU" (manufactured by DIC Corporation) using a laminator so that the porous film side protruded 5 mm from each end in the machine direction, and then wound around a core to be taken up into a roll, thereby obtaining a laminated porous film 1. The obtained laminated porous film 1 was unwound with the copper foil facing the ground side and transported using a roll-to-roll film transport device set at a transport speed of 2 m / min. During the transport process, a high-voltage application device equipped with staggered needle electrodes (200 mm x 1000 mm) was used to apply a voltage of -15 kV to the porous film, thereby performing a charging process. The gap between the needle electrodes and the ground roll was 20 mm. The voltage application time to the porous film was calculated based on the relationship between the transport speed, the number of needle electrodes, and the length of the electrodes in the transport direction, and was found to be approximately 6 seconds. Then, before winding the film, an AC ionizer was installed to destaticize the film, yielding an electret film. The obtained electret film was 50 m long and wound around a core into a roll. The ambient temperature during the experiment was 21°C, humidity was 51%, and the surface temperature of the earth roll was 19°C.

[0088] It was confirmed from Examples 1 and 2 that the laminated piezoelectric sheets having the configurations defined by the present invention exhibited good piezoelectric properties and little variation in output voltage. The sensor device of Example 3 fabricated from these laminated piezoelectric sheets also had good signal strength. On the other hand, in the laminated piezoelectric sheet of Comparative Example 1, electrodes were laminated on both sides of the electret film, which resulted in a non-uniform electric field applied to the electret film and large variations in output voltage. The sensor device of Comparative Example 4, which was made from this laminated piezoelectric sheet, had low signal strength because the electret film was unable to retain sufficient charge due to the shielding effect of the electrodes. Furthermore, in the laminated piezoelectric sheets of Comparative Examples 2 and 3, the electret film and the electrode were not bonded together, and therefore the adhesive strength was insufficient, resulting in large variations in output voltage. Furthermore, it was confirmed from Production Example 2 that the laminated piezoelectric sheet having the configuration of the present invention can be continuously produced by a roll-to-roll method.

Claims

1. A laminated piezoelectric sheet in which an electrode is laminated on only one side of an electret film, the adhesion strength between the electret film and the electrode is 1 N / cm or more, and the output voltage variation measured by the following method is less than 0.36 V. <Output voltage variation> The laminated piezoelectric sheet was placed with the electrode facing downwards, and a 100 μm thick polyester film with aluminum vapor deposition was placed on top of it with the vapor deposition side facing downwards. Both were sandwiched between 100 μm thick polyester film clear files, and the laminated piezoelectric sheet and the vapor deposition side were each connected to an oscilloscope. A ping-pong ball with a diameter of 40 mm and a weight of 2.7 g was dropped onto the laminate from a height of 40 mm, and the maximum voltage of the generated pulse was measured with the oscilloscope. This procedure was performed five times, and the standard deviation of the measured values ​​was taken as the "output voltage variation."

2. The laminated piezoelectric sheet according to claim 1 , wherein the electret film is a porous electret film.

3. 3. The laminated piezoelectric sheet according to claim 2, wherein the porous electret film is composed mainly of a polyolefin resin.

4. 4. The laminated piezoelectric sheet according to claim 3, wherein the polyolefin resin is a polypropylene resin having a β-crystal generating ability of 80% or more.

5. 5. The laminated piezoelectric sheet according to claim 2, wherein the porous electret film has a porosity of 5% or more and 60% or less.

6. 6. The laminated piezoelectric sheet according to claim 1, wherein the thickness of the electret film is 10 μm or more and 200 μm or less.

7. A wound body formed by winding a laminated piezoelectric sheet described in any one of claims 1 to 6, in which an electrode is laminated on only one side of the electret film and the adhesion strength between the electret film and the electrode is 1 N / cm or more.

8. A first electrode, a first electret film, a second electret film, and a second electrode are laminated in this order, and the adhesion strength between the first electret film and the first electrode and the adhesion strength between the second electret film and the second electrode are 1 N / cm or more. The laminated piezoelectric sheet according to any one of claims 1 to 6.

9. A method for producing a piezoelectric sheet, comprising: (A) a step of obtaining two laminated piezoelectric sheets each having an electrode laminated on one side of an electret film; (B) a step of positively charging the surface of one of the laminated piezoelectric sheets facing the electret film to obtain a first laminated piezoelectric sheet, and negatively charging the surface of the other laminated piezoelectric sheet facing the electret film to obtain a second laminated piezoelectric sheet; and (C) a step of stacking the first laminated piezoelectric sheet and the second laminated piezoelectric sheet so that the surfaces facing the electret films are in contact with each other.

10. 10. The method for manufacturing a piezoelectric sheet according to claim 9, wherein in the step (A), a first electrode, a first electret film, a second electret film, and a second electrode are laminated in this order, and the first electret film and the first electrode, and the second electret film and the second electrode are laminated via an adhesive.

11. The method for producing a piezoelectric sheet according to claim 9 or 10, wherein in the step (C), surfaces of the laminated piezoelectric sheets on the electret film side are laminated together via an adhesive.

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