JP2022118650A - Laminate piezoelectric sheet - Google Patents

Abstract

To provide a laminate piezoelectric sheet having excellent water resistance and good piezoelectric properties while enabling mass production.SOLUTION: A laminate piezoelectric sheet according to an embodiment of the present invention has a laminated structure in which at least an electret film, an electrode, and a protective film are provided in this order, and the protective film is a porous film.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a laminated piezoelectric sheet, and more particularly to a laminated piezoelectric sheet that can be suitably used for sensors such as vibration power generators, water level gauges, acoustic detectors, mat sensors, and robot hands.

An electret is a semi-permanently polarized material obtained by thermally or electrically treating a polymeric material or an inorganic material that does not readily conduct electricity.
For example, porous electrets using porous resin films are known to exhibit an excellent piezoelectric effect, and are widely used in vibration power generation, sensor devices, and the like.
However, the porous electret film has a problem that the piezoelectric properties are greatly reduced when forming an electrode for incorporation into a device.
In addition, the porous electret is vulnerable to humidity, and there is a problem that the piezoelectric properties are dramatically lowered when the electrification is lost due to moisture.

To address these problems, it has been reported that high piezoelectric properties can be achieved by embossing the surface of a porous electret film and devising contact points with electrodes (Patent Document 1). Moreover, it has been reported that the moisture resistance of electrets can be improved by using a special fluororesin (Patent Document 2).

JP 2013-214932 A International publication 2013-157505A1 pamphlet

However, the porous electret described in Patent Literature 1 has a complicated manufacturing process, and the electret described in Patent Literature 2 uses a special fluororesin, which has a large environmental impact.
As described above, conventional electrets have problems in terms of mass production, and further improvements are required in terms of water resistance and piezoelectric properties.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a laminated piezoelectric sheet that is excellent in water resistance and has good piezoelectric properties while enabling mass production.

The present inventors have made intensive studies to solve the above problems, and as a result, have completed the present invention focusing on solving the above problems. That is, the gist of the present invention is as follows.

[1] A laminated piezoelectric sheet having a laminated structure in which at least an electret film, an electrode, and a protective film are provided in this order, and wherein the protective film is a porous film.
[2] The laminated piezoelectric sheet according to [1] above, wherein the electret film has a porosity of 0% or more and 50% or less.
[3] The laminated piezoelectric sheet according to [1] or [2] above, wherein the electret film is a porous film.
[4] The laminated piezoelectric sheet according to any one of [1] to [3] above, wherein the electret film and/or protective film contains a polyolefin resin as a main component.
[5] The laminated piezoelectric sheet according to [4] above, wherein the polyolefin resin is a polypropylene resin having a β-crystal forming ability of 80% or more.
[6] The laminated piezoelectric sheet according to any one of [1] to [5] above, wherein the protective film has a porosity of more than 0% and not more than 50%.
[7] The laminated piezoelectric sheet according to any one of [1] to [6] above, wherein the electret film has a thickness of 10 μm or more and 200 μm or less.
[8] The laminated piezoelectric sheet according to any one of [1] to [7] above, wherein the protective film has a thickness of 1 μm or more and 100 μm or less.
[9] The laminated piezoelectric sheet according to any one of the above [1] to [8], wherein the ends of the protective film are fused to form a bag and seal the inside.
[10] A sensor device using the laminated piezoelectric sheet according to any one of [1] to [9] above.

The laminated piezoelectric sheet of the present invention can be mass-produced, is excellent in water resistance, and has good piezoelectric properties.

Embodiments of the present invention will be described in detail below. However, the contents of the present invention are not limited to the embodiments described below.

<Laminated Piezoelectric Sheet>
The laminated piezoelectric sheet of the present invention (hereinafter also referred to as "this laminated piezoelectric sheet") has a laminated structure in which at least an electret film, an electrode, and a protective film are provided in this order, and the protective film is a porous film. characterized by
The present laminated piezoelectric sheet has the above structure, and thus has excellent water resistance and excellent piezoelectric properties.
In addition, since the present laminated piezoelectric sheet can be obtained by simply laminating at least the above three members, it is possible to mass-produce the sheet without complicated manufacturing processes or the use of special materials.
The characteristics of this laminated piezoelectric sheet will be described below.

(1) Output Voltage The output voltage of the laminated piezoelectric sheet of the present invention is preferably 0.01 V or more and 1000 V or less, more preferably 0.1 V or more and 500 V or less, and even more preferably 0.5 V or more and 100 V or less. Above all, the output voltage of the laminated piezoelectric sheet is more preferably 1 V or higher, more preferably 3 V or higher, and even more preferably 5 V or higher.
By setting the output voltage of the present laminated piezoelectric sheet to 0.01 V or more, it is possible to obtain sufficient sensitivity when used as a sensor, for example. Moreover, since the output voltage of the present laminated piezoelectric sheet is 1000 V or less, the risk of spark discharge when incorporated as a sensor or actuator can be reduced.
Incidentally, the output voltage of the laminated piezoelectric sheet of the present invention is measured by the method described later.

(2) Thickness The thickness of the laminated piezoelectric sheet of the present invention is preferably 20 μm or more and 600 μm or less, more preferably 30 μm or more and 500 μm or less, even more preferably 50 μm or more and 400 μm or less, even more preferably 70 μm or more and 300 μm or less, and particularly preferably 200 μm or less. .
When the thickness of the present laminated piezoelectric sheet is 20 μm or more, the responsiveness is improved. In addition, since the thickness of the present laminated piezoelectric sheet is 600 μm or less, it can be transported and wound in a roll-to-roll manner, and subsequent processing is facilitated, resulting in higher productivity.

Next, the structure of each layer of this laminated piezoelectric sheet will be described.

1. Electret Film The laminated piezoelectric sheet of the present invention includes at least one electret film.
The type of electret film is not particularly limited as long as it has piezoelectric properties, but a porous film is preferable from the viewpoint of further enhancing the piezoelectric properties. Moreover, as the electret film, it is more preferable to use a charged porous film.
When the electret film is a porous film, the method for making it porous is not particularly limited, but examples thereof include chemical or physical foaming, and making porous by stretching. Among them, drawing to form a porous film is preferable because a dense porous structure can be obtained and the shape of the pores can be easily controlled.

Electret film materials include polyolefin resins, fluorine resins, vinyl chloride resins, polystyrene resins, butadiene resins, polyester resins, and acrylic resins. In view of this, polyolefin-based resins are preferably used.

(1) Polyolefin-based resin The electret film of the present invention preferably contains a polyolefin-based resin as a main component, and more preferably a polypropylene-based resin as a main component.
When the electret film contains a polyolefin resin as a main component, the content of the polyolefin resin in the electret film is 50% by mass or more, preferably 70% by mass or more and 99.9999% by mass or less, more preferably 80% by mass or more. It is 99.999% by mass or less, more preferably 90% by mass or more and 99.99% by mass or less.

Further, when the electret film contains a polypropylene resin as a main component, the content of the polypropylene resin in the electret film is 50% by mass or more, preferably 70% by mass or more and 99.9999% by mass or less, more preferably 80% by mass. 99.999% by mass or more, more preferably 90% by mass or more and 99.99% by mass or less.
Examples of polypropylene-based resins include homopolypropylene (propylene homopolymer), or propylene and ethylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene or alpha such as 1-decene. - random copolymers or block copolymers with olefins; Among these, homopolypropylene is more preferably used from the viewpoint of mechanical strength.

In addition, the polypropylene resin preferably has an isotactic pentad fraction of 80% or more and 99% or less, more preferably 83% or more and 98% or less, and still more preferably 85% or more and 97%. It is below.
If the isotactic pentad fraction is 80% or more, the mechanical strength is good. On the other hand, the upper limit of the isotactic pentad fraction is defined as the upper limit that can be obtained industrially at present. is not. The isotactic pentad fraction is a three-dimensional structure in which all five side chain methyl groups are located in the same direction with respect to the main chain formed by a carbon-carbon bond composed of any five consecutive propylene units. Or it means the ratio. The assignment of the signal in the methyl group region is A. Zambelli et al. (Macromol. 8, 687 (1975)).

Moreover, it is preferable that Mw/Mn, which is a parameter indicating the molecular weight distribution, of the polypropylene-based resin is 1.5 or more and 10.0 or less. It is more preferably 2.0 or more and 8.0 or less, and still more preferably 2.0 or more and 6.0 or less.
A smaller Mw/Mn means a narrower molecular weight distribution, but by setting the Mw/Mn to 1.5 or more, sufficient extrudability can be obtained and industrial mass production is possible. On the other hand, by setting Mw/Mn to 10.0 or less, sufficient mechanical strength can be ensured.
Mw/Mn is measured as a polystyrene conversion value by GPC (gel per emission chromatography) method.

The melt flow rate (MFR) of the polypropylene resin is not particularly limited, but usually the MFR is preferably 0.5 g/10 min or more and 15 g/10 min or less, and 1.0 g/10 min. More preferably, it is 10 g/10 minutes or less.
By setting the MFR to 0.5 g/10 minutes or more, it is possible to have a sufficient melt viscosity at the time of molding and to ensure high productivity. On the other hand, by setting the MFR to 15 g/10 minutes or less, sufficient strength can be secured.
The MFR is measured under conditions of a temperature of 230° C. and a load of 2.16 kg in accordance with JIS K7210-1 (2014).

In addition, the method for producing a polypropylene resin is not particularly limited, and a known polymerization method using a known polymerization catalyst, such as a multi-site catalyst represented by a Ziegler-Natta type catalyst or a metallocene catalyst. and a polymerization method using a single-site catalyst.

Polypropylene resins that can be suitably used in the present invention include, for example, trade names "Novatec PP", "WINTEC", "WAYMAX" (manufactured by Japan Polypropylene Corporation), "Versify", "Notio", and "Tafmer XR" (Mitsui Chemicals, Inc.). (manufactured by Mitsubishi Chemical Co., Ltd.), “Zelath”, “Thermoran” (manufactured by Mitsubishi Chemical Co., Ltd.), “Sumitomo Noblen”, “Tafselene” (manufactured by Sumitomo Chemical Co., Ltd.), “Prime Polypro”, “Prime TPO” (manufactured by Prime Polymer Co., Ltd.), “Adflex”, “Adsyl "HMS-PP (PF814)" (manufactured by SunAllomer), "Inspire" (Dow Chemical) and other commercially available products can be used.

[β crystal activity]
The electret film of the present invention is preferably made of a resin composition containing a polypropylene-based resin containing a large amount of β-crystal, which is one of the crystal forms, as a main component. A nonporous film made of a resin composition mainly composed of a polypropylene-based resin containing a large amount of β crystals exhibits excellent piezoelectricity after electrification treatment. You get sex. Formation of a porous structure using β-crystals is a process in which β-crystals in the polypropylene resin transition to α-crystals during the stretching process. Compared to making porosity by adding a soluble organic substance, it is advantageous for preparing a porous structure because it does not depend on the particle size or dispersion diameter.
A film made porous using β-crystals has a dense porous structure and a large surface area of the pores, so that more charges are likely to be trapped in the charging process. Since the porous electret film exhibits piezoelectricity due to charges trapped at the interface between the pores and the matrix, the denser the porous structure of the film, the better the piezoelectric characteristics tend to be. In addition, when the porous structure is dense, the distance between vacancies becomes very short, and the trapped charges are easily fixed by mutual Coulomb forces. As a result, the trapped charges are less likely to be discharged, and the properties of the electret film are less likely to deteriorate.

The β-crystal activity of the electret film of the present invention can be regarded as an index indicating that the polypropylene-based resin generated β-crystals in the nonporous film before stretching. If the polypropylene-based resin in the nonporous film-like material before stretching has β crystals, many fine and uniform pores are formed by stretching after that, so it has excellent mechanical properties and fine and uniform pores. Excellent voltage resistance can be obtained by pore formation.

The presence or absence of β-crystal activity in the electret film of the present invention is determined by performing differential thermal analysis of the electret film using a differential scanning calorimeter, and whether or not the crystal melting peak temperature derived from the β-crystal of the polypropylene resin is detected. It is judged by
Specifically, the laminated porous film was heated from 40° C. to 200° C. at a heating rate of 10° C./min with a differential scanning calorimeter and held for 1 minute, and then cooled from 200° C. to 40° C. at a rate of 10° C./min. After the temperature was lowered at 1 minute, the temperature was held for 1 minute, and when the temperature was reheated from 40 ° C. to 200 ° C. at a heating rate of 10 ° C./min, the crystal melting peak temperature (Tmβ ) is detected, it is determined to have β-crystal activity.

The presence or absence of β-crystal activity can also be determined from the diffraction profile obtained by X-ray diffraction measurement of an electret film subjected to a specific heat treatment. Specifically, the electret film is subjected to heat treatment at 170 to 190 ° C., which is a temperature exceeding the crystal melting peak temperature of polypropylene resin, and slowly cooled to generate and grow β crystals. If a diffraction peak derived from the (300) plane of the β-crystal of the resin is detected in the range of 2θ=16.0° to 16.5°, it is determined that the β-crystal activity is present. For details on the β-crystal structure of polypropylene resin and X-ray diffraction measurement, see 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.

As a method of obtaining the β-crystal activity of the polypropylene-based resin described above, there is a method of not adding a substance that promotes the formation of α-crystals of the polypropylene-based resin, and a method of generating peroxide radicals as described in Japanese Patent No. 3739481. and a method of adding a β-crystal nucleating agent. In the present invention, it is particularly preferable to add a β-crystal nucleating agent to obtain β-crystal activity. . By adding a β-crystal nucleating agent, the formation of β-crystals in a polypropylene-based resin can be promoted more homogeneously and efficiently, and an electret film having β-crystal activity can be obtained.

The degree of β crystal activity can be quantified by measuring the β crystal formation ability. The electret film has a β-crystal formation ability of preferably 80% or more, more preferably 85% or more, and still more preferably 90% or more.
When the β-crystal forming ability is 80% or more, suitable piezoelectricity can be exhibited when a laminated piezoelectric sheet is formed. Although there is no particular upper limit, it is preferably 100% or less.
The β-crystal forming ability is calculated by the method described later.

(2) β Crystal Nucleating Agent The electret film of the present invention preferably contains a β crystal nucleating agent in order to obtain excellent piezoelectricity. β-crystal activity can be obtained by containing the β-crystal nucleating agent. The β-crystal nucleating agent used in the present invention includes the following. If necessary, two or more kinds of β-crystal nucleating agents may be mixed and used.

Examples of β-crystal nucleating agents include amide compounds; tetraoxaspiro compounds; quinacridones; iron oxides having a nanoscale size; potassium 1,2-hydroxystearate, magnesium benzoate or magnesium succinate, magnesium phthalate, and the like. alkali or alkaline earth metal salts of carboxylic acids represented by; aromatic sulfonic acid compounds represented by sodium benzenesulfonate or sodium naphthalenesulfonate; phthalocyanine-based pigments represented by such as; a two-component compound consisting of component A, which is an organic dibasic acid, and component B, which is an oxide, hydroxide or salt of a Group 2 metal of the periodic table; a cyclic phosphorus compound and magnesium A composition composed of a compound and the like can be mentioned.

Among these β crystal nucleating agents, amide compounds are preferred. Piezoelectric properties can be enhanced by using amide compounds in electret films. Examples of the amide compound include N,N'-dicyclohexyl-2,6-naphthalenedicarboxamide, N,N'-dicyclohexylterephthalamide, N,N'-diphenylhexanediamide and the like, among which N,N'-dicyclohexyl -2,6-naphthalene dicarboxamide is preferred. Since an amide compound has a highly polar amide group, it is believed that it is possible to localize charges in the crystal structure and have high piezoelectric properties.
On the other hand, a highly polar compound such as an amide compound has a problem of poor dispersibility due to electrostatic interaction with a polypropylene-based resin having a low polarity, and tends to agglomerate. However, common β-crystal nucleating agents have the property of dissolving in polypropylene-based resins in a certain temperature range. Due to this characteristic, the β-crystal nucleating agent is uniformly dispersed in the polypropylene-based resin, and crystals derived from the β-crystal nucleating agent are easily precipitated uniformly. Therefore, it is considered that crystals of the amide compound with high polarity are uniformly dispersed in the polypropylene resin with low polarity, and high piezoelectric properties can be obtained.

Specific examples of the commercially available β-crystal nucleating agent include the β-crystal nucleating agent “Enjestar NU-100” manufactured by Shin Nippon Rika Co., Ltd., and a specific example of the propylene-based resin to which the β-crystal nucleating agent is added is Aritech Co., Ltd. polypropylene “Bepol B-022SP” manufactured by Borealis, polypropylene “Beta (β)-PP BE60-7032” manufactured by Mayzo, and polypropylene “BNX BETAPP-LN” manufactured by mayzo.

The content of the β-crystal nucleating agent in the electret film of the present invention can be appropriately adjusted depending on the type of β-crystal nucleating agent or the composition of the polypropylene-based resin. 5.0 parts by mass is preferable, 0.001 to 3.0 parts by mass is more preferable, and 0.01 to 1.0 parts by mass is even more preferable. If it is 0.0001 parts by mass or more, the β-crystal of the polypropylene-based resin can be generated and grown sufficiently during production, and sufficient β-crystal activity can be secured, and sufficient β-crystal activity can be secured even when a porous film is formed. can. Therefore, a porous electret film having desired piezoelectricity can be obtained by subjecting the porous film to charging treatment. On the other hand, addition of 5.0 parts by mass or less is economically advantageous and is preferable because the β crystal nucleating agent does not bleed to the film surface.

The electret film of the present invention contains additives such as heat stabilizers, antioxidants, ultraviolet absorbers, light stabilizers, crystal nucleating agents, coloring agents, antistatic agents, and anti-hydrolysis agents, to the extent that their properties are not impaired. Various additives such as agents, lubricants, flame retardants, conductive agents, and elastomers may be included as appropriate.

The porosity of the electret film of the present invention is preferably 0% or more and 50% or less, more preferably 0% or more and 40% or less, still more preferably 5% or more and 30% or less, and even more preferably 10% or more.
By setting the porosity of the electret film within the above range, the impact resistance of the laminated piezoelectric sheet is improved.

The thickness of the electret film of the present invention is preferably 10 μm or more and 200 μm or less, more preferably 15 μm or more and 150 μm or less, and even more preferably 20 μm or more and 120 μm or less.
By setting the thickness of the electret film within the above range, the piezoelectric characteristics of the laminated piezoelectric sheet can be improved without increasing the thickness more than necessary.

2. Electrode The laminated piezoelectric sheet of the present invention has at least one electrode. At least two layers of electrodes are preferably provided so as to sandwich the electret film.
The layer to be the electrode should just have conductivity, and aluminum foil, copper foil, silver foil, gold foil, nickel foil, tin foil, carbon sheet and the like are preferably used.

The thickness of the electrode is preferably 2 μm or more and 100 μm or less, more preferably 3 μm or more and 50 μm or less, and even more preferably 5 μm or more and 30 μm or less.
When the thickness of the electrode is 2 μm or more, the electrode can exhibit conductivity stability. On the other hand, when the thickness of the electrode is 100 μm or less, the flexibility of the laminated piezoelectric sheet can be enhanced.

3. Protective Film The laminated piezoelectric sheet of the present invention has at least one protective film, and the protective film is a porous film.
Since the laminated piezoelectric sheet has a protective film, deterioration of the electret film due to moisture can be prevented.
The protective film is preferably provided so as to cover the electret film and the electrodes from the viewpoint of enhancing the water resistance of the laminated piezoelectric sheet.
Moreover, it is more preferable that at least two protective films are provided in the laminated piezoelectric sheet in order to protect both the front and back surfaces of the electret film and the electrodes.

In addition, since the protective film of the present invention is a porous film, the pressure applied to the laminated piezoelectric sheet is easily dispersed, so the force is easily propagated over a wide range of the electret film, and the piezoelectric properties are improved. The method for making the protective film porous is not particularly limited, but examples thereof include chemical or physical foaming and stretching. Among them, drawing to form a porous film is preferable because a dense porous structure can be obtained and the shape of the pores can be easily controlled.

As the protective film, a resin film such as a polyester resin film, a polyolefin resin film, an acrylic resin film, a polystyrene resin film, a polycarbonate resin film, or a fluorine resin film can be preferably used. From the point of view, a film obtained by laminating a hot-melt resin on these films can also be used. These films are also readily available as commercially available laminated films.
Above all, it is preferable that the protective film is made of the same material as the electret film. Therefore, when a polyolefin resin is used as the material of the electret film, it is preferable to use a polyolefin resin film as the protective film. Moreover, when a polypropylene-based resin is used as the material of the electret film, it is preferable to use a polypropylene-based resin film as the protective film.
When the same material is used for the protective film and the electret film, the amount of dimensional change due to temperature becomes the same, so even if the laminated piezoelectric sheet is subjected to temperature changes such as thermal processing, wrinkles and deflection are less likely to occur.

Specifically, the protective film preferably contains a polyolefin resin as a main component, more preferably a polypropylene resin as a main component. The details of the content when the polyolefin resin or polypropylene resin in the protective film is the main component are the same as those described for the electret film.
Among them, it is preferable that the protective film is made of a resin composition mainly composed of a polypropylene-based resin containing a large amount of β-crystals, and therefore preferably has β-crystal activity, and more preferably contains a β-crystal nucleating agent. . Forming a porous structure using β crystals in the protective film is advantageous in preparing a porous structure.
The physical properties and characteristics of the main component resin of the protective film may be the same as those of the electret film. Therefore, the details of the polypropylene-based resin used for the protective film are as described for the electret film.
In addition, details of the protective film having β-crystal activity, such as the method for determining the presence or absence of β-crystal activity in the protective film, the method for obtaining β-crystal activity, the value of β-crystal formation ability, and the type and content of β-crystal nucleating agent, are available at It is the same as that explained in the electret film, and the explanation thereof is omitted. In addition, the protective film has a dense porous structure by increasing the β-crystal formation ability to 80% or more.

In addition, additives such as heat stabilizers, antioxidants, ultraviolet absorbers, light stabilizers, crystal nucleating agents, coloring agents, antistatic agents, and hydrolysis inhibitors may be added to the protective film to the extent that the properties thereof are not impaired. , lubricants, flame retardants, conductive agents, and elastomers.

The porosity of the protective film of the present invention is preferably more than 0% and 50% or less, more preferably 5% or more and 40% or less, still more preferably 10% or more and 30% or less, and even more preferably 15% or more.
When the porosity of the protective film is more than 0%, the pressure applied to the laminated piezoelectric sheet is easily dispersed, so the force is easily propagated over a wide range of the electret film, and the piezoelectric properties are improved. Also, by setting the porosity of the protective film to 50% or less, the impact resistance of the present laminated piezoelectric sheet is improved.

The thickness of the protective film of the present invention is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less, still more preferably 10 μm or more and 40 μm or less, and even more preferably 20 μm or more and 30 μm or less.
When the protective film has a thickness of 1 μm or more, it is possible to sufficiently impart water resistance to the present laminated piezoelectric sheet. Further, when the thickness of the protective film is 100 μm or less, the pressure applied to the laminated piezoelectric sheet can be easily propagated to the electret film, and the flexibility of the laminated piezoelectric sheet can be ensured.

4. Others The laminated piezoelectric sheet of the present invention is provided with an electrode tab, a shield layer, a spacer, an adhesive layer, a buffer layer, etc., in addition to the above constituent members, for the purpose of improving handling properties and electrical properties when used as a device. good too.

This laminated piezoelectric sheet preferably has an electrode tab in order to connect the electrode to other electronic components. The electrode tab may have any configuration as long as it is provided so as to be connected to the electrode. For example, it may be formed on the protective film, may be formed on the electret film, or may be sandwiched between the protective film and the electret film. may be arranged so that
In addition, for example, when a plurality of electrodes are provided, the electrode tabs are preferably provided according to the number of electrode layers. For example, when two electrodes are provided, two electrode tabs are provided according to the number of layers. should be provided.

In addition, the laminated piezoelectric sheet preferably has an adhesive layer in order to prevent the piezoelectric properties from deteriorating due to positional displacement of the electrodes.
When the laminated piezoelectric sheet has an adhesive layer, the adhesive layer is preferably interposed between the electrode and the protective film so as not to degrade the piezoelectric properties of the electret film.

The material of the adhesive layer is not particularly limited. Examples include adhesives and the like. When the adhesive layer is used as an adhesive layer, it is preferable to use various adhesives such as acrylic adhesives, urethane adhesives, synthetic rubber adhesives, natural rubber adhesives, and silicone adhesives.
In addition, the adhesive layer preferably has electrical conductivity from the viewpoint of enhancing the electrical continuity stability of the electrode. Examples of methods for imparting conductivity include metal powder particles such as gold, silver, copper, nickel, and aluminum; conductive carbon particles such as carbon and graphite; and the surface of core materials such as resins, solid glass beads, and hollow glass beads. A method of incorporating conductive particles such as particles having a metal coating into the adhesive or pressure-sensitive adhesive.
Among them, from the viewpoint of conductivity and adhesiveness, an acrylic conductive pressure-sensitive adhesive containing metal powder particles such as nickel powder particles, copper powder particles, silver powder particles, etc. is preferable.

Although the thickness of the adhesive layer is not particularly limited, it is preferably 1 μm or more and 100 μm or less, and 2 μm or more and 50 μm or less from the viewpoint of ensuring adhesion between the protective film and the electrode without making the present laminated piezoelectric sheet thicker than necessary. is more preferable, and more preferably 4 μm or more and 35 μm or less.
Alternatively, an adhesive tape having an adhesive layer preliminarily laminated on one side of the electrode may be used. For example, an adhesive tape having an adhesive layer laminated on one side of a metal foil such as copper foil or aluminum foil may be used. may Such adhesive tapes may be commercially available products such as DIC's "E-2300ND", "E20CU", "E30CU", "E40CU", "E50CU", "E65CU", and "52050AD". etc. can be used.

6. Layer Structure of Laminated Piezoelectric Sheet As described above, the laminated piezoelectric sheet of the present invention has a laminated structure in which at least an electret film, an electrode, and a protective film are provided in this order.
At least one of the outermost layers of the laminated piezoelectric sheet is preferably a protective film in order to improve water resistance.
Moreover, as described above, the present laminated piezoelectric sheet preferably has a structure in which an electret film is arranged between two electrodes.
Furthermore, the present laminated piezoelectric sheet more preferably has a laminated structure in which at least a protective film, an electrode, an electret film, an electrode, and a protective film are provided in this order.
In the laminated structure having two layers of protective films, two protective films may be provided, and the protective film constituting each layer may be formed from another protective film. The protective film may be folded, for example, so that one protective film forms two layers of protective film.

In addition, when the present laminated piezoelectric sheet has two electrodes and an adhesive layer, at least one of the electrodes may be adhered to the protective film via the adhesive layer. From the viewpoint of preventing this and improving the piezoelectric properties, it is preferable to interpose an adhesive layer between each of the two electrodes and each of the protective films.
Therefore, when having two electrodes, the present laminated piezoelectric sheet preferably has a laminated structure in which at least a protective film, an adhesive layer, an electrode, an electret film, an electrode, an adhesive layer and a protective film are provided in this order.
At least a protective film, an electrode, an electret film, an electrode, an adhesive layer, and a protective film are provided in this order, and an adhesive layer may not be provided between one of the protective films and the electrode. In this case, one of the protective films and the electrodes are preferably laminated directly.

In addition, the electret film and the electrode may be directly laminated without intervening other layers, or may be interposed with other layers such as spacers other than adhesive layers. Therefore, the electrodes do not have to be adhered to the electret film. The spacer is a member for maintaining a constant gap between the electrode and the electret film, and is, for example, a circular or rectangular resin frame.
Therefore, when the laminated piezoelectric sheet has two electrodes, both electrodes may be directly laminated on the electret film, or both electrodes may be laminated on the electret film with a layer other than an adhesive layer such as a spacer. It may be laminated with intervening layers.
Alternatively, one electrode and the electret film may be directly laminated, and the other electrode and the electret film may be laminated with a layer other than an adhesive layer such as a spacer interposed therebetween.

<Manufacturing Method of Laminated Piezoelectric Sheet>
The method of manufacturing the laminated piezoelectric sheet of the present invention will be described. The following description is an example of the method of manufacturing the laminated piezoelectric sheet of the present invention. It is not limited to sheets.

The laminated piezoelectric sheet of the present invention is produced by preparing an electret film and a protective film and laminating these and electrodes. Note that other processes and processes may be further provided.

1. Manufacturing Method of Electret Film The electret film of the present invention is preferably manufactured through a film-forming process, a stretching process and an electrifying process. However, the stretching step may be omitted.
The film-forming process, the stretching process, and the electrification process will be sequentially described below.

(1) Film-forming process In the film-forming process, a nonporous film-like material made of the material constituting the electret film is formed. The film-forming method is not particularly limited, but an example thereof includes a method of heating and melting a resin (material resin) constituting the electret film to form a film. Specifically, the T-die method, the inflation method, and the like can be mentioned, and among them, it is preferable to adopt the T-die method. Practically, it is preferable to melt-extrude the material resin from a T-die and cast it with a cast roll (chill roll, cast drum, etc.).
In addition, the material resin may be mixed with additives as appropriate, and two or more resin components may be mixed to form a film as a resin composition containing two or more components.

The materials constituting the electret film may be kneaded in a kneading device and then formed into a film. The kneading device used for 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 necessity, a pressure reducing machine may be connected to the vent port of the extruder to remove moisture and low-molecular-weight substances from the material constituting the electret film.

When adopting the T-die method as a specific method for forming a film, a sheet-shaped molten resin extruded from the T-die is extruded onto a cast roll, and a film-shaped material is taken off while being brought into close contact with the rotating cast roll. A method of molding can be mentioned.
A touch roll, an air knife, an electric contact device, or the like may be attached to the cast roll in order to adhere the film material to the cast roll.

Moreover, when forming a film while cooling the molten resin (resin composition), the temperature of the cast roll is preferably 100° C. or higher. It is more preferably 110° C. or higher, and still more preferably 120° C. or higher. In the present invention, it is possible to increase the porosity even by opening the pores in the crystalline portion and the amorphous portion of the polypropylene resin during the stretching process. It is preferred to obtain non-porous membranes.
Moreover, the upper limit of the cast roll temperature is preferably 140° C. or less. It is more preferably 135° C. or lower, still more preferably 130° C. or lower. By setting the temperature of the cast roll to 140° C. or lower, the film can be easily peeled off from the cast roll during film formation.

In the resulting nonporous film-like material, the thickness of the effective portion excluding both end portions is preferably 30 μm or more and 500 μm or less, more preferably 40 μm or more and 300 μm or less, and even more preferably 50 μm or more and 200 μm or less. .
If the thickness of the nonporous membrane is 30 μm or more, breakage during stretching can be prevented, and if the thickness of the nonporous membrane is 500 μm or less, stretching of the nonporous membrane can be facilitated. can.
The layer structure of the nonporous film-like electret film is not limited to the single layer structure described above, and may be a structure in which other layers are combined.

(2) Stretching step The obtained nonporous membrane may be subjected to charging treatment as it is, or may be subjected to stretching treatment. By stretching the nonporous membrane, the nonporous membrane can be easily formed into a porous film.
In the stretching step, the non-porous membrane may be uniaxially stretched or biaxially stretched, but uniaxial stretching is preferable in order to make the porosity of the electret film more than 0% and 50% or less. The uniaxial stretching may be vertical uniaxial stretching or horizontal uniaxial stretching. Biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching.
The stretching of the film in the machine direction (MD) is called "longitudinal stretching", and the stretching in the direction perpendicular to the machine direction (TD) is called "transverse stretching".

The stretching temperature should be appropriately selected depending on the composition of the resin composition to be used, the crystalline melting peak temperature, the degree of crystallinity, etc. The longitudinal stretching temperature is preferably 60 to 140°C, more preferably 80 to 120°C. is.
By setting the longitudinal stretching temperature to 140° C. or lower, stretching can be performed at a temperature not higher than the melting point of the polypropylene-based resin, which is a suitable main component, without breakage, which is preferable. On the other hand, it is preferable to set the longitudinal stretching temperature to 60° C. or higher because breakage during stretching can be suppressed.
The transverse stretching temperature is preferably 90 to 160°C, more preferably 100 to 150°C. When the transverse stretching temperature is within the specified range, the pores generated during the longitudinal stretching are enlarged, the porosity is increased, and sufficient piezoelectric properties can be obtained.
The temperature described above is the temperature in the case of uniaxial stretching or sequential biaxial stretching, but the stretching temperature in the case of simultaneous biaxial stretching is preferably 90 ° C. or higher and 140 ° C. or lower, more preferably 90 ° C. or higher and 140 ° C. or lower, from the above viewpoint. The temperature may be adjusted within the range of 100° C. or higher and 120° C. or lower.

The draw ratio may be arbitrarily selected according to the desired porosity, but the draw ratio per uniaxial stretching is preferably 1.1 times or more and 10 times or less, more preferably 1.5 times or more and 9.0 times. or less, more preferably 1.5 times or more and 8.0 times or less.
By setting the draw ratio per uniaxial drawing to 1.1 times or more, whitening progresses and sufficient porosity is obtained by drawing. In addition, by setting the draw ratio per uniaxial drawing to 10 times or less, the porosity does not become too high, and an electret film having excellent pressure resistance can be obtained.
Further, in the case of sequential biaxial stretching, by stretching at the above-specified stretching ratio for each axis, voids generated during the previous stretching are not deformed during the subsequent stretching.

(3) Charging Treatment The electret film of the present invention can be obtained by subjecting the nonporous membrane obtained in the film-forming step or the porous film obtained through the stretching step to charging treatment. The charging treatment may be of a continuous type or a batch type. The electrification treatment may be performed by passing a film between electrodes such as a needle electrode, a wire electrode, a roll electrode, or a plate electrode, and applying an electric field between the electrodes. A method of applying an electric field after forming is also possible.
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 still more preferably 0.3 MV/m or more and 6 MV/m or less. When it is 0.1 MV/m or more, excellent piezoelectric properties can be obtained. When it is 10 MV/m or less, dielectric breakdown during electrification can be reduced.

Furthermore, the electret film of the present invention may be subjected to surface treatment such as corona treatment, plasma treatment, printing, coating, vapor deposition, etc., and perforation, etc., as necessary. It is also possible to stack several

2. Method for Producing Protective Film The protective film of the present invention is preferably produced through a film-forming process and a stretching process. However, the stretching step may be omitted.
The film-forming process and the stretching process of the protective film are the same as the film-forming process and the stretching process described in the above "1. Electret film manufacturing method". However, the following points are different.

The thickness of the effective portion of the non-porous membranous material obtained in the process of forming the protective film is preferably 10 μm or more and 500 μm or less, more preferably 20 μm or more and 300 μm or less, and more preferably 30 μm or more and 200 μm. More preferably:
If the thickness of the nonporous membrane is 10 μm or more, breakage during stretching can be prevented, and if the thickness of the nonporous membrane is 500 μm or less, stretching of the nonporous membrane can be facilitated. can.
The layer structure of the nonporous protective film is not limited to a single layer structure, and may be a structure in which other layers are combined.

3. Method for Producing Laminated Piezoelectric Sheet The laminated piezoelectric sheet of the present invention can be obtained by laminating the electret film and protective film produced by the above-described method, and an electrode. The order of lamination is preferably the protective film, the electrode, and the electret film from the outside.
For example, in the case of a laminated structure in which a protective film, an electrode, an electret film, an electrode, and a protective film are provided in this order, after bonding the two electrodes and the protective film using an adhesive, the two electrodes are formed into the electret film. It is preferable that the protective films are overlapped so as to face each other with the sandwiched between them, and the ends of the protective films are heat-sealed to be sealed.

Moreover, it is preferable that the present laminated piezoelectric sheet is made into a bag shape by laminating the electret film, the electrodes and the protective film, and then fusing the ends of the protective film. A method for fusing the ends is not particularly limited, and a heat sealer or the like may be used.
When forming a bag, it is preferable to provide two layers each of the electrode and the protective film. For example, the protective film, the electrode, the electret film, the electrode, and the protective film are laminated in this order, and the ends of the protective films are heated. It is preferable to heat-seal and form a bag.
When two protective films are used, the ends of the two protective films may be partially fused in advance before lamination.
Also, the ends of the protective film may be bonded together by a method other than heat sealing.

<Sensor device>
The laminated piezoelectric sheet of the present invention can be made into a sensor device by providing lead wires or circuit mounting.
Since the laminated piezoelectric sheet is excellent in water resistance and piezoelectric properties, a vibration power generator, a water level gauge, an acoustic detector, a mat sensor, a robot hand, and the like are preferable as a sensor device provided with the laminated piezoelectric sheet.

Examples and Comparative Examples are shown below to describe the laminated piezoelectric sheet of the present invention in more detail, but the present invention is not limited in any way.

Materials used in Examples and Comparative Examples are as follows.
(polypropylene resin)
・ A-1; homopolypropylene (Novatec PP FY6HA, MFR: 2.4 g/10 minutes [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 (manufactured by Shin Nippon Chemical Co., Ltd., NU-100)
(Antioxidant)
· C-1; tris (2,4-di-t-butylphenyl) phosphite and tetrakis [3-(3',5'-di-t-butyl-4'-hydroxyphenyl) propionic acid] pentaerythritol 1: 1 mixture of (IRGANOX-B225, manufactured by BASF)

(Formation of electret film)
100 parts by mass of polypropylene resin (A-1), 0.2 parts by mass of β-crystal nucleating agent (B-1), and 0.1 part by mass of antioxidant (C-1) are mixed and fed into a twin-screw extruder. A mixture 1 for an electret film was obtained by melt-extrusion at 280°C. The electret film mixture 1 was put into an extruder connected to a T-die with a lip opening of 1 mm, molded, and guided to cast rolls to obtain a non-porous film having a thickness 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 (manufactured by Kyoto Kikai Co., Ltd.) to obtain a porous film.
The obtained porous film was placed on an earth plate, and an electret film was obtained by applying a voltage of 10 kV using a needle-like electrode with a distance between the electrodes of 20 mm and performing an electrification treatment.

(Example 1)
100 parts by mass of polypropylene resin (A-1), 0.2 parts by mass of β-crystal nucleating agent (B-1), and 0.1 part by mass of antioxidant (C-1) are mixed and fed into a twin-screw extruder. A protective film mixture 1 was obtained by melt-extrusion at 280°C. The protective film mixture 1 was put into an extruder connected to a T-die with a lip opening of 1 mm, molded, and led to a cast roll to obtain a non-porous membrane 1 having a thickness of 55 μm. Thereafter, the film was stretched 7 times in the horizontal direction at a stretching temperature of 100° C. using a film tenter (manufactured by Kyoto Kikai Co., Ltd.) to obtain a porous film (protective film 1) having a thickness of 10 μm.
Inside the protective film 1, as an electrode having an adhesive layer on one side, two pieces of conductive copper foil adhesive tape "E20CU" (manufactured by DIC, electrode thickness 9 μm, adhesive layer thickness 11 μm) were cut into 19 cm squares. After sticking together, an electrode tab connecting to the two electrodes was formed on the protective film.
Thereafter, an electret film cut into 20 cm squares was sandwiched between two electrodes so as not to protrude from the electrodes, and the ends of the protective films were heat-sealed using a heat sealer to produce a laminated piezoelectric sheet. .
The electrode tab extended from the inside to the outside of the bag formed by fusing the ends of the protective film.

(Example 2)
A nonporous membrane 2 was obtained in the same manner as in Example 1 except that the thickness of the nonporous membrane was 100 μm. Thereafter, the film was stretched 7 times in the horizontal direction at a stretching temperature of 100° C. using a film tenter (manufactured by Kyoto Kikai Co., Ltd.) to obtain a porous film (protective film 2) having a thickness of 20 μm.
A laminated piezoelectric sheet was produced in the same manner as in Example 1 using the protective film 2 described above.

(Comparative example 1)
A laminated piezoelectric sheet was produced in the same manner as in Example 1, using a polyethylene terephthalate film (manufactured by Mitsubishi Chemical Corporation, non-porous film) with a thickness of 40 μm as a protective film.

The thickness, porosity, β-crystal forming ability, output voltage, and water resistance of the laminated piezoelectric sheets obtained in Examples and Comparative Examples were measured and evaluated by the following methods.

(1) Thickness Using a dial gauge of 1/1000 mm, the thickness of the protective film and the electret film was randomly measured at 10 points, and each average value was obtained.

(2) Porosity Each of the protective film and the electret film was cut into a width of 100 mm and a length of 100 mm to obtain a sample for measurement. The actual weight W1 of the measurement sample was measured, the weight W0 when the porosity was 0% was calculated based on the density of the resin composition, and the porosity was calculated from these values according to the following formula.
Porosity (%) = {(W0-W1)/W0} x 100
Since the protective film of Comparative Example 1 is a non-porous film, it is indicated as "-" without measuring the porosity.

(3) β-Crystal Formation Ability A protective film and an electret film were subjected to DSC measurement by the following method. First, the temperature was raised from 40° C. to 200° C. at a rate of 10° C./min in a nitrogen atmosphere, held for 1 minute, and then cooled to 40° C. at a rate of 10° C./min. After holding for 1 minute, the melting peaks observed when the temperature was raised again at 10 ° C./min. Using the melting peak of the α crystal as the melting peak, the base line drawn based on the flat part on the high temperature side and the area of the region surrounded by the peak are used to obtain the respective heats of fusion, and the heat of fusion of the α crystals is ΔHα, β crystal was calculated by the following formula, with the heat of fusion of ΔHβ.
β-crystal formation ability (%) = [ΔHβ/(ΔHα+ΔHβ)]×100

(4) Output Voltage A laminated piezoelectric sheet was placed on a horizontal table, a polyethylene foam plate of 10 mm thickness was placed thereon, and a polycarbonate plate of 2 mm thickness was placed thereon. Both the foamed polyethylene plate and the polycarbonate plate are 20 cm square. At this time, a softball with a diameter of 9.7 cm and a weight of 187.8 g was vertically dropped from a point of 30 cm in height from the laminated piezoelectric sheet, and the peak value of the output voltage generated at that time was measured using an oscilloscope. The measurement was performed 5 times, and the average of the output voltage peak values was calculated.

(5) Water resistance The laminated piezoelectric sheet was immersed in water at 25°C for 10 minutes, air-dried, and the output voltage was measured in the same manner as in (4) above. rate was calculated.
Retention rate = [(output voltage after water immersion test)/(output voltage of blank (before water immersion test))] x 100 (1)
A case where the retention rate was 80% or more was evaluated as A (good), and a case where the retention rate was less than 80% was evaluated as B (poor).

Table 1 shows the evaluation results of Examples and Comparative Examples.

It was confirmed that the laminated piezoelectric sheets of Examples 1 and 2 had water resistance without a significant drop in output voltage even after being submerged in water due to the protective film.
In addition, in the laminated piezoelectric sheets of Examples 1 and 2, since the protective film is a porous film, the pressure applied to the laminated piezoelectric sheet is dispersed, and the force is easily propagated over a wide range of the electret. The output voltage was higher than that of Comparative Example 1 used, and the piezoelectric characteristics were good.

Claims (10)

A laminated piezoelectric sheet having a laminated structure in which at least an electret film, an electrode and a protective film are provided in this order, wherein the protective film is a porous film. The laminated piezoelectric sheet according to claim 1, wherein the electret film has a porosity of 0% or more and 50% or less. 3. The laminated piezoelectric sheet according to claim 1, wherein the electret film is a porous film. 4. The laminated piezoelectric sheet according to any one of claims 1 to 3, wherein the electret film and/or protective film contains a polyolefin resin as a main component. 5. The laminated piezoelectric sheet according to claim 4, wherein the polyolefin resin is a polypropylene resin having a β-crystal forming ability of 80% or more. The laminated piezoelectric sheet according to any one of claims 1 to 5, wherein the protective film has a porosity of more than 0% and not more than 50%. The laminated piezoelectric sheet according to any one of claims 1 to 6, wherein the electret film has a thickness of 10 µm or more and 200 µm or less. The laminated piezoelectric sheet according to any one of claims 1 to 7, wherein the protective film has a thickness of 1 µm or more and 100 µm or less. The laminated piezoelectric sheet according to any one of claims 1 to 8, wherein the end portions of the protective film are fused to form a bag and seal the inside. A sensor device using the laminated piezoelectric sheet according to any one of claims 1 to 9.