Laminated piezoelectric body and manufacturing method therefor

Abstract

This laminated piezoelectric body comprises: a piezoelectric film containing a fluorine-based resin as a main component; an antistatic layer disposed on at least one surface of the piezoelectric film; and a moisture-proof layer disposed on the reverse side of the antistatic layer from the piezoelectric film. The water vapor permeability of the laminated piezoelectric body in an environment of 40ºC and 90% RH is 0.00 g / m2 / day / atm to 4.00 g / m2 / day / atm.

Description

Multilayer piezoelectric element and its manufacturing method

[0001] The present invention relates to a laminated piezoelectric element and a method for manufacturing the same.

[0002] Various touch-type input devices have been proposed, in which a touch panel is attached to the display of an electronic device and operation input is detected by touching the operation surface. As such touch-type input devices, a three-dimensional touch sensor (a touch sensor that detects position (two dimensions) and pressure (one dimension)) that detects position and pressure has been proposed.

[0003] As a three-dimensional touch sensor, a touch sensor that detects position coordinates has been proposed that is equipped with a pressure-sensitive sensor, thereby enabling simultaneous detection of the magnitude of pressure in addition to the detection of position coordinates (see Patent Documents 1 and 2). Such touch panels require accurate visual recognition of images and high detection sensitivity, so the development of a piezoelectric film with high transparency and a high piezoelectric constant is anticipated.

[0004] Known piezoelectric films include those containing polymeric materials such as polylactic acid and fluororesin. While polymeric piezoelectric films have excellent transparency, they are prone to charging due to friction and vibration during transportation, generating static electricity. This can lead to poor appearance and performance degradation of piezoelectric devices due to the static electricity itself or the static electricity attracting dust to the piezoelectric film. To address these issues, a piezoelectric element has been proposed in which an antistatic layer is laminated on a piezoelectric film containing polylactic acid (see Patent Document 3).

[0005] Among piezoelectric films containing polymeric materials, those containing fluororesin not only have a higher piezoelectric constant than those containing polylactic acid, but also tend to be piezoelectrically polarized in the same direction as the stress direction, making them more suitable for touch panels.

[0006] Patent Document 1: JP 2010-26938 A, International Publication No. 2010 / 102635, International Publication No. 2016 / 098597

[0007] Touch panels are used in a variety of environments. For example, when used in car navigation systems and other in-vehicle applications, touch panels are required to maintain high transparency even under high temperature and humidity conditions.

[0008] However, it has been found that when a laminated piezoelectric body in which an antistatic layer is laminated on a piezoelectric film containing a fluorine-based resin is used under high temperature and high humidity conditions, the piezoelectric film is prone to discoloration over time and its transparency decreases.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laminated piezoelectric element that can suppress discoloration of the piezoelectric film under high temperature and high humidity conditions and maintain high transparency, and a method for manufacturing the same.

[0010] [1] A laminated piezoelectric body comprising: a piezoelectric film containing a fluorine-based resin as a main component; an antistatic layer disposed on at least one surface of the piezoelectric film; and a moisture-proof layer disposed on the opposite side of the piezoelectric film with the antistatic layer interposed therebetween, wherein the moisture-proof layer has a water vapor permeability of 0.00 g / m or more in an environment of 40°C and 90% RH. ２ / day / atm or more 4.00g / m ２ / day / atm or less. [2] The laminated piezoelectric element according to [1], wherein no pressure-sensitive adhesive layer is disposed between the antistatic layer and the moisture-proof layer. [3] The laminated piezoelectric element according to [1] or [2], wherein the antistatic layer contains a cured product of a curable composition containing a conductive material and an amine-based material. [4] The laminated piezoelectric element according to any one of [1] to [3], wherein the moisture-proof layer is composed of an inorganic oxide. [5] The laminated piezoelectric element according to any one of [1] to [4], further comprising at least one of a hard coat layer and an optical adjustment layer disposed between the antistatic layer and the moisture-proof layer. [6] The laminated piezoelectric element according to [5], wherein the total thickness of the hard coat layer and the optical adjustment layer is 0.30 μm or more and 4.0 μm or less. [7] When the laminated piezoelectric element is maintained in an environment of 85°C and 85% RH for 500 hours, the hue difference Δb before and after the maintenance in the environment is ＊[8] The laminated piezoelectric element according to any one of [1] to [7], wherein the antistatic layer has a thickness of 0.010 μm or more and 0.40 μm or less. [9] The laminated piezoelectric element according to any one of [1] to [8], wherein the fluororesin contains, as a main component, a polymer containing a structural unit derived from vinylidene fluoride.

[10] A method for producing the laminated piezoelectric element according to any one of [1] to [9], comprising the steps of: applying a curable composition containing a conductive material and an amine-based material onto the piezoelectric film, and curing the composition to form an antistatic layer; and forming a moisture-proof layer on the antistatic layer by a reactive sputtering method.

[0011] According to the present invention, it is possible to provide a laminated piezoelectric element in which the piezoelectric film undergoes little discoloration and maintains high transparency even under high temperature and high humidity conditions, and a method for producing the same.

[0012] Fig. 1 is a schematic cross-sectional view showing a laminated piezoelectric material according to the present embodiment. Figs. 2A and 2B are schematic cross-sectional views showing a laminated piezoelectric material according to another embodiment. Fig. 3 is a schematic cross-sectional view showing a laminated piezoelectric material according to another embodiment.

[0013] As described above, it has become clear that when a laminated piezoelectric element including a piezoelectric film containing a fluorine-based resin and an antistatic layer is used under high temperature and high humidity conditions, the piezoelectric film discolors.

[0014] The mechanism behind this is unclear, but is speculated as follows. In a laminated piezoelectric element containing an antistatic layer, bases such as amines may be generated due to, for example, decomposition of the constituent materials of the antistatic layer. When the fluororesin contained in the piezoelectric film comes into contact with a base in a high-temperature, high-humidity environment, a dehydrofluorination reaction occurs, which easily forms conjugated double bonds within the molecule. As a result, piezoelectric films containing fluororesin are prone to discoloring to a yellowish color.

[0015] In response to this problem, the present inventors have discovered that providing a moisture-proof layer on the antistatic layer, either directly or via another layer, can suppress the dehydrofluorination reaction caused by contact between the fluorine-based resin and a base, thereby suppressing discoloration of the piezoelectric film. As a result, a laminated piezoelectric body can be obtained that exhibits minimal discoloration of the piezoelectric film and maintains high transparency even when stored under high temperature and high humidity conditions. The laminated piezoelectric body of the present invention will be described in detail below.

[0016] 1. Laminated Piezoelectric The laminated piezoelectric body includes a piezoelectric film, an antistatic layer, and a moisture-proof layer. If necessary, the laminated piezoelectric body may further include other layers, such as a hard coat layer or an optical adjustment layer, to prevent scratches on the antistatic layer or adjust the hue. However, from the viewpoint of further suppressing discoloration of the piezoelectric film, it is preferable that the laminated piezoelectric body does not include a layer (e.g., a pressure-sensitive adhesive layer) that is easily penetrated by moisture from the outside between the antistatic layer and the moisture-proof layer on the side of the piezoelectric film where the antistatic layer is disposed.

[0017] A laminated piezoelectric element according to one embodiment of the present invention and a method for manufacturing the same will be specifically described below.

[0018] 1 is a schematic cross-sectional view showing a laminated piezoelectric element 10 according to this embodiment. As shown in FIG. 1, the laminated piezoelectric element 10 according to this embodiment includes a piezoelectric film 11, an antistatic layer 12, a hard coat layer 13, and a moisture-proof layer 14.

[0019] 1-1. Piezoelectric Film 11 The piezoelectric film contains a fluororesin as a main component. "Containing a fluororesin as a main component" means that the content of the fluororesin in the piezoelectric film is 50% by mass or more, and preferably 60% by mass or more.

[0020] Among fluororesins, the fluororesin is preferably a polymer containing a structural unit derived from vinylidene fluoride, from the viewpoint that a high piezoelectric effect can be easily obtained.

[0021] The content of the vinylidene fluoride-derived structural units in the polymer is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, still more preferably 80% by mass or more and 100% by mass or less, and particularly preferably 90% by mass or more and 100% by mass or less, relative to the total amount of the structural units in the polymer. The higher the content of the vinylidene fluoride-derived structural units, the more likely it is that a higher piezoelectric effect will be obtained.

[0022] The polymer may further contain a structural unit derived from a monomer copolymerizable with vinylidene fluoride, provided that the effects of the present invention are not impaired. Examples of the monomer copolymerizable with vinylidene fluoride include fluorine-containing monomers such as trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, and vinyl fluoride. Two or more of these monomers may be contained.

[0023] Among them, the piezoelectric constant d ３３ From the viewpoint of increasing the value of the piezoelectric constant and making it easier to obtain the piezoelectric effect, a homopolymer of vinylidene fluoride is preferred.

[0024] Piezoelectric constant d of the piezoelectric film ３３ The piezoelectric constant d of the piezoelectric film is preferably 7 pC / N or more and 40 pC / N or less. ３３ When the piezoelectric constant d of the piezoelectric film is 7 pC / N or more, the amount of electric charge generated by the piezoelectric effect is larger, and therefore the pressure sensitivity can be further improved. ３３ When the piezoelectric constant d of the piezoelectric film is 40 pC / N or less, it is possible to make it less likely that the appearance of the piezoelectric film will be deteriorated due to the decrease in the flatness of the surface of the piezoelectric film caused by the polarization treatment. ３３ is more preferably 10 pC / N or more and 40 pC / N or less, further preferably 13 pC / N or more and 35 pC / N or less, and particularly preferably 15 pC / N or more and 30 pC / N or less.

[0025] Piezoelectric constant d of the piezoelectric film ３３ is the direct quasi-static method (d ３３ The piezoelectric constant d of the piezoelectric ceramic measured by the Meter method and the Berlincoat method ３３The piezoelectric constant can be measured in accordance with the test method ISO 19622: 2018. Specifically, the piezoelectric constant can be measured by using a piezoelectric constant measuring device (for example, Piezometer System PM300 manufactured by PIEZOTEST) to clip a sample with 1.0 N and read the generated charge when a force of 0.15 N, 110 Hz is applied.

[0026] Piezoelectric constant d of the piezoelectric film ３３ can be adjusted mainly by the type of resin contained in the piezoelectric film and the manufacturing conditions (conditions for polarization and stretching). For example, among fluorine-based resins, the more structural units derived from vinylidene fluoride a resin contains, the lower the piezoelectric constant d ３３ In addition, by strengthening the polarization process or stretching process, the piezoelectric constant d ３３ tends to become large.

[0027] The thickness of the piezoelectric film can be adjusted by the film-forming conditions and stretching ratio of the resin film. The thickness of the piezoelectric film is preferably, for example, 25 μm or more and 120 μm or less. When the thickness of the piezoelectric film is 25 μm or more, the amount of charge generated by the piezoelectric effect increases, making it easier to obtain higher piezoelectricity. The thickness of the piezoelectric film is more preferably 30 μm or more, and even more preferably 35 μm or more. When the thickness of the piezoelectric film is 120 μm or less, the transparency of the piezoelectric film is less likely to be impaired, and 100 μm or less is more preferable, and 80 μm or less is even more preferable. From the same viewpoint, the thickness of the piezoelectric film is more preferably 35 μm or more and 80 μm or less.

[0028] 1-2. Antistatic Layer 12 The antistatic layer is disposed on at least one surface of the piezoelectric film. In this embodiment, the antistatic layer is disposed on one surface of the piezoelectric film (see FIG. 1). The antistatic layer is preferably in contact with the piezoelectric film. The antistatic layer can make it difficult for static electricity to be generated in the laminated piezoelectric element.

[0029] The antistatic layer may contain a cured product of a curable composition containing a conductive material. The curable composition containing a conductive material may contain a conductive material, a curable resin, and optionally a crosslinking agent, or may contain a conductive material and a crosslinking agent.

[0030] The conductive material may be an ion-conductive conductive material or an electron-conductive conductive material.

[0031] Examples of the ion-conductive conductive material include: (a) cationic antistatic agents having a cationic group such as a quaternary ammonium salt, a pyridinium salt, or a primary, secondary, or tertiary amino group; (b) anionic antistatic agents having an anionic group such as a sulfonate group, a sulfate ester group, a phosphate ester group, or a phosphonate group; (c) amphoteric antistatic agents such as amino acid-based or amino sulfate-based; and (d) nonionic antistatic agents such as amino alcohol-based, glycerin-based, or polyethylene glycol-based.

[0032] Examples of electron-conductive conductive materials include conductive polymers and other conductive materials. Examples of conductive polymers include polyacetylene or its derivatives, polythiophene or its derivatives, polypyrrole or its derivatives, and polyaniline or its derivatives. Among them, polythiophene or its derivatives are preferred from the viewpoint of high transparency and high conductivity. These conductive polymers may have functional groups such as sulfonic acid groups, amino groups, amide groups, hydroxyl groups, mercapto groups, and carboxyl groups. Examples of conductive materials other than conductive polymers include carbon nanotubes and graphene.

[0033] Among these, it is preferable to include an electron-conductive conductive material, and carbon nanotubes are more preferable, from the viewpoint of easily reducing the surface resistivity of the laminated piezoelectric body and making bleed-out less likely to occur.

[0034] Examples of the curable resin include acrylic resins, epoxy resins, polyurethane resins, polyimide resins, melamine resins, polyester resins, polyvinyl alcohol resins, polystyrene resins, polyvinyl acetate resins, silicone compounds, polyethylene glycols, and the like, which have functional groups such as hydroxyl groups, methylol groups, carboxyl groups, sulfonyl groups, epoxy groups, and amino groups.

[0035] The crosslinking agent may be any agent that reacts with functional groups of the curable resin or conductive polymer to form crosslinks, and examples of the crosslinking agent include melamine-based crosslinking agents, polycarbodiimide-based crosslinking agents, polyoxazolidine-based crosslinking agents, polyepoxy-based crosslinking agents, and polyisocyanate-based crosslinking agents.

[0036] As described above, curable resins and crosslinking agents having amino groups, such as melamine resins (also referred to as "amine-based materials"), are sometimes used as curable resins and crosslinking agents for obtaining antistatic layers. Among these, melamine resins are sometimes preferred because of their low-temperature curing properties. However, such amine-based materials tend to generate amines (bases) that cause dehydrofluorination of fluorine-based resins under high-temperature and high-humidity conditions, which can particularly cause discoloration of the piezoelectric film. Even in such cases, discoloration of the piezoelectric film in the laminated piezoelectric element can be suppressed by providing the moisture-proof layer described above.

[0037] The surface resistivity of the antistatic layer is, for example, 1.0×10 ４ Ω / sq. More than 1.0×10 ９ Ω / sq. or less, preferably 1.0×10 ５ Ω / sq. More than 1.0×10 ８ It is more preferable that the surface resistivity of the antistatic layer is 1.0×10 / sq. or less. ９ When the surface resistivity is Ω / sq. or less, the surface resistivity of the laminated piezoelectric body can be further reduced, and sufficient antistatic properties can be imparted. The surface resistivity can be measured in accordance with JIS K 6911 using, for example, a known resistivity meter (for example, a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800, URS probe)).

[0038] The thickness of the antistatic layer is not particularly limited, but is preferably, for example, 0.010 μm or more and 0.40 μm or less. By making the thickness of the antistatic layer 0.010 μm or more, the generation of static electricity in the laminated piezoelectric body can be further suppressed. If the thickness of the antistatic layer is 0.40 μm or less, discoloration of the piezoelectric film in the laminated piezoelectric body can be more unlikely to occur. From the same viewpoint, the thickness of the antistatic layer is more preferably 0.030 μm or more and 0.40 μm or less, and even more preferably 0.045 μm or more and 0.30 μm or less.

[0039] The thickness of the antistatic layer can be measured using a spectral interference film thickness meter (for example, Optical NanoGauge C13027-11 manufactured by Hamamatsu Photonics K.K.). The thickness of each layer is measured at three points in an area including the center of the surface of the laminated piezoelectric body, and the thickness can be calculated as the arithmetic mean value. The thickness of each of the following layers can also be measured in a similar manner.

[0040] 1-3. Hard Coat Layer 13 The hard coat layer is preferably disposed between the antistatic layer and the moisture-proof layer. The hard coat layer can suppress an increase in haze by making the surface of the antistatic layer less susceptible to scratches during the production of the laminated piezoelectric element. Only one hard coat layer may be included, or two or more hard coat layers may be included. When two or more hard coat layers are included, one of the two or more hard coat layers may be disposed on one side of the piezoelectric film, and the other may be disposed on the other side.

[0041] The hard coat layer preferably contains a cured product of a curable composition containing a polymerizable compound.

[0042] (Polymerizable Compound) The polymerizable compound may be any one of a monomer, an oligomer, or a polymer. The polymerizable compound may be a thermosetting compound or an ionizing radiation compound, but is preferably an ionizing radiation compound. The ionizing radiation may usually be ultraviolet (UV) or an electron beam (EB).

[0043] The ionizing radiation-curable compound is a compound having an ionizing radiation-curable functional group. Examples of the ionizing radiation-curable functional group include ethylenically unsaturated bond groups such as (meth)acryloyl groups, vinyl groups, and allyl groups, and ring-opening polymerizable groups such as epoxy groups and oxetanyl groups. Among these, compounds having an ethylenically unsaturated bond group are preferred, compounds having two or more ethylenically unsaturated bond groups are more preferred, and polyfunctional (meth)acrylate compounds are even more preferred. The term "(meth)acrylate" refers to either or both of acrylate and methacrylate.

[0044] Among the polyfunctional (meth)acrylate compounds, examples of bifunctional (meth)acrylate monomers include ethylene glycol di(meth)acrylate, bisphenol A tetraethoxydiacrylate, bisphenol A tetrapropoxydiacrylate, 1,6-hexanediol diacrylate, etc. Examples of trifunctional or higher functional (meth)acrylate monomers include trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol tetra(meth)acrylate, isocyanuric acid-modified tri(meth)acrylate, etc. Furthermore, the above-mentioned (meth)acrylate monomers may be partially modified in the molecular skeleton, and those modified with ethylene oxide, propylene oxide, caprolactone, isocyanuric acid, alkyl, cyclic alkyl, aromatic, bisphenol, etc. can also be used.

[0045] Examples of polyfunctional (meth)acrylate oligomers include acrylate polymers such as urethane (meth)acrylate, epoxy (meth)acrylate, polyester (meth)acrylate, polyether (meth)acrylate, etc. Urethane (meth)acrylate can be obtained, for example, by reacting a polyhydric alcohol and an organic diisocyanate with a hydroxy (meth)acrylate.

[0046] (Polymerization Initiator) When the ionizing radiation-curable compound is an ultraviolet-curable compound, the curable composition preferably contains a photopolymerization initiator. Examples of the photopolymerization initiator include one or more selected from acetophenone, benzophenone, α-hydroxyalkylphenone, Michler's ketone, benzoin, benzyl methyl ketal, benzoyl benzoate, α-acyloxime ester, thioxanthones, etc.

[0047] (Other Components) The curable composition may further contain other components in addition to those described above, as necessary. For example, from the viewpoint of suppressing blocking during the production of the laminated piezoelectric body or adjusting the refractive index, the curable composition may further contain particles. The particles may be inorganic particles or organic particles.

[0048] Examples of inorganic particles include particles of silica (silicon oxide), titanium oxide, zirconium oxide, aluminum oxide, zinc oxide, tin oxide, etc., diamond powder, sapphire particles, boron carbide particles, silicon carbide particles, antimony pentoxide particles, etc. Examples of organic particles include resin particles of acrylic resin, acrylic-styrene copolymer, silicone resin, etc. Among these, inorganic particles are preferred, and silica particles are more preferred, from the viewpoint of further suppressing the decrease in transparency of the hard coat layer. The surfaces of the inorganic particles may be treated with a surface modifier such as a silane coupling agent.

[0049] The thickness of the hard coat layer is not particularly limited, but is, for example, 0.05 μm or more, preferably 0.3 μm to 3.0 μm, more preferably 0.5 μm to 2.0 μm, and even more preferably 0.5 μm to 1.5 μm.

[0050] As described below, the laminated piezoelectric element may further include an optical adjustment layer between the antistatic layer and the moisture-proof layer (see FIG. 2B ). The total thickness of the hard coat layer and the optical adjustment layer is preferably 0.30 μm or more and 4.0 μm or less. When the total thickness is 4.0 μm or less, moisture penetration from the outside through these layers can be further suppressed, and the effect of suppressing discoloration of the piezoelectric film by providing a moisture-proof layer can be more easily maintained. Furthermore, when the total thickness is 0.30 μm or more, the color tone of the laminated piezoelectric element can be further improved and its transparency can be further increased. From the same perspective, the total thickness is more preferably 0.50 μm or more and 3.0 μm or less, and even more preferably 0.60 μm or more and 2.0 μm or less.

[0051] 1-4. Moisture-proof layer 14 The moisture-proof layer is disposed on the opposite side of the piezoelectric film, with the antistatic layer sandwiched between them. In this embodiment, it is disposed on the hard coat layer (see FIG. 1). The moisture-proof layer can suppress the penetration of moisture, which causes discoloration of the piezoelectric film. Only one moisture-proof layer may be included, or two or more moisture-proof layers may be included. When two or more moisture-proof layers are included, one of the two or more moisture-proof layers may be disposed on one side of the piezoelectric film, and the other may be disposed on the other side.

[0052] The material constituting the moisture-proof layer is not particularly limited as long as it is transparent and can prevent moisture from penetrating. For example, the moisture-proof layer is preferably made of an inorganic oxide, and more preferably a thin film of an inorganic oxide (such as a vapor-deposited film).

[0053] The inorganic oxide is a metal oxide, a nonmetal oxide, or a submetal oxide. Examples of the inorganic oxide include aluminum oxide, zinc oxide, antimony oxide, indium oxide, indium tin oxide, calcium oxide, cadmium oxide, silver oxide, gold oxide, chromium oxide, silicon oxide, cobalt oxide, zirconium oxide, tin oxide, titanium oxide, iron oxide, copper oxide, nickel oxide, platinum oxide, palladium oxide, bismuth oxide, magnesium oxide, manganese oxide, molybdenum oxide, vanadium oxide, and barium oxide, and indium tin oxide and silicon oxide are particularly preferred.

[0054] The thickness of the moisture-proof layer is not particularly limited, but from the viewpoint of achieving a higher degree of both moisture resistance and transparency, it is preferably from 0.005 μm to 0.100 μm, more preferably from 0.010 μm to 0.060 μm, and even more preferably from 0.015 μm to 0.050 μm.

[0055] 1-5. Physical Properties of the Laminated Piezoelectric Body 10 (Water Vapor Permeability) The laminated piezoelectric body can suppress moisture permeation by including a moisture-proof layer. Specifically, the water vapor permeability of the laminated piezoelectric body in an environment of 40°C and 90% RH is 0.00 g / m ２ / day / atm or more 4.00g / m ２ / day / atm or less, and 0.00 g / m ２ / day / atm or more 2.00g / m ２ / day / atm or less, and 0.05 g / m ２ / day / atm or more 2.00g / m ２ / day / atm or less is more preferable, and 0.05 g / m ２ / day / atm or more 1.00g / m ２ It is particularly preferable that the water vapor permeability of the laminated piezoelectric body is 4.00 g / m or less. ２ When the moisture permeability of the laminated piezoelectric body is 0.05 g / m / day / atm or less, the penetration of moisture into the laminated piezoelectric body can be reduced, and discoloration of the piezoelectric film can be suppressed. ２ When the moisture-proof layer is heated to 10000 kJ / day / atm or more, the thickness of the moisture-proof layer can be made thinner, and coloration caused by inorganic compounds contained in the moisture-proof layer can be further suppressed.

[0056] The water vapor permeability of the laminated piezoelectric body can be measured under conditions of 40° C. and 90% RH using a water vapor permeability measuring device in accordance with JIS K7129-2.

[0057] The water vapor permeability of the laminated piezoelectric body can be adjusted by the layer structure and the composition or thickness of the moisture-proof layer. For example, if a hard coat layer or an optical adjustment layer is further provided between the antistatic layer and the moisture-proof layer, the water vapor permeability tends to be lower. Furthermore, if the thickness of the moisture-proof layer is increased, the water vapor permeability tends to be lower.

[0058] (b ＊ value, Δb ＊ ) b of the laminated piezoelectric body before storage ＊ The value is preferably, for example, −5.0 or more and 5.0 or less. When the laminated piezoelectric element is stored in an environment of 85° C. and 85% RH for 500 hours, the hue difference Δb ＊ is preferably 4.0 or less, and more preferably 3.5 or less. ＊ When the value is 4.0 or less, the laminated piezoelectric body undergoes little change in hue under high temperature and high humidity conditions, and therefore transparency and visibility can be maintained at a good level.

[0059] b of the laminated piezoelectric body ＊ The value can be measured using a spectrophotometer (for example, SD7000 manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z 8722. ＊ The value is the thickness of the laminated piezoelectric body 25 cm ２ The laminated piezoelectric body was cut into a square shape, and the four corners of the laminated piezoelectric body were fixed to a SUS plate with tape. The laminated piezoelectric body was then placed in a thermo-hygrostat chamber set at a temperature of 85°C and a humidity of 85% RH for 500 hours. After the laminated piezoelectric body was taken out of the thermo-hygrostat chamber, the thickness of the laminated piezoelectric body was measured. ＊ The value may be measured in the same manner as above.

[0060] b of the laminated piezoelectric body ＊ value and Δb ＊ can be adjusted by the thickness of the antistatic layer and the composition and thickness of the moisture-proof layer. For example, if the thickness of the antistatic layer is reduced, b ＊ value and Δb ＊ In addition, by increasing the thickness of the moisture-proof layer, discoloration of the piezoelectric film due to moisture penetration can be reduced. ＊ value and Δb ＊ can be made smaller.

[0061] (Piezoelectric constant) Piezoelectric constant d of the laminated piezoelectric body ３３ The piezoelectric constant d of the laminated piezoelectric material is preferably 7 pC / N or more and 40 pC / N or less. ３３ When the piezoelectric constant d of the laminated piezoelectric material is 7 pC / N or more, higher pressure sensitivity is likely to be obtained. ３３From the same viewpoint, when the piezoelectric constant d of the laminated piezoelectric body is 40 pC / N or less, the above-mentioned appearance defects can be further reduced. ３３ is more preferably 10 pC / N or more and 40 pC / N or less, further preferably 13 pC / N or more and 35 pC / N or less, and particularly preferably 15 pC / N or more and 30 pC / N or less. ３３ can be measured in the same manner as above.

[0062] Piezoelectric constant d of the laminated piezoelectric material ３３ is the piezoelectric constant d of a piezoelectric film, which is one of the components of the laminated piezoelectric body. ３３ The piezoelectric constant d of the piezoelectric film can be adjusted by ３３ When the piezoelectric constant d ３３ It is also likely to be expensive.

[0063] (Total Light Transmittance) The laminated piezoelectric body preferably has high transparency from the viewpoint of application to, for example, a touch panel. Specifically, the total light transmittance of the laminated piezoelectric body is preferably 80% or more, and more preferably 85% or more.

[0064] The total light transmittance of the laminated piezoelectric body can be measured using a haze meter (for example, NDH7000SP II manufactured by Nippon Denshoku Industries Co., Ltd.) based on the method described in JIS K 7361-1.

[0065] The total light transmittance of the laminated piezoelectric body can be adjusted by the layer structure and the refractive index and thickness of each layer. For example, if the laminated piezoelectric body includes a hard coat layer, the haze of the laminated piezoelectric body can be further reduced, and therefore the total light transmittance tends to be higher.

[0066] 2. Manufacturing Method of Laminated Piezoelectric Body The laminated piezoelectric body can be manufactured by any method. For example, the laminated piezoelectric body shown in FIG. 1 can be manufactured through the following steps: (1) preparing a piezoelectric film containing a fluorine-based resin; (2) forming an antistatic layer on the piezoelectric film; (3) forming a hard coat layer on the antistatic layer; and (4) forming a moisture-proof layer on the hard coat layer. Note that if the laminated piezoelectric body does not include a hard coat layer, the step (3) can be omitted.

[0067] (1) Step of Preparing Piezoelectric Film The piezoelectric film containing a fluororesin may be a pre-produced one or may be newly produced. For example, the piezoelectric film containing a fluororesin can be obtained through a step of producing a film containing a fluororesin (film-forming step), a step of stretching the film containing a fluororesin (stretching step), and a step of polarizing the film (polarization step).

[0068] (Film Forming Process) The film containing a fluororesin can be produced by any method, such as a melt extrusion method, a heat pressing method, a solution casting method, etc. Among these, the film containing a fluororesin is preferably produced by a melt extrusion method, from the viewpoint of easily obtaining a piezoelectric film having a predetermined thickness or more.

[0069] (Stretching step) In the stretching step, the produced film containing a fluororesin is stretched. A film containing a fluororesin has a structure in which α-type crystals (the main chain has a helical structure) and β-type crystals (the main chain has a planar zigzag structure) are mixed. β-type crystals have a large polarization structure. By stretching a film containing a fluororesin, a transition from α-type crystals to β-type crystals occurs, and the proportion of β-type crystals can be increased. Therefore, the stretching step is preferably performed as necessary to increase the proportion of β-type crystals in the fluororesin. The stretching direction may be either the TD direction or the MD direction, and the MD direction is more preferable.

[0070] The stretching method is not particularly limited, and can be a known stretching method such as a tenter method or a drum method.

[0071] The stretching ratio can be, for example, 3.0 times or more and 6.0 times or less. When the stretching ratio is 3.0 times or more, it is easy to adjust the thickness and polarity of the film to a more appropriate range. When the stretching ratio is 3.0 times or more, the rearrangement of the β-type crystals progresses more, and not only is it easy to exhibit higher piezoelectricity, but transparency can also be further improved. When the stretching ratio is more than 6.0 times, there is a risk that the film will break due to stretching, which is not preferable.

[0072] (Polarization Step) In the polarization step, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. By applying a DC voltage to the fluororesin film in which the proportion of polar β crystals has been increased in the stretching step, a piezoelectric film with high piezoelectricity can be obtained. The DC voltage to be applied may be adjusted depending on the thickness of the stretched film, and may be, for example, 1 kV or more and 50 kV or less.

[0073] (2) Step of forming an antistatic layer The above-described curable composition for the antistatic layer is applied to the obtained piezoelectric film, and then dried and cured to form an antistatic layer. From the viewpoint of improving low-temperature curability, the curable composition for the antistatic layer is preferably a curable composition containing a conductive material and an amine-based material, and more preferably a curable composition containing a conductive material and a melamine resin.

[0074] The curable composition may further contain water or a solvent, examples of which include alcohol-based solvents such as methanol, ethanol, and isopropyl alcohol.

[0075] The method for applying the curable composition is not particularly limited, and may be any of spin coating, gravure coating, die coating, bar coating, dip coating, and the like.

[0076] The curable composition can be dried by heating the applied curable composition. The heating temperature is preferably a temperature at which the solvent can be removed or higher and a temperature at which the fluorine-based resin constituting the piezoelectric film is not higher than the heat distortion temperature, and can be, for example, 100° C. or higher and 150° C. or lower. The heat distortion temperature can be measured, for example, in accordance with JIS K 7191-2:2015.

[0077] (3) Step of Forming a Hard Coat Layer The above-described curable composition for the hard coat layer is applied onto the obtained antistatic layer, and then dried and cured to form a hard coat layer.

[0078] The curable composition may further contain a dilution solvent. The dilution solvent preferably has a polarity similar to that of the particles. Examples of the dilution solvent include organic solvents such as alcohol-based solvents, ketone-based solvents, ester-based solvents, carbonate-based solvents, and aromatic solvents.

[0079] The curable composition may be applied and dried in the same manner as described above. The heating temperature may be set to a temperature within a range that allows the solvent to be volatilized and is equal to or lower than the heat distortion temperature of the fluororesin constituting the piezoelectric film, for example, 60°C or higher and 100°C or lower.

[0080] The curable composition may be cured by heat or by ionizing radiation. Curing by ionizing radiation can be carried out by irradiation with ultraviolet rays or electron beams. Alternatively, curing by heat and curing by ionizing radiation may be used in combination.

[0081] (4) Step of Forming Moisture-Proof Layer A moisture-proof layer is formed on the obtained antistatic layer.

[0082] The method for forming the moisture-proof layer is not particularly limited, but examples thereof include vapor deposition (dry process). The vapor deposition may be a physical vapor deposition (PVD) method such as vacuum deposition, sputtering, or ion plating, or a chemical vapor deposition (CVD) method. Among these, vacuum deposition and sputtering are preferred, and reactive sputtering is more preferred from the viewpoint of ease of forming a thin film containing an oxide or nitride.

[0083] 3. Applications The laminated piezoelectric body can be used for a variety of applications. In particular, since the laminated piezoelectric body exhibits high piezoelectricity while having high transparency and visibility, it can be preferably used as a piezoelectric sensor for touch panels mounted on various electronic devices.

[0084] 4. Modifications Although the above embodiment shows an example in which the laminated piezoelectric body has the layer structure shown in FIG. 1, the layer structure of the laminated piezoelectric body is not limited to this.

[0085] 2A, 2B, and 3 are schematic cross-sectional views showing laminated piezoelectric elements according to other embodiments.

[0086] 2A, the laminated piezoelectric element 10 does not necessarily have to include the hard coat layer 13. That is, it does not necessarily have to include any other layer between the antistatic layer 12 and the moisture-proof layer 14.

[0087] The laminated piezoelectric body 10 may further include another layer instead of or in addition to the hard coat layer 13. For example, as shown in FIG. 2B , the laminated piezoelectric body 10 may further include an optical adjustment layer 15.

[0088] The optical adjustment layer 15 is preferably disposed between the antistatic layer and the moisture-proof layer (see FIG. 2B ). In FIG. 2B , the optical adjustment layer is disposed between the hard coat layer and the moisture-proof layer. The optical adjustment layer can further improve the color of the laminated piezoelectric element by appropriately adjusting the refractive index and thickness. The optical adjustment layer may include only one layer, or two or more layers. For example, when two or more optical adjustment layers are included, one of the two or more optical adjustment layers may be disposed on one side of the piezoelectric film, and the other may be disposed on the other side.

[0089] The refractive index of the optical adjustment layer is preferably higher than that of the antistatic layer and lower than that of the moisture-proof layer. In Fig. 2B, the refractive index of the optical adjustment layer is preferably higher than that of the hard coat layer and lower than that of the moisture-proof layer. A laminated piezoelectric element including such an optical adjustment layer can further suppress poor appearance caused by interference between light incident on and reflected from the hard coat layer and light reflected at the interface between the optical adjustment layer and the hard coat layer.

[0090] From the above viewpoint, the refractive index of the optical adjustment layer is preferably 1.60 or more and less than 1.80, more preferably 1.63 or more and less than 1.78, and even more preferably 1.65 or more and 1.75 or less.

[0091] In this specification, the refractive index refers to the refractive index at a wavelength of 589 nm. The refractive index of the optical adjustment layer can be determined by measuring psi (Ψ) and delta (Δ) using a multi-angle high-speed spectroscopic ellipsometer (for example, M-2000 manufactured by J.A. Woollam Co., Ltd.), and calculating the refractive index at a wavelength of 589 nm from the measured values.

[0092] The material of the optical adjustment layer may be any material that satisfies such a refractive index. For example, the refractive index may be adjusted by adding metal oxide particles to the curable composition exemplified as the material of the hard coat layer. The metal oxide particles are preferably a refractive index material having a refractive index of 1.50 or more. Examples of such metal oxide particles include aluminum oxide, titanium oxide, zirconium oxide, zinc oxide, and tin oxide, and among these, titanium oxide and zirconium oxide are preferred.

[0093] The thickness of the optical adjustment layer is not particularly limited, but may be 0.050 μm or more. For example, the thickness of the optical adjustment layer may be 0.050 μm or more and 0.50 μm or less, or 0.080 μm or more and 0.18 μm or less. When the thickness of the optical adjustment layer is within the above range, the color tone of the laminated piezoelectric body can be further improved.

[0094] As shown in FIG. 3 , the laminated piezoelectric element 10 may further include a transparent conductive film 16 (a laminate of a substrate film 16 a and a transparent electrode 16 b) on the other surface of the piezoelectric film 11 (the surface opposite to the surface on which the antistatic layer 12 is disposed) via a transparent adhesive layer 17.

[0095] The present invention will be further described below with reference to examples and comparative examples, but the technical scope of the present invention is not limited thereto.

[0096] 1. Evaluation of Physical Properties (1) Thickness of Each Layer The thickness of each layer was measured using a spectroscopic interference film thickness meter (Optical NanoGauge C13027-11, manufactured by Hamamatsu Photonics K.K.) The thickness of each layer was measured at three points in an area including the center of the surface of the laminated piezoelectric body, and the arithmetic mean value was calculated.

[0097] (2) Surface Resistivity The surface resistivity of the antistatic layer was measured using a URS probe with a high resistivity meter (manufactured by Nitto Seiko Analytech Co., Ltd., Hirester UX, model number: MCP-HT800).

[0098] 2. Preparation and Evaluation of Laminated Piezoelectric Body [Example 1] (1) Preparation of Piezoelectric Film A polyvinylidene fluoride film (manufactured by Kureha Corporation, consisting of 100% by mass of a homopolymer of vinylidene fluoride) was stretched in the MD direction at a stretching ratio of 4.2 times, and then a DC voltage was applied between the ground electrode and the needle electrode while increasing from 0 kV to 11.0 kV, thereby performing a polarization treatment, and a piezoelectric film with a thickness of 42 μm was obtained.

[0099] (2) Formation of Antistatic Layer A solution containing an aqueous dispersion of carbon nanotubes and melamine resin was applied to side A of the obtained piezoelectric film using a multi-coater (manufactured by Hirano Tecseed Co., Ltd.), and the film was heat-treated at 130°C for 1 minute to form an antistatic layer having a thickness of 0.050 μm.

[0100] (3) Formation of Hard Coat Layer On the obtained antistatic layer, an ultraviolet-curable composition containing an acrylic compound and amorphous silica was applied using a multi-coater, and then heat-treated at 80°C for 2 minutes. ２ The coating was photocured by irradiating it with UV rays of 1000 kJ / cm to form a hard coat layer having a thickness of 0.70 μm.

[0101] (4) Formation of Moisture-Proof Layer Silicon dioxide (SiO ２ A 0.025 μm thick moisture-proof layer was formed on the surface of the piezoelectric film. This resulted in a laminated piezoelectric element having a laminated structure of piezoelectric film / antistatic layer / hard coat layer / moisture-proof layer.

[0102] Example 2 A laminated piezoelectric body was obtained in the same manner as in Example 1, except that the thickness of the moisture-proof layer was changed to 0.020 μm.

[0103] Example 3 A laminated piezoelectric body was obtained in the same manner as in Example 1, except that the thickness of the moisture-proof layer was changed to 0.040 μm.

[0104] [Example 4] A piezoelectric film was prepared, an antistatic layer was formed, and a hard coat layer was formed in the same manner as in Example 1. An ultraviolet-curable composition containing an acrylic compound and zirconium oxide particles was applied to the obtained hard coat layer using a multi-coater, and the coating was dried by heating at 80°C for 2 minutes. Thereafter, an integrated light dose of 400 mJ / cm was applied. ２ The laminate was photocured by irradiating it with ultraviolet light of 1000 kJ / cm 2 to form an optical adjustment layer having a thickness of 0.10 μm. An indium tin oxide film was formed on the obtained optical adjustment layer by a reactive sputtering method to form a moisture-proof layer having a thickness of 0.030 μm, thereby obtaining a laminated piezoelectric body.

[0105] Example 5 A laminated piezoelectric body was obtained in the same manner as in Example 4, except that the thickness of the antistatic layer was changed to 0.38 μm.

[0106] Comparative Example 1 A laminated piezoelectric body was obtained in the same manner as in Example 1, except that the hard coat layer and the moisture-proof layer were not formed.

[0107] Comparative Example 2 A laminated piezoelectric body was obtained in the same manner as in Example 5, except that the hard coat layer, the optical adjustment layer, and the moisture-proof film were not formed.

[0108] Comparative Example 3 A laminated piezoelectric body was obtained in the same manner as in Example 5, except that the optical adjustment layer and the moisture-proof layer were not formed.

[0109] [Evaluation] (1) Piezoelectric constant: Direct quasi-static method (d ３３ The piezoelectric constant d of the piezoelectric ceramic measured by the Meter method and the Berlincoat method ３３ Test method: Piezoelectric constant d according to ISO 19622:2018 ３３ Specifically, the piezoelectric constant d ３３ The piezoelectric constant d was measured by using a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST Co., Ltd.) to clip the sample with 1.0 N and apply a force of 0.15 N at 110 Hz. ３３ The measured value is measured at a temperature of 25° C. and may be a positive or negative value depending on whether the film is on the front or back side, but in this specification the absolute value is taken as the magnitude of the piezoelectric constant.

[0110] (2) Water Vapor Permeability The water vapor permeability of the laminated piezoelectric body was measured under conditions of 40° C. and 90% RH using a water vapor permeability measuring device (Aqua Sense Model 7101, manufactured by Systech Illinois) in accordance with JIS K7129-2.

[0111] (3) Total Light Transmittance The total light transmittance of the laminated piezoelectric body was measured using a haze meter (NDH7000SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) based on the method described in JIS K 7361-1.

[0112] (4) Hue difference Δb ＊ The hue b of the laminated piezoelectric body before storage in a humid and heated environment was measured using a spectrocolorimeter (SD7000, manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS Z 8722. ＊ Next, the laminated piezoelectric body was ２ The laminated piezoelectric material was cut into a square shape, and the four corners of the laminated piezoelectric material were fixed to a SUS plate with tape. The laminated piezoelectric material was then placed in a thermo-hygrostat (LH44-14 manufactured by Nagano Science Co., Ltd.) set to a temperature of 85°C and a humidity of 85% RH. After being held under the above conditions for 500 hours, the laminated piezoelectric material was removed from the thermo-hygrostat and the color b of the laminated piezoelectric material after storage was measured. ＊ The value was measured in the same manner as above. ＊ Value and hue before saving b ＊ The difference between the values ​​is Δb ＊ The value was set as

[0113] The evaluation results of the laminated piezoelectric bodies of Examples 1 to 5 and Comparative Examples 1 to 3 are shown in Table 1.

[0114]

[0115] As shown in Table 1, it can be seen that the laminated piezoelectric elements of Comparative Examples 1 to 3, which do not include a moisture-proof layer, all show a large change in hue. In contrast, it can be seen that the laminated piezoelectric elements of Examples 1 to 5, which include a moisture-proof layer, all show a small change in hue.

[0116] These findings show that the inclusion of a moisture-proof layer suppresses discoloration of the piezoelectric film in the laminated piezoelectric body, thereby reducing changes in the hue of the laminated piezoelectric body.

[0117] This application claims priority from Japanese Patent Application No. 2023-216252, filed December 21, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0118] The laminated piezoelectric element of the present invention exhibits minimal discoloration of the piezoelectric film and maintains high transparency even under high temperature and high humidity conditions, making it suitable for use as a piezoelectric sensor in a touch panel.

[0119] REFERENCE SIGNS LIST 10 laminated piezoelectric body 11 piezoelectric film 12 antistatic layer 13 hard coat layer 14 moisture-proof layer 15 optical adjustment layer 16 transparent conductive film 16a substrate film 16b transparent electrode 17 transparent adhesive layer

Claims

1. A laminated piezoelectric body comprising a piezoelectric film containing a fluorine-based resin as a main component, an antistatic layer disposed on at least one surface of the piezoelectric film, and a moisture-proof layer disposed on the opposite side of the piezoelectric film with the antistatic layer interposed therebetween, the moisture-proof layer having a water vapor transmission rate of 0.00 g / m2 in an environment of 40°C and 90% RH. ２ / day / atm or more 4.00g / m ２ / day / atm or less.

2. The laminated piezoelectric element according to claim 1, wherein no adhesive layer is disposed between said antistatic layer and said moisture-proof layer.

3. The laminated piezoelectric element according to claim 1 or 2, wherein the antistatic layer comprises a cured product of a curable composition containing a conductive material and an amine-based material.

4. The laminated piezoelectric element according to any one of claims 1 to 3, wherein the moisture-proof layer is made of an inorganic oxide.

5. The laminated piezoelectric element according to any one of claims 1 to 4, further comprising at least one of a hard coat layer and an optical adjustment layer disposed between said antistatic layer and said moisture-proof layer.

6. The laminated piezoelectric element according to claim 5, wherein the total thickness of the hard coat layer and the optical adjustment layer is 0.30 μm or more and 4.0 μm or less.

7. When the sample is stored in an environment of 85°C and 85% RH for 500 hours, the hue difference Δb before and after storage in the environment ＊ The laminated piezoelectric element according to claim 1 , wherein the piezo-electric constant is 4.0 or less.

8. The laminated piezoelectric element according to any one of claims 1 to 7, wherein the antistatic layer has a thickness of 0.010 µm or more and 0.40 µm or less.

9. The laminated piezoelectric element according to any one of claims 1 to 8, wherein the fluororesin contains, as a main component, a polymer containing a structural unit derived from vinylidene fluoride.

10. A method for producing a laminated piezoelectric body according to any one of claims 1 to 9, comprising the steps of: applying a curable composition containing a conductive material and an amine-based material onto the piezoelectric film, and curing the composition to form an antistatic layer; and forming a moisture-proof layer on the antistatic layer by a reactive sputtering method.

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