Conductive piezoelectric film, device, and method for manufacturing conductive piezoelectric film

Abstract

The present invention provides a conductive piezoelectric film in which variation in transparency is suppressed.　The present invention relates to a conductive piezoelectric film comprising a piezoelectric film and a conductive layer that is laminated on one surface of the piezoelectric film, wherein total light transmittance is not less than 80%, the standard deviation of total light transmittance is not more than 1.4%, and the conductive layer includes at least one material selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene.

Description

[Technical Field]

[0001] The present invention relates to a conductive piezoelectric film, a device including the conductive piezoelectric film, and a method for manufacturing the conductive piezoelectric film. [Background technology]

[0002] Touch panels are widely used as devices that allow users to input information by directly touching an image display unit. A typical example of such a touch panel is a capacitive touch panel, which utilizes the change in current capacity that occurs between a transparent electrode and a finger (Patent Document 1).

[0003] Transparent conductive films are used in the position detection sensors used in touch panels. For example, ITO (indium tin oxide) is formed on a PET film by dry coating such as sputtering, and then the crystallinity of the ITO is increased by heat treatment (at around 150°C), thereby achieving low resistance and high transmittance.

[0004] Recently, there has been growing interest in technology that can simultaneously detect the position and pressure when an operating surface is touched by a finger or other object, and a method has been proposed in which a pressure detection sensor made of a piezoelectric sensor is combined with a position detection sensor.However, the piezoelectric film used in piezoelectric sensors, like PET film, deteriorates in resistance and loses its color when heated at high temperatures, making it difficult to use conductive piezoelectric film on which ITO is deposited by dry coating such as sputtering in touch panels. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-324203 Summary of the Invention [Problem to be solved by the invention]

[0006] The inventors investigated a wet coating method, which involves applying a solution containing a conductive material such as metal nanowires or a conductive polymer, as a method for forming a conductive layer that does not require high-temperature heat treatment. However, they found that applying the wet coating method to a piezoelectric film to form a conductive layer resulted in variations in performance, particularly in transparency.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a conductive piezoelectric film with reduced variation in transparency, a device including the conductive piezoelectric film, and a method for manufacturing the conductive piezoelectric film. [Means for solving the problem]

[0008] The present inventors have discovered that the above-mentioned problems can be solved by a conductive piezoelectric film that includes a conductive layer containing metal nanowires or the like and that has a total luminous transmittance and a standard deviation of the total luminous transmittance that satisfy specific ranges, and have thereby completed the present invention. Specifically, the present invention relates to the following.

[0009] The present invention relates to a conductive piezoelectric film comprising a piezoelectric film and a conductive layer laminated on one surface of the piezoelectric film, wherein the conductive layer has a total light transmittance of 80% or more and a standard deviation of the total light transmittance of 1.4% or less, and the conductive layer contains at least one material selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene.

[0010] It is preferable that the surface waviness Wa of the piezoelectric film at the interface with the conductive layer is 30 μm or less. In the conductive piezoelectric film, it is preferable that the absolute value of the heat shrinkage rate when heat treated at 80° C. for 30 minutes is less than 1.0% at most. The adhesiveness of the conductive layer as evaluated in accordance with ASTM D3359 is preferably 4B or higher. The metal nanowires are preferably silver nanowires. The piezoelectric film preferably contains a fluorine-based resin.

[0011] The present invention also relates to a device comprising the conductive piezoelectric film.

[0012] Furthermore, the present invention relates to a method for producing a conductive piezoelectric film, which includes a step of forming a conductive layer having a total light transmittance of 80% or more by applying a solution containing a conductive substance including at least one selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene to the surface of a piezoelectric film on the side opposite to the protective film on which the protective film is laminated.

[0013] In the above step, the surface onto which the solution is applied preferably has a water contact angle of 70° or less. The protective film preferably has an elastic modulus of 1.0 GPa or more. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a conductive piezoelectric film with reduced variation in transparency, a device including the conductive piezoelectric film, and a method for manufacturing the conductive piezoelectric film. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view schematically showing a conductive piezoelectric film 1, which is one embodiment of the conductive piezoelectric film of the present invention. [Figure 2] FIG. 2 is a cross-sectional view schematically showing a conductive piezoelectric film 2, which is another embodiment of the conductive piezoelectric film of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings, but the present invention is not limited to this and various modifications are possible within the scope of the gist thereof.

[0017] <Conductive piezoelectric film> The conductive piezoelectric film according to the present invention comprises a piezoelectric film and a conductive layer laminated on one surface of the piezoelectric film, wherein the conductive layer has a total light transmittance of 80% or more and a standard deviation of the total light transmittance of 1.4% or less, and the conductive layer contains at least one material selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene. In this specification, the term "laminated" refers to the layers being laminated in order, and other layers may be laminated between the layers.

[0018] <Characteristics of conductive piezoelectric film> The conductive piezoelectric film has a total light transmittance of 80% or more, and the standard deviation of the total light transmittance is 1.4% or less.

[0019] The higher the total light transmittance of the conductive piezoelectric film, the higher the transparency, and therefore the more preferable. The total light transmittance of the film is more preferably 84% or more, even more preferably 86% or more, and particularly preferably 90% or more. The upper limit of the total light transmittance is not particularly limited.

[0020] The smaller the standard deviation of the total light transmittance of the conductive piezoelectric film, the smaller the variation in transparency, and therefore the more preferable. The standard deviation of the total light transmittance of the film is preferably 1.4% or less, more preferably 1.1% or less, even more preferably 0.8% or less, and particularly preferably 0.5% or less. The lower limit of the standard deviation of the total light transmittance is not particularly limited.

[0021] In this specification, the total light transmittance of a conductive piezoelectric film is a value measured in accordance with JIS K 7361-1, and specifically, can be measured by the method described in the Examples below. The standard deviation of the total light transmittance is a value calculated from the measured total light transmittance values.

[0022] The standard deviation of the total light transmittance can be reduced, for example, by laminating a protective film on the surface of the piezoelectric film opposite the surface to which the solution containing a conductive material is applied before applying the solution to the piezoelectric film. Piezoelectric films have lower surface smoothness than films such as PET, and applying the solution to such a piezoelectric film surface is thought to cause variations in transparency. Therefore, laminating a protective film is thought to improve the smoothness of the piezoelectric film. Furthermore, the standard deviation of total light transmittance can also be reduced by, for example, performing a surface modification treatment (such as corona treatment, plasma treatment, flame treatment, or ultraviolet irradiation treatment) on the surface of the piezoelectric film to be coated before applying a solution containing a conductive substance to the surface of the piezoelectric film, or by forming a transparent coating layer on the surface of the piezoelectric film to be coated, thereby reducing the water contact angle of the surface to be coated to 70° or less. Piezoelectric films have low wettability with the above-mentioned aqueous solutions, and it is thought that the repelling of the above-mentioned solutions causes unevenness, which in turn leads to variations in transparency. Therefore, it is thought that reducing the water contact angle of the surface to be coated to 70° or less can eliminate uneven application of the above-mentioned solutions.

[0023] The surface waviness Wa of the piezoelectric film at the interface with the conductive layer is preferably 30 μm or less, more preferably 25 μm or less. There is no particular lower limit for the surface waviness Wa. When the surface waviness Wa is 30 μm or less, the surface smoothness of the piezoelectric film is good, the coating film of the conductive layer is more likely to be uniform, and variations in transparency are more likely to be suppressed. In this specification, the surface waviness Wa is the arithmetic mean height over the reference length of a waviness curve as a contour curve. The waviness curve does not represent the minute irregularities represented by a roughness curve, but represents larger-scale irregularities (i.e., waviness). Specifically, the surface waviness Wa can be measured by the method described in the examples below.

[0024] In a conductive piezoelectric film, the absolute value of the heat shrinkage percentage when heat-treated at 80°C for 30 minutes is preferably less than 1.0% at most (in either direction), and more preferably less than 0.5%. There is no particular lower limit to the absolute value of the heat shrinkage percentage. An absolute value of the heat shrinkage percentage within the above range means that the absolute value of the heat shrinkage percentage of the piezoelectric film used is low, and the piezoelectric film is less likely to shrink during the drying process of the coating film, making it easier to obtain a more uniform conductive layer and reducing variations in transparency. In this specification, the heat shrinkage rate can be measured specifically by the method described in the examples below. The thermal shrinkage rate of the conductive piezoelectric film can be reduced by subjecting the polarization-treated piezoelectric film to heat treatment, for example, at 130° C. for 1 minute.

[0025] In the conductive piezoelectric film, the adhesiveness of the conductive layer evaluated in accordance with ASTM D3359 is preferably 4B or higher. In this specification, the adhesiveness of the conductive layer refers to the adhesiveness between the conductive layer and a layer adjacent to the conductive layer on the piezoelectric film side. For example, when the piezoelectric film and the conductive layer are adjacent to each other, the adhesiveness refers to the adhesiveness between the piezoelectric film and the conductive layer. When a transparent coating layer is disposed between the piezoelectric film and the conductive layer, the adhesiveness refers to the adhesiveness between the transparent coating layer and the conductive layer. Specifically, the adhesiveness of the conductive layer can be measured by the method described in the Examples below.

[0026] The haze value of the conductive piezoelectric film is preferably 3.0% or less, more preferably 2.5% or less, even more preferably 2.0% or less, and particularly preferably 1.5% or less. There is no particular lower limit to the haze value. In this specification, the haze value of the conductive piezoelectric film is a value measured in accordance with JIS K 7136, and specifically, can be measured by the method described in the examples below.

[0027] The sheet resistance of the conductive piezoelectric film is preferably 1000 Ω / sq or less, more preferably 600 Ω / sq or less, even more preferably 400 Ω / sq or less, and particularly preferably 200 Ω / sq or less. The lower limit of the sheet resistance is not particularly limited, but from the viewpoint of transparency, it is preferably 10 Ω / sq or more, more preferably 20 Ω / sq or more, and particularly preferably 40 Ω / sq or more. Furthermore, the lower limit of the sheet resistance is not particularly limited, but as a combination of numerical ranges, it is preferably 10 Ω / sq to 1000 Ω / sq or less, more preferably 10 Ω / sq to 600 Ω / sq or less, even more preferably 20 Ω / sq to 400 Ω / sq or less, and particularly preferably 40 Ω / sq to 200 Ω / sq or less. In this specification, the sheet resistance of the conductive piezoelectric film is a value measured in accordance with JIS K 7194, and specifically, can be measured by the method described in the examples below.

[0028] The thickness of the conductive 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, a larger amount of charge is generated due to the piezoelectric effect, 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. The thickness of the piezoelectric film is more preferably 100 μm or less, and even more preferably 80 μm or less. From the same perspective, the thickness of the piezoelectric film is more preferably 35 μm or more and 80 μm or less. In this specification, the thickness of the conductive piezoelectric film and each layer is determined by measuring the thickness at 10 random locations on a cross-sectional photograph of the layer taken at 10,000 to 100,000 magnifications using a scanning electron microscope (SEM), and the arithmetic average of the thicknesses measured at 10 locations.

[0029] Next, each layer of the conductive piezoelectric film will be described with reference to the drawings.

[0030] 1 is a cross-sectional view schematically showing a conductive piezoelectric film 1, which is one embodiment of the conductive piezoelectric film. The conductive piezoelectric film 1 has a piezoelectric film 11 and a conductive layer 31 laminated on one surface of the piezoelectric film 11. 2 is a cross-sectional view schematically showing a conductive piezoelectric film 2, which is another embodiment of the conductive piezoelectric film. The conductive piezoelectric film 2 differs from the conductive piezoelectric film 1 in that the conductive piezoelectric film 2 includes a pair of transparent coating layers (a first transparent coating layer 21 and a second transparent coating layer 22) sandwiching the piezoelectric film 11. The other configurations are the same as those of the conductive piezoelectric film 1.

[0031] <Piezoelectric film> The piezoelectric film 11 is a film (thin film) having piezoelectricity (the property of converting an applied force into a voltage, or the property of converting an applied voltage into a force).

[0032] Materials constituting the piezoelectric film 11 include polarized polar polymers, which can exhibit piezoelectricity by polarizing molecular dipoles through a polarization process generally known as thermal poling, and stretched chiral polymers, which can exhibit piezoelectricity by stretching chiral polymers. Examples of polarized polar polymers include fluorine-based resins; vinylidene cyanide polymers; vinyl acetate polymers; odd-numbered nylons such as nylon 9 and nylon 11; and polyurea. Examples of stretched chiral polymers include helical chiral polymers such as polylactic acid; polyhydroxycarboxylic acids such as polyhydroxybutyrate; and cellulose derivatives. These can be used alone or in combination of two or more. Among these, the piezoelectric constant d of the piezoelectric film is particularly important. 33 From the viewpoint of easily increasing the value of the piezoelectric layer and more easily obtaining the piezoelectric effect, fluorine-based resins are preferred, and homopolymers of vinylidene fluoride are more preferred. Piezoelectric constant d of the piezoelectric film 33 The piezoelectric constant d of the piezoelectric film is preferably 10 pC / N or more and 40 pC / N or less. 33 When the piezoelectric constant d of the piezoelectric film is 10 pC / N or more, the amount of charge generated by the piezoelectric effect is larger, and the pressure sensitivity can be further improved. 33When the piezoelectric constant d of the piezoelectric film is 40 pC / N or less, the deterioration of the surface flatness of the piezoelectric film caused by the polarization treatment can be further reduced, and the standard deviation of the total light transmittance can be further reduced. 33 is more preferably 13 pC / N or more and 35 pC / N or less, and further preferably 15 pC / N or more and 30 pC / N or less. 33 is the charge generated when a piezoelectric constant measurement device ("Piezometer System PM300" manufactured by PIEZOTEST) is used to clip the sample with 0.2 N and apply a force of 0.15 N, 110 Hz. The piezoelectric constant d 33 The measured values ​​of may be positive or negative depending on the front and back of the film being measured, but in this specification, absolute values ​​are given.

[0033] Examples of fluorine-based resins include polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers (for example, vinylidene fluoride / trifluoroethylene copolymer, vinylidene fluoride / trifluoroethylene / chlorotrifluoroethylene copolymer, hexafluoropropylene / vinylidene fluoride copolymer, perfluorovinyl ether / vinylidene fluoride copolymer, tetrafluoroethylene / vinylidene fluoride copolymer, hexafluoropropylene oxide / vinylidene fluoride copolymer, hexafluoropropylene oxide / tetrafluoroethylene / vinylidene fluoride copolymer, hexafluoropropylene / tetrafluoroethylene / vinylidene fluoride copolymer); tetrafluoroethylene polymers; and chlorotrifluoroethylene polymers. These may be used alone or in combination of two or more. Among these, polyvinylidene fluoride and / or vinylidene fluoride copolymers are more preferred from the viewpoints of the high piezoelectricity, weather resistance, heat resistance, etc. obtained.

[0034] Piezoelectric constant d of piezoelectric film 11 33can be adjusted mainly by the type of resin contained in the piezoelectric film 11 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, the piezoelectric constant d of the piezoelectric film tends to increase by strengthening the polarization process or stretching process. 33 tends to become large.

[0035] The thickness of the piezoelectric film 11 is preferably, for example, 25 μm or more and 120 μm or less. When the thickness of the piezoelectric film 11 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 11 is more preferably 30 μm or more, and even more preferably 35 μm or more. When the thickness of the piezoelectric film 11 is 120 μm or less, the transparency of the piezoelectric film 11 is less likely to be impaired. The thickness of the piezoelectric film is more preferably 100 μm or less, and even more preferably 80 μm or less. From the same perspective, the thickness of the piezoelectric film is more preferably 35 μm or more and 80 μm or less.

[0036] <Conductive layer> The conductive layer 31 contains at least one material selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene. By including such a conductive material, both conductivity and transparency can be achieved.

[0037] Metal nanowires are conductive materials made of metal, shaped like needles or threads, and measuring nanometers in diameter. Metal nanowires may be linear or curved. Examples of metal nanowires include silver nanowires, gold nanowires, copper nanowires, and nickel nanowires. These may be used alone or in combination of two or more. Of these, silver nanowires are preferred.

[0038] The average diameter, average length, and aspect ratio (average length / average diameter) of the metal nanowires are not particularly limited as long as they are within a range that provides good conductivity and transparency. For example, the average diameter is preferably 10 nm to 500 nm, more preferably 10 nm to 100 nm. The average length is preferably 1 μm to 300 μm, more preferably 1 μm to 100 μm. The aspect ratio (average length / average diameter) is preferably 10 to 10,000, more preferably 100 to 5,000. The average diameter and average length of the metal nanowires are determined by measuring the dimensions of 100 randomly selected metal nanowires using a scanning electron microscope and taking the arithmetic mean value.

[0039] Known manufacturing methods can be used to manufacture metal nanowires. For example, silver nanowires can be synthesized by reducing silver nitrate in the presence of polyvinylpyrrolidone using the polyol method (see Chem. Mater., 2002, 14, 4736-4745). Gold nanowires can also be synthesized by reducing chloroauric acid hydrate in the presence of polyvinylpyrrolidone (see J. Am. Chem. Soc., 2007, 129, 1733-1742).

[0040] Examples of conductive polymers that can be used include polythiophene, polypyrrole, polyaniline, polyacetylene, polyparaphenylene, polyparaphenylenevinylene, and polyfluorene. These may be used alone or in combination of two or more. Among these, polythiophene is preferred, and a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT / PSS) is more preferred.

[0041] The carbon nanotubes are not particularly limited as long as they provide good electrical conductivity and transparency. For example, they may be single-walled carbon nanotubes or multi-walled carbon nanotubes such as double-walled carbon nanotubes. These may be used alone or in combination of two or more.

[0042] The graphene is not particularly limited as long as it has good conductivity and transparency. For example, it may be single-layer graphene or multi-layer graphene. Graphene oxide, graphene nanoribbon, etc. may also be used. These may be used alone or in combination of two or more.

[0043] The conductive layer may contain other components in addition to these conductive materials, provided that their functions are not impaired. Examples of such other components include surfactants, surface conditioners, corrosion inhibitors, pH adjusters, binder resins, and thickeners. The conductive layer may also contain trace components or unavoidable components, provided that their functions are not impaired.

[0044] The surfactant may be any compound having a surface-active function. Examples of the surfactant include nonionic surfactants, anionic surfactants, cationic surfactants, and amphoteric surfactants. These surfactants may be used alone or in combination of two or more.

[0045] The surface conditioner may be any that can prevent surface defects such as bubbles, repelling, pinholes, and mottle in the coating film. Examples of the surface conditioner include acrylic compounds, vinyl compounds, silicone compounds, and fluorine compounds. These can be used alone or in combination of two or more.

[0046] The corrosion inhibitor may be any compound capable of preventing metals from rusting, and specific examples include imidazoles, benzimidazoles, benzotriazoles, tetrazoles, thiazoles, benzothiazoles, thiadiazoles, thiazolines, etc. These may be used alone or in combination of two or more.

[0047] The pH adjuster is a compound used to adjust the pH of a solution containing a conductive substance, and specific examples include sulfuric acid, acetic acid, sodium hydroxide, potassium hydroxide, ammonia, dimethylamine, ethanolamine, dimethylethanolamine, etc. These can be used alone or in combination of two or more.

[0048] The binder resin can be any resin that can be dissolved or dispersed in the solvent used in the solution containing the conductive material. Specific examples include acrylic resin, epoxy resin, oxetane resin, polyurethane resin, polyimide resin, melamine resin, silicone resin, vinyl acetate resin, polyvinylpyrrolidone, polyester resin, polysaccharides and their derivatives. These can be used alone or in combination of two or more.

[0049] The thickener may be any compound capable of increasing the viscosity of a solution containing a conductive substance, and specific examples thereof include alkaline thickeners, urethane thickeners, etc. These may be used alone or in combination of two or more.

[0050] The thickness of the conductive layer is preferably 100 nm or more and 2 μm or less, and more preferably 200 nm or more and 1 μm or less. Within the above numerical range, good conductivity is easily obtained and the visible light transmittance is likely to be high. Thus, to obtain a conductive piezoelectric film with better transparency, it is preferable to reduce the thickness of the conductive layer. However, as mentioned above, piezoelectric films have lower surface smoothness than films such as PET, so if the thickness of the conductive layer is thin, the influence of film thickness variations will be greater, which is thought to lead to variations in transparency, etc. In other words, within the above numerical range, variations in transparency are likely to occur, and the effects of the present invention are easily achieved. In the case of a conductive layer containing metal nanowires, the sparse dispersion of the metal nanowires makes it difficult to measure the thickness. In such cases, the thickness of the coating film containing other components such as a thickener and a binder resin is used as the thickness of the conductive layer.

[0051] <Transparent coating layer> The conductive piezoelectric film according to the present invention may be provided with a transparent coating layer. The conductive piezoelectric film 2 has a pair of transparent coating layers: a first transparent coating layer 21 on the surface of the piezoelectric film 11 facing the conductive layer 31, and a second transparent coating layer 22 on the surface of the piezoelectric film 11 opposite the conductive layer 31. That is, the conductive piezoelectric film 2 has a layered structure in which the second transparent coating layer 22, the piezoelectric film 11, the first transparent coating layer 21, and the conductive layer 31 are layered in this order. The first transparent coating layer 21 and the second transparent coating layer 22 may be the same in thickness, structure, and composition, or may be different. Furthermore, it is not necessary to have both the first transparent coating layer 21 and the second transparent coating layer 22; only one of them may be provided.

[0052] Examples of the transparent coating layer include a hard coat layer, an undercoat layer, and an overcoat layer. The transparent coating layer may be a single layer or a combination of multiple layers. The hard coat layer or undercoat layer not only protects the surface of the piezoelectric film from scratches and dirt, but also improves transparency (reducing haze) by filling in scratches on the surface of the piezoelectric film. Furthermore, by providing a hard coat layer or undercoat layer between the conductive layer and the piezoelectric film, the water contact angle of the surface to which the solution containing the conductive substance is applied can be reduced to 70° or less, thereby reducing the standard deviation of the total light transmittance. The overcoat layer can prevent oxidation of the metal nanowires due to water, air, and light.

[0053] The transparent coating layer contains, for example, a cured resin obtained by curing a resin composition. The resin composition preferably contains at least one selected from a thermosetting resin composition, an ultraviolet-curable resin composition, and an electron beam-curable resin composition. The resin composition may contain at least one selected from a (meth)acrylic acid ester resin, a urethane resin, an epoxy-based resin, a phenoxy-based resin, and a melamine-based resin.

[0054] The resin composition is a composition containing a curable compound having an energy ray-reactive group such as a (meth)acryloyl group or a vinyl group. The term "(meth)acryloyl group" means at least one of an acryloyl group and a methacryloyl group. The curable compound preferably contains a polyfunctional monomer or oligomer containing two or more, preferably three or more, functional groups that react with energy rays in one molecule. When an energy ray-curable resin composition is used, the resin composition can be cured by irradiating it with energy rays such as ultraviolet rays, and therefore, the use of such a resin composition is preferable from the viewpoint of the manufacturing process.

[0055] The curable compound preferably contains an acrylic monomer, such as a urethane-modified acrylate or an epoxy-modified acrylate.

[0056] The resin composition preferably contains fine particles (organic and / or inorganic fine particles) from the viewpoints of increasing the strength of the coating film and improving adhesion to the conductive layer. Examples of organic fine particles include organosilicon fine particles, crosslinked acrylic fine particles, and crosslinked polystyrene fine particles. Examples of inorganic fine particles include silica particles, calcium carbonate particles, aluminum oxide fine particles, zirconium oxide fine particles, titanium oxide fine particles, and iron oxide fine particles. These may be used alone or in combination of two or more.

[0057] If the transparent coating layer is too thin, it may be difficult for the transparent coating layer to adequately cover the fine irregularities on the surface of the piezoelectric film, and the effect of reducing the haze of the piezoelectric film may not be sufficiently achieved. Furthermore, if the transparent coating layer is too thick, the piezoelectricity of the conductive piezoelectric film may be insufficient. Therefore, the thickness of the transparent coating layer is preferably 0.20 μm or more and 3.5 μm or less, more preferably 0.35 μm or more and 2.5 μm or less, and even more preferably 0.50 μm or more and 1.5 μm or less. By ensuring that the thickness of the transparent coating layer is within the above range, the conductive piezoelectric film can achieve both sufficient piezoelectricity and transparency depending on the application.

[0058] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, the conductive piezoelectric film 2 described above has a pair of transparent coating layers, but it may have only one of the first transparent coating layer 21 and the second transparent coating layer 22. Furthermore, the conductive piezoelectric films 1 and 2 may have a transparent coating layer as an overcoat layer on the surface of the conductive layer 31 opposite the piezoelectric film 11, from the viewpoint of improving the durability of the conductive layer 31. Furthermore, the conductive piezoelectric films 1 and 2 may have any layer other than the above-mentioned layers at any position, as long as the function of the films is not significantly impaired.

[0059] <Applications of conductive piezoelectric film> The conductive piezoelectric film according to the present invention is suitable for use in devices such as piezoelectric panels including capacitive and resistive touch panels, pressure sensors, actuators for haptic devices, piezoelectric vibration power generators, and flat speakers. The piezoelectric panel further includes a general display panel unit such as an LCD under the conductive piezoelectric film. The devices are suitable for use in mobile phones, smartphones, personal digital assistants, tablet PCs, laptops, ATMs, factory automation equipment, office automation equipment, medical equipment, car navigation systems, and the like.

[0060] <Method for manufacturing conductive piezoelectric film> The conductive piezoelectric film of the present invention can be manufactured by a method including a step of forming a conductive layer having a total light transmittance of 80% or more by applying a solution containing a conductive substance to the surface of a piezoelectric film having a protective film laminated thereon opposite the protective film.

[0061] As the piezoelectric film, those described in <Piezoelectric film> can be used. When a piezoelectric film containing a fluorine-based resin is used, the piezoelectric film can be produced by the following method.

[0062] When producing a piezoelectric film containing a fluororesin, the piezoelectric film can be obtained through a process of poling a film containing a fluororesin. The film containing a fluororesin may be a stretched film or an unstretched film. In this embodiment, from the viewpoint of exhibiting a high piezoelectric effect, it is preferable to stretch the film containing a fluororesin and then perform a polarization process.

[0063] The film containing a fluororesin can be produced by any method, such as melt extrusion, hot pressing, or solution casting. Among these, the film containing a fluororesin is preferably produced by melt extrusion, from the viewpoint of easily obtaining a piezoelectric film having a predetermined thickness or more. In the melt extrusion method, the fluororesin and any additives are heated and melted in the cylinder of an extruder, and then extruded through a die to obtain a film.

[0064] The obtained film has a structure in which α-type crystals (main chain has a helical structure) and β-type crystals (main chain has a planar zigzag structure) are mixed. β-type crystals have a large polarization structure. Stretching the film can convert α-type crystals into β-type crystals, and in the stretching process, it is preferable to adjust the stretching ratio as necessary to convert the fluororesin into β-type crystals. The stretching direction may be either the TD direction or the MD direction, with the MD direction being more preferable.

[0065] The stretching method is not particularly limited, and can be a known stretching method such as a tenter method or a drum method.

[0066] 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 becomes more sufficient, and not only is it easy to exhibit higher piezoelectricity, but transparency can also be further improved. When the stretching ratio is 6.0 times or less, breakage due to stretching can be further suppressed.

[0067] The resulting stretched film is preferably subjected to a polarization treatment. The polarization treatment can be carried out, for example, by applying a DC voltage between a ground electrode and a needle-like electrode. The voltage may be adjusted depending on the thickness of the stretched film, and can be, for example, 1 kV to 50 kV.

[0068] In this manner, in this embodiment, a piezoelectric film can be obtained by subjecting a stretched film to polarization treatment.

[0069] The piezoelectric film laminated with the protective film can be obtained by, for example, laminating the protective film and the piezoelectric film using a laminator or a multi-coater.

[0070] The elastic modulus of the protective film is preferably 1.0 GPa or more, more preferably 2.0 GPa or more, and even more preferably 2.5 GPa or more. By bonding a protective film having an elastic modulus of 1.0 GPa or more to a piezoelectric film, the surface of the piezoelectric film can be suitably flattened. This can further reduce variations in the transparency of the conductive piezoelectric film. In this specification, the modulus of elasticity of the protective film means the tensile modulus of elasticity of the substrate, and is a value measured in accordance with JIS K 7127:1999.

[0071] The substrate of the protective film is not particularly limited as long as its elastic modulus satisfies the above range, and examples thereof include polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate; polyolefins such as polyethylene (PE) and polypropylene (PP); halogen-containing polymers such as polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF); acrylic polymers such as polymethyl methacrylate; and styrene polymers such as polystyrene and styrene-methyl methacrylate copolymer. Among these, PET and PP are preferred, with PET being more preferred from the viewpoint of a higher elastic modulus. Furthermore, these films may be unstretched films, or more preferably biaxially stretched films.

[0072] The thickness of the substrate of the protective film is preferably 30 μm or more, more preferably 50 μm or more. Furthermore, the thickness of the substrate is preferably 300 μm or less, more preferably 150 μm or less. Within the above range, the smoothness of the piezoelectric film is further improved, and the variation in transparency of the conductive piezoelectric film can be further suppressed.

[0073] The conductive substance is not particularly limited, but is preferably a conductive substance (transparent conductive substance) that forms a transparent conductive layer (for example, a total light transmittance of 80% or more), such as the above-mentioned silver nanowires, conductive polymers, carbon nanotubes, graphene, etc. It is preferable that the conductive substance does not contain carbon black, graphite, etc., which form an opaque conductive layer.

[0074] The solvent used in the solution containing the conductive substance is not particularly limited as long as it can disperse the conductive substance, and can be appropriately selected depending on the desired viscosity, drying speed, etc. For example, in the case of silver nanowires, water or alcohol-based solvents such as ethanol, methanol, or 1-propanol can be used. In the case of conductive polymers, in addition to water or alcohol-based solvents, ether-based solvents such as propylene glycol dimethyl ether, nitrogen-containing solvents such as N-methylpyrrolidone, and sulfur-containing solvents such as dimethyl sulfoxide can be used. These solvents can be used alone or in combination of two or more. In addition to the conductive substance, the solution may contain other components as described above.

[0075] The content of the conductive material in 100% by mass of the solution is not particularly limited and can be set appropriately taking into consideration the coating properties and drying properties, and is, for example, 0.01% by mass to 5% by mass.

[0076] The method for applying the solution containing the conductive material is not particularly limited and can be any known method, such as an extrusion nozzle method, a blade method, a knife method, a bar coating method, a kiss coating method, a kiss reverse method, a gravure roll method, a dip method, a reverse roll method, a direct roll method, a curtain method, and a squeeze method. Any appropriate drying method (e.g., natural drying, air drying, or heat drying) can be used as the drying method after coating. For example, in the case of heat drying, the drying temperature is, for example, 50°C to 130°C, and the drying time is, for example, 0.5 to 10 minutes, in order to prevent the resistance of the piezoelectric film from deteriorating or the color of the film from being damaged.

[0077] Since the piezoelectric film has low wettability with a solution containing a conductive substance, and the solution is repelled, causing unevenness and resulting in variable transparency, it is preferable that the water contact angle of the surface to which the solution is applied be 70° or less. A water contact angle of 70° or less also tends to improve the adhesion of the conductive layer. In this specification, the water contact angle is the arithmetic average of 10 measurements of the contact angle 3 seconds after a pure water droplet (2.0 μL) is dropped onto the surface using a contact angle meter FACE CA-V (manufactured by Kyowa Interface Science Co., Ltd.).

[0078] Methods for reducing the water contact angle of the surface to which the solution is to be applied to 70° or less include surface modification treatments such as corona treatment, plasma treatment, flame treatment, and ultraviolet irradiation treatment on the surface of the piezoelectric film to be applied, and forming a transparent coating layer on the surface of the piezoelectric film to be applied.

[0079] The transparent coating layer can be produced by applying a solution or dispersion of the resin composition onto the surface of the piezoelectric film, drying the solution, and curing the resin composition. The application can be performed in the same manner as the application method for the solution containing the conductive material described above.

[0080] After the conductive layer is formed by the above process, the protective film may be peeled off from the piezoelectric film. The peeling method is not particularly limited. After the protective film is peeled off, a transparent coating layer may be formed on the surface of the piezoelectric film on the peeled side or on the surface of the conductive layer opposite the piezoelectric film. [Example]

[0081] The present invention will be further explained below with reference to examples and comparative examples, but the present invention is not limited to these examples. The properties of the conductive piezoelectric film of the present invention were measured by the following methods, and the results are shown in Table 1.

[0082] <Physical property evaluation> (surface waviness) The surface waviness (Wa) (μm) of the conductive layer side of the conductive piezoelectric films of the examples and comparative examples was measured using a 3D shape measuring instrument (Keyence Corporation, "VR-5000") and a multi-file analysis application (Keyence Corporation, "VR-H3CA"). A test specimen cut to a size of 50 mm x 50 mm was placed on a stage with the conductive layer facing outward, and the four corners of the test specimen were fixed with tape to avoid applying tension. As pretreatment, a reference plane was set by specifying multiple points, and surface shape correction was performed to remove waviness with a correction strength of 5. The surface was smoothed using a simple 3 x 3 average. Measurements were taken using a low-magnification camera. Measurements were performed in multi-line roughness mode with five peripheral lines, 70 lines spaced apart, horizontal (TD) direction, and no cutoff. The measurements were performed three times, and the arithmetic mean was taken as the surface waviness (Wa).

[0083] (Thermal shrinkage rate) Using a test method conforming to JIS K 7133, a conductive piezoelectric film was cut into a 100 mm x 100 mm piece, and the pre-test distances (L0 and T0) between two benchmark lines marked on the cut-out test piece in the longitudinal and transverse directions were measured. The machine direction was the flow direction of the film during the formation of the conductive layer, and the perpendicular direction was the transverse direction. The test piece was then heated to a specified temperature (80°C) for a specified time (30 minutes) in a hot air drying oven, and then conditioned at room temperature for at least 30 minutes. The machine and transverse distances (L and T) were then measured again, and the changes in the machine and transverse distances (ΔL and ΔT) for the test piece were calculated using the following equations (1) and (2). ΔL = [(L - L0) / L0] × 100(%) (1) ΔT = [(T-T0) / T0] × 100(%) (2) The change in the gauge line distance in the machine direction (ΔL) was taken as the heat shrinkage rate in the machine direction (MD) of the film, and the change in the gauge line distance in the transverse direction (ΔT) was taken as the heat shrinkage rate in the transverse direction of the film. In all of the Examples and Comparative Examples, the absolute value of the heat shrinkage rate in the transverse direction was smaller than the absolute value of the heat shrinkage rate in the machine direction, so the absolute value of the heat shrinkage rate in the machine direction was the largest.

[0084] (adhesion) The adhesiveness of each conductive piezoelectric film was evaluated using a cross-cut method. 11 lines were cut with a cutter at 1 mm intervals vertically and horizontally on the conductive layer of each conductive piezoelectric film to create 100 grids. Tape (Nichiban Cellotape®, adhesive strength 4.01 N / 10 mm) was then applied and peeled off. The percentage of the conductive layer that peeled off from the conductive piezoelectric film was evaluated according to ASTM D3359 using the following criteria: 0B: Peeling of 65% or more 1B: Peeling of 35% or more and less than 65% 2B: Peeling of 15% or more and less than 35% 3B: Peeling of 5% or more and less than 15% 4B: Less than 5% peeling 5B: No peeling

[0085] (sheet resistance) The resistance (Ω / sq) of each of the conductive piezoelectric films of the examples and comparative examples was measured using a resistivity meter (LorestaGP MCP-T610, manufactured by Nitto Seiko Analytech Co., Ltd.) in accordance with JIS K 7194. The resistance was measured three times, and the arithmetic mean of the three measurements was determined as a representative value.

[0086] <Optical property evaluation> (Total light transmittance) The total light transmittance (%) of each of the conductive piezoelectric films of the examples and comparative examples was measured in accordance with JIS K 7361-1 using a haze meter ("NDH7000SP II", manufactured by Nippon Denshoku Industries Co., Ltd.) Fifteen measurements were taken on a 10 cm × 15 cm test piece, and the arithmetic mean value was determined as a representative value.

[0087] The standard deviation of the total light transmittance was calculated based on the following formula.

number

[0088] (Haze value) The haze value (%) of each of the conductive piezoelectric films of the examples and comparative examples was measured using a haze meter ("NDH7000SP II", manufactured by Nippon Denshoku Industries Co., Ltd.) in accordance with JIS K 7136. Measurements were taken at 15 points on a 10 cm × 15 cm test piece, and the arithmetic mean value was determined as a representative value.

[0089] Example 1 A resin film (thickness 120 μm) made from polyvinylidene fluoride (manufactured by Kureha Corporation) with an inherent viscosity of 1.1 dl / g was uniaxially stretched to a stretch ratio of 4.2 times. After stretching, the film was subjected to a polarization treatment by applying a DC voltage between a ground electrode and a needle-shaped electrode while increasing it from 0 kV to 12.0 kV. The film after the polarization treatment was further heat-treated at 130°C for 1 minute to obtain a piezoelectric film with a thickness of 40 μm. The d of the piezoelectric film 33 The surface contact angle with water was 84°. Next, a protective film (base material: polyethylene terephthalate, elastic modulus: 3800 MPa, thickness: 125 μm) and the piezoelectric film were bonded together using a multi-coater, and then wound into a roll to obtain a laminated film. The multi-coater was set to a line speed of 5 m / min and a laminating roll contact pressure of approximately 0.3 N. Furthermore, a corona treatment was performed on the surface of the laminated film facing the piezoelectric film, resulting in a laminated film with a water contact angle of 70° on the surface facing the piezoelectric film. Silver nanowires with an average diameter of 30 nm and an average length of 9.5 μm were prepared with reference to the method described in Chem. Mater., 2002, 14, 4736-4745. The silver nanowires were filtered out using a filter, washed with water, and then redispersed in ethanol to prepare a silver nanowire dispersion (silver nanowire content: 0.2% by mass). Furthermore, a silver nanowire ink containing 63% by mass of the silver nanowire dispersion and 37% by mass of a dispersion medium (ultrapure water) containing a thickener and a surfactant was prepared. The prepared silver nanowire ink was applied to the piezoelectric film side of the laminated film using a bar coater to a wet thickness of 18 μm, and then dried at 80°C for 4 minutes. Next, the protective film was peeled off from the piezoelectric film, and then an amorphous silica-containing ultraviolet-curable resin composition made of an acrylic resin was applied to the surface of the piezoelectric film from which the protective film had been peeled off, and dried at 80°C for 2 minutes. The dried coating film was irradiated with 400 mJ / cm using a UV irradiation device CSOT-40 (manufactured by GS Yuasa Corporation). 2 By irradiating the piezoelectric film with UV light at an integrated light intensity of 1000 nm, a conductive piezoelectric film was obtained having a conductive layer with a thickness of 200 nm on one side of the piezoelectric film and a transparent coating layer with a thickness of 0.7 μm on the other side.

[0090] Example 2 A conductive piezoelectric film having a conductive layer with a thickness of 160 nm on one side of the piezoelectric film and a transparent coating layer with a thickness of 0.7 μm on the other side was obtained in the same manner as in Example 1, except that the wet thickness of the silver nanowire ink was changed to 11 μm.

[0091] Comparative Example 1 In the same manner as in Example 1, a piezoelectric film having a thickness of 40 μm was obtained. The prepared silver nanowire dispersion was applied to one side of a piezoelectric film using a bar coater to a wet thickness of 25 μm, and then dried at 80°C for 4 minutes to obtain a conductive piezoelectric film.

[0092] Comparative Example 2 A conductive piezoelectric film was obtained in the same manner as in Comparative Example 1, except that the wet thickness of the silver nanowire dispersion was changed to 18 μm.

[0093] Example 3 An amorphous silica-containing ultraviolet-curable resin composition made of an acrylic resin was applied to one side of a piezoelectric film obtained in the same manner as in Example 1, and dried at 80°C for 2 minutes. The dried coating film was irradiated with 400 mJ / cm using a UV irradiation device CSOT-40 (manufactured by GS Yuasa Corporation). 2to form a transparent coating layer a. Next, a protective film was attached to the surface of the transparent coating layer a in the same manner as in Example 1 to obtain a laminated film. An amorphous silica-containing ultraviolet-curable resin composition made of acrylic resin was applied to the piezoelectric film side of the laminated film and dried at 80°C for 2 minutes. The dried coating film was irradiated with 400 mJ / cm using a UV irradiation device CSOT-40 (manufactured by GS Yuasa Corporation). 2 The film was irradiated with UV light at an integrated light intensity of 1000 to form a transparent coating layer b. Next, a conductive polymer (PEDOT:PSS) dispersion with a solid content of 2.5% by mass was applied to the surface of the transparent coating layer b using a bar coater to a wet thickness of 25 μm, and then dried at 80°C for 4 minutes to obtain a conductive piezoelectric film having transparent coating layers with a thickness of 0.7 μm on both surfaces of the piezoelectric film and a conductive layer with a thickness of 0.7 μm on one surface.

[0094] Example 4 In the same manner as in Example 1, a laminated film was obtained by bonding a protective film and a piezoelectric film together. An amorphous silica-containing ultraviolet-curable resin composition made of acrylic resin was applied to the piezoelectric film side of the laminated film and dried at 80°C for 2 minutes. The dried coating film was irradiated with 400 mJ / cm using a UV irradiation device CSOT-40 (manufactured by GS Yuasa Corporation). 2 A transparent coating layer was formed by irradiating the piezoelectric film with UV light at an integrated light intensity of 10 μm. Next, a conductive polymer (PEDOT:PSS) dispersion with a solid content of 2.5% by mass was applied to the surface of the transparent coating layer using a bar coater to a wet thickness of 10 μm, and the film was dried at 80°C for 4 minutes to obtain a conductive piezoelectric film having a transparent coating layer with a thickness of 0.7 μm and a conductive layer with a thickness of 0.3 μm on one side of the piezoelectric film.

[0095] Comparative Example 3 In the same manner as in Example 1, a piezoelectric film having a thickness of 40 μm was obtained. A conductive polymer (PEDOT:PSS) dispersion with a solid content of 2.5% was applied to one side of the piezoelectric film using a bar coater to a wet thickness of 52 μm, and then dried at 80°C for 4 minutes to obtain a conductive piezoelectric film. However, it was difficult to measure the sheet resistance.

[0096] [Table 1]

[0097] As shown in Table 1, the standard deviation of the total light transmittance was low in the examples, which confirmed that the present invention can provide a conductive piezoelectric film with reduced variation in transparency. [Explanation of symbols]

[0098] 1, 2: conductive piezoelectric film, 11: piezoelectric film, 21: first transparent coating layer, 22: second transparent coating layer, 31: conductive layer

Claims

1. a piezoelectric film and a conductive layer laminated on one surface of the piezoelectric film; The total light transmittance is 80% or more, The standard deviation of the total light transmittance is 1.4% or less, the piezoelectric film contains a fluorine-based resin, A conductive piezoelectric film, wherein the conductive layer comprises at least one selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene.

2. The conductive piezoelectric film of claim 1 , further comprising a protective film laminated on a surface of the piezoelectric film opposite the conductive layer.

3. the conductive piezoelectric film further comprises a transparent coating layer; The conductive piezoelectric film according to claim 1 or 2, wherein the transparent coating layer is provided on at least one surface of the piezoelectric film.

4. the transparent coating layer is a transparent coating layer containing a cured resin obtained by curing a resin composition, 4. The conductive piezoelectric film according to claim 3, wherein the resin composition comprises at least one resin selected from the group consisting of (meth)acrylic acid ester resins, urethane resins, epoxy-based resins, phenoxy-based resins, and melamine-based resins.

5. 3. The conductive piezoelectric film according to claim 1, wherein the piezoelectric film has a surface waviness Wa of 30 μm or less at the interface with the conductive layer.

6. 3. The conductive piezoelectric film according to claim 1, wherein the absolute value of the thermal shrinkage rate when heat-treated at 80°C for 30 minutes is less than 1.0% at most.

7. 3. The conductive piezoelectric film according to claim 1, wherein the adhesiveness of the conductive layer is 4B or higher as evaluated in accordance with ASTM D3359.

8. The conductive piezoelectric film according to claim 1 or 2, wherein the metal nanowires are silver nanowires.

9. A device comprising the conductive piezoelectric film according to claim 1 or 2.

10. A method for producing the conductive piezoelectric film according to claim 1, comprising: a step of applying a solution containing a conductive material, the conductive material being at least one selected from the group consisting of metal nanowires, conductive polymers, carbon nanotubes, and graphene, to a surface of a piezoelectric film on the side opposite to the protective film on which the protective film is laminated, to form a conductive layer.

11. The method for producing a conductive piezoelectric film according to claim 10 , wherein in the step, the surface to which the solution is applied has a water contact angle of 70° or less.

12. The method for producing a conductive piezoelectric film according to claim 10 or 11, wherein the protective film has an elastic modulus of 1.0 GPa or more.

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