Piezoelectric film and method for manufacturing same

Abstract

Provided is a piezoelectric film having excellent piezoelectric properties and high transparency. This piezoelectric film contains a fluorine-based resin as a main component, has a piezoelectric constant d31 of 29-50 pC / N, an internal haze of 0.01-1.5%, and a retardation of 100-3000 nm.

Description

Piezoelectric film and method for manufacturing the same

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

[0002] Display and input devices with touch panels mounted on the front of display devices such as liquid crystal displays are widely used, including in mobile phones and other portable devices, as well as in the control panels of home electrical appliances. In touch-panel display and input devices, users operate the device by pressing on the display on the screen. Therefore, the piezoelectric film used in touch panels requires high piezoelectricity to enhance detection sensitivity and high transparency to accurately view the display image.

[0003] Films primarily composed of fluororesins such as polyvinylidene fluoride (PVDF) can be subjected to polarization treatment to produce piezoelectric films with high piezoelectric and pyroelectric constants. Patent Document 1 describes a method for polarization treatment of a film of PVDF or a copolymer of vinylidene fluoride (VDF) and trifluoroethylene (TFE) while stretching at 60°C to 110°C, thereby increasing the piezoelectric constant d ３１ It is stated that they manufactured piezoelectric films with a capacitance of 40-60 pC / N.

[0004] U.S. Patent No. 5,254,296

[0005] As described in Patent Document 1, a piezoelectric film can be manufactured by simultaneously heating, stretching, and polarization of a fluororesin film. However, according to the inventors' findings, while the piezoelectric film obtained in this way exhibits excellent piezoelectric properties, it has low transmittance in the visible light region, posing a challenge in terms of transparency. High transparency is required for piezoelectric films used in touch panels and the like, so there is a need to develop a fluororesin piezoelectric film that has both high piezoelectricity and high transparency.

[0006] The present invention has been made in view of the above problems, and aims to provide a piezoelectric film having high piezoelectricity and high transparency, and a method for manufacturing the same.

[0007] An embodiment of the present invention for solving the above problems relates to the piezoelectric film and the method for manufacturing the same described below [1] to

[15] . [1] A piezoelectric film mainly composed of a fluororesin, wherein the piezoelectric constant d ３１[1] A piezoelectric film having a density of 29 pC / N or more and 50 pC / N or less, an internal haze of 0.01% or more and 1.5% or less, and a retardation of 100 nm or more and 3000 nm or less. [2] The piezoelectric film according to [1], wherein the storage modulus measured parallel to the slow axis in in-plane birefringence is 2.7 GPa or more and 10.0 GPa or less. [3] The piezoelectric film according to [1] or [2], wherein the piezoelectric film is mainly composed of vinylidene fluoride resin. [4] The piezoelectric film according to any one of [1] to [3]. [5] A method for manufacturing a piezoelectric film according to any one of [1] to [4], comprising a stretching step of stretching a fluororesin film at least twice, and a polarization step of polarizing the fluororesin film. [6] The method for manufacturing a piezoelectric film according to [5], wherein the stretching step is performed such that the total stretching ratio is 4.0 times or more and 10.0 times or less, and the stretching temperature (T1) in the first stretching step and the stretching temperature (T2) in the second stretching step satisfy the following relationship: 110°C ≤ T1°C ≤ (T2 - 5)°C T2 < 186°C. [7] The method for manufacturing a piezoelectric film according to [5] or [6], wherein in the first stretching step, the fluororesin film is stretched to a ratio of 3.0 times or more and 5.0 times or less at a temperature (T1) of 110°C or more and less than 150°C. [8] The method for manufacturing a piezoelectric film according to any one of [5] to [7], wherein in the second stretching step, the fluororesin film is stretched to a ratio of 1.1 times or more and 2.0 times or less in the same direction as the stretching direction in the first stretching step at a temperature (T2) of T1 + 5°C or more and less than 186°C. [9] The method for manufacturing a piezoelectric film according to any one of [5] to [8], wherein the polarization step is performed simultaneously with the first stage of stretching, or after the first stage of stretching, by applying a voltage of 5 kV to 30 kV at a temperature of 50°C to less than 150°C.

[10] The method for manufacturing a piezoelectric film according to any one of [5] to [9], wherein the polarization step is performed after the second stage of stretching.

[11] The method for manufacturing a piezoelectric film according to any one of [5] to

[10] , wherein the polarization step is performed before the second stage of stretching.

[12] The method for manufacturing a piezoelectric film according to any one of [5] to

[11] , wherein the polarization step is performed simultaneously with the second stage of stretching.

[13] A method for manufacturing a piezoelectric film according to any one of [5] to

[12] , wherein the polarization step is performed simultaneously with the first stage stretching.

[14] A method for manufacturing a fluororesin piezoelectric film according to any one of [5] to

[13] , wherein the first stage stretching, the second stage stretching, and the polarization step are all performed while conveying the fluororesin film.

[15] A method for manufacturing a fluororesin piezoelectric film according to any one of [5] to

[13] , wherein in the stretching step the fluororesin film is stretched in a roll-to-roll manner, and in the polarization step the fluororesin film is polarized in a roll-to-roll manner.

[0008] According to the present invention, a piezoelectric film having high piezoelectricity and high transparency is provided, as well as a method for manufacturing the same.

[0009] [Piezoelectric Film] One embodiment of the present invention relates to a piezoelectric film mainly composed of a fluororesin.

[0010] The piezoelectric film may contain a fluororesin as its main component. A fluororesin is a resin obtained by polymerizing a monomer containing a fluorine-containing olefin. Containing a fluororesin as its main component means that the proportion of the fluororesin to the total mass of the piezoelectric film is 50% by mass or more. The proportion of the fluororesin to the total mass of the piezoelectric film 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, and even more preferably 90% by mass or more and 100% by mass or less.

[0011] Fluorine-based resins can be homopolymers or copolymers obtained by polymerizing TFE, VDF, etc. Examples of fluorine-based resins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP). Examples of fluorine-based resins obtained by polymerizing VDF include homopolymers of VDF, as well as copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), CTFE, TFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether.

[0012] Of these, from the viewpoint of facilitating polarization of the fluororesin film, fluororesins obtained by polymerizing VDF are preferred, with homopolymers of VDF, copolymers of VDF and TrFE, copolymers of VDF and HFP, copolymers of VDF and TFE, copolymers of VDF, TrFE and TFE, copolymers of VDF, TrFE and TFE and CTFE, and copolymers of VDF, TrFE and TFE and 1-chloro-1-fluoroethylene being more preferred, and homopolymers of VDF being even more preferred. These fluororesins may be used individually or in combination of multiple types.

[0013] The fluororesin included as a component of the fluororesin film is preferably a fluororesin mainly composed of VDF, and more preferably a homopolymer of VDF, because high piezoelectricity is expected. A fluororesin mainly composed of VDF means that the VDF content relative to the total mass of the fluororesin is 50% by mass or more. The VDF content is the ratio of the mass of constituent units derived from VDF to the total mass of the fluororesin. The content of constituent units derived from VDF relative to the total mass of the fluororesin 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, even more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less.

[0014] Among them, since a fluororesin film is expected to have high piezoelectricity, it is more preferable that the content of the homopolymer of VDF is higher. The content of the homopolymer with respect to the total mass of the fluororesin film 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 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less.

[0015] The content of the resin having VDF as a constitutional unit contained in the fluororesin film and the fluororesin piezoelectric film １９ can be measured by quantitative analysis using F-NMR with an internal standard.

[0016] The fluororesin film has a melt viscosity (hereinafter, unless otherwise specified, the melt viscosity measured under the above conditions is referred to as "melt viscosity") measured at a measurement temperature of 260 ° C and a shear rate of 50 s －１ of preferably 600 Pa·s or more and 4000 Pa·s or less, more preferably 1000 Pa·s or more and 3500 Pa·s or less, and still more preferably 1000 Pa·s or more and 2400 Pa·s or less. Since the melt viscosity of the fluororesin does not change even after passing through the steps of film formation, stretching, and polarization, the melt viscosity of the piezoelectric film is an index for knowing the melt viscosity of the resin composition containing the fluororesin as a raw material (hereinafter, the resin composition containing the fluororesin is referred to as "resin composition") and the fluororesin film. Since the fluororesin film is produced by forming the resin composition into a film, a resin composition having a melt viscosity of 4000 Pa·s or less can be formed into a film by extrusion at a relatively low temperature. Further, since the molecular chains of the resin in the resin composition are short and there is little entanglement of the molecular chains, the film produced from the resin composition can be stretched at a high magnification without breaking. Furthermore, since the molecular chains are short, the mobility of the molecular chains due to heat is improved, and the selective arrangement of the molecular chains progresses during cooling, resulting in an improvement in crystallinity. By stretching the film with a high crystallinity, the orientation of the molecular chains progresses and the proportion of β-phase crystals increases. By polarizing the film in which the β-phase crystals are developed, the dipoles of the molecules are arranged in one direction, and high piezoelectricity is exhibited. Therefore, a resin film with a lower melt viscosity is more advantageous as a raw material film for a piezoelectric film having higher piezoelectricity.

[0017] Also, in order to obtain a piezoelectric film with few foreign substances, it is preferable to filter the resin composition melted during film production with a filter and form a resin composition from which foreign substances have been removed. For a resin composition having a melt viscosity of 4000 Pa·s or less, since the viscosity of the resin easily decreases by heating, the filter is less likely to become clogged and filtration becomes easier. Therefore, it is not necessary to raise the melting temperature to enable filtration, and it is possible to less likely cause spot-like irregularities on the film surface due to the resin being deteriorated by heat and foreign substances being generated. On the other hand, for a resin composition having a melt viscosity exceeding 4000 Pa·s, since the molecular weight of the resin is large and the movement of the molecular chains is suppressed, it is difficult to form an orientation structure of the molecular chains and crystallization hardly progresses. Therefore, a film made from the resin composition has a uniform structure, is less likely to cause light scattering due to the difference in refractive index between the crystalline part and the non-crystalline part, has excellent transparency, and various physical properties of the film are likely to be stable. However, the resin composition needs to be at a relatively high temperature to have a viscosity for filtration, and the resin composition is likely to be deteriorated or decomposed. A resin composition having a melt viscosity of less than 600 Pa·s contains many molecular chains with a small molecular weight, so various physical properties of the obtained film are unlikely to be stable.

[0018] The melt viscosity is measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho Co., Ltd., Capillograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, at a measurement temperature of 260°C and a shear rate of 50 s －１ the viscosity measured under these conditions is used.

[0019] The retardation of the piezoelectric film is 100 nm or more and 3000 nm or less, preferably 500 nm or more and 3000 nm or less, and more preferably 750 nm or more and 3000 nm or less. The larger the retardation, the higher the degree of orientation of the molecular chains of the fluororesin, the sufficiently high ratio of β-phase crystals, and the easier it is to increase the piezoelectricity.

[0020] Retardation is measured using a light source with a wavelength of 589.1 nm and the parallel nicol rotation method. The retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the piezoelectric film is measured, and this value is taken as the representative value of the piezoelectric film's retardation. If the length of one side of the piezoelectric film is 300 mm or more, a rectangular area with sides of 300 mm is arbitrarily assumed, and the retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular area is measured. If both sides of the piezoelectric film are 300 mm or more, a square with sides of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above.

[0021] The piezoelectric film has a piezoelectric constant d. ３１ The ratio is 29 pC / N or more and 50 pC / N or less, preferably 30 pC / N or more and 50 pC / N or less, more preferably 32 pC / N or more and 50 pC / N or less, even more preferably 35 pC / N or more and 50 pC / N or less, and particularly preferably 35 pC / N or more and 45 pC / N or less.

[0022] Piezoelectric constant d of piezoelectric film ３１It is measured by a static method. Specifically, Al vapor deposition electrodes are formed on both sides of the film, and a rectangular sample with a length of 60 mm parallel to the slow axis detected by the in-plane birefringence of the film during retardation measurement and a width of 10 mm perpendicular to the slow axis is cut out as a test piece from near the center of the planar portion of the piezoelectric film with the electrodes. Next, the test piece is installed in a tensile testing machine so that the distance between the chucks is 50 mm, and the induced charge amount Q (pC) when the test piece is stretched in the longitudinal direction (slow axis direction) with a tensile speed of 10 mm / min at room temperature with a force F (N) is measured with an electrometer. The relationship between different forces F (N) and the induced charge amounts Q (pC) for each is measured at 128 points in this way, with the force F (N) on the horizontal axis and the induced charge amount Q (pC) on the vertical axis and plotted, and the slope (pC / N) of the generated charge amount Q (pC) with respect to the force F (N) is obtained by the least squares method. The measurement of the charge amount is to measure the charge amount in the range including the intersection of the diagonals of the polarization plane of the piezoelectric film by the above method, and the absolute value of the value is taken as the representative value Q (pC) of the charge amount of the piezoelectric film. At this time, when the length of one side of the piezoelectric film is 300 mm or more, a rectangular area with one side being 300 mm is arbitrarily set, and the range including the intersection of the diagonals of the above rectangular area is measured. Also, when both sides of the piezoelectric film are 300 mm or more, a square with one side being 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the above method. Piezoelectric constant d ３１ (pC / N) can be obtained by Equation 1.

[0023] Here, A is the area defined by the product of the distance between the chucks and the width of the test piece, which is 50×10 = 500 (mm ２ ), t (mm) is the thickness of the test piece, and w is the width of the test piece, which is 10 mm.

[0024] The piezoelectric film has an internal haze of 0.01% or more and 1.5% or less, preferably 0.01% or more and 1.0% or less, and more preferably 0.01% or more and 0.8% or less. The lower the internal haze, the higher the transparency of the piezoelectric film.

[0025] The internal haze of the piezoelectric film is determined by filling in any scratches or other irregularities that cause external haze with silicone oil to eliminate the influence of external haze, and then measuring the haze in a range including the center of the piezoelectric film in accordance with ISO 14782:1999. Specifically, silicone oil (Nikon, MXA22179) is placed between two glass plates (thickness: 0.15 mm, flatness: 0.01 mm or less), and the haze (H2) caused by the glass plates and silicone oil is measured using a haze meter (Nippon Denshoku Industries Co., Ltd., NDH2000). Next, a 50 mm x 50 mm rectangular film is cut out so as to include the intersection of the diagonals of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with sides of 300 mm is arbitrarily assumed, and the rectangular film is cut out from a range including the intersection of the diagonals of the rectangular film. If both sides of the piezoelectric film are 300 mm or longer, an arbitrary square with sides of 300 mm is set on the piezoelectric film, the measurement range is set according to the method described above, and the rectangular film is cut out. Both sides of the cut piezoelectric film are uniformly wet with the silicone oil described above, and the haze (H3) is measured using the haze meter while sandwiched between the two glass plates. H2 is subtracted from H3 to obtain the internal haze (H1) of the piezoelectric film. The internal haze (H1) of the piezoelectric film is calculated using the following formula: H1 = H3 - H2

[0026] The piezoelectric film preferably has a storage modulus of 1.0 GPa or more and 10.0 GPa or less, more preferably 2.7 GPa or more and 10.0 GPa or less, even more preferably 2.7 GPa or more and 8.8 GPa or less, even more preferably 2.7 GPa or more and 6.0 Pa or less, and particularly preferably 2.7 GPa or more and 5.0 GPa or less. As shown in the formula described later, the larger the storage modulus, the higher the elastic compliance s １１ The electromechanical coupling coefficient k becomes smaller. ３１ This increases. Therefore, the higher the storage modulus, the higher the conversion efficiency between electrical energy and mechanical energy. On the other hand, if the storage modulus is 10.0 GPa or less, the sensitivity to load when using a piezoelectric film as a sensor does not decrease easily.

[0027] The storage modulus is measured in accordance with JIS K7244-4:1999 (ISO 6721-4:1994), using a sample cut parallel to the slow axis in the in-plane birefringence of the piezoelectric film, with a length of 25 mm and a width of 3 mm, as the test specimen. If one side of the piezoelectric film is 300 mm or longer, a rectangular area with sides of 300 mm is arbitrarily defined, and the test specimen is cut from the area including the intersection of the diagonals of this rectangular area. If both sides of the piezoelectric film are 300 mm or longer, a square with sides of 300 mm is arbitrarily defined on the piezoelectric film, and the test specimen is cut according to the above method. Specifically, the storage modulus of the test specimen is measured using a TA Instruments RSA-III with a chuck distance of 15 mm, an initial load of 1.5 N, a frequency of 10 Hz, a strain of 0.1%, and a temperature of 23°C, and the obtained value is defined as the storage modulus of the piezoelectric film.

[0028] Dielectric constant ε of fluororesin piezoelectric film ３３ The pressure is preferably 60 pF / m to 150 pF / m, more preferably 80 pF / m to 140 pF / m, even more preferably 90 pF / m to 130 pF / m, particularly preferably 100 pF / m to 130 pF / m, and most preferably 110 pF / m to 130 pF / m.

[0029] Dielectric constant ε ３３ The measurement is performed in accordance with JIS C2138:2007. Specifically, electrodes with a diameter of 19.5 mm are formed on both sides of a piezoelectric film by aluminum vapor deposition, and the relative permittivity is measured at room temperature, with a voltage of 1 V and a frequency of 1 kHz using an LCR meter manufactured by HIOKI Corporation. The obtained relative permittivity is then multiplied by the permittivity of vacuum (8.854 × 10⁻¹⁴). －１２ The dielectric constant ε is calculated by multiplying by F / m. This is the dielectric constant in the range including the center of the polarization surface of the piezoelectric film (or the range including the intersection of the diagonals in the case of a rectangular piezoelectric material). ３３ The dielectric constant ε of the piezoelectric film is measured using the method described above, and the absolute value of that value is taken as the dielectric constant ε of the piezoelectric film. ３３This shall be a representative value. If the length of one side of the piezoelectric film is 300 mm or more, a rectangular area with sides of 300 mm shall be arbitrarily set, and the range including the intersection of the diagonals of the above rectangular area shall be measured. If both sides of the piezoelectric film are 300 mm or more, a square with sides of 300 mm shall be arbitrarily set on the piezoelectric film, and the measurement range shall be set according to the above method.

[0030] Furthermore, the piezoelectric film has an electromechanical coupling coefficient k. ３１ 0.14 or higher is preferred, 0.16 to 0.40 is more preferred, 0.17 to 0.35 is even more preferred, and 0.18 to 0.30 is particularly preferred. Electromechanical coupling coefficient k ３１ The higher the value of k, the higher the conversion efficiency between electrical and mechanical energy. On the other hand, the electromechanical coupling coefficient k ３１ By setting this value to 0.40 or less, it becomes unnecessary to increase the stretching ratio and polarization voltage during the manufacturing of fluorine-based piezoelectric films, and the film haze is less likely to increase during film formation.

[0031] Electromechanical coupling coefficient k ３１ d is the piezoelectric constant ３１ , dielectric constant ε ３３ , and elastic compliance １１ The elastic compliance s is calculated using the following formula. １１ The reciprocal of the storage modulus is used as the value.

[0032]

[0033] The thickness of the fluororesin piezoelectric film is not particularly limited, but is preferably 10 μm to 200 μm, more preferably 20 μm to 80 μm, and even more preferably 30 μm to 80 μm. The thicker the film, the more advantageous it is in terms of insulation and mechanical properties. The thinner the film, the more advantageous it is in terms of optical properties such as transparency and cost. According to this embodiment, even if the thickness of the piezoelectric film is 30 μm or more, it is possible to increase both piezoelectricity and transparency.

[0034] The thickness of piezoelectric film is generally measured using a micrometer (JIS C 2151:2019), but it can also be measured using known methods such as a laser displacement meter, a capacitance displacement meter, or infrared light. Specifically, the thickness of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the piezoelectric film is measured, and this value is taken as a representative value of the piezoelectric film's thickness. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with sides of 300 mm is arbitrarily assumed, and the thickness of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with sides of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above.

[0035] Piezoelectric film has a minimum size of 100 μm, and the number of foreign objects larger than 100 μm is 7 per 0.25 m when viewed from above. ２ The following is the result: 0 pieces / 0.25 m ２ 5 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following are even more preferable.

[0036] Piezoelectric film has zero foreign particles larger than 200 μm when viewed from above. ２ 3 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 2 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following is even more preferable: 0 pieces / 0.25 m ２ That is particularly preferable.

[0037] Piezoelectric film has zero foreign particles less than 100 μm in size when viewed from above. ２ More than 50 pieces / 0.25m ２The following is preferable: 0 pieces / 0.25 m ２ More than 25 pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following are even more preferable.

[0038] The fewer these impurities there are, the better the transparency of the piezoelectric film, and the more uniform the stretching and polarization processes become during the manufacturing process of the fluororesin film, which is the material for the piezoelectric film. The number of impurities can be reduced by filtering the molten fluororesin during film manufacturing.

[0039] The number of these foreign objects is determined by cutting out four rectangular pieces of film (observation pieces) from the piezoelectric film so that they are adjacent to each other, and then summing the number of foreign objects measured from each of these observation pieces. Specifically, the pieces are cut out at a distance of 0.010 m so that they are adjacent to each other from the piezoelectric film. ２ Four observation pieces measuring (100 mm x 100 mm) are cut out. Then, the sum of the number of foreign objects measured from each observation piece is calculated, and the sum of the number of foreign objects is multiplied by 25 / 4 and rounded to the first decimal place to obtain 0.25 m. ２ This is the number of foreign objects per sample. At this time, the foreign objects are observed using transmitted light and marked, and the size of the foreign object is determined by observing the marked area under a microscope. The size of the foreign object is the arithmetic mean of the maximum and minimum widths of the foreign object.

[0040] [Method for manufacturing a fluororesin piezoelectric film] The method for manufacturing the piezoelectric film described above is not particularly limited, but it is preferable to include a stretching step of stretching a fluororesin film, which mainly consists of a fluororesin, at least twice, and a polarization step of polarizing the fluororesin film.

[0041] (Fluorine-based resin film) The fluorine-based resin film is a film made by forming the above-mentioned fluorine-based resin. The fluorine-based resin film may be made by melt extrusion of the resin composition or by solvent casting.

[0042] In the melt extrusion method, a fluoropolymer resin film is produced by extruding a molten resin. In the solvent casting method, a liquid composition in which the resin composition is dissolved in a solvent is cast (coated) onto a substrate, and the solvent is vaporized by heating and drying to produce a fluoropolymer resin film.

[0043] In the melt extrusion method, a fluororesin film can be produced by heating and melting the resin composition described above (melting step) and forming a film from the molten resin composition (film formation step). At this time, a step of filtering the molten resin composition (filtration step) may be further performed between the melting step and the film formation step.

[0044] In the melting step, the resin composition is melted. This step can be carried out, for example, by melting and kneading the resin composition using an extruder.

[0045] The resin composition melted in the melting process may be any resin composition mainly composed of the fluororesin described above. However, if the resin composition contains solvent components, the residual solvent components that do not volatilize may hinder polarization in subsequent processes. Therefore, it is preferable that the content of the solvent components in the resin composition be small, preferably 1.0% by mass or less, and more preferably 0.1% by mass or less, relative to the total mass of the resin composition. In particular, it is preferable that the solvent content be 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less, relative to the total mass of the resin composition.

[0046] Furthermore, the resin composition melted in the melting process preferably has a melt viscosity of 600 Pa·s or more and 4000 Pa·s or less. Resin compositions with a melt viscosity of 4000 Pa·s or less can be formed by extrusion at relatively low temperatures. In addition, because the resin composition has short molecular chains and less entanglement of molecular chains, the film made from the resin composition can be stretched at high magnification without breaking. Moreover, the short molecular chains improve the mobility of the molecular chains due to heat, and selective alignment of the molecular chains progresses during cooling, resulting in improved crystallinity. Stretching the highly crystallinity film promotes molecular chain orientation, increasing the proportion of β-phase crystals. Polarization treatment of the film with well-developed β-phase crystals causes the molecular dipoles to align in one direction, resulting in high piezoelectricity. Therefore, resin films with lower melt viscosity are advantageous as raw materials for piezoelectric films with high piezoelectricity. Furthermore, to obtain a piezoelectric film with fewer impurities, it is preferable to filter the heat-melted resin composition during film manufacturing to remove impurities and then form a film using the resulting resin composition. Resin compositions with a melt viscosity of 4000 Pa·s or less readily decrease in viscosity upon heating, making it less likely for the filter to clog and thus easier to filter. Therefore, it is not necessary to raise the melting temperature to enable filtration, and it is less likely for the resin to change due to heat and generate impurities, resulting in mottled irregularities on the film surface. On the other hand, resin compositions with a melt viscosity exceeding 4000 Pa·s have a large molecular weight, and the movement of molecular chains is suppressed, making crystallization difficult. Therefore, films made from such resin compositions have a uniform structure, are less prone to light scattering due to differences in refractive index between crystalline and amorphous parts, exhibit excellent transparency, and have stable various physical properties. However, these resin compositions require relatively high temperatures to achieve the viscosity necessary for filtration, which can easily lead to deterioration and decomposition of the resin composition. Resin compositions with a melt viscosity of less than 600 Pa·s contain many molecular chains with small molecular weights, which makes it difficult to maintain the stability of various physical properties in the resulting films.

[0047] The melting temperature shall be the highest temperature in the conduit from the extruder to the filtration device.

[0048] The melting temperature of the resin composition is preferably 20°C (mp + 20°C) to 130°C (mp + 130°C) above the melting point (mp) of the resin composition, more preferably (mp + 50°C) to (mp + 100°C), even more preferably (mp + 80°C) to (mp + 100°C), and particularly preferably (mp + 85°C) to (mp + 95°C). By setting the melting temperature to 20°C (mp + 20°C) or higher above the melting point of the resin composition, the viscosity of the resin composition can be reduced to a degree that allows for extrusion. By setting the melting temperature to 130°C (mp + 130°C) or lower above the melting point of the resin composition, decomposition and condensation of the resin due to heating can be suppressed, and the generation of decomposition products and the like can be suppressed. By suppressing the generation of the above-mentioned decomposition products, the amount of foreign matter in the fluororesin piezoelectric film can be reduced, thereby improving the transparency and smoothness of the fluororesin piezoelectric film.

[0049] The melting point of the resin composition shall be the value measured in accordance with JIS K 7121:1987. Specifically, 5 mg of the sample shall be sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation). Under a nitrogen atmosphere, the temperature shall be raised from room temperature to 230°C at a heating rate of 10°C / min, and the maximum melting peak temperature in the DSC curve shall be defined as the melting point of the resin composition.

[0050] (Filtration process) In the filtration process, the resin composition, which has been melted and reduced in viscosity in the melting process, is filtered. The filtration method is not particularly limited; it is sufficient to pass the molten resin composition through a filter, and known types of filters such as pleated filters and leaf disc filters can be used.

[0051] In the filtration process, the resin composition is filtered using a filter with a filtration accuracy of 10 μm to 40 μm. Using a filter with a filtration accuracy of 10 μm or more facilitates the filtration of the resin composition that has been heated and melted to the above temperature, and also prevents the filtration pressure from becoming too high, thus shortening the filtration time. Using a filter with a filtration accuracy of 40 μm or less allows for sufficient removal of foreign matter from the resin composition, resulting in a fluororesin film with fewer foreign matter particles. The filtration accuracy of the filter is preferably 10 μm to 30 μm, and more preferably 15 μm to 30 μm.

[0052] The filter may be placed between the extruder and the die used in the film formation process. Alternatively, the filter may be placed in a different extruder or melt-kneading device than the one used in the film formation process, and the resin composition filtered through this filter may be introduced into the extruder used in the melting process for film formation.

[0053] In the film formation process, the resin composition, which has been heated and melted in the melting process and filtered in the filtration process as needed, is formed into a film. The film formation method is not particularly limited, and known methods such as extruding the melted and filtered resin composition from a T-die and cooling it by contacting it with a cooling roll can be used.

[0054] The film formation method is not particularly limited, and known methods such as extruding the molten and filtered resin composition from a T-die and cooling it in contact with a cooling roll can be used. The surface temperature of the cooling roll is preferably 125°C or lower. By rapidly cooling the film of the resin composition extruded by a cooling roll with a low surface temperature, crystal growth can be suppressed, and the internal haze of the resulting fluororesin film and the fluororesin piezoelectric film manufactured from the film can be reduced. The surface temperature of the cooling roll is preferably 5°C to 115°C, more preferably 20°C to 80°C, and even more preferably 30°C to 60°C. By setting the surface temperature of the cooling roll to 5°C or higher, the film does not become too hard, and a decrease in the stretchability and transportability of the film due to a decrease in adhesion between the guide roll or stretching roll and the film is less likely to occur.

[0055] The fluororesin film obtained in this way may be stored after being wound up, or it may be used continuously in the stretching and polarization processes as is.

[0056] (Stretching Process) The stretching process consists of a first stretching stage (first stretching process) in which the fluororesin film is stretched while being heated, and a second stretching stage (second stretching process) in which the fluororesin film stretched in the first stretching stage is further stretched while being heated. In addition, further stretching, such as a third stretching process, may be performed after the second stretching stage.

[0057] In each stretching process, the stretching direction is preferably the MD direction (film transport direction) when stretching is performed while transporting the film. When both the first and second stretching processes are performed by batch processing, the stretching direction only needs to be the same for the first and second stretching processes, and the stretching direction can be any direction.

[0058] The total stretching ratio in the stretching process (the product of the stretching ratios in each stretching process) is preferably 4.0 to 10.0 times, more preferably 4.5 to 8.0 times, even more preferably 4.7 to 7.0 times, and particularly preferably 5.0 to 7.0 times. The total stretching ratio can be determined as the product of the stretching ratios in each stretching process. By setting the total stretching ratio to 4.0 times or higher, the crystal ratio of the β phase in the film increases, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through polarization treatment. By setting the total stretching ratio to 10.0 times or lower, it is possible to reduce the occurrence of crazes and whitening due to stress concentration, as well as film breakage during the stretching process.

[0059] Furthermore, it is preferable that the stretching process be carried out such that the stretching temperature (T1) in the first stretching process and the stretching temperature (T2) in the second stretching process satisfy the following relationship: 110°C ≤ T1°C ≤ (T2 - 5)°C and T2 < 186°C

[0060] Patent Document 1 describes a method in which stretching and polarization are performed simultaneously in the range of 25°C to 110°C. However, high-magnification stretching in such a temperature range tends to cause film whitening and an increase in haze value. In contrast, by performing a first stretching step at a temperature of 110°C or higher (T1) and a second stretching step at a temperature of T1+5°C or higher but less than 186°C, multiple stretching processes can be performed to suppress film whitening and an increase in haze value. This is thought to be because performing the first stretching at a temperature of 110°C or higher softens the polymer, suppressing the occurrence of craze and whitening due to stress concentration. Furthermore, by performing the second stretching at a temperature higher than the first stretching step, the stretching ratio can be further increased while suppressing whitening. Increasing the stretching ratio increases the degree of orientation of the molecular chains, resulting in a higher storage modulus and an electromechanical coupling coefficient k. ３１ A high piezoelectric film can also be obtained.

[0061] Each stretching process can be performed using a stretching apparatus having multiple rolls and pinch rolls for holding the fluororesin film. In this case, the stretching ratio can be controlled by controlling the ratio of the rotational speed of the feed-side roll in the preceding stage to the rotational speed of the take-up-side roll in the subsequent stage. For example, when performing the first stretching process and the second stretching process with four rolls, the stretching can be performed by changing the rotational speed ratio of the feed-side and take-up-side rolls and the temperature of each roll in each of the first and second stretching processes. In the case of a roll-to-roll method, the stretching temperature is the higher of the surface temperatures of the feed-side roll in the preceding stage and the take-up-side roll in the subsequent stage. Specifically, in the first stretching process, the surface temperature of the roll with the higher temperature among the surface temperatures of the preceding and subsequent rolls is set to T1. Also, in the second stretching process, the surface temperature of the roll with the higher temperature among the surface temperatures of the preceding and subsequent rolls is set to T2. In batch processing, the stretching temperature is defined as the temperature at which the film temperature reaches its highest point within the stretching process. When the film is heated and stretched by other methods, the stretching temperature is defined as the temperature at which the film surface temperature reaches its highest point within the stretching process. The film temperature can be measured using either a contact or non-contact method.

[0062] In the first stretching step, the stretching temperature (T1) is preferably 110°C or higher, more preferably 110°C or higher and less than 150°C, even more preferably 115°C or higher and less than 150°C, particularly preferably 120°C or higher and less than 150°C, very preferably 125°C or higher and less than 150°C, extremely preferably 125°C or higher and 145°C or lower, and most preferably 130°C or higher and 145°C or lower. By increasing the temperature T1 of the fluororesin film in the first stretching step, the temperature inside the film is increased during stretching, making it easier to deform in the stretching direction (MD direction), and thus reducing the haze of the film. By moderately lowering the surface temperature of the roll during stretching, the stretching stress of the fluororesin film is increased, and crystals of the highly piezoelectric β phase can be efficiently formed. Furthermore, by stretching while heating, the film is stretched uniformly, localized crystallization is suppressed, and molecular chains are uniformly arranged throughout the film, which suppresses light scattering caused by differences in the degree of crystallinity within the film, and thus reduces internal haze. By keeping the surface temperature of the roll during stretching below 150°C, the efficiency of molecular chain alignment during film stretching can be increased, the proportion of β-phase structures that contribute to piezoelectricity can be increased, and deformation of the film due to melting of the fluororesin can be suppressed.

[0063] The stretching ratio in the first stretching step is preferably 3.0 times or more and 5.0 times or less, more preferably 3.5 times or more and 5.0 times or less, even more preferably 3.5 times or more and 4.7 times or less, even more preferably 3.5 times or more and 4.5 times or less, and particularly preferably 3.5 times or more and 4.3 times or less. By setting the stretching ratio in the first stretching step to 3.0 times or more, the crystal ratio of the β phase in the film increases, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through polarization treatment. By setting the stretching ratio in the first stretching step to 5.0 times or less, it is possible to suppress the increase in haze due to the generation of excessive stretching stress.

[0064] In the second stretching step, the stretching temperature (T2) is preferably T1 + 5°C or higher, more preferably T1 + 5°C or higher and less than 186°C, even more preferably T1 + 10°C or higher and less than 186°C, still more preferably T1 + 15°C or higher and less than 186°C, still more preferably T1 + 15°C or higher and 172°C or lower, and particularly preferably T1 + 15°C or higher and 165°C or lower. By making the temperature T2 of the fluororesin film in the second stretching step higher than the temperature T1 in the first stretching step, the film can be softened, and the occurrence of crazes and whitening due to stress concentration can be suppressed. Furthermore, by setting the temperature T2 of the fluororesin film in the second stretching step to less than 186°C, the stretching stress of the fluororesin can be appropriately increased, and crystals of the highly piezoelectric β phase can be efficiently formed.

[0065] The stretching ratio in the second stretching step is preferably 1.1 to 2.0 times, more preferably 1.18 to 2.0 times, even more preferably 1.18 to 1.7 times, particularly preferably 1.25 to 1.7 times, and most preferably 1.25 to 1.5 times. By setting the stretching ratio in the second stretching step to 1.1 times or higher, a film with a higher proportion of piezoelectric β-phase crystals and a higher elastic modulus can be obtained. By setting the stretching ratio in the second stretching step to 2.0 times or lower, the occurrence of crazing and whitening due to stress concentration can be suppressed, and a highly transparent film can be obtained.

[0066] The total stretching ratio obtained by the stretching process, including the first and second stretching steps, is preferably 4.0 to 10.0 times, more preferably 4.5 to 8.0 times, even more preferably 4.7 to 7.0 times, and particularly preferably 5.0 to 7.0 times. The total stretching ratio can be determined as the product of the stretching ratios in each stretching step. By setting the total stretching ratio to 4.0 times or more, the proportion of β phase in the film increases, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through polarization treatment. By setting the total stretching ratio to 10.0 times or less, it is possible to reduce the occurrence of crazes and whitening due to stress concentration, as well as film breakage during the stretching process.

[0067] (Polarization Process) The polarization process is performed simultaneously with the first stretching process or after the first stretching process. By performing a polarization treatment on a fluororesin film whose β-phase crystal ratio has been increased by the first stretching process, a fluororesin piezoelectric film with high piezoelectricity can be manufactured. The polarization process can be performed in batch processing or as continuous polarization processing while the film is being transported, and from the viewpoint of productivity, continuous processing while the film is being transported is preferred. As a method of performing the process while the film is being transported, for example, a roll-to-roll method is preferred, and the polarization treatment can be performed by discharging DC current from a needle-shaped electrode onto the metal roll performing the first stretching process, the metal roll performing the second stretching process, or a different metal roll. The fluororesin film, which is transported in close contact with the surface of the metal roll, is polarized as it passes between the needle-shaped electrode discharging DC current and the metal roll. At this time, the metal roll from which the DC current is being discharged is electrically grounded.

[0068] When performing polarization in a batch process, the film can be heated by a heating plate method, where the film is placed on a heating plate; a hot air circulation method, where the film is placed in a heating chamber and heated by circulating hot air; or an infrared heating method, where the film is heated non-contact using an infrared heater. Here, the polarization temperature when polarization is performed in a batch process is the highest temperature of the film within the range where a DC electric field is applied. When polarization is performed continuously while the film is being transported, the hot air circulation method or infrared heating method can be applied to heat the film, but from the viewpoint of simplifying the apparatus and facilitating temperature control, it is preferable to heat the film to be polarized by heating a metal roll that discharges DC current from the electrode. When heating with the metal roll, the polarization temperature is the surface temperature of the metal roll, and when the film is heated by other methods, the polarization temperature is the highest temperature of the film within the range where a DC electric field is applied. Here, the film temperature can be measured by either a contact method or a non-contact method.

[0069] In the polarization process, it is preferable to apply a voltage of 5kV to 30kV to the fluororesin film at a temperature of 50°C to less than 170°C. The temperature in the polarization process is preferably 80°C to 160°C, more preferably 100°C to 150°C, even more preferably 115°C to 150°C, and particularly preferably 130°C to 150°C. The DC voltage applied in the polarization process is preferably 3kV to 30kV, more preferably 5kV to 30kV, even more preferably 5kV to 25kV, particularly preferably 5kV to 20kV, and most preferably 10kV to 17kV.

[0070] The polarization process may be performed simultaneously with the first stretching process, between the first and second stretching processes, simultaneously with the second stretching process, or after the second stretching process. Performing the polarization process after the second stretching process is preferable from the viewpoint of manufacturing stability because it reduces the likelihood of film breakage during flashover compared to performing stretching and polarization simultaneously. The same applies when the polarization process is performed between the first and second stretching processes. Furthermore, performing the polarization process simultaneously with the first stretching process makes it easier to obtain a piezoelectric film with high piezoelectric properties because the orientation of molecular chains due to stretching and the orientation of dipoles due to the electric field proceed simultaneously. The same applies when the polarization process is performed simultaneously with the second stretching process.

[0071] In this process, any one or all of the first stretching step, the second stretching step, and the polarization step may be performed in batch or continuous processing, or the process may be performed in combination with batch and continuous processing. From the viewpoint of productivity, it is preferable to perform the process continuously while conveying the film. As for the continuous processing method, it is preferable to perform the process while conveying the film. For example, in a roll-to-roll method, the fluororesin film supplied from a supply roll can be conveyed by multiple guide rolls while the first stretching step, the second stretching step, and the polarization step are performed in continuous processing, and the film can be wound onto a winding roll and stored. Alternatively, the first stretching step, the second stretching step, and the polarization step may be divided into multiple lines and the fluororesin piezoelectric film may be manufactured by performing multiple roll-to-roll processes corresponding to each line.

[0072] By performing the first stretching process, the second stretching process, and the polarization process while conveying the film, these processes can be carried out in a short time of a few seconds while heating. If stretching and polarization processes are performed over a long period of time using batch processing, the film is prone to haze and discoloration due to the prolonged heating and polarization processes, but by performing the processes in a short time, it is possible to reduce the likelihood of haze and discoloration in the film. From the viewpoint of mass production, it is preferable to perform the stretching and polarization processes continuously using a roll-to-roll method.

[0073] The polarization treatment time is preferably 0.001 seconds to 60 minutes, more preferably 0.01 seconds to 30 minutes, even more preferably 0.1 seconds to 15 minutes, particularly preferably 1 second to 10 minutes, very preferably 1 second to 1 minute, extremely preferably 1 second to 30 seconds, and most preferably 1 second to 10 seconds. By setting the time to 0.001 seconds or more, sufficient polarization treatment can be performed, and the piezoelectric constant can be increased. Furthermore, by setting it to 60 minutes or less, good productivity can be achieved. Here, the polarization treatment time refers to the time during which the film passes through the region sandwiched between the electrode to which a high voltage is applied and the roll when the polarization treatment is performed on the roll while the film is being transported. In the case of batch processing, it refers to the time during which the film is subjected to polarization treatment at a predetermined voltage, and when polarization treatment is performed multiple times, the sum of the respective polarization treatment times is defined as the polarization time.

[0074] The piezoelectric film obtained by the stretching and polarization processes can be wound into a roll for storage and transport.

[0075] [Applications] The piezoelectric film described above can be used for various applications such as piezoelectric films for touch sensors and touch panels, actuators, protective films, and phase difference films.

[0076] [Other Embodiments] It goes without saying that the embodiments described above are exemplary embodiments of the present invention, and the present invention may include embodiments other than those described above within the scope of its core technical concept.

[0077] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0078] 1. Fabrication of Piezoelectric Films Piezoelectric films 1 to 8 were fabricated according to the following procedure.

[0079] The melt viscosity of the resin composition used as the material was measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (Capillograph 1D, manufactured by Toyo Seiki Seisakusho Co., Ltd.) was used, and a capillary die with an inner diameter of 1 mmφ and a tube length of 10 mm was used, at a measurement temperature of 260°C and a shear rate of 50 s. －１ The viscosity was measured using [the specified method].

[0080] Furthermore, the melting point of the resin composition was measured in accordance with JIS K 7121:1987. Specifically, the melting point of the resin composition was determined by placing 5 mg of the sample in an aluminum pan and setting it in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation). Under a nitrogen atmosphere, the temperature was raised from room temperature to 230°C at a heating rate of 10°C / min, and the maximum melting peak temperature in the resulting DSC curve was defined as the melting point of the resin composition.

[0081] Furthermore, the melting temperature was defined as the highest temperature in the conduit from the extruder to the filtration device.

[0082] 1-1. Piezoelectric Film 1 A PVDF with a melt viscosity of 2500 Pa·s and a melting point (mp) of 173°C was melted using a single-screw extruder with a diameter of φ35 mm to a melting temperature of 270°C (mp + 97°C). The molten resin was then extruded into a film using a T-die and brought into contact with a cooling roll at 110°C to obtain a fluororesin film with a thickness of 150 μm, a width of 240 mm, and a length of over 10,000 mm.

[0083] The fluororesin film was stretched to a predetermined magnification by adjusting the rotation speed of the take-up metal roll relative to the rotation speed of the feed-side metal roll using a multi-stage uniaxial stretching apparatus equipped with multiple metal rolls and pinch rolls. In the first stage of stretching, the film was stretched to a magnification of 4.0 times at a stretching temperature of 130°C, and in the second stage of stretching, it was stretched to a magnification of 1.25 times at a stretching temperature of 145°C. The total magnification (product of the magnification of the first stage and the magnification of the second stage) was 5.0 times. Polarization treatment was performed on the feed-side metal roll during the first stage of stretching. The feed-side metal roll was heated to 130°C, and a DC voltage of 13.5 kV was applied to multiple needle electrodes positioned 10 mm away from the surface of the metal roll. By discharging from the needle electrodes towards the metal roll, the fluororesin film passing between the metal roll and the needle electrodes was polarized. The polarization time, determined from the time it took for the film to pass through the space between the needle electrode and the metal roll, was 5 seconds. In this way, a piezoelectric film 1 with a thickness of 40 μm and a length of over 5000 mm was obtained.

[0084] 1-2. Piezoelectric film 2 Piezoelectric film 2 was obtained in the same manner as in the preparation of piezoelectric film 1, except that the applied voltage during polarization treatment was 15.0 kV.

[0085] 1-3. Piezoelectric film 3 was obtained in the same manner as in the preparation of piezoelectric film 1, except that the applied voltage during polarization treatment was 17.0 kV.

[0086] 1-4. Piezoelectric film 4 was obtained in the same manner as the preparation of piezoelectric film 3, except that in the second stretching stage, the film was stretched at a stretching temperature of 140°C and the stretching ratio was 1.18 times.

[0087] 1-5. Piezoelectric Film 5 A fluororesin film that had been stretched and prepared in the same manner as the piezoelectric film 1, except for the polarization treatment, was subjected to the following polarization treatment. A metal roll for polarization was placed behind the second stage take-up metal roll of the uniaxial stretching apparatus used in the preparation of piezoelectric film 1, and multiple needle electrodes were placed 10 mm away from the surface of the polarization metal roll. The surface temperature of the polarization metal roll was heated to 90°C, a DC voltage of 13.5 kV was applied to the needle electrodes, and the stretched fluororesin film was passed between the metal roll and the needle electrodes while a discharge was performed from the needle electrodes toward the polarization metal roll, thereby performing the polarization treatment. The polarization time, determined from the time it took for the film to pass through the space between the needle electrodes and the metal roll, was 5 seconds. In this way, a piezoelectric film 5 with a thickness of 39 μm and a length of more than 5000 mm was obtained.

[0088] 1-6. Piezoelectric film 6 was obtained in the same manner as the production of piezoelectric film 1, except that the temperature of the first stage feed metal roll was heated to 90°C, the first stage stretching treatment was performed at a stretching temperature of 90°C and a stretching ratio of 4.0 times, the second stage stretching treatment was not performed, and the polarization temperature was set to 90°C.

[0089] 1-7. Piezoelectric film 7 was obtained in the same manner as the production of piezoelectric film 1, except that the second stretching process was not performed.

[0090] 1-8. Piezoelectric Film 8 A PVDF with a melt viscosity of 2500 Pa·s and a melting point of (mp) 173°C was melted using a single-screw extruder with a diameter of φ35 mm to a melting temperature of 270°C (mp + 97°C). The molten resin was then extruded into a film using a T-die and brought into contact with a cooling roll at 110°C to obtain a fluororesin film with a thickness of 40 μm, a width of 240 mm, and a length of 5000 mm. The obtained fluororesin film was cut into a square with sides of 120 mm and placed on a grounded 120 mm × 120 mm metal plate electrode. Multiple needle electrodes were placed 10 mm away from the surface of the metal plate electrode. A DC voltage of 10.0 kV was applied to the needle electrodes at 25°C for 5 minutes to perform polarization treatment and obtain a piezoelectric film 8 with a width of 120 mm, a length of 120 mm, and a thickness of 40 μm.

[0091] 2. Evaluation of Piezoelectric Films Piezoelectric films 1 to 8 were left to stand at room temperature for 14 days after manufacturing and then subjected to characteristic evaluation. For piezoelectric films 1 to 7, unless otherwise specified, the following measurements were performed on a range including the intersection of the diagonals of a rectangle cut out of the piezoelectric film, with dimensions of 100 mm in the TD direction and 300 mm in the MD direction, including the center of the manufactured piezoelectric film in the TD direction. The results obtained were used as representative values ​​for the piezoelectric film. For piezoelectric film 8, the following measurements were performed on a range including the intersection of the diagonals of a piezoelectric film with a width of 120 mm and a length of 120 mm, and the results obtained were used as representative values ​​for the piezoelectric film.

[0092] 2-1. Retardation: Using a KOBRA-HB measuring instrument manufactured by Oji Instruments, the retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular film was measured by the parallel nicol rotation method. The value at the measurement wavelength of 589.1 nm was defined as the retardation of the film. The lagging axis was determined from the in-plane birefringence of the film.

[0093] 2-2. Piezoelectric constant d ３１ Piezoelectric constant d by static method ３１ The following measurements were taken. Specifically, Al-deposited electrodes were formed on both sides of each piezoelectric film, and rectangular samples with a length of 60 mm parallel to the slow phase axis detected by in-plane birefringence of the film during retardation measurement and a width of 10 mm perpendicular to the slow phase axis were cut out as test specimens from the range including the intersection of the diagonals of the rectangular film with electrodes. The slow phase axis was identified during retardation measurement. Next, the test specimens were placed on a tensile testing machine with a chuck distance of 50 mm, and the amount of induced charge Q (pC) was measured using an electrometer when the test specimens were stretched in the longitudinal direction (slow phase axis direction) with a force F (N) at a tensile speed of 10 mm / min at room temperature. The relationship between different forces F (N) and the amount of induced charge Q for each was measured at 128 points, and the force F (N) was plotted on the horizontal axis and the amount of induced charge Q (pC) on the vertical axis. The slope (pC / N) of the generated charge Q (pC) with respect to the force F (N) was determined by the least squares method. Piezoelectric constant d ３１ This was obtained using Equation 1.

[0094] Here, A is the area defined by the product of the distance between the chucks and the width of the test piece, which is 50 × 10 = 500 (mm²). ２ ), t (mm) is the thickness of the test specimen, and w is the width of the test specimen, which is 10 mm.

[0095] 2-3. Internal Haze The internal haze of the piezoelectric film was determined by filling in scratches and other irregularities with silicone oil to eliminate the influence of external haze, and then performing haze measurement in accordance with ISO 14782:1999 in the area including the center of the piezoelectric film. Haze measurement was performed using a turbidimeter (NDH2000, manufactured by Nippon Denshoku Industries Co., Ltd.) to measure the haze at three different points within the sample, and the arithmetic mean of the three points was calculated as the haze value of the sample. Specifically, silicone oil (Nikon, MXA22179) was placed between two glass plates (thickness: 0.15 mm, flatness: 0.01 mm or less), and the haze (H2) caused by the glass plates and silicone oil was measured. Next, a rectangular film measuring 50 mm x 50 mm was cut out so as to include the intersection of the diagonals of the above rectangular film. Both sides of this rectangular film were uniformly wet with the above silicone oil, and the haze (H3) was measured by sandwiching it between the two glass plates. H1 was then subtracted from H3 (using the following formula) to obtain the internal haze (H1) of the piezoelectric film. H1 = H3 - H2

[0096] 2-4. Storage Modulus A sample measuring 25 mm in length and 3 mm in width was cut from the range including the intersection of the diagonals of the rectangular film, parallel to the slow axis in the in-plane birefringence of the film, to be used as a test specimen. The storage modulus of the film was measured using a TA Instruments RSA-III with a chuck distance of 15 mm, an initial load of 1.5 N, a frequency of 10 Hz, a strain of 0.1%, and a temperature of 23 °C.

[0097] 2-5. Dielectric constant ε ３３ Electrodes with a diameter of 19.5 mm were formed on both sides of the film by aluminum vapor deposition, and the relative permittivity was measured at room temperature, with a voltage of 1 V and a frequency of 1 kHz, using an LCR meter manufactured by HIOKI Corporation. The obtained relative permittivity was multiplied by the permittivity of vacuum (8.854 × 10⁻⁴). －１２ Multiply by F / m to obtain the dielectric constant ε ３３ The result was calculated.

[0098] 2-6. Electromechanical coupling coefficient k ３１Piezoelectric constant d ３１ , dielectric constant ε ３３ , and elastic compliance １１ Using the following formula, the electromechanical coupling coefficient k of the film is calculated. ３１ The elastic compliance s was calculated. １１ The reciprocal of the storage modulus was used as the value.

[0099]

[0100] 3. The manufacturing conditions for each piezoelectric film are shown in Table 1, and the evaluation results are shown in Table 2.

[0101]

[0102]

[0103] This application claims priority to Japanese Patent Application No. 2024-180732 and No. 2024-180734, both filed on 16 October 2024. The matters described in the original specifications, claims and drawings of those applications are incorporated herein by reference.

[0104] The piezoelectric film of the present invention is useful as a piezoelectric film having high piezoelectricity and high transparency.

Claims

1. A piezoelectric film mainly composed of a fluororesin, wherein the piezoelectric constant d ３１ A piezoelectric film having a density of 29 pC / N or more and 50 pC / N or less, an internal haze of 0.01% or more and 1.5% or less, and a retardation of 100 nm or more and 3000 nm or less.

2. The piezoelectric film according to claim 1, wherein the storage modulus measured parallel to the slow axis in in-plane birefringence is 2.7 GPa or more and 10.0 GPa or less.

3. The piezoelectric film according to claim 1, wherein the thickness is 30 μm or more.

4. The piezoelectric film according to claim 1, wherein the piezoelectric film is mainly composed of vinylidene fluoride resin.

5. A method for manufacturing a piezoelectric film according to any one of claims 1 to 4, comprising: a stretching step of stretching a fluororesin film at least twice; and a polarization step of polarizing the fluororesin film.

6. The method for manufacturing a piezoelectric film according to claim 5, wherein the stretching step is performed such that the total stretching ratio is 4.0 times or more and 10.0 times or less, and the stretching temperature (T1) in the first stretching stage and the stretching temperature (T2) in the second stretching stage satisfy the following relationship: 110°C ≤ T1°C ≤ (T2 - 5)°C and T2 < 186°C.

7. The method for manufacturing a piezoelectric film according to claim 6, wherein in the first stretching stage, the fluororesin film is stretched to a length of 3.0 to 5.0 times at a temperature (T1) of 110°C or higher and less than 150°C.

8. The method for manufacturing a piezoelectric film according to claim 6, wherein in the second stretching stage, the fluororesin film is stretched by 1.1 times or more and 2.0 times or less in the same direction as the stretching direction in the first stretching step at a temperature (T2) of T1 + 5°C or more and less than 186°C.

9. The method for manufacturing a piezoelectric film according to claim 5, wherein the polarization step is performed simultaneously with the first stage of stretching, or after the first stage of stretching, by applying a voltage of 5 kV to 30 kV at a temperature of 50°C to less than 150°C.

10. The method for manufacturing a piezoelectric film according to claim 5, wherein the polarization step is performed after the second stretching step.

11. The method for manufacturing a piezoelectric film according to claim 5, wherein the polarization step is performed before the second stretching step.

12. The method for manufacturing a piezoelectric film according to claim 5, wherein the polarization step is performed simultaneously with the second stretching step.

13. The method for manufacturing a piezoelectric film according to claim 5, wherein the polarization step is performed simultaneously with the first stage stretching.

14. The method for manufacturing a fluororesin piezoelectric film according to claim 5, wherein the first stretching step, the second stretching step, and the polarization step are all performed while conveying the fluororesin film.

15. A method for manufacturing a fluororesin piezoelectric film according to claim 5, wherein in the stretching step, the fluororesin film is stretched using a roll-to-roll method, and in the polarization step, the fluororesin film is polarized using a roll-to-roll method.

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