Fluorine-based resin piezoelectric film and manufacturing method thereof

Abstract

Provided is a fluorine-based resin piezoelectric film which is obtained from a fluorine-based resin having a low melt viscosity and has high breaking strength. The fluorine-based resin piezoelectric film according to the present invention has a retardation of 50-3000 nm, an internal haze of less than 1.2%, a breaking strength of 310-550 MPa, and a piezoelectric constant d33 of 5.0-40.0 pC / N.

Description

Fluorine-based resin piezoelectric film and its manufacturing method

[0001] The present invention relates to a fluorine-based resin piezoelectric film and a method for producing the same.

[0002] Display and input devices with a touch panel installed on the front of a display device such as a liquid crystal display are widely used, and are used in mobile devices such as mobile phones and operation panels for home appliances. Display and input devices using a touch panel allow users to operate the device by pressing the display on the screen. Therefore, the piezoelectric film used in touch panels must have high piezoelectricity to increase detection sensitivity and high transparency to allow accurate viewing of the display image.

[0003] As described in Patent Documents 1 and 2, piezoelectricity can be imparted to a fluororesin film by polarization treatment. Known methods for producing a fluororesin film include a solution casting method using a solution in which a fluororesin is dissolved (Patent Document 1, etc.) and an extrusion molding method in which a fluororesin is thermally melted (Patent Document 2).

[0004] International Publication No. 2015 / 064324 Japanese Patent Application Laid-Open No. 05-102548 International Publication No. 2022 / 091828

[0005] Piezoelectric films are produced from fluororesin films produced by solution casting by poling without stretching, whereas piezoelectric films are produced from fluororesin films formed by hot melt extrusion by stretching the film and then poling it.

[0006] The solution casting method described in this paper uses a large amount of polar organic solvent to dissolve the fluororesin, which requires solvent recovery, which not only increases production costs but also requires consideration of the impact of the organic solvent on the working environment and the natural environment. Furthermore, if the polar organic solvent used to dissolve the fluororesin remains in the film, it can hinder polarization of the piezoelectric film and impair the smoothness of the film surface as the solvent evaporates.

[0007] To increase the detection sensitivity of the touch panel, the piezoelectric constant d ３３ It is necessary to increase the piezoelectric constant d ３３ To increase the film's internal haze, it is necessary to increase the stretch ratio to increase the proportion of the β phase, which is likely to exhibit piezoelectricity. However, increasing the film's stretch ratio tends to increase the internal haze due to light scattering caused by the difference in refractive index between the crystalline parts grown as crystallization progresses and the amorphous parts, and light scattering caused by defects caused by stretching.

[0008] As described above, from the viewpoint of enhancing the piezoelectricity of the piezoelectric film, a higher stretching ratio is preferable. However, if the stretching ratio is too high, the film may break. Therefore, it is preferable that the piezoelectric film has high breaking strength so that it will not break even when stretched at a high stretching ratio. Furthermore, piezoelectric films are used by laminating them with functional films such as electrode films using a roll-to-roll method or the like. In the roll-to-roll method, these films are bonded to each other under tension, so high breaking strength is preferable. Furthermore, polymer-based piezoelectric films such as fluororesin piezoelectric films are flexible, and therefore, when attached to various materials, the piezoelectric film may be subjected to large forces when bent. Considering such potential uses, high breaking strength is preferable for piezoelectric films. However, the piezoelectric films in Patent Documents 1 and 2 are not stretched, and therefore high breaking strength cannot be expected. Patent Document 3 discloses a fluororesin piezoelectric film that has been stretched and then polarized. However, the piezoelectric film disclosed in Patent Document 3 has high haze and does not disclose breaking strength.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a fluorine-based resin piezoelectric film having high breaking strength and high transparency, and a method for producing the same.

[0010] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric film [1] to [5]: [1] The retardation is 50 nm or more and 3000 nm or less, the internal haze is less than 1.2%, the breaking strength is 310 MPa or more and 550 MPa or less, and the piezoelectric constant d３３ [2] A fluorine-based resin piezoelectric film having a shear strength of 5.0 pC / N or more and 40.0 pC / N or less. －１ [3] The fluororesin piezoelectric film according to [1] or [2], which contains a vinylidene fluoride resin as a main component. [4] The number of foreign particles having a size of 100 μm or more, which is the arithmetic mean of the maximum and minimum widths when viewed from above, of the film is 7 / 0.25 m. ２ [5] The fluororesin piezoelectric film according to any one of [1] to [4], wherein the surface roughness in surface height Rz of the surface on the side where the Rz is smaller as measured in accordance with JIS B 0601:2001 is 0.50 μm or less.

[0011] One embodiment of the present invention for solving the above problems relates to a method for producing a piezoelectric film according to the following [6] to [8]. [6] A method for producing a fluororesin piezoelectric film according to any one of [1] to [5], comprising the steps of heating and melting a resin composition containing a fluororesin, extruding the molten resin composition to form a film, stretching the formed film at a stretching temperature of 125°C to 165°C at a stretching ratio of 4.0 to 8.0 times, and polarizing the formed film at an applied voltage of 7.0 kV to 50.0 kV. [7] A method for producing a fluororesin piezoelectric film according to [6], comprising the step of filtering the resin composition melted in the melting step through a filter having a filtration accuracy of 10 μm to 40 μm. [8] A method for producing a fluororesin piezoelectric film according to [6] or [7], wherein, in the film-forming step, the extruded resin composition is cooled by contacting it with a cooling roll having a surface temperature of 125°C or less.

[0012] According to the present invention, a fluororesin piezoelectric film having high breaking strength and high transparency, and a method for producing the same are provided.

[0013] [Fluorocarbon Resin Piezoelectric Film] One embodiment of the present invention relates to a fluorocarbon resin piezoelectric film.

[0014] A fluororesin piezoelectric film can be produced by stretching and poling a fluororesin film, which is the raw material. By stretching the fluororesin film at a high ratio, the piezoelectricity of the produced fluororesin piezoelectric film can be increased, and the breaking strength can also be increased. Furthermore, by increasing the temperature of the fluororesin film during stretching, the molecular motion of the molecular chains during stretching can be facilitated, making it easier for the molecular chains to adopt a highly oriented structure parallel to the stretching direction of the film. By more fully forming this oriented structure, it is believed that the bonding energy between the atoms constituting the molecular chains can be fully reflected in the breaking strength of the film, thereby increasing the breaking strength of the film.

[0015] The fluororesin piezoelectric film contains a fluororesin.

[0016] The fluororesin can be a homopolymer or copolymer obtained by polymerizing tetrafluoroethylene (TFE), vinylidene fluoride (VDF), or the like. Examples of fluororesins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, hexafluoropropylene (HFP), or the like. Examples of fluororesins obtained by polymerizing VDF include homopolymers of VDF, and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), TFE, tetrafluoropropene, HFP, perfluoroalkyl vinyl ether, or the like.

[0017] Among these, from the viewpoint of facilitating polarization of the fluororesin film, VDF resin obtained by polymerizing VDF is preferred, and a VDF homopolymer, a copolymer of VDF and HFP, a copolymer of VDF and TrFE, a copolymer of VDF and TFE, and a copolymer of VDF, TrFE, and TFE are more preferred, with a VDF homopolymer being even more preferred. These fluororesins may be used alone or in combination.

[0018] When the VDF resin is a copolymer, it is preferably a resin containing VDF as the main component, specifically a resin in which the proportion of VDF-derived structural units is 50% by mass or more. The proportion of the VDF-derived structural units relative to the total mass of the VDF resin 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.

[0019] The fluororesin piezoelectric film preferably contains a VDF resin, preferably a VDF homopolymer, as a main component. "Containing such a resin as a main component" means that the content of VDF-derived structural units relative to the total mass of the fluororesin piezoelectric film is 50% by mass or more. The content of such a resin relative to the total mass of the fluororesin 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.

[0020] The content of the resin having VDF as a constituent unit contained in the fluororesin film and the fluororesin piezoelectric film is １９ It can be measured by quantitative analysis using an internal standard using F-NMR.

[0021] The fluorine-based resin piezoelectric film was measured at a temperature of 260°C and a shear rate of 50 s －１ (Hereinafter, unless otherwise specified, the measurement temperature is 260°C, and the shear rate during measurement is 50 s －１ The melt viscosity (measured by the method described above and referred to as "melt viscosity") is preferably 600 Pa·s or more and 4000 Pa·s or less. Since the piezoelectric film is produced by melt-forming, the melt viscosity of the piezoelectric film reflects the melt viscosity of the fluororesin film before piezoelectric treatment (stretching and polarization treatment), and serves as an index of the melt viscosity of a resin composition containing a fluororesin as a raw material resin (hereinafter, a resin composition containing a fluororesin is referred to as a "resin composition"). Furthermore, the melt viscosity is an index of the degree of polymerization of a resin composition; the lower the melt viscosity, the lower the degree of polymerization.

[0022] Resin compositions with a melt viscosity of 4000 Pa·s or less have short molecular chains, which reduces entanglement and makes the film less susceptible to breakage even at high stretch ratios. Furthermore, their low melting temperatures reduce resin degradation, decomposition products, and foreign matter associated with heating and melting the resin. On the other hand, resin compositions with a piezoelectric film melt viscosity of 600 Pa·s or more have long molecular chains, which increases shear stress during stretching and makes the film less susceptible to breakage. Furthermore, the resin composition crystallizes during film formation. As crystals grow in the resulting film, the crystallites become larger, and the difference in refractive index between the crystalline and amorphous portions tends to cause light scattering, resulting in poor film transparency. Resin compositions with particularly low melt viscosity have short molecular chains, which facilitate free movement and selective alignment of the molecular chains, making crystal growth more likely, leading to increased internal haze in the film. The resin composition's molecular chains are long enough to suppress crystal growth, thereby reducing internal haze. The melt viscosity of the piezoelectric film is more preferably 600 Pa·s to 3500 Pa·s, even more preferably 600 Pa·s to 2400 Pa·s, particularly preferably 600 Pa·s to 2000 Pa·s, and most preferably 600 Pa·s to 1500 Pa·s.

[0023] Furthermore, the lower the melt viscosity of a resin composition, the lower the melting temperature can be kept, which makes it less likely that spots will appear on the film surface due to deterioration of the resin composition, less likely that foreign matter will be generated due to thermal decomposition, and easier to filter.If foreign matter is contained in the film, stress will concentrate around the foreign matter when the film is stretched, which may promote breakage of the film.From the above viewpoints as well, the lower the melt viscosity of the fluororesin piezoelectric film, the more preferable it is.

[0024] Furthermore, by forming a molten resin composition into a film and then rapidly cooling it before crystallization progresses, for example, crystallization of the fluororesin can be suppressed, reducing the scattering of light that occurs between crystalline and amorphous parts, and increasing the transparency of the resulting fluororesin piezoelectric film.

[0025] The melt viscosity is 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.) is used, and the melt viscosity is measured at a measurement temperature of 260°C and a shear rate of 50 s －１ The viscosity measured is taken as the viscosity.

[0026] The breaking strength of the fluorine-based resin piezoelectric film is 310 MPa or more and 550 MPa or less, preferably 330 MPa or more and 500 MPa or less, more preferably 350 MPa or more and 500 MPa or less, even more preferably 380 MPa or more and 500 MPa or less, particularly preferably 400 MPa or more and 500 MPa or less, particularly preferably 420 MPa or more and 500 MPa or less, and particularly preferably 440 MPa or more and 500 MPa or less.

[0027] The breaking strength of the fluororesin piezoelectric film was measured in accordance with JIS K 7127:1999 by cutting a strip of film A measuring 50 mm by 10 mm in width and a strip of film B measuring 50 mm by 10 mm in width from the film so that the longitudinal direction was perpendicular to that of strip of film A. Next, strip of film A and strip of film B were used as test pieces, and the breaking strength of each test piece was measured at a tension rate of 50 mm / min using a tensile tester (Tensilon RTC-1210A manufactured by Orientec Co., Ltd.) in an environment with a measurement temperature of 23°C, and the larger breaking strength value was taken as the breaking strength of the fluororesin piezoelectric film.

[0028] The fluororesin piezoelectric film has a retardation of 50 nm to 3000 nm, preferably 100 nm to 3000 nm, more preferably 500 nm to 2500 nm, and even more preferably 700 nm to 2000 nm. The larger the retardation, the higher the degree of molecular orientation of the fluororesin film and the more sufficiently the proportion of the β phase, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity by the polarization step.

[0029] The retardation is measured by the parallel Nicol rotation method using a light source with a wavelength of 587.8 nm, using a film cut into a 20 mm x 20 mm area from the area including the intersection of the diagonals of the fluororesin piezoelectric film, and the measured value is used as the representative value of the retardation of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, an arbitrary rectangle with one side of 300 mm is assumed, and the retardation of the film cut into a 20 mm x 20 mm area 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, an arbitrary square with one side of 300 mm is set on the piezoelectric film, and the measurement range is set according to the method described above.

[0030] The fluororesin piezoelectric film has an internal haze of less than 1.2%, preferably 0.1% to 1.1%, more preferably 0.1% to 1.0%, even more preferably 0.1% to 0.8%, particularly preferably 0.1% to 0.6%, and very preferably 0.1% to 0.4%. The lower the internal haze, the more improved the transparency of the fluororesin piezoelectric film.

[0031] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 5.0%, even more preferably 0.0% to 3.0%, and particularly preferably 0.0% to 2.0%. The lower the haze, the more improved the transparency of the fluororesin piezoelectric film.

[0032] The internal haze of a fluororesin piezoelectric film is determined by forming coating layers on both sides of the film, removing external haze due to scratches, etc., and measuring the haze of the film in accordance with ISO 14782:2021. Specifically, a 50 mm x 50 mm rectangular film is cut out of the fluororesin piezoelectric film so as to include the intersection of the diagonals. If the length of one side of the piezoelectric film is 300 mm or more, an arbitrary rectangle with a side of 300 mm is assumed, and the rectangular film is cut out from an area including the intersection of the diagonals of the rectangular film. If both sides of the piezoelectric film are 300 mm or more, an arbitrary square with a side of 300 mm is set on the piezoelectric film, and the measurement range is set according to the above method, and the rectangular film is cut out. Next, a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) is applied to one surface (side A) of the rectangular film using a bar coater and dried at 80°C for 30 minutes. Thereafter, an ultraviolet (UV) irradiation device (GS NIPPON DENCHI, CSOT040) was used to irradiate the sample with a target cumulative light dose of 400 mJ / cm. ２ The film is irradiated with UV light so that a coating layer with a thickness of 2 μm is formed. A coating layer similar to that on side A is also formed on the other surface (side B) of the rectangular film with side A coated, and an internal haze measurement film is prepared in which external haze due to scratches on the film surface has been removed. The haze is measured in accordance with ISO 14782:2021 using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) for an area including the intersection of the diagonals of the obtained measurement film, and the value obtained is used as a representative value of the internal haze of the fluororesin piezoelectric film.

[0033] The haze is measured in the same manner as in the internal haze measurement method, except that no surface coating layer is formed on the rectangular film, and the measured value is used as a representative value.

[0034] The fluorine-based resin piezoelectric film has a piezoelectric constant d ３３is 5.0 pC / N or more and 40.0 pC / N or less, preferably 8.0 pC / N or more and 40.0 pC / N or less, more preferably 10.0 pC / N or more and 35.0 pC / N or less, even more preferably 15.0 pC / N or more and 35.0 pC / N or less, and particularly preferably 20.0 pC / N or more and 30.0 pC / N or less.

[0035] The piezoelectric constant of the fluororesin piezoelectric film was measured by the direct quasi-static method (d ３３ Piezoelectric constant d by Mehta method, Berlincourt method ３３ The piezoelectric constant d is measured in accordance with ISO 19622:2018, a test method for determining the piezoelectric constant. Specifically, a piezoelectric film test piece is held at a measurement temperature of 25°C with a holding force of 1.0 N using a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST), and an alternating force of 0.15 N and a frequency of 110 Hz is applied to measure the electric charge generated. ３３ is the piezoelectric constant d calculated from the electric charge measured by the above method in the range including the intersection of the diagonal lines of the polarization surface of the fluorine-based resin piezoelectric film. ３３ The absolute value of the piezoelectric constant d ３３ The representative value of the measurement range is set as follows. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set, and the area including the intersection of the diagonal lines of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the above method.

[0036] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles having a size of 100 μm or more is 0 / 0.25 m. ２ More than 7 pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 5 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following are particularly preferred:

[0037] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles larger than 200 μm is 0 / 0.25 m ２ 3 or more pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ 2 or more pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following is even more preferred:

[0038] When the fluorine-based resin piezoelectric film is viewed in plan, the number of foreign particles less than 100 μm in size is 0 / 0.25 m ２ More than 50 pieces / 0.25m ２ Preferably, 0 pieces / 0.25m or less ２ More than 25 pieces / 0.25m ２ More preferably, 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following is even more preferred:

[0039] The fewer these foreign matters there are, the more transparent the fluororesin piezoelectric film will be, and the more uniform the stretching and polarization will be when the fluororesin piezoelectric film is stretched or polarized.

[0040] The number of these foreign particles is measured by cutting four rectangular films (observation pieces) adjacent to each other from the fluorine-based resin piezoelectric film, and then calculating the sum of the number of foreign particles measured from each observation piece. ２ Four observation pieces each measuring 100 mm x 100 mm were cut out. The sum of the number of foreign particles measured from each observation piece was calculated, and the sum was multiplied by 25 / 4, and the result was rounded to the nearest tenth to obtain a value of 0.25 m. ２ The number of foreign particles per unit area is measured. The foreign particles are observed using transmitted light, marked, and the marked areas are observed under a microscope to determine the size of the foreign particles. The size of the foreign particles is the arithmetic mean value of the maximum and minimum widths of the foreign particles.

[0041] The surface height roughness Rz of the fluororesin piezoelectric film is preferably 0.50 μm or less, more preferably 0.05 μm or more and 0.50 μm or less, even more preferably 0.05 μm or more and 0.40 μm or less, and particularly preferably 0.05 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin piezoelectric film, the less likely it is that haze will occur on the surface of the film and the less likely the film will wrinkle.

[0042] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter conforming to JIS B 0601:2001 (Keyence Corporation, Shape Analysis Laser Microscope VK-X260) is used. Measurement is then performed over an area including the intersection of the diagonals of the fluororesin piezoelectric film, and the resulting value is used as the representative value of the surface height roughness Rz of the fluororesin piezoelectric film. When the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set, and the area including the intersection of the diagonals of the rectangular film is measured. When both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the above method. The surface of the fluororesin piezoelectric film that comes into contact with the chill roll tends to have a smaller surface height roughness Rz than the surface that does not come into contact with the chill roll. This is because the surface irregularities are reduced by pressing the fluororesin piezoelectric film against the cooling roll. This tendency is maintained even after stretching and polarization. Here, the measurement result for the surface with the smallest surface height roughness Rz (the surface in contact with the cooling roll) is taken as the surface height roughness Rz.

[0043] 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. A thicker film is more advantageous in terms of electrical properties such as insulation and piezoelectric properties. A thinner film is more advantageous in terms of optical properties such as transparency and cost.

[0044] The thickness of a fluororesin piezoelectric film is generally measured using a micrometer (JIS C 2151:2019), but measurements can also be made using known methods such as a laser displacement meter, a capacitance displacement meter, or an infrared method over an area including the intersection of the diagonals of the fluororesin piezoelectric film, and the resulting value is used as the representative thickness of the fluororesin piezoelectric film. When the length of one side of the piezoelectric film is 300 mm or more, a rectangle with a side of 300 mm is arbitrarily set, and the area including the intersection of the diagonals of the rectangular film is measured. When both sides of the piezoelectric film are 300 mm or more, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement area is set according to the above method.

[0045] The fluororesin piezoelectric film may contain resins other than fluororesin or other additives as long as the above physical properties can be satisfied.

[0046] [Method of manufacturing fluororesin piezoelectric film]

[0047] The method for producing the above-mentioned fluororesin piezoelectric film is not particularly limited, but preferably includes the steps of: a step of heating and melting a resin composition containing a fluororesin (hereinafter, a resin composition containing a fluororesin will be referred to as a "resin composition") (melting step); a step of forming the heat-melted resin composition into a film by extrusion molding to produce a fluororesin film (film formation step); a step of stretching the formed fluororesin film (stretching step); and a step of polarizing the formed fluororesin film (polarization step).

[0048] At this time, a step of filtering the molten resin composition (filtration step) may be carried out as needed.

[0049] (Melting Step) In the melting step, the resin composition containing the fluororesin is heated and melted. In this step, the resin composition can be melt-kneaded using an extruder, for example.

[0050] The resin composition melted in the melting step may be any resin composition containing the above-mentioned fluororesin. If the resin composition contains a solvent component, the solvent component remaining without volatilization may interfere with polarization in a subsequent step. Therefore, the content of the solvent component in the resin composition is preferably low, preferably 1% by mass or less, more preferably 0.1% by mass or less, based on the total mass of the resin composition. In particular, the content of the polar solvent is preferably 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less, based on the total mass of the resin composition.

[0051] The resin composition melted in the melting step was measured at a temperature of 260°C and a shear rate of 50 s －１ The melt viscosity measured by is 600 Pa·s or more and 4000 Pa·s or less, preferably 600 Pa·s or more and 3500 Pa·s or less, more preferably 600 Pa·s or more and 2400 Pa·s or less, even more preferably 600 Pa·s or more and 2000 Pa·s or less, and particularly preferably 600 Pa·s or more and 1500 Pa·s or less. In the production of a fluororesin piezoelectric film, the higher the stretching ratio of the fluororesin film, the higher the piezoelectricity tends to be. A resin composition having a melt viscosity of 4000 Pa·s or less has short molecular chains, which reduces entanglement of the molecular chains. This makes it difficult for the film to break even at high stretching ratios, and allows it to be stretched at high stretching ratios. Therefore, from the viewpoint of stretching a fluororesin film at a high ratio, the lower the melt viscosity of the resin composition, the more preferable it is. Furthermore, the lower the melt viscosity of the resin composition, the easier it is to filter it and the lower the melting temperature can be kept, which reduces the likelihood of spot-like irregularities on the film surface due to resin deterioration and the generation of low-molecular-weight foreign matter due to thermal decomposition. This prevents a decrease in visibility due to foreign matter, and also reduces the risk of film breakage due to the concentration of stretching stress on the film around the foreign matter if foreign matter is present in the film during stretching. Using a resin composition with a melt viscosity of 600 Pa·s or higher reduces internal haze due to the progression of crystallization.

[0052] The melting temperature of the resin composition is preferably 75°C or more higher than the melting point of the resin composition but not higher than 105°C, more preferably 75°C or more higher but not higher than 100°C, even more preferably 80°C or more higher but not higher than 100°C, and particularly preferably 85°C or more higher but not higher than 95°C. By setting the melting temperature at 75°C or more higher than the melting point of the resin composition, the viscosity of the resin composition can be reduced to a level that allows filtration in the subsequent process. By setting the melting temperature at 105°C or less higher than the melting point of the resin composition, decomposition and condensation of the resin composition due to heating can be suppressed, and the resulting generation of decomposition products can be suppressed. By suppressing the generation of the decomposition products, the amount of foreign matter in the fluororesin piezoelectric film can be reduced, and the transparency and smoothness of the fluororesin piezoelectric film can be improved. Furthermore, by suppressing the generation of the decomposition products, the clogging of the filter caused by these products can be suppressed, and the filtration efficiency of the resin composition can be improved. The melting point of the resin composition is a value measured in accordance with JIS K 7121: 1987. Specifically, 5 mg of a measurement sample is sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation), and the temperature is raised from room temperature to 230°C at a heating rate of 10°C / min in a nitrogen atmosphere to measure a DSC curve, and the maximum melting peak temperature is taken as the melting point of the resin composition.

[0053] (Filtration Step) In the filtration step, the resin composition melted and reduced in viscosity in the melting step is filtered. By setting the melting temperature of the resin composition containing a fluororesin within the above range and filtering the resin composition whose viscosity has been reduced by melting, it becomes possible to filter the resin composition without using a polar solvent. Furthermore, by not using a solvent, polarization due to the polar solvent remaining in the fluororesin film is less likely to be inhibited during polarization treatment. Furthermore, since a polar solvent is not basically used in the film formation step, it is possible to reduce the burden on the working environment and the natural environment and to reduce the production cost for recovering the polar solvent.

[0054] The filtration method is not particularly limited, and the molten resin composition may be passed through a filter, and any known filter type such as a pleated type or leaf disc type filter may be used.

[0055] In the filtration step, the resin composition is preferably filtered through a filter with a filtration accuracy of 10 μm or more and 40 μm or less. Using a filter with a filtration accuracy of 10 μm or more makes it easier to filter the resin composition that has been heated to the above temperature and melted, and also prevents the filtration pressure from becoming too high, allowing the filtration time to be shortened. Using a filter with a filtration accuracy of 40 μm or less allows foreign matter in the resin composition to be sufficiently removed, resulting in a fluororesin film with little foreign matter. The filtration accuracy of the filter is preferably 10 μm or more and 30 μm or less, and more preferably 15 μm or more and 30 μm or less.

[0056] The resin composition is filtered using a multilayer filter consisting of multiple layers with different shapes, mesh sizes, etc. The filtration accuracy of the filter used to filter the resin composition refers to the filtration efficiency of the filter, i.e., the filter's ability to filter out particles of a given size with a given filtration efficiency. For example, in this specification, a filtration accuracy of 10 μm means that the filter can filter out particles of 10 μm or larger with a filtration efficiency of 95% or higher.

[0057] In this step, the resin composition may be passed through multiple filters. For example, a filter with low filtration accuracy (large filtration accuracy value) may be used in the first stage to remove coarse foreign matter, and then a filter with high filtration accuracy (small filtration accuracy value) may be used in the second stage to remove finer foreign matter. In this case, the filtration accuracy is the value of the filter with the highest filtration accuracy.

[0058] The filter may be disposed between the extruder used in the film-forming step and the die. Alternatively, the filter may be disposed in an extruder or melt-kneading apparatus different from the extruder used in the film-forming step, and the resin composition filtered through the filter may be fed into the extruder used in the melting step to form a film.

[0059] (Film Forming Step) In the film forming step, the resin composition melted in the melting step and filtered, if necessary, in the filtration step is formed into a film.

[0060] The film-forming method is not particularly limited, and a known method can be used, such as extruding a molten and filtered resin composition through a T-die and cooling it by contacting it with a cooling roll.

[0061] The surface temperature of the chill roll is preferably 125°C or lower. By rapidly cooling the resin composition film extruded by the chill roll with a low surface temperature, crystal growth can be suppressed and the internal haze of the resulting fluororesin film can be reduced. The surface temperature of the chill roll is preferably 5°C or higher and 115°C or lower, more preferably 10°C or higher and 100°C or lower, even more preferably 20°C or higher and 90°C or lower, particularly preferably 20°C or higher and 80°C or lower, very preferably 30°C or higher and 70°C or lower, and most preferably 30°C or higher and 60°C or lower.

[0062] The fluorine-based resin film thus obtained may be stored after being wound up, or may be transported to a subsequent process such as a stretching process or a polarization process.

[0063] (Stretching step) In the stretching step, the formed fluororesin film is stretched. In the stretching step, the fluororesin film formed in the film formation step may be stretched as is, or the fluororesin film that has been wound up and stored may be stretched. Furthermore, the film may be stretched after being heated as necessary.

[0064] The stretching can be performed in the conveying direction (MD direction) and, if necessary, in the direction perpendicular to the conveying direction (TD direction) while the fluororesin film is conveyed by multiple rolls. In this embodiment, the stretching ratio (ratio in the MD direction) is set to 4.0 times or more and 8.0 times or less. By increasing the stretching ratio, the molecular chains of the fluororesin in the film can be stretched in the stretching direction, thereby increasing the orientation of the molecular chains and facilitating the alignment of the crystals. This increases the rigidity of the film and increases the retardation of the fluororesin piezoelectric film. This also reduces the internal haze of the fluororesin piezoelectric film, making the film more transparent.

[0065] When a balance between high breaking strength and high transparency is desired, the stretching ratio is preferably 4.3 to 8.0 times, more preferably 4.3 to 7.7 times, even more preferably 4.5 to 7.7 times, particularly preferably 5.0 to 7.7 times, and very preferably 5.5 to 7.5 times. The higher the stretching ratio, the higher the proportion of β crystals, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through the polarization step. By setting the stretching ratio to 8.0 times or less, the fluororesin film is less likely to break during the stretching step.

[0066] Furthermore, by stretching the formed film at a high magnification, the lamellae that make up the crystals can be elongated and at the same time, the orientation of the molecular chains that were insufficiently oriented can be promoted. Therefore, by stretching the film at a high magnification, the film can be made rigid, and the breaking strength of the fluororesin piezoelectric film can be increased.

[0067] Furthermore, the surface temperature of the roll during stretching is preferably 125°C or higher and 165°C or lower, more preferably 125°C or higher and 160°C or lower, even more preferably 130°C or higher and 160°C or lower, particularly preferably 130°C or higher and 155°C or lower, and most preferably 130°C or higher and 150°C or lower. By increasing the surface temperature of the roll that comes into direct contact with the fluororesin film during stretching, the temperature of the fluororesin film during stretching can be increased. By maintaining the surface temperature of the roll at 125°C or higher during stretching, molecular motion of the molecular chains is facilitated, and the stretching stress generated during stretching can be reduced. This makes it possible to reduce scission of molecular chains due to stretching stress, and the film can be stretched at a high ratio without breaking. The molecular chains constituting the film stretched at a high ratio have a highly oriented structure parallel to the stretching direction of the film, and the high bonding energy between atoms in the molecular chains efficiently works to improve the breaking strength of the film, thereby increasing the breaking strength of the film. Furthermore, by stretching the film while heating, the film is stretched uniformly, preventing localized crystallization and aligning the molecular chains uniformly throughout the film, thereby preventing light scattering due to differences in the degree of crystallinity within the film and reducing internal haze. By keeping the roll surface temperature at 165°C or lower during stretching, the efficiency of molecular chain alignment due to film stretching can be increased, the proportion of the β-phase structure that contributes to the development of piezoelectricity can be increased, and deformation of the film due to melting of the fluororesin can be suppressed.

[0068] (Polarization Step) In the polarization step, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. Fluororesin containing a VDF homopolymer or copolymer undergoes a transition from α phase to β phase during the stretching step, increasing the proportion of the β phase. By applying a DC voltage to a fluororesin film with an increased proportion of the polar β phase, a fluororesin piezoelectric film with high piezoelectricity can be obtained.

[0069] The applied DC voltage is preferably 7.0 kV or more and 50.0 kV or less, more preferably 7.5 kV or more and 30.0 kV or less, even more preferably 8.0 kV or more and 30.0 kV or less, and particularly preferably 8.5 kV or more and 30.0 kV or less.

[0070] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0071] The fluororesin film after the film-forming step or the fluororesin piezoelectric film after the polarization step can be wound into a roll for storage, transportation, and the like.

[0072] [Uses] The above-described fluorine-based resin piezoelectric film can be used in various applications such as touch sensors and touch panels, piezoelectric films for actuators, protective films, and retardation films.

[0073] [Other Embodiments] It goes without saying that the above-described embodiments are exemplary embodiments of the present invention, and the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept.

[0074] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0075] The melt viscosity of the fluororesin 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 Seisaku-sho, Ltd.) was used, and the melt viscosity was measured at a temperature of 260°C and a shear rate of 50 s−1 using a capillary die with an inner diameter of 1 mm and a tube length of 10 mm. －１ The viscosity was measured at 1000 kJ / min.

[0076] The melting point of the resin composition was measured in accordance with JIS K 7121: 1987. Specifically, 5 mg of a measurement sample was sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation), and the temperature was raised from room temperature to 230°C at a heating rate of 10°C / min in a nitrogen atmosphere to measure a DSC curve. The maximum melting peak temperature in the DSC curve was determined as the melting point of the resin composition.

[0077] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0078] 1. Preparation of Fluorine-Based Resin Piezoelectric Films Films 1 to 12, all of which were fluorine-based resin piezoelectric films, were prepared by the following procedure.

[0079] 1-1. Film 1 A VDF homopolymer (PVDF) with a melt viscosity of 800 Pa s and a melting point of 173°C was melted at a melting temperature of 260°C in a single-screw extruder with a diameter of φ50 mm, and then filtered through a sintered metal nonwoven filter element with a filtration accuracy of 20 μm (Fuji Metal Fiber, manufactured by Fuji Filter Industrial Co., Ltd.). The filtered resin was extruded into a film from a T-die and cooled by contacting it with the surface of a cooling roll whose surface was heated to 70°C, yielding an unstretched film with a thickness of 160 μm.

[0080] The unstretched film was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and the surface temperature and rotation speed ratio of each roll was adjusted to stretch the film 5.9 times in the machine direction (MD direction) at 130° C. A voltage of 9.6 kV was applied from the surface of the film in the thickness direction to obtain Film 1.

[0081] 1-2. Film 2 Film 2 was obtained in the same manner as Film 1, except that an unstretched film having a thickness of 190 μm, which had been produced at a surface temperature of the cooling roll of 50° C., was stretched 7.5 times at a stretching temperature of 140° C. and an applied voltage of 12.8 kV.

[0082] 1-3. Film 3 Film 3 was obtained in the same manner as in film 2, except that the film was stretched 7.1 times at a stretching temperature of 150° C. and the applied voltage was 12.0 kV.

[0083] 1-4. Film 4 Film 4 was obtained in the same manner as in Film 1, except that PVDF having a melt viscosity of 2500 Pa·s and a melting point of 173°C was used, the stretching ratio was 5.2 times, and the applied voltage was 7.5 kV.

[0084] 1-5. Film 5 Film 5 was obtained in the same manner as in the production of Film 4, except that the stretching ratio was 4.5 times and the applied voltage was 7.2 kV.

[0085] 1-6. Film 6 Film 6 was obtained in the same manner as in the production of Film 4, except that the stretching temperature was 150° C., the stretching ratio was 4.9 times, and the applied voltage was 7.6 kV.

[0086] 1-7. Film 7 Film 7 was obtained in the same manner as in the production of Film 4, except that the surface temperature of the cooling roll was 110° C., the draw ratio was 4.4 times, and the applied voltage was 7.1 kV.

[0087] 1-8. Film 8 Film 8 was obtained in the same manner as in the production of Film 1, except that the stretching temperature was 110° C., the stretching ratio was 3.5 times, and the applied voltage was 8.2 kV.

[0088] 1-9. Film 9 Film 9 was obtained in the same manner as in Film 7, except that a 190 μm thick unstretched film was stretched at a temperature of 120° C., a stretch ratio of 4.8 times, and an applied voltage of 7.4 kV.

[0089] 1-10. Film 10 Film 10 was obtained in the same manner as in the production of Film 4, except that the surface temperature of the cooling roll was 130°C, the stretching temperature was 110°C, the stretching ratio was 4.2 times, and the applied voltage was 7.5 kV.

[0090] 1-11. Film 11 Film 11 was obtained in the same manner as film 10, except that PVDF having a melt viscosity of 4500 Pa s and a melting point of 173°C was used, the melting temperature was 280°C, no polymer filter was used, the stretching temperature was 130°C, the stretching ratio was 3.6 times, and the applied voltage was 8.8 kV.

[0091] 1-12. Film 12: 100 g of PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was weighed and added to 900 ml of n-methylpyrrolidone (NMP). The temperature was raised to 60°C while stirring with a stirrer, and stirring was continued for 6 hours to produce a resin solution. This resin solution was filtered through a sintered metal nonwoven filter element (Fuji Metal Fiber, manufactured by Fuji Filter Industrial Co., Ltd.) with a filtration accuracy of 40 μm. The filtered resin solution was placed in an automatic coater to produce a coating film with a liquid thickness of 600 μm. The film was dried at 120°C for 1 hour, and a voltage of 9.1 kV was applied in the thickness direction from the surface of the resulting film to obtain Film 12.

[0092] 2. Evaluation of Fluoroplastic Resin Piezoelectric Films Unless otherwise specified, Films 1 to 12 were cut into squares with sides of 300 mm, and the area including the intersection of the diagonals of the square film was measured for breaking strength, retardation, internal haze, and piezoelectric constant d in the MD direction using the following methods. ３３ The surface height roughness Rz, the number of foreign particles, and the thickness were measured, and the values ​​were taken as representative values.

[0093] 2-1. Breaking strength of fluororesin piezoelectric film According to JIS K 7127, strip-shaped film A, 50 mm long and 10 mm wide, was cut out arbitrarily (regardless of the area including the intersection of the diagonals) from the square film, and strip-shaped film B, 50 mm long and 10 mm wide, was cut out from each film so that its longitudinal direction was perpendicular to that of strip-shaped film A. Next, strip-shaped film A and strip-shaped film B were each used as test pieces, and the breaking strength of each test piece was measured at a tensile speed of 50 mm / min using a tensile tester (Tensilon RTC-1210A manufactured by Orientec Co., Ltd.) in an environment with a measurement temperature of 23°C, and the larger breaking strength value of each test piece was taken as the breaking strength of the film.

[0094] 2-2. Retardation Using a KOBRA-HB manufactured by Oji Scientific Instruments, the retardation of a film cut into a 20 mm x 20 mm area from the area including the intersection of the diagonal lines of the square film was measured by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8 nm was taken as the retardation of the film.

[0095] 2-3. Internal haze A rectangular film was cut into a size of 50 mm x 50 mm so as to include the intersection of the diagonals of the square film. One surface (side A) of the rectangular film was coated with a hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) using a bar coater, and dried at 80°C for 30 minutes. Thereafter, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to apply a hard coating agent to the surface of the rectangular film (side A) until the target integrated light dose reached 400 mJ / cm. ２ The film was irradiated with UV light so that the thickness of the coating layer was 2 μm. A coating layer similar to that on the A side was also formed on the other surface (side B) of the rectangular film with the A side coated. Coating layers were formed on both sides of the film by the above method, and external haze due to scratches on the film surface, etc., was removed. The internal haze of the fluororesin piezoelectric film was measured in accordance with ISO 14782:2021 using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) for a range including the intersection of the diagonals of the film for internal haze measurement, and the result was used as a representative value.

[0096] 2-4. Haze Measurement was performed using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) In accordance with ISO 14782:2021, the haze was measured in an area including the intersection of the diagonal lines of the rectangular film before the coating layer was formed, and the measured value was used as the representative haze value of the fluororesin piezoelectric film.

[0097] 2-5. Piezoelectric constant d ３３ Piezoelectric constant d ３３ is the direct quasi-static method (d ３３ The piezoelectric constant d of the piezoelectric ceramic measured by the Meter method and the Berlincoat method ３３ The measurement was performed in accordance with the test method ISO 19622:2018. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) was used, and the test piece, the measurement film, was held at a measurement temperature of 25°C with a holding force of 1.0 N. An alternating force of 0.15 N and a frequency of 110 Hz was applied, and the generated charge was measured on the polarization surface. The piezoelectric constant d ３３ The absolute value of the piezoelectric constant d ３３ was used as a representative value.

[0098] 2-6. Surface Height Roughness Rz The surface height roughness Rz of the measurement film was measured using a surface roughness meter (Keyence Corporation, shape analysis laser microscope VK-X260) conforming to JIS B 0601:2001. The surface height roughness Rz was measured in an area including the intersection of the diagonal lines of the fluororesin piezoelectric film, and this value was used as the representative value of the surface height roughness Rz of the film. The Rz measurement was performed on the surface of the fluororesin piezoelectric film that had been in contact with the cooling roll.

[0099] 2-7. Number of foreign objects: 0.010 m so that each film is cut out from adjacent positions in succession. ２ Four rectangular films (observation pieces) each measuring 100 mm x 100 mm were cut out from the fluorine-based resin piezoelectric film. The sum of the number of foreign particles measured by observing each observation piece was multiplied by 25 / 4, and the result was rounded to the nearest whole number to obtain a value of 0.25 m. ２ The number of foreign particles per unit area was calculated. At this time, foreign particles were observed using transmitted light, marked, and the marked areas were observed under a microscope to determine the size of the foreign particles. The size of the foreign particles was calculated as the arithmetic mean value of the maximum and minimum widths of the foreign particles. In this way, the number of foreign particles larger than 200 μm, foreign particles with a size of 100 μm to 200 μm, and foreign particles with a size of less than 100 μm was calculated.

[0100] 2-8. Thickness A digital linear gauge (DG525H, manufactured by Ono Sokki Co., Ltd.) and a gauge stand (SH-022, manufactured by Ono Sokki Co., Ltd.) were used. The thickness was measured in the area including the intersection of the diagonal lines of the square film and the area including the intersection of the diagonal lines of the fluororesin piezoelectric film, and the measured value was used as the representative value of the thickness of the film.

[0101] 3. Results The production conditions and evaluation results for each film are shown in Tables 1 and 2. Film 12 had a high haze and was very opaque, making it impossible to measure the number of foreign particles.

[0102]

[0103]

[0104] This application claims priority to Japanese Patent Application No. 2024-124972 filed on July 31, 2024, and Japanese Patent Application No. 2024-124983 filed on July 31, 2024. The matters described in the specification, claims, and drawings of those applications as originally filed are incorporated herein by reference.

[0105] The fluororesin piezoelectric film according to the present invention is useful as a piezoelectric film that is highly transparent and resistant to breakage.

Claims

1. The retardation is 50 nm or more and 3000 nm or less, the internal haze is less than 1.2%, the breaking strength is 310 MPa or more and 550 MPa or less, and the piezoelectric constant d ３３ A fluorine-based resin piezoelectric film having a piezoelectric constant of 5.0 pC / N or more and 40.0 pC / N or less.

2. Measurement temperature: 260°C, shear rate: 50 s －１ The fluorine-containing resin piezoelectric film according to claim 1 , wherein the melt viscosity η measured by the method is 600 Pa·s or more and 4000 Pa·s or less.

3. The fluororesin piezoelectric film according to claim 1, which is primarily composed of vinylidene fluoride resin.

4. The number of foreign particles whose size, which is the arithmetic mean value of the maximum and minimum widths when the film is viewed in a plane, is 100 μm or more is 7 / 0.25 m ２ The fluorine-based resin piezoelectric film according to claim 1 , wherein:

5. The fluorine-based resin piezoelectric film according to claim 1, wherein the surface roughness in surface height Rz measured in accordance with JIS B 0601:2001 on the surface having a smaller Rz is 0.50 μm or less.

6. A method for producing a fluorine-based resin piezoelectric film according to any one of claims 1 to 5, comprising the steps of: heating and melting a resin composition containing a fluorine-based resin; extruding the molten resin composition to form a film; stretching the formed film at a stretching temperature of 125°C to 165°C at a stretching ratio of 4.0 to 8.0 times; and polarizing the formed film at an applied voltage of 7.0 kV to 50.0 kV.

7. The method for producing a fluorine-based resin piezoelectric film according to claim 6, further comprising a step of filtering the resin composition melted in the melting step through a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

8. The method for producing a fluorine-based resin piezoelectric film according to claim 6, wherein in the film-forming step, the extruded resin composition is cooled by contacting it with a cooling roll having a surface temperature of 125°C or less.

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