Fluororesin piezoelectric film and method for producing same

Abstract

To provide a fluororesin piezoelectric film which is obtained from a fluororesin having low melt viscosity and has high breaking strength.SOLUTION: Wherein the fluororesin piezoelectric film has a retardation of 50nm or more and 3000nm or less, an internal haze of less than 1.2%, a breaking strength of 310MPa or more and 550MPa or less, and a piezoelectric coefficient d33 of 5. 0pC / N or more and 40. 0pC / N or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[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 Document 1 and Patent Document 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). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2015 / 064324 [Patent Document 2] Japanese Patent Application Publication No. 05-102548 [Patent Document 3] International Publication No. 2022 / 091828 Summary of the Invention [Problem to be solved by the invention]

[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 mentioned above, from the perspective of enhancing the piezoelectricity of a 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 have 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, so when they are 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 its 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. [Means for solving the problem]

[0010] One embodiment of the present invention for solving the above problems relates to the following fluorine-based resin piezoelectric films [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 d 33 Fluorine-based resin piezoelectric film having a dielectric 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 [1], having a melt viscosity η measured by a method of 600 Pa·s or more and 4000 Pa·s or less. [3] A fluorine-based resin piezoelectric film according to [1] or [2], which contains vinylidene fluoride resin as a main component. [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 any one of [1] to [3] below: [5] The fluorine-based resin 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 the following methods for producing a piezoelectric film [6] to [8]. [6] A method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [5], comprising the steps of: heating and melting a resin composition containing a fluorine-based resin; extruding the melted resin composition to form a film; stretching the formed film at a stretching temperature of 125°C or higher and 165°C or lower at a stretching ratio of 4.0 times or higher and 8.0 times or lower; and polarizing the formed film at an applied voltage of 7.0 kV or higher and 50.0 kV or lower. [7] The method for producing a fluorine-based resin piezoelectric film according to [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 [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. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Fluoroplastic piezoelectric film] An embodiment of the present invention relates to a fluorine-based 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] Fluorine-based resins can be homopolymers or copolymers obtained by polymerizing tetrafluoroethylene (TFE), vinylidene fluoride (VDF), etc. Examples of fluorine-based resins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, hexafluoropropylene (HFP), etc. Examples of fluorine-based resins obtained by polymerizing VDF include VDF homopolymers and copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), TFE, tetrafluoropropene, HFP, perfluoroalkyl vinyl ether, etc.

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

[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 VDF-derived structural units to the total mass of the VDF resin is more preferably 50% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and particularly 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 resin whose constituent unit is VDF contained in the fluororesin film and the fluororesin piezoelectric film is as follows: 19 It can be measured by quantitative analysis using an internal standard using F-NMR.

[0021] The fluorine 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 -1The melt viscosity (measured by the method described above) is preferably 600 Pa·s or more and 4000 Pa·s or less. Because 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 processing (stretching and polarization processing) and serves as an index of the melt viscosity of the resin composition containing the fluororesin raw material resin. Resin compositions that produce piezoelectric films with a melt viscosity of 4000 Pa·s or less have short molecular chains and facilitate molecular motion, making them easy to extrude into films at low melting temperatures. This minimizes resin degradation, decomposition products, and foreign matter that occur when the resin composition is heated and melted. Furthermore, because the molecular chains of the resin composition are less entangled, the film can be stretched at a high stretch ratio. The higher the stretch ratio, the more the molecular chain orientation structure develops, resulting in higher breaking strength of the piezoelectric film. On the other hand, piezoelectric films made from resin compositions with a melt viscosity of 600 Pa·s or higher have long molecular chains, which increase shear stress during stretching, making the film less likely to break. Furthermore, piezoelectric films made from these resin compositions can suppress crystal growth, resulting in less internal haze caused by light scattering due to differences in refractive index between crystalline and amorphous regions. The melt viscosity of the piezoelectric film is preferably 600 Pa·s or higher but not higher than 3500 Pa·s, more preferably 600 Pa·s or higher but not higher than 2400 Pa·s, particularly preferably 600 Pa·s or higher but not higher than 2000 Pa·s, and most preferably 600 Pa·s or higher but not higher than 1500 Pa·s.

[0022] Melt viscosity is an index of the degree of polymerization of a fluororesin, and the lower the melt viscosity, the lower the degree of polymerization. Fluororesin with low melt viscosity has short molecular chains, which means that the molecular chains are less likely to entangle, making the film less likely to break even when stretched at a high ratio, and easier to stretch at a high ratio.

[0023] Furthermore, the lower the melt viscosity, the lower the melting temperature can be kept, which makes it less likely that speckled irregularities will occur on the film surface due to resin deterioration, 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 during stretching, 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] Fluorine-based resins crystallize during the formation of a fluororesin film. As crystallization progresses, the crystallites become larger, and the difference in refractive index between the crystalline and amorphous parts causes light scattering, which tends to reduce the transparency of the film. Fluorine-based resins with low melt viscosity in particular tend to crystallize easily because of their low degree of polymerization, which facilitates free movement of molecular chains and facilitates selective alignment of molecular chains, and therefore tend to increase the internal haze of the fluororesin film. However, by forming the molten resin into a film and then rapidly cooling it before crystallization progresses, the crystallization of the fluororesin can be suppressed, reducing the scattering of light between the crystalline and amorphous parts, and improving the transparency of the resulting fluororesin 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 using a capillary die with an inner diameter of φ1 mm and a tube length of 10 mm. -1 The viscosity measured is taken as the viscosity.

[0026] The breaking strength of the fluororesin 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 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 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 through the polarization step.

[0029] The retardation is measured using a light source with a wavelength of 587.8 nm by the parallel Nicol rotation method on a 20 mm x 20 mm piece of film cut from an area including the intersection of the diagonals of a fluororesin piezoelectric film, and this value is used as the representative retardation value of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, an arbitrary 300 mm rectangle is assumed, and the retardation of a 20 mm x 20 mm piece of film cut from an 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 300 mm square is cut 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%, further preferably 0.1% to 0.8%, particularly preferably 0.1% to 0.6%, and most preferably 0.1% to 0.4%. The lower the internal haze, the more transparent 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 transparent 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 that it includes the intersection of the diagonals. If the length of one side of the piezoelectric film is 300 mm or longer, a rectangle with a side of 300 mm is arbitrarily assumed, and the rectangular film is cut out from the area including the intersection of the diagonals of the rectangular film. If both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above, 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. Then, an ultraviolet (UV) irradiation device (GS NIPPON DENCHI, CSOT040) was used to achieve a target cumulative light dose of 400 mJ / cm 2The film is irradiated with UV light so that the thickness becomes 2 μm, forming a coating layer. 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, to prepare a film for internal haze measurement in which external haze due to scratches on the film surface, etc., has been removed. Haze is measured in accordance with ISO 14782:2021 for an area including the intersection of the diagonal lines of the obtained measurement film using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.), and the value obtained is used as the 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] Fluorine resin piezoelectric film has a piezoelectric constant d 33 is 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 measurement is performed in accordance with ISO 19622:2018, which is a test method for determining the piezoelectric constant. Specifically, a piezoelectric constant measuring device (Piezometer System PM300, manufactured by PIEZOTEST) is used to hold the piezoelectric film as a test piece at a measurement temperature of 25°C with a holding force of 1.0 N, and measure the charge generated when an alternating force of 0.15 N and a frequency of 110 Hz is applied. The charge of the piezoelectric film is measured by the above method in the range including the intersection of the diagonal lines of the polarization surface of the fluororesin piezoelectric film, and the piezoelectric constant d 33 The absolute value of the piezoelectric constant d 33If the length of one side of the piezoelectric film is 300 mm or more, assume a rectangle with a side of 300 mm, and calculate the piezoelectric constant d 33 If both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm shall be arbitrarily set on the piezoelectric film, and the measurement range shall be set according to the above method.

[0036] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles 100 μm or larger is 0 per 0.25 m 2 More than 7 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 5 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 3 pieces / 0.25m 2 The following is even more preferable: 0 pieces / 0.25m 2 More than 1 piece / 0.25m 2 The following are particularly preferred:

[0037] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles larger than 200 μm is 0 per 0.25 m 2 More than 3 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 2 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 2 More than 1 piece / 0.25m 2 The following is even more preferred:

[0038] When the fluorine-based resin piezoelectric film is viewed in a plane, the number of foreign particles less than 100 μm in size is 0 / 0.25 m 2 More than 50 pieces / 0.25m 2 The following is preferred: 0 pieces / 0.25m 2 More than 25 pieces / 0.25m 2 Less than 0 pieces / 0.25m is more preferable. 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 fluororesin piezoelectric film, and then calculating the sum of the number of foreign particles measured from each observation piece. Specifically, four rectangular films (observation pieces) adjacent to each other from the fluororesin film are cut out at 0.010 m 2 Four observation pieces each measuring 100 mm x 100 mm are cut out. The sum of the number of foreign particles measured from each observation piece is calculated, and the sum is multiplied by 25 / 4. The result is rounded to the nearest tenth and equals 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 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. Note that 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 fluororesin piezoelectric film becomes less uneven on the surface when pressed against the cooling roll. This tendency remains the same even after stretching and polarization. Here, the measurement results for the surface with the smallest surface height roughness Rz (the surface in contact with the cooling roll) are 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 taken of the area including the intersection of the diagonals of the fluororesin piezoelectric film using known methods such as a laser displacement meter, a capacitance displacement meter, or an infrared method, and the resulting value is used as the representative thickness of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or longer, 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. If both sides of the piezoelectric film are 300 mm or longer, a square with a side of 300 mm is arbitrarily set on the piezoelectric film and the measurement area is determined according to the method described above.

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

[0046] [Method of manufacturing fluororesin piezoelectric film] 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 may 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).

[0047] At this time, a step of filtering the molten resin composition (filtration step) may be carried out as needed.

[0048] (Melting process) 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, for example, an extruder.

[0049] 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 remaining solvent component 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.

[0050] The resin composition melted during the melting process was measured at a temperature of 260°C and a shear rate of 50 s -1 The melt viscosity measured by HPLC 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 fluororesin piezoelectric films, the higher the stretching ratio of the fluororesin film, the higher the piezoelectricity tends to be. Resin compositions with a melt viscosity of 4000 Pa·s or less have short molecular chains, which reduces entanglement of the molecular chains. This makes the film less likely to break even at high stretching ratios, enabling it to be stretched at high stretching ratios. Therefore, the lower the melt viscosity of the resin composition, the more preferable it is for producing fluororesin piezoelectric films with high piezoelectricity and high breaking strength. Furthermore, resin compositions with a melt viscosity of 4000 Pa·s or less are easy to filter and can maintain a low melting temperature, reducing the risk of spotted irregularities on the film surface due to resin deterioration and the generation of low-molecular-weight impurities due to thermal decomposition. This prevents reduced visibility due to impurities and also reduces the risk of film breakage caused by the concentration of stretching stress on the film around the impurities if they are present in the film during stretching. Using a resin composition with a melt viscosity of 600 Pa·s or more reduces internal haze due to the progression of crystallization.

[0051] The melting temperature of the resin composition is preferably 75°C or more higher than the melting point of the resin composition but not exceeding 105°C, more preferably 75°C or more higher but not exceeding 100°C, even more preferably 80°C or more higher but not exceeding 100°C, and particularly preferably 85°C or more higher but not exceeding 95°C. By setting the melting temperature at least 75°C 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 most 105°C 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. Suppressing the generation of decomposition products can reduce the amount of foreign matter in the fluororesin piezoelectric film and improve the transparency and smoothness of the fluororesin piezoelectric film. Suppressing the generation of decomposition products can also suppress filter clogging and improve the filtration efficiency of the resin composition. 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.

[0052] (filtration process) 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 manufacturing cost for recovering the polar solvent.

[0053] 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.

[0054] 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 facilitates filtration of the resin composition that has been heated to the above temperature and melted, and also prevents the filtration pressure from becoming too high, allowing for a shorter 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 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.

[0055] 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 certain size with a predetermined 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.

[0056] 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.

[0057] 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.

[0058] (Film forming process) In the film-forming step, the resin composition that has been melted in the melting step and, if necessary, filtered in the filtration step, is formed into a film.

[0059] 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.

[0060] The surface temperature of the cooling roll is preferably 125°C or lower. By rapidly cooling the resin composition film extruded using a cooling 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 cooling roll is preferably 5°C to 115°C, more preferably 10°C to 100°C, even more preferably 20°C to 90°C, particularly preferably 20°C to 80°C, very preferably 30°C to 70°C, and most preferably 30°C to 60°C.

[0061] 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.

[0062] (Stretching process) 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.

[0063] Stretching can be performed in the machine direction (MD) and, if necessary, in the direction perpendicular to the machine direction (TD) while transporting the fluororesin film using 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. Increasing the stretching ratio can elongate the molecular chains of the fluororesin in the film in the stretching direction, thereby enhancing the molecular chain orientation and facilitating crystal alignment. This increases the rigidity of the film and the retardation of the fluororesin piezoelectric film. The stretching ratio is preferably 4.3 times or more and 8.0 times or less, more preferably 4.3 times or more and 7.7 times or less, even more preferably 4.5 times or more and 7.7 times or less, particularly preferably 5.0 times or more and 7.7 times or less, and extremely preferably 5.5 times or more and 7.5 times or less. The higher the stretching ratio, the higher the proportion of the β phase, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through the polarization process. On the other hand, setting the stretching ratio to approximately 8.0 times or less can suppress breakage during the stretching process.

[0064] 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.

[0065] Furthermore, the stretching temperature of the fluororesin film can be controlled by controlling the surface temperature of the stretching roll during the stretching process. Here, the surface temperature of the stretching roll is referred to as the stretching temperature. The stretching temperature 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 maintaining the surface temperature of the roll at 125°C or higher during stretching, molecular chain motion is facilitated, thereby reducing the stretching stress generated during stretching. This reduces molecular chain scission due to stretching stress, allowing the film to be stretched at a high ratio without breaking. The molecular chains constituting a film stretched at a high ratio have a highly oriented structure parallel to the stretching direction of the film, and the high bond energy between atoms in the molecular chains efficiently improves the film's breaking strength, thereby increasing the film's breaking strength. Furthermore, by stretching the film while heating, the film is stretched uniformly, preventing localized crystallization and aligning the molecular chains uniformly throughout the film, which suppresses light scattering caused by differences in crystallinity within the film and reduces 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 is improved, the proportion of the β-phase structure that contributes to the development of piezoelectricity is increased, and film deformation due to softening of the resin composition is suppressed.

[0066] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. Fluororesins containing vinylidene fluoride homopolymers or copolymers undergo a transition from α phase to β phase during the stretching process, 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.

[0067] 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.

[0068] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0069] 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.

[0070] [Application] The above-mentioned 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.

[0071] [Other embodiments] It should be noted that the above-described embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-described embodiments within the scope of its core technical concept. [Example]

[0072] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0073] The melt viscosity of the fluororesin 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 the melt viscosity was measured at a temperature of 260 °C and a shear rate of 50 s using a capillary die with an inner diameter of φ1 mm and a tube length of 10 mm. -1 The viscosity was measured at 1000 kJ / min.

[0074] 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.

[0075] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0076] 1. Preparation of Fluoropolymer Piezoelectric Film Films 1 to 10, all of which were fluorine-based resin piezoelectric films, were prepared by the following procedure.

[0077] 1-1.Film 1 Vinylidene fluoride 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 50mm diameter single-screw extruder and then filtered through a sintered metal nonwoven filter element (Fuji Metal Fiber, manufactured by Fuji Filter Industrial Co., Ltd.) with a filtration accuracy of 20μm. The filtered resin was extruded into a film from a T-die and cooled by contacting it with the surface of a cooling roll heated to 70°C, yielding an unstretched film with a thickness of 160μm.

[0078] The unstretched film was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and stretched 5.9 times in the machine direction (MD) at 130°C by adjusting the surface temperature and rotation speed ratio of each roll. Furthermore, a voltage of 9.6 kV was applied from the surface of the film to the thickness direction to obtain Film 1.

[0079] 1-2.Film 2 Film 2 was obtained in the same manner as Film 1, except that PVDF with 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.

[0080] 1-3.Film 3 Film 3 was obtained in the same manner as in the production of Film 2, except that the stretching ratio was 4.5 times and the applied voltage was 7.2 kV.

[0081] 1-4.Film 4 Film 4 was obtained in the same manner as in the production of Film 2, except that the stretching temperature was 150° C., the stretching ratio was 4.9 times, and the applied voltage was 7.6 kV.

[0082] 1-5.Film 5 Film 5 was obtained in the same manner as in Film 2, 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.

[0083] 1-6.Film 6 Film 6 was obtained in the same manner as Film 1, except that the cooling roll temperature was 50°C, the stretching temperature was 140°C, the stretching ratio was 7.5 times, and the applied voltage was 12.8 kV.

[0084] 1-7.Film 7 Film 7 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.

[0085] 1-8.Film 8 Film 8 was obtained in the same manner as in the production of Film 2, except that the cooling roll temperature was 130°C, the stretching temperature was 110°C, and the stretching ratio was 4.2 times.

[0086] 1-9.Film 9 Film 9 was obtained in the same manner as Film 5, except that PVDF with 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 cooling roll temperature was 130°C, the draw ratio was 3.6 times, and the applied voltage was 8.8 kV.

[0087] 1-10.Film 10 100 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 mixture was heated to 60°C while stirring with a stirrer and continued stirring 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 then placed in an automatic coating machine to produce a 600 μm thick coating film. The resulting film was dried at 120°C for 1 hour, and a voltage of 10.0 kV was applied from the surface to the thickness direction to produce Film 10.

[0088] 2. Evaluation of fluororesin piezoelectric film Unless otherwise specified, the obtained films 1 to 10 were cut into squares with sides of 300 mm, and the area including the intersection of the diagonal lines of the square films was measured for breaking strength in the MD direction, retardation, internal haze, and piezoelectric constant d 33 The surface height roughness Rz, the number of foreign particles, and the thickness were measured, and the values ​​were used as representative values.

[0089] 2-1. Breaking strength of fluororesin piezoelectric film According to JIS K 7127, strip-shaped film A having a length of 50 mm and a width of 10 mm was cut out arbitrarily (regardless of the range including the intersection of the diagonals) from the square film, and strip-shaped film B having a length of 50 mm and a width of 10 mm 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 where the measurement temperature was 23°C, and the larger breaking strength value of each test piece was taken as the breaking strength of the film.

[0090] 2-2.Retardation The retardation of a 20mm x 20mm film cut out from the area including the intersection of the diagonal lines of the square film was measured using a KOBRA-HB made by Oji Scientific Instruments by the parallel Nicol rotation method. The value at a measurement wavelength of 587.8nm was taken as the retardation of the film.

[0091] 2-3. Internal haze A rectangular film was cut to 50 mm x 50 mm so as to include the intersection of the diagonals of the square film. A hard coating agent (BS CH271, manufactured by Arakawa Chemical Industries, Ltd.) was applied to one surface (side A) of the film using a bar coater and dried at 80°C for 30 minutes. Then, an ultraviolet (UV) irradiation device (CSOT040, manufactured by GS NIPPON DENCHI Co., Ltd.) was used to apply a target cumulative light dose of 400 mJ / cm. 2 The film was irradiated with UV light so that the thickness was 2 μm, forming a coating layer. A coating layer similar to that on side A was also formed on the other surface (side B) of the rectangular film with side A coated. Coating layers were formed on both surfaces of the film using the method described above, 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.

[0092] 2-4. Piezoelectric constant d 33 Piezoelectric constant d 33 is the direct quasi-static method (d 33 Piezoelectric constant d of piezoelectric ceramics measured by Meter method and Berlincourt 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 to hold the test film as a test piece at a measurement temperature of 25°C with a holding force of 1.0 N, and apply an alternating force of 0.15 N and a frequency of 110 Hz, measuring the electric charge generated. The piezoelectric constant d 33 The absolute value of the piezoelectric constant d 33 was used as a representative value.

[0093] 2-5. Surface height roughness Rz The surface height roughness Rz 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 square 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 come into contact with the cooling roll.

[0094] 2-6. Number of foreign objects 0.010m so that each film is cut from adjacent positions in succession 2 Four rectangular films (observation pieces) 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 first decimal place to obtain a value of 0.25 m. 2 The number of foreign particles per unit area was calculated. The foreign particles were observed using transmitted light, marked, and the marked areas were then 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 between 100 μm and 200 μm, and foreign particles smaller than 100 μm were calculated.

[0095] 2-7. 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 this value was used as the representative value of the film thickness.

[0096] 3.Results The production conditions and evaluation results for each film are shown in Tables 1 and 2. Film 10 had a high haze and was very opaque, making it impossible to measure the number of foreign particles.

[0097] [Table 1]

[0098] [Table 2] [Industrial Applicability]

[0099] 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, Piezoelectric constant d ３３ is 5.0 pC / N or more and 40.0 pC / N or less, Fluorine-based resin piezoelectric film.

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

3. The main component is vinylidene fluoride resin. The fluorine-based resin piezoelectric film according to claim 1 .

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

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

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

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

8. 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. The method for producing the fluorine-based resin piezoelectric film according to claim 6 .

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