Fluororesin piezoelectric film and method for producing same

Abstract

To provide a fluorine-based resin piezoelectric film having a low shrinkage factor during heating and high transparency.SOLUTION: A fluororesin piezoelectric film having a maximum shrinkage of 2.5% or less when held at 80 °C for 30 minutes, a retardation of 50nm or more and 3000nm or less, an internal haze of less than 1.2%, 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 subjecting it to a polarization treatment. Known methods for producing a fluororesin film include a method in which a solution in which a fluororesin is dissolved in an organic solvent is produced by solution casting (Patent Document 1, etc.), and a method in which a fluororesin is thermally melted and extruded (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 formed by solution casting by poling without stretching, whereas piezoelectric films are produced from fluororesin films formed by hot melt extrusion by stretching the films and then poling them.

[0006] Fluoropolymer films produced by the solution casting method are characterized by their inherently low shrinkage upon heating, since they are not stretched. However, the solution casting method requires the use of a large amount of polar organic solvent to dissolve the fluoropolymer. This requires the recovery of the evaporated solvent during the drying process, 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 fluoropolymer remains in the film, it can hinder the polarization of the film and impair the smoothness of the film surface as the solvent evaporates.

[0007] In contrast, the method of extrusion molding a fluororesin by thermal melting allows the production of a fluororesin film without using an organic solvent, which is less likely to cause deterioration of the working environment or the natural environment, or to cause insufficient polarization or a decrease in the smoothness of the film surface due to polar organic solvents remaining in the film.

[0008] On the other hand, when imparting piezoelectricity to a fluororesin film formed by the thermal melting method, the film is usually stretched. From the perspective of increasing piezoelectricity, a higher stretching ratio is preferable. However, increasing the stretching ratio increases strain in the film, resulting in greater heat shrinkage. Piezoelectric films are used in sensors and other applications by laminating them with electrode films. The piezoelectric film and electrode film are used by adhering them with adhesives, and drying the adhesive usually involves heat treatment. If the piezoelectric film shrinks due to heat treatment, it can cause a decrease in adhesive strength, curvature of the laminated film, and a decrease in dimensional accuracy. For this reason, piezoelectric films are required to have a low shrinkage rate when heated.

[0009] Patent Document 3 discloses a fluororesin piezoelectric film that has been subjected to a stretching process and then a polarization process. However, the piezoelectric film disclosed in Patent Document 3 has high haze, and there is no disclosure of the thermal shrinkage rate when heated.

[0010] According to the investigations of the present inventors, a fluororesin piezoelectric film produced at a high stretch ratio has a large thermal shrinkage rate, and further, as the crystallization of the film progresses, light scattering due to the difference in refractive index between the crystalline and non-crystalline parts increases, and transparency tends to decrease.

[0011] The present invention has been made in view of the above problems, and an object of the present invention is to provide a fluororesin piezoelectric film that has a low shrinkage rate when heated and high transparency, and a method for producing the same. [Means for solving the problem]

[0012] 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 maximum shrinkage rate when kept at 80°C for 30 minutes is 2.5% or less, the retardation is 50nm or more and 3000nm or less, the internal haze is less than 1.2%, 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.

[0013] 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 and 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]

[0014] According to the present invention, a fluororesin piezoelectric film having a low shrinkage rate when heated and high transparency, and a method for producing the same are provided. DETAILED DESCRIPTION OF THE INVENTION

[0015] [Fluoroplastic piezoelectric film] An embodiment of the present invention relates to a fluorine-based resin piezoelectric film.

[0016] Fluorine-based resin piezoelectric films can be produced by stretching and poling a fluororesin film, which is the raw material. The higher the stretching ratio of the fluororesin film, the higher the piezoelectricity of the resulting fluororesin piezoelectric film. However, strain in the film increases, which tends to result in a non-uniform and high shrinkage rate upon heating. In contrast, increasing the temperature of the fluororesin film during stretching facilitates the movement of the polymer molecular chains that make up the film, suppressing the generation of strain in the film during stretching and thereby reducing the shrinkage rate upon heating.

[0017] The fluororesin piezoelectric film contains a fluororesin.

[0018] 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.

[0019] Among these, from the viewpoint of facilitating polarization of the fluororesin film that is the material for the fluororesin piezoelectric film, VDF resin obtained by polymerizing VDF is preferred, with VDF homopolymer, copolymer of VDF and HFP, copolymer of VDF and TrFE, copolymer of VDF and TFE, and copolymer of VDF, TrFE and TFE being more preferred, and VDF homopolymer being even more preferred. These fluororesins may be used alone or in combination.

[0020] 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.

[0021] 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 proportion 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.

[0022] 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.

[0023] 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 hereinafter referred to as "melt viscosity") is preferably 600 Pa·s or more and 4000 Pa·s or less, more preferably 600 Pa·s or more and 3500 Pa·s or less, even 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. The melt viscosity of a fluororesin film hardly changes even after stretching or polarization treatment, so the melt viscosity of a fluororesin piezoelectric film serves as an indicator of the melt viscosity of a resin composition containing the fluororesin raw material resin. Resin compositions that produce a piezoelectric film with a melt viscosity of 4000 Pa·s or less have short molecular chains, which reduces entanglement of the molecular chains and reduces the stretching stress between molecular chains that occurs during stretching, making it less likely for distortion to occur in the film during stretching. In this way, the film made from the resin composition has high piezoelectricity due to the increased proportion of β phase by increasing the stretching ratio, and the shrinkage rate upon heating is small because the generation of distortion can be suppressed even when the stretching ratio is increased.

[0024] Furthermore, resin compositions with a melt viscosity of 4000 Pa·s or less for the piezoelectric film are easy to filter and can maintain a low melting temperature, 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 also has the advantage of preventing reduced visibility due to foreign matter. Meanwhile, resin compositions with a melt viscosity of 600 Pa·s or more for the piezoelectric film have long molecular chains that suppress crystal growth, thereby suppressing light scattering due to the difference in refractive index between the crystalline and amorphous regions caused by the enlargement of crystallites, resulting in low internal haze.

[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 fluororesin piezoelectric film has a maximum shrinkage rate of 2.5% or less when kept at 80°C for 30 minutes, preferably 0.0% to 2.3%, more preferably 0.0% to 2.1%, even more preferably 0.0% to 1.9%, particularly preferably 0.0% to 1.7%, very preferably 0.0% to 1.5%, and most preferably 0.0% to 1.3%.

[0027] The maximum shrinkage rate when maintained at 80°C for 30 minutes can be determined by the following method. A fluororesin piezoelectric film is cut into a roughly square shape approximately 100 mm x 100 mm from within an area including the intersection of the diagonals of the fluororesin piezoelectric film to prepare a shrinkage measurement film (sometimes referred to as the "measurement film"). If the length of one side of the piezoelectric film is 300 mm or longer, a rectangle with a side length of 300 mm is arbitrarily assumed, and the measurement film is cut from within an 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 length of 300 mm is arbitrarily set on the piezoelectric film, and the measurement film is cut out according to the above method. On one side of the measurement film, line segments A and B are marked, passing through the intersection of the diagonals of the measurement film (the center of the film) and approximately parallel to the two sides of the measurement film, connecting both ends of the measurement film and intersecting at right angles at the center of the measurement film. The lengths of the marked line segments A and B are measured using an image dimension measuring instrument (Keyence Corporation, One-Shot 3D Shape Measuring Machine VR-5200). Next, the measurement film is placed in an oven heated to 80°C and held there for 30 minutes. The measurement film is then removed from the oven and allowed to cool to room temperature, and the lengths of line segments A and B of the measurement film are measured using the same image dimension measuring instrument. The length of line segment A after heating is subtracted from the length of line segment A before heating, and this is divided by the length of line segment A before heating to obtain a value expressed as a percentage, which is the shrinkage rate of line segment A. Next, the shrinkage rate of line segment B is calculated in the same manner as for line segment A. The shrinkage rates of line segment A and line segment B are compared, and the larger shrinkage rate is taken as the maximum shrinkage rate 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 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 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 2 The 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 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] Piezoelectric constant d of fluororesin piezoelectric film 33 is the direct quasi-static method (d 33 Piezoelectric constant d by Mehta method, Berlincourt method 33 Measurements are made in accordance with ISO 19622:2018, the test method for piezoelectric constants. Specifically, a piezoelectric constant measuring device (PIEZOTEST, Piezometer System PM300) is used to hold the piezoelectric film specimen 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 piezoelectric constant d 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 fluororesin piezoelectric film. 33 The absolute value of the piezoelectric constant d 33 If 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 2More 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. 2The 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, VK-X260 shape analysis laser microscope) is used. Measurements are 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 measurements are performed over an area including the intersection of the diagonals of the rectangular film. 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 method described above. Note that the surface of the fluororesin piezoelectric film that comes into contact with the chill roll tends to have a smaller surface 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 roughness Rz (the surface in contact with the cooling roll) are taken as the above-mentioned 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 (melting step); a step of extruding the heat-melted resin composition to form a film to produce a fluororesin film (film-forming 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. This step can be carried out, for example, by melt-kneading the resin composition using an extruder.

[0049] The resin composition melted in the melting step may be any resin composition containing the above-mentioned fluororesin (hereinafter, a resin composition containing a fluororesin will be referred to as a "resin composition"). If the resin composition contains a solvent component, the solvent component that remains unvolatilized 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] Furthermore, the resin composition melted during the melting process has a melt viscosity of 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. Resin compositions with a melt viscosity of 4000 Pa·s or less are easy to filter and can maintain a low melting temperature, which reduces the likelihood of spot-like irregularities on the film surface due to resin deterioration and the generation of low-molecular-weight impurities due to thermal decomposition. This also has the advantage of preventing reduced visibility due to impurities. Meanwhile, resin compositions with a melt viscosity of 600 Pa·s or more have long molecular chains that suppress crystal growth, thereby suppressing light scattering due to the difference in refractive index between the crystalline and amorphous portions caused by large crystallites, thereby reducing internal haze.

[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 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 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] Resin compositions containing fluororesins crystallize during the formation of a fluororesin film. As crystallization progresses, the crystalline particles become larger, and the difference in refractive index between the crystalline and amorphous regions causes light scattering, which can increase the internal haze of the fluororesin film and reduce the transparency of the film. Fluorine-based resins with low melt viscosity, in particular, have a low degree of polymerization, allowing for easy free movement of molecular chains and easy selective alignment of molecular chains, making them prone to crystallization and increasing the internal haze of the fluororesin film. However, by forming the molten resin into a film and then rapidly cooling it before crystallization progresses, for example, the crystallization of the fluororesin can be suppressed, reducing the scattering of light between the crystalline and amorphous regions, and improving the transparency of the resulting fluororesin film.

[0061] 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.

[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 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 may be stretched after being wound up and stored. Furthermore, the film may be stretched under heating as necessary.

[0064] 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 preferably 4.0 times or more and 8.0 times or less. Increasing the stretching ratio elongates the molecular chains of the fluororesin in the film in the stretching direction, improving the molecular chain orientation and facilitating crystal alignment. This increases the film's rigidity while increasing the retardation of the fluororesin piezoelectric film. This also reduces the internal haze of the fluororesin piezoelectric film, making the film more transparent. 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, by setting the stretching ratio to 8.0 times or less, it is possible to suppress the occurrence of distortion during the stretching process and to suppress an increase in the shrinkage rate of the fluorine-based resin piezoelectric film when heated.

[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 stretching at a high surface temperature of the stretching roll of 125°C or higher, stresses generated during the selective alignment of molecular chains are alleviated, thereby reducing the shrinkage rate of the film when heated. Furthermore, by stretching the film while heating, the film is uniformly stretched, preventing localized crystallization and aligning molecular chains uniformly throughout the film, thereby suppressing light scattering due to differences in crystallinity within the film and reducing internal haze. By keeping the surface temperature of the rolls below 165°C during stretching, the efficiency of molecular chain alignment due to stretching of the film is improved, the proportion of the β-phase structure that contributes to the expression of piezoelectricity is increased, and deformation of the film due to melting of the fluororesin can be 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 kV or less, even more preferably 8.0 kV or more and 30 kV or less, and particularly preferably 8.5 kV or more and 30 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] 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.

[0074] 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.

[0075] 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.

[0076] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[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 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 4.5 times, and the applied voltage was 7.2 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 temperature was 150° C., the stretching ratio was 4.9 times, and the applied voltage was 7.6 kV.

[0081] 1-4.Film 4 Film 4 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.

[0082] 1-5.Film 5 The surface temperature of the cooling roll was set to 50°C, and the unstretched film having a thickness of 190 μm was stretched at a stretching temperature of 140°C and a stretching ratio of 7.5 times. Film 5 was obtained in the same manner as film 1, except that the film was polarized at an applied voltage of 12.8 kV.

[0083] 1-6.Film 6 Film 6 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 5.0 times, and the applied voltage was 8.2 kV.

[0084] 1-7.Film 7 Film 7 was obtained in the same manner as Film 6, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used, the stretching ratio was 3.5 times, and the applied voltage was 7.2 kV.

[0085] 1-8.Film 8 Film 8 was obtained in the same manner as film 4, except that no polymer filter was used, the cooling roll temperature was 130°C, the stretching temperature was 110°C, the stretching ratio was 4.2 times, and the applied voltage was 7.5 kV.

[0086] 1-9.Film 9 Film 9 was obtained in the same manner as Film 4, 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 stretching 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 stirring was continued for 6 hours to produce a resin solution. This resin solution was filtered through a filter with a filtration accuracy of 40 μm. The filtered resin solution was added to an automatic coating machine 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 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 each cut into a square film with a side of 300 mm, and the area including the intersection of the diagonal lines of the square film was measured for the maximum shrinkage rate, 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. Maximum shrinkage rate The maximum shrinkage rate when maintained at 80°C for 30 minutes was determined using the following method. A fluororesin piezoelectric film was cut into a roughly square shape measuring approximately 100 mm × 100 mm from within an area including the intersection of the diagonals of the square film to prepare a measurement film. On one side of the measurement film, line segments A and B were marked, passing through the intersection of the diagonals (the center of the film) of the measurement film, approximately parallel to the two sides of the measurement film, connecting both ends of the measurement film, and perpendicular to the center of the measurement film. The lengths of the marked line segments A and B were measured using an image dimension measuring instrument (Keyence Corporation, One-Shot 3D Shape Measuring Instrument VR-5200). The measurement film was then placed in an oven heated to 80°C and maintained there for 30 minutes, then removed from the oven and allowed to cool to room temperature. The lengths of line segments A and B were then measured using the same image dimension measuring instrument. The length of line segment A after heating was subtracted from the length of line segment A before heating, and the resulting value, expressed as a percentage, was defined as the shrinkage rate of line segment A. Next, the shrinkage rate of line segment B was calculated in the same manner as for line segment A. The shrinkage rates of line segment A and line segment B were compared, and the larger shrinkage rate was defined as the maximum shrinkage rate.

[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. 2The 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 2Four 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 resulting value 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 was used as a 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 of the present invention is useful as a piezoelectric film having a small thermal shrinkage rate when heated and high transparency.

Claims

1. The maximum shrinkage rate when maintained at 80°C for 30 minutes is 2.5% or less, The retardation is 50 nm or more and 3000 nm or less, The internal haze is less than 1.2%; 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 and 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 .

Images