Method for producing fluororesin piezoelectric film

Abstract

To provide a method for producing a fluororesin piezoelectric film having high transparency.SOLUTION: A method for producing a fluorine-based resin piezoelectric film includes a step of thermally melting a resin composition containing a fluorine-based resin, a step of extruding the thermally melted resin composition to form a film, a step of stretching the formed fluorine-based resin film at a stretching ratio of 4.0 times or more and 8.0 times or less at a stretching temperature of 125 °C or more and 160 °C or less, and a step of polarizing the stretched fluorine-based resin film with an applied voltage of 7.0 kV or more and 50.0 kV or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a fluorine-based resin piezoelectric film. [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 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] The solution casting method described in Patent Document 1 uses a large amount of polar organic solvent to dissolve the fluororesin, which requires solvent recovery, resulting in higher production costs and the need to consider 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 interfere with polarization of the piezoelectric 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, as described in Patent Document 2, allows for the production of a fluororesin film without using an organic solvent, thereby preventing deterioration of the working environment and the natural environment and making it less likely to suffer from insufficient polarization or a decrease in the smoothness of the film surface due to polar organic solvents remaining in the film.

[0008] 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.

[0009] 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.

[0010] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a fluorine-based resin piezoelectric film having high transparency and high breaking strength. [Means for solving the problem]

[0011] One embodiment of the present invention for solving the above problems relates to the following methods for producing a fluorine-based resin piezoelectric film [1] to [7]. [1] A method for producing a fluorine-based resin piezoelectric film, comprising the steps of: heat-melting a resin composition containing a fluorine-based resin; extruding the heat-melted resin composition to form a film; stretching the formed fluorine-based resin film at a stretching temperature of 125°C to 165°C at a stretching ratio of 4.0 to 8.0; and polarizing the stretched fluorine-based resin film by applying a voltage of 7.0 to 50.0 kV. [2] The method for producing a fluorine-based resin piezoelectric film according to [1], wherein the cooling roll temperature in the film-forming step is 125°C or lower. [3] The resin composition was measured at a temperature of 260°C and a shear rate of 50 s -1 The method for producing a fluorine-based resin piezoelectric film according to [1] or [2], wherein the melt viscosity η measured by the method is 600 Pa·s or more and 4000 Pa·s or less. [4] The method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [3], further comprising a step of filtering the heat-melted resin composition through a filter having a filtration accuracy of 10 μm or more and 40 μm or less, and the film-forming step is carried out after the filtering step. [5] The method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [4], wherein the resin composition contains vinylidene fluoride resin as a main component. [6] Piezoelectric constant d 33 The method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [5], wherein a film having a modulus of elasticity of 7.0 pC / N or more and 40.0 pC / N or less is produced. [7] 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 method for producing a fluorine-based resin piezoelectric film according to any one of [1] to [6], wherein the following film is produced: [Effects of the Invention]

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

[0013] [Method of manufacturing fluororesin piezoelectric film] An embodiment of the present invention relates to a method for producing a fluorine-based resin piezoelectric film.

[0014] The fluororesin piezoelectric film is produced by thermally melting a resin composition containing a fluororesin (thermal melting process), extruding the thermally melted resin composition containing a fluororesin to form a film (film formation process), stretching the formed film (stretching process), and polarizing the stretched film (polarization process).

[0015] At this time, a step of filtering the resin composition containing the thermally molten fluorine-based resin (filtration step) may be carried out as needed.

[0016] (Thermal melting process) In the heat-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.

[0017] The resin composition melted in the melting step may be any resin composition containing a 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, and the content of the solvent component relative to the total mass of the resin composition is preferably 1% by mass or less, more preferably 0.1% by mass or less. 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, relative to the total mass of the resin composition.

[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, VDF resin obtained by polymerizing VDF is preferred, and a VDF homopolymer, a copolymer of VDF and HFP, a copolymer of VDF and TrFE, a copolymer of VDF and TFE, and a copolymer of VDF, TrFE and TFE are more preferred, with a VDF homopolymer being even more preferred. These fluororesins may be used alone or in combination.

[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 resin composition preferably contains a VDF resin, preferably a VDF homopolymer, as a main component. The term "containing such a resin as a main component" means that the proportion of VDF-derived structural units relative to the total mass of the resin composition is 50% by mass or more. The proportion of such a resin relative to the total mass of the resin composition 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 the resin having VDF as a constituent unit contained in the fluororesin composition is 19 It can be measured by quantitative analysis using an internal standard using F-NMR.

[0023] The resin composition may contain resins other than fluorine-based resins or other additives.

[0024] The resin composition containing fluororesin 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) is preferably between 600 and 4000 Pa·s. Resin compositions with a melt viscosity of 4000 Pa·s or less can be extruded at low melting temperatures and are therefore easy to form into films. Furthermore, the lower the melt viscosity, the easier it is to filter the film. Furthermore, the lower the melt temperature, the less likely the film surface will develop spots due to resin degradation and the less likely low-molecular-weight impurities will be generated due to thermal decomposition. Furthermore, the lower the melt viscosity, the shorter the molecular chains, which reduces entanglement and makes the film less susceptible to breakage even at high stretch ratios. This allows for high stretching ratios. On the other hand, piezoelectric films made from resin compositions with a melt viscosity of 600 Pa·s or more have low internal haze because they can suppress light scattering due to the difference in refractive index between the crystalline and amorphous regions as crystallization progresses. The melt viscosity of the resin composition containing a fluororesin is more preferably 600 Pa·s or more and 4000 Pa·s or less, even more preferably 600 Pa·s or more and 3500 Pa·s or less, still more preferably 600 Pa·s or more and 2400 Pa·s or less, particularly preferably 600 Pa·s or more and 2000 Pa·s or less, and very preferably 600 Pa·s or more and 1500 Pa·s or less.

[0025] Fluoropolymers with low melt viscosity have a low degree of polymerization, allowing molecular chains to move freely. This facilitates selective alignment of molecular chains, making them prone to crystallization during the production of fluoropolymer films. As the crystals grow, the crystallites become larger, and the difference in refractive index between the crystalline and amorphous regions causes light scattering, which tends to reduce the transparency of the film. In contrast, piezoelectric films with a melt viscosity of 600 Pa·s or higher can suppress the free movement of molecular chains, thereby reducing internal haze.

[0026] However, even in the case of a fluororesin with a low melt viscosity, by forming the molten resin into a film and then rapidly cooling it before crystallization progresses, it is possible to suppress the crystallization of the fluororesin, reduce the scattering of light that occurs between the crystalline and amorphous parts, and increase the transparency of the resulting fluororesin film.

[0027] 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.

[0028] 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.

[0029] According to the findings of the present inventors, fluororesins have high melt viscosities, and high temperatures are required to reduce the viscosity to a level suitable for filtration. However, heating to high temperatures is prone to decomposition, condensation, and other reactions, which can easily clog filters. Therefore, it has been difficult to melt and filter a resin composition containing a fluororesin. In contrast, by controlling the melting temperature of the resin composition within the above range, the resin composition containing a fluororesin can be efficiently filtered. By controlling the melting temperature of the resin composition containing a fluororesin within the above range and filtering the resin composition with a reduced viscosity due to melting, the resin composition can be filtered without using a polar solvent. Furthermore, the absence of a solvent reduces the likelihood of polarization being inhibited during polarization treatment by polar solvents remaining in the fluororesin film. Furthermore, since polar solvents are not generally used in the film formation process, this reduces the burden on the working environment and the natural environment, and also reduces the production costs associated with recovering the polar solvent.

[0030] (filtration process) In the filtration step, the resin composition that has been melted and reduced in viscosity in the melting step is filtered. The filtration method is not particularly limited, and the molten resin composition may be passed through a filter, and known filter types such as pleated filters and leaf disc filters may be used.

[0031] 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.

[0032] 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.

[0033] In this step, the fluororesin composition may be passed through multiple filters. For example, a filter with low filtration accuracy (large filtration accuracy value) placed in the front stage may remove coarse foreign matter, and then a filter with high filtration accuracy (small filtration accuracy value) placed in the rear stage may remove finer foreign matter. In this case, the filtration accuracy is the value of the filter with the highest filtration accuracy.

[0034] 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.

[0035] (Film forming process) In the film-forming step, the resin composition filtered in the filtration step is formed into a film.

[0036] The film formation method is not particularly limited, and known methods can be used, such as extruding a molten and filtered resin composition through a T-die and contacting it with a cooling roll to cool it. The surface temperature of the cooling roll is preferably 125°C or lower. Rapidly cooling the extruded resin composition film using a cooling roll with a low surface temperature can form a large number of fine crystals, thereby increasing the transparency of the resulting fluororesin film. 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.

[0037] 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.

[0038] (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 taken up and stored may be stretched, if necessary, by heating or the like.

[0039] 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 fluororesin crystals in the film in the stretching direction, facilitating molecular chain alignment. This reduces the haze of the fluororesin piezoelectric film and makes 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 very 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 in the polarization process. Setting the stretching ratio to 8.0 times or less makes the fluororesin film less likely to break during the stretching process.

[0040] 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 125°C or higher and 165°C or lower, preferably 125°C or higher and 160°C or lower, more preferably 130°C or higher and 160°C or lower, even more preferably 130°C or higher and 155°C or lower, and particularly preferably 130°C or higher and 150°C or lower. Stretching at high temperatures facilitates molecular motion of the molecular chains, thereby reducing the stretching stress generated during stretching. This reduces scission of the molecular chains 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 bonding energy between the atoms of the molecular chains efficiently improves the breaking strength of the film, thereby increasing the breaking strength of the film. Furthermore, by increasing the surface temperature of the rolls during stretching, the film is stretched uniformly, preventing localized crystallization and arranging the molecular chains uniformly throughout the film, which suppresses light scattering caused by differences in the degree of crystallinity within the film and reduces internal haze.

[0041] (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.

[0042] 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.

[0043] The stretching step and the polarization step may be carried out simultaneously, or the polarization step may be carried out after the stretching step.

[0044] 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.

[0045] [Fluoroplastic piezoelectric film] The fluororesin piezoelectric film thus produced has a high piezoelectric constant and is also highly transparent.

[0046] Fluorine resin piezoelectric film has a piezoelectric constant d 33 is preferably 7.0 pC / N or more and 40.0 pC / N or less, more preferably 8.0 pC / N or more and 40.0 pC / N or less, even more preferably 10.0 pC / N or more and 35.0 pC / N or less, still 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.

[0047] 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 Measurements are made in accordance with ISO 19622:2018, the test method for piezoelectric constant measurement. Specifically, a piezoelectric constant measurement 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 of the fluororesin piezoelectric film is 33 The measurement is carried out on the polarization surface of the fluorine-based resin piezoelectric film. The electric charge in the range including the intersection of the diagonal lines of the piezoelectric film is measured by the above method, and the piezoelectric constant d is calculated from the measured value. 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 33If 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.

[0048] 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%, particularly preferably 0.1% to 0.8%, and most preferably 0.1% to 0.6%. The lower the internal haze, the more transparent the fluororesin piezoelectric film.

[0049] The haze of the fluororesin piezoelectric film is preferably 0.0% to 10.0%, more preferably 0.0% to 5.0%, and even more preferably 0.0% to 3.0%. The lower the haze, the more transparent the fluororesin piezoelectric film.

[0050] The internal haze of a fluororesin piezoelectric film is determined by forming a coating layer 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.

[0051] The haze of the fluororesin piezoelectric film is measured in the same manner as the internal haze, except that no coating layer is formed on both sides of the piezoelectric film.

[0052] 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.

[0053] The breaking strength of the fluororesin piezoelectric film was measured in accordance with JIS K 7127:1999 by cutting a strip of film A 50 mm long and 10 mm wide, and a strip of film B 50 mm long and 10 mm wide 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.

[0054] The retardation of the fluororesin piezoelectric film is preferably 100 nm or more and 3000 nm or less, more preferably 500 nm or more and 2500 nm or less, and more preferably 700 nm or more and 2000 nm or less. 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 process.

[0055] 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.

[0056] 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:

[0057] 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 2Less than 0 pieces / 0.25m is more preferable. 2 More than 1 piece / 0.25m 2 The following is even more preferred:

[0058] 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:

[0059] 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.

[0060] 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 marked using transmitted light, 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.

[0061] 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.

[0062] The thickness of fluororesin piezoelectric film is typically measured using a micrometer (JIS C 2151:2019), but can also be measured using other known methods, such as a laser displacement meter, a capacitance displacement meter, or an infrared meter. Specifically, the thickness of a 20mm x 20mm piece of film cut from the area including the intersection of the diagonal lines of the fluororesin piezoelectric film is measured, and this value is used as the representative thickness of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300mm or longer, a 300mm square is arbitrarily assumed, and the thickness of a 20mm x 20mm piece of film cut from the area including the intersection of the diagonal lines of the rectangular film is measured. If both sides of the piezoelectric film are 300mm or longer, a 300mm square is arbitrarily cut on the piezoelectric film, and the measurement range is determined according to the method described above.

[0063] [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.

[0064] [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]

[0065] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0066] 1. Preparation of Fluoropolymer Piezoelectric Film Films 1 to 8, all of which were fluorine-based resin piezoelectric films, were prepared by the following procedure.

[0067] 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.

[0068] 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.

[0069] The melt temperature was the maximum temperature in the conduit from the extruder to the filtration device.

[0070] 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 the surface with the surface of a cooling roll set at 70°C, yielding an unstretched film with a thickness of 190μm.

[0071] The unstretched film was introduced into a uniaxial stretching device equipped with multiple metal rolls and pinch rolls, and stretched 6.5 times in the machine direction (MD) at 130°C by adjusting the surface temperature and rotation speed ratio of each roll. A voltage of 10.8 kV was then applied from the surface of the film in the thickness direction to obtain Film 1.

[0072] 1-2.Film 2 Film 2 was obtained in the same manner as film 1, except that the surface temperature of the cooling roll was 50°C, the stretching temperature was 140°C, the stretching ratio was 7.5 times, and the applied voltage was 12.8 kV.

[0073] 1-3.Film 3 Film 3 was obtained in the same manner as in film 2, except that the stretching temperature was 150° C., the stretching ratio was 7.1 times, and the applied voltage was 12.0 kV.

[0074] 1-4.Film 4 Film 4 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 and extruded into a film from a T-die without using a polymer filter, the surface temperature of the cooling roll was set to 110°C, the draw ratio was set to 5.3 times, and the applied voltage was set to 8.4 kV.

[0075] 1-5.Film 5 Film 5 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 surface temperature of the cooling roll was 110°C, the stretching temperature was 130°C, the stretching ratio was 4.4 times, and the applied voltage was 7.1 kV.

[0076] 1-6.Film 6 Film 6 was obtained in the same manner as Film 5, except that the surface temperature of the cooling roll was 130°C, the stretching temperature was 110°C, the stretching ratio was 4.2 times, and the applied voltage was 7.5 kV.

[0077] 1-7.Film 7 Film 7 was obtained in the same manner as in the production of Film 5, except that the stretching temperature was 120° C., the stretching ratio was 4.8 times, and the applied voltage was 7.4 kV.

[0078] 1-8. Film 8 (unable to be produced) An attempt was made to produce Film 8 in the same manner as Film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used; however, the resin clogged the filter, and no film could be obtained.

[0079] 2. Evaluation of fluororesin piezoelectric film Unless otherwise specified, each of the obtained films 1 to 7 was cut into a square film with a side of 300 mm, and the piezoelectric constant d 33 The haze, number of foreign particles, and thickness were measured, and the values ​​were used as representative values.

[0080] 2-1. 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.

[0081] 2-2. Hayes The haze was measured using a haze meter (NDH7700SP II, manufactured by Nippon Denshoku Industries Co., Ltd.) In accordance with ISO 14782:2021, the haze was measured in an area including the intersection of the diagonals of the square film on which no coating layer was formed, and the measured value was used as the representative haze value of the fluororesin piezoelectric film.

[0082] 2-3. 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.

[0083] 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 square 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.

[0084] 2-5. 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 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.

[0085] 2-6. 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.

[0086] 3.Results The preparation conditions and evaluation results of each film are shown in Tables 1 and 2.

[0087] [Table 1]

[0088] [Table 2] [Industrial Applicability]

[0089] The fluororesin piezoelectric film according to the present invention is useful as a highly transparent piezoelectric film.

Claims

1. a step of thermally melting a resin composition containing a fluorine-based resin; a step of extruding the heat-molten resin composition to form a film; stretching the formed fluorine-based resin 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; a step of polarizing the stretched fluororesin film by applying a voltage of 7.0 kV or more and 50.0 kV or less; having A method for manufacturing a fluorine-based resin piezoelectric film.

2. The cooling temperature in the film-forming step is 125°C or less. A method for producing the fluorine-based resin piezoelectric film according to claim 1 .

3. The resin composition was subjected to a constant temperature of 260°C and a shear rate of 50 s －１ 3. The method for producing a fluorine-based resin piezoelectric film according to claim 1, wherein the melt viscosity η measured by the method is 600 Pa·s or more and 4000 Pa·s or less.

4. a step of filtering the heat-melted resin composition through a filter having a filtration accuracy of 10 μm or more and 40 μm or less, The film-forming step is performed after the filtering step. A method for producing the fluorine-based resin piezoelectric film according to claim 1 .

5. The resin composition contains vinylidene fluoride resin as a main component. A method for producing the fluorine-based resin piezoelectric film according to claim 1 .

6. Piezoelectric constant d ３３ Produce a film having a modulus of 7.0 pC / N or more and 40.0 pC / N or less. A method for producing the fluorine-based resin piezoelectric film according to claim 1 .

7. 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 Manufacture a film that is: A method for producing the fluorine-based resin piezoelectric film according to claim 1 .

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