Fluorine-based resin film and method for producing the same

Abstract

To provide a fluorine-based resin film having high transparency.SOLUTION: A fluorine-based resin film has melt viscosity η measured at a measurement temperature of 260°C and a shear rate 50 s-1 at the time of measurement of 600 Pa s or more and 4,000 Pa s or less, and a lamellar long period determined by a small angle X-ray scattering method of 11.5 nm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Fluorine-based resin films have excellent properties such as weather resistance, heat resistance, contamination resistance, chemical resistance, solvent resistance, mechanical properties, and ease of secondary processability, and are therefore used in a variety of applications such as interior and exterior building components, surface materials for molded products, surface or back surface protective materials for solar cell modules, fuel cell components, etc. In particular, because of their high weather resistance, contamination resistance, and chemical resistance, they are expected to be used in protective films and optical components, and there is a high demand for highly transparent fluororesin films.

[0003] It is also known that a fluororesin film can be transformed into a piezoelectric film by polarization treatment. The piezoelectric film is required to be transparent since it is used in touch panels and the like. The transparency of the piezoelectric film is largely determined by the transparency of the fluororesin film before the piezoelectric treatment, so there is a demand for the development of a highly transparent fluororesin film. In this specification, the term "fluororesin piezoelectric film" or "piezoelectric film" refers to a film having a piezoelectric constant d measured by the method described below. 33 This means a fluorine-based resin film having a modulus of 1.0 pC / N or more.

[0004] Known methods for producing fluororesin films 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]

[0005] [Patent Document 1] International Publication No. 2015 / 064324 [Patent Document 2] Japanese Patent Application Publication No. 05-102548

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the solution casting method, since a large amount of polar organic solvent is used for dissolving the fluororesin, it is necessary to recover the solvent, which not only increases the manufacturing cost, but also requires consideration of the impact of the organic solvent on the working environment and the natural environment. In addition, since a polar organic solvent is used for dissolving the fluororesin, if the solvent remains in the fluororesin film, there is a risk that polarization in the manufacturing process of the piezoelectric film will be hindered. Therefore, it is preferable to form a film by extruding a heat-melted fluororesin without using an organic solvent.

[0007] On the other hand, in the method of heat-melting a fluororesin and extruding it, if it is heated to a high temperature, there is a risk that the resin will be denatured, resulting in problems such as the generation of decomposition products (foreign substances).

[0008] In order to perform extrusion molding by the heat melting method while suppressing denaturation such as decomposition of the resin, it is preferable to use a fluororesin having a low melt viscosity. However, the fluororesin film formed by extruding the heat-melted resin has a problem of low transparency.

[0009] The present invention has been made in view of the above problems, and an object thereof is to provide a fluororesin film having high transparency, particularly high transparency even after stretching and polarization treatment, and a method for producing the same.

Means for Solving the Problems

[0010] One embodiment of the present invention for solving the above problems relates to a fluororesin film of the following [1] to [5]. [1] A fluororesin film having a melt viscosity η measured at a measurement temperature of 260 ° C and a shear rate of 50 s -1 during measurement of 600 Pa·s or more and 4000 Pa·s or less, wherein the lamellar long period determined by the small-angle X-ray scattering method is 11.5 nm or less, Fluorine-based resin film. [2] The fluorine-based resin film according to [1], which is a film for a piezoelectric film and has a thickness of 80 μm or more and 1000 μm or less. [3] The fluorine-containing resin film according to [1] or [2], which contains a structural unit derived from vinylidene fluoride 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-containing resin film according to any one of [1] to [3] below: [5] The fluorine-containing resin film according to any one of [1] to [4], wherein the surface roughness in height Rz measured in accordance with JIS B 0601:2001 on the surface with a smaller roughness is 0.50 μm or less.

[0011] One embodiment of the present invention for solving the above problems relates to the following methods for producing a fluorine-based resin film [6] to [7]. [6] Measurement temperature: 260°C, shear rate: 50 s -1 a step of heating and melting a fluororesin having a melt viscosity η measured by the method of 600 Pa s or more and 4000 Pa s or less; a step of extruding the heated and melted fluorine-based resin to form a film; a step of cooling the formed film by contacting it with a cooling roll having a surface temperature of 125°C or less; The method for producing a fluorine-based resin film according to any one of [1] to [5], comprising: [7] In the heat-melting step, the fluororesin is melted at a temperature that is 75°C or more higher than the melting point of the fluororesin and 105°C or less higher than the melting point of the fluororesin; The method further comprises a step of filtering the fluorine-based resin melted at the temperature through a filter with a filtration accuracy of 10 μm or more and 40 μm or less. [6] A method for producing a fluorine-based resin film according to [6]. [Effects of the Invention]

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

Brief Description of the Drawings

[0013]

Figure 1

Embodiments for Carrying Out the Invention

[0014] [Fluororesin Film] One embodiment of the present invention relates to a fluororesin film.

[0015] The fluororesin film may be a film mainly composed of a fluororesin. The fluororesin is a resin obtained by polymerizing a monomer composed of an olefin containing fluorine. Containing a fluororesin as a main component means that the content ratio of the structural unit derived from the monomer composed of an olefin containing fluorine to the total mass of the fluororesin film is 50% by mass or more. The ratio of the content of the fluororesin to the total mass of the fluororesin film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

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

[0017] Among these, from the viewpoint of facilitating the polarization of the fluororesin film, fluororesins obtained by polymerizing monomers containing VDF are preferred, and homopolymers of VDF, copolymers of VDF and HFP, copolymers of VDF and trifluoroethylene, copolymers of VDF and TFE, and copolymers of VDF, trifluoroethylene, TFE, and CTFE are more preferred, and homopolymers of VDF are even more preferred. These fluororesins may be used alone or in combination of multiple types.

[0018] The fluororesin film preferably contains a resin having a structural unit derived from VDF as a main component, and a homopolymer of VDF is most preferred. Containing a structural unit derived from VDF as a main component means that the content ratio of the structural unit derived from VDF to the total mass of the fluororesin film and the fluororesin piezoelectric film is 50% by mass or more. The content ratio of the structural unit derived from VDF to the total mass of the fluororesin film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

[0019] Among them, since a fluororesin film is expected to have high piezoelectricity, it is more preferable that the content of the homopolymer of vinylidene fluoride is higher. The content of the homopolymer with respect to the total mass of the fluororesin film is preferably 50% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less.

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

[0021] The fluororesin film has a melt viscosity measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 of 600 Pa·s or more and 4000 Pa·s or less, preferably 600 Pa·s or more and 3500 Pa·s or less, and more preferably 600 Pa·s or more and 2400 Pa·s or less. The lower the melt viscosity of the resin, the lower the temperature at which it can be extruded into a film. However, for a resin with a low melt viscosity, since the movement of the molecular chain is easy, the selective arrangement of the molecular chain progresses during film formation, and crystals are likely to grow. When the crystals of the resin grow, the refractive indices of light in the crystalline part and the amorphous part are different, so the light scattering at the interface between the crystalline part and the amorphous part increases, and the transparency of the film decreases. On the other hand, even for a fluororesin with a low melt viscosity, after film formation, by rapidly cooling the film, the crystal growth of the film can be suppressed, the light scattering can be reduced, and a film with high transparency can be obtained. The lamellar periodic length determined by X-ray diffraction method is a good index indicating the crystallite size of the film, and the smaller the crystallite size, the smaller the lamellar long period. Also, the lower the melt viscosity of the resin, the easier it is to filter through a filter, and the melting temperature for film formation can be kept low, so that spot-like irregularities on the film surface due to resin modification are less likely to occur.

[0022] The melt viscosity is measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho, Ltd., Capillograph 1D), with a capillary die having an inner diameter of φ1 mm and a tube length of 10 mm, the viscosity is measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 as the measured viscosity.

[0023] The fluororesin film has a lamellar long period determined by the small-angle X-ray scattering method of 11.5 nm or less, preferably 8.0 nm or more and 11.0 nm or less, and more preferably 8.5 nm or more and 10.8 nm or less. The shorter the lamellar long period, the smaller the crystallite size of the resin constituting the fluororesin film. By suppressing crystal growth and reducing the crystallite size, the transparency of the fluororesin film and the transparency of the piezoelectric film to be produced therefrom can be enhanced. Further, the inventors have found that the shorter the lamellar long period, the higher the piezoelectric constant d of the piezoelectric film obtained by polarizing the fluororesin film 33 is exhibited. This is presumably because the shorter the film with a lamellar long period, the higher the amount of orientation polarization in the polarization process, so that the piezoelectric constant d of the obtained piezoelectric film 33 can be increased.

[0024] The lamellar long period can be calculated by measuring the X-ray diffraction pattern of the fluororesin film with a small-angle X-ray scattering measuring device and using the black reflection method. Specifically, the fluororesin film is placed on the small-angle X-ray scattering measuring device, and irradiated with CuKα rays (wavelength λ = 0.15418 nm) monochromatized by a Ni filter as the X-ray source, and a two-dimensional diffraction image is measured using a two-dimensional detector. Then, the X-ray scattering intensity (B(2θ)) due to air is subtracted from the X-ray scattering (diffraction) intensity (A(2θ)) measured by the above method, and the X-ray scattering (diffraction) intensity (I(2θ)) with respect to the diffraction angle 2θ in the range of diffraction angle 2θ of 0.200° to 2.000° is obtained. The background of I(2θ) is obtained by linear approximation from the diffraction angle 2θ showing the lowest X-ray intensity and its X-ray intensity in the range of 2θ = 0.200° to 0.450°, and the X-ray intensity at 2θ = 2.000°. Next, in the X-ray scattering (diffraction) intensity curve obtained by subtracting the background intensity from I(2θ), the diffraction angle 2θ at which the X-ray intensity becomes maximum (peak top) is obtained, and the lamellar long period L is obtained by the black reflection method shown in Equation 1.

[0025]

Number

[0026] The thickness of the fluororesin film is preferably 80 μm or more and 1000 μm or less, more preferably 100 μm or more and 500 μm or less, still more preferably 100 μm or more and 300 μm or less, still more preferably 120 μm or more and 200 μm or less, and particularly preferably 120 μm or more and 180 μm or less. The thicker the film, the more advantageous for mechanical physical properties such as wear resistance, durability, and insulation. On the other hand, the thinner the film, the more advantageous for optical properties such as transparency and cost.

[0027] The thickness of the fluororesin film is generally measured by a method using a micrometer (JIS C 2151:2019), but it can also be measured by known methods such as a method using a laser displacement meter, a method using a capacitance displacement meter, and a method using infrared rays. Specifically, when the intersection of the diagonals of a rectangular fluororesin film is taken as the center point A and the long side direction is taken as the width direction, thickness measurements are performed at a total of three measurement points: the center point A (measurement point), and two measurement points each set at positions 30 mm away from the center point A in both end directions on the line segment passing through the center point A and the midpoint A parallel to the long side. The average value of these is taken as the thickness of the fluororesin film. In the case where the shape of the fluororesin film is circular or polygonal, the film is cut out into a rectangle so as to have the maximum area, and the thickness is measured by the above method.

[0028] Foreign substances in the resin tend to be more likely to be detected by visual inspection of the film surface as the film becomes thinner. Therefore, for the quantification of the amount of foreign substances in the film, the film thickness used for foreign substance measurement is set to 40 μm or less. When a fluororesin film with a film thickness of 40 μm or less is viewed in plan view, the number of foreign substances with a size of 100 μm or more is 7 pieces / 0.25 m 2 The following is preferable, 0 pieces / 0.25 m 2 5 pieces / 0.25 m or less 2 The following is more preferable, 0 pieces / 0.25 m 2 3 pieces / 0.25 m or less 2 The following is even more preferable, 0 pieces / 0.25 m 2 1 piece / 0.25 m or more 2 The following is particularly preferable.

[0029] When a fluororesin film with a film thickness of 40 μm or less is viewed in plan view, the number of foreign substances with a size larger than 200 μm is 0 pieces / 0.25 m 2 3 pieces / 0.25 m or more 2 The following is preferable, 0 pieces / 0.25 m 2 2 pieces / 0.25 m or more 2 The following is more preferable, 0 pieces / 0.25 m 2 1 piece / 0.25 m or more 2 The following is even more preferable, 0 pieces / 0.25 m2 is particularly preferred.

[0030] When a fluororesin film with a film thickness of 40 μm or less is viewed in a plan view, the number of foreign matters with a size of less than 100 μm is 0 pieces / 0.25 m 2 or more and 50 pieces / 0.25 m 2 or less is preferred, and 0 pieces / 0.25 m 2 or more and 25 pieces / 0.25 m 2 or less is more preferred, and 0 pieces / 0.25 m 2 or more and 16 pieces / 0.25 m 2 or less is even more preferred.

[0031] The fewer these foreign matters are, the more the transparency of the fluororesin film is improved, and when stretching treatment or polarization treatment of the fluororesin film is performed, stretching and polarization can be made uniform.

[0032] The number of these foreign matters is obtained by continuously cutting out 25 rectangular films (observation pieces) from a fluororesin film with a film thickness of 40 μm or less so that they are adjacent to each other, and obtaining the sum of the numbers of foreign matters measured from each of the observation pieces. For a fluororesin film with a thickness exceeding 40 μm, after stretching until the film thickness becomes 35 μm to 40 μm or less, the number of foreign matters is measured by the method described below. Specifically, 25 observation pieces having a size of 0.010 m 2 (100 mm × 100 mm) are cut out from the fluororesin film so as to be in adjacent positions continuously. Then, the sum of the numbers of foreign matters measured from each of the observation pieces is obtained. At this time, the foreign matters are marked by observing with transmitted light, and the size of the foreign matters is obtained by observing the marked portions with a microscope. The size of the foreign matters is the arithmetic mean value of the maximum width and the minimum width of the foreign matters.

[0033] The surface height roughness Rz of the fluororesin film is preferably 0.50 μm or less, more preferably 0.01 μm or more and 0.50 μm or less, still more preferably 0.01 μm or more and 0.45 μm or less, and particularly preferably 0.01 μm or more and 0.40 μm or less. The smoother the surface of the fluororesin film, the less likely it is to generate haze on the film surface and the better the transparency obtained. Also, a small surface height roughness Rz indicates that there are few irregularities such as wrinkles on the film.

[0034] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter (Keyence Corporation, Shape Analysis Laser Microscope VK-X260) compliant with JIS B 0601:2001 is used. Specifically, when the intersection of the diagonal lines of a rectangular fluororesin film is taken as the center point A and the long side direction is taken as the width direction, for a total of three measurement points including the center point A (measurement point) and two measurement points set at positions 30 mm away from the center point A in both end directions on the line segment passing through the center point A and the midpoint A parallel to the long side, the surface height roughness Rz is measured, and the average value of these is taken as the surface height roughness of the fluororesin film. In the case where the shape of the fluororesin film is circular or polygonal, the film is cut out into a rectangle so as to have the maximum area, and the surface height roughness is measured by the above method. Also, the contact surface of the fluororesin film with the cooling roll tends to have a smaller surface roughness Rz compared to the non-contact surface with the roll. This is because the irregularities on the surface of the fluororesin film become smaller due to being pressed against the cooling roll. Here, the measurement result for the surface with a small surface roughness Rz (the contact surface with the cooling roll) is taken as the above surface height roughness Rz.

[0035] The fluororesin film preferably has a haze per unit thickness of 0.00% / μm or more and less than 0.35% / μm, more preferably 0.00% / μm or more and 0.30% / μm or less, and even more preferably 0.00% / μm or more and 0.25% / μm or less. When it is a piezoelectric film subjected to stretching and polarization treatment, the haze of the fluororesin film is preferably 10.0% or less, more preferably 0.0% or more and 5.0% or less, and even more preferably 0.0% or more and 3.0% or less. The lower the haze, the higher the transparency of the fluororesin film and the piezoelectric film produced therefrom.

[0036] The haze per unit thickness of the fluororesin film is obtained by dividing the haze measured in accordance with JIS K 7136:2000 by the thickness of the film. Specifically, a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II) is used. When the intersection of the two diagonal lines of the fluororesin film is the center point A and the long side direction is the width direction, the haze is measured at a total of three measurement points: the center point A (measurement point), and two measurement points set at positions 30 mm away from the center point A in both end directions on the line segment passing through the center point A and the midpoint A parallel to the long side. The average value of these is taken as the haze of the fluororesin film. When the shape of the fluororesin film is circular or polygonal, the film is cut out into a rectangle so as to have the maximum area, and the haze is measured by the above method. The haze of the fluororesin film is divided by the thickness of the fluororesin film to obtain the haze per unit thickness of the fluororesin film.

[0037] The haze of the piezoelectric fluororesin film is measured basically in the same manner as the above method except for the setting of the measurement points. Specifically, the advancing axis direction obtained by measuring the birefringence of the fluororesin film is defined as the width direction. An arbitrary point on the midpoint line connecting both ends in the width direction of the film is taken as point A. The haze is measured at a total of three measurement points: point A (measurement point), and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A. The average value of these is taken as the haze of the fluororesin piezoelectric film.

[0038] The measurement of the above birefringence is performed by the parallel Nicol rotation method using a light source having a wavelength of 587.8 nm.

[0039] In addition, the fluororesin film may contain a resin other than the fluororesin or other additives as long as the above physical properties can be satisfied.

[0040] Examples of the resin other than the fluororesin include polyesters such as polycarbonate, polyethylene terephthalate, and polyethylene naphthalate that can be added to enhance flexibility, silicone resin, polyether, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, acrylic resin, epoxy resin, polyethylene oxide, polypropylene oxide, polyphenylene oxide, polyphenylene sulfide, polyamide, polyimide, polyamideimide, polystyrene, and polybenzimidazole that can be added to enhance strength, and odd polyamides, cyanopurans, and copper phthalocyanine-based polymers that can be added to further enhance dielectric properties.

[0041] [Manufacturing Method of Fluororesin Film] The manufacturing method of the above fluororesin film is not particularly limited, but preferably includes a step of forming a film from the heat-melted resin by extrusion molding.

[0042] For example, the fluororesin film can be produced by performing a step of heating and melting the above-described fluororesin (melting step) and a step of forming the filtered fluororesin into a film (film-forming step). At this time, a step of filtering the fluororesin melted in the melting step (filtering step) may be further performed.

[0043] The obtained fluororesin film may be subjected to a stretching step (stretching step) as needed. In addition, a polarization step (polarization step) may be performed as needed to impart piezoelectricity to the film. The fluororesin film to which piezoelectricity has been imparted can be used as a piezoelectric film.

[0044] (Melting process) In the melting step, the fluororesin is heated and melted. This step can be carried out, for example, by melt-kneading the fluororesin using an extruder.

[0045] If the fluororesin melted in the melting step contains a solvent component, the remaining solvent component without volatilization may hinder polarization in a subsequent step, so the content of the solvent component in the fluororesin is preferably small, preferably 1 mass % or less, more preferably 0.1 mass % or less, based on the total mass of the fluororesin. 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 fluororesin.

[0046] The melting temperature of the fluororesin is preferably at least 75°C higher than the melting point of the fluororesin but not more than 105°C higher than the melting point, more preferably at least 75°C higher but not more than 100°C higher, even more preferably at least 80°C higher but not more than 100°C higher, and particularly preferably at least 85°C higher but not more than 95°C higher. Setting the melting temperature at least 75°C higher than the melting point of the fluororesin can reduce the viscosity of the fluororesin to a level that allows it to be filtered in the subsequent process. Setting the melting temperature at least 105°C higher than the melting point of the fluororesin can suppress decomposition and condensation of the fluororesin due to heating, thereby suppressing the generation of decomposition products and the like. Suppressing the generation of the decomposition products and the like reduces the amount of foreign matter in the fluororesin film and improves the transparency and smoothness of the fluororesin film. Furthermore, suppressing the generation of the decomposition products and the like can suppress filter clogging caused by these products when filtering the fluororesin, thereby improving the filtration efficiency of the fluororesin.

[0047] According to the findings of the present inventors, fluororesins need to be heated to a high temperature in order to reduce the viscosity of resins with a high melt viscosity to a filterable viscosity. When heated to a high temperature, decomposition, condensation, etc. are likely to occur, and clogging of the filter is likely to occur. Therefore, it has been difficult to melt and filter fluororesins. In contrast, the present inventors have found that fluororesins can be efficiently filtered by setting the melting temperature of fluororesins having an appropriate melt viscosity within the above range. By setting the melting temperature of the fluororesin within the above range and filtering the fluororesin whose viscosity has been reduced by melting, the fluororesin can be filtered without using a polar solvent. Further, by not using a solvent, it is less likely to cause inhibition of polarization by the solvent during polarization treatment due to the polar solvent remaining in the fluororesin film. Furthermore, since a polar solvent is basically not used in the film-forming step, the burden on the working environment and the natural environment can be reduced, and the manufacturing cost for recovering the polar solvent can be reduced.

[0048] (Filtering step) In the filtering step, the fluororesin melted and reduced in viscosity by the melting step is filtered. The filtering method is not particularly limited, and it is only necessary to pass the melted fluororesin through a filter, and known filter types such as pleated type and leaf disk type filters can be used.

[0049] The filtration accuracy of the filter used in the filtering step is preferably 10 μm or more and 40 μm or less, more preferably 10 μm or more and 35 μm or less, and even more preferably 15 μm or more and 30 μm or less. When a filter with a filtration accuracy of 10 μm or more is used, the filtration of the melted fluororesin becomes easy, and the filtration pressure does not become too high, so that the filtration time can be shortened. Further, by using a filter with a filtration accuracy of 10 μm or more, the filtration pressure does not become too high, and the filtration time can be shortened. By using a filter with a filtration accuracy of 40 μm or less, foreign matters in the fluororesin can be sufficiently removed, and a fluororesin film with few foreign matters can be obtained.

[0050] Note that the filtration of the fluororesin is performed by a filter having a multilayer structure composed of a plurality of layers with different shapes, mesh sizes, etc. The filtration accuracy of the filter for filtering the fluororesin indicates the filtration efficiency of the filter, that is, the ability of the filter to separate particles of a given size with a predetermined filtration efficiency. For example, in this specification, a filtration accuracy of 10 μm means that the filter can separate particles with a size of 10 μm or more with a filtration efficiency of 95% or more.

[0051] In this step, the filtration of the fluororesin, which may involve passing the resin composition through a plurality of filters, may be performed multiple times. For example, coarse foreign matter may be removed using a filter with a low filtration accuracy (a large numerical value of filtration accuracy) arranged in the front stage, and then finer foreign matter may be removed using a filter with a high filtration accuracy (a small numerical value of filtration accuracy) arranged in the rear stage. The filtration accuracy at this time shall be the value of the filter with the highest filtration accuracy.

[0052] The filter may be arranged between the extruder and the die for performing the film-forming process. Alternatively, the filter may be arranged in an extruder or a melt-kneading device different from the extruder for performing the film-forming process, and the fluororesin filtered by the filter may be introduced into the extruder for performing the film-forming process.

[0053] (Film-forming process) In the film-forming process, the fluororesin filtered in the filtration process is formed into a film.

[0054] The film-forming method is not particularly limited, and known methods such as extruding the melted and filtered fluororesin from a T-die and cooling it by bringing it into contact with a cooling roll can be used. In this embodiment, the surface temperature of the cooling roll at this time is set to 125°C or lower. By rapidly cooling the film of the fluororesin extruded by the cooling roll with a low surface temperature, a large number of minute crystals can be formed, and the lamellar long period of the obtained fluororesin film can be shortened. According to the findings of the present inventors, in the fluororesin having a low melt viscosity used in this embodiment, the crystallization rate is different from that of the fluororesin having a higher melt viscosity, and the lamellar long period of the obtained fluororesin film is likely to change greatly depending on the temperature of the cooling roll. Therefore, in this embodiment, it is considered important to adjust the temperature of the cooling roll. The surface temperature of the cooling roll is preferably 5°C or higher and 110°C or lower, more preferably 20°C or higher and 80°C or lower, and even more preferably 30°C or higher and 60°C or lower.

[0055] In this way, the fluororesin film according to this embodiment can be obtained. The produced fluororesin film may be wound up as it is and stored, or may be conveyed to a stretching step, a polarization step, or the like, which are subsequent steps.

[0056] (Stretching step) In the stretching step, the above-mentioned formed fluororesin film is uniaxially stretched. In the stretching step, the fluororesin film formed in the film-forming step may be stretched as it is, or the fluororesin film wound up and stored once may be stretched by heating or the like as necessary.

[0057] In the case of uniaxial stretching, the stretching direction is not limited. In the stretching process for mass production, it is preferable to continuously uniaxially stretch the fluororesin film. Specifically, while conveying the fluororesin film by a plurality of rolls, it can be uniaxially stretched in the conveying direction (MD direction) by passing between a plurality of rolls with different rotational speeds. The stretching ratio (the ratio in the MD direction) is preferably 2.5 times or more and 6.0 times or less, more preferably 3.5 times or more and 5.0 times or less. By setting the stretching ratio to 2.5 times or more, the ratio of the β-crystals described later increases sufficiently, and it becomes easier to obtain a fluororesin film with a high piezoelectric constant through the polarization process. By setting the stretching ratio to 6.0 times or less, breakage of the fluororesin film in the stretching process is less likely to occur. In the film uniaxially stretched in the MD direction, the MD direction is the slow axis, and the direction perpendicular to the stretching direction (TD direction) is the fast axis direction.

[0058] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. In the fluororesin containing a homopolymer or copolymer of VDF, a transition from α-crystals to β-crystals occurs due to the stretching process, and the ratio of β-crystals increases. By applying a DC voltage to the fluororesin film with an increased ratio of polar β-crystals, a fluororesin film with a high piezoelectric constant can be obtained.

[0059] The DC voltage to be applied 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.

[0060] The stretching process and the polarization process may be performed simultaneously, or the polarization process may be performed after the stretching process.

[0061] After the polarization process, the fluororesin film can be wound into a roll for storage, transfer, etc.

[0062] [Applications] The above-mentioned fluororesin film can be used for various applications such as touch sensors, touch panels, piezoelectric films for actuators, interior and exterior materials for automobiles, furniture, and building materials, protective films, and retardation films.

[0063] [Other Embodiments] It should be noted that the above-mentioned embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-mentioned embodiments within the scope of the core technical idea.

Examples

[0064] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0065] 1. Preparation of Fluororesin Film Films 1 to 7 and Film 10, all of which are fluororesin films, were prepared according to the following procedures.

[0066] 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 (manufactured by Toyo Seiki Seisakusho Co., Ltd., Capilograph 1D) was used, and the viscosity was measured at a measurement temperature of 260°C and a shear rate of 50 s -1 using a capillary die with an inner diameter of φ1 mm × tube length of 10 mm.

[0067] The melting point of the fluororesin was determined by enclosing 5 mg of the measurement sample in an aluminum pan and placing it in a differential scanning calorimeter (manufactured by Shimadzu Corporation, DSC-60A). Under a nitrogen atmosphere, the temperature was raised from room temperature to 230°C at a rate of 10°C / min, and the maximum peak temperature of the endothermic curve in the obtained DSC curve was taken as the melting point.

[0068] The melting temperature during film production was taken as the maximum temperature in the conduit from the extruder to the filtration device.

[0069] 1-1. Film 1 A polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 800 Pa·s and a melting point of 173°C was melted using a single-screw extruder with a diameter of φ50 mm, then filtered through a pleated polymer filter with a filtration accuracy of 20 μm, and extruded in film form from a T-die. At this time, the melting temperature and the filtration temperature were 260°C. The extruded film was brought into contact with a cooling roll having a surface temperature of 50°C while maintaining a temperature of 150°C or higher, to obtain Film 1, which is an unstretched fluororesin film with a thickness of 183 μm.

[0070] The obtained unstretched film was led to a uniaxial stretching device equipped with a plurality of metal rolls and pinch rolls, and the rotation speed ratio of each roll was adjusted to stretch it 4.6 times in the flow direction. Further, a voltage of 8.0 kV was applied from the surface of the film in the thickness direction to obtain Piezoelectric Film 1.

[0071] 1-2. Film 2 Film 2 was obtained in the same manner as the production of Film 1, except that no filter was attached to the single-screw extruder. Also, stretching and polarization treatment were performed in the same manner as the production of Piezoelectric Film 1, except that the voltage applied after stretching was set to 8.8 kV, to obtain Piezoelectric Film 2 from Film 2.

[0072] 1-3. Film 3 Film 3 was obtained in the same manner as the production of Film 1, except that no filter was attached to the single-screw extruder and the surface temperature of the cooling roll was set to 70°C. Also, stretching and polarization treatment were performed in the same manner as the production of Piezoelectric Film 1, except that the voltage applied after stretching was set to 8.8 kV, to obtain Piezoelectric Film 3 from Film 3.

[0073] 1-4. Film 4 A film 4 was obtained in the same manner as in the preparation of film 1, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used, a pleated polymer filter with a filtration accuracy of 15 μm was used, and the surface temperature of the cooling roll was set to 110°C. Also, stretching and polarization treatments were carried out in the same manner as in the preparation of piezoelectric film 1, except that the stretching ratio was 4.0 times and the voltage applied after stretching was 8.8 kV, to obtain piezoelectric film 4 from film 4.

[0074] 1-5. Film 5 A film 5 was obtained in the same manner as in the preparation of film 1, except that a polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 3400 Pa·s and a melting point of 173°C, which was prepared by mixing PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C and PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C at a mass ratio of 1:1, was used, and the surface temperature of the cooling roll was set to 110°C. Also, stretching and polarization treatments were carried out in the same manner as in the preparation of piezoelectric film 1, except that the stretching ratio was 4.0 times and the voltage applied after stretching was 8.8 kV, to obtain piezoelectric film 5 from film 5.

[0075] 1-6. Film 6 A film 6 was obtained in the same manner as in the preparation of film 1, except that the surface temperature of the cooling roll was set to 130°C. Also, stretching and polarization treatments were carried out in the same manner as in the preparation of piezoelectric film 1, except that the voltage applied after stretching was 8.8 kV, to obtain piezoelectric film 6 from film 6.

[0076] 1-7. Film 7 A film 7 was obtained in the same manner as in the preparation of film 1, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used, the melting temperature was set to 280°C, no filter was attached to the single-screw extruder, and the surface temperature of the cooling roll was set to 140°C. Also, stretching and polarization treatments were carried out in the same manner as in the preparation of piezoelectric film 1, except that the stretching ratio was 4.0 times and the voltage applied after stretching was 8.8 kV, to obtain piezoelectric film 7 from film 7.

[0077] 1-8. Film 8 (Could not be produced) A film 8 was attempted to be produced in the same manner as that of film 1, except that PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C was used. However, the filter became clogged with resin, and a film could not be obtained.

[0078] 1-9. Film 9 (Could not be produced) A film 9 was attempted to be produced in the same manner as that of film 1, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used and a pleated polymer filter with a filtration accuracy of 5 μm was used. However, the filter became clogged with resin, and a film could not be obtained.

[0079] 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 put into 900 ml of n-methylpyrrolidone (NMP). The temperature was raised to 60°C while stirring with a stirrer, and stirring was continued for 6 hours as it was to prepare a resin solution. This resin solution was filtered through a filter with a filtration accuracy of 40 μm. This resin solution was put into an automatic coater to prepare a coated film with a liquid thickness of 600 μm, and it was dried at 120°C for 1 hour to obtain film 10. The surface height roughness Rz of film 10 was large, and it was difficult to accurately detect the amount of foreign matter.

[0080] 2. Evaluation of fluororesin films For the obtained films 1 to 7 and film 10, the lamellar long period, thickness, number of foreign matters, surface height roughness Rz, and haze were measured by the following methods. The lamellar long period, thickness, number of foreign matters, and surface height roughness Rz are the measured values of the unstretched film, and the haze is the measured value of the piezoelectric film.

[0081] 2-1. Lamellar long period The X-ray diffraction patterns of each film were measured with a small-angle X-ray scattering measurement device, and the lamellar long period was calculated by the black reflection method.

[0082] Specifically, each film was placed on a small-angle X-ray scattering measurement device (NANO-Viewer, manufactured by Rigaku Corporation), and irradiated with CuKα rays (wavelength λ = 0.15418 nm) monochromatized by a Ni filter as the X-ray source, and the two-dimensional diffraction image of the film was measured. Then, from the X-ray diffraction (scattering) intensity, the X-ray diffraction intensity (A(2θ)) with respect to the diffraction angle 2θ in the range of 0.200° to 2.000° of the diffraction angle 2θ was obtained. Since the X-ray scattering intensity of air (B(2θ)) is included in A(2θ), B(2θ) was measured in a state where no film was placed, and B(2θ) was subtracted from A(2θ) to obtain the X-ray diffraction intensity (I(2θ)) of the film.

[0083] The background of I(2θ) was determined by linear approximation from the diffraction angle 2θ showing the lowest X-ray intensity and its X-ray intensity in the range of 2θ = 0.200° to 0.450°, and the X-ray intensity at 2θ = 2.000°. Next, the background was subtracted from I(2θ), the diffraction angle 2θ at which the X-ray intensity becomes maximum was obtained, and the lamellar period length was obtained by the black reflection formula of Equation 1.

[0084]

Equation

[0085] (Measurement method of small-angle X-ray scattering (diffraction)) · Measuring device: NANO-Viewer · X-ray source: CuKα (monochromatization: Ni filter) · Applied current: 20 mA, applied voltage: 40 kV · 1st slit: φ0.7 mm, 2nd slit: φ0.6 mm, 3rd slit: φ0.8 mm · Camera length: 830 mm · Detector: Imaging plate · Measurement time: 2 hours · Correction of diffraction angle: Si(111) plane · Shape and size of the sample: With the stretching direction (MD) as the length direction of the sample, the film was cut into samples with a length of 30 mm and a width of 10 mm. The samples were set so that the longitudinal direction of the measurement folder was the same as the sample length (the stretching direction of the film). At this time, one or more samples were stacked and adjusted so that the total thickness was 200 ± 20 μm. · When converting from a two-dimensional diffraction image obtained by an X-ray two-dimensional detector to an X-ray scattering (diffraction) intensity profile with respect to the diffraction angle 2θ, the sampling width was set to 0.001°.

[0086] 2-2. Thickness A digital linear gauge (manufactured by Ono Sokki Co., Ltd., DG525H) and a gauge stand (manufactured by Ono Sokki Co., Ltd., SH-022) were used. Then, with the intersection of the diagonal lines of the rectangular fluororesin film as the center point A and the long side direction as the width direction, thickness measurements were taken at a total of three measurement points: the center point A (measurement point), and two measurement points set at positions 30 mm away from the center point A in both directions along the line segment passing through the center point A and the midpoint A parallel to the long side. The average value of these measurements was taken as the thickness of the fluororesin film.

[0087] 2-3. Number of foreign matters Uniaxial stretching was performed so that the thickness of the fluororesin-based piezoelectric film was 35 μm to 40 μm, and 25 rectangular films (observation pieces) with a size of 0.010 m 2 (100 mm × 100 mm) were cut out from the fluororesin film. The sum of the number of foreign matters measured by observing each observation piece was used to determine the number of foreign matters per 0.250 m 2 At this time, foreign matters were marked by observing with transmitted light, and the size of the marked foreign matters was determined by observing with a microscope. The size of the foreign matter was taken as the arithmetic mean value of the maximum width and the minimum width of the foreign matter. In this way, the number of foreign matters larger than 200 μm, the number of foreign matters with a size of 100 μm or more and 200 μm or less, and the number of foreign matters smaller than 100 μm were determined respectively.

[0088] 2-4. Surface height roughness Rz A surface roughness meter (Keyence Corporation, shape analysis laser microscope VK-X260) compliant with JIS B 0601:2001 was used. Then, the surface roughness of the contact surface of the fluororesin film with the cooling roll (the surface with a smaller Rz) was measured. Specifically, when the intersection point of the diagonal lines of the fluororesin film was taken as the center point A and the long side direction was taken as the width direction, the surface height roughness Rz was measured at a total of three measurement points, namely, the center point A (measurement point), and two measurement points set at positions 30 mm away from the center point A in both end directions on the line segment passing through the center point A and the midpoint A parallel to the long side. The average value of these was taken as the surface height roughness of the fluororesin film.

[0089] 2-5. Haze 2-5-1. Haze per unit thickness of the fluororesin film The haze per unit thickness of the fluororesin film was obtained by dividing the haze measured in accordance with JIS K 7136:2000 by the thickness of the film. Specifically, a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II) was used. Then, when the intersection point of the two diagonal lines of the fluororesin film was taken as the center point A and the long side direction was taken as the width direction, the haze was measured at a total of three measurement points, namely, the center point A (measurement point), and two measurement points set at positions 30 mm away from the center point A in both end directions on the line segment passing through the center point A and the midpoint A parallel to the long side. The average value of these was taken as the haze of the fluororesin film. The haze of the fluororesin film was divided by the thickness of the fluororesin piezoelectric film to obtain the haze per unit thickness of the fluororesin film.

[0090] 2-5-2. Haze of the fluororesin piezoelectric film The haze of the piezoelectric film was measured in accordance with ISO 14782:2021 using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II). Using a KOBRA-HB manufactured by Oji Scientific Instruments, the birefringence of the film cut into 20 mm × 20 mm was measured by the parallel Nicol rotation method using a light source having a wavelength of 587.8 nm, and the fast axis and slow axis were determined from the in-plane birefringence of the film. The slow axis direction coincides with the average direction of the molecular chains that have migrated depending on stretching or extrusion. Since the film used this time was stretched in the flow direction (machine direction) of the film, the MD direction and the slow axis direction coincide. The fast axis direction determined by measuring the birefringence was defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film was defined as point A. Haze was measured at a total of three measurement points: point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, and the average value of these was taken as the haze of the film.

[0091] 3. Results The production conditions and evaluation results of each film are shown in Tables 1 and 2. Note that Film 10 had a large haze and was opaque, and the number of foreign substances could not be measured.

[0092] [Table 1]

[0093] [Table 2]

[0094] 4. Relationship between lamellar long period and piezoelectric constant Film 1 and Film 6 were guided to a uniaxial stretching apparatus equipped with a plurality of metal rolls and pinch rolls, and the rotation speed ratio of each roll was adjusted to stretch 4.6 times in the flow direction. Further, a voltage of 7.0 kV to 10.0 kV was applied from the surface of the film in the thickness direction to polarize the film.

[0095] In accordance with ISO 19622:2018, the piezoelectric constant d of the polarized film was measured. 33 Specifically, a piezoelectric constant measuring device (PiezoTest, piezometer system PM300) was used to hold the fluororesin film as the test piece with a holding force of 1.0 N, and the charge generated when an alternating force with a vibration force of 0.15 N and a frequency of 110 Hz was applied was measured. The charge was measured on the polarization plane of the film, and the piezoelectric constant was calculated using the absolute value of the measured value. The advance axis direction obtained by measuring the birefringence was defined as the width direction, and an arbitrary point on the midpoint line of the line segment connecting both ends in the width direction of the film was defined as point A. For a total of three measurement points, namely point A (measurement point) and two measurement points set at positions 30 mm away from point A in both end directions on the line segment in the width direction passing through point A, the piezoelectric constant d was measured. 33 The average value of the piezoelectric constants obtained from these measurements was taken as the piezoelectric constant d of the film. 33

[0096] The relationship between the applied voltage and the piezoelectric constant d of Film 1 (lamellar long period is 9.6 nm) and Film 6 (lamellar long period is 12.4 nm) is shown in FIG. 1. 33 As shown in FIG. 1, Film 1 with a smaller lamellar long period had a higher piezoelectric constant d than Film 6 with a larger lamellar long period.

[0097] 33

Industrial Applicability

[0098] The fluororesin film according to the present invention has high transparency.​​

Claims

1. Measurement temperature: 260°C, shear rate during measurement: 50 s －１ a fluororesin film having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less A fluororesin film having a lamellar long period of 11.5 nm or less determined by small-angle X-ray scattering method.

2. A film for a piezoelectric film, having a thickness of 80 μm or more and 1000 μm or less. The fluororesin film according to Claim 1.

3. The fluororesin film according to Claim 1, containing a structural unit derived from vinylidene fluoride as a main component.

4. The number of foreign matters having a size, which is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in a plan view, of 100 μm or more is 7 pieces / 0.25 m ２ is as follows The fluororesin film according to Claim 1.

5. The Rz of the surface on the smaller side of the surface height roughness Rz measured according to JIS B 0601:2001 is 0.50 μm or less. The fluororesin film according to Claim 1.

6. Measurement temperature: 260°C, shear rate during measurement: 50 s －１ A step of heating and melting a fluororesin having a melt viscosity η measured at 600 Pa·s or more and 4000 Pa·s or less A step of extruding the heat-melted fluororesin to form a film, and A step of cooling the formed film by bringing it into contact with a cooling roll having a surface temperature of 125°C or less. A method for producing a fluororesin film according to any one of Claims 1 to 5, having the above steps.

7. In the step of heating and melting, the fluororesin is melted at a temperature 75°C or more higher than the melting point of the fluororesin and 105°C or lower. The method for producing a fluororesin film according to Claim 6, further having a step of filtering the fluororesin melted at the above temperature with a filter having a filtration accuracy of 10 μm or more and 40 μm or less. ​

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