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, has thickness of 80 μm or more and 1,000 μm or less, and a haze per unit thickness of 0.35% / μm or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a fluororesin film and a method for producing the same.

Background Art

[0002] Fluororesin films are used in various applications such as interior and exterior building members, surface materials for molded products, front and back surface protective materials for solar cell modules, and fuel cell components because of their high properties such as weather resistance, heat resistance, stain resistance, chemical resistance, solvent resistance, mechanical properties, and secondary processability. Among them, due to their high weather resistance, stain resistance, and chemical resistance, they are expected to be applied to protective films and optical members, and there is a high demand for highly transparent fluororesin films.

[0003] 33 It is also known that a fluororesin film can be made into a piezoelectric film by polarization treatment. Since the piezoelectric film is used for a touch panel or the like, transparency is required. The transparency of the piezoelectric film is greatly influenced by the transparency of the fluororesin film before the piezoelectric treatment, and thus the development of a highly transparent fluororesin film is desired. In this specification, the fluororesin piezoelectric film or the piezoelectric film means a fluororesin film having a piezoelectric constant d

[0004] As methods for producing a fluororesin film, a method of producing by a solution casting method using a solution in which a fluororesin is dissolved (Patent Document 1, etc.) and a method of hot-melting a fluororesin and performing extrusion molding (Patent Document 2) are known.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

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.

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

[0008] In order to perform extrusion molding by the hot-melting method while suppressing the 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 hot-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 manufacturing 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, having a thickness of 80 μm or more and 1000 μm or less, and a haze per unit thickness of 0.35% / μm or less, fluororesin film. [2] When the film is viewed in a plan view, the number of foreign substances with an arithmetic mean value of the maximum width and the minimum width of 100 μm or more is 7 pieces / 0.25 m 2 The fluororesin film according to [1], wherein the number is 7 or less. [3] The fluororesin film according to [1] or [2], wherein the surface roughness Rz on the side with a smaller surface height roughness Rz measured in accordance with JIS B 0601: 2001 is 0.50 μm or less. [4] The fluororesin film according to any one of [1] to [3], which contains a structural unit derived from vinylidene fluoride as a main component. [5] The fluororesin film according to any one of [1] to [4], which is a film for a piezoelectric film.

[0011] One embodiment of the present invention for solving the above problems relates to a method for producing a fluororesin film according to the following [6] to [7]. [6] A step of heating and melting a fluororesin having a melt viscosity η measured at a measurement temperature of 260 ° C. and a shear rate of 50 s -1 is 600 Pa·s or more and 4000 Pa·s or less, and a step of extruding the heated and 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 lower. The method for producing a fluororesin film according to any one of [1] to [5], which comprises [7] In the step of heating and melting, the fluororesin is heated and melted at a temperature 75 ° C. or higher and 105 ° C. or lower than the melting point of the fluororesin, The method for producing a fluororesin film according to [6], further comprising a step of filtering the fluororesin melted at the temperature with a filter having a filtration accuracy of 10 μm or more and 40 μm or less.

Advantages 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

DETAILED DESCRIPTION OF THE INVENTION

[0014] [Fluorine-based resin film] One embodiment of the present invention relates to a fluorine-based resin film.

[0015] The fluorine-based resin film may be a film mainly composed of a fluorine resin. The fluorine resin is a resin obtained by polymerizing a monomer composed of an olefin containing fluorine. Containing a fluorine resin as a main component means that the content ratio of the structural unit derived from the monomer of the fluorine resin to the total mass of the fluorine-based resin film is 50% by mass or more. The ratio of the content of the fluorine-based resin to the total mass of the fluorine-based resin 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 fluorine-based resin can be a homopolymer or copolymer obtained by polymerizing tetrafluoroethylene (TFE) or vinylidene fluoride (VDF). Examples of fluorine-based resins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoro-ethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP). Examples of fluorine-based resins 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, a fluororesin obtained by polymerizing a monomer containing VDF is preferable, and a homopolymer of VDF, a copolymer of VDF and HFP, a copolymer of VDF and trifluoroethylene, a copolymer of VDF and TFE, a copolymer of VDF, trifluoroethylene and TFE, a copolymer of VDF, trifluoroethylene, TFE and CTFE, and a copolymer of VDF, trifluoroethylene, TFE and 1-chloro-1-fluoro-ethylene are more preferable, and a homopolymer of VDF is even more preferable. These fluororesins may be used alone or in combination of multiple types.

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

[0019] Among them, since high piezoelectricity is expected for the fluororesin film and the fluororesin piezoelectric film, the higher the content of the homopolymer of vinylidene fluoride, the more preferable. The content of the homopolymer in 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 in the range 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 fluororesin, the easier it is to form a film by extrusion at a lower melting temperature. However, since a resin with a low melt viscosity has easy molecular chain movement, the selective arrangement of molecular chains progresses during film formation, and crystals tend 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 light scattering at the interface between the crystalline part and the amorphous part increases, and the haze of the film tends to increase and the transparency decreases. On the other hand, even for a fluororesin with a low melt viscosity, by rapidly cooling it after film formation before crystallization progresses, crystal growth in the film can be suppressed, light scattering can be reduced, and a film with low haze and high transparency can be obtained. Also, the lower the melt viscosity, the easier it is to filter by a filter, and the melting temperature can be kept low, so it is less likely to cause spot-like unevenness on the film surface due to resin modification.

[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., Capilograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, the viscosity measured at a measurement temperature of 260°C and a shear rate of 50 s -1 shall be the viscosity.

[0023] The thickness of the fluororesin film is 80 μm or more and 1000 μm or less, preferably 100 μm or more and 500 μm or less, 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 properties such as abrasion resistance, durability, and insulation. On the other hand, the thinner the film, the more advantageous for optical properties such as transparency and cost.

[0024] 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. 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, the thickness is 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 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.

[0025] The fluororesin film has a haze per unit thickness of less than 0.35% / μm, preferably 0% / μm or more and 0.30% / μm or less, and more preferably 0% / μm or more and 0.25% / μm or less. The lower the haze, the higher the transparency of the fluororesin film. Further, according to the findings of the present inventors, the smaller the haze per unit thickness, the easier it is to increase the piezoelectric constant d when the fluororesin film is polarized. 33 This suggests that the smaller the haze per unit thickness, the smaller the crystal size in the film, and it is considered that this appropriate crystal size can increase the amount of orientation polarization in the polarization process and increase d. 33

[0026] ​The haze of the fluororesin film is measured in accordance with JIS K 7136:2000. Specifically, a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II) is used. When the intersection point of the two diagonals 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 30 mm intervals in both end directions from the center point A 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 is divided by the thickness of the fluororesin film to obtain the haze per unit thickness of the fluororesin film.

[0027] Since foreign substances in the resin increase in proportion to be present near the surface as the film becomes thinner, there is a tendency that foreign substances are more easily detected in the plan view of the film. 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 the 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 more 2 The following is more preferable, 0 pieces / 0.25 m 2 3 pieces / 0.25 m or more 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.

[0028] When the 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 μ is 3 pieces / 0.25 m 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.

[0029] When a fluororesin film with a film thickness of 40 μm or less is viewed in plan view, the number of foreign matters with a size of less than 100 μm is 50 pieces / 0.25 m 2 The following is preferred, 0 pieces / 0.25 m 2 25 pieces / 0.25 m or more 2 The following is more preferred, 0 pieces / 0.25 m 2 16 pieces / 0.25 m or more 2 The following is even more preferred.

[0030] The fewer these foreign matters are, the more the transparency of the fluororesin film is improved, and when stretching treatment or polarization treatment is performed on the fluororesin film, stretching and polarization can be made uniform.

[0031] 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 as to be adjacent to each other, and obtaining the sum of the number 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 so as to be adjacent to each other continuously from the fluororesin film. Then, the sum of the number of foreign matters measured from each observation piece is obtained. At this time, the foreign matters are marked by observing with transmitted light, and the size of the foreign matter is obtained by observing the marked portion with a microscope. The size of the foreign matter is the arithmetic mean value of the maximum width and the minimum width of the foreign matter.

[0032] 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 is, 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.

[0033] 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. Then, with the intersection of the two diagonals of the fluororesin film as the center point A and the long side direction 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 of the surface with a small surface roughness Rz (contact surface with the cooling roll) is taken as the above surface height roughness Rz.

[0034] Note that the fluororesin film may contain a resin other than the fluororesin or other additives as long as it can satisfy the above physical properties.

[0035] Examples of resins other than the above fluororesin include polycarbonate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, silicone resin, polyether, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene, which can be added to enhance flexibility; acrylic resin, epoxy resin, polyethylene oxide, polypropylene oxide, polyphenylene oxide, polyphenylene sulfide, polyamide, polyimide, polyamideimide, polystyrene, and polybenzimidazole, etc., which can be added to enhance strength; and odd polyamides, cyanopurans, and copper phthalocyanine-based polymers, etc., which can be added to further enhance dielectric properties.

[0036] [Method for manufacturing fluororesin film] The method for manufacturing the above-mentioned fluororesin film is not particularly limited, but preferably includes a step of forming a film by extrusion molding of the heat-melted resin.

[0037] For example, the fluororesin film can be produced by performing a step of heating and melting the above-mentioned 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.

[0038] The obtained fluororesin film may be stretched (stretching step) as necessary. Also, a step of polarizing (polarizing step) may be performed as necessary to impart piezoelectricity. The fluororesin film imparted with piezoelectricity can be used as a piezoelectric film.

[0039] (Melting step) In the melting step, the fluororesin containing the fluororesin is heated and melted. This step can be carried out, for example, by melt-kneading the fluororesin with an extruder.

[0040] When the fluororesin melted in the melting process contains a solvent component, there is a risk that the polarization in the subsequent process may be hindered by the solvent component remaining without volatilization. Therefore, the content of the solvent component in the fluororesin is preferably low, preferably 1% by mass or less, more preferably 0.1% by 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.

[0041] The melting temperature of the fluororesin is preferably 75°C or more higher than the melting point of the fluororesin and 105°C or lower higher than the melting point, more preferably 75°C or more higher than the melting point and 100°C or lower higher than the melting point, even more preferably 80°C or more higher than the melting point and 100°C or lower higher than the melting point, and particularly preferably 85°C or more higher than the melting point and 95°C or lower higher than the melting point. By setting the melting temperature 85°C or more higher than the melting point of the fluororesin, the viscosity of the fluororesin can be reduced to such an extent that it can be filtered in the next process. By setting the melting temperature 105°C or lower higher than the melting point of the fluororesin, decomposition, condensation, etc. of the fluororesin due to heating can be suppressed, and the generation of decomposition products, etc. can be suppressed. By suppressing the generation of the above decomposition products, etc., the amount of foreign substances in the fluororesin film can be reduced, and the transparency and smoothness of the fluororesin film can be enhanced. Also, by suppressing the generation of the above decomposition products, etc., when filtering the fluororesin, clogging of the filter due to these can be suppressed, and the filtration efficiency of the fluororesin can be enhanced.

[0042] According to the findings of the present inventors, fluororesins need to be heated to a high temperature to make resins with a high melt viscosity have 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 lowered 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 the 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 process, 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.

[0043] (Filtering step) In the filtering step, the fluororesin melted and made to have a lower viscosity in the melting step is filtered. The filtering method is not particularly limited, and the melted fluororesin may be passed through a filter, and known filter types such as pleated type and leaf disk type filters can be used.

[0044] 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 30 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, and 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.

[0045] 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.

[0046] In this step, the filtration of the fluororesin may be performed multiple times. For example, coarse foreign matters may be removed by a filter with a low filtration accuracy (a large numerical value of filtration accuracy) arranged in the previous stage, and then, finer foreign matters may be removed by a filter with a high filtration accuracy (a small numerical value of filtration accuracy) arranged in the subsequent stage. The filtration accuracy at this time shall be the value of the filter with the highest filtration accuracy.

[0047] The filter may be arranged between the extruder and the die for performing the film-forming step. Alternatively, the filter may be arranged in an extruder different from the extruder for performing the film-forming step or a melt-kneading device, and the fluororesin filtered by the filter may be introduced into the extruder for performing the film-forming step.

[0048] (Film-forming step) In the film-forming step, the fluororesin filtered in the filtration step is formed into a film.

[0049] The film-forming method is not particularly limited, and known methods such as extruding the melted and filtered fluororesin from a T-die and contacting it with a cooling roll for cooling 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 haze per unit thickness of the obtained fluororesin film can be reduced. According to the findings of the present inventors, the fluororesin having a low melt viscosity used in this embodiment has a different crystallization rate from the fluororesin having a higher melt viscosity, so the haze 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.

[0050] The fluororesin film thus obtained may be wound up once and stored, or may be directly conveyed to the stretching step.

[0051] (Stretching step) In the stretching step, the 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 may be heated as necessary and then stretched.

[0052] 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). The stretching ratio (the ratio in the MD direction) is preferably 2.5 times or more and 6.0 times or less, and 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 proportion of β-crystals described later is sufficiently increased, 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.

[0053] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film to impart piezoelectricity to the fluororesin film. In a fluororesin containing a homopolymer or copolymer of VDF, a transition from α-crystals to β-crystals occurs in the stretching process, and the proportion of β-crystals increases. By applying a DC voltage to a fluororesin film with an increased proportion of polar β-crystals, a fluororesin film with a high piezoelectric constant can be obtained.

[0054] The DC voltage to be applied is preferably 7.0 kV or more and 50.0 kV or less, and more preferably 7.5 kV or more and 30.0 kV or less.

[0055] The stretching process and the polarization process may be performed simultaneously, or the polarization process may be performed after the stretching process.

[0056] The fluororesin film after the polarization process can be wound into a roll for storage, transfer, etc.

[0057] In this way, the fluororesin film according to this embodiment can be obtained.

[0058] [Applications] The fluororesin film described above 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.

[0059] [Other Embodiments] Note 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 the core technical idea.

Examples

[0060] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0061] 1. Preparation of Fluororesin Film Films 1 to 7 and Film 10, all of which are fluororesin films, were prepared according to the following procedure.

[0062] 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, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho Co., Ltd., Capilograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, the viscosity was measured at a measurement temperature of 260 °C and a shear rate of 50 s -1 and used as the measured viscosity.

[0063] The melting point of the fluororesin was obtained 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 heating rate of 10 °C / min, and the maximum peak temperature of the endothermic curve in the obtained DSC curve was used as the melting point.

[0064] The melting temperature was defined as the highest temperature in the polymer filter in the filtration device or in the conduit from the extruder to the filtration device.

[0065] 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 in 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.

[0066] 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 and the surface temperature of the cooling roll was set to 70°C.

[0067] 1-3. Film 3 Film 3 was obtained in the same manner as the production 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.

[0068] 1-4. Film 4 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. Film 4 was obtained in the same manner as the production of Film 1, except that the surface temperature of the cooling roll was set to 110°C.

[0069] 1-5. Film 5 Film 5 was obtained in the same manner as the production of Film 1, except that the surface temperature of the cooling roll was set to 130°C.

[0070] 1-6. Film 6 Film 6 was obtained in the same manner as the production of Film 1, except that PVDF with a melt viscosity of 2500 Pa·s and a melting point of 173°C was used and the surface temperature of the cooling roll was set to 130°C.

[0071] 1-7. Film 7 Except for using PVDF with a melt viscosity of 4500 Pa·s and a melting point of 173°C, setting the melting temperature to 280°C, not attaching a filter to the single-screw extruder, and setting the surface temperature of the cooling roll to 140°C, Film 7 was obtained in the same manner as the production of Film 1.

[0072] 1-8. Film 8 (Could not be produced) Film 8 was attempted to be produced in the same manner as the production 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 it was not possible to obtain a film.

[0073] 1-9. Film 9 (Could not be produced) Film 9 was attempted to be produced in the same manner as the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 5 μm was used. However, the filter became clogged with resin and it was not possible to obtain a film.

[0074] 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 coating film with a film thickness of 600 μm, and it was dried at 120°C for 1 hour to obtain Film 10. For Film 10, the surface height roughness Rz was large and it was difficult to accurately detect the amount of foreign matter.

[0075] 2. Evaluation of Fluorine-based Resin Films For the obtained Films 1 to 7 and Film 10, the thickness, haze, number of foreign matters, and surface height roughness Rz were measured by the following methods.

[0076] 2-1. 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. When the intersection of the diagonals of a rectangular fluororesin film was taken as the center point A and the long side direction was taken 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 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 thickness of the fluororesin film.

[0077] 2-2. Haze Using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., NDH7700SP II), in accordance with ISO 14782:2021, the haze of the fluororesin film was measured. Specifically, when the intersection of the two diagonals of the fluororesin film was taken as the center point A and the long side direction was taken as the width direction, haze measurements were taken at a total of three measurement points: the center point A (measurement point), and two measurement points set at intervals of 30 mm 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. Next, the value obtained by dividing the haze of the fluororesin film by the thickness of the film (2-1) was taken as the haze per unit thickness of the fluororesin film.

[0078] 2-3. Number of foreign substances Uniaxial stretching was performed so that the thickness of the fluororesin-based piezoelectric film became 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 substances measured by observing each observation piece was 0.250 m 2It was determined as the number of foreign substances per hit. At this time, the foreign substances were observed with transmitted light and marked, and the marked locations were observed with a microscope to determine the size of the foreign substances. The size of the foreign substances was taken as the arithmetic mean value of the maximum width and the minimum width of the foreign substances. In this way, the number of foreign substances larger than 200 μm, the number of foreign substances with a size of 100 μm or more and 200 μm or less, and the number of foreign substances smaller than 100 μm were determined respectively.

[0079] 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 line of the rectangular 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: the center point A (measurement point), and two measurement points set at intervals of 30 mm in both end directions from the center point A on the line segment passing through the center point A and the midpoint A parallel to the long side, and the average value of these was taken as the surface height roughness of the fluororesin film.

[0080] 3. Results The production conditions and evaluation results of each film are shown in Table 1 and Table 2. Note that Film 10 had a large haze and was opaque, and the measurement of the number of foreign substances could not be performed.

[0081]

Table 1

[0082]

Table 2

[0083] 4. Relationship between haze and piezoelectric constant Film 1 and Film 5 were 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 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.

[0084] In accordance with ISO 19622:2018, the piezoelectric constant d of the polarized film 33 was measured. Specifically, using a piezoelectric constant measuring device (PiezoTest, piezometer system PM300), a fluororesin film as a test piece was held with a holding force of 1.0 N, and the charge generated when an alternating 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 advancing 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 33 was measured, and the average value of the piezoelectric constants obtained from these was taken as the piezoelectric constant d of the film 33 and used.

[0085] Using a KOBRA-HB manufactured by Oji Scientific Instruments and a light source having a wavelength of 587.8 nm, the birefringence of a film cut into 20 mm × 20 mm was measured by the parallel Nicol rotation method, and the advancing axis and the retarding axis were determined from the in-plane birefringence of the film. The direction of the retarding axis coincides with the average direction of the molecular chains shifted by 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 retarding axis direction coincide.

[0086] The relationship between the applied voltage and the piezoelectric constant d of Film 1 (haze per unit thickness is 0.20% / μm) and Film 5 (haze per unit thickness is 0.51% / μm) 33 is shown in FIG. 1.

[0087] As shown in FIG. 1, the film 1 having a small haze per unit thickness has a higher piezoelectric constant d than the film 5 having a large haze per unit thickness. 33 It was higher.

Industrial Applicability

[0088] The fluororesin film according to the present invention has high transparency.

Claims

1. Measured 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 thickness of 80 μm or more and 1000 μm or less, and a haze per unit thickness of 0.35% / μm or less.

2. 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 and which is 100 μm or more is 7 pieces / 0.25 m ２ is as follows The fluororesin film according to Claim 1.

3. The Rz of the surface on the side with a small surface height roughness Rz measured in accordance with JIS B 0601:2001 is 0.50 μm or less. The fluororesin film according to Claim 1.

4. Containing a structural unit derived from vinylidene fluoride as a main component. The fluororesin film according to Claim 1.

5. A film for a piezoelectric film. The fluororesin film according to Claim 1.

6. Measuring 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 at a shear rate of 50 s during measurement at a measurement temperature of 260 °C; 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. The method for producing a fluororesin film according to any one of Claims 1 to 5, comprising the steps.

7. In the step of heating and melting, the fluororesin is heated and 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 comprising a step of filtering the fluororesin melted at the above temperature through a filter having a filtration accuracy of 10 μm or more and 40 μm or less. ​

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