Fluorine-based resin film and method for producing the same, and method for producing piezoelectric film

Abstract

To provide a fluorine-based resin film which has less foreign matters and irregularity of the surface.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. The fluorine-based resin film has surface height roughness Rz measured according to JIS B 0601: 2001 of 0.50 μm or less, and the number of foreign matters having an arithmetic average value of a maximum width and a minimum width of 100 μm or more when the film is viewed in plan view is 7 pieces / 0.25 m2 or less.SELECTED DRAWING: None

Description

Technical Field

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

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 protection materials for solar cell modules, and fuel cell members because they have 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 fluororesin films with few foreign substances and high transparency.

[0003] Also, it is known that some fluororesin films exhibit piezoelectricity by polarization treatment. Piezoelectrics made of fluororesin films are being considered for use in transparent touch panels and the like because they are expected to have high transparency and flexibility. In this specification, the fluororesin piezoelectric film or piezoelectric film means a fluororesin film having a piezoelectric constant d 33 of 1.0 pC / N or more measured by the method described later.

[0004] Foreign substances such as polymer masses may occur during the production of fluororesin. Such foreign substances may reduce the transparency of the fluororesin film or make the polarization non-uniform. Therefore, foreign substances may be removed from the fluororesin before film production. For example, in Patent Document 1, a solution in which a fluororesin is dissolved in methyl ethyl ketone as a solvent is filtered through a filter to remove foreign substances. Also, in Patent Document 2, a solution in which a fluororesin is dissolved in a fluorine-containing aliphatic solvent as a solvent is filtered through a filter to remove foreign substances.

Prior Art Documents

Patent Documents

[0005] Patent Document 1 International Publication No. 2015 / 064324 Patent Document 2 Japanese Unexamined Patent Application Publication No. 2020-164781 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] As described in Patent Document 1 and Patent Document 2, a method of dissolving a fluororesin in a solvent and filtering it is known. When filtering is performed by these methods, the solvent volatilizes during subsequent film formation, and unevenness is likely to occur on the film surface. Furthermore, since a large amount of polar organic solvent is used for dissolving the fluororesin, consideration for the working environment and the natural environment is also required, and manufacturing costs such as solvent recovery become relatively high.

[0007] In addition, in the solution casting method, since a large amount of polar organic solvent is used for dissolving the fluororesin, not only is solvent recovery necessary and manufacturing costs become relatively high, but consideration for the impact of the organic solvent on the working environment and the natural environment is also required. Furthermore, since a polar organic solvent is used for dissolving the fluororesin, if the solvent remains in the fluororesin film, polarization in the manufacturing process of the piezoelectric film may be hindered. Therefore, it is preferable to form a film by extruding a heat-melted fluororesin without using an organic solvent.

[0008] On the other hand, in a method of heat-melting a fluororesin and performing extrusion molding, there is a risk that problems such as the resin being denatured and decomposition products (foreign substances) being generated may occur when heated to a high temperature.

[0009] The present invention has been made in view of the above problems, and an object thereof is to provide a fluororesin film with few foreign substances and few surface irregularities, a method for manufacturing the same, and a method for manufacturing a piezoelectric film. MEANS FOR SOLVING THE PROBLEMS

[0010] One embodiment of the present invention for solving the above problems relates to a fluororesin film as described in [1] to [4] below. [1] A fluororesin film having a melt viscosity η measured at a measurement temperature of 260 ° C. and a shear rate of 50 s during measurement -1 is 600 Pa·s or more and 4000 Pa·s or less, and the Rz of the surface on the side with a small surface height roughness Rz measured according to JIS B 0601:2001 is 0.50 μm or less, and the number of foreign matters having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view is 7 pieces / 0.25 m 2 or less, fluororesin film. [2] The fluororesin film according to [1], which contains a structural unit derived from vinylidene fluoride as a main component. [3] A film for a piezoelectric film, having a thickness of 80 μm or more and 1000 μm or less, the fluororesin film according to [1] or [2]. [4] Piezoelectric constant d measured according to ISO 19622:2018 33 is 5.0 pC / N or more and 40.0 pC / N or less, the fluororesin film according to any one of [1] to [3].

[0011] One embodiment of the present invention for solving the above problems relates to a method for manufacturing a fluororesin film as described in [5] to [6] below. [5] 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 during measurement at a temperature 75 ° C. or more and 105 ° C. or less higher than the melting point of the fluororesin, -1 and 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, a step of forming the filtered fluororesin into a film shape, The method for manufacturing a fluororesin film according to any one of [1] to [3], which has A step of stretching the fluororesin film according to any one of [6][1] to [3] at a stretching ratio of 2.5 times or more and 6.0 times or less, A step of polarizing the stretched film by applying a DC voltage of 7.0 kV or more and 50.0 kV or less, The method for producing a piezoelectric film according to [4], which comprises the above steps.

Effect of the Invention

[0012] According to the present invention, there are provided a fluororesin film with few foreign matters and few surface irregularities, a method for producing the same, and a method for producing a piezoelectric film.

Embodiment for Carrying Out the Invention

[0013] [Fluororesin Film] One embodiment of the present invention relates to a fluororesin film. The fluororesin film may be an unstretched film or a stretched film.

[0014] 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 of the fluororesin to the total mass of the fluororesin film is 50% by mass or more. The content ratio of the fluororesin in the piezoelectric film to the total mass 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.

[0015] The fluororesin can be a homopolymer or copolymer obtained by polymerizing tetrafluoroethylene (TFE), vinylidene fluoride (VDF), or the like. Examples of fluororesins obtained by polymerizing TFE include copolymers of ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, 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-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene, TFE, CTFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether.

[0016] 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 hexafluoropropylene, copolymers of VDF and trifluoroethylene, copolymers of VDF and TFE, copolymers of VDF, trifluoroethylene, TFE, and CTFE, and copolymers of vinylidene fluoride VDF, trifluoroethylene, TFE, and 1-chloro-1-fluoroethylene are more preferred, and homopolymers of VDF are even more preferred. These fluororesins may be used alone or in combination of multiple types.

[0017] The fluororesin film preferably contains a resin having a constituent unit derived from VDF as a main component, and a homopolymer of VDF is most preferred. Containing a constituent unit derived from VDF as a main component means that the content rate of the constituent unit derived from VDF with respect to the total mass of the fluororesin film is 50% by mass or more. The content rate of the constituent unit derived from VDF 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.

[0018] Among them, since a fluororesin film is expected to have high piezoelectricity, the higher the content ratio of the homopolymer of vinylidene fluoride, the more preferable it is. The content ratio 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.

[0019] The content ratio of the resin having VDF as a structural unit contained in the fluororesin film and the fluororesin piezoelectric film is 19 It can be measured by quantitative analysis using F-NMR with an internal standard.

[0020] 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, the less likely the filter is to be clogged and the easier the filtration becomes. Therefore, it is not necessary to increase the melting temperature to enable filtration, and it is possible to suppress the generation of spot-like irregularities on the film surface due to the resin being denatured by heat and foreign substances being generated. Also, the higher the melt viscosity, the larger the molecular weight of the resin, so it becomes difficult to selectively arrange the molecular chains and crystallization is difficult. Therefore, light scattering due to the difference in refractive index between the crystalline part and the non-crystalline part is less likely to occur, the transparency of the film is increased, and various physical properties of the film are likely to be stable.

[0021] 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 Seisaku-sho, Ltd., Capillograph 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 is taken as the viscosity.

[0022] The fluororesin film has a surface height roughness Rz of 0.50 μm or less, preferably 0.01 μm or more and 0.50 μm or less, more preferably 0.01 μm or more and 0.40 μm or less, and even more preferably 0.01 μm or more and 0.30 μm or less. The smoother the surface of the fluororesin film, the less likely it is to cause 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.

[0023] The surface height roughness Rz is measured in accordance with JIS B 0601:2001. Specifically, a surface roughness meter (VK-X260, a shape analysis laser microscope manufactured by Keyence Corporation) compliant with JIS B 0601:2001 is used. Then, with the intersection of the diagonals of the rectangular 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 height roughness Rz compared to the non-contact surface with the roll. This is because the irregularities on the surface become smaller due to the fluororesin film being pressed against the cooling roll. Here, the measurement result for the surface with a small surface height roughness Rz (the contact surface with the cooling roll) is taken as the above surface height roughness Rz.

[0024] Since foreign substances in the resin increase in proportion to be present near the surface as the film becomes thinner, foreign substances tend to be easily detected by viewing the film in a plan view. Therefore, for quantifying the amount of foreign substances in the film, the film thickness for foreign substance measurement is set to 40 μm or less. When viewing a fluororesin film with a film thickness of 40 μm or less in a plan view, the number of foreign substances with a size of 100 μm or more is 2 7 pieces / 0.25 m 25 or more per 0.25 m 2 Preferably 0 or less per 0.25 m 2 3 or more per 0.25 m 2 More preferably 0 or less per 0.25 m 2 1 or more per 0.25 m 2 Even more preferably 0 or less per 0.25 m

[0025] The number of foreign matters larger than 200 μm in size when the fluororesin film with a film thickness of 40 μm or less is viewed in a plan view is 0 per 0.25 m 2 3 or more per 0.25 m 2 Preferably 0 or less per 0.25 m 2 2 or more per 0.25 m 2 More preferably 0 or less per 0.25 m 2 1 or more per 0.25 m 2 Even more preferably 0 or less per 0.25 m 2 is particularly preferred

[0026] The number of foreign matters with a size of less than 100 μm when the fluororesin film with a film thickness of 40 μm or less is viewed in a plan view is 0 per 0.75 m 2 50 or more per 0.25 m 2 Preferably 0 or less per 0.25 m 2 25 or more per 0.25 m 2 More preferably 0 or less per 0.25 m 2 16 or more per 0.25 m 2 Even more preferably 0 or less per 0.25 m

[0027] The fewer these foreign matters are, the more the transparency of the fluororesin film can be improved, and when stretching or polarizing the fluororesin film, stretching and polarization can be made uniform

[0028] The number of these foreign substances is obtained by successively 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 calculating the sum of the numbers of foreign substances measured from each of the said observation pieces. For a fluororesin film with a thickness exceeding 40 μm, after stretching the film until its thickness becomes 35 μm to 40 μm or less, the number of foreign substances is measured by the method described below. Specifically, 0.010 m 2 (100 mm × 100 mm) of 25 of the said observation pieces are cut out. Then, the sum of the numbers of foreign substances measured from each of the observation pieces is obtained. At this time, the foreign substances are marked by observing them with transmitted light, and the size of the foreign substances is determined by observing the marked portions with a microscope. The size of the foreign substances is the arithmetic mean value of the maximum width and the minimum width of the foreign substances.

[0029] The thickness of the fluororesin film is not particularly limited. However, for a film for a piezoelectric film before stretching and polarization treatment, it 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 still more preferably 120 μm or more and 180 μm or less. The thicker the film, the more advantageous it is for electrical properties such as insulation and piezoelectric properties. The thinner the film, the more advantageous it is for optical properties such as transparency and cost.

[0030] When it is a piezoelectric film that has been stretched and polarized, the thickness of the fluororesin film is preferably 20 μm or more and 100 μm or less, more preferably 25 μm or more and 80 μm or less, and still more preferably 30 μm or more and 60 μm or less.

[0031] 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, or a method using infrared rays. When the intersection of the two diagonals of the fluororesin film is defined as the center point A and the long side direction is defined as the width direction, the thickness 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 measurements is taken as the thickness 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 thickness is measured by the above method.

[0032] The fluororesin film may be polarized to impart piezoelectricity. The piezoelectric constant d 33 is preferably 5.0 pC / N or more and 40.0 pC / N or less, more preferably 8.0 pC / N or more and 40.0 pC / N or less, and even more preferably 10.0 pC / N or more and 30.0 pC / N or less.

[0033] The piezoelectric constant of the fluororesin film is measured in accordance with ISO 19622:2018, which is a test method for the piezoelectric constant d 33 by the direct quasi-static method (d 33 meter method, Berlincourt method). Specifically, a piezoelectric constant measuring device (manufactured by PIEZOTEST, piezometer system PM300) is used. The fluororesin film, which is the test piece, is held 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 is applied is measured. The charge is measured on the polarization surface of the fluororesin film, and the piezoelectric constant is calculated using the absolute value of the measured value. At this time, the advancing axis direction obtained by measuring the birefringence of the fluororesin piezoelectric film is 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 is defined as point A. The piezoelectric constant d 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.33 Measure these, and take the average value of the piezoelectric constants obtained therefrom as the piezoelectric constant d of the fluororesin film. 33 Let it be so.

[0034] The haze per unit thickness of the fluororesin film is preferably 0.00% / μm or more and 0.35% / 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 still more preferably 0.0% or more and 3.0% or less. The lower the haze, the higher the transparency of the fluororesin film.

[0035] 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. Then, when the intersection of the diagonal lines of the rectangular 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 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.

[0036] The haze of the piezoelectric fluororesin film is also measured 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, and an arbitrary point on the midpoint line of the line segment 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.

[0037] The measurement of the above-mentioned birefringence is performed by the parallel Nicol rotation method using a light source having a wavelength of 587.8 nm.

[0038] The fluororesin film may contain a resin other than the fluororesin or other additives as long as it can satisfy the above physical properties.

[0039] Examples of the resin other than the above-mentioned fluororesin include polycarbonate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, silicone resin, polyether, polyvinyl acetate, and polyolefins such as polyethylene and polypropylene that can be added to enhance flexibility; 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, cyanopullulan, and copper phthalocyanine-based polymers that can be added to further enhance dielectric properties.

[0040] [Manufacturing method of fluororesin film] The manufacturing method of 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.

[0041] For example, the fluororesin film can be produced by a manufacturing method including a step of heating and melting the above-mentioned fluororesin (melting step), a step of filtering the melted fluororesin (filtering step), and a step of forming the filtered fluororesin into a film (film forming step).

[0042] The obtained fluororesin film may be subjected to a stretching process (stretching step) as necessary. Further, a polarization process (polarization step) can be performed on the stretched fluororesin film as necessary to impart piezoelectricity. The fluororesin film imparted with piezoelectricity can be used as a piezoelectric film.

[0043] (Melting step) 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.

[0044] If the fluororesin melted in the melting step contains a solvent component, there is a risk that the polarization in the subsequent step will be hindered by the remaining solvent component that does not volatilize. 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.

[0045] The heating temperature of the fluororesin in the melting step is preferably 75°C or more higher than the melting point of the fluororesin and 105°C or less higher than the melting point, more preferably 75°C or more higher and 100°C or less higher than the melting point, even more preferably 80°C or more higher and 100°C or less higher than the melting point, and particularly preferably 85°C or more higher and 95°C or less higher than the melting point. By setting the melting temperature to 75°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 step. By setting the melting temperature to 105°C or less 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 and the like can be suppressed. By suppressing the generation of the decomposition products and the like, the amount of foreign matter in the fluororesin film can be reduced, and the transparency and smoothness of the fluororesin film can be improved. Further, by suppressing the generation of the decomposition products and the like, clogging of the filter due to these can be suppressed, and the filtration efficiency of the fluororesin can be improved.

[0046] According to the findings of the present inventors, in order to make a fluororesin with a high melt viscosity into a viscosity that can be filtered, it is necessary to heat the resin to a high temperature. On the other hand, when the resin is 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 by setting the melting temperature of a fluororesin having an appropriate melt viscosity within the above range, the fluororesin can be efficiently filtered. 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 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 step, it is possible to reduce the burden on the working environment and the natural environment and to reduce the manufacturing cost for recovering the polar solvent.

[0047] (Filtering step) In the filtering step, the fluororesin is heat-melted by the melting step and then 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.

[0048] In the filtration process, the fluororesin is filtered using a filter with a filtration accuracy of 10 μm or more and 40 μm or less. When using a filter with a filtration accuracy of 10 μm or more, the filtration of the molten fluororesin becomes easier, and the filtration pressure does not become too high, enabling the filtration time to be shortened. By using a filter with a filtration accuracy of 10 μm or more, it becomes easier to pass the filter through the fluororesin heated and melted to the above temperature. Also, 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 substances in the fluororesin can be sufficiently removed, and a fluororesin film with few foreign substances can be obtained. The filtration accuracy of the filter is preferably 10 μm or more and 35 μm or less, and more preferably 15 μm or more and 30 μm or less.

[0049] Note that the filtration of the fluororesin is performed using a filter having a multilayer structure composed of a plurality of layers having different shapes, mesh sizes, etc. And 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 having a size of 10 μm or more with a filtration efficiency of 95% or more.

[0050] In this step, the filtration of the fluororesin may be performed multiple times. For example, coarse foreign substances may be removed using a filter with a low filtration accuracy (a large numerical value of filtration accuracy) arranged in the previous stage, and then finer foreign substances may be removed using 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.

[0051] The filter may be arranged between the extruder and the die for performing the film forming process. Also, the filter may be arranged in an extruder different from the extruder for performing the film forming process or a melt kneading device, and the fluororesin filtered by the filter may be introduced into the extruder for performing the film forming process.

[0052] (Film forming process) In the film forming step, the fluororesin filtered in the filtration step is formed into a film.

[0053] The film forming method is not particularly limited, and known methods such as extruding the melted and filtered fluororesin from a T-die and winding it around a roll can be used. By bringing the film extruded and formed from the T-die and maintained at a temperature of 150 °C or higher into contact with a cooling roll, the film can be rapidly cooled, and the progress of crystallization of the fluororesin in the fluororesin can be suppressed. When the crystallization of the resin progresses, the refractive indices of light in the crystallized part and the amorphous part are different, so the haze of the film made of the resin tends to increase due to light scattering. To increase the transparency of the film, it is effective to suppress the progress of the crystallization. To suppress the crystallization, it is preferable that the surface temperature of the cooling roll is 125 °C or lower, more preferably 110 °C or lower, still more preferably 80 °C or lower, and particularly preferably 50 °C or lower.

[0054] In this way, the fluororesin film according to this embodiment can be obtained. The manufactured fluororesin film may be wound up as it is and stored, or may be conveyed to a stretching step or a polarization step, which are subsequent steps.

[0055] (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.

[0056] 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 transporting the fluororesin film by a plurality of rolls, it can be uniaxially stretched in the transport 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, 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 β-crystals described later sufficiently increases, and it becomes easier to obtain a fluororesin film with a high piezoelectric constant by 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.

[0057] (Polarization process) In the polarization process, a DC voltage is applied to the fluororesin film stretched in the stretching process 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 ratio of β-crystals increases. By applying a DC voltage to a fluororesin film with an increased ratio of polar β-crystals, a fluororesin film with a high piezoelectric constant can be obtained.

[0058] 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.

[0059] The stretching process and the polarization process may be performed simultaneously, or the polarization process may be performed after the stretching process.

[0060] After the film formation process or after the polarization process, the fluororesin film can be wound into a roll for storage, transfer, etc.

[0061] [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.

[0062] [Other Embodiments] It should be noted that the above-described embodiments are exemplary embodiments of the present invention, and it goes without saying that the present invention may include embodiments other than the above-described embodiments within the scope of the core technical idea.

Examples

[0063] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0064] 1. Preparation of Fluororesin Film Films 1 to 10 and Film 13, all of which are fluororesin films, were prepared according to the following procedures.

[0065] 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, Ltd., Capillograph 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.

[0066] 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.

[0067] The melting temperature was taken as the maximum temperature in the polymer filter in the filtration device or in the conduit from the extruder to the filtration device.

[0068] 1-1. Film 1 A polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 2500 Pa·s and a melting point of 173°C was melted using a single-screw extruder with a diameter of φ50 mm. The molten resin was filtered through a pleated polymer filter with a filtration accuracy of 20 μm, extruded from a T-die, and brought into contact with a cooling roll for cooling to obtain an unstretched film with a thickness of 160 μm. At this time, the melting temperature and the filtration temperature were 260°C.

[0069] The obtained unstretched film was guided 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 4.0 times in the flow direction. Furthermore, a voltage of 11.6 kV was applied from the surface of the film in the thickness direction to obtain Film 1 with a thickness of 39 μm.

[0070] 1-2. Film 2 Film 2 was obtained in the same manner as the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 40 μm was used.

[0071] 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 800 Pa·s and a melting point of 173°C was used.

[0072] 1-4. Film 4 Film 4 was obtained in the same manner as the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was set to 275°C.

[0073] 1-5. Film 5 Film 5 was obtained in the same manner as the production of Film 1, except that PVDF with a melt viscosity of 3400 Pa·s and a melting point of 173°C was used.

[0074] 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 800 Pa·s and a melting point of 173°C was used and the melting temperature was set to 250°C.

[0075] 1-7. Film 7 Film 7 was obtained in the same manner as the production of Film 1, except that a filter was not attached to the single-screw extruder.

[0076] 1-8. Film 8 Film 8 was obtained 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 and a filter was not attached to the single-screw extruder.

[0077] 1-9. Film 9 Film 9 was obtained in the same manner as the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 60 μm was used.

[0078] 1-10. Film 10 Film 10 was obtained in the same manner as the production of Film 1, except that a pleated polymer filter with a filtration accuracy of 10 μm was used and the melting temperature was set to 280°C. The surface height roughness Rz of Film 10 was large, and it was difficult to accurately detect the amount of foreign matter.

[0079] 1-11. Film 11 (Could not be produced) An attempt was made to produce Film 11 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.

[0080] 1-12. Film 12 (Could not be produced) An attempt was made to produce Film 12 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.

[0081] 1-13. Film 13 100 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). While stirring with a stirrer, the temperature was raised to 60°C, 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. The filtered resin solution was put into an automatic coater to prepare a coating film with a liquid thickness of 600 μm, and it was dried at 120°C for 1 hour to obtain Film 13. Film 13 had a large surface height roughness Rz and it was difficult to accurately detect the amount of foreign matter.

[0082] 2. Evaluation of Fluororesin Films For the obtained Film 1 to Film 10 and Film 13, the surface height roughness Rz, the number of foreign matters, the thickness, the piezoelectric constant d 33 , and the haze were measured by the following methods.

[0083] 2-1. Birefringence Using KOBRA-HB manufactured by Oji Scientific Instruments, with a light source having a wavelength of 587.8 nm, the birefringence of a film cut out to 20 mm × 20 mm was measured by the parallel Nicol rotation method, and the fast axis and the slow axis were determined from the in-plane birefringence of the film. The direction of the slow axis coincides with the average direction of the molecular chains oriented by stretching or extrusion. Since the films used this time were stretched in the flow direction (machine direction) of the film, the MD direction and the slow axis direction coincide.

[0084] 2-2. Surface Height Roughness Rz 2-2-1. Fluororesin Films (Other than Piezoelectric Films) A surface roughness meter (Keyence Corporation's shape analysis laser microscope VK-X260) compliant with JIS B 0601:2001 was used. Then, the surface roughness of the contact surface (the surface with a smaller Rz) of the fluororesin film with the cooling roll was measured. Specifically, when the intersection of the diagonals 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, 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.

[0085] 2-2-2. Fluororesin piezoelectric film The surface height roughness Rz of the fluororesin piezoelectric film was also measured in the same manner as the method in 2-2-1, except for the setting of the measurement points. Specifically, the direction of the optical axis obtained by measuring the birefringence of the piezoelectric film 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 taken as point A. The surface height roughness Rz was measured at a total of three measurement points, namely, point A (measurement point), and two measurement points set at intervals of 30 mm from point A in both end directions on the line segment in the width direction passing through point A. The average value of these was taken as the surface height roughness Rz of the fluororesin film.

[0086] 2-3. Number of foreign matters Uniaxial stretching was performed so that the thickness of the fluororesin-based 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 matters 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 marked by observing with transmitted light, and the size of the foreign substances was determined by observing the marked locations with a microscope. 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 in size, 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 in size were determined respectively.

[0087] 2-4. Thickness 2-4-1. Fluorine-based resin film (other than piezoelectric film) 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. Specifically, when the intersection of the diagonals of a rectangular fluorine-based resin film was taken as the center point A and the long side direction was taken as the width direction, thickness measurements were performed 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 thickness of the fluorine-based resin film.

[0088] 2-4-2. Fluorine-based resin piezoelectric film The thickness of the fluorine-based resin piezoelectric film was also measured in the same manner as in the method of 2-4-1 except for the setting of the measurement points. Specifically, the advancing axis direction obtained by measuring the birefringence of the piezoelectric film 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 taken as point A. Thickness measurements were performed at 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 average value of these was taken as the thickness of the fluorine-based resin piezoelectric film.

[0089] 2-5. Haze 2-5-1. Fluorine-based resin film (other than piezoelectric film) A haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., model NDH7700SP II) was used. Haze was measured in accordance with ISO 14782:2021. Specifically, 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, haze was 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 measurements was taken as the haze of the fluororesin film. The obtained haze was divided by the thickness of the film measured in accordance with 2-3-1 to obtain the haze per unit thickness of the fluororesin film.

[0090] 2-5-2. Fluororesin Piezoelectric Film The haze of the fluororesin piezoelectric film was measured in the same manner as in the method of 2-5-1, except for the setting of the measurement points. Specifically, the advancing axis direction obtained by measuring the birefringence of the piezoelectric film 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 taken as point A. Haze was measured at a total of three measurement points: point A (measurement point), and two measurement points set at intervals of 30 mm from point A in both end directions on the line segment in the width direction passing through point A. The average value of these measurements was taken as the haze of the fluororesin piezoelectric film.

[0091] 2-6. Piezoelectric Constant d 33 The piezoelectric constant d was measured in accordance with ISO 19622:2018 33was measured. 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 surface of the fluororesin 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 33 of the film.

[0092] 3. Results The production conditions and evaluation results of each film are shown in Table 1 and Table 2. Note that for Film 10, the unevenness of the leaf surface was very large, and the number of foreign substances could not be measured. Also, for Film 13, the haze was large and opaque, and the number of foreign substances could not be measured.

[0093]

Table 1

[0094]

Table 2

Industrial Applicability

[0095] The fluororesin film according to the present invention has few foreign substances and few surface irregularities.

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 The Rz of the surface on the side where the surface height roughness Rz measured in accordance with JIS B 0601:2001 is small is 0.50 μm or less, and the number of foreign matters having a size that is the arithmetic mean value of the maximum width and the minimum width when the film is viewed in plan view is 7 pieces / 0.25 m ２ is as follows A fluororesin film.

2. The fluororesin film according to claim 1, comprising a constituent unit derived from vinylidene fluoride as a main component.

3. The fluororesin film according to claim 1, which is a film for a piezoelectric film and has a thickness of 80 μm or more and 1000 μm or less.

4. The fluororesin film according to claim 1.

5. Piezoelectric constant d measured in accordance with ISO 19622:2018 ３３ is 5.0 pC / N or more and 40.0 pC / N or less, 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, and a step of forming the filtered fluororesin into a film shape. The method for producing a fluororesin film according to any one of claims 1 to 3, comprising the steps.

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 －１ of 600 Pa·s or more and 4000 Pa·s or less at a temperature 75 °C or more higher than the melting point of the fluororesin and at a temperature not higher than 105 °C higher; A step of stretching the fluororesin film according to any one of claims 1 to 3 at a stretching ratio of 2.5 times or more and 6.0 times or less, and a step of polarizing the stretched film by applying a DC voltage of 7.0 kV or more and 50.0 kV or less. The method for producing a fluororesin film according to claim 4, comprising the steps. ​ ​ ​ ​ ​ ​

Images