Method for manufacturing fluorine-based resin piezoelectric film

Abstract

Provided is a method for manufacturing a piezoelectric film having excellent piezoelectric properties and high transparency. The present invention is a method for manufacturing a piezoelectric film having a piezoelectric constant d31 of 20-50 pC / N and a retardation of 100-3000 nm, the method comprising: a step for preparing a piezoelectric film that has a fluorine-based resin as a main component and has a piezoelectric constant d31 of 5-50 pC / N; and a step for stretching the piezoelectric film to at least 103% and less than 200% of the original size at a temperature of at least 110°C and less than 186°C.

Description

Method for manufacturing a fluororesin piezoelectric film

[0001] The present invention relates to a method for manufacturing a fluororesin piezoelectric film.

[0002] Display and input devices having a touch panel installed on the front surface of a display device such as a liquid crystal display are widely used and are used for operation panels of portable devices such as mobile phones and operation panels of home electric appliances. In a display and input device using a touch panel, a user operates the device by pressing a display on the screen. Therefore, the piezoelectric film used for the touch panel is required to have high piezoelectricity for enhancing detection sensitivity and high transparency for accurately viewing an image of the display.

[0003] A film mainly composed of a fluororesin such as polyvinylidene fluoride (PVDF) can be made into a piezoelectric film having a high piezoelectric constant and pyroelectric constant by a polarization treatment. In Patent Document 1, a film of a copolymer of PVDF or vinylidene fluoride (VDF) and trifluoroethylene (TFE) is stretched at 60°C to 110°C while being subjected to a polarization treatment, and the piezoelectric constant d ３１ is described as having produced a piezoelectric film of 40 to 60 pC / N. [[ID=_{12}]]

[0004] U.S. Patent No. 5254296

[0005] As described in Patent Document 1, a piezoelectric film can be manufactured by simultaneously performing stretching and polarization treatment while heating a fluororesin film. However, according to the findings of the present inventors, the piezoelectric film obtained in this way exhibits excellent piezoelectric characteristics, but has a low transmittance in the visible light region and has a problem with transparency. Since a high transparency is required for a piezoelectric film used for a touch panel or the like, it is required to enhance piezoelectricity while suppressing a decrease in optical transparency.

[0006] The present invention has been made in view of the above problems, and an object thereof is to provide a method for manufacturing a fluororesin piezoelectric film having high piezoelectricity while suppressing a decrease in optical transparency.

[0007] An embodiment of the present invention for solving the above problems relates to a method for manufacturing a fluororesin piezoelectric film as described below [1] to [6]. [1] A method for manufacturing a piezoelectric film using a fluororesin as the main component and having a piezoelectric constant d ３１ The piezoelectric constant d comprises the steps of: preparing an initial piezoelectric film having a piezoelectric constant of 5 pC / N or more and 50 pC / N or less; and stretching the initial piezoelectric film at a stretching temperature of 110°C or more and less than 186°C, with a stretching ratio of 103% or more and less than 200%. ３１ [1] A method for manufacturing a piezoelectric film, wherein the internal haze value is 20 pC / N or more and 50 pC / N or less, and the retardation is 100 nm or more and 3000 nm or less. [2] The method for manufacturing a piezoelectric film according to [1], wherein in the preparation step, an initial piezoelectric film having an internal haze value of 0.01% or more and 1.50% or less is prepared, and in the stretching step, a treated piezoelectric film having an internal haze value of 0.01% or more and 1.50% or less is obtained. [3] The method for manufacturing a piezoelectric film according to [1] or [2], wherein the initial piezoelectric film is a stretched film, and in the stretching step, the initial piezoelectric film is stretched in the main stretching direction during the production of the initial piezoelectric film. [4] The method for manufacturing a piezoelectric film according to any one of [1] to [3], performed by a roll-to-roll method. [5] The method for manufacturing a piezoelectric film according to any one of [1] to [4], wherein the piezoelectric film is mainly composed of vinylidene fluoride resin. [6] The fluororesin is measured at a temperature of 260°C and a shear rate of 50 s during measurement. －１ A method for manufacturing a piezoelectric film according to any one of [1] to [5], wherein the melt viscosity η measured is 1000 Pa·s or more and 3500 Pa·s or less.

[0008] The present invention provides a method for manufacturing a fluororesin piezoelectric film having high piezoelectricity and high transparency.

[0009] One embodiment of the present invention is a fluororesin which is the main component and which also has a piezoelectric constant d ３１ This invention relates to a method for manufacturing a fluororesin piezoelectric film, comprising stretching a piezoelectric film having a density of 5 pC / N or more and 50 pC / N or less.

[0010] [Stretching of Piezoelectric Film] In this embodiment, a piezoelectric film that has already undergone polarization treatment and possesses predetermined piezoelectric properties is stretched while being heated.

[0011] The stretching temperature is 110°C or higher and less than 186°C, preferably 115°C or higher and less than 186°C, more preferably 120°C or higher and less than 186°C, even more preferably 125°C or higher and 172°C or lower, and particularly preferably 125°C or higher and 165°C or lower. Stretching at a temperature of 110°C or higher softens the film, suppressing the occurrence of crazes and whitening due to stress concentration. Stretching at a temperature of less than 186°C moderately increases the stretching stress of the fluororesin, allowing for the efficient formation of highly piezoelectric β-phase crystals.

[0012] The stretching ratio during stretching is 103% to 200%, preferably 118% to 200%, more preferably 118% to 170%, even more preferably 125% to 170%, and most preferably 125% to 150%. By setting the stretching ratio to 103% or higher, a film with a higher proportion of piezoelectric β-phase crystals and a higher elastic modulus can be obtained. By setting the stretching ratio to 200% or lower, the occurrence of crazing and whitening due to stress concentration can be suppressed, and a highly transparent film can be obtained.

[0013] When stretching a film while it is being transported, the stretching direction should be the MD direction (the direction in which the film is transported). When stretching is performed by batch processing, the stretching direction can be any direction. However, for films stretched as piezoelectric films (for example, when stretching is performed before or after imparting piezoelectricity), it is preferable to stretch them in the direction of the main stretching direction in the previous stretch, specifically in the direction of the slow phase axis determined from the in-plane birefringence of the film.

[0014] The stretching process may be performed continuously while the film is being transported, or in batches. Continuous stretching while the film is being transported can be performed using a stretching device having multiple rolls and pinch rolls for holding the fluororesin film. In this case, the stretching ratio can be controlled by controlling the ratio of the rotation speed of the feed-side rolls in the preceding stage to the rotation speed of the take-up-side rolls in the subsequent stage, and the stretching temperature can be controlled by changing the temperature of each roll. For example, in stretching while the film is being transported, a roll-to-roll method can be used, in which the fluororesin film supplied from the supply roll is transported by multiple guide rolls while being stretched, and then wound onto a winding roll for storage. By stretching while the film is being transported, the stretching process with heating can be performed in a short time of a few seconds. In the case of the roll-to-roll method, the stretching temperature is set to the higher of the surface temperatures of the feed-side rolls in the preceding stage and the take-up-side rolls in the subsequent stage. In batch processing, the stretching temperature is defined as the temperature at which the film temperature reaches its highest point within the range of the stretching process. When the film is heated and stretched by other methods, the stretching temperature is defined as the temperature at which the film surface temperature reaches its highest point within the range of the stretching process. Here, the film temperature can be measured by either a contact method or a non-contact method. The process may be carried out in either batch or continuous processing, and from the viewpoint of productivity, it is preferable to carry out the process continuously while transporting the film. When stretching is performed over a long period of time using batch processing, the film is prone to haze and discoloration due to prolonged heating. In contrast, by performing the stretching process in a short time using a roll-to-roll method, it is possible to reduce the likelihood of haze and discoloration in the film. From the viewpoint of mass production, it is preferable to perform the stretching process continuously using a roll-to-roll method.

[0015] When stretching is performed to impart piezoelectricity to a piezoelectric film, it is preferable to adjust the stretching ratio so that the total stretching ratio of the piezoelectric film is within an appropriate range. The total stretching ratio is preferably 400% to 1000%, more preferably 450% to 800%, even more preferably 470% to 700%, and particularly preferably 500% to 700%. The total stretching ratio can be determined as the product of the stretching ratios in each stretching step. By setting the total stretching ratio to 400% or more, the crystallization ratio of the β phase in the film increases, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through polarization treatment. By setting the total stretching ratio to 1000% or less, it is possible to reduce the occurrence of crazes and whitening due to stress concentration, as well as film breakage during the stretching process.

[0016] [Piezoelectric film to be stretched] The piezoelectric film to be stretched can be any piezoelectric film mainly composed of a fluororesin. A fluororesin is a resin obtained by polymerizing a monomer containing a fluorine-containing olefin. To mainly contain a fluororesin means that the proportion of the fluororesin to the total mass of the piezoelectric film is 50% by mass or more. The proportion of the fluororesin content to the total mass of the 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.

[0017] Fluorine-based resins can be homopolymers or copolymers obtained by polymerizing TFE, VDF, etc. Examples of fluorine-based resins obtained by polymerizing TFE include copolymers of TFE with ethylene, perfluoroalkyl vinyl ether, VDF, 1-chloro-1-fluoroethylene, chlorotrifluoroethylene (CTFE), and hexafluoropropylene (HFP). Examples of fluorine-based resins obtained by polymerizing VDF include homopolymers of VDF, as well as copolymers of VDF with 1-chloro-1-fluoroethylene, 1-chloro-2-fluoroethylene, trifluoroethylene (TrFE), CTFE, TFE, tetrafluoropropene, HFP, and perfluoroalkyl vinyl ether.

[0018] Of these, from the viewpoint of facilitating polarization of the fluororesin film, fluororesins obtained by polymerizing VDF are preferred, with homopolymers of VDF, copolymers of VDF and TrFE, copolymers of VDF and HFP, copolymers of VDF and TFE, copolymers of VDF, TrFE and TFE, copolymers of VDF, TrFE and TFE and CTFE, and copolymers of VDF, TrFE and TFE and 1-chloro-1-fluoroethylene being more preferred, and homopolymers of VDF being even more preferred. These fluororesins may be used individually or in combination of multiple types.

[0019] The fluororesin included as a component of the fluororesin film is preferably a fluororesin mainly composed of VDF, and more preferably a homopolymer of VDF, because high piezoelectricity is expected. A fluororesin mainly composed of VDF means that the VDF content relative to the total mass of the fluororesin is 50% by mass or more. The VDF content is the ratio of the mass of constituent units derived from VDF to the total mass of the fluororesin. The content of constituent units derived from VDF relative to the total mass of the fluororesin 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, even more preferably 90% by mass or more and 100% by mass or less, and particularly preferably 95% by mass or more and 100% by mass or less.

[0020] In particular, since high piezoelectric properties can be expected from fluororesin films, a higher content of the VDF homopolymer is preferable. The content of the homopolymer relative to the total mass of the fluororesin film is preferably 50% to 100% by mass, more preferably 70% to 100% by mass, even more preferably 90% to 100% by mass, and particularly preferably 95% to 100% by mass.

[0021] The content of resins with VDF as a constituent unit in fluororesin films is: １９ It can be measured by quantitative analysis using an internal standard with F-NMR.

[0022] The fluoropolymer film was measured at a temperature of 260°C and a shear rate of 50 s. －１ The melt viscosity was measured (hereinafter, unless otherwise specified, the measurement temperature was 260°C and the shear rate during measurement was 50 s). －１The melt viscosity measured by [method / method] is referred to as "melt viscosity," preferably between 600 Pa·s and 4000 Pa·s, more preferably between 1000 Pa·s and 3500 Pa·s, and even more preferably between 1000 Pa·s and 2400 Pa·s. Since the melt viscosity of the fluororesin does not change even after going through the processes of film formation, stretching, and polarization, the melt viscosity of the piezoelectric film serves as an indicator of the melt viscosity of the resin composition containing the fluororesin (hereinafter, the resin composition containing the fluororesin is referred to as "resin composition") and the fluororesin film. Since the fluororesin film is made by forming the resin composition, a resin composition with a melt viscosity of 4000 Pa·s or less can be formed by extrusion at a relatively low temperature. Furthermore, since the resin composition has short molecular chains and little entanglement of molecular chains, the film made from the resin composition can be stretched at high magnification without breaking. Furthermore, the short molecular chains improve the mobility of the molecular chains due to heat, leading to selective alignment of the molecular chains during cooling, and consequently improving the degree of crystallinity. Stretching the highly crystallinity film promotes molecular chain orientation, increasing the proportion of β-phase crystals. Polarizing the film, in which β-phase crystals have developed, causes the molecular dipoles to align in one direction, resulting in high piezoelectricity. Therefore, resin films with lower melt viscosity are advantageous as raw materials for piezoelectric films with high piezoelectricity. In addition, to obtain a piezoelectric film with fewer impurities, it is preferable to filter the heat-melted resin composition during film manufacturing to remove impurities and then form the film. Resin compositions with a melt viscosity of 4000 Pa·s or less easily decrease in viscosity upon heating, making it less likely for the filter to clog and facilitating filtration. Therefore, it becomes unnecessary to raise the melting temperature to enable filtration, and it is possible to reduce the occurrence of mottled irregularities on the film surface caused by the alteration of the resin due to heat and the generation of impurities. On the other hand, resin compositions with a melt viscosity exceeding 4000 Pa·s have a large molecular weight, and the movement of molecular chains is suppressed, making crystallization difficult. Therefore, films made from such resin compositions have a uniform structure, less light scattering due to differences in refractive index between crystalline and amorphous regions, excellent transparency, and stable various physical properties of the film.However, the resin composition needs to be at a relatively high temperature to achieve a viscosity for filtration, and it is likely to cause deterioration and decomposition of the resin composition. A resin composition with a melt viscosity of less than 600 Pa·s contains many short molecular chains with a small molecular weight, so various physical properties of the resulting film are less likely to be stable.

[0023] The melt viscosity is measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, using a capillary rheometer (manufactured by Toyo Seiki Seisakusho, Ltd., Capillograph 1D), with a capillary die having an inner diameter of φ1 mm × tube length of 10 mm, at a measurement temperature of 260°C and a shear rate of 50 s －１ shall be the viscosity measured.

[0024] [Step of preparing an initial piezoelectric film] The initial piezoelectric film can be manufactured by stretching and polarizing a fluororesin film, or by imparting piezoelectricity to a fluororesin film by polarizing the fluororesin film. Alternatively, a piezoelectric film mainly composed of a fluororesin having predetermined characteristics may be purchased.

[0025] (Fluororesin film) The fluororesin film to be polarized may be one produced by the melt extrusion method of the resin composition mainly composed of the above-mentioned fluororesin, or one produced by the solvent casting method.

[0026] In the melt extrusion method, the heat-melted resin composition is formed into a film by extrusion molding to produce a fluororesin film. In the solvent casting method, a liquid composition in which the resin composition is dissolved in a solvent is cast (coated) onto a substrate, and the solvent is vaporized by heating and drying to produce a fluororesin film.

[0027] In the melt extrusion method, the fluororesin film can be produced by performing a step of heating and melting the above-mentioned resin composition (melting step) and a step of forming the melted resin composition into a film (film-forming step). At this time, a step of filtering the melted resin composition (filtering step) may be further performed between the melting step and the film-forming step.

[0028] In the melting step, the resin composition is melted. This step can be carried out, for example, by melting and kneading the resin composition using an extruder.

[0029] The resin composition melted in the melting process may be any resin composition mainly composed of the fluororesin described above. However, if the resin composition contains solvent components, the residual solvent components that do not volatilize may hinder polarization in subsequent processes. Therefore, it is preferable that the content of the solvent components in the resin composition be small, preferably 1.0% by mass or less, and more preferably 0.1% by mass or less, relative to the total mass of the resin composition. In particular, it is preferable that the solvent content be 100 ppm or less, more preferably 10 ppm or less, and even more preferably 1 ppm or less, relative to the total mass of the resin composition.

[0030] Furthermore, the resin composition melted in the melting process preferably has a melt viscosity of 600 Pa·s or more and 4000 Pa·s or less. Resin compositions with a melt viscosity of 4000 Pa·s or less can be formed by extrusion at relatively low temperatures. In addition, because the resin composition has short molecular chains and less entanglement of molecular chains, the film made from the resin composition can be stretched at high magnification without breaking. Moreover, the short molecular chains improve the mobility of the molecular chains due to heat, and selective alignment of the molecular chains progresses during cooling, resulting in improved crystallinity. Stretching the highly crystallinity film promotes molecular chain orientation, increasing the proportion of β-phase crystals. Polarization treatment of the film with well-developed β-phase crystals causes the molecular dipoles to align in one direction, resulting in high piezoelectricity. Therefore, resin films with lower melt viscosity are advantageous as raw materials for piezoelectric films with high piezoelectricity. Furthermore, to obtain a piezoelectric film with fewer impurities, it is preferable to filter the heat-melted resin composition during film manufacturing to remove impurities and then form a film using the resulting resin composition. Resin compositions with a melt viscosity of 4000 Pa·s or less readily decrease in viscosity upon heating, making it less likely for the filter to clog and thus easier to filter. Therefore, it becomes unnecessary to raise the melting temperature to enable filtration, and it is possible to reduce the occurrence of mottled irregularities on the film surface caused by the alteration of the resin due to heat and the generation of impurities. On the other hand, resin compositions with a melt viscosity exceeding 4000 Pa·s have a large molecular weight, and the movement of molecular chains is suppressed, making it difficult to form an oriented molecular chain structure and thus crystallization is less likely to proceed. Therefore, films made from such resin compositions have a uniform structure, are less prone to light scattering due to differences in refractive index between crystalline and amorphous parts, exhibit excellent transparency, and have stable various physical properties. However, these resin compositions require relatively high temperatures to achieve the viscosity necessary for filtration, which can easily lead to alteration and decomposition of the resin composition. Resin compositions with a melt viscosity of less than 600 Pa·s contain many molecular chains with small molecular weights, which makes it difficult to maintain the stability of various physical properties in the resulting films.

[0031] The melting temperature shall be the highest temperature in the conduit from the extruder to the filtration device.

[0032] The melting temperature of the resin composition is preferably 20°C (mp + 20°C) to 130°C (mp + 130°C) above the melting point (mp) of the resin composition, more preferably (mp + 50°C) to (mp + 100°C), even more preferably (mp + 80°C) to (mp + 100°C), and particularly preferably (mp + 85°C) to (mp + 95°C). By setting the melting temperature to 20°C (mp + 20°C) or higher above the melting point of the resin composition, the viscosity of the resin composition can be reduced to a degree that allows for extrusion. By setting the melting temperature to 130°C (mp + 130°C) or lower above the melting point of the resin composition, decomposition and condensation of the resin due to heating can be suppressed, and the generation of decomposition products can be suppressed. By suppressing the generation of the above-mentioned decomposition products, the amount of foreign matter in the fluororesin piezoelectric film can be reduced, thereby improving the transparency and smoothness of the fluororesin piezoelectric film.

[0033] The melting point of the resin composition shall be the value measured in accordance with JIS K 7121:1987. Specifically, 5 mg of the sample shall be sealed in an aluminum pan and placed in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation). Under a nitrogen atmosphere, the temperature shall be raised from room temperature to 230°C at a heating rate of 10°C / min, and the maximum melting peak temperature in the DSC curve shall be defined as the melting point of the resin composition.

[0034] In the filtration step, the resin composition, which has been melted and reduced in viscosity in the melting step, is filtered. The filtration method is not particularly limited; it is sufficient for the molten resin composition to pass through the filter, and known types of filters such as pleated filters and leaf disc filters can be used.

[0035] In the filtration process, the resin composition is filtered using a filter with a filtration accuracy of 10 μm to 40 μm. Using a filter with a filtration accuracy of 10 μm or more facilitates the filtration of the resin composition that has been heated and melted to the above temperature, and also prevents the filtration pressure from becoming too high, thus shortening the filtration time. Using a filter with a filtration accuracy of 40 μm or less allows for sufficient removal of foreign matter from the resin composition, resulting in a fluororesin film with fewer foreign matter particles. The filtration accuracy of the filter is preferably 10 μm to 30 μm, and more preferably 15 μm to 30 μm.

[0036] The filter may be placed between the extruder and the die used in the film formation process. Alternatively, the filter may be placed in a different extruder or melt-kneading device than the one used in the film formation process, and the resin composition filtered through this filter may be introduced into the extruder used in the melting process for film formation.

[0037] In the film formation process, the resin composition, which has been heated and melted in the melting process and filtered in the filtration process as needed, is formed into a film. The film formation method is not particularly limited, and known methods such as extruding the melted and filtered resin composition from a T-die and cooling it by contacting it with a cooling roll can be used.

[0038] The film formation method is not particularly limited, and known methods such as extruding a molten and filtered resin composition from a T-die into a film and cooling it in contact with a cooling roll can be used. The surface temperature of the cooling roll is preferably 125°C or lower. By rapidly cooling the film of the resin composition extruded by a cooling roll with a low surface temperature, crystal growth can be suppressed, and the internal haze of the resulting fluororesin film and the fluororesin piezoelectric film produced therefrom can be reduced. The surface temperature of the cooling roll is preferably 5°C to 115°C, more preferably 20°C to 80°C, and even more preferably 30°C to 60°C. By setting the surface temperature of the cooling roll to 5°C or higher, the film does not become too hard, and a decrease in the stretchability and transportability of the film due to a decrease in adhesion between the guide roll or stretching roll and the film is less likely to occur.

[0039] The fluororesin film obtained in this manner may be stored after being wound up, or it may be used continuously in the stretching and polarization processes. Alternatively, it may be used in the polarization process without performing the stretching process.

[0040] (Stretching process for fluororesin film) The stretching process can be performed as needed. The stretching method is not particularly limited and can be performed, for example, by a stretching device having multiple rolls and pinch rolls for holding the fluororesin film. In this case, the stretching ratio can be controlled by controlling the ratio of the rotation speed of the feed-side roll in the preceding stage to the rotation speed of the take-up-side roll in the subsequent stage. The stretching temperature is, for example, in the case of a roll-to-roll method, the higher of the surface temperatures of the feed-side roll in the preceding stage and the take-up-side roll in the subsequent stage. In batch processing, the stretching temperature is the temperature at which the film temperature becomes the highest within the range of the stretching process. When the film is heated and stretched by other methods, the stretching temperature is the temperature at which the film surface temperature becomes the highest within the range of the stretching process. Here, the film temperature can be measured by either a contact method or a non-contact method.

[0041] The stretching of the fluororesin film can be performed while heating as needed, and the stretching process may be carried out multiple times.

[0042] Stretching is preferably performed by stretching the fluororesin film to 300% to 500% at a temperature of 110°C or higher but less than 150°C. When stretching is performed while the film is being transported, the stretching direction is preferably the MD direction (the direction in which the film is transported). When stretching is performed by batch processing, the stretching direction may be any direction. Furthermore, when performing stretching on multiple films, it is preferable that the stretching directions are the same.

[0043] The temperature during stretching is preferably 110°C or higher and less than 150°C, more preferably 115°C or higher and less than 150°C, even more preferably 120°C or higher and less than 150°C, even more preferably 125°C or higher and less than 150°C, particularly preferably 125°C or higher and 145°C or lower, and most preferably 130°C or higher and 145°C or lower. By increasing the temperature of the fluororesin film during the first stretching, the temperature inside the film is increased during stretching, making it easier to deform in the stretching direction (MD direction), and thus reducing the haze of the film. By moderately lowering the surface temperature of the roll during stretching, the stretching stress of the fluororesin film is increased, and crystals of the highly piezoelectric β phase can be efficiently formed.

[0044] The stretching ratio is preferably 300% to 500%, more preferably 350% to 500%, even more preferably 350% to 470%, even more preferably 350% to 450%, and particularly preferably 350% to 430%. By setting the stretching ratio in the first stretching to 300% or more, the crystal ratio of the β phase in the film increases, making it easier to obtain a fluororesin piezoelectric film with high piezoelectricity through polarization treatment. By setting the stretching ratio to 500% or less, it is possible to suppress the increase in haze due to the generation of excessive stretching stress.

[0045] (Polarization treatment of fluororesin film) Polarization treatment of fluororesin film can be performed, for example, by discharging a DC current from a needle-shaped electrode onto a metal roll that conveys the fluororesin film. The fluororesin film, which is conveyed in close contact with the surface of the metal roll, is polarized as it passes between the needle-shaped electrode discharging DC current and the metal roll. At this time, the metal roll on which the DC current is discharged is grounded. Also, at this time, the metal roll on which the DC current is discharged is heated to heat the fluororesin film.

[0046] In the polarization process, it is preferable to apply a voltage of 5kV to 30kV to the fluororesin film at a temperature of 50°C to less than 170°C. The temperature in the polarization process is preferably 80°C to 160°C, more preferably 100°C to 150°C, even more preferably 115°C to 150°C, and particularly preferably 130°C to 150°C. The DC voltage applied in the polarization process is preferably 3kV to 30kV, more preferably 5kV to 30kV, even more preferably 5kV to 25kV, particularly preferably 5kV to 20kV, and most preferably 10kV to 17kV.

[0047] The polarization process may be performed simultaneously with stretching or after stretching. Alternatively, the polarization process may be performed without stretching. Performing the polarization process after the first stretching is preferable from the viewpoint of manufacturing stability because it reduces the likelihood of film breakage during flashover compared to performing stretching and polarization simultaneously. Furthermore, performing the polarization process simultaneously with the first stretching allows for the orientation of molecular chains due to stretching and the orientation of dipoles due to the electric field to proceed simultaneously, making it easier to obtain a piezoelectric film with high piezoelectric properties.

[0048] (Initial piezoelectric film) The initial piezoelectric film can be prepared as described above.

[0049] Piezoelectric constant d of the initial piezoelectric film ３１ The piezoelectric constant d of the initial piezoelectric film is between 5 pC / N and 50 pC / N. ３１ A higher value is preferable because it results in a higher piezoelectric constant after stretching. Piezoelectric constant d of the initial piezoelectric film ３１ The piezoelectric constant d is preferably 10 pC / N or more and 50 pC / N or less, more preferably 15 pC / N or more and 50 pC / N or less, even more preferably 20 pC / N or more and 50 pC / N or less, and particularly preferably 25 pC / N or more and 50 pC / N or less. ３１ If the ratio is less than 5 pC / N, it is undesirable because a sufficiently high piezoelectric constant cannot be obtained even after stretching.

[0050] Piezoelectric constant d of the initial piezoelectric film ３１This is measured using a static method. Specifically, Al-deposited electrodes are formed on both sides of the piezoelectric film, and a rectangular sample with a length of 60 mm parallel to the slow axis and a width of 10 mm perpendicular to the slow axis is cut out as a test piece from near the center of the planar portion of the piezoelectric film with electrodes, using a method similar to the method for measuring retardation of the processed piezoelectric film described later. Next, the test piece is placed in a tensile testing machine with a chuck distance of 50 mm, and the amount of induced charge Q (pC) is measured using an electrometer when the test piece is stretched in the longitudinal direction (slow axis direction) with a force F (N) at a tensile speed of 10 mm / min at room temperature. The relationship between different forces F(N) and the induced charge Q(pC) for each is measured at 128 points using the method described above. The force F(N) is plotted on the horizontal axis and the induced charge Q(pC) on the vertical axis. The slope (pC / N) of the generated charge Q(pC) with respect to the force F(N) is determined by the least squares method. The charge is measured by the method described above within a range including the intersection of the diagonals of the polarization surface of the fluororesin piezoelectric film. The absolute value of this measurement is taken as the representative value Q(pC) of the charge of the fluororesin piezoelectric film. If one side of the fluororesin piezoelectric film is 300 mm or longer, a rectangular area with sides of 300 mm is arbitrarily set, and the range including the intersection of the diagonals of the rectangular area is measured. If both sides of the fluororesin piezoelectric film are 300 mm or longer, a square with sides of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above. Piezoelectric constant d ３１ (pC / N) can be calculated using Equation 1.

[0051] Here, A is the area defined by the product of the distance between the chucks and the width of the test piece, which is 50 × 10 = 500 (mm²). ２ ), t (mm) is the thickness of the test specimen, and w is the width of the test specimen, which is 10 mm.

[0052] The initial internal haze value of the piezoelectric film is preferably 0.01% to 1.50%, more preferably 0.01% to 1.20%, even more preferably 0.01% to 1.00%, and particularly preferably 0.01% to 0.80%. The lower the initial internal haze value of the piezoelectric film, the better the transparency of the piezoelectric film after stretching.

[0053] The initial internal haze of the piezoelectric film is determined by filling in any scratches or other irregularities that cause external haze with silicone oil to eliminate the influence of external haze, and then measuring the haze in the area including the center of the piezoelectric film in accordance with ISO 14782:1999. Specifically, silicone oil (Nikon, MXA22179) is placed between two glass plates (thickness: 0.15 mm, flatness: 0.01 mm or less), and the haze (H2) caused by the glass plates and silicone oil is measured using a haze meter (Nippon Denshoku Industries Co., Ltd., NDH2000). Next, a 50 mm x 50 mm rectangular film is cut out so as to include the intersection of the diagonals of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with sides of 300 mm is arbitrarily assumed, and the rectangular film is cut out from the area including the intersection of the diagonals of the rectangular film. Furthermore, if both sides of the piezoelectric film are 300 mm or longer, a square with sides of 300 mm is arbitrarily set on the piezoelectric film, the measurement range is set according to the method described above, and the rectangular film is cut out. Both sides of the cut piezoelectric film are uniformly wet with the silicone oil described above, and the haze (H3) is measured using the haze meter while sandwiched between the two glass plates. H2 is subtracted from H3 to obtain the internal haze (H1) of the piezoelectric film. The internal haze (H1) of the piezoelectric film is calculated using the following formula: H1 = H3 - H2

[0054] The initial piezoelectric film had 7 foreign objects with a size of 100 μm or more when viewed from above. ２ The following is the result: 0 pieces / 0.25 m ２ 5 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m２ 1 piece or more / 0.25m ２ The following are even more preferable.

[0055] The initial piezoelectric film had zero foreign particles larger than 200 μm when viewed from above. ２ 3 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 2 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following is even more preferable: 0 pieces / 0.25 m ２ That is particularly preferable.

[0056] The initial piezoelectric film has zero foreign particles with a size of less than 100 μm when viewed from above. ２ More than 50 pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ More than 25 pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following are even more preferable.

[0057] The fewer these impurities there are, the better the transparency of the initial piezoelectric film, and the more uniform the stretching and polarization processes become during the creation of the initial piezoelectric film. Furthermore, by suppressing stress concentration during the stretching process of the initial piezoelectric film, film breakage can be reduced. Filtering the molten fluororesin during the manufacturing of the broken film can further reduce the number of impurities.

[0058] The number of these foreign objects is determined by cutting out four rectangular pieces of film (observation pieces) from the initial piezoelectric film so that they are adjacent to each other, and then summing the number of foreign objects measured from each of these observation pieces. Specifically, the pieces are cut out at a distance of 0.010 m so that they are adjacent to each other from the initial piezoelectric film. ２Four observation pieces measuring (100 mm x 100 mm) are cut out. Then, the sum of the number of foreign objects measured from each observation piece is calculated, and the sum of the number of foreign objects is multiplied by 25 / 4 and rounded to the first decimal place to obtain 0.25 m. ２ This is the number of foreign objects per sample. At this time, the foreign objects are observed using transmitted light and marked, and the size of the foreign object is determined by observing the marked area under a microscope. The size of the foreign object is the arithmetic mean of the maximum and minimum widths of the foreign object.

[0059] The initial piezoelectric film preferably has a melt viscosity of 600 Pa·s to 4000 Pa·s, more preferably 600 Pa·s to 3500 Pa·s, even more preferably 600 Pa·s to 2400 Pa·s, particularly preferably 600 Pa·s to 2000 Pa·s, and very preferably 600 Pa·s to 1500 Pa·s.

[0060] [Processed piezoelectric film] In this way, the processed piezoelectric film according to this embodiment can be obtained.

[0061] The treated piezoelectric film has a piezoelectric constant d ３１ The ratio is 20 pC / N or more and 50 pC / N or less, preferably 30 pC / N or more and 50 pC / N or less, more preferably 32 pC / N or more and 50 pC / N or less, even more preferably 35 pC / N or more and 50 pC / N or less, and particularly preferably 35 pC / N or more and 45 pC / N or less.

[0062] Piezoelectric constant d of fluororesin piezoelectric film ３１ The piezoelectric constant d of the initial piezoelectric film is ３１ It can be measured in the same manner.

[0063] The treated piezoelectric film has a retardation of 100 nm to 3000 nm, preferably 500 nm to 3000 nm, and more preferably 750 nm to 3000 nm. The greater the retardation, the higher the degree of orientation of the molecular chains of the fluororesin, the sufficiently high crystal ratio of the β phase, and the easier it is to achieve high piezoelectricity.

[0064] Retardation is measured using a light source with a wavelength of 589.1 nm and the parallel nicol rotation method. The retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the treated piezoelectric film is measured, and this value is taken as the representative value of the retardation of the treated piezoelectric film. If the length of one side of the treated piezoelectric film is 300 mm or more, a rectangular area with sides of 300 mm is arbitrarily assumed, and the retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular area is measured. If both sides of the treated piezoelectric film are 300 mm or more, a square with sides of 300 mm is arbitrarily set on the treated piezoelectric film, and the measurement range is set according to the method described above.

[0065] The internal haze of the treated piezoelectric film is preferably 0.01% to 1.50%, more preferably 0.01% to 1.20%, even more preferably 0.01% to 1.00%, and particularly preferably 0.01% to 0.80%. The lower the internal haze, the better the transparency of the piezoelectric film.

[0066] The internal haze of the treated piezoelectric film can be measured in the same manner as the original piezoelectric film.

[0067] The storage modulus of the treated piezoelectric film is preferably 1.0 GPa or more and 10.0 GPa or less, more preferably 1.5 GPa or more and 8.8 GPa or less, even more preferably 2.0 GPa or more and 6.0 Pa or less, and particularly preferably 2.7 GPa or more and 5.0 GPa or less. As shown in the formula described later, the larger the storage modulus, the higher the elastic compliance s １１ The electromechanical coupling coefficient k becomes smaller. ３１ This increases. Therefore, the higher the storage modulus, the higher the conversion efficiency between electrical energy and mechanical energy. On the other hand, if the storage modulus is 10.0 GPa or less, the sensitivity to load when the treated piezoelectric film is used as a sensor does not decrease easily.

[0068] The storage modulus is measured in accordance with JIS K7244-4:1999 (ISO 6721-4:1994), using a sample cut parallel to the slow axis in the in-plane birefringence of the piezoelectric film, with a length of 25 mm and a width of 3 mm, as the test specimen. If one side of the treated piezoelectric film is 300 mm or longer, a rectangular area with sides of 300 mm is arbitrarily defined, and the test specimen is cut from the area including the intersection of the diagonals of this rectangular area. If both sides of the treated piezoelectric film are 300 mm or longer, a square with sides of 300 mm is arbitrarily defined on the piezoelectric film, and the test specimen is cut according to the above method. Specifically, the storage modulus of the test specimen is measured using a TA Instruments RSA-III with a chuck distance of 15 mm, an initial load of 1.5 N, a frequency of 10 Hz, a strain of 0.1%, and a temperature of 23°C, and the obtained value is defined as the storage modulus of the piezoelectric film.

[0069] Dielectric constant ε of the treated piezoelectric film ３３ Preferably, the pressure is 60 pF / m to 150 pF / m, more preferably 80 pF / m to 140 pF / m, more preferably 90 pF / m to 130 pF / m, even more preferably 100 pF / m to 130 pF / m, and particularly preferably 110 pF / m to 130 pF / m.

[0070] Dielectric constant ε ３３ The measurement is performed in accordance with JIS C2138:2007. Specifically, electrodes with a diameter of 19.5 mm are formed on both sides of a piezoelectric film by aluminum vapor deposition, and the relative permittivity is measured at room temperature, with a voltage of 1 V and a frequency of 1 kHz using an LCR meter manufactured by HIOKI Corporation. The obtained relative permittivity is then multiplied by the permittivity of vacuum (8.854 × 10⁻¹⁴). －１２ The dielectric constant ε is calculated by multiplying by F / m. This is the dielectric constant in the range including the center of the polarization surface of the treated piezoelectric film (or the range including the intersection of the diagonals in the case of a rectangular piezoelectric body). ３３ The dielectric constant ε of the treated piezoelectric film is measured using the method described above, and the absolute value of that value is taken from the dielectric constant ε of the treated piezoelectric film. ３３This shall be used as a representative value. Furthermore, if the length of one side of the processed piezoelectric film is 300 mm or more, a rectangular area with sides of 300 mm shall be arbitrarily set, and the range including the intersection of the diagonals of the above rectangular area shall be measured. If both sides of the processed piezoelectric film are 300 mm or more, a square with sides of 300 mm shall be arbitrarily set on the piezoelectric film, and the measurement range shall be set according to the above method.

[0071] Furthermore, the electromechanical coupling coefficient k of the treated piezoelectric film. ３１ The electromechanical coupling coefficient k is preferably 0.14 or higher, more preferably 0.16 to 0.40, even more preferably 0.17 to 0.35, and particularly preferably 0.18 to 0.30. ３１ The higher the value of k, the higher the conversion efficiency between electrical and mechanical energy. On the other hand, the electromechanical coupling coefficient k ３１ If the value is set to around 0.40 or less, there is no need to increase the stretching ratio and polarization voltage during the manufacturing of fluorine-based piezoelectric films, and the haze of the film is less likely to increase during film formation.

[0072] Electromechanical coupling coefficient k ３１ d is the piezoelectric constant ３１ , dielectric constant ε ３３ , and elastic compliance １１ The elastic compliance s is calculated using the following formula. １１ The reciprocal of the storage modulus is used as the value.

[0073]

[0074] The thickness of the treated piezoelectric film is not particularly limited, but is preferably 10 μm to 200 μm, more preferably 20 μm to 80 μm, and even more preferably 30 μm to 80 μm. The thicker the film, the more advantageous it is in terms of insulation and mechanical properties. The thinner the film, the more advantageous it is in terms of optical properties such as transparency and cost.

[0075] The thickness of a processed piezoelectric film is generally measured using a micrometer (JIS C 2151:2019), but it can also be measured using known methods such as a laser displacement meter, a capacitance displacement meter, or infrared light. Specifically, the thickness of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the processed piezoelectric film is measured, and this value is taken as a representative value of the thickness of the fluororesin piezoelectric film. If the length of one side of the piezoelectric film is 300 mm or more, a rectangle with sides of 300 mm is arbitrarily assumed, and the thickness of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular film is measured. If both sides of the piezoelectric film are 300 mm or more, a square with sides of 300 mm is arbitrarily set on the piezoelectric film, and the measurement range is set according to the method described above.

[0076] The treated piezoelectric film had 7 foreign objects per 0.25 m² when viewed from above, with a size of 100 μm or larger. ２ The following is the result: 0 pieces / 0.25 m ２ 5 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 3 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following are even more preferable.

[0077] The treated piezoelectric film had zero foreign particles larger than 200 μm when viewed from above. ２ 3 or more pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ 2 or more pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ 1 piece or more / 0.25m ２ The following is even more preferable: 0 pieces / 0.25 m ２ That is particularly preferable.

[0078] The treated piezoelectric film has zero foreign particles less than 100 μm in size when viewed from above. ２More than 50 pieces / 0.25m ２ The following is preferable: 0 pieces / 0.25 m ２ More than 25 pieces / 0.25m ２ The following is more preferable: 0 pieces / 0.25 m ２ More than 16 pieces / 0.25m ２ The following are even more preferable.

[0079] The fewer these impurities there are, the better the transparency of the treated piezoelectric film, and the more uniform the stretching and polarization processes become during the manufacturing process of fluororesin piezoelectric films. Filtering the molten fluororesin during film production can reduce the number of impurities.

[0080] The number of these foreign objects is determined by cutting out four rectangular pieces of film (observation pieces) from the fluororesin piezoelectric film so that they are adjacent to each other, and then summing the number of foreign objects measured from each of these observation pieces. Specifically, the pieces are cut out at a distance of 0.010 m so that they are adjacent to each other from the fluororesin piezoelectric film. ２ Four observation pieces measuring (100 mm x 100 mm) are cut out. Then, the sum of the number of foreign objects measured from each observation piece is calculated, and the sum of the number of foreign objects is multiplied by 25 / 4 and rounded to the first decimal place to obtain 0.25 m. ２ This is the number of foreign objects per sample. At this time, the foreign objects are observed using transmitted light and marked, and the size of the foreign object is determined by observing the marked area under a microscope. The size of the foreign object is the arithmetic mean of the maximum and minimum widths of the foreign object.

[0081] The fluororesin piezoelectric film preferably has a melt viscosity of 600 Pa·s to 4000 Pa·s, more preferably 600 Pa·s to 3500 Pa·s, even more preferably 600 Pa·s to 2400 Pa·s, particularly preferably 600 Pa·s to 2000 Pa·s, and very preferably 600 Pa·s to 1500 Pa·s.

[0082] [Applications] The fluororesin piezoelectric film described above can be used for various applications such as piezoelectric films for touch sensors and touch panels, actuators, protective films, and phase difference films.

[0083] [Other Embodiments] It goes without saying that the embodiments described above are exemplary embodiments of the present invention, and the present invention may include embodiments other than those described above within the scope of its core technical concept.

[0084] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0085] 1. Preparation of initial piezoelectric films Initial piezoelectric films 1-1 to 1-4 were prepared according to the following procedure.

[0086] The melt viscosity of the resin composition used as the material was measured in accordance with ASTM D 3835:2016 (ISO 11443:2021, JIS K 7199:1999). Specifically, a capillary rheometer (Capillograph 1D, manufactured by Toyo Seiki Seisakusho Co., Ltd.) was used, and a capillary die with an inner diameter of 1 mmφ and a tube length of 10 mm was used, at a measurement temperature of 260°C and a shear rate of 50 s. －１ The viscosity was measured using [a specific method / tool].

[0087] Furthermore, the melting point of the resin composition was measured in accordance with JIS K 7121:1987. Specifically, the melting point of the resin composition was determined by placing 5 mg of the sample in an aluminum pan and setting it in a differential scanning calorimeter (DSC-60A, manufactured by Shimadzu Corporation). 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 melting peak temperature in the resulting DSC curve was defined as the melting point of the resin composition.

[0088] Furthermore, the melting temperature was defined as the highest temperature in the conduit from the extruder to the filtration device.

[0089] 1-1. Initial Piezoelectric Film 1-1 A polyvinylidene fluoride homopolymer (PVDF) with a melt viscosity of 2500 Pa·s and a melting point (mp) of 173°C was melted using a single-screw extruder with a diameter of φ35 mm to a melting temperature of 270°C (mp + 97°C). The molten resin was then extruded into a film using a T-die and brought into contact with a cooling roll at 110°C to obtain a fluorine-based resin film for an initial piezoelectric film with a thickness of 150 μm and a width of 240 mm.

[0090] Using a multi-stage uniaxial stretching apparatus equipped with multiple metal rolls and pinch rolls, the fluororesin film for the initial piezoelectric film was stretched at a stretching temperature of 130°C and a stretching ratio of 400% by adjusting the speed of the take-up roll relative to the speed of the feed-side roll. In addition, a DC voltage of 13.5 kV was applied to a needle electrode positioned approximately 10 mm away from the surface of the feed-side roll during stretching. In this way, the initial piezoelectric film 1-1 was obtained.

[0091] 1-2. Initial piezoelectric film 1-2 Initial piezoelectric film 1-2 was obtained in the same manner as initial piezoelectric film 1-1, except that the applied voltage during polarization treatment was 15.0 kV.

[0092] 1-3. Initial piezoelectric film 1-3 Initial piezoelectric film 1-3 was obtained in the same manner as initial piezoelectric film 1-1, except that the applied voltage during polarization treatment was 17.0 kV.

[0093] 1-4. Initial piezoelectric film 1-4 Initial piezoelectric film 1-4 was obtained in the same manner as initial piezoelectric film 1-1, except that it was stretched at a stretching temperature of 90°C and a stretching ratio of 400%, and polarized at 90°C simultaneously with stretching.

[0094] 2. Preparation of processed piezoelectric films Processed piezoelectric films 2-1 to 2-6 were prepared according to the following procedure.

[0095] 2-1. Processed piezoelectric film 2-1 Using a multi-stage uniaxial stretching apparatus equipped with multiple metal rolls and pinch rolls, the initial piezoelectric film 1-1 was stretched at a stretching temperature of 145°C with a stretching ratio of 125% by adjusting the speed of the take-up roll relative to the speed of the feed-side roll to obtain a processed piezoelectric film 2-1 with a thickness of 40 μm. The total stretching ratio (product of the stretching ratio of the first stage and the stretching ratio of the second stage) is 500%.

[0096] 2-2. Processed piezoelectric film 2-2 The initial piezoelectric film 1-2 was stretched at a stretching temperature of 145°C and a stretching ratio of 125% using the same apparatus as that used to produce the processed piezoelectric film 2-1, to obtain the processed piezoelectric film 2-2.

[0097] 2-3. Processed piezoelectric film 2-3 A processed piezoelectric film 2-3 was obtained in the same manner as processed piezoelectric film 2-2, except that the initial piezoelectric film 1-2 was used as the initial piezoelectric film 1-3.

[0098] 2-4. Processed piezoelectric film 2-4 The initial piezoelectric film 1-3 was stretched at a stretching temperature of 140°C with a stretching ratio of 118% using the same apparatus as that used to produce the processed piezoelectric film 2-1, to obtain the processed piezoelectric film 2-4. The total stretching ratio (product of the stretching ratio of the first stage and the stretching ratio of the second stage) is 472%.

[0099] 2-5. Processed piezoelectric film 2-5 The initial piezoelectric film 1-4 was used as the processed piezoelectric film 2-5 without stretching.

[0100] 2-6. Processed film 2-6 The initial piezoelectric film 1-1 was used as the processed piezoelectric film 2-6 without stretching.

[0101] 3. Evaluation of the treated piezoelectric films Treated piezoelectric films 2-1 to 2-6 were left to stand at room temperature for 14 days after manufacturing and then subjected to characterization. Unless otherwise specified, the following measurements were performed within the range including the intersection of the diagonals of a rectangle cut out of the fabricated treated piezoelectric film, with dimensions of 100 mm in the TD direction and 300 mm in the MD direction, so as to include the central part of the treated piezoelectric film in the TD direction. The results obtained were taken as representative values ​​for the treated piezoelectric film.

[0102] 3-1. Retardation Using a KOBRA-HB measuring instrument manufactured by Oji Instruments, the retardation of a 20 mm x 20 mm section of film cut from the area including the intersection of the diagonals of the rectangular film was measured by the parallel nicol rotation method. The value at the measurement wavelength of 589.1 nm was defined as the retardation of the film. The lagging axis was determined from the in-plane birefringence of the film.

[0103] 3-2. Piezoelectric constant d ３１ Piezoelectric constant d by static method ３１The following measurements were taken. Specifically, Al-deposited electrodes were formed on both sides of each treated piezoelectric film, and a rectangular sample with a length of 60 mm parallel to the slow phase axis detected by in-plane birefringence of the film during retardation measurement and a width of 10 mm perpendicular to the slow phase axis was cut out as a test specimen from the area including the intersection of the diagonals of the rectangular film with electrodes. The slow phase axis was identified during retardation measurement. Next, the test specimen was placed in a tensile testing machine with a chuck distance of 50 mm, and the amount of induced charge Q (pC) was measured using an electrometer when the test specimen was stretched in the longitudinal direction (slow phase axis direction) with a force F (N) at a tensile speed of 10 mm / min at room temperature. The relationship between different forces F(N) and the induced charge Q(pC) for each force was measured at 128 points. The force F(N) was plotted on the x-axis and the induced charge Q(pC) on the y-axis, and the slope (pC / N) of the generated charge Q(pC) with respect to the force F(N) was determined by the least squares method. The piezoelectric constant d ３１ This was calculated using Equation 1.

[0104] Here, A is the area defined by the product of the distance between the chucks and the width of the test piece, which is 50 × 10 = 500 (mm²). ２ ), t (mm) is the thickness of the test specimen, and w is the width of the test specimen, which is 10 mm.

[0105] Furthermore, the same measurement method was used for initial piezoelectric films 1-1 to 1-4, and the piezoelectric constant d was determined. ３１ They sought it.

[0106] 3-3. Storage Modulus A sample measuring 25 mm in length and 3 mm in width was cut from the range including the intersection of the diagonals of the rectangular film described above, parallel to the slow axis in the in-plane birefringence of the film, and used as a test specimen. The storage modulus of the piezoelectric film was measured using a TA Instruments RSA-III with a chuck distance of 15 mm, an initial load of 1.5 N, a frequency of 10 Hz, a strain of 0.1%, and a temperature of 23 °C.

[0107] 3-4. Dielectric constant ε ３３An electrode with a diameter of 19.5 mm was formed on both sides of the film by aluminum evaporation, and the relative permittivity was measured at room temperature, with a voltage of 1 V and a frequency of 1 kHz, using an LCR meter manufactured by HIOKI. The obtained relative permittivity was multiplied by the permittivity of vacuum (8.854×10 －１２ F / m) to calculate the permittivity ε ３３ .

[0108] 3-5. Electromechanical coupling coefficient k ３１ Piezoelectric constant d ３１ , permittivity ε ３３ , and elastic compliance s １１ were used to calculate the electromechanical coupling coefficient k ３１ of the film by the following formula. The reciprocal of the storage modulus was used as the elastic compliance s １１ .

[0109]

[0110] 3-6. The internal haze of the internally haze-treated piezoelectric film was determined by measuring the haze in the range including the center of the piezoelectric film in accordance with ISO 14782:1999, after filling the unevenness such as scratches with silicone oil to eliminate the influence of external haze. The haze was measured using a turbidimeter (NDH2000 manufactured by Nippon Denshoku Industries Co., Ltd.). The haze at three different points in the measurement sample was measured, and the arithmetic mean value of the three points was obtained as the haze value of the measurement sample. Specifically, the haze (H2) caused by the glass plates and silicone oil was measured with silicone oil (Nikon, MXA22179) sandwiched between two glass plates (thickness: 0.15 mm, flatness: 0.01 mm or less). Next, both sides of a rectangular film cut out to 50 mm × 50 mm so as to include the intersection of the diagonals of the above rectangular film were uniformly wetted with the above silicone oil, and the haze (H3) was measured with the two glass plates sandwiched therebetween. The internal haze (H1) of the film was obtained by subtracting H2 from H3 (the following formula). H1 = H3 - H2

[0111] The internal haze was also measured for the initial piezoelectric films 1-1 to 1-4 in the same manner to obtain the internal haze.

[0112] 4. Results The production conditions of each film, and the piezoelectric constant d of the initial piezoelectric film３１ The internal haze values ​​are shown in Table 1, and the evaluation results are shown in Table 2.

[0113]

[0114]

[0115] This application claims priority to Japanese Patent Application No. 2024-180738, filed on 16 October 2024. The matters set forth in the original specification and claims of said application are incorporated herein by reference.

[0116] The fluororesin piezoelectric film produced by the manufacturing method of the present invention is useful as a piezoelectric film having high piezoelectricity and high transparency.

Claims

1. A fluororesin is the main component, with a piezoelectric constant d ３１ The piezoelectric constant d comprises the steps of: preparing an initial piezoelectric film having a piezoelectric constant of 5 pC / N or more and 50 pC / N or less; and stretching the initial piezoelectric film at a stretching temperature of 110°C or more and less than 186°C, with a stretching ratio of 103% or more and less than 200%. ３１ A method for manufacturing a piezoelectric film, wherein the density is 20 pC / N or more and 50 pC / N or less, and the retardation is 100 nm or more and 3000 nm or less.

2. The method for manufacturing a piezoelectric film according to claim 1, wherein in the preparation step, an initial piezoelectric film having an internal haze value of 0.01% or more and 1.50% or less is prepared, and in the stretching step, a treated piezoelectric film having an internal haze value of 0.01% or more and 1.50% or less is obtained.

3. The method for manufacturing a piezoelectric film according to claim 1, wherein the initial piezoelectric film is a stretched film, and in the stretching step, the initial piezoelectric film is stretched in the main stretching direction used when manufacturing the initial piezoelectric film.

4. A method for manufacturing a piezoelectric film according to claim 1, carried out by a roll-to-roll method.

5. The piezoelectric film is mainly composed of vinylidene fluoride resin, the method for manufacturing a piezoelectric film according to claim 1.

6. The fluororesin was measured at a temperature of 260°C and a shear rate of 50 s² during measurement. －１ A method for manufacturing a piezoelectric film according to claim 1, wherein the melt viscosity η measured is 1000 Pa·s or more and 3500 Pa·s or less.

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