WO2021024868A1

WO2021024868A1 - Energy-conversion film and production method therefor, and production method for energy-conversion element

Info

Publication number: WO2021024868A1
Application number: PCT/JP2020/028953
Authority: WO
Inventors: 小池　弘; 祐太郎菅俣; 誠一郎飯田
Original assignee: 株式会社ユポ・コーポレーション
Priority date: 2019-08-02
Filing date: 2020-07-29
Publication date: 2021-02-11
Also published as: JP7240504B2; JPWO2021024868A1

Abstract

Provided is a method for producing an energy-conversion film that exhibits no variation in in-plane piezoelectric performance and has excellent uniformity in in-plane piezoelectric performance. This method for producing an energy-conversion film, which is a porous resin film, comprises: a kneading step for kneading a resin composition that contains a thermoplastic resin and a pore-forming agent, and then extruding the obtained kneaded product; an extrusion step for forming a non-stretched resin film by extruding the kneaded product into a film shape; and a stretching step for stretching the non-stretched resin film at least in one axial direction, wherein the kneading step and/or the extrusion step further comprises a step for filtering the kneaded product using a filtration filter having a filtration grain size of 40-300 μm, the stretching step includes a stress-relaxation treatment in which the degree of stretch of the resin film in the stretching axial direction is relaxed in mid-course of the stretching step, and the result of calculation of formula (1): (B-A)/A falls between -10.0% and -0.1%, where A represents the maximum value of the degree of stretch when the resin film is stretched in the stretching axial direction, and B represents the value of the degree of stretch at the time when the degree of stretch is lowered by the stress relaxation treatment.

Description

Energy conversion film and its manufacturing method, and manufacturing method of energy conversion element

The present invention relates to an energy conversion film, an energy conversion element, and a method for manufacturing an energy conversion film.

Conventionally, a porous resin film having piezoelectricity has been used as an energy conversion film that converts mechanical energy into electrical energy. For example, a dielectric film in which a plastic film having air bubbles inside is pulled two-dimensionally and the surface thereof is coated with a conductive layer is applied to a microphone or the like (see Patent Document 1).

In addition, by retaining a large amount of electric charge in the pores inside the film, the piezoelectric performance and stability of the porous resin film can be improved, so that the size and number of pores using the pore-forming nucleating agent are optimized. Has been proposed (see Patent Document 2).

Japanese Unexamined Patent Publication No. 61-148404 Japanese Unexamined Patent Publication No. 2011-84735

However, although the energy conversion film described in Patent Document 2 exhibits excellent piezoelectric performance, for example, when electrodes or the like are arranged on both sides and applied to a large-area sensor or the like, in the plane of the energy conversion film. Piezoelectric performance may vary, and there is room for improvement in terms of obtaining elements of stable quality.

Therefore, an object of the present invention is to provide a method for producing an energy conversion film in order to obtain an energy conversion film having no in-plane variation in piezoelectric performance and excellent in-plane piezoelectric performance uniformity.

As a result of diligent studies to solve the above problems, the present inventors, when producing an energy conversion film, when the kneaded product of the resin composition is filtered using a specific filtration filter and the resin film is stretched. The energy conversion film obtained by the manufacturing method can solve the above-mentioned problems by loosening the degree of stretching the resin film in the stretching axis direction in the middle of the stretching step and performing the stress relaxation treatment at a specific stretching degree. The present invention has been completed.
That is, the present invention is as follows.

[1] A method for producing an energy conversion film, which is a porous resin film.
A kneading step of kneading a resin composition containing a thermoplastic resin and a pore-forming agent and extruding the obtained kneaded product,
An extrusion step of forming a non-stretched resin film by extruding the kneaded product into a film, and
A stretching step of stretching the unstretched resin film in at least a uniaxial direction is included.
At least one of the kneading step and the extrusion step comprises a step of filtering the kneaded product using a filtration filter having a filtration particle size of 40 to 300 μm.
The stretching step includes a stress relaxation treatment that loosens the degree to which the resin film is stretched in the stretching axis direction in the middle of the stretching step.
The following formula (1) is used using the maximum value (A) of the degree of stretching when the resin film is stretched in the stretching axis direction and the value (B) of the degree of stretching when the degree of stretching is reduced by the stress relaxation treatment. ) Satisfies -10.0 to -0.1%.
(1) (BA) / A
A method for producing an energy conversion film, which is characterized in that.
[2] The method for producing an energy conversion film according to the above [1], wherein the filtration filter is a plain weave wire mesh, a twill weave wire mesh, a flat woven wire mesh, or a twill woven wire mesh.
[3] The method for producing an energy conversion film according to the above [1] or [2], wherein the treatment temperature of the resin film in the stress relaxation treatment is not less than the temperature of Tma-20 ° C. and not more than the temperature of Tma + 10 ° C. However, the melting point Tm of the thermoplastic resin having the highest content among the thermoplastic resins constituting the resin film is defined as Tma.
[4] The method for producing an energy conversion film according to any one of [1] to [3], wherein the processing time of the resin film in the stress relaxation treatment is 1 to 300 seconds.
[5] The method for producing an energy conversion film according to any one of [1] to [4] above, wherein the pore-forming agent is a filler or a foaming agent.
[6] With respect to the energy conversion film produced by the method for producing an energy conversion film according to any one of the above [1] to [5], an electrode is formed on at least one surface of the energy conversion film. A method for manufacturing an energy conversion element.
[7] An energy conversion film composed of a porous resin film containing a thermoplastic resin and a pore-forming agent, and having a fluctuation value of a voltage generated by a ball drop test of 7 or less.

According to the present invention, it is possible to provide a method for producing an energy conversion film for obtaining an energy conversion film having no in-plane variation in piezoelectric performance and excellent in-plane piezoelectric performance uniformity.

It is sectional drawing which shows the structural example of the energy conversion film of this embodiment. It is the schematic which shows an example of the arrangement of the filtration filter in a kneading process. It is the schematic which shows an example of each zone of preheating, stretching, relaxation, and cooling in a tenter oven used for tenter stretching. It is a graph which shows the rail pattern of the tenter oven used in the tenter stretching of an Example. It is a schematic diagram which shows an example of the manufacturing process of an energy conversion film. It is a schematic diagram which shows an example of an electret making apparatus. Is. It is sectional drawing which shows an example of an energy conversion element. It is a schematic diagram which shows the manufacturing process of the energy conversion film which provided the conductive layer on one side produced in an Example. It is a schematic diagram which shows the manufacturing process of the electretized energy conversion film produced in an Example. It is a schematic diagram which shows the manufacturing process of the energy conversion film which provided the conductive layer on both sides produced in an Example. It is a schematic diagram which shows the falling ball test apparatus.

Hereinafter, the method for producing the energy conversion film and the method for producing the energy conversion element of the present invention will be described in detail, but the description of the constituent requirements described below is an example (typical example) as one embodiment of the present invention. Yes, it is not specified in these contents. Further, the dimensional ratio of the drawings is not limited to the ratio shown.

In addition, in this specification, the description of "(meth) acrylic" indicates both acrylic and methacrylic.

[Manufacturing method of energy conversion film]
The present invention defines a method for producing an energy conversion film which is a porous resin film.
The method for producing an energy film of the present invention includes a kneading step, an extrusion step, and a stretching step.
The method for producing an energy film of the present invention includes a step of filtering the kneaded product using a filtration filter having a filtration particle size of 40 to 300 μm in at least one of a kneading step and an extrusion step.
The method for producing an energy film of the present invention includes a stress relaxation treatment in which the stretching step relaxes the degree of stretching the resin film in the stretching axis direction in the middle of the stretching step.
In the method for producing an energy film of the present invention, the maximum value (A) of the degree of stretching when the resin film is stretched in the stretching axis direction and the value of the degree of stretching (B) when the degree of stretching is reduced by stress relaxation treatment. ) Is calculated by the following formula (1), which satisfies -10.0 to -0.1%.
(1) (BA) / A

The manufacturing method of the present invention will be described in detail later. First, the energy conversion film which is the target of the production method of the present invention will be described.

(Energy conversion film)
The energy conversion film according to the present invention is a porous resin film having a large number of pores inside. An energy conversion film having an electro-mechanical energy conversion performance can be obtained by subjecting the porous resin film to an electretization treatment in which an electric charge is injected. The electrical-mechanical energy conversion performance includes not only the ability to convert mechanical energy (kinetic energy) into electrical energy, but also the ability to convert electrical energy into mechanical energy (kinetic energy).

The energy conversion film according to the present invention may be a porous resin film before the electretization treatment or a porous resin film after the electretization treatment. The porous resin film after the electretization treatment is a film into which charges are intentionally injected, and has a large amount of electric charges as compared with those before the electretization treatment.

<Porosity resin film>
The porous resin film according to the present invention is a layer containing at least one porous resin layer, and the porous resin layer is obtained by film-molding a resin composition containing a thermoplastic resin and a pore-forming agent. Be done. The porous resin film may have a single-layer structure containing only the porous resin layer described below, or may have a multilayer structure having at least one of the porous resin layers.
The porous resin film is preferably a stretched film containing a pore-forming agent because it is easy to form the desired pores.

<Shape and size of holes>
The shape and size of the pores in the porous resin film (more specifically, in the porous resin layer) may be appropriately set according to the required performance and the like, and are not particularly limited.
It is considered that different charges are held in pairs on the opposite inner surfaces of the individual pores inside the porous resin film, as in the case of a single-plate capacitor. Therefore, in order to accumulate electric charges inside like a single plate type capacitor, it is preferable that the pores of the porous resin film have an area and a height of a certain value or more. If the area of the pores is a certain amount or more, a sufficient capacitance can be obtained and an electret having excellent performance can be easily obtained. Further, when the height (distance) of the hole is equal to or higher than a certain level, the discharge (short circuit) inside the hole is suppressed and the electric charge is easily accumulated. Here, the "area" of the pores means the maximum value of the pore area in the cross section parallel to the surface of the porous resin layer, and the "height" of the pores is in the thickness direction of the porous resin layer. It means the maximum value of the pore diameter. From these points, it can be said that the larger the size (area) of the individual pores inside the porous resin film, the more effective the function, but the discharge (short circuit) generated by the communication between the adjacent pores is reduced and accumulated. From the viewpoint of increasing the electric charge, the size of the pores is preferably not more than a certain value. Further, when the height (distance) of the pores is equal to or less than a certain value, the electric charge is easily polarized and an electret having excellent charge stability is easily obtained.

Therefore, from the viewpoint of stably accumulating more electric charges, the porous resin film preferably has a specific amount of pores of a specific size effective for accumulating electric charges. Specifically, when the porous resin film is observed in an arbitrary cross section, the height of the film in the thickness direction is 3 to 30 μm, and the diameter of the film in the surface direction is 50 to 50. It is preferable to have 100 / mm ² or more of pores having a size of 500 μm, more preferably 150 / mm ² or more, further preferably 200 / mm ² or more, and particularly preferably 300 / mm ² or more.

On the other hand, from the viewpoint of suppressing short circuits due to communication between adjacent pores and film strength, the porous resin film has a height of 3 to 30 μm in the thickness direction of the film and a diameter of the film in the surface direction. pores is 50 ~ 500 [mu] m, preferably has 3,000 / mm ² or less, and more preferably has 2,500 / mm ² or less, further preferably has 2,000 / mm ² or less, It is particularly preferable to have 1,500 pieces / mm ² or less.

As the number of pores of the specific size increases, the charge storage capacity tends to improve and the energy conversion efficiency tends to improve. On the other hand, as the number of holes of the specific size is smaller, it is possible to prevent the adjacent holes from communicating with each other and causing a discharge (short circuit) between the adjacent holes. In addition, it is possible to suppress a decrease in restoring force against external stress such as compression caused by a decrease in strength of the film itself. Insufficient compression recovery of the porous resin film causes adverse effects such as a decrease in the restoration rate while the film is repeatedly compressed and restored. Therefore, for example, when an energy conversion film is used as an energy conversion element such as a piezoelectric element, the product life may be shortened. In consideration of these balances, it is preferable to adjust the size of the pores in the porous resin film within the above range.

<Vacancy rate>
The porosity of the porous resin film may be appropriately set according to the required performance and the like, and is not particularly limited, but is preferably 20 to 80%. Such a vacancy rate correlates with the number of valid vacancy described above. The porosity of the porous resin film means the ratio (volume fraction) of the volume occupied by the pores in the film to the total volume of the film. The pore ratio of the porous resin film is equal to the ratio of the area occupied by the pores (area ratio) in the cross section of the film in the thickness direction on the premise that the pores are uniformly distributed throughout the film.

Therefore, the porosity of the porous resin film in the present invention was calculated by observing the cross section of the material in the thickness direction with a scanning electron microscope, capturing the observation image into an image analyzer, and analyzing the observation area. , Can be obtained as the area ratio of the pores on the cross section. Specifically, a sample for cross-section observation is prepared from the porous resin film by a method such as a gallium focused ion beam so that the pores are not crushed. The cross-sectional observation of this sample is performed at an appropriate magnification (for example, 2000 times, etc.) using a scanning electron microscope (manufactured by JEOL Ltd., trade name: JSM-6490) or the like. Using an image analyzer (manufactured by Nireco Corporation, trade name: LUZEX AP) or the like for the observation area of the obtained cross-sectional photograph, the ratio (area ratio) of the area occupied by the pores in the sample cross section was calculated. This can be used as the porosity.

The porosity of the porous resin film is preferably 20% or more, preferably 25% or more, from the viewpoint of ensuring the charge storage capacity by providing a large number of pores having a size suitable for accumulating charges inside the film. % Or more, more preferably 30% or more, and particularly preferably 35% or more. On the other hand, the porosity of the porous resin film is 80 from the viewpoint of suppressing short circuits due to the communication of pores, reducing the elastic modulus of the porous resin film, and suppressing the decrease in durability due to the decrease in compression recovery in the thickness direction. It is preferably% or less, more preferably 70% or less, further preferably 60% or less, and particularly preferably 55% or less.

<Raw materials used for the porous resin layer>
As described above, the porous resin layer contains a thermoplastic resin and a pore forming agent. From the viewpoint of pore-forming property, it is preferable to contain a pore-forming agent, and from the viewpoint of improving chargeability and heat resistance, it is preferable to contain a metal soap.

<< Thermoplastic resin >>
The thermoplastic resin is a matrix resin having a porous resin layer, and imparts a piezoelectric effect and compression recovery.
As the thermoplastic resin of the energy conversion film, an insulating polymer material that does not easily conduct electricity can be preferably used. Examples of such thermoplastic resins include polyethylene-based resins, polypropylene-based resins, polybutenes, and polyolefin-based resins such as 4-methyl-1-pentene (co) copolymers; ethylene-vinyl acetate copolymers and ethylene- (meth). ) Acrylic acid copolymer, metal salt (ionomer) of ethylene- (meth) acrylic acid copolymer, ethylene- (meth) acrylic acid alkyl ester copolymer (the number of carbon atoms of the alkyl group may be 1 to 8). (Preferably), functional group-containing olefin resins such as maleic acid-modified polyethylene and maleic acid-modified polypropylene; aromatic polyesters (polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc.), aliphatic polyesters (polybutylene succinate, polylactic acid, etc.) ) Etc. Polyester resin; Nylon-6, Nylon-6,6, Nylon-6,10, Nylon-6,12, etc. Polyethylene resin; Syndiotactic polystyrene, tactic polystyrene, acrylonitrile-styrene (AS) Examples thereof include styrene resins such as polymers, styrene-butadiene (SBR) copolymers and acrylonitrile-butadiene-styrene (ABS) copolymers; polyvinyl chloride resins; polycarbonate resins; polyphenylene sulfides and the like.

Examples of the polyethylene-based resin include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, low-crystalline or amorphous ethylene / α-olefin copolymer, and ethylene-cyclic olefin copolymer. And so on.
Examples of polypropylene-based resins include crystalline polypropylene, low-crystalline polypropylene, amorphous polypropylene, propylene / ethylene copolymer (random copolymer or block copolymer), propylene / α-olefin copolymer, propylene / (Ethylene / α-olefin copolymer, etc.) and the like.
The α-olefin is not particularly limited as long as it can be copolymerized with ethylene and propylene. For example, ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1 -Heptene, 1-octene, etc. can be mentioned.
Among these thermoplastic resins, polyolefin-based resins or functional group-containing olefin-based resins having excellent insulating properties and processability are preferable.

As the thermoplastic resin, one type may be selected from the above-mentioned thermoplastic resins and used alone, or two or more types may be selected and used in combination.
Further, among the above-mentioned polyolefin resins, polypropylene resins are particularly preferable from the viewpoints of insulation, processability, water resistance, chemical resistance, cost and the like. From the viewpoint of film moldability, the polypropylene-based resin preferably contains a resin having a melting point lower than that of the propylene homopolymer in an amount of 2 to 25% by mass based on the total amount of the thermoplastic resin. Examples of such a resin having a low melting point include polyethylene-based resins, and among them, high-density or low-density polyethylene is preferable.

The content (content rate) of the thermoplastic resin in the porous resin layer is not particularly limited, and for example, as a matrix resin of the porous resin layer, communication between the pores is formed while forming a sufficient pore interface in the same layer. It may be appropriately set from the viewpoint of suppressing the above and ensuring the mechanical strength of the porous resin film. Specifically, based on the total mass of the porous resin layer, the thermoplastic resin is preferably contained in an amount of 50% by mass or more, more preferably 60% by mass or more, further preferably 65% by mass or more, and 70% by mass. It is particularly preferable to contain more than mass%. On the other hand, the thermoplastic resin is preferably contained in an amount of 98% by mass or less, more preferably 97% by mass or less, further preferably 96% by mass or less, and particularly preferably 85% by mass or less.

<< Pore-forming agent >>
Examples of the pore forming agent include fillers and foaming agents.
Examples of the filler include an inorganic filler and an organic filler.
By stretching the film (layer) containing the filler, it becomes easy to form a large number of pores inside the film with the filler as the starting point (nucleus). By controlling the content or stretching conditions of the filler, it is possible to control the size or existence frequency of the pores, and by controlling the particle size or stretching conditions of the filler, the size of the pores (high). It is possible to control the diameter). In addition, since the filler can function as a support in the pores even after the pores are formed, the pores are not easily crushed, and the obtained electret is likely to exhibit sufficient compression recovery even if a compressive force is repeatedly applied. Furthermore, stabilization of piezoelectric performance (pillar effect) can be expected.

The content of the filler in the porous resin layer is preferably 2% by mass or more, more preferably 4% by mass or more, further preferably 10% by mass or more, and 14% by mass from the viewpoint of forming sufficient pores. % Or more is particularly preferable. On the other hand, from the viewpoint of suppressing communication between pores in the porous resin layer, the content is preferably 50% by mass or less, more preferably 40% by mass or less, further preferably 30% by mass or less, 25. Mass% or less is particularly preferable.

When the content of the filler is equal to or higher than the lower limit of the above-mentioned preferable range, it is easy to obtain pores of a size suitable for accumulating a sufficient number of charges in the stretching step described later, and desired piezoelectric performance can be obtained. Easy to get. On the other hand, when the content of the filler is not more than the upper limit of the above-mentioned preferable range, the decrease in film strength due to the formation of excessive pores is likely to be suppressed. In the obtained energy conversion film, sufficient compression recovery is likely to be exhibited even if a compressive force is repeatedly applied, and it can be expected that the piezoelectric performance will be stable.

When a filler is used as the pore forming agent, for example, the inorganic filler can be used alone, the organic filler can be used alone, or the inorganic filler and the organic filler can be combined. When the inorganic filler and the organic filler are used in combination, the content ratio of each is not particularly limited. For example, the content ratio of the inorganic filler can be 10 to 99% by mass, 20 to 90% by mass, or 30 to 80% by mass with respect to the total amount of the pore forming agent. Good.

<<< Inorganic filler >>
Among the pore-forming agents, the inorganic filler is preferable because a large number of products having different particle sizes are commercially available at low cost. Specific examples of usable inorganic fillers include heavy calcium carbonate, light calcium carbonate, calcined clay, silica, diatomaceous earth, white clay, talc, titanium oxide, barium sulfate, alumina, zeolite, mica, sericite, bentonite, etc. Examples thereof include sepiolite, vermiculite, dolomite, wallastonite, glass fiber and the like, but the present invention is not particularly limited thereto. Of these, heavy calcium carbonate, light calcium carbonate or titanium oxide is preferable, and heavy calcium carbonate is more preferable, from the viewpoint of pore forming property and cost. These inorganic fillers can be used alone or in combination of two or more.

<<< Organic Filler >>>
Among the pore-forming agents, the organic filler is available as spherical particles having a uniform particle size, and the pores formed in the porous resin layer also tend to have a uniform size and shape.

As the organic filler, it is preferable to select resin particles of a type different from that of the thermoplastic resin which is the main component of the porous resin layer. For example, when the thermoplastic resin is a polyolefin-based resin, preferable organic fillers include resin particles that are incompatible with polyolefin and do not have fluidity during kneading and stretching molding of the polyolefin-based resin. Specific examples thereof include crosslinked acrylic resin, crosslinked methacrylic resin, crosslinked styrene resin, and crosslinked urethane resin, but the present invention is not particularly limited thereto. The resin particles made of these crosslinked resins are available as spherical particles having a uniform particle size in advance, and are particularly preferably used because the size of the pores can be easily adjusted.

The organic filler is incompatible with the thermoplastic resin, which is the main component of the porous resin layer, but is melt-kneaded together with the thermoplastic resin to form a sea-island structure, and the organic filler, which is an island, is empty during stretch molding. It may become the core of the hole and form the desired hole. For example, when the thermoplastic resin is a polyolefin resin, specific examples of such an organic filler include polyethylene terephthalate, polybutylene terephthalate, polycarbonate, nylon-6, nylon-6,6, cyclic olefin polymer, and polystyrene. , Polymethacrylate, etc., which has a melting point higher than the melting point of the polyolefin resin (for example, 170 to 300 ° C.) or a glass transition temperature (for example, 170 to 280 ° C.), and is a matrix resin by melt-kneading. Examples thereof include resin particles that can be finely dispersed in the based resin. The organic filler may be used alone or in combination of two or more.

The volume average particle size of the inorganic filler or organic filler (median diameter (D ₅₀ ) measured by a particle size distribution meter by laser diffraction) is appropriately selected in consideration of forming pores of a size suitable for accumulating electric charges. Can be, and is not particularly limited. From the viewpoint that pores of an appropriate size can be easily obtained and the desired piezoelectric performance can be easily obtained, the volume average particle size of the inorganic filler is preferably 3 μm or more, more preferably 4 μm or more, and 5 μm or more. It is more preferable to have. On the other hand, the formation of coarse pores suppresses the decrease in chargeability due to the communication of the pores, suppresses the decrease in film strength, and exhibits sufficient compression recovery even when a repeated compressive force is applied. From the viewpoint that stabilization of piezoelectric performance can be expected, the volume average particle size of the inorganic filler is preferably 30 μm or less, more preferably 20 μm or less, and further preferably 15 μm or less.

When the inorganic filler and the organic filler are used in combination, the volume average particle diameter may be a combination of the inorganic filler and the organic filler having individual particle diameters within the same range, or the inorganic filler and the organic filler are mixed. A particle size having the same volume average particle size as measured by a particle size distribution meter by laser diffraction may be used.

<<< Foaming Agent >>
As the pore forming agent, a foaming agent can be used instead of the above filler or together with the filler.
Examples of the foaming agent include a thermal decomposition type foaming agent. As the pyrolysis type foaming agent, gas may be generated by decomposition, for example, azodicarbonamide, benzenesulfonylhydrazide, dinitrosopentamethylenetetramine, toluenesulfonylhydrazide, 4,4-oxybis (benzenesulfonylhydrazide) and the like. Can be mentioned.
For example, a resin composition containing a thermoplastic resin and a pyrolysis foaming agent is melt-kneaded, and then extruded into a sheet to obtain a foamable resin film, and the foamable resin film is above the decomposition temperature of the pyrolysis foaming agent. A porous resin film can be produced by heating and foaming.
The content of the foaming agent in the porous resin layer is preferably 1% by mass or more, more preferably 3% by mass or more, still more preferably 5% by mass or more, from the viewpoint of forming sufficient pores. On the other hand, from the viewpoint of suppressing communication between the pores in the film, the content is preferably 25% by mass or less, more preferably 20% by mass or less, and further preferably 15% by mass or less.

<< Metal soap >>
The porous resin film preferably contains a metal soap from the viewpoint of further enhancing the charge retention performance of the energy conversion film and suppressing the deterioration of the piezoelectric performance even when stored or used in a high temperature environment.

As the metal soap used in the present invention, the metal soap which is melted and uniformly dispersed in the thermoplastic resin at the kneading stage of the raw material and which is solid at the environmental temperature after the formation of the porous resin layer has high charge retention performance. Easy to exert and preferable. Therefore, assuming that the melting point of the thermoplastic resin is Tm (° C.), the melting point of the metal soap is preferably in the range of 50 ° C. or higher and (Tm + 50) ° C. or lower, and is in the range of 70 ° C. or higher and (Tm + 40) ° C. or lower. It is more preferably within the range of 100 ° C. or higher and (Tm + 30) ° C. or lower. For example, when a polypropylene resin (melting point 160 to 170 ° C.) is used as the thermoplastic resin, it is preferable to use a metal soap having a melting point of 50 to 220 ° C., and it is preferable to use a metal soap having a melting point of 70 to 210 ° C. It is more preferable to use a metal soap having a melting point of 100 to 200 ° C. When the porous resin layer contains two or more kinds of thermoplastic resins, the melting point of the thermoplastic resin contained most in the layer is Tm.

Since the metal soap has a melting point in the above temperature range, it melts and uniformly disperses in the thermoplastic resin during layer molding of the porous resin layer, and maintains its dispersed state in the thermoplastic resin after layer molding. It solidifies as it is and becomes difficult to flow. At the time of electretization treatment, the metal soap is oriented by the dipoles in the molecule, and it is presumed that the orientation of the metal soap enhances the charge retention performance.

The metal soap is preferably a metal salt of a fatty acid, and more preferably a metal salt of a higher fatty acid. Examples of fatty acids include saturated fatty acids, unsaturated fatty acids and structural isomers thereof. The number of carbon atoms per fatty acid molecule is usually 5 to 30, preferably 6 to 28 carbon atoms, more preferably 8 to 24 carbon atoms, and further preferably 10 to 20 carbon atoms.

Among them, the metal salt of saturated fatty acid is preferable because it has a high melting point and tends to obtain a porous resin film having excellent heat resistance.

Further, the metal element of the metal soap is not particularly limited as long as it is a metal that forms a stable salt with a fatty acid. From the viewpoint of melting point and charge retention performance of metal soap, sodium (Group 1), magnesium (Group 2), calcium (Group 2), barium (Group 2), zinc (Group 12) and aluminum (Group 12) At least one of Group 13) is more preferred. Among them, at least one of calcium, zinc and aluminum is particularly preferable from the viewpoint of safety, calcium or aluminum is particularly preferable, and aluminum is most preferable from the viewpoint of further enhancing the charge retention performance. Moreover, the metal soap may be a basic salt.

The most preferably used metal soap is a saturated higher fatty acid aluminum salt. Examples of the saturated higher fatty acid aluminum salt include dihydroxyaluminum octadecanoate, hydroxyaluminum dioctadecanoate, aluminum trioctadecanoate, dihydroxyaluminum dodecanoate, hydroxyaluminum didodecanoate, aluminum tridodecanoate, dihydroxyaluminum 2-ethylhexanoate, and di-2. Examples thereof include hydroxyaluminum ethylhexanate and aluminum tri-2-ethylhexanoate.

The above-mentioned metal soaps are generally used in the plastic industry as various additives (for example, stabilizers, lubricants, filler dispersants, anti-shear agents, fluidity improvers, nucleating agents or anti-blocking agents). ing. However, the metal soap in the porous resin film is added to enhance the chargeability of the film, and is added as a functional agent for suppressing the deterioration of the piezoelectric performance especially in a high temperature environment. Therefore, from the viewpoint of suppressing the deterioration of the piezoelectric performance of the energy conversion element, the amount of metal is relatively larger than the blending amount (for example, 0.01% by mass) when used as the above-mentioned general various additives. It is preferable to add soap.

Therefore, the content of the metal soap in the porous resin layer is 0.02% by mass with respect to 100% by mass of the composition composed of the thermoplastic resin and the metal soap constituting the porous resin layer from the viewpoint of charge retention ability. % Or more is preferable, 0.03% by mass or more is more preferable, 0.05% by mass or more is further preferable, 0.1% by mass or more is particularly preferable, and 0.2% by mass or more is most preferable.

On the other hand, the effect of the metal soap reaches a plateau even if it is added excessively, and from the viewpoint of avoiding bleed-out etc., the metal soap is made up of 100% by mass of the composition composed of the thermoplastic resin constituting the porous resin layer and the metal soap. On the other hand, it is preferably 20% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, particularly preferably 3% by mass or less, and most preferably 0.7% by mass or less.

<< Additives >>
The porous resin layer can contain additives such as a dispersant, a heat stabilizer (antioxidant), and a light stabilizer, if necessary.

The content of the dispersant in the porous resin layer is preferably 0.01% by mass or more from the viewpoint of improving the dispersibility of the pore-forming agent and suppressing the generation of coarse pores or communication pores. 03% by mass or more is more preferable, and 0.05% by mass or more is further preferable. On the other hand, from the viewpoint of moldability and charge retention of the porous resin layer, the content is preferably 10% by mass or less, more preferably 5% by mass or less, and further preferably 2% by mass or less. Examples of the dispersant include, but are not limited to, dispersants such as fatty acids, glycerin fatty acids, polyglycerin fatty acid esters, sorbitan fatty acid esters, silane coupling agents, poly (meth) acrylic acid or salts thereof.

The content of the heat stabilizer in the porous resin layer is usually 0.001 to 1% by mass. Examples of the heat stabilizer include, but are not limited to, steric hindrance phenol-based, phosphorus-based, amine-based and other heat stabilizers.
Originally, the melting point of the heat stabilizer is preferably high from the viewpoint of charge retention performance, but from the viewpoint of uniformly dispersing the heat stabilizer in the porous resin layer, the melting point of the heat stabilizer is preferably low. Therefore, it is preferable that the melting point of the heat stabilizer has a melting point in the same temperature range as that of metal soap.

The content of the light stabilizer in the porous resin layer is usually 0.001 to 1% by mass. Examples of the light stabilizer include, but are not limited to, steric hindrance amine-based, benzotriazole-based, and benzophenone-based light stabilizers.

<Layer structure of energy conversion film>
The energy conversion film may have a single-layer structure consisting of only a porous resin layer having the above composition, or may have a multilayer structure having at least one porous resin layer. From the viewpoint of enhancing the energy conversion performance, the porous resin film preferably has a multi-layered structure having at least a core layer and a skin layer, and preferably has a three-layer structure of a skin layer / core layer / skin layer. More preferred.

The multi-layered energy conversion film is more preferably provided with a skin layer made of a stretched resin film on at least one surface of the core layer made of the porous resin film described above, and on both sides of the core layer made of the porous resin film described above. It is more preferable to provide a skin layer made of a stretched resin film.

FIG. 1 shows a configuration example of an energy conversion film 1 having a multilayer structure as an embodiment of the energy conversion film according to the present invention.
As shown in FIG. 1, the energy conversion film 1 includes a core layer 2 and a skin layer 3 provided on one surface of the core layer 2. The energy conversion film 1 may also include a skin layer 4 on the other surface of the core layer 2, if necessary.

<< Core layer >>
The multilayer structure of the core layer and the skin layer can be formed by using the above-mentioned porous resin layer as the core layer and providing the skin layer on the surface of the core layer.

<< Skin layer >>
From the viewpoint of protecting the core layer, the skin layer is preferably laminated on at least one surface of the core layer (porous resin layer), and more preferably laminated on both sides of the core layer. By covering the surface of the core layer with the skin layer, it is possible to prevent the electric charges stored inside through the pores in the core layer from being discharged to the atmosphere. In addition, the surface strength of the film can be improved, and by smoothing the surface, the adhesiveness with the electrode is likely to be improved.

The skin layer is preferably a resin film containing a thermoplastic resin. The skin layer may be a porous resin film like the core layer. As the thermoplastic resin constituting the skin layer, for example, it can be selected from the resins listed as the thermoplastic resin of the porous resin film and used.

The skin layer may or may not contain metal soap like the core layer. From the viewpoint of using the skin layer as a protective layer for the core layer, it is preferable that the skin layer does not contain metal soap. When the skin layer contains metal soap, it is preferable that the content thereof is lower than that of the core layer.

It is preferable that the skin layer has a composition that makes it more difficult to form pores than the core layer, or that the pore ratio is lower than that of the core layer. The formation of such a skin layer is a method of reducing the content of the pore-forming agent from that of the core layer, and the volume average particle diameter of the pore-forming agent used for the skin layer is used for the core layer. This can be achieved by a method of making the particle size smaller than the volume average particle size, a method of forming the core layer by biaxial stretching, and a method of forming the skin layer by uniaxial stretching to make a difference in the draw ratios between the two.

When the energy conversion film of the present invention is used as an energy conversion element, electrodes are provided on the surface of the skin layer. From the viewpoint of improving the adhesion to the electrodes, it is preferable that the skin layer contains a filler. An undulating structure can be formed on the surface of the skin layer by the filler, and an anchoring effect that enhances the adhesion to the electrodes can be obtained. In addition, the dielectric constant of the skin layer can be improved and the electrical characteristics of the core layer can be modified. As the filler of the skin layer, the fillers listed in the above-mentioned section of the porous resin layer can be used. The filler of the skin layer may be of the same type as the pore-forming agent of the porous resin layer, or may be of a different type. Among the pore-forming agents, the inorganic filler is preferable because it is easy to obtain an anchoring effect.

When the pore forming agent is contained in the skin layer, it is preferable to use the same dispersant as the dispersant used for the porous resin layer.
From the viewpoint of improving the physical strength of the skin layer and improving the durability of the core layer, it is preferable that the skin layer does not contain a pore-forming agent.

The skin layer is preferably a stretched film. By stretching, the uniformity of the thickness (film thickness) of the skin layer and the uniformity of electrical characteristics such as dielectric strength can be improved. If the thickness of the skin layer is highly uniform, local discharge concentration in the thin part of the skin layer is unlikely to occur during charge injection using a high voltage, so high voltage application for effective charge injection is applied. Easy to do.

The skin layer may have a single layer structure or a multi-layer structure having two or more layers. In the case of a multi-layer structure, by changing the type and content of the thermoplastic resin, pore-forming agent, and dispersant used for each layer, it is possible to design a multi-layer porous resin film with higher charge retention performance. It will be easy.

When the skin layers are provided on both sides of the core layer, the types, contents, thicknesses, etc. of the components constituting each skin layer may be the same or different.

<Thickness>
The thickness of the core layer is preferably 10 μm or more, more preferably 20 μm or more, further preferably 30 μm or more, and particularly preferably 40 μm or more. By setting the thickness of the core layer to the lower limit or higher, it is easy to secure the volume required for the accumulation of internal charges that function effectively for energy conversion, and in particular, an empty space of an appropriate size for the accumulation of internal charges in the porous resin layer. It is easy to form holes uniformly in a desired quantity. On the other hand, the thickness of the core layer is preferably 500 μm or less, more preferably 300 μm or less, further preferably 150 μm or less, and particularly preferably 120 μm or less. By setting the thickness of the core layer to the upper limit or less, it becomes possible for the electric charge to reach the inside of the layer during the electret treatment described later, and the energy conversion film of the present invention tends to exhibit the desired performance.

The thickness of the skin layer is not particularly limited, but is preferably 0.1 μm or more, more preferably 0.3 μm or more, further preferably 0.5 μm or more, and 0.7 μm or more. Is particularly preferable. When it is 0.1 μm or more, the thickness of the skin layer can be easily made uniform, the charge injection amount can be made uniform, and the dielectric strength can be easily improved. On the other hand, the thickness of the skin layer is preferably 100 μm or less, more preferably 50 μm or less, further preferably 30 μm or less, and particularly preferably 10 μm or less. When the thickness of the skin layer is 100 μm or less, when the electric charge is injected into the porous resin film having a multilayer structure, the electric charge can easily reach the core layer inside the film.

The skin layer is preferably thinner than the core layer. Since the skin layer is a layer that is relatively less likely to be elastically deformed in the thickness direction than the core layer, by suppressing the thickness of the skin layer, the compressive elastic modulus of the porous resin film or the like does not decrease, and the energy conversion efficiency is improved. It will be easier to maintain.

Therefore, the ratio of the thickness of the core layer to the thickness of the skin layer (core layer / skin layer) is preferably 1.1 to 1000, more preferably 2 to 300, and preferably 5 to 150. More preferably, it is particularly preferably 10 to 50. The same ratio when there are a plurality of skin layers is calculated using the total thickness of each layer.

The thickness of the single-layer structure porous resin film can be in the same range as that of the core layer.

In the present specification, the film thickness is a value obtained by measuring the total film thickness using a thickness gauge based on JIS K7130: 1999 “Plastic-Film and Sheet-Thickness Measuring Method”.
The thickness of each layer constituting the multi-layered film is measured as follows. The sample to be measured is cooled to a temperature of −60 ° C. or lower with liquid nitrogen, and a razor blade is applied at a right angle to the sample placed on the glass plate to cut the sample to prepare a sample for cross-sectional measurement. The cross-sectional observation of the obtained sample is performed with a scanning electron microscope, the boundary line of each layer is discriminated from the pore shape and the appearance of the composition, and the ratio of each layer thickness obtained from the observation image to the total thickness of the film is determined. The thickness of each layer is defined as a value calculated by multiplying the total thickness of the film obtained by using a thickness gauge by the above ratio of the thickness of each layer.

<Surface resistivity of film>
The energy conversion film is preferably insulating, and the surface resistivity of at least one surface is preferably 1 × 10 ¹³ Ω / □ or more, and more preferably 5 × 10 ¹³ Ω / □ or more. By setting the surface resistivity to the above lower limit value or more, when the electretization treatment is performed, the injected charge is difficult to escape along the surface, and efficient charge injection can be easily performed. On the other hand, the surface resistivity of at least one surface of the energy conversion film is preferably 9 × 10 ¹⁷ Ω / □ or less, and more preferably 5 × 10 ¹⁶ Ω / □ or less. By setting the surface resistivity to the above upper limit or less, it is possible to prevent the adhesion of foreign matter such as dust and dirt, and it is possible to prevent the electretization treatment from being generated by local discharge through the foreign matter during the electretization treatment. Easy to suppress.

In the present specification, the surface resistivity of the film is measured under the conditions of a temperature of 23 ° C. and a relative humidity of 50% using a double ring method electrode according to JIS K6911: 1995 “General Test Method for Thermosetting Plastics”. The value is calculated based on the following formula (2) from the surface resistivity measured in the above.
(2) Kf = RS × π × (D + d) / (Dd)
Kf: Surface resistivity (Ω / □)
RS: Surface resistance (Ω)
π: Pi d: Outer diameter (cm) of the inner circle of the surface electrode
D: Inner diameter (cm) of the annular electrode on the surface

(Manufacturing method of energy conversion film)
The production method of the present invention is a production method for an energy conversion film which is a porous resin film.
In the present invention, an energy conversion film is produced through a kneading step, an extrusion step, and a stretching step.
The kneading step refers to a step of kneading a resin composition containing at least a thermoplastic resin and a pore-forming agent and extruding the obtained kneaded product.
The extrusion step is a step of forming a non-stretched resin film by extruding the kneaded product into a film.
The stretching step refers to a step of stretching a non-stretched resin film in at least a uniaxial direction.

A preferred embodiment of the energy conversion film manufacturing process in the present invention includes, for example, the following manufacturing process (i).
(I) A resin composition containing at least a thermoplastic resin and a pore-forming agent is melt-kneaded by an extrusion kneader, and the obtained kneaded product is extruded into an extruder in an extrusion step (kneading step). Next, the kneaded product is extruded from the T-die with an extruder to eject a film-like unstretched resin film from the die (extrusion step). Next, the unstretched resin film is stretched with a roll, a tenter, or the like (stretching step).

Further, in the production method (i) above, the kneaded product that has undergone the kneading step may be produced as pellets, and the pellets may be subjected to the extrusion step. In that case, as a preferred embodiment, for example, the following manufacturing step (ii) can be mentioned.
(Ii) A resin composition containing at least a thermoplastic resin and a pore-forming agent is melt-kneaded by an extrusion kneader, and the obtained kneaded product is extruded into a strand shape (kneading step), cooled, and then cut with a cutter. To make pellets. Next, the obtained pellets are put into a feeder of an extruder, melt-kneaded, and then the kneaded product is extruded against a T-die by an extruder to discharge a film-like unstretched resin film from the die. (Extrusion process). Next, the unstretched resin film is stretched with a roll, a tenter, or the like (stretching step).

In the present invention, it is possible to provide an energy conversion film having energy conversion performance by subjecting an energy conversion film obtained through a kneading step, an extrusion step, and a stretching step to an electretization treatment.
Further, in the present invention, the energy conversion film after the stretching step may be subjected to a step of oxidizing at least one surface.

Various conventionally known methods can be used for producing the porous resin film. For example, in the case of a porous resin film having a single layer structure, the resin composition containing the above raw materials may be melt-kneaded, extruded from a single die, and stretched if necessary. Further, in the case of a multi-layered porous resin film having a core layer and a skin layer, each layer is laminated by a coextrusion method using a multi-layer die using a feed block or a multi-manifold, an extrusion lamination method using a plurality of dies, or the like. The film can be produced. A coextrusion method using a multilayer die and an extrusion lamination method may be combined.

<Filtration process>
One of the features of the production method of the present invention is that the kneaded product is filtered using a filtration filter having a filtration particle size of 40 to 300 μm in at least one of the kneading step and the extrusion step.
A filtration filter showing a specific filtration particle size can effectively remove coarse particles, foreign substances, agglomerates of pore-forming agents, and the like from the kneaded product. By removing these coarse particles and the like, it is possible to make the performance of the resin film uniform in the plane.
If the pressure fluctuates when the kneaded product is extruded from the die, the uniformity of the thickness of the film obtained by fluctuating the discharge amount decreases, and the performance of the energy conversion film tends to vary. The filtration particle size of the filtration filter used in the present invention is 40 μm or more, but from the viewpoint of reducing pressure fluctuation due to the load of the filtration filter itself and reducing pressure fluctuation due to clogging of the filtration filter, the filtration particle size is 50 μm. The above is preferable, and 100 μm or more is more preferable. On the other hand, the filtration particle size is 300 μm or less, but from the viewpoint of removing coarse particles and the like, 200 μm or less is preferable, and 160 μm or less is more preferable.
The filter has no particular limitation on the material and shape as long as it exhibits the desired filtration particle size, and can be selected according to the purpose. For example, it has heat resistance such as metallic or ceramic. A filtration filter made of a material is preferable. Further, a woven wire mesh having high uniformity of filtration accuracy, specifically, a plain woven wire mesh, a twill woven wire mesh, a flat woven wire mesh, or a twill woven wire mesh is preferably used.

The filtration particle size of the filtration filter can be determined by the bubble point test described in JIS K3832-1990 (ASTM316-86).
For example, specifically, it can be obtained as follows.
The filter element soaked in isopropyl alcohol (surface tension at 25 ° C., 21 dyne / cm) for 10 minutes or more in advance is placed horizontally in a tank containing isopropyl alcohol, and first continuously by the bubble point method conforming to JIS K3832. the pressure P _{a (bubble} point pressure) when the bubble is generated in the measuring. Request filter particle size from the measured pressure _{P A} by Washburn equation represented by the following formula (3).
_{(3) D = 4Scosθ / P} A
D: Filtration particle size (μm)
S: Surface tension of isopropyl alcohol θ: Contact angle (Since isopropyl alcohol has a sufficiently low contact angle, θ = 0, cos θ = 1 here)

The filtration filter showing a specific filtration particle size may be provided in at least one of the kneading step and the extrusion step. A filtration filter can be provided only in the kneading step, a filtration filter can be provided only in the extrusion step, or a filtration filter can be provided in both the kneading step and the extrusion step.
Further, even when a filtration filter is provided in both the kneading step and the extrusion step, if a filtration filter having a filtration particle size of 40 to 300 μm specified in the present invention is used in one of the steps, the other In the step, a filtration filter having a filtration particle size outside the range of 40 to 300 μm may be used.
In the present invention, when the energy conversion film is a multi-layered porous resin film having a core layer and a skin layer, both the core layer and the skin layer are formed by using a filtration filter showing the specific filtration particle size. It is more preferable that at least the core layer is formed by using a filtration filter exhibiting the above-mentioned specific filtration particle size.

The location of the filtration filter in the kneading step and the extrusion step is not particularly limited as long as coarse particles, foreign substances, agglomerates of the pore forming agent, etc. are effectively removed from the kneaded product, and should be appropriately set according to the purpose. Can be done.
For example, in the manufacturing steps (i) and (ii), the installation location of the filtration filter in the kneading step includes the front of the breaker plate of the extrusion kneader. The location of the filtration filter in the extrusion process includes the front of the breaker plate installed at the outlet of the extruder, the back of the outlet of the extruder, or the front of the die or feed block.
A schematic view of the extrusion kneader 21 is shown in FIG. 2 in order to explain the arrangement location of the filtration filter in the kneading step. In FIG. 2, the material 22 of the resin composition is extruded from the nozzle die 25 through the breaker plate 24 while being kneaded through the screw portion 23. Here, the filtration filter 26 can be installed in front of the breaker plate 24.

The porous resin film is preferably a stretched film stretched at least in the uniaxial direction. Stretching improves the uniformity of the thickness of the porous resin film and facilitates the formation of a large number of pores inside. Further, in the case of a multi-layered porous resin film having a core layer and a skin layer, it is preferable that the skin layer is laminated on the core layer and then stretched at least in the uniaxial direction. By laminating the skin layer on the core layer and then stretching the film, the uniformity of the film thickness is improved as compared with the case where the stretched films are laminated, and as a result, the electrical characteristics are improved.

It is desirable that the pores formed in the porous resin film by stretching have a relatively large volume, a relatively large number of pores, and independent shapes from the viewpoint of retaining electric charges. The size of the pores tends to be larger when stretched in the biaxial direction than when stretched only in the uniaxial direction. In particular, by stretching the film in the biaxial directions of the vertical direction (MD: Machine Direction) and the horizontal direction (TD: Transverse Direction), the disk-shaped pores are stretched in the plane direction around the pore forming agent. Can be formed, so that positively and negatively polarized charges can be easily accumulated in the pores by electretization treatment, and the charge retention performance is improved. Therefore, the porous resin film is preferably a biaxially stretched film.

Stretching can be performed by various known methods. Specifically, a longitudinal stretching method using the difference in peripheral speed of the roll group, a transverse stretching method using a tenter oven, a sequential biaxial stretching method in which the longitudinal stretching and the transverse stretching are performed in the forward or reverse order, a rolling method, Examples include a simultaneous biaxial stretching method using a combination of a tenter oven and a linear motor, a simultaneous biaxial stretching method using a combination of a tenter oven and a pantograph, and the like. In addition, a simultaneous biaxial stretching method by the tubular method, which is a stretching method of an inflation film, can be mentioned.

<< Stress relaxation treatment >>
One of the features of the production method of the present invention is that, in the stretching step, stress relaxation treatment is performed in which the degree of stretching the resin film in the stretching axis direction is loosened in the middle of the stretching step.
The stress relaxation treatment is a treatment for reducing the residual strain generated in the resin film by stretching by applying heat to loosen the tension.
By the stress relaxation treatment, the residual strain localized in the vertical direction and / or the horizontal direction of the resin film can be reduced, and the in-plane piezoelectric performance of the obtained energy conversion film can be improved and made uniform. In addition, it has the effect of reducing the increase in strain generated by repeating processing such as electrode forming processing and charge injection processing on the energy conversion film. When the obtained resin film is wound while the strain generated in the stretching process is present, the natural relaxation of the strain generated in the stretching process is added to the residual stress generated in the winding process, and the film uniformity is obtained. It is desirable that the stress relaxation treatment be performed before the winding process, because the stress tends to decrease.
In the stress relaxation treatment, the ratio of loosening the degree of stretching is the maximum value (A) of the degree of stretching when the resin film is stretched in the stretching axis direction and the degree of stretching when the degree of stretching is reduced by the stress relaxation treatment. The result calculated by the following formula (1) using the value (B) of -10.0 to -0.1% is satisfied.
(1) (BA) / A
The value represented by the above formula (1) represents the rate of change in the stretching dimension before and after the stress relaxation treatment. Hereinafter, the value represented by the above formula (1) may be referred to as a relaxation machine magnification.

The relaxation mechanical magnification represented by the above formula (1) is -10.0%, but the stress relaxation treatment is performed so that the stress relaxation treatment is uniformly performed and the piezoelectric performance of the obtained energy conversion film does not vary. From the viewpoint of maintaining the tension applied to the film, -8.0% or more is preferable, and -5.0% or more is more preferable. On the other hand, the relaxation machine magnification is -0.1% or less, but it depends on the treatment temperature and the treatment time, but in order to obtain a sufficient effect, it is preferably -1.0% or less, preferably -1.5%. The following is more preferable.

As a more specific mode for obtaining the above formula (1), for example, the following cases (iii) and (iv) can be mentioned.
(Iii) When the stretching treatment is tenter stretching, the maximum width of stretching in the stretching zone is the above-mentioned value (A), and the width of stretching at the end of the relaxation treatment in the relaxation zone is described above in the stretching performed using the tenter oven. The above equation (1) is calculated as the value (B).
(Iv) When the stretching treatment is roll stretching, the above formula (1) is calculated with the maximum speed of the stretched roll as the above value (A) and the speed of the roll at the end of the relaxation treatment as the above value (B). ..

When the stretching step is performed by using, for example, a biaxial stretching method of longitudinal stretching and transverse stretching, the stress relaxation treatment may be performed in at least one of the longitudinal stretching step and the transverse stretching step. .. The stress relaxation treatment may be performed in the longitudinal stretching step, the stress relaxation treatment may be performed in the transverse stretching step, or the stress relaxation treatment may be performed in both the longitudinal stretching step and the transverse stretching step.

In the present invention, as a preferred embodiment when the resin film having a multilayer structure is stretched by using the biaxial stretching method of longitudinal stretching and transverse stretching, for example, the method shown in the following (v) can be mentioned.
(V) The kneaded product is extruded by three extruders, and in the feed block, the skin layer, the core layer, and the skin layer are laminated in this order and co-extruded from the T die. The co-extruded laminated film of the skin layer / core layer / skin layer is stretched in the longitudinal direction (MD) by a stretching device such as a roll, and then further stretched in the transverse direction (TD) by a stretching device such as a tenter. ..
In this case, the stress relaxation treatment may be performed in a longitudinal stretching step using a roll, in a transverse stretching step using a tenter, or in both a roll and a tenter step. From the viewpoint of ease of adjustment of stress relaxation treatment, it is preferable to perform stress relaxation treatment in the transverse stretching step using a tenter.

Further, in the present invention, when the resin film having a multilayer structure is a laminated film of a skin layer (uniaxially stretched) / core layer (biaxially stretched) / skin layer (uniaxially stretched), a preferred embodiment of the stretching method is For example, the method shown in the following (vi) can be mentioned.
(Vi) The kneaded product forming the core layer is extruded from the T-die by an extruder to form a resin film (core layer). The resin film (core layer) is stretched in the longitudinal direction (MD) by a stretching device such as a roll. Next, the kneaded product forming the skin layer is extruded from the T-die by two extruders, and the extruded resin film (skin layer) is applied to both surfaces of the stretched resin film (core layer) described above. Stacked. The obtained laminated film of the skin layer / core layer / skin layer is stretched in the lateral direction (TD) by a stretching device such as a tenter.
In this case, the stress relaxation treatment may be performed in a longitudinal stretching step using a roll, in a transverse stretching step using a tenter, or in both a roll and a tenter step. From the viewpoint of ease of adjustment of stress relaxation treatment, it is preferable to perform stress relaxation treatment in the transverse stretching step of the tenter.

<<< Stress relaxation treatment in tenter stretching >>>
The stress relaxation treatment in tenter stretching will be described in more detail.
When stretching the tenter, for example, the resin film is stretched using a tenter oven as shown in FIG. In the tenter oven, for example, a plurality of zones are connected in the longitudinal direction of the resin film, and the resin film is preheated, stretched, relaxed, cooled, etc. while being held at a predetermined temperature in each zone. It is configured to perform processing. The above zone is a section corresponding to processing processes such as preheating, stretching, relaxation, and cooling, and is composed of a preheating zone a, a stretching zone b, a relaxation zone c, and a cooling zone d as shown in FIG. ing.
Both ends of the resin film are supported by left and right clips installed in the width direction (horizontal direction), and the resin film is stretched in the lateral direction through the preheating zone a and the stretching zone b. Next, in the relaxation zone c, stress relaxation treatment is performed, and the tensile strength is relaxed as compared with the case of the maximum stretched width, so that the stretched width is narrowed. The width of the stretch when the relaxation zone c ends, after passing through the cooling zone d, the clip is released from the resin film, and the lateral stretching process of the resin film ends.
As the rail pattern of the tenter oven, for example, the five rail patterns A to E shown in Table 1 below can be mentioned. The rail patterns A to E in Table 1 are graphed with the rail width / 2 (mm) on the vertical axis and the distance (zone length) (mm) from the entrance where the preheating zone started on the horizontal axis. Shown in. In the examples described later, experiments were conducted using the five rail patterns A to E shown in Table 1 and FIG.

The rail pattern of B shown in Table 1 above will be described in detail below.
When the resin film is stretched using the rail pattern B, the maximum width of the stretched resin film in the stretch zone (corresponding to t2 in FIG. 3) is 1,215 mm. The stretching width when the degree of stretching is reduced by stress relaxation treatment (stretching width at the end of the relaxation zone (corresponding to t3 in FIG. 3)) is 1,195 mm. Therefore, when the equation (1) is calculated using these values, it becomes as follows.
(T3-t2) / t2 × 100 (%) =
(1,195-1,215) / 1,215 x 100 (%) =
-1.65% (2 digits after the decimal point)
That is, the relaxation mechanical magnification of the rail pattern of B is −1.65%, and when stretched with the rail pattern of B, the requirement of the relaxation mechanical magnification of -10.0 to −0.1% specified in the present invention is satisfied. You can be satisfied. Similarly, the relaxation machine magnification can be obtained for the other rail patterns A and C to E (see the values of the relaxation machine magnification shown in Table 1).
In Table 1, the stretching machine magnification, which indicates how much the width of the resin film changed before and after starting stretching, is also described for reference. This stretching machine magnification indicates the ratio of the resin width at the outlet (corresponding to t4 in FIG. 3) to the resin width at the inlet (corresponding to t1 in FIG. 3) of the tenter oven.
Explaining with the rail pattern of B in Table 1, it is obtained as follows using the resin width (160 mm) at the entrance when entering the preheating zone a and the resin width (1,195 mm) at the cooling zone d at the outlet. ..
t4 / t1 = 1,195 / 160 = 7.47 (2 digits after the decimal point)

The temperature during stretching is preferably 1 to 70 ° C. lower than the melting point Tm of the crystal portion of the main thermoplastic resin from the glass transition point of the main (mostly used in terms of mass ratio) thermoplastic resin used for the resin film. Specifically, when the main thermoplastic resin is a propylene homopolymer (melting point 155 to 167 ° C.), it is preferably in the range of 100 to 166 ° C., and high-density polyethylene (melting point 121 to 136 ° C.). In some cases, it is preferably in the range of 70 to 135 ° C. When a resin film having a multilayer structure is stretched, the stretching efficiency of the layer having the largest set basis weight (usually the core layer) or the layer having the highest set porosity (usually the core layer) is taken into consideration. It is preferable to set the temperature. Of course, if the stretching temperature is determined by using thermoplastic resins having different melting points or glass transition points for the core layer and the skin layer, the porosity of each layer can be adjusted.

The treatment temperature of the resin film in the stretching step, particularly in the stress relaxation treatment, is Tma-20 ° C. when the melting point Tm of the thermoplastic resin having the highest content among the thermoplastic resins constituting the resin film is Tma. It is preferable that the temperature range is equal to or higher than the temperature and lower than the temperature of Tma + 10 ° C. The higher the temperature of the stress relaxation treatment, the easier it is to obtain the effect. Therefore, the temperature of Tma-15 ° C. or higher is more preferable, Tma-10 ° C. or higher is further preferable, and the temperature of Tma-5 ° C. or higher is particularly preferable. On the other hand, if the resin film is melted, the uniformity of the thickness of the resin film is impaired, and the formed pores are crushed, so that the uniformity of the performance as an energy conversion film is lowered, but the stretched resin film The melting point of is higher than the general melting point of the thermoplastic resin (measured by JIS K7121-1987) that composes it due to the thermal history until stretching and crystallization by stretching orientation. Therefore, the treatment temperature of the stress relaxation treatment is more preferably Tma + 8 ° C. or lower, further preferably Tma + 5 ° C. or lower, and particularly preferably Tma + 3 ° C. or lower. The processing time of the resin film in the stress relaxation treatment is preferably in the range of 1 to 300 seconds.

The draw ratio is not particularly limited, and may be appropriately determined in consideration of the draw characteristics of the thermoplastic resin used for the resin film, the desired porosity, and the like. For example, when a propylene homopolymer or a copolymer thereof is used as the main thermoplastic resin, the draw ratio is preferably 1.2 times or more, more preferably 2 times or more when stretched in the uniaxial direction. On the other hand, the draw ratio is preferably 12 times or less, more preferably 10 times or less. Further, in the case of stretching in the biaxial direction, the area stretching ratio (product of the longitudinal magnification and the horizontal magnification) is preferably 1.5 times or more, and more preferably 4 times or more. On the other hand, the same area stretching ratio is preferably 60 times or less, more preferably 50 times or less.

When using other thermoplastic resins, the draw ratio is preferably 1.2 times or more, more preferably 2 times or more when stretching in the uniaxial direction. On the other hand, the draw ratio is preferably 10 times or less, more preferably 5 times or less. Further, in the case of stretching in the biaxial direction, the area stretching ratio is preferably 1.5 times or more, more preferably 4 times or more. On the other hand, the same area stretching ratio is preferably 20 times or less, more preferably 12 times or less.

When stretching in the biaxial direction in a porous resin film, setting the longitudinal magnification and the lateral magnification to the same magnification as much as possible forms a disk-shaped pore that easily accumulates electric charges, and a cross section in an arbitrary direction. It is easy to adjust the shape or frequency of the pores observed in the above to the above-mentioned preferable range. Therefore, when stretching in the biaxial direction, the ratio of the vertical magnification to the horizontal magnification (vertical magnification / horizontal magnification) is preferably 0.40 or more, more preferably 0.45 or more, and 0. It is particularly preferably .50 or more. On the other hand, the ratio of the vertical magnification to the horizontal magnification (vertical magnification / horizontal magnification) is preferably 2.5 or less, more preferably 2.0 or less, and further preferably 1.5 or less. , 1.3 or less is particularly preferable. The stretching speed is preferably 10 to 350 m / min from the viewpoint of stable stretch molding.

The method for producing an energy conversion film of the present invention includes a filtration step of filtering a kneaded product of a resin composition using a specific filtration filter, and a stress relaxation treatment for relaxing the degree of stretching to a specific ratio when the resin film is stretched. It is a feature to do both of. As shown in the following examples, by performing both of these treatments, it is possible to produce an energy conversion film in which the effects of both are combined, there is no variation in the in-plane piezoelectric performance, and the in-plane performance uniformity is excellent. it can.

<Oxidation treatment>
In the present invention, one surface or both sides of the porous resin film may be oxidized. Further, a step of performing a washing treatment after the oxidation treatment and a step of performing a washing treatment and then a drying treatment may be performed.
The oxidation treatment is not particularly limited as long as the oxygen atom concentration on the surface can be increased, and a known oxidation treatment can be used. Specific examples of the oxidation treatment include dielectric barrier discharge treatment, frame treatment, ozone treatment and the like.

FIG. 5 shows an example of the manufacturing process of the energy conversion film 1. This manufacturing process is an example, and the process differs depending on the layer structure of the film, the number of stretched axes, and the like.

According to the example shown in FIG. 5, the resin composition is kneaded and extruded by three kneading extruders 29 to 31. Next, the three-layer structure energy conversion film 1 can be manufactured by the extruders 32 to 34. For example, the kneaded product of the resin composition of each layer is extruded by three extruders 32 to 34, and the skin layer, the core layer, and the skin layer are laminated in this order in the feed block 35 and co-extruded from the T die 36.

The co-extruded skin layer / core layer / skin layer laminated film is cooled by the cooling roll 37, stretched in the longitudinal direction (MD) by the stretching device 38, and then further laterally (TD) by the stretching device 39. It is stretched. The stretched film is wound as a take-up roll 41 via an oxidation treatment device 40 which may be appropriately set.

<Pressure treatment>
The pores inside the porous resin film can be further expanded by pressure treatment. In the pressure treatment, a porous resin film is placed in a pressure vessel, and the inside of the vessel is pressurized with a non-reactive gas to allow the non-reactive gas to permeate into the pores, and then the porous resin film is placed under non-pressurization. Do it by releasing it.

Specific examples of the non-reactive gas used include an inert gas such as argon and helium, nitrogen, carbon dioxide or a mixed gas thereof, air and the like. Although the expansion effect can be obtained even when a gas other than the non-reactive gas is used, it is desirable to use the non-reactive gas from the viewpoint of safety during the pressurization treatment and the safety of the porous resin film. The processing pressure during the pressurization treatment is not particularly limited, but is preferably in the range of 0.2 to 10 MPa, more preferably 0.3 to 8 MPa, and even more preferably 0.4 to 6 MPa. When it is 0.2 MPa or more, the pressure is sufficient and a sufficient expansion effect can be easily obtained. On the other hand, when it is 10 MPa or less, when the porous resin film is released under non-pressurization, it is possible to prevent the pore wall from breaking because it cannot withstand the internal pressure, and the pores tend to maintain the state of independent holes. The treatment time of the pressurization treatment is not particularly limited, but is preferably in the range of 1 hour or more, more preferably 1 to 50 hours. In the case of the porous resin film of the present invention, if the treatment time is 1 hour or more, the pores can be sufficiently filled with the non-reactive gas. In a porous resin film in which the pores are sufficiently filled with the non-reactive gas in less than one hour, the non-reactive gas is dissipated during the heat treatment described later, and a stable expansion effect is obtained. Tends to be difficult to obtain.

When the take-up roll of the porous resin film is pressure-treated, it is desirable to take it together with a buffer sheet and then pressurize it so that the non-reactive gas easily permeates into the inside of the take-up roll. Examples of the cushioning sheet include a foamed polystyrene sheet, a foamed polyethylene sheet, a foamed polypropylene sheet, a non-woven fabric, a woven fabric, a sheet having a communication gap such as paper.

<Heat treatment>
The pressure-treated porous resin film is preferably heat-treated from the viewpoint of maintaining its expansion effect. The porous resin film expands by performing a pressure treatment and releasing it under non-pressurization. However, if left as it is, the non-reactive gas that has permeated into the pores will gradually escape, and the porous resin film may return to its original thickness. Therefore, it is desirable to heat-treat the expanded porous resin film to promote the crystallization of the thermoplastic resin so that the expansion effect can be maintained even after the inside of the pores drops to atmospheric pressure.

The heat treatment can be performed within a temperature range equal to or higher than the glass transition point of the main thermoplastic resin of the porous resin film and lower than the melting point of the crystal part. Specifically, for example, when the main thermoplastic resin is a propylene homopolymer (melting point 155 to 167 ° C.), it is in the range of 80 to 160 ° C. Moreover, a known method can be used as a heating method. Specific examples include, but are not limited to, hot air heating by hot air from a nozzle, radiant heating by an infrared heater, contact heating by a roll with a temperature control function, and the like. During the heat treatment, the elastic modulus of the porous resin film decreases and the pores are easily crushed when a load is applied. Therefore, non-contact heat treatment such as hot air heating or radiant heating tends to maintain a high expansion ratio. There is a tendency.

<Electret processing>
The electretization process is a process of injecting an electric charge into an energy conversion film.
As the electretization process, there are several processing methods. For example, there are known an electro-electret method in which both sides of a film are held by a conductor and a DC high voltage or a pulsed high voltage is applied, and a radio electret method in which the film is irradiated with γ-rays or electron beams to form an electret. Is.

Among these, the electro-electretization method using DC high-voltage discharge has a small device, a small load on workers and the environment, and is suitable for electretization of polymer materials such as porous resin films. It is preferable.

FIG. 6 shows an electretizing device by DC high voltage discharge as an example of the electretizing device. As shown in FIG. 6, in this electretizing device, the energy conversion film 1 is fixed between the needle-shaped electrode 11 and the ground electrode 12 connected to the DC high-voltage power supply 10 and a predetermined voltage is applied. By applying a voltage, the energy conversion film 1 can accumulate a large amount of electric charges inside the film.

The applied voltage during the electretization process is the film thickness, pore ratio, material of the thermoplastic resin or pore forming agent used, processing speed, shape and material of the electrode used, size, and the amount of charge desired for the energy conversion film. It may be set appropriately in consideration of the above. The applied voltage is not particularly limited, but is preferably 5 kV or higher, more preferably 6 kV or higher, and even more preferably 7 kV or higher. By setting the value to the above lower limit or more, a sufficient amount of electric charge can be injected, and the desired piezoelectric performance tends to be easily exhibited. On the other hand, the applied voltage of the electret formation treatment is preferably 100 kV or less, more preferably 70 kV or less, and further preferably 50 kV or less. By setting it to the above upper limit or less, a phenomenon that local spark discharge occurs during the electretization process and partial destruction such as pinholes occurs in the film, and the ground electrode is transmitted from the film surface to the end face during the electretization process. There is a tendency to avoid the phenomenon that the current flows to the electret and the efficiency of the electret processing deteriorates.

The treatment temperature at the time of electrification treatment may be appropriately set and is not particularly limited, but it is preferably performed at the glass transition point or higher and the melting point of the crystal portion or lower of the main thermoplastic resin used for the energy conversion film. When the treatment temperature is above the glass transition point, the molecular motion of the amorphous part of the thermoplastic resin is active, and the molecular arrangement is suitable for the given charge, so that efficient electrification processing becomes possible. .. Further, when the energy conversion film contains metal soap, if the processing temperature is equal to or higher than the melting point of the metal soap, the metal soap molecules also form an arrangement suitable for the given electric charge, so that more efficient electrifying treatment is possible. It becomes. On the other hand, in order to avoid that the energy conversion film itself cannot maintain its structure and it becomes difficult to obtain the desired performance, it is preferable that the processing temperature does not exceed the melting point of the main thermoplastic resin used for the energy conversion film. ..

In the electretization process, an excessive charge may be injected into the energy conversion film intentionally or unintentionally. In this case, from the viewpoint of preventing the energy conversion film from being discharged after the electretization treatment and causing inconvenience in the post-processing process, it is preferable to perform the static elimination treatment of the excess charge after the electretization treatment.

As the static elimination treatment, a known method using a voltage application type static eliminator (ionizer), a self-discharge type static eliminator, or the like can be used. In the static elimination treatment using these general static eliminators, the surface charge of the energy conversion film can be removed, but the charge accumulated inside the film, particularly in the pores of the core layer, cannot be completely removed. Therefore, the static elimination treatment does not significantly reduce the performance of the electret material. Therefore, it is possible to prevent the discharge phenomenon of the energy conversion film by performing such a static elimination treatment to remove the excess charge on the film surface.

[Manufacturing method of energy conversion element]
The method for manufacturing an energy conversion element of the present invention is a method for forming electrodes on at least one surface of the energy conversion film manufactured by the above method for manufacturing an energy conversion film.

(Energy conversion element)
The energy conversion element according to the present invention includes the above-mentioned energy conversion film and electrodes provided on at least one surface of the energy conversion film. The energy conversion element according to the present invention inputs / outputs electric power or an electric signal, and from the viewpoint of more efficient input / output, usually a pair of electrodes are provided on both sides of the energy conversion film. preferable.

FIG. 7 shows the configuration of the energy conversion element 5 including the energy conversion film 1 shown in FIG. 1 as an embodiment of the energy conversion element according to the present invention.
As shown in FIG. 7, the energy conversion element 5 includes an energy conversion film 1 and an electrode 6 on one surface thereof. The energy conversion element 5 can include an electrode 7 on the other surface of the energy conversion film 1.

The timing of installing the electrodes is not particularly limited, and may be, for example, before or after the electretization process. After the electretization process, it is possible to prevent a part of the injected charge from being dissipated through the electrodes during the electretization process. However, when a load such as heat is applied to the film during the subsequent electrode installation, a part of the injected charge is dissipated, and the piezoelectric performance may be slightly deteriorated. At present, judging from the performance of the energy conversion element finally obtained, an electrode is provided in advance on at least one surface of the energy conversion film before the electretization treatment, and then the above-mentioned electretization treatment is performed. Is preferable.

<Types of electrodes and method of forming electrodes>
Examples of the electrode include a thin film formed of a known conductive material such as metal particles, conductive metal oxide particles, carbon-based particles, or a conductive resin. Examples of the electrode include a coating film obtained by printing or coating a conductive paint, a metal vapor deposition film, and the like.

Examples of the conductive material include metal particles such as gold, silver, platinum, copper, and silicon; tin-doped indium oxide (ITO), antimon-doped tin oxide (ATO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide, and the like. Conductive metal oxide particles; Examples thereof include materials obtained by mixing carbon-based particles such as graphite, carbon black, Ketjen black, carbon nanofiller, and carbon nanotubes with a solution or dispersion of a binder resin component. Examples of the binder resin component include acrylic resin, urethane resin, ether resin, ester resin, epoxy resin, vinyl acetate resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, amide resin, and melamine resin. Examples thereof include phenol resin, vinyl alcohol resin, and modified polyolefin resin. Further, examples of the conductive material include a solution or dispersion of a conductive resin such as a polyaniline resin, a polypyrrole resin, and a polythiophene resin.

Examples of the printing method when the electrode is provided by printing using the ink of the conductive material include screen printing, flexo printing, gravure printing, inkjet printing, letterpress printing, offset printing and the like. Further, as a coating device when an electrode is provided by coating using a paint of a conductive material, for example, a die coater, a bar coater, a comma coater, a lip coater, a roll coater, a curtain coater, a gravure coater, a spray coater, and a blade. Examples include coaters, reverse coaters, and air knife coaters.

As the metal vapor deposition film, for example, a metal such as aluminum, zinc, gold, silver, platinum, nickel is vaporized under reduced pressure and vapor-deposited on the surface of the energy conversion film, and a metal thin film formed directly on the surface is used, as well as polyethylene terephthalate. (PET) A metal thin film formed by depositing a metal such as aluminum, zinc, gold, silver, platinum, nickel on a carrier such as a film, and a metal thin film transferred to the surface of an energy conversion film. Be done.

The electrode is a laminate in which an electrode such as a coating film of the conductive paint or a metal vapor deposition film is previously formed on a dielectric film such as a polyethylene terephthalate film or a polypropylene film, and an energy conversion film so that the electrode is on the outside. It may be provided by laminating. Examples of the laminating method include known methods such as dry laminating, wet laminating, and extruded laminating.

The thickness of the electrode is not particularly limited, but is preferably 0.1 μm or more, more preferably 1 μm or more, and further preferably 5 μm or more. The thickness of the electrode is preferably 200 μm or less, more preferably 50 μm or less, and further preferably 20 μm or less.

<Surface resistivity of electrodes>
The surface resistivity of the electrode is preferably 1 × 10 ^-3 Ω / □ or more, and preferably 1 × 10 ^-1 or more, for the purpose of easily inputting and outputting electric power. When an electrode of 1 × 10 ^-3 Ω / □ or more is provided and the electrode is provided by coating, it is not necessary to provide the electrode thickly, and it is porous due to the heat of drying and sintering after coating. Deformation such as crushing of pores in the resin film and heat shrinkage of the porous resin film can be suppressed. Further, when the electrodes are provided by metal vapor deposition, it is possible to suppress film deformation due to the heat of the deposited metal. On the other hand, the surface resistivity of the electrode is preferably 9 × 10 ⁷ Ω / □ or less, and more preferably 9 × 10 ⁴ Ω / □ or less. When the resistance value of the electrode is 9 × 10 ⁷ Ω / □ or less, the transmission efficiency of the electric signal is high, and the performance as a material for electric and electronic input / output devices tends to be improved.

In the present specification, the resistivity of the electrode surface is calculated by the following formula (4) from the resistivity value measured by the four-probe method according to JIS K7194: 1994 "Resistance test method by the four-probe method of conductive plastic". The value calculated based on this.
(4) Ke = F × R
Ke: Surface resistivity (Ω / □)
F: Correction coefficient (described in JIS K7194)
R: Resistance value (Ω)

<Plan view area>
Since the energy conversion film and the energy conversion element according to the present invention use a porous resin film, they are relatively inexpensive, unlike semiconductor materials that have been widely used as materials for electric-mechanical energy conversion. For example, it is easy to increase the area of the film by about 10 to 50,000 cm ^{2 in} a plan view. When a large-area energy conversion film and an energy conversion element are configured, the plan-viewing area thereof may be appropriately set in consideration of desired performance, physical restrictions on the installation location, etc., and is not particularly limited, but 20 to 20 to 30,000 cm ² is preferable, and 50 to 25,000 cm ² is more preferable.

<Generated voltage>
The voltage (average value) generated by the impact of the energy conversion element is preferably 10 mV or more, more preferably 30 mV or more, and more preferably 50 mV or more from the viewpoint of practical performance of the energy conversion element. Is more preferable, and 100 mV or more is particularly preferable. The upper limit is not particularly limited, but is preferably 1000 mV or less, more preferably 700 mV or less, further preferably 500 mV or less, and particularly preferably 300 mV or less.

The generated voltage can be measured by a ball drop test. Specifically, an iron ball having a height of 8 mm in the vertical direction, a diameter of 9.5 mm, and a mass of 3.5 g is placed on an energy conversion element placed on a horizontal plane in an environment of a temperature of 23 ° C. and a relative humidity of 50%. The generated voltage generated by the impact when naturally dropped is measured 10 times, and the value obtained by calculating the average value of the generated voltage is defined as the generated voltage.
The energy conversion film of the present invention has no in-plane variation in piezoelectric performance and has high in-plane piezoelectric performance uniformity. Specifically, an energy conversion film having a fluctuation value of the voltage generated by the falling ball test of 7 or less can be obtained. The fluctuation value of the voltage generated by the falling ball test of the energy conversion film of the present invention is preferably less than 5. The fluctuation value of the generated voltage is preferably low, but the lower limit is usually about 1. The method of measuring the fluctuation value of the generated voltage is as described in Examples described later.
As a result, by using the energy conversion film of the present invention, it is possible to obtain an energy conversion element having no in-plane performance variation and no piezoelectric performance variation even when an energy conversion element having a large area is formed, for example. Can be done.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. In addition, the description of "part", "%", etc. in the Examples means the description of the mass standard unless otherwise specified.
It was.

(Resin composition a)
Propropylene homopolymer (manufactured by Nippon Polypro Co., Ltd., trade name: Novatec PP FY4, MFR (230 ° C, 2.16 kg load): 5 g / 10 minutes, melting point: 162 ° C, density: 0.91 g / cm ³ ) 71. 8 parts by mass, high density polyethylene (manufactured by Nippon Polyethylene Co., Ltd., trade name: Novatec HD HJ360, MFR (190 ° C, 2.16 kg load): 5.5 g / 10 minutes, melting point: 131 ° C, density: 0.95 g / cm ³ ) 10 parts by mass, heavy calcium carbonate powder (manufactured by Bikita Powder Industry Co., Ltd., trade name: BF100, average particle size 10.1 μm (median diameter D50), density: 2.7 g / cm ³ ) 18 parts by mass , And 0.2 parts by mass of oleic acid (trade name: Lunac OA, manufactured by Kao Co., Ltd.) as a dispersant, and melt-kneaded with a twin-screw kneader set at 210 ° C. Next, the resin composition a was extruded into a strand shape with a kneading extruder set at 230 ° C., cooled, and then cut with a strand cutter to prepare pellets of the resin composition a.
A filtration filter having a filtration particle size of 360 μm was installed in front of the breaker plate of the kneading extruder.

(Resin composition b)
Propropylene homopolymer (manufactured by Nippon Polypro Co., Ltd., trade name: Novatec PP FY4, MFR (230 ° C, 2.16 kg load): 5 g / 10 minutes, melting point: 162 ° C, density: 0.91 g / cm ³ ) 71. 8 parts by mass, high density polyethylene (manufactured by Nippon Polyethylene Co., Ltd., trade name: Novatec HD HJ360, MFR (190 ° C, 2.16 kg load): 5.5 g / 10 minutes, melting point: 131 ° C, density: 0.95 g / cm ³ ) 10 parts by mass, heavy calcium carbonate powder (manufactured by Bikita Powder Industry Co., Ltd., trade name: BF100, average particle size 10.1 μm (median diameter D50), density: 2.7 g / cm ³ ) 18 parts by mass , And 0.2 parts by mass of oleic acid (trade name: Lunac OA, manufactured by Kao Co., Ltd.) as a dispersant, and melt-kneaded with a twin-screw kneader set at 210 ° C. Next, the resin composition b was extruded into a strand shape with a kneading extruder set at 230 ° C., cooled, and then cut with a strand cutter to prepare pellets of the resin composition b.
A filtration filter having a filtration particle size of 125 μm was installed in front of the breaker plate of the kneading extruder.

(Resin composition c)
Propylene homopolymer (manufactured by Japan Polypropylene Corporation, trade name: Novatec PP FY4, MFR (230 ° C, 2.16 kg load): 5 g / 10 minutes, melting point: 162 ° C, density: 0.91 g / cm ³ ) 99 mass 1 part by mass of heavy calcium carbonate powder (manufactured by Bikita Powder Industry Co., Ltd., trade name: BF100, average particle size 10.1 μm (median diameter D50), density: 2.7 g / cm ³ ) The mixture was melt-kneaded with a twin-screw kneader set at 210 ° C. Next, the resin composition c was extruded into a strand shape with a kneading extruder set at 230 ° C., cooled, and then cut with a strand cutter to prepare pellets of the resin composition c.
A filtration filter having a filtration particle size of 150 μm was installed in front of the breaker plate of the kneading extruder.

Table 2 below shows the specifications of the composition of the resin compositions a to c and the specifications of the filtration filter used when the kneaded product of the resin compositions a to c was filtered.

(Production example of porous resin film)
The pellets of the resin composition for the skin layer 1, the resin composition for the core layer, and the resin composition for the skin layer 2 shown in Table 3 below were charged into the feeders of the three extruders, respectively, at 230 ° C. After melt-kneading with, the mixture was supplied to a feed block type multilayer die set at 250 ° C., laminated in the order of laminating skin layer / core layer / skin layer, and extruded into a sheet. This was cooled to 60 ° C. by a cooling device to obtain a three-layer unstretched sheet. A screen changer was installed between the extruder of the core layer and the feed block, and the extrusion process filtration filter shown in Table 3 was arranged. Further, a flat weave # 120 was arranged as an extrusion process filtration filter in front of the breaker plate in front of the outlet of the extruder of the skin layer 1 and the skin layer 2.

The obtained unstretched sheet is heated to the temperature in the vertical direction shown in Table 3 using a heating roll, and the vertical direction shown in Table 3 is used in the vertical direction (MD direction) by utilizing the peripheral speed difference of the roll group. A uniaxially stretched sheet was obtained by stretching at a magnification of. Next, the obtained uniaxially stretched sheet was cooled to 60 ° C. and set to the stretching temperature and rail pattern (corresponding to the rail pattern shown in Table 1) shown in Table 3 in the lateral direction (TD direction) using a tenter. It was stretched at the lateral magnification shown in Table 3. Then, stress relaxation treatment was performed at the relaxation temperature and relaxation machine magnification shown in Table 3 to obtain a biaxially stretched sheet.

The obtained biaxially stretched sheet was cooled to 60 ° C., the ears were slit, and then both sides were subjected to surface oxidation treatment by dielectric barrier discharge, and Examples 1 to 5 having the physical characteristics shown in Table 3 and Comparative Examples. Energy conversion films 1 to 4 were obtained.

(Example 1)
An energy conversion element was manufactured as follows.
<Manufacturing of energy conversion elements>
Thickness 12μm PET film (trade name: E5200, manufactured by Toyobo Co., Ltd.) using a vacuum vapor deposition apparatus of roll-to-roll process in a vacuum condition of 1 × ¹⁰ -2 Pa, as the thickness of the deposited film is 30nm Aluminum vapor deposition was performed to prepare a metal vapor deposition film having a surface resistance of 1Ω / □ on the vapor deposition surface. On the other hand, 100 parts of a polyester-based adhesive (trade name: ROBOND L-292, manufactured by Toyo Morton, solid content concentration 45% by mass) and an isocyanate-based curing agent (trade name: ROBOND CR3A, manufactured by Toyo Morton, solid content concentration). 2 parts (100% by mass) and 55 parts of water were mixed to prepare an adhesive coating material having a solid content concentration of 30% by mass.

Using the laminating device shown in FIG. 8, a gravure coater 44 was used on the surface of the previously produced metal-deposited film 42 that had not been vapor-deposited so that the coating amount after drying was 2 g / m ^2. The adhesive paint was applied, dried in an oven 46 at 80 ° C., and then pressure-bonded to the energy conversion film 41 with nip rolls (47 and 48) to obtain an energy conversion film 43 having a conductive layer on one side.

Using the manufacturing apparatus shown in the schematic view of FIG. 9, the energy conversion film 43 having the conductive layer on one side is unwound, and the corona processing apparatus 49 is used on the surface on which the conductive layer is not installed to use a DC type corona. A charge injection process by discharge was carried out to obtain an electretized energy conversion film 51.
As the conditions for the charge injection process, the distance between the needle-shaped electrode 50 and the counter electrode roll 45 in FIG. 9 was set to 1 cm, the discharge voltage was set to 12 kV, the processing width was set to 300 mm, and the processing speed was set to 10 m / min.

Using the bonding device shown in FIG. 10, the gravure coater 44 was used on the surface of the previously prepared metal-deposited film 42 that had not been vapor-deposited so that the coating amount after drying was 2 g / m ^2. After applying the adhesive paint and drying in an oven 46 at 80 ° C., the electrified energy conversion film 51 is pressure-bonded to the surface of the energy conversion film 51 not provided with the metal-deposited film with nip rolls (47 and 48) to form conductive layers on both sides. An energy conversion film 52 provided was obtained.
The energy conversion film 52 provided with the obtained conductive layer was cut out to a width of 300 mm and a length of 3 m to prepare a sample for performance evaluation.

(Examples 2 to 5, Comparative Examples 1 to 4)
In the same manner as in Example 1, for the energy conversion films of Examples 2 to 5 and Comparative Examples 1 to 4, an electretized energy conversion film having conductive layers on both sides was obtained, and further for performance evaluation. A sample was prepared.

(Performance when used for energy conversion elements)
The energy conversion elements of each Example and Comparative Example were manufactured, and the power generation performance of the energy conversion elements was evaluated.
<Generated voltage>
The generated voltage was measured in an environment of a temperature of 23 ° C. and a relative humidity of 50% using the ball drop test apparatus shown in FIG. First, a lead wire 17 and a lead wire 17 and a conductive tape (trade name: AL-25BT, manufactured by Sumitomo 3M Ltd.) are used for the electrodes on the front and back surfaces of the performance evaluation sample 20 (energy conversion film 5) having a width of 30 cm and a length of 3 m. One end of 18 was attached, and the other ends of

lead wires

17 and 18 were connected to a high-speed recorder 19 (trade name: GR-7000, manufactured by Keyence).

Next, the performance evaluation sample 20 was placed on the insulating sheet 15 (soft vinyl chloride sheet, thickness 1 mm) of the ball drop test apparatus shown in FIG. 11 with the surface facing up. A glass plate 14 (thickness 8 mm) was placed on the upper surface of the sample 20, and an iron ball 16 having a diameter of 9.5 mm and a mass of 3.5 g was placed on the glass plate 14.
Next, the iron ball 16 is naturally dropped from the glass plate 14 onto the performance evaluation sample 20 from a height of 8 mm in the vertical direction, and the voltage signal from the performance evaluation sample 20 is taken into the high-speed recorder 19 and generated by the impact of the falling ball. The generated voltage was measured 5 times per measurement point, and the average value of the generated voltage (mV) was used as a representative value of one measurement point. This measurement was performed by measuring a total of 90 points, 3 points in the width direction and 30 points in the flow direction of the sample for performance evaluation, and the mean value (Ave), standard deviation (σ), and volatility (CV = σ / Ave * 100) of the 90 points. ) Was calculated.

Table 4 shows the evaluation results of measuring the generated voltage of the energy conversion elements of Examples 1 to 5 and Comparative Examples 1 to 4.
The volatility CV was evaluated according to the following criteria.
◎ Less than 5 〇 5-7
Greater than x7

As shown in Table 4, the energy conversion film of the example has no in-plane performance variation and excellent in-plane performance uniformity even when a large-area energy conversion element is formed. It turned out to be a good thing.

This application claims priority based on Japanese Patent Application No. 2019-143297, which is a Japanese patent application filed on August 2, 2019, and incorporates all the contents of the Japanese patent application.

1 ... energy conversion film, 2 ... core layer, 3,4 ... skin layer, 5 ... energy conversion element, 6,7 ... electrode

Claims

A method for manufacturing an energy conversion film, which is a porous resin film.
A kneading step of kneading a resin composition containing a thermoplastic resin and a pore-forming agent and extruding the obtained kneaded product,
An extrusion step of forming a non-stretched resin film by extruding the kneaded product into a film, and
A stretching step of stretching the unstretched resin film in at least a uniaxial direction is included.
At least one of the kneading step and the extrusion step comprises a step of filtering the kneaded product using a filtration filter having a filtration particle size of 40 to 300 μm.
The stretching step includes a stress relaxation treatment that loosens the degree to which the resin film is stretched in the stretching axis direction in the middle of the stretching step.
The following formula (1) is used using the maximum value (A) of the degree of stretching when the resin film is stretched in the stretching axis direction and the value (B) of the degree of stretching when the degree of stretching is reduced by the stress relaxation treatment. ) Satisfies -10.0 to -0.1%.
(1) (BA) / A
A method for producing an energy conversion film, which is characterized in that.
The method for producing an energy conversion film according to claim 1, wherein the filtration filter is a plain weave wire mesh, a twill weave wire mesh, a flat woven wire mesh, or a twill woven wire mesh.
The method for producing an energy conversion film according to claim 1 or 2, wherein the processing temperature of the resin film in the stress relaxation treatment is not less than the temperature of Tma-20 ° C. and not more than the temperature of Tma + 10 ° C. However, the melting point Tm of the thermoplastic resin having the highest content among the thermoplastic resins constituting the resin film is defined as Tma.
The method for producing an energy conversion film according to any one of claims 1 to 3, wherein the processing time of the resin film in the stress relaxation treatment is 1 to 300 seconds.
The method for producing an energy conversion film according to any one of claims 1 to 4, wherein the pore-forming agent is a filler or a foaming agent.
An energy conversion characterized by forming electrodes on at least one surface of the energy conversion film with respect to the energy conversion film produced by the method for producing an energy conversion film according to any one of claims 1 to 5. Method of manufacturing the element.
An energy conversion film composed of a porous resin film containing a thermoplastic resin and a pore forming agent, and having a fluctuation value of a voltage generated by a ball drop test of 7 or less.