Stretched film

Abstract

The present invention addresses the problem of providing a film containing polylactic acid and having a thickness of 20 μm or less, with reduced variation in piezoelectric constant. The problem is solved by a stretched film containing polylactic acid, the stretched film having a thickness of 5-20 μm, a standard deviation of thickness in a first direction of 0.5 μm or less, and a standard deviation of thickness in a second direction orthogonal to the first direction of 0.8 μm or less.

Description

Stretched film

[0001] The present invention relates to a stretched film that can be used as a piezoelectric film, and the like.

[0002] Polylactic acid films do not require poling treatment to exhibit piezoelectricity, and have high piezoelectric constants, high transparency, low pyroelectricity and other properties, so their application to piezoelectric films and piezoelectric elements has been studied. In Patent Document 1, in order to obtain a polylactic acid-based resin piezoelectric film in which the molecular orientation direction is aligned in one direction even when biaxial stretching is performed, the film exhibits high piezoelectricity, and can be increased in area or reduced in thickness, a technique of adjusting the angle formed between the principal axis direction of the orientation angle of the film and the TD direction to within 5° has been proposed.

[0003] On the other hand, with the popularization of IoT (Internet of Things), wearable devices and the like, there are increasing demands for size reduction and thickness reduction of sensors such as piezoelectric elements.

[0004] Japanese Unexamined Patent Application Publication No. 2022-063539

[0005] The inventor of the present invention has conducted intensive research aimed at reducing the thickness of polylactic acid films used as piezoelectric films, and has found that polylactic acid films with a thickness of 20 µm or less have large variations in piezoelectric constant.

[0006] An object of the present invention is to provide a film containing polylactic acid having a thickness of 20 µm or less, in which variation in piezoelectric constant is suppressed.

[0007] As a result of intensive research conducted by the inventor of the present invention in view of the above problems, the above problems can be solved by a stretched film that contains polylactic acid, has a thickness of 5 µm or more and 20 µm or less, has a standard deviation of thickness in a first direction of 0.5 µm or less, and has a standard deviation of thickness in a second direction orthogonal to the first direction of 0.8 µm or less. The inventor of the present invention has further advanced research based on this finding, and as a result completed the present invention. That is, the present invention includes the following aspects.

[0008] Item 1. A stretched film containing polylactic acid, having a thickness of 5 μm or more and 20 μm or less, a standard deviation of thickness in a first direction of 0.5 μm or less, and a standard deviation of thickness in a second direction perpendicular to the first direction of 0.8 μm or less.

[0009] Item 2. The stretched film according to Item 1, wherein the polylactic acid content is 50% by mass or more relative to 100% by mass of the stretched film.

[0010] Item 3. The stretched film according to item 1 or 2, wherein the standard deviation of the thickness in the first direction is 0.4 μm or less, and the standard deviation of the thickness in the second direction is 0.6 μm or less.

[0011] Item 4. A stretched film according to any one of items 1 to 3, which is a single-layer film.

[0012] Item 5. A stretched film according to any one of items 1 to 4, which is a uniaxially oriented film.

[0013] Item 6. A stretched film according to any one of items 1 to 5, which is a piezoelectric film.

[0014] Item 7. The stretched film according to Item 6, wherein the piezoelectric constant d14 is 6.0 pC / N or more and 7.8 pC / N or less.

[0015] Item 8. The stretched film according to item 6 or 7, wherein the standard deviation of the piezoelectric constant d14 is less than 0.20 pC / N.

[0016] Item 9. A laminated film comprising a stretched film as described in any of Items 1 to 8, and other layers disposed on a first surface and / or a second surface of the stretched film.

[0017] Item 10. The laminated film according to item 9, wherein the other layer includes a protective film.

[0018] Item 11. The laminated film according to item 9 or 10, wherein the other layer includes a conductive layer.

[0019] Item 12. A piezoelectric element comprising the laminated film described in Item 11.

[0020] According to the present invention, it is possible to provide a film containing polylactic acid with a thickness of 20 μm or less in which variations in the piezoelectric constant are suppressed. Furthermore, the film has less unevenness in density in sheet form and less unevenness in roll form, resulting in a good appearance.

[0021] In this specification, the terms “contains” and “includes” include the concepts of “contains,” “includes,” “substantially consist of,” and “consist solely of.”

[0022] In specifying the inventions contained herein, the components (properties, structures, functions, etc.) described in each embodiment of this disclosure may be combined in any way. That is, this disclosure encompasses all subject matter consisting of any combination of the combinatable components described herein.

[0023] 1. Stretched Film The present invention relates in one embodiment to a stretched film (which may also be referred to as "the stretched film of the present invention" in this specification) that contains polylactic acid, has a thickness of 5 μm or more and 20 μm or less, has a standard deviation of thickness in a first direction of 0.5 μm or less, and has a standard deviation of thickness in a second direction perpendicular to the first direction of 0.8 μm or less.

[0024] The type of polylactic acid is not particularly limited, and known polylactic acids can be widely used, for example, polylactic acid obtained by condensation polymerization of lactic acid components as raw material monomers. Polylactic acid may contain only one optical isomer, either L-lactic acid (L-isomer) or D-lactic acid (D-isomer), or both. When polylactic acid contains both L-isomers and D-isomers, the D-isomer content is, for example, 15.0 mol% or less, preferably 0.01 mol% to 10.0 mol%, more preferably 0.01 mol% to 5.0 mol%, even more preferably 0.01 mol% to 2.0 mol%, even more preferably 0.01 mol% to 1.0 mol%, particularly preferably 0.01 mol% to 0.8 mol%, and especially preferably 0.01 mol% to 0.6 mol%. A lower D-isomer ratio makes crystallization easier, thus suppressing dimensional changes and warping during cast sheet molding (cooling), and making it easier to achieve a more uniform film thickness profile. The lower limit of the D-isomer content can be, for example, 0.02 mol% or more, 0.05 mol% or more, 0.1 mol% or more, or 0.2 mol% or more.

[0025] The content of the D-isomer of polylactic acid is the value measured according to the method described in "(1) Film Production" of the Examples below. In this method, the above content can also be measured by using a test piece of stretched film containing polylactic acid instead of polylactic acid pellets.

[0026] The glass transition temperature, crystallization temperature, molecular weight, etc., of polylactic acid are not particularly limited.

[0027] The method for producing polylactic acid is not particularly limited; for example, polylactic acid can be obtained by known manufacturing methods. Polylactic acid can also be obtained from commercially available products. Representative commercially available polylactic acid products include, for example, NatureWorks' "2500HP" and "4032D," and TotalCorbion's Luminy series, specifically "L175," "LX175," and "LX575."

[0028] The stretched film of the present invention contains polylactic acid as its main component. The polylactic acid content is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and especially preferably 97% by mass or more, based on 100% by mass of the stretched film of the present invention. The polylactic acid content is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and especially preferably 97% by mass or more, based on 100% by mass of the resin component contained in the stretched film of the present invention. There is no particular upper limit to the polylactic acid content, and it is, for example, 100% by mass or less, 99.9% by mass or less, or 99.5% by mass or less, based on 100% by mass of the stretched film of the present invention or 100% by mass of the resin component contained in the stretched film of the present invention. Other than polylactic acid, the resin components are not particularly limited as long as they do not significantly impair the piezoelectric properties of the stretched film of the present invention.

[0029] The resin component content is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 97% by mass or more, based on 100% by mass of the stretched film of the present invention. There is no particular upper limit to the resin component content, and it may be, for example, 100% by mass or less, 99.9% by mass or less, or 99.5% by mass or less, based on 100% by mass of the stretched film of the present invention.

[0030] The stretched film of the present invention may also contain additives in addition to the resin component. Examples of additives include heat stabilizers, antioxidants, organic and inorganic lubricants, chlorine scavenging agents, antistatic agents, antifogging agents, and hydrolysis inhibitors.

[0031] The additive content is, for example, 0% to 20% by mass, preferably 0% to 10% by mass, more preferably 0% to 5% by mass, and even more preferably 0% to 3% by mass, based on 100% by mass of the stretched film of the present invention.

[0032] The thickness of the stretched film of the present invention is 5 μm or more and 20 μm or less. Preferably, the thickness is 5 μm or more and 18 μm or less, more preferably 5 μm or more and 16 μm or less, and even more preferably 6 μm or more and 16 μm or less.

[0033] The thickness of the stretched film of the present invention is the value measured according to the average thickness measurement method described in "(2) Measurement of film thickness" of the examples below. If the stretched film of the present invention is a multilayer film as described below, the thickness of the stretched film of the present invention refers to the sum of the thicknesses of each layer.

[0034] The standard deviation of the thickness in the first direction of the stretched film of the present invention is 0.5 μm or less, and the standard deviation of the thickness in the second direction perpendicular to the first direction is 0.8 μm or less. By satisfying these requirements, the stretched film of the present invention, while being a thin film as described above, has suppressed variations in the piezoelectric constant, and also exhibits good appearance with less unevenness in density in the sheet form and less irregularity in the roll form.

[0035] The first direction is any direction of the stretched film of the present invention, and may be the TD direction, the MD direction, or any other direction (for example, an oblique direction such as the 45° direction). Preferably, the first direction is the stretching direction in the case of uniaxial stretching (preferably the TD direction or the MD direction, more preferably the TD direction), and in the case of biaxial stretching, it is the stretching direction with the higher stretching ratio (preferably the TD direction or the MD direction, more preferably the TD direction). The second direction is a direction perpendicular to the first direction; for example, if the first direction is the TD direction, the second direction is the MD direction, and if the first direction is the MD direction, the second direction is the TD direction.

[0036] The standard deviation of the thickness in the first direction is preferably 0.4 μm or less, more preferably 0.3 μm or less, and even more preferably 0.2 μm or less, from the viewpoint of further suppressing variations in the piezoelectric constant and / or improving the appearance. The lower limit of this standard deviation is not particularly limited and may be, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0037] The standard deviation of the thickness in the second direction is preferably 0.7 μm or less, more preferably 0.6 μm or less, even more preferably 0.5 μm or less, particularly preferably 0.4 μm or less, and especially preferably 0.3 μm or less, from the viewpoint of further suppressing variations in the piezoelectric constant and / or improving the appearance. The lower limit of this standard deviation is not particularly limited and may be, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0038] The standard deviation of the thickness is a value measured according to the standard deviation measurement method described in "(2) Measurement of film thickness" of the example below. By capturing such microscopic variations in thickness and adjusting the standard deviation to a certain range, the effects of the present invention can be obtained.

[0039] The difference (Δmax) between the maximum value of the measured values ​​in the first direction and the average thickness in the first direction in the above standard deviation measurement method is preferably 1.0 μm or less, more preferably 0.7 μm or less, even more preferably 0.4 μm or less, and even more preferably 0.2 μm or less. The lower limit of this difference is not particularly limited and may be, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0040] The difference (Δmin) between the minimum value of the measured values ​​in the first direction and the average thickness in the first direction in the above standard deviation measurement method is preferably 1.0 μm or less, more preferably 0.7 μm or less, even more preferably 0.4 μm or less, and even more preferably 0.2 μm or less. The lower limit of this difference is not particularly limited and may be, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0041] The difference (Δmax) between the maximum value of the measurement values ​​in the second direction and the average thickness in the second direction in the above standard deviation measurement method is preferably 1.7 μm or less, more preferably 1.3 μm or less, even more preferably 1.0 μm or less, and even more preferably 0.7 μm or less. The lower limit of this difference is not particularly limited and may be, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0042] The difference (Δmin) between the minimum value among the measured values in the second direction and the average thickness in the second direction in the above standard deviation measurement method is preferably 1.6 μm or less, more preferably 1.2 μm or less, still more preferably 0.8 μm or less, and even more preferably 0.5 μm or less. The lower limit of the difference is not particularly limited, and is, for example, 0.0 μm or more, 0.05 μm or more, or 0.1 μm or more.

[0043] The average thickness in the first direction and the average thickness in the second direction are values measured in accordance with the method for measuring the average thickness in each direction described in "(2) Measurement of Film Thickness" in the Examples described later.

[0044] The layer structure of the stretched film of the present invention is not particularly limited. The stretched film of the present invention may be a single-layer film or a multi-layer film. The stretched film of the present invention is preferably a single-layer film.

[0045] The stretched film of the present invention may be a uniaxially stretched film formed by uniaxial stretching, or may be a biaxially stretched film formed by biaxial stretching. The stretched film of the present invention is preferably a uniaxially stretched film.

[0046] The stretching direction of the stretched film of the present invention is not particularly limited, and the film may be stretched in any of the TD direction, the MD direction, and directions other than these (for example, an oblique direction such as a 45° direction). When the stretched film of the present invention is a uniaxially stretched film, the stretching direction is preferably the TD direction or the MD direction, and more preferably the TD direction. When the stretched film of the present invention is a biaxially stretched film, the stretching directions are preferably the TD direction and the MD direction.

[0047] The stretching ratio of the stretched film of the present invention is not particularly limited as long as piezoelectric performance can be exhibited. The stretching ratio (in the case of biaxial stretching, the higher stretching ratio) is preferably 3.5 or more, more preferably 4.0 or more, still more preferably 4.5 or more, and the upper limit thereof is preferably 6.0 or less, more preferably 5.5 or less, still more preferably 5.0 or less. In the case of biaxial stretching, from the viewpoint of easily exhibiting piezoelectric performance, the ratio of the higher stretching ratio to the lower stretching ratio is preferably 2 or more.

[0048] The stretched film of the present invention can be used as a piezoelectric film.

[0049] The piezoelectric constant d₁₄ of the stretched film of the present invention is, for example, 5.6 pC / N or more, preferably 5.8 pC / N or more, more preferably 6.0 pC / N or more, still more preferably 6.2 pC / N or more, even more preferably 6.4 pC / N or more, particularly preferably 6.6 pC / N or more, even more particularly preferably 6.8 pC / N or more, and most preferably 7.0 pC / N or more. The upper limit of the piezoelectric constant d₁₄ is not particularly limited, and is, for example, 8.2 pC / N or less, 8.0 pC / N or less, or 7.8 pC / N or less.

[0050] The piezoelectric constant d₁₄ is a value measured in accordance with the method for measuring an average value described in "(3) Measurement of Piezoelectric Constant d₁₄" in the Examples described later.

[0051] In the stretched film of the present invention, variation in piezoelectric constant is suppressed. The standard deviation of the piezoelectric constant d₁₄ of the stretched film of the present invention is, for example, 0.24 pC / N or less, preferably 0.20 pC / N or less, more preferably 0.16 pC / N or less, still more preferably 0.14 pC / N or less, even more preferably 0.12 pC / N or less, particularly preferably 0.10 pC / N or less, and most preferably 0.08 pC / N or less. The lower limit of the standard deviation is not particularly limited, and is, for example, 0 pC / N or more, 0.005 pC / N or more, 0.01 pC / N or more, or 0.02 pC / N or more.

[0052] The standard deviation of the piezoelectric constant d₁₄ is a value measured in accordance with the method for measuring a standard deviation described in "(3) Measurement of Piezoelectric Constant d₁₄" in the Examples described later.

[0053] 2. Manufacturing Method The method for producing the stretched film of the present invention is not particularly limited, and for example, widely known methods can be employed. For example, the stretched film of the present invention can be produced by a production method comprising a step of stretching a sheet containing polylactic acid.

[0054] More specifically, a resin sheet can be obtained by extruding a resin raw material containing at least polylactic acid, and the stretched film of the present invention can be manufactured by stretching this resin sheet. This manufacturing method will be abbreviated as "Manufacturing Method A".

[0055] The resin raw material used in manufacturing method A mainly consists of polylactic acid, and may also contain other resins as needed. Furthermore, the resin raw material may also contain various additives as needed. The method for preparing the resin raw material can be the same as known preparation methods, for example, by dry blending polylactic acid pellets or powder using a batch-type kneading device such as a tumbler or mixer, or a continuous weighing-type kneading device; or by supplying polylactic acid pellets or powder together with pellets or powder of other resins and / or additives as needed to a kneading machine and melt-kneading to obtain a melt-blended resin composition; and so on. Among these, it is preferable to prepare the resin raw material by melt-kneading.

[0056] For the mixing process, known mixing machines can be used, and single-screw type, twin-screw type, or multi-screw type with more than one screw may be used. Furthermore, in the case of twin-screw type machines, either co-rotating or staggered-rotating mixing types may be used.

[0057] The mixing temperature for melt mixing is preferably in the range of 200°C to 300°C, and more preferably in the range of 220°C to 280°C. To prevent deterioration of the resin during melt mixing, an inert gas such as nitrogen can be purged. The melt-mixed resin can be pelletized to an appropriate size using a generally known granulator to obtain melt-blend resin composition pellets.

[0058] In manufacturing method A, a resin sheet can be obtained using the resin raw material obtained as described above. Specifically, the resin raw material is supplied to an extruder, heated and melted, and if necessary, minute foreign matter is removed using a filter or the like. Then, the resin sheet can be obtained by melt-extruding it into a sheet shape from a T-die.

[0059] For obtaining the resin sheet, any known extruder can be widely used. There are no restrictions on the screw type of the extruder; a single-screw type, a twin-screw type, or a multi-screw type with more than one screw may be used. When the resin raw material is prepared using the dry blend described above, using a twin-screw type or a multi-screw type with more than one screw tends to provide better kneading and dispersibility. The extrusion temperature is preferably in the range of 200°C to 300°C, and more preferably in the range of 220°C to 280°C. To prevent thermal degradation of the resin during extrusion, purging with an inert gas such as nitrogen can be performed.

[0060] The melt-extruded resin sheet is formed into a sheet by adhering it to a cast roll, and thus obtained as a so-called raw material sheet. By adhering it to the cast roll under predetermined conditions using an electrostatic pinning method, the adhesion to the cast roll during casting can be improved, the smoothness of the resin sheet can be improved, and it becomes easier to adjust the standard deviation of the thickness in each direction after stretching to a predetermined range.

[0061] Electrostatic pinning is a method of making a resin sheet adhere to a cast roll by applying a voltage to a wire electrode positioned along the path of the resin sheet emerging from the extrusion site (die lip) toward the surface of the cast roll.

[0062] The voltage applied to the wire electrode is preferably 2kV to 6kV, more preferably 3kV to 6kV, even more preferably 4kV to 6kV, even more preferably 4kV to 5.5kV, and particularly preferably 4.5kV to 5.5kV, from the viewpoint of easily adjusting the standard deviation of the thickness in each direction after stretching to a predetermined range.

[0063] The distance between the die lip and the wire electrode is preferably 25 mm to 48 mm, more preferably 30 mm to 45 mm, even more preferably 30 mm to 40 mm, even more preferably 33 mm to 40 mm, and particularly preferably 33 mm to 37 mm, from the viewpoint of easily adjusting the standard deviation of the thickness in each direction after stretching to a predetermined range. By adjusting this distance, the landing point of the resin extruded from the die can be adjusted. If the distance is too far, the timing at which the electrostatic effect of the wire electrode sufficiently acts on the resin will be after the resin has landed on the cast roll, reducing the effect of electrostatic pinning. On the other hand, if the distance is too close, the electrostatic effect of the wire electrode acts too strongly on the resin before the resin lands on the cast roll, resulting in a strong voltage being applied in the air, which raises concerns such as surface disturbance, melted resin coming into contact with the wire, deterioration of film thickness due to wire vibration, and generation of foreign matter.

[0064] The temperature of the cast roll is preferably 30°C to 45°C, more preferably 33°C to 45°C, and even more preferably 33°C to 42°C, from the viewpoint of easily adjusting the standard deviation of the thickness in each direction after stretching to a predetermined range.

[0065] In one embodiment, the present invention relates to a method for manufacturing a stretched film, which includes a step of obtaining a raw sheet before stretching by bringing a resin sheet coming out of an extrusion section (die lip) into close contact with a cast roll by an electrostatic pinning method. In this manufacturing method, it is preferable to adjust at least one selected from the group consisting of the voltage applied to the wire electrode, the distance between the die lip and the wire electrode, and the temperature of the cast roll to the above range.

[0066] Manufacturing method A may further include a lamination step as needed. For example, when attempting to obtain the multilayer stretched film of the present invention described above, it is preferable that manufacturing method A includes a lamination step. In the lamination step, for example, conventional lamination methods can be widely used, and examples include films obtained by lamination using co-extrusion, lamination, heat sealing, etc.

[0067] When manufacturing a multilayer stretched film, a resin sheet having a laminated structure can be obtained by co-extruding two or more dry blend and / or melt blend resin compositions (the composition of each resin composition may be different or the same). Alternatively, a multilayer stretched film can also be manufactured by laminating a single-layer stretched film with another film. Furthermore, a stretched film having a laminated structure can also be manufactured by stretching a multilayer unstretched film (the composition of the resin composition constituting each layer may be different or the same) obtained by laminating two or more unstretched films extruded as single layers together.

[0068] Co-extrusion methods include pre-die lamination, where molten resin is brought into contact within a feed block before the mold; in-die lamination, where contact occurs within a path inside the mold, such as a multi-manifold die; and off-die lamination, where the molten resin is extruded and brought into contact from multiple concentric lips. For example, in the case of in-die lamination, a multi-layer die such as a three-layer multi-manifold die can be used.

[0069] Lamination methods include the extrusion lamination method, which uses equipment for molten extrusion molding used in the T-die method to directly extrude a film of molten resin onto another film to form a laminated film.

[0070] Heat sealing methods include external heating methods, in which a heated metal object is pressed against multiple films to be bonded together from the outside of the films, and the conducted heat melts and bonds the films, and internal heating methods, in which heat is generated in the films using high-frequency radio waves or ultrasound to bond them.

[0071] In manufacturing method A, the above lamination methods can be used individually or in combination. If manufacturing method A includes a lamination process, the resin sheet may have a laminated structure; if manufacturing method A does not include a lamination process, the resin sheet may have a single-layer structure. By performing the stretching described below using a resin sheet having a single-layer structure, a stretched film consisting of a layer containing polylactic acid can be obtained.

[0072] In manufacturing method A, the resin sheet (raw material sheet) having the single-layer or laminated structure described above is stretched. As for the stretching method, known methods such as stretching between rolls with a difference in peripheral speed, the tenter method, and the tubular method can be used. As for the stretching direction, uniaxial stretching, biaxial stretching, and biaxial stretching in an oblique direction are possible, and for stretching with two or more axes, both sequential stretching and simultaneous stretching can be applied. Of these, uniaxial stretching is preferred because it is easier to obtain the desired stretched film. When uniaxial stretching is used, for example, the stretching temperature can be 60°C to 90°C.

[0073] In manufacturing method A, the stretching of the resin sheet (raw material sheet) can be carried out according to the desired stretching ratio described above.

[0074] As a heat treatment after stretching, a heat-setting treatment can be performed. The temperature for the heat-setting treatment is preferably 90°C to 130°C, more preferably 100°C to 125°C, and even more preferably 110°C to 120°C.

[0075] 3. Laminated Film, Piezoelectric Element In one embodiment, the present invention relates to a laminated film (which may be referred to herein as "the laminated film of the present invention") comprising a stretched film of the present invention and other layers disposed on a first surface and / or a second surface of the stretched film.

[0076] The first surface is one of the two main surfaces of the stretched film of the present invention, and the second surface is the other surface.

[0077] Other layers are not particularly limited and include, for example, a protective film. The material of the protective film is not particularly limited and includes polyolefins such as polyethylene and polypropylene.

[0078] Other layers include, for example, a conductive layer. The conductive layer is not particularly limited as long as it can be used as an electrode in a piezoelectric element. Examples of materials for the conductive layer include conductive polymers (e.g., PEDOT:PSS), carbon nanotubes, graphene, ITO, ZnO, IGZO, aluminum, zinc, tin, lead, nickel, iron, copper, etc.

[0079] In one embodiment, the present invention relates to a piezoelectric element (which may also be referred to herein as "the piezoelectric element of the present invention") comprising a laminated film of the present invention, which includes a conductive layer as another layer.

[0080] The piezoelectric element of the present invention may include only one laminated film of the present invention, or it may include multiple laminated films of the present invention. The piezoelectric element of the present invention may include a wound body of the laminated film of the present invention, or it may include a laminate formed by stacking multiple laminated films of the present invention.

[0081] The piezoelectric element of the present invention can be used in various fields, such as speakers, headphones, touch panels, remote controllers, microphones, underwater microphones, ultrasonic transducers, ultrasonic measuring instruments, piezoelectric vibrators, mechanical filters, piezoelectric transformers, delay devices, sensors, acceleration sensors, shock sensors, vibration sensors, pressure sensors, tactile sensors, electric field sensors, sound pressure sensors, displays, fans, pumps, variable focus mirrors, sound insulation materials, soundproofing materials, keyboards, audio equipment, information processing machines, measuring instruments, and medical equipment.

[0082] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0083] (1) Film Manufacturing (Example 1) Ingeo Biopolymer: 2500HP (manufactured by Nature Works) was prepared as polylactic acid, and was fed from a hopper into a screw-type extruder to melt it and extruded from a single-layer die. The extruded resin layer was brought into close contact with a cast roll controlled to 35°C, with the electrostatic pinning voltage set to 5kV, the distance between the electrostatic pinning wire and the die lip set to 35mm, and the distance between the die lip and the cast roll set to 20mm, to obtain a raw material sheet. This raw material sheet was then subjected to stretching. The stretching was performed under conditions of a stretching temperature of 75°C and a heat setting temperature of 115°C, with the stretching direction being the TD direction. This resulted in a uniaxially oriented film with a stretching ratio of 4.9 times and a thickness of 14.6 μm.

[0084] (Examples 2-14, Comparative Examples 1-6) Uniaxially oriented films were obtained using the same method as in Example 1, except that the manufacturing conditions were changed as shown in Table 1. In Comparative Examples 5 and 6, the raw material sheets were obtained by pressing them onto a cooling drum using an air knife instead of electrostatic pinning. In Comparative Example 3, the voltage was too high, causing sparks and preventing film formation.

[0085] (Example 15) A uniaxially oriented film was obtained in the same manner as in Example 1, except that Ingeo Biopolymer: 4032D (manufactured by Nature Works) was used as the polylactic acid.

[0086] (Method for measuring the content of the D-isomer of polylactic acid) The content of the D-isomer of polylactic acid used in the examples and comparative examples was measured. The measurement method is as follows. The measurement results are shown in Table 1 below. After freeze-grinding the polylactic acid pellets, hydrolysis was performed with a sodium hydroxide solution. After neutralization with hydrochloric acid, the solution was filtered through a membrane filter and measured by HPLC. The ratio of L-lactic acid to D-lactic acid was calculated from the peak area ratio of L-lactic acid and D-lactic acid measured by HPLC. <HPLC measurement conditions> Apparatus: Agilent 1100 series Column: SUMICHIRAL OA5000 manufactured by Sumika Analysis Center Column temperature: 30℃ Mobile phase: 2 mM copper sulfate aqueous solution / 2-propanol (98 / 2 (v / v)) Mobile phase flow rate: 1.0 mL / min Detector: PDA, detection wavelength: 254 nm.

[0087] (2) Measurement of film thickness The thickness of the uniaxially oriented films in the examples and comparative examples was measured using a desktop offline contact thickness measuring device (TOF-5R01, manufactured by Yamabun Electric Co., Ltd.). Specifically, the following was done: The thickness was measured at 500 points at 1 mm intervals along a virtual line extending in the width direction (TD direction) of the film, and 500 TD measurement values ​​were obtained. On the other hand, the thickness was measured at 3000 points at 1 mm intervals along a virtual line extending in the flow direction (MD direction) at the center of the film in the width direction (TD direction), and 3000 MD measurement values ​​were obtained.

[0088] The average thickness was calculated by dividing the sum of 500 TD measurements and 3000 MD measurements by the total number of measurements (3500).

[0089] The average TD thickness was calculated by dividing the sum of the TD measurements by the number of TD measurements, and the average MD thickness was calculated by dividing the sum of the MD measurements by the number of MD measurements. Using the individual measurements in the TD direction, the average TD thickness, and the number of measurements in the TD direction, the standard deviation of the thickness in the TD direction was calculated according to the following formula (a). Similarly, using the individual measurements in the MD direction, the average MD thickness, and the number of measurements in the MD direction, the standard deviation of the thickness in the MD direction was calculated according to the following formula (a).

[0090] The difference between the maximum TD measurement value and the average TD thickness was defined as Δmax in the TD direction, and the difference between the minimum TD measurement value and the average TD thickness was defined as Δmin in the TD direction. Similarly, the difference between the maximum MD measurement value and the average MD thickness was defined as Δmax in the MD direction, and the difference between the minimum MD measurement value and the average MD thickness was defined as Δmin in the MD direction.

[0091] (3) Measurement of Piezoelectric Constant d14 The method for measuring the piezoelectric constant d14 of the uniaxially oriented film in the examples and comparative examples is as follows. The uniaxially oriented film was cut to 150 mm in a direction 45° with respect to the MD direction and to 50 mm in a direction perpendicular to that direction to obtain five rectangular test pieces. After forming a conductive layer by coating both sides of each test piece with the organic electrode conductive polymer PEDOT:PSS, it was cut to 10 mm in a direction perpendicular to the direction 45° with respect to the MD direction and to 70 mm or more in a direction perpendicular to that direction to obtain measurement samples. Two measurement samples were obtained from one test piece. The measurement samples were set on a test stand (JSV-H1000, manufactured by Nippon Keisoku System Co., Ltd.) with a handy force gauge (HF-1, manufactured by Nippon Keisoku System Co., Ltd.) attached, with a chuck distance of 70 mm, and the charge was determined when the sample reciprocated within an arbitrary load range using an electrometer (KEITHLEY 6514 / J). The piezoelectric constant d14 was calculated according to the following equation (b): d14 = (2 × t) / L × ΔQ / ΔF (wherein equation (b) is the thickness of the sample to be measured (m), L is the distance between the chucks (m), and ΔQ / ΔF is the ratio of the change in charge to the change in force). The piezoelectric constant d14 was calculated for each of the 10 samples to be measured, and the mean and standard deviation were calculated.

[0092] (4) Evaluation of Appearance The evaluation method for the appearance of the uniaxially oriented films of the examples and comparative examples is as follows. Using a strain tester manufactured by Shinto Kagaku Co., Ltd., the sample to be observed was placed between two polarizing plates arranged in a crossed nicol state, and the appearance of the film was observed with the backlight turned on. As samples to be observed, one sheet of uniaxially oriented film and a 300m roll of the film were prepared, respectively. The appearance of the film was evaluated according to the following criteria.

[0093] <Evaluation Criteria> ◎: There were no variations in film density or unevenness in the roll, and the film appearance was good. ○: There were some variations in film density, but the produced roll was smooth. △: There were variations in film density, and slight unevenness was observed in the produced roll. ×: There were variations in film density, and unevenness was observed in the produced roll.

[0094] (5) Results Table 1 shows the manufacturing conditions, measurement results, and evaluation results for the uniaxially oriented films of the examples and comparative examples.

[0095]

Claims

1. A stretched film containing polylactic acid, having a thickness of 5 μm or more and 20 μm or less, a standard deviation of thickness in a first direction of 0.5 μm or less, and a standard deviation of thickness in a second direction perpendicular to the first direction of 0.8 μm or less.

2. The stretched film according to claim 1, wherein the polylactic acid content is 50% by mass or more based on 100% by mass of the stretched film.

3. The stretched film according to claim 1, wherein the standard deviation of the thickness in the first direction is 0.4 μm or less, and the standard deviation of the thickness in the second direction is 0.6 μm or less.

4. The stretched film according to claim 1, which is a single-layer film.

5. The stretched film according to claim 1, which is a uniaxially oriented film.

6. The stretched film according to claim 1, which is a piezoelectric film.

7. The stretched film according to claim 6, wherein the piezoelectric constant d14 is 6.0 pC / N or more and 7.8 pC / N or less.

8. The stretched film according to claim 6, wherein the standard deviation of the piezoelectric constant d14 is less than 0.20 pC / N.

9. A laminated film comprising a stretched film according to any one of claims 1 to 8, and other layers disposed on a first surface and / or a second surface of the stretched film.

10. The laminated film according to claim 9, wherein the other layer includes a protective film.

11. The laminated film according to claim 9, wherein the other layer includes a conductive layer.

12. A piezoelectric element comprising the laminated film described in claim 11.

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