Electret sheet, piezoelectric sensor, pressure detection system, and method for manufacturing an electret sheet

Abstract

To provide an electret sheet which reduces coming-out of charges over time, and can maintain excellent piezoelectricity over a long period of time.SOLUTION: An electret sheet contains a foam sheet that contains a synthetic resin and is charged, wherein arithmetic average roughness Ra of the first surface of the foam sheet is 0.5-10 μm, which reduces coming-out of charges over time, and can stably maintain excellent piezoelectricity over a long period of time.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electret sheet, a piezoelectric sensor, a pressure detection system, and a method for producing an electret sheet. [Background Art]

[0002] An electret sheet is a material that is imparted with permanent internal charge by injecting charge into an insulating polymer material.

[0003] It is known that a foamed sheet made of synthetic resin exhibits extremely high piezoelectricity comparable to that of ceramics by charging the cell membranes forming the cells and the vicinity thereof. Applications of such electret sheets using synthetic resin foamed sheets to acoustic pickups, various pressure sensors, and the like have been proposed utilizing their excellent sensitivity.

[0004] As an electret sheet, Patent Document 1 discloses an electret sheet comprising a charged propylene-based resin foamed sheet having an average thickness of 90 to 150 µm, an expansion ratio of 3.5 to 9 times, and a gel fraction of 30 to 50% by weight, wherein the propylene-based resin foamed sheet contains a resin composition containing 60% by weight or more of a crosslinked product of a propylene-α-olefin random copolymer having a weight average molecular weight of 370000 to 420000. [Prior Art Literature] [Patent Literature]

[0005] [Patent Document 1] WO2015 / 137351 [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] However, the electret sheet disclosed in Patent Document 1 has a problem that the charge injected into the foamed sheet leaks out over time, and the piezoelectricity tends to decrease.

[0007] The present invention provides an electret sheet that reduces charge loss over time and maintains excellent piezoelectric properties over a long period of time, a method for manufacturing the same, and a piezoelectric sensor and pressure detection system using the electret sheet. [Means for solving the problem]

[0008] The electret sheet of the present invention comprises a foamed sheet containing a synthetic resin and charged, characterized in that the arithmetic mean roughness Ra of the first surface of the foamed sheet is 0.5 to 10 μm.

[0009] The piezoelectric sensor of the present invention is The above electret sheet and, A signal electrode is laminated on one side of the electret sheet mentioned above, The invention is characterized by including a ground electrode laminated on the other side of the electret sheet.

[0010] The present invention provides a method for manufacturing an electret sheet, A step of removing the surface portion of a foamed sheet containing synthetic resin and having air bubbles formed inside, opening the air bubble cross-sections on the surface of the foamed sheet, and making the arithmetic mean roughness Ra of the surface portion of the foamed sheet 0.5 to 10 μm, The method is characterized by including the step of electrostatically charging the foamed sheet described above. [Effects of the Invention]

[0011] The electret sheet of the present invention exhibits reduced charge loss over time even when using the same synthetic resin composition, and can stably maintain excellent piezoelectric properties over a long period of time.

[0012] According to the present invention's method for manufacturing electret sheets, by removing the surface portion including the skin layer of the foamed sheet, the cross-sections of the air bubbles are opened on the surface of the foamed sheet, and an electret sheet having an arithmetic mean roughness Ra within a predetermined range can be easily manufactured. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing a cross-section of the foam sheet before the surface layer is removed. [Figure 2] This is a schematic diagram showing a cross-section of the foam sheet after the surface layer has been removed. [Modes for carrying out the invention]

[0014] (Electret Seat) The electret sheet includes a foamed sheet containing a synthetic resin. The synthetic resin contained in the foamed sheet is not particularly limited and includes, for example, polyethylene resins, polyolefin resins such as polypropylene resins, polyvinylidene fluoride, polylactic acid, and liquid crystal resins. It is preferable that the sheet contains a polyolefin resin, and more preferably a polypropylene resin.

[0015] The synthetic resin is preferably one with excellent insulating properties. The synthetic resin should have a volume resistivity (hereinafter simply referred to as "volume resistivity") of 1.0 × 10⁻¹⁰ after 1 minute of voltage application at 500V, in accordance with JIS K6911. 10 A synthetic resin with a mass of Ω·m or greater is preferred.

[0016] The volume resistivity of the synthetic resin is 1.0 × 10⁻⁶ because the electret sheet has superior piezoelectric properties. 14 Preferably Ω·m or greater, 1.0 × 10 16 Ω·m or greater is more preferable, and 1.0 × 10 17 A value of Ω·m or greater is preferable.

[0017] Examples of polyethylene resins include ethylene homopolymers, or copolymers of ethylene containing more than 50% by mass of an ethylene component and at least one α-olefin having 3 to 20 carbon atoms. Examples of ethylene homopolymers include low-density polyethylene (LDPE) obtained by radical polymerization under high pressure, and medium- and low-pressure method high-density polyethylene (HDPE) polymerized in the presence of a catalyst at medium and low pressure. Linear low-density polyethylene (LLDPE) can be obtained by copolymerizing ethylene and an α-olefin. Examples of the α-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-nonene, 1-decene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene, with α-olefins having 4 to 10 carbon atoms being preferred. The content of α-olefin in linear low-density polyethylene is usually 1 to 15% by mass.

[0018] As the polypropylene resin, it is sufficient that the resin contains more than 50% by mass of a propylene component, and propylene homopolymers (homopolypropylene) and copolymers of propylene and at least one α-olefin other than propylene having 20 or less carbon atoms are preferred. The polypropylene resin may be used alone or in combination of two or more. Further, the copolymer of propylene and at least one α-olefin other than propylene having 20 or less carbon atoms may be either a block copolymer or a random copolymer, and a random copolymer is preferred. As the polypropylene resin, a propylene-α-olefin random copolymer is preferred. The content of the propylene-α-olefin random copolymer in the polyolefin resin is preferably 60 to 100% by mass, more preferably 70 to 100% by mass, particularly preferably 85 to 100% by mass, and most preferably 96 to 100% by mass. When the content of the propylene-α-olefin random copolymer is within the above range, the electret sheet can maintain high piezoelectricity even in a high-temperature environment.

[0019] Examples of the α-olefin to be copolymerized with propylene include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-tetradecene, 1-hexadecene, 1-octadecene and 1-eicosene.

[0020] The arithmetic average roughness Ra of the first surface of the foamed sheet is 0.5 µm or more, preferably 1.0 µm or more, and more preferably 2.0 µm or more. When the arithmetic average roughness Ra of the first surface is 0.5 µm or more, the volume resistance value of the electret sheet can be increased, charge leakage of the electret sheet over time can be reduced, and excellent piezoelectricity can be maintained over a long period of time.

[0021] The arithmetic average roughness Ra of the first surface of the foamed sheet is 10 µm or less, preferably 8 µm or less, more preferably 6 µm or less, and still more preferably 4 µm or less. When the arithmetic average roughness Ra of the first surface is 10 µm or less, the volume resistance value of the electret sheet can be increased, charge leakage of the electret sheet over time can be reduced, and excellent piezoelectricity can be maintained over a long period of time. Further, while controlling the thickness of the electret sheet to be thin, the number of air bubbles present in the thickness direction can be increased, the occurrence of charge unevenness in the permanently charged portion in the plane direction of the first surface of the electret sheet is suppressed, and the amount of charge generated when pressure is applied to the electret sheet can be made more uniform in the plane direction of the first surface.

[0022] The arithmetic mean roughness Ra of the second surface of the foamed sheet is not particularly limited, but is preferably 10 μm or less, preferably 8 μm or less, more preferably 6 μm or less, and more preferably 4 μm or less. When the arithmetic mean roughness Ra of the second surface is 10 μm or less, the volume resistivity of the electret sheet is increased, reducing the time-dependent charge loss of the electret sheet and maintaining excellent piezoelectric properties over a long period of time. Furthermore, while controlling the thickness of the electret sheet to be thin, the number of bubbles present in the thickness direction can be increased, suppressing the occurrence of charge unevenness in the permanently charged portion in the surface direction of the second surface of the electret sheet, and making the amount of charge generated when pressure is applied to the electret sheet more uniform in the surface direction of the second surface.

[0023] The calculated mean roughness Ra of the second surface of the foam sheet is preferably less than 0.5 μm. In a foam sheet, by setting the arithmetic mean roughness of the first surface to 0.5 μm or more, and the arithmetic mean roughness of the second surface to less than 0.5 μm, the time-dependent charge loss of the electret sheet is reduced, excellent piezoelectric properties are maintained over a long period of time, the occurrence of unevenness in the permanently charged portion in the planar direction is suppressed, and the amount of charge output by the pressure applied portion can be made more uniform.

[0024] The arithmetic mean roughness Ra and the maximum height Rz (described later) of the foamed sheet surface were measured five times in accordance with JIS B0601 (2001), and the arithmetic mean of the obtained measurements was taken as the arithmetic mean roughness Ra and the maximum height Rz (described later).

[0025] In this invention, the "first surface" is defined as "either the surface of the foamed sheet having the largest area (main surface) or the surface of the foamed sheet opposite the first surface in the thickness direction (the surface facing the main surface)," and the other surface is defined as "the second surface." That is, when the "main surface" of the foamed sheet is defined as the "first surface," the "surface facing the main surface" of the foamed sheet is defined as the "second surface." When the "surface facing the main surface" of the foamed sheet is defined as the "first surface," the "main surface" of the foamed sheet is defined as the "second surface."

[0026] The maximum height Rz of the first surface of the foamed sheet is preferably 5 μm or more, more preferably 6 μm or more, more preferably 7 μm or more, more preferably 8 μm or more, more preferably 9 μm or more, more preferably 10 μm or more, and more preferably 18 μm or more. When the maximum height Rz of the first surface is 5 μm or more, the volume resistivity of the electret sheet is increased, reducing the charge loss of the electret sheet over time and allowing excellent piezoelectric properties to be maintained for a long period of time.

[0027] The maximum height Rz of the first surface of the foamed sheet is preferably 100 μm or less, more preferably 80 μm or less, more preferably 70 μm or less, and more preferably 60 μm or less. When the maximum height Rz of the first surface is 100 μm or less, the volume resistivity of the electret sheet is increased, reducing the time-dependent charge loss of the electret sheet and maintaining excellent piezoelectric properties over a long period of time. Furthermore, while controlling the thickness of the electret sheet to be thin, the number of bubbles present in the thickness direction can be increased, suppressing the occurrence of charge unevenness in the permanently charged portion in the surface direction of the first surface of the electret sheet, and making the amount of charge generated when pressure is applied to the electret sheet more uniform in the surface direction of the first surface.

[0028] The maximum height Rz of the second surface of the foamed sheet is not particularly limited, but is preferably 100 μm or less, more preferably 80 μm or less, more preferably 70 μm or less, and more preferably 60 μm or less. When the maximum height Rz of the second surface is 100 μm or less, the volume resistivity of the electret sheet is increased, reducing the charge loss of the electret sheet over time and allowing excellent piezoelectricity to be maintained for a long period of time. Furthermore, while controlling the thickness of the electret sheet to be thin, the number of bubbles present in the thickness direction can be increased, suppressing the occurrence of charge unevenness in the permanently charged portion in the planar direction of the second surface of the electret sheet, and making the amount of charge generated when pressure is applied to the electret sheet more uniform in the planar direction of the second surface.

[0029] Furthermore, it is particularly preferable that the maximum height Rz of the second surface of the foam sheet be less than 5 μm. In the foam sheet, by setting the maximum height Rz of the first surface to 5 μm or more and the maximum height Rz of the second surface to less than 0.5 μm, the time-dependent charge loss of the electret sheet is reduced, excellent piezoelectricity is maintained over a long period of time, the occurrence of unevenness in the permanently charged portion in the planar direction is suppressed, and the amount of charge output by the pressure applied portion can be made more uniform.

[0030] The foaming ratio of the foamed sheet is preferably 40 times or less, more preferably 30 times or less, more preferably 25 times or less, more preferably 20 times or less, and more preferably 15 times or less. When the foaming ratio of the foamed sheet is 40 times or less, when a piezoelectric sensor using an electret sheet is used in an application where a load is applied to the piezoelectric sensor, the deformation associated with the use of the electret sheet is suppressed, and the measurement accuracy of the piezoelectric sensor can be improved. In addition, when the foaming ratio of the foamed sheet is 40 times or less, the amount of resin contained in the foamed sheet increases, so the amount of charge that can be permanently charged increases, and the output of the piezoelectric sensor can be improved. Therefore, the foaming ratio of the foamed sheet is preferably between 3.5 and 40 times because it provides a good balance between the amount of charge of the electret sheet (output of the piezoelectric sensor) and the amount of deformation (amount of strain).

[0031] The foaming ratio of the foamed sheet refers to the value measured according to the following procedure. A test specimen is prepared by cutting a 10 cm square from the foamed sheet. The thickness of the foamed sheet is measured in accordance with ISO 1923 (1981) "Method for measuring the linear dimensions of foamed plastics and rubber". The thickness is measured at any five points on the test specimen, and the arithmetic mean of the five thicknesses is taken as the thickness of the test specimen. The apparent volume (cm³) of the test specimen is calculated using the obtained thickness of the test specimen. 3 Calculate the apparent volume (cm³) of the test specimen. Measure the mass of the test specimen using an electronic balance. 3 The foaming ratio of the foamed sheet is obtained by dividing the value obtained by the mass (g) of the test specimen. For measuring the mass of the test specimen, an example of an electronic balance is the one commercially available from AS ONE Corporation under the product name "Precision Electronic Balance with Built-in Calibration Weight".

[0032] From the viewpoint of improving the charge-retaining properties, the foamed sheet is preferably free of through-holes and preferably contains closed cells.

[0033] The foamed sheet constituting the electret sheet only needs to be manufactured such that the arithmetic mean roughness Ra of the first surface is within a predetermined range, and can be manufactured using general methods. For example, it can be manufactured by foaming a synthetic resin in a known manner to produce a foamed sheet, and then removing the surface of the foamed sheet. The foamed sheet obtained by foaming a synthetic resin in a known manner has a thin layer called a skin layer formed on its surface, which is either unfoamed or hardly foamed. By removing the surface including the skin layer formed on the first surface of the foamed sheet, the cross-section of the bubbles is exposed, and an electret sheet with an arithmetic mean roughness Ra of the first surface within the above range can be manufactured. The method for removing the surface of the foamed sheet is not particularly limited, and examples include methods of cutting the surface of the foamed sheet. As an apparatus for removing the skin layer of the foamed sheet, for example, an apparatus commercially available from Nippi Machinery Co., Ltd. under the product name "NP-120RS" can be used.

[0034] The foamed sheet 1 has numerous air bubbles 11, 11... inside it. Since a skin layer 12 is formed on the surface of the foamed sheet 1, the air bubbles 11 are not open (exposed) on the surface 1a of the foamed sheet 1 (see Figure 1).

[0035] As shown in Figure 2, by removing the surface portion 14 including the skin layer 12 of the foamed sheet 1, a portion of the bubbles 11 formed inside the foamed sheet is cut along the surface of the foamed sheet, and the cross-section 11a of the bubbles is opened (exposed) on the surface 1a of the foamed sheet, thereby enabling the production of an electret sheet with an arithmetic mean roughness Ra of the first surface 1a of the foamed sheet 1 within the above range. In Figure 2, the case in which the cross-section of the bubbles is exposed on the first surface of the foamed sheet is illustrated, but the cross-section of the bubbles may also be exposed on the first and second surfaces of the foamed sheet by removing the surface portion including the skin layer 12 from the first and second surfaces of the foamed sheet.

[0036] When manufacturing an electret sheet by removing the surface portion, including the skin layer, from a foamed sheet, the arithmetic mean roughness Ra and maximum height Rz of the foamed sheet can be controlled by adjusting the average bubble diameter of the foamed sheet being manufactured. Increasing the average bubble diameter of the foamed sheet generally increases the arithmetic mean roughness Ra and maximum height Rz of the foamed sheet.

[0037] By exposing the cross-section of the foamed sheet's air bubbles and adjusting the arithmetic mean roughness Ra of the first surface to a predetermined range, the contact area between the first surface of the foamed sheet and the electrode laminated to this first surface (or the fixing agent if the electrode is laminated to the first surface of the foamed sheet via a fixing agent) is reduced. This increases the volume resistivity of the foamed sheet, making it less likely for charge to escape over time, and thus allowing the excellent piezoelectric properties of the electret sheet to be maintained more effectively over a long period of time.

[0038] Furthermore, the method for manufacturing an electret sheet is not limited to the method of removing the skin layer of the foam sheet described above. The electret sheet may also be manufactured by controlling the surface of the foam sheet so that the arithmetic mean roughness Ra of the first surface of the foam sheet falls within a predetermined range. For example, the following methods (1) and (2) can be cited. (1) The surface of the foamed sheet may be made uneven using an arbitrary abrasive material so that the arithmetic mean roughness Ra of the first surface is within a predetermined range. (2) During the foaming process in the manufacturing of the foam sheet, when the surface of the foam sheet is softening, a molded plate having irregularities may be pressed onto the surface of the foam sheet to form an irregular shape on the surface of the resulting foam sheet, and the arithmetic mean roughness Ra of the first surface may be controlled to be within a predetermined range.

[0039] It is preferable to uniaxially stretch the foam sheet along the first surface. By uniaxially stretching the foam sheet, a thinner foam sheet can be obtained. The thinner the foam sheet, the lower the voltage can be controlled when charging the foam sheet to obtain an electret, and dielectric breakdown of the foam sheet can be suppressed. As a result, permanent charging becomes easier to achieve.

[0040] The stretching ratio for uniaxial stretching of the foamed sheet is not particularly limited, but it is preferable to stretch it to an arbitrary thickness. For example, the stretching ratio for uniaxial stretching of the foamed sheet is 1.1 to 10 times, preferably 2 to 8 times, and more preferably 3 to 5 times. The uniaxial stretching ratio of the foamed sheet refers to the value obtained by dividing the length after uniaxial stretching by the length before uniaxial stretching in the uniaxial stretching direction.

[0041] The method for uniaxial stretching of a foamed sheet is not particularly limited, and examples include heating the foamed sheet to soften it, and then uniaxially stretching the softened foamed sheet using a known uniaxial stretching apparatus.

[0042] The thickness of the foam sheet is not particularly limited. Preferably, the thickness of the foam sheet is 500 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. When the thickness of the foam sheet is 500 μm or less, the absolute value of the voltage used during electret treatment can be reduced, making it easier to generate permanent charge when electret treatment is performed at the same voltage. Furthermore, because electret treatment can be performed at a lower absolute voltage, dielectric breakdown of the resin contained in the foam sheet is suppressed, making it easier to permanently charge the foam sheet. Therefore, the electret sheet exhibits reduced charge loss over time, and can stably maintain excellent piezoelectric properties over a long period.

[0043] The thickness of the foamed sheet is preferably 20 μm or more, more preferably 50 μm or more, and even more preferably 70 μm or more. When the thickness of the porous sheet is 20 μm or more, the tensile strength increases, and the decrease in handling properties such as tearing or ripping can be suppressed. In particular, when uniaxially stretched to the desired thickness, handling properties are improved. In addition, since the amount of resin contained in the foamed sheet is moderately increased, the shortage of charged area when electret treatment is suppressed, and the output when used as a piezoelectric sensor is improved.

[0044] Electret sheets can be manufactured by electrostatically charging a foamed sheet to induce piezoelectric properties in the foamed sheet. The order in which the steps of removing the surface from the foamed sheet and electrostatically charging the foamed sheet are performed does not matter; either step can be performed first. However, it is preferable to perform the step of removing the surface from the foamed sheet first, followed by the step of electrostatically charging the foamed sheet.

[0045] The method for electrostatically charging the foam sheet is not particularly limited, and examples include applying DC electrolysis to the foam sheet.

[0046] The method for applying a DC electric field to the foam sheet is not particularly limited, and examples include the following methods. (1) A method of charging a foam sheet by sandwiching it between a pair of flat electrodes, connecting the flat electrode in contact with the surface to be charged to a high-voltage DC power supply and grounding the other flat electrode, and applying a DC or pulsed high voltage to the foam sheet to inject charge into the synthetic resin. (2) A method in which a grounded flat electrode is placed in close contact with the first surface of a foam sheet, and a needle-shaped electrode or wire electrode electrically connected to a DC high-voltage power supply is placed on the second surface of the foam sheet at a predetermined distance. Corona discharge is generated by concentrating an electric field near the tip of the needle-shaped electrode or the surface of the wire electrode, ionizing air molecules, and the foam sheet is charged by repelling the air ions generated by the polarity of the needle-shaped electrode or wire electrode.

[0047] The absolute value of the DC processing voltage when applying a DC electric field to the foam sheet is preferably 5 to 40 kV, and more preferably 10 to 30 kV. By adjusting the DC processing voltage within the above range, the amount of charge in the resulting electret sheet can be increased, and the output of the electret sheet can be increased.

[0048] (Piezoelectric sensor) A piezoelectric sensor is constructed by laminating or integrating a signal electrode on one of the first or second surfaces of an electret sheet, and laminating or integrating a ground electrode on the other surface. By measuring the potential of the signal electrode using the ground electrode as a reference electrode, the potential generated in the electret sheet can be measured.

[0049] By using a piezoelectric sensor with an electret sheet, it is possible to easily measure biological signals such as respiratory rate, heart rate, and body movement without burdening the person whose biological signals are being measured.

[0050] Preferably, the signal electrode is laminated and integrated with the first surface of the electret sheet via a fixing agent as needed. Similarly, preferably, the ground electrode is laminated and integrated with the second surface of the electret sheet via a fixing agent as needed. The signal electrode and ground electrode are not particularly limited as long as they are conductive, and examples include metal sheets such as copper foil and aluminum foil, and conductive films.

[0051] When the signal electrode and ground electrode are made of a conductive film, the conductive film may be formed on an electrical insulating sheet and then laminated and integrated onto an electret sheet, or it may be formed directly on the surface of the electret sheet. Methods for forming a conductive film on an electrical insulating sheet or electret sheet include, for example, (1) applying a conductive paste containing conductive fine particles in a binder to an electrical insulating sheet or electret sheet and drying it to form a conductive film, or (2) forming a conductive film on an electrical insulating sheet or electret sheet by vapor deposition.

[0052] The electrical insulating sheet is not particularly limited as long as it has electrical insulating properties, and examples include polyimide sheets, polyethylene terephthalate sheets, polyethylene naphthalate sheets, and polyvinyl chloride sheets.

[0053] The fixing agent constituting the fixing layer can be a reactive, solvent-based, water-based, or hot-melt adhesive or tack. From the viewpoint of maintaining the sensitivity of the electret sheet, a fixing agent with a low dielectric constant is preferred. [Examples]

[0054] The following examples of the present invention will be described, but the present invention is not limited to the following examples.

[0055] The raw materials used in the examples and comparative examples are shown below. (Polypropylene resin) • Propylene-ethylene random copolymer (manufactured by Nippon Polypropylene Co., Ltd., product name "Novatec EG7FTB")

[0056] (Polyethylene resin) • Low-density polyethylene (Prime Polymer Co., Ltd. product name "Moretec 0238CN")

[0057] Thermal decomposition type foaming agent: Azodicarbonamide Copper damage inhibitor: Methylbenzotriazole Antioxidant: Tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane Crosslinking agent: Trimethylolpropane trimethacrylate

[0058] (Examples 1-10, 12) Polypropylene resin, polyethylene resin, pyrolysis-type foaming agent, copper pollution inhibitor, antioxidant, and crosslinking aid were supplied to an extruder in the proportions shown in Table 1, melted and kneaded at 150°C, and extruded from a T-die into a sheet while being wound at a predetermined winding speed to obtain a long foamed resin sheet.

[0059] The foamed resin sheet described above was crosslinked by irradiating it with an electron beam at an acceleration voltage of 300 kV for 25 kGy. The foamed resin sheet was heated to 250°C to foam it, and a first foamed sheet having the thickness shown in Table 1 was obtained. The first foamed sheet contained closed cells.

[0060] A skin layer was formed over the entire surface of the first and second surfaces of the obtained first foamed sheet, and no air bubbles were exposed on the first and second surfaces of the first foamed sheet. The surface portion including the skin layer was completely removed from the first surface of the first foamed sheet along the first surface to obtain a second foamed sheet having the thickness shown in Table 1. Air bubble cross-sections were exposed on the first surface of the second foamed sheet, while a skin layer was formed over the entire second surface of the second foamed sheet. The air bubble cross-sections exposed on the first surface of the second foamed sheet were scattered throughout the first surface.

[0061] The second foamed sheet was heated to 130°C to soften it, and then uniaxially stretched using an automatic uniaxial stretching machine (Imoto Seisakusho Co., Ltd., product name "IMC-18C6") in the direction perpendicular to the extrusion direction (width direction) at a stretching speed of 250 mm / min to the stretching ratio shown in Table 1. Uniaxially stretched foamed sheets with the thickness shown in Table 1 were obtained.

[0062] The obtained uniaxially oriented foam sheet was subjected to corona discharge treatment using a charge imparting device (Simco Japan Co., Ltd., product name "ECM-30N") under the conditions of voltage -15kV, discharge distance 40mm, and voltage application time 1 minute, thereby injecting charge into the uniaxially oriented foam sheet and electret sheet containing the charged foam sheet was obtained. The obtained electret sheet was cured at 80°C for 4 hours.

[0063] (Example 11) A first foamed sheet was obtained in the same manner as in Example 2. The surface portion including the skin layer was completely removed from the first and second surfaces of the first foamed sheet along the first and second surfaces to obtain a second foamed sheet having the thickness shown in Table 1 from the first foamed sheet. Cellular cross-sections were exposed on the first and second surfaces of the second foamed sheet. The exposed cellular cross-sections on the first and second surfaces of the second foamed sheet were scattered throughout the first and second surfaces, respectively.

[0064] The second foamed sheet was uniaxially stretched in the same manner as in Example 2 to produce the first stretched foamed sheet. Then, an electric charge was injected into this first stretched foamed sheet in the same manner as in Example 2 to charge the uniaxially stretched foamed sheet, and an electret sheet containing the charged foamed sheet was obtained. The obtained electret sheet was cured at 80°C for 4 hours.

[0065] (Comparative Examples 1-5) An electret sheet containing a charged foam sheet was obtained in the same manner as in Examples 1 to 5, except that the skin layer formed on the first and second surfaces of the first foam sheet was not removed. The procedures for Comparative Examples 1 to 5 corresponded, in order, to those of Examples 1 to 5.

[0066] For the obtained electret sheets, the arithmetic mean roughness Ra and maximum height Rz of the first and second surfaces, as well as the foaming ratio and thickness of the foamed sheet (which were the same as the thickness of the first stretched foamed sheet), were measured in the manner described above, and the results are shown in Tables 1 and 2.

[0067] The volume resistivity, initial piezoelectric constant d33, and endurance piezoelectric constant d33 of the obtained electret sheets were measured according to the following procedure, and the results are shown in Tables 1 and 2.

[0068] (Volume resistivity) The volume resistivity of the electret sheet was measured in accordance with JIS C2170 (2004) "Test Method for Resistivity and Resistivity of Solid Planar Materials to Prevent Electrostatic Charge Accumulation". Specifically, a 10 cm square test specimen was cut from the electret sheet. The volume resistivity of the obtained test specimen was measured by measuring a minute current under a voltage of 500 V using a volume resistivity measuring device (ADC Corporation, product name "8340A").

[0069] (Initial piezoelectric constant d33 and endurance piezoelectric constant d33) A 10cm square test specimen was cut from the electret sheet. A vibration exciter was used to apply pressure to the specimen under the conditions of a load F of 1N, a dynamic load of 1.5N, and a frequency of 1Hz. The resulting charge Q (coulomb) was measured using a Piezo leader (manufactured by Lead Techno Co., Ltd.) to determine the piezoelectric constant d33 (charge amount). Note that the piezoelectric constant dij represents the load in the j direction and the charge in the i direction, while d33 represents the load and charge in the thickness direction of the electret sheet.

[0070] For an electret sheet immediately after manufacturing, the piezoelectric constant d33 was measured at five arbitrary locations, and the arithmetic mean of these measurements was defined as the initial piezoelectric constant d33.

[0071] Next, the electret sheet was wrapped in aluminum foil and left in a constant temperature and humidity chamber at 50°C for 500 hours. After that, the electret sheet was left in a constant temperature and humidity chamber at 23°C for 1 hour to reach a steady state. The piezoelectric constant d33 (charge amount) was measured at five arbitrary locations on this electret sheet, and the arithmetic mean of these values ​​was defined as the endurance piezoelectric constant d33.

[0072] The d33 retention rate (%) was calculated based on the following formula using the obtained initial piezoelectric constant d33 and endurance piezoelectric constant d33. d33 retention rate (%) = 100 × initial piezoelectric constant d33 / endurance piezoelectric constant d33

[0073] [Table 1]

[0074] [Table 2] [Explanation of Symbols]

[0075] 1a First face 1. Foam sheet 11 bubbles 12 skin layers 14 Surface part

Claims

1. The invention comprises a foamed sheet containing a synthetic resin and being charged, wherein the arithmetic mean roughness Ra of the first surface of the foamed sheet is 0.5 to 10 μm. The above-mentioned foamed sheet is an electret sheet characterized by having air cell cross-sections open on its surface.

2. The electret sheet according to claim 1, characterized in that the maximum height Rz of the first surface of the foamed sheet is 2 to 100 μm.

3. The electret sheet according to claim 1 or 2, characterized in that the synthetic resin contains a polyolefin resin.

4. The electret sheet according to claim 1 or 2, characterized in that the foamed sheet contains closed cells.

5. The electret sheet according to claim 1 or 2, characterized in that the synthetic resin contains a polypropylene resin.

6. The electret sheet according to claim 1 or 2, characterized in that the foaming ratio of the foamed sheet is 3.5 to 40 times.

7. An electret sheet according to claim 1 or claim 2, A signal electrode is laminated on one side of the electret sheet mentioned above, A piezoelectric sensor characterized by including a ground electrode laminated on the other side of the electret sheet.

8. A pressure detection system characterized by comprising the piezoelectric sensor described in Claim 7.

9. The pressure detection system according to claim 8, characterized by detecting biological signals.

10. A step of removing the surface portion of a foamed sheet containing synthetic resin and having air bubbles formed inside, opening the air bubble cross-sections on the surface of the foamed sheet, and making the arithmetic mean roughness Ra of the removed surface portion of the foamed sheet 0.5 to 10 μm, A method for manufacturing an electret sheet, characterized by including a step of electrostatically charging the foamed sheet described above.

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