JP2014143369A - Electret sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electret sheet for holding high piezoelectricity even if used under a high temperature.SOLUTION: An electret sheet is provided in which at least two layers of a porous sheet containing a synthetic resin are integrally laminated and charges are injected into a laminated porous sheet having eleven or more of the average number of vacancies in the cross section in the thickness direction to be charged. The electret sheet exhibits excellent piezoelectricity and can maintain such an excellent piezoelectricity over a long period of time even under a high temperature.

Description

  The present invention relates to an electret sheet that retains high piezoelectricity even when used at high temperatures.

  An electret is a material that is charged inside by injecting a charge into an insulating polymer material. Electrets are widely used as dust collection filters formed into fibers.

  Further, it is known that the synthetic resin sheet exhibits very high piezoelectricity comparable to ceramics by charging it. An electret sheet using such a synthetic resin sheet has been proposed for application to acoustic pickups, various pressure sensors, and the like by utilizing its excellent sensitivity.

  Patent Document 1 discloses an electret formed by injecting a charge into a polymer porous body having pores, and has an average aspect ratio of pores of 7 or more and 30 or less, a porosity of 80% or more, and an average void in the thickness direction. An electret sheet having a pore number of 1 or more and 10 or less and an average pore diameter in the thickness direction of 30 μm or more and 200 μm or less is disclosed. The polymer porous body having pores is obtained by biaxially stretching an organic polymer foam.

JP 2010-186960 A

  However, the conventional electret sheet has a problem that the piezoelectricity decreases with time when used at a high temperature.

  Therefore, the present invention provides an electret sheet that retains high piezoelectricity even when used at high temperatures.

  The electret sheet of the present invention injects a charge into a laminated porous sheet in which at least two layers of a porous sheet containing a synthetic resin are integrated and the average number of pores in the cut surface in the thickness direction is 11 or more. It is characterized by being charged.

  By injecting the charge into the laminated porous sheet, the charge is apparently accumulated at the interface between the pores and the synthetic resin and is polarized into a positive charge and a negative charge. And an electrical response arises by making the positive charge and negative charge which have accumulated in the polarized state change relatively, and an electret sheet exhibits piezoelectricity. In the electret sheet of the present invention, the number of pores in the thickness direction can be greatly increased by using a laminated porous sheet obtained by laminating and integrating at least two layers of porous sheets. While exhibiting excellent piezoelectricity, it is possible to maintain such excellent power generation properties over a long period of time even at high temperatures.

  The porous sheet contains a synthetic resin. Synthetic resins include olefin resins, polyetherimide resins, polystyrene resins, polyetherimide resins, polyester resins, polyvinyl chloride resins, polytetrafluoroethylene resins, ethylene tetrafluoride and perfluoroalkoxyethylene. And a copolymer of tetrafluoroethylene and hexafluoropropylene, a polyvinylidene fluoride resin, and a polyether ether ketone resin. Especially, since it is excellent in insulation and can provide the electret sheet | seat excellent in electric charge retention, an olefin resin is preferable and a propylene resin is more preferable.

  The propylene-based resin is not particularly limited as long as it contains 50% by weight or more of a propylene component. For example, propylene homopolymer (homopolypropylene), having 2 to 20 carbon atoms other than propylene and at least one kind of propylene. Examples thereof include copolymers with olefins. In addition, propylene resin may be used independently or 2 or more types may be used together. Further, the copolymer of propylene and at least one kind of olefin having 2 to 20 carbon atoms other than propylene may be either a block copolymer or a random copolymer.

  Examples of α-olefin copolymerized with propylene include, for example, ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-decene, Examples include tetradecene, 1-hexadecene, 1-octadecene, and 1-eicocene.

  The porous sheet may contain additives such as an antioxidant, a metal damage inhibitor, an ultraviolet absorber, a pigment, a dye, and an antiblocking agent.

  The electret sheet of the present invention uses a laminated porous sheet obtained by laminating and integrating at least two porous sheets described above.

  The average number of pores in the cut surface in the thickness direction of the laminated porous sheet is limited to 11 or more, preferably 11 to 30, and more preferably 15 to 20. If the average number of vacancies in the thickness direction of the laminated porous sheet is too small, the piezoelectricity of the electret sheet using such a laminated porous sheet is reduced, so that excellent piezoelectricity can be maintained over a long period of time. There is a risk of disappearing.

  In the present invention, the average number of pores in the cut surface in the thickness direction of the laminated porous sheet is a value measured according to the following method. First, the cut surface in the thickness direction of the laminated porous sheet (direction perpendicular to the surface of the laminated porous sheet) was photographed at a magnification of 50 times using a scanning electron microscope, and in the obtained photographed image, A straight standard line parallel to the thickness direction of the laminated porous sheet is drawn. Next, the number of pores in the thickness direction of the laminated porous sheet is obtained by measuring the number of pores existing across the standard line. Then, the number of pores in the thickness direction of the laminated porous sheet was measured at least 10 or more so as to draw a standard line with an interval of 60 μm or more between each other, and the arithmetic average value of these was calculated for the laminated porous sheet. The average number of holes in the cut surface in the thickness direction.

  The average pore diameter in the cut surface in the thickness direction of the laminated porous sheet is preferably 5 to 50 μm, more preferably 10 to 20 μm. According to the laminated porous sheet having an average pore diameter within the above range, an electret sheet having excellent piezoelectric performance can be provided.

  The variation coefficient of the pore diameter in the cut surface in the thickness direction of the laminated porous sheet is preferably 30% or less, and more preferably 20% or less. If the variation coefficient of the hole diameter is too large, the piezoelectric performance in the electret sheet may be uneven. There is a possibility that an electret sheet with non-uniform piezoelectric performance cannot be used for applications that require high sensitivity such as sensors.

In addition, the measurement of the average hole diameter in the cut surface of the thickness direction of a laminated porous sheet and the variation coefficient of a hole diameter can be measured as follows. A cut surface in the thickness direction of the laminated porous sheet (direction perpendicular to the surface of the laminated porous sheet) was photographed at a magnification of 500 times using a scanning electron microscope, and at least 100 in the obtained photographed image. The average pore diameter can be calculated by measuring the diameter of the pores and obtaining the arithmetic mean value. The diameter of the hole is the diameter of the smallest perfect circle that can surround the cross section of the hole. Then, the average particle diameter (D [μm]) and the standard deviation (σ [μm]) are calculated from the photographed image, and the variation coefficient of the hole diameter can be calculated from these values based on the following formula (1). .
Variation coefficient of pore diameter (%) = (σ / D) × 100 (1)

  The thickness of the laminated porous sheet is preferably 50 to 300 μm, more preferably 80 to 150 μm. If the laminated porous sheet is too thick, there is a possibility that electric charges cannot be sufficiently injected into the laminated porous sheet. Further, if the stacked porous sheet is too thin, noise may occur in the electret sheet.

  In addition, the thickness of a laminated porous sheet measures the thickness of a laminated porous sheet 15 or more places using a film thickness measuring device (for example, DIGIMICRO MFC-101 by NIKON), and calculates the arithmetic mean value thereof It is set as the value obtained by this.

  The number of laminated porous sheets in the laminated porous sheet is at least 2 layers, preferably 2 to 10 layers, more preferably 3 to 5 layers. If the number of laminated porous sheets is too large, the uniformity of the thickness of the laminated porous sheet may be reduced.

  Next, as a method for producing the electret sheet of the present invention, a laminated porous sheet is obtained by laminating and integrating at least two porous sheets, and a charge is injected into the laminated porous sheet to obtain a laminated porous sheet. A method of charging the sheet is exemplified.

  As a method for producing a porous sheet, (I) a T-die that is supplied with a synthetic resin and a pyrolytic foaming agent to an extruder, melt-kneaded at a temperature lower than the decomposition temperature of the pyrolytic foaming agent, and attached to the extruder The foamable synthetic resin sheet is extruded from, and the foamable synthetic resin sheet is cross-linked as necessary, and then the foamable synthetic resin sheet is heated to a temperature higher than the decomposition temperature of the thermal decomposable foaming agent to cause foaming. The method of manufacturing is mentioned.

  Examples of the thermally decomposable foaming agent include azodicarbonamide, benzenesulfonylhydrazide, dinitrosopentamethylenetetramine, toluenesulfonylhydrazide, 4,4-oxybis (benzenesulfonylhydrazide), and the like.

  The foamable synthetic resin sheet preferably contains a polyfunctional monomer. Therefore, it is preferable to further supply a polyfunctional monomer to the extruder in addition to the synthetic resin and the pyrolytic foaming agent. Polyfunctional monomers include divinylbenzene, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol di (meth) acrylate, and triaryl trimellitic acid Examples include esters, triethylene glycol diacrylate, tetraethylene glycol diacrylate, cyanoethyl acrylate, bis (4-acryloxypolyethoxyphenyl) propane, and trimethylolpropane trimethacrylate and divinylbenzene are preferred. In addition, (meth) acrylate means a methacrylate or an acrylate.

  The porous sheet obtained by foaming of the pyrolytic foaming agent is preferably stretched. By performing stretching, the aspect ratio of the bubbles can be improved, and the piezoelectric performance of the electret sheet can be improved.

  Examples of the method for stretching the porous sheet include a uniaxial stretching method in which stretching is performed in the length direction (extrusion direction) or the width direction of the porous sheet, and both the length direction (extrusion direction) and width direction of the porous sheet. A biaxial stretching method in which stretching is performed in a stretched state, a stretching method in which stretching is performed in the length direction (extrusion direction) while the width direction of the porous sheet is fixed, and a length direction (extrusion direction) of the porous sheet is fixed. Examples of the stretching method include stretching in the width direction.

  The stretching of the porous sheet may be carried out by stretching the laminated porous sheet after the porous sheet is laminated and integrated into at least two layers, and each of the porous sheets of at least two layers is stretched. After stretching, these may be integrated to form a laminated porous sheet.

  Moreover, as a method for producing a porous sheet, (II) a synthetic resin sheet is obtained by extruding a resin composition obtained by mixing a synthetic resin and a filler, and the synthetic resin sheet is crosslinked as necessary. In addition, a method of obtaining a porous sheet in which pores are formed by uniaxially stretching or biaxially stretching the synthetic resin sheet to peel off the interface between the synthetic resin and the filler is also included.

  Examples of the filler include calcium carbonate, magnesium carbonate, talc, mica, carbon black, silica, microballoon, and synthetic resin particles. The synthetic resin particles preferably have a higher melting point than the synthetic resin used as the base resin of the laminated porous sheet.

  The synthetic resin sheet preferably contains a polyfunctional monomer. Therefore, the resin composition is preferably prepared by mixing a synthetic resin, a filler, and a polyfunctional monomer. Examples of the polyfunctional monomer include the same polyfunctional monomers as described above, but trimethylolpropane trimethacrylate and divinylbenzene are preferable.

  Lamination integration of the porous sheet can be performed using a known method such as a thermal lamination method, a hot press method, or a lamination integration method using an adhesive. Examples of the adhesive include hot melt adhesives, solvent-based adhesives, and water-based adhesives.

  And the electret sheet | seat of this invention can be manufactured by inject | pouring an electric charge into the laminated porous sheet mentioned above in the general purpose method, and charging a laminated porous sheet.

  The method for injecting electric charge into the laminated porous sheet is not particularly limited. For example, (1) the laminated porous sheet is sandwiched between a pair of flat plate electrodes, one flat plate electrode is grounded, and the other flat plate electrode is connected to a high voltage. A method of charging a laminated porous sheet by connecting a direct current power source and applying a direct current or pulsed high voltage to the laminated porous sheet to inject charges into the laminated porous sheet; (2) A flat electrode grounded on one side is superposed in close contact, and a needle-like electrode or wire electrode electrically connected to a DC high-voltage power source is disposed on the other side of the laminated porous sheet with a predetermined interval. The corona discharge is generated by the electric field concentration near the tip of the needle electrode or the surface of the wire electrode, the air molecules are ionized, and the air ions generated by the polarity of the needle electrode or the wire electrode are repelled and accumulated. A method for charging a laminated porous sheet by injecting an electric charge into the porous sheet, (3) A portion near the laminated porous sheet by irradiating the laminated porous sheet with ionizing radiation such as an electron beam or X-ray or ultraviolet rays And a method of charging the laminated porous sheet by injecting charges into the laminated porous sheet by ionizing the air molecules. Since the charge can be easily injected into the laminated porous sheet in the above method, the methods (1) and (2) are preferable, and the method (2) is more preferable.

  In the methods (1) and (2) above, the absolute value of the voltage applied to the laminated porous sheet is preferably 3 to 100 kV, more preferably 5 to 50 kV. If the absolute value of the voltage is small, it may not be possible to sufficiently inject charges into the laminated porous sheet, and an electret sheet having high piezoelectricity may not be obtained. On the other hand, if the absolute value of the voltage is large, arc discharge occurs, and on the contrary, it is not possible to sufficiently inject a charge into the laminated porous sheet, and an electret sheet having high piezoelectricity may not be obtained.

  The electret sheet of the present invention is formed by injecting a charge into a laminated porous sheet in which at least two porous sheets containing a synthetic resin are laminated and integrated, and the average number of pores in the thickness direction is 11 or more. Thus, high piezoelectricity can be maintained even when used at high temperatures.

  Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto.

Example 1
100 parts by weight of a propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 7 parts by weight of azodicarbonamide as a thermal decomposable foaming agent, and crosslinking aid 1 (trade name “Light Ester TND” manufactured by Kyoei Chemical Co., Ltd.) -46K15 ") and 5 parts by weight of crosslinking aid 3 (trade name" ADEKA STAB # SB-1002RG "manufactured by ADEKA) are supplied to an extruder, melted and kneaded, and extruded from a T die into a sheet. Thus, a foamable synthetic resin sheet was produced. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The cross-linked foamable synthetic resin sheet is placed in a 250 ° C. hot air oven to decompose the pyrolytic foaming agent and foam the foamable synthetic resin sheet. ) Three porous sheets were produced according to such a procedure, and these were designated as porous sheets A, B, and C, respectively.

  The porous sheet A was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet A was heated. Next, the porous sheet C was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet C was heated. Then, the porous sheet A is laminated on one surface of the porous sheet B so that the one surface of the porous sheet B and the heating surface of the porous sheet A face each other. The porous sheet B is fused and the porous sheet C is laminated on the other surface of the porous sheet B so that the other surface of the porous sheet B and the heating surface of the porous sheet C face each other. Then, the porous sheet C and the porous sheet B were fused. Thereafter, by cooling them, a laminated body in which the porous sheet A, the porous sheet B, and the porous sheet C were laminated and integrated in this order was obtained.

  The laminate was stretched only in the length direction (extrusion direction) at a stretching ratio of 3 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C., and then at a stretching speed of 200 mm / min. The film was stretched only in the width direction at a stretch ratio of 2 to obtain a laminated porous sheet.

(Example 2)
50 parts by weight of propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 50 parts by weight of propylene-based resin 2 (product name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), azo as a pyrolytic foaming agent 9 parts by weight of dicarbonamide, 5 parts by weight of crosslinking aid 2 (trade name “ADEKA STAB # SB-1002RG” manufactured by ADEKA), and 2 parts by weight of crosslinking aid 3 (trade name “ADEKA STAB # SB-1002RG” manufactured by ADEKA) Was supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet to produce a foamable synthetic resin sheet. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The cross-linked foamable synthetic resin sheet is placed in a hot air oven at 250 ° C., the pyrolyzable synthetic resin sheet is foamed by decomposing the pyrolytic foaming agent, and then cooled to obtain a porous sheet (thickness 400 μm). ) Two porous sheets were produced according to such a procedure, and these were designated as porous sheets A and C, respectively.

  100 parts by weight of a propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 7 parts by weight of azodicarbonamide as a thermal decomposable foaming agent, and crosslinking aid 1 (trade name “Light Ester TND” manufactured by Kyoei Chemical Co., Ltd.) -46K15 ") and 5 parts by weight of crosslinking aid 3 (trade name" ADEKA STAB # SB-1002RG "manufactured by ADEKA) are supplied to an extruder, melted and kneaded, and extruded from a T die into a sheet. Thus, a foamable synthetic resin sheet was produced. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The cross-linked foamable synthetic resin sheet is put into a hot air oven at 250 ° C., the pyrolytic foaming agent is decomposed to foam the foamable synthetic resin sheet, and then cooled, so that the porous sheet B (thickness) 500 μm) was obtained.

  The porous sheet A was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet A was heated. Next, the porous sheet C was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet C was heated. Then, the porous sheet A is laminated on one surface of the porous sheet B so that the one surface of the porous sheet B and the heating surface of the porous sheet A face each other. The porous sheet B is fused and the porous sheet C is laminated on the other surface of the porous sheet B so that the other surface of the porous sheet B and the heating surface of the porous sheet C face each other. Then, the porous sheet C and the porous sheet B were fused. Thereafter, by cooling them, a laminated body in which the porous sheet A, the porous sheet B, and the porous sheet C were laminated and integrated in this order was obtained.

  The laminate was stretched in the length direction (extrusion direction) at a stretch ratio of 6 times at a stretching speed of 200 mm / min with the surface temperature of 135 ° C. and both ends in the width direction fixed. Thus, a laminated porous sheet was obtained.

(Example 3)
100 parts by weight of a propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 7 parts by weight of azodicarbonamide as a thermal decomposable foaming agent, and crosslinking aid 1 (trade name “Light Ester TND” manufactured by Kyoei Chemical Co., Ltd.) -46K15 ") and 5 parts by weight of crosslinking aid 3 (trade name" ADEKA STAB # SB-1002RG "manufactured by ADEKA) are supplied to an extruder, melted and kneaded, and extruded from a T die into a sheet. Thus, a foamable synthetic resin sheet was produced. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The cross-linked foamable synthetic resin sheet is placed in a hot air oven at 250 ° C., the pyrolytic foaming agent is decomposed to foam the foamable synthetic resin sheet, and then cooled, so that a porous sheet (thickness 600 μm) is obtained. ) Two porous sheets were prepared according to such a procedure, and these were designated as porous sheets A and B, respectively.

  The porous sheets A and B were each stretched only in the length direction (extrusion direction) at a stretching ratio of 6 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C.

  The stretched porous sheet A is supplied to a roll heated to a surface temperature of 150 ° C. and heated on one surface of the stretched porous sheet A, and then stretched on the heated surface of the stretched porous sheet A. After laminating the porous sheet B, the laminated porous sheet was obtained by fusing them and then cooling.

(Example 4)
50 parts by weight of propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 50 parts by weight of propylene-based resin 2 (trade name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), and crosslinking aid 3 (ADEKA) The product name “Adeka Stub # SB-1002RG” manufactured by the company, and 2 parts by weight, and 50 parts by weight of calcium carbonate as a filler are supplied to an extruder, melt-kneaded, and extruded from a T-die into a sheet to obtain a synthetic resin. A sheet (thickness 200 μm) was produced. The synthetic resin sheet was crosslinked by irradiating the synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. In the same procedure, two crosslinked synthetic resin sheets were produced. These two crosslinked synthetic resin sheets were each stretched only in the length direction (extrusion direction) at a stretch ratio of 4 times at a stretching speed of 200 mm / min, with the surface temperature being 135 ° C., Porous sheets A and C in which pores were formed by peeling the interface between propylene resins 1 and 2 and calcium carbonate were obtained.

  50 parts by weight of propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 50 parts by weight of propylene-based resin 2 (trade name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), and crosslinking aid 3 (ADEKA) The product name “Adeka Stub # SB-1002RG” manufactured by the company, and 2 parts by weight, and 50 parts by weight of calcium carbonate as a filler are supplied to an extruder, melt-kneaded, and extruded from a T-die into a sheet to obtain a synthetic resin. A sheet (thickness 150 μm) was produced. The synthetic resin sheet was crosslinked by irradiating the synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The crosslinked synthetic resin sheet was stretched only in the width direction at a stretching ratio of 3 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C., and propylene-based resins 1 and 2 and carbonic acid A porous sheet B having pores formed by peeling the interface with calcium was obtained.

  The porous sheet A was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet A was heated. Next, the porous sheet C was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet C was heated. Then, the porous sheet A is laminated on one surface of the porous sheet B so that the one surface of the porous sheet B and the heating surface of the porous sheet A face each other. The porous sheet B is fused and the porous sheet C is laminated on the other surface of the porous sheet B so that the other surface of the porous sheet B and the heating surface of the porous sheet C face each other. Then, the porous sheet C and the porous sheet B were fused. Thereafter, by cooling these, a laminated porous sheet in which the porous sheet A, the porous sheet B, and the porous sheet C were laminated and integrated in this order was obtained.

(Example 5)
20 parts by weight of propylene-based resin 2 (trade name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), 80 parts by weight of propylene-based resin 3 (product name “Novatech # EG9” manufactured by Nippon Polypro Co., Ltd.), and 20 parts by weight of calcium carbonate as a filler The part was supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet to produce a synthetic resin sheet (thickness 150 μm). Next, the synthetic resin sheet was stretched only in the length direction (extrusion direction) at a stretching ratio of 3 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C., and the propylene-based resin 2 And the porous sheet in which the void | hole was formed by peeling the interface of 3 and calcium carbonate was obtained. Two porous sheets were produced according to such a procedure, and these were designated as porous sheets A and C, respectively.

  20 parts by weight of propylene-based resin 2 (trade name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), 80 parts by weight of propylene-based resin 3 (trade name “Novatech # EG9” manufactured by Nippon Polypro Co., Ltd.), and 50 parts by weight of calcium carbonate as a filler The part was supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet to produce a synthetic resin sheet (thickness 150 μm). Next, the synthetic resin sheet was stretched only in the width direction at a stretch ratio of 4 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C., and the propylene-based resins 2 and 3, A porous sheet B having pores formed by peeling the interface with calcium was obtained.

  The porous sheet A was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet A was heated. Next, the porous sheet C was supplied to a roll whose surface temperature was heated to 150 ° C., and one surface of the porous sheet C was heated. Then, the porous sheet A is laminated on one surface of the porous sheet B so that the one surface of the porous sheet B and the heating surface of the porous sheet A face each other. The porous sheet B is fused and the porous sheet C is laminated on the other surface of the porous sheet B so that the other surface of the porous sheet B and the heating surface of the porous sheet C face each other. Then, the porous sheet C and the porous sheet B were fused. Thereafter, by cooling these, a laminated porous sheet in which the porous sheet A, the porous sheet B, and the porous sheet C were laminated and integrated in this order was obtained.

(Comparative Example 1)
50 parts by weight of propylene-based resin 1 (trade name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 50 parts by weight of propylene-based resin 2 (product name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), azo as a pyrolytic foaming agent 5 parts by weight of dicarbonamide, 5 parts by weight of crosslinking aid 1 (trade name “Light Ester TND-46K15” manufactured by Kyoei Chemical Co., Ltd.), and 2 parts by weight of crosslinking aid 3 (trade name “ADEKA STAB # SB-1002RG” manufactured by ADEKA) The part was supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet to produce a foamable synthetic resin sheet. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. A porous sheet (thickness of 750 μm) is obtained by placing the crosslinked foamed synthetic resin sheet in a hot air oven at 250 ° C., decomposing the pyrolytic foaming agent to foam the foamable synthetic resin sheet, and then cooling the foamed synthetic resin sheet. ) Two porous sheets were prepared according to such a procedure, and these were designated as porous sheets A and B, respectively.

  The porous sheet A is supplied between a pair of rolls heated to a surface temperature of 150 ° C. to heat one surface of the porous sheet A, and the porous sheet B is laminated on the heated surface of the porous sheet A. These were then fused and cooled. Thereby, the laminated body in which the porous sheet A and the porous sheet B were laminated and integrated in this order was obtained.

  The laminate was stretched only in the length direction (extrusion direction) at a stretching ratio of 3 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C., and then at a stretching speed of 200 mm / min. The film was stretched only in the width direction at a stretch ratio of 2 to obtain a laminated porous sheet.

(Comparative Example 2)
30 parts by weight of propylene-based resin 1 (product name “Wintech # WFW4” manufactured by Nippon Polypro Co., Ltd.), 70 parts by weight of propylene-based resin 2 (product name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), azo as a pyrolytic foaming agent 5 parts by weight of dicarbonamide, 3 parts by weight of crosslinking aid 2 (trade name “ADEKA STAB # SB-1002RG” manufactured by ADEKA), and 2 parts by weight of crosslinking aid 3 (trade name “ADEKA STAB # SB-1002RG” manufactured by ADEKA) Was supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet to produce a foamable synthetic resin sheet. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The cross-linked foamable synthetic resin sheet is put into a hot air oven at 250 ° C., the pyrolyzable synthetic resin sheet is foamed by decomposing the pyrolyzable foaming agent, and then cooled. ) Two porous sheets were prepared according to such a procedure, and these were designated as porous sheets A and B, respectively.

  The porous sheet A is supplied between a pair of rolls heated to a surface temperature of 150 ° C. to heat one surface of the porous sheet A, and the porous sheet B is laminated on the heated surface of the porous sheet A. These were then fused and cooled. Thereby, the laminated body in which the porous sheet A and the porous sheet B were laminated and integrated in this order was obtained.

  The laminate was stretched only in the length direction (extrusion direction) at a stretch ratio of 6 times at a stretch rate of 200 mm / min with the surface temperature of 135 ° C. and both ends in the width direction fixed. Thus, a laminated porous sheet was obtained.

(Comparative Example 3)
100 parts by weight of propylene-based resin 2 (trade name “Novatech # EG8B” manufactured by Nippon Polypro Co., Ltd.), 5 parts by weight of azodicarbonamide as a thermal decomposable foaming agent, and crosslinking aid 1 (trade name “Light Ester TND- manufactured by Kyoei Chemical Co., Ltd.) 46K15 ") 3 parts by weight, and crosslinking aid 3 (trade name" ADEKA STAB # SB-1002RG "manufactured by ADEKA) 2 parts by weight are supplied to an extruder, melted and kneaded, and extruded from a T-die into a sheet form. Thus, a foamable synthetic resin sheet was produced. The expandable synthetic resin sheet was crosslinked by irradiating the expandable synthetic resin sheet with 20 kGy of an electron beam under the condition of an acceleration voltage of 300 kV. The crosslinked foamable synthetic resin sheet is put into a hot air oven at 250 ° C., the pyrolytic foaming agent is decomposed to foam the foamable synthetic resin sheet, and then cooled to obtain a porous sheet (thickness 800 μm). ) Two porous sheets were prepared according to such a procedure, and these were designated as porous sheets A and B, respectively.

  Each of the porous sheets A and B was stretched only in the length direction (extrusion direction) at a stretching ratio of 6 times at a stretching speed of 200 mm / min so that the surface temperature was 135 ° C.

  The stretched porous sheet A is supplied between a pair of rolls heated at a surface temperature of 150 ° C. to heat one surface of the porous sheet A, and the stretched porous sheet A is heated on the heated surface of the porous sheet A. After the quality sheets B were laminated and fused, they were cooled. Thereby, the porous sheet A and the porous sheet B were laminated and integrated in this order to obtain a laminated porous sheet.

(Evaluation)
The average number of pores and average pore diameter at the cut surface in the thickness direction of the laminated porous sheet, and the thickness of the laminated porous sheet were measured according to the above procedure. Moreover, according to the following procedure, the electret sheet | seat was produced using the laminated porous sheet, and the piezoelectric performance of this electret sheet | seat was measured. The results are shown in Table 1.

(Piezoelectric performance of electret sheet)
A grounded flat plate electrode is placed in close contact with one side of the laminated porous sheet, and a needle-like electrode electrically connected to a DC high-voltage power source is arranged on the other side of the laminated porous sheet with a predetermined interval. The corona discharge is generated under the conditions of a voltage of −10 kV, a discharge distance of 10 mm, and a voltage application time of 1 minute due to the electric field concentration near the surface of the needle electrode to ionize the air molecules. Air ions generated by polarity were repelled to inject charges into the laminated porous sheet to charge the laminated porous sheet. Then, the static electricity which exists on the surface of a lamination | stacking porous sheet was removed by hold | maintaining the laminated porous sheet which inject | poured the charge for 3 hours in the state wrapped with the aluminum foil earth | grounded, and obtained the electret sheet | seat.

  By cutting the electret sheet, a flat square test piece having a side of 5 cm was obtained. A test specimen was obtained by disposing a flat square copper foil having a side of 4.5 cm and a thickness of 40 μm at the center of each of the front and back surfaces of the test piece. A pressing force was applied to this test body under the conditions of a load F of 2 N, a dynamic load of ± 0.25 N, and a frequency of 110 Hz using a vibrator, and the charge Q (C) generated at that time was measured. Then, the piezoelectric performance (pC / N) of the test piece was calculated by dividing the charge Q (C) by the load F (N).

The electret sheet immediately after production was allowed to stand in a temperature environment of 23 ° C. for 1 hour, and then the piezoelectric performance (P ₀ ) of the electret sheet was measured according to the above procedure and evaluated according to the following criteria. The results are shown in the column of “Piezoelectric performance (initial)” in Table 1.
A: The piezoelectric performance was 20 pC / N or more.
X: The piezoelectric performance was less than 20 pC / N.

Further, the electret sheet was left in an oven at 80 ° C. for 7 days, and then the electret sheet was left in a constant temperature and humidity chamber at 23 ° C. for 1 hour. The piezoelectric performance (P ₁ ) of this electret sheet is measured according to the above procedure, and the rate of change with respect to the piezoelectric performance (P ₀ ) of the electret sheet immediately after manufacture is expressed by the formula [rate of change (%) = 100 × (P ₀ −P ₁ ) / P ₀ ] and evaluated according to the following criteria.
A: Change rate was less than 50%.
X: The rate of change was 50% or more.

Claims (4)

  A porous sheet containing a synthetic resin is laminated and integrated with at least two layers, and is charged by injecting a charge into a laminated porous sheet having an average number of pores of 11 or more in the cut surface in the thickness direction. Characteristic electret sheet.   The electret sheet according to claim 1, wherein the porous sheet contains a propylene-based resin.   The electret sheet according to claim 1 or 2, wherein an average pore diameter in a cut surface in the thickness direction of the laminated porous sheet is 5 to 50 µm.   The electret sheet according to any one of claims 1 to 3, wherein the thickness of the laminated porous sheet is 50 to 300 µm.