JP2014093313A - Electret sheet - Google Patents

Abstract

An electret sheet that retains high piezoelectricity even when used at high temperatures is provided.
An electret sheet of the present invention includes a surface layer containing an olefin resin on both sides of a porous sheet containing an olefin resin, having an air permeability of 800 sec / 100 mL or more and a thickness of 10 to 40 μm. Is characterized in that the porous sheet is charged by injecting electric charge into the porous sheet of the laminated sheet formed by laminating and integrating. In the electret sheet of the present invention, the surface layers provided on both sides of the porous sheet prevent electric charges from being discharged at high temperatures, so that the piezoelectricity does not deteriorate over time even when used at high temperatures. Have excellent piezoelectricity over a long period of time.
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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.

  For example, in Patent Document 1, a porous sheet in which pores are formed by stretching a thermoplastic resin film containing at least one of inorganic powder and organic filler, and at least one surface of the porous sheet is provided. An electret sheet having a surface layer made of a stretched resin film is disclosed. The surface layer is used as a protective layer that prevents leakage of charges stored in the porous sheet.

International Publication No. 2011-034186

  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.

  As a result of intensive studies in view of the above problems, the present inventors have found that when the electret sheet is placed at a high temperature, gas such as water vapor present in the surroundings reaches the porous sheet via the surface layer, and the porous sheet It is considered that the piezoelectricity of the electret sheet decreases with time in order to promote the leakage of the electric charge stored in.

  Therefore, the electret sheet of the present invention has a surface layer containing an olefin resin, an air permeability of 800 sec / 100 mL or more, and a thickness of 10 to 40 μm on both surfaces of a porous sheet containing an olefin resin. It is characterized by charging the porous sheet by injecting electric charge into the porous sheet of an integrated laminated sheet.

  In such an electret sheet of the present invention, gas permeation into the porous sheet is highly reduced by using a surface layer having excellent gas barrier properties. Therefore, the electret sheet of the present invention does not deteriorate in piezoelectricity over time even when used at high temperatures, and can maintain excellent piezoelectricity over a long period of time.

[Porous sheet]
The porous sheet contains an olefin resin. Examples of the olefin resin contained in the porous sheet include an ethylene resin and a propylene resin. Of these, propylene-based resins are preferred because of their excellent insulating properties and high charge retention.

  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.

  As the propylene-based resin, a propylene-ethylene copolymer is more preferable, and a propylene-ethylene random copolymer is particularly preferable.

  The flexural modulus of the propylene-based resin is preferably 150 MPa or more, and more preferably 150 to 3000 MPa. If the flexural modulus of the propylene-based resin is small, the charge retention of the electret sheet may be reduced, and the performance of the electret sheet may not be stably maintained over a long period of time. In addition, the bending elastic modulus of propylene-type resin says the value measured based on JISK7171.

  10-200 micrometers is preferable and, as for the thickness of a porous sheet, 60-100 micrometers is more preferable. If the porous sheet is too thick, there is a possibility that charges cannot be sufficiently injected into the porous sheet. Moreover, when a porous sheet is too thin, there exists a possibility that the mechanical strength of a lamination sheet may fall.

[Surface layer]
On both surfaces of the porous sheet described above, a surface layer containing an olefin resin, having an air permeability of 800 sec / 100 mL or more and a thickness of 10 to 40 μm is laminated and integrated.

  Examples of the olefin resin contained in the surface layer include ethylene resins and propylene resins. Of these, propylene-based resins are preferable. Since the propylene-based resin is excellent in heat resistance, it can form a surface layer that can exhibit excellent gas barrier properties even at high temperatures.

  The propylene 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.

  As the propylene-based resin, a propylene-ethylene copolymer is more preferable, and a propylene-ethylene random copolymer is particularly preferable.

  In the surface layer, the olefin resin is preferably crosslinked. By cross-linking the olefin resin, a surface layer having a high gas barrier property can be obtained.

  The air permeability of the surface layer is limited to 800 sec / 100 mL or more, but is preferably 900 sec / 100 mL or more, and more preferably 1000 sec / 100 mL or more. If the gas barrier property of the surface layer is too low, the piezoelectricity of the electret sheet may be lowered when used at high temperatures.

  The air permeability of the surface layer can be controlled by the type of olefin resin used for the surface layer and the degree of crosslinking.

  The air permeability of the surface layer can be measured as follows. First, only the surface layer is peeled from the laminated sheet using a cutter. When a part of the porous sheet is adhered to the peeled surface layer, the part of the adhered porous sheet is removed by a cutter without damaging the surface layer. The air permeability of the obtained surface layer was measured at 10 points at intervals of 10 cm in the length direction of the surface layer in an atmosphere of a temperature of 23 ° C. and a relative humidity of 65% in accordance with JIS P8117. Is a value obtained by calculating.

  Although the thickness of a surface layer is limited to 10-40 micrometers, 10-30 micrometers is preferable. If the thickness of the surface layer is too thick, there is a possibility that a sufficient charge cannot be injected into the porous sheet. Moreover, when the surface layer is too thin, there is a possibility that the piezoelectricity of the electret sheet is lowered during use at a high temperature.

  The thickness of the surface layer in the laminated sheet was obtained by taking a cross-section of the laminated sheet at a magnification of 500 times using a scanning electron microscope and measuring the thickness of the surface layer in at least 10 locations in the obtained photographed image. , And the value obtained by calculating the arithmetic mean value. Moreover, the measurement of the thickness of the porous sheet in a lamination sheet can be performed similarly.

  The porous sheet and the surface layer may contain other additives such as an antioxidant, a metal damage inhibitor, an ultraviolet absorber, a pigment, a dye, and an antiblocking agent. Moreover, the porous sheet and the surface layer may be uniaxially stretched or biaxially stretched in a general manner.

  As a method for producing a laminated sheet in which the surface layers are laminated and integrated on both surfaces of the porous sheet, a coextrusion method, a thermal lamination method, or the like is used, and the coextrusion method is preferable. In order to manufacture a laminated sheet by the co-extrusion method, first, a manufacturing apparatus in which the first extruder, the second extruder, and the third extruder are all connected to the same die is prepared. An olefin resin and a thermally decomposable foaming agent are supplied and melt-kneaded at a temperature lower than the decomposition temperature of the thermally decomposable foaming agent to obtain a foamable resin composition, and to the first extruder and the third extruder After obtaining the composition for forming the surface layer by supplying the olefin resin and melt-kneading, the foamable resin composition and the composition for forming the surface layer are obtained from the joining die connecting the first to third extruders. Is extruded to obtain a molten laminated sheet in which a molten surface layer made of a surface layer forming composition is laminated and integrated on both sides of a molten foamable resin sheet made of a foamable resin composition. Next, after subjecting the melt-foamable resin sheet and the melt surface layer to a crosslinking treatment as necessary, the melt-laminated sheet is heated to a temperature equal to or higher than the decomposition temperature of the thermally decomposable foaming agent to foam the melt-foamable resin sheet. After obtaining the molten porous sheet, the molten surface layer and the molten porous sheet are cooled. Thereby, the lamination sheet by which a surface layer is laminated and integrated on both surfaces of a porous sheet is obtained.

  In the case of obtaining a porous sheet or a surface layer that has been subjected to a crosslinking treatment, the foamable resin composition or the surface layer forming composition preferably contains a polyfunctional monomer as a crosslinking aid. 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.

  In addition, as a thermal decomposition type foaming agent, what is necessary is just to generate | occur | produce gas by decomposition | disassembly, for example, azodicarbonamide, benzenesulfonyl hydrazide, dinitroso pentamethylene tetramine, toluenesulfonyl hydrazide, 4, 4-oxybis (benzenesulfonyl hydrazide). Etc.

  Examples of a method for producing a laminated sheet by a thermal laminating method include a method in which a porous sheet and a surface layer are separately prepared, and then the surface layer is thermally laminated on both surfaces of the porous sheet.

  When a laminated sheet is produced by the thermal laminating method, the porous sheet is produced by supplying an olefin resin and a pyrolytic foaming agent to an extruder and melting at a temperature lower than the decomposition temperature of the pyrolytic foaming agent. A foamable resin sheet is extruded from a T-die kneaded and attached to an extruder, and the foamable resin sheet is cross-linked as necessary, and then the foamable resin sheet is heated to a temperature higher than the decomposition temperature of the thermally decomposable foaming agent. A method for producing a porous sheet by foaming; a resin film is obtained by extruding a resin composition obtained by mixing an olefin resin and a filler, and the resin film is uniaxially or biaxially stretched to obtain an olefin resin. And a method of obtaining a porous sheet in which voids are formed by peeling the interface between the filler and the filler.

  When a laminated sheet is produced by a thermal lamination method, the surface layer is produced by supplying an olefin resin to an extruder, melt-kneading and extruding the resin sheet from a T-die attached to the extruder. The method of bridge | crosslinking as needed is mentioned.

(Electret sheet)
The electret sheet | seat of this invention can be manufactured by inject | pouring a charge into a porous sheet in the laminated sheet mentioned above in the general purpose point, and charging a porous sheet.

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

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

  In the electret sheet of the present invention, electric charge is injected into the porous sheet and charged. The porous sheet contains many voids. By injecting electric charge into such a porous sheet, the electric charge apparently accumulates at the interface between the void and the olefin resin and is polarized in a positive charge and a negative charge. And the electrical response arises by making the positive charge and the negative charge which have accumulated in the polarized state change relatively, and the electret sheet exhibits excellent piezoelectricity.

  In the electret sheet of the present invention, the discharge of electric charges at high temperatures is prevented by the surface layers provided on both sides of the porous sheet, and therefore the piezoelectricity decreases with time even when used at high temperatures. No, it has excellent piezoelectricity over a long period of time.

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

(Examples 1-2 and Comparative Examples 1-4)
A production apparatus was prepared in which the first extruder, the second extruder, and the third extruder were all connected to the same die. In the second extruder, propylene-ethylene random copolymer (ethylene content: 4.5 wt% or less, melting start temperature: 97 ° C., flexural modulus: 850 MPa), trimethylolpropane trimethacrylate (TMPT) as a polyfunctional monomer Azodicarbonamide as pyrolytic foaming agent, tetrakis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxymethyl] methane as antioxidant, and methylbenzotriazole as metal hazard inhibitor Were supplied at the blending amounts shown in the column of “blending amount (porous sheet)” in Table 1, and melt-kneaded at 160 ° C. to obtain a foamable resin composition for forming a porous sheet. . In each of the first extruder and the third extruder, a propylene-ethylene random copolymer (ethylene content: 4.5% by weight or less, melting start temperature: 97 ° C., flexural modulus: 850 MPa), polyfunctional monomer As trimethylolpropane trimethacrylate (TMPT), azodicarbonamide as a pyrolytic foaming agent, tetrakis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionyloxymethyl] methane as an antioxidant, And a composition for forming a surface layer by supplying methylbenzotriazole as a metal harm-preventing agent at a blending amount shown in the column of “blending amount (surface layer)” in Table 1 and melt-kneading at 160 ° C. Got. Next, by extruding the foamable resin composition and the surface layer forming composition from the joining die connecting the first to third extruders, on both sides of the melt-foamable resin sheet comprising the foamable resin composition, A melt-laminated sheet obtained by laminating and integrating a melt surface layer made of the surface layer forming composition was obtained.

  The molten laminated sheet is irradiated with an electron beam at 25 kGy under an acceleration voltage of 300 kV to crosslink the propylene-ethylene random copolymer contained in the molten foamable resin sheet and the molten surface layer, respectively, The sheet was put into a hot air oven at 250 ° C., and the molten foamable resin sheet obtained by decomposing and cross-linking the pyrolytic foaming agent was foamed to obtain a molten porous sheet, which was then cooled. Thus, a laminated sheet in which the surface layers were laminated and integrated on both surfaces of the porous sheet was obtained.

  Then, a plate electrode grounded on one surface of the obtained laminated sheet is superposed in a close contact state, and acicular electrodes electrically connected to a DC high-voltage power source with a predetermined interval on the other surface side of the laminated sheet are provided. By arranging the electric field near the surface of the needle electrode, a 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 to ionize air molecules, and the needle electrode The porous sheet was charged by repelling air ions generated due to the polarity of and injecting charges into the porous sheet in the laminated sheet. Thereafter, the laminate sheet containing the porous sheet into which the electric charge has been injected is held for 3 hours in a state of being wrapped in a grounded aluminum foil, thereby removing static electricity present on the surface of the laminate sheet and removing the electret sheet. Obtained.

(Evaluation)
The thickness of the porous sheet and the surface layer in the laminated sheet, and the air permeability of the surface layer were measured according to the methods described above. Moreover, the piezoelectric performance of the electret sheet was measured in the following manner. The obtained results are shown in Table 1.

(Piezoelectric performance)
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 aluminum foil having a side of 3 cm and a thickness of 0.3 mm 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 piezoelectricity (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 left in a constant temperature and humidity chamber at 23 ° C. and 50% relative humidity for 1 hour, and then the piezoelectric performance of the electret sheet was measured. The result is shown in “Piezoelectric performance (initial)” in Table 1. It described in the column of.

  Further, after the electret sheet is left in a constant temperature and humidity chamber at 50 ° C. and a relative humidity of 50% for 7 days, the electret sheet is placed in a constant temperature and humidity chamber at 23 ° C. and a relative humidity of 50% for 1 hour. I left it alone. The piezoelectric performance of this electret sheet was measured, and the result is shown in the column of “Piezoelectric performance (50 ° C. durability)” in Table 1.

Claims (4)

  The above-mentioned laminated sheet in which a surface layer containing an olefin resin, having an air permeability of 800 sec / 100 mL or more and having a thickness of 10 to 40 μm is laminated and integrated on both surfaces of a porous sheet containing an olefin resin. An electret sheet obtained by injecting electric charge into a porous sheet to charge the porous sheet.   The electret sheet according to claim 1, wherein the olefin resin contained in the porous sheet contains a propylene resin.   The electret sheet according to claim 1 or 2, wherein the olefin resin contained in the surface layer contains a propylene resin.   The electret sheet according to any one of claims 1 to 3, wherein electric charge is injected by corona discharge treatment.