JP7221582B2 - electret sheet - Google Patents

Description

The present invention relates to an electret sheet.

An electret sheet is a material that is internally permanently charged by injecting an electric charge into an insulating polymer material.

It is known that a foamed sheet made of synthetic resin exhibits extremely high piezoelectricity comparable to that of ceramics by electrifying the cell membrane forming the cells and the vicinity thereof. Electrets using such synthetic resin foam sheets have been proposed to be applied to acoustic pickups, various pressure sensors, etc. by utilizing their excellent sensitivity.

As an electret sheet, Patent Document 1 discloses an electret sheet which is a sheet to which chlorinated polyolefin is applied and which has a surface charge density of 1×10 ⁻¹⁰ Coulomb/cm ² or more. there is

JP-A-8-284063

However, the electret sheet of Patent Document 1 has a problem that the piezoelectricity is lowered in a high-temperature environment.

The present invention provides an electret sheet that retains high piezoelectricity even in a high temperature environment.

The electret sheet of the present invention comprises a charged porous sheet, and has a volume resistivity of 1.0×10 ¹⁵ Ω·cm or more at 25° C. and 60% relative humidity and at 25° C. and 60% relative humidity It is characterized by having a dielectric breakdown voltage of 6 kV or higher.

Since the electret sheet of the present invention has the structure as described above, the discharge of electric charges is reduced even in a high-temperature environment, and excellent piezoelectricity is maintained.

The electret sheet of the present invention includes a charged porous sheet. The porous sheet is not particularly limited as long as it has voids inside, but a synthetic resin foam sheet is preferable. The synthetic resin constituting the synthetic resin foam sheet is not particularly limited, and examples thereof include polyolefin resins such as polyethylene resins and polypropylene resins, polyvinylidene fluoride, polylactic acid, and liquid crystal resins. It preferably contains a polypropylene-based resin, and more preferably contains a polypropylene-based resin.

It is preferable that the synthetic resin has excellent insulating properties, and the synthetic resin has a volume specific resistance value after 1 minute of voltage application at an applied voltage of 500 V in accordance with JIS K6911 (hereinafter simply referred to as "volume specific resistance value" ) of 1.0×10 ¹⁰ Ω·m or more is preferred.

The volume resistivity of the synthetic resin is preferably 1.0×10 ¹² Ω·m or more, more preferably 1.0×10 ¹⁴ Ω·m or more, because the electret sheet has superior piezoelectricity.

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

The polypropylene-based resin is not particularly limited as long as it contains more than 50% by mass of the propylene component. Examples thereof include copolymers with the following olefins. Incidentally, the polypropylene-based resin may be used alone or in combination of two or more. The copolymer of propylene and at least one olefin having 20 or less carbon atoms other than propylene may be either a block copolymer or a random copolymer.

Examples of α-olefins 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, 1-eicosene and the like.

The expansion ratio of the synthetic resin foam sheet is preferably 3 to 15 times, more preferably 4 to 10 times. When the expansion ratio of the synthetic resin foam sheet is 3 times or more, the flexibility of the electret sheet is improved, the degree of deformation against pressure is increased, and the piezoelectricity of the electret sheet is increased, which is preferable. When the expansion ratio of the synthetic resin foam sheet is 15 times or less, the mechanical strength of the electret sheet is improved, the compression set is small, and the electret sheet maintains excellent piezoelectricity for a long period of time, which is preferable. The expansion ratio of a synthetic resin foamed sheet refers to a value obtained by dividing the density of the entire synthetic resin forming the synthetic resin foamed sheet by the density of the synthetic resin foamed sheet.

The thickness of the synthetic resin foam sheet is preferably 10-300 μm, more preferably 30-200 μm. When the thickness of the synthetic resin foam sheet is 10 µm or more, the number of cells in the thickness direction can be secured, and the piezoelectricity of the electret sheet is improved, which is preferable. When the thickness of the synthetic resin foam sheet is 300 μm or less, the cell walls of the electret sheet can be effectively charged in a polarized state, and the piezoelectric stability of the electret sheet is improved, which is preferable.

The method for producing the synthetic resin foam sheet is not particularly limited. A foamable synthetic resin sheet is extruded from a T-die attached to a melt-kneading extruder at a temperature of , and after cross-linking the foamable synthetic resin sheet as necessary, the foamable synthetic resin sheet is treated with a thermally decomposable foaming agent. A method of producing a synthetic resin foamed sheet by heating to a decomposition temperature or higher to foam is exemplified.

Thermal decomposition type blowing agents include, for example, azodicarbonamide, benzenesulfonylhydrazide, dinitrosopentamethylenetetramine, toluenesulfonylhydrazide, 4,4-oxybis(benzenesulfonylhydrazide) and the like.

The synthetic resin foam sheet is preferably crosslinked using a polyfunctional monomer. By using a polyfunctional monomer, the cross-linking efficiency of the synthetic resin can be improved, and the electret sheet exhibits excellent piezoelectricity even under weak stress.

Polyfunctional monomers include divinylbenzene, trimethylolpropane tri(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, triallyl trimellitate, tri ethylene glycol diacrylate, tetraethylene glycol diacrylate, cyanoethyl acrylate, bis(4-acryloxypolyethoxyphenyl)propane and the like. Among them, trimethylolpropane tri(meth)acrylate, divinylbenzene, and 1,9-nonanediol di(meth)acrylate are preferred. (Meth)acrylate means methacrylate or acrylate.

The amount of the polyfunctional monomer is preferably 0.1 to 10 parts by mass, preferably 0.5 to 8 parts by mass, per 100 parts by mass of the synthetic resin. When the amount of the polyfunctional monomer is 0.1 parts by mass or more, the cross-linking efficiency of the synthetic resin can be sufficiently improved. When the amount of the polyfunctional monomer is 10 parts by mass or less, the electret sheet exhibits excellent piezoelectricity even under weak stress.

In the above manufacturing method, it is preferable to cure the expandable synthetic resin sheet. By curing the foamable synthetic resin sheet, the residual strain in the synthetic resin is released, and the cells of the synthetic resin foamed sheet obtained become uniform and fine. retain their sexuality.

The ambient temperature for curing the expandable synthetic resin sheet is preferably 20 to 70°C, more preferably 20 to 50°C. When the curing temperature of the foamable synthetic resin sheet is 20° C. or higher, the curing time can be shortened, and the manufacturing efficiency of the synthetic resin foamed sheet is improved. When the curing temperature of the foamable synthetic resin sheet is 70° C. or less, the cells of the foamed synthetic resin sheet become uniform, and the resulting electret sheet maintains high piezoelectricity even in a high-temperature environment.

The curing time of the expandable synthetic resin sheet is preferably 1 to 120 hours, more preferably 2 to 72 hours, and particularly preferably 20 to 72 hours. When the foamable synthetic resin sheet is cured for 1 hour or more, the resulting synthetic resin foamed sheet has uniform cells, and the resulting electret sheet retains high piezoelectricity even in a high-temperature environment. When the curing time of the foamable synthetic resin sheet is 120 hours or less, the synthetic resin foamed sheet having excellent surface smoothness can be obtained, and the electret sheet retains high piezoelectricity even in a high temperature environment.

Since the synthetic resin foam sheet can improve the charge retention of the synthetic resin foam sheet, it is preferably stretched, more preferably uniaxially stretched, only in the direction perpendicular to the extrusion direction. It is particularly preferred that the film is uniaxially stretched. Examples of methods for stretching the synthetic resin foam sheet include (1) a uniaxial stretching method in which the synthetic resin foam sheet is stretched in the length direction (extrusion direction) or the width direction (direction perpendicular to the extrusion direction); (3) A biaxial stretching method in which the resin foam sheet is stretched in both the length direction (extrusion direction) and the width direction (perpendicular to the extrusion direction), (3) the width direction of the synthetic resin foam sheet (perpendicular to the extrusion direction) and (4) the width direction (perpendicular to the extrusion direction) with the length direction (extrusion direction) of the synthetic resin foam sheet fixed. direction), and the like.

An electret sheet is constructed by electrifying a porous sheet. A method for charging the porous sheet is not particularly limited, and examples thereof include a method of applying a DC electric field to the porous sheet.

The method of applying a DC electric field to the porous sheet is not particularly limited, and includes, for example: (1) The porous sheet is sandwiched between a pair of flat plate electrodes, and the flat plate electrodes in contact with the surface to be charged are connected to a high-voltage DC power supply. (2) A method of charging the porous sheet by injecting electrical charges into the synthetic resin by applying a direct current or pulse-like high voltage to the porous sheet while grounding the other flat electrode. A needle-like electrode or a wire electrode having a grounded flat plate electrode superposed on one surface in close contact with the second surface of the porous sheet with a predetermined space therebetween and electrically connected to a DC high-voltage power supply. is arranged to generate corona discharge by electric field concentration near the tip of the needle electrode or the surface of the wire electrode, ionize the air molecules, and repel the air ions generated by the polarity of the needle electrode or wire electrode. a method of charging the porous sheet with a

The absolute value of the DC treatment voltage when applying a DC electric field to the porous sheet is preferably 5 to 40 kV, more preferably 10 to 30 kV. By adjusting the DC processing voltage within the above range, the retention of piezoelectricity of the resulting electret sheet at high temperatures is improved.

The volume resistivity of the electret sheet at 25° C. and 60% relative humidity is 1.0×10 ¹⁵ Ωcm or more, preferably 1.0×10 ¹⁵ to 1.0×10 ¹⁸ Ωcm. 0×10 ¹⁶ to 1.0×10 ¹⁸ Ω·cm is more preferable. When the electret sheet has a volume resistivity of 1.0×10 ¹⁵ Ω·cm or more at 25° C. and a relative humidity of 60%, the electret sheet retains excellent piezoelectricity in a high-temperature atmosphere. If the electret sheet has a volume resistivity of 1.0×10 ¹⁸ Ω·cm or less at 25° C. and a relative humidity of 60%, static electricity is less likely to occur on the surface of the electret sheet, and discharge of internal charges is suppressed. This is preferable because it improves the retention of the piezoelectricity of the electret sheet.

The volume resistivity of the electret sheet at 25° C. and 90% relative humidity is preferably 1.0×10 ¹³ Ω·cm or more, more preferably 1.0×10 ¹⁴ to 1.0×10 ¹⁸ Ω·cm, 1.0×10 ¹⁵ to 1.0×10 ¹⁸ Ω·cm is particularly preferable. When the electret sheet has a volume resistivity of 1.0×10 ¹³ Ω·cm or more at 25° C. and a relative humidity of 90%, the electret sheet retains excellent piezoelectricity in a high-temperature atmosphere. If the electret sheet has a volume resistivity of 1.0×10 ¹⁸ Ω·cm or less at 25° C. and a relative humidity of 90%, static electricity is less likely to occur on the surface of the electret sheet, and discharge of internal charges is suppressed. This is preferable because it improves the retention of the piezoelectricity of the electret sheet.

The volume resistivity of the electret sheet at 25° C. under the relative humidity to be measured is measured in the following manner. Specifically, a 10 cm square planar test piece was obtained, and this test piece was placed on the back electrode in an atmosphere of 25° C. and a predetermined relative humidity and allowed to stand for 10 minutes. Place the electrodes and measure the resistance value (R [Ω]) one minute after applying a voltage of 500 V between the electrodes, and calculate the volume resistivity (ρ [Ω cm]) based on the following formula. bottom. During measurement, the load applied to the test piece by the front and back electrodes was adjusted to 5 kPa or less.
ρ=(π×d ² /4t)×R
ρ: Volume resistivity of electret sheet [Ω cm]
π: Circumference ratio (3.14)
d: Diameter of surface electrode (5 [cm])
t: thickness of test piece (0.01 [cm])
R: Resistance value ([Ω])
The volume resistivity of the electret sheet is measured using, for example, a detector (digital ultra-high resistance/micro current meter 8340A manufactured by ADC) and a temperature-controllable measuring electrode (resistivity chamber 12708 manufactured by ADC). be able to.

The method for controlling the volume resistivity of the electret sheet at 25° C. and the relative humidity to be measured within the above range is not particularly limited. Examples thereof include a method of adjusting the weight average molecular weight and a method of adjusting the melt flow rate of the synthetic resin forming the porous sheet forming the electret sheet.

The dielectric breakdown voltage of the electret sheet at 25° C. and 60% relative humidity is 6 kV or more, preferably 6 to 12 kV, more preferably 6 to 10 kV. When the electret sheet has a dielectric breakdown voltage of 6 kV or more at 25° C. and a relative humidity of 60%, the electret sheet has excellent retention of piezoelectricity in a high-temperature atmosphere. When the electret sheet has a dielectric breakdown voltage of 12 kV or less at 25° C. and a relative humidity of 60%, the inside of the electret sheet is easily electrified, and the electret sheet has excellent electrification properties, which is preferable.

The dielectric breakdown voltage of the electret sheet at 25° C. and 90% relative humidity is preferably 4 kV or more, more preferably 4 to 10 kV, and particularly preferably 4 to 8 kV. When the electret sheet has a dielectric breakdown voltage of 4 kV or more at 25° C. and a relative humidity of 90%, the electret sheet retains excellent piezoelectricity in a high-temperature atmosphere. When the dielectric breakdown voltage of the electret sheet at 37° C. is 10 kV or less, the inside of the electret sheet is easily electrified, and the electret sheet has excellent electrification properties, which is preferable.

Incidentally, the dielectric breakdown voltage of the electret sheet at 25° C. and relative humidity to be measured refers to the voltage measured in the following manner. Specifically, a planar square test piece having a side of 50 mm is cut out from the electret sheet. A flat square metal plate and an acrylic plate having a side of 50 mm are prepared. A test piece is sandwiched between a metal plate and an acrylic plate in an atmosphere of 25° C. and relative humidity to be measured. The test piece is sandwiched between the metal plate and the acrylic plate to such an extent that the thickness of the test piece does not decrease (constant pressure load of 5 kPa). Next, an electrode is inserted into a through hole formed in the center of the acrylic plate, the electrode is brought into contact with the test piece, and a DC voltage is applied to the test piece. If there is no current for 30 seconds due to the applied DC voltage, the DC voltage applied to the test piece is increased by 0.5 kV. It is the dielectric breakdown voltage. For the dielectric breakdown voltage of the electret sheet at the measurement temperature, for example, a measuring device commercially available from Kikusui Denshi Co., Ltd. under the trade name "TOS5101" can be used.

Although the method for controlling the dielectric breakdown voltage of the electret sheet at 25° C. and a predetermined relative humidity within the above range is not particularly limited, for example, the synthetic resin constituting the porous sheet constituting the electret sheet and a method of adjusting the weight average molecular weight of the synthetic resin forming the porous sheet forming the electret sheet.

If the volume resistivity and dielectric breakdown voltage of the electret sheet are within the above-specified ranges at 25°C and a predetermined relative humidity, the mechanism by which the retention of the electret sheet at high temperatures is improved is not clearly known. And when the volume resistivity and dielectric breakdown voltage under a predetermined relative humidity are within the above ranges, even if the molecular motion of the constituent materials of the porous sheet constituting the electret sheet increases due to high temperature, intermolecular It is believed that this is because the gap between the porous sheets can be maintained at a predetermined size, and as a result, the charge charged in the porous sheet can be retained well even at high temperatures.

A piezoelectric sensor is constructed by laminating and integrating a signal electrode on a first surface of an electret sheet and a ground electrode on a second surface of the electret sheet. By measuring the potential of the signal electrode using the ground electrode as a reference electrode, the potential generated in the electret sheet of the piezoelectric sensor can be measured.

The signal electrode is laminated and integrated on the first surface of the electret sheet via a fixing agent as necessary. Similarly, the ground electrode is integrally laminated on the second surface of the electret sheet via a fixing agent as required. The signal electrode and the ground electrode are not particularly limited as long as they have conductivity, and examples thereof include metal sheets such as copper foil and aluminum foil, and conductive films.

When the signal electrode and the ground electrode are composed of a conductive film, the conductive film may be formed on an electrically insulating sheet and then laminated and integrated on the electret sheet, or directly on the surface of the electret sheet. , may be formed. Methods for forming a conductive film on an electrical insulating sheet or electret sheet include, for example, (1) coating a conductive paste containing conductive fine particles in a binder on the electrical insulating sheet or electret sheet, and drying the paste; (2) a method of forming electrodes on an electrically insulating sheet or an electret sheet by vapor deposition;

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

The fixing agent that constitutes the fixing agent layer is composed of reactive, solvent, water, or hot-melt adhesives or pressure-sensitive adhesives. agents are preferred.

EXAMPLES Next, examples of the present invention will be described, but the present invention is not limited to the following examples.

The following polypropylene resins A to E and polyethylene resins A and B were prepared.
[Polypropylene resin]
Propylene-ethylene random copolymer (polypropylene resin A, trade name "Novatec EG8B" manufactured by Japan Polypropylene Corporation, ethylene unit content: 5% by mass)
Propylene-ethylene random copolymer (polypropylene resin B, trade name “Wintec WFW4” manufactured by Japan Polypropylene Corporation, ethylene unit content: 2% by mass)
Propylene-ethylene random copolymer (polypropylene resin C, trade name “Wintech WFX4T” manufactured by Japan Polypropylene Corporation, ethylene unit content: 4% by mass)
Propylene-ethylene random copolymer (polypropylene resin D, trade name “Wintec WEG7T” manufactured by Japan Polypropylene Corporation, ethylene unit content: 1% by mass)
Propylene-ethylene random copolymer E (polypropylene resin E, trade name “Prime Polypro B241” manufactured by Prime Polymer Co., ethylene unit content: 2.5% by mass)

[Polyethylene resin]
Linear low-density polyethylene (polyethylene resin A, trade name “EXACT3027” manufactured by Exxon Chemical)
Low-density polyethylene (polyethylene resin B, trade name “Novatec LE520H” manufactured by Japan Polypropylene)

(Examples 1 to 5, Comparative Examples 1 and 2)
Polypropylene resins A to E, polyethylene resins A and B, trimethylolpropane trimethacrylate, azodicarbonamide and phenolic antioxidant are supplied to an extruder in predetermined amounts shown in Table 1, melt-kneaded, and T-die. A foamable resin sheet having a thickness of 180 μm was produced by extruding the resin into a sheet form. A flat square having a side of 30 cm was cut out from the expandable resin sheet.

The obtained foamable resin sheet was cured at an ambient temperature of 25° C. for 48 hours. Both surfaces of the resulting foamable resin sheet were irradiated with electron beams at an acceleration voltage of 500 kV and an intensity of 25 kGy to crosslink the polyolefin resin constituting the foamable resin sheet. The crosslinked foamable resin sheet was heated to 250° C. to foam the foamable resin sheet to obtain a foamed polyolefin resin sheet. The obtained polyolefin resin foam sheet was stretched in the extrusion direction until the thickness became 200 μm using an automatic uniaxial stretching device (trade name “IMC-18C6 type” manufactured by Imoto Seisakusho Co., Ltd.) while the surface temperature was maintained at 130°C. It was uniaxially stretched at a stretching speed of 900 mm/min in a direction orthogonal to the other to obtain a 200 μm polyolefin resin foamed sheet. Table 1 shows the expansion ratio and thickness of the polyolefin resin foam sheet.

A grounded flat plate electrode is placed in close contact with the first surface of the polyolefin resin foam sheet. A connected needle-like electrode is arranged, and an electric field is concentrated near the surface of the needle-like electrode to generate a corona discharge under the conditions of a voltage of -10 kV, a discharge distance of 30 mm, and a voltage application time of 10 seconds, thereby generating air molecules. After ionization, air ions generated by the polarity of the needle-like electrode were repulsed, and a DC electric field was applied to the polyolefin resin foam sheet to inject electric charge, thereby electrifying the polyolefin resin foam sheet as a whole. While maintaining the surface temperature of the polyolefin resin foam sheet at 40° C. using a heat gun, the polyolefin resin foam sheet was subjected to the charging treatment. After that, the polyolefin resin foam sheet into which the electric charge was injected was wrapped in a grounded aluminum foil and held for 3 hours to obtain an electret sheet.

Regarding the obtained electret sheet, the volume resistivity at 25 ° C. and 60% relative humidity, the volume resistivity at 25 ° C. and 90% relative humidity, the dielectric breakdown voltage at 25 ° C. and 60% relative humidity, and 25 ° C. and relative humidity The dielectric breakdown voltage at 90% was measured as described above, and the initial piezoelectric constant d33 and high-temperature piezoelectric constant d33 were measured as described below.

(piezoelectric constant d33)
A flat square test piece having a side of 10 mm was cut out from the electret sheet, and gold was deposited on both sides of the test piece to prepare a test piece.

Using a vibrator, a pressing force was applied to the specimen under the conditions of a load F of 1 N, a dynamic load of ±0.5 N, and a frequency of 30 Hz, and the charge Q (coulomb) generated at that time was measured. The piezoelectric constant d33 was calculated by dividing the charge Q (coulomb) by the load F (N). The piezoelectric constant dij means the load in the j direction and the charge in the i direction, and d33 is the load in the thickness direction and the charge in the thickness direction of the electret sheet.

The piezoelectric constant d33 of the electret sheet immediately after production was measured and taken as the initial piezoelectric constant d33.

After the electret sheet was wrapped in aluminum foil and left in a constant temperature and humidity chamber at 50°C and a relative humidity of 60% for one week, the electret sheet was placed in a constant temperature and humidity chamber at 23°C for 24 hours. I left it. The piezoelectric constant d33 of this electret sheet was measured and defined as the high-temperature piezoelectric constant d33.

Claims (3)

It contains a charged synthetic resin foam sheet, and has a volume resistivity of 1.0×10 ¹⁵ Ω·cm or more at 25° C. and 60% relative humidity, and a dielectric breakdown voltage of 6 kV or more at 25° C. and 60% relative humidity. and an electret sheet characterized by having an expansion ratio of 7.5 times or more, and wherein the synthetic resin foam sheet does not contain inorganic fine particles. A volume resistivity of 1.0×10 ¹³ Ω·cm or more at 25° C. and 90% relative humidity, and a dielectric breakdown voltage of 4 kV or more at 25° C. and 90% relative humidity. Electret sheet described in . 3. The electret sheet according to claim 1, wherein the synthetic resin foam sheet has a thickness of 10 to 300 μm.