Electret sheet

Abstract

The present invention provides an electret sheet that exhibits excellent piezoelectricity even by light stress. The electret sheet of the invention is characterized by including a charged porous sheet, in which the electret sheet has a compressive elastic modulus of 80 to 300 MPa when compressively deformed at 25° C. and a 50% compression stress of 120 to 300 kPa at 25° C., and thus has the excellent piezoelectricity for light stress and exhibits the excellent piezoelectricity even by light stress (0.5 N or less) caused by a pulse wave or a breathing.

Claims

1. An electret sheet comprising a charged porous sheet, the electret sheet having a compressive elastic modulus of 80 to 300 MPa when the electret sheet is compressively deformed at 25° C. and a 50% compression stress of 130 to 300 kPa at 25° C.

2. The electret sheet according to claim 1, having a compressive elastic modulus of 60 to 250 MPa when the electret sheet is compressively deformed at 37° C. and a 50% compression stress of 110 to 250 kPa at 37° C.

3. The electret sheet according to claim 1, having a compressive elastic modulus of 40 to 180 MPa when the electret sheet is compressively deformed at 50° C. and a 50% compression stress of 90 to 200 kPa at 50° C.

4. The electret sheet according to claim 1, having a compressive elastic modulus of 120 to 250 MPa when the electret sheet is repetitively compressed 100 times under a stress of 100 kPa and then compressively deformed at 25° C.

5. The electret sheet according to claim 1, having a compressive elastic modulus of 80 to 200 MPa when the electret sheet is repetitively compressed 1,000 times under a stress of 100 kPa and then compressively deformed at 25° C.

6. The electret sheet according to claim 1, wherein the porous sheet is a foamed polypropylene-based resin sheet.

Description

EXAMPLES

(1) Next, Examples of the present invention will be described, however the present invention is not limited to the following Examples.

(2) The following polypropylene-based resins A to E and polyethylene-based resins A and B were prepared.

(3) [Polypropylene-Based Resins]

(4) Propylene-ethylene random copolymer (a polypropylene-based resin A, trade name “Novatec EG8B” manufactured by Japan Polypropylene Corp., ethylene unit content: 5% by mass)

(5) Propylene-ethylene random copolymer (a polypropylene-based resin B, trade name “WINTEC WFW4” manufactured by Japan Polypropylene Corp., ethylene unit content: 2% by mass)

(6) Propylene-ethylene random copolymer (a polypropylene-based resin C, trade name “WINTEC WFX4T” manufactured by Japan Polypropylene Corp., ethylene unit content: 4% by mass)

(7) Propylene-ethylene random copolymer (a polypropylene-based resin D, trade name “WINTEC WEG7T” manufactured by Japan Polypropylene Corp., ethylene unit content: 1% by mass)

(8) Propylene-ethylene random copolymer E (a polypropylene-based resin E, trade name “Prime Polypro B241” manufactured by Prime Polymer Co., Ltd., ethylene unit content: 2.5% by mass)

(9) [Polyethylene-Based Resins]

(10) Linear low-density polyethylene (a polyethylene-based resin A, trade name “EXACT3027” manufactured by Exxon Chemical Co., Ltd.)

(11) Low-density polyethylene (a polyethylene-based resin B, trade name “Novatec LE520H” manufactured by Japan Polypropylene Corp.)

Examples 1 to 5, Comparative Examples 1 to 3, 5, and 6

(12) The polypropylene-based resins A to E, the polyethylene-based resins A and B, trimethylolpropane trimethacrylate, azodicarbonamide, and a phenolic antioxidant were supplied to an extruder in respective predetermined amounts indicated in Table 1, molten and kneaded, and extruded into a sheet form through a T-die, to produce a foamable resin sheet having a thickness of 180 μm. The foamable resin sheet was cut into a flat square shape of which one side was 30 cm.

(13) The resulting foamable resin sheet was aged at an atmospheric temperature of 25° C. for 48 hours. Both surfaces of the resulting foamable resin sheet were irradiated with an electron beam under conditions of an accelerating voltage of 500 kV and an intensity of 25 kGy, to crosslink the polyolefin-based resin constituting the foamable resin sheet. The crosslinked foamable resin sheet was heated at 250° C. to foam the foamable resin sheet, thereby obtaining a foamed polyolefin-based resin sheet. The resulting foamed polyolefin-based resin sheet was uniaxially stretched at a stretching rate of 900 mm/min in a direction orthogonal to an extrusion direction to a thickness of 200 μm using an automatic uniaxial stretching apparatus (trade name “IMC-18C6” manufactured by Imoto Machinery Co., Ltd.) while a surface temperature of the foamed polyolefin-based resin sheet was maintained at 130° C. The foamed polyolefin-based resin sheet having a thickness of 200 μm was thus obtained. Note that a foaming ratio and thickness of the foamed polyolefin-based resin sheet were shown in Table 1.

(14) A grounded plate electrode was overlaid on a first surface of the foamed polyolefin-based resin sheet in tight contact with each other. Needle-like electrodes electrically connected to a direct-current high-voltage power supply were arranged on a second surface of the foamed polyolefin-based resin sheet at predetermined intervals. Corona discharge was generated by electric field concentration near the surfaces of the needle-like electrodes under conditions of a voltage of −10 kV, a discharge distance of 30 mm, and voltage application time of 10 seconds to ionize air molecules. A direct-current electric field was applied to the foamed polyolefin-based resin sheet by repulsion of air ions generated by the polarity of the needle-like electrodes, to inject electric charges into the foamed polyolefin-based resin sheet. The foamed polyolefin-based resin sheet was entirely charged in this manner. The foamed polyolefin-based resin sheet was subjected to the above-described charging treatment while a surface temperature of the foamed polyolefin-based resin sheet was maintained at 40° C. using a heat gun. Then, the foamed propylene-based resin sheet having the electric charges injected therein was maintained in a state of being wrapped with a grounded aluminum foil for 3 hours to obtain an electret sheet.

Comparative Example 4

(15) The polypropylene-based resin A, trimethylolpropane trimethacrylate, and a phenolic antioxidant were supplied to the extruder in respective predetermined amounts indicated in Table 1, molten and kneaded, and extruded into a sheet form through the T-die, to produce a polypropylene-based resin sheet having a thickness of 0.24 mm. The polypropylene-based resin sheet was cut into a flat square shape of which one side was 30 cm.

(16) Both surfaces of the resulting polypropylene-based resin sheet were irradiated with an electron beam under conditions of an accelerating voltage of 300 kV and an intensity of 25 kGy to crosslink the polypropylene-based resin constituting the polypropylene-based resin sheet.

(17) A grounded plate electrode was overlaid on a first surface of the polypropylene-based resin sheet in tight contact with each other. Needle-like electrodes electrically connected to the direct-current high-voltage power supply were arranged on a second surface of the polypropylene-based resin sheet at predetermined intervals. Corona discharge was generated by electric field concentration near the surfaces of the needle-like electrodes under conditions of a voltage of −20 kV, a discharge distance of 10 mm, and voltage application time of 1 minute to ionize air molecules. A direct-current electric field was applied to the polypropylene-based resin sheet by repulsion of air ions generated by the polarity of the needle-like electrodes, to inject electric charges into the polypropylene-based resin sheet. The polypropylene-based resin was entirely charged in this manner. Then, the polypropylene-based resin sheet having the electric charges injected therein was maintained in a state of being wrapped with a grounded aluminum foil for 3 hours to obtain an electret sheet.

(18) The compressive elastic modulus obtained by compressively deforming each resulting electret sheet at 25° C., 37° C., and 50° C., and the 50% compression stress of each resulting electret sheet at 25° C., 37° C., and 50° C. were measured as described above, while an initial piezoelectric constant d33 and a high-temperature piezoelectric constant d33 of each resulting electret sheet were measured as described below. Results of the measurements were shown in Table 1.

(19) The compressive elastic modulus obtained by repetitively compressing each resulting electret sheet 100 times or 1,000 times under a stress of 100 kPa and then compressively deforming the electret sheet at 25° C. was measured as described above, and results of the measurements were shown in Table 1.

(20) (Piezoelectric Constant d33)

(21) The electret sheet was cut into a test piece having a flat square shape of which one side was 10 mm. Both surfaces of the test piece were subjected to a gold vapor-deposition to prepare a test object.

(22) A pressing force was applied to the test object using a vibration exciter under conditions of a load F of 1.0 N or 10 N, a dynamic load of ±0.25 N, a frequency of 110 Hz, and an atmospheric temperature of 25° C., and an electric charge Q (coulomb) generated in these conditions was measured. The piezoelectric constant d33 was calculated by dividing the electric charge Q (coulomb) by the load F (N). Note that, in a piezoelectric constant dij, j denotes a direction of the load and i denotes a direction of the electric charge, and thus the d33 represents the piezoelectric constant for the load in a thickness direction of the electret sheet and the electric charge in a thickness direction of the electret sheet.

(23) The initial piezoelectric constant d33 was obtained by measuring the piezoelectric constant d33 of the electret sheet immediately after production.

(24) The high-temperature piezoelectric constant d33 was obtained by measuring the piezoelectric constant d33 in the same manner as the initial piezoelectric constant d33 except that the atmospheric temperature was changed to 50° C.

(25) TABLE-US-00001 TABLE 1 EXAMPLE COMPARATIVE EXAMPLE 1 2 3 4 5 1 2 3 4 5 6 COMPO- POLY- A NOVATEC 100 0 0 0 0 100 100 100 100 0 0 SITION PRO- EG8B [PARTS PYLENE- B WINTEC 0 100 0 0 0 0 0 0 0 0 0 BY BASED WFW4 MASS] RESINS C WINTEC 0 0 100 0 0 0 0 0 0 0 0 WFX4T D WINTEC 0 0 0 100 0 0 0 0 0 0 0 WEG7T E PRIME 0 0 0 0 100 0 0 0 0 0 0 POLYPRO B241 POLY- A EXACT3027 0 0 0 0 0 0 0 0 0 100 0 ETH- B NOVATEC 0 0 0 0 0 0 0 0 0 0 100 YLENE- LE520H BASED RESINS AZODICARBONAMIDE 6 6 6 6 6 1 1.5 10 0 6 6 TRIMETHYLOLPROPANE 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 0 0 TRIMETHACRYLATE PHENOLIC ANTIOXIDANT 2 2 2 2 2 2 2 2 2 2 2 ELEC- FOAMING RATIO 7.8 8.2 8.3 7.5 7.8 1.8 2.5 14.2 1 7.7 8.2 TRET (TIMES) SHEET THICKNESS (mm) 0.23 0.21 0.23 0.22 0.20 0.23 0.23 0.27 0.24 0.21 0.21 EVALU- COMPRES- 25° C. 153 163 140 175 160 850 551 101 3240 110 98 ATION SIVE 37° C. 132 138 129 139 139 775 504 88 3050 95 85 ELASTIC 50° C. 111 120 103 121 109 722 485 67 2774 75 80 MODULUS (25° C.) AFTER 148 151 144 160 172 350 262 50 581 108 101 (MPa) REPEATING 100 TIMES (25° C.) AFTER 122 123 121 141 151 332 241 45 556 72 51 REPEATING 1,000 TIMES 50% 25° C. 175 173 152 201 189 722 630 108 5210 111 106 COMPRES- 37° C. 168 165 141 199 174 636 589 99 4980 105 100 SION 50° C. 141 158 130 182 153 535 511 87 4730 101 91 STRESS (kPa) INITIAL 1.0N 287 208 211 223 188 41 73 341 33 224 161 PIEZO-  10N 95 69 77 80 54 17 21 28 22 30 31 ELECTRIC CONSTANT d33 (pC/N) HIGH- 1.0N 162 150 108 111 91 20 19 130 15 12 11 TEMPER-  10N 78 61 61 31 22 15 15 16 14 8 3 ATURE PIEZO- ELECTRIC CONSTANT d33 (pC/N)

INDUSTRIAL APPLICABILITY

(26) The electret sheet of the present invention has the excellent piezoelectricity even for light stress, and thus can be suitably used as a sensor for detecting a biological signal, such as a pulse wave and a breathing.

CROSS-REFERENCE TO RELATED APPLICATIONS

(27) The present application claims the priority under Japanese Patent Application No. 2016-19532 filed on Feb. 4, 2016 and Japanese Patent Application No. 2016-19533 filed on Feb. 4, 2016, which are hereby incorporated in their entirety by reference.