JP2016039275A - Electret material and using method thereof, and sensor and using method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electret material which achieves good sensitivity even in a low frequency region by selecting a thermoplastic resin having the elastic modulus that decreases in the low frequency region; and a using method of such an electret material; a sensor arranged by use of such an electret material; and a using method of such a sensor.SOLUTION: An electret material comprises a thermoplastic resin. In the electret material, the content of the thermoplastic resin is 50 mass% or more. The electret material includes a porous resin film. When measuring a loss modulus (E'') of the porous resin film at a frequency of 1 Hz in a temperature range of -100 to 100°C with a temperature rising rate of 1°C/min, the peak temperature of the loss modulus (E'') is 0-40°C.SELECTED DRAWING: Figure 6

Description

The present invention is mechanical-electrical energy conversion that converts mechanical energy such as vibration and pressure changes into electrical energy, thermal-electrical energy conversion that converts thermal energy such as infrared rays and temperature changes into electrical energy, and mechanical energy is converted into thermal energy. The present invention relates to an electret material that can be used for mechanical-thermal energy conversion.
In particular, the electret material of the present invention obtained by accumulating charges in the film has excellent electro-mechanical energy conversion performance in the audible frequency range.

An electret is a material that forms an electric field to the outside (applies an electric force) while maintaining electric polarization semi-permanently even in the absence of an external electric field, and has been difficult to conduct electricity in the past. A material that is semi-permanently polarized (macroscopically charged with static charge) by thermally or electrically treating a material or inorganic material.
Conventionally, electrets made of a polymer material are used in various forms such as a film, a sheet, a fiber, and a non-woven fabric depending on the use mode. In particular, electret filters formed by molding electrets have been widely used for applications such as air filters that efficiently adsorb minute dust, allergens, and the like by an electric field. In addition, the electret is widely used in various applications as a material for electro-mechanical energy conversion such as a speaker, a headphone, a microphone, an ultrasonic sensor, a pressure sensor, an acceleration sensor, and a vibration control device.

An electret using a porous resin film is known to exhibit a piezoelectric effect, and can be used for sound detection, sound generation, vibration measurement, vibration control, and the like.
For example, a foamed film is proposed in which a foamable plastic material is extruded onto a film and foamed at the same time to obtain an elastic film having a porous structure, and then the film is stretched two-dimensionally (Patent Document 1). ).
However, since a normal foamed film gradually loses its gas, it has been difficult to maintain a constant porosity. Therefore, Patent Document 1 discloses a method of fixing the shape by applying heat treatment while the foamed film is expanded to promote crystallization of the thermoplastic resin.
However, at a temperature higher than the phase transition temperature or glass transition temperature of the plastic material, the gas permeability of the thermoplastic resin is improved, so that the gas inside the foamed film tends to escape and the energy conversion efficiency is reduced. was there.

On the other hand, the present inventors proposed a film for energy conversion having a certain pore size using a thermoplastic resin and an inorganic fine powder or organic filler having a specific volume average particle size as a piezoelectric material with a small environmental load. (Cited document 2, cited document 3).
However, from the viewpoint of selecting a resin having high charging characteristics, a conventional electret material has been considered to be a thermoplastic resin having a low dielectric constant and dielectric loss. Since such a resin has no frequency dependence of dielectric constant and dielectric loss, the capacitance of the thermoplastic resin is constant with respect to the frequency. Here, since the impedance of the capacitor (capacitor) is inversely proportional to the capacitance and the frequency, the conventional electret material has a characteristic that the impedance increases as the frequency decreases.
Therefore, when the conventional electret material is used for a sensor targeting a low frequency region such as a vibration sensor or a microphone, there is a drawback that the sensitivity of the sensor is likely to be lowered due to impedance characteristics.

JP-A-61-148044 JP 2011-084735 A JP 2011-086924 A

  In the present invention, instead of selecting a thermoplastic resin having a low dielectric constant and dielectric loss as in the prior art, by selecting a thermoplastic resin whose elastic modulus decreases in the low frequency region, the sensitivity is good even in the low frequency region. An object of the present invention is to provide an electret material and a method of using the same, a sensor using the electret material, and a method of using the sensor.

As a result of diligent studies to solve these problems, the inventors of the present invention are electret materials containing 50% by mass or more of a thermoplastic resin, and the porous resin film contained in the electret materials is −100 to When the loss elastic modulus (E ″) is measured at a frequency of 1 Hz at a temperature increase rate of 1 ° C./min in a temperature range of 100 ° C., the peak temperature of the loss elastic modulus (E ″) is 0 to 40 ° C. As a result, it has been found that an electret material having good sensitivity can be provided even in a low frequency region at room temperature, and the present invention has been achieved.
That is, the present invention is as follows.

(1) An electret material containing a thermoplastic resin,
The content of the thermoplastic resin in the electret material is 50% by mass or more,
The electret material comprises a porous resin film,
When the loss elastic modulus (E ″) of the porous resin film was measured at a frequency of 1 Hz at a temperature increase rate of 1 ° C./min in the temperature range of −100 to 100 ° C., the loss elastic modulus (E ″) An electret material having a peak temperature of 0 to 40 ° C.
(2) An electret material containing a thermoplastic resin,
An electret material wherein a resonance frequency appears in a range of 10 Hz to 1 MHz when the electret material subjected to charge injection at a voltage that does not reach a dielectric breakdown voltage by direct current corona discharge is measured by the following method.
Resonance frequency measurement method: the ratio (C) and dielectric loss (tan δ) of the electret material measured in the frequency range of 1 kHz to 1 MHz in an environment of 25 ° C. and 50% relative humidity The dielectric constant (ε _r ) and the dielectric loss (tan δ) obtained by the above measurement are plotted against the logarithm of the frequency, and the frequency at which the peak of the dielectric loss (tan δ) is observed is 0.5 to 1.5 times. The frequency at which the change in relative permittivity (ε _r ) is maximum within the frequency range is defined as the resonance frequency (Sf).
ε _r = C × t / A / ε ₀ (Expression 1)
C: Capacitance of electret material [F]
ε _r : relative permittivity [−]
ε ₀ : Dielectric constant of vacuum (8.854 × 10 ⁻¹² ) [F / m]
A: Electrode area [m ² ]
t: thickness of electret material [m]
(3) The electret material according to (2) above, wherein a dielectric constant change rate (Se) obtained from the following formula 2 is 0.1 to 5.0% in the measurement of the resonance frequency.
_{Se = (ε rB -ε rA)} / ε rS × 100 ··· ( Formula 2)
Se: Dielectric constant change rate [%]
ε _rS : relative dielectric constant at resonance frequency [−]
ε _rB : relative permittivity [−] at a frequency 0.5 times the resonance frequency
ε _rA : relative dielectric constant [−] at a frequency 2.0 times the resonance frequency
(4) In the measurement of the resonance frequency, the slope when the relative dielectric constant (ε _rf ) in the range from 1 kHz to 0.5 times the resonance frequency is approximated by the following equation 3 using the least square method ( The electret material according to (2), wherein the frequency dependency represented by a) is -0.0200 to -0.0001.
[epsilon] _rf = a * log (Feq) + b (Formula 3)
ε _rf : relative permittivity [−]
Feq: Frequency [Hz]
a: inclination b: Y section (5) An electret material containing a thermoplastic resin,
Storage modulus (E ′ _(0.01) ) measured at a frequency of _0.01 Hz and storage elastic modulus measured at a frequency of 100 Hz for the electret material in an environment of 25 ° C. and a relative humidity of 50% according to JIS-K-7244-4. An electret material having a storage elastic modulus ratio (A _e ) calculated from (E ′ ₍₁₀₀₎ ) according to the following formula 4 from 1.10 to 2.00:
A _e = E ′ ₍₁₀₀₎ / E ′ _(0.01) (Expression 4)
A _e : Storage elastic modulus ratio [−]
E ′ ₍₁₀₀₎ : Storage elastic modulus [Pa] at a frequency of 100 Hz
E ′ _(0.01) : Storage elastic modulus [Pa] at a frequency of 0.01 Hz
(6) The electret material according to (1) to (5) above,
The electret material contains 50 to 98% by mass of a thermoplastic resin, 49.99 to 1.99% by mass of a pore forming nucleating agent, and 0.01 to 10% by mass of a dispersant (a thermoplastic resin and a pore forming nucleating agent). And the total content of the dispersant is 100%), electret material.
(7) The electret material according to (6), wherein the electret material has a porosity of 10 to 70%, calculated by the following formula 5.

ρ_０：ＪＩＳ−Ｋ−７１１２：１９９９で測定した樹脂フィルムの真密度
ρ ：ＪＩＳ−Ｋ−７２２２：２００５で測定した樹脂フィルムの密度
（８）上記（１）に記載のエレクトレット材料を該エレクトレット材料の前記損失弾性率（Ｅ’’）ピーク温度に対して−２０〜２０℃の範囲で動作させることを特徴とするエレクトレット材料の使用方法。
（９）上記（１）〜（７）の何れかに記載のエレクトレット材料を使用したセンサー。
（１０）上記（９）に記載のセンサーを、該センサーを構成するエレクトレット材料の前記損失弾性率（Ｅ’’）ピーク温度に対して−２０〜２０℃の範囲で動作させることを特徴とするセンサーの使用方法。

ρ ₀ : True density of resin film measured according to JIS-K-7112: 1999 ρ: Density of resin film measured according to JIS-K-7222: 2005 (8) The electret material described in (1) above is the electret material. The method of using an electret material, wherein the loss elastic modulus (E ″) is operated in the range of −20 to 20 ° C. with respect to the peak temperature.
(9) A sensor using the electret material according to any one of (1) to (7).
(10) The sensor according to (9) is operated in a range of −20 to 20 ° C. with respect to the loss elastic modulus (E ″) peak temperature of the electret material constituting the sensor. How to use the sensor.

  Since the electret material of the present invention has good sensitivity in a low frequency region, particularly in an audible frequency region, at a temperature in the vicinity of the loss elastic modulus (E ″) peak temperature of the electret material, an acoustic sensor, a vibration sensor, and an impact sensor It is useful for manufacturing measuring instruments, control devices, abnormality diagnosis systems, crime prevention devices, stabilizers, robots, percussion instruments, game machines and the like using them.

It is a partially expanded sectional view of the one aspect | mode of the electret material of this invention. It is a schematic diagram of an example of a batch type electretization apparatus. It is a schematic diagram of an example of a continuous electretization apparatus. It is a schematic diagram of an example of a batch type electretization apparatus. It is a schematic diagram of an example of a continuous electretization apparatus. It is a temperature dependence measurement result of the storage elastic modulus (E ') and loss elastic modulus (E' ') in the measurement of the phase transition temperature of the porous resin film produced in Example 1 of this specification. It is a measurement result of the frequency dependence of the dielectric constant (ε _r ) and the dielectric loss tangent (tan δ) in the measurement of the storage elastic modulus ratio of the porous resin film prepared in Example 1 of the present specification. It is a measurement result of the frequency dependence of the storage elastic modulus (E ') of the porous resin film produced in Example 1 of this invention.

Below, the electret material of the present invention and its method of use, and the sensor and its method of use will be described in detail.
In the present specification, a numerical range represented by using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
Further, in this specification, the “low frequency region” includes an “audible frequency region”.

[Loss modulus (E '') peak temperature]
In the electret material of the present invention, the content of the thermoplastic resin in the electret material is 50% by mass or more, the electret material includes a porous resin film, and the porous resin film is -100 to 100 ° C. When the loss elastic modulus (E ″) is measured at a frequency of 1 Hz at a rate of temperature increase of 1 ° C./min in the temperature range, the loss elastic modulus (E ″) peak temperature is 0 to 40 ° C.
When determining the phase transition temperature, the tensile storage elastic modulus (E ′) and loss elastic modulus (E ″) are measured according to JIS-K-7244-1: 1998 “Plastics? Part 1: General Rules ”and JIS-K-7244-4: 1999“ Plastics—Test Method for Dynamic Mechanical Properties—Part 4: Tensile Vibration—Non-Resonance Method ”. At this time, in the temperature range of −110 to 130 ° C., a 1 Hz sinusoidal vibration was applied to the sample while increasing the temperature at a constant rate of 1 ° C./min, and the tensile storage elastic modulus (E ′) [GPa] was measured with a dynamic viscoelasticity measuring device. ] And a loss elastic modulus (E '') [MPa].
Further, the loss elastic modulus (E ″) is plotted against the temperature, and the peak temperature of the loss elastic modulus (E ″) is read from the plot.

The range of the peak temperature of the loss modulus (E ″) of the porous resin film comprising the electret material is preferably 0 to 40 ° C., preferably 5 to 35 ° C., more preferably 10 to 30 ° C., and 15 to 25. More preferred is ° C. If the peak temperature of the loss elastic modulus (E ″) is within the above range due to the characteristics of the electret material, the intended performance of the present invention can be obtained at room temperature.
Since the electret material of the present invention has a peak of loss elastic modulus (E ″) at a temperature near the use environment temperature, frequency dependence occurs in the tensile storage elastic modulus (E ′) at the temperature.
The peak of the loss modulus (E ″) observed varies depending on the characteristics of the thermoplastic resin mainly used for the electret material. Here, the “mainly used thermoplastic resin” refers to the type of thermoplastic resin that is contained most in mass ratio among the thermoplastic resins contained in the electret material.

[Resonance frequency]
The electret material of the present invention contains a thermoplastic resin, and when the electret material subjected to charge injection at a voltage that does not reach the dielectric breakdown voltage by DC corona discharge is measured by the method described later, the resonance phenomenon due to the piezoelectric characteristics is 1 kHz to Appears in the 1 MHz range. Normally, the resonance phenomenon of the piezoelectric material is observed in the width direction, the thickness direction, and the thickness direction, and the frequency at which the resonance phenomenon appears varies depending on the size and thickness of the sample to be measured. In the electret material, the resonance phenomenon in the thickness direction appears in the range of 1 kHz to 1 MHz. Therefore, the resonance frequency does not change because the thickness of the electret material does not change even if the sample size changes.
In addition, the resonance frequency of the normal piezoelectric material having the thickness range (10 to 500 μm) of the present invention exceeds 1 MHz, but the resonance frequency of the electret material of the present invention is much lower than that, which is the electret material of the present invention. However, it shows that the flexibility in the thickness direction is higher than that of a normal piezoelectric material.

In the present invention, the resonance frequency is measured by measuring the capacitance (C) and dielectric loss (tan δ) of the electret material subjected to charge injection in the frequency range of 1 kHz to 1 MHz, and calculating the relative dielectric constant according to the following formula 1. The ratio (ε _r ) and the dielectric loss (tan δ) obtained by the above measurement are plotted against the logarithm of the frequency to obtain the frequency at which the change in the relative permittivity (ε _r ) is maximum.
Here, the charge injection is performed on the electret material, as will be described later, by DC corona discharge at a voltage that does not reach the dielectric breakdown voltage.
Further, in order to measure the electrostatic capacity (C) of the electret material, it is necessary to provide electrodes on both surfaces of the electret material, and the surface resistivity should be 1 Ω / □ or less by aluminum deposition by sputtering. An electrode is provided.
In addition, an impedance analyzer is used for measurement in the frequency range of 1 kHz to 1 MHz.
The measurement is performed in an environment of 25 ° C. and a relative humidity of 50%. From the relation of plotting the relative dielectric constant (ε _r ) and the dielectric loss (tan δ) obtained in the above measurement against the logarithm of the frequency, the frequency of 1 kHz to 1 MHz is obtained. The measurement is performed by scanning the range logarithmically at equal intervals.
From the capacitance obtained by the measurement, the relative dielectric constant (ε _r ) is calculated by the following equation (1).
ε _r = C × t / A / ε ₀ (Expression 1)
C: Capacitance of electret material [F]
ε _r : relative permittivity [−]
ε ₀ : Dielectric constant of vacuum (8.854 × 10 ⁻¹² ) [F / m]
A: Electrode area [m ² ]
t: thickness of electret material [m]
Then, from the plotted graph, the frequency at which the change in relative permittivity (ε _r ) near the frequency at which the peak is observed in the dielectric loss (tan δ) is obtained as the resonance frequency. More specifically, the frequency at which the change in relative permittivity (ε) is maximum within the frequency range of 0.5 to 2.0 times the frequency at which the peak of dielectric loss (tan δ) is observed is the resonance frequency. And

  The lower limit of the resonance frequency of the electret material of the present invention is preferably 10 kHz or more, preferably 50 kHz or more, and more preferably 100 kHz or more. On the other hand, the upper limit of the resonance frequency of the electret material of the present invention is preferably 1000 kHz (1 MHz) or less, preferably 700 kHz or less, and more preferably 500 kHz or less. There is a close relationship between the flexibility of the electret material of the present invention and the resonance frequency. When the flexibility of the electret material is high, the resonance frequency tends to decrease. If the resonance frequency is 10 kHz or more, rigidity that does not change the thickness of the electret material due to load or air temperature is obtained, and if the resonance frequency is 1 MHz or less, the electret material has a performance as a piezoelectric body particularly in a low frequency region. It has the flexibility that can be obtained.

In the measurement of the resonance frequency, the dielectric constant change rate (Se) is obtained from the following formula 2.
_{Se = (ε rB -ε rA)} / ε rS × 100 ··· ( Formula 2)
Se: Dielectric constant change rate [%]
ε _rS : relative dielectric constant at resonance frequency [−]
ε _rB : relative permittivity [−] at a frequency 0.5 times the resonance frequency
ε _rA : relative dielectric constant [−] at a frequency 2.0 times the resonance frequency
According to the above equation 1, since the dielectric constant of the vacuum, the electrode area, and the thickness of the electret material do not change in the measurement of the resonance frequency, the relative dielectric constant is proportional to the capacitance of the electret material. It can be rewritten and calculated as follows.
Se = (C _B -C _A ) / C _S × 100 (Equation 2 ′)
Se: Dielectric constant change rate [%]
C _S : capacitance at resonance frequency [F]
C _B : Capacitance [F] at a frequency 0.5 times the resonance frequency
C _A : capacitance [F] at a frequency 2.0 times the resonance frequency
The change in dielectric constant observed before and after the resonance frequency occurs when a sample that vibrates in an unconstrained state at a frequency lower than the resonance frequency becomes restricted and does not vibrate at a frequency higher than the resonance frequency. . Therefore, the dielectric constant change rate (Se) represents the magnitude of vibration in an unconstrained state. The larger the dielectric constant change rate (Se), the higher the performance of the electret material.
Se of the electret material of the present invention is preferably 0.1% or more, more preferably 0.2% or more, and still more preferably 0.3% or more. Thereby, when the obtained electret material is used as a piezoelectric body, there exists a tendency which exhibits sufficient performance. On the other hand, Se of the electret material of the present invention is preferably 5.0% or less, more preferably 3.0% or less, and further preferably 1.0% or less. It is technically difficult to obtain an electret material in which Se exceeds 5.0% using a thermoplastic resin.

In the measurement of the resonant frequency in a range from 1kHz to 0.5 times the frequency of the resonance frequency, the frequency of the frequency is plotted the relative dielectric constant (epsilon _rf) versus log of (Feq), the relative dielectric constant (epsilon _rf) Dependencies can show linearity.
That is, when the linear portion of the plot is approximated by the following equation 3 using the least square method, the frequency dependence of the relative dielectric constant (ε _rf ) is expressed by the slope (a).
[epsilon] _rf = a * log (Feq) + b (Formula 3)
ε _rf : relative permittivity [−]
Feq: Frequency [Hz]
a: slope b: Y intercept Normally, a thermoplastic resin used as an electret material exhibits a substantially constant relative dielectric constant (ε _rf ) in the week region lower than the resonance frequency, and its frequency dependency (a) is zero. .
Here, the value of the dielectric constant change rate (Se _(Feq) ) at the frequency Feq obtained by the following formula 2 ″ applied to the frequency Feq [Hz] by modifying the above formula 2 is resonant from the frequency Feq of 1 kHz. It is constant regardless of the frequency (Feq) in the region up to 0.5 times the frequency, and is equivalent to Se in the above equation 2.
_{Se (Feq) = (ε r} (Feq) -ε rA) / ε rS × 100 ··· ( Equation 2 '')
Se _(Feq) : Dielectric constant change rate [%] at frequency Feq
ε _rS : relative dielectric constant at resonance frequency [−]
_{εr (Feq)} : relative permittivity [−] at frequency Feq
ε _rA : dielectric constant [−] at a frequency 1.5 times the resonance frequency
On the other hand, the electret material of the present invention is characterized by a higher relative dielectric constant as the frequency is lower, and the frequency dependency (a) takes a negative value.
Here, when the characteristics of the electret material of the present invention are applied to the above formula 2 ″, the lower the Feq (frequency) [Hz], the larger the dielectric constant change rate (Se _(Feq) ) and the piezoelectric performance increases. It has the characteristics to go. This phenomenon is a phenomenon observed at the temperature near the peak of the loss elastic modulus (E ″) of the electret material, that is, the elastic region of the electret material, and shows the excellent characteristics of the electret material of the present invention.

  The frequency dependency (a) of the relative dielectric constant in the electret material of the present invention is preferably −0.0001 or less, more preferably −0.0002 or less, and further preferably −0.0003 or less. When the frequency dependency (a) of the relative dielectric constant takes a value equal to or lower than the upper limit value, practically good piezoelectric performance can be obtained. On the other hand, the frequency dependency (a) of the relative dielectric constant is preferably −0.0200 or more, more preferably −0.0150 or more, and further preferably −0.0100 or more. When the frequency dependency (a) of the relative permittivity takes a value equal to or higher than the lower limit value, the piezoelectric performance in the low frequency region tends to be good. When the frequency dependence (a) of the relative permittivity of the electret material is set to −0.0050 or less by lowering the viscoelasticity of the thermoplastic resin to be used or using a resin having a high dielectric constant, the electret material As a result, the charge storage performance may be reduced, and the piezoelectric performance itself may be reduced.

[Storage modulus ratio]
The electret material of the present invention contains a thermoplastic resin, and the storage modulus (E ′ ₍ E ′ ₍₎₎ measured at a frequency of 0.01 Hz according to JIS-K-7244-4 in an environment of 25 ° C. and 50% relative humidity. _0.01) ) and the storage elastic modulus ratio (E e ₍₁₀₀₎ ) measured at a frequency of 100 Hz, the storage elastic modulus ratio (A _e ) determined by the following formula 4 is in the range of 1.10 to 2.00.
A _e = E ′ ₍₁₀₀₎ / E ′ _(0.01) (Expression 4)
A _e : Storage elastic modulus ratio [−]
E ′ ₍₁₀₀₎ : Storage elastic modulus [Pa] at a frequency of 100 Hz
E ′ _(0.01) : Storage elastic modulus [Pa] at a frequency of 0.01 Hz
In determining the phase transition temperature, the tensile storage modulus (E ′) is measured according to JIS-K-7244-1: 1998 “Plastics—Testing method for dynamic mechanical properties—Part 1: General rules” and JIS-K. -7244-4: 1999 "Plastics-Test method for dynamic mechanical properties-Part 4: Tensile vibration-Non-resonant method" At this time, a sinusoidal vibration of 0.01 Hz was added to the sample in an environment of 25 ° C. and a relative humidity of 50%, and the tensile storage elastic modulus (E ′ _(0.01) ) at 0.01 Hz with a dynamic viscoelasticity measuring device. GPa] is measured. Similarly, a sinusoidal vibration of 100 Hz is applied to the sample, and the tensile storage elastic modulus (E ′ ₍₁₀₀₎ ) [GPa] at 100 Hz is measured with a dynamic viscoelasticity measuring device.
Further, the storage elastic modulus ratio is calculated (A _e ) based on the above formula 4.
An electret material having a high storage elastic modulus ratio has a high frequency dependency of the storage elastic modulus and tends to have a high frequency dependency of the dielectric constant, and is preferable as the electret material of the present invention.

The storage elastic modulus ratio in the electret material of the present invention is preferably 1.10 or more, preferably 1.15 or more, and more preferably 1.20 or more. An electret material having a storage elastic modulus ratio equal to or greater than the lower limit value tends to have a high frequency dependence of dielectric constant, and can obtain the desired piezoelectric performance. On the other hand, the storage elastic modulus ratio in the electret material of the present invention is preferably 2.00 or less, preferably 1.70 or less, and more preferably 1.50 or less. It is difficult to manufacture an electret material having a storage elastic modulus ratio exceeding the upper limit with the current technology.
In addition, when electret material has anisotropy within a plane, it measures by changing the direction of a measurement sample, and employ | adopts the value with the highest storage elastic modulus ratio.

[Porous resin film]
(Vacancy)
The electret material comprises a porous resin film in which pores are formed by stretching a thermoplastic resin film containing a thermoplastic resin, a pore-forming nucleating agent, and a dispersant. In the porous resin film, pores having a shape suitable for accumulating electric charges and a shape that provides high compression recovery to the porous resin film are formed.
It is considered that different charges are held in one set on the inner surfaces of the individual pores inside the porous resin film. Therefore, in order to accumulate electric charges inside the holes, the area and height of a certain amount or more are required as in the case of the single plate capacitor. If there is no area larger than a certain area, sufficient electrostatic capacity cannot be obtained, and an electret with excellent performance cannot be obtained. If there is no height (distance) above a certain level, a discharge (short circuit) occurs inside the hole, and charge cannot be accumulated. However, if the height (distance) is too large, it is disadvantageous for charge polarization, and an electret having excellent stability cannot be obtained. For this reason, it was considered that the larger the size (area) of the individual pores inside the core layer (A), the more effectively it functions. However, if the size of the vacancies is excessively increased, adjacent vacancies communicate with each other, and a discharge (short circuit) occurs between the adjacent vacancies, making it difficult to accumulate charges.
Therefore, the porous resin film has a height of 3 to 30 μm in the thickness direction of the film and in the surface direction of the film from the viewpoint of accumulating a large amount of charge on the observed image when observed in an arbitrary cross section. The number of holes having a diameter of 50 to 500 μm is preferably 100 / mm ² or more, more preferably 150 / mm ² or more, still more preferably 200 / mm ² or more, and 300 / mm ^2. It is particularly preferable to have the above. On the other hand, from the viewpoint of suppressing a short circuit between adjacent holes, 3,000 holes / having a height of 3 to 30 μm in the thickness direction of the film and a diameter of 50 to 500 μm in the surface direction of the film / It is preferable to have mm ² or less, more preferably 2500 pieces / mm ² or less, still more preferably 2,000 pieces / mm ² or less, and particularly preferably 1,500 pieces / mm ² or less.

  In addition, as the number of effective vacancies in the porous resin film increases, it is considered that the charge storage capacity is improved and the energy conversion efficiency is improved. The possibility that adjacent air holes communicate with each other increases, the strength of the film itself decreases, and this is not preferable in that the structure is difficult to recover against an external force such as compression. Insufficient compression recovery results in adverse effects such as a decrease in the recovery rate during repeated compression and decompression, and causes inconvenience as a mechanical-electrical energy conversion material that converts mechanical energy into electrical energy. There is a case.

Formation of such pores in the present invention is achieved by adding an inorganic fine powder and an organic filler to a thermoplastic resin, which is a polymer material having excellent insulating properties, and performing stretch molding described later.
In particular, by stretching and molding at a temperature lower than the glass transition point or melting point of the thermoplastic resin, pores starting from the organic filler are formed.
The electret material having such a porous resin film preferably has a porosity calculated by the following formula 5 of 10 to 70%.

ρ_０：ＪＩＳ−Ｋ−７１１２で測定した樹脂フィルムの真密度
ρ ：ＪＩＳ−Ｋ−７２２２で測定した樹脂フィルムの密度
電荷を蓄積するのに適したサイズの空孔を数多く得て、電荷の蓄積容量を確保する観点から、エレクトレット材料の空孔率は１０％以上であり、１５％以上であることがより好ましく、２５％以上であることが更に好ましく、３０％以上であることが特に好ましい。一方、空孔が連通して電荷が短絡することを抑制したり、多孔質樹脂フィルムの弾性率が
極端に劣り、厚み方向の復元性が低下し、耐久性に劣ることを抑制したりする観点から、エレクトレット材料の空孔率は７０％以下であり、６５％以下であることがより好ましく、６０％以下であることが更に好ましく、５５％以下であることが特に好ましい。

ρ ₀ : True density of the resin film measured according to JIS-K-7112 ρ: Density of the resin film measured according to JIS-K-7222 From the viewpoint of securing the capacity, the porosity of the electret material is 10% or more, more preferably 15% or more, further preferably 25% or more, and particularly preferably 30% or more. On the other hand, it is possible to suppress the short circuit of the charge due to the communication of the pores, the extremely low elastic modulus of the porous resin film, the decrease in the restoring property in the thickness direction, and the suppression of the poor durability. Therefore, the porosity of the electret material is 70% or less, more preferably 65% or less, still more preferably 60% or less, and particularly preferably 55% or less.

(Materials used)
The electret material of the present invention contains 50 to 98% by mass of a thermoplastic resin, 49.99 to 1.99% by mass of a pore-forming nucleating agent, and 0.01 to 10% by mass of a dispersing agent (thermoplastic resin and pores). The total content of the nucleating agent and the dispersing agent is preferably 100%).
The thermoplastic resin is required to be contained in an amount of 50% by mass or more, and more preferably 70% by mass or more, from the viewpoint of sufficiently obtaining the effect of the phase transition temperature aimed at by the present invention. On the other hand, from the viewpoint of forming sufficient pores in the obtained electret material, it is necessary to contain 98% by mass or less, and more preferably 96% by mass or less.
When inorganic fine powder is used as the pore-forming nucleating agent, it is more preferably 12.99% by mass or more, and 14.99% by mass or more from the viewpoint of forming sufficient pores in the obtained electret material. Is more preferable. On the other hand, from the viewpoint of providing independence of the holes formed in the electret material, it is necessary to contain 49.99% by mass or less, and more preferably 29.99% by mass or less.
When using an organic filler as a pore-forming nucleating agent, it is necessary to contain 1.99% by mass or more and 3.99% by mass or more from the viewpoint of forming sufficient pores in the obtained electret material. More preferably. On the other hand, from the viewpoint of giving the independence of the holes formed in the electret material, it is more preferably 29.99% by mass or less, and further preferably 19.99% by mass or less.
The dispersant must be contained in an amount of 0.01% by mass or more, and 0.03% by mass or more from the viewpoint of suppressing a short circuit of electric charge due to generation of large pores or communication pores due to poor dispersion of the pore-forming nucleating agent. More preferably, it is more preferably 0.05% by mass or more. On the other hand, from the viewpoint of moldability and charge retention, it is necessary to contain 10% by mass or less, more preferably 5.00% by mass or less, and still more preferably 2.00% by mass or less.

Thermoplastic resin:
The thermoplastic resin suitable for use in the porous resin film is preferably an insulating polymer material that is difficult to conduct electricity. For example, polyolefin resins such as high density polyethylene, medium density polyethylene, ethylene resin including low density polyethylene, propylene resin, polymethyl-1-pentene, cyclic polyolefin; ethylene / vinyl acetate copolymer, ethylene / acrylic acid copolymer Polymers, functional group-containing polyolefin resins such as maleic acid-modified polyethylene and maleic acid-modified polypropylene; polyamide resins such as nylon-6 and nylon-6,6; polyethylene terephthalate and copolymers thereof, polybutylene terephthalate, polybutylene Polyester resins such as succinate, polylactic acid, and aliphatic polyester; polycarbonate, atactic polystyrene, syndiotactic polystyrene, and the like can be used. Among these thermoplastic resins, it is preferable to use a polyolefin resin or a functional group-containing polyolefin resin having a low hygroscopic property and a high insulating property.

  Examples of polyolefin resins include homopolymers of olefins such as ethylene, propylene, butene, butylene, butadiene, isoprene, chloroprene, methylpentene, cyclobutenes, cyclopentenes, cyclohexenes, norbornenes, tricyclo-3-decenes, And copolymers comprising two or more of these olefins. Specific examples of the polyolefin resin include high density polyethylene, medium density polyethylene, propylene resin, a copolymer of ethylene and another olefin, and a copolymer of propylene and another olefin.

Among these polyolefin-based resins, isotactic or syndiotactic and propylene homopolymers having various degrees of stereoregularity, or propylene as a main component, ethylene, 1-butene, 1-hexene, -Propylene-based resin containing propylene-based copolymer copolymerized with α-olefin such as heptene, 4-methyl-1-pentene is non-hygroscopic and insulating, processability, Young's modulus, durability From the viewpoints of performance and cost.
The propylene-based copolymer may be a binary system or a ternary system or may be a random copolymer or a block copolymer.

Specific examples of the functional group-containing polyolefin resin include a copolymer of a functional group-containing monomer copolymerizable with the olefins.
Such functional group-containing monomers include styrenes such as styrene and α-methylstyrene, vinyl acetate, vinyl alcohol, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl caproate, vinyl laurate, vinyl stearate, benzoic acid. Carboxylic acid vinyl esters such as vinyl, vinyl butylbenzoate and vinyl cyclohexanecarboxylate, (meth) acrylic acid, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, hexyl (meth) acrylate, octyl (Meth) acrylate, 2-ethylhexyl (meth) acrylate, stearyl (meth) acrylate, benzyl (meth) acrylate, cyclohexyl (meth) acrylate, isobornyl (meth) acrylate, dicyclopentani Acrylic esters such as (meth) acrylate, acrylic amides such as (meth) acrylamide, N-metalol (meth) acrylamide, methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, cyclopentyl vinyl ether, cyclohexyl vinyl ether, benzyl vinyl ether Vinyl ethers such as phenyl vinyl ether are particularly representative. From these functional group-containing monomers, one or two or more types appropriately selected and polymerized can be used as necessary.

Furthermore, it is also possible to use those polyolefin-based resins and functional group-containing polyolefin-based resins which are optionally graft-modified.
A known technique can be used for graft modification, and specific examples include graft modification with an unsaturated carboxylic acid or a derivative thereof. Examples of the unsaturated carboxylic acid include (meth) acrylic acid, maleic acid, fumaric acid, itaconic acid and the like. As the unsaturated carboxylic acid derivative, acid anhydrides, esters, amides, imides, metal salts and the like can also be used. Specifically, maleic anhydride, itaconic anhydride, citraconic anhydride, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, glycidyl (meth) acrylate, monoethyl maleate, Maleic acid diethyl ester, fumaric acid monomethyl ester, fumaric acid dimethyl ester, itaconic acid monomethyl ester, itaconic acid diethyl ester, (meth) acrylamide, maleic acid monoamide, maleic acid diamide, maleic acid-N-monoethylamide, maleic acid- N, N-diethylamide, maleic acid-N-monobutylamide, maleic acid-N, N-dibutylamide, fumaric acid monoamide, fumaric acid diamide, fumaric acid-N-monoethylamide, fumaric acid-N, N-diethylamide , Fumaric acid-N-mono Chiruamido, fumaric acid -N, N- dibutylamide, maleimide, N- butyl maleimide, N- phenylmaleimide, (meth) sodium acrylate, and (meth) potassium acrylate.
Examples of the graft-modified product include those obtained by graft-modifying the graft monomer by adding generally 0.005 to 10% by mass, preferably 0.01 to 5% by mass with respect to the polyolefin-based resin and the functional group-containing polyolefin-based resin. It is done.

As a thermoplastic resin suitable for use in the porous resin film, one of the above thermoplastic resins may be selected and used alone, or two or more may be selected and used in combination. May be.
Among these, it is preferable to use a thermoplastic resin having a glass transition point near room temperature, and it is more preferable to use a thermoplastic resin having a glass transition point of -20 to 20 ° C. Thereby, the peak temperature of the loss elastic modulus (E '') of the porous resin film contained in the electret material is at room temperature, and the electret material is obtained from the loss elastic modulus of the porous resin film ( E ″) is preferable from the viewpoint of operating in the range of −20 to 20 ° C. with respect to the peak temperature. Specific examples of the thermoplastic resin having a glass transition point near room temperature include polypropylene (Tg: −10 ° C.). By using such a thermoplastic resin alone or in combination of two or more, an electret material having a peak elastic modulus (E ″) peak temperature of 0 to 40 ° C. can be obtained. In addition, it was obtained not only when the peak temperature of the loss elastic modulus (E ″) of the porous resin film is in the range of 0 to 40 ° C. but also when it is outside the range of 0 to 40 ° C. The effect of the present invention can also be obtained by operating the electret material in the range of −20 to 20 ° C. with respect to the loss elastic modulus (E ″) peak temperature of the electret material.

Pore-forming nucleating agent:
The pore-forming nucleating agent used for the porous resin film is added to form pores in the film using this as a nucleus. Examples of the pore-forming nucleating agent suitable for use in the porous resin film include inorganic fine powders and organic fillers. By adding these pore-forming nucleating agents and the stretching step described later, it becomes possible to form pores in the film. It is possible to control the frequency of pores by controlling the content of pore-forming nucleating agent, and control the height and diameter of the pores by controlling the particle size of the pore-forming nucleating agent. Is possible.
Although the content rate of a void | hole formation nucleating agent is as above-mentioned, if the content rate of a void | hole formation nucleating agent is more than the said lower limit, it will be 3-30 micrometers height in the thickness direction of a film at the extending process mentioned later. And a sufficient number of holes having a diameter of 50 to 500 μm in the surface direction of the film are generated, and the required piezoelectric performance can be obtained. On the other hand, if the content rate of the pore-forming nucleating agent is not more than the above upper limit value, even if a compressive force is repeatedly applied to the obtained electret, the compression recoverability can be suppressed in order to suppress a decrease in film strength due to too many pores Can be expected to stabilize the piezoelectric performance.

Among the pore-forming nucleating agents, the inorganic fine powder is characterized by low cost and abundant volume average particle size.
The volume average particle diameter of the inorganic fine powder (median diameter (D50) measured with a particle size distribution meter by laser diffraction) has a height of 3 to 30 μm in the thickness direction of the film and 50 to 500 μm in the surface direction of the film. A hole having a diameter is selected in consideration of forming a necessary amount. The volume average particle size is preferably 3 μm or more, and more preferably 4 μm or more, from the viewpoint that the size of the formed pores becomes sufficiently large and the required piezoelectric performance can be obtained. On the other hand, it is possible to prevent adjacent holes from communicating with each other and short-circuiting the charge, making it difficult to accumulate charges, and suppressing reduction in film strength due to too large holes, and repeatedly compressing the obtained electret From the standpoint that compression recovery works even when a force is applied and the piezoelectric performance is stabilized, the volume average particle size is preferably 30 μm or less, more preferably 20 μm or less, and 15 μm or less. Further preferred.
Specific examples of the inorganic fine powder include calcium carbonate, calcined clay, silica, diatomaceous earth, white clay, talc, titanium oxide, barium sulfate, alumina, zeolite, mica, sericite, bentonite, sepiolite, vermiculite, dolomite, wallast. A knight, glass fiber, etc. are mentioned, Among these, 1 or more types can be used individually or in combination.

Among the pore-forming nucleating agents, organic fillers are available as spherical particles having a uniform volume average particle size (median diameter), which can sharpen the pore size distribution of the porous resin film, and in addition, provide pores. Even after formation, the organic filler becomes a pillar in the pores, and even if a compressive force is repeatedly applied to the obtained electret, the compression recovery property works and the piezoelectric performance can be expected to stabilize (pillar effect). Have.
The volume average particle diameter of the organic filler (median diameter (D50) measured with a particle size distribution meter by laser diffraction) has a height of 3 to 30 μm in the thickness direction of the film and a diameter of 50 to 500 μm in the surface direction of the film. Is selected in consideration of forming a required amount of pores having a diameter. The volume average particle size is preferably 3 μm or more, and more preferably 4 μm or more, from the viewpoint that the size of the formed pores becomes sufficiently large and the required piezoelectric performance can be obtained. On the other hand, it is possible to prevent adjacent holes from communicating with each other and short-circuiting the charge, making it difficult to accumulate charges, and suppressing reduction in film strength due to too large holes, and repeatedly compressing the obtained electret From the standpoint that compression recovery works even when a force is applied and the piezoelectric performance is stabilized, the volume average particle size is preferably 30 μm or less, more preferably 20 μm or less, and 15 μm or less. Further preferred.

As the organic filler, it is preferable to select a different type of resin from the thermoplastic resin that is the main component of the porous resin film. For example, when the thermoplastic resin is a polyolefin resin, the organic filler is a cross-linked acrylic resin, a cross-linked methacryl resin, a cross-linked styrene resin, a cross-linked urethane resin, or the like. It is preferable to use a material that does not have fluidity during kneading and stretching. In addition, resin particles made of these cross-linked resins are more preferable because they are available as spherical particles having a predetermined particle diameter, and the pore size can be easily adjusted.
In addition, it is preferable to select an organic filler for the purpose of obtaining a desired pore diameter as a result of the organic filler not being deformed in the stretching step. For example, when the thermoplastic resin is a polyolefin resin, it is a polymer such as polyethylene terephthalate, polybutylene terephthalate, polycarbonate, nylon-6, nylon-6,6, cyclic olefin polymer, polystyrene, polymethacrylate, etc. By finely dispersing a material having a melting point (for example, 170 to 300 ° C.) or a glass transition temperature (for example, 170 to 280 ° C.) higher than the melting point of the polyolefin resin into the polyolefin resin as a matrix resin by melt kneading. Can be used.
One or more of these organic fillers can be used alone or in combination.

  When using inorganic fine powder and organic filler in combination as a pore-forming nucleating agent, use one or more of the inorganic fine powders listed above and one or more of the organic fillers listed above. can do. Also in this case, the volume average particle size is preferably 3 to 30 μm for the same purpose as described above. When using an organic filler and an inorganic powder in combination, the volume average particle size may be combined with the same range of inorganic powders individually. You may use that whose volume average particle diameter measured with the distribution meter is 3-30 micrometers. The volume average particle size of the mixed state of the organic filler and the inorganic powder measured with a particle size distribution meter by laser diffraction is preferably 3 to 30 μm, more preferably 4 to 20 μm, and more preferably 4 to 15 μm. Is more preferable.

Dispersant:
The dispersant used for the porous resin film is used for the purpose of improving the uniformity of the performance of the electret material as a result of uniformly dispersing the pore-forming nucleating agent in the resin.
When a dispersant is not used, agglomerates of pore-forming nucleating agents remain in the film, and large pores exceeding 30 μm are likely to be generated by the stretching process. Charge short-circuiting due to the formation of continuous voids and compression of the porous resin film Although there is a tendency that the piezoelectricity is lowered due to a decrease in the recoverability, the film has a height of 3 to 30 μm in the thickness direction of the film by uniformly dispersing the pore-forming nucleating agent using a dispersant, and the film 100-30 holes having a diameter of 50-500 μm in the surface direction of
It is easy to obtain a porous resin film having 00 pieces / mm ² .

  Specific examples of the dispersant include caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, palmitooleic acid, oleic acid, elaidic acid, paxenoic acid, erucic acid Fatty acids such as linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic acid, docosahexaenoic acid; glycerin monostearate, glycerin monobehenate, glycerin mono12-hydroxystearate, glycerin monooleate, glycerin monocaprylate Glycerol fatty acid such as glycerol monolaurate, glycerol distearate, glycerol dibehenate, glycerol dioleate, diglycerol caprylate; ester diglycerol laurate, diglycerol mi Polyglycerin fatty acid esters such as state, diglyceryl stearate, diglycerin oleate, tetraglyceryl stearate, decaglycerin laurate, decaglycerin stearate, decaglycerin oleate, polyglycerin polyricinoleate; sorbitan laurate, sorbitan palmitate Sorbitan fatty acid esters such as sorbitan stearate, sorbitan tristearate, sorbitan oleate, sorbitan trioleate, sorbitan tribehenate, sorbitan caprylate; ethylene oxide such as polyoxyethylene sorbitan trioleate, polyoxyethylene glycerin monostearate Adducts: vinyltrimethoxysilane, vinyltriethoxysilane, 2- (3,4 epoxycyclohexyl) ethyl Rutrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropyl Silane coupling agents such as methyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane; polyacrylic acid Or a salt thereof, polymethacrylic acid or a salt thereof, and the like. These dispersants can be used alone or in combination of two or more. Among these dispersants, it is preferable to use a fatty acid that has a small effect on the electrical properties of the added resin.

Other materials:
A thermal stabilizer (antioxidant), a stabilizer and the like can be optionally added to the thermoplastic resin composition used for the porous resin film, if necessary.
When adding a heat stabilizer, it is preferable to add within 0.001 to 1 weight% with respect to a thermoplastic resin composition. As specific examples of the heat stabilizer, sterically hindered phenol-based, phosphorus-based, amine-based stabilizers can be used.
When a light stabilizer is added, it is usually added within a range of 0.001 to 1% by weight with respect to the thermoplastic resin composition. Specific examples of the light stabilizer include sterically hindered amine-based, benzotriazole-based, and benzophenone-based light stabilizers.

(Surface layer)
The porous resin film preferably has a surface layer made of a stretched resin film on at least one surface of the porous resin film for the purpose of functioning as a protective layer.
By providing a surface layer on the surface of the porous resin film, it is possible to prevent pores formed in the porous resin film from being discharged into the atmosphere through the outside, and in the case of electretization by charge injection. In addition, the dielectric strength of the porous resin film can be improved, and more charges can be injected under a high voltage.
It is preferable that the surface layer has a composition in which pores are harder to form than the porous resin film or a structure with a low porosity. The formation of such a surface layer can be achieved by reducing the content of the pore-forming nucleating agent in the surface layer compared to the porous resin film, or by setting the volume average particle size of the pore-forming nucleating agent used in the surface layer to be porous. The stretching ratio of both, such as a method of making the pore size nucleating agent used for the resin film smaller than the volume average particle size, a method of forming the porous resin film by biaxial stretching, and forming the surface layer by uniaxial stretching, etc. This can be achieved by making a difference.
As a thermoplastic resin which comprises a surface layer, the kind enumerated by the term of the thermoplastic resin used for a porous resin film can be used.

The surface layer may or may not contain a pore-forming nucleating agent, but from the viewpoint of modifying the electrical properties such as the dielectric constant of the surface layer, the surface layer is preferably contained. . When the surface layer contains a pore-forming nucleating agent, the same types as those listed in the section of the pore-forming nucleating agent used for the porous resin film can be used. May be the same as or different from the pore-forming nucleating agent for the porous resin film.
In particular, since the organic filler generally has a higher dielectric constant than the thermoplastic resin, it is suitable for modifying the electrical characteristics of the surface layer. In particular, when using a low dielectric constant resin such as polyolefin resin as the thermoplastic resin for the surface layer, the porous resin film can be made to contain an organic filler due to its dielectric effect when high voltage is applied during electretization. Charges can reach the inside. On the other hand, after the electretization treatment, the effect of retaining the electric charge inside the porous resin film without escaping can be obtained due to the low dielectric properties of the polyolefin resin as the main component.
When the surface layer contains a pore-forming nucleating agent, it is preferable to use a dispersant similar to the type listed in the section of the dispersant used for the porous resin film.

The surface layer is preferably stretched. The surface layer can improve the uniformity of thickness (film thickness) and the uniformity of electrical characteristics such as withstand voltage by a stretching process described in detail later. If the thickness of the surface layer is not uniform, local discharge concentration is likely to occur in the thin portion of the surface layer during charge injection using a high voltage, making it difficult to raise the voltage for effective charge injection. Prone.
The surface layer may have not only a single layer structure but also a multilayer structure of two or more layers. In the case of a multilayer structure, it is possible to design a porous resin film with higher charge retention performance by changing the type and content of thermoplastic resin, pore-forming nucleating agent, and dispersing agent used in each layer It becomes.
The surface layer is preferably provided on at least one side of the porous resin film, and more preferably provided on both sides. When the surface layer is provided on both surfaces of the porous resin film, the composition and configuration of the front and back surfaces may be the same or different.

  When the surface layer is provided on the surface of the porous resin film, the thickness is preferably 0.1 μm or more, more preferably 0.3 μm or more, still more preferably 0.5 μm or more, and It is especially preferable that it is 7 micrometers or more. As a result, it is easy to provide a uniform surface layer, and uniform charge injection and improvement in dielectric strength can be expected. On the other hand, the thickness of the surface layer is preferably 200 μm or less, more preferably 100 μm or less, still more preferably 50 μm or less, and particularly preferably 30 μm or less. Thereby, when the charge is injected into the porous resin film, the charge tends to easily reach the inside of the porous resin film. Also, since the surface layer is a layer that is relatively difficult to elastically deform in the thickness direction, suppressing the thickness of the surface layer tends to maintain the piezoelectricity without reducing the compression elastic modulus of the porous resin film. There is.

(Manufacture of porous resin film)
Various conventionally known methods can be used for the production of the porous resin film. For example, when the porous resin film is a single layer film, it may be extruded from a single die and stretched in at least one axial direction. In addition, when the porous resin film has a surface layer, a co-extrusion method using a multilayer die using a feed block or a multi-manifold, an extrusion lamination method using a plurality of dies, and the like can be mentioned. Furthermore, a method of combining a coextrusion method using a multilayer die and an extrusion lamination method can be mentioned.
The thickness uniformity of the porous resin film is important because the dielectric breakdown voltage is improved, so that the charge injection efficiency is improved, and as a result, the piezoelectric efficiency is improved. When the porous resin film has a surface layer, it is preferable that the surface layer is stretched and then stretched in at least one axial direction. By stretching the surface layer after laminating, the uniformity of the film thickness is improved, and as a result, the electrical characteristics are improved, compared with laminating stretched films.

  In the present invention, the porous resin film is a stretched resin film. Many pores are formed in the porous resin film by stretching. It is desirable that the pores formed in the porous resin film have a large individual volume, a large number, and shapes independent from each other from the viewpoint of maintaining electric charge. The size of the pores can be increased by extending in the biaxial direction rather than extending in only one direction. In particular, those stretched in the biaxial direction of the film in the width direction and the flow direction can form disk-like vacancies that are stretched in the plane direction. The charge retention performance is excellent. Therefore, the porous resin film is preferably biaxially stretched.

  The stretching of the porous resin film can be performed by various known methods. Specifically, a longitudinal stretching method using the peripheral speed difference of the roll group, a lateral stretching method using a tenter oven, a sequential biaxial stretching method in which the longitudinal stretching and the lateral stretching are performed in the normal order or reverse order, a rolling method, Examples thereof include a simultaneous biaxial stretching method using a combination of a tenter oven and a linear motor, and a simultaneous biaxial stretching method using a combination of a tenter oven and a pantograph. Moreover, the simultaneous biaxial stretching method by the tubular method which is a stretching method of an inflation film can be mentioned.

From the viewpoint of pore formation, the stretching of the porous resin film may be performed at a temperature from the glass transition temperature of the main thermoplastic resin used for the porous resin film to the melting point of the crystal part of the main thermoplastic resin. preferable. In addition, when stretching a porous resin film that is a multilayer laminate, it is appropriate to set the stretching temperature in consideration of the stretching efficiency of the layer with the largest set basis weight or the layer with the highest set porosity. is there.
The temperature during stretching is preferably higher than the glass transition temperature of the thermoplastic resin most frequently used in the mass ratio for the porous resin film and lower by 1 to 70 ° C. than the melting point of the thermoplastic resin. Specifically, when the thermoplastic resin of each layer is a propylene homopolymer (melting point 155 to 167 ° C.), 100 to 166 ° C. is preferable, and when it is a high density polyethylene (melting point 121 to 136 ° C.), 70 to 135. ° C is preferred.
Of course, if the stretching temperature is determined using thermoplastic resins having different melting points or glass transition points for the inner layer and the surface layer of the porous resin film, the porosity of each layer can be adjusted. .

The draw ratio is not particularly limited, and is appropriately determined in consideration of the draw characteristics of the thermoplastic resin used for the porous resin film, the set porosity described above, and the like.
When a propylene homopolymer or a copolymer thereof is used as the thermoplastic resin, the stretching ratio is preferably 1.2 times or more and more preferably 2 times or more when stretching in a uniaxial direction. On the other hand, 12 times or less is preferable and 10 times or less is more preferable. Moreover, when extending | stretching to a biaxial direction, 1.5 times or more are preferable at an area magnification (product of a vertical magnification and a horizontal magnification), and 4 times or more are more preferable. On the other hand, 60 times or less is preferable and 50 times or less is more preferable.
When other thermoplastic resins are used, the draw ratio is preferably 1.2 times or more and more preferably 2 times or more when uniaxially drawn. On the other hand, 10 times or less is preferable and 5 times or less is more preferable. Moreover, when extending | stretching to a biaxial direction, 1.5 times or more are preferable at an area magnification, and 4 times or more are more preferable. On the other hand, 20 times or less is preferable and 12 times or less is more preferable.
When stretching in the biaxial direction, it is possible to set the vertical and horizontal magnifications to the same magnification as much as possible.
It is easy to adjust the shape and frequency of vacancies observed in a cross section in an arbitrary direction to a preferable range of the present invention by forming disk-like vacancies that easily accumulate electric charges. Therefore, when stretching in the biaxial direction, the ratio of the vertical magnification and the horizontal magnification is preferably 0.4 or more, more preferably 0.5 or more, and further preferably 0.7 or more. Preferably, it is 0.8 or more. On the other hand, it is preferably 2.5 or less, more preferably 2.0 or less, still more preferably 1.5 or less, and particularly preferably 1.3 or less.
The stretching speed is preferably in the range of 20 to 350 m / min from the viewpoint of stable stretch molding.

(Anchor coat layer)
On the surface of the porous resin film, an anchor coat layer may be provided on one side or both sides in order to improve adhesion to other materials (film, vapor-deposited metal film, etc.).
For the anchor coat layer, a polymer binder is preferably used from the viewpoint of adhesion to other materials. Specific examples of the polymer binder include polyethyleneimine and alkyl-modified polyethyleneimine having 1 to 12 carbon atoms. Polyethyleneimine polymers such as poly (ethyleneimine-urea); polyamine polyamide polymers such as polyamine polyamide ethyleneimine adducts and polyamine polyamide epichlorohydrin adducts; acrylic acid amide-acrylic acid ester copolymers, Acrylic acid ester-based polymers such as acrylic amide-acrylic acid ester-methacrylic acid ester copolymers, polyacrylamide derivatives, and oxazoline group-containing acrylic acid ester polymers; polyvinyl alcohols containing polyvinyl alcohol and modified products thereof Polymers; water-soluble resins such as polyvinylpyrrolidone and polyethylene glycol; polypropylene polymers such as chlorinated polypropylene, maleic acid-modified polypropylene, and acrylic acid-modified polypropylene, as well as polyvinyl acetate, polyurethane, ethylene-vinyl acetate copolymer, poly Examples thereof include organic solvent-diluted resins or water-diluted resins of thermoplastic resins such as vinylidene chloride, acrylonitrile-butadiene copolymers, and polyesters. Among these, a polyethyleneimine polymer, a polyamine polyamide polymer, a polyvinyl alcohol polymer, and a polypropylene polymer are preferable because of excellent adhesion to the porous resin film.

The basis weight of the anchor coat layer provided on the porous resin film, in terms of solid content, preferably 0.001 g / ^{m 2} or more from the viewpoint of expressing the adhesion between the porous resin film, 0.005 g / ^{m 2} or more Is more preferable, and 0.01 g / m ² or more is particularly preferable. On the other hand, 5 g / m ² or less is preferable, 3 g / m ² or less is more preferable, and 1 g / m ² or less is particularly preferable from the viewpoint of keeping the thickness of the anchor coat layer that is the coating layer uniform. If the thickness of the anchor coat layer that is the coating layer cannot be kept uniform, the uniformity of the electrical properties of the porous resin film may be impaired due to fluctuations in the thickness, or the anchor coat layer itself may be porous due to insufficient cohesive strength. Since the adhesion with the resin film is reduced, or the surface resistance value of the anchor coat layer is reduced to less than 1 × 10 ¹³ Ω, and the charge easily escapes through the surface, the electretization of the porous resin film In some cases, it is difficult to inject electric charges, and the electric charges cannot reach the porous resin film, and the desired performance of the present invention may not be exhibited.

As a method of providing the anchor coat layer on the porous resin film, a method of coating the coating liquid containing the polymer binder on the porous resin film is preferable. Specifically, it can be formed by forming a coating film of the coating solution on a porous resin film using a known coating apparatus and drying the coating film.
Specific examples of coating apparatuses include, for example, die coaters, bar coaters, comma coaters, lip coaters, roll coaters, curtain coaters, gravure coaters, spray coaters, squeeze coaters, blade coaters, reverse coaters, air knife coaters, A size press coater etc. are mentioned.
The timing at which the anchor coat layer is provided on the porous resin film may be before or after the electret treatment described in detail later.

(Thickness of porous resin film)
The thickness of the porous resin film is measured using a thickness meter based on JIS-K-7130: 1999 “Plastic-film and sheet-thickness measuring method”. When the porous resin film is a multilayer film, the sample to be measured is cooled to a temperature of −60 ° C. or lower with liquid nitrogen, and cut with a razor blade perpendicular to the sample placed on the glass plate. Samples for cross-section measurement were prepared, and the obtained samples were observed for cross-section using a scanning electron microscope, and the boundary lines of each layer were determined from the pore shape and composition appearance, and each layer obtained from the observed image The ratio of the thickness to the thickness of the porous resin film is determined. Furthermore, it calculates | requires by multiplying the said ratio of each layer thickness to the film total thickness calculated | required using the thickness meter.
The thickness of the porous resin film is preferably 10 μm or more, more preferably 20 μm or more, and particularly preferably 30 μm or more. As a result, it is possible to uniformly form a desired number of holes having a size that effectively functions for energy conversion. On the other hand, the thickness of the porous resin film is preferably 500 μm or less, more preferably 300 μm or less, and particularly preferably 150 μm or less. As a result, when the charge injection (DC high-voltage discharge treatment) is performed to make the electret, the charge can reach the inside of the layer, and the expected performance of the present invention can be exhibited.

(Surface resistance value of porous resin film)
The surface resistance value [Ω] of the porous resin film was measured under the conditions of a temperature of 23 ° C. and a relative humidity of 50% using a double ring electrode according to JIS-K-6911: 1995 “General Test Method for Thermosetting Plastics”. Measure below.
The surface resistance of at least one surface of the porous resin film is preferably 1 × 10 ¹³ Ω or more, and more preferably 5 × 10 ¹³ Ω or more. As a result, when the electretization process is performed, electric charges are difficult to escape through the surface, and sufficient charge injection is performed. On the other hand, the surface resistance of at least one surface is preferably 9 × 10 ¹⁷ Ω or less, and more preferably 5 × 10 ¹⁶ Ω or less. Thereby, dust and dust adhere to the porous resin film, and a local discharge is caused to propagate through the electret treatment, thereby suppressing the phenomenon of partial destruction of the porous resin film.

[Electret material]
The electret material of the present invention can be obtained by electretizing the porous resin film.
In the electretized material of the present invention, a resonance phenomenon appears in the range of 10 kHz to 1 MHz as described above. This resonance phenomenon is measured at a storage elastic modulus (E ′ _(0.01) ) measured at a frequency of 0.01 Hz according to JIS-K-7244-4 in an environment of an electret material at 25 ° C. and a relative humidity of 50%, and at a frequency of 100 Hz. This is closely related to the storage elastic modulus ratio (A _e ) obtained by the above equation 4 from the storage elastic modulus (E ′ ₍₁₀₀₎ ) of 1.10 to 2.00.
The resonance phenomenon is caused by the fact that the thermoplastic resin mainly contained in the electret material is electret in a temperature environment higher than the phase transition temperature required as an inflection point at which the rate of change of the tensile storage modulus (E ′) with respect to temperature changes. Observed when operating the material.

(Electretization)
As such electretization processing, there are several processing methods. For example, a method in which both sides of a porous resin film are held by a conductor and DC high voltage or pulsed high voltage is applied (electro-electretization method) or γ-ray or electron beam irradiation is performed (radio-electretization) Etc.) are known.
Among these, the electretization method (electroelectretization method) using DC high-voltage discharge has a small apparatus and a small burden on workers and the environment, and is like the energy conversion film (i) of the present invention. It is suitable for the electretization treatment of a high polymer material.

  As a preferred example of an electretization apparatus that can be used in the present invention, a porous resin film is fixed between a needle electrode 6 connected to a DC high voltage power source 5 and a ground electrode 7 and a predetermined voltage is applied as shown in FIG. 4, a method of fixing the porous resin film between the wire electrode 10 connected to the DC high voltage power source 5 and the ground electrode 7 as shown in FIG. 4 and moving the wire electrode 10 while applying a predetermined voltage, FIG. A method of passing a porous resin film while applying a predetermined voltage between a needle electrode 8 connected to a DC high voltage power source 5 and a roll 9 connected to the ground, as shown in FIG. For example, a method of passing a porous resin film while applying a predetermined voltage between the wire electrode 11 connected to the power supply 5 and the roll 9 connected to the ground, and the like can be mentioned.

The porous resin film can accumulate more electric charges inside by electretization treatment by direct current high voltage discharge.
The applied voltage of the electret treatment is the thickness of the porous resin film, the porosity, the material of the resin and filler, the treatment speed, the shape, material and size of the electrode used, the charge amount of the electret material to be finally obtained, etc. However, 5 kV or more is preferable, 6 kV or more is more preferable, and 7 kV or more is more preferable. Thereby, a sufficient amount of charge can be injected, and piezoelectric performance can be exhibited. On the other hand, the applied voltage for the electretization treatment is preferably 100 kV or less, more preferably 70 kV or less, and even more preferably 50 kV or less. As a result, a local spark discharge occurs during the electretization process, causing partial destruction such as pinholes in the porous resin film, or the earth electrode that travels from the surface of the porous resin film to the end face during the electretization process. It is easy to avoid the phenomenon that current flows easily and the efficiency of electretization deteriorates.

In the electretization treatment, an excessive charge may be injected into the porous resin film. In this case, the electret material after processing may cause electric discharge, which may cause inconvenience in the post-processing process. Therefore, after the electretization process, the charge removal process for surplus charges may be performed.
As such a charge removal process, a known method such as a voltage application charge remover (ionizer) or a self-discharge charge remover can be used. These general static eliminators can remove the surface charge of the porous resin film, but cannot remove the charges accumulated in the porous resin film, particularly in the pores. Accordingly, the performance of the electret material is not greatly reduced by the charge removal process.
By performing the charge removal process, it is possible to remove the charge applied excessively by the electretization process and prevent the discharge phenomenon of the electret material.

  The electretization treatment is desirably performed at a temperature not lower than the glass transition temperature of the main thermoplastic resin used for the porous resin film and not higher than the melting point of the crystal part. If the glass transition point or higher, the molecular motion of the amorphous portion of the thermoplastic resin is active, and a molecular arrangement suitable for a given charge is formed, so that an efficient electret treatment is possible. On the other hand, if the melting point of the main thermoplastic resin used for the porous resin film is exceeded, the porous resin film itself cannot maintain its structure, and the desired performance of the present invention may not be obtained. .

[Conductive layer (D)]
The electret material obtained by electrifying the porous resin film can input and output electric power by providing a conductive layer on at least one surface. Since the conductive layer is used as an electrode, a conductive layer is provided on at least one side of the electret material, and a four-terminal method is performed in accordance with JIS-K-7194: 1994 “Resistivity test method for conductive plastics by four-probe method”. It is preferable that the surface resistance value measured by is set to 1 × 10 ^{−2 to} 9 × 10 ⁷ Ω.
When the surface resistance value of the electret material exceeds 9 × 10 ⁷ Ω, the electric signal transmission efficiency is poor, and the performance as a material for an electric / electronic input / output device tends to be lowered. On the other hand, providing a conductive layer of less than 1 × 10 ⁻² Ω means that in the case of coating, it is necessary to provide a thick conductive layer, and after coating, the porous resin film undergoes heat shrinkage due to heat during drying. There is. Moreover, when vapor-depositing, the void | hole formed in the porous resin film inside may be crushed by the heat | fever of the metal to vapor-deposit, or a porous resin film may raise | generate a heat shrink.

Examples of the conductive layer include a coating film formed by applying a conductive paint and a metal vapor deposition film.
Examples of conductive materials that can be applied include metal particles such as gold, silver, platinum, copper, and silicon; tin-doped indium oxide (ITO), antimony-doped tin oxide (ATO), fluorine-doped tin oxide (FTO), aluminum Conductive metal oxide particles such as doped zinc oxide; carbon particles or the like mixed in a solution or dispersion of a binder resin component such as polyacrylate, polyurethane, polyepoxy, polyether, polyester, and the like. In addition, a solution or a dispersion of a conductive resin such as polyaniline, polypyrrole, or polythiophene can be given.
The conductive coating is applied by a conventionally known coating apparatus such as a die coater, bar coater, comma coater, lip coater, roll coater, curtain coater, gravure coater, spray coater, blade coater, reverse coater, air knife coater, etc. Can be implemented.
As a specific example of the metal vapor deposition film, a metal such as aluminum, zinc, gold, silver, platinum, nickel is vaporized under reduced pressure, and a thin film is formed on the surface of the porous resin film, or Examples thereof include a metal thin film formed by vapor-depositing a metal such as aluminum, zinc, gold, silver, platinum, or nickel on a carrier such as a polyethylene terephthalate (PET) film and transferred onto the surface of the porous resin film.

The conductive layer may be installed on the surface of the porous resin film either before or after the electret treatment.
If the conductive layer is installed on the electret material after the electret treatment, it is possible to prevent the charge from being dissipated through the conductive layer during the electret treatment. However, when the conductive layer is installed, a load such as heat is applied to the porous resin film, and the electric charge escapes and the performance may deteriorate. Judging from the performance of the finally obtained electret material, the conductive layer is preferably provided before the electret treatment.

The present invention will be described more specifically with reference to the following examples. The materials, amounts used, ratios, operations, and the like shown below can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.
In addition,% described below is mass% unless otherwise specified. Moreover, the volume average particle diameter in this specification is described as the median diameter D50.
Table 1 summarizes materials used in production examples and examples of porous resin films.

[Composition example]
As described in Table 1, thermoplastic resin (71.8% by mass of propylene homopolymer and 10% by mass of high-density polyethylene), pore-forming nucleating agent (18% by mass of calcium carbonate) and dispersing agent (oleic acid) 2% by mass was mixed, melted and kneaded with a twin-screw kneader set at 210 ° C., then extruded into a strand shape with an extruder set at 230 ° C., cooled, and cut with a strand cutter to obtain a resin composition ( The pellet of a) was prepared.

[Example 1]
The resin composition a for porous resin film is melt-kneaded in one extruder set at 230 ° C., and then supplied to the middle layer of a feed block type multilayer die set at 250 ° C., and at the same time, becomes a thermoplastic surface layer. The resin composition b (propylene homopolymer) was melt-kneaded in two extruders set at 230 ° C., and then supplied to the surface layer of a feed block type multilayer die set at 250 ° C. to obtain b / a / b. Were laminated in a die so as to be in this order and extruded into a sheet shape, which was cooled to 60 ° C. by a cooling device to obtain a three-layer unstretched sheet.
This unstretched sheet was heated to 140 ° C. and stretched 4 times in the machine direction (MD) using the peripheral speed difference of the roll group. Next, the uniaxially stretched sheet was cooled to 60 ° C., heated again to 150 ° C. using a tenter oven and stretched 8 times in the transverse direction (TD), and further heated to 160 ° C. in the oven for heat treatment. .
Next, after cooling to 60 ° C. and slitting the ears, both surfaces were subjected to corona surface discharge treatment, and both sides of the film were treated with polyamine polyamide polymer solution (an epichlorohydrin adduct solution of polyamine polyamide (Starlight PMC) as anchor agent A. (Product name: WS4024, solid content concentration: 25% by mass) obtained by diluting 25 times with a mixed solution of water / 2-propanol = 9/1) using a squeeze coater. The coating amount was 0.02 g / m ² and dried in an oven at 80 ° C. to provide an anchor coat layer to obtain a porous resin film.
The obtained porous resin film has a thickness of 70 μm, the thickness of each layer is 1 μm / 68 μm / 1 μm, the number of stretching axes of each layer is 2 axes / 2 axes / 2 axes, the porosity is 39%, and the surface resistance value is on both sides. It was 10 ¹⁴ Ω. In addition, from the cross-sectional observation of the film, the number of holes generated in the porous resin film having a height of 3 to 30 μm in the thickness direction of the film and a diameter of 50 to 500 μm in the surface direction of the film is The film flow direction was 554 / mm ² , and the film width direction was 872 / mm ² .

[Measurement of phase transition temperature]
One test piece 70 mm long and 3 mm wide from the porous resin film prepared in Example 1 so that the MD direction of the porous resin film is vertical to the test piece, and the TD direction of the porous resin film is the test piece. One piece was collected so as to be vertical.
Next, according to JIS-K-7244-1 and JIS-K-7244-4, the test piece was set to the dynamic viscoelasticity measuring apparatus (TA Instruments company make: RSA3). The distance between clamps at this time was 20 mm.
Subsequently, the tensile storage elastic modulus (E ′) and the tensile loss elastic modulus (E ″) are set to 1 at a frequency of 1 Hz while heating at a temperature increase rate of 1 ° C./min in the range of −110 ° C. to 130 ° C. Measured at intervals of ° C. Next, the temperature was plotted on the horizontal axis, and the tensile storage elastic modulus (E ′) value and loss elastic modulus (E ″) value were plotted on the vertical axis. The obtained result is shown in FIG.
As shown in the lower part of FIG. 6, the loss elastic modulus (E ″) peak temperature in the porous resin film of Example 1 is 18 ° C. with the MD direction being the length of the test piece, and the TD direction is the length of the test piece. The orientation was 16 ° C.

[Measurement of storage modulus ratio]
One test piece 70 mm long and 3 mm wide from the porous resin film prepared in Example 1 so that the MD direction of the porous resin film is vertical to the test piece, and the TD direction of the porous resin film is the test piece. One piece was collected so as to be vertical.
Next, according to JIS-K-7244-1 and JIS-K-7244-4, the test piece was set to the dynamic viscoelasticity measuring apparatus (TA Instruments company make: RSA3). The distance between clamps at this time was 20 mm.
Subsequently, the storage elastic modulus (E ′) was measured at 16 points in a logarithmic uniform interval in a frequency range of 0.01 Hz to 100 Hz in an environment of 25 ° C. and a relative humidity of 50%. The obtained result is shown in FIG.
As shown in FIG. 8, the storage elastic modulus (E ′) showed good linearity in the frequency region.
Next, the storage elastic modulus (E ′) at two points of 0.01 Hz and 100 Hz was measured for each of the direction in which the MD direction is the length of the test piece and the direction in which the TD direction is the length of the test piece.
The storage elastic modulus (E ′) with the MD direction being the length of the test piece was 1.04 GPa at 100 Hz and 0.74 GPa at 0.01 Hz.
The storage elastic modulus (E ′) with the TD direction as the length of the test piece was 2.39 GPa at 100 Hz and 1.83 GPa at 0.01 Hz.
Next, the storage elastic modulus ratio was calculated | required by Formula (4) about each of MD direction and TD direction.
A _e = E ′ ₍₁₀₀₎ / E ′ _(0.01) (Expression 4)
A _e : Storage elastic modulus ratio [−]
E ′ ₍₁₀₀₎ : Storage elastic modulus [Pa] at a frequency of 100 Hz
E ′ _(0.01) : Storage elastic modulus [Pa] at a frequency of 0.01 Hz
The storage elastic modulus ratio in the direction in which the MD direction is the longitudinal direction of the test piece is 1.41, and the storage elastic modulus ratio in the direction in which the TD direction is the vertical direction of the test piece is 1.31, and these values are the second decimal place. Indicated.

[Creation of electret materials]
Aluminum was vapor-deposited on one side of the porous resin film prepared in Example 1 using a vacuum deposition apparatus (trade name: VE-2030, manufactured by Hitachi High-Tech) to form a conductive layer.
Next, the aluminum vapor deposition surface of the vapor deposition film obtained above is in contact with the ground electrode surface on the ground electrode board of the electretization apparatus shown in FIG. 2 set to a distance between the main electrodes of 10 mm and a distance between the main electrode and the ground electrode of 10 mm. A DC voltage was applied.
The applied voltage was gradually increased from 1 kV, and the voltage (dielectric breakdown voltage) at which the deposited film was destroyed by local spark discharge was measured.
Electric charge is injected into a vapor-deposited film separately produced at a voltage 1 kV lower than the dielectric breakdown voltage, electretization is performed, and then a vacuum vapor deposition apparatus (made by Hitachi High-Tech, product name: VE) is formed on the surface where aluminum vapor deposition is not formed. -2030) was used to deposit aluminum to form a conductive layer to produce an electret material.
When the surface resistance value of the front and back surfaces of the electret material obtained was measured by the 4-terminal method according to JIS-K-7194: 1994 “Resistivity test method by conductive probe 4-probe method”, the surface resistance value was 1Ω or less. there were.

[Measurement of resonance frequency]
Capacitance between front and back (C) in an environment of 25 ° C. and 50% relative humidity connected to an impedance analyzer (Agilent Technology: 4194A) on the electret material subjected to charge injection by DC corona discharge by the method described above. The dielectric loss (tan δ) is measured at 201 points by the electrode contact method so that the frequency range of 1 kHz to 1 MHz is logarithmically uniform, and the relative permittivity (ε _r ) Was calculated.
ε _r = C × t / A / ε ₀ (Expression 1)
C: Capacitance of electret material [F]
ε _r : relative permittivity [−]
ε ₀ : Dielectric constant of vacuum (8.854 × 10 ⁻¹² ) [F / m]
A: Electrode area [m ² ]
t: thickness of electret material [m]
Next, the relative dielectric constant (ε _r ) obtained by the above equation 1 and the dielectric loss (tan δ) obtained by the above measurement were plotted against the logarithm of the frequency in the frequency range of 1 kHz to 1 MHz. The obtained results are shown in FIG.
As shown in the lower part of FIG. 7, since the peak of dielectric loss (tan δ) was observed at 318 kHz, the relative dielectric constant (ε _r ) of the frequency range (159 to 636 kHz) was 0.5 to 2.0 times. It was 326 kHz when the frequency (Sf) with the maximum change was determined as the resonance frequency.

[Measurement of dielectric constant change rate]
From the measurement result of the resonance frequency, the dielectric constant change rate (Se) was calculated by Equation 2.
_{Se = (ε rB -ε rA)} / ε rS × 100 ··· ( Formula 2)
Se: Dielectric constant change rate [%]
ε _rS : relative dielectric constant at resonance frequency [−]
ε _rB : relative permittivity [−] at a frequency 0.5 times the resonance frequency
ε _rA : relative dielectric constant [−] at a frequency 2.0 times the resonance frequency
As shown in FIG. 7, the resonance frequency is 326 kHz, ε _rS is 1.431 [−], ε _rB is 1.438 [−] in relative permittivity at 163 kHz, and ε _rA is 1.426 in relative permittivity at 652 kHz. Since it was [−], the dielectric constant change rate (Se) was estimated to be 0.84%.

[Measurement of frequency dependence of relative permittivity]
In the measurement of the resonance frequency, the relative dielectric constant (ε _rf ) against the logarithm of the frequency (Feq) is plotted in the range from 1 kHz to a frequency that is 0.5 times the resonance frequency. As shown in FIG. The frequency dependence of the dielectric constant (ε _rf ) showed linearity.
Furthermore, when the linear portion of the plot was approximated by the following equation 3 using the least square method, the frequency dependence (a) of the relative permittivity (ε _rf ) was calculated to be −0.0042.
[epsilon] _rf = a * log (Feq) + b (Formula 3)
ε _rf : relative permittivity [−]
Feq: Frequency [Hz]
a: inclination = −0.0042
b: Y intercept = 1.4591
On the other hand, when the frequency dependence of the relative permittivity (ε _rf ) was measured in the same manner for the deposited film on which charge was not injected, the relative permittivity was constant at 1.26 and the frequency dependence (a) was 0. there were.

The measurement results are summarized in Table 2.

From Table 2, the loss elastic modulus (E ″) peak temperatures of the porous resin film of Example 1 are 16 ° C. (TD) and 18 ° C. (MD), and good piezoelectric performance is obtained at the measurement temperature (25 ° C.). You can see that it works.
Further, the storage elastic modulus ratio between the frequency of 0.01 Hz and the frequency of 100 Hz obtained from Equation 4 is in the range of 1.10 to 2.00, and the storage elastic modulus is increased in the low frequency region. It can be seen that good piezoelectric performance can be obtained in a region (for example, an audible frequency region).
Subsequently, when the resonance frequency of the electret material subjected to charge injection was measured, a resonance peak was observed at 326 kHz. It is well known that no dielectric frequency appears in the range of 10 kHz to 1 MHz in a dielectric that is not subjected to charge injection, and it can be seen that the electret material of the present invention functions effectively as a piezoelectric body by charge injection. Further, it can be seen that the dielectric constant difference is 0.84% and the performance as a piezoelectric body is high. Furthermore, the frequency dependence of the dielectric constant (ε _rf ) is −0.0042, which indicates that the piezoelectricity is higher at lower frequencies.
From the above results, the usage method of operating the electret material of the present invention in the range of −20 to 20 ° C. with respect to the loss elastic modulus (E ″) peak temperature of the electret material is useful. , A method of using the sensor in the audible frequency region, and the sensor in a range of −20 to 20 ° C. with respect to a peak temperature of loss elastic modulus (E ″) of the electret material constituting the sensor. It can be seen that the operating method of operation is possible.

  The electret material of the present invention exhibits piezoelectricity at a temperature higher than the phase transition temperature of the electret material. In addition, because it has good sensitivity in the low frequency region, especially in the audible frequency region, it has excellent characteristics such as acoustic sensors, vibration sensors, impact sensors, etc., and measuring instruments, control devices, abnormality diagnosis systems, security devices, It can be used for the manufacture of stabilizers, robots, percussion instruments, game machines and the like, and makes a great contribution to these industrial fields.

Claims (10)

An electret material containing a thermoplastic resin,
The content of the thermoplastic resin in the electret material is 50% by mass or more,
The electret material comprises a porous resin film,
When the loss elastic modulus (E ″) of the porous resin film was measured at a frequency of 1 Hz at a temperature increase rate of 1 ° C./min in the temperature range of −100 to 100 ° C., the loss elastic modulus (E ″) An electret material having a peak temperature of 0 to 40 ° C. An electret material containing a thermoplastic resin,
An electret material wherein a resonance frequency appears in a range of 10 kHz to 1 MHz when the electret material subjected to charge injection at a voltage that does not reach a dielectric breakdown voltage by direct current corona discharge is measured by the following method.
Resonance frequency measurement method: the ratio (C) and dielectric loss (tan δ) of the electret material measured in the frequency range of 1 kHz to 1 MHz in an environment of 25 ° C. and 50% relative humidity The dielectric constant (ε _r ) and the dielectric loss (tan δ) obtained by the above measurement are plotted against the logarithm of the frequency, and the frequency at which the peak of the dielectric loss (tan δ) is observed is 0.5 to 1.5 times. The frequency at which the change in relative permittivity (ε _r ) is maximum within the frequency range is defined as the resonance frequency (Sf).
ε _r = C × t / A / ε ₀ (Expression 1)
C: Capacitance of electret material [F]
ε _r : relative permittivity [−]
ε ₀ : Dielectric constant of vacuum (8.854 × 10 ⁻¹² ) [F / m]
A: Electrode area [m ² ]
t: thickness of electret material [m] The electret material according to claim 2, wherein in the measurement of the resonance frequency, a dielectric constant change rate (Se) obtained from the following formula 2 is 0.1 to 5.0%.
_{Se = (ε rB -ε rA)} / ε rS × 100 ··· ( Formula 2)
Se: Dielectric constant change rate [%]
ε _rS : relative dielectric constant at resonance frequency [−]
ε _rB : relative permittivity [−] at a frequency 0.5 times the resonance frequency
ε _rA : relative dielectric constant [−] at a frequency 2.0 times the resonance frequency In the measurement of the resonance frequency, the relative permittivity (ε _rf ) in the range from 1 kHz to 0.5 times the resonance frequency is expressed by the slope (a) when approximated by the following equation 3 using the least square method. The electret material according to claim 2, wherein the expressed frequency dependence is −0.0200 to −0.0001.
[epsilon] _rf = a * log (Feq) + b (Formula 3)
ε _rf : relative permittivity [−]
Feq: Frequency [Hz]
a: inclination b: Y intercept An electret material containing a thermoplastic resin,
Storage modulus (E ′ _(0.01) ) measured at a frequency of _0.01 Hz and storage elastic modulus measured at a frequency of 100 Hz for the electret material in an environment of 25 ° C. and a relative humidity of 50% according to JIS-K-7244-4. An electret material having a storage elastic modulus ratio (A _e ) calculated from (E ′ ₍₁₀₀₎ ) according to the following formula 4 from 1.10 to 2.00:
A _e = E ′ ₍₁₀₀₎ / E ′ _(0.01) (Expression 4)
A _e : Storage elastic modulus ratio [−]
E ′ ₍₁₀₀₎ : Storage elastic modulus [Pa] at a frequency of 100 Hz
E ′ _(0.01) : Storage elastic modulus [Pa] at a frequency of 0.01 Hz The electret material according to claim 1,
The electret material contains 50 to 98% by mass of a thermoplastic resin, 49.99 to 1.99% by mass of a pore forming nucleating agent, and 0.01 to 10% by mass of a dispersant (a thermoplastic resin and a pore forming nucleating agent). And the total content of the dispersant is 100%), electret material. The electret material according to claim 6, wherein a porosity of the electret material calculated by the following formula 5 is 10 to 70%.

ρ ₀ : True density of resin film measured according to JIS-K-7112: 1999 ρ: Density of resin film measured according to JIS-K-7222: 2005   The method of using the electret material according to claim 1, wherein the electret material according to claim 1 is operated in a range of -20 to 20 ° C with respect to the loss elastic modulus (E '') peak temperature of the electret material.   A sensor using the electret material according to claim 1.   A method for using a sensor, wherein the sensor according to claim 9 is operated in a range of -20 to 20 ° C with respect to a peak temperature of the loss elastic modulus (E '') of an electret material constituting the sensor. .

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