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JP2014100868

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DESCRIPTION JP2014100868
Abstract: The present invention relates to an electret sheet which retains excellent
piezoelectricity over a long period of time and has excellent mechanical strength. SOLUTION: The
electret sheet of the present invention is an electret sheet formed by injecting a charge into a
laminated sheet formed by laminating and integrating a plurality of porous sheets, and the
porous sheet is stretched, and the laminating direction is The main stretching directions of the
porous sheets adjacent to each other are substantially orthogonal to each other, and further,
between the porous sheets adjacent to each other, the stretching ratio E of the main stretching
direction of one porous sheet and the other porous It is characterized in that a ratio (E / E) to a
stretching ratio E in a direction coincident with the main stretching direction of the one porous
sheet in the quality sheet is 0.7 or less. [Selected figure] Figure 2
エレクトレットシート
[0001]
TECHNICAL FIELD The present invention relates to an electret sheet which retains excellent
piezoelectricity despite temperature change of use environment and is excellent in mechanical
strength.
[0002]
The electret is a material having an internal charge by injecting a charge into an insulating
polymer material.
04-05-2019
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Electrets are formed into fibers and are widely used as dust filters and the like.
[0003]
Also, it is known that the synthetic resin sheet exhibits very high piezoelectricity comparable to
ceramics by charging it. The electret using such a synthetic resin sheet is proposed to be applied
to an acoustic pickup, various pressure sensors, and the like by utilizing its excellent sensitivity.
[0004]
As an electret sheet, Patent Document 1 is an electret formed by injecting a charge into a porous
polymer having pores, and the average aspect ratio of the pores is 7 to 30, the porosity is 80% or
more, and the thickness An electret sheet having an average number of pores in a direction of 1
or more and 10 or less and an average pore size in a thickness direction of 30 μm or more and
200 μm or less is disclosed. This electret sheet biaxially stretches an organic polymer foam It is
described that it is preferable to inject a charge into the polymer porous body obtained by the
method described above, and also in the example, the charge is injected into the polymer porous
body obtained by simultaneous biaxial stretching of a polypropylene foam. It is manufactured by.
[0005]
However, the electret sheet is formed by injecting a charge into a biaxially stretched single-layer
polymer porous body, and distortion due to stretching remains in the polymer porous body. With
the temperature change, the polymer porous body tends to shrink due to the strain remaining in
the polymer porous body.
[0006]
In the electret sheet, the charge is held in a polarized state in the hole portion, and when the
contraction occurs in the electret sheet, the charge held in the polarized state in the hole portion
disappears, and the piezoelectric property of the electret sheet is While the problem of falling
arises, the surface nature of an electret sheet falls, and when an electret sheet is used as a sensor,
the problem that the accuracy of a sensor falls arises.
[0007]
Although it is conceivable to increase the draw ratio of biaxial stretching in order to improve the
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mechanical strength of the electret sheet, raising the draw ratio of biaxial stretching causes a
problem that the electret sheet is more easily shrunk.
[0008]
JP, 2010-186960, A
[0009]
The present invention provides an electret sheet that retains excellent piezoelectricity despite
temperature changes in the usage environment and has excellent mechanical strength.
[0010]
The electret sheet of the present invention is an electret sheet formed by injecting a charge into a
laminated sheet formed by laminating and integrating a plurality of porous sheets, ie, two or
three or more porous sheets, and the porous sheet is stretched The main draw directions of the
porous sheets adjacent to each other in the laminating direction are substantially orthogonal to
each other, and further, between the porous sheets adjacent to each other, the draw ratio E1 of
the main draw direction of one porous sheet A ratio (E2 / E1) of the draw ratio E2 in the
direction coincident with the main stretch direction of the one porous sheet in the other porous
sheet is 0.7 or less.
[0011]
The porous sheet is not particularly limited as long as it is a sheet having a void (void) inside and
capable of injecting a charge and holding the charge in a polarized state, for example, synthesis A
resin foam sheet, a porous synthetic resin non-foam sheet, etc. may be mentioned.
[0012]
It does not specifically limit as a synthetic resin which comprises a synthetic resin foam sheet
and a porous synthetic resin non-foam sheet, For example, polyethylene-type resin,
polypropylene-type resin, poly -4- methyl pentene, ethylene propylene rubber etc. Polyolefin
resin, ethylene-vinyl acetate copolymer, polycycloolefin resin, diene resin such as ethylenepropylene-diene copolymer, norbornene resin, polyvinyl chloride, chlorinated polyvinyl chloride,
chlorinated polyethylene Chlorinated resins such as, polytetrafluoroethylene,
polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, perfluoroalkoxy fluorine
resin, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene
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Copolymer, tetrafluoroethylene Fluorinated resins such as perfluoroalkoxyethylene copolymer,
tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-chlorotrifluoroethylene copolymer,
cyano-based resins such as polyvinyl polycyanide and polyvinylidene cyanide, polyethylene
terephthalate, Polyester resins such as polybutylene terephthalate and polylactic acid resin,
polyamide resins such as nylon 6, nylon 66 and nylon 11, epoxy resins, polyurethane resins,
acrylic resins, polyetherimide resins, polystyrene resins, poly Ether ether ketone etc. are
mentioned.
The synthetic resin may be used alone or in combination of two or more.
[0013]
As the above-mentioned synthetic resin, since it is excellent in the piezoelectricity of an electret
sheet, it is preferred to contain polyolefin system resin, it is more preferred to contain
polypropylene system resin, and to contain homo polypropylene Is particularly preferred.
[0014]
The synthetic resin is preferably excellent in insulation, and as the synthetic resin, the volume
specific resistance value after application of a voltage of 500 V and one minute after the
application of voltage according to JIS K6911 (hereinafter simply referred to as "volume specific
resistance value" The synthetic resin which is 1.0 * 10 <10> ohm * m or more is preferable.
[0015]
The volume specific resistance value of the synthetic resin is preferably 1.0 × 10 <13> Ω · m or
more, and 1.0 × 10 <15> Ω · m or more, since the electret sheet has more excellent
piezoelectricity. Is more preferred.
[0016]
As a polyethylene-based resin, an ethylene homopolymer, or a copolymer of ethylene and at least
one α-olefin having a carbon number of 3 to 20 other than ethylene, containing more than 50%
by weight of an ethylene component It can be mentioned.
Examples of the ethylene homopolymer include low density polyethylene resin (LDPE) radically
polymerized under high pressure, and medium and low pressure high density polyethylene resin
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(HDPE) polymerized in the presence of a catalyst at medium and low pressure.
A linear low density polyethylene (LLDPE) can be obtained by copolymerizing ethylene and an
α-olefin, and a linear low density polyethylene is preferable.
Examples of the α-olefins include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene,
1-octene, 1-nonene, 1-decene, 1-tetradecene, 1-hexadecene, 1 -Octadecene, 1-eicosene etc. is
mentioned, A C4-C10 alpha-olefin is preferable.
In addition, content of the alpha olefin in linear low density polyethylene is 1 to 15 weight%
normally.
[0017]
The polypropylene-based resin is not particularly limited as long as it contains more than 50% by
weight of a propylene component, and, for example, propylene homopolymer
(homopolypropylene), propylene and at least one kind of propylene having 3 to 3 carbon atoms
other than propylene Copolymers with 20 olefins may, for example, be mentioned, with
preference given to homopolypropylene.
The polypropylene resins may be used alone or in combination of two or more.
The copolymer of propylene and at least one olefin having 3 to 20 carbon atoms other than
propylene may be either a block copolymer or a random copolymer.
[0018]
Examples of α-olefins copolymerized with propylene include ethylene, 1-butene, 1-pentene, 4methyl-1-pentene, 1-hexene, 1-octene, 1-nonene, 1-decene, 1-tetradecene, 1-hexadecene, 1octadecene, 1-eicosene and the like can be mentioned, with preference given to ethylene.
[0019]
As a method of producing a porous sheet, for example, (1) a synthetic resin and a thermal
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decomposition type foaming agent are supplied to an extruder, and melted and kneaded at a
temperature lower than the decomposition temperature of the thermal decomposition type
foaming agent. The resin sheet is extruded, and the foamable resin sheet is irradiated with
ionizing radiation such as electron beam, α-ray, β-ray, γ-ray, if necessary, crosslinked and then
heated to decompose the thermal decomposition type foaming agent A method of producing a
synthetic resin foam sheet by foaming, (2) a method of supplying a synthetic resin and a physical
type foaming agent to an extruder, melting and kneading, extruding and foaming from the
extruder to produce a synthetic resin foam sheet, (3) A porous synthetic resin non-foamed sheet
formed by forming pores (voids) by causing peeling at the interface between the particles and the
synthetic resin by stretching the synthetic resin sheet containing the particles Etc.
[0020]
In addition, the thermal decomposition type foaming agent is not particularly limited as long as it
is used for producing a foam, and for example, azodicarbonamide, benzenesulfonylhydrazide,
dinitrosopentamethylenetetramine, toluenesulfonylhydrazide, 4 , 4- oxybis (benzenesulfonyl
hydrazide) and the like.
Moreover, as a physical type foaming agent, propane, normal butane, isobutane, normal pentane,
isopentane, hexane etc. are mentioned, for example.
[0021]
The particles contained in the synthetic resin sheet are not particularly limited, and, for example,
inorganic fine particles such as talc, calcium carbonate and barium sulfate, organic fine particles
such as polystyrene resin particles and polyacrylic acid ester resin particles, etc. are mentioned
Be
[0022]
In addition, in the range which does not impair the physical property, the porous sheet may
contain additives, such as an antioxidant, a metal damage inhibitor, a ultraviolet absorber, a
pigment, a dye, and an antiblocking agent.
[0023]
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The electret sheet of the present invention is formed by integrally laminating a plurality of
porous sheets.
Each porous sheet is stretched, and the plurality of porous sheets are laminated and integrated
such that the main stretching directions of the porous sheets adjacent to each other in the
stacking direction are substantially orthogonal.
In addition, uniaxial stretching or biaxial stretching is mentioned as extending | stretching of a
porous sheet, and extending | stretching of a porous sheet should just be performed using a
general purpose method.
[0024]
Here, when the porous sheet is stretched in a plurality of directions like biaxial stretching, the
main stretching direction of the porous sheet means the stretching direction with the highest
stretching ratio, and the porous sheet is uniaxially stretched. If it has been, it means the direction
of stretching.
When the porous sheet is stretched in a plurality of directions, the porous sheet is stretched so as
to have only one stretching direction with the highest stretching ratio.
Moreover, the draw ratio of a porous sheet means the value which multiplied 100 by the value
which remove | divided the dimension difference before and behind extending | stretching in the
extending | stretching direction by the dimension before extending | stretching.
Stretching ratio (%) = 100 × dimensional difference before and after stretching / dimension
before stretching
[0025]
Further, that the stretching directions of the porous sheets are substantially orthogonal means
that, as shown in FIGS. 1 and 2, straight lines L1, L2 parallel to the stretching directions D1, D2
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of the porous sheets F1, F2. It means that an angle α formed by each other is 70 to 90 °.
The angle α takes a value of 90 ° C. or an acute angle.
[0026]
Since the porous sheets constituting the electret sheet are respectively stretched, the porous
sheets are excellent in mechanical strength such as tear strength and toughness, and such porous
sheets are excellent in mechanical strength. The electret sheet formed by integrating a plurality
of sheets is excellent in mechanical strength such as tear strength and toughness, and can be
used stably without breakage for a long period of time, It is also excellent in processability such
as cutting.
[0027]
In addition, the porous sheet is stretched to improve mechanical strength such as tear strength
and toughness, distortion due to the stretching remains, and the porous sheet tends to shrink in
the stretched direction.
Therefore, the electret sheets of the present invention are configured such that the stretching
directions of the porous sheets laminated and integrated adjacent to each other are substantially
orthogonal, preferably orthogonal, and adjacent to each other. Ratio between the stretch ratio E1
in the main stretching direction of one porous sheet and the stretch ratio E2 in the direction
coinciding with the main stretching direction of the one porous sheet in the other porous sheet
E2 / E1) is configured to be 0.7 or less.
As described above, a plurality of porous sheets are integrally laminated such that the main
stretching directions of the porous sheets adjacent to each other are substantially orthogonal and
the ratio (E2 / E1) of stretching ratios is 0.7 or less, By adjusting so that the direction in which
one porous sheet tends to shrink and the direction in which the other porous sheet tends to
shrink do not overlap between adjacent porous sheets, one porous sheet can While the shrinkage
of the porous sheet is suppressed as much as possible, the other porous sheet suppresses the
shrinkage of one porous sheet as much as possible, and as a result, the electret sheet of the
present invention is constructed so as not to shrink as a whole. There is.
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[0028]
And since the porous sheet which adjoins this porous sheet generally suppresses shrinkage of
the porous sheet which constitutes an electret sheet, the draw ratio of each porous sheet can be
made high, and as a result The mechanical strength of the entire electret sheet can be increased.
[0029]
When the stretching ratio in the main stretching direction of the porous sheet is low, the
mechanical strength of the electret sheet may be lowered, and when it is high, the electret sheet
is easily shrunk, and the piezoelectric property is lowered by the heat contraction. Therefore, it is
preferably 1.1 to 10 times, more preferably 2 to 8 times, and particularly preferably 2 to 7 times.
[0030]
When the porous sheet is stretched in a plurality of directions, when the stretching ratio in a
direction different from the main stretching direction of the porous sheet is low, the mechanical
strength of the electret sheet may decrease, and when it is high, the electret is The sheet tends to
be shrunk, and the heat shrinkage may reduce the piezoelectricity, so 1.1 to 1.9 times is
preferable, and 1.1 to 1.5 times is more preferable.
[0031]
Furthermore, between porous sheets adjacent to each other, a direction in which the draw ratio
E1 in the main stretching direction of one porous sheet F1 matches the main stretching direction
of the one porous sheet F1 in the other porous sheet F2. If the ratio (E2 / E1) to the draw ratio
E2 is large, the other porous sheet F2 does not exhibit the effect of suppressing the shrinkage in
the main drawing direction of one porous sheet F1, and is therefore limited to 0.7 or less. 0.5 or
less is preferable.
When the other porous sheet F2 is not stretched in the direction coincident with the main
stretching direction of one porous sheet F1, the stretch ratio in the direction coincident with the
main stretching direction of one porous sheet F1 is 1 Do.
[0032]
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When uniaxial stretching of the porous sheet is performed while permitting dimensional change
in the direction orthogonal to the stretching direction of the porous sheet, shrinkage occurs in
the porous sheet in the direction orthogonal to the stretching direction of the porous sheet. The
porous sheet may be stretched while being constrained in the direction orthogonal to the
stretching direction so that the dimension in the direction orthogonal to the stretching direction
of the quality sheet does not change before and after stretching.
Only in this case, exceptionally, in calculating the draw ratio in the direction orthogonal to the
drawing direction in the porous sheet, the dimensional difference before and after drawing of the
porous sheet changes in the direction perpendicular to the drawing direction of the porous sheet.
This refers to the difference between the dimensions of the porous sheet after stretching when
allowed to be allowed and the dimensions of the porous sheet after stretching when constrained
so as not to change in size.
The dimension before the stretching of the porous sheet refers to the dimension after the
stretching of the porous sheet when the dimensional change in the direction perpendicular to the
stretching direction of the porous sheet is allowed.
[0033]
Preferably, the porous sheet is stretched and then subjected to an annealing treatment. By
subjecting the porous sheet to an annealing treatment, it is possible to alleviate the residual
strain during stretching that has occurred in the porous sheet, and to reduce the occurrence of
shrinkage due to temperature changes in the porous sheet, and hence to the electret sheet. The
excellent piezoelectricity of the electret sheet can be stably maintained over a long period of time
by reducing the occurrence of contraction due to temperature change.
[0034]
If the heating dimensional change rate in the main stretching direction after standing at 120 °
C. for 24 hours in the porous sheet is large, the electret sheet may easily shrink and the
piezoelectricity may be reduced in a short period of time 7% or less is preferable and 5% or less
is more preferable.
[0035]
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In addition, the heating dimensional change rate of the main extending | stretching direction
after leaving to stand at 120 degreeC in the porous sheet for 24 hours says the value measured
in the following way.
A straight line L having a length of 50 mm is drawn on the surface of the porous sheet so that
the length direction coincides with the main stretching direction of the porous sheet. Next, the
porous sheet is left in an oven maintained at 120 ° C. for 24 hours. The porous sheet is taken
out of the oven, and the length L1 of the straight line L drawn on the surface of the porous sheet
is measured. The heating dimensional change rate is calculated based on the following equation.
Heating dimensional change rate (%) = 100 × (L1-50) / 50
[0036]
The electret sheet of the present invention is obtained by laminating and integrating a plurality
of porous sheets, but the method of laminating and integrating a plurality of porous sheets is not
particularly limited. For example, (1) plural porous sheets A method of overlapping and heating
the sheets in the thickness direction and pressing the porous sheet in the thickness direction if
necessary to thermally fuse and integrate the porous sheets, (2) hot melt adhesion of the porous
sheets The method of carrying out adhesion | attachment integration with adhesives, such as an
agent, a solvent type adhesive agent, and a water-based adhesive agent, etc. are mentioned.
[0037]
An electret sheet is configured by injecting a charge into a laminated sheet formed by laminating
and integrating a plurality of porous sheets and holding the charge in a polarized state.
The method for injecting charges into the laminated sheet is not particularly limited. For
example, (1) the laminated sheet is sandwiched between a pair of flat plate electrodes, the first
flat plate electrode is grounded, and the second flat plate electrode is a high voltage DC power
supply (2) Ionizing radiation such as electron beam, X-ray, and ultraviolet light by charging the
laminated sheet by applying a direct current or pulsed high voltage to the laminated sheet to
inject charges into the synthetic resin. A method of injecting charges into the synthetic resin by
irradiating the laminated sheet and ionizing air molecules in the vicinity of the laminated sheet to
charge the laminated sheet, (3) the first surface of the laminated sheet is grounded A flat
electrode is stacked in close contact, and a needle electrode or a wire electrode electrically
connected to a DC high voltage power supply is disposed on the second surface of the laminated
04-05-2019
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sheet with a predetermined interval, and the tip of the needle electrode Or the surface of the wire
electrode A method of generating corona discharge by concentration of electric field to the side,
ionizing air molecules, repelling air ions generated by the polarity of needle electrode or wire
electrode, injecting electric charge into synthetic resin, and charging laminated sheet Etc. Among
the above methods, since the charge can be easily injected into the laminated sheet, the methods
(2) and (3) are preferable, and the method (2) is more preferable.
[0038]
In the methods (1) and (3), when the absolute value of the voltage applied to the laminated sheet
is small, electric charges can not be sufficiently injected into the laminated sheet, and an electret
sheet having high piezoelectricity can be obtained. If it is large, arc discharge may occur, and on
the contrary, electric charge may not be sufficiently injected into the laminated sheet, and it may
not be possible to obtain an electret sheet having high piezoelectricity. Is preferred, and more
preferably 5 to 50 kV.
[0039]
In the method (2), when the absolute value of the accelerating voltage of the ionizing radiation to
be applied to the laminated sheet is small, molecules in the air can not be sufficiently ionized, and
a sufficient charge is injected to the laminated sheet. In some cases, it is impossible to obtain an
electret sheet having high piezoelectricity. In many cases, since ionizing radiation transmits air, it
may not be possible to ionize molecules in the air. 15 kV is preferred.
[0040]
In the electret sheet of the present invention, as described above, the stretched porous sheets are
integrally laminated, excellent in mechanical strength such as tear resistance and toughness, and
shrinkage due to temperature change under use environment is substantially suppressed.
Therefore, it is excellent in cutting processability and can be suitably used for various
applications, and the shrinkage due to the temperature change of the use environment does not
reduce the surface property and the piezoelectricity is not reduced, Excellent piezoelectricity can
be stably maintained despite temperature changes in the usage environment.
[0041]
In the electret sheet, when the main stretching directions of the porous sheets adjacent to each
other in the stacking direction are orthogonal to each other, the shrinkage accompanying the
temperature change of the use environment can be suppressed more reliably, and the excellent
piezoelectricity is achieved. It can be held more stably.
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[0042]
It is the disassembled perspective view which showed typically the extending | stretching
direction of the porous sheet which is comprising the electret sheet | seat of this invention.
It is the top view which showed typically the extending | stretching direction of the porous sheet
when the electret sheet | seat of this invention is seen from a plane.
[0043]
Next, examples of the present invention will be described, but the present invention is not limited
to the following examples.
[0044]
Polypropylene resin A (trade name "Wintech # WFW4" manufactured by Japan Polypropylene
Corp.), polypropylene resin B (trade name "Novatec # EG8 B" manufactured by Japan
Polypropylene Corp.), azodicarbonamide, assistant amount shown in Table 1 Agent A (Kyoeisha
Chemical Co., Ltd. trade name “Light Ester TND-46K15”) and Auxiliary Agent B (ADEKA trade
name “Adekastab # SB-1002RG”) are supplied to an extruder and melt-kneaded to obtain a
sheet from T-die Was extruded to produce a foamable resin sheet having a thickness of 300 μm.
[0045]
The both sides of the foamable resin sheet were crosslinked by irradiating an electron beam at an
accelerating voltage of 300 V and an irradiation dose of 20 kGy.
The foamable resin sheet was heated at 250 ° C. for 2 minutes in a vertically suspended state
with its upper end supported and the foamable resin sheet was foamed to obtain a flat
rectangular foam sheet.
The foamed sheet was left to be annealed for 20 minutes so that the surface temperature was
135 ° C.
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The expansion ratio of the obtained foam sheet is shown in Table 1.
The expansion ratio of the foam sheet is a value obtained by dividing the density of the foamable
resin sheet by the density of the foam sheet.
The heating dimensional change rate after leaving at 120 ° C. for 24 hours in the foamed sheet
after the annealing treatment is shown in Table 1.
[0046]
Example 1 Two types of foam sheets F1 and F2 were produced according to the above-described
manner with the formulations shown in Table 1. The foamed sheets F1 and F2 are each stretched
at a stretching speed of 200 mm / min at 135 ° C. so that the stretching ratio in the width
direction (direction orthogonal to the extrusion direction) is 1.5 times, and then stretched in the
extrusion direction Biaxial stretching was performed in two steps by stretching at a stretching
speed of 200 mm / min at 135 ° C. so that the magnification was tripled.
[0047]
The biaxially stretched foam sheets F1 and F2 are heated to 135 ° C. to give 50 mm / 50 mm
while applying tension in the width direction and the extrusion direction so that no wrinkles are
generated in the biaxially stretched foam sheets F1 and F2. The foamed sheets F1 and F2 were
subjected to annealing treatment by shrinking at a rate of shrinkage of minutes. The heating
dimensional change rates in the main stretching direction after leaving for 24 hours at 120 ° C.
in the foam sheets F1 and F2 are shown in Table 1.
[0048]
One side of the foam sheets F1 and F2 is heated using a heating roller at 150 ° C., and the foam
sheets F1 and F2 are superimposed on each other with the heated faces facing each other to
thermally fuse the foam sheets F1 and F2 together. The lamination sheet was obtained. The foam
04-05-2019
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sheets F1 and F2 were superimposed such that the main stretching directions of the foam sheets
F1 and F2 were orthogonal to each other. In the laminated sheet, a portion where one foam sheet
protrudes from the other foam sheet was cut off.
[0049]
A grounded flat plate electrode is placed in close contact with one side of the resulting laminated
sheet, and a needle electrode electrically connected to a DC high-voltage power source is
provided on the other side of the laminated sheet at a predetermined interval. The corona
discharge is generated under the condition of voltage -10kV, discharge distance 10mm and
voltage application time 1 minute by concentration of electric field near the surface of needle
electrode to ionize air molecules, and needle electrode Air ions generated due to polarity were
repelled to inject charges into the laminated sheet to charge the laminated sheet. Thereafter, the
laminated sheet in which the charge was injected was held for 3 hours in a state of being
wrapped in a grounded aluminum foil to obtain an electret sheet.
[0050]
(Example 2) Two types of foam sheets F1 and F2 were manufactured in the manner described
above with the formulations shown in Table 1. After holding both ends in the width direction so
that the width dimensions of the foam sheets F1 and F2 do not change due to stretching, the
foam sheets F1 and F2 were stretched in the extrusion direction at a draw ratio of 6 times. The
draw ratio in the width direction (direction orthogonal to the extrusion direction) of the foam
sheets F1 and F2 was 1.25. The stretching direction of the foam sheets F1 and F2 coincided with
the extrusion direction. The biaxially stretched foam sheets F1 and F2 were subjected to an
annealing treatment in the same manner as in Example 1. The heating dimensional change rates
in the main stretching direction after leaving for 24 hours at 120 ° C. in the foam sheets F1 and
F2 are shown in Table 1. The foamed sheets F1 and F2 were laminated and integrated in the
same manner as in Example 1 to produce a laminated sheet. Electric charges were injected into
the laminated sheet in the same manner as in Example 1 to charge the laminated sheet to obtain
an electret sheet.
[0051]
Comparative Example 1 Two types of foamed sheets F1 and F2 were produced according to the
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15
above-described manner with the formulations shown in Table 1. The foamed sheets F1 and F2
are each stretched at a stretching speed of 200 mm / min at 135 ° C. so that the stretching ratio
in the width direction (direction orthogonal to the extrusion direction) is 1.5 times, and then
stretched in the extrusion direction Biaxial stretching was performed in two steps by stretching
at a stretching speed of 200 mm / min at 135 ° C. so that the magnification was tripled. No
annealing treatment was performed on the foam sheets F1 and F2. The heating dimensional
change rates in the main stretching direction after leaving for 24 hours at 120 ° C. in the foam
sheets F1 and F2 are shown in Table 1.
[0052]
A laminated sheet was obtained in the same manner as Example 1, except that the foam sheets
F1 and F2 were superimposed so that the main stretching directions of the foam sheets F1 and
F2 were the same. Electric charges were injected into the laminated sheet in the same manner as
in Example 1 to charge the laminated sheet to obtain an electret sheet.
[0053]
Comparative Example 2 Two types of foamed sheets F1 and F2 were produced according to the
above-described manner with the formulations shown in Table 1. After holding both ends in the
width direction so that the width dimensions of the foam sheets F1 and F2 do not change due to
stretching, the foam sheets F1 and F2 were stretched in the extrusion direction at a draw ratio of
6 times. The draw ratio in the width direction (direction orthogonal to the extrusion direction) of
the foam sheets F1 and F2 was 1.25. The stretching direction of the foam sheets F1 and F2
coincided with the extrusion direction. No annealing treatment was performed on the foam
sheets F1 and F2. The heating dimensional change rates in the main stretching direction after
leaving for 24 hours at 120 ° C. in the foam sheets F1 and F2 are shown in Table 1. A laminated
sheet was produced by laminating and integrating the foamed sheets F1 and F2 in the same
manner as in Example 1 except that the foamed sheets F1 and F2 were superimposed in a state
in which the main stretching directions of the foamed sheets F1 and F2 matched. . Electric
charges were injected into the laminated sheet in the same manner as in Example 1 to charge the
laminated sheet to obtain an electret sheet.
[0054]
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Comparative Example 3 Two types of foam sheets F1 and F2 were produced in the manner
described above with the formulations shown in Table 1. The foamed sheet F1 was uniaxially
stretched at a stretching ratio of 3 in the extrusion direction. After drawing the foamed sheet F2
at a drawing speed of 200 mm / min at 135 ° C. so that the draw ratio in the width direction
(direction orthogonal to the extrusion direction) is 1.5 times, the draw ratio is 3 times in the
extrusion direction Biaxial stretching was performed in two steps by stretching at 135 ° C. at a
stretching speed of 200 mm / min. No annealing treatment was performed on the foam sheets F1
and F2. The heating dimensional change rates in the main stretching direction after leaving for
24 hours at 120 ° C. in the foam sheets F1 and F2 are shown in Table 1. A laminated sheet was
manufactured by laminating and integrating in the same manner as in Example 1 except that the
foam sheets F1 and F2 were superimposed in a state in which the main stretching directions of
the foam sheets F1 and F2 matched. Electric charges were injected into the laminated sheet in
the same manner as in Example 1 to charge the laminated sheet to obtain an electret sheet.
[0055]
Comparative Example 4 Three types of foam sheets F1 to F3 were produced in the manner
described above with the formulations shown in Table 1. The foamed sheets F1 and F3 were
uniaxially stretched in the extrusion direction at a stretching ratio of 2 times. After drawing the
foamed sheet F2 at a drawing speed of 200 mm / min at 135 ° C. so that the draw ratio in the
width direction (direction orthogonal to the extrusion direction) is doubled, the draw ratio
becomes 4 times in the extrusion direction Biaxial stretching was performed in two steps by
stretching at 135 ° C. at a stretching speed of 200 mm / min. No annealing treatment was
performed on the foam sheets F1 to F3. The heating dimensional change rate in the main
stretching direction after leaving at 120 ° C. for 24 hours in the foam sheets F1 to F3 is shown
in Table 1.
[0056]
One side of each of the foam sheets F1 and F3 and both sides of the foam sheet F2 were heated
using 150 ° C. heating rolls. The foam sheets F1 to F3 were stacked in this order, and the foam
sheets F1 to F3 were heat-fused and integrated with each other to obtain a laminated sheet. The
foam sheets F1 to F3 were superimposed so that all the main stretching directions of the foam
sheets F1 to F3 coincide with each other. Electric charges were injected into the laminated sheet
in the same manner as in Example 1 to charge the laminated sheet to obtain an electret sheet.
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[0057]
The heating dimensional change rate after leaving at 120 ° C. for 24 hours in the obtained
electret sheet is measured in the manner described above, and the tearing strength and the rate
of change in piezoelectricity of the obtained electret sheet are measured in the following manner.
The results are shown in Table 1.
[0058]
(Tear strength) The tear strength of the entire electret sheet was measured according to JIS K
7128-1 (Trouser tear method).
Specifically, it was measured in accordance with JIS K 7128-1 (Trouser Tear Method). Two flat
rectangular test pieces each having a width of 50 mm and a length of 150 mm were cut out from
the electret sheet. The length direction of one of the two test pieces is adjusted to match the main
stretching direction of any one foam sheet constituting the electret sheet, and the length of the
other test piece is adjusted. The longitudinal direction and the longitudinal direction of one of the
test pieces were adjusted to make an angle of 90 ° C. with each other. Each test piece was cut
from the end face in the length direction in parallel in the length direction to a length of 75 mm
in the constant temperature chamber at 23 ° C. to form a divided piece over the entire length in
the thickness direction. Cutting of the test piece was performed at the central portion in the
width direction. The tear force (N) of each test piece was measured by pulling the divided pieces
of the test piece in the width direction of the test piece under the condition of a speed of 200 mm
/ min. Among the tearing forces of the two test pieces, the case where the smaller one exceeds
1.2 N is regarded as “o”, and the case less than 1.2 N is regarded as “x”.
[0059]
(Percent Change in Piezoelectricity) Gold was vapor-deposited on both sides of the electret sheet
to prepare a test body. A pressing force was applied to the test body under the conditions of a
load F of 2 N, a dynamic load of ± 0.25 N and a frequency of 110 Hz using a shaker, and the
charge Q (coulomb) generated at that time was measured. Initial charge d33 (Q / N) was
calculated by dividing charge Q (coulomb) by load F (N).
[0060]
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The specimens were then left at 80 ° C. for a week. The electric charge d33 (Q / N) generated
after heat treatment of the test body was calculated in the same manner as described above.
Based on the following equation, the rate of change in piezoelectricity (the rate of change in
piezoelectricity) was calculated. It evaluated based on the following criteria. Piezoelectric change
(%) = 100 × (initially generated charge−heat generated after charge) / initially generated
charge ◎... The change in piezoelectricity was less than 50%. ・ ・ ・: The change rate of
piezoelectricity was 50% or more and less than 80%. X: The rate of change in piezoelectricity was
80% or more.
[0061]
[0062]
F1, F2 porous sheet
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