TECHNICAL FIELD
The present invention relates to a piezoelectric
element for use in touch-type input devices and pointing
devices. More specifically, it relates to a piezoelectric
element capable of generating a sufficient electric output
10 as a touch sensor simply by rubbing the surface or a
piezoelectric element which functions as an actuator which
changes its shape according to an electric signal applied
thereto.
15 BACKGROUND ART
The number of so-called input devices employing touch
panel system, that is, touch-type input devices is now
significantly increasing. Along with the development of
thin display technology, the touch panel system as an input
20 interface is increasingly employed in not only bank ATM's
and ticket vending machines at stations but also mobile
phones, portable game machines and mobile music players.
In recent mobile phones and smart phones, system
capable of direct input into the screen by mounting a
25 touch-type input device on a display making use of liquid
crystals or organic electroluminescence is often employed.
In order to further improve the convenience of portable
devices such as smart phones which are being upgraded, it
is preferred that not only an input device should be mounted
30 on the screen but also a plurality of touch-type input means
should be made available.
For instance, in the case of a smart phone, to input
into the display screen with fingers, the smart phone must
be held by one hand and the fingers of the other hand must
2
be used for input. Therefore, the smart phone must be
operated with both hands. Meanwhile, if a touch sensor is
incorporated into the housing of the smart phone, the smart
phone can be operated with one hand.
5 As an example.of this, JP-A 2001-189792 (Patent
Document 1) discloses system for selecting an item or anchor
point out of screen information with a touch sensor
incorporated into the housing of a non-display screen part
such as the rear side of the display screen which is normally
10 not used as a sensor. Examples of the input device which
realizes the touch sensor of Patent Document 1 include those
employing capacitance system, resistance film system,
optical system, electromagnetic induction system and
piezoelectric sheet system.
15 Meanwhile, an example of the input device employing
piezoelectric sheet system is disclosed by JP-A 2011-253517
(Patent Document 2). Unlike touch sensors employing
capacitance system and resistance film system, a touch sensor
employing piezoelectric sheet system can detect both
20 pressure applied to the sensor and position information at
the same time by itself and can contribute to the
diversification of input information.
Patent Document 2 discloses an example of a
piezoelectric sheet member making use of polylactic acid
25 which is a piezoelectric polymer. As disclosed by Patent
Document 2, the piezoelectric sheet comprising polylactic
acid can be made flexible and is an excellent element capable
of detecting position information and stress at the same time
by itself. However, in order to obtain a sufficient electric
30 output, the piezoelectric sheet must be bent to some extent
with its stress at the time of input.
Although the piezoelectric sheet comprising
polylactic acid generates an electric output with shearing
stress applied to the sheet, a sufficient electric output
3
cannot be obtained with tension or compression. Therefore,
to obtain a large electric output, the sheet must be bent
with pressing force in a direction perpendicular to the plane
of the piezoelectric sheet.
5 For example, when it is considered that this
piezoelectric sheet is attached to the housing on the rear
side of a smart phone or integrated with the housing before
use, it is difficult to bend the sheet spatially with pushing
pressure applied to the sheet in the vertical direction, and
10 a piezoelectric element which generates a sufficient
electric output simply by rubbing the surface has been
desired. Since the surface of the housing of a smart phone
is not always flat and there are many 3-D irregularities in
shape to ensure its design, the piezoelectric element for
15 use in the smart phone has been desired to be flexible.
A piezoelectric fiber technology in which a
piezoelectric polymer is tv/isted and oriented is disclosed
by Japanese Patent No. 354028 (Patent Document 3). A
piezoelectric fiber disclosed by Patent Document 3 obtains
20 an electric output with the tension and compression of the
fiber by twisting the fiber by a special production method
in advance. However, Patent Document 3 is silent about a
technology for generating a sufficient electric output v/ith
shearing stress produced by rubbing the surface of the fiber
25 and extracting the electric output.
Therefore, it is extremely difficult to extract a
sufficient electric output only with relatively small
application stress produced by rubbing the surface with a
finger by incorporating this piezoelectric fiber element
30 into the housing of the above-mentioned smart phone.
In general, it is known that a polylactic acid fiber
which has been uniaxially stretched and oriented rarely
produces polarization by stretching in the stretching axis
and a direction perpendicular to the stretching axis and
4
compression stress with the result that an electric output
is hardly obtained with relatively small application stress
produced by rubbing the surface with a finger.
Meanwhile, it is known that polarization is produced
5 by applying force in a direction neither parallel nor
perpendicular to the stretching axis of the polylactic acid
piezoelectric fiber, that is, shearing stress so that the
polylactic acid piezoelectric fiber develops a function as
a piezoelectric body.
10 {Patent Document 1) JP-A 2001-189792
(Patent Document 2) JP-A 2011-253517
{Patent Document 3) Japanese Patent No. 354 0208
DISCLOSURE OF THE INVENTION
15 Problem to Be Solved by the Invention
It is an object of the invention to provide a fibrous
piezoelectric element which can extract an electric output
with relatively small application stress produced by rubbing
the surface with a finger.
20 Means for Solving the problems
The inventors of the present invention found that a
combination of two conductive fibers and one piezoelectric
fiber may function as a piezoelectric element and
accomplished the present invention.
25 That is, the present invention includes the following
inventions.
1. A piezoelectric element comprising a piezoelectric
unit including two conductive fibers and one piezoelectric
fiber all of which are arranged substantially on the same
30 plane while they have contact points between them.
2. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric unit includes a conductive fiber,
a piezoelectric fiber and a conductive fiber all of which
are arranged in this order.
5
3. The piezoelectric element in the above paragraph 2,
wherein the piezoelectric unit includes a conductive fiber,
a piezoelectric fiber and a conductive fiber all of which
are arranged substantially parallel to one another.
5 4. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric unit includes an insulating fiber
which is arranged such that the conductive fibers in the
piezoelectric unit are not in contact with conductive fibers
and a piezoelectric fiber in another piezoelectric unit.
10 5. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric fiber comprises polylactic acid
as the main component.
6. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric fiber comprises poly-L-lactic acid
15 or poly-D-lactic acid as the main component and the optical
purities of these components are 99 % or more.
7. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric fiber is uniaxially oriented and
contains a crystal.
20 8. The piezoelectric element in the above paragraph 1,
wherein the conductive fiber is a carbon fiber.
9. The piezoelectric element in the above paragraph 4,
wherein the insulating fiber comprises a polyethylene
terephthalate-based fiber as the main component.
25 10. The piezoelectric element in the above paragraph 3
which is a woven or knitted fabric comprising a plurality
of parallel piezoelectric units.
11. The piezoelectric element in the above paragraph 10
which is a woven fabric comprising a plurality of parallel
30 piezoelectric units and having a satin weave structure.
12. The piezoelectric element in the above paragraph 11,
v/herein the piezoelectric units are arranged in the weft
direction.
13. The piezoelectric element in the above paragraph 11,
6
wherein the step number of a piezoelectric fiber in the
piezoelectric unit is 3 to 7.
14 . A piezoelectric element which includes a conductive
fiber, a piezoelectric polymer covering the surface of the
5 fiber, and a surface conductive layer formed on the surface
of the piezoelectric polymer.
15. A piezoelectric element including at least two covered
fibers obtained by covering the surfaces of conductive fibers
with a piezoelectric polymer, wherein the covered fibers are
10 arranged substantially parallel to each other, and the
piezoelectric polymers on the surfaces are in contact with
each other.
16. The piezoelectric element in any one of the above
paragraphs 1 to 15 which is a sensor for detecting the size
15 of stress applied to the piezoelectric element and/or the
application position.
17. The piezoelectric element in the above paragraph 16,
wherein stress applied to the piezoelectric element to be
detected is rubbing force to the surface of the piezoelectric
20 element.
18. The piezoelectric element in any one of the above
paragraphs 1 to 15 which is an actuator that changes its shape
according to an electric signal applied to the piezoelectric
element.
25
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view of the piezoelectric element
of Example 1 which is an example of the constitution of the
piezoelectric element of the present invention;
30 Fig. 2 is a schematic view of an evaluation system for
the piezoelectric elements of Examples 1 and 7 and
Comparative Example 1;
Fig. 3 is a graph showing the piezoelectric
characteristics of the piezoelectric element of Example 1;
7
Fig. 4 is a schematic view of the piezoelectric element
of Example 2 which is an example of the constitution of the
piezoelectric element of the present invention;
Fig. 5 is a schematic view of an evaluation system for
5 the piezoelectric element of Example 2;
Fig. 6 is a graph showing the piezoelectric
characteristics of the piezoelectric element of Example 2;
Fig. 7 is a schematic view of the piezoelectric element
of Example 3 which is an example of the constitution of the
10 piezoelectric element of the present invention;
Fig. 8 is a graph showing the piezoelectric
characteristics (rubbing) of the piezoelectric element of
Example 3;
Fig. 9 is a graph showing the piezoelectric
15 characteristics (bending) of the piezoelectric element of
Example 3;
Fig. 10 is a schematic view of the piezoelectric element
of Example 4 which is an example of the constitution of the
piezoelectric element of the present invention;
20 Fig- 11 is a graph showing the piezoelectric
characteristics of the piezoelectric element of Example 4;
Fig. 12 is a schematic view of the piezoelectric element
of Example 5 which is an example of the constitution of the
piezoelectric element of the present invention;
25 Fig. 13 is a schematic view of the piezoelectric element
of Example 6 which is an example of the constitution of the
piezoelectric element of the present invention; and
Fig. 14 is a graph showing the piezoelectric
characteristics of the piezoelectric element of Example 6.
30
Effect of the Invention
The piezoelectric element of the present invention is
flexible and can extract an electric output simply by rubbing
the surface of the piezoelectric element with a finger.
8
The piezoelectric element of the present invention can
be advantageously used as a touch sensor. By incorporating
the piezoelectric element of the present invention into the
housing of a smart phone, the smart phone can be operated
5 with one hand. Since the piezoelectric element of the
present invention is in the form of a flexible fiber, it can
be woven or knitted to produce cloth, whereby a cloth touch
panel which can be folded like a handkerchief can be
materialized. Further, since the piezoelectric element of
10 the present invention can extract an electric output simply
by rubbing, it can be used in a micro-generator.
Further, since the piezoelectric element of the
present invention changes its shape when an electric signal
is applied thereto, it can be used as an actuator as well.
15 For example, by applying an electric signal to a cloth
piezoelectric element, an object mounted on the surface of
the cloth can be moved or wrapped. Also, an electric signal
to be applied to the piezoelectric element constituting cloth
can be controlled.
20
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention is attained by a piezoelectric
element comprising a piezoelectric unit including two
conductive fibers and one piezoelectric fiber all of which
25 are arranged substantially on the same plane while they have
contact points between them. The constitution of the
piezoelectric element will be described hereinbelow.
(conductive fiber)
30 The diameter of the conductive fiber is preferably 1
um to 10 mm, more preferably 10 urn to 5 mm, much more preferably
0.1 to 2 mm. When the diameter is small, strength degrades
and handling becomes difficult. When the diameter is large,
flexibility is sacrificed. The sectional shape of the
9
conductive fiber is preferably circular or elliptic from the
viewpoints of the design and production of the piezoelectric
element but not limited to these.
Any material may be used as the material of the
5 conductive fiber if it exhibits conductivity. A conductive
polymer is preferred as it needs to be formed fibrous. As
the conductive polymer may be used polyaniline,
polyacetylene, poly(p-phenylene vinylene), polypyrrole,
polythiophene, poly(p~phenylene sulfide) and carbon fiber.
10 A conductive polymer comprising a polymer as a matrix and
a fibrous or granular conductive filler may be used. From
the viewpoints of flexibility and the stability of electric
characteristics in the longitudinal direction, a carbon
fiber is preferred. To extract an electric output from the
15 piezoelectric polymer efficiently, electric resistance is
preferably low with a volume resistivity of preferably 10"1
£J*cm or less, more preferably 10 n*cm or less, much more
preferably 10~3 H'cm or less.
An ordinary carbon fiber is generally a multifilament
20 which is a bundle of filaments. This multifilament may be
used, or only one monofilament may be used. Use of a
multifilament is preferred from the viewpoint of the
stability of electric characteristics in the longitudinal
direction. The diameter of the monofilament is 1 to 5,000
25 um, preferably 2 to 100 \xmr more preferably 3 to 10 um. The
filament count is preferably 10 to 100,000, more preferably
100 to 50,000, much more preferably 500 to 30,000.
(piezoelectric fiber)
30 The piezoelectric fiber is a fiber having
piezoelectric properties. The piezoelectric fiber is
preferably composed of a piezoelectric polymer. Although
any polymer which exhibits piezoelectric properties, such
as vinylidene polyfluoride or polylactic acid, may be used
10
as the piezoelectric polymer, it preferably comprises
polylactic acid as the main component. Polylactic acid is
easily oriented by stretching after melt spinning to exhibit
piezoelectric properties and is excellent in productivity
5 as it does not require an electric field orientation
treatment which is required for vinylidene polyfluoride.
Further, since the piezoelectric fiber comprising polylactic
acid has small polarization with tension or compression
stress in the axial direction, it is difficult to make it
10 function as a piezoelectric element. However, this is
preferred for the piezoelectric element of the present
invention having a constituent body which readily applies
shearing stress to a piezoelectric polymer since it obtains
a relatively large electric output with shearing stress.
15 The piezoelectric polymer preferably comprises
polylactic acid as the main component. The expression "as
the main component" means that the content of polylactic acid
is preferably 90 mol% or more, more preferably 95 mol% or
more, much more preferably 98 mol% or more.
20 As the polylactic acid, there are poly-L-lactic acid
obtained by polymerizing L-lactic acid or L-lactide,
poly-D-lactic acid obtained by polymerizing D-lactic acid
or D-lactide and stereocomplex polylactic acid having the
hybrid structure of these according to the crystal structure.
2 5 Any polylactic acid is acceptable if it exhibits
piezoelectric properties. From the viewpoint of high
piezoelectricity, poly-L-lactic acid and poly-D-lactic acid
are preferred. Since the polarizations of poly-L-lactic
acid and poly-D-lactic acid are opposite to each other with
30 respect to the same stress, it is possible to use a combination
of these according to purpose. The optical purity of
polylactic acid is preferably 99 % or more, more preferably
99.3 % or more, much more preferably 99.5 % or more. When
the optical purity is lower than 99 %, piezoelectricity may
11
significantly drop, thereby making it difficult to obtain
a sufficient electric output with rubbing force to the
surface of the piezoelectric element. Preferably, the
piezoelectric polymer comprises poly-L-lactic acid or
5 poly-D-lactic acid as the main component, and the optical
purities of these components are 99 % or more.
Preferably, the piezoelectric polymer is uniaxially
oriented in the fiber axis direction of a covered fiber and
contains a crystal. More preferably, it is uniaxially
10 oriented polylactic acid having a crystal. This is because
polylactic acid exhibits great piezoelectric properties in
the crystalline state and the uniaxially oriented state.
Since polylactic acid is a polyester which is
hydrolyzed relatively quickly, when it has a problem with
15 moist heat resistance, a known hydrolysis inhibitor such as
isocyanate compound, oxazolinc compound, epoxy compound or
carbodiimide compound may be added. An antioxidant such as
a phosphoric acid-based compound, plasticizer and optical
deterioration inhibitor may be added as required to improve
20 physical properties.
Further, polylactic acid may be used as an alloy with
another polymer. When polylactic acid is used as the main
piezoelectric polymer, it is contained in an amount of
preferably at least 50 wt% or more, more preferably 70 wt%
25 or more, most preferably 90 wt% or more based on the total
weight of the alloy.
In the case of a polylactic acid alloy, preferred
examples of a polymer other than polylactic acid include
polybutylene terephthalate, polyethylene terephthalate,
30 polyethylene naphthalate copolymers and polymethacrylate.
However, the polymer is not limited to these and any polymer
may be used as lonq as a piezoelectric effect which is the
object of the present invention is obtained.
The piezoelectric fiber is generally a multifilament
12
which is a bundle of filaments. This multifilament may be
used, or only one monofilament may be used. Use of the
multifilament is preferred from the viewpoint of the
stability of piezoelectric characteristics in the
5 longitudinal direction. The diameter of the monofilament
is 1 to 5, 000 Jim, preferably 5 to 500 \xva. It is more preferably
10 to 100 |im. The filament count is preferably 1 to 100,000,
more preferably 10 to 50,000, much more preferably 100 to
10,000.
10 In order to produce a piezoelectric fiber from this
piezoelectric polymer, any known technique for fiberizing
a polymer may be employed as long as the effect of the present
invention is obtained. Examples of the technique include
one in which a piezoelectric polymer is extrusion molded to
15 be fiberized, one in which a piezoelectric polymer is melt
spun to be fiberized, one in which a piezoelectric polymer
is fiberized by dry or wet spinning, and one in which a
piezoelectric polymer is fiberized by electrostatic spinning.
As for these spinning conditions, a known technique may be
20 used according to the piezoelectric polymer in use, and a
melt spinning technique which facilitates industrial-scale
production may be generally employed.
As described above, when the piezoelectric polymer is
polylactic acid, it exhibits great piezoelectric properties
25 if it is uniaxially oriented and contains a crystal.
Therefore, its fiber is preferably stretched.
(contact points)
Two conductive fibers and one piezoelectric fiber need
30 to have contact points between them. These fibers may have
contact points between them in any manner as long as these
fibers are in contact with each other. For example, two
conductive fibers are arranged parallel to each other and
one piezoelectric fiber intersects with the two conductive
13
fibers . Further, two conductive fibers are arranged as warps
(or wefts) and one piezoelectric fiber is arranged as a weft
(or a warp) . In this case, the two conductive fibers are
preferably not in contact with each other and an insulating
5 material, for example, a polyester fiber having insulating
properties is interposed between the two conductive fibers,
or only the easy contact surfaces of the conductive fibers
are covered with an insulating material and the conductive
fibers are in direct contact with the piezoelectric fiber.
10
(substantially on the same plane)
In the piezoelectric element of the present invention,
two conductive fibers and one piezoelectric fiber are
arranged substantially on the same plane. The expression
15 "substantially on the same plane" means that the fiber axes
of the three fibers are arranged substantially on a flat
surface. The word "substantially'' means that this includes
a case where the intersections between the fibers become
thick.
20 For example, when one piezoelectric fiber is arranged
parallel to two parallel conductive fibers between the
conductive fibers, they have contact points between them and
are existent substantially on the same plane. Even when the
fiber axis of one piezoelectric fiber is inclined so that
25 it is not parallel to two parallel conductive fibers, they
are substantially on the same plane. Further, even when one
conductive fiber and one piezoelectric fiber are arranged
parallel to each other and the other conductive fiber is
arranged to intersect with the conductive fiber and the
30 piezoelectric fiber, they are substantially on the same
plane.
When they are not "substantially on the same plane",
two conductive fibers have contact points at a position away
from the surface of one piezoelectric fiber (excluding
14
contact with the point symmetrical parts of the fiber axis
of the piezoelectric fiber which is aligned and contacted)
and the two conductive fibers do not intersect with each
other.
5 When they are arranged substantially on the same plane,
a fibrous or cloth piezoelectric element is easily formed
by combining the piezoelectric units, and the degree of
freedom in the shape design of a stress sensor or an actuator
can be increased by using the fibrous or cloth piezoelectric
10 element.
(arrangement order)
In the piezoelectric unit, preferably, a conductive
fiber, a piezoelectric fiber and a conductive fiber are
15 arranged in this order. When they are arranged in this order,
the two conductive fibers of the piezoelectric unit are not
in contact with each other, thereby making it possible for
the piezoelectric unit to function effectively without using
a technique for covering the conductive fibers with another
20 means, for example, an insulating material. In the
piezoelectric unit, preferably, the conductive fiber, the
piezoelectric fiber and the conductive fiber are arranged
substantially parallel to one another.
25 (insulating fiber)
The piezoelectric unit of the present invention
includes an insulating fiber which is preferably arranged
such that the conductive fibers of this piezoelectric unit
are not in contact with the conductive fibers and
30 piezoelectric fiber of another piezoelectric unit. Since
the arrangement order of the present invention is generally
[conductive fiber/piezoelectric fiber/conductive fiber],
the insulating fiber is arranged in the order of [insulating
fiber/conductive fiber/piezoelectric fiber/conductive
15
fiber] or [insulating fiber/conductive fiber/piezoelectric
fiber/conductive fiber/insulating fiber].
Even when a plurality of piezoelectric units are used
in combination by arranging the insulating fiber in the
5 piezoelectric units as described above, it is possible to
improve the performance (detection resolution of a detection
sensor, small shape change of an actuator) of the
piezoelectric element without contact between the conductive
fibers.
10 This insulating fiber should have a volume resistivity
of 106 Q-cm or more, preferably 108 Q-cm or more, more
preferably 1010 £i*cm or more.
Examples of the insulating fiber include polyester
fibers, nylon fibers, acrylic fibers, polyethylene fibers,
15 polypropylene fibers, vinyl chloride fibers, aramid fibers,
polysulfone fibers, polyether fibers and polyurethane fibers,
natural fibers such as silk, semi-synthetic fibers such as
acetate fibers and regenerated fibers such as rayon and cupra.
The insulating fiber is not limited to these and any known
20 insulating fiber may be used. Further, these insulating
fibers may be used in combination, and a combination of an
insulating fiber and a fiber having no insulating properties
may be used as a fiber having insulating properties as a whole.
In consideration of production ease, handling ease and
25 strength, the insulating fiber preferably contains a
polyethylene terephthalate-based fiber as the main component.
The expression "as the main component" means that the fiber
is contained in an amount of more than 50 %, preferably 75 %
or more, more preferably 90 % or more, particularly
30 preferably 99% or more, most preferably 100 % based on the
insulating fiber. The expression vvpolyethylene
terephthalate-based" means that polyethylene terephthalate
is contained in the fiber in an amount of more than 50 %,
preferably 75 % or more, more preferably 90 % or more,
16
particularly preferably 99 % or more, most preferably 100 %
based on the component constituting the fiber.
(combination of piezoelectric units)
5 In the present invention, a woven or knitted fabric
comprising a plurality of parallel piezoelectric units is
preferred. Because of this, it is possible to improve the
degree of freedom in the shape change (flexibility) of the
piezoelectric element.
10 There is no limitation to the shape of this woven or
knitted fabric as long as it comprises a plurality of parallel
piezoelectric units and exhibits the function of a
piezoelectric element. To obtain a woven or knitted form,
it may be woven by using an ordinary loom or knitted by using
15 a knitting machine.
Examples of the weave structure of the woven fabric
include three foundation weaves which are plain weave, twill
weave and satin weave, derivative weave, single double weaves
such as warp-backed weave and weft-backed weave, and warp
20 velvet.
As for the type of the knitted fabric, the knitted
fabric may be a circular knitted fabric (weft knitted fabric)
or warp knitted fabric. Preferred examples of the structure
of the circular knitted fabric (weft knitted fabric) include
25 plain stitch, rib stitch, interlock stitch, pearl stitch,
tuck stitch, float stitch, single rib stitch, lace stitch
and plating stitch. Examples of the structure of the warp
knitted fabric include single Denbigh stitch, single atlas
stitch, double cord stitch, half-tricot stitch, fleeced
30 stitch and jacquard stitch. The number of layers may be one,
or two or more. Further, a napped woven fabric or napped
knitted fabric comprising a napped part composed of cut piles
and/or loop piles and a ground structure part may also be
used.
17
Although a bent part is existent in the piezoelectric
fiber itself when the piezoelectric units are incorporated
in a weave structure or knit structure, to develop the
piezoelectric performance of the piezoelectric element
5 efficiently, the bent part of the piezoelectric fiber is
preferably small. Therefore, a woven fabric is more
preferred than a knitted fabric.
From the viewpoint of balance among the strength,
handling ease and production ease of the woven fabric, the
10 piezoelectric units are arranged in the weft direction.
Another fiber, for example, a polyethylene
terephthalate-based fiber which is an insulating fiber is
preferably arranged in the warp direction.
Even in this case, as described above, piezoelectric
15 performance is developed efficiently when the bent part of
the piezoelectric fiber is small. Therefore, as a weave
structure, twill weave is more preferred than plain weave,
and satin weave is more preferred than twill weave. When
satin weave has a step number of 3 to 7, the weave structure
20 can be kept and a high level of piezoelectric performance
can be obtained advantageously.
Further, since the piezoelectric fiber tends to be
electrified, an erroneous operation is apt to occur. In this
case, the piezoelectric fiber which is to extract a signal
25 may be earthed before use. As an earthing method, another
conductive fiber is preferably arranged in addition to the
conductive fiber for extracting a signal. In this case, the
volume resistivity of the conductive fiber is preferably 10"1
Q'cm or less, more preferably 10~2 £2*cm or less, much more
30 preferably 10~3 H'cm or less.
(another embodiment 1 of piezoelectric element)
The piezoelectric element of the present invention
includes the following piezoelectric element as another
18
embodiment.
1. A piezoelectric element including a conductive fiber,
a piezoelectric polymer which covers the surface of the
conductive fiber and a surface conductive layer formed on
5 the surface of the piezoelectric polymer.
2. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric polymer comprises polylactic acid
as the main component.
3. The piezoelectric element in the above paragraph 1 or
10 2, wherein the piezoelectric polymer comprises poly-L-lactic
acid or poly-D-lactic acid as the main component, and the
optical purities of these components are 99 % or more.
4. The piezoelectric element in the above paragraph 2 or
3, wherein the piezoelectric polymer is uniaxially oriented
15 and contains a crystal.
5. The piezoelectric element in any one of the above
paragraphs 1 to 4, wherein the conductive fiber is a carbon
fiber.
6. The piezoelectric element in any one of the above
20 paragraphs 1 to 5 which is a sensor for detecting stress
applied to the piezoelectric element and/or the application
position of stress.
7 . The piezoelectric element in the above paragraph 6,
wherein stress applied to the piezoelectric element to be
25 detected is rubbing force to the surface of the piezoelectric
element.
(conductive fiber)
The diameter of the conductive fiber is preferably 1
30 urn to 10 mm, more preferably 10 urn to 5 mm, much more'preferably
0.1 to 2 mm. When the diameter is small, strength degrades
and handing becomes difficult. When the diameter is large,
flexibility is sacrificed. The sectional shape of the
conductive fiber is preferably circular or elliptic from the
19
viewpoint of the design and production of the piezoelectric
element. However, the sectional shape is not limited to
these. Although the piezoelectric polymer and the
conductive fiber are preferably adhered to each other as
5 tightly as possible, an anchor layer or an adhesive layer
may be formed between the conductive fiber and the
piezoelectric polymer to improve adhesion between them.
Any material may be used as the material of the
conductive fiber if it exhibits conductivity. A conductive
10 polymer is preferred as it needs to be formed fibrous. As
the conductive polymer may be used polyaniline,
polyacetylene, poly(p-phenylene vinylene), polypyrrole,
polythiophene, poly(p-phenylene sulfide) and carbon fiber.
A conductive polymer comprising a polymer as a matrix and
15 a fibrous or granular conductive filler may be used. From
the viewpoints of flexibility and the stability of electric
characteristics in the longitudinal direction, a carbon
fiber is preferred.
To extract an electric output from the piezoelectric
20 polymer efficiently, electric resistivity is preferably low
with a volume resistivity of preferably 10"1 n*cm or less,
more preferably 10-2 n*cm or less, much more preferably 10~3
Q*cm or less.
An ordinary carbon fiber is generally a multifilament
25 which is a bundle of filaments.' This multifilament may be
used, or only one monofilament may be used. Use of a
multifilament is preferred from the viewpoint of the
stability of electric characteristics in the longitudinal
direction. The diameter of the monofilament is 1 to 5,000
30 pirn, preferably 2 to 100 j.un, more preferably 3 to 10 (jm. The
filament count is preferably 10 to 100,000, more preferably
100 to 50,000, much more preferably 500 to 30,000.
(piezoelectric polymer)
20
The thickness of the piezoelectric polymer covering
the conductive fiber is preferably 1 urn to 5 mm, more
preferably 5 um to 3 mm, much more preferably 10 \xm to 1 mm,
most preferably 20 um to 0.5 mm. When the thickness is too
5 small, a strength problem may occur, and when the thickness
is too large, it may be difficult to extract an electric
output.
As for the covering state of the conductive fiber with
this piezoelectric polymer, the conductive fiber and a fiber
10 composed of the piezoelectric polymer are preferably as
concentric as possible in order to keep a constant distance
between the conductive fiber and the surface conductive layer.
Although the method of forming the conductive fiber and the
fiber composed of the piezoelectric polymer is not
15 particularly limited, there is one in which the conductive
fiber on the inner side and the piezoelectric polymer on the
outer side are co-extruded, melt spun and stretched. When
the conductive fiber is a carbon fiber, a method in which
the outer surface of the conductive fiber is covered with
20 the piezoelectric polymer which has been melt extruded and
stretching stress is applied to stretch and orient the
piezoelectric polymer at the time of covering may be employed.
Further, a method in which a fiber composed of a hollow
stretched piezoelectric polymer is prepared and the
25 conductive fiber is inserted into the fiber may also be used.
Moreover, a method in which the conductive fiber and
a fiber composed of a stretched piezoelectric polymer are
formed by separate steps and the fiber composed of a
piezoelectric polymer is wound round the conductive fiber
30 may be employed as well.
In this case, the conductive fiber is preferably
covered with the above fiber to ensure that these fibers are
arranged as concentrically as possible. For example, a
method in which the conductive fiber on the inner side, the
21
piezoelectric polymer and the surface conductive layer are
co-extruded, melt spun and stretched may be employed to form
three layers at a time.
When the conductive fiber and the fiber composed of
5 a stretched piezoelectric polymer are formed by separate
steps and polylactic acid is used as the piezoelectric
polymer/ as preferred spinning and stretching conditions,
the melt spinning temperature is preferably 150 to 250°C,
the stretching temperature is preferably 40 to 150°C, the
10 draw ratio is preferably 1.1 to 5.0 times, and the
crystallization temperature is preferably 80 to 170°C.
Although any polymer which exhibits piezoelectric
properties, such as vinylidene polyfluoride or polylactic
acid, may be used as the piezoelectric polymer, it preferably
15 comprises polylactic acid as the main component. Polylactic
acid is easily oriented by stretching after melt spinning
to exhibit piezoelectric properties and is excellent in
productivity as it does not require an electric field
orientation treatment which is required for vinylidene
20 polyfluoride. Further, since the piezoelectric fiber
comprising polylactic acid has small polarization with
tension or compression stress in the axial direction, it is
difficult to make it function as a piezoelectric element.
However, this is preferred for the piezoelectric element of
25 the present invention having a constituent body which readily
applies shearing stress to a piezoelectric polymer since it
obtains a relatively large electric output with shearing
stress.
When the piezoelectric polymer fiber is wound round
30 the conductive fiber to cover it, a multifilament v/hich is
a bundle of filaments or a monofilament may be used as the
piezoelectric polymer fiber.
To wind the piezoelectric polymer fiber round the
conductive fiber, for example, the piezoelectric polymer
22
fiber is formed into a braided tube and the conductive fiber
as a core is inserted into the tube to be covered, or when
the piezoelectric polymer fiber is to be braided to produce
a braided cord, a braided cord which includes the conductive
5 fiber as core yarn and the piezoelectric polymer fiber
arranged around the core yarn is produced to cover the
conductive fiber.
The single filament diameter is 1 urn to 5 mm, preferably
5 um to 2 mm, more preferably 10 urn to 1 mm. The filament
10 count is preferably 1 to 100, 000, more preferably 50 to 50, 000,
much more preferably 100 to 20,000.
The piezoelectric polymer preferably comprises
polylactic acid as the main component. The expression "as
the main component" means that the content of polylactic acid
15 is preferably 90 mol% or more, more preferably 95 mol% or
more, much more preferably 98 mol% or more.
When a multifilament is used as the conductive fiber,
the piezoelectric polymer may cover the multifilament in such
a manner that it is in contact with at least part of the surface
20 {fiber outer surface) of the multifilament, and may or may
not cover the surfaces (fiber outer surfaces) of all the
filaments constituting the multifilament. The covering
state of each inside filament constituting the multifilament
is suitably set in consideration of the performance and
25 handling ease of the piezoelectric element.
As the polylactic acid, there are poly-L-lactic acid
obtained by polymerizing L-lactic acid or L-lactide,
poly-D-lactic acid obtained by polymerizing D-lactic acid
or D-lactide and stereocomplex polylactic acid having the
30 hybrid structure of these according to the crystal structure.
Any polylactic acid is acceptable if it exhibits
piezoelectric properties. From the viewpoint of high
piezoelectricity, poly-L-lactic acid and poly-D-lactic acid
are preferred. Since the polarizations of poly-L-lactic
23
acid and poly-D-lactic acid are opposite to each other with
respect to the same stress, it is possible to use a combination
of these according to purpose. The optical purity of
polylactic acid is preferably 99 % or more, more preferably
5 99.3 % or more, much more preferably 99.5 % or more. When
the optical purity is lower than 99 %, piezoelectricity may
significantly drop, thereby making it difficult to obtain
a sufficient electric output by rubbing force to the surface
of the piezoelectric element.
10 Preferably, the piezoelectric polymer comprises
poly-L-lactic acid or poly-D-lactic acid as the main
component, and the optical purities of these components are
99 % or more.
Preferably, the piezoelectric polymer is uniaxially
15 oriented and contains a crystal. More preferably, it is
uniaxially oriented polylactic acid having a crystal. This
is because polylactic acid exhibits great piezoelectric
properties in the crystalline state and the uniaxially
oriented state.
20 Since polylactic acid is a polyester which is
relatively quickly hydrolyzed, when it has a problem with
moist heat resistance, a known hydrolysis inhibitor such as
an isocyanate, epoxy or carbodiimide compound may be added.
An antioxidant such as a phosphoric acid-based compound,
25 plasticizer and optical deterioration inhibitor may be added
as required to improve physical properties. Further,
polylactic acid may be used as an alloy with another polymer.
When polylactic acid is used as the main piezoelectric
polymer, it is contained in an amount of preferably at least
30 50 wt% or more, more preferably 70 wt% or more, most preferably
90 wt% or more.
In the case of a polylactic acid alloy, preferred
examples of a polymer other than polylactic acid include
polybutylene terephthalate, polyethylene terephthalate,
24
polyethylene naphthalate copolymers and polymethacrylate.
However, the polymer is not limited to these and any polymer
may be used as long as the effect of the present invention
is obtained.
5
(surface conductive layer)
Any material may be used as the material of the surface
conductive layer if it exhibits conductivity. Examples of
the material include coats of paste containing a metal such
10 as silver or copper, vapor-deposited films of silver, copper
and indiumtin oxide, and conductive polymers such as
polyaniline, polyacetylene, poly(p-phenylene vinylene),
polypyrrole, polythiophene, poly(p-phenylene sulfide) and
carbon fiber. To keep high conductivity, the volume
15 resistivity is preferably 10_1 Q"cm or less, more preferably
10"2 Q*cm or less, much more preferably 10~3 Q*cm or less.
The thickness of this surface conductive layer is
preferably 10 nm to 100 (am, more preferably 20 nm to 10 urn,
much more preferably 30 nm to 3 urn. When the thickness is
20 too small, conductivity degrades and an electric output may
be hardly obtained and when the thickness is too large,
flexibility may be lost.
The surface conductive layer may be formed on the entire
surface of the piezoelectric polymer or discretely. Since
25 this arrangement method may be designed according to purpose,
this arrangement is not particularly limited. By arranging
this surface conductive layer discretely and extracting an
electric output from the discrete surface conductive layers,
the strength and position of stress applied to the
30 piezoelectric element can be detected.
In order to protect the surface conductive layer, that
is, prevent the surface conductive layer which is the
outermost layer from contact with a human hand, some
protective layer may be formed. This protective layer is
25
preferably insulating, more preferably made of a polymer from
the viewpoint of flexibility. As a matter of course, the
protective layer is rubbed in this case and is not
particularly limited if shearing stress reaches the
5 piezoelectric polymer by this rubbing and can induce its
polarization. The protective layer is not limited to a
protective layer which is formed by coating a polymer but
may be a film or a combination of films. An epoxy resin and
an acrylic resin are preferably used for the protective
10 layer.
The thickness of the protective layer should be as small
as possible since shearing force can be easily transmitted
to the piezoelectric polymer. However, when the thickness
is too small, a problem such as destruction tends to occur.
15 Therefore, it is preferably 10 nm to 200 Jim, more preferably
50 nm to 50 urn, much more preferably 70 nm to 30 \xm, most
preferably 100 nm to 10 urn.
Although there is a case where only one piezoelectric
element is used, a plurality of piezoelectric elements may
20 be used in combination, woven or knitted into cloth, or
braided. Thereby, a cloth or braided piezoelectric element
can be obtained. To produce a cloth or braided piezoelectric
element, as long as the object of the present invention is
attained, a fiber other than the piezoelectric element may
25 be used in combination to carry out mixing, interweaving or
interknitting, or incorporated into the resin of the housing
of a smart phone.
(another embodiment 2 of piezoelectric element)
30 The piezoelectric element of the present invention
includes the following piezoelectric element as another
embodiment.
1. A piezoelectric element includes at least two covered
fibers which are prepared by covering the surfaces of
26
conductive fibers with a piezoelectric polymer and are
arranged substantially parallel to each other, wherein the
piezoelectric polymers on the surfaces are in contact with
each other.
5 2. The piezoelectric element in the above paragraph 1,
wherein the piezoelectric polymer comprises polylactic acid
as the main component.
3. The piezoelectric element in the above paragraph 1 or
2, wherein the piezoelectric polymer comprises poly-L-lactic
10 acid or poly-D-lactic acid as the main component and the
optical purities of these components are 99 % or more.
4. The piezoelectric element in any one of the above
paragraphs 1 to 3, wherein the piezoelectric polymer is
uniaxially oriented and contains a crystal.
15 5. The piezoelectric element in any one of the above
paragraphs 1 to 4, wherein the conductive fiber is a carbon
fiber.
6. The piezoelectric element in any one of the above
paragraphs 1 to 5 which is a sensor for detecting the size
20 of stress applied to the piezoelectric element and/or the
application position.
7. The piezoelectric element in the above paragraph 6,
wherein stress applied to the piezoelectric element to be
detected is rubbing force to the surface of the piezoelectric
25 element.
(covered fiber)
The piezoelectric element of the present invention
includes at least two covered fibers prepared by covering
30 the surfaces of conductive fibers v/ith a piezoelectric
polymer.
Fig. 4 is a schematic view showing one embodiment of
the piezoelectric element of the present invention. In Fig.
4, reference numeral 1 denotes the piezoelectric polymer and
27
2 the conductive fiber.
Although the length of the piezoelectric element is
not particularly limited, the piezoelectric element is
produced continuously and then may be cut to a desired length
5 before use. For the actual use of the piezoelectric element,
the length is 1 mm to 10 m, preferably 5 mm to 2 m, more
preferably 1 cm to 1 m. When the length is small, convenience
that the piezoelectric element has a fibrous shape is lost
and when the length is large, there occurs a problem such
10 as a drop in electric output due to the resistance value of
the conductive fiber.
{conductive fiber)
Any material may be used as the material of the
15 conductive fiber if it exhibits conductivity. A conductive
polymer is preferred as it needs to be formed fibrous. As
the conductive polymer may be used polyaniline,
polyacetylene, poly(p-phenylene vinylene), polypyrrole,
polythiophene, poly(p-phenylene sulfide) and carbon fiber.
20 A conductive polymer comprising a polymer as a matrix and
a fibrous or granular conductive filler may be used. From
the viewpoints of flexibility and the stability of electric
characteristics in the longitudinal direction, a carbon
fiber is preferred.
25 To extract an electric output from the piezoelectric
polymer efficiently, electric resistance is preferably low
with a volume resistivity of preferably 10"1 Q*cm or less,
more preferably 10"2 n*cm or less, much more preferably 10"3
H'cm or less.
30 The diameter of the conductive fiber is preferably 1
urn to 10 mm, more preferably 10 \xm to 5 mm, much more preferably
0.1 to 2 mm. When the diameter is small, strength degrades
and handling becomes difficult. When the diameter is large,
flexibility is sacrificed.
28
The sectional shape of the conductive fiber is
preferably circular or elliptic from the viewpoints of the
design and production of the piezoelectric element but not
limited to these. As a matter of course, only one conductive
5 fiber may be used, or a bundle of conductive fibers may be
used.
An ordinary carbon fiber is generally a multifilament
which is a bundle of filaments. This multifilament may be
used, or only one monofilament may be used. Use of a
10 multifilament is preferred from the viewpoint of the
stability of electric characteristics in the longitudinal
direction.
The diameter of the monofilament is 1 to 5,000 urn,
preferably 2 to 100 urn, more preferably 3 to 10 urn. The
15 filament count is preferably 10 to 100,000, more preferably
100 to 50,000, much more preferably 500 to 30,000.
(piezoelectric polymer)
Although a polymer which exhibits piezoelectric
2 0 properties such as vinylidene polyfluoride or polylactic
acid may be used as the piezoelectric polymer, it preferably
comprises polylactic acid as the main component. Polylactic
acid is easily oriented by stretching after melt spinning
to exhibit piezoelectric properties and is excellent in
25 productivity as it does not require an electric field
orientation treatment which is required for vinylidene
polyfluoride. Further, since the piezoelectric fiber
comprising polylactic acid has small polarization with
tension or compression stress in the axial direction, it is
30 difficult to make it function as a piezoelectric element.
However, this is preferred for the piezoelectric element of
the present invention having a constituent body which readily
applies shearing stress to a piezoelectric polymer since it
obtains a relatively large electric output with shearing
29
stress.
The piezoelectric polymer preferably comprises
polylactic acid as the main component. The expression "as
the main component" means that the content of polylactic acid
5 is preferably 90 mol% or more, more preferably 95 mol% or
more, much more preferably 98 mol% or more.
As the polylactic acid, there are poly-L-lactic acid
obtained by polymerizing L-lactic acid or L-lactide,
poly-D-lactic acid obtained by polymerizing D-lactic acid
10 or D-lactide, and stereocomplex polylactic having a hybrid
structure of these according to the crystal structure. Any
polylactic acid is acceptable if it exhibits piezoelectric
properties. From the viewpoint of high piezoelectricity,
poly-L-lactic acid and poly-D-lactic acid are preferred.
15 Since the polarizations of poly-L-lactic acid and
poly-D-lactic acid are opposite to each other with respect
to the same stress, it is possible to use a combination of
these according to purpose. The optical purity of polylactic
acid is preferably 99 % or more, more preferably 99.3 % or
20 more, much more preferably 99.5 % or more. When the optical
purity is lower than 99 %, piezoelectricity may significantly
drop, thereby making it difficult to obtain a sufficient
electric output with rubbing force to the surface of the
piezoelectric element. Preferably, the piezoelectric
25 polymer comprises poly-L-lactic acid or poly-D-lactic acid
as the main component and the optical purities of these
components are 99 % or more.
Preferably, the piezoelectric polymer is uniaxially
oriented in the fiber axis direction of the covered fiber
30 and contains a crystal. More preferably, it is uniaxially
oriented polylactic acid having a crystal. This is because
polylactic acid exhibits great piezoelectric properties in
the crystalline state and the uniaxially oriented state.
Since polylactic acid is a polyester which is
30
relatively quickly hydrolyzed, when it has a problem with
moist heat resistance, a known hydrolysis inhibitor such as
isocyanate compound, oxazoline compound, epoxy compound or
carbodiimide compound may be added. An antioxidant such as
5 a phosphoric acid-based compound, plasticizer and optical
deterioration inhibitor may be added as required to improve
physical properties.
Further, polylactic acid may be used as an alloy with
another polymer. When polylactic acid is used as the main
10 piezoelectric polymer, it is contained in an amount of
preferably at least 50 wt% or more, more preferably 70 wt%
or more, most preferably 90 wt% or more.
In the case of a polylactic acid alloy, preferred
examples of a polymer other than polylactic acid include
15 polybutylene terephthalate, polyethylene terephthalate,
polyethylene naphthalate copolymers and polymethacrylate.
However, the polymer is not limited to these, and any polymer
may be used as long as the effect of the present invention
is obtained.
20
(covering)
The surface of each conductive fiber is covered with
the piezoelectric polymer. The thickness of the
piezoelectric polymer covering the conductive fiber is
25 preferably 1 um to 10 mm, more preferably 5 f.irn to 5 mm, much
more preferably 10 (.im to 3 mm, most preferably 20 um to 1
mm. When the thickness is too small, a strength problem may
occur, and when the thickness is too large, it may be difficult
to extract an electric output.
30 Although the piezoelectric polymer and the conductive
fiber are preferably adhered to each other as tightly as
possible, an anchor layer or an adhesive layer may be formed
between the conductive fiber and the piezoelectric polymer
to improve adhesion between them.
31
The covering method and the shape are not particularly
limited as long as an electric output generated by
application stress can be extracted.
For example, like the manufacture of an electric wire,
5 the conductive fiber is covered with the molten piezoelectric
polymer, piezoelectric polymer yarn is wound round the
conductive fiber, or the conductive fiber is sandwiched
between piezoelectric polymer films to be bonded. Three or
more conductive fibers may be prepared when the conductive
10 fibers are to be covered with the piezoelectric polymer as
described above, or after only one conductive fiber is
covered with the piezoelectric polymer, the surface of the
piezoelectric polymer is bonded, thereby making it possible
to obtain the piezoelectric element of the present invention.
15 The adhesion method is not particularly limited but use of
an adhesive or welding may be employed. The conductive fiber
and the piezoelectric polymer may be merely adhered to each
other.
As for the covering state of' the conductive fiber with
20 the piezoelectric polymer, although the shapes of the
conductive fiber and the piezoelectric polymer are not
particularly limited, for example, to obtain the
piezoelectric element of the present invention by bonding
a fiber prepared by covering one conductive fiber with the
25 piezoelectric polymer afterward, it is preferred that they
should be arranged as concentrically as possible in order
to keep a constant distance between conductive fibers.
When a multifilament is used as the conductive fiber,
the piezoelectric polymer may cover the multifilament in such
30 a manner that it is in contact with at least part of the surface
(fiber outer surface) of the multifilament, and may or may
not cover the surfaces {fiber outer surfaces) of all the
filaments constituting the multifilament. The covering
state of each inside filament constituting the multifilament
32
is suitably set in consideration of the performance and
handling ease of the piezoelectric element.
The piezoelectric element of the present invention
includes at least two conductive fibers, and the number of
5 conductive fibers is not limited to two and may be more.
(parallelism)
The conductive fibers are arranged substantially
parallel to each other. The distance between the conductive
10 fibers is preferably 1 um to 10 mm, more preferably 5 am to
5 mm, much more preferably 10 urn to 3 mm, most preferably
20 um to 1 mm. When the distance is too small, a strength
problem may occur and when the distance is too large, it may
be difficult to extract an electric output. The expression
15 "substantially parallel to each other" means that a plurality
of conductive fibers are arranged without contacting each
other, and the permissible deviation angle differs according
to the fiber length of the conductive fiber.
20 (contact)
The piezoelectric polymers on the surfaces of the
covered fibers are in contact with each other. There is an
embodiment in which covered fibers, each comprising the
conductive fiber as a core and the piezoelectric polymer as
25 a cover layer, are in contact with each other at the surface
cover layers. There is another embodiment in which a
plurality of conductive fibers arranged parallel to each
other are sandwiched between two piezoelectric polymer films
to be covered.
30
(production method (i))
The piezoelectric element can be manufactured by
bonding together at least two covered fibers prepared by
covering the surfaces of conductive fibers with the
33
piezoelectric polymer. Examples of this method are given
below.
(i-1) A method comprising the steps of: coextruding
a conductive fiber on the inner side and a piezoelectric
5 polymer on the outer side, melt spinning the co-extruded
product and stretching it.
(ii-2) A method comprising the steps of: melt extruding
a piezoelectric polymer onto a conductive fiber to cover it
and applying stretching stress at the time of covering to
10 orient the piezoelectric polymer.
(iii-3) A method comprising the steps of: preparing a
fiber composed of a hollow stretched piezoelectric polymer
and inserting a conductive fiber into the fiber.
(iv-4) A method comprising the steps of: preparing a
15 conductive fiber and a fiber composed of a stretched
piezoelectric polymer by separate steps and winding the fiber
composed of a piezoelectric polymer round the conductive
fiber to cover the conductive fiber. In this case, the
conductive fiber is preferably covered to ensure that both
20 fibers are arranged as concentrically as possible.
In this case, as preferred spinning and stretching
conditions when polylactic acid is used as the piezoelectric
polymer, the melt spinning temperature is preferably 150 to
250°C, the stretching temperature is preferably 40 to 150°C,
25 the draw ratio is preferably 1.1 to 5.0 times, and the
crystallization temperature is preferably 80 to 170°C.
A multifilament which Is a bundle of filaments or a
monofilament may be used as the piezoelectric polymer fiber
to be wound.
30 To wind the piezoelectric polymer fiber round the
conductive fiber, for example, the fiber composed of a
piezoelectric polymer is formed into a braided tube and the
conductive fiber as a core is inserted into the tube to be
covered, or when the fiber composed of a piezoelectric
34
polymer is braided to produce a braided cord, a braided cord
which includes the conductive fiber as core yarn and the
piezoelectric polymer fiber arranged around the core yarn
is produced to cover the conductive fiber. The single
5 filament diameter of the fiber composed of a piezoelectric
polymer is 1 |im to 5 mm, preferably 5 um to 2 mm, more
preferably 10 um to 1 mm. The number of filaments is
preferably 1 to 100,000, more preferably 50 to 50,000, much
more preferably 100 to 20,000.
10 The piezoelectric element of the present invention can
be obtained by bonding together a plurality of fibers
prepared by covering the surfaces of the conductive fibers
with the piezoelectric polymer according to the above method.
15 (production method (ii))
The piezoelectric element of the present invention can
be obtained by covering a plurality of conductive fibers
arranged parallel to each other with a piezoelectric polymer.
For example, the piezoelectric element of the present
20 invention can be obtained by sandwiching a plurality of
conductive fibers arranged parallel to each other between
two piezoelectric polymer films. Also, a piezoelectric
element having excellent flexibility can be obtained by
cutting this piezoelectric element in a strip.
25
(protective layer)
A protective layer may be formed on the outermost
surface of the piezoelectric element of the present invention.
This protective layer is preferably insulating, more
30 preferably made of a polymer from the viewpoint of
flexibility. As a matter of course, the protective layer
is rubbed in this case, and the protective layer is not
particularly limited if shearing stress produced by this
rubbing reaches the piezoelectric polymer and can induce its
35
polarization. The protective layer is not limited to one
which is formed by coating a polymer but may be a film or
a combination of films. An epoxy resin and an acrylic resin
are preferably used for the protective layer.
5 The thickness of the protective layer should be as small
as possible since shearing force can be easily transmitted
to the piezoelectric polymer. When the thickness is too
small, a problem such as the destruction of the protective
layer tends to occur. Therefore, the thickness is preferably
10 10 nm to 200 am, more preferably 50 nm to 50 am, much more
preferably 70 nm to 30 am, most preferably 100 nm to 10 um.
The shape of the piezoelectric element can be formed by this
protective layer.
15 (a plurality of piezoelectric elements)
A plurality of piezoelectric elements may be used in
combination before use. They may be arranged in one level
one-directionally, stacked two-directionally, further
woven or knitted into cloth, or braided. Thereby, a cloth
20 or braided piezoelectric element can be obtained. To produce
a cloth or braided piezoelectric element, as long as the
object of the present invention is attained, a fiber other
than the piezoelectric element may be used in combination
to carry out mixing, interweaving or interknitting, or
25 incorporated into the resin of the housing of a smart phone.
When a plurality of the piezoelectric elements of the present
invention are used in combination before use, as the
piezoelectric elements of the present invention do not have
an electrode on the surface, the arrangement and braiding
30 of these can be selected from wide ranges.
(application technology of piezoelectric element)
The piezoelectric element according to any one of the
above embodiments of the present invention can be used as
36
a sensor for detecting the size of stress produced by rubbing
the surface of the piezoelectric element and/or the
application position. The piezoelectric element of the
present invention can extract an electric output when
5 shearing stress is applied to the piezoelectric polymer by
pressing other than rubbing as a matter of course.
The expression "application stress" means stress
produced by rubbing with the surface of a finger as described
in the object of the invention. The level of stress produced
10 by rubbing with the surface of a finger is approximately 1
to 100 Pa. As a matter of course, it is needless to say that
if the stress is larger than this range, it is possible to
detect application stress and the application position
thereof.
15 In the case of input with a finger, the piezoelectric
element operates under a load of preferably 1 to 50 gf (100
to 500 mmN) , more preferably 1 to 10 gf (10 to 100 mmN) . As
a matter of course, the piezoelectric element operates under
a load larger than 50 gf (500 mmN) as described above.
20 The piezoelectric element according to any one of the
above embodiments of the present invention can be used as
an actuator by applying an electric signal thereto.
Therefore, the piezoelectric element of the present
invention can be used as a cloth actuator. In the actuator
25 of the present invention, by controlling an electric signal
to be applied, a concave or convex part can be formed in part
of the surface of the cloth, or the whole cloth can be rolled.
The actuator of the present invention can hold goods. When
it is wound round a human body (arm, leg, hip, etc. ) , it can
30 function as a supporter.
EXAMPLES
The following examples are provided for the purpose
of further illustrating the present invention but are in no
37
way to be taken as limiting.
Example 1
{production of polylactic acid)
5 0.005 part by weight of tin octylate was added to 100
parts by weight of L-lactide (manufactured by Musashino
Chemical Laboratory, Ltd., optical purity of 100 %) to carry
out a reaction in a nitrogen atmosphere at 180°C for 2 hours
in a reactor equipped with a stirring blade, phosphoric acid
10 was added in an amount which was 1.2 times the equivalent
of tin octylate, the residual lactide was removed under a
reduced pressure of 13.3 Pa, and the resulting product was
formed into a chip to obtain poly-L-lactic acid (PLLA1) . The
obtained PLLA1 had a weight average molecular weight of
15 152, 000, a glass transition point (Tg) of 55°C and a melting
point of 175°C.
(evaluation of piezoelectric element)
The piezoelectric element was evaluated as follows in
20 Example 1.
A finger was caused to touch the surface (gold deposited
surface) of a surface conductive layer and to rub the surface
at a velocity of about 0.5 m/s in a direction parallel to
the longitudinal direction of the piezoelectric element so
25 as to evaluate piezoelectric characteristics (substantially
the same load of 50 gf (500 mmN) or less was set in all Examples
and Comparative Examples). The evaluation system of
Examples is shown in Fig. 2. For the evaluation of voltage,
the DL6000 series digital oscilloscope (trade name of DL6000)
30 of Yokokawa Electric Corporation was used.
(production of piezoelectric element)
A carbon fiber multifilament manufactured by Toho
Tenax Co., Ltd. (trade name of HTS40 3K) was used as the
38
conductive fiber, covered with PLLAl which was molten at a
resin temperature of 200°C concentrically and cooled in air
right away to obtain a covered fiber 1 having a length of
10 m.
5 The carbon fiber in the covered fiber 1 was the
conductive fiber in the present invention. This carbon fiber
was a multifilament consisting of 3,000 filaments having a
diameter of 7. 0 \im and having a volume resistivity of 1. 6
x 10~3 Q*cm. The diameter of this conductive fiber was 0.6
10 mm, and the thickness of the PLLAl layer covering the
conductive fiber was 0. 3 mm (the diameter of the covered fiber
1 was 1.2 mm).
Then, this covered fiber 1 was cut to a fiber length
of 12 cm, and both ends of only the carbon fiber (conductive
15 fiber) on the inner side was removed 1 cm to prepare a covered
fiber 2 having a length of the carbon fiber (conductive fiber)
on the inner side of 10 cm and a length of the PLLAl layer
on the outer side of 12 cm. Thereafter, this covered fiber
2 was placed into a tensile tester set at a temperature of
20 80°C, and portions (1cm end portions) composed of only the
PLLAl layer at the both ends of the covered fiber 2 were held
with a nip to stretch only the PLLAl layer on the outer side
uniaxially. The stretching rate was 200 mrn/min, and the draw
ratio was 3 times. Subsequently, while the covered fiber
25 was held with the nip, the temperature was raised to 140°C
to carry out a heat treatment for 5 minutes, and the covered
fiber 2 was crystallized, quenched and taken out from the
tensile tester.
The obtained covered fiber 2 was composed of two
30 concentric layers and had a diameter of 0. 8 mm and a thickness
of the PLLAl layer of 0.1 mm. Gold was coated on the half
of the surface of this covered fiber to a thickness of about
100 nm by a vapor-deposition method to obtain the
piezoelectric element of the present invention. The volume
39
resistivity of the gold surface conductive layer was 1.0 x
10~4 Q-cm.
Fig. 1 is a schematic view of this piezoelectric element.
Four of the piezoelectric elements were prepared by the same
5 method and arranged parallel to one another as shown in Fig.
2 to evaluate the piezoelectric characteristics.
The evaluation result of the piezoelectric element is
shown in Fig. 3. It was understood that an extremely large
voltage of 2V or more was obtained simply by rubbing the
10 surface. It was confirmed that this piezoelectric element
functions as a piezoelectric element (sensor).
Example 2
(production of polylactic acid)
15 0.005 part by weight of tin octylate was added to 100
parts by weight of L-lactide (manufactured by Musashino
Chemical Laboratory, Ltd., optical purity of 100 %) to carry
out a reaction in a nitrogen atmosphere at 180°C for 2 hours
in a reactor equipped with a stirring blade, phosphoric acid
2 0 was added in an amount which was 1.2 times the equivalent
of tin octylate, the residual lactide was removed under a
reduced pressure of 13.3 Pa, and the resulting product was
formed into a chip to obtain poly-L-lactic acid (PLLAl) . The
obtained PLLAl had a weight average molecular weight of
25 152,000, a glass transition point (Tg) of 55°C and a melting
point of 175°C.
(evaluation of piezoelectric element)
The piezoelectric element was evaluated as follows in
30 Example 2.
A finger was caused to touch the surface and to rub
the surface at a velocity of about 0.5 m/s in a direction
parallel to the longitudinal direction of the piezoelectric
element so as to evaluate piezoelectric characteristics.
40
The evaluation system of Example 2 is shown in Fig. 5. For
the evaluation of voltage, the DL6000 series digital
oscilloscope (trade name of DL6000) of Yokokawa Electric
Corporation was used.
5
(production of piezoelectric element)
A carbon fiber multifilament manufactured by Toho
Tenax Co., Ltd. (trade name of HTS40 3K) was used as the
conductive fiber, covered with PLLAl which was molten at a
10 resin temperature of 200°C concentrically and cooled in air
right away to obtain a covered fiber 1 having a length of
10 m.
The carbon fiber in the covered fiber 1 was the
conductive fiber in the present invention. This carbon fiber
15 was a multifilament consisting of 3,000 filaments having a
diameter of 7 .0 jam and having a volume resistivity of 1.6
x 10"3 n*cm. The diameter of this conductive fiber was 0.6
mm, and the thickness of the PLLAl layer covering the
conductive fiber was 0. 3 mm (the diameter of the covered fiber
20 1 was 1.2 mm).
Then, this covered fiber 1 was cut to a fiber length
of 12 cm, and both ends of only the carbon fiber {conductive
fiber) on the inner side was removed 1 cm to prepare a covered
fiber 2 having a length of the carbon fiber {conductive fiber)
25 on the inner side of 10 cm and a length of the PLLAl layer
on the outer side of 12 cm. Thereafter, this covered fiber
2 was placed into a tensile tester set at a temperature of
80°C, and portions (1cm end portions) composed of only the
PLLAl layer at the both ends of the covered fiber 2 were held
30 with a nip to stretch only the PLLAl layer on the outer side
uniaxially. The stretching rate was 200 mm/min, and the draw
ratio was 3 times. Subsequently, while the covered fiber
was held with the nip, the temperature was raised to 140°C
to carry out a heat treatment for 5 minutes, and the covered
41
fiber 2 was crystallized, quenched and taken out from the
tensile tester.
The obtained covered fiber 2 was composed of two
concentric layers and had a diameter of 0. 9 mm and a thickness
5 of the PLLAl layer of 0.15 mm. Further/ two of the covered
fibers 2 were welded together, and end portions of the
piezoelectric polymers on the surfaces were removed to expose
the conductive fibers so as to obtain a piezoelectric element
shown in Fig. 4.
10 The piezoelectric characteristics of this
piezoelectric element were evaluated with constitution shown
in Fig. 5. The evaluation result of the piezoelectric
element is shown in Fig. 6. It was found that an extremely
large voltage of about 6V is obtained simply by rubbing the
15 surface. It was confirmed that this piezoelectric element
functioned as a piezoelectric element (sensor).
Examples 3 to 7
(production of polylactic acid)
20 Polylactic acid used in Examples 3 to 7 was produced
by the following method in Examples 3 to 7.
0.005 part by weight of tin octylate was added to 100
parts by weight of L-lactide (manufactured by Musashino
Chemical Laboratory, Ltd., optical purity of 100 %) to carry
25 out a reaction in a nitrogen atmosphere at 180°C for 2 hours
in a reactor equipped with a stirring blade, phosphoric acid
was added in an amount which was 1.2 times the equivalent
of tin octylate, the residual lactide was removed under a
reduced pressure of 13.3 Pa, and the resulting product was
30 formed into a chip to obtain poly-L-lactic acid (PLLAl) . The
obtained PLLAl had a weight average molecular weight of
152,000, a glass transition point (Tg) of 55°C and a melting
point of 175°C.
42
(evaluation of piezoelectric element)
The piezoelectric element was evaluated as follows in
Examples 3 to 7.
The piezoelectric characteristics of the
5 piezoelectric element were evaluated by transforming the
piezoelectric element. The evaluation system is shown in
Fig. 2. For the evaluation of voltage, the DL6000 series
digital oscilloscope (trade name of DL6000) of Yokokawa
Electric Corporation was used.
10 The piezoelectric fiber, the conductive fiber and the
insulating fiber used in Examples 3 to 7 were manufactured
by the following methods.
(piezoelectric fiber)
15 PLLA1 molten at 240°C was discharged from a cap having
24 holes at a rate of 20 g/min and taken up at a rate of 887
m/min. This unstretched multifilament yarn was stretched
to 2.3 times at 80°C and heat set at 100°C to obtain
multifilament uniaxially stretched yarn 1 having a fineness
20 of 84 dTex/24 filaments. 8 of the multifilament uniaxially
stretched yarns were bundled to obtain a piezoelectric fiber
1.
(conductive fiber)
25 A carbon fiber multifilament manufactured by Toho
Tenax Co., Ltd. (trade name of HTS40 3K) was used as a
conductive fiber 1. This conductive fiber 1 was a
multifilament consisting of 3,000 filaments having a
diameter of 7.0 um and having a volume resistivity of 1.6
30 x 10"3 n*cm.
(insulating fiber)
PET1 molten at 280 °C was discharged from a cap having
48 holes at a rate of 45 g/min and taken up at a rate of 800
43
m/min. This unstretched yarn was stretched to 2 . 5 times at
80°C and heat set at 180°C to obtain multifilament stretched
yarn having a fineness of 167 dTex/48 filaments. 4 of the
multifilament stretched yarns were bundled to obtain an
5 insulating fiber 1.
Example 3
As shown in Fig. 7, a plain woven fabric was
manufactured by arranging the insulating fiber 1 as a warp
10 and the piezoelectric fiber 1 and the conductive fiber 1
alternately as wefts. A pair of the conductive fibers
sandwiching the piezoelectric fiber in the plain woven fabric
were connected as signal lines to an oscilloscope, and the
other conductive fibers are connected to an earth. By
15 rubbing the piezoelectric fiber sandwiched between the
conductive fibers connected to the signal lines with a finger,
a voltage signal shown in Fig. 8 was obtained. By bending
the fibers, a voltage signal shown in Fig. 9 was obtained.
Thus, it was confirmed that this plain woven fabric
20 functioned as a piezoelectric element (sensor).
Example 4
A plain woven fabric was manufactured by arranging the
piezoelectric fiber 1 and the insulating fiber 1 alternately
25 as warps and the conductive fiber 1 and the insulating fiber
1 alternately as wefts as shown in Fig. 10. A pair of
conductive fibers which were 20 mm apart from each other in
this woven fabric were connected as signal lines to an
oscilloscope and the other conductive fibers were connected
30 to an earth. By rubbing the piezoelectric fiber sandwiched
between the conductive fibers connected to the signal lines
with a finger, a voltage signal shown in Fig. 11 was obtained.
It was confirmed that this plain woven fabric functioned as
a piezoelectric element (sensor).
44
Example 5
A plain woven fabric was manufactured by arranging the
insulating fiber 1 as a warp and the piezoelectric fiber 1
5 and the conductive fiber 1 alternately as v/efts as shown in
Fig. 12. When a pair of conductive fibers sandwiching the
piezoelectric fiber close to the both ends of this woven
fabric were connected as signal lines to a voltage source
and a voltage was applied to this plain woven fabric, the
10 whole woven fabric was twisted. It was confirmed that this
plain woven fabric functioned as a piezoelectric element
(actuator).
Example 6
15 A satin woven fabric was manufactured by arranging the
insulating fiber 1 as a warp and the insulating fiber 1, the
conductive fiber 1, the piezoelectric fiber 1 and the
conductive fiber 1 as wefts in this order as shown in Fig.
13. When a pair of the conductive fibers sandwiching the
20 piezoelectric fiber of this woven fabric were connected as
signal lines to an oscilloscope and the woven fabric was
twisted, a voltage signal shown in Fig. 14 was obtained. It
was confirmed that this satin woven fabric functioned as a
piezoelectric element (sensor).
25
Example 7
Two braids were manufactured by using the HTS4 0 3K which
is a carbon fiber multifilament manufactured by Toho Tenax
Co., Ltd. as a core and multifilament uniaxially stretched
30 yarn 1 as a braided cord.
These two braids were welded together by melting part
of the fiber surface of the multifilament uniaxially
stretched yarn using dichloromethane to obtain a
piezoelectric element shown in Fig. 1.
45
The piezoelectric characteristics of this
piezoelectric element were evaluated with constitution shown
in Fig. 2.
It was found that an extremely large voltage of 5V could
5 be obtained by rubbing the surface of this piezoelectric
element, and it was confirmed that this piezoelectric element
functioned as a piezoelectric element (sensor).
Comparative Example 1
10 PLLAl was molded at a resin temperature of 200°C by
using a film melt extruder having a T die and quenched with
a 40°C cooling roll to obtain a unstretched film.
Subsequently, the film was stretched in a transverse
direction at a draw ratio of 2.5 times and 80°C by using a
15 tenter type transversely stretching machine and then
crystallized in a heat setting zone at 140°C to obtain a
stretched film having a width of 70 cm. This film was cut
to a width of 1 cm and a length of 10 cm, and gold was vapor
deposited on the both sides thereof to manufacture a
20 piezoelectric element. The volume resistivity of the gold
surface conductive layer was 1.0 x 10~4 n*cin. This
piezoelectric element was evaluated in the same manner as
in Example 1 except that the piezoelectric element was
changed to this film piezoelectric element in Fig. 2.
25 When this piezoelectric element was evaluated, it was
found that only a voltage lower than about 0 . IV was obtained
and that the rubbing force of the surface of the piezoelectric
element could not be completely converted into voltage. It
was confirmed that this piezoelectric element could not
30 function as a piezoelectric element (sensor) which is the
object of the present invention.
Explanation of symbols in Figs. 1 and 2
11 piezoelectric polymer
46
12
13
21
22
23
24
25
26
27
conductive fiber
surface conductive layer
oscilloscope
wiring for evaluation
wiring for evaluation
conductive fiber
metal electrode
piezoelectric polymer
surface conductive layer
10
Explanation of symbols in Fig. 4 and 5
1 piezoelectric polymer
2 conductive fiber
3 iezoelectric element fixing plate
15 4 wiring for evaluation
5 osci.l 1 oscope
Explanation of symbols in Figs. 1, 10/ 12 and 13
A piezoelectric fiber
20 B conductive fiber
C insulating fiber

CLAIMS
1. A piezoelectric element comprising a piezoelectric
unit including two conductive fibers and one piezoelectric
5 fiber all of which are arranged substantially on the same
plane while they have contact points between them.
2. The piezoelectric element according to claim 1,
wherein the piezoelectric unit includes a conductive fiber,
10 a piezoelectric fiber and a conductive fiber all of which
are arranged in this order.
3 . The piezoelectric element according to claim 2,
wherein the piezoelectric unit includes a conductive fiber,
15 a piezoelectric fiber and a conductive fiber all of which
are arranged substantially parallel to one another.
4 . The piezoelectric element according to claim 1,
wherein the piezoelectric unit includes an insulating fiber
20 which is arranged such that the conductive fibers in the
piezoelectric unit are not in contact with conductive fibers
and a piezoelectric fiber in another piezoelectric unit.
5. The piezoelectric element according to claim 1,
25 wherein the piezoelectric fiber comprises polylactic acid
as the main component.
6. The piezoelectric element according to claim 1,
wherein the piezoelectric fiber comprises poly-L-lactic acid
30 or poly-D-lactic acid as the main component and the optical
purities of these components are 99 % or more.
7 . The piezoelectric element according to claim 1,
wherein the piezoelectric fiber is uniaxially oriented and
48
contains a crystal.
8 . The piezoelectric element according to claim 1,
wherein the conductive fiber is a carbon fiber. '
5
9. The piezoelectric element according to claim 4,
wherein the insulating fiber comprises a polyethylene
terephthalate-based fiber as the main component.
10 10. The piezoelectric element according to claim 3 which
is a woven or knitted fabric comprising a plurality of
parallel piezoelectric units.
11. The piezoelectric element according to claim 10 which
15 is a woven fabric comprising a plurality of parallel
piezoelectric units and having a satin weave structure.
12. The piezoelectric element according to claim 11,
wherein the piezoelectric units are arranged in the weft
20 direction.
13. The piezoelectric element according to claim 11,
wherein the step number of a piezoelectric fiber in the
piezoelectric unit is 3 to 7.
25
14 . A piezoelectric element which includes a conductive
fiber, a piezoelectric polymer covering the surface of the
fiber, and a surface conductive layer formed on the surface
of the piezoelectric polymer.
30
15. A piezoelectric element including at least two covered
fibers obtained by covering the surfaces of conductive fibers
with a piezoelectric polymer, wherein the covered fibers are
arranged substantially parallel to each other and the
49
piezoelectric polymers on the surfaces are in contact with
each other.
16. The piezoelectric element according to any one of
5 claims 1 to 15 v/hich is a sensor for detecting the size of
stress applied to the piezoelectric element and/or the
application position.
17. The piezoelectric element according to claim 16,
10 wherein stress applied to the piezoelectric element to be
detected is rubbing force to the surface of the piezoelectric
element.
18. The piezoelectric element according to any one of
15 claims 1 to 15 which is an actuator that changes its shape
according to an electric signal applied to the piezoelectric
element.