U.S. patent application number 13/718144 was filed with the patent office on 2013-05-16 for method for manufacturing polymeric piezoelectric film and polymeric piezoelectric film.
This patent application is currently assigned to KUREHA CORPORATION. The applicant listed for this patent is Kureha Corporation. Invention is credited to Nobuhiro Moriyama, Ken'ichi Nakamura, Kazuyuki Suzuki, Keitarou Suzuki.
Application Number | 20130122275 13/718144 |
Document ID | / |
Family ID | 39608677 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130122275 |
Kind Code |
A1 |
Moriyama; Nobuhiro ; et
al. |
May 16, 2013 |
METHOD FOR MANUFACTURING POLYMERIC PIEZOELECTRIC FILM AND POLYMERIC
PIEZOELECTRIC FILM
Abstract
A method for producing a polymer piezoelectric film, comprising:
a process of moving and stretching a crystalline polar polymer
sheet in contact with a conductive stretching roller having a
diameter of at least 30 mm and a surface friction coefficient which
has been reduced to such a level as to allow a relative
displacement of the crystalline polar polymer sheet in contact with
the conductive stretching roller: and a step in the process of
applying a polarization voltage between an electrode disposed
opposite to the crystalline polar polymer sheet and the conductive
stretching roller to polarize the crystalline polar polymer sheet.
As a result, it is possible to stably produce a polymer
piezoelectric film exhibiting stable piezoelectricity over a large
area. Especially, it is possible to obtain polymer piezoelectric
film exhibiting a temperature-dispersion peak temperature of
d.sub.31 piezoelectricity coefficient at at least 120.degree. C.
and surface scratches extending in one direction.
Inventors: |
Moriyama; Nobuhiro; (Tokyo,
JP) ; Nakamura; Ken'ichi; (Fukushima-ken, JP)
; Suzuki; Kazuyuki; (Fukushima-ken, JP) ; Suzuki;
Keitarou; (Fukushima-ken, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kureha Corporation; |
Tokyo |
|
JP |
|
|
Assignee: |
KUREHA CORPORATION
Tokyo
JP
|
Family ID: |
39608677 |
Appl. No.: |
13/718144 |
Filed: |
December 18, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12448856 |
Jul 10, 2009 |
8356393 |
|
|
PCT/JP2008/050084 |
Jan 8, 2008 |
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13718144 |
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Current U.S.
Class: |
428/220 ;
526/255 |
Current CPC
Class: |
H01L 41/45 20130101;
C08F 114/22 20130101; H01L 41/257 20130101; Y10T 29/49162 20150115;
Y10T 29/42 20150115; Y10T 428/24471 20150115 |
Class at
Publication: |
428/220 ;
526/255 |
International
Class: |
C08F 114/22 20060101
C08F114/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2007 |
JP |
002366/2007 |
Claims
1-10. (canceled)
11. A polymer piezoelectric film, exhibiting a
temperature-dispersion peak temperature of d.sub.31
piezoelectricity coefficient at at least 120.degree. C., and
surface scratches extending in one direction.
12. The polymer piezoelectric film according to claim 11,
exhibiting the temperature-dispersion peak temperature of d.sub.31
piezoelectricity coefficient at at least 120.degree. C. over an
area thereof covering a full width in a direction perpendicular to
a direction of developing the d.sub.31 piezoelectricity coefficient
and a length of at least 1 m in the direction of developing the
d.sub.31 piezoelectricity coefficient.
13. The polymer piezoelectric film according to claim 11,
exhibiting a d.sub.31 piezoelectricity coefficient distribution
giving an average of 15-35 pC/N and a local deviation thereof
within .+-.20% or less from the average.
14. The polymer piezoelectric film according to claim 11,
comprising a polyvinylidene fluoride resin.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for stably
producing a polymer piezoelectric film having a stably large
piezoelectricity (and pyroelectricity), and a polymer piezoelectric
film thus formed.
BACKGROUND ART
[0002] As a typical method for producing a high-performance
piezoelectric film as represented by a large piezoelectricity
(usually developing also pyroelectricity as is well known), there
is known a type of method, comprising: polarizing a crystalline
polymer sheet of, e.g., a polyvinylidene fluoride resin by corona
discharge from a pair of electrodes, at least one of which
comprises a needle electrode, disposed opposite to each other so as
to place the crystalline polymer sheet therebetween, while
stretching the crystalline polymer sheet (Patent documents 1 and
2). Incidentally, the terms "sheet" and "film" are used herein so
that the former refers to a state before stretching and the latter
refers to a state after stretching, regardless of a conventional
use thereof in the art that they are differentiated from each other
with reference to 250 .mu.m as a boundary thickness. In the
above-mentioned methods of Patent documents 1 and 2, needle
electrodes are principally used as both of the par of electrodes
disposed opposite to each other between which the crystalline
polymer sheet is placed, whereas there is also shown a n example
wherein a roller electrode is used as one of them (Example 2 of
Patent document 2). However, the polymer piezoelectric films thus
obtained through the above-mentioned methods still involve
irregularities in thickness and piezoelectric performance in a
planar direction, and the production conditions thereof have not
been satisfactorily stabilized, so that as a present state, it is
impossible to obtain a polymer piezoelectric film having a stably
large piezoelectricity over a wide area.
[0003] Patent document 1: JP-A 55-157801
[0004] Patent document 2: EP-A 018802
DISCLOSURE OF INVENTION
[0005] In view of the above-mentioned situation, a principal object
of the present invention is to provide a method for stably
producing a polymer piezoelectric film having a stably large
piezoelectricity over a large area, and a polymer piezoelectric
film thus formed and exhibiting stable piezoelectric
performances.
[0006] The method for producing a polymer piezoelectric film of the
present invention has been developed in order to accomplish the
above-mentioned object, and more specifically, includes: a process
of moving and stretching a crystalline polar polymer sheet in
contact with a conductive stretching roller having a diameter of at
least 30 mm and a surface friction coefficient which has been
reduced to such a level as to allow a relative displacement of the
crystalline polar polymer sheet in contact with the conductive
stretching roller, and a step in the process of applying a
polarization voltage between an electrode disposed opposite to the
crystalline polar polymer sheet and the conductive stretching
roller to polarize the crystalline polar polymer sheet.
[0007] A history through which the present inventors have studied
with the above-mentioned object to arrive at the present invention,
will be briefly described.
[0008] As a result of earnest study about the methods for producing
a high-performance polymer piezoelectric film, the present
inventors have arrived at a conclusion that the reason why the
above-mentioned production methods of Patent documents 1 and 2 have
failed in stable production of a polymer piezoelectric film having
a stably large piezoelectricity over a large area is attributable
to a failure in control of the conditions for neck stretching which
is necessary for development of stable piezoelectric performances
of a crystalline polymer film. More specifically, in the process of
stretching a crystalline polymer sheet at a temperature below the
melting point thereof while moving the crystalline polymer sheet,
the stretching is caused not in a form that the thickness and width
thereof are gradually decreased in the moving direction but usually
in a form (of neck stretching) that the thickness and width are
abruptly decreased to form a constriction (a neck or a necking
portion) at a point in the moving direction. In the case where a
polyvinylidene fluoride resin (hereinafter sometimes
representatively abbreviated as "PVDF") is used as a crystalline
and also polar polymer, the neck stretching causes a transformation
from a substantially non-polar .alpha.-from crystal to a polar
.beta.-form crystal and if a DC-high voltage is effectively applied
to the neck in this instance, a high degree of piezoelectricity is
developed. However, in the production methods of Patent documents 1
and 2, it has been difficult to stably control the neck stretching.
If a first consideration is made on the case of polarization using
non-contact electrodes, such as a pair of needle electrode rows
principally disclosed in Patent documents 1 and 2, the heating
condition for a crystalline polymer sheet moved at a prescribed
speed (while the case of batch treatment is set aside) cannot be
made uniform because of thickness irregularity or other causes
present in the sheet, thus making it difficult to form a necking
portion, so that it becomes difficult to form a polymer
piezoelectric film having a stable piezoelectricity over a large
area. Further, the sheet or film during the stretching is liable to
be sucked to touch one side of needle electrodes, thus causing
breakage thereof.
[0009] On the other hand, Patent document 2 discloses, as shown in
FIG. 1 attached hereto (corresponding to FIG. 6 of Patent document
2), an example (Example 2) wherein a PVDF sheet 11 was stretched
while being moved along a heating roller (also acting as a counter
electrode) 12 and was polarized by applying a DC voltage of 8 kV
between the heating roller 12 and needle electrodes 13 connected to
a DC high-voltage power supply 14 and 8 mm spaced apart from the
sheet 11 to obtain a polarized PVDF piezoelectric film 11'.
According to this embodiment, a more stable neck stretching has
become possible because one counter electrode was a heating roller
compared with the case of using a pair of needle electrodes to rely
on the environmental temperature as a heating source. Also in this
case, however, it was still difficult to form a polymer
piezoelectric film having a sufficiently stable piezoelectricity in
the width direction or length direction of the film 11'. According
to the present inventors' study, this is also attributable to a
failure in stable control of neck stretching. This point is
explained with reference to FIG. 2 which is a schematic view
showing a state of the PVDF sheet 11 moving over the roller 12 as
viewed in the direction of an arrow A in FIG. 1.
[0010] With reference to FIG. 2 and FIG. 1, a PVDF sheet 11 sent at
a speed R from an upstream side of a roller 12 having a diameter D1
causes necking (to form a neck N) on the roller 12 to cause a
crystal transformation from .alpha.-form to .beta.-form while
reducing its width and thickness, and is taken up at a speed R'
larger than R to be wound about a take-up roller (not shown)
disposed on the downstream side. (R and R' are vectors representing
moving direction and speed of the PVDF sheet 11 or PVDF film 11'.)
In this operation, the PVDF sheet 11 contacts the roller 12 over a
length (arc) L1 of the roller 12 and, during the contact period, is
polarized to be provided with a high piezoelectricity due to a DC
(direct current) voltage applied between the needle electrodes 13
and the roller 12 which also functions as a counter electrode. In
this instance, it is ideal for the neck line (constriction line) at
which the width and thickness of the sheet 11 is abruptly reduced
to be formed as an apparently stationary linear line as denoted by
NL in FIG. 2, which is parallel to the roller axis O and keeps its
relative position in the sheet moving direction R (and R') with
respect to the roller axis O, within the lengthwise range L1 of
contact with the roller 12.
[0011] However, as a result of the present inventors' earnest study
for reducing the fluctuation in piezoelectricity of the polymer
piezoelectric film obtained by the method of Patent document 2 and
detailed observation of neck stretching state of the sheet 11 on
the roller 12, it has been discovered that the neck line NL, which
should ideally form a stationary linear line, causes (i) a movement
to an upstream side or a downstream side as represented by arrows V
and V' in FIG. 2 and (ii) a deformation from a linear line NL to
change the positions of neck stretching along the width of the
sheet 11, most typically, to form a bellow-like neck line NL' by a
largest deviation to a downstream side at a position around the
center of the sheet width. Such instability of neck stretching is
considered to be partly attributable to a high electric field
simultaneously applied with the neck stretching for polarization,
which restricts the molecular movement of PVDF constituting the
sheet and exerts a force obstructing the neck stretching. Further,
it has become clear that the non-ideal neck stretching state has
obstructed the stable development of a high-degree of
piezoelectricity. This is because if the neck line NL goes outside
the range of contact length L1 due to such a non-ideal neck
stretching state, the polarization due to application of a DC
voltage between the electrodes 13 and 12 is not effectively
accomplished.
[0012] Then, the present inventors tried to use a larger diameter
roller having a diameter D.sub.2 (at a glance, similar to a roller
22 shown in FIG. 3 and used in examples of the present invention)
instead of the smaller diameter roller 12 having a diameter D.sub.1
usually used as a stretching roller, so as to provide an increased
length L2 (>L1) of contact between the sheet 11 and the roller
12, thereby retaining the neck line NL or NL' within the limits of
length L2 of contact with the roller, even when the above-mentioned
non-ideal neck stretching phenomena (i) and (ii) occurred. In this
case, however, the breakage of PVDF sheet having a small thickness
on the order of 100-500 .mu.m occurred frequently, so that a
commercial production was found to be impossible (Comparative
Example 2 appearing hereinafter). This is presumably because, in
the case of a neck stretching giving a velocity ratio R'/R
exceeding 2 between the downstream and upstream of the neck line,
it was difficult obtain a roller peripheral speed harmonized with
both R and R'. This is in line with the conventional practice in
the art that small diameter rollers are generally used in
stretching processes (not only in the case of neck stretching)
where plastic sheet are moved at different speeds between the
upstream and downstream of the rollers. The above-mentioned
breakage of PVDF sheet is understood to be promoted by other
factors, such that the application of a high electric field applied
in the direction of the sheet thickness simultaneously with the
neck stretching exerts a pressure in the direction of the electric
field due to the piezoelectricity of the sheet itself, and the
electric field exerts a force obstructing the molecular movement of
PVDF to obstruct the flexible deformability of the sheet.
[0013] On the other hand, the present inventors have acquired the
following knowledge. Even a roller having a large diameter D2 can
be used as a stretching roller contacting the neck stretching
portion of a thin polymer piezoelectric stock sheet while obviating
the breakage of the sheet if its surface friction coefficient has
been reduced to such a level as to allow a relative displacement of
the polymer piezoelectric stock sheet in contact with the roller
instead of mirror surface-finished rollers generally used in
plastic sheet transportation and used as small-diameter roller (D1)
or large-diameter roller (D2) in the above examples. Further, by
using such an improved large-diameter roller, among the
above-mentioned non-ideal neck stretching phenomena, (i) the
movement of neck line position still remains but can be
sufficiently stably retained within the range of contact length L2
with the roller, and (ii)the formation of a non-linear neck line
NL' can be remarkably suppressed to stably obtain a substantially
linear neck line NL, whereby commercial production of a polymer
piezoelectric film stably exhibiting a high-degree of
piezoelectricity has become possible.
[0014] The above-mentioned method for producing a polymer
piezoelectric film of the present invention has been developed as a
result of a series of knowledge described above.
[0015] Further, a characteristic of the polymer piezoelectric film
obtained by the polarization (stretching-polarization concurrent
processing) by the DC (direct-current) high-voltage application at
the neck stretching portion adopted by the method for producing the
polymer piezoelectric film of the present invention, is that it has
a remarkably improved thermal resistance of piezoelectricity
(represented by, e.g., a piezoelectricity coefficient in the
stretching direction (i.e., d.sub.31 piezoelectricity
coefficient)), compared with a polymer piezoelectric film obtained
through conventional stretching.fwdarw.polarization sequential
processing. For example, compared with a temperature-dispersion
peak temperature of d.sub.31 piezoelectricity coefficient of ca.
100.degree. C. obtained through conventional
stretching.fwdarw.polarization sequential processing, a value of at
least 120.degree. C. can be obtained. This means that the polymer
piezoelectric film can withstand heating up to 120.degree. C. and
can retain its piezoelectricity even thereafter. Particularly, in
the case of using a surface-roughened conductive stretching roller
according to a preferred embodiment of the production method
according to the present invention, the resultant piezoelectric
film is provided with surface scratches extending in one direction,
as a result of contact with the roller through neck stretching
accompanied with relative movement with the roller.
[0016] Thus, the polymer piezoelectric film of the present
invention is characterized by exhibiting a temperature-dispersion
peak temperature of d.sub.31 piezoelectricity coefficient at at
least 120.degree. C., and surface scratches extending in one
direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic view showing an outline of neck
stretching-polarization concurrent processing device including a
small-diameter heating roller according to a conventional
method.
[0018] FIG. 2 is a schematic top view for illustrating a neck
stretching state on a heating roller under application of a
polarization electric field.
[0019] FIG. 3 is a schematic view showing an outline of an
embodiment of neck stretching-polarization concurrent processing
device including a large-diameter heating roller used in a method
of the invention.
[0020] FIG. 4 is a schematic view showing an outline of another
embodiment of neck stretching-polarization concurrent processing
device including a large-diameter heating roller used in a method
of the invention.
[0021] FIG. 5 is a graph showing a temperature-dispersion curve of
d.sub.31 piezoelectric coefficient.
BEST MODE FOR PRACTICING THE INVENTION
[0022] Hereinbelow, the method for producing the polymer
piezoelectric film of the present invention will be described in
the order of steps involved therein with reference to a preferred
embodiment thereof.
[0023] (Crystalline Polar Polymer Sheet)
[0024] A starting material used in the production method according
to the present invention is a crystalline polar polymer sheet. It
is known that if the polarization of a crystalline polar polymer is
carried out under moderate conditions, piezoelectricity is
developed (and also pyroelectricity in many cases). Examples of
crystalline polar polymers used in the present invention may
include: polyvinylidene fluoride resin and nylons having
odd-numbered carbon chain, such as nylon 11, etc., but it is
preferred to use a polyvinylidene fluoride resin in view of
resultant high piezoelectricity, weatherability, heat resistance,
etc.
[0025] Examples of the polyvinylidene fluoride resin may include:
vinylidene fluoride homopolymer, and copolymers of vinylidene
fluoride containing at least 50 mol %, preferably at least 70 mol
%, of vinylidene fluoride units and units of other monomers
copolymerizable with vinylidene fluoride, such as
trifluoroethylene, tetrafluoroethylene, hexafluoro-propylene,
trifluorochloroethylene, and vinyl fluoride.
[0026] The polyvinylidene fluoride resin may preferably be one
having a high molecular weight so as to provide a high strength
durable against neck stretching, more specifically one showing an
inherent viscosity of at least 1.0 dl/g (when measured as a
dimethylformamide solution at a concentration of 0.4 g/dl at a
temperature of 30.degree. C.). In order to provide a high strength
and good stretchability in combination, it is particularly
preferred to use a polyvinylidene fluoride resin having an inherent
viscosity of 1.1-2.0 dl/g. A sheet of a polyvinylidene fluoride
resin having an inherent viscosity below 1.0 dl/g is liable to
cause breakage at a high probability due to an intense shearing
force applied with a large-diameter roller (while it may be
alleviated due to the use of a low-friction coefficient roller)
during the neck stretching step on the large-diameter roller
adopted in the present invention.
[0027] A sheet of the crystalline polar polymer represented by the
above-mentioned polyvinylidene fluoride resin is treated by the
method of the present invention. While the sheet formation may be
performed by melt extrusion, solution casting, etc., it is
preferred to use a sheet formed by melt extrusion or further
subjected to an appropriate thickness adjustment as by stretching,
etc. Especially, the thickness of the crystalline polar polymer
sheet before the neck stretching-polarization concurrent processing
by the method of present invention may preferably be in the range
of 20-2500 .mu.m, particularly 40-1500 .mu.m. Less than 20 .mu.m,
the sheet is liable to have an insufficient strength. In excess of
2500 .mu.m, the sheet becomes too rigid, and it becomes difficult
to move the sheet in contact with a pre-heating roller before the
stretching.
[0028] In accordance with the method of the present invention, a
polymer piezoelectric film of the present invention may be obtained
through processing by a neck stretching-polarization concurrent
processing device, of which an outline is, for example, shown in
FIG. 3.
[0029] Hereinbelow, the method of the present invention will be
described more specifically with reference to the case where
polyvinylidene fluoride resin, especially PVDF (vinylidene fluoride
homopolymer), is used as a crystalline polar polymer. It will be
easily understood to a person skilled in the art that a similar
processing is possible for other crystalline polar polymers by some
modification of conditions in consideration of difference in
characteristics, such as the melting point and Curie point.
[0030] With reference to FIG. 3, a PVDF sheet 11 of which the
thickness has been adjusted to 20-2500 .mu.m, is fed at a speed R
to a conductive heating roller 22 having a diameter of at least 30
mm, usually 30-500 mm, preferably 100-300 mm, and a reduced surface
friction coefficient, to contact the roller for a length of L2,
where the sheet 11 is subjected to neck stretching and is
concurrently polarized (or poled) under the action of a high DC
(direct current) electric field applied between a non-contact acute
electrode 23 connected to a DC high-voltage power supply 24 and the
grounded roller 22 (counter electrode). A film 11a after the neck
stretching (which is already substantially equivalent to a polymer
piezoelectric film of the present invention), leaves the roller 22
at a speed R' (>R) and receives post-processing, such as heat
treatment for dimensional stabilization, as desired, to be wound
about a take-up roller (not shown).
[0031] If the diameter of the roller 22 is less than 30 mm, the
effect of increasing the contact length L2 with the sheet 11 is not
sufficient, and the neck line can go out of the L2 range due to
fluctuation (V, V') of the neck line in the sheet feeding
direction, whereby the effect of stabilizing the piezoelectricity
becomes insufficient. If the diameter exceeds 500 mm, the rotation
torque becomes smaller and the stretching becomes difficult.
[0032] The surface friction coefficient of the roller 22 can be
decreased in various ways. The easiest way may be to apply a
lubricant, such as optionally-emulsified fine particles of
polytetrafluoroethylene, higher fatty acid salts or esters, and
fluorine-containing lubricants, to the peripheral face of the
metallic roller 22. By this friction coefficient-reduction
treatment, it has been confirmed that the breakage of sheet caused
by a large-diameter mirror-finished metal roller can be prevented
and the formation of a linear neck line NL becomes possible, so
that it is possible to form a high-performance piezoelectric film
of ca. 50 m in length. However, the lubricant is inevitably worn
out as the continuation of the production, the formation of
nonlinear neck line NL' gradually occurs until the sheet 22 is
finally broken.
[0033] A more persistent reduction of the surface friction
coefficient of the roller 22 can be accomplished by using a roller
22 having a roughened peripheral surface. A roughened surface may
be provided by sandpaper processing of a mirror-finished metal
roller surface or by use of a roller of a non-metallic conductive
material formed originally with a rough surface. Generally, the
large-diameter roller suited for the purpose of the present
invention may have a ten-point average roughness Ra (JIS
B0601-1994) of 0.1-30 .mu.m, preferably 0.2-2 .mu.m. Below 0.1
.mu.m, the surface friction coefficient-reduction effect is not
sufficient, and it is difficult to avoid the breakage of the sheet
11 or film 11a during the neck stretching of the crystalline polar
polymer sheet accompanied with a relative displacement on the
surface of the large-diameter roller. On the other hand, if the
roughness is excessive, the sheet 11 is liable to be broken due to
projections on the roller, and the scratches caused by contact with
the rough surface roller are liable to occur to a level
unacceptable for the product piezoelectric film.
[0034] The roller 22 has to be electroconductive so that it can
operate as a counter electrode receiving a DC voltage of at least 5
kV, preferably 7-50 kV, applied to an acute or tip electrode 23 for
polarization of the sheet 11. However, the required conductivity
need not be so large and may be satisfied with an intrinsic
electrical resistance on the order of 10.sup.4-10 .sup.5 ohmcm or
below that can be realized by titanium oxide-based or alumina-based
ceramic.
[0035] The acute electrode 23 (that is a kind of a counter
electrode with respect to the conductive roller electrode 22) is
disposed to generate electric charges by corona discharge caused at
its acute end and impart the charges to the surface of the film 11a
for forming a DC electric field to polarize the PVDF sheet 11 (or
film 11a) under neck stretching. In order to cause an effective
corona discharge, it is preferred that the electrode 23 has an
acute end. As the acute electrode, in addition to a needle
electrode (which has a literally needle-like tip) explained with
reference to the device of FIG. 1, a wire electrode (namely, a
wire-form electrode having its portion closest to the roller 22
extending in parallel with and over an almost identical length as
the roller-axis length of the roller 22) can also be used
preferably. While it is possible to perform the polarization by
using an electrode directly contacting the sheet 11 together with
the roller electrode 22 instead of such a non-contact electrode,
the sheet 11 to be processed is also subjected to neck stretching
and is therefore liable to cause dielectric breakdown leading to a
shutdown of the power supply in that case, so that the use of a
non-contact electrode is preferred.
[0036] The spacing between the acute electrode 23 and the roller 22
surface, may generally be preferably on the order of 5-30 mm. At
too small a spacing, dielectric breakdown of the film 11a is liable
to occur, whereas if excessive, corona discharge is suppressed to
reduce the polarization effect. The acute electrode 23 has to be
provided in a number large enough to cover the polarized area of
the sheet 11, i.e., the full width x the contact length L2 with the
roller 22, and more specifically, it is desirable to dispose ca.
0.5-2 electrodes/cm when the acute electrode 23 is a wire
electrode, and at a density of ca. 0.5-3 electrodes/cm.sup.2 in the
case of needle electrodes
[0037] The length L2 where the sheet 11 contacts the roller 22
(therefore, almost the same length as the length of the sheet 11
covered with the acute electrodes 23 and receiving a polarization
process) on the roller 22, may preferably be at least 30 degrees in
terms of a corresponding central angle .theta. of the roller 22,
and at most 210 degrees. At too small .theta., the
piezoelectricity-stabilizing effect due to an increased L2 becomes
scarce even if a large-diameter roller is used, and at too large
.theta., the force of pressing the sheet 11 against the roller 22
becomes large so that the sheet 11 (or film 11') is liable to be
broken.
[0038] In order to cause the neck stretching effectively, the
surface temperature of the roller 22 (regarded as almost the same
as the sheet 11 temperature) may suitably be from room temperature
to below the melting point, especially ca. 70-130.degree. C. In
order to obviate intense heating on the roller 22, it is possible
to dispose a preheating means, such as a preheating roller having a
surface temperature lower than that of the roller 22 or an infrared
heater, at a position upstream from the roller 22.
[0039] The entire stretching ratio around the roller 22 (i.e., a
region between a send roller on the upstream side and a receiver
roller on the downstream side) including the neck stretching ratio
on the roller 22 (occupying most of the entire stretching ratio),
may preferably be in the range of 2.5 to 6 times. Below 2.5 times,
the necking is not stabilized but above 6 times, the sheet or film
is liable to be broken.
[0040] FIG. 4 illustrates an outline of the stretching-polarization
concurrent processing device for practicing another preferred
embodiment of the polymer piezoelectric film production method of
the present invention. Referring to FIG. 4, a right half of the
device for providing a piezoelectric film 11a (including a heating
and counter electrode roller 22, an acute electrode 23, and a DC
high-voltage power supply 24) is almost identical to what has been
described with reference to FIG. 3.
[0041] In the device of FIG. 4, the piezoelectric film 11a formed
through the above-described neck stretching-polarization concurrent
processing as in the device of FIG. 3, is further subjected to a
polarization processing with a reverse polarity of voltage applied
to the surface thereof under heating by means of a second heating
roller 22a, a second acute electrode 23a, and a second high-voltage
power supply 24a which supplies a voltage of reverse polarity with
respect to the voltage supplied from the power supply 24. As a
result of such an additional polarization process, even for a thick
piezoelectric film, stable polarization becomes possible and the
piezoelectricity is stabilized.
[0042] The piezoelectric film 11a (or 11b) produced in the
above-described manner may be subjected to an optional
post-treatment, such as heat treatment for dimensional
stabilization, and wound about a take-up roller as a product
piezoelectric film for storage or circulation in the market, or
provided with an electrode on one or both surfaces thereof by vapor
deposition or via an adhesive to provide a film-form piezoelectric
(or pyroelectric) element product.
[0043] The thus-produced polymer piezoelectric film of the present
invention may exhibit, as representative properties, a thickness:
10-500 .mu.m, and a d.sub.31 piezoelectricity coefficient in the
stretched direction (average): 15-35 pC/N, including only a local
deviation thereof within .+-.20% or less from the average over
almost the entire region, as a characteristic. Further, as an
important feature of the piezoelectric film of the present
invention obtained through the stable neck stretching-polarization
concurrent processing method, the piezoelectric film exhibits an
excellent heat resistance of piezoelectricity stably over a large
area. This point is explained with reference to FIG. 5 showing a
temperature-dispersion (temperature dependence) curve of d.sub.31
piezo-electricity coefficient. Thus, an example of d.sub.31
temperature- dispersion curve of the piezoelectric film obtained
through the conventional stretching.fwdarw.polarization sequential
processing is shown in FIG. 5 as dotted line 1 showing a peak
temperature up to nearly 100.degree. C. at the maximum. This means
that as the measurement temperature rises, the d.sub.31
piezoelectric coefficient of the film gradually increases up to
100.degree. C. as the peak temperature but rapidly decreases above
100.degree. C., and the once-decreased piezoelectric coefficient
does not recover even if the temperature is restored to below
100.degree. C. or less. Therefore, it is difficult or impossible to
use such a piezoelectric film at a temperature above 100.degree. C.
On the other hand, the piezoelectric film of the present invention
obtained through the neck stretching-polarization concurrent
processing shows a temperature dispersion curve 2 as shown in FIG.
5 including a peak temperature of at least 120.degree. C.
Accordingly, the piezoelectric film can be used in a temperature
region exceeding 100.degree. C., and it is permanently usable in a
temperature region up to 120.degree. C. A further important feature
of the piezoelectric film of the present invention is that such a
good heat resistance of piezoelectricity is secured uniformly over
an extremely large area, as a result of stabilization of the neck
stretching-polarization process according to the production method
of the present invention. More specifically, a characteristic
feature of the piezoelectric film of the present invention is that
it exhibits a temperature-dispersion peak temperature of d.sub.31
piezoelectricity coefficient at at least 120.degree. C. over an
area thereof covering a full width in a direction perpendicular to
a direction of developing the d.sub.31 piezoelectricity coefficient
(i.e., the stretching direction) and a length of at least 1 m in
the direction of developing the d.sub.31 piezoelectricity
coefficient.
[0044] Further, according to a preferred embodiment, the
piezoelectric film of the present invention has an additional
feature of having surface scratches extending in one direction in
agreement with the direction of developing d.sub.31
piezoelectricity coefficient. Such surface scratches are obtained
in the production method of the present invention as a result of
using a surface-roughened roller as the heating and counter
electrode roller 22. More specifically, as explained with reference
to FIG. 2, when a sheet 11 is subjected to a stable neck stretching
over contact length L2 on a large-diameter roller 22, the sheet 11
moves at a speed R before the neck line NL and at R' after the NL
giving a ratio R'/R as large as 2 or more. While the sheet 11 moves
at speeds ranging R to R', the roller 22 can only rotate at a fixed
peripheral speed within the range of R to R'. Therefore, the sheet
11 and the roller 22 move over the contact length L2 with a certain
speed difference along a substantial portion of L2 (usually a
portion contacting the film 11a after NL). This is the reason why
the piezoelectric film of the present invention obtained by using a
surface-roughed roller is provided with surface scratches
consistent with the direction of developing the d.sub.31
piezoelectric coefficient
[0045] Such surface scratches cause some rise of haze value of the
piezoelectric film and can be disadvantageous in a use requiring a
transparent piezoelectric film. However, a principal use of the
piezoelectric film of the present invention is a piezoelectric (or
pyroelectric) element obtained by attaching an electrode on at
least one surface thereof by vapor deposition or an adhesive. In
this use, the surface roughening by formation of surface scratches
is preferable since it contributes to a remarkable increase in
attached strength of the surface electrode.
EXAMPLES
[0046] Hereinbelow, the present invention will be described more
specifically based on Examples and Comparative Examples. The
physical properties described in the present specification
including the following examples are based on values measured
according to the following methods.
[0047] (1) Piezoelectricity Coefficient d.sub.31 (and the
Temperature Dispersion Thereof)
[0048] A polymer piezoelectric film was provided with a 100-800
nm-thick Al vapor deposition electrode on both surfaces thereof,
and a sample measuring 7 mm.times.30 mm was cut out at a prescribed
spot from the piezoelectric film. The sample was set by clamping in
a sample chamber of a piezoelectric coefficient measuring device
("RHEOLOGRAPH SOLID", made by K. K. Toyo Seiki Seisakusho), and the
d.sub.31 piezoelectricity coefficient was measured under the
conditions of a tension:1 N (Newton), and a frequency: 10 Hz. The
measurement was repeated at an increment of 1.degree. C. while
raising a temperature at a rate of 2.degree. C./min. from room
temperature to 150.degree. C. to obtain a temperature dispersion
curve of d.sub.31 piezoelectricity coefficient
[0049] (2) Surface Roughness Coefficient Ra
[0050] A roller surface roughness Ra was measured by means of a
surface roughness meter ("SURFCORDER SE1700", made by K. K. Kosaka
Kenkyusho) according to JIS B0601-1994.
[0051] (3) Electrical Resistance of a Roller
[0052] An ordinary tester was used to measure an electrical
resistance between the roller axis and the surface.
Comparative Example 1
[0053] A 160 .mu.m-thick sheet of PVDF having an inherent viscosity
(.eta.i) of 1.2 dl/g (polyvinylidene fluoride: made by Kureha
Kagaku Kogyo K. K.) was supplied to a device having an organization
substantially as shown in FIG. 1 and subjected to a neck
stretching-polarization concurrent processing. More specifically,
the above PVDF sheet was moved over a hard chrome mirror-finished
heating roller 12 (with a surface roughness Ra <0.03 .mu.m) so
as to provide a contact center angle .theta.=ca. 40 deg. and
stretched under the conditions of a sheet supply speed of 10
cm/min. and a take-up speed of 40 cm/min. In this state, a DC
voltage applied to needle electrodes 13 spaced apart from the
roller 12 by ca. 10 mm and disposed at a density of 2
electordes/cm.sup.2 was increase form 0 kV to 9 kV to effect a neck
stretching-polarization concurrent processing.
[0054] At the initial period when the applied voltage was 0 kV,
neck stretching was stably performed on the roller 12 surface, and
a stretched film of ca. 40 .mu.m in thickness and ca. 300 mm in
width was obtained. However, when 9 kV was applied, it changed into
a state where a non-linear neck line NL' was formed by gradual
bellow-like elongation of the central portion and the whole neck
line moved toward the downstream on the roller surface until the
neck line went out of the roller surface.
[0055] As a result of the measurement of d.sub.31 piezoelectric
coefficient, the thus-formed piezoelectric film exhibited
d.sub.31=25 pC/N (and a dispersion peak temperature
.gtoreq.120.degree. C.) with respect to a portion (of ca. 0.5 m in
length) which was supposed to be formed in a relatively early stage
when the neck line occurred on the roller surface but exhibited
only d.sub.31.ltoreq.1 pC/N with respect to the portions which were
supposed to be formed where the neck line went out of the roller
surface including a side marginal portion outside the ca. 0.5
m-lenth portion formed in the relatively early stage.
Comparative Example 2
[0056] A neck stretching-polarization concurrent processing of a
160 .mu.m-thick PVDF sheet was performed in the same manner as in
Comparative Example 1 except for using a hard chrome
mirror-finished roller (Ra<0.03 .mu.m) of 200 mm in diameter as
the heating roller.
[0057] In the course of gradually increasing the voltage applied
between the roller and the needle electrodes up to 10 kV, the neck
line moved downwards and became non-linear until the breakage of
the stretched film at the time of application of 10 kV, so that a
long period of production was impossible.
[0058] With respect to the portion supplied with 10 kV before the
breakage, thickness irregularities were noticeable with eyes, and a
relatively good portion (of ca. 1 m in length) exhibited
d.sub.31=27 pC/N (and a dispersion peak temperature 120.degree. C.)
but side portions outside the relatively good portion included a
portion exhibiting only d.sub.31.ltoreq.1 pC/N.
Example 1
[0059] A neck stretching-polarization concurrent processing of a
160 .mu.m-thick PVDF sheet was performed in the same manner as in
Comparative Example 2 except for using a heating roller
(corresponding to one denoted by 22 in FIG. 3) formed by uniformly
applying a lubricant comprising polytetrafluoroethylene fine
particles in the form of an aerosol for spraying ("DAIFRE GA6010",
made by Daikin Kogyo K. K.) onto the surface of a hard chrome
mirror-finished roller used in Comparative Example 2.
[0060] The processing could be continued until the voltage applied
between the roller(22) and the needle electrodes (23) was increased
up to 12 kV. Under this condition, a linear neck line NL was formed
sufficiently within the roller-sheet contact length L2 (=ca. 70 mm)
while some up-and-down fluctuation of the neck line was
observed
[0061] The processing was continued until ca. 20 m of a
piezoelectric film was produced, when the instability of neck line
(i.e., non-linear formation and deviation out of the contact length
L2 region of the neck line) started to be recognized, so that the
processing was terminated thereafter.
[0062] The film obtained under the stable processing conditions
exhibited a piezoelectricity coefficient d.sub.31=25 pC/N (and a
dispersion peak temperature 120.degree. C.) over the full width and
no irregularity in piezoelectricity coefficient or thickness.
Example 2
[0063] A neck stretching-polarization concurrent processing of a
160 .mu.m-thick PVDF sheet was performed in the same manner as in
Example 1 except for using a heating roller (of surface roughness
Ra=1 .mu.m; corresponding to one denoted by 22 in FIG. 3) formed by
abrading the surface of a hard chrome mirror-finished roller used
in Comparative Example 2, successively with sandpaper (of #40) and
sandpaper (of #240).
[0064] Under the voltage application condition of 12 kV between the
roller(22) and the needle electrodes (23), a stable production of a
piezoelectric film under a stable neck stretching state was
possible while some fluctuation of a linear neck line within the
contact length L2 region was observed, and the stable production
state was maintained even after production of 100 m of
piezoelectric film.
[0065] The thus-obtained piezoelectric film exhibited a
piezoelectricity coefficient d.sub.31=30 pC/N (and a dispersion
peak temperature .gtoreq.120.degree. C.) over the full width, and
no irregularity in piezoelectricity coefficient or thickness was
observed.
Example 3
[0066] A neck stretching-polarization concurrent processing of a
160 .mu.m-thick PVDF sheet was performed in the same manner as in
Example 2 except for using a heating roller (22) of 200 mm in outer
diameter surfaced with a titanium oxide-based ceramic layer (of
surface roughness Ra=1 .mu.m and a resistance between the roller
axis and the surface=77 ohm).
[0067] Under the voltage application condition of 12 kV between the
roller(22) and the needle electrodes (23), a stable production of a
piezoelectric film under a stable neck stretching state was
possible while some fluctuation of a linear neck line within the
contact length L2 region was observed, and the stable production
state was maintained even after production of 200 m of
piezoelectric film.
[0068] The thus-obtained film exhibited a piezoelectricity
coefficient d.sub.31=30 pC/N (and a dispersion peak temperature
.gtoreq.120.degree. C.) over the full width, and no irregularity in
piezoelectricity coefficient or thickness was observed.
Example 4
[0069] A neck stretching-polarization concurrent processing of a
1000 .mu.m-thick PVDF sheet was performed by using a device having
an organization as roughly shown in FIG. 4. Referring to FIG. 4,
rollers 22 and 22a were each identical to the 200 .mu.m-dia.
ceramic-coated roller used in Example 3. Needle electrodes 23 and
23a were respectively disposed at a density of 2 electrodes/cm2,
and the spacings between the needle electrodes (23,23a) and the
rollers (22,22a) were respectively set at 10 mm.
[0070] For the processing, 1000 .mu.m-thick PVDF sheet was fed to
the heating roller 22 with surface temperature of 120.degree. C. at
a speed of 50 cm/min., and a DC voltage of +25 kV was applied from
a power supply 24 via needle electrodes 23 to perform
stretching-polarization concurrent processing on the roller 22.
[0071] The film 11a having left the roller 22 was fed to a roller
22a with a surface temperature of 120.degree. C., and a DC voltage
of -25 kV was applied from a power supply 24a to the film 11a on
the roller 22a while the film 11a was taken up at a take-up speed
of 210 cm/min.
[0072] On the roller 22, a stable neck stretching state was formed
while some fluctuation of a linear neck line within the contact
length L2 region was observed, and including the additional
polarization processing on the roller 22a, stable formation of a
piezoelectric film was possible as a whole. The stable production
state was maintained even after production of 200 m of
piezoelectric film
[0073] The thus-obtained film exhibited a piezoelectricity
coefficient d.sub.31=30 pC/N (and a dispersion peak temperature
.gtoreq.120.degree. C.) over the full width, and no irregularity in
piezoelectricity coefficient or thickness was observed.
[0074] The production conditions of the above Examples and a
Comparative Examples, and the properties of the resultant
piezoelectric films are inclusively summarized in the following
Table 1.
TABLE-US-00001 TABLE 1 Stretch roller (12 or 22) Piezoelectric film
Surface Sheet 11 Applied Stable (stably produced portion) Diameter
treatment thickness voltage production Thickness d.sub.31 Surface
Example (mm) Material (state) (.mu.m) (kV) length (m) (.mu.m)
(pC/N) scratches Comp. 1 20 Mirror- None 160 9 0.3 40 (25) No
finished (Ra < 0.03 .mu.m) metal Comp. 2 200 do do 160 10 1 40
(27) No 1 200 do lubricant 160 12 20 40 30 No 2 200 do sandpaper
160 12 .gtoreq.200 40 30 Yes (Ra = 1 .mu.m) 3 200 Ceramic (Ra = 0.4
.mu.m) 160 15 .gtoreq.200 40 30 Yes 4 20 Ceramic (Ra = 0.4 .mu.m)
1000 25*1 .gtoreq.200 250 30 Yes *1: A reverse-polarity voltage was
additionally applied from the back on the roller 22a (FIG. 4).
[0075] Incidentally, in all the Examples described above,
large-diameter rollers of 200 mm in diameter were used, but it was
confirmed in preliminary tests, a substantial improvement over
Comparative Example 1 could be obtained even by using a 30 mm-dia.
roller coated with the lubricant or surface-roughened by sandpaper
treatment, and a remarkable improvement could be obtained if a
roller of 50 mm or larger in diameter was used.
INDUSTRIAL APPLICABILITY
[0076] As is understood from the above-described results of
Examples and Comparative Examples including those shown in Table 1,
according to the present invention, there is provided a stable
method for producing a polymer piezoelectric film showing stable
piezo-electricity over a large area, wherein a neck
stretching-polarization concurrent processing is realized by using
a large-diameter roller with a reduced surface friction coefficient
as a heating roller for performing the neck stretching-polarization
concurrent processing. Also, a polymer piezoelectric film thus
produced and particularly improved in stable heat resistance of
piezoelectricity over a large area., is provided.
* * * * *