JPS6041274A - Polymer piezoelectric film - Google Patents

Abstract

PURPOSE:To provide vertical electromechanical connecting coefficient to a relatively high film surface by adding 60-87mol% of vinylidene fluoride 10- 40mol% of ethylene trifluoride and 3-20mol% of vinyl fluoride. CONSTITUTION:A vinylidene fluoride copolymer contains as indispensable components 60-87mol% or preferably 65-85mol% of vinylidene fluoride, 10- 40mol% or preferably 15-30mol% of ethylene trifluoride, and 3-20mol% or preferably 5-15mol% of vinyl fluoride. In addition, small amounts of one or more of fluorine-containing monomers such as ethylene tetrafluoride, propylene hexafluroide, ethylene trifluoride chloride may be added as constituent units.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polymer piezoelectric film that has high piezoelectric performance even in the high frequency range and has excellent device manufacturing performance, and in particular, it relates to a high-performance ultrasonic transformer. This article relates to a polymer piezoelectric film suitable for juicers.

As a polymeric piezoelectric material having high piezoelectric performance, for example, polyvinylidene fluoride is disclosed in Japanese Patent Publication No. 45-83771, and polyvinylidene fluoride is disclosed in Japanese Patent Publication No. 50-29159 and Japanese Patent Application Laid-open No. 56-1989.
Vinylidene fluoride copolymers have been reported in publications such as No. 111281 and JP-A-58-60585. Moreover, these vinylidene fluoride resins were shown to have high piezoelectric performance even in the high frequency range.
It is reported in the publication No. 1-23439.

These polymeric piezoelectric materials are generally made into a film by roll rolling or casting, and then the electromechanical coupling coefficient ki (hereinafter often simply rk) in the direction perpendicular to the film surface is
Abbreviated as t and J. ) is heat-treated to increase the piezoelectric film, and an electric field is applied in a direction perpendicular to the film to perform an orientation polarization process, thereby forming a piezoelectric film. Among these series of vinylidene fluoride resins, polyvinylidene fluoride is
It has a kt of approximately 0.2 and good moldability, and has been considered the most useful polymer element material for ultrasonic transducers, but further improvement in the electromechanical coupling coefficient kt is desired. There is.

On the other hand, the vinylidene fluoride copolymer described in the above patent publication,
In particular, a copolymer of vinylidene fluoride and ethylene trifluoride is used to overlay polyvinylidene fluoride in roll-rolled products and cast films having a composition of around 75 mol% vinylidene fluoride and 25 mol% ethylene trifluoride. , is obtained. However, when a film-like molded product of this copolymer is molded into a concave surface in order to increase the wave transmission/reception sensitivity of an ultrasonic transducer, cracks are likely to occur, resulting in a very low device manufacturing yield.

The reason why cracks are more likely to occur is that in the case of vinylidene fluoride-ethylene trifluoride copolymer, the degree of crystallinity is extremely increased by heat treatment to increase kt compared to polyvinylidene fluoride. . By the way, k when the heat treatment time is reduced to a level that does not deteriorate the manufacturing yield of the device.

This only slightly overlays polyvinylidene fluoride, and there is no point in purposefully converting it into a copolymer.

In view of the problems of the prior art described above, the main object of the present invention is to provide excellent secondary processing characteristics or deformation resistance, which is necessary for increasing the yield of manufacturing elements for ultrasonic transducers, for example. Another object of the present invention is to provide a vinylidene fluoride copolymer piezoelectric film having a relatively high kt.

As a result of intensive research into the properties of vinylidene fluoride copolymers for the above-mentioned purpose, the present inventors have found that reduction in deformation resistance such as cracking during bending due to heat treatment to improve kt does not necessarily occur with vinylidene fluoride copolymers. It was found that the problem was not due to any inherent property of the polymer film, but rather due to an inappropriate alignment of the polymer molecular chains in the film. and,
In particular, the film obtained by forming a terpolymer of vinylidene fluoride-ethylene trifluoride-vinyl fluoride having a molecular weight of 100 ml or more has a molecular chain axis oriented parallel to the film surface, i.e. This is thought to be due to the high degree of plane orientation, but it has been found that after heat treatment and orientation polarization, it not only has a relatively high kt but also has practically sufficient secondary workability.

The polymer piezoelectric film of the present invention is based on such knowledge, and more specifically, the piezoelectric polymer film of the present invention has a concentration of 0.
．． Inherent viscocity measured as a dimethylformamide solution of 4 g/di is 1.8 dl/g or more, and vinylidene fluoride is 60 to 87 mol%, ethylene trifluoride is 710 to 40 mol%, and vinyl fluoride is 3 to 40 mol%. 20
It is characterized by consisting of an oriented polarized body of a vinylidene fluoride copolymer film consisting of mol%.

The present invention will be explained in more detail below.

The vinylidene fluoride copolymer used as a raw material in the present invention contains 60 to 8 vinylidene fluoride as a core component.
7 mol%, preferably 65 to 85 mol%, 10 to 40 mol% of trifufluorinated ethylene, preferably 15 to 30 mol%
, 3 to 20 mol% of vinyl fluoride, preferably 5 to 15
Contained in mole%. The raw material copolymer is preferably a ternary copolymer consisting of the above three components, but in addition, a small amount of fluorine-containing monomers such as tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, etc. One or more of these may be added as a structural unit.

In any of the above cases, if vinylidene fluoride, ethylene trifluoride, and vinyl fluoride are outside the above range,
In order to reduce kt or lose flexibility of the resulting piezoelectric film, it is necessary to keep the three components of the core area within the above composition range.

In the present invention, among the above-mentioned vinylidene fluoride-trifluoroethylene-vinyl fluoride copolymers, the temperature is 30°C.
Inherent viscocity (herein, simply referred to as "inherent viscocity" is based on this definition) measured as a dimethylformamide solution with a concentration of 0.4 g/di is 1.8 dl.
/g or more, preferably 2.0 dl/g or more, more preferably 2.2 dl/g or more, even more preferably 3.0 dl/g
g or more is used. There is no particular upper limit for inherent viscosity, but if this value becomes large, molding becomes difficult.For example, when forming a film using a solvent casting method, it is necessary to reduce the resin concentration in the solution, resulting in a thick Preferably it is less than io, odl/g, more preferably 9 d.
i/g or less, particularly preferably 8 dl/g or less.

In order to obtain a film from the above-mentioned copolymer, a method that easily causes plane orientation is preferably used, such as a casting method using a solvent, a T-die extrusion method, or a press molding method.
Among these, a casting method using a solvent is particularly preferably used. The solvent used for forming the solution may be any solvent as long as it can dissolve the copolymer at room temperature or under heating. For example, polar organic solvents such as dimethylacetamide, dimethylformamide, and dimethyl sulfoxide are preferably used. Although the resin concentration in the solution varies depending on the degree of polymerization of the copolymer, it is practically used in the range of 0.3 to 15% by weight, particularly 0.5 to 10% by weight. Film formation by the solvent casting method involves applying a uniform solution of a predetermined concentration as described above to, for example, a glass plate,
After forming a wet film by casting or coating on a substrate such as a synthetic board such as aluminum, or a polymer film that is difficult to dissolve in the solvent in the solution, the solvent is evaporated, and if necessary, This is done by peeling it off from the base material. The obtained copolymer film may be stretched as appropriate, and usually has a thickness in the range of several IL to about 500p.

The copolymer film obtained by the above method is then heat treated, preferably at a temperature between 5° C. below its crystal transition temperature and its melting point. Here, the melting point refers to the heating rate of 4℃
It is the temperature that gives the maximum endothermic peak in the DSC (differential, scanning, calorimeter) curve when the temperature is raised at a rate of /min, and the crystal transition temperature is a peak or shoulder that appears on the lower temperature side than the maximum endothermic peak. When there is more than one, it is defined as the temperature that gives a peak or shoulder close to the melting point. This heat treatment
This is done to increase film crystallinity and improve kt. The treatment time is preferably an appropriate time until the degree of crystallinity is almost saturated, but usually about 10 minutes to 2 hours is employed. Further, tension heat treatment (that is, heat treatment under conditions in which the periphery is fixed to such an extent as to suppress shrinkage during heat treatment) is preferably used.

After or together with this heat treatment, the piezoelectric film of the present invention can be obtained by subjecting the film to orientation and polarization treatment by applying an electric field. The conditions for applying the electric field are the same as those used for the polyvinylidene fluoride piezoelectric film. The higher the electric field strength is, the more desirable it is as long as it does not cause dielectric breakdown, and the longer the electric field application time is, the more desirable it is, but production efficiency -1-
, usually 300-1000Kv/Cm, about 10 minutes ~
The 2 o'clock book range is often used.

The piezoelectric film of the present invention obtained as described above can be deformed into a shape suitable for the device or subjected to secondary processing, for example in an ultrasonic transducer device, in order to improve the characteristics of the device. provided for use.

As described above, according to the method of the present invention, the film is composed of an oriented polarized vinylidene fluoride-ethylene trifluoride-vinyl fluoride copolymer film having a sufficiently large molecular weight, has a relatively high kt, and has good secondary processing characteristics. A polymer piezoelectric film with excellent properties can be obtained. Therefore, by performing secondary processing on this piezoelectric film, an electromechanical transducer element with generally high conversion efficiency, such as an element for an ultrasonic transducer with high ultrasonic receiving ability, can be obtained at a high yield.

Examples and comparative examples of the present invention are shown below.

Put an aqueous solution containing methyl cellulose as a suspending agent into a stainless steel autoclave equipped with a stirrer, cool it to 5°C, and add n-propyl peroxydicarbonate as a polymerization initiator.
Other polymerization aids were added, and the mixture was thoroughly stirred after replacing with N2.

After this, the autoclave was cooled from the outside with a methanol-dry ice system, and vinylidene fluoride, ethylene trifluoride, and vinyl fluoride were placed in the autoclave at molar ratios of 70%, 20%, and 10%, respectively. It was press-fitted from the cylinder. Next, the temperature inside the autoclave was raised to initiate polymerization, and then the outside temperature of the autoclave was maintained at about 25° C. to continue polymerization. The initial pressure of polymerization is 36
K g/c tn', a pressure drop was observed over time, and when it finally reached about 8 K g/cm', the residual pressure was purged to terminate the polymerization, and a white powder was formed. Obtained. It was thoroughly washed with water and dried to obtain a white copolymer powder.

The yield was 90% or more, which means that a terpolymer having a raw composition was obtained. The inherent viscocity η crystal of this copolymer was 5.5 dl/g.

This powder was mixed with dimethylformamide as a solvent.
A solution having a concentration of about 2% by weight was obtained by dissolving at a temperature of about 70°C, which was then cast and dried at 80°C to form a 30I
A cast film of approximately L size was obtained.

After drying the obtained unstretched film in air at 145°C for 1 hour, electrodes were formed on both sides by aluminum vapor deposition, and the electric field strength was 650 K at 85°C.
A DC voltage of V/cm was applied for 30 minutes, and while the voltage was being applied, it was cooled to room temperature to perform a polarization treatment. In order to calculate the piezoelectric constant d 81 of this film, the result of measurement at 10H2 using a rheolograph made by Toyo Seiki is d31 + d32 = 2
It was 5 pC/N. Note that the sample used here is an unstretched polymer film that has been polarized, so d 9
1='d32. In addition, piezoelectric constants d31, d
32 is defined as follows. That is, in the case of a polymer that exhibits piezoelectricity, the x, y, and z axes are generally determined by taking the y axis in the stretching direction, the y axis in the film stretching direction perpendicular to the stretching direction, and the two axes perpendicular to the film surface. The piezoelectric constant indicating biaxial polarization when stress is applied in the axial direction is d i
ll, 2 when stress is applied in the y-axis direction and the Z-axis direction
The piezoelectric constants indicating axial polarization are d32 and d, respectively.
83.

Furthermore, the electric machine coupling coefficient kt (z-X axis direction) was determined by analyzing the dependence of the electrical admittance and phase angle on the circumferential liquid number near the free resonance point of the piezoelectric film, and found that kt=0. It was 275. 1 of this piezoelectric film
As a result of the 80 degree bending test, no breakage occurred even after 50 or more reciprocating bends, and the product could be stretched with Tensilon at 25°C, 50% humidity, sample length 4 cm1, width 1 cm, and tensile speed 1 cm 7 minutes. When measured, it showed an elongation of over 180%.

2 and 3 The electric field strength during orientation polarization was set to 800 KV/c, respectively.
A piezoelectric film was obtained in the same manner as in Example 1 except that the voltage was 950 KV/cm.

A piezoelectric film was obtained in the same manner as in Example 1, except that a ternary copolymer having an η crystal of 0.92 dl/g and the same composition was used.

The results of Examples 2 and 3 and Comparative Example 1 obtained in the same manner as in Example 1 are shown together with the results of Example 1 in Table 1 below.

2 Table 1 *: Number of bends before cutting

Claims (1)

[Claims] 1. Inherent viscosity measured as a dimethylformamide solution with a concentration of 0.4 g/di at a temperature of 30° C. is 1.8 dl/1 or more, vinylidene fluoride 60 to 87 mol%, Trifluoroethylene 10-40
1. A polymeric piezoelectric film comprising an oriented polarized body of a vinylidene fluoride copolymer film containing mol% and 3 to 20 mol% of hynyl fluoride. 2.2 The polymer piezoelectric film according to claim 1, which is obtained by forming a hynylidene fluoride copolymer film by a casting method.