JPH051636B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a polymer piezoelectric material that has high piezoelectric performance even in a high frequency range and has excellent device fabrication performance. The present invention relates to a method of manufacturing a polymer piezoelectric material for an ultrasonic transducer.

As a polymeric piezoelectric material having high piezoelectric performance, for example, polyvinylidene fluoride is disclosed in Japanese Patent Publication No. 50-40720, and Japanese Patent Publication No. 50-29159, Japanese Patent Application Laid-open No. 56-111281, Japanese Patent Application Publication No. 58-60585, etc. A vinylidene fluoride copolymer has been reported in the publication. Moreover, these polypyridine fluoride resins are
It is reported in Japanese Patent Publication No. 51-23439 that it has high piezoelectric performance even in the high frequency range. These polymeric piezoelectric materials are generally made into films by roll rolling or casting, and then the electromechanical coupling coefficient in the direction perpendicular to the film surface is
A piezoelectric film is formed by heat treatment to increase Kt (hereinafter often simply referred to as "Kt") and by applying an electric field perpendicular to the film. Among these series of vinylidene fluoride resins, polyvinylidene fluoride has a Kt of approximately 0.2 and good moldability, and has been considered the most useful polymer element material for ultrasonic transducers. , further improvements in ultrasonic transmission and reception capabilities are desired.

On the other hand, the vinylidene fluoride copolymer described in the above patent publication, particularly the copolymer of vinylidene fluoride and ethylene trifluoride, has a composition of around 75 mol% vinylidene fluoride and 25 mol% ethylene trifluoride. Kt higher than polyvinylidene fluoride can be obtained in roll-rolled products and cast films. 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. Furthermore, even if the device is made without cracking, it may be prone to cracking over time, or even if it does not crack, it may become large.
The improvement in ultrasonic transmission and reception ability was not seen as much as with Kt.

Vinylidene fluoride is more likely to crack.
This is because, in the case of ethylene trifluoride copolymers, the degree of crystallinity is extremely increased by heat treatment to increase Kt, compared to polyvinylidene fluoride. By the way, Kt when the heat treatment time is reduced to a level that does not deteriorate the device manufacturing yield.
is only slightly superior to polyvinylidene fluoride, and there is no point in purposefully converting it into a copolymer. As shown above, in the conventional technology, element manufacturing performance and
I couldn't get a balanced version of Kt.

In view of the above-mentioned problems of the prior art, the main object of the present invention is to provide a material with excellent secondary processing characteristics or deformation resistance and a relatively high Kt, for example, in order to increase the yield of manufacturing ultrasonic transducer elements. An object of the present invention is to provide a method for producing a vinylidene fluoride copolymer piezoelectric film that maintains high ultrasonic wave transmission and reception capabilities.

For the above-mentioned purpose, the present inventors have conducted extensive research on the properties of vinylidene fluoride-ethylene trifluoride copolymers, and have found that the resistance to deformation, such as cracking during bending, due to heat treatment to improve Kt. The decline is
This is not necessarily due to the inherent properties of the vinylidene fluoride-ethylene trifluoride copolymer film, but is due to the inappropriate arrangement of the polymer molecular chains in the film, and it is believed that increasing the molecular weight of the copolymer may result in an inappropriate alignment of the polymer molecular chains within the film. , and if an appropriate film formation method is adopted to arrange the polymer molecular chains parallel to the film surface, a sufficiently high Kt can be maintained after heat treatment, and practically sufficient secondary processability can be obtained. I found out.

In light of such knowledge, when looking at the conventional manufacturing technology of vinylidene fluoride-ethylene trifluoride copolymer piezoelectric films, JP-A-56-111281 discloses extrusion, press roll rolling, etc. as specific examples. However, these film-forming methods are not preferable methods for essentially arranging polymer molecular chains in a direction parallel to the film surface. On the other hand, casting from polymer solution is
Although potentially providing good alignment of the polymer molecular chains, in order to actually obtain good alignment the polymer needs to be of fairly high molecular weight. This point was discovered by the present inventors when casting vinylidene fluoride polymers, and was clarified in the specification of Japanese Patent Application No. 57-214249. - It was also observed for trifluoroethylene copolymer. On the other hand, in the conventional technology that employs film formation by casting this copolymer, the above-mentioned meaning of molecular weight is not recognized at all, and the above-mentioned copolymer having a sufficient molecular weight is not used. I can't imagine there being any. For example, the inherent viscocity of the copolymer used in the specific example of Japanese Patent Publication No. 50-29159 is about 1.0 dl/g. Further, in the specific example shown in JP-A-58-60585, a highly concentrated solution is used, and considering the solubility of this copolymer, it is considered that the copolymer is related to a relatively small molecular weight polymer. The present invention is based on the above series of findings, uses a high molecular weight vinylidene fluoride-ethylene trifluoride copolymer, and maximizes the arrangement effect of polymer molecular chains by the casting method.
The object of the present invention is to produce a vinylidene fluoride-ethylene trifluoride copolymer piezoelectric film having a good molecular chain arrangement, and thus a good Kt and good secondary processability even after heat treatment. That is, the method for producing the vinylidene fluoride copolymer piezoelectric film of the present invention is carried out at a temperature of 30
The inherent viscocity measured as a dimethylformamide solution with a concentration of 0.4 g/dl at °C is 3.0 dl/g or more, and vinylidene fluoride 40
~90 mol% and trifluoroethylene 10-60 mol%
The resulting copolymer film is heat-treated at a temperature between 5°C below the crystal transition temperature and the melting point, and simultaneously or after the heat treatment, The process is characterized by applying a direct current electric field substantially perpendicular to the .

The present invention will be explained in more detail below.

The vinylidene fluoride copolymer used as a raw material in the present invention contains vinylidene fluoride in an essential component of 40 to 90 mol%, preferably 65 to 85 mol%,
10 to 60 mol% of ethylene trifluoride, preferably 15
Contains at a rate of ~30 mol%. The raw material copolymer is preferably a binary copolymer consisting of the above two components, but in addition, small amounts of vinyl fluoride, ethylene tetrafluoride, propylene hexafluoride, ethylene trifluoride chloride, etc. One or more types of element-containing monomers may be added as structural units. In any of the above cases, if vinylidene fluoride and ethylene trifluoride are outside the above range, the resulting piezoelectric film will deteriorate.
Since Kt becomes small, the ultrasonic wave transmission/reception ability becomes small, and flexibility is lost, it is necessary to keep the above-mentioned two essential components within the above-mentioned composition range.

In the present invention, among the vinylidene fluoride-ethylene trifluoride copolymers mentioned above, the inherent viscocity (herein simply referred to as "inherent viscocity") measured as a dimethylformamide solution with a concentration of 0.4 g/dl at a temperature of 30 ° C. When referring to "healent viscocity", it is based on this definition) that is 3.0 dl/g or more.
There is no particular upper limit for inherent viscocity, but as this value increases, it becomes necessary to reduce the concentration of the solution during casting, which becomes an obstacle when producing thick films, so it is usually
10.0 dl/g or less is often used.

A copolymer as described above is made into a solution and formed into a film by casting. The solvent used for solution molding may be any solvent that can dissolve the copolymer at room temperature or under heating. For example, dimethylacetamide, dimethylformamide,
Polar organic solvents such as dimethyl sulfoxide are preferably used. The resin concentration in the solution varies depending on the degree of polymerization of the copolymer, but in practical terms,
A content of about 0.3 to 15% by weight, particularly about 0.5 to 10% by weight is adopted. Film formation is carried out by casting a uniform solution of a predetermined concentration as described above according to a known method. For example, after forming a wet film by casting or coating on a substrate such as a glass plate, a synthetic plate such as aluminum, or a polymer film that is difficult to dissolve in the solvent in the solution, the solvent is evaporated, A copolymer film can be obtained by peeling it from the base material if necessary. This copolymer membrane is
It may be stretched as appropriate, and the thickness range is usually from several microns to about 500 microns. The copolymer film obtained by the above method is then heat treated at a temperature between 5° C. below its crystal transition temperature and its melting point. Here, the melting point is the temperature that gives the maximum endothermic peak in the DSC (differential scanning, calorimeter) curve when the temperature is raised at a heating rate of 4°C/min, and the crystal transition temperature is
A peak or shoulder that appears on the lower temperature side than the maximum endothermic peak, and when there are multiple peaks or shoulders, it is defined as the temperature that gives a peak or shoulder close to the melting point. This heat treatment is performed to increase the crystallinity of the film and improve the 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.

After or together with this heat treatment, an electric field is applied. The electric field application conditions are essentially the same as those used for polyvinylidene fluoride piezoelectric films. The electric field strength is preferably as high as possible without causing dielectric breakdown, and the electric field application time is also preferably long.
For production efficiency, normally 300-1000KV/cm, approx.
The 10 minute/2 hour range is often used. 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.

The piezoelectric film obtained in the above manner may be deformed into a shape suitable for the device or secondary Processed and used.

As mentioned above, according to the method of the present invention, a vinylidene fluoride-ethylene trifluoride copolymer having a sufficiently large molecular weight is used, and after forming a film by casting, heat treatment and electric field treatment are performed. Vinylidene fluoride has a high Kt and excellent secondary processing characteristics, so it can be used to produce electromechanical transducer elements with high conversion efficiency in general, and ultrasonic transducer elements with high ultrasonic wave receiving ability in high yield. A copolymer piezoelectric membrane is obtained.

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

Example 1 An aqueous solution containing methylcellulose as a suspending agent was placed in a stainless steel autoclave equipped with a stirrer, and after cooling to 5°C, n-propyl peroxydicarbonate as a polymerization initiator and other polymerization aids were added, and N ₂ After the substitution, the mixture was thoroughly stirred. After that, the autoclave was cooled from the outside with a methanol-dry ice system, and vinylidene fluoride and ethylene trifluoride were added to the autoclave at a molar ratio of 75
%, and was press-fitted from the cylinder so that it was 25%. Next, the temperature inside the autoclave was raised to initiate polymerization, and then the outside temperature of the autoclave was maintained at about 20° C. to continue polymerization. The initial pressure of polymerization is 32Kg/cm ²
A decrease in pressure was observed over time, and the pressure eventually reached approximately 8
When the residual pressure reached Kg/cm ² , the residual pressure was purged to terminate the polymerization, and a white powder was 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 binary copolymer having almost the same composition as the initial composition was obtained. The inherent viscocity ηinh of this copolymer is 5.9dl/g
It was hot. This powder was casted using dimethylformamide as a solvent to obtain a cast film of about 30 μm. This unstretched film
After drying in air at 132℃ for 1 hour, electrodes were formed on both sides by aluminum evaporation, and a DC voltage with an electric field strength of 650KV/cm was applied at 85℃.
The voltage was applied for 30 minutes, and the voltage was cooled to room temperature while the voltage was being applied to perform polarization treatment. The piezoelectric constant d ₃₁ of this film was measured at 10 Hz using a rheograph manufactured by Toyo Seiki Co., Ltd., and the result was d ₃₁ +d ₃₂ =25 pC/N. Note that since the sample used here is an unstretched polymer film that has been subjected to polarization treatment, d ₃₁ =d ₃₂ . Moreover, the piezoelectric constants d ₃₁ and d ₃₂ are defined as follows. That is, in the case of polymers exhibiting piezoelectricity, the x-axis is generally in the stretching direction, the y-axis is in the film stretching direction perpendicular to the stretching direction, and the z-axis is perpendicular to the film surface.
Determine the y and z axes, and calculate the piezoelectric constant that indicates the polarization in the z-axis direction when stress is applied in the x-axis direction as d ₃₁ , and the polarization in the z-axis direction when stress is applied in the y-axis and z-axis directions. Let the piezoelectric constants representing d ₃₂ and d ₃₃ be respectively. The electromechanical coupling coefficient Kt (z-z axis direction) is
The result obtained by analyzing the frequency dependence of the electrical admittance and phase angle near the free resonance point of the piezoelectric film was Kt=0.275. As a result of a 180 degree bending test of this piezoelectric film, the reciprocating
Even if it is bent more than 50 times, it will not break, and the
℃, 50% humidity, sample length 4cm, width 1cm, tensile speed 1
Tensilon elongation measurements in cm/min showed an elongation of 165%.

In order to examine the ultrasonic wave transmitting and receiving ability of this piezoelectric film, an ultrasonic transducer 1 whose vertical cross-sectional view is shown in FIG. 1 was used. That is, this transducer 1 includes a bakelite rod 2 having a concave shape with a radius of curvature of 7 mm at one end, a copper plate 3 having a quarter wavelength thickness (i.e. 60 μm) serving as an acoustic reflector, and a piezoelectric film 4 on the concave surface of the bakelite rod 2. These are successively bonded together using epoxy adhesive. Note that the ultrasonic transducer has electrodes (not shown) with a diameter of 10 mm on both sides. As shown in FIG.
It was connected to AEROTECH UTA-3) and an oscilloscope 9, and the ultrasonic transmission and reception ability was measured by the pulse echo method. receiver gain
When I set it to 40dB and read the pulse echo voltage, it was 43V at Vp.p. (peak.to.peak voltage).
The value of was obtained.

Comparative Example 1 Vinylidene fluoride obtained by a method according to Example 1
A binary copolymer consisting of 75 mol% and 25 mol% of ethylene trifluoride (inherent viscosity ηinh).
A cast film with a thickness of about 30 μm prepared from a solution of 1.29 dl/g) was subjected to heat treatment and polarization treatment under exactly the same conditions as in Example 1. The obtained film had a d ₃₁ +d ₃₂ constant of 20 pC/N and a Kt of 0.217. As a result of a 180 degree bending test, this piezoelectric film could be cut by bending it back and forth once.
Furthermore, the elongation of Tensilon at room temperature was zero, making it a very fragile piezoelectric film. Using this piezoelectric film, an ultrasonic transducer was manufactured in the same manner as in Example 1, and the output voltage (Vp.
p.) was measured and found to be 28V. In manufacturing the ultrasonic transducer, the piezoelectric body and the copper plate were bonded together under the same conditions as in Example 1, but due to the piezoelectric body being fragile, cracks often occurred during the manufacturing process, resulting in a large amount of damage. A loss has occurred.

Comparative Example 2 Vinylidene fluoride obtained by a method similar to Example 1
Binary copolymer consisting of 75 mol% and trifluoroethylene 25 mol% (inherent viscocity ηinh=
1.82 dl/g) far above the melting point.
Although it was applied to a melt indexer at 270°C, the resin did not fall easily. This indicates a high molecular weight that is clearly unsuitable for hot melt molding.

Next, the above copolymer was dissolved, and a cast film with a thickness of about 30 μm obtained from the solution in the same manner as in Example 1 was subjected to heat treatment and polarization treatment under exactly the same conditions as in Example 1. d ₃₁ of the obtained film
The +d ₃₂ constant was 21 pC/N and the Kt was 0.229. The number of samples for this piezoelectric film was 3 (n=
In 3), a 180 degree bending test was performed, and as a result, two sheets were cut after one round trip, and the remaining sheet was also cut after two round trips. Furthermore, the elongation due to Tensilon at room temperature was almost zero. Using this piezoelectric film, an ultrasonic transducer was manufactured in the same manner as in Example 1, and the output voltage (Vp.p.
When I measured it, it was 30V. In manufacturing the ultrasonic transducer, the piezoelectric body and the copper plate were bonded under the same conditions as in Example 1, but cracks could not be generated during the manufacturing process due to the lack of strength of the piezoelectric body. Nakatsuta.

As can be seen from the above examples, the piezoelectric film obtained by the method of the present invention has an elongation of 20% or more by the measurement method shown in the above example, 30% or more under preferable conditions, and even 40% or more. It also has excellent secondary processability or deformation resistance to improve conversion performance as an electro-mechanical conversion element, including an element for an ultrasonic transducer. In addition, as Kt=0.275 was obtained in the example, Kt
is generally 0.22 or more, and under preferable conditions it can be 0.23 or more, and even 0.24 or more. In addition, this piezoelectric film has various performances common to polyvinylidene fluoride piezoelectric materials, including elongation piezoelectricity and pyroelectricity, and is also characterized by excellent mechanical properties. .

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view of an ultrasonic transducer using a piezoelectric film according to the method of the present invention, and Fig. 2 is a block diagram of a device system for investigating the ultrasonic wave transmission and reception ability of this ultrasonic transducer. It is a diagram. DESCRIPTION OF SYMBOLS 1... Ultrasonic transducer, 2... Bakelite rod, 3... Copper plate, 4... Piezoelectric film, 6... Acrylic resin board, 7... Matching circuit, 8... Pulsar receiver, 9... Oscilloscope.

Claims (1)

[Claims] 1 Inherent viscocity measured as a dimethylformamide solution with a concentration of 0.4 g/dl at a temperature of 30°C is 3.0 dl/g or more, and vinylidene fluoride 40 to 90 mol% and ethylene trifluoride 10
A copolymer consisting of ~60 mol% is casted, and the resulting copolymer film is heated to a crystal transition temperature of 5°C.
A piezoelectric film made of a vinylidene fluoride copolymer, which is heat-treated at a temperature between the temperature below and the melting point, and is treated by applying a DC electric field substantially perpendicular to the copolymer film simultaneously with or after the heat treatment. Production method.