JPH0414517B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a polymeric piezoelectric material having high piezoelectric performance even in a high frequency range, and particularly to a method for manufacturing a polymeric piezoelectric material for a high-performance ultrasonic transducer. As polymeric piezoelectric materials having high piezoelectric performance, for example, polyvinylidene fluoride has been reported in Japanese Patent Publication No. 45-83771, and vinylidene fluoride copolymer has been reported in Japanese Patent Publication No. 50-29159. Moreover, these resins have been shown to have high piezoelectric performance even in the high frequency range, as reported in the 51st Japanese Patent Publication.
It is reported in JP-A No. 23439 or Japanese Patent Application Laid-open No. 111281-1981. Among these, polyvinylidene fluoride has an electromechanical coupling coefficient k _t of 0.20 in the direction perpendicular to the film surface and has good formability, and has been considered the most useful polymer material for ultrasonic transducers. However, further improvements in ultrasonic transmission and reception capabilities are desired. On the other hand, regarding the vinylidene fluoride copolymer described in JP-A-56-111281, especially the copolymer of vinylidene fluoride and ethylene trifluoride, for example, 75 mol% of vinylidene fluoride, ethylene trifluoride At a composition of 25 mol %, a k _t exceeding that of polyvinylidene fluoride is obtained. In addition, in order to increase the electromechanical coupling coefficient k _t in the thickness direction of such a copolymer molded product, the intensity of the Pauling electric field is increased, and the temperature of the crystal transition temperature (Tm') -5 (℃) and the melting point are increased. A method is also disclosed in which a poling treatment is performed after heat treatment in the temperature range, or a poling treatment is performed simultaneously with the heat treatment in this temperature range. The present invention further improves the electromechanical coupling coefficient k _t in the thickness direction from the above-mentioned vinylidene fluoride copolymer, and makes it easier to produce a polymeric piezoelectric material with high ultrasonic transmitting and receiving capabilities. The main objective is to provide a method for manufacturing. As a result of research for the above-mentioned purpose, the present inventors have found that it is effective to improve k _t by crystallizing a molten vinylidene fluoride copolymer as it is at a temperature close to its melting point and then subjecting it to poling treatment. It has been found that this method eliminates the need for another heat treatment step. The method for producing a polymeric piezoelectric material of the present invention is based on such knowledge.
A melt of a copolymer consisting of ~40 mol% and vinyl fluoride 0 to 20 mol% is processed without intermediate cooling.
This copolymer is characterized by being crystallized in a temperature range determined by less than the melting point temperature Tm and more than Tm -30°C, and then subjected to a poling treatment. Incidentally, the above-mentioned Japanese Patent Application Laid-open No. 56-111281 also discloses that after molding and prior to poling, heat treatment is performed at a high temperature near the melting point.
However, in this prior art, a combination of processes is employed in which, after the copolymer is once molded, it is subjected to another heat treatment, and thereafter, or at the same time, a poling treatment is performed. However, this method requires cooling to room temperature or its vicinity for molding, and then heat treatment again, which not only increases the number of steps and significantly reduces thermal efficiency, but also impairs the properties of the resulting piezoelectric material. It has become clear that this is not preferable, especially from the viewpoint of improving k _t . The reason for this is not necessarily clear, but as in the prior art described above,
Prior to heat treatment near the melting point, when forming is performed by intermediate cooling to room temperature, the temperature range that promotes rapid crystallization is passed through, and the partial crystal structure thus formed is then formed near the melting point. It is thought that this may inhibit crystallization during heat treatment or produce a non-uniform crystal structure. On the other hand,
As in the present invention, if the melted vinylidene fluoride copolymer is maintained at a temperature near its melting point without intermediate cooling, crystallization proceeds slowly and uniformly, which is desirable for improving k _t . It is estimated that a homogeneous crystal structure will develop. The present invention will be explained in more detail below. The main constituent monomers of the vinylidene fluoride copolymer used in the method of the present invention are vinylidene fluoride and ethylene trifluoride, or trifluoride containing vinylidene fluoride.
Vinylidene fluoride in this copolymer contains 60
~90 mol%, preferably 65-85%, trifluoroethylene 10-40 mol%, preferably 15-35 mol%, and vinyl fluoride 0-20 mol%, preferably 0-15 mol%. It is mole%. The vinylidene fluoride copolymer is preferably a binary or ternary copolymer consisting of the above components, but in addition, a small amount of
For example, up to about 5 mol % of one or more fluorine-containing monomers such as tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene 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, k _t becomes small, and the ultrasonic transmitting and receiving ability becomes small, and flexibility is lost. The above-mentioned constituent monomers are within the above-mentioned composition range. Such a vinylidene fluoride copolymer can be obtained by a polymerization method generally known for producing polyvinylidene fluoride. In the present invention, when the melt of the vinylidene fluoride copolymer is crystallized and molded, the melting point temperature of the copolymer is within a range of less than Tm and Tm - 30°C or more without intercooling the melt. It is held and crystallized. Intermediate cooling here refers to substantially maintaining the temperature below the crystallization temperature range mentioned above;
means. As mentioned above, such intercooling should be avoided since it causes undesirable crystal growth in the process. The crystallization temperature may be within the above range, but preferably Tm-5°C to Tm-
The temperature is preferably 25°C, more preferably Tm-10°C to Tm-20°C. A temperature gradient can also be applied within the above temperature range. Crystallization is substantially completed by maintaining the melted vinylidene fluoride copolymer at a temperature within the above range for 5 minutes or more, preferably about 10 minutes to 5 hours. The method for performing a series of steps from melting to crystallization and molding is not particularly limited as long as the necessary temperature conditions and shape retention conditions are met, but examples include press molding, blow molding, etc. , injection molding method is preferably used. Next, double-sided electrodes are formed on the sheet or film-like crystallized product thus obtained,
A piezoelectric body according to the present invention can be obtained by performing a poling process using a method such as applying a DC electric field. The poling conditions are essentially the same as those used for polyvinylidene difluoride piezoelectric films. In other words, the higher the applied 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 better, but from the viewpoint of production efficiency, it is usually
The range of 300 to 1000 KV/cm and about 10 seconds to 2 hours is often used. Further, the poling treatment is carried out at a temperature ranging from room temperature to the melting point, and is particularly preferably carried out at a crystal transition temperature (Tm') of -30°C to Tm'. The k _t of the polarized body (piezoelectric body) thus obtained according to the present invention can be determined by poling a molded body obtained by a method that involves crystallization during a normal intercooling process after or simultaneously with heat treatment. A larger k _t can be obtained than that of the polarized body obtained by Moreover,
As is clear from the above, in the method of the present invention, there is no need to perform heat treatment again after molding, so piezoelectric bodies can be manufactured simply and with high thermal efficiency with a small number of steps. Since the polarized body obtained by the present invention has a large electro-mechanical coupling coefficient _kt , it is particularly useful as an ultrasonic transducer. Further, this polarized body has various properties known in polarized bodies of polyvinylidene fluoride, such as elongation piezoelectricity and pyroelectricity, and is used for the same purposes as oriented polarized bodies of polyvinylidene fluoride. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. 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 and other polymerization aids were added as a polymerization initiator,
After replacing with _N2 , the mixture was stirred well. After this, the autoclave was cooled from the outside with a methanol-dry ice system, and vinylidene fluoride, ethylene trifluoride, and vinyl fluoride were added to the autoclave in molar ratios.
It was press-fitted from the cylinder so that it was 70%, 20%, and 10%. Next, increase the temperature inside the autoclave,
After polymerization, the outside temperature of the autoclave was maintained at about 25°C to continue polymerization. The initial polymerization pressure was 36Kg/ ^cm2 , and a decrease in pressure was observed over time, and the final pressure was about 8Kg/cm2.
The residual pressure was purged at the cm ² stage to terminate the polymerization, and the polymer was separated, thoroughly washed with water, and dried to obtain a white copolymer powder. The yield was 90% or more, and a copolymer having almost the composition as charged was obtained. The inherent viscocity ηinh of this copolymer was 0.92 under the measurement conditions of a concentration of 0.4 g/dl, a temperature of 25°C, and dimethylformamide as a solvent.
It was dl/g. In addition, the temperature (i.e., the melting point referred to herein) Tm at which the maximum value of the melting curve observed when this copolymer is measured with a differential scanning calorimeter (Model DSC2B manufactured by PerkinElmer) is 150.
It was warm at ℃. This copolymer was melt-pressed at 200℃ using a normal hot press equipment, and then
The mixture was cooled to 138°C, held at 138°C for 2 hours, and then cooled to room temperature to obtain a pressed sheet with a thickness of about 60 μm. Electrodes were formed on this press sheet by aluminum vapor deposition, a DC voltage with an electric field strength of 650 KV/cm was applied at 90° C. for 30 minutes, and the sheet was cooled to room temperature while the voltage was being applied to perform a poling treatment. The piezoelectric constant d ₃₁ of the obtained film was measured at 10Hz using a rheolograph made by Toyo Seiki. As a result, d ₃₁
=12.7pC/N. Note that since the sample used here is an unstretched polymer that has been subjected to a poling treatment, d ₃₁ =d ₃₂ . Note that the piezoelectric constants d ₃₁ and d ₃₂ are defined as follows. In other words, in the case of polymers that exhibit piezoelectricity, the x-axis is generally in the stretching direction, and the y-axis is perpendicular to the stretching direction.
axis, the z-axis is taken perpendicular to the film surface, and x, y, z
z when the axis is determined and stress is applied in the x-axis direction
A piezoelectric constant indicating polarization in the axial direction is d ₃₁ , and piezoelectric constants indicating polarization in the z-axis direction when stress is applied in the y-axis direction and the z-axis direction are d ₃₂ and d ₃₃ , respectively. The electromechanical comprehensive coefficient k _t (in the z-z axis direction) was found to be k _t =0.273 by analyzing the frequency dependence of the electrical admittance and phase angle near the free resonance point of the piezoelectric film. Example 2 Vinylidene fluoride 75
A binary copolymer containing 25 mol% of ethylene trifluoride was obtained. The Tm of this copolymer is 149℃
ηinh was 1.29 dl/g. A press sheet of this copolymer was produced under exactly the same conditions as in Example 1,
As a result of polling under the same conditions, the d ₃₁ constant is
It was 11.5 pC/N, and k _t was 0.261. Comparative Examples 1 and 2 The ternary and binary copolymers used in Examples 1 and 2 were melt-pressed at 200°C using the hot press equipment used in Example 1, and then transferred to a cooling press. The mixture was cooled to room temperature to obtain a pressed sheet with a thickness of about 60 μm. After heat-treating these untreated press sheets in air at 138°C for 2 hours, electrodes were formed by aluminum vapor deposition, and then poling was performed under the same conditions as in Examples 1 and 2 to obtain d ₃₁ and k. When we measured _t , we obtained the results shown in the following table.

[Table] The results of Comparative Examples 1 and 2 are summarized in Example 1.
Comparing the results of 2 and 2, the piezoelectric material obtained by the method of the present invention is superior in both aspects of d ₃₁ and k _t to that obtained by the conventional manufacturing method from a polymer of the same composition. I understand that there are.

Claims (1)

[Claims] 1. A melt of a copolymer consisting of 60 to 90 mol% of vinylidene fluoride, 10 to 40 mol% of ethylene trifluoride, and 0 to 20 mol% of vinyl fluoride is heated to the melting point of this copolymer without intermediate cooling. Temperature below Tm, Tm
1. A method for producing a polymer piezoelectric material, which comprises performing a poling treatment after substantially completing crystallization in a temperature range determined by -30° C. or higher.