Manufacturing method of piezoelectric material

Abstract

To provide a method for manufacturing a sheet material containing PVDF and having piezoelectricity on its outermost surface while maintaining the mechanical properties of the sheet material as a base material, in a low-cost and highly productive manner.SOLUTION: A manufacturing method of piezoelectric material including a resin component mainly composed of polyvinylidene fluoride includes, in this order, a step I of applying a coating liquid containing the polyvinylidene fluoride and titanium dioxide as a β-crystal nucleating agent for the polyvinylidene fluoride to a sheet-like material that is a substrate, a drying step II, and a polarization treatment step III, and the content of titanium dioxide in the coating liquid is 1 pt.mass or more and 20 pts.mass or less with respect to a total of 100 pts.mass of polyvinylidene fluoride, and the solvent of the coating liquid is a solvent having a boiling point of 100°C or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a piezoelectric material.

Background Art

[0002] Due to the impact of the spread of coronavirus infection, the importance of imparting hygienic functions such as antibacterial and antiviral properties to materials has been increasing.

[0003] In these techniques, a method of coating an antibacterial agent or an antiviral agent on the material surface is well known. Recently, antibacterial materials that utilize the piezoelectricity of specific materials (the property of generating a voltage according to the amount of strain applied to the material) have been developed and have attracted attention. Patent Document 1 discloses such a technique, and a fiber material having an antibacterial function that utilizes the fact that a stretched material of polylactic acid has piezoelectricity is described.

[0004] There are several polymer materials that exhibit piezoelectricity. As a typical example, polyvinylidene fluoride (hereinafter sometimes referred to as PVDF) is known. Patent Document 2 discloses a piezoelectric material using PVDF. By utilizing the piezoelectricity of this PVDF, it is possible to create a material having the same hygienic function as the polylactic acid of Patent Document 1.

[0005] Thus, various studies have been conducted on piezoelectric materials. However, the polylactic acid disclosed in Patent Document 1 is a material with low heat resistance and cannot be used in applications that require heat resistance, such as automotive applications. In addition, although PVDF described in Patent Document 2 exhibits higher heat resistance than polylactic acid, unlike polylactic acid that can exhibit piezoelectricity simply by performing a stretching process, complex processes must be carried out to develop piezoelectricity. Usually, PVDF takes the form of α-crystals and does not have piezoelectricity in this state. However, as disclosed in Patent Document 2 and Patent Document 3, by performing a stretching process or casting from a specific solvent such as hexamethylphosphoric triamide to transfer α-crystals to β-crystals and then further performing a polarization process called poling, it is known that PVDF with piezoelectricity can be obtained.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when attempting to form β-crystals by stretching, heat treatment is required to grow β-crystals after stretching, and this heat treatment requires a significant amount of time. In addition, stretching is difficult if the PVDF before stretching does not take the form of a molded product such as a film or thread. For example, when it is desired to impart a sanitary function utilizing piezoelectricity to an existing sheet-like material, the molded product of PVDF will be used in combination with the existing sheet-like material, and there is a problem that its mechanical properties will be impaired.

[0008] On the other hand, when casting from a specific solvent such as hexamethylphosphoric triamide, if a solution containing PVDF is coated on a sheet-like material as a substrate, piezoelectricity can be imparted only to the outermost surface while maintaining the mechanical properties of the substrate. However, since the solvent is expensive or has a high boiling point, cost and productivity become issues.

[0009] Therefore, in view of such problems, the present invention aims to provide a method for manufacturing a piezoelectric material that contains PVDF, maintains the mechanical properties of a sheet-like material as a substrate, and realizes a piezoelectric material having piezoelectricity on the outermost surface in a form excellent in low cost and productivity.

Means for Solving the Problems

[0010] In order to solve such problems, the present invention is a method for manufacturing a sheet-like material that adopts the following configuration. That is, (1) A method for manufacturing a piezoelectric material containing a resin component mainly composed of polyvinylidene fluoride, comprising a step I of applying a coating liquid containing polyvinylidene fluoride and titanium dioxide as a β crystal nucleating agent for the polyvinylidene fluoride to a sheet-like material, a drying step II, and a poling treatment step III in this order, wherein the content of titanium dioxide in the coating liquid is 1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass in total of the polyvinylidene fluoride, and the solvent of the coating liquid is a solvent having a boiling point of 100°C or lower. This is a method for manufacturing a piezoelectric material. (2) Preferably, the drying temperature in the drying step II is 50°C to 90°C, and this is the method for manufacturing a piezoelectric material according to (1).

Effects of the Invention

[0011] According to the present invention, it is possible to provide a method for manufacturing a piezoelectric material for realizing a piezoelectric material that contains PVDF, maintains the mechanical properties of a sheet-like material as a substrate, and has piezoelectricity on the outermost surface in a form excellent in low cost and productivity.

Modes for Carrying Out the Invention

[0012] The manufacturing method of the piezoelectric material of the present invention will be described in detail below. The present invention is a method for manufacturing a piezoelectric material containing a resin component mainly composed of polyvinylidene fluoride (hereinafter referred to as PVDF), comprising a step I of applying a coating liquid containing PVDF and titanium dioxide as a β crystal nucleating agent for this PVDF to a sheet-like material as a base material, a drying step II, and a poling treatment step III in this order. The content of titanium dioxide in the above coating liquid is 1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass in total of the above PVDF, and the solvent of the coating liquid is a solvent having a boiling point of 100°C or lower. It is a manufacturing method of a piezoelectric material, characterized by this.

[0013] In the present invention, since a β crystal nucleating agent of PVDF is contained in the coating liquid, when the coating liquid is applied to the base material and the solvent is removed by the drying step II, β crystals are rapidly formed around the β crystal nucleating agent. Further, since the boiling point of the solvent used for the coating liquid is as low as 100°C or lower, removal by evaporation of the solvent becomes easy. As a result, the manufacturing method of the present invention is low-cost and has excellent productivity.

[0014] <pvdf> The PVDF of the present invention means a polymer having vinylidene fluoride as the main monomer. Specifically, it means that the proportion of vinylidene fluoride is 90 mol% or more with respect to 100 mol% of all the monomers constituting PVDF. Since the piezoelectricity of PVDF depends on the crystallinity of PVDF, if a monomer component other than vinylidene fluoride is included, the crystallinity will decrease, and as a result, the desired piezoelectricity cannot be obtained. Therefore, it is preferable that the proportion of vinylidene fluoride is higher, and it is preferably 95 mol% or more with respect to 100 mol% of all the monomers constituting PVDF.

[0015] In addition, the PVDF of the present invention may contain one or more monomers other than vinylidene fluoride as long as the above-mentioned crystallinity is not impaired. Examples of other monomers include tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, 2,3,3,3-tetrafluoropropene, pentafluoropropene, perfluoromethyl vinyl ether, and perfluoropropyl vinyl ether.

[0016] <β crystal nucleating agent> The β crystal nucleating agent of the present invention is an additive that promotes the formation of β crystals of PVDF. Regarding the β crystals of PVDF, it is known from the research report of the Faculty of Engineering, University of Yamanashi (No. 45, December 1994, pages 16 - 20) etc. that the formation is promoted by melt-kneading and blending polymethyl methacrylate (PMMA) and PVDF and then performing aging. However, in this method, a large amount of PMMA is required to promote the formation of β crystals, and as a result, the crystallinity of PVDF greatly decreases, so that sufficient piezoelectricity cannot be obtained when used as a piezoelectric material. On the other hand, when casting a solution containing PVDF, the inventors have found that the addition of a small amount of titanium dioxide promotes the formation of β crystals, that is, titanium dioxide functions effectively as a β crystal nucleating agent.

[0017] In the present invention, the content of titanium dioxide in the coating liquid described later is 1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass in total of PVDF. If the content of titanium dioxide is less than 1 part by mass, a sufficient β crystal nucleating agent effect cannot be obtained. On the other hand, if the content of titanium dioxide is more than 20 parts by mass, the amount of PVDF on the surface of the piezoelectric material becomes relatively small, and thus the desired piezoelectricity cannot be obtained. Thus, from the viewpoint that a certain amount or more of titanium dioxide is required to promote β crystal formation while a certain amount or more of PVDF is required to obtain the desired piezoelectricity, the content of titanium dioxide in the coating liquid described later is preferably 3 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass in total of PVDF.

[0018] <Solvent and Coating Liquid> In the method for manufacturing the piezoelectric material of the present invention, a solvent having a boiling point of 100°C or lower is used as the solvent of the coating liquid containing PVDF and the β crystal nucleating agent of PVDF. Specific examples of such solvents include acetone and methyl ethyl ketone. Other solvents capable of dissolving PVDF include dimethylformamide, dimethyl sulfoxide, dimethylacetamide, gamma-butyrolactone, hexamethylphosphoric triamide, etc. In particular, hexamethylphosphoric triamide is known to have an effect of promoting β crystal formation during casting as described at the beginning. However, since these solvents have a high boiling point, when removing the solvent by drying after coating on the sheet-like material as the substrate, heating at a high temperature for a long time is required, which impairs the productivity of the piezoelectric material, or due to the heat resistance problem of the sheet-like material as the substrate, heating at a high temperature for a long time cannot be performed in the first place, and the solvent cannot be completely removed. Conversely, if heating is performed at a high temperature for a long time to remove the solvent, there is a problem that the mechanical strength of the sheet-like material itself is impaired. Also, it is known that β crystals will transform into α crystals when exposed to high temperatures. When heating is performed at a high temperature for a long time to remove the solvent, there is a problem that the β crystals will disappear.

[0019] After such coating, the solvent contained in the coating liquid can be efficiently heated and removed, and by setting the heating temperature to a low temperature of 100°C or lower, it is possible to suppress thermal deterioration and thermal deformation of the sheet-like material itself, and maintain the mechanical strength of the sheet-like material. From the viewpoint of suppressing the transition of the above-mentioned β-crystals to α-crystals, in the method for producing a piezoelectric material of the present invention, it is preferable to use acetone and / or methyl ethyl ketone as a solvent having a boiling point of 100°C or lower.

[0020] In the method for producing a piezoelectric material of the present invention, the content of PVDF in the coating liquid is preferably 1% by mass or more and 20% by mass or less with respect to 100% by mass of the total solvent constituting the coating liquid. If the content of PVDF is less than 1% by mass, the amount of PVDF that can be coated on the sheet-like material as the base material becomes too small, and thus the desired piezoelectricity may not be obtained. On the other hand, if the content of PVDF is higher than 20% by mass, PVDF may not be completely dissolved in the solvent. From such a viewpoint, the content of PVDF is more preferably 3% by mass or more and 10% by mass or less.

[0021] <Sheet-like material> The sheet-like material used in the method for producing a piezoelectric material of the present invention is a material having a sheet-like shape, and specifically, examples include resin films, foam sheets, non-woven fabric sheets, woven fabric sheets, knitted fabric sheets, and the like.

[0022] As the material constituting the sheet-like material, a resin material is preferable from the viewpoint of the interlayer adhesion with PVDF, and among resin materials, from the viewpoints of resistance to the solvent and resistance to the drying temperature described later, polypropylene, cycloolefin copolymer, polyethylene terephthalate, polybutylene terephthalate, poly naphthalene terephthalate, 6-nylon, 6,6-nylon, PVDF, ethylene tetrafluoroethylene copolymer, polytetrafluoroethylene, etc. are preferable.

[0023] <Step I of applying the coating liquid and Step II of drying> The manufacturing method of the piezoelectric material of the present invention includes step I of applying a coating liquid containing PVDF and titanium dioxide as a β crystal nucleating agent of PVDF and composed of a solvent having a boiling point of 100°C or lower to a sheet-like material serving as a base material. As methods for applying the coating liquid to the above sheet-like material, there are a knife coating method, a comma coating method, a gravure coating method, a dipping method, etc., and a coating method adapted to the type of each base material and the viscosity of the coating liquid may be adopted.

[0024] Further, in the manufacturing method of the piezoelectric material of the present invention, following the above step I, it has a drying step II for removing the solvent. In the drying step II, while it is easier to remove the solvent when the drying temperature is set higher, if the drying temperature is too high, the molecular mobility of PVDF increases, and as a result, a part of the β crystals formed during drying may transfer to α crystals, and thus a so-called β crystal inhibition may occur. From the viewpoints of drying efficiency and promotion of β crystal formation, the drying temperature is preferably 50°C or higher and 100°C or lower.

[0025] <Polarization treatment> In the manufacturing method of the piezoelectric material of the present invention, following the above drying step II, it has a polarization treatment step III. By performing the polarization treatment in this step III, the dipoles in the β crystals of PVDF are oriented in the direction of the electric field, and piezoelectricity can be exhibited in the obtained piezoelectric material.

[0026] Examples of the polarization treatment step III include a method of sandwiching both surfaces of the sheet-like material after finishing step II with electrodes and applying a pulse voltage, and a method of applying a voltage from the coated surface side of the sheet-like material after finishing step II using a corona discharge device.

Examples

[0027] Hereinafter, the present invention will be described more specifically using examples. The measurement methods of the characteristic values shown in the examples are as follows.

[0028] A. Content of β crystals Using a Fourier transform infrared spectrophotometer "IR Prestige-21 (manufactured by Shimadzu Corporation)", the infrared absorption spectrum of the surface coated with PVDF was measured, and the peak intensity of 840 cm -1 which is a peak attributed to the β crystal, and the peak intensity of 765 cm -1 which is a peak attributed to the α crystal were used to calculate the ratio of the β crystal. Specifically, 815 cm -1 and 860 cm -1 in the obtained infrared absorption spectrum were connected by a straight line to create a baseline. Then, the difference between the absorbance at 840 cm -1 of this baseline and the absorbance at 840 cm -1 in the infrared absorption spectrum was taken as the peak intensity of 840 cm -1 . Similarly, for the peak of 765 cm -1 , using the baseline obtained by connecting 780 cm -1 and 750 cm -1 by a straight line, the difference between the absorbance at 765 cm -1 of the baseline and the absorbance at 765 cm -1 in the infrared absorption spectrum was taken as the peak intensity of 765 cm -1 . Based on these values, the ratio of the β crystal was calculated using the following formula. Ratio of β crystal (%) = Peak intensity of 840 cm -1 / (Peak intensity of 840 cm -1 + Peak intensity of 765 cm -1 ) × 100 The measurement conditions of the infrared absorption spectrum were as follows.

[0029] · Unit: ATR method unit (MIRacleA) · Measurement wave number: 400 - 4000 cm -1 .

[0030] B. Retention rate of mechanical properties The retention rate of the mechanical properties of the sheet-like material was measured by the method of JIS K7127 (1999) using TENSILON (registered trademark) UCT-100 (manufactured by Orientec). The measurement method is as follows.

[0031] · Sample shape: Strip shape with a length of 150 mm and a width of 10 mm in the measurement direction · Initial chuck distance: 50 mm · Tensile speed: 200 mm / min.

[0032] For each sample, five strip-shaped samples were prepared, and the average value of the tensile strength obtained from five measurements was taken as the strength of that sample. Also, the above measurements were performed on the sheet-shaped base material before coating PVDF and the sheet-shaped base material before and after coating, including the sheet-shaped material after coating, and the strength retention rate was calculated based on the following formula. Strength retention rate (%) = (Tensile strength of the base material - Tensile strength of the sheet-shaped material after coating) / Tensile strength of the base material × 100.

[0033] C. Piezoelectricity (Change amount of surface potential) Using the tensile testing machine and surface potential measuring instrument "Digital Low Potential Measuring Instrument MODEL KSD-3000 manufactured by Kasuga Electric Co., Ltd." used in B., the presence or absence of piezoelectricity was confirmed by measuring the change in potential when strain was applied. First, a sample cut into a strip shape with a length of 150 mm and a width of 20 mm was set in a tensile testing machine with a chuck distance of 50 mm. Next, the surface potential of the side of the sample sandwiched between the chucks where the coating solution containing PVDF was applied was measured using a surface potential measuring instrument. Next, with a 10% strain applied to the sample, the surface potential was measured in the same manner, and the absolute value of the change amount of the surface potential was evaluated. The measurement method was as follows.

[0034] · Measurement range: High · Distance between sample and probe: 10 mm It was judged that the greater the change amount of the surface potential with strain application, the greater the piezoelectricity. When the change amount was 1 V or less, it was considered that there was no piezoelectricity. D. Titanium dioxide content in the coating solution The uniformly stirred coating solution was placed in a petri dish and heated above the boiling point of the solvent to remove the solvent, thereby producing a film composed of the solid content in the coating solution. Subsequently, using an XRF1800 manufactured by Shimadzu Corporation, the fluorine content and titanium content in the obtained film were measured by the fluorescence X-ray method, and the content (parts by mass) of titanium dioxide with respect to 100 parts by mass of PVDF was calculated. The conditions for the fluorescence X-ray measurement are as follows.

[0035] 〔X-ray generation section〕 · X-ray tube target (tube): Rh · Voltage (kv): 40 · Current (ma): 95 · Diaphragm: 30 〔Spectrometer system〕 · Slit; Standard · Crystal: LiF 〔Atmosphere〕 · Vacuum [Example 1] After dry-blending 5 parts by mass of titanium dioxide ("R-104" manufactured by Ti-pure) with respect to 100 parts by mass of PVDF ("Solef6010" (registered trademark) manufactured by Solvay), acetone was added and adjusted so that the concentration of PVDF with respect to the mass of acetone was 5% by mass. This solution was heated under reflux at 60 °C to prepare a coating solution in which PVDF was dissolved.

[0036] Next, as a sheet-like material as the substrate, a polyester knit (basis weight 216 g / m 2 , thickness 0.82 µm) was prepared, and the above coating solution was applied to one side thereof at a dry weight of 1 g / m 2 and then dried at 80 °C for 10 minutes to evaporate and remove acetone.

[0037] Furthermore, corona treatment (polarization treatment step III) was performed 3 times on the coating surface of the sheet-like material under the conditions of an output of 100 W × 1 m / min. The physical properties of the obtained piezoelectric material are shown in Table 1.

[0038] [Example 2] A piezoelectric material was produced in the same manner as in Example 1, except that the content of titanium dioxide in the coating liquid was 1 part by mass. The physical properties of the obtained piezoelectric material are shown in Table 1.

[0039] [Example 3] A piezoelectric material was produced in the same manner as in Example 1, except that the content of titanium dioxide was 20 parts by mass. The physical properties of the obtained piezoelectric material are shown in Table 1.

[0040] [Comparative Example 1] A piezoelectric material was produced in the same manner as in Example 1, except that titanium dioxide was not used. The physical properties of the obtained piezoelectric material are shown in Table 2.

[0041] [Comparative Example 2] A piezoelectric material was produced in the same manner as in Example 1, except that the content of titanium dioxide was 25 parts by mass. The physical properties of the obtained piezoelectric material are shown in Table 2.

[0042] [Example 4] A piezoelectric material was produced in the same manner as in Example 1, except that the drying temperature after coating was 100 °C. The physical properties of the obtained piezoelectric material are shown in Table 2.

[0043] [Example 5] A piezoelectric material was produced in the same manner as in Example 1, except that the drying temperature after coating was 120 °C. The physical properties of the obtained piezoelectric material are shown in Table 1.

[0044] [Example 6] A piezoelectric material was produced in the same manner as in Example 1, except that the solvent of the coating liquid was methyl ethyl ketone. The physical properties of the obtained piezoelectric material are shown in Table 1.

[0045] [Comparative Example 3] The piezoelectric material was attempted to be fabricated in the same manner as in Example 1, except that the solvent of the coating solution was dimethylformamide. However, since the drying temperature was much lower than the boiling point of dimethylformamide, which is 153°C, the solvent could not be removed by drying, and the subsequent corona treatment could not be performed. As a result, a piezoelectric material could not be obtained. The physical properties of the obtained piezoelectric material are shown in Table 2.

[0046] [Comparative Example 4] A piezoelectric material was fabricated in the same manner as in Example 1, except that the solvent of the coating solution was dimethylformamide and the drying temperature was 200°C. The physical properties of the obtained piezoelectric material are shown in Table 2.

[0047] From the comparison between Examples 1 to 3 and Comparative Examples 1 and 2, it can be seen that when the content of titanium dioxide in the coating solution is 1 to 20 parts by mass with respect to 100 parts by mass of PVDF in total, piezoelectricity is exhibited. Also, from the comparison between Examples 1 to 3, when the content of titanium dioxide is within the preferable range, that is, within the range of 3% by mass or more and 10% by mass or less, it can be seen that the change amount of the surface potential, which is an index of the magnitude of piezoelectricity, also increases. Further, from the comparison between Examples 1, 4, and 5, it can be seen that when the drying temperature is within the preferable range, the formation of β-crystals is promoted and the change amount of the surface potential also increases. From the comparison between Example 1 and 6 and Comparative Examples 3 and 4, it can be seen that when the boiling point of the solvent of the coating solution is 100°C or lower, the solvent can be removed in the drying process and the mechanical properties of the sheet-like material as the base material can be maintained.

[0048]

Table 1

[0049]

Table 2

Industrial Applicability

[0050] Since the present invention relates to a method for manufacturing a piezoelectric material having piezoelectricity on its outermost surface, it can be used for applications that require hygiene due to the antibacterial action utilizing piezoelectricity. Specifically, it can be used for interior surface materials such as seats, mats, and handles in various mobilities such as automobiles, railways, buses, and ships.< / pvdf>

Claims

1. A method for manufacturing a piezoelectric material containing a resin component mainly composed of polyvinylidene fluoride, comprising: a step I of applying a coating liquid containing polyvinylidene fluoride and titanium dioxide as a β crystal nucleating agent of the polyvinylidene fluoride to a sheet-like material as a base material; a drying step II; and a polarization treatment step III in this order, wherein the content of titanium dioxide in the coating liquid is 1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass in total of the polyvinylidene fluoride, and the solvent of the coating liquid is a solvent having a boiling point of 100°C or lower. A method for manufacturing a piezoelectric material, characterized in that.

2. The method for manufacturing a piezoelectric material according to claim 1, characterized in that the drying temperature in the drying step II is 50°C to 100°C.

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