JP2024031038A - Electret membrane and its manufacturing method - Google Patents

Abstract

An object of the present invention is to provide an electret film with excellent charge retention performance and a method for manufacturing the same. [Solution] A step of preparing a liquid crystal resin containing 93 to 100 mol% of an asymmetric aromatic monomer residue having an asymmetric molecular structure and having an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g. This is a method for producing an electret film, which includes step A, step B of dissolving a liquid crystal resin in a solvent to prepare a liquid crystal resin solution, and step C of forming a film using the liquid crystal resin solution. [Selection diagram] None

Description

The present invention relates to an electret film using a liquid crystal resin and a method for manufacturing the same.

Electret is a material that generates a charge on its surface when an electric field is applied, and retains the charge semi-permanently even after the electric field is removed.It is used as an electric-kinetic energy conversion material (electrostatic induction type conversion element) for speakers and headphones. It is used in electroacoustic conversion materials such as microphones, and various sensors such as ultrasonic sensors, pressure sensors, and acceleration sensors.

Among electrets, ferroelectric materials such as polyvinylidene fluoride and polytetrafluoroethylene are known as electrets made of polymer materials.

On the other hand, liquid crystal resins are widely used as high-performance engineering plastics because they have excellent fluidity, mechanical strength, heat resistance, chemical resistance, electrical properties, etc. in a well-balanced manner. Therefore, it would be useful if the electret could be composed of liquid crystalline resin. As electrets made of liquid crystalline resins, those obtained by thermal electretization of molded bodies such as polyesters containing aromatic hydroxycarboxylic acid residues and exhibiting anisotropy when melted are known (see Patent Document 1). .

Japanese Unexamined Patent Publication No. 62-224090

However, when the electret described in Patent Document 1 is formed into a film, even if an electric field is applied to the film to generate a charge, the charge tends to disappear in a short time. That is, the conventional electret film has insufficient charge retention performance, and there remains room for improvement.

The present invention has been made in view of the above-mentioned conventional problems, and its object is to provide an electret film with excellent charge retention performance and a method for manufacturing the same.

One aspect of the present invention that solves the above problem is as follows.
(1) Step A of preparing a liquid crystalline resin containing 93 to 100 mol% of asymmetric aromatic monomer residues having an asymmetric molecular structure and having an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g and,
Step B of preparing a liquid crystal resin solution by dissolving the liquid crystal resin in a solvent;
Step C of forming a film using the liquid crystal resin solution;
A method for producing an electret membrane, comprising:

(2) The method for producing an electret film according to (1) above, wherein in step C, the film is formed by a solution casting method using the liquid crystal resin solution.

(3) The method for producing an electret film according to (1) above, wherein in the step C, the liquid crystal resin solution is gelled, and the resulting gel is formed into a film by heating and drying to form a film.

(4) Contains a liquid crystal resin that contains 93 to 100 mol% of asymmetric aromatic monomer residues having an asymmetric molecular structure and has an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g, and An electret film having a crystallinity of 35% or less as measured by a diffraction method.

According to the present invention, it is possible to provide an electret film with excellent charge retention performance and a method for manufacturing the same.

FIG. 2 is a diagram schematically showing the state of polymer molecules before (a) and after (b) corona discharge treatment of the liquid crystal resin in the present embodiment. It is a graph showing the relationship between the surface charge density and the elapsed time of the electret films produced in Examples and Comparative Examples.

<Method for manufacturing electret membrane>
The method for producing an electret membrane of the present embodiment includes 93 to 100 mol% of an asymmetric aromatic monomer residue having an asymmetric molecular structure, and an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g. The method includes a step A of preparing a liquid crystal resin, a step B of dissolving the liquid crystal resin in a solvent to prepare a liquid crystal resin solution, and a step C of forming a film using the liquid crystal resin solution.

Simply put, the method for producing an electret film of this embodiment is to prepare a liquid crystal resin solution as described above, and form a film using the liquid crystal resin solution to obtain an electret film. It is presumed that by forming the film in this manner, countless trap sites capable of trapping charges are created inside the obtained film, and the charge retention performance is improved.
Each step will be explained in detail below.

[Process A]
In step A, a liquid crystal resin containing 93 to 100 mol% of asymmetric aromatic monomer residues having an asymmetric molecular structure and having an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g is prepared.

In the present embodiment, the liquid crystal resin contains 93 to 100 mol% of asymmetric aromatic monomer residues, but if the content is less than 93 mol%, dielectric properties deteriorate. The asymmetric aromatic monomer residue is preferably contained in an amount of 97 to 100 mol%.

In addition, the liquid crystal resin has an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g, but if it is less than 1.0 dL/g, it will not exhibit ferroelectricity and will not exhibit ferroelectricity, In this case, it becomes difficult to control the orientation by corona discharge. The limiting viscosity is preferably 2.5 to 5.5 dL/g, more preferably 4 to 5 dL/g.
Note that the intrinsic viscosity can be obtained by measuring according to JIS K7367 using a 50/50 mixed solvent of pentafluorophenol and chloroform as the solvent.

In this embodiment, a wholly aromatic polyester or a wholly aromatic polyester amide can be used as the liquid crystal resin.

In this embodiment, the liquid crystal resin has a predetermined ratio of asymmetric aromatic monomer residues having an asymmetric molecular structure, as described above. The monomer from which the asymmetric aromatic monomer residue is derived is at least one compound selected from the group consisting of aromatic hydroxycarboxylic acids and polymerizable derivatives thereof, aromatic aminocarboxylic acids, and polymerizable derivatives thereof. At least one compound selected from the group consisting of derivatives is included.

The asymmetric aromatic monomer residue preferably has a structure of at least one type of structural units represented by the following structural formulas (I) to (V). In other words, in this embodiment, the monomer from which the asymmetric aromatic monomer residue is derived preferably has one of the structural units represented by the following structural formulas (I) to (V).

Examples of monomers having a structural unit represented by structural formula (I) include 4-hydroxybenzoic acid (hereinafter also referred to as "HBA").
Examples of monomers having a structural unit represented by structural formula (II) include 3-hydroxybenzoic acid.
Examples of monomers having a structural unit represented by structural formula (III) include 6-hydroxy-2-naphthoic acid (hereinafter also referred to as "HNA").
Examples of monomers having a structural unit represented by structural formula (IV) include 4-hydroxy-4'-biphenylcarboxylic acid (hereinafter also referred to as "HBCA").
Examples of the monomer having a structural unit represented by structural formula (V) include N-acetyl-aminobenzoic acid.
It should be noted that any of the monomers may form structural units represented by structural formulas (I) to (V) after polymerization, and is not limited to the monomers exemplified above. For example, monomers exemplified above whose hydroxyl or amino groups are acylated may be used.

In this embodiment, examples of the liquid crystalline resin include those obtained by copolymerizing two or more monomers having any of the structural units represented by the above structural formulas (I) to (V). For example, it is preferable to copolymerize a monomer having a structural unit represented by structural formula (I) and a monomer represented by structural formula (III). Specifically, examples include copolymerization of HBA and HNA, and in that case, the copolymerization ratio (HBA:HNA) is preferably 60:40 to 90:10.

In the liquid crystal resin according to the present embodiment, it is preferable that among the asymmetric aromatic monomer residues, monomer residues having a kink structure account for 0.1 mol% or more and 45 mol% or less. When the monomer having a kink structure is within the above range, processability is improved and melting point can be easily controlled.
Monomer residues having a kink structure include 3-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, 2-hydroxybenzoic acid, 3-hydroxy-2-naphthoic acid, 1-hydroxy-2-naphthoic acid, 5 Examples include residues of various monomers such as -hydroxybiphenyl-3-carboxylic acid, among which 3-hydroxybenzoic acid residues or 6-hydroxy-2-naphthoic acid residues are preferred.

[Process B]
In step B, a liquid crystal resin solution is prepared by dissolving the liquid crystal resin in a solvent. That is, in step B, the liquid crystal resin prepared in step A is dissolved in a solvent to prepare a liquid crystal resin solution to be used in step C, which will be described later.

The solvent used in step B may be any solvent that can dissolve the liquid crystal resin, and examples thereof include pentafluorophenol (PFP) and 3,5-bistrifluoromethylphenol (BTFMP).

The concentration of the liquid crystal resin in the liquid crystal resin solution is preferably 0.1 to 5% by mass, and 0.5 to 3% by mass, from the viewpoint of improving the film formability in step C and generating countless trap sites. It is more preferably 0.7 to 1.5% by mass, and even more preferably 0.7 to 1.5% by mass. From another point of view, the liquid crystal resin solution is preferably a saturated solution at the coating temperature in step C.

The liquid crystalline resin can be dissolved in a solvent according to a conventional method. That is, the liquid crystal resin can be dissolved in a state in which it is easily dissolved by heating, stirring, etc., if necessary.

[Process C]
In step C, the liquid crystal resin solution prepared in step B is used to form a film. That is, in step C, a liquid crystalline resin solution is formed into a film by coating it on a supporting substrate, and then heated and dried to form a film to obtain an electret film.

In step C, a liquid crystal resin solution is used to form a film, and two aspects will be described below. The first embodiment is an embodiment in which a film is formed by a solution casting method, and the second embodiment is an embodiment in which a film is formed through a gel state.
First, a first aspect of forming a film by a solution casting method will be described.

In the solution film forming method, a film can be formed by coating a liquid crystalline resin solution on a supporting substrate to form a coating film, and then heating and drying this coating film. The film formed in the first embodiment is presumed to have a polycrystalline structure, with countless trap sites existing at grain boundaries, and is thought to have excellent charge retention performance.

The coating method for coating the liquid crystal resin solution on the support substrate is not particularly limited, and examples include spin coating, gravure coating, bar coating, die coating, spray coating, air knife coating, and dip coating. A known coating method such as the like can be adopted.

The thickness of the coating film is determined in consideration of the thickness of the electret film that is the final resultant, and can be, for example, 0.1 to 100 μm.

A liquid crystalline resin solution is applied onto a supporting substrate by the above-described coating method to form a coating film, and then the coating film is dried by heating. The heating temperature at this time is preferably near the boiling point of the solvent, and specifically can be 140 to 160°C.

Next, a second mode of forming a film through a gel state, that is, a mode of forming a film by turning a liquid crystal resin solution into a gel state will be described. In the second embodiment, a liquid crystal resin solution is gelled, and the resulting gel is formed into a film (hereinafter, this film-like gel is also referred to as a "gel-like film").The film is formed by heating and drying. In the second aspect, when the liquid crystal resin solution is gelatinized, the crystallinity of the liquid crystal resin, which had been uniaxially oriented, decreases and becomes an amorphous state (non-oriented), resulting in the development of countless trap sites. By forming a film in this state, it becomes a polydomain structure while retaining the trap sites, and is considered to have excellent charge retention performance.

Gelation of a liquid crystalline resin solution can be performed by adding a poor solvent, cooling a saturated solution at high temperature, and causing precipitation. Examples of the poor solvent include chloroform, dichloromethane, bromodichloromethane, bromoform, dibromochloromethane, and the like.

The thickness of the gel-like film is determined in consideration of the thickness of the final resultant electret film, as in the first embodiment, and can be, for example, 80 to 100 μm.

An example of the process from gelling a liquid crystal resin solution to forming a film is shown. For example, 10 drops of chloroform are added dropwise to a pentafluorophenol (hereinafter also referred to as "PFP") solution of a liquid crystal resin every 5 minutes. Then, the solution separates into two layers, with a gel and PFP layer in the upper layer and a chloroform and PFP layer in the lower layer. Then, the lower layer is removed and the upper layer is placed on the supporting substrate. At this time, if the upper layer of gel and PFP is soft and difficult to place on the support base, it is preferable to evaporate a certain amount of PFP to make it hard to some extent. Further, when PFP is evaporated, a gel-like film is obtained. Then, a membrane is obtained by heating and drying the obtained gel-like membrane. The heating temperature at this time is preferably 140 to 160°C.
In the second embodiment, when heating and drying the gel-like film, it is optional whether or not to pressurize the gel-like film, but it is preferable to heat the gel-like film as it is without applying pressure.

In either embodiment, a release layer can be provided to facilitate peeling of the resulting film from the supporting substrate. That is, it is preferable to first form a release layer on the supporting substrate, and then apply a liquid crystal resin solution to the release layer.

In this embodiment, an electret film is obtained by subjecting the film obtained in Step C as described above to polarization treatment. For example, it is preferable to carry out the polarization treatment by performing corona discharge treatment so that the corona current value is −30 μA or less or 30 μA or more. Through this treatment, electric charges are injected into the liquid crystal resin so that it functions as an electret film, and polarization can be increased by orienting the aromatic rings in the vertical direction.

FIG. 1 schematically shows the state of molecules on the surface of the liquid crystal resin before (a) the corona discharge treatment and (b) after the corona discharge treatment. FIG. 1 shows a state in which liquid crystal resin molecules 12 are formed in a film shape on a substrate 10. As shown in FIG. In FIG. 1(a), the aromatic ring 12A of the liquid crystal resin molecule 12 is oriented parallel to the substrate, but in FIG. 1(b) after corona discharge treatment, the aromatic ring 12A near the surface is oriented parallel to the substrate. oriented vertically.

Corona discharge treatment is performed not only to polarize the liquid crystal resin but also to orient polymer molecules, particularly aromatic rings, in the vertical direction. More specifically, the corona discharge treatment is preferably performed so that the corona current value is −30 μA or less or 30 μA or more. Therefore, the corona discharge treatment is different from the corona discharge treatment performed solely for the purpose of polarization, and various conditions are set so that the polymer molecules, especially the aromatic rings, are oriented in the vertical direction. As for the conditions, it is preferable that the applied voltage is -4.5 to -6 kV or 4.5 to 6 kV, the stage temperature is 30 to 130° C., the sweep speed is 1 to 10 mm/s, and the number of sweeps is 0 to 40 times.

In this embodiment, the thickness of the electret film is preferably 0.1 to 100 μm, more preferably 0.1 to 70 μm, and 0.1 to 40 μm from the viewpoint of facilitating charge accumulation. It is even more preferable.

<Electret membrane>
The electret film of this embodiment is a liquid crystal resin that contains 93 to 100 mol% of asymmetric aromatic monomer residues having an asymmetric molecular structure and has an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g. and has a crystallinity of 35% or less as measured by X-ray diffraction. Such an electret film can be manufactured, for example, by the method for manufacturing an electret film of the present embodiment described above. That is, in the method for manufacturing an electret film of this embodiment, the degree of crystallinity is reduced because the liquid crystal resin is not formed into a film as it is, but is formed as a solution. Furthermore, the electret film of this embodiment has excellent charge retention properties as described above.

In this embodiment, the crystallinity of the electret film is 35% or less, preferably 10 to 30%, more preferably 15 to 25%. In this embodiment, the degree of crystallinity can be calculated by performing wide-angle X-ray diffraction measurement according to the method of W. Ruland, Acta Cryst., 14, 1180 (1961).

Further, in this embodiment, it is preferable that the electret film and the electrode are bonded to each other. By connecting the electrodes, the amount of charge increases, and it becomes difficult to discharge, furthermore, it has excellent charge retention characteristics. The electrode may be bonded to one surface or both surfaces of the liquid crystal resin.

Examples of the material for the electrode to be bonded to the electret film include aluminum, gold, silver, copper, platinum, and conductive polymers, among which aluminum, gold, and copper are preferred. When the electrode is in the form of a film, the film thickness is preferably 10 nm to 1 mm, more preferably 50 nm to 0.1 mm.

The present embodiment will be described in more detail below with reference to examples, but the present embodiment is not limited to the following examples.

(Method for manufacturing liquid crystalline polyester LCP1)
A polymerization vessel equipped with a stirrer, a reflux column, a monomer inlet, a nitrogen inlet, and a vacuum/outflow line was charged with the following raw material monomers, a fatty acid metal salt catalyst, and an acylating agent, and nitrogen substitution was started.
(material)
4-hydroxybenzoic acid; 1660g (73mol%)
2-hydroxy-6-naphthoic acid; 837g (27mol%)
Acylating agent (acetic anhydride); 1714g

After charging the raw materials into the polymerization vessel, the temperature of the reaction system was raised to 140°C, and the reaction was carried out at 140°C for 1 hour. Thereafter, the temperature was further increased to 325°C over 3.5 hours, and the pressure was then reduced to 10 Torr (i.e. 1330 Pa) over 15 minutes, while acetic acid, excess acetic anhydride, and other low-boiling components were distilled out. Melt polymerization was performed. After the stirring torque reached a predetermined value, nitrogen was introduced, the pressure was reduced to normal pressure, and then the pressure was increased, and the polymer was discharged from the lower part of the polymerization vessel.
The obtained polymer had an intrinsic viscosity of 4.7 dL/g when measured in a 50/50 mixed solvent of pentafluorophenol and chloroform at 60° C. at a concentration of 0.1% by mass.
Note that the amount of asymmetric aromatic monomer residues in the obtained polymer was 100 mol%, and the amount of monomer residues having a kink structure was 27 mol%.

[Example 1]
21.5 mg of LCP1 was added to 2000 mg of pentafluorophenol (PFP) and heated to 60° C. to prepare a 1.08% by mass LCP1 solution. The prepared LCP1 solution was kept at 40° C., and chloroform was added dropwise every 5 minutes, 10 drops per time, for a total of 36 times. The total amount dropped was 3.85 g. After dropping chloroform, the phases were separated, with the upper layer being a layer of LCP1 gel and PFP, and the lower layer being a layer of PHP and chloroform. Next, the lower layer was removed, and the mixture was left to stand at room temperature for 17 hours, and the solvent in the upper layer was evaporated to obtain a gel-like film of LCP1. Further, the gel-like membrane was heated at 160° C. for 5 minutes while applying pressure (compression) of 1.29 kg/cm ² to obtain a membrane-like body.
A 100 nm thick aluminum electrode was vacuum deposited on the obtained film-like body under conditions of 4×10 ⁻⁴ Pa to form a liquid crystal resin film with electrodes.

After forming the liquid crystal resin film, a corona discharge treatment was performed using a high voltage power supply manufactured by Trek Japan Co., Ltd. under the following conditions to obtain an electret film. Note that the corona current value was determined by measuring the current value generated during the corona discharge treatment.
(Corona discharge treatment conditions)
Applied voltage: -5.5kV
Temperature: 30℃
Sweep speed: 10mm/s
Number of sweeps: 40 times

(Measurement of crystallinity)
The degree of crystallinity of the obtained electret film was measured as follows. Measurement was performed using a sample horizontal multipurpose X-ray diffractometer "Ultima IV RINT-21" manufactured by Rigaku, using CuKα rays as the radiation source and scanning at a scanning speed of 3°/min. Then, the obtained diffraction curve was subjected to regression analysis using a Gaussian function, and the ratio of the total area of the peaks appearing from 15° to 30° to the sharp peak appearing around 19° was calculated as the degree of crystallinity. The measurement results are shown in Table 1.

[Example 2]
An electret membrane was obtained in the same manner as in Example 1 except that no pressure was applied to the gel membrane. Then, in the same manner as in Example 1, the crystallinity of the electret film was measured. The measurement results are shown in Table 1.

[Example 3]
1.1 g of chloroform was added dropwise to the LCP1 solution obtained in the same manner as in Example 1 and mixed to obtain a 0.7% by mass LCP1 chloroform mixed solution. A mixed solution of LCP1 in chloroform was dropped onto the surface of a glass substrate on which a polyvinyl alcohol film was formed to form a film. Leave at room temperature for 14 hours. Thereafter, it was immersed in water for 3 hours. Thereafter, the membrane was taken out of the water to obtain an electret membrane. Then, in the same manner as in Example 1, the crystallinity of the electret film was measured. The measurement results are shown in Table 1.

[Comparative example 1]
A film was obtained by melt-forming the polymer of LCP1. The melt film forming machine used was a Laboplast Mill manufactured by Toyo Seiki Seisakusho Co., Ltd. equipped with a single-screw extruder and a T-die. A 100 nm thick aluminum electrode was vacuum deposited on the obtained film under conditions of 4×10 ⁻⁴ Pa to form a liquid crystalline resin film with electrodes. Thereafter, corona discharge treatment was performed in the same manner as in Example 1 to obtain an electret film. Then, in the same manner as in Example 1, the crystallinity of the electret film was measured. The measurement results are shown in Table 1.

[evaluation]
(1) Measurement of surface charge density The charge density on the surface of the obtained electret (immediately after the corona discharge treatment) was measured in the atmosphere using a Monroe Electronics surface potentiometer Model 244A manufactured by Kansai Electronics Co., Ltd. The measurement results are shown in Table 1.
(2) Measurement of surface charge density 24 hours after corona discharge treatment The charge density on the surface of the electret 24 hours after corona discharge treatment was measured in the same manner as in (1) above. The measurement results are shown in Table 1. Note that during the aging process, the sample was left in an environment of 23° C. and 50% RH without being placed in a dry box.

From Table 1, in Examples 1 to 3, electret films with a crystallinity of 35% or less were obtained, and in all cases, the surface charge density after 24 hours after the corona discharge treatment was higher than that in Comparative Example 1. That is, the electret films of Examples 1 to 3 have high charge retention properties.

On the other hand, FIG. 2 shows a graph of changes in surface charge density with respect to elapsed time for the electret films of Examples 1 to 3 and Comparative Example 1. In FIG. 2, the solid line graph represents Example 1, the dashed line graph represents Example 2, the dashed line graph represents Example 3, and the dotted line graph represents Comparative Example 1. It can also be seen from FIG. 2 that Examples 1 to 3 have higher surface charge retention characteristics than Comparative Example 1.

10 Substrate 12 Liquid crystal resin molecule 12A Aromatic ring

Claims (4)

Step A of preparing a liquid crystal resin containing 93 to 100 mol% of an asymmetric aromatic monomer residue having an asymmetric molecular structure and having an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g;
Step B of preparing a liquid crystal resin solution by dissolving the liquid crystal resin in a solvent;
Step C of forming a film using the liquid crystal resin solution;
A method for producing an electret membrane, comprising: The method for producing an electret film according to claim 1, wherein in the step C, the film is formed by a solution casting method using the liquid crystal resin solution. The method for producing an electret membrane according to claim 1, wherein in the step C, the liquid crystal resin solution is gelatinized, and the resulting gel is formed into a membrane by heating and drying to form a membrane. Contains a liquid crystalline resin that contains 93 to 100 mol% of asymmetric aromatic monomer residues having an asymmetric molecular structure and has an intrinsic viscosity of 1.0 dL/g or more and less than 6.0 dL/g, and is determined by X-ray diffraction method. An electret film having a measured crystallinity of 35% or less.

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