JP2817238B2 - Method for producing electretized porous sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application fields) Electrets are used in audio equipment, various sensors, electret motors, electret switches, air filters, fracture treatment materials, etc. by utilizing their semi-permanent charge trapping characteristics. ing.

(Prior art) A porous dielectric sheet, unlike a substantially dielectric sheet such as a film, has a space in a material, so that a high voltage cannot be applied during electretization, or poor contact with an earth surface. For example, there was a problem that injection of effective charges could not be performed, so that electrets with high charge could not be formed. To solve this problem, for example,
The method of corona charging a nonwoven fabric described in JP-A-151671 on a green film is to increase the green breakdown voltage of the charged space by the green film, increase the corona charging voltage, and increase the electret of the nonwoven. This is an attempt to improve performance. However, even if this method is used, most of the electric charges pass through the space of the nonwoven fabric, and there is a limit to the amount of charge retained in the nonwoven fabric.

The method of corona-loading a fiber sheet using a liquid electrode as an earth electrode described in Japanese Patent Application Laid-Open No. Sho 61-102476 attempts to improve the contact between the fiber and the earth electrode to improve the charge injection property. However, the green electrode breakdown voltage of the liquid electrode was low, no effective charge injection occurred, and the amount of charge held by the fiber sheet was also limited. From such a background, there has been a long-awaited demand for a method of retaining charges at a higher density on a porous dielectric sheet.

(Problems to be Solved by the Invention) The present invention is to solve the uneven electrified state or the low electrified state of the electret porous sheet by a novel electretization method.

(Means for Solving the Problems) In order to solve the problems of the conventional method of forming a porous dielectric sheet into an electret, various charge methods have been intensively studied. As a result, a charged medium dielectric substance sheet is charged with an electret material. When a certain porous dielectric sheet is superimposed and a self-discharge type electrode is further provided on the intermediate substantially dielectric sheet and charge is injected by electric discharge, an electretized porous sheet having a high charge amount can be obtained. I found That is, the present invention provides a method for producing an electretized porous sheet, comprising superposing a porous dielectric sheet on a medium-substrate dielectric sheet surface having a surface potential of at least 4 kv, and injecting charges by discharging the dielectric sheet. It is about.

The reason why a porous dielectric sheet having a high charge amount can be obtained by the method of the present invention will be described with reference to the drawings. FIG. 1 shows a state in which the medium substantially dielectric sheet 5 is charged. The electric charge 3 emitted from the electrode 2 advances toward the ground 6, and a large amount of electric charge is accumulated on the medium dielectric sheet 5. Next, as shown in FIG. 2, the porous dielectric sheet 7 is superimposed on the medium dielectric sheet immediately after charging by corona charging, and at the same time, the self-discharge electrode 9
Is provided, the charge 4 is released toward the self-discharge type electrode 9, that is, a discharge occurs. At this time, charges are trapped uniformly and semi-permanently in the plane direction in the porous dielectric sheet 7. The reason that the electretization method according to the present invention can be electretized to a higher charge amount than the conventional electretization method is that, in the conventional method, for example, in the method by corona charging, the concentration of the electric charge immediately below the tip of the needle that generates corona. Is extremely high and is electretized to a high charge amount, but only a low charge amount is obtained in the part other than the needle tip,
As a result, unevenness of the charge amount occurs on the porous dielectric sheet.

In the electret porous sheet formed by the method of the present invention, the charges uniformly accumulated on the surface of the medium-substrate dielectric sheet are superimposed and simultaneously discharge to the porous dielectric sheet having a low surface potential, resulting in extremely uniform charge injection. Therefore, an electretized porous sheet having a high charge amount and a uniform charge state, which cannot be seen in the conventional method, can be obtained.

In the present invention, a highly dielectric material is preferably used as the porous dielectric sheet.
Olefin-based polymers such as polyolefins, polyesters, polystyrene, polyvinylidene fluoride, Teflon, polycarbonate, polysulfone, polyacrylonitrile, polyvinyl chloride, synthetic resins such as polyvinylidene chloride, copolymers and blend compositions of two or more thereof, Non-polar glass etc. are mentioned.

Examples of the form of the porous dielectric sheet used in the present invention include a sponge-like sheet, a woven fabric, a nonwoven fabric, a particle sintered sheet and a composite sheet thereof, and have a thickness of 0.05 to 10
mm is preferred. The packing density can be preferably 0.01 to 0.9 cc / cc, more preferably 0.03 to 0.3 cc / cc, and still more preferably 0.05 to 0.2 cc / cc.

The medium-substrate dielectric sheet used in the present invention is preferably a material having a large charge accumulating property and a discharge that occurs quickly. Olefin polymers such as polyethylene, polypropylene and α-polyolefin, polyester, polystyrene, polyvinylidene fluoride And synthetic resins such as Teflon, polycarbonate, polysulfone, polyacrylonitrile, polyvinyl chloride, and polyvinylidene chloride; co-selected and blended compositions of two or more thereof; semiconductor materials such as silicon; and nonpolar glass.

In the present invention, the surface potential of the medium dielectric sheet is preferably at least 4 kv, more preferably 6 kv or more, and further preferably 9 kv or more. This surface potential is
It is an electrostatic charge voltage at a position of 1 cm above the middle substance dielectric sheet, and is a value measured using a current collecting potential meter.

In the present invention, a needle, a wire, a flat plate, a knife, a bar, or the like is used as the self-discharge type electrode, and the polarity of the electrode is preferably set to OV, that is, ground, or reversed from the polarity applied in the charging step. .

In the present invention, examples of the method for charging the medium dielectric sheet include corona charge, electric field charging, and electron beam irradiation.

In the present invention, when the medium dielectric sheet having a surface potential and the porous dielectric sheet are superposed, it is preferable that the medium dielectric sheet is superimposed on the porous dielectric sheet immediately after the charging step. In order to achieve this preferable condition, there is a method in which the medium substantial dielectric sheet and the porous dielectric sheet are overlapped in advance, and the charging step and the discharging step of the medium substantial dielectric sheet are continued. FIG. 3, FIG.
The figure shows an outline of a method for performing the above steps continuously. FIG. 3 shows an apparatus comprising a roller 11 having a medium-substrate dielectric sheet lined in a layer and a charging processor 10. The medium dielectric layer is provided so as to be continuously superimposed on the porous dielectric sheet 12 immediately after the charging process. FIG. 4 shows the device of FIG.
The porous dielectric sheet is unwound from the position of 12a or 12b and superposed on the roller 11. Here, the mode of the method of continuously stacking the medium-substrate dielectric sheets has been described, but any configuration and shape may be used as long as the apparatus can basically perform the same steps.

(Function) In the present invention, the porous dielectric sheet is uniformly electretized to a high charge amount by a uniform discharging action in a planar direction from the medium-substrate dielectric sheet having a surface potential of at least 4 kv. This method for producing an electretized porous sheet is an unprecedented new method.

(Example) Example 1 A nonwoven fabric having a basis weight of 25 g / m ² and a thickness of 0.3 mm made of polypropylene ultrafine fibers (average fiber diameter: 1.9 μm) was electretized by an apparatus shown in FIG. The charging treatment of the medium dielectric sheet was performed by corona charging using a needle electrode. At that time, the applied voltage was 20 kv and the application time was 10 seconds. The surface potential of the dielectric sheet was 12 kv. A 60 μm-thick polyvinyl chloride sheet was used as the dielectric sheet. The nonwoven fabric was used as an air filter, and its removal efficiency was measured using a salt particle (mean particle size: 0.3 μm, geometric standard deviation: 1.2) using a filter removal efficiency measuring device shown in FIG. The electretized non-woven fabric 17 is installed in a duct 16 and controls the flow meter 18 so that the ventilation velocity is 5.3 cm / sec. The number of salt particles upstream and downstream of the non-woven fabric is measured by a laser particle counter (RC KC-14). Measured by The removal efficiency was 99.8%. The removal efficiency was calculated using the following equation.

Removal efficiency (%) = (1− (downstream particle individuality) / (upstream particle number) × 100 Example 2 The same polypropylene nonwoven fabric as in Example 1 was electretized using the apparatus shown in FIG. The body sheet was charged by corona charging using a needle electrode, the applied voltage at that time was 10 kv, the application time was 10 seconds, and the surface potential of the dielectric sheet was 5 kv. Used a polyvinyl chloride sheet having a thickness of 60 μm, and the removal efficiency of the nonwoven fabric was 91.5%.

Comparative Example 1 The same polypropylene nonwoven fabric as in Example 1 was electretized by the apparatus shown in FIG. The charging treatment of the medium dielectric sheet is performed by corona charge using a needle electrode,
The applied voltage at that time was 6 kv, and the application time was 10 seconds. The surface potential of the dielectric sheet was 3 kv. A 60 μm-thick polyvinyl chloride sheet was used as the dielectric sheet. The removal efficiency of the nonwoven fabric was 49%.

Comparative Example 2 The same polypropylene nonwoven fabric as in Example 1 was not electretized at all. 45% removal efficiency of the non-woven fabric
Met.

In the case of the electretized filter, the higher the removal efficiency, the larger the charge retention amount, and the effect of the present invention was clear.

Example 3 Fabric weight of polystyrene fine particles (particle size: 2 μm) 20
A particle-filled porous sheet having a g / cm ² and a thickness of 0.15 mm was electretized by an apparatus shown in FIG. The charging treatment of the medium-substrate dielectric sheet was performed by corona charging using a 0.1 mmφ tungsten wire, at which time the applied voltage was 30 kv and the application time was 10 seconds. The surface potential of the dielectric sheet is 10
kv. As a self-discharge type electrode, a flat stainless steel plate was used, and the operation was performed at 3 kv for 5 seconds with a polarity opposite to the corona charging. A 0.3 mm thick Teflon sheet was used as the medium dielectric sheet. When the surface potential distribution of the porous sheet was measured using a surface potentiometer (S-211 manufactured by Kawaguchi Electric Co., Ltd.), the porous sheet had a very uniform surface potential distribution of 1800 ± 100 V in the plane direction.

Comparative Example 3 The same polystyrene microparticle-filled porous sheet as in Example 2 was corona-charged under the same conditions as in Example 2, and was separated from the Teflon sheet after the charging treatment. The surface potential of the porous sheet was 600 ± 500V.

Comparative Example 4 The same polystyrene microparticle-filled porous sheet as in Example 2 was electretized by the apparatus shown in FIG. The charging treatment of the medium dielectric sheet was performed by corona charging using a 0.1 mmφ tungsten wire. At that time, the applied voltage was 10 kv and the application time was 10 seconds. The surface potential of the dielectric sheet was 4 kv. As a self-discharge type electrode, a flat stainless steel plate is used, and the polarity is opposite to that of corona charging.
The test was performed at 3 kv for 5 seconds. As a medium dielectric sheet,
A 0.3 mm thick Teflon sheet was used. The surface potential distribution of the porous sheet was 1100 ± 150V.

In the case of the electret sheet, the higher the surface potential, the larger the charge holding amount, and the smaller the variation in the surface potential distribution, the more uniform the charged state, and the present invention The effect was clear.

From the above Examples and Comparative Examples, it can be seen that the method for producing an electretized porous sheet according to the present invention is excellent.

(Effect of the Invention) The electretization method for a porous dielectric sheet according to the present invention has a high charge retention amount that cannot be obtained by a conventional charging method and has a uniform charge state. You can get a sheet.

[Brief description of the drawings]

FIG. 1 shows the charge accumulation state of the middle dielectric sheet at the time of corona charging, and FIG. 2 shows that the middle dielectric sheet and the porous dielectric sheet are overlapped with each other immediately after the completion of corona charging, and the self FIG. 3 and FIG. 4 are schematic views of an apparatus for forming a porous dielectric sheet into an electret according to the present invention, showing a discharge state of electric charges when a discharge electrode is provided.
The figure is a schematic diagram of a filter removal efficiency measuring device.

Claims (2)

(57) [Claims] 1. A porous dielectric sheet is superimposed on a medium dielectric sheet surface having a surface potential of at least 4 kv,
A method for producing an electretized porous sheet, comprising injecting charges by discharging the dielectric sheet. 2. A porous dielectric sheet is superimposed on a medium dielectric sheet surface having a surface potential of at least 4 kv.
A method for producing an electretized porous sheet, comprising: placing a self-discharge type electrode in a direction opposite to a surface of the substantially substantially dielectric sheet when charging by discharging the dielectric sheet.