JPS61289177A - Production of electret fibrous sheet - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for continuously producing an electret fibrous sheet that maintains stable polarization charge over a long period of time.

(Prior Art) Conventionally, as described in Japanese Patent Publication No. 59-15168, there is a method of continuously rolling an electret fiber sheet.
There is a method in which a fiber sheet is continuously applied while moving on a fixed earth electrode to form an electret.

However, in this method, the uneven fiber sheet is moved while being in contact with a fixed earth electrode, so it does not come into contact with the earth electrode due to the unevenness of the fiber sheet, and contact pressure cannot be obtained. It is difficult to obtain compensation charge from the side. For this reason, there is a drawback that sufficient continuous electret formation cannot be achieved.

Moreover, since the fiber sheet moves on a fixed earth electrode, frictional electrification occurs, which inhibits continuous electret formation, resulting in insufficient characteristics.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned drawbacks, that is, insufficient compensation charge due to poor contact caused by a fixed earth electrode and a moving uneven fiber sheet, and inhibition of electretization caused by frictional charging, and to provide a manufacturing method for continuous electretization. It provides:

(Means for Solving the Problems) The present invention applies a high voltage using a non-contact type application electrode while moving the earth electrode and the fiber sheet together with the ll1i fibrous sheet in contact with the earth electrode. The present invention relates to a method for manufacturing an electret fibrous sheet, which is characterized in that the electret fiber sheet is continuously converted into an electret by performing the following steps.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 shows an example of an embodiment of the invention. In Fig. 1, the fibrous sheet ■ is brought into contact with a grounded rotating drum ■ rotating in the six directions of arrows while being continuously supplied.
Non-contact application electrode■, for example, a high voltage generator that provides negative polarity■
After applying voltage, the fiber sheet is then brought into contact with a grounded rotating drum ■ that rotates in the direction of arrow B, and a high-pressure generator ■ gives positive polarity, for example, to the back side of the fiber sheet.
The electret fibrous sheet (2) can be continuously obtained by injecting charge from the non-contact application electrode (2) using the electret fibrous sheet (2). Furthermore, the present invention will not be impaired even if this method is repeated further.

Moreover, as another embodiment, it is possible to use either the upper or lower stage of the electretization apparatus shown in FIG. 1 and apply high pressure to continuously obtain an electret fibrous sheet. It is also possible to continuously obtain an electret fibrous sheet by further applying a high pressure with the opposite polarity to the back side of the electret fibrous sheet thus obtained.

Further, as another embodiment, a method as shown in FIG. 2 may be used.

That is, the fiber sheet (■) is moved in contact with a grounded conveyor (■) while being applied with a non-contact application electrode [phase], for example, with negative polarity, in a heating box (■), and then passed through a cooling box (■). Further earthed conveyor■
While moving the electret fibrous sheet in contact with the above, the opposite polarity (positive) is applied using the non-contact application electrode in the heating box [phase], and then the electret fibrous sheet is passed through the cooling box [phase] again. (2) This is a method of continuously manufacturing.

Comparing the methods of the embodiments shown in Fig. 1 and Fig. 2, the rotating drum type shown in Fig. 1 makes it easier for the fiber sheet to adhere uniformly to the drum, which is the earth electrode, and allows uniform and continuous electret formation. This is preferable in this respect.

Note that electretization increases when heat is applied using a heating box or the like. In this case, it is preferable to carry out the test at a temperature equal to or higher than the Tg of the sheet material polymer. Further, after applying heat, the electret state is fixed by immediately cooling it using a cooling box or the like. This fixation allows the subsequent application of opposite charges to be performed more efficiently.

In the present invention, even if a dielectric material or a semiconductor material is inserted between the fiber sheet and the drum or conveyor serving as the ground electrode, there is nothing that particularly impairs the effects of the present invention.

Especially semiconductor materials, e.g. volume resistivity of 10-3 to 10
10Ω・cm material, more preferably 10-1 to 10
Inserting or attaching a material of 6 Ω·cm is preferable since sufficient continuous electret formation can be achieved.

Specifically, as a semiconductor material, a polyethylene sheet mixed with carbon (volume resistivity 103 to 106 Ω・cm)
, paint mixed with carbon, material impregnated with liquid, etc. can be used.

It is also possible to use the semiconductor material itself for the ground electrode. In this case, stable continuous electret formation can be achieved by preventing spark discharge.

The fiber sheet used has a basis weight of 80 CJ/2 to facilitate charge injection and promote sufficient electret formation.
Tr is preferably 12 or less. Further, the cover factor is preferably 40% or more from the viewpoint of preventing spark discharge. In addition, the diameter of the constituent fibers of the fibrous sheet has less unevenness,
20. To promote sufficient electret formation by bringing it into close contact with the earth electrode. The average diameter is preferably 10 μm or less, and more preferably 10 μm or less in average diameter.

Examples of the form of the fibrous sheet include nonwoven fabric, woven fabric, knitted fabric, paper, perforated film, and sponge.

In particular, electretization in the form of a nonwoven fabric is efficient, and melt-blown nonwoven fabrics made of ultrafine fibers are especially suitable.

As the material of the fibrous sheet, a material having a volume resistivity of 1013 Ω·cm or more can be used, such as polyethylene,
Inorganic compounds such as polyolefin such as polypropylene, polycarbonate, polyfluorine resin, polyester, polyvinyl chloride, and glass can be used.

In particular, polyolefin resins are non-polar and have a high specific resistance, and electrets are stable and preferred.

Next, the applied strength is an important factor as a condition for continuous electret formation.

In order to continuously obtain a sufficient number of electret fiber sheets, the applied strength should be 0.2 to 2000 W min/112,
Preferably it is 2 to 1001000W/1T12.

That is, the applied intensity is a numerical representation of the relationship among the applied voltage, applied current, and application time necessary for continuous electret formation.

That is, the applied intensity is expressed by the following formula.

Applied strength (w-min/1712) = VX I XT
V: Applied voltage (volt) I: Applied current per unit area of the fibrous sheet (Ampere/Tn2) T: Application time (min) If the applied intensity exceeds 2000W-1llin/T12, spark discharge occurs and the electret cannot be converted into

Moreover, if it is less than 0.2 W-min/m2, sufficient electret formation cannot be achieved.

Here, a method for calculating the applied current (I) will be described.

It is determined by measuring the discharge current emitted from the non-contact type application electrode and dividing it by the area of the fiber sheet through which the discharge current flows.

As the non-contact application electrode, a needle electrode, a wire electrode, etc. can be used.

The electrode arrangement is as shown in Figure 3, with electrodes arranged vertically and horizontally.
It is preferable to arrange them and take the application time.

The application time is preferably 300 seconds or less; if the application is applied for too long, the electrified layer of the electret will deteriorate.

Concerning the polarity of application during continuous electret formation, an electret fiber sheet with a higher surface charge density can be obtained by first applying a negative voltage and then applying a positive voltage.

Furthermore, in this case, it is possible to obtain an electret fibrous sheet with a higher surface electrification density by applying a higher negative applied voltage than a positive applied voltage.

〔Example〕

Next, the measurement method used in Examples will be explained.

(1) Volume resistivity was measured according to JIS-C2318.

(2) The cover factor was determined using a magnifying fluoroscope for the sample (2
cm x 2 cm) was created, and the bright area where the light was transmitted was B, and the shadow area where the light was blocked by the fiber was C, which was calculated using the following formula.

Cover factor (%) = C/B + Cx100 (3)
Apparent Density of Sheet The apparent density of the sheet was determined from the following formula.

Sheet apparent density (g/cnf> = D/ExFD: Weight per unit area CI/cJ) E: Thickness (
cm) F: unit area (2d) The thickness was measured at 50q/-.

(4) Surface charge density The surface charge density was measured as shown in Figure 4.
Charges existing on the sample surface are accumulated in gold O by electrostatic induction, the potential is measured with an electrometer O, and the surface charge density on the sample surface is determined by the following formula.

Surface charge density (C/cd) = CXV/AC: Capacitance of capacitor (Farad) = Potential (volt) A: Sample area (cJ) Example 1 A stainless steel material with a diameter of 100 cm and a width of 100 cm as shown in Figure 1. (volume resistivity 10-5 Ω cm) was used as a ground electrode, and a polypropylene melt-blown nonwoven fabric with a fabric weight of 20 Q/m2, a cover factor of 98%, an average fineness of 3.5 μm, and a sheet width of 100 cm was brought into contact with the drum. In this state, move the drum at a circumferential speed of 1 m/min, -30 K V (7) After applying high DC voltage with a distance between electrodes of 3 cm using a needle-shaped 1ffi, use a second drum similar to the front drum to A voltage of +18 KV was further applied to the back side of the nonwoven fabric to continuously obtain an electret nonwoven fabric. In both drums, five needle-like electrodes were arranged in the sheet width direction and five needle-like electrodes were arranged in the length direction to apply the voltage.

The applied strength at the first drum is 150W-min/m2
．． In the second drum it was 10 W-min/112.

In addition, the temperature at the time of application was carried out at room temperature. As a result,
The surface charge density of the obtained electret nonwoven fabric was 7.5
It was X10-10 coulombs/-.

This was left indoors for two months, but almost no charge decay was observed. In addition, a DC high voltage generator manufactured by Kasugadendono was used.

Example 2 Using a device similar to that used in Example 1, a carbon-containing polyethylene sheet (volume resistivity 104 Ω cm) was attached as a semiconductor material between the nonwoven fabric and the drum earth electrode, and the same device as Example 1 was prepared. Similar experiments were conducted using nonwoven fabric.

However, the applied voltage is 138KV at the beginning and 2KV at the second.
It was carried out at 1KV.

The applied intensity was initially 238W-min/Tl'12.2
The second one is 12 W −+nin/l112 and ivy.

The surface charge density of the obtained electret nonwoven fabric was 8.5
It was X10'coulombs/-.

Although it was left indoors for two months, the charge hardly decreased.

Example 3 An experiment was conducted using only the lower device of the device used in Example 2.

A voltage of 40 KV was applied to the same nonwoven fabric as in Example 2 at a drum circumferential speed of 2 m/mrn and a drum surface temperature of 40°C, and then wound up into a roll. After a while, a voltage of +20 KV was applied to the surface of the nonwoven fabric at a drum speed of 1 m/min to obtain an electret nonwoven fabric.

The first applied intensity was 135 W-min/112.
The applied intensity for the second time was 11 W-min/112. The surface N loading density of the obtained electret nonwoven fabric was 9.
It was 5×10 −10 coulombs/clTf. Also, 2
Almost no charge decay was observed even after being left indoors for several months.

Comparative Example 1 Fixed earth electrode plate with a width of 4Qcm and a length of 4Ωcm
A device using a stainless steel plate with two needle-like electrodes arranged horizontally and two in the length direction, and a similar device but with the fixed ground electrode and needle-like electrodes in opposite positions. Then, the same nonwoven fabric as in Example 1 was heated at 4 Qcm/min.
The application was performed by moving at a speed of . Initially, the distance between the electrodes is 3cm.
Then, -30 KV was applied between the same electrodes, and then +18 KV was applied between the same electrodes.

As a result, the surface charge density of the obtained nonwoven fabric was 0°9×1
It is as low as 0-10 coulombs/d, and after being left indoors for 2 months, it becomes 0.4 x 10-10 coulombs/-.
Significant charge decay was observed.

Example 4 Using a device as shown in Fig. 2, an average fineness of 14 μm,
The test was conducted using 1m polypropylene film with a cover factor of 97% and a size of 40g/m2.

A conveyor using a stainless steel plate with a volume resistivity of 10 −5 Ω·cm was used as a ground electrode.

The conveyor used had a width of 4Qcm and a length of 2m.

The electrode arrangement was 2 horizontally and 5 vertically on both the upper and lower sides.

First, a voltage of -28 KV was applied in a 45° C. atmosphere, and after cooling, a voltage of +15 KV was applied in a 40° C. atmosphere, and then the coil was continuously wound up.

The first applied intensity was 112 W-min/m2. The second applied intensity was 7.5 W-min/112 F atta.

The surface charge density of the obtained electret nonwoven fabric was 6.
0x10'' coulomb/-, and no decrease in charge was observed even after being left for 2 months.

(Effects of the Invention) The present invention continuously applies high voltage to the electret by keeping the fibrous sheet in contact with the earth electrode and moving the earth electrode and the fibrous sheet together using a non-contact type application electrode. Therefore, the conventional drawbacks such as insufficient compensation charge due to poor contact and inhibition of electret formation due to frictional charging are eliminated. Therefore, the electret fibrous sheet obtained in the present invention can maintain stable electret properties over a long period of time, and can be used in a wide range of applications such as filters, adsorbents, and antithrombotic materials.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of an electretization apparatus used in the method for producing an electret fibrous sheet of the present invention, and FIG. FIG. 3 is a schematic diagram showing an example of the arrangement of non-contact application electrodes, and FIG. 4 is a schematic diagram showing a surface charge density measuring device. 1.8: Fibrous sheet 2.5: Grounded rotating drum (earth electrode) 3.7
．． 12, 17: Non-contact application electrode 10. 15: Grounded conveyor 11. 16: Heating box 13. 18: Cooling box Patent applicant Azuma Shi Co., Ltd. Figure 1

Claims (9)

[Claims] (1) It is a body in which a fibrous sheet is brought into contact with an earth electrode, and while the earth electrode and the fibrous sheet are moved together, high voltage is applied using a non-contact application electrode to continuously convert the electret into electret. A method for producing an electret fibrous sheet. (2) The method for producing an electret fibrous sheet according to claim (1), wherein high voltage application electrodes are applied using non-contact application electrodes on the front and back sides of the fibrous sheet. (3) Production of an electret fibrous sheet according to claim (1) or (2), wherein high voltage is applied with a non-contact type application electrode with negative polarity and then with positive polarity. Law. (4) The applied electrode applied to the fiber sheet by the non-contact applied electrode and the ground electrode is 0.2 to 2000 W-mi.
A method for producing an electret fibrous sheet according to claim (1), wherein the electret fibrous sheet is n/m^2. (5) The method for producing an electret fibrous sheet according to claim (1), wherein the earth electrode is heated. (6) The method for producing an electret fibrous sheet according to claim (1), wherein the fibrous sheet has a basis weight of 80 g/m^2 or less. (7) The method for producing an electret fibrous sheet according to claim (1), wherein the fibrous sheet has a cover factor of 40% or more. (8) The method for producing an electret fibrous sheet according to claim (1), wherein the fiber sheet has a fiber diameter of 20 μm or less. (9) A method for producing an electret fibrous sheet according to claim (1), wherein a semiconductor material is interposed between the fiber sheet and the earth electrode.