JPS61102476A - Electret fiber sheet and its production - 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 an electret fiber that can be applied to filters and the like, and a method for producing the same.

[Prior Art] Japanese Patent Publication No. 56-47299 has been proposed as a technique for obtaining an electret fiber sheet. However, the sheet obtained by this method has a structure in which the fibrous slit film 1 having positive electrodes and negative electrodes is randomly dispersed within the web 2, as shown in the model diagram in FIG. The disadvantage is that the electric field strength of the part is canceled out by the fibrous slit film with positive and negative charges, resulting in a small electric field strength, and when this is used as a filter, sufficient electrostatic force does not act on the collected particles. Therefore, there was a drawback that the improvement in collection efficiency was poor.

In addition, Japanese Patent Publication No. 59-124 proposes a technique in which the melt-blown fibers are made into electret by applying high pressure directly below the die, and then collected on a conveyor to make wear. Similar to the one shown in FIG. 2, the electret fibers are randomly dispersed inside, so there is a drawback that the internal electric field becomes small.

[Problems to be Solved by the Invention] The present invention solves these problems of the prior art and makes it possible to maintain high electric field strength within the web reliably and for a long time. An object of the present invention is to provide a fiber sheet and a method for manufacturing the same.

[Means for solving the problem 1] That is, the present invention consists of a m-fiber sheet with a basis weight of 80 Q/m2 or less, a cover factor of 60% or more, a surface charge density of 5X 10-'IC10tF or more, and a sheet One side of an electret fiber sheet having different polarities on the front and back sides of the electret fiber sheet and a m-fiber sheet consisting of fibers having a volume resistivity of 10 Ω・Cff1 or more with a volume resistivity of 1
This is achieved by a method for producing an electret tsra sheet, which is characterized by contacting a liquid electrode under 08 Ω·cmlX and applying a high DC voltage.

In the present invention, the fibers constituting the fiber sheet include polyolefin,
Synthetic fibers such as polyester, polycarbonate, polyfluorine compounds, and vinyl chloride are preferably used, and those having a volume resistivity of 10 Ω·cm or more are preferable.

In the present invention, it is important to use a fiber sheet with a basis weight of 80 g/m" or less in order to be able to perform the electret treatment efficiently and to obtain an electret fiber sheet with a high surface charge density. If the basis weight exceeds 80 holes/m2, sufficient compensation charge cannot be obtained from the earth electrode and the surface charge density decreases, which is not preferable.

Further, in the present invention, in order to obtain electret fibers with a large electret effect and a large surface charge density, it is important that the cover factor of the fiber sheet is 60% or more. In order to increase the cover factor with as low a basis weight as possible, it is preferable to use fibers with extremely fine fineness, and the average fineness is preferably 1 d or less.

The electret fiber sheet of the present invention is made of a fiber sheet that satisfies the above-mentioned specific basis weight and cover factor, and has a surface charge density of 5 x 10-
It is important that the polarity of the electret fiber sheet is greater than ``/c1'' and that the front and back surfaces of the sheet have different polarities, and the electret fiber sheet is used alone, in a stack of sheets of the same type, or in a stack with other sheets.

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

FIG. 1 is a schematic diagram illustrating an electret fiber sheet according to the present invention in a model manner, and shows a laminated state. That is, two electret fiber sheets 2 are made of fibers 1 having a volume resistivity of 1013Ω·01 or more, have a cover factor of 60% or more, and have uniform charges with different polarities on both surfaces. The laminated electret fiber sheet 3 is formed by laminating the above multilayers.

In the electret fiber sheet having such a structure, charges exist in layers within the laminated sheet, so they do not cancel each other out, and the positive charges and negative charges existing on the surface of each fiber sheet 2 A high internal electric field strength can be maintained within each sheet, and when used as a filter, for example, a large electrostatic force can be applied to the collected particles, significantly improving the collection performance.

Next, in the sheet lamination method of the present invention, as shown in FIG. 3, the electret 8M sheets 2 can be laminated so that the polarities thereof become the same on the laminated surfaces. This lamination method is different from the case where the laminated surfaces shown in FIG. The residence time of the collected particles can be increased, and the collection performance can be further improved.

In the present invention, the surface charge density refers to a value measured by the following method.

That is, the schematic diagram of the surface charge density measuring device is shown in FIG. 4, in which a sample 7 is placed on a grounded metal plate 6, and then another metal plate 8 is approached from above and brought into contact with the sample. The electric charge existing on the sample surface is generated on a metal plate 8 by electrostatic induction, this electric charge is stored in a capacitor 9, and the potential is measured with an electrometer 10, and the surface charge density on the sample surface is determined by the following formula. It is.

Surface charge density (c/c/) = CXV/AC: Capacitance (Farad) V: Potential (Port) A: Sample area (ffl) The electret 1M sheet of the present invention determined by the above method is 5X10-''/QTF However, the electret fiber sheet according to the prior art as shown in FIG. 2 shows almost no surface charge density.

This is because positive and negative polarities coexist on the sheet surface, as shown in FIG. 2, which mutually weaken the electric field strength.

It is particularly desirable for the electret sheet according to the present invention to be used as a laminated electret fiber sheet by laminating multiple layers of electret fiber sheets having different polarities and high surface charge density.

That is, the laminated electret fiber sheet of the present invention, which has a multilayer structure in which positive and negative charges are distributed in layers, can maintain stable charges for a long period of time.

To obtain this effect, it is preferable to increase the number of laminated electret fiber sheets as much as possible, and the basis weight of the laminated electret fiber sheets should be as low as 80 Q/tn" or less, especially for clean room applications. 50Q/m when used as a filter
'The following is preferable.

Note that the number of stacked layers may be two, but from the viewpoint of long-term stability of charge, it is preferable to stack three or more layers.

Next, the method for manufacturing the electret fiber sheet of the present invention will be explained.

That is, the manufacturing method of the present invention has a volume resistivity of 10'3 Ω·cm.
One side of a fiber sheet made of jlAN having the above is brought into contact with a liquid electrode with a volume resistivity of 108 Ω cm or less, and a DC high voltage is applied to form an electret fiber sheet.The resulting sheets are laminated. By laminating them into a laminated electret fiber sheet, highly electret fibers can be obtained.

FIG. 5 is a schematic diagram showing the manufacturing method of the present invention, in which 11 is a grounded metal container, 12 is an electrode liquid placed in the metal container 11, and one side of the fiber sheet 13 is placed on the liquid. The electret fiber sheet is manufactured by bringing only the fibers into contact with each other, and applying a voltage generated by the DC high voltage generator 14 from the high voltage electrode 15 to the fiber sheet 13 in this state.

Previously, as an electret technology
As seen in Publication No. 99, there is a method in which a sample is made into an electret between a wire electrode loaded with high voltage and a ground electrode using a flat metal plate.
As shown in the figure (unlike a film, the surface of the sheet 17 on the ground electrode 16 has unevenness, so the contact area with the ground electrode is small, and this tendency is particularly noticeable when using a fiber sheet with a low basis weight. Therefore, when a high voltage is applied, sufficient complementary @ charges cannot be obtained from the earth electrode, resulting in the disadvantage that an electret l1M sheet with a large surface charge density cannot be obtained.

Therefore, the present inventors used a liquid electrode as a ground electrode to increase the contact area between the fiber and the ground liquid electrode as shown in FIG. We succeeded in obtaining an electret fiber sheet with high density and low basis weight.

In the present invention, in order to efficiently perform the electret treatment and obtain an electret fiber sheet with a high surface charge density, an il with a basis weight of 80 Q/rn2 or less is required.
It is better to use 1 sheet.

One side of the fiber sheet is brought into contact with a liquid electrode and a predetermined high voltage is applied. For this purpose, it is preferable that the cover factor of the fiber sheet is 60% or more. This is because if the surface of the fiber sheet is coated with liquid, the electretization effect will be reduced.

In addition, to prevent liquid from penetrating the IM receipt surface,
It is preferable to increase the apparent density of the $111 sheet.
0.05g/a+f or more indicates excellent electret performance.

Generally speaking, the fiber sheet used in the present invention is preferably one in which the constituent fibers are arranged in one direction. HAM1 like this
This is because the sheet has few irregularities, so the adhesion area with the liquid electrode can be increased, and therefore a sufficient compensation charge can be obtained when high voltage is applied, and an electret fiber sheet with a high surface charge density can be obtained.

Therefore, the degree of sheet orientation is preferably 2 or more.

Here, the degree of sheet orientation refers to the ratio of the tensile strength in the orientation direction of the sheet to the tensile strength in the direction perpendicular to the orientation direction.

The treatment time for the liquid electrode can be selected as appropriate within a range that does not reduce the electretization effect, but it is usually 20
A range of seconds to 80 seconds is used.

The liquid temperature of the liquid electrode is preferably set to a temperature range above the glass transition temperature of the fiber sheet and below the melting point.If high pressure is applied under such temperature conditions, a surface charge density with a large surface charge density and a deep trap charge can be obtained. , a stable electret fiber sheet that is less affected by changes over time can be obtained.

That is, when the temperature is higher than the glass transition temperature, molecular motion becomes active, more charges are injected, and a deep trap is formed. Note that the trap depth can be determined by thermally stimulated current measurement, which measures the depolarization current by increasing the temperature at a constant rate.

In the present invention, the liquid used for the electrode is preferably one that has a volume resistivity of 108 Ω・CI or less and can easily conduct a current, such as water, an organic solvent such as alcohol or acetone, or water containing a surfactant. can.

Two or more m miscellaneous sheets that have been subjected to a predetermined electret treatment are then laminated to form a laminated electret.

The laminated electret fiber sheet of the present invention can be subjected to post-processing such as needle punching, stitching, water punching, embossing, high-frequency welding, etc., as necessary, to improve sheet strength.

Further, the sheet of the present invention can be further laminated with other sheets for use.

In addition, as the fiber sheet used in the present invention, short II fiber nonwoven fabric, spunbond nonwoven fabric, melt blown nonwoven fabric, other fabrics, TA fibrous porous film, etc. can be used.

The laminated electret fiber sheet of the present invention that satisfies the above-mentioned preferable conditions maintains a high internal electric field strength over a long period of time, and therefore can be widely used in filters, wipers, masks, and the like.

In the present invention, the cover factor and sheet apparent density are each measured by the following methods.

The cover factor was calculated by measuring the sample (10 cm) using a magnifying fluoroscope.
x 10cm), and the bright area created by the light passing through is B,! The shadow area created by blocking the light by a[ is defined as C, and is calculated using the following formula.

Cover factor (%)=-Ω-×100β+C The apparent density of the sheet was calculated from the formula below.

D2 Weight per unit area q/+o#) E: Thickness (cm
) F: unit area (1 d) The thickness is 50-glo (measured under load).

[Effects of the Invention] Since the present invention is configured as described above, it is possible to efficiently obtain electret fibers that have a high surface charge density and can stably retain the charge for a long time. It was made.

[Example] Hereinafter, the present invention will be specifically explained with reference to Examples.

Example 1 A melt-blown fiber sheet made of polypropylene with a volume resistivity of 1016 Ω·cm was used. The sheet is 14 inches 2
0 Q/m2, cover factor 99%, sheet apparent density 0.13g/a+f, average fineness 0.2d.

The degree of sheet orientation was 3.9.

The sheet has a volume resistivity of 103Ω, which is the manufacturing method of the present invention.
- One side was brought into contact with an electrode using C1l of water and treated with a needle electrode for 60 seconds at an applied voltage of -30 KV. The temperature of the water at this time was 40° C., and the distance between the needle electrode and the mM receipt to be applied was 3 cm.

The surface charge density of the obtained electret m-fiber sheet was 1.5×10−9°10n with one charge on one side, and 1.OX 10−9′−/ffl with ten charges on the other side.

Five of these sheets were laminated to obtain a laminated electret fiber sheet.

The stacking method was such that the stacked surfaces of each cm sheet had the same polarity.

Here, the filter performance of the sheet of the present invention was evaluated in comparison with the conventional sheet.

The measurement conditions were as follows: 0.33μ polystyrene standard latex particles were used, the measurement wind speed was 1.5m/1n, and the number of particles before and after the measurement sample was counted using a light scattering type dust counter to determine the collection efficiency. As a result, the collection efficiency of the sheet of the present invention was 99.999%.

As a comparison sample, an electret fiber sheet according to the prior art as shown in FIG. 2 (area weight 100 (1/m2)) was measured, and the result was 98.200%, confirming that the sheet of the present invention is significantly superior.

Example 2 Filter performance was evaluated by changing the lamination method of the electret fiber sheets produced in Example 1. One was an electret fiber sheet in which two electret fiber sheets were laminated so that the polarities of the laminated surfaces were different. On the other hand, a laminated electret fiber sheet was created in which the polarities of the laminated surfaces of the electret fiber sheets were made to be the same polarity.

When comparing the two, the former has a collection efficiency of 99.7.
23%, the latter was 99.955%, and the latter was clearly superior.

Example 3 A fiber sheet made of polypropylene mH with a volume resistivity of 10 Ω·Cl 1 l was used. The sheet has a basis weight of 50 Q
/ m2, cover factor 99%, sheet apparent density 0
．． 10g/a+! , average fineness 0.8d, sheet orientation degree 3
．． It is 0.

Using the sheet, the volume resistivity is 1 by the manufacturing method of the present invention.
03Ω・cm water/ethyl alcohol (mixing ratio 90/1
0) The mixed solution was heated at 10KV/3cm using a liquid electrode for 5 minutes.
The voltage was applied for 0 seconds to create an electret fiber sheet.

In addition, as a comparative example, a 110l1 thick stainless steel plate was used as the ground electrode in the conventional method, and the fiber sheet was placed on top of this.
An electret fiber sheet was produced by processing under the same application conditions as above. The surface inherent brightness of the electret fiber sheet according to the present invention is 2.5X 10-IOC on one side.
/ln, and the other side was +1,5x10-phantom C/af.

In contrast, the surface charge density according to the conventional method is -
2,3X 10- I+'/arI, other side is +1.2X
10-11'/ci, and the former invention was clearly superior.

Example 4 Made of polypropylene fiber, fineness 0.9d, basis weight 30
Fiber sheets with Q/rn2 and sheet orientation degrees of 1.1 and 4.5 were used to form electrets by the method of the present invention. The applied voltage was +20 KV/3 cm, and a water/ethyl alcohol (mixing ratio 90/10) mixed solution was used as the liquid electrode for 50 seconds.

As a result, the average surface charge density on both sides of the sheet made into electret using a fiber sheet with a degree of sheet orientation of 1.1 was 2 x 10- lo'-/at, but that with a degree of sheet orientation of 4.5! The surface charge density on both sides of the electret-treated sheet using I-fiber receipt is 8.0X10-9°
/d, and it can be seen that the surface charge density differs depending on the degree of sheet orientation.

Example 5 Made of polyethylene terephthalate fiber with a volume resistivity of 1014 Ω・cm, fineness 3d, basis weight 30 Q/tn2
, cover factor 81%, sheet apparent density 0.21
An electret fiber sheet was prepared under the same conditions as in Example 1 using a fiber sheet with an orientation degree of 2.3 g/GlIN sheet.

The surface charge density of this sheet is -g x 10-1Qe on one side
lcd, the other side was +7 x 10-''/c/.

Next, three of these sheets were laminated to obtain a laminated electret fiber sheet.

The lamination method was such that the polarities of the laminated surfaces were the same. When the obtained laminated electret sheet was used as a wiper, the dust adsorption rate was extremely good.

Example 6 Fabric weight 30 Q/rn2 made of polypropylene fiber
, cover factor 99%, sheet apparent density 0.080
An electret fiber sheet was obtained under the conditions of Example 1 using an m-fiber sheet with /a+t and a fineness of 0.5d.

The surface charge density of this sheet is −8.1×10−” on one side.
10TF, other side +6.2 x 10- IQc/ci
Met.

Next, two of these sheets were laminated, and the sheet made in Example 1 with a high apparent density was placed on top of the laminated sheet. Two electret fiber sheets were laminated to obtain a laminated electret fiber sheet with a density gradient.

When the obtained sheet was used as a filter with the side of the sheet with a lower apparent density serving as the collection surface, large particles were collected on the side with a lower apparent density of the sheet, so it could not be used for a long period of time. I was able to do that.

Example 7 Three fiber sheets having a basis weight of 20 Q/vn2 used in Example 1 were stacked, and the laminated sheet was subjected to an electret treatment under the same conditions as in Example 1.

The surface charge density of the obtained electret fiber sheet is
-5x 10-”eloff on one side, +4X1 on the other side
0-"/d. Among the three-ply sheets, the surface charge density of the middle fiber sheet was less than 1Q-11c/d. This is because both surfaces of the three-ply sheet had a large surface charge density. However, it shows that the inside is small.

Next, two of the above three-ply sheets were laminated to produce a laminated electret m-fiber sheet. When this sheet was evaluated using a filter, it showed a high collection efficiency of 99.95%. However, the results of the laminated electret fiber sheet of Example 1, in which each sheet was subjected to electret processing, were inferior.

Example 8 Polypropylene made in Example 7! ! Fabric weight 6
The long-term stability of the three-ply electret fiber sheet of 0g/1T12 and the laminated electret fabricated in Example 1 that satisfies the present invention was investigated.

That is, both of the above-mentioned sheets were left at 20° C. and 60% humidity for one month, and then taken out and the rate of decrease in surface charge density of both was measured. The former showed a 20% decrease, but the latter sheet of the present invention decreased by 20%. Almost no decrease was observed.

This shows that the multilayered sheet of the present invention has excellent stability over time.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram illustrating the electret fiber sheet according to the present invention as a model, and is a schematic diagram showing an example of a laminated structure sheet. FIG. 2 is a schematic diagram illustrating a conventional electret fiber sheet as a model. Figure 4 is a schematic diagram explaining another example of the electret a-fiber sheet according to the present invention, Figure 4 is a schematic diagram explaining a surface charge density measuring device, Figure 5 is a schematic diagram explaining the method of the present invention, FIG. 6 is a schematic diagram illustrating the state of contact between the conventional ground electrode and the sheet, and FIG. 7 is a schematic diagram illustrating the state of contact between the liquid electrode and the fiber sheet in the present invention. 2: Electret m fiber sheet 3: Laminated electret fiber sheet 11: Metal container 12: Electrode liquid 13: Fiber sheet 14: High pressure generator 15: High voltage electrode

Claims (8)

[Claims] (1) Made of a fiber sheet with a basis weight of 80g/m^2 or less, a cover factor of 60% or more, and a surface charge density of 5x
An electret fiber sheet having a polarity of 10-^1^1c/cm^2 or more and having different polarities on both the front and back sides of the sheet. (2) A laminated electret fiber sheet formed by laminating two or more electret fiber sheets. (3) The electret fiber sheet according to claim (1) or (2), wherein the fibers constituting the fiber sheet have a fineness of 1 d or less. (4) The electret fiber sheet according to claim (1) or (2), wherein the fiber sheet has an apparent density of 0.05 g/cm^2 or less. (5) The electret fiber sheet according to claim (2), wherein the fiber sheets are laminated so that the polarities of the laminated surfaces thereof are the same. (6) One side of a fiber sheet made of fibers with a volume resistivity of 10^1^3 Ω/cm or more has a volume resistivity of 10^8 Ω/cm.
A method for producing an electret fiber sheet, which comprises bringing it into contact with a liquid electrode of cm or less and applying a high DC voltage. (7) The method for producing an electret fiber sheet according to claim (6), wherein the liquid temperature of the liquid electrode is maintained at a temperature higher than the glass transition point and lower than the melting point of the fibers constituting the fiber sheet. (8) The method for producing an electret fiber sheet according to claim (6), characterized in that a high voltage is applied to the fiber sheet in a non-contact state with the application side electrode.