JPH0674545B2 - Method for manufacturing electret fiber sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an electret fiber sheet having a high surface charge density.

[Prior Art] A conventional method for producing an electretized fiber sheet is described in Japanese Patent Publication No. 56-47299, in which a wire electrode and a conductor earth electrode, for example, iron having a volume resistivity of 10 ⁻⁶ Ω · cm are used. A method of forming an electret by applying a high voltage using such a metal plate has been used. When using this method, if dust, especially conductive matter, is attached to the fiber sheet,
When a high voltage was applied, a spark discharge occurred and the fiber sheet was perforated and a high surface charge density was not obtained, which was unsuitable for use in a filter or the like.

[Problems to be Solved by the Invention] An object of the present invention is to obtain an electret fiber sheet having a high surface charge density by preventing perforation due to spark discharge due to dust, which is a drawback of the above-mentioned known electret method. Is provided.

[Means for Solving Problems] In the present invention, the non-contact type application electrode and the volume resistivity are 10 ^-1 to 10 ⁸
80 g / m ² or less in basis weight between the semiconductor ground electrode of Ω ・ cm,
The present invention relates to a method for producing an electret fiber sheet, which comprises performing electret with an applied-side electric field strength of 3 KV / cm or more with a fibrous sheet having a cover factor of 60% or more interposed.

Hereinafter, the method for producing the electret fiber sheet of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing an embodiment of an electretization apparatus used in the method for producing an electret fiber sheet of the present invention, 1 is a fiber sheet, 2 is a semiconductor ground electrode, 3
Is a non-contact type needle-shaped application electrode, 4 is a DC high voltage generator, and 5 is ground.

A needle-shaped electrode as a non-contact type applying electrode used in the present invention,
Although there are wire electrodes and the like, the material for these electrodes is preferably a conductive metal having a volume resistivity of 10 ⁻⁴ Ω · cm or less.

The volume resistivity of the semiconductor ground electrode must be 10 ^-1 to 10 ⁸ Ω · cm.

If it is less than 10 ^-1 Ω · cm, a spark discharge is generated due to the semiconductor attached to the fiber sheet, dust on the conductor and other deposits, and holes are formed in the fiber sheet, and electretization is not sufficiently performed.

If it exceeds 10 ⁸ Ω ・ cm, the compensating charge, which is considered to be injected from the ground electrode when injecting electrons or ions generated from the non-contact type application electrode into the fiber, cannot be sufficiently obtained, so that stable electretization is achieved. Can not.

Particularly preferred semiconductor ground electrode material has a volume resistivity of 10 ^-1 to
10 ⁶ Ω · cm.

As the material having such a volume resistivity, for example, one having a semiconductor property made of a synthetic resin mixed with carbon particles or metal particles, and an organic polymer satisfying the above volume resistivity, for example, polyalkylene glycol and nylon Examples thereof include copolymers and blends.

As a form, a film, a sheet, a plate-like material or the like can be used.

Next, as a condition for electretization, it is necessary to set the applied-side electric field strength to 3 KV / cm or more, and preferably 8 KV / cm.
cm or more, at least the gap between the electrode on the application side and the fiber sheet
It should be 5 mm or more. If the applied field strength is lower than 3 KV / cm, it will be difficult to increase the surface charge density, and if the gap between the applied electrode and the fiber sheet is less than 5 mm, spark discharge will easily occur. I can't go up.

Further, the applied temperature is preferably higher than or equal to the glass transition point of the fiber sheet, and when the applied temperature is low, the surface charge density is low and it becomes difficult.

Furthermore, the discharge current per unit area of the fiber sheet is 1
It is preferable to apply at × 10 ⁻⁶ mA / cm ^{2 to} 5 × 10 ⁻³ mA / cm ² . If it is less than 1 × 10 ^-6 mA / cm ² , sufficient electretization cannot be achieved, and if it exceeds 5 × 10 ^-3 mA / cm ² , spark discharge due to dust or the like is likely to occur.

The distance between the non-contact type applying electrode and the earth electrode made of a semiconductor material is preferably 10 mm or more, and when it is less than 10 mm, generally the applied electric field strength is increased and spark discharge is generated, and the surface charge density of the obtained electret fiber sheet is increased. Can not be increased. Therefore, more preferably 20
mm or more is recommended.

From the standpoint of charge injection, the fiber sheet must have a basis weight of 80 g / cm ² or less. This has a basis weight of 80 g / cm
^This is because when the number exceeds ² , the electric charge is not injected into the inside of the fiber sheet and remains only on the surface, and the surface charge density does not increase.

The cover factor of the fiber sheet must be 60% or more. If it is less than 40%, the injected charges pass through the gaps between the fibrous sheets, so that they are not trapped in the fibrous sheets and the surface charge density does not increase.

The apparent density of the fiber sheet is preferably 0.01 g / cm ² or more. 0.0
If it is less than 1 g / cm ² , the injected charge does not enter the inside.

The average fineness of the fiber sheet is preferably 5d or less, and when it exceeds 5d, the surface charge density is small. More preferably, it is 1d or less.

As the fibrous sheet, non-woven fabrics, woven fabrics, papers, knits and the like can be used, but in particular, a melt blown non-woven fabric is preferably used as a uniform fibrous sheet having a fine fiber sheet with a fineness and a large cover factor. be able to.

As the material of the fiber sheet, polyolefin such as polyethylene and polypropylene, polycarbonate, polyfluorine-based resin, polyvinyl chloride, polyester, fibrous material made of glass or an inorganic compound can be used, but the volume resistivity is particularly 10 ¹³ Ω. A fiber sheet of cm or more is preferable.

Among them, the polyolefin resin has a large specific resistance and a large surface charge density because it is non-polar.

The surface charge density of the electret fiber sheet according to the present invention obtained by satisfying the above conditions is 7 × 10 ^-11 coulomb / c.
m ^2, and more preferably 2 × 10 ^-10 coulombs / cm ^2, most preferably a 5 × 10 ^-10 coulombs / cm ² or more.

[Effect of the Invention] The electret fiber sheet obtained by the present invention is
It has no discharge holes due to dust and is stable with a high surface charge density. Therefore, it can be used as a filter as well as in a useful application such as an adsorbent or a self-adhesive with a stable charge.

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

(1) Cover factor Make a shadow of the sample (2 cm x 2 cm) using a magnifying projector, and let B be the bright area created by the transmission of light and C be the shadow area created by the light being blocked by the fibers. It is obtained by the following formula.

(2) Apparent Density of Sheet The apparent density of the sheet is obtained from the following formula.

D: Weight per unit area (g / cm ² ) E: Thickness (cm) F: Unit area (2 cm ² ) The thickness is measured at 50 g / cm ² .

(3) Surface charge density The schematic diagram of the apparatus for measuring the surface charge density is as shown in FIG. 2. The sample 7 is placed on the grounded metal plate 6, and then another metal plate 8 is approached from above. Contact the sample, sample 7
The electric charge existing on the surface is generated in the metal plate 8 by electrostatic induction, this electric charge is stored in the capacitor 9, the electric potential is measured by the electrometer 10, and the surface charge density of the sample surface is obtained by the following formula. is there.

Surface charge density (C / cm ² ) = C × V / AC: Condenser capacity (farad) V: Potential (volt) A: Sample area (cm ² ) [Example] The present invention will be described in more detail below with reference to Examples. Explained.

Example 1 A polypropylene meltblown nonwoven fabric having a basis weight of 20 g / m ² , an apparent density of 0.125 g / cm ³ , a cover factor of 99%, an average fineness of 0.1 d and a volume resistivity of 10 ¹⁶ Ω · cm was used as a fiber sheet. , Was electretized using the apparatus shown in FIG.

One needle electrode made of stainless steel with a volume resistivity of 10 ^-5 Ω · cm is used as the applying electrode, and the volume resistivity of a 20 cm square is 10 ³ Ω · cm.
The carbon particle-containing polyethylene sheet of 1 was used as a ground electrode and a fiber sheet of the same shape was placed thereon. Applied voltage -30kv / cm (ie, electric field strength 10KV / cm) in 25 ℃ atmosphere
The treatment was performed for 30 seconds at a distance of 3 cm between the needle-shaped application electrode and the ground electrode. The discharge current during the treatment was 1 × 10 ⁻⁶ mA / cm ² per area of the fiber sheet.

The surface charge density of the obtained electret fiber sheet is
+ 6.7 × 10 ^-10 Coulomb / cm ² on the front side, -8.1 × on the back side
It was 10 ⁻¹⁰ coulomb / cm ² . Furthermore, no spark discharge phenomenon was observed.

Example 2 After the procedure of Example 1, the treated fiber sheet was turned upside down and further treated with an applied voltage of +20 KV (electric field strength of 6.6 KV / cm, discharge current of 2 × 10 ⁻⁵ mA / cm ² ) for 30 seconds. In this case, a polyethylene sheet mixed with carbon particles having a volume resistivity of 10 ⁴ Ω · cm was used as the ground electrode. This thickness is 0.9 mm.

As a result, the surface charge density of the obtained fiber sheet was + 8.5 × 10 ^-10 coulomb / cm ² on the front side and −9.8 × 10 ^-10 on the back side.
It was coulomb / cm ² .

Furthermore, no spark discharge phenomenon was observed.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing one embodiment of an electretization device used in the method for producing an electret fiber sheet of the present invention, and FIG. 2 is a schematic diagram showing a device for measuring surface charge density. 1: Fiber sheet 2: Semiconductor earth electrode 3: Non-contact type needle-shaped application electrode 4: DC high voltage generator 5: Earth

Claims (1)

[Claims] 1. A non-contact type application electrode and a volume resistivity of 10 ^-1 to 10 ^-1.
It is characterized in that a fibrous sheet having a basis weight of 80 g / m ² or less and a cover factor of 60% or more is interposed between the semiconductor ground electrode of ⁸ Ω · cm and an electret with an applied-side electric field strength of 3 KV / cm or more. Method for manufacturing electret fiber sheet.