JP3061534B2 - Method for producing electret fiber sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an electret fiber sheet which can be used for filters, masks, wiping materials, dustproof clothing, and the like.

[0002]

2. Description of the Related Art Conventionally, electret fiber sheets have been disclosed in Japanese Patent Publication No. 3-54620 and Japanese Patent Publication No. 4-8539.
As shown in FIG. 3 and Japanese Patent Publication No. 5-83283, and as shown in FIG. 3, a state in which a fiber sheet 3 such as a nonwoven fabric is brought into contact with a ground electrode 5 composed of a water electrode or a steel drum or the like. There has been known an electret formed by applying a high DC voltage from a discharge electrode 6 such as a wire electrode or a needle electrode. In these electret fiber sheets, the electric charge is held as polarized electric charges in the constituent fibers, and is applied in the same polarity as the applied voltage on the surface to which a high DC voltage is applied, and to the applied voltage on the surface in contact with the ground electrode. Are charged to the opposite polarity, and particles such as dust can be collected by electrostatic force due to polarization charge, and the force is maintained for a relatively long time.

However, in the conventional method of applying a high DC voltage, corona discharge is unlikely to occur unless a wire electrode or a needle electrode is used. On the other hand, if these electrodes are used, an uneven electric field is formed, so that a high voltage is applied. In this case, there is a problem that a spark is easily generated. On the contrary,
If an attempt is made to prevent sparking, a sufficient charge cannot be applied to the fiber sheet because the voltage that can be applied is limited.

[0004] In the conventional method of applying a high DC voltage, the conditions for generating corona discharge greatly depend on the nature of the space between a wire electrode or a needle electrode and the ground electrode. Immediately shifts to spark discharge, and if the ambient temperature is too high, stable corona discharge may not be obtained, and the stability of corona generated depends on conditions such as the type and temperature of gas existing in the space There was a problem.

Further, in the conventional method of applying a high DC voltage, the direct current corona discharge depends on the electric field structure between the discharge electrode and the ground electrode. It works in the direction of suppressing corona discharge by changing the electric field in the vicinity. For this reason, in a thick fiber sheet in which the distance between the corona discharge electrode and the surface of the fiber sheet is short,
Since corona discharge is suppressed only by accumulation of a small amount of charge on the sheet surface, there is a problem that a sufficient amount of charge cannot be provided.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and is less likely to cause a spark, and can impart a large amount of electric charge to a fiber sheet. It is an object of the present invention to provide a method for manufacturing an electret fiber sheet that can be made into an electret without being affected by the ambient temperature and the type of gas.

[0007]

SUMMARY OF THE INVENTION The object of the present invention is to produce an electret fiber sheet according to the present invention, wherein unipolar ions extracted from an AC creeping discharge are electretized by acting on the fiber sheet. Solved by the method.

That is, in the present invention, unlike the prior art, in which a direct current corona discharge is directly applied to a fiber sheet, a unipolar ion extracted by an extraction electrode from an alternating surface creeping discharge formed in an alternating surface creeping discharge element is used. Since it acts on the fiber sheet, the mechanism that substantially generates the creeping discharge and the mechanism that charges the fiber sheet work independently. Therefore, even if the voltage value or frequency is increased to generate a large amount of ions in AC creeping discharge, or if the electric field strength of the charged atmosphere is increased by increasing the voltage applied to the extraction electrode, sparks can be generated immediately. Thus, a large amount of charge can be imparted to the fiber sheet.

Further, since the creeping discharge portion and the charged portion of the fiber sheet work substantially independently, the stability of the creeping discharge depends on the temperature of the space between the AC creeping discharge element and the extraction electrode and the type of gas. For example, even when the temperature of the fiber sheet is changed, the surface discharge can be kept stable even when the ambient temperature is changed, and the electret of the fiber sheet can be formed without limiting the type of gas.

Furthermore, in the present invention, since the creeping discharge generating portion and the charged portion of the fiber sheet work substantially independently,
The potential gradient between the surface of the charged fiber sheet and the discharge electrode is zero.
Since the fiber sheet can be electretized by extracting monopolar ions from the creeping discharge as long as the discharge does not occur, there is no great limitation due to the thickness of the fiber sheet as in the case of the conventional DC corona discharge.

The present invention will be described below with reference to the drawings. FIG.
FIG. 2 is a schematic cross-sectional view showing an example of an apparatus used in the method for producing an electret fiber sheet of the present invention, and FIG. 2 is a schematic sectional view showing another example of an apparatus used in the method for producing an electret fiber sheet of the present invention. is there.

An AC creeping discharge element 1 for generating an AC creeping discharge is formed by disposing a discharge electrode 12 on one surface of a dielectric 11 and an induction electrode 13 on the other surface. The discharge electrode 12 and the induction electrode 13 are connected to an AC power supply 14,
By applying an AC high voltage, ionization occurs from the discharge electrode 12 to the surface of the dielectric 11 on the side where the discharge electrode is disposed, and ions of both positive and negative polarities are generated. The applied AC voltage is preferably 0.1 KV or more, more preferably 1 KV or more, and the frequency is 10 ² to 10 ⁵ Hz, more preferably 1 Hz.
The frequency is preferably in the range of 0 ^{3 to} 2 × 10 ⁴ Hz.

A plate or a sheet of glass, ceramic, plastic, or the like is used for the dielectric 11. Although the thickness of the dielectric 11 is not particularly limited, if it is too thick, a very high voltage is required to discharge, and if it is too thin, the capacitance between the discharge electrode and the induction electrode increases, making it difficult to apply an AC high voltage. Therefore, those having a thickness of about 0.1 to 5 mm are preferable. The discharge electrode 12 is formed by applying a conductive paint on a surface of a plastic or the like, forming a metal layer or a conductive resin layer, or a grid electrode or a mesh formed from a metal such as aluminum or copper. Electrodes are suitable. Preferably, the discharge electrode 12 has a structure having an opening so that the dielectric 11 is exposed on the surface. As the induction electrode 13, a material in which a conductive paint is applied to the surface of plastic or the like, a metal layer or a conductive resin layer is formed, or a flat electrode or a net-like electrode formed of a metal such as aluminum or copper is suitable. ing. The shape of the AC creeping discharge element 1 is usually a flat plate as shown in the figure, but may be deformed according to the shape of the extraction electrode. For example, when the extraction electrode 2 is a roll shape such as a steel drum, It may be curved so as to follow this roll shape.

In the AC surface discharge element 1, the length of the dielectric 11 is longer than that of the discharge electrode 12 and the induction electrode 13 so that no direct discharge occurs between the discharge electrode and the induction electrode at the end of the AC surface discharge element. It is desirable. In addition, a dielectric thin film such as a ceramic thin film or a polymer thin film may be coated on the discharge electrode 12, so that the discharge electrode can be prevented from being consumed by creeping discharge.

In the present invention, the extraction electrode 2 is provided at a position facing the discharge electrode 12 side of the AC surface discharge element 1 at an interval. The extraction electrode 2 is connected to a DC power supply 21 and functions to extract unipolar ions having a polarity opposite to that of the applied DC high voltage from the AC creeping discharge by applying a high DC voltage. For example, when a positive charge is applied as a DC high voltage, only negative ions are extracted from the AC surface discharge and move from the AC surface discharge element 1 to the extraction electrode 2. If the DC voltage applied to the extraction electrode 2 is too large, a spark discharge may occur between the discharge electrode 12 and the discharge electrode 12. If the DC voltage is too small, the amount of charge is insufficient. For example, in air, the absolute value (because the applied voltage may be positive or negative) may be 1 to 15 KV / cm, more preferably 3 to 10 KV / cm.

The extraction electrode 2 is obtained by applying a conductive paint on a surface of a plastic or the like, forming a metal layer or a conductive resin layer on the surface, a plate-like electrode made of a metal such as aluminum or copper, a net-like electrode, or the like. Electrodes, roll-shaped electrodes and the like are preferred. This extraction electrode 2 and AC surface discharge element 1
Is appropriately determined depending on the thickness and density of the fiber sheet to be electretized, the magnitude of the DC high voltage applied to the extraction electrode 2, or the AC voltage and frequency applied to the discharge electrode 12 and the induction electrode 13. However, if the distance is too large, a very high voltage is required for effective charging, so there is a safety problem. If the distance is too short, there is a danger of short circuit. Is preferably in the range of 5 to 30 mm.

According to the present invention, the unipolar ions are extracted by the extraction electrode 2 from the AC surface discharge generated in the AC surface discharge element 1 as described above, and the unipolar ions are extracted from the AC surface discharge element 1 to the extraction electrode 2. Therefore, if the fiber sheet 3 is arranged in the space between the AC creeping discharge element 1 and the extraction electrode 2, the fiber sheet 3 is electretized by the action of the unipolar ions.

When electretization is performed with the extraction electrode 2 and the fiber sheet 3 in contact with each other as shown in FIG.
Compared to the case of electretization without contact,
A larger charge amount can be given to the fiber sheet 3.

Further, as shown in FIG. 2, electretization may be performed in a state where a dielectric such as ceramic, plastic or glass is interposed between the extraction electrode 2 and the fiber sheet 3.

In the method for producing an electret fiber sheet of the present invention, it is preferable to charge the constituent fibers by applying unipolar ions in a high-temperature atmosphere having a glass transition point or higher and lower than the melting point of the constituent fibers, and then cool the fibers. In particular, when measuring the thermal stimulation current of the constituent fibers, the current value of the current-temperature curve rises to a temperature range from the rise to the peak,
Thereafter, it is preferable to rapidly cool the temperature to the temperature at the time of rising of the current.
The measurement of the thermal stimulation current of the constituent fibers is performed by electretizing a film made of the resin forming the constituent fibers, heating the electretized film at a constant speed, and measuring the generated current. This temperature range is about 60-130 ° C for polypropylene and about 100-150 ° C for polyester. For example, if the constituent fibers of the fiber sheet are polypropylene fibers, 60 to 130
After the unipolar ions are allowed to act in an atmosphere at a temperature of ° C., it is preferable to rapidly cool to about room temperature. In this way, the ions are held at a deeper position inside the constituent fibers, so that they are less likely to be electrically neutralized or fall off from the constituent fibers, and more excellent electret performance is maintained over a long period of time. Can be expected stably. In the present invention, since the discharge portion and the charged portion of the fiber sheet work substantially independently, even if electretization is performed in such a high temperature atmosphere, the surface discharge is hardly affected. The fiber sheet can be stably formed into an electret.

The fiber sheet 3 used in the present invention includes nonwoven fabrics such as dry nonwoven fabric, spunbonded nonwoven fabric, melt blown nonwoven fabric, hydroentangled nonwoven fabric, and wet nonwoven fabric.
Fabrics such as woven and knitted fabrics, and fibrous porous films are suitable. In particular, a fiber sheet such as a melt-blown non-woven fabric or a hydro-entangled non-woven fabric to which no fiber oil agent or adhesive is attached may be easily formed into an electret.

As the fibers constituting these fiber sheets, dielectric fibers such as polyolefins such as polyethylene and polypropylene, polyesters, polycarbonates and polyvinyl chlorides are suitable. Is desirably 10 ¹⁴ Ωcm or more.
When the volume resistivity of the constituent fibers is within this range, unipolar ions can be easily applied to the fibers, and the fibers can be stably electretized for a long period of time.

[0023]

【Example】

Example 1 A glass plate having a thickness of 1.1 mm was used as a dielectric, and a grid electrode made of aluminum foil with a width of 5 mm and an interval of 5 mm was attached to one surface, and a plate electrode made of aluminum foil was attached to the other surface. Then, the grid-like electrode and the plate-like electrode were connected to an AC power supply to form an AC creeping discharge element. An extraction electrode made of a copper plate was arranged so as to face the grid-like electrode side surface of this AC creeping discharge element and to have a distance of 17 mm from the AC creeping discharge element, and a DC power supply was connected to this. A polypropylene meltblown nonwoven fabric having a basis weight of 45 g / m ² and a thickness of 0.6 mm was placed in contact with the extraction electrode, placed therein, and charged for 3 minutes to form an electret. In addition, 3 KHz, 2.5 KV is applied between the above-mentioned grid electrode and the plate electrode.
And a DC high voltage of -8 KV was applied to the extraction electrode. Atmospheric dust is passed through the obtained electret meltblown nonwoven fabric at a wind speed of 5 cm / sec, and the number of dust particles having a particle size of 0.3 to 0.5 μm before and after passing is measured by a particle counter to improve the collection efficiency. I asked. The collection efficiency of dust with a particle size of 0.3 to 0.5 μm is 91%.
It showed high performance as an air filter or a filter for a mask.

Example 2 The DC high voltage applied to the extraction electrode was changed from -8 KV to +8 KV.
The electret melt-blown non-woven fabric was obtained in exactly the same manner as in Example 1 except that this was changed to. When the collection efficiency of the obtained electret meltblown nonwoven fabric was measured in the same manner as in Example 1, the collection efficiency was as high as 87%, indicating that it had high performance as an air filter or a filter for a mask.

COMPARATIVE EXAMPLE 1 As shown in FIG. 3, a discharge electrode formed by applying gold plating to a tungsten wire having a diameter of 60 μm was arranged so as to face a ground electrode made of a copper plate having a thickness of 0.5 mm. In addition, unlike FIG. 3, 15 discharge electrodes were arranged at 10 mm intervals, and the distance between each discharge electrode and the ground electrode was set to 20 mm. The same melt blown nonwoven fabric as used in Example 1 was placed on this ground electrode, and a high DC voltage of -9.7 KV was applied to the discharge electrode for 3 minutes to obtain an electret meltblown nonwoven fabric. When the collection efficiency of the obtained electret meltblown nonwoven fabric was measured in the same manner as in Example 1, it was 71%.
And lower than those of Examples. In the method of this comparative example, it is difficult to generate a corona discharge stably due to sparking, so that the DC high voltage could not be made higher (in absolute value). In Examples 1 and 2, spark did not occur even if the DC high voltage applied to the extraction electrode was further increased.

Example 3 A glass plate having a thickness of 3 mm was used as a dielectric, and a grid electrode made of aluminum foil with a width of 5 mm and an interval of 5 mm was attached to one surface, and a plate electrode made of aluminum foil was attached to the other surface. Then, the grid electrode and the plate electrode were connected to an AC power supply to form an AC creeping discharge element. An extraction electrode made of a copper plate was arranged so as to face the surface of the AC creeping discharge element facing the grid electrode and to have a distance of 17 mm from the AC creeping discharge element, and a DC power supply was connected thereto. Basis weight 4
A 5 g / m ² , 0.6 mm thick polypropylene meltblown nonwoven fabric is placed in contact with the extraction electrode and placed at 120 ° C.
After electrification by charging for 3 minutes in a temperature atmosphere of, the mixture was quenched with liquid nitrogen while charging. In addition, 3 KHz, 4.5 K is provided between the grid electrode and the plate electrode.
A high AC voltage of V was applied, and a high DC voltage of -8 KV was applied to the extraction electrode. When the collection efficiency of the obtained electret meltblown nonwoven fabric was measured in the same manner as in Example 1, the collection efficiency was as high as 95%, indicating high performance as an air filter or a filter for a mask.

[0027]

According to the method for producing an electret fiber sheet of the present invention, unlike the conventional method in which a direct current corona discharge is directly applied to a fiber sheet, the direct current corona discharge is drawn from an AC surface discharge formed in an AC surface discharge element. Since the unipolar ions extracted by the electrodes act on the fiber sheet, the discharge portion and the portion for charging the fiber sheet substantially work independently. Therefore, even if the voltage value or frequency is increased to generate a large amount of ions in the AC creeping discharge, even if the applied voltage of the extraction electrode is increased to increase the amount of monopolar ions extracted, a spark is immediately generated. That is, a large amount of charge can be imparted to the fiber sheet. Therefore, according to the present invention, it is possible to obtain an electret fiber sheet which has an excellent ability to collect particles such as dust and is suitable for an air filter, a filter for a mask, a wiping material, dustproof clothing, and the like.

Further, in the manufacturing method of the present invention, since the creeping discharge section and the charging section of the fiber sheet work substantially independently, the creeping discharge does not occur even when the atmosphere temperature is changed when charging the fiber sheet. It maintains a stable state and is not easily affected by the composition of the gas around the fiber sheet. In particular, when electretization is performed at a temperature equal to or higher than the glass transition point and equal to or lower than the melting point of the constituent fibers of the fiber sheet and rapidly cooled, excellent electret performance can be maintained for a long period of time.

Further, according to the production method of the present invention, it is possible to form an electret without being so limited by the thickness and density of the fiber sheet as in the case of the conventional electret formation by DC corona discharge.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an apparatus used for a method for producing an electret fiber sheet of the present invention.

FIG. 2 is a schematic cross-sectional view showing another example of an apparatus used in the method for producing an electret fiber sheet of the present invention.

FIG. 3 is a schematic cross-sectional view showing an example of an apparatus used in a conventional method for manufacturing an electret fiber sheet.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... AC creeping discharge element 11 ... Dielectric 12 ... Discharge electrode 13 ... Induction electrode 14 ... AC power supply 2 ... Extraction electrode 21 ... DC power supply 3 ...・ Fiber sheet

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-49-4433 (JP, A) JP-A-5-226187 (JP, A) JP-A-3-69663 (JP, A) JP-A-62 104957 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) D06M 10/00

Claims (4)

(57) [Claims] 1. A method for producing an electret fiber sheet, wherein unipolar ions extracted from an AC creeping discharge are applied to a fiber sheet to form an electret. 2. A discharge electrode is provided on one surface of a dielectric, and an induction electrode is provided on the other surface, and an AC high voltage is applied between the discharge electrode and the induction electrode to cause a discharge from the discharge electrode to the dielectric surface. An AC creeping discharge element for generating a creeping discharge along, and a unipolar ion is provided from the creeping discharge generated in the AC creeping discharge element by being provided at a position facing the discharge electrode and applying a high DC voltage. The method for producing an electret fiber sheet according to claim 1, wherein a fiber sheet is disposed between the fiber sheet and a drawing electrode for drawing, and the fiber sheet is electretized by allowing monopolar ions to act on the fiber sheet. 3. The method for producing an electret fiber sheet according to claim 2, wherein the fiber sheet is brought into contact with a drawing electrode. 4. The electret fiber sheet according to claim 2, wherein unipolar ions act on the fiber sheet in an atmosphere at a temperature equal to or higher than the glass transition point of the constituent fibers of the fiber sheet and lower than the melting point, and the fiber sheet is cooled. Production method.