JPH0670301B2 - Method for producing electret melt blown nonwoven fabric - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an electret meltblown nonwoven fabric having a stable polarization charge, which can be very effectively used as a high-performance filter, an adsorbent or the like.

[Prior Art] Conventionally, as a method for making a meltblown nonwoven fabric into an electret, as described in Japanese Patent Publication No. 59-124, when a meltblown fiber from an orifice travels in a jet stream, a high pressure is applied to the fiber. Has been proposed to produce an electret meltblown nonwoven fabric by collecting the fibers into an electret.

However, the electrified meltblown non-woven fabric obtained by this method had a random charge direction, as shown in FIG.

For this reason, the fibers having the polarization charge inside the nonwoven fabric weaken the charges to each other, and the stability of the charges is not good. Therefore, for example, when used as a filter, there is a drawback that a high collection efficiency cannot be obtained because an effective electric force cannot be applied to the aerosol to be collected.

Further, when the meltblown nonwoven fabric is electretized, it is difficult to obtain a high-performance electret meltblown nonwoven fabric because the electretization is performed in one step.

[Problems to be Solved by the Invention] The purpose of the present invention is to improve the collection efficiency when it is used as a filter, for example, in order to improve the efficiency of a conventional melt-blown nonwoven fabric in which the directions of polarization charges are random (Fig. 3). Which makes it possible to produce an electret meltblown non-woven fabric having good electret properties with polarization charges oriented in the thickness direction as shown in FIG. A method for producing a high-performance electret meltblown non-woven fabric having a particularly stable electric charge for a long period of time, which is capable of continuously producing such an electret melt-blown non-woven fabric in Is intended to be provided.

[Means for Solving the Problems] In the method for producing an electret meltblown nonwoven fabric of the present invention, which achieves the above-mentioned object, a collecting surface for collecting jetted meltblown fibers as a meltblown nonwoven fabric and a collecting surface transferred from the collecting surface. During the transfer of the meltblown nonwoven fabric using the transfer surface for further transferring the meltblown nonwoven fabric,
The melt-blown non-woven fabric is subjected to high-voltage application treatment with negative polarity by using the non-contact type electrode with the collecting surface as an earth electrode, and further, using the non-contact type electrode with the transfer surface as the earth electrode, A method for producing an electret meltblown nonwoven fabric, which comprises subjecting a surface opposite to a surface facing the non-contact type electrode to a high voltage application process having a positive polarity again in a high voltage application process having a negative polarity.

[Operation] Hereinafter, the present invention will be described in more detail with reference to the drawings showing the embodiment of the method for producing an electret meltblown nonwoven fabric of the present invention.

FIG. 1 is a schematic view showing an example of a method for manufacturing an electret meltblown nonwoven fabric of the present invention. 1 is a melt blow base, 2 is a collection drum, 3 is ground, 4 is an application electrode,
5 is a melt blown nonwoven fabric, 6 is a melt blown fiber, 7 is an electret melt blown nonwoven fabric, and 8 is a high voltage generator.

Meltblown fibers 6 sprayed from meltblown spinneret 1
Has a suction 10 and is collected on a collecting drum 2 rotating in the direction of arrow A to form a meltblown nonwoven fabric 5. Thereafter, the melt-blown non-woven fabric 5 that is in contact with the grounded collecting surface 9 is subjected to a negative electrode and high-voltage application treatment by the application electrode 4 to generate corona discharge, and charges are injected into the fibers forming the melt-blown non-woven fabric. The first stage electret. Then, the nonwoven fabric is further transferred from the collecting surface 9 to the transfer surface (on the transfer conveyor 11) and further transferred, and further on the back surface of the meltblown nonwoven fabric 7 '(the above-mentioned first step). (A surface opposite to the surface facing the electrode 4 side in the electret processing of the eye), but this time, using a positive application electrode 4 ', is grounded 3'and has a suction conveyor 10' Electret meltblown nonwoven fabric 7'having a stable polarization state and high electret performance by subjecting the meltblown nonwoven fabric 5 in contact with 11 to high pressure application processing.
Is what you get.

The electret meltblown nonwoven fabric obtained by the manufacturing method of the present invention shown in FIG. 1 is an electret fiber having a polarization charge state as shown in FIG.
It is composed of 20 and the direction of the polarization charge is
Compared with the prior art product, the composition is oriented remarkably and orderly in the thickness direction of the nonwoven fabric as shown by the vector line in FIG.

In the method of the present invention, for the collecting surface and the transferring surface, for example, a drum, a conveyor belt, etc. can be used, and a material having holes can also be used. For example, the net,
Punch metal etc. can be used. As a material for the collecting surface and the transferring surface, it is necessary to ground, so that a material having a low volume resistivity such as a metal material can be used. Further, a material coated with a material having a volume resistivity higher than that of a metal material does not hinder the practice of the present invention, but the collection surface or the transfer surface may be grounded directly or indirectly. It is essential.

It is appropriate that the electric field strength for applying the high DC voltage is within the range of 3 KV / cm to 30 KV / cm. It is necessary to use a non-contact type electrode for the applied electrode in order to facilitate corona discharge and perform good electret formation continuously.For example, a needle electrode or a wire electrode is optimally used. Can be done. The gap between the electrode and the collecting surface and the transfer surface on the side of the ground electrode is preferably in the range of 5 to 100 mm, more preferably in the range of 150 to 70 mm.

On the other hand, the contact type electrode cannot be used in the method of the present invention which requires continuous and reliable electret processing because it is difficult for corona discharge to occur.

In carrying out the method of the present invention, the melt polymer is melt-blown into fibers and collected, and therefore the melt-blown nonwoven fabric has a temperature higher than room temperature, which is a preferable condition for electretization. However, if the temperature is too high, electretization may be hindered. Therefore, in such a case, the high voltage application process may be performed after cooling the meltblown nonwoven fabric.

Regarding such a point, according to various findings of the present inventors, it is preferable that the temperature of the melt-blown nonwoven fabric at the time of application is within 100 ° C. above and below the glass transition point of the polymer.

In addition, during the application treatment, it is desirable that the meltblown nonwoven fabric is in intimate contact with the grounded collecting surface or the transfer surface, and therefore the contactability is improved by giving a suction action to the meltblown nonwoven fabric. It is preferable to devise a means to make it.

Further, the basis weight of the meltblown nonwoven fabric is preferably 100 g / m ² or less from the electret property, and further 60
g / m ² is good.

This is because when the basis weight of the non-woven fabric is large, the applied electric field strength cannot be increased due to the discharge, and it becomes difficult for the compensation charge from the ground side of the collection surface or the transfer surface to enter. Because it will not be done to.

The electret performance can be known by measuring a thermal stimulation current described later.

Furthermore, it is important to bring the above-mentioned melt-blown nonwoven fabric into close contact with and contact with the collecting surface in order to obtain a compensation charge from the ground surface during electretization by applying a high voltage. It is preferable to use the melt-blown non-woven fabric thus collected.

In other words, the distance between the melt blow base and the collecting surface is 15 cm.
It is preferable that it is in the range of -100 cm in order to improve the adhesion. This is because at such a distance, the meltblown fibers can be collected in a still flexible state, so that they can be closely attached along the collection surface.

For the same reason as above, it is more preferable that the surface temperature of the melt-blown nonwoven fabric at the time of collection is higher than at least 60 ° C. below the glass transition point. Further, in the state where the collection distance is short, the wind pressure due to the jet flow is also strong, which is effective for the adhesion.

The volume resistivity of the polymer used for the melt blown nonwoven fabric is
From the electret property, 10 ¹³ Ω · cm or more is preferable, and for example, as such a polymer, polyolefin (polypropylene, polyethylene, polystyrene), polyester, acrylic resin, nylon, polycarbonate,
Fluorine-based resin or the like can be used.

The average fineness of the meltblown fibers is preferably 10 μm or less. This is effective in increasing the fiber surface area and increasing the charge density.

The electret meltblown nonwoven fabric obtained by the method of the present invention described above has a very high electret performance.

As a means for determining such electret performance, there is a method of measuring the above-mentioned thermally stimulated current, and from this method, the activation energy and the amount of polarization charge can be obtained as a representation of the trap depth of polarization charge. .

Incidentally, according to the knowledge of the present inventors, as the activation energy, or 0.2 eV, preferably at least ^{0.4eV, 7 × 10- 11 c /} cm 2 or more as a polarization charge, preferably 2 × 10-
It is preferable to have ¹⁰ c / cm ² or more from the standpoint of the performance and long-term stability of the electret, and it is preferable that the filter characteristics also satisfy these numerical values.

As shown in FIG. 5, the measurement of the thermal stimulation current is performed by using electrodes on both sides of a sample electret meltblown non-woven fabric 14 (the sample is a circular one having a diameter of 2 cm) installed in a heating tank 13 having a temperature control device 12. It is strongly sandwiched between 15 and 16,
This electrode is connected to the high-sensitivity ammeter 17 for measurement. That is, when the temperature of the heating tank is raised at a constant rate, for example, from room temperature to near the melting point at 5 ° C./minute, the trapped charges are depolarized and a current flows. This current is applied to the data processor 18
When recorded in the recorder 19 via the, the current curves for various temperature regions are obtained. FIG. 6 shows an example of the relationship between the depolarizing current and the temperature of the electret meltblown nonwoven fabric obtained by the manufacturing method of the present invention, which is measured by the thermal stimulation current. The quotient obtained by dividing the area of this current curve by the area of the measurement sample is the polarization charge amount.

Since the rising of each peak of this chart follows the following equation, the activation energy of the polarization charge can be calculated from the slope of the obtained straight line by plotting InJ vs. 1 / T for the rising part of the peak.

InJ = C- △ E / KT [In the formula, J: depolarization current (A), C: constant, △ E activation energy (eV), K: Boltzmann constant, T: temperature (° K)] activity In addition to the activation energy and the amount of polarization charge, the higher the peak temperature of the activation energy curve, the longer the electret life.
As described above, those having a temperature of 80 ° C or higher are preferable in terms of durability and stability.

On the other hand, FIG. 7 shows the depolarization of the electret meltblown nonwoven fabric obtained by the conventional electretization method of the meltblown fiber in which a high pressure is applied to the meltblown fiber running just below the spinneret in the meltblown method. An example of the relationship between current and temperature is shown (Comparative Example 1 described later). In this case, no clear current curve is obtained, and it is estimated that the directions of the polarization charges are very random.

[Examples] Specific configurations and effects of the present invention will be described below with reference to Examples.

Example 1 Using the apparatus of the embodiment used in FIG. 1, volume resistivity of 10 ¹⁶ Ω
・ Cm polypropylene has a melting temperature of 330 ° C and an air temperature of 3
Meltblown at 35 ° C and collect 120 mesh collection drum (1
(A diameter of 00 cm) and then transferred to a transfer collecting conveyor to obtain a melt blown nonwoven fabric having a basis weight of 20 g / m ² . The average diameter of the meltblown fibers was 3.5 μm, the collection distance was 30 cm, and the surface temperature of the meltblown nonwoven fabric immediately after collection was 60 ° C.

This non-woven fabric is electretized by the two-stage method as shown in Fig. 1. First, on the collecting drum, needle electrodes arranged in two rows are used, and the applied electric field strength is minus 8 KV / cm, needle electrode-collection surface gap of 3 cm. The surface temperature of the meltblown nonwoven fabric at the time of application was 30 ° C. After that, using needle electrodes arranged in two rows on the conveyor transfer surface, the applied voltage is plus 6 KV / cm, and the needle electrodes
The application process was performed again with a transfer surface gap of 3 cm. In this case, the meltblown nonwoven fabric temperature was 25 ° C.

Contact was improved by using suction on both the drum collection and the conveyor.

The surface temperature of the melt-blown nonwoven fabric was 30 ° C. when cold air was used when applied on the drum surface. The temperature was 25 ° C when applied on the conveyor.

Here, the characteristics of the thermal stimulation current of the electret melt blown nonwoven fabric were as follows.

The peak temperatures were 91 ° C and 154 ° C, and their activation energies were 0.52 eV and 0.81 eV, respectively. Further, the amount of polarization charge is ^{1.8 × 10- 9 c / cm 2} , a sufficiently high value, the effect of the present invention a method from each of these data were those can be evaluated as very high.

Five non-woven fabrics according to the present invention thus obtained are laminated,
The filter performance was evaluated before and after immersion in water for 1 day.

The filter performance is as follows: The particle counts before and after the sample are measured by an aerosol monitor by generating 0.33 μm polystyrene aerosol (polystyrene uniform latex particles manufactured by Dow Chemical Co., USA) using an atomizer at a wind speed of 1.5 m / min. (Hitachi) to measure the collection efficiency.

As a result, the collection efficiency before dipping was 99.999% and that after dipping was 99.9%.
Almost no decrease was observed at 97%, indicating excellent performance.

Comparative Example 1 In the melt-blowing method described in Example 1, a melt-blowing non-woven fabric electret-treated by applying a high voltage of −40 KV to the melt-blowing fiber running just below the mouthpiece with a space between the needle electrode and the ground plate of 5 cm. Got

However, five needle electrodes were arranged in a row in the width direction at intervals of 10 cm.

As a result of measuring the thermal stimulation current of the thus obtained nonwoven fabric, the result is as shown in FIG. 7, a clear curve is not obtained, and it is presumed that the directions of the polarization charges are very random. It was

When the filter performance of the non-woven fabric thus obtained was evaluated by the method shown in Example 1, the collection efficiency before dipping was 99.82%, and after dipping 98.25%,
The level of collection efficiency was originally low, and the decrease after immersion was also large in proportion.

Comparative Example 2 In the melt-blowing method described in Example 1, only the second-stage electretization process described in Example 1 (1
Was performed) to obtain an electret meltblown nonwoven fabric.

Activation energy 0.33eV and 0.51eV Further, the amount of polarization charge is ^{6.1 × 10- 10 c / cm 2} , compared to the ones in Example 1,
The electret performance was inferior. In addition,
It was far superior to that of Comparative Example 1.

However, when this electret meltblown nonwoven fabric was also evaluated for filter performance by the method shown in Example 1,
The collection efficiency before immersion is 99.98%, after immersion 99.85%,
Although it was significantly better than that of Comparative Example 1, it was judged to be clearly inferior to that of Example 1.

[Effects of the Invention] According to the method of the present invention as described above, it is possible to manufacture an ideal electret meltblown nonwoven fabric having high electret performance and good charge stability over a long period of time.

The present invention collects meltblown fibers jetted from a meltblowing die on a collecting surface to form a meltblown non-woven fabric in close contact with the grounded collecting surface. By performing the electretization treatment of the stage, it is possible to perform electretization simultaneously with the production of the meltblown nonwoven fabric. further,
Since the melt-blown nonwoven fabric is electretized in a state in which it is in close contact with the grounded collecting surface, the compensating charge easily enters from the collecting surface, and the electretization can be performed with high efficiency.

Furthermore, in the transfer surface transferred from the collecting surface, the non-woven fabric on the opposite side of the non-woven fabric surface facing the non-contact electrode in the electretization treatment of the first step is grounded on the transfer surface in the second stage with the positive electrode. The electret-melting nonwoven fabric having a very stable and high electret performance can be produced by performing the electret-forming treatment.

In particular, therefore, the electret meltblown nonwoven fabric obtained by the production method of the present invention has a structure in which the polarization charges are oriented, and the trap charges are deep and the amount of the charges is large, so that stable electret performance can be exhibited for a long period of time. .

Therefore, high-performance filters, adsorbents, masks, dust-proof clothing,
It can be used for various purposes such as clean futon.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a method for manufacturing an electret meltblown nonwoven fabric according to the present invention. FIG. 2 is a schematic cross-sectional view showing the charge state of the polarization charge of the electret meltblown nonwoven fabric obtained by the present invention, and FIG. 3 is the vector line showing the direction of the polarization charge of the electret meltblown nonwoven fabric obtained by the conventional method. It is the schematic diagram shown by. FIG. 4 is a schematic diagram showing the direction of polarization charge of the electret meltblown nonwoven fabric obtained by the present invention by a vector line. FIG. 5 is a schematic diagram showing an activation energy measuring device, and FIGS. 6 and 7 are diagrams showing an example of the relationship between depolarization current and temperature. 1: Melt blown cap 2: Collection drum 3, 3 ': Earth 4, 4': Applying electrode 5: Melt blown nonwoven fabric 6: Melt blown fiber 7, 7 ', 14: Electret melt blown nonwoven fabric 8, 8': High voltage generator 9 : Collection surface 10, 10 ': Suction 11: Collection conveyor 20: Electret fiber

Claims (5)

[Claims] 1. A meltblown non-woven fabric is being transported using a collection surface for collecting the sprayed meltblown fibers as a meltblown nonwoven fabric and a transport surface for further transporting the meltblown nonwoven fabric transferred from the collection surface. A non-woven fabric is subjected to a high-voltage application treatment with a negative polarity by using the non-contact type electrode with the collecting surface as the ground electrode, and the negative polarity by using the non-contact type electrode with the transfer surface as the ground electrode. The method for producing an electret meltblown nonwoven fabric, characterized in that the surface opposite to the surface facing the non-contact type electrode is subjected to high voltage application processing again with positive polarity in the high voltage application processing in 1. 2. The method for producing an electret meltblown nonwoven fabric according to claim 1, wherein a collecting surface having an opening is used. 3. When collecting the melt-blown nonwoven fabric on the collecting surface, a suction action is given to the melt-blown nonwoven fabric so that the melt-blown nonwoven fabric is brought into close contact with the collecting surface. Method for manufacturing electret meltblown nonwoven fabric. 4. The melt-blown non-woven fabric having a basis weight of 100 g / m ² or less. Claim (1)
Item 8. A method for producing an electret meltblown nonwoven fabric according to item. 5. A high-voltage application treatment is carried out within a range of 100 ° C. above and below the glass transition point of the synthetic organic polymer forming the meltblown nonwoven fabric. A method for producing the described electret meltblown nonwoven fabric.