Electret apparatus

Abstract

To provide an electret apparatus that enables formation, within a short time, of an electret having uniformly mixed regions processed to positive and negative polarities, and that allows achievement of high collection efficiency of a nonwoven fabric.SOLUTION: An electret apparatus comprising a grounding electrode 2 and an applying electrode 3 that directly clamp a nonwoven fabric s to be processed, wherein the applying electrode 3 is in a roller shape composed of a metallic core member 3a to which a negative voltage is applied from a direct current power supply 4, and a resistor layer 3b provided on a surface of the core member 3a, such that corona discharge is generated from the applying electrode 3 and induced discharge is generated from the grounding electrode 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electret device for performing electret treatment on nonwoven fabric. [Background technology]

[0002] It has been known for some time that electret-treated nonwoven fabrics are used in medical masks and the like. The electret-treated nonwoven fabric maintains an electric charge, allowing fine particles to be captured by the nonwoven fabric through electrostatic force. In order to maintain a high fine particle collection efficiency, the entire surface of the nonwoven fabric must be sufficiently electret-treated. As an electret device for treating nonwoven fabric, for example, as shown in FIG. 10, there is known a device that includes a needle-shaped discharge electrode 1 and a ground electrode 2 on which the nonwoven fabric s to be treated is placed, and that applies a high DC voltage from a DC power supply 4 to the discharge electrode 1 to generate a corona discharge, thereby irradiating the nonwoven fabric s with ions (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] FIG. 7 of JP 2001-118749 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the conventional electret device using the needle-shaped discharge electrode 1, the irradiated charge is distributed depending on the distance from the tip of the discharge electrode 1 to the object to be treated. The area directly facing the tip of the discharge electrode 1 is an area with a high density of irradiated charges, and the charge density decreases as the area moves away. This can cause a distribution in the electret effect of the entire nonwoven fabric s. In particular, to achieve a high capture rate for nonwoven fabrics used in masks and filters, electret treatment is required to mix positively and negatively charged areas throughout the nonwoven fabric. However, with conventional electret devices, the mixture of charged areas in the nonwoven fabric s becomes uneven, or the polarity of the charged areas is partially biased to one side, making it difficult to achieve uniform and sufficient electret treatment of the entire nonwoven fabric s. An object of the present invention is to provide an electret device that can achieve high collection efficiency by mixing charged areas treated with positive and negative polarities across the entire nonwoven fabric in a short time. [Means for solving the problem]

[0005] The first invention comprises a ground electrode and an application electrode that directly clamp the nonwoven fabric to be treated, the application electrode being in the form of a roller consisting of a metal core material to which a negative DC voltage is applied and a resistor layer provided on the surface of the core material, and is configured to generate an induced discharge from the ground electrode.

[0006] In a second aspect of the present invention, the ground electrode is in the form of a roller that rotates while sandwiching the processing target together with the application electrode.

[0007] In a third aspect of the present invention, the resistor layer has an electrical resistance value of 50 [MΩ / cm] to 300 [MΩ / cm].

[0008] In a fourth aspect of the present invention, at least the surface of the ground electrode is made of a metal having a work function of 4.8 eV or less. [Effects of the Invention]

[0009] According to the first invention, a stable discharge can be generated between the application electrode and the ground electrode by applying a negative DC voltage to the application electrode while maintaining a small and uniform distance between them. Furthermore, because the distance between the application electrode and the ground electrode is only the thickness of the sandwiched nonwoven fabric, the electric field strength between the electrodes is high. Therefore, between these electrodes, a corona discharge from the application electrode and an inductive discharge from the surface of the ground electrode simultaneously occur. The charges generated by these discharges are of opposite polarity, and because both electrodes are in contact with the surface of the nonwoven fabric, the charges generated by the discharges are directly irradiated into the interior of the nonwoven fabric. Therefore, the nonwoven fabric is homogeneously electret-treated with a mixture of positive and negative charges. Therefore, when nonwoven fabric treated with the device of the present invention is used in filters, masks, etc., the fine particle collection efficiency can be increased.

[0010] According to the second invention, continuous treatment of long materials becomes possible. Furthermore, since the distance between the pair of electrodes can be easily maintained constant in the longitudinal direction of the electrodes (the width direction of the nonwoven fabric), uniform corona discharge and uniform induction discharge can be maintained. Therefore, uniform electret treatment can be achieved across the entire width of the nonwoven fabric.

[0011] According to the third aspect of the present invention, by maintaining the electrical resistance of the application electrode at an appropriate value, the discharge current between the application electrode and the ground electrode is stabilized, and stable corona discharge and induction discharge are maintained, thereby enabling more uniform electret treatment.

[0012] According to the fourth aspect of the present invention, the ground electrode has a low electrical resistance and a low work function for the surface material, which facilitates electrons to escape from the surface of the ground electrode and induces an inductive discharge. This allows for smoother injection of positive charges into the nonwoven fabric, resulting in a uniform and sufficient electret treatment. Furthermore, metal ground electrodes are strong and resistant to breakage and deformation, making it easy to maintain their electrode shape. This maintains a constant distance between the ground electrode and the applied electrode, ensuring stable corona discharge and inductive discharge. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of the electret device of the first embodiment. [Figure 2] FIG. 2 is a perspective view of the application electrode of the first embodiment. [Figure 3] FIG. 3 is a photograph of a sample of nonwoven fabric treated with the electret device of the first embodiment with charged toner attached thereto. [Figure 4] Figure 4 shows photographs of comparative example samples with charged toner attached, where (a) is a photograph of the sample of comparative example 1 in which a positive voltage was applied to the application electrode of the electret device of Figure 1, and (b) is a photograph of the sample of comparative example 2 processed with a conventional device. [Figure 5] FIG. 5 shows the measurement results of the thermally stimulated surface potential decay characteristics of a sample that was treated once by the electret device of the first embodiment. [Figure 6] FIG. 6 is a table showing the measurement results of the collection efficiency of samples treated by the electret device of the first embodiment. [Figure 7] FIG. 7 is a schematic diagram of the electret device according to the second embodiment. [Figure 8] FIG. 8 is a schematic diagram of the electret device according to the third embodiment. [Figure 9] FIG. 9 is a table showing the collection efficiency of nonwoven fabric samples treated with the electret device of the third embodiment. [Figure 10] FIG. 10 is a schematic diagram of a conventional electret device using a needle-shaped discharge electrode. DETAILED DESCRIPTION OF THE INVENTION

[0014] [First embodiment] An electret device according to a first embodiment of the present invention will be described with reference to Figures 1 to 6. Figure 1 is a schematic diagram of the first embodiment. Figure 2 is a perspective view of an application electrode. 3 to 6 are diagrams showing data confirming the effect of the electret treatment of the first embodiment.

[0015] The electret device shown in FIG. 1 includes a ground electrode 2 on which the nonwoven fabric s to be treated is placed, and an application electrode 3 that is provided above the nonwoven fabric s and directly sandwiches the nonwoven fabric s between the ground electrode 2. 2, the application electrode 3 is in the form of a roller, with a resistor layer 3b provided on the outer periphery of a core material 3a made of a metal such as stainless steel. A DC power source 4 is connected to the core material 3a so that a negative voltage is applied thereto. The ground electrode 2 is a flat plate made of stainless steel.

[0016] The resistor layer 3b is made of a conductive resin or ceramic containing a conductive material and having an electrical resistance adjusted to 50 MΩ / cm to 300 MΩ / cm. Because the resistor layer 3b is harder than the nonwoven fabric s, when the nonwoven fabric s is sandwiched between the resistor layer 3b and the ground electrode 2, no gap is formed between the resistor layer 3b and the nonwoven fabric s, and the contact area with the nonwoven fabric s does not deform, so the distance between the resistor layer 3b and the ground electrode 2 can be kept constant in the axial direction of the application electrode 3.

[0017] A negative voltage is applied to the core material 3a of the application electrode 3 from a DC power source 4, and the application electrode 3 moves over the nonwoven fabric s while rotating in the direction of the arrow shown in FIG. 1, thereby carrying out electret treatment. The nonwoven fabric s to be treated has a thickness of a few mm or less. In particular, when used for a mask, breathability is required, so a fabric that is too thick is not suitable. In this embodiment, a polypropylene nonwoven fabric s (P03070 manufactured by Asahi Kasei Corporation) having an average thickness of approximately 0.48 mm and a void volume ratio of 89% is used.

[0018] [Actions, effects, etc.] In this first embodiment, the ground electrode 2 and the application electrode 3 face each other at a distance equivalent to the thickness of the nonwoven fabric s, i.e., 0.48 mm or less, resulting in high electric field strength. Furthermore, the electric field on the surface of the ground electrode 2, which has the highest electric field strength, can generate an inductive discharge such as a corona discharge in the voids in the nonwoven fabric s. In this way, a configuration in which the electric field on the surface of the ground electrode 2 is high in strength and there is space (voids in the nonwoven fabric s) in which a discharge can be sustained is a configuration in which an inductive discharge from the ground electrode occurs.

[0019] Furthermore, if the roller-shaped application electrode 3 is placed so as to apply uniform pressure to the nonwoven fabric s so as to lightly crush it, a small, uniform distance between the electrodes can be maintained without creating a gap between the ground electrode 2 and the application electrode 3 and the surface of the nonwoven fabric s. Therefore, a corona discharge from the application electrode 3 and an inductive discharge from the ground electrode 2 can be generated simultaneously and uniformly. In addition, since the nonwoven fabric s has sufficient voids, the air in the voids can be ionized to sustain the corona discharge caused by the application electrode 3 and the inductive discharge generated by the electric field on the surface of the ground electrode 2.

[0020] In this way, the ground electrode 2 and the application electrode 3 are in close contact with both surfaces of the nonwoven fabric s, and discharge occurs on both the front and back sides of the nonwoven fabric s under conditions of high electric field strength. The electric charges generated by the discharge gain large kinetic energy in the high-strength electric field and are irradiated (poured) into the interior of the nonwoven fabric s, resulting in a sufficiently high electret effect (charge retention capacity).

[0021] The procedure for electret-treating the nonwoven fabric s using the device of the first embodiment is as follows. A rectangular nonwoven fabric s (P03070 manufactured by Asahi Kasei Corp.) measuring 18 cm x 24 cm is placed flat on the ground electrode 2, and both sides of the nonwoven fabric s are fixed to the ground electrode 2 with tape (not shown).

[0022] Next, a voltage of −3.5 kV is applied to the application electrode 3, and the application electrode 3 is rolled over the nonwoven fabric s. A handle made of an insulating material (not shown) is attached to the core material 3a of the application electrode 3 at the portion protruding from the resistor layer 3b. The operator holds this handle and manually moves the application electrode 3. At this time, the pressing force of the application electrode 3 is set to about 0.8 kgw, and the moving speed is set to about 10 cm / s. The direction of movement of the application electrode 3 is the longitudinal direction of the nonwoven fabric s. Then, the nonwoven fabric s was subjected to electret treatment after each of the movements 1, 2, 3, 4 and 5 times, and five types of samples were prepared.

[0023] Furthermore, as Comparative Example 1, an electret treatment was performed in which +3.5 kV was applied to the application electrode 3 in Fig. 1, and as Comparative Example 2, an electret treatment was performed using the conventional device shown in Fig. 10 to create samples. In the treatment using the conventional device, which is Comparative Example 2, the voltage applied to the needle-shaped discharge electrode 1 was +20 kV, the inter-electrode distance between the tip of the discharge electrode 1 and the ground electrode 2 was 30 mm, and the ion irradiation time was 3 seconds.

[0024] In both Comparative Examples 1 and 2, the treatment object was the same nonwoven fabric s as above, and the temperature of the nonwoven fabric s during treatment was about 25°C. The nonwoven fabric samples treated as described above were subjected to measurements of charge distribution, thermally stimulated surface potential decay characteristics, and collection efficiency.

[0025] [Charge distribution measurement] To confirm the charge distribution of the nonwoven fabric s sample treated as described above, positively charged red toner and negatively charged blue toner were electrostatically attached to the surface of the nonwoven fabric s. The charge pattern of the nonwoven fabric s with the color toners attached was then observed and photographed.

[0026] The results are as follows: Figure 3 shows a photograph of a sample processed once using the device of the first embodiment. A uniform mixture of red and blue toners is attached to the entire surface of the sample. The entire sample appears purple. This indicates that tiny positively and negatively charged areas are uniformly distributed.

[0027] In the first embodiment, the reason why charged areas of both positive and negative polarities are formed in a mixed state even though a negative DC high voltage is applied to the application electrode 3 can be assumed as follows. In the above device, the distance between the application electrode 3 and the ground electrode 2 is small and constant, and the application electrode 3 is pressed against the nonwoven fabric s. Therefore, the corona discharge generated by the negative application electrode 3 is uniform not only on the surface of the nonwoven fabric s but also inside the nonwoven fabric s, and negative charges are directed from the application electrode 3 to the ground electrode 2. Furthermore, due to an induced discharge caused by a strong electric field on the surface of the ground electrode 2, positive charges of the opposite polarity to the application electrode 3 are generated from the ground electrode 2 and directed toward the application electrode 3. In this way, negative charges are poured into the interior of the nonwoven fabric s from the application electrode 3 due to corona discharge, and positive charges are poured into the interior of the nonwoven fabric s from the ground electrode 2 due to induced discharge. Therefore, positive and negative charges are uniformly mixed throughout the entire nonwoven fabric s. As a result, a uniform electret effect is obtained.

[0028] On the other hand, Figure 4(a) shows the charging pattern of the sample of Comparative Example 1. Compared to the sample of Figure 3, there are clearly separate reddish areas E1 (negatively charged areas) and bluish areas E2 (positively charged areas), and both areas have a certain size. Furthermore, a white area with no toner attached was observed at the boundary between the two areas E1 and E2. The white area is an uncharged area (uncharged area). In other words, in Comparative Example 1, it was difficult to uniformly and sufficiently treat the entire nonwoven fabric as shown in Figure 3, and it was found that electret treatment in which tiny positively charged areas and negatively charged areas are uniformly distributed was not possible. Therefore, in Comparative Example 1, a uniform and sufficient electret effect cannot be expected.

[0029] In the embodiment in which a negative DC voltage is applied to the application electrode 3, a process in which positive and negative charges are mixed uniformly can be achieved, whereas in Comparative Example 1 in which a positive voltage is applied to the application electrode 3, the areas of the negatively charged area E1 and the positively charged area E2 each become larger, and the mixing of positive and negative charges becomes uneven. The reason for this is currently unknown, but this state is highly reproducible, and it has been found that the polarity of the application electrode 3 must be negative.

[0030] FIG. 4(b) shows a sample of Comparative Example 2 using a conventional device. In the sample of Comparative Example 2, compared to the first embodiment and Comparative Example 1, the toner adhesion on the sample surface of the nonwoven fabric s is thin, but there is a bluish area E2 along a circle centered at a position directly facing the needle-shaped discharge electrode 1, and within that circle there are a reddish area E1 (negatively charged area), a bluish area E2 (positively charged area), and a purple area E3 that is a mixture of red and blue. However, the purple area E3 is small, and both the reddish negatively charged area E1 and the bluish positively charged area E2 have a certain amount of area, and a white area (uncharged area) with no toner adhesion can be seen at the boundary between the two areas E1 and E2.

[0031] In the conventional device used in Comparative Example 2, the distance between the discharge electrode 1 and the nonwoven fabric s is large, so that a layer of positive charge is deposited by corona discharge on the surface of the nonwoven fabric s at the position directly facing the needle-shaped discharge electrode 1, which interferes with the supply of positive charge to the inside of the nonwoven fabric s. Furthermore, the deposition of positive charge is reduced at a position slightly away from the discharge electrode 1, and further outside, almost no charge is deposited.

[0032] If the accumulated positive charge becomes excessive near the center of the deposition layer directly facing the discharge electrode 1, a discharge (back ionization) occurs from the excess positive charge toward the upper space, causing the accumulated charge to disappear, and a negative charge due to back ionization remains in that area, making the negatively charged area E1 with red toner attached noticeable. At the contour of the above-mentioned deposition layer, a short distance from the discharge electrode 1, the electric field strength between the discharge electrode 1 and the ground electrode 2 is low, making it difficult for corona discharge or the above-mentioned back ionization from the discharge electrode 1 to occur. For this reason, it can be assumed that a positively charged area E2, where blue toner adheres to the area where the positive charge remains, is formed.

[0033] Furthermore, outside the annular bluish positively charged area E2, the electric field strength is even lower, and no electret effect can be expected. Thus, in Comparative Example 2, unlike the sample of the first embodiment shown in Fig. 3, there are almost no dark purple areas where tiny positively and negatively charged areas are uniformly mixed, and the charge is uneven, so a uniform electret effect cannot be expected. Therefore, even if an electret device with multiple needle-shaped discharge electrodes 1 arranged is manufactured and an attempt is made to treat the entire nonwoven fabric s, it is expected that a uniform electret effect will not be obtained. 3, 4(a) and 4(b) show the surface facing the application electrode 3 or the discharge electrode 1, but it has been confirmed that a similar charging pattern to that on the surface can also be seen on the back side.

[0034] [Measurement of thermally stimulated surface potential decay characteristics] The thermally stimulated surface potential decay characteristics of the electret-treated nonwoven fabrics were measured to confirm their charge retention ability. This measurement involves measuring the decay characteristics of the surface potential with respect to temperature while continuously heating a sample of electret-treated nonwoven fabric s at a constant temperature gradient from 35°C to 170°C.

[0035] FIG. 5 is a graph showing the measurement results of the thermally stimulated surface potential decay characteristics of a sample of nonwoven fabric s that was subjected to a single electret treatment using the device of the first embodiment. As shown in FIG. 5, the peak is at about 130° C., and the sample treated with the electret device of the first embodiment has sufficient charge retention capacity. Although not shown, it was also confirmed that the peak value was approximately 130°C for Comparative Examples 1 and 2. In other words, no difference was observed in the measured values ​​of the thermally stimulated surface potential attenuation characteristics depending on the treatment device. However, in Comparative Examples 1 and 2, the charging pattern was uneven, and therefore it is expected that the charge retention ability also varies depending on the measurement position.

[0036] [Collection efficiency] FIG. 6 shows the results of measuring the fine particle collection efficiency of a sample of electret-treated nonwoven fabric s. This collection efficiency was measured by passing a gas containing fine particles (atmospheric dust) through the nonwoven fabric s, and then measuring the number of particles with a particle counter before and after the passage of the gas through the nonwoven fabric s. The measurement targets were electret-treated nonwoven fabrics s as in the first embodiment, comparative example 1, and comparative example 2, and the samples of the first embodiment and comparative example 1 were each subjected to one to five treatments and then the measurement was performed. The particles to be measured are those with a diameter of 0.3 μm or less.

[0037] The measurement results of the collection efficiency are shown in Figure 6. The values ​​shown in Figure 6 are the average values ​​obtained by measuring the same sample twice. The collection efficiency of the nonwoven fabric s treated with the device of the first embodiment was 96.45% after one treatment, and increased as the number of treatments increased. In contrast, the collection efficiency in Comparative Example 1 was approximately 85% to approximately 90%, and it cannot be expected that the collection efficiency will increase significantly even if the number of treatments is increased. In Comparative Example 2, the collection efficiency was 89.10% even when the treatment time was 3 seconds. In both Comparative Examples 1 and 2, the collection efficiency was less than 95%. The collection efficiency of the untreated nonwoven fabric s was 74.00%.

[0038] Thus, the nonwoven fabric s treated with the electret device of the first embodiment has a collection efficiency of 95% or more even after one treatment, exhibiting sufficient performance as a mask and satisfying the so-called N95 standard. Furthermore, when the collection efficiency of the same sample was measured after 9 months, it was 97.42% after two treatments and 97.35% after four treatments, showing no significant decrease in collection efficiency. In other words, it was confirmed that the first embodiment can achieve a sufficient electret effect in a short period of time and that this effect is sustained. The high collection efficiency of the treated sample of the first embodiment can be inferred to be due to the fact that, as shown in Figure 3, the electret treatment, which creates a fine mixture of positive and negative charges not only on the surface but also inside the nonwoven fabric s, creates electric fields in various directions throughout the nonwoven fabric s, allowing fine particles to be collected in all parts.

[0039] In contrast, in Comparative Examples 1 and 2, the charge distribution is non-uniform and there are uncharged areas (white areas), which is thought to result in a low efficiency of capturing fine particles. The nonwoven fabric to be treated (P03070 manufactured by Asahi Kasei Corporation) is breathable and relatively inexpensive. While it is difficult for breathable nonwoven fabrics like this to capture fine particles without treatment, electret treatment increases the collection efficiency, making it ideal for disposable masks and the like.

[0040] [Second embodiment] FIG. 7 is a schematic diagram of the second embodiment. The electret device of the second embodiment has a configuration in which a nonwoven fabric s to be treated is sandwiched between a roller-shaped ground electrode 5 and an application electrode 6 to which a negative DC voltage is applied. The core material 5a and outer layer 5b of the ground electrode 5 are both made of stainless steel and are grounded via the core material 5a. The application electrode 6 has a resistor layer 6b on the outer periphery of the metal core material 6a. The configurations of the core material 6a and resistor layer 6b are the same as those of the core material 3a and resistor layer 3b of the first embodiment. However, the ground electrode 5 and the application electrode 6 are rotatably supported in opposing positions as shown in the figure. Therefore, the nonwoven fabric s sandwiched between the ground electrode 5 and the application electrode 6 is transported in the direction of the arrow while undergoing electret treatment. In addition, the application electrode 6 is provided with a gap adjustment mechanism (not shown) to adjust the pressing force of the application electrode 6 against the nonwoven fabric s to approximately 0.8 kgw, which is the same as that of the electret device of the first embodiment, and to keep the distance between the electrodes uniform.

[0041] [Actions, effects, etc.] In the electret device of the second embodiment, the shape of the ground electrode 5 is roller-shaped, which is different from the ground electrode 2 of the first embodiment, but the other configurations are the same as those of the first embodiment. Therefore, similar to the first embodiment, it is possible to achieve a charge distribution in which fine positive and negative areas are uniformly dispersed on the nonwoven fabric s. In particular, since the roller-shaped electrodes 5 and 6 face each other while sandwiching the nonwoven fabric s, it is easier to maintain a constant inter-electrode distance in the longitudinal direction of the application electrode 6 than in the first embodiment, which uses the flat ground electrode 2. Therefore, a more uniform treatment effect can be expected. Furthermore, since the nonwoven fabric s can be transported by a pair of roller-shaped electrodes 5, 6, it is possible to process a long nonwoven fabric or a plurality of nonwoven fabrics s continuously.

[0042] [Third embodiment] FIG. 8 is a schematic diagram of the third embodiment. As shown in FIG. 8, the electret device of the third embodiment includes a large-diameter roller-shaped ground electrode 7 having a core material 7a and an outer layer 7b made of metal, and five small-diameter roller-shaped application electrodes 8, 9, 10, 11, and 12 arranged circumferentially around the ground electrode 7. The electret device of the third embodiment is installed between a feed roller (not shown) around which the nonwoven fabric s is wound and a take-up roller that takes up the nonwoven fabric s, and is capable of continuously processing a long length of nonwoven fabric s.

[0043] Similar to the application electrodes 3 and 6 of the first and second embodiments, all of the application electrodes 8, 9, 10, 11, and 12 have resistor layers 8b, 9b, 10b, 11b, and 12b provided around metal cores 8a, 9a, 10a, 11a, and 12a, respectively. These resistor layers 8b, 9b, 10b, 11b, and 12b are the same as the resistor layer 3b of the first embodiment. Furthermore, each of the application electrodes 8, 9, 10, 11, and 12 is provided with a gap adjustment mechanism (not shown) that adjusts the pressing force of the application electrodes 8, 9, 10, 11, and 12 against the nonwoven fabric s to approximately 0.8 kgw, which is the same as that of the electret devices of the first and second embodiments, and keeps the distance from the ground electrode 7 uniform.

[0044] The roller-shaped application electrodes 8, 9, 10, 11, and 12 are pressed against the ground electrode 7 to sandwich the nonwoven fabric s, and rotate in the direction of the arrow. A DC power supply 4 is connected to the application electrodes 8, 9, 10, 11, and 12, and a negative DC voltage is applied to each of the application electrodes 8, 9, 10, 11, and 12. In the electret device of the third embodiment, the electret treatment is carried out at the positions of the application electrodes 8, 9, 10, 11, and 12, respectively.

[0045] [Actions, effects, etc.] In the electret device of the third embodiment, the ground electrode 7 and the application electrodes 8, 9, 10, 11, and 12 that sandwich the nonwoven fabric s to be treated are also in close contact with the surface of the nonwoven fabric s, so that no partial gaps are formed between the ground electrode 7, the application electrodes 8, 9, 10, 11, and 12, and the nonwoven fabric s. Therefore, the voltage applied to the application electrodes 8, 9, 10, 11, and 12 can be made to function effectively, and the entire nonwoven fabric s can be uniformly electret-treated by corona discharge from the application electrodes 8, 9, 10, 11, and 12 and inductive discharge from the ground electrode 7. In this embodiment, the application electrodes 8, 9, 10, 11, and 12 also serve as conveying means, making it suitable for treating large nonwoven fabrics s while conveying them. Furthermore, the electret treatment is repeated five times, so a stronger electret effect can be expected.

[0046] [Collection efficiency] FIG. 9 shows the measurement results of the collection efficiency of a sample of nonwoven fabric s treated with the electret device of the third embodiment. During electret treatment, -3.5 kV was applied to each of the application electrodes 8, 9, 10, 11, and 12, and the line speed v, which is the conveying speed of the nonwoven fabric s, was set to 5 m / min, 10 m / min, 15 m / min, and 20 m / min. The values ​​shown in Fig. 9 are the average values ​​obtained by measuring the same sample three times. The method for measuring the collection efficiency was the same as in the first embodiment shown in Fig. 6. The results shown in Figure 9 indicate that the collection efficiency tends to decrease as the line speed v of the equipment increases, but that a high collection efficiency of over 90% can be achieved even at a line speed of 20 m / min.

[0047] In the first to third embodiments, the electrical resistance values ​​of the resistor layers 3b, 6b, 8b, 9b, 10b, 11b, and 12b of the application electrodes 3, 6, 8, 9, 10, 11, and 12 are set to 50 MΩ / cm to 300 MΩ / cm, thereby achieving a more stable processing effect. If the electrical resistance of the applied electrode is too low, the current flowing through the nonwoven fabric s may be too large, causing damage to the nonwoven fabric s, while if the electrical resistance is too high, the current flowing through the nonwoven fabric s may be too small, resulting in insufficient processing. Therefore, it is preferable to set the appropriate electrical resistance value of the resistor layers 3b, 6b, 8b, 9b, 10b, 11b, and 12b within the above range depending on the thickness of the nonwoven fabric s, the applied voltage, and the like. Furthermore, by forming the resistor layers 3b, 6b, 8b, 9b, 10b, 11b, and 12b from a foam and exposing the fine bubbles of the foam on the surface, it is possible to form many discharge points. The more discharge points there are, the more efficient the uniform treatment.

[0048] Furthermore, in the above embodiment, the ground electrodes 2, 5, and 7 are made of stainless steel, which has a low electrical resistance and a small work function of 4.8 eV or less, which allows for smooth supply of charge from the ground and makes it easier for induced discharge to occur. Furthermore, by making the ground electrode out of a metal such as stainless steel, the ground electrode is less likely to be damaged or deformed, and the distance between the ground electrode and the application electrode is maintained, resulting in a stable discharge. However, the material of the ground electrode is not limited to stainless steel or other metals. Regardless of the material, the electrical resistance is, for example, 1 × 10 -7 It has been confirmed that if the work function of the ground electrode surface material is 4.8 eV or less, an inductive discharge can be generated efficiently.

[0049] Note that a neutralization process for neutralizing the surface charge of the nonwoven fabric s may be provided after the electret treatment process using the electret devices of the first to third embodiments. The neutralization process can prevent the occurrence of electrostatic discharge from the surface of the nonwoven fabric s after the electret treatment to the outside. If there is excessive charge on the surface of the nonwoven fabric s that could have an external impact, brush discharge or creeping discharge may occur toward, for example, a metal conveying roller while the nonwoven fabric s is being conveyed. If brush discharge or creeping discharge occurs, it can damage the uniform charging of the nonwoven fabric s or the conveying roller, but this can be prevented by providing the neutralization process. The neutralization process provided downstream of the electret process may be of any configuration as long as it can reduce the excessive surface potential of the nonwoven fabric s.

[0050] Furthermore, a cleaning process for removing foreign matter such as dust and paper powder adhering to the surface of the nonwoven fabric s may be provided prior to the electret treatment process using the electret devices of the first to third embodiments. By cleaning, the electret treatment can be performed in a state in which foreign matter such as dust adhering to the surface of the nonwoven fabric s has been removed. Furthermore, adhesion of foreign matter from the nonwoven fabric s to the application electrode and ground electrode can be prevented, so a stable discharge state can be maintained for a long period of time. As a result, it is possible to uniformly treat the entire nonwoven fabric with electrets, thereby improving the productivity of electret nonwoven fabrics with high collection efficiency. [Industrial Applicability]

[0051] Nonwoven fabrics that can be used for masks, filters, etc. can be efficiently electret-treated. [Explanation of symbols]

[0052] 2,5,7 ground electrode 3,6,8,9,10,11,12 Application electrode 3a, 6a, 8a, 9a, 10a, 11a, 12a core material 3b,6b,8b,9b,10b,11b,12b Resistor layer 4 DC power supply s Nonwoven fabric

Claims

1. The nonwoven fabric treatment device includes a ground electrode and an application electrode that directly sandwich the nonwoven fabric to be treated, The application electrode is A roller-shaped device comprising a metal core to which a negative DC voltage is applied and a resistor layer provided on the surface of the core, The ground electrode is configured to generate an inductive discharge. Electret device.

2. 2. The electret device according to claim 1, wherein the ground electrode is in the form of a roller that rotates while sandwiching the object to be treated together with the application electrode.

3. 2. The electret device according to claim 1, wherein the electrical resistance value of the resistor layer is 50 MΩ / cm to 300 MΩ / cm.

4. At least the surface of the ground electrode is made of a metal having a work function of 4.8 eV or less. The electret device according to any one of claims 1 to 3.

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