JP2012239995A - Electret filter medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electret filter medium which develops stable collection efficiency while maintaining general filter properties such as pressure drops, filtering efficiency and strength of an air filter.SOLUTION: In the electret filter medium used for the pleat type filter unit that develops 50%-99% of the JIS 11 type colorimetry method collection efficiency at the initial stage among the performance evaluation methods defined by JIS B 9908:2001, it is characterized by bonding a sheet (A) which can be electretted with a web (B) that contains a short fiber which can be electretted. The portion (B layer)composing the short fiber that can be electretted after the heat pressure welding satisfies the following requirements (a)-(c): (a) the bulk density is 0.03-0.20 g/cmand the thickness is 1.5 mm or less, (b) the bending rebound property by the Gurley method is 100-500 mg and (c) the attachment amount of an oil agent is 0.1 wt.% or less.

Description

  TECHNICAL FIELD The present invention relates to an air filter unit, mainly for air conditioning, and a filter medium used for a filter unit that can be used for an air conditioner, and particularly for slightly fine dust to which type 2 of JIS B 9908: 2001 is applied. The present invention relates to a filter medium used for a filter unit.

  The electret filter medium can effectively remove fine particles in the air by the electrostatic force of electric charges semi-permanently fixed to the fibers constituting the filter medium, and is used for air conditioning, air conditioning, air purifier filters, OA equipment. Used in exhaust filter units. In such fields, it is necessary to efficiently collect fine particles in the atmosphere while keeping the pressure loss of the filter unit low, so it is used as a pleated filter in which the filter medium is folded in a zigzag to form pleats. On the other hand, as the fibrous nonwoven fabric used as the electret, there are a melt blown nonwoven fabric, a thin spunbond nonwoven fabric, and a thin spunlace nonwoven fabric. As a method for solving such a problem, a laminated filter medium composed of a thin polypropylene melt blown nonwoven fabric and a fiber web composed of core-sheath fibers having a large fineness, and a heated portion that is highly rigid and a portion that can be charged is used. It has been devised (Patent Document 1).

  In addition, a multilayer filter medium has been devised in which a synthetic fiber network is thermally bonded to an electret nonwoven fabric to improve mechanical strength (Patent Document 2). Such a heat-bonded laminated filter medium is bonded to a thin non-woven fabric that has been electretized beforehand and a rigid base material, so that it is neutralized by heat, and the dust removal efficiency by the electret may decrease.

  In addition, a thin non-woven fabric that can be electret and a member with mechanical strength may be charged after being thermally bonded in advance, but organic components such as oil agents contained in the member act on the non-woven fabric that can be electret. There was a problem to block. Moreover, about the oil agent which alienates an electret, there exists description of the electret nonwoven fabric using the polyolefin-type heat-fusion fiber whose reduction rate of the oil agent adhesion amount after heat processing is 60% or more (patent document 3). Although this document describes that electret characteristics are obtained by reducing the oil agent by heating and dust collection efficiency is high, the characteristics as filter media for filters and the specific collection conditions in the atmosphere There is no mention about

JP-A-62-83017 JP-A-1-194912 JP 2002-339256 A

  The present invention provides an electret filter medium capable of expressing stable collection efficiency while maintaining general filter characteristics such as pressure loss, dust collection efficiency and strength of an air filter.

The present invention has been obtained as a result of intensive studies in view of such problems. That is, the present invention is as follows.
(1) Electret filter media, particularly electrets used in pleated filter units that exhibit 50% to 99% JIS11 colorimetric collection efficiency in the early stage of the performance evaluation method defined in JIS B 9908: 2001 In the filter medium, an electret filter medium in which the electret-capable sheet (A) and the electret-capable short fiber web (B) are joined by heat-pressure welding, and the portion (B layer) constituting the short-fiber web after heat-pressure welding is as follows: An electret filter medium that satisfies the requirements of (a) to (c).
(A) The bulk density of the filter medium is 0.03 to 0.20 g / cm ³ and the thickness is 1.5 mm or less.
(B) The bending resilience in the Gurley method is 100 to 500 mg.
(C) The amount of oil applied is 0.1% by weight or less.
(2) In the web containing short fibers that can be electret, the electret fibers constituting the web are polyolefin heat-sealing fibers, and the heat-sealing fibers are contained in 60 to 100% by weight in the above (1) The electret filter medium described.

  According to the present invention, while maintaining general filter characteristics such as pressure loss, dust collection efficiency and strength of an air filter, it is possible to obtain an appropriate strength necessary for manufacturing and using the filter, and stable collection. Efficiency can be obtained.

Hereinafter, the present invention will be described in detail.
The present invention relates to an electret filter medium, particularly an electret used for a pleated filter unit in which the initial JIS11 colorimetric collection efficiency is 50% to 99% of the performance evaluation method defined in JIS B 9908: 2001. Applied to filter media. The filter medium is characterized in that it is an electret filter medium in which an electret-capable sheet (A) and a web (B) containing electret-capable short fibers are joined by heat-pressure welding. Specifically, the electret after heat-pressure welding is possible. The site | part (B layer) which comprises a short fiber must satisfy the requirements of the following (a)-(c).
(A) The bulk density is 0.03 to 0.20 g / cm ³ and the thickness is 1.5 mm or less.
(B) The bending repulsion characteristic in the Gurley method is 100 to 500 mg.
(C) The amount of oil applied is 0.1% by weight or less.
Moreover, in order to implement | achieve this invention, the electret short fiber which comprises a web is a polyolefin-type heat-fusion fiber, and a web needs to contain 60-100 weight% of this heat-fusion fiber.

  In the electret filter medium in the present invention, the electret filter medium is characterized in that an electret-capable sheet (A layer) and an electret-capable short fiber web (B layer) are joined by heat-pressure welding. In this case, the electret-capable sheet (A layer) is a fiber made of polypropylene, polyethylene, poly-3-methyl-1-butene, poly-4-methyl-1-pentene, polyvinylidene fluoride, and polycarbonate. Woven fabrics and knitted fabrics, spunbonds, melt blown nonwoven fabrics, wet papers made with binders, etc. For example, polypropylene fibers, spunbonded nonwoven fabrics based on polyethylene / polypropylene heat-bonded fibers, and polypropylene meltblown nonwoven fabrics are preferably used.

Further, the other web (B) containing short fibers that can be electret is obtained by defibrating the above-mentioned electret fibers as a raw cotton by a known method and putting it on a card to form a layer. The electretable fiber in this case is a polyolefin-based heat-sealing fiber, and is obtained by laminating the web (B) containing the heat-sealing fiber and the electretable sheet (A) and pressing them by heating. In this case, about the site | part (B layer) which comprises the short fiber web after heat press-contacting, when considering the balance of the dust collection efficiency and ventilation resistance which a bulk density has, and the workability of a filter medium, 0.03- 0.20 g / cm ^3, preferably 0.04~0.18g / cm ^3. When the bulk density is smaller than 0.03 g / m ³ , the amount of fibers is small, so the dust collection efficiency after electret is lowered. When the bulk density is higher than 0.20 g / cm ³ , the air resistance of the filter medium is high and the filter unit is made. In some cases, the pressure loss increases, and the dust holding amount decreases, which is not preferable.

  Since the thickness greatly affects the pressure loss of the filter unit after pleating, it is adjusted to 1.5 mm or less, preferably 1.4 mm or less. When the thickness is larger than 1.5 mm, the structural resistance at the time of ventilation in the filter unit after pleating increases, so that the pressure loss increases, which is not preferable. Furthermore, it is necessary for maintaining the appropriate pleating processability that the bending repulsion characteristics in the Gurley method of the portion after heating and pressure welding in the present invention is 100 to 500 mg, preferably 120 to 480 mg. When the bending resilience characteristic is lower than 100 mg, the bending strength of the filter medium is lowered, so that a beautiful mountain-shaped pleat is not formed at the time of pleating, and an irregular shape is formed. On the other hand, if it is larger than 500 mg, the strength is high and it cannot be bent during pleating, and the apex is formed in an irregular shape.

  In the present invention, it is necessary for the electret filter medium for the pleated filter unit that the amount of the oil agent contained in the portion (B layer) constituting the short fiber web after heating and pressure welding is 0.1% by weight or less. It is important in obtaining performance. The raw cotton used for the short fiber nonwoven fabric contains an oil mainly composed of organic components in order to stabilize the web-forming property by suppressing static electricity generated during carding. While such an oil agent stabilizes the formation of the web, it has an effect of suppressing even when electretized by performing a charging process after forming into a sheet. Therefore, the adhesion amount of the oil agent in the present invention is important for obtaining a charging effect that is required to be 0.1% by weight or less with respect to the total weight of the part (B layer). When the amount of the oil agent adhered is more than 0.1% by weight, the charge is not stable even if it is charged, and the required dust collection efficiency cannot be obtained, which is not preferable.

  In order to obtain such an electret filter medium, it is necessary to be a non-woven fabric that can be electretized, particularly a heat-bonded non-woven fabric containing 60 to 100% by weight of a polyolefin-based heat-bonding fiber with a polyolefin-based component. When the content of the polyolefin-based heat-fusible fiber is lower than 60% by weight, a necessary charging effect cannot be obtained, and a necessary strength as a sheet cannot be obtained.

  A specific manufacturing method for realizing the present invention will be described. In the present invention, in particular, the web (B) containing short fibers that can be electreted, a core-sheath fiber composed of polypropylene as a core and polyethylene as a sheath is used to form a web by a known method. It is preferably used from the viewpoint of rationality. When considering the strength and pressure loss of the sheet, the core-sheath fiber is preferably a fiber of 2.2 to 22 dtex, and if the fiber has a fineness greater than that, the fusion strength when the flame retardant is contained is The strength of the sheet is weakened due to the decrease. Moreover, when it is less than 2.2 dtex, since the fiber diameter becomes thin, the pressure loss at the time of making it into a sheet becomes high, and it is not preferable. The ratio of the main component polypropylene fiber to polyethylene as an adhesive component varies depending on the blending method, but in order to obtain sufficient adhesion, polypropylene (PP) fiber and polyethylene (PE) are in a weight ratio of 1: 4 to It is preferably adjusted to 4: 1. After blending these polyolefin-based core-sheath fibers and other fibers into a web using a known method, laminating the electret-capable sheet (A) obtained by a known method, and applying heat-pressure welding Go to produce filter media. Particularly for the web (B) containing short fibers that can be electreted, other fibers mixed with the polyolefin-based core-sheath fibers are selected according to the function required of the sheet. For example, in order to impart antibacterial properties, high moldability and flame retardancy, fibers to which known additives having such functions are added may be mixed to such an extent that electret properties are not impaired.

  A known method may be used to produce the web (B) containing short fibers that can be electret. However, in order to improve handling, for example, a method in which the card web is subjected to a needle punch, a card sheet is entangled by a water flow, and a rough sheet After the configuration, a method of laminating with (A) and thermocompression bonding may be used. However, in order to obtain the required strength, the laminated layers are heated in a heat-through oven for a time during which heat is sufficiently distributed at a temperature equal to or higher than the melting point of the sheath component of the heat-sealing fiber, specifically, for 10 to 60 seconds. It is necessary to melt the part and bond between the fibers. In this case, in order to obtain the required thickness, the nonwoven fabric entangled with the web is sandwiched between the nets adjusted to a predetermined gap and heated and pressed. It is also possible to heat a nonwoven fabric entangled with a web through a press roller adjusted to a predetermined gap after heating in an oven. In order to obtain the required bending resilience characteristics as a pleated filter medium, it is necessary that the heat-bonding fiber component of the B layer is joined to the A layer in a heated state, adjusted to a predetermined thickness and cooled. If it is the method of adjusting thickness by, it will not specifically limit.

  A method for removing the oil necessary for obtaining the required electret properties will be described. The oil agent is obtained by impregnating a hot bath at about 50 to 80 ° C. for several minutes after the formation of the sheet and dehydrating it. It is also possible to continuously obtain an unheated B layer by passing it through a warm bath, drying it at a predetermined temperature, and heating and pressing the A layer to a temperature not lower than the melting point. Moreover, you may use the fiber (patent document 3) from which an oil agent component reduces by heating. The sheet from which the oil agent has been removed is charged by the charging process and becomes an electret filter medium. The charging method is not particularly limited, and may be a known method such as a method using corona discharge, a method obtained by passing a liquid such as water, or a method of generating static electricity by rubbing the sheet.

Next, the present invention will be specifically described using examples and comparative examples, but the present invention is not limited to the examples. In addition, the measuring method was implemented with the following method.
Regarding the physical property evaluation of the B layer after thermocompression bonding, the A layer was peeled off after thermocompression bonding, and the following (1) to (5) were evaluated.
(1) Weight per unit area (g / m ² ): Cut out with a size of 200 mm square, weighed and divided by the size.
(2) Thickness (mm): The thickness at a load of 0.7 kPa was read.
(3) Bulk density (g / cm ³ ): A value obtained by dividing the basis weight by the thickness was adopted.
(4) Bending resilience: Conforms to JIS L 1096 (1990) 6.20.1 A method.
(5) Oil agent adhesion amount: 2 g of a sample was collected and impregnated in 50 cc of an extract of ethanol / methanol: weight mixing ratio 2/1 to extract the oil agent. The weight before and after extraction was measured to calculate the amount of extraction (% by weight) per unit weight.
(6) Characteristics of the filter unit A filter medium of 605 mm width × 20 m length is manufactured, and bead resin is poured at a 25 mm interval and molded so as to be folded at a peak height of 58 mm to a peak pitch of 6 mm. Formed. A 610 mm square aluminum frame member was loaded with pleats and installed in a measuring apparatus so that the B layer of the filter medium was on the upstream side, and pressure loss, initial collection efficiency of JIS 11 species, and dust holding amount were measured. As the measuring apparatus, an apparatus based on type 2 of JIS B 9908 (2001) was used. The air flow was 56 m ³ / min, and the dust load was finished at 294 Pa.

The sheet charging method used in the examples and comparative examples is shown below.
(A) Charging device Ground side: A silicon sheet having a thickness of 2 mm was laid on an aluminum plate.
Electrode side: Needle electrode with 10 mm needle spacing Electrode-earth distance: 10 mm from needle tip to silicon sheet
(B) Charging method Applied voltage: 20 kV
Charging time: 30 seconds

[Example 1]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. As the A layer side, a polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared. 2.2 dtex short fibers composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio of 1: 1) and 6.6 dtex short fibers (both manufactured by Ube Nitto Kasei Co., Ltd.) Heat-resistant deoiling fiber HR-LE) was mixed at a ratio of 8: 2 and applied to a card machine to make a web. The web was continuously laminated with a cross layer, and needle punching was applied to make a B layer. Layers A and B are stacked, sandwiched between upper and lower stainless steel meshes, transported in a state where the distance between the upper and lower meshes is set to 1.0 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 107 g / A laminated filter medium with m ² and a thickness of 1.0 mm was obtained. The nonwoven fabric was charged through a charging device to obtain an electret filter medium. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

[Example 2]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 95% filter according to JIS B 9908: 2001 type 2. As the layer A side, a polypropylene melt-blown nonwoven fabric having a basis weight of 30 g / m ² , a thickness of 0.30 m, and an average fiber diameter of 2 μm was prepared. As the B layer, the layer manufactured in Example 1 was used. Layers A and B are laminated and sandwiched between stainless meshes, transported in a state where the distance between the upper and lower meshes is set to 1.0 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 115 g / m ^2. A laminated filter medium having a thickness of 1.1 mm was obtained. The nonwoven fabric was charged through a charging device to obtain an electret filter medium. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

[Example 3]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. As the A layer side, a polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared. 2.2 dtex short fiber (heat-resistant deoiled fiber HR-LE made by Ube Nitto Kasei Co., Ltd.) composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio is 1: 1), and A 6.6 dtex polyester short fiber (T201 manufactured by Toray Industries, Inc.) is mixed at a ratio of 8: 2 and applied to a card machine to make a web. The web is continuously laminated with a cross layer, and a needle punch is applied to form a B layer. did. Layers A and B are laminated and sandwiched between stainless meshes, conveyed in a state where the distance between the upper and lower meshes is set to 1.0 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 117 g / m ^2. A laminated filter medium having a thickness of 1.2 mm was obtained. The nonwoven fabric was charged through a charging device to obtain an electret filter medium. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

[Example 4]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. A polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared as the A layer side. 2.2 dtex short fibers (HR-CK manufactured by Ube Nitto Kasei Co., Ltd.) composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio is 1: 1), and 6.6 dtex polyester Short fibers (T201 manufactured by Toray Industries, Inc.) were mixed at a ratio of 8: 2 and applied to a card machine to make a web. The web was continuously laminated with a cross layer, and a doll punch was applied to form a B layer. Layers A and B are laminated, sandwiched from above and below with stainless steel mesh, transported in a state where the distance between the top and bottom mesh is set to 1.0 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 112 g / A laminated filter medium with m ² and a thickness of 1.0 mm was obtained. The sheet was impregnated in a warm bath set at 70 ° C. for 5 minutes, thoroughly rinsed and washed so that moisture spread throughout the nonwoven fabric, and then dehydrated to remove the oil agent. After removing the oil agent twice, it was dried and charged through a charging device to obtain an electret filter medium.
The filter unit was manufactured by folding the filter medium and the unit characteristics were evaluated. Unit weight on the B layer side, thickness, bulk density, amount of oil adhesion, filter unit pressure loss, dust retention, initial JIS 11 species collection efficiency Is shown in Table 1.

[Comparative Example 1]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. As the A layer side, a polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared. 2.2 dtex short fibers composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio of 1: 1) and 6.6 dtex short fibers (both manufactured by Ube Nitto Kasei Co., Ltd.) Heat-resistant deoiling fiber HR-LE) was mixed at a ratio of 8: 2 and applied to a card machine to make a web. The web was continuously laminated with a cross layer, and needle punching was applied to make a B layer. Layers A and B are laminated and sandwiched between upper and lower stainless steel meshes, transported in a state where the distance between the upper and lower meshes is set to 2.5 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 107 g / A laminated filter medium with m ² and a thickness of 2.5 mm was obtained. The nonwoven fabric was charged through a charging device to obtain an electret filter medium. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

[Comparative Example 2]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. As the A layer side, a polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared. 2.2 dtex short fibers composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio of 1: 1) and 6.6 dtex short fibers (both manufactured by Ube Nitto Kasei Co., Ltd.) Heat-resistant deoiling fiber HR-LE) was mixed at a ratio of 8: 2 and applied to a card machine to make a web. The web was continuously laminated with a cross layer, and needle punching was applied to make a B layer. Layers A and B are stacked and sandwiched between upper and lower stainless steel meshes, transported in a state where the distance between the upper and lower meshes is set to 0.7 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 67 g / A laminated filter medium with m ² and a thickness of 0.8 mm was obtained. The nonwoven fabric was charged through a charging device to obtain an electret filter medium. Although this filter medium was fold-folded, it could not be processed well due to the low rigidity of the filter medium.
Table 1 shows the weight per unit area, thickness, bulk density, and oil adhesion amount.

[Comparative Example 3]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. After obtaining a laminated filter medium under the same conditions as in Example 4, it was charged through a charging device without washing, and an electret filter medium was obtained. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

[Comparative Example 4]
An electret filter medium was manufactured by the method described below as a filter medium for an efficiency 90% filter according to JIS B 9908: 2001 format 2. As the A layer side, a polypropylene melt-blown nonwoven fabric having a basis weight of 22 g / m ² , a thickness of 0.23 m, and an average fiber diameter of 3.5 μm was prepared. 2.2 dtex short fibers (HR-CK manufactured by Ube Nitto Kasei Co., Ltd.) composed of polypropylene (core component) / polyethylene (sheath component) (polypropylene: polyethylene weight ratio is 1: 1), and 6.6 dtex polyester Short fibers (T201 manufactured by Toray Industries, Inc.) were mixed at a ratio of 1: 1 and applied to a card machine to make a web. The web was continuously laminated with a cross layer, and a doll punch was applied to form a B layer. Layers A and B are stacked, sandwiched from above and below with stainless steel mesh, transported in a state where the distance between the top and bottom mesh is set to 1.0 mm, passed through an air-through oven, heated and fused at a temperature of 150 ° C. for 1 minute, and a basis weight of 172 g / A laminated filter medium with m ² and a thickness of 1.2 mm was obtained. The sheet was impregnated in a warm bath set at 70 ° C. for 5 minutes, thoroughly rinsed and washed so that moisture spread throughout the nonwoven fabric, and then dehydrated to remove the oil agent. After removing the oil agent twice, it was dried and charged through a charging device to obtain an electret filter medium. This filter medium was folded and a filter unit was manufactured to evaluate the unit characteristics.
Table 1 shows the basis weight, thickness, bulk density, oil agent adhesion amount, filter unit pressure loss, dust holding amount, and initial JIS 11 species collection efficiency on the B layer side.

  Since the electret filter medium of the present invention has charge retention in the cover layer for maintaining strength, the overall charge retention of the filter medium is stabilized, and a higher dust collection efficiency can be obtained than in the past.

Claims (2)

Among the performance evaluation methods defined in JIS B 9908: 2001, in an electret filter medium used in a pleated filter unit that exhibits 50% to 99% JIS11 species colorimetric collection efficiency in the initial stage, the filter medium is an electret This is an electret filter medium characterized in that a sheet (A) capable of being electret and a web (B) containing short fibers capable of electret are joined by heat pressure welding, and a portion (B Layer) satisfies the following requirements (a) to (c).
(A) The bulk density is 0.03 to 0.20 g / cm ³ and the thickness is 1.5 mm or less.
(B) The bending repulsion characteristic in the Gurley method is 100 to 500 mg.
(C) The amount of oil applied is 0.1% by weight or less.   2. The electret filter medium according to claim 1, wherein the electret short fibers constituting the web are polyolefin heat-sealing fibers, and the web contains 60 to 100% by weight of the fusion fibers.