JP7349414B2 - Filter medium for air filter and its manufacturing method - Google Patents

Description

The present disclosure relates to a filter medium for air filters used in air filters installed in clean rooms, building air conditioners, air cleaners, etc. for the semiconductor, liquid crystal, and food industries.

In order to collect and remove submicron or micron particles in the air, an air filter equipped with an air filter medium is generally used. Air filters are classified into coarse dust filters, medium-performance filters, HEPA filters, and ULPA filters depending on the particle size that can be collected and collection efficiency, and the latter have higher performance.

Pressure loss and collection efficiency are characteristics that indicate the filtration performance of air filter media. The higher the pressure loss, the higher the energy consumption required for ventilation and the higher the running cost of the filter, so a filter medium with low pressure loss and high particle collection efficiency is desirable. As an index of filtration performance shown from this viewpoint, there is a PF value defined by the formula of Equation 1. Note that transmittance [%] = 100 - collection efficiency [%], and a filter medium having a high PF value is desirable.

Methods for improving the PF value of air filter media include a method of adding a cationic surfactant, which is a quaternary ammonium salt, to the air filter media (see, for example, Patent Document 1), and a method of adding a cationic surfactant, which is a quaternary ammonium salt, to the air filter media. A method of applying an agent (for example, see Patent Document 2) and a method of applying a mixture of a binder resin and a fluorosurfactant (see, for example, Patent Document 3) have been proposed.

Furthermore, a method has also been proposed in which glass fibers to which a nitrogen-containing fluorine-based surfactant is attached in advance are used as part of the constituent fibers (see, for example, Patent Document 4).

On the other hand, water repellency is another physical property required of air filter media. By using water-repellent filter media, we prevent the problem of sealants and hot melt used when processing the filter media into air filter units seeping in, and the problem of water droplets caused by condensation blocking the pores of the filter media. be able to. Furthermore, by using a filter medium with high water repellency, it is possible to prevent sea salt particles from entering the filter when the filter is used in coastal areas.

Japanese Patent Application Publication No. 2010-94580 WO2014-171165 publication Japanese Patent Application Publication No. 2015-85250 JP2019-51481A

As described above, the techniques disclosed in Patent Documents 1 to 3 have been proposed as methods for improving the PF value of filter media for air filters, but there is a need for further improvement in the PF value from the viewpoint of energy conservation. For example, in the method of applying a fluororesin and a surfactant presented in Patent Document 2, an example is shown in which anionic surfactant is attached at a ratio of 20 parts by mass or less to 100 parts by mass of fluororesin. . Surfactants have the effect of improving the PF value by adsorbing to the filter medium and improving the dispersibility of the fibers, but because glass fibers are negatively charged, anionic surfactants are difficult to adsorb to the fibers. Since the surfactant is preferentially adsorbed to the fluororesin adsorbed to the fibers, the effect of increasing the PF value by increasing the proportion of the surfactant is limited, and there is a problem in that water repellency tends to decrease. Furthermore, since the method of Patent Document 4 requires a step of adhering a surfactant to glass fibers, there is a problem that the manufacturing efficiency is low and the manufacturing cost is high.

As mentioned above, filter media for air filters are required to have a high PF value and water repellency, and also to be easy to manufacture. Therefore, an object of the present disclosure is to provide a filter medium for an air filter that has a high PF value and has practically sufficient water repellency, and to provide the filter medium by an easy manufacturing method.

A filter medium for an air filter according to the present invention is a filter medium for an air filter made of a wet-laid nonwoven fabric containing glass fibers, the filter medium contains a fluororesin and a cationic surfactant, and does not contain a binder resin, The solid content mass ratio of the surfactant and the surfactant is in the range of 40/60 to 80/20. By adsorbing both the fluororesin and the cationic surfactant on glass fibers in the above ratio, a filter medium for an air filter having a high PF value and sufficient water repellency for practical use can be obtained.

In the air filter medium according to the present invention, it is preferable that the wet-laid nonwoven fabric further includes melt-bonded binder fibers, and that the melt-bonded binder fibers are melt-bonded to the glass fibers in a fibrous state . The strength of the filter medium can be further increased.

In the air filter medium according to the present invention, the fluororesin is preferably nonionic or cationic. Since glass fiber has a negative surface charge, the fluororesin is more easily adsorbed to the glass surface, improving the water repellent effect.

In the air filter medium according to the present invention, the total solid mass content of the fluororesin and the surfactant contained in the filter medium may be 0.01 to 2.00% with respect to the entire filter medium. preferable. It becomes possible to achieve both high strength and high PF value of the filter medium.

In the air filter medium according to the present invention, the wet nonwoven fabric includes, as the glass fibers, glass wool fibers with a fiber diameter of 1 to 10 μm, glass wool fibers with a fiber diameter of less than 1 μm, and chopped glass fibers with a fiber diameter of 4 to 30 μm. Preferably, it contains fibers. It becomes easier to obtain a high PF value and high strength.

The method for producing a filter medium for an air filter according to the present invention includes the steps of: forming a wet sheet by forming a slurry containing glass fibers into a sheet by a wet papermaking method; impregnation in an aqueous dispersion containing a surfactant, in which the solid content mass ratio of the fluororesin and the surfactant is in the range of 40/60 to 80/20, and not containing a binder resin; The method is characterized by comprising a step of drying a wet sheet impregnated with a dispersion liquid to obtain a dry sheet. As a result, a filter medium for an air filter having a high PF value and sufficient water repellency for practical use can be obtained.

According to the present disclosure, it is possible to provide a filter medium for an air filter that has a high PF value and a practically sufficient water repellency, and to provide the filter medium using an easy manufacturing method.

It is a graph showing the relationship between the solid content mass ratio of fluororesin/cationic surfactant and the PF value of 0.10 to 0.15 μm. It is a graph showing the relationship between the solid content mass ratio of fluororesin/cationic surfactant and water repellency.

Next, the present invention will be described in detail by showing embodiments, but the present invention should not be interpreted as being limited to these descriptions. The embodiments may be modified in various ways as long as the effects of the present invention are achieved.

The filter medium for an air filter according to the present embodiment is a filter medium for an air filter made of a wet-laid nonwoven fabric containing glass fiber, and the filter medium contains a fluororesin and a cationic surfactant, and does not contain a binder resin. The solid content mass ratio of the surfactant and the surfactant is in the range of 40/60 to 80/20. In addition, in the manufacturing process of the filter medium, it contains a fluororesin and a cationic surfactant, the solid content mass ratio of the fluororesin and the surfactant is in the range of 40/60 to 80/20, and a binder resin is included. A wet sheet obtained by forming a slurry containing glass fibers into a sheet by a wet paper-making method is impregnated with an impregnating liquid of an aqueous dispersion containing no water. The fluororesin and surfactant are both adsorbed onto the glass fiber through impregnation. The impregnated sheet is then dried. It is estimated that in filter media that has undergone this process, the glass fibers are uniformly dispersed, preventing agglomeration of glass fibers, increasing the surface area of the filter media, improving collection efficiency, and resulting in a filter media with a high PF value. be done.

The fluororesin used in this embodiment is a resin containing a fluoroalkyl group in its molecule, and has properties such as water repellency, oil repellency, and non-adhesion due to the repulsive force provided by fluorine atoms. It is preferable to use an aqueous emulsion type or aqueous dispersion type agent made of a perfluoroalkyl group-containing resin that is commercially available as a water repellent, oil repellent, or antifouling agent. The fluororesin is preferably nonionic or cationic, and more preferably a cationic fluororesin that is easily adsorbed to glass fibers. Since glass fiber has a negative surface charge, the fluororesin is more easily adsorbed to the glass surface, improving the water repellent effect. The term cationic here means that the fluororesin itself or the emulsifier is cationic, and the colloidal particles of the fluororesin dispersed in water have a positive surface charge. By using a cationic fluororesin, it becomes easier to adsorb to glass fibers having a negative surface charge in water.

The surfactant used in this embodiment is selected from cationic ones, and includes, for example, primary to tertiary amine salts, quaternary ammonium salts, and the like.

In this embodiment, the impregnating liquid containing the fluororesin and the surfactant does not contain a binder resin. The binder resins mentioned here are polyacrylic ester resins, polystyrene butadiene resins, polyvinyl acetate resins, and polyurethane resins that have the effect of improving the strength of nonwoven fabrics when applied to nonwoven fabrics using methods such as impregnation or coating. It is an aqueous dispersion consisting of resins such as. In order to obtain sufficient strength using such a binder resin, it is necessary to use a larger amount of binder resin than the fluororesin and surfactant used in this embodiment. hinder. Note that the fluororesin used in this embodiment is not included in the binder resin referred to herein because, as described above, the effect on imparting strength is small due to the repulsive force of fluorine atoms.

In this embodiment, the solid content mass ratio of the fluororesin and surfactant in the filter medium (fluororesin/surfactant) is 40/60 to 80/20. By setting this mass ratio, it is possible to obtain a filter medium for an air filter that has both a very high PF value (for example, 13 or more) and practically sufficient water repellency (for example, 150 mm water column height or more). If the solid content mass ratio of the fluororesin is less than 40 parts and the solid content mass ratio of the surfactant exceeds 60 parts, water repellency will not be obtained because the fluororesin will not be sufficiently adsorbed to the glass fibers, and the solid content of the fluororesin will decrease. If the mass ratio exceeds 80 parts and the solid content mass ratio of the surfactant is less than 20 parts, the surfactant will not be sufficiently adsorbed to the glass fibers, resulting in a low PF value.

The total solid mass content of the fluororesin and surfactant in the filter medium is preferably 0.01 to 2.00%, and preferably 0.10 to 1.00% based on the entire filter medium. More preferred. If the content of these components is lower than 0.01%, the effect of improving the PF value will not be sufficient, and if the content is higher than 2.00%, further improvement in the PF value cannot be expected as the amount is increased.

The air filter medium in this embodiment is made of a wet nonwoven fabric containing glass fibers. Since glass fiber has high rigidity, it is possible to maintain sufficient voids in the filter medium for air to pass through, and a high PF value can be obtained. Glass wool fibers and chopped glass fibers can be used as the glass fibers. The glass wool fibers referred to herein are amorphous, discontinuous, wool-like glass fibers whose fiber diameters have a certain distribution width and are produced by drawing by a flame drawing method or a rotary method. The fiber diameter range is generally about 0.1 to about 10 μm, and since it has a certain degree of distribution width, the fiber diameter value is generally expressed as an average fiber diameter. The fiber diameter of the fiber is also the average fiber diameter. On the other hand, chopped glass fiber is a regular-shaped, straight glass fiber obtained by cutting a continuous glass fiber spun from a spindle with a predetermined diameter into a predetermined fiber length, and the fiber diameter range is generally about 4 to about 30 μm, with fiber lengths generally ranging from about 1.5 to about 25 mm. In the filter medium of this embodiment, the glass wool fibers having a small fiber diameter and an irregular shape have the effect of increasing the collection efficiency and retaining voids in the filter medium. Chopped glass fibers with a thick fiber diameter and a straight shape have the effect of imparting the strength and rigidity required during processing and use of the filter unit, but the fibers tend to accumulate horizontally during the manufacturing of the filter medium. A high blending ratio of chopped glass fiber tends to increase the density of the filter medium. In this embodiment, when chopped glass fibers are used as the glass fibers, the blending ratio of the chopped glass fibers is preferably 1 to 50% by mass, and 3 to 30% by mass, based on the total fiber mass of the fibers in the filter medium. More preferably, 5 to 10% by mass is even more preferred.

In this embodiment, the average fiber diameter of the glass wool fibers is not particularly limited, but is preferably 0.1 to 10 μm, more preferably 0.2 to 7 μm. Furthermore, in order to obtain a high PF value, it is preferable that at least a portion of the glass fibers be glass fibers with a fiber diameter of less than 1 μm.

In this embodiment, binder fibers can be used to provide the required strength to the air filter medium. Binder fibers are fibers that are added to a slurry containing glass fibers to impart strength through melt adhesion, hydrogen bonding, physical entanglement, etc., and can be freely selected within a range that does not impair the effects of this embodiment. Examples include polyvinyl alcohol fibers, polyester fibers, polyolefin fibers, etc. In this embodiment, it is preferable to use melt adhesive binder fibers among these. The forms of melt-bonded binder fibers include side-by-side type binder fibers in which a melting part and non-melting part are composited side by side, core-sheath type binder fibers having a core part that does not melt and a sheath part that melts, and There are fully meltable binder fibers that melt and contribute to adhesion between main fibers such as glass fibers. The blending ratio of the binder fiber is preferably 10 to 70% by mass, more preferably 20 to 60% by mass, and even more preferably 30 to 50% by mass, based on the total fiber mass of the fibers in the filter medium. Melt adhesive binder fibers include side-by-side type binder fibers, core-sheath type binder fibers, and all-melting type binder fibers, in which any one type is included in the filter medium for air filters, or two or three types thereof may be included. good. Examples of including two types include a combination of a side-by-side type binder fiber and a core-sheath type binder fiber, a combination of a side-by-side type binder fiber and a fully fused type binder fiber, or a combination of a core-sheath type binder fiber and a fully fused type binder fiber. be.

In the manufacturing process of the filter medium for an air filter according to the present embodiment, raw material fibers are dispersed in water to obtain a raw material slurry, and this is formed into a sheet by a wet papermaking method to obtain a wet sheet. The water used for dispersion and paper making is preferably acidic, and more preferably has a pH of 2 to 4. By carrying out dispersion and paper making under acidic conditions, glass fibers can be easily dispersed, and the strength of the wet paper can be increased, making operations easier.

In the manufacturing process of the filter medium for an air filter according to the present embodiment, the sheet in a wet state before drying obtained by the above method is impregnated with an aqueous dispersion containing a fluororesin and a surfactant, and the sheet is dried. In this way, a filter medium for air filters can be obtained. The effects of this embodiment can be enhanced by impregnating the sheet before drying with a fluororesin and a surfactant. The sheet is dried at a temperature of, for example, 100 to 170°C, more preferably 120°C, using a multi-tube dryer, Yankee dryer, or hot air dryer in a paper machine, or a rotary dryer or circulation dryer in a hand paper machine. It is preferred to dry at a temperature of ~160°C.

In this embodiment, additives such as antifoaming agents can be appropriately added to the aqueous dispersion containing a fluororesin and a surfactant within a range that does not impede the effects of the present invention.

The present invention will be described below with reference to specific examples, but the present invention is not limited to these descriptions. In addition, "parts" in the examples indicate the solid content mass ratio of the fibers in the raw material slurry or the solid content mass ratio of the components in the impregnation liquid, and the total amount of all fibers in the raw material slurry is 100 parts, In the impregnation liquid, the total amount of fluororesin and surfactant was 100 parts. Further, "%" in the examples indicates the solid mass concentration of the component in the impregnating liquid or the solid mass content of the component in the filter medium.

<Example 1>
42 parts of glass wool fibers with an average fiber diameter of 0.65 μm (B-06-F, manufactured by Unifrax Co.), 5 parts of glass wool fibers with an average fiber diameter of 2.44 μm (B-26-R, manufactured by Unifrax Co.), 5 parts of chopped glass fiber (EC-6-6-SP, manufactured by Unifrax Co.) with a fiber diameter of 6 μm and a cut length of 6 mm, a core/sheath binder fiber whose core/sheath is polyester/polyester with a fineness of 1.1 dtex and a cut length of 5 mm. (Melty 4080, manufactured by Unitika Co., Ltd.) and 30 parts of polyester fully fused binder fiber (Melty 4000, manufactured by Unitika Co., Ltd.) with a fineness of 2.2 dtex and a cut length of 5 mm were heated to pH 3.0 using a table disintegrator. Disintegration was performed using acidic water to obtain a raw material slurry. Next, the raw material slurry was put into a hand paper machine filled with acidic water of pH 3.0, and dehydrated to obtain a wet sheet. Next, 40 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 60 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water were mixed to a concentration of 0. A wet sheet was impregnated with an impregnating solution adjusted to 10% and dried using a rotary dryer at 130° C. to obtain a filter medium for an air filter having a basis weight of 75 g/m ² . The content of impregnated components in the filter medium was 0.12%.

<Example 2>
Mix 60 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 40 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that the prepared impregnating liquid was used. The content of impregnated components in the filter medium was 0.30%.

<Example 3>
Mix 80 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 20 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that the prepared impregnating liquid was used. The content of impregnated components in the filter medium was 0.39%.

<Example 4>
42 parts of glass wool fibers with an average fiber diameter of 0.65 μm (B-06-F, manufactured by Unifrax Co.), 53 parts of glass wool fibers with an average fiber diameter of 2.44 μm (B-26-R, manufactured by Unifrax Co.), Five parts of chopped glass fiber (EC-6-6-SP, manufactured by Unifrax Co.) with a fiber diameter of 6 μm and a cut length of 6 mm were disintegrated using acidic water with a pH of 3.0 in a table disintegrator, and fluororesin (Asahi Impregnation liquid prepared by mixing 60 parts of Guard AG-E060 (manufactured by AGC Co., Ltd.), 40 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 70 g/m ² was obtained in the same manner as in Example 1, except that . The content of impregnated components in the filter medium was 0.27%.

<Example 5>
Mix 60 parts of a fluororesin (Asahi Guard AG-E550D, manufactured by AGC Co., Ltd.), 40 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that the prepared impregnating liquid was used. Note that the content of impregnated components in the filter medium was 0.20%.

<Comparative example 1>
The same procedure as in Example 1 was carried out, except that an impregnating solution prepared by mixing 100 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and water to a concentration of 0.10% was used. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained. Note that the content of impregnated components in the filter medium was 0.20%.

<Comparative example 2>
Mix 20 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 80 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that the prepared impregnating liquid was used. The content of impregnated components in the filter medium was 0.12%.

<Comparative example 3>
Mix 90 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 10 parts of a cationic surfactant (Cationogen TMP, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water to a concentration of 0.10%. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that the prepared impregnating liquid was used. The content of impregnated components in the filter medium was 0.92%.

<Comparative example 4>
A sample with a basis weight of 75 g was prepared in the same manner as in Example 1, except that an impregnating solution prepared by mixing 100 parts of fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.) and water to a concentration of 0.10% was used. /m ² of air filter media was obtained. Note that the content of impregnated components in the filter medium was 1.0%.

<Comparative example 5>
A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1 except that no impregnation was performed.

<Comparative example 6>
60 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 40 parts of a nonionic surfactant (Noigen EA-137, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and water were mixed to give a concentration of 0.10. A filter medium for an air filter having a basis weight of 75 g/m ² was obtained in the same manner as in Example 1, except that an impregnating liquid adjusted to 50% was used. The content of impregnated components in the filter medium was 0.39%.

<Comparative example 7>
60 parts of a fluororesin (Asahi Guard AG-E060, manufactured by AGC Co., Ltd.), 40 parts of an anionic surfactant (Hitenol 330T, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) and water were added to a concentration of 0.10%. When mixed, aggregates were generated. Therefore, no air filter was manufactured using this impregnation liquid.

Evaluation of the filter media for air filters obtained in Examples and Comparative Examples was performed using the method shown below.

<Pressure loss>
The pressure loss is calculated as the differential pressure when air is passed through an air filter medium with an effective area of 100 ^cm2 at a surface wind speed of 5.3 cm/sec using a manometer (Manostar Gauge WO81, manufactured by Yamamoto Denki Seisakusho Co., Ltd.). It was measured.

<Transmittance>
The transmittance is the upstream and downstream PAO when air containing polydisperse polyalphaolefin (PAO) particles generated by a Ruskin nozzle is passed through an air filter medium with an effective area of 100 ^cm2 at a surface wind velocity of 5.3 cm/sec. The number of particles was measured using a laser particle counter (KC-18, manufactured by Rion Co., Ltd.) and determined from the ratio of the number of particles upstream and downstream. The target particle diameters were 0.10 to 0.15 μm and 0.30 to 0.40 μm.

<PF value>
The PF value was calculated from the pressure loss and permeability values using the formula shown in Equation 1. The target particle diameters were 0.10 to 0.15 μm and 0.30 to 0.40 μm.

<Tensile strength>
The tensile strength was measured using Autograph AGX-S (manufactured by Shimadzu Corporation) under the conditions of a test width of 1 inch, a test length of 100 mm, and a tensile speed of 15 mm/min.

<Water repellency>
Water repellency was measured in accordance with MIL-STD-282.

Tables 1 and 2 show the evaluation results of the air filter media performed by the above method. In addition, using the results of Examples 1 to 3 and Comparative Examples 1 to 4, the relationship between the solid content mass ratio of fluororesin/cationic surfactant, PF value of 0.10 to 0.15 μm, and water repellency is shown. The graphs obtained are shown in FIGS. 1 and 2, respectively.

From Examples 1 to 4, when the mass ratio of cationic fluororesin and cationic surfactant is in the range of 40/60 to 80/20, very high PF value (13 or more) and high water repellency ( 200 mm or more) could be obtained. From Example 5, when a nonionic fluororesin and a cationic surfactant were used, a filter medium having a very high PF value (13 or more) and practically necessary water repellency (150 mm or more) could be obtained.

According to Comparative Examples 1 and 2, when the mass ratio of the fluororesin was less than 40 parts, sufficient water repellency was not obtained. According to Comparative Examples 3 and 4, when the mass ratio of the fluororesin exceeded 80 parts, the PF value was low. According to Comparative Example 5, when both a fluororesin and a surfactant were not contained, water repellency was not obtained and the PF value was low. According to Comparative Example 6, when the surfactant was nonionic, the PF value was low. According to Comparative Example 7, when the surfactant was anionic, the impregnating liquid agglomerated due to the compatibility between the fluororesin used this time and the surfactant, and therefore a filter medium could not be produced.

Claims (6)

In air filter media made of wet-laid nonwoven fabric containing glass fibers,
The filter medium contains a fluororesin and a cationic surfactant, and does not contain a binder resin,
A filter medium for an air filter, characterized in that the solid content mass ratio of the fluororesin and the surfactant is in the range of 40/60 to 80/20. The wet-laid nonwoven fabric further includes melt adhesive binder fibers,
The filter medium for an air filter according to claim 1 , wherein the melt-bonded binder fiber is melt-bonded to the glass fiber in a fibrous state . The filter medium for an air filter according to claim 1 or 2, wherein the fluororesin is a nonionic or cationic fluororesin. Any one of claims 1 to 3, wherein the total solid mass content of the fluororesin and the surfactant contained in the filter medium is 0.01 to 2.00% with respect to the entire filter medium. The filter medium for air filters described in one of the above. The wet-laid nonwoven fabric is characterized in that the glass fibers include glass wool fibers with a fiber diameter of 1 to 10 μm, glass wool fibers with a fiber diameter of less than 1 μm, and chopped glass fibers with a fiber diameter of 4 to 30 μm. The filter medium for an air filter according to any one of 1 to 4. forming a slurry containing glass fibers into a sheet using a wet papermaking method to form a wet sheet;
The wet sheet contains a fluororesin and a cationic surfactant, the solid content mass ratio of the fluororesin and the surfactant is in the range of 40/60 to 80/20, and a binder resin is used. a step of impregnating it in an aqueous dispersion free of
A method for producing a filter medium for an air filter, comprising the step of drying a wet sheet impregnated with the aqueous dispersion to obtain a dry sheet.

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