KR100638499B1 - Filter medium for air filters - Google Patents

Abstract

The present invention relates to a filter medium for an air filter in which the collection performance of a polytetrafluoroethylene (PTFE) membrane is not damaged even after processing, for example, pleating and bonding to a breathable support member. .

The filter media of the present invention comprise three or more made of porous PTFE membrane, wherein the pressure drop of the porous PTFE membrane 4 positioned as the outermost layer is no more than 1/2 times the pressure drop of the porous PTFE membrane 5 positioned as the at least one intermediate layer. A laminate 2 composed of layers. The breathable support member 3 is adhered to both outermost layers of the laminate 4 to obtain a filter medium 1 for an air filter.

Filter media for air filters, porous polytetrafluoroethylene membranes, breathable support members, laminates, pressure drops

Description

Filter medium for air filters

1 is a cross-sectional view showing an example of the configuration of a filter medium for an air filter according to the present invention.

2 is a cross-sectional view showing still another example of the configuration of a filter medium for an air filter according to the present invention.

In the figures, each reference character represents:

(1), (10): filter media for air filter

(2): porous PTFE membrane laminate

(3): breathable support member

(4): porous PTFE membrane as outermost layer

(5): porous PTFE membrane as intermediate layer

The present invention relates to a filter medium for an air filter using a porous polytetrafluoroethylene (hereinafter referred to simply as "PTFE") membrane.

As filter media for air filters, filter media made by blending glass fibers with binders and processing to paper have often been used. However, such filter media suffers from several problems, for example, dusting caused by the microfibers contained therein, itself due to dusting and deterioration due to chemicals such as hydrofluoric acid. Thus, recently, porous PTFE membranes that are clean materials and have high chemical resistance have been used as filter media for air filters in the semiconductor industry and the like. Porous PTFE membranes can be prepared by molding PTFE into a sheet and then stretching the sheet to make it porous (see, for example, JP-A Nos. 7-196831 and JP-A No. 6-816802). number; The term "JP-A" as used herein means "unexamined Japanese Laid-Open Patent Publication." When the porous PTFE membrane thus obtained is used alone, the rigidity is insufficient. Thus, in many cases porous PTFE membranes are used by adhering the breathable support member to one surface. The filter media so reinforced by the breathable support member is pleated to provide a continuous W-shaped structure (hereinafter referred to as "pleating") and used as filter media units.

On the other hand, various methods have been proposed for producing laminates of porous PTFE membranes (see, for example, JP-A 57-131236 and JP-A 3-179038). JP-A No. 9-504737 proposes a filter medium using such a laminate.

That is, JP-A No. 9-504737 describes a high recovery filter medium comprising a laminate comprising five layers of porous PTFE membranes having the same pressure drop and breathable support members located on both sides of the laminate. In this filter medium, the breathable support member is loosely pleated but not bonded to the porous PTFE membrane laminate.

However, in a filter medium having a breathable support member adhered to one surface of a porous PTFE membrane, some of the pores of the porous PTFE membrane are blocked by adhesion, thereby increasing the pressure drop significantly. On the other hand, the filter medium having the breathable support members located on both sides of the stack does not suffer any difficulty in reducing the pressure drop since the support members are not bonded to the stack. In this case, however, the laminate and the support member are not integrated, and thus the handling characteristics are poor. In this case, since the support member is also not adhered to the stack, this sometimes deviates from the pleat processing step, thereby causing undesirable bending in the stack or the support member.

The present invention seeks to overcome the above problems encountered in the prior art.

Accordingly, it is an object of the present invention to improve the porous PTFE membrane laminate, thereby exhibiting inherent collection performance on the porous PTFE membrane even after processing (e.g., pleat processing, bonding to a breathable support member, etc.) and excellent processability and handling. It is to provide a filter medium for an air filter that can exhibit characteristics.

In order to achieve the above object, the present invention uses a porous membrane capable of sufficiently alleviating the effects of the processing as described above as the outermost layer of the laminate of the porous PTFE membrane.                         

The filter medium for air filters according to the invention comprises a laminate consisting of at least three layers consisting of porous PTFE membranes as a collecting layer, wherein the laminate is a first porous PTFE membrane and an outermost layer positioned as the outermost layers. And a second porous PTFE membrane positioned as one or more intermediate layers, wherein the pressure drop of the first porous PTFE membrane is no more than 1/2 times the pressure drop of the second porous PTFE membrane.

In the filter medium for an air filter according to the present invention, the first porous PTFE membrane positioned as the outermost layer reduces the effects of processing (processing to adhere to the breathable support member, pleating processing, etc.) to the second porous PTFE membrane positioned as the intermediate layer. It works. Bonding the breathable support member to the first porous PTFE membrane positioned as the outermost layer increases the pressure drop in the present invention, but the extent of the increase is small compared to conventional cases. In other words, the collection performance of the filter media is not severely affected by the processing. In addition, the first porous PTFE membrane positioned as the outermost layer is physically impacted during the pleat processing step, thereby mitigating the impact on the second porous PTFE membrane positioned as the intermediate layer, thereby reducing the influence on the collection performance as a filter medium. Let's do it.

In the filter medium for air filters according to the present invention, the breathable support member is preferably adhered to both outermost layers of the laminate described above comprising a porous PTFE membrane. This bonded structure facilitates the handling of the filter media and makes it possible to obtain stable pleating.

In the filter medium for an air filter of the present invention, not only when the breathable support member is bonded to the laminate by an adhesive or the like, but also when the breathable support member is fused to the laminate, any excessive increase in pressure drop due to the adhesion of the breathable support member is achieved. Can also be prevented. That is, the pressure drop of the filter medium for the air filter is 1.15 times or less (1 to 1.15 times the pressure drop of the laminate, even when the filter medium comprises a breathable support member adhered to the outermost layer of the laminate by suitable means. Can be adjusted to the level of.

Embodiments of the present invention will be described with reference to the accompanying drawings.

1 is a cross-sectional view showing one example of the configuration of a filter medium for an air filter according to the present invention. In the filter medium 1 for air filter of FIG. 1, the breathable support members 3, 3 are bonded to both outermost layers of the stack 2 of porous PTFE membrane. The laminate 2 comprises a porous PTFE membrane 4, 4 positioned as the outermost layers and a porous PTFE membrane 5 positioned as the intermediate layer. Porous PTFE membranes 4, 4 located as outermost layers alleviate the effect of pleat processing on adhesion and collection performance to breathable support members 3, 3. In addition, a porous PTFE membrane laminate 2 can be used as the filter medium.

Porous PTFE membranes can be obtained by forming a sheet by extruding and / or rolling a mixture of fine PTFE powder and a liquid lubricant, removing the liquid lubricant from the unsintered sheet thus obtained, and then stretching the sheet to make it porous. . The strength of the membrane can be further enhanced by sintering this stretched sheet by heating above the melting point of PTFE. The above-mentioned porous PTFE membrane laminate 2 can be prepared by adding the step of laminating the porous PTFE membrane in any step as a conventional method for producing the porous PTFE membrane. An example of a method of making a porous PTFE membrane comprising a lamination step is described below.

1) A preform is prepared in which a plurality of layers containing fine PTFE powder and a liquid lubricant are laminated. This preform is subsequently subjected to extrusion, rolling, stretching or the like to obtain a porous PTFE membrane.

2) Laminate a number of unsintered PTFE sheets containing liquid lubricant. The laminate thus obtained is successively treated by rolling, stretching or the like as in a conventional production method to obtain a laminate of porous PTFE membrane.

3) Compress multiple unsintered porous PTFE membranes to form a laminate.

The porous structure of the porous PTFE membrane is affected by the materials and conditions used in the rolling and stretching steps. In porous PTFE membrane laminates, materials and processing conditions are adjusted for each layer. Such adjustment is not particularly limited, but is carried out by adjusting, for example, the molecular weight of the fine PTFE powder, the rolling conditions for forming the unsintered PTFE sheet, the stretching conditions for producing the unsintered porous PTFE membrane, and the like.

It is difficult to measure the pressure drop of each layer of the porous PTFE membrane laminate thus obtained. However, the pressure drop of each layer can be assessed by separately preparing the layers used using the material used to make the laminate and the conditions such as rolling, elongation (ie, the first and second porosities constituting the laminate). PTFE membranes are prepared separately under the same conditions as used to prepare the laminate and the evaluation is performed on the state of the membrane thus prepared].

The pressure drop of the porous PTFE membrane 4 provided as the outermost layer and the pressure drop of the porous PTFE membrane 5 provided as the intermediate layer can be measured respectively using the method as described later. It is preferable that the pressure drop of the porous PTFE membrane 4 provided as the outermost layer is 2 to 15 mmH ₂ O, and the pressure drop of the porous PTFE membrane 5 provided as the intermediate layer is 10 to 30 mmH ₂ O. It has been found that the pressure drop of the porous PTFE membrane as the outermost layer is preferably in the range of 10 to 50% of the pressure drop of the porous PTFE membrane as the intermediate layer.

The porous PTFE membrane 4 provided as the outermost layer does not always have to have the same pressure drop as long as each porous PTFE membrane 4 is not more than 1/2 of the pressure drop of the porous PTFE membrane 5 provided as the intermediate layer. In addition, the number of porous PTFE membranes of the laminate 2 is not limited to three, and the laminate may be composed of four layers or five or more layers (see Fig. 2). In the case of the filter medium 10 for an air filter having at least two porous PTFE membranes 5 as the intermediate layer, the pressure drop of the porous PTFE membrane 4 as the outermost layer exhibits the maximum pressure drop among the porous PTFE membranes provided as the intermediate layer. It may be less than 1/2 of the pressure drop of the porous PTFE membrane 5.

The intermediate layer preferably comprises a porous PTFE membrane having a PF (performance of filter) value indicating collection performance of greater than 20, preferably 21 to 40. PF value is calculated according to the following equation (1).

In Equation 1,

L means the pressure drop, and more specifically, the sample (porous PTFE membrane, etc.) is placed in a circular holder with an effective area of 100 cm ² and through this sample a face vleocity of 5.3 cm / sec. Pressure drop value (in mmH ₂ O) measured by a pressure gauge by allowing air to pass through,

E is the collection efficiency, more specifically the sample is placed in the same holder measuring the pressure drop, and polydisperse dioctyl phthalate with a particle size of 0.1 to 0.2 μm as an aerosol while permeating air through the sample at the same plane speed (DOP) was fed at a rate providing a concentration of about 10 ⁸ particles / l, and then the laser particle counter (LPC) was used to measure the particle concentration of the upstream portion and the particle concentration of the downstream portion. The ratio calculated according to 2.

In Equation 2,

C _D means the particle concentration of the downstream portion,

C _U means the particle concentration of the upstream portion.

The breathable support member 3 can be any member, as long as the breathability is better than that of the porous PTFE membrane. Examples of such member materials include nonwovens, woven fabrics, meshes and other porous materials. The breathable support member 3 is not limited in material, structure or form, but it is preferable to use a nonwoven fabric for the member in view of strength, flexibility and workability. Since such nonwoven fabrics can be easily bonded, it is more preferable to use nonwoven fabrics comprising composite fibers having a core-shell structure in which the melting point of the core component is higher than the melting point of the shell component.

The breathable support member is bonded to the laminate of porous PTFE members using an adhesive member such as an adhesive. Alternatively, the breathable support member may be heated to partially melt and then fused to the laminate. When such an adhesion method is applied to a conventional porous PTFE membrane, the pores of the porous PTFE membrane are blocked by a molten breathable support member or an adhesive member to reduce air permeability, thereby deteriorating the inherent performance of the porous PTFE membrane. In contrast, the laminate of the porous PTFE member described above can reduce the influence of lowering the air permeability.

The influence of the lowering of breathability can thereby be alleviated, so that although the breathable support member is adhered to both outermost layers of the porous PTFE membrane laminate, the present invention provides a filter medium for an air filter having a PF value of 17 or more. Will be.

In the filter medium for an air filter, it is preferable that the porous PTFE members be adhered to each other because the filter medium having such a structure can be easily processed and pleated in a stable state.

The invention will be described in more detail with reference to the following examples, but it should be understood that the invention is not to be construed as limited thereto. In these examples, the pressure drop, collection efficiency and PF value are measured by the respective methods described above. On the other hand, leakage performance is measured by the following method.

(Leakage performance)

The sample (filter medium) is pleated using a reciprocating pleat machine. The collection performance of each sample pleated in this way is measured in the same manner as described above. The particle concentration of the upstream portion and the leaked particle concentration of the downstream portion are measured with a particle counter, and the particle transmittance is calculated according to the following Chemical Formula 3.

In Equation 3,

P _D means the particle transmission of the downstream portion,

P _U means the particle transmission of the upstream portion.

Particle transmittance (P _0.1 ) having a particle size of 0.1 to 0.2 μm and particle transmittance (P _0.2 ) having a particle size of 0.2 to 0.3 μm are measured. When the following expression (4) is satisfied, it is considered that leakage occurs.

Leakage performance is assessed by measuring particle permeability at 60 points in each sample and calculating the leak frequency.

Example 1

Fine PTFE powder [Fluon CD-014, manufactured by Asahi-ICI Fluoropolymers; Referred to below as "fine PTFE powder (A)" 100 parts by weight are uniformly mixed with 35 parts by weight of the liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste-molded in rod form by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet ("rolled sheet A") having a thickness of 500 mu m.

Individually, another fine PTFE powder [Fluon CD-123, manufactured by Asahi-ICI Fluoropolymers; Hereinafter referred to as "fine PTFE powder (B)" 100 parts by weight are mixed uniformly with 25 parts by weight of the liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste molded into rods by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet ("rolled sheet B") having a thickness of 300 mu m.

After superimposing the rolled sheet A on both surfaces of the rolled sheet B, the resultant composite is integrated by passing between a pair of metal rolls to obtain a molded article in the form of a sheet having a thickness of 900 m. The liquid lubricant is removed from the sheet by the extrusion method with Triclene to obtain an unsintered PTFE laminate. The unsintered PTFE laminate is stretched by rolling 20 plies in the longitudinal direction of the sheet at a stretching temperature of 290 ° C. The laminate is further stretched by tentering 30 plies in the transverse direction of the sheet at a stretching temperature of 80 ° C. to obtain an unsintered porous PTFE membrane. A filter medium comprising a three layer porous PTFE membrane laminate having a thickness of about 42 μm while being heated to 400 ° C. for 20 seconds while the unsintered porous PTFE membrane of this three layer structure was fixed [“filter medium 1). "] Is obtained.

A pair of rolls of polyethylene terephthalate (PET) / polyester (PE) core-shell nonwoven fabric (ELEVES TO303WDO, manufactured by Unitika Ltd., shell: 129 ° C) having a thickness of 150 µm and a basis weight of 30 g / m 2 (Roll temperature: 140 DEG C) by lamination on both surfaces of the filter medium by thermal lamination, so that the porous PTFE membrane laminate had the same structure as shown in Fig. 1 sandwiched between nonwoven fabrics ("filter media"). (2) "] is obtained.

In order to evaluate the properties of each layer constituting the filter medium 1, the rolled sheet A alone is passed between a pair of metal rolls to obtain a molded article in the form of a sheet having a thickness of 350 m. The liquid lubricant is removed from the molded article, each stretched and heat treated in the same manner as described above to obtain a porous PTFE membrane ("outermost layer membrane (A)") having a thickness of about 16 µm.

Similarly, the rolled sheet B alone is passed between a pair of metal rolls to obtain a molded article in sheet form having a thickness of 200 mu m. The liquid lubricant is removed from the molded article, each stretched and heat treated in the same manner as described above to obtain a porous PTFE membrane ("intermediate layer membrane (A)") having a thickness of about 8 µm.

Example 2

The outermost layer film (A) is superposed on both surfaces of the interlayer film (A) obtained in the same manner as in Example 1. Further, the same PET / PE core-shell nonwoven fabric as used in Example 1 was laminated on both surfaces thereof by thermal lamination (roll temperature: 140 ° C.) to have a filter medium having a multilayer porous PTFE membrane sandwiched between the nonwoven fabrics [ "Filter medium 3"] is obtained.

Example 3

100 parts by weight of fine PTFE powder (A) is uniformly mixed with 35 parts by weight of a liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste molded into rods by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet (“rolled sheet C”) having a thickness of 350 μm.

Individually, 100 parts by weight of fine PTFE powder (B) is mixed uniformly with 25 parts by weight of a liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste molded into rods by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet ("rolled sheet D") having a thickness of 200 mu m.

The liquid lubricant is removed from each of the rolled sheets (C) and (D) by extrusion using trickle to yield an unsintered PTFE sheet. This unsintered PTFE sheet is drawn by rolling 20 plies in the longitudinal direction at a drawing temperature of 290 ° C. Thus, the stretched PTFE sheet (C) and the stretched PTFE sheet (D) are obtained from the rolled sheet (C) and the rolled sheet (D), respectively. The stretched PTFE sheet (C) is superimposed on both surfaces of the stretched PTFE sheet (D), and the resulting composite sheet is stretched by tentering 30 plies in the transverse direction at a stretching temperature of 80 ° C. to thereby obtain an unsintered porous PTFE membrane. To obtain. This three-layer unsintered porous PTFE membrane was heated to 400 ° C. for 20 seconds while being dimensionally fixed to include a desired filter medium comprising a sintered three-layer porous PTFE membrane laminate having a thickness of about 39 μm [ "Filter medium 4"] is obtained.

The above-mentioned PET / PE core-shell nonwoven fabric having a thickness of 150 µm and a basis weight of 30 g / m 2 is laminated on both surfaces of the filter medium 4 by thermal lamination (roll temperature: 140 ° C.), thereby sandwiching the sandwich between the nonwoven fabrics. A filter medium ["filter medium 5") with a porous PTFE membrane laminate is obtained.

In order to evaluate the properties of each layer constituting the filter medium 4, the stretched PTFE sheet (C) was stretched by tentering and heated in the same manner as above to obtain a porous PTFE membrane having a thickness of about 16 µm [" Layer film (B) "].

Similarly, the stretched PTFE sheet (D) is stretched by tentering and heated to obtain a porous PTFE membrane ("intermediate layer membrane (B)") having a thickness of about 8 μm.

Example 4

100 parts by weight of fine PTFE powder (A) is uniformly mixed with 35 parts by weight of a liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste molded into rods by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet having a thickness of 500 mu m. This molded article in the form of a sheet containing a liquid lubricant was stretched by rolling two plies in the longitudinal direction at a stretching temperature of 50 ° C. to obtain a stretched PTFE sheet.

Filter media having a thickness of about 29 μm comprising a three-layer porous PTFE membrane laminate sintered by repeating the process of Example 1, except that an elongated PTFE sheet was used as an alternative to the rolled sheet (A). ["Filter medium 6"] is obtained. The nonwoven fabric is thermally laminated on both surfaces of the filter medium 6 in the same manner as in Example 1 to obtain another filter medium ("filter medium 7").

In order to evaluate the properties of each layer constituting the filter medium 6, the stretched PTFE sheet alone is passed between the metal rolls to obtain a continuous sheet having a thickness of 230 mu m. This molded article in the form of a sheet is subjected to stretching or the like as in the manufacture of the filter medium 6 to obtain a porous PTFE membrane ("outermost layer membrane C") having a thickness of about 10 mu m. In order to evaluate the intermediate layer of this example, the intermediate layer film (A) is used in the same manner as in Example 1.

Comparative Example 1

As a substitute for fine PTFE powder (A), fine PTFE powder [Polyflon F-104U, manufactured by Daikin Industries, Ltd .; Filter media comprising a sintered three layer porous PTFE membrane laminate having a thickness of about 38 μm by repeating the process of Example 1, except using the following, referred to as “fine PTFE powder (C)”. ["Filter medium 8"] is obtained. In addition, another filter medium ("filter medium 9") with a filter medium 8 sandwiched between nonwovens is obtained.

In order to evaluate the properties of each layer constituting the filter medium 8, the stretched sheet obtained using the fine PTFE powder (C) is passed between the metal rolls to obtain a continuous sheet having a thickness of 350 mu m. This molded article in sheet form is subjected to stretching or the like as in the manufacture of the filter medium 8 to obtain a porous PTFE membrane (“outermost layer membrane D”) having a thickness of about 16 μm. In order to evaluate the interlayer in this example, the interlayer film A is used in the same manner as in Example 1.

Comparative Example 2

A continuous sheet having a thickness of 300 µm was repeated by repeating the manufacturing process of the rolled sheet B of Example 1, except that the fine PTFE powder (A) was used as a substitute for the fine PTFE powder (B). (E) "]. Except for superimposing the rolled sheet A on both surfaces of the rolled sheet E, the process of Example 1 was repeated to include a sintered three-layer porous PTFE membrane laminate having a thickness of about 43 μm. A filter medium ("filter medium 10") is obtained. The nonwoven fabric is laminated on both surfaces of the filter medium 10 by thermal lamination as in Example 1 to obtain another filter medium ("filter medium 11").

The rolled sheet alone is passed between metal rolls to obtain a continuous sheet having a thickness of 320 탆. In the same manner as in the production of the interlayer film A in Example 1, this molded article in the form of a sheet is treated with stretching or the like to obtain a porous PTFE film ("interlayer film C") having a thickness of about 10 m. For evaluation of the outermost layer in this example, the outermost layer film A is used in the same manner as in Example 1.

Comparative Example 3

100 parts by weight of fine PTFE powder (B) is uniformly mixed with 25 parts by weight of a liquid lubricant (liquid paraffin). The resulting mixture is preformed at 20 kg / cm 2 and then paste molded into rods by extrusion. The shaped article in the form of a rod is passed between a pair of metal rolls to obtain a continuous sheet having a thickness of 250 μm. From the molded article in the form of a sheet, liquid lubricant is removed by extrusion using triclen to obtain an unsintered PTFE sheet. This unsintered PTFE sheet is stretched by rolling 15 ply longitudinally at a stretching temperature of 290 ° C. Subsequently, the sheet is further stretched by tentering 30 plies in the transverse direction at a stretching temperature of 80 ° C. to obtain an unsintered porous PTFE membrane. This unsintered porous PTFE membrane was heated to 400 ° C. for 20 seconds while being dimensionally fixed to obtain a sintered porous PTFE membrane (“single layer porous PTFE membrane”) having a thickness of about 13 μm.

The above-described PET / PE core-shell nonwoven fabric having a thickness of 150 µm and a basis weight of 30 g / m 2 is sandwiched between the nonwoven fabrics by laminating on both surfaces of such a single layer porous PTFE membrane by thermal lamination (roll temperature: 140 ° C.). A filter medium ("filter medium 12") with a porous PTFE membrane laminate is obtained.

Table 1 shows the pressure drop and collection efficiencies of the filter media, outermost layer membranes, interlayer membranes and monolayer porous PTFE membranes obtained in the above examples and comparative examples.

Sample Pressure drop (mmH ₂ O) Collection efficiency (%) Pressure drop ratio of outermost layer and middle layer (pressure drop of outermost layer / pressure drop of middle layer) Example 1 Filter media (1) 37.3 99.999993               0.33 Outermost layer film (A)  7.3 90 Interlayer film (A) 22.4 99.9992 Example 2 Outermost layer film (A)  7.3 90              0.33 Interlayer film (A) 22.4 99.9992 Example 3 Filter media (4) 36.8 99.999996               0.33 Outermost layer film (B)  7.3 92 Interlayer film (B) 22.4 99.999 Example 4 Filter media (6) 32.8 99.99993               0.24 Outermost layer film (C)  5.3 78 Interlayer film (A) 22.4 99.9992 Comparative Example 1 Filter media (8) 51.6 > 99.999999               0.69 Outermost layer film (D) 15.5 99.94 Interlayer film (A) 22.4 99.992 Comparative Example 2 Filter media (10) 22.5 99.91               1.07 Outermost layer film (A)  7.3 90 Interlayer film (C)  6.8 85 Comparative Example 3 Single Layer Porous PTFE Membrane 37.8 99.999998               -

Table 2 shows the pressure drop, collection efficiency and PF values of the filter media obtained in the above examples and comparative examples.
Pressure drop (mmH ₂ O) Collection efficiency (%) % Increase in pressure drop during thermal lamination PF value Example 1 Filter media (1) 37.3 99.999993 8.0 17.9 Filter media (2) 40.3 99.999994 Example 2 Filter media (3) 39.8 99.999991 7.6 17.7 Example 3 Filter media (4) 36.8 99.999996 7.9 17.6 Filter media (5) 39.7 99.99999 Example 4 Filter media (6) 32.8 99.99993 7.9 18.1 Filter media (7) 35.4 99.99996 Comparative Example 1 Filter media (8) 51.6 > 99.999999 18.3 - Filter media (9) 61.0 > 99.999999 Comparative Example 2 Filter media (10) 22.5 99.91 8.2 12.3 Filter media (11) 24.3 99.9 Comparative Example 3 Single Layer Porous PTFE Membrane 37.8 99.999998 19.5 - Filter media (12) 45.2 > 99.999999

delete

The "% increase in pressure drop in thermal lamination" of Example 2 overlaps the pressure drop of the filter medium 4 and the outermost layer film A on both surfaces of the interlayer film A obtained in Example 1. Calculate from the pressure drop observed by

As shown in Table 2, each of the filter media (1) to (7) in which the pressure drop of the outermost layer film is adjusted to a level of 1/2 times or less of the pressure drop of the interlayer film is a thermal lamination of the nonwoven fabric used as the breathable support member. It shows a small increase in pressure drop (ie less than 10%), resulting in a small pressure drop (45 mmH ₂ O) of each filter medium. In addition, filter media 1 to 7 each exhibit a collection efficiency of at least 99.9999% and a PF value of greater than 17. In contrast, the filter media 9 and 12 show a large increase in pressure drop (greater than 18%) upon thermal lamination of the nonwoven fabric, and thus an excessively large pressure drop. The filter medium 11 exhibits a small pressure drop during thermal lamination but only achieves a low collection efficiency, thereby making the medium unsuitable as a filter medium.

Table 3 shows the leakage performance of the filter media obtained in the above examples and comparative examples.

Sample Leak Frequency (Leakage / Measurement Point) Example 1 Filter media (2) 0/60 Example 2 Filter media (3) 3/60 Example 3 Filter media (5) 8/60 Example 4 Filter media (7) 0/60 Comparative Example 1 Filter media (9) 15/60 Comparative Example 2 Filter media (11) 22/60 Comparative Example 3 Filter media (12) 33/60

As shown in Table 3, the filter media (2), (3), (5) and (7) are superior to the filter media (9), (11) and (12) also in the leakage performance.

As mentioned above, this invention uses the membrane which can fully reduce the influence of the process for obtaining the filter medium for air filters as a membrane located as outermost layer in the porous PTFE membrane laminated body which consists of three or more layers. This provides a filter medium for an air filter that can exhibit a collection performance inherent in porous PTFE membranes.

Claims (4)

A filter medium for an air filter comprising as a collection layer a laminate consisting of at least three layers of porous polytetrafluoroethylene membrane, wherein in the laminate, the first porous polytetrafluoroethylene membrane is positioned as the outermost layers and the second The porous polytetrafluoroethylene membrane is positioned as one or more intermediate layers except the outermost layers, the pressure drop of the first porous polytetrafluoroethylene membrane is 2 to 15 mmH ₂ O and the pressure drop of the second porous polytetrafluoroethylene membrane is 10 To 30 mmH ₂ O, wherein the pressure drop of the first porous polytetrafluoroethylene membrane is 1/2 or less times the pressure drop of the second porous polytetrafluoroethylene membrane. The filter medium of claim 1, further comprising a breathable support member adhered to both outermost layers of the laminate. The filter medium for air filters of Claim 2 whose pressure drop of a breathable support member is 1.15 times or less of the pressure drop of a laminated body. The filter medium for an air filter according to claim 2 or 3, wherein the breathable support member is a nonwoven fabric comprising a composite fiber having a core-shell structure in which the melting point of the core component is higher than the melting point of the shell component.

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