KR20000011539A - Filter medium for air filters - Google Patents

Abstract

PURPOSE: A filter medium for an air filter is provided not to be damaged after a pleating process and a process for sticking to a ventilating support member. CONSTITUTION: A filter medium(1) for an air filter includes: a first porosity poly tetra fluoro-ethylene film(4) being located as an extremely outer angled layer of a laminated body; a second porosity poly tetra fluoro-ethylene film(5) being located as more than one middle layer except the extremely outer angled layer of the laminated body; the pressure falling of the first porous poly tetra fluoro-ethylene film being less than half of the pressure falling of the second porous poly tetra fluoro-ethylene film; and a ventilating support member(3) being stuck to the both extremely outer angled layer of the laminated body. Therefore, the filter medium for the air filter represents an excellent processing property and manipulating characteristic.

Description

Filter medium for air filters

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

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

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 efficiency filter medium comprising a laminate comprising five layers of porous PTFE membrane 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 the 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, since the filter medium having the breathable support members located on both sides of the laminate is not adhered to the laminate, it does not suffer from any pressure drop. In this case, however, the laminate and the support member are not integrated, and thus the handling characteristics are poor. In this case also because the support member is not adhered to the laminate, this sometimes deviates from the pleat processing step, thereby causing undesirable bending in the laminate 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 comprising at least three porous PTFE membranes as a collecting layer, wherein the laminate excludes at least three first porous PTFE membranes and the outermost layer positioned as the outermost layer. And a second porous PTFE membrane positioned as at least one intermediate layer, 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.

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.

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

In the drawings, each reference numeral represents:

(1), (10): filter medium 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

In the filter medium for air filters according to the present invention, the first porous PTFE membrane positioned as the outermost layer reduces the influence of processing (processing to adhere to the breathable support member, pleating processing, etc.) to the second porous PTFE membrane positioned as the intermediate layer. To act. 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 in the pleat processing step, thus reducing the impact on the second porous PTFE membrane positioned as the intermediate layer.

In the filter medium for air filters according to the present invention, the breathable support member is preferably bonded 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 adhered to the laminate by an adhesive or the like, but also when bonded to the laminate, any excessive increase in pressure drop due to the adhesion of the breathable support member can be prevented. have. 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 includes 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 filters of FIG. 1, the breathable support members 3, 3 are bonded to the outermost layer of the laminate 2 of porous PTFE membrane. The laminate 2 comprises a porous PTFE membrane 4,4 positioned as the outermost layer and a porous PTFE membrane positioned as the intermediate layer. Porous PTFE membranes 4, 4 located as the outermost layer alleviate the effect of pleat processing on the adhesion and collection performance to the breathable support members 3, 3. It is also possible to use porous PTFE laminate 2 as the filter medium.

The porous PTFE membrane is formed into a sheet by extruding and / or rolling a mixture of PTFE fine powder and a liquid lubricant, to remove the liquid lubricant from the thus obtained unsintered sheet and then stretch the sheet to be porous. It can be obtained by. The strength of the membrane can be further enhanced by sintering this stretched sheet by heating above the melting point of PTFE. The porous PTFE membrane laminate 2 can be prepared by adding the step of laminating the porous PTFE membrane in a voluntary step in a conventional manner for producing the porous PTFE membrane. An example of a method for producing a porous PTFE membrane comprising a lamination step is described below.

1) A preform is manufactured in which a plurality of layers containing PTFE fine 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 subjected to continuous treatment by rolling, stretching or the like as in a conventional manufacturing method to obtain a 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 conditions used in the rolling and stretching steps and the materials used. In porous PTFE membrane laminates, materials and processing conditions are adjusted for each layer. Such adjustment is not particularly limited, but is performed by adjusting, for example, the molecular weight of the PTFE fine 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 using materials and rolling, elongation, etc. conditions used to make the laminate (ie, the first and second porous 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. 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 preferably 10 to 30 mmH ₂ O. It has been found that the pressure drop of the porous PTFE membrane provided as the outermost layer is preferably in the range of 10 to 50% of the pressure drop of the porous PTFE membrane 5 provided 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). For filter media 10 for air filters having two or more porous PTFE membranes 5 as the intermediate layer, the pressure drop of the porous PTFE membrane 4 as the outermost layer is the maximum pressure drop in the porous PTFE membrane 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 the above formula,

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 (in mmH ₂ O) measured in nanometers by permeating air at

E is the recovery efficiency, more specifically the polydisperse dioctyl having a particle size of 0.1 to 0.2 μm as an aerosol while placing the sample in the same holder as in the measurement of pressure drop and permeating air through the sample at the same plane velocity Phthalate (DOP) was fed at a rate providing a concentration of about 10 ⁸ particles / l, and then using a laser particle counter (LPC) to determine the particle concentration of the upstream portion and the particles of the downstream portion. It is a ratio calculated according to the following equation (2) by measuring the concentration.

In the above formula,

C _D is the particle concentration of the bottom stream portion,

C _U is the particle concentration of the upper stream portion.

The breathable support member 3 may be voluntary 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 adhesive 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 the breathability, thereby deteriorating the inherent performance of the porous PTFE membrane. In contrast, the laminate of the porous PTFE member described above can alleviate the effect of lowering breathability.

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

In the filter medium for the air filter, it is preferable that the porous PTFE members be adhered to each other, since 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 each method 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 pleated sample is measured in the same manner as described above. The particle concentration of the upper stream portion and the leaked particle concentration of the lower stream portion are measured with a particle counter, and the particle permeability is calculated according to the following formula (3).

In the above formula,

P _D means particle permeability of the lower stream portion,

P _U means the particle permeability of the upper stream portion.

The transmittance (P _0.1 ) of the particles having a particle size of 0.1 to 0.2 μm and the transmittance (P _0.2 ) of the particles having a particle size of 0.2 to 0.3 μm are measured. When the relationship of 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

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

Individually, another PTFE fine powder [Fluon CD-123, manufactured by Asahi-ICI Fluoropolymers; Hereinafter referred to as "PTFE fine powder (B)" 100 parts by weight are 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 extruded to paste mold into a rod. The rod-shaped shaped article is pressed between a pair of metal rolls to obtain a continuous sheet ("rolled sheet B") having a thickness of 300 mu m.

After the rolled sheet A is superimposed on both surfaces of the rolled sheet B, the resulting composite is passed through a pair of metal rolls and integrated to obtain a molded article in the form of a sheet having a thickness of 900 m. Extrusion with Triclene removes the liquid lubricant from the sheet 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. This three-layer unsintered porous PTFE membrane was heated to 400 ° C. for 20 seconds while being dimensionally fixed to filter media comprising a three-layer porous PTFE membrane laminate having a thickness of about 42 μm ["filter medium (1)"). ] Is obtained.

A pair of polyethylene terephthalate (PET) / polyester (PE) core-shell nonwoven fabric (ELEVES TO303WDO, manufactured by Unitika Ltd., melting point of the shell: 129 ° C.) having a thickness of 150 μm and a basis weight of 30 g / m 2. A filter medium having the same structure as shown in FIG. 1 in which the porous PTFE membrane laminate was sandwiched between nonwoven fabrics by laminating to both surfaces of the filter medium by thermal lamination using a roll (roll temperature: 140 ° C.) ["filter medium (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 mu m. The liquid lubricant is removed from the molded article, each drawn in the same manner as described above, and heat treated to obtain a porous PTFE membrane ("outer 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 drawn in the same manner as described above, and heat treated to obtain a porous PTFE membrane ("intermediate layer membrane (A)") having a thickness of about 8 µm.

Example 2

The outer layer film A is superimposed on both surfaces of the intermediate layer 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 sides 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 PTFE fine 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 extruded to paste mold into a rod. 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 mu m.

Individually, 100 parts by weight of PTFE fine 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 extruded to paste mold into a rod. 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 give an unsintered PTFE sheet. These unsintered PTFE sheets are stretched by rolling 20 plies in the longitudinal direction at a stretching 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) was superimposed on both surfaces of the stretched PTFE sheet (D) and the resulting composite sheet was stretched by tentering 30 plies in the transverse direction at a stretching temperature of 80 ° C. to obtain an unsintered porous PTFE membrane. do. This three-layer unsintered porous PTFE membrane was heated to 400 ° C. for 20 seconds while being dimensionally fixed to provide a desired filter medium comprising a sintered three layer porous PTFE membrane laminate having a thickness of about 39 μm [" Filter media 4 "].

The above-described PET / PE core-shell type nonwoven fabric having a thickness of 150 μm and a basis weight of 30 g / m 2 was laminated on both surfaces of the filter medium 4 by thermal lamination (roll temperature: 140 ° C.), so that A filter medium ("filter medium 5") with sandwiched porous PTFE membrane laminates 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 a porous PTFE membrane ["outer layer membrane ( B) "].

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

Example 4

100 parts by weight of PTFE fine 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 extruded to paste mold into a rod. 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. The molded article in the form of a sheet containing a liquid lubricant is 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 three layers of porous PTFE membrane laminates sintered by repeating the process of Example 1 except for using the stretched PTFE sheet as a replacement for 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 sheet-shaped molded article is treated by drawing or the like as in the case of producing the filter medium 6 to obtain a porous PTFE membrane ("outer 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

PTFE fine powder as a substitute for PTFE fine powder (A) [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 “PTFE fine (C)”]. Filter media 8 "]. 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 PTFE fine powder (C) alone was passed between metal rolls to obtain a continuous sheet having a thickness of 350 mu m. . This sheet-shaped molded article is treated with stretching or the like as in the manufacture of the filter medium 8 to obtain a porous PTFE membrane (“outer layer membrane D”) having a thickness of about 16 μm. In order to evaluate the intermediate layer in this example, the intermediate layer film (A) is used in the same manner as in Example 1.

Comparative Example 2

Repeating the manufacturing process of the rolled sheet B of Example 1, except that the PTFE fine powder (A) was used as a substitute for the PTFE fine powder (B), the continuous sheet having a thickness of 300 µm ["Rolled Sheet (E) ) "]. Filter comprising a sintered three-layer porous PTFE membrane laminate having a thickness of about 43 μm by repeating the process of Example 1 except for overlapping the rolled sheet A on both surfaces of the rolled sheet E. 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 mu m. In the same manner as in the production of the intermediate layer membrane A in Example 1, this molded article in the form of a sheet is treated by stretching or the like to obtain a porous PTFE membrane (“intermediate layer membrane C”) having a thickness of about 10 μm. For evaluation of the outer layer in this example, the outer layer film (A) is used in the same manner as in Example 1.

Comparative Example 3

100 parts by weight of PTFE fine 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 drawn by rolling 15 plies in the longitudinal direction at a draw temperature of 290 ° C. Subsequently, the sheet is further stretched by exposing 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 (“monolayer porous PTFE membrane”) having a thickness of about 13 μm.

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

Table 1 shows the pressure drop and collection efficiency of the filter media, outer layer membrane, middle layer membrane and single layer porous PTFE membrane obtained in the above examples and comparative examples.

Sample Pressure drop (mmH ₂ O) Collection efficiency (%) Pressure drop ratio of outer layer and middle layer (pressure drop of outer layer / pressure drop of middle layer) Example 1 Filter media (1) 37.3 99.999993 0.33 Outer layer membrane (A) 7.3 90 Middle layer film (A) 22.4 99.9992 Example 2 Outer layer membrane (A) 7.3 90 0.33 Middle layer film (A) 22.4 99.9992 Example 3 Filter media (4) 36.8 99.999996 0.33 Outer layer membrane (B) 7.3 92 Middle layer film (B) 22.4 99.999 Example 4 Filter media (6) 32.8 99.99993 0.24 Outer layer membrane (C) 5.3 78 Middle layer film (A) 22.4 99.9992 Comparative Example 1 Filter media (8) 51.6 〉 99.999999 0.69 Outer layer membrane (D) 15.5 99.94 Middle layer film (A) 22.4 99.992 Comparative Example 2 Filter media (10) 22.5 99.91 1.07 Outer layer membrane (A) 7.3 90 Middle layer film (C) 6.8 85 Comparative Example 3 Monolayer 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

The "% increase in pressure drop in thermal lamination" of Example 2 overlaps the pressure drop of the filter medium 4 and the outer layer membrane A on both surfaces of the intermediate layer membrane 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 outer layer membrane is adjusted to a level not more than 1/2 times the pressure drop of the middle layer membrane is a thermal lamination of the nonwoven fabric used as the breathable support member. A small increase in pressure drop (ie less than 10%) is shown, with the result that 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 99.99999% 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, filter media (2), (3), (5) and (7) are superior to filter media (9), (11) and (12) also in leakage performance.

As described above, the present invention provides a porous material by using a membrane capable of sufficiently alleviating the effect of processing for obtaining the filter medium for an air filter as a membrane located in a porous PTFE membrane laminate composed of at least three layers as the outermost layer. Filter media for air filters are provided that can exhibit inherent collection performance in PTFE membranes.

Claims (4)

The laminate comprises a first porous polytetrafluoroethylene membrane positioned as the outermost layer and a second porous polytetrafluoroethylene membrane positioned as one or more intermediate layers except for the outermost layer, wherein the first porous polytetrafluoroethylene A filter medium for an air filter comprising a laminate of at least three layers of a porous polytetrafluoroethylene membrane as a collecting layer, wherein the pressure drop of the ethylene membrane is 1/2 times or less of 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. 3. The filter medium for air filters according to claim 2, wherein the pressure drop of the breathable support member is not more than 1.15 times the pressure drop of the laminate. The filter medium for air filters 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.