JP2020163311A - Method for manufacturing air filter medium - Google Patents

Abstract

To provide a method for manufacturing an air filter medium which can improve homogeneity of a pressure loss.SOLUTION: A method for manufacturing an air filter medium having a porous film of a fluorine resin includes a step of increasing a stretch length speed (m/min) in a predetermined stretch direction during stretching of the fluorine resin sheet in the predetermined stretch direction. Preferably, during the stretching of the fluorine resin sheet in the predetermined stretch direction, a change in the stretch length speed (m/min) in the predetermined stretch direction is continuous. More preferably, in the stretching of the fluorine resin sheet in the predetermined stretch direction, the stretching is performed in a state where the stretch length speed (m/min) in the predetermined stretch direction is lowered after the stretch length speed (m/min) in the predetermined stretch direction is increased and the sheet is stretched and before the stretching in the predetermined stretch direction is finished.SELECTED DRAWING: Figure 4

Description

The present disclosure relates to a method for producing an air filter filter medium.

Conventionally, polytetrafluoroethylene (hereinafter, may be simply referred to as PTFE) has been used for various purposes. For example, PTFE fine particles wet with a liquid lubricant are compressed into a rod shape, extruded from a ram extruder into a sheet shape, rolled to an appropriate thickness by a pair of rolls, stretched in the longitudinal direction, and further width. A porous membrane is formed by stretching in a direction or the like.

For example, in the method described in Patent Document 1 (Japanese Unexamined Patent Publication No. 2011-105895), in order to obtain a PTFE porous membrane having high strength and a small increase in pressure loss due to pressing, an unfired PTFE sheet is set to a temperature equal to or higher than the melting point of PTFE. At the temperature of, the sheet is stretched in the longitudinal direction at a stretching ratio of 50 to 100 times, and further, at a temperature below the melting point of PTFE, the sheet is stretched in the width direction at a stretching ratio of 4 to 10 times, and the stretching ratio and width in the longitudinal direction. A method for producing a PTFE porous membrane has been proposed in which the product with the draw ratio in the direction is 400 or more.

However, conventionally, in the porous membrane obtained by stretching, the homogeneity of the pressure loss may not be ensured in the entire surface through which the fluid to be treated passes, and there is room for improvement in the homogeneity.

The present disclosure has been made in view of the above points, and an object of the present disclosure is to provide a method for producing an air filter filter medium capable of improving the homogeneity of pressure loss.

The method for producing an air filter filter medium according to the first aspect is a method for producing an air filter filter medium having a porous film of fluororesin, which is a stretching length in a predetermined stretching direction while the fluororesin sheet is being stretched in a predetermined stretching direction. It is equipped with a step of increasing the speed (m / min).

Here, the fluororesin is not particularly limited, and examples thereof include polytetrafluoroethylene (PTFE).

In this method of manufacturing the air filter filter medium, the pressure loss of the obtained air filter filter medium is increased in order to increase the stretching length velocity (m / min) in the predetermined stretching direction while the fluororesin sheet is being stretched in the predetermined stretching direction. It becomes possible to improve the homogeneity.

The method for manufacturing the air filter filter medium according to the second aspect is the method for manufacturing the air filter filter medium according to the first aspect, and during stretching the fluororesin sheet in the predetermined stretching direction, the stretching length velocity in the predetermined stretching direction ( The change of m / min) is continuous.

This method for producing an air filter filter medium suppresses the sudden action of tension on the fibers and suppresses the breakage of the fibers constituting the porous membrane.

The method for producing the air filter filter medium according to the third aspect is the method for producing the air filter filter medium according to the first aspect or the second aspect, and in the stretching of the fluororesin sheet in the predetermined stretching direction, the stretching length in the predetermined stretching direction. After stretching at an increased rate (m / min) and before finishing stretching in a predetermined stretching direction, stretching is performed in a state where the stretching length velocity (m / min) in the predetermined stretching direction is decreased.

In this method for producing an air filter filter medium, the stretching is not completed immediately after the stretching in which the stretching length rate is increased, but the stretching is performed in a state where the stretching length rate (m / min) is lowered. Residual stress can be relaxed in the fibers constituting the film.

The method for producing the air filter filter medium according to the fourth aspect is the method for producing the air filter filter medium according to any one of the first to third viewpoints, and in the stretching of the fluororesin sheet in the predetermined stretching direction, the predetermined stretching direction. Stretching at a stretching length velocity (m / min) of 0.5 (m / min) or more and 4.0 (m / min) or less is performed for 0.5 seconds or longer, and then the stretching length in a predetermined stretching direction. Stretching at a speed (m / min) of 4.0 (m / min) or more and 12.0 (m / min) or less is performed for 1.0 second or more.

This method for producing an air filter filter medium makes it possible to secure a sufficient thickness and obtain a porous membrane having a low filling rate.

The method for manufacturing the air filter filter medium according to the fifth aspect is the method for producing the air filter filter medium according to any one of the first to fourth viewpoints, and the first stretching step of stretching the fluororesin sheet in the first direction is performed. After that, a second stretching step of stretching at least in the second direction intersecting the first direction is performed. Stretching of the fluororesin sheet in a predetermined stretching direction is stretching in the second direction in the second stretching step.

This method for producing an air filter filter medium can obtain a wide porous membrane with improved homogeneity of pressure loss.

The method for producing the air filter filter medium according to the sixth aspect is the method for producing the air filter filter medium according to any one of the first to fifth aspects, and produces a nodule having an average size larger than a circle having a diameter of 1 μm.

In this method for producing an air filter filter medium, since the obtained porous membrane has a nodule portion having an average size larger than a circle having a diameter of 1 μm, it is easy to secure a sufficient thickness of the porous membrane.

It is a schematic block diagram in the side view of the apparatus which stretches in a longitudinal direction. It is a schematic block diagram in the side view of the apparatus which stretches in a width direction. It is a schematic perspective block diagram of the apparatus which performs stretching in a width direction. It is a top view explaining the difference in the drawing speed in the width direction due to the difference in the line shape of a continuous clip. It is the schematic sectional drawing which shows the layer structure of the air filter filter medium. It is an external perspective view of a filter pack. It is an external perspective view of an air filter unit. It is an SEM photograph of the fluororesin multinomial film of Example 1.

Hereinafter, the method for manufacturing the air filter filter medium according to the present embodiment will be described with reference to examples.

(1) Method for manufacturing air filter filter medium In the method for manufacturing an air filter filter medium having a fluororesin porous film according to the present disclosure, the stretching length velocity in a predetermined stretching direction while the fluororesin sheet is being stretched in a predetermined stretching direction. It is provided with a step of increasing (m / min). Hereinafter, preparation of a fluororesin sheet using a raw material, stretching thereof, a porous film of the obtained fluororesin, and physical properties of an air filter filter medium will be described with reference to examples.

(1-1) Preparation of Fluororesin Sheet The fluororesin sheet used in the method for producing the air filter filter medium according to the present disclosure is not particularly limited as long as it is in the form of a sheet containing fluororesin.

The fluororesin sheet preferably mainly contains a fluororesin, and the type of the fluororesin contained may be only one type or two or more types. The fluororesin sheet may contain 50% by weight or more of the fluororesin, preferably 80% by weight or more of the fluororesin, and more preferably 95% by weight or more of the fluororesin. , It may be composed only of fluororesin.

Examples of the fluororesin contained in the fluororesin sheet include PTFE (corresponding to the component A described later) that can be fibrous. The fluororesin contained in the fluororesin sheet can be further subjected to fluororesin (corresponding to component B described later), which is a non-fibrotic non-thermal melt processable component, and / or, non-fibrotic heat melt process having a melting point of less than 320 ° C. Fluororesin (corresponding to the C component described later) may be contained.

When the fluororesin sheet contains a component different from that of the fluororesin, examples of the different component include thermosetting resin, inorganic filler, polystyrene, polyethylene terephthalate (PET), polyester, polyamide and the like.

The fluororesin sheet is capable of fibrous PTFE (hereinafter, also referred to as component A), a non-fibrous non-thermal melt processable component (hereinafter, also referred to as component B), and a non-fibrous heat melt process having a melting point of less than 320 ° C. It preferably contains a mixture of three components (hereinafter, also referred to as C component). According to the fluororesin sheet containing these three components, a porous film having many voids and a large film thickness can be obtained by stretching, and fine particles in the gas can be collected in a wide region in the thickness direction of the filter medium and maintained. A porous membrane having an excellent amount of dust can be obtained (see International Publication 2013/157647).

(1-1-1) Component A: PTFE that can be fibrous
The PTFE that can be fibrous is, for example, one that has stretchability and non-melt processability. In addition, "non-melt processability" means that since it has a high melt viscosity, it does not easily flow in a molten state and it is difficult to perform melt process. The PTFE which may be fiberized, it preferably has a melt viscosity at 380 ° C. is 1 × ¹⁰ 8 Pa · S or more.

The PTFE that can be fibrous is, for example, high molecular weight PTFE obtained from emulsion polymerization or suspension polymerization of tetrafluoroethylene (TFE). The high molecular weight referred to here is one that easily becomes fibrous during stretching during the formation of a porous film and can obtain fibrils having a long fiber length, and has a standard specific gravity (SSG) of 2.130 to 2.230. , A molecular weight having a size that does not substantially melt and flow due to its high melt viscosity. The SSG of PTFE that can be fibrous is preferably 2.130 to 2.190, and more preferably 2.140 to 2.170, from the viewpoint that it is easy to be fibrous and a fibril having a long fiber length can be obtained. If the SSG is too high, the stretchability may be deteriorated, and if the SSG is too low, the rollability may be deteriorated, the homogeneity of the porous film may be deteriorated, and the pressure loss of the porous film may be increased. The standard specific gravity (SSG) is measured according to ASTM D 4895.

Further, PTFE obtained by emulsion polymerization is preferable from the viewpoint of easily forming fibers and obtaining fibrils having a long fiber length. Emulsion polymerization can generally be carried out in TFE or an aqueous medium containing a monomer other than TFE and TFE, a dispersant and a polymerization initiator. It is preferable that the emulsion polymerization is carried out by gently stirring under the stirring conditions set so that the produced PTFE fine particles do not aggregate. In emulsion polymerization, the polymerization temperature is generally 20 to 100 ° C., preferably 50 to 85 ° C., and the polymerization pressure is generally 0.5 to 3.0 MPa. As the polymerization initiator in emulsion polymerization, a radical polymerization initiator, a redox-based polymerization initiator and the like are preferable. The smaller the amount of the polymerization initiator, the more the formation of low molecular weight PTFE is suppressed, and it is preferable that PTFE having a low SSG can be obtained. However, if the amount is too small, the polymerization rate tends to be too low, and if it is too large, the polymerization rate tends to be too low. PTFE with high SSG tends to be produced.

PTFE may constitute a fine powder obtained by emulsion polymerization. The fine powder can be obtained by recovering PTFE fine particles from the PTFE aqueous dispersion obtained by the above-mentioned emulsion polymerization, coagulating them, and then drying them. The fine powder made of PTFE has good extrusion processability, and for example, paste can be extruded at an extrusion pressure of 20 MPa or less. The extrusion pressure was measured when the paste was extruded through an orifice (diameter 2.5 cm, land length 1.1 cm, introduction angle 30 °) under the conditions of reduction ratio 100, extrusion speed 51 cm / min, and 25 ° C. It is a thing. In the paste extrusion molding, generally, the fine powder and an extrusion aid (lubricant) are mixed, then preformed and extruded. The extrusion aid is not particularly limited, and conventionally known ones can be used, but petroleum-based hydrocarbons having a boiling point of 150 ° C. or higher, such as naphtha, are preferable. The amount of the extrusion aid added can be an amount corresponding to 10 to 40% by mass of the total mass of the fine powder and the extrusion aid. Pre-molding and extrusion can be performed by conventionally known methods, and conditions can be appropriately selected.

The presence or absence of fibrosis, that is, whether or not it can be fibrous, can be determined by whether or not paste extrusion, which is a typical method for molding a high molecular weight PTFE powder made from a TFE polymer, is possible. Usually, paste extrusion is possible because the high molecular weight PTFE has fibrous properties. If the unbaked molded product obtained by paste extrusion does not have substantial strength or elongation, for example, if the elongation is 0% and it breaks when pulled, it can be considered to be non-fibrous.

The high molecular weight PTFE may be modified polytetrafluoroethylene (hereinafter referred to as modified PTFE), homopolytetrafluoroethylene (hereinafter referred to as homo-PTFE), or modified PTFE and homo PTFE. It may be a mixture. Homo PTFE is not particularly limited, and Japanese Patent Application Laid-Open No. 53-60979, Japanese Patent Application Laid-Open No. 57-135, Japanese Patent Application Laid-Open No. 61-16907, Japanese Patent Application Laid-Open No. 62-104816, Japanese Patent Application Laid-Open No. 62- 190206, Japanese Patent Application Laid-Open No. 63-137906, Japanese Patent Application Laid-Open No. 2000-143727, Japanese Patent Application Laid-Open No. 2002-201217, International Publication No. 2007/046345 Pamphlet, International Publication No. 2007/11829 Pamphlet, International Publication No. Homo PTFE disclosed in Pamphlet 2009/001894, Pamphlet 2010/1193950, Pamphlet 2013/027850, etc. can be preferably used. Among them, JP-A-57-135, JP-A-63-137906, JP-A-2000-143727, JP-A-2002-201217, and International Publication No. 2007/046345, which have high stretching characteristics, Homo PTFE disclosed in International Publication No. 2007/11929 pamphlet, International Publication No. 2010/1193950 pamphlet, etc. is preferable.

The modified PTFE is composed of TFE and a monomer other than TFE (hereinafter referred to as a modified monomer). Examples of the modified PTFE include those uniformly modified by a modified monomer, those modified at the initial stage of the polymerization reaction, those modified at the final stage of the polymerization reaction, and the like, but are not particularly limited thereto. The modified PTFE is preferably a TFE copolymer obtained by subjecting a trace amount of a monomer other than TFE to the polymerization together with TFE within a range that does not significantly impair the properties of the TFE homopolymer. The modified PTFE is, for example, JP-A-60-42446, JP-A-61-16907, JP-A-62-104816, JP-A-62-190206, JP-A-64-1711. , JP-A-2-261810, JP-A-11-240917, JP-A-11-240918, International Publication No. 2003/033555 Pamphlet, International Publication No. 2005/061567 Pamphlet, International Publication No. 2007/005361 Pamphlet, International Publication Those disclosed in Pamphlet No. 2011/055824, Pamphlet No. 2013/027850, etc. can be preferably used. Among them, Japanese Patent Application Laid-Open No. 61-16907, Japanese Patent Application Laid-Open No. 62-104816, Japanese Patent Application Laid-Open No. 64-1711, Japanese Patent Application Laid-Open No. 11-240917, International Publication No. 2003/033555 Pamphlet, International Publication No. Modified PTFE disclosed in Publication No. 2005/061567 Pamphlet, International Publication No. 2007/005361 Pamphlet, International Publication No. 2011/055824, etc. is preferable.

Modified PTFE includes a TFE unit based on TFE and a modified monomer unit based on a modified monomer. The modified monomer unit is a part of the molecular structure of the modified PTFE and is a portion derived from the modified monomer. The modified PTFE preferably contains 0.001 to 0.500% by weight of the total monomer unit of the modified monomer unit, and preferably 0.01 to 0.30% by weight. The total monomeric unit is the portion derived from all the monomers in the molecular structure of the modified PTFE.

The modified monomer is not particularly limited as long as it can be copolymerized with TFE, and is, for example, a perfluoroolefin such as hexafluoropropylene (HFP); a chlorofluoroolefin such as chlorotrifluoroethylene (CTFE); trifluoro. Hydrogen-containing fluoroolefins such as ethylene and vinylidene fluoride (VDF); perfluorovinyl ether; perfluoroalkylethylene (PFAE), ethylene and the like can be mentioned. The modified monomer used may be one kind or a plurality of kinds.

The perfluorovinyl ether is not particularly limited, and examples thereof include perfluorounsaturated compounds represented by the following general formula (1).

CF ₂ = CF-ORf ... (1)
In the formula, Rf represents a perfluoroorganic group.

In the present specification, the perfluoroorganic group is an organic group in which all hydrogen atoms bonded to carbon atoms are replaced with fluorine atoms. The perfluoroorganic group may have ether oxygen.

Examples of the perfluorovinyl ether include perfluoro (alkyl vinyl ether) (PAVE), which is a perfluoroalkyl group having Rf of 1 to 10 carbon atoms in the above general formula (1). The number of carbon atoms of the perfluoroalkyl group is preferably 1 to 5. Examples of the perfluoroalkyl group in PAVE include a perfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, a perfluorobutyl group, a perfluoropentyl group, a perfluorohexyl group and the like. As the PAVE, perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE) are preferable.

The perfluoroalkylethylene (PFAE) is not particularly limited, and examples thereof include perfluorobutylethylene (PFBE) and perfluorohexylethylene (PFHE).

The modified monomer in the modified PTFE is preferably at least one selected from the group consisting of HFP, CTFE, VDF, PAVE, PFAE and ethylene.

Homo PTFE is preferably contained in an amount of more than 50% by weight of PTFE that can be fibrous, particularly from the viewpoint of easily fibring and obtaining a fibril having a long fiber length.

The PTFE that can be fibrous may be a combination of a plurality of the above-mentioned components.

From the viewpoint of maintaining the fibrous structure of the porous membrane, PTFE that can be fibrous is preferably contained in an amount of more than 50% by weight of the porous membrane.

As the fluororesin sheet, not only the above-mentioned PTFE (A component) that can be fibrous, but also a non-fibrous non-thermal melt processable component (B component) and a non-fibrous heat melt processable component (C component) having a melting point of less than 320 ° C. ) Is also included, the following can be used as each B component and C component.

(1-1-2) Component B: Non-thermal melt-processable component that does not become fibrous The non-thermal melt-processable component that does not become fibrous is unevenly distributed as non-fibrous particles mainly at the nodules, and PTFE that can be fibrous is a fiber. It works to prevent it from being transformed.

Examples of non-thermal melt processable components that do not become fibrous include thermoplastic components such as low molecular weight PTFE, thermosetting resins, inorganic fillers, and mixtures thereof.

The thermoplastic component preferably has a melting point of 320 ° C. or higher and a high melt viscosity. For example, low molecular weight PTFE has a high melt viscosity, so that it can stay in the nodule even when processed at a temperature equal to or higher than the melting point. In the present specification, the low molecular weight PTFE, the number average molecular weight of 600,000 or less, a melting point of 320 ° C. or higher 335 ° C. or less, 380 melt viscosity at ° C. is ^{100Pa · s~7.0 × 10 5 Pa ·} s PTFE (See Japanese Patent Application Laid-Open No. 10-147617).

As a method for producing low molecular weight PTFE, high molecular weight PTFE powder (molding powder) obtained from suspension polymerization of TFE or high molecular weight PTFE powder (FP: fine powder) obtained from emulsion polymerization of TFE and a specific fluoride are used. A method of thermally decomposing by contact reaction at a high temperature (see JP-A-61-162503) and a method of irradiating the above-mentioned high-molecular-weight PTFE powder or molded body with ionizing radiation (see JP-A-48-78252). ), And a method of directly polymerizing TFE together with a chain transfer agent (see International Publication No. 2004/050727, International Publication No. 2009/020187, International Publication No. 2010/114033, etc.) and the like. The low molecular weight PTFE may be a homo-PTFE or a modified PTFE containing the above-mentioned modified monomer, like the PTFE that can be fibrous.

Low molecular weight PTFE is not fibrous. The presence or absence of fibrosis can be determined by the method described above. The low molecular weight PTFE has no substantial strength or elongation in the unbaked molded product obtained by paste extrusion, for example, the elongation is 0%, and it breaks when pulled.

The low molecular weight PTFE is not particularly limited, but the melt viscosity at 380 ° C. is preferably 1000 Pa · s or more, more preferably 5000 Pa · s or more, and further preferably 10,000 Pa · s or more. As described above, when the melt viscosity is high, even if the non-fibrous heat-meltable component melts as the C component during the production of the porous membrane, the non-fibrous non-heat-meltable component may remain in the nodule. It can suppress fibrosis.

Examples of the thermosetting resin include resins such as epoxy, silicone, polyester, polyurethane, polyimide, phenol, and mixtures thereof. As the thermosetting resin, a resin dispersed in water in an uncured state is preferably used from the viewpoint of workability of co-coagulation. All of these thermosetting resins can also be obtained as commercial products.

Examples of the inorganic filler include talc, mica, calcium silicate, glass fiber, calcium carbonate, magnesium carbonate, carbon fiber, barium sulfate, calcium sulfate, and a mixture thereof. Of these, talc is preferably used from the viewpoint of affinity and specific gravity with high molecular weight PTFE that can be fibrous. As the inorganic filler, those having a particle diameter of 3 μm or more and 20 μm or less are preferably used from the viewpoint of being able to form a stable dispersion during the production of a porous membrane. The particle size is an average particle size and is measured by a laser diffraction / scattering method. All of these inorganic fillers can also be obtained as commercial products.

The non-melt processable component that does not become fibrous may be a combination of a plurality of the above-mentioned components.

The non-thermal melt processable component that does not become fibrous is preferably contained in an amount of 1% by weight or more and 50% by weight or less of the porous membrane. When the content of the non-thermal melt processable component that does not become fibrous is 50% by weight or less, it is easy to maintain the fibrous structure of the porous membrane. The non-thermal melt processable component that does not become fibrous is preferably contained in an amount of 20% by weight or more and 40% by weight or less, and more preferably 30% by weight or less. When it is contained in an amount of 20% by weight or more and 40% by weight or less, the fibrosis of PTFE that can be fibrous can be more effectively suppressed.

(1-1-3) C component: a non-fibrous heat-meltable component having a melting point of less than 320 ° C. A non-fibrous heat-meltable component having a melting point of less than 320 ° C. (Also referred to as) has fluidity at the time of melting, so that it can be melted at the time of manufacturing (stretching) of the porous film and solidified at the nodule, increasing the strength of the entire porous film and being compressed in a subsequent step. Even if this happens, deterioration of filter performance can be suppressed.

The non-fibrous, heat-meltable component preferably exhibits a melt viscosity of less than 10,000 Pa · s at 380 ° C. The melting point of the non-fibrous heat-meltable component is heated to a temperature higher than the melting point at a temperature rise rate of 10 ° C./min by a differential scanning calorimeter (DSC), melted once completely, and melted at 10 ° C./min. After cooling to the following, the peak top of the heat of fusion curve obtained when the temperature is raised again at 10 ° C./min is used.

The heat-meltable components that do not become fibrous include heat-meltable fluoropolymers, polystyrene, polyethylene terephthalate (PET), polyester, polyamide, and other resins, or mixtures thereof, and the stretching temperature during the production of the porous film. Examples include those that can sufficiently exhibit the meltability and fluidity in the above. Among them, a heat-meltable fluoropolymer is preferable from the viewpoint of excellent heat resistance at the stretching temperature during the production of the porous film and excellent chemical resistance. The heat-meltable fluoropolymer has the following general formula (2).
RCF = CR ₂ ... (2)
(In the formula, R is H, F, Cl, an alkyl having 1 to 8 carbon atoms, an aryl having 6 to 8 carbon atoms, a cyclic alkyl having 3 to 10 carbon atoms, and 1 to 8 carbon atoms, respectively. It is selected from a number of perfluoroalkyls. In this case, all Rs may be the same, or any two Rs may be the same and the remaining one R may be different, all Rs. May be different from each other), and examples thereof include fluoropolymers containing copolymerization units derived from at least one fluorinated ethylenically unsaturated monomer, preferably two or more monomers.

Useful examples of the compound represented by the general formula (2) include, but are not limited to, perfluoroolefins such as fluoroethylene, VDF, trifluoroethylene, TFE and HFP, and chlorofluoroolefins such as CTFE and dichlorodifluoroethylene. Examples thereof include (perfluoroalkyl) ethylene such as PFBE and PFHE, perfluoro-1,3-dioxol and mixtures thereof.

Further, the fluoropolymer is composed of at least one kind of monomer represented by the general formula (2) and
The above general formula (1) and / or the following general formula (3)
R ₂ C = CR ₂ ... (3)
(In the formula, R is independently selected from H, Cl, an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 8 carbon atoms, and a cyclic alkyl group having 3 to 10 carbon atoms. In this case, all Rs may be the same, or any two or more Rs may be the same, and these two or more Rs may be different from the remaining other Rs, and all Rs are different from each other. The other Rs may also include copolymers derived from copolymerization with at least one copolymerizable comonomer represented by (if there is more than one, they may be different from each other).

A useful example of the compound represented by the general formula (1) is perfluoro (alkyl vinyl ether) (PAVE). As this PAVE, perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE) are preferable.

Useful examples of the compound represented by the general formula (3) include ethylene, propylene and the like.

More specific examples of fluoropolymers include polyfluoroethylene derived from the polymerization of fluoroethylene, polyvinylidene fluoride (PVDF) derived from the polymerization of vinylidene fluoride (VDF), and chlorotrifluoroethylene (CTFE). Polychlorotrifluoroethylene (PCTFE) derived from polymerization, a fluoropolymer derived from the copolymerization of two or more different monomers represented by the above general formula (2), and at least one kind of the above general formula (2). Included are fluoropolymers derived from the copolymerization of the monomer and at least one of the general formulas (1) and / or at least one of the monomers represented by the general formula (3).

Examples of such polymers are polymers with copolymer units derived from VDF and hexafluoropropylene (HFP), polymers derived from at least one copolymerizable comonomer (at least 3% by weight) other than TFE and TFE. Is. The latter type of fluoropolymer includes TFE / PAVE copolymer (PFA), TFE / PAVE / CTFE copolymer, TFE / HFP copolymer (FEP), TFE / ethylene copolymer (ETFE), TFE /. Examples thereof include HFP / ethylene copolymer (EFEP), TFE / VDF copolymer, TFE / VDF / HFP copolymer, TFE / VDF / CTFE copolymer and the like, or a mixture thereof.

The component that can be heat-melted without fiberization may be a combination of a plurality of the above-mentioned components.

The content of the non-fibrous heat-meltable component in the porous membrane is preferably 0.1% by weight or more and less than 20% by weight. When it is less than 20% by weight, it is possible to prevent the heat-meltable component that does not become fibrous from being dispersed in a portion other than the nodule portion in the porous membrane and increasing the pressure loss of the porous membrane. Further, when it is less than 20% by weight, it becomes easy to perform stretching at a high magnification of 40 times or more in the stretched area ratio. Since the content of the non-fibrous heat-meltable component in the porous membrane is 0.1% by weight or more, deterioration of the filter performance of the porous membrane is sufficiently suppressed even if a compressive force or the like is applied in a subsequent process. It will be easier. The content of the non-fibrous heat-meltable component in the porous membrane is preferably 15% by weight or less, and more preferably 10% by weight or less. Further, the content of the non-fibrous heat-meltable component in the porous membrane is preferably 0.5% by weight or more from the viewpoint of ensuring the strength of the porous membrane. Above all, it is particularly preferable that it is about 5% by weight.

The content of the heat-meltable component that does not become fibrous is preferably 10% by weight or less in order to perform good stretching at an elongation area magnification of 40 times or more and 800 times or less.

(1-1-4) Fluororesin Sheet The fluororesin used in the production of the fluororesin porous film can use the above-mentioned component A, but further contains three types of components including component B and component C. It is preferable to use it.

The form of the fluororesin is not particularly limited, and is, for example, a composition, a mixed powder, or a molding material. First, the composition, the mixed powder, and the molding material that are the raw materials of the porous membrane will be described.

The composition, the mixed powder, and the molding material all appropriately contain the above-mentioned A component, B component, and C component. Here, the C component is preferably contained, for example, 0.1% by weight or more and less than 20% by weight of the whole.

The molding material is, for example, a porous membrane molding material for molding a porous membrane used as a filter medium for a filter that collects fine particles in a gas.

The form of the raw material of the porous membrane may be a mixed powder, a non-powdered mixture, or a molding material or composition. Examples of the mixed powder include fine powder obtained by coagulation and cocoagulation, and a powder obtained by mixing two of the three raw materials by cocoagulation and mixing the other component using a mixer. Examples thereof include powder obtained by mixing three kinds of raw materials with a mixer. Examples of the non-powder mixture include a molded product such as a porous body (for example, a porous membrane) and an aqueous dispersion containing three kinds of components.

The molding material refers to a material that has been adjusted for processing in order to mold the composition, for example, a material to which a processing aid (liquid lubricant, etc.) has been added, a material having an adjusted particle size, or a preliminary material. It is made by performing various moldings. The molding material may contain, for example, known additives and the like in addition to the above three types of components. Examples of known additives include carbon materials such as carbon nanotubes and carbon black, pigments, photocatalysts, activated carbon, antibacterial agents, adsorbents, deodorants and the like.

The composition can be produced by various methods. For example, when the composition is a mixed powder, the powder of the component A, the powder of the component B, and the powder of the component C are mixed in a general mixer or the like. Method, method of obtaining co-coagulation powder by co-segregating three aqueous dispersions containing A component, B component, and C component, and water containing any two components of A component, B component, and C component. It can be produced by a method of mixing the mixed powder obtained by co-segregating the dispersion liquid in advance with the remaining one-component powder by a general mixer or the like. With such a method, a suitable stretched material can be obtained by any production method. Among them, the composition is obtained by co-coagulation of three aqueous dispersions containing the A component, the B component, and the C component, respectively, in that three different components are easily dispersed uniformly. Is preferable.

The size of the mixed powder obtained by coagulation and cocoagulation is not particularly limited, and for example, the average particle size is preferably 100 μm or more and 1000 μm or less, and preferably 300 μm or more and 800 μm or less. In this case, the average particle size is measured according to JIS K6891. The apparent density of the mixed powder obtained by coagulation and cocoagulation is not particularly limited, and is, for example, 0.40 g / ml or more and 0.60 g / ml or less, and 0.45 g / ml or more and 0.55 g / ml. The following is preferable. The apparent density is measured according to JIS K6892.

As the method of co-coagulation, for example, a method of mixing an aqueous dispersion of component A, an aqueous dispersion of component B, and an aqueous dispersion of component C and then coagulating is used to uniformly disperse the three components. It is preferable because it is easy to do.

The form of the component A before mixing is not particularly limited, but may be the above-mentioned aqueous dispersion of PTFE that can be fibrotic or a powder. Examples of the powder (particularly, the above-mentioned FP: fine powder) include "Teflon 6-J" (hereinafter, Teflon is a registered trademark), "Teflon 6C-J", and "Teflon 62-J" manufactured by Mitsui DuPont Fluorochemical. Etc., Daikin Industries, Ltd. "Polyflon F106" (hereinafter, Polyflon is a registered trademark), "Polyflon F104", "Polyflon F201", "Polyflon F302", etc., Asahi Glass Co., Ltd. "Furuon CD123" (hereinafter, Furuon is a registered trademark), " "Full-on CD1", "Full-on CD141", "Full-on CD145", etc., "Teflon 60", "Teflon 60 X", "Teflon 601A", "Teflon 601 X", "Teflon 613A", "Teflon 613A", "Teflon 613A", "Teflon 613A" Examples thereof include "Teflon 669 X". The fine powder may be obtained by coagulating and drying an aqueous dispersion of PTFE (an aqueous dispersion after polymerization) that can be fibrous obtained from the emulsion polymerization of TFE.

The aqueous dispersion of PTFE that can be fibrous may be the above-mentioned aqueous dispersion after polymerization, or a commercially available aqueous dispersion. A preferred method for producing an aqueous dispersion of PTFE that can be made into fibers after polymerization includes the production methods disclosed in the above-mentioned publications and the like listed as those that disclose homo-PTFE. Commercially available aqueous dispersions of PTFE that can be fibrous include Daikin Industries, Ltd. "Polyflon D-110", "Polyflon D-210", "Polyflon D-210C", "Polyflon D-310", etc. Examples thereof include aqueous dispersions such as "Teflon 31-JR" and "Teflon 34-JR" manufactured by Fluorochemical Co., Ltd. and "Furuon AD911L", "Furuon AD912L" and "AD938L" manufactured by Asahi Glass Co., Ltd.

The form of the component B before mixing is not particularly limited, but when the component B is low molecular weight PTFE, the form before mixing is not particularly limited, but it may be an aqueous dispersion or a powder (generally). It may be (called PTFE micropowder, or micropowder). Examples of the low molecular weight PTFE powder include "MP1300-J" manufactured by Mitsui DuPont Fluorochemical Co., Ltd., "Lubron L-5" manufactured by Daikin Industries, Ltd. (hereinafter, Lubron is a registered trademark), "Lubron L-5F" and the like. , Asahi Glass Co., Ltd. "Furuon L169J", "Furuon L170J", "Furuon L172J", etc., Kitamura Co., Ltd. "KTL-F", "KTL-500F" and the like.

The aqueous dispersion of low molecular weight PTFE may be a polymerized aqueous dispersion obtained from the emulsion polymerization of TFE described above, or a commercially available aqueous dispersion. In addition, micropowder dispersed in water by using a surfactant can also be used. Preferred methods for producing a PTFE aqueous dispersion liquid that can be made into fibers after polymerization include JP-A-7-165828, JP-A-10-147617, JP-A-2006-063140, JP-A-2009-1745, and International. Publication No. 2009/020187 The production method disclosed in the pamphlet and the like can be mentioned. Examples of the commercially available aqueous dispersion of PTFE that can be made into fibers include an aqueous dispersion such as "Lubron LDW-410" manufactured by Daikin Industries, Ltd.

Further, when an inorganic filler is used as the B component, the form before mixing is not particularly limited, but an aqueous dispersion is preferable. Examples of the inorganic filler include "talc P2" manufactured by Japan Talc Co., Ltd. and "LMR-100" manufactured by Fuji Tarku Kogyo Co., Ltd. These are used by appropriately performing surface treatment with a silane coupling agent or the like and dispersing the powder in water. Of these, a secondary pulverized product using a jet mill (such as "talc P2") is preferably used because of its dispersibility in water.

Examples of the C component include fluororesins such as FEP and PFA, and resins such as acrylic, urethane, and PET. The form before mixing is not particularly limited, but an aqueous dispersion is preferable. As the aqueous dispersion, in the case of a resin obtained by emulsion polymerization, the polymerized dispersion can be used as it is, or a resin powder dispersed in water using a surfactant or the like can also be used. A predetermined amount of the C component is dispersed in water so that the porous membrane contains 0.1% by weight or more and less than 20% by weight to prepare an aqueous dispersion.

The method of co-coagulation is not particularly limited, but it is preferable to mix the three aqueous dispersions and then apply a mechanical stirring force.

After coagulation and cocoagulation, dehydration and drying are performed, a liquid lubricant (extrusion aid) is mixed, and extrusion is performed.

The liquid lubricant is particularly limited as long as it is a substance that can wet the surface of the PTFE powder and can be removed after molding the product obtained by coagulation or cocoagulation into a film. Not done. Examples thereof include hydrocarbon oils such as liquid paraffin, naphtha, white oil, toluene and xylene, alcohols, ketones and esters.

The product obtained by coagulation and co-coagulation is mixed with a liquid lubricant and then extruded and rolled by a conventionally known method to form a film-like product. Extrusion can be carried out by paste extrusion, ram extrusion or the like, but is preferably carried out by paste extrusion. The sheet-shaped extruded product extruded by paste extrusion is rolled under heating, for example, under a temperature condition of 40 ° C. or higher and 80 ° C. or lower, using a calendar roll or the like. The thickness of the obtained rolled film-like product is set based on the thickness of the target porous film, and is usually 100 μm or more and 400 μm or less.

The liquid lubricant is then removed from the unfired film, which is a rolled product. The removal of the liquid lubricant is carried out by a heating method or an extraction method, or a combination thereof. The heating temperature in the case of the heating method is not particularly limited as long as it is lower than the melting point of the heat-melt processable component that does not become fibrous when the three components A to C are used, and is, for example, 100 ° C. or higher and 250 ° C. or lower. , 180 ° C. or higher and 200 ° C. or lower.

Here, from the viewpoint of reducing the filling rate of the obtained fluororesin porous film and reducing the pressure loss, the rolled product from which the liquid lubricant has been removed has a temperature atmosphere of 250 ° C. or higher and 325 ° C. or lower before stretching. It is preferable to perform a heat treatment of heating under the heat for 1 minute or more. The temperature of the heat treatment may be, for example, 320 ° C. or lower, preferably lower than the melting point of the fluororesin used for producing the fluororesin porous film, and the temperature of the rolled product is raised by using a differential scanning calorimeter. If multiple heat absorption curves (primary melting point, secondary melting point) appear on the crystal melting curve when the temperature is raised at a rate of 10 ° C./min, even if the temperature is lower than the maximum peak temperature (primary melting point). Good. Further, the temperature of the heat treatment may be, for example, 260 ° C. or higher or 280 ° C. or higher from the viewpoint of sufficiently reducing the filling rate and the pressure loss, and is heated from an unfired film which is a rolled product. It may be higher than the temperature at which the liquid lubricant is removed by the method, or may be higher than the stretching temperature (in the case of biaxial stretching, the temperature of the primary stretching performed first). The duration of the heat treatment is not particularly limited, but may be, for example, 1 minute or more and 2 hours or less, or 30 minutes or more and 1 hour or less, depending on the desired effect of the heat treatment.

As described above, the fluororesin sheet before stretching can be obtained.

(1-2) Stretching of Fluororesin Sheet A porous film can be obtained by stretching the fluororesin sheet before stretching obtained as described above in a predetermined direction orthogonal to the thickness direction.

Stretching includes stretching in a first direction and preferably in a second direction that intersects or is orthogonal to the first direction. From the viewpoint of being able to increase the total area magnification after stretching without breaking the fluororesin sheet and from the viewpoint of suppressing the increase in film thickness due to the alignment of the directions of the fibrils (fibers), the first direction is adopted. It is preferable to perform not only stretching in the second direction but also stretching in the second direction. Here, stretching in the first direction may be followed by stretching in the second direction, or stretching in the first direction and stretching in the second direction may be realized at the same time. After stretching in the first direction, stretching in the first direction and stretching in the second direction may be realized at the same time. In the present embodiment, the first direction can be the longitudinal direction of the rolled product (vertical direction: MD direction), and the second direction is the width direction of the rolled product (horizontal direction: TD direction). Can be done. In addition, the stretching may be carried out in a state where a plurality of fluororesin sheets are stacked and stretched at the same time.

Here, the stretching in the first direction can be, for example, 2 times or more and 20 times or less, and preferably 3 times or more and 10 times or less. Further, when stretching in the second direction is also performed, the stretching in the second direction is preferably larger than the magnification of stretching in the first direction, for example, 10 times or more and 50 times or less. It is preferable that the amount is 20 times or more and 45 times or less, and more preferably 30 times or more and 40 times or less. As a result, the fluororesin sheet can be stretched to a total area magnification of 40 times or more and 800 times or less through stretching in the first direction and stretching in the second direction. The multiple here indicates the state after stretching with reference to the state before stretching. For example, when a product having a length of 1 is stretched to a length of 3, it is tripled.

When the fluororesin sheet contains a heat-melt processable component that does not become fibrous and a non-heat-melt processable component that does not become fibrous, the non-thermal melt that is above the melting point of the non-fibrous heat-melt processable component and does not become fibrous. Stretching is performed at a temperature equal to or lower than the decomposition temperature of the processable component. When a heat-melt processable component that does not become fibrous is used in the production of the fluororesin porous film, the non-fibrous heat-melt processable component melts during this stretching process and later hardens at the nodule. The strength of the porous membrane in the thickness direction can be enhanced. The stretching temperature at this time may be set by the temperature of the furnace for stretching or the temperature of the heating roller for transporting the rolled product, or may be realized by combining these settings.

The temperature at the time of stretching in the first direction can be, for example, 200 ° C. or higher and 300 ° C. or lower, and preferably 230 ° C. or higher and 270 ° C. or lower. Further, when stretching in the second direction is also performed, the temperature at the time of stretching in the second direction can be 200 ° C. or higher and 310 ° C. or lower, preferably 250 ° C. or higher and 300 ° C. or lower. ..

From the viewpoint of further reducing the filling rate of the fluororesin porous film obtained by stretching, it is preferable to preheat the fluororesin sheet before stretching.

Here, regarding the stretching of the fluororesin sheet (also referred to as unfired fluororesin) obtained by rolling, the temperature at the time of stretching, the stretching ratio, and the stretching speed (stretching length velocity and stretching ratio velocity) are used. (Including) is known to affect the physical properties of the stretched product. The SS curve (graph showing the relationship between tensile tension and elongation) of the unfired fluororesin shows unique characteristics different from those of other resins. Normally, the tensile tension of a resin material increases as it stretches. While the range of elastic region, breaking point, etc. differ depending on the material and evaluation conditions, it is extremely general that the tensile tension tends to increase with the amount of elongation. On the other hand, in the unfired fluororesin, the tensile tension shows a peak at a certain elongation amount and then gradually decreases. This indicates that the unfired fluororesin has a "region in which the unstretched portion is stronger than the stretched portion".

Explaining the behavior that appears during stretching due to the above general properties of resin, in the case of general resin, the weakest part in the stretched surface begins to stretch during stretching, but the stretched portion is stretched. Since it becomes stronger than the non-existent portion, the next weak unstretched portion is stretched, so that the stretched region expands and tends to be stretched as a whole. On the other hand, to explain the behavior that appears during stretching due to the properties of the fluororesin unfired product, in the case of the fluororesin unfired product, the region where the stretched portion starts to grow is stronger in the unstretched portion than in the stretched portion. As a result, the already stretched portion is further stretched, and as a result, the unstretched portion tends to remain as a node (nodule, unstretched portion). When the stretching speed becomes slow, this phenomenon becomes remarkable, and the nodes (nodules, unstretched portions) tend to remain large. Further, as the stretching speed increases, the fibrils (fibers) tend to stretch so that the nodes (nodules, unstretched portions) become smaller.

Therefore, in the present embodiment, the stretching length velocity (m / min) in the predetermined stretching direction is increased while the fluororesin sheet is being stretched in the predetermined stretching direction. As a result, the stretching at a relatively slow stretching length rate (m / min) is performed first, so that the fibrils (fibers) are mainly stretched, and the nodes (nodules, unstretched parts) are considerably large. It is possible to leave up to. Later, by stretching at a relatively fast stretching length rate (m / min), fibrils (fibers) extend from the nodes (nodules, unstretched portions) that remain large, resulting in fibrils (fibers). ) Can be stretched while suppressing breakage, and the nodes (nodule, unstretched portion) do not become too small even at the stage of finishing the stretching, and the size of the node (nodule, unstretched portion) is reduced to some extent. It will be easier to secure. This suppresses "the node (nodule, unstretched portion) becomes smaller and makes it impossible to secure a large thickness of the porous membrane after stretching", and increases the thickness of the porous membrane after stretching to increase the thickness of the porous membrane. Can be reduced in density.

Compared with the above, when first stretching at a relatively slow stretching length velocity (m / min) and then subsequent stretching at a relatively slow stretching length velocity (m / min), mainly fibrils ( If the fiber) continues to stretch, it tends to break.

Further, as compared with the above, first, stretching at a relatively high stretching length rate (m / min) is performed, and then subsequent stretching at a relatively high stretching length rate (m / min) is performed. At the stage, a large number of relatively small nodes (nodule, unstretched part) are generated, and then each node (nodule, unstretched part) becomes smaller, so that the thickness of the obtained porous film is sufficient. Not only can the dust retention amount not be increased, but also the coefficient of variation CV of the pressure loss tends to be large because the film cannot be made thin.

Further, as compared with the above, if the stretching is first performed at a relatively high stretching length velocity (m / min) and then the stretching is performed at a relatively slow stretching length velocity (m / min). , A large number of relatively small nodes (nodule, unstretched part) are generated, so if you try to stretch more sufficiently from that state, you will no longer have a node (nodule, unstretched part) of a certain size. It is difficult to leave, and the thickness of the porous membrane becomes small.

Based on the above, in the present embodiment, the stretching length velocity (m / min) in the predetermined stretching direction is increased while the fluororesin sheet is being stretched in the predetermined stretching direction. Here, as the timing for increasing the stretching length velocity (m / min) in the predetermined stretching direction, for example, the stretching length velocity (m / min) may be increased during stretching in the first direction. Alternatively, when stretching in the second direction is performed, the stretching length velocity (m / min) may be increased during stretching in the second direction, or in the second direction. When the stretching is performed, the stretching length velocity (m / min) may be increased in each of the stretching in the first direction and the stretching in the second direction. In particular, when the two-step stretching is performed by the stretching in the second direction, the stretching length velocity (m / min) should be increased during the stretching in the second direction. It is preferable to do so. As a result, when the stretching length velocity (m / min) is increased during stretching in the second direction, the nodes (nodule portion, unstretched portion) generated by the stretching in the first direction have already been performed. ), The fibrils (fibers) can be stretched from the above () to suppress the breakage of the fibrils (fibers) caused by the stretching in the first direction, and the drawing with a sufficiently increased area magnification becomes possible. It is preferable that nodes (nodules, unstretched portions) having an average size larger than a circle having a diameter of 1 μm remain in the porous membrane obtained in the state where the stretching in the second direction is completed. Here, the presence or absence of nodes (nodules, unstretched portions) having an average size larger than a circle having a diameter of 1 μm can be confirmed by an SEM photograph of the fluororesin porous membrane. For example, even if it is judged by comparing the average area of a plurality of nodes (nodules, unstretched portions) existing in an arbitrary region of 15 μm × 15 μm in the fluororesin porous membrane with the area of a circle having a diameter of 1 μm. Good. It is preferable that more than half of the plurality of nodes (nodules, unstretched portions) existing in an arbitrary region of 15 μm × 15 μm in the fluororesin porous membrane is larger than a circle having a diameter of 1 μm.

It is sufficient that the stretching length velocity (m / min) increases during stretching in the predetermined stretching direction. For example, the stretching length velocity (m / min) in stretching in the second direction is increased in the first direction. It may or may not be increased above the stretching length velocity (m / min) in the stretching of. Among them, when the two-step stretching is performed by the stretching in the second direction, the node (nodule, unstretched) having a sufficient size when the stretching in the second direction is performed. From the viewpoint of leaving a portion), the stretching in the first direction is preferably performed at a relatively slow stretching length velocity (m / min). When the stretching length velocity (m / min) is increased in the stretching in the second direction, the stretching speed in the first direction is the high stretching length velocity (m / min) increased in the stretching in the second direction. ) Slower stretch length velocity (m / min), that is, a stretch length velocity (m / min) slower than the maximum stretch length velocity (m / min) in stretching in the second direction. It is preferable that this is done.

The process of increasing the stretching length rate (m / min) of the fluororesin sheet is performed during stretching by intermittently applying tension by stopping the stretching process and then restarting the stretching process. This is because a sudden tension acts on the fluororesin sheet at the time of restarting, which may cause the nodes (nodule portion, unstretched portion) to become smaller or break. Therefore, in the stretching step, it is preferable that the stretching length velocity (m / min) is continuously changed.

Further, after increasing the draw length velocity (m / min) as described above, the draw length velocity (m / min) is continuously increased from the viewpoint of relaxing the tension in the fibrils (fibers) and reducing the residual stress. Therefore, it is preferable to finish the stretching after stretching while gradually lowering.

In particular, when the fluororesin sheet wound in a roll shape is first stretched in the longitudinal direction and then stretched in the width direction, the stretching length velocity (m / min) is increased during stretching in the width direction. This is preferable in terms of the productivity of the porous membrane.

Further, in the above stretching, as long as the stretching length velocity (m / min) increases, the stretching ratio velocity (% / min) may or may not increase. The stretching length velocity (m / min) is the velocity related to the absolute value of the change in length in the stretching direction per unit time, and the stretching ratio velocity (% / min) is the length in the stretching direction per unit time. The speed with respect to the rate of change.

From the above, among the stretching of the fluororesin sheet in a predetermined direction, the stretching at a relatively slow speed before increasing the stretching length velocity (m / min) is, for example, 0.5 (m / min) or more. Stretching at a stretching length rate (m / min) in the range of 0 (m / min) or less is preferably performed for 0.5 seconds or longer, more preferably 1.0 seconds or longer, and 1.0 (m). Stretching at a stretching length velocity (m / min) in the range of / min) or more and 2.0 (m / min) or less is more preferably performed for 0.5 seconds or longer, and more preferably 1.0 seconds or longer. More preferred. Further, among the stretching of the fluororesin sheet in a predetermined direction, the stretching at a relatively high speed after increasing the stretching length velocity (m / min) is, for example, 4.0 (m / min) or more 12. Stretching at a stretching length rate (m / min) in the range of 0 (m / min) or less is preferably performed for 1.0 second or longer, more preferably 3.0 seconds or longer, and 6.0 (m). Stretching at a stretching length velocity (m / min) in the range of / min) or more and 10.0 (m / min) or less is more preferably performed for 1.0 second or longer, and more preferably 3.0 seconds or longer. More preferred. Further, in order to suppress the stress in the fluororesin porous film obtained by stretching to a small value, after stretching in a state where the stretching length velocity (m / min) is increased, for example, 1.0 (m / min) or more. Stretching at a stretching length velocity (m / min) in the range of 4.0 (m / min) or less is preferably performed for 0.5 seconds or more and 3.0 seconds or less.

The porous membrane thus obtained is preferably heat-fixed in order to obtain mechanical strength and dimensional stability. The temperature at the time of heat fixing is preferably 250 to 420 ° C, more preferably 350 to 400 ° C.

The fluororesin sheet wound in a roll shape can be stretched by using, for example, the apparatus shown in FIG. 1 and the apparatus shown in FIGS. 2 and 3.

Here, in the apparatus shown in FIG. 1, the fluororesin sheet wound in a roll shape is set as a roll 41, and the longitudinally stretched sheet stretched in the longitudinal direction (longitudinal direction) is wound on the winding roll 42. Taken. In addition, 43 to 45 are rolls, 46 and 47 are heat rolls, and 48 to 52 are rolls, respectively.

Next, the longitudinally stretched sheet is stretched in the width direction by the apparatus shown in FIGS. 2 and 3. Specifically, the wound longitudinally stretched sheet is set on the roll 61 and stretched in the width direction by the tenter 65 while being sequentially fed. In the tenter 65, both ends of the stretched sheet in the width direction are sandwiched by continuous clips (not shown) facing each other, and the stretched sheets are stretched in the width direction by being separated from each other while being fed out. To. The continuous clip is configured to be fed from upstream to downstream along a line that can be adjusted to a particular shape. The tenter 65 preheats the stretched sheet in the preheating region 62 on the upstream side, stretches the stretched sheet in the width direction while the stretched sheet is heated to a predetermined temperature in the intermediate stretching region 63, and heat-fixing region on the downstream side. In 64, the stretched sheet can be heat-fixed. The fluororesin porous membrane that has been heat-fixed is laminated with a breathable support membrane 33, which will be described later, as needed, and is thermally laminated by a heating roll 66 to become an air filter filter medium 30 and wound around a roll 67.

Here, for example, by changing the shape of the line for feeding the continuous clip described above, even if the supply speed of the stretched sheet fed from the roll 61 is maintained at a predetermined speed, the stretching speed in the width direction (stretching length speed). And the draw rate rate) can be adjusted. For example, as shown in the plan view of FIG. 4 showing the state of stretching in the width direction, the shape of the line that guides the continuous clip can be changed to each shape of (i), (ii), and (iii). it can. Here, in FIG. 4, as shown by a large arrow, the stretched sheet is fed at a predetermined speed from the left side to the right side. Further, in FIG. 4, as shown by a small arrow, the stretched sheet is stretched in the vertical direction, which is the width direction. Here, for example, as shown in (i), when the shape of the lines arranged so as to face each other is a linear shape, the stretch length in the width direction is long when the speed at which the stretch sheet is fed is constant. The speed (m / min) will also be constant. On the other hand, as shown by (ii), the shapes of the lines arranged so as to face each other are arranged so as to be close to each other in the upstream portion of the extension line and far from each other in the downstream portion of the extension line. In the case of the shapes arranged in such a manner, when the speed at which the stretched sheet is fed is constant, the stretch length speed (m / min) in the width direction is relatively slow in the initial stage, and is compared in the intermediate stage. It will be faster and slower again in the final stages. Further, as shown by (iii), when the shapes of the lines arranged so as to face each other are arranged so as to be far from each other in the upstream portion of the extension line, the speed at which the extension sheet is fed is increased. In certain cases, the stretch length velocity (m / min) in the width direction will be relatively high in the initial stage and relatively slow thereafter.

In any of the above cases, the stretching speed in the width direction can be increased as a whole by increasing the speed at which the stretched sheet is fed, and the stretching in the width direction can be increased by reducing the speed at which the stretched sheet is fed. It is possible to slow down the overall speed.

(2) Fluororesin Porous Membrane The fluororesin porous membrane produced as described above produces a porous membrane structure having fibrils (fibers) and nodes (knots, unstretched portions) connected to the fibrils. It becomes possible to make it. Then, this fluororesin porous membrane has excellent homogeneity of pressure loss evaluated by passing an air flow through a plurality of places.

The homogeneity of the pressure drop of the produced fluororesin porous film is, for example, that the coefficient of variation CV (coefficient of variation of pressure loss) obtained by dividing the standard deviation of the pressure loss distribution by the average value is 9.0% or less. Is more preferable, 8.5% or less is more preferable, and 8.0% or less is further preferable. This makes it possible to avoid a usage state in which a portion having a low pressure loss is intensively used (dust tends to collect in a portion having a low pressure loss). The lower limit of the coefficient of variation CV is not particularly limited, but is, for example, 1%.

The pressure loss distribution of the fluororesin porous membrane can be obtained, for example, by dividing the fluororesin porous membrane into 100 lattices and measuring the pressure loss in a region having a diameter of 100 mm in each lattice. The pressure loss is measured, for example, by using a measuring device equipped with a manometer for measuring both sides of the fluororesin porous membrane in a state of being close to the surface of the fluororesin porous membrane, the manometer determines on the surface on the downstream side of each region. It can be done by manipulating it to move along the route. Then, the coefficient of variation CV (%) can be obtained by calculating the standard deviation from the pressure loss distribution consisting of the measured pressure drops in each region and dividing this by the average value of the pressure losses in all the measured regions. it can. The size of the fluororesin porous membrane is not particularly limited, but is, for example, a length in the longitudinal direction of 100 to 1000 m and a length in the width direction of 600 to 2000 mm.

When the fluororesin porous membrane is produced using the above-mentioned three components as a raw material, the fibril is mainly composed of the A component, and the nodule is composed of the A to C components. Such nodules are formed relatively large in the porous membrane, thereby forming a thick porous membrane. In addition, such a nodule is relatively hard because it contains a component that does not become fibrous and can be heat-melted, and acts like a pillar that supports the porous membrane in the thickness direction. Even if a compressive force in the thickness direction is applied in a post-process such as pleating, it is possible to suppress deterioration of the filter performance of the porous membrane.

The film thickness of the fluororesin porous membrane can be, for example, 1.0 μm or more, preferably 10.0 μm or more, and more preferably 50.0 μm or more. By increasing the film thickness of the fluororesin porous film, it is possible to increase the amount of dust retention. The film thickness of the fluororesin porous membrane is, for example, 100.0 μm or less. When the film thickness is thin, the homogeneity of the pressure loss at a plurality of points in the porous membrane tends to be more problematic, but even in that case, the pressure loss is more likely to occur in the porous membrane produced as described above. Has excellent homogeneity. For the thickness of the fluororesin porous membrane, for example, a film thickness meter (1D-110MH type, manufactured by Mitutoyo Co., Ltd.) is used to measure the total film thickness by stacking five measurement targets and dividing the value by 5. The numerical value can be grasped as the film thickness of one sheet.

The fluororesin porous membrane may have a particle collection efficiency of 99.00% or more when air containing NaCl particles having a particle diameter of 0.1 μm is passed at a flow velocity of 5.3 cm / sec, and may be 99.99%. % Or more is preferable.

The fluororesin porous membrane preferably has a pressure loss of 250 Pa or less, and more preferably 20 Pa or more and 200 Pa or less.

(3) Breathable Support Membrane The fluororesin porous membrane is preferably used in a state in which the breathable support membrane is laminated.

The breathable support membrane can be laminated on the upstream side or the downstream side or both the upstream side and the downstream side of the fluororesin porous membrane to support the fluororesin porous membrane. Therefore, even if it is difficult to stand on its own due to the thin film thickness of the fluororesin porous membrane, it is possible to make the fluororesin porous membrane stand by supporting the breathable support membrane. In addition, the strength as an air filter filter medium is secured, and it becomes easy to handle.

The material and structure of the breathable support membrane are not particularly limited, and examples thereof include non-woven fabrics, woven fabrics, metal meshes, and resin nets. Of these, a non-woven fabric having heat-sealing properties is preferable from the viewpoints of strength, collectability, flexibility, and workability. The non-woven fabric is a non-woven fabric in which some or all of the constituent fibers have a core / sheath structure, a two-layer non-woven fabric consisting of two layers of a fiber layer made of a low melting point material and a fiber layer made of a high melting point material, and heat fusion to the surface. A non-woven fabric coated with a sex resin is preferable. Examples of such non-woven fabrics include spunbonded non-woven fabrics. Further, the non-woven fabric having a core / sheath structure preferably has a core component having a higher melting point than the sheath component. For example, as a combination of each material of the core / sheath, for example, PET / PE, high melting point polyester / low melting point polyester can be mentioned. Examples of the combination of the low melting point material / high melting point material of the two-layer non-woven fabric include PE / PET, PP / PET, PBT / PET, and low melting point PET / high melting point PET. Examples of the non-woven fabric in which the thermosetting resin is coated on the surface include a PET non-woven fabric coated with EVA (ethylene vinyl acetate copolymer resin) and a PET non-woven fabric coated with an olefin resin.

The material of the non-woven fabric is not particularly limited, and polyolefin (PE, PP, etc.), polyamide, polyester (PET, etc.), aromatic polyamide, or a composite material thereof can be used.

The breathable support membrane is made of fluorine by melting a part of the breathable support membrane by heating, or by melting the hot melt resin, utilizing the anchor effect, or using the adhesion of a reactive adhesive or the like. It can be bonded to a resin porous membrane.

The breathable support membrane has extremely low pressure loss, collection efficiency, and dust retention as compared with the above-mentioned fluororesin porous membrane, and may be considered to be substantially zero. The pressure loss of the breathable support membrane is, for example, preferably 10 Pa or less, more preferably 5 Pa or less, and further preferably 1 Pa or less. Further, each collection efficiency of NaCl having a particle size of 0.1 μm in the breathable support membrane may be, for example, substantially 0 or substantially 0. The thickness of the breathable support membrane is, for example, preferably 0.3 mm or less, and more preferably 0.25 mm or less. The basis weight of the breathable support membrane is preferably, for example, 20 g / m ² or more and 50 g / m ² or less.

(4) Air filter filter medium The air filter filter medium includes a fluororesin porous membrane, but the specific layer structure thereof is not particularly limited, and examples thereof include those shown below.

For example, as in the air filter filter medium 30 shown in FIG. 5, the fluororesin porous membrane 31 and the breathable support membrane 33 may be laminated in the air flow direction. The breathable support membrane 33 may be provided on the leeward side of the fluororesin porous membrane 31, may be provided on the leeward side, or may be provided on both the leeward side and the leeward side.

As the fluororesin porous membrane, those having the same physical properties or those having different physical properties may be laminated and used.

Further, the method of superimposing each of these films and layers is not particularly limited, and may be a bonding using the anchor effect due to partial melting by heating or melting of hot melt resin, or reactive adhesion. It may be bonded using an agent or the like, or it may be simply stacked. The thickness of each film or layer does not substantially change due to bonding.

For any of the above air filter filter media, the coefficient of variation CV of the pressure loss as the air filter filter medium (the coefficient of variation CV obtained by dividing the standard deviation of the pressure loss distribution of the air filter filter medium by the average value) is described above. Similar to the coefficient of variation CV of the fluororesin porous film, it is preferably 9.0% or less, more preferably 8.5% or less, and further preferably 8.0% or less. The lower limit of the coefficient of variation CV of the pressure loss as the air filter filter medium is not particularly limited, but is, for example, 1%.

The pressure loss of the air filter filter medium is not particularly limited, but is preferably 300 Pa or less, and more preferably 20 Pa or more and 250 Pa or less.

The PF value of the air filter filter medium is preferably 26.0 or more.

The thickness of the air filter filter medium is not particularly limited, but is preferably 50 μm or more and 1000 μm or less, and 400 μm or more and 800 μm or less.

(5) Filter Pack Next, the filter pack of the present embodiment will be described with reference to FIG.

FIG. 6 is an external perspective view of the filter pack 20 of the present embodiment.

The filter pack 20 includes the air filter filter medium described above (for example, the air filter filter medium 30). The air filter filter medium of the filter pack 20 is a processed filter medium processed (pleated) into a zigzag shape in which mountain folds and valley folds are alternately repeated. The pleating process can be performed by, for example, a rotary folding machine. The folding width of the filter medium is not particularly limited, but is, for example, 25 mm or more and 280 mm or less. Since the filter pack 20 is pleated, the folding area of the filter medium when used in the air filter unit can be increased, whereby an air filter unit with high collection efficiency can be obtained. ..

In addition to the filter medium, the filter pack 20 may further include a spacer (not shown) for holding the pleated spacing when used in the air filter unit. The material of the spacer is not particularly limited, but a hot melt resin can be preferably used. Further, the air filter filter medium 30 may have a plurality of embossed protrusions, and the pleated spacing may be maintained by the embossed protrusions.

(6) Air Filter Unit Next, the air filter unit 1 will be described with reference to FIG. 7.

FIG. 7 is an external perspective view of the air filter unit 1 of the present embodiment.

The air filter unit 1 includes the air filter filter medium or filter pack described above, and a frame body 25 for holding the air filter filter medium or filter pack. In other words, the air filter unit may be manufactured so that the filter medium which is not folded in a mountain fold or valley is held by the frame body, or the filter pack 20 may be manufactured so as to be held by the frame body 25. .. The air filter unit 1 shown in FIG. 7 is manufactured by using the filter pack 20 and the frame body 25.

The frame body 25 is made, for example, by combining plate materials or molding a resin, and the space between the filter pack 20 and the frame body 25 is preferably sealed with a sealant. The sealing agent is for preventing leakage between the filter pack 20 and the frame body 25, and for example, a resin material such as epoxy, acrylic, or urethane is used.

The air filter unit 1 including the filter pack 20 and the frame body 25 is a mini-pleated type air filter unit in which one filter pack 20 extending in a flat plate shape is held so as to be housed inside the frame body 25. It may be a V-bank type air filter unit or a single header type air filter unit in which a plurality of filter packs extending in a flat plate shape are arranged and held in a frame.

(7) Examples of Applications The air filter filter media, filter pack, and air filter unit manufactured by the method for manufacturing the air filter filter media according to the present embodiment are used for the following applications, for example.

ULPA filter (Ultra low Penetration Air Filter) (for semiconductor manufacturing), HEPA filter (for hospital, semiconductor manufacturing), cylindrical cartridge filter (for industrial use), bag filter (for industrial use), heat-resistant bag filter (for exhaust gas treatment), heat-resistant Pleated filter (for exhaust gas treatment), SINBRAN (registered trademark) filter (for industrial use), catalyst filter (for exhaust gas treatment), filter with HEPA (for HDD built-in), vent filter with HEPA (for HDD built-in), vent filter (For HDD built-in, etc.), Vacuum cleaner filter (for vacuum cleaner), general-purpose multi-layer felt material, gas turbine cartridge filter (for gas turbine compatible products), cooling filter (for electronic equipment housing), etc.

Ventilation / internal pressure adjustment for freeze-drying materials such as freeze-drying containers, automobile ventilation materials for electronic circuits and lamps, container applications such as container caps, protective ventilation applications for electronic devices, medical ventilation applications, etc. Field.

Hereinafter, the contents of the present disclosure will be specifically described with reference to Examples and Comparative Examples.

(Example 1)
Add 290 g of hydrocarbon oil ("IP Solvent 2028" manufactured by Idemitsu Kosan Co., Ltd.) as an extruded liquid lubricant per 1 kg of PTFE fine powder ("Polyflon fine powder F106" manufactured by Daikin Industries, Ltd.) with an average molecular weight of 6.5 million. And mixed. Next, the obtained mixture was extruded using a paste extruder to obtain a round bar-shaped molded product. This round bar-shaped molded body was molded into a sheet by a calendar roll heated to 70 ° C. to obtain a fluororesin sheet. The fluororesin sheet was passed through a hot air drying furnace at 250 ° C. to evaporate and remove the hydrocarbon oil to obtain a strip-shaped unfired fluororesin sheet having an average thickness of 200 μm and an average width of 150 mm.

Next, the unfired fluororesin sheet was stretched in the longitudinal direction at a stretching ratio of 5 times. The stretching temperature in stretching in the longitudinal direction was 250 ° C. Here, the stretching ratio rate (% / s) in the longitudinal direction was 150 (% / s).

Next, the stretched unfired fluororesin sheet was stretched in the width direction at a stretching ratio of 36 times using a tenter capable of continuously clipping. The stretching temperature in stretching in the width direction was 290 ° C.

Here, in the first embodiment, by adjusting the line shape of the tenter to a predetermined gently curved shape, the “extension length in the width direction” corresponding to the length speed at which the opposing continuous clips are separated from each other in the width direction. The initial, middle, and late stages of "velocity (m / min)" and "stretching rate in the width direction (% / s)" corresponding to the rate at which the opposing continuous clips separate from each other in the width direction. Stretching was performed in each of the steps according to the conditions shown in Table 1 below. The moving speed (including both vertical and horizontal components) of the continuous clip moving on the tenter line was 15 (m / min).

Here, the early, middle, and late stages in each table are divided into three stages of stretching in the width direction. The initial stage is a period from the start of stretching to a predetermined time, and the latter stage is a period from the end time of stretching to the end of stretching from a predetermined time before, in Examples 1 to 3 and Comparative Example 1. Then, it is set to 2 seconds in each case. The middle period is a period including an intermediate time point between the time when the drawing is started and the time when the drawing is finished, and is 2.5 seconds in Examples 1 to 3 and Comparative Example 1 (before and after the intermediate time point). 1.25 seconds). For example, the stretching length velocity (m / min) in the width direction in the initial stage of Example 1 is the distance between continuous clips facing each other in 2 seconds, and the distance is divided by 2 seconds. It is calculated by.

In each of the examples and comparative examples, the acceleration in the stretching step in the width direction was always maintained in the range of −0.08 (m / s ² ) or more and 0.08 (m / s ² ) or less.

Further, the fluororesin sheet that had been stretched was heat-fixed. The heat fixing temperature was 390 ° C. (atmospheric temperature around the sheet). As a result, a fluororesin porous membrane was obtained. In addition, about Example 1, the SEM (scanning electron microscope) photograph of the obtained fluororesin multinomial film is shown in FIG. Here, it was confirmed that nodes (nodules, unstretched portions) having an average size larger than a circle having a diameter of 1 μm were formed in the fluororesin porous membrane.

A breathable support membrane was bonded to both sides of the obtained fluororesin porous membrane by heat fusion using a laminating apparatus to obtain an air filter filter medium. As each of these breathable support membranes, a spunbonded non-woven fabric (average fiber diameter 24 μm, mesh size 40 g / m ² , thickness 0.2 mm) composed of core / sheath structure fibers using PET as a core and PE as a sheath is used. Using.

With respect to the air filter filter medium thus obtained, the pressure loss and the collection efficiency were measured, and the PF value was calculated. Moreover, the coefficient of variation CV of the pressure loss was measured. The thickness of the air filter filter medium of Example 1 was 10.0 μm.

(Example 2)
In Example 2, by adjusting the line shape of the tenter differently from that in Example 1, "stretch length velocity in the width direction (m / min)" and "stretch ratio velocity in the width direction (% / s)". Each of the above was the same as in Example 1 except that stretching was performed for each of the initial, middle, and late stages under the conditions shown in Table 2 below.

(Example 3)
In Example 3, by adjusting the line shape of the tenter differently from Examples 1 and 2, "stretch length velocity in the width direction (m / min)" and "stretch ratio velocity in the width direction (% / s)" ) ”, The same as in Example 1 except that stretching was performed for each of the initial, middle, and late stages under the conditions shown in Table 3 below.

(Comparative Example 1)
In Comparative Example 1, by making the line shape of the tenter a linear shape so that the continuous clips are separated toward the downstream side, "stretching length velocity in the width direction (m / min)" and "stretching ratio velocity in the width direction" (% / S) ”was the same as in Example 1 except that stretching was performed for each of the initial, middle, and late stages under the conditions shown in Table 4 below.

(Comparative Example 2)
In Comparative Example 2, the line shape of the tenter is the same as that of Comparative Example 1, and the moving speed of the continuous clips on the tenter line is made faster than that of Comparative Example 1, so that the “stretch length speed (m) in the width direction is increased. / Min) ”and“ stretching ratio rate in the width direction (% / s) ”are not stretched under the conditions shown in Table 5 below for each of the initial, middle, and late stages. Was the same as in Comparative Example 1. Specifically, in Examples 1 to 3 and Comparative Example 1, the speed at which the continuous clip moves on the tenter line (including both vertical and horizontal components) was 15 (m / min), but Comparative Example 2 Then, the speed at which the continuous clip moves on the line of the tenter (including both vertical and horizontal components) is set to 20 (m / min).

The physical properties measured in Examples and Comparative Examples are as follows.

(Pressure loss of air filter filter media)
A circular test sample having an effective area of 100 cm ² is taken out from an air filter filter medium having a fluororesin porous film, the test sample is set in a cylindrical filter filter medium holder, the inlet side is pressurized by a compressor, and air passes through the filter medium. The air flow was adjusted so that the speed was 5.3 cm / sec, the pressure was measured on the upstream side and the downstream side of the test sample using a manometer, and the difference in pressure between the upstream and downstream sides was determined as the pressure loss.

(Collection efficiency of air filter filter media (NaCl particles with a particle size of 0.1 μm))
According to the method described in JIS B9928 Annex 5 (Specification) NaCl aerosol generation method (pressurized spray method), the NaCl particles generated by the atomizer are subjected to an electrostatic classifier (manufactured by TSI) to obtain a particle size of 0. After classifying to 1 μm and neutralizing the particle charge with aerosol 241 the permeated flow rate was adjusted to 5.3 cm / sec, and using a particle counter (CNC manufactured by TSI), the filter medium as the measurement sample The number of particles before and after was calculated, and the collection efficiency was calculated by the following formula.

Collection efficiency (%) = (CO / CI) x 100
CO = Number of particles of NaCl 0.1 μm collected by the measurement sample CI = Number of particles of NaCl 0.1 μm supplied to the measurement sample (PF value of air filter filter medium)
The PF value of the air filter filter medium was calculated using the following relational expression with the above-mentioned collection efficiency and pressure loss.

PF value = {-log ((100-collection efficiency (%)) / 100)} / (pressure loss (Pa) / 1000)
(Floating coefficient of pressure loss of air filter filter media)
From a long filter medium (length 650 mm in the width direction) wound into a roll, a portion of about 5 m including the tip is pulled out, divided into 25 pieces every 200 mm in the longitudinal direction of the filter medium, and both ends in the width direction. The pressure loss was measured using a filter holder having a diameter of 100 mm at 100 grid-like locations divided into four pieces every 130 mm. The pressure loss here is measured by moving the filter medium in the longitudinal direction and continuously measuring a plurality of grid-like points by using a measuring device provided with five or more filter holders in the width direction of the filter medium. Was done by. Next, the standard deviation was obtained from the pressure loss distribution consisting of these measured pressure losses, and the coefficient of variation (%) was obtained by dividing the obtained standard deviation by the average value of the pressure losses at all the measured points.

The physical properties of the air filter filter media of each example and each comparative example are shown in the table below.

Although the embodiments of the present disclosure have been described above, it will be understood that various modifications of the forms and details are possible without departing from the purpose and scope of the present disclosure described in the claims. ..

1 Air filter unit 20 Filter pack 25 Frame body 30 Air filter Filter material 31 Fluororesin porous membrane 33 Breathable support membrane

Japanese Unexamined Patent Publication No. 2011-105895

Claims (6)

A method for manufacturing an air filter filter medium having a fluororesin porous membrane.
A step of increasing the stretching length velocity (m / min) in the predetermined stretching direction is provided while the fluororesin sheet is being stretched in the predetermined stretching direction.
A method for manufacturing an air filter filter medium. While the fluororesin sheet is being stretched in the predetermined stretching direction, the change in the stretching length velocity (m / min) in the predetermined stretching direction is continuous.
The method for producing an air filter filter medium according to claim 1. The fluororesin sheet is stretched in the predetermined stretching direction after the stretching length velocity (m / min) in the predetermined stretching direction is increased and before the stretching in the predetermined stretching direction is completed. Stretching is performed in a state where the stretching length velocity (m / min) in a predetermined stretching direction is reduced.
The method for producing an air filter filter medium according to claim 1 or 2. In stretching the fluororesin sheet in the predetermined stretching direction,
Stretching at a stretching length velocity (m / min) of 0.5 (m / min) or more and 4.0 (m / min) or less in the predetermined stretching direction is performed for 0.5 seconds or longer, and then,
Stretching at a stretching length velocity (m / min) of 4.0 (m / min) or more and 12.0 (m / min) or less in the predetermined stretching direction is performed for 1.0 second or longer.
The method for producing an air filter filter medium according to any one of claims 1 to 3. After performing the first stretching step of stretching the fluororesin sheet in the first direction, a second stretching step of stretching at least in the second direction intersecting with the first direction is performed.
The stretching of the fluororesin sheet in the predetermined stretching direction is the stretching in the second direction in the second stretching step.
The method for producing an air filter filter medium according to any one of claims 1 to 4. Produces nodules with average dimensions larger than circles with a diameter of 1 μm,
The method for producing an air filter filter medium according to any one of claims 1 to 5.

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