JP7780307B2 - Air filter and its manufacturing method - Google Patents

Description

The present invention relates to an air filter that purifies air containing oil mist and dust, and a manufacturing method thereof. More specifically, it relates to an air filter in which a multifunctional coating film with weak hydrophilicity and oil repellency is formed on the fiber surface of a nonwoven fabric, and a manufacturing method thereof. In this specification, "weakly hydrophilic" refers to a property in which, after 0.1 mL of water is dropped from above onto a horizontally placed air filter, the water is retained for 60 seconds or more, and the water penetrates into the air filter after 1 hour.

When cutting machines, turning machines, and other machine tools that process metal products using cutting oil operate at high speeds, the cutting oil scatters, generating oil mist and dust. This oil mist and dust worsen the working environment and reduce work efficiency. For this reason, air filter media have been proposed that can purify air containing oil mist and dust, not only by filtering out dust suspended in the air, but also by preventing clogging caused by oil mist (see, for example, Patent Document 1 (Claim 1, paragraphs [0006], [0021], [0045], [0053] to [0060])).

This air filter medium includes a first PTFE (polytetrafluoroethylene) porous membrane and a second PTFE porous membrane, and the airflow passes through the first PTFE porous membrane and the second PTFE porous membrane in this order from the first main surface of the air filter medium to the second main surface of the air filter medium. The thickness of the first PTFE porous membrane is in the range of 4 to 40 μm, the specific surface area of the first PTFE porous membrane is 0.5 m ² /g or less, and the specific surface area of the second PTFE porous membrane is 1.5 to 10 m ² /g or less, which is larger than that of the first PTFE porous membrane.

The first and second porous PTFE membranes are each formed into a sheet-like compact from a mixture of PTFE fine powder and liquid lubricant. The first porous PTFE membrane is produced by heating and stretching the sheet-like compact in the longitudinal (MD) direction at a temperature above the melting point of PTFE (327°C) and at a stretching ratio of 50 times or more, and then heating and stretching it in the transverse (TD) direction at a temperature of 130-400°C to 5-8 times its original length. The second porous PTFE membrane is produced by heating and stretching a sheet-like compact of PTFE in the MD direction at a temperature below the melting point of PTFE (270-290°C) to 15-40 times its original length, and then heating and stretching it in the TD direction at a temperature of 120-130°C to 15-40 times its original length, the same stretching ratio as in the MD stretching.

Japanese Patent Application Laid-Open No. 2018-51546

In the air filter medium disclosed in Patent Document 1, the first PTFE porous membrane is produced at a higher stretching temperature and a larger stretch ratio than the second PTFE porous membrane, thereby reducing the specific surface area of the first PTFE porous membrane to 0.5 m ² /g or less, thereby capturing large particle diameter dust and oil mist, while the second PTFE porous membrane has a larger specific surface area of 1.5 to 10 m ² /g, thereby capturing small particle diameter dust and oil mist.

However, in the air filter medium disclosed in Patent Document 1, even though the first and second PTFE porous membranes capture dust particles and oil mist of different particle sizes, the PTFE porous membranes are prone to generating static electricity, which is difficult to remove, making it difficult to process into a filter shape. Furthermore, because the PTFE porous membrane is more water-repellent than oil-repellent, moisture in the air can clog the PTFE porous membrane, making it easy for dust particles to adhere to it. Therefore, with continued use of the air filter medium, oil mist continues to remain inside the air filter medium, easily clogging the air filter medium. As a result, the volume of air passing through the air filter is likely to decrease, posing the issue of having to frequently replace the air filter with a new one.

An object of the present invention is to provide an air filter that purifies air containing oil mist and dust and prevents clogging. Another object of the present invention is to provide a simple method for manufacturing an air filter that purifies air containing oil mist and dust and prevents clogging.

A first aspect of the present invention is an air filter comprising a nonwoven fabric having a large number of pores formed among fibers, penetrating between one side into which air containing oil mist and dust flows in and another side opposite to the one side through which the air flows out, wherein a multifunctional coating film having weak hydrophilicity and water repellency is formed on the fiber surface of the nonwoven fabric, the multifunctional coating film comprising metal oxide particles (B) having an average particle size of 2 nm to 90 nm and bonded to a fluorine-based functional group component (A) containing a perfluoroether structure represented by the following general formula (1) or formula (2), a self-reactive carboxyl group- and/or acetyl group-containing substance (C), and a thickener (D), wherein, when the multifunctional coating film is taken as 100 mass%, the fluorine-based functional group component (A) is contained in the multifunctional coating film in a proportion of 1.5 mass% to 15 mass%, and the thickener (D) is contained in a proportion of 0.3 mass% to 3.0 mass%, The fluorine-based functional group component (A) and the metal oxide particles (B) are contained in a total amount of 15 to 70% by mass when the multifunctional coating film is taken as 100% by mass, the mass ratio (A/B) of the fluorine-based functional group component (A) to the metal oxide particles (B) is in the range of 0.02 to 0.5, and the air permeability of the air filter is 1 ml/cm ² /sec to 30 ml/cm ² /sec.

In the above formulas (1) and (2), p, q, and r are each the same or different integers of 1 to 6, and the carbon skeleton may be linear or branched. Furthermore, in the above formulas (1) and (2), X is a hydrocarbon group having 2 to 10 carbon atoms, and may contain one or more bonds selected from an ether bond, a CO-NH bond, an O-CO-NH bond, and a sulfonamide bond. Furthermore, in the above formulas (1) and (2), Y is a silane hydrolyzate or the main component of a silica sol-gel.

Explaining Y further, Y is the moiety that bonds to the metal oxide particles (B). Specific examples include structures in which the Z moiety is hydrolyzed in Formula (3) or Formula (4) described below. Y also includes the main component of a silica sol-gel obtained by mixing a silane compound of Formula (3) or Formula (4) with a silicon alkoxide such as tetraethoxysilane or tetramethoxysilane and hydrolyzing and polymerizing the mixture. Y also includes the main component of a silica sol-gel obtained by mixing a silane compound of Formula (3) or Formula (4) with a silicon alkoxide such as tetraethoxysilane or tetramethoxysilane and a silane containing an epoxy group, a vinyl group, or an ether group and hydrolyzing and polymerizing the mixture. Incidentally, the carboxyl group-containing material (C) is used as a binder component for adhering the metal oxide particles (B) to which the fluorine-based functional group component (A) is bonded to the nonwoven fabric substrate.

A second aspect of the present invention is an air filter based on the first aspect, in which the metal oxide particles (B) are oxide particles of one metal selected from the group consisting of Si, Al, Mg, Ca, Ti, Zn, and Zr.

A third aspect of the present invention is an air filter based on the first aspect, in which the self-reactive carboxyl group-containing substance (C) is an ethylene-acrylic acid copolymer, an ethylene-vinyl acetate copolymer, or an ethylene-vinyl acetate-acrylic acid copolymer.

A fourth aspect of the present invention is an air filter based on the first aspect, in which the thickener (D) is a layered inorganic compound or a thickening polysaccharide.

A fifth aspect of the present invention is an air filter based on the first aspect, in which the nonwoven fabric is composed of a single layer or a laminate of multiple layers.

A sixth aspect of the present invention is an air filter based on either the first or fifth aspect, wherein the fibers constituting the nonwoven fabric are one or more types of fibers selected from the group consisting of polyethylene terephthalate (PET), polypropylene (PP), polytetrafluoroethylene (PTFE), glass, alumina, carbon, cellulose, pulp, nylon, and metal.

A seventh aspect of the present invention is a method for producing an air filter according to any one of the first to sixth aspects of the present invention, which method includes the steps of: preparing a multifunctional coating film-forming liquid composition (hereinafter , sometimes simply referred to as a liquid composition) by mixing an aqueous dispersion of fluorine-containing metal oxide particles, a self-reactive carboxyl group-containing substance, a mixed solvent of water and an alcohol having 1 to 4 carbon atoms, and an aqueous thickener solution or an aqueous thickener dispersion, as shown in FIG. 3 ; dipping a nonwoven fabric into the multifunctional coating film-forming liquid composition; and dewatering and drying the dipped nonwoven fabric.

An eighth aspect of the present invention is a method for producing an air filter based on the seventh aspect, in which the aqueous dispersion of fluorine-containing metal oxide particles is prepared by adding and mixing a fluorine-based compound to an aqueous dispersion of metal oxide particles.

An air filter according to a first aspect of the present invention comprises a nonwoven fabric comprising a multifunctional coating film formed on the fiber surface thereof, the multifunctional coating film comprising metal oxide particles (B) having an average particle size of 2 nm to 90 nm and having a fluorine-based functional group component (A) represented by the general formula (1) or (2) described above bonded thereto, a self-reactive carboxyl group-containing substance or the like (C), and a thickener (D), wherein the fluorine-based functional group component (A) accounts for 1.5 mass % to The multifunctional coating film contains the fluorine-based functional group component (A) in a proportion of 15% by mass, the thickener (D) in a proportion of 0.3% by mass to 3.0% by mass, the fluorine-based functional group component (A) and the metal oxide particles (B) are contained in a proportion of 15% by mass to 70% by mass when the total mass of the multifunctional coating film is 100% by mass, the mass ratio (A/B) of the fluorine-based functional group component (A) to the metal oxide particles (B) is in the range of 0.02 to 0.5, and the air permeability of the air filter is 1 ml/cm ² /sec to 30 ml/cm ² /sec. Therefore, when air containing oil mist and dust flows into the air filter from one side, the oil mist and dust are captured by the nonwoven fabric, and only the air passes through the pores of the nonwoven fabric and flows out from the other side of the air filter, making the air clean and preventing clogging.

At this time, due to the oil-repellent properties of the multifunctional coating and the air permeability of the air filter being 1 ml/ ^cm² /sec to 30 ml/ ^cm² /sec, the oil mist is not adsorbed by the multifunctional coating on the fiber surface of the nonwoven fabric but is instead repelled and adheres to it. As the amount of oil mist trapped inside the nonwoven fabric increases with continued use of the air filter, if the air filter is placed horizontally, the oil mist liquefies and is carried along with the passing air, collecting on the other side of the air filter. If the air filter is placed vertically, the captured oil mist collects at the bottom of the air filter due to its own weight, and does not block the pores of the nonwoven fabric. This prevents the oil mist from clogging the pores.

On the other hand, because the air permeability of the air filter is 1 ml/cm ² /sec to 30 ml/cm ² /sec, dust adheres either directly to the multifunctional coating on the fiber surface of the nonwoven fabric or to the oil mist that has adhered to the multifunctional coating. Therefore, when an air filter becomes clogged with dust or the like after long-term use, the dust that has adhered together with the oil mist can be easily removed by impacting the air filter with an air knocker or the like, and the air filter can be regenerated.

In the air filter of the second aspect of the present invention, the metal oxide particles (B) contained in the multifunctional coating are oxide particles of one metal selected from the group consisting of Si, Al, Mg, Ca, Ti, Zn, and Zr, making it possible to select a metal oxide suitable for the environment in which the air filter will be used from among the many types of metal oxide particles.

In the air filter of the third aspect of the present invention, the self-reactive carboxyl group-containing substance (C) is an ethylene-acrylic acid copolymer, an ethylene-vinyl acetate copolymer, or an ethylene-vinyl acetate-acrylic acid copolymer, so that the copolymer acts as a binder for the metal oxide particles and, when a multifunctional coating film is formed on the fiber surface of the nonwoven fabric, can firmly bind the film to the surface of the nonwoven fabric.

In the air filter of the fourth aspect of the present invention, the thickener (D) contained in the multifunctional coating film is a layered inorganic compound or a thickening polysaccharide, which increases the thickness of the multifunctional coating film and reduces the air permeability of the air filter, thereby preventing oil mist from passing through the air filter. It is also possible to impart weak hydrophilicity to the multifunctional coating film.

In the air filter of the fifth aspect of the present invention, if the nonwoven fabric is composed of a single layer, the air filter will have a simple configuration. If the nonwoven fabric is composed of a laminate of multiple layers, each layer can be configured according to the properties of the incoming dust particle size, the size of the oil particles in the oil mist, etc.

In the air filter of the sixth aspect of the present invention, the material of the fibers making up the nonwoven fabric can be selected from polyethylene terephthalate (PET), polypropylene (PP), polytetrafluoroethylene (PTFE), glass, alumina, carbon, cellulose, pulp, nylon, and metal, depending on the particle size of the inflowing dust, the size of the oil particles in the oil mist, and other properties.

In the method of the seventh aspect of the present invention, as shown in Figure 3, a multifunctional coating film-forming liquid composition is prepared by mixing an aqueous dispersion of fluorine-containing metal oxide particles, a self-reactive carboxyl group-containing material, a mixed solvent of water and an alcohol having 1 to 4 carbon atoms, and an aqueous thickener solution or thickener dispersion. An air filter is manufactured by dipping a nonwoven fabric into this multifunctional coating film-forming liquid composition, dewatering the nonwoven fabric, and drying it, thereby enabling the formation of a uniform multifunctional coating film on the fiber surface of the nonwoven fabric. Furthermore, the presence of fluorine-containing metal oxide particles in the carboxyl group-containing material and the inclusion of a thickener in the multifunctional coating film makes it easy to reduce the air permeability of the nonwoven fabric while maintaining weak hydrophilicity and oil repellency. Furthermore, unlike the PTFE porous membrane of Patent Document 1, the multifunctional coating is weakly hydrophilic. Therefore, firstly, static electricity is less likely to occur in the multifunctional coating, making it easier to manufacture air filters; secondly, by absorbing the water vapor contained in the oil mist, the multifunctional coating exhibits an oil-repellent effect, further improving the anti-clogging effect of the air filter.

In the air filter manufacturing method according to the eighth aspect of the present invention, a fluorine-based compound is added and mixed into an aqueous dispersion of metal oxide particles, thereby obtaining a dispersion in which fluorine-containing metal oxide particles are uniformly dispersed.

FIG. 1 is a side view of a single layer nonwoven fabric of the present embodiment. FIG. 2 is a side view of the two-layer nonwoven fabric of the present embodiment. FIG. 2 is a flow chart showing a method for manufacturing the air filter of the present embodiment.

Next, an embodiment of the present invention will be described with reference to the drawings.

[Air filter]
As shown in FIG. 1 , the air filter 10 of this embodiment comprises a nonwoven fabric 20 and a multifunctional coating 21 formed on the fiber surface of the nonwoven fabric, which has weak hydrophilicity and oil repellency. The nonwoven fabric 20, which is the main component of the air filter 10, is a single layer having a first surface 20a through which air containing oil mist and dust enters and a second surface 20b opposite to the first surface 20a through which the air exits. As shown in FIG. 2 , an air filter 50 may be constructed as a two-layer laminate consisting of an upper nonwoven fabric 30 and a lower nonwoven fabric 40. In this case, the upper surface of the upper nonwoven fabric 30 corresponds to the first surface 30a through which air containing oil mist and dust enters, and the lower surface of the lower nonwoven fabric 40 corresponds to the second surface 40b opposite the first surface 30a. The laminate is not limited to two layers, and may be constructed of multiple layers, such as three or four layers.

As shown in the enlarged view in the center of Figure 1, the nonwoven fabric 20 is formed by intertwining numerous fibers 20c, with pores 20d formed between the fibers. The pores 20d penetrate between the first surface 20a and the second surface 20b of the nonwoven fabric 20. A multifunctional coating film 21 is formed on the surface of the fibers 20c of the nonwoven fabric. The basis weight of the nonwoven fabric is preferably in the range of 100 g/ ^m2 to 400 g/ ^m2 , but is not limited to this range. The multifunctional coating film 21 contains metal oxide particles (B), a self-reactive carboxyl group-containing substance or the like (C) as a binder component, and a thickener (D). The metal oxide particles (B) are bonded to a fluorine-based functional group component (A) represented by the aforementioned general formula (1) or (2). When the multifunctional coating film is taken as 100% by mass, the fluorine-based functional group component (A) is contained in the multifunctional coating film 21 in a proportion of 1.5% by mass to 15% by mass, and the thickener (D) is contained in a proportion of 0.3% by mass to 3.0% by mass. Furthermore, when the multifunctional coating film is taken as 100% by mass, the fluorine-based functional group component (A) and the metal oxide particles (B) are contained in a total proportion of 15% by mass to 70% by mass. Furthermore, the mass ratio (A/B) of the fluorine-based functional group component (A) to the metal oxide particles (B) is in the range of 0.02 to 0.5.

As shown in the further enlarged view at the top of Figure 1, the multifunctional coating film 21 is composed of numerous metal oxide particles 21a, the particle surfaces of which are coated with fluorine-based functional group components, bound together by a binder component 21b, such as a carboxyl group-containing material. Because the multifunctional coating film 21 contains the metal oxide particles 21a and thickener (D), it appears thick, narrowing the pores 20d between the fibers. Furthermore, the film thickness can be controlled by varying the particle size of the metal oxide particles and the proportion of thickener in the film components. Furthermore, the inclusion of thickener (D) imparts weak hydrophilic properties to the multifunctional coating film, which, first, makes it less susceptible to static electricity generation, and, second, absorbs water vapor contained in oil mist, providing the multifunctional coating with an oil-repellent effect, further enhancing the anti-clogging effect of the air filter.

If the basis weight of the nonwoven fabric is less than 100 g/ ^m2 , the pores between the fibers will be too large, and the dust collection ability will be insufficient. If it exceeds 400 g/ ^m2 , the air permeability will be less than 1 ml/ ^cm2 /sec, and dust will easily clog the pores between the fibers, or the air permeability will be too low, causing pressure loss in the air filter due to resistance of the air sent into the air filter, and the efficiency of the air blowing energy will be easily reduced. It is more preferable that the basis weight of the nonwoven fabric is in the range of 200 g/ ^m2 to 350 g/ ^m2 .

In the state of the air filter 10 in which the multifunctional coating film 21 is formed on the fiber surface, the nonwoven fabric 20 is manufactured to have an air permeability of 1 ml/cm ² /sec to 30 ml/cm ² /sec. Air permeability less than 1 ml/cm ² /sec results in poor breathability, making it difficult for air containing oil mist and dust to pass through. Air permeability greater than 30 ml/cm ² /sec results in the size of the pores 20d of the nonwoven fabric being much larger than the particle sizes of the oil particles 22 and dust particles 23 of the oil mist in the incoming air, causing the oil particles 22 and dust particles 23 to pass through the pores of the nonwoven fabric along with the air, preventing the oil mist and dust from being captured. The air permeability is preferably 1.5 ml/cm ² /sec to 40 ml/cm ² /sec. Air permeability is measured using a Frazier-type testing machine as described in JIS-L1913:2000.

When the multifunctional coating film 21 is taken as 100% by mass, if the proportion of the fluorine-based functional group component (A) in the multifunctional coating film is less than 1.5% by mass, the oil repellency effect is poor and the ability to repel oil mist is insufficient. In other words, when oil mist reaches the air filter, it spreads over the fiber surface and is likely to clog the pores 20d. If the proportion of the fluorine-based functional group component (A) in the multifunctional coating film exceeds 15% by mass, the adhesion of the multifunctional coating film to the nonwoven fabric deteriorates. The content of the fluorine-based functional group component (A) in the multifunctional coating film 21 is preferably 1.2% to 12% by mass.

The thickener (D) contained in the multifunctional coating film 21 is contained in a proportion of 0.3% to 3.0% by mass, assuming the multifunctional coating film to be 100% by mass. If it is less than 0.3% by mass, the film will be water-repellent and will not exhibit weak hydrophilicity. If it exceeds 3.0% by mass, the oil-repellency of the multifunctional coating film will deteriorate. It is preferable that the thickener (D) contained in the multifunctional coating film 21 be 0.5% to 2.8% by mass.

The operation of this air filter 10 will now be explained. As shown in Figure 1, air containing oil mist and dust particles reaches one surface 20a of the nonwoven fabric 20 that constitutes the air filter 10. Because the air filter 10 has a predetermined air permeability and the multifunctional coating 21 exhibits oil repellency, oil particles 22 of the oil mist cannot pass through the air filter 10, even if they are larger than the pore size of the pores 20d, or even if they are slightly smaller than the pore size of the pores 20d. Instead, they are repelled by the multifunctional coating 21 and adhere to and remain on the multifunctional coating 21 between the fibers 20c of the nonwoven fabric 20. At the same time, dust particles 23 also adhere to and remain on the multifunctional coating 21. The inclusion of fluorine-containing metal oxide particles 21a in the multifunctional coating 21 creates an uneven film, which reduces the adhesion of oil particles 22 to the film while making it easier for dust particles 23 to adhere. As a result, oil particles 22 from the oil mist and dust particles 23 are captured by the nonwoven fabric, and the air containing the oil mist and dust passes through the pores 20d formed between the fibers 20c shown in the enlarged view of Figure 1 to reach the other surface 20b, where it becomes oil mist- and dust-free air and passes through the nonwoven fabric 20.

As the amount of oil mist trapped inside the nonwoven fabric increases with continued use of the air filter, if the air filter is positioned horizontally, oil mist that does not adhere to the membrane to a large extent liquefies and is carried along with the passing air, collecting on the other side of the air filter. If the air filter is positioned vertically, the captured oil mist collects at the bottom of the air filter due to its own weight and does not block the pores of the nonwoven fabric. This prevents the oil mist from clogging the pores. Dust adheres directly to the multifunctional coating on the fiber surface of the nonwoven fabric, or to the oil mist that has adhered to the multifunctional coating. Oil mist and dust that has accumulated in the nonwoven fabric 20 can be removed from the air filter 10 by periodically impacting it with an air knocker or similar device. Furthermore, the inclusion of a thickener imparts weak hydrophilic properties to the multifunctional coating, which, firstly, makes the multifunctional coating less prone to static electricity, and secondly, absorbs the water vapor contained in the oil mist, giving the multifunctional coating an oil-repellent effect and further improving the anti-clogging effect of the air filter.

[Air filter manufacturing method]
The air filter is generally manufactured by the following method.
As shown in FIG. 3 , an aqueous dispersion 54 of fluorine-containing metal oxide particles is prepared by mixing a fluorine-based compound 52 containing a fluorine-based functional group component (A) with an aqueous dispersion 51 of metal oxide particles. If the pH of the dispersion is neutral, the fluorine-based compound 52 can be mixed, followed by adding and mixing a catalyst 53 to prepare the aqueous dispersion 54 of fluorine-containing metal oxide particles. A carboxyl group-containing substance or the like 55 serving as a binder component is mixed with this aqueous dispersion 54 to prepare a mixture 56. A mixed solvent 57 of water and an alcohol having 1 to 4 carbon atoms is then added to dilute the mixture 56, and a thickener aqueous solution or thickener aqueous dispersion 58 is further added to prepare a multifunctional coating film-forming liquid composition 60. A nonwoven fabric 20 is dipped into this liquid composition 60. The nonwoven fabric 20 is then dewatered and dried to produce an air filter 10.

The method for manufacturing the air filter will be described in detail below.
[Preparing the nonwoven fabric]
First, a nonwoven fabric having an air permeability of 1.1 ml/cm ² /sec to 40 ml/cm ² /sec is prepared. Specifically, a nonwoven fabric having an air permeability of 1 ml/cm ² /sec to 30 ml/cm ² /sec is prepared in a state in which a multifunctional coating film, described below, is formed on the fiber surface of the nonwoven fabric to form an air filter. When the multifunctional coating film is to be formed into a thick film, a nonwoven fabric with high air permeability is selected, and when the multifunctional coating film is to be formed into a thin film, a nonwoven fabric with low air permeability is selected. The air permeability of the nonwoven fabric to be prepared is also selected depending on the content of a thickener that acts to thicken the multifunctional coating film.

Examples of such nonwoven fabrics include cellulose-mixed ester membrane filters, glass fiber filter paper, nonwoven fabrics made from a mixture of polyethylene terephthalate and glass fibers (product name: 336 or product name: 356, both manufactured by Azumi Filter Paper Co., Ltd.), nonwoven fabrics made from polyethylene terephthalate fibers (product name: G2260-1S, manufactured by Toray Industries, Inc., or product name: 191001, manufactured by Toyobo Co., Ltd.), nonwoven fabrics made from polypropylene fibers (product name: M03150, manufactured by Mitsui Chemicals, Inc.), and nonwoven fabrics made from metal fibers (manufactured by Nichie Techno Co., Ltd.). Nonwoven fabrics are made from one or more fibers selected from the group consisting of polyethylene terephthalate (PET), polypropylene (PP), polytetrafluoroethylene (PTFE), glass, alumina, carbon, cellulose, pulp, nylon, and metal. The fibers may also be a mixture of two or more fibers. The fiber thickness (fiber diameter) is preferably 0.01 μm to 10 μm to achieve the above-mentioned air permeability. The thickness of the nonwoven fabric is preferably 0.2 mm to 0.8 mm if the air filter is a single layer, and a thickness that results in a laminate thickness of 0.2 mm to 5 mm if the air filter is a multi-layer laminate.

[Method for producing a multifunctional coating film-forming liquid composition]
[Preparation of Aqueous Dispersion of Metal Oxide Particles]
First, an aqueous dispersion of metal oxide particles is prepared by dispersing metal oxide particles in an aqueous solvent. The metal oxide particles have an average particle size of 2 nm to 90 nm, preferably 2 nm to 85 nm. If the average particle size is less than 2 nm, the metal oxide particles are likely to aggregate and become difficult to disperse in the medium. If the average particle size exceeds 90 _nm , the metal oxide particles are likely to fall off from the multifunctional coating film when the liquid composition is formed into a film. Examples of metal oxide particles include particles of _SiO2 , _Al2O3 , MgO, CaO, _TiO2 , ZnO, and _ZrO2 . Examples of commercially available products include aqueous dispersions of silicon dioxide (trade name: ST-ZL (manufactured by Nissan Chemical Industries, Ltd., _SiO2 concentration 40%), trade name: ST-C (manufactured by Nissan Chemical Industries, Ltd., _SiO2 concentration 20%), trade name: ST-CM (manufactured by Nissan Chemical Industries, Ltd., _SiO2 concentration 30%)), aqueous dispersions of zirconium dioxide (trade name: SZR-W (manufactured by Sakai Chemical Industries, Ltd., _ZrO2 concentration 30%)), aqueous dispersions of titanium dioxide (trade name: TKS-203 (manufactured by Teika Corporation, _TiO2 concentration 20%)), and aqueous dispersions of zinc oxide (trade name: MZ-500 (manufactured by Teika Corporation, ZnO concentration 30%)).

Examples of aqueous solvents include water or a mixed solvent of water and an alcohol having 1 to 4 carbon atoms. It is preferable to use pure water such as ion-exchanged water or distilled water as the water to prevent the inclusion of impurities. The reason why an aqueous solvent is used instead of an organic solvent is to ensure safety in handling. In this specification, the average particle size of metal oxide particles refers to the average value of the particle sizes measured by image analysis at 200 points among the particle shapes observed with a scanning electron microscope (SEM).

[Preparation of Aqueous Dispersion of Fluorine-Containing Metal Oxide Particles]
Next, a fluorine-based compound containing a fluorine-based functional group component represented by the above formula (1) or (2) is added to the prepared aqueous dispersion of metal oxide particles to synthesize a nanocomposite material of the metal oxide particles and the fluorine-based functional group component. A catalyst may be added to further promote the reaction. This results in the preparation of an aqueous dispersion of fluorine-containing metal oxide particles.

When a catalyst is used, examples of the catalyst include organic acids, inorganic acids, alkalis, and titanium compounds. Examples of organic acids include formic acid and oxalic acid. Examples of inorganic acids include hydrochloric acid, nitric acid, and phosphoric acid. Examples of alkalis include sodium hydroxide, lithium hydroxide, magnesium hydroxide, potassium hydroxide, calcium hydroxide, and ammonia. Examples of titanium compounds include tetrapropoxytitanium, tetrabutoxytitanium, tetraisopropoxytitanium, and titanium lactate. Catalysts are not limited to those listed above.

Fluorine-based compounds containing fluorine-based functional group components are represented by the following general formula (3) or formula (4). More specific examples of the perfluoroether group in formula (3) or formula (4) include perfluoroether structures represented by the following formulas (5) to (13).

X in the above formulas (3) and (4) can include structures represented by the following formulas (14) to (18). Note that formula (14) below shows an example containing an ether bond, formula (15) below shows an ester bond, formula (16) below shows an amide bond, formula (17) below shows a urethane bond, and formula (18) below shows a sulfonamide bond.

In the above formulas (14) to (18), ^R2 and ^R3 are hydrocarbon groups having 0 to 10 carbon atoms, and ^R4 is a hydrogen atom or a hydrocarbon group having 1 to 6 carbon atoms. Examples of the hydrocarbon group for ^R3 include alkylene groups such as a methylene group and an ethylene group, and examples of the hydrocarbon group for ^R4 include alkyl groups such as a methyl group and an ethyl group, as well as a phenyl group.

In the above formulas (3) and (4), R ¹ may be a methyl group, an ethyl group, or the like.

In addition, in the above formulas (3) and (4), Z is not particularly limited as long as it is a hydrolyzable group that can be hydrolyzed to form an Si-O-Si bond. Specific examples of such hydrolyzable groups include alkoxy groups such as methoxy, ethoxy, propoxy, and butoxy; aryloxy groups such as phenoxy and naphthoxy; aralkyloxy groups such as benzyloxy and phenethyloxy; and acyloxy groups such as acetoxy, propionyloxy, butyryloxy, valeryloxy, pivaloyloxy, and benzoyloxy. Of these, the ethoxy group is preferred.

Specific examples of fluorine-based compounds containing a fluorine-based functional group component having a perfluoroether structure represented by formula (3) or (4) above include structures represented by formulas (19) to (27) below. In formulas (19) to (27) below, R represents a methyl group or an ethyl group.

As described above, the fluorine-based compound contained in the multifunctional coating film-forming liquid composition of this embodiment has a perfluoroether group in which multiple short-chain perfluoroalkyl groups and perfluoroalkylene groups with six or fewer carbon atoms are bonded to an oxygen atom within the molecule, and the high fluorine content within the molecule can impart excellent oil repellency to the formed film.

[Self-reactive carboxyl group-containing substances, etc.]
The self-reactive carboxyl group-containing material (C) as a binder component is an ethylene-acrylic acid copolymer, an ethylene-vinyl acetate copolymer, or an ethylene-vinyl acetate-acrylic acid copolymer. Commercially available ethylene-vinyl acetate-based products include Sepolsion VA406N and Sepolsion VA407N (all manufactured by Sumitomo Seika Chemicals), Sumikaflex S-201HQ, S-355HQ, S-401HQ, S-465HQ, S-951HQ, and S3950 (all manufactured by Sumika Chemtex Co., Ltd.), and Aquatex EC-1800 and EC-1200 (all manufactured by Japan Coating Resins Co., Ltd.). Examples of ethylene-acrylic acid copolymers include Zaixen A and Zaixen B. Examples of suitable acrylic resins include Xen L and Zaixen N (both manufactured by Sumitomo Seika Chemicals Co., Ltd.), and examples of ethylene-vinyl acetate-acrylic acid-based resins include Sumikaflex S-900HL (manufactured by Sumika Chemtex Co., Ltd.). Examples of suitable acrylic resins include Sumikaflex S-830, an ethylene-vinyl acetate-vinyl chloride copolymer, and Sumikaflex S-950HQ, an ethylene-vinyl acetate-vinyl versatate copolymer (both manufactured by Sumika Chemtex Co., Ltd.). Examples of suitable acrylic resins include TOCRYL BCX-1160R-2 (manufactured by Toyochem Co., Ltd.).

After preparing a mixed solution by mixing the above-mentioned self-reactive carboxyl group-containing material with an aqueous dispersion of fluorine-containing metal oxide particles, this mixed solution is diluted by mixing a mixed solvent of water and an alcohol having 1 to 4 carbon atoms. The mixed solvent contains 40% or less alcohol by mass. The reason for keeping the alcohol content at 40% or less by mass is to ensure safety in handling and the storage stability of the liquid composition. Furthermore, using a mixed solvent containing water and an alcohol having 1 to 4 carbon atoms improves the drying speed and film-forming properties. Examples of alcohols having 1 to 4 carbon atoms include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-2-propanol.

After diluting the mixture, an aqueous thickener solution or aqueous thickener dispersion is added and mixed. In addition to water-insoluble layered inorganic compounds, water-soluble thickening polysaccharides can also be used as thickeners. Aqueous thickener solutions are prepared by dissolving thickening polysaccharides in water, while aqueous thickening polysaccharide dispersions are prepared by dispersing layered inorganic compounds in water. Examples of water-soluble thickening polysaccharides include xanthan gum, hydroxyethyl cellulose, hydroxymethyl cellulose, carboxymethyl cellulose, starch, dextrin, carrageenan, guar gum, and pectin. Examples of water-insoluble layered inorganic compounds include bentonite (trade name: Moistonite S (manufactured by Kunimine Kogyo Co., Ltd.)), smectonite (trade name: Sumecton SA (manufactured by Kunimine Kogyo Co., Ltd.)), and hectorite (trade name: Laponite RDS (manufactured by Rockwood Co., Ltd.)). These layered inorganic compounds are particulate and preferably have an average particle size of 0.1 μm to 10 μm. If the average particle size is less than 0.1 μm or more than 10 μm, it is difficult to form an aqueous dispersion. An example of xanthan gum is Xanthan G (manufactured by Sansho Co., Ltd.). The thickener aqueous solution or thickener aqueous dispersion is prepared by mixing the above-mentioned thickener with water and stirring. The preferred content of the thickener is 0.08% to 0.45% by mass, assuming the multifunctional coating film-forming liquid composition to be 100% by mass.

[Multifunctional coating film forming liquid composition]
The multifunctional coating film-forming liquid composition of this embodiment is produced by the above-described production method and includes metal oxide particles (B) bonded to the fluorine-based functional group component (A), a self-reactive carboxyl group-containing material (C) or the like as a binder component, a mixed solvent, and a thickener aqueous solution or thickener aqueous dispersion. The fluorine-based functional group component (A) has a perfluoroether structure represented by the general formula (1) or (2) above. The liquid composition contains 1.5 to 15% by mass of the fluorine-based functional group component (A) when the total amount of all components excluding the solvent is taken as 100% by mass. If the fluorine-based functional group component is less than 1.5% by mass, the formed film cannot be imparted with oil repellency, while if it exceeds 15% by mass, the film will repel water, resulting in poor film-forming properties. The preferred content of the fluorine-based functional group component (A) is 2 to 13% by mass. The total content of the fluorine-based functional group component (A) and the metal oxide particles (B) is 15 to 70% by mass, preferably 20 to 65% by mass, when the total amount of all components excluding the solvent is taken as 100% by mass. Furthermore, the mass ratio of the fluorine-based functional group component (A) to the metal oxide particles (B) (A/B) is in the range of 0.02 to 0.5, preferably 0.05 to 0.4.

If the total amount of component (A) and particle (B) is less than 15% by mass, the oil-repellent properties of the multifunctional coating film will decrease. Furthermore, if the total amount exceeds 70% by mass, the content of the carboxyl group-containing substance (C) will be relatively low, and when the liquid composition is applied to a substrate, the multifunctional coating film will not adhere firmly to the fiber surface of the nonwoven fabric. Furthermore, if the mass ratio (A/B) is less than 0.02, the multifunctional coating film will have poor oil-repellent properties, and if it exceeds 0.5, the adhesion of the multifunctional coating film to the fiber surface of the nonwoven fabric will decrease.

[Method for forming a multifunctional coating film on the fiber surface of a nonwoven fabric]
To form a multifunctional coating film on the fiber surface of a nonwoven fabric according to this embodiment, the nonwoven fabric is dipped into a multifunctional coating film-forming liquid composition, removed from the diluted solution, and then spread on a horizontal wire mesh or the like in the air at room temperature to allow the nonwoven fabric to drain until a certain amount of liquid is reached. Alternatively, the removed nonwoven fabric can be shaken off to remove excess liquid, or the removed nonwoven fabric can be passed through a mangle roll (wringer) to drain the liquid. The drained nonwoven fabric is then dried in the air at a temperature of 25°C to 140°C for 0.5 to 24 hours. This results in the formation of a multifunctional coating film 21 on the surface of the fibers 20c constituting the nonwoven fabric 20, as shown in the enlarged view in the center of Figure 1. When the amount of liquid removed is small, a thick multifunctional coating film is formed on the fiber surface of the nonwoven fabric; when the amount of liquid removed is large, a thin multifunctional coating film is formed on the fiber surface of the nonwoven fabric.

Next, examples of the present invention will be described in detail along with comparative examples. First, Synthesis Examples 1 to 6 and Comparative Synthesis Examples 1 to 3, which prepare aqueous dispersions of fluorine-containing metal oxide particles, will be described. Next, Examples 1 to 7 and Comparative Examples 1 to 4, which relate to the preparation of multifunctional coating film-forming liquid compositions and the production of air filters using these Synthesis Examples and Comparative Synthesis Examples, will be described.

[Synthesis Examples 1 to 6 and Comparative Synthesis Examples 1 to 3 for Preparing Aqueous Dispersions of Fluorine-Containing Metal Oxide Particles]
<Synthesis Example 1>
To a beaker containing 50 g of an aqueous dispersion of silicon dioxide having an average particle size of 70 nm (product name: ST-ZL (Nissan Chemical Industries, Ltd., _SiO concentration 40%)), 2.00 g of the fluorine-based compound represented by formula (19) was added and mixed. The mixture was then mixed at 40°C for 2 hours to obtain an aqueous dispersion of silicon dioxide (silica) particles in which silicon dioxide particles were bonded to the fluorine-based compound (aqueous dispersion of fluorine-containing silica particles). The mass ratio (A/B) of the fluorine-based functional group component (A) to the silicon dioxide, or metal oxide particles (B), was 0.09.
Table 1 below shows the conditions for preparing the aqueous dispersion of fluorine-containing metal oxide particles in Synthesis Example 1.

<Synthesis Examples 2 to 6 and Comparative Synthesis Examples 1 to 3>
In Synthesis Examples 2 to 6 and Comparative Synthesis Examples 1 to 3, the type of metal oxide particles was changed to a different type from that in Synthesis Example 1, and the type of fluorine-based compound was changed to a different type from that in Synthesis Example 1. In Synthesis Examples 4 to 6 and Comparative Synthesis Example 1, the fluorine-based compounds were mixed, and then nitric acid was used as a catalyst. The amount of catalyst used was the value shown in Table 1. The mass ratio (A/B) of the fluorine-based functional group component (A) to the metal oxide particles (B) was changed to a different value from that in Synthesis Example 1. Otherwise, aqueous dispersions of fluorine-containing metal oxide particles in Synthesis Examples 2 to 6 and Comparative Synthesis Examples 1 to 3 were prepared in the same manner as in Synthesis Example 1, as shown in Table 1 above. Note that in Table 1, all of the R in the formulas of the fluorine-containing silanes represented by formulas (19) to (21) and (27) as the fluorine-based compounds is an ethyl group.

[Examples 1 to 7 and Comparative Examples 1 to 4 for the preparation of a multifunctional coating film-forming liquid composition and the production of an air filter]
Example 1
A mixed solution was prepared by mixing 13.0 g of the aqueous dispersion of fluorine-containing metal oxide particles obtained in Synthesis Example 1 with 10.00 g of an acetyl-containing ethylene-vinyl acetate-based carboxyl group-containing binder component (trade name: Sumikaflex S-355HQ (Sumitomo Chemical Co., Ltd.)). This mixed solution was mixed with 23.7 g of a mixed solvent (a solvent obtained by mixing 19.9 g of water and 3.8 g of industrial alcohol (trade name: Solmix AP-7 (Japan Alcohol Sales Co., Ltd.))) and an aqueous dispersion containing 0.55 g of a thickener (trade name: Moistonite S (Kunimine Industries Co., Ltd.)), to prepare a multifunctional coating film-forming liquid composition. The thickener was contained in this liquid composition in a proportion of 0.28% by mass, based on 100% by mass of the liquid composition. The base material for the air filter was a nonwoven fabric G2260-1S (manufactured by Toray Industries, Inc.) made of PET fiber and having an air permeability of 15 ml/ ^m² /s. The nonwoven fabric was dipped in the multifunctional coating film-forming liquid composition, excess liquid was shaken off, and the nonwoven fabric was dried at room temperature for 24 hours to produce an air filter with an air permeability of 6.0 ml/ ^cm² /s. The results are shown in Tables 2 to 4 below.

Table 2 shows the type and weight of the "fluorine-containing metal oxide particles" used to prepare the multifunctional coating film-forming liquid composition, the type and weight of the "self-reactive carboxyl group-containing substance (C)", details of the "mixed solvent", and the type, weight, and content in the liquid composition of the "thickener".
Table 3 shows the content of the "fluorine-based functional group component (A) in the liquid composition excluding the solvent," the content of the "carboxyl group-containing substance, etc. (C) excluding the solvent," and the content of the "total of the fluorine-based functional group component (A) excluding the solvent and the metal oxide particles (B)."

<Examples 2 to 7 and Comparative Examples 1 to 4>
As shown in Table 2, in Examples 2 to 7 and Comparative Examples 1 to 4, the respective weights were determined using the aqueous dispersions of fluorine-containing metal oxide particles obtained in Synthesis Examples 1 to 6 or Comparative Synthesis Examples 1 to 3 shown in Table 1. In Examples 2 to 7 and Comparative Examples 1 to 4, the respective weights were determined using binder components such as a self-reactive carboxyl group-containing substance, a mixed solvent, and a thickener, as shown in Table 2. The content of the thickener indicates the proportion of the thickener contained in the liquid composition when the liquid composition is taken as 100% by mass. In this manner, each of the multifunctional coating-forming liquid compositions of Examples 2 to 7 and Comparative Examples 1 to 4 was prepared.

Nonwoven fabrics with different breathability shown in Table 3 and the type of air filter substrate were selected, and the nonwoven fabric substrates were dipped in each of the multifunctional coating film-forming liquid compositions of Examples 2 to 7 and Comparative Examples 1 to 4 in the same manner as in Example 1, followed by dewatering and drying to obtain air filters with the properties shown in Table 4.

<Comparative testing and evaluation>
(1) Air Permeability The air permeability of the 11 types of air filters obtained in Examples 1 to 7 and Comparative Examples 1 to 4 was measured using the Frazier type testing machine described in JIS-L1913:2000, as described above.

(2) Oil Repellency The oil repellency of the 11 types of air filters obtained in Examples 1 to 7 and Comparative Examples 1 to 4 was measured and evaluated by the following method. To simulate the oil mist and dust dispersed from machine tools that process metal products using cutting oil, n-hexadecane and iron (III) oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were added to a planetary centrifugal mixer (ARE-310, manufactured by Thinky Corporation) in a mass ratio of 80:20 and stirred and mixed to obtain a simulated liquid. 1 ml of the resulting simulated liquid was dripped from above onto each of the 11 types of horizontally placed air filters obtained in Examples 1 to 7 and Comparative Examples 1 to 4, and the air filters were then stood vertically to check the ability of the simulated liquid to fall. If the simulated liquid passes through the air filter after being captured on the air filter, the oil repellency of the air filter is deemed to be "poor." If the simulated liquid passes through the air filter to a certain extent after being captured on the air filter and falls off the surface of the air filter, but the amount of the simulated liquid that falls is reduced, the oil repellency of the air filter is deemed to be "fairly good." If the simulated liquid is captured on the air filter but does not pass through the air filter and falls off the surface of the air filter, the oil repellency of the air filter is deemed to be "good."

(3) Weak Hydrophilicity After 0.1 mL of water was dropped from above onto each of the 11 types of horizontally placed air filters obtained in Examples 1 to 7 and Comparative Examples 1 to 4, the water was retained for 60 seconds or more. If the water had penetrated the air filter after one hour, the weak hydrophilicity was judged to be "good." If the water had penetrated the air filter after a retention time of less than 60 seconds, the air filter was judged to be hydrophilic. If the water was retained after one hour, the air filter was judged to be water-repellent, and the weak hydrophilicity was judged to be "poor."

(4) Film Strength The strength of the multifunctional coating film on the surface of the nonwoven fabric fibers constituting the 11 types of air filters obtained in Examples 1 to 7 and Comparative Examples 1 to 4 was measured in accordance with JIS K7136 using a static and dynamic friction tester (TL201Tt, manufactured by Trinity Labs). The measurement conditions were: contact: urethane rubber, load: 500 gf, movement speed: 50 mm/sec, and movement distance: 30 mm. After the contact was moved back and forth 10 times on the surface of the horizontally placed air filter, the simulant liquid used in measuring the oil repellency was dropped, and the oil repellency was evaluated using the same method as in (2) above. If the oil repellency did not change after the contact was moved, the film strength was considered "good." If the simulant liquid penetrated into the air filter and changed the oil repellency, the film strength was considered "poor."

As is clear from Table 3, the air filter of Comparative Example 1 was made by preparing a multifunctional coating film-forming liquid composition from Comparative Synthesis Example 1 containing metal oxide (silicon dioxide) particles with an average particle size of 230 nm, dipping a nonwoven fabric into this liquid composition, dewatering, and drying. As a result, the average particle size of the metal oxide particles was too large, making it difficult for the metal oxide particles to adhere to the fiber surface of the nonwoven fabric due to the binder component, such as a carboxyl group-containing substance, and the air permeability was too low. As a result, the simulated liquid did not fall off the air filter, and the oil repellency was "poor." Furthermore, the film strength was also "poor."

In the air filter of Comparative Example 2, the content of the fluorine-based functional group component (A) in the liquid composition excluding the solvent was too low at 0.4 mass%, and the "(A)/(B)" ratio in the aqueous dispersion of fluorine-containing metal oxide particles (aqueous dispersion of fluorine-containing silica particles) was 0.005, meaning the content of the fluorine-based functional group component was too low. Furthermore, the content of "(A) + (B)" in the liquid composition excluding the solvent was 80 mass%, meaning the content of carboxyl group-containing substances and other binders in the film was relatively low. As a result, the simulated liquid soaked into the air filter and did not fall off, resulting in a "poor" oil repellency. Furthermore, due to the low content of the binder component, the film strength was low and rated "poor."

In the air filter of Comparative Example 3, the ratio of the fluorine-based functional group component (A) to the metal oxide particles (B) was too high at 4.7% by mass, resulting in polymerization of only the fluorine-based functional group component, resulting in an inhomogeneous coating liquid. Furthermore, the content of "(A) + (B)" in the liquid composition excluding the solvent was 10% by mass, resulting in an insufficient amount of fluorine-based functional group component in the film. As a result, the multifunctional coating liquid composition was not uniformly applied to the nonwoven fabric, and the multifunctional coating film was not uniformly formed on the fiber surface of the nonwoven fabric. As a result, the multifunctional coating film became porous, resulting in an excessively high air permeability of 36.0 ml/cm2/sec. The multifunctional coating film did not exhibit oil repellency, and the simulated liquid passed through the air filter, resulting in a "poor" oil repellency rating. Furthermore, because the formed film was inhomogeneous, the weak hydrophilicity rating was "poor." Furthermore, for the same reasons as above, the film strength was low and rated "poor."

In the air filter of Comparative Example 4, the amount of thickener added was too high at 9.1% by mass, resulting in a "good" weak hydrophilicity rating. However, the fluorine-based functional group component was too high at 16.6% by mass, resulting in poor adhesion of the multifunctional coating to the nonwoven fabric and a "poor" oil repellency rating. Furthermore, because the amount of fluorine-based functional group component was too high, the film was uneven for the same reason as in Comparative Example 3, resulting in weak film strength and a "poor" rating.

In contrast, the air filters of Examples 1 to 7 fulfilled the scope of the first aspect of the invention, and as such, the simulated liquid was captured and fell off the air filter, and all of the oil repellency was "good." Furthermore, because the multifunctional coating film was uniformly formed, all of the weak hydrophilicity was "good," and furthermore, all of the film strength was "good."

The air filter of the present invention is used in work environments where machine tools such as cutting machines and turning machines are used to process metal products using cutting oil.

REFERENCE SIGNS LIST 10 Air filter 20 Nonwoven fabric 20a One surface of nonwoven fabric 20b Other surface of nonwoven fabric 20c Fiber of nonwoven fabric 20d Pores of nonwoven fabric 21 Multifunctional coating film 21a Fluorine-containing metal oxide particles 21b Carboxyl group-containing substance, etc. 22 Oil particles of oil mist 23 Dust particles

Claims (8)

An air filter including a nonwoven fabric having a large number of pores formed between fibers, the pores penetrating between one side into which air containing oil mist and dust flows in and another side opposite to the one side through which the air flows out,
a multifunctional coating film having weak hydrophilicity and water repellency is formed on the fiber surface of the nonwoven fabric,
The multifunctional coating film comprises metal oxide particles (B) having an average particle size of 2 nm to 90 nm and bonded to a fluorine-based functional group component (A) containing a perfluoroether structure represented by the following general formula (1) or (2), a self-reactive carboxyl group- and/or acetyl group-containing substance (C), and a thickener (D),
When the multifunctional coating film is taken as 100% by mass, the fluorine-based functional group component (A) is contained in the multifunctional coating film in a proportion of 1.5% by mass to 15% by mass, and the thickener (D) is contained in a proportion of 0.3% by mass to 3.0% by mass,
the fluorine-based functional group component (A) and the metal oxide particles (B) are contained in a total amount of 15% by mass to 70% by mass when the multifunctional coating film is taken as 100% by mass,
the mass ratio (A/B) of the fluorine-based functional group component (A) to the metal oxide particles (B) is in the range of 0.02 to 0.5;
The air filter has an air permeability of 1 ml/cm ² /sec to 30 ml/cm ² /sec.
In the above formulas (1) and (2), p, q, and r are each the same or different and are integers of 1 to 6, and the carbon skeleton may be linear or branched. Furthermore, in the above formulas (1) and (2), X is a hydrocarbon group having 2 to 10 carbon atoms and may contain one or more bonds selected from an ether bond, a CO—NH bond, an O—CO—NH bond, and a sulfonamide bond. Furthermore, in the above formulas (1) and (2), Y is a silane hydrolyzate or a main component of a silica sol-gel. The air filter according to claim 1, wherein the metal oxide particles (B) are oxide particles of one metal selected from the group consisting of Si, Al, Mg, Ca, Ti, Zn, and Zr. The air filter according to claim 1, wherein the self-reactive carboxyl group and/or acetyl group-containing material (C) is an ethylene-acrylic acid copolymer, an ethylene-vinyl acetate copolymer, or an ethylene-vinyl acetate-acrylic acid copolymer. The air filter according to claim 1, wherein the thickener (D) is a layered inorganic compound or a thickening polysaccharide. The air filter of claim 1, wherein the nonwoven fabric is composed of a single layer or a laminate of multiple layers. An air filter according to claim 1 or 5, wherein the fibers constituting the nonwoven fabric are one or more types of fibers selected from the group consisting of polyethylene terephthalate (PET), polypropylene (PP), polytetrafluoroethylene (PTFE), glass, alumina, carbon, cellulose, pulp, nylon, and metal. An air filter manufacturing method for manufacturing the air filter according to any one of claims 1 to 6, comprising:
a step of preparing a multifunctional coating film-forming liquid composition by mixing an aqueous dispersion of fluorine-containing metal oxide particles, a self-reactive carboxyl group- and/or acetyl group-containing substance, a mixed solvent of water and an alcohol having 1 to 4 carbon atoms, and an aqueous thickener solution or aqueous thickener dispersion;
a step of dipping a nonwoven fabric into the multifunctional coating film-forming liquid composition;
and a step of dehydrating and drying the dipped nonwoven fabric. The method for manufacturing an air filter according to claim 7, wherein the aqueous dispersion of fluorine-containing metal oxide particles is prepared by adding and mixing a fluorine-based compound to an aqueous dispersion of metal oxide particles.