Filter, filter unit, and method for producing filter

Abstract

The purpose of the present invention is to provide a filter having excellent rigidity.　This filter comprises nonwoven fabric (100) having single-fiber parts (B1, B2) which are made of an organic resin and which are fused to thereby bond constituent fibers (1) of the nonwoven fabric to each other. That is, most of the portions where adjacent constituent fibers of the nonwoven fabric are bonded to each other are connected to each other not only by themselves but also by the single-fiber parts. In other words, the portions lying between the adjacent bonded portions are reinforced with not only the constituent fibers but also the single-fiber parts.　Because of this, even when external force is applied to the filter, the portions lying between the adjacent bonded portions are less apt to break and exhibits excellent rigidity.　This filter hence has excellent rigidity.

Description

Filter, filter unit, and method for manufacturing a filter

[0001] The present invention relates to a filter, a filter unit including the filter, and a method for manufacturing the filter.

[0002] Conventionally, air filters including nonwoven fabrics have been used to remove dust particles from the air and purify the air, and liquid filters including nonwoven fabrics have been used to remove impurities from liquids and purify the liquid.

[0003] As such a filter, the present applicant has provided a filter having the configuration described in, for example, Japanese Patent Application Laid-Open No. 2024-151790 (Japanese Patent Application No. 2023-65500: Patent Document 1). That is, the filter includes, as constituent fibers, core-sheath composite fibers made of a core component that is an organic resin and a sheath component that is an organic resin having a lower melting point than the core component, and also includes a nonwoven fabric having another granular organic resin having a lower melting point than the sheath component, and the constituent fibers of the nonwoven fabric are bonded to each other by the sheath component, and the constituent fibers of the nonwoven fabric are bonded to each other by the other granular organic resin.

[0004] In a filter having the configuration described in Patent Document 1, the constituent fibers of the nonwoven fabric are bonded together by a separate granular organic resin in addition to the sheath component, making it possible to provide an air filter in which the constituent fibers of the nonwoven fabric are firmly bonded together.

[0005] Patent Document 1 describes a method for preparing a filter having the above-mentioned configuration, in which a fiber web prepared by blending a core-sheath type composite fiber with a single fiber composed of another organic resin is subjected to a two-step heating treatment, in which (Step 1) the fiber web is heated at a temperature equal to or higher than the melting point of the other organic resin and lower than the melting point of the sheath component to melt the single fiber and form a granular organic resin, and then the fiber web is cooled; and (Step 2) the fiber web that has been subjected to (Step 1) is heated at a temperature equal to or higher than the melting point of the sheath component and lower than the melting point of the core component to melt the sheath component that constitutes the core-sheath type composite fiber, and then the fiber web is cooled to prepare a nonwoven fabric.

[0006] JP 2024-151790 A (Patent Application No. 2023-65500)

[0007] However, the rigidity of the filter described in Patent Document 1 was still insufficient. For example, when an external force was applied to the filter by transporting or folding the filter to provide a pleated filter unit using the filter, cracks sometimes occurred on the main surface of the filter. The cracks on the main surface of the filter were thought to be caused by the breaking of bonds between the constituent fibers. When a filter with such poor rigidity was used, it was sometimes impossible to pleat the filter into the desired shape. The present invention aims to provide a filter with excellent rigidity.

[0008] The present invention provides: "(1) A filter comprising a nonwoven fabric containing, as constituent fibers, sheath-core composite fibers composed of a core component which is an organic resin and a sheath component which is an organic resin having a melting point lower than that of the core component, wherein the filter has portions where constituent fibers of the nonwoven fabric are bonded together by the sheath component, and has monofilament portions made of the organic resin that are fused together to bond the constituent fibers of the nonwoven fabric. (2) The filter according to (1), wherein the constituent fibers of the nonwoven fabric are only the sheath-core composite fibers. (3) The filter according to (1), wherein the percentage of the mass of the monofilament portions in the mass of the nonwoven fabric is more than 0% by mass and less than 30% by mass. (4) The filter according to (1), wherein the thickness of the nonwoven fabric is greater than 1.2 mm. (5) A filter unit comprising the filter according to any one of (1) to (4), folded into a pleated shape." (6) A filter comprising a frictionally charged nonwoven fabric layer between adjacent nonwoven fabrics according to any one of (1) to (4), in which two or more types of fibers having different resins constituting the fiber surfaces are mixed. (7) A filter unit comprising the filter according to (6), folded into a pleated shape. (8) A method for producing a filter, comprising: (Step 1) preparing a core-sheath type composite fiber comprising a core component that is an organic resin and a sheath component that is an organic resin having a melting point lower than that of the core component; (Step 2) preparing a single fiber composed of an organic resin having a melting point equal to or higher than the melting point of the sheath component and lower than that of the core component; (Step 3) blending the core-sheath type composite fiber with the single fiber to prepare a fiber web; (Step 4) heating the fiber web to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component to melt the sheath component and the organic resin constituting the single fiber; and (Step 5) cooling the fiber web that has been subjected to (Step 4) to prepare a nonwoven fabric.

[0009] As a result of continued research, the applicant has found that the rigidity of a nonwoven fabric is further improved when the nonwoven fabric has a portion where constituent fibers are bonded to each other by the sheath component of a core-sheath type composite fiber, and the nonwoven fabric also has "single fiber portions made of an organic resin that are fused to bond the constituent fibers to each other," as defined in the present invention.

[0010] Although the reason for this is not fully understood, it is believed to be due to the following effect. The nonwoven fabric constituting the filter disclosed in Patent Document 1 has portions where the constituent fibers of the nonwoven fabric are bonded together by another granular organic resin. Therefore, adjacent bonded portions exist independently and without contact because the other organic resin is present in a granular form. As a result, most of the adjacent bonded portions are bonded only by the constituent fibers. In contrast, the nonwoven fabric constituting the filter disclosed in the present invention has monofilament portions made of an organic resin that bond the constituent fibers of the nonwoven fabric together by fusion. In other words, most of the bonded portions between adjacent constituent fibers of the nonwoven fabric are bonded not only by the constituent fibers but also by the monofilament portions. In other words, the spaces between adjacent bonded portions are reinforced by not only the constituent fibers but also the monofilament portions. Therefore, even when an external force is applied to the filter, the spaces between adjacent bonded portions are less likely to break, resulting in excellent rigidity. From the above, the filter disclosed in the present invention has excellent rigidity.

[0011] In a preferred embodiment of the present invention, the constituent fibers of the nonwoven fabric are only core-sheath composite fibers, thereby more effectively bonding the constituent fibers of the nonwoven fabric. Therefore, the filter has even better rigidity. Furthermore, when the percentage of the mass of the monofilament portion of the mass of the nonwoven fabric is greater than 0% by mass and less than 30% by mass, the rigidity of the nonwoven fabric increases. Therefore, the filter has even better rigidity. Furthermore, when the thickness of the nonwoven fabric is greater than 1.2 mm, the filter has even better rigidity.

[0012] As described above, the filter according to the present invention can include, between rigid nonwoven fabrics, a layer of tribo-electrically charged nonwoven fabric in which two or more types of fibers having different resins constituting the fiber surfaces are mixed. With this configuration, the tribo-electrically charged nonwoven fabric layer is sandwiched and protected by the rigid nonwoven fabrics. Therefore, the filter has excellent collection performance and excellent rigidity due to the presence of the electrically charged fiber layer.

[0013] Next, since the filter according to the present invention has excellent rigidity, it can be pleated into a desired shape to form a filter unit.

[0014] Furthermore, as a result of further investigations, the applicant has found that a nonwoven fabric having the configuration according to the present invention can be prepared by preparing a fiber web by blending a sheath-core composite fiber with a single fiber composed of an organic resin having a melting point equal to or higher than the melting point of the sheath component of the core composite fiber and lower than the melting point of the core component, and then heating the fiber web to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component. Therefore, a filter having excellent rigidity can be produced by the manufacturing method according to the present invention.

[0015] 3 is an optical microscope photograph of a portion of the nonwoven fabric prepared in Example 1. The constituent fibers of the nonwoven fabric shown in FIG. 1 are only one type of sheath-core composite fiber. The symbols shown in the (Explanation of Symbols) column have been added to FIG. 1. The symbols shown in the (Explanation of Symbols) column have been added to FIG. 1. The symbols given in FIG. 2 are not shown in FIG. 3. An optical microscope photograph of a portion of the nonwoven fabric prepared in Comparative Example 1. The constituent fibers of the nonwoven fabric shown in FIG. 4 are only one type of sheath-core composite fiber. The symbols shown in the (Explanation of Symbols) column have been added to FIG. 4.

[0016] In the present invention, various configurations can be appropriately selected, for example, the following configurations. Note that, unless otherwise specified, the various measurements described in the present invention are performed under atmospheric pressure. Furthermore, measurements are performed under a temperature condition of 25°C. Unless otherwise specified, the various measurement results described in the present invention are measured to a value one digit smaller than the desired value, and the value is calculated by rounding off the value. As a specific example, when the desired value is expressed to one decimal place, the value is measured to two decimal places, and the obtained value is rounded to one decimal place to calculate the value to one decimal place, and this value is used as the desired value. The upper and lower limit values ​​exemplified in the present invention can be combined in any combination.

[0017] The nonwoven fabric of the filter of the present invention includes, as its constituent fibers, sheath-core composite fibers, which have a core component (hereinafter sometimes abbreviated as the core component) that is an organic resin, and a sheath component (hereinafter sometimes abbreviated as the sheath component) that is an organic resin having a melting point lower than that of the core component.

[0018] The core component and the sheath component may be, for example, a known organic resin such as a polyolefin resin (e.g., polyethylene, polypropylene, or a polyolefin resin in which a portion of a hydrocarbon is substituted with a cyano group or a halogen such as fluorine or chlorine), polymethylpentene, a styrene resin, a polyvinyl alcohol resin, a polyether resin (e.g., polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone), a polyester resin (e.g., polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycarbonate, polyarylate, wholly aromatic polyester resin), a polyamideimide resin, a polyamide resin (e.g., aromatic polyamide resin, aromatic polyetheramide resin, nylon resin), a resin having a nitrile group (e.g., polyacrylonitrile), a urethane resin, an epoxy resin, a polysulfone resin (e.g., polysulfone, polyethersulfone), or a fluorine-containing resin (e.g., polytetrafluoroethylene, polyvinylidene fluoride).

[0019] These resins may be either linear polymers or branched polymers. They may also be block copolymers or random copolymers. The organic resins may have any three-dimensional structure or may be crystalline, but are not particularly limited. They may also be mixed resins containing multiple organic resins. To obtain a filter with excellent heat resistance, it is preferable that both the core component and the sheath component be polyester resins.

[0020] The sheath component has a lower melting point than the core component. The difference in melting point may be greater than 0°C. The difference in melting point can be 5°C or more, preferably 10°C or more, more preferably 20°C or more, and most preferably 30°C or more. The upper limit of the melting point difference can be adjusted as appropriate, but is preferably 200°C or less, more preferably 150°C or less. An example of such a combination of the core component and the sheath component is a combination in which the core component is a polyester resin (melting point: 250°C) and the sheath component is a polyester copolymer resin (melting point: 110°C). The "melting point" referred to here means the value obtained by Method A specified in JIS L1015:2021 "Test Methods for Chemical Fiber Staples," 8.16.1 (Melting Point).

[0021] The fineness and fiber length of the sheath-core composite fiber are not particularly limited. The fineness of the sheath-core composite fiber can be 0.01 to 30 dtex, 5 to 25 dtex, or 10 to 20 dtex. Note that the "fineness" in the present invention refers to the value obtained by Method A (normal method) specified in JIS L1015:2021 "Test methods for synthetic fiber staples," 8.5.1 (corrected fineness).

[0022] The sheath-core composite fiber may be a short fiber cut to a specific length. The fiber length of the short fiber may be 5 to 130 mm, 20 to 110 mm, or 35 to 80 mm. Alternatively, the sheath-core composite fiber may be a fiber (continuous fiber) having a continuous length, which does not have a specific length like directly spun fibers such as melt-blown fibers or electrospun fibers. In the present invention, "fiber length" refers to the average fiber length measured by Method C (direct method) specified in JIS L 1015:2021 "Test Methods for Staple Chemical Fibers," 8.4.1 (Average Fiber Length).

[0023] The core-sheath type composite fiber can be obtained by known methods such as melt spinning, dry spinning, wet spinning, and direct spinning (melt-blowing, spunbonding, electrostatic spinning, etc.).

[0024] The constituent fibers of the nonwoven fabric constituting the filter according to the present invention (excluding the monofilament portion according to the present invention) may be only sheath-core composite fibers. Specifically, it is preferred that the constituent fibers of the nonwoven fabric do not contain any fibers other than sheath-core composite fibers (for example, drawn fibers made of a single component such as drawn polyester fibers). This configuration provides stronger bonds between the constituent fibers of the nonwoven fabric, resulting in a filter with excellent rigidity, which is preferred.

[0025] Alternatively, other fibers may be included as constituent fibers for the purpose of improving filter performance, such as improving the filter's collection performance and reducing pressure loss. Such other fibers may be fibers composed of an organic resin exemplified as being capable of constituting the sheath-core composite fiber and / or other sheath-core composite fibers. The melting point of the other fibers may be equal to or higher than the melting point of the organic resin constituting the single fiber portion of the present invention, and may be adjusted as appropriate. The fineness and fiber length of the other fibers may also be the fineness and fiber length exemplified as being capable of being taken by the sheath-core composite fiber. The other fibers may be prepared using the manufacturing method exemplified as the manufacturing method for the sheath-core composite fiber.

[0026] The percentage of the mass of the core-sheath composite fibers in the mass of the constituent fibers is appropriately adjusted so as to obtain a nonwoven fabric with improved rigidity. To efficiently bond the constituent fibers to each other to obtain a nonwoven fabric with improved rigidity, the percentage of the mass is preferably higher than 50% by mass, more preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and most preferably 90% by mass or more.

[0027] Similarly, the percentage of the mass of the core-sheath composite fibers in the mass of the nonwoven fabric is appropriately adjusted so as to obtain a nonwoven fabric with improved rigidity. Specifically, the percentage is preferably higher than 40% by mass, more preferably higher than 50% by mass, more preferably 60% by mass or more, more preferably 70% by mass or more, and most preferably higher than 70% by mass. The upper limit is less than 100% by mass, preferably 90% by mass, and more preferably 80% by mass.

[0028] Next, the configuration of the nonwoven fabric included in the filter of the present invention will be described with reference to Figures 1 to 3. The constituent fibers of the nonwoven fabric shown in Figures 1 to 3 (excluding the monofilament portion according to the present invention) are only one type of core-sheath type composite fiber.

[0029] The nonwoven fabric (100) provided in the filter according to the present invention has a monofilament portion (C, shown in FIG. 3 as a dashed rectangle; hereinafter, sometimes referred to as the monofilament portion) made of an organic resin that bonds the constituent fibers (1) of the nonwoven fabric (100) together by fusion. In other words, the bonded portions of adjacent constituent fibers (1) of the nonwoven fabric are bonded not only by the constituent fibers but also by the monofilament portion (C). In other words, the bonded portions between adjacent constituent fibers are bonded and reinforced by the organic resin that makes up the monofilament portion (C). Therefore, even when an external force is applied to the filter, the bonded portions between adjacent constituent fibers are unlikely to break. The filter according to the present invention has excellent rigidity and can be pleated into a desired shape.

[0030] The single fiber portion (C) in the present invention is derived from a molten single fiber. Therefore, the single fiber portion (C) is composed of the organic resin (usually one type of organic resin) that constitutes the single fiber. Note that when the single fiber portion (C) is composed of one type of mixed resin and only one type of mixed resin is continuously present in the length direction, the single fiber portion (C) is considered to be composed of one type of organic resin.

[0031] The monofilament portion (C) has a plurality of portions (B1, shown by a dashed circle in FIG. 2 ) that are bonded to the constituent fibers (1) of the nonwoven fabric (100), and also connects the bonded portions (B1) together. Therefore, the monofilament portion (C) can have a portion (B1) and a portion (B2) that connects the portions (B1) together. Note that the portion (B1) is fused to the sheath component of the core-sheath composite fiber. Therefore, the portion (B1) is usually wider than the portion (B2). Furthermore, to obtain a filter with superior rigidity, the portion (B2) is preferably thicker than the constituent fibers (1) of the nonwoven fabric (100).

[0032] In addition, whether or not the nonwoven fabric (100) has a single fiber portion (C) can be confirmed by cutting the nonwoven fabric (100) in the thickness direction and photographing the cross section of the cut sample using an optical microscope at 30x magnification.

[0033] First, on the optical microscope photograph, a portion (B1) where the constituent fibers of the sample are bonded by an organic resin other than the sheath component, another adjacent portion (B1), and a portion (B2) where these are connected by the same organic resin as the organic resin in question are selected. Then, the smallest rectangle enclosing these portions is drawn. For reference, a schematic drawing of this smallest rectangle is shown in Figure 3. The aspect ratio of the length of the long side to the short side of the rectangle is then calculated. Similarly, smallest rectangles are drawn at 19 arbitrary locations. If smallest rectangles can be drawn at 20 or more locations on the same optical microscope photograph, the average aspect ratio of the length of the long side to the short side of the rectangle at any 20 of these locations is calculated.

[0034] Alternatively, if the smallest rectangle can be drawn in fewer than 20 locations in the same optical microscope photograph, multiple optical microscope photographs are taken. Then, the aspect ratio of the length of the long side to the short side of the rectangle is calculated for any 20 locations in each of the optical microscope photographs. The average of the obtained 20 aspect ratios is then calculated.

[0035] If the average aspect ratio obtained is 4 or more, the nonwoven fabric (100) is determined to have a monofilament portion (C). On the other hand, if the average aspect ratio obtained is less than 4, the nonwoven fabric (100) is determined to not have a monofilament portion (C). The upper limit of the average aspect ratio of the monofilament portion is 50.

[0036] The organic resin constituting the single fiber portion can be selected from the organic resins listed as being capable of constituting the core component and the sheath component. In particular, the type of organic resin constituting the single fiber portion is preferably the same as the type of organic resin constituting the sheath component so that the single fiber portion is more firmly bonded to the core-sheath composite fiber constituting the nonwoven fabric. Specifically, it is preferable that both the organic resin constituting the single fiber portion and the organic resin constituting the sheath component be polyester-based resins.

[0037] It is preferable that the temperature difference between the melting point of the organic resin constituting the monofilament portion and the melting point of the sheath component constituting the core-sheath type composite fiber is small, since this makes it easier to realize a nonwoven fabric that satisfies the configuration of the present invention. Specifically, the temperature difference is preferably 30°C or less, more preferably 10°C or less, and most preferably 5°C or less. It is most preferable that the melting point of the organic resin constituting the monofilament portion and the melting point of the sheath component are the same.

[0038] It is preferable that the nonwoven fabric has multiple types of sheath-core conjugate fibers as its constituent fibers, as this results in a nonwoven fabric with improved rigidity. In this case, the sheath-core conjugate fiber having the sheath component with the lowest melting point among the multiple types of sheath-core conjugate fibers is used as the sheath-core conjugate fiber for melting point comparison. When multiple types of sheath-core conjugate fibers all have sheath components with the same melting point, all of the multiple types of sheath-core conjugate fibers are used as the sheath-core conjugate fibers for melting point comparison.

[0039] The portions where the constituent fibers of the nonwoven fabric are bonded together by the sheath component and the monofilament portions may be present anywhere in the nonwoven fabric. In particular, if these portions are present throughout the entire nonwoven fabric, the overall rigidity is further improved, making the nonwoven fabric easier to pleat into the desired shape, which is preferable. These portions may also be present uniformly throughout the entire nonwoven fabric. Alternatively, they may be present so that their number gradually increases from one main surface of the nonwoven fabric to the other main surface of the nonwoven fabric.

[0040] The percentage of the mass of the constituent fibers, including the core-sheath composite fibers, and the mass of the monofilament portion of the nonwoven fabric is appropriately adjusted so that the constituent fibers of the nonwoven fabric can be sufficiently bonded to each other. To efficiently bond the constituent fibers to each other, the mass of the constituent fibers of the nonwoven fabric is preferably greater than the mass of the monofilament portion constituting the nonwoven fabric. Specifically, the percentage of the mass of the constituent fibers in the sum of the mass of the constituent fibers and the mass of the monofilament portion is less than 100% by mass, preferably greater than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, and most preferably 90% by mass or more.

[0041] As will be described later, in a nonwoven fabric prepared using a sheath-core conjugate fiber having a mass A and a single fiber (forming the single fiber portion) having a mass B, the mass percentage of the sheath-core conjugate fiber and the mass percentage of the single fiber portion in the sum of the masses of the sheath-core conjugate fiber and the single fiber portion contained in the nonwoven fabric can be calculated as A / (A+B) mass%:B / (A+B) mass%.

[0042] Similarly, the percentage of the mass of the monofilament portion in the mass of the nonwoven fabric is appropriately adjusted to obtain a nonwoven fabric with improved rigidity. Specifically, the percentage is preferably 30% by mass or less, more preferably less than 30% by mass, more preferably 25% by mass or less, and even more preferably 20% by mass or less. The percentage is preferably greater than 0% by mass, with the lower limit being 10% by mass.

[0043] The various configurations of the nonwoven fabric, such as thickness and basis weight, are not particularly limited. They can be appropriately adjusted to obtain a nonwoven fabric with more efficiently improved rigidity. The thickness can be 0.1 to 50 mm, 0.3 to 30 mm, 0.5 to 20 mm, 1 to 10 mm, or greater than 1.2 mm and 5 mm or less. In particular, a nonwoven fabric with a thickness of 1.2 mm or greater is preferred as it has more efficiently improved rigidity. The basis weight is 10 to 1000 g / m ２ and 100 to 700 g / m ２ and 200 to 500 g / m ２In the present invention, the thickness is the value measured using a high-precision digital length measuring device when a load of 98 Pa is applied in the direction perpendicular to the main surface. The basis weight is the weight per square meter of the main surface having the widest area of ​​the object to be measured. ２ This refers to the mass per unit mass.

[0044] Next, an example of a method for manufacturing a filter according to the present invention will be described. The filter can be manufactured by a manufacturing method including the following steps: (Step 1) preparing a core-sheath composite fiber composed of a core component which is an organic resin and a sheath component which is an organic resin having a melting point lower than that of the core component; (Step 2) preparing a single fiber composed of an organic resin having a melting point equal to or higher than the melting point of the sheath component and lower than that of the core component; (Step 3) blending the core-sheath composite fiber with the single fiber to prepare a fiber web; (Step 4) heating the fiber web to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than that of the core component to melt the sheath component and the organic resin constituting the single fiber; and (Step 5) cooling the fiber web that has been subjected to Step 4 to prepare a nonwoven fabric.

[0045] First, (Step 1) will be described. The sheath-core composite fiber can have the above-described configuration. The fiber length of the sheath-core composite fiber can be adjusted as appropriate, but the sheath-core composite fiber can be uniformly mixed with the monofilaments, particularly when the monofilament is a short fiber. As a result, a nonwoven fabric can be prepared in which the fibers constituting the nonwoven fabric are uniformly bonded to each other by the sheath component and the monofilament portion throughout the fabric. This is preferable because it allows the preparation of a nonwoven fabric that can provide a filter with superior rigidity.

[0046] Next, (Step 2) will be explained. The term "single fiber" used here refers to a fiber composed of one type of organic resin. Note that when a single fiber is composed of one type of mixed resin and only one type of mixed resin is present continuously in the length direction, the single fiber is considered to be composed of one type of organic resin. Therefore, only one type of organic resin or one type of mixed resin is exposed in the fiber cross section formed by cutting the single fiber in a direction perpendicular to the fiber length direction.

[0047] The organic resin constituting the single fiber can be selected from the organic resins listed as being capable of constituting the core component and the sheath component. In particular, the type of organic resin constituting the single fiber is preferably the same as the type of organic resin constituting the sheath component so that the single fiber can be more firmly bonded to the core-sheath composite fiber constituting the nonwoven fabric. Specifically, it is preferable that both the organic resin constituting the single fiber and the organic resin constituting the sheath component be polyester-based resins.

[0048] The melting point of the organic resin constituting the single fiber may be adjusted appropriately as long as it is within a temperature range equal to or higher than the melting point of the sheath component and lower than the melting point of the core component in the sheath-core composite fiber. In particular, it is preferable that the temperature difference between the melting point of the organic resin constituting the single fiber and the melting point of the sheath component is small so that the molten single fiber and the sheath component mix to form a strong bond. Specifically, the temperature difference between the two melting points is preferably 30°C or less, more preferably 10°C or less, and even more preferably 5°C or less, and it is most preferable that the melting point of the organic resin constituting the single fiber and the melting point of the sheath component are the same.

[0049] The fiber length of the single fibers can be adjusted as appropriate. In particular, short single fibers can be uniformly mixed with the core-sheath composite fibers. This allows the preparation of a nonwoven fabric in which the fibers constituting the nonwoven fabric are uniformly bonded to each other by the sheath component and the single fiber portion throughout the fabric. As a result, a nonwoven fabric that can provide a filter with superior rigidity can be prepared, which is preferable.

[0050] The fineness of the single fibers can be adjusted as appropriate. To provide a filter with superior rigidity, it is preferably 1 dtex or more, more preferably 5 dtex or more, even more preferably 10 dtex or more, and even more preferably 20 dtex or more. The upper limit of the fineness can be adjusted as appropriate, but is preferably 70 dtex or less. Furthermore, the single fiber portions that bond the constituent fibers together are formed thick by the molten single fibers, and to provide a filter with superior rigidity, it is preferable that the fineness of the single fibers be larger than that of core-sheath type composite fibers.

[0051] Next, (Step 3) will be described. The method for preparing a fiber web by blending the sheath-core composite fibers and single fibers can be selected appropriately. For example, the fiber web can be produced by a dry method in which the sheath-core composite fibers and single fibers are fed into a carding device or an air-laying device, or by a wet method in which the above-mentioned fibers are dispersed in a solvent and paper-formed into a sheet. It is preferable to use a fiber web prepared by a dry method, so that the filter has excellent collection capacity due to its bulkiness.

[0052] The fiber web formed as described above may be entangled with a water jet or needles for easier handling. When entangling the fiber web with needles, the entanglement conditions are not particularly limited. However, since a high needle density makes it easier to prepare a nonwoven fabric with improved rigidity, the needle density is preferably 10 needles / cm. ２ It is preferable that the number of fibers is 50 or more per cm. ２ The upper limit of the needle density can be adjusted as appropriate, but it is more preferable that the needle density is 1000 needles / cm. ２ The needle density is preferably 600 needles / cm or less. ２ More preferably, it is:

[0053] The mass of the blended sheath-core conjugate fiber and the mass of the single fiber are appropriately adjusted so that the constituent fibers can be sufficiently bonded to each other. To efficiently bond the constituent fibers to each other, the mass of the blended sheath-core conjugate fiber is preferably greater than the mass of the blended single fiber. Specifically, the percentage of the mass of the blended sheath-core conjugate fiber in the sum of the mass of the blended sheath-core conjugate fiber and the mass of the single fiber is less than 100% by mass, preferably greater than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, and most preferably 90% by mass or more.

[0054] Finally, (Step 4) and (Step 5) will be described. The method for heating the fiber web can be selected as appropriate. For example, a method of heating the fiber web using a heater such as an oven dryer, far-infrared heater, dry heat dryer, or hot air dryer can be used. The heating temperature is adjusted to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component. Specifically, the heating temperature can be equal to or higher than the melting point of the organic resin constituting the single fiber (a temperature equal to or higher than the melting point of the sheath component), 10°C higher than the melting point of the organic resin constituting the single fiber, 20°C higher than the melting point of the organic resin constituting the single fiber, or 30°C higher than the melting point of the organic resin constituting the single fiber. The upper limit of the heating temperature may be lower than the melting point of the organic resin constituting the core component of the core-sheath composite fiber. The temperature is adjusted as appropriate to prevent unintended deterioration of the components constituting the filter due to melting, deformation, decomposition, or the like.

[0055] This (Step 4) allows the sheath component of the sheath-core composite fiber and the organic resin constituting the single fiber to melt simultaneously. The melting point of the organic resin constituting the single fiber used in this production method is equal to or higher than the melting point of the organic resin constituting the sheath component. Therefore, when the fiber web is heated to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component, the sheath component melts simultaneously with the melting of the single fiber. In other words, the thermal energy applied to the fiber web in this (Step 4) is also consumed to melt the sheath component, which is as easily melted as or more easily melted than the organic resin constituting the single fiber. Therefore, the sheath component is also melted simultaneously, preventing the single fiber from melting alone. As a result, as described above, it is believed that the single fiber melts to the extent that single fiber portions with an average aspect ratio of 4 or more remain. Furthermore, it is believed that using single fibers with a larger fineness and thickness than the core-core composite fiber further facilitates melting of the single fiber to the extent that single fiber portions with an average aspect ratio of 4 or more remain, as defined in the above-mentioned (Confirmation Method).

[0056] Then, in (Step 5), the fiber web heated through (Step 4) is cooled to form a nonwoven fabric having a portion where the constituent fibers of the nonwoven fabric are bonded together by the sheath component of the molten core-sheath composite fibers and a monofilament portion where the constituent fibers of the nonwoven fabric are bonded together by the molten monofilament.

[0057] The nonwoven fabric obtained as described above can be used alone as a filter. Alternatively, the nonwoven fabric may be used as a filter by providing other layers (e.g., nonwoven fabric layers) such as a pre-filter layer or a back-filter layer on the nonwoven fabric. In this case, the materials constituting the pre-filter layer and the back-filter layer can be appropriately selected. For example, a newly prepared fabric (e.g., a newly prepared nonwoven fabric, woven fabric, or knitted fabric), a porous film, or a breathable foam can be used.

[0058] A specific example of a filter having another layer may be a filter having a layer of tribo-electrically charged nonwoven fabric between two or more nonwoven fabrics according to the present invention, in which two or more types of fibers having different resins forming the fiber surface are mixed. Here, "different resins forming the fiber surface" means that the resins forming the surfaces of the two types of fibers being compared other than the fiber cross sections are different from each other. The two or more types of fibers having different resins forming the fiber surface may be any known combination of fibers that can be charged by rubbing their fiber surfaces together, and can be selected appropriately. For example, if the tribo-electrically charged nonwoven fabric layer contains two types of fibers, the combination of fibers may be a combination of olefin-based fibers (particularly polyolefin fibers) and acrylic-based fibers.

[0059] The nonwoven fabric of the present invention can sandwich and protect a frictionally charged nonwoven fabric layer. As a result, the laminated filter has excellent collection performance and rigidity due to the presence of the charged nonwoven fabric layer. Because both main surfaces of the laminated filter are highly rigid, it can be pleated into a desired shape.

[0060] The filter of the present invention may contain functional particles, such as particles of a radioactive substance adsorbent (e.g., zeolite, Prussian blue, etc.), a photocatalyst (e.g., titanium oxide, manganese dioxide, or platinum-supported alumina, etc.), a humidity conditioner (e.g., silica gel, silica microcapsules, etc.), a deodorizer such as activated carbon or carbon black, a flame retardant (e.g., a halogen-based flame retardant, a metal hydroxide-based flame retardant, a phosphorus-based flame retardant, etc.), a deodorant, an insect repellent, a cation exchange resin, an anion exchange resin, a dye, a pigment, a fragrance, an antiviral agent, an antibacterial agent, or an antifungal material.

[0061] The functional particles may be supported on the surface and / or inside of the nonwoven fabric constituting the filter. The functional particles may be supported on the nonwoven fabric in the following manner: simply present on the surface of the constituent fibers of the nonwoven fabric or between the constituent fibers; adhered to the surface of the constituent fibers of the nonwoven fabric or between the constituent fibers with a binder; or fixed to the molten sheath component and / or single fiber portion. Alternatively, the functional particles may be present in a state kneaded into the constituent fibers of the nonwoven fabric.

[0062] The filter of the present invention can be pleated to prepare a filter unit. The pleat height of the pleated filter can be 5 to 50 mm, 10 to 45 mm, or 15 to 30 mm. The pleat spacing of the pleated filter can be 2 to 20 mm, 3 to 15 mm, or 5 to 10 mm.

[0063] The pleated filter can be used as is. However, a filter unit may be prepared by providing edge bands on both sides of the pleated filter that are perpendicular to the pleat fold lines when viewed from the main surface side. Alternatively, a filter unit may be prepared by providing edge bands around the four periphery sides.

[0064] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0065] (Comparative Example 1) 90% by mass of core-sheath type composite fibers (core component: polyester resin (melting point: 250°C), sheath component: polyester copolymer resin (melting point: 210°C), fineness: 17 dtex, fiber length: 76 mm) and 10% by mass of single fibers (composed only of polyester copolymer resin with a melting point of 110°C, fineness: 22 dtex, fiber length: 38 mm) were blended and fed to a carding machine to prepare a fiber web. Thereafter, needles were inserted into one main surface of the prepared fiber web at a needle density of 50 needles / cm. ２ As a result, the fibers were entangled with each other to prepare a needle-punched fiber web.

[0066] The needle-punched fiber web was then heated for 3 minutes in a dryer adjusted to a heating temperature of 110°C. The heated needle-punched fiber web was then cooled by being left in a 25°C atmosphere for 5 minutes to prepare a nonwoven fabric. The nonwoven fabric thus prepared was used as a filter. An optical microscope photograph of a portion of the filter (nonwoven fabric) thus prepared is shown in Figure 4.

[0067] Comparative Example 2 A filter was prepared in the same manner as in Comparative Example 1, except that the heating temperature was changed to 210°C.

[0068] Comparative Example 3 The nonwoven fabric prepared in Comparative Example 1 was heated for 3 minutes in a dryer adjusted to a heating temperature of 210°C. The heated needle-punched fiber web was then cooled by being left in an atmosphere of 25°C for 5 minutes to prepare a further heated nonwoven fabric. The nonwoven fabric prepared in this manner was used as a filter.

[0069] (Example 1) Instead of the sheath-core conjugate fiber used in Comparative Example 1, another sheath-core conjugate fiber having a sheath component with a different melting point (core component: polyester resin (melting point: 250°C), sheath component: polyester copolymer resin (melting point: 110°C), fineness: 17 dtex, fiber length: 76 mm) was used. Except for this, a filter was prepared in the same manner as in Comparative Example 1. An optical microscope photograph of a portion of the filter (nonwoven fabric) prepared in this manner is shown in Figure 1.

[0070] (Example 2) Instead of the sheath-core conjugate fiber used in Comparative Example 2, another sheath-core conjugate fiber used in Example 1 was used. Except for this, a filter was prepared in the same manner as in Comparative Example 2. In the filters prepared in Examples 1 and 2, the thickness of the portion (B2) in the single fiber portion that connects the bonded portions of the constituent fibers of the nonwoven fabric derived from the molten single fibers was thicker than the constituent fibers of the nonwoven fabric.

[0071] Electron microscope photographs of the prepared filter (nonwoven fabric) were taken. The presence or absence of portions where the constituent fibers of the filter (nonwoven fabric) were bonded together by the organic resin that constitutes the sheath component of the core-sheath composite fiber was confirmed. The bonding state due to the organic resin derived from the single fiber was also confirmed. The results of the observations are summarized in Table 1. If there were portions where the constituent fibers of the nonwoven fabric were bonded together by the sheath component, "Present" was entered in the "Bonds derived from sheath component" column in the table. On the other hand, if there were no such bonded portions, "Absent" was entered in the "Bonds derived from sheath component" column in the table.

[0072] When there is a monofilament portion (a portion derived from a molten monofilament) made of organic resin that bonds the constituent fibers of the nonwoven fabric together by fusion, as shown in (C) in Figure 3, "monofiber portion" is entered in the "bonding mode derived from monofilament" column in the table. On the other hand, when there is no monofilament portion made of organic resin and there are granular portions (a portion derived from a completely molten monofilament, each with an aspect ratio of less than 4) made of organic resin, as shown in (C) in Figure 5, "granular" is entered in the "bonding mode derived from monofilament" column in the table.

[0073] Furthermore, the prepared filter (nonwoven fabric) was folded so that the shortest distance between adjacent pleat valleys and pleat peaks on the main surface of the filter was 19 mm, and the spacing between adjacent pleat peaks was 15 mm. This resulted in a pleated filter having multiple pleats and a length of 15 cm in the direction in which the pleats were formed. The surface of the pleated filter prepared in this manner was then visually inspected. If cracks were found to have occurred on the main surface of the filter as a result of inspection, an "X" was entered in the "Crack occurrence after pleating" column in the table. On the other hand, if cracks were not found on the main surface of the filter, an "O" was entered in the "Crack occurrence after pleating" column in the table.

[0074]

[0075] Comparative Example 1 was compared with Example 1, and Comparative Example 2 with Example 2. The results showed that a nonwoven fabric having the configuration according to the present invention can be prepared by preparing a fiber web in which a core-sheath type composite fiber is blended with a single fiber made of an organic resin having a melting point equal to or higher than the melting point of the sheath component of the core-sheath type composite fiber and lower than the melting point of the core component, and then heating the fiber web to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component.

[0076] Furthermore, from the results of Comparative Example 3, it was found that as long as a nonwoven fabric manufacturing method according to the conventional technology such as that of Patent Document 1 is used, that is, a fiber web is prepared by blending core-sheath composite fibers with single fibers composed of an organic resin having a melting point below the melting point of the sheath component of the core-sheath composite fibers, and the fiber web is then heated to a temperature equal to or higher than the melting point of the organic resin constituting the single fibers and lower than the melting point of the sheath component, and then heated at a temperature equal to or higher than the melting point of the sheath component and lower than the melting point of the core component, it is impossible to prepare a nonwoven fabric having the configuration according to the present invention.

[0077] It was also found that a filter having excellent rigidity can be realized by including a nonwoven fabric having the configuration according to the present invention.

[0078] (Preparation of fibers) The following fibers were prepared: - Sheath-core composite fiber A1 (core component: polyester resin (melting point: 250°C), sheath component: polyester copolymer resin (melting point: 110°C), fineness: 15 dtex, fiber length: 51 mm) - Sheath-core composite fiber A2 (core component: polyester resin (melting point: 250°C), sheath component: polyester copolymer resin (melting point: 110°C), fineness: 6 dtex, fiber length: 51 mm) - Single fiber (composed only of polyester copolymer resin with a melting point of 110°C, fineness: 22 dtex, fiber length: 38 mm)

[0079] (Examples 3 to 6) The fibers listed in the "Fiber Preparation" column were blended together to achieve the mass percentages listed in Table 2, and the blended fibers were fed to a carding machine to prepare a fiber web. Then, the prepared fiber web was carded from one main surface side with a needle density of 50 needles / cm. ２ The fibers were entangled by applying needles at a temperature of 150°C to prepare a needle-punched fiber web. The needle-punched fiber web was then heated for 3 minutes in a dryer adjusted to a heating temperature of 150°C. The heated needle-punched fiber web was then cooled by being left in a 25°C atmosphere for 5 minutes to prepare a nonwoven fabric. The nonwoven fabric thus prepared was used as a filter. In the filters prepared in Examples 3 to 6, the thickness of the portion (B2) in the single fiber portion connecting the bonded portions of the constituent fibers of the nonwoven fabric derived from the molten single fibers was thicker than the constituent fibers of the nonwoven fabric.

[0080]

[0081] The physical properties of the filters (nonwoven fabrics) prepared in Examples 3 to 6 are summarized in Table 3. The prepared filters (nonwoven fabrics) were subjected to JIS L1913:2010 "General nonwoven fabric testing methods" 6.7.4 "Gurley method" to measure their bending resistance (mN). A higher bending resistance indicates a more rigid filter (nonwoven fabric).

[0082]

[0083] The results of Examples 3 to 6 demonstrate that a filter having excellent rigidity can be achieved by using a nonwoven fabric having a configuration according to the present invention.

[0084] (Preparation of Tribo-Electrified Nonwoven Fabric Comprising a Mixture of Two or More Types of Fibers with Different Constituent Fibers) The fibers listed in the (Fiber Preparation) column were blended with the fibers listed below in the mass percentages shown in Table 4, and the blend was fed into a carding machine to prepare a fiber web. - Acrylic fiber (melting point: none, fineness: 1.7 dtex, fiber length: 51 mm) - Polypropylene fiber (melting point: 165°C, fineness: 1.7 dtex, fiber length: 51 mm) Then, needles were inserted into one main surface of the prepared fiber web at a needle density of 50 needles / cm. ２ The fibers were entangled by needle action at 1000 kJ / min, to prepare tribo-electrically charged fiber webs A to E.

[0085]

[0086] (Example 7) The fibers were mixed so as to have the same fiber composition and mass percentage as in Example 4, and the mixture was subjected to a carding machine to produce a fiber web (basis weight: 65 g / m ２ Two sheets of the fiber web prepared in this manner were prepared. Then, the tribo-electrically charged fiber web A was sandwiched between the two fiber webs to prepare a laminate. Thereafter, needles were inserted from one main surface of the prepared laminate at a needle density of 50 needles / cm. ２ The constituent fibers of each nonwoven fabric were entangled by applying needles at a temperature of 110°C to prepare a needle-punched fiber web. The needle-punched fiber web was then heated for 3 minutes in a dryer adjusted to a heating temperature of 110°C. The heated needle-punched fiber web was then cooled by being left in a 25°C atmosphere for 5 minutes to prepare a nonwoven fabric. The prepared nonwoven fabric was then washed with hot water at 60°C for 10 minutes to remove the oil, and then air-dried. An external force was then applied from one main surface of the air-dried nonwoven fabric to rub the acrylic fibers and polypropylene fibers together. The nonwoven fabric thus prepared was used as a filter.

[0087] (Example 8) The fibers were mixed so as to have the same fiber composition and mass percentage as in Example 4, and the mixture was subjected to a carding machine to produce a fiber web (basis weight: 50 g / m ２ ) was prepared. Two sheets of the fiber web prepared in this manner were also prepared. Then, tribo-electrically charged fiber web B was sandwiched between the two prepared fiber webs to prepare a laminate. A filter was prepared in the same manner as in Example 7, except that the laminate prepared in this manner was used.

[0088] Example 9 A filter was prepared in the same manner as in Example 8, except that tribo-electrically charged fibrous web C was used instead of tribo-electrically charged fibrous web B.

[0089] Example 10 A filter was prepared in the same manner as in Example 8, except that tribo-electrically charged fibrous web D was used instead of tribo-electrically charged fibrous web B.

[0090] (Example 11) The fibers were mixed so as to have the same fiber composition and mass percentage as in Example 6, and the mixture was subjected to a carding machine to produce a fiber web (basis weight: 50 g / m ２ ) was prepared. Two sheets of the fiber web prepared in this manner were also prepared. Then, the tribo-electrically charged fiber web E was sandwiched between the two prepared fiber webs to prepare a laminate. A filter was prepared in the same manner as in Example 7, except that the laminate prepared in this manner was used.

[0091] (Example 12) The fibers were blended so that the mass percentages of the core-sheath type composite fiber A1 were 40 mass%, the core-sheath type composite fiber A2 were 10 mass%, and the single fiber was 50 mass%. Then, the fibers were fed to a carding machine to form a fiber web (basis weight: 50 g / m ２ ) was prepared. Two sheets of the fiber web prepared in this manner were also prepared. Then, the tribo-electrically charged fiber web E was sandwiched between the two prepared fiber webs to prepare a laminate. A filter was prepared in the same manner as in Example 7, except that the laminate prepared in this manner was used.

[0092] In the filters prepared in Examples 7 to 12, the thickness of the portions (B2) in the monofilament portion connecting the bonded portions of the constituent fibers of the nonwoven fabric derived from the molten monofilaments was thicker than the constituent fibers of the nonwoven fabric. Furthermore, the filters prepared in Examples 7 to 12 had, between the nonwoven fabrics having a configuration according to the present invention, a layer of tribo-electrically charged nonwoven fabric in which two or more types of fibers having different resins constituting the fiber surface were mixed (a layer of tribo-electrically charged nonwoven fabric in which acrylic fibers and polypropylene fibers were mixed). The physical properties of the filters (nonwoven fabrics) prepared in Examples 7 to 12 are summarized in Table 5.

[0093]

[0094] The results of Examples 7 to 12 demonstrate that a filter having excellent rigidity can be realized by using a nonwoven fabric having a configuration according to the present invention.

[0095] Example 13 A nonwoven fabric (thickness: 1.14 mm, basis weight: 150 g / m) was produced in the same manner as in Example 1, except that only the total mass of the blended fibers fed to the carding machine was increased, and the needle-punched fiber web was heated by feeding it to a dryer device adjusted to a heating temperature of 110°C for 3 minutes, and a load was applied between both main surfaces of the web to reduce the thickness. ２ ) was prepared.

[0096] The nonwoven fabric thus prepared was then folded so that the shortest distance between adjacent pleat valleys and pleat peaks on the filter main surface was 19 mm, and the spacing between adjacent pleat peaks was 15 mm. This resulted in a pleated nonwoven fabric having a plurality of pleats and a length of 15 cm in the direction in which the pleats were formed. Next, edge bands were applied to both sides of the pleated nonwoven fabric perpendicular to the pleat fold lines when viewed from the main surface side, thereby fixing the pleat shape of the pleated nonwoven fabric and preparing a filter unit. In this way, a filter unit (thickness of the filter (nonwoven fabric) constituting the filter unit: 1.14 mm, basis weight of the filter (nonwoven fabric) constituting the pleated filter: 150 g / m) was prepared. ２ ) was created.

[0097] (Example 14) The filter unit prepared in Example 13 was subjected to a dry heat dryer adjusted to a heating temperature of 80°C, thereby expanding the thickness of the filter (nonwoven fabric) constituting the filter unit. The filter unit removed from the dry heat dryer was allowed to cool, and then another filter unit (thickness of the filter (nonwoven fabric) constituting the pleated filter: 1.44 mm, basis weight of the filter (nonwoven fabric) constituting the pleated filter: 150 g / m) was produced. ２ ) was created.

[0098] In the filters (nonwoven fabrics) constituting the filter units prepared in Examples 13 and 14, the thickness of the portion (B2) connecting the bonded portions of the constituent fibers of the nonwoven fabric derived from the molten single fibers in the single fiber portion was thicker than the constituent fibers of the nonwoven fabric. In the filters (nonwoven fabrics) constituting the filter units prepared in Examples 13 and 14, the bonding mode derived from the single fibers was the "single fiber portion," and the occurrence of cracks after pleating was evaluated as "good."

[0099] A metal rod (width: 10 mm, length: 250 mm) was placed on the main surface of the pleated filters prepared in Examples 13 and 14 so that it was perpendicular to the direction in which the pleat peaks continued and in contact with each pleat peak when viewed from the main surface side. A load was then applied to the metal rod, thereby applying a load to the filter unit in its thickness direction. The load at which the pleats of the filter (nonwoven fabric) of the filter unit first buckled was confirmed. The load was 789 g for the filter unit prepared in Example 13 and 987 g for the filter unit prepared in Example 14. These results demonstrated that the filter unit prepared in Example 14 was more rigid than the filter unit prepared in Example 13. This was thought to be due to the filter (nonwoven fabric) constituting the filter unit being thicker than 1.2 mm.

[0100] The filter of the present invention can be used as a flat or pleated filter, or as a roll filter, and can be incorporated into air conditioning equipment installed in buildings, factories, ordinary homes, vehicles, etc.

[0101] 100: Nonwoven fabric 1: Constituent fibers of nonwoven fabric A: Portion where constituent fibers of nonwoven fabric are bonded together by the sheath component of core-sheath composite fiber B1: Portion derived from molten single fiber where constituent fibers of nonwoven fabric are bonded together B2: Portion derived from molten single fiber that connects the bonded portions of constituent fibers of nonwoven fabric C: Single fiber portion composed of B1 and B2 D: Granular organic resin

Claims

1. A filter comprising a nonwoven fabric containing, as constituent fibers, core-sheath composite fibers consisting of a core component which is an organic resin and a sheath component which is an organic resin having a lower melting point than the core component, wherein the constituent fibers of the nonwoven fabric have portions where they are bonded together by the sheath component, and the filter has monofilament portions made of organic resin that are fused together to bond the constituent fibers of the nonwoven fabric.

2. The filter according to claim 1, wherein the nonwoven fabric is composed solely of the core-sheath type composite fibers.

3. The filter according to claim 1, wherein the percentage of the mass of the monofilament portion relative to the mass of the nonwoven fabric is greater than 0 mass % and less than 30 mass %.

4. The filter of claim 1, wherein the thickness of the nonwoven fabric is greater than 1.2 mm.

5. A filter unit comprising a filter according to any one of claims 1 to 4 folded into a pleated shape.

6. A filter comprising a layer of frictionally charged nonwoven fabric between two or more layers of the nonwoven fabric according to any one of claims 1 to 4, the layer being a mixture of two or more types of fibers whose fiber surfaces are made of different resins.

7. A filter unit comprising the filter of claim 6 folded into a pleated shape.

8. A method for manufacturing a filter, comprising: (Step 1) preparing a core-sheath composite fiber composed of a core component which is an organic resin and a sheath component which is an organic resin having a melting point lower than that of the core component; (Step 2) preparing a single fiber composed of an organic resin having a melting point equal to or higher than the melting point of the sheath component and lower than that of the core component; (Step 3) blending the core-sheath composite fiber with the single fiber to prepare a fiber web; (Step 4) heating the fiber web to a temperature equal to or higher than the melting point of the organic resin constituting the single fiber and lower than the melting point of the core component to melt the sheath component and the organic resin constituting the single fiber; and (Step 5) cooling the fiber web that has been subjected to (Step 4) to prepare a nonwoven fabric.

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