Nonwoven fabric and air-filter filter material

Abstract

This nonwoven fabric is composed of at least three kinds of organic fibers having different fiber diameters. The nonwoven fabric is characterized by containing ultrafine fibers having a fiber diameter of 0.01-0.90 μm and a degree of irregularity of 1.1-5.0.　Provided is a nonwoven fabric that has both high collection-efficiency and low pressure-loss and that is suitable for an air-filter filter material.

Description

Nonwoven fabrics and air filter media

[0001] The present invention relates to a nonwoven fabric suitable for an air filter medium and an air filter medium.

[0002] In recent years, there has been an increasing demand for clean spaces, and air filters that remove fine dust from the air are used in a wide range of fields, from living environments to industry, to address health problems caused by dust particles of 2.5 μm or less in particle size, to eliminate dust in the manufacture of semiconductors and pharmaceuticals, etc. In particular, in spaces that require extremely clean air, such as clean rooms and semiconductor manufacturing equipment, high-performance air filters, such as HEPA filters (High Efficiency Particulate Air filters) and ULPA filters (Ultra Low Penetration Air filters), are used.

[0003] Conventionally, the air filter material of HEPA filter or ULPA filter is made of glass fiber nonwoven fabric or PTFE (polytetrafluoroethylene) porous membrane.But the air filter material made of glass fiber nonwoven fabric and PTFE porous membrane is disposed of in landfill after use, and there is a problem that it causes heavy environmental impact.

[0004] In response to the recent increase in environmental awareness, various nonwoven fabrics have been proposed that are made of ultrafine polyester fibers and have excellent filtering properties, which have a smaller environmental impact.

[0005] For example, Patent Document 1 proposes an air filter medium using a nonwoven fabric containing three types of polyester fibers: nanofibers with a fiber diameter of 200 to 800 nm, fibers thicker than the nanofibers, and binder fibers. This proposal claims to provide an air filter medium that has excellent pleating properties and wind pressure deformation resistance, low pressure loss, and high collection performance.

[0006] Patent Document 2 proposes an air filter medium using a nonwoven fabric containing three types of polyester fibers: nanofibers with a fiber diameter of 100 to 1,000 nm, fibers with a fiber diameter of 4 to 12 μm, and binder fibers. This proposal claims to provide an air filter medium with excellent texture uniformity, thin film pleatability, low pressure loss, and high collection performance.

[0007] JP 2015-140495 A JP 2022-105839 A

[0008] The method described in Patent Document 1 can produce a nonwoven fabric that is excellent in pleatability and resistance to wind pressure deformation, but does not achieve the collection efficiency required for HEPA filters or ULPA filters, and is also insufficient in terms of achieving both low pressure loss and high collection efficiency.

[0009] Furthermore, the method described in Patent Document 2 can obtain a thin nonwoven fabric that is excellent in pleatability and can further achieve the collection efficiency required for HEPA filters and ULPA filters, but the pressure loss is high and the method is insufficient in terms of achieving both low pressure loss and high collection efficiency.

[0010] An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a nonwoven fabric that is suitable for air filter media and has both high collection efficiency and low pressure loss, and an air filter media using the same.

[0011] As a result of intensive research into the above-mentioned problems, the present inventors have discovered that a nonwoven fabric composed of at least three types of organic fibers with different fiber diameters, which contains ultrafine fibers having a fiber diameter and degree of irregularity within a specific range, can have both high collection efficiency and low pressure loss and can be suitably used as an air filter medium, thereby completing the present invention.

[0012] The above-mentioned problems can be solved by the present invention, that is, a nonwoven fabric composed of at least three types of organic fibers having different fiber diameters, characterized in that the nonwoven fabric contains ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm and a degree of irregularity of 1.1 to 5.0, microfibers, and binder fibers.

[0013] It is also preferable that the variation in the degree of irregularity of the ultrafine fibers is 1.0 to 10.0%.

[0014] The nonwoven fabric can be suitably used as an air filter medium, and the air filter medium can be suitably used as an air filter.

[0015] The air filter can also be suitably used in fan filter units, clean rooms, and semiconductor manufacturing equipment.

[0016] According to the present invention, a nonwoven fabric suitable for an air filter medium can be obtained, which has both high collection efficiency and low pressure loss.

[0017] FIG. 1 is a diagram showing an example of the cross-sectional shape of an ultrafine fiber according to the present invention.

[0018] The nonwoven fabric of the present invention is a nonwoven fabric composed of at least three types of organic fibers having different fiber diameters, and is characterized by containing ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm and a degree of irregularity of 1.1 to 5.0, microfibers, and binder fibers.

[0019] The nonwoven fabric of the present invention will be described in detail below. The ultrafine fibers, microfibers and binder fibers used in the nonwoven fabric of the present invention are all short fibers.

[0020] The ultrafine fibers used in the nonwoven fabric of the present invention have a fiber diameter of 0.01 to 0.90 μm. The fiber diameter in this invention refers to the value measured by the method described in the Examples section. Smaller fiber diameters are preferable because they result in a higher specific surface area and exhibit high collection performance when made into a nonwoven fabric. However, a fiber diameter of 0.01 μm or more not only provides high collection performance, but also improves handleability and molding processability during nonwoven fabric processing, resulting in a nonwoven fabric with excellent durability during use. The fiber diameter is more preferably 0.05 μm or more, and even more preferably 0.10 μm or more. On the other hand, a fiber diameter of 0.90 μm or less exhibits excellent collection performance when made into a nonwoven fabric due to the high specific surface area resulting from the small fiber diameter. The fiber diameter is more preferably 0.70 μm or less, and even more preferably 0.50 μm or less.

[0021] Because ultrafine fibers have a smaller fiber diameter than ordinary fibers, when they are made into short fibers, the ratio of fiber diameter to fiber length (= aspect ratio) is large. Due to the large aspect ratio, ultrafine fibers have a problem of entanglement and aggregation in the solution when stirring during the process of obtaining ultrafine fibers by dissolving and removing the sea component from sea-island fibers (described below) or during the process of preparing a fiber dispersion containing ultrafine fibers and making paper. A nonwoven fabric in which ultrafine fibers are entangled and aggregated has low texture uniformity. Furthermore, in air filter media such as HEPA filters and ULPA filters, fine dust particles collide with the ultrafine fibers that make up the air filter media, and the particles adhere to the surfaces of the ultrafine fibers, thereby achieving high collection performance. However, in a nonwoven fabric in which the ultrafine fibers are entangled and aggregated, the surface area of ​​the ultrafine fibers that is effective for the adhesion of fine dust particles is reduced, resulting in insufficient collection performance.

[0022] As a result of intensive research into the above-mentioned problems, the inventors have found that by forming ultrafine fibers with an irregular cross section, entanglement and aggregation of the ultrafine fibers can be suppressed, leading to the completion of the present invention, which allows the production of nonwoven fabrics suitable for use as air filter media. Specifically, when the ultrafine fibers have a circular cross section (meaning a cross section that is a perfect circle or a circular cross section close to a perfect circle; the same applies hereinafter), the ultrafine fibers can bend in any direction 360° in the solution when stirred, resulting in entanglement and aggregation of the ultrafine fibers. In contrast, when the ultrafine fibers have an irregular cross section, for example, an equilateral triangle, the directions in which the ultrafine fibers can bend can be limited to three directions, thereby suppressing entanglement and aggregation of the ultrafine fibers compared to a circular cross section with a perfect circle. As a result, the resulting nonwoven fabric has a uniform texture, and when used as an air filter media, the surface area of ​​the ultrafine fibers that is effective for adhering fine dust particles is increased, resulting in high collection performance.

[0023] In addition, when the blending ratio (weight%) of ultrafine fibers in the nonwoven fabric increases, the nonwoven fabric becomes denser, and the pressure loss increases when used as an air filter material. However, by making the ultrafine fibers irregular cross-section, the surface area of ​​each ultrafine fiber can be increased compared to a circular cross-section with the same fiber cross-sectional area. Therefore, assuming that the cross-sectional area of ​​the fiber is the same as that of a circular cross-section, and that the same collection performance is achieved when made into a nonwoven fabric, the blending ratio (weight%) of ultrafine fibers in the nonwoven fabric can be reduced by making the cross-section irregular, and the pressure loss can be reduced.

[0024] From the above, it is important that the ultrafine fibers according to the present invention have a cross-sectional shape that is not a perfect circular cross-section but an irregular cross-section.

[0025] The ultrafine fibers according to the present invention are not particularly limited as long as the cross-sectional shape of the fibers is other than a perfect circular cross section, and examples thereof include polygonal shapes (e.g., FIGS. 1(a) and 1(b)), multi-lobed shapes (e.g., FIG. 1(c)), flat shapes (e.g., FIG. 1(d)), elliptical shapes (e.g., FIG. 1(e)), cross shapes (e.g., FIG. 1(f)), X-shapes, and Y-shapes (e.g., FIG. 1(g)), but are not limited to these.

[0026] The ultrafine fibers according to the present invention have a non-circularity of 1.1 to 5.0. The non-circularity in the present invention refers to a value measured by the method described in the Examples section. A non-circularity of 1.1 or more can prevent entanglement and aggregation of ultrafine fibers in a solution during stirring in the process of obtaining ultrafine fibers by dissolving and removing the sea component from a sea-island composite fiber, as described below, or in the process of preparing a fiber dispersion containing ultrafine fibers and making paper from the dispersion, as described below. This prevents the ultrafine fibers from becoming entangled and agglomerated when the solution is stirred, resulting in excellent filtering performance when the nonwoven fabric is made into a nonwoven fabric. Furthermore, since the surface area of ​​each ultrafine fiber can be increased compared to a perfectly circular cross section, assuming that the cross-sectional area of ​​the fiber is equivalent to that of a perfectly circular cross section and that the nonwoven fabric is made into a nonwoven fabric with equivalent filtering performance, the blending ratio (wt%) of the ultrafine fibers in the nonwoven fabric can be reduced, thereby reducing pressure loss. The non-circularity is more preferably 1.2 or more, even more preferably 1.5 or more, and particularly preferably 2.0 or more. On the other hand, if the degree of irregularity is 5.0 or less, abrasion during processing of the nonwoven fabric or during use can be suppressed, and a nonwoven fabric with excellent durability can be obtained. The degree of irregularity is more preferably 4.5 or less, even more preferably 4.0 or less, and particularly preferably 3.5 or less.

[0027] The ultrafine fibers according to the present invention preferably have a non-circularity variation of 1.0 to 10.0%. The non-circularity variation in the present invention refers to the value measured by the method described in the Examples section. The smaller the non-circularity variation, the better, from the viewpoint of the uniformity of the texture of the obtained nonwoven fabric and the uniformity of the filter performance when used as an air filter medium. At present, the technical lower limit is 1.0%. On the other hand, if the non-circularity variation is 10.0% or less, the uniformity of the texture of the obtained nonwoven fabric is good, and the filter performance when used as an air filter medium is stable, which is preferable. The non-circularity variation is more preferably 7.0% or less, and even more preferably 5.0% or less.

[0028] The nonwoven fabric of the present invention contains microfibers. By including microfibers, it is possible to prevent ultrafine fibers, which are thinner than microfibers, from falling off during the nonwoven fabric processing by wet papermaking, which will be described later, and to achieve high collection performance by the ultrafine fibers. In addition, the ultrafine fibers are not excessively densified, and high pressure loss can be suppressed.

[0029] The microfibers according to the present invention preferably have a fiber diameter of 1.0 to 30.0 μm. The fiber diameter in the present invention refers to the value measured by the method described in the Examples section. Microfibers having a fiber diameter of 1.0 μm or more are preferred because they serve as aggregates in the nonwoven fabric to maintain the shape of the nonwoven fabric, and because the ultrafine fibers are not excessively densified, suppressing high pressure loss and enabling the production of a nonwoven fabric that combines high collection performance with low pressure loss. The fiber diameter of the microfibers is more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. On the other hand, microfibers having a fiber diameter of 30.0 μm or less are preferred because they suppress the shedding of ultrafine fibers, which are thinner than microfibers, during the nonwoven fabric processing by wet papermaking described below, and because the microfibers serve as a scaffold for the ultrafine fibers in the resulting nonwoven fabric, allowing the formation of three-dimensionally uniform microspaces. The fiber diameter of the microfibers is more preferably 28.0 μm or less, and even more preferably 25.0 μm or less.

[0030] The nonwoven fabric of the present invention contains binder fibers. The inclusion of binder fibers physically bonds the fibers constituting the nonwoven fabric together by thermal bonding, thereby preventing the fibers constituting the nonwoven fabric from falling off during the wet-laid papermaking process described below. This also helps maintain the three-dimensional structure of the nonwoven fabric and improve its strength. The binder fibers are not particularly limited, but suitable examples include sheath-core fibers in which a thermoplastic polymer with a melting point of 150°C or less is disposed in the sheath. When such sheath-core fibers are used, the nonwoven fabric is preferably subjected to a drying process using a Yankee dryer or air-through dryer, or a heat treatment process using a calendar, after formation, whereby the sheath component of the binder fiber on the surface melts and bonds to the other fibers constituting the nonwoven fabric, thereby enhancing the rigidity of the nonwoven fabric. Furthermore, the core component of the binder fiber is preferred because it contributes to ensuring the strength of the nonwoven fabric. It is preferable that the melting point of the core component of the binder fiber is higher than that of the sheath component, and the difference in melting points is 20°C or more, because this allows the sheath component on the surface of the binder fiber to melt sufficiently easily, prevents the orientation of the core component from decreasing, and ensures both sufficient thermal adhesiveness and high rigidity.

[0031] The binder fiber according to the present invention preferably has a fiber diameter of 5.0 to 50.0 μm. The fiber diameter in the present invention refers to a value measured by the method described in the Examples section. A binder fiber diameter of 5.0 μm or more is preferred because the strength of the nonwoven fabric can be ensured by the adhesion between the binder fiber and other fibers constituting the nonwoven fabric. A binder fiber diameter of 7.0 μm or more is more preferred, and 10.0 μm or more is even more preferred. On the other hand, a binder fiber diameter of 50.0 μm or less is preferred because the binder fiber can be uniformly and firmly bonded to other fibers constituting the nonwoven fabric. A binder fiber diameter of 45.0 μm or less is more preferred, and 40.0 μm or less is even more preferred.

[0032] In the nonwoven fabric of the present invention, the blending ratio (wt%) of each fiber is not particularly limited and can be appropriately selected depending on the fiber diameter of each fiber and the intended use and required properties of the nonwoven fabric. However, the blending ratio of ultrafine fibers is preferably 5 to 40 wt%. A blending ratio of ultrafine fibers of 5 wt% or more is preferred because the high specific surface area resulting from the small fiber diameter of the ultrafine fibers results in excellent collection performance when made into a nonwoven fabric. A blending ratio of ultrafine fibers of 10 wt% or more is more preferred, and 15 wt% or more is even more preferred. On the other hand, a blending ratio of ultrafine fibers of 40 wt% or less is preferred because the ultrafine fibers do not become excessively dense in the nonwoven fabric, suppressing high pressure loss and enabling the production of a nonwoven fabric that combines high collection performance and low pressure loss. A blending ratio of ultrafine fibers of 35 wt% or less is more preferred, and 30 wt% or less is even more preferred.

[0033] In the nonwoven fabric of the present invention, the binder fiber content is preferably 5 to 50 wt %. A binder fiber content of 5 wt % or more is preferable because it ensures adhesion between fibers in the nonwoven fabric and improves the strength of the resulting nonwoven fabric. A binder fiber content of 10 wt % or more is more preferable, and 15 wt % or more is even more preferable. On the other hand, a binder fiber content of 50 wt % or less is preferable because it prevents the adhesion of the binder fibers from reducing the size or blocking of the microscopic spaces in the nonwoven fabric, thereby preventing the air flow from being obstructed when air is passed through the nonwoven fabric and suppressing high pressure loss. A binder fiber content of 45 wt % or less is more preferable, and 40 wt % or less is even more preferable.

[0034] In the nonwoven fabric of the present invention, the blending ratio of microfibers is preferably 10 to 90% by weight. A blending ratio of microfibers of 10% by weight or more is preferable because it suppresses the shedding of ultrafine fibers, which are thinner than microfibers, during the nonwoven fabric processing by wet papermaking described below. Therefore, in the resulting nonwoven fabric, the microfibers serve as a scaffold for the ultrafine fibers, forming three-dimensionally uniform microspaces. A blending ratio of microfibers of 15% by weight or more is more preferable, and 20% by weight or more is even more preferable. On the other hand, a blending ratio of microfibers of 90% by weight or less is preferable because it allows for the production of a nonwoven fabric that exhibits both the excellent collection performance of the ultrafine fibers and the good strength of the binder fibers. A blending ratio of microfibers of 80% by weight or less is more preferable, and 70% by weight or less is even more preferable.

[0035] When the three types of organic fibers with different fiber diameters constituting the nonwoven fabric of the present invention are ultrafine fibers, microfibers, and binder fibers, the blending ratio thereof is preferably ultrafine fibers / microfibers / binder fibers = 5 to 40 / 10 to 90 / 5 to 50 (wt %). A blending ratio within this range is preferable because the microfibers serve as a scaffold for the ultrafine fibers, forming three-dimensionally uniform microspaces, thereby enabling the ultrafine fibers to exhibit high collection performance and preventing the ultrafine fibers from becoming excessively dense, thereby suppressing high pressure loss. Furthermore, the binder fibers thermally bond the fibers constituting the nonwoven fabric, thereby maintaining the three-dimensional structure of the nonwoven fabric and improving the strength of the nonwoven fabric.

[0036] The fibers used in the nonwoven fabric of the present invention are organic fibers. In the present invention, organic fibers are fibrous materials primarily composed of organic matter. Specific examples of organic fibers include, but are not limited to, cellulose produced from wood pulp, natural fibers such as cotton, hemp, wool, and silk, regenerated fibers such as rayon, semi-synthetic fibers such as acetate, and synthetic fibers such as polyester, nylon, and acrylic. Among these, synthetic fibers made of thermoplastic polymers are preferred from the viewpoint of mechanical properties and dimensional stability. Specific examples of thermoplastic polymers include, but are not limited to, polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid; polyamides such as polyamide 6, polyamide 66, and polyamide 610; polyolefins such as polyethylene, polypropylene, and polymethylpentene; thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane, and copolymers thereof. Among these, polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate, polyamides such as polyamide 6 and polyamide 66, and polyphenylene sulfide are preferred because they have both mechanical properties and heat resistance.

[0037] The organic fiber in the present invention may be modified in various ways by adding secondary additives, as long as the effects of the present invention are not impaired. Specific examples of secondary additives include, but are not limited to, compatibilizers, plasticizers, antioxidants, ultraviolet absorbers, infrared absorbers, fluorescent brighteners, release agents, antibacterial agents, nucleating agents, heat stabilizers, flame retardants, antistatic agents, coloring inhibitors, adjusters, matting agents, defoaming agents, preservatives, gelling agents, latex, fillers, inks, colorants, dyes, pigments, and fragrances. These secondary additives may be used alone or in combination.

[0038] The basis weight of the nonwoven fabric of the present invention is 10 to 200 g / m ２ The basis weight of the nonwoven fabric in the present invention refers to a value measured by the method described in the Examples section. ２If the weight is 20 g / m or more, the handling property and molding processability during nonwoven fabric processing will be good, and a uniform nonwoven fabric with little density difference and excellent durability during use can be obtained, which is preferable. ２ More preferably, it is 30 g / m or more. ２ On the other hand, it is more preferable that the basis weight is 200 g / m or more. ２ If the weight is 150 g / m or less, it is possible to suppress high pressure loss due to densification of the nonwoven fabric, and also it is preferable because it has good molding processability such as pleating when used as an air filter medium. ２ More preferably, it is 100 g / m or less. ２ It is more preferable that:

[0039] The thickness of the nonwoven fabric of the present invention is preferably 0.05 to 1.0 mm. The thickness of the nonwoven fabric in the present invention refers to the value measured by the method described in the Examples section. A thickness of 0.05 mm or more is preferable because it provides good handleability and molding processability during nonwoven fabric processing, allowing for the production of a nonwoven fabric with excellent durability during use. Furthermore, when used as an air filter medium, it is preferable because it provides good molding processability, such as pleating. The thickness is more preferably 0.1 mm or more, and even more preferably 0.2 mm or more. On the other hand, a thickness of 1.0 mm or less is preferable because it can suppress high pressure loss due to densification of the nonwoven fabric. Furthermore, when used as an air filter medium, when pleated to form an air filter, the thickness of the filter medium reduces the contact area between adjacent filter materials, thereby ensuring a sufficient filtration area and suppressing an increase in pressure loss, which is preferable. The thickness is more preferably 0.9 mm or less, and even more preferably 0.8 mm or less.

[0040] The porosity of the nonwoven fabric of the present invention is preferably 70% or more. The porosity of the nonwoven fabric in the present invention refers to the value measured by the method described in the Examples section. A porosity of 70% or more is preferable because it suppresses high pressure loss due to densification of the nonwoven fabric, allows air to flow efficiently into the fine spaces in the nonwoven fabric, improves filtration performance, and allows a nonwoven fabric to be obtained that combines high collection performance and low pressure loss. A porosity of 80% or more is more preferable, and a porosity of 90% or more is even more preferable.

[0041] Next, an example of a method for producing the nonwoven fabric of the present invention will be described below.

[0042] First, fibers other than ultrafine fibers are added to an aqueous medium and stirred with a disintegrator to uniformly disperse the fibers, thereby preparing a fiber dispersion. In this process, the dispersibility of the fibers can be adjusted by adjusting the amount of fiber charged, the amount of aqueous medium, the stirring time, etc., and it is preferable that each short fiber is dispersed as uniformly as possible in the aqueous medium. Furthermore, a dispersant may be added to improve the dispersibility of the fibers in the aqueous medium, but it is preferable to keep the amount of dispersant added to the minimum necessary so as not to affect the processability when the nonwoven fabric is subjected to post-processing.

[0043] Next, a fiber dispersion of ultrafine fibers in which the ultrafine fibers are uniformly dispersed in an aqueous medium is prepared according to the method described below. This fiber dispersion of ultrafine fibers is mixed with the above-mentioned fiber dispersion of fibers other than ultrafine fibers to obtain a papermaking solution, which is then wet-laid to obtain a nonwoven fabric in which the ultrafine fibers, the thinnest of all the fibers constituting the nonwoven fabric, are uniformly distributed.

[0044] The ultrafine fibers of the present invention can be produced by using a sea-island fiber made of two or more polymers with different dissolution rates in a solvent. The sea-island fiber is a fiber having a structure in which island components made of a slightly soluble polymer are scattered in a sea component made of a readily soluble polymer. The sea component is dissolved to leave only the island components, thereby obtaining ultrafine fibers.

[0045] The sea-island composite spinning method by melt spinning is suitable for spinning the sea-island composite fiber because it has high productivity and can be produced continuously. Furthermore, a method using a sea-island composite spinneret is preferred because it allows excellent control of the fiber diameter and cross-sectional shape of the island component fibers.

[0046] Specific examples of the hardly soluble polymer used for the island components include, but are not limited to, polyesters such as polyethylene terephthalate (PET), polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polylactic acid; polyamides such as polyamide 6, polyamide 66, and polyamide 610; polyolefins such as polyethylene, polypropylene, and polymethylpentene; thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane; and copolymers thereof.

[0047] From the viewpoint of simplifying the dissolution step of the sea component, the readily soluble polymer used for the sea component preferably exhibits readily solubility in an aqueous solvent, hot water, etc. As the readily soluble polymer in the present invention, it is preferable to use a copolymerized polyester, polylactic acid, polyvinyl alcohol, etc., and in particular, it is preferable to use a polyester copolymerized with polyethylene glycol or sodium 5-sulfoisophthalate, either alone or in combination, or polylactic acid, from the viewpoint of ease of handling and easy solubility in a low-concentration aqueous solvent.

[0048] Easily soluble means that the dissolution rate ratio (easily soluble polymer / slightly soluble polymer) is 100 or more when the slightly soluble polymer is used as a reference in the solvent used in the dissolution treatment. In consideration of simplifying the dissolution treatment and shortening the time, this dissolution rate ratio is preferably large, more preferably 1,000 or more, and even more preferably 10,000 or more. This range is preferable because the dissolution treatment can be completed in a short time and ultrafine fibers suitable for the present invention can be obtained without unnecessary deterioration of the slightly soluble polymer.

[0049] Furthermore, from the viewpoints of solubility in aqueous solvents and simplification of treatment of waste liquid generated during dissolution, polyesters copolymerized with polylactic acid, 3 to 20 mol % of sodium 5-sulfoisophthalate, and polyesters copolymerized with the aforementioned sodium 5-sulfoisophthalate and 5 to 15 wt % of polyethylene glycol having a weight-average molecular weight of 500 to 3,000 are particularly preferred.

[0050] As a result, examples of suitable polymer combinations for the aforementioned islands-in-sea fiber include, but are not limited to, a sea component made of either a polyester copolymerized with 3 to 20 mol % of sodium 5-sulfoisophthalate and 5 to 15 wt % of polyethylene glycol having a weight average molecular weight of 500 to 3000, or polylactic acid, and an island component made of either polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or copolymers thereof.

[0051] The spinning temperature for the sea-island composite fiber is preferably set to a temperature at which the poorly soluble polymer and the easily soluble polymer, mainly those with high melting points and high viscosity, exhibit fluidity, as determined from the above-mentioned viewpoints. The temperature at which the fluidity is exhibited varies depending on the properties and molecular weight of the polymer, but the melting point of the polymer is used as a guide, and the spinning temperature may be set to a temperature not higher than 60°C above the melting point. This range is preferable because it prevents thermal decomposition of the polymer in the spinning head or spin pack, thereby preventing a decrease in molecular weight and enabling a good sea-island composite fiber to be produced.

[0052] As described above, in the present invention, ultrafine fibers are obtained by dissolving and removing the sea component from the sea-island composite fiber, and therefore, to obtain ultrafine fibers with irregular cross sections, the island component fibers must have irregular shapes. To obtain sea-island composite fibers with irregular island component shapes, the sea-island composite spinneret exemplified in Japanese Patent No. 5740877 can be suitably used, but is not limited to this.

[0053] The melted yarn extruded from the sea-island composite spinneret is cooled and solidified, and converged by adding an oil or the like, and taken up by a roller with a specified peripheral speed. The take-up speed can be determined based on the discharge rate and the target fiber diameter, and is preferably 100 to 7000 m / min from the viewpoint of stable production of the sea-island composite fiber. The spun sea-island composite fiber is preferably drawn from the viewpoint of improving mechanical properties and thermal stability. The drawn multifilament may be drawn after being wound up, or may be drawn immediately after spinning without being wound up.

[0054] The sea-island composite fiber is preferably formed into a tow of several tens to several millions of fibers, and then cut to the desired fiber length using a cutting machine such as a guillotine cutter, a slicer, or a cryostat. The fiber length after cutting is preferably set so that the ratio (fiber length / fiber diameter) of the sea-island composite fiber to the diameter of the island component fiber (corresponding to the fiber diameter of the ultrafine fiber) is 1,000 to 6,000. This range is preferable because it increases the number of contact points between fibers in a nonwoven fabric, promotes the formation of a bridging structure between fibers, and enhances the reinforcing effect of the nonwoven fabric. A fiber length / fiber diameter ratio of 1,000 or more is preferable because it prevents the ultrafine fibers from falling out of the nonwoven fabric during wetlaid papermaking. A fiber length / fiber diameter ratio of 1,500 or more is more preferable, and a ratio of 2,000 or more is even more preferable. On the other hand, a fiber length / fiber diameter ratio of 6,000 or less is preferable because it prevents the ultrafine fibers from aggregating in an aqueous medium, resulting in a highly uniform nonwoven fabric. The fiber length / fiber diameter ratio is more preferably 5,500 or less, and even more preferably 5,000 or less.

[0055] Ultrafine fibers can be produced by dissolving and removing the sea part from the sea-island composite fiber. That is, the sea-island composite fiber after cutting can be immersed in a solvent capable of dissolving the soluble polymer of the sea part to remove the soluble polymer. When the soluble polymer is copolymerized polyethylene terephthalate or polylactic acid in which sodium 5-sulfoisophthalate or polyethylene glycol is copolymerized, an alkaline aqueous solution such as a sodium hydroxide aqueous solution can be used. When using an alkaline aqueous solution, the bath ratio of the sea-island composite fiber to the alkaline aqueous solution (weight (g) of the sea-island composite fiber:weight (g) of the alkaline aqueous solution) is preferably 1:5 to 1:10,000, more preferably 1:10 to 1:5,000. This range is preferable because it prevents unnecessary entanglement of the ultrafine fibers when the soluble polymer of the sea part dissolves.

[0056] The alkali concentration of the alkaline aqueous solution is preferably 0.1 to 5 wt %, more preferably 0.5 to 3 wt %. This range is preferable because dissolution of the readily soluble sea component polymer is completed in a short time, and a fiber dispersion in which ultrafine fibers are uniformly dispersed can be obtained without unnecessarily deteriorating the sparingly soluble island component polymer. The temperature of the alkaline aqueous solution is not particularly limited, but a temperature of 50°C or higher is preferable because dissolution of the readily soluble sea component polymer can be accelerated.

[0057] In the present invention, an aqueous solution of the readily soluble sea component polymer from the sea-island composite fiber may be used as a fiber dispersion of ultrafine fibers as is, or may be used after adjusting the pH by adding an acid or alkali, or after diluting with water. A dispersant may also be added to the fiber dispersion to prevent the ultrafine fibers from aggregating over time. Examples of dispersants include cationic compounds, nonionic compounds, and anionic compounds. Among these, anionic compounds are preferred for improving dispersibility in an aqueous medium due to electrical repulsion. The amount of dispersant added is preferably 0.001 to 10 times the weight of the ultrafine fibers. This range is preferred because it ensures the dispersibility of the ultrafine fibers without impairing the processability during wetlaid nonwoven fabric fabrication.

[0058] In the present invention, when the fibers other than the ultrafine fibers (microfibers, binder fibers, etc.) are synthetic fibers made of thermoplastic polymers, they can be produced by melt spinning using a known method, stretching as needed, and then cutting to the desired fiber length as described above.Here, the fiber length of the fibers other than the ultrafine fibers is preferably 30 mm or less.If the fiber length is 30 mm or less, the formation of fiber agglomerates caused by the fibers being tightly entangled with each other during dispersion in an aqueous medium is suppressed, and a homogeneous nonwoven fabric can be obtained, which is preferable because it can be used as an air filter material.

[0059] The fiber dispersion of ultrafine fibers prepared in this manner is mixed with the fiber dispersion of fibers other than ultrafine fibers, diluted to a certain concentration, and adjusted to prepare a papermaking stock solution, which is then dewatered on an inclined wire, cylinder, or the like to form a nonwoven fabric by wet papermaking. Apparatuses used for wet papermaking include, but are not limited to, a cylinder papermaking machine, a Fourdrinier papermaking machine, an inclined short-wire papermaking machine, or a combination thereof. In the papermaking process, a three-dimensionally homogeneous nonwoven fabric can be produced by adjusting the papermaking speed, the amount of fiber, and the aqueous medium, as well as the dispersibility of the fibers in the papermaking stock solution, to control the accumulation of fibers during drainage.

[0060] The nonwoven fabric formed by wet papermaking is subjected to a drying process to remove moisture. As a drying method, a method using hot air ventilation (air through) or a method of contacting with a heated rotating roll (heated calendar roll, etc.) can be suitably adopted, from the viewpoint that the nonwoven fabric can be dried and the heat-bonding of the heat-bondable fibers can be carried out simultaneously.

[0061] The nonwoven fabric of the present invention has both high collection efficiency and low pressure loss, so the air filter material using the nonwoven fabric of the present invention can be suitably used as the air filter material for air purifiers, air conditioners, building air conditioners, industrial clean rooms, and the cabins of automobiles, trains, etc. In addition, in spaces that require extremely high clean air, such as clean rooms and semiconductor manufacturing equipment, it can be suitably used as the air filter material for, for example, the air conditioner air filter that takes in outside air into the clean room, the air conditioner air filter that circulates the air in the clean room, and the air filter material for the fan filter unit that is installed on the ceiling of the clean room or semiconductor manufacturing equipment.The clean rooms and semiconductor manufacturing equipment equipped with the air filter that uses these air filter material of the present invention are useful in various industries.

[0062] The present invention will now be described in detail with reference to examples. However, the present invention is not limited to these examples. The respective property values ​​in the examples were determined by the following methods.

[0063] A. Dispersion State of Fiber Dispersion The fiber dispersions of ultrafine fibers prepared in the Examples and Comparative Examples were used as samples, and 20 evaluators visually evaluated the dispersion state of the ultrafine fibers in the dispersion, assigning 5 points to "absolutely no floating matter or sediment due to entanglement or aggregation of ultrafine fibers," 4 points to "almost no floating matter or sediment due to entanglement or aggregation of ultrafine fibers," 3 points to "a small amount of floating matter or sediment due to entanglement or aggregation of ultrafine fibers," 2 points to "a large amount of floating matter or sediment due to entanglement or aggregation of ultrafine fibers," and 1 point to "a very large amount of floating matter or sediment due to entanglement or aggregation of ultrafine fibers." The scores assigned by the 20 evaluators were averaged, and an average score of 3.0 or more was considered to be acceptable.

[0064] B. Fiber diameter The sea-island composite fibers obtained in the examples and comparative examples were used as samples, embedded in epoxy resin, frozen using a Reichert FC-4E cryosectioning system, and cut using a Reichert-Nissei Ultracut N (ultramicrotome) equipped with a diamond knife. The cut surface was then imaged at 5000x magnification using a transmission electron microscope (H-7100FA, Hitachi High-Technologies Corporation). The diameters of the perfect circles circumscribing 100 island component fibers randomly selected from the image (circumscribed circle diameter) were measured to two decimal places in μm units, and the average value was calculated and rounded off to two decimal places to obtain the diameter of the island component fibers of the sea-island composite fiber (corresponding to the fiber diameter of the ultrafine fibers used as raw fiber).

[0065] The microfibers or binder fibers used as raw material fibers in the Examples and Comparative Examples were used as samples, and images were taken with a scanning electron microscope (SU-1510 manufactured by Hitachi High-Technologies Corporation) at a magnification sufficient to observe 150 to 300 fibers, and the fiber diameters of 100 fibers randomly selected from the images were measured. The fiber diameter was determined by measuring the fiber width in the direction perpendicular to the fiber axis from the two-dimensionally captured image to one decimal place in μm units, and the average value was calculated and rounded to one decimal place to obtain the fiber diameter of the microfibers or binder fibers.

[0066] The nonwoven fabrics obtained in the Examples and Comparative Examples were used as samples, and images of the surface of the nonwoven fabric were taken with a scanning electron microscope (SU-1510 manufactured by Hitachi High-Technologies Corporation) at a magnification sufficient to observe 300 to 3,000 fibers. The fiber diameters of 100 fibers randomly selected from the images were measured. The fiber diameters were measured in units of μm to two decimal places from the two-dimensionally captured images, taking the fiber width in the direction perpendicular to the fiber axis as the fiber diameter. The above procedure was repeated for 10 images taken in the same manner, and a graph showing the fiber diameter distribution was created with the fiber diameter on the horizontal axis and the number on the vertical axis for a total of 1,000 fibers measured from the 10 images. Next, for each of the upwardly protruding fiber diameter distributions, within a fiber diameter range of ±10% of the peak value with the highest number of fibers present in the fiber diameter distribution, the fiber diameter was calculated using the following formula: Fiber diameter (μm) = {Sum of (fiber diameters × number of fibers constituting the fiber diameter distribution)} ÷ (Sum of the number of fibers constituting the fiber diameter distribution), and the value was rounded to two decimal places to obtain the fiber diameter. Here, the smallest value among the calculated fiber diameters was determined as the fiber diameter of the ultrafine fibers. Furthermore, the fiber diameter calculated for fibers having five or more bonding points with other fibers in the captured image was determined as the fiber diameter of the binder fibers. Note that the bonding points do not simply refer to points where fibers overlap, but rather to points where fibers are bonded to each other and integrated. The fiber diameters of fibers other than the ultrafine fibers and binder fibers were determined as the fiber diameter of the microfibers.

[0067] C. Non-circularity Using the sea-island composite fiber image taken in B above, the diameters of the circles circumscribing 100 randomly selected island component fibers (circumscribing circle diameters) were measured in μm units to two decimal places, and the diameters of the circles inscribing those island component fibers (inscribed circle diameters) were measured in μm units to two decimal places. The circumscribing circle diameter of each island component fiber was then divided by the inscribed circle diameter and rounded to two decimal places to calculate the non-circularity of each island component fiber. The average value of the non-circularities of the 100 island component fibers was then calculated and rounded to two decimal places to obtain the non-circularity of the island component fibers of the sea-island composite fiber (corresponding to the non-circularity of the ultrafine fibers used as raw fiber fibers). When the number of inscribed circles was not fixed, as in Fig. 1(d) and (e), the inscribed circle with the largest diameter was used as the inscribed circle diameter.

[0068] The nonwoven fabrics obtained in the Examples and Comparative Examples were used as samples, and images of the cross sections of the nonwoven fabrics were taken with a scanning electron microscope (SU-1510, manufactured by Hitachi High-Technologies Corporation) at a magnification sufficient to observe 30 to 300 fibers. Ten fibers were randomly selected from the taken image, and the diameter of the perfect circle circumscribing the fiber cross section (circumscribing circle diameter) was measured in μm units to two decimal places. Furthermore, the diameter of the perfect circle inscribing each of these fibers (inscribed circle diameter) was measured in μm units to two decimal places. The above procedure was repeated for 10 similarly taken images, and the circumscribing circle diameter and inscribing circle diameter were measured for a total of 100 fibers. Next, the circumscribed circle diameter of each fiber was divided by the inscribed circle diameter, and the result was rounded to two decimal places to calculate the irregularity of each fiber. The average irregularity of 100 fibers was then calculated, and the average irregularity was rounded to two decimal places to determine the irregularity. Note that, when there is not a single inscribed circle, as in Figures 1(d) and (e), the inscribed circle with the largest diameter was used as the inscribed circle diameter.

[0069] D. Irregularity Variation Using the irregularities of 100 island component fibers or 100 fibers calculated in the above section C, the standard deviation and average irregularity were calculated, and then the calculated value was calculated using the following formula: Irregularity Variation (%) = (Standard deviation of irregularity / Average irregularity) x 100. The calculated value was rounded to one decimal place to obtain the irregularity variation of island component fibers in the sea-island composite fiber set (corresponding to the irregularity variation of the ultrafine fibers used as raw fiber) and the irregularity variation (%) of the ultrafine fibers in the nonwoven fabric.

[0070] E. Basis Weight The nonwoven fabric obtained in each of the Examples and Comparative Examples was used as a sample. The nonwoven fabric was cut into a 250 mm x 250 mm square and weighed. ２ The value converted to the weight per unit area (g / m) is rounded to the nearest tenth to obtain the basis weight of the nonwoven fabric (g / m ２ Measurements were carried out at three randomly selected locations per sample, and the average value was rounded off to one decimal place to obtain the basis weight.

[0071] F. Thickness The nonwoven fabric used in the measurement in the above item E was used as a sample, and the thickness of the nonwoven fabric was measured using a dial thickness gauge (SM-114 manufactured by TECLOCK Corporation, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at any five points per sample, and the average value was rounded to two decimal places to calculate the thickness (mm) of the nonwoven fabric.

[0072] G. Porosity Using the basis weight and thickness of the nonwoven fabric calculated in the above items E and F, the value calculated by the following formula was rounded to one decimal place to obtain the porosity (%) of the nonwoven fabric. Porosity (%) = 100 - [basis weight (g / m ２ ) / {thickness (mm) × fiber density (g / cm ３ )}] × 0.1 The fiber density may be the density of the fibers constituting the nonwoven fabric, and in the case of PET, it is 1.38 g / cm ３ It was calculated as:

[0073] H. Collection efficiency The nonwoven fabrics obtained in the examples and comparative examples were used as samples. The nonwoven fabrics were cut into circles with a diameter of 200 mm and an effective opening area of ​​0.1 m. ２ and polystyrene latex particles with a particle size of 0.15 to 0.50 μm were blown in the vertical direction at a surface air velocity of 3.3 m / min at a rate of 10,000 to 25,000 particles / m ３ Air containing the particles was passed through the filter, and the number of atmospheric dust particles with particle diameters of 0.3 to 0.5 μm was measured upstream and downstream of the filter using a particle counter (KC-01D manufactured by RION Corporation), and the collection efficiency was calculated using the following formula: Collection efficiency (%) = {1 - (number of particles downstream / number of particles upstream)} x 100 Measurements were carried out at three randomly selected locations per sample, and the average value was rounded to two decimal places to obtain the collection efficiency (%).

[0074] I. Pressure Loss The nonwoven fabrics obtained in the Examples and Comparative Examples were used as samples. The nonwoven fabrics were cut into circles with a diameter of 200 mm and an effective opening area of ​​0.1 m. ２ The sample was placed in a holder, and air was passed through it vertically at a surface velocity of 3.3 m / min, and the pressure difference between the upstream and downstream sides of the filter was measured with a differential pressure meter. Measurements were taken at three randomly selected locations per sample, and the average value was rounded off to the nearest tenth to obtain the pressure loss (Pa).

[0075] J. Performance Index Using the collection efficiency and pressure loss of the nonwoven fabric calculated in Sections H and I above, the following formula was used to calculate performance index (1 / Pa) = -ln[(1 - collection efficiency (%) / 100)} / pressure loss (Pa)], and the value was rounded to three decimal places to obtain the performance index (1 / Pa) of the nonwoven fabric.

[0076] K. Quality The nonwoven fabrics obtained in the Examples and Comparative Examples were used as samples, and 20 evaluators evaluated the 10 images taken in Section B above. Five points were assigned to "absolutely no areas where ultrafine fibers have aggregated or been scraped off," four points to "almost no areas where ultrafine fibers have aggregated or been scraped off," three points to "a few areas where ultrafine fibers have aggregated or been scraped off," two points to "many areas where ultrafine fibers have aggregated or been scraped off," and one point to "very many areas where ultrafine fibers have aggregated or been scraped off," and the average score assigned by the 20 evaluators was calculated, and an average score of 3.0 or more was deemed acceptable.

[0077] Example 1 Polyethylene terephthalate (PET) was used as island components, and a copolymerized PET prepared by copolymerizing 8.0 mol % of 5-sodium sulfoisophthalate and 10 wt % of polyethylene glycol having a molecular weight of 1,000 was used as a sea component, and each was vacuum dried for 12 hours at 150° C. Subsequently, the island components and sea components were supplied to an extruder-type multi-component spinning machine in a blending ratio of 50 wt % and 50 wt % and melted separately, and the melts were fed into a spinning pack equipped with a sea-island composite spinneret (number of island components: 2,000, cross-sectional shape of island components: triangular, Figure 1(a)) at a spinning temperature of 285° C., and the composite polymer stream was discharged from the discharge holes at a throughput rate of 12 g / min to obtain a spun yarn. The spun yarn was cooled with cooling air at a temperature of 20°C and a speed of 20 m / min, oiled with an oiling device to converge the yarn, taken up by a first godet roller rotating at 1000 m / min, passed through a second godet roller rotating at the same speed as the first godet roller, and wound by a winder to obtain an undrawn yarn. The undrawn yarn was then drawn 3.4 times between rollers heated to 85°C and 130°C using a drawing machine to produce a sea-island fiber (island component diameter 0.60 µm, island component cross-sectional area 0.115 µm). ２The irregularity of the island component fiber was 1.89. The diameter of the circle circumscribing the island component fiber (circumscribing circle diameter) was defined as the diameter of the island component fiber.

[0078] The resulting sea-island composite fiber was cut to a fiber length of 0.6 mm, treated in a 1 wt% aqueous sodium hydroxide solution at a bath ratio of 1:100 at 90°C for 30 minutes, and then neutralized with acetic acid to a pH of 7 to obtain a fiber dispersion of ultrafine fibers.

[0079] Next, PET staple fibers (fiber diameter 3.0 μm, fiber length 3.0 mm) were used as the microfibers at a blending ratio of 55% by weight (compounding ratio in the papermaking dope), and core-sheath PET staple fibers (core component: PET, sheath component: a copolymerized polyester having a melting point of 110°C copolymerized with 60 mol% of terephthalic acid and 40 mol% of isophthalic acid as dicarboxylic acid components, and 85 mol% of ethylene glycol and 15 mol% of diethylene glycol as diol components, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.0 μm, fiber length 5.0 mm) were used as the binder fibers at a blending ratio of 30% by weight (compounding ratio in the papermaking dope), and these were uniformly mixed and dispersed with water using a disintegrator to prepare a fiber dispersion of microfibers and binder fibers.

[0080] The papermaking stock solution was prepared by homogeneously mixing the above-mentioned ultrafine fiber dispersion with this fiber dispersion of microfibers and binder fibers so that the blending ratio of the ultrafine fibers was 15 wt % (blending ratio in the papermaking stock solution). This papermaking stock solution was wet-formed using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., and then dried and heat-treated in a rotary dryer with a roller temperature set to 110°C, to obtain a nonwoven fabric.

[0081] The evaluation results of the ultrafine fiber dispersion and the obtained nonwoven fabric are shown in Table 1. The obtained nonwoven fabric had a basis weight of 50.1 g / m ２ The cross-sectional shape of the ultrafine fibers is triangular, which prevents the ultrafine fibers from entangling or agglomerating in the fiber dispersion and nonwoven fabric, resulting in both high collection efficiency and low pressure loss, and the air filter medium also showed a good performance index representing the balance between collection efficiency and pressure loss.

[0082] Examples 2 and 3, Comparative Example 1 Nonwoven fabrics were prepared in the same manner as in Example 1, except that the fiber diameter of the ultrafine fibers was changed as shown in Table 1.

[0083] The evaluation results of the ultrafine fiber dispersion and the resulting nonwoven fabric are shown in Table 1. In Examples 2 and 3, the collection efficiency and pressure loss tended to increase as the fiber diameter of the ultrafine fibers decreased, but the performance index showed good values. In Comparative Example 1, the pressure loss of the ultrafine fibers showed a low value. However, because the fiber diameter of the ultrafine fibers was large, the collection performance of the ultrafine fibers was not exhibited, and the collection efficiency was low and the performance index also showed an extremely low value.

[0084] Example 4 A nonwoven fabric was produced in the same manner as in Example 1, except that the cross-sectional shape of the island component fibers was changed to a Y-shape (FIG. 1(g)).

[0085] The evaluation results of the fiber dispersion of ultrafine fibers and the obtained nonwoven fabric are shown in Table 2. Even when the cross section of the ultrafine fibers was Y-shaped, entanglement and aggregation of the ultrafine fibers was suppressed in the fiber dispersion and nonwoven fabric, and the fabric had both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0086] Examples 5 and 6, Comparative Examples 2 and 3 Nonwoven fabrics were prepared in the same manner as in Example 4, except that the degree of deformation of the ultrafine fibers was changed as shown in Table 2.

[0087] The evaluation results of the fiber dispersion of ultrafine fibers and the resulting nonwoven fabric are shown in Table 2. Examples 5 and 6 combined high collection efficiency and low pressure loss, and the performance index also showed good values. In Comparative Example 2, because the degree of irregularity of the ultrafine fibers was small, entanglement and aggregation of the ultrafine fibers were observed in the fiber dispersion and nonwoven fabric, and the pressure loss was low, but the collection performance of the ultrafine fibers was not exhibited, and the collection efficiency and performance index also showed low values. In Comparative Example 3, because the degree of irregularity of the ultrafine fibers was large, the dispersion state of the ultrafine fibers in the fiber dispersion was good, but areas where the ultrafine fibers had been scraped were observed in the nonwoven fabric, resulting in poor quality. Furthermore, although the collection efficiency showed a high value, the pressure loss was somewhat high and the performance index showed somewhat low values.

[0088] Example 7 A nonwoven fabric was produced in the same manner as in Example 1, except that the cross-sectional shape of the island component fibers was changed to an elliptical shape (FIG. 1( e )).

[0089] The evaluation results of the fiber dispersion of ultrafine fibers and the obtained nonwoven fabric are shown in Table 3. Even when the cross-sectional shape of the ultrafine fibers was elliptical, entanglement and aggregation of the ultrafine fibers was suppressed in the fiber dispersion and nonwoven fabric, and the fabric exhibited both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0090] Comparative Example 4 A nonwoven fabric was produced in the same manner as in Example 1, except that the cross-sectional shape of the island component fibers was changed to a circular shape.

[0091] The evaluation results of the fiber dispersion of ultrafine fibers and the obtained nonwoven fabric are shown in Table 3. Because the cross-sectional shape of the ultrafine fibers is circular, entanglement and aggregation of the ultrafine fibers were observed in the fiber dispersion and nonwoven fabric, and although the pressure drop showed a low value, the filtering performance of the ultrafine fibers was not expressed, the filtering efficiency was low, and the performance index also showed a low value.

[0092] Comparative Example 5 A nonwoven fabric was prepared in the same manner as in Comparative Example 4, except that the fiber diameter of the ultrafine fibers was changed as shown in Table 3.

[0093] The evaluation results of the ultrafine fiber dispersion and the resulting nonwoven fabric are shown in Table 3. Because the cross-sectional shape of the ultrafine fibers is circular, entanglement and aggregation of the ultrafine fibers were observed in the fiber dispersion and nonwoven fabric. Because the ultrafine fibers have a small fiber diameter, the collection efficiency was high, but the pressure loss was somewhat high and the performance index was somewhat low.

[0094] Comparative Examples 6 to 8 Nonwoven fabrics were produced in the same manner as in Example 1, except that in Comparative Example 6, no ultrafine fibers were used, in Comparative Example 7, no microfibers were used, and in Comparative Example 8, no binder fibers were used, and the blending ratios of the three types of fibers were changed as shown in Table 3.

[0095] The evaluation results of the ultrafine fiber dispersion and the resulting nonwoven fabric are shown in Table 3. Comparative Example 6 exhibited a low pressure drop. However, because ultrafine fibers, which have a small fiber diameter and a high specific surface area, were not used, dust collection performance was not achieved, the collection efficiency was extremely low, and the performance index also exhibited a low value. Comparative Example 7 exhibited a low pressure drop. However, because microfibers, which serve as a scaffold for the ultrafine fibers, were not used in the nonwoven fabric, most of the ultrafine fibers fell off during wet-laid papermaking processing. As a result, the collection performance of the ultrafine fibers was not fully achieved, and the collection efficiency and performance index also exhibited a low value. Comparative Example 8 exhibited no binder fibers, which serve to bond the fibers constituting the nonwoven fabric, and some of the ultrafine fibers fell off during wet-laid papermaking processing. As a result, the collection performance of Fiber A was not fully achieved, and the collection efficiency and performance index also exhibited a slightly low value.

[0096]

[0097]

[0098]

[0099] 1: Outer shape of ultrafine fiber 2: Circumscribed circle 3: Inscribed circle

Claims

1. A nonwoven fabric composed of at least three types of organic fibers having different fiber diameters, characterized in that it contains ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm and a degree of irregularity of 1.1 to 5.0, microfibers, and binder fibers.

2. The nonwoven fabric according to claim 1, wherein the variation in the degree of irregularity of the ultrafine fibers is 1.0 to 10.0%.

3. An air filter medium comprising the nonwoven fabric according to claim 1 or 2.

4. An air filter comprising the air filter medium according to claim 3.

5. A fan filter unit comprising the air filter according to claim 4.

6. A clean room or semiconductor manufacturing equipment equipped with the air filter according to claim 4.

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