Nonwoven fabric and air filter medium

Abstract

To provide a nonwoven fabric having both high collection efficiency and low pressure loss and suitable for an air filter medium.SOLUTION: A nonwoven fabric comprising ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm, heat-shrinkable fibers, and heat-fusible fibers.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, there has been an increasing demand for cleaner spaces, and air filters that remove fine dust from the air are being used in a wide range of fields, from residential environments to industry, to address health problems caused by dust particles 2.5 μm or smaller in diameter, and to eliminate dust in the manufacturing of semiconductors and pharmaceuticals. 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, nonwoven glass fiber fabrics and porous PTFE (polytetrafluoroethylene) membranes have been used as air filter media for HEPA filters and ULPA filters. However, both air filter media made of nonwoven glass fiber fabrics and porous PTFE membranes have been disposed of in landfills after use, which poses a significant environmental burden.

[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 nanofibers, and heat-fused 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 1000 nm, fibers with a fiber diameter of 4 to 12 μm, and heat-fused fibers. This proposal claims to provide an air filter medium with excellent texture uniformity, thin film pleatability, low pressure loss, and high collection performance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-140495 [Patent Document 2] Japanese Patent Publication No. 2022-105839 Summary of the Invention [Problem to be solved by the invention]

[0008] The method described in Patent Document 1 can produce a nonwoven fabric with excellent pleatability and resistance to wind pressure deformation, but it 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. [Means for solving the problem]

[0011] The above problems can be solved by the present invention, that is, by a nonwoven fabric characterized by containing ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm, heat-shrinkable fibers, and heat-fusible fibers.

[0012] It is also preferable that the ratio (R2 / R1) of the fiber diameter (R1) of the ultrafine fiber to the fiber diameter (R2) of the heat-shrinkable fiber is 10 to 200, and the heat-shrinkable fiber is selected from the group consisting of polyester-based fibers, polyamide-based fibers, polyolefin-based fibers, and combinations thereof.

[0013] 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.

[0014] The air filter can also be suitably used in fan filter units, clean rooms, and semiconductor manufacturing equipment.

[0015] Furthermore, a clean room or semiconductor manufacturing equipment equipped with the air filter can be suitably employed in the manufacture of semiconductors. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0017] [Figure 1] Figure 1 shows examples of cross-sectional shapes of ultrafine fibers according to the present invention: (a) triangular, (b) rectangular, (c) multilobal, (d) flat, (e) oval, (f) cross, and (g) Y-shaped. DETAILED DESCRIPTION OF THE INVENTION

[0018] The nonwoven fabric of the present invention is characterized by containing ultrafine fibers with a fiber diameter of 0.01 to 0.90 μm, heat-shrinkable fibers, and heat-fusible fibers.

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

[0020] The ultrafine fibers contained in the nonwoven fabric of the present invention have a fiber diameter of 0.01 to 0.90 μm. The fiber diameter in the present invention refers to a 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] The ultrafine fibers contained in the nonwoven fabric of the present invention may have a cross-sectional shape of a perfect circle (circular cross-section) or a modified cross-section. The modified cross-sectional shape of the ultrafine fibers is preferred because it can prevent entanglement and aggregation of the ultrafine fibers in a solution when stirring the solution in the process of obtaining ultrafine fibers by dissolving and removing the sea component from sea-island composite fibers, which will be described later, and in the process of preparing a fiber dispersion containing the ultrafine fibers and making paper from the dispersion. As a result, the ultrafine fibers are uniformly dispersed in the nonwoven fabric, which exhibits excellent filtering performance and prevents high pressure loss due to the densification of the ultrafine fibers. This makes it possible to obtain a nonwoven fabric that combines high filtering performance and low pressure loss.

[0022] Specific examples of the irregular cross section of the ultrafine fibers of the present invention include, but are not limited to, polygonal (for example, see FIGS. 1(a) and 1(b)), multilobal (for example, see FIG. 1(c)), flat (for example, see FIG. 1(d)), elliptical (for example, see FIG. 1(e)), cross (for example, see FIG. 1(f)), X-shape, and Y-shape (for example, see FIG. 1(g)).

[0023] When the ultrafine fibers of the present invention have an irregular cross section, the irregularity is preferably 1.1 to 5.0. The irregularity in the present invention refers to a value measured by the method described in the Examples section. An irregularity of 1.1 or more is preferable because it can prevent the ultrafine fibers from becoming entangled and agglomerated in the solution when stirred during the process of obtaining ultrafine fibers by dissolving and removing the sea component from a sea-island composite fiber, as described below, or during the process of preparing a fiber dispersion containing ultrafine fibers and making paper from the dispersion, thereby achieving excellent collection performance when made into a nonwoven fabric. The irregularity 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, an irregularity of 5.0 or less is preferable because it can prevent abrasion during nonwoven fabric processing and use, resulting in a nonwoven fabric with excellent durability. The irregularity is more preferably 4.5 or less, even more preferably 4.0 or less, and particularly preferably 3.5 or less.

[0024] The ultrafine fibers contained in the nonwoven fabric of the present invention are preferably fibrous materials primarily composed of organic matter (hereinafter referred to as organic fibers). 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 combine mechanical properties and heat resistance.

[0025] 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.

[0026] The nonwoven fabric of the present invention contains heat-shrinkable fibers. By including the heat-shrinkable fibers, the nonwoven fabric can be formed by wet papermaking, as described below, and then the heat-shrinkable fibers can be shrunk in a drying process to expand the voids between the fibers constituting the nonwoven fabric, thereby obtaining a bulky nonwoven fabric. As a result, a nonwoven fabric can be obtained that has high collection performance due to the ultrafine fibers and that suppresses high pressure loss due to the densification of the ultrafine fibers.

[0027] The heat-shrinkable fiber according to the present invention is preferably selected from the group consisting of polyester fibers, polyamide fibers, polyolefin fibers, and combinations thereof. Among these, polyester fibers and polyamide fibers are preferred due to their high mechanical strength. A specific example of a polyester fiber is polyethylene terephthalate fiber copolymerized with isophthalic acid and 2,2-bis{4-(2-hydroxyethoxy)phenyl}propane, but is not limited thereto. A specific example of a polyamide fiber is polyamide fiber copolymerized with nylon 6 and nylon 6,6, but is not limited thereto. A specific example of a polyolefin fiber is polyolefin fiber copolymerized with ethylene and an α-olefin, but is not limited thereto. Specific examples of α-olefins include, but are not limited to, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, and 1-dodecene.

[0028] The heat-shrinkable fiber according to the present invention is different from the heat-fusible fiber described below. Heat-fusible fibers have bonding points with other fibers, such as ultrafine fibers and heat-shrinkable fibers, whereas heat-shrinkable fibers do not have bonding points with other fibers. The above-mentioned bonding points do not refer to the points where fibers simply overlap, but rather to the points where fibers are bonded and integrated by the thermal adhesion of the heat-fusible fiber. Furthermore, the heat-shrinkable fiber according to the present invention is different from the crimped fiber. While the crimped fiber has a crimped shape such as a crimped shape, a curled shape, or a spiral shape, the heat-shrinkable fiber does not have the above-mentioned crimped shapes.

[0029] The shrinkage initiation temperature of the heat-shrinkable fiber according to the present invention is preferably lower than the melting point of the heat-fusible fiber described below. If the shrinkage initiation temperature of the heat-shrinkable fiber is lower than the melting point of the heat-fusible fiber, the heat-shrinkable fiber can be shrunk in the drying step after forming a nonwoven fabric by wet papermaking to expand the voids between the fibers constituting the nonwoven fabric, thereby forming a bulky nonwoven fabric, and then the fibers can be bonded together by the heat-fusible fiber, thereby maintaining the three-dimensional structure of the nonwoven fabric in a bulky state, which is preferable.

[0030] The heat-shrinkable fibers contained in the nonwoven fabric of the present invention preferably have a fiber diameter of 1.0 to 30.0 μm. The fiber diameter in the present invention refers to a value measured by the method described in the Examples section. A heat-shrinkable fiber diameter of 1.0 μm or more is preferable because the expansion of voids between the fibers constituting the nonwoven fabric prevents the ultrafine fibers from becoming excessively dense, suppressing high pressure loss and enabling a nonwoven fabric that combines high collection performance with low pressure loss. The heat-shrinkable fiber diameter is more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. On the other hand, a heat-shrinkable fiber diameter of 30.0 μm or less is preferable because it can prevent the shedding of ultrafine fibers, which are thinner than the heat-shrinkable fibers, during the nonwoven fabric processing by wet papermaking described below, thereby enabling the ultrafine fibers to exhibit high collection performance. The heat-shrinkable fiber diameter is more preferably 28.0 μm or less, and even more preferably 25.0 μm or less.

[0031] In the nonwoven fabric of the present invention, the ratio (R2 / R1) of the fiber diameter (R1) of the ultrafine fibers to the fiber diameter (R2) of the heat-shrinkable fibers is preferably 10 to 200. A fiber diameter ratio (R2 / R1) of 10 or more expands the interfiber spaces between the fibers constituting the nonwoven fabric due to the shrinkage of the heat-shrinkable fibers, preventing the ultrafine fibers from becoming excessively dense. This prevents high pressure loss and allows for the production of a nonwoven fabric that combines high collection performance with low pressure loss. The fiber diameter ratio (R2 / R1) is more preferably 15 or more, and even more preferably 20 or more. On the other hand, a fiber diameter ratio (R2 / R1) of 200 or less is preferable because it prevents the ultrafine fibers, which are thinner than the heat-shrinkable fibers, from falling off during the nonwoven fabric processing by wet papermaking, as described below, thereby enabling the ultrafine fibers to exhibit high collection performance. The fiber diameter ratio (R2 / R1) is more preferably 170 or less, and even more preferably 150 or less.

[0032] The nonwoven fabric of the present invention contains thermally fused fibers. The inclusion of thermally fused fibers allows the fibers constituting the nonwoven fabric to be physically bonded together by thermal bonding, thereby preventing the fibers constituting the nonwoven fabric from falling off during the wet-laid papermaking process described below. Furthermore, the three-dimensional structure of the nonwoven fabric can be maintained and the strength of the nonwoven fabric can be improved. While the thermally fused fibers are not particularly limited, for example, sheath-core fibers in which a thermoplastic polymer with a melting point of 150°C or less is disposed in the sheath can be suitably used. When such core-sheath 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 on the surface of the thermally fused fibers melts and thermally bonds to the other fibers constituting the nonwoven fabric, thereby increasing the rigidity of the nonwoven fabric. Furthermore, the core component of the thermally fused fibers is preferred because it contributes to ensuring the strength of the nonwoven fabric. In addition, if the melting point of the core component of the heat-fusible fiber is higher than that of the sheath component, and the difference in melting points is 20°C or more, the sheath component on the surface of the heat-fusible fiber is easily melted sufficiently, and the decrease in the orientation of the core component is suppressed, thereby achieving both sufficient thermal adhesion and high rigidity, which is preferable.

[0033] The thermally fusible fibers contained in the nonwoven fabric of the present invention preferably have 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 fiber diameter of 5.0 μm or more is preferable because the strength of the nonwoven fabric can be ensured by thermal bonding between the thermally fusible fibers and other fibers constituting the nonwoven fabric. The fiber diameter of the thermally fusible fibers is more preferably 7.0 μm or more, and even more preferably 10.0 μm or more. On the other hand, a fiber diameter of 50.0 μm or less is preferable because the thermally fusible fibers can be uniformly and firmly thermally bonded to other fibers constituting the nonwoven fabric. The fiber diameter of the thermally fusible fibers is more preferably 45.0 μm or less, and even more preferably 40.0 μm or less.

[0034] 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% of the nonwoven fabric. A blending ratio of ultrafine fibers of 5 wt% or more is preferable 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 7.5 wt% or more is more preferable, and 10 wt% or more is even more preferable. On the other hand, a blending ratio of ultrafine fibers of 40 wt% or less is preferable because the ultrafine fibers do not become excessively dense in the nonwoven fabric, suppressing high pressure loss and resulting in 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 preferable, and 30 wt% or less is even more preferable.

[0035] In the nonwoven fabric of the present invention, the blending ratio of the heat-shrinkable fiber is preferably 10 to 90 wt% based on the weight of the nonwoven fabric. If the blending ratio of the heat-shrinkable fiber is 10 wt% or more, the shrinkage of the heat-shrinkable fiber in the nonwoven fabric expands the voids between the fibers constituting the nonwoven fabric, preventing the ultrafine fibers from becoming excessively dense. This prevents high pressure loss and allows for the production of a nonwoven fabric that combines high collection performance with low pressure loss. The blending ratio of the heat-shrinkable fiber is more preferably 15 wt% or more, and even more preferably 20 wt% or more. On the other hand, if the blending ratio of the heat-shrinkable fiber is 90 wt% or less, this 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 heat-fusible fibers. The blending ratio of the heat-shrinkable fiber is more preferably 80 wt% or less, and even more preferably 70 wt% or less.

[0036] In the nonwoven fabric of the present invention, the blending ratio of the thermally fusible fibers is preferably 5 to 50 wt% based on the weight of the nonwoven fabric. A blending ratio of the thermally fusible fibers of 5 wt% or more is preferable because it ensures thermal adhesion between the fibers in the nonwoven fabric and improves the strength of the resulting nonwoven fabric. The blending ratio of the thermally fusible fibers is more preferably 10 wt% or more, and even more preferably 15 wt% or more. On the other hand, a blending ratio of the thermally fusible fibers of 50 wt% or less is preferable because it prevents the microscopic spaces in the nonwoven fabric from becoming smaller or blocked by the thermal adhesion of the thermally fusible fibers, thereby preventing the air flow from being obstructed when air is passed through the nonwoven fabric and suppressing high pressure loss. The blending ratio of the thermally fusible fibers is more preferably 45 wt% or less, and even more preferably 40 wt% or less.

[0037] When the nonwoven fabric of the present invention is composed of ultrafine fibers, heat-shrinkable fibers, and heat-fusible fibers, the blending ratio of these fibers to the weight of the nonwoven fabric is preferably ultrafine fibers / heat-shrinkable fibers / heat-fusible fibers = 5-40 / 10-90 / 5-50 (wt%). A blending ratio within this range is preferred because the ultrafine fibers exhibit high collection performance, and the shrinkage of the heat-shrinkable fibers expands the voids between the fibers that make up the nonwoven fabric, preventing the ultrafine fibers from becoming excessively dense and suppressing high pressure loss. Furthermore, the heat-fusible fibers thermally bond the fibers that make up the nonwoven fabric, maintaining the three-dimensional structure of the nonwoven fabric and improving the strength of the nonwoven fabric.

[0038] The 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 the value measured by the method described in the Examples section. 2 If 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 prevent high pressure loss due to densification of the nonwoven fabric, and also to provide good molding processability such as pleating when used as an air filter medium, which is preferable. 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 improves handleability and molding processability during nonwoven fabric processing, allowing for the production of a nonwoven fabric with excellent durability during use. Furthermore, it is preferable because it provides good molding processability, such as pleating, when used as an air filter medium. A thickness of 0.1 mm or more is more preferable, and 0.2 mm or more is even more preferable. 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. A thickness of 0.9 mm or less is more preferable, and 0.8 mm or less is even more preferable.

[0040] The nonwoven fabric of the present invention preferably has a porosity of 70% or more. The porosity of the nonwoven fabric in the present invention refers to a 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 for a nonwoven fabric 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 and / or sodium 5-sulfoisophthalate, 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 waste liquid treatment generated during dissolution, polyesters copolymerized with 3 to 20 mol % of polylactic acid, sodium 5-sulfoisophthalate, and polyesters copolymerized with 5 to 15 wt % of polyethylene glycol having a weight-average molecular weight of 500 to 3,000 in addition to the aforementioned sodium 5-sulfoisophthalate 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 polyester copolymerized with 3-20 mol% of sodium 5-sulfoisophthalate and 5-15 wt% of polyethylene glycol having a weight average molecular weight of 500-3000, 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] 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 to improve 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.

[0053] 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 1000 to 6000. 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 1000 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 1500 or more is more preferable, and a ratio of 2000 or more is even more preferable. On the other hand, a fiber length / fiber diameter ratio of 6000 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.

[0054] Ultrafine fibers can be produced by dissolving and removing the sea part from the aforementioned islands-in-sea fiber. That is, the sea part can be removed by immersing the cut sea-island fiber in a solvent capable of dissolving the soluble polymer of the sea part. 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 fiber to the alkaline aqueous solution (weight (g) of the sea-island 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.

[0055] 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 it can accelerate the dissolution of the readily soluble sea component polymer.

[0056] 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. Anionic compounds are preferred for improving dispersibility in aqueous media through 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 of the nonwoven fabric during wetlay papermaking.

[0057] In the present invention, when the fibers other than the ultrafine fibers (heat-shrinkable fibers, heat-fused fibers, etc.) are synthetic fibers made of thermoplastic polymers, they can be produced by melt-spinning them using a known method, stretching them as needed, and then cutting them 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 clumps 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.

[0058] The ultrafine fiber dispersion thus prepared is mixed with the aforementioned fiber dispersion of fibers other than ultrafine fibers, diluted to a certain concentration, and adjusted to form a papermaking stock solution, which is then dewatered on an inclined wire or cylinder to form a nonwoven fabric by wet papermaking. Examples of equipment 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 of these. In the papermaking process, a three-dimensionally uniform nonwoven fabric can be produced by adjusting the papermaking speed, the amount of fiber, and the aqueous medium, in addition to the dispersibility of the fibers in the papermaking stock solution, to control the accumulation of fibers during drainage.

[0059] The nonwoven fabric formed by wet papermaking is subjected to a drying process to remove moisture, shrink the heat-shrinkable fibers, and thermally bond the fabric with the heat-fusible fibers. As a drying method, a method using hot air ventilation (air-through) or a method of contacting the fabric with a heated rotating roll (such as a heated calendar roll) can be suitably adopted, from the viewpoint of simultaneously achieving the above three steps. However, a method using hot air ventilation (air-through) is more preferable in order not to impair the bulkiness of the nonwoven fabric obtained by shrinking the heat-shrinkable fibers.

[0060] 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 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 for taking in outside air into clean rooms, the air conditioner air filter for circulating the air in clean rooms, and the air filter material for the fan filter unit installed on the ceiling of clean rooms and semiconductor manufacturing equipment.The clean rooms and semiconductor manufacturing equipment equipped with the air filter using these air filter material of the present invention, and the semiconductors obtained using the air filter using these air filter material of the present invention and semiconductor manufacturing equipment are useful in various industries. [Example]

[0061] 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.

[0062] A. Fiber diameter <Fiber diameter of ultrafine fibers> 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 sectioned using a Reichert-Nissei Ultracut N (ultramicrotome) equipped with a diamond knife. The sectioned surfaces were then imaged at 5000x magnification using a transmission electron microscope (Hitachi High-Technologies Corporation H-7100FA). The diameters of the circles circumscribing 100 island component fibers randomly selected from the image were measured to two decimal places in μm units, and the average was calculated and rounded off to two decimal places to determine the diameter of the island component fibers in the sea-island composite fiber (corresponding to the fiber diameter of the ultrafine fiber used as the raw fiber).

[0063] <Fiber diameter of heat-shrinkable and heat-sealable fibers> The heat-shrinkable or heat-fusible fibers used as raw materials in the examples and comparative examples were used as samples, and images were taken using a scanning electron microscope (SU-1510, manufactured by Hitachi High-Technologies Corporation) at a magnification sufficient to observe 150 to 300 fibers. The fiber diameters of 100 fibers randomly selected from the images were measured. The fiber diameters were determined by measuring the fiber width perpendicular to the fiber axis in the two-dimensionally captured images in μm units to one decimal place, and the average value was calculated and rounded to one decimal place to obtain the fiber diameter of the heat-shrinkable or heat-fusible fiber.

[0064] <Fiber diameter in nonwoven fabric> 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 μm units 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 horizontal axis representing the fiber diameter and the vertical axis representing the number of fibers. Next, for each of the upwardly convex fiber diameter distributions, the following formula was used within a fiber diameter range of ±10% of the peak value with the highest number of fibers present in the fiber diameter distribution: Fiber diameter (μm) = {sum of (fiber diameter × number of fibers constituting the fiber diameter distribution)} ÷ (sum of the number of fibers constituting the fiber diameter distribution) The value calculated by the above method 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 fiber. Furthermore, the fiber diameter calculated for a fiber having five or more bonding points with other fibers in the photographed image was determined as the fiber diameter of the heat-sealable fiber. Note that the above 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 the fibers other than the ultrafine fibers and heat-sealable fibers were determined as the fiber diameter of the heat-shrinkable fiber.

[0065] B. Fiber diameter ratio The fiber diameter of the ultrafine fiber in the nonwoven fabric calculated in Section A above was defined as R1, and the fiber diameter of the heat-shrinkable fiber was defined as R2. R2 / R1 was calculated and rounded to one decimal place to obtain the fiber diameter ratio R2 / R1.

[0066] C. Metsuke The nonwoven fabrics obtained in the examples and comparative examples were used as samples. The nonwoven fabrics were cut into 250 mm x 250 mm squares, and the weights of the pieces were measured to determine the unit area (1 m 2 ) and round off to the nearest tenth to get the weight per unit area of ​​the nonwoven fabric (g / m 2Measurements were carried out on three randomly selected locations per sample, and the average value was rounded off to one decimal place to obtain the basis weight.

[0067] D. Thickness The nonwoven fabric used in the measurement in item C above was used as a sample, and the thickness of the nonwoven fabric was measured using a dial thickness gauge (TECLOCK SM-114, probe shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). Measurements were taken at five randomly selected points per sample, and the average value was rounded to two decimal places to calculate the thickness (mm) of the nonwoven fabric.

[0068] E. Porosity Using the basis weight and thickness of the nonwoven fabric calculated in sections C and D above, the value calculated using the following formula was rounded to one decimal place to determine the porosity (%) of the nonwoven fabric. Porosity (%) = 100 - [weight (g / m 2 ) / {thickness (mm) × fiber density (g / cm 3 )}]×0.1 The fiber density can be determined by the density of the fibers that make up the nonwoven fabric, and in the case of PET, it is 1.38 g / cm 3 , and 1.14 g / cm for nylon. 3 It was calculated as:

[0069] F. Collection efficiency The nonwoven fabrics obtained in the examples and comparative examples were used as samples. The nonwoven fabrics were cut into circular pieces with a diameter of 200 mm and an effective opening area of ​​0.1 m. 2 The holder was set and the 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 The number of atmospheric dust particles with particle diameters of 0.3 to 0.5 μm upstream and downstream of the filter was measured 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 off to two decimal places to determine the collection efficiency (%).

[0070] G. Pressure loss The nonwoven fabrics obtained in the examples and comparative examples were used as samples. The nonwoven fabrics were cut into circular pieces with a diameter of 200 mm and an effective opening area of ​​0.1 m. 2 The filter was set in a holder, and air was passed vertically at a surface velocity of 3.3 m / min, and the pressure difference between upstream and downstream 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 to the nearest decimal place to obtain the pressure loss (Pa).

[0071] H. Performance indicators Using the collection efficiency and pressure loss of the nonwoven fabric calculated in items F and G above, the following formula is used: Performance index (1 / Pa) = -ln[{1-Collection efficiency (%) / 100)} / Pressure loss (Pa)] The value calculated by the above was rounded to three decimal places to obtain the performance index (1 / Pa) of the nonwoven fabric.

[0072] I. Heteromorphic degree Using the sea-island composite fiber image taken in Section A above, the diameters of the circles circumscribing 100 randomly selected island component fibers (circumscribed circle diameter) were measured in μm units to two decimal places, and the diameters of the circles inscribing those island component fibers (inscribed circle diameter) were also measured in μm units to two decimal places. Next, the circumscribing circle diameter of each island component fiber was divided by the inscribed circle diameter and rounded to two decimal places to calculate the non-circularity of each island component fiber. The average non-circularity of the 100 island component fibers was then calculated and rounded to two decimal places to determine the non-circularity of the island component fibers of the sea-island composite fiber (corresponding to the non-circularity of the ultrafine fiber used as the raw fiber). When there was not a single inscribed circle, as in Figure 1(d) and (e), the inscribed circle with the largest diameter was used as the inscribed circle diameter.

[0073] The nonwoven fabrics obtained in the Examples and Comparative Examples were used as samples. 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 images, each with a diameter of a circle circumscribing the fiber cross section (circumscribed circle diameter) of less than 1.0 μm. The circumscribed circle diameters of these fibers were measured in μm units to two decimal places. Furthermore, the diameters of the circles inscribed in these fibers (inscribed circle diameters) were measured in μm units to two decimal places. The above procedure was repeated for 10 images taken in the same manner, resulting in measurements of the circumscribed circle diameters and inscribed circle diameters of a total of 100 fibers. Next, the circumscribed circle diameter of each fiber was divided by the inscribed circle diameter and rounded to two decimal places to calculate the irregularity of each fiber.Then, the average irregularity of 100 fibers was calculated and rounded to two decimal places to determine the irregularity.In cases where there was not a single inscribed circle, as in Figure 1(d) and (e), the inscribed circle with the largest diameter was used as the inscribed circle diameter.

[0074] Example 1 Polyethylene terephthalate (PET) was used as the island component, and copolymerized PET, prepared by copolymerizing 8.0 mol% of sodium 5-sulfoisophthalate and 10 wt% of polyethylene glycol with a molecular weight of 1000, was used as the sea component, and each was vacuum dried at 150°C for 12 hours. Subsequently, the island component and sea component were fed at a blending ratio of 50 wt% to 50 wt% into an extruder-type composite spinning machine and melted separately, and then the melted polymer streams were fed into a spinning pack equipped with a sea-island composite spinneret (number of island components: 2000, cross-sectional shape of island components: round) 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 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, 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 obtain an islands-in-sea fiber (island component diameter 0.21 μm).

[0075] 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.

[0076] Next, copolymerized PET staple fibers (copolymerized polyester copolymerized with 92.9 mol% terephthalic acid and 7.1 mol% isophthalic acid as dicarboxylic acid components, 95.6 mol% ethylene glycol and 4.4 mol% 2,2-bis{4-(2-hydroxyethoxy)phenyl}propane as diol components, fiber diameter 6.18 μm, fiber length 3.0 mm) were mixed at a blending ratio of 60 wt% (compounding ratio in papermaking solution) as heat-shrinkable fibers, and core-sheath PET staple fibers (core component: PET, sheath component: A copolymer polyester with a melting point of 110°C, copolymerized with 60 mol% terephthalic acid and 40 mol% isophthalic acid as dicarboxylic acid components, 85 mol% ethylene glycol and 15 mol% diethylene glycol as diol components, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.02 μm, fiber length 5.0 mm, was adjusted to a blend ratio of 30 wt% (compound ratio in papermaking solution), and uniformly mixed and dispersed with water using a disintegrator to prepare a fiber dispersion of heat-shrinkable fiber and heat-fusible fiber.

[0077] The papermaking solution was prepared by homogeneously mixing the above-mentioned ultrafine fiber dispersion with this heat-shrinkable fiber and heat-fusible fiber dispersion so that the ultrafine fiber content was 10% by weight (compound ratio in the papermaking solution). This papermaking 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.

[0078] The evaluation results of the obtained nonwoven fabric are shown in Table 1. The obtained nonwoven fabric had a basis weight of 50.1 g / m 2The thickness was 0.40 mm. Because it is a nonwoven fabric containing ultrafine fibers, it has high collection performance, and because it is a nonwoven fabric containing heat-shrinkable fibers, it is bulky and has low pressure loss. In addition, the performance index that represents the balance between collection efficiency and pressure loss in air filter media also showed good values.

[0079] Examples 2 and 3, Comparative Example 1 A nonwoven fabric was produced in the same manner as in Example 1, except that the fiber diameter of the ultrafine fibers was changed as shown in Table 1.

[0080] The evaluation results of the obtained nonwoven fabrics are shown in Table 1. In Examples 2 and 3, the filtering efficiency and pressure loss tended to decrease as the fiber diameter of the ultrafine fibers increased, but the performance index showed good values. In Comparative Example 1, the pressure loss showed a low value, but because the fiber diameter of the ultrafine fibers was large, the filtering performance of the ultrafine fibers was not expressed, and the filtering efficiency and performance index showed extremely low values.

[0081] Example 4 A nonwoven fabric was prepared in the same manner as in Example 1, except that the fiber diameter of the heat-shrinkable fiber was changed as shown in Table 1.

[0082] The evaluation results of the obtained nonwoven fabric are shown in Table 1. Because the ratio (R2 / R1) of the fiber diameter of the ultrafine fibers (R1) to the fiber diameter of the heat-shrinkable fibers (R2) was large, some of the ultrafine fibers fell off during the nonwoven fabric processing by wet papermaking. As a result, the collection performance of the ultrafine fibers was not fully realized, and the collection efficiency was somewhat low. However, due to the bulkiness created by the heat-shrinkable fibers, the pressure loss was low and the performance index also showed good values.

[0083] Examples 5 and 6 Nonwoven fabrics were produced in the same manner as in Example 1, except that the heat-shrinkable fibers were changed to copolymer polyamide staple fibers (copolymer polyamide obtained by copolymerizing 85% by weight of nylon 6 and 15% by weight of nylon 66, fiber diameter 6.09 μm, fiber length 3.0 mm) in Example 5, and to copolymer polyolefin staple fibers (copolymer polyolefin obtained by copolymerizing 3% by weight of ethylene and 97% by weight of propylene, fiber diameter 6.03 μm, fiber length 3.0 mm) in Example 6.

[0084] The evaluation results of the obtained nonwoven fabric are shown in Table 1. When short copolymer polyamide fibers or short copolymer polyolefin fibers were used as the heat-shrinkable fibers, high collection efficiency and low pressure loss were achieved, and the performance index also showed good values.

[0085] Examples 7 and 8 The heat-fusible fibers were changed to core-sheath PET staple fibers in Example 7 (core component: PET, sheath component: polyethylene with a melting point of 135°C, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.08 μm, fiber length 5.0 mm), and to core-sheath PET staple fibers in Example 8 (core component: PET, sheath component: copolymerized polyester with a melting point of 150°C, obtained by copolymerizing 78 mol% of terephthalic acid and 22 mol% of isophthalic acid as dicarboxylic acid components, and 52 mol% of ethylene glycol and 48 mol% of tetramethylene glycol as diol components, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.05 μm, fiber length 5.0 mm), and the roller temperature was changed to 135°C in Example 7 and 150°C in Example 8. Except for this, nonwoven fabrics were produced in the same manner as in Example 1.

[0086] The evaluation results of the obtained nonwoven fabric are shown in Table 2. Even when the melting point of the heat-fusible fiber was changed, high collection efficiency and low pressure loss were achieved, and the performance index also showed good values.

[0087] Comparative Examples 2 to 4 Nonwoven fabrics were produced in the same manner as in Example 1, except that in Comparative Example 2, no ultrafine fibers were used, in Comparative Example 3, no heat-shrinkable fibers were used, and in Comparative Example 4, no heat-fusible fibers were used, and the blending ratios of the three types of fibers were changed as shown in Table 2.

[0088] The evaluation results of the obtained nonwoven fabrics are shown in Table 2. In Comparative Example 2, the pressure drop was low, but the fiber diameter was small and ultrafine fibers, which serve to capture dust by the effect of a high specific surface area, were not used, so collection performance was not exhibited, and the collection efficiency and performance index were also very low. In Comparative Example 3, the collection efficiency was high, but the heat-shrinkable fibers, which serve to increase the bulk of the nonwoven fabric, were not used, resulting in a thin and dense structure, and the pressure drop was high and the performance index was also low. In Comparative Example 4, the heat-fusible fibers, which serve to thermally bond the fibers constituting the nonwoven fabric, were not used, so some of the ultrafine fibers fell off during the nonwoven fabric processing by wet papermaking. As a result, the collection performance of the ultrafine fibers was not fully exhibited, and the collection efficiency was somewhat low. In addition, the three-dimensional structure of the nonwoven fabric could not be maintained, and bulkiness was lost, so the pressure drop was somewhat high and the performance index was also low.

[0089] Comparative Example 5 A nonwoven fabric was produced in the same manner as in Example 1, except that PET short fibers (fiber diameter 6.14 μm, fiber length 3.0 mm) were used instead of the heat-shrinkable fibers.

[0090] The evaluation results of the obtained nonwoven fabric are shown in Table 2. Although the collection efficiency was high, the absence of heat-shrinkable fibers, which play a role in increasing the bulk of the nonwoven fabric, resulted in a thin and dense structure, resulting in a high pressure loss and somewhat low performance index.

[0091] Example 9 A nonwoven fabric was produced in the same manner as in Example 2, except that in addition to the three types of fibers used in Example 2, PET short fibers (fiber diameter 3.05 μm, fiber length 3.0 mm) were used and the blending ratio of the four types of fibers was changed as shown in Table 2.

[0092] The evaluation results of the obtained nonwoven fabric are shown in Table 2. When four types of fibers were used, high collection efficiency and low pressure loss were achieved, and the performance index also showed good values.

[0093] Example 10 A nonwoven fabric was produced in the same manner as in Example 2, except that the cross-sectional shape of the island component fibers was changed to a triangle (FIG. 1(a)).

[0094] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the cross-sectional shape of the ultrafine fibers was triangular, it had both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0095] Examples 11 and 12 A nonwoven fabric was produced in the same manner as in Example 10, except that the degree of deformation of the ultrafine fibers was changed as shown in Table 3.

[0096] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the degree of irregularity of the ultrafine fibers was changed, the fabric exhibited both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0097] Example 13 A nonwoven fabric was produced in the same manner as in Example 2, except that the cross-sectional shape of the island component was changed to a Y-shape (FIG. 1(g)).

[0098] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the cross section of the ultrafine fibers was Y-shaped, it had both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0099] Examples 14 and 15 Nonwoven fabrics were produced in the same manner as in Example 13, except that the degree of deformation of the ultrafine fibers was changed as shown in Table 3.

[0100] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the degree of irregularity of the ultrafine fibers was changed, the fabric exhibited both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0101] Example 16 A nonwoven fabric was produced in the same manner as in Example 2, except that the cross-sectional shape of the island component was changed to an elliptical shape (FIG. 1(e)).

[0102] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the cross-sectional shape of the ultrafine fibers was elliptical, it had both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0103] Examples 17 and 18 Nonwoven fabrics were produced in the same manner as in Example 16, except that the degree of deformation of the ultrafine fibers was changed as shown in Table 3.

[0104] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the degree of irregularity of the ultrafine fibers was changed, the fabric exhibited both high collection efficiency and low pressure loss, and also showed good values ​​for the performance index.

[0105] [Table 1]

[0106] [Table 2]

[0107] [Table 3] [Explanation of symbols]

[0108] 1: Outer shape of ultra-fine fibers 2: Circumscribed circle 3: Inscribed circle

Claims

1. A nonwoven fabric comprising ultrafine fibers having a fiber diameter of 0.01 to 0.90 μm, heat-shrinkable fibers, and heat-fusible fibers.

2. 2. The nonwoven fabric according to claim 1, wherein the ratio (R2 / R1) of the fiber diameter (R1) of the ultrafine fibers to the fiber diameter (R2) of the heat-shrinkable fibers is 10 to 200.

3. 2. The nonwoven fabric according to claim 1, wherein the heat-shrinkable fiber is selected from the group consisting of polyester fibers, polyamide fibers, polyolefin fibers, and combinations thereof.

4. An air filter medium comprising the nonwoven fabric according to any one of claims 1 to 3.

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

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

7. A clean room or semiconductor manufacturing equipment equipped with the air filter according to claim 5.

8. A semiconductor obtained using a clean room or semiconductor manufacturing equipment equipped with the air filter according to claim 5.