Nonwoven cloth and air filter medium

Abstract

To provide a nonwoven cloth preferred for an air filter medium excellent in pleat processability, and wind pressure deformation resistance while having a low basis weight, and having both of high collection efficiency and low pressure loss.SOLUTION: A nonwoven cloth includes an ultrafine organic fiber having a fiber diameter of 0.01-0.90 μm, and a reinforcing fiber, where the basis weight is 10-90 g / m2, and the stiffness is 100-500 mgf.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 Art

[0002] In recent years, the demand for space purification has been increasing. Air filters that remove fine dust in the air are used in a wide range of fields from living environments to industries, such as measures against health problems caused by dust with a particle size of 2.5 μm or less and dust-free production in the manufacture 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 typified by HEPA filters (High Efficiency Particulate Air filters) and ULPA filters (Ultra Low Penetration Air filters) are used.

[0003] Conventionally, nonwoven fabrics made of glass fibers and porous membranes made of PTFE (polytetrafluoroethylene) have been used for air filter media of HEPA filters and ULPA filters. However, air filter media composed of nonwoven fabrics made of glass fibers and porous membranes made of PTFE both have the problem of being disposed of in landfills after use, resulting in a large environmental burden. In addition to such problems, due to the increasing environmental awareness in recent years, various nonwoven fabrics made of ultrafine polyester fibers, which have a smaller environmental burden and excellent filter performance, have been proposed.

[0004] Here, from the perspective of making the air filter lightweight and compact, an air filter medium with a low basis weight and a small thickness is required. Further, the nonwoven fabric used for the air filter medium is usually pleated and then used as an air filter in order to increase the contact area with air and improve the collection efficiency. Therefore, the air filter medium is required to have pleat-forming properties that can form pleats with an arbitrary number of folds, fold height, and fold interval. Furthermore, when the air filter is used under a high wind speed, if the pleat shape is deformed and adjacent folds come into contact with or adhere to each other, the pressure loss will increase. Therefore, wind pressure deformation resistance that can maintain the pleat shape even under a high wind speed is required.

[0005] In order to enhance the pleat-forming properties and wind pressure deformation resistance, it is effective to increase the stiffness and flexibility of the air filter medium, and various proposals have been made.

[0006] For example, Patent Document 1 proposes an air filter medium obtained by embossing a nonwoven fabric containing three types of polyester fibers: nanofibers with a fiber diameter of 100 to 1000 nm, fibers thicker than the nanofibers, and binder fibers. According to this proposal, it is said that an air filter medium excellent in pleat-forming properties in addition to collection performance can be provided.

[0007] Further, Patent Document 2 proposes an air filter medium obtained by impregnating 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 binder fibers with resin. According to this proposal, it is said that an air filter medium having excellent ground uniformity, excellent pleat-forming properties with a thin film, low pressure loss, and high collection performance can be provided.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

[0009] In the method described in Patent Document 1, by performing embossing, the stiffness and softness of the nonwoven fabric can be increased. However, in order to obtain sufficient stiffness and softness even when the nonwoven fabric is thin, it is necessary to have a high basis weight, and it was difficult to obtain a thin nonwoven fabric with a low basis weight.

[0010] Also, in the method described in Patent Document 2, by performing resin impregnation processing, the stiffness and softness of the nonwoven fabric can be increased. However, since the resin is impregnated into the voids in the nonwoven fabric, there is a problem that the pressure loss increases and the nonwoven fabric has low filter performance.

[0011] An object of the present invention is to solve the problems of the above-mentioned conventional technologies, and to provide a nonwoven fabric suitable for an air filter medium that has a low basis weight, is excellent in pleat processability and wind pressure deformation resistance, and has both high collection efficiency and low pressure loss, and an air filter medium using the same. [Means for Solving the Problems]

[0012] The above problems can be solved by the present invention, that is, a nonwoven fabric containing ultra-fine organic fibers having a fiber diameter of 0.01 to 0.90 μm and reinforcing fibers, having a basis weight of 10 to 90 g / m 2 and having a stiffness and softness of 100 to 500 mgf.

[0013] Further, it is preferable that the reinforcing fibers are selected from the group consisting of polyarylate fibers, aramid fibers, polyketone fibers, polyparaphenylene benzobisoxazole fibers, ultra-high molecular weight polyethylene fibers, carbon fibers, glass fibers, and combinations thereof.

[0014] Further, the nonwoven fabric preferably contains microfibers and binder fibers.

[0015] The nonwoven fabric can be suitably used as an air filter medium, and further, the air filter medium can be suitably used in an air filter.

[0016] In addition, the air filter can be suitably used in a fan filter unit, a clean room, and a semiconductor manufacturing apparatus.

Advantages of the Invention

[0017] According to the present invention, it is possible to obtain a nonwoven fabric suitable for an air filter medium, which has a low basis weight, excellent pleat processability and wind pressure deformation resistance, and combines high collection efficiency and low pressure loss.

Embodiments for Carrying Out the Invention

[0018] The nonwoven fabric of the present invention contains ultra-fine organic fibers having a fiber diameter of 0.01 to 0.90 μm and reinforcing fibers, and has a basis weight of 10 to 90 g / m 2 and a stiffness-flexibility of 100 to 500 mgf.

[0019] Hereinafter, the nonwoven fabric of the present invention will be described in detail.

[0020] The ultra-fine organic fibers used in the non-woven 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 the value measured by the method described in the Examples section. The smaller the fiber diameter, the higher the specific surface area, which is preferable because it exhibits high collection performance when made into a non-woven fabric. However, if the fiber diameter is 0.01 μm or more, in addition to high collection performance, the handleability and molding processability during non-woven fabric processing will be good, and a non-woven fabric with excellent durability during use can be obtained. The fiber diameter is more preferably 0.05 μm or more, and even more preferably 0.10 μm or more. On the other hand, if the fiber diameter is 0.90 μm or less, due to the effect of the high specific surface area resulting from the small fiber diameter, excellent collection performance will be exhibited when made into a non-woven fabric. The fiber diameter is more preferably 0.70 μm or less, and even more preferably 0.50 μm or less.

[0021] The ultra-fine organic fibers used in the non-woven fabric of the present invention are fibrous materials mainly composed of organic substances. Specific examples of organic fibers include cellulose produced from wood pulp, etc., natural fibers such as cotton, hemp, wool, silk, etc., regenerated fibers such as rayon, semi-synthetic fibers such as acetate, and synthetic fibers represented by polyester, nylon, acrylic, etc., but are not limited thereto. Among them, from the viewpoints of mechanical properties and dimensional stability, synthetic fibers made of thermoplastic polymers are preferable. Specific examples of thermoplastic polymers include polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, polyamides such as polyamide 6, polyamide 66, polyamide 610, polyolefins such as polyethylene, polypropylene, polymethylpentene, and thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, thermoplastic polyurethane, and their copolymers, but are not limited thereto. Among them, polyesters such as polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyamides such as polyamide 6, polyamide 66, and polyphenylene sulfide are preferable because they have both mechanical properties and heat resistance.

[0022] Here, as described above, various proposals have been made to increase the stiffness and flexibility of the nonwoven fabric in order to improve the pleating processability and wind pressure deformation resistance. However, when embossing is performed, the basis weight becomes high, and when resin impregnation is performed, there is a problem of high pressure loss. In addition, various proposals have been made to increase the wind pressure deformation resistance by laminating nonwoven fabrics, but there is a problem that the thickness is thick and the basis weight becomes high. As a result of intensive studies on the above problems, the inventors of the present invention have found that by increasing the stiffness and flexibility of the nonwoven fabric with reinforcing fibers, resin impregnation is unnecessary and the nonwoven fabric is a single layer, with a thin thickness and a low basis weight, yet excellent in pleating processability and wind pressure deformation resistance, and having both high collection efficiency and low pressure loss, thus completing the present invention, which is that a nonwoven fabric suitable for an air filter medium can be obtained. That is, it is important that the nonwoven fabric of the present invention contains reinforcing fibers.

[0023] The reinforcing fibers used in the nonwoven fabric of the present invention are preferably selected from the group consisting of polyarylate fibers, aramid fibers, polyketone fibers, polyparaphenylene benzobisoxazole fibers, ultrahigh molecular weight polyethylene fibers, carbon fibers, glass fibers, and combinations thereof. These fibers have a high tensile elastic modulus and can increase the stiffness and flexibility of the nonwoven fabric. Therefore, the obtained nonwoven fabric can suppress the occurrence of wrinkles and tears during pleating, and can maintain the pleated shape when used under high wind speeds after being made into an air filter, which is preferable. Among them, polyarylate fibers, aramid fibers, polyketone fibers, polyparaphenylene benzobisoxazole fibers, and ultrahigh molecular weight polyethylene fibers are preferable because they are organic fibers like the above-mentioned ultra-fine organic fibers. When the ultra-fine organic fibers are made of polyester, using polyarylate fibers, which are polyester-based reinforcing fibers, can bring the polarities of the ultra-fine organic fibers and the reinforcing fibers closer together and improve the affinity. Therefore, aggregation between the ultra-fine organic fibers and between the reinforcing fibers in the nonwoven fabric can be suppressed, and a uniform nonwoven fabric with less difference in coarseness and density and excellent durability during use can be obtained, which is preferable. Similarly, when the ultra-fine organic fibers are made of polyamide, it is preferable to use aramid fibers, which are polyamide-based reinforcing fibers.

[0024] The reinforcing fibers used in the nonwoven fabric of the present invention are preferably fibers having a fiber diameter of 1.0 to 30.0 μm. If the fiber diameter of the reinforcing fibers is 1.0 μm or more, the reinforcing effect by the reinforcing fibers is exhibited in the nonwoven fabric, and the occurrence of wrinkles and tears can be suppressed when the nonwoven fabric is pleated, and the pleated shape can be maintained when used under high wind speed after being made into an air filter, which is preferable. The fiber diameter of the reinforcing fibers is more preferably 2.0 μm or more, and even more preferably 3.0 μm or more. On the other hand, if the fiber diameter of the reinforcing fibers is 30.0 μm or less, the formability during pleating is good, the occurrence of tears can be suppressed when the nonwoven fabric is pleated, and pleats can be formed at any number of pleats, pleat height, and pleat interval, which is preferable. The fiber diameter of the reinforcing fibers is more preferably 28.0 μm or less, and even more preferably 25.0 μm or less.

[0025] The nonwoven fabric of the present invention may contain microfibers. The microfibers in the present invention are organic fibers having a larger fiber diameter than ultra-fine organic fibers and different from the reinforcing fibers. By containing microfibers, it is possible to suppress the shedding of ultra-fine organic fibers thinner than the microfibers during the processing of the nonwoven fabric by wet papermaking described later, and to exhibit the high collection performance by the ultra-fine organic fibers, which is preferable. In addition, it is preferable because the ultra-fine organic fibers are not excessively densified and the increase in pressure loss can be suppressed. Here, in the present invention, although the ultra-fine organic fibers and the reinforcing fibers are greatly different in fiber diameter and tensile elastic modulus, the microfibers have a tensile elastic modulus close to that of the ultra-fine organic fibers and a fiber diameter close to that of the reinforcing fibers. Therefore, when the nonwoven fabric contains microfibers, the microfibers play an intermediate role between the ultra-fine organic fibers and the reinforcing fibers, the fibers are uniformly dispersed in the nonwoven fabric, the reinforcing effect is exhibited over the entire nonwoven fabric, and a nonwoven fabric excellent in pleating processability and wind pressure resistance deformation can be obtained, which is preferable.

[0026] When the nonwoven fabric of the present invention contains microfibers, it is preferably a fiber having a fiber diameter of 1.0 to 30.0 μm. If the fiber diameter of the microfiber is 1.0 μm or more, in the nonwoven fabric, it serves as an aggregate for maintaining the shape of the nonwoven fabric, and the ultra-fine organic fibers are not overly densified, and an increase in pressure loss can be suppressed, so that it is possible to obtain a nonwoven fabric having both high collection performance and low pressure loss, which is preferable. The fiber diameter of the microfiber is more preferably 2.0 μm or more, and still more preferably 3.0 μm or more. On the other hand, if the fiber diameter of the microfiber is 30.0 μm or less, when processing the nonwoven fabric by wet papermaking described later, it is possible to suppress the shedding of ultra-fine organic fibers thinner than the microfibers, and in the obtained nonwoven fabric, the microfibers serve as a scaffold for the ultra-fine organic fibers, and a three-dimensionally homogeneous fine space can be formed, which is preferable. The fiber diameter of the microfiber is more preferably 28.0 μm or less, and still more preferably 25.0 μm or less.

[0027] The microfibers used in the nonwoven fabric of the present invention can preferably be heat-shrinkable fibers. By being heat-shrinkable fibers, after forming the nonwoven fabric by wet papermaking, by shrinking the heat-shrinkable fibers in the drying process, the voids between the fibers constituting the nonwoven fabric are expanded, and a bulky nonwoven fabric can be obtained, which is preferable. As a result, it is possible to obtain a nonwoven fabric having high collection performance due to ultra-fine fibers and suppressing an increase in pressure loss caused by densification of the ultra-fine fibers, which is preferable.

[0028] The heat-shrinkable fiber used in the present invention is preferably selected from the group consisting of polyester fibers, polyamide fibers, polyolefin fibers, and combinations thereof. Among them, polyester fibers and polyamide fibers are preferred because of their high mechanical strength. Specific examples of polyester fibers include, but are not limited to, polyethylene terephthalate fibers copolymerized with isophthalic acid and 2,2-bis{4-(2-hydroxyethoxy)phenyl}propane. Specific examples of polyamide fibers include, but are not limited to, polyamide fibers copolymerized with nylon 6 and nylon 6,6. Specific examples of polyolefin fibers include, but are not limited to, polyolefin fibers copolymerized with ethylene and α-olefins. Here, 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, 1-dodecene, and the like.

[0029] The shrinkage start temperature of the heat-shrinkable fiber used in the present invention is preferably lower than the melting point of the binder fiber described later. If the shrinkage start temperature of the heat-shrinkable fiber is lower than the melting point of the binder fiber, in the drying process after forming a nonwoven fabric by wet papermaking, the heat-shrinkable fiber is shrunk to expand the voids between the fibers constituting the nonwoven fabric, and after making the nonwoven fabric bulky, the binder fiber is used to join the fibers, and the three-dimensional structure of the nonwoven fabric can be maintained in a bulky state, which is preferable.

[0030] The nonwoven fabric of the present invention may contain binder fibers. By containing binder fibers, the fibers constituting the nonwoven fabric can be physically adhered to each other by thermal adhesion, so that it is possible to suppress the shedding of each fiber constituting the nonwoven fabric during the processing of the nonwoven fabric by wet papermaking described later, and to maintain the three-dimensional structure of the nonwoven fabric and improve the strength of the nonwoven fabric. The binder fibers are not particularly limited, but for example, core-sheath fibers having a thermoplastic polymer with a melting point of 150°C or lower disposed in the sheath can be preferably employed. When such core-sheath fibers are used, after forming the nonwoven fabric, through a drying process such as a Yankee dryer or an air-through dryer, or a heat treatment process such as a calendar, the sheath component on the surface of the binder fibers melts and adheres to other fibers constituting the nonwoven fabric, which is preferable because the rigidity of the nonwoven fabric can be increased. Furthermore, it is preferable because the core component of the binder fibers can contribute to ensuring the strength of the nonwoven fabric. In addition, if the melting point of the core component of the binder fibers is higher than the melting point of the sheath component and the difference in melting points is 20°C or more, the sheath component on the surface of the binder fibers is likely to melt sufficiently, and the decrease in the orientation of the core component is suppressed, so that sufficient thermal adhesiveness and high rigidity can be achieved simultaneously, which is preferable.

[0031] When the nonwoven fabric of the present invention contains binder fibers, the fiber diameter is preferably 5.0 to 50.0 μm. If the fiber diameter of the binder fibers is 5.0 μm or more, it is preferable because the strength of the nonwoven fabric can be ensured by the adhesion between the binder fibers and other fibers constituting the nonwoven fabric. The fiber diameter of the binder fibers is more preferably 7.0 μm or more, and even more preferably 10.0 μm or more. On the other hand, if the fiber diameter of the binder fibers is 50.0 μm or less, it is preferable because the binder fibers and other fibers constituting the nonwoven fabric can be adhered uniformly and firmly. The fiber diameter of the binder fibers is more preferably 45.0 μm or less, and even more preferably 40.0 μm or less.

[0032] The microfibers and binder fibers in the nonwoven fabric of the present invention are preferably organic fibers. The organic fibers in the present invention are fibrous materials mainly composed of organic substances. Specific examples of the organic fibers and thermoplastic polymers include the specific examples described in the above-mentioned ultra-fine organic fibers.

[0033] In the nonwoven fabric of the present invention, the blending ratio (weight % based on all fibers) of each fiber is not particularly limited and can be appropriately selected according to the fiber diameter of each fiber, the use of the nonwoven fabric, and the required characteristics. However, the blending ratio of the ultra-fine organic fibers is preferably 5 to 40% by weight. If the blending ratio of the ultra-fine organic fibers is 5% by weight or more, it is preferable because the excellent collection performance can be exhibited when made into a nonwoven fabric due to the effect of the high specific surface area derived from the small fiber diameter of the ultra-fine organic fibers. The blending ratio of the ultra-fine organic fibers is more preferably 10% by weight or more, and still more preferably 15% by weight or more. On the other hand, if the blending ratio of the ultra-fine organic fibers is 40% by weight or less, in the nonwoven fabric, the ultra-fine organic fibers are not overly densified, the increase in pressure loss can be suppressed, and a nonwoven fabric with both high collection performance and low pressure loss can be obtained, so it is preferable. The blending ratio of the ultra-fine organic fibers is more preferably 35% by weight or less, and still more preferably 30% by weight or less.

[0034] In the nonwoven fabric of the present invention, the blending ratio of the reinforcing fibers is preferably 5 to 60% by weight. If the blending ratio of the reinforcing fibers is 5% by weight or more, the reinforcing effect by the reinforcing fibers is exhibited in the nonwoven fabric, the occurrence of wrinkles and breakage can be suppressed when the nonwoven fabric is pleated, and the pleated shape can be maintained when used at a high wind speed after being made into an air filter, so it is preferable. The blending ratio of the reinforcing fibers is more preferably 10% by weight or more, and still more preferably 15% by weight or more. On the other hand, if the blending ratio of the reinforcing fibers is 60% by weight or less, the formability during pleating is good, the occurrence of breakage can be suppressed when the nonwoven fabric is pleated, and pleats can be formed at an arbitrary number of pleats, pleat height, and pleat interval, so it is preferable. The blending ratio of the reinforcing fibers is more preferably 55% by weight or less, and still more preferably 50% by weight or less.

[0035] In the nonwoven fabric of the present invention, when microfibers are included, the blending ratio of the microfibers is preferably 10 to 60% by weight. If the blending ratio of the microfibers is 10% by weight or more, when processing the nonwoven fabric by wet papermaking described later, the ultra-fine organic fibers thinner than the microfibers are suppressed from falling off. Therefore, in the obtained nonwoven fabric, the microfibers serve as a scaffold for the ultra-fine organic fibers, and it is possible to form a three-dimensionally uniform fine space, which is preferable. The blending ratio of the microfibers is more preferably 15% by weight or more, and still more preferably 20% by weight or more. On the other hand, if the blending ratio of the microfibers is 60% by weight or less, it is preferable because it is possible to obtain a nonwoven fabric that exhibits both excellent collection performance by ultra-fine organic fibers and good strength by binder fibers. The blending ratio of the microfibers is more preferably 55% by weight or less, and still more preferably 50% by weight or less.

[0036] In the nonwoven fabric of the present invention, when binder fibers are included, the blending ratio of the binder fibers is preferably 5 to 50% by weight. If the blending ratio of the binder fibers is 5% by weight or more, the adhesiveness between the fibers in the nonwoven fabric can be ensured, and the strength of the obtained nonwoven fabric is also good, which is preferable. The blending ratio of the binder fibers is more preferably 10% by weight or more, and still more preferably 15% by weight or more. On the other hand, if the blending ratio of the binder fibers is 50% by weight or less, it is possible to suppress the reduction or blockage of the fine space in the nonwoven fabric due to the adhesion of the binder fibers. Therefore, when air passes through the nonwoven fabric, the air flow is not hindered, and an increase in pressure loss can be suppressed, which is preferable. The blending ratio of the binder fibers is more preferably 45% by weight or less, and still more preferably 40% by weight or less.

[0037] Each fiber used in the nonwoven fabric of the present invention may be subjected to various modifications by adding secondary additives as long as the effects of the present invention are not impaired. Specific examples of the secondary additives include compatibilizers, plasticizers, antioxidants, ultraviolet absorbers, infrared absorbers, fluorescent brighteners, mold release agents, antibacterial agents, nucleating agents, heat stabilizers, flame retardants, antistatic agents, anti-coloring agents, regulators, matting agents, defoaming agents, preservatives, gelling agents, latexes, fillers, inks, colorants, dyes, pigments, fragrances, etc., but are not limited thereto. These secondary additives may be used alone or in combination of two or more.

[0038] The basis weight of the nonwoven fabric of the present invention is 10 to 90 g / m 2 2. Herein, the basis weight of the nonwoven fabric refers to the value measured by the method described in the Examples section. If the basis weight is 10 g / m 2 2 or more, the handleability and moldability during nonwoven fabric processing will be good, and a uniform nonwoven fabric with a small difference in thickness and density and excellent durability during use can be obtained. In addition, when pleating the nonwoven fabric, the occurrence of wrinkles and tears can be suppressed, and the pleated shape can be maintained when used under high wind speed after being used as an air filter. The basis weight is more preferably 15 g / m 2 2 or more, and even more preferably 20 g / m 2 2 or more. On the other hand, if the basis weight is 90 g / m 2 2 or less, an increase in pressure loss due to densification of the nonwoven fabric can be suppressed, and the moldability such as pleating during use as an air filter filter medium will be good. In addition, the air filter can be made lightweight and compact. The basis weight is more preferably 80 g / m 2 2 or less, and even more preferably 70 g / m 2 2 or less.

[0039] The thickness of the non-woven fabric of the present invention is preferably 0.05 to 1.0 mm. The thickness of the non-woven fabric in the present invention refers to the value measured by the method described in the column of Examples. If the thickness is 0.05 mm or more, the handleability and moldability during non-woven fabric processing are good, and a non-woven fabric excellent in durability during use can be obtained, which is preferable. In addition, when pleating the non-woven fabric, the occurrence of wrinkles and tearing can be suppressed, and the pleat shape can be maintained when used under high wind speed after being made into an air filter, which is preferable. The thickness is more preferably 0.1 mm or more, and still more preferably 0.2 mm or more. On the other hand, if the thickness is 1.0 mm or less, it is preferable because an increase in high pressure loss due to densification of the non-woven fabric can be suppressed. Further, when used as an air filter filter medium and pleated to form an air filter, the contact portion between adjacent filter media is reduced due to the thickness of the filter medium, so that the filtration area can be ensured and an increase in pressure loss can be suppressed, which is preferable. In addition, it is preferable because the air filter can be made lightweight and compact. The thickness is more preferably 0.9 mm or less, and still more preferably 0.8 mm or less.

[0040] The stiffness-flexibility of the non-woven fabric of the present invention is 100 to 500 mgf. The stiffness-flexibility of the non-woven fabric in the present invention refers to the value measured by the method described in the column of Examples. If the stiffness-flexibility is 100 mgf or more, the occurrence of wrinkles and tearing can be suppressed when pleating the non-woven fabric. Further, when used under high wind speed after being made into an air filter, the pleat shape can be maintained, and the contact and adhesion between adjacent folds can be suppressed, so that an increase in pressure loss can be suppressed. The stiffness-flexibility is more preferably 150 mgf or more, and still more preferably 200 mgf or more. On the other hand, if the stiffness-flexibility is 500 mgf or less, the formability during pleating is good, the occurrence of tearing can be suppressed when pleating the non-woven fabric, and pleats can be formed with an arbitrary number of folds, fold height, and fold interval. The stiffness-flexibility is more preferably 450 mgf or less, and still more preferably 400 mgf or less.

[0041] The porosity of the nonwoven fabric of the present invention is preferably 80% 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. If the porosity is 80% or more, an increase in high pressure loss due to densification of the nonwoven fabric is suppressed, air efficiently flows into the fine spaces in the nonwoven fabric, filtration performance is enhanced, and a nonwoven fabric with both high collection performance and low pressure loss can be obtained, which is preferable. More preferably, the porosity is 85% or more, and even more preferably, 90% or more.

[0042] Next, an example of the method for manufacturing the nonwoven fabric of the present invention is shown below.

[0043] First, fibers other than ultra-fine organic fibers are put into an aqueous medium, and a fiber dispersion is prepared by stirring with a disintegrator so as to be uniform. In this step, the dispersibility of the fibers can be adjusted by the amount of fibers charged, the amount of the aqueous medium, the stirring time, etc., and it is preferable that each short fiber is as uniformly dispersed as possible in the aqueous medium. Further, a dispersant may be added to improve the dispersibility of the fibers in the aqueous medium, but when post-processing is performed on the nonwoven fabric, the addition amount of the dispersant is preferably kept to the minimum necessary so as not to affect the processability.

[0044] Next, according to the method described below, a fiber dispersion of ultra-fine organic fibers in which the ultra-fine organic fibers are uniformly dispersed in the aqueous medium is prepared. By mixing this fiber dispersion of ultra-fine organic fibers with the fiber dispersion of fibers other than the ultra-fine organic fibers described above to obtain a papermaking stock solution and subjecting this to wet papermaking, a nonwoven fabric in which the ultra-fine organic fibers, which are the thinnest among the fibers constituting the nonwoven fabric, are evenly arranged can be obtained.

[0045] The ultra-fine organic fibers in the present invention can be manufactured by using sea-island fibers composed of two or more types of polymers having different dissolution rates in a solvent. The sea-island fibers in the present invention are fibers having a structure in which island components made of a hardly soluble polymer are dispersed in a sea component made of an easily soluble polymer. By dissolving the sea component and leaving only the island components, ultra-fine organic fibers are obtained.

[0046] As a method for spinning this sea-island fiber, sea-island composite spinning by melt spinning is suitable from the viewpoint of high productivity and continuous production. Further, from the viewpoint of excellent control of the fiber diameter and cross-sectional shape of the island component, a method using a sea-island composite die is preferable.

[0047] Specific examples of the hardly soluble polymer used for the island component in the present invention include 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, and thermoplastic polymers such as polycarbonate, polyacrylate, polyphenylene sulfide, and thermoplastic polyurethane, and copolymers thereof, but are not limited thereto.

[0048] The easily soluble polymer used for the sea component in the present invention preferably exhibits easy solubility in an aqueous solvent or hot water or the like from the viewpoint of simplifying the elution step of the sea component. As the easily soluble polymer in the present invention, it is preferable to use a copolyester, polylactic acid, polyvinyl alcohol, etc. In particular, polyethylene glycol, a polyester copolymerized with sodium 5-sulfoisophthalate alone or in combination, or polylactic acid is preferable from the viewpoints of handleability and easy dissolution in a low-concentration aqueous solvent.

[0049] The term "easily soluble" in the present invention means that the dissolution rate ratio (easily soluble polymer / hardly soluble polymer) is 100 or more when the hardly soluble polymer is used as a reference with respect to the solvent used for the dissolution treatment. Considering the simplification and time shortening of the dissolution treatment, it is preferable that this dissolution rate ratio is large. The dissolution rate ratio is more preferably 1000 or more, and even more preferably 10000 or more. Within such a range, the dissolution treatment can be completed in a short time, and ultrafine organic fibers suitable for the present invention can be obtained without unnecessarily deteriorating the hardly soluble polymer, which is preferable.

[0050] From the viewpoints of solubility in an aqueous solvent and simplification of waste liquid treatment during dissolution, a polyester obtained by copolymerizing 3 to 20 mol% of sodium 5-sulfoisophthalate with polylactic acid, and a polyester obtained by copolymerizing 5 to 15 wt% of polyethylene glycol having a weight average molecular weight of 500 to 3000 in addition to the aforementioned sodium 5-sulfoisophthalate are particularly preferred.

[0051] From the above, as an example of a suitable polymer combination of the aforementioned sea-island fiber, either the sea component is a polyester obtained by copolymerizing 3 to 20 mol% of sodium 5-sulfoisophthalate and copolymerizing 5 to 15 wt% of polyethylene glycol having a weight average molecular weight of 500 to 3000, or polylactic acid, and the island component is any one of polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and their copolymers, but is not limited thereto.

[0052] The spinning temperature of the aforementioned sea-island fiber is preferably set to the temperature at which the polymer having mainly a high melting point or high viscosity among the poorly soluble polymer and the easily soluble polymer, which is determined from the above viewpoints, exhibits fluidity. This temperature at which fluidity is exhibited varies depending on the polymer properties and molecular weight, but the melting point of the polymer can be used as a reference, and the spinning temperature may be set to be equal to or lower than the melting point + 60°C. Within such a range, since thermal decomposition of the polymer and the like are suppressed in the spinning head or spinning pack, a decrease in molecular weight is suppressed, and it is preferable because sea-island fiber can be produced favorably.

[0053] The filament melt-extruded from the sea-island composite die is cooled and solidified, converged by applying an oil agent or the like, and taken up by a roller having a specified peripheral speed. The take-up speed can be determined from the discharge amount, the target fiber diameter, etc., and from the viewpoint of stably producing sea-island fiber, it is preferably 100 to 7000 m / min. The spun sea-island fiber is preferably drawn from the viewpoints of improving mechanical properties and thermal stability, and may be drawn after once winding up the spun multifilament, or may be drawn continuously following spinning without winding up.

[0054] The above-mentioned sea-island fibers are preferably bundled into dozens to millions of units and then cut into a desired fiber length using a cutting machine such as a guillotine cutter, a slicing machine, a cryostat, etc. The fiber length after cutting is preferably such that the ratio (fiber length / fiber diameter) of the fiber length to the diameter of the island component of the sea-island fiber (corresponding to the fiber diameter of the ultrafine organic fiber) is 1000 to 6000. Within such a range, when made into a nonwoven fabric, the contact points between the fibers increase, the formation of the bridging structure between the fibers is promoted, and the reinforcing effect of the nonwoven fabric can be enhanced, which is preferable. If the fiber length / fiber diameter is 1000 or more, the dropout of the ultrafine organic fibers from the nonwoven fabric during the processing of the nonwoven fabric by wet papermaking is suppressed, which is preferable. The fiber length / fiber diameter is more preferably 1500 or more, and even more preferably 2000 or more. On the other hand, if the fiber length / fiber diameter is 6000 or less, the aggregation of the ultrafine organic fibers in the aqueous medium is suppressed, and a nonwoven fabric with high homogeneity can be obtained, which is preferable. The fiber length / fiber diameter is more preferably 5500 or less, and even more preferably 5000 or less.

[0055] By dissolving and removing the sea component from the above-mentioned sea-island fibers, ultrafine organic fibers can be produced. That is, the above-mentioned sea-island fibers after the cutting process may be immersed in a solvent capable of dissolving the easily soluble polymer of the sea component, and the easily soluble polymer may be removed. When the easily soluble polymer is a copolymerized polyethylene terephthalate or polylactic acid copolymerized with sodium 5-sulfoisophthalate, polyethylene glycol, etc., an aqueous alkali solution such as an aqueous sodium hydroxide solution can be used. In the case of the aqueous alkali solution, the bath ratio of the sea-island fiber to the aqueous alkali solution (weight of sea-island fiber (g): weight of aqueous alkali solution (g)) is preferably 1:5 to 1:10000, and more preferably 1:10 to 1:5000. Within such a range, when the easily soluble polymer of the sea component dissolves, the entanglement of the ultrafine organic fibers is unnecessarily suppressed, which is preferable.

[0056] Further, the alkali concentration of the aqueous alkali solution is preferably 0.1 to 5% by weight, more preferably 0.5 to 3% by weight. Within such a range, the dissolution of the easily soluble polymer of the sea component is completed in a short time, and a fiber dispersion liquid in which ultrafine organic fibers are homogeneously dispersed can be obtained without unnecessarily deteriorating the hardly soluble polymer of the island component, which is preferable. Also, although the temperature of the aqueous alkali solution is not particularly limited, setting it at 50°C or higher is preferable because it can accelerate the progress of the dissolution of the easily soluble polymer of the sea component.

[0057] In the present invention, an aqueous solution in which the easily soluble polymer of the sea component is dissolved from the sea-island fiber may be used as it is as a fiber dispersion liquid of ultrafine organic fibers, or an acid or alkali may be added to adjust the pH, or it may be diluted with water and used. Further, in order to suppress the aggregation of ultrafine organic fibers over time in the fiber dispersion liquid, a dispersant may be added. Examples of the type of dispersant include cationic compounds, nonionic compounds, anionic compounds, etc. Among them, from the viewpoint of improving dispersibility due to the electrical repulsive force in an aqueous medium, it is preferable to use anionic compounds. The addition amount of the dispersant is preferably 0.001 to 10 times the weight of the ultrafine organic fibers. Within such a range, the dispersibility of the ultrafine organic fibers is ensured without impairing the processability during nonwoven fabric processing by wet papermaking, which is preferable.

[0058] Fibers other than the ultrafine organic fibers in the present invention (reinforcing fibers, microfibers, binder fibers, etc.), when they are synthetic fibers made of a thermoplastic polymer, can be produced by melt spinning by a well-known method, followed by stretching as necessary, and then cutting to a desired fiber length as described above. Here, the fiber length of the fibers other than the ultrafine organic fibers is preferably 30 mm or less. If the fiber length is 30 mm or less, the formation of fiber lumps associated with the strong entanglement of the fibers during dispersion in an aqueous medium is suppressed, and a homogeneous nonwoven fabric can be obtained, which is preferable because it can be suitably used as an air filter medium.

[0059] The fiber dispersion of the ultrafine organic fibers prepared in this way is mixed with the fiber dispersion of fibers other than the aforementioned ultrafine organic fibers, diluted to a certain concentration and adjusted to obtain a papermaking stock solution, and then dehydrated on an inclined wire, a cylinder net, etc. to form a nonwoven fabric by wet papermaking. Examples of the apparatus used for wet papermaking include, but are not limited to, a cylinder paper machine, a fourdrinier paper machine, an inclined short wire paper machine, or a paper machine combining these. In the papermaking process, in addition to the dispersibility of the fibers in the papermaking stock solution, by adjusting the papermaking speed, the amount of fibers and the water medium, and controlling the accumulation of fibers during drainage, a three-dimensionally homogeneous nonwoven fabric can be produced.

[0060] The nonwoven fabric formed by wet papermaking is passed through a drying process to remove moisture. As the drying method, from the viewpoint that the drying of the nonwoven fabric and the thermal adhesion of the thermally adhesive fibers can be carried out simultaneously, a method using hot air ventilation (air through) or a method of contacting a heat rotating roll (such as a heat calendar roll) can be preferably adopted.

[0061] The nonwoven fabric of the present invention has excellent pleat processability and wind pressure deformation resistance while having a low basis weight, and has both high collection efficiency and low pressure loss. Therefore, the air filter medium using the nonwoven fabric of the present invention can be preferably used as an air filter medium for air purifiers, air conditioners, building air conditioners, industrial clean rooms, and vehicle compartments such as automobiles and trains. Further, in a space that requires extremely clean air, such as a clean room or a semiconductor manufacturing apparatus, for example, an air filter of an air conditioner for taking in outside air into the clean room, an air filter of an air conditioner for circulating the air in the clean room, and an air filter of a fan filter unit installed on the ceiling of a clean room or a semiconductor manufacturing apparatus can be preferably used as the air filter medium. A clean room or a semiconductor manufacturing apparatus equipped with an air filter using these air filter media of the present invention will be useful in various industries.

Examples

[0062] Next, the present invention will be described in detail based on examples. However, the present invention is not limited only to these examples. Each characteristic value in the examples was obtained by the following method.

[0063] A. Fiber diameter Using the nonwoven fabrics obtained in the examples and comparative examples as samples, the surface of the nonwoven fabric was photographed with a scanning electron microscope (SU-1510 manufactured by Hitachi High-Technologies Corporation) at a magnification that allows 300 to 3000 fibers to be observed. The fiber diameters of 100 fibers randomly extracted from the photographed image were measured. For the fiber diameter, from the two-dimensionally photographed image, the fiber width in the direction perpendicular to the fiber axis was taken as the fiber diameter and measured in μm units up to the second decimal place. The above operations were performed on 10 images photographed in the same manner. Regarding the fiber diameters of a total of 1000 fibers measured from the 10 images, a graph of the fiber diameter distribution was created with the fiber diameter on the horizontal axis and the number of fibers on the vertical axis. Subsequently, for each upwardly convex fiber diameter distribution, in the range of fiber diameters of ±10% of the peak value with the largest number of occurrences in the fiber diameter distribution, the value calculated by the following formula was rounded to the third decimal place to obtain the fiber diameter. Fiber diameter (μm) = {Sum of (fiber diameter of fibers constituting the fiber diameter distribution × number of fibers)} / {Sum of the number of fibers constituting the fiber diameter distribution}

[0064] B. Basis weight Using the nonwoven fabrics obtained in the examples and comparative examples as samples, the weight of the nonwoven fabric cut out into a 200 mm × 200 mm square was weighed, and the value converted to the weight per unit area (1 m 2 ) was rounded to the first decimal place to calculate the basis weight (g / m 2 ) of the nonwoven fabric. The measurement was carried out by cutting out three arbitrary locations for each sample, and the first decimal place of the average value was rounded to obtain the basis weight.

[0065] C. Thickness Using the nonwoven fabric used for the measurement in Item B above as a sample, the thickness of the nonwoven fabric was measured using a dial thickness gauge (SM-114 manufactured by TECLOCK: measuring head shape 10 mmφ, graduation 0.01 mm, measuring force 2.5 N or less). The measurement was performed at five arbitrary locations for each sample, and the thickness (mm) of the nonwoven fabric was calculated by rounding off the third decimal place of the average value.

[0066] D. Porosity Using the basis weight and thickness of the nonwoven fabric calculated in Items B and C above, the porosity (%) of the nonwoven fabric was obtained by rounding off the second decimal place of the value calculated by the following formula. The fiber density was calculated from the density and blending ratio of each fiber constituting the nonwoven fabric. Porosity (%) = 100 - [basis weight (g / m 2 ) / {thickness (mm) × fiber density (g / cm 3 )}] × 0.1 Regarding the density of each fiber, the PET fiber was 1.38 g / cm 3 , the polyarylate fiber was 1.39 g / cm 3 , the aramid fiber was 1.42 g / cm 3 , and the glass fiber was 2.49 g / cm 3 . When impregnated with resin, the fiber density was calculated from the density and blending ratio of each fiber and resin constituting the nonwoven fabric. The density of the polyurethane-based resin was 1.10 g / cm 3 .

[0067] E. Collection efficiency Using the nonwoven fabrics obtained in the examples and comparative examples as samples, the nonwoven fabric cut into a circle with a diameter of 200 mm was set in a holder with an effective aperture area of 0.1 m 2 . Air containing 10,000 to 25,000 polystyrene latex particles with a particle size of 0.15 to 0.50 μm was passed vertically at a face velocity of 3.3 m / min. The number of airborne dust particles with a particle size of 0.3 to 0.5 μm upstream and downstream of the filter was measured using a particle counter (KC-01D manufactured by RION), and the collection efficiency was calculated by the following formula. 3 Collection efficiency (%) = {1 - (number of downstream particles / number of upstream particles)} × 100 ​For each sample, measurements were carried out by cutting out any three locations, and the third decimal place of the average value was rounded off to obtain the collection efficiency (%).

[0068] F. Pressure Loss Using the nonwoven fabrics obtained in the examples and comparative examples as samples, the nonwoven fabrics cut out into a circle with a diameter of 200 mm were set in a holder with an effective aperture area of 0.1 m 2 Air was passed vertically at a face velocity of 3.3 m / min, and the pressure difference between the upstream and downstream of the filter was measured with a differential pressure gauge. For each sample, measurements were carried out by cutting out any three locations, and the first decimal place of the average value was rounded off to obtain the pressure loss (Pa).

[0069] G. Performance Index Using the collection efficiency and pressure loss of the nonwoven fabric calculated in items E and F above, the fourth decimal place of the value calculated by the following formula was rounded off to obtain the performance index (1 / Pa) of the nonwoven fabric. Performance Index (1 / Pa) = -ln[{1 - collection efficiency (%) / 100} / pressure loss (Pa)]

[0070] H. Stiffness and Softness Using the nonwoven fabrics obtained in the examples and comparative examples as samples, measurements were carried out in accordance with the Gurley method in 6.7.4 of JIS L1913:2010 (General Test Methods for Nonwoven Fabrics). For each sample, measurements were carried out by cutting out any five locations, and the first decimal place of the average value was rounded off to obtain the stiffness and softness (mgf).

[0071] I. Pleating Processability The nonwoven fabrics obtained in the examples and comparative examples were used as samples, cut into a width of 100 mm × a length of 200 mm, and using a reciprocating pleating machine, the width was set to 100 mm, and mountain folds and valley folds were repeated at 5 mm intervals to perform pleating with a pleat height of 5 mm. In each example and comparative example, three nonwoven fabrics subjected to pleating were produced. Subsequently, 20 evaluators evaluated three nonwoven fabrics after pleating, and "there are no wrinkles or tears, and the pleat heights are uniform" was rated 5 points, "there are almost no wrinkles or tears, and the pleat heights are almost uniform" was rated 4 points, "there are slight wrinkles or tears, and / or the pleat heights are slightly uneven" was rated 3 points, "there are many wrinkles or tears, and / or the pleat heights are uneven" was rated 2 points, and "there are very many wrinkles or tears, and / or the pleat heights are very uneven" was rated 1 point. The average score of the scores given by each of the 20 evaluators was calculated, and a passing grade was set when the average score was 3.0 points or more.

[0072] J. Wind pressure deformation resistance Using the nonwoven fabric after pleating produced in item I above as a sample, a polyester nonwoven fabric (basis weight 260 g / m 2 , thickness 1 mm) was attached as an outer frame to the four outer sides thereof to produce a filter unit with a width of 100 mm, a length of 100 mm, and a thickness of 6 mm (number of pleats 20, pleat height approximately 4 mm, pleat interval approximately 5 mm). In each example and comparative example, three filter units were produced. Subsequently, for each of the three filter units, 20 evaluators visually observed the pleat shape when ventilating at a face velocity of 3.3 m / min, and "there is no disturbance in the pleat interval, and there is no contact or adhesion between adjacent pleats" was rated 5 points, "there is almost no disturbance in the pleat interval, and there is almost no contact or adhesion between adjacent pleats" was rated 4 points, "there is slight disturbance in the pleat interval, and there is slight contact or adhesion between adjacent pleats" was rated 3 points, "there is disturbance in the pleat interval, and there is contact or adhesion between adjacent pleats" was rated 2 points, and "there is very much disturbance in the pleat interval, and there is very much contact or adhesion between adjacent pleats" was rated 1 point. The average score of the scores given by each of the 20 evaluators was calculated, and a passing grade was set when the average score was 3.0 points or more.

[0073] Example 1 Using polyethylene terephthalate (PET) as the island component and copolymerized PET copolymerized with 8.0 mol% of sodium 5-sulfoisophthalate and 10 wt% of polyethylene glycol with a molecular weight of 1000 as the sea component, each was vacuum dried at 150 °C for 12 hours. Subsequently, the island component was supplied to an extruder-type composite spinning machine at a mixing ratio of 50 wt% and the sea component at 50 wt% and melted separately, and at a spinning temperature of 285 °C, it was made to flow into a spinning pack incorporating a sea-island composite die (number of island components: 2000, cross-sectional shape of island component: round), and a composite polymer stream was discharged from the discharge holes at a discharge rate of 12 g / min to obtain a spun yarn. This spun yarn was cooled with cooling air at an air temperature of 20 °C and a wind speed of 20 m / min, oil was applied by an oiling device to converge it, it was taken up by a first godet roller rotating at 1000 m / min, and wound up by a winder via a second godet roller rotating at the same speed as the first godet roller to obtain an undrawn yarn. Thereafter, using a stretching machine, the obtained undrawn yarn was stretched 3.4 times between rollers heated to 85 °C and 130 °C to obtain sea-island fibers (diameter of island component: 0.30 μm).

[0074] The obtained sea-island fibers were subjected to cutting so that the fiber length became 0.6 mm. After the cutting process, the sea-island fibers were 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 to pH = 7 with acetic acid to obtain a fiber dispersion of ultrafine organic fibers.

[0075] Next, polyarylate staple fibers (fiber diameter 15.0 μm, fiber length 5.0 mm) were used as reinforcing fibers at a mixing ratio of 55 wt% (mixing ratio in the papermaking stock solution), and core-sheath PET staple fibers (core component: PET, sheath component: copolymerized polyester with a melting point of 110 °C copolymerized at a ratio of 60 mol% of terephthalic acid and 40 mol% of isophthalic acid as the dicarboxylic acid component and 85 mol% of ethylene glycol and 15 mol% of diethylene glycol as the diol component, core-sheath ratio (weight ratio) = 50:50, fiber diameter 10.0 μm, fiber length 5.0 mm) were used as binder fibers at a mixing ratio of 30 wt% (mixing ratio in the papermaking stock solution), and they were adjusted to be uniformly mixed and dispersed with water by a disintegrator to prepare a fiber dispersion of reinforcing fibers and binder fibers.

[0076] For this fiber dispersion of reinforcing fibers and binder fibers, the fiber dispersion of the aforementioned ultra-fine organic fibers was homogeneously mixed so that the blending ratio of the ultra-fine organic fibers was 15% by weight (blending ratio in the papermaking stock solution), thereby preparing the papermaking stock solution. After wet papermaking this papermaking stock solution using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., drying and heat treatment were performed with a rotary dryer having a roller temperature set at 110°C to obtain a non-woven fabric.

[0077] The evaluation results of the obtained non-woven fabric are shown in Table 1. The obtained non-woven fabric had a basis weight of 50 g / m 2 and a thickness of 0.48 mm. It had both high collection efficiency and low pressure loss, and since it was a non-woven fabric containing reinforcing fibers, the stiffness and flexibility of the non-woven fabric were high, and both the pleating processability and wind pressure deformation resistance were good.

[0078] Example 2 The blending ratio of the reinforcing fibers was changed to 15% by weight (blending ratio in the papermaking stock solution), and PET short fibers (fiber diameter 3.0 μm, fiber length 3.0 mm) were added as microfibers so that the blending ratio was 40% by weight (blending ratio in the papermaking stock solution). A non-woven fabric was produced in the same manner as in Example 1 except that a fiber dispersion composed of reinforcing fibers, microfibers, and binder fibers was prepared.

[0079] The evaluation results of the obtained non-woven fabric are shown in Table 1. Since it was a non-woven fabric containing microfibers in addition to the reinforcing fibers, it had high collection efficiency, and because the stiffness and flexibility of the non-woven fabric were high, the pleating processability and wind pressure deformation resistance were also good.

[0080] Examples 3 and 4 A non-woven fabric was produced in the same manner as in Example 2 except that the reinforcing fibers were changed to aramid short fibers (fiber diameter 15.0 μm, fiber length 5.0 mm) in Example 3 and glass short fibers (fiber diameter 15.0 μm, fiber length 5.0 mm) in Example 4.

[0081] Table 1 shows the evaluation results of the obtained nonwoven fabric. When aramid staple fibers or glass staple fibers were used as the reinforcing fibers, they had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, as well as good pleating processability and wind pressure deformation resistance.

[0082] Examples 5 and 6 A nonwoven fabric was produced in the same manner as in Example 2, except that the blending ratio of the reinforcing fiber and the microfiber was changed as shown in Table 1.

[0083] Table 1 shows the evaluation results of the obtained nonwoven fabric. Even when the blending ratio of the reinforcing fiber and the microfiber was changed, it had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, as well as good pleating processability and wind pressure deformation resistance.

[0084] Comparative Example 1 A nonwoven fabric was produced in the same manner as in Example 2, except that no reinforcing fiber was used and the blending ratio of the microfiber was changed as shown in Table 1.

[0085] Table 1 shows the evaluation results of the obtained nonwoven fabric. While it had both high collection efficiency and low pressure loss, since it was a nonwoven fabric without reinforcing fibers, the stiffness and flexibility of the nonwoven fabric were low, and it was inferior in both pleating processability and wind pressure deformation resistance.

[0086] Examples 7 and 8, Comparative Example 2 A nonwoven fabric was produced in the same manner as in Example 2, except that the fiber diameter of the ultra-fine organic fiber was changed as shown in Table 2.

[0087] Table 2 shows the evaluation results of the obtained nonwoven fabric. In Examples 7 and 8, they had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, as well as good pleating processability and wind pressure deformation resistance. In Comparative Example 2, the nonwoven fabric had high stiffness and flexibility, as well as good pleating processability and wind pressure deformation resistance, and showed a low value of pressure loss. However, since the fiber diameter of the ultra-fine organic fiber was large, the collection performance by the ultra-fine organic fiber was not exhibited, and the collection efficiency showed a low value.

[0088] Comparative Examples 3 and 4 By adjusting the fiber content of the stock solution for papermaking, the basis weight of the nonwoven fabric was changed to 5 g / m² in Comparative Example 3 2 and 93 g / m² in Comparative Example 4 2 except for this change, nonwoven fabrics were produced in the same manner as in Example 2.

[0089] The evaluation results of the obtained nonwoven fabrics are shown in Table 2. In Comparative Example 3, since the basis weight of the nonwoven fabric was low, the pressure loss showed a low value, while the collection efficiency showed a low value. Also, the stiffness and softness of the nonwoven fabric were low, and both the pleating processability and the wind pressure resistance deformation property were inferior. In Comparative Example 4, since the basis weight of the nonwoven fabric was high, the collection efficiency showed a high value, while the pressure loss showed a high value. Also, the stiffness and softness of the nonwoven fabric were high, and the wind pressure resistance deformation property was good, but the pleating processability was inferior.

[0090] Comparative Example 5 The nonwoven fabric produced in the same manner as in Comparative Example 1 was immersed in a resin solution containing a polyurethane resin and N,N-dimethylformamide, and then the excessively adhered resin solution was squeezed with a press roll. Then, it was immersed in a coagulating liquid consisting of water at room temperature to coagulate the resin and obtain a nonwoven fabric impregnated with a polyurethane resin. The basis weight of the nonwoven fabric was 50.5 g / m² before resin impregnation 2 and 55.6 g / m² after resin impregnation 2 .

[0091] The evaluation results of the obtained nonwoven fabrics are shown in Table 2. The stiffness and softness of the nonwoven fabric were high, and both the pleating processability and the wind pressure resistance deformation property were good, and the collection efficiency also showed a high value. On the other hand, since the resin was impregnated into the voids in the nonwoven fabric, the porosity of the nonwoven fabric was low, and the pressure loss showed a high value.

[0092] Examples 9, 10 By adjusting the fiber content of the stock solution for papermaking, the basis weight of the nonwoven fabric was changed to 10 g / m² in Example 9 2 and 25 g / m² in Example 10 2 except for this change, nonwoven fabrics were produced in the same manner as in Example 1.

[0093] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the basis weight was changed, it had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, and was also good in pleating processability and wind pressure deformation resistance.

[0094] Examples 11 and 12 Except that the basis weight of the nonwoven fabric was adjusted by changing the fiber amount in the papermaking stock solution to 15 g / m in Example 11 2 and 25 g / m in Example 12 2 a nonwoven fabric was produced in the same manner as in Example 2.

[0095] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the basis weight was changed, it had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, and was also good in pleating processability and wind pressure deformation resistance.

[0096] Examples 13 and 14 Except that the fiber diameter of the reinforcing fiber was changed to 7.5 μm in Example 13 and 30.0 μm in Example 14, a nonwoven fabric was produced in the same manner as in Example 2.

[0097] The evaluation results of the obtained nonwoven fabric are shown in Table 3. Even when the fiber diameter of the reinforcing fiber was changed, it had both high collection efficiency and low pressure loss, and the nonwoven fabric had high stiffness and flexibility, and was also good in pleating processability and wind pressure deformation resistance.

[0098] Example 15 Except that the microfiber was changed to a copolymerized PET staple fiber which is a heat-shrinkable fiber (a copolymerized polyester copolymerized at a ratio of 92.9 mol% of terephthalic acid and 7.1 mol% of isophthalic acid as dicarboxylic acid components, and 95.6 mol% of ethylene glycol and 4.4 mol% of 2,2-bis{4-(2-hydroxyethoxy)phenyl}propane as diol components, shrinkage start temperature 70 °C, fiber diameter 6.0 μm, fiber length 3.0 mm), a nonwoven fabric was produced in the same manner as in Example 2.

[0099] Table 3 shows the evaluation results of the obtained nonwoven fabric. Since the microfibers are heat-shrinkable fibers, the nonwoven fabric is bulky, has both high collection efficiency and low pressure loss, has high stiffness and softness, and is also good in pleating processability and wind pressure deformation resistance.

[0100]

Table 1

[0101]

Table 2

[0102]

Table 3

Claims

1. It contains ultrafine organic fibers with a fiber diameter of 0.01 to 0.90 μm and reinforcing fibers, and has a basis weight of 10 to 90 g / m ２ and a bending resistance of 100 to 500 mgf.

2. 2. The nonwoven fabric according to claim 1, wherein the reinforcing fibers are selected from the group consisting of polyarylate fibers, aramid fibers, polyketone fibers, polyparaphenylene benzobisoxazole fibers, ultra-high molecular weight polyethylene fibers, carbon fibers, glass fibers, and combinations thereof.

3. 2. The nonwoven fabric of claim 1, comprising microfibers and binder fibers.

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 comprising the air filter according to claim 5.

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