filter media

Abstract

To provide a filter medium which comprises a non-woven fabric layer, and is excellent in balance between pressure loss and trapping efficiency.SOLUTION: In a filter medium comprising a non-woven fabric layer including modified cross-section fibers, it has been found that balance between pressure loss and trapping efficiency is affected by a mode of bore diameter distribution in the non-woven fabric layer and a mode of most frequent bore diameter in the non-woven fabric layer. Specifically, an applicant of the present application found that with respect to the mode of bore diameter distribution in the non-woven fabric layer including the modified cross-section fibers, a difference between a bore diameter of 5% cumulative value of relative frequency (hereinafter, referred to as P5, unit: μm) and a bore diameter of 95% cumulative value of relative frequency (hereinafter, referred to as P95, unit: μm) is larger than 14.1 μm, and with respect to a size of most frequent bore diameter in the non-woven fabric layer, the most frequent bore diameter (unit: μm) falls within a range of P5% or more and P95% or less and is smaller than 24.8 μm, and therefore, balance between pressure loss and trapping efficiency of the non-woven fabric layer including the modified cross-section fibers is improved.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a filter medium having a nonwoven fabric layer containing modified cross-section fibers. [Background technology]

[0002] Conventionally, air filters and masks have been required to have low pressure loss and excellent breathability, as well as excellent efficiency in capturing atmospheric dust particles, dust particles such as PM2.5, and pollen. There is a demand for air filters and masks made using filter media with excellent filtration performance that can satisfy both of these contradictory performance requirements.

[0003] As a prior art filter medium, Japanese Patent Laid-Open Publication No. 2021-169059 (Patent Document 1) discloses a nonwoven filter medium comprising polyolefin adhesive fibers with a tensile strength of 3 cN / dtex or more and other fibers. Patent Document 1 also discloses the finding that it is preferable to use other fibers with small fiber diameters to improve the strength and filtration performance of the nonwoven filter medium, and that these other fibers preferably have fibril-like portions. Furthermore, the working examples of Patent Document 1 disclose the application of a strong water flow to splittable fibers having an orange-shaped cross section that can be split into 16 parts, thereby forming irregular cross-section fibers. Furthermore, the application of a strong water flow to the irregular cross-section fibers until they become fibril-like is also disclosed, and the production of a nonwoven filter medium comprising the other fibers having fibril-like portions prepared in this manner. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-169059 Summary of the Invention [Problem to be solved by the invention]

[0005] The applicant of the present application has studied filter media according to the prior art, such as that disclosed in Patent Document 1. In particular, based on the disclosure of Patent Document 1, a fiber web containing split fibers was fed to a hydroentanglement device to form irregular cross-section fibers formed by splitting the split fibers and entangling the constituent fibers, thereby preparing a filter media having a nonwoven fabric layer. Furthermore, based on the disclosure of Patent Document 1 that the filtration performance of the filter media is improved, a strong water flow was applied to the split fibers in the entanglement step to completely split the splittable fibers, so that a filter media made of irregular cross-section fibers with a small fiber diameter could be prepared.

[0006] However, filter media having a nonwoven fabric layer produced based on such conventional technology are not necessarily filter media that have an excellent balance between pressure loss and collection efficiency, contrary to the findings disclosed in the conventional technology.

[0007] Therefore, there was a need to develop a filter medium that offers an excellent balance between pressure loss and collection efficiency. [Means for solving the problem]

[0008] The present invention provides "A filter medium having a nonwoven fabric layer containing irregular cross-section fibers, In a histogram prepared by arranging the pore sizes of the nonwoven fabric layer in order from smallest to largest, the difference between the pore size at 5% cumulative relative frequency (hereinafter abbreviated as P5, unit: μm) and the pore size at 95% cumulative relative frequency (hereinafter abbreviated as P95, unit: μm) is greater than 14.1 μm; the most frequent pore size (unit: μm) in the nonwoven fabric layer is within the range of P5 or more and P95 or less and is smaller than 24.8 μm; The modified cross-section fiber includes a split fiber in a partially split state. filter medium. ” is. [Effects of the Invention]

[0009] As a result of continued research, the applicant of the present application discovered that in a filter medium having a nonwoven fabric layer containing irregular cross-section fibers, the balance between pressure loss and collection efficiency is affected by the pore size distribution pattern in the nonwoven fabric layer and the most frequently occurring pore size pattern in the nonwoven fabric layer.

[0010] Specifically, the applicant: Regarding the pore size distribution in the nonwoven fabric layer containing modified cross-section fibers, in a histogram created by arranging the pore sizes of the nonwoven fabric layer in order from the smallest pore size to the largest pore size, the difference between the pore size at 5% cumulative value of relative frequency (hereinafter abbreviated as P5, unit: μm) and the pore size at 95% cumulative value of relative frequency (hereinafter abbreviated as P95, unit: μm) is greater than 14.1 μm. and, The most frequent pore size in the nonwoven fabric layer (unit: μm) is within the range of P5% to P95% and is smaller than 24.8 μm. It was found that the balance between pressure loss and collection efficiency is improved when using a nonwoven fabric layer containing irregular cross-section fibers.

[0011] Furthermore, the applicant has found that the filter medium described in claim 1 can be realized by including split fibers, part of which has been split, as modified cross-section fibers in the nonwoven fabric layer that constitutes the filter medium.

[0012] As described above, the present invention can provide a filter medium having a nonwoven fabric layer that has an excellent balance between pressure loss and collection efficiency. DETAILED DESCRIPTION OF THE INVENTION

[0013] In the present invention, various configurations can be appropriately selected, such as the following configurations. Note that, unless otherwise specified, the various measurements described in the present invention were performed under atmospheric pressure. Furthermore, measurements were performed at a temperature of 25°C. Furthermore, unless otherwise specified, the various measurement results described in the present invention were measured to a value one decimal place smaller than the desired value, and the value was calculated by rounding the value. As a specific example, when the desired value is measured to one decimal place, the value was measured to two decimal places, and the obtained value was rounded to one decimal place to calculate the value to one decimal place, and this value was used as the desired value. Furthermore, the respective upper and lower limit values ​​described below can be arbitrarily combined as desired to determine a usable numerical range.

[0014] The filter medium according to the present invention comprises a nonwoven fabric layer containing modified cross-section fibers.

[0015] The term "modified cross-section fiber" as used herein refers to a fiber in which the cross-sectional contour shape of the fiber cut in a direction perpendicular to the fiber length direction has a portion that is not a solid circle or a solid oval. The cross-sectional contour shape of the modified cross-section fiber can be adjusted as appropriate, and can be a roughly fan-shaped, T-shaped, Y-shaped, +-shaped, hollow (doughnut-shaped), or polygonal shape. It can also be a fiber with an irregular cross-sectional shape, such as fibril fiber.

[0016] Such modified cross-section fibers can be prepared by splitting split fibers. For example, splitting split fibers with an orange cross-section can produce modified cross-section fibers with a cross-sectional shape that is roughly fan-shaped. Split fibers that are not completely split but are partially split, prepared by applying a water flow to the splittable fibers so that they do not split completely, can also be used as modified cross-section fibers.

[0017] The type of resin constituting the modified cross-section fiber can be appropriately selected, and examples thereof include polyolefin resins (polypropylene resin, polyethylene resin, poly-4-methylpentene resin, etc.), styrene resins, polyvinyl alcohol resins, polyether resins (e.g., polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone, etc.), polyester resins (e.g., polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycarbonate, polyarylate, wholly aromatic polyester resin, etc.), polyimide resins, polyamide-imide resins, polyamide It can be constructed using known resins such as amide-based resins (e.g., aromatic polyamide resins, aromatic polyetheramide resins, nylon resins, etc.), resins having nitrile groups (e.g., polyacrylonitrile, etc.), urethane-based resins, epoxy-based resins, polysulfone-based resins (e.g., polysulfone, polyethersulfone, etc.), fluorine-based resins (e.g., polytetrafluoroethylene, polyvinylidene fluoride, etc.), cellulose-based resins, polybenzimidazole resins, and acrylic-based resins (e.g., polyacrylonitrile-based resins copolymerized with acrylic acid esters or methacrylic acid esters, modacrylic-based resins copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride, etc.).

[0018] These resins may be either linear or branched polymers, may be block or random copolymers, may have any three-dimensional structure, may have any crystallinity, or may be a mixture of multiple resin components.

[0019] In addition, the modified cross-section fiber may be a single fiber composed of a single component, or may be composed of multiple components. For example, it can be a form generally called a composite fiber, such as a core-sheath type, an island-in-the-sea type, a side-by-side type, an orange type, a bimetal type, etc. As an example, it can be a composite fiber such as a core-sheath type adhesive fiber, which has a low melting point resin in the sheath part and a resin with a melting point higher than the low melting point resin in the core part. In such a composite fiber, even if the low melting point resin is softened or melted, the fiber shape of the modified cross-section fiber is maintained due to the presence of the core, and the fiber bonding between the constituent fibers in the nonwoven fabric layer is performed. As a result, it is possible to provide a filter medium with excellent strength and a nonwoven fabric layer with an excellent balance between pressure loss and collection efficiency.

[0020] When filter material is required to have flame retardancy, it is preferable that modified cross-section fiber contains flame-retardant resin.As such flame-retardant resin, for example, modacrylic resin, vinylidene resin, polyvinyl chloride resin, polyvinylidene fluoride resin, novoloid resin, polychlor resin, polyester resin copolymerized with phosphorus compound, acrylic resin copolymerized with halogen-containing monomer, aramid resin, resin kneaded with halogen-based, phosphorus-based or metal compound-based flame retardant, etc.Furthermore, modified cross-section fiber can contain functional substances such as moisture absorbent, matting agent, pigment, flame retardant, stabilizer, antistatic agent, colorant, dye, conductive agent, hydrophilic agent, deodorizer or antibacterial agent.

[0021] In order to provide a filter medium with an excellent balance between pressure loss and collection efficiency, the average fiber diameter of the modified cross section fibers may be 1 to 20 μm, 2 to 18 μm, or 3 to 16 μm.

[0022] The "average fiber diameter" as used herein refers to the arithmetic mean value of the fiber diameters of 50 fibers measured on an electron microscope photograph taken at 5000x magnification of the cross section or surface of the object to be measured. Furthermore, if the fiber diameter is too small to measure, it can be measured on an electron microscope photograph at a magnification higher than 5000x. Furthermore, if the cross-sectional shape of the fiber is not a solid circle, the diameter of a circle having the same area as the cross-sectional area of ​​the cross-sectional shape of the fiber can be considered to be the fiber diameter.

[0023] The modified cross-section fibers may be continuous long fibers or short fibers cut to a predetermined length. To provide a filter medium with an excellent balance between pressure loss and collection efficiency, the fiber length is preferably 0.5 to 20 mm. The "fiber length" refers to the value measured in accordance with JIS L1015 (2010), 8.4.1c) direct method (C method).

[0024] The filter medium of the present invention contains modified cross-section fibers, and therefore provides a filter medium that has an excellent balance between pressure loss and collection efficiency.

[0025] The nonwoven fabric layer of the filter material of the present invention may contain fibers other than modified cross-section fibers (hereinafter, sometimes referred to as other fibers) as its constituent fibers. The cross-sectional shape of the other fibers can be appropriately selected, and may be a solid circle or a solid ellipse, a fiber that has a part that is not a solid circle or a solid ellipse, or a fiber with an irregular cross-sectional shape such as fibril fiber.

[0026] The type of resin constituting the other fibers can be selected appropriately, and the resins listed as being capable of constituting modified cross-section fibers can be used. These resins may be either linear or branched polymers, may be block copolymers or random copolymers, and may have any three-dimensional structure or crystallinity, without any particular limitations. Furthermore, a mixture of multi-component resins may also be used.

[0027] In addition, the other fibers may be single fibers composed of a single component, or may be composed of multiple components. For example, they can be generally called composite fibers, such as core-sheath type, sea-island type, side-by-side type, orange type, bimetal type, etc. As an example, they can be composite fibers such as core-sheath type adhesive fibers, which have a low melting point resin in the sheath and a resin with a melting point higher than the low melting point resin in the core. In such composite fibers, even if the low melting point resin is softened or melted, the fiber shape of the other fibers is maintained due to the presence of the core, and the component fibers in the nonwoven fabric layer are bonded to each other. As a result, a filter medium can be provided that has excellent strength and a nonwoven fabric layer with an excellent balance between pressure loss and collection efficiency.

[0028] When the filter material is required to have flame retardancy, it is preferable that the other fibers contain flame retardant resin.As such flame retardant resin, for example, modacrylic resin, vinylidene resin, polyvinyl chloride resin, polyvinylidene fluoride resin, novoloid resin, polyclar resin, polyester resin copolymerized with phosphorus compound, acrylic resin copolymerized with halogen-containing monomer, aramid resin, resin kneaded with halogen-based, phosphorus-based or metal compound-based flame retardant, etc.

[0029] Furthermore, the other fibers may contain functional substances such as moisture absorbents, matting agents, pigments, flame retardants, stabilizers, antistatic agents, colorants, dyes, conductive agents, hydrophilizing agents, deodorizing agents, or antibacterial agents.

[0030] In order to provide a filter medium with an excellent balance between pressure loss and collection efficiency, the average fiber diameter of the other fibers may be 1 to 20 μm, 2 to 18 μm, or 3 to 16 μm.

[0031] The other fibers may be continuous long fibers or short fibers cut to a predetermined length. The fiber length is preferably 0.5 to 20 mm to provide a filter medium with an excellent balance between pressure loss and collection efficiency.

[0032] Regarding the fibers constituting the nonwoven fabric layer according to the present invention, the percentage of the mass of the irregular cross-section fibers in the mass of the fibers constituting the nonwoven fabric layer (for example, the sum of the masses of the irregular cross-section fibers and other fibers) is greater than 0% by mass and less than 100% by mass. This percentage is appropriately adjusted to provide a filter medium with an excellent balance between pressure loss and collection efficiency. Specifically, this percentage can be 5% by mass or more and less than 100% by mass, 10% by mass or more and less than 90% by mass, 20% by mass or more and less than 80% by mass, 30% by mass or more and less than 80% by mass, 40% by mass or more and less than 70% by mass, or 50% by mass or more and less than 60% by mass.

[0033] The nonwoven fabric layer of the filter material of the present invention is characterized in that, in a histogram created by arranging the pore sizes of the nonwoven fabric layer in order from smallest to largest (i.e., the pore sizes of the nonwoven fabric layer in order of size), the difference between the pore size at 5% cumulative value of relative frequency (hereinafter abbreviated as P5, unit: μm) and the pore size at 95% cumulative value of relative frequency (hereinafter abbreviated as P95, unit: μm) is greater than 14.1 μm.

[0034] Whether or not the nonwoven fabric layer of the filter medium satisfies this constitution can be confirmed by the following method.

[0035] (How to calculate the difference between P5 and P95) 1. Collect a sample from the nonwoven fabric layer of the filter material. Alternatively, you can collect the nonwoven fabric layer from the filter material as a sample. 2. Using a porometer (manufactured by Coulter), measure the pore size (unit: μm) of the sample by the bubble point method. The pore size of the sample refers to the flow pore size (unit: μm) measured by the bubble point method. The measurement result is taken as the pore size of the nonwoven fabric layer. 3. A histogram showing the relationship between pore size and the number of pores is created by arranging the pore sizes of the measured sample in order from smallest to largest. The histogram is a graph showing the number of pores for each pore size class (data interval). The width of the pore size class (width of the data interval) is set to 0.1 μm. Specifically, the pore size range from greater than 0 μm to 0.1 μm or less is set as the first data interval, and the pore size range from greater than 0.1 μm to 0.2 μm or less is set as the second data interval, with the width of the pore size classes set in the same manner. 4. The maximum pore diameter in the class containing the 5%-th pore diameter when the total number of measured pore diameters is calculated from the smallest to the largest pore diameters and converted to 100% is defined as the P5 pore diameter. Specifically, if the total number of pore diameters in the measured sample is 2n or 2n + 1 (n is an integer greater than or equal to 50), the pore diameter in the class containing the pore diameter is calculated from the smallest to the largest pore diameter. The maximum pore diameter in the class containing that pore diameter (for example, if the class is in the pore diameter range of greater than 14.9 μm and less than or equal to 15.0 μm, the maximum pore diameter is 15.0 μm) is defined as the P5 pore diameter. 5. Next, the pore sizes are sorted from smallest to largest, and the total number of measured pore sizes is converted to 100%. The maximum pore size in the class containing the 95th pore size is defined as the P95 pore size. Specifically, if the measured sample has a total of 2n or 2n + 1 pore sizes (n is an integer greater than or equal to 50), the pore size is sorted from smallest to largest, and the pore size that is 2n × 0.95th is determined. The maximum pore size in the class containing that pore size (for example, if the class is in the pore size range of greater than 42.7 μm and less than or equal to 42.8 μm, the maximum pore size is 42.8 μm) is defined as the P95 pore size. 6. The value obtained by subtracting P5 from P95 as described above is the difference between P5 and P95 (unit: μm).

[0036] The larger the difference between P5 and P95, the broader the pore size distribution of the nonwoven fabric layer. The filter medium of the present invention has a nonwoven fabric layer with this pore size distribution, and therefore has an excellent balance between pressure loss and collection efficiency.

[0037] In addition, the nonwoven fabric layer of the filter material of the present invention is characterized in that the most frequent pore size (unit: μm) in the nonwoven fabric layer is in the range of P5 or more and P95 or less, and is smaller than 24.8 μm. In addition, in the histogram created by the above-mentioned (method of determining the difference between P5 and P95), the maximum value of the pore size in the class containing the most pore sizes (for example, if the class is in the pore size range of 22.9 to 23.0 μm, the maximum value of the pore size is 23.0 μm) is the most frequent pore size (unit: μm) in the nonwoven fabric layer.

[0038] The applicant of the present application discovered that by having a structure in which the difference between P5 and P95 is greater than 14.1 μm and the most frequent pore size is in the range of P5 or greater and P95 or less and smaller than 24.8 μm, the balance between the pressure loss and collection efficiency of the nonwoven fabric layer is improved, and as a result, a filter material equipped with this nonwoven fabric layer has an excellent balance between pressure loss and collection efficiency.

[0039] The applicant has found that providing a nonwoven fabric layer with a broad pore size distribution (a nonwoven fabric layer with a large difference between P5 and P95) makes it easier to provide a filter medium with an excellent balance between pressure loss and collection efficiency. In order to provide a filter medium with an even better balance between pressure loss and collection efficiency, the difference between P5 and P95 in the nonwoven fabric layer is preferably 15 to 100 μm, preferably 16 to 60 μm, preferably 17 to 50 μm, preferably 18 to 40 μm, preferably 19 to 30 μm, and preferably 19.8 to 27.8 μm.

[0040] The present applicant has also found that a filter medium with an excellent balance between pressure loss and filtering efficiency can be easily provided by using a nonwoven fabric in which the most frequent pore size (unit: μm) in the nonwoven fabric layer is in the range of P5 to P95 and smaller than 24.8 μm. To provide a filter medium with an even better balance between pressure loss and filtering efficiency, the most frequent pore size in the nonwoven fabric is preferably 1 μm or larger, more preferably 3 μm or larger, more preferably 5 μm or larger, more preferably 10 μm or larger, and more preferably larger than 12.8 μm. The upper limit can be appropriately adjusted to be smaller than 24.8 μm, but is preferably 24.0 μm or smaller, and preferably 23.5 μm or smaller. The most frequent pore size is preferably in the range of 13.9 to 23.0 μm.

[0041] In addition, the percentage (frequency (unit: %)) of the number of the most frequently occurring pore size in the total number of pore sizes used to create the histogram of the present invention can be adjusted appropriately. The lower the frequency, the broader the pore size distribution in the nonwoven fabric tends to be, which is preferable because it is easier to provide a filter medium that has an excellent balance between pressure loss and collection efficiency.

[0042] Other configurations of the nonwoven fabric layer, such as thickness and basis weight, are not particularly limited and can be adjusted appropriately. The thickness can be 0.1 to 10 mm, 0.3 to 5 mm, or 0.5 to 3 mm. The basis weight can be, for example, 10 to 500 g / m 2 and can be 30 to 300 g / m 2 and 40 to 100 g / m 2 In the present invention, the thickness is 20 g / cm in the direction perpendicular to the main surface. 2 It refers to the length in the vertical direction when a compressive load is applied, and the basis weight is the weight per square meter on the surface (principal surface) with the widest area of ​​the object being measured. 2 This refers to the mass per unit mass.

[0043] The filter material of the present invention can be used as the filter material that is only made up of the above-mentioned nonwoven fabric, but can also be made into the filter material that is laminated with other material (for example, porous body, porous film or non-porous film, foam, nonwoven fabric, woven fabric or knitted fabric, etc.), which is made into charged filter material by being subjected to charging treatment.In addition, the shape of filter material can be adjusted appropriately according to use, and can be the filter material that is pleated or corrugated other than flat plate.

[0044] The filter medium according to the present invention can be produced by a known method, but as an example, it can be produced by a production method comprising the following steps. Step 1: A step of preparing modified cross-section fibers or a fiber web having modified cross-section fibers and other fibers as constituent fibers. Step 2: A process in which irregular cross-section fibers are entangled with each other, or with other fibers, by applying a water current to the fiber web to hydroentangle it. Step 3: A step of drying the fiber web in which the constituent fibers are entangled to prepare a nonwoven fabric. Step 4: A step of preparing a filter medium using the prepared nonwoven fabric.

[0045] First, step 1 will be described. A known method can be used to prepare a fiber web containing modified cross-section fibers or modified cross-section fibers and other fibers as constituent fibers. Examples of methods that can be used include feeding the modified cross-section fibers or the modified cross-section fibers and other fibers into a carding machine and blending them to prepare a fiber web; collecting fibers (such as modified cross-section fibers) spun using a direct spinning method to prepare a fiber web having the fiber configuration; and forming a fiber web from a dispersion containing the modified cross-section fibers or modified cross-section fibers and other fibers. The dispersion medium can be removed by feeding the fiber web prepared by forming the dispersion into a dryer (for example, an air-through dryer).

[0046] Next, step 2 will be described. The mode of hydroentanglement applied to the fiber web is appropriately adjusted so that the constituent fibers of the fiber web can be entangled to produce a nonwoven fabric having the pore size distribution of the present invention. The water pressure can be 1 MPa or more, less than 9 MPa, 7 MPa or less, or 5 MPa or less. The fiber web may also be subjected to hydroentanglement multiple times. While the water stream is being applied to the fiber web, it is preferable to remove excess water from below the fiber web by suction.

[0047] Next, step 3 will be described. The method for drying the fibrous web in which the constituent fibers are entangled can be selected as appropriate, and examples include a method of subjecting the web to a dryer such as a dry heat dryer. The heating temperature is adjusted as appropriate, but is adjusted to a temperature that can remove water from the fibrous web and does not unintentionally denature or deteriorate the constituent components of the fibrous web. In this process, the fiber adhesive components of the modified cross-section fibers and other fibers may be melted to bond the constituent fibers of the fibrous web to each other. Furthermore, if the fibrous web contains an adhesive, the adhesive may be melted to bond the constituent fibers to each other.

[0048] Next, step 4 will be explained. The nonwoven fabric prepared through step 3 can be used as a flat filter material as it is, or can be subjected to secondary processing to prepare a filter material.For example, the nonwoven fabric is provided with functional materials such as activated carbon or chemicals to prepare a filter material; the nonwoven fabric is punched or cut into the desired shape to prepare a filter material; the nonwoven fabric is subjected to charging treatment such as corona charging treatment, plasma charging treatment, or charging treatment with polar liquid such as water to charge the nonwoven fabric; the nonwoven fabric is laminated or laminated with another material (for example, porous body, porous film, non-porous film, foam, nonwoven fabric, woven fabric or knitted fabric) to prepare a filter material; the nonwoven fabric is folded into pleats or corrugated shape to prepare a three-dimensional filter material; the nonwoven fabric is provided on a filter frame or the like to prepare a filter unit, and other secondary processing can be adopted.

[0049] The filter medium according to the present invention can also be produced by another production method that includes the following steps 1' and 2' instead of the above steps 1 and 2. Step 1': A step of preparing split fibers that can be split to form modified cross-section fibers, or a fiber web that includes the split fibers and other fibers as constituent fibers. Step 2': A process in which a water stream is applied to the fiber web to hydroentangle it, thereby splitting the split fibers to form irregular cross-section fibers, and entangling the irregular cross-section fibers with each other, or with other fibers.

[0050] First, step 1' will be described. A known method can be used to prepare split fibers that can be split to form modified cross-section fibers, or a fiber web containing the split fibers and other fibers as constituent fibers, such as a method of feeding the split fibers or constituent fibers such as the split fibers and other fibers to a carding machine and blending them to prepare a fiber web, a method of collecting fibers (such as split fibers) spun using a direct spinning method to prepare a fiber web having the fiber configuration, or a method of preparing a fiber web by papermaking a dispersion containing the modified cross-section fibers or the modified cross-section fibers and other fibers. The dispersion medium can be removed by feeding the fiber web prepared by papermaking the dispersion into a dryer (for example, an air-through dryer).

[0051] Next, step 2' will be described. The mode of hydroentanglement applied to the fiber web is appropriately adjusted so that the split fibers are split to form modified cross-section fibers and the constituent fibers of the fiber web are entangled with each other, thereby producing a nonwoven fabric having the pore size distribution according to the present invention. The water pressure can be 1 MPa or more, less than 9 MPa, 7 MPa or less, or 5 MPa or less. The fiber web may also be subjected to hydroentanglement multiple times. While the water stream is applied to the fiber web, it is preferable to remove excess water from below the fiber web by suction.

[0052] The applicant of the present application has unexpectedly discovered that if the split fibers are completely split in step 2' as disclosed in Patent Document 1, it is difficult to provide a filter medium that has an excellent balance between pressure loss and collection efficiency.

[0053] In contrast, the applicant of the present application has discovered that a filter medium having a nonwoven fabric layer containing split fibers that are not completely divided but are incompletely divided, that is, a filter medium having a nonwoven fabric layer containing split fibers that are partially divided into fibers with modified cross-sections, is a filter medium that has an excellent balance between pressure loss and collection efficiency, unexpectedly compared to the findings disclosed in Patent Document 1. The applicant has also discovered that in step 2', by adjusting the mode of hydroentanglement, such as the strength of the water flow applied to the fibrous web, the number of hydroentanglement treatments applied to the fibrous web, the treatment time, and the strength of the suction of excess water from below the fibrous web, it is possible to prepare a nonwoven fabric containing split fibers that are not completely divided but are incompletely divided.

[0054] Whether or not the nonwoven fabric layer in the filter material contains split fibers in a partially split state as modified cross-section fibers can be determined by taking microscopic photographs of both main surfaces and cross sections of the nonwoven fabric layer and examining the photographs. In other words, if the photograph shows split fibers that are partially split but not completely split, it can be determined that the nonwoven fabric shown in the photograph contains split fibers in a partially split state as modified cross-section fibers. [Example]

[0055] The present invention will be specifically described below with reference to examples, but these examples are not intended to limit the scope of the present invention.

[0056] (Preparation of component fibers) A split fiber having the following configuration was prepared. Fiber length: 5mm Average fiber diameter: 15.4 μm Fiber cross-section: Solid orange shape that can be divided into 16 sections The ratio of the area of ​​one polypropylene resin part to the area of ​​one poly-4-methylpentene resin part in the fiber cross section is 1:1. Fibers produced by splitting: Polypropylene resin (melting point: 165°C) irregular cross-section fiber with a solid, roughly fan-shaped cross-section (average fiber diameter: 3.8 μm), and poly-4-methylpentene resin (melting point: 240°C) irregular cross-section fiber with a solid, roughly fan-shaped cross-section (average fiber diameter: 3.8 μm). As other fibers, core-sheath type adhesive fibers having the following configuration were prepared. Fiber length: 5mm Average fiber diameter: 10.6 μm Fiber cross-sectional shape: solid circular core-sheath shape Core: Polypropylene resin (melting point: 165°C) Sheath: Polyethylene resin (melting point: 135°C)

[0057] (Comparative Example 1) A dispersion containing 40% by mass of split fibers and 60% by mass of core-sheath type adhesive fibers was paper-formed to prepare a fiber web. The fiber web was then dried by being placed in an air-through heater adjusted to a heating temperature of 140°C, and at the same time, only the adhesive component (sheath component) of the core-sheath adhesive fiber was melted, thereby bonding the constituent fibers together via the adhesive component (sheath component) of the core-sheath adhesive fiber, and the fiber was then allowed to cool to prepare a nonwoven fabric. The nonwoven fabric thus produced was then used as a flat filter material. The fibers of the filter material (nonwoven fabric) thus prepared were composed of unsplit split fibers and core-sheath type adhesive fibers.

[0058] Example 1 A dispersion containing 40% by mass of split fibers and 60% by mass of core-sheath type adhesive fibers was paper-formed to prepare a fiber web. The fiber web was then dried by being placed in an air-through heater adjusted to a heating temperature of 140°C, and at the same time, only the adhesive component (sheath component) of the core-sheath adhesive fiber was melted, thereby bonding the constituent fibers together via the adhesive component (sheath component) of the core-sheath adhesive fiber, and the fiber web was then allowed to cool. After cooling, the fiber web was fed to a hydroentanglement device to split the split fibers contained in the fiber web and simultaneously entangle the constituent fibers. The hydroentanglement conditions applied to the fiber web in the hydroentanglement device were a water flow pressure of 3 MPa, a suction pressure of 45 kPa, and four water flow passes. Thereafter, the fiber web was dried in an air-through heater adjusted to a heating temperature of about 100°C to prepare a nonwoven fabric. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split).

[0059] Example 2 A nonwoven fabric was prepared in the same manner as in Example 1, except that the pressure of the water jet applied to the fiber web after cooling was changed to 5 MPa. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split).

[0060] Example 3 A nonwoven fabric was prepared in the same manner as in Example 1, except that the pressure of the water jet applied to the fiber web after cooling was changed to 7 MPa. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split).

[0061] (Comparative Example 2) A nonwoven fabric was prepared in the same manner as in Example 1, except that the pressure of the water jet applied to the fiber web after cooling was changed to 9 MPa. Then, the nonwoven fabric thus produced is made into flat filter material as it is.Note that the composition fiber of the filter material (nonwoven fabric) thus prepared does not contain the split fiber that is partly split as irregular cross section fiber (the split fiber that is not completely split and is incompletely split), and is composed of the irregular cross section fiber that splits split fiber and core-sheath type adhesive fiber.

[0062] The physical properties of each filter material manufactured as described above are summarized in Table 1. In addition, the pressure loss (unit: Pa) and collection efficiency (unit: %) were measured using the following measurement method. Furthermore, the QF value, which can evaluate the balance between the pressure loss and collection efficiency of the filter material, was calculated from the values ​​of the pressure loss and collection efficiency that were measured.

[0063] (Method for measuring pressure loss and collection efficiency) Test pieces were taken from the filter media and attached to a measuring device "AP-9000" manufactured by Shibata Scientific Co., Ltd. to measure the collection efficiency and pressure loss. First, the effective filtration area of ​​the test piece is 44.12 cm 2 The test flow rate was adjusted to 40 liters per minute, and the differential pressure between the upstream and downstream of the test piece was measured, and the pressure loss (unit: Pa) of the test piece was calculated from the measured differential pressure. Next, the effective filtration area of ​​the test piece is 44.12 cm 2 The test flow rate was adjusted to 30 liters per minute, and sodium chloride particles (particle size distribution number standard median: 0.06 to 0.10 μm, geometric standard deviation: 1.8 or less) were measured at a concentration of 50 mg / m 3 A test airflow containing the following sodium chloride particles (concentration fluctuation: ±15% or less) was supplied upstream of the test piece. After supplying the test airflow for one minute, the concentrations of the sodium chloride particles present on the upstream and downstream sides of the test piece were measured using a light scattering dust concentration meter, and the concentration of sodium chloride particles captured on the test piece was calculated from the two measured concentrations. The percentage of the concentration of sodium chloride particles captured on the test piece relative to the concentration of sodium chloride particles supplied upstream of the test piece was then calculated, and this value was taken as the collection efficiency of the test piece (unit: %).

[0064] (How to calculate QF value) The QF value (unitless) was calculated by substituting the pressure drop and collection efficiency values ​​calculated as described above into the following formula. Note that a higher QF value indicates a filter medium with an excellent balance between low pressure drop and high collection efficiency. QF value = -Ln(1-A / 100) / B Ln: natural logarithm A: Collection efficiency (unit: %) B: Pressure loss (unit: Pa)

[0065] In Tables 1 and 2, the difference between P5 and P95 is shown in the "P95% - P5% [μm]" column. The most frequently occurring pore size is shown in the "Most frequently occurring pore size [μm]" column, and the frequency of that pore size is shown in the "Frequency [%]" column.

[0066] [Table 1]

[0067] Example 4 A nonwoven fabric was prepared in the same manner as in Example 1, except that the conditions for hydroentanglement applied to the cooled fiber web were changed to a water pressure of 3 MPa, a suction pressure of 35 kPa, and four times the water was applied. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split). Example 5 A nonwoven fabric was prepared in the same manner as in Example 1, except that the conditions for hydroentanglement applied to the cooled fiber web were changed to a water pressure of 1 MPa, a suction pressure of 45 kPa, and two times of water application. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split).

[0068] Example 6 A nonwoven fabric was prepared in the same manner as in Example 1, except that the conditions for hydroentanglement applied to the cooled fiber web were changed to a water pressure of 3 MPa, a suction pressure of 45 kPa, and two water stream applications. The nonwoven fabric thus produced is then used as a flat filter material.The constituent fibers of the filter material (nonwoven fabric) thus prepared include the split fibers that are partially split as modified cross-section fibers (split fibers that are not completely split but are incompletely split).

[0069] The physical properties of each filter medium produced as described above are summarized in Table 2.

[0070] [Table 2]

[0071] The filter material of the embodiment prepared as described above has high QF value, so it is the filter material that has good balance between low pressure loss value and high collection efficiency.On the other hand, the filter material of comparative example 1-2 is low QF value compared with these embodiments, so it is the filter material that has poor balance between low pressure loss value and high collection efficiency.

[0072] These results show that when the difference between P5 and P95 in the nonwoven fabric layer containing irregular cross-section fibers in the filter material is greater than 14.1 μm, and when the most frequent pore size (unit: μm) is in the range of P5 or greater and P95 or less and smaller than 24.8 μm, a filter material with a uniquely excellent balance between low pressure loss and high collection efficiency can be achieved.

[0073] In addition, the filter material of the embodiment prepared as described above all comprises the split fiber of the state of being partially split as irregular cross-section fiber (the split fiber of the state of being not completely split, but incompletely split).On the other hand, compared with these embodiments, the filter material of comparative example 2 does not comprise the split fiber of the state of being partially split as irregular cross-section fiber (the split fiber of the state of being not completely split, but incompletely split).

[0074] Therefore, by including split fibers in a partially split state (split fibers that are not completely split but are incompletely split) in the nonwoven fabric layer that constitutes the filter material, it is possible to provide a filter material that has an excellent balance between pressure loss and collection efficiency. [Industrial Applicability]

[0075] The filter material of the present invention can be suitably used as a gas filter or liquid filter in, for example, food or medical product production factories, precision equipment manufacturing factories, indoor agricultural cultivation facilities, general households or industrial facilities such as office buildings, electrical appliances such as air purifiers and office equipment, various vehicles such as automobiles and aircraft, and equipment such as masks and medical equipment.

Claims

[Claim 1] A filter medium comprising a nonwoven fabric layer containing modified cross-section fibers, In a histogram prepared by arranging the pore sizes of the nonwoven fabric layer in order from smallest to largest, the difference between the pore size at 5% cumulative value of relative frequency (hereinafter abbreviated as P5, unit: μm) and the pore size at 95% cumulative value of relative frequency (hereinafter abbreviated as P95, unit: μm) is greater than 14.1 μm, the most frequent pore size (unit: μm) in the nonwoven fabric layer is in the range of P5 or more and P95 or less and is smaller than 24.8 μm; The modified cross-section fiber includes a split fiber in a partially split state. filter medium.

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