Tribo-electrically charged nonwoven fabric and filter medium comprising said tribo-electrically charged nonwoven fabric

Abstract

The present invention relates to a tribo-electrically charged nonwoven fabric, and a filter medium comprising said tribo-electrically charged nonwoven fabric. [Solution] As a result of continued research, the applicant of this application has found that tribo-electrically charged nonwoven fabrics in accordance with the present invention in which the calculated value y is less than 0.57 have excellent moldability. This means that the smaller the calculated value y of the tribo-electrically charged nonwoven fabric, the easier it is to deform in the CMD direction of the tribo-electrically charged nonwoven fabric relative to the ease with which the tribo-electrically charged nonwoven fabric deforms in the MD direction. Furthermore, because the tribo-electrically charged nonwoven fabric according to the present invention has excellent formability, filter media formed from this tribo-electrically charged nonwoven fabric have the desired shape without wrinkles.

Description

[Technical Field]

[0001] The present invention relates to a tribo-electrically charged nonwoven fabric, and a filter medium comprising said tribo-electrically charged nonwoven fabric. [Background technology]

[0002] Air filters and masks have traditionally been required to have low pressure loss and excellent breathability, as well as excellent efficiency in capturing atmospheric dust, dust particles such as PM2.5, and pollen. In order to satisfy both of these contradictory performance requirements, air filters and masks that use tribo-electrically charged nonwoven fabric as a filtering material have been studied.

[0003] As an example of such a tribo-electrically charged nonwoven fabric, Japanese Patent Laid-Open No. 2006-218342 (Patent Document 1) discloses a tribo-electrically charged nonwoven fabric in which first fibers and second fibers (hereinafter sometimes referred to as tribo-electrically charged fibers) that are made of different component resins are mixed together and can become electrically charged when they rub against each other, and the first fibers and second fibers are electrically charged. Specifically, the examples of Patent Document 1 disclose that a tribo-electrically charged nonwoven fabric can be prepared by subjecting a web made of tribo-electrically charged fibers to a hydroentanglement process and a needle punching process, causing the tribo-electrically charged fibers to rub against each other. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2006-218342 Summary of the Invention [Problem to be solved by the invention]

[0005] The applicant of the present application has investigated the prior art such as that disclosed in the above-mentioned Patent Document 1. However, tribo-electrically charged nonwoven fabrics prepared based on the knowledge disclosed in the prior art have often exhibited poor formability for use as a filter medium.

[0006] For example, when attempting to use a tribo-electrically charged nonwoven fabric as a filter medium for cup-shaped masks, the tribo-electrically charged nonwoven fabric must be deformed into a cup shape with a raised centre. Specifically, when attempting to continuously produce filter medium for cup-shaped masks using tribo-electrically charged nonwoven fabric, the tribo-electrically charged nonwoven fabric is generally deformed into a cup shape by pressing the tribo-electrically charged nonwoven fabric against a cup-shaped mold while applying tension in the production direction of the tribo-electrically charged nonwoven fabric (generally the direction parallel to the main surfaces of the tribo-electrically charged nonwoven fabric that is strongest; hereafter sometimes referred to as the MD direction).

[0007] However, when tribo-electrically charged nonwoven fabrics made with conventional technology were deformed into a cup shape, wrinkles appeared that were pulled in a direction parallel to the MD. As a result, it was not possible to form the tribo-electrically charged nonwoven fabric into the desired shape, such as one where no wrinkles were generated.

[0008] Furthermore, in air filters and masks that include as a filter material a tribo-electrically charged nonwoven fabric that cannot be formed into the desired shape, distortion occurs in the air filter or mask due to the inclusion of this filter material, and there is a risk that problems such as leaks will occur or the mask will fit poorly to the human face.

[0009] For this reason, there was a demand for the realization of a tribo-electrically charged nonwoven fabric with excellent formability. [Means for solving the problem]

[0010] The first aspect of the present invention is "A tribo-electrically charged nonwoven fabric in which first fibers and second fibers, which are made of different constituent resins and can be charged by rubbing against each other, are mixed together, and the first fibers and second fibers are electrically charged, A tribo-electrically charged nonwoven fabric, in which the value y calculated from the following formula is less than 0.57: Note y=b / a a: Tensile strength of the tribo-electrically charged nonwoven fabric in the direction parallel to the main surface of the tribo-electrically charged nonwoven fabric, in which the tensile strength is greatest (unit: N / 50 mm). b: The tensile strength (unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric in a direction parallel to the main surface of the tribo-electrically charged nonwoven fabric, which is perpendicular to the direction in which the tensile strength is greatest. is.

[0011] The second aspect of the present invention is "A molded filter medium comprising the tribo-electrically charged nonwoven fabric according to claim 1." is. [Effects of the Invention]

[0012] As a result of continued research, the applicant of this application found that tribo-electrically charged nonwoven fabrics according to the present invention in which the calculated value y is less than 0.57 have excellent formability. Furthermore, because the tribo-electrically charged nonwoven fabrics according to the present invention have excellent formability, filter media formed from this tribo-electrically charged nonwoven fabric have the desired wrinkle-free shape. [Brief explanation of the drawings]

[0013] [Figure 1] (a): A schematic cross-sectional view of the tip of a needle that can produce a tribo-electrically charged nonwoven fabric according to the present invention, formed by cutting the tip of the needle in a direction parallel to the length of the needle and across the barb. (b): A schematic cross-sectional view showing the length from the surface of the needle to the bottom of the depression (depth of the depression) in Figure 1(a). Note that the reference numerals assigned in Figure 1(a) have been omitted. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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 are performed under normal pressure and a temperature condition of 25°C. Furthermore, unless otherwise specified, the various measurement results described in the present invention are measured to a value one decimal place smaller than the desired value, and the value is calculated by rounding off the value. As a specific example, if the desired value is expressed to one decimal place, the value is measured to two decimal places, and the obtained value is rounded to one decimal place to calculate the value to one decimal place, and this value is used as the desired value. Furthermore, the upper and lower limits exemplified in the present invention can be combined in any combination.

[0015] The tribo-electrically charged nonwoven fabric according to the present invention contains a mixture of first fibers and second fibers that are made of different constituent resins and that can be charged by rubbing against each other. Here, "a mixture of first fibers and second fibers that are made of different constituent resins and that can be charged by rubbing against each other" means that The tribo-electrically charged nonwoven fabric contains a mixture of two or more types of fibers, and in these two or more types of fibers, the constituent resin on the surface (excluding both ends) of the first fibers of the first type is different from the constituent resin on the surface (excluding both ends) of the second fibers of the second type, and A combination of first fibers and second fibers in which the surfaces of the first fibers and the second fibers rub against each other, causing the surfaces of the fibers to become electrically charged; The first fiber and the second fiber are in contact with each other; means.

[0016] Furthermore, in the tribo-electrically charged nonwoven fabric, the first fibers and the second fibers are charged by their surface portions rubbing against each other. The types of tribo-electrically charged fibers can be selected as appropriate, as long as they are a combination of fibers that become charged when rubbed against each other. Examples of combinations of the first fiber and the second fiber include a combination of a polyolefin fiber and an acrylic fiber; a combination of a fluorine-based fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of a urethane fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of a vinyl chloride fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of a polyolefin fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of an acrylic fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of a vinylon fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of a polyester fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; a combination of an acetate fiber and a polyamide fiber, wool, glass fiber, silk, or rayon fiber; and a combination of a polyolefin fiber and a polyester fiber. Of these, a combination of polyolefin fibers and acrylic fibers is preferable because it allows the tribo-electrically charged fibers to be rubbed against each other, thereby enabling a large amount of charging to be achieved, and this allows for the production of a tribo-electrically charged nonwoven fabric that can be used to create air filters and masks with excellent collection efficiency.

[0017] Examples of the constituent resins of polyolefin fibers include polypropylene resin, polyethylene resin, polystyrene resin, vinyl acetate copolymer resins of these resins, ethylene-propylene copolymers, and resins in which a portion of these resins has been substituted with a nitrile group, a cyano group, or a halogen. Polyolefin fibers can be made of one or more of these constituent resins. For example, they may be core-sheath composite fibers in which the sheath component is made of a polyolefin resin.

[0018] Furthermore, the constituent resin of the polyolefin fiber (particularly the resin that constitutes the surface of the polyolefin fiber) preferably contains a phosphorus-based additive or a sulfur-based additive. The inclusion of a phosphorus-based additive or a sulfur-based additive is preferable because it allows for the realization of a tribo-electrically charged nonwoven fabric that is highly electrostatically charged. Note that, because a large total amount of these additives can deteriorate spinnability, the total amount of additives is preferably 5% by mass or less of the polyolefin fiber, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0019] Examples of phosphorus-based additives include trisnonylphenyl phosphite, tris(2,4-di-t-butylphenyl) phosphite, distearyl pentaerythritol diphosphite, bis(2,4-di-t-butylphenyl)pentaerythritol phosphite, bis(2,6,di-t-butyl-4-methylphenyl)pentaerythritol phosphite, and 2,2-methylenebis(4,6-di-t-butylphenyl)octyl phosphite. Examples of phosphorus-based antioxidants include tetrakis(2,4-di-t-butylphenyl)-4,4'-biphenylene-diphosphonite, bis(2,4-bis(1,1-dimethylethyl)-6-methylphenyl)ethyl ester phosphorous acid, tetrakis(2,4-di-t-butylphenyl)(1,1-biphenyl)-4,4'-diyl bisphosphonite, and bis(bis(2,4-di-t-butyl-5-methylphenoxy)phosphino). The phosphorus-based additive is contained in the polyolefin fiber in an amount of preferably 0.01% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and even more preferably 0.6% by mass or more.

[0020] Suitable sulfur-based additives include sulfur-based antioxidants such as dilauryl-3,3'-thiodipropionate, dimyristyl-3,3'-thiodipropionate, distearyl-3,3'-thiodipropionate, and pentaerythritol tetrakis. The sulfur-based additive is preferably contained in an amount of 0.01% by mass or more, and more preferably 0.1% by mass or more, in the polyolefin fiber.

[0021] The acrylic fibers can be either polyacrylonitrile fibers containing acrylonitrile as the main component (85% or more) or modacrylic fibers containing 35% to less than 85% acrylonitrile. There are two types of acrylic fibers: those spun using organic solvents and those spun using inorganic solvents, and either type of acrylic fiber can be used. The use of acrylic fibers spun using organic solvents is particularly preferred, as it allows for the realization of a tribo-electrically charged nonwoven fabric that is highly electrostatically charged. Many acrylic fibers spun using organic solvents have constricted portions on the fiber surface, and the fiber cross section is not circular but irregular. Therefore, it is believed that having such an irregular cross section has an advantageous effect on the charge state after tribo-electric charging.

[0022] Tribo-electrically charged fibers can be obtained by known methods, such as melt spinning, dry spinning, wet spinning, direct spinning (melt-blowing, spunbonding, electrostatic spinning, etc.), methods of extracting fibers with a small fiber diameter by removing one or more resin components from composite fibers, and methods of beating fibers to obtain split fibers.

[0023] The fiber length of the tribo-electrically charged fibers is not particularly limited as long as the objectives of the present invention can be achieved. They may be fibers cut to a specific length, such as short fibers, or fibers (continuous fibers) that are not cut to a specific length and have a continuous length, such as those produced by direct spinning. However, using tribo-electrically charged fibers cut to a specific length makes it easier to realize a tribo-electrically charged nonwoven fabric that is highly charged due to the efficient friction between the tribo-electrically charged fibers. Furthermore, it makes it easier to realize a tribo-electrically charged nonwoven fabric in which the tribo-electrically charged fibers are uniformly distributed. This is preferable because it allows for the realization of a tribo-electrically charged nonwoven fabric that has uniform physical properties such as strength and pore size, and excellent mechanical collection capabilities. Specifically, the fiber length of the tribo-electrically charged fibers is preferably 3 to 150 mm, more preferably 10 to 100 mm, and even more preferably 30 to 80 mm. Note that "fiber length" refers to the average fiber length measured by the method specified in JIS L1015 (Testing Method for Staple Fibers of Chemical Fibers): 2021, Section 8.4.1, Method B (Corrected Staple Diagram Method).

[0024] The fineness (unit: dtex) of the tribo-electrically charged fiber is adjusted as appropriate to achieve a tribo-electrically charged nonwoven fabric with excellent filtration performance. The fineness of the tribo-electrically charged fiber is preferably 0.1 to 10 dtex, preferably 0.3 to 7 dtex, preferably 0.6 to 5 dtex, and most preferably 0.8 to 3 dtex. Note that "fineness" refers to the value measured by the method specified in JIS L1015 (Testing Methods for Staple Chemical Fibers):2021, Section 8.5.1, Method A (normal method).

[0025] The mass ratio of the first fibers to the second fibers that make up the tribo-electrically charged nonwoven fabric can be adjusted as appropriate. For example, the mass ratio of the first fibers to the second fibers can be 5% by mass:95% to 95% by mass:5% by mass, 15% by mass:85% to 85% by mass:15% by mass, 25% by mass:75% to 75% by mass:25% by mass, or 40% by mass:60% to 60% by mass:40% by mass.

[0026] The tribo-electrically charged nonwoven fabric may also include fibers other than the first fibers and second fibers. One example of a tribo-electrically charged nonwoven fabric that includes fibers other than the first fibers and second fibers is a tribo-electrically charged nonwoven fabric prepared by entangling the first fibers, second fibers, and other fibers. Alternatively, the tribo-electrically charged nonwoven fabric may be prepared by layering and entangling a web composed of the first fibers and second fibers with another fabric (such as a spunbond nonwoven fabric). However, so as to achieve a tribo-electrically charged nonwoven fabric that has excellent filtration performance, it is preferable that the constituent fibers of the tribo-electrically charged nonwoven fabric be only the first fibers and second fibers.

[0027] The fibers that make up the tribo-electrically charged nonwoven fabric may be held together by a binder or fiber adhesive. However, in order to achieve a tribo-electrically charged nonwoven fabric that is rich in charge due to the tribo-electrically charged fibers rubbing against each other efficiently, and to achieve a tribo-electrically charged nonwoven fabric in which the tribo-electrically charged fibers rub against each other even when gas passes through the tribo-electrically charged nonwoven fabric, it is preferable that the fibers that make up the tribo-electrically charged nonwoven fabric are not held together by a binder or fiber adhesive, but are simply entangled with each other. In addition, a tribo-electrically charged nonwoven fabric in which the fibers are simply entangled with each other is preferable because it makes it possible to achieve a tribo-electrically charged nonwoven fabric that is less susceptible to contamination and less susceptible to deterioration in texture.

[0028] The constituent fibers of the tribo-electrically charged nonwoven fabric may contain an oil on their surfaces. The type of oil can be selected as appropriate, and hydrophilic or non-hydrophilic oils can be used. For example, hydrophilic oils include lubricants such as mineral oil or synthetic oil containing a wetting agent such as an anionic surfactant, cationic surfactant, or nonionic surfactant. Furthermore, for example, non-hydrophilic oils include lubricants such as mineral oil or synthetic oil containing a fluorine-based or silicone-based component.

[0029] The mass percentage of the oil contained in the constituent fibers of the tribo-electrically charged nonwoven fabric can be adjusted as appropriate, but the mass percentage of the oil relative to the fiber mass is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, even more preferably 0.08% by mass or more, even more preferably 0.10% by mass or more, and particularly preferably 0.11% by mass or more. The upper limit can be adjusted as appropriate, but a realistic value is 1% by mass or less, so as to achieve a tribo-electrically charged nonwoven fabric that is highly efficiently charged by the tribo-electrically charged fibers rubbing against each other.

[0030] The basis weight of the tribo-electrically charged nonwoven fabric can be adjusted appropriately according to the application and desired physical properties, and is not particularly limited, but is preferably 15 to 600 g / m 2 It is preferable that the thickness is 25 to 400 g / m 2 More preferably, it is 50 to 300 g / m 2 It is more preferable that the "weight" is the weight per square meter of the main surface, which is the widest surface. 2 This refers to the "mass per unit area" measured by the method specified in JIS L1085 (Testing method for nonwoven fabric interlining):1998, Section 6.2.

[0031] The thickness of the tribo-electrically charged nonwoven fabric can be adjusted appropriately in accordance with the application and the desired physical properties, and is not particularly limited, but can be 0.5 to 6 mm, 1 to 5 mm, or 1.5 to 4 mm. Note that "thickness" in the present invention refers to the thickness of an area of ​​5 cm2 relative to the main surface of the tribo-electrically charged nonwoven fabric. 2 The thickness in the load area is measured at five randomly selected locations by applying a load of 0.98 N (=100 gf) in the thickness direction per unit area, and the arithmetic mean value of the thicknesses is calculated. Such thickness measurements can be performed using, for example, a high-precision digital length measuring machine (Litematic (registered trademark), manufactured by Mitutoyo Corporation).

[0032] A tribo-electrically charged nonwoven fabric according to the present invention is characterized in that the value y calculated using the following formula is less than 0.57: Note y=b / a a: Tensile strength (unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric in the direction (MD direction) parallel to the main surfaces of the tribo-electrically charged nonwoven fabric in which the tensile strength is greatest. b: Tensile strength (unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric in a direction (CMD direction) that is parallel to the main surface of the tribo-electrically charged nonwoven fabric and is perpendicular to the direction in which the tensile strength is greatest.

[0033] As a result of further investigations, the applicant of the present application found that tribo-electrically charged nonwoven fabrics in which the calculated value y is less than 0.57 have excellent formability. Note that, as will be explained in detail below, there is no direct correlation between the tensile strength in the CMD of a tribo-electrically charged nonwoven fabric and the formability of the tribo-electrically charged nonwoven fabric.

[0034] The values ​​of a and b used to calculate the above-mentioned calculated value y can be obtained by the following method. (How to find the values ​​of a and b) (Step 1) Eighteen rectangular samples (length: 150 mm, width: 50 mm) are taken from the tribo-electrically charged nonwoven fabric so that their length directions are in directions that differ by 10° from each other relative to the direction parallel to the main surface of the tribo-electrically charged nonwoven fabric. (Step 2) Each sample is subjected to a tensile test in accordance with JIS P8113:2006 "Paper and paperboard - Test methods for tensile properties - Part 2: Constant rate of extension" using a tensile tester (Orientec Co., Ltd., UCT-500) to determine the maximum tensile load (tensile strength, unit: N / 50 mm) measured in the longitudinal direction of each sample before breakage. Measurements are performed under conditions of a gripping distance (length between the chucks holding the sample) of 100 mm and a pulling speed of 50 mm / min. (Step 3) As a result of carrying out (Step 2) above, the sample with the strongest maximum tensile load (tensile strength, units: N / 50 mm) measured before breaking is selected. Then, ten new rectangular samples (length: 150 mm, width: 50 mm) are taken from the tribo-electrically charged nonwoven fabric, with the length direction of the selected sample parallel to the length direction of the samples to be taken. (Step 4) In the same manner as in (Step 2) above, the maximum tensile load (tensile strength, unit: N / 50 mm) in the longitudinal direction of each of the newly collected samples is measured. (Step 5) The average value of the maximum tensile load in the length direction (tensile strength, unit: N / 50 mm) for a total of 10 samples measured as above is taken as the tensile strength (a, unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric in the direction (MD direction) parallel to the main surfaces of the tribo-electrically charged nonwoven fabric that has the greatest tensile strength. (Step 6) Ten new rectangular samples (length: 150 mm, width: 50 mm) are obtained from the tribo-electrically charged nonwoven fabric, with the length direction of the samples to be obtained parallel to the direction (CMD direction) that is perpendicular to the direction (MD direction) that is parallel to the main surfaces of the tribo-electrically charged nonwoven fabric and the direction in which the tensile strength is greatest. (Step 7) In the same manner as in (Step 2) above, the maximum tensile load (tensile strength, unit: N / 50 mm) in the longitudinal direction of each of the newly collected samples is measured. (Step 8) The average value of the maximum tensile load in the length direction (tensile strength, unit: N / 50 mm) for a total of 10 samples measured as above is taken as the tensile strength (b, unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric in the direction (CMD direction) perpendicular to the direction in which the tensile strength is greatest (MD direction), of all directions parallel to the main surfaces of the tribo-electrically charged nonwoven fabric.

[0035] In order to achieve a tribo-electrically charged nonwoven fabric that has excellent formability, a is preferably no more than 250 N / 50 mm, more preferably no more than 200 N / 50 mm, and even more preferably no more than 100 N / 50 mm. The lower limit can also be adjusted as appropriate, but it is most preferably no less than 5 N / 50 mm.

[0036] Furthermore, in order to achieve a tribo-electrically charged nonwoven fabric that exhibits excellent formability, b is preferably no more than 50 N / 50 mm, more preferably no more than 40 N / 50 mm, and even more preferably no more than 30 N / 50 mm. The lower limit can also be adjusted as appropriate, but it is most preferably no less than 5 N / 50 mm.

[0037] The calculated value y is a value that represents the degree to which the tensile strength of the tribo-electrically charged nonwoven fabric in the CMD direction, relative to the tensile strength of the tribo-electrically charged nonwoven fabric in the MD direction. Low tensile strength in a certain direction means that the tribo-electrically charged nonwoven fabric is easily stretched and deformed in that direction, and so the calculated value y can be said to be a value that represents the degree to which the tribo-electrically charged nonwoven fabric is easily deformed in the CMD direction, relative to the ease with which the tribo-electrically charged nonwoven fabric is easily deformed in the MD direction.

[0038] For this reason, the smaller the calculated value y of a tribo-electrically charged nonwoven fabric, the easier it is to deform in the CMD direction relative to the ease with which the tribo-electrically charged nonwoven fabric deforms in the MD direction. As a result of studies, the applicant of the present application has found that tribo-electrically charged nonwoven fabrics in which the calculated value y is less than 0.57 have excellent moldability.

[0039] To obtain a tribo-electrically charged nonwoven fabric with even better formability, a tribo-electrically charged nonwoven fabric in which the calculated value y is 0.5 or less is preferred, a tribo-electrically charged nonwoven fabric in which it is 0.4 or less is more preferred, and a tribo-electrically charged nonwoven fabric in which it is 0.3 or less is most preferred. Meanwhile, the lower limit can also be adjusted as appropriate, but is preferably 0.1 or greater so that it can be formed into a fabric with good handleability.

[0040] In order to achieve a tribo-electrically charged nonwoven fabric that has excellent moldability, the elongation in the MD direction (unit: %) is preferably at least 30%, more preferably at least 40%, and most preferably at least 50%. The upper limit can also be adjusted as appropriate, but is preferably no more than 80%.

[0041] The elongation in the MD direction of the tribo-electrically charged nonwoven fabric can be calculated by subjecting 10 rectangular samples (length: 200 mm, width: 50 mm) taken from the tribo-electrically charged nonwoven fabric so that their length directions are parallel to the MD direction to the method described in (Step 2) of (How to determine the values ​​of a and b) above, and using the length (units: mm) between the chucks when each sample breaks. In other words, the elongation (%) of the sample is the result of calculating 100 × (D-100) / 100 (where D means the length (units: mm) between the chucks when the tribo-electrically charged nonwoven fabric breaks). The average value of the elongations (%) calculated from a total of 10 samples measured in this way is then regarded as the elongation (units: %) in the MD direction of the tribo-electrically charged nonwoven fabric.

[0042] Furthermore, in order to achieve a tribo-electrically charged nonwoven fabric that has excellent moldability, the elongation in the CMD direction (unit: %) is higher than the elongation in the MD direction, and is preferably 85% or higher, more preferably 90% or higher, and most preferably 100% or higher. The upper limit can also be adjusted as appropriate, but is preferably 150% or lower.

[0043] The elongation in the CMD direction of the tribo-electrically charged nonwoven fabric can be calculated by subjecting 10 rectangular samples (length: 200 mm, width: 50 mm) taken from the tribo-electrically charged nonwoven fabric so that the MD direction and the length direction are perpendicular to each other to the method described in (Step 2) of (How to determine the values ​​of a and b) above, and using the length (units: mm) between the chucks when each sample breaks. That is, the elongation (%) of the sample is the calculation result of 100 × (D-100) / 100 (where D means the length (units: mm) between the chucks when the tribo-electrically charged nonwoven fabric breaks). The average value of the elongations (%) calculated from a total of 10 samples measured in this way is then taken as the elongation (units: %) of the tribo-electrically charged nonwoven fabric in the CMD direction.

[0044] Furthermore, in order to achieve a tribo-electrically charged nonwoven fabric that has excellent formability, the stress at 10% elongation in the MD direction (unit: N / 50 mm) is preferably 70 N / 50 mm or less, more preferably 50 N / 50 mm or less, even more preferably 30 N / 50 mm or less, and most preferably 10 N / 50 mm or less. The lower limit can also be adjusted as appropriate, but is preferably 5 N / 50 mm or more.

[0045] Note that the stress (unit: N / 50 mm) at 10% elongation in the MD direction of the tribo-electrically charged nonwoven fabric is determined by subjecting 10 rectangular samples (length: 200 mm, width: 50 mm) taken from the tribo-electrically charged nonwoven fabric so that their length directions are parallel to the MD direction to the method described in (step 2) of (Method for determining the values ​​of a and b) above, and measuring the stress (unit: N / 50 mm) for each sample when the length between the chucks (unit: mm) is 110 mm. The average value of the stresses (unit: N / 50 mm) determined for a total of 10 samples measured in this way is then regarded as the stress (unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric at 10% elongation in the MD direction.

[0046] Furthermore, in order to achieve a tribo-electrically charged nonwoven fabric that has excellent formability, the stress at 10% elongation in the CMD direction (unit: N / 50 mm) is lower than the stress at 10% elongation in the MD direction (unit: N / 50 mm), and is preferably 5 N / 50 mm or less, more preferably 3 N / 50 mm or less, and most preferably 1 N / 50 mm or less. The lower limit can also be adjusted as appropriate, but is preferably 0.1 N / 50 mm or more.

[0047] Note that the stress (unit: N / 50 mm) at 10% elongation in the CMD direction of the tribo-electrically charged nonwoven fabric is determined by subjecting 10 rectangular samples (length: 200 mm, width: 50 mm) taken from the tribo-electrically charged nonwoven fabric so that the MD direction and length direction are perpendicular to each other to the method described in (step 2) of (How to determine the values ​​of a and b) above, and measuring the stress (unit: N / 50 mm) for each sample when the length between the chucks (unit: mm) is 110 mm. The average value of the stresses (unit: N / 50 mm) determined for a total of 10 samples measured in this way is then regarded as the stress (unit: N / 50 mm) of the tribo-electrically charged nonwoven fabric at 10% elongation in the CMD direction.

[0048] Next, an example of a method for producing a tribo-electrically charged nonwoven fabric according to the present invention will be described. Note that a description of the configuration explained above will be omitted. The method for producing a tribo-electrically charged nonwoven fabric according to the present invention includes: (Step 1) preparing a web including a mixture of first fibers and second fibers; (Step 2) needle-punching the web using needles having at least one of the characteristics described below, thereby entangling and rubbing the first fibers and the second fibers together; It can have:

[0049] First, (Step 1) will be explained.

[0050] The method for preparing a web containing a mixture of the first and second fibers can be selected as appropriate. Examples of methods that can be used include a method in which the first and second fibers are mixed in a desired mass ratio and then fed to a carding device to prepare a web; a method in which the first and second fibers, prepared to have a desired mass ratio, are fed to an air-laying device and deposited to prepare a web; and a method in which the first and second fibers are spun into a desired mass ratio using direct spinning such as a melt-blown nonwoven fabric, a spunbond nonwoven fabric, or an electrospun nonwoven fabric, and the first and second fibers are collected in the same collector to prepare a web.

[0051] Alternatively, a method may be employed in which a first fiber is spun by direct spinning, and second fibers cut to a specific length are supplied to the spun first fiber in flight, mixed with the spun first fiber, and collected in the same collector to form a web; or a method may be employed in which a first fiber is spun by direct spinning, the spun first fiber is collected in a collector, and second fibers cut to a specific length are supplied to the collector to mix with the first fiber, to form a web.

[0052] The web preferably has a structure in which a fiber layer in which the fibers are oriented in one direction is laminated with a fiber layer in which the fiber orientation is different from that one direction. A specific example is a criss-cross web in which a parallel web containing tribo-electrically charged fibers is laminated with a cross-lay web in which the fiber orientations are different.

[0053] A cross-lay web is a fiber web formed by folding a unidirectional fiber web (parallel web) so that the fiber orientation is not parallel to the production direction.

[0054] A tribo-electrically charged nonwoven fabric made from a web having such a layer structure is preferable because it allows the tribo-electrically charged fibers to rub against each other more efficiently, thereby enabling greater charging, and allows for the production of a tribo-electrically charged nonwoven fabric that can be used to create air filters and masks with excellent collection efficiency.

[0055] Various values ​​such as the basis weight and thickness of the web are adjusted as appropriate so that a tribo-electrically charged nonwoven fabric according to the present invention can be prepared.

[0056] Furthermore, the constituent fibers of the web may include adhesive fibers, but so that the tribo-electrically charged fibers can rub against each other efficiently, resulting in a tribo-electrically charged nonwoven fabric that is highly charged, it is preferable that the constituent fibers of the web are not bonded together by adhesive fibers, and a web whose constituent fibers are only first fibers and second fibers is preferred.

[0057] Next, (Step 2) will be explained.

[0058] Through investigations, the applicant of the present application has found that it is possible to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y is less than 0.57 by needle-punching a web using needles that have at least one of the characteristics listed below.

[0059] The characteristics of a needle that can realize a tribo-electrically charged nonwoven fabric according to the present invention will be explained using Figures 1(a) and (b), which are schematic cross-sectional views of the tip of a needle that can realize a tribo-electrically charged nonwoven fabric according to the present invention, formed by cutting the tip of the needle in a direction parallel to the length of the needle and across the barb.

[0060] A needle (10) that can produce a tribo-electrically charged nonwoven fabric according to the present invention has a slit (1) formed in the tip portion of the needle, located on the left side of the page in Figure 1(a), facing away from the tip of the needle. The presence of this slit (1) forms a barb (2) in the needle (10) that points towards the tip of the needle, and also forms a depression (3) in the needle (10). Note that, as an example, Figure 1(a) shows only one barb (2) present on the needle (10).

[0061] Figure 1(b) is a schematic cross-sectional view showing the length from the surface of the needle to the bottom of the depression (depth of the depression) in Figure 1(a). Note that the reference numerals given in Figure 1(a) are omitted. In Figure 1(b), the depth of the depression is represented by a double-headed arrow D. Note that the depth of the depression refers to the length of a line segment that satisfies the following features 1 to 3. (Configuration 1) In the above-mentioned cross section, the line segment is parallel to a direction perpendicular to the longitudinal direction of the needle, (Configuration 2) A line segment connecting the surface of the needle in the cross section when it is assumed that the notch portion is not formed (the surface is supplemented by a dashed line in FIG. 1(b)) and the surface of the needle present in the depression, (Configuration 3) The longest line segment among the line segments that satisfy the above-mentioned configurations 1 and 2.

[0062] (Feature 1) Whereas the depth of the depression (3) formed in the needle (10) is 0.08 mm or less in commonly used needles, the depth (a) of the depression in the needle (10) that can produce a tribo-electrically charged nonwoven fabric according to the present invention is greater.

[0063] (Feature 2) With regard to the number of barbs (2) that the needle has, whereas commonly used needles have four or fewer barbs within a range of 1 cm from the tip of the needle, the needles (10) that can produce tribo-electrically charged nonwoven fabrics in accordance with the present invention have a greater number of barbs (2).

[0064] In particular, using a needle that has multiple of the above-mentioned characteristics makes it easier to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y is less than 0.57, and therefore it is preferable to use a needle that has two or more of the above-mentioned characteristics.

[0065] It is not clear why using needles with the characteristics described above makes it possible to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y is less than 0.57. However, as is clear from the examples described below, needle-punching using needles with these characteristics makes it possible to achieve a tribo-electrically charged nonwoven fabric in which the tensile strength in the CMD direction is significantly weaker than in the MD direction. For this reason, it is thought that needle-punching using needles with these characteristics changes the fiber orientation of the web to something unique compared to when needle-punching is performed using ordinary needles.

[0066] In step 2, the needle punching may be performed on only one main surface of the web, or on both main surfaces of the web, and the number of times the needle punching may be performed may be one or more.

[0067] Furthermore, after undergoing the above-mentioned (step 2), tension may be applied to the needle-punched web in its production direction (MD). By undergoing this step (sometimes referred to as the stretching step hereinafter), the orientation of the constituent fibers can be made to follow the production direction of the needle-punched web, and as a result, it is easier to prepare a tribo-electrically charged nonwoven fabric that has a lower calculated value y and better moldability.

[0068] Next, a filter medium comprising a tribo-electrically charged nonwoven fabric will be described.

[0069] The tribo-electrically charged nonwoven fabric produced as described above can be used alone as a filter medium, but the filter medium can also be constructed by laminating the tribo-electrically charged nonwoven fabric with a cover material, support, and / or a pre-filter or backup filter. Known materials can be used for the cover material, support, and / or pre-filter or backup filter, and examples of such materials include nonwoven fabrics, woven fabrics, or knitted fabrics, as well as porous films and breathable foams. Note that the filter medium can be one formed by simply overlaying the exemplified material with the tribo-electrically charged nonwoven fabric, or it can be a laminated filter medium in which the layers are bonded together using a binder or hot-melt material (powder, sheet (e.g., hot-melt web, hot-melt film, etc.)) or fiber bonding, or a laminated filter medium in which the layers are bonded together by subjecting the material to an adhesive treatment such as heat sealing or ultrasonic welding.

[0070] Filter media and laminated filter media made of tribo-electrically charged nonwoven fabric can be used in a two-dimensional sheet shape. Furthermore, they can have cutouts, punched-out sections, or slits depending on the purpose. It is preferable to use filter media and laminated filter media made of tribo-electrically charged nonwoven fabric as molded filter media by deforming them into a cup shape with a raised center, or by folding them into a pleated shape. The outer shape of the molded filter media can be adjusted as appropriate and is not particularly limited, but can be, for example, a cup shape, a three-dimensional corrugated shape, a pleated shape, a cylindrical shape, or the like. Furthermore, they can have cutouts, punched-out sections, or slits depending on the purpose.

[0071] The filter material of the present invention can be used to provide bifold masks and flat masks. Alternatively, the molded and processed filter material can be used to provide, for example, cup-shaped masks with a raised cup shape in the center and pleated masks. [Example]

[0072] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0073] (Preparing the fiber) As shown below, two types of polypropylene fibers A to B were prepared as the first fibers, and three types of acrylic fibers a to c were prepared as the second fibers. Polypropylene fiber A: Fiber cross section: circular, fineness: 1.7 dtex, fiber length: 51 mm Polypropylene fiber B: Fiber cross section: circular, fineness: 2.2 dtex, fiber length: 51 mm Acrylic fiber a: Fiber cross section: circular, fineness: 1.7 dtex, fiber length: 51 mm Acrylic fiber b... Fiber cross section: circular, fineness: 1.0 dtex, fiber length: 38 mm Acrylic fiber c... Fiber cross-sectional shape: irregular cross-section, fineness: 2.2 dtex, fiber length: 70 mm

[0074] (Comparative Example 1, Example 1) Polypropylene fiber A was used as the first fiber, and acrylic fiber a was used as the second fiber. The first fiber and the second fiber were mixed in a mass ratio of 60% by mass:40% by mass and opened by a carding machine to prepare a parallel web. Parallel webs prepared in the same manner were laminated so that the fiber orientations were different to prepare a cross-lay web. The parallel web and the cross-lay web were then laminated to prepare a criss-cross web. Next, the prepared criss-cross web was subjected to a hydroentanglement treatment on each of its two main surfaces to wash the first fibers and the second fibers with a water stream and entangle the first fibers and the second fibers with each other. The web was then dried in a dry heat dryer to prepare a hydroentangled web. The hydroentangled web was then subjected to a needle punching process from the main surface side of the crosslay web at a needle density of 50 / cm2. As a result, the first fibers and second fibers were rubbed strongly against each other and the first fibers and second fibers that make up the hydroentangled web were entangled, producing a tribo-electrically charged nonwoven fabric. The prepared tribo-electrically charged nonwoven fabric had a band-like shape with its longer sides in the production direction, and this production direction was the direction (MD) in which the strength was greatest of all directions parallel to the main surfaces of the tribo-electrically charged nonwoven fabric. In Example 1 and Comparative Example 1, needles having the following characteristics were used in the needle punching process. Comparative Example 1: (Feature 1) The depth of the recess formed in the needle: 0.08 mm (Feature 2) There are four barbs within 1 cm of the needle tip. Example 1: (Feature 1) The depth of the recess formed in the needle: 0.09 mm (Feature 2) There are eight barbs within 1 cm of the needle tip. Example 2 The tribo-electrically charged nonwoven fabric prepared in Example 1 was wound around a roll so that the winding direction was parallel to the direction in which the fabric was strongest (MD direction) of all directions parallel to its main surfaces, to form a roll. Tension was then applied to the tribo-electrically charged nonwoven fabric in the MD direction, and the tribo-electrically charged nonwoven fabric was unwound from the roll. In this way, a tribo-electrically charged nonwoven fabric was obtained in which tension was applied in the MD direction. Furthermore, even after the adjusted tension had been applied to the tribo-electrically charged nonwoven fabric, the direction in which the tension was applied remained the direction in which the strength was greatest (MD direction).

[0075] Example 3 A tribo-electrically charged nonwoven fabric was prepared in the same manner as in Example 1, except that the first fibers and the second fibers were blended in a mass ratio of 50% by mass:50% by mass.

[0076] The construction and physical properties of each of the Examples and Comparative Examples prepared as described above are summarized in Table 1. In the table, "HE" stands for hydroentanglement treatment, and "NP" stands for needle punch treatment. The "Moldability evaluation" column in the table also lists the results of subjecting the tribo-electrically charged nonwoven fabric to the following evaluation methods.

[0077] (Formability evaluation) A mask substrate was prepared that was made of a smooth plate and a nonwoven fabric with a cup shape that raised in the middle. When a mask including the mask substrate was worn on a person's face, the direction connecting the two ends closest to the nose and mouth was the up-down direction, and the direction perpendicular to this and connecting the two ends closest to the left and right ears was the left-right direction. Then, the mask substrate was placed on a flat and smooth plate so that the side of the mask substrate where the protrusion was located was exposed. Next, the tribo-electrically charged nonwoven fabric was pulled by applying tension (tensile strength: 10 N) in the direction parallel to its main surfaces in which it had the greatest strength (MD direction). The tribo-electrically charged nonwoven fabric in this pulled state was then supplied onto a mask substrate, and thereafter pressed against the mask substrate from above at a temperature of 25°C under normal pressure, thereby deforming the tribo-electrically charged nonwoven fabric into the cup shape of the mask substrate. At this time, the MD direction of the tribo-electrically charged nonwoven fabric was made parallel to the left-right direction of the mask substrate. In this way, the entire mask substrate was covered with tribo-electrically charged nonwoven fabric. At this time, no tension was applied to pull the tribo-electrically charged nonwoven fabric in the direction (CD direction) that is parallel to its main surfaces and perpendicular to the direction in which it is strong. Ultrasonic waves were applied to the periphery of the mask substrate that faced the smooth plate, and the periphery of the smooth plate side of the mask substrate was fused and integrated with the tribo-electrically charged nonwoven fabric. In this way, a cup-shaped mask was prepared in which the tribo-electrically charged nonwoven fabric was layered on the mask substrate, and the periphery was fused and integrated. The main surface derived from the tribo-electrically charged nonwoven fabric that was exposed in the prepared cup-shaped mask was then visually inspected. If, as a result of this check, wrinkles appearing on the main surface as if pulled in a direction parallel to the MD direction were found, it was determined that the tribo-electrically charged nonwoven fabric used was difficult to deform and would not be able to conform sufficiently to the cup-shaped mask substrate. Tribo-electrically charged nonwoven fabrics for which this determination was made were marked with an "X" in the "Moldability evaluation" column in the table. On the other hand, if the check showed that no wrinkles had occurred on the main surface, it was determined that the tribo-electrically charged nonwoven fabric used was easy to deform and could adequately conform to the cup-shaped mask substrate. Tribo-electrically charged nonwoven fabrics for which this determination was made were marked with a "○" in the "Moldability evaluation" column in the table.

[0078] [Table 1]

[0079] The results of comparing Comparative Example 1 with Examples 1 and 2 show that the tribo-electrically charged nonwoven fabric prepared in the Example had a smaller calculated value of y than the tribo-electrically charged nonwoven fabric prepared in Comparative Example 1, which involves the prior art (calculated value of y was 0.57). The fabric also had excellent moldability. Similarly, the tribo-electrically charged nonwoven fabric prepared in Example 3 had a smaller calculated value of y than the tribo-electrically charged nonwoven fabric prepared in Comparative Example 1, which involves the prior art (calculated value of y was 0.57). The fabric also had excellent moldability.

[0080] (Comparative Example 2) A tribo-electrically charged nonwoven was prepared in the same manner as in Comparative Example 1, except that instead of being subjected to needle punching, a hydroentanglement treatment was again carried out under the same conditions.

[0081] The composition and physical properties of Comparative Example 2 prepared as described above are summarized in Table 2. Components that were not present are marked with "-" in the table. For ease of understanding, the results of Comparative Example 1 are also shown in the table.

[0082] [Table 2]

[0083] The results of comparing Comparative Example 1 and Comparative Example 2 show that even when only hydroentanglement treatment was used as the fiber entanglement treatment, it was not possible to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y was less than 0.57. Furthermore, the tribo-electrically charged nonwoven fabrics prepared in this manner had poor moldability.

[0084] (Comparative Example 3) Polypropylene fiber B was used as the first fiber, and acrylic fiber B was used as the second fiber. The first fiber and second fiber were mixed in a mass ratio of 70% by mass:30% by mass, and the mixture was opened in a carding machine to prepare a web. A tribo-electrically charged nonwoven fabric was prepared in the same manner as in Comparative Example 1, except that the web prepared in this manner was used.

[0085] The composition and physical properties of Comparative Example 3 prepared as described above are summarized in Table 3. For ease of understanding, the results of Comparative Example 1 are also shown in the table.

[0086] [Table 3]

[0087] The results of comparing Comparative Example 1 and Comparative Example 3 show that even if the fiber composition was changed and the tensile strength and elongation were improved, it was not possible to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y was less than 0.57. Furthermore, the results of comparing Comparative Example 3 and Comparative Example 1 show that, conversely, even if the fiber composition was changed and the tensile strength and elongation were reduced, it was not possible to achieve a tribo-electrically charged nonwoven fabric in which the calculated value y was less than 0.57. Furthermore, the tribo-electrically charged nonwoven fabric prepared in this manner had poor formability.

[0088] Example 4 Polypropylene fiber A was used as the first fiber, and acrylic fiber c was used as the second fiber. The first fiber and the second fiber employed were mixed in a mass ratio of 50% by mass:50% by mass, and the mixture was fed to a carding machine to open the fibers and prepare a web. A tribo-electrically charged nonwoven fabric was prepared in the same manner as in Example 1, except that the web prepared in this manner was used.

[0089] Example 5 A tribo-electrically charged nonwoven fabric was prepared in the same manner as in Example 4, except that the basis weight of the web was made lighter.

[0090] Example 6 Polypropylene fiber A was used as the first fiber, and acrylic fiber a was used as the second fiber. The first fiber and the second fiber employed were mixed in a mass ratio of 60% by mass:40% by mass, and the mixture was opened by a carding machine to prepare a web. A tribo-electrically charged nonwoven fabric was prepared in the same manner as in Example 1, except that the web prepared in this manner was used.

[0091] Example 7 Tension was applied to the tribo-electrically charged nonwoven fabric prepared in Example 6 in the direction (MD) in which the strength was greatest of the directions parallel to the main surfaces of the fabric. In this way, a tribo-electrically charged nonwoven fabric to which tension had been applied was prepared. Furthermore, even in the prepared tribo-electrically charged nonwoven fabric to which tension had been applied, the direction in which the tension was applied remained the direction in which the strength was greatest (MD direction).

[0092] Table 4 shows the compositions and properties of Examples 4 to 7 prepared as described above.

[0093] [Table 4]

[0094] The tribo-electrically charged nonwoven fabrics prepared in Examples 4 to 7 had calculated values ​​of y that were smaller than 0.57, and were also excellent in moldability.

[0095] The Comparative Examples are attempts to produce tribo-electrically charged nonwoven fabrics in which the calculated value of y is less than 0.57, by, for example, changing the fiber blend, a method that has traditionally been studied in the technical field of nonwoven fabrics. However, insofar as this study was carried out, it was not possible to realize a tribo-electrically charged nonwoven fabric in which the calculated value of y is less than 0.57.

[0096] In contrast, the tribo-electrically charged nonwoven fabrics in Examples 1 to 7 had calculated values ​​of y that were smaller than 0.57.

[0097] The reason for this is thought to be that by using a needle-punching process using needles that have at least one of the characteristics listed above, it was possible to prepare a tribo-electrically charged nonwoven fabric in which the fiber orientation was changed to something unique compared to when needle-punching was performed using ordinary needles.

[0098] Furthermore, low tensile strength in a certain direction means that the fabric is easily stretched and deformed in that direction. For this reason, it was inferred that a tribo-electrically charged nonwoven fabric with low tensile strength in the CMD is easily stretched and deformed in the CMD, and therefore is a tribo-electrically charged nonwoven fabric with excellent formability that makes it easy to form into a desired shape.

[0099] However, the tribo-electrically charged nonwoven fabric of Example 6, which had a CMD tensile strength of 42.6 N / 50 mm, had excellent formability. However, the tribo-electrically charged nonwoven fabrics of Comparative Examples 1 and 2 and 4, which had an even lower CMD tensile strength, had poor formability, and it was found that just because a tribo-electrically charged nonwoven fabric has a low CMD tensile strength does not necessarily mean that it has excellent formability.

[0100] Furthermore, the tribo-electrically charged nonwoven fabric of Comparative Example 6 (tensile strength: 52.4 N / 50 mm), which has a higher tensile strength in the CMD direction than the tribo-electrically charged nonwoven fabric of Example 6, had inferior formability, and it was therefore clear that even if a tribo-electrically charged nonwoven fabric has a high tensile strength in the CMD direction, it does not necessarily have excellent formability.

[0101] Furthermore, because the tribo-electrically charged nonwoven fabrics of Examples 1 to 5, which have lower tensile strength in the CMD direction than the tribo-electrically charged nonwoven fabrics of Comparative Examples 1 and 2 and 4, had excellent formability, it was also found that it is not the case that a tribo-electrically charged nonwoven fabric with excellent formability cannot be obtained unless the tensile strength in the CMD direction is 42.6 N / 50 mm or greater and less than 52.4 N / 50 mm.

[0102] For this reason, it was found that there is no direct correlation between the tensile strength in the CMD of a tribo-electrically charged nonwoven fabric and the formability of the tribo-electrically charged nonwoven fabric. [Industrial Applicability]

[0103] By using a tribo-electrically charged nonwoven fabric according to the present invention, it is possible to prepare air filters for use in food and medical product production factories, precision equipment manufacturing factories, indoor crop cultivation facilities, ordinary households or industrial facilities such as office buildings, electrical appliances such as air purifiers, office equipment, and medical equipment, and various vehicles such as automobiles and aircraft. Furthermore, by using a tribo-electrically charged nonwoven fabric according to the present invention, it is possible to prepare masks. [Explanation of symbols]

[0104] 10: Needle capable of realizing the tribo-electrically charged nonwoven fabric according to the present invention 1: Cutout part 2: Barb 3: Depression formed in the needle D: Length from the needle surface to the bottom of the depression (depth of the depression)

Claims

1. A tribo-electrically charged nonwoven fabric comprising a mixture of first fibers and second fibers that are made of different constituent resins and that can be charged by rubbing against each other, and the first fibers and the second fibers are electrically charged, A tribo-electrically charged nonwoven fabric, in which the value y calculated by the following formula is less than 0.57: Note y = b / a a: tensile strength of the tribo-electrically charged nonwoven fabric in the direction parallel to the main surfaces of the tribo-electrically charged nonwoven fabric, in which the tensile strength is greatest (unit: N / 50 mm). b: tensile strength of the tribo-electrically charged nonwoven fabric in a direction parallel to the main surfaces of the tribo-electrically charged nonwoven fabric, which is perpendicular to the direction in which the tensile strength is greatest (unit: N / 50 mm).

2. A shaped filter medium comprising the tribo-electrically charged nonwoven fabric of claim 1.

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