Fibrillated fiber, manufacturing method of the same, and filtering medium for filter

Abstract

To provide a fibrillated fiber constituting a filtering medium for a filter, and having both high collection efficiency and low pressure loss, and small environmental load at disposal.SOLUTION: A fibrillated fiber to be used for a filtering medium for a filter comprising a wet type nonwoven fabric is such that: a value of a fine fiber ratio B / A represented by an equation 1 is 2.0 or more. (equation 1): a fine fiber ratio B / A= a fine fiber ratio B [%] / a fine fiber ratio A [%], herein, the fine fiber ratio A [%] and the fine fiber ratio B [%] are determined by the following equations 1a, 1b: (equation 1a) the fine fiber ratio A [%]=(a total project area of a fine material having a length of less than 0.2 mm and a width of less than 75 μm / a total project area of all measured objects)×100; and (equation 1b) the fine fiber ratio B [%]=(a total length of a fine material having a length of more than 0.2 mm and a width of less than 10 μm / a total length of all measured objects having a length of more than 0.2 mm)×100.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to fibrillated fibers constituting a filter medium used to remove dust particles contained in a fluid to obtain a clean fluid, a method for producing the same, and a filter medium using the same. [Background technology]

[0002] As a filter medium used to remove dust particles contained in liquid or air, a filter medium made of paper composed of natural fibers, organic synthetic fibers, or inorganic fibers, or a nonwoven fabric composed of natural fibers, organic synthetic fibers, or inorganic fibers, is widely used.

[0003] As fine fibers used in filter media, glass fibers are widely used.

[0004] Fibrillated fibers have been proposed as fine fibers constituting a filter medium other than glass fibers (see, for example, Patent Documents 1 to 4). The fibrillated fibers referred to here are fine fibers obtained by applying shear force to fibers having a fibril structure consisting of an aggregate of many fibrils (primary fibers) to gradually peel off the fibrils from the main fibers. At this time, the phenomenon in which the entire or part of the fibrils peel off from the main fibers is called fibrillation. Fibrillated fibers include fine fibers that have been completely peeled off from the main fibers and fine fibers that have been partially peeled off from the main fibers.

[0005] Patent Document 1 proposes a filter medium containing fibrillated fibers with a degree of beating of 150 ml CSF or less. Patent Document 2 proposes a filter medium having a submicron fiber layer and a support region, the submicron fiber layer containing fibrillated fibers with a degree of beating of 20 to 95°SR, an average fiber length of 0.1 to 5 mm, and an average diameter of 20 to 1000 μm. Patent Document 3 proposes a filter medium composed of a nonwoven fabric and a short fiber layer laminated on the nonwoven fabric, the short fiber layer containing fibrillated fibers having a sedimentation water retention of 500 ml / g or more. Patent Document 4 proposes a filter medium for liquid filtration, in which a filter medium layer containing (A) fibrillated Lyocell fibers with a fiber diameter of 1 μm or less separated from the trunk by applying a shear force, and (B) fibrillated Lyocell fibers with a fiber diameter of 1 μm or less generated from the trunk by applying a shear force, is laminated and integrated with a support layer. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-70305 [Patent Document 2] Special Publication No. 2015-520666 [Patent Document 3] JP 2011-36763 A [Patent Document 4] JP 2004-188409 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to obtain a filter medium with high collection efficiency, it is effective to use fine fiber, but if fiber is packed too densely, the pressure loss becomes higher than necessary, and the filter life, that is, the filtering time until the pressure loss reaches a certain level, decreases.In addition, in air filters, the power consumption of the fan used for ventilation increases.For these reasons, it is strongly required that filter medium has both high collection efficiency and low pressure loss.

[0008] Filter media made of glass fibers often combine high collection efficiency with low pressure loss, but because they are non-flammable, they are disposed of in landfills as industrial waste after use, which poses a large environmental burden when disposed of.

[0009] The degree of beating is an index showing the degree of fibrillation, and methods for measuring it include the Canadian Standard Freeness Method (CSF) and the Schopper-Riegler Method (SR). These degrees of beating are index values ​​showing the freeness of fibrillated fibers, and do not indicate the shape of the fibers themselves.

[0010] As mentioned above, both high collection efficiency and low pressure loss are required for filter media. In order to achieve this, the shape of the fibers constituting the filter media is important. Specifically, a shape with a small fiber diameter and a long fiber length is preferable. The smaller the fiber diameter, the larger the surface area contributing to filtration, so that high collection efficiency is easily obtained. In addition, the longer the fiber length, the easier it is to form a network structure that maintains sufficient voids around the fibers, so that high collection efficiency is easily obtained. On the other hand, fibers with short fiber length are difficult to form a network structure and are prone to clogging in the voids, which may cause unnecessary pressure loss to increase. In addition, fibers with thick fiber diameters have a small surface area contributing to filtration, so that high collection efficiency is difficult to obtain.

[0011] In the proposals in Patent Documents 1 to 4, the amounts and ratios of fibers with small fiber diameters and long fiber lengths, which are advantageous for filtration, and fibers with short fiber lengths and large fiber diameters, which are disadvantageous for filtration, are not clear.

[0012] The problem to be solved by the present disclosure is to provide a fibrillated fiber constituting a filter medium used to remove dust particles contained in a fluid to obtain a clean fluid, which has both high collection efficiency and low pressure loss and further has a small environmental impact when disposed of, a manufacturing method thereof, and a filter medium using the same. [Means for solving the problem]

[0013] The fibrillated fiber according to the present invention is used in a filter medium made of a wetlaid nonwoven fabric, and is characterized in that the fine fiber ratio B / A, as shown in the formula 1 below and measured using a fiber image analyzer conforming to ISO 16065-2:2014, is 2.0 or more. This allows for the production of a filter medium that has both high collection efficiency and low pressure loss. (Number 1) Fine fiber rate B / A = Fine fiber rate B [%] / Fine fiber rate A [%] Here, the fine fiber rate A [%] and the fine fiber rate B [%] are calculated by the following Equations 1a and 1b, respectively. (Equation 1a) Fine fiber rate A [%] = (total projected area of ​​fine objects less than 0.2 mm long and less than 75 μm wide / total projected area of ​​all measured objects) × 100 (Equation 1b) Fine fiber rate B [%] = (total length of fine objects with a length of more than 0.2 mm and a width of less than 10 μm / total length of all measured objects with a length of more than 0.2 mm) × 100

[0014] The fibrillated fiber according to the present invention is preferably a fibrillated lyocell fiber, which can provide a fibrillated fiber having a small fiber diameter and a long fiber length.

[0015] In the fibrillated fiber according to the present invention, the fine fiber ratio A is preferably 10 to 50%, which allows for an appropriate progression of fibrillation while providing a filter medium with low pressure loss.

[0016] In the fibrillated fiber according to the present invention, the fine fiber ratio B is preferably 60% or more, which makes it possible to obtain a filter medium with higher filtering performance.

[0017] The method for producing fibrillated fibers according to the present invention is a method for producing fibrillated fibers according to the present invention, which includes a step of fibrillating fibers in a slurry in which fibers having a fibrillar structure are dispersed in water, and is characterized in that the fibrillation process fibrillates the fibers by hydrodynamic shock waves or mechanical abrasion.

[0018] In the method for producing fibrillated fibers according to the present invention, the solid content of the fibers in the slurry is preferably 0.5% by mass or more and less than 3.0% by mass, which makes it easy to adjust the value of B / A to a predetermined range.

[0019] The filter medium according to the present invention is characterized in that it is made of a wet-laid nonwoven fabric containing the fibrillated fibers according to the present invention and non-fibrillated fibers, and does not contain glass fibers. This makes it possible to obtain a filter medium that has both high collection efficiency and low pressure loss, and has a small environmental impact when disposed of.

[0020] In the filter medium according to the present invention, the mass ratio of the fibrillated fibers to the non-fibrillated fibers (fibrillated fibers / non-fibrillated fibers) is preferably 5 / 95 to 80 / 20, which makes it easier to obtain high collection efficiency and low pressure loss.

[0021] In the filter medium according to the present invention, the non-fibrillated fibers preferably have a fiber diameter of 5 to 40 μm and a fiber length of 3 to 15 mm, which makes it easier to obtain high collection efficiency and low pressure loss. Effect of the Invention

[0022] According to the present disclosure, it is possible to obtain a fibrillated fiber constituting a filter medium used for removing dust particles contained in a fluid to obtain a clean fluid, which has both high collection efficiency and low pressure loss and further has a small environmental impact when disposed of, a manufacturing method thereof, and a filter medium using the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Next, the present invention will be described in detail with reference to the embodiments, but the present invention is not limited to these descriptions. Various modifications of the embodiments may be made as long as the effects of the present invention are achieved.

[0024] The fibrillated fiber in this embodiment is a fiber that has been fibrillated by applying a mechanical shear force to a fiber having a fibrillated structure. The fibrillated fiber in this embodiment is a fibrillated fiber used in a filter medium made of a wet-laid nonwoven fabric, and has a fine fiber ratio B / A value of 2.0 or more, which is expressed by the formula 1 and measured using a fiber image analyzer conforming to ISO 16065-2:2014. (Number 1) Fine fiber rate B / A = Fine fiber rate B [%] / Fine fiber rate A [%] Here, the fine fiber rate A [%] and the fine fiber rate B [%] are calculated by the following Equations 1a and 1b, respectively. (Equation 1a) Fine fiber rate A [%] = (total projected area of ​​fine objects less than 0.2 mm long and less than 75 μm wide / total projected area of ​​all measured objects) × 100 (Equation 1b) Fine fiber rate B [%] = (total length of fine objects with a length of more than 0.2 mm and a width of less than 10 μm / total length of all measured objects with a length of more than 0.2 mm) × 100

[0025] In this embodiment, unless otherwise specified, the term "fibrillated fiber" does not refer to each individual fine fiber produced by the fibrillation process, but refers to a mixture of objects produced by the fibrillation process of fibers. Objects produced by the fibrillation process include fine fibers generally called fibrillated fibers, as well as non-fibrous substances (impurities) such as non-fibrillated fibers or flocks or vessels that have not been fibrillated. In the formula 1a, the fine objects are those objects produced by these fibrillation processes that have a length of less than 0.2 mm and a width of less than 75 μm. In the formula 1b, the fine objects are those objects produced by these fibrillation processes that have a length of more than 0.2 mm and a width of less than 10 μm. In the formulas 1a and 1b, the length and width of the fine objects are values ​​measured by a fiber image analyzer conforming to ISO 16065-2, and are measured values ​​of the fiber thickness measured by direct observation of a planar image. In this specification, each fine fiber produced by the fibrillation process may be referred to as a "fine fiber." Here, fibrillation refers to completely separating fibrils (also called microfibrils or minute fibers) that are components of the main fiber from the main fiber, or partially separating the fibrils that constitute the main fiber from the fiber to branch it. Fibrillated fibers include flaked fine fibers that have completely peeled off from the main fiber, and branched fine fibers that have partially peeled off from the main fiber. In this embodiment, it is preferable that the fibrillated fibers include both flaked fine fibers and branched fine fibers.

[0026] The present inventors have noticed that the mixture obtained by fibrillating fibers contains fibers with long fiber length and small fiber width, which are fibers that work advantageously for filtration, and fibers with short fiber length and large fiber width, which are fibers that work disadvantageously for filtration.Then, the value of the fine fiber ratio B / A shown in (Mathematical formula 1) is used as an index of the ratio of fibers that work advantageously for filtration to fibers that work disadvantageously for filtration contained in the fibrillated fibers, and it has been found that high collection efficiency and low pressure loss can be achieved by forming a filter medium using fibrillated fibers with this value of 2.0 or more, and the present invention has been completed.The higher the value of the fine fiber ratio B / A, the more fibers that work advantageously for filtration are present, which indicates that a filter medium with high collection efficiency and reduced pressure loss can be obtained.

[0027] If the value of the fine fiber ratio B / A is less than 2.0, the collection efficiency is low and the pressure loss is high. The lower limit of the fine fiber ratio B / A is preferably 2.1 or more, and more preferably 2.2 or more. The upper limit of the fine fiber ratio B / A is not particularly limited, but in consideration of operability such as not requiring a long beating time, it is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less.

[0028] The fine fiber rate A [%] and the fine fiber rate B [%] of the fibrillated fiber in the present invention can be calculated by taking an image of the fibrillated fiber and analyzing it using a fiber image analyzer conforming to ISO 16065-2:2014 "Pulp-Determination of fiber length by automated optical analysis-Part 2: Non-polarized method". For example, when a Valmet fiber image analyzer (Valmet FS5, manufactured by Valmet) is used as a fiber image analyzer conforming to ISO 16065-2, the fine fiber rate A [%] is a value calculated as "Fines A% (Fine Fiber A%)", and the fine fiber rate B [%] is a value calculated as "Fines B% (Fine Fiber B%)". "Fines A% (Fine Fiber A%)" is a value expressed as the ratio (%) of the projected part of the measured particle of thin-flaky fine fibers (A type), and fine fibers A include particles shorter than 0.2 mm. "Fines B%" is the percentage of length of thin layered fine fibers (Type B), where fine fibers B include particles with a width of less than 10 μm and a length of more than 0.2 mm. "Fines B%" is calculated by dividing the total length of such particles by the total length of all particles longer than 0.2 mm, then multiplying the result by 100.

[0029] The fine fiber rate A is calculated as a percentage of the ratio of the total projected area of ​​fine objects with a fiber length of less than 0.2 mm and a fiber width of less than 75 μm to the total projected area of ​​all measured objects. All measured objects are all objects with a length of 0.01 to 10.00 mm and a width of 1 to 1000 μm among objects that can be acquired by an image analysis device. The length refers to the distance of the longest part of the object, and the width refers to the distance of the shortest part of the object in a direction perpendicular to the length. In addition to fine fibers, all measured objects include impurities such as non-fibrillated fibers that have not been fibrillated or flocks or vessels. The impurities here include non-fibrous substances originally contained in the fibers before fibrillation and components that do not contribute to web formation by a wet method that are generated by fibrillation treatment of the fibers. Fine objects with a fiber length of less than 0.2 mm and a fiber width of less than 75 μm are fibers with a fiber length of 0.01 mm or more and less than 0.2 mm and a fiber width of 1 μm or more and less than 75 μm among objects that can be acquired by an image analysis device. The fine objects of the number 1a include fibrillated fibers with short fiber length and non-fibrillated fibers with short fiber length. A large value of the fine fiber ratio A means that there are many dust-like fibers with short fiber length. Here, dust-like fibers refer to fibers that hardly contribute to collection efficiency. These fibers are difficult to form a network structure and are prone to clogging in voids, which may cause unnecessary increase in pressure loss. The region of all measured objects with fiber length less than 0.2 mm and fiber width 75 μm or more includes a region with a small ratio of fiber length to fiber width. Objects that fall into such a region are considered to have a shape close to a particle and are not considered to be fibers. For this reason, the number 1a is specified so that the ratio of such objects is not included in the fine fiber ratio A. The fine fiber ratio A is preferably 10 to 50%, more preferably 20 to 45%, and even more preferably 30 to 40%. If the fine fiber ratio A is less than 20%, fibrillation may be insufficient. If the fine fiber ratio A exceeds 50%, the pressure loss becomes too high, which may result in a decrease in filter performance.

[0030] The fine fiber ratio B is calculated as a percentage of the ratio between the total length of fine objects having a fiber length of more than 0.2 mm and a fiber width of less than 10 μm and the total length of all measured objects having a fiber length of more than 0.2 mm. All measured objects having a fiber length of more than 0.2 mm are all objects having a length of more than 0.2 mm and less than 7.6 mm and a width of 1 to 1000 μm among objects that can be acquired by an image analysis device. Fine objects having a fiber length of more than 0.2 mm and less than 10 μm are fibers having a fiber length of more than 0.2 mm and less than 7.6 mm and a fiber width of 1 μm or more and less than 10 μm among objects that can be acquired by an image analysis device. A large value of the fine fiber ratio B means that there are many fibers having a small fiber diameter and a long fiber length. These fibers have a large surface area that contributes to filtration and are easy to form a network structure that maintains sufficient voids around the fibers, so that a high collection efficiency can be obtained. The fine fiber ratio B is preferably 60% or more, more preferably 70% or more. If the fine fiber ratio B is less than 60%, sufficient pressure loss and filter performance may not be obtained. The upper limit of the fine fiber ratio B is not particularly limited, but is preferably, for example, 100% or less.

[0031] The method for producing fibrillated fibers according to this embodiment includes a step of fibrillating fibers in a slurry in which fibers having a fibrillated structure are dispersed in water, the fibrillating process fibrillating the fibers by hydrodynamic shock waves or mechanical abrasion.

[0032] Examples of fibrillation methods for obtaining such fibrillated fibers include a method in which a slurry of fibers having a fibrillated structure dispersed in water is passed between a blade rotating at high speed and a fixed blade, such as a single disc refiner or double disc refiner, to fibrillate the fibers through mechanical wear; and a method in which a slurry of fibers having a fibrillated structure dispersed in water is passed through a ring-shaped blade with fine slits rotating at high speed, such as a Top Finer (manufactured by Aikawa Iron Works) or a Full Disintegrator VF type (manufactured by Shinhama Pump Works), to fibrillate the fibers by the hydrodynamic shock waves generated when the slurry passes through the slits.

[0033] The conditions for fibrillation vary depending on the device used, but in order to obtain fibers with a fine fiber ratio B / A value of 2.0 or more, it is preferable to fibrillate gradually by lowering the slurry concentration during fibrillation as much as possible. The slurry concentration is preferably 0.5% by mass or more and less than 3.0% by mass, more preferably 0.8% by mass or more and less than 2.5% by mass, and even more preferably 1.0% by mass or more and less than 2.0% by mass. If the slurry concentration is less than 0.5% by mass, the fibrillation takes time, so the probability of the fibers being cut increases. If the slurry concentration is 3.0% by mass or more, the concentration is too high and the fibers are easily cut. When fibrillation is performed using hydrodynamic shock waves, the slurry concentration and the processing speed of fibrillation are preferably in the following ranges. That is, the slurry concentration is preferably 0.5% by mass or more and less than 3.0% by mass, more preferably 0.5% by mass or more and less than 2.5% by mass, and even more preferably 0.5% by mass or more and less than 2.0% by mass. The fibrillation treatment speed is preferably 50 to 250 L / min, more preferably 100 to 200 L / min. When fibrillation is performed by mechanical abrasion, the slurry concentration, fibrillation treatment speed and treatment time are preferably in the following ranges. That is, the slurry concentration is preferably 0.5% by mass or more and less than 2.5% by mass, more preferably 0.5% by mass or more and 2.0% by mass or less, and even more preferably 0.5% by mass or more and 1.5% by mass or less. The fibrillation treatment speed is preferably 50 to 200 L / min, more preferably 100 to 150 L / min. In addition, since fibrillation by mechanical abrasion is prone to fiber breakage due to friction, it is preferable to perform fibrillation by hydrodynamic shock waves.

[0034] The degree of beating of the fibrillated fiber is not particularly specified, but the Canadian Standard Freeness (CSF) is preferably 20 to 100 ml, more preferably 20 to 80 ml, and even more preferably 20 to 60 ml. If the CSF is lower than 20 ml, the fibrillation may be excessive in the use of the present invention, and extra time and energy may be required for the fibrillation. If the CSF is higher than 100 ml, the amount of fine fibers required to obtain a high collection efficiency may be insufficient.

[0035] The fibrillated fiber in the present invention may be any fiber obtained by fibrillating a fiber having a fibrillated structure, and may be, for example, fibrillated lyocell fiber, fibrillated aramid fiber, or fibrillated cellulose fiber. Of these, in the present embodiment, the fibrillated fiber is preferably fibrillated lyocell fiber.

[0036] Here, the Lyocell fiber is a regenerated cellulose fiber spun by an organic solvent spinning method using N-methylmorpholine-N-oxide as a solvent. In the organic solvent spinning method, cellulose is dissolved in an organic solvent and spun, so that the molecular breakage is small, the average degree of polymerization is higher than other regenerated cellulose fibers, the fiber has high rigidity, and the cross-sectional shape of the fiber is close to a circle. This rigidity and cross-sectional shape make it easier to maintain the voids in the filter medium, and the Lyocell fiber after fibrillation also maintains the above-mentioned rigidity and cross-sectional shape, so that unnecessary pressure loss is unlikely to increase.

[0037] The filter medium according to the present embodiment is made of a wet-laid nonwoven fabric containing the fibrillated fiber according to the present embodiment and the non-fibrillated fiber, but not containing glass fiber. This makes it possible to obtain a filter medium that has both high collection efficiency and low pressure loss, and has a small environmental impact when disposed of.

[0038] In the filter medium according to the present embodiment, the mass ratio of fibrillated fibers to non-fibrillated fibers (fibrillated fibers / non-fibrillated fibers) is preferably within the range of 5 / 95 to 80 / 20, more preferably 10 / 90 to 70 / 30, and even more preferably 10 / 90 to 60 / 40. In other words, the mass ratio of fibrillated fibers to 100 parts by mass of the total mass of fibrillated fibers and non-fibrillated fibers is preferably 5 to 80 parts by mass, more preferably 10 to 70 parts by mass, and even more preferably 10 to 60 parts by mass. If the mass ratio of fibrillated fibers is lower than 5 / 95, the required collection efficiency may not be obtained. In addition, if the mass blending ratio of fibrillated fibers is higher than 80 / 20, the structure of the filter medium may become too dense, and the pressure loss may become too high.

[0039] The non-fibrillated fibers in the present invention are fibers that have not been fibrillated and have not been fibrillated or have a slightly fluffy surface. Here, "slightly fluffy surface" refers to a state in which the physical properties such as freeness are not affected. The average fiber diameter of the non-fibrillated fibers is preferably 5 to 40 μm, more preferably 7 to 35 μm. If the average fiber diameter is smaller than 5 μm, the voids for uniformly distributing the fibrillated fibers cannot be maintained, which may cause an increase in pressure loss. On the other hand, if the average fiber diameter is larger than 40 μm, the voids may become too large, which may cause a decrease in collection efficiency. The average fiber length of the non-fibrillated fibers is preferably 3 to 15 mm, more preferably 3 to 10 mm. If the average fiber length is shorter than 3 mm, the voids for uniformly distributing the fibrillated fibers cannot be maintained, which may cause an increase in pressure loss. On the other hand, if the average fiber length is longer than 15 mm, the uniformity of the filter medium may decrease due to poor dispersion in water, such as twisting, which may cause a decrease in collection efficiency. The average fiber diameter of non-fibrillated fibers can be determined as follows. The surface of the fiber was photographed using an electron microscope, a straight line was drawn horizontally on the obtained electron microscope photograph, and the width of the fiber at the intersection between the straight line and the fiber was measured. The average fiber diameter was taken as the arithmetic average value of 200 measurement points. The average fiber length of non-fibrillated fibers can be determined as follows. The surface of the fiber was photographed using an electron microscope, a straight line was drawn horizontally on the obtained electron microscope photograph, and the fiber length at the intersection between the straight line and the fiber was measured. The average fiber length was taken as the arithmetic average value of 200 measurement points. The average fiber diameter and average fiber length of non-fibrillated fibers obtained by the above-mentioned method are both length-weighted average values.

[0040] The non-fibrillated fiber in the present invention is preferably a biodegradable fiber that is made of non-fibrillated fibers other than glass fibers. The biodegradable fiber is, for example, regenerated cellulose fiber, natural cellulose fiber, polylactic acid-based fiber, polybutylene succinate fiber, or polyhydroxyalkanoate fiber. Of these, the non-fibrillated fiber is more preferably one or more types of biodegradable fiber selected from regenerated cellulose fiber, natural cellulose fiber, and polylactic acid fiber. The fiber does not melt during heat drying, and the melted fiber blocks the pores of the filter medium, thereby preventing a decrease in collection efficiency. In this embodiment, non-fibrillated fibers having different types and / or different fiber diameters may be mixed and used.

[0041] Here, the regenerated cellulose fiber refers to viscose rayon fiber spun by the viscose method using cellulose as the raw material, lyocell fiber spun by the organic solvent spinning method, etc. These are renewable materials made from wood pulp as the raw material, and are degradable in soil burial and biodegradable in the ocean.

[0042] Natural cellulose fibers are fibers that are primarily made of cellulose extracted from plants, and include wood pulp, cotton linter pulp, hemp pulp, kenaf pulp, mercerized pulp obtained by treating wood pulp with alkali, etc. These are renewable materials made from plants and are biodegradable when buried in soil.

[0043] Polylactic acid fiber is a fiber spun mainly from a polymer obtained by chemically polymerizing lactic acid obtained by saccharification and fermentation of starch derived from biomass. It is a renewable material made from corn and the like, and is biodegradable when buried in soil. In addition, unlike cellulose-based fibers, polylactic acid fiber has thermoplasticity, so that it can impart tensile strength or thermoformability to a filter medium by heat fusion. Polylactic acid fiber includes polylactic acid-based fibers having a substantially uniform composition in cross section and polylactic acid core-sheath fibers having a core and a sheath in cross section.

[0044] The filter material of the present invention is preferably manufactured by using wet papermaking method.That is, according to the wet papermaking method, the fiber that constitutes the filter material is dispersed in water by using a dispersing machine such as a pulper, the fiber slurry obtained is piled on a wire and dehydrated to form a sheet, and the wet sheet obtained is dried by using a dryer such as a hot air dryer or a cylinder dryer, thereby obtaining a wet nonwoven fabric as a dry sheet, and this wet nonwoven fabric is suitable as a filter material.

[0045] The filter material according to the present embodiment includes the form of air filter material or liquid filter material.The filter material according to the present embodiment is a component that constitutes the member that is fixed to a frame or the like and is incorporated into various filters such as air filters or liquid filters, and includes the form of fixing the filter material that is a wet nonwoven fabric to a frame to make a filter, and the form of fixing the filter material that is a wet nonwoven fabric to a frame after laminating another member or coating the filter material that is a wet nonwoven fabric to make a filter.

[0046] As long as it does not impair the effect of the present invention, a chemical agent for the purpose of imparting functionality such as a binder, a water repellent, or a flame retardant may be added to the filter medium. As a method of addition, a method of dispersing the chemical agent together with the fiber in water to form a sheet, or a method of spraying or impregnating the chemical agent on a wet or dry sheet can be used. EXAMPLES

[0047] The present invention will be specifically described below with reference to examples and comparative examples. However, the present invention is not limited to these. In the examples, "parts" and "%" respectively refer to "parts by mass" and "% by mass" unless otherwise specified. The number of parts added is a value calculated as a solid content.

[0048] <Example 1> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 1.5 mass %, and the resulting slurry was fibrillated for 30 minutes at a processing speed of 150 L / min using a Top Finer (manufactured by Aikawa Iron Works Co., Ltd.) to obtain fibrillated fiber 1 in Table 1. The Canadian Standard Freeness (CSF) of fibrillated fiber 1 was 40 mL.

[0049] <Example 2> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 1.0 mass %, and the resulting slurry was fibrillated for 55 minutes at a processing speed of 100 L / min using a single disc refiner (manufactured by Aikawa Iron Works Co., Ltd.) to obtain fibrillated fiber 2 in Table 1. The Canadian Standard Freeness (CSF) of fibrillated fiber 2 was 35 mL.

[0050] <Comparative Example 1> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 2.0 mass %, and the resulting slurry was fibrillated for 20 minutes at a processing speed of 150 L / min using a double disc refiner (manufactured by Aikawa Iron Works Co., Ltd.) to obtain fibrillated fiber 3 in Table 1. The Canadian Standard Freeness (CSF) of fibrillated fiber 3 was 35 mL.

[0051] <Comparative Example 2> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 2.0 mass %, and the resulting slurry was fibrillated for 35 minutes at a processing speed of 150 L / min using a single disc refiner (manufactured by Aikawa Iron Works Co., Ltd.) to obtain fibrillated fiber 4 in Table 1. The Canadian Standard Freeness (CSF) of fibrillated fiber 4 was 40 mL.

[0052] The fine fiber rate A [%] and the fine fiber rate B [%] of the fibrillated fibers 1 to 4 obtained in the examples and comparative examples were determined using a fiber image analyzer Valmet FS5 (manufactured by Valmet Inc.) conforming to ISO 16065-2. The dispersion concentration of the fibrillated fiber used in the analysis was 1.0 mass%.

[0053] [Table 1]

[0054] <Examples 3 and 4 and Comparative Examples 3 and 4> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 2 so that the pressure loss was about 11 Pa, water was added so that the slurry concentration was 0.5 mass%, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 2.

[0055] [Table 2]

[0056] <Examples 5 and 6 and Comparative Examples 5 and 6> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 3 so that the pressure loss was about 20 Pa, water was added so that the slurry concentration was 0.5 mass%, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 3.

[0057] [Table 3]

[0058] <Examples 7 and 8 and Comparative Examples 7 and 8> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 4 so that the pressure loss was about 40 Pa, water was added so that the slurry concentration was 0.5 mass%, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 4.

[0059] [Table 4]

[0060] <Examples 9 and 10 and Comparative Examples 9 and 10> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 5 so that the pressure loss was about 100 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 5.

[0061] [Table 5]

[0062] <Examples 11 and 12 and Comparative Examples 11 and 12> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 6 so that the pressure loss was about 200 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 6.

[0063] [Table 6]

[0064] <Examples 13 and 14 and Comparative Examples 13 and 14> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 7 so that the pressure loss was about 300 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 7.

[0065] [Table 7]

[0066] <Examples 15 and 16 and Comparative Examples 15 and 16> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2The fibers were mixed in the ratio of parts shown in Table 8 so that the pressure loss was about 500 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 8.

[0067] [Table 8]

[0068] <Examples 17 and 18 and Comparative Examples 17 and 18> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) were used with a basis weight of 70 g / m 2 The ratio of fibrillated fiber / non-fibrillated fiber was 60 / 40, water was added to the mixture so that the slurry concentration was 0.5% by mass, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 9.

[0069] [Table 9]

[0070] <Examples 19 and 20 and Comparative Examples 19 and 20> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) were used with a basis weight of 70 g / m 2 The ratio of fibrillated fiber / non-fibrillated fiber was 5 / 95, water was added to the mixture so that the slurry concentration was 0.5% by mass, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 10.

[0071] <Comparative Example 21> As a non-fibrillated fiber, 100 parts of Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, Lenzing AG) were used with a basis weight of 70 g / m 2 Water was added to the mixture so that the slurry concentration was 0.5% by mass, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a hand-made papermaking machine, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 10.

[0072] [Table 10]

[0073] <Examples 21 and 22 and Comparative Examples 22 and 23> Fibrillated fibers 1 to 4 and 10 parts of non-fibrillated fibers, namely, Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) and polylactic acid core-sheath fiber (fineness 2.2 dtex (fiber diameter 15 μm), fiber length 5 mm, core melting point 170°C, sheath melting point 130°C, product name: Terramac PL80, manufactured by Unitika Co., Ltd.) were mixed together to form a spunbonded fiber having a basis weight of 70 g / m. 2 The fibers were mixed in the ratio of parts shown in Table 11 so that the pressure loss was about 40 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was passed through a rotary dryer at 120°C and dried to obtain the air filter medium shown in Table 11.

[0074] [Table 11]

[0075] <Examples 23 and 24 and Comparative Examples 24 and 25> Fibrillated fibers 1 to 4 and non-fibrillated fibers, mainly polylactic acid fibers (fiber size 1.7 dtex (fiber diameter 13 μm), fiber length 5 mm, melting point 170°C, product name: Terramac PL01, manufactured by Unitika Co., Ltd.) were used with a basis weight of 70 g / m. 2 The fibers were mixed in the ratio of parts shown in Table 12 so that the pressure loss was about 20 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was passed through a rotary dryer at 120°C and dried to obtain the air filter medium shown in Table 12.

[0076] [Table 12]

[0077] <Examples 25 and 26 and Comparative Examples 26 and 27> Fibrillated fibers 1 to 4 and non-fibrillated fibers, mainly polylactic acid fibers (fiber size 1.7 dtex (fiber diameter 13 μm), fiber length 5 mm, melting point 170°C, product name: Terramac PL01, manufactured by Unitika Co., Ltd.) were used with a basis weight of 70 g / m. 2 The fibers were mixed in the ratio of parts shown in Table 13 so that the pressure loss was about 150 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 13.

[0078] [Table 13]

[0079] <Examples 27 and 28 and Comparative Examples 28 and 29> Fibrillated fibers 1 to 4 and mercerized pulp fibers (average fiber diameter 34 μm, average fiber length 2.8 mm, product name: Porocenia, manufactured by Rayonier Inc.) as non-fibrillated fibers were used with a basis weight of 70 g / m. 2The fibers were mixed in the ratio of parts shown in Table 14 so that the pressure loss was about 20 Pa, water was added so that the slurry concentration was 0.5 mass%, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 14.

[0080] [Table 14]

[0081] <Examples 29 and 30 and Comparative Examples 30 and 31> Fibrillated fibers 1 to 4 and mercerized pulp fibers (average fiber diameter 34 μm, average fiber length 2.8 mm, product name: Porocenia, manufactured by Rayonier Inc.) as non-fibrillated fibers were used with a basis weight of 70 g / m. 2 The fibers were mixed in the ratio of parts shown in Table 15 so that the pressure loss was about 150 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was passed through a rotary dryer at 120°C and dried to obtain the air filter medium shown in Table 15.

[0082] [Table 15]

[0083] <Examples 31 and 32 and Comparative Examples 32 and 33> Fibrillated fibers 1 to 4 and 20 parts of non-fibrillated fibers, including lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) and polylactic acid core-sheath fiber (fineness 2.2 dtex (fiber diameter 15 μm), fiber length 5 mm, core melting point 170°C, sheath melting point 130°C, product name: Terramac PL80, manufactured by Unitika Co., Ltd.), were mixed together to form a 120 g / m2 fiber bundle. 2The fibers were mixed in the ratio of parts shown in Table 16 so that the pressure loss was about 3000 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain a liquid filter medium shown in Table 16.

[0084] [Table 16]

[0085] <Example 33> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 0.5 mass %, and the resulting slurry was fibrillated for 90 minutes at a processing speed of 100 L / min using Top Finer (manufactured by Aikawa Iron Works Co., Ltd.) to obtain Fibrillated Fiber 5 in Table 17. The Canadian Standard Freeness (CSF) of Fibrillated Fiber 5 was 50 mL.

[0086] <Comparative Example 34> Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) was dispersed in water at a dispersion concentration of 3.0 mass %, and the resulting slurry was fibrillated for 12 minutes at a face speed of 5000 rpm using a PFI mill (manufactured by Kumagai Riki Kogyo Co., Ltd.) to obtain fibrillated fiber 6 in Table 17. The Canadian Standard Freeness (CSF) of fibrillated fiber 6 was 50 mL.

[0087] The fine fiber rate A [%] and the fine fiber rate B [%] of the fibrillated fibers 5 to 6 obtained in Example 33 and Comparative Example 34 were determined in the same manner as for the fibrillated fibers 1 to 4.

[0088] [Table 17]

[0089] <Example 34 and Comparative Example 35> Fibrillated fibers 5 to 6 and non-fibrillated fibers, Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) with a basis weight of 70 g / m 2 The fibers were mixed in the ratio of parts shown in Table 18 so that the pressure loss was about 11 Pa, and water was added so that the slurry concentration was 0.5 mass %, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a manual papermaking device, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 18.

[0090] [Table 18]

[0091] <Examples 35 and 36 and Comparative Examples 36 and 37> Fibrillated fibers 1 to 4 and non-fibrillated Lyocell fibers (fiber size 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufactured by Lenzing AG) were used with a basis weight of 70 g / m 2 The ratio of fibrillated fiber / non-fibrillated fiber was 80 / 20, water was added to the mixture so that the slurry concentration was 0.5% by mass, and the mixture was defibrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into a wet sheet using a hand-made papermaking machine, and the wet sheet was dried through a rotary dryer at 120°C to obtain the air filter medium shown in Table 19.

[0092] <Comparative Example 38> The fibrillated fiber 1 was added with water so that the slurry concentration was 0.5% by mass, and disintegrated using a pulper to obtain a fiber slurry. Next, the fiber slurry was made into paper using a manual papermaking device to obtain a wet sheet, which was passed through a rotary dryer at 120°C and dried to obtain a filter medium for air filters shown in Table 19. However, the sheet was severely cut into the papermaking wire, and a uniform sheet could not be produced.

[0093] [Table 19]

[0094] The air filter media and liquid filter media obtained in the examples and comparative examples were evaluated by the following methods.

[0095] The fine fiber rate A [%] and the fine fiber rate B [%] of fibrillated fibers 1 to 7 were determined as follows. Fibrillated fibers 1 to 7 were dispersed in water to a dispersion concentration of 1.0 mass% to obtain a dispersion. Measurement was performed using a fiber image analyzer Valmet FS5 (Valmet Inc.) conforming to ISO 16065-2, and the value calculated as "Fines A%" was the fine fiber rate A [%], and the value calculated as "Fines B%" was the fine fiber rate B [%].

[0096] (1) Basis weight [g / m 2 ] Measurement was performed according to JIS P 8124:2011 "Paper and paperboard -- Determination of basis weight."

[0097] (2) Pressure loss [Pa] Effective area: 0.01m 2 The differential pressure when air was passed through the air filter medium at a surface velocity of 5.3 cm / sec was measured using a micro-differential pressure gauge.

[0098] (3)PAO transmittance [%] Air containing polydisperse polyalphaolefin (PAO) particles generated by a Ruskin nozzle was blown into a nozzle with an effective area of ​​0.01 m. 2 The number of PAO particles upstream and downstream of the air filter medium was measured using a laser particle counter KC-22B (Rion Co., Ltd.) when the air flow rate was 5.3 cm / sec. The transmittance was calculated by the formula: downstream number / upstream number x 100. The target particle size was 0.10-0.15 μm. The PAO collection efficiency [%] is 100-PAO transmittance [%].

[0099] (4) PF value The PF value was calculated from the measured values ​​of pressure loss and permeability using the formula 2. A higher PF value means that the filter medium has a higher collection efficiency (i.e., a lower permeability) with a lower pressure loss.

[0100]

number

[0101] (5) Liquid filtration efficiency [%] Liquid filtration efficiency is measured with a 12.5 cm effective area. 2 A liquid (hereinafter referred to as the stock solution) in which silicon carbide fine powder (product name: Green Desic GC#8000, manufacturer: Showa Denko K.K.) was dispersed in distilled water to a mass concentration of 100 ppm was passed through the filter medium at a flow rate of 20 mL / min and filtered, and 50 mL of the filtrate was collected 1 minute after the start of the liquid passing. The turbidity [%] of the stock solution and the filtrate was then measured using a color / turbidity simultaneous measuring device COH400 (manufactured by Nippon Denshoku Kogyo Co., Ltd.), and the liquid filtration efficiency [%] was calculated from the turbidity [%] of the obtained stock solution and filtrate using the formula 3.

[0102]

number

[0103] As shown by the Examples and Comparative Examples, when comparing filter media having similar pressure loss, filter media using fibrillated fibers with a fine fiber ratio B / A of 2.0 or more with filter media using fibrillated fibers with a fine fiber ratio B / A of less than 2.0, filter media using fibrillated fibers 1, 2 or 5 with a fine fiber ratio B / A of 2.0 or more had a higher PF value in air filter media and a higher liquid filtration efficiency in liquid filter media than filter media using fibrillated fibers 3, 4 or 6 with a fine fiber ratio B / A of less than 2.0. [Industrial Applicability]

[0104] The filter medium of the present invention can be used as a filter medium for filters used in various fields such as factory and building air conditioning, automobile passenger compartments, air conditioners, air purifiers, personal protective equipment, water filtration, and oil filtration.

Claims

1. The fibrillated fiber is used for a filter medium made of a wet nonwoven fabric, and is characterized in that the value of the fine fiber ratio B / A, which is expressed by the formula 1 and measured using a fiber image analyzer conforming to ISO 16065-2:2014, is 2.0 or more. (Equation 1) Fine fiber rate B / A=Fine fiber rate B [%] / Fine fiber rate A [%] Here, the fine fiber rate A [%] and the fine fiber rate B [%] can be calculated by the following Equations 1a and 1b, respectively. (Equation 1a) Fine fiber rate A [%] = (total projected area of ​​fine objects with a length of less than 0.2 mm and a width of less than 75 μm / total projected area of ​​all measured objects) × 100 (Equation 1b) Fine fiber rate B [%] = (total length of fine objects having a length of more than 0.2 mm and a width of less than 10 μm / total length of all measured objects having a length of more than 0.2 mm) × 100

2. 2. The fibrillated fiber of claim 1, wherein the fibrillated fiber is a lyocell fiber.

3. 2. The fibrillated fiber according to claim 1, wherein the fine fiber rate A is 10 to 50%.

4. 2. The fibrillated fiber according to claim 1, wherein the fine fiber ratio B is 60% or more.

5. A method for producing the fibrillated fiber according to claim 1 or 2, comprising the steps of: The method includes a step of fibrillating fibers in a slurry in which fibers having a fibril structure are dispersed in water, The method for producing fibrillated fibers, wherein the fibrillation treatment comprises fibrillating the fibers by hydrodynamic shock waves or mechanical abrasion.

6. 6. The method for producing fibrillated fibers according to claim 5, wherein the solid content of the fibers in the slurry is 0.5% by mass or more and less than 3.0% by mass.

7. A filter medium comprising a wet-laid nonwoven fabric containing the fibrillated fibers according to any one of claims 1 to 4 and non-fibrillated fibers, and containing no glass fibers.

8. The filter medium according to claim 7, characterized in that a mass ratio of the fibrillated fibers to the non-fibrillated fibers (the fibrillated fibers / the non-fibrillated fibers) is 5 / 95 to 80 / 20.

9. 8. The filter medium according to claim 7, wherein the non-fibrillated fibers have a fiber diameter of 5 to 40 μm and a fiber length of 3 to 15 mm.

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