JP7627205B2 - Filter material for liquid filters and method for producing same - Google Patents

Description

This disclosure relates to a filter medium for liquid filters, which is used to remove solid particles contained in a liquid to obtain a clean liquid, and a method for manufacturing the same.

Filter media used to remove solid particles contained in liquids are generally made of paper or nonwoven fabric composed of natural fibers, organic synthetic fibers, or inorganic fibers. To obtain a filter medium with high filtering efficiency, it is effective to use a filter medium densely packed with fine fibers. However, if the fiber diameter is too fine or the fiber packing is too dense, the pressure loss increases, making it difficult to ensure the amount of liquid passing through, and there is a problem that the filter life, i.e., the filtering time until a certain pressure loss is reached, is reduced.

Fibrillated lyocell fibers are widely used as fibers constituting filter media for liquid filters because they provide high filtration efficiency. For example, filter media containing fibrillated lyocell fibers with a fiber diameter of 1 μm or less and organic fibers with a fiber diameter of 1 to 30 μm (see, for example, Patent Document 1) and filter media containing about 20 to about 80% by weight of bicomponent staple fibers and about 10 to about 50% by mass of fibrillated lyocell staple fibers (see, for example, Patent Document 2) have been proposed. However, there is a demand for filter media that have even higher filtration efficiency and longer filtration life while maintaining low pressure loss.

JP 2004-188409 A Special Publication No. 2021-521000

As described above, there is a demand for a filter medium that can efficiently filter solid particles contained in liquids while maintaining low pressure loss and that has a long filtration life. However, with conventional technology, it has been difficult to obtain a filter medium that combines all of these characteristics. Therefore, the objective of this disclosure is to provide a filter medium for liquid filters that combines low pressure loss, high filtration efficiency, and a long filtration life, and a method for manufacturing the same.

The filter medium for liquid filters according to the present invention is characterized in that it contains fibrillated lyocell fibers and heat-fused fibers having a fiber diameter of 35 μm or more, and has a wet-laid nonwoven fabric in which at least the fibrillated lyocell fibers form a network structure.

In the air filter medium of the present invention, it is preferable that the fibrillated lyocell fiber has an average fiber diameter of 0.3 μm or more, a maximum fiber diameter of 8 μm or less, and a length-weighted average fiber length of 1 mm or more. This allows for high filtration efficiency.

In the liquid filter medium of the present invention, the wet nonwoven fabric includes a form that contains non-thermally fused fibers that are not fibrillated. This makes it possible to obtain a filter medium with a good balance of physical properties.

In the liquid filter medium according to the present invention, the amount of fibrillated lyocell fiber is preferably 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium. This allows pressure loss to be reduced while ensuring sufficient filtration efficiency.

In the liquid filter medium according to the present invention, the blending amount of the heat-sealing fibers is preferably 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium. This allows sufficient filtering efficiency to be obtained while ensuring the strength of the filter medium.

In the liquid filter medium according to the present invention, the heat-sealing fibers are preferably composite heat-sealing fibers having a non-heat-sealing component that does not melt when heated and a heat-sealing component that melts when heated. The composite heat-sealing fibers are also added to the network structure formed by the fibrillated lyocell fibers, so that sufficient filtering efficiency can be obtained while maintaining the strength of the filter medium.

In the liquid filter medium according to the present invention, the blending amount of the non-thermal fusion fiber is preferably up to 70 parts per 100 parts of the total fiber mass constituting the filter medium. This allows a filter medium with a good balance of physical properties to be obtained. In addition, the fibers constituting the filter medium may only include the fibrillated lyocell fiber and the thermal fusion fiber having a fiber diameter of 35 μm or more .

In the liquid filter material according to the present invention, it is preferable that the non-thermal fusion fibers have a fiber diameter of 5 μm or more. This can prevent the pressure loss from becoming too high.

The method for producing a filter medium for liquid filters according to the present invention is characterized by comprising the steps of: preparing a raw material slurry containing fibrillated lyocell fibers and heat-sealed fibers having a fiber diameter of 35 μm or more; forming a wet sheet by papermaking the raw material slurry; and drying the wet sheet to obtain a dry sheet to be a filter medium for liquid filters . The heat-sealed fibers are preferably fibers containing a heat-sealing component that melts when heated in the step of obtaining the dry sheet.

This disclosure makes it possible to obtain a filter medium for liquid filters that combines low pressure loss, high filtration efficiency, and a long filtration life.

1 is a graph comparing the relationship between pressure loss and filtration efficiency in Examples and Comparative Examples. 1 is a graph comparing the relationship between pressure loss and filter life in Examples and Comparative Examples.

Next, the present invention will be described in detail with reference to an embodiment, but the present invention should not be interpreted as being limited to these descriptions. Various modifications of the embodiment may be made as long as the effects of the present invention are achieved.

The lyocell fiber in this embodiment 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 as it is and spun, so there is little molecular breakage, 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 circular. This rigidity and cross-sectional shape make it easier to maintain the voids in the filter medium. In addition, the fibrillated lyocell fiber after beating also maintains the above-mentioned characteristics of rigidity and cross-sectional shape. Furthermore, when fibrillated by beating, the surface area of the fiber that contributes to particle collection increases, thereby increasing the filtration efficiency.

Beating machines such as Niagara beaters, PFI mills, single disc refiners, and double disc refiners can be used to fibrillate lyocell fibers. During beating, it is preferable to beat without applying too strong a load so as not to shorten the fiber length of the lyocell too much.

As the beating of the lyocell fibers proceeds, the fibers are cut and the fiber length shortens. If the fiber length becomes too short, it may fill voids after the sheet is formed, resulting in high pressure loss. The fibrillated lyocell fibers used in this embodiment preferably have a length-weighted average fiber length of 1 mm or more, more preferably 1 to 3 mm, and even more preferably 1 to 2 mm.

The length-weighted average fiber length of fibrillated lyocell fibers was measured according to ISO 16065-2:2007 "Determination of fiber length by automated optical analysis - Part 2."

When the lyocell fiber is beaten, fibrillation progresses and the fiber diameter becomes smaller. The average fiber diameter of the fibrillated lyocell fiber used in this embodiment is preferably 0.3 μm or more, more preferably 0.3 to 1.0 μm, and even more preferably 0.3 to 0.8 μm. If the average fiber diameter is less than 0.3 μm, the fibers may be cut as the fibrillation progresses, making it impossible to maintain voids in the filter medium, and pressure loss may increase. On the other hand, if the average fiber diameter exceeds 1.0 μm, the surface area of the fiber that contributes to particle collection may become smaller, and filtration efficiency may decrease.

The maximum fiber diameter of the fibrillated lyocell fiber is preferably 8 μm or less, more preferably 6 μm or less, and even more preferably 4 μm or less. If the maximum fiber diameter exceeds 8 μm, the surface area of the fiber that contributes to particle collection will be small, and the filtration efficiency may decrease.

In this embodiment, the fiber diameter was measured by taking a photograph of the surface of the filter medium using an electron microscope, drawing a line horizontally on the obtained electron microscope photograph, and measuring the fiber diameter at the intersection of the line and the fiber. The average fiber diameter was calculated as the arithmetic mean value of 200 measurement points.

The amount of fibrillated lyocell fiber in this embodiment can be adjusted as appropriate depending on the physical properties required for the filter medium, but is preferably 20 to 80 parts, and more preferably 30 to 70 parts, assuming that the total fiber mass constituting the filter medium is 100 parts. If the amount of fibrillated lyocell fiber is too small, it may be difficult to obtain sufficient filtration efficiency. If the amount of fibrillated fiber is too large, the structure of the filter medium may become dense, resulting in high pressure loss.

The heat-sealing fiber in this embodiment is a fiber containing a heat-sealing component that melts when heated in the manufacturing process of the wet nonwoven fabric. By adding the heat-sealing fiber, the strength of the wet nonwoven fabric can be imparted by melt adhesion. The melt adhesion includes a form in which the heat-sealing fiber is point-bonded to the fibrillated lyocell fiber. Examples of the heat-sealing fiber include polyvinyl alcohol fiber, polyester fiber, polyolefin fiber, etc. The form of the heat-sealing fiber includes a side-by-side type heat-sealing fiber in which a melting part and a non-melting part are combined side by side, a core-sheath type heat-sealing fiber having a non-melting core part and a melting sheath part, and a full melt type heat-sealing fiber in which the entire fiber melts and contributes to the adhesion between the main fibers such as glass fibers. Among these, composite heat-fusible fibers, including side-by-side heat-fusible fibers and core-sheath heat-fusible fibers, are preferred, which are made up of a non-heat-fusible component that does not melt when heated and a heat-fusible component that melts when heated, and core-sheath heat-fusible fibers, which have a non-heat-fusible component as the core and a heat-fusible component as the sheath, are more preferred. Examples of non-heat-fusible components include polyesters, polyolefins, polyamides, polyurethanes, polyacrylonitrile, cellulose-based polymers, etc., and among these, polyesters such as polyethylene terephthalate and polyolefins such as polypropylene are preferred. Examples of heat-fusible components include polyesters, polyolefins, poly(ethylene-vinyl acetate), poly(ethylene-vinyl alcohol), etc., and among these, polyesters such as copolymerized polyethylene terephthalate and polyolefins such as polyethylene are preferred.

The heat-sealing fibers may be any one of side-by-side type heat-sealing fibers, core-sheath type heat-sealing fibers, and full-melt type heat-sealing fibers, or may be two or three types. Examples of two types include a combination of side-by-side type heat-sealing fibers and core-sheath type heat-sealing fibers, a combination of side-by-side type heat-sealing fibers and full-melt type heat-sealing fibers, or a combination of core-sheath type heat-sealing fibers and full-melt type heat-sealing fibers.

When the heat-sealed fiber is in a wet state, it maintains the shape it had when it was mixed, but when the heat-sealed fiber is heated in the heat-drying process, it melts in part or in whole and bonds with the fibrillated Lyocell fiber or the non-fibrillated non-heat-sealed fiber, which is the main fiber, or bonds with each other. The melted heat-sealed fiber contained in the filter material that has been subjected to the heat-drying process maintains its fiber shape, or although there are some broken parts, the fiber shape can be observed, and only the shape of a dot or line is observed. More specifically, when the fully melted heat-sealed fiber is heated, it melts in its entirety and bonds with the fibrillated Lyocell fiber or the non-fibrillated non-heat-sealed fiber, or bonds with each other. The fully melted heat-sealed fiber that has been melted may have broken, crushed, bent, or other deformed parts, but the fiber shape can be observed, and only the shape of a line or dot is observed. In this embodiment, the only difference between the fully melted heat-fusible fibers is that they are fibrous before and after the heating and drying process, whereas the fully melted heat-fusible fibers have traces of fibrous form, so the fully melted heat-fusible fibers before melting and the melted fully melted heat-fusible fibers are referred to as "fully melted heat-fusible fibers". When the side-by-side heat-fusible fibers or the core-sheath type heat-fusible fibers are heated, the melting part of the side-by-side heat-fusible fibers or the sheath part of the core-sheath type heat-fusible fibers melts and bonds with the fibrillated lyocell fibers, or the side-by-side heat-fusible fibers bond with each other, or the core-sheath type heat-fusible fibers bond with each other. In the melted side-by-side heat-fusible fibers or the melted core-sheath type heat-fusible fibers, the unmelted part of the side-by-side heat-fusible fibers or the core part of the core-sheath type heat-fusible fibers maintain the shape of the fibers. In this embodiment, since the side-by-side type heat-fused fiber or the core-sheath type heat-fused fiber is in a fibrous form both before and after the heat drying process, both the side-by-side type heat-fused fiber or the core-sheath type heat-fused fiber before melting and the melted side-by-side type heat-fused fiber or the core-sheath type heat-fused fiber are referred to as "side-by-side type heat-fused fiber or the core-sheath type heat-fused fiber." When the binder resin is heated by the heat drying process or the like, it melts or the emulsion particles fuse together to form a film, which spreads over the entire filter medium and is distributed in a flat shape.

The heat-sealed fibers in this embodiment have a fiber diameter of 35 μm or more, preferably 35 to 80 μm, and more preferably 38 to 50 μm. By having a fiber diameter of 35 μm or more, a filter medium having a high filtration efficiency and a long filtration life can be obtained when compared with a heat-sealed fiber having a smaller diameter at the same pressure loss. The reason for this is unclear, but it is presumed to be due to the difference in the effect on the fine network structure formed by the fibrillated lyocell fiber. On the other hand, if the fiber diameter exceeds 80 μm, there is a risk that sufficient strength of the filter medium cannot be obtained. Of the heat-sealed fibers contained in the filter medium for liquid filters in this embodiment, the heat-sealed fibers having a fiber diameter of 35 μm or more are preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 100 mass%.

The liquid filter medium according to this embodiment includes fibrillated lyocell fibers and heat-sealed fibers having a fiber diameter of 35 μm or more, and has a wet nonwoven fabric in which at least the fibrillated lyocell fibers form a network structure. Here, the word "at least" is defined because in the following several aspects, fibrillated lyocell fibers are always included as fibers that constitute the network structure. In the first aspect, when the heat-sealed fibers are full-melt heat-sealed fibers and are melted as a whole to serve as binding, a network structure of fibrillated lyocell fibers exists in the wet nonwoven fabric. Here, the fibers are bonded together by the melted heat-sealed fibers. In the second aspect, when the heat-sealed fibers are full-melt heat-sealed fibers and are partially melted to serve as binding while retaining their fiber shape, a network structure formed by fibrillated lyocell fibers and heat-sealed fibers that maintain some of the fiber shape exists in the wet nonwoven fabric. Here, the fibers are bonded together by the remaining melted heat-sealed fibers. As a third aspect, when non-thermal fusion fibers that are not fibrillated are blended, and the thermal fusion fibers are full-melt type thermal fusion fibers that melt as a whole and play a role of binding, a network structure formed by fibrillated lyocell fibers and non-thermal fusion fibers exists in the wet nonwoven fabric. Here, the fibers are bonded to each other by the melted thermal fusion fibers. As a fourth aspect, when non-thermal fusion fibers that are not fibrillated are blended, and the thermal fusion fibers are full-melt type thermal fusion fibers that melt partially and play a role of binding while retaining the fiber shape, a network structure formed by fibrillated lyocell fibers, non-thermal fusion fibers, and thermal fusion fibers that maintain a part of the fiber shape exists in the wet nonwoven fabric. Here, the fibers are bonded to each other by the remaining melted thermal fusion fibers. As a fifth aspect, when the thermal fusion fibers are composite type thermal fusion fibers, a network structure formed by fibrillated lyocell fibers and composite type thermal fusion fibers exists in the wet nonwoven fabric. In the sixth embodiment, when non-fibrillated non-thermal fusion fibers are blended and the thermal fusion fibers are composite thermal fusion fibers, a network structure formed by fibrillated lyocell fibers, non-thermal fusion fibers, and composite thermal fusion fibers exists in the wet-laid nonwoven fabric. The fiber diameter of the thermal fusion fibers is the same or has increased by up to +40% compared to before melting, in all cases where the fiber shape is maintained, the fiber shape can be observed although there are interrupted parts, and only the dot or line shape is observed.

The amount of heat-sealed fibers in this embodiment can be adjusted as appropriate depending on the physical properties required for the filter medium, but is preferably 20 to 80 parts, and more preferably 30 to 70 parts, assuming that the total fiber mass constituting the filter medium is 100 parts. If the amount of heat-sealed fibers is too small, it may be difficult to obtain the strength of the filter medium required during processing and use. If the amount of heat-sealed fibers is too large, the amount of fibrillated lyocell will be small, and it may be difficult to obtain sufficient filtration efficiency.

In this embodiment, non-fibrillated non-thermal fusion fibers can be used. The non-thermal fusion fibers referred to here are fibers made of non-thermal fusion components that do not melt when heated during the manufacturing process of the wet nonwoven fabric. Examples of non-thermal fusion fibers include synthetic fibers such as polyester, polyolefin, polyamide, polyurethane, and polyacrylonitrile; natural fibers such as wood pulp, cotton linter pulp, hemp pulp, kenaf pulp, and mercerized pulp obtained by alkali treatment of wood pulp; and regenerated fibers such as viscose rayon fibers spun by the viscose method using cellulose as a raw material, and lyocell fibers spun by the organic solvent spinning method.

In this embodiment, the non-thermally fusible fibers preferably have a fiber diameter of 5 μm or more, more preferably 5 to 30 μm, and even more preferably 8 to 25 μm. If the fiber diameter is too small, the pressure loss may become too high. If the fiber diameter is too large, the filtration efficiency may decrease.

The amount of non-thermally fusible fibers in this embodiment can be adjusted as appropriate depending on the physical properties required for the filter medium, but it is preferably 0 to 70 parts, and more preferably 5 to 50 parts, assuming that the total fiber mass constituting the filter medium is 100 parts.

In this embodiment, a filter aid such as diatomaceous earth, celite, or perlite may be blended to take advantage of the powder retention ability of the fibrillated lyocell fiber, as long as it does not impede the effects of the present invention. These filter aids can be contained in an amount of about 1 to 40% by mass relative to the fibrillated lyocell fiber.

The basis weight of the filter medium in this embodiment is not particularly limited, but is preferably 20 to 400 g/m ² , and more preferably 30 to 300 g/m ² .

The filter medium of this embodiment is manufactured using a wet papermaking method. Here, the fibers that make up the filter medium are dispersed in water using a dispersing machine such as a pulper, and the resulting raw material slurry is deposited on a wire and dehydrated to form a sheet. The resulting wet sheet is dried using a dryer such as a hot air dryer or a cylinder dryer to obtain the filter medium as a dry sheet that will become the filter medium for liquid filters. The drying temperature is preferably 80 to 180°C, and more preferably 100 to 160°C.

In this embodiment, papermaking aids such as dispersants, defoamers, viscosity agents, retention aids, and paper strength agents may be used in the production using the wet papermaking method as long as they do not impede the effects of the present invention.

The liquid filter medium according to this embodiment can be used as a filter medium using the wet nonwoven fabric alone, but a reinforcing material such as a nonwoven fabric may also be bonded to the wet nonwoven fabric.

The method for producing a filter material for liquid filters according to the present embodiment includes the following aspects.
(1) The fibrillated lyocell fiber has an average fiber diameter of 0.3 μm or more, a maximum fiber diameter of 8 μm or less, and a length-weighted average fiber length of 1 mm or more;
(2) The wetlaid nonwoven fabric contains non-thermofusible fibers that are not fibrillated.
(3) The amount of the fibrillated lyocell fiber is 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium;
(4) The amount of the heat-fusible fiber is 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium.
(5) The heat-fusible fiber is a composite heat-fusible fiber having a non-heat-fusible component that does not melt when heated and a heat-fusible component that melts when heated.
(6) A form in which the blending amount of the non-thermal fusion fibers is 0 to 70 parts per 100 parts of the total fiber mass constituting the filter medium, or (7) a form in which the non-thermal fusion fibers are fibers having a fiber diameter of 5 μm or more.

The present invention will be described below with specific examples, but the present invention is not limited to these descriptions. In the examples, "parts" indicates the solid mass ratio of the fibers in the raw material slurry, and the total amount of all fibers is 100 parts.

Example 1
Lyocell fiber (fineness 1.7 dtex (fiber diameter 12 μm), fiber length 4 mm, manufacturer: Lenzing AG) was beaten using a Niagara beater to obtain fibrillated lyocell fiber with an average fiber diameter of 0.7 μm, a maximum fiber diameter of 3.5 μm, and a length-weighted average fiber length of 1.1 mm.
Next, 50 parts of the obtained fibrillated lyocell fiber and 50 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.) were used as raw fiber, and disintegrated in water in a standard disintegrator to obtain raw material slurry. The obtained raw material slurry was made into paper in a manual papermaking device to obtain wet paper. The obtained wet paper was dried in a rotary dryer at 130 ° C. to obtain a liquid filter medium with a basis weight of 140 g / m ² .

Example 2
As raw fibers, 50 parts of fibrillated lyocell fiber obtained in Example 1, 40 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.), and 10 parts of non-heat-fused PET fiber (product name: N081, fineness 1.6 dtx (fiber diameter 12 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.) as non-heat-fused fiber that is not fibrillated are used, and otherwise, by the same method as in Example 1, a filter material for liquid filters with a basis weight of 141 g/ ^m2 is obtained.

Example 3
As raw fibers, 50 parts of fibrillated lyocell fiber obtained in Example 1, 40 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.), and 10 parts of lyocell fiber (fineness 1.7 dtx (fiber diameter 12 μm), fiber length 4 mm, manufacturer: Lenzing AG) as non-fibrillated non-heat-fused fiber were used, and otherwise, a filter material for liquid filters with a basis weight of 142 g/ ^m2 was obtained by the same method as in Example 1.

Example 4
As raw material fibers, 25 parts of fibrillated lyocell fiber obtained in Example 1 and 75 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.) are used, and otherwise, by the same method as in Example 1, a filter material for liquid filters with a basis weight of 142 g/ ^m2 is obtained.

Example 5
As raw material fibers, 75 parts of fibrillated lyocell fiber obtained in Example 1 and 25 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.) were used, and otherwise, the same method as in Example 1 was used to obtain a filter medium for liquid filters with a basis weight of 140 g/ ^m2 .

Example 6
As raw fibers, 20 parts of fibrillated lyocell fiber obtained in Example 1, 20 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 17 dtx (fiber diameter 40 μm), fiber length 10 mm, manufacturer: Unitika Co., Ltd.), and 60 parts of lyocell fiber (fineness 1.7 dtx (fiber diameter 12 μm), fiber length 4 mm, manufacturer: Lenzing AG) as non-fibrillated non-heat-fused fiber were used, and otherwise, a filter medium for liquid filters with a basis weight of 140 g/ ^m2 was obtained by the same method as in Example 1.

<Comparative Example 1>
As raw material fibers, 50 parts of fibrillated lyocell fiber obtained in Example 1 and 50 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.) are used, and otherwise, by the same method as in Example 1, a filter medium for liquid filters with a basis weight of 140 g/ ^m2 is obtained.

<Comparative Example 2>
As raw fibers, 50 parts of fibrillated lyocell fiber obtained in Example 1, 40 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.), and 10 parts of non-heat-fused PET fiber (product name: N081, fineness 1.6 dtx (fiber diameter 12 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.) as non-heat-fused fiber that is not fibrillated are used, and otherwise, by the same method as in Example 1, a filter material for liquid filters with a basis weight of 141 g/ ^m2 is obtained.

<Comparative Example 3>
As raw fibers, 50 parts of fibrillated lyocell fiber obtained in Example 1, 40 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.), and 10 parts of lyocell fiber (fineness 1.7 dtx (fiber diameter 12 μm), fiber length 4 mm, manufacturer: Lenzing AG) as non-fibrillated non-heat-fused fiber were used, and otherwise, a filter medium for liquid filters with a basis weight of 140 g/ ^m2 was obtained by the same method as in Example 1.

<Comparative Example 4>
As raw material fibers, 25 parts of fibrillated lyocell fiber obtained in Example 1 and 75 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.) are used, and otherwise, by the same method as in Example 1, a filter material for liquid filters with a basis weight of 141 g/ ^m2 is obtained.

<Comparative Example 5>
As raw material fibers, 75 parts of fibrillated lyocell fiber obtained in Example 1 and 25 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.) were used, and otherwise, the same method as in Example 1 was used to obtain a liquid filter medium with a basis weight of 142 g/ ^m2 .

<Comparative Example 6>
As raw fibers, 20 parts of fibrillated lyocell fiber obtained in Example 1, 20 parts of core-sheath type heat-fused PET fiber (product name: Melty 4080, fineness 1.7 dtx (fiber diameter 13 μm), fiber length 5 mm, manufacturer: Unitika Co., Ltd.), and 60 parts of lyocell fiber (fineness 1.7 dtx (fiber diameter 12 μm), fiber length 4 mm, manufacturer: Lenzing AG) as non-fibrillated non-heat-fused fiber were used, and otherwise, a filter medium for liquid filters with a basis weight of 140 g/ ^m2 was obtained by the same method as in Example 1.

The liquid filter media obtained in the examples and comparative examples were evaluated using the methods described below.

<Basic weight [g/m ² ]>
The basis weight was measured according to JIS P 8124:2011.

<Thickness [mm]>
The thickness was measured using a digital thickness gauge (manufacturer: Sagawa Corporation) in accordance with JIS P 8118; 1998. The measurement pressure was 50 kPa.

<Density [g/cm ³ ]>
The density was measured in accordance with JIS P 8118;1998.

<Pressure loss [Pa]>
The pressure loss was measured by using a differential pressure gauge (product name: Manostar Gauge, manufacturer: Yamamoto Electric Works, Ltd.) to measure the differential pressure when air was passed through a liquid filter medium having an effective area of 100 ^cm2 at a face velocity of 5.3 cm/sec.

<Filtration efficiency [%]>
Filtration efficiency is measured by passing the aqueous dispersion (hereinafter referred to as "raw solution" ⁾ of test particles (product name: Green Densic GC#8000, manufacturer: Showa Denko Co., Ltd.) with a mass concentration of 100 ppm through a filter material for liquid filters with an effective area of 12.5 cm2 at a filtration flow rate of 20 ml/min, and filtrate 50 ml after 1 minute of passing the liquid through the filter material. The turbidity [%] of the raw solution and the filtrate is measured using a turbidity meter (product name: COH400, manufacturer: Nippon Denshoku Kogyo Co., Ltd.), and filtration efficiency is calculated using the formula shown in Math.

(Equation 1)
Filtration efficiency [%] = (raw solution turbidity [%] - filtrate turbidity [%]) / (raw solution turbidity [%]) × 100

<Filtration life [min]>
In the measurement of the filtration efficiency, the upstream pressure of the filter medium was measured using a pressure gauge (product name: KDM30, manufacturer: Krone Co., Ltd.), and the time required for the pressure to reach 100 kPa from the start of liquid passage was measured.
<Tensile strength [kN/m]>
The tensile strength was measured using a universal testing machine (product name: Autograph AGS-X, manufacturer: Shimadzu Corporation) in accordance with JIS P 8113-2006 "Paper and paperboard-Testing methods for tensile properties-Part 2: Constant speed extension method."

The evaluation results of the filter media for liquid filters performed using the above-mentioned method are shown in Tables 1 and 2. Based on the results of Examples 1 to 6 and Comparative Examples 1 to 6, the relationship between pressure loss and filtration efficiency is shown in Figure 1, and the relationship between pressure loss and filtration life is shown in Figure 2.

The results in Tables 1 and 2 and Figures 1 and 2 show that, in terms of the relationship between pressure loss and filtration efficiency, and between pressure loss and filtration life, the Examples have higher filtration efficiency and longer filtration life than the Comparative Examples.

Claims (11)

A filter medium for liquid filters, comprising fibrillated lyocell fibers and heat-fused fibers having a fiber diameter of 35 μm or more, and characterized in having a wet-laid nonwoven fabric in which at least the fibrillated lyocell fibers form a network structure. The filter medium for liquid filters according to claim 1, characterized in that the fibrillated lyocell fibers have an average fiber diameter of 0.3 μm or more, a maximum fiber diameter of 8 μm or less, and a length-weighted average fiber length of 1 mm or more. The liquid filter medium according to claim 1 or 2, characterized in that the wet-laid nonwoven fabric contains non-fibrillated non-thermal fusion fibers. A filter medium for liquid filters according to any one of claims 1 to 3, characterized in that the amount of the fibrillated lyocell fibers is 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium. A filter medium for liquid filters according to any one of claims 1 to 4, characterized in that the blending amount of the heat-sealing fibers is 20 to 80 parts per 100 parts of the total fiber mass constituting the filter medium. The filter material for liquid filters according to any one of claims 1 to 5, characterized in that the heat-sealing fiber is a composite heat-sealing fiber having a non-heat-sealing component that does not melt when heated and a heat-sealing component that melts when heated. The filter medium for liquid filters according to claim 3, characterized in that the blending amount of the non-thermal fusion fibers is a maximum of 70 parts per 100 parts of the total fiber mass constituting the filter medium. 3. The filter medium for liquid filters according to claim 1, characterized in that the filter medium contains only the fibrillated lyocell fibers and the heat-fused fibers having a fiber diameter of 35 μm or more as fibers constituting the filter medium . The filter material for liquid filters according to claim 3 or 7, characterized in that the non-thermal fusion fibers have a fiber diameter of 5 μm or more. A step of preparing a raw material slurry containing fibrillated lyocell fiber and a heat-fusible fiber having a fiber diameter of 35 μm or more;
forming a wet sheet from the raw slurry;
and drying the wet sheet to obtain a dry sheet which will become a filter material for liquid filters. The method for producing a filter medium for liquid filters according to claim 10, wherein the heat-fusible fibers are fibers containing a heat-fusible component that melts when heated in the step of obtaining the dry sheet.