JP2024147916A - Branched aramid fiber, sheet using same, and manufacturing method thereof - Google Patents

Abstract

To provide a branched aramid fiber having a specific specific-surface area, and a sheet made of the fiber that is excellent in a dust shedding property, and has a high tensile strength and breaking elongation.SOLUTION: There is provided a branched aramid fiber that is a wholly aromatic polyamide fiber, has a BET specific surface area of 25 m2/g or more and 100 m2/g or less, and has a branched structure.SELECTED DRAWING: Figure 1

Description

The present invention relates to branched aramid fibers, sheets made using the same, and methods for producing the same.

The specific surface area of a fiber is one of the important indicators of the fineness of the fiber shape. Fibers with a large specific surface area have an increased number of contact points where fibers intertwine due to their fine structure, and therefore have the characteristic that it is easy to obtain paper or sheet-like products with excellent strength, for example. In addition, when fibers are combined with functional materials, such as functional material particles such as silica aerogel or activated carbon, the number of contact points with the functional material particles increases, so a composite of a fiber with a large specific surface area and a functional material can be obtained that holds more functional material, and the functional material can be held with a strong grip, which is characterized by the effect of easily suppressing the functional material from falling off.

On the other hand, it is also known that there is a practical upper limit to the size of the specific surface area. For example, when a composite with functional particles is produced using fibers with an excessively large specific surface area, the fiber shape becomes excessively fine compared to the size of the functional particles, and the composite shape may not be maintained. In addition, when a sheet-like composite is produced using a wet papermaking method, for example, the structure becomes too dense, resulting in poor dewatering and requiring a long time for production, which causes problems in productivity. Fibers with a moderate specific surface area are required, and the range is, for example, about 25 to 100 m ² /g.

It is known that there are natural fibers such as cellulose fibers with a specific surface area of about 25 to 100 m ² /g. However, natural fibers are insufficient in physical properties such as heat resistance and flame retardancy, and are often unable to be used. In contrast, fully aromatic polyamide (aramid) fibers are known to be excellent in physical properties such as heat resistance, flame retardancy, and strength. However, due to the high rigidity and crystallinity derived from the chemical structure, it has been difficult to improve the specific surface area to the level of natural fibers. For example, among fully aromatic polyamide fibers, a pulp-shaped fiber shape is an example of a fiber shape with a large specific surface area. As a method for obtaining fully aromatic polyamide fibers in a pulp-shaped form, for example, a method for para-type fully aromatic polyamide pulp disclosed in Patent Document 1 can be mentioned. Patent Document 1 discloses a method in which para-type fully aromatic polyamide fibers are swollen with a swelling liquid containing an amide-based solvent and an inorganic salt, and then wet-pulverized. However. According to the examples, the specific surface area of the para-type wholly aromatic polyamide fiber obtained by the method disclosed in Patent Document 1 is 3.64 g/m2, which can be said to be significantly lower than that of natural fibers.

Another form of fully aromatic polyamide fiber with a large specific surface area is nanofibers. Patent Document 2 discloses aramid nanofibers that are produced by using fully aromatic polyamide fibers as a raw material, immersing and swelling the fibers in a solvent with high affinity, and then adding a strong base to break the hydrogen bonds. According to the method disclosed in Patent Document 2, fully aromatic polyamide fibers with a highly fine fiber shape can be obtained, but if the strong base is not sufficiently removed after obtaining the aramid nanofibers, the fibers may continue to be finely sized, resulting in an excessive increase in the specific surface area.

JP 2014-196575 A JP 2022-17796 A

The present invention was made to solve the above problems, and its purpose is to obtain branched aramid fibers with a suitable specific surface area and a sheet using the same.

As a result of intensive research, the present inventors have found that a branched aramid fiber, which is a wholly aromatic polyamide fiber, has a BET specific surface area of 25 ^m2 /g or more and 100 ^m2 /g or less and is characterized by having a branched structure, can solve the above-mentioned problems.

The fully aromatic polyamide fiber may be a hydrous molded product obtained by a process of introducing a fully aromatic polyamide polymer solution into a water-based coagulation liquid, and may be a branched aramid fiber having a moisture content of 5 to 99%.

Furthermore, the sheet may contain 10 to 90% of the branched aramid fibers.
The sheet contains the branched aramid fibers and a functional material having an average maximum side length of 1 nm to 100 μm, and may contain 10 to 90% of the functional material.

Alternatively, the sheet may be a sheet containing the branched aramid fibers and aramid nanofibers having an average fiber diameter of 100 nm or less.
Alternatively, the sheet may be a sheet containing the branched aramid fibers and aramid nanofibers having an average fiber diameter of 100 nm or less, and having a tensile strength of 50 MPa or more and 70 MPa or less.
Alternatively, the sheet may have an elastic modulus of 1,000 MPa or more and 10,000 MPa or less.

The method for producing branched aramid fibers is characterized in that the wholly aromatic polyamide fibers have a BET specific surface area of 25 ^m2 /g or more and 100 ^m2 /g or less and have a branched structure, the wholly aromatic polyamide fibers being a hydrous molded product obtained through a process of introducing a wholly aromatic polyamide polymer solution into an aqueous coagulation liquid, and the branched aramid fibers are obtained by beating open the hydrous molded product.

By using the above method, it is possible to obtain branched aramid fibers with a high specific surface area and sheets using the same.

1 shows a typical example of the branched aramid fibers of the present invention. 1 shows an example of an observation image of the structure of an aramid nanofiber. FIG. 3 is an example of a structural observation image enlarged from FIG. 2 for calculating the average fiber diameter. FIG. 2 is an explanatory diagram regarding a method for calculating an average fiber diameter.

The details of the present invention are described below.

<Branched aramid fiber>
The branched aramid fiber of the present invention has a BET specific surface area of 25 ^m2 /g or more and 100 ^m2 /g or less (preferably 30 ^m2 /g or more and 100 ^m2 /g or less, more preferably 30 ^m2 /g or more and 65 ^m2 /g or less, and further preferably 40 ^m2 /g or more and 60 ^m2 /g or less). If the BET specific surface area is smaller than the lower limit, the surface area available for entanglement with other functional materials (e.g., aramid nanofibers, etc.) becomes low, and the ability to hold other functional materials becomes insufficient, which is not preferable.

The branched aramid fiber of the present invention is a wholly aromatic polyamide fiber. Examples of wholly aromatic polyamide fibers include para-type wholly aromatic polyamides (poly-p-phenylene terephthalamide, poly-p-benzamide, poly-p-amide hydrazide, poly-p-phenylene terephthalamide-3,4-diphenyl ether terephthalamide, etc.), meta-type wholly aromatic polyamides (poly-m-phenylene isophthalamide, etc.), and copolymers of para-type wholly aromatic polyamides and meta-type wholly aromatic polyamides. Para-type wholly aromatic polyamides are preferred because of their high elasticity and strength.

The branched aramid fiber of the present invention is a fiber having a branched structure. The branched aramid fiber of the present invention is composed of a trunk portion formed by the aggregation of many components, and branch portions branching from the trunk portion. The branched aramid fiber of the present invention may further have a structure including a portion in which the components are arranged in a streamlined manner after being opened flat. Figure 1 shows a typical example of the fiber of the present invention.

<Water-containing molding>
The hydrous molded product in the present invention refers to a thin leaf-like or scale-like piece having fine fibrils, a randomly fibrillated micro short fiber, or a granular particulate material, which is prepared by a method such as mixing an organic polymer solution with a coagulating solution of the polymer solution in a system in which shear force is present, as described in WO2004/099476A1, JP-B-35-11851, JP-B-37-5732, etc. Here, amorphous generally refers to a structure before the formation of a crystal structure based on hydrogen bonds. Furthermore, a hydrous molded product is a product in which water is contained in an amorphous structure. Generally, after coagulation, a polymer is crystallized by drying or stretching, but the crystallization is suppressed by containing a certain amount of water in the coagulated polymer, and therefore the crystallization degree of the hydrous molded product can be said to be correlated to a certain degree with the water content. Although it cannot be said in general, it is estimated that the higher the water content, the lower the crystallization degree, and the lower the water content, the higher the crystallization degree. The lower the crystallinity, the more flexible the fiber is, and the more gripping ability (binder-like properties) it has by entangling with other fiber materials such as aramid nanofibers.

Furthermore, when the water is removed from the hydrous molding through a drying process or the like, the progress of polymer crystallization makes it difficult for a large amount of water to be present in the polymer again, and as a result, the binder-like properties expected in the present invention are not exhibited, and high strength is not achieved, which is undesirable.

Furthermore, the presence of moisture in the hydrous molded product inhibits the crystallization of the polymer, making the polymer itself flexible and not too rigid. When a sheet is obtained in this state by papermaking or the like and then dried or hot pressed, the water is removed from the hydrous molded product in the sheet, promoting the crystallization of the polymer, and the polymer deforms smoothly during this process, which is thought to result in the sheet being highly strong.

For these reasons, the moisture content of the hydrous molding is preferably 5 to 99%. If it is less than 5%, the degree of crystallization will be high and the high strength of the present invention will not be obtained. If it is more than 99%, it is not preferable because it will be mostly water and will be less efficient. More preferably, it is 10% to 99%, and most preferably 20% to 99%.

The hydrous molded product of the present invention is a wholly aromatic polyamide fiber. Examples of wholly aromatic polyamide fibers include para-type wholly aromatic polyamide, meta-type wholly aromatic polyamide, and copolymers of para-type wholly aromatic polyamide and meta-type wholly aromatic polyamide, and para-type wholly aromatic polyamide is preferred because of its high elasticity and strength.

The wholly aromatic polyamide used as the raw material for the wholly aromatic polyamide polymer solution to obtain the hydrous molded product does not have to be a virgin product, and may be, for example, a wholly aromatic polyamide obtained by recycling used products.

<Sheet>
In the present invention, a sheet containing the branched aramid fiber described above can be formed. Such a sheet has excellent powder shedding properties (powder shedding prevention properties) and has good tensile strength and breaking elongation. The tensile strength of the sheet is preferably 30 MPa to 70 MPa, more preferably 40 MPa to 70 MPa, even more preferably 45 MPa to 70 MPa, and particularly preferably 55 MPa to 70 MPa. If the tensile strength exceeds the upper limit, the stiffness increases and flexibility may be impaired.

The breaking elongation of the sheet is preferably 5% or more and 25% or less, more preferably 8% or more and 22% or less, even more preferably 10% or more and 20% or less, and particularly preferably 10% or more and 15% or less.

The tensile modulus of the sheet is preferably 1000 MPa or more and 10,000 MPa or less, more preferably 1000 MPa or more and 5,000 MPa or less, even more preferably 1000 MPa or more and 2,000 MPa or less, and particularly preferably 1500 MPa or more and 2,000 MPa or less.

The sheet in the present invention may be, for example, a sheet made by using a wet papermaking method using only branched aramid fibers, or a mixture of branched aramid fibers and a functional material (e.g., aramid nanofibers) in any ratio, but is not limited thereto.

In addition, the functional material of the present invention preferably has an average length of the maximum side of 1 nm to 100 μm, more preferably 1 nm to 50 μm, even more preferably 1 nm to 10 μm, and particularly preferably 100 nm to 10 μm. The functional material may be in any shape, such as a sphere, a rod, a cylinder, or an indefinite shape. For example, it may be in a rod shape with a specified aspect ratio, such as aramid nanofiber, a spherical shape, such as silica aerogel, or an indefinite shape, such as activated carbon. A network structure is formed by entangling the branch portions of the branched aramid fibers, and the network structure has a high degree of freedom and can sufficiently follow the shape of the functional material, so that it shows a strong grip (binding property) on the functional material. In addition, the functional material may be a single material as described above, or a combination of multiple materials.

If the branched aramid fiber of the present invention further includes a portion in which the components are arranged in a streamlined manner by being opened flat, this structural portion can act as a tray to support the functional material, further improving the gripping force, i.e., improving the powder shedding properties.

That is, the sheet of the present invention may be a sheet containing the branched aramid fibers described above and aramid nanofibers having an average fiber diameter of 100 nm or less as a functional material, in which case the sheet has excellent powder shedding properties and is particularly good in tensile strength and breaking elongation. The tensile strength of the sheet is preferably 50 MPa to 70 MPa, more preferably 55 MPa to 70 MPa, and even more preferably 60 MPa to 70 MPa. The breaking elongation of the sheet is preferably 5% to 25%, more preferably 8% to 22%, even more preferably 10% to 20%, and especially preferably 10% to 15%.

Sheets containing aramid nanofibers have excellent strength, elasticity, thermal stability, etc., and can be used as functional materials for a variety of applications. In particular, sheets containing branched aramid fibers and aramid nanofibers as functional materials, as in the present invention, have excellent tensile strength and elastic modulus, etc., and are expected to be used in fields such as speaker diaphragms and electronic materials.

<Aramid nanofiber>
The aramid nanofibers used in the present invention have an average fiber diameter of 100 nm or less, preferably 50 nm or less, and more preferably 25 nm or less. The lower limit of the average diameter is preferably 1 nm or more, and preferably 3 nm or more. In addition, it is preferable that the aramid nanofibers do not have a diameter of 500 nm or more. If the average fiber diameter exceeds 100 nm, it becomes difficult to achieve the high specific surface area that is a characteristic of nanofibers, and the entanglement density between the nanofibers decreases, which may result in a decrease in the film formability and mechanical properties of the sheet.

The aramid nanofibers of the present invention preferably have an aspect ratio, expressed as fiber length/fiber diameter, of 10 or more and 1,000 or less, more preferably 10 or more and 500 or less, and even more preferably 10 or more and 100 or less. If the aspect ratio is less than 10, it is difficult for the fibers to form an entangled structure, and therefore it may be difficult to achieve the expected characteristics. If the aspect ratio exceeds 1,000, the viscosity of the dispersion liquid or disintegration liquid prepared during sheet production becomes very high, making production difficult.

The aramid nanofibers of the present invention are fibers made of wholly aromatic polyamides, and examples of wholly aromatic polyamides include para-type wholly aromatic polyamides, meta-type wholly aromatic polyamides, and copolymers of para-type wholly aromatic polyamides and meta-type wholly aromatic polyamides. Para-type wholly aromatic polyamides are preferred because of their high elasticity and strength.

The aramid nanofibers of the present invention are produced by immersing and swelling the fibers in a solvent with high affinity, and then adding a strong base to break the hydrogen bonds. The aromatic polyamides that can be preferably used in the present invention are polymers in which one or more divalent aromatic groups are linked by amide bonds, and the aromatic groups may have two or more aromatic rings, and the aromatic rings may be directly bonded or may be bonded via oxygen or sulfur. The hydrogen atoms of the divalent aromatic groups may be substituted with halides, lower alkyl groups, or phenyl groups. Examples of solvents used in producing the aramid nanofibers include dimethyl sulfoxide, dimethylacetamide, and N-methyl-2-pyrrolidone. Examples of strong bases used in producing the aramid nanofibers include potassium hydroxide, sodium hydroxide, barium hydroxide, and calcium hydroxide.

The aramid nanofibers of the present invention can be produced by immersing fully aromatic polyamide (e.g., poly-p-phenylene terephthalamide) fibers or pulp in alkaline dimethyl sulfoxide.

Fully aromatic polyamides (e.g., poly-p-phenylene terephthalamide) are obtained by forming domains of a liquid crystal structure in a spinning solution, discharging the solution from a capillary, and then washing off the spinning solvent with water. The weak bonds between the liquid crystal domains that make up the resulting fiber are cut under alkaline conditions using the above-mentioned method, and the resulting fiber is then liberated in a highly compatible solvent to obtain cut fibers with high elasticity and strength. The cut fibers are then placed in a poor solvent (water, alcohol, acetone, etc.) that serves as a dispersion solvent, making it possible to isolate aramid nanofibers.

As for the fully aromatic polyamide fiber used as the raw material for the aramid nanofiber in the present invention, examples of the fiber using para-type fully aromatic polyamide include poly-p-phenylene terephthalamide fiber (commercially available products include "Twaron" (trademark) manufactured by Teijin Limited and "Kevlar" (trademark) manufactured by Toray DuPont Co., Ltd.) and coparaphenylene-3,4'-oxydiphenylene terephthalamide fiber (commercially available products include "Technora" (trademark) manufactured by Teijin Limited), and examples of the fiber using meta-type fully aromatic polyamide include poly-m-phenylene isophthalamide fiber (commercially available products include "Conex" (trademark) manufactured by Teijin Limited and "Nomex" (trademark) manufactured by DuPont), but are not limited thereto. The fully aromatic polyamide fiber used as the raw material for obtaining the aramid nanofiber does not have to be a virgin product, and may be, for example, a fully aromatic polyamide fiber obtained by recycling used products.

The branched aramid fibers and sheets obtained by the present invention can be used extremely effectively in many applications, including industrial materials, electrical and electronics, agricultural materials, optical materials, and aircraft, automobile, and marine applications. Examples of industrial materials include resin reinforcement materials, functional paper, paper reinforcement materials, thickeners, thickeners, filters, paints, and speaker diaphragms, while electrical and electronic applications include composite paper structures, separators, electrode sheets, and electrode binders for lithium secondary batteries and capacitors, but are not limited to these.

The present invention will be described in detail below with reference to examples and comparative examples, but the scope of the present invention is not limited to the following examples and comparative examples. The characteristic values in the examples were measured by the following methods.

<Average fiber diameter>
The structure of the sample was observed using a scanning electron microscope (manufactured by JEOL Ltd., plate: JSM-6330F). From an image observed at a magnification setting of 50,000 times, an image region of 1,800 nm to 2,000 nm in width and 1,200 nm to 1,500 nm in length was selected, and the image region was further divided vertically into 4 and horizontally into 4 to define a total of 16 grid regions A1-D4. One sample present within each grid region was selected, and the average value of the fiber diameters of the selected samples measured on the image was adopted as the average length of the average fiber diameter (see Figures 3 and 4).

<Average length of the longest side of the functional material>
The structure of the sample was observed using a transmission electron microscope (manufactured by FEI, product number: TECNAI 12 BIO TWIN). In order to prevent the samples to be observed from overlapping with each other and being unable to measure the maximum side length, the sample was sufficiently diluted with a solvent, dropped onto a film, and then dried to obtain a thin slice for observation. From the image observed at a magnification setting of 100 times, an image area of 1.0 mm to 1.2 mm wide and 0.8 mm to 1.0 mm long was selected, and the image area was further divided vertically into four and horizontally into four to define a total of 16 grid areas A1-D4, one sample (functional material) present in each grid area was selected, and the average value of the maximum side length of the selected sample measured on the image was adopted as the average length of the maximum side.

<Tensile test>
A tensile test was carried out according to the test method of JIS P 8113 (2006) except that the sheet obtained in the present invention was cut into strips with a width of 10 mm. A Tensilon universal material testing machine (RTF series) was used as the tensile tester, and the tensile strength, tensile modulus, and elongation at break were measured.

<Specific surface area>
The resulting fibers were freeze-dried to obtain samples for measuring the specific surface area, which were then measured using an automatic flow-type specific surface area measuring device (Shimadzu Corporation, FlowSorbIII).

<Powder shedding>
Using a wet papermaking method, fibers and functional particles were mixed in an arbitrary ratio, and the obtained sheet was dried in an oven. Two points on the long side of the dried sheet were fixed with clips and vibrated for one minute. The change in weight before and after vibration was defined as the amount of functional particles that fell off, and was used to evaluate the powder shedding.

[Example 1]
According to the method described in Japanese Patent No. 4852328, a hydrous extrusion made of para-type wholly aromatic polyamide fiber was obtained. Water was added to this hydrous extrusion to adjust the concentration to 1% by weight, and the extrusion was defibrated to obtain a hydrous extrusion disintegrated liquid. This disintegrated liquid was beaten with a millstone refiner (Hypermass Colloider MKZA-20JA, manufactured by Masuko Sangyo Co., Ltd.) to obtain a fiber (Refined pulp-A-1). The beating was performed three times with a clearance of 100 μm and a refiner rotation speed of 1,500 rpm. The specific surface area of this fiber (Refined pulp-A-1) was measured. It was confirmed that the fiber had a higher specific surface area than Comparative Example 2, which is a hydrous extrusion that was not beaten. It was also confirmed that the fiber had a higher specific surface area than Comparative Example 3, which is an unbeaten aramid pulp, and Comparative Example 4, which is a beaten aramid pulp. The evaluation results are shown in Table 1.

[Example 2]
A disintegrated liquid of defibrated hydrous extrusion was obtained in the same manner as in Example 1. This disintegrated liquid was beaten with a high-pressure homogenizer (N-2000 110, manufactured by COS 21) to obtain a fiber (Refined pulp-A-2). The beating treatment was performed three times with a clearance of 70 μm and a pressure of 150 MPa. The specific surface area of this fiber (Refined pulp-A-2) was evaluated. It was confirmed that a fiber having a higher specific surface area than that of Comparative Example 2, which is an unbeaten hydrous extrusion, was obtained. It was also confirmed that the fiber had a higher specific surface area than that of Comparative Example 3, which is an unbeaten aramid pulp. The evaluation results are shown in Table 1.

[Example 3]
Water was added to Twaron (registered trademark) pulp manufactured by Teijin Aramid Co., Ltd. to prepare a 1% by weight concentration, and the pulp was defibrated to obtain a Twaron pulp disintegration liquid. This disintegration liquid was beaten in a millstone refiner (Hypermass Colloider MKZA-20JA manufactured by Masuko Sangyo Co., Ltd.) to obtain fibers (Refined pulp-B). The beating process was performed three times with a clearance of 100 μm and a refiner rotation speed of 1,500 rpm. The specific surface area of the obtained fibers (Refined pulp-B) was evaluated, and the results are shown in Table 1.

[Comparative Example 1]
Aramid nanofibers were isolated according to the method described in JP 2022-17796 A. The average fiber diameter of the aramid nanofibers was 42 nm, and the average length of the maximum side was 3.0 μm. Water was added to the aramid nanofibers to prepare a concentration of 1% by weight, and the mixture was dispersed to obtain an aramid nanofiber disintegrated liquid. The disintegrated liquid was beaten with a millstone refiner (Hypermasscolloider MKZA-20JA, manufactured by Masuko Sangyo Co., Ltd.), and the specific surface area of the obtained fiber (ANF) was evaluated. The results are shown in Table 1.

[Comparative Example 2]
The specific surface area of the hydrous molded product (pulp-A) made of para-type wholly aromatic polyamide fibers obtained according to the method described in Japanese Patent No. 4,852,328 was evaluated. The results are shown in Table 1.

[Comparative Example 3]
As an example of existing aramid fiber pulp, the specific surface area of Twaron (registered trademark) pulp (Pulp-B) manufactured by Teijin Aramid Ltd. was evaluated. The results are shown in Table 1.

[Example 4]
A sheet was produced by mixing 75% by weight of the fiber (Refined pulp-A-1) obtained in Example 1 and 25% by weight of activated carbon particles as functional material (particles) by a wet papermaking method using a square sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd. The average length of the maximum side of the activated carbon particles was 7.5 μm. The obtained sheet was dried in an oven and then evaluated for dusting. No falling off of the activated carbon particles was confirmed. The evaluation results are shown in Table 2.

[Example 5]
A sheet was produced by mixing 75% by weight of the fiber (Refined pulp-B) obtained in Example 3 and 25% by weight of activated carbon particles as a functional material (particles) by a wet papermaking method using a square sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd. The average length of the maximum side of the activated carbon particles was 7.3 μm. The obtained sheet was dried in an oven and then evaluated for dusting. No falling off of the activated carbon particles was confirmed. The evaluation results are shown in Table 2.

[Comparative Example 4]
A sheet was produced by a wet papermaking method using a square sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd., by mixing 75% by weight of the fiber (pulp-A) of Comparative Example 2 and 25% by weight of activated carbon particles as a model of a functional material (particles). The average length of the maximum side of the activated carbon particles was 7.6 μm. The obtained sheet was dried in an oven and then evaluated for dusting. Falling off of activated carbon particles was confirmed.

[Comparative Example 5]
A sheet was produced by a wet papermaking method using a square sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd., by mixing 75% by weight of the fiber (pulp-B) of Comparative Example 3 and 25% by weight of activated carbon particles as a model of a functional material (particles). The average length of the maximum side of the activated carbon particles was 7.4 μm. The obtained sheet was dried in an oven, and then the dusting property was evaluated. Falling off of the activated carbon particles was confirmed.

[Example 6]
A sheet was produced by mixing 25% by weight of the fiber (Refined pulp-A-1) obtained in Example 1 and 75% by weight of the aramid nanofiber (ANF) obtained in Comparative Example 1 by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd. The obtained sheet was dried in an oven and then evaluated by a tensile test. It showed a higher tensile strength than the sheet containing 100% by weight of aramid nanofiber (ANF) shown in Comparative Example 7, and it was confirmed that the fiber obtained in Example 1 can be a support that can improve the tensile strength of a sheet containing aramid nanofiber. The evaluation results are shown in Table 3.

[Example 7]
A sheet was produced by mixing 75% by weight of the fiber (Refined pulp-A-1) obtained in Example 1 and 25% by weight of the aramid nanofiber (ANF) obtained in Comparative Example 1 by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd. The obtained sheet was dried in an oven and then evaluated by a tensile test. Although the tensile strength was slightly lower than that of the sheet containing 100% by weight of aramid nanofiber (ANF) shown in Comparative Example 7, a large improvement in the breaking strength was confirmed. It was confirmed that the fiber obtained in Example 1 can be a support that can improve the breaking elongation of a sheet containing aramid nanofiber. The evaluation results are shown in Table 3.

[Example 8]
A sheet was produced by mixing 75% by weight of the fiber (Refined pulp-B) obtained in Example 3 and 25% by weight of the aramid nanofiber (ANF) obtained in Comparative Example 1 by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd. The obtained sheet was dried in an oven and then evaluated by a tensile test. It showed a higher tensile strength than the sheet containing 100% by weight of aramid nanofiber (ANF) shown in Comparative Example 7, and it was confirmed that the fiber obtained in Example 3 is a support that improves the tensile strength of the sheet containing aramid nanofiber. The evaluation results are shown in Table 3.

[Example 9]
The fiber (Refined pulp-A-1) obtained in Example 1 was made into a sheet by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd., so that the fiber was 100% by weight. The obtained sheet was dried in an oven and then evaluated by a tensile test. The evaluation results are shown in Table 3.

[Comparative Example 6]
The fiber (Refined pulp-B) obtained in Example 3 was made into a sheet by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd., so that the fiber was 100% by weight. The obtained sheet was dried in an oven and then evaluated by a tensile test. The evaluation results are shown in Table 3.

[Comparative Example 7]
The fiber (ANF) obtained in Comparative Example 1 was made into a sheet by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd., so that the fiber was 100% by weight. The obtained sheet was dried in an oven and then evaluated by a tensile test. The evaluation results are shown in Table 3.

[Comparative Example 8]
The fiber (pulp-A) obtained in Comparative Example 2 was made into a sheet by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd., so that the fiber (pulp-A) obtained in Comparative Example 2 was 100% by weight. The obtained sheet was dried in an oven and then evaluated by a tensile test. The sheet showed a lower tensile strength than the sheet shown in Comparative Example 7. The evaluation results are shown in Table 3.

[Comparative Example 9]
A sheet was produced by mixing 75% by weight of the fiber (pulp-A) obtained in Comparative Example 2 and 25% by weight of the aramid nanofiber (ANF) obtained in Comparative Example 1 by a wet papermaking method using a square sheet machine (250 mm square, No. 2555) manufactured by Kumagai Riki Kogyo Co., Ltd. The obtained sheet was dried in an oven and then evaluated by a tensile test. It showed a lower tensile strength than the sheet containing 100% by weight of aramid nanofiber (ANF) shown in Comparative Example 7, and it was confirmed that the fiber obtained in Comparative Example 2 was a support that reduced the tensile strength of the sheet containing aramid nanofiber. The evaluation results are shown in Table 3.

1 Fiber 11 Trunk portion of fiber 12 Branch portion of fiber 13 Part of fiber in which components are flattened and arranged in a streamlined manner

Claims (8)

A branched aramid fiber is a wholly aromatic polyamide fiber, characterized in that it has a BET specific surface area of 25 m ² /g or more and 100 ^{m 2} /g or less and has a branched structure. The branched aramid fiber according to claim 1, wherein the wholly aromatic polyamide fiber is a hydrous molded product obtained by a process of introducing a wholly aromatic polyamide polymer solution into a water-based coagulation liquid, and the moisture content of the fiber is 5 to 99%. A sheet containing 10 to 90% of the branched aramid fiber described in claim 1. A sheet comprising the branched aramid fiber according to claim 1 and a functional material having an average maximum side length of 1 nm to 100 μm, the sheet comprising 10 to 90% of the functional material. A sheet comprising the branched aramid fiber according to claim 1 and aramid nanofibers having an average fiber diameter of 100 nm or less. A sheet comprising the branched aramid fiber according to claim 1 and aramid nanofibers having an average fiber diameter of 100 nm or less, the sheet having a tensile strength of 50 MPa or more and 70 MPa or less. The sheet according to claims 3 to 6, having an elastic modulus of 1000 MPa or more and 10000 MPa or less. A method for producing a branched aramid fiber, which is a wholly aromatic polyamide fiber having a BET specific surface area of 25 ^m2 /g or more and 100 ^m2 /g or less and has a branched structure, wherein the wholly aromatic polyamide fiber is a hydrous molded product obtained through a process of introducing a wholly aromatic polyamide polymer solution into an aqueous coagulation liquid, and the branched aramid fiber is obtained by beating open the hydrous molded product.