JP6226572B2 - Nonwoven fabric and method for producing the same - Google Patents

Description

  The present invention relates to a nonwoven fabric and a method for producing the same. More specifically, the present invention relates to a nonwoven fabric excellent in heat resistance and chemical resistance, including ultrafine fibers mainly composed of polybenzimidazole, and a method for producing the same. Since the nonwoven fabric of the present invention is excellent in heat resistance and chemical resistance, it can be suitably used as a filter, separator, support, cushioning material, or protective clothing.

  Polybenzimidazole has a thermal decomposition temperature exceeding 600 ° C., a high glass transition temperature of about 430 ° C., excellent heat resistance, and excellent mechanical and electrical characteristics. , And can be suitably used for applications requiring these characteristics.

  For example, a fiber comprising a blend of polybenzimidazole and an aromatic polyamide, an aromatic polyamide-hydrazide or an aromatic polyamide containing a heterocyclic bond is post-treated with heat and a sulfonating agent to be thermally cured and later heated. It discloses minimizing shrinkage when exposed and enhancing resistance to solvents and acids (Patent Document 1). However, when thermosetting by such a method, the sulfonating agent may be detached by the organic solvent, and it cannot be said that the chemical resistance is sufficient. For example, when a polymer electrolyte membrane is produced using a dispersion solution in which the fibers are dispersed in a solution in which a polymer electrolyte is dissolved, the fibers are dissolved, and it is expected that the reinforcing action cannot be sufficiently exhibited. It was done. Moreover, when it was set as the ultrafine fiber with a small fiber diameter, it tried to strengthen the tolerance with respect to a solvent and an acid by heat-processing after processing with a sulfonating agent, but the ultrafine fiber melt | dissolved and it was excellent in desired tolerance. It was difficult to produce ultrafine fibers.

JP 2012-238590 A

  This invention is made | formed under the above situations, and aims at providing the nonwoven fabric excellent in heat resistance and chemical resistance, and its manufacturing method.

The present invention provides: “In addition to polybenzimidazole, a thermosetting resin or a compound having an epoxy group that can be cross-linked with polybenzimidazole has a mass of the polybenzimidazole of 100, and the thermosetting resin or A step of preparing a spinning solution mainly composed of polybenzimidazole containing an epoxy compound in a mass of 6 to 100, spinning the spinning solution to form ultrafine fibers having an average fiber diameter of 2 μm or less, and direct accumulation The step of forming a fiber web, and heat treatment of the fiber web, comprising polybenzimidazole as a main component and a thermosetting resin crosslinked with the polybenzimidazole or a compound having an epoxy group, A process for forming a non-woven fabric composed of 100 mass% of ultrafine fibers with an average fiber diameter of 2 μm or less A method for producing a nonwoven fabric having the spinning solution, in addition to the polybenzimidazole prepared solvent capable of dissolving the thermosetting resin or the epoxy compound, by dissolving in the solvent A method for producing a nonwoven fabric, which is a prepared spinning solution, wherein the mass of the nonwoven fabric after being immersed in an N-methyl-2-pyrrolidone solution at a temperature of 80 ° C. for 30 minutes is 85% or more of the mass before immersion. . "

The present invention can produce a nonwoven fabric that is excellent in heat resistance and chemical resistance, and also has a good balance of chemical resistance in addition to the heat resistance, mechanical characteristics, and electrical characteristics of PBI.

  Since the nonwoven fabric of the present invention contains ultrafine fibers mainly composed of polybenzimidazole and having an average fiber diameter of 2 μm or less, it is excellent in heat resistance, and is excellent in mechanical properties and electrical properties.

  The polybenzimidazole (hereinafter sometimes referred to as “PBI”) that constitutes the ultrafine fiber of the present invention is not particularly limited. For example, the repeating unit represented by the following formula (I) or (II) It can have.

式（ＩＩ）において、ＹはＯ及びＳから選択される置換元素、又は炭素間結合（例えば、−Ｏ−、−ＣＯ−、−ＳＯ_２−などの二価の基）である。

In the formula (II), Y is a substituent selected from O and S, or a carbon-carbon bond (for example, a divalent group such as —O—, —CO—, —SO ₂ —).

Z is divalent C ₁ -C ₁₀ alkanediyl, divalent C ₂ -C ₁₀ alkene diyl, divalent C ₆ -C ₁₅ aryl, divalent C ₅ -C ₁₅ heteroaryl, divalent C ₅ -C ₁₅ heterocyclyl. A divalent group selected from the group consisting of divalent C ₆ -C ₁₉ aryl sulfone and divalent C ₆ -C ₁₉ aryl ether, and having at least one aromatic ring. For example, functional groups having the following groups are preferred.

  The ultrafine fiber of the present invention is mainly composed of polybenzimidazole (PBI) as described above (50 mass% or more), but preferably contains a thermosetting resin or a compound having an epoxy group. . This is because when the thermosetting resin is contained, the thermosetting resin reacts with polybenzimidazole to crosslink the polymer chain and has excellent chemical resistance. In addition, if it contains a compound having an epoxy group (hereinafter sometimes referred to as “epoxy compound”), the epoxy compound reacts with polybenzimidazole to crosslink the polymer chain and has excellent chemical resistance. Because.

  The former thermosetting resin is not particularly limited, and examples thereof include a phenol resin and an epoxy resin.

  The latter epoxy compound is not particularly limited, but epoxy silane, glycidyl phenyl ether, 1,4-butanediol diglycidyl ether, butyl glycidyl ether, allyl glycidyl ether, glycidyl methacrylate, 2-ethylhexyl glycidyl ether, ethylene Glycol glycidyl ether, 1,4-bis (glycidyl) oxybenzene, and propylene glycol diglycidyl ether can be exemplified, but the boiling point is higher than that of the solvent constituting the spinning solution at the time of spinning so that it can effectively act as a curing agent Is preferably an epoxy compound having a boiling point higher than 10 ° C., more preferably higher than 20 ° C., more preferably higher than 30 ° C. If the boiling point of the epoxy compound is less than or equal to the boiling point of the solvent constituting the spinning solution, for example, the spinning solution containing the epoxy compound is volatilized together with the solvent when spinning to fiberize, effectively acting as a curing agent. This is because the chemical resistance tends to be inferior as a result. For example, when dimethylacetamide (DMAc) is used as the solvent for the spinning solution, since the boiling point of DMAc is 165 ° C., the boiling point is higher than 165 ° C. (preferably 175 ° C. or higher, more preferably 185 ° C. or higher, more preferably Is preferably an epoxy compound (for example, epoxy silane, glycidyl phenyl ether, 1,4-butanediol diglycidyl ether).

  The ultrafine fiber of the present invention is mainly composed of polybenzimidazole (PBI) as described above, and preferably contains a thermosetting resin or an epoxy compound (particularly, it consists of PBI and a thermosetting resin or an epoxy compound). In addition to heat resistance, mechanical properties, and electrical properties due to PBI, it also has excellent chemical resistance. Thus, when the thermosetting resin or epoxy compound is contained, the thermosetting resin or epoxy compound has a mass of PBI of 100 in the ultrafine fiber so that the balance of the performance is excellent. The mass is preferably 6 to 100, and more preferably 7 to 50.

  The fibers constituting the nonwoven fabric of the present invention are excellent in various performances such as separation performance, liquid retention performance, wiping performance, concealment performance, insulation performance, flexibility, and contribution to miniaturization. Is 2 μm or less. For example, if the nonwoven fabric of the present invention is used as a membrane support, it can contribute to the production of a strong thin film, and if it is used as a separator for an electrochemical element, it can provide electrical insulation performance and electrolyte retention performance. It is excellent, and if it is used as a filter material for a filter, a filter with excellent filtration accuracy can be obtained.

  The smaller the average fiber diameter is, the better the performance is. Therefore, it is preferably 1 μm or less, more preferably 800 nm or less, further preferably 600 nm or less, and further preferably 400 nm or less. . On the other hand, the lower limit of the average fiber diameter is not particularly limited, but is preferably 1 nm or more so as to be excellent in strength. Further, “fiber diameter” in the present invention means a fiber diameter obtained by measurement from an electron micrograph on the surface of the nonwoven fabric, and “average fiber diameter” means an arithmetic average value of fiber diameters at 50 locations.

  The fiber length of the ultrafine fiber of the present invention is not particularly limited, but is preferably 0.1 mm or more so that the ultrafine fiber is not easily dropped from the nonwoven fabric. In particular, it is preferable that the ultrafine fiber is a continuous fiber because it has excellent fiber drop prevention properties and has almost no fiber end portion, and the risk of damaging other materials by the fiber end portion is extremely low.

  The nonwoven fabric of the present invention contains the ultrafine fibers as described above, but the more ultrafine fibers, the better the heat resistance, mechanical properties, electrical properties, and chemical resistance. 50 mass% or more, preferably 70 mass% or more, more preferably 90 mass% or more, and most preferably 100 mass% ultrafine fibers.

  In addition, fibers other than the ultrafine fibers constituting the nonwoven fabric can be fibers that do not have PBI as a main component (less than 50 mass%) and fibers that have PBI as a main component and have an average fiber diameter of more than 2 μm. Examples of the resin constituting the fiber not mainly composed of PBI include synthetic resins such as polyamide resin, polyester resin, acrylic resin, polyolefin resin (polypropylene resin, polyethylene resin, etc.).

  The nonwoven fabric of the present invention contains the above-described ultrafine fibers, but the nonwoven fabric is contained in an N-methyl-2-pyrrolidone solution (hereinafter sometimes referred to as “NMP solution”) at a temperature of 80 ° C. The mass of the non-woven fabric after being immersed for 30 minutes has excellent chemical resistance of 85% or more of the mass before immersion. That is, although the NMP solution is a solution that is also used as a solvent for polybenzimidazole, the nonwoven fabric containing ultrafine fibers mainly composed of PBI of the present invention has a temperature of 80 ° C., in which PBI is easily dissolved by the NMP solution. Even when exposed to an environment of 30 minutes, only 15 mass% or less dissolves, that is, 85 mass% or more does not dissolve, and has excellent chemical resistance that the mass can be maintained. It means that the greater the mass of the nonwoven fabric after immersion, the better the chemical resistance. Therefore, the mass of the nonwoven fabric after immersion is preferably 90 mass% or more of the mass of the nonwoven fabric before immersion, and 92 mass%. More preferably, it is more preferably 94 mass% or more.

  In addition, the measurement of the mass change before and after the immersion of the nonwoven fabric in the NMP solution is performed while maintaining the temperature of the NMP solution at 80 ° C. Moreover, as long as the whole nonwoven fabric can be immersed in a NMP solution, what quantity may be sufficient as a nonwoven fabric amount and a NMP solution amount, It does not specifically limit. Further, after immersion for 30 minutes, the nonwoven fabric is washed with pure water, dried in an oven set at a temperature of 130 ° C., and then the mass is measured.

  The nonwoven fabric of the present invention exhibits excellent chemical resistance, but also exhibits excellent chemical resistance with respect to other than N-methyl-2-pyrrolidone. For example, it exhibits excellent chemical resistance against N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, and toluene, which can be used as a solvent for PBI. That is, instead of NMP, when chemical resistance is evaluated using N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, or toluene, it is immersed in any solvent. The mass of the subsequent nonwoven fabric can be 85% or more of the mass of the nonwoven fabric before immersion.

The basis weight of the nonwoven fabric of the present invention (value measured as 10 cm × 10 cm according to JIS L1085) is not particularly limited, but is preferably 0.1 to ²⁰ g / m ² , and preferably 0.5 to 18 g / m 2. ² is more preferable, and 1 to 15 g / m ² is even more preferable. The thickness of the nonwoven fabric is a value (μm) measured using an outer micrometer at 5N load, preferably 2 to 100 μm, and more preferably 3 to 80 μm.

  Such a nonwoven fabric excellent in heat resistance, mechanical properties, electrical properties and chemical resistance of the present invention includes, for example, polybenzimidazole containing a thermosetting resin or a compound having an epoxy group in addition to polybenzimidazole. A step of preparing a spinning solution mainly comprising: a step of spinning the spinning solution to form an ultrafine fiber having an average fiber diameter of 2 μm or less, a direct accumulation to form a fiber web, and a heat treatment of the fiber web. It can manufacture by the process of forming a nonwoven fabric.

  First, polybenzimidazole, a thermosetting resin, or an epoxy compound is prepared, and a spinning solution mainly composed of PBI containing PBI and a thermosetting resin or a compound having an epoxy group is prepared. As described above, a phenol resin or an epoxy resin can be used as the thermosetting resin, and the epoxy compound does not volatilize at the time of spinning, so that it can effectively act as a curing agent than the solvent of the spinning solution. It is preferable to use an epoxy compound having a high boiling point (preferably a boiling point higher by 10 ° C. or higher, more preferably a boiling point higher by 20 ° C. or higher, more preferably a boiling point higher by 30 ° C. or higher). For example, when the solvent of the spinning solution is DMAc, since the boiling point of DMAc is 165 ° C., the boiling point is higher than 165 ° C. (preferably 175 ° C. or higher, more preferably 185 ° C. or higher, more preferably 195 ° C. or higher). It is preferable to use an epoxy compound (for example, epoxy silane, glycidyl phenyl ether, 1,4-butanediol diglycidyl ether, etc.).

  The spinning solution can be prepared by preparing a solvent capable of dissolving a thermosetting resin or an epoxy compound in addition to PBI and dissolving it in the solvent. Since this solvent varies depending on the thermosetting resin or epoxy compound, it is not particularly limited. For example, N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, Toluene can be mentioned. A mixed solvent in which two or more kinds of solvents are mixed may be used.

  The spinning solution preferably contains a salt in addition to the thermosetting resin or the epoxy compound. It is because the fiber diameter of the ultrafine fiber is likely to be reduced and the variation in the fiber diameter can be reduced by increasing the conductivity of the spinning solution containing salt.

  Examples of such salts include chloride salts such as lithium chloride, sodium chloride, potassium chloride, calcium chloride, and ammonium chloride; phosphates such as sodium phosphate, calcium phosphate, and sodium dihydrogen phosphate; nitrous acid Nitrites such as sodium; nitrates such as aluminum nitrate, potassium nitrate, calcium nitrate and sodium nitrate; acetates such as calcium acetate and sodium acetate; carbonates such as potassium carbonate, calcium carbonate and sodium carbonate; aluminum sulfate, ammonium sulfate and potassium sulfate And sulfates such as calcium sulfate, sodium sulfate, and barium sulfate.

  In addition, the spinning solution can be prepared by dissolving PBI and a thermosetting resin or epoxy compound in the same solvent, or after dissolving PBI and the thermosetting resin or epoxy compound in another solvent to prepare a solution, respectively. Can be prepared by mixing. When preparing a spinning solution containing PBI and a thermosetting resin or an epoxy compound, mixing PBI with a thermosetting resin or an epoxy compound so that the PBI in the resulting ultrafine fiber is the main component (50 mass% or more). Adjust the ratio.

  Furthermore, the concentration of the spinning solution is not particularly limited as long as it is a concentration at which PBI and the thermosetting resin or epoxy compound can be dissolved, and can be 5 to 40 mass%. In addition, in order to manufacture an ultrafine fiber with a thinner average fiber diameter, it is preferable that it is 10-30 mass%.

  Next, the spinning solution is spun to form ultrafine fibers having an average fiber diameter of 2 μm or less, and directly accumulated to form a fiber web. Thus, the method of forming ultrafine fibers having an average fiber diameter of 2 μm or less and directly accumulating them to form a fiber web is not particularly limited. For example, an electrostatic spinning method, Japanese Patent Application Laid-Open No. 2009-287138. The method of spinning by the shearing action of gas as disclosed in the publication, or the spinning by applying the shearing force of gas in addition to the action of the electric field as disclosed in JP 2011-32593 A The fiber spun by the method can be directly accumulated on a collecting body such as a drum or a net to form a fiber web. Among these, the electrospinning method is suitable because it is easy to spin ultrafine fibers having an average fiber diameter of 2 μm or less, the fiber diameters are uniform, and continuous ultrafine fibers are easily spun.

  Next, by heating the fiber web, a nonwoven fabric having a mass after being immersed in an N-methyl-2-pyrrolidone solution at a temperature of 80 ° C. for 30 minutes is 85% or more of the mass before the immersion. This heat treatment is a heat treatment in which the thermosetting resin is thermally cured when a thermosetting resin is contained in the ultrafine fiber, and an epoxy compound when the epoxy compound is contained in the ultrafine fiber. Is a heat treatment that acts as a curing agent. Thus, since the heat treatment varies depending on the type of the thermosetting resin or epoxy compound, it is not particularly limited. For example, when the thermosetting resin is an epoxy resin, the heat treatment is performed at a temperature of 180 ° C. for 30 minutes. It can carry out and can manufacture the nonwoven fabric of this invention.

  Thus, according to the method for producing a nonwoven fabric of the present invention, a nonwoven fabric excellent in heat resistance, mechanical properties, electrical properties and chemical resistance can be obtained by simply heat-treating a fiber web in which spun fibers are directly accumulated. Since it can manufacture, it is a simple manufacturing method of a nonwoven fabric. Further, since it can be produced by heat treatment at a relatively low temperature, it is advantageous in terms of energy or cost.

  Although the nonwoven fabric of this invention can be manufactured by the above methods, if necessary, various post-processing can be implemented so that a nonwoven fabric may suit various uses. For example, calendar treatment, hydrophilic treatment, water repellency treatment, surfactant application treatment, pure water washing treatment, and the like can be performed.

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

(Examples 1-4, Comparative Examples 1-3)
Polybenzimidazole (PBI, manufactured by Sato Light Industrial Co., Ltd., PBI DOPE S26) was added to dimethylacetamide (DMAc, boiling point: 165 ° C.) to prepare a dissolution solution having a concentration of 19 mass%. Then, 1,4-butanediol diglycidyl ether (BDE, boiling point: 266 ° C.) was 10 (Example 1) and 7 (Example 2), respectively, with respect to the solid mass (100) of PBI of the dissolved solution. 20 (Example 3), 100 (Example 4), 3 (Comparative Example 1), 5 (Comparative Example 2) or 400 (Comparative Example 3) were dissolved to prepare a spinning solution.

  Subsequently, the spinning solution was electrostatically spun into fibers by the following conditions, and directly accumulated on a drum as a collecting body to form a fiber web.

<Electrostatic spinning conditions>
(1) Discharge amount per nozzle: 1 g / hour (2) Distance between nozzle tip and collector (drum) surface: 8 cm
(3) Applied voltage: 20 kV
(4) Spinning space atmosphere: temperature 25 ° C./relative humidity 30% RH

Then, the fiber web was heat-treated for 30 minutes in an oven set at a temperature of 180 ° C., and a non-woven fabric consisting only of ultrafine continuous fibers having an average fiber diameter shown in Table 1 (weight per unit: 5 g / m ² , thickness: 25 μm) ) Were produced respectively.

  Table 1 also shows the percentage of the mass of the nonwoven fabric with respect to the mass before immersion after immersion of the nonwoven fabric in an N-methyl-2-pyrrolidone solution at a temperature of 80 ° C. for 30 minutes. In addition, after immersing the nonwoven fabric of Examples 1-4 in NMP solution, although the surface state was observed with the electron microscope, the wrinkle by shrinkage | contraction of a nonwoven fabric was not observed, On the other hand, When the surface state was observed with an electron microscope after the nonwoven fabric was immersed in the NMP solution, wrinkles due to shrinkage of the nonwoven fabric were observed. In Comparative Example 3, an attempt was made to produce a non-woven fabric, but the ultrafine continuous fibers could not be formed or a part was dissolved after the ultrafine continuous fibers were formed. Therefore, measurement of the average fiber diameter and evaluation of chemical resistance were performed. There wasn't.

  As is clear from Table 1, the nonwoven fabric of the present invention was excellent in chemical resistance with a mass after immersion of 85% or more of the mass before immersion.

(Examples 5-7, Comparative Examples 4-5)
In dimethylacetamide (DMAc), a polybenzimidazole (PBI, manufactured by Sato Light Industry Co., Ltd., PBI DOPE S26) having a concentration of 19 mass% was prepared. Thereafter, with respect to the solid mass (100) of PBI of the dissolved solution, phenol resin [Asahi Organic Industry Co., Ltd., HP3000A (Example 5)], epoxy group-containing silane coupling agent [Toray, SH6040, boiling point: 290 ° C. (Example 6)], epoxy resin [manufactured by DIC, EPICLON N-660 (Example 7)], butyl glycidyl ether (boiling point: 165 ° C., comparative example 4), allyl glycidyl ether (boiling point: 153.9 ° C.) , Comparative Example 5), except that 20 were dissolved, and the fiber web was formed and heat treated in the same manner as in Example 1 to form a nonwoven fabric composed of ultrafine continuous fibers (weight: 6 g / m ² , thickness Respectively: 28 μm). Table 2 shows the average fiber diameter of the ultra-fine continuous fibers constituting these nonwoven fabrics and the percentage of the mass of the nonwoven fabrics after immersion in an NMP solution at a temperature of 80 ° C. for 30 minutes. In addition, after immersing the nonwoven fabric of Examples 5-7 in a NMP solution, the surface state was observed with the electron microscope, but the wrinkle by shrinkage | contraction of a nonwoven fabric was not observed. Moreover, when the nonwoven fabric of Comparative Example 4 or Comparative Example 5 was immersed in the NMP solution, it was completely dissolved.

  From Table 2, it was found that the nonwoven fabric containing an epoxy compound having a boiling point higher than that of the thermosetting resin or dimethylacetamide is excellent in chemical resistance with a mass after immersion of 85% or more of the mass before immersion. .

(Comparative Example 6)
Polybenzimidazole (PBI, manufactured by Sato Light Industry Co., Ltd., PBI DOPE S26) was dissolved in dimethylacetamide (DMAc) to prepare a spinning solution having a concentration of 19 mass%.

Subsequently, the spinning solution was electrostatically spun under the same conditions as in Example 1 to form a fiber web, and then the formed fiber web was heat-treated in an electric furnace set at a temperature of 300 ° C. for 60 minutes to obtain an average fiber diameter. A non-woven fabric (weighing: 6 g / m ² , thickness: 28 μm) consisting only of 300 nm ultrafine continuous fibers was produced. After this nonwoven fabric was immersed in an NMP solution at a temperature of 80 ° C. for 30 minutes, the percentage of the mass of the nonwoven fabric with respect to the mass before immersion was 50%, which was inferior in chemical resistance. Moreover, after immersing a nonwoven fabric in a NMP solution, when the surface state was observed with the electron microscope, the wrinkles resulting from shrinkage | contraction of a nonwoven fabric were observed.

(Comparative Example 7)
A fiber web formed in the same manner as in Comparative Example 6 was immersed in a sulfuric acid solution (concentration: 5 mass%) for 1 hour at room temperature and then dried in an electric furnace set at a temperature of 350 ° C., and the fiber was dissolved. Therefore, it could not be handled as a sheet.

  Since the nonwoven fabric of the present invention is excellent in heat resistance, mechanical properties, electrical properties and chemical resistance, it can be suitably used as a filter, separator, support, cushioning material, heat insulating material, or protective clothing. More specifically, filters for engines such as space aircraft, filters for bag filters for dust collection equipment such as industrial fuel furnaces, separators for electrochemical elements such as separators for secondary batteries, separators for capacitors, fuel cells, etc. Support materials for electrolyte membranes; cushion materials such as cushion materials used in the manufacturing process in the field of steel and non-ferrous metals; heat insulating materials for precision equipment and various industrial equipment; protective clothing or fire protection that requires heat resistance and solvent resistance It can be suitably used as clothes.

Claims (1)

In addition to polybenzimidazole, a thermosetting resin or a compound having an epoxy group that can crosslink with polybenzimidazole has a mass of the thermosetting resin or epoxy compound when the mass of the polybenzimidazole is 100. A step of preparing a spinning solution mainly composed of polybenzimidazole containing 6 to 100, spinning the spinning solution to form an ultrafine fiber having an average fiber diameter of 2 μm or less, and directly collecting the fiber web; The step of forming and heat-treating the fiber web so that the average fiber diameter is 2 μm, which is mainly composed of polybenzimidazole and composed of a compound having a thermosetting resin or epoxy group crosslinked with the polybenzimidazole. A step of forming a non-woven fabric composed of 100 mass% of the following ultrafine fibers: In the method for producing a woven fabric, the spinning solution is prepared by preparing a solvent capable of dissolving the thermosetting resin or the epoxy compound in addition to the polybenzimidazole and dissolving the solvent in the solvent. A method for producing a nonwoven fabric, which is a spinning solution, wherein the mass of the nonwoven fabric after being immersed in an N-methyl-2-pyrrolidone solution at a temperature of 80 ° C for 30 minutes is 85% or more of the mass before immersion.