Method for producing polyacrylonitrile fibers with controlled morphology

Abstract

The present disclosure generally relates to a method for producing polymer fibers, typically polyacrylonitrile-based fibers, whose morphology is controlled by using polymer additives to form polymer blends with polyacrylonitrile, which are then subjected to specific coagulation and washing conditions. The present disclosure also relates to carbon fibers produced by processing the produced polymer fibers.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority to U.S. Provisional Patent Application No. 63 / 129891, filed on 23 December 2020, the entire contents of which are incorporated herein by reference.

[0002] This disclosure generally relates to methods for producing polymer fibers, typically polyacrylonitrile-based fibers, whose morphology is controlled by forming a polymer blend with polyacrylonitrile using polymer additives and then subjecting it to specific coagulation and washing conditions. This disclosure also relates to carbon fibers produced by processing the polymer fibers produced. [Background technology]

[0003] Carbon fibers have been used in a wide variety of applications due to their desirable properties, such as high strength and rigidity, high chemical resistance, and low thermal expansion. For example, carbon fibers can be molded into structural components that combine high strength and rigidity while being significantly lighter than metal components with comparable properties. Carbon fibers are increasingly being used as structural components in composite materials, particularly for aerospace and automotive applications. In particular, composite materials have been developed in which carbon fibers function as reinforcing materials in resin or ceramic matrices.

[0004] Over 90% of carbon fibers are derived from polyacrylonitrile (PAN) precursors. Generally, the conversion process of PAN to carbon fibers involves solvent spinning, coagulation, oxidation, stabilization, and then carbonization.

[0005] During solidification, non-solvents, typically water, flow into the polymer solution, while solvents (typically DMF, DMSO, etc.) flow into the bath, generating fiber filaments through interdiffusion. The fiber skin and core structure are initially formed in the first few seconds of solidification, and the filaments are stretched using a continuous bath to remove residual solvent. While the polymer chains can be aligned and crystalline domains added by stretching, manipulating the fiber structure (or morphology), reforming the skin-core structure, or introducing new features after solidification is difficult. Furthermore, there is growing interest in introducing porosity into fibers in the carbon fiber industry. Porous fibers can offer the advantage of allowing resin to penetrate deeper into the fiber, creating larger interfacial phase regions, thereby improving mechanical adhesion and conversion properties in composite materials. Another possible advantage of porous fibers is their application in gas barrier technology, where gas diffusion and / or separation are facilitated by the fibers. Porous fibers can provide lighter and more compressible materials suitable for advanced membranes used in greenhouse gas separation, self-sustaining energy storage materials, and hydrogen production. Furthermore, porous fibers have lower densities, demonstrating potential for producing lighter carbon fibers and potentially serving as an alternative to hollow fibers.

[0006] However, porous fibers are generally considered to have insufficient mechanical properties due to the presence of voids and defects within the fiber. The production of porous fibers is possible by purposefully selecting solidification conditions that promote solvent interdiffusion and rapidly cool the fiber structure to a porous state. However, macrovoids formed during solidification can hinder the stretching and drawability of the fiber. Furthermore, macrovoids formed during the initial stages of spinning can amplify defects that arise if they form very early.

[0007] Techniques for producing porous carbon fibers are known, including physical or chemical activation, carbonization of polymer blends, and molding using nanoparticles and block copolymers. Carbonization of polymer blends requires blending of incompatible polymers that undergo microphase separation into a) matrix-forming carbon source polymers and b) dispersed pore-forming sacrificial polymers. Such sacrificial polymers are then typically burned off by thermal decomposition during the process of forming the porous carbon material. Not only are the pore-forming sacrificial polymers unrecoverable and unusable, but the removal of such polymers during oxidation and carbonization makes the carbon material or fibers more susceptible to further damage, resulting in a decrease in mechanical properties.

[0008] Therefore, there is a continuous need for the development of processes to control the fiber structure (or morphology), such as introducing and manipulating porosity, in polymer fibers, in order to mitigate the impact on the mechanical properties of the manufactured fibers and the carbon fibers subsequently produced therefrom. In this specification, a novel method for controlling fiber morphology is described, which involves forming a polymer blend with polyacrylonitrile using polymer additives and then subjecting it to specific coagulation and washing conditions. [Overview of the Initiative]

[0009] Advantageously, it has been discovered that the morphology of carbon fibers can be controlled when a polymer blend is formed with polyacrylonitrile using polymer additives. The polymer blend is then subjected to specific coagulation and washing conditions to remove the polymer additives in a controlled manner and introduce porosity into the resulting fibers having a controlled morphology. Such fibers can then be converted into carbon fibers. The polymer additives can be recovered and reused, and since the removal of the polymer blend is carried out before oxidation and carbonization, damage and degradation of mechanical properties are avoided.

[0010] In a first aspect, the present disclosure relates to a method for producing polyacrylonitrile fibers having controlled morphology, the method comprising: a) Polyacrylonitrile polymer (polymer A), A polymer different from polyacrylonitrile-based polymers (polymer B), A first liquid containing a solvent for polymer A A step of forming a homogeneous solution containing polymer B and a first liquid in which polymer B is soluble, b) A step of co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid containing a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) A step of selectively removing polymer B from a polyacrylonitrile material by contacting the polyacrylonitrile material with a third liquid containing a non-solvent for polymer A and in which polymer B is soluble, thereby producing a polyacrylonitrile fiber having controlled morphology. Includes.

[0011] In a second aspect, the disclosure relates to polyacrylonitrile fibers manufactured by the methods described herein.

[0012] In a third aspect, the disclosure relates to a method for producing carbon fiber, the method being: (i) A step of producing polyacrylonitrile fibers according to the method described herein, (ii) A step to produce carbon fibers by oxidizing the polyacrylonitrile fibers produced in step (i) to form stabilized carbon fiber precursor fibers, and then carbonizing the stabilized carbon fiber precursor fibers. Includes.

[0013] In a fourth aspect, the disclosure relates to carbon fibers manufactured by the methods described herein.

[0014] In a fifth aspect, the present disclosure relates to a composite material comprising carbon fibers produced according to the method described herein and a matrix resin.

[0015] In a sixth aspect, the present disclosure relates to a composite article obtained by curing the composite material described herein. DETAILED DESCRIPTION OF THE INVENTION

[0016] As used herein, the terms "a", "an", or "the" mean "one or more" or "at least one" unless specifically stated otherwise, and can be used interchangeably with each other.

[0017] As used herein, the term "and / or" used in phrases of the form "A and / or B" means A alone, B alone, or A and B together.

[0018] As used herein, the term "comprises" includes "consists essentially of" and "consists of". The term "comprising" includes "consisting essentially of" and "consists of". "Comprising" is synonymous with "including", "containing", or "characterized by", is intended to be inclusive or open-ended, and does not exclude additional, unrecited elements or steps. The transitional phrase "consisting essentially of" encompasses the specified materials or steps, as well as those that do not materially affect the basic characteristics or functions of the described composition, process, method, or product. The transitional phrase "consisting of" excludes any unrecited element, step, or component.

[0019] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art in which this specification relates.

[0020] Where used herein, unless otherwise specified, the terms “about” or “approximately” mean an acceptable error of a particular value as determined by those skilled in the art, which depends in part on how the value is measured or determined. In certain embodiments, the terms “about” or “approximately” mean within 1, 2, 3, or 4 standard deviations. In certain embodiments, the terms “about” or “approximately” mean within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0021] Furthermore, it will be understood that any numerical range described herein is intended to include all sub-ranges contained therein. For example, the range "1 to 10" is intended to include all sub-ranges between the listed minimum value of 1 and the listed maximum value of 10, and including those sub-ranges, i.e., having a minimum value of 1 or more and a maximum value of 10 or less. Since the disclosed numerical ranges are continuous, they include all values ​​between the minimum and maximum values. Unless otherwise specified, the various numerical ranges specified in this application are approximate values.

[0022] Through this disclosure, various publications may be incorporated by reference. If the meaning of any language in such incorporated publications conflicts with the meaning of the language in this disclosure, the meaning of the language in this disclosure shall prevail unless otherwise indicated.

[0023] The processes described herein typically utilize polymer blends containing PAN and polymer additives, designed to control morphology through appropriate control of coagulation and washing conditions. In particular, the polymer additives are polymers soluble in both the non-solvent and solvent aspects of PAN. Solubility in the solvent is necessary to form a homogeneous solution in the viscosity spinning "dope" before spinning. This is important because if neither is soluble, the polymer additive and PAN cannot form a blend. Solubility in the non-solvent is unique and provides an opportunity to intentionally control the kinetics of interdiffusion for fibril formation. Furthermore, if the polymer additive blended with PAN can be dissolved, for example by changing the conditions of the coagulation or washing bath, it provides the possibility of manipulating the structure beyond the initial stages of coagulation.

[0024] Therefore, a first aspect of the present disclosure relates to a method for producing polyacrylonitrile fibers having controlled morphology, the method comprising: a) Polyacrylonitrile polymer (polymer A), A polymer different from polyacrylonitrile-based polymers (polymer B), A step of forming a homogeneous solution comprising a first liquid containing a solvent for polymer A, and a first liquid in which polymer B is soluble; b) A step of co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid containing a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) A step of selectively removing polymer B from a polyacrylonitrile material by contacting the polyacrylonitrile material with a third liquid containing a non-solvent for polymer A and in which polymer B is soluble, thereby producing a polyacrylonitrile fiber having controlled morphology. Includes.

[0025] In step a) of the method, a homogeneous solution is formed comprising a polyacrylonitrile polymer (polymer A), a polymer different from the polyacrylonitrile polymer (polymer B), and a first liquid containing a solvent for polymer A, wherein polymer B is soluble in the first liquid.

[0026] A polyacrylonitrile polymer, polymer A, may be any polymer containing repeating units derived from acrylonitrile. Suitable polyacrylonitrile polymers may be homopolymers consisting of repeating units derived from acrylonitrile or copolymers containing repeating units derived from acrylonitrile and one or more comonomers. Such polymers may be obtained from commercially available sources or prepared according to methods known to those skilled in the art. For example, polymer A may be produced by any polymerization method, including but not limited to solution polymerization, dispersion polymerization, precipitation polymerization, suspension polymerization, emulsion polymerization, and variations thereof.

[0027] Polyacrylonitrile polymers comprise repeating units derived from acrylonitrile and at least one comonomer selected from the group consisting of vinyl acids, vinyl esters, vinyl amides, vinyl halides, ammonium salts of vinyl compounds, sodium salts of sulfonic acids, and mixtures thereof.

[0028] In one embodiment, the polyacrylonitrile polymer is composed of acrylonitrile and methacrylic acid (MAA), acrylic acid (AA), itaconic acid (ITA), methacrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), propyl methacrylate, butyl methacrylate, β-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, 2-ethylhexyl acrylate, isopropyl acetate, vinyl acetate The material comprises repeating units derived from at least one comonomer selected from the group consisting of acetone (VA), vinyl propionate, vinylimidazole (VIM), acrylamide (AAm), diacetone acrylamide (DAAm), allyl chloride, vinyl bromide, vinyl chloride, vinylidene chloride, sodium vinylsulfonate, sodium p-styrenesulfonate (SSS), sodium methallylsulfonate (SMS), sodium 2-acrylamido-2-methylpropanesulfonate (SAMPS), and mixtures thereof.

[0029] The comonomer ratio (the amount of one or more comonomers relative to the amount of acrylonitrile) is not particularly limited. However, a suitable comonomer ratio is 0–20%, typically 1–5%, and more typically 1–3%.

[0030] The molar weight of polyacrylonitrile polymers suitable for use in the above process ranges from 60 to 500 kg / mol, typically from 90 to 250 kg / mol, and more typically from 115 to 180 kg / mol.

[0031] The first liquid contains a solvent for polymer A. At the same time, polymer B is soluble in the first liquid.

[0032] As used herein, the term “solvent” means any compound that, at the temperature in which the solvent is used, can typically completely dissolve each polymer. On the other hand, the term “non-solvent” means any compound that, at the temperature in which the non-solvent is used, cannot dissolve each polymer by itself. It will be understood by those skilled in the art that a combination of a solvent and a non-solvent, which are typically miscible, can form a liquid in which the solubility of each polymer differs from that of the solvent alone or the non-solvent alone.

[0033] As used herein, the term “soluble” when used to describe a material means that the material can be dissolved in a particular solvent or liquid at a concentration of 1% by weight or more, typically 5% by weight or more, relative to the weight of the solvent or liquid. As used herein, the term “insoluble” when used to describe a material means that the material can be dissolved in a particular non-solvent or liquid at a concentration of less than 1% by weight or less, typically 0.5% by weight, relative to the weight of the non-solvent or liquid.

[0034] A suitable solvent for polymer A may be selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl2) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof, and typically can be selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), and N-methyl-2-pyrrolidone (NMP).

[0035] In step a), the temperature of the first liquid is maintained at a temperature higher than room temperature, i.e., above 25°C. In one embodiment, the temperature of the first liquid is approximately 40°C to approximately 85°C.

[0036] The homogeneous solution produced typically does not contain gels and / or agglomerating polymers. The presence of gels and / or agglomerating polymers can be determined using any method well known to those skilled in the art. For example, the presence of gels and / or agglomerating polymers may be determined using a Hegman gauge. The homogeneous solution produced is generally stable and does not exhibit gel formation over time.

[0037] A homogeneous solution may have a polymer concentration of at least 10% by weight, typically about 16% to about 28% by weight, and more typically about 19% to about 24% by weight, based on the total weight of the solution.

[0038] Step b) is a step in which polymer A and polymer B are coprecipitated by contacting the homogeneous solution formed in step a) with a second liquid containing a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B.

[0039] The second liquid contains a solvent for polymer A and a non-solvent for polymer A, and polymer B is insoluble in the second liquid. As a result, when the homogeneous solution formed in step a) is brought into contact with the second liquid, polymers A and B coprecipitate in the form of a polyacrylonitrile-based material, which is typically in solid form such as a film, discrete particles, or fibers.

[0040] The second liquid used in the process is a mixture of a solvent and a non-solvent for polymer A. Suitable solvents include those described herein. In one embodiment, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or a mixture thereof is used as the solvent. In another embodiment, dimethyl sulfoxide is used as the solvent.

[0041] The non-solvent for polymer A can be any compound known to those skilled in the art that does not dissolve polymer A at the temperature in which it is used. Typical non-solvents for polymer A include water and C1-C6 alkanols, such as methanol, ethanol, n-propanol, isopropanol, etc. In one embodiment, the non-solvent for polymer A is water.

[0042] The ratio of solvent to non-solvent and the bath temperature are not particularly limited and can be adjusted according to known methods to achieve the desired solidification rate. However, the second liquid preferably contains 85% by weight or less of the solvent for polymer A and 15% by weight or more of the non-solvent for polymer A, based on the total weight of the second liquid.

[0043] In another embodiment, the second liquid contains 40% to 85% by weight of one or more solvents, with the remainder being a non-solvent. In one embodiment, the second liquid contains 40% to 70% by weight of one or more solvents, with the remainder being a non-solvent. In yet another embodiment, the second liquid contains 50% to 85% by weight of one or more solvents, with the remainder being a non-solvent.

[0044] Typically, the temperature of the second liquid is between 0°C and 80°C. In one embodiment, the temperature of the second liquid is between 30°C and 80°C. In another embodiment, the temperature of the second liquid is between 0°C and 20°C.

[0045] In one embodiment, step b) includes spinning the homogeneous solution formed in step a) in or into a coagulation bath having a second liquid containing a solvent for polymer A and a non-solvent for polymer A to form one or more fibers of a polyacrylonitrile-based material.

[0046] In this embodiment, a homogeneous solution is spun in or into a coagulation bath. After removing air bubbles by vacuum, conventional wet spinning and / or air gap spinning can be performed on the homogeneous solution (i.e., “spin dope”). In wet spinning, the spinning stock is filtered through the holes of a spinneret (typically metal) and extruded into a liquid coagulation bath to form filaments. The spinneret holes determine the desired filament count of the fiber (e.g., 3,000 holes for 3K carbon fiber). In air gap spinning, a vertical air gap of 1 to 50 mm, typically 2 to 10 mm, is provided between the spinneret and the coagulation bath. In one embodiment, the polymer solution is filtered through the spinneret into air and extruded, and the extruded filaments are then coagulated in a coagulation bath.

[0047] The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid may be the same or different, and each may be selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl2) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof, and typically selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), and N-methyl-2-pyrrolidone (NMP).

[0048] In one embodiment, the solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are the same.

[0049] In step c), polymer B is selectively removed from the polyacrylonitrile-based material by contacting the polyacrylonitrile-based material with a third liquid containing a non-solvent for polymer A, in which polymer B is soluble.

[0050] The third liquid contains a non-solvent for polymer A, while polymer B is soluble in the third liquid. Therefore, in step c), polymer B can be selectively removed from the PAN polymer fibers to produce polyacrylonitrile fibers with controlled morphology.

[0051] The temperature of the third liquid is 0 to 100°C, typically 0 to 30°C, and more typically 10 to 25°C.

[0052] The non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid may be the same or different. In one embodiment, the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are the same.

[0053] In one embodiment, the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are both water.

[0054] In one embodiment, the first liquid is a solvent for polymer A.

[0055] In another embodiment, the second liquid consists of a solvent for polymer A and a non-solvent for polymer A.

[0056] In yet another embodiment, the third liquid is a non-solvent for polymer A.

[0057] In one embodiment, step c) includes the step of stretching one or more fibers through one or more stretching baths and washing baths, wherein at least one bath contains a third liquid containing a non-solvent for polymer A.

[0058] The solidified polymer fibers are stretched, for example, by transporting them through one or more stretching and washing baths using rollers. The solidified polymer fibers are transported through one or more washing baths to remove excess solvent, and then stretched in a hot (e.g., 40°C to 100°C) water bath to impart molecular orientation to the filaments as a first step in controlling the fiber diameter. The resulting stretched polymer fibers are substantially solvent-free.

[0059] Therefore, in one embodiment, step c) includes stretching one or more fibers through a plurality of stretching baths and washing baths, where the first bath contains a third liquid containing a non-solvent for the polymer, and the temperature of the first bath is 0 to 30°C, typically 10 to 25°C. The first bath means the bath immediately following the bath used in step b). The bath after the first bath may have a temperature up to 100°C.

[0060] A polymer additive, i.e., polymer B, which is combined with polymer A in the manner described herein to form a homogeneous solution, is a polymer different from polymer A. A suitable polymer for use as polymer B is a polymer that is soluble in a first liquid, insoluble in a second liquid, and soluble in a third liquid at the temperature in which it is used, and may be a homopolymer or copolymer. One suitable polymer is given by formula (I): [ka] (In the formula: R 1 is hydrogen or methyl, R 2 and R 3 Each is a polymer comprising one or more repeating units derived from at least one monomer, each independently of H or alkyl (typically H or (C1-C6) alkyl).

[0061] As used herein, the term "Cx~Cy" or "(Cx~Cy)" (where x and y are integers) in relation to an organic group means that the group can contain x to y carbon atoms per group.

[0062] As used herein, the term "alkyl" means a monovalent linear or branched saturated hydrocarbon group, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, or hexyl.

[0063] As used herein, the term “derived from” means that the repeating units in the polymer are formed by polymerization of monomers described herein in accordance with methods known to those skilled in the art. The polymer may, in some cases, undergo subsequent chemical modification. For example, a polymer produced by polymerization of acyl group-containing monomers can be hydrolyzed to form a polymer having hydroxyl groups.

[0064] In one embodiment, polymer B is a homopolymer derived from a monomer according to formula (I).

[0065] In another embodiment, polymer B is poly(N-isopropylacrylamide).

[0066] In one embodiment, polymer B is a copolymer containing monomer units derived from monomers according to formula (I), more typically, about 50 weight percent ("wt%") or more of the repeating units of the polymer are derived from monomers according to formula (I).

[0067] Another suitable polymer is given by formula (II): [ka] (In the formula: R 4 is hydrogen or methyl, R 5 It is a polymer comprising one or more repeating units derived from at least one monomer (typically H or acyl).

[0068] As used herein, the term “acyl” means a substituent characterized by the formula -(C=O)-R (where R is an alkyl group).

[0069] In one embodiment, polymer B is a homopolymer derived from a monomer according to formula (II).

[0070] In another embodiment, polymer B is polyvinyl alcohol.

[0071] In another embodiment, polymer B is a copolymer comprising monomer units derived from monomers according to formula (II), more typically, about 50 weight percent ("wt%") or more of the repeating units of the polymer are derived from monomers according to formula (II).

[0072] In step a), the amount of polymer B that combines with polymer A to form a homogeneous solution is not particularly limited. However, suitable results are obtained when the homogeneous solution contains polymer B at a rate of 50% by weight or less, typically 20% by weight or less, and more typically 10% by weight or less, relative to the total weight of the homogeneous solution.

[0073] The process may further include step d) drying a substantially solvent-free stretched polymer fiber on, for example, a drying roll. The drying roll may consist of a plurality of rotatable rolls arranged in series and in a meandering configuration, the filament passing over the rolls sequentially from roll to roll and under sufficient tension to stretch or relax the filament on the rolls. At least some of the rolls are heated by pressurized steam circulating inside or through the rolls, or by an electric heating element inside the rolls. A finishing oil may be applied to the stretched fiber before drying to prevent the filaments from sticking together in downstream processes.

[0074] The processes of this disclosure may be carried out sequentially or in batches. As used herein, a process “carried out sequentially” means a process in which a single unit of work is carried through one or more processing steps at a time without any interruption in time, material, or sequence. This is in contrast to the batch method, which is understood as a process comprising one or more steps in a sequence in which a finite amount of material is processed or manufactured at the end of a sequence, and which is carried out in a defined order and must be repeated to process or manufacture another batch of material. In one embodiment, the process is carried out sequentially.

[0075] Advantageously, the polymer additive, polymer B, can be recovered and reused. The ability to recover polymer B from the process provides advantages over the "sacrificial polymer" that is volatilized during thermal decomposition and lost during the downstream carbon fiber formation process. Therefore, in one embodiment, the process further includes step e) recovering polymer B at least partially. Polymer B can be recovered from any of the steps in the process using any separation method known to those skilled in the art. For example, polymer B can be recovered from any of the liquids described herein using vacuum distillation, thin-film evaporation, etc.

[0076] In a second aspect, the disclosure relates to polyacrylonitrile fibers produced by the methods described herein. Polyacrylonitrile fibers produced by the methods described herein can be used as precursor fibers, so-called white fibers, for the production of carbon fibers.

[0077] Therefore, in a third aspect, the disclosure relates to a process for manufacturing carbon fibers, the process being (i) A step of producing polyacrylonitrile fibers according to the method described herein, (ii) A step to produce carbon fibers by oxidizing the polyacrylonitrile fibers produced in step (i) to form stabilized carbon fiber precursor fibers, and then carbonizing the stabilized carbon fiber precursor fibers. Includes.

[0078] After producing polyacrylonitrile fibers according to the method described herein, the polyacrylonitrile fibers can be oxidized to form stabilized carbon fiber precursor fibers, and then the stabilized carbon fiber precursor fibers can be carbonized to produce carbon fibers.

[0079] During the oxidation stage, the polymer fibers are supplied under tension through one or more specialized ovens, each having a temperature of 150–300°C, typically 200–280°C, and more typically 220–270°C, where heated air is supplied to each oven.

[0080] The oxidation process initiates crosslinking of polymer chains by combining oxygen molecules from the air with the fibers, thereby increasing the fiber density to 1.30 g / cm³. 3 ~1.45g / cm 3 This increases the amount. Such oxidized PAN fibers have an insoluble ladder-type aromatic molecular structure and are ready for carbonization at any time.

[0081] Carbonization leads to the crystallization of carbon molecules, resulting in finished carbon fibers with a carbon content of over 90 percent. The carbonization of oxidized or stabilized carbon fiber precursor fibers is carried out in an inert (oxygen-free) atmosphere, typically in a nitrogen atmosphere, in one or more specially designed ovens. The oxidized carbon fiber precursor fibers pass through one or more ovens, each heated to a temperature of 300°C to 1650°C, typically 1100°C to 1450°C.

[0082] Adhesion between the matrix resin and carbon fibers is a critical criterion in carbon fiber reinforced polymer composite materials. Therefore, during the manufacturing of carbon fibers, surface treatments can be performed after oxidation and carbonization to enhance this adhesion.

[0083] Surface treatment may include stretching the carbonized fibers through an electrolytic bath containing an electrolyte such as ammonium bicarbonate or sodium hypochlorite. The chemicals in the electrolytic bath etch or roughen the surface of the fibers, thereby increasing the surface area available for interfacial fiber / matrix bonding and adding reactive chemical groups.

[0084] Next, the carbon fibers can be subjected to a sizing treatment in which a size coating (e.g., an epoxy coating) is applied to the fibers. The sizing treatment can be carried out by passing the fibers through a size bath containing a liquid coating material. The sizing treatment protects the carbon fibers during handling and during processing into intermediate forms such as dry fabrics and prepregs. The sizing treatment also holds the filaments together in individual tows to reduce fuzz, improve processability, and increase the interfacial shear strength between the fibers and the matrix resin.

[0085] After sizing, the coated carbon fibers are dried and then wound onto bobbins.

[0086] Those skilled in the art will understand that processing conditions (such as the composition of the spinning solution and coagulation bath, the total volume of the bath, the stretching, the temperature, and the filament speed) are relevant to obtaining a filament of the desired structure and denier.

[0087] In a fourth aspect, the disclosure relates to carbon fibers manufactured by the methods described herein.

[0088] Carbon fibers produced according to the processes described herein can be characterized by mechanical properties such as tensile strength and tensile modulus as determined by the ASTM D4018 test method.

[0089] Manufactured carbon fibers generally have a tensile strength of 300-1000 ksi, typically 400-600 ksi.

[0090] The produced carbon fiber generally has a tensile modulus of 30 to 50 msi, typically 35 to 40 msi.

[0091] The produced carbon fibers can be characterized by their density. Generally, the carbon fibers formed according to the method described herein have a lower density than conventional carbon fibers. Advantageously, the present disclosure provides low-density, lightweight carbon fibers. The carbon fiber produced according to the present disclosure has a density of 1.80 g / cm 3 the following , typically 1.79 g / cm 3 the following , typically 1.78 g / cm 3 the following . In one embodiment, the density is 1.50 to 1.77 g / cm 3 . In another embodiment, the density is 1.70 to 1.77 g / cm 3 or 1.74 to 1.79 g / cm 3 .

[0092] The carbon fiber produced herein is suitable for use in the manufacture of composite materials. Accordingly, in a fifth aspect, the present disclosure relates to a composite material comprising a carbon fiber produced according to the method described herein and a matrix resin.

[0093] In many liquid molding processes, a composite material can be produced by molding a preform containing the carbon fiber produced according to the method described herein, and injecting a thermosetting resin into the preform. Usable liquid molding processes include, but are not limited to, Vacuum Assisted Resin Transfer Molding (VARTM), which injects resin into the preform using a differential pressure generated in a vacuum. Another method is Resin Transfer Molding (RTM), which injects resin into the preform under pressure in a closed mold. A third method is Resin Film Infusion (RFI), in which a semi-solid resin is placed under or on top of the preform, a suitable tool is placed on the component, the component is bagged, and then placed in an autoclave to dissolve and inject the resin into the preform.

[0094] The matrix resin for impregnating or injecting the preforms described herein is a curable resin. In this disclosure, “curing” or “cure” means the solidification of a polymer material by chemical crosslinking of polymer chains. With respect to a composition, the term “curable” means that the composition can be subjected to conditions that cause the composition to solidify or thermoset. The matrix resin is typically a solidifying or thermosetting resin containing one or more uncured thermosetting or thermoplastic resins. Suitable thermosetting resins include, but are not limited to, epoxy resins, oxetanes, imides (such as polyimides or bismaleimides), vinyl ester resins, cyanate ester resins, isocyanate-modified epoxy resins, phenolic resins, furan resins, benzoxazines, formaldehyde condensation resins (such as urea, melamine, or phenol), polyesters, acrylics, hybrids, blends, and combinations thereof. Suitable thermoplastic resins include, but are not limited to, polyolefins, fluoropolymers, perfluorosulfonic acids, polyamide-imides, polyamides, polyesters, polyketones, polyphenylene sulfide, polyvinylidene chloride, sulfone polymers, hybrids, blends, and combinations thereof.

[0095] Suitable epoxy resins include aromatic diamines, aromatic monoprimary amines, aminophenols, polyhydric phenols, polyhydric alcohols, glycidyl derivatives of polycarboxylic acids, and non-glycidyl resins produced by peroxidation of olefinic double bonds. Examples of suitable epoxy resins include polyglycidyl ethers of bisphenols such as bisphenol A, bisphenol F, bisphenol S, bisphenol K, and bisphenol Z; polyglycidyl ethers of cresol and phenolic novolacs; glycidyl ethers of phenol-aldehyde adducts; glycidyl ethers of aliphatic dials; diglycidyl ethers; diethylene glycol diglycidyl ethers; aromatic epoxy resins; aliphatic polyglycidyl ethers; epoxidized olefins; brominated resins; aromatic glycidylamines; heterocyclic glycidylimides and amides; glycidyl ethers; fluorinated epoxy resins; or combinations thereof.

[0096] Specific examples include tetraglycidyl derivatives of 4,4'-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, bromobisphenol F diglycidyl ether, tetraglycidyl derivatives of diaminodiphenylmethane, trihydroxyphenylmethane triglycidyl ether, polyglycidyl ether of phenol-formaldehyde novolac, polyglycidyl ether of o-cresol novolac, or tetraglycidyl ether of tetraphenylethane.

[0097] Suitable oxetane compounds that contain at least one oxetano group per molecule include, for example, 3-ethyl-3[[(3-ethyloxetane-3-yl)methoxy]methyl]oxetane, oxetane-3-methanol, 3,3-bis-(hydroxymethyl)oxetane, 3-butyl-3-methyloxetane, 3-methyl-3-oxetanemethanol, 3,3-dipropyloxetane, and 3-ethyl-3-(hydroxymethyl)oxetane.

[0098] The curable matrix resin may optionally contain one or more additives, such as curing agents, curing catalysts, comonomers, rheology control agents, tackifiers, inorganic or organic fillers, thermoplastic and / or elastic polymers as reinforcing agents, stabilizers, inhibitors, pigments, dyes, flame retardants, reactive diluents, UV absorbers, and other additives known to those skilled in the art for improving the properties of the matrix resin before and / or after curing.

[0099] Examples of suitable curing agents include, but are not limited to, aromatic, aliphatic, and alicyclic amines or guanidine derivatives. Suitable aromatic amines include 4,4'-diaminodiphenylsulfone (4,4'-DDS) and 3,3'-diaminodiphenylsulfone (3,3'-DDS), 1,3-diaminobenzene, 1,4-diaminobenzene, 4,4'-diammodiphenylmethane, and benzenediamine (BDA); suitable aliphatic amines include ethylenediamine (EDA), 4,4'-methylenebis(2,6-diethylaniline) (M-DEA), m-xylylenediamine (mXDA), diethylenetriamine (DETA), triethylenetetramine (TETA), trioxatridecanediamine (TTDA), polyoxypropylenediamine, and further homologues, diamino Examples of suitable curing agents include alicyclic amines such as cyclohexane (DACH), isophorone diamine (IPDA), 4,4'-diaminodicyclohexylmethane (PACM), bisaminopropylpiperazine (BAPP), and N-aminoethylpiperazine (N-AEP); other suitable curing agents include anhydrides, typically polycarboxylic acid anhydrides, such as nadic anhydride, methylnadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, pyromellitic dianhydride, chloridenic anhydride, and trimellitic anhydride.

[0100] Furthermore, other curing agents are Lewis acid:Lewis base complexes. Suitable Lewis acid:suitable base complexes include, for example, BCl3:amine complexes, BF3:amine complexes, for example, BF3:monoethylamine, BF3:propylamine, BF3:isopropylamine, BF3:benzylamine, BF3:chlorobenzylamine, BF 3: Examples of complexes include trimethylamine, BF3:pyridine, BF3:THF, AlCl3:THF, AlCl3:acetonitrile, and ZnCl2:THF.

[0101] Additional curing agents include polyamides, polyamines, amidoamines, polyamidoamines, polyalicyclic compounds, polyetheramides, imidazoles, dicyandiamides, substituted ureas and urones, hydrazines and silicones.

[0102] Urea-based curing agents include materials available under the trade name DYHARD (sold by Alzchem), and urea derivatives such as those commercially available as UR200, UR300, UR400, UR600, and UR700. Examples of uron accelerators include 4,4-methylenediphenylenebis(N,N-dimethylurea) (available from Onmicure as U52M).

[0103] If present, the total weight of the curing agent is in the range of 1% to 60% by weight of the resin composition. Typically, the curing agent is present in the range of 15% to 50% by weight, and more typically in the range of 20% to 30% by weight.

[0104] Suitable reinforcing agents may include, but are not limited to, polyamides, copolyamides, polyimides, aramids, polyketones, polyetherimides (PEI), polyetherketones (PEK), polyetherketone ketones (PEKK), polyetheretherketones (PEEK), polyethersulfones (PES), polyetherethersulfones (PEES), polyesters, polyurethanes, polysulfones, polysulfides, polyphenylene oxide (PPO) and modified PPO, poly(ethylene oxide) (PEO) and polypropylene oxide, polystyrene, polybutadiene, polyacrylates, polystyrene, polymethacrylates, polyacrylics, polyphenylsulfones, high-performance hydrocarbon polymers, liquid crystal polymers, elastomers, segmented elastomers, and homopolymers or copolymers of core-shell particles, either alone or in combination.

[0105] If present, reinforcing particles or reinforcing agents may be present in an amount of 0.1% to 30% by weight of the resin composition. In one embodiment, reinforcing particles or reinforcing agents may be present in an amount of 10% to 25% by weight. In another embodiment, reinforcing particles or reinforcing agents may be present in an amount of 0.1% to 10% by weight. Suitable reinforcing particles or reinforcing agents include, for example, Virantage VW10200FRP, VW10300FP and VW10700FRP from Solvay, and BASF UltrasonE2020 and Sumikaexcel5003P from Sumitomo Chemical.

[0106] The reinforcing particles or reinforcing agents may be in the form of particles having a diameter greater than 20 microns to prevent them from being incorporated into the fiber layer. The size of the reinforcing particles or reinforcing agents can be selected so as not to be filtered by the fiber reinforcing material. Optionally, the composition may also include inorganic ceramic particles, microspheres, microballoons, and clay.

[0107] The resin composition may contain conductive particles as described in International Publication No. 2013 / 141916, International Publication No. 2015 / 130368, and International Publication No. 2016 / 048885, etc.

[0108] The mold for resin injection may be a two-component sealed mold or a single-sided mold sealed with a vacuum bag. After injecting the matrix resin into the mold, the mold is heated to cure the resin.

[0109] During heating, the resin reacts with itself to form crosslinks in the matrix of the composite material. After initial heating, the resin gels. Once gelled, the resin no longer flows and behaves more like a solid. After gelling, the temperature or curing may be increased to the final temperature to complete curing. The final curing temperature varies depending on the properties and characteristics of the selected thermosetting resin. Therefore, in a preferred method, the composite material is heated to a first temperature suitable for gelling the matrix resin, and then the temperature is increased to a second temperature and held at the second temperature for a period of time to complete curing. This yields a composite article.

[0110] The methods and carbon fibers produced therefrom according to this disclosure are further illustrated by the following non-limiting examples. [Examples]

[0111] Example 1. PAN / pNIPAM film A polyacrylonitrile polymer having repeating units derived from methacrylic acid (MAA) was used. Poly(N-isopropylacrylamide (pNIPAM; available from Sigma Aldrich, with a number-average molecular weight of approximately 40,000 kDa)) was used as a polymer additive. A blend of the two polymers was prepared using a Thinky AR-100 centrifuge mixer (2000 rpm) in a sample size of approximately 6 grams, using either 1% by weight or 10% by weight of pNIPAM relative to the concentration of the PAN-based polymer in DMSO (approximately 15% by weight). A polymer film was prepared by spreading the solution on a glass plate onto a thin film and air-drying it. The film was then extracted by one of the following methods:

[0112] [Table 1]

[0113] Using FTIR, two peaks were observed, approximately 1540 and 1640 cm², corresponding to the CN (stretching) and C=O (stretching) amide groups, respectively. -1 The presence of pNIPAM was confirmed by this method. Approximately 1640 cm -1 The peak at this point overlaps with the polymer baseline peak related to the carboxylic acid functional value, so it is approximately 1540 cm. -1 The peak at this point is used as a quantitative measure of the presence of pNIPAM in the polymer.

[0114] Method 2 showed the greatest reduction in pNIPAM compared to other extraction methods. Method 6 was the second most effective extraction method. Wash extraction (Methods 1, 3, and 7) was the least effective and retained the highest amount of pNIPAM. Among the water-based methods, Method 6 was found to be the most efficient in removing pNIPAM.

[0115] To further investigate the effect of temperature, water immersion was performed for 2-3 hours at either 10°C or 50°C for both the 1 wt% pNIPAM film and the 10 wt% pNIPAM film (wt% is with respect to PAN-based polymers). Cold water washing was performed at approximately 1540 cm² at both loading levels. -1 It showed a small peak at that point.

[0116] The physical characteristics of the film were examined before and after extraction using scanning electron microscopy (SEM). Before extraction, the film appeared smooth, granular at higher magnifications, and at 50,000x magnification, it appeared to have a spongy network structure. After extraction, lower magnification images showed many surface features not present in the control film. The surface was riddled with small depressions or pores left over from the PAN-based polymer by pNIPAM extraction. At higher magnifications, the surface features appeared to be submicron cavities.

[0117] Example 2. PAN / pNIPAM white fiber A polymer blend prepared using a PAN-based polymer and polymer additives was used in Example 1. The PAN-based polymer and pNIPAM were dissolved using a 15-gallon Myers mixer with a disperser at 500 rpm and a sweeper at 60 rpm (3.16 kg of PAN-based polymer and 30 g of pNIPAM in 14.49 kg of DMSO). The temperature was raised to 80°C and run for 2 hours, then cooled to 45°C. The polymer solution ("doped") was then spun into a coagulation bath (65% DMSO). The coagulation bath was varied between 40 and 50°C, and the first stretching bath was either 60°C or cooled to below 10°C with ice. As a control, the same spinning process was performed with the PAN-based polymer without pNIPAM.

[0118] All filaments imaged using standard sample preparation techniques appeared to be standard and showed no signs of deviation from the control process. Optical images did not show macrovoids. Therefore, the introduction of 1 wt% pNIPAM does not significantly alter the preferred solidification region from the basic process.

[0119] However, SEM revealed that the fibrous structure showed clear differences at the submicron scale. For example, samples taken from the coagulation bath showed filaments with a smooth surface and an internal core structure filled with porosity throughout the entire cross-section. Surprisingly, the surface remained intact and showed no surface defects. The core structure appeared more porous and spongy compared to the standard coagulated sample, and in particular, the network structure contained many of the small cavities previously observed within the film structure. It was unexpected that the coagulated sample would show such remarkable porosity when the bath concentration was 65 wt% DMSO (outside the solubility range of pNIPAM), but it is presumed that pNIPAM precipitates at a different rate than PAN-based polymers and undergoes phase separation upon coagulation.

[0120] The solidified filaments were stretched, and the porous structure was found to remain intact. The densified spongy core and the roughened epidermal layer as fibers were stretched. Although the pore size decreased, the core still retained many small indentations on the order of 100 nm in size within the structure. The pNIPAM concentration in the fibers was found to be higher in the solidification bath compared to the washing bath and decreased after the washing process, indicating that pNIPAM was removed from the fibers during washing.

[0121] Another difference found in the pNIPAM-blend fibers compared to the control process fibers lies in their swelling behavior. The degree of swelling of the pNIPAM-blend fibers and the control fibers was measured according to the following procedure. A sample was taken and first centrifuged at 3000 rpm for 15 minutes to remove any adhering liquid from the filament surface. The collected sample was then submerged in a glass beaker / flask containing deionized water and "washed" for at least 15 minutes. This washing process was then repeated two more times with fresh deionized water to ensure that the sample was completely solidified and the solvent was removed. As soon as the final washing was complete, the sample was centrifuged again at 3000 rpm for 15 minutes and weighed to determine the weight, or wt, after washing. a The sample was then placed in a circulating air oven at 110°C for 3 hours. After drying, the sample was removed from the oven and placed in a desiccator for at least 10 minutes. The dried sample, placed in the desiccator, was reweighed, and the final weight was determined to be W f This was recorded as follows. The degree of swelling was then calculated using the following formula: Swelling degree (%) = (W a -W f ) × (100 / W) f )

[0122] This method links the porosity of fibers to liquid absorption.

[0123] The swelling of the sample blended with pNIPAM was similar to that of the solidified sample and the first stretched sample (163% vs. 181%), but much lower (183%) than that of the control sample (192%). This difference suggests that pNIPAM may affect the kinetics of interdiffusion of the solvent into and out of the fiber.

[0124] Interestingly, the mechanical properties of the pNIPAM-blended fibers appeared unaffected by the coagulation bath and the first stretching bath conditions, although structural differences, namely the presence of porosity and swelling behavior, were noted. Toughness, elongation, and Young's modulus were all within the range of measurement and process errors for these experiments.

[0125] Example 3. Carbon fiber manufactured from PAN / pNIPAM white fiber Carbon fibers were formed by oxidizing and carbonizing PAN / pNIPAM white fibers produced according to the procedure described in Example 2. Carbon fibers were also formed by oxidizing and carbonizing white fibers produced from PAN-based polymers without pNIPAM. It was observed that the addition of pNIPAM did not significantly affect the mechanical properties. For six carbonization experiments to form control fibers, the average tensile strength was 482 ± 32 ksi and the average tensile strength was 39.1 ± 0.4 Msi, while the carbon fibers produced from PAN / pNIPAM white fibers showed a tensile strength exceeding 500 ksi and a tensile strength exceeding 38.3 Msi. This result indicates that the presence of 1% pNIPAM did not impair the mechanical load-bearing capacity of the carbon fibers produced from the PAN / pNIPAM blend. The density of the carbon fibers was a typical 1.74–1.79 g / cm³. 3 It was lower than that.

[0126] Interestingly, the strand fractography of the carbon fibers of the present invention revealed extremely fine pores in the cross-section of the fibers. All of the pores were less than 100 nanometers in size and were almost entirely concentrated near the surface of the fiber's epidermis. This demonstrates that the porosity introduced during spinning is retained through carbonization.

[0127] Example 4. PAN / PVOH white fiber Spin dope was prepared according to the procedure described in Example 2, except that pNIPAM was replaced with polyvinyl alcohol (PVOH; available from Sigma Aldrich). The final spin dope contained approximately 17.5% by weight of solids (PAN-based polymer + pNIPAM) and approximately 5% by weight of PVOH (relative to the PAN-based polymer). The zero shear viscosity at 45°C was approximately 56 Pa*sec.

[0128] The coagulation bath was set to 50°C, and the spinning dope was spun to form PAN / PVOH white fibers as in Example 2.

[0129] Fiber samples were taken after coagulation and after the first stretching bath. The swelling of the sample taken after coagulation was 241%, which is much higher than typical for fibers made solely from the same PAN-based polymer, which is generally around 190-200%. The swelling of the sample after the first stretching was 174%, which is also much higher than typical for fibers made solely from the same PAN-based polymer, which is generally around 120-140%. Similar to pNIPAM, the difference suggests that PVOH may affect the kinetics of interdiffusion of the solvent into and out of the fiber. Furthermore, fiber samples extracted after the washing step show fibers with even greater pore density and cavity throughout the fiber core compared to Example 2 due to the higher concentration of PVOH relative to the PAN-based polymer. Importantly, this indicates that pore density and pore volume can be controlled by the polymer blend properties of the polymer blend concentration.

[0130] Example 5. Carbon fiber manufactured from PAN / PVOH white fiber PAN / PVOH white fibers, manufactured according to the procedure described in Example 4, were oxidized and carbonized to successfully form carbon fibers.

[0131] (According to ASTM methods) the tensile strength was 340 ± 12 ksi and the tensile modulus was 31 ± 2.5 Msi. Furthermore, the density of carbon fibers produced from PAN / PVOH was a typical 1.70–1.77 g / cm³. 3 Lower.

[0132] It will be apparent to those skilled in the art that the conditions for carrying out the methods of the present invention described herein can be optimized based on the intended use and environment without departing from the spirit of this disclosure.

Claims

1. A method for producing a porous polyacrylonitrile fiber, a) Polyacrylonitrile polymer (polymer A), A homopolymer (polymer B) different from the aforementioned polyacrylonitrile-based polymer, A step of forming a homogeneous solution comprising a first liquid containing a solvent for polymer A and in which polymer B is soluble, wherein the homogeneous solution contains polymer B at a concentration of 50% by weight or less relative to the total weight of the homogeneous solution. b) A step of spinning the homogeneous solution in or into a solidification bath containing a second liquid which includes a solvent for polymer A and a non-solvent for polymer A and in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) The polyacrylonitrile material removed from the coagulation bath is brought into contact with a third liquid consisting of water in which polymer B is soluble, thereby selectively removing polymer B from the polyacrylonitrile material. This process involves manufacturing porous polyacrylonitrile fibers. Includes, The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are each selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl₂) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof. The second liquid comprises, in proportion to the total weight of the second liquid, the solvent for polymer A at 85% by weight or less, and the non-solvent for polymer A at 15% by weight or more. Polymer B is given by formula (I): 【Chemistry 1】 (In the formula: R1 is hydrogen or methyl, A method for producing a homopolymer derived from a monomer (where R2 and R3 are each independently H or alkyl).

2. The method according to claim 1, wherein the solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are the same.

3. The method according to claim 1 or 2, wherein the homogeneous solution has polymer concentrations of polymer A and polymer B of 16% to 28% by weight, based on the total weight of the solution.

4. The method according to any one of claims 1 to 3, wherein the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are the same.

5. The method according to any one of claims 1 to 4, wherein the non-solvent for polymer A in the second liquid is water.

6. The method according to any one of claims 1 to 5, wherein the first liquid comprises the solvent for polymer A.

7. The method according to any one of claims 1 to 6, wherein the second liquid comprises the solvent for polymer A and water.

8. Polymer A is composed of acrylonitrile and methacrylic acid (MAA), acrylic acid (AA), itaconic acid (ITA), methacrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), propyl methacrylate, butyl methacrylate, β-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, 2-ethylhexyl acrylate, isopropyl acetate, vinyl acetate (VA), vinyl propionate, and The method according to any one of claims 1 to 7, comprising a repeating unit derived from nilimidazole (VIM), acrylamide (AAm), diacetone acrylamide (DAAm), allyl chloride, vinyl bromide, vinyl chloride, vinylidene chloride, sodium vinyl sulfonate, sodium p-styrene sulfonate (SSS), sodium methallyl sulfonate (SMS), sodium 2-acrylamido-2-methylpropanesulfonate (SAMPS), and at least one comonomer selected from the group consisting of mixtures thereof.

9. The method according to claim 1, wherein polymer B is poly(N-isopropylacrylamide).

10. A method for producing a porous polyacrylonitrile fiber, a) Polyacrylonitrile polymer (polymer A), A polymer (polymer B) different from the aforementioned polyacrylonitrile-based polymer, A step of forming a homogeneous solution comprising a first liquid containing a solvent for polymer A and in which polymer B is soluble, wherein the homogeneous solution contains polymer B at a concentration of 50% by weight or less relative to the total weight of the homogeneous solution. b) A step of co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid containing a solvent for polymer A and a non-solvent for polymer A, and in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) By contacting the polyacrylonitrile material with a third liquid containing a non-solvent for polymer A and in which polymer B is soluble, polymer B is selectively removed from the polyacrylonitrile material. This process involves manufacturing porous polyacrylonitrile fibers. Includes, The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are each selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl₂) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof. The second liquid comprises, in proportion to the total weight of the second liquid, the solvent for polymer A at 85% by weight or less, and the non-solvent for polymer A at 15% by weight or more. Polymer B is given by formula (I): 【Chemistry 1】 (In the formula: R １ is hydrogen or methyl, R ２ and R ３ A method comprising one or more repeating units derived from at least one monomer (where each is independently H or alkyl).

11. The method according to claim 10, wherein polymer B is a copolymer comprising monomer units derived from a monomer according to formula (I), and 50 weight percent ("weight %) or more of the repeating units of polymer B are derived from a monomer according to formula (I).

12. The method according to any one of claims 1 to 11, wherein step c) comprises stretching one or more fibers through one or more stretching baths and washing baths, wherein at least one bath contains the third liquid which contains a non-solvent for polymer A.

13. The method according to any one of claims 1 to 12, wherein in step a), the homogeneous solution contains polymer B in an amount of 10% by weight or less relative to the total weight of the homogeneous solution.

14. The method according to any one of claims 1 to 13, further comprising step d) drying the polyacrylonitrile fiber produced in step c).

15. The method according to any one of claims 1 to 14, further comprising step e) recovering polymer B at least partially.

16. A method for producing carbon fibers having a low density, (i) A step of producing a polyacrylonitrile fiber according to the method of any one of claims 1 to 15, (ii) A step of oxidizing the polyacrylonitrile fiber produced in step (i) to form stabilized carbon fiber precursor fibers, and then carbonizing the stabilized carbon fiber precursor fibers to produce carbon fibers having a density of 1.80 g / cm³ or less. Methods that include...

17. The density of the carbon fiber is 1.50 to 1.77 g / cm³. ３ The method according to claim 16.

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