PAN-BASED CARBON FIBER AND PRODUCTION METHOD THEREFOR

Abstract

 Provided are: a PAN-based carbon fiber comprising three or more phases different in crystal size, preferably having a sheath-core structure with three or more layers; and a production method therefor. By configuring a carbon fiber from three or more phases different in crystal size, or by a production method wherein a specified polymer for spinning is spun, stabilization is performed under specified conditions and thereafter carbonization is performed, time for the stabilization can be greatly shortened and productivity can be improved, and a PAN-based carbon fiber capable of exhibiting a high degree of elongation while maintaining a sufficiently high strength can be obtained.

Description

Technical Field of the Invention

[0001] The present invention relates to a polyacrylonitrile (hereinafter, referred to as PAN)-based carbon fiber comprising three or more phases different in crystal size, and a production method therefor.

Background Art of the Invention

[0002] Carbon fiber is broadly used in various uses, for example, for aerospace materials for airplanes, rockets, etc., and for sport articles such as tennis rackets and golf shafts, and further, is being used also for transportation and mechanical fields such as ships and vehicles, from its properties such as mechanical and chemical properties and lightness in weight. Further, in recent years, from high conductivity or high radiation property of carbon fiber, application to uses for parts for electronic equipment such as housings of portable telephones or personal computers, or for electrodes of fuel cells, is strongly required. In particular, PAN-based carbon fiber, because of its high specific strength, is used in particular for aerospace materials for airplanes, space satellites, etc., for members for vehicles, etc., and recently, application to vehicle members is being remarkably increased. Therefore, it is desired to improve the productivity of carbon fiber.

[0003] The PAN-based carbon fiber at the present time can be obtained by inducing a polymer solution dissolved with mainly PAN into a solvent to a PAN-based fiber by spinning, and burning it at a high temperature under a condition of an inert atmosphere. In case where the PAN-based fiber is made into a carbon fiber, the PAN-based fiber is passed through a process of air stabilization (cyclization reaction and oxidation reaction of PAN) which heats the PAN-based fiber in air at a high temperature such as 200 to 300°C. It is general to obtain a carbon fiber by further treating it in a carbonization furnace at 2,000°C to 3,000°C for several minutes. However, because an exothermic reaction progresses in the stabilization process, heat removal is required when a large amount of PAN-based fibers are stabilized. Therefore, for the temperature control, a long-time treatment is required, and it is necessary to restrict the fineness of the PAN-based precursor fiber to a small fineness of a specified value or less in order to finish the air stabilization in a desired period of time. Thus, in the process for producing a carbon fiber known at the present time, the stabilization process known at the present time is a rate-limiting factor, and it cannot be said to be a sufficiently efficient process.

[0004] Further, although a carbon fiber is excellent in specific strength and specific elastic modulus, it has a defect of a very low degree of elongation. Increase of degree of elongation of carbon fiber is strongly desired accompanying with increase of demand for carbon fiber. So far, in order to increase degree of elongation of carbon fiber, although a fiber spun with a raw material composition, the main component of which is a polymer compound compounded with an aromatic sulfonic group or a salt thereof via a methylene-type bond, is disclosed (Patent document 1), there is a defect that the cost of the main raw material is too high. Further, although technologies for intending to improve properties of carbon fiber by making a hollow carbon fiber or a dual-structured carbon fiber are also known (Patent documents 2 to 4), the degree of elongation thereof is still insufficient. Therefore, a long fiber of carbon fiber having a sufficient degree of elongation relative to its strength has not been obtained.

[0005] Namely, it is required to greatly shorten the time for stabilization of a fiber and obtain a carbon fiber having a high degree of elongation.

Prior art documents

Patent documents

[0006] 

Patent document 1: JP-A-6-173122

Patent document 2: JP-A-2008-169511

Patent document 3: JP-A-2007-291557

Patent document 4: JP-A-2001-73230

Summary of the Invention

Problems to be solved by the Invention

[0007] Accordingly, in order to satisfy the above-described requirements, an object of the present invention is to provide a PAN-based carbon fiber capable of greatly shortening the time for stabilization of a fiber and exhibiting a high degree of elongation while maintaining a sufficiently high strength, and a production method therefor.

Means for solving the Problems

[0008] To achieve the above-described object, a PAN-based carbon fiber according to the present invention comprises three or more phases different in crystal size.

[0009] In the above-described PAN-based carbon fiber according to the present invention, it is preferred that the respective phases are layered.

[0010] Further, it is preferred that this PAN-based carbon fiber has a sheath-core structure having three or more layers, and satisfies the following conditions of A to D:

A: in a sectional area in a direction perpendicular to a fiber axis, an area occupied by a core occupies 10 to 70% of the whole of the sectional area,

B: a thickness of a sheath is in a range of 100 nm to 10,000 nm,

C: a thickness of an intermediate layer is more than 0 nm and 5,000 nm or less, and

D: a diameter in the direction perpendicular to the fiber axis is 2 µm or more.

[0011] Further, it is preferred that the above-described PAN-based carbon fiber has a sheath-core structure having three or more layers, and satisfies the following conditions of E to H:

where, a crystal size of a core is referred to as Lc1, a crystal size of a sheath is referred to as Lc2, and a crystal size of an intermediate layer is referred to as Lc3.

E: Lc1/Lc3 ≧ 1.05,

F: Lcl/Lc2 ≧ 1.05,

G: 1.0 ≦ Lc1 ≦ 7.0 nm, and

H: Lc2 ≠ Lc3.

[0012] Further, in the above-described PAN-based carbon fiber having a sheath-core structure with three or more layers, it is preferred that an orientation degree f of a crystal of a core is 0.7 or less.

[0013] Further, it is preferred that the PAN-based carbon fiber according to the present invention is one obtained by carbonizing a fiber spun from a single kind of polymer solution for spinning.

[0014] Further, it is preferred that the PAN-based carbon fiber according to the present invention is one obtained by spinning a fiber from a polymer solution for spinning satisfying the following two points of A and B and carbonizing the spun fiber:

A: a polymer in the polymer solution for spinning is a polymer prepared by modifying PAN with an amine-based compound and oxidizing it with a nitro compound, and

B: the nitro compound is not contained in the polymer solution for spinning.

[0015] Further, in such a PAN-based carbon fiber, in the above-described A relating to a polymer in the polymer solution for spinning, it is preferred that it is obtained using a polymer solution for spinning which contains PAN oxidized using a nitro compound, in particular, nitrobenzene, at an amount of 10 wt% or more relative to PAN.

[0016] Furthermore, it is preferred that the PAN-based carbon fiber according to the present invention is one obtained using a polymer solution for spinning which has a divergent structure in which a gradient a is 0.1 or more and 0.3 or less as the result determined by GPC (Gel Permeation Chromatography).

[0017] Where, the gradient a means a gradient a represented by Mark Houwink-Sakurada equation (equation (1)):

where, [η] is an intrinsic viscosity, K is a constant inherent for a material, and Mw is a weight average molecular weight.

[0018] A method for producing a PAN-based carbon fiber according to the present invention comprises the steps of: spinning the above-described polymer solution for spinning; performing stabilization in air at 280°C or higher and 400°C or lower for 10 seconds or more and 15 minutes or less; and thereafter, performing carbonization. In case of this method, it is preferred that the stabilization is performed using an infrared heater (for example, a ceramic heater) and a hot air drier (for example, a hot air circulation drier) together.

Effect according to the Invention

[0019] In the PAN-based carbon fiber and the production method therefor according to the present invention, by configuring the carbon fiber from three or more phases different in crystal size, or by the production method wherein a specified polymer for spinning is spun, stabilization is performed under specified conditions and thereafter carbonization is performed, time for the stabilization can be greatly shortened and productivity can be improved, and a PAN-based carbon fiber capable of exhibiting a high degree of elongation while maintaining a sufficiently high strength can be obtained.

Brief explanation of the drawings

[0020] 

[Fig. 1] Fig. 1 shows a schematic sectional view in a direction perpendicular to a fiber axis showing an example of a sheath-core structure having three layers, and a partially enlarged view thereof.

[Fig. 2] Fig. 2 shows diagrams exemplifying electron diffractions of TEM (Transmission Electron Microscope) in a core, an intermediate layer and a sheath of a sheath-core structure.

[Fig. 3] Fig. 3 is a characteristic diagram showing distribution curves converted from the light and shade of the electron diffraction diagrams depicted in Fig. 2.

[Fig. 4] Fig. 4 is a schematic vertical sectional view showing an example of a hot air circulation furnace equipped with an infrared heater which is used for stabilization referred in the present invention.

Embodiments for carrying out the Invention

[0021] Hereinafter, embodiments of the present invention will be explained in detail. In the present invention, a carbon fiber means a fiber composed of 90% or more with C (carbon) component. It is possible to determine the content of C component by elemental analysis.

[0022] It is necessary that the PAN-based carbon fiber according to the present invention is a carbon fiber comprising three or more phases different in crystal size. By forming three or more phases, high functions can be provided to the carbon fiber. Further, the carbon fiber according to the present invention is preferably a carbon fiber in which the above-described respective phases are layered. By the layered structure, it tends that the strength of the carbon fiber is maintained and the carbon fiber has a high degree of elongation.

[0023] Further, the carbon fiber according to the present invention preferably forms a sheath-core structure having three or more layers in order to exhibit the properties of the present invention. For example, as shown in Fig. 1, the sheath-core structure 1 having three or more layers is a structure having an intermediate layer 3 (for example, a plurality of intermediate layers) between a core 2 and a sheath 4, which is a structure formed in three or more layers as a whole, and in particular, it is preferred to be a structure of three layers. In the sheath-core structure having three or more layers, it is more preferred that a crystal size of the core Lc1, a crystal size of the sheath Lc2, and a crystal size of the intermediate layer Lc3 have relationships of Lc1/Lc3 ≧ 1.05, Lcl/Lc2 ≧ 1.05, and 1.5 ≦ Lc1 ≦ 7.0 nm. More preferably, the relationships are Lc1/Lc3 ≧ 1.10 and Lcl/Lc2 ≧ 1.08. Further preferably, the relationships are Lc1/Lc3 ≧ 1.15 and Lcl/Lc2 ≧ 1.1. Lc referred here indicates an overlap thickness of graphite moment in a direction of fiber axis. The crystal size Lc of each layer can be determined by converting from the light and shade of the electron diffraction diagrams of TEM (Transmission Electron Microscope) exemplified in Fig. 2 to the distribution curves as shown in Fig. 3, and calculating Lc using a half-value width of each peak. For example, a crystal size can be calculated as a relative value of the known Lc of T300 (carbon fiber supplied by Toray Industries, Inc.). Where, in Fig.2, a portion appearing in a rod-like form is a shade of a measuring device.

[0024] Furthermore, in order that a core becomes in a softer condition, an orientation degree f of the core is preferably 0.7 or less, and more preferably 0.6 or less.

[0025] By forming such a structure, a high degree of elongation of a carbon fiber can be achieved. The reason of a high degree of elongation is supposed in that, by forming an intermediate layer as a hard layer, relatively soft sheath and core take charge of impact caused when the intermediate layer is broken, and the carbon fiber elongates without reaching breakage.

[0026] A high degree of elongation of a carbon fiber in the present invention means one in a range of 1.1% or more and 2.5% or less, more preferably in a range of 1.2 to 2.5%, and particularly preferably in a range of 1.3 to 2.5%. To the contrary, a low degree of elongation means one of 1.0% or less. Here, the higher the degree of elongation is, the better molding processing property becomes, thereby suppressing occurrence of fluffs in a process for obtaining a final product.

[0027] Next, the thicknesses of the respective layers in the carbon fiber will be explained. It is preferred that the core occupies 10 to 70% relative to the cross-sectional area of the fiber, the thickness of the sheath is in a range of 100 nm to 10,000 nm in a direction perpendicular to the fiber axis so as to cover the core, and the thickness of the intermediate layer is more than 0 nm and 5,000 nm or less. More preferably, the thickness of the intermediate layer is in a range of 100 nm to 5,000 nm. Further, it is preferred that the core occupies 30 to 50% relative to the cross-sectional area of the fiber.

[0028] A flame resistant fiber in the present invention is liable to be flattened in section at an initial stage of carbonization, and tends to become a fiber bundle intermingled with flat yarns. By flattening, because the surface area of the fiber increases, the fiber bundle easily radiates heat, and the time for stabilization tends to be able to be shortened. The cross-sectional shape of a fiber can be observed by a laser microscope. The rate of interminglement of flat yarns was determined by counting the numbers of non-circular ones and circular ones, respectively, in a photograph taken at 1,000 times in magnification of a section of a fiber bundle using a laser microscope. Here, the counting was performed by referring a single yarn with a ratio of a minor axis to a major axis in a range of 1 to 0.8 as a circular one, and a single yarn with a ratio of a minor axis to a major axis in a range of 0.1 or more and less than 0.8 as a flat yarn.

[0029] Next, several characteristics of the production method for obtaining a carbon fiber according to the present invention will be raised.

[0030] In the carbon fiber according to the present invention, because it is possible to obtain a carbonized yarn having a sheath-core structure with three or more layers by wet spinning a single kind of polymer and burning it, there is a merit that it is not necessary to perform compounding, coating, etc., after spinning. Further, because the respective layers are strongly combined by performing spinning and burning and forming three or more layers from a single kind of polymer, achieved is a structure in which it is possible to supplement poor points of the layers each other, as aforementioned.

[0031] Next, a polymer solution for spinning will be described. The polymer solution for spinning is preferred to be a polymer prepared by modifying PAN with an amine-based compound and oxidizing it with a nitro compound.

[0032] In the present invention, by using a polymer solution for spinning which does not contain a nitro compound, it tends to become possible that an exothermic reaction in a stabilization of a spun fiber is suppressed, thereby realizing a stabilization of the fiber within a shorter period of time. Furthermore, by using a polymer solution for spinning which does not contain a nitro compound, because nitrobenzene does not exist in the spun coagulated yarn and/or dried fiber, it is possible to form a carbon fiber having a three-layer structure through stabilization and carbonization. In case where a nitro compound is left in a polymer solution for spinning, it is supposed that the nitro compound in the fiber operates as an oxidant even in the process of stabilization, and it is considered that this oxidation during formation of a structure of a fiber is a cause of becoming a carbon fiber with a two-layer structure. As a method for controlling an amount of the nitro compound left in a polymer solution for spinning to 0%, there are two kinds of methods of a method for removing it by washing with ethanol after PAN is modified with an amine-based compound and a nitro compound, and a method for making a nitro compound easily react by increasing an amount of an amine-based compound. Since the washing takes time and cost and there is a possibility of being left in the polymer, more preferred is the latter method for controlling an amount of the residual nitro compound to 0% in the reaction system. Concrete explanation of such a method will be described later.

[0033] In PAN composed of only acrylonitrile, a long period of time is required for stabilization of a fiber after spinning, further, burning and fusion and the like are caused during the stabilization of the fiber, and the properties of a carbon fiber finally made tend to be lowered.

[0034] As a state "modified with an amine-based compound" referred here, exemplified is a state where an amine-based compound is chemically reacted with PAN as a raw material, or a state where an amine-based compound is incorporated into a polymer by hydrogen bonding or an interaction such as van der Waals force.

[0035] It is determined by the following methods whether a polymer for spinning is modified with an amine-based compound or not.

A. Method for analyzing a difference in structure with a polymer which is not modified, by spectroscopic manner, for example, using NMR spectrum, infrared absorption (IR) spectrum, etc. aforementioned .

B. Method for determining masses of a polymer before and after making a polymer for spinning by a method described later and confirming whether the mass of the polymer for spinning is increased relatively to the mass of PAN as a raw material or not.

[0036] In case of the former method, a section originating from an amine-based compound used as a modifier is added as a new spectrum in a spectrum of a polymer for spinning modified with the amine-based compound, relatively to a spectrum of PAN as a raw material.

[0037] The mass of a polymer for spinning modified with an amine-based compound increases by 1.1 times or more, preferably 1.2 times or more, particularly preferably 1.3 times or more, relatively to PAN as a raw material. Further, in case of being increased, the upper limit is preferably 3 times or less, more preferably 2.6 times or less, and further preferably 2.2 times or less. If the change in mass is smaller or greater than such a range, there is a possibility that the spinning property is damaged and the strength or the degree of elongation of a carbon fiber is reduced.

[0038] As an amine-based compound capable of being used for modifying a polymer for spinning, although any of compounds having primary to quaterary amino group may be employed, concretely, polyethylene polyamines such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine and N-aminoethyl piperazine, and ortho, meta and para phenylene diamines can be exemplified.

[0039] In particular, it is also preferred to have a functional group having an element of oxygen, nitrogen, sulfur, etc. such as a hydroxyl group except an amino group, and it is preferably a compound having two or more functional groups including an amino group and such a functional group except the amino group, from the viewpoint of reactivity, etc. Concretely, ethanol amine group such as monoethanol amine, diethanol amine, triethanol amine and N-aminoethyl ethanol amine can be exemplified. Among these, in particular, monoethanol amine is more preferred. These can be used solely or at a combination of two or more kinds. In case of a compound having a functional group except an amino group, for example, having a hydroxyl group, there is a possibility that the hydroxyl group modifies a polymer for spinning.

[0040] The nitro compound in the present invention is an oxidant, and oxidizes PAN. Therefore, the fiber spun using PAN modified with an amine and oxidized by a nitro compound tends to be able to be finished with stabilization in a very short period of time of 10 seconds or more and 15 minutes or less. As the nitro compound, concretely, an oxidant of nitro-based, nitroxide-based, etc. can be exemplified. Among these, as particularly preferable ones, aromatic nitro compounds such as nitrobenzene, o, m, p-nitrotoluene, nitroxylene, o, m, p-nitrophenol and o, m, p-nitrobenzoic acid can be exemplified. In particular, nitrobenzene having a simple structure is most preferably used, since it is little in risk, and a quick oxidation is possible because of less steric hindrance.

[0041] Although the amount to be added of these oxidants is not particularly restricted, in order that PAN is sufficiently oxidized in the present invention, it is preferred to use a nitro compound at 10 wt% or more relative to PAN, more preferably 15 wt% or more. Further, as the amount to be added of a nitro compound, in order to control the remaining rate of the nitro compound in the aforementioned polymer solution for spinning at 0%, it is preferred to use 1 to 50 parts by mass relatively to 100 parts by mass of an amine-based compound to be employed. It is more preferred to use 20 to 45 parts by mass. At that time, the reaction temperature is preferably in a range of 130 to 300°C, and more preferably in a range of 130 to 250°C. The reaction time is preferably 4 hours or more and 10 hours or less, and more preferably 5 hours or more and 8 hours or less. If heated for a time more than 10 hours, a polymer is too damaged, and finally the strength of a carbon fiber is reduced. In case of a time less than 4 hours, the nitro compound is liable to be left in the system, the structure of a carbon fiber finally obtained does not become three layers, and the degree of elongation tends to be reduced.

[0042] In case where PAN is modified under a condition present with an amine-based compound after being dissolved in a polar organic solvent, the amine-based compound and the polar organic solvent and an oxidant may be mixed before addition of PAN and may be simultaneously with addition of PAN. It is preferred that first PAN, an amine-based compound and a polar organic solvent are mixed, and after dissolution by heating, a polymer for spinning is prepared by adding an oxidant, from the viewpoint of less insoluble substances. Of course, it is not obstructed to mix a component other than PAN, an oxidant, an amine-based compound and a polar organic solvent with such a solution.

[0043] Where, in a polymer for spinning used in the present invention, inorganic particles such as alumina or zeolite, a pigment such as carbon black, an antifoaming agent such as silicone, stabilizer· flame retardant such as a phosphorus compound, various kinds of surfactants, and other additives may be contained. Further, for the purpose of improving the solubility of a polymer for spinning, an inorganic compound such as lithium chloride or calcium chloride can be contained. These may be added before expediting the reaction, and may be added after expediting the reaction.

[0044] Further, the molecular weight and the shape of a polymer for spinning used in the present invention are determined by GPC, and it is preferred that the value of the gradient a (hereinafter, referred to as "a") is in a range of 0.1 to 0.3. The "a" determined by GPC means "a" represented by Mark Houwink-Sakurada equation (equation (1)).

Where, [η] is an intrinsic viscosity, K is a constant inherent for a material, and Mw is a weight average molecular weight.

[0045] It is known that a polymer exists in a polymer solution as a rod-like polymer as the value of this gradient "a" is closer to 2, as a random coil-like polymer as closer to 0.7, and as a spherical polymer as closer to 0.

[0046] It is preferred that the "a" of a polymer for spinning used in the present invention is in a range of 0.1 to 0.3, and it is understood that the polymer for spinning becomes a divergent structure much closer in shape to a spherical shape than to a rod-like shape. By employing a divergent structure, molecules are more intertwined with each other as compared with case of employing a straight-chain structure. Accordingly, in case where stabilization of a spun fiber is performed, molecules of the polymer are easily combined with each other, and the time for the stabilization of the fiber tends to be able to be shortened. Therefore, when the "a" exceeds 0.3, the stabilization becomes insufficient, there are a tendency to be decomposed in a carbonization process and a tendency that the differences between the "Lc"s and between the orientation degree "f"s of three layers of a carbon fiber are smallened and the degree of elongation is reduced. Further, when the "a" becomes less than 0.1, because the molecular weight itself is being greatly decreased, spinning becomes difficult. Further, even if spinning can be carried out, the strength of the fiber tends to be fairly reduced.

[0047] Next, PAN as a raw material will be explained.

[0048] PAN used in the present invention may be a homo PAN and may be a copolymerized PAN. With the copolymerized PAN, from the view point of the solubility of a polymer and the flame resistant property of a fiber, the structural unit originating from acrylonitrile (hereinafter, referred to as AN) is preferably 85 mol% or more, more preferably 90 mol% or more, and further preferably 92 mol% or more.

[0049] As concrete copolymerization components, allyl sulfonic acid metal salt, methallyl sulfonic acid metal salt, acrylic ester, methacrylic ester, acrylic amide, etc. can be also copolymerized. Further, except the above-described copolymerization components, as components for accelerating stabilization, components containing a vinyl group, concretely, acrylic acid, methacrylic acid, itaconic acid, etc., can also be copolymerized, and a part or the whole amount thereof may be neutralized with an alkali component such as ammonia.

[0050] Further, in PAN as a raw material, it is preferred that the "a" determined by GPC is 0.4 or more and 0.7 or less.

[0051] In case where PAN is dissolved in a polar organic solvent, the shape and form of the PAN may be any of powder, flake and fiber, and polymer waste, yarn waste, etc. generated during polymerization or at the time of spinning can also be used as recycled raw material. Desirably, it is preferred to be in a form of powder, in particular, microparticles of 100 µm or less, from the viewpoint of solubility into solvent.

[0052] The polymer solution for spinning used in the present invention can be made by dissolving a polymer for spinning in an organic solvent. With respect to the concentration of the polymer solution for spinning, in case where the concentration is low, the productivity at the time of spinning tends to be low although the effect due to the present invention itself is not damaged, and in case where the concentration is high, the flowability is poor and it tends to be hard to be spun. In consideration of being served to spinning, it is preferably in a range of 8 to 30 mass%. Here, the concentration of the polymer for spinning can be determined by the following method.

[0053] The polymer solution for spinning is weighed, the solution of about 4g is put into distilled water of 500 ml, and this is boiled up. A solid material is once taken out, it is again put into distilled water of 500 ml, and this is boiled up. A residual solid component is placed on an aluminum pan, dried for one day by an oven heated at a temperature of 120°C, and a polymer for spinning is isolated. The isolated solid component is weighed, and the concentration is determined by calculating a ratio with the mass of the original polymer solution for spinning.

[0054] Further, the polymer for spinning used in the present invention tends to be easily made into a solution when employing, in particular, a polar organic solvent as the solvent among organic solvents. This is because the polymer for spinning modified with an amine-based compound is high in polarity and the polymer is well dissolved by a polar organic solvent.

[0055] Here, the polar organic solvent means a solvent having an amino group, an amide group, a sulfonyl group, a sulfone group, etc. and further having a good compatibility with water, and as concrete examples, ethylene glycol, diethylene glycol, triethylene glycol, a polyethylene glycol having a molecular weight of about 200 to 1,000, dimethyl sulfoxide (hereinafter, also abbreviated as DMSO), dimethyl formamide, dimethyl acetamide, N-methyl pyrrolidone, etc. can be used. These may be used solely, and may be used as a mixture of two or more kinds. In particular, DMSO is preferably used because of its high dissolvability relative to PAN.

[0056] The viscosity of the polymer solution for spinning in the present invention can be set in respective preferable ranges depending upon a forming method or a molding method using the polymer, a molding temperature, a kind of a die or a mold and the like, etc. Generally, it can be used in a range of 1 to 1,000 Pa· s in the measurement at 50°C. More preferably, it is in a range of 10 to 100 Pa· s, and further preferably, it is in a range of 20 to 600 Pa· s. Such a viscosity can be measured by various viscosity measuring devices, for example, a rotary-type viscometer, a rheometer, a B-type viscometer, etc. The viscosity determined by any one method may be controlled in the above-described range. Further, even if out of such a range, by heating or cooling at the time of spinning, it can be used as an appropriate viscosity.

[0057] As the method for obtaining a polymer solution for spinning in the present invention, the following methods are exemplified.

A. A method for modifying PAN with an amine and oxidizing with a nitro compound in a solution as described above.

B. A method for isolating PAN modified with an amine and oxidized with a nitro compound, and directly dissolving it in a solvent.

[0058] In case of directly dissolving PAN spun after modification and oxidation in an organic solvent, the dissolution may be performed under an atmospheric pressure, and as the case may be, it may be performed under a pressurized or pressure-reduced condition. As an apparatus used for the dissolution, except a usual reaction vessel with an agitator, a mixer such as an extruder or a kneader can be used solely or at a form of combination thereof.

[0059] In this case, the dissolution is preferably performed using an amine-based compound and a polar organic solvent at the sum thereof in a range of 100 to 1,900 parts by mass, preferably in a range of 150 to 1,500 parts by mass, relative to 100 parts by mass of an acrylic-based polymer.

[0060] Although it is preferred that non-reacted substances, insoluble substances, gel, etc. are not contained in the polymer solution for spinning obtained by the above-described method and used in the present invention, there is a possibility that they are left at a fine amount. As the case may be, it is preferred to filtrate or disperse non-reacted substances or insoluble substances using a sintered filter or the like before formation into fibers.

[0061] Next, the method for producing a flame resistant fiber suitable for obtaining a carbon fiber according to the present invention will be explained.

[0062] As the method for spinning the polymer solution for spinning into a fiber, a wet spinning or a dry/wet spinning is employed in order to improve the productivity of process. Preferably, a wet spinning is used.

[0063] Concretely, the spinning can be performed by preparing the aforementioned polymer solution for spinning as a polymer solution for spinning, elevating the pressure through a pipe by a booster pump, etc., extruding with metering by a gear pump, etc., and discharging from a die. Here, as the material of the die, SUS (stainless), gold, platinum, etc. can be appropriately used.

[0064] Further, it is preferred that, before the polymer solution for spinning flows into holes of the die, the polymer solution for spinning is filtrated or dispersed using a sintered filter of inorganic fibers or using a woven fabric, a knitted fabric, a nonwoven fabric, etc. comprising synthetic fibers such as polyester or polyamide as a filter, from the viewpoint that the fluctuation of the cross-sectional areas of single fibers in a fiber aggregate to be obtained can be reduced.

[0065] As the hole diameter of the die, an arbitrary one in a range of 0.01 to 0.5 mmϕ can be employed, and as the hole length, an arbitrary one in a range of 0.01 to 1 mm can be employed. Further, as the number of the holes of the die, an arbitrary one in a range of 10 to 1,000,000 can be employed. As the hole arrangement, an arbitrary one such as a staggered arrangement can be employed, and the holes may be divided in advance so as to realize easy yarn dividing.

[0066] Coagulated yarns are obtained by discharging the polymer solution for spinning from the die directly or indirectly into a coagulation bath. It is preferred that the liquid for the coagulation bath is formed from a solvent used for the polymer solution for spinning and a coagulation acceleration component, from the viewpoint of convenience, and it is more preferred to use water as the coagulation acceleration component. Although the rate of the solvent for spinning to the coagulation acceleration component in the coagulation bath and the temperature of the liquid for the coagulation bath are appropriately selected and set in consideration of denseness, surface smoothness, spinnability, etc. of the coagulated yarns to be obtained, in particular, as the concentration of the coagulation bath, an arbitrary concentration can be employed within a range of solvent/water = 0/100 to 95/5, and a range of 30/70 to 70/30 is preferable, and a range of 40/60 to 60/40 is particularly preferable. Further, as the temperature of the coagulation bath, an arbitrary temperature in a range of 0 to 100°C can be employed. Further, as the coagulation bath, if an alcohol such as propanol or butanol reducing an affinity with water is employed, it can also be used as 100% bath.

[0067] Here, in the method for producing a carbon fiber according to the present invention, the degree of swelling of the coagulated yarn obtained is controlled preferably in a range of 50 to 1,000 mass%, more preferably in a range of 200 to 900 mass%, and further preferably in a range of 300 to 800 mass%. The degree of swelling of the coagulated yarn controlled in such a range greatly relates to the toughness and easiness in deformation of the coagulated yarn and affects the spinnability. The degree of swelling is decided from the viewpoint of spinnability, and affects a stretching property in bath at a later process, and if in such a range, the coefficient of variation of the cross-sectional areas of single fibers can be made small in the carbon fibers to be obtained. Where, the degree of swelling of the coagulated yarn can be controlled by the affinity between the polymer for spinning forming the coagulated yarn and the coagulation bath and the temperature or the concentration of the coagulation bath, and a degree of swelling in the above-described range can be achieved by controlling the temperature of the coagulation bath or the concentration of the coagulation bath in the aforementioned range relatively to a specified polymer for spinning.

[0068] Next, it is preferred that the coagulated yarn is stretched in a stretching bath or washed in a water washing bath. Of course, it may be stretched in a stretching bath as well as washed in a water washing bath. The draw ratio for the stretching is preferably in a range of 1.05 to 5 times, more preferably in a range of 1.1 to 3 times, and further preferably in a range of 1.15 to 2.5 times. For the stretching bath, hot water or solvent/water is used, and the concentration of solvent/water for the stretching bath can be set at an arbitrary concentration in a range of 0/100 to 80/20. Further, for the water washing bath, usually hot water is used, and the temperature of both the stretching bath and the water washing bath is preferably in a range of 30 to 100°C, more preferably in a range of 50 to 95°C, and particularly preferably in a range of 65 to 95°C.

[0069] In the present invention, the fiber completed with coagulation is dried, and as needed, stretched to become a carbon fiber through stabilization and carbonization.

[0070] As the drying method, a drying method for bringing the fiber into contact directly with a plurality of dried and heated rollers, a drying method for sending hot air or water vapor, a drying method for irradiating infrared rays or electromagnetic rays with a high frequency, a drying method for making a pressure reduced condition, etc. can be appropriately selected and combined. Usually, in case of a drying method due to hot air, hot air is sent in a direction parallel or perpendicular to the running direction of the fiber. For the infrared rays of radiation-heating type, far infrared rays, mid infrared rays or near infrared rays can be employed, and radiation of microwaves can also be employed. Although the temperature for the drying can be employed arbitrarily in a range of approximately 50 to 250°C, generally, the drying takes a long time in case of a low temperature and a short time in case of a high temperature.

[0071] In case where stretching is carried out after drying, the specific gravity of the fiber after drying is usually in a range of 1.15 to 1.5, preferably in a range of 1.2 to 1.4, and more preferably in a range of 1.2 to 1.35. The coefficient of variation of the cross-sectional areas of single fibers in the fiber aggregate after drying is preferably in a range of 5 to 30%, more preferably in a range of 7 to 28%, and further preferably in a range of 10 to 25%. Further, the elongation of the single fiber in the fiber aggregate after drying is preferably in a range of 0.5 to 20%. Furthermore, in the fiber aggregate after drying, oxidation calorific value (J/g) determined by differential scanning calorimetry (DSC) is preferably in a range of 50 to 4,000 J/g. As the case may be, not a continuous drying but a batch drying can be carried out.

[0072] For such a stretching process, because a fiber is plasticized with moisture, it is preferred to use a method for heating the fiber at a condition of containing water in the fiber such as a bath stretching using warm water or hot water, a stretching using steam (water vapor), or a heat stretching by a dryer or rolls after providing water to the fiber in advance, and heating/stretching by steam stretching is particularly preferred.

[0073] In case of using a bath stretching, it is preferred that the stretching is carried out at a temperature of, preferably 70°C or higher, more preferably 80°C or higher, and further preferably 90°C or higher. At this stage, the fiber structure is already densified, even if the temperature is elevated, there is no fear of generating micro voids, and a stretching at a temperature as high as possible is preferred because a high effect due to molecular orientation can be obtained. Although it is preferred to use water for the bath, the stretching property may be further enhanced by adding a solvent or other additives.

[0074] Although a higher stretching temperature is preferred, in a bath stretching, basically 100°C becomes the upper limit. Accordingly, a stretching using steam is employed more preferably. Although the temperature for the stretching is preferred to be higher, in case where a saturated vapor is used, because the internal pressure of the apparatus is high, there is a possibility that the fiber is damaged by blow of the vapor. For the purpose of obtaining a carbon fiber with a degree of orientation of the sheath of 65% or more, a saturated vapor with a temperature of 100°C or higher and 150°C or lower may be used. If the temperature exceeds 150°C, the effect due to the plasticization gradually gets to the top, and the damage of the fiber due to the blow of vapor becomes greater than the effect due to the plasticization. As the stretching treatment apparatus using a saturated vapor, an apparatus devising to pressurize the inside of the treatment apparatus by providing a plurality of apertures at the fiber inlet and outlet is preferably used.

[0075] In order to prevent the damage of the fiber due to the blow of vapor, it is also possible to use a super-heated atmospheric high-temperature steam. This becomes possible by heating an atmospheric steam using electric heating, water vapor heating, induction heating, etc., and thereafter, introducing it into the stretching treatment apparatus. Although it is possible to employ a range of 100°C or higher and 170°C or lower for the temperature, it is preferred to be 110°C or higher and 150°C or lower. If the temperature is too high, the moisture contained in the steam is reduced, and the effect for plasticizing the fiber becomes hard to be obtained.

[0076] The draw ratio for the bath stretching and the draw ratio for the stretching by steam are preferably 1.5 times or more, and more preferably 2.0 times or more. In order to promote the molecular orientation, the draw ratio for the stretching is preferred to be higher, and an upper limit thereof is not particularly present. However, from restriction on stability for spinning, it is frequently difficult to exceed about 6 times.

[0077] Further, in the method for stretching the fiber in the present invention, the means thereof is not restricted to the bath stretching or the steam stretching. For example, heat stretching by a drying furnace or a hot roller, etc. after providing moisture may be possible.

[0078] A non-contact type stretching machine using a drying furnace, further, a contact type stretching machine using a contact plate, a hot roller, etc., can also be used. However, in case of a contact type stretching machine, evaporation of moisture is fast, and further, there is a high possibility that a fiber is mechanically scratched at a point occurred with stretching. Further, in case of a non-contact type stretching machine, a required temperature becomes 250°C or higher, and as the case may be, thermal decomposition of the polymer starts. Furthermore, in case where a non-contact type stretching machine or a contact type stretching machine is used, the effect due to stretching is low, and it is more difficult to obtain a carbon fiber with a high orientation than the stretching method using moisture. From these reasons, it is more preferred to use a bath stretching or a steam stretching.

[0079] The stretched yarn thus stretched is preferably dried again, as needed. The moisture percentage of the fiber is preferably 10% or less, and more preferably 5% or less. As this drying method, bringing the fiber into contact directly with a plurality of dried and heated rollers or hot plates, sending hot air or water vapor, irradiating infrared rays or electromagnetic rays with a high frequency, making a pressure reduced condition, etc. can be appropriately selected and combined. It is preferred to employ drying due to rollers in order to perform an efficient drying. The number of the rollers is not restricted. The temperature of the rollers is preferably 100 °C or higher and 250°C or lower, and more preferably 150°C or higher and 200°C or lower. If the drying at this process is insufficient, there is a possibility to cause a fiber breakage when a tension is applied to the fiber at a heat treatment process carried out later.

[0080] In the present invention, to the coagulate yarn, or the fiber at a water swelling state after being water washed and stretched, an oil component can be appropriately provided depending upon the necessity of a higher-order processing. In case where an oil component is provided, usually the concentration of the oil is set at 0.01 to 20 mass%. As the method for providing, it may be appropriately selected and employed in consideration of being provided uniformly up to the interior of the yarn. Concretely, a method such as dipping of the yarn into an oil bath or spray or dropping onto the running yarn is employed. Here, the oil comprises, for example, a main oil component such as silicone and a diluent component for diluting it. The concentration of oil means a content of the main oil component relative to the whole of the oil. The kind of the oil component is not particularly restricted, polyether-based one, polyester surfactant, silicone, amino-modified silicone, epoxy-modified silicone or polyether-modified silicone can be provided solely or at a mixture thereof, and other oil components may be provided.

[0081] The adhesion amount of such an oil component is determined as a rate relative to the dried mass of the fiber included with the oil component, and it is preferably in a range of 0.05 to 5 mass%, more preferably in a range of 0.1 to 3 mass%, and further preferably in a range of 0.1 to 2 mass%. If the adhesion amount of an oil component is too little, there is a possibility that fusion of single fibers to each other occurs and the tensile strength of an obtained carbon fiber is reduced, and if too much, there is a possibility that it becomes difficult to obtain the effect due to the present invention.

[0082] The fiber obtained by the above-described process is transferred to a process for stabilization. Where, the fiber before being transferred to the stabilization process is preferably in a dried condition. As the method for stabilization, in particular, it is preferred to use a dry-heating apparatus in order to control chemical reaction and suppress unevenness in fiber structure, and concrete equipment thereof will be described later. The temperature and the treatment length are appropriately selected depending upon the oxidation degree of the used polymer for spinning, the fiber orientation degree and the required properties for a final product. Concretely, the treatment temperature for the stabilization is preferably 280°C or higher and 400°C or lower. More preferably, it is 300 °C or higher and 360 °C or lower, and particularly preferably, it is in a range of 300°C to 330°C. If the temperature is lower than 280°C, a problem tends to occur in a carbonization process. If the temperature exceeds 400°C, the fiber tends to be decomposed in a stabilization furnace. The treatment time of the stabilization is preferably 10 seconds or longer in order to prevent decomposition in a carbonization process. Further, in case where the treatment time of the stabilization exceeds 15 minutes, because the merit for shortening the time for stabilization becomes small and besides the fiber is fuzzed to cause reduction of strength and degree of elongation, it is preferred that the treatment time of the stabilization is 15 minutes or shorter. From the viewpoint of suppressing occurrence of fluffs, more preferably it is 5 minutes or shorter.

[0083] Further, it is preferred to perform a stretching when the heat treatment is carried out. By carrying out the stretching treatment, the molecular orientation can be further enhanced. The draw ratio for this stretching is preferably in a range of 1.05 to 4 times. The draw ratio is set from required strength and fineness of the flame resistant fiber, process passing-through property and the temperature of the heat treatment. Concretely, the draw ratio for the stretching is set in a range of 1.1 to 4 times, preferably in a range of 1.2 to 3 times, and more preferably in a range of 1.3 to 2.5 times. Further, it is also important to perform heat treatment at the time of stretching, and as the time for the heat treatment, an arbitrary value in a range of 1 to 15 minutes can be employed depending upon the temperature. The stretching and the treatment for stabilization may be performed either simultaneously or separately.

[0084] Among dry-heating apparatuses, in particular, it is preferred to use an infrared heater and a hot air drier together. By employing heating due to an infrared heater and a hot air drier together, the treatment time for stabilization tends to be shortened.

[0085] Here, to use an infrared heater and a hot air drier together includes to treat separately from each other, and it is particularly preferred to provide an infrared heater in a hot air circulation drier and perform simultaneous treatment of emission (radiation) and heat transfer by the integrated hot air circulation drier equipped with the infrared heater. By using the integrated apparatus, high temperature-elevation· short-time treatment due to the infrared heater and uniform treatment of single fibers due to hot air can be achieved simultaneously. Although a metal, a ceramic, etc. can be used as the material of the infrared heater, it is preferred to be made from a ceramic from its high heat radiation rate and high thermal stability.

[0086] A schematic structure of a hot air circulation drier equipped with an infrared heater is exemplified in Fig. 4, and as shown in the figure, it can be manufactured, for example, by providing two or more openings 15a, 15b to a forced-type hot air circulation drier 11 sold on the market so as to be able to treat a fiber continuously, and further, attaching an electric ceramic heater 16 sold on the market (for example, a ceramic plate heater "PLC-323", supplied by NORITAKE CO., LTD.) inside the drier. It is preferred that two or more ceramic heaters are installed, and further, it is particularly preferred that they are installed so as to be able to irradiate the infrared rays to the fiber from both directions of upper and lower sides or left and right sides in order to irradiate the infrared rays to the fiber uniformly. With respect to the treatment by the hot air circulation drier 11, for example, a non-treated fiber 12 (fiber before treatment) is introduced into hot air circulation drier 11 from opening 15a while being guided by a roller 14a, it is irradiated with the infrared rays from both directions of upper and lower sides by ceramic heaters 16 attached to, for example, punching metals 17 for attaching ceramic heaters, and at the same time, heat transfer treatment due to hot air (the flow of the hot air is shown by arrows 18) is performed, and a stabilized fiber 13 (fiber after treatment) is sent out from opening 15b while being guided by a roller 14b.

[0087] Here, as the circulation system of the hot air circulation drier, both a down flow system and an up flow system can be applied. As a fan for controlling the circulation amount of hot air, although a propeller fan and a sirocco fan can be used, it is preferred to use a sirocco fan from the viewpoint of its good wind resistance. It is preferred to rotate this fan by a motor after conversion to a direct current by an inverter. As a concrete inverter, "FR-E720-0.2K" supplied by Mitsubishi Electric Corporation can be exemplified, and as an induction motor, "5IK60A-SF" supplied by ORIENTAL MOTOR Co., Ltd. can be exemplified. Further, as the rotational speed of the fan, it is preferably in a range of 500 to 1,500 rpm, and in order to shorten the treatment time within a range which does not cause to fuzz, particularly preferably it is in a range of 800 to 1,200 rpm.

[0088] Furthermore, in the present invention, by suppressing exothermic reaction at the time of stabilization, it is enabled to shorten the treatment time for stabilization and to perform stabilization, which has been performed by two furnaces, by a single furnace.

[0089] The fibers having been spun are in a bundle form comprising a plurality of single fibers, the number of single fibers included in a single bundle can be appropriately selected depending upon the purpose of use, and in order to control the aforementioned preferred number, it can be adjusted by the number of holes of a die, and a plurality of spun fibers may be doubled.

[0090] Further, in order to control the fineness of the single fiber in the aforementioned preferable range, it can be controlled by selecting the hole diameter of a die or appropriately deciding the discharge amount from a die.

[0091] Further, in case where the fineness of a single fiber is made greater, making the time for drying longer, or elevating the temperature for drying higher, is preferred from the viewpoint of reduction of the amount of residual solvent.

[0092] Further, the cross-sectional shape of a single fiber can be controlled by the shape of a discharge hole of a die such as a circular hole, an oval hole or a slit and the condition at the time of removing a solvent.

[0093] Next, a production method suitable for obtaining a carbon fiber according to the present invention using the obtained flame resistant fiber will be explained.

[0094] A carbon fiber is obtained by heat treating the flame resistant fiber obtained in the present invention at a high temperature in an inert atmosphere, so-called carbonizing. As a concrete method for obtaining a carbon fiber, a carbon fiber can be obtained by treating the aforementioned flame resistant fiber according to the present invention at a highest temperature in an inert atmosphere in a range of 1,000°C or higher and lower than 2,000°C. More preferably, as the lower side of the highest temperature, 1,000 °C or higher, 1,200 °C or higher and 1,300 °C or higher are preferred in order, and as the upper side of the highest temperature, 1,800°C or lower can also be employed. Further, by further heating such a carbon fiber in an inert atmosphere at a temperature in a range of 2,000 to 3,000°C, a carbon fiber developing in graphite structure can also be obtained.

[0095] In the carbon fiber according to the present invention, the density is preferably in a range of 1.6 to 1.9 g/cm³, and more preferably in a range of 1.7 to 1.9 g/cm³. If such a density is too small, there is a possibility that many pores are present in a single fiber and the fiber strength is reduced, and on the contrary, if too great, there is a possibility that the denseness becomes too high and the degree of elongation is reduced. Such a density can be determined utilizing immersion method or sink-float method based on JIS R 7603(1999).

[0096] In the carbon fiber according to the present invention, usually, the single fibers of the carbon fibers are gathered to form an aggregate such as a fiber bundle. In case of forming the fibers as a bundle, although the number of single fibers per one bundle is appropriately decided depending on the purpose of use, from the viewpoint of higher-order processing property, it is preferably in a range of 50 to 100,000/bundle, more preferably in a range of 100 to 80,000/bundle, and further preferably in a range of 200 to 60,000/bundle.

[0097] In the carbon fiber according to the present invention, the tensile strength of a single fiber is preferably in a range of 1.0 to 10.0 GPa, more preferably in a range of 1.5 to 7.0 GPa, and further preferably in a range of 2.0 to 7.0 GPa. Such a tensile strength can be determined based on JIS R7606(2000) using a universal tensile testing machine (for example, small-sized desk-top tester EZ-S, supplied by Shimadzu Corporation).

[0098] In the carbon fiber according to the present invention, it is desired that the diameter of the single fiber is 2 µm or more, in particular, in a range of to 2 µm to 70 µm, preferably in a range of 2 to 50 µm, and more preferably in a range of 3 to 20 µm. If such a diameter of the single fiber is less than 2 µm, there is a possibility that the fiber is liable to be broken, and if more than 70 µm, a defect rather tends to be caused. Where, the single fiber of the carbon fiber may be one having a hollow portion. In this case, the hollow portion may be either continuous or discontinuous.

[0099] From the viewpoint of reducing cost, it is preferred to produce a carbon fiber continuously by one process from a polymer for spinning to the carbon fiber.

[0100] The carbon fiber according to the present invention tends to have a peak nearly at 26° in X-ray diffraction (XRD) similarly in a general PAN-based carbon fiber.

Examples

[0101] Next, the present invention will be explained more concretely by Examples. Where, in the Examples, the respective properties and characteristics were determined by the following methods.

<Preparation of polymer solutions for spinning (a, c to e)>

[0102] A thermometer, a cooler, an agitator and a nitrogen introducing tube were attached to a three neck flask having a sufficient capacity. In this flask, PAN was dissolved in DMSO at the rate described in Table 1, an amine-based compound and a nitro compound were added, and while stirring by a stirring blade at 300 rpm, heating was carried out in an oil bath at 150°C for the time described in Table 1 to perform an reaction.

<Preparation of polymer solution for spinning (b)>

[0103] PAN and DMSO were put into a polyethylene bottle of 2L, and they were stirred at 80°C for the time described in Table 1 to dissolve PAN.

<Isolation of polymer for spinning >

[0104] The obtained polymer solution for spinning was washed with ethanol or hot water, and the precipitate was dried to obtain a polymer for spinning.

<Spinning>

[0105] By the above-described method, the obtained polymer solution for spinning was served to a wet spinning apparatus as it was, thereby forming fibers. The dried fiber was 1 denier.

<Determination of molecular weight by GPC>

[0106] It was dissolved in N-methyl pyrrolidone (added with 0.01 N-lithium bromide) so that the concentration of a polymer for spinning to be determined became 2 mg/mL to prepare a specimen solution. With respect to the prepared specimen solution, a distribution curve of the absolute molecular weight was determined from the GPC curve measured at the following conditions using a GPC apparatus, and a weight average molecular weight Mw were calculated. The measurement was carried out at n=1.

• GPC apparatus: PROMINAICE (supplied by Shimadzu Corporation)
• Column: polar organic solvent-system GPC column TSK-GEL-α-M (x2) (supplied by Tosoh Corporation)
• Detector: (viscosity detection and R1 detection system) Viscotek Model 305TDA Detectors (supplied by Malvern Corporation)
• Flow rate: 0.6 mL/min.
• Temperature: 40°C
• Filtration of sample: membrane filter (0.45 µm cut)
• Amount of injection: 100 µL

<Determination of residual amount of nitro compound by GC-MS>

[0107] First, a calibration curve of an added nitro compound was made. The method for determining a sample is as follows.

[0108] A polymer extract extracted with ethanol was determined by GC-MS (Gas Chromatography-Mass Spectroscopy), and compounds present in the extract were identified by automatic analysis. The measurement was carried out at n=1.

[0109] The conditions of the determination of GC-MS are as follows.

• System: GCMS-QP2010 Ultra (supplied by Shimadzu Corporation)
• Column oven temperature: 500°C
• Column flow rate: 1 mL/min.
• Column: PtxR Amine, film thickness: 1 µm, length: 30 cm, inner diameter: 0.25 mm GC determination program:
• Temperature elevation speed: 10°C/ min.
• Range of determination: 50°C (maintained for 1 min.)→280°C (maintained for 1 min.)

M/Z (M: mass of molecule, Z: number of electric charge) determination program:

• Scanning speed: 1250
• Starting time: 8 min.
• Finishing time: 25 min.
• Scanning speed: 1250
• Starting m/z: 50
• Finishing m/z: 400

<Stabilization>

[0110] The treatment was carried out under a condition of air at predetermined temperature and temperature elevation speed, using one furnace of a hot air circulation drier incorporated with an infrared heater as shown in Fig. 4. Here, the hot air circulation drier was a down flow-system one, a sirocco fan having a diameter of 200 mm was controlled by an inverter (FR-E720-0.2K) supplied by Mitsubishi Electric Corporation, and further, it was rotated by an induction motor (5IK60A-SF) supplied by ORIENTAL MOTOR Co., Ltd. The wind direction of the hot air was a cross flow, and the rotational speed of the fan was 1,200 rpm. Furthermore, as the infrared heater in the hot air circulation drier, and six electric ceramic plate heaters (PLC-323) supplied by NORITAKE CO., LTD. were installed at each of the upper side and the lower side relative to a yarn path, respectively. Here, the temperature of the hot air in the furnace and the temperature of the infrared heater were set at an identical temperature.

<Carbonization>

[0111] The treatment was carried out under a nitrogen atmosphere at a predetermined temperature and at a tensile condition. The carbonization was carried out by two furnaces. In the first furnace, the treatment was carried out at a temperature in a range of 700 to 800°C, and in the second furnace, the treatment was carried out at a temperature of 1,300°C. The temperature elevation speed was in a range of 50 to 200 °C.

<Determination of density of fiber>

[0112] It was determined based on the sink-float method of JIS R 7603(1999).

< Determination of areal weight of fiber bundle>

[0113] The mass of a sample cut out by 1m from 12,000 carbon fibers was measured, and it was determined as the areal weight. The unit of the areal weight is g/m.

<Calculation of diameter of single fiber>

[0114] An average value calculate from the above-described density of fiber and areal weight of fiber bundle by the following equation (Equation 1) was calculated as a diameter of a cross section of a single fiber.

[0115] In the above-described Equation 1, represented are 1: diameter of single fiber (µm), Mf: areal weight of 12,000 carbon fibers (g/m), and p: density (g/cm³).

<Determination of strength and degree of elongation of single fiber by tensing single fiber>

[0116] The strength and degree of elongation of a single fiber were determined under the following conditions based on JIS R7606 (2000). Further, the strength was calculated by dividing a maximum load in an S-S curve with the cross section calculated from the density and the areal weight. Further, the degree of elongation was calculated from a displacement. The number of n was set at 5 or more.

[0117] The conditions for the determination are as follows.

• System: small-sized desk-top tester EZ-S (supplied by Shimadzu Corporation)
• Load cell: 20N (PEG50NA)
• Operation for control: loading
• Testing control: stroke
• Testing speed: 1 mm/min.
• Sampling: 50 msec.
• Free length pace between grippers: 25 mm

<TEM observation>

[0118] After a specimen was embedded with a resin on a Si base plate, two protective layers of Pt-based (conductive treatment) and C-based layers were deposited. This specimen was chipped in a fiber axis direction by the following method to prepare a thin-film test piece having a thickness of several-hundred µm. Further, it was chipped in parallel to the fiber axis direction so as to be able to pick up a center of a fiber, thereby preparing a thin-film test piece having a thickness of several-hundred µm. If hitting a void present in a fiber when a thin film for TEM is prepared, a sample is prepared at another position with no voids.

• Method: FIB (Focused Ion Beam)
• System: SMI3200SE supplied by SINT Corporation, FB-2000A supplied by Hitachi, Ltd., STRATA400S supplied by FEI Corporation
• System: transmission electron microscope; H-9000UHR No. 2 machine supplied by Hitachi, Ltd.
• Acceleration voltage: 300 kV
• Diaphragm of restricted visual field: about 300 nmϕ

<Making of intensity distribution graph and Calculation of crystal size and orientation degree from TEM image>

[0119] Intensity distribution graph was made from shades of colors by image analysis of TEM image. Further, from the intensity distribution graph, a crystal size Lc was calculated from a half-value width of a peak corresponding to (002) plane by the following equation (Equation 2), and an orientation degree of a crystal was calculated from a total width of a half-value of the intensity distribution in each orientation direction by the following equation (Equation 3).

[0120] In the above-described equation (Equation 2), θh: high angle side of (002) plane, and θ1: low angle side of (002) plane.

[0121] In the above-described equation (Equation 3), FWHM is a total width of a half-value of intensity distribution in each orientation direction.

<Elemental analysis>

[0122] Measurement was carried out with n number of 2, and an average value of these two values was determined as the measured value. However, in case where a difference between the two values (the respective elemental rates of C, H and N) was more than ±0.4%, the measurement was repeated until it became ±0.4% or less.

[0123] The conditions for the measurement are as follows.

• System: small-sized elemental analysis device, EuroEA3000 supplied by Evisa Corporation
• Cup: Tin capsules Pressed 5 x 9 mm Code E12007
• Reaction tube: Packed reactor single for CHNS/S 18/6 mm Code E13040
• Carrier: 60 kPa
• Purge: 80 mL/min.
• Oxygen: 15 mL
• ΔP O₂: 35 kPa
• Oxygen Time: 6.6 sec.
• Sample Delay: 5 sec.
• Run Time: 320 sec.
• Front Furnace: 980°C
• Oven: 100°C

<Observation of fiber bundle by SEM>

[0124] SEM determination was carried out at the following conditions.

• System: VK-9800 (supplied by KEYENCE Corporation)
• Acceleration voltage: 10 kV
• Spot diameter: 4

<Laser microscope>

[0125] The observation of a fiber in a laser microscope was carried out at the following conditions.

• System: VK-X210 (supplied by KEYENCE Corporation)
• Lens: 50x (integrated lens: 20x), observed at a total magnification of 1,000 times.

(Example 1)

[0126] Polymer solution for spinning (a) was wet spun at a number of filaments of 12,000 to obtain fibers through a drying process. The obtained fibers were served to stabilization at conditions of 300°C and 5 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0127] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.1 GPa, the degree of elongation was 1.7%, and they were good results.

(Example 2)

[0128] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 320°C and 5 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0129] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.1 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 3)

[0130] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 340 °C and 5 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0131] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.2 GPa, the degree of elongation was 1.5%, and they were good results.

(Example 4)

[0132] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 360 °C and 5 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0133] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.2 GPa, the degree of elongation was 1.5%, and they were good results.

(Example 5)

[0134] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 3 00°C and 10 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0135] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.2 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 6)

[0136] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 360°C and 10 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0137] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.4 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 7)

[0138] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 300°C and 15 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0139] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.0 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, the intermediate layer was oriented at 0.89 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.3 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 8)

[0140] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 3 00°C and 15 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0141] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.1 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.85, the intermediate layer was oriented at 0.88 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.4 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 9)

[0142] Polymer solution for spinning (d) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 3 00°C and 15 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0143] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.4 nm at the sheath, 1.6 nm at the intermediate layer and 1.8 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.82, the intermediate layer was oriented at 0.84 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 2.0 GPa, the degree of elongation was 1.3%, and they were good results.

(Example 10)

[0144] Polymer solution for spinning (e) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 3 00°C and 15 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0145] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.4 nm at the sheath, 1.6 nm at the intermediate layer and 1.8 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.82, the intermediate layer was oriented at 0.84 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 1.6 GPa, the degree of elongation was 1.6%, and they were good results.

(Example 11)

[0146] Polymer solution for spinning (a) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to stabilization. For the obtained fibers, the stabilization was carried out at conditions of 360°C and 30 minutes, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0147] As the result of TEM observation, the obtained carbon fiber had a 3-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath, 1.8 nm at the intermediate layer and 2.0 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.79, the intermediate layer was oriented at 0.81 and the core was oriented at 0.6 or less. As the result of tensing a single fiber, because the time for stabilization was too long, the fiber was fuzzed and the thickness thereof became small, and therefore, the tensile strength was reduced to 1.7 GPa, the degree of elongation was reduced to 1.5%, but they were good results.

(Comparative Example 1)

[0148] Polymer solution for spinning (a) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. The obtained fibers were served to stabilization at conditions of 260°C and 15 minutes. Although the stabilized fiber was tried to be carried out with carbonization at a carbonization temperature of 1,300°C , the fiber was burnt and broken immediately after being introduced into a furnace, and it could not be carbonized as a carbon fiber.

(Comparative Example 2)

[0149] Polymer solution for spinning (a) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. The obtained fibers were served to stabilization at conditions of 260°C and 15 minutes. Although the stabilized fiber was tried to be carried out with carbonization at a carbonization temperature of 1,300°C , the fiber was burnt and broken immediately after being introduced into a furnace, and it could not be carbonized as a carbon fiber.

(Comparative Example 3)

[0150] Polymer solution for spinning (b) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. The obtained fibers were served to stabilization at conditions of 240°C and 15 minutes. Although the stabilized fiber was tried to be carried out with carbonization at a carbonization temperature of 1,300°C , the fiber was burnt and broken immediately after being introduced into a furnace, and it could not be carbonized as a carbon fiber.

(Comparative Example 4)

[0151] Polymer solution for spinning (b) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. The obtained fibers were served to stabilization. The fiber was stabilized at conditions of 280°C and 15 minutes. Although a fusion happened at the stage of the stabilization, the fiber was carbonized as it was. Although the stabilized fibers were tried to be carried out with carbonization at a carbonization temperature of 1,300°C, most of the fibers were burnt and broken in a furnace. As the result of tensing a single fiber with respect to parts barely taken as carbon fibers, the tensile strength was reduced to 1.3 GPa, the degree of elongation was 1.0%, and they were very low tensile strength and degree of elongation to cause poor results.

(Comparative Example 5)

[0152] Polymer solution for spinning (b) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. Although the obtained fibers were tried to be carried out with stabilization at conditions of 300°C and 15 minutes, they were burnt and broken in a furnace for stabilization.

(Comparative Example 6)

[0153] Polymer solution for spinning (b) was wet spun in a manner similar to that in Example 1 to obtain fibers through a drying process. Although the obtained fibers were tried to be carried out with stabilization at conditions of 360°C and 15 minutes, they were burnt and broken in a furnace for stabilization.

(Comparative Example 7)

[0154] Polymer solution for spinning (c) was treated in a manner similar to that in Example 1 to obtain fibers. The obtained fibers were served to burning at conditions similar to those in Example 7 to obtain carbon fibers. Because a nitro compound was left in the polymer solution for spinning, as the result of TEM observation, the obtained carbon fiber had a 2-layer sheath-core structure. With respect to Lc, it was 1.7 nm at the sheath and 1.5 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.86, and the core was oriented at 0.83 or less. As the result of tensing a single fiber, the tensile strength was 1.9 GPa, and the degree of elongation was 0.8%. In particular, the degree of elongation was greatly reduced as compared with Example 8, and it was a poor result.

(Comparative Example 8)

[0155] Polymer solution for spinning (a) was wet spun at a number of filaments of 12,000 to obtain fibers through a drying process, in a manner similar to that in Example 1. The obtained fibers were served to stabilization at conditions of 300°C and 5 minutes similar to those in Example 1, using a hot air circulation drier equipped with no infrared heater, and carbonization was carried out at a carbonization temperature of 1,300°C.

[0156] As the result of TEM observation, the obtained carbon fiber had substantially a 2-layer sheath-core structure. With respect to Lc, it was 1.6 nm at the sheath and 2.2 nm at the core. With respect to orientation degree f, the sheath was oriented at 0.80, and the core was oriented at 0.6 or less. As the result of tensing a single fiber, the tensile strength was 1.8 GPa, and the degree of elongation was 1.0% and much lower than that in Example 1, and occurrence of fluffs was also much.

(Comparative Example 9)

[0157] Polymer solution for spinning (a) was wet spun at a number of filaments of 12,000 to obtain fibers through a drying process, in a manner similar to that in Example 1. The obtained fibers were served to stabilization at conditions of 300 °C and 5 minutes similar to those in Example 1, using only an infrared heater (without hot air circulation), and carbonization was carried out at a carbonization temperature of 1,300 °C, but yarn breakage happened because of unevenness of treatment.

[0158] The polymer solutions for spinning (a) to (e) used in the above-described respective Examples and Comparative Examples are shown in Table 1, the conditions and results of Examples 1 to 11 are shown in Table 2, and the conditions and results of Comparative Examples 1 to 9 are shown in Table 3, respectively.

[Table 1]

			Polymer solution for spinning
			a	b	c	d	e
Raw material	acrylonitrile homopolvmer	part by weight part by welg	11	15	11	10	11
	nitrobenzene		2		2	1.5	1.5
	monoethanol amine		5		2	3	7
Polar solvent	dimethyl sulfoxide		82	85	85	85.5	80.5
Conditions for reaction	dissolution or reaction temperature	°C	150	80	150	151	152
	dissolution or reaction time	h	6	6	6	10	7
Properties of polymer solution	residual rate of nitro	%	0	0	24	0	0
	Mark-Houwink a		0.21	0.5	0.22	0.4	0.07

[0159] [Table 2]

Table 2

		Unit	Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7	Example 8	Example 9	Example 10	Example 11
	Kind of polymer solution for spinning		PAN(a)	PAN(a)	PAN(a)	PAN(a)	PAN(a)	PAN(a)	PAN(a)	PAN(a)	PAN(d)	PAN(e)	PAN(a)
Conditions for burning	Temperature for stabilization	°C	300	320	340	360	300	360	300	360	360	360	360
	Time for stabilization	min	5	5	5	5	10	10	15	15	15	15	30
	Time for carbonization	°C	1300	1300	1300	1300	1300	1300	1300	1300	1300	1300	1300
TEM analysis	Structure		3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core	3-layer sheath /core
Crystal size Lc	Sheath	nm	1.6	1.6	1.6	1.6	1.6	1.6	1.6	1.6	1.4	1.5	1.5
	Intermediate layer		1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.8	1.6	1.8	1.8
	Core		2.1	2.1	2.1	2.2	2.1	2.1	2.0	2.1	1.8	2.1	2.1
Orientation degree f	Sheath		0.86	0.86	0.86	0.86	0.86	0.86	0.86	0.85	0.82	0.80	0.80
	Intermediate layer		0.89	0.89	0.89	0.89	0.89	0.89	0.89	0.88	0.84	0.82	0.82
	Core		0.56	0.55	0.55	0.55	0.55	0.55	0.56	0.54	0.54	0.54	0.54
Rate of flat yarn			70%	80%	70%	80%	80%	80%	70%	70%	60%	90%	70%
Tensile strength of single fiber	Strength	GPa	2.1	2.1	2.2	2.2	2.2	2.4	2.3	2.4	2.0	1.6	1.7
	Degree of elongation	%	1.7	1.6	1.5	1.5	1.6	1.6	1.6	1.6	1.3	1.6	1.5

[0160] [Table 3]

Table 3

		Unit	Comparative Example 1	Comparative Example 2	Comparative Example 3	Comparative Example 4	Comparative Example 5	Comparative Example 6	Comparative Example 7	Comparative Example 8	Comparative Example 9
Raw material	Kind of polymer solution for spinning		PAN(a)	PAN(a)	PAN(b)	PAN(b)	PAN(b)	PAN(b)	PAN(c)	PAN(a)	PAN(a)
Conditions for burning	Apparatus		A	A	A	A	A	A	A	B	C
	Temperature for stabilization	°C	240	260	240	280	300	360	300	300	300
	Time for stabilization	min	15	15	15	15	15	15	15	5	5
	Time for carbonization	°C	1300	1300	1300	1300	1300	1300	1300	1300	1300
TEM analysis	Structure		F2	F2	F2	hollow	F1	F1	2-layer sheath / core	2-layer sheath / core	F2
Crystal size Lc	Sheath	nm	-	-	-	1.8	-	-	1.7	1.6	-
	Intermediate layer		-	-	-	none	-	-	none	none	-
	Core		-	-	-	none	-	-	1.5	2.2	-
Orientation degree f	Sheath		-	-	-	0.86	-	-	0.86	0.86	-
	Intermediate layer		-	-	-	0.89	-	-	none	none	-
	Core		-	-	-	-	-	-	0.83	0.56	-
Rate of flat yarn			-	-	-	0%	-	-	40%	30%	-
Tensile strength of single fiber	Strength	GPa	-	-	-	1.3	-	-	1.9	1.8	-
	Degree of elongation	%	-	-	-	1.0	-	-	0.8	1	-

Apparatus for burning conditions : A; hot air circulation drier equipped with infrared heater, B; hot air circulation drier, C; infrared heater F1: fused or cut by being molten, impossible in stabilization as fiber bundle, F2: burnt in furnace, impossible in carbonization

Industrial Applicability

[0161] The PAN-based carbon fiber and the production method therefor according to the present invention can be applied to production of any PAN-based carbon fiber required with shortening of time for stabilization and a high degree of elongation.

Explanation of symbols

[0162] 

1:

sheath-core structure having three or more layers

2:

core

3:

intermediate layer

4:

sheath

11:

hot air circulation drier

12:

non-treated fiber (fiber before treatment)

13:

stabilized fiber (fiber after treatment)

14a, 14b:

roller

15a, 15b:

opening

16:

ceramic heater

17:

punching metal for attaching ceramic heater

18:

flow of hot air

Claims

1. A polyacrylonitrile (hereinafter, referred to as PAN)-based carbon fiber comprising three or more phases different in crystal size.

2. The PAN-based carbon fiber according to claim 1, wherein respective phases are layered.

3. The PAN-based carbon fiber according to claim 2, wherein said carbon fiber has a sheath-core structure having three or more layers, and satisfies the following conditions of A to D:

A: in a sectional area in a direction perpendicular to a fiber axis, an area occupied by a core occupies 10 to 70% of the whole of said sectional area,

B: a thickness of a sheath is in a range of 100 nm to 10,000 nm,

C: a thickness of an intermediate layer is more than 0 and 5,000 nm or less, and

D: a diameter in said direction perpendicular to said fiber axis is 2 µm or more.

4. The PAN-based carbon fiber according to claim 2 or 3, wherein said carbon fiber has a sheath-core structure having three or more layers, and satisfies the following conditions of E to H:

where, a crystal size of a core is referred to as Lc1, a crystal size of a sheath is referred to as Lc2, and a crystal size of an intermediate layer is referred to as Lc3.

E: Lc1/Lc3 ≧ 1.05,

F: Lcl/Lc2 ≧ 1.05,

G: 1.0 ≦ Lc1 ≦ 7.0 nm, and

H: Lc2 ≠ Lc3.

5. The PAN-based carbon fiber according to claim 3 or 4, wherein an orientation degree f of a crystal of a core is 0.7 or less.

6. The PAN-based carbon fiber according to any of claims 1 to 5 obtained by carbonizing a fiber spun from a single kind of polymer solution for spinning.

7. The PAN-based carbon fiber according to any of claims 1 to 6 obtained by spinning a fiber from a polymer solution for spinning satisfying the following two points of A and B and carbonizing the spun fiber:

A: a polymer in said polymer solution for spinning is a polymer prepared by modifying PAN with an amine-based compound and oxidizing it with a nitro compound, and

B: said nitro compound is not contained in said polymer solution for spinning.

8. The PAN-based carbon fiber according to claim 7, wherein said nitro compound is nitrobenzene.

9. The PAN-based carbon fiber according to claim 7 or 8 obtained using a polymer solution for spinning which contains PAN oxidized using nitrobenzene at an amount of 10 wt% or more relative to PAN in A of claim 7.

10. The PAN-based carbon fiber according to any of claims 1 to 9 obtained using a polymer solution for spinning which has a divergent structure in which a gradient a is 0.1 or more and 0.3 or less as the result determined by gel permeation chromatography. Where, said gradient a means a gradient a represented by Mark Houwink-Sakurada equation (equation (1)):

where, [η] is an intrinsic viscosity, K is a constant inherent for a material, and Mw is a weight average molecular weight.

11. A method for producing a PAN-based carbon fiber according to any of claims 1 to 10 comprising the steps of:

spinning a polymer solution for spinning according to any of claims 6 to 10;

performing stabilization in air at 280°C or higher and 400°C or lower for 10 seconds or more and 15 minutes or less; and thereafter,

performing carbonization.

12. The method for producing a PAN-based carbon fiber according to claim 11, wherein said stabilization is performed using an infrared heater and a hot air drier together.

Drawing