Lightweight carbon fiber, lightweight carbon fiber strand, carbon fiber reinforced composite material, and methods for manufacturing the same, and microwave heating furnace

Abstract

To provide a light-weight carbon fiber which is light-weight and has excellent tensile strength and tensile elastic modulus both, a light-weight carbon fiber strand, a carbon fiber-reinforced composite material, a manufacturing method thereof, and a microwave oven.SOLUTION: A polyacrylonitrile-based light-weight carbon fiber having a density by a sink-and-float method of 1.60-1.75[g / cm3], a tensile strength of 3.00 [GPa] or over, a tensile elastic modulus of 200 [GPa] or over, a specific strength of 1.50-2.50 [kNm / kg], and a relative elastic modulus of 120-150 [MNm / kg] is manufactured by microwave heating under a prescribed condition.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to lightweight carbon fibers, lightweight carbon fiber strands, carbon fiber reinforced composite materials, methods for manufacturing these, and microwave heating furnaces. Specifically, it relates to lightweight carbon fibers, lightweight carbon fiber strands, carbon fiber reinforced composite materials having a density of 1.60 to 1.75 [g / cm 3 obtained by a flotation method, methods for manufacturing these, and microwave heating furnaces having a structure particularly suitable for this manufacturing.

Background Art

[0002] Carbon fibers have excellent specific strength and specific modulus compared to other fibers, and are widely used industrially as reinforcing fibers for composite materials with resins, etc., taking advantage of their lightweight and excellent mechanical properties.

[0003] Conventionally, carbon fibers have been manufactured as follows. First, the precursor fiber is heat-resistant treated by heating in hot air at 230 to 260°C for 30 to 100 minutes. This heat-resistant treatment causes a cyclization reaction of acrylic fibers and increases the amount of oxygen bonds to obtain heat-resistant fibers. These heat-resistant fibers are carbonized, for example, using a firing furnace at 300 to 800°C in a nitrogen atmosphere while applying a temperature gradient (first carbonization treatment). Then, they are further carbonized using a firing furnace at 800 to 2100°C in a nitrogen atmosphere while applying a temperature gradient (second carbonization treatment). Thus, carbon fibers are manufactured by heating the heat-resistant fibers from the outside in a heated firing furnace.

[0004] The carbon fibers manufactured in this way usually have a density of about 1.8 to 2.2 [g / cm 3 , but further weight reduction has been attempted.

[0005] Non-Patent Document 1 discloses carbon fibers having a hollow structure with a density of 1.2 g / cm 3Low-density carbon fibers (apparent density including the hollow part) have been disclosed. However, although carbon fibers with a hollow structure have excellent elastic modulus, their strength is insufficient. Also, the true density of this carbon fiber excluding the hollow part is 1.85 g / cm 3 or so.

[0006] Patent Document 1 discloses carbon fibers with a density of 1.75 g / cm 3 or less, and resin-impregnated strand strength and elastic modulus of 650 kgf / mm 2 or more and 35 t / mm 3 or more, respectively.

[0007] Patent Document 2 discloses carbon fibers with a density of 1.79 g / cm 3 or less, La ≥ 1 [nm], and when the tensile elastic modulus TM [GPa] is 170 ≤ TM ≤ 230, La ≤ -0.5 + 0.01 × TM [nm].

[0008] Non-Patent Document 2 discloses carbon fibers with a specific gravity of 1.5 - 1.6, a tensile strength of 0.5 - 0.7 GPa, and a tensile elastic modulus of 20 - 30 GPa.

[0009] As described above, at present, carbon fibers that are lightweight and have excellent tensile strength and tensile elastic modulus have not been obtained.

Prior Art Documents

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Patent Document

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] The problem of the present invention is to provide lightweight carbon fibers, lightweight carbon fiber strands, carbon fiber reinforced composite materials, manufacturing methods thereof, and a microwave heating furnace that are lightweight and excellent in both tensile strength and tensile modulus of elasticity.

Means for Solving the Problems

[0013] As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by carbonizing the fiber to be heated by microwave heating under predetermined conditions, and have completed the present invention.

[0014] The present invention for solving the above problems is as described below.

[0015] 〔1〕 A lightweight carbon fiber made of a polyacrylonitrile-based material, having a density of 1.60 to 1.75 [g / cm 3 , a tensile strength of 3.00 [GPa] or more, a tensile modulus of elasticity of 200 [GPa] or more, a specific strength of 1.50 to 2.50 [MNm / kg], and a specific modulus of elasticity of 120 to 150 [MNm / kg].

[0016] 〔2〕 The lightweight carbon fiber according to 〔1〕, having a crystal size La of 1.80 to 3.00 [nm].

[0017] 〔3〕 The lightweight carbon fiber according to 〔1〕 or 〔2〕, having a crystal size Lc of 1.00 to 3.00 [nm].

[0018] 〔4〕 The lightweight carbon fiber according to any one of 〔1〕 to 〔3〕, having a solid structure.

[0019] 〔5〕 The carbon fiber density (A) [g / cm 3 measured by the helium filling method in the state of filaments, and the carbon fiber density (B) [g / cm 3 measured by the helium filling method after pulverizing the filaments to a volume average particle diameter of 0.2 to 0.5 μm satisfy the following formula (1) 1.15 > A / B > 1.03 ··· Formula (1) The lightweight carbon fiber according to any one of 〔1〕 to 〔4〕.

[0020] 〔6〕 A lightweight carbon fiber strand, made of the lightweight carbon fiber according to any one of 〔1〕 to 〔5〕, and having a fineness of 1000 [tex] or more.

[0021] The lightweight carbon fiber according to 〔1〕 above is lightweight with a density of 1.60 to 1.75 [g / cm 3 , and has high physical properties such as a tensile strength of 3.00 [GPa] or more and a tensile modulus of elasticity of 200 [GPa] or more. This lightweight carbon fiber preferably has a crystal size La of 1.80 to 3.00 [nm] (the above [2]), and preferably has a crystal size Lc of 1.00 to 3.00 [nm] (the above [3]). Also, this lightweight carbon fiber preferably has a solid structure rather than a hollow structure (the above [4]). Furthermore, it is preferable that the density measured in the state of the filament of this lightweight carbon fiber and the density measured in the state where the filament is pulverized to a predetermined particle size are within a predetermined range (the above [5]). This lightweight carbon fiber may be a strand formed by bundling a plurality of fibers (the above [6]).

[0022] [7] A carbon fiber reinforced composite material comprising the lightweight carbon fiber or lightweight carbon fiber strand according to any one of [1] to [6].

[0023] [8] A method for producing carbon fiber, wherein while running a fiber to be heated in a microwave heating furnace, microwaves are introduced into the microwave heating furnace to directly irradiate the fiber to be heated with microwaves, thereby heating the fiber to be heated, characterized in that, as a heating means for the fiber to be heated, heating is further performed using a heating means other than direct irradiation of microwaves on the fiber to be heated.

[0024] [9] The method for producing carbon fiber according to [8], wherein the heating means is a heating means for heating the fiber to be heated by causing the microwave absorption tube disposed in a part of the microwave heating furnace to generate heat by introducing microwaves into the microwave heating furnace while running the fiber to be heated through the microwave absorption tube.

[0025]

[10] The method for producing carbon fiber according to [9], wherein a microwave absorption tube is disposed on one end side of the microwave heating furnace, the fiber to be heated is preheated by causing the microwave absorption tube to generate heat, and then the fiber to be heated is directly irradiated with microwaves.

[0026] The carbon fiber obtained by the production method according to any one of

[11] , [8] to

[10] .

[0027] 〔12〕 A carbon fiber reinforced composite material comprising the carbon fiber according to

[11] .

[0028] 〔13〕 A microwave heating furnace in which a cylindrical microwave absorption tube is provided on one end side in the microwave heating furnace, and the fiber to be heated is configured to travel through the microwave absorption tube. By introducing microwaves into the microwave heating furnace, the microwave absorption tube is heated to preheat the fiber to be heated traveling through the microwave absorption tube, and then the fiber to be heated is directly irradiated with microwaves outside the microwave absorption tube for further heating. A microwave heating furnace characterized by being configured as such.

Effects of the Invention

[0029] The lightweight carbon fiber of the present invention is lightweight and has high strength, so that a carbon fiber reinforced composite material manufactured using the same can be made lighter than before.

Brief Description of the Drawings

[0030]

Figure 1

Embodiments for Carrying Out the Invention

[0031] Hereinafter, the lightweight carbon fiber and the like of the present invention will be described in detail. In the present invention, the density means the value at 25°C.

[0032] (1) Lightweight carbon fiber The lightweight carbon fiber of the present invention has a density of 1.60 to 1.75 [g / cm 3 by the floating method, a tensile strength of 3.00 [GPa] or more, a tensile elastic modulus of 200 [GPa] or more, a specific strength of 1.50 to 2.50 [MNm / kg], and a specific elastic modulus of 120 to 150 [MNm / kg]. It is a lightweight polyacrylonitrile-based carbon fiber.

[0033] The lightweight carbon fiber of the present invention has a density of 1.60 to 1.75 [g / cm 3 by the sedimentation method, preferably 1.62 to 1.73 [g / cm 3 , and more preferably 1.63 to 1.72 [g / cm 3 . When it exceeds 1.75 [g / cm 3 , the weight reduction is insufficient. When it is less than 1.60 [g / cm 3 , the carbon fiber may have a hollow structure or large voids may be formed inside the fiber, and the tensile strength is likely to decrease.

[0034] The lightweight carbon fiber of the present invention has a strand tensile strength of 3.00 [GPa] or more measured by the resin impregnation method described later, preferably 3.05 to 5.00 [GPa], and particularly preferably 3.50 to 4.50 [GPa]. When the tensile strength is less than 3.00 [GPa], the physical properties of the carbon fiber reinforced composite material may not be sufficiently high.

[0035] The lightweight carbon fiber of the present invention has a strand tensile modulus of elasticity of 200 [GPa] or more measured by the resin impregnation method described later, preferably 205 to 300 [GPa], and more preferably 210 to 250 [GPa]. When the tensile modulus of elasticity is less than 200 [GPa], the physical properties of the carbon fiber reinforced composite material may not be sufficiently high.

[0036] The lightweight carbon fiber of the present invention has a specific strength of 1.50 to 2.50 [MNm / kg], preferably 1.70 to 2.50 [MNm / kg], and more preferably 1.80 to 2.40 [MNm / kg]. The specific strength is the value obtained by dividing the strand tensile strength [GPa] measured by the resin impregnation method described above by the density [g / cm 3 by the sedimentation method.

[0037] The lightweight carbon fiber of the present invention has a specific modulus of elasticity of 120 to 150 [MNm / kg], preferably 124 to 146 [MNm / kg], and more preferably 126 to 144 [MNm / kg]. The specific strength is the value obtained by dividing the strand tensile elastic modulus [GPa] measured by the resin impregnation method described later by the density [g / cm 3 measured by the sedimentation method.

[0038] The lightweight carbon fiber of the present invention preferably has a crystal size La of 1.80 to 3.00 [nm], more preferably 2.12 to 2.81 [nm], and even more preferably 2.25 to 2.80 [nm]. When La is outside the above range, the tensile strength and tensile elastic modulus are likely to decrease.

[0039] The lightweight carbon fiber of the present invention preferably has a crystal size Lc of 1.00 to 3.00 [nm], more preferably 1.20 to 2.20 [nm], and even more preferably 1.40 to 2.00 [nm]. When Lc is outside the above range, the tensile strength and tensile elastic modulus are likely to decrease.

[0040] The lightweight carbon fiber of the present invention preferably has a solid structure. In the present invention, the solid structure means that there is no cavity having a diameter of 1 / 20 or more of the filament diameter in the cross section of the filament. Carbon fiber having a hollow structure is lightweight but does not have a sufficiently high tensile strength.

[0041] The lightweight carbon fiber of the present invention has a carbon fiber density (A) [g / cm 3 measured by the helium filling method in the state of the filament, and a carbon fiber density (B) [g / cm 3 measured by the helium filling method after freeze-crushing the filament to a volume average particle size of 0.2 to 0.5 μm, and satisfies the following formula (1) 1.15 > A / B > 1.03 ··· Formula (1) preferably. When it is above 1.15, since the carbon fiber has a hollow structure or there are many large voids inside the carbon fiber, the tensile strength of the carbon fiber is likely to decrease. When it is below 1.03, the density by the flotation method may not be sufficiently low.

[0042] Freeze grinding is carried out by ball mill grinding in liquid nitrogen. Freeze grinding is carried out until the volume average particle diameter of the carbon fiber becomes 0.2 to 0.5 μm. When it exceeds 0.5 μm, the voids inside the fiber are not sufficiently exposed.

[0043] The lightweight carbon fiber of the present invention preferably has a carbon content of 90 to 98 [mass%], more preferably 91 to 96 [mass%], and even more preferably 91 to 95 [mass%].

[0044] (2) Lightweight carbon fiber strand The lightweight carbon fiber of the present invention may be a strand in which a plurality of strands are bundled. The fineness of the strand is preferably 1000 [tex] or more, and more preferably 1600 [tex] or more.

[0045] The lightweight carbon fiber of the present invention is preferably a continuous fiber having a fiber length of 10 [cm] or more, and more preferably a continuous fiber of 1 [m] or more.

[0046] (3) Method for producing lightweight carbon fiber or lightweight carbon fiber strand The lightweight carbon fiber or lightweight carbon fiber strand of the present invention can be produced by irradiating the fiber to be heated with microwaves under predetermined conditions and heating.

[0047] The fiber to be heated is a polyacrylonitrile-based flame-retardant fiber, and its carbon content is preferably 66 to 72 [mass%], and more preferably 67 to 71 [mass%]. The polyacrylonitrile-based flame-retardant fiber is usually produced by subjecting a PAN-based fiber, which is a precursor fiber, to a flame-retardant treatment.

[0048] 〈Raw material fiber〉 As the raw material fiber, a spinning solution obtained by homopolymerizing acrylonitrile or copolymerizing a monomer composition containing 90% by mass or more, preferably 95% by mass or more of acrylonitrile can be used. After spinning the spinning solution by a wet or dry-wet spinning method, a PAN-based fiber obtained by washing with water, drying, and stretching can be used. As the monomers to be copolymerized, polar monomers such as methyl acrylate, itaconic acid, methyl methacrylate, methacrylic acid, and acrylic acid are preferred.

[0049] The raw material fiber, which is the precursor fiber of the carbon fiber, is bundled to form a precursor fiber strand. The number of single fibers in the precursor fiber strand is preferably 1000 to 48000, more preferably 3000 to 24000, from the viewpoint of production efficiency.

[0050] 〈Flame resistance treatment〉 Using the above PAN-based fiber as the precursor fiber, the fiber is subjected to a flame resistance treatment by oxidizing it in heated air at 200 to 300 °C for 10 to 100 minutes. By this flame resistance treatment, an intramolecular cyclization reaction of the PAN-based fiber occurs, and further the amount of oxygen bonds increases, resulting in a flame-resistant fiber. In this flame resistance treatment step, the PAN-based fiber is preferably stretched in the range of a draw ratio of 0.90 to 1.20. Further, after the flame resistance treatment, a preliminary carbonization treatment may be performed in an inert atmosphere, preferably at a temperature of 500 to 1000 °C in a nitrogen atmosphere. By performing such a treatment, the carbon content of the fiber to be heated can be adjusted to an appropriate range.

[0051] 〈Carbonization〉 The lightweight carbon fiber of the present invention can be produced by microwave heating the above PAN-based flame-resistant fiber, which is the fiber to be heated, under predetermined conditions.

[0052] The frequency of the microwave is not particularly limited, but generally 915 MHz or 2.45 GHz is used. The output of the microwave oscillator is not particularly limited, but 300 to 2400 W is appropriate, and 500 to 2000 W is more appropriate.

[0053] As manufacturing conditions for obtaining the lightweight carbon fibers of the present invention, the time until the fiber to be heated changes into carbon fibers by heating is particularly important. As methods for adjusting this time, examples include a method of changing the resonance length of microwaves, a method of using heating means other than microwaves in combination, and a method of using electric field heating and magnetic field heating in combination.

[0054] By changing the resonance length of microwaves, the time until the fiber to be heated changes into carbon fibers can be adjusted, and the density, La, and Lc of the obtained carbon fibers can be adjusted. The said conditions can be determined by manufacturing carbon fibers by changing the resonance length of microwaves and measuring their density, La, and Lc.

[0055] In the carbonization of the fiber to be heated, it is also preferable to use heating means other than microwaves in combination. Specifically, it can be heated using an electric heater, plasma, etc. Further, a microwave heating furnace is used, which is configured such that a microwave absorber is provided on one end side in the microwave heating furnace and the fiber to be heated travels near the microwave absorber. After heating through the microwave absorber in the microwave heating furnace, it is also preferable to directly irradiate the fiber to be heated with microwaves for further heating. At this time, as the microwave absorber, it is preferable to use a cylindrical microwave absorption tube and configure it such that the fiber to be heated travels inside this tube. Examples of the material of the microwave absorber include silicon carbide, silicon nitride, a resin containing carbon particles, a resin containing soft magnetic metal particles, etc. By using heating other than microwaves in combination, it becomes easier to control the crystallite size La. It is preferable that the inner wall of the microwave absorber and the fiber to be heated are separated by 1 to 50 [mm], more preferably separated by 3 to 30 [mm], and even more preferably separated by 5 to 20 [mm]. Note that the separation distance between the inner wall of the microwave absorber and the fiber to be heated refers to the distance between the inner wall of the microwave absorber and the center point of the cross-section (transverse cross-section) in the direction perpendicular to the fiber axis of the fiber bundle to be heated. Also, from the viewpoint of heat insulation efficiency, the distance between the inner wall of the following microwave absorption tube 21 and the outer periphery of the fiber bundle is preferably 0.1 to 50 [mm], more preferably 1 to 20 [mm].

[0056] In addition, for heating by microwaves, it is also possible to use both electric field heating and magnetic field heating in combination. Examples of methods for using them in combination include a method of heating using a plurality of microwave heating furnaces, a method of causing the fiber to be heated to travel obliquely with respect to the axis of the furnace body of the microwave heating furnace, and the like.

[0057] (4) Microwave heating furnace The microwave heating furnace of the present invention is configured such that a cylindrical microwave absorption tube is provided on one end side inside the microwave heating furnace, and the fiber to be heated travels inside the microwave absorption tube. By introducing microwaves into the microwave heating furnace, the microwave absorption tube is heated to preheat the fiber to be heated traveling inside the microwave absorption tube, and then microwaves are directly irradiated onto the fiber to be heated outside the microwave absorption tube for further heating.

[0058] FIG. 1 is an explanatory diagram showing a configuration example of the microwave heating furnace of the present invention. In FIG. 1, 100 is a microwave heating furnace, and 11 is a microwave oscillator. One end of a connection waveguide 12 is connected to the microwave oscillator 11, and the other end of the connection waveguide 12 is connected to one end of a carbonization furnace 17. A circulator 13 and a matcher 15 are interposed in the connection waveguide 12 in order from the side of the microwave oscillator 11.

[0059] One end of the carbonization furnace 17 is closed, and the other end is coupled to the connection waveguide 12. The carbonization furnace 17 is composed of a waveguide. A fiber introduction port 17a for introducing the fiber to be heated into the carbonization furnace is formed at one end of the carbonization furnace 17, and a fiber outlet 17b for taking out the carbonized fiber is formed at the other end. A short-circuit plate 17c is disposed at the inner end portion on the fiber outlet 17b side of the carbonization furnace 17. A cylindrical microwave absorption tube 21 is disposed on the fiber introduction port 17a side of the carbonization furnace 17. One end of a connection waveguide 14 is connected to the circulator 13, and a dummy load 19 is connected to the other end of the connection waveguide 14.

[0060] Next, the operation of this microwave heating furnace 100 will be described. In FIG. 1, 31b is the fiber to be heated, and is introduced into the carbonization furnace 17 through the fiber inlet 17a, through the microwave absorption tube 21 in the carbonization furnace 17, by means of a fiber conveying means (not shown), through the inlet 22a formed in the connecting waveguide 12. The microwave oscillated by the microwave oscillator 11 is introduced into the carbonization furnace 17 through the connecting waveguide 12. The microwave that has reached the inside of the carbonization furnace 17 is reflected by the short-circuit plate 17c and reaches the circulator 13 via the matcher 15. Also, the microwave introduced into the carbonization furnace 17 is absorbed by the microwave absorption tube 21, causing the microwave absorption tube 21 to generate heat. Due to this heat generation, the fiber 31b to be heated is preheated. The reflected microwave (hereinafter also referred to as "reflected wave") has its direction changed by the circulator 13 and is absorbed by the dummy load 19 through the connecting waveguide 14. At this time, matching is achieved between the matcher 15 and the short-circuit plate 17c using the matcher 15, and a standing wave is generated inside the carbonization furnace 17. By this standing wave, the fiber 31b to be heated is carbonized to become carbon fiber 31c. At this time, the inside of the carbonization furnace 17 is at normal pressure and is in an inert atmosphere by means of an inert gas supply means (not shown). The carbon fiber 31c is led out of the carbonization furnace 17 through the fiber outlet 17b by means of a fiber conveying means (not shown). By continuously introducing the fiber to be heated into the carbonization furnace 17 from the fiber inlet 17a, irradiating the fiber to be heated with microwaves in the carbonization furnace 17 for carbonization, and continuously leading it out from the fiber outlet 17b, carbon fiber can be continuously produced. The carbon fiber led out from the fiber outlet 17b is subjected to surface treatment or sizing treatment as necessary. The methods of surface treatment and sizing treatment may follow known methods.

[0061] (5) Fiber Reinforced Composite Material The fiber reinforced composite material of the present invention is configured to include the above lightweight carbon fiber. As the matrix resin, known thermosetting resins or thermoplastic resins can be used. A known method can be adopted for the manufacturing method of the fiber reinforced composite material. The fiber-reinforced composite material of the present invention can be reduced in weight by 5 to 15 [%] when various properties are kept constant, as compared with the fiber-reinforced composite material using ordinary carbon fibers.

Example

[0062] Hereinafter, the present invention will be specifically described with reference to examples. The evaluation of the physical properties of the fibers in each example and comparative example was carried out by the following method.

[0063] [1] Strand strength, elastic modulus In accordance with JIS R 7608, the tensile strength and tensile elastic modulus of the cured epoxy resin-impregnated strand were measured.

[0064] [2] Crystal Lc, La The measurement of the crystal sizes Lc and La was carried out by the transmission method using an X-ray diffractometer RINT2000 manufactured by Rigaku Corporation, with a sample stage on which the fibers were set attached. As the X-ray, CuKα rays generated at an acceleration voltage of 40 (kV) and a current of 30 (mA) were used. When measuring the crystal size Lc, the fiber axis direction of the fiber bundle was perpendicular to the equatorial plane, and when measuring the crystal size La, the fiber axis direction of the fiber bundle was parallel to the equatorial plane. Also, diffraction patterns in the range of diffraction angle 2θ from 10° to 60° were taken respectively, and curves passing near 10°, 20°, 35°, and 60° of the diffraction pattern were used as the baseline. The crystal size Lc is the half-value width β of the diffraction peak of the plane index (002) obtained by the above method 002 and can be calculated using the following formula (2). Crystal size Lc [nm] = 0.9λ / (β 002 cosθ 002 ) ··· Formula (2) 〔In the formula, λ: wavelength of X-ray, β 002 : half-value width of the diffraction peak of the plane index (002), θ 002 : diffraction angle of the plane index (002).〕 The crystal size La can be calculated using the following formula (3) from the half-value width β10 of the diffraction peak of the plane index (10) obtained by the above method. Crystallite size La [nm] = 0.9λ / (β 10 cosθ 10 ) ··· Equation (3) [where λ: wavelength of X-ray, β 10 : half-value width of diffraction peak of plane index (10), θ 10 : diffraction angle of plane index (10).]

[0065] [3] Density Measured by the helium filling method using AccuPyc 1330 manufactured by Micromeritics. A 10 cc measurement cell was used and measured with approximately 0.5 g of the sample. Freeze grinding was performed by ball mill grinding in liquid nitrogen. After freeze grinding the filament, it was measured by the helium filling method. Freeze grinding was performed until the volume average particle diameter reached 0.2 - 0.5 μm.

[0066] (Example 1) PAN-based fibers composed of 24,000 filaments were heated in a flame-resistant furnace at a maximum temperature of 270°C in air for oxidation treatment and then heated to 600°C in a nitrogen atmosphere. The obtained precursor fiber bundle (carbon content 66%) was continuously conveyed and introduced into a microwave carbonization furnace, and conductive heating was carried out under atmospheric pressure in a nitrogen atmosphere using the electric field formed in the furnace by an applied power of 0.5 kW for carbonization. The temperature of the fiber during heating measured by a radiation thermometer was 1100°C. The carbon content of the obtained fibrous carbonized material was 92% and carbon fibers were obtained. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.71 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.51 nm. The bulk density of the carbon fiber was 1.69 g / cm 3 and the ratio of the density after freeze grinding to the density before freeze grinding was 1.10. The tensile strength of the resin-impregnated strand tensile test was 4.03 GPa, the tensile elastic modulus was 223 GPa, and the specific strength and specific elastic modulus, which are the mechanical properties per density as an index of light weight, were 2.39 MNm / kg and 132 MNm / kg, respectively. Other results are shown in Table 1.

[0067] (Example 2) A high dielectric loss body (silicon carbide pipe) was disposed at the fiber inlet of the furnace to partially absorb microwaves and generate heat, thereby preheating the fibers. Carbonization was carried out in the same manner as in Example 1 except for this. The temperature of the fibers during heating measured by a radiation thermometer was 1200 °C. The carbon content of the obtained fibrous carbonized material was 93%, and carbon fibers were obtained. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.65 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.52 nm. The bulk density of the carbon fibers was 1.66 g / cm 3 and the ratio of the density after cryogenic grinding to the density before cryogenic grinding was 1.12. The tensile strength of the resin-impregnated strand tensile test was 3.80 GPa, the tensile elastic modulus was 229 GPa, and the specific strength and specific elastic modulus, which are the mechanical properties per density as an index of lightness, were 2.29 GN m / kg and 138 MN m / kg, respectively. Other results are shown in Table 1.

[0068] (Example 3) The number of filaments in the PAN-based precursor fiber bundle was changed to 48,000, and carbonization was carried out in the same manner as in Example 1 except that the applied power was 0.8 kW. The temperature of the carbon fibers during heating measured by a radiation thermometer was 1100 °C. The carbon content of the obtained fibrous carbonized material was 93%, and carbon fibers were obtained. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.73 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.51 nm. The bulk density of the carbon fibers was 1.70 g / cm 3 and the ratio of the density after cryogenic grinding to the density before cryogenic grinding was 1.05. The tensile strength of the resin-impregnated strand tensile test was 3.08 GPa, the tensile elastic modulus was 218 GPa, and the specific strength and specific elastic modulus, which are the mechanical properties per density as an index of lightness, were 1.81 MN m / kg and 128 MN m / kg, respectively. Other results are shown in Table 1.

[0069] (Example 4) A PAN-based precursor fiber bundle consisting of 24,000 filaments (carbon content 66%), the same as in Example 1, was continuously conveyed and introduced into a microwave carbonization furnace. Using the electric field formed in the furnace by an applied power of 0.52 kW, conductive heating was carried out in a nitrogen atmosphere under atmospheric pressure for carbonization to convert it into an intermediate. At this time, a high dielectric loss body (a silicon carbide pipe) was arranged at the fiber inlet of the furnace to partially absorb microwaves and generate heat to preheat the fibers. The resonance length of the microwave heating furnace was adjusted to 103% compared to Example 1. The temperature of the fibers during heating measured by a radiation thermometer was 1000 °C. The obtained intermediate was introduced into a second microwave carbonization furnace that was connected equipment-wise as the next step, and induction heating was carried out in a nitrogen atmosphere under atmospheric pressure using the magnetic field formed in the furnace by an applied power of 0.65 kW for carbonization. The temperature of the fibers during heating measured by a radiation thermometer was 1100 °C. The carbon content of the obtained fibrous carbonaceous material was 93%, and carbon fibers were obtained. The bulk density of the carbon fibers was 1.71 g / cm 3 and the ratio of the density after cryogenic grinding to the density before cryogenic grinding was 1.12. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.66 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.49 nm. The tensile strength of the resin-impregnated strand tensile test was 3.78 GPa, and the tensile elastic modulus was 222 GPa. The specific strength and specific elastic modulus, which are the mechanical properties per density as an index of light weight, were 2.21 MNm / kg and 130 MNm / kg, respectively. Other results are shown in Table 1.

[0070] (Comparative Example 1) A PAN-based precursor fiber bundle consisting of 24,000 filaments (carbon content 66%), the same as in Example 1, was continuously conveyed and introduced into a resistance heating carbonization furnace set at 1100 °C, and external heat heating was carried out in a nitrogen atmosphere under atmospheric pressure for carbonization. The carbon content of the obtained fibrous carbonaceous material was 94%, and carbon fibers were obtained. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.59 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.57 nm. The bulk density of the carbon fibers was 1.80 g / cm 3It was 1.01 for the ratio of the density after cryogenic milling to the density before cryogenic milling. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.59 nm. The tensile strength of the resin-impregnated strand tensile test was 4.00 GPa, and the tensile elastic modulus was 240 GPa. The specific strength and specific elastic modulus, which are the mechanical properties per density as an index of light weight, were 2.22 MNm / kg and 133 MNm / kg, respectively. Other results are shown in Table 1.

[0071] (Comparative Example 2) Carbonization was carried out by external heating in the same manner as in Comparative Example 1, except that the same 48,000-filament precursor fiber bundle as in Example 3 was used as the PAN-based precursor fiber bundle. The carbon content of the obtained fibrous carbonized material was 94%, and carbon fibers were obtained. The bulk density of the carbon fiber was 1.78 g / cm 3 It was 1.02 for the ratio of the density after cryogenic milling to the density before cryogenic milling. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.11 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.71 nm. The tensile strength of the resin-impregnated strand tensile test was 4.00 GPa, and the tensile elastic modulus was 240 GPa. The specific strength and specific elastic modulus, which are the mechanical properties per density as an index of light weight, were 2.25 MNm / kg and 135 MNm / kg, respectively. Other results are shown in Table 1.

[0072] (Comparative Example 3) Carbonization was carried out by heating in the same manner as in Example 2, except that the resonance length of the microwave heating furnace was adjusted to 103% compared to Example 1. The temperature of the fiber during heating measured by a radiation thermometer was 1200 °C. The carbon content of the obtained fibrous carbonized material was 93%, and it was carbon fiber. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 2.82 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.58 nm. The bulk density of the carbon fiber was 1.76 g / cm 3The ratio of the density after cryogenic grinding to the density before cryogenic grinding was 1.06. The tensile strength of the resin-impregnated strand tensile test was 3.78 GPa, and the tensile elastic modulus was 221 GPa. The specific strength and specific elastic modulus, which are the mechanical properties per unit density as indicators of light weight, were 2.14 MNm / kg and 126 MNm / kg, respectively. Other results are shown in Table 1.

[0073] (Comparative Example 4) As the PAN-based precursor fiber bundle, the same 48,000-filament precursor fiber bundle as in Example 3 was used, and carbonization was carried out in the same manner as in Comparative Example 3 except that the applied power was 0.8 kW. The temperature of the fiber during heating measured by a radiation thermometer was 1300 °C. The carbon content of the obtained fibrous carbonized material was 95%, and carbon fibers were obtained. The bulk density of the carbon fibers was 1.66 g / cm 3 The ratio of the density after cryogenic grinding to the density before cryogenic grinding was 1.05. The crystallite size La reflecting the development in the fiber axis direction of the carbon structure was 3.11 nm, and the crystallite size Lc reflecting the development in the direction perpendicular to the fiber axis was 1.63 nm. The tensile strength of the resin-impregnated strand tensile test was 1.97 GPa, and the tensile elastic modulus was 216 GPa. The specific strength and specific elastic modulus, which are the mechanical properties per unit density as indicators of light weight, were 1.19 MNm / kg and 130 MNm / kg, respectively. Other results are shown in Table 1.

[0074]

Table 1

Description of Symbols

[0075] 100 ··· Microwave heating furnace 11 ··· Microwave oscillator 12 ··· Connecting waveguide 13 ··· Circulator 14 ··· Connecting waveguide 15 ··· Tuner 15 17 ··· Carbonization furnace 17a ··· Fiber inlet 17b ··· Fiber outlet 17c ··· Short - circuit board 19 ··· Dummy load 21 ··· Microwave absorption tube 22a ··· Inlet 31b ··· Fibers to be heated 31c ··· Carbon fiber

Claims

1. By the floating and sinking method, the density is 1.66 to 1.71 [g / cm ３ , the tensile strength is 3.00 [GPa] or more, the tensile modulus is 200 [GPa] or more, the specific strength is 1.50 to 2.50 [kNm / kg], and the specific modulus is 120 to 150 [MNm / kg], and The carbon fiber density (A) [g / cm³] by the helium filling method measured in the state of the filament, and the carbon fiber density (B) [g / cm³] by the helium filling method measured after pulverizing the filament to a volume average particle diameter of 0.2 to 0.5 μm satisfy the following formula (1): 1.15 > B / A > 1.03... Formula (1) A lightweight carbon fiber of polyacrylonitrile type, characterized by satisfying the above.

2. The lightweight carbon fiber according to Claim 1, wherein the crystal size La is 1.80 to 3.00 [nm].

3. The lightweight carbon fiber according to Claim 1 or 2, wherein the crystal size Lc is 1.00 to 3.00 [nm].

4. The lightweight carbon fiber according to any one of Claims 1 to 3, having a solid structure.

5. A lightweight carbon fiber strand, comprising the lightweight carbon fiber according to any one of Claims 1 to 4, and having a fineness of 1000 [tex] or more.

6. A carbon fiber reinforced composite material, comprising the lightweight carbon fiber or the lightweight carbon fiber strand according to any one of Claims 1 to 5.

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