JP2023003361A - Light-weight carbon fiber, light-weight carbon fiber strand, carbon fiber fiber-reinforced composite material, manufacturing method thereof, and microwave oven - Google Patents

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 The present invention relates to lightweight carbon fibers, lightweight carbon fiber strands, carbon fiber reinforced composite materials, production methods thereof, and microwave heating furnaces. Specifically, a lightweight carbon fiber, a lightweight carbon fiber strand, a carbon fiber reinforced composite material having a density of 1.60 to 1.75 [g/cm ³ ] by the floating and sinking method, a method for producing the same, and a method particularly suitable for the production It relates to a microwave heating furnace having a structure.

Carbon fiber has excellent specific strength and specific modulus compared to other fibers, and is widely used in industry as a reinforcing fiber that is compounded with resin, taking advantage of its light weight and excellent mechanical properties. used for purposes.

Conventionally, carbon fibers are manufactured as follows. First, the precursor fiber is flameproofed by heating it in heated air at 230-260° C. for 30-100 minutes. This flameproofing treatment causes a cyclization reaction of the acrylic fiber to increase the amount of oxygen bonds to obtain a flameproof fiber. This flameproof fiber is carbonized, for example, in a nitrogen atmosphere using a firing furnace at 300 to 800° C. while applying a temperature gradient (first carbonization treatment). Then, it is further carbonized in a nitrogen atmosphere using a firing furnace at 800 to 2100° C. while applying a temperature gradient (second carbonization treatment). Thus, carbon fibers are produced by externally heating flameproof fibers in a heated kiln.

Carbon fibers produced in this way usually have a density of about 1.8 to 2.2 [g/cm ³ ], but efforts are being made to further reduce their weight.

Non-Patent Document 1 discloses a lightweight carbon fiber having a hollow structure and a density of about 1.2 g/cm ³ (apparent density including the hollow portion). However, hollow-structured carbon fibers have an excellent elastic modulus but are insufficient in strength. In addition, this carbon fiber has a true density of about 1.85 g/cm ³ excluding the hollow portion.

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

In Patent Document 2, when the density is 1.79 g/cm ³ or less, La≧1 [nm], and the tensile elastic modulus TM [GPa] is 170≦TM≦230, La≦−0.5+0. 01×TM [nm] are disclosed.

Non-Patent Document 2 discloses a carbon fiber having a specific gravity of 1.5 to 1.6, a tensile strength of 0.5 to 0.7 GPa, and a tensile modulus of elasticity of 20 to 30 GPa.

As described above, the current situation is that no carbon fiber that is lightweight and excellent in both tensile strength and tensile modulus has been obtained.

Low-density and high-modulus carbon fibers from polyacrylonitrile with honeycomb structure, Prabhakar V.; Gulgunje, Carbon Volume 95, December 2015, Pages 710-714, An International Journal Founded in Connection with the American Carbon Society carbon fiber, H.I. Zushi, Y. Suzuki, N. Sugiura, TANSO No. 296, 2021, Pages 9-33, The Carbon Society of Japan

Japanese Patent Publication No. 8-6210 Japanese Patent No. 6469212

An object of the present invention is to provide a lightweight carbon fiber, a lightweight carbon fiber strand, a carbon fiber reinforced composite material, a manufacturing method thereof, and a microwave heating furnace, which are lightweight and excellent in both tensile strength and tensile modulus. It is to be.

As a result of intensive studies aimed at solving the above problems, the present inventors found that the above problems can be solved by carbonizing the fibers to be heated under predetermined conditions by microwave heating. Arrived.

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

[1] Density by floating and sinking method of 1.60 to 1.75 [g/cm ³ ], tensile strength of 3.00 [GPa] or more, tensile modulus of elasticity of 200 [GPa] or more, specific strength of 1.50 or more 2. A lightweight polyacrylonitrile-based carbon fiber characterized by having a specific elastic modulus of 2.50 [MNm/kg] and a specific elastic modulus of 120 to 150 [MNm/kg].

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

[3] The lightweight carbon fiber according to [1] or [2], which has a crystal size Lc of 1.00 to 3.00 [nm].

[4] The lightweight carbon fiber according to any one of [1] to [3], which has a solid structure.

[5] Carbon fiber density (A) [g/cm ³ ] by helium filling method measured in filament state, and helium filling method measured after pulverizing the filament to a volume average particle size of 0.2 to 0.5 μm. The carbon fiber density (B) [g/cm ³ ] by the following formula (1)
1.15>A/B>1.03 Expression (1)
The lightweight carbon fiber according to any one of [1] to [4], which satisfies

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

The lightweight carbon fiber described in [1] above has a density of 1.60 to 1.75 [g/cm ³ ] according to the floating-sink method, which is lightweight, yet has a tensile strength of 3.00 [GPa] or more, and a tensile modulus of elasticity of 3.00 [GPa] or more. has high physical properties of 200 [GPa] or more.
The lightweight carbon fiber preferably has a crystal size La of 1.80 to 3.00 [nm] ([2] above), and a crystal size Lc of 1.00 to 3.00 [nm]. It is preferable (above [3]).
Also, the lightweight carbon fiber preferably has a solid structure rather than a hollow structure ([4] above).
Furthermore, it is preferable that the density of the lightweight carbon fiber measured in the filament state and the density measured in the state of pulverizing the filament to a predetermined particle size fall within a predetermined range ([5] above).
This lightweight carbon fiber may be a strand formed by bundling a plurality of fibers (above [6]).

[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].

[8] Directly irradiating the fibers to be heated with microwaves by introducing microwaves into the microwave heating furnace while running the fibers to be heated in the microwave heating furnace, thereby heating the fibers to be heated. A method for producing a carbon fiber that
A method for producing carbon fibers, wherein the fibers to be heated are heated by using a heating means other than direct irradiation of microwaves to the fibers to be heated.

[9] The heating means introduces microwaves into the microwave heating furnace while causing the fibers to be heated to run in a microwave absorption tube disposed in a portion of the microwave heating furnace. The method for producing carbon fiber according to [8], wherein the heating means heats the fiber to be heated by heating the microwave absorption tube.

[10] A microwave absorbing tube is provided at one end of the microwave heating furnace, and after preheating the fibers to be heated by heating the microwave absorbing tube, the fibers to be heated are directly irradiated with microwaves. The method for producing carbon fiber according to [9].

[11] A carbon fiber obtained by the production method according to any one of [8] to [10].

[12] A carbon fiber-reinforced composite material comprising the carbon fiber of [11].

[13] A microwave heating furnace comprising a tubular microwave absorption tube provided at one end of the microwave heating furnace, and a fiber to be heated running in the microwave absorption tube,
By introducing microwaves into the microwave heating furnace, the microwave absorbing tube is heated to preheat the fibers to be heated running in the microwave absorbing tube, and then the coated fiber is heated outside the microwave absorbing tube. A microwave heating furnace characterized in that it is configured to directly irradiate heating fibers with microwaves to further heat them.

Since the lightweight carbon fiber of the present invention is lightweight and has high strength, it is possible to reduce the weight of the carbon fiber reinforced composite material produced by using it.

FIG. 1 is a schematic diagram showing an example of a carbonization furnace.

The lightweight carbon fiber and the like of the present invention will be described in detail below. In addition, in this invention, a density means the value in 25 degreeC.

(1) Lightweight carbon fiber The lightweight carbon fiber of the present invention has a density of 1.60 to 1.75 [g/cm ³ ], a tensile strength of 3.00 [GPa] or more, and a tensile modulus of elasticity 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].

The lightweight carbon fiber of the present invention has a density of 1.60 to 1.75 [g/cm ³ ], preferably 1.62 to 1.73 [g/cm ³ ], as measured by a floating/sinking method. It is more preferably 63 to 1.72 [g/cm ³ ]. If it exceeds 1.75 [g/cm ³ ], weight reduction is insufficient. If it is less than 1.60 [g/cm ³ ], the carbon fiber will have a hollow structure, or large voids will be formed inside the fiber, and the tensile strength will tend to decrease.

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

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

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 1.80 to 2.50 [MNm/kg]. It is more preferably 40 [MNm/kg]. The specific strength is a value obtained by dividing the strand tensile strength [GPa] measured by the resin impregnation method described later by the density [g/cm ³ ] by the floating and sinking method.

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

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], more preferably 2.25 to 2.81 [nm]. It is more preferably 0.80 [nm]. If La is outside the above range, the tensile strength and tensile modulus are likely to decrease.

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 more preferably 1.40 to 2.0 [nm]. It is more preferably 0.00 [nm]. If Lc is outside the above range, the tensile strength and tensile modulus are likely to decrease.

The lightweight carbon fibers of the present invention preferably have a solid structure. In the present invention, a solid structure means that the cross section of the filament does not have a cavity having a diameter of 1/20 or more of the filament diameter.
A carbon fiber having a hollow structure is lightweight, but does not have a sufficiently high tensile strength.

The lightweight carbon fiber of the present invention has a carbon fiber density (A) [g/cm ³ ] measured by the helium filling method measured in the state of a filament, and freeze-pulverized the filament to a volume average particle size of 0.2 to 0.5 μm. The carbon fiber density (B) [g/cm ³ ] by the helium filling method to be measured later is expressed by the following formula (1)
1.15>A/B>1.03 Expression (1)
is preferably satisfied.
If it is 1.15 or more, the carbon fiber has a hollow structure or many large voids are present inside the carbon fiber, so the tensile strength of the carbon fiber tends to decrease. If it is 1.03 or less, the density by the floating-sinking method may not be sufficiently low.

Freeze pulverization is performed by ball mill pulverization in liquid nitrogen. Freeze pulverization is performed until the volume average particle size of the carbon fibers reaches 0.2 to 0.5 μm. If it exceeds 0.5 μm, voids inside the fiber are not sufficiently exposed.

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 more preferably 91 to 95 [mass%]. More preferred.

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

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

(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 heating the fiber to be heated by irradiating it with microwaves under predetermined conditions. can.

The fibers to be heated are polyacrylonitrile-based flameproof fibers, and the carbon content thereof is preferably 66 to 72 [mass %], more preferably 67 to 71 [mass %]. Polyacrylonitrile-based flame-resistant fibers are usually produced by subjecting PAN-based fibers, which are precursor fibers, to a flame-resistant treatment.

<Raw material fiber>
As the raw material fiber, a spinning solution obtained by homopolymerizing acrylonitrile or copolymerizing a monomer composition containing 90 [mass %] or more, preferably 95 [mass %] or more of acrylonitrile, is wet-type or dry-wet. A PAN-based fiber obtained by washing, drying and drawing after spinning in a spinning method can be used. Monomers to be copolymerized are preferably polar monomers such as methyl acrylate, itaconic acid, methyl methacrylate, methacrylic acid and acrylic acid.

Raw fibers, which are precursor fibers of carbon fibers, are bundled to form precursor fiber strands. The number of single fibers in the precursor fiber strand is preferably 1,000 to 48,000, more preferably 3,000 to 24,000 in terms of production efficiency.

<Flame resistant>
The above PAN-based fiber is used as a precursor fiber, and this fiber is oxidized in heated air at 200 to 300° C. for 10 to 100 minutes to be flameproofed. This flameproofing treatment causes an intramolecular cyclization reaction of the PAN-based fiber, further increasing the amount of oxygen bonding, and obtaining a flameproof fiber. In this flameproofing treatment step, the PAN-based fiber is preferably drawn at a draw ratio of 0.90 to 1.20. Further, after performing the flameproofing treatment, a preliminary carbonization treatment may be performed at a temperature of 500 to 1000° C. in an inert atmosphere, preferably in a nitrogen atmosphere. By performing such treatment, the carbon content of the fiber to be heated can be adjusted to an appropriate range.

<Carbonization>
The lightweight carbon fiber of the present invention can be produced by subjecting the PAN-based flameproof fiber, which is the fiber to be heated, to microwave heating under predetermined conditions.

Although the frequency of microwaves is not particularly limited, 915 MHz and 2.45 GHz are generally used. The output of the microwave oscillator is not particularly limited, but 300 to 2400W is suitable, and 500 to 2000W is more suitable.

As a production condition for obtaining the lightweight carbon fiber of the present invention, the time required for the heated fiber to change into carbon fiber by heating is particularly important. Examples of methods for adjusting this time include a method of changing the resonance length of microwaves, a method of using heating means other than microwaves, and a method of using both electric field heating and magnetic field heating.

By changing the resonance length of the microwave, it is possible to adjust the time required for the heated fibers to change to carbon fibers, and to adjust the density, La, and Lc of the obtained carbon fibers. The conditions can be determined by changing the resonance length of microwaves to produce carbon fibers and measuring their densities, La, and Lc.

In carbonizing the fibers to be heated, it is also preferable to use heating means other than microwaves. Specifically, it can be heated using an electric heater, plasma, or the like. Further, a microwave heating furnace is provided with a microwave absorber on one end side in the microwave heating furnace, and the fibers to be heated are configured to run near the microwave absorber. It is also preferable to heat the fiber through the microwave absorber in step 1 and then directly irradiate the fiber to be heated with microwaves to further heat the fiber. In this case, it is preferable that the microwave absorber uses a tubular microwave absorption tube, and is configured such that the fibers to be heated run inside the tube. Silicon carbide, silicon nitride, carbon particle-containing resin, soft magnetic metal particle-containing resin, etc. are exemplified as the material of the microwave absorber. The combined use of heating other than microwaves makes it easier to control the crystallite size La. The inner wall of the microwave absorber and the heated fiber are preferably separated by 1 to 50 [mm], more preferably by 3 to 30 [mm], and further by 5 to 20 [mm]. More preferably. The distance between the inner wall of the microwave absorber and the heated fiber refers to the distance between the inner wall of the microwave absorber and the center point of the cross section (cross section) of the heated fiber bundle in the direction perpendicular to the fiber axis. . From the viewpoint of heat insulation efficiency, the distance between the inner wall of the microwave absorbing tube 21 described below and the outer periphery of the fiber bundle is preferably 0.1 to 50 [mm], more preferably 1 to 20 [mm]. ].

Electric field heating and magnetic field heating can also be used together for heating by microwaves. As a method of using these together, a method of heating using a plurality of microwave heating furnaces, a method of running the fiber to be heated obliquely with respect to the axis of the furnace body of the microwave heating furnace, and the like can be mentioned.

(4) Microwave heating furnace In the microwave heating furnace of the present invention, a cylindrical microwave absorption tube is provided at one end of the microwave heating furnace, and the fibers to be heated run inside the microwave absorption tube. By introducing microwaves into the microwave heating furnace, the microwave absorption tube is heated to preheat the fibers to be heated running in the microwave absorption tube, and then the microwave absorption It is configured to directly irradiate microwaves to the fibers to be heated outside the tube to further heat them.

FIG. 1 is an explanatory view showing one structural 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 the 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 the carbonization furnace 17 . A circulator 13 and a matching device 15 are interposed in this connection waveguide 12 in order from the microwave oscillator 11 side.

One end of the carbonization furnace 17 is closed and the other end is coupled to the connecting waveguide 12 . Carbonization furnace 17 consists of a waveguide. One end of the carbonization furnace 17 is formed with a fiber inlet 17a for introducing the heated fibers into the carbonization furnace, and the other end is formed with a fiber outlet 17b for taking out the carbonized fibers. ing. A short-circuit plate 17c is provided at the inner end portion of the carbonization furnace 17 on the side of the fiber outlet port 17b. A cylindrical microwave absorption tube 21 is arranged 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 .

Next, the operation of this microwave heating furnace 100 will be described. In FIG. 1, 31b is a fiber to be heated, and the microwave absorption tube in the carbonization furnace 17 is fed from the fiber introduction port 17a through the introduction port 22a formed in the connection waveguide 12 by the fiber conveying means (not shown). 21 and introduced into the carbonization furnace 17 . Microwaves oscillated by the microwave oscillator 11 are introduced into the carbonization furnace 17 through the connecting waveguide 12 . The microwave reaching the carbonization furnace 17 is reflected by the short circuit plate 17 c and reaches the circulator 13 via the matching device 15 . Further, 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. This heat generation preheats the fibers to be heated 31b. The reflected microwave (hereinafter also referred to as “reflected wave”) is redirected by the circulator 13 , passes through the connecting waveguide 14 and is absorbed by the dummy load 19 . At this time, the matching device 15 and the short-circuit plate 17 c are matched using the matching device 15 , and a standing wave is generated in the carbonization furnace 17 . The standing wave carbonizes the heated fibers 31b to form carbon fibers 31c. At this time, the pressure inside the carbonization furnace 17 is normal, and the atmosphere is made inert by an inert gas supply means (not shown). The carbon fibers 31c are led out of the carbonization furnace 17 through the fiber outlet 17b by a fiber conveying means (not shown). The fibers to be heated are continuously introduced into the carbonization furnace 17 from the fiber introduction port 17a, are carbonized by irradiating the fibers to be heated in the carbonization furnace 17 with microwaves, and are continuously discharged from the fiber outlet 17b. Thus, carbon fibers can be continuously produced. The carbon fibers drawn out from the fiber outlet 17b are subjected to surface treatment and size treatment as necessary. Surface treatment and size treatment may be carried out according to known methods.

(5) Fiber Reinforced Composite Material The fiber reinforced composite material of the present invention contains the above lightweight carbon fibers.
A known thermosetting resin or thermoplastic resin can be used as the matrix resin.
A known method can be adopted as a method for producing the fiber-reinforced composite material.
The fiber-reinforced composite material of the present invention can be reduced in weight by 5 to 15 [%] compared to a fiber-reinforced composite material using ordinary carbon fibers, provided that various performances are kept constant.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to Examples. The physical properties of the fibers in each example and comparative example were evaluated by the following methods.

[1] Strand strength and elastic modulus The tensile strength and tensile elastic modulus of the cured epoxy resin-impregnated strand were measured according to JIS R 7608.

[2] Crystals Lc, La
The crystal sizes Lc and La are measured using an X-ray diffractometer RINT2000 manufactured by Rigaku Co., Ltd., with a sample stage on which fibers are set, and by a transmission method. CuKα rays generated at an acceleration voltage of 40 (kV) and a current of 30 (mA) are used as X-rays. Regarding the orientation of the sample, when measuring the crystal size Lc, the fiber axis direction of the fiber bundle was set to be perpendicular to the equatorial plane, and when measuring the crystal size La, the fiber axis direction of the fiber bundle was set to the equator. Parallel to the plane. Diffraction patterns with diffraction angles 2θ in the range of 10° to 60° are taken, and curves passing around 10°, 20°, 35°, and 60° of the diffraction patterns are used as baselines.
The crystal size Lc can be calculated using the following formula (2) from the half width β ₀₀₂ of the diffraction peak of the plane index (002) obtained by the above method.
Crystal size Lc [nm] = 0.9λ/(β ₀₀₂ cos θ ₀₀₂ ) Equation (2)
[In the formula, λ: X-ray wavelength, β ₀₀₂ : half width of diffraction peak of plane index (002), θ ₀₀₂ : diffraction angle of plane index (002). ]
The crystal size La can be calculated from the half-value width β10 of the diffraction peak of plane index (10) obtained by the above method using the following formula (3).
Crystal size La [nm] = 0.9λ/(β ₁₀ cos θ ₁₀ ) Equation (3)
[In the formula, λ: X-ray wavelength, β ₁₀ : half width of diffraction peak of plane index (10), θ ₁₀ : diffraction angle of plane index (10). ]

[3] Density Measured by a helium filling method using AccuPyc 1330 manufactured by Micromeritics. A 10 cc measuring cell was used, and a sample of about 0.5 g was measured.
Freeze pulverization was performed by ball mill pulverization in liquid nitrogen. After the filaments were freeze-milled, they were measured by the helium filling method. Freeze pulverization was performed until the volume average particle size reached 0.2 to 0.5 μm.

(Example 1)
A PAN-based fiber consisting of 24000 filaments was heated in air in a flameproofing furnace with a maximum temperature of 270° C. 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 an electric field formed in the furnace with an applied power of 0.5 kW was used to perform carbonization under atmospheric pressure in a nitrogen atmosphere. Conductive heating was performed to carbonize. The temperature of the fiber during heating measured with 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, which reflects the development of the carbon structure along the fiber axis, was 2.71 nm, and the crystallite size Lc, which reflects the development 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 density after freeze-grinding to 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 mechanical performance per density as an index of lightness, were 2.39 MNm / kg and 132 MNm, respectively. / kg. Other results are shown in Table 1.

(Example 2)
Carbonization was carried out in the same manner as in Example 1, except that a high dielectric loss body (silicon carbide pipe) was placed at the fiber inlet of the furnace to partially absorb microwaves to generate heat, thereby preheating the fibers. . The temperature of the fiber during heating measured with 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, which reflects the development of the carbon structure along the fiber axis, was 2.65 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.52 nm. The bulk density of the carbon fiber was 1.66 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-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 mechanical performance per density as an index of lightness, were 2.29 GNm / kg and 138 MNm, respectively. / kg. Other results are shown in Table 1.

(Example 3)
Carbonization was carried out in the same manner as in Example 1, except that the number of filaments in the PAN-based precursor fiber bundle was changed to 48000 and the applied power was changed to 0.8 kW. The temperature of the carbon fiber during heating measured with 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, which reflects the development of the carbon structure along the fiber axis, was 2.73 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.51 nm. The bulk density of the carbon fiber was 1.70 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-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 mechanical performance per density as an index of lightness, were 1.81 MNm / kg and 128 MNm, respectively. / kg. Other results are shown in Table 1.

(Example 4)
A PAN-based precursor fiber bundle (carbon content: 66%) composed of 24,000 filaments, which is the same as in Example 1, was continuously conveyed and introduced into a microwave carbonization furnace. It was converted to an intermediate by conducting conductive heating in a nitrogen atmosphere under atmospheric pressure. At this time, a high dielectric loss body (silicon carbide pipe) is placed at the fiber inlet of the furnace to partially absorb the microwaves to generate heat, thereby preheating the fibers and increasing the resonance length of the microwave heating furnace. was adjusted to 103% compared to Example 1. The temperature of the fiber during heating measured with a radiation thermometer was 1000°C. The obtained intermediate was introduced into a second microwave carbonization furnace connected in the next step, and was subjected to carbonization under atmospheric pressure in a nitrogen atmosphere using a magnetic field formed in the furnace with an applied power of 0.65 kW. It was carbonized by induction heating. The temperature of the fiber during heating measured with a radiation thermometer was 1100°C. The carbon content of the obtained fibrous carbonized material was 93%, and carbon fibers were obtained. The bulk density of the carbon fiber was 1.71 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-grinding was 1.12. The crystallite size La, which reflects the development of the carbon structure along the fiber axis, was 2.66 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.49 nm. The tensile strength of the resin-impregnated strand tensile test was 3.78 GPa, the tensile elastic modulus was 222 GPa, and the specific strength and specific elastic modulus, which are mechanical performance per density as an index of lightness, were 2.21 MNm / kg and 130 MNm, respectively. / kg. Other results are shown in Table 1.

(Comparative example 1)
A PAN-based precursor fiber bundle (carbon content: 66%) consisting of 24,000 filaments, which is the same as in Example 1, was continuously transported and introduced into a resistance heating carbonization furnace set at 1100 ° C., and then extruded in a nitrogen atmosphere under atmospheric pressure. It was heated and carbonized. The carbon content of the obtained fibrous carbonized material was 94%, and carbon fibers were obtained. The crystallite size La, which reflects the development of the carbon structure along the fiber axis, was 2.59 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.57 nm. The bulk density of the carbon fiber was 1.80 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-grinding was 1.01. The crystallite size La, which reflects the growth of the carbon structure along the fiber axis, was 2.59 nm. The tensile strength of the resin-impregnated strand tensile test was 4.00 GPa, the tensile elastic modulus was 240 GPa, and the specific strength and specific elastic modulus, which are mechanical performance per density as an index of lightness, were 2.22 MNm / kg and 133 MNm, respectively. / kg. Other results are shown in Table 1.

(Comparative example 2)
Carbonization was performed by external heating in the same manner as in Comparative Example 1, except that the same precursor fiber bundle of 48000 filaments 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 and the ratio of the density after freeze-grinding to the density before freeze-grinding was 1.02. The crystallite size La, which reflects the development of the carbon structure along the fiber axis, was 2.11 nm, and the crystallite size Lc, which reflects the development 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 modulus was 240 GPa. / kg. Other results are shown in Table 1.

(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% of that in Example 1. The temperature of the fiber during heating measured with a radiation thermometer was 1200°C. The obtained fibrous carbonized material had a carbon content of 93% and was carbon fiber. The crystallite size La, which reflects the development of the carbon structure along the fiber axis, was 2.82 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.58 nm. The bulk density of the carbon fiber was 1.76 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-grinding was 1.06. The tensile strength of the resin-impregnated strand tensile test was 3.78 GPa, the tensile modulus was 221 GPa, and the specific strength and specific modulus, which are mechanical performance per density as an index of lightness, were 2.14 MNm / kg and 126 MNm, respectively / kg. Other results are shown in Table 1.

(Comparative Example 4)
As the PAN-based precursor fiber bundle, the same 48000 filament precursor fiber bundle as in Example 3 was used, and carbonization was performed by heating in the same manner as in Comparative Example 3 except that the applied power was set to 0.8 kW. The temperature of the fiber during heating measured with 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 fiber was 1.66 g/cm ³ and the ratio of the density after freeze-grinding to the density before freeze-grinding was 1.05. The crystallite size La, which reflects the development of the carbon structure along the fiber axis, was 3.11 nm, and the crystallite size Lc, which reflects the development perpendicular to the fiber axis, was 1.63 nm. The tensile strength of the resin-impregnated strand tensile test was 1.97 GPa, the tensile elastic modulus was 216 GPa, and the specific strength and specific elastic modulus, which are mechanical performance per density as an index of lightness, were 1.19 MNm / kg and 130 MNm, respectively. / kg. Other results are shown in Table 1.

DESCRIPTION OF SYMBOLS 100... Microwave heating furnace 11... Microwave oscillator 12... Connection waveguide 13... Circulator 14... Connection waveguide 15... Matching device 15
DESCRIPTION OF SYMBOLS 17... Carbonization furnace 17a... Fiber introduction port 17b... Fiber outlet 17c... Short-circuit plate 19... Dummy load 21... Microwave absorption tube 22a... Introduction port 31b...・Heated fiber 31c: carbon fiber

Claims (13)

Density by floating-sink method is 1.60 to 1.75 [g/cm ³ ], tensile strength is 3.00 [GPa] or more, tensile modulus is 200 [GPa] or more, specific strength is 1.50 to 2.50. [kNm/kg] and a specific elastic modulus of 120 to 150 [MNm/kg], a polyacrylonitrile-based lightweight carbon fiber. The lightweight carbon fiber according to claim 1, having a crystal size La of 1.80 to 3.00 [nm]. 3. The lightweight carbon fiber according to claim 1, 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. Carbon fiber density (A) [g/cm ³ ] by helium filling method measured in filament state, and carbon fiber by helium filling method measured after pulverizing the filament to a volume average particle size of 0.2 to 0.5 μm. Density (B) [g/cm ³ ] is expressed by the following formula (1)
1.15>A/B>1.03 Expression (1)
The lightweight carbon fiber according to any one of claims 1 to 4, satisfying: A lightweight carbon fiber strand comprising the lightweight carbon fiber according to any one of claims 1 to 5 and having a fineness of 1000 [tex] or more. A carbon fiber reinforced composite material comprising the lightweight carbon fiber or lightweight carbon fiber strand according to any one of claims 1 to 6. A carbon fiber that heats the fibers to be heated by introducing microwaves into the microwave heating furnace while running the fibers to be heated in the microwave heating furnace and directly irradiating the fibers to be heated with microwaves. A manufacturing method of
A method for producing carbon fibers, wherein the fibers to be heated are heated by using a heating means other than direct irradiation of microwaves to the fibers to be heated. The heating means introduces microwaves into the microwave heating furnace while running the fibers to be heated in a microwave absorption tube disposed in a part of the microwave heating furnace, thereby absorbing the microwaves. 9. The method for producing carbon fiber according to claim 8, wherein the heating means heats the fiber by generating heat in a tube. A microwave absorbing tube is disposed at one end of the microwave heating furnace, and after preheating the fibers to be heated by heating the microwave absorbing tube, the fibers to be heated are directly irradiated with microwaves. 9. The method for producing carbon fiber according to 9. A carbon fiber obtained by the manufacturing method according to any one of claims 8 to 10. A carbon fiber reinforced composite material comprising the carbon fibers of claim 11 . A microwave heating furnace in which a cylindrical microwave absorption tube is provided at one end side of the microwave heating furnace, and a fiber to be heated runs in the microwave absorption tube,
By introducing microwaves into the microwave heating furnace, the microwave absorbing tube is heated to preheat the fibers to be heated running in the microwave absorbing tube, and then the coated fiber is heated outside the microwave absorbing tube. A microwave heating furnace characterized in that it is configured to directly irradiate heating fibers with microwaves to further heat them.

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