JP7675329B2 - Carbon fiber composite material and its manufacturing method - Google Patents

Description

The present invention relates to a low-hardness carbon fiber composite material and a method for producing the same.

In recent years, composite materials that use carbon nanotubes to improve mechanical strength have been attracting attention. However, carbon nanotubes have strong cohesive properties and tend to form clumps, making it very difficult to use them as a fibrous reinforcing material in composite materials.

In response to this, a carbon fiber composite material has been proposed in which the elastomer molecules bond with the radicals at the ends of the carbon nanotubes during the process of kneading the carbon nanotubes into the elastomer, thereby weakening the cohesive force of the carbon nanotubes and compounding the carbon nanotubes in a defibrillated state (for example, Patent Document 1).

Carbon fiber composite materials using fluorine-containing elastomers as elastomers have also been reported, and a heat-resistant sealant with high heat resistance that can be used in downhole equipment for exploring underground resources such as oil and natural gas has been proposed (for example, Patent Document 2).

In addition to the carbon fiber composite materials mentioned above, carbon fiber composite materials using fluorine-containing elastomers have also been proposed that reduce the amount of expensive carbon nanotubes used, making them more cost-competitive, and that use carbon nanotubes but make it easier to adjust the rubber hardness than before (Patent Document 3).

JP 2005-97525 A International Publication No. WO2009/125503 Patent No. 6415284

Generally, rubber products such as O-rings and diaphragms that use elastomers tend to have a hardness of 75 or less, but carbon fiber composite materials made by blending fluorine-containing elastomers with carbon nanotubes to improve mechanical strength, etc., with a hardness of 75 or less are not available on the market. On the other hand, low-hardness rubber products tend to be easily damaged when demolded, so it is desirable to improve yields by reducing molding defects.

Therefore, the present invention aims to provide a carbon fiber composite material using a fluorine-containing elastomer that has a hardness of 75 degrees or less and excellent mechanical strength, and a method for producing the same.

The present invention has been made to solve at least some of the problems described above, and can be realized in the following aspects or application examples.

[1] One embodiment of the carbon fiber composite material according to the present invention is
A carbon fiber composite material containing a carbon-based reinforcing material in a crosslinked fluorine-containing elastomer,
In a cross section of the carbon fiber composite material, the carbon fiber composite material has a plurality of aggregate structures in which two or more adjacent carbon-based reinforcing materials are in close proximity to or in contact with each other at a distance of 100 nm or less,
the aggregate structure includes the carbon-based reinforcement material;
the carbon-based reinforcing material includes carbon nanotubes having an average diameter of 0.7 nm to 30 nm and carbon black having an average particle size of 35 nm to 300 nm , and the average distance between adjacent carbon-based reinforcing materials in the aggregate structure is 10 nm or more and less than 100 nm;
a circumscribing circle circumscribing the assembly structure has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm;
The area of the cross section occupied by the aggregate structure is 5% to 40%.

[2] In one embodiment of the carbon fiber composite material,
The carbon black may include high-structure carbon black having an average particle size of 35 nm to 80 nm.

[3] In one embodiment of the carbon fiber composite material,
The carbon black further includes carbon black other than high structure carbon black,
The other carbon black may have an average particle size of 100 nm to 300 nm.

[4] In one embodiment of the carbon fiber composite material,
The carbon nanotube composition may contain 0.1 to 3 parts by mass of the carbon nanotubes, 1 to 10 parts by mass of the high structure carbon black, and 0 to 4 parts by mass of the other carbon black, relative to 100 parts by mass of the fluorine-containing elastomer.

[5] One embodiment of the carbon fiber composite material according to the present invention is
the composition contains, relative to 100 parts by mass of a fluorine-containing elastomer, 0.1 to 3 parts by mass of carbon nanotubes, 1 to 10 parts by mass of high-structure carbon black, and 0 to 4 parts by mass of carbon black other than the high-structure carbon black;
The carbon nanotubes have an average diameter of 0.7 nm to 30 nm;
The high structure carbon black has an average particle size of 35 nm to 80 nm,
The other carbon black has an average particle size of 100 nm to 300 nm,
The mass ratio of the total amount of the high-structure carbon black and the other carbon black to the carbon nanotubes is 1:140 to 3:1.

[6] In one embodiment of the carbon fiber composite material,
The carbon nanotubes may include multi-walled carbon nanotubes and single-walled carbon nanotubes.

[7] In one embodiment of the carbon fiber composite material,
The carbon fiber composite material may have a tear strength of 39.0 N/mm or more in a tear test at room temperature in accordance with JIS K6252.

[8] In one embodiment of the carbon fiber composite material,
The carbon fiber composite material may have a tear strength of 10.0 N/mm or more in a tear test at 200° C. in accordance with JIS K6252.

[9] In one embodiment of the carbon fiber composite material,
The carbon fiber composite material may have a breaking frequency of 20 or more in a tear fatigue test at 200° C. with a maximum tensile stress of 1.9 N/mm and a frequency of 1 Hz.

[10] In one embodiment of the carbon fiber composite material,
The carbon fiber composite material may have a stress at 100% elongation of 6.0 MPa or more.

[11] One embodiment of the method for producing a carbon fiber composite material according to the present invention comprises:
The method comprises the steps of: mixing the fluorine-containing elastomer before crosslinking with the carbon-based reinforcing material to obtain an uncrosslinked carbon fiber composite material; and crosslinking the fluorine-containing elastomer in the uncrosslinked carbon fiber composite material to obtain one embodiment of the carbon fiber composite material.
[12] One embodiment of the method for producing a carbon fiber composite material according to the present invention comprises:
A method for producing a carbon fiber composite material, comprising thin-passing a carbon fiber composite material containing a carbon-based reinforcing material in a crosslinked fluorine-containing elastomer using open rolls with a roll gap of 0 mm to 0.5 mm and a roll temperature set to 0° C. to 50° C.,
In a cross section of the carbon fiber composite material, the carbon fiber composite material has a plurality of aggregate structures in which two or more adjacent carbon-based reinforcing materials are in close proximity to or in contact with each other at a distance of 100 nm or less,
the aggregate structure includes the carbon-based reinforcement material;
The carbon-based reinforcing material includes carbon nanotubes having an average diameter of 0.7 nm to 30 nm and carbon black having an average particle size of 35 nm to 300 nm,
the average distance between adjacent carbon-based reinforcing materials in the aggregate structure is 10 nm or more and less than 100 nm;
a circumscribing circle circumscribing the assembly structure has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm;
The area of the cross section occupied by the aggregate structure is 5% to 40%.

FIG. 2 is a schematic diagram illustrating a cross-sectional structure of a carbon fiber composite material. 1 is a SEM image of a tensile fracture surface of a carbon fiber composite material. 1 is a SEM image of a tensile fracture surface of a carbon fiber composite material to illustrate the assembly structure. FIG. 1 is a diagram illustrating a method for producing a carbon fiber composite material. FIG. 1 is a diagram illustrating a method for producing a carbon fiber composite material. FIG. 1 is a diagram illustrating a method for producing a carbon fiber composite material.

Below, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention described in the claims. Furthermore, not all of the configurations described below are necessarily essential components of the present invention.

1. Carbon Fiber Composite Material According to a First Embodiment A carbon fiber composite material according to a first embodiment of the present invention is a carbon fiber composite material comprising a carbon-based reinforcing material in a cross-linked fluorine-containing elastomer, wherein a cross-section of the carbon fiber composite material has a plurality of aggregate structures in which two or more adjacent carbon-based reinforcing materials are in close proximity to or in contact with each other at a distance of 100 nm or less, the carbon-based reinforcing materials comprise carbon nanotubes having an average diameter of 0.7 nm to 30 nm and carbon black having an average particle size of 35 nm to 300 nm, the average distance between adjacent carbon-based reinforcing materials in the aggregate structure is 10 nm or more and less than 100 nm, a circumscribed circle circumscribing the aggregate structure has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm, and the aggregate structure occupies 5% to 40% of the cross-section.

The carbon fiber composite material according to the first embodiment may contain high-structure carbon black having an average particle size of 35 nm to 80 nm as the carbon black. The carbon black may further contain other carbon blacks besides the high-structure carbon black, and the other carbon blacks may have an average particle size of 100 nm to 300 nm.

1.1. Assembly structure The inventors of the present invention performed measurements of carbon fiber composite materials using a scanning electron microscope and found that carbon fiber composite materials that have a hardness of 75 degrees or less and excellent mechanical strength have common structural features.

The structural features of the carbon fiber composite material 50 will now be described in detail with reference to Figures 1 to 3. Figure 1 is a schematic diagram illustrating the structure in an image 90 of a cross section of the carbon fiber composite material 50, Figure 2 is an SEM image 90 of a tensile fracture surface of the carbon fiber composite material 50, and Figure 3 is an image 90 in which the aggregate structure 84 and the circumscribing circle C1 are indicated by dashed lines in the image 90 of Figure 2.
may be a freeze-fractured surface or a tensile fracture surface. In addition, "SEM" is an abbreviation for Scanning Electron Microscope.

The carbon fiber composite material 50 shown in Figures 1 to 3 contains a carbon-based reinforcing material in a cross-linked fluorine-containing elastomer 30. The carbon-based reinforcing material contains carbon nanotubes 81 with an average diameter of 0.7 nm to 30 nm and carbon black with an average particle size of 35 nm to 300 nm. The carbon black preferably contains high-structure carbon black 82 with an average particle size of 35 nm to 80 nm, which will be described later in "3.2. High-structure carbon black", and other carbon black 83 with an average particle size of 100 nm to 300 nm, which will be described later in "3.4. Other carbon black", and this example is shown in Figures 1 to 3.

As shown in Figures 1 to 3, the cross section of the carbon fiber composite material 50 has a plurality of aggregate structures 84 in which two or more adjacent carbon-based reinforcing materials are in close proximity or in contact with each other at a distance L1 of 100 nm or less. Distance L1 is the distance at the position where adjacent carbon-based reinforcing materials are closest to each other, and when adjacent carbon-based reinforcing materials are in contact, the distance L1 is measured as 0 nm. The shaded area in Figure 1 is a range of 100 nm around each carbon-based reinforcing material, and indicates the aggregate structure 84. Also, the dashed free curve in Figure 3 indicates the aggregate structure 84 by surrounding each carbon-based reinforcing material with a perimeter of 100 nm.

The average value of the distance L1 between adjacent carbon-based reinforcing materials in the aggregate structure 84 is 10 nm or more and less than 100 nm. Furthermore, the average value of the distance L1 can be 10 nm to 95 nm. In FIG. 1, the distance L1 indicates the distance between adjacent high-structure carbon blacks 82, 82, but is not limited to this, and can also be the distance between other adjacent carbon blacks 83, 83, the distance between adjacent carbon nanotubes 81, 81, or the distance between adjacent carbon nanotubes 81 and carbon black. If the average value of the distance L1 is less than 10 nm, the hardness of the carbon fiber composite material 50 exceeds 75 degrees and the flexibility decreases. Furthermore, if the average value of the distance L1 is 100 nm or more, the interaction between the carbon-based reinforcing materials decreases, and the reinforcing effect decreases. The distance L1 is measured on the image 90 of a scanning electron microscope.

The circumscribing circle C1 circumscribing the aggregate structure 84 in Figures 1 and 3 has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm. The circumscribing circle C1 can be drawn as a circle circumscribing the aggregate structure 84 on the scanning electron microscope image 90. The area of the cross section of the carbon fiber composite material 50 that is occupied by the aggregate structure 84 is 5% to 40%. The area can be measured on the image 90.

The aggregate structure 84 has a higher elastic modulus than the phase of the fluorine-containing elastomer 30 alone, and is therefore presumed to behave like a single structure within the carbon fiber composite material 50. As a result, the aggregate structure 84 bears the stress on the carbon fiber composite material 50, and exhibits high resistance to tearing, tension, and repeated loads at room temperature and high temperatures. It is believed that interactions occur within the aggregate structure 84 due to the stress field of adjacent carbon-based reinforcing materials. In addition, multiple aggregate structures 84 are independently scattered within the carbon fiber composite material 50, thereby maintaining flexibility during deformation.

2. Carbon Fiber Composite Material According to a Second Embodiment A carbon fiber composite material according to a second embodiment of the present invention contains, per 100 parts by mass of fluorine-containing elastomer, 0.1 to 3 parts by mass of carbon nanotubes, 1 to 10 parts by mass of high structure carbon black, and 0 to 4 parts by mass of other carbon black other than the high structure carbon black, wherein the carbon nanotubes have an average diameter of 0.7 nm to 30 nm, the high structure carbon black has an average particle size of 35 nm to 80 nm, the other carbon black has an average particle size of 100 nm to 300 nm, and the mass ratio of the total amount of the high structure carbon black and the other carbon black to the carbon nanotubes is 1:140 to 3:1.

The carbon fiber composite material according to the second embodiment forms the same aggregate structure as that according to the first embodiment. The carbon fiber composite material according to the second embodiment will be described below, but the carbon fiber composite material according to the first embodiment will also have the same performance if it is made with the same raw materials and blend as the second embodiment.

Carbon fiber composite materials can have a hardness (JIS-A) of 75 degrees or less. Most rubber products such as general O-rings and diaphragms have a hardness of 75 degrees or less, so by making the hardness similar to these, it is possible to expand the range of application, uses, and markets of carbon fiber composite materials, which have high mechanical strength and heat resistance. It is also more preferable that the hardness (JIS-A) of carbon fiber composite materials is in the range of 75 degrees to 65 degrees.

In addition, because the carbon fiber composite material has a hardness of 75 degrees or less and excellent mechanical strength, it can suppress breakage during demolding during molding. Specifically, when the carbon fiber composite material is molded into products such as O-rings and diaphragms, the product may stick to or get caught on the mold during the process of removing it from a high-temperature mold (demolding), and may be pulled. However, because the carbon fiber composite material has excellent mechanical strength, it can suppress the occurrence of molding defects such as tearing or breaking of the product. Here, mechanical strength refers to the tear strength in a tear test, particularly at high temperatures. In addition, in recent years, there has been a demand for further improvement in durability of rubber products for various industrial and commercial applications, and the number of uses in harsh environments such as high temperatures is increasing. In particular, rubber products using fluorine-containing elastomers, which are often used in harsh environments, are desired to have durability, heat resistance, and mechanical strength.

The carbon fiber composite material has the above-mentioned hardness and mechanical strength while maintaining elongation and flexibility. Here, elongation and flexibility can be evaluated by the elongation at break in a tensile test. A carbon fiber composite material having these properties can be achieved by blending a predetermined amount of carbon black in addition to a relatively small amount of defibrated carbon nanotubes. In addition, it is preferable that the carbon black contains at least high-structure carbon black, and it may further contain carbon black other than high-structure carbon black.

In carbon fiber composite materials, carbon nanotubes are dispersed throughout in a defibrated state. Carbon fiber composite materials do not contain agglomerates of carbon nanotubes. If agglomerates exist, they can become the starting point of fracture, resulting in a decrease in mechanical strength.

The carbon fiber composite material can have a tear strength of 39.0 N/mm or more and 54.0 N/mm or less in a tear test conforming to JIS K6252 at room temperature. When the carbon fiber composite material has a tear strength of 39.0 N/mm or more at room temperature, the mechanical strength is excellent. Therefore, the carbon fiber composite material can suppress breakage during demolding in molding. Furthermore, the carbon fiber composite material more preferably has a tear strength of 39.5 N/mm or more and even more preferably has a tear strength of 40.0 N/mm or more in a tear test conforming to JIS K6252 at room temperature.

The carbon fiber composite material may have a tear strength of 10.0 N/mm or more and 16.0 N/mm or less in a tear test according to JIS K6252 at 200°C. When the carbon fiber composite material has a tear strength of 10.0 N/mm or more at a high temperature of 200°C, the material has excellent mechanical strength at high temperatures and excellent heat resistance. Therefore, the carbon fiber composite material can be prevented from being broken when it is removed from a high-temperature mold during molding. In addition, it is more preferable that the carbon fiber composite material has a tear strength of 11.0 N/mm or more in a tear test according to JIS K6252 at 200°C.

The carbon fiber composite material may have a number of breaks of 20 or more in a tear fatigue test at 200°C with a maximum tensile stress of 1.9 N/mm and a frequency of 1 Hz. By having a number of breaks of 20 or more at 200°C, the repeated tensile fatigue strength of the carbon fiber composite material under heat is improved. Therefore, the carbon fiber composite material can be prevented from being broken when it is removed from a high-temperature mold during molding. It is more preferable that the carbon fiber composite material has a number of breaks of 50 or more in a tear fatigue test at 200°C with a maximum tensile stress of 1.9 N/mm and a frequency of 1 Hz.

The carbon fiber composite material can have a stress at 100% elongation (M100) of 6.0 MPa or more and 10.5 MPa or less. When the stress at 100% elongation of the carbon fiber composite material is 6.0 MPa or more, it has high deformation resistance and is effective in preventing tearing when pulled.

Next, the raw materials constituting the carbon fiber composite material according to the second embodiment will be described. Note that the raw materials constituting the carbon fiber composite material according to the first embodiment are basically the same, so duplicated descriptions will be omitted.

The fluorine-containing elastomer in the present invention is a vinylidene fluoride-based synthetic rubber (FKM) or a tetrafluoroethylene-propylene-based synthetic rubber (FEPM) that contains fluorine atoms in the molecule. The fluorine-containing elastomer includes binary and ternary types, types with improved low-temperature properties, and base-resistant grades.

Examples of binary and ternary fluorine-containing elastomers include vinylidene fluoride (VDF)-hexafluoropropylene (HFP) binary copolymer (VDF-HFP), tetrafluoroethylene (TFE)-propylene (P) binary copolymer (TFE-P), vinylidene fluoride (VDF)-hexafluoropropylene (HFP)-tetrafluoroethylene (TFE) terpolymer (VDF-HFP-TFE), etc. Examples of fluorine-containing elastomers with improved low temperature properties include those in which the hexafluoropropylene in the terpolymer is replaced with fluorinated vinyl ether (FVE), perfluoro(methyl vinyl ether) (PMVE), etc. Examples of base-resistant fluorine-containing elastomers include vinylidene fluoride (VDF)-hexafluoropropylene (HFP)-tetrafluoroethylene (TFE)-ethylene (E)-perfluoro(methyl vinyl ether) (PMVE) 5-polymer (VDF-HEP-TFE-E-PMVE).

Examples of fluorine-containing elastomers include Viton (trade name) manufactured by Chemours, Dai-el (trade name) manufactured by Daikin Industries, Tecnoflon (trade name) manufactured by Solvay Specialty Polymers, Dyneon (trade name) manufactured by 3M, and AFLAS (trade name) manufactured by AGC. In the following explanation, fluorine-containing elastomers may be abbreviated as FKM or FEPM.

The weight-average molecular weight of the fluorine-containing elastomer is preferably 50,000 to 300,000. When the molecular weight of the fluorine-containing elastomer is within this range, the fluorine-containing elastomer molecules are entangled and connected to each other, so that the fluorine-containing elastomer can have good elasticity for dispersing carbon nanotubes. The fluorine-containing elastomer has viscosity, so that aggregated carbon nanotubes can easily penetrate each other, and furthermore, because of its elasticity, the carbon nanotubes can be separated from each other. When the weight-average molecular weight of the fluorine-containing elastomer is smaller than 50,000, the fluorine-containing elastomer molecules cannot be sufficiently entangled with each other, and even if a shear force is applied in a later process, the elasticity is small, so the effect of dispersing the carbon nanotubes tends to be small. Also, when the weight-average molecular weight of the fluorine-containing elastomer is larger than 300,000, the fluorine-containing elastomer tends to become too hard and difficult to process.

The fluorine-containing elastomer can be crosslinked using a known crosslinking agent, for example, polyamine crosslinking, polyol crosslinking, or peroxide crosslinking, with peroxide crosslinking being preferred. By crosslinking the fluorine-containing elastomer with a crosslinking agent, a carbon fiber composite material with excellent heat resistance and chemical resistance can be produced.

The fluorine-containing elastomer has a fluorine content of 57% by mass or more, and can be from 57% to 72% by mass, and can be particularly from 65% to 72% by mass. If the fluorine-containing elastomer has a fluorine content of 57% by mass or more, it can have excellent heat resistance. In addition, taking into account chemical resistance, it is preferable that the fluorine-containing elastomer has a fluorine content of 65% by mass or more.

The fluorine-containing elastomer has a halogen group that has an affinity for carbon nanotubes, especially for the radicals at their ends. Carbon nanotubes usually have a closed structure with six-membered rings of carbon atoms on the sides and a five-membered ring at the end, but this is structurally unreasonable and in practice it is prone to defects, making it easy to generate radicals and functional groups at those parts. In addition, by having a halogen group that has a high affinity (reactivity or polarity) with the radicals of carbon nanotubes in at least one of the main chain, side chain, and end chain of the fluorine-containing elastomer, it is possible to bond the fluorine-containing elastomer and the carbon nanotubes. This makes it possible to overcome the cohesive force of the carbon nanotubes and easily disperse them.

3.2. High Structure Carbon Black The average particle size of high structure carbon black (hereinafter referred to as "HS carbon") can be 35 nm to 80 nm. Here, structure refers to the state of particle aggregation, and in the case of a high structure, the degree of connection between particles is high, and the cohesive force of the particles directly acts to reinforce the force, which affects tensile stress, etc. The average particle size of HS carbon can be calculated as the arithmetic average value by measuring the particle diameter of 2000 or more particles by imaging them with a scanning electron microscope, regarding the small spherical components that make up the HS carbon aggregate as single particles (basic particles).

The DBP absorption of HS carbon (Method A) can further be 140 cm ³ /100g to 160 ^{cm 3} /100g. The DBP absorption is the amount of DBP (dibutyl phthalate) absorbed by 100g of HS carbon (cm ³ /100g) and is measured according to JIS K6217-4 (ASTM D 2414). The DBP absorption can indirectly quantify the structure, which is the degree of development of aggregates formed by fusion of HS carbon particles. The DBP absorption of HS carbon referred to here is a measurement value before being blended with a fluorine-containing elastomer.

The carbon fiber composite material contains 1 to 10 parts by mass of HS carbon relative to 100 parts by mass of the fluorine-containing elastomer. The carbon fiber composite material can contain 2 to 8 parts by mass of HS carbon relative to 100 parts by mass of the fluorine-containing elastomer. Compared with other carbon blacks described below, HS carbon has a higher degree of particle-to-particle bonding and is prone to interact with other carbon blacks even in small amounts, which contributes to the formation of an aggregate structure with carbon nanotubes in the composite material and contributes to improving mechanical strength, so it is preferable to blend 1 part by mass or more. Furthermore, it is preferable to limit the amount of HS carbon to 10 parts by mass or less in terms of ease of hardness adjustment.

3.3. Carbon nanotubes The carbon nanotubes used in one embodiment of the present invention may have an average diameter (fiber diameter) of 0.7 nm to 30 nm, and may further have a diameter of 2 nm to 20 nm. Such carbon nanotubes have a relatively small average diameter, so that they have a large specific surface area, and the surface reactivity with the elastomer matrix is improved, and the poor dispersion of the carbon nanotubes in the elastomer tends to be improved. Carbon nanotubes with a diameter of 0.7 nm or more are commercially available, and those with a diameter of 30 nm or less have the effect of being excellent in tear fatigue property and abrasion resistance. The carbon nanotubes may be subjected to a known activation treatment in order to improve the reactivity of their surface with the elastomer. The average diameter of the carbon nanotubes can be measured by observation with an electron microscope. In the detailed description of the present invention, the average diameter and average length of the carbon nanotubes can be obtained by measuring the diameter and length at 200 or more points from an image taken with an electron microscope at, for example, 5,000 times (the magnification can be changed appropriately depending on the size of the carbon nanotubes), and calculating the arithmetic average value.

Carbon nanotubes are at least one of so-called multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT), which have a shape formed by rolling up one surface (graphene sheet) of carbon hexagonal mesh graphite into a cylindrical shape, and carbon materials partially having a carbon nanotube structure can also be used. Carbon nanotubes may include multi-walled carbon nanotubes and single-walled carbon nanotubes. In addition to the name carbon nanotubes, they are also called graphite fibril nanotubes and vapor-grown carbon fibers.

Carbon nanotubes can be obtained by vapor phase growth, also known as catalytic chemical vapor deposition (CCVD), which is a method of producing unprocessed carbon nanotubes by pyrolyzing a gas such as a hydrocarbon in the presence of a metal catalyst. To explain the vapor phase growth method in more detail, for example, the floating reaction method uses organic compounds such as benzene and toluene as raw materials and organic transition metal compounds such as ferrocene and nickelcene as metal catalysts, and introduces them together with a carrier gas into a reactor set at a high reaction temperature, for example, 400°C to 1000°C, to generate carbon nanotubes in a floating state or on the reactor wall, or the substrate reaction method, in which metal-containing particles previously supported on ceramics such as alumina or magnesium oxide are contacted with a carbon-containing compound at high temperature to generate carbon nanotubes on a substrate. For example, carbon nanotubes with an average diameter of 9 nm to 20 nm can be obtained by the catalyst support reaction method, and carbon nanotubes thicker than this can be obtained by the floating reaction method. The diameter of the carbon nanotubes can be adjusted, for example, by the size of the metal-containing particles and the reaction time.

The amount of carbon nanotubes can be adjusted together with the amount of HS carbon or other carbon black, and 0.1 to 3 parts by mass are blended with respect to 100 parts by mass of the fluorine-containing elastomer. It is more preferable to blend 0.3 to 3 parts by mass of carbon nanotubes with respect to 100 parts by mass of the fluorine-containing elastomer, and it is even more preferable to blend 0.5 to 3 parts by mass of carbon nanotubes. If the amount of carbon nanotubes is 3 parts by mass or less, it is easy to adjust the rubber hardness to 75 degrees or less while maintaining elongation and flexibility, and by using 3 parts by mass or less of carbon nanotubes in combination with carbon black, it is possible to provide cost competitiveness while increasing mechanical strength. In addition, by blending 0.1 parts by mass or more of carbon nanotubes with 100 parts by mass of the fluorine-containing elastomer and compounding them in a defibrated state, a microcellular structure is formed so as to be dotted, and a reinforcing effect is exhibited. The microcellular structure can be formed so that the matrix material is surrounded by a network structure in which the carbon nanotubes are spread out three-dimensionally. Research results to date have shown that the maximum diameter of a single cell is approximately 2 to 10 times the average diameter of the carbon nanotubes. In addition, by adding a certain amount of carbon black-based reinforcing agent, it is possible to form an aggregate structure together with the carbon nanotubes in the composite material, thereby increasing the mechanical strength.

3.4. Other carbon black As the other carbon black other than HS carbon used in the carbon fiber composite material, various grades of other carbon black made from various raw materials can be used. The other carbon black can have an average particle size of 100 nm to 300 nm. The average particle size of the other carbon black can be determined by observing the particle diameters of 2000 or more basic constituent particles by imaging with a scanning electron microscope and taking the arithmetic average.

As such other carbon black, for example, reinforcing carbon black such as FT grade or MT grade can be used. By using carbon black with a relatively large particle size, it is possible to reinforce the carbon fiber composite material while maintaining its flexibility. In addition, other carbon black with a relatively large particle size has a small specific surface area, so it provides a flexible reinforcing effect while suppressing the increase in hardness and decrease in elongation in the carbon fiber composite material. In addition, other carbon black with a relatively large particle size fills the gaps between the carbon nanotubes and HS carbon, which contributes to the formation of an aggregate structure and provides a reinforcing effect while suppressing the decrease in elongation.

The amount of the other carbon black is 0 to 4 parts by mass per 100 parts by mass of the fluorine-containing elastomer. The amount of the other carbon black is preferably 1 to 4 parts by mass, more preferably 2 to 4 parts by mass, per 100 parts by mass of the fluorine-containing elastomer. By using the other carbon black within the above range, the elongation and flexibility of the carbon fiber composite material can be maintained while enhancing the reinforcing properties through the interaction with the fluorine-containing elastomer and carbon nanotubes. It also becomes easier to adjust the hardness.

3.5. Mass ratio In the carbon fiber composite material, within the above-mentioned ranges of each blend amount, the mass ratio of the total amount of HS carbon and other carbon black to the carbon nanotubes (hereinafter referred to as "total amount of carbon black") is 1:140 to 3:1. A mass ratio of 1:140 is a minimum amount of carbon nanotubes of 0.1 parts by mass and a maximum amount of carbon black of 14 parts by mass. In addition, a mass ratio of 3:1 is a maximum amount of carbon nanotubes of 3 parts by mass and a minimum amount of carbon black of 1 part by mass. In addition, in the carbon fiber composite material, the mass ratio of the total amount of carbon black to the carbon nanotubes is preferably 1:47 to 3:1, more preferably 1:28 to 2.5:1, and even more preferably 1:22 to 1:1.

When the mass ratio of HS carbon and other carbon black to carbon nanotubes is 1:140 to 3:1 within the range of each of the above blend amounts, the carbon fiber composite material forms a microcellular structure scattered throughout the composite material, and an aggregate structure is formed through the interaction between the carbon black-based reinforcing agent and the carbon nanotubes, thereby increasing the mechanical strength at a rubber hardness of 75 degrees or less.

3.6. Other compounding agents For the fluorine-containing elastomer, reinforcing agents such as carbon black and silica, crosslinking agents such as peroxides, polyfunctional unsaturated compound co-crosslinking agents, metal oxides such as zinc oxide, fillers such as calcium carbonate, talc, clay, graphite, calcium silicate, processing aids such as stearic acid, palmitic acid, paraffin wax, plasticizers, liquid elastomers, and other antioxidants, which are generally used as compounding agents for rubber, can be appropriately added as necessary. Bituminous coal pulverized material, which is obtained by pulverizing general coal including bituminous coal, a type of coal known as high-grade coal, can also be used. Furthermore, the average particle size of the bituminous coal pulverized material can be 1 μm to 10 μm, and in particular, the average particle size of the bituminous coal pulverized material can be 3 μm to 8 μm. Such compounding agents may be used to adjust the rubber hardness of the carbon fiber composite material.

4. Manufacturing Method of Carbon Fiber Composite Material The manufacturing method of the carbon fiber composite material will be described in detail with reference to FIGS.

Figures 4 to 6 are schematic diagrams showing a method for producing a carbon fiber composite material by an open roll method according to one embodiment of the present invention. The method for producing a carbon fiber composite material includes a step of mixing a carbon-based reinforcing material with a fluorine-containing elastomer before crosslinking to obtain an uncrosslinked carbon fiber composite material, and a crosslinking step of crosslinking the fluorine-containing elastomer in the uncrosslinked carbon fiber composite material to obtain a carbon fiber composite material. The step of obtaining an uncrosslinked carbon fiber composite material can include, for example, a mixing step of mixing a fluorine-containing elastomer with a carbon-based reinforcing material to obtain a mixture, and a thin-passing step of feeding the mixture into an open roll with a roll gap of 0.5 mm or less and thin-passing the mixture to obtain an uncrosslinked carbon fiber composite material. Note that the raw materials and the amounts of the materials are the same as those in 2 and 3 above, so duplicated explanations will be omitted.

As shown in Figures 4 to 6, the first roll 110 and the second roll 120 in the two-roll open roll 100 are arranged with a predetermined roll spacing d, for example, 0.5 mm to 1.5 mm, and rotate forward or backward at rotation speeds V1 and V2 in the directions shown by the arrows in Figures 4 to 6.

First, as shown in FIG. 4, the fluorine-containing elastomer 30 before crosslinking that is wound around the first roll 110 is masticated to appropriately cut the fluorine-containing elastomer molecular chains and generate free radicals. The free radicals of the fluorine-containing elastomer generated by mastication are in a state that makes them easy to bond with the carbon nanotubes.

5, a filler 80 such as HS carbon, carbon nanotubes, and other carbon blacks is added to the bank 34 of the fluorine-containing elastomer 30 wound around the first roll 110 and kneaded to obtain a mixture. The temperature of the fluorine-containing elastomer 30 in this kneading can be, for example, 0° C. to 100° C., and can also be 0° C. to 50° C. The step of mixing the fluorine-containing elastomer 30 and the filler 80 is not limited to the open roll method, and for example, an internal kneading method or a multi-screw extrusion kneading method can also be used.

6, the roll gap d between the first roll 110 and the second roll 120 is set to, for example, 0.5 mm or less, more preferably, greater than 0 mm and 0.5 mm or less, and the mixture 36 is fed into the open roll 100 to perform thin passing.

The thin pass can be performed, for example, 1 to 10 times.

If the surface speed of the first roll 110 is V1 and the surface speed of the second roll 120 is V2, the surface speed ratio (V1/V2) between the two during thin passing can be 1.05 to 3.00, and preferably 1.05 to 1.2. By using such a surface speed ratio, the desired shear force can be obtained.

The carbon fiber composite material 50 extruded between the narrow rolls in this way is significantly deformed as shown in FIG. 6 by the restoring force due to the elasticity of the fluorine-containing elastomer, and at that time, the carbon nanotubes move significantly together with the fluorine-containing elastomer.

The carbon fiber composite material 50 obtained by thin-threading may be rolled with a roll to be cut into a sheet of a predetermined thickness.

In this thin-threading process, in order to obtain the highest possible shear force, the roll temperature is set to a relatively low temperature, for example, 0 to 50°C, more preferably 5 to 30°C, and the actual measured temperature of the fluorine-containing elastomer can also be adjusted to 0 to 50°C.

The shear force thus obtained acts on the fluorine-containing elastomer, and the aggregated carbon nanotubes are separated from each other and defibrated as if they are pulled out one by one by the fluorine-containing elastomer molecules, and are dispersed in the fluorine-containing elastomer. In particular, the fluorine-containing elastomer has elasticity, viscosity, and chemical interaction with the carbon nanotubes, so that the carbon nanotubes can be easily dispersed. Thus, a carbon fiber composite material 50 having excellent dispersibility and dispersion stability of the carbon nanotubes (resistance of the carbon nanotubes to re-aggregation) can be obtained.

More specifically, when the fluorine-containing elastomer and carbon nanotubes are mixed on an open roll, the viscous fluorine-containing elastomer penetrates the carbon nanotubes, and specific parts of the fluorine-containing elastomer bond to the highly active parts of the carbon nanotubes through chemical interactions. If the surface activity of the carbon nanotubes is moderately high, they can bond particularly easily with the fluorine-containing elastomer molecules. Next, when a strong shear force is applied to the fluorine-containing elastomer, the carbon nanotubes move along with the movement of the fluorine-containing elastomer molecules, and further, the restoring force of the fluorine-containing elastomer due to its elasticity after shearing separates the aggregated carbon nanotubes and disperses them in the fluorine-containing elastomer.

According to this embodiment, when the carbon fiber composite material is extruded between narrow rolls, the restoring force due to the elasticity of the fluorine-containing elastomer causes the carbon fiber composite material to deform to a thickness greater than the roll gap. It is presumed that this deformation causes the carbon fiber composite material, which is subjected to a strong shear force, to flow in a more complex manner, dispersing the carbon nanotubes in the fluorine-containing elastomer. Once dispersed, the carbon nanotubes are prevented from re-aggregating due to chemical interactions with the fluorine-containing elastomer, and can have good dispersion stability.

The thin-threading process is not limited to the open roll method, and may also use a closed kneading method or a multi-screw extrusion kneading method, as long as the carbon nanotubes can be defibrated in the fluorine-containing elastomer by shear force. In short, this process may be performed by applying a shear force to the fluorine-containing elastomer that can separate and defibrate the aggregated carbon nanotubes. In particular, the open roll method is preferable because it allows not only the roll temperature to be controlled, but also the actual temperature of the mixture to be measured and controlled. A crosslinking agent can be mixed into the separated carbon fiber composite material before or during mixing of the fluorine-containing elastomer and the carbon nanotubes, or after thin-threading. For crosslinking of the fluorine-containing elastomer, for example, peroxide crosslinking, which has excellent heat resistance, can be used.

4.3. Crosslinking process In the crosslinking process, the fluorine-containing elastomer in the uncrosslinked carbon fiber composite material obtained in the thin-passing process is crosslinked to obtain a carbon fiber composite material. In the crosslinking process, for example, the carbon fiber composite material containing a crosslinking agent is placed in a mold, and the mold is heated to crosslink the fluorine-containing elastomer and press-process the carbon fiber composite material to form a rubber product of a desired shape. The carbon fiber composite material obtained in the crosslinking process has the above-mentioned aggregate structure that can be observed in a scanning electron microscope image of its cross section.

As described above, the embodiments of the present invention have been described in detail, but it will be readily apparent to those skilled in the art that many modifications are possible that do not substantially depart from the novel features and effects of the present invention. Therefore, all such modifications are intended to be included within the scope of the present invention.

(1) Preparation of Samples The samples of Examples 1 to 7 and Comparative Examples 1 to 5 were prepared by the following steps.

Kneading process: 100 parts by mass (phr) of the fluorine-containing elastomer shown in Tables 1 and 2 (referred to as "FKM" and "FEPM" in Tables 1 and 2) was placed on an open roll with a roll diameter of 6 inches (roll temperature 10 to 20°C) and wound around the roll (see Figure 4).

Next, the compounding agents such as HS carbon (listed as "MT-CB", "HS-CB", "MWCNT", and "SWCNT" in Tables 1 and 2) in the parts by mass (phr) shown in Tables 1 and 2 were added to the fluoroelastomer (see Figure 5). At this time, the roll distance d was set to 1.5 mm.

Thin-threading process: The mixture of fluorine-containing elastomer mixed with carbon nanotubes was removed from the rolls, and the roll gap d was narrowed from 1.5 mm to 0.3 mm, and the mixture was put into an open roll for thin-threading to obtain a first mixture (see Figure 6). At this time, the surface speed ratio of the two rolls was set to 1.1. Thin-threading was repeated five times.

Furthermore, an organic peroxide was added as a cross-linking agent, and a polyfunctional unsaturated compound co-cross-linking agent was added, and the rolls were set at a predetermined interval (1.0 mm to 2.5 mm) to separate the uncross-linked carbon fiber composite material.

Molding process: For Examples 1 to 6 and Comparative Examples 1 and 4, the uncrosslinked carbon fiber composite material (for Comparative Examples 2 and 3, the uncrosslinked rubber composition) was placed in a vacuum press and press molded at 160°C for 5 minutes (primary crosslinking). For Example 7, the uncrosslinked carbon fiber composite material (for Comparative Example 5, the uncrosslinked rubber composition) was placed in a vacuum press and press molded at 170°C for 5 minutes (primary crosslinking).

The carbon fiber composite material was then transferred to an oven and subjected to secondary crosslinking at 230°C for 4 hours to obtain the peroxide-crosslinked sheet-shaped carbon fiber composite material samples of Examples 1 to 6 and Comparative Examples 1 and 4 (Comparative Examples 2 and 3 were peroxide-crosslinked rubber composition samples), and the peroxide-crosslinked sheet-shaped carbon fiber composite material sample of Example 7 (Comparative Example 5 was a peroxide-crosslinked rubber composition sample) was obtained by secondary crosslinking at 200°C for 4 hours.

For each sample of Examples 1 to 7 and Comparative Examples 1 to 5, the amount of carbon-based reinforcing material was adjusted so that the rubber hardness (Hs) was 75 degrees or less, or about 70 degrees ±5 degrees.

The details of the fluorine-containing elastomer and various compounding ingredients in the compounding column of the table are as follows: In addition, the mass ratio of the total amount of HS-CB and MT-CB to the carbon nanotubes is recorded as "mass ratio".

FKM: ternary FKM, Mooney viscosity ML ₁₊₁₀ 121°C (center value) 48,
FEPM: FEPM, Mooney viscosity ML ₁₊₁₀ 121°C (center value) 85,
MT-CB: MT grade carbon black, average diameter 200 nm;
HS-CB: High structure SRF grade carbon black, average diameter 70 nm, DBP absorption (method A) 152 cm ³ /100 g,
MWCNT: multi-wall carbon nanotube, average diameter 15.3 nm;
SWCNT: single-wall carbon nanotube, average diameter 5 nm;
It was.

The test samples of Examples 1 to 7 and Comparative Examples 1 to 5 were subjected to the various tests described below, and the test results are shown in Tables 1 and 2.

(2) Basic Property Tests For each sample of the examples and comparative examples, rubber hardness (Hs (JIS-A)) was measured in accordance with JIS K 6253.

Furthermore, for the test pieces punched out into a JIS No. 3 dumbbell shape for each sample of the examples and comparative examples, a tensile test was carried out in accordance with JIS K6251 at 23±2°C and a tensile speed of 500 mm/min using a Shimadzu Autograph AG-X tensile tester, and the tensile strength (TS (MPa)), breaking elongation (Eb (%)), 50% stress (M50 (MPa)), 100% stress (M100 (MPa)), and breaking energy (breaking E (J)) were measured. The measurement results are shown in each column of the table.

(3) Tear test Each sample of the examples and comparative examples was punched out into an angle-shaped test piece without a slit according to JIS K6252, and a tear test was carried out in accordance with JIS K6252 at room temperature and 200°C at a tensile speed of 500 mm/min using an autograph AG-X manufactured by Shimadzu Corporation, and the tear strength (N/mm) was calculated. The measurement results are shown in each column of the table.

(4) Tear fatigue test Each sample of the examples and comparative examples was punched out into a rectangular test piece of 20 mm × 4 mm width × 1 mm thickness, and a 1 mm deep cut was made in the width direction from the center of the long side of the test piece. A tear fatigue test was performed using a TMA/SS6100 tester manufactured by SII Corporation, with repeated tensile loads (0 N/mm to 1.9 N/mm) being applied under conditions of 200°C, a frequency of 1 Hz, and a maximum tensile stress of 1.9 N/mm in an air atmosphere, and the number of pulls (fatigue life (number of times)) until the test piece broke was measured. The measurement results are shown in the "Fatigue life (number of times)" column in the table.

(5) SEM Measurement Images of the fracture surface of each sample after the tensile test of the examples and comparative examples were taken with a SEM. The distance between adjacent carbon-based reinforcements (MT-CB, HS-CB, MWCNT, and SWCNT) in each image was measured, and the aggregate structure was estimated based on the adjacent carbon-based reinforcements at 100 nm or less, and the area of the aggregate structure was measured. The circumscribing circle of each aggregate structure was drawn on the image, and the diameter of the circumscribing circle was measured. The measurement results are shown in Tables 1 and 2. In Tables 1 and 2, the "average circumscribing circle diameter" is the average value of the diameter of the circumscribing circle of the aggregate structure, the "maximum circumscribing circle diameter" and the "minimum circumscribing circle diameter" are the maximum and minimum values of the diameter of the circumscribing circle of the aggregate structure, the "average distance" and the "standard deviation of distance" are calculated from the distance between adjacent carbon-based reinforcements in the aggregate structure, and the "area" is the percentage (%) of the area of the aggregate structure in the image.

According to Tables 1 and 2, the carbon fiber composite material samples of Examples 1 to 7 and Comparative Examples 1 to 5 had a hardness of 74 degrees or less.

According to Tables 1 and 2, the carbon fiber composite material samples of Examples 1 to 7 had a 100% stress (M100) exceeding 6.1 MPa. The samples of Comparative Examples 1 to 5 had a 100% stress (M100) of 5.6 MPa or less.

According to Tables 1 and 2, the carbon fiber composite material samples of Examples 1 to 7 had tear strengths of 39.5 N/mm or more at room temperature. The samples of Comparative Examples 1 to 5 had tear strengths of 38.0 N/mm or less at room temperature.

According to Tables 1 and 2, the carbon fiber composite material samples of Examples 1 to 7 had a tear strength of 11.1 N/mm or more at 200°C. The samples of Comparative Examples 1 to 5 had a tear strength of 9.4 N/mm or less at 200°C.

According to Tables 1 and 2, the carbon fiber composite material samples of Examples 1 to 7 had fatigue lives of 53 or more in the tear fatigue test. The samples of Comparative Examples 1 to 5 had fatigue lives of 3 or less in the tear fatigue test.

According to Tables 1 and 2, the diameter of the circumscribed circle in the aggregate structure of the carbon fiber composite material samples of Examples 1 to 7 was 0.38 μm to 1.89 μm, the average diameter was 0.66 μm to 1.16 μm, the average distance was 56.3 nm to 65.5 nm, the standard deviation of the distance was 23.3 nm to 31.2 nm, and the area was 15.3% to 24.5%. The diameter of the circumscribed circle in the aggregate structure of the samples of Comparative Examples 1 to 5 was 0.55 μm to 2.43 μm, the average diameter was 1.21 μm to 1.35 μm, the average distance was 51.8 nm to 60.5 nm, the standard deviation of the distance was 28.8 nm to 31.9 nm, and the area was 19.8% to 32.6%.

30...Fluorine-containing elastomer, 34...Bank, 36...Mixture, 50...Carbon fiber composite material, 80...Filler, 81...Carbon nanotube, 82...High structure carbon black, 83...Other carbon black, 84...Aggregate structure, 90...Image, 100...Open roll, 110...First roll, 120...Second roll, d...Roll spacing, C1...Circumscribed circle, L1...Distance, V1, V2...Rotational speed

Claims (12)

A carbon fiber composite material containing a carbon-based reinforcing material in a crosslinked fluorine-containing elastomer,
In a cross section of the carbon fiber composite material, the carbon fiber composite material has a plurality of aggregate structures in which two or more adjacent carbon-based reinforcing materials are in close proximity to or in contact with each other at a distance of 100 nm or less,
the aggregate structure includes the carbon-based reinforcement material;
the carbon-based reinforcing material includes carbon nanotubes having an average diameter of 0.7 nm to 30 nm and carbon black having an average particle size of 35 nm to 300 nm , and the average distance between adjacent carbon-based reinforcing materials in the aggregate structure is 10 nm or more and less than 100 nm;
a circumscribing circle circumscribing the assembly structure has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm;
The area of the cross section occupied by the aggregate structure is 5% to 40%. In claim 1,
The carbon fiber composite material, wherein the carbon black comprises high-structure carbon black having an average particle size of 35 nm to 80 nm. In claim 2,
The carbon black further includes carbon black other than the high structure carbon black,
The carbon fiber composite material, wherein the other carbon black has an average particle size of 100 nm to 300 nm. In claim 3,
A carbon fiber composite material comprising, relative to 100 parts by mass of the fluorine-containing elastomer, 0.1 to 3 parts by mass of the carbon nanotubes, 1 to 10 parts by mass of the high-structure carbon black, and 0 to 4 parts by mass of the other carbon black. the composition contains, relative to 100 parts by mass of a fluorine-containing elastomer, 0.1 to 3 parts by mass of carbon nanotubes, 1 to 10 parts by mass of high-structure carbon black, and 0 to 4 parts by mass of carbon black other than the high-structure carbon black;
The carbon nanotubes have an average diameter of 0.7 nm to 30 nm;
The high structure carbon black has an average particle size of 35 nm to 80 nm,
The other carbon black has an average particle size of 100 nm to 300 nm,
A carbon fiber composite material, wherein a mass ratio of the total amount of the high structure carbon black and the other carbon black to the carbon nanotubes is 1:140 to 3:1. In any one of claims 1 to 5,
The carbon nanotubes include multi-walled carbon nanotubes and single-walled carbon nanotubes. In any one of claims 1 to 6,
The carbon fiber composite material has a tear strength of 39.0 N/mm or more at room temperature in a tear test in accordance with JIS K6252. In any one of claims 1 to 7,
The carbon fiber composite material has a tear strength of 10.0 N/mm or more in a tear test at 200°C in accordance with JIS K6252. In any one of claims 1 to 8,
The carbon fiber composite material has a breaking frequency of 20 or more in a tear fatigue test at 200° C. with a maximum tensile stress of 1.9 N/mm and a frequency of 1 Hz. In any one of claims 1 to 9,
The carbon fiber composite material has a stress at 100% elongation of 6.0 MPa or more. A method for producing a carbon fiber composite material, comprising: a step of mixing the carbon-based reinforcing material with the fluorine-containing elastomer before crosslinking to obtain an uncrosslinked carbon fiber composite material; and a crosslinking step of crosslinking the fluorine-containing elastomer in the uncrosslinked carbon fiber composite material to obtain the carbon fiber composite material according to any one of claims 1 to 10. A method for producing a carbon fiber composite material , comprising thin-passing a carbon fiber composite material containing a carbon-based reinforcing material in a crosslinked fluorine-containing elastomer using open rolls with a roll gap of 0 mm to 0.5 mm and a roll temperature set to 0° C. to 50° C. ,
In a cross section of the carbon fiber composite material, the carbon fiber composite material has a plurality of aggregate structures in which two or more adjacent carbon-based reinforcing materials are in close proximity to or in contact with each other at a distance of 100 nm or less,
the aggregate structure includes the carbon-based reinforcement material;
The carbon-based reinforcing material includes carbon nanotubes having an average diameter of 0.7 nm to 30 nm and carbon black having an average particle size of 35 nm to 300 nm,
the average distance between adjacent carbon-based reinforcing materials in the aggregate structure is 10 nm or more and less than 100 nm;
a circumscribing circle circumscribing the assembly structure has a diameter of 10 nm to 4 μm and an average diameter of 50 nm to 1.2 μm;
the aggregate structure accounts for 5% to 40% of the area of the cross section.

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