JP7590880B2 - Conductive composition and molded body or coating film containing said conductive composition - Google Patents

Description

The present invention relates to an electrically conductive composition in which a carbonaceous material is dispersed in a base material, and more specifically, to an electrically conductive composition in which the electrical conductivity is increased by using a group of carbonaceous materials of different shapes in combination, and to a molded body or coating film containing the electrically conductive composition, or a paint for forming the electrically conductive composition.

Carbon fibers and carbon fiber reinforced plastics (CFRP) are lightweight and have both strength and flexibility, and therefore are widely used in various molded products as substitutes for metals, etc.
The shortest carbon fibers are also called milled fibers, and generally have an average fiber length of 70 μm to 200 μm and an average fiber diameter of about 3 μm to 20 μm (3,000 nm to 20,000 nm). Because of their small shape (size), they are often used as abrasives, reinforcing/auxiliary agents, etc.

Chopped fiber and long fiber, which have a longer average fiber length than milled fiber, are primarily used in molded products such as prepregs and forged products.
In addition, fine CFRP made by further pulverizing milled fibers to shorten the fiber length is used, for example, as a concrete reinforcing material.
However, the shorter the particle diameter, the more difficult it becomes to process the particles, and the particles are more likely to aggregate after being pulverized. Therefore, at present, the particles are of little value in fields where good dispersion is required.

If we define carbon fibers simply by their shape (size), without considering their actual dispersion (existence of each individual fiber) or the utility of those fibers, the currently common definitions are "carbon fibers defined as having a diameter of 3 μm to 10 μm and a length of 500 μm to 10,000 μm," "carbon nanofibers with a diameter of 50 nm to 1,000 nm and a length of 0.2 μm to 200 μm," and "carbon nanotubes with a diameter of 0.4 nm to 100 nm and a length of 50 nm to 20,000 nm."

Although there are documents describing the diameter and aspect ratio of carbon fibers (e.g., Patent Documents 1 and 2), currently, there are almost no carbon nanofiber groups like the "(1) carbon nanofiber group" in the present invention, which are actually small in shape (size), have a particularly large aspect ratio, and are in a state where they can be isolated (separated) approximately one by one, or can be (stably) dispersed in a dispersion medium, etc., approximately one by one.
At present, there are almost no carbon nanofiber groups having the shape (size) defined above that can be easily separated and dispersed. In addition, the utilization rate of carbon fibers of the carbon nanofiber size is currently extremely low due to poor dispersibility or redispersibility, etc.

Furthermore, nano-sized carbon fibers with large aspect ratios are not present in the market (commercially), at least, in a state in which they are dispersed or in which they can be dispersed. This is thought to be because the raw carbon fibers are specifically and neatly split vertically when they are pulverized, in other words, the raw filaments cannot be suitably peeled off from the filaments.
Therefore, there is no known composition, molded body, coating film, dispersion liquid, or paint in which "carbon fibers such as carbon nanofibers" having a large aspect ratio, such as "(1) carbon nanofiber group" in the present invention, are stably and well dispersed.
In other words, carbon nanofibers, which have a large aspect ratio, are dispersible, and have dispersion stability, are expected to have a wide variety of demands (applications), but good ones have not been realized so far.

On the other hand, it is known to use a mixture of carbonaceous materials having different shapes.
Patent Document 3 describes a "carbon material for the negative electrode of a secondary battery" that contains scaly graphite particles (A) and carbonaceous particles (B). The carbonaceous particles (B) in this case have characteristic surface shape and surface physical properties, and are not carbon nanofibers in the first place, but are on the order of μm in size.

Patent Document 4 describes a resin composition for sliding members that contains an amorphous resin, flake graphite, and carbon fibers, but the purpose is to improve physical properties, and the carbon fibers in this composition have a number-average fiber diameter of 5 μm to 20 μm, and are not carbon nanofibers in the first place.

There is a strong demand for carbonaceous material-containing compositions with excellent electrical performance, as well as molded bodies, coating films, and paints that contain them, but excellent products have not yet been realized, and there is room for development.

JP 2012-188790 A JP 2017-066546 A JP 2014-165018 A JP 2015-067726 A

The present invention was made in consideration of the above-mentioned background art, and its objective is to provide a highly conductive "conductive composition containing a group of carbonaceous materials of different shapes," and a molded body, coating film, or paint containing the conductive composition.

The present inventors have conducted extensive research to solve the above problems and have found that a specific group of carbon nanofibers can be dispersed in a base material in an unprecedentedly favorable manner.
Furthermore, they have found that by using (1) the carbon nanofiber group having a specific range of size and shape in combination with "(2a) the flake graphite group or (2b) the approximately spherical carbon particle group," they are suitably dispersed in the base material, resulting in extremely good electrical conductivity.
The present inventors also found that the combined use of at least two of these carbonaceous materials improves electrical conductivity compared with the use of either one alone, and thus completed the present invention.

That is, the present invention provides an electrically conductive composition characterized by containing the following carbonaceous material group (1) and "(2a) or (2b)" in a dispersed state in a base material.
(1) A group of carbon nanofibers obtained by pulverizing carbon fibers, in which 50% or more by number of all carbon nanofibers are distributed in the range of "diameter 30 nm or more and 1000 nm or less, and length 0.2 μm or more and 70 μm or less." (2a) A group of flake-like graphite in which, when D1 is the number average of the maximum diameter length on the basal plane of the scaly graphite and D2 is the number average of the minimum diameter length, D1 and D2 are both 1 μm or more and 300 μm or less, D1/D2 is 3 or less, and when d is the number average thickness of the scaly graphite, the number average aspect ratio [(D1+D2)/2]/d is 10 or more. (2b) A group of approximately spherical carbon particles in which L1 is the number average of the maximum diameter of the approximately spherical carbon particles and L2 is the number average of the minimum diameter, L1/L2 is 3 or less.

In addition, the present invention provides a method for producing a conductive composition comprising the steps of: (1) carbon nanofiber group [A×r] mass % and “(2a) flake graphite group and/or (2b) substantially spherical carbon particle group” “B×(1−r)” mass % (r is a real number greater than 0 and less than 1) co-dispersing the (1) carbon nanofiber group [A×r] mass % and “(2a) flake graphite group and/or (2b) substantially spherical carbon particle group” “B×(1−r)” mass % (r is a real number greater than 0 and less than 1) in the base material,
(1) The electrical conductivity is made higher than that of a composition in which only the carbon nanofiber group is dispersed in an amount of A% by mass, and
The present invention provides an electrically conductive composition having a higher electrical conductivity than a composition in which only "the (2a) flake graphite group and/or the (2b) approximately spherical carbon particle group" are dispersed in an amount of B mass %.

The present invention also provides the above-mentioned conductive composition, in which the number-average aspect ratio of the carbon nanofibers contained in the carbon nanofiber group (1) is 3 or more and 200 or less.

The present invention also provides the conductive composition, in which the (1) carbon nanofiber group is obtained by dry-pulverizing carbon fibers until the number-average length is 100 μm or less, and then wet-pulverizing the carbon fibers.

The present invention also provides the above-mentioned conductive composition, in which the raw material for the carbon nanofiber group (1) is mesophase pitch-based carbon fiber.

The present invention also provides the electrically conductive composition, in which the (1) carbon nanofiber group is obtained by dry grinding followed by wet grinding, and the wet grinding is bead mill grinding or ball mill grinding in an aqueous medium in the presence of a surfactant.

The present invention also provides the aforementioned conductive composition, wherein (1) the carbon nanofiber groups have a volume resistivity of 7×10 ⁻⁴ Ω·cm or less.

The present invention also provides the conductive composition, which contains the (2a) flake graphite group or the (2b) approximately spherical carbon particle group in an amount of 5% by mass or more and 70% by mass or less based on the total carbonaceous material group.

The present invention also provides a molded body that contains the conductive composition.

The present invention also provides a coating film that contains the conductive composition.

The present invention also provides a paint that contains the above-mentioned conductive composition and a liquid dispersion medium.

The conductive composition of the present invention solves the above problems and issues, and when (1) carbon nanofibers with a specific shape and large aspect ratio, such as a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm, are contained in a base material, one at a time, they can be well dispersed. When the dispersion is mixed and dispersed with a specific (2a) flake graphite group or a specific (2b) roughly spherical carbon particle group to standardize the content so that it can be compared, it is surprisingly possible to improve the electrical conductivity compared to the use of either of them alone.

Here, "conductivity increases when dispersed in combination compared to when dispersed alone" is defined as follows. Note that hereafter, "conductivity increases" can be read as "volume conductivity increases" or "volume resistivity decreases", etc.

The measured values of the volume conductivity (logarithm) are plotted with the volume conductivity or volume resistivity (logarithm) of the conductive composition on the vertical axis and the mixture ratio of "(1) carbon nanofiber group" and "(2a) scaly graphite group or (2b) roughly spherical carbon particle group" "conductive composition in which 100 mass % of the total carbonaceous material is dispersed and contained in the base material" on the horizontal axis (see Figure 10 of Example 1 and Figure 11 of Example 2).

The left end of the horizontal axis represents "100% by mass of (1) carbon nanofiber group relative to the entire carbonaceous material," and the right end of the horizontal axis represents "100% by mass of (2a) flake graphite or 100% by mass of (2b) roughly spherical carbon particle group relative to the entire carbonaceous material," or "100% by mass as the total amount of (2a) flake graphite and (2b) roughly spherical carbon particle group." In other words, the left and right ends of the horizontal axis are conductive compositions that contain only either (1) or "(2a) or (2b)." The left and right ends may be reversed.

Here, the content ratio of the carbonaceous material group in the base material is arbitrary for both the left and right ends. Usually, an appropriate content is set in consideration of mechanical strength, thermal properties, electrical properties, dispersibility, dispersion stability, etc., when used alone.
The concentration of (1) carbon nanofibers in the conductive composition at the left end and the concentration of "(2a) flake graphite and/or (2b) roughly spherical carbon particles" in the conductive composition at the right end may be the same or different, so long as the above-mentioned physical properties (when used alone) are good.
At both the left and right ends, the content ratio of the carbonaceous material group in the conductive composition containing the base material (concentration of the single carbonaceous material in the conductive composition) is arbitrary, so even if the concentration relative to the base material at the left or right end (i.e., the content concentration in the conductive composition) is a single point, it can be said that "conductivity increases when used in combination" as long as the conditions below are met.

Also, as shown in Example 4, Table 3, and Figure 12, the "total amount of carbonaceous materials relative to the entire conductive composition (total carbon amount)" is kept constant (constant at 40 mass% in Example 4), and within that, "(1) carbon nanofiber group" and "(2a) scaly graphite and/or (2b) roughly spherical carbon particle group" are allocated and contained, and the volume resistivity and electrical conductivity are measured to see whether, for example, the minimum value (minimum value) of volume resistivity is present in a mixture part that is neither at the left nor the right end.

Under the above conditions, "the electrical conductivity is increased by using them in combination compared to when they are used individually" means that there is a maximum value of volume conductivity (or a minimum value of volume resistivity) in the above plot for the mixed portion (combined portion) where the "carbonaceous material at the left end" does not have a volume conductivity of 100 mass% and the "carbonaceous material at the right end" does not have a volume conductivity of 100 mass%.
"Conductivity increases when used in combination" means that even when the vertical axis is the logarithm of the volume resistivity (of the conductive composition), there is a minimum value in the plot for the above combination (the middle part, not the left end or the right end).

The (1) carbon nanofibers of the present invention, even when used alone as a dispersed material in a base material, exhibit superior thermal, mechanical and electrical effects compared to other carbon nanofibers.
Generally, the more carbonaceous materials are contained in a base material, the higher the electrical conductivity becomes, but this usually leads to deterioration in physical properties such as mechanical strength, dispersibility, dispersion stability, cost, etc. According to the present invention, it is possible to overcome this contradictory relationship, and further, by using the specific (2a) flake graphite group or the specific (2b) substantially spherical carbon particle group in combination, the electrical properties are further improved.

In general, when a base material having an overwhelmingly low electrical conductivity is mixed with a plurality of types of fine particles (carbonaceous material groups) having different electrical conductivities, the electrical conductivity of the resulting composition (molded body) will be at most the electrical conductivity when the fine particle group (carbonaceous material group) having the higher electrical conductivity is contained alone, when compared at the same content level.
However, in the present invention, when a comparison is made at the same content level (when the content is made so that the electrical conductivity can be compared), the dispersion of a specific plurality of types of carbonaceous materials in a base material was able to increase the electrical conductivity more than the dispersion of either one alone.

In particular, with regard to (1) carbon nanofiber group, by using specific raw carbon fibers, a specific crushing method, a crushing method while performing a specific surface treatment, or a specific treatment, it was possible to obtain (1) carbon nanofiber group having an unprecedentedly high dispersibility in the base material.
Furthermore, by using such (1) carbon nanofibers and mixing and dispersing them with "(2a) flake graphite groups or (2b) roughly spherical carbon particle groups," it is possible to increase the electrical conductivity (particularly the volume electrical conductivity) compared to using them alone.

In the first place, the carbon nanofiber group (1) in which 50% or more of the total carbon nanofibers are distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less" as in the present invention has not existed in the past.
Even if the above numerical values (ranges) existed (were known) as the definition of "carbon nanofiber," they would actually be non-dispersible or lacking dispersion stability. Therefore, if we limit the definition to carbon nanofibers that have dispersibility, etc., the above "(1) carbon nanofiber group" is novel.

Carbon nanofibers are generally known in the broad sense of the word, but multiple fibers are firmly bound to the surroundings to form a single unit. Conventionally, it has not been possible to produce carbon nanofibers by separating or peeling them while maintaining a high aspect ratio.
In other words, conventional techniques have not been able to produce carbon nanofibers by pulverization or other methods while maintaining a high aspect ratio, and to make them reproducibly and stably separable or dispersible without causing agglomeration.
As described above, the reason why the electrical conductivity is improved compared to the case where the carbon nanofibers are dispersed alone is due to the good dispersibility of the carbon nanofiber groups (1).

The (1) carbon nanofiber group in the present invention has extremely good dispersibility in the base resin, so that each carbon nanofiber can come into contact with and entangle with the flake graphite or roughly spherical carbon particles, and in particular, by connecting flake graphite or roughly spherical carbon particles that are not in direct contact with each other and ensuring electrical continuity, the electrical conductivity is increased synergistically (resistivity is reduced).

Since the electrical conductivity increases when used in combination, the electrically conductive composition of the present invention and molded bodies and coatings containing the electrically conductive composition of the present invention can achieve higher electrical conductivity even if the total content of carbonaceous materials such as (1) carbon nanofibers, (2) flake graphite, and (3) roughly spherical carbon particles in the base material is less or the same, so they are particularly suitable for use in molded bodies and coatings that require high electrical conductivity, high electromagnetic wave absorption, etc., or in types of molded bodies and coatings where it is desired to reduce the content of carbonaceous materials.

(1) Scanning electron microscope (SEM) photographs of the cross section of pitch-based carbon fiber, the raw material before pulverization in the production of carbon nanofibers. (Left) Radial mesophase pitch-based carbon fiber (middle) Random mesophase pitch-based carbon fiber (right) Onion mesophase pitch-based carbon fiber (1) Schematic diagrams of cross sections of pitch-based carbon fibers, which are raw materials before pulverization in producing carbon nanofibers. (a) (a') Random mesophase pitch-based carbon fibers (b) Radial mesophase pitch-based carbon fibers (c) Onion mesophase pitch-based carbon fibers (1) A scanning electron microscope (SEM) photograph of the cross section of one random mesophase pitch carbon fiber, which is preferably used as a raw material for carbon nanofibers. (1) Scanning electron microscope (SEM) photographs of random mesophase pitch-based carbon fibers (before grinding) that are preferably used as a raw material for carbon nanofibers. (a) 400x (b) 1300x. (1) Scanning electron microscope (SEM) photographs of carbon fibers after dry grinding in the production of carbon nanofibers. (a) 600x (b) 1500x. (1) This is a 700x optical microscope (biological microscope) photograph of the wet grinding process in the production of carbon nanofibers (reference image). (1) Scanning electron microscope (SEM) photographs after wet grinding in the production of carbon nanofibers. (a) 1500x magnification (b) 6500x magnification FIG. 1 is a schematic diagram showing a process for producing filaments of (random type) mesophase pitch-based carbon fibers, which are preferably used as a raw material for carbon nanofibers. FIG. 1 is a schematic diagram showing an example of an apparatus used for dry pulverization in producing carbon nanofibers. 1 is a graph of Example 1, showing the change in volume resistivity [Ω·cm] (vertical axis) when the horizontal axis represents the mixture ratio of "(1) a material containing carbon nanofiber groups and polyimide" and "(2a) a material containing flake graphite (graphene) and polyimide." (a) The vertical axis (volume resistivity) is on a logarithmic scale. (b) The vertical axis (volume resistivity) is on a linear scale. 1 is a graph of Example 2, showing the change in volume resistivity [Ω·cm] (vertical axis) when the horizontal axis shows the mixture ratio of "(1) a material containing carbon nanofibers and polyimide" and "(2b) a material containing approximately spherical carbon particles (Ketjen Black) and polyimide." (a) Vertical axis (volume resistivity) is on a logarithmic scale (b) Vertical axis (volume resistivity) is on a linear scale 13 is a graph showing the measurement results of the volume resistivity of Example 4 (a mixture of (1) carbon nanofiber groups and (2b) approximately spherical carbon particle groups, with a polyester base material). 1 is a graph showing the results of measuring the volume resistivity of Comparative Example 1 ((1) a mixture of carbon nanofiber groups and (2a') flake graphite having a small aspect ratio, with a polyester base material). 1 is a graph showing the measurement results of volume resistivity in Example 5 and Comparative Example 2. 1 is a 1200x scanning electron microscope (SEM) photograph of (2a) flake graphite used in Examples 1 and 2. 1 is a 5,000x scanning electron microscope (SEM) photograph of (2a) flake graphite used in Examples 1 and 2. 1 is a 600x scanning electron microscope (SEM) photograph of (2a') (spherical) flake graphite used in Comparative Example 1. 1 is a 4000x scanning electron microscope (SEM) photograph of (2a') (spherical) flake graphite used in Comparative Example 1.

The present invention is described below, but is not limited to the specific embodiments below and can be modified as desired within the scope of the technical concept.

The conductive composition of the present invention is characterized in that it contains the following carbonaceous material group (1) and (2a) or (2b) in a dispersed state in a base material.
(1) A group of carbon nanofibers obtained by pulverizing carbon fibers, in which 50% or more by number of all carbon nanofibers are distributed in the range of "diameter 30 nm or more and 1000 nm or less, and length 0.2 μm or more and 70 μm or less." (2a) A group of flake-like graphite in which, when the number average of the maximum diameter length of the basal plane of the scaly graphite is D1 and the number average of the minimum diameter length is D2, both D1 and D2 are 1 μm or more and 300 μm or less, D1/D2 is 3 or less, and when the number average thickness of the scaly graphite is d, the number average aspect ratio [(D1+D2)/2]/d is 10 or more. (2b) A group of approximately spherical carbon particles in which, when the number average of the maximum diameter of the approximately spherical carbon particles is L1 and the number average of the minimum diameter is L2, L1/L2 is 3 or less.

The length and diameter of the carbon nanofibers in the present invention; the length and diameter of the carbon nanofibers during production; (1) the number average diameter and number average length of the carbon nanofiber groups in the present invention; (2a) the number average value of the maximum and minimum diameter lengths on the basal surface of the flake graphite groups in the present invention; (2b) the number average value of the maximum and minimum diameters of the roughly spherical carbon particle groups in the present invention; etc., the length and average length are measured and calculated by randomly selecting 100 nanofibers with a scanning electron microscope (SEM), measuring the diameter and length of each nanofiber, and taking the arithmetic mean.

In the present invention, the sizes and average sizes such as diameter, length, aspect ratio, etc. are defined as those measured and calculated as described above.
Since the carbon nanofibers in the present invention have a large aspect ratio, the diameter and length of each fiber are measured using a microscope rather than using a particle size distribution meter. Since the diameter and length cannot be suitably measured using an automatic particle size distribution meter, the above method is the only option.

[(1) Carbon nanofiber(s)]
As described above, the (1) carbon nanofiber (group) in the present invention is essentially one obtained by pulverizing carbon fibers, in which 50% or more of the total carbon nanofibers are distributed in the range of "diameter 30 nm or more and 1000 nm or less, and length 0.2 μm or more and 70 μm or less"

In the present invention, "carbon fiber" refers to any long and thin carbonaceous material (carbon material) having a graphene structure, and includes carbon fiber, carbon nanofiber, and carbonaceous materials (carbon materials) of a similar size. In addition, "carbon fiber" includes filaments 10 of carbonaceous materials (carbon materials) having a graphene structure, the elementary filaments 20 that constitute them, and strands in which the filaments 10 are arranged vertically in parallel.

The carbon nanofiber group in the present invention has a diameter of 30 nm or more and 1000 nm or less, and 50% or more of the total number of the carbon nanofiber group is distributed in the range of a length of 0.2 μm or more and 70 μm or less.
It is essential that 50% or more of the total is distributed within the above range, but preferably 70% or more, more preferably 80% or more, even more preferably 90% or more, particularly preferably 95% or more, and most preferably 98% or more. According to the present invention, since it is possible to produce particles having the above-mentioned percentage or more, the sharper the distribution, the better in terms of dispersibility and the properties of the molded body and coating material containing the particles, but a broader distribution is acceptable in terms of productivity, etc.

The diameter of the "carbon nanofibers in which 50% or more by number is distributed" is 30 nm to 1000 nm, preferably 50 nm to 850 nm, and particularly preferably 100 nm to 700 nm. According to the present invention, carbon nanofibers having a diameter in the above range can be produced with good yield.
If the diameter is too small, it may be difficult to manufacture, or the length may be too short, resulting in a small aspect ratio, etc. On the other hand, if the diameter is too large, the performance at the intended use may be poor, or the applications of the carbon nanofiber group may be limited.

The length of the "carbon nanofibers in which 50% or more by number is distributed" is 0.2 μm to 70 μm, more preferably 1 μm to 50 μm, even more preferably 2 μm to 30 μm, and particularly preferably 5 μm to 20 μm. According to the present invention, carbon nanofibers having a length in the above range can be produced with good yield.
If the length is too short, the aspect ratio may become small, the performance may be poor in the application (e.g., molded body, paint, etc.), the applications of the carbon nanofibers may be limited, it may be difficult to prevent aggregation, etc. On the other hand, if the length is too long, the applications of the carbon nanofibers may be limited, and it may be difficult to manufacture while maintaining a large aspect ratio, etc.

The above size distribution is determined from the diameter and length ranges in which many (e.g., 50% or more by number, 70% or more by number, etc.) carbon nanofibers are distributed within the carbon nanofiber group. When determined on average, it is preferable that the number average diameter is 30 nm or more and 1000 nm or less, and the number average length is 0.2 μm or more and 70 μm or less.
In other words, in the conductive composition of the present invention, it is preferable that (1) the number average diameter of the carbon nanofibers contained in the carbon nanofiber group is 30 nm or more and 1000 nm or less, and the number average length is 0.2 μm or more and 70 μm or less.

The number average diameter is more preferably from 50 nm to 850 nm, particularly preferably from 100 nm to 700 nm.
The number average length is more preferably from 1 μm to 50 μm, further preferably from 2 μm to 30 μm, and particularly preferably from 5 μm to 20 μm.

The carbon nanofibers which can be isolated and dispersed in the present invention have a diameter and length within the above ranges (preferably within the above ranges), and therefore one of their characteristics is that they have a large aspect ratio.
The number average aspect ratio of the carbon nanofibers is preferably 3 or more and 200 or less, more preferably 5 or more and 160 or less, even more preferably 7 or more and 130 or less, particularly preferably 15 or more and 100 or less, and most preferably 20 or more and 70 or less.

If the aspect ratio is too small, it may not be possible to suitably increase the electrical conductivity when used in combination with the flake graphite group (2a) or the approximately spherical carbon particle group (2b), and the performance of, for example, conductive compositions, molded bodies, coating films, paints, etc., in which the carbon nanofiber group is contained, may be inferior or applications may be limited.
On the other hand, if the aspect ratio is too large, manufacturing may become difficult or the part may be wasted.

When pitch-based carbon fibers are used as the raw material and dry-milled and then wet-milled, the raw carbon fibers are specifically and neatly split vertically during milling; in other words, the filaments 20 can be favorably peeled off from the filaments 10 without shortening their length too much. In the present invention, this resulted in carbon nanofibers that are dispersible and have a favorable aspect ratio, which has never been seen before. It is believed that the large aspect ratio and favorable size and shape have improved the physical properties of the molded body, coating film, and paint in which the obtained carbon nanofiber groups are dispersed. In addition, when used in combination with flake graphite groups (2a) or approximately spherical carbon particle groups (2b), the electrical conductivity can be favorably increased.

When the carbon nanofibers are contained/dispersed in the base material, they may be contained in the form of a solid (powder) or may be contained in the form of a dispersion liquid.
When it exists in a solid (powder) form, for example, as shown in FIG. 7, it is in a state in which each carbon nanofiber can be isolated or dispersed individually.

When present in a dispersion (paint), molded body, or coating, the carbon nanofibers are substantially dispersed individually. Although it is not necessarily possible to separate the carbon nanofibers into individual filaments as described below, it is sufficient if the carbon nanofibers can be dispersed individually. "Substantially dispersed individually" means "dispersed approximately individually," and refers to a state in which the carbon nanofibers are not strongly aggregated in parallel, and the base material is embedded between the carbon nanofibers.
The (1) carbon nanofiber group in the present invention can be made to have the dispersibility and dispersed state as described above.

<(1) Production of Carbon Nanofiber(s)>
In the present invention, the carbon nanofibers having the above-mentioned shape (diameter and length) and the carbon nanofiber group having the above-mentioned distribution are obtained by at least dry pulverization and then wet pulverization. Furthermore, by specifying the carbon fibers as the raw material, the (re)dispersible carbon nanofiber group having the above-mentioned shape and distribution can be obtained.
It is also preferable to add other treatments (operations) as necessary before the dry grinding, between or during the two types of grinding, or after the wet grinding. Moreover, the dry grinding and the wet grinding may each be carried out in one stage or two or more stages.

In particular, "carbon nanofibers obtained by dry-pulverizing and then wet-pulverizing pitch-based carbon fibers" can be suitably dispersed in a base material, and the molded body, coating film, or paint in which they are dispersed has excellent physical properties as described below, and can be suitably used in combination with (2a) flake graphite or (2b) roughly spherical carbon particles to increase electrical conductivity.

A "particularly preferred manufacturing process" for the (1) carbon nanofiber group of the present invention is described below, but the (1) carbon nanofiber group of the present invention is not limited to only those actually manufactured by the "preferred manufacturing process" described below.
However, the (1) carbon nanofiber group in the present invention can be produced by changing the production condition items such as the raw material carbon fiber (carbon material), the grinding method and grinding conditions, the type of dispersant, surfactant, etc. used, and the type of treatment, which are specified in the "preferred production method" described below.

Regarding the manufacturing process, when the following processes are carried out, it is particularly preferable to carry out them in the following order, but other processes may be interposed between them.
That is, preparation of raw carbon fiber, pre-grinding, dry grinding, heat treatment, moistening treatment, wet grinding, anti-agglomeration treatment, and water removal treatment.

Of the above, at least preparing specific raw carbon fibers, dry grinding, and wet grinding are particularly important for obtaining the (1) carbon nanofiber group in the present invention, and by doing so, the conductive composition of the present invention, and a molded body or coating film or paint having the same can be suitably produced.
Of the above, pre-grinding, heat treatment, wetting treatment, aggregation prevention treatment, and water removal treatment are not essential, but in order to produce the product satisfactorily, it is preferable to carry out some or all of them as necessary. In particular, heat treatment is preferable when the raw material contains a resin such as a sizing material, but may not be carried out when the raw material has not been subjected to a sizing treatment or the like.
Each processing step will be described below in the order of preferred processing.

<<Raw carbon fiber>>
In the present invention, the carbon fibers before pulverization (raw carbon fibers) are not particularly limited, but it is preferable to use pitch-based carbon fibers, more preferably mesophase pitch-based carbon fibers, and particularly preferably random mesophase pitch-based carbon fibers (see FIG. 4).
With PAN-based carbon fibers, it is difficult to obtain a group of carbon nanofibers having a diameter of 30 nm or more and 1000 nm or less and a length of 0.2 μm or more and 70 μm or less, and having the above-mentioned high number average aspect ratio, regardless of the pulverization method used.

In addition, even if pitch-based carbon fibers are used, it may be difficult to obtain carbon nanofibers with the above size, shape, and distribution if the carbon fibers are isotropic pitch-based carbon fibers.

Mesophase pitch-based carbon fibers, on the other hand, are classified based on their cross-sectional shape (i.e., internal shape) into at least radial type (Figure 1 (left), Figure 2 (b)), random type (Figure 1 (middle), Figures 2 (a) (a'), Figure 3), and onion type (Figure 1 (right), Figure 2 (c)). A "random type" is one in which the fiber cross section is random. For this reason, it was named random type.

In the present invention, even if the mesophase pitch-based carbon fiber is a radial type mesophase pitch-based carbon fiber or an onion type mesophase pitch-based carbon fiber, it may be somewhat difficult to obtain a carbon nanofiber group having the above size, shape, and distribution. If a radial type or an onion type is used as the raw carbon fiber, it may not be possible to preferably obtain the above shape, form, and distribution, and in particular, it may not be possible to preferably obtain a carbon nanofiber group having a large number average aspect ratio (for example, only those with an aspect ratio of less than 3 may be obtained).

The "filaments constituting the random mesophase pitch carbon fiber" are made up of smaller rod-shaped or plate-shaped (sheet-shaped) filaments. For example, in FIG. 3, plate-shaped (sheet-shaped) filaments are vertically assembled.
In this specification, the “smaller rod-shaped or plate-shaped (sheet-shaped) object” that constitutes the filament 10 is abbreviated as “bare filament” 20.
It is thought that within one elementary filament, graphene structures in which benzene rings are condensed are stacked together facing the same direction to form one unit, or one or more carbon nanotubes are bundled together facing the same direction to form one unit, etc.

In the present invention, the carbon fiber raw material has a number average thickness or number average thickness of the filaments 20 constituting the filament 10 of preferably 10 nm or more and 200 nm or less, more preferably 15 nm or more and 150 nm or less, even more preferably 20 nm or more and 100 nm or less, and particularly preferably 30 nm or more and 70 nm or less.
When the size of the raw carbon fiber filaments is equal to or larger than the lower limit, a synergistic effect can be achieved by using the raw carbon fiber in combination with the flake graphite group (2a) or the approximately spherical carbon particle group (2b), thereby favorably increasing the electrical conductivity (reducing the resistance value).
On the other hand, when it is equal to or less than the upper limit, it is easy to prepare "filaments 10 or carbon fibers" containing such raw filaments 20 as a raw material.

8 shows an outline of the manufacturing process for mesophase pitch-based carbon fiber filaments 10. Random mesophase pitch-based carbon fibers can also be obtained by adjusting the spinning viscosity, nozzle shape, flow state of the raw material pitch, etc. in FIG.
The manufacturing process of mesophase pitch-based carbon fiber does not include a drawing process. The microstructure controlled in the spinning process becomes the crystal structure of the filament 10 almost as it is, and a boundary between the crystal structures with different orientations is formed, and the raw filament 20 becomes visible (exists).

In FIG. 8, the thickness of the raw material filament 10 is usually 4000 nm to 20000 nm, and often 5000 nm to 12000 nm. On the other hand, the number average thickness or number average thickness of the raw filaments 20 in the raw material carbon fiber filament 10 in the present invention is preferably 10 nm or more and 200 nm or less, more preferably 15 nm or more and 100 nm or less. Therefore, in the raw material carbon fiber, one filament 10 contains usually 40 to 700 raw filaments 20, and in most cases 60 to 400 raw filaments 20.
When one elementary filament 20 is divided (defined) at a boundary where the crystal structure orientations are different, the above number of elementary filaments 20 are bundled together to form one filament 10. Note that in Fig. 8, for ease of viewing, it is depicted as if one filament were made up of three elementary filaments when viewed from the front.

In the present invention, when random mesophase pitch carbon fibers are used as the raw material, the above-mentioned embodiment is particularly preferred, although there are no limitations thereon.
The shape, distribution, etc. of the (1) carbon nanofiber group in the present invention, as well as whether or not such (1) carbon nanofiber group can be formed in the first place, depend on the type of carbon fiber that is the raw material.

<<<<Relationship between carbon nanofibers and raw filaments according to the present invention>>>
The carbon nanofiber in the present invention is preferably configured by gathering together 2 to 20 "bare filaments constituting the raw filament," more preferably 3 to 16, and particularly preferably 4 to 12. The number of raw filaments is the average value for each carbon nanofiber in the (1) carbon nanofiber group.
When producing carbon nanofibers, the crystallinity and external shape of the filaments may be slightly deformed, but this is also included in the above-mentioned "number of filaments." Even if the filaments are originally plate-shaped, they become elongated after being crushed, and are therefore referred to as the "number of filaments" as above.

<<Pre-grinding>>
The carbon fiber as the raw material may be in the form of chopped fiber or milled fiber, but is preferably in the form of chopped fiber. In the case of milled fiber, many short fibers with a length of about 1 μm are mixed in, so chopped fiber is preferable in order to obtain carbon nanofibers with a large aspect ratio. In the case of milled fiber, for example, even if it is said (publicly announced) that the "average length is 70 μm," many short fibers with a length of about 1 μm may be mixed in.

Although not limited, it is preferable to pre-grind the raw carbon fiber to an average size of 1 mm to 15 mm, more preferably 2 mm to 10 mm, and particularly preferably 5 mm to 8 mm. For example, in the case of long fiber bobbin type, pre-grinding may be necessary. If the size is within the above range from the beginning, it is also preferable not to perform pre-grinding.

There are no particular limitations on the grinding method used for pre-grinding, and any commercially available dry grinding machine can be used, such as a cutter mill.

<<Dry grinding>>
In the present invention, "dry grinding" is not particularly limited, but is preferably airflow grinding, cutter grinding, or "grinding in which both airflow grinding and cutter grinding are performed simultaneously.""Grounding in which both airflow grinding and cutter grinding are performed simultaneously" means "grinding that has both an airflow grinding mechanism/function and a cutter grinding mechanism/function at the same time."

<<<<Airflow grinding>>>
Examples of the airflow pulverization include pulverization using an airflow pulverizer such as a cyclone mill, and a jet mill.
Airflow pulverization using a cyclone mill produces an airflow by rotating an impeller (rotating blade) and pulverizes the object thrown into the airflow in a dry state to produce fine particles, while airflow pulverization using a jet mill produces fine particles by colliding the object with a collision plate in a dry state to pulverize the object.
In order to obtain (1) carbon nanofiber groups of a predetermined shape by wet grinding, an airflow grinder having a rotating body such as an impeller, rotor, blade, or rotary blade (such as a jet mill) or a grinder described later in <<Grinding performing both airflow grinding and cutter grinding simultaneously>>> is preferable as the "dry grinding before wet grinding".

As the cyclone mill, commercially available devices can also be suitably used. Examples of commercially available products include the Cyclone Mill manufactured by Shizuoka Plant Co., Ltd., the Cyclone Mill manufactured by Shizuoka Seiki Co., Ltd., the Super Powder Mill manufactured by Nishimura Machinery Manufacturing Co., Ltd., the Tornado Mill manufactured by Sansho Industry Co., Ltd., and the Dream Mill manufactured by Furukawa Industrial Machinery Systems Co., Ltd.

The structure of the cyclone mill is not particularly limited, but it is particularly preferred that the cyclone mill has one or more impellers and crushes the materials to be crushed by colliding them with each other using the swirling air current generated by the impeller, as this makes it easier to achieve the effects of using the air current crusher; there is very little metal contamination; etc.

Commercially available jet mills can also be used. Commercially available manufacturers include, for example, Seishin Enterprise Co., Ltd., Hosokawa Micron Corporation, Nippon Pneumatic Co., Ltd., and Nisshin Engineering Inc.

<<<<Cutter-type crushing>>>
Examples of cutter-type pulverization include pulverization using a crusher mill, a pin mill, a cutter mill, a hammer mill, an axial flow mill, and the like.

<<<<Crushing using both airflow and cutter crushing at the same time>>>
As the dry pulverization in the present invention, pulverization in which both airflow pulverization and cutter pulverization are carried out simultaneously is particularly preferred.
In particular, the dry grinding in the present invention is preferably carried out using a dry grinder having a blade and capable of applying shear and impact, or is preferably carried out using a dry grinder that applies "shear and impact by a blade".
A schematic diagram of an example of such a dry grinding machine is shown in FIG.

In the case of "full airflow mills that do not use impellers for grinding" such as jet mills, cyclone mills, tornado mills, and Dream Mills (registered trademark), depending on the raw material, the diameter may become too small during the dry grinding stage, resulting in an aspect ratio that is too small (leading to a round shape) during the subsequent wet grinding. This means that, although not limited to this, they may not be very suitable for grinding prior to wet grinding.

The ambient temperature or set temperature in dry grinding is not particularly limited and may be in accordance with the method of use of the apparatus used, but is preferably 0°C or higher and 50°C or lower, and particularly preferably 5°C or higher and 35°C or lower.
The impeller rotation speed may also be determined in accordance with the method of use of the device used, but is preferably 4,000 rpm or more and 20,000 rpm or less, and particularly preferably 8,000 rpm or more and 15,000 rpm or less.

It is preferable to use the above-mentioned apparatus and the above-mentioned grinding method to dry grind the material until the number average length is 100 μm or less, and then to provide the material for the next step.
By dry-pulverizing the carbon nanofibers to a particle size of 100 μm or less and then wet-pulverizing the carbon nanofibers, the diameter, length, aspect ratio, shape distribution, etc., tend to fall within the essential or preferred ranges described above.

The number average length is preferably adjusted by dry grinding to 100 μm or less, more preferably 5 μm to 70 μm, further preferably 7 μm to 50 μm, and particularly preferably 10 μm to 40 μm.
If the number average length after dry grinding is too long, it may be difficult to achieve a final diameter and length of the carbon nanofiber within the above-mentioned range in the subsequent wet grinding step, even if the conditions for the wet grinding are adjusted.
On the other hand, if the length after dry pulverization is too short, the length of the carbon nanofibers after wet pulverization cannot be longer than that, and therefore it may be difficult for the final length and number-average aspect ratio of the carbon nanofibers to fall within the above-mentioned preferred ranges, etc. In particular, the aspect ratio of the final carbon nanofibers may be too small.

Regarding the number average diameter after dry milling, it is difficult to make it small by dry milling alone, that is, it is difficult to mill so as to increase the aspect ratio. Furthermore, if the diameter is forcibly reduced by dry milling, the length also becomes shorter, and it becomes difficult to keep the final aspect ratio after wet milling within a suitable range.
The diameter after dry pulverization is preferably 3000 nm or more, more preferably 5000 nm to 15000 nm, and particularly preferably 7000 nm to 12000 nm. It is desirable to carry out dry pulverization so that the diameter falls within this range.

The number average aspect ratio after dry grinding is not particularly limited, but is preferably 10 or less, more preferably 1.2 or more and 7 or less, and particularly preferably 1.5 or more and 5 or less.
In the dry grinding, it is difficult to increase the average aspect ratio above the upper limit, i.e., it is difficult to reduce the average fiber diameter to a degree that allows the average aspect ratio to be increased above the upper limit.
In the present invention, it has been found that by performing dry pulverization, the number average length is preferably set to 100 μm or less, and a relatively large diameter and a relatively small number average aspect ratio are achieved, or by performing this in the same manner, a group of carbon nanofibers having the above-mentioned suitable "diameter, length, and large number average aspect ratio" can be finally obtained by the subsequent wet pulverization.

<<Heat treatment>>
In the conductive composition of the present invention, the (1) carbon nanofiber group is preferably substantially free of any resin that is mixed in the raw carbon fibers before pulverization.
Here, the case where a mixed resin is included includes, for example, a case where a sizing agent or the like is contained in the carbon fiber as a raw material. That is, the mixed resin is not limited, but includes, for example, a sizing agent or the like.

In the present invention, when a resin is present in the dry-ground product, it is preferable to remove the resin by carrying out a heat treatment.
In other words, the conductive composition of the present invention contains the (1) carbon nanofiber group obtained by dry grinding and then wet grinding, and it is preferable that a heat treatment is performed between the dry grinding and the wet grinding to remove any resins present in the raw materials.
In addition, when no resin such as a sizing agent is mixed in, the heat treatment can be omitted.

The conditions for the heat treatment are not limited, but for example, the material is heated in a furnace at a temperature of 320° C. to 480° C. for 5 to 15 minutes, and the resin content is reduced to preferably 0.1% by mass or less, and particularly preferably 0.01% by mass or less. The term "substantially free of mixed resins" refers to a state in which the resin content is reduced to the above-mentioned level (by the above-mentioned heat treatment, etc.).
By carrying out the heat treatment, preferably after dry grinding and before wet grinding, the grinding and dispersion effects of the wetting agent or surfactant used in the subsequent wet treatment or wet grinding step (if wet treatment is performed) are improved.

<<Wet Treatment>>
Although not limited thereto, it is preferable to further carry out a moistening treatment, and it is particularly preferable to carry out the moistening treatment after the dry grinding or heat treatment.
The wet treatment is preferably carried out by immersing the product obtained above in an aqueous solution containing an anionic surfactant, a cationic surfactant, or an amphoteric surfactant, which can also be suitably used in the subsequent wet grinding.

The anionic surfactant is preferably a polymeric anionic surfactant ("polymer" also includes oligomers), and more preferably an alkali metal salt, ammonium salt, alkylammonium salt, alkylolammonium salt, or the like, of a (co)polymer having an acid group.
The above-mentioned anionic surfactants may be used alone or in combination of two or more kinds.

The above-mentioned "(co)polymer having an acid group" is particularly preferably at least one (co)polymer selected from the group consisting of (co)polymers of (meth)acrylic acid, (co)polymers of (anhydride) phthalic acid, (co)polymers of vinylbenzenesulfonic acid, and (co)condensates of naphthalenesulfonic acid. Here, the terms "(co)", "(meth)", and "(anhydride)" refer to the presence or absence of parentheses. In the case of copolymers, the copolymerizing monomer is not particularly limited, and examples thereof include (meth)acrylic acid alkyl esters, (meth)acrylic acid hydroxyalkyl esters, styrene, and vinyl acetate.
The (co)condensation product of naphthalenesulfonic acid includes those in which the rings are bonded with an aldehyde such as formaldehyde, etc. In the case of the co-condensation product, examples of the co-condensation monomer include phenol, cresol, naphthol, etc.

The cationic surfactant is preferably a surfactant in which quaternary ammonium is a hydrophilic group, and the substituent on the "N ⁺ " of the quaternary ammonium is not particularly limited, but is preferably an alkyl group (which may have a substituent) such as a stearyl group, a palmityl group, a dodecyl group, a methyl group, a benzyl group, a butyl group, etc. Furthermore, a long-chain alkyl group having 12 or more carbon atoms is more preferable.
The counter anion is not particularly limited, but a halogen ion such as a chloride ion or a bromide ion is particularly preferred.

Examples of amphoteric surfactants include alkyl betaine type, fatty acid amidopropyl betaine type, alkyl imidazole type, amino acid type, and amine oxide type.

Among these, it is preferable to use an anionic surfactant or an amphoteric surfactant.
The content of the surfactant and the content of the carbon fibers (after dry grinding) in the aqueous dispersion medium are in the same range as the numerical range of <wet grinding> described below.

By using a surfactant, and in particular the preferred anionic or amphoteric surfactants described above, the carbon fibers can be unraveled vertically, reducing the diameter while maintaining a long length, resulting in carbon nanofibers (groups) with a large average aspect ratio.

<<Wet grinding>>
In the present invention, the method for producing the carbon nanofiber group (1) is preferably a method for performing wet pulverization after dry pulverization. The wet pulverization is not particularly limited, but is preferably bead mill pulverization or ball mill pulverization. Bead mill pulverization or ball mill pulverization performed in an aqueous medium in the presence of a surfactant is particularly preferable.
Between the dry milling and the wet milling, the above-mentioned "other treatments" such as heat treatment and wet treatment may be performed. The "other treatments" further include premixing, preliminarily preparing a solution, and the like.

The surfactant may be any of those described in the <Wetting Treatment> section above, whether the wetting treatment is performed or not.
The same surfactants are also preferred. That is, the anionic surfactants, cationic surfactants, or amphoteric surfactants as described above in the section on <Wet Treatment> are preferred. The surfactant used in the wet treatment may be used as is in the wet grinding, or a new surfactant may be added or a different surfactant from that used in the wet treatment may be added during the wet grinding.

The amount of the surfactant used is not particularly limited, but is preferably 0 parts by mass or more and 100 parts by mass or less, more preferably 0 parts by mass or more and 50 parts by mass or less, and particularly preferably 0 parts by mass or more and 10 parts by mass or less (when two or more types of surfactants are used in combination, the total amount) relative to 100 parts by mass of the carbon fibers (carbon nanofibers in the process of being ground) to be ground and dispersed.
If too much surfactant is used, the carbon fibers may aggregate as they are unraveling vertically. As a result, the carbon nanofibers will be in an aggregated state in the dispersion, which may affect the physical properties of the resulting product when applied to an object.

When the above-mentioned wetting treatment is carried out, a new surfactant may be added in wet pulverization, or the surfactant that was mixed in the wetting treatment may be used as it is.
When a surfactant is added during wet grinding, the surfactant may be the same as or different from the surfactant used in the wet treatment.

<<<<Wet grinding method, equipment and conditions>>>
By carrying out wet pulverization, it is possible to produce (1) carbon nanofibers that have the specific shape (diameter, length, aspect ratio) and particle size distribution as described above and are also dispersible.
The conditions of the wet milling are adjusted so as to obtain carbon nanofibers (group) having the above-mentioned specific shape (diameter, length, aspect ratio) of the present invention.

Preferred materials for the grinding media used in wet grinding include glass, alumina, zircon (zirconia-silica ceramics), zirconia, metal (steel), etc.

Taking a bead mill as an example, the diameter of the beads used is preferably 0.1 mm or more and 3 mm or less, more preferably 0.2 mm or more and 2 mm or less, and particularly preferably 0.3 mm or more and 1 mm or less.
If the bead diameter is too large, the number of beads in the bead mill container will decrease, and the number of contact points will decrease, which may result in failure to crush and disperse properly, failure to crush to a sufficiently small diameter, etc. On the other hand, if the bead diameter is too small, it may result in failure to crush and disperse properly, or crushing may take too long, etc.

The bead filling rate used in the bead mill is preferably from 45% to 90%, more preferably from 55% to 87%, and particularly preferably from 65% to 85%.
If the bead filling rate is too low, the carbon fibers are less likely to split vertically, and it may be difficult to produce carbon nanofibers with a large aspect ratio, etc. On the other hand, if the bead filling rate is too high, it may be difficult for the stirring blades of the bead mill to rotate, etc.

The carbon fiber content after dry grinding is preferably 1% by mass or more and 20% by mass or less, more preferably 3% by mass or more and 15% by mass or less, and particularly preferably 5% by mass or more and 10% by mass or less, of the entire slurry to be subjected to bead mill processing.

The shape of the stirring blade used in the bead mill treatment is not particularly limited.
The rotation speed of the stirring blade (agitator) depends on the span length of the stirring blade and the capacity of the bead mill, but is preferably 600 rpm or more and 4500 rpm or less, more preferably 800 rpm or more and 4000 rpm or less, and particularly preferably 1000 rpm or more and 3500 rpm or less, calculated based on a capacity of 2 L.
The peripheral speed of the tip of the stirring blade (agitator) depends on the span length of the stirring blade, but is preferably within the range that can be calculated from the above rotation speed assuming a diameter of 20 cm. Specifically, it is preferably 5 m/s to 40 m/s, more preferably 7 m/s to 30 m/s, and particularly preferably 9 m/s to 20 m/s.

The operation method of the bead mill for grinding and dispersing may be either a circulation type or a batch type, but the circulation type is preferred. In the case of the circulation type, the material is not transferred to a container as in the batch type, so that aggregation does not progress.
When using a circulation system, the degree of pulverization varies depending on the number of passes. For example, when the residence time per pass is increased, the particle size distribution becomes sharper due to the absence of short passes of the treated material, but the aspect ratio of the carbon nanofibers also becomes smaller. Therefore, for example, when converted to 4L, the bead mill treatment is performed by circulation for preferably 70 minutes or more and 270 minutes or less, more preferably 80 minutes or more and 230 minutes or less, and particularly preferably 90 minutes or more and 180 minutes or less.

The temperature in the wet treatment is preferably from 0° C. to 50° C., particularly preferably from 5° C. to 35° C. The bead mill may be of either a vertical or horizontal type.
In addition, a commercially available bead mill can be used, such as the Dyno Mill by Willy & Bachofen (WAB) and the Bead Mill by Netsch (USA).

The time per pass (continuous operation time), number of passes, and total time may depend on the equipment structure, slurry concentration, grinding and dispersion conditions, type of surfactant, etc., so it is preferable to withdraw the material after each pass or during wet grinding, observe it one by one with a particle size distribution measuring device, optical microscope, scanning electron microscope (SEM), etc., and adjust as appropriate.

By using the above-mentioned "production method using both dry and wet grinding" and adjusting the grinding conditions etc. appropriately within the above-mentioned range, it is possible to obtain carbon nanofibers (groups) with a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm.

In addition, a group of carbon nanofibers (1) is obtained in which the carbon nanofibers are in a state in which they can be isolated one by one, or in a state in which they are substantially dispersed one by one in the base material described below. Here, "substantially dispersed one by one" means "dispersed approximately one by one," and refers to a state in which the carbon nanofibers are not in close contact with each other in parallel, and the base material is inserted between the carbon nanofibers.

In addition, by using the above-mentioned "production method using both dry and wet grinding" and adjusting the grinding conditions, etc. appropriately within the above-mentioned range, it is possible to obtain carbon nanofibers (groups) with a number average aspect ratio of 3 or more and 200 or less.

<<Anti-aggregation treatment>>
After the wet grinding, it is particularly preferable to carry out an anti-aggregation treatment, although this is not limited thereto. The anti-aggregation treatment is preferably carried out by blending an anti-aggregation agent such as a (composite) metal chelate compound, a (composite) metal oxide fine particle, a wax containing a metal, or a (composite) metal ion water, or a surfactant, although this is not limited thereto.
The aggregation prevention treatment may be carried out immediately after the above-mentioned wet pulverization, or after the water removal treatment described below, or may be carried out at both stages.

The anti-aggregation agent used in the anti-aggregation treatment is preferably a metal-containing anti-aggregation agent as described above, more preferably a (composite) metal chelate compound, and further preferably a metal salt such as HEDTA, EDTA, PDTA, NTA, ethylenediamine, bipyridine, phenanthroline, or porphyrin.
Among these, metal salts of acetic acid derivatives such as hydroxyethyl ethylene diamine triacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA), 1,3-propanediamine tetraacetic acid (PDTA), and nitrilo triacetic acid (NTA) are particularly preferred.

As the surfactant used in the anti-aggregation treatment, an anionic surfactant, a cationic surfactant, or an amphoteric surfactant is preferred, and an anionic surfactant or an amphoteric surfactant is more preferred. Although not limited thereto, particularly preferred specific surfactants include those similar to those described in the above sections on <Wet treatment> and <Wet grinding>.

The types of anti-aggregation agents such as metal-containing anti-aggregation agents and surfactants are determined taking into consideration the surface condition of the object to be prevented from agglomeration immediately before the anti-aggregation treatment.
When the types of the aggregation inhibitor and surfactant are as described above, aggregation is less likely to occur during the removal treatment step of the dispersion medium (water) and over time thereafter, and storage stability is improved.

The amount of the anti-aggregation agent such as a (composite) metal chelate compound to be used is not particularly limited, but is preferably added in an amount of 0.01% by mass or more and 1% by mass or less, and particularly preferably 0.03% by mass or more and 0.3% by mass or less, based on the total amount of the slurry after wet pulverization.
It is also preferable to add 0.01 to 5 parts by mass, and particularly preferably 0.1 to 1 part by mass, per 100 parts by mass of the object to be prevented from agglomerating, such as carbon nanofiber groups.

The amount of surfactant used in the anti-aggregation treatment is not particularly limited, but when two or more surfactants are used in combination, the total amount is preferably 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, and particularly preferably 0 to 1 part by mass, per 100 parts by mass of the object to be prevented from agglomeration.

When the amount of anti-aggregation agent or surfactant used is within the above range, aggregation is less likely to occur during the process of removing the dispersion medium (water) and over time thereafter, improving storage stability.

The stirring during the aggregation prevention treatment is not particularly limited, but may be, for example, stirring with a hand mixer, etc. The stirring speed is not particularly limited, but is preferably 300 to 1200 rpm, and particularly preferably 500 to 1000 rpm.
The temperature for the aggregation prevention treatment is not particularly limited, but is preferably from 20°C to 100°C, more preferably from 40°C to 90°C, and particularly preferably from 60°C to 80°C.

<<Water removal treatment>>
(1) In order to obtain the carbon nanofiber group as a powder, a water removal process is carried out.
However, the (1) carbon nanofibers dispersed in the base material may be carbon nanofibers contained in a slurry after wet grinding, may be carbon nanofibers contained in a slurry after the above-mentioned anti-aggregation treatment, or may be powdered carbon nanofibers after water is removed from the slurry.
Regardless of whether the (1) carbon nanofiber group is in the above-mentioned state, it can be used in various applications as an electrically conductive composition by combining it with (2a) flake graphite or (2b) approximately spherical carbon particles.

The method for the water removal treatment is not particularly limited, and the treatment can be carried out by reducing pressure and/or increasing temperature. It is particularly preferred to semi-dry up the mixture by a cyclone separation and recovery method, and then remove (dry) the water by reducing pressure and/or increasing temperature in an oven.
The temperature for the water removal treatment is not particularly limited, but is preferably 40°C to 160°C, and particularly preferably 70°C to 130°C.

Furthermore, it is also preferable to perform baking at about 400° C. in order to remove the surfactant.
The carbon nanofibers constituting the carbon nanofiber group produced by the production method of the present invention have good dispersibility themselves, so it is also preferable that no dispersant or surfactant is attached to the surface of the carbon nanofibers.

It is also preferable to carry out the above-mentioned anti-agglomeration treatment after the above-mentioned water removal treatment. In other words, it is also preferable to mix the above-mentioned anti-agglomeration agent and surfactant into the concentrated slurry or powder after the above-mentioned water removal treatment.

The (1) carbon nanofiber group preferably has a volume resistivity of 2×10 ⁻³ Ω·cm or less, more preferably 7×10 ⁻⁴ Ω·cm or less, and particularly preferably 4×10 ⁻⁴ Ω·cm or less.
Here, the volume resistivity is measured in accordance with JIS R 7222, a method for measuring physical properties of graphite materials, and is defined as the value measured in this manner.

According to the above-mentioned manufacturing method, it is possible to manufacture a material having the above-mentioned volume resistivity (electrical conductivity).
The (1) carbon nanofiber group having the above volume resistivity, when used in combination with the (2a) flake graphite group or the (2b) substantially spherical carbon particle group described below, synergistically increases the electrical conductivity (particularly the volume electrical conductivity) and makes it easier to achieve the above-mentioned effects. In other words, the resistivity (particularly the volume resistivity) can be lowered, making it easier to achieve the above-mentioned effects.

The powdered (1) carbon nanofiber group produced by the above-mentioned production method, even if solidified, can be easily dispersed in a base material such as a resin emulsion or a resin, and can be diffused without agglomeration to obtain a conductive composition. FIG. 7 shows an SEM photograph of the final (1) carbon nanofiber group.
The resin may be a thermoplastic resin or a thermosetting resin. In the case of a two-component thermosetting resin, the resin disperses well in both the base resin and the curing agent.

[(2a) Flake-like graphite group]
The conductive composition of the present invention is characterized in that it contains the following carbonaceous material group (1) and (2a) or (2b) in a dispersed state in a base material.
(1) Carbon nanofiber group described above; (2a) scaly graphite group described below; (2b) roughly spherical carbon particle group described below.

Here, (2a) the flake graphite group must have, when the number average value of the maximum diameter length on the basal plane of the flake graphite is D1 and the number average value of the minimum diameter length is D2, both D1 and D2 are 1 μm or more and 300 μm or less, D1/D2 is 3 or less, and when the number average thickness of the flake graphite is d, the number average aspect ratio [(D1+D2)/2]/d is 10 or more.

Here, the above-mentioned "minimum diameter length" refers to the minimum diameter length on the basal surface, provided that it passes through the midpoint of the line segment that gives the above-mentioned "maximum diameter length".
It is essential that both D1 and D2 are 1 μm or more and 300 μm or less, but D1 is preferably 2 μm or more and 300 μm or less, more preferably 3 μm or more and 100 μm or less, and particularly preferably 4 μm or more and 30 μm or less.
Moreover, D2 is preferably 1 μm or more and 100 μm or less, more preferably 1.5 μm or more and 50 μm or less, and particularly preferably 2 μm or more and 30 μm or less.

D1/D2 must be 3 or less, but is preferably 2 or less, more preferably 1.7 or less, and particularly preferably 1.4 or less.

There are no particular limitations on d, so long as the number-average aspect ratio [(D1+D2)/2]/d satisfies the range below, but d is preferably 3 nm or more and 1000 nm or less, more preferably 5 nm or more and 500 nm or less, and particularly preferably 7 nm or more and 300 nm or less.

The number average aspect ratio [(D1+D2)/2]/d must be 10 or more, but is preferably 50 or more, more preferably 100 or more, even more preferably 300 or more, and particularly preferably 700 or more.

If D1, D2, D1/D2, d, and [(D1+D2)/2]/d are within the above ranges, the combined use with the carbon nanofiber group (1) described above can synergistically increase the electrical conductivity (particularly the volume electrical conductivity) and facilitate the achievement of the above-mentioned effects. In other words, the resistivity (particularly the volume resistivity) can be lowered, and the above-mentioned effects can be easily achieved.
Specifically, within the above range, each carbon nanofiber in the carbon nanofiber group (1) contacts the basal surface to ensure electrical conductivity, and these carbon nanofibers overlap and connect to each other, resulting in overall contact and electrical conductivity, which can synergistically increase electrical conductivity (reduce resistivity).
The lower limit of d and the upper limit of [(D1+D2)/2]/d are also limited by factors such as ease of production, availability, and stability.

Flake graphite may be natural or crushed natural flake graphite, synthetic, or natural material that has been physically or chemically processed (semi-synthetic product). Natural graphite is produced in a variety of places, including China, India, Brazil, Ukraine, and Japan, but there is no particular limit to where it is produced. Graphite with high crystallinity is preferred, and graphite with large anisotropy in physical properties such as electrical conductivity in the directions parallel and perpendicular to the basal plane is preferred.

(2a) Examples of the flake graphite include natural graphite, artificial graphite, graphene, etc., and particularly preferred examples include artificial graphite, graphene, etc.
In addition, commercially available products can also be used as long as they meet the above requirements.

The flake graphite group (2a) preferably has a volume resistivity of 1×10 ⁻³ Ω·cm or less, more preferably 5×10 ⁻⁴ Ω·cm or less, and particularly preferably 1.5×10 ⁻⁴ Ω·cm or less. The above "volume resistivity of the flake graphite group (2a)" is a value measured on the basal plane (A-B plane).
Here, the volume resistivity is measured according to the method described in the section "(1) Carbon nanofiber group" above, and is defined as the volume resistivity thus measured.

The flake graphite group (2a) with the above volume resistivity, when used in combination with the carbon nanofiber group (1), synergistically increases the electrical conductivity (particularly the volume electrical conductivity), making it easier to achieve the above-mentioned effects. In other words, the resistivity (particularly the volume resistivity) can be lowered, making it easier to achieve the above-mentioned effects.

[(2b) Nearly spherical carbon particle group]
The conductive composition of the present invention is characterized in that it contains the following carbonaceous material group (1) and (2a) or (2b) in a dispersed state in a base material.
The above-mentioned (1) carbon nanofiber group, the above-mentioned (2a) scaly graphite group, and the below-mentioned (2b) roughly spherical carbon particle group.

Here, (2b) the approximately spherical carbon particle group must have an L1/L2 ratio of 3 or less, where L1 is the number average value of the maximum diameter of the approximately spherical carbon particles and L2 is the number average value of the minimum diameter.

The "minimum diameter" refers to the smallest diameter (diameter) that passes through the midpoint of the line segment that gives the "maximum diameter" above.

L1 is more preferably 5 nm or more and 200 nm or less, and particularly preferably 10 nm or more and 100 nm or less.
L2 is preferably 5 nm or more and 100 nm or less, more preferably 10 nm or more and 70 nm or less, and particularly preferably 20 nm or more and 50 nm or less.

L1/L2 must be 3 or less, but is preferably 2 or less, more preferably 1.7 or less, and particularly preferably 1.4 or less.

If L1, L2, and L1/L2 are within the above ranges, the combined use with the carbon nanofiber group (1) described above can synergistically increase the electrical conductivity (particularly the volume electrical conductivity) and facilitate the achievement of the above-mentioned effects. In other words, the resistivity (particularly the volume resistivity) can be lowered, and the above-mentioned effects can be easily achieved.
Specifically, within the above range, the carbon nanofibers in the carbon nanofiber group as described above (1) are in contact with each other via the approximately spherical carbon particles, and the carbon nanofibers in the carbon nanofiber group that are not in direct contact are connected to ensure conductivity, and the carbon nanofibers are connected in a series, thereby synergistically increasing the electrical conductivity (reducing the resistivity).

The roughly spherical carbon particles may be natural or synthetic, or may be natural particles that have been crushed and physically or chemically processed (semi-synthetic products) if necessary. They may also be flake graphite that has been physically or chemically spheroidized. In the case of naturally derived particles, there are no particular limitations on the place of origin, etc.

(2b) Examples of the roughly spherical carbon particles include furnace black, channel black, acetylene black, thermal black, and ketjen black, and particularly preferred are ketjen black and acetylene black.
In addition, commercially available products can be used as long as they satisfy the above requirements. As commercially available products, those commercially available as (highly) conductive carbon black and the like are preferable, and specific examples thereof include Ketjen Black and the like.

The (2b) roughly spherical carbon particle group having the above volume resistivity, when used in combination with the (1) carbon nanofiber group, synergistically increases the electrical conductivity (particularly the volume electrical conductivity) and makes it easier to achieve the above-mentioned effects. In other words, the resistivity (particularly the volume resistivity) can be lowered, making it easier to achieve the above-mentioned effects.

[Base material]
In the conductive composition of the present invention, the base material is not particularly limited, and specific examples thereof include organic materials such as resins; and inorganic materials such as glass, ceramics, metals, and alloys.
<Base resin>
A particularly preferred base material is a resin, and the resin may be a thermoplastic resin or a thermosetting resin.

The thermoplastic resin is particularly preferably one or more thermoplastic resins selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl acetate, thermoplastic polyurethane, polytetrafluoroethylene, acrylonitrile butadiene styrene resin, acrylonitrile styrene resin, (meth)acrylic resin, polyamide, polyacetal, polycarbonate, (modified) polyphenylene ether, polyester, cyclic polyolefin, polyphenylene sulfide, polytetrafluoroethylene, polysulfone, polyether sulfone, polyarylate, polyether ether ketone, thermoplastic polyimide, and polyamide imide.

When the resin is in the form of a resin emulsion, the resin is not limited, but examples thereof include (meth)acrylic resins, styrene-maleic anhydride (anhydride) resins, and urethane resins, which are particularly good at dispersing carbonaceous materials.

The carbon nanofiber(s) of the present invention are suitably dispersed in a thermoplastic resin and, as described above, exhibit superior thermal, mechanical, and electrical effects compared to other carbon nanofiber(s). In particular, when used in combination with (2a) flake graphite group or (2b) roughly spherical carbon particle group, a conductive composition can be obtained that exhibits synergistically high electrical conductivity.

The thermosetting resin is particularly preferably one or more thermosetting resins selected from the group consisting of phenolic resins, epoxy resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, thermosetting polyurethanes, and thermosetting polyimides.

The (1) carbon nanofiber(s) of the present invention is suitably dispersed in the base material (containing unreacted resin having functional groups) of the thermosetting resin and/or the curing agent (which crosslinks, reacts, and polymerizes the functional groups), and as described above, exhibits superior thermal, mechanical, and electrical effects compared to other carbon nanofiber(s). In particular, it can be used in combination with (2a) flake graphite group or (2b) roughly spherical carbon particle group to obtain a conductive composition with high electrical conductivity.

<Other ingredients>
The conductive composition of the present invention, a molded body or coating film comprising "the conductive composition of the present invention," or a coating material for forming the conductive composition may contain "other components" as necessary within the range in which the effects of the present invention can be obtained.

Examples of the "other components" include colorants such as inorganic pigments and organic dyes; antioxidants; crystallization regulators such as nucleating agents; release agents such as wax; lubricants; antistatic agents; light stabilizers; ultraviolet absorbers; particles such as inorganic fillers and organic fillers; processing aids for each base material; flame retardants; plasticizers, etc. These "other components" may be used alone or in combination of two or more types. The content may be determined appropriately.

[Concentration of each carbonaceous material group in the conductive composition containing the base material and the mixing ratio of each carbonaceous material group]
(1) Carbon nanofiber group, and (2a) scaly graphite group or (2b) approximately spherical carbon particle group are contained in a base material to form an electrically conductive composition, but the concentration of each carbonaceous material group relative to the entire electrically conductive composition is not particularly limited as long as the electrical conductivity is increased synergistically.
That is, it depends on whether the base material is an organic or inorganic material, and if the organic material is a resin, it also depends on the type of the resin, and also on whether the product containing the conductive composition is a molded body or a coating film, and also on whether the important conductivity is volume conductivity (volume resistivity) or surface conductivity (surface resistance).

However, in any case, the following ranges are preferred.
The total carbon content (total amount of carbonaceous materials) is preferably 6 mass % or more and 90 mass % or less, more preferably 10 mass % or more and 85 mass % or less, still more preferably 15 mass % or more and 65 mass % or less, and particularly preferably 20 mass % or more and 55 mass % or less, based on the entire conductive composition, the entire molded body, or the entire coating film.

(1) The carbon nanofiber group preferably accounts for 6 mass % or more and 90 mass % or less, more preferably 10 mass % or more and 85 mass % or less, and particularly preferably 14 mass % or more and 65 mass % or less, of the entire conductive composition, the entire molded body, or the entire coating film.
In the case where (2b) approximately spherical carbon particle groups are not used, (2a) the flake graphite groups are preferably 5 mass % or more and 60 mass % or less, more preferably 10 mass % or more and 50 mass % or less, and particularly preferably 15 mass % or more and 40 mass % or less, of the entire conductive composition.
(2a) In the case where flake graphite groups are not used, (2b) the approximately spherical carbon particle groups are preferably 3 mass% or more and 20 mass% or less, more preferably 4 mass% or more and 15 mass% or less, and particularly preferably 5 mass% or more and 10 mass% or less, of the entire conductive composition.
When both (2a) the flake graphite group and (2b) the approximately spherical carbon particle group are used in combination with (1) the carbon nanofiber group, the total mass of (2a) the flake graphite group and (2b) the approximately spherical carbon particle group is preferably 3 mass% or more and 40 mass% or less, more preferably 4 mass% or more and 20 mass% or less, and particularly preferably 5 mass% or more and 15 mass% or less, based on the entire conductive composition.

The mixing ratio of each carbonaceous material group is preferably as follows:
That is, the (2a) flake graphite group or the (2b) approximately spherical carbon particle group is contained in an amount of preferably 3 mass % or more and 50 mass % or less, more preferably 5 mass % or more and 45 mass % or less, still more preferably 10 mass % or more and 40 mass % or less, and particularly preferably 15 mass % or more and 35 mass % or less, relative to the entire (total carbon amount) of "(1) carbonaceous material group including carbon nanofiber group."

The conductive composition of the present invention is not particularly limited, but is preferably one produced without applying an external orientation treatment to the carbonaceous material contained therein, such as (1) carbon nanofibers. In the present invention, the volume resistivity can be reduced by mixing (to a minimum value) without applying an orientation treatment for the purpose of orientation.
That is, (1) carbon nanofibers, (2a) flake graphite, and (2b) approximately spherical carbon particles may be naturally oriented and dispersed in the conductive composition of the present invention, or may be randomly dispersed. Carbonaceous materials may be oriented by coating, kneading, etc., even without external treatment for the purpose of orientation, and such orientation is included as a preferred embodiment of the present invention.

[Increase in electrical conductivity of conductive compositions and the like by combined use and absolute value of electrical conductivity (volume resistivity)]
The conductive composition of the present invention is obtained by mixing and dispersing two or more kinds of carbonaceous materials (combined dispersion) in a base resin, and the phrase "the electrical conductivity is improved by the combined dispersion as compared to the single dispersion" is defined as described in the "Effects of the Invention" above.

In a preferred embodiment of the present invention, the (1) carbon nanofiber group [A×r] mass % and "the (2a) scaly graphite group and/or the (2b) substantially spherical carbon particle group""B×(1−r)" mass % (r is a real number greater than 0 and less than 1) are co-dispersed in the base material, with respect to the entire base material,
(1) The electrical conductivity is made higher than that of a composition in which only the carbon nanofiber group is dispersed in an amount of A% by mass, and
The conductive composition has a higher electrical conductivity than a composition in which only "the (2a) flake graphite group and/or the (2b) approximately spherical carbon particle group" are dispersed in an amount of B mass %.

The above "r" is preferably from 0.30 to 0.98, more preferably from 0.40 to 0.94, even more preferably from 0.50 to 0.90, and particularly preferably from 0.60 to 0.86.
If the content of the mixture is within this range, the effect described in the "Effects of the Invention" is easily achieved. Alternatively, if the content of the mixture is within this range, the "maximum value of electrical conductivity (minimum value of volume resistivity) due to mixing" described in the "Effects of the Invention" appears.

In addition to satisfying the above-mentioned "preferred aspects," a composition that satisfies the above-mentioned concentrations of each carbonaceous material group in the "conductive composition containing a base material" and/or the mixing ratios of each carbonaceous material group is preferred.

The volume resistivity of the conductive composition (solid) obtained by dispersing the optimal amount of "(1) carbon nanofiber group, and (2a) scaly graphite group or (2b) roughly spherical carbon particle group" in a suitable base resin, which is an insulator, taking into consideration various physical properties, is 0.01 to 100 [Ω cm]. By further optimization, it can be made 0.1 to 10 [Ω cm], and by further optimization, it can be made 0.5 to 1 [Ω cm].

The conductive composition of the present invention is most characterized by the (1) carbon nanofiber group contained therein, and the special properties of the (1) carbon nanofiber group synergistically increase the electrical conductivity of the "conductive composition in which carbonaceous materials are mixed and dispersed." However, in terms of electrical conductivity alone, the (2a) flake graphite group or the (2b) roughly spherical carbon particle group may be higher than the (1) carbon nanofiber group.

[Molded body, coating film, or coating material for forming conductive composition]
The conductive composition of the present invention can be incorporated into a molded article or coating film, and the molded article or coating film also has synergistically improved electrical conductivity compared to when a carbonaceous material is used alone (contains the carbonaceous material).
Furthermore, when a coating film is formed using a paint containing the conductive composition of the present invention and a liquid dispersion medium, the coating film has synergistically improved electrical conductivity compared to the coating film that uses (contains) a carbonaceous material alone.

The manufacturing method of the above molded body or coating film is not particularly limited, but examples include injection molding, extrusion molding, blow (air) molding, vacuum molding, compressed air molding, roll molding, cast molding, compression molding, coating and drying of paint, impregnation into paint, etc. If necessary, the composition can be kneaded with a kneader, mixer, etc. before molding.

The method for producing the paint is not particularly limited, but the paint can be obtained by placing the conductive composition ((1) carbon nanofiber group; (2a) scaly graphite group or (2b) roughly spherical carbon particle group; base material; and; "other components"), liquid dispersion medium, and, if necessary, paint additives, etc., in a container, and mixing, stirring, and dispersing them in the liquid dispersion medium with a stirrer or the like.

[Application]
The conductive composition of the present invention can be suitably used as a molded body, coating film, or paint, and further, the conductive composition of the present invention can also be used in the form of a molded body called fiber reinforced plastics (FRP) by impregnating or penetrating the mesh of a woven or knitted cloth (sheet) made of carbon fiber, glass fiber, organic fiber, etc. In other words, the conductive composition of the present invention can also be suitably used as a matrix resin for fiber reinforced plastics (FRP).

Molded bodies or coating films, paints, fiber reinforced plastics (FRP), etc., containing the conductive composition of the present invention are suitable for use in a variety of applications and fields, including those requiring electrical conductivity, and those requiring electromagnetic wave absorption due to high electrical conductivity.

[Action/Principle]
In the present invention, the mechanism by which the combined use of carbonaceous materials synergistically increases the electrical conductivity (or synergistically decreases the volume resistivity) of the conductive composition (or its molded body or coating film) containing them is not particularly clear, but is thought to be as follows.
That is, it is believed that the (1) carbon nanofiber group in the present invention has extremely good dispersibility despite its large aspect ratio, and therefore the carbon nanofibers are broken down into individual fibers in the base material, come into contact with the (basal surface of) flake graphite or the (surface of) approximately spherical carbon particles, and are entangled to obtain conductivity.
In other words, it is believed that the flake graphite and roughly spherical carbon particles that are not in direct contact with each other are connected by carbon nanofibers that have a large aspect ratio and are well-dispersed, thereby ensuring conductivity and linking them together, thereby synergistically increasing conductivity (reducing resistivity).

The present invention will be explained in more detail below with reference to manufacturing examples, examples, and comparative examples, but the present invention is not limited to these as long as they do not exceed the gist of the invention.

Production Example 1
[(1) Carbon fibers as raw materials for carbon nanofibers]
<Dry grinding>
"Chopped fibers of about 6 mm," which are random mesophase pitch-based carbon fibers that have not been subjected to sizing treatment, were dry-pulverized in an amount of 1000 g at 30° C. for 10 minutes without pre-pulverization using a "pulverizer using shear and impact with blades" as shown in FIG. 9 so that 90% of the total particles were in the range of 10 μm in diameter and 10 μm to 40 μm in length.

<Wet treatment>
A slurry was obtained by mixing and stirring 100 parts by mass of dry-milled carbon fiber (e.g., FIG. 5(a) and (b)) that had not been subjected to a heat treatment, 1 part by mass of a wetting agent (surfactant), and 1,500 parts by mass of purified water.
Here, the wetting agent (surfactant) was an ammonium salt of a compound having an acid group.

The above slurry was wetted by stirring it at 800 rpm with a hand mixer for 10 minutes at 30°C.

<Wet grinding>
Using a bead mill with a volume of 0.6 L, beads with a diameter of 0.3 mm, a bead filling rate of 60%, a vessel motor rotation speed of 1500 rpm, and a tubular pump as a circulation pump with a transfer rate of 500 mL per minute, 4 L of the slurry obtained above was circulated for 90 minutes or more.

<Anti-aggregation treatment>
3500 mL of the obtained slurry was transferred from the container of the bead mill to a separate container, and then 0.01% by mass of a coblock polymer was added as an anti-agglomeration agent relative to the entire slurry (3500 mL).
After the addition, the mixture was stirred at room temperature (15 to 25° C.) at 800 rpm for 5 minutes using a hand mixer to carry out an anti-aggregation treatment.

<Water removal treatment>
Heating and decompression were applied, and the semi-dried material was collected using a cyclone separation and collection method, and heated in an oven at 120° C. for 240 minutes to remove water, thereby preparing solid carbon nanofibers.

<Evaluation of the carbon nanofiber group obtained in Production Example 1>
The obtained solid (1) carbon nanofiber group and the carbon nanofiber group in the dispersion liquid (slurry) before the water removal treatment were observed with an optical microscope and a scanning electron microscope and measured as described above. As a result, the obtained (1) carbon nanofiber group had a diameter of 30 nm or more and 1000 nm or less and a length of 0.2 μm or more and 70 μm or less, with 90% by number of the total carbon nanofibers distributed within the above range.

The carbon nanofibers contained in the carbon nanofiber group (1) obtained as described above had a number average diameter of 700 nm, a number average length of 5 μm, and a number average aspect ratio of 7.

The obtained solid carbon nanofiber group (1) was added to an aqueous emulsion of acrylic resin, an aqueous emulsion of styrene-maleic anhydride resin, and an aqueous emulsion of polyurethane resin, and stirred in the usual manner, so that the carbon nanofibers from the solid carbon nanofiber group (1) were dispersed in the water approximately one by one. They did not aggregate during dispersion, and did not aggregate over time.

The solid carbon nanofibers obtained were dispersed in a matrix resin made of any general-purpose thermoplastic resin using a conventional kneader, and the fibers were dispersed appropriately in the matrix resin, approximately one by one.

Furthermore, when the obtained solid carbon nanofibers were dispersed in the curing agent of any general-purpose thermosetting resin using a conventional stirrer, the fibers were dispersed favorably in the curing agent approximately one by one.
When the curing agent in which the carbon nanofibers were dispersed was mixed with a base resin containing either an epoxy resin or a urethane resin, the dispersion was maintained in either case without agglomeration, and the resin could be suitably used as a thermosetting resin.

A carbon nanofiber group was prepared in the same manner as in Production Example 1, except that isotropic pitch-based carbon fiber, radial mesophase pitch-based carbon fiber, and onion mesophase pitch-based carbon fiber were used as raw materials instead of random mesophase pitch-based carbon fiber.

The ease of production of "carbon nanofibers having a diameter of 30 nm or more and 1000 nm or less, a length of 0.2 μm or more and 70 μm or less, and preferably a number average aspect ratio of 3 or more" was as follows.
In addition, the further up (to the left) you go, the more superior the result is. The degree of superiority is also indicated by ">>", ">", and ">>".
Random type mesophase pitch carbon fiber >> Radial type mesophase pitch carbon fiber ≒ Onion type mesophase pitch carbon fiber >> PAN carbon fiber

The order of superiority was also roughly the same for dispersibility, dispersion stability, high-concentration dispersibility, thermal properties, mechanical properties, and electrical properties. The order of superiority also seemed to be the same as above, with ">>" >" and ">" approximately equal.

Production Example 2
(1) Carbon nanofiber groups were obtained in the same manner as in Production Example 1, except that milled fibers having a diameter of 10 μm and a length of 70 μm were used as the raw material instead of the chopped fibers.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm, a number average length of 5 μm, and a number average aspect ratio of 7.

Since there were many short fibers with lengths of about 1 μm, the number average aspect ratio tended to be small, but a good group of carbon nanofibers was obtained and the dispersibility and other aspects were also evaluated as good.

Production Example 3
Instead of the "6 mm chopped fiber" in Production Example 1, a long fiber bobbin type was used as the raw material.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

Although pre-pulverization using a cutter mill was necessary, (1) carbon nanofiber groups were successfully obtained in the same manner as in Production Example 1, and the dispersibility and other aspects were also evaluated as being good.

Production Example 4
The same treatment as in Production Example 1 was carried out except that in the dry grinding step of Production Example 1, a full airflow type grinder that does not use impellers, blades, etc. (or does not use impellers, etc.), such as a jet mill, cyclone mill, tornado mill, or dream mill, was used instead of the "grinding machine using shear and impact with blades."

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

(1) Carbon nanofibers were obtained satisfactorily, and the evaluation of dispersibility, etc. was also favorable. However, there was a tendency for the number average aspect ratio to become slightly smaller during the dry grinding stage, and even after the subsequent wet grinding, this tendency for the number average aspect ratio to become slightly smaller remained.

Production Example 5
In Production Example 1, a group of carbon nanofibers was obtained in the same manner as in Production Example 1, except that a random mesophase pitch-based carbon fiber that had been subjected to a sizing treatment was used as a raw material instead of the random mesophase pitch-based carbon fiber that had not been subjected to a sizing treatment, and further, a heat treatment was performed after dry grinding.
The heat treatment was performed in an electric furnace at a temperature of 400° C. for 10 minutes.

The epoxy resin contained in the raw materials was reduced to 0.01% by mass or less. As a result, the surfactant (wetting agent) was effectively absorbed into the filaments (in the gaps between the raw filaments), resulting in the production of carbon nanofibers with a large number-average aspect ratio.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

Production Example 6
In Production Example 1, the wetting treatment was not carried out. Instead, the same wetting agent (surfactant) as that used in the wetting treatment in Production Example 1 was used as a surfactant in the wet grinding.
Other than that, the same procedure as in Production Example 1 was carried out to obtain carbon nanofiber groups.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

The effect of the wetting agent (surfactant) was slightly less than in Production Example 1, and the number average aspect ratio tended to be smaller, but good carbon nanofiber groups were obtained, and the evaluation of dispersibility, etc. was also good.

Production Example 7
A plurality of types of carbon nanofiber groups were obtained in the same manner as in Production Example 6, except that a carbobetaine type, imidazoline type, amidobetaine type, amidosulfobetaine type, or amidoamine oxide type amphoteric surfactant was used instead of the surfactant used in Production Example 6 (the wetting agent (surfactant) used in the wetting treatment in Production Example 1). Those using an amphoteric surfactant showed good high foaming properties in hard water and over a wide pH range.
As the carbobetaine type amphoteric surfactant, Softazoline (manufactured by Kawaken FinChemical Co., Ltd.) was used.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

As in Production Examples 1 and 6, carbon nanofiber groups were obtained well, and the evaluation of dispersibility, etc. was also good.

Production Example 8
Carbon nanofiber groups were obtained in the same manner as in Production Examples 1 and 5, except that the raw material in Production Example 5 was used and the aggregation prevention treatment in Production Example 1 was not carried out.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

In all cases, (1) carbon nanofiber groups were obtained well, and the evaluation of dispersibility, etc. was also good. However, the obtained carbon nanofiber dispersion tended to aggregate slightly over time compared to the dispersion obtained in Production Example 1, but this was not at a level that was problematic.

Production Example 9
A carbon nanofiber dispersion was obtained without performing the water removal treatment in Production Example 1. The dispersion had excellent dispersibility and could be used for the next application as it was.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

Production Example 10
In the aggregation prevention treatment of Production Example 1, highly condensed sodium naphthalenesulfonate, which is an anionic surfactant, was added in an amount of 5 mass% to the entire slurry (3500 mL) instead of the composite metal chelate used in Production Example 1. That is, 50 mass parts were added to 100 mass parts of the target. Carbon nanofiber groups were obtained in the same manner as in Production Example 1 except for the above.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

(1) Carbon nanofiber groups were obtained well, and the evaluation of dispersibility and dispersion stability was also good.

Production Example 11
In Production Examples 1 and 10, the order of the anti-agglomeration treatment and the water removal treatment was reversed, and the target material that had been subjected to the water removal treatment to turn into a thick slurry or powder was mixed with the "surfactant that is an anionic surfactant" in Production Example 10 or the "composite metal chelate that is an anti-agglomeration agent" in Production Example 1. Carbon nanofiber groups were obtained in the same manner as in Production Examples 1 and 10, except that the order of the anti-agglomeration treatment and the water removal treatment was reversed, and the target material that had been subjected to the water removal treatment to turn into a thick slurry or powder was mixed with the "surfactant that is an anionic surfactant" in Production Example 10 or the "composite metal chelate that is an anti-agglomeration agent" in Production Example 1.

In the carbon nanofiber group (1) obtained as described above, 90% by number of all the carbon nanofibers were distributed in the range of "diameter 30 nm or more and 1000 nm or less and length 0.2 μm or more and 70 μm or less."
The carbon nanofibers contained therein had a number average diameter of 700 nm and a number average length of 5 μm.

(1) Carbon nanofiber groups were obtained well, and the evaluation of dispersibility and dispersion stability was also good.

Production Example 12
In Production Example 1, an attempt was made to obtain carbon nanofibers by dry grinding alone, without wet grinding, but it was difficult to obtain a group of carbon nanofibers in which 50% or more of the total carbon nanofibers were distributed within the above range, with a diameter of 30 nm or more and 1000 nm or less and a length of 0.2 μm or more and 70 μm or less.
The carbon nanofibers contained therein had a number average diameter of 7000 nm and a number average length of 150 μm.

Production Example 13
In Production Example 1, an attempt was made to obtain carbon nanofibers by wet grinding only, without performing dry grinding, but the grinding did not progress well, and it was difficult to obtain a group of carbon nanofibers in which 50% or more of the total carbon nanofibers were distributed within the above range, with a diameter of 30 nm or more and 1000 nm or less and a length of 0.2 μm or more and 70 μm or less.

Example 1
<[(1)+(2a)]/Polyimide>
A thermosetting polyimide varnish (U-Varnish-A (registered trademark), manufactured by Ube Industries, Ltd.) was prepared in an amount equivalent to 70 parts by mass of polyimide.
To the above amount of polyimide varnish (containing 70 parts by mass of polyimide), 30 parts by mass of the (1) carbon nanofiber group obtained in Production Example 2 was added as a solid, and NMP (N-methylpyrrolidone) was further added as a dispersion medium. The mixture was stirred at room temperature using a planetary stirrer to disperse the (1) carbon nanofiber group.
The resultant is abbreviated as "CNF30%".

On the other hand, the above-mentioned polyimide varnish was prepared in an amount equivalent to 90 parts by mass of polyimide.
To the above amount of polyimide varnish (containing 90 parts by mass of polyimide), 10 parts by mass of a solid of (2a) flake graphite group sold as "flake graphite" (SEM photographs are shown in Figs. 15 and 16) was added and dispersed in the same manner.
The result is abbreviated as "GRF 10%."

In the "flake graphite" used here, when the number average value of the maximum diameter length on the basal surface was D1 and the number average value of the minimum diameter length was D2, both D1 and D2 were 10 μm, and D1/D2 was 1. In other words, when the basal surface was viewed, there was no distortion (vertical and horizontal directionality) and it was a substantially perfect circle.
The number average thickness was d, which was 13 nm. Therefore, the number average aspect ratio [(D1+D2)/2]/d was 770.

The paints were prepared by varying the mixture ratio of "CNF 30%" and "GRF 10%" stepwise from 0 part by mass/1 part by mass to 1 part by mass/0 part by mass, as shown on the horizontal axis of Figures 10(a) and (b).
The obtained coating material was applied to a glass substrate using a bar coater so as to give a dry film thickness of 50 μm for measuring the volume resistivity, and then the coating film was formed by heating and drying at 200° C. The coating film is both the “conductive composition” and the “molded body” in the present invention.

The volume resistivity of the resulting coating film (conductive composition) was measured in accordance with JIS K 7194 "Test method for resistivity of conductive plastics using the four-probe method."

The results are shown in Table 1 and Figures 10(a) and (b). In this specification and drawings, the total mass% of the carbonaceous material contained in the entire coating film (conductive composition) may be referred to as "total carbon amount [mass%]".
In Figures 10(a) and (b), "CNF 30%"/"GRF 10%" = 0 part by mass/1 part by mass is shown at the left end of the horizontal axis, and "CNF 30%"/"GRF 10%" = 1 part by mass/0 part by mass is shown at the right end of the horizontal axis.

As can be seen from Table 1 and Figures 10(a) and (b), the volume resistivity of the coating film prepared by mixing "CNF 30%" and "GRF 10%" at a ratio of 0.5 parts by mass/0.5 parts by mass was smaller than the volume resistivity of the coating film prepared by mixing "CNF 30%" and "GRF 10%" at a ratio of 0 parts by mass/1 part by mass (left end), and was also smaller than the volume resistivity of the coating film prepared by mixing "CNF 30%" and "GRF 10%" at a ratio of 1 part by mass/0 part by mass (right end).
That is, the mixed use (combined use of two types of carbonaceous materials) resulted in a lower volume resistivity than either of the materials used alone. In other words, the minimum value of the volume resistivity was found near the above content ratio. In other words, the minimum value of the volume resistivity was found at the point where 0<r<1.

Example 2
<[(1)+(2b)]/Polyimide>
Using the same (1) carbon nanofiber group as in Example 1, “CNF 30%” was prepared in the same manner as in Example 1.

On the other hand, the same polyimide varnish as in Example 1 was prepared in an amount equivalent to 90 parts by mass of polyimide.
To the above amount of polyimide varnish (containing 90 parts by weight of polyimide), 10 parts by weight of (2b) roughly spherical carbon particles commercially available as "Ketjen black" was added as a solid, and the mixture was dispersed using a disperser and a roll mill.
The resultant is abbreviated as "KTJ10%."

The "Ketjen Black" used here was a group of nearly spherical carbon particles with a maximum diameter of 30 nm, L1 being the number average diameter, and a minimum diameter of 30 nm, L2 being the number average diameter, and L1/L2 being 1.

Paints were prepared by varying the mixing ratio of "CNF 30%" and "KTJ 10%" stepwise from 0 part by mass/1 part by mass to 1 part by mass/0 part by mass as shown in Figures 11(a) and (b).
The obtained coating material was applied to a substrate using a bar coater so as to give a dry film thickness of 50 μm for measuring the volume resistivity, and then dried to form a coating film. The coating film is both the "conductive composition" and the "molded body" in the present invention.

The volume resistivity of the resulting coating film (conductive composition) was measured in accordance with JIS K7194.
The results are shown in Table 2 and Figures 11(a) and 11(b), in which "CNF 30%"/"KTJ 10%" = 0 part by mass/1 part by mass is plotted on the left end of the horizontal axis, and "CNF 30%"/"KTJ 10%" = 1 part by mass/0 part by mass is plotted on the right end of the horizontal axis.

As can be seen from Table 2 and Figures 11(a) and (b), the volume resistivity of the coating film prepared by mixing "CNF 30%" and "KTJ 10%" at a ratio of 0.75 parts by mass and 0.25 parts by mass was smaller than the volume resistivity of the coating film prepared by mixing "CNF 30%" and "KTJ 10%" at a ratio of 0 parts by mass and 1 part by mass (left end), and was also smaller than the volume resistivity of the coating film prepared by mixing "CNF 30%" and "KTJ 10%" at a ratio of 1 part by mass and 0 part by mass (right end).
That is, the mixed use (combined use of two types of carbonaceous materials) resulted in a lower volume resistivity than either of the materials used alone. In other words, the minimum value of the volume resistivity existed at the point where 0<r<1.

Example 3
<[(1)+(2a)]/Polyimide>
A conductive composition was prepared and a coating film for measuring volume resistivity was formed in the same manner as in Examples 1 and 2, except that "(1) carbon nanofiber group obtained in Production Examples 1, 3 to 11" was used instead of "(1) carbon nanofiber group obtained in Production Example 2" used in Examples 1 and 2.

As in Examples 1 and 2, the mixing ratio of the carbonaceous material was varied and the volume resistivity was measured in the same manner as in Examples 1 and 2. The minimum value of the volume resistivity was found in the mixed portion.
When the (1) carbon nanofiber group obtained in Production Examples 1, 3 to 11 was used, a significant decrease in volume resistivity (synergistic effect) due to the combined use of (2a) flake graphite or (2b) approximately spherical carbon particles was observed in both combinations of (2a) and (2b), similar to Examples 1 and 2 in which the (1) carbon nanofiber group obtained in Production Example 2 was used.

Example 4
<[(1)+(2b)]/Polyester>
A commercially available polyester-based varnish (FS-7733 (solid content: 28 mass%) manufactured by Arakawa Paint Co., Ltd.) was mixed with the total amount (total amount of carbonaceous material, total carbon amount) of "(1) carbon nanofiber group obtained in Production Example 2" used in Examples 1 and 2 and "(2b) roughly spherical carbon particle group sold as 'Ketjen black'" at a constant amount of 40 mass% relative to the total solid content of the coating film, and the ratio of (1) to (2b) was varied. In other words, 60 mass% of resin was mixed with 40 mass% of the total amount of carbonaceous material (total carbon amount).

(1) The carbon nanofiber group was varied to 40.0-27.5% by mass relative to the total solid content of the coating film, and (2b) the roughly spherical carbon particle group was varied to 0-12.5% by mass relative to the total solid content of the coating film, and the paint was prepared by diluting with a solvent (toluene/butyl acetate = 7/3) to obtain the appropriate viscosity.

The "Ketjen Black" used here was a group of nearly spherical carbon particles with a maximum diameter of 30 nm, L1 being the number average diameter, and a minimum diameter of 30 nm, L2 being the number average diameter, and L1/L2 being 1.

The coating material prepared as described above was applied to a dry film thickness of 30 μm in the same manner as in Examples 1 and 2, and the volume resistivity of the resulting coating film was measured in the same manner. The coating film is both the "conductive composition" and the "molded body" in the present invention.
The measurement results are shown in Table 3 below and FIG.

As can be seen from Table 3 and Figure 12, even though the total carbon amount (sum of the carbonaceous material amounts) is the same, there is a point where the volume resistivity decreases (a point where it becomes minimal), demonstrating the synergistic effect of the two types of carbonaceous material.

The conductive composition (coating) is obtained by co-dispersing (1) carbon nanofiber (group) [40×r] mass % and (2b) approximately spherical carbon particle (group) “40×(1−r)” mass % (r is a real number greater than 0 and less than 1) in the base material relative to the entire conductive composition (coating in Example 4).
That is, for the above A and B, A=B=40.

12, (1) for the carbon nanofiber (group), there is a minimum value near 40×r=35. Therefore, r=35/40=0.88.
On the other hand, although it is the same, as is clear from Table 3 and FIG. 12, for (2b) substantially spherical carbon particles (group), there is a minimum value in the vicinity of 40×(1−r)=5.
Therefore, 1-r=5/40, r=1-5/40=35/40=0.88.

The volume resistivity in the above-mentioned "r=0.88 blend (carbonaceous material content)" was lower (higher electrical conductivity) than (1) a blend in which only carbon nanofiber groups were dispersed at 40% by mass, and was also lower (higher electrical conductivity) than (2b) a blend in which only roughly spherical carbon particle groups were dispersed at 40% by mass.

Comparative Example 1
<[(1)+(2a') Group of (spherical) flake-like graphite particles having a number-average aspect ratio of 4]
/Polyester>
A coating material was prepared by using (2a') (spherical) scaly graphite group having a number average aspect ratio [(D1+D2)/2]/d of 4 (SEM photographs are shown in Figs. 17 and 18) instead of the (2a) scaly graphite group.

A commercially available polyester varnish (FS-7733, solid content 28% by mass, manufactured by Arakawa Paint Co., Ltd.) was mixed with the total amount (total amount of carbonaceous materials, total carbon amount) of "(1) carbon nanofiber group obtained in Manufacturing Example 2 used in Examples 1 and 2" and "(2a') (spherical) flake graphite (highly purified natural graphite from Mozambique, D50 = 9 μm, manufactured by NSC Corporation)" at a constant 40% by mass relative to the total solid content of the coating film, and the "(1) carbon nanofiber group" was varied between 40 and 30% by mass relative to the total solid content of the coating film, and "(2a') (spherical) flake graphite" was varied between 0 and 10% by mass relative to the total solid content of the coating film, and the paint was prepared by diluting with a solvent (toluene/butyl acetate = 7/3) to obtain an appropriate viscosity.

The coating material prepared as described above was applied in the same manner as in Examples 1 and 2 so that the dry film thickness was 30 μm, and the volume resistivity of the resulting coating film was measured in the same manner.
The measurement results are shown in Table 4 below and FIG.

As shown in Table 4 and Figure 13, unlike Example 4, (1) the volume resistivity of the coating film containing only carbon nanofiber groups was the lowest, and there was no point at which the volume resistivity decreased (minimum point) due to mixing, and no synergistic effect of the two types of carbonaceous materials was observed.

Example 5, Comparative Example 2
<[(1)+(2b)]/Polyester>
A commercially available polyester-based varnish (FS-7733 manufactured by Arakawa Paint Co., Ltd., solid content 28 mass %) was mixed with the "(1) carbon nanofiber group obtained in Production Example 2" used in Examples 1 and 2 in the content [mass %] shown in "Example 5" in Table 5, and 5.0 mass % of "(2b) approximately spherical carbon particle group" sold as "Ketjen black".
As shown in "Example 5" in Table 5, 27.5 mass % to 32.5 mass % of (1) carbon nanofiber group was mixed and diluted with a solvent (toluene/butyl acetate = 7/3) to obtain a suitable viscosity to prepare a paint.

The paint was applied in the same manner as in Examples 1 and 2 so that the dry film thickness was 30 μm, and the volume resistivity of the resulting coating was measured in the same manner.

As Comparative Example 2, in Example 5 above, (1) only carbon nanofiber groups were blended so that the total carbon content (amount of carbonaceous matter) in the coating film was 30% by mass to 50% by mass, and the paint was prepared by diluting with a solvent (toluene/butyl acetate = 7/3) to obtain an appropriate viscosity (see Table 5).

The paint was applied in the same manner as in Examples 1 and 2 so that the dry film thickness was 30 μm, and the volume resistivity of the resulting coating was measured in the same manner.

As a result, the coating film in which (1) carbon nanofiber groups and (2b) roughly spherical carbon particle groups were added at 5 mass % (Example 5) had a lower volume resistivity than the coating film in which (1) carbon nanofiber groups were used alone (Comparative Example 2) when compared at the same total carbon amount (see Figure 14).

Example 6
An electrically conductive composition was prepared in the same manner as in Example 2, except that the base material in Example 2 was changed to a thermoplastic resin, polycarbonate (manufactured by Mitsubishi Engineering Plastics Corporation, Iupilon (registered trademark)), and a coating film for measuring volume resistivity was formed.

When evaluated in the same manner as in Example 2, the lowest volume resistivity was found in the mixture of the two types of carbonaceous material.

Example 7
In Example 2, the base material was replaced with a thermosetting epoxy resin for evaluation.
That is, the base material was made by mixing an epoxy resin base material (jER828 (registered trademark) manufactured by Mitsubishi Chemical Corporation) and a dedicated epoxy resin hardener (jER Cure ST14 (registered trademark) manufactured by the same company) in the ratio specified in the instructions.
Except for the above, an electrically conductive composition was prepared in the same manner as in Example 2, and a coating film for measuring the volume resistivity was formed.

When evaluated in the same manner as in Example 2, the lowest volume resistivity was found in the mixture of the two types of carbonaceous material.

The conductive composition of the present invention contains (1) carbon nanofibers that have excellent shape, size, distribution, etc., and a surface state with good dispersibility. In addition, since the carbon nanofibers are a collection of individual carbon nanofibers and have good dispersibility in base materials and dispersion media, they can be widely used in various fields requiring various performances, such as electrically conductive objects and electromagnetic wave absorption, in the form of dispersions, molded bodies, coatings, paints, films, paints, layers (membranes), circuits, powders, etc.

10: Filament 20: Plain filament

Claims (22)

1. An electrically conductive composition comprising the following carbonaceous material group (1) and (2a) or (2b) dispersed in a base material:
(1) A group of carbon nanofibers obtained by pulverizing carbon fibers, in which 50% or more by number of all carbon nanofibers are distributed in the range of "diameter 30 nm or more and 1000 nm or less, and length 0.2 μm or more and 70 μm or less." (2a) A group of flake-like graphite in which, when the number average of the maximum diameter length of the basal plane of the scaly graphite is D1 and the number average of the minimum diameter length is D2, both D1 and D2 are 1 μm or more and 300 μm or less, D1/D2 is 3 or less, and when the number average thickness of the scaly graphite is d, the number average aspect ratio [(D1+D2)/2]/d is 10 or more. (2b) A group of approximately spherical carbon particles in which, when the number average of the maximum diameter of the approximately spherical carbon particles is L1 and the number average of the minimum diameter is L2, L1/L2 is 3 or less. The (1) carbon nanofiber group [A×r] mass % and the (2a) scaly graphite group and/or the (2b) substantially spherical carbon particle group “B×(1−r)” mass % (r is a real number greater than 0 and less than 1) are co-dispersed in the base material with respect to the entire conductive composition,
(1) The electrical conductivity is made higher than that of a composition in which only the carbon nanofiber group is dispersed in an amount of A% by mass, and
2. The conductive composition according to claim 1, which has a higher electrical conductivity than a composition in which only "the (2a) flake graphite group and/or the (2b) approximately spherical carbon particle group" are dispersed in an amount of B mass %. The conductive composition according to claim 1 or 2, wherein the number average aspect ratio of the carbon nanofibers contained in the carbon nanofiber group (1) is 3 or more and 200 or less. The conductive composition according to any one of claims 1 to 3, wherein the number average diameter of the carbon nanofibers contained in the carbon nanofiber group (1) is 30 nm or more and 1000 nm or less, and the number average length is 0.2 μm or more and 70 μm or less. The conductive composition according to any one of claims 1 to 4, wherein the (1) carbon nanofiber group is obtained by dry-pulverizing carbon fibers until the number-average length is 100 μm or less, and then wet-pulverizing the carbon fibers. The conductive composition according to any one of claims 1 to 5, wherein the (1) carbon nanofiber group is dispersed substantially individually in the base material. The conductive composition according to any one of claims 1 to 6, wherein the (1) carbon nanofiber group is obtained by dry grinding and then wet grinding pitch-based carbon fibers as a raw material. The conductive composition according to claim 7, wherein the number average thickness or number average width of the filaments constituting "the filaments of the pitch-based carbon fiber, which is the raw material of the carbon nanofiber group (1)" is 10 nm or more and 200 nm or less. The conductive composition according to claim 7 or 8, wherein the carbon nanofibers of the carbon nanofiber group are constituted by an assembly of 2 to 20 individual filaments constituting "the filaments of the pitch-based carbon fiber that is the raw material of the carbon nanofiber group (1)." The conductive composition according to any one of claims 7 to 9, wherein the pitch-based carbon fiber, which is the raw material for the carbon nanofiber group (1), is a mesophase pitch-based carbon fiber. The conductive composition according to claim 10, wherein the mesophase pitch-based carbon fiber is a random mesophase pitch-based carbon fiber. The conductive composition according to any one of claims 1 to 11, wherein the (1) carbon nanofiber group is substantially free of mixed resin that is mixed into the raw carbon fibers before pulverization. The conductive composition according to any one of claims 1 to 12, wherein the (1) carbon nanofiber group is obtained by dry grinding followed by wet grinding, and a heat treatment is performed between the dry grinding and the wet grinding to remove the mixed resin present in the raw material. The conductive composition according to any one of claims 1 to 13, wherein the (1) carbon nanofiber group is obtained by dry grinding followed by wet grinding, and the wet grinding is bead mill grinding or ball mill grinding carried out in an aqueous medium in the presence of a surfactant. The conductive composition according to any one of claims 1 to 14, wherein the carbon nanofiber group (1) has a volume resistivity of 7×10 ⁻⁴ Ω·cm or less. The conductive composition according to any one of claims 1 to 15, wherein the number average thickness d of the flake graphite groups (2a) is 3 nm or more and 1000 nm or less. The conductive composition according to any one of claims 1 to 16, wherein the (2a) flake graphite group or the (2b) approximately spherical carbon particle group is contained in an amount of 5% by mass to 70% by mass with respect to the entire carbonaceous material group. The conductive composition according to any one of claims 1 to 17, wherein the (2a) flake graphite group or the (2b) approximately spherical carbon particle group is contained in an amount of 10% by mass or more and 50% by mass or less with respect to the entire carbonaceous material group. The conductive composition according to any one of claims 1 to 18, wherein the base material is one or more thermoplastic resins selected from the group consisting of polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl acetate, thermoplastic polyurethane, polytetrafluoroethylene, acrylonitrile butadiene styrene resin, acrylonitrile styrene resin, (meth)acrylic resin, polyamide, polyacetal, polycarbonate, (modified) polyphenylene ether, polyester, cyclic polyolefin, polyphenylene sulfide, polytetrafluoroethylene, polysulfone, polyether sulfone, polyarylate, polyether ether ketone, thermoplastic polyimide, and polyamide imide; or one or more thermosetting resins selected from the group consisting of phenolic resin, epoxy resin, melamine resin, urea resin, unsaturated polyester resin, alkyd resin, thermosetting polyurethane, and thermosetting polyimide. A molded body comprising the conductive composition according to any one of claims 1 to 19. A coating film comprising the conductive composition according to any one of claims 1 to 19. A coating material comprising the conductive composition according to any one of claims 1 to 19 and a liquid dispersion medium.

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