A method for producing a group of carbon nanofibers, a group of carbon nanofibers produced by the said method, and a resin containing the carbon nanofibers and a molded article using the same.

Abstract

To provide a method for producing a group of carbon nanofibers in which carbon nanofibers having a large aspect ratio and specified in a predetermined size can be isolated substantially one by one or dispersed substantially one by one.SOLUTION: There is provided a method for producing a group of carbon nanofibers each having a diameter of not less than 30 nm and not more than 1000 nm and a length of not less than 0.2 μm and not more than 70 μm, in which not less than 50 carbon nanofibers in total are distributed in the above range. The method for producing a group of carbon nanofibers uses random mesophase pitch-based carbon fibers as a raw material, and achieves the above distribution by dry-grinding until the number-average length becomes 100 micrometers or less and thereafter wet-grinding. Also there are provided the group of carbon nanofibers produced by the production method, a carbon nanofiber dispersion containing the group of carbon nanofibers, a resin containing the carbon nanofibers, and a molded product using the same.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a group of carbon nanofibers having carbon nanofibers with a specific shape, a group of carbon nanofibers produced by the production method, a carbon nanofiber dispersion containing the group of carbon nanofibers, a resin containing the carbon nanofibers, and a molded body using the same.

Background Art

[0002] Carbon fibers and carbon fiber reinforced plastics (CFRP) are lightweight and have both toughness and flexibility, so they are widely used in various molded products as a substitute for metals. Among them, the shortest carbon fibers are also called mild fibers. Generally, the average fiber length is not less than 70 μm and not more than 200 μm, and the average fiber diameter is about 3 to 10 μm. Due to their fine size, they are often used as abrasives, reinforcing aids, etc.

[0003] Chopped fibers and long fibers having an average fiber length longer than that of mild fibers are mainly used in molded products such as prepregs and forgings. In addition, as other sizes, fine CFRP obtained by further pulverizing mild fibers to shorten the fiber length is used, for example, as a reinforcing material for concrete. However, these become more difficult to process as they become shorter, and are likely to aggregate even after being pulverized. Therefore, in fields where good dispersion is required, they are considered to have little current utility value.

[0004] Setting aside the actual dispersibility of carbon fibers (the existence of each individual fiber) and their utility, if we simply define them by their shape (size), the current general definition includes carbon fibers with 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.

[0005] While there are documents describing the diameter and aspect ratio of carbon fibers (for example, Patent Documents 1 and 2), currently there are almost no carbon nanofibers that are small in size, have a large aspect ratio, and are in a state where they can be isolated (separated) almost individually, or are in a state where they can be dispersed almost individually, as in the present invention. In other words, the utilization rate of carbon fibers of the carbon nanofiber size defined above is currently extremely low due to poor dispersibility or redispersibility.

[0006] Furthermore, there are no commercially available materials on the market that contain or are capable of dispersing nano-sized carbon fibers with a large aspect ratio, and no dispersion liquids in which such carbon fibers are stably dispersed are known.

[0007] Carbon nanofibers, which have a large aspect ratio, are dispersible, and have dispersion stability, suggest a wide range of potential uses, such as incorporating them into thermoplastic or thermosetting resins to create carbon nanofiber-containing resins (composite materials). However, satisfactory applications have not yet been realized. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2012-188790 [Patent Document 2] Japanese Patent Publication No. 2017-066546 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The present invention has been made in view of the above-mentioned background art, and its objective is to provide a method for producing a group of carbon nanofibers having a large aspect ratio and a specified size, which can be isolated approximately one by one, or which can be dispersed or dispersed approximately one by one. Furthermore, the present invention provides a group of carbon nanofibers produced by this method, and a carbon nanofiber dispersion containing the group of carbon nanofibers.

[0010] Furthermore, the objective is to provide a carbon nanofiber-containing resin containing the carbon nanofiber group and a molded article using the same. [Means for solving the problem]

[0011] As a result of diligent research to solve the above problems, the inventors have found that by using specific carbon fibers as raw materials, and then performing dry grinding followed by wet grinding, it is possible to isolate carbon nanofibers of a specified size almost individually, or to disperse them almost individually or in a dispersed state. Furthermore, by further limiting the carbon fibers used as raw materials, further limiting the dry grinding method and / or wet grinding method, and limiting the surfactants and dispersants used at each stage, we discovered that carbon nanofibers (groups) can be more preferably dispersed (possibly) into the state described above, thus completing the present invention.

[0012] In other words, the present invention is a method for producing a group of carbon nanofibers in which carbon nanofibers having a diameter of 30 nm or more and a length of 0.2 μm or more and 70 μm or less are distributed within the above range, with 50 percent or more of the total being. This invention provides a method for producing a group of carbon nanofibers characterized by using random-type mesophase pitch carbon fibers as raw materials, dry grinding them until the number average length is 100 μm or less, and then wet grinding them to achieve the above distribution.

[0013] Furthermore, the present invention provides a method for producing the carbon nanofiber group having a number-average aspect ratio of 3 or more and 200 or less.

[0014] Furthermore, the present invention provides a method for producing the carbon nanofiber group described above, wherein the number-average thickness or number-average diameter of the elementary filaments constituting the filaments constituting the random-type mesophase-pitch carbon fiber is 10 nm or more and 200 nm or less.

[0015] Furthermore, the present invention provides a method for producing the carbon nanofiber group in which the elementary filaments constituting the filament are assembled in a range of 2 to 20 individuals.

[0016] Furthermore, the present invention provides a method for producing the carbon nanofiber group described above, wherein the dry grinding is performed by airflow grinding, cutter grinding, or both airflow grinding and cutter grinding simultaneously.

[0017] Furthermore, the present invention provides a method for producing the carbon nanofiber group, wherein the wet grinding is performed in an aqueous medium containing a surfactant, using either a bead mill or a ball mill.

[0018] Furthermore, the present invention provides a method for producing the carbon nanofiber group by performing a heat treatment between the dry grinding and the wet grinding to remove the mixed resin.

[0019] Furthermore, the present invention further provides a method for producing the above-described carbon nanofiber group, which comprises performing an anti-aggregation treatment by adding one or more anti-aggregation agents selected from the group consisting of a metal-containing anti-aggregation agent, a coblock polymer, a comb-shaped coblock polymer, and a surfactant to the slurry of the carbon nanofiber group obtained after the wet pulverization.

[0020] The present invention also provides a carbon nanofiber group produced by the method for producing the carbon nanofiber group described above.

[0021] The present invention also provides a carbon nanofiber dispersion containing the above-described carbon nanofiber group.

[0022] The present invention also provides a carbon nanofiber-containing resin containing the above-described carbon nanofiber group and a thermoplastic resin or a thermosetting resin, and a molded article using the same.

[0023] The present invention also provides a matrix resin for fiber reinforced plastics containing the above-described carbon nanofiber group and a thermoplastic resin or a thermosetting resin, and a molded article using the same.

Advantages of the Invention

[0024] According to the production method of the present invention, the above problems and issues are solved, and carbon nanofibers having an aspect ratio as large as 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 can be produced into a carbon nanofiber group that can be isolated one by one (substantially) or can be in a dispersed state or a dispersible state one by one (substantially).

[0025] The present invention is characterized in that the carbon fiber as a raw material is a random-type mesophase pitch-based carbon fiber. That is, in the case of PAN-based carbon fibers, no matter how they are pulverized, those having the above shape, form, and properties cannot be obtained. Furthermore, even with pitch-based carbon fibers, isotropic pitch-based carbon fibers cannot be obtained with any grinding method, and the above-mentioned shape, form, and properties can only be achieved by using mesophase pitch-based carbon fibers. In PAN-based carbon fibers and isotropic pitch-based carbon fibers, the aspect ratio becomes particularly small during the wet grinding stage to ultimately produce the carbon nanofibers of the present invention, and for example, it is not possible to obtain those with an aspect ratio of 3 or more.

[0026] Furthermore, even with mesophase pitch carbon fibers, onion-type mesophase pitch carbon fibers and radial-type mesophase pitch carbon fibers cannot be obtained with the above-mentioned shape, form, and properties, and only those with a small aspect ratio can be obtained (for example, only those with an aspect ratio of less than 3 can be obtained). Surprisingly, according to the present invention, the above-mentioned shape, form, and properties were obtained for the first time by using random-type mesophase pitch carbon fibers as the raw material.

[0027] Carbon nanofibers were tightly bonded to their surroundings and integrated into the broader definition of carbon fibers as commonly known (see, for example, Figures 1-3). Conventionally, it was not possible to manufacture carbon nanofibers from this structure while maintaining a high aspect ratio. In other words, with current technology, it has not been possible to produce carbon nanofibers by crushing or other methods while maintaining a high aspect ratio, and to reproducibly and stably make them into an isolated (separable) or (re)dispersible state without aggregation.

[0028] The carbon nanofibers obtained using the manufacturing method of the present invention have suppressed re-aggregation, and the carbon nanofibers are in a state where they can be isolated one by one, or in a dispersed or dispersible state, and are suitable for various applications as they are.

[0029] Specifically, for example, even if a slurry obtained by wet grinding in an aqueous medium is solidified (dried up) by removing the water medium, it can be redispersed into resin emulsions, aqueous solutions of water-soluble resins, or matrix resins, depending on the subsequent application. For example, even if a semi-dried product is obtained by distilling off the water from the slurry and then dried to a solid state at, for example, 100°C or higher, the mass can be redispersed into individual pieces. If the product is used for subsequent applications in the slurry or semi-dried stage without solidification, (re)dispersion becomes even more possible.

[0030] Furthermore, carbon nanofiber-containing resins obtained by incorporating the carbon nanofiber group of the present invention into thermoplastic resins or thermosetting resins exhibit excellent mechanical and electrical properties. In particular, the resin containing the carbon nanofiber group of the present invention is excellent as a matrix (resin) for fiber-reinforced plastics, and especially excellent as a matrix (resin) for carbon fiber-reinforced plastics. [Brief explanation of the drawing]

[0031] [Figure 1] These are scanning electron microscope (SEM) images of cross-sections of mesophase pitch carbon fibers, the raw material, before grinding. (Left) Radial type mesophase pitch carbon fiber (Center) Random type mesophase pitch carbon fiber used as a raw material in this invention (Right) Onion type mesophase pitch carbon fiber [Figure 2] This is a schematic diagram of a cross-section of a mesophase pitch carbon fiber. (a)(a') Random-type mesophase pitch carbon fiber used as a raw material in the present invention (b) Radial-type mesophase pitch carbon fiber (c) Onion-type mesophase pitch carbon fiber [Figure 3] This is a scanning electron microscope (SEM) image of a cross-section of a single random mesophase pitch carbon fiber used as a raw material in the present invention. [Figure 4]These are scanning electron microscope (SEM) images of random-type mesophase-pitch carbon fibers (before grinding), which are preferable for use as raw materials. (a) 400x magnification (b) 1300x magnification [Figure 5] These are scanning electron microscope (SEM) images of carbon fibers after dry grinding according to the present invention. (a) 600x magnification (b) 1500x magnification [Figure 6] This is a 700x magnification photograph taken with an optical microscope (biological microscope) during the wet grinding process in the present invention (reference figure). [Figure 7] These are scanning electron microscope (SEM) images of the wet-milled material used in the present invention. (a) 1500x magnification (b) 6500x magnification [Figure 8] This is a schematic diagram showing the manufacturing process of (random type) mesophase pitch carbon fiber filaments used as raw materials in the present invention. [Figure 9] This is a schematic diagram showing an example of an apparatus used for dry grinding in the present invention. [Modes for carrying out the invention]

[0032] The present invention will be described below, but it is not limited to the following specific forms and can be modified as desired within the scope of the technical idea.

[0033] The present invention's "method for producing a group of carbon nanofibers" is A method for producing a group of carbon nanofibers in which 50% or more of the total carbon nanofibers are distributed within the above range, and the carbon nanofibers have a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm. The method is characterized by using random-type mesophase pitch carbon fibers as raw materials, dry grinding them until the number-average length is 100 μm or less, and then wet grinding them to achieve the above distribution.

[0034] <Carbon nanofibers (group)> In this invention, "carbon fiber" refers to any elongated carbonaceous material having a graphene structure, and includes carbon fibers, carbon nanofibers, and carbonaceous materials of similar size. Furthermore, "carbon fiber" includes filaments of carbonaceous materials having a graphene structure, elementary filaments that constitute them, and strands in which such filaments are arranged in parallel in a longitudinal order.

[0035] The carbon nanofiber group produced by this invention has a diameter of 30 nm to 1000 nm, and more than 50 percent of the total carbon nanofibers have a length of 0.2 μm to 70 μm. It is essential that at least 50 percent of the total is distributed within the above range, preferably at least 70 percent, more preferably at least 80 percent, even more preferably at least 90 percent, and particularly preferably at least 95 percent. Since it is possible to manufacture quantities exceeding the above percentages, a sharper distribution is preferable, but a broader distribution is preferable from a productivity standpoint.

[0036] The diameter of the carbon nanofiber is between 30 nm and 1000 nm, but preferably between 50 nm and 900 nm, more preferably between 100 nm and 850 nm, and particularly preferably between 300 nm and 800 nm. A good yield can be achieved with diameters within the above range. If the diameter is too small, manufacturing may become difficult, and the length will also be shortened, resulting in a smaller aspect ratio. On the other hand, if the diameter is too large, the performance at the intended application may be inferior, or the applications of the carbon nanofiber group may be limited.

[0037] The length of the carbon nanofibers is between 0.2 μm and 70 μm, more preferably between 1 μm and 50 μm, even more preferably between 2 μm and 30 μm, and particularly preferably between 5 μm and 20 μm. If the length is within the above range, it can be manufactured in good yield. If the length is too short, the aspect ratio may become small, the performance at the intended application may be inferior, the applications of the carbon nanofiber group may be limited, and it may become difficult to prevent aggregation. On the other hand, if the length is too long, the applications of the carbon nanofiber group may be limited, and it may become difficult to manufacture while maintaining a large aspect ratio.

[0038] The isolated and dispersible carbon nanofibers produced by this invention are characterized by a large aspect ratio. The number-average aspect ratio of the carbon nanofibers is preferably 3 to 200, more preferably 5 to 160, even more preferably 7 to 130, particularly preferably 15 to 100, and most preferably 20 to 70. If the aspect ratio is too small, the performance of the carbon nanofibers may be inferior or their applications may be limited. On the other hand, if the aspect ratio is too large, manufacturing may become difficult.

[0039] The diameter and length of the carbon nanofibers contained in the carbon nanofiber group produced by this invention are determined by randomly selecting 100 nanofibers using an optical microscope or scanning electron microscope (SEM), measuring the diameter and length of each nanofiber, and taking the arithmetic mean. An optical microscope equipped with a size measuring gauge is preferable for improving measurement accuracy and reducing measurement time. If measurement is difficult without increasing the magnification, a scanning electron microscope (SEM) image may be used instead of an optical microscope. Because the carbon nanofibers produced by this invention have a large aspect ratio, their diameter and length are measured individually using a microscope rather than by measuring with a particle size analyzer. Since automatic particle size analyzers cannot adequately measure the diameter and length, the above method is necessary. In this invention, sizes such as diameter, length, and aspect ratio are defined as those measured as described above.

[0040] The carbon nanofiber group may exist as a solid (powder) or in a dispersion. If it exists as a solid (powder), it may be in a state where each individual fiber can be isolated or dispersed individually. If it exists in a dispersion, it may be in a dispersed state where each individual fiber is dispersed. The carbon nanofiber group produced by this invention can be in any of the above states.

[0041] <Manufacturing process> In the present invention, carbon nanofibers having the above-described shape (diameter and length), and carbon nanofiber groups having the above-described distribution, are obtained by at least dry grinding followed by wet grinding. By specifying the carbon fibers used as raw materials, (re)dispersible carbon nanofiber groups having the above-described shape and distribution can be obtained. It is also preferable to add other processing (operations) as needed before the dry grinding, during the grinding, or after the wet grinding. Furthermore, the dry grinding and wet grinding may each be carried out in one stage or in two or more stages.

[0042] The most preferred steps are listed below in order. Preparation of raw carbon fiber, pre-pulverization, dry pulverization, heat treatment, wetting treatment, wet pulverization, anti-coagulation treatment, water removal treatment. Of the above, at a minimum, the preparation of specific raw material carbon fibers, dry grinding, and wet grinding are essential, and if these are done, the carbon nanofibers of the present invention can be manufactured. Of the above, pre-pulverization, heat treatment, wetting treatment, anti-coagulation treatment, and water removal treatment are not essential, but it is preferable to perform some or all of them as needed in order to manufacture the product well. Heat treatment is particularly preferable when the raw material contains resin such as a sizing agent, but it is not necessary when the raw material has not undergone sizing treatment or the like. The following describes each processing step in order of execution.

[0043] <Preparation of raw carbon fiber> In this invention, random-type mesophase pitch carbon fibers are prepared as carbon fibers before pulverization (raw material carbon fibers). Figure 4 shows scanning electron microscope (SEM) images of an example of random-type mesophase pitch carbon fiber (before grinding) used as a raw material. Figure 4(a) is at 400x magnification, and Figure 4(b) is at 1300x magnification. With PAN-based carbon fibers and isotropic pitch-based carbon fibers, regardless of the grinding method used, it is not possible to obtain a group of carbon nanofibers with a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm that also possess a high number-average aspect ratio.

[0044] Mesophase pitch carbon fibers are classified into at least three types based on their cross-sectional shape (i.e., internal shape): radial type (Figure 1 (left), Figure 2 (b)), random type (Figure 1 (center), Figure 2 (a)(a'), Figure 3), and onion type (Figure 1 (right), Figure 2 (c)). The "random type" refers to fibers whose cross-section is random in shape. This is the origin of the name "random type."

[0045] In the present invention, even with mesophase pitch carbon fibers, radial mesophase pitch carbon fibers or onion mesophase pitch carbon fibers may be somewhat difficult to obtain the carbon nanofiber group with the above size, shape, and distribution. When radial or onion type carbon fibers are used as raw materials, the above shape, form, and distribution may not be readily available, and in particular, carbon nanofiber groups with a large number-average aspect ratio may not be readily available (for example, only those with an aspect ratio of less than 3 may be available).

[0046] The "filaments constituting the above-mentioned random-type mesophase-pitch carbon fibers" are made up of even smaller rod-shaped or plate-shaped (sheet-shaped) pieces that are aggregated together. For example, in Figure 3, plate-shaped (sheet-shaped) pieces are aggregated vertically. In this specification, the "even smaller rod-shaped or plate-shaped (sheet-shaped) components" that make up the filament will be abbreviated as "elemental filament". Within a single elementary filament, it is thought that one structure consists of fused benzene rings forming graphene structures stacked in the same direction, or one or more carbon nanotubes bundled together in the same direction.

[0047] In the present invention, the carbon fiber used as a raw material preferably has a number-average thickness or number-average diameter of the elemental filaments constituting the filament of 10 nm or more and 200 nm or less. More preferably it is 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. If the size of the raw material carbon fiber filament is greater than or equal to the lower limit mentioned above, the thermal conductivity [W / (m·K)] of the filament containing the filament increases sharply, and therefore, the various physical properties of the carbon nanofiber of the present invention containing the filament, including thermal conductivity, also improve. On the other hand, if the value is below the above upper limit, the material can be prepared using "filaments or carbon fibers" containing the elemental filament.

[0048] Figure 8 shows a schematic of the manufacturing process for mesophase pitch carbon fiber filament 10. Random-type mesophase pitch carbon fiber can also be obtained by adjusting the spinning viscosity, nozzle shape, and flow state of the raw material pitch, as shown in Figure 8. The manufacturing process for mesophase pitch carbon fibers does not include a drawing step. The microstructure controlled during the spinning process becomes almost directly the crystal structure of the filament 10, creating boundaries with different crystal orientations, and allowing the elementary filaments 20 to be observed (exist).

[0049] In Figure 8, the thickness of the raw material filament 10 is typically 4000nm to 10000nm, mostly 5000nm to 7000nm. On the other hand, in the present invention, the number average thickness or number average diameter of the elemental filaments 20 in the raw material carbon fiber filament 10 is preferably 10nm to 200nm, more preferably 15nm to 100nm. Therefore, in the raw material carbon fiber, there are typically 40 to 700 elemental filaments 20, and in most cases 60 to 400, in one filament 10. When a single elementary filament 20 is separated (defined) at a boundary where the orientation of the crystal structure is different, the above number of elementary filaments 20 are bundled together to form a single filament 10. Note that in Figure 8, for the sake of a schematic and easier-to-understand representation, it is depicted as if a single filament 10 is formed from three elementary filaments 20.

[0050] In the present invention, when using random-type mesophase pitch carbon fibers as raw materials, the invention is not limited to the above-described embodiment, but is particularly preferred. In a method for producing a group of carbon nanofibers as described in the present invention, the shape, distribution, and even whether such a group of carbon nanofibers can be produced at all depend heavily on the type of carbon fiber used as the raw material.

[0051] <Relationship between carbon nanofibers and elementary filaments in this invention> The carbon nanofibers produced by the present invention are preferably composed of two to twenty elementary filaments that constitute the raw material filaments. More preferably, there are three to sixteen elementary filaments, and particularly preferably four to twelve. The number of elementary filaments is the average value taken for each carbon nanofiber in the carbon nanofiber group. When manufacturing carbon nanofibers, the crystallinity and external shape of the individual filaments may be slightly altered, but this is still referred to as the "number of filaments." Furthermore, even if the original filaments are plate-shaped, they become roughly elongated after crushing, and are therefore also referred to as the "number of filaments."

[0052] <Pre-crushing> The carbon fibers used as raw materials may be in the form of chopped fibers or milled fibers, but the chopped fiber form is preferable. In the case of milled fibers, many short fibers of about 1 μm in length are mixed in, so in order to obtain carbon nanofibers with a large aspect ratio, the chopped fiber form is preferable. In the case of milled fibers, even if it is stated that the "average length is 70 μm," many short fibers of about 1 μm in length may be mixed in.

[0053] While not limited to this, it is preferable that the carbon fibers used as raw material be made to an average size of 1 mm to 15 mm during pre-pulverization, 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-pulverization may be necessary. If the fibers are within the above range from the beginning, it is preferable not to perform pre-pulverization.

[0054] There are no particular limitations on the pre-grinding method; any commercially available dry grinder can be used, but examples of devices include cutter mills.

[0055] <Dry grinding> In the present invention, dry grinding is preferably air-jet grinding, cutter grinding, or "grinding that performs both air-jet grinding and cutter grinding simultaneously." "Grinding that performs both air-jet grinding and cutter grinding simultaneously" means "grinding that has both an air-jet grinding mechanism / function and a cutter grinding mechanism / function at the same time." Figure 5 shows scanning electron microscope (SEM) images of carbon fibers after dry grinding. Figure 5(a) is at 600x magnification, and Figure 5(b) is at 1500x magnification.

[0056] <<Airflow type grinding>> Examples of air-jet pulverization include pulverizers such as cyclone mills and pulverization using jet mills. Airflow pulverization using a cyclone mill generates an airflow by rotating an impeller (rotating blade), and the material introduced into the airflow is dry-pulverized to produce fine particles. Airflow pulverization using a jet mill, on the other hand, produces fine particles by impacting the material against a collision plate to dry-pulverize it. Compared to pulverizers that do not have rotating parts such as impellers, rotors, blades, or rotating blades (such as jet mills), air-jet pulverizers that have such parts, or pulverizers that perform both air-jet and cutter-type pulverization simultaneously (described later), are preferable as "dry pulverization before wet pulverization" in order to obtain carbon nanofiber groups of a predetermined shape through wet pulverization.

[0057] Commercially available devices can also be suitably used as cyclone mills. Examples of commercially available products include the cyclone mill manufactured by Shizuoka Seiki Co., Ltd., the Super Powder Mill manufactured by Nishimura Machine Works Co., Ltd., the Tornado Mill manufactured by Sansho Industry Co., Ltd., and the Dream Mill manufactured by Furukawa Sangyo Systems Co., Ltd.

[0058] The structure of the cyclone mill described above is not particularly limited, but it is especially preferable to have one or more impellers and to crush the materials to be crushed mainly by causing them to collide with each other using the swirling airflow generated by the impellers, as this makes it easier to achieve the effects of using the airflow crusher and results in very little metal contamination.

[0059] Commercially available jet mills can also be suitably used. Examples of manufacturers that offer such equipment include Seishin Corporation, Hosokawa Micron Corporation, Nippon Pneumatic Co., Ltd., and Nisshin Engineering Co., Ltd.

[0060] <<Cutter-type crushing>> Furthermore, cutter-type grinding methods include grinding using crusher mills, pin mills, cutter mills, hammer mills, axial flow mills, and the like.

[0061] <> In the present invention, the dry grinding method is particularly preferably one that performs both air-jet grinding and cutter-type grinding simultaneously. In particular, the dry grinding in the present invention is preferably carried out using a dry grinder that has blades and applies shearing and impacting. Alternatively, it is preferable to carry out the dry grinding using a dry grinder that applies "shearing and impacting with blades". A schematic diagram of an example of such a dry grinder is shown in Figure 9.

[0062] In the case of "all-airflow grinders that do not use impellers to grind" such as Jet Mill, Cyclone Mill, Tornado Mill, and Dream Mill (registered trademark), depending on the raw material, the diameter may become too small during the dry grinding stage, which may result in a smaller aspect ratio (a rounder shape) during the subsequent wet grinding stage. Therefore, although not limited to these, they may not be very suitable as a grinding step before wet grinding.

[0063] The ambient temperature or set temperature for dry grinding is not particularly limited and should be determined according to the operating method of the equipment used, but is preferably 0°C to 50°C, and particularly preferably 5°C to 35°C. Furthermore, the impeller rotation speed should be determined according to the operating method of the device being used, but preferably it is 4000 rpm to 20000 rpm, and particularly preferably 8000 rpm to 15000 rpm.

[0064] Using the apparatus described above and the grinding method described above, the material is dry-ground until the number-average length is 100 μm or less, and then subjected to the next process. By dry grinding the material to a size of 100 μm or less, followed by wet grinding, the diameter, length, aspect ratio, shape distribution, etc., of the carbon nanofibers are more likely to fall within the essential or preferred ranges described above.

[0065] Dry grinding is essential to reduce the number average length to 100 μm or less, but preferably it should be between 5 μm and 70 μm, more preferably between 7 μm and 50 μm, and particularly preferably between 10 μm and 40 μm. If the number-average length after dry grinding is too long, even if the conditions for the subsequent wet grinding process are adjusted, it may be difficult for the final diameter and length of the carbon nanofibers to fall within the aforementioned range. On the other hand, if the length after dry grinding is too short, the length of the carbon nanofibers cannot exceed that after wet grinding, making it difficult for the final carbon nanofiber length and average aspect ratio to reach the desirable range mentioned above. In particular, the aspect ratio of the final carbon nanofibers may become too small.

[0066] Regarding the number-average diameter after dry grinding, it is difficult to reduce it significantly with dry grinding alone, meaning it is difficult to grind it in a way that increases the aspect ratio. Furthermore, if the diameter is forcibly reduced through dry grinding, the length will also be shortened, making it difficult for the final aspect ratio after wet grinding to fall within a desirable range. The diameter after dry grinding 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 perform dry grinding to achieve this range.

[0067] The number-average aspect ratio after dry grinding is not particularly limited, but is preferably 10 or less, more preferably 1.2 to 7, and most preferably 1.5 to 5. With dry grinding, it is inherently difficult to increase the number-mean aspect ratio beyond the lower limit mentioned above. In other words, it is difficult to reduce the average fiber diameter to a degree that allows the number-mean aspect ratio to exceed the lower limit mentioned above. This invention was made based on the discovery that by dry grinding, the number average length can be reduced to 100 μm or less, and by having a relatively large diameter and a relatively small number average aspect ratio, or by doing so, a group of carbon nanofibers with the aforementioned suitable "diameter, length, and large number average aspect ratio" can be obtained through subsequent wet grinding.

[0068] <Heat treatment> If resin is present in the dry-pulverized material, it is preferable to remove the resin by heat treatment. In other words, in the method for producing carbon nanofibers of the present invention, it is preferable to remove the mixed resin by performing heat treatment between the dry-pulverization and wet-pulverization steps. Here, the case in which resin is present includes, for example, cases where the raw materials contain sizing agents, etc. That is, the mixed resin can be the sizing agent, etc. If there is no presence of resins such as sizing agents, the heat treatment can be omitted.

[0069] The heat treatment conditions are not limited, but for example, heating in air or an inert atmosphere at a furnace temperature of 320°C to 480°C for 5 to 15 minutes reduces the resin content to preferably 0.1% by mass or less, and particularly preferably 0.01% by mass or less. Heat treatment enhances the grinding and dispersion effects of wetting agents and surfactants used in subsequent processes (such as wet treatment or wet grinding if wet treatment is performed).

[0070] <Wetting treatment> While not limited to this, further wetting treatment is preferable. In particular, wetting treatment after the dry grinding or after the heat treatment is especially preferable. The wetting treatment preferably involves immersing the material obtained above in an aqueous solution containing an anionic surfactant, a cationic surfactant, or an amphoteric surfactant. The surfactant can also be suitably used in subsequent wet grinding.

[0071] The above anionic surfactant is preferably a high molecular weight anionic surfactant (the term "high molecular weight" includes oligomers), and more preferably an alkali metal salt, ammonium salt, alkylammonium salt, alkylol ammonium salt, etc., of an (co)polymer having an acid group. The anionic surfactants described above may be used individually or in combination of multiple types.

[0072] The above-mentioned "(co)polymer having an acid group" is particularly preferably at least one (co)polymer selected from the group consisting of (meth)acrylic acid (co)polymers, (phthalic anhydride) (co)polymers, vinylbenzenesulfonic acid (co)polymers, and naphthalenesulfonic acid (co)condensates. Here, the notations "(co)", "(meth)", and "(anhydride)" indicate whether or not they are enclosed in parentheses. In the case of copolymers, there are no particular limitations on the copolymer monomers, but examples include alkyl (meth)acrylates, hydroxyalkyl (meth)acrylates, styrene, vinyl acetate, etc. (Co)condensates of naphthalene sulfonic acid include those in which the ring is bonded with an aldehyde such as formaldehyde. Examples of cocondensation monomers in the case of cocondensates include phenol, cresol, and naphthol.

[0073] Furthermore, the cationic surfactant is preferably a surfactant in which the quaternary ammonium is the hydrophilic group, and the quaternary ammonium is "N + There are no particular limitations on the substituents to ", but alkyl groups (which may have substituents) such as stearyl, palmityl, dodecyl, methyl, benzyl, and butyl groups are preferred. Long-chain alkyl groups with 12 or more carbon atoms are also preferred. There are no particular limitations on the counter-anion, but halogen ions such as chloride ions and bromide ions are especially preferred.

[0074] Examples of amphoteric surfactants include alkyl betaine type, fatty acid amidopropyl betaine type, alkylimidazole type, amino acid type, and amine oxide type.

[0075] In particular, it is preferable to use an anionic surfactant or an amphoteric surfactant. The surfactant content and the carbon fiber content (after dry grinding) in the aqueous dispersion medium are the same as the numerical ranges for <wet grinding> described below.

[0076] By using a surfactant, and further by using the preferred anionic surfactant or amphoteric surfactant described above, it is possible to unravel the carbon fibers lengthwise, reduce their diameter while maintaining their length, and obtain carbon nanofibers (groups) with a large average aspect ratio.

[0077] <Wet rum> In the present invention, wet grinding is essential after dry grinding. The wet grinding is not particularly limited, but is preferably bead mill grinding or ball mill grinding. Particularly preferably, bead mill grinding or ball mill grinding is performed in an aqueous medium containing a surfactant. Furthermore, other treatments besides the wetting treatment described above may be inserted between dry grinding and wet grinding. Examples of such "other treatments" include pre-mixing and pre-preparation of liquids.

[0078] The surfactants mentioned above are those listed in the <Wetting Treatment> section above, regardless of whether the above wetting treatment is performed or not. Similar types of surfactants are also preferred. Specifically, anionic surfactants, cationic surfactants, or amphoteric surfactants, as described above in the <Wetting Treatment> section, are preferred. The surfactant used during the wetting treatment may be used as is in the wet grinding, or a new surfactant may be added, or a different type of surfactant may be added during the wet grinding process.

[0079] The amount of surfactant used is not particularly limited, but it is preferably 30 parts by mass or less, more preferably 0.1 parts by mass or more and 20 parts by mass or less, and particularly preferably 0.5 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of carbon fibers (carbon nanofibers in the process of being crushed) to be crushed and dispersed (the total amount when two or more types of surfactants are used in combination). In other words, in the method for producing the carbon nanofiber group of the present invention, it is desirable that the amount of the above surfactant present is 0.1 parts by mass or more and 30 parts by mass or less per 100 parts by mass of carbon fibers.

[0080] Using too much surfactant can cause the carbon fibers to aggregate as they dissolve vertically. As a result, the carbon nanofibers become dispersed in an aggregated state, which can affect the physical properties of the resulting material when applied to an object.

[0081] When a wet treatment is performed, a new surfactant may be added during wet grinding, or the surfactant that was used during the wet treatment may be used as is. When adding a surfactant during wet grinding, it may be the same surfactant as used in the wet treatment, or it may be a different one.

[0082] <<Methods, equipment, and conditions for wet grinding>> By wet grinding, it is possible to produce carbon nanofibers with the specific shapes (diameter, length, aspect ratio) and particle size distributions described above. The conditions for wet grinding are adjusted to obtain carbon nanofibers (groups) of the specific shape (diameter, length, aspect ratio) described above in the present invention. Figure 6 shows an optical microscope (biological microscope) image taken during wet grinding, and Figure 7 shows an example of a scanning electron microscope (SEM) image taken after wet grinding. Figure 7(a) is at 1500x magnification, and Figure 7(b) is at 6500x magnification.

[0083] Preferred materials for the grinding media used in wet grinding include glass, alumina, zircon (zirconia-silica ceramics), zirconia, and metal (steel).

[0084] Taking a bead mill as an example, the diameter of the beads used is preferably 0.1 mm to 3 mm, more preferably 0.2 mm to 2 mm, and particularly preferably 0.3 mm to 1 mm. If the bead diameter is too large, the number of beads in the bead mill container decreases, reducing the contact points, which can lead to problems such as inadequate grinding and dispersion, or inability to grind to a sufficiently small diameter. On the other hand, if the bead diameter is too small, it can lead to problems such as inadequate grinding and dispersion, or inadequate grinding time.

[0085] The bead filling rate used in the bead mill is preferably 45% to 90%, more preferably 55% to 87%, and particularly preferably 65% ​​to 85%. If the bead packing density is too low, the carbon fibers may not split lengthwise easily, making it difficult to create carbon nanofibers with a large aspect ratio. On the other hand, if the bead packing density is too high, the stirring blades of the bead mill may not rotate easily.

[0086] For the entire slurry to be treated with the bead mill, the carbon fibers after dry grinding are preferably 1% to 20% by mass, more preferably 3% to 15% by mass, and particularly preferably 5% to 10% by mass.

[0087] The shape of the stirring blade used in the bead milling process described above is not particularly limited. The rotational speed of the agitator blade depends on the length of the blade and the capacity of the bead mill, but when converted to a capacity of 2L, it is preferably 600 rpm to 4500 rpm, more preferably 800 rpm to 4000 rpm, and particularly preferably 1000 rpm to 3500 rpm. The peripheral speed at the tip of the agitator blade depends on the span of the blade, but it is preferable to use a range that can be calculated from the rotational speed assuming a diameter of 20 cm. Specifically, it is preferable to use a range of 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.

[0088] The operation method for grinding and dispersing in a bead mill can be either a circulating system or a batch system, but the circulating system is preferred. In the circulating system, there is no transfer to a container as in the batch system, so agglomeration does not occur during that process. When using a circulating system, the degree of refinement changes depending on the number of passes. For example, if the residence time per pass is increased, the particle size distribution becomes sharper because there are no short passes of the processed material, but the aspect ratio of the carbon nanofibers also becomes smaller. Therefore, for example, when converted to 4L, the bead milling process is preferably carried out by circulating for 70 minutes to 270 minutes, more preferably 80 minutes to 230 minutes, and particularly preferably 90 minutes to 180 minutes.

[0089] The temperature during the wet process is preferably 0°C to 50°C, and particularly preferably 5°C to 35°C. The bead mill may be vertical or horizontal. In addition, commercially available bead mills can also be used. Examples of commercially available devices include the Dyno Mill from Willy e Bakoffen (WAB) and the bead mill from Netsch (USA).

[0090] The time per pass (continuous operation time), the number of passes, and the total time may depend on the apparatus structure, slurry concentration, grinding and dispersion conditions, the type of surfactant, etc. Therefore, it is preferable to observe the sample after wet grinding or midway through the process using a particle size distribution analyzer, optical microscope, scanning electron microscope (SEM), etc., and adjust the settings accordingly.

[0091] By using the aforementioned "manufacturing method requiring dry and wet grinding" and appropriately adjusting the grinding conditions within the aforementioned range, carbon nanofibers (groups) with a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm can be obtained. Carbon nanofibers can be obtained that are either individually separable or dispersed individually or in a dispersible state. Furthermore, carbon nanofibers (groups) with a number-average aspect ratio of 5 to 200 can be obtained. The present invention also refers to a group of carbon nanofibers produced by the above-described method for producing a group of carbon nanofibers.

[0092] <Agglomeration prevention treatment> After wet grinding, it is particularly preferable, though not limited to, to further apply an anti-aggregation treatment. The anti-aggregation treatment is not limited to the above, but it is desirable to further include in the slurry of carbon nanofibers obtained after the wet grinding of the above-mentioned material one or more anti-aggregates selected from the group consisting of metal-containing anti-aggregates, coblock polymers, comb-type coblock polymers, and surfactants to perform the anti-aggregation treatment.

[0093] Specifically, although not particularly limited, it is preferable to use a metal-containing anti-coagulant such as a (complex) metal chelate compound, (complex) metal oxide fine particles, metal-containing wax, or (complex) metal ion water; a coblock polymer having polyester, polyacrylate, polyurethane, etc. as units; a comb-type coblock polymer having the polymer as units; or a surfactant. This anti-coagulation treatment may be performed immediately after the wet grinding described above, after the water removal treatment described later, or at both stages.

[0094] When using a metal-containing anti-coagulant as an anti-coagulant in an anti-coagulation treatment, (complex) metal chelate compounds are more preferred, and metal salts such as HEDTA, EDTA, PDTA, NTA, ethylenediamine, bipyridine, phenanthroline, and porphyrin are even more preferred. Among these, metal salts such as HEDTA (hydroxyethyl ethylene diamine triacetic acid), ethylenediaminetetraacetic acid (EDTA (ethylenediaminetetraacetic acid)), PDTA (1,3-propanediamine tetraacetic acid), and NTA (nitrilo triacetic acid) are particularly preferred.

[0095] Furthermore, preferred surfactants for use in the anti-aggregation treatment include anionic surfactants, cationic surfactants, or amphoteric surfactants, with anionic surfactants or amphoteric surfactants being more preferred. While not limited to these, particularly preferred specific surfactants include those similar to those described in the sections on <Wetting Treatment> and <Wet Grinding>.

[0096] For metal-containing anti-flocculating agents, surfactants used in anti-flocculating treatments, and anti-flocculating agents such as coblock polymers, the type is determined by considering the surface condition of the object to be flocculated immediately before the anti-flocculating treatment. When the type of anti-coagulant used is one of the above, such as a metal-containing anti-coagulant, surfactant, or co-block polymer, coagulation becomes less likely during the removal process of the dispersion medium (water) and over time, resulting in improved storage stability.

[0097] The amount of "(composite) metal-containing anti-flocculants such as metal chelate compounds; surfactants used in anti-flocculant treatment; coblock polymers; and other anti-flocculants" used is not particularly limited, but it is preferable to add them in an amount of 0.1% by mass or less relative to the total amount of slurry after wet grinding (or the total amount when two or more anti-flocculants are used in combination), more preferably in an amount of 0.0001% by mass or more and 0.06% by mass or less, and particularly preferably in an amount of 0.0002% by mass or more and 0.03% by mass or less.

[0098] Furthermore, while there are no particular limitations on the amount of the above-mentioned anti-flocculation agent used, it is preferable to add it at a rate of 1% by mass or less relative to the total amount of the material to be prevented from flocculating, such as carbon nanofibers (or the total amount when two or more anti-flocculation agents are used in combination), more preferably at 0.001% by mass or more and 0.5% by mass or less, even more preferably at 0.002% by mass or more and 0.3% by mass or less, and particularly preferably at 0.003% by mass or more and 0.1% by mass or less.

[0099] In other words, in the method for producing the carbon nanofiber group of the present invention, it is desirable to add the above-mentioned anti-aggregation agent in an amount of 0.0001% by mass or more and 0.1% by mass or less to the entire slurry after wet grinding, or in an amount of 0.001% by mass or more and 1% by mass or less to the total amount of the carbon nanofiber group, which is the object to be prevented from agglomerating.

[0100] When the amount of "anti-coagulants such as metal-containing anti-coagulants, surfactants, and coblock polymers" used is within the above range, coagulation becomes less likely during the removal process of the dispersion medium (water) and over time, resulting in improved storage stability.

[0101] There are no particular limitations on stirring during the anti-coagulation treatment, but examples include stirring with a hand mixer. There are no particular limitations on the stirring speed, but 300 to 1200 rpm is preferred, and 500 to 1000 rpm is particularly preferred. The temperature for the anti-aggregation treatment is not particularly limited, but is preferably 20°C to 100°C, more preferably 40°C to 90°C, and especially preferably 60°C to 80°C.

[0102] <Water removal treatment> The present invention also refers to a group of carbon nanofibers produced by the above-described method for producing a group of carbon nanofibers. In other words, the carbon nanofibers may be those contained in the slurry after wet grinding, those contained in the slurry after the above-mentioned anti-aggregation treatment, or those in powder form after water has been removed from the slurry. Carbon nanofibers in any of the above states can be used as finished products for a variety of applications. The morphology of the carbon nanofiber group (after water removal treatment) is almost identical to that after wet grinding; therefore, the carbon nanofiber group of the present invention is similar to the scanning electron microscope (SEM) image after wet grinding shown in Figure 7. Figure 7(a) is at 1500x magnification, and Figure 7(b) is at 6500x magnification.

[0103] The method for water removal is not particularly limited and can be carried out by reducing pressure and / or increasing temperature. It is particularly preferable to partially dry the material using a cyclone separation and recovery method, and then remove (dry) the water in an oven by reducing pressure and / or increasing temperature. The temperature for the water removal treatment is not particularly limited, but is preferably 40°C to 160°C, more preferably 55°C to 150°C, and particularly preferably 70°C to 130°C.

[0104] Furthermore, to remove surfactants and other contaminants, it is preferable to calcine the product at a temperature of, for example, 250°C to 400°C. The carbon nanofibers that make up the group of carbon nanofibers produced by the manufacturing method of the present invention have good dispersibility, so it is also preferable that no dispersants or surfactants are attached to their surface.

[0105] It is also preferable to perform the aforementioned anti-coagulation treatment after the water removal treatment. That is, it is also preferable to incorporate the aforementioned anti-coagulation agent or surfactant into the concentrated slurry or powder after the water removal treatment.

[0106] The powdered carbon nanofibers produced by the manufacturing method of the present invention (even in solid form) readily disperse and diffuse without agglomerating in resin emulsions or resins of the intended application. Examples of such resins include thermoplastic resins and thermosetting resins, and in the case of thermosetting resins, they disperse well in both the main component and the curing agent. While not limited to specific resin emulsions, examples of resins with particularly good dispersibility include (meth)acrylic resins, styrene-(anhydride)maleic acid resins, and urethane resins. Among thermoplastic and thermosetting resins, those listed below are preferred.

[0107] <Carbon nanofiber dispersion> The present invention is also a carbon nanofiber dispersion containing "a group of carbon nanofibers produced by the method for producing the carbon nanofiber group described above." The carbon nanofiber dispersion may be the slurry itself after the wet grinding or anti-aggregation treatment described above, or it may be obtained by adding a new dispersion medium to the slurry or by replacing the dispersion medium with a new dispersion medium, or it may be obtained by re-dispersing the carbon nanofiber powder after the water removal treatment described above.

[0108] Carbon nanofiber dispersions can also be applied to resins or resin emulsions at the application site without agglomerating, maintaining their dispersed state.

[0109] <Carbon nanofiber-containing resin and molded articles using the same> The present invention also relates to a carbon nanofiber-containing resin and a molded article using the same, characterized by containing the carbon nanofiber group described above and a thermoplastic resin or 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, polyethersulfone, polyarylate, polyetheretherketone, thermoplastic polyimide, and polyamideimide. The carbon nanofibers(s) of the present invention are suitably dispersed in thermoplastic resins and exhibit the superior effects described above compared to other carbon nanofibers(s).

[0110] Examples of thermosetting resins include phenolic resins, epoxy resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, thermosetting polyurethanes, and thermosetting polyimides. The carbon nanofibers of the present invention are suitably dispersed in the main component (unreacted resin having functional groups) and / or the curing agent (which crosslinks, reacts, and polymerizes the functional groups) of a thermosetting resin, exhibiting the aforementioned superior mechanical, electrical, and thermal effects compared to other carbon nanofibers (groups).

[0111] Whether it is a thermoplastic resin or a thermosetting resin, it is preferable that the carbon nanofibers (group) of the present invention be contained in an amount of 1% by mass or more and 80% by mass or less of the total carbon nanofiber-containing resin, more preferably 2% by mass or more and 70% by mass or less, and particularly preferably 5% by mass or more and 60% by mass or less.

[0112] <Matrix resin for fiber-reinforced plastics> The present invention is also a fiber-reinforced carbon nanofiber-containing resin characterized by containing the carbon nanofiber group and resin described above. If we refer to the resin to be reinforced by the fiber substrate as the "matrix resin," then, to rephrase the above, the present invention is also a matrix resin for fiber-reinforced plastics characterized by containing the carbon nanofiber group and resin described above.

[0113] Thermosetting resins are preferred as the resin because they have excellent mechanical strength and other properties, and their characteristics are more easily exhibited when reinforced with a fiber base material. The carbon nanofibers(s) of the present invention are dispersed in a resin, preferably in a thermosetting resin, and impregnated into various fibrous substrates as a matrix (resin) to exhibit the effects described above.

[0114] The (particularly) preferred content of the carbon nanofiber group of the present invention in the entire matrix (resin) for fiber-reinforced plastics is within the range described above. Furthermore, the aforementioned materials are examples of preferred thermosetting resins. [Examples]

[0115] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples unless it exceeds the essence of the invention.

[0116] Example 1 <Preparation of raw carbon fiber> <Dry grinding> Approximately 6mm chopped fibers, which are random-type mesophase-pitch carbon fibers that have not undergone sizing treatment, were dry-ground at 30°C for 10 minutes using a shearing and impact grinder with blades, as shown in Figure 9, without pre-grinding. The goal was to create fibers with a diameter of 10μm and a length range of 10μm to 40μm, with 90% of the total fibers being in that range.

[0117] <Wetting treatment> A slurry was obtained by mixing and stirring 100 parts by mass of dry-ground carbon fibers (e.g., Figure 5(a)(b)) that had not been heat-treated, 1 part by mass of a wetting agent (surfactant), and 1500 parts by mass of purified water. Here, the wetting agent (surfactant) was an ammonium salt of a compound having an acidic group.

[0118] The slurry described above was subjected to a wetting treatment by stirring it with a hand mixer at 800 rpm for 10 minutes at 30°C.

[0119] <Wet rum> Using a 0.6L bead mill, with beads of 0.3mm diameter, a bead filling rate of 60%, a vessel motor rotation speed of 1500 rpm, and a tubular circulation pump, the 4L slurry obtained above was circulated for more than 90 minutes at a transfer rate of 500mL per minute.

[0120] <Agglomeration prevention treatment> After transferring 3500 mL of the obtained slurry from the bead mill container to another container, 0.01% by mass of Coblock polymer was added as an anti-flocculation agent relative to the total slurry volume (3500 mL). After addition, the mixture was stirred at room temperature (15-25°C) using a hand mixer at 800 rpm for 5 minutes to prevent flocculation.

[0121] <Water removal treatment> The partially dried material was collected using a cyclone separation and recovery method by applying heat and reduced pressure. The material was then heated in a 150°C oven for 240 minutes to remove water and prepare a solid group of carbon nanofibers. Subsequently, it was heated in a 260°C oven for 1 hour to remove surfactants and other contaminants.

[0122] <Rating> When the obtained solid carbon nanofibers and the carbon nanofibers in the dispersion (slurry) before water removal treatment were observed using an optical microscope and a scanning electron microscope and measured as described above, a carbon nanofiber group was obtained in which approximately 90% of the total carbon nanofibers were distributed within the range of a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm.

[0123] The obtained solid carbon nanofibers were added to aqueous emulsions of acrylic resin, styrene-(anhydrous) maleic acid resin, and polyurethane resin, respectively, and stirred normally. As a result, approximately one carbon nanofiber at a time was suitably dispersed in the water from the solid carbon nanofibers. No aggregation occurred during dispersion, nor over time.

[0124] Furthermore, when the obtained solid carbon nanofibers were dispersed in a general-purpose thermoplastic resin using a conventional kneader, approximately one nanofiber at a time was found to be suitably dispersed in the matrix resin.

[0125] Furthermore, when the obtained solid carbon nanofibers were dispersed in the curing agent side of any general-purpose thermosetting resin using a standard stirrer, approximately one nanofiber at a time was found to be suitably dispersed in the curing agent. When a curing agent containing dispersed carbon nanofibers was mixed with a main component containing either epoxy resin or urethane resin, the dispersion was maintained without aggregation, and both mixtures could be suitably used as thermosetting resins.

[0126] Furthermore, the same results as above were obtained when, in the aforementioned <water removal treatment>, heating and reduced pressure were applied, the partially dried material was recovered using a cyclone separation and recovery method, and the water was removed by heating in a 120°C oven for 240 minutes to prepare a group of solid carbon nanofibers.

[0127] Carbon nanofibers were prepared in the same manner as described above, except that, for comparison, isotropic pitch carbon fibers, radial mesophase pitch carbon fibers, and onion mesophase pitch carbon fibers were used instead of random mesophase pitch carbon fibers as raw materials.

[0128] The ease of manufacturing "carbon nanofibers having a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm, preferably with a number-average aspect ratio of 3 or more" was as follows: Regarding ">>", ">", and ≒", the higher (left) they are, the better they are. The degree of superiority or inferiority is also indicated by ">>", ">", and ≒. Random-type mesophase pitch carbon fiber >>Radial type mesophase pitch carbon fiber ≒ Onion-type mesophase pitch carbon fiber >>Isotropic pitch-based carbon fiber (Comparative Example 1 (described later), actually could not be manufactured) ≒ PAN-based carbon fiber (Comparative Example 1 (described later), actually could not be manufactured)

[0129] Furthermore, the order of dispersibility, dispersion stability, high-concentration dispersibility, thermal properties, mechanical properties, and electrical properties was generally as described above. The degree of superiority or inferiority was also as indicated by ">>", ">", and "≒" above.

[0130] Comparative Example 1 In an attempt to prepare a group of carbon nanofibers in the same manner as in Example 1, except that PAN-based carbon fibers, isotropic pitch-based carbon fibers, radial-type mesophase-pitch-based carbon fibers, and onion-type mesophase-pitch-based carbon fibers were used as raw materials instead of random-type mesophase-pitch-based carbon fibers, however, in all cases, a group of carbon nanofibers was not obtained in which the number-average aspect ratio was less than 3, the diameter was between 30 nm and 1000 nm, and the length was between 0.2 μm and 70 μm, with more than 50 percent of the total distributed within the above range.

[0131] Example 2 A group of carbon nanofibers was obtained in the same manner as in Example 1, except that milled fibers with a diameter of 10 μm and a length of 70 μm were used as the raw material instead of chopped fibers. Because many short nanofibers, approximately 1 μm in length, were present, the number-average aspect ratio tended to be small. However, a good group of carbon nanofibers was obtained, and the evaluation of dispersibility and other properties was also good.

[0132] Example 3 In this example, instead of using the "6mm chopped fiber" from Example 1, a long fiber bobbin type was used as the raw material. Although pre-grinding with a cutter mill was necessary, a good group of carbon nanofibers was obtained in the same manner as in Example 1, and the evaluation of dispersibility and other properties was also good.

[0133] Example 4 The process was carried out in the same manner as in Example 1, except that, in the dry grinding stage, instead of using a "shearing and impact grinder with blades," an all-airflow grinder such as a jet mill, cyclone mill, tornado mill, or dream mill, which does not grind with impellers, blades, etc. (or does not use impellers, etc.), was used.

[0134] While a suitable group of carbon nanofibers was obtained and the dispersibility and other properties were evaluated favorably, there was a slight tendency for the number-average aspect ratio to decrease during the dry grinding stage, and this tendency persisted even after the subsequent wet grinding.

[0135] Example 5 In Example 1, a group of carbon nanofibers was obtained in the same manner as in Example 1, except that sized random mesophase pitch carbon fibers were used as raw materials instead of unsized random mesophase pitch carbon fibers, and that heat treatment was performed after dry grinding. The heat treatment was performed by heating in an electric furnace at a temperature of 400°C for 10 minutes in an inert atmosphere.

[0136] The epoxy resin content in the raw materials was reduced to less than 0.01% by mass. As a result, it was suggested that the surfactant (wetting agent) effectively penetrated the filaments (in the gaps between the individual filaments), resulting in the formation of carbon nanofibers with a favorably large number-average aspect ratio.

[0137] Example 6 In Example 1, no wetting treatment was performed. Instead, the same wetting agent (surfactant) used in the wetting treatment in Example 1 was added as a surfactant in the wet milling process, at a concentration of 0.06 parts by mass relative to the total amount of the material to be prevented from flocculating (slurry). Otherwise, the carbon nanofiber group was obtained in the same manner as in Example 1, including the mixing ratio (blending amount), etc.

[0138] Although the effect of the wetting agent (surfactant) was slightly reduced compared to Example 1, and the number-average aspect ratio tended to be smaller, a good group of carbon nanofibers was obtained, and the evaluation of dispersibility and other properties was also good.

[0139] Example 7 Except for using a carbobetaine-type, imidazoline-type, amidebetaine-type, amidesulfobetaine-type, or amideamine oxide-type amphoteric surfactant instead of the surfactant used in Example 6 (the wetting agent (surfactant) used in the wetting treatment in Example 1), several types of carbon nanofibers were obtained in the same manner as in Example 6. Those using amphoteric surfactants showed good foaming properties in hard water and across a wide pH range. As the carbobetaine-type amphoteric surfactant, softazoline (manufactured by Kawaken Fine Chemicals Co., Ltd.) was used.

[0140] Similar to Examples 1 and 6, a good group of carbon nanofibers was obtained, and the evaluation of dispersibility and other properties was also good.

[0141] Example 8 Using the raw materials of Example 5, a group of carbon nanofibers was obtained in the same manner as in Example 1, except that the anti-aggregation treatment was not performed.

[0142] A good group of carbon nanofibers was obtained, and the evaluation of dispersibility and other properties was also satisfactory. However, the resulting carbon nanofiber dispersion showed a slight tendency to aggregate over time compared to the dispersion obtained in Example 1, but this was at a level that did not pose a problem.

[0143] Example 9 In Example 1, a carbon nanofiber dispersion was obtained without water removal treatment. It exhibited excellent dispersibility and could be provided as a dispersion for the following applications.

[0144] Example 10 In Example 1, the order of the anti-aggregation treatment and the water removal treatment was reversed, and the target material obtained by the water removal treatment to a concentrated slurry or powder was blended with the "Coblock polymer, which is an anti-aggregation agent" from Example 1. Otherwise, a group of carbon nanofibers was obtained in the same manner as in Example 1.

[0145] A good group of carbon nanofibers was obtained, and evaluations of dispersibility and dispersion stability were also favorable.

[0146] Comparative Example 2 In Example 1, an attempt was made to obtain carbon nanofibers by dry grinding alone without wet grinding, but no carbon nanofiber group was obtained in which more than 50% of the total carbon nanofibers were distributed within the above range, with a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm.

[0147] Comparative Example 3 In Example 1, an attempt was made to obtain carbon nanofibers by wet grinding alone without dry grinding, but the grinding did not proceed, and a group of carbon nanofibers in which more than 50% of the total carbon nanofibers were distributed within the above range, with a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm, could not be obtained.

[0148] Example 11 The carbon nanofibers obtained in Example 1 and Example 8 were mixed with polycarbonate (Teijin Limited, Panlite L-1225Z 100®) as a thermoplastic resin to obtain a carbon nanofiber-containing resin at a concentration of 10% by mass relative to the total.

[0149] The resulting molded articles of carbon nanofiber-containing resin were subjected to mechanical strength measurements, specifically bending strength and rigidity, using a dedicated measuring device. As a result, the 10% by mass blend showed an improvement of 20% or more in the above physical properties compared to a similar polycarbonate that does not contain carbon nanofibers. [Industrial applicability]

[0150] The carbon nanofiber group obtained using the carbon nanofiber group manufacturing method of the present invention is an aggregate of individual carbon nanofibers with extremely excellent shapes, such as a large aspect ratio and fineness. Furthermore, it contains almost no impurities other than the carbon nanofibers themselves, has good dispersibility in both liquids (aqueous media, etc.) and (molten) solids (matrix resins, etc.), is resistant to aggregation, and can be incorporated into resins for molding. Therefore, it can be widely used in various fields where various properties are required, such as heat-resistant objects, thermally conductive objects, electrically conductive objects, high mechanical strength objects, wear-resistant objects, radio wave shielding / absorbing objects, and jet-black objects, in the form of dispersions, films, paints, structures, layers (membranes), circuits, powders, etc. [Explanation of symbols]

[0151] 10... Filament 20... Raw filament

Claims

1. A method for producing a group of carbon nanofibers in which carbon nanofibers having a diameter of 30 nm to 1000 nm and a length of 0.2 μm to 70 μm are distributed within the above range, with 50% or more of the total being. This method involves using random-type mesophase pitch carbon fibers as raw materials, dry grinding them until the number average length is 100 μm or less, and then wet grinding them to produce the carbon nanofiber group with the above distribution. A method for producing a group of carbon nanofibers, characterized in that the number-average thickness or number-average diameter of the elementary filaments constituting the filaments constituting the random-type mesophase-pitch carbon fiber is 10 nm or more and 200 nm or less.

2. The method for producing a group of carbon nanofibers according to claim 1, wherein the carbon nanofibers are in a state in which they can be isolated one by one, or in a dispersed state or a dispersible state.

3. A method for producing a group of carbon nanofibers according to claim 1, wherein the number-average aspect ratio of the carbon nanofibers is 3 or more and 200 or less.

4. A method for producing a group of carbon nanofibers according to claim 1, wherein the elementary filaments constituting the filament are assembled in a range of 2 to 20 to form the carbon nanofiber.

5. The method for producing a group of carbon nanofibers according to claim 1, wherein the dry grinding is performed by air-jet grinding, cutter grinding, or grinding that is performed simultaneously by both air-jet grinding and cutter grinding.

6. The method for producing a group of carbon nanofibers according to claim 1, wherein the dry pulverization is performed using a dry pulverizer having blades and applying shearing and impact.

7. A method for producing a group of carbon nanofibers according to claim 1, wherein a heat treatment is performed between the dry grinding and the wet grinding to remove the mixed resin.

8. The method for producing a group of carbon nanofibers according to claim 1, wherein the wet grinding is performed in an aqueous medium containing a surfactant, and the method is to produce a group of carbon nanofibers using a bead mill or a ball mill.

9. The method for producing a group of carbon nanofibers according to claim 8, wherein the surfactant is an anionic surfactant, a cationic surfactant, or an amphoteric surfactant.

10. The method for producing a group of carbon nanofibers according to claim 8, wherein the amount of the surfactant present is 0.1 parts by mass or more and 30 parts by mass or less per 100 parts by mass of carbon fiber.

11. Furthermore, the method for producing a group of carbon nanofibers according to claim 1, wherein the slurry of the group of carbon nanofibers obtained after wet grinding is treated to prevent flocculation by adding one or more anti-flocculation agents selected from the group consisting of metal-containing anti-flocculation agents, coblock polymers, comb-type coblock polymers, and surfactants.

12. The method for producing a group of carbon nanofibers according to claim 11, wherein the anti-flocculation agent is added to the entire slurry after wet grinding in an amount of 0.0001% by mass or 0.1% by mass, or added to the total amount of the carbon nanofiber group, which is the object to be prevented from flocculating.

13. The method for producing a group of carbon nanofibers according to claim 1, wherein the wet grinding is performed in an aqueous medium containing a surfactant, and the method is to produce a group of carbon nanofibers using a bead mill or a ball mill.

14. A method for producing a carbon nanofiber dispersion, characterized by containing a group of carbon nanofibers produced by the method for producing a group of carbon nanofibers described in any one of claims 1 to 13.

15. A method for producing a carbon nanofiber-containing resin, characterized by containing a group of carbon nanofibers produced by the method for producing a group of carbon nanofibers described in any one of claims 1 to 13, and a thermoplastic resin or a thermosetting resin.

16. A method for manufacturing a molded article, characterized by molding a carbon nanofiber-containing resin produced by the method for manufacturing a carbon nanofiber-containing resin described in Claim 15.

17. A method for producing a matrix resin for fiber-reinforced plastics, characterized by containing a group of carbon nanofibers produced by the method for producing a group of carbon nanofibers described in any one of claims 1 to 13, and a thermoplastic resin or a thermosetting resin.

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