JP7402631B2 - Ultrafine carbon fiber mixture, manufacturing method thereof, and carbon-based conductive aid - Google Patents

Description

The present invention relates to a mixture consisting of ultrafine carbon fibers and a modified thermoplastic resin, and a method for producing the same. The present invention also relates to a carbon-based conductive additive containing this ultrafine carbon fiber mixture.

Carbon nanomaterials, especially ultrafine carbon fibers with an average fiber diameter of less than 1 μm, have excellent properties such as high crystallinity, high conductivity, high strength, high modulus of elasticity, and light weight, so they can be used for high performance. It is used as a nanofiller in composite materials. In addition to reinforcing nanofillers for the purpose of improving mechanical strength, the nanofillers can be used as additives for electrodes of various batteries and capacitors, and as electromagnetic shielding materials by taking advantage of the high electrical conductivity and high thermal conductivity of carbon materials. , conductive nanofiller for antistatic materials, nanofiller to be added to electrostatic paint for resins, and as an additive to heat dissipation materials are being considered. Furthermore, by taking advantage of its chemical stability, thermal stability, and fine structure characteristics as a carbon material, it is also expected to be used as a field emission material for flat displays and the like.

Patent Document 1 describes (1) a step of forming a precursor fiber from a mixture consisting of 100 parts by mass of a thermoplastic resin and 1 to 150 parts by mass of a thermoplastic carbon precursor such as pitch; (2) a step of forming a precursor fiber; (3) subjecting the fiber to a stabilization treatment to stabilize the thermoplastic carbon precursor in the precursor fiber to form a stabilized resin composition; (3) extracting the thermoplastic resin from the stabilized resin composition under reduced pressure; A method for producing carbon fibers is disclosed, which includes the steps of removing the fibrous carbon precursor to form a fibrous carbon precursor, and (4) carbonizing or graphitizing the fibrous carbon precursor. In this carbon fiber manufacturing method, by setting specific conditions when removing the thermoplastic resin, a stabilized mesophase pitch modified product having predetermined properties can be efficiently obtained. The obtained stabilized mesophase pitch modified product does not contain hard carbon derived from a thermoplastic resin as a foreign substance even when it is converted into carbon fiber through a carbonization step and/or a graphitization step. Therefore, ultrafine carbon fibers with high physical properties can be provided.

Patent Document 2 discloses a lithium ion secondary battery configured by including a fibrous carbon material and granular carbon as a conductive additive in a positive electrode mixture.

International Publication No. 2009/125857 Japanese Patent Application Publication No. 11-176446

The manufacturing method described in Patent Document 1 efficiently forms a fibrous carbon precursor by setting specific conditions when removing the thermoplastic resin. That is, in the step of removing the thermoplastic resin, by reducing the residual amount of the thermoplastic resin as much as possible, hard carbon derived from the thermoplastic resin is prevented from remaining as foreign matter in the ultrafine carbon fibers. However, since the pressure reduction process requires a long time, there is also concern that it may be disadvantageous in terms of cost.

In the method described in Patent Document 2, it is necessary to mix fibrous carbon and particulate carbon at an appropriate ratio in order to improve the performance of the lithium ion secondary battery, which complicates the manufacturing process.

An object of the present invention is to provide a mixture containing ultrafine carbon fibers and a modified thermoplastic resin, which is amorphous carbon, at a low cost. Another object of the present invention is to provide a mixture of ultrafine carbon fibers that has excellent handling properties and allows the ultrafine carbon fibers to be randomly oriented three-dimensionally in an electrode without any special operations when used as a conductive additive. .

The present inventors have made extensive studies in view of the above-mentioned prior art. In general, amorphous carbon, such as a modified thermoplastic resin, has low conductivity and is not expected to have much effect on increasing the conductivity of electrodes or batteries. However, by mixing the modified product with a predetermined ultrafine carbon fiber, the highly linear ultrafine carbon fibers can be bundled, or the ultrafine carbon fibers can be prevented from being oriented in one direction in the electrode, and can be oriented randomly. We have found that it is possible to make it exist. As a result, it was discovered that conductive paths could be formed in various directions in the electrode, and that the conductivity of the electrode could be increased without preparing a mixture with a carbon material such as particulate carbon. That is, the present inventors have discovered that excellent performance can be exhibited by coexisting a modified thermoplastic resin with ultrafine carbon fibers, and have completed the present invention.

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

[1] Ultrafine carbon fibers with an average fiber diameter of 10 to 900 nm,
A modified thermoplastic resin,
An ultrafine carbon fiber mixture characterized by containing.

[2] The ultrafine carbon fiber mixture according to [1], wherein the modified thermoplastic resin is a carbide and/or hard carbon of a thermoplastic resin oxide.

The invention described in [1] and [2] above is an ultrafine carbon fiber mixture characterized in that ultrafine carbon fibers and amorphous carbon, which is a modified product of a thermoplastic resin, coexist.

[3] The ultrafine carbon fiber mixture according to [1] or [2], wherein some or all of the ultrafine carbon fibers are in the form of fiber bundles.

[4] The ultrafine carbon fiber mixture according to [3], wherein some or all of the ultrafine carbon fibers are bonded via a modified thermoplastic resin.

In the invention described in [3] and [4] above, at least a part of the ultrafine carbon fibers exists in the form of fiber bundles, and the fiber bundles are bundled with amorphous carbon, etc., which is a modified product of thermoplastic resin. It is a mixture of ultrafine carbon fibers.

[5] The ultrafine carbon fiber mixture according to any one of [1] to [4], wherein the mass ratio of the ultrafine carbon fiber to the modified thermoplastic resin is 1:0.05 to 1:1.5. .

[6] The modified thermoplastic resin is a modified product of at least one polyolefin selected from the group consisting of polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these [1] The ultrafine carbon fiber mixture according to any one of [5].

[7] The ultrafine carbon fiber according to any one of [1] to [6], wherein the average interplanar spacing d002 of the (002) planes measured by X-ray diffraction is in the range of 0.335 to 0.350 nm. Ultra-fine carbon fiber mixture.

[8] (1) A mesophase pitch composition consisting of a thermoplastic resin and 1 to 150 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin is molded in a molten state to form the mesophase pitch into fibers. a molding process to obtain a resin composite fiber;
(2) Stabilizing the mesophase pitch by bringing a gas containing oxygen into contact with the resin composite fiber, and oxidizing a part of the thermoplastic resin to generate a thermoplastic resin oxide to stabilize the resin composite. a stabilization/oxidation step to obtain fiber;
(3) a removing step of removing the unoxidized thermoplastic resin from the resin composite stabilized fiber to obtain an ultrafine carbon fiber precursor mixture consisting of an ultrafine carbon fiber precursor and a thermoplastic resin oxide;
(4) a high temperature heating step of heating the ultrafine carbon fiber precursor mixture to obtain ultrafine carbon fibers and a modified thermoplastic resin;
The method for producing an ultrafine carbon fiber mixture according to any one of [1] to [7], which comprises:

[9] Production of the ultrafine carbon fiber mixture according to [8], wherein the thermoplastic resin is at least one selected from the group consisting of polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these. Method.

[10] The ultrafine carbon fiber according to [8] or [9], wherein the thermoplastic resin is polyethylene with a melt mass flow rate (MFR) of 0.1 to 25 g/10 min as measured in accordance with JIS K 7210. Method of manufacturing the mixture.

[11] The mesophase pitch composition comprises the thermoplastic resin and 1 to 100 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin according to any one of [8] to [10]. A method for producing an ultrafine carbon fiber mixture.

[12] The method for producing an ultrafine carbon fiber mixture according to any one of [8] to [11], wherein the resin composite fiber has an average fiber diameter of 10 to 200 μm.

[13] The method for producing an ultrafine carbon fiber mixture according to any one of [8] to [12], wherein the stabilization/oxidation step includes heat treatment at 250 to 400° C. for 1 to 10 hours.

[14] The method for producing an ultrafine carbon fiber mixture according to any one of [8] to [13], wherein the heating temperature in the high-temperature heating step is 500 to 3500°C.

The inventions described in [9] to [14] above obtain resin composite fibers by fiberizing a resin composition consisting of a thermoplastic resin and mesophase pitch, which is a carbon fiber precursor, and the mesophase pitch in the resin composite fibers. At the same time, some thermoplastic resins are also oxidized. Thereafter, the unoxidized thermoplastic resin is removed, and then the resin is fired. Since some of the thermoplastic resin is oxidized, hard carbon (amorphous carbon), which is a modified product of the thermoplastic resin, remains in the ultrafine carbon fiber mixture.

[15] A carbon-based conductive additive comprising the ultrafine carbon fiber mixture according to any one of [1] to [7].

According to the present invention, an ultrafine carbon fiber mixture consisting of ultrafine carbon fibers and a modified thermoplastic resin can be provided at low cost. Since this ultrafine carbon fiber mixture contains a modified thermoplastic resin, it not only has high handling properties but also allows the ultrafine carbon fibers to be oriented in random directions. Therefore, when used as an electrode material (a conductive aid or a negative electrode) of a secondary battery, it can exhibit excellent conductivity. Furthermore, for the same reason, it is estimated that when incorporated into a filler of a composite material, it becomes easier to improve the isotropy of the material.

1 is a SEM photograph (500x magnification) of the ultrafine carbon fiber precursor mixture of Example 1. 1 is a SEM photograph (500x magnification) of the ultrafine carbon fiber mixture of Example 1. 1 is a SEM photograph (2000x magnification) of the ultrafine carbon fiber mixture of Example 1. 1 is a SEM photograph (2000x magnification) of the ultrafine carbon fiber mixture of Example 1. 1 is a SEM photograph (500x magnification) of the ultrafine carbon fiber precursor mixture of Example 2. It is a SEM photograph (500 times) of the ultrafine carbon fiber mixture of Example 2. It is a SEM photograph (2000 times) of the ultrafine carbon fiber mixture of Example 2. It is a SEM photograph (500 times) of the ultrafine carbon fiber precursor of Comparative Example 1. This is a SEM photograph (500x magnification) of the ultrafine carbon fiber of Comparative Example 1.

The present invention will be explained in detail below.
The present invention is an ultrafine carbon fiber mixture containing ultrafine carbon fibers having an average fiber diameter of 10 to 900 nm and a modified thermoplastic resin. It is preferable that some of the ultrafine carbon fibers constituting the ultrafine carbon fiber mixture of the present invention exist in the form of fiber bundles.

<Ultra-fine carbon fiber>
The ultrafine carbon fiber in the present invention has an average fiber diameter (D) of 100 to 900 nm. Ultrafine carbon fibers with an average fiber diameter in this range have excellent conductivity and durability, and can contribute to the formation of efficient conductive paths within electrodes, so they are particularly useful as conductive aids in lithium-ion secondary batteries, for example. It is. If the average fiber diameter is less than 100 nm, the bulk density will be very small and the handling properties will be poor. Furthermore, due to the fineness of the fibers, the fibers may be entangled with each other due to stirring during preparation of the slurry, resulting in a decrease in dispersibility. Furthermore, when the electrode mixture layer is produced, the electrode strength tends to decrease. When the average fiber diameter exceeds 900 nm, gaps tend to occur in the electrode mixture layer, and it may be difficult to increase the electrode density. The lower limit of the average fiber diameter (D) is preferably greater than 150 nm, more preferably greater than 200 nm. The upper limit of the average fiber diameter (D) is preferably 600 nm or less, more preferably 400 nm or less. Here, the fiber diameter in the present invention means a value measured from a photograph taken with a scanning electron microscope at a magnification of 5000 times.

In the present invention, the ultrafine carbon fibers may exist as a fiber bundle consisting of two or more ultrafine carbon fibers. By existing as fiber bundles, handling properties can be improved and handling properties are excellent. Note that the fiber bundle referred to herein is not limited to a general fiber bundle in which all single fibers are arranged in substantially the same direction, but also includes aggregates of single fibers arranged in various directions.

The aspect ratio (L/D), which is the ratio of the average fiber length (L) to the average fiber diameter (D) of the ultrafine carbon fibers, is preferably 30 or more, more preferably 50 or more, and 100 or more. It is even more preferable that there be. By setting the aspect ratio (L/D) to 30 or more, a conductive path is efficiently formed within the electrode, and the cycle characteristics of the resulting battery can be improved. If it is less than 30, the formation of a conductive path within the electrode tends to be insufficient, and the resistance value in the thickness direction of the electrode may not be sufficiently reduced. The upper limit of the aspect ratio (L/D) is not particularly limited, but is generally 10,000 or less, preferably 1,000 or less, and more preferably 800 or less.

Note that the ultrafine carbon fibers only need to have a fibrous form as a whole, and for example, those having an aspect ratio less than the preferred range mentioned above may come into contact with or combine with each other and have an integral fibrous shape. (For example, fibers with multiple extremely short fibers connected by fusing or the like) are also included.

It is preferable that the ultrafine carbon fiber in the present invention has a linear structure having substantially no branched structure. Here, "having substantially no branched structure" means that the degree of branching is 0.01 pieces/μm or less. Note that ultrafine carbon fibers (carbon nanofibers) having a branched structure include, for example, vapor-grown carbon fibers (vapor-grown carbon fibers produced by a vapor-phase method in which hydrocarbons such as benzene are vaporized in a high-temperature atmosphere in the presence of iron as a catalyst). For example, VGCF (registered trademark) manufactured by Showa Denko Co., Ltd. is known.

The ultrafine carbon fiber in the present invention preferably has an average interplanar spacing d002 of (002) planes in the range of 0.335 to 0.350 nm as measured by X-ray diffraction. d002 is more preferably from 0.335 to 0.345, even more preferably from 0.335 to 0.340. The crystallite size Lc002 measured by X-ray diffraction is preferably 1.0 to 130 nm. If it is less than 1.0 nm, the electrical conductivity will drop significantly, which is not preferable. The lower limit of Lc002 is more preferably 5 nm or more, even more preferably 10 nm or more, even more preferably 30 nm or more, and particularly preferably 50 nm or more. The upper limit of Lc002 is preferably 150 nm or less, more preferably 130 nm or less, and particularly preferably 100 nm or less. This ultrafine carbon fiber has high crystallinity, so it has excellent electrical conductivity and thermal conductivity.

The average fiber length of the ultrafine carbon fibers is preferably 1 to 100 μm, more preferably 1 to 50 μm, and even more preferably 5 to 20 μm. If it is less than 1 μm, it is not preferable because the conductivity in the electrode mixture layer, the strength of the electrode, and the electrolyte retention property decrease. If it exceeds 100 μm, the dispersibility of the ultrafine carbon fibers is likely to be impaired.

The ultrafine carbon fiber of the present invention may be hollow (tubular) or porous, but it is preferable that the ultrafine carbon fiber be processed into a resin composite fiber in the process of producing the ultrafine carbon fiber. Therefore, the ultrafine carbon fiber of the present invention is substantially solid, and its surface is basically smooth.

<Modified thermoplastic resin>
The modified thermoplastic resin in the present invention is a carbide of an oxide of the thermoplastic resin, or hard carbon (amorphous carbon) in which graphite crystals are difficult to grow, but graphite crystals may be partially present. The modified thermoplastic resin is preferably a modified thermoplastic resin that is the same as the thermoplastic resin used in the manufacturing process of ultrafine carbon fibers.

That is, the thermoplastic resin is preferably one that can be mixed with mesophase pitch and can be molded into a thread or film shape. Specifically, at least one polyolefin selected from the group consisting of polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these is preferred. From the viewpoint of removing the unoxidized thermoplastic resin after the stabilization (infusibility)/oxidation step, it is preferable to use polyethylene.
Furthermore, in the present invention, thermoplastic resins other than polyolefins may be used. Specific examples include polyacrylate polymers such as polymethacrylate and polymethylmethacrylate, polystyrene, polycarbonate, polyarylate, polyester, polyamide, polyester carbonate, polysulfone, polyimide, polyetherimide, polyketone, and polylactic acid.

In the present invention, the modified thermoplastic resin in the ultrafine carbon fiber mixture may be present in any form, for example, in the form of particles, threads, rods, membranes, plates, or blocks. Also good. Further, it may be in contact with or adhere to a part of the ultrafine carbon fiber, or it may be entirely covered. Two or more ultrafine carbon fibers may be bonded in parallel or at different angles.

In the present invention, it is preferable that the modified thermoplastic resin in the ultrafine carbon fiber mixture functions as a sizing agent that bundles a plurality of ultrafine carbon fibers. That is, it is preferable that it adheres to some or all of the plurality of ultrafine carbon fibers or covers some or all of the plurality of ultrafine carbon fibers. It is possible to prevent the entire ultrafine carbon fiber from becoming bulky, and it is easy to handle.
A part of the ultrafine carbon fibers constituting the ultrafine carbon fiber mixture of the present invention may exist in the form of fiber bundles. In other words, a plurality of ultrafine carbon fibers are assembled to form a bundle. In this case, the modified thermoplastic resin exists within the fiber bundle containing two or more ultrafine carbon fibers or around the fiber bundle. Moreover, it may be attached in the form of particles to the inside or outside of the fiber bundle as shown in FIG. 3, or it may be adhered to the inside or outside of the fiber bundle in the form of a layer as shown in FIGS. 4 and 7. The fiber bundle of ultrafine carbon fibers may be bonded so that the ultrafine carbon fibers constituting the fiber bundle are in direct contact with each other, or may be bonded via a modified thermoplastic resin. Furthermore, the inside of this fiber bundle does not necessarily need to contain a modified thermoplastic resin. That is, the fiber bundle may be made of only ultrafine carbon fibers, and as shown in the center of FIG. 3, there may be gaps or voids in part or all of the inside of the fiber bundle.

By including a modified thermoplastic resin in the fiber bundle, the ultrafine carbon fibers can be oriented in various directions. Further, the plurality of fiber bundles are oriented in various directions in a disorderly manner. Therefore, the ultrafine carbon fibers constituting the fiber bundle are oriented three-dimensionally in random directions as a whole. Therefore, the resulting electrode can be expected to have improved conductivity. That is, it is thought that when the ultrafine carbon fiber mixture of the present invention is used as a conductive aid, it also contributes to the formation of efficient conductive paths regardless of whether the distance is long or short.

The plurality of ultrafine carbon fibers may include fibers that are bonded at different angles. By bonding at different angles, the ultrafine carbon fibers can be randomly oriented in various directions. Therefore, for example, when used in an electrode, it becomes possible to form conductive paths in multiple directions within the electrode.

The mass ratio of the ultrafine carbon fibers and the modified thermoplastic resin in the ultrafine carbon fiber mixture of the present invention is preferably within the range of 1:0.05 to 1:1.5, and preferably 1:0.05 to 1:1.5. It is more preferably within the range of 1:1.2, even more preferably within the range of 1:0.05 to 1:0.7, and even more preferably within the range of 1:0.05 to 1:0.2. It is particularly preferable that the When the ratio of the modified thermoplastic resin is less than 0.05, the effect of adding the modified thermoplastic resin is difficult to exhibit. If the ratio of the modified thermoplastic resin exceeds 1.5, it becomes difficult for the ultrafine carbon fibers to come into contact with each other, making it difficult to exhibit properties such as improved conductivity. Furthermore, the effect of improving conductivity and the like with respect to the amount of the ultrafine carbon fiber mixture added becomes low. By adjusting this mass ratio, desired properties such as electrical conductivity and thermal conductivity can be exhibited.

The ultrafine carbon fiber mixture of the present invention essentially contains ultrafine carbon fibers and a modified thermoplastic resin, but may contain other components. When using an ultrafine carbon fiber mixture as a conductive additive, it contains known conductive additives such as particulate carbon such as acetylene black; fibrous carbon such as vapor grown carbon fiber (VGCF (registered trademark)) and carbon nanotubes; It's okay to do so. In addition, when the ultrafine carbon fiber mixture is used as a filler, it may contain a known inorganic filler or the like.

<Method for producing ultrafine carbon fiber mixture>
The ultrafine carbon fiber mixture of the present invention can be produced, for example, by the following method.
First, a mesophase pitch composition in which mesophase pitch is dispersed in a thermoplastic resin is prepared. Next, this mesophase pitch composition is formed into a thread or film in a molten state. In particular, spinning is preferred. By spinning, the mesophase pitch dispersed within the thermoplastic resin is stretched inside the thermoplastic resin, and the mesophase pitch composition is made into fibers to obtain resin composite fibers. This resin composite fiber has a sea-island structure in which the thermoplastic resin is the sea component and the mesophase pitch is the island component.

Next, the obtained resin composite fiber is brought into contact with a gas containing oxygen to stabilize the mesophase pitch, and at least a portion of the thermoplastic resin is oxidized to generate a thermoplastic resin oxide, thereby stabilizing the resin composite. Obtain synthetic fibers. This resin composite stabilized fiber has a sea-island structure in which the thermoplastic resin is the sea component and the stabilized mesophase pitch is the island component. Note that the thermoplastic resin oxide is estimated to exist as a sea component at this stage.

Next, from among the thermoplastic resins that are the sea component of this resin composite stabilized fiber, the unoxidized thermoplastic resin is removed to obtain an ultrafine carbon fiber precursor consisting of an ultrafine carbon fiber precursor and a thermoplastic resin oxide. Get a mixture.

Furthermore, this ultrafine carbon fiber precursor mixture is heated at a high temperature to obtain an ultrafine carbon fiber mixture consisting of ultrafine carbon fibers and a carbide or graphitized product of a thermoplastic resin oxide (a modified product of a thermoplastic resin).

That is, the ultrafine carbon fiber mixture of the present invention can be manufactured through the following steps.
(1) A mesophase pitch composition consisting of a thermoplastic resin and 1 to 150 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin is molded in a molten state to form fibers from the mesophase pitch to produce a resin. Molding process to obtain composite fibers,
(2) A gas containing oxygen is brought into contact with the resin composite fiber to stabilize the mesophase pitch, and a part of the thermoplastic resin is oxidized to generate a thermoplastic resin oxide, thereby forming the resin composite stabilized fiber. a stabilization/oxidation step to obtain
(3) a removal step of removing the unoxidized thermoplastic resin from this resin composite stabilized fiber to obtain an ultrafine carbon fiber precursor mixture consisting of an ultrafine carbon fiber precursor and a thermoplastic resin oxide;
(4) A high temperature heating step in which the ultrafine carbon fiber precursor mixture is heated at a high temperature to obtain ultrafine carbon fibers and a modified thermoplastic resin.

Next, each step will be explained in detail.
<Molding process>
In the molding step, a mesophase pitch composition consisting of a thermoplastic resin and 1 to 150 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin is molded in a molten state to form the mesophase pitch into fibers. Obtain resin composite fibers.

<Mesophase pitch composition>
The mesophase pitch composition comprises a thermoplastic resin and 1 to 150 parts by weight of mesophase pitch per 100 parts by weight of the thermoplastic resin. The content of mesophase pitch is preferably 1 to 100 parts by mass, more preferably 5 to 100 parts by mass. When the content of mesophase pitch exceeds 150 parts by mass, it is difficult to obtain resin composite fibers having a desired dispersion diameter, and when the content is less than 1 part by mass, the manufacturing cost of ultrafine carbon fibers increases.

In order to produce ultrafine carbon fibers with an average fiber diameter of 10 to 900 nm, the dispersed diameter of mesophase pitch in the thermoplastic resin is preferably 0.01 to 50 μm, and preferably 0.01 to 30 μm. More preferred. If the diameter of the mesophase pitch dispersed in the thermoplastic resin is outside the range of 0.01 to 50 μm, it may be difficult to produce the desired ultrafine carbon fiber. Note that in the mesophase pitch composition, mesophase pitch forms spherical or elliptical island components. The dispersion diameter in the present invention means the diameter when the island component is spherical, and the long axis diameter when the island component is elliptical.

The dispersion diameter of mesophase pitch is preferably maintained within the above range even after the mesophase pitch composition is held at 300°C for 3 minutes, and is preferably maintained even after held at 300°C for 5 minutes. More preferably, it is particularly preferable that the temperature is maintained even after being held at 300°C for 10 minutes. Generally, when a mesophase pitch composition is kept in a molten state, the mesophase pitch aggregates in the thermoplastic resin over time. If the mesophase pitch aggregates and its dispersed diameter exceeds 50 μm, it may be difficult to produce the desired ultrafine carbon fiber. The aggregation rate of mesophase pitch in a thermoplastic resin varies depending on the type of thermoplastic resin and mesophase pitch used.

A mesophase pitch composition can be produced by kneading a thermoplastic resin and mesophase pitch in a molten state. Melt-kneading of the thermoplastic resin and mesophase pitch can be performed using a known device. For example, one or more types selected from the group consisting of a single-screw kneader, a twin-screw kneader, a mixing roll, and a Banbury mixer can be used. Among these, it is preferable to use a twin-screw kneader for the purpose of finely dispersing mesophase pitch in the thermoplastic resin. In particular, it is preferable to use a two-screw kneader in which each shaft rotates in the same direction.

The kneading temperature is not particularly limited as long as the thermoplastic resin and mesophase pitch are in a molten state, but it is preferably 100 to 400°C, and preferably 150 to 350°C. If the kneading temperature is less than 100°C, the mesophase pitch will not be in a molten state and it will be difficult to microdisperse it in the thermoplastic resin, which is not preferable. On the other hand, if the temperature exceeds 400°C, decomposition of the thermoplastic resin and mesophase pitch progresses, which is not preferable. Further, the melt-kneading time is preferably 0.5 to 20 minutes, more preferably 1 to 15 minutes. When the melt-kneading time is less than 0.5 minutes, it is difficult to microdisperse mesophase pitch. On the other hand, if the heating time exceeds 20 minutes, the productivity of ultrafine carbon fibers decreases.

The mesophase pitch used in the present invention is modified by reacting with oxygen during melt-kneading, which may inhibit microdispersion into the thermoplastic resin. For this reason, it is preferable to perform melt-kneading under an inert atmosphere to suppress the reaction between oxygen and mesophase pitch. The melt-kneading is preferably performed in an inert atmosphere with an oxygen gas content of less than 10% by volume. More preferably, the oxygen gas content is less than 5% by volume, particularly preferably less than 1% by volume.

<Mesophase pitch>
Mesophase pitch is pitch that can form an optically anisotropic phase (liquid crystal phase) in a molten state. Examples of the mesophase pitch used in the present invention include those made from distillation residues of coal or petroleum, and those made from aromatic hydrocarbons such as naphthalene. For example, mesophase pitch derived from coal can be obtained by a treatment mainly consisting of hydrogenation and heat treatment of coal tar pitch, a treatment mainly consisting of hydrogenation, heat treatment, and solvent extraction, etc.

The optical anisotropy content (mesophase ratio) of mesophase pitch is preferably 80% or more, more preferably 90% or more.

The softening point of mesophase pitch is preferably 100 to 400°C, more preferably 150 to 350°C.

<Thermoplastic resin>
The thermoplastic resin used in the present invention preferably has a melt mass flow rate (MFR) of 0.1 to 25 g/10 min, measured in accordance with JIS K 7210 (1999), and preferably 0.1 to 15 g/10 min. It is more preferably 10 min, and particularly preferably 0.1 to 10 g/10 min. When the MFR is within the above range, mesophase pitch can be favorably microdispersed in the thermoplastic resin. Further, when spinning the resin composite fiber, the mesophase pitch is stretched, so that the fiber diameter of the obtained ultrafine carbon fiber can be made smaller and the linearity of the obtained ultrafine carbon fiber can be increased. Since the thermoplastic resin used in the present invention can be easily melted and kneaded with mesophase pitch, the glass transition temperature is preferably 250°C or lower when it is amorphous, and the melting point is 300°C or lower when it is crystalline. preferable.

Furthermore, the thermoplastic resin used in the present invention can maintain its shape in the stabilization process, can be oxidized, and can be easily converted into unoxidized thermoplastic resin in the process of obtaining the ultrafine carbon fiber precursor mixture described below. Must be able to be removed.

Examples of such thermoplastic resins include polyolefins such as polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these. From the viewpoint of ease of removal in the thermoplastic resin removal process, it is preferable to use polyethylene. Examples of polyethylene include low-density polyethylene such as high-pressure low-density polyethylene, gas-phase, solution-process, and high-pressure linear low-density polyethylene, homopolymers such as medium-density polyethylene, and high-density polyethylene, or ethylene and α-olefin. copolymers of ethylene and other vinyl monomers, such as ethylene/vinyl acetate copolymers.

<Resin composite fiber>
An example of a method for producing a resin composite fiber from the mesophase pitch composition described above is a method in which the mesophase pitch composition is melt-spun using a spinneret. Thereby, the initial orientation of the mesophase pitch contained in the resin composite fiber can be increased.

When the mesophase pitch composition is melt-spun using a spinneret, the number of spinning holes in the spinneret directly corresponds to the number of fibers in the fiber bundle. The number of fibers is preferably in the range of 100 to 3,000, more preferably 200 to 2,000, even more preferably 300 to 1,500. If the number is less than 100, productivity decreases, and if it exceeds 3000, process stability tends to decrease.

The resin composite fiber thus obtained has an average fiber diameter of 10 to 200 μm. The lower limit of the average fiber diameter is preferably 50 μm or more, more preferably 70 μm or more, and even more preferably 80 μm or more. The upper limit of the average fiber diameter is preferably 150 μm or less, more preferably 130 μm or less, and even more preferably 120 μm or less. If it exceeds 200 μm, it becomes difficult for the reactive gas to come into contact with the mesophase pitch dispersed inside the resin composite fiber during the stabilization/oxidation step described below. Therefore, productivity decreases. Furthermore, the oxidation state of the thermoplastic resin may become non-uniform between the center and the outer periphery of the resin composite fiber. On the other hand, if the thickness is less than 10 μm, the strength of the resin composite fiber may decrease and process stability may decrease.

The temperature when manufacturing (spinning) resin composite fibers from the mesophase pitch composition needs to be higher than the melting temperature of mesophase pitch, preferably 150 to 400°C, and preferably 180 to 350°C. is more preferable. When the temperature exceeds 400°C, the deformation relaxation rate of mesophase pitch increases, making it difficult to maintain the fiber shape.

Moreover, the manufacturing process of the resin composite fiber may include a cooling process. Examples of the cooling step include, in the case of melt spinning, a method of cooling the atmosphere downstream of a spinneret. By providing a cooling step, the region where the mesophase pitch deforms due to elongation can be adjusted, and the rate of strain can be adjusted. Further, by providing a cooling step, the resin composite fiber after spinning is immediately cooled and solidified, thereby enabling stable molding.

The resin composite fiber obtained through these steps is made into a fiber in a state in which mesophase pitch is microdispersed in a thermoplastic resin.

Note that a resin composite fiber bundle may be obtained by heat-sealing a plurality of resin composite fibers to each other. By heat-sealing, the resin composite fiber becomes easier to handle in the subsequent stabilization/oxidation step, and it becomes easier to uniformly process it in the stabilization/oxidation step and the removal step.

A method for forming a thermal bond is, for example, by heating one or more gases selected from air and nitrogen with a heater and spraying it onto a bundle of resin composite fibers being conveyed by a roller or belt conveyor. An example is a method of melting a thermoplastic resin and fusing resin composite fibers together. Another example is a method in which a hot plate such as an iron is pressed against the resin composite fiber bundle. Although it depends on the heating temperature and time, the former method is preferable because the stabilization described below can be performed more efficiently if there is a space inside the resin composite fiber bundle. Note that these methods may be used in combination.

The temperature near the resin conjugate fibers when the resin conjugate fibers are fused together depends on the spinning speed, but is preferably from the melting point of the thermoplastic resin to 400°C, more preferably in the range of 200 to 350°C. Even if heated below the melting point, resin composite fibers cannot be fused together. On the other hand, if heating exceeds 400° C., the mesophase pitch within the resin composite fiber may melt and the fiber shape may be lost.

<Stabilization/oxidation process>
The resin composite stabilized fiber can be produced by contacting the above-mentioned resin composite fiber with a reactive gas containing oxygen. By contacting the reactive gas, the mesophase pitch contained within the resin composite fiber is stabilized (infusible).

This process includes a stabilization process in which a gas containing oxygen such as air is brought into contact with the resin composite fiber to stabilize the mesophase pitch, and an oxidation process in which the thermoplastic resin is oxidized to generate a thermoplastic resin oxide. are carried out simultaneously.

In the stabilization/oxidation step, the reactive gas may contain an oxidizing gas other than oxygen or an inert gas. Examples of the oxidizing gas include nitrogen dioxide, nitrogen monoxide, and sulfur dioxide. Examples of the inert gas include carbon dioxide, nitrogen, and argon. The preferred oxygen concentration is 0.1 to 21 vol%, although it varies depending on the type of mesophase pitch and the fiber diameter of the resin composite fiber. The amount of thermoplastic resin oxidized in this stabilization/oxidation step is reflected in the amount of modified thermoplastic resin present in the final product. Therefore, by appropriately adjusting the oxygen concentration and temperature in this stabilization/oxidation step, the amount of thermoplastic resin to be oxidized can be adjusted.

According to the present invention, mesophase pitch is stabilized in the state of a resin composite fiber composited with a thermoplastic resin. Therefore, compared to stabilizing a fiber formed by melt-spinning only mesophase pitch, process stability is not impaired even if the fiber diameter of mesophase pitch is made smaller.

The stabilization/oxidation reaction temperature is preferably 250 to 400°C, more preferably 300 to 350°C. By performing the stabilization/oxidation treatment at a temperature in this range, it is possible to quickly stabilize the mesophase pitch in the resin composite stabilized fiber and to generate an oxide of the thermoplastic resin. The production of this thermoplastic resin oxide can be confirmed by, for example, infrared spectroscopy or thermogravimetric differential thermal analysis (TG-DTA).

The stabilization/oxidation treatment time is preferably 1 to 10 hours, more preferably 2 to 7 hours, and even more preferably 3 to 6 hours.

The softening point of the mesophase pitch increases significantly through the above stabilization treatment, but for the purpose of obtaining the desired ultrafine carbon fibers, the softening point of the mesophase pitch is preferably 400°C or higher, more preferably 500°C or higher. preferable.

In this step, the stabilization reaction of the mesophase pitch and the oxidation reaction of the thermoplastic resin proceed simultaneously. Stabilization of mesophase pitch and oxidation of thermoplastic resin can be achieved by appropriately adjusting the conditions described above, especially the type of thermoplastic resin, type of oxidizing gas, concentration, wind speed, reaction temperature, and treatment time. can be controlled. Note that, in the present invention, the thermoplastic resin that has not been oxidized is removed by a removal process described below. It is not necessary that all of the thermoplastic resin be oxidized, but the thermoplastic resin should be in an amount such that the mass ratio of the ultimately obtained ultrafine carbon fiber to the modified thermoplastic resin is 1:0.05 to 1:1.5. It is sufficient if an oxide of the resin is generated. As such conditions, when polyethylene is used as the thermoplastic resin, a method of heating at 250° C. for 5 hours or more in the atmosphere under natural convection is exemplified. When the heating temperature is 300°C or higher, the heating time is preferably about 5 minutes to 5 hours. When the wind speed of the atmosphere is extremely low or when the heating temperature is high, the proportion of modified thermoplastic resin tends to increase. The shape and size of the fiber bundles also change. When polypropylene is used as the thermoplastic resin, a method of heating it in the atmosphere at 250° C. for 1 hour or more is exemplified. The upper limit of the heating temperature is usually the above-mentioned stabilization reaction temperature.

<Removal process>
The ultrafine carbon fiber precursor mixture is obtained by removing the thermoplastic resin that was not oxidized in the stabilization step from the resin composite stabilized fiber. Thermoplastic resin oxides include modified products derived from thermoplastic resin oxides. In the removal step, the thermoplastic resin is decomposed and removed while suppressing thermal decomposition of the resin-stabilized fibers. Examples of the method of decomposing and removing the thermoplastic resin include a method of removing the thermoplastic resin using a solvent, and a method of removing the thermoplastic resin by thermally decomposing it. Among these methods, the method of removing with a solvent requires a large amount of solvent and has to be recovered, increasing the process cost. Therefore, the latter removal by thermal decomposition is practical and preferable.

It is preferable to thermally decompose the thermoplastic resin under an inert gas atmosphere. The inert gas atmosphere here refers to a gas atmosphere such as carbon dioxide, nitrogen, water vapor, argon, etc., and the oxygen concentration thereof is preferably 30 volume ppm or less, more preferably 20 volume ppm or less. As the inert gas used in this step, it is preferable to use carbon dioxide or nitrogen from the viewpoint of cost, and it is particularly preferable to use nitrogen.

When the thermoplastic resin is removed by thermal decomposition, it can also be carried out under reduced pressure. The thermoplastic resin can be removed more efficiently by thermal decomposition under reduced pressure. The lower the atmospheric pressure, the more preferable it is, but it is preferably 50 kPa or less, more preferably 30 kPa or less, even more preferably 10 kPa or less, and particularly preferably 5 kPa or less. On the other hand, since a complete vacuum is difficult to achieve, the lower limit of the pressure is generally 0.01 kPa or more.

When the thermoplastic resin is removed by thermal decomposition, trace amounts of oxygen or inert gas may be present as long as the above atmospheric pressure is maintained. Note that under a trace amount of oxygen atmosphere here means that the oxygen concentration is 30 volume ppm or less, and under a trace amount of inert gas atmosphere means that the inert gas concentration is 20 volume ppm or less. . The type of inert gas used is as described above.

The thermal decomposition temperature is preferably 350 to 600°C, more preferably 380 to 550°C. If the thermal decomposition temperature is less than 350° C., although the thermal decomposition of the ultrafine carbon fiber precursor can be suppressed, the thermoplastic resin may not be sufficiently thermally decomposed. On the other hand, when the temperature exceeds 600°C, although the thermoplastic resin can be sufficiently thermally decomposed, the ultrafine carbon fiber precursor and the oxide of the thermoplastic resin may also be thermally decomposed, and the yield tends to decrease. . The thermal decomposition time is preferably 0.05 to 5 hours, more preferably 0.05 to 3 hours. Note that the shape and size of the fiber bundle can be changed by lowering the pyrolysis temperature or controlling the degree of pressure reduction.

<High temperature heating process>
The ultrafine carbon fiber mixture is obtained by heating an ultrafine carbon fiber precursor mixture in an inert gas atmosphere to carbonize or graphitize the ultrafine carbon fiber precursor and carbonize the thermoplastic resin oxide.

Examples of the inert gas used in the high temperature heating step include nitrogen, argon, and the like. The oxygen concentration in the inert gas is preferably 20 volume ppm or less, more preferably 10 volume ppm or less. The heating and firing temperature during carbonization and/or graphitization is preferably 500 to 3,500°C, more preferably 800 to 3,000°C. In particular, the firing temperature during graphitization is preferably 1200 to 3000°C. The heating time is preferably 0.1 to 24 hours, more preferably 0.2 to 10 hours.

The container used in the high-temperature heating step may be made of metal, ceramic, or graphite depending on the heating temperature, but a crucible-shaped container made of graphite is preferable.

<Crushing process>
The obtained ultrafine carbon fiber mixture may be subjected to pulverization treatment in order to improve handling properties. The pulverization treatment is preferably carried out after the thermoplastic resin removal step and/or after the high-temperature heating step. As the pulverization method, it is preferable to use a pulverizer such as a jet mill, a ball mill, a bead mill, an impeller mill, or a cutter mill. If necessary, classification may be performed after pulverization. In the case of wet pulverization, the dispersion medium is removed after pulverization, but if significant secondary aggregation occurs at this time, subsequent handling becomes extremely difficult. In such a case, after drying, it is preferable to perform a crushing operation using a ball mill, jet mill, or the like.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples. Various measurements and analyzes in the examples were performed according to the following methods.

(1) Morphological observation of ultrafine carbon fibers Observation was performed using a scanning electron microscope (JCM-6000 manufactured by JEOL Ltd.) at an accelerating voltage of 10 kV.
The average fiber diameter of ultrafine carbon fibers, etc. is determined by randomly selecting 300 locations from the obtained electron micrograph, measuring the fiber diameter, and taking the average value of all the measurement results (n = 300) as the average fiber diameter. did. The average fiber length was also measured in the same manner.

(2) Calculation method of mass ratio of modified product of thermoplastic resin Resin composite stabilized fiber was heated to 500°C at a rate of 5°C/min and held for 60 min using TGA (model TG 8120 manufactured by Thermo plus Rigaku). It was calculated using the following formula from the value of the mass reduction rate and the mass content of mesophase pitch in the resin composite stabilized fiber. The value obtained by this measurement was defined as the mass ratio of the modified thermoplastic resin. Although this value is the mass ratio of the oxide of the thermoplastic resin, it can be regarded as the mass ratio of the modified thermoplastic resin in the ultimately obtained ultrafine carbon fiber mixture.
Mass ratio of modified thermoplastic resin = ((100 - mass reduction rate (%)) - mesophase pitch mass content (%)) / mesophase pitch mass content (%)

(3) X-ray diffraction measurement of ultrafine carbon fiber composite X-ray diffraction measurement was performed using RINT-2100 manufactured by Rigaku Corporation in accordance with the JIS R7651 method, and the lattice spacing (d002) and crystallite size (Lc002) were measured. did.

[Reference Example 1] (Mesophase pitch manufacturing method)
Coal tar pitch with a softening point of 80°C from which quinoline insoluble matter was removed was hydrogenated at a pressure of 13 MPa and a temperature of 340°C in the presence of a Ni-Mo catalyst to obtain hydrogenated coal tar pitch. This hydrogenated coal tar pitch was heat treated at 480° C. under normal pressure, and then the low boiling point components were removed under reduced pressure to obtain mesophase pitch. This mesophase pitch was filtered using a filter at a temperature of 340° C. to remove foreign substances in the pitch and obtain purified mesophase pitch.

[Example 1]
90 parts by mass of low-density polyethylene (UBE Polyethylene (registered trademark) R300, manufactured by Ube Maruzen Polyethylene, MFR = 0.35 g/10 min) as a thermoplastic resin, and mesophase pitch obtained in Reference Example 1 (mesophase ratio 90.9%) , softening point 303.5°C) in a codirectional twin-screw extruder (Toshiba Machine Co., Ltd. "TEM-26SS", barrel temperature 300°C, under nitrogen stream) to obtain a mesophase pitch composition. Prepared.

The above mesophase pitch composition was spun into a resin composite fiber (mesophase pitch A sea-island type composite fiber containing fiber as an island component was produced. The average fiber diameter of the single yarn at that time was 182 μm.

The obtained resin composite fibers were cut with scissors to about 50 mm, and then 2 g of the fibers was uniformly placed on a 150 mm square SUS304 wire mesh (0.34×14 mesh manufactured by Nippon Kinami Shoko Co., Ltd.). This was heated in an electric furnace in an air atmosphere at a rate of 5°C/min, and after reaching 340°C, was held for 5 hours to stabilize the inside of the resin composite fiber and obtain a resin composite stabilized fiber. . Here, the atmospheric wind speed inside the electric furnace was substantially 0 m/s.

Next, the resin composite stabilized fiber was heated to 500°C at a temperature increase rate of 5°C/min in a vacuum gas replacement furnace whose pressure was reduced to 1 kPa after nitrogen substitution, and then at 500°C for 1 hour. held. As a result, the low density polyethylene was removed to obtain an ultrafine carbon fiber precursor mixture consisting of an ultrafine carbon fiber precursor and low density polyethylene oxide. Figure 1 shows an electron micrograph taken here. As can be seen from FIG. 1, in addition to the ultrafine carbon fiber precursor, a large number of solid substances embedding a plurality of stabilized ultrafine carbon fiber precursors were observed. These solids were composites of low density polyethylene oxide and stabilized ultrafine carbon fiber precursors.

The ultrafine carbon fiber precursor mixture prepared above was added to a mixed solvent of ethanol/ion-exchanged water (volume ratio 1/1) and stirred for 10 minutes with a mixer to disperse the ultrafine carbon fiber precursor and low density polyethylene oxide. I let it happen. The resulting dispersion was filtered. This ultrafine carbon fiber precursor and low-density polyethylene oxide were charged into a cubic graphite crucible so that the bulk density was 0.1 g/ml. Thereafter, the temperature was raised from room temperature to 1000°C at a rate of 5°C/min under nitrogen at a flow rate of 1 l/min, and after reaching 1000°C, carbonization was carried out by holding the temperature for 30 minutes. Thereafter, the temperature was raised from room temperature to 3000° C. in 3 hours under an argon gas atmosphere to produce ultrafine carbon fibers and modified low-density polyethylene. The obtained ultrafine carbon fibers were crushed using a dry jet mill.

The average fiber diameter of the obtained ultrafine carbon fibers was 400 nm, the average fiber length was 14 μm, and no branched structure was observed (degree of branching: 0). The average interplanar spacing d002 of the (002) plane measured by X-ray diffraction was 0.3377 nm, and the crystallite size Lc002 was 21.7 nm. Moreover, the mass ratio of the modified thermoplastic resin to the ultrafine carbon fiber was 1.2. Electron micrographs taken here are shown in Figures 2 to 4. As can be seen from FIG. 2, ultrafine carbon fibers and fiber bundles containing ultrafine carbon fibers were observed. In FIG. 3, particulate hard carbon, which is a modified low-density polyethylene, and fiber bundles were confirmed. In FIG. 4, a fiber bundle containing a modified thermoplastic resin and embedding ultrafine carbon fibers was observed.

[Example 2]
The thermoplastic resin in Example 1 was changed to linear low-density polyethylene (MORETEC (registered trademark) 0248Z, manufactured by Prime Polymer Co., Ltd., MFR = 2 g/10 min), and the conditions in the stabilization step were changed to an oxygen concentration of 7000 ppm. The operation was carried out in the same manner as in Example 1, except that the temperature was changed to 340° C. for 3 hours under a nitrogen gas atmosphere adjusted to . An electron micrograph of the ultrafine carbon fiber precursor mixture is shown in FIG.

The average fiber diameter of the ultrafine carbon fibers finally obtained was 400 nm, the average fiber length was 15 μm, and no branched structure was observed (degree of branching: 0).
Further, the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction was 0.3374 nm, and the crystallite size Lc002 was 37.1 nm. Electron micrographs taken here are shown in FIGS. 6 and 7. Furthermore, the mass ratio of the modified thermoplastic resin (modified low density polyethylene) was 0.1.

[Comparative example 1]
The thermoplastic resin in Example 1 was changed to linear low-density polyethylene (MORETEC (registered trademark) 0248Z, manufactured by Prime Polymer Co., Ltd., MFR = 2 g/10 min), and the conditions in the stabilization step were changed to nitrogen dioxide and The same operation as in Example 1 was performed except that the mixture was held at room temperature for 0.5 hours in a mixed gas atmosphere of 2:98 (vol%/vol%) with air. An electron micrograph of the ultrafine carbon fiber precursor is shown in FIG.

The average fiber diameter of the ultimately obtained ultrafine carbon fibers was 297 nm, the average fiber length was 15 μm, and no branched structure was observed (degree of branching: 0).
Further, the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction was 0.3373 nm, and the crystallite size Lc002 was 40.7 nm. FIG. 9 shows an electron micrograph taken here. No modified thermoplastic resin was observed. This ultrafine carbon fiber was extremely bulky and difficult to handle.

[Example 3] (Negative electrode capacity of ultrafine carbon fiber mixture)
The ultrafine carbon fiber mixtures (abbreviated as CNF mixtures) obtained in Examples 1 and 2, the ultrafine carbon fibers (abbreviated as CNFs) obtained in Comparative Example 1, and PVdF manufactured by Kureha (W#7200) as a binder were used. , CNF mixture:PVdF was mixed at a mass ratio of 9:1 (in the case of Comparative Example 1, CNF:PVdF was mixed at a mass ratio of 9:1) to prepare slurries, respectively. The obtained slurry was applied onto a copper foil, dried at 80° C. for 1 hour or more, and then dried at 120° C. for 2 hours or more to prepare an electrode (negative electrode). The prepared electrode was roll pressed and punched into a circular shape, dried at 150°C for 1 hour, and then dried at 120°C under vacuum for 6 hours or more. Using these electrodes, coin batteries were fabricated to evaluate each electrode. The electrolytic solution used was EC:EMC=3:7 (v/v) containing 1 mol/L LiPF ₆ . Lithium was used as a counter electrode. The charging and discharging characteristics of each of the manufactured coin batteries were evaluated. Charging and discharging are performed between 0.0V and 2.0V, charging at 0.4mA (0.26mA/cm ² ) CCCV (0.01mA cutoff), and discharging at 0.4mA (0.26mA/cm 2 ). cm ² ) CC conditions. The results obtained are shown in Table 1.

When using the ultrafine carbon fiber mixtures obtained in Examples 1 and 2, the irreversible capacity was smaller than in Comparative Example 1. Furthermore, the charging capacity and discharging capacity were high despite the low electrode density. That is, it is presumed that by using the ultrafine carbon fiber mixtures of Examples 1 and 2, the ultrafine carbon fibers are oriented in multiple directions within the electrode. In addition, the electrode density is low and the weight can be reduced.

Claims (13)

Ultrafine carbon fibers having an average fiber diameter of 10 to 900 nm,
A modified thermoplastic resin,
including;
The modified thermoplastic resin is hard carbon,
An ultrafine carbon fiber mixture characterized in that the mass ratio of the ultrafine carbon fiber to the modified thermoplastic resin is 1:0.05 to 1:1.5 . The ultrafine carbon fiber mixture according to claim 1, wherein some or all of the ultrafine carbon fibers are in the form of fiber bundles. The ultrafine carbon fiber mixture according to claim 2, wherein some or all of the ultrafine carbon fibers are bonded via a modified thermoplastic resin. The ultrafine carbon fiber according to any one of claims 1 to 3 , wherein the average interplanar spacing d002 of the (002) planes measured by X-ray diffraction of the ultrafine carbon fiber is in the range of 0.335 to 0.350 nm. blend. Any one of claims 1 to 4, wherein the modified thermoplastic resin is a modified product of at least one polyolefin selected from the group consisting of polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these. The ultrafine carbon fiber mixture according to item 1. (1) A mesophase pitch composition consisting of a thermoplastic resin and 1 to 150 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin is molded in a molten state to form the mesophase pitch into fibers to produce a resin. A forming process to obtain composite fibers;
(2) Stabilizing the mesophase pitch by bringing a gas containing oxygen into contact with the resin composite fiber, and oxidizing a part of the thermoplastic resin to generate a thermoplastic resin oxide to stabilize the resin composite. a stabilization/oxidation step to obtain fiber;
(3) a removing step of removing the unoxidized thermoplastic resin from the resin composite stabilized fiber to obtain an ultrafine carbon fiber precursor mixture consisting of an ultrafine carbon fiber precursor and a thermoplastic resin oxide;
(4) a high temperature heating step of heating the ultrafine carbon fiber precursor mixture to obtain ultrafine carbon fibers and a modified thermoplastic resin;
The method for producing an ultrafine carbon fiber mixture according to any one of claims 1 to 5 , comprising: The method for producing an ultrafine carbon fiber mixture according to claim 6 , wherein the thermoplastic resin is at least one selected from the group consisting of polyethylene, polypropylene, poly-4-methylpentene-1, and copolymers containing these. The method for producing an ultrafine carbon fiber mixture according to claim 6 or 7 , wherein the thermoplastic resin is polyethylene having a melt mass flow rate (MFR) of 0.1 to 25 g/10 min as measured in accordance with JIS K 7210. The ultrafine carbon fiber according to any one of claims 6 to 8 , wherein the mesophase pitch composition comprises a thermoplastic resin and 1 to 100 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin. Method of manufacturing the mixture. The method for producing an ultrafine carbon fiber mixture according to any one of claims 6 to 9, wherein the resin composite fiber has an average fiber diameter of 10 to 200 μm. The method for producing an ultrafine carbon fiber mixture according to any one of claims 6 to 10, wherein the stabilization/oxidation step includes heat treatment at 250 to 400°C for 1 to 10 hours. The method for producing an ultrafine carbon fiber mixture according to any one of claims 6 to 11, wherein the heating temperature in the high temperature heating step is 500 to 3500°C. A carbon-based conductive additive comprising the ultrafine carbon fiber mixture according to any one of claims 1 to 5 .