JPH10273882A - Carbon fiber mill and its manufacturing method - Google Patents

Abstract

(57) [Summary] [Problem] A carbon fiber mill excellent in fluidity and dispersibility and suitable for reinforcement of composite materials and functional materials. A carbon fiber having a fiber length of less than 1 mm has a length of 0.0 mm.
Carbon fiber mill to which less than 5% by weight of a sizing agent has adhered. 0.005-0.05% by weight of sizing agent
Is preferred. It is effective for a carbon fiber mill having a fiber length of 3 to 500 μm. The aspect ratio (L / D) is preferably from 1 to 70, and polyacrylonitrile-based carbon fibers are suitable. It has excellent fluidity and dispersibility, and is suitable for reinforcing fibers and functional materials of carbon fiber composite materials, for example, materials for battery electrodes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon fiber mill used as a reinforcing fiber for a reinforced composite material or a material for a battery electrode, and a method for producing the same. More specifically, the present invention relates to a method for providing an easy-to-use carbon fiber mill excellent in fluidity and dispersibility with good productivity by controlling the amount of the sizing agent attached. In the present invention, the term “carbon fiber milled” refers to carbon fiber having a fiber length shorter than 1 mm and generally less than 1 mm, which is generally referred to as chopped carbon fiber. The sizing agent refers to a substance applied to the surface of a carbon fiber for the purpose of making it easier to handle.

[0002]

2. Description of the Related Art Carbon fibers are supplied in various forms depending on the purpose of use, such as long fibers, chopped fibers having a fiber length of several millimeters, and milled fibers having a fiber length of several μm to several hundred μm. It is used as a functional fiber. Of these, milled carbon fiber is relatively easy to uniformly disperse in various matrices, so it goes beyond simply strengthening the matrix mechanically, as well as heat resistance, electrical conductivity, abrasion resistance, dimensional stability, etc. It is also used for the purpose of improving various physical properties of the matrix. Recently, its use has been expanding to, for example, materials for negative electrodes of lithium ion secondary batteries.

[0003]

[Problems to be Solved by the Invention] With the expansion of applications,
The demands of users for improvement of various properties and uniformity of carbon fiber mill, cost, and the like have become severe. The present inventor studied carbon fiber mills in order to meet these demands. As a result, carbon fiber mills using a large amount of a sizing agent have: (1) a large tendency of the carbon fiber mill to agglomerate, and low fluidity and dispersibility; (2) It was found that the adhesion to the matrix was not sufficient. Agglomeration of the carbon fiber mill is likely to occur even in the manufacturing process of the carbon fiber mill, and the carbon fiber mill once agglomerated into a lump is difficult to re-disperse. (2) It is found that the fiber length distribution of the carbon fiber mill tends to be non-uniform (3) The yield and productivity are reduced.

To solve such a problem, Japanese Patent Laid-Open Publication No.
Japanese Patent No. 7021 discloses a method for producing a chopped fiber strand in which a volatile sizing agent is provided at the time of cutting, and the sizing agent is removed by natural drying or low-temperature heating after cutting. However, this method can cope with chopped fibers having a fiber length of 1 mm or more, but in the production of a milled carbon fiber, smooth processing may be hindered due to aggregation during pulverization. An object of the present invention is to provide a carbon fiber mill and a method for producing a carbon fiber mill with good productivity, which is excellent in fluidity and dispersibility and is suitable as a raw material of a product having high uniformity as a reinforcing fiber for a composite material or a functional material. It was completed.

[0005]

According to the present invention, as a means for solving the above-mentioned problems, a sizing agent not exceeding 0.05% by weight adheres to a carbon fiber having a fiber length not exceeding 1 mm. A carbon fiber mill. Here, the adhesion amount of the sizing agent is 0.005 to 0.0
The content is preferably 5% by weight, and the present invention is particularly effective for a milled carbon fiber having a fiber length of 3 to 500 μm. The aspect ratio (L / D) is preferably in the range of 1 to 70, and carbon fibers derived from polyacrylonitrile fibers are preferable. Further, according to the present invention, after attaching at least 0.1% by weight of the sizing agent to the carbon fiber, the amount of the sizing agent attached is reduced to less than 0.05% by weight, and cut to a fiber length not exceeding 1 mm. A method for producing a carbon fiber mill is provided.

[0006] Further, the present invention provides a carbon fiber multifilament having at least 0.1% by weight of a sizing agent attached thereto, and then heating the carbon fiber to reduce the amount of the sizing agent attached to less than 0.05% by weight. And a method for producing a milled carbon fiber characterized by cutting into a fiber length not exceeding 1 mm. The heat treatment is performed at 300 to 70
The carbon fiber surface 1 was placed in a heating furnace in an air atmosphere maintained at 0 ° C.
It is very effective to continuously supply and heat the carbon fiber while maintaining the amount of carbon fiber per m ² (hereinafter referred to as a carbon fiber inner surface density) in the range of 0.1 to 8 kg. As the carbon fiber to be used, a carbon fiber derived from polyacrylonitrile fiber is desirable.

In the present invention, the fiber length (L) and the fiber diameter (D) of the carbon fiber mill are observed by using a scanning electron microscope, and the length and diameter of the carbon fiber mill in the fiber direction are measured. The calculated values were obtained by measuring at least 20 carbon fiber mills and averaging the measured values. The amount of sizing agent (W)
Was determined as follows. About 10 g of a carbon fiber mill is weighed (referred to as w ₁ (g)), and the thickness is 5 m in a metal container.
m or less. The carbon fiber mill was heated to 500 ° C. in advance, placed in an electric furnace in which the inside of the furnace was sufficiently purged with nitrogen, and heated at 500 ° C. for 15 minutes. After the heating, it was taken out immediately, placed in a closed container which had been sufficiently purged with nitrogen in advance, and cooled to around room temperature. The weight of the carbon fiber mill after cooling was precisely weighed (referred to as w ₂ (g)) and calculated by the following equation. W (% by weight) = {(w ₁ −w ₂ ) / w ₂ } × 100 When W was negative, the weight was set to 0% by weight.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has found the above-mentioned problem which is considered to be caused by a sizing agent, and as a result of research on a method for solving the problem, it has been found that it is important to control the amount of sizing agent adhered. The conclusion has been reached. Hereinafter, details of the present invention will be described. The carbon fiber mill of the present invention comprises carbon fibers having a fiber length not exceeding 1 mm and a sizing agent not exceeding 0.05% by weight adhering to the carbon fibers. The type of the sizing agent may be selected according to the type of the matrix, the characteristics of the carbon fiber, the purpose of using the carbon fiber mill, and the state of the process. Generally, a sizing agent containing an epoxy resin as a main component is used, but is not limited thereto. A sizing agent mainly containing a polyester resin, a polyurethane resin, or the like may be used, or a plurality of sizing agents may be mixed and used. The sizing agent containing an epoxy resin as a main component preferably has an epoxy resin content of 40% by weight or more. The sizing agent may contain additives such as a surfactant and a solvent such as water used for dilution and dispersion.

In the carbon fiber mill of the present invention, it is not preferable that the sizing agent adhesion amount exceeds 0.05% by weight. Preferably it is less than 0.03% by weight, more preferably less than 0.01% by weight. The amount of sizing agent attached is 0.
If the content exceeds 05% by weight, the aggregation of the carbon fiber mill becomes intense, so that not only the flow and dispersion become non-uniform during the production process or during mixing with the matrix, but also the adhesion to the matrix decreases. In addition, in use, for example, when used for a battery electrode, its characteristics may be degraded. The smaller the amount of the sizing agent attached, the better in terms of fluidity, dispersibility, and adhesiveness to the resin. However, it is preferable to leave a trace amount of the sizing agent for reasons such as suppressing the scattering of the carbon fiber mill in the manufacturing process. In general, the residual amount of the sizing agent is preferably 0.005% by weight or more, though it depends on the properties and process of the carbon fiber mill.

The fiber length L of the carbon fiber mill of the present invention is 1 m
m or less and can be selected according to the application. It is effective for those having a thickness of 3 to 500 μm from the viewpoint of fluidity and dispersibility. It is more effective for those having a fiber length of 3 to 400 μm, which is easy to aggregate, and particularly effective for those having a short fiber length of 3 to 300 μm, which is easily aggregated. Fiber length is 3μ
If it is less than m, for example, the properties expected when used for reinforcing fibers may not be obtained. If the fiber length is 500 μm or more, the dispersibility tends to decrease. Diameter D of carbon fiber mill
Is generally in the range of 1 to 30 μm, and currently 4 to 12 μm is easy to use, but the present invention is not limited to this.

The aspect ratio L of the milled carbon fiber of the present invention
/ D can be determined according to the application, but fluidity,
From the viewpoint of dispersibility, it is usually in the range of 1 to 70. It is more preferably 1 to 50, and still more preferably 1 to 30. If the aspect ratio is less than 1, the properties as a fiber cannot be maintained, and if it exceeds 70, the fluidity and dispersibility tend to decrease.

The carbon fibers used in the present invention may be derived from any of polyacrylonitrile (hereinafter, referred to as PAN), pitch, rayon, etc., but PAN-based carbon fibers are preferred in view of strength characteristics. For use as a reinforcing fiber, one having a tensile strength of 1 to 5 GPa and a tensile modulus of 10 to 30 GPa is suitable. next,
The method for producing a carbon fiber mill of the present invention will be described.
The raw carbon fiber has a fiber length of 1 mm or more and 0.1
A fiber bundle having a sizing agent of about 5% by weight attached thereto to enhance convergence is preferable. Above all, 100-500 continuous carbon fiber
A so-called multifilament bundle of about 00 bundles is preferable in terms of productivity and easy handling. The multifilament is preferably supplied in the state of being wound on a bobbin in operation, but may be one that has been dropped in a ring shape in a container,
It can also be supplied continuously from the carbon fiber production process.

In the present invention, usually 0.1 to 5% by weight of the sizing agent attached to the raw carbon fiber is added to 0.05% by weight.
The weight is adjusted to less than 0.03% by weight, more preferably less than 0.01% by weight. Generally, it is better to reduce the amount of sizing agent adhered to the carbon fiber mill. If the amount of the sizing agent adhered is 0.05% by weight or more, coagulation occurs during milling, and the operability is significantly reduced. When agglomeration occurs, not only the production efficiency, the yield, and the fiber length uniformity are reduced, but also an overload is applied to the manufacturing apparatus, and in the worst case, the milling itself may not be possible. There is no particular limitation on the method for adjusting the amount of the carbon fiber sizing agent to be attached, and for example, thermal decomposition of the sizing agent by heat treatment, washing and removal using a solvent, and the like can be used. Heat treatment is preferable because the treatment can be continuously performed in a short time.

Hereinafter, the heat treatment will be described as an example. Examples of the heat treatment include a method of continuously supplying carbon fibers using a heating furnace, a method of intermittently supplying carbon fibers, and a method of directly energizing and heating carbon fibers. Generally, a method of continuously supplying carbon fibers using a heating furnace is advantageous in terms of productivity and operability. For the temperature distribution of the heating furnace to be used, it is efficient to provide a long soaking zone and to provide a low temperature region on the outlet side for cooling the heated carbon fiber multifilament. Heating temperature is furnace atmosphere, heat resistance of sizing agent,
Although it is appropriately selected according to the supply amount of the carbon fiber, it is sufficient that the surface of the carbon fiber to be heated can be heated to the thermal decomposition temperature of the sizing agent. The thermal decomposition temperature of the sizing agent can be easily known using a commercially available thermobalance or the like. The thermal decomposition initiation temperature of a general sizing agent containing an epoxy resin as a main component is about 300 to 500 ° C., and heating to about 500 ° C. can reduce the amount of the sizing agent attached. The heating time after the temperature is raised to the predetermined temperature may be set according to the type of the sizing agent, the target amount of adhesion, the ultimate temperature,
Generally, it is good to cool immediately after the temperature rises.

The atmosphere in the heating furnace is not particularly limited,
Usually, air is sufficient. When heating in air, the temperature of the heating section in the furnace is 300 to 700 ° C., preferably 400 to 650.
° C, more preferably 500-600 ° C. Although it depends on the atmosphere, if the temperature inside the heating furnace is 700 ° C. or higher, the carbon fiber is generally disadvantageously increased in weight loss. Also, 300
If the temperature is lower than 0 ° C, productivity is undesirably reduced. The heating time only needs to be able to raise the temperature of the carbon fiber surface to the thermal decomposition temperature of the sizing agent.

It is also important to reduce the amount of sizing agent deposited uniformly. For that purpose, the furnace inner surface density of the carbon fibers is kept within a certain range. Usually, the furnace inner surface density may be maintained in the range of 0.1 to 8 kg / m ² , preferably 0.3 to 6 kg / m ² , more preferably 0.5 to 4 kg / m ² .
kg / m ² . When the furnace inner surface density is low, the production efficiency is low, and when the furnace internal surface density is high, a temperature difference is generated between the inside and the outside of the carbon fiber bundle, making uniform heating and cooling difficult. Conditions will be selected based on a balance between quality and cost. In order to improve the productivity of the heating furnace, the carbon fiber may be divided into a plurality of surfaces and supplied in multiple stages. In this case, the furnace inner surface density is basically the surface density of each stage. In addition, as a method of supplying to a heating furnace, there is a method in which carbon fibers are shaken down on a conveyor net, passed through the furnace and heated. After reducing the amount of the sizing agent attached to the carbon fiber to less than 0.05% by weight, milling is performed. When the sizing agent is reduced in weight using a solvent, it is preferable to completely remove the solvent.

The milling means is not limited, but a mechanical pulverization method is effective. It can be appropriately selected from atters and pulverizers such as a hammer mill, a roll mill, an orient mill, and an impeller mill. The same device may be used for pulverizing a plurality of times, or a combination of different types of devices may be used. When manufacturing a carbon fiber mill having a fiber length of 3 to 100 μm,
After coarsely pulverizing to about 300 μm with a hammer mill or an oriental mill or the like, fine grinding to a required fiber length with a roll mill or an impeller mill or the like is preferable.

[0018]

EXAMPLES The effects of the present invention will be specifically described with reference to examples. Examples 1 to 4 After a PAN-based carbon fiber multifilament to which a sizing agent containing an epoxy resin as a main component was attached by 1.0% by weight was adjusted to reduce the amount of the sizing agent attached in a heating furnace, a hammer mill and a roll mill were used. Then, it was cut and pulverized to obtain a carbon fiber mill of the present invention. Table 1 summarizes the heating conditions, pulverizing conditions, evaluation results of the obtained carbon fiber mill, and the like. The yield (%) in the table is: Yield (%) = (weight of milled carbon fiber product) / (weight of milled carbon fiber fiber) × 100. is there.

[0019]

[Table 1] Comparative Examples 1-2 A carbon fiber mill was manufactured using the same carbon fiber multifilament as in Example 1 and in the same process, but under the conditions shown in Table 2. The results were evaluated in the same manner as in the examples, and are shown in Table 2.

[0020]

[Table 2]

[0021]

The carbon fiber mill of the present invention is excellent in fluidity and dispersibility by having a sizing agent adhesion amount of 0.05% by weight or less, thereby reinforcing fibers and functional materials of carbon fiber composite materials, such as battery electrodes. Is not only suitable for
The method for producing a carbon fiber mill of the present invention can be produced with high productivity. It is not clear why the reduction in the amount of the sizing agent adhered improves the fluidity, dispersibility, adhesiveness to a resin, and various functional properties, but is presumed as follows. It is considered that the fluidity and the dispersibility are improved because the interaction between the carbon fiber surfaces is smaller than the interaction between the sizing agents.
It is considered that the sizing agent was acting as an impurity in the adhesive force with the resin. The carbon fiber mill of the present invention is also suitable as a material for battery electrodes. For example, an electrode manufactured by mixing a carbon fiber mill with a conductive material, a binder, a solvent, and the like to form a paste and applying the paste on a metal foil is suitable for a negative electrode for a lithium ion secondary battery. By using the carbon fiber mill of the present invention, a paste dispersed uniformly and at high density can be obtained, and a battery having excellent discharge characteristics and high energy density can be manufactured.

Claims (9)

[Claims] 1. A carbon fiber having a fiber length not exceeding 1 mm,
A milled carbon fiber characterized by having a sizing agent not exceeding 0.05% by weight adhere thereto. 2. The carbon fiber mill according to claim 1, wherein the amount of the sizing agent attached is 0.005 to 0.05% by weight. 3. The milled carbon fiber according to claim 1, wherein the fiber length is 3 to 500 μm. 4. The milled carbon fiber according to claim 1, wherein the aspect ratio (L / D) is in the range of 1 to 70. 5. The milled carbon fiber according to claim 1, wherein the carbon fiber is derived from a polyacrylonitrile fiber. 6. After at least 0.1% by weight of the sizing agent is applied to the carbon fiber, the amount of the sizing agent is reduced to 0.1%.
A method for producing a milled carbon fiber, comprising adjusting the weight to less than 05% by weight and cutting the fiber to a fiber length not exceeding 1 mm. 7. At least 0.1% by weight of a sizing agent is applied to the multifilament of carbon fibers, and then the carbon fiber is heated to reduce the amount of the sizing agent to 0.05% by weight.
A method for producing a milled carbon fiber, comprising adjusting the weight to less than 1 mm and cutting into a fiber length not exceeding 1 mm. 8. The method according to claim 1, wherein the heat treatment is performed in the range of 300 to 7
00 a heating furnace in an air atmosphere maintained at ° C., claim 7, characterized in that from 0.1 carbon fiber per ² carbon fiber surface 1m continuously fed while maintaining the range of 8 kg, heat treatment A method for producing a carbon fiber mill according to the above. 9. A carbon fiber derived from polyacrylonitrile-based fibers is used.
A method for producing a carbon fiber mill according to the above.