JPH04222275A - Highly electrically conductive carbon fiber and its production - Google Patents

Abstract

PURPOSE:To obtain carbon fiber, having a coating layer composed of expanded graphite on the surface and further a high electric conductivity and excellent in mechanical characteristics and an efficient method for producing the carbon fiber. CONSTITUTION:Graphite powder is heated with an acid such as nitric acid or sulfuric acid to provide oxidized graphite, which is further heated and expanded to afford expanded graphite powder. The resultant expended graphite powder is then applied onto the surface of carbon fiber and the obtained carbon fiber is normally press formed under 10-100kg/cm<2> pressure to afford highly electrically conductive carbon fiber having a coating layer of the expanded graphite formed on the surface thereof by utilizing mutual cohesive force of the expanded graphite.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a carbon fiber having a coating layer made of expanded graphite on its surface and having high electrical conductivity and excellent mechanical properties, and an efficient manufacturing method thereof. Regarding. The highly conductive carbon fiber of the present invention can be effectively used as a material for electrical equipment, particularly as a static elimination material such as a grounding material for lightning protection in buildings or a grounding material for telephones.

0002

[Prior Art and Problems to be Solved by the Invention] Carbon fibers have excellent specific strength, specific modulus of elasticity, and chemical resistance, and are therefore used in the fields of space and aeronautics, sporting goods, and other industrial materials. Its uses are about to expand even further.

The electrical properties of carbon fibers are significantly influenced by their firing temperature. Generally 1000℃~14
The electrical resistance of carbon fiber fired at a temperature of about 00°C is 1
It is approximately 0-2 to 10-3 Ω·cm, but in order to lower the electrical resistance more than this, it is necessary to bake at a temperature of 2000°C to 2500°C, or in some cases close to 3000°C. By graphitizing carbon fiber through heat treatment at high temperatures, its electrical resistance can be reduced to 10-4Ω・cm.
Although it is possible to reduce the temperature to a certain extent, in this case it is inevitable that a large amount of thermal energy and special baking equipment are required. Therefore, if the electrical resistance of carbon fibers fired at a temperature of about 1000°C can be lowered to about 10-4 Ωcm without heat treatment at high temperatures, it would be an extremely effective technology from an economic point of view. become.

[0004] Various methods have heretofore been attempted in order to improve the electrical resistance of carbon fibers fired at a temperature of about 1000°C. For example, a method has been proposed in which a metal film is formed on the surface of carbon fiber by plating.
There are problems with waste liquid treatment, etc. In addition, methods have been proposed to improve the electrical properties of carbon fibers by forming a film by vapor deposition, sputtering, or ion implantation, but these methods have the problem of requiring large-scale equipment. there were.

[0005] The present inventor solved these conventional problems and lowered the electrical resistance of carbon fiber fired at a temperature of about 1000° C. to about 10 −4 Ω·cm without heat treatment at high temperatures. We have conducted extensive research into possible methods. As a result, by adhering expanded graphite powder onto the surface of carbon fibers and then press-molding this, a coating layer made of expanded graphite is formed on the surface, which has high electrical conductivity and is machined. The inventors have discovered that carbon fibers with excellent physical properties can be obtained, and have completed the present invention based on this knowledge.

[0006]

[Means for Solving the Problems] That is, the present invention provides a highly conductive carbon fiber having a coating layer of expanded graphite formed on the surface of the carbon fiber. There are no particular restrictions on the carbon fibers constituting the highly conductive carbon fibers of the present invention, and there are various known carbon fibers, such as pitch-based carbon fibers obtained from coal pitch, petroleum pitch, synthetic pitch, etc., and polyacrylonitrile-based ( PAN-based carbon fibers can be used.

[0007] It is sufficient that the carbon fiber is fired at a temperature of about 1000°C to 1400°C, and there is no need to use one fired at a high temperature exceeding 2000°C. Furthermore, the thread diameter, length, etc. of carbon fibers vary depending on the shape, and are difficult to determine unambiguously.

[0008] Here, the shape of the carbon fibers may be strand, nonwoven fabric, woven fabric, or the like. For example, in the case of strand-like carbon fiber, the thread diameter is usually 0.1 to
10 μm, preferably 7 to 10 μm, 1000
A bundle of ~12,000 pieces is used, and in the case of a nonwoven fabric, the length is 3 to 60 mm, preferably 5 to 25 mm.
In the case of a woven fabric, a woven fabric of the above-mentioned strand-shaped carbon fibers is preferably used.

On the other hand, expanded graphite, which constitutes the highly conductive carbon fiber of the present invention and forms the coating layer formed on the surface of the carbon fiber, is one of the carbon materials that has high conductivity. Graphite oxide is made by heating the powder with nitric acid, sulfuric acid or other acids;
It is inflated.

[0010] The processing conditions for producing graphite oxide are as follows: graphite powder is heated in the presence of nitric acid, sulfuric acid, or other acids;
It is sufficient to perform the treatment at room temperature for about 1 to 10 hours. In addition, in order to produce expanded graphite from the graphite oxide obtained in this way, the graphite oxide is placed in an electric furnace or the like and heated to a temperature of 600 to 1000°C, preferably 750 to 850°C. It is sufficient to process for about 5 minutes.

[0011] The expansion ratio can be changed as appropriate by changing the ratio of concentrated nitric acid to concentrated sulfuric acid or by changing the treatment time, but it is usually 1 to 5 times, preferably 2 to 3 times. shall be. Here, if the expansion ratio is too low, the coating will peel off when coated on carbon fibers. Also,
The higher the expansion ratio is, the more preferable it is because a thin coating layer can be formed on the carbon fiber.

The expanded graphite thus obtained is in powder form with an average particle size of 10 to 100 μm, preferably 30 to 50 μm. If the average particle size is less than 10 μm, the workability will decrease, while if the average particle size is less than 10 μm, the workability will decrease.
If it exceeds m, it will be easy to peel off when coating the carbon fiber surface. When such expanded graphite is made into a sheet by pressing or passing between rolls,
Usually, it has an electrical resistance of about 10-4 Ωcm.

The highly conductive carbon fiber of the present invention has such expanded graphite on the surface of the carbon fiber, usually 50 to 500 μm thick.
Preferably, it is coated with a thickness of 100 to 150 μm. If the thickness of this coating layer is too small, it will be difficult to obtain a highly conductive material, and if the thickness of this coating layer is too large, the expanded graphite will easily peel off during bending or deformation, which is not preferable. When the amount of coverage of expanded graphite is expressed as a composition ratio, expanded graphite is usually 99 to 1% by weight, preferably 96 to 60% by weight, and carbon fiber is usually 1 to 1% by weight.
99% by weight, preferably 4-40% by weight.

The highly conductive carbon fiber of the present invention, which has a coating layer of expanded graphite formed on the surface of the carbon fiber, can be produced by various methods. After adhering to the carbon fiber surface, it can be manufactured efficiently by press-molding it. In order to form expanded graphite powder on the carbon fiber surface, a method using phenol resin, furan resin, or epoxy resin can be considered, but in the method of the present invention, it is possible to form expanded graphite powder on the carbon fiber without using any of these resins. It is produced by pressing expanded graphite powder into close contact with each other and utilizing the cohesive force between expanded graphite particles.

[0015] Therefore, it is possible to sufficiently control the electrical resistance of carbon fibers by adjusting the amount and thickness of expanded graphite deposited. Such a coating layer has a weight of 10 to 100 kg/cm2
, preferably at a pressure of 20-50 kg/cm2 and a temperature of 0-200°C, preferably 20-30°C,
It can be obtained by pressure molding using a method such as a press for 10 seconds to 5 minutes, preferably 30 seconds to 2 minutes. If the pressure in pressure molding is too low or the pressurization time is too short, it will be difficult to form a coating layer with good adhesion, while if the pressure is too high, there is a risk of damaging the carbon fibers. be. Further, if the pressurization time is too long, it is not preferable because it causes a decrease in productivity.

[0016] In this manner, the highly conductive carbon fiber of the present invention can be obtained. The expanded graphite layer on the surface of the carbon fiber, which constitutes the highly conductive carbon fiber of the present invention, may peel off if it is repeatedly bent to a large extent, but if the deformation is small, this will not occur at all, and usually There is no problem in using it. The present invention makes it possible to reduce the electrical resistance of the carbon fiber surface by forming an expanded graphite layer with high conductivity on the carbon fiber surface.

[0017]

EXAMPLES Next, the present invention will be explained in detail by examples.

Production Example 1 (Production of expanded graphite) Graphite powder 50
g was placed in a beaker, 100 g of concentrated sulfuric acid (concentration 95%) and 50 g of concentrated nitric acid (concentration 61%) were added, and the mixture was thoroughly stirred at room temperature for 2 hours. Thereafter, it was diluted with a large amount of water, and then washed with water several times by decantation or suction filtration, and this process was repeated until the pH of the washing solution reached 5. Finally, the mixture was filtered to produce oxidized graphite powder. This graphite powder 3
When the sample was placed in a crucible and placed in an electric furnace heated to 800° C. for 2 minutes, graphite powder (expanded graphite) expanded by about 4 times was obtained.

Production Examples 2 to 5 (Production of Expanded Graphite) The degree of expansion of the produced graphite oxide was studied when the ratio of concentrated sulfuric acid and concentrated nitric acid was varied. The amount of graphite used is 1
5g, the total amount of concentrated sulfuric acid and concentrated nitric acid used is about 150g, and the ratio of concentrated sulfuric acid to concentrated nitric acid is 2:1, 3:1.
, 4:1 and 5:1 (Production Example 2 respectively)
, 3, 4, 5) processed. Note that the treatment temperature was room temperature in all cases, and the treatment time was 2 hours. Further, washing with water, drying, etc. after treatment were performed in the same manner as in Production Example 1. Even if the ratio in the mixed acid used was different, there was no difference in the appearance of the produced graphite oxide.

[0020] Then, take 3 g of graphite oxide and add it to 80
It was placed in an electric furnace at 0° C. and heat treated for 1 minute, 2 minutes, and 3 minutes to cause expansion. The apparent volume of the graphite powder before and after this treatment was roughly measured to determine the expansion ratio. As a result, it was found that the expansion ratio was highest when the ratio of concentrated nitric acid to concentrated sulfuric acid was 2:1, which was a preferable blending ratio. Regarding the heating time at 800°C, 1 minute was insufficient because the graphite did not fully expand, but the expansion ratio was almost the same for 2 minutes and 3 minutes, so heat treatment for 2 minutes ,
It was confirmed that the graphite expanded completely. The results are shown in Table 1.

Table 1

Example 1 The following operation was performed to form expanded graphite powder on the surface of carbon fibers. The carbon fiber is a non-woven fabric (width 3 cm, length 6 cm, thickness 0.3 mm, basis weight 33
g/m2) was used. A predetermined amount of the expanded graphite powder obtained in Production Example 2 was uniformly spread over this carbon fiber nonwoven fabric through a sieve. Next, put a Teflon film on top of this,
It was attached to the carbon fiber by applying light pressure. Next, the carbon fiber nonwoven fabric was turned over and the same amount of expanded graphite powder was applied.

[0023] This is sandwiched between two stainless steel plates,
It was molded by applying a pressure of 30 kg/cm2 for 1 minute using a hydraulic press. The thickness of the carbon fiber nonwoven fabric after pressurization varies depending on the amount of expanded graphite attached, but it ranges from about 0.4 mm to 1 mm.
．． It increased to about 1mm. Moreover, the bulk density was also increased. These results are shown in Table 2 together with the specific resistance values obtained by the following method.

The electrical resistance of the prepared sample piece was measured using an ordinary tester by changing the distance between the two probes from 6 cm to 1 cm.
The electrical resistance at zero distance was estimated and determined as the specific resistance of the sample.

The electrical resistance of the carbon fiber nonwoven fabric is 20 to 40Ω as measured by a tester, which is 0 when converted to specific resistance.
．． 95Ω・cm, but by attaching expanded graphite particles to the carbon fiber nonwoven fabric, the measured value of the tester becomes 0.95.
It gradually decreased from Ω to a value so small that it could not be measured with a normal tester. Therefore, when we determined the electrical resistance of a commercially available expanded graphite sheet (specific resistance is about 10-4 Ω cm), we found that the electrical resistance was exactly the same as that of a commercially available expanded graphite sheet. Resistance is also 10-
It is estimated to be about 4Ω·cm. Further, the expanded graphite powder did not peel off from the carbon fiber nonwoven fabric, and exhibited a good adhesion state.

Table 2

Example 2 Strand-shaped petroleum pitch carbon fiber (thread diameter: 10 μm
, 3,000 pieces) were cut into lengths of about 5 cm, expanded graphite powder was attached to both sides of the carbon fibers, and then pressed to produce an expanded graphite powder-carbon fiber composite material. The amount of expanded graphite attached to carbon fiber was increased by 4 times, 9 times, 16 times, 24 times, 32 times, 49 times, and 99 times (sample 8,
9, 10, 11, 12, 13, 14), and was uniformly adhered to both sides of the carbon fiber. The expanded graphite powder also entered between the carbon fiber filaments and sufficiently filled the voids. Even when the carbon fiber strand was bent or curved, the expanded graphite attached to the surface did not peel off.

[0028] The electrical resistance of the carbon fiber surface in this expanded graphite powder-carbon fiber composite material was measured at a distance between the probes of 4 cm. The specific resistance before attaching the expanded graphite powder is
It was approximately 1Ω, but as the amount of expanded graphite increased, Table 3
As shown in Figure 2, it was found that the specific resistance value was reduced and the conductivity was improved.

Table 3

[0030]

[Effects of the Invention] The carbon fiber of the present invention not only has extremely good conductivity but also excellent mechanical properties. Further, according to the method of the present invention, the carbon fiber coated with expanded graphite described above can be easily heated without being subjected to heat treatment at high temperature.
And it can be obtained economically.

Claims (2)

[Claims] 1. A highly conductive carbon fiber comprising a coating layer of expanded graphite formed on the surface of the carbon fiber. 2. The method for producing highly conductive carbon fibers according to claim 1, wherein the expanded graphite powder is adhered onto the surface of the carbon fibers and then pressure-molded.