JP3698156B2 - Carbon fiber - Google Patents

Description

The present invention relates to a carbon fiber having a high electrical resistance of 5 × 10 ⁻² Ωcm or more and a tensile strength of 90 kg / mm ² or more. This carbon fiber is particularly useful as a carbon fiber for reinforcing a thermoplastic resin.

  Carbon fibers are classified into PAN (polyacrylonitrile) type, pitch type, and the like depending on their raw materials. Among these, the pitch-based carbon fibers are roughly classified into high-performance carbon fibers and general-purpose carbon fibers according to the raw material preparation method. High-performance carbon fiber uses pitch obtained from coal, petroleum, etc. as a raw material, and by applying means such as heating to this, it produces a liquid crystalline optically anisotropic part that is a precursor of the carbon structure. It is obtained by spinning, making it infusible in an oxidizing atmosphere, then carbonizing, and if necessary, graphitizing. Here, the reason why the optically anisotropic part is generated is that the optically anisotropic part having liquid crystallinity has a good orientation, so that the obtained carbon fiber also has an excellent orientation and has a high strength. This is because it becomes easy to express a high elastic modulus. On the other hand, it is known that pitch-based carbon fibers used as a general-purpose grade are difficult to obtain high characteristics because optically isotropic pitches are spun, infusibilized, and fired.

  Conventionally, fiber reinforced resin compositions made by mixing and dispersing carbon fibers as short fibers in various matrices have been evaluated for mechanical properties such as high strength, high rigidity, low specific gravity, high electrical conductivity, and high wear resistance. It has been used for various purposes as an industrially important material. In general, when carbon fiber is mixed and dispersed in various resins to obtain a fiber reinforced resin material, a large number of carbon short fibers are sized in advance to facilitate handling of carbon short fibers and improve workability in the mixing and dispersing process. A short carbon fiber aggregate formed by bundling with an agent or the like is used.

Recently, however, the electrical resistance (10 ⁵ Ωcm or more) exceeds the electrical resistance (about 10 ⁻¹ to 10 ⁻² Ωcm) of the conventional fiber reinforced thermoplastic resin composition, particularly as an antistatic material for electronic parts.
There has been a demand for a fiber reinforced thermoplastic resin composition having a high carbon resistance and a high strength as a reinforcing fiber. The relationship between the carbonization temperature and the electrical resistance has been conventionally known, and the electrical resistance decreases as the carbonization temperature increases (see Non-Patent Document 1). However, when conventional raw materials are used, when carbon fibers are produced at a firing temperature within a range where a desired electrical resistance can be obtained, the temperature is low, so pitch-based carbon fibers cannot provide high tensile strength. Moreover, in the PAN-based carbon fiber, the nitrogen is not completely removed and the carbon content is low, and a desired carbon fiber cannot be obtained.
“Carbon Fiber”, published by Modern Editing Company, p. 83

As a result of intensive investigations to solve such problems, the present inventors have thoroughly eliminated the factors that cause defects that can cause a decrease in strength, and identified the optical anisotropy ratio of the spinning pitch. The inventors have found a production condition in which high strength is exhibited even though the firing temperature is low, and have found a carbon fiber having high electrical resistance and high strength.
That is, a liquid crystal pitch having an optical anisotropy ratio of 80% or more, a carbon content of 93% or more, and an ash content of 30 ppm or less is spun and infusible, and then fired at 700 to 1000 ° C. to obtain a tensile strength of 90 kg / mm. ² or more and a tensile modulus of 3 ton / mm ² or more and an electrical resistance 5 × 10 ^-2 Ωcm ~3.5Ωcm, in which it was possible to obtain more than 85% of carbon fiber carbon content.

  If the carbon fiber according to the present invention is used, a high-strength carbon fiber-reinforced resin composition can be obtained while having a high electrical resistance required as an antistatic material for electronic parts, for example.

Hereinafter, the present invention will be described in more detail.
In order to produce a fiber having high electrical resistance and high tensile strength according to the present invention, (i) thoroughly removing ash that causes a physical / chemical decrease in strength in the fiber. (B) The strength is improved by reducing the proportion of the optically isotropic portion in the spinning pitch stage that causes the strength reduction as a carbon crystal, and at the minimum temperature necessary for the strength development. It is necessary to fire and maintain high electrical resistance.

  Examples of the raw material pitch used in the production of the carbon fiber according to the present invention include, for example, coal-based coal tar, coal-tar pitch, coal liquefaction, petroleum-based heavy oil, pitch, petroleum resin and thermal polycondensation reaction product thereof. And carbonaceous raw materials such as polymerization reaction products by catalytic reaction of naphthalene or anthracene. Further, the carbonaceous raw material is subjected to a pretreatment by, for example, a method in which a soluble component is extracted with a specific solvent after heat treatment, or a method in which a hydrogenation treatment is performed in the presence of a hydrogen donating solvent and hydrogen gas. You may keep it.

Usually, the raw pitch contains ash (Ash component) as an insoluble substance. This then causes heat treatment of the raw material pitch to produce an optically anisotropic liquid crystal pitch that becomes a carbon fiber precursor, resulting in a heterogeneous precursor of the structure. Further, after spinning, physical defects are generated in the fiber obtained by infusibilization and firing, which adversely affects strength and elastic modulus.
In the present invention, the tensile strength of the carbon fiber can be remarkably improved by using a pitch purified to have an ash content of usually 30 ppm or less, preferably 20 ppm or less at the stage of the pitch used for spinning.

Carbon fiber produced from a spinning pitch having an ash content of more than 30 ppm is selectively oxidized around the ash that is exposed to the fiber surface in the oxidation reaction with the surrounding air atmosphere in the infusibilization reaction. As a result, so-called “pits” are generated, and defects that cause a decrease in strength are generated.
The timing for removing the ash may be any time before spinning, and for example, it may be removed at the raw material pitch stage or spinning pitch stage. The method for removing ash is not particularly limited, and a known method may be used. For example, there are gravity sedimentation method, centrifugal separation method, filtration method, adsorption method, washing method with acid, alkali, solvent, etc., but each may be performed independently, and combination with removal method suitable for each according to pitch form It may be repeated repeatedly. Further, in order to increase the removal efficiency, a porous inorganic substance (such as a filter aid) may be added. Industrially, it is preferable to use a gravity sedimentation method, a centrifugal separation method, and a filtration method because they can be processed continuously and in large quantities.

The pitch purified as described above is converted into a liquid crystal pitch that exhibits optical anisotropy according to a conventional method. In the present invention, the optical anisotropy ratio of the pitch used for spinning needs to be 80% or more, preferably 90% or more, and more preferably 95% or more.
If the optical anisotropy ratio is less than 80%, the strength of the carbon fiber is reduced, and if the required tensile strength is to be obtained, the firing temperature must be increased, and therefore the electrical resistance is inevitably reduced. Therefore, the desired high electrical resistance and high strength carbon fiber cannot be obtained.

The pitch optical anisotropy ratio referred to in the present invention is a value obtained by obtaining a portion showing the optical anisotropy in the pitch sample under normal temperature and a polarizing microscope as an area ratio. Specifically, for example, a sample obtained by pulverizing a pitch sample into several mm squares is embedded in almost the entire surface of a resin having a diameter of about 2 cm according to a conventional method, and after polishing the surface, the entire surface is polarized. It is obtained by observing under a microscope (magnification 100 times) and measuring the area ratio of the optically anisotropic portion in the total surface area of the sample.

The method for producing the optically anisotropic liquid crystal pitch is not particularly limited, and a known method may be used. For example, the purified pitch is usually 350 to 500 ° C., preferably 380 to 450 ° C. for 2 minutes to 50 hours, preferably 5 minutes to 5 hours, in an inert gas atmosphere such as nitrogen, argon, water vapor, or the like, or There is a method of performing heat treatment under blowing or under reduced pressure. As another example, a method for polymerizing condensed polycyclic hydrocarbons such as naphthalene in the presence of a catalyst such as HF / BF ₃ or solvent splitting using a solvent having a certain solubility parameter as a raw material pitch. There is a method of performing a desired pitch.

The carbon content of the spinning pitch is 93% or more, preferably 95% or more. If the carbon content is less than 93%, like the above-mentioned ash, foreign elements such as nitrogen, sulfur, oxygen and the like cause a decrease in strength and decrease the tensile strength of the carbon fiber.
Using the spinning pitch as described above, melt spinning is performed to obtain pitch fibers. Since this pitch fiber has low breaking strength as a single fiber, 1000 to 20000 pitch fibers are converged with a sizing agent to prevent generation of fluff in a guide, a roller or the like, and a pitch fiber tow is obtained. Here, as the sizing agent, it is necessary to dissolve a part of the pitch fibers or to use a material that does not cause the fibers to be bonded or fused together during the infusibilization treatment. For example, a water emulsion of silicone oil Is preferred. In order to more effectively avoid the fusion, inorganic fine particles such as carbon black and SiC may be added to the sizing agent.

Next, the pitch fiber tow is infusibilized by heating to 160 to 400 ° C. in an oxidizing gas atmosphere by continuous / batch processing.
The obtained infusible fiber tow is baked in an inert gas atmosphere such as nitrogen or argon. In order to obtain a desired carbon fiber in the present invention, firing is usually performed at 700 to 1000 ° C, preferably 730 to 900 ° C, and more preferably 750 to 850 ° C. When the firing temperature is lower than 700 ° C., a high electrical resistance can be obtained, but the tensile strength is not sufficiently developed. When the firing temperature exceeds 1000 ° C., the tensile strength is high but the electrical resistance is low. Only carbon fiber can be obtained.

The carbon fiber according to the present invention is usually obtained by the above method, and the carbon fiber obtained in this way has the properties of high strength and high electrical resistance, but usually has a tensile strength of 90 kg / mm ² or more. , Preferably 100 kg / mm ² or more, more preferably 110 kg / mm ²
Or more, the tensile modulus 3 ton / mm ² or more, preferably 4 ton / mm ² or more, more preferably 5 ton / mm ² or more, a volume electrical resistivity is 5 × 10 ^-2 Ωcm or more,
Preferably, it is 1 × 10 ⁻¹ Ωcm or more. Tensile strength is 90kg / mm ² and tensile modulus is 3
If it is less than ton / mm ² , since the reinforcing effect of the resin is small when used as a carbon fiber for resin reinforcement, the resulting molding material cannot have sufficient strength and elastic modulus. Further, if the volume specific electric resistance is smaller than 5 × 10 ⁻² Ωcm, when the molding material is similarly formed, the electric resistance is small, that is, the conductivity is improved. Can no longer be used. Here, the tensile strength and the tensile elastic modulus are values measured using a single fiber sample according to JIS R7601, and the volume resistivity is a value measured using a yarn sample according to JIS R7601.

Next, a method for producing a carbon short fiber aggregate for resin reinforcement from the carbon fiber will be described. In the carbon fiber tow thus obtained, the sizing agent with respect to the total amount of the carbon fiber is usually at 0.
2-10 wt.%, Preferably 0.5-7 wt. When the amount of sizing agent is less than 0.2% by weight, the carbon fiber has poor convergence, and when it is later aligned into 1 to 30 mm short fibers, the bulk density of carbon short fibers is low and the handling property is poor. In addition, when the amount exceeds 10% by weight, the short carbon fiber aggregate obtained later is too good in convergence, and the dispersibility in the resin is lowered, and the short carbon fiber reinforced thermoplastic resin is obtained. This is not preferable because the physical properties of the resin deteriorate.

  As a method for attaching the sizing agent, there is a method in which 1,000 to 20,000 carbon fiber tows are impregnated with the sizing agent and then dried. The form of the sizing agent for impregnation may be dissolved in an appropriate solvent or dispersed in water as an emulsion using a surfactant. Examples of the solvent used include 2-butanone, tetrahydrofuran, N, N-dimethylformamide, acetone, chloroform, and dichloromethane.

  As the sizing agent, any material usually used for this purpose can be used, and a carbon fiber tow with good converging property may be selected from them. Specifically, homopolymers or copolymers such as epoxy compounds, saturated or unsaturated polyesters, polyphenylene sulfites, polyphenylene ethers, polycarbonates, polyoxymethylenes, polystyrenes, polyolefins, polyurethane resins, acrylic resins, vinyl acetate resins, polyamide resins, etc. Etc. Among these, an epoxy compound, a water-soluble polyamide compound, or a polyurethane compound is particularly preferable.

  The short carbon fiber aggregate is obtained by cutting carbon fiber tows focused by a sizing agent using a known cutting method, and usually has an arbitrary length of 1 to 30 mm, preferably 3 to 10 mm, or the range thereof. It is a mixture of lengths. If the fiber length is shorter than 1 mm, the yarn is scattered to the lowest single yarn level due to the shearing force applied to the fiber tow at the time of cutting, and a concentrated short fiber aggregate cannot be obtained. When used for reinforcing a thermoplastic resin, it becomes difficult to obtain a uniform mixture upon mixing with the resin.

In addition, the obtained short carbon fiber aggregate, particularly when used for reinforcing a thermoplastic resin, has a bulk density of usually 300 g / l or more, preferably 350 g / l or more, more preferably 400 g / l or more. Is desirable. When the bulk density is less than 300 g / l, the feed property at the time of blending with the resin is lowered due to its bulkiness.
EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to an Example, unless the summary is exceeded.

After adding 1 part by weight of a pre-hydrogenated aromatic oil having a boiling range of 240 to 290 ° C. to 1 part by weight of coal tar and mixing, a commercially available diatomaceous earth filter aid “Celite 505” (trade name) And 0.01 parts by weight of Celite), and filtered through a candle filter having an opening of 10 μm. The obtained filtrate was continuously supplied to an autoclave maintained at a temperature of 450 ° C. and a hydrogen pressure of 150 kg / cm ² . The average residence time was 60 minutes. The obtained reaction product was further filtered through a sintered filter having an opening of 0.5 μm, and the filtrate was distilled under reduced pressure to obtain a hydrogenated pitch. The obtained hydrogenated pitch was heat-treated at 430 ° C. for 140 minutes under nitrogen gas bubbling, and a spinning pitch having an optical anisotropy ratio of 100%, a Mettler softening point of 302 ° C., a carbon content of 96% by weight and an ash content of 20 ppm. Was prepared.

Next, spinning was performed at a die temperature of 330 ° C. while concentrating the spinning pitch with a silicon-based oil, and a continuous long pitch fiber tow having 8000 filaments and a fiber diameter of 13 μm was obtained.
Next, the pitch fiber tow was infusibilized in air and then fired in nitrogen gas at 770 ° C. under a residence time of 2 minutes to prepare carbon fibers. The obtained carbon fiber has a carbon content of 89% and a fiber diameter of 1
The volume resistivity was 2.4 μ, the tensile strength was 100 kg / mm ² , the tensile modulus was 5.0 ton / mm ² , and a high volume specific electrical resistance of 3.5 Ωcm was exhibited. After 6% by weight of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting device to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 510 g / l.

After 15 parts by weight of this short carbon fiber aggregate and 85 parts by weight of polycarbonate resin pellets were dry blended, they were charged into a screw extruder, melted and mixed, extruded into strands, cooled to water, and cut into pellets. The feeding into the extruder was smooth, and the short carbon fiber aggregate and the resin were uniformly dispersed. The carbon short fiber reinforced molding material thus obtained was dried at 90 ° C. for 4 hours and then injection molded to prepare a test piece. The bending strength (according to ASTM-D790) of this test piece was 1350 kg / cm ² , and the volume specific electric resistance was 1 × 10 ⁸ Ωcm.

An infusible fiber tow prepared in exactly the same manner as in Example 1 was baked in nitrogen gas at 820 ° C. for a residence time of 2 minutes to prepare a carbon fiber. The obtained carbon fiber has a high volume resistivity of 1.7 × 10 ⁻¹ Ωcm with a carbon content of 91%, a fiber diameter of 12.3 μ, a tensile strength of 120 kg / mm ² and a tensile elastic modulus of 6.5 ton / mm ^2. showed that.
After 5 wt% of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting device to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 500 g / l.

Further, the short carbon fiber reinforced molding material prepared by dispersing the fibers in the same manner as in Example 1 in a weight of 15% by weight is 1400 kg / cm ² , and the volume resistivity is 2 × 10. ⁷ Ωcm.

An infusible fiber tow prepared in exactly the same manner as in Example 1 was baked in nitrogen gas at 950 ° C. for a residence time of 0.07 minutes to prepare carbon fibers. The obtained carbon fiber has a high volume resistivity of 1.8 × 10 ⁻¹ Ωcm with a carbon content of 91%, a fiber diameter of 12.2 μ, a tensile strength of 120 kg / mm ² and a tensile elastic modulus of 6.2 ton / mm ^2. showed that.
After 5 wt% of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting device to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 520 g / l.

Comparative Example 1
The hydrogenated pitch obtained in the same manner as in Example 1 was heat-treated at 430 ° C. for 40 minutes under nitrogen gas bubbling, with an optical anisotropy ratio of 30%, a Mettler softening point of 280 ° C., and a carbon content of 91% by weight. A spinning pitch with an ash content of 20 ppm was prepared.
Next, the spinning pitch was spun at a die temperature of 310 ° C. while converging with a silicon-based oil agent, and a continuous long pitch fiber having 8000 filaments and a fiber diameter of 13 μm was obtained.

Next, the pitch fiber was infusibilized in air and then fired in nitrogen gas at 820 ° C. for 2 minutes in residence time to prepare carbon fiber. The resulting carbon fiber had a low tensile strength of 40 kg / mm ² . The tensile modulus was 3.0 ton / mm ² and the volume resistivity was 3.5 Ωcm.
After 5 wt% of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting apparatus to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 490 g / l.

Further, a carbon fiber reinforced molding material prepared by the same procedure as in Example 1 and having the above fibers dispersed in a polycarbonate resin by 15% by weight has a bending strength of 1150 kg / cm ² and a volume specific electrical resistance of 3 × 10 ^8. The reinforcing effect was small due to the low strength of the carbon fiber.
Comparative Example 2
Carbon fibers were prepared in exactly the same manner as in Example 3 except that filtration with a 0.5 μ sintered filter after the hydrogenation reaction was not performed. The physical properties of the spinning pitch were an optical anisotropy ratio of 100%, a Mettler softening point of 302 ° C., a carbon content of 96% by weight, and an ash content of 80 ppm. The tensile strength of the carbon fiber obtained from this spinning pitch was as low as 60 kg / mm ² . The tensile modulus was 6.0 ton / mm ² and the volume resistivity was 1.7 × 10 ⁻¹ Ωcm.

After 5 wt% of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting apparatus to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 490 g / l.
The short carbon fiber reinforced molding material prepared by dispersing the fibers in the same manner as in Example 1 and having a weight of 15% by weight dispersed in polycarbonate resin has a bending strength of 1250 kg / cm ² and a volume specific electrical resistance of 2 × 10. ^The reinforcing effect was small due to the low strength of the carbon fiber.

Comparative Example 3
An infusible fiber tow prepared in exactly the same manner as in Example 1 was baked in nitrogen gas at 1200 ° C. and a residence time of 7 seconds to prepare carbon fibers. The obtained carbon fiber has a carbon content of 99%, a fiber diameter of 11.5 μ, a tensile strength of 200 kg / mm ² , a tensile elastic modulus of 19.0 ton / mm ² , and a volume specific electrical resistance of 2 × 10 ⁻³ Ωcm. there were.

After 6 wt% of an epoxy sizing agent was attached to the obtained carbon fiber, it was continuously fed to a cutting apparatus to obtain a short carbon fiber aggregate having a cut length of 6 mm. The bulk density of the obtained product was 510 g / l.
Further, a carbon fiber reinforced molding material prepared by dispersing in the same manner as in Example 1 in which 15% by weight of this fiber is dispersed in a polycarbonate resin has a bending strength of 1450 kg / cm ² and a volume specific electrical resistance of 1 × 10 ⁻ Only ² Ωcm, which had high strength but low electrical resistance, was obtained.
The results are shown in Table 1.

Claims (4)

Carbon fiber having a carbon content of 85% or more, having a tensile strength of 90 kg / mm ² or more, a tensile modulus of 3 ton / mm ² or more, and an electrical resistance of 5 × 10 ⁻² Ωcm to 3.5 Ωcm. Carbon fiber having a carbon content of 85% or more, having a tensile strength of 100 kg / mm ² or more, a tensile modulus of 4 ton / mm ² or more, and an electrical resistance of 1 × 10 ⁻¹ Ωcm to 3.5 Ωcm. Carbon having a tensile strength of 90 to 120 kg / mm ² , a tensile elastic modulus of 3 to 6.5 ton / mm ² and an electric resistance of 5 × 10 ^{−2 to} 3.5 Ωcm and a carbon content of 85% or more. fiber. A carbon fiber tow obtained by focusing 1000 to 20000 carbon fibers according to any one of claims 1 to 3.