JPH04257321A - Production of pitch carbon fiber having high tensile elastic modulus and high compression strength - Google Patents

Abstract

PURPOSE:To produce a pitch carbon fiber having a high tensile elastic modulus and a high compression strength, the compression strength being largely improved in comparison with conventional carbon fibers while the tensile elastic modulus is maintained in the high state. CONSTITUTION:A liquid crystalline pitch having an optically anisotropic phase content of >=955 and an average mol.wt. of <=1600 is melt-spun through a fine diameter nozzle having a diameter of 0.10-0.25mm under a shear stress of 0.2-7.0kg/cm<2>, followed by subjecting the produced pitch fiber to a thermal treatment to provide a pitch carbon fiber having a high tensile elastic modulus and a high compression strength.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to pitch-based carbon fibers, and more particularly to a method for producing pitch-based carbon fibers that have a high tensile modulus and a high compressive strength. High tensile modulus obtained in the present invention,
High compressive strength pitch-based carbon fibers can be suitably used as reinforcing fibers for composite materials used in various industrial fields such as the space and aviation industry, the automobile industry, and the construction industry.

0002

[Prior Art] In recent years, there has been a strong demand for low-cost, high-performance carbon fibers as reinforcing fibers for lightweight, high-strength, and high-elasticity composite materials in the space/aircraft, automobile, construction, and other various industrial fields. There is.

Conventionally, for example, PAN-based carbon fibers made from polyacrylonitrile or rayon-based carbon fibers have been widely used as carbon fibers having high tensile strength and high tensile modulus, but the raw materials are expensive and However, the carbonization yield is poor and there are problems in economic efficiency.

On the other hand, research and development of carbon fibers using petroleum-based pitch or coal-based pitch as raw materials has been actively conducted in recent years because the raw materials are inexpensive and the carbonization yield is higher. In order to obtain, for example, high-strength, high-modulus carbon fiber from carbonaceous raw material pitch such as petroleum-based pitch or coal-based pitch, the optically anisotropic phase must be 95% or more, substantially 100%. It is known that it is necessary to obtain a liquid crystal pitch that is For example, Japanese Patent Application Laid-open No. 54-55625 discloses that using inert gas bubbling and stirring together,
JP-A-54-160427 discloses a method of obtaining liquid crystal pitch in which the optically anisotropic phase is substantially 100% by performing long-term thermal decomposition polycondensation. A method for obtaining a liquid crystal pitch with substantially 100% orthogonal phase is disclosed. Also, special public service No. 61-38755
The publication discloses that a liquid crystal pitch containing substantially 100% of the optically anisotropic phase is produced by heat-treating raw material pitch to produce a pitch containing an optically anisotropic phase, and then performing specific gravity separation. discloses how to obtain it. According to this method,
The softening point is said to be about 230 to 320°C, and the
Compared to liquid crystal pitch for pitch-based carbon fibers as described in Japanese Patent Application Laid-Open No. 55625 or Japanese Patent Application Laid-Open No. 54-160427, the optically anisotropic phase with an extremely low softening point is substantially 100% % liquid crystal pitch is obtained,
Therefore, it has the advantage that spinning can be stably performed at a spinning temperature of 280 to 380°C.

The liquid crystal pitch thus obtained is melt-spun, for example, at a temperature of 280 to 380°C to form pitch fibers, which are then spun at 200° C. in an oxidizing gas atmosphere.
It is made infusible at ~350°C and further heated under an inert gas atmosphere for 5
Carbonization or graphitization was performed by high-temperature firing at a temperature of 00 to 3,000°C to produce carbon fibers. Further, if necessary, in a heat treatment process such as infusibility or high temperature firing, stretching treatment may be performed simultaneously with the heat treatment.

In the pitch-based carbon fiber produced in this way, the fiber axis orientation of the pitch component is strengthened by melt spinning, thereby achieving a high degree of axial orientation of the crystals, and liquid crystal by high temperature firing. The polycondensation reaction of methyl and naphthenic groups in pitch promotes graphitization and improves crystal size, resulting in high tensile strength (σt) and high tensile modulus (
Et) is achieved. Furthermore, such pitch-based carbon fibers also have high thermal conductivity and high electrical conductivity.

[0007]

[Problems to be Solved by the Invention] As described above, pitch-based carbon fibers exhibit an improvement in the degree of graphite crystallinity of the carbon fibers due to high-temperature firing, and an accompanying significant improvement in tensile properties, especially tensile modulus (Et). On the other hand, it has a major problem in that the compressive strength (σc) becomes extremely low as the degree of crystallinity increases. The current pitch-based carbon fiber has a tensile modulus of 50
T/mm2, the compressive strength (σc) is 40 to 50 kg/mm2, and the tensile modulus is 7.
If it is about 0T/mm2, the compressive strength is 35~
It becomes very low at 40 kg/mm2.

[0008] Recently, with the advancement of technology, in the space/aircraft, automobile, architectural, and other various industrial fields, low-cost products that are lightweight, have high tensile strength, high tensile modulus, and high compressive strength are being used. High-performance carbon fiber is desired as a material for composite materials.

[0009] The present inventors conducted many research experiments in order to obtain a carbon fiber having particularly high tensile modulus and high compressive strength, which met these demands.

[0010] Conventionally, the disadvantage of low compressive strength of pitch-based carbon fibers is due to high crystallization accompanying high-temperature firing, and in general, the tensile modulus improves with high-temperature firing. Since the compressive strength decreases, in order to maintain the tensile modulus at a high level and improve the compressive strength, it is necessary to optimize the manufacturing conditions after the liquid crystal pitch spinning process and increase the tensile modulus through low-temperature heat treatment as much as possible. It is considered essential to

[0011] In the course of many research experiments, the present inventors found that the tensile modulus of pitch-based carbon fibers is generally controlled by the heat treatment temperature of the pitch fibers, as described above. We have found that the spinning process is also a very important process in order to obtain pitch-based carbon fibers with high tensile modulus.

In other words, control in the spinning process is generally carried out according to the melt viscosity of the liquid crystal pitch, but the present inventors have attempted to reduce the nozzle diameter of the spinning nozzle and reduce the nozzle shear stress ( τ) is 0.2 to 7.0 kg/cm2
By spinning under extremely high conditions such as The produced pitch-based carbon fiber has a lower tensile modulus (Et)/compressive strength (σc) ratio than that produced by conventional methods, and as a result, pitch-based carbon fiber with high tensile modulus and high compressive strength is produced. I found out that I can get it. The present invention has been made based on this new knowledge.

Therefore, an object of the present invention is to optimize the nozzle and spinning conditions in the liquid crystal pitch spinning process, and to maintain the tensile modulus of the manufactured carbon fiber at a high level.
An object of the present invention is to provide a method for producing pitch-based carbon fibers with high tensile modulus and high compressive strength, which can significantly improve compressive strength compared to conventional carbon fibers.

[0014]

[Means for Solving the Problems] The above objects are achieved by a method for producing pitch-based carbon fibers with high tensile modulus and high compressive strength according to the present invention. In summary, the present invention provides liquid crystal pitch having an optically anisotropic phase content of 95% or more and an average molecular weight of 1600 or less, by using a small nozzle with a nozzle diameter of 0.10 to 0.25 mm to reduce shear stress. 0.2-7.0kg/cm2
This is a method for producing pitch-based carbon fibers with high tensile modulus and high compressive strength, which is characterized by spinning pitch fibers by melt-spinning the pitch fibers and then heat-treating the pitch fibers. Preferably, the average molecular weight of the liquid crystal pitch is 1000 to 15
00, and the shear stress in the nozzle is 1.0 to 5.0 kg
/cm2. Further, it is preferable that the optically anisotropic phase content of the liquid crystal pitch used is 98% or more.

The carbon fiber thus obtained has a tensile modulus of 25 T/mm2 or more and a compressive strength of 45 kg/mm2.
mm2 or more, and tensile modulus (T/mm2
)/compressive strength (kg/mm2) is made smaller than 1.6. More specifically, the pitch-based carbon fiber produced according to the present invention has a compressive strength (σc) of 65 kg/mm when the tensile modulus is about 50 T/mm2.
When the tensile modulus is about 70 T/mm2, the compressive strength is about 47 kg/mm2, which is a very high physical property value.

To further explain, as mentioned above, when producing pitch-based carbon fibers with a high tensile modulus, the heat treatment temperature of the pitch fibers is generally controlled, but in the present invention, in particular, the pitch fiber spinning process is Conditions are controlled. That is, the nozzle diameter of the spinning nozzle used for spinning pitch fibers is reduced, and a small diameter nozzle having a nozzle diameter of 0.10 to 0.25 mm, preferably 0.15 to 0.20 mm is used. If a spinning nozzle with a nozzle diameter of less than 0.10 mm is used, thread breakage occurs frequently during spinning, making spinning difficult. Moreover, even if carbon fibers are obtained by heat-treating the obtained pitch fibers, radial cracks occur in the carbon fibers, making it impossible to obtain high-performance carbon fibers. On the other hand, when a spinning nozzle with a nozzle diameter exceeding 0.25 mm is used, the fiber axis orientation of the mesophase molecules of the liquid crystal pitch imparted during spinning is not sufficiently performed, and as a result, carbon fibers are The crystal axis orientation of carbon fibers is poor, making it impossible to obtain carbon fibers with high tensile modulus.

Further, according to the present invention, the nozzle shear stress (τ) applied when the liquid crystal pitch passes through the spinning nozzle
is 0.2 to 7.0 kg/cm2, preferably 1.
It is considered to be a very high value of 0 to 5.0 kg/cm2. If the nozzle shear stress is less than 0.2 kg/cm2, even if a small diameter nozzle is used as the spinning nozzle, the fiber axis orientation of mesophase molecules will not be achieved sufficiently. Furthermore, if the nozzle shear stress exceeds 7.0 kg/cm2, yarn breakage occurs frequently during spinning, making spinning difficult. Moreover, even if carbon fibers are obtained by heat-treating the obtained pitch fibers, radial cracks occur in the carbon fibers, making it impossible to obtain high-performance carbon fibers.

According to the present invention, petroleum-based pitch, coal-based pitch, or pitch made from aromatic hydrocarbons can be used as the liquid crystal pitch, and this pitch can be used as a liquid crystal pitch using the above-mentioned conventional methods. By this method, a liquid crystal pitch having an optically anisotropic phase content of 95% or more and an average molecular weight of 1600 or less is prepared. Preferably, the liquid crystal pitch has an average molecular weight of 100
0 to 1500, and the optically anisotropic phase content is 98
% or more, ie, substantially 100%. When the optical anisotropy content of the liquid crystal pitch used is less than 95%, or when the average molecular weight exceeds 1600, pitch-based carbon fibers with high tensile strength and high tensile modulus are produced. This makes it difficult and undesirable.

According to the present invention, the liquid crystal pitch having such properties is, for example, 280 to 3
Pitch fibers are obtained by melt spinning at a temperature of 80°C. The pitch fibers are then made infusible at 200 to 350°C in an oxidizing gas atmosphere, and then carbonized or graphitized by firing at a high temperature of 500 to 3000°C in an inert gas atmosphere. Considered to be carbon fiber. Further, if necessary, in a heat treatment process such as infusibility or high-temperature firing, a stretching treatment is performed simultaneously with the heat treatment.

As described above, the pitch-based carbon fiber produced according to the present invention has a compressive strength (σc) of approximately 65 kg/m2 when the tensile modulus is approximately 50 T/mm2.
m2, and when the tensile modulus is about 70 T/mm2, the compressive strength is approximately 47 kg/mm2, which is extremely high compared to the conventional pitch-based carbon fibers mentioned above. Therefore, the pitch-based carbon fiber according to the present invention has improved tensile modulus (Et)/compressive strength (σc), that is, lower than 1.6, compared to those produced by the conventional method, and has high tensile strength and high compressive strength. A strong pitch-based carbon fiber can be obtained.

[0021] It should be noted that the meaning of the phrase "optically anisotropic phase" used in this specification is not necessarily uniformly used in academic circles or in various technical documents, so the meaning of the phrase "optically anisotropic phase" used in this specification The "optically anisotropic phase" is one of the constituent components of pitch, and when a cross section of a pitch lump solidified near room temperature is polished and observed under crossed Nicols with a reflective polarizing microscope, it shows that the sample or It means the part where brightness is observed by rotating the orthogonal nicols, that is, the part that is optically anisotropic, and on the other hand,
A portion where no brightness is observed, that is, an optically isotropic portion is called an optically isotropic phase.

The optically anisotropic phase is mainly composed of molecules with a chemical structure in which the flatness of polycyclic aromatic condensed rings is more developed than that of the optically isotropic phase, and the optically anisotropic phase aggregates in the form of a stack of planes. , and are considered to be in a kind of liquid crystal state at the melting temperature. Therefore, when extruded from a thin spinneret (spinning nozzle) and spun, the planes of the molecules are aligned nearly parallel to the direction of the fiber axis, so carbon fibers made from this optically anisotropic pitch have high strength and It shows high elasticity. or,
The optically anisotropic phase is quantified by observing it under a polarizing microscope under crossed nicols, taking a photograph, and measuring the area ratio occupied by the optically anisotropic portion, which essentially represents volume %.

Regarding the method of measuring "average molecular weight",
The molecular weight of the pitch component is measured by dissolving the soluble part in a chloroform solvent and measuring it by vapor pressure equilibrium (VPO), and measuring the insoluble part by solubilizing it by a mild hydrogenation reaction using metallic lithium and ethylenediamine. The average molecular weight was determined by the vapor pressure equilibrium method described above.

[0024] Furthermore, the measurement of "nozzle shear stress" assumes that the flow of liquid crystal pitch in the spinning nozzle is a laminar flow in a circular tube,
Hagen-Poiseuille (Hagen
-Poiseulle) formula. τ=32μQ/gc・π・D3 Here, τ: Shear stress μ: Pitch melt viscosity Q: Pitch discharge amount gc: Gravitational acceleration D: Spinning nozzle diameter Furthermore, “compressive strength” is The compressive strength is measured according to ASTM D3410.
This was done in accordance with the. In addition, in this specification, compressive strength is expressed as Vf (Volume fraction) 60% (
The composite compressive strength value (meaning that carbon fibers are present in the composite material in an amount of 60% in terms of volume) was used.

Next, the present invention will be explained in more detail with reference to examples.

[0026]

[Example] Example 1 In producing liquid crystal pitch, 40% of the optically anisotropic phase was
% and a softening point of 220° C. was used as the precursor pitch. This precursor pitch was centrifuged to successively separate pitches containing many optically anisotropic phases and pitches containing many optically isotropic phases, and each was extracted.

The obtained liquid crystal pitch A containing a large amount of optically anisotropic phase contains 100% optically anisotropic phase, has an average molecular weight of 1150, a softening point of 270°C, and a quinoline insoluble content of 25%.
．． It was 0% by weight. The liquid crystal pitch A is determined by nozzle 1 (nozzle diameter D = 0.15 mm, nozzle length L = 0.60 mm)
using, shear stress τ = 1.45 kg/cm2, temperature 3
Spinning was performed at 10°C to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundle was heated to a maximum temperature of 2 in an oxidizing atmosphere.
After being infusible at 95° C., carbonization was performed at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The tensile properties and compressive strength of the obtained carbon fiber are shown in Table 1, and it can be seen that the ratio of compressive strength to tensile modulus is improved by about 20 to 30% compared to the conventional product. Furthermore, σt
represents tensile strength.

[0028]

[Table 1]

Example 2 The liquid crystal pitch A used in Example 1 was used with nozzle 2 (nozzle diameter D = 0.20 mm, nozzle length L = 0.80 mm), shear stress τ = 4.0 kg/cm2, and temperature. Spinning was performed at 290°C to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundles were infusible in an oxidizing atmosphere at a maximum temperature of 295° C., and then carbonized at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The tensile properties and compressive strength of the obtained carbon fiber are shown in Table 2, and it can be seen that the ratio of compressive strength to tensile modulus is improved by about 20% compared to the conventional product. Note that σt represents tensile strength.

[0030]

[Table 2]

Comparative Example 1 In producing liquid crystal pitch, 50% of the optically anisotropic phase was
% and a softening point of 235° C. was used as the precursor pitch. This precursor pitch was centrifuged to successively separate pitches containing many optically anisotropic phases and pitches containing many optically isotropic phases, and each was extracted.

The obtained liquid crystal pitch B containing a large amount of optically anisotropic phase contains 100% optically anisotropic phase, has an average molecular weight of 1700, a softening point of 310°C, and a quinoline insoluble content of 40%.
% by weight. The liquid crystal pitch B was adjusted using nozzle 1 (nozzle diameter D = 0.15 mm, nozzle length L = 0.60 mm), shear stress τ = 5.00 kg/cm2, and temperature 320 mm.
The fibers were spun at ℃ to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundle was heated to a maximum temperature of 295°C in an oxidizing atmosphere.
After infusibility, carbonization was performed at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The tensile properties and compressive strength of the obtained carbon fibers are as shown in Table 3, and the ratio of compressive strength to tensile modulus was low. Note that σt represents tensile strength.

[0033]

[Table 3]

Comparative Example 2 In producing liquid crystal pitch, an optically anisotropic phase of 40
% and a softening point of 215° C. was used as the precursor pitch. This precursor pitch was centrifuged to successively separate pitches containing many optically anisotropic phases and pitches containing many optically isotropic phases, and each was extracted.

The obtained liquid crystal pitch C containing a large amount of optically anisotropic phase contains 85% optically anisotropic phase, has an average molecular weight of 1100, a softening point of 265°C, and a quinoline insoluble content of 22.
It was 0% by weight. The liquid crystal pitch C was prepared using nozzle 1 (nozzle diameter D = 0.15 mm, nozzle length L = 0.60 mm), shear stress τ = 1.50 kg/cm2, and temperature 30.
Spinning was carried out at 0°C to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundle was heated to a maximum temperature of 295 ml under an oxidizing atmosphere.
After being infusible at 0.degree. C., carbonization was performed at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 .mu.m. The tensile properties and compressive strength of the obtained carbon fibers are as shown in Table 4,
The ratio of compressive strength to tensile modulus was low. Note that σt represents tensile strength.

[0036]

[Table 4]

Comparative Example 3 Using the liquid crystal pitch A used in Example 1, using nozzle 3 (nozzle diameter D = 0.30 mm, nozzle length L = 1.20 mm), shear stress τ = 1.00 kg/cm2, temperature 325℃
The fibers were spun to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundles were infusible in an oxidizing atmosphere at a maximum temperature of 295° C., and then carbonized at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The tensile properties and compressive strength of the obtained carbon fibers are as shown in Table 5, and the ratio of compressive strength to tensile modulus was low. Note that σt represents tensile strength.

[0038]

[Table 5]

Comparative Example 4 The liquid crystal pitch A used in Example 1 was changed using nozzle 4 (nozzle diameter D = 0.10 mm, nozzle length L = 0.4 mm).
When spinning was performed at a shear stress τ = 9.10 kg/cm2 and a temperature of 310°C, yarn breakage occurred frequently and spinning was difficult. Further, the obtained pitch fiber bundle was made infusible in an oxidizing atmosphere at a maximum temperature of 295°C, and then carbonized at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The resulting carbon fibers had radial cracks, and high-performance carbon fibers could not be obtained.

Comparative Example 5 Using the liquid crystal pitch A used in Example 1, using nozzle 1 (nozzle diameter D = 0.15 mm, nozzle length L = 0.60 mm), shear stress τ = 0.10 kg/cm2, temperature 335℃
The fibers were spun to obtain pitch fibers with a diameter of 13 μm. The obtained pitch fiber bundles were infusible in an oxidizing atmosphere at a maximum temperature of 295° C., and then carbonized at various temperatures in an inert atmosphere to obtain carbon fibers with a diameter of about 10 μm. The tensile properties and compressive strength of the obtained carbon fibers are as shown in Table 6, and the ratio of compressive strength to tensile modulus was low. Note that σt represents tensile strength.

[0041]

[Table 6]

[0042]

Effects of the Invention According to the manufacturing method of the present invention configured as described above, the nozzle and spinning conditions in the liquid crystal pitch spinning process are optimized, and the compressive strength is lower than that of the conventional one while maintaining the tensile modulus at a high level. It is possible to suitably produce pitch-based carbon fibers with high tensile modulus and high compressive strength.

Claims (4)

[Claims] [Claim 1] Liquid crystal pitch having an optically anisotropic phase content of 95% or more and an average molecular weight of 1600 or less is processed using a small diameter nozzle with a nozzle diameter of 0.10 to 0.25 mm at a shear stress of 0.2. A method for producing pitch-based carbon fibers with high tensile modulus and high compressive strength, which comprises spinning pitch fibers by melt spinning under ~7.0 kg/cm2, and then heat-treating the pitch fibers. [Claim 2] The obtained carbon fiber has a tensile modulus of 25
Claim 1: T/mm2 or more, compressive strength is 45kg/mm2 or more, and tensile modulus (T/mm2)/compressive strength (kg/mm2) is less than 1.6.
manufacturing method. [Claim 3] The average molecular weight of the liquid crystal pitch is 1000~
1500, and the shear stress in the nozzle is 1.0-5.0
The manufacturing method according to claim 1 or claim 2, wherein the amount is kg/cm2. 4. The manufacturing method according to claim 1, wherein the optically anisotropic phase content of the liquid crystal pitch is 98% or more.