Process for the production of carbon fibers

Details Process for the production of carbon fibers Process for the production of carbon fibers

1999-06-18

2001-06-05

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

Filaments

C264S029600, C264S029700, C423S447600, C423S447700, C423S447800

Reexamination Certificate

active

06241923

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process of producing high-performance pitch-based carbon fibers continuously and stably for a long time and a process for the production of high-performance pitch-based carbon fibers having high strength and high elastic modulus.
PRIOR ART OF THE INVENTION
Generally, high-performance carbon fibers are industrially produced from PAN (polyacrylonitrile) as a raw material, while PAN is expensive and its carbonization yield is low. In recent years, it has been found that high-performance carbon fibers can be produced from pitch as a raw material.
Pitch for carbon materials includes optically isotropic pitch and optically anisotropic pitch. Carbon fibers produced from isotropic pitch are inexpensive, while such carbon fibers are poor in strength due to its poor molecular orientation. Therefore, high-performance carbon fibers can not be obtained from the isotropic pitch. In contrast, carbon fibers produced from anisotropic pitch called “mesophase pitch” have a higher degree of molecular orientation so that such carbon fibers show excellent mechanical properties in strength and elastic modulus.
Thus, there have been developed methods of producing mesophase pitch as a raw material for high-performance carbon fibers from pitch obtained from catalystic cracked oil of petroleum, petroleum tar pitch or coal tar pitch.
When fibers are produced from this mesophase pitch by a melt-spinning method, the developed aromatic planar molecules are brought into alignment with the fiber-axis because of the shear stress exerted when the pitch passes through nozzle holes. This structure is maintained in subsequent infusibilization and carbonization without being disturbed, so that highly oriented, high-performance carbon fibers can be obtained.
The pitch-based carbon fibers have characteristic features that yield in a carbonization step is large and that the elastic modulus is high, while the pitch-based carbon fibers are extremely inferior in spinning properties to PAN-based carbon fibers. The reason is as follows. Since the mesophase pitch is composed of complicated compounds, a spinning nozzle is soiled due to the alteration by heat and smoking at spinning. As a result, it is difficult to perform spinning for a long time. Further, the above fact also makes it difficult to impart sufficient strength to the carbon fibers.
As for the production of pitch-based carbon fibers, therefore, there are some attempts of producing carbon fibers more stably. However, no carbon fibers equivalent to PAN-based carbon fibers have yet obtained.
The present inventors have proposed a process for the production of mesophase pitch which is excellent in infusibilization properties and has a low softening point and high mesophase content, bypolymerizing a condensedpolycyclic aromatic hydrocarbon such as naphthalene by the use of HF-BF3 (Japanese Patent Number 2,562,585, Japanese Patent Number 2,621,253). However, although this pitch has a low softening point, this pitch still has large contents to be vaporized. As a result, improvements are required for carrying out a continuous spinning for a long time.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the production of pitch-based carbon fibers, which process can produce high-performance carbon fibers continuously and stably for a long time.
It is another object of the present invention to provide a process for the production of pitch-based carbon fibers, which process can stably produce high-performance carbon fibers having high strength and high elastic modulus.
According to the present invention, there is provided a process for the production of carbon fibers, comprising melt-spinning mesophase pitch having an optically anisotropic content of at least 90%, a softening point of 190° C. to 280° C. and a heating weight loss of 0.7% by weight or less at a spinning temperature to form a spun fiber, infusibilizing the spun fiber to obtain an infusible fiber and carbonizing the infusible fiber.
According to the present invention, further, there is provided a process for the production of carbon fibers, comprising melt-spinning mesophase pitch having an optically anisotropic content of at least 90%, a softening point of 190° C. to 280° C. and a difference of 30° C. or less between a flow starting temperature and the softening point, measured with a flow tester, to form a spun fiber, infusibilizing the spun fiber to obtain an infusible fiber and carbonizing the infusible fiber.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have made diligent studies on a process for the production of pitch-based carbon fibers, which process can continuously and stably produce high-performance carbon fibers for a long time and, as a result, found the following. Continuous spinning excellent in spinning properties can be stably carried out for a long time by spinning mesophase pitch having a high optically anisotropic content, a softening point in the specific range and a low heating weight loss at a spinning temperature to form a spun fiber, infusibilizing the spun fiber to obtain an infusible fiber and carbonizing the infusible fiber. Thereby, pitch-based carbon fibers excellent in fiber properties can be obtained.
That is, for the stabilization of spinning, the mesophase pitch must be composed of thermally stable molecules and light contents must be removed completely. It has been found that these features are particularly closely related to the heating weight loss at a spinning temperature. As a result, the present invention has been accomplished.
That is, the present invention 1 is directed to a process for the production of carbon fibers, comprising melt-spinning mesophase pitch having an optically anisotropic content of at least 90%, a softening point of 190° C. to 280° C. and a heating weight loss of 0.7% by weight or less at a spinning temperature to forma spun fiber, then infusibilizing the spun fiber to obtain an infusible fiber and carbonizing the infusible fiber.
Further, the present inventors have made more studies and as a result found that, for obtaining carbon fibers having high strength and high elastic modulus, the softening point and the distribution of component molecules are also important other than the optically anisotropic content of mesophase pitch. That is, the present inventors have found that mesophase pitch having an optically anisotropic content, a softening point and a difference between a flow starting point and the softening point, which difference is the index of the distribution of the component molecules, in their specific ranges, has a high orientation, a narrow distribution of component molecules and an excellent flowability. Further, it has been found that high-performance carbon fibers can be stably produced by spinning, infusibilizing and carbonating pitch having such properties. As a result, the present invention has been accomplished.
That is, the present invention 2 provides a process for the production of carbon fibers, comprising melt-spinning mesophase pitch having an optically anisotropic content of at least 90%, a softening point of 190° C. to 280° C. and a difference of 30° C. or less between a flow starting temperature and the softening point, measured with a flow tester, to form a spun fiber, infusibilizing the spun fiber to obtain an infusible fiber and carbonizing the infusible fiber.
Mesophase pitch of a high optically anisotropic content is used as a raw material for the carbon fibers to be produced by the process of the present invention 1 or the present invention 2. A condensed polycyclic aromatic hydrocarbon such as naphthalene, methyl naphthalene or anthracene, various petroleum fractions having these skeletons, the residual oil originating from petroleum processing steps and petroleum tar fractions are used as a raw material for the mesophase pitch.
The mesophase pitch may be produced by a conventional thermal polymerization, while the production process in which polymerization is carried out by using hydrogen fluoride and

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