Process for preparing pitch-type carbon fibers

Details Process for preparing pitch-type carbon fibers Process for preparing pitch-type carbon fibers

1987-11-18

1989-08-22

Lorin, Hubert

Plastic and nonmetallic article shaping or treating: processes

Carbonizing to form article

Filaments

264 83, 26417711, 26417713, 26417718, 26417719, 26421111, 26421115, 26421117, D01F 900

Patent

active

048593818

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a process for preparing pitch-type carbon fibers.


BACKGROUND ART

Because carbon fibers prepared from pitch-type materials involve lower production costs than carbon fibers produced from organic synthetic fibers such as polyacrylonitrile as a precursor and because articles of high elastic modulus can be easily produced from the pitch-type carbon fibers, it is expected that pitch-type carbon fibers can be less expensive then high-performance materials. However, commercially available pitch-type carbon fibers have a tensile strength of not more than about 200 kg/mm.sup.2 and poor quality stability, hence not satisfactory.
Generally the cohesion stage of molecules in section of pitch-type carbon fibers (hereinafter referred to as high-order sectional structure) widely varies with spinning conditions. More specifically, this structure is basically classified into the type in which molecules form crystals along the concentric circles of fiber (so-called onion type), the type in which molecules form crystals radially of center of fiber (radial type), and the type in which molecules are randomly distributed without orientation (random type). Yet there actually exist fibers having a structure of mixtures of these types. Further, flaws such as longitudinal fractures, cracks, voids and the like may exist in part or whole of the fibers. Accordingly the high-order sectional structures of pitch-type carbon fibers including such flaws are complicated and abundantly various. The presence of various flaws and high-order sectional structures is one of the main causes of lowering the quality stability of pitch-type carbon fibers.
While depending on the properties of pitch for use in spinning, the occurrence of the above-mentioned flaws and the formation of high-order sectional structures are most greatly affected and varied by the spinning conditions. Accordingly, in order to improve the quality and stability of carbon fibers, it is necessary to establish spinning techniques capable of invariable producing carbon fibers substantially free of flaws and having a specific high-order sectional structure even if these are some irregularities in properties of pitch for spinning. In other words, techniques are required for stably forming the onion type and/or random type of high-order sectional structures which are effective in exhibiting a high tensile strength without creating flaws such as longitudinal fractures, cracks, voids or the like.


DISCLOSURE OF THE INVENTION

In view of the above-mentioned state of the art, we conducted extensive research and found that when during melt-spinning operation, a molten pitch-type material, prior to passage through the final nozzle orifice, is passed through a capillary portion having specific shape to thereby exert a shearing stress exceeding a specific value on the material, then temporarily maintained in a state particularly free of shearing stress, and passed through the nozzle orifice for spinning, the problems arising from the conventional techniques can be practically obviated or alleviated to a great extent. The present invention provides a process for preparing pitch-type carbon fibers by melt-spinning a pitch-type material, rendering the material infusible and carbonizing it, characterized by the steps of passing molten pitch, before reaching the final nozzle, through a capillary portion of circular, otherwise shaped, or slit type to thereby apply a shearing stress at least 1/2 as high as the level of shearing stress to exerted during the passage through the final nozzle orifice, maintaining the molten pitch in a state substantially free of shearing stress and then passing the pitch through the nozzle orifice for spinning.
According to the present invention, when the pitch is graphitized in the carbonization step described above, a graphite fiber can be produced. The term "carbonization" in the specification refers also to graphitization and term "carbon fibers" is intended to encompass graphite fibers.
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