Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2001-01-22
2002-08-06
Edwards, N. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S394000, C428S367000, C264S662000
Reexamination Certificate
active
06428891
ABSTRACT:
TECHNICAL FIELD
This invention relates to polyacrylonitrile-based precursor fibers for carbon fibers and a process for preparing the same.
BACKGROUND ART
Carbon fibers and graphite fibers (herein referred to collectively as “carbon fibers”) formed by using polyacrylonitrile-based fibers as precursors have excellent mechanical properties and are hence being commercially produced and sold as fibrous reinforcements of high-performance composite materials for use in aerospace applications, sports and leisure applications, and the like. Moreover, in recent years, the demand for carbon fibers is growing in general industrial applications such as automobile and marine applications and building material applications. Thus, in order to enhance the performance of such composite materials, inexpensive carbon fibers having high quality are desired in the market.
In contrast to acrylic fibers for clothing use, acrylonitrile-based fibers for use as precursors of carbon fibers are no more than intermediate products for the formation of carbon fibers as final products. Accordingly, it is not only desirable to provide acrylonitrile-based fibers capable of yielding carbon fibers having excellent quality and performance, but it is also very important that the acrylonitrile-based fibers have good stability during spinning of precursor fibers, exhibit high productivity in forming carbon fibers, and can be provided at low cost.
From this point of view, a large number of propositions have been made in order to provide acrylonitrile-based fibers capable of yielding carbon fibers having high strength and high elasticity. These propositions include, for example, an increase in the polymerization degree of the copolymer, and a decrease in the content of copolymerized components other than acrylonitrile. As to the spinning method, dry-wet spinning is commonly employed.
However, when the content of copolymerized components other than acrylonitrile is decreased, the solubility of the resulting copolymer in solvents is generally reduced. This not only detracts from the stability of the spinning solution, but also causes an extreme increase in the viscosity of the spinning solution, making it necessary to reduce the copolymer concentration in the spinning solution correspondingly. Consequently, the copolymer shows a marked tendency toward precipitation and coagulation, so that the resulting fibers may frequently undergo devitrification or develop a large number of voids therein. Thus, this production method cannot be regarded as a stable one.
Since the dry-wet spinning process comprises extruding a polymer solution through a nozzle into air and then passing it continuously through a coagulating bath to form filaments, it is easy to obtain dense coagulated filaments. On the other hand, a decrease in the pitch of nozzle holes will cause a problem in that adjacent filaments may adhere to each other. Thus, there is a limit to the number of nozzle holes.
Generally, an increased density of nozzle holes is advantageous for the low-cost production of acrylonitrile-based precursor fibers. Accordingly, the wet spinning process is being employed, partly because it requires a relatively low cost of production equipment. However, the resulting filament tow generally include many broken filaments and much fluff. Thus, the resulting precursor fibers have a low tensile strength and a low elastic modulus, and the fiber structure of the precursor fibers is less dense and has a low degree of orientation. Consequently, the mechanical properties of the carbon fibers obtained by carbonizing them are generally unsatisfactory.
For precursor fibers used to form high-quality carbon fibers, it is very important that they are free of minute defects which will be responsible for breakage after they are converted to carbon fibers. In order to minimize such defects, it is necessary that the precursor fibers have a high tensile strength and a high elastic modulus, their fiber structure be highly dense, the copolymer be highly oriented in the direction of the fiber axis, and the degree of variation in tow size be small.
For example, Japanese Patent Laid-Open No. 214518/'83 makes mention of the denseness of the fiber structure while employing the wet spinning process. As measures of the denseness, the amount of iodine adsorbed and the thickness of the skin layer to which iodine is adsorbed are defined therein. However, the precursor fiber thus obtained has a low density as demonstrated by an iodine adsorption of about 1-3% by weight, and also has a low tensile strength and a low elastic modulus. Consequently, it is very difficult to produce a carbon fiber having high quality.
On the other hand, Japanese Patent Laid-Open No. 35821/'88 discloses a precursor fiber which has been prepared by the dry-wet spinning process and which has a highly densified surface structure. Moreover, Japanese Patent Laid-Open Nos. 21905/'85 and 117814/'87 disclose precursor fibers which have also been prepared by the dry-wet spinning process and which have a high tensile strength and a high elastic modulus and comprise a copolymer highly oriented in the direction of the fiber axis. Although an improvement in the quality of the resulting carbon fibers can be achieved by using these precursor fibers, their productivity is low owing to the use of the dry-wet spinning process. Moreover, the fibers prepared by dry-wet spinning have a smoother surface as compared with the fibers prepared by wet spinning. The former fibers exhibit good bundling properties, but also have several disadvantages in that they tend to fuse together in the oxidation step and in that they tend to show poor spreadability in the formation of a sheet-like prepreg. Furthermore, the polymers used in these inventions practically have an acrylonitrile content of not less than 99.0% by weight. Accordingly, from the viewpoint of the stability of the spinning solution and the tendency of the copolymer toward precipitation and coagulation, these processes are unsatisfactory for the stable preparation of a precursor fiber.
In order to obtain a precursor fiber having a densified surface structure while employing the wet spinning process, pressurized steam drawing has been investigated as a drawing method for achieving a higher draw ratio.
For example, Japanese Patent Laid-Open No. 70812/'95 discloses a precursor fiber which has been prepared by the wet spinning process but has a densified surface structure. In this patent, the densification of a precursor fiber has been achieved by using a copolymer having a specific composition and a coagulated fiber having specific properties, in combination with pressurized steam drawing. However, since no consideration is given to the appropriate range of drawing conditions after coagulation, this process is unsatisfactory for the purpose of preparing a precursor fiber having a high degree of denseness and a high degree of orientation. Moreover, since no mention is made of the strength, elastic modulus, degree of crystal orientation, and degree of variation in tow fineness of the resulting precursor fiber, the properties of a precursor fiber which are required for the formation of a carbon fiber having excellent quality have been still unknown. Furthermore, it has been difficult to spin a precursor fiber stably at a high spinning speed of not less than 100 m per minute.
Thus, all conventional techniques have failed to provide a satisfactory precursor fiber for the formation of a high-quality and inexpensive carbon fiber and a satisfactory process for preparing the same.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide an acrylonitrile-based precursor fiber for carbon fiber which has a high strength, a high elastic modulus, a high degree of denseness, a high degree of orientation, and a low degree of variation in tow fineness, and can hence be used to form a high-quality carbon fiber inexpensively by carbonizing for a shorter period of time
Amakawa Takeaki
Hamada Mitsuo
Kageyama Yoshitaka
Okuya Takahiro
Edwards N.
Mitsubishi Rayon Co. Ltd.
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