In recent years, with the rapid growth of aircraft space and missile industries and the like, materials having extraordinary excellent mechanical properties have been needed. Further, as materials for sporting goods, particularly those of high quality, there have come to be desired materials which are lighter and have superior mechanical properties as compared with conventional materials.
Since these materials are required to have high strength, high Young's modulus and light weight, the search for materials now appears to be concentrated upon composite materials.
One of the most hopeful materials suggested as a constituent of composite materials was carbon fibers having high strength and high Young's modulus. Such carbon fibers were incorporated into plastics and metal matrixes to give composite materials having very high strength, and high modulus-to-weight ratio, and other special properties. However, the high production cost of the carbon fibers having high strength and high Young's modulus used as constituent of such composite materials is a serious obstacle to wide use of the carbon fibers though the composite materials have excellent characteristics.
Most carbon fibers having high strength and high Young's modulus which are now available are derived from acrylic fibers, and are intrinsically expensive because precursors of the acrylic fibers are expensive. In addition to the expensiveness of the starting materials, a low carbon yield (about 45%) attained from such precursors and a complicated production process raise the price of the final product.
Further, such PAN based carbon fibers not only require a high cost but also are disadvantageous in that though they can be easily given a high strength, they cannot easily be given a high Young's modulus and require an additional special treatment step for giving them high Young's modulus.
On the other hand, as carbon fibers used in place of such PAN based carbon fibers, pitch based carbon fibers have come to noted because of their low material cost, high carbon yield and the like.
In the case of the pitch based carbon fibers, pitches obtained from coal, petroleum and the like are used as starting materials. When the precursor pitch is a so-called mesophase pitch containing 40% or more, preferably 80% or more of mesophase, the resulting carbon fibers can have high Young's modulus.
It is common knowledge that high Young's modulus of pitch based carbon fiber can be attained by promoting its graphitization, and a typical pitch based carbon fiber having high Young's modulus obtained by applying this principle and a process for producing the same are disclosed in Japanese Patent Kokai (Laid Open Publn) No. 19127/74. Such a pitch based carbon fiber has a strength of 210 kg.multidot.mm.sup.-2, a Young's modulus of 70 ton.multidot.mm.sup.-2 and an elongation of 0.3%, and a preferred orientation parameter (HWHM) of 5.degree. or less, a crystallite size (L.sub.c (002)) of 100 nm or more and an interlayer spacing (d002) of 0.337 nm or less as determined by X-ray diffraction, a electrical resistivity of 2.5.times.10.sup.4 .multidot..OMEGA..multidot..cm or less, and an X-ray diffraction pattern characterized by the presence of the (112) line and resolved (100) and (101) lines and thereby it is shown that in the carbon fiber, graphite crystallite are grown three-dimensionally, namely, in both the fiber axis direction and a direction of section perpendicular to the fiber axis. When the graphite crystallite are grown three-dimensionally, the magnetoresistance is generally positive.
However, carbon fibers having a high degree of three-dimensional graphitization and a high Young's modulus besides the carbon fiber described above are also said to have two defects.
One of the defects is that though such carbon fibers have a high Young's modulus, they are very brittle. Here, the term "brittleness" means brittleness against forces other than tensile force, for example, torsional stress and stress in a direction perpendicular to fiber. One cause of such brittless seems to be an increase of the cleavage of graphite crystallites with their three-dimensional growth.
What is important here is as follows. According to the analysis results obtained by the present inventors, the Young's modulus increases with a growth of graphite crystallites in the fiber axis direction, namely, a decrease of the preferred orientation parameter (HWHM) determined by X-ray analysis, but the growth of graphite crystallites in a direction of section perpendicular to the fiber axis to give a three-dimentional structure, resulting in a large crystallite size (Lc(002)), a small interlayer-spacing (d002), and a positive high magnetoresistance, makes no contribution to the Young's modulus and further embrittles the carbon fibers.
However, it is not easy to overcome such brittleness without lowering the Young's modulus of a pitch based carbon fiber. This is because it is generally difficult to control the growth of graphite crystallites in the fiber axis direction and that in a direction of section perpendicular to the axis independently of each other.
The second defect of the carbon fibers having three-dimensionally grown graphite crystallites is that when they are spun into fibers by a conventional method, the structures of section perpendicular to the axis of each carbon fiber tend to be of a so-called radial type in which molecules are aligned facing in the direction of the center of the section, so that the carbon fibers become liable to split longitudinally along the fiber axis direction with a growth of graphite crystallites in the section direction. Needless to say, such carbon fibers are greatly lowered in commercial value by the split.
Here, the second defect can be overcome either by changing the section structure of radial type to a section structure of another type, or by depressing the growth of graphite crystallite in a direction of section perpendicular to the fiber axis. In practice, the former is easy, and therefore processes for producing a carbon fiber having a section structure of a type other than radial type is investigated without considering the latter.
For example, as disclosed in Japanese Patent Koakai (Laid Open Publn) Nos. 76925/84 and 53717/84, such processes comprise raising spinning temperature and thereby obtaining a section structure of random type or onion type.
Carbon fibers produced by these conventional processes have a section structure of a type other than radial type and hence can overcome the second defect, i.e., the split, sufficiently. However, they can not overcome the first defect, i.e., the brittleness. The reason for this is as follows. In order to overcome the first defect, the brittleness, the growth of graphite crystallites in a direction of section perpendicular to the fiber axis should be depressed sufficiently though graphite crystallites are satisfactorily grown in the direction of fiber axis of the carbon fiber. For this, molecules constituting pitch should be satisfactorily oriented in the fiber axis direction at a pitch fiber stage, and in the section direction domains where such molecules are oriented in the same direction should be subdivided. According to conventional spinning methods, subdivision of such domains in the section of pitch fiber is not conducted sufficiently, so that a carbon fiber obtained by heat-treating the pitch fiber has three-dimensionally grown graphite crystallite. Therefore, though the second problem, the split could be solved, the first problem, the brittleness has not yet been able to be overcome.