At present, production of carbon fibers having excellent strength and excellent modulus of elasticity using pitches as a raw material is roughly classified into two processes, namely, (1) a process which comprises stretching carbon fibers composed of isotropic carbon under tension at a high temperature of 2,500.degree. C. or more and (2) a process which comprises using an anisotropic pitch as a raw material.
A typical example of the process (2) is that which comprises producing carbon fibers using a pitch containing a large amount of mesophase as a raw material. U.S. Pat. No. 4,115,527 has desclosed a process for producing carbon fibers which comprises using polynuclear aromatic compounds having a high plane structure formed by condensation of 7 or more rings as raw materials. When these raw materials are formed in fibers, orientation in the filament axis of plane molecules is observed by polarizing microspcopic observation of X-ray observation of an abrasion face parallel to the direction of filament axis. Further, in materials subjected to carbonization or graphitization, it is said that the same orientation is observed by X-ray observation after they are subjected to infusibility treatment.
A process for producing a pitch containing a large amount of mesophase is disclosed in U.S. Pat. No. 4,026,788. The disclosed process comprises carrying out thermal modification of a precursor material (substance which becomes a raw material for producing pitches) while blowing an inert gas to form a mesophase.
Further, U.S. Pat. No. 4,026,788 has disclosed a process for producing carbon fibers having high modulus of elasticity and high strength from a mesophase pitch having a mesophase content of 40 to 90% by weight. The mesophase in this case is defined as a state which can be optically observed by an examination by means of a polarizing microscope and it substantially insoluble in organic solvents such as quinoline and pyridine, etc.
So far as these patents desclose, realization of carbon fibers having high strength and high modulus of elasticity has been done using pitches containing a large amount of mesophase. It has been confirmed that carbon fibers having high strength and high modulus of elasticity can be produced if a pitch containing a large amount of mesophase is spun and infusiblization, carbonization and graphitization are carried out under a suitable condition. However, it is very difficult to carry out melt spinning of pitches containing a large amount of mesophase (components to be insoluble in quinoline and pyridine), particularly pitches containing 40% by weight or more of mesophase. Accordingly, in order to stably and industrially produce carbon fibers having high strength and high modulus of elasticity using pitches containing a large amount of mesophase as a raw material, it is necessary to overcome spinning difficulties.
On the one hand, Japanese Patent Application (OPI) 160427/79 (The term "OPI" herein refers to a "published unexamined Japanese Patent Application".) corresponding to U.S. Pat. Appln. Ser. No. 813,931 filed on July 8, 1977 has disclosed a process that when a fraction separated from an isotropic carbonaceous pitch by a solvent extraction process is heated at 230.degree. to 400.degree. C. for 10 minutes or less, the fraction forms a pitch having more than 75% of optical anisotrophic phase. Although pitch fibers per se and production of carbon fibers as well as the process for producing pitches is described in the above Japanese Patent Application (OPI) 160427/79, the fact that carbon fibers having high strength and high modulus of elasticity can be obtained from pitches according to the process of the present invention is not described therein.
Further, Japanese Patent Application (OPI) 88016/82 has disclosed a process for producing a mesophase containing pitch which comprises carrying out thermal modification of a precursor material and then concentrating mesophase by a gravity settling method. Further, Japanese Patent Application (OPI) 57881/81 (corresponding to U.S. Pat. Appln. Ser. No. 79,891 filed on Sept. 28, 1979) has disclosed a process which obtained a component containing a large amount of mesophase from a pitch as a precursor material by an extracting operation using a solvent. However, in this case, it is also required that the caronaceous precursor material of the pitch is a precursor material capable of forming a pitch having a large portion of mesophase by a thermal process as described in U.S. Pat. No. 4,005,183.
In any case, these processes comprise an operation of thermal modification in the pitch production process. Generally, there are few cases where a chemically pure compound is used as a precursor material, and petroleum or coal heavy oils are used as the precursor material in many cases. These petroleum or coal heavy oils contain some impurities though the amounts are very small. When they are subjected to thermal modification, dehydrogenation reactions proceed to form substances which are near carbon and difficult to fuse even if heated. Accordingly, it has been generally required to treat at a temperature as low as possible and lengthen the thermal modification time as long as possible as described in the above described U.S. Pat. Nos. 4,026,788 and 4,032,430. However, formation of small amounts of substances which are difficult to fuse is an inevitable problem.
When producing carbon fibers using a pitch containing such substances which are difficult to fuse, troubles such as breaking of filaments or blockade of spinning nozzles are caused when the pitch is spun. Of course, it is possible to remove such impurities from the raw material of pitch by means of filtration or others before spinning. However, when the amount thereof is large, filtration becomes difficult due to high viscosity of the raw material of pitch and, at the same time, it is necessary to frequently carry out cleaning of the filter. Consequently, much time is required which deteriorates economization, making the process undesirable industrially.
On the other hand, before carbonization of a pitch, infusibilization is generally carried out in order to prevent deformation caused by fusion of the pitch. Particularly, in case of carbon fibers, infusibilization is often carried out by oxidizing pitch fibers spun in an oxidative atmosphere such as air, etc. In this case, if infusibilization is insufficiently carried out, adhesion by fusion of fibers themselves is caused or shrinkage of fibers is caused. If such fibers having insufficient infusibility are carbonized, carbon fibers having excellent strength and excellent modulus of elasticity cannot be obtained.
As described above, the mesophase is defined hitherto as a state in which optical anisotropy can be optically observed by a polarizing microscopic examination and it is substantially insoluble in organic solvents such as quinoline or pyridine. In Japanese Patent Applications (OPI) 57881/81 and 88016/82, the mesophase is defined as a state in which optical anisotropy can be optically observed by polarizing microscopic examination. However, it is impossible to recognize qualitative features of the mesophase by only the fact that optical anisotropy is shown in the polarizing microscopic examination. Further, it is quantitatively difficult to discriminate a main component composing the mesophase and the other components, when they are coexistent. Particularly, in pitches produced from mixtures such as petroleum heavy oils as precursor material, it is right to think that various substances are coexistent.
As a result of various studies relating to pitches suitable as raw materials for carbon fibers, we have found that, in pitches which not only produce carbon fibers having high strength and high moddulus of elasticity as a final product but also have excellent processability and, particularly excellent spinnability, and are difficult to cause adhesion by fusion in case of carrying out infusiblization, amounts of n-heptane insoluble component, quinoline insoluble component and toluene insoluble component are in a quite limited range, and a pitch having such properties can be produced economically.