EP0276840A2

EP0276840A2 - Pitch-based carbon fibers and their production method

Info

Publication number: EP0276840A2
Application number: EP88101195A
Authority: EP
Inventors: Yoshinori Cf. Kashima Seiyusho Of Suto; Hideyuki Cf. Kashima Seiyusho Of Nakajima; Toshiyuki Cf. Kashima Seiyusho Of Ito
Original assignee: Petoca Ltd
Current assignee: Petoca Ltd
Priority date: 1987-01-28
Filing date: 1988-01-27
Publication date: 1988-08-03
Anticipated expiration: 2008-01-27
Also published as: EP0276840A3; CA1311883C; EP0276840B1; JPH0651928B2; DE3850419T2; JPS63303123A; DE3850419D1

Abstract

Pitch-based carbon fibers on which silicone type spin finish oils are remained in the amount of 0.1 - 2.0% by weight of said fibers and having a stack height of graphitic layers L_c (002) of 16 - 28Å, a graphitic interlayer spacing distance d002 of 3.46 - 3.49Å, a tensile strength of 5 - 50 Kgf/mm² and an elongation of 0.3 - 8.0% are provided. They show good cohesiveness and lubricity of bundles and superior in adaptation to various kinds of processes.

Description

Background of the Invention

Field of the Invention

This invention relates to pitch-based high performance carbon fibers having superior workability and their production method. More particularly, it relates to pitch-based carbon fibers obtained by mitigating carbonization treatment to such an extent that silicone types spin finish oils coated at the time of spinning of pitch are remained, having a high cohesiveness of bundles and lubricity by the remaining silicone type spin finish oils and having a superior workability and to their producing method.
The carbon fibers obtained from a pitch according to a method of the present invention is incomplete in crystallization and orientation structure of carbon hexagonal network and yet have capability of increasing their tensile strength and modulus of elasticity greatly by an additive heat treatment carried out under a relaxed state whereby the growth of graphitic crystalline and orientation structure proceed.
The carbon fibers of the present invention are superior in adaptation to various kinds of processes such as taking up on bobbins, transportation to a higher grade of carbonization or graphitization step, weaving,knitting and working for the reinforcement of resin.
The pitch-based carbon fibers of the present invention are easy in working because of their lower carbonization grade, and their cost is lower than those of a higher carbonization grade. Thus even when working loss is produced, they have an advantage because the influence upon the cost of product is small.
The carbon fibers derived from a pitch according to the present invention are patient against bending of small radius of curvature compared with carbon fibers subjected to higher grade of carbonization and have superior characteristic properties because their bent portions receive stress relaxation by the carbonization treatment applied thereafter and show superior resistance to abrasion, flexion and scratching.
A method for obtaining carbon fibers by subjecting a pitch having a high softening point to melt-spinning, oxidizing the resulting fibers to make them infusible, followed by carbonization carried out in an inert gas atmosphere, is disclosed in Japanese official gazette of examined application (Tokuko) 15728 of 1966. This is certainly a superior production method of pitch-based carbon fibers but according to the disclosed method, it is necessary to keep fibers in a stretched state during the carbonization to obtain fibers having high modulus of elasticity. Since thermoset pitch fibers are extremely brittle, it is difficult to hold them in a stretched state. It is considered actually to be impossible to obtain high modulus carbon fibers by this method.
In order to work out a solution to this problem, a method in which optically anisotropic pitch is used has been proposed as disclosed in Japanese official gazette of examined application (Tokuko) 8634 of 1974 and Japanese official gazette of unexamined application (Tokukai) 19127 of 1974. An optically anisotropic pitch is easily graphitizable material and has superior properties as raw material for high strength, high modulus carbon fibers. Particularly, there is no need of being kept in a stretched state during the carbonization; it is considered to be an advantageous method in view of cost and quality.
Carbon fibers from an optically anisotropic pitch can be easily made into a high strength and high modulus fibers but on the other hand, they have such weak points, that they are liable to be flawed, e.g. liable to be broken at the time of working. Such weak points exist more or less in case of brittle fibers. Glass fibers, PAN-based carbon fibers, etc. are coated by sizing agents to give lubricity and cohesiveness of bundles. In case of carbon fibers from optically anisotropic pitch, there is a tendency of repelling a sizing agent due to harmful effect of easily graphitizable property. Since uniform coating is difficult, shortages of lubricity and cohesiveness of bundles are also weak points.
In order to solve these problems, Japanese unexamined patent application (Tokukai) 21911 of 1985 discloses a method in which light grade of carbonization is carried out at a temperature of 400 - 650°C after thermosetting. This method is effective to some-extent for keeping the modulus of elasticity of carbon fibers small and for preventing them from being flawed but since bundles of the fibers have no cohesiveness and no lubricity, there are problems in insufficiency of workability. In order to solve such problems, it is a general method to coat an oiling agent after carbonization but in case of lightly carbonized pitch fibers, a tendency of repelling an oiling agent is created, and there is a problem of the fibers being liable to be flawed at the time of coating of the oiling agent on one hand because strength of the fibers is not increased.
In the treatment of pitch-based carbon fibers, the most severe condition for spin finish oils is a thermosetting which is a heat treatment carried out in the oxidative atmosphere. In order to cover the oxidative decomposition of spin finish oils, it is considered advantageous to impart second oils after thermosetting process. The problem of this method is the liability of being flawed at the time of imparting the second oils because the pitch fibers after thermosetting are equally or more brittle than the pitch fibers after spinning.
For imparting the second oils at this step, a spray method can be adopted, but the loss of the second oils by scattering is so great that there is an economical problem specially in the case of expensive silicone type oils.
It is an object of the present invention to overcome the brittleness, shortage of lubricity and cohesiveness of bundles of carbon fibers produced from an optically anisotropic pitch or from a high softening point pitch having characteristic carbonization properties similar to the optically anisotropic pitch.
The carbonization of pitch-based fibers is carried out generally by the heat treatment in the atmosphere of an inert gas and its effect is considered, in general, to be depend on temperature and time. However, when a detailed investigation is carried out for workability, it has become clear that there is the effect of spin finish oils remained. Particularly, it has become clear that the effect for lubricity and cohesiveness of bundles are notable. Further there seems to be a difference of effectiveness between apparatus for carbonization.
Though the reason is not clear, it is inferred that a shape of bundles, which is formed at the stage of bundles of pitch fibers possessing a good lubricity and cohesiveness, is maintained by spin finish oils remaining in a slight amount and that this gives a large influence upon workability.

Summary of the Invention

In a method for producing pitch-based carbon fibers by subjecting a pitch having a high softening point to melt-spinning, thermosetting and carbonization or carbonization and then graphitization, the present invention comprises coating spun pitch fibers with silicone type spin finish oils, introducing said pitch fibers in the oxidative atmosphere at a maximum temperature of 200 - 400°C to effect the thermosetting and subsequently subjecting said pitch fibers to the carbonization treatment in the atmosphere of an inert gas at a temperature of 400 - 1000°C under the condition that the silicone type finish oils remained on said pitch fibers are in the range of 0.1% - 2.0% by weight of said fibers and transferring them to a next working step.

Detailed Description of the Preferred Embodiment of the Invention

A high softening point pitch referred to in the present invention is an easily graphitizing pitch such as an optically anisotropic pitch. The easily graphitizing pitch forms needle cokes by dry distillation. Further, at the time of carbonization of pitch fibers, it produces high modulus carbon fibers even under a tensionless condition.
The easily graphitizing pitches include, besides optically anisotropic pitches, dormant mesophase pitches, and premesophase carbonaceous materials which show similar graphitizing property.
The silicone type oil used in the present invention is preferably a matter having a heat resistance of 500°C or more. The heat resistance of oils, referred to herein, is defined as a temperature at which a reduction of the weight of oils measured by using a thermobalance (TG high temperature type CN 8068 AZ manufactured by Rigaku Denki was used; Sample size 10 mg, flow amount of nitrogen; 40 mℓ/min. cell diameter; 5mm, cell depth 2.5 mm) at a heating rate of 10°C/min. in the stream of nitrogen becomes practically zero. (It means that the change of the weight in a temperature range of 100°C becomes less than the sensitivity. The sensitivity is adjusted as 0.1% of the initial weight.)
As the silicone type spin finish oils, those which show smaller amount of decomposition sludge by heating is preferable. Polysiloxane type and polyaminosiloxane type are preferable. At the time of coating spin finish oils on pitch fibers it is possible to mix therewith, besides a solvent as a diluent, those such as a surfactant which is not a silicone type, a lubricant or an antioxidant.
With regard to the measurement of the remaining amount of oils, fibers are burnt out to ash and silicon content was measured by IPC emission spectrochemical analysis, and by regarding its numerical value as silicon content of principal constituent molecule of the silicone type oils, the remaining amount of the oils was calculated.
Further by an elemental analysis, carbon fibers of the present invention were determined to contain 2.0 - 15.0% by weight of oxygen, 0.07 - 0.7% by weight of sulfur. If these contents are too little, those inferior in workability are resulted. On the contrary, if they are too much, there is a tendency that the properties of ultimate products which are carbonized at a temperature of 2000°C or more, are lowered.
The remaining amount of the oils is preferably in the range of 0.2 - 1.0% by weight of fibers. In case of the remaining amount of the oils being too small, the cohesiveness and lubricity of bundle of filament yarns become poor and are liable to produce trouble by static electricity. In case of the remaining amount of the oils being too much, it is not preferable not only because of the increase of the amount of oils imparted at the time of spinning which is disadvantageous in view of the cost but also because of reduction of thermosetting velocity. The reason for reduction of thermosetting velocity is not clear completely but it seems to be caused by the prevention of diffusion of oxygen by the film of the oils and reduction of the effective oxygen concentration caused by the large amount of vapour due to the oils generated in the inside of a furnace which drives oxygen out of the furnace.
Carbon fibers produced by the present invention have a tensile strength of 5 - 50 Kgf/mm², an elongation of 0.3 - 8.0% and a capability of increasing its tensile strength to 150 Kgf/mm² or more and its modulus of elasticity to 30,000 Kgf/mm² or more by the additive heat treatment carried out in the relaxed state. If the tensile strength becomes smaller than this range, it is not preferable because fibers become liable to be flawed at the step of next working. If the tensile strength becomes greater than this range, it is not preferable because fibers become liable to be broken at the time of working and abrasion resistance is reduced. The tensile strength is preferably in the range of 10 - 45 Kgf/mm². If the elongation of fibers is smaller than above-mentioned range, it is not preferable because fibers become liable to be flawed. If the elongation is greater than above-mentioned range, it is not preferable because the shape and dimensional stability of final products become worse. The elongation is preferably in the range of 0.6 - 5.0%.
Increase of tensile strength and increase of modulus of elasticity by the additive heat treatment carried out in the relaxed state are phenomena usually observable in case of easily graphitizing pitch but those having a tensile strength smaller than above-mentioned range after an additive heat treatment is not preferable because resistance to fatique and resistance to oxidation are inferior.
Those having a tensile strength smaller than above-mentioned range after the additive heat treatment have a smaller tendency of repelling a sizing agent after the heat treatment. Therefore, necessity of using the method of the present invention is smaller. The tensile strength after the additive heat treatment is preferably in the range of 200 - 450 Kgf/mm². Those having a modulus of elasticity smaller than above-mentioned range are not preferable because resistance to fatique and resistance to oxidation is inferior and change of dimension at the time of working is greater. The modulus of elasticity after the additive heat treatment is preferably in the range of 40,000 - 100,000 Kgf/mm².
The carbon fibers produced according to the present invention have, preferably a specific gravity of 1.30 - 1.70, a specific electric resistance of 5 × 10⁸ - 5 Ω·cm, a stack height of graphitic layers L_c (002) of 8 - 32Å, a graphitic interlayer spacing distance d 002 of 3.46 - 3.49Å and after strength and modulus have been increased by the additive heat treatment, a stack height of graphitic layers L_c (002) of 36Å or more, increase of a stack height L_c (002) of 5Å or more, a graphitic interlayer spacing distance d 002 is 3.46Å or less and decrease of interlayer spacing distance d 002 is 0.03Å or more. Most preferably, a specific gravity is 1.35 - 1.60, a specific electric resistance is 1 × 10⁸ ∼ 1 × 10² Ω·cm and after strength and modulus have been increased by the additive heat treatment, a stack height of graphitic layers Lc (002) of 70 -240Å and a graphitic interlayer spacing distance d 002 is 3.36 - 3.44Å.
After a high softening point pitch is subjected to melt-spinning in the present invention, preferably resulting pitch fibers are wound up on bobbins or without being wound up on bobbins and introduced continuously into an oxidative atmosphere at a maximum temperature of 200 - 400°C while being placed on a transportation belt for thermosetting, subsequently the fibers are subjected to carbonizing treatment in the atmosphere of an inert gas at a temperature of 400 - 1000°C while being placed on a transportation belt, under the condition to make the silicone type oils remaining on the pitch fibers in the adhered state in the amount of 0.1% - 2.0% by weight of said fibers and said fibers are transferred to a next step. The spin finish oils and a sizing agent are imparted during the spinning step before the pitch fibers are placed on a transportation belt. The remaining of these chemical agents is effective in improving handling property at the time of winding up of fibers after carbonization or various kind of working.
Though the reason for the fact hereinafter described is not clear, handling property is different between apparatus for carbonization, and those which have been carbonized on a transportation belt is most superior in handling property. Those which have been carbonized while the pitch fibers were wound up on heat-proof bobbins, those carbonized in cans and those carbonized on a belt have been examined. All showed values which are not greatly different each other in a tensile strength, an elongation, and a modulus of elasticity but at the time of working such as winding, weaving and knitting, those which have been carbonized while the pitch fibers were placed on a blet, were superior in cohesiveness of bundles.
With regard to the way of placement of spun pitch fibers on a transportation belt, any way is allowable so long as reversal of order of fiber-bundles does not occur e.g. such a way where it does not occur that fibers placed afterwards get into the previously formed fiber layers and order of fibers is disturbed. It is preferable to use a porous transportation belt and to press and adhere the fibers by suction from the back side so as to prevent the fibers placed on the transportation belt from moving due to vibration or gas flow. In this case, it is preferable that a transportation belt is a net conveyer When the fiber-bundles are delivered from a direction close to the vertical to the transportation belt surface, it often happens that they get into the holes of belt or previously formed fiber layers. It is preferable to make the angle between the direction of delivery of the fibers and a surface of belt, smaller by swinging the running fibers so as to perform a circular movement or a movement which draws a figure "8". At the time of collision of the fibers with the belt, it often happens that the fiber-bundles are opened by shock and becomes a cause of reversal of order of the fibers or a cause of drawback during a working after carbonization.
The fibers are subjected to thermosetting in the oxidative atmosphere at a maximum temperature of 200° - 400°C preferably while being placed on a transportation belt after spinning. As for heating temperature, it is preferable to select a temperature lower than 200°C at the inlet and to elevate the temperature slowly to give the highest temperature at the outlet, rather than to keep a fixed temperature throughout the whole process. Most preferably the maximum temperature is 250 - 350°C.
Since the pitch fibers after thermosetting are extremely weak, they cannot be subjected to a treatment in which a force is applied to the fibers. It is preferable to send them into a carbonization apparatus as they are in the state placed on the transportation belt. During the treatment carried out in the state placed on the transportation belt, there is no need of imparting oils or sizing agents.
The carbonization treatment is carried out at a temperature of 400 - 1000°C in an inert gas atmosphere under the condition in which silicone type oils are remaining in the state adhered on the pitch-based fibers in an amount of 0.1% - 2.0% by weight based upon said fibers. In the beginning of carbonization treatment, it is preferable to start from the substitution of the oxidative atmosphere by an inert gas at a temperature close to about 400°C. If the substitution by the inert gas is insufficient, a problem such as a decrease of fiber diameter, insufficiency of an increase of strength or the like may occur. Treatment time varies according to the diameter of fibers but it is preferable to elevate the temperature slowly at a rate of 10 - 100°C/min at the beginning and carry out the substitution of the atmosphere sufficiently by an inert gas and to maintain at a constant temperature for several seconds or several hundred seconds in the final stage.
Resulting fibers are subsequently taken up on bobbin or the like and subjected to a next processing. If necessary after subjecting to a further processing, such as weaving, knitting or the like, an additional carbonization can be applied to produce high strength, high modulus carbon fibers. Further it is possible to treat the fibers at higher temperature to graphitize the fibers. At the time of the additional carbonization, it is possible to stretch the fibers to increase a tensile strength and a modulus of elasticity.
In case of winding up of resulting fivers from the transportation belt onto bobbins or the like or sending to the next higher temperature treatment, it is necessary to pull out the bundles of fibers through rollers or the like. At this time, it is preferable to reverse the fiber layers on the transportation belt and then pull out the fibers to correct the shape of bundles to a straight line form. In order to reverse the fiber layers on the transportation belt in the direction of thickness, various kinds of processes may be adopted, but it is most preferable to use a second belt. The second belt is caused to contact the fiber layers from upper side, and while putting the fiber layers between both the belts, the top and the bottom are reversed. Therefore, the fiber layers are placed on the second belt and resulting fibers are pulled out from the top thereof.
The carbon fibers obtained according to the present invention, differently from the fibers highly carbonized, have a smaller modulus of elasticity, a superiority in cohesiveness of bundles and a superiority in workability to such works as those containing a step of bending at a small radius of curvature e.g. weaving and knitting. Further since the fibers of the present invention are of lower cost than fibers of advanced carbonization state, they are extremely advantageous in case of products which cause a large amount of working loss. Since relaxation of strain occurs at the time of carbonization, they are superior in abrasion resistance and fatique resistance of bent parts of small radium of curvature. Further they show a resistance against a fluff forming by abrasion and against a flexion and a scratching.
The carbon fibers obtained according to the present invention are liable to be wetted by a resin prepolymer, an adhesive, an oiling agent and a sizing agent and have superior workability.
The graphitic interlayer spacing distance of the pitch-based carbon fibers of the present invention was measured by using a X-ray diffraction apparatus. Fibers were ground to powder. About 10% by weight of high purity silicon powder for X-ray standard was admixed as a internal standard substance, and mixture was filled in a sample cell. By a X-ray diffractometer method, in which Cu Kα ray was used as a source of ray, carbon 002 diffraction line and the standard. silicon 111 diffraction line were measured,then the diffraction angle of(ϑ)of carbon 002 plane was calculated from 002 diffraction peak position to which correction relating to Lorentz polarization factor, atomic scattering factor and absorption factor have been made. And d002 was calculated from a formula of d002=1.5418 Å/ 2Sin ϑ. L_c (002) was obtained by calculating the half value width (β) of carbon 002 diffraction peak in the above-mentioned X ray diffraction curve, to which correction for Kα₁ Kα₂ doublet lines have been applied and by using a formula of L_c = 91/β.
Hereinafter the present invention will be more fully explained. All "%" are percentages by weight unless otherwise described.

Example 1

A distillate fraction of a residual oil of a thermal catalytic cracking (FCC) having an initial fraction of 450°C and a final fraction of 560°C (converted to an atmospheric pressure) was subjected to heat treatment at a temperature of 400°C for 6 hours while sending methane gas therein and further heated at a temperature of 330°C for 8 hours to grow mesophase and the mesophase pitch was separated by sedimentation taking advantage of difference of specific gravities. This pitch had an optically anisotropic portion of 100%, a quinoline insoluble portion of 43% and a toluene insoluble portion of 82%.
This pitch was spun through a spinning hole having an enlarged part at an outlet. After spin finish oils were coated upon the spun fibers according to a common procedure, the pitch fibers were taken up at a rate of 270 m/min. and piled up on a transportation belt while giving waving motion so as to form spiral shape locus. As the spin finish oils, silicone type oils having a heat resistance of 630°C and a viscosity of 230 centi-Stokes was used. The amount of adhered spin finish oils was 3.0% based upon the weight of the pitch fibers.
Subsequently, resulting fibers were subjected to thermosetting by an oxidation treatment with air while elevating a temperature at a rate of 3°C/min. in a furnace having a temperature of 160°C at an inlet and 320°C at an outlet. The fibers which came out of a furnace were sent into a carbonization furnace while being kept on the transportation belt. The temperature at the inlet was 420°C. While elevating temperature at a rate of 5°C/min. till 500°C and at a rate of 20°C/min. till 580°C, substitution of the atmosphere with an inert gas was carried out. After the treatment at 580°C was continued for 45 seconds, the fibers were taken out of the furnace and after reversing the upper and lower layer while putting the fibers between the transportation belt and a second belt, the fibers were wound up on bobbins.
The amount of the oils remaining on the resulting fibers was 0.25%. A tensile strength, a modulus of elasticity, an elongation, a specific gravity and a specific electric resistance were, 27 Kgf/mm², 820 Kgf/mm², 3.3%, 1.52 and 2 × 10⁷ Ω·cm, respectively.
When said fibers were treated in the atmosphere of argon at a temperature of 2800°C for 2 minutes, high strength, high modulus carbon fibers having a tensile strength of 290 Kgf/mm² a modulus of elasticity of 75,000 Kgf/mm² and an elongation of 0.4 were obtained.
By using the fibers before and after the heat treatment in the atmosphere of argon, their weaving properties were investigated. In case of a plain weave, there were no notable difference between the two. In case of a double weave, the fibers before the heat treatment were easier in weaving. In case of a multiple axis weaving and a three dimensional weaving, weaving of the fibers after the heat treatment were difficult.
The properties of plain woven fabrics before and after the heat treatment in the atmosphere of argon were investigated. The woven fabrics of the fibers before the heat treatment were compared after an additive heat treatment carried out in the atmosphere of argon. Both did not show big difference in a tensile strength, an elongation and a modulus of elasticity but the woven fabrics made from the fibers after the heat treatment were slightly bulky, had a tendency of being fluffy by abrasion, and were slightly inferior in resistance to flexion and scratching and the resistance to abrasion of their selvage parts was inferior greatly.

Comparative example 1

The pitch fibers spun as in Example 1 were wound up on bobbins made of alumina porcelain and subjected to a thermosetting and a carbonization treatment under a condition for heating similar to Example 1. The amount of the oils remaining was 0.09%. It seems that a cooling rate after the carbonization treatment was slow and on this account decomposition loss was large.
The tensile strength, the elongation, the modulus of elasticity and the crystalline state did not show much difference from Example 1 but the weaving property was greatly inferior, and weaving of the multiple axial fabrics and the three-dimensional fabrics were difficult.
Further when the amount of the spin finish oils after spinning was increased than in Example 1 and the remaining amount was 0.25%, the weaving property became close to Example 1, but the wound-up shape of filaments was worse, breakage of filaments occurred frequently and it was difficult to pass through the preparation step for weaving.

Comparative example 2

Pitch fibers spun as Example 1 were taken in a cans made of a heat-proof alloy and subjected to a thermosetting and a carbonization treatment under a temperature-elevating condition similar to Example 1. The remaining amount of the oils was 0.08%. Since a cooling rate after the carbonization treatment was slow as in Comparative example 1, it seems that decomposition loss increased. The tensile strength, the elongation, the modulus of elasticity and the crystalline state of the fibers were not different greatly from these values of Example 1. But because taking out of the cans was difficult, the estimation of the weaving property was difficult.
Further when the amount of adhesion of the spin finish oils after spinning was increased to about 0.25% which was more than that of Example 1 and the pitch fibers were taken in a cans, because the amounts of the remaining oils of the surface part and the bottom were greatly different, there was formed an unequality in working characteristics and woven fabrics having a good quality could not be produced.

Example 2

By using the pitch same with Example 1, spinning was carried out under the same spinning condition with Example 1. The fibers after a thermosetting in the state piled up on a transportation belt were subjected to a carbonization treatment by changing the maximum temperature of a carbonization furnace. Then the fibers were wound up on bobbins and the remaining amount of the oils was measured and the working property was evaluated by weaving. The result thereof is shown in Table 1.

Effectiveness of the Invention

The pitch-based carbon fibers produced according to the present invention are superior in cohesiveness and lubricity of bundles and in processability or workability even when second oils or the like are not imparted after the thermosetting or the carbonization.
The pitch-based carbon fibers produced according to the present invention can be easily processed than conventional products with no remaining spin finish oils and having higher carbonization grade and are inexpensive in cost because there is no need of an additional imparting of second oils or the like. Compared with carbon fibers with no remaining spin finish oils and having higher carbonization grade, the carbon fibers produced according to the present invention, are patient against a bending of small radius of curvature and are superior in resistance to abrasion, to flexion and to scratching of bent parts because the bent parts receive a stress relaxation by the carbonization treatment carried out in the later stage.
The carbon fibers produced according to the present invention can be used in various kind of fiber reinforced composite materials as they are or after the carbonization treatment or the graphitization treatment. Further they can be used as raw materials for activated carbon fibers.

Claims

1. Pitch-derived carbon fibers containing 0.1%-2.0% of remaining silicone type oiling agent based upon the weight of the fibers and having a stack height of graphite crystal Lc(002) of 16-28 Å,an interlayer spacing distance of graphite crystal d002 of 3.46-3.49 Å, a tensile strength of 5-50Kgf/mm and an elongation of 0.3-8.0%.

2. In the method for subjecting a pitch having a high softening point to melt-spinning and then to thermosetting treatment and carbonization or carbonization and a further graphitization, an improvement which comprises coatng said spun pitch-derived fibers with a silicone type oiling agent, introducing said coated fibers into an oxidative atomosphere at a maximum temperature of 200-400 °C to effect the thermosetting of said pitch-derived fibers, subsequently subjecting said pitch-derived fibers to the carbonization treatment at an inert gas atmosphere of 400-1000°C under the condition of the silicone type oiling agent remaining in said pitch-derived fibers being in the range of 0.1%-2.0% based upon the weight of said fibers and transferring said pitch-derived fibers to the next step of working.