GB2107297A

GB2107297A - Process for the production of high-strength and high-modulus carbon fibers

Info

Publication number: GB2107297A
Application number: GB08224605A
Authority: GB
Inventors: Mikio Oyabu; Kenji Fukuda; Keiichi Hirata; Keisuke Takesue
Original assignee: Mitsui Coke Co Ltd
Current assignee: Mitsui Coke Co Ltd
Priority date: 1981-08-29
Filing date: 1982-08-27
Publication date: 1983-04-27
Also published as: DE3231682C2; FR2512076A1; GB2107297B; JPS6030365B2; JPS5841914A; DE3231682A1; FR2512076B1

Abstract

A process for the production of high-strength and high-modulus carbon fibers involves melting and spinning a mesophase pitch containing 1 to 40% by weight of a mesophase, rendering the resulting fiber infusible in air, carbonizing the resulting infusible fiber in an atmosphere of an inert gas, and optically graphitizing the carbonized fiber. The mesophase pitch is prepared by hydrogenating a pitch having an aromaticity index of not less than 0.6 in a hydrocarbon solvent under the influence of a hydrogenation catalyst, removing the catalyst, insoluble solid matter, and hydrocarbon solvent from the reaction mixture, and then heat- treating the resulting hydrogenated pitch under reduced pressure.

Description

SPECIFICATION Process for the production of high-strength and high-modulus carbon fibers This invention relates to a process for the production of high-strength and high-modulus carbon fibers. More particularly, the carbon fibers are prepared from mesophase pitches derived from pitches having a high degree of aromaticity.

Carbon fibers can be classified into two groups, according to mechanical strength: generalperformance (or GP) carbon fibers and highperformance (or HP) carbon fibers.

GP carbon fibers have a strength of 70 to 140 kg/mm2 and a modulus of 3 to 5 tons/mm2 (3040 to 5080 kg/mm2) and are typically produced by using optically isotropic pitches as the raw material. Principal applications of GP carbon fibers include abrasion materials, heat insulating materials, antistatic materials, sliding materials, filters, packings, and the like. These applications involves the utilization of certain properties of carbon in the form of fibers and, in most cases, do not require carbon fibers having high strength and high modulus.

On the other hand, HP carbon fibers have a high strength of 200 to 350 kg/mm2 and a high modulus of 10 to 40 tons/mm2 (10,140 to 40,700 kg/mm2) and are typically produced by using polyacrylonitrile as the raw material.

Principal applications of HP carbon fibers are composite materials made by combining them with resins and the like. These composite materials derived from HP carbon fibers are significantly excellent in both strength and modulus per unit weight as compared with other industrial materials and, therefore, are being used as special materials for the manufacture of rocket and aircraft parts and in leisure goods such as golf clubs, tennis rackets, and fishing rods. It is expected that the demand for HP carbon fibers as structural materials for automobiles, buildings, and so on will markedly increase in the future.

However, since HP carbon fibers produced from polyacrylonitrile are very expensive, composite materials derived therefrom have seldom been used in ordinary industrial applications despite their high mechanical strength. Accordingly, it is highly desirable to develop a method which enables HP carbon fibers to be produced at low cost.

A number of processes for the production of low-cost HP carbon fibers by using, as the raw material, mesophase pitches obtained by heat treatment of inexpensive pitches have already been proposed in Japanese Patent Application Nos. 8634/'74, 7533fi78, and 181 0/'79 as well as Japanese Patent Laid-Open Nos. 55625/'79 and 11 330/r79. According to these processes, HP carbon fibers can be produced by heat-treating a pitch at a relatively low temperature in the vicinity of 4000C for a period of time ranging from several hours to several tens of hours to prepare a mesophase pitch containing 40 to 100% by weight of a mesophase, melting and spinning the mesophase pitch to form a raw fiber having the mesophase oriented along the fiber axis, rendering the raw fiber infusible in air, carbonizing the resulting infusible fiber, and optionally graphitizing the carbonized fiber.

However, the preparation of a mesophase pitch in the above-described processes involves the acceleration of polycondensation of the pitch by heat-treating the pitch for a period of time ranging from several hours to several tens of hours, so that the degree of condensation of the mesophase produced in early stages of the heat treatment is higher than that of the mesophase produced in late stages thereof. In consequence, the degree of condensation of the mesophase shows a very wide distribution, resulting in a lack of uniform meltability of the mesophase pitch.

This leads eventually to a decrease in spinnability of the mesophase pitch. Moreover, when the pitch is heat-treated at a temperature of 4300C or above, the polycondensation of the mesophase is often accelerated to such an extent that the resulting high degree of condensation makes it impossible to melt the mesophase pitch. For this reason, it may be necessary to remove such an insoluble and infusible mesophase from the mesophase pitch prior to its spinning.

As described above, the prior art method for the preparation of a mesophase pitch is far from being the best method for the preparation of a raw material for HP carbon fibers.

It is an object of the present invention to provide an improved process for the production of HP carbon fibers, which uses a mesophase pitch having a mesophase content lower than those of mesophase pitches heretofore in use, and which possesses excellent spinnability therefore.

Another object of the present invention is to provide an improved process for the production of HP carbon fibers which includes the preparation of a mesophase pitch as described above.

According to the present invention, there is provided a process for the production of HP carbon fibers which comprises the steps of melting and spinning a mesophase pitch containing 1 to 40% by weight of a mesophase, rendering the spun fiber infusible in air, carbonizing the resulting infusible fiber in an atmosphere of an inert gas. Optionally the carbonized fiber is graphitized.

The term "mesophase content" as used herein denotes the content of quinoline-insoluble matter as determined by the method of Japanese Industrial Standard (JIS) K-2425.

The mesophase pitch used as the preferred raw material in the process of the present invention has a low mesophase content of at least 1% and below 40% by weight, as contrasted with the mesophase pitches which have been used in prior art processes.

Also according to the invention, there is provided a process for the production of high performance carbon fibers which comprises the steps of (a) hydrogenating a pitch having an aromaticity index of not less than 0.6 in a hydrocarbon solvent under the influence of a hydrogenation catalyst and removing the catalyst, insoluble solid matter, and hydrocarbon solvent from the resulting reaction mixture to obtain a hydrogenated pitch.

(b) heat-treating the hydrogenated pitch under reduced pressure to obtain a mesophase pitch containing 1 to 40% by weight of a mesophase; (c) melting and spinning the mesophase pitch; (d) rendering the spun fiber infusible in air; and (e) carbonizing the resulting infusible fiber in an atmosphere of an inert gas.

Thus, the mesophase pitch of the present invention shows a marked improvement in spinnability.

The invention will be described in more detail by way of example with reference to the accompanying illustrations, in which: Fig. 1 is an optical photomicrograph of a mesophase pitch used in the process of the present invention; Fig. 2 is an optical photomicrograph of a raw fiber obtained by melt spinning of the mesophase pitch of Fig. 1; and Fig. 3 is a scanning electron micrograph of a carbon fiber produced by the process of the present invention.

In accordance with one preferred embodiment of the present invention, pitches having a high degree of aromaticity, such as depolymerized coal, coal tar, coal-tar pitch, ethylene bottom oil pitch, etc., are used as raw materials for the preparation of mesophase pitches. The term "depolymerized coal" as used herein denotes any pitch-like material that is obtained by depolymerizing coal in a hydrocarbon solvent under a pressure of hydrogen and then removing any undissolved residue and the solvent from the resulting product. Such a highly aromatic pitch is hydrogenated in a hydrocarbon solvent under the influence of a hydrogenation catalyst. In the presently preferred process, it is not adequate merely to subject the pitch to hydrogenation, but it is important to enhance the degree of hydrogenation of the pitch and, at the same time, suppress its polycondensation.Moreover, the hydrogen content of the hydrogenated pitch should be at least 10%, and preferably 15 to 50%, higher than that of the starting pitch, and at least 90% of the hydrogenated pitch should have a molecular weight in the range of 400 to 600. The term "molecular weight" as used herein denotes the molecular weight determined by gel permeation chromatography. The selection of proper hydrogenation conditions and a proper catalyst is essential for the satisfaction of these requirements. Thereafter, the insoluble solid matter originating from the starting pitch and the hydrocarbon solvent are removed from the reaction mixture obtained as a result of the above-described hydrogenation.Alternatively, prior to the hydrogenation, the insoluble solid matter originating from the starting pitch may be removed by suitable techniques such as melt filtration, centrifugation, solvent extraction, and the like.

Next, the hydrogenated pitch is heat-treated at elevated temperature and under reduced pressure for a short period of time. This heat treatment is preferably carried out under such conditions that the temperature is not lower than 4800C, the holding time at that temperature is not more than 30 minutes, and the pressure is not higher than 40 mmHg (abs.). As a result, there is obtained a mesophase pitch containing 1 to 40% by weight of a mesophase.

As described above, the heat treatment of the hydrogenated pitch is carried out at elevated temperature and under reduced pressure for a short period of time, so that the formation of a mesophase occurs intensively during a short period of time and results in a mesophase having a very uniform degree of condensation. In addition, opticaliy isotropic components of the hydrogenated pitch, which can hardly be converted into a mesophase and cannot form a homogeneous melt with the mesophase, are removed as a distillate oil. Thus, the resulting mesophase pitch can be melted uniformly and, moreover, is endowed with excellent spinnability.

Furthermore, according to the present invention, the mesophase content of the mesophase pitch can be reduced to significantly less than the mesophase contents, which range from 40 to 100% by weight, of the mesophase pitches used in prior art processes. This also contributes iargely to the improvement in spinnability of the mesophase pitch.

The mesophase pitch prepared by the abovedescribed procedure is melted and spun, and the spun fiber is rendered infusible in air. The resulting infusible fiber is carbonized in an atmosphere of an inert gas and, if desired, the carbonized fiber is subsequently graphitized. As a result, there is obtained an HP carbon fiber having a strength of not less than 200 kg/mm2 and a modulus of not less than 10 tons/mm2 (10,140 kg/mm2).

Thus, mesophase pitches which serve as raw materials for the production of HP carbon fibers having high strength and high modulus can be prepared from quite common and inexpensive pitches without using any special solvent, reagent, or technique.

In the process of the present invention, a pitch having a high degree of aromaticity is used as the starting material. Suitable examples of the pitch include depolymerized coal, coal tar, coal-tar pitch, (derived from coal) as well as ethylene bottom oil pitch (derived from petroleum). Pitches having a high degree of aromaticity can also be prepared by heat-treating a heavy oil rich in aliphatic hydrocarbons at a temperature of 350 to 4500C for a period of time ranging from 15 minutes to 10 hours and then removing any insoluble solid matter. The pitches which can suitably be used as the starting material in the process of the present invention have an aromaticity index of not less than 0.6.The term "aromaticity index" as used herein denotes a parameter which has been proposed by Takeya et al. [Journal of the Japan Fuel Society, 46,927 (1967)] and is defined by the following equation.

C/H-Hjx-H y Aromaticity Index= C/H where C is the total number of carbon atoms, H is the total number of hydrogen atoms, Ha is the number of hydrogen atoms situated in the a positions, and Ho is the number of hydrogen atoms situated in the p or higher positions. For the purpose of the present invention, both x and y are assumed to be equal to 2.

Pitches having an aromaticity index of less than 0.6 are unsuitabie because they result in a low yield of the mesophase pitch, produce a mesophase pitch lacking uniform meltability and are responsible for poor strength of the final product or HP carbon fiber.

No particular limitation is placed on the type of hydrocarbon solvent used in the process of the present invention, and any solvent that can substantially dissolve the above-described starting material may be used. Solvents having a high degree of aromaticity are suitable for this purpose and specific examples thereof include heavy oils derived from coal, such as absorbing oil, creosote oil, tar middle oil, and anthracene oil, etc., and iighter fractions of heavy oils derived from petroleum, such as ethylene bottom oil, FCC cracking oil, etc. On the other hand, solvents rich in aliphatic hydrocarbons are unsuitable because they cannot dissolve the starting material of the present invention to a sufficient degree and do not permit the succeeding hydrogenation to be carried out smoothly.

The amount of hydrocarbon solvent used is preferably chosen so that the weight ratio of the starting pitch to the hydrocarbon solvent is 1:2 or greater and more preferably in the range of 1:3 to 1:10. If the weight ratio of the starting pitch to the hydrocarbon solvent is less than 1:2, it is difficult to remove the catalyst and insoluble solid matter from the reaction mixture obtained as a result of the hydrogenation.

The preferred conditions for the hydrogenation in the process of the present invention are such that the temperature is in the range of 400 to 5000C and more preferably 430 to 4800C, the hydrogen pressure is in the range of 50 to 300 kg/cm2G and more preferably 70 to 200 kg/cm2G, and the holding time at that temperature is not more than 240 minutes and more preferably in the range of 5 to 60 minutes. The spinnability of the resulting mesophase pitch is improved according as the hydrogenation is carried out at a higher temperature for a shorter period of time. If the hydrogenation temperature is lower than 4000C and the hydrogen pressure is lower than 50 kg/cm2G, the starting pitch is not hydrogenated to a full degree and it becomes difficult to obtain a hydrogenated pitch having properties suitable for use in the process of the present invention.On the other hand, if the hydrogenation temperature is higher than 500"C and the holding time is more than 240 minutes, the starting pitch undergoes an excessive degree of polycondensation and it becomes difficult to obtain a hydrogenated pitch having properties useful in the process of the present invention.

Moreover, the mesophase pitch so prepared for use as the raw material is not susceptible of uniform melting. Furthermore, if the hydrogen pressure is higher than 300 kg/cm2G, the resulting hydrogenated pitch does not provide a much better raw material for the preparation of a mesophase pitch as compared with hydrogenated pitches obtained at a hydrogen pressure of less than 300 kg/cm2G.

The hydrogen content of the hydrogenated pitch obtained by hydrogenation under the abovedescribed conditions is at least 10%, and preferably 1 5 to 50% higher than that of the starting pitch, and at least 90% of the hydrogenated pitch has a molecular weight in the range of 400 to 600.

If the starting pitch is hydrogenated under conditions other than those described above and the resulting hydrogenated pitch does not satisfy all of the above requirements, it is difficult to obtain a mesophase pitch which exhibits uniform melting behaviour. This leads to not only a decrease in spinnability of the mesophase pitch but also a loss in strength of the carbon fiber obtained as the final product If the starting pitch is not hydrogenated but is merely heat-treated directly, the resulting mesophase pitch cannot be melted uniformly and is entirely devoid of spinnability. Accordingly, hydrogenation is essential for the preparation of mesophase pitches suitable for use in the process of the present invention.

The hydrogenation catalyst which is used in the process of the present invention preferably comprises at least one of the following metals, to wit iron, cobalt, molybdenum, copper, tungsten, nickel, platinum, and rhodium, or at least one of the oxides and sulfides of the foregoing metals.

The hydrogenation catalyst is preferably added to the starting pitch in an amount of 1 to 20% by weight and more preferably 2 to 10% by weight of said pitch. If the amount of catalyst added is less than 1% by weight, an unduly long time is required for the hydrogenation, while if it is greater than 20% by weight, no additional catalytic effect is produced.

After completion of the hydrogenation under the above-described conditions, the hydrogenation catalyst and the insoluble solid matter originating from the starting pitch are removed from the reaction mixture, for example, by filtration or centrifugation. Thereafter, the hydrocarbon solvent is removed, for example, by vacuum distillation to obtain a hydrogenated pitch suitable for use in the process of the present invention. No particular limitation is placed on the conditions for the removal of the hydrocarbon solvent, and it is not necessary to remove all of the hydrocarbon solvent used. However, in order to assure that the resulting hydrogenated pitch has the above-described properties, the distillation conditions may be chosen so that the bottom temperature is in the range of 200 to 3000C and the pressure is in the range of 5 to 20 mmHg (abs.).

The hydrogenated pitch thus obtained is then heat-treated at elevated temperature and under reduced pressure for a short period of time to prepare a mesophase pitch containing 1 to 40% by weight, and preferably 5 to 40% by weight, of a mesophase. The heat treatment conditions should be chosen so that the resulting mesophase pitch has a mesophase content of 1 to 40% by weight. This step is preferably carried out under such conditions that the temperature is not lower than 480"C and more preferably in the range of 500 to 5500C, the holding time at that temperature is not more than 30 minutes and more preferably in the range of 2 to 1 5 minutes, and the pressure is not higher than 40 mmHg (abs.) and more preferably in the range of 3 to 20 mmHg (abs.).If the mesophase content is less than 1% by weight, it is impossible to produce an HP carbon fiber from the mesophase pitch according to the conventional procedure, while if it is greater than 40% by weight, the mesophase pitch shows a marked decrease in spinnability.

Moreover, if the heat treatment temperature is lower than 4800 C, the holding time at that temperature is more than 30 minutes, and the pressure is higher than 40 mmHg (abs.), it is impossible to obtain a mesophase pitch which has uniform meltability and excellent spinnability.

When obssrved under a reflected polarized light microscope, at least 60% of the mesophase pitch obtained by the above-described heat treatment is optically anisotropic.

Thus in the heat treatment step involved in the process of the present invention, a hydrogenated pitch which has been prepared under special hydrogenation conditions is heat-treated at elevated temperature for a short period of time and, moreover, the time required to heat the hydrogenated pitch to a predetermined temperature is as short as 1 or 2 minutes, so that the formation of a mesophase occurs intensively during a short period of time. This serves to prevent the formation of an insoluble and infusible mesophase and, therefore, gives a mesophase pitch having uniform meltability and excellent spinnability.

The mesophase pitch containing 1 to 40% by weight of a mesophase is then melted and spun at a temperature of 320 to 4000 C. Its spinnability is as good as that of optically isotropic pitches used for the production of GP carbon fibers.

Moreover, the diameter of the resulting raw fiber is very uniform along the fiber axis, indicating the good meltability of the mesophase pitch of the present invention.

Although less than 40% of the mesophase pitch of the present invention is optically isotropic, this mesophase pitch can be melted uniformly. Moreover, the mesophase is oriented along the fiber axis during the course of spinning and this orientation is taken over by the resulting carbon or graphite fiber.

The raw fibar so formed is then rendered infusible in air. This step is preferably carried out under such conditions that the temperature is in the range of 260 to 3400C and more preferably 270 to 3200 C, the holding time at that temperature is not more than 60 minutes and more preferably in the range of 5 to 30 minutes, and the heating rate is not greater than 3.30C/min. and more preferably in the range of 0.5 to 2.00C/min. If the temperture is lower than 2600C, the raw fiber is not rendered infusible.

As a result, the fiber melts or fuses together during the succeeding carbonization and hence fails to give an HP carbon fiber. If the temperature is higher than 3400C and the holding time is more than 60 minutes, the fiber is oxidized to an undue extent and the resulting carbon fiber suffers a loss in strength.

The resulting infusible fiber is then carbonized in an atmosphere of an inert gas. This step is preferably carried out under such conditions that the temperature is not lower than 8000C and more preferably 1,000 to 1 ,5000C, the holding time at that temperature is not less than 5 minutes and more preferably in the range of 10 to 30 minutes, and the heating rate is not greater than 70C/min. and more preferably in the range of 2 to 5 C/min. If the temperature is lower than 8000C and the holding time is less than 5 minutes, the fiber is not carbonized to a full degree and hence fails to give a carbon fiber having high strength. if the heating rate is greater than 70C/min., the resulting carbon fiber undergoes a partial fusion and hence suffers a loss in strength.Furthermore, where it is desired to enhance the modulus of the carbon fiber, it is graphitized in an atmosphere of an inert gas, preferably at a temperature of 2,000 to 3,0000 C.

The carbon fibers (including graphitized fibers) produced by the process of the present invention have a strength of 200 to 350 kg/mm2, a modulus of 10 to 40 tons/mm2 (10,140 to 40,700 kg/mm2), and hence compare favorably with the HP carbon fibers produced from polyacrylonitrile, with respect to both external appearance and mechanical strength. Moreover, the process of the present invention makes it possibie to produce HP carbon fibers from inexpensive and readily available pitches at low cost, with ease, and without using any special solvent, reagent, or technique.

The present invention will be more fully understood by reference to the following examples, which are intended merely to illustrate the preferred practice of the invention.

Example 1 A coal-tar pitch (which was composed, on a weight basis, of 91.43% carbon, 4.68% hydrogen, 0.97% nitrogen, 1.09% sulfur, and 1.83% oxygen according to the method of Japanese Industrial Standard M-8813 and had an aromaticity index of 0.95) was crushed to particle size of 12 mesh or finer, and 5% by weight of an iron oxide catalyst and a threefold amount of an absorbing oil were added thereto. After intimate mixing, the resulting mixture was hydrogenated by heating it to a temperature of 4600C under a hydrogen pressure of 100 kg/cm2G and holding it at that temperature for 10 minutes. The catalyst and the insoluble solid matter originating from the coaltar pitch were removed from the reaction mixture by filtration.The resulting filtrate was distilled at a bottom temperature of 2500C and a pressure of 10 mmHg (abs.) to obtain a hydrogenated pitch.

This hydrogenated pitch was composed, on a weight basis of 91.97% carbon, 5.61% hydrogen, 0.88% nitrogen, 0.41% sulfur, and 1.13% oxygen and its hydrogen content showed an increase of approximately 20% as compared with that of the coal-tar pitch used as the raw material. Moreover, its molecular weight distribution was measured by gel permeation chromatography using quinoline as the solvent, with the result that 92% by weight of the hydrogenated pitch had a molecular weight in the range of 400 to 600.

The above hydrogenated pitch was immersed in a salt bath at 51 00C, immediately exposed to a reduced pressure of 10 mmHg (abs.), and held at that temperature for 10 minutes. The resulting mesophase pitch had a mesophase content of 27.6% by weight.

This mesophase pitch was melted and spun at a temperature of 3500C and a draw rate of 800 meters/min. The resulting raw fiber was rendered infusible by heating it in air to a temperature of 2800C at a heating rate of 1 .00C/min. and holding it at that temperature for 5 minutes. The resulting infusible fiber was heated in an atmosphere of argon gas to a temperature of 1,000 C at a heating rate of 5 C/min, and held at that temperature for 1 5 minutes to form a carbon fibers The yield of this carbon fiber was 88.9% by weight based on the raw fiber.

The carbon fiber thus obtained had an average diameter of 12.8 zz, a strength of 235 kg/mm2, and a modulus of 16.2 tons/mm2 (16,470 kg/mm2).

Example 2 The hydrogenated pitch obtained in Example 1 was heat-treated in the same manner as in Example 1, except that the heat-treatment temperature was 5200C and the holding time at that temperature was 5 minutes. The resulting mesophase pitch had a mesophase content of 17.5% by weight.

This mesophase pitch was spun at a temperature of 3500C and a draw rate of 1,000 meters/min. to obtain a raw fiber, which was worked up in the same manner as in Example 1 to form a carbon fiber. The yield of this carbon fiber was 88.2% by weight based on the raw fiber.

The carbon fiber thus obtained had an average diameter of 11.2 , a strength of 288 kg/mm2, and a modulus of 14.9 tons/mm2 (15,150 kg/mm2).

Example 3 The hydrogenated pitch obtained in Example 1 was heat-treated in the same manner as in Example 1, except that the heat treatment temperature was 5300C and the holding time at that temperature was 4 minutes. The resulting mesophase pitch had a mesophase content of 11.6% by weight.

This mesophase pitch was spun at a temperature of 3450C and a draw rate of 1,400 meters/min. to obtain a raw fiber, which was worked up in the same manner as in Example 1 to form a carbon fiber. The yield of this carbon fiber was 89.4% by weight based on the raw fiber.

The carbon fiber thus obtained had an average diameter of 10.1 u, a strength of 310 kg/mm2, and a modulus of 17.9 tons/mm2 (18,200 kg/mm2).

A comparison of the results given in Examples 1 to 3 reveals that the spinnability of a mesophase pitch (which can be evaluated by the fiber diameter and the draw rate) and the strength of the resulting carbon fiber are improved by raising the heat treatment temperature (at which the hydrogenated pitch is heat-treated under reduced pressure) and reducing the heat treatment time.

Example 4 The carbon fiber obtained in Example 3 was graphitized by heating it in an atmosphere of argon gas to a temperature of 2,8000C at a heating rate of 1 00C/min. and holding it at that temperature for 5 minutes. The yield of the resulting graphite fiber was 85.9% by weight based on the raw fiber.

The graphite fiber thus obtained had an average diameter of 9.8 4, a strength of 210 kg/mm2, and a modulus of 38.5 tons/mm2 (39,150 kg/mm2).

Example 5 An Australian lignite was crushed to particle sizes of 60 mesh or finer and added to a threefold amount of tar middle oil. The resulting mixture was heated to a temperature of 41 00C under a hydrogen pressure of 50 kg/cm2G and held at that temperature for 60 minutes to dissolve the solvent-soluble components of the coal to a full degree. Any undissolved residue was removed from the reaction mixture by filtration and the resulting filtrate was distilled under reduced pressure to obtain a depolymerized coal. This distillation was carried out at a bottom temperature of 3500C and a pressure of 10 mmHg (abs.).The resulting depolymerized coal was composed, on a weight basis, of 89.27% carbon, 5.14% hydrogen, 0.96% nitrogen, 0.34% sulfur, and 4.29% oxygen and had an aromaticity index of 0.80.

To the above depolymerized coal were added 2% by weight of a cobalt-molybdenum catalyst and a threefold amount of a light fraction of ethylene bottom oil. After intimate mixing, the resulting mixture was hydrogenated by heating it to a temperature of 4500C under a hydrogen pressure of 1 50 kg/cm2G and holding it at that temperature for 10 minutes. After the separation of solid and liquid phases by filtration, the resulting filtrate was distilled at a bottom temperature of 2000C and a pressure of 10 mmHg (abs.) to obtain a hydrogenated pitch.This hydrogenated pitch was composed, on a weight basis, of 89.41% carbon, 6.24% hydrogen, 1.04% nitrogen, 0.28% sulfur, and 3.03% oxygen and its hydrogen content showed an increase of approximately 21% as compared with that of the depolymerized coal used as the raw material.

Moreover, its molecular weight distribution was measured by gel permeation chromatography using quinoline as the solvent, with the result that 91% by weight of the hydrogenated pitch had a molecular weight in the range of 400 to 600.

The above hydrogenated pitch was immersed in a salt bath at 5200 C, immediately exposed to a reduced pressure of 10 mmHg (abs.), and held at that temperature for 5 minutes. The resulting mesophase pitch had a mesophase content of 7.19/0 by weight.

This mesophase pitch was spun at a temperature of 3350C and a draw rate of 1,400 meters/min. The resulting raw fiber was rendered infusibie by heating it in air to a temperature of 3000C at a heating rate of 2.00C/min. and holding it at that temperature for 5 minutes. The resulting infusible fiber was heated in an atmosphere of argon gas to a temperature of 1 0000C at a heating rate of 5 C/min. and held at that temperature for 1 5 minutes to form a carbon fiber. The yield of this carbon fiber was 87.4% by weight based on the raw fiber.

The carbon fiber thus obtained had an average diameter of 10.0,u, a strength of 262 kg/mm2, and a modulus of 21.2 tons/mm2 (21450 kg/mm2).

Example 6 To an ethylene bottom oil pitch freed of light fractions (which was composed, on a weight basis, of 94.26% carbon, 5.53% hydrogen, 0.009/0 nitrogen, 0.07% sulfur, and 0.14% oxygen and had an aromaticity index of 0.76) were added 7% by weight of an iron oxide catalyst and a threefold amount of anthracene oil. After intimate mixing, the resulting mixture was hydrogenated by heating it to a temperature of 4700C under a hydrogen pressure of 100 kg/cm2G and holding it at that temperature for 5 minutes, the catalyst and the insoluble solid matter were removed from the reaction mixture by filtration, and the resulting filtrate was distilled at a bottom temperature of 300"C and a pressure of 10 mmHg (abs.) to obtain a hydrogenated pitch.

This hydrogenated ptich was composed, on a weight basis, of 92.89% carbon, 6.91% hydrogen, 0.05% nitrogen, 0.03% sulfur, and 0.12% oxygen and its hydrogen content showed an increase of approximately 25% as compared with that of the ethylene bottom oil pitch free of light fractions.

Moreover, its molecular weight distribution was measured by gel permeation chromatography using quinoline as the solvent, with the result that 95% by weight.of the hydrogenated pitch had a molecular weight in the range of 400 to 600.

The above hydrogenated pitch was immersed in a salt bath at 5300C, immediately exposed to a reduced pressure of 10 mmHg (abs.), and held at that temperature for 4 minutes. The resulting mesophase pitch had a mesophase content of 13.1% by weight This mesophase pitch was spun at a temperature of 3400C and a draw rate of 1 ,200 meters/min. to obtain a raw fiber, which was worked up in the same manner as in Example 1 to form a carbon fiber. The yield of this carbon fiber was 89.6% by weight.

The carbon fiber thus obtained had an average diameter of 11.4,u, a strength of 239 kg/mm2, and a modulus of 14.2 tons/mm2 (14500 kg/mm2).

In most cases, the mesophase pitch of the present invention does not consist entirely of an optically anistropic structure, but contains an optically isotropic structure.

In fact, all of the mesophase pitches described in Examples 1 to 6 consist partly of an optically isotropic structure, but they can be melted uniformly. the mesophase is oriented along the fiber axis during the course of spinning and this orientation is taken over by the resulting carbon fiber. By way of example, the mesophase pitch illustrated in Fig. 1 contains an optically isotropic structure in an amount of approximately 10 to 1 5%. However, the raw fiber obtained by spinning this mesophase pitch shows an orientation along the fiber axis, as illustrated in Fig. 2. In addition, it can be seen from Fig. 3 that the orientation of the raw fiber is taken over by the carbon fiber.

Claims

1. A process for the production of a highperformance carbon fiber which comprises the steps of melting and spinning a mesophase pitch containing 1 to 40% by weight of a mesophase, rendering the spun fiber infusible in air, and carbonizing the resulting infusible fiber in an atmosphere of an inert gas to form said carbon fiber.

2. A process according to claim 1 wherein, subsequently to the carbonization of the resulting infusible fiber in an atmosphere of an inert gas, the carbonized fiber is graphitized.

3. A process for the production of highperformance carbon fibers which comprises the steps of (a) hydrogenating a pitch having an aromaticity index of not less than 0.6 in a hydrocarbon solvent under the influence of a hydrogenation catalyst and removing the catalyst, insoluble solid matter, and hydrocarbon solvent from the resulting reaction mixture to obtain a hydrogenated pitch; (b) heat-treating the hydrogenated pitch under reduced pressure to obtain a mesophase pitch containing 1 to 40% by weight of a mesophase; (c) melting and spinning the mesophase pitch; (d) rendering the spun fiber infusible in air; and (e) carbonizing the resulting infusible fiber in an atmosphere of an inert gas.

4. A process according to claim 3 wherein, subsequently to the carbonization of the resulting infusible fiber in an atmosphere of an inert gas, the carbonized fiber is graphitized.

5. A process according to claim 3 or 4, wherein the catalyst comprises at least one member selected from iron, cobalt, molybdenum, copper, tungsten, nickel, platinum, and rhodium as well as oxides and sulfides of the foregoing metals.

6. A process according to claim 5 wherein the catalyst is used in an amount of 1 to 20% by weight based on the starting pitch.

7. A process according to claims, 3, 4, 5 or 6, wherein the hydrogenation in step (a) is carried out under such conditions that the temperature is in the range of 400 to 5000C, the holding time at that temperature is not more than 240 minutes, and the hydrogen pressure is in the range of 50 to 300 kg/cm2G.

8. A process according to any of claims 3 to 7, wherein the hydrogen content of the hydrogenated pitch is at least 10% higher than that of the starting pitch and at least 90% of the hydrogenated pitch has a molecular weight in the range of 400 to 600.

9. A process according to any of claims 3 to 8, wherein, in step (b) the hydrogenated pitch is subjected to such conditions that the temperature is not lower than 4800C, the pressure is not higher than 40 mmHg (abs), and the holding time at that temperature is not more than 30 minutes.

1 0. A process according to claim 9, wherein the heat treatment is carried out at a temperature of 500 to 5500C and a pressure of 3 to 20 mmHg (abs.) for a period of 2 to 1 5 minutes.

11. A process according to any of claims 3 to 10, wherein the starting pitch is selected from depolymerized coal, coal tar, coal-tar pitch, and ethylene bottom oil pitch.

12. A process according to any of claims 3 to 11, wherein in step (a), the starting pitch and the hydrocarbon solvent are used in a weight ratio of 1:2 or greater.

13. A process according to any of claims 3 to 12, wherein in step (a), the hydrocarbon solvent is removed by distillation under such conditions that the bottom temperature of the distillation zone is in the range of 200 to 3000C and the pressure is in the range of 5 to 20 mmHg (abs.).

14. A process according to any of claims 3 to 13, wherein, in step (c), the mesophase pitch is melted and spun at a temperature of 320 to 4000C.

1 5. A process according to any of claims 3 to 14, wherein, in step (d), the spun fiber is rendered infusible under such conditions that the heating rate is not greater than 3.30C/min., the temperature is in the range of 260 to 3400C, and the holding time at that temperature is not more than 60 minutes.

1 6. A process according to any of claims 3 to 15, wherein the infusible fiber resulting from step (d) is carbonized in an atmosphere of an inert gas under such conditions that the heating rate is not greater than 70C/min., the temperature is not lower than 8000C, and the holding time at that temperature is not less than 5 minutes.

1 7. A process according to claim 4 or any claim dependent thereon, wherein the graphitization is carried out at a temperature of 2,000 to 3,0000C.

18. Carbon fibres prepared by the process according to any of the preceding claims, which have a strength of 200 to 350 kg/mm2 and an elastic modulus of 10 to 40 tons/mm2 (10140 to 40,700 kg/mm2).

1 9. A process of producing high performance carbon fibres substantially as herein specifically described and with reference to the Examples.