DE69007941T2 - Mesophase pitch for the production of carbon materials. - Google Patents

Description

The present invention relates to a mesophase pitch for use in the production of high performance carbon fibers and other carbon materials.

High performance carbon fibers are produced commercially mainly from PAN (polyacrylonitrile). However, PAN is expensive and does not have a high carbonization yield. Recently, it has been discovered that carbon fibers with properties comparable to or (even) better than those made from PAN can be produced from inexpensive pitch. Subsequently, extensive efforts have been made to commercialize this process.

Two types of pitch can be used as starting materials for the production of carbon materials: isotropic pitch and anisotropic pitch.

Carbon fibers made from isotropic pitch are inexpensive but suffer from low strength due to low molecular orientation. Consequently, high performance carbon products cannot be made from isotropic pitch. In contrast, carbon fibers made from anisotropic pitch, so-called "mesophase pitch", have a higher degree of molecular orientation and exhibit improved mechanical properties in terms of strength and elastic modulus. Consequently, with a view to producing high performance carbon fibers, extensive research is being conducted on the production of mesophase pitch from catalytically cracked petroleum pitch, petroleum tar pitch or coal tar pitch. When fibers are produced by a melt spinning process using mesophase pitch, the developed aromatic planar molecules aligned with the fiber axis by applying a shearing force as the pitch passes through nozzle holes. This oriented structure is maintained without being destroyed during the subsequent "stabilization" stage, in which the surface of the fibers is oxidized by gradual heating under an air stream, and during the "carbonization" stage, in which the stabilized fibers are heat-treated in an inert gas atmosphere at temperatures not lower than 1000ºC. It has been confirmed by numerous studies that this effect contributes to the production of highly oriented, high-performance carbon fibers.

The portion of pitch exhibiting an optically anisotropic phase (this portion is hereinafter referred to as "mesophase") is insoluble in polar solvents, such as quinoline and pyridine, and it has been previously assumed that the mesophase is identical with the component insoluble in polar solvents. However, recent studies have shown that the portion exhibiting anisotropy does not necessarily correspond to the insoluble content when observed under a polarizing microscope, but that the mesophase contains both components insoluble in polar solvents and components soluble in the solvents. Consequently, the term "mesophase" as used herein means that a portion of a phase which exhibits optical anisotropy when observed under a polarizing microscope, and the portion occupied by the area of this optically anisotropic phase when observed with a polarizing microscope shall be referred to as "the content of a mesophase" or simply "the mesophase content".

If the mesophase content of pitch is low, the anisotropic phase and the isotropic phase in molten pitch will separate, disrupting the spinning process. Consequently, the mesophase content of pitch is at least 90%, preferably 100%. An increase in the mesophase content However, this generally causes an increase in the softening point and viscosity of the pitch and makes it difficult to carry out spinning with consistent results. A high softening point and high viscosity make spinning at elevated temperatures necessary. In this case, however, the pitch is prone to thermal decomposition or condensation, with the resulting gases and infusible high molecular weight substances making it difficult to continue spinning operations for a long time with consistent results.

To solve these problems with mesophase pitch, various methods have been proposed. US-A-4,472,265 proposes a method in which the mesophase pitch is partially hydrogenated to reduce the degree of stacking of its molecules to an appropriate level, and then the resulting "isotropic pitch" is spun. JP-A-18421/1983 discloses a method characterized by the use of a single type of pitch or "premesophase" pitch which is isotropic during spinning but becomes anisotropic during carbonization. US-A-4,208,267 discloses a method in which an isotropic pitch is subjected to solvent extraction, and then the insoluble material is heated to 230 to 400°C. JP-A-136835/1983 discloses a process in which isotropic pitch is heat-treated, the resulting mesophase is filtered off and the remaining pitch is subjected to further heat treatment. US-A-4 533 461 discloses a process in which pitch is heat-treated to adjust the mesophase content to a range of 20 to 80%, precipitated and the mesophase recovered.

Although these methods have been improved in one way or another with regard to the use of mesophase pitch, they still suffer from the problems described below and are not yet able to provide completely satisfactory results.

In the processes described in US-A-4 472 265 and JP-A-18421/1983, an isotropic pitch which is not highly oriented is spun. Thus, the molecular orientation in the fibers is not as high as in the fibers spun from an anisotropic pitch and the fiber performance in terms of strength and elastic modulus is rather low. In addition, the process in which hydrogenation of high-viscosity pitch in which the polymerized molecules of a condensed polycyclic aromatic compound are stacked on one another is carried out is complicated and not favorable for industrial applications.

In the process described in US-A-4,208,267, only a small amount of the insoluble material is extracted with a solvent, so that the yield of mesophase pitch is low. The process described in JP-A-136835/1983 has the disadvantage of a complex filtration procedure that has to be carried out after the heat treatment. The process described in US-A-4,533,461 has technical difficulties in obtaining the mesophase and suffers from a low yield in the carbonization.

As mentioned above, the mesophase content of mesophase pitch for use in the production of carbon materials must be increased to achieve high performance in terms of strength and elastic modulus. The mesophase content of the pitch must also be increased to facilitate spinning operations in the production of carbon fibers. Additional requirements are high thermal stability during spinning operations, high stabilization reactivity of the spun fibers and high carbonization yield. In the case of the production of carbon materials, the yield of carbon material produced by carbonization of the pitch must also be high.

Consequently, the mesophase pitch for use in the production of carbon materials must meet the following conditions:

(1) high mesophase content,

(2) high thermal stability during the spinning processes,

(3) high stabilization reactivity and

(4) high carbonization yield.

The present inventors have found that mesophase pitch having a significant degree of polymerization in which large portions of the total carbon atoms are occupied by methyl groups and aromatic ring structures exhibits excellent performance and satisfies the above-mentioned four requirements. It is thus suitable for use in the production of high-performance carbon products and can be spun into fibers in a simple and consistent manner. Furthermore, the spun fibers can be effectively stabilized, with the additional advantage of high yield in subsequent carbonization. The present invention has been completed on the basis of these findings.

The present invention relates to a mesophase pitch for use in the production of carbon materials, the pitch having an average molecular weight of at least 1000 and a hydrogen/carbon atomic ratio of 0.5 to 1.0 with an aromatic carbon content (fa) of at least 0.7, a methyl carbon content of at least 4% of the total carbon atoms, a naphthenic (i.e. cycloparaffin) carbon content of less than 7% of the total carbon atoms and at least 90% of an optically anisotropic phase.

The inventors of the present invention have previously obtained a mesophase pitch from a condensed polycyclic hydrocarbon containing naphthenic (ie, cycloparaffin) carbon in an amount of at least 7% of total carbon atoms. Since this pitch satisfying the four conditions described above, the inventors filed a patent application therefor, which is US-A-4,891,126. As a result of their further investigations on mesophase pitch, the inventors of the present invention found that when polymerizing naphthalene derivatives, for example methylnaphthalene having at least one methyl group, a mesophase pitch containing less than 7% naphthenic carbon atoms and still having high performance could be produced when the methyl carbon content is less than 4% of the total carbon atoms. In particular, this mesophase pitch has a higher stabilization reactivity than that described in US-A-4,891,126.

The average molecular weight of the mesophase pitch of the present invention is determined using a vapor pressure osmometer using chloroform as a solvent. The solvent-soluble portion of the pitch is dissolved in chloroform and its molecular weight is determined using a vapor pressure osmometer. The insoluble portion is made soluble by a hydrogenation reaction under mild conditions using metallic lithium and ethylenediamine and its molecular weight is determined using the same vapor pressure osmometer. The results of the two determinations were used to determine the average molecular weight of the mesophase pitch. The mesophase pitch of the present invention has an average molecular weight of at least 1000, preferably 1000 to 1700 when determined using the method described above. If the average molecular weight is below about 1000, the achievable degree of polymerization is too low to produce pitch with a high mesophase content.

The carbon and hydrogen content of the mesophase pitch of the invention is determined by means of an automatic analyzer (CHN coding device) using a detection technique that uses thermal conductivity of combustion gases. The proportion of aromatic carbon (fa) is determined by an IR absorption technique and the content of methyl carbon by NMR.

The hydrogen/carbon atomic ratio of the mesophase pitch for use in producing carbon materials of the present invention is in the range of 0.5 to 1.0, preferably 0.6 to 1.0. With a hydrogen/carbon atomic ratio of less than about 0.5, the resulting pitch suffers from excessive dehydration as compared with polymerization, and its softening point is so greatly increased that subsequent spinning and other treatments become difficult. With a hydrogen/carbon atomic ratio of more than about 1.0, the resulting pitch has a low degree of orientation due to an insufficient degree of polymerization, making it impossible to obtain carbon fibers or other carbon materials having desired performance in such matters as strength and elastic modulus.

The aromatic carbon content (fa) is the ratio of the number of carbon atoms in aromatic ring structures to the total number of carbon atoms present. The pitch of the invention has a fa of at least 0.7, preferably between 0.75 and 0.87. At a fa value of less than about 0.7, the molecules forming the mesophase do not have a high degree of planar structure, making consistent production of pitch with a high content of an optically anisotropic phase difficult.

The methyl carbon content of the pitch according to the invention is at least 4%, preferably at least 5% of the total carbon atoms present. At a methyl carbon content of less than about 4%, the stabilization reactivity is so low that the completion of the stabilization treatment takes an unreasonably long time, with a difference between the stabilized Fibers are more likely to fuse.

As already mentioned, the optically anisotropic phase (mesophase) of the pitch according to the invention is determined using a polarizing microscope. The pitch according to the invention has a mesophase content of at least 90%, preferably at least 95%. In a particularly preferred manner, substantially all of the pitch consists of a mesophase. If the mesophase content of the pitch is less than about 90%, the carbon fibers or other carbon materials formed therefrom have only a low performance in terms of strength and elastic modulus. From the point of view of spinning, the mesophase content must also be at least about 90%.

The mesophase pitch of the present invention can be prepared by polymerizing naphthalene derivatives having at least one methyl group in the presence of hydrogen fluoride and boron trifluoride. Examples of naphthalene derivatives that can be used as starting materials are methylnaphthalene, dimethylnaphthalene and mixtures thereof. Materials containing these naphthalene derivatives can also be used. Examples of these are various petroleum fractions, residual oil from petroleum treatment stages and coal tar fractions.

As mentioned above, a hydrogen fluoride/boron trifluoride catalyst is used as the catalyst for polymerizing these naphthalene derivatives. In this respect, particularly suitable starting materials are those having low contents of nitrogen, sulfur and oxygen-containing compounds, all of which are basic compounds that bind strongly to the hydrogen fluoride/boron trifluoride catalyst. The polymerization catalyst is preferably used in an amount of 0.1 to 20 moles of hydrogen fluoride and 0.05 to 1.0 moles of boron trifluoride per mole of naphthalene derivative. Even with more than 20 moles of hydrogen fluoride or more than 1.0 mole of boron trifluoride, there is no corresponding increase in the reaction rate. On the contrary, the circulation of the catalyst is increased, which necessarily leads to the use of a large-sized reactor. With less than about 0.1 mole of hydrogen fluoride or less than about 0.05 mole of boron trifluoride, mesophase pitch having a mesophase content of at least 90% cannot be obtained. Hydrogen fluoride or boron trifluoride alone is not effective as a polymerization catalyst, so that they must be used in combination according to the invention.

Hydrogen fluoride (HF) when used together with boron trifluoride (BF3) forms a strong protic acid which reacts with the basic naphthalene derivative to form a complex.

The temperature for obtaining the desired mesophase by polymerization reaction is in the range of 180 to 400°C, preferably 250 to 320°C. At a temperature above about 400°C, excessive polymerization occurs, causing the resulting pitch to have an excessively high softening point. At a temperature below 180°C, mesophase pitch having a mesophase content of at least 90% cannot be obtained.

The time required to complete the polymerization reaction varies with the type of starting material used, the temperature and the amount of catalyst used, but typically it is in the range of 5 to 300 minutes, preferably 30 to 240 minutes. The pressure in the polymerization reaction is generally in the range of about 5 to about 100 bar, preferably about 20 to about 50 bar.

The polymerization reaction is carried out by mixing and stirring the starting material and catalyst charged in a corrosion-resistant reactor equipped with a stirrer. The reaction can be carried out batchwise or continuously.

The naphthalene derivative (Nd) introduced as starting material forms when mixed with the catalyst a complex and undergoes rapid polymerization to form a polymer in complex form according to the following scheme:

HF + BF3 + (Nd)n < -> H+(Nd)nBF₄- (1)

The obtained polymer in complex form is in equilibrium as shown in equation (1), so that after completion of the polymerization, the volatile components, HF and BF3, are distilled off at the polymerization temperature and recovered as catalyst components. At the same time, some volatile fractions are recovered and the polymerized pitch is separated.

Specific procedures for separating and recovering the catalyst from the pitch are the following:

Catalyst separation by a batch system consists of maintaining the polymerization temperature after completion of the polymerization reaction and removing HF and BF3 as a vapor phase from the reactor, recovering the polymer as molten pitch. The heating used for this purpose can be indirect heating (external heating through a jacket, etc.) or direct heating (by introducing heated vapor of a diluent, e.g. benzene, toluene or a halogenated hydrocarbon that is relatively inert to the catalyst).

Catalyst separation can also be carried out by a continuous process in a distillation column, wherein the refluxed diluent continuously fed with the polymerization reaction solution for extracting the HF and BF3 vapors from the top of the column is recovered with the pitch from the bottom of the column in the form of a solution in the diluent.

Whatever process is used, the temperature required to recover the catalyst corresponds to the polymerization temperature, whereas the pressure for catalyst recovery generally in the range of about 0 to about 30 atmospheres, preferably about 1 to about 5 atmospheres.

The pitch obtained by the process described above is characterized by a high mesophase content and the presence of numerous carbon atoms in aromatic ring structures and methyl groups. This pitch also has a low softening point, which is in the range of 200 to 250ºC when determined by a micromelting point method.

The pitch described above is a mesophase pitch which contains essentially no HF and BF3 and has an anisotropic phase of at least about 90%. It can be used as a raw material for producing carbon fibers and other carbon materials without being subjected to any special treatment. For example, this mesophase pitch can be spun into fibers without difficulty at a spinning temperature of about 280 to about 340°C. The spun pitch fibers have such a high stabilization reactivity that they can be sufficiently stabilized by heating to a temperature of about 300°C at a rate of about 7°C/min under an air stream. This easy stabilization can be attributed to the high content of methyl-derived carbon atoms in the pitch.

Carbon fibres can be produced from the pitch of the invention as follows: The pitch is first extruded through a die (ca. 0.25 µm) in a nitrogen atmosphere at a pressure of about 1 to about 3 bar and a temperature of about 280 to about 340°C, whereupon the fibres are wound around a reel at a winding speed of, for example, about 500 m/min. The fibres are then dried by heating from room temperature to a temperature of between about 200 and about 350°C at a typical rate of about 1 to 7ºC/min under a stream of air. Finally, the stabilized fibers are carbonized or graphitized by heating to about 1000ºC or more at a typical rate of about 10ºC/min in a stream of inert gas, e.g. nitrogen.

The mesophase pitch according to the invention has the following advantages:

(1) The mesophase pitch of the present invention can be stabilized without any complex and expensive steps such as filtration of the high temperature infusible material and solvent extraction thereof. The pitch of the present invention consists of a substantially homogeneous mesophase. It can be spun into carbon fibers at a temperature between about 280 and about 340°C, which is significantly lower than the temperature range commonly used.

(2) The mesophase pitch of the present invention can be spun into fibers at a temperature significantly lower than the point at which significant thermal decomposition or polycondensation occurs (400°C). Consequently, the spinnability of the pitch is good enough to resist deterioration during spinning. Furthermore, carbon fibers of consistent quality can be produced.

(3) Without releasing any decomposition gases or forming an infusible material, the pitch of the present invention can be spun at a high speed, and the spun pitch fibers have so few defects that high-strength carbon fibers can be produced.

(4) The mesophase pitch of the present invention has a high anisotropic phase content of at least about 90%, so that carbon fibers made from this pitch are characterized by a well-developed orientation in the direction of the fiber axis and have a high elastic modulus.

(5) Despite this high anisotropic phase content, the mesophase pitch of the present invention has a high H/C atomic ratio, with the proportion of methyl carbon in the total carbon content being high enough to increase the adaptability of the pitch to stabilization. As a result, completely infusible (stabilized) fibers can be obtained in a shorter time.

(6) The mesophase pitch according to the invention ensures a high carbonization yield due to its high degree of polymerization.

Due to these features (1) to (6), the present invention offers great industrial utility. The following examples are intended to further illustrate the present invention, but are not intended to limit it in any way.

example 1

An acid-resistant autoclave of 0.5 liter capacity was charged with α-methylnaphthalene (1 mole), HF (0.5 mole) and BF3 (0.2 mole). After raising the temperature in the autoclave to 270°C, a reaction was carried out for 4 hours. Thereafter, the release valve on the autoclave was opened so that substantially all of the charged HF and BF3 could be recovered in gaseous form at atmospheric pressure. Nitrogen was then blown into the autoclave to remove the low boiling point components. The yield of pitch was 76% by weight based on the charged α-methylnaphthalene. When observed with a polarizing microscope, this pitch was found to be 100% anisotropic mesophase pitch with a softening point of 240ºC, an average molecular weight of 1360, an atomic ratio H/C of 0.65 and an aromatic carbon content (fa) of 0.82, with the methyl carbon content being 6% of the total carbon atoms. The naphthenic carbon content of this pitch was 3% of the total carbon. All of these parameters except the naphthenic carbon content were measured according to the method described above. The naphthenic carbon content was determined by an NMR spectrum technique.

This mesophase pitch could be spun into fibers at 310ºC and a take-up speed of 500 m/min without any fiber breaking during spinning. The fibers could be stabilized without difficulty by heating to 300ºC at a rate of 7ºC/min. The stabilized fibers showed no fusion.

The stabilized fibers were heated to 1000°C at a rate of 100/min in an inert gas atmosphere to produce carbon fibers of 10 μm diameter. The carbonization yield was 90%, and the resulting carbon fibers had a tensile strength of 2747 N/mm² (280 kgf/mm²) and a modulus of elasticity of 215.8 kN/mm² (22 tf/mm²).

Example 2

An acid-resistant 3-liter autoclave was charged with a mixture of 60% α-methylnaphthalene and 40% β-methylnaphthalene (7 moles), HF (3 moles) and BF3 (1.0 mole). A reaction was then carried out at an elevated temperature of 265°C for 5 hours. By repeating the following procedures in Example 1, pitch was obtained in a yield of 76% by weight based on the charged methylnaphthalene mixture. When observed with a polarizing microscope, this pitch was found to be 100% anisotropic mesophase pitch. It had a softening point of 212°C, an average molecular weight of 1220, an H/C atomic ratio of 0.68 and a fa of 0.81, with methyl and naphthenic carbon contents of 7% and 4% of the total carbon atoms, respectively. All determinations were carried out according to the procedures of Example 1.

This mesophase pitch could be spun into fibers at 280ºC and a take-up speed of 500 m/min without any fiber breaking during spinning. The fibers could be stabilized without difficulty by heating to 280ºC at a rate of 7ºC/min. The stabilized fibers did not show any fusion.

The stabilized fibers were heated to 1000°C at a rate of 10°/min in an inert gas atmosphere to produce carbon fibers of 8 µm diameter. The carbonization yield was 90%, and the resulting carbon fibers had a tensile strength of 3139 N/mm² (320 kgf/mm²) and a modulus of elasticity of 196.2 kN/mm² (20 tf/mm²).

Example 3

An acid-resistant 3-liter autoclave was charged with a mixture of 60% 2,6-dimethylnaphthalene and 40% 1,4-dimethylnaphthalene (7 moles), HF (3.5 moles) and BF3 (1.4 moles). A reaction was then carried out at an elevated temperature of 275°C for 5 hours. By repeating the subsequent procedures of Example 1, pitch was obtained in a yield of 68% by weight based on the dimethylnaphthalene mixture charged. When observed with a polarizing microscope, this pitch was found to be 100% anisotropic mesophase pitch. It had a softening point of 230ºC, an average molecular weight of 1330, an H/C atomic ratio of 0.70 and a fa of 0.80, with the methyl and naphthenic carbon contents being 9% and 3% of the total carbon atoms, respectively.

This mesophase pitch could be spun into fibers at 310ºC and a take-up speed of 500 m/min without any fiber breaking during spinning. The fibers could be stabilized without difficulty by heating to 270ºC at a rate of 7ºC/min. The stabilized fibers showed no fusion.

The stabilized fibers were heated to 1000°C at a rate of 100/min in an inert gas atmosphere to produce carbon fibers of 10 µm diameter. The carbonization yield was 90%, and the carbon fibers formed had a tensile strength of 2845 N/mm² (290 kgf/mm²) and an elastic modulus of 225.6 kN/mm² (23 tf/mm²).

Comparison example 1

An acid-resistant 3-liter autoclave was charged with α-methylnaphthalene (1 mole), HF (3 moles) and BF3 (0.5 moles). A reaction was then carried out at an elevated temperature of 80°C for 3 hours. The release valve on the autoclave was then opened and gradually heated to 180-200°C at one atmosphere so that substantially all of the charged HF and BF3 could be recovered in gaseous form. The pitch was then discharged from the autoclave in a molten state. This pitch had a softening point of 72°C and contained no mesophase.

This pitch was first heat treated for 50 min at 1 bar and 47 sac, then for 30 min at a reduced pressure of 133.3 mbar and 420ºC to give 100% mesophase pitch (softening point 250ºC) in a yield of 50% based on α-methylnaphthalene.

The obtained mesophase pitch had an average molecular weight of 900, an H/C atomic ratio of 0.51 and a fa of 0.93, with the methyl and naphthenic carbon contents being 2% and 6% of the total carbon atoms, respectively.

This pitch could be spun into fibres at a take-up speed of 300 m/min and at 360ºC, but not at a higher take-up speed of 500 m/min. The pitch fibres obtained by spinning at a take-up speed of 300 m/min could not be stabilized by heating to 270ºC at a rate of 5ºC/min.

In this comparative example, α-methylnaphthalene was polymerized in the presence of an HF/BF3 catalyst, and the resulting pitch was converted into a mesophase pitch by subsequent heat treatments. However, this pitch was found to be unsuitable for high-speed spinning and stabilization because it had a low average molecular weight and a low methyl carbon content.

Claims (8)

1. Mesophase pitch for use in the production of carbon materials, the pitch having an average molecular weight of at least 1000, a hydrogen/carbon atomic ratio of 0.5 to 1.0, a proportion of aromatic carbon atoms of at least 0.7, a methyl carbon content of at least 4% of the total carbon atoms, a naphthenic carbon content of at least 7% of the total carbon atoms and containing at least 90% of an optically anisotropic phase. 2. Mesophase pitch according to claim 1, wherein the average molecular weight is in a range of 1000 to 1700. 3. Mesophase pitch according to claim 1 or claim 2, wherein the hydrogen/carbon atomic ratio is in a range of 0.6 to 1.0. 4. Mesophase pitch according to one of the preceding claims, wherein the proportion of aromatic carbon atoms is in a range from 0.75 to 0.87. 5. Mesophase pitch according to any one of the preceding claims, wherein the methyl carbon content is at least 5 % of the total carbon atoms. 6. Mesophase pitch according to any one of the preceding claims having a softening point in the range of 200 to 250°C. 7. Mesophase pitch according to any one of the preceding claims, which contains at least 95% of an optically anisotropic phase. 8. The mesophase pitch according to claim 7, wherein the content of an optically anisotropic phase is substantially 100%.