GB2131781A - Process for producing carbon fibers using pitch - Google Patents

Abstract

High-performance carbon fibers are obtained using a pitch having a continuous phase which is optically isotropic at a melt-spinning temperature, and has a random or onion like structure (Fig. 1b and 1c). The random or onion-like structure remains after infusibilisation. <IMAGE>

Description

SPECIFICATION Process for producing carbon fibers using pitch Background of the invention The present invention relates to a process for producing carbon fibers using pitch as a starting material.

Recently, many processes have been made public for the production of high-performance carbon fibers using pitch as a starting material. All of these processes take note of the tensile modulus of elasticity and tensile strength of carbon fibers as well as elongation expressed in terms of a ratio of the two. In addition to these characteristics, the structure of a section perpendicular to the fiber axis has come to be recognized as an important characteristic required of carbon fibers.According to the basic structure of carbon fibers, graphite layer planes are developed parallely in the direction of the fiber axis, and it is presumed that such characteritics as the tensile modulus of elasticity and tensile strength depend on the degree of development of the said structure.On the other hand, in the section perpendicular to the fiber axis, there mainly appear planes parallel to the C axis of graphite crystallites.

It has become clear that this configuration does not affect the tensile modulus of elasticity and tensile strength of carbon fibers, but seriously affects the easiness of cracking in the direction of the fiber axis.

Carbon fibers wherein the configuration of graphite crystallites at a section perpendicular to the fiber axis takes the form of a radial structure are cracked easily, while carbon fibers wherein the said configuration takes the form of a random or onion structure are not cracked and therefore preferable.

Heretofore, a process for producing carbon fibers having a random or onion structure by using a spinning nozzle special in shape has been proposed in U.S. Patent No. 4,376,747. However, there has been made public no technique concerning the method of producing carbon fibers of a random or onion structure without using such a special shaped spinning nozzle. Establishment of the technique for eliminating a radial structure is keenly desired.

Brief description of the drawing Figure 1 is a view showing a typical state of configuration of graphite crystallites at a section perpendicular to the fiber axis of carbon fiber, in which (a) shows a radial structure, (b) a random structure and (c) an onion structure; Figure 2 is a sectional view showing a typical cracked state of carbon fiber of a radial structure; and Figure 3 is a sectional view showing a typical cracked state of pitch fiber.

Carbon fiber having the radial structure of Figure 1 (a) is apt to undergo such a crack as shown in Figure 2 in comparison with carbon fiber having the random structure of Figure 1 (b) or the onion structure of Figure 1(c).

Summary of the invention It is the object of the present invention to provide a process for producing high-performance carbon fibers having a random or onion structure and not having a radial structure by using pitch as a starting material.

The present invention resides in a process for producing a high-performance carbon fiber by meltspinning a carbonaceous precursor pitch, rendering the resultant pitch fiber infusible and subjecting the fiber thus rendered infusible to a carbonization treatment and, if required, to a subsequent graphitization treatment, characterized in that the continuous phase of the precursor pitch is optically isotropic at the melt-spinning temperature and in that the structure of a section perpendicular to the fiber axis of the pitch fiber is a random or onion structure and the structure of a section perpendicular to the fiber axis of the infusiblized fiber and that of the carbon fiber are also a random or onion structure.

Carbon fiber which has a random or onion structure and which is difficult to be cracked can be produced by melt-spinning a specific pitch under specific conditions and subjecting the resultant pitch fiber to infusiblization and carbonization treatments and, if required, to a subsequent graphitization treatment in accordance with the process of the present invention.

Description of preferred embodiments Having made detailed studies as to in which of the melt-spinning step for a precursor pitch and the subsequent infusiblization and carbonization steps the "configuration of graphite crystallites at a section perpendicular to the fiber axis" (hereinafter referred to as the "sectional structure") of carbon fiber is determined, the present inventors could confirm that the sectional structure of carbon fiber is determined already at the stage of melt spinning. This has already been pointed out by the description of pages 1 67-1 69 of "Applied Polymer Symposium", No. 29 (1976). There arises the next question why such three kinds of sectional structures as shown in Figure 1 are determined at the stage of melt spinning. On this point, no information has heretofore been available.Of more importance is to clarify which of various factors, for example, the following factors, affects the sectional structure of carbon fiber and how it affects such structure: properties of the precursor pitch, the melt-spinning temperature, the shape of a spinning nozzle, the extrusion speed of the precursor pitch from a nozzle, cooling conditions for the precursor pitch which has become fibrous after leaving a die, and the take-up speed when winding the cooled pitch fiber onto a drum. By so doing, it becomes possible to establish a technique for avoiding such a crack of carbon fiber as shown in Figure 2.

Another point which should not be overlooked is that if the sectional structure of carbon fiber is determined already at the stage of melt spinning, then in the very melt-spun pitch fiber, not graphite crystallites but condensed polycyclic aromatic planar molecules (i.e., a precursor of graphite crystallites), which may be a main constituent of the precursor pitch, are presumed to take such a configuration as shown in Figure 1. In the case of carbon fiber of a radial structure, therefore, it is presumed that the aromatic planar molecules are arranged radially (planes are arranged in parallel with the fiber axis) as shown in Figure 1 (a) already at the previous stage, namely, at the stage of pitch fiber.

In this case, a crack is often produced already at the stage of pitch fiber. Thus, prior to the problem that carbon fibers are easily cracked, the easiness of cracking at the stage of pitch fiber, the easiness of cracking at the subsequent infusiblization step and that at the carbonization step are seriously involved in the production of carbon fibers. Therefore, if there is established a technique for eliminating the radial structure throughout the entire process from the melt-spinning step up to the production of carbon fiber, there can be attained not only the advantage that the resultant carbon fiber is difficult to crack in the aspect of its performance, but also the advantage that the difficulty of cracking is maintained throughout the entire manufacturing process beginning with melt spinning and ending in the production of carbon fiber, thus permitting a smooth production.

Having made detailed studies, the present inventors reached the conclusion that the sectional structure of carbon fiber is determined by properties of pitch, particularly whether the continuous phase at the spinning temperature is optically isotropic or anisotropic.

More particularly, it has become clear that if the continuous phase of the precursor pitch at the spinning temperature is optically anisotropic, the resultant carbon fiber is apt to have the radial structure and that an optically isotropic continuous phase affords carbon fibers of a random or onion structure. The "spinning temperature" referred to herein indicates the temperature of the precursor pitch before leaving the nozzle of a spinning apparatus.As to the method of observing optical properties of the precursor pitch at room temperature, it is described, for example, in "The Formation of Some Graphitizing Carbon" (Chemistry and Physics of Carbon, Vol. 4, pp. 243-268). Further, the method of producing carbon fiber by melt-spinning a precursor pitch rich in an optical anisotropy is disclosed in Japanese Patent Publication No. 37611/1980. In these conventional methods, however, the content and texture of an optically anisotropic phase at room temperature and mainly referred to.

But, as a result of studies made by the present inventors, it became clear that in order to produce carbon fibers having a random or onion structure, the very optical properties of a precursor pitch at the spinning temperature are important and optical properties of the precursor pitch at room temperature as in the prior art are not important at all.

The relation between optical properties of a precursor pitch at room temperature and at the spinning temperature will now be described. First, it is to be noted that the proportion of an optically anisotropic phase in a precursor pitch varies according to temperature of the precursor pitch and the state of that change quite differs, depending on the kind of the precursor pitch used.For example, in the case of one precursor pitch containing 80% of an optically anisotropic phase at room temperature, the content of the optically anisotropic phase decreases to 20% when observed directly with a polarizing microscope after raising the temperature to 4000 C. In the case of another precursor pitch containing 80% of an optically anisotropic phase at room temperature, the content of that phase decreases to 70% when observed directly with a polarizing microscope after raising the temperature to 4000C. If the melt-spinning temperature is 4000 C, this difference between the two precursor pitches is serious. Because, in the case of the former, the continuous phase at the spinning temperature is optically isotropic, while in the case of the latter, the continuous phase at the spinning temperature is optically anisotropic.That is, in the case of the former, carbon fiber of a random or onion structure is obtained, while in the case of the latter, carbon fiber of a radial structure is obtained. Thus, the content of an optically anisotropic phase at room temperature does not affect at all the determination of the sectional structure of carbon fiber. As to the relation between the content of an optically anisotropic phase and the temperature, it is referred to, for example, in the "Carbon", Voi. 16, p. 503 (1978) and in the '82 Carbonaceous Materials Society Seminar Drafts, p. 23.

As will be appreciated from the above description, the present invention resides in a process for producing a high-performance carbon fiber by melt-spinning a carbonaceous precursor pitch, rendering the resultant pitch fiber infusible and subjecting the fiber thus rendered infusible to a carbonization treatment and, if required, to a subsequent graphitization treatment, characterized in that the continuous phase of the precursor pitch is optically isotropic at the melt-spinning temperature and in that the section perpendicular to the fiber axis of the pitch fiber has a random or onion structure and the section perpendicular to the fiber axis of the infusiblized fiber and that of the carbon fiber also have a random or onion structure.

There are many pitches having an isotropic continuous phase at the spinning temperature, of which those having a reflectivity of the optically isotropic phase in the range of 8.5% to 11.0% exhibit an outstanding effect.

Because of restrictions on reflectivity measuring means, it is difficult to directly measure the reflectivity of a precursor pitch at the spinning temperature, so the reflectivity of a precursor pitch is measured in the following manner.

The precursor pitch held at the spinning temperature is quenched. This quenching permits cooling of the precursor pitch while substantially retaining the its state at the spinning temperature. The thusquenched precursor pitch is embedded in a resin, then polished and measured for reflectivity.

The reflectivity is measured in air by means of a reflectivity measuring device. More particularly, 30 or more points on an optically isotropic portion on a sample plane are selected at random, then measured for reflectivity and an average value is adopted as the reflectivity of the optically isotropic portion of the precursor pitch. The method of measuring the reflectivity of coal samples has been standarized (JIS M8816-1979), which has heretofore been adopted widely. The measurement of the reflectivity of pitches is conducted basically in accordance with the above standardized measuring method.

If the reflectivity is smaller than 8.5%, it becomes difficult to effect a smooth spinning probably because of a too large difference in viscosity between the optically isotropic portion as the continuous phase and the opticaliy anisotropic portion as the discontinuous phase. Further, an optically isotropic phase having a reflectivity larger than 1 1% is difficult to manufacture and not effective.

If the precursor pitch to be used satisfies the above-mentioned conditions on reflectivity, its manufacturing method is not specially limited.

The spinning temperature may be selected from a range in which the continuous phase of the precursor pitch used exhibits isotropy. The temperature at which the precursor pitch used exhibits isotropy differs according to the kind of pitch, but whether or not the precursor pitch is actually exhibiting isotropy at that temperature can be confirmed easily by observation with a polarizing microscope. It is to be noted that if the absolute value of the spinning temperature is too high, for example, if it is above 4000 C, the evolution-of gas due to a thermal decomposition of the precursor pitch is unavoidable, which may lead to a disadvantage, for example, the formation of a cavity in the pitch fiber. Therefore, the adoption of an unnecessarily high temperature should be avoided. A preferred range of the spinning temperature is from 3000 to 4000C.

The following examples are given to further illustrate the present invention, but it is to be understood that these examples are for the aid of understanding of the present invention and are not intended to limit the same in any manner.

Example 1 A vacuumdistilied gas oil (VGO) from Arabic crude oil was subjected to a hydrogenation treatment, and the thus-hydrogenated oil was subjected to a catalytic cracking at 5000C using a silicaalumina catalyst to obtain a heavy oil (A) having a boiling range above 2000C, properties of which are shown in Table 1.

A heavy oil (B) having a boiling range above 2000C was obtained as a by-product in steam cracking of naphtha at 8300 C. Table 2 shows properties of the heavy oil (B). The heavy oil (B) was heat-treated at a temperature of 4000C and a pressure of 1 5 kg/cm2 G for 3 hours. This heat-treated oil (C) was distilled at 2500C/1 .0 mmHg to obtain a fraction (D) having a boiling range of 1600 to 4000 C, properties of which are shown in Table 3. The fraction (D) was contacted with hydrogen at a temperature of 3300C, a pressure of 35 kg/cm2, G and a liquid hourly space velocity (LHSV) of 1.5 in the presence of a nickel-molybdenum catalyst (NM-502), thereby ailowing a partial nuclear hydrogenation to proceed, to obtain a hydrogenated oil (E). The percent nuclear hydrogenation was 31%.Table 4 shows properties of the hydrogenated oil (E).

60 parts by weight of the heavy oil (A), 30 parts by weight of the heavy oil (B) and 10 parts by weight of the hydrogenated oil (E) were mixed and heat-treated at a temperature of 4300C and a pressure of 20 kg/cm2 G for 3 hours. The oil thus heat treated was distilled at 2500C/1 .0 mmHg to distill off the light fraction to obtain pitch (1 ) having a softening point of 800C.

Table 1 Properties of heavy oil (A)

Specific gravity (1 50C/40C) 0.965 Distillation Initial boiling point 3200C Property 5% 3400C 10% 3530C 20% 3700C 30% 3850C 40% 3990C 50% 4150C 60% 4270C 70% 4450C 80% 4670C 90% 5120C Viscosity cSt # 50 C 500C 18.21 Table 2 Properties of heavy oil (B)

Specific gravity (1 50C/40 C) 1.039 Distillation Initial boiling point 1 920C Property 5% 2000C 10% 2060C 20% 2170C 30% 2270C 40% 2410C 50% 2630C 60% 2900C 70% 3600C Table 3 Properties of fraction (D)

Specific gravity (1 50C/40C) 0.991 Refractive index (n25/D) 0 1.5965 Molecular weight 145 Distillation Initial boiling point 1600C Property 10% 2000C 30% 2150C 50% 2300C 70% 2560C 90% 3050C Table 4 Properties of fraction (E)

Specific gravity (1 50C/40C) 1.02 Refractive index (nod5) 1.5867 Distillation Initial boiling point 1 630C Property 10% 208"C 30% 2260C 50% 2390C 70% 2620C 90% 3170C The pitch (1) was treated by means of a film evaporator at 3450C under a reduced pressure of 1 mmHg for 1 5 minutes and then heat-treated at 3700C under atmospheric pressure for 20 minutes to obtain a precursor pitch (2) having a softening point of 261 OC. The continuous phase of the precursor pitch (2) proved to be isotropic at a temperature not lower than 350"C. The precursor pitch (2) was melt-spun from a 0.3 mm-dia. die having a length/diameter (VD) ratio of 2 at a spinning temperature of 3600C to obtain pitch fiber having a diameter of 12 jum. The reflectivity of an optically isotropic portion of the precursor pitch was 9.0%.

The pitch fiber was rendered infusible in air by a conventional method, then the fiber thus rendered infusible was carbonized at 1 ,0000C in an inert gas atmosphere by a conventional method and subsequently graphitized at 2,5000C in an inert gas atmosphere by a conventional method to obtain carbon fiber having a diameter of 10 zm. The carbon fiber, upon observation with a scanning electron microscope, proved to have a section of such a typical random structure as shown in Figure 1 (b). The tensile modulus of elasticity and tensile strength of the carbon fiber were 40 ton/mm2 and 300 kg/mm2, respectively.

Comparative Example 1 The same precursor pitch as that described in Example 1 was melt-spun in the same way as in Example 1 at a spinning temperature of 3250C to obtain pitch fiber having a diameter of 12 ym. The continuous phase of the precursor pitch was optically anisotropic at 3250C.

The pitch fiber was subsequently treated in the same manner as in Example 1 to obtain carbon fiber having a diameter of 10 ,um. The carbon fiber proved to have a section of such a typical radial structure as shown in Figure 1 (a) and was partially cracked.

Example 2 1 50 ml. of a high-temperature tar (properties of which are shown in Table 5) was heated after removal of quinoline insoiubles up to 4400C at a heating rate of 3cC/min at an initial hydrogen pressure of 100 kg/cm2 - G in an autoclave equipped with a stirrer and having a content volume of 300 ml, and was held at 4400C for 3 hours. Thereafter, the heating was stopped and the temperature was allowed to drop to room temperature. The resultant liquid product was distilled at 2500C/1 mmHg to distill off the light fraction to obtain a starting pitch (3) having a softening point of 700C and a quinoline insolubles content of 3%. The yield was 40 wt.%.

The pitch (3) was treated by means of a film evaporator at 3450C under a reduced pressure of 1 mmHg for 1 5 minutes and then heat-treated at 3500C under atmospheric pressure for 1 5 minutes to obtain a precursor pitch (4).

Table 5 Properties of the high-temperature tar

Density | 1.18 Carbon content (%) 91.3 Hydrogen content (%) 5.1 Sulfur content (%) 1.2 Nitrogen content (%) 0.7 Ash content (%) 0.03 Toluene insolubles content (%) 9.1 Viscosity,* mole-sec &commat; 600C L 680 measured by a Redwood viscometer The continuous phase of the precursor pitch (4) was optically isotropic at above 3500 C. The precursor pitch (4) was melt-spun at 3600C to obtain pitch fiber having a diameter of 12 Hm. The reflectivity of the optically isotropic phase of this precursor pitch was 9.3%. The pitch fiber was subsequently treated in the same manner as in Example 1 to obtain carbon fiber having a diameter of 10 ym. The carbon fiber proved to have a section of such a typical random structure as shown in Figure 1(b). The tensile modulus of elasticity and tensile strength of the carbon fiber were 39 ton/mm2 and 290 kg/mm2, respectively.

Comparative Example 2 The same precursor pitch as that described in Example 2 was melt-spun at 3400 C. The continuous phase of this precursor pitch was optically anisotropic at 3400C. The resultant carbon fiber proved to have a section of such a typical radial structure as shown in Figure 1 (a). At the stage of pitch fiber, cracks like the one shown in Figure 3 were observed, and also in the carbon fiber there were observed cracks like the one shown in Figure 2.

Comparative Example 3 DC0 pitch was heat-treated at 4000C for 7 hours while introducing nitrogen gas. The continuous phase of the resultant precursor pitch was optically isotropic at above 3700 C. But its reflectivity was 8.2%, and although a melt spinning was tried at 3800C, it was impossible to obtain pitch fiber having a uniform diameter.

Claims (7)

Claims

1. A process for producing carbon fiber by melt-spinning a carbonaceous precursor pitch, rendering the resultant pitch fiber infusible and subjecting the pitch fiber thus rendered infusible to a carbonization treatment and, if required, to a subsequent graphitization treatment to obtain carbon fiber, characterized in that the continuous phase of the precursor pitch is optically isotropic at the temperature of said melt spinning and in that the structure of a section perpendicular to the fiber axis of said pitch fiber is a random structure or an onion structure and the structure of a section perpendicular to the fiber axis of said infusiblized fiber and that of said carbon fiber are also a random structure or an onion structure.

2. A process for producing carbon fiber as claimed in claim 1 , wherein said carbonaceous precursor pitch subjected to said melt-spinning has a reflectivity of the optically isotropic portion in the range of 8.5% to 11.0%.

3. A process for producing carbon fiber as claimed in claim 1 , wherein said carbonaceous precursor pitch has a continuous phase which is optically isotropic at a temperature in the range of 3000 to 4000 C, and has a reflectivity of the optically isotropic portion in the range of 8.5% to 11.0%, and it is subjected to said melt spinning at said temperature.

4. A process as claimed in claim 1, substantially as hereinbefore described with particular reference to the Examples.

5. A process as claimed in claim 1 , substantially as illustrated in any one of the Examples.

6. Carbon fiber obtained by the process claimed in any one of the preceding claims.

7. Graphite fiber obtained by the process claimed in any one of claims 1 to 5.