Process for producing formed carbon products

Abstract: A process for producing a formed carbon product comprises: (i) mixing a pitch or a pitch precursor and at least one polymer which is capable of becoming compatible with the pitch or the pitch precursor by heat treating and which has an aromatic ring and at least one sulfur, oxygen or nitrogen atom in its repeating unit; (ii) heat treating the mixture to obtain a raw material pitch; and (iii) thereafter forming, infusibilizing and carbonizing the raw material pitch.

Description: The present invention relates to a process for producing a formed carbon product. In general, a formed carbon product is manufactured by forming a coal pitch, a petroleum pitch or a synthetic pitch and subsequently by infusibilizing and carbonizing the formed pitch product. In the manufacturing process, an infusibilizing step is so far the step requiring most time and has been a principal factor in lowering the production efficiency of formed carbon products. In order to reduce the infusibilizing time, a variety of catalysts and accelerators have been investigated. For instance, in a method where a certain kind of metal salt was added to a pitch as a catalyst, although there were some salts showing a recognizable effect in reducing the infusibilizing time, some of the physical properties of the carbon products obtained had deteriorated. In another method infusibilization was carried out in an atmosphere of a halogen molecule or nitrogen oxide, which was thought to be an accelerator. Although there was no deterioriation of the physical properties of the carbon products, there was no significant reduction in the time for infusibilization. Accordingly, the methods so far investigated were unsatisfactory and it has been unavoidable to build a large scale infusibilizing apparatus because of its present ineffectiveness. There are other infusibilizing methods, wherein pitch fibers are dusted with powdery solid lubricants such as molybdenum disulfide or talc and heat-treated thereafter. Such methods are disclosed in JP-A-61-160,422 (1986), 62-57,929 (1987) and 62-110-923 (1987). These methods are effective in preventing fibers adhering together but show almost no effect in reducing infusibilizing time. I. Mochida, H. Toshima, Y. Korai & T. Naito, J. Mat. Sci, 23 , pp 678-686 (1988) discloses the blending of polyphenyleneoxide in amounts of 5 or 10 wt% into petroleum-derived meso-phase pitch to reinforce the pitch fibre before the oxidative stabilization to achieve better handling properties. The present inventors have found that when a pitch or a pitch precursor (hereinafter referred to as "pitch et al.") is mixed with a polymer having no aromatic ring in its repeating unit, for example polyethylene; heated to react with the polymer, further heated under reduced pressure to remove volatile components (hereinafter the reaction heat-treatment and the heat-treatment to remove volatile components are referred to as the "heat-treatment") ; formed in an optional form; and infusibilized, there is no improvement in the infusibilizing time. On the other hand, the present inventors have further found that when a pitch et al. is mixed with a certain polymer having an aromatic ring in its repeating unit, for example polyphenylene sulfide; heat-treated; formed; and infusibilized, the infusibilizing time is reduced remarkably and the carbonized product not only maintains its initial shape but also has physical properties which are not inferior to those of a carbon product infusibilized for a long time and carbonized. An object of the present invention is to provide a process for manufacturing formed carbon products using simple apparatus having low energy consumption because of the reduced infusibilizing'time or low infusibilizing temperature. Another object of the present invention is to provide a process for the manufacture of formed carbon products which comprises mixing a pitch et al. with a polymer having an aromatic ring and at least one sulfur or nitrogen atom in its repeating unit and becoming compatible with the pitch et al. by heat-treatment; heat-treating; forming; infusibilizing; and carbonizing. Accordingly, the present invention provides a process for producing a formed carbon product, which process comprises (i) mixing a pitch or a pitch precursor and at least one polymer which is capable of becoming compatible with the pitch or the pitch precursor by heat treating and which has an aromatic ring and at least one sulfur atom in its repeating unit; (ii) heat treating the mixture to obtain a raw material pitch; and (iii) thereafter forming, infusibilizing and carbonizing the raw material pitch. Pitch precursor means a substance which becomes a pitch by heat-treatment, for example, coal tar, petroleum tar or synthetic tar. However, it is preferable to select an appropriate precursor according to the shape of the formed carbon product and its application. In addition, a raw material pitch of the present invention, which includes a pitch obtained by heat-treatment of said precursor with the polymer, can be an isotropic one or an anisotropic one obtained by hydrogenation or a separate heat-treatment from aforesaid heat-treatment with or without solvent-fractionation or air-blowing. To a pitch et al., a polymer, becoming compatible therewith by heat-treating and having an aromatic ring and at least one sulfur atom in its repeating unit, is mixed. In the present specification, "a polymer becoming compatible with a pitch et al. by heat-treating" means a polymer which is not compatible with the pitch et al. at a relatively low temperature, i.e. at a temperature lower than the temperature of the first step of heat treatment, but becomes compatible therewith at a higher temperature, i.e. at the temperature of the first step of heat treatment. In addition, "becoming compatible" herein means a state in which no separated layer can be recognized by eye-sight. So far, according to the common view in the art, use of a pitch having a small H/C value, namely, a pitch rich in aromaticity, was believed to result in a long time being required for infusibilization. Accordingly, it has been out of the question to add a substance having an aromatic ring to a pitch in order to reduce the infusibilizing time of the pitch. Contrary to the common view, the present inventors have tried to add a polymer having an aromatic ring to a pitch et al. and reached the unexpected result of reducing the time taken for infusibilization. As the polymer having an aromatic ring and at least one sulfur atom in its repeating unit, either a polymer having aromatic rings at its main chain as exemplified by polyarylene sulfide or a polymer having aromatic rings at its side chains can be used. Preferred is for example a polyarylene sulfide such as polyphenylene sulfide. Further, a single polymer or a mixture of two or more polymers can be used. The amount added can be optionally determined with the limit being the amount which is compatible with the pitch et al. by heat-treating. Usually an addition amount of from 0.5 to 50% by weight is preferable, from 1 to 10% by weight is more preferable and from 2 to 5% by weight is further preferable. The pitch et al. mixed with the polymer are then heat treated to be converted into a raw material pitch for forming. The temperature and period of time for the heat-treatment are adjusted mainly according to the types and properties of the pitch et al. In the heat-treatment, a first step, namely the heating reaction of the pitch et al. and the polymer, is performed at a temperature higher than the temperature which is necessary to proceed the reaction, however it is usually performed at a temperature of from 350 to 450°C and for from 0.5 to 10 hours in a flow of nitrogen and a second step for removing volatile components is performed until the pitch et al. becomes possible to be formed and infusibilized in the subsequent steps. Whether the reaction in the first step is completed or not can be judged from the aforementioned layer separation. As for the second heat-treating step, its conditions to remove volatile components of the pitch et al. are determined based on the necessity to make a formed product that has enough strength to handle, not to be deformed the product and not to adhere each other at an initial temperature of infusibilization, however, it is usually performed at a temperature of from 280 to 330°C and for from 0.5 to 3 hours. The raw material pitch is formed, infusibilized, carbonized and graphitized, if necessary, according to conventional and publicly known methods. In the following Examples, these steps will be explained with respect to spinning the pitch. Needless to say, the Examples are also applicable to the cases where the raw material pitch is formed into any desired form, such as a film of sphere. The object of the infusibilizing step is to oxidize a pitch fiber to convert it into an infusibilized fiber having no thermoplasticity and to prevent the fiber from melt-deforming during carbonization step. In the infusibilizing step, although the highest temperature for infusibilization is dependent on the properties of the formed pitch, usually a formed pitch is heated in an atmosphere of an oxidizing gas at a rate of temperature rise of from 0.5 to 2.0°C/minute up to a temperature of from 200 to 350°C and kept for 0 to 60 minutes at that temperature. In the present invention, the formed pitch contains a polymer having aromatic rings. Therefore, it is possible to reduce the total infusibilizing time by an increased rate of temperature raise. For example, the infusibilizing time according to the present invention can be reduced to not more than 70% of that of formed pitch containing no polymer having aromatic rings. As far as conventional pitch fibers are concerned, if their infusibilizing times are reduced, the fibers are welded together during the carbonizing step. Therefore, they are prone to stick together and to be deformed in their shapes. The infusibilized fibers produced in accordance with the present invention can be then carbonized by heating in an inert gas, e.g., nitrogen gas, to 900°C at a temperature raising rate of 5 to 50°C/minute to obtain carbon fibers. Further, graphite fibers can be obtained from said carbon fibers by carbonizing at the temperature not lower than 2,000°C in an inert gas. According to the present invention, comparing with a pitch containing no polymer having aromatic rings, it is possible to increase the temperature raising rate in the infusibilizing step while maintaining the original shape of the formed pitch and to reduce remarkably the infusibilizing time. On the other hand, when the temperature raising rate is kept at the same level as in the conventional step, it is possible for the raw material pitch used in the present invention to have the infusibilizing temperature lowered. Accordingly, the present invention permits compact infusibilizing apparatus as well as reduced energy consumption during production. In the following Examples, shape retention of the fibers is evaluated by comparing the roundness of a cross section of the pitch fiber with that of the resulting carbon fiber and the same evaluation method can be applied to other shapes. For examples, the shape retention is evaluated by comparing the roundness of pitch and carbon spheres for spherical carbons and for carbon films by comparing the surface flatness of the films. The present invention is now further described in the following Examples. EXAMPLE 1: 5 parts by weight of polyphenylene sulfide was added to 100 parts by weight of ethylene tar and then, after heat-treating the mixture at 400°C for one hour in a flow of nitrogen gas, the resulting mixture was treated under a reduced pressure of 10 torr (1.3 kPa) at 300°C for one hour to obtain a pitch having a softening point of 180°C and showing no separation of any layer. The pitch was spun through a single-pore nozzle with a diameter of 0.3 mm to obtain a pitch fiber having a diameter of 15 µm. The fiber was infusibilized by heating starting from 100°C to the highest temperature of 250°C at a temperature raising rate of 6°C/minute in an atmosphere containing 1% by volume of nitrogen dioxide. The infusibilizing time was 25 minutes. Then the infusibilized fiber was heated to 900°C for carbonization and a carbon fiber was obtained. The cross section of the obtained carbon fiber was almost round and retained the shape of the pitch fiber originally formed. The tensile strength and tensile elongation of the carbon fiber were 95 kg/mm 2 and 2.8%, respectively. In comparison with the results of COMPARATIVE EXAMPLE 1, the infusibilizing time was reduced about 75% and the properties of the carbon fiber were almost the same as in the COMPARATIVE EXAMPLE 1. EXAMPLE 2: Ethylene tar was air blown at 180°C and consumed about 15 l oxygen/kg tar. To 100 parts by weight of air blown ethylene tar, 5 parts by weight of polyphenylene sulfide was added and then, after heat-treating the mixture at 400°C for one hour in a flow of nitrogen, the product was treated under reduced pressure of 10 torr (1.3 kPa) at 300°C to obtain a pitch having a softening point of 180°C. Except for the above conditions, the same operations as in EXAMPLE 1 were performed and the infusibilizing time required for the pitch fiber was 25 minutes and the tensile strength and tensile elongation of the obtained carbon fiber were 92 kg/mm 2 and 2.7%, respectively. EXAMPLE 3: Using the same pitch fiber obtained in EXAMPLE 1, except that infusibilization was performed by heating the fiber starting from 100°C to the final temperature of 180°C at a temperature raising rate of 1.5°C/minute, the pitch fiber was infusibilized and carbonized as in EXAMPLE 1. Infusibilizing time was about 36 minutes. The tensile strength and the tensile elongation of the carbon fiber were 98 kg/mm 2 and 2.9%, respectively. In comparison with COMPARATIVE EXAMPLE 1, when the temperature raising rate in both infusibilizing processes are the same, the maximum temperature in the infusibilization of this Example can be lowered and carbon fibers having similar properties can be obtained. COMPARATIVE EXAMPLE 1: A pitch having a softening point of 180°C was obtained from an ethylene tar. Except for adding no polymer having aromatic rings, such as polyphenylene sulfide, the tar was spun, infusibilized and carbonized in a similar manner, namely, an infusibilizing time of 25 minutes, as in EXAMPLE 1. Since the infusibilization was insufficient, the fibers melted together and could not be isolated to a single fiber. The conditions for infusibilizing the pitch fiber to obtain carbon fibers maintaining the cross sectional shape of pitch fibers originally formed were a temperature raising rate of 1.5°C/minute and the time for infusibilization of 100 minutes. The tensile strength and the tensile elongation were 95 kg/mm 2 and 2.9%, respectively. COMPARATIVE EXAMPLE 2: 5 parts by weight of polyethylene was added to 100 parts by weight of ethylene tar and after heat-treating the mixture at 400°C for one hour in a flow of nitrogen gas, the mixture was treated under a reduced pressure to obtain a pitch having a softening point of 181°C. The pitch was spun in a similar manner as in EXAMPLE 1 to obtain a pitch fiber having a diameter of 15µm. The pitch fiber was infusibilized and carbonized in a similar manner as in EXAMPLE 1. However, the fibers melted together and could not be isolated in a single fiber. The conditions for infusibilizing the pitch fiber to obtain a carbon fiber maintaining the cross sectional shape of the pitch fiber originally formed were a temperature raising rate of 1.5°C/minute and the time for infusibilization of 100 minutes. The tensile strength and the tensile elongation of the obtained carbon fiber were 87 kg/mm 2 and 2.6%, respectively.

Claims: A process for producing a formed carbon product, which process comprises (i) mixing a pitch or a pitch precursor and at least one polymer which is capable of becoming compatible with the pitch or the pitch precursor by heat treating and which has an aromatic ring and at least one sulfur atom in its repeating unit; (ii) heat treating the mixture to obtain a raw material pitch; and (iii) thereafter forming, infusibilizing and carbonizing the raw material pitch. A process according to claim 1, wherein the amount of said polymer mixed in step (i) is from 0.5 to 50% by weight of the resulting mixture. A process according to claim 1 or 2, wherein said polymer is a polyarylene sulfide. A process according to claim 3, wherein said polyarylene sulfide is polyphenylene sulfide. A process according to any one of the preceding claims, wherein the amount of said polymer mixed in step (i) is from 2 to 5% by weight of the resulting mixture. A process according to any one of the preceding claims, wherein the raw material pitch is formed into fibers, a film or spheres.

Inventors: MURAKAMI YOSINOBU, SEO IKUO, MURAKAMI, YOSINOBU, SEO, IKUO
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: D01F9
IPC codes: D01F9, C01B31, C04B35