PITCH-BASED CARBON OR GRAPHITE FIBRE

Abstract: A carbonaceous pitch can be produced by thermal cracking and polycondensation to have a softening point of 230-320°C; preferably it is fractionated to give an optically anisotropic pitch, which is spun from nozzles in an inert gas atmosphere to form fibres which are taken up on a bobbin. A spinning treatment oil is applied, the fiber bundles are doubled, e.g. by rewinding, a non-aqueous heat-resistant treatment oil is applied (e.g. an alkyl phenyl polysiloxane of viscosity 10-­ 1000 cst or dimethyl polysiloxane of viscosity 5-1000 cst, containing an antioxidant) in amount of 0.01-­ 10 wt%, the bundles are passed continuously into an oxygen-rich gas of 30% or more O₂ content in a furnace at not above 350° (300-330°C); the atmosphere being preferably continuously stirred and exchanged, so as to infusibilize the fibres and finally the fibres are heated in inert gas to 500-1000°C to precarbonise and then to 1000-2000°C to form carbon fibres or 2000-­ 3000° to form graphite fibres. The carbon or graphite fibres formed have a good appearance, high elasticity and strength and little fluff, and the process is rapid and continuous.

Description: The present invention relates to a process for producing a carbon fibre or graphite fiber from a carbonaceous pitch fiber, in particular, it relates to a process for obtaining a long filament of carbon fiber and graphite fiber, by spinning an optically anisotropic carbonaceous pitch, infusibilizing the pitch fibers, then carbonizing and graphitizing them. Hitherto, it has been much desired to develop a high performance light material having high strength and high elasticity for use in various industries such as the manufacture of automobiles or aircraft. Thus, carbonaceous fibers or molded carbonaceous materials have been used as a composite material. In particular, a process for producing carbon fiber from carbonaceous pitch has been regarded as important in order to obtain a high performance carbon fiber at a low cost. It is however, quite difficult to obtain by conventional technology a long filament carbon fiber needed for high performance in the products, due to the fragility of the pitch precursor because of its small tensile strength which may be as low as 0.01 GPa. A process disclosed in Japanese Patent Publication No. 12740/76 produces a long filament carbon fiber from a pitch fiber, by dropping and accumulating spun yarn into a wire net basket as it is, furthermore conducting a primary heat treatment at a temperature of 700°C or more, thus making the tensile strength of the yarn to be 0.2 GPa or more, in addition, pulling the yarn upwards from said bucket to wind it out or while winding it, carbonizing it at a temperature of about 1,500°C, thus obtaining a carbon fiber. This process, however, tends to generate kinks or twists, and is liable to cause curvature of the yarn when the yarn is accumulated, which causes much irregularity of the yarn surface of the final product carbon fiber, and causes the yarn to be of a poor appearance, in addition, it results in greatly reducing the strength of the area of curvature, and causes the yarn to break frequently; thus it becomes difficult to obtain high quality products. Such disadvantage has been impossible to substantially overcome. The production of carbon and graphite fibres from pitch is described in "Carbon & Graphite Fibres, Manufacture and Applications", ed by M. Sittig, U.S.A., 1980. The treatment of the spun pitch fibres to make them infusible and convert them to carbon or graphite fibres is described at pages 144-147, e.g. by treatment in oxygen-rich air at a temperature (e.g. up to 300°C) below the spinning temperature (this infusibilization can also be referred to as thermosetting). U.S. Patent 4,138,525 discloses a process of producing a carbon yarn by carbonizing after the treatment wherein by melt spinning of mesophase pitch, and once winding the spun yarn on a wire net dish, and oxidizing it under an oxidizing atmosphere at a temperature of 250 to 500°C so as to increase the strength of the yarn, thus making the yarn easy to be processed. However, in this method oxidization is conducted at a temperature of 400 to 500°C, that is at an excessively high temperature, which results in reducing the yarn strength of the carbon fiber a the final products; furthermore, first the yarn is once wound up, next a part of the yarn is oxidized while pulling it upwards, which reduces the efficiency of production. A process disclosed in published Japanese Patent Applications Nos. 81320/85 and 21911/85 conducts a primary heat treatment (preliminary carbonizing) under a non-oxidizing atmosphere at a given temperature or less, after infusibilizing a wound bobbin as it is. However, this causes insufficient gas permeability during infusibilization or preliminary carbonization when the winding thickness of the pitch fiber on the bobbin becomes thicker, which results in fusion and sticking among the filaments, making it difficult to rewind the wound yarn on the bobbin after the primary carbonization, and making it liable to make fluff of carbon around the yarn on rewinding, thus remarkably reducing the commercial value of thus obtained carbon fiber or graphite fiber. Furthermore, insufficiency of gas permeability would increase the irregularity of the degree of infusibilization, and remarkably enhances the inhomogeneity of the strength of the final products of carbon fiber or graphite fiber. These disadvantages were largely improved by a method of utilizing a gas-permeable bobbin as disclosed in Japanese Laid-open Application No. 173121/85; the efficiency of manufacturing in this method, however, is still not satisfactory. A process disclosed in Japanese Laid-open Patent Application No. 128020/80 obtains carbon fiber by melt spinning the yarn, drawing it with a godet roller, and passing it continuously through an air-heating furnace for infusibilization at a yarn rate of 0.15 m/min, subsequently passing it continuously also through a carbonizing furnace. Although this method can infusibilize the yarn homogeneously so as to reduce the irregularity of its properties, thus obtaining a good appearance of carbon fibers, it has a disadvantage in making the operation difficult to continue due to breakage of the fiber bundle during its infusibilization, since the spinning treatment oil (finish) which is added to the fiber bundle is decomposed while the infusibilization temperature increases. EP-A-175200 discloses a process to produce a carbon pitch fiber by melt spinning pitch material in a gaseous atmosphere to form precursory pitch fibers, bundling the precursory pitch fibers after or while an aqueous emulsion of a silicone oil is applied thereon as an oiling agent, and then subjecting the fibers to infusiblilization treatment and carbonisation treatment in accordance with conventional methods. Although an aqueous emulsion of a silicone oil has high facility in handling, can make it easy to handle brittle fibers and can prevent the adhesion or fusion of monofilaments due to an infusible treatment under mild conditions, it has a disadvantage in making it impossible to continue an infusiblization operation under severe conditions, adopted with the intention of providing a great reduction in-operation time, since it contains as an essential component a surfactant inferior in heat-resistance and, therefore, readily decomposes under a severe condition, such as a high-temperature oxygen-rich condition, to cause a sharp increase of viscosity or to solidify, resulting in breakage of the fiber bundles. Methods are also known such as disclosed in Japanese Patent Publication No. 42696/73, wherein air containing 0.1 to 10% NO₂ is used as the atmospheric gas at infusibilization, and as disclosed in Japanese Laid-open Patent Application 75828/74 wherein a mixed gas of chlorine and oxygen is used to enhance the infusibilization speed. Although these methods have an advantage in increasing the infusibilization speed, they have disadvantages not only in generation of breakage of fiber bundles at infusibilization by passing them through linearly and continuously as thread line, but also in that explosion or burning is liable to occur since the reaction rushes in a treatment under a high temperature, in addition, the treatment with strong oxidizing gas makes the apparatus liable to corrode, which results in a short life of the apparatus. As stated above, it has been desired to find a method which is free from breakage of fiber bundles due to disturbance of sizing performance of doubling treatment oil, which provides a rapid infusibilization so as to increase the amount of production per hour, and which can obtain a high quality of the final product pitch based carbon fiber in long filaments without inhomogeneity of their strength, in high strength, in high elasticity, in a good appearance, and with a very small amount of fluff in treatment. Accordingly, the first object of the present invention is to provide a method for producing, from pitch, carbon fiber or graphite fiber with long filaments in a good appearance, in high strength, in high elasticity and in high quality. The second object of the present invention is to provide a method for rapid infusibilization suited for a linearly and continuously integrated process as thread line between infusibilization and heat treatment for producing carbon fiber or graphite fiber. The aforementioned objects of the present invention have been attained by a method for producing carbon fiber and graphite fiber comprising infusibilizing the pitch fiber obtained by spinning carbonaceous pitch and carbonizing or graphitizing said infusibilized fibers, characterised by (a) doubling said spun pitch fiber bundles into strands so that the number of filaments constituting one strand is from 100 to 100,000, (b) during or after said doubling processing adding a heat-resistant non-aqueous doubling treatment oil having a viscosity less than 100.10⁻⁶ m² / s (100 cst) at 25°C (measured after a heat treatment of the oil in an air atmosphere while raising the temperature from 100°C to 330°C at a rate of 0.5°C/min), and then (c) infusibilizing said doubled fiber bundles by passing them continuously and linearly as a thread line through an oxygen-rich gas of 30% to 80% oxygen content while applying 9.81x10⁻⁶N to 1,962x10⁻⁶N (0.001 to 0.2 g) tension per filament and heating the fiber bundles at a temperature of 300 to 350°C. This method reduces the time of infusibilization with an increase in the speed of infusibilization reaction as a result of the high partial oxygen pressure, i.e. by the use of oxygen-rich gas. In addition, this method reduces the time of infusibilization with a selective increase in the speed of infusibilization of fiber surface as a result of high oxygen concentration, since the distribution of oxygen concentration towards the direction of the fiber radius in infusibilization varies depending on the oxygen concentration of the infusibilization atmosphere. According to the two effects stated above, this method can rapidly infusibilize the pitch fibers, at a high temperature and in a short time, while preventing fusion of the fibers. The invention will be described in detail under the following headings: a) Carbonaceous pitch Carbonaceous pitch used in this invention is not limited specifically, but covers various pitches such as coal tar pitch obtained by carbonization (dry distillation) of coal, coal pitch such as liquefied coal substance, tar pitch from naphtha cracking, tar pitch from catalyic cracking, petroleum pitch from atmospheric distillation residue and vacuum distillation residue, and synthesized pitch obtained by decomposition of a synthetic resin, a hydrogenized product of an aforementioned pitch obtained by hydrogen or a hydrogen donor, and a reformed product of said pitch obtained by heat treatment or solvent extraction. The carbonaceuous pitch used in the invention, can be optically isotropic pitch or anisotropic pitch, also the pitch so called neomesophase or premesophase can be used; the softening point, however, is preferable about 230 to 320°C. In particular, an optically anisotropic pitch described below is preferable. b-1) Optically anisotropic pitch The term "optically anisotropic carbonaceous pitch" used in the present invention means a pitch, the most part of which is substantially optically anisotropic, that is, its brightness is recognized by rotating a Nicol prism with a reflection polarized microscope after polishing a cross section of the solidified pitch block at an atmospheric temperature, and an optically isotropic pitch in which no brightness is recognized is called "optically isotropic carbonaceous pitch". Therefore, the term "optically anisotropic carbonaceous pitch" is used to mean not only pure optically anisotropic carbonaceous pitch, but also a dispersion in which an optically isotropic phase is contained in a spherical shape or undefined shape of islands dispersed in the optically anisotropic phase. In addition, a "substantially optically anisotropic" phase is a mixture of optically anisotropic carbonaceous pitch and optically isotropic carbonaceous pitch, however, the amount of optically isotropic pitch is so small that no optically isotropic pitch phase (referred to as IP hereinafter) can be observed with said reflection polarized microscope, and only the optically anisotropic phase (referred to as AP hereinafter) can be observed. In this connection, in general a clear boundary is observed between AP and IP. The term AP in this specification may be considered to be the same as a so called "mesophase"; however "mesophase" includes two kinds, one containing mainly components substantially insoluble in quinoline or pyridine and the other containing mainly components soluble in quinoline or pyridine, the AP referred to in the present invention mean the latter "mesophase". The aforementioned AP phase and IP phase are largely distinguished not only by optical properties but also viscosity, accordingly, in general, it is not desirable to spin a pitch which includes both the phases together because it will cause breakage of fiber or inhomogeneity of the size of the fiber. Therefore even if the optically isotropic phase pitch includes no foreign matter undesirable for spinning, when the IP phase is not dispersed homogeneously in the AP phase, the pitch does not give any satisfactory results at all. From this point of view, the optically anisotropic pitch used in the present invention is required to be substantially homogeneous. Such homogeneous optically anisotropic pitch means that its IP content is 20% or less, and no solid particles of 1 µm or more in diameter is detected on its cross - section with a reflection microscope, in addition, in its melt spinning temperature substantially no foaming occurs caused by any volatile matter. Quantitative determination of AP or IP is performed by measuring an area ratio of the AP or IP portion by observation and photographing of AP or IP under a Nicol prism using a polarized microscope. The area ratio statistically represents substantially a volume %. However, a difference in specific gravity between AP and IP is as small as about 0.05. Therefore, the volume % is considered to be almost the same as the weight %. It is preferred for the softening point of the optically anisotropic pitch used to be low; the term "softening point of a pitch" as used herein refers to a solid-liquid transition temperature of a pitch. The softening point is determined by measuring the peak temperature of absorption or emission of latent heat at which the pitch melts or solidifies, using a differential scanning calorimeter. The softening point measured by this method is identical with a temperature measured by other methods, such as a ring-and-bell method, or a melting point method, with an error of ± 10°C. Visual spinning technology can be used in the spinning in this invention. The spinning temperature suited for melt spinning is, in general, one 60 to 100°C higher than the softening point of the material to be spun. On the other hand, the optically anisotropic pitch used in the present invention may suffer thermal cracking and polycondensation at a temperature of 380°C or higher, which results in generation of decomposed gas or in formation of non-melted matter. The optically anisotropic pitch used therefore preferably has a softening point not above 320°C, and usually of at least 230°C for satisfactory infusibilization as described later. b-2) Process for producing optically anisotropic pitch The optically anisotropic pitch used in this invention may be produced, e.g. by a conventional method of stirring heavy hydrocarbon oil, tar, or commercial pitch which is generally used for producing pitch in a reactor at a temperature range of 380 to 500°C, performing thermal cracking and polycondensation to sufficient extent while removing volatiles with an inert gas, thus enhancing the AP of the residue pitch. However, when an optically anisotropic pitch containing - 80% or more AP (by measurement of a polarized microscope) is produced, the reaction of thermal decomposition polycondensation proceeds too far, and in some case quinoline-insoluble content will be as large as 70 weight % or more, and softening point will be 330°C or more; in addition, the IP is difficult to be in a state of fine dispersion, thus said method is not necessarily favourable. We thus prefer our process for producing an optically anisotropic pitch having a large AP content, which comprises discontinuing the thermal cracking and polycondensation half way, settling the thermally cracked, polycondensed product while maintaining at a temperature in the range of from 350 to 400°C, precipitating AP having a large density in the lower layer while growing and ripening AP, separating the AP precipitate from IP having a small density in the upper layer and withdrawing the AP precipitate see Japanese Laid-open Patent Application No. 119984/82. A more preferable method for producing the optically anisotropic pitch used, as described in Japanese Laid-open Patent No. 180585/83, comprises a process for producing an optically anisotropic carbonaceous pitch having a low softening point and a high AP content which comprises subjecting a molten carbonaceous pitch containing AP to an appropriate degree but not rendered excessively heavy, to centrifugal separation with an acceleration of centrifugal force, whereby the AP portion rapidly precipitates. The AP phase is collected in the lower layer (a layer toward the centrifugal force) while being combined and grown, and the AP amounts to be a continuous layer in about 80% or more, in which a pitch containing a small amount of IP in an island-shape or in a fine spherical shape forms the lower layer whereas the upper layer is formed by a pitch composed mainly of IP in a shape wherein AP is dispersed as fine spherical droplets. In this case, the boundary between both the layers is so distinct that only the lower layer can be separated from the upper layer, accordingly, an easily spinnable optically anisotropic pitch provided with a large content of AP is possible to be produced. According to this method, a carbonaceous pitch having an AP content of 95% or more and having a softening point ranging from 230 to 320°C can be obtained economically in a short period of time. Such an optically anisotropic carbonaceous pitch is excellent in spinning characteristic in melt spinning. Due to its homogeneous property and high orientation of molecules, carbon fibers and graphite fibers prepared therefrom are excellent, particularly in tensile strength and elastic modulus. c) Process for producing fibers i) spinning A pitch with a high AP content and a low softening point as described above can be spun in accordance with any known method. For instance, a pitch is charged into a metal-made spinning vessel equipped with 1 to 1,000 spinning nozzles having a diameter of 0.1 to 0.5 mm at the bottom. The pitch is maintained at a definite temperature between 280 and 380°C in an inert gas. When the pressure of the inert gas is increased to several hundred mm Hg while keeping the pitch in a melted state, the pitch melt is extruded from the nozzle and flows down. While controlling the temperature and atmosphere at the extruded portion, the pitch fibers produced are taken up around a bobbin rotating at a high speed. Furthermore, a method can be applied wherein pitch fibers spun from a spinneret are collected into a collecting box at the bottom while focusing and taking over by a gaseous flow. In this case, it is possible to continuously spin, when the pitch is fed into the spinning vessel in a previously melted state under pressure using a gear pump. Further in the above method, it is also possible to withdraw the pitch fibers around the spinneret while stretching the fibers with a gas flowing down at a high speed at a controlled temperature and to prepare long fibers, on a melt conveyor located below. A spinning method also can be applied which comprises rotating a cylindrical spinning vessel having a spinning nozzle around the wall at a high speed, continuously feeding a melt pitch thereto, and collecting pitch fibers extruded from the wall of the cylindrical spinning vessel by centrifugal force and stretched by the action of the rotation. In the present invention, melt spun pitch fibers are preferably introduced into an oiling roller while focusing by passing through an air sucker, and further sized by adding a spinning treatment oil (finish). Examples of a spinning treatment oil are water or alcohol such as ethyl alcohol, isopropyl alcohol, n-propyl alcohol or butyl alcohol, or polysiloxane such as dimethyl polysiloxane, alkyl phenyl polysiloxane alkylchloro polysiloxane or phenyl chloro polysiloxane, of 3.10⁻⁶ m² / s to 300.10⁻⁶ m² / s (3 to 300 cst) viscosity (at 25°C), diluted by a solvent such as a silicone oil (polysiloxane) or paraffin oil with a low boiling point, or dispersed into water by adding an emulsifier, or in the same way, graphite or polyethylene glycol and hindered esters dispersed in water, surface active agent diluted by water, and various spinning treatment oils not damaging pitch fiber used for other textile fibers such as polyester fiber. In addition, the same treatment oil as the heat-resistant one added after doubling, which will be described later, can be added as a spinning treatment oil. In general, 0.01 to 10 wt% of spinning treatment oil is added to the fibers; in particular, 0.05 to 5 wt% is preferable. When a pitch fiber bundle is wound up around a bobbin, a traverse as large as 2 to 100 mm/(one revolution of a bobbin) at winding, and 1 to 100 mm thickness of winding, preferably 5 to 50 mm, is efficient, in order to give stable and continuous rewinding for a long time from a state of winding. From a viewpoint of rewinding efficiency of a pitch fiber bundle from a bobbin, the pitch of traverse is preferably 5 to 20 mm/per revolution of a bobbin. The term "fiber bundle" used herein is synonymous with multifilament bundle, yarn, tow or strand. ii) Doubling of pitch fiber bundles In the present invention, at infusibilization, pitch fiber bundles are doubled prior to the infusibilization, for the purpose of strengthening the fiber bundles, and passing said bundles continuously and stably into the infusibilizing furnace. The number of pitch fiber filaments of one fiber bundle spun from a melt spinning machine (a spinneret) is restricted because of the melt spinning, and it is in general 1 to 2,000, in particular, 50 to 1,000 filaments. We use 2 to 50 pitch fiber bundles obtained by melt spinning, and double them into 100 to 100,000, particular, 500 to 50,000 filaments. When the number of filaments is less than 100, the strength of the fiber bundles is small, and the fiber bundles are liable to be broken at passing through the high temperature infusibilizing furnace, and the production efficiency is found to be poor. When the number of filaments exceeds 100,000, heat is accumulated in the fiber bundles at infusibilization, and the fibers are liable to be melted and broken. Doubling can be performed by rewinding spun pitch fiber bundles on a plurality of bobbins at a time, doubling them into a fiber bundle, then winding said fiber bundle up around a bobbin. The pitch of traverse at doubling is preferable to be 5 to 100 mm per revolution of a bobbin. A larger pitch of traverse is preferable to improve the smoothness of rewinding from a bobbin; an excessively large pitch of traverse, however, is not preferable since it will be liable to damage the fibers. Doubling maybe performed by taking up pitch fiber bundles from a plurality of collecting baskets or cases. Doubling may be performed not only by rewinding from a plurality of bobbins, but also by focusing pitch fiber bundles spun from a plurality of spinning machines or spinnerets at a time. Doubling of 2 to 50 pitch fiber bundles at one time maybe performed; another method, however, can be adopted wherein first 2 to 10 pitch fiber bundles are doubled, and 2 to 10 of them are redoubled in a second step. For the purpose of improving the doubling efficiency and focusing properties of fiber bundles during infusibilization, twisting can be applied to the filaments 0.1 to 30 times/m, in particular, 1 to 5 times/m, during the doubling stage, if required. The thickness of winding after doubling can be set as desired; from a viewpoint of working and operation, however, 10 to 100 mm is preferable. In the doubling, the heat-resistance doubling treatment oil (finish) of non-aqueous type is applied, in order to improve the sizing properties of fiber bundles, at infusibilization, to pass the bundles stably into an infusibilizing furnace under a high temperature of 300 to 350°C. As the result of the good sizing effect, the pitch fiber bundles and infusibilized bundles are soft, pliable and well lubricated. The heat-resistant treatment oil is one of which the residue has a viscosity of 1000 .10⁻⁶ m² / s (1,000 cst) or less at a temperature of 25°C when 0.5 g of it is taken into a 50 ml beaker and heated at a rate of increase of 0.5°C/min from 100 to 330°C under an atmosphere of air. The viscosity can be measured using a rotating viscometer (Contrabus Rheomat 30) or a capillary viscometer. A suitable heat-resistant oil is an alkyl phenyl polysiloxane containing 5 to 80 mol%, in particular, 10 to 50 mol% phenyl group and having a viscosity of 10.10⁻⁶ m² / s to 1000.10⁻⁶ m² / s (10 to 1,000 cst) at a temperature of 25°C. The alkyl group is preferably methyl, ethyl or propyl; two or more kinds of alkyl groups may be contained in a molecule. Another suitable treatment oil is a dimethyl polysiloxane preferably of viscosity 5.10⁻⁶ m² / s to 1000.10⁻⁶ m² / s (5 to 1,000 cst) at 25°C, to further enhance the heat-resistability. These oils should have an added antioxidant such as amines, organic selenium compounds, phenols, such as phenyl-a-naphthylamine, dilauryl selenide, phenothiazine and iron octalate. While infusibilizing the fiber bundles under a high temperature of 300 to 350°C, these treatment oils show a remarkably small amount of cracking and degradation, and the fiber bundles are free from breakage of fiber bundles during infusibilization; in addition, there is a small amount of fluff of the filament, and the bundles can be passed into the infusibilizing furnace continuously and linearly. The treatment oil can be applied by any method such as roller contact, spray application, foam application or dipping in a treating oil bath. The amount of application of these treatment oils to a fiber is 0.01 to 10 wt%, preferably 0.05 to 5 wt%. iii) Infusibilization of pitch fibers In the present invention, fiber bundles are doubled, in order to enhance their strength, and heat-resistant doubling treatment oil is applied aiming at improving sizing properties of fiber bundles during infusibilization which is performed by passing the fiber bundles linearly and continuously through the oxygen rich gas atmosphere at a temperature of 350°C or less, preferably 300 to 330°C, by passing the fiber bundles into an infusibilization furnace, and rewinding the doubled fiber bundles on bobbins; or by passing the fiber bundles into the infusibilization furnace and doubling the pitch fiber bundles. This method reduces the time of the infusibilization with an increase in the speed of infusibilization reaction as a result of the high temperature and high partial oxygen pressure, since the fiber is infusibilized at a high temperature by the use of oxygen-rich gas. In addition, this method reduces the time of infusibilization with a selective increase in the speed of infusibilization of fiber surface as a result of high oxygen concentration, since the distribution of oxygen concentration towards the direction of the fiber radius in infusibilization varies depending on the oxygen concentration of the infusibilization atmosphere. Due to these two effects, this method can rapidly infusibilize the pitch fibers, at a high temperature and in a short time while preventing fusion of the fibers. The oxygen-rich gas used herein means an oxygen gas or a mixed gas of 30% or more oxygen content consisting of oxygen gas and an inert gas (e.g. a rare gas, nitrogen or carbon dioxide). Examples of the mixed gas are oxygen and air or oxygen and nitrogen. To avoid damage to the furnace seals, the concentration of oxygen in the oxygen-rich gas is 30 to 80%. If the concentration of oxygen exceeds 80%, the infusibilization reaction becomes rushed by heat accumulation in fiber bundles, which is undesirable because breakage of fiber bundles, burning of fiber or explosion of the furnace is liable to occur. Less than 30% is also undesirable since the delaying of the reaction has no satisfactory effect on the result. At infusibilization, it is preferable to exchange a fresh gas of the same kind as the atmosphere into the furnace, at a rate of 0.1 to 5 times a minute and to exhaust the old atmosphere, so as to replace with each other. Part of the exhaust gas can be recycled, or reused after refining it. The atmosphere at infusibilization is preferably stirred continuously using a fan, at a wind speed of 0.1 to 10 m/sec, in particular, 0.5 to 5 m/sec. Such stirring promotes the permeability of gas into the fiber bundles and filaments, eliminates inhomogeneity of temperature in the infusibilization furnace and results in homogeneous infusibilization. Infusibilization can be effected while applying 9.81x10⁻⁶N to 1,962x10⁻⁶N (0.001 to 0.2 g) tension per filament, in general to prevent the occurrence of flaws made by rubbing the furnace bottom and wall due to the sagging of fiber bundles in the infusibilization furnace, and to improve the carbon fiber properties such as tensile strength and tensile elasticity modulus. Infusibilization in the present invention as described in the above can reduce the time required for it by 1/2 to 1/5 of the time required for the conventional infusibilization conducted under an air atmosphere, which will make the time as long as the time in the subsequent heat treatment processing, and will enable the infusibilization processing to be conducted linearly and continuously as a thread line with the heat treatment processing. iv) Heat treatment processing Next, the infusibilized carbonaceous pitch fiber is put into an inert atmosphere of argon or nitrogen gas, and heated to a temperature of 500 to 1,000°C, then precarbonation is performed to obtain a precarbonization carbon fiber. Subsequently the precarbonization carbon fiber is carbonized by increasing the temperature in a range of 1,000 to 2,000°C, to obtain carbon fiber, and by increasing the temperature up to 2,000 to 3,000°C, to obtain so called graphite fiber. Other details of the carbonizing and graphitizing are as in the published methods. Effect of the Invention The present invention doubles the carbonaceous pitch fiber bundles so as to increase the strength of fiber bundles, and infusibilizes the fiber bundles linearly and continuously as a thread line treatment after applying heat-resistant doubling treatment oil, therefore breakage of fiber bundles cannot be seen during infusibilization, and the strength of the fiber bundles is increased, which results in enhancing the speed of the production. In addition, infusibilization is performed under the oxygen-rich atmosphere, and at a temperature of infusibilization increased up to 350°C, accordingly the time of infusibilization can be reduced. Such a processing can reduce the difference of the times between the infusibilization and the subsequent heat treatment, as a result the size of the infusibilization furnace can be shortened in length, and infusibilization can be made economically without affecting the handling properties of the pitch fiber bundles and infusibilized pitch fiber bundles, and infusibilization and heat treatment processing can be performed linearly and continuously as a thread line treatment. The fiber bundles are passed into the infusibilization furnace linearly and continuously, therefore not only good appearance of fibers can be obtained but also fibers free from irregularity of infusibilization and from inhomogeneity of the strength of the final products of carbon fiber or graphite fiber can be obtained. In particular, when an optically anisotropic carbonaceous pitch is used, carbon fibers or graphite fibers with high strength and high elasticity modulus can be obtained. The present invention is explained in more detail by the following examples. Example 1 A carbonaceous pitch containing about 55% optically anisotropic phase (AP) and having a softening point of 232°C was used as a precursory pitch. This precursory pitch contained 16.1 wt% quinoline-insoluble component and 0.26 wt% ash, and exhibited a viscosity of 2.8 poises at 370°C. The pitch was melted in a melting tank having a capacity of 20 l, the temperature was controlled to be 370°C, and the pitch was fed at a flow rate of 20 ml per minute to a cylindrical continuous centrifuge having an effective rotor capacity of 200 ml. While the rotor temperature was being controlled at 370°C, a centrifugal force of 30,000 G was applied. A pitch having a large proportion of the optically anisotropic phase (pitch A) was continuously taken out through an AP outlet, and a pitch having a large proportion of the optically isotropic phase (pitch I) through an IP outlet. The obtained optically anisotropic pitch (pitch A) contained 98% of optically anisotropic phase. The softening point of the pitch was 265°C and the amount of the quinoline-insoluble component contained in the pitch was 29.5%. The obtained optically anisotropic pitch was introduced into a melt spinning machine having a spinneret of 500 holes (diameter of nozzles: 0.3 mm), then the spinning was carried out at 355°C under a nitrogen pressure of 26700 Pa (200 mm Hg). The obtained pitch fiber was wound on a stainless steel mesh bobbin of 200 mm width and 210 mm diameter for 10 minutes with a winding speed of 500 m/min. A pitch of traverse per one revolution of bobbin was 10 mm. No breakage of fibers was observed during the spinning. During the spinning, filaments were gathered into one fiber bundle by an air sucker and introduced to an oiling roller to supply a spinning treatment oil in an amount of about 0.5 wt% to the fiber. The oil used was a methylphenylpolysiloxane having a viscosity of 14.10⁻⁶ m² / s (14 cst) at 25°C. Pitch fibers were unwound from six bobbins, then wound into one fiber bundle of 3,000 filaments on a stainless steel bobbin under a condition of traverse pitch of 20 mm per one revolution of bobbin, wherein a methylphenylpolysiloxane (phenyl group content was 45 mole%) having a viscosity of 40.10⁻⁶ m² / s (40 cst) at 25°C was used as a doubling treatment oil. The viscosity of the oil after a test of heat-resistant property at 300°C (the test described in this specification) was changed to 140.10⁻⁶ m² / s (140 cst) at 25°C, therefore the heat-resistant property of the oil was sufficient. The quantity of the supplied oil was 0.2% per the fiber. The pitch fiber bundle wound on a bobbin thus obtained was unwound and introduced linearly and continuously as thread line treatment into an infusibilization furnace, having oxygen-rich atmosphere (oxygen:nitrogen = 1:1), a gradient of temperature of the infusibilization furnace was arranged so as to have 180°C at an entrance of the furnace and the highest temperature of 330°C, wherein hot wind was forced to circulate by a fan. The temperature was raised from 180°C to 330°C at a rate of 10°C/min. The infusibilization treatment was carried out for 15 minutes. During this infusibilization treatment, the atmosphere was replaced at a rate of 0.5 times/min. Wind velocity was 0.7 m/sec, and tension for the fiber bundle was 68.67x10⁻⁶N (0.007g) per one filament. The unwinding of the pitch fiber bundles from bobbins during the infusibilization, as well as infusibilization treatment itself was carried out smoothly since no breakage of fiber bundles occurred. After finishing the infusibilization, the same oil as used for doubling processing was supplied to the infusibilized pitch fiber bundles by roller contact method. Thus obtained infusibilized pitch fiber was heated to 1,500°C in a nitrogen gas atmosphere to make carbon fiber. The diameter of the carbon fiber was 9.8 µm, having a tensile strength of 3.0 G Pa and a tensile elastic modulus of 280 G Pa, respectively. By heating the carbon fiber to 2,500°C in an argon gas atmosphere, a graphite fiber having a diameter of 9.7 µm, a tensile strength of 3.4 G Pa and tensile elastic modulus of 700 G Pa, respectively, was obtained. EXAMPLE 2 The infusibilization treatment was carried out in the same manner as in Example 1, except using an oil consisting of a dimethylpolysiloxane having a viscosity of 40.10⁻⁶ m² / s (40 cst) at 25°C and an iron octalate as an antioxidant. No breakage of the fiber occurred in the infusibilization treatment, which was smoothly carried out linearly and continuously. The viscosity of the oil after the test of heat resisting property at 330°C was 160.10⁻⁶ m² / s (160 cst) at 25°C. The diameter of a carbon fiber obtained by heating the above obtained infusibilized fiber at 1,500°C was 9.8 µm, and tensile strength and tensile elastic modulus were 2.9 G Pa and 275 G Pa respectively. Comparative Example 1 An infusibilization treatment was carried out in the same manner as in Example 1, except using air as the atmosphere gas. In this case, the fiber bundles were fused and fell into tatters; thus, the fiber bundles had broken in the infusibilization furnace and long fiber bundles could not be obtained. Comparative Example 2 An infusibilization treatment was carried out in the same manner as in Example 1, except using air as atmosphere gas and raising the temperature at a rate of 2.5°C/min. Although no breakage of fiber bundles occurred during the infusibilization treatment and long fibers were obtained, a long time such as 60 minutes was required to make the fiber infusible. A carbon fiber was obtained by heating the above obtained infusibilized pitch fiber at 1,500°C in a nitrogen gas atmosphere. The diameter of the carbon fiber was 9.8 µm and tensile strength and tensile elastic modulus were 2.8 G Pa and 280 G Pa, respectively. Comparative Example 3 An infusibilization treatment was carried out in the same manner as in Example 1, except omitting the doubling proces of the pitch fiber bundles. However, no long infusibilized fiber bundles were obtained since the pitch fiber bundles has been broken in the furnace. Comparative Example 4 An infusilization treatment was carried out in the same manner as in Example 2, except adding no antioxidant in the oil. The fiber bundle had fallen into tatters in the infusibilization furnace, then the fiber bundle had broken and a long fiber bundle was not obtained. As a result of heat-resistant property test at 330°C, the oil was gelled completely, therefore its viscosity could not be measured. Comparative Example 5 An infusibilization treatment was carried out in the same manner as in Example 1, except for using a methylphenyl-polysiloxane (2 mole % content of phenyl group) having a viscosity of 90.10⁻⁶ m² / s (90 cst) at 25°C as a doubling treatment oil. The viscosity of the treatment oil after a heat-resistant property test at 330°C was 2,100.10⁻⁶ m² / s (2,100 cst) at 25°C. In this case, a long fiber bundle was obtained since the fiber bundle was not broken in the furnace during the treatment, however, a lot of fluff was observed on the surface of the bundle. The diameter of the fibers after carbonization at 1,500°C was 9.8 µm and tensile strength and tensile elastic modulus were 2.5 G Pa and 260 G Pa, respectively.

Claims: A process for producing carbon fibers or graphite fibers which comprises infusibilizing pitch fibers obtained by spinning carbonaceous pitch into fiber filaments and gathering the spun fiber filaments into bundles, and carbonizing or graphiting said infusibilized fibers, characterised by (a) doubling said spun pitch fiber bundles into strands so that the number of filaments constituting one strand is from 100 to 100,000, (b) during or after said doubling processing adding a heat resistant non-aqueous doubling treatment oil having a viscosity less than 1000.10⁻⁶ m² / s (1000 cst) at 25°C (measured after a heat treatment of the oil in an air atmosphere while raising the temperature from 100°C to 330°C at a rate of 0.5°C/min), and then (c) infusibilizing said doubled fiber bundles by passing them continuously and linearly as a thread line through an oxygen-rich gas of 30% to 80% oxygen content while applying 9.81x10⁻⁶N to 1,962x10⁻⁶N (0.001 to 0.2 g) tension per filament and heating the fiber bundles at a temperature of 300 to 350°C. A process as claimed in Claim 1, wherein the number of filaments after doubling the fiber bundles is 500 to 50,000. A process as claimed in Claim 1 or 2, wherein the infusibilization temperature (c) is 300 to 330°C. A process as claimed in Claim 1, 2 or 3, wherein the heat-resistant doubling treatment oil is an alkyl phenyl polysiloxane containing 5 to 80 mol% of phenyl group or is dimethylsiloxane, each containing an antioxidant. A process as claimed in Claim 4, wherein the heat-resistant doubling treatment oil is an alkyl phenyl polysiloxane containing 10 to 50 mol% of phenyl group. A process as claimed in any preceding claim, wherein the filaments are twisted 0.1 to 30 times/metre during the doubling step.

Inventors: HINO, TAKASHI, KATO, HISAO, KOMIME, KIKUJI, KURODA, HIROYUKI, NAITO, TSUTOMU
Applicants: TOA NENRYO KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: D01F9
IPC codes: D06M13, D06M15, D01F9, D06M101