PROCESS FOR THE PRODUCTION OF PITCH-TYPE CARBON FIBERS

Abstract: A process for producing pitch-type carbon fibers, which comprises melt spinning pitch material through spinning nozzles, followed by infusible treatment and carbonization and optionally by graphitization, characterized in that a packing layer is provided ad an upstream portion of each nozzle, and the pitch material is passed first through the packing layer and then through the nozzle for spinning.

Description: The present invention relates to a process for producing pitch-type carbon fibers according to the preamble of Claim 1. Such a process is known from US-A-4376747. Carbon fibers have high specific strength and high specific modulus, and they are expected to be most prospective as filler fibers for high performance composite materials. Among them, pitch-type carbon fibers have various advantages over polyacrylonitrile-type carbon fibers in that the raw material is abundantly available, the yield in the carbonization step is high, and the elastic modulus of fibers is high. Various studies have been made on pitch material as the raw material for pitch-type carbon fibers having such advantages. Namely, various studies have been made for the preparation of pitch material having good orientation properties for spinning, since it has been reported that it is possible to obtain pitch-type carbon fibers having high quality by using a pitch wherein carbonaceous raw material is heat-treated to develop anisotropy and readily orientable molecular seeds are formed, instead of an isotropic pitch which has been commonly used as the pitch material for spinning (Japanese Examined Patent Publication No. 8634/1974). It is well known that when a carbonaceous raw material such as heavy oil, tar or pitch is heated at a temperature of from 350 to 500 o C, there form, in the material, small spherical particles which have a particle size of from a few microns to a few hundred microns and which exhibit an optical anisotropy under polarized light. When further heated, these small spherical particles grow and are integrated to form a structure having an optical anisotropy. This anisotropic structure is considered to be a precursor for a graphite crystal structure, wherein planar polymeric aromatic hydrocarbon layers formed by the thermal polycondensation of the carbonaceous raw material are laminated and oriented. A heat-treated product including such an anisotropic structure is generally called mesophase pitch. As a method for using such mesophase pitch as the pitch material for spinning, there has been proposed a method wherein e.g. petroleum pitch is subjected to heat treatment at a temperature of from about 350 to about 450 o C under a stand-still condition to obtain a pitch containing from 40 to 90% by weight of a mesophase, which is used as the pitch material for spinning (Japanese Unexamined Patent Publication No. 19127/1974). However, it takes a long period of time to convert an isotropic carbonaceous raw material to the mesophase pitch by such a method. Under the circumstances, there has been proposed a method wherein the carbonaceous raw material is preliminarily treated with a sufficient amount of a solvent to obtain an insoluble component, which is then subjected to heat treatment at a temperature of from 230 to 400 o C for a short period of time, i.e. for 10 minutes or less, to form a so-called neomesophase pitch which is highly oriented and contains at least 75% by weight of the optical anisotropic component and at most 25% by weight of quinoline-insoluble components, and the neomesophase pitch is used as the pitch material for spinning (Japanese Unexamined Patent Publication No. 160427/1979). As other pitch materials having good orientation properties for the production of high performance carbon fibers, there have been proposed a so-called premesophase pitch, i.e. a pitch which is obtainable by subjecting e.g. coal tar pitch to hydrogenation treatment in the presence of tetrahydroquinoline, followed by heat treatment at a temperature of about 450 o C for a short period of time and which is optically isotropic and capable of being changed to have an optical anisotropy when heated at a temperature of at least 600 o C (Japanese Unexamined Patent Publication No. 18421/1983), or a so-called dormant mesophase, i.e. a pitch which is obtainable by subjecting a mesophase pitch to hydrogenation treatment e.g. by the Birch reduction method and which is optically isotropic and, when an external force is applied, exhibits an orientation to the direction of the external force (Japanese Unexamined Patent Publication No. 100186/1982). It is possible to obtain pitch fibers by melt spinning such pitch material through spinning nozzles. Then, the pitch fibers may be subjected to infusible treatment and carbonization, and optionally to graphitization, to obtain pitch-type high performance carbon fibers. When the above-mentioned pitch material having good orientation properties is melt-spun by a conventional method, the laminar structure of planar polymeric hydrocarbon in the resulting pitch fibers is likely to have radial orientation in the cross-section of each fiber. As a result, there have been drawbacks such that when tensile stress is exerted in the circumferential direction of the cross-section of each fiber due to the carbonization shrinkage during the subsequent infusible treatment and carbonization treatment, wedge-shaped cracks extending in the axial direction of each fiber are likely to form in the cross-section of the resulting carbon fiber, whereby the commercial value of the carbon fibers is impaired. The present inventors have conducted extensive researches to solve the above difficulties, and have found that such drawbacks can be overcome by passing the pitch material through a packing layer prior to supplying it to the spinning nozzles. The present invention has been accomplished on the basis of this discovery. Namely, the object of the present invention is to produce pitch-type carbon fibers having a cross-sectional structure which is substantially free from radial orientation, whereby the formation of wedge-shaped cracks extending in the axial direction of each fiber is substantially suppressed. The above object can readily be attained by a process for producing pitch-type carbon fibers, as defined in claim 1. Now, the present invention will be described in detail with reference to the preferred embodiments. In the accompanying drawings, Figures 1 to 5 are enlarged cross-sectional diagramatic representations of various spinnerets to be used in the present invention. There is no particular restriction for the pitch material to be used in the present invention, so long as it gives an optically anisotropic pitch wherein readily orientable molecular seeds are formed. Various pitch materials as mentioned above may be employed. However, amorphous pitch may also be employed in a case where not so high specific strength and specific modulus are required. As the carbonaceous raw material to obtain such pitch material, there may be mentioned, for instance, coal-originated coal tar, coal tar pitch or liquefied coal, or petroleum-originated heavy oil, tar or pitch. These carbonaceous raw materials usually contain impurities such as free carbon, non-dissolved coal or ash contents. It is desired that these impurities are preliminarily removed by a conventional method such as filtration, centrifugal separation or sedimentation separation by means of a solvent. Further, the above-mentioned carbonaceous raw material may be pre-treated by a method wherein it is subjected to heat treatment, and then soluble components are extracted with a certain solvent, or by a method wherein it is subjected to hydrogenation treatment in the presence of a hydrogen-donative solvent and hydrogen gas. In the present invention, the above-mentioned carbonaceous raw material or pre-treated carbonaceous raw material is heat treated usually at a temperature of from 350 to 500 o C, preferably from 380 to 450 o C for from 2 minutes to 50 hours, preferably from 5 minutes to 5 hours in an inert gas atmosphere such as nitrogen or argon or while introducing such an inert gas, to obtain a pitch containing at least 40% by weight, particularly more than 70% by weight of an optically anisotropic structure, which is suitable for use as the pitch material for spinning. The proportion of the optically anisotropic structure of the pitch material for spinning in the present invention is a value obtained as the proportion of the surface area of the portion exhibiting an optical anisotropy in the pitch material for spinning, as observed by a polarizing microscope at normal temperature. Specifically, for instance, a pitch sample is crushed into particles having a size of a few millimeter, and the sample particles are embedded on the almost entire surface of a resin having a diameter of about 2 cm in accordance with a conventional method, and the surface was polished and then the entire surface was thoroughly observed by a polarizing microscope (100 magnifications), and the ratio of the surface area of the optically anisotropic portion to the entire surface area of the sample is obtained. In the present invention, the above-mentioned pitch material for spinning is passed through a net packing layer, and then supplied to spinning nozzles for spinning. Here, the packing layer is provided at an upstream portion of each spinning nozzle, in the flow passageway of the pitch material. When the molten pitch material passes through the packing layer, the flow of the pitch material is finely divided, and the laminar state of the mesophase of the pitch material is disturbed during the passage through the packing layer, whereby pitch fibers having a cross-sectional structure having no substantial radial orientation will be formed. The packing layer of the present invention is provided independently of the filter used in the production of e.g. synthetic fibers to remove foreign matters present in the raw material for spinning. Namely, as mentioned above, the packing layer of the present invention is provided at an upstream portion of each spinning nozzle and has a function to sufficiently disturb the laminar state of the mesophase of the pitch material. As the packing material constituting the net packing layer, there may be mentioned woven fabric or a net made of a metal material such as stainless steel, copper or aluminum, or an inorganic material such as ceramics, glass, or graphite, which is sufficiently durable at a temperature of from 350 to 400 o C. The packing material is selected from those having a shape capable of finely dividing the flow of the pitch material during the passage of the material therethrough and capable of providing a shearing force to disturb the laminar state of the mesophase of the pitch material. When a net is used as the packing material constituting the packing layer, it is preferred to employ a net obtained by weaving fine fibers of the above-mentioned metal or inorganic material by plain weave, twill weave or tatami weave. However, it is also possible to employ a net obtained by punching out a flat metal plate to form numerous perforations, or a net like an expanded metal obtained by expanding a metal plate provided with a number of slits. If the openings of the net are too large, the effects for finely dividing the cross-sectional structure of the fibers to avoid the radial orientation, tend to diminish. Therefore, the smaller the net openings, the better. Specifically, it is usual to employ the one having openings smaller than 50 mesh, preferably smaller than 100 mesh, more preferably smaller than 200 mesh. Such a net may be used in a single sheet. However, it is also possible to use a plurality of nets in a laminated state. Figures 1 to 5 show enlarged views of the portions in the vicinity of the spinning nozzles in various embodiments in which the packing layers of the present invention are provided. Reference numeral 1 designates a spinneret, numeral 2 designates a spinning nozzle, numeral 3 designates a supply hole, numeral 4 designates a packing layer, and numeral 5 designates a space. The thickness of the packing layer 4 may vary depending upon the type or configuration of the packing material. In general, however, the thicker, the better. However, if the packing layer is too thick, the flow resistance of the pitch material increases. On the other hand, if the packing layer is too thin, the desired effects can not be obtained. Therefore, it is usual to employ a thickness within a range of from 1 to 300 mm, preferably from 3 to 200 mm. Especially when the above-mentioned pitch material is spun by means of a spinneret having a number of holes, it is difficult to uniformly pack the above-mentioned packing material to the respective holes. If the packing material is not uniformly packed, there will be differences in the construction of the packing layers among the respective holes. As a result, the flow rates of pitch material discharged from the respective spinning nozzles differ from one another, thus leading to non-uniformity in the size, i.e. non-uniformity in the diameter of the pitch fibers spun from the respective spinning nozzles, whereby it becomes difficult to produce pitch fibers constantly while maintaining the uniform fiber diameter from every spinning nozzle. Therefore, in order to minimize such non-uniformity of the size and to avoid the breakage of fibers, it is important to provide a space between the packing layer and each spinning nozzle. Namely, it is possible to control the fiber structure to avoid the formation of cracks by providing the packing layer within the supply hole of the spinneret. However, if the packing layer is located immediately in front of the inlet of the spinning nozzle, it is difficult to minimize the variation in the flow rate of pitch material due to the difference in the construction of the packing layer of every hole, and thus leading to non-uniformity of the size. Whereas, by providing a space between the packing layer and the spinning nozzle, the non-uniformity of the size can be substantially overcome. The size of the space is represented by a value obtained by dividing the time required for the pitch material passed through the packing layer to reach the spinning nozzle, i.e. the internal volume from the terminal end of the packing layer to the upper end of the inlet of the spinning nozzle, by the discharge amount of the pitch material, and said time is selected within 10 minutes, preferably within 1 minute, more preferably from 0.05 to 15 seconds, most preferably from 0.1 to 5 seconds. Also in the case where such a space is provided, the above-mentioned various packing materials may be used as the packing material constituting the packing layer. Various shapes may be employed for the space, as shown in Figures 1 to 5. However, in the case where a net is used as the packing material, it is preferred to adjust the angle ϑ from the space to the inlet of the spinning nozzle to be at least 90 o , preferably at least 120 o , whereby the effects for finely dividing the cross-sectional structure of the resulting fibers to avoid the radial orientation, can be increased. The joint portion of the space and the inlet of the spinning nozzle may be curved so far as such effects will not be diminished. The thickness of the packing layer is preferably greater from the aspect of the effects against crystal orientation. However, such effects can be obtained sufficiently if the thickness corresponds to about five particles when provided immediately above each nozzle. If the packing layer is too thick, the resistance against the flow of the pitch material tends to increase, and the installation costs likewise increase. Thus, the thickness of the packing layer is at most about 20 mm. Needless to say, the thickness of the packing layer for each hole should be as uniform as possible. Preferably, the total amount of the packing material constituting the packing layers is made up with the packing material as specified in the above range. However, so long as the object of the invention can be accomplished, certain amounts of other packing material than as defined above may be incorporated. However, in the vicinity of the spinning nozzles, it is preferred to employ, e.g. at least 10 layers of the packing material as specified above. The thickness of the packing layer is selected within the above-mentioned range. The packing layer is located immediately above the spinning nozzle for the reason that if the pitch material passed through the packing layer is maintained in the molten state for a long period of time, the finely divided flow units of the pitch material are likely to be integrated again to return to the original state prior to the passage through the packing layer. Namely, it is intended that after the passage through the packing layer, the pitch material swiftly reaches the spinning nozzle. There is no particular restriction as to the spinning nozzles to be used in the present invention. They may be of a straight tubular type or of a type wherein the center portion of the nozzle is expanded, or of a type wherein the lower portion of the nozzle is expanded. Spinning nozzles having a nozzle hole diameter of from 0.05 to 0.5 mm, preferably from 0.1 to 0.3 mm, are used. The length of the spinning nozzles is preferably selected within a range of from 0.01 to 5 mm. The spinning nozzle means a fine hole through which the pitch material passes through immediately prior to being spun and which determines the fiber diameter, and the nozzle hole diameter means the diameter of the fine hole discharging the pitch material. The pitch material passes through the packing layer 4 and is discharged from the spinning nozzle 2 to be spun. By providing the packing layer 4, it is possible to conduct the spinning while exerting a pressure of at least 1.96 bar (2 kg/cm²G), preferably at least 4.9 bar (5 kg/cm²G), more preferably at least 9.81 bar (10 kg/cm²G) to the pitch material, at the time of discharging the pitch material. In the present invention, when the pitch material in a molten state passes through the packing layer 4, the flow of the pitch material is finely divided and the laminar state of the mesophase is disturbed by the packing layer 4, whereby pitch fibers, or consequential pitch-type carbon fibers, having a cross-secitonal fiber structure with no substantial radial orientation can be obtained. Accordingly, the flowability of the pitch material can be improved by the packing layer 4, and at the same time, the formation of gas or bubbles generated from the pitch material at the spinning temperature can be suppressed by the pressurizing operation within the above-mentioned range during the spinning, whereby the stability for spinning is improved, and pitch fibers having improved properties can be produced constantly for a long period of time as uniform fibers having no size deviation among the nozzle holes. The obtained pitch fibers are then subjected to infusible treatment and carbonization, and optionally graphitization, whereby high performance pitch-type carbon fibers having a cross-sectional fiber structure with random orientation or onion-like orientation, free from wedge-shaped cracks extending in the axial direction of the fibers, are obtainable. Here, the onion-like orientation means that the main portion of the cross-section of the fiber has a concentric molecular orientation, and includes the one wherein a part, particularly the peripheral portion, has a radial orientation to such an extent that no cracks will be formed by the subsequent carbonization or graphitization treatment. These cross-sectional fiber structures are as measured by a polarizing microscope. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by these specific Examples. Ref-EXAMPLE Into a 5 liter autoclave, 2 kg of coal tar pitch and 2 kg of a hydrogenated aromatic oil were introduced and heat-treated at 450 o C for 1 hour. The treated product was distilled under reduced pressure to obtain residual pitch. Then, 200 g of this residual pitch was subjected to heat treatment at 430 o C for 125 minutes while bubbling nitrogen gas. The mesophase pitch thereby obtained had an optical anisotropy of 100%. EXAMPLES 1 to 3 In the same manner as in Ref.-Example, a mesophase pitch having an optical anisotropy of 100% was prepared. Then, by using a spinneret as shown in Figure 1, a stainless steel metal net (i.e. a network layer) 4 having the size as identified in Table 1 was provided in each supply hole 3 thereof. The position of the metal net was adjusted so that the time required for the pitch material passed through the network layer 4 to reach the spinning nozzle 2, i.e. the retention time in the space 5, was as shown in Table 1. Then, by using this spinneret, the above-mentioned mesophase pitch was melt-spun within a temperature range of from 325 to 360 o C. In each case, pitch fibers having a diameter as small as 7 µm were obtained constantly over a long period of time by adjusting the winding up speed at the optimum temperature. Pitch fibers obtained by melt spinning at a temperature of 336 o C, were subjected to infusible treatment in air at 310 o C, and then carbonization treatment in an argon atmosphere at 1400 o C, to obtain carbon fibers. The tensile strength and the cross-sectional structure of the carbon fibers were measured. The results are shown in Table 1. EXAMPLE 4 The melt spinning and carbonization treatment were conducted in the same manner as in Example 1 except that by using a spinneret as shown in Figure 2 (i.e. a spinning nozzle 2 having a diameter of 0.2 mm and a length of 0.1 mm), a 200 mesh stainless steel metal net was provided as a network layer 4 in each supply hole 3 thereof, at a position where the retention time of pitch material in the space 5 was 3.8 seconds. In the spinning, pitch fibers having a diameter as small as 7 µm were obtained constantly over a long period of time. The results thereby obtained are shown in Table 1. EXAMPLE 5 The melt spinning and carbonization treatment were conducted in the same manner as in Example 1 except that by using a spinneret as shown in Figure 3 (i.e. a spinning nozzle 2 having a diameter of 0.1 mm and a length of 0.1 mm), a 635 mesh stainless steel metal net was provided as a network layer 4 in each supply hole 3 thereof, at a position where the retention time of pitch material at the space 5 was 0.2 second. In the spinning, pitch fibers having a diameter as small as 7 µm were obtained constantly over a long period of time. The results thereby obtained are shown in Table 1. COMPARATIVE EXAMPLE The melt spinning was conducted in the same manner as in Example 1 except that no network layer was employed, whereby pitch fibers having a diameter of 7 µm or less could not be obtained constantly. The physical values of the carbon fibers obtained in the same manner as in Example 1 are shown in Table 1.

Claims: A process for producing carbon fibers having a cross-sectional structure which is substantially free from radial orientation, orientated as observed by a polarizing microscope, which comprises melt-spinning pitch material through spinning nozzles, followed by infusible treatment and carbonization and optionally by graphitization, characterized in that a net packing layer composed of a woven fabric or a perforated metal plate is provided as an upstream portion of each nozzle, and the pitch material is passed first through the net packing layer and then through the nozzle for spinning, whereby the net packing layer has openings smaller than 50 mesh. The process according to Claim 1, wherein the pitch material is a pitch having an optical anisotropy of at least 40 %. The process according to Claim 1, wherein the net packing layer is composed of a net made of metal or inorganic material. The process according to Claim 1, wherein the time required for the pitch material to pass through the net packing layer to reach the nozzle is not longer than 1 minute. The process according to Claim 1, wherein the time required for the pitch material to pass through the net packing layer to reach the nozzle is not longer than 15 seconds. The process according to Claim 1, wherein the pitch material is melt-spun under a pressure of at least 1.96 bar (2 kg/cm²) G.

Inventors: HARA, RYUICHI MITSUBISHI CHEMICAL IND. LTD., KAGIZAKI, MASAMI MITSUBISHI CHEMICAL IND. LTD., TAKAKURA, TSUYOSHI MITSUBISHI CHEMICAL IND. LTD., TOMONO, SHIGEKI MITSUBISHI CHEMICAL IND. LTD.
Applicants: MITSUBISHI KASEI CORPORATION
Application year: 1985
Main classification: D01D4
IPC codes: D01F9, D01D4