PROCESS FOR PRODUCING CARBON FIBERS

Abstract: A process for producing carbon fibers, which comprises continuously preoxidizing fiber bundles of acrylonitrile polymer containing at least 90 wt % of acrylonitrile in a plura­ lity of preoxidizing furnaces operated at different treating temperatures in an oxidative atmosphere at 200 to 350°C, in such a manner that the fiber density, ρ n , of the preoxidized fibers satisfies the specific condition and the fiber density, ρ k , after completion of the treatment is 1.34 to 1.40 g/ml, and conducting carbonizing treatment in an inert atmosphere.

Description: TECHNICAL FIELD The present invention relates to a multistage flame-resisting treatment and carbonization of acrylic fibers in tow form, whereby it is possible to produce carbon fibers in tow form which have properties of high tenacity and high elasticity and are superior in homogeneity throughout the monofilaments and have less yarn defects including nap, etc. BACKGROUND ART As is known generally, the usual process for producing carbon fibers is roughly divided into the step of flame-resisting wherein acrylic fibers are subjected to heat treatment in an oxidizing atmosphere and the step of carbonization wherein fibers from the flame-resisting treatment are subjected to heat treatment in an inert atmosphere The step of imparting flame-resistance to acrylic fibers is practiced in an oxidizing atmosphere at temperatures of 200 to 300°C over a period generally of 2 to 4 hours. This flame-resistance imparting step occupies 90% or more of the total time required for the process to produce carbon fibers. Accordingly it is said that the reduction of carbon fiber production cost resides in shortening the time required for this flame-resistance imparting reaction. One of the methods for shortening the flame-resisting step is to raise the temperature of flame-resisting as disclosed in Japanese Patent Publication No. 35938/72. However, since the flame-resisting reaction is exothermic as shown in Textile Res. J., 30 , 882-896 (1960), this method when adopted may cause a vigorous incontrollable reaction, inducing the inflammation of acrylic fibers. Even if the inflammation is not induced, the acrylic fibers treated by this method will have flame-resisting structure at the peripheral part of each filament but have insufficiently flame-resisting structure in the inner part thereof, thus turning into flame-resistant fibers of nonuniformly flame-resisting structure. Such flame-resisting fibers in the later carbonization step will develop undesirable phenomena such as napping and fiber break, hence being difficult to undergo effective carbonizing reaction and unable to form high-performance carbon fibers. Against this, Japanese Patent Publication No. 25487/76 discloses a method free of such difficulties, whereby the time for the flame-resisting treatment of acrylic fibers is reduced to 5 -30 minutes. This method comprises subjecting acrylic fibers to flame-resisting treatment under such conditions that the heat treatment time until the equilibrium moisture content of the acrylic fibers reaches 4% may be from 5 to 20 minutes, followed by carbonizing the fibers at a temperature of at least 1000°C. However, the flame-resisting fibers having an equilibrium moisture content of 4%, as can be seen in a number of known literatures, are insufficient in flame-resistant structure and the cross section of each filament shows an outstanding double structure. Such flame-resisting fibers undergo pyrolysis in the later step of carbonization and micro-voids are formed in the resulting fibers. Hence it is difficult to convert these fibers into high-tenacity carbon fibers having a tensile strength of 3.923 GPa (400 kg/mm²) or more. In this way, the incontrollable reaction and nonuniform flame-resisting reaction of acrylic fibers in the flame-resisting step become more remarkable with an increase in the number of acrylic monofilaments constituting a tow. Japanese Patent Application Laid-Open No. 163729/83 discloses an effective method for the flame-resisting such a tow constituted of a large number of acrylic monofilaments. This method comprises heating acrylic tows, each constructed of 1000 to 30,000 filaments of 0.055 to 0.166 tex (0.5 to 1.5 deniers) in monofilament size in a flame-resisting oven at temperatures of 200 to 260°C to convert the filaments into incompletely flame-resisting filaments having an oxygen content of 3 to 7% (thus preventing the filaments from fusion during the later flame-resisting treatments of higher degrees), treating then the filaments under high-temperature flame-resisting conditions to convert them into completely flame-resisting filaments having an oxygen content of at least 9.5%, followed by carbonizing the filaments. However, in this method, while napping or breaking of filaments does not occur, conditions of the treatment for converting the incompletely flame-resisting filaments into the completely flame-resisting ones are harsh, hence microvoids being liable to develop in each filament, and moreover the oxygen content in the completely flame-resisting filaments is as high as 9.5% at least and crosslinked structure caused by oxygen develops therein to a high degree, so that it is impossible to apply stretching treatment effective for enhancing performance characteristics of carbon fibers obtained in the carbonization step and thus the tensile strength of the product carbon fibers is up to 3.432 GPa (350 kg/mm²). In recent years, carbon-fiber reinforced composites have been in extensive use for sporting, astronautical, industrial, and other applications and the expansion of their consumption has been remarkable. In response to such circumstances, performance characteristics of carbon fibers for use have been under improvements to great extents. As regards the elastic modulus, it was 196.14 GPa (20 ton/mm²) 10 years ago, and improved to standard values of 225.56 to 235.37 GPa (23 to 24 ton/mm²) several years ago, and lately carbon fibers having an elastic modulus of about 294.21 GPa (30 ton/mm²) have been aimed at. There is pointed out the possibility that such carbon fibers will be dominant for the future. However, if such improvement of the elastic modulus of carbon fibers is achieved while the strength of the fibers is kept constant, a decrease in the elongation of the fibers will be brought about, as a matter of course, and composites reinforced with such carbon fibers will be brittle. Accordingly, there is an intense demand for carbon fibers of high elasticity and high elongation, that is, carbon fibers having high elongation and high strength at the same time. The conventional method for improving the elastic modulus has been to raise the carbonization temperature, i.e. the temperature of the final heat treatment. This method, however, has a drawback in that, as the elastic modulus is increased, the strength and consequently the elongation decrease. For instance, a carbonization temperature of about 1800°C is necessary in order to maintain an elastic modulus of 274.59 GPa (28 ton/mm²), but this temperature results in a strength at least 0.980 GPa (100 kg/mm²) lower than the value resulting from a carbonization temperature of 1300°C; thus a high strength cannot be achieved at all. Such a decrease in the strength with an increase in the carbonization temperature corresponds well with the decrease in the density. This is assumed to be caused by the development of micro-voids, which will bring about a decrease in the strength in the fibers during elevation of the carbonization temperature. When acrylic tows each having a whale filament size of 111.1 to 2222 tex (1000 to 20,000 denier) after flame-resisting treatment are subjected to carbonizing treatment, it is also impossible to produce carbon fibers in tow form having high strength and high elongation from such tows since napping or filament breaking takes place frequently in the carbonization step. The causes thereof are exemplified by significant uneveness of the flame-resisting degree throughout the monofilaments constructing the tow, high unevenness in the longitudinal direction of each monofilament subjected to flame-resisting treatment, and minute flaws present in each monofilament itself subjected to flame-resisting treatment. PROBLEMS TO SOLVE ACCORDING TO THE INVENTION As described above, it is the present situation that no technique has still be established that enables acrylic fibers in tow form, each tow consisting of as large a number of monofilaments as 1000 to 15,000, particularly a precursor consisting of such tows arranged in parallel in sheet form, to be subjected to a high-speed flame-resisting treatment for a period of up to 60 minutes and to stretching treatment for enhancing performance characteristics of the carbon fibers in the subsequent carbonization step. When the fibers of high elasticity are produced, the carbonizing treatment has hitherto been carried out at high temperatures, but it is extremely difficult by this method to obtain carbon fibers of high strength and high elongation. For example, those carbon fibers have the drawback of significant variation in the tensile strength which are obtained by subjecting flame-resisting fibers of 1.37 g/ml in density to treatment under tension in an inert atmosphere at a temperature of 200 to 800°C and then heat-treating the resulting fibers in an inert atmosphere at a temperature of 1300 to 1800°C. According to the present inventors' study, it is considered that there is a problem in the inter-filament and filament lengthwise unevenness of flame-resisting degree. According to the conventional flame-resisting method, however, the unevenness of flame-resisting degree is difficult to reduce. As regards methods for flame-resisting of acrylic fibers, a method is known wherein the treatment temperature is raised, thereby increasing the gradient of temperature rise in the earlier stage of the flame-resisting step and decreasing the gradient of temperature rise in the latter half of the step (see Japanese Patent Publication No. 35938/72). According to this method, however, fusion or agglutination occurs frequently among filaments, further a vigorous incontrollable reaction is caused, and inflammation is liable to take place. There is also known a method wherein the gradient of temperature rise is decreased in the earlier stage of the flame-resisting step and increased in the latter half of the step (see Japanese Patent Application Laid-Open No. 163729/83). According to this method, the occurrence of fusion or agglutination among filaments is relatively limited but the flame-resisting reaction proceeds rapidly in the latter half stage, hence increasing the interfilament and filament axis directional unevenness of the flame-resisting degree and causing frequently napping and filament breaking phenomena. In addition, this method is extremely inferior in step passableness and is difficult to provide high-performance carbon fibers. Methods for the carbonizing treatment were also investigated, among which a method is known wherein fibers subjected to flame-resisting treatment are treated at a temperature of 250 to 600°C, then at a temperature of 400 to 800°C, and finally at a temperature of 800 to 1300°C (see Japanese Patent Application Laid-Open No. 150116/84). But, carbon fibers having satisfactory performance characteristics are also difficult to obtain according to this method. MEANS FOR SOLVING PROBLEMS Therefore, the present inventors made intensive studies in order to solve the above noted problems, thus acquiring the following knowledge: (i) In the prior art, the permeation of oxygen into acrylic monofilaments tends to delay because of the inadequate rate of oxygen diffusion into interstices between the monofilaments in tow form. (ii) In consequence, it has become necessary that the density of the fibers treated for flame-resisting to be fed to the carbonization step should be increased to 1.40 g/ml or more, thus causing such undesirable matters as stated above. (iii) Based on this finding, flame-resisting conditions are chosen so as to increase the rate of oxygen diffusion into tows of acrylic fibers, whereby the above undesirable matters can be markedly inhibited and carbon fibers exhibiting extremely-high performance can be produced from the flame-resisting fibers obtained in this way. The present invention has been accomplished on the basis of the above knowledge. Thus, the substance of the invention is a process for producing carbon fibers whereby acrylic fibers in bundle form containing at least 90% by weight of acrylonitrile are continuously subjected to a multistage flame-resisting treatment in an oxidizing atmosphere at temperatures of 200 to 350°C by using a plurality of flame-resisting furnaces different in treatment temperature. The said process, known per se from FR-A- 2 488 917, is characterised in that the of flame-resisting treatment stages is at least three and the treatment is carried out under such conditions that the fiber density ρ n after each stage of flame-resisting treatment may be maintained on the level defined by the following equation (1) and so that the fiber density ρ k after completion of the flame-resisting treatment may be from 1.34 to 1.40 g/ml, and successively the fibres are subjected to carbonizing treatment in an inert atmosphere; wherein ρ n is the density (g/ml) of the fibers after the n-th treatment stage, ρ o is the density (g/ml) of the feedstock acrylic fibers, ρ k is the density of the fibers after completion of the flame-resisting treatment and is a value ranging from 1.34 to 1.40 g/ml, t n is the period of flame-resisting treatment at the n-th stage, and k is the number of flame-resisting treatment stages. BRIEF DESCRIPTION OF DRAWINGS The accompanying drawings are explained below. Fig. 1 is a graph showing the relation between the density of flame-resisting fibers and the period of flame-resisting treatment, for the purpose of explaining the treatment method of the present invention. In Fig. 1, curve A is in the case of high-temperature treatment, curve B is in the case of low-temperature treatment followed by high-temperature treatment, and curve C is in the case of treatment according to the process of the invention. Fig. 2 is a graph showing temperature profiles in low-temperature carbonization, with furnace length as abscissa and temperature as ordinate. Straight lines 1 and 3 show such a temperature profile in the invention. Fig. 3, which also shows furnace length as abscissa, shows modes of increasing the gradient of furnace temperature in the case of high-temperature heat treatments for carbonization. In Fig. 3, numeral 4 shows said mode in a high-temperature carbonization heat treatment method according to the prior art, 5 and 6 said modes in the present inventive process, and 7 to 9 those, given for comparison, in high-temperature carbonizing treatment methods. BEST MODE FOR CARRYING OUT THE INVENTION The polymer constructing acrylic fibers used in carrying out the invention is a copolymer of 90% by weight or more of acrylonitrile and 10% by weight or less of other copolymerizable vinyl monomer(s). This polymer can be produced by various methods including solution polymerization method, suspension polymerization method, emulsion polymerization method, etc., and is desired to have a reduced viscosity ranging from 1.0 to 10.0. Fibers formed from a polymer containing less than 90% by weight of acrylonitrile units have low flame-resisting reaction activity. Hence, when such fibers are used, the temperature of initiating the flame-resisting reaction needs to be raised and once the flame-resisting reaction is initiated, a vigorous incontrollable reaction is liable to occur on the contrary. The polymer is preferred to contain 95% by weight or more of acrylonitrile units polymerized. The other copolymerizable vinyl monomer to be copolymerized with acrylonitrile is a constituent which accelerates the flame-resisting reaction of the acrylic fibers and contributes to reduction in the flame-resisting reaction period. Such usable monomers include, for example, hydroxyethylacrylonitrile, methyl vinyl ketone, methyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, and t-butyl methacrylate. The total amount of these constituents copolymerized is desirably up to 10%, preferably up to 5%, by weight. The above defined acrylonitrile-based polymer is normally spun by a wet spinning method or a dry-wet spinning method to form tows of acrylic fibers of desirably 0.033 to 0.166 tex (0.3 to 1.5 deniers) in monofilament size, each tow having a whole fiber size of 111.1 to 2222 tex (1000 to 20,000 deniers). Fibers of less than 0.033 tex (0.3 denier) in monofilament size are undesirable since their strength is insufficient for use as feedstock fibers to produce carbon fibers. On the contrary when the size exceeds 0.166 tex (1.5 deniers), a tendency is observed to lower the rate of oxygen diffusion into the monofilament in the flame-resisting step and make it difficult to prepare fibers flame-resisted uniformly. On the other hand, tows of less than 111.1 tex (1000 deniers) in the whole fiber size of each tow have good passableness through the flame-resisting step but exhibit rapidly-lowered productivity for flame-resisting fibers. When the whole fiber size of each tow, on the contrary, exceeds 2222 tex (20,000 deniers), the diffusion of oxygen into the inner part of the acrylic tow will be retarded in the flame-resisting step and this tends to develop difference in flame resistance between outer-side filaments and inner-side filaments in each tow. Properties necessary for the fibers subjected to flame-resisting treatment to have from which high-performance carbon fibers can be produced include that; nap should not be developed; 2% or more, preferably 5% or more, stretch should be possible in the initial stage of the carbonization step; and the amount of tar formed should be limited. The tow of flame-resisting fibers provided with such properties need to have no large difference in the density of filaments subjected to flame-resisting treatment between the filaments located in the outer side of the tow of 111.1-2222 tex (1000-20,000 deniers) and the filaments located in the central part of the tow and the degree of flame-resisting in each treated filament should be uniformed as far as possible. In order to prepare tows of flame-resisting fibers provided with such properties as stated above by oxidizing treatment of 111.1-to 2222-tex (1000- to 20,000- denier) tows of acrylic fibers, it is necessary that the condition defined by equation (1) given above should be satisfied by the fiber density which indicates the degree of improving the flame resistance of the fibers passed through the n-th one of plural flame-resisting furnaces. When ρ n is larger than the value of the right side of equation (1) in the former half of the flame-resisting step, high-temperature treatment becomes necessary in order to increase the fiber density in the initial stage as shown by line A of Fig. 1 appended. Accordingly, the inflammation or fusion of filaments is liable to occur on account of an incontrollable run of reaction and the flame-resisting step becomes difficult to shorten. In order to avoid the vigorous incontrollable reaction accompanying the high-temperature treatment, in the prior art, it is necessary to treat fibers at relatively low temperatures in the former half of the flame-resisting step and increase rapidly the density of the treated fibers, as shown by line B of Fig. 1, in the latter half step, wherein the incontrollable run of reaction may rarely occur. In consequence, micro-voids are formed in each resulting flame-resisting filament and the filament has large difference in the degree of flame-resisting between its outer side and its inner side of itself. It can be seen that such fibers subjected for a short time to flame-resisting treatment do not exhibit stretchability at all in the later carbonizing treatment step and are liable to develop nap because a structure which is not stable to heat is formed partially in the interior of the fibres. In the present invention, as opposed to this, the flame-resisting reaction, when such flame-resisting treatment conditions that ρ n may be in the range defined by equation (1) are applied, proceeds so as to hold a nearly linear relation between the density ρ ox of the fibers subjected to flame-resisting treatment and the period t n of flame-resisting treatment as shown by line C in Fig. 1, and even when the total period t n of flame-resisting treatment is limited to 60 minutes or less, the difference between ρ ox of outer side filaments of each tow resulting from the flame-resisting treatment and ρ ox of inner side filaments of the tow can be reduced in the extreme. Moreover, it can be seen that uniform flame-resisting in each filament can be carried out effectively and the thus treated tows have slightest inter-filament fusion or agglutination. The value of ρ o is normally about 1.18 g/ml, and ρ k in the present invention needs to lie in the range of 1.34 to 1.40 g/ml, preferably 1.35 to 1.38 g/ml. Flame-resisting fibers having ρ k values of less than 1.34 g/ml undergo rapid pyrolysis and tend to develop nap, in the carbonization step, and hence cannot be converted into carbon fibers having good performance characteristics. On the contrary, those having ρ k values exceeding 1.40 g/ml are difficult to provide high-performance carbon fibers having tensile strengths of at least 3.923 GPa (400 kg/mm²). Against this, the present inventive fibers subjected to flame-resisting treatment, having ρ k values ranging from 1.35 to 1.40 g/ml, can be stretched by as much as 3 to 25% without undergoing abnormal pyrolysis in the carbonization step, providing carbon fibers having excellent performance characteristics. The invention produces distinguished effect when the flame-resisting treatment period is up to 90 minutes, particularly in the range of 20 to 60 minutes. The number of stages in the multistage flame-resisting furnace used in the invention is at least 3, preferably 3 to 6. A too large number of these stages is undesirable, since such a furnace is uneconomical and much restricted with respect to the installation thereof and has adverse effect on the workability. The multistage flame-resisting method of the invention is effective in baking a single or plural acrylic tows of 0.033 to 0.166 tex (0.3 to 1.5 deniers) in monofilament size and 111.1 to 2222 tex (1000 to 20,000 deniers) in each tow size, particularly effective in baking dozens to hundreds of acrylic tows arranged in parallel and in sheet form. When acrylic tows arranged in sheet form are baked, the objects of the present invention can be fully achieved by spacing the tows so suitably that the diffusion of oxygen into each tow may not be hindered and by controlling the rate of heating so that the rate of flame-resisting may satisfy equation (1). Fibers obtained by flame-resisting treatment in this way can be baked in the carbonization step while being streched sufficiently and can be converted into carbon fibers having excellent performance characteristics. Additionally, the period of flame-resisting treatment can be reduced notably in this way of baking as compared with the case of the conventional way. In the invention, it is desirable to conduct the flame-resisting treatment, while stretching the fibers to a stretch percentage of up to 30% until the fiber density reaches 1.22 g/ml and then to a total stretch percentage of up to 50% until the fiber density reaches 1.26 g/ml. Fibers subjected to flame-resisting treatment which are convertible into high-performance carbon fibers are those having highly-oriented structure which tend to form graphite net planes. Before the acrylic fiber density, which is usually about 1.18 g/ml, reaches 1.22 g/ml, about 50% stretch of acrylic fibers is possible, but when the stretch percentage exceeds 30%, the unevenness of the fibers resulting from flame-resisting treatment may increase and simultaneously yarn defects may develop. The growth of graphite crystal structure in the carbonization step is facilitated and highly oriented defect-free carbon fibers can be obtained, by the stretch at a draw to give a total stretch percentage of up to 50% until the fiber density reaches 1.26 g/ml. It may be noted that the flame-resisting treatment in the region where the fiber density exceeds 1.26 g/ml needs to be conducted under such conditions that the substantial stretch of the fibers may not take place. If the substantial stretch of the fibers takes place in this region, numerous micro-voids will be contained in the carbon fibers and performance characteristics of these fibers will be deteriorated. Shrinkage of the fibers when caused in this step induces disorder in the fine structure of the fibers subjected to flame-resisting treatment and decreases the strength of the resulting carbon fibers. An example of methods for stretching the fibers is that the fibers are brought into contact with a number of rotating rolls, the speeds of which are increased for a while until the density reaches 1.26 g/ml and thereafter are maintained constant. In the carbonization according to the present invention, the fibers of 1.34 to 1.40 g/ml density subjected to flame-resisting treatment are heat-treated in an inert atmosphere at a starting temperature of 300 ± 50°C, final temperature of 450 ± 50°C, and heating rate of 50 to 300°C/min. When the starting temperature of the heat treatment is below 250°C, the tarry component formed in the fibers subjected to flame-resisting treatment is difficult to remove effectively. When the starting temperature exceeds 350°C, rapid pyrolysis of the flame-resisting fibers followed by frequent filament breaking or napping will take place, deteriorating the step passableness and tending to provide fibers which contain numerous micro-voids, making it impossible to produce high-performance carbon fibers. The final heat treatment temperature in this step needs to be 450 ± 50°C. When the final temperature is below 400°C, a formed tarry component may remain in the fibers. When the final temperature exceeds 500°C, performance characteristics of the resulting carbon fibers are rapidly deteriorated. The rate of heating needs to be from 50 to 300°C/min within the above temperature range. When the rate of heating exceeds 300°C/min, performance characteristics of the resulting carbon fibers are rapidly deteriorated. When the rate of heating is less than 50°C/min, it becomes necessary to increase the furnace length markedly, this being economically unfavorable. In the next place, the fibers are heat-treated in an inert atmosphere at a temperature of 400 to 800°C. When this treatment temperature is below 400°C or higher than 800°C, it is impossible to produce carbon fibers excellent in strength and elastic modulus. The treatment period is desirably up to 3 minutes, preferably in the range of 0.1 to 1 minute. As will be shown in Examples, the treatment period exceeding 3 minutes is undesirable since deterioration is observed in performance characteristics of the resulting carbon fibers. The above stated low-temperature carbonization treatment can be carried out with ease by using, for example, a 300 ± 50°C to 450°C ±50°C increasing temperature furnace and a 400 to 800°C constant temperature furnace. The relation between the treatment temperature and the furnace length in this case is explained with reference to a drawing. Fig. 2 is a graph showing temperature profiles in low-temperature carbonizing treatments, with abscissa as furnace length and temperature as ordinate. Straight line 1 shows the profile in case of the heat treatment wherein the starting temperature is 300°C and the final temperature is 450°C and straight line 3 shows the profile in case of the heat treatment at a constant temperature of 600°C. Dotted line 2 shows the profile in case of the heat treatment at the same rate of raising temperature as in the case of straight line 1, in the temperature range of 450 to 600°C. The treatment to conduct as shown by straight line 1 and dotted line 2 requires a markedly larger furnace length than does the treatment to conduct as shown by straight lines 1 and 3. In the former case, high-performance carbon fibers cannot be obtained. For the purpose of producing carbon fibers having a high elastic modulus, the following way of stretching is preferable. That is, the fibers resulting from flame-resisting treatment according to the above described method are treated under tension in an inert atmosphere at temperatures of 300 to 500°C. This operation step is necessary to convert the flame-resisting fibers into a carbon fiber structure having excellent performance characteristics. Carbon fibers produced without this step have many yarn defects such as voids and are inferior in performance characteristics. Then, the fibers are heat-treated in an inert atmosphere at a temperature of 500 to 800°C while being stretched at a stretch percentage of 0 to 10%. When the fibers subjected to such heat treatment under stretch are fed to the step of carbonization at a temperature of 1000°C or higher, carbon fibers having an elastic modulus of 254.98 GPa (26 ton/mm²) or more can be obtained without heat treatment at a high temperature of at least 2000°C, since the growth of graphite net planes is good. The following conditions are also preferable for the purpose of producing carbon fibers having a high elastic modulus. That is, the fibers subjected to low-temperature heat treatment as stated above are heat-treated in an inert atmosphere in a high-temperature heat treating furnace where the starting temperature of heat treatment is from 1000 to 1300°C, the maximum temperature of heat treatment from 1350 to 1900°C, the maximum temperature zone on the furnace exit side of the middle part of the furnace as shown by 5 and 6 in Fig. 3, and thus the gradient of temperature rise is low, so that the nitrogen content of the resulting carbon fibers will be from 0.5 to 5.0% by weight. In the carbonizing treatment step, temperature rise in the region of a rapid denitrifying reaction, which starts usually at about 1000°C, becomes steep when the starting temperature of heat-treating the fibers exceeds 1300°C. This results in a fiber structure abundant in void, making it difficult to produce carbon fibers having excellent performance characteristics. On the contrary, it is not much effective to lower the starting temperature of heat treatment than 1000°C since substantial carbonizing reaction has not yet occurred. In this high-temperature heat treatment step, the maximum temperature of heat treatment is from 1350 to 1900°C, preferably from 1450 to 1850°C. When the maximum temperature is below 1350°C, an elastic modulus of 254.98 to 323.63 GPa (26 to 33 ton/mm²) or more cannot be provided to the resulting carbon fibers. On the other hand, when this temperature exceeds 1900°C, the tensile strength of the resulting carbon fibers decreases to a large extent below 3.923 GPa (400 kg/mm²). When the maximum temperature zone in the high-temperature heat treating furnace is positioned on the fiber entrance side of the middle part of the furnace, the gradient of temperature rise from the starting temperature to the maximum is extremely high as shown by 7 in Fig. 3. Hence, an excessive amount of gas evolves during this temperature rise and the fiber structure is set in a state wherein numerous micro-voids are formed. Therefore, no high-strength and high-elasticity carbon fibers can be produced in this case. When a step such that the gradient of temperature rise is high, for example, as shown by 8 in Fig. 3, is involved between the initiation of high-temperature fiber treatment and the maximum temperature arrival, an excessive gas evolution is caused and high-performance carbon fibers also cannot be obtained. In the present invention, in contrast to this, a low gradient of temperature rise is applied as shown by 5 or 6 in Fig. 3. Therefore, not so much gas is evolved along with the growth of carbon net planes, unusual void formation does not take place in the course of raising the fiber temperature, and the action of restoring from voids is added. Thus, high-performance carbon fibers can he produced. In the invention, it is desirable to control the temperature in the high-temperature heat treatment step so that the nitrogen content of the resulting carbon fibers may be in the range of 0.5 to 5.0% by weight. Such high-temperature treatment in this step as to give a less nitrogen content than 0.5% by weight may lower the strength of the resulting carbon fibers. On the other hand, such high-temperature treatment as to give a nitrogen content exceeding 5.0% by weight makes it difficult to grow the structure sufficiently in carbon fibers. EFFECT OF THE INVENTION According to the invention, it is possible to produce effectively high-performance carbon fibers of at least 4.413 GPa (450 kg/mm²) tensile strength and at least 254.98 GPa (26 ton/mm²) elastic modulus which are free of yarn defects and have a highly oriented graphite crystal structure, because the fibers subjected to flame-resisting treatment which have a frame-resisting degree uniform from the outer side of the filaments to the inner side of the filaments in each tow of the fibers as well as uniform in the filament axial direction are treated to be carbonized under specific conditions. Having high elasticity and high strength, carbon fibers obtained according to the invention can be used for a wide variety of applications; those as primary construction materials for aircraft; sporting goods such as fishing rods and golf shafts; industrial applications to high-speed centrifuges, robots, etc., high-speed land transporting vehicle applications; and so forth. EXAMPLES The following examples illustrate the present invention. In the examples, the strength and elastic modulus of strands were measured in accordance with the method of JIS R7601. The density was measured by the density gradient tube method. Example 1 The range of fiber density after each of the following flame-resisting treatment stages was calculated by using equation (1). That is, tows each consisting of 12,000 acrylic monofilaments of 1.18 g/ml in density and 0.144 tex (1.3 d) in size are subjected to flame-resisting treatment for a treating period of 30 minutes by using a hot-air circulating type of flame-resisting furnace which has 5 different temperature stages, the 1st to 4th stages being each 8 m long and the 5th stage being 5.3 m long, so that the fiber density after completion of the flame-resisting treatment may become 1.36 g/ml. The calculated density ranges were as shown in Table 1. Then, the respective treatment temperatures necessary to attain the above calculated ranges of fiber densities were read out from curves previously drawn by plotting fiber density vs. flame-resisting treatment period at various given temperatures. The determined temperature conditions are shown in Table 1. Under these temperature conditions, 50 said acrylic tows arranged at suitable intervals were subjected to 30 minutes' flame-resisting treatment while being stretched by substantially 10% at a feed speed of 67.8 m/hr and a take-off speed of 74.6 m/hr. This flame-resisting treatment was operated continuously for 24 hours, during which no inflammation due to an incontrollable run of reaction took place, and the flame-resisting tows obtained were free of fusion and nap, thus being satisfactory. After operation for 24 hours, fibers resulting from each stage of treatment were sampled and the density thereof was measured by using density gradient tubes. The found densities of fibers from all the stages were in the respective ranges of calculated densities as shown in Table 1. Tows treated for flame-resisting were then carbonized in an atmosphere of nitrogen by passing them continuously through a precarbonization furnace at 600°C and a carbonization furnace at 1400°C. In this case, the percentage of stretch in the precarbonization furnace was changed until nap developed, wherein nap did not develop at all up to 12% stretch and slight nap was observed on 14% stretch. Then, the carbonization was carried out while setting the percentage of stretch in the precarbonization furnace at 8%. The resulting carbon fibers showed very little napping and high performance characteristics such as a tensile strength of 4.707 GPa (480 kg/mm²) and an elastic modulus of 235.37 GPa 24 ton/mm²). Comparative Example 1 Flame-resisting treatment was carried out according to the procedure of Example 1 but changing temperature conditions as shown in Table 2. The flame-resisting treatment was stable without causing napping or fusion. Then, carbonizing treatment was conducted according to the procedure of Example 1, but napping occurred frequently in the precarbonization furnace and the stretch could not be performed at all. Therefore the carbonizing treatment was tried without stretch, but napping took place frequently in the carbonization furnace and the resulting carbon fibers were unworthy of evaluation. The density of fibers from each stage of flame-resisting treatment was also measured in the same manner as in Example 1. As shown in Table 2, the result was that the found densities of fibers from the 1st through 3rd stages departed from the respective ranges of calculated densities shown in Table 1. Comparative Example 2 In the same manner as in Example 1, flame-resisting treatment temperatures were determined which satisfy equation (1) when the treatment is conducted for 30 minutes so that the fiber density after completion of the treatment may be 1.36 g/ml, as in Example 1 but using only the 1st and 2nd stages. The calculated temperatures of the 1st and 2nd stages were 245°C and 265°C, respectively. Flame-resisting treatment was tried at these temperatures for a treatment period of 30 minutes at a take-off speed of 74.6 m/hr, but the treatment was infeasible as tow break was caused in the 2nd stage by an incontrollable run of reaction. Treatment stage No. Calculated density range (g/ml) Treatment temperature (°C) Found density (g/ml) 1 1.2086 - 1.2286 241 1.2254 2 1.2472 - 1.2672 245 1.2618 3 1.2858 - 1.3058 253 1.2978 4 1.3244 - 1.3444 261 1.3307 5 1.3500 - 1.3700 272 1.3546 Treatment stage No. Treatment temperature (°C) Found density (g/ml) 1 223 1.2020 2 228 1.2250 3 247 1.2638 4 264 1.3252 5 278 1.3617 Example 2 The range of fiber density after each of the following flame-resisting treatment stages was calculated by using equation (1). That is, tows each consisting of 12,000 acrylic monofilaments of 1.18 g/ml in density and 0.144 tex (1.3 d) in size are subjected to flame-resisting treatment for a treating period of 45 minutes by using a hot-air circulating type of flame-resisting furnace which has 5 different temperature stages, the 1st to 4th stages being each 8 m long and the 5th stage being 5.3 m long, so that the fiber density after completion of the flame-resisting treatment may become 1.36 g/ml. The calculated density ranges were as shown in Table 3. Then, the respective treatment temperatures necessary to attain the above calculated ranges of fiber densities were read out from curves previously drawn by plotting fiber density vs. flame-resisting treatment period at various given temperatures. The determined temperature conditions are shown in Table 3. Under these temperature conditions, 50 said acrylic tows arranged at suitable intervals were subjected to 45 minutes' flame-resisting treatment while being stretched by 20% in the 1st stage and by 8% in the 2nd stage at a take-off speed of 50 m/hr. This flame-resisting treatment was operated continuously for 24 hours, during which no inflammation due to an incontrollable run of reaction took place, and the flame-resisting tows obtained were free of fusion and nap, thus being satisfactory. After operation for 24 hours, fibers resulting from each stage of treatment were sampled and the density thereof was measured by using density gradient tubes. The found densities of fibers from all the stages were in the respective ranges of calculated densities as shown in Table 3. Tows treated for flame-resisting were then carbonized in an atmosphere of nitrogen by passing them continuously through a precarbonization furnace at a maximum temperature of 600°C and a carbonization furnace at a maximum temperature of 1500°C. In this case, the percentage of stretch in the 600°C carbonization furnace was changed until nap developed, wherein nap did not develop at all up to 20% stretch and slight nap was observed on 22% stretch. Then, the carbonization was carried out while setting the percentage of stretch in the 600°C carbonization furnace at 8% and then giving a shrinkage of 4% at 1600°C. The resulting carbon fibers showed very little napping and excellent performance characteristics such as a tensile strength of 5.247 GPa (535 kg/mm²) and an elastic modulus of 279.50 GPa (28.5 ton/mm²). Treatment stage No. Calculated density range (g/ml) Treatment temperature (°C) Found density (g/ml) 1st Stage 1.2086 - 1.2286 228 1.2233 2nd Stage 1.2472 - 1.2672 237 1.2654 3rd Stage 1.2858 - 1.3058 244 1.3007 4th Stage 1.3244 - 1.3444 252 1.3345 5th Stage 1.3500 - 1.3700 262 1.3604 Comparative Example 3 Flame-resisting treatment was conducted according to the procedure of Example 2 but the temperature conditions were changed as shown in Table 4. This flame-resisting treatment was stable without causing napping or fusion. Then carbonizing treatment was conducted in the same manner as in Example 1, but napping occurred frequently in the carbonization furnace of maximum temperature 600°C and the stretch could not be performed at all. Also the passage through the carbonization furnace at zero percentages of stretch caused napping frequently in the furnace and the resulting carbon fibers were unworthy of evaluation. The fiber density after each stage of flame-resisting treatment was measured according to the method of Example 2. As shown in Table 4, the results were that the densities of fibers from the 1st to 4th stages departed from the respective calculated density ranges shown in Table 3. Treatment stage No. Treatment temperature (°C) Found density (g/ml) 1st Stage 215 1.1993 2nd Stage 220 1.2184 3rd Stage 232 1.2500 4th Stage 255 1.3155 5th Stage 270 1.3648 Example 3 The treatment procedure of Example 2 was followed except that the fibers were 20% stretched in the 1st stage of flame-resisting treatment until the treated fiber density reached 1.22 g/ml and further 15% stretched in the 2nd stage until the fiber density reached 1.26 g/ml, thereby giving a total stretch of 38% in the flame-resisting treatment step. The obtained carbon fibers exhibited a tensile strength of 5.443 GPa (555 kg/mm²) and an elastic modulus of 286.36 GPa (29.2 ton/mm²). Comparative Example 4 The procedure of Example 2 was followed, but the fibers were 38% stretched in the 1st stage of flame-resisting treatment to a treated fiber density of 1.22 g/ml. This caused frequent napping and further break of tows in the stretch zone. Example 4 Multifilament tows each consisting of 12,000 filaments of 0.166 tex (1.5 d) in monofilament size were prepared from an acrylonitrile/methacrylic acid (98/2) copolymer by a dry-wet spinning process. These tows were subjected to flame-resisting treatment for a period of about 45 minutes in air having a temperature gradient of from 230 to 270°C while being stretched to a total stretch of 20%, giving flame-resisting fibers of 1.35 - 1.36 g/ml in density. The flame-resisting fibers were treated under 8% stretch in an inert atmosphere having a profile of temperature raised linearly from 300 to 500°C, then under 4% stretch in an inert atmosphere having a temperature profile with a maximum of 800°C, and in an inert atmosphere having a temperature profile with a maximum of 1600°C without stretch. Table 5 shows performance characteristics of the thus obtained carbon fibers and conditions of the experiments. No. Rate of raising temperature from 300 to 500°C (°C min) Treatment period at 400-800°C (min) Strand strength GPa(kg/mm²) Strand elastic modulus GPa(ton/mm²) 1 (Comparative) 20 0.3 5.276(538) 313.82(32.0) 2 50 0.3 5.355(546) 315.78(32.2) 3 100 0.3 5.285(539) 311.86(31.8) 4 200 0.3 5.099(520) 306.96(31.3) 5 300 0.3 4.884(498) 302.05(30.8) 6 (Comgparative) 450 0.3 4.639(473) 288.32(29.4) 7 200 0.7 5.168(527) 304.01(31.0) 8 200 1.0 4.962(506) 302.05(30.8) 9 (Comparative) 200 1.3 4.727(482) 298.13(30.4) 10 (Comparative) 200 1.9 4.599(469) 295.19(30.1) 11 (Comparative) 200 3.8 4.413(450) 290.28(29.6) Nos. 1 and 6 are comparative examples different in the rate of raising temperature in the range of 300 to 500°C and Nos. 9, 10, and 11 are comparative examples different in the treatment period at temperatures of 400 to 800°C. Example 5 Multifilament tows each consisting of 12,000 filaments of 0.166 tex (1.5 d) in monofilament size were prepared from a polymer of 0.25 specific viscosity [ηsp] constituted of 98 wt% acrylonitrile and 2 wt% of acrylic acid by a dry-wet spinning process. These tows were arranged in sheet form wherein multifilaments were in intimate contact one with another. These tows in sheet form were subjected to flame-resisting treatment by using a flame-resisting furnace having 5 zones which were maintained under an oxidizing atmosphere by forced circulation of air and were adjusted to temperatures of 232, 240, 248, 255, and 266°C, respectively. The treatment period was 8 minutes in each of the 1st to 4th zones and 5.3 minutes in the 5th zone, amounting to 37.3 minutes. In this way, the density of fibers passed through each zone satisfied the condition of equation (1) and the fiber density after completion of the flame-resisting treatment became 1.35 - 1.36 g/ml. The percentage of stretch was 15% in the 1st zone, 5% in the 2nd zone, and 0% in the other zones. The thus flame-resisted fibers were subjected to precarbonization treatment in two stages, one having a gradient of temperature raised from 300 to 500°C and the other having a temperature of 600°C, while being stretched as shown in the following table. Thereafter, the fibers were subjected to carbonizing treatment in an inert atmosphere having a gradient of temperature raised from 1300 to 1800°C while being shrinked by 4%. For comparison, carbon fibers were produced in the same manner except that the precarbonization was conducted in an inert atmosphere having a temperature gradient of from 300 to 700°C. Table 6 shows strand strengths and elastic moduli of the obtained carbon fibers. It can be seen from this table that great elastic modulus increasing effect is achieved by dividing the precarbonizing treatment into two stages and distributing the stretch between the two stages in particular when the amount of stretch is large. While napping was observed in the case of single-stage treatment when the percentage of stretch was 14%, it has been revealed that in the present invention, no napping is observed even when the total percentage of stretch in the precarbonizing treatment is 14%, and higher stretch can be achieved. Example 6 Acrylic tows each consisting of 12,000 filament of 1.18 g/ml in density and 0.144 (1.3 d) in monofilament size were subjected to flame-resisting treatment by using a hot-air circulating type of multistage flame-resisting furnace having 5 different temperature stages, the 1st to 4th stages being each 8 m long and the 5th stage being 5.3 m long, so that a total stretch of 20% might be achieved during a treatment period of 45 minutes and the fiber density might become 1.36 g/ml after completion of the flame-resisting treatment. Table 7 shows treatment temperatures preset in this case so that the fiber density after each stage of treatment might be in the density range calculated according to equation (1) and the fiber densities found under the above temperature conditions. It can be seen from Table 7 that the found densities after all the stages lie in the respective calculated density ranges. Successively, tows from the above flame-resisting treatment were treated under an atmosphere of nitrogen in a heat-treating furnace having a maximum temperature of 600°C and a temperature gradient of 200°C/min from 300 to 600°C, while being 8% stretched. Then, the tows were subjected to high-temperature treatment under the same atmosphere in a furnace of temperature profile (5 in Fig. 3) having a heat treatment starting temperature of 1200°C, a maximum treatment temperature of 1600°C and the maximum temperature zone on the fiber exit side of the middle part of the furnace. The resulting carbon fibers exhibited a tensile strength of 5.345 GPa (545 Kg/mm²) and an elastic modulus of 282.44 GPa (28.8 ton/mm²), being of such considerably high performance, and the nitrogen content thereof was 2.1%. Treatment stage No. Calculated density range (g/ml) Treatment temperature (°C) Found density (g/ml) 1st Stage 1.2086 - 1.2286 228 1.2235 2nd Stage 1.2472 - 1.2672 237 1.2660 3rd Stage 1.2858 - 1.3058 244 1.3024 4th Stage 1.3244 - 1.3444 252 1.3348 5th Stage 1.3500 - 1.3700 262 1.3598 Example 7 The treatment was conducted under the same conditions as applied in Example 6 except that the maximum heat treatment temperature in the high temperature carbonization was changed to 1350°C. The obtained carbon fibers exhibited a tensile strength of 5.540 GPa (565 kg/mm²), elastic modulus of 266.75 GPa (27.2 ton/mm²), and nitrogen content of 4.3%. Comparative Example 5 The treatment was conducted under the same conditions as applied in Example 6 but using a temperature profile (7 of Fig. 3) having the maximum temperature zone on the fiber entrance side of the middle part of the furnace in the high-temperature carbonizing treatment. The obtained carbon fibers exhibited a tensile strength of 4.393 GPa (448 kg/mm²) and an elastic modulus of 270.67 GPa (27.6 ton/mm²), which were much lower than those of carbon fibers obtained in Example 6. Comparative Example 6 The treatment was conducted under the same conditions as applied in Example 6 except that the heat treatment starting temperature in the high-temperature carbonizing treatment was changed to 1400°C (9 of Fig. 3). The obtained carbon fibers exhibited a tensile strength of 4.511 (460 kg/mm²) and an elastic modulus of 268.71 GPa (27.4 ton/mm²), which were much lower than those of carbon fibers obtained in Example 6.

Claims: A process for producing carbon fibers whereby acrylic fibers in bundle form containing at least 90% by weight of acrylonitrile are continuously subjected to a multistage flame-resisting treatment in an oxidizing atmosphere at temperatures of 200 to 350°C by using a plurality of flame-resisting furnaces different in treatment temperature characterized in that the number of flame-resistant treatment stages is at least 3 and the treatment is carried out under such conditions that the fiber density ρ n after each stage of flame-resisting treatment may be maintained on the level defined by the following equation (1) and so that the fiber density ρ k after completion of the flame-resisting treatment may be from 1.34 to 1.40g/ml, and successively the fibers are subjected to a carbonizing treatment in an inert atmosphere; wherein ρ n is the density (g/ml) of the fibers after the n-th treatment stage, ρ o is the density (g/ml) of the feedstock acrylic fibers, ρ k is the density of the fibers after completion of the flame-resisting treatment and is a value ranging from 1.34 to 1.40 g/ml, t n is the period of flame-resisting treatment at the n-th stage and k (≧3) is the number of flame-resisting treatment stages. The process for producing carbon fibers as set forth in claim 1, characterized in that the overall period of flame resisting treatment is at least 20 minutes and less than 90 minutes. The process for producing carbon fibers as set forth in claim 2, characterized in that the overall period of flame-resisting treatment is at least 20 minutes and not more than 60 minutes. The process for producing carbon fibers as set forth in claim 1, characterized in that the fibers are subjected to flame-resisting treatment while being stretched under controlling the percentage of stretch to 30% or less until the density of the fibers reaches 1.22 g/ml, then the fibers are stretched so that the total percentage of stretch may not exceed 50 % until the density of the fibers reaches 1.26 g/ml, and thereafter the flame-resisting treatment is conducted while inhibiting the shrinkage of the fibers so that the fiber density after completion of the flame-resisting treatment may be from 1.34 to 1.40 g/ml. The process for producing carbon fibers asset forth in claim 1, characterized in that the fibers treated for flame-resisting are subjected to precarbonizing treatment in an inert atmosphere under the conditions of heat treatment starting temperature 300 ± 50°C, final heat treatment temperature 450 ± 50°C, and rate of temperature raise 50 - 300°C/min, and then are heat-treated in an inert atmosphere within the temperature range of 400 to 800°C. The process for producing carbon fibers asset forth in claim 5, characterized in that the treatment period in the temperature range of 400 to 800°C is up to 3 minutes. The process for producing carbon fibers asset forth in claim 6, characterized in that the treatment period is from 0.1 to 1 minute. The process for producing carbon fibers as set forth in claim 1, characterized in that the fibers treated for flame-resisting are treated under tension in an inert atmosphere at temperatures of 300 to 500°C, then heat-treated in an inert atmosphere at temperatures of 500 to 800°C while being stretched at a percentage of stretch of 0 to 10%, and are subjected to carbonizing treatment at temperatures of 1300 to 1800°C. The process for producing carbon fibers as set forth in claim 1, characterized in that the fibers treated for flame-resisting are heat-treated in an inert atmosphere by using a low-temperature heat treating furnace kept at temperature of 300 to 800°C and are then heat-treated in an inert atmosphere by using a high-temperature heat treating furnace in which the heat treatment starting temperature is from 1000 to 1300°C, the maximum heat treatment temperature is from 1350 to 1900°C, the maximum temperature zone is positioned on the furnace exit side of the middle part of the furnace, and the gradient of rising temperature in the range from the heat treatment starting temperature to the maximum heat treatment temperature has a gentle slope.

Inventors: IMAI, YOSHITAKA MITSUBISHI RAYON CO., LTD., KOBAYASHI, TOHA MITSUBISHI RAYON CO., LTD., NAKATANI, MUNETSUGU MITSUBISHI RAYON CO., LTD., SASAKI, SUSUMU MITSUBISHI RAYON CO., LTD., YAMAMOTO, NOBUYUKI MITSUBISHI RAYON CO., LTD.
Applicants: MITSUBISHI RAYON CO., LTD.
Application year: 1986
Main classification: D01F9
IPC codes: D01F9