A precursor fibre bundle for production of a carbon fibre bundle, a carbon fibre bundle, and a process for producing thereof

Abstract: This invention relates to a precursor fiber bundle for production of a carbon fiber bundle, a process for producing the precursor fiber bundle, a carbon fiber bundle produced with the precursor fiber bundle, and a process for producing the carbon fiber bundle. A percursor fiber bundle according to the invention has a fineness being in the range of from 300,000 denier to 1,500,000 denier and has a potential dividability to sub-tows each of which has a fineness being in the range of from 50,000 denier to 250,000 denier. The precursor fiber bundle is obtained by dividing a multifilament spun from a spinnerette into a plurality of sub-tows, drawing the sub-tows in this state, and after that converging the sub-tows into one tow with overlapping adjacent sub-tows at a side edge portion thereof. The converging is practiced typically by passing the overlapped sub-tows through a crimping apparatus. A carbon fiber bundle according to the invention is produce by dividing the precursor fiber bundle into the sub-tows and after that treating the sub-tows by passing thereof through stabilizing and carbonizing processes.

Description: Background of the Invention The present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, a process for producing it, a carbon fiber bundle, and a process for producing it. In more detail, the present invention relates to a precursor fiber bundle to be processed into a carbon fiber bundle, which is low in production cost, excellent in productivity, and less in the occurrences of fiber breakage and fuzz, and can be transformed into an optimum fiber bundle style when it is supplied to a process for producing a carbon fiber bundle, and also relates to a process for producing thereof, a carbon fiber bundle obtained by using the precursor fiber bundle, and a process for producing the carbon fiber bundle. Furthermore, the present invention relates to a precursor fiber bundle made of an acrylic polymer to be processed into a carbon fiber bundle, a process for producing thereof, a carbon fiber bundle obtained by using the precursor fiber bundle, and a process for producing the carbon fiber bundle. As a conventional precursor fiber bundle made of an acrylic polymer to be processed into a carbon fiber bundle, a fiber bundle having the number of filaments of from 3,000 to 20,000 or having a fineness of from 1,000 deniers to 24,000 deniers which has less occurrences of fiber breakage and fuzz and excellent quality has been used for production of a carbon fiber bundle having a high strength and high modulus. The precursor fiber bundle made of an acrylic polymer to be processed into a carbon fiber bundle is processed into a carbon fiber bundle which has been widely used as reinforcing fibers for members and implements in the fields of aerospace, sports, etc. On the conventional carbon fiber bundle, it has been mainly examined to enhance the strength and elastic modulus of carbon fibers. Specific examination items include the degree of crystallite orientation and densifying property of the precursor fibers, single filament breakage, fuzz, adhesion between filaments, acceleration of stabilization of the precursor fibers, etc. Recently, the utilization of carbon fibers is being expanded at a rapid pace into general industrial fields of automobiles, civil engineering, architecture, energy, compounds, etc., and it is demanded to supply a raw fiber bundle (precursor fiber bundle) to be processed into a carbon fiber bundle as a multifilament higher in strength and elastic modulus at lower cost at higher productivity. However, the raw fiber bundle (precursor fiber bundle) to be processed into a carbon fiber bundle is actually produced as a multifilament and wound on a drum or bobbin, and supplied in this style to a process for producing a carbon fiber bundle. So, due to the restriction in the process for producing the carbon fiber bundle, particularly the restriction in the thickness (fineness) of the raw fiber bundle (precursor fiber bundle) in the stabilizing process, the productivity is remarkably kept low. That is, the precursor fiber bundle made of an acrylic polymer to be processed into a carbon fiber bundle are heated in an oxidizing atmosphere having a temperature of from 200°C to 350 °C for stabilizing treatment prior to carbonizing treatment. The stabilization is a treatment causing oxidization and cyclization, but since it generates heat, the heat stored in the fiber bundle comes into question. If the heat stored in the fiber bundle is excessive, fiber breakage and adhesion between filaments are caused. So, the stored heat must be kept lower than a certain level. Therefore, a precursor fiber bundle having too large thickness, i.e., total fineness cannot be supplied into the stabilizing furnace, and in industrial production, the precursor fiber bundle made of an acrylic polymer to be processed into a carbon fiber bundle is restricted in thickness (fineness). The restriction is a cause to keep the productivity low in the production process of the precursor fiber bundle to be processed into the carbon fiber bundle, hence an obstacle in reducing the production cost of the carbon fiber bundle. As a process for producing a thermoplastic synthetic fiber bundle as a raw fiber bundle to be processed into a spun yarn or a non-woven fabric, not as a precursor fiber bundle to be processed into a carbon fiber bundle, a process for producing a dividable crimped tow is disclosed in Japanese Patent Laid-Open (Kokai) No. 56-4724. In this process, a tow running into a crimping apparatus is divided by dividing pins provided at a position being close to the entrance of the crimping apparatus, and the plurality of divided sub-tows are simultaneously supplied into the crimping apparatus, so that the plurality of sub-tows may be crimped as a whole, to be collected as one tow being capable of being potentially divided into sub-tows later. However, if this process is applied to a production of a precursor fiber bundle to be processed into a carbon fiber bundle, fiber breakage occurs often to lower the grade, since it is need to divide a precursor fiber bundle having a fineness of not less than 300,000 deniers in which filaments are engaged with each other by mutual oblique crossing and are closed up each other, into a plurality of sub-tows, and this also adversely affects the production of carbon fibers. Summary of the Invention The object of the present invention is to provide a precursor fiber bundle to be processed into a carbon fiber bundle which can be larger in thickness, i.e., in fineness to allow a high productivity and a lower production cost to be achieved when the precursor fiber bundle to be processed into a carbon fiber bundle is produced, and which can be easily divided into sub-tows each of which has a thickness (fineness) required in a process for producing the carbon fiber bundle, considering the restriction in the thickness (fineness) of a fiber bundle in the process for producing the carbon fiber bundle. The object of the present invention also includes to provide a process for producing the precursor fiber bundle, a carbon fiber bundle obtained by using the precursor fiber bundle to be processed into a carbon fiber bundle, and a process for producing the carbon fiber bundle. Hereinafter in this specification, a precursor fiber bundle means a precursor fiber bundle to be processed into a carbon fiber bundle or a precursor fiber bundle for production of a carbon fiber bundle. The precursor fiber bundle the present invention developed for achieving the above object is a precursor fiber bundle which can keep the form of one tow when packed in a container and potentially can be divided in the crosswise direction into a plurality of sub-tows when taken out of the container to be used for producing a carbon fiber bundle. The precursor fiber bundle of the present invention is an acrylic polymer fiber tow having the total fineness of from not less than 300,000 deniers to not more than 1,500,000 deniers or preferably having the number of filaments of from not less than 50,000 to not more than 1,000,000, which can be potentially divided into sub-tows each of which has a fineness of from 50,000 deniers to 250,000 deniers. The precursor fiber bundle may also be a crimped tow or non-crimped tow. In the case of a non-crimped tow, a moisture content is preferably in the range of from 10 % to 50 %. Furthermore, the degree of entanglement of each of the sub-tows divided from the precursor fiber bundle is preferably in the range of from not less than 10 m -1 to not more than 40 m -1 , measured according to the hook drop testing method. Where the degrees of entanglement are in that range, the precursor fiber bundle e.g. the original tow can be easily divided into a plurality of each of which is used for producing a carbon fiber bundle. The process for producing a precursor fiber bundle having the above properties in the present invention comprises the steps of dividing a fiber bundle consisting of a plurality of spun filaments into a plurality of sub-tows in such a way that each sub-tow may consist of a predetermined number of filaments; drawing the filaments with this state of division maintained; collecting the plurality of drawn sub-tows into one tow potentially capable of being divided into the plurality of sub-tows when used for producing a carbon fiber bundle; and packing it into a container. In this process, a plurality of groups each of which consists of the plurality of sub-tows may also be arranged to run in parallel each other. The process for producing a carbon fiber bundle of the present invention may also comprise the steps of dividing the precursor fiber bundle into a plurality of sub-tows; and being subjected to a stabilizing process and a carbonizing process. According to the present invention, a number of filaments taken up from a spinnerette are once divided into a plurality of sub-tows, and the respective sub-tows are collected into one tow capable of being potentially divided into the plurality of sub-tows later when used for producing a carbon fiber bundle, before they are packed in a container. The precursor fiber bundle formed as one tow is once packed in a container, since the production speed is greatly different the treatment speed of the following carbonizing process. In the carbon fiber production process, the precursor fiber bundle formed as one tow is taken out from the container and fed to the stabilizing process. In this case, it is divided into a plurality of sub-tows each of which has a predetermined thickness, before it is fed to the stabilizing process. Therefore, the problem of excessively stored heat as described before can be prevented from occurring, and carbon fibers with a desired high strength and high modulus can be produced efficiently. In the final stage of the process for producing the precursor fiber bundle, the filaments are formed as one fiber bundle having a large total fineness, but when the carbon fiber bundle are produced, it is divided into a plurality of sub-tows each of which has a fineness being suitable to stabilizing and carbonizing. So, the production of the precursor fiber bundle, and the production of the carbon fiber bundle can be carried out in efficient conditions. The precursor fiber bundle of the present invention is preferably made of the following acrylic polymer. Preferable acrylic polymer: An acrylic polymer containing acrylonitrile, one or more unsaturated monomers enumerated in the following group A and one or more unsaturated monomers enumerated in the following group B by amounts shown with the following equations (1), (2) and (3). Group A: One or more unsaturated monomers selected from a group consisting of vinyl acetate, methyl acrylate, methyl methacrylate and styrene. Group B: One or more unsaturated monomers selected from a group consisting of itaconic acid and acrylic acid. AN (wt%) ≧ 86 3 ≦ A (wt%) ≦ 10 0.25A - 0.5 ≦ B (wt%) ≦ 0.43A - 0.29 The symbols in the above formulae stand for the following: AN: Acrylonitrile content (wt%) in the acrylic polymer A : Content (wt%) of the unsaturated monomer selected from said group A in the acrylic polymer (total weight of unsaturated monomers when a plurality of unsaturated monomers are contained) B : Content (wt%) of the unsaturated monomer selected from said group B in the acrylic polymer (total weight of unsaturated monomers when a plurality of unsaturated monomers are contained) As shown by the formula (2), the weight percent (content) of the unsaturated monomer selected from said group A is in the range of from 3 wt% to 10 wt%. If the amount is less than 3 wt%, the filaments are slightly less likely to be stretched when drawn, and the tension in the stabilizing process is too high unpreferably. If more than 10 wt%, more filaments adhere to each other when stabilized, and carbonization at a lower temperature at a lower speed is required for preventing it, to raise the production cost unpreferably. Furthermore, as shown in the formula (3), the weight percent (content) B of the unsaturated monomer selected from said group B is in the range of from (0.25 x A - 0.5) wt% to (0.43 x A - 0.29) wt%. If the amount is less than the lower limit, the effect of accelerating the stabilization at the initial critical temperature of stabilization dominated by this component is not substantially observed, and if more than the upper limit, the effect of accelerating the stabilization is less efficient to raise the production cost unpreferably. The acrylic polymer may be produced by any publicly known polymerization method such as suspension polymerization, solution polymerization or emulsion polymerization, etc. The polymerization degree is preferably 1.0 or more as intrinsic viscosity ([η]). The upper limit of intrinsic viscosity ([η]) is desirably 3.0 or less since otherwise the production of the spinning dope itself is difficult and since otherwise the spinning stability is also remarkably lowered. The intrinsic viscosity in this case refers to the value measured at 25 °C with dimethylformamide as the solvent. The solution of the acrylic polymer, i.e., the spinning dope is spun using a coagulating bath of an organic solvent or water, into an acrylic polymer fiber bundle. The spinning method may be wet spinning in which a spinning dope is ejected from a spinnerette immersed in a coagulating bath, semi-wet spinning in which a spinning dope is ejected from a spinnerette installed above the liquid surface of a coagulating bath with a distance between them, into air or inactive gas and introduced into the coagulating bath, or melt spinning. In the spinning using a solvent and plasticizer, the spun filaments may be drawn in a bath immediately, or after having been washed with water to remove the solvent and plasticizer. The acrylic polymer fiber bundle obtained by any of these methods is drawn with a draw ratio being in the range of from 2 times to 8 times in a drawing bath having a temperature of from 50 °C to 98 °C. If the drawing ratio is too low, the densifying property cannot be obtained to leave voids, and the physical properties are likely to be low. If more than 8 times, the tension during carbonization increases to require a larger apparatus unpreferably. Drawing in a steam tube may be used with drawing in a bath, but in the case of drawing in a steam tube, it is preferable to keep the drawing ratio low for suppressing the orientation of fibers. However, drawing in a bath only is preferable. As for the number of filaments of the acrylic polymer fiber bundle, it is preferable to use a multifilament comprising the number of filaments of in the range of from 5 x 10 4 filaments to 10 x 10 5 filaments to enhance the production efficiency for cost reduction. Subsequently, the filaments are dried by a gentle air flow having a temperature being in the range of from 110°C to 180°C or a heating roller under tension or relaxation, and densified simultaneously. Prior to the drying and densifying, it is desirable to apply a proper oiling treatment to prevent the adhesion between filaments or to facilitate the handling of the dried and densified fiber bundle. The dried and densified fiber bundle is treated to be shrunken with a ratio of from 5 % to 18 %. The shrinking treatment is intended to shrink the filaments under proper tension using a heating roller or any other heating means such as hot air, and this is effective to decrease the tension acting on the fiber bundle in the subsequent stabilizing process. For decreasing the tension, a treatment of shrink having a ratio of from 5 % to 18 % is important. The heating temperature is in the range of from 80 °C to 120 °C, and as for the tension, it is preferable to maintain substantially at no tension, but some tension may act for the convenience of process if it can allow the above percentage of shrinkage to be achieved. The percentage of shrinkage may be controlled by combining the heat treatment temperature, residence time and proper tension. The fineness (d) of each of the filaments finally obtained is preferably in the range of from 1 denier to 2.0 deniers, more preferably of from 1.0 denier to 1.5 deniers, in view of higher productivity. The precursor fiber bundle obtained as described above may be processed into a carbon fiber bundle by any conventional method. The stabilizing conditions in this case may be as in the conventional methods. The fiber bundle is treated in an oxidizing atmosphere having a temperature being in the range of from 200 °C to 300 °C under tension or while being drawn. The shrinkage stress during stabilization of the fiber bundle made of an acrylic polymer has correlativity with the potential physical properties of the obtained carbon fiber bundle. When the raw fibers are higher in strength, that is, more highly oriented to be higher in shrinkage stress, the potential physical properties of the carbon fibers obtained are higher. However, for obtaining such a potential in physical properties, it is desirable to control the shrinkage of fibers or rather to apply a high tension treatment to the fibers by drawing. To manifest the physical properties of reinforcing carbon fibers for general industrial applications, the high tension treatment is not required so much, and the problem in commodity design is to produce carbon fibers high in cost performance which can substitute the conventional materials such as glass fibers, iron and aluminum in price. The present invention using the acrylic polymer has been developed to overcome the conventional limit in this situation. Conventionally, carbon fibers with a high tensile strength are generally produced by stabilizing precursor fibers with a high capability of shrinkage stress at a high tension, to produce oxidized fibers (stabilized fibers) with a high degree of crystallite orientation and a high tensile strength as an intermediate product. In such a high tension process, the occurrences of fuzz and breakage of fibers are likely to lower the grade and processability, and to prevent them, the production conditions and equipment conditions are variously designed. However, such approach usually raises the production cost of carbon fibers. On the contrary, according to the present invention, styrene, methyl acrylate or methyl methacrylate as a polymerizable unsaturated monomer is added to the acrylic polymer fibers, to manifest less shrinkage stress, thereby allowing the tension in the stabilizing process to be lowered. As a result, the tension.in the stabilizing process can be kept low, to allow the occurrences of fiber breakage and fuzz in the stabilizing process to be prevented. Furthermore, a carbon fiber bundle of 25,000 deniers or more in fineness, substantially having no twist, and of from 10 m -1 to 100 m -1 in the degree of entanglement measured according to the hook drop testing method can be obtained, and its physical properties being in the range of from 2.0 GPa to 5.0 GPa, preferably from 3.0 GPa to 4.5 GPa in tensile strength and in the range of from 200 GPa to 300 GPa in elastic modulus can be obtained. These carbon fibers may be used for general purpose. "Substantially no twist" means a state where the twisting count per 1 m is not more than 1 turn. It is preferable that the tension T in the stabilizing process satisfies the following formula (4). 30 ≦ T (mg/d) ≦ 120 More preferably, the tension T is in the range of from 60 mg/d to 100 mg/d. If the tension T is less than 30 mg/d, the tension is so low as to shrink the fibers, and to lower the degree of crystallite orientation, and the fibers obtained are low in tensile strength. If more than 120 mg/d, high physical properties can be obtained, but since the tension is too high, return rollers high in strength or return rollers large in diameter, etc. are required, to make the equipment heavy industrially undesirably. If return rollers large in diameter are installed for the stabilizing furnace, it is difficult to achieve a high frequency of return, making mass processing difficult. Also in view of this, it is not preferable to keep the tension excessive. In the present invention, since the tension T in the stabilizing process is kept in a low range of from 30 mg/d to 120 mg/d, the load per unit filaments acting on rollers is small, and an unprecedented consistent carbon fiber production process to allow mass processing can be established. Therefore, no excessive equipment is necessary, and general purpose carbon fibers can be produced by inexpensive equipment, advantageously in view of production cost reduction. As a result, carbon fibers may be used for applications where they could not be used because of high cost. The effect of cost reduction by low tension is further described below. Firstly, the cost reduction effect can be obtained through process stability. A lower tension is effective for decreasing the occurrences of fuzz and fiber breakage in the strand formed as an aggregate of many short fibers during processing, and hence very effective to decrease production troubles such as the seizure of filaments and the strand on the rollers caused by such occurrences. The amount of fuzz has good correlatively with the processability and the tension also has good correlativity with the amount of fuzz. The amount of fuzz is a good indicator for evaluating the processability. Secondly, the cost reduction effect can be obtained through the enhanced volume availability in the stabilizing furnace. In the carbon fiber production process, since a strand to be processed is continuously processed, a series of rollers are usually used. Since these rollers are deflected by the tension of the strand, the deflection which poses no problem in equipment or process stability is secured by design. In the case of a cylindrical roller uniform in diameter, the maximum deflection is proportional to the product of the tension and the 4th power of (roller length L/roller diameter D). Therefore, in general, if the tension is doubled, the deflection is doubled, and to lower the doubled deflection to the original deflection, the diameter must be increased to 1.2 times. Especially the diameter of a roller directly affects the volume availability of a stabilizing furnace, and if the diameter of a roller is smaller, the volume availability of a stabilizing furnace is higher, to enhance the carbon fiber productivity. Brief Description of the Drawings Fig. 1 is a schematic side view showing an apparatus for implementing the process for producing a precursor fiber bundle of the present invention. Fig. 2 is a plan view showing typically a portion of running state of the divided sub-tows in the coagulating bath in the spinning step in the apparatus shown in Fig. 1. Fig. 3 is a schematic side view showing an apparatus for implementing the process for producing carbon fibers of the present invention. Fig. 4 is a plan view showing typically a portion of running state of the sub-tows collected to one tow in the apparatus shown in Fig. 1. Description of the Preferred Embodiments The precursor fiber bundle of the present invention maintains the form of one tow when packed in a container, and potentially can be divided into two or more sub-tows when taken out of the container, to be supplied to the stabilizing process. The precursor fiber bundle is produced, for example, with a process for producing an acrylic precursor fiber bundle as shown in Fig. 1. In a spinning step 1, a plurality of filaments are spun from a spinnerette. The spinning method is not especially limited, and may be, for example, any known wet spinning in which many filaments spun from a spinnerette are coagulated in a coagulating bath. The plurality of the spun filaments are divided into a plurality of sub-tows each of which has a predetermined number of filaments. This division is carried out in the coagulating bath or at the outlet of the coagulating bath in the case of wet spinning. The division may be practiced by using a dividing bar. Fig. 1 does not illustrate the divided state since it is a side view. If the process is viewed from above, the divided state can be identified. Fig. 2 is a plan view showing typically a portion of running state of the divided sub-tows in the coagulating bath in the spinning step in the apparatus shown in Fig. 1. In Fig. 2, it is shown that the spun multifilament is divided into the plurality of sub-tows 2, 2 by the dividing bar 18 comprising a pole having a elliptic cross section and they run in the direction shown with arrow 19, 19. The sub-tow group 2 comprising plurality of sub-tows divided from the spun multifilament is fed to a filament drawing step 3 and a finish oiling step 4 in the divided state. In this example, the sub-tow group 8 delivered from the oiling step 4 are fed to a crimping step 5 equipped with a crimper, and the sub-tow group 8 are crimped, so that each of sub-tows in the sub-tow group 8 is collected into the form of one tow 9. This convergence of sub-tows are formed by weak entanglement of filaments located in the side edge portion of each of adjacent sub-tows due to the crimping. The entanglement along with the length direction of the filaments at the side edge portions of the adjacent sub-tows is weak. Therefore, the fiber bundle formed as one tow 9 can be re-divided into the sub-tows forming the sub-tow group 8 at the side edge portions of the sub-tows at supplying them to a process for producing a carbon fiber bundle. That is, the precursor fiber bundle 10 having the form of one tow delivered from a drying step 6 subsequent to the crimping step 5 has potential dividability into a plurality of sub-tows in the crosswise direction of the precursor fiber bundle 10. The precursor fiber bundle 10 formed like this is packed in a can 12 (see Fig. 3) in a packing step 7. In the process for producing the precursor fiber bundle shown in Fig. 1, it is also possible to divide a spun multifilament into a plurality of groups 8 each of which comprises a plurality of sub-tows for preparing a plurality of precursor fiber bundles 9 in parallel each of which is dividable into a plurality of sub-tows in desired number. Furthermore, as the container for packing the precursor fiber bundle 10, a bale may also be used instead of a can. The precursor fiber bundle 11 produced through the above respective steps is sent to a carbon fiber production process, as packed in a can 12. The reason why it is once packed in a container is that the process for producing the precursor fiber bundle is greatly different in fiber processing speed from the process for producing carbon fibers. A carbon fiber bundle can be produced, for example, according to the process shown in Fig. 3. In the process for producing a carbon fiber bundle shown in Fig. 3, the precursor fiber bundle 11 is supplied as packed in the can 12. Where processing simultaneously a plurality of the precursor fiber bundles 11, cans as many as necessary are prepared. The precursor fiber bundle 11 taken out from the can 12 is divided into sub-tows in a dividing step 13 upstream of a stabilizing furnace 14. The division can be practiced by using, for example, a grooved roll or dividing bar. Since the sub-tows are collected or converged with weak entanglement of filaments placed at the side edge portion of the sub-tows along with its lengthwise, the division can be practiced very easily. In the division step little fuzz and fiber breakage occur. Each divided sub-tow is treated to be stabilized in a stabilizing step 14. The stabilization is effected by heat treatment in an oxidizing atmosphere having a temperature being in the range of from 200°C to 350°C in the stabilizing furnace 14. Since each of sub-tows having a predetermined size is treated to be stabilized, excessive heat storage does not occur, and the fiber breakage and the adhesion between filaments in the stabilizing treatment can be prevented. The stabilized sub-tows are then fed to a carbonizing step 15 and further, as required, to a surface treatment step 16 such as sizing step, to be formed as a carbon fiber bundle, and it is wound in a winding step 17. Since the stabilizing treatment is effected against sub-tows each of which has a proper thickness, the carbon fibers obtained are excellent in strength and elastic modulus. It is preferable that the precursor fiber bundle has a total fineness of from 300,000 deniers to 1,500,000 deniers, more preferably from 400,000 deniers to 1,200,000 deniers, and it is preferable that each of the sub-tows finally obtained from the precursor fiber bundle having potential dividability has a fineness of from 50,000 deniers to 250,000 deniers, more preferably from 80,000 deniers to 150,000 deniers. If the precursor fiber bundle has a fineness of less than 300,000 deniers, the degree of entanglement between filaments is likely to be less than 10 m -1 , and the property of entanglement of the filaments is little and it causes deformation of tow, and where such tow is supplied into a stabilizing step and stabilized, irregular tension occurs due to dislocation between filaments, to cause fiber breakage. If more than 1,500,000 deniers, the adhesion between filaments becomes strong, to increase drawing nonuniformity and fiber breakage, thus lowering the productivity in filament drawing and carbonization. If fineness of each of the divided sub-tows is less than 50,000 deniers, the productivity in the carbonizing step is too low, and if more than 250,000 deniers, irregular carbonization occurs to lower the grade. If the precursor fiber bundle is crimped, the adhesion between filaments is likely to be dissolved and the strength of carbon fibers is likely to be,manifested. A desirable number of crimps is in the range of from 8 peaks per 25 mm to 13 peaks per 25 mm, preferably from 10 peaks per 25 mm to 12 peaks per 25 mm. If it is less than 8 peaks per 25 mm, the adhesion between filaments is likely to persist, and the strength of carbon fibers is unlikely to be manifested. If more than 13 peaks per 25 mm, the filaments are buckled to lower the strength. The number of crimp is obtained as a mean value of 20 measuring samples each of which number of crimp is measured as follows. A single filament as a measuring sample is taken out from a precursor fiber bundle and is weighted 2 mg/d. Number of peaks of the weighted sample is counted at predetermined length taking along the straight lengthwise direction of the sample and the resultant is turned into at a length of 25 mm. The precursor fiber bundle in the present invention can also be a non-crimped tow ( a straight tow having substantially no crimp). In the case of the non-crimped tow, since the degree of entanglement of filaments is too small, it is desirable to let the filaments contain moisture for enhancing the collectability. The moisture content in this case is desirably in the range of from 10 % to 50 %. If less than 10 %, collectability is too low, and if more than 50 %, the packing rate may become too low. The moisture content is obtained by the resultant of equation of (10 - B) x 100/B. The B is a weight obtained by the following measurement. A tow of 10 g as a measuring sample taken out from a precursor fiber bundle is dried by a hot-air dryer for 2 hours at 105°C and after that the sample is left in a desiccator having a drying agent therein for 10 minutes and then a weight of the sample is measured. The obtained value of the weight is used as the B in the above. In the process for producing a precursor fiber bundle, after spinning a polymer solution through a spinnerette for forming a multifilament and coagulating the spun multifilament, the multifilament can be divided as desired. It is preferable that the dividing bar used in this case does not allow any frictional force to act on the tow, not to damage the tow as much as possible, but the dividing bar is not especially limited in material or form. However, the width of the dividing portion of the bar is important. It is preferable that the dividing portion has such a width as to ensure that the side edge portions of adjacent divided sub-tows are overlapped each other with about 1 mm when they are finally collected as a tow, if the tow is non-crimped. This also applies to a crimped tow, and it is preferable that the guide width ensures that the side edge portions of the adjacent sub-tows are engaged with each other by about 1 mm before they are crimped. If such a divided state cannot be ensured by the division in the coagulating step only, further dividing operation may be added in another step, to make the side edge portions of the adjacent sub-tows engage with each other by about 1 mm, before they are crimped. The cross section of the dividing bar is preferably formed to be ellipsoidal or rhombic, etc. and as small as possible in contract area, for ensuring that the filaments constituting the tow is less rubbed or damaged by the bar. Especially in the case of a bar having an ellipsoidal cross section, it is preferable to place the bar of which major axis and the running direction of tow make substantially right angle. Such a state is shown in Fig. 2 with a dividing bar 18. And Fig. 4 is a plan view showing typically the state of overlapping. In Fig. 4, the portion of the overlapping is shown with the mark OL. For example, when a tow is divided into sub-tows each of which has a fineness of 50,000 deniers or more, the running space, which is shown with the mark D in Fig. 2, between adjacent sub-tows divided in the drawing step is preferably in the range of from 1.5 cm to 2 cm. If less than 1.5 cm, the adjacent divided sub-tows are engaged too intensively with each other at the side edge portions and it causes increase of fiber breakage and fuzz when the tow is re-divided in the stabilizing step, and it causes troubles in the carbonizing step or lowering a grade of the carbon fiber bundle. If more than 2 cm, the sub-tows are less engaged with each other at the side edge portions, and the sub-tows are taken up irregularly in a step of forming the non-crimped tow or in a step of forming the crimped tow, and it causes dislocation of filaments in the longitudinal direction. Furthermore, the tow itself is deformed. Examples 1 to 10, and Comparative Example 1 A dimethyl sulfoxide (DMSO) solution of an acrylic polymer consisting of acrylonitrile (AN)/methyl acrylate (MEA)/sodium methacrylsulfonate (SMAS)/itaconic acid (IA) = 93.5/5.5/0.5/0.5 (by weight) was introduced into 60% DMSO aqueous solution of 30 °C, and a fiber bundle of 400,000 deniers was wet-spun, and divided into four sub-tows each of which has a fineness of 100,000 deniers at the outlet of the coagulating bath. In this process, an elliptical dividing bar 18 (see Fig. 2) having a length of the major axis (LMA) of 1.5 cm was used in Example 1, a length of the major axis of 1 cm was used in Example 2, and a length of the major axis of 2.5 cm was used in Example 3. They were drawn, washed with water, oiled, and crimped with a conventional stuffing box type crimper. In Comparative Example 1, the fiber bundle was not divided in the coagulating step and divided only just before it was crimped. Non-crimped sub-tows obtained after washing with water in Example 1 were treated with finish-oil to adjust their moisture content of 2.5 %, 40 % and 60 % respectively in Examples 4, 5 and 6. A fiber bundle of 270,000 deniers was wet-spun and divided into three sub-tows each of which has a fineness of 90,000 deniers at the outlet of the coagulating bath. In this process, as Example 7 an elliptical dividing bar 18 (see Fig. 2) having a length of the major axis of 1.5 cm was used. A fiber bundle of 400,000 deniers was wet-spun and divided into 10 sub-tows each of which has a fineness of 40,000 deniers at the outlet of the coagulating bath. In this process, as Example 8 an elliptical dividing bar 18 (see Fig. 2) having a length of the major axis of 1.5 cm was used. A fiber bundle of 1,600,000 deniers was wet-spun and divided into 16 sub-tows each of which has a fineness of 100,000 deniers at the outlet of the coagulating bath. In this process, as Example 9 an elliptical dividing bar 18 (see Fig. 2) having a length of the major axis of 1.5 cm was used. A fiber bundle of 1,600,000 deniers was wet-spun and divided into 40 sub-tows each of which has a fineness of 40,000 deniers at the outlet of the coagulating bath. In this process, as Example 10 an elliptical dividing bar 18 (see Fig. 2) having a length of the major axis of 1.5 cm was used. In Examples 7-10, the sub-tows were respectively drawn, washed with water, oiled, crimped and dried. Sample having a length of 5,000 m was taken in each of Examples 1-10 and Comparative Example 1 for evaluating a dividability, the degree of entanglement and an adhesion thereof. The results are shown in Table 1. The methods for evaluating the respective properties in the examples were as described below. (i) Dividability: For evaluating the dividability, a crimped tow was taken by 5000 m, and divided manually from end to end. A sample which was poor in dividability and had to be divided forcibly by scissors, etc. was evaluated as "Δ"; a sample which could not be divided due to fiber breakage or defective division, "x"; and a sample which could be simply manually divided over the entire length, "○". (ii) Degree of entanglement of a precursor fiber bundle, measured according to the hook drop testing method: A precursor fiber bundle (tow) is hanged on a horizontal setting bar with a fineness of 20,000 deniers/cm and fixed at the upper end portion of the bundle on the bar with an adhesive tape. On the lower end portion, a weighing bar of 20 g/10,000 deniers was fixed with an adhesive tape. A wire having a diameter of 1 mm and its tip portion having a length of 2 cm bent at right angle and fixed a weight of 100 g at its lower end is prepared. The wire is hooked on the hanged bundle with the bent tip portion and let the wire fall in downward freely. A falling distance X (in meter) of the wire is measured.. Such falling distance X (in meter) is measured at 20 different positions with substantially equal interval along the width of the hanged bundle. The mean value (Xm) of the 20 measuring data (X) is calculated. The degree (CFP) (in 1/m = m -1 ) of entanglement of a precursor fiber bundle is obtained by the following formula. Degree of entanglement (CFP) = 1/Xm (iii) Adhesion: A volume of filaments having a length of 5 mm which is obtained by cutting a precursor fiber bundle is prepared as a measuring sample so that the volume is equal to about 10,000 filaments in a precursor bundle (where a fineness of single filament is 1.5 denier, the volume becomes 0.0084 g). A rotor and 100 ml of 0.1% Noigen SS were put in a beaker, and the sample was added. They were stirred by a magnetic stirrer for 1 minute, and the mixture was suction-filtered using black filter paper, to visually judge the dispersibility of fibers in reference to six grades. The 1st grade is the best in adhesion and the 6th grade, the worst. As described above, according to the present invention, a precursor fiber bundle can maintain the form of one tow when packed in a container, and can be easily divided in crosswise direction into sub-tows each of which has a desired fineness when used for producing carbon fibers (when supplied to the stabilizing step). So, a thick (large in fineness) precursor fiber bundle can be produced at a very high productivity, and in the carbon fiber production process, it can be divided into sub-tows each of which has a predetermined thickness to allow stable stabilizing treatment. Therefore, both the productivity improvement of the precursor fiber bundle and the stable production of carbon fibers having excellent properties can be simultaneously achieved which contribute to the reduction of cost for producing carbon fibers. Examples 11 to 13 and Comparative Examples 2 to 6 Example 11 Ninety two point three weight percent of acrylonitrile, 6.3 wt% of methyl acrylate and 1.4 wt% of itaconic acid were polymerized in nitrogen gas atmosphere at 60 °C for 11 hours and furthermore at 73°C for 9 hours by the solution polymerization with dimethyl sulfoxide as the solvent. The polymer solution obtained as a spinning dope was 22.5% in concentration and 240 cps in viscosity. It was extruded from a spinnerette with 70,000 holes of 0.055 mm in diameter into 55 % dimethyl sulfoxide aqueous solution of 40°C, to be coagulated. The fiber bundle obtained here was drawn to 5 times in hot water while being washed, subsequently oiled, dried and densified by a drying drum, and treated to be shrunken by 15 % in 113 °C air, to obtain a precursor fiber bundle, made of an acrylic polymer and of 1.5 d in filament fineness. Then, it was treated to be stabilized in air at 210 °C to 250 °C, and heated up to 1,400°C in nitrogen atmosphere, to obtain carbon fibers. In succession, they were electrolyzed at 10 coulombs/g with a sulfuric acid aqueous solution of 0.1 mole/liter in concentration as the electrolyte, washed with water and dried in 150 °C air. The carbon fibers obtained here were impregnated with an epoxy resin according to the method specified in JIS R 7601, to measure the tensile strength and elastic modulus of the strand by a tensile tester. The conditions in this case and the physical properties of the obtained carbon fibers are shown in Tables 2a and 2b. It can be seen that even if the tension during stabilization is low, the physical properties of carbon fibers are satisfactory. Example 12 Carbon fibers were obtained as described in Example 11, except that 96.1 wt% of acrylonitrile, 3.2 wt% of methyl acrylate and 0.7 wt% of itaconic acid were polymerized, and that the shrinkage percentage was 7 %. The conditions in this case and the physical properties of the obtained carbon fibers are shown in Tables 2a and 2b. Example 13 Carbon fibers were obtained as described in Example 11, except that 86 wt% of acrylonitrile, 10 wt% of methyl acrylate and 4 wt% of itaconic acid were polymerized, and that the shrinkage percentage was 18 %. The conditions in this case and the physical properties of the obtained carbon fibers are shown in Tables 2a and 2b. Comparative Examples 2 and 3 Carbon fibers were obtained as described in Example 11, except that 99.3 wt% of acrylonitrile and 0.7 wt% of itaconic acid were polymerized, and that the shrinkage percentage was 5%. The conditions in this case and the physical properties of the obtained carbon fibers are shown in Tables 2a and 2b. Since the monomer as the second component (group A) was not contained, the physical properties of carbon fibers were low when the tension during stabilization was low. Comparative Example 4 Carbon fibers were obtained as described in Example 11, except that the fiber bundle was drawn in a bath and in steam by 12 times in total. The conditions in this case and the physical properties of the obtained carbon fibers are shown in Tables 2a and 2b. Comparative Example 5 Carbon fibers were obtained and evaluated as described in Example 12, except that the drawn fiber bundle was not treated to be shrunken. The results are shown in Tables 2a and 2b. Comparative Example 6 Carbon fibers were obtained as described in Example 12, except that the drawn fiber bundle was treated to be shrunken by 2 %. The results are shown in Tables 2a and 2b. The methods for evaluating the properties in the examples were as described below. (iv) Number of fuzz: From a precursor fiber bundle, ten 1 m long samples were taken. From each of the samples, a fiber bundle consisting of from 1,000 filaments to 2,000 filaments was divided and taken, and the number of fuzz in a length range of 0.5 m at the center was counted on an illuminated cloth inspection table. The mean value of 10 samples was calculated in numbers/m 10K (number of fuzz existing in 10,000 filaments of 1 m in length), and the value was adopted as the number of fuzz. The numbers of fuzz of the precursor fiber bundles made of an acrylic polymer used in Examples 11 to 13 were 8 to 9 numbers/m 10K. (v) Degree of entanglement of carbon fiber bundle measured according to the hook drop testing method: A carbon fiber bundle is hanged on a horizontal setting bar and fixed at the upper end portion of the bundle on the bar with an adhesive tape. On the lower end portion, a weighing bar of 200 g was fixed with an adhesive tape. A crochet needle with a weight of 10 g was pierced through the carbon fiber bundle, and the crochet needle falling distance X (in cm) was measured 50 times. Of the measured values, 10 largest values and 10 smallest values were excluded, and the mean value Xm (in cm) of the remaining measured values was used, to obtain the degree of entanglement (CFC) (in 1/m = m -1 ) of the carbon fiber bundle according to the hook drop testing method from the following formula: Degree of entanglement (CFC) = 100/Xm LMA of Dividing bar (cm) Moisture Content (%) Dividability Degree of Entanglement Adhesion (grade) Productivity of Carbonization Example 1 1.5 - ○ 22.2 1.5 ○ Example 2 1.0 - Δ 17.3 1.5 ○ Example 3 2.5 - ○ 28.3 1.5 ○ Example 4 1.5 2.5 ○ 8.3 3.0 Δ Example 5 1.5 40 ○ 11.9 3.0 ○ Example 6 1.5 60 ○ 13.4 3.0 Δ Example 7 1.5 - ○ 8.2 1.5 Δ Example 8 1.5 - ○ 23.4 1.5 Δ Example 9 1.5 - Δ 42.5 6.0 Δ Example 10 1.5 - Δ 43.5 6.0 Δ Comparative Example 1 dividing just before crimping: could not be divided due to too often fiber breakage X - - -

Claims: A precursor fiber bundle to be processed into carbon fibers, comprising filaments capable of being formed as one tow when packed in a container and capable of being divided in crosswise direction into a plurality of sub-tows when taken out of the container, to be used for producing carbon fibers, wherein said one tow is a tow made of an acrylic polymer, having a total fineness being in the range of from 300,000 deniers to 1,500,000 deniers and capable of being divided into sub-tows each of which has a fineness being in the range of from 50,000 deniers to 250,000 deniers and wherein said filaments are crimped and have the number of crimps being in the range of from 8 per 25 mm to 13 per 25 mm or said filaments are non-crimped and have a moisture content being in the range of from 10% to 50%. A precursor fiber bundle to be processed into carbon fibers, according to claim 1, wherein a fineness of each of the filaments constituting the respective sub-tows is 1 denier to 2.0 deniers. A precursor fiber bundle to be processed into carbon fibers, according to claim 1, wherein a fineness of each of the filaments constituting the respective sub-tows is 1 denier to 1.5 deniers. A precursor fiber bundle to be processed into carbon fibers, according to any one of claims 1 through 3, wherein each of the sub-tows has a degree of entanglement being in the range of from 10 m -1 to 40 m -1 according to the hook drop testing method. A precursor fiber bundle to be processed into carbon fibers, according to any one of claims 1 through 4, wherein (a) said acrylic polymer consists of acrylonitrile, one or more unsaturated monomers of group A and one or more unsaturated monomers of group B; (b) said one or more unsaturated monomers of group A is or are one or more unsaturated monomers selected from a group consisting of vinyl acetate, methyl acrylate, methyl methacrylate and styrene; (c) said one or more unsaturated monomers of group B is or are one or more unsaturated monomers selected from a group consisting of itaconic acid and acrylic acid; (d) the content AN (wt%) of the acrylonitrile in the acrylic polymer satisfies the following formula (1): AN (wt%) ≥ 86 and (e) the content A (wt%) of the unsaturated monomer(s) selected from group A in the acrylic polymer and the content B (wt%) of the unsaturated monomer(s) selected from group B in the acrylic polymer satisfy the following formulae (2) and (3): 3 ≤ A (wt% ) ≤ 10 0.25A - 0.5 ≤ B (wt%) ≤ 0.4A - 0.29 A precursor fiber bundle to be processed into carbon fibers, according to any one of claims 1 through 5, wherein the number of said filaments is 5 x 10 4 to 10 x 10 5 . A process for producing a precursor fiber bundle to be processed into carbon fibers, comprising forming a group comprising a plurality of sub-tows each of which is produced by separation of multifilaments spun from a spinnerette; drawing the sub-tows in this state; collecting the plurality of drawn sub-tows into a form of one tow capable of being divided into said plurality of sub-tows when used for producing carbon fibers later; and packing the tow into a container. wherein said fiber bundle collected in the form of one tow capable of being divided into a plurality of sub-tows when used for producing carbon fibers is a tow made of an acrylic polymer and having a total fineness being in the range of from 300,000 deniers to 1,500,000 deniers, and a fineness of each of the sub-tows is in the range of from 50,000 deniers to 250,000 deniers, and wherein the means for collecting in a form to allow division into said plurality of sub-tows is crimping. A process for producing a precursor fiber bundle to be processed into carbon fibers, according to claim 7, wherein (a) said acrylic polymer consists of acrylonitrile, one or more unsaturated monomers of group A and one or more unsaturated monomers of group B; (b) said one or more unsaturated monomers of group A is or are one or more unsaturated monomers selected from a group consisting of vinyl acetate, methyl acrylate, methyl methacrylate and styrene; (c) said one or more unsaturated monomers of group B is or are one or more unsaturated monomers selected from a group consisting of itaconic acid and acrylic acid; (d) the content AN (wt%) of the acrylonitrile in the acrylic polymer satisfies the following formula (1): AN (wt%) ≥ 86 and (e) the content A (wt%) of the unsaturated monomer(s) selected from group A in the acrylic polymer and the content B (wt%) of the unsaturated monomer(s) selected from group B in the acrylic polymer satisfy the following formulae (2) and (3): 3 ≤ A (wt%) ≤ 10 0.25A - 0.5 ≤ B (wt%) ≤ 0.4A - 0.29 A process for producing a precursor fiber bundle to be processed into carbon fibers, according to claim 8, wherein the spun filaments are drawn at a ratio being in the range of from 2 times to 8 times, and in succession treated to be shrunken in the range of from 5 % to 18 %. A process for producing carbon fibers, comprising the steps of dividing the precursor fiber bundle to be processed into carbon fibers defined in any one of claims 1 through 6, into sub-tows; supplying the sub-tows into a stabilizing process, to treat them for stabilization; and supplying them into a carbonizing process, to treat them for carbonization. A process for producing carbon fibers, according to claim 10, wherein the stabilizing process is carried out in an oxidizing atmosphere having a temperature being in the range of 6 from 200 °C to 300 °C while being under tension or while being drawn and the carbonizing process is carried out in an inactive atmosphere having a temperature being in the range of from 500 °C to 1,500 °C . A process for producing carbon fibers, in which the precursor fiber bundle to be processed into carbon fibers stated in claim 5 is divided into a plurality of sub-tows and in which the sub-tows are supplied into a stabilizing process and treated to be stabilized, and supplied into a carbonizing process and treated to be carbonized, comprising said stabilizing process is being carried out in the following conditions: (a) the stabilizing treatment time is in the range of from 45 minutes to 180 minutes, (b) the drawing ratio is in the range of from 0.9 to not larger than the D defined by D = 1 + (Dmax - 1) x 0.6 where Dmax is the maximum drawing ratio, and (c) tension T satisfies 30 ≤ T (mg/d) ≤ 120. A process for producing carbon fibers, according to claim 12, wherein the stabilizing process is carried out in an oxidizing atmosphere having a temperature being in the range of from 200 °C to 300 °C and the carbonizing process is carried out in an inactive atmosphere having a temperature being in the range of from 500 °C to 1,500 °C. A carbon fiber bundle having a total fineness of not less than 25,000 deniers, substantially no twist, and a degree of entanglement being in the range of from 10 m -1 to 100 m -1 according to the hook drop testing method. A carbon fiber bundle, according to claim 14, wherein the tensile strength is in the range of from 2.0 GPa to 5.0 GPa and the elastic modulus is in the range of from 200 GPa to 300 GPa.

Inventors: ENDO MAKATO, MIYOSHI TOSHIYUKI, MORIKAWA HARUKI, ONO KEIZO, SHINTO MASAKATSU, YAMANAKA SHUICHI, YAMAZAKI JUN, ENDO, MAKATO, MIYOSHI, TOSHIYUKI, MORIKAWA, HARUKI, ONO, KEIZO, SHINTO, MASAKATSU, YAMANAKA, SHUICHI, YAMAZAKI, JUN
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1997
Main classification: D01F9
IPC codes: D01F9, D01F6