JP2012082541A - Method for producing carbon fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high-performance carbon fiber that satisfies both excellent tensile strength and excellent tensile modulus of elasticity without deteriorating productivity and processability.SOLUTION: In a method for producing a carbon fiber, a polyacrylonitrile-based fiber for a carbon fiber is subjected to heat treatment at a temperature of 220°C or more in air, and an integrated value of heat quantity given in the heat treatment is 100 J h/g-800 J h/g.

Description

  The present invention relates to a method for producing a high-performance carbon fiber satisfying both an excellent tensile elastic modulus and an excellent tensile strength with high productivity.

  Carbon fiber is increasingly used as a reinforced fiber for composite materials due to increasing environmental problems, and its importance is increasing. Further, high performance is strongly demanded. Conventionally, for carbon fibers, there has been a central demand for improvement of single properties such as tensile properties, and various properties of carbon fibers have been improved in response to such requirements. In recent years, however, carbon fibers have been required to satisfy a plurality of characteristics simultaneously and at a high level. In particular, in carbon fibers applied to parts subjected to stress from various directions, such as aircraft structural members, simultaneously improving the tensile modulus and tensile strength is important for achieving further weight reduction. It has become a challenge.

  The most widely used polyacrylonitrile-based carbon fiber is a polyacrylonitrile-based fiber for carbon fibers (precursor for carbon fiber) that is heat-treated in an oxidizing atmosphere at a temperature of 200 to 300 ° C. to convert it into flame-resistant fibers. It is industrially manufactured through a flameproofing step to be performed and a carbonization step in which carbonization is performed by heat treatment in an inert atmosphere at a temperature of 300 to 3000 ° C. At this time, it is generally known that the tensile elastic modulus of carbon fibers can be increased as the maximum temperature in the carbonization process is increased. However, increasing the maximum temperature of the carbonization process promotes the growth of graphite crystals and reduces the tensile strength of the resulting carbon fiber. That is, as long as the carbonization temperature is adjusted using the same polyacrylonitrile fiber for carbon fiber (precursor for carbon fiber), the tensile modulus and tensile strength of carbon fiber are in a trade-off relationship. Is inevitable to be sacrificed. Therefore, in applications that require tensile strength, a technique for increasing the tensile modulus by means other than control of the carbonization temperature is required.

  In industrial applications, there is a strong demand for low cost as well as performance. In the carbon fiber production process, it is necessary to improve the productivity of the flameproofing process, which requires a long processing time.

  Up to now, as a method of shortening the process time in flame resistance, a method of increasing the flame resistance temperature by using a plurality of furnaces having different temperatures, shortening the process time in flame resistance and further pulling the carbon fiber As a method for improving the strength and elastic modulus, a method of extending in accordance with the density of the carbon fiber precursor fiber that has passed through each furnace in a flameproofing furnace composed of a plurality of furnaces has been proposed (Patent Document 1). -3)).

  However, in the proposal of Patent Document 1, the temperature control region of the flameproofing process is set to 2 to 3, and processing is performed at as high a temperature as possible in each region. However, the processing time is 44 to 60 minutes. Further, in the proposal of Patent Document 2, similarly, the temperature control region of the flameproofing process is set to 2 to 3, and the heat treatment time in the high temperature region is made longer to perform the flameproofing in a short time. In the method of increasing the residence time of the region, continuous flameproofing treatment cannot be performed, and in order to perform the treatment continuously, it is necessary to have a device configuration that can arbitrarily change the time in the middle. Further, in the proposal of Patent Document 3, the apparatus is enlarged and the apparatus mechanism is complicated, such as setting a plurality of stages of extension in the flameproofing furnace and requiring 3 to 6 furnaces for shortening the flameproofing time. Further, the total elongation of the flame resistant yarn is as high as about 50%, and there is a problem of deterioration in quality.

  These conventional techniques are particularly aimed at shortening the flame resistance process time, and the performance of the obtained carbon fiber is not sufficiently high. As described above, the flameproofing method that is excellent in productivity without impairing the performance and quality of the carbon fiber is insufficient with the prior art.

  In addition, a method has been proposed in which flameproofing is completed in a short time by performing a flameproofing treatment in the heat medium particles and actively removing heat generated during the flameproofing (see Patent Document 4). However, in this proposal, equipment for removing the heat medium particles adhering to the fibers is necessary, and furthermore, since the heat medium particles become defects of the carbon fibers as impurities, sufficient strength is not obtained.

JP 58-163729 A JP-A-6-294020 JP-A-62-257422 JP-A-3-33220

  Accordingly, an object of the present invention is to provide a method for producing high-performance carbon fibers that simultaneously satisfy excellent tensile strength and excellent tensile modulus without impairing productivity and processability.

  The present invention is intended to achieve the above object, and the carbon fiber production method of the present invention is such that the polyacrylonitrile fiber for carbon fiber is heat-treated in air at a temperature of 220 ° C. or higher, and the amount of heat given at that time. The carbon fiber production method is characterized in that an integrated value of 100 J · h / g to 800 J · h / g.

  Further, in the method for producing carbon fiber of the present invention, the step of heat-treating the polyacrylonitrile fiber for carbon fiber in air is continuously performed in a plurality of stages at a temperature of 220 ° C. or more, and the heat treatment temperature in each stage is the preceding stage. The temperature is at least 20 ° C. lower than the temperature at which the polyacrylonitrile fiber heat-treated in the heat treatment is reduced, and the integrated value of the amount of heat given to the polyacrylonitrile fiber is 100 J · h / g or more and 500 J · h / g or less. This is a method for producing a carbon fiber.

  In the method for producing carbon fiber of the present invention, the step of heat-treating the polyacrylonitrile fiber for carbon fiber in air is continuously performed in a single step at a temperature of 220 ° C. or higher, and the heat treatment temperature at that time is polyacrylonitrile-based. Carbon having a temperature lower by 20 ° C. or more than the fiber weight loss starting temperature, and an integrated value of heat given to the polyacrylonitrile fiber is 100 J · h / g or more and 700 J · h / g or less It is a manufacturing method of a fiber.

  According to a preferred aspect of the method for producing carbon fiber of the present invention, when the step of heat-treating the polyacrylonitrile fiber for carbon fiber in air is performed in a plurality of stages, the heat treatment time in each stage is the heat treatment in the subsequent stage. The time is the same as or shorter than the heat treatment time of the previous stage, and the specific gravity of the polyacrylonitrile fiber heat-treated in the first heat treatment step is 1.32 g / cc or less.

  According to a preferred embodiment of the method for producing carbon fiber of the present invention, the polyacrylonitrile fiber for carbon fiber is made of a polyacrylonitrile polymer having an intrinsic viscosity in the range of 1.0 to 10.

  According to a preferred aspect of the method for producing carbon fiber of the present invention, the polyacrylonitrile fiber for carbon fiber has an intrinsic viscosity in the range of 1.0 to 5.0, and has a Z average molecular weight Mz and a weight average molecular weight Mw. The polydispersity Mz / Mw indicated by the ratio is 2.7 to 10.0 and is made of a polyacrylonitrile-based polymer.

  According to a preferred aspect of the method for producing carbon fiber of the present invention, the polyacrylonitrile fiber for carbon fiber is spun by discharging a polyacrylonitrile polymer from a spinneret by a wet spinning method or a dry-wet spinning method. It is a fiber manufactured through a process, a drying heat treatment process for drying and heat treating the fiber obtained in the spinning process, and a steam stretching process for steam stretching the fiber obtained in the drying heat treatment process.

  According to the preferable aspect of the manufacturing method of the carbon fiber of this invention, the flame-proofing process which makes the said polyacrylonitrile-type fiber for carbon fibers flame-resistant in the air, and the fiber obtained by this flame-proofing process are 300-800 degreeC. A pre-carbonization step of pre-carbonizing in an inert atmosphere at a temperature of 5 and a carbonization step of carbonizing the fibers obtained in the pre-carbonization step in an inert atmosphere at a temperature of 1,000 to 2,000 ° C. It is.

  According to a preferred aspect of the method for producing carbon fiber of the present invention, the carbonization is carried out at a tension in the carbonization step of 4.0 mN / dTex to 35.0 mN / dTex.

  According to the present invention, carbon fibers having both tensile strength and tensile elastic modulus can be produced without impairing productivity and processability.

  The present inventors addressed the problem of producing a flame resistant yarn suitable for producing a high-performance carbon fiber capable of achieving both tensile strength and tensile elastic modulus without impairing productivity and processability. It has been found that carbon fibers that stably exhibit high tensile strength can be obtained by reducing the integrated value of the amount of heat given to the polyacrylonitrile fiber by controlling the temperature and the time, and have reached the present invention. .

  The method for producing carbon fiber of the present invention is such that the integrated value of the amount of heat given when the polyacrylonitrile fiber for carbon fiber is heat-treated in air at a temperature of 220 ° C. or higher is 100 J · h / g or more and 800 J · h / g. It is as follows.

  That is, in the present invention, the integrated value of the amount of heat given when the polyacrylonitrile fiber for carbon fiber is heat-treated in air is 100 J · h / g or more and 800 J · h / g or less, preferably 120 J · h / g. It is 750 J · h / g or less, more preferably 150 J · h / g or more and 700 J · h / g or less. If the integrated value of heat is less than 100 J · h / g, yarn breakage will occur in the subsequent preliminary carbonization process and carbon fibers cannot be obtained, and if the integrated value of heat exceeds 800 J · h / g, it will be obtained. The tensile strength of the carbon fiber is not increased.

The integrated value of heat can be obtained more accurately by actually measuring the temperature of the polyacrylonitrile fiber to be heat-treated, but it is technically difficult to measure the temperature of the running polyacrylonitrile fiber. Moreover, when a polyacrylonitrile fiber bundle is introduced into an oven set at a flameproofing temperature and the temperature of the fiber bundle is measured, the atmosphere in the oven is sufficiently short in comparison with the normal flameproofing time (several tens of minutes). The temperature and yarn bundle temperature matched. Therefore, the integrated value of the heat amount referred to here is that the temperature of the polyacrylonitrile fiber in the flameproofing furnace is regarded as the same as the flameproofing temperature, and the flameproofing temperature T (K) and the residence time t (h) of the flameproofing furnace. ), And a polyacrylonitrile fiber heat capacity of 1.507 J / g · ° C.
・ Integrated value of heat (J · h / g) = T × t × 1.507
T: Furnace flameproof temperature (K)
t: Furnace residence time (h).

Here, flameproofing can be performed using one or more flameproofing furnaces, and when the temperature and residence time are different for each furnace, the following equation is obtained.
・ Integrated value of heat (J · h / g) = (T1 × t1 +... + Tn × tn) × 1.507
T1: First furnace flameproof temperature (K)
t1: First furnace residence time (h)
Tn: nth furnace flameproof temperature (K)
tn: nth furnace residence time (h).

In addition, when the flameproofing furnace is not individually divided and divided into regions (stages or sections) having different temperatures, the following equation is obtained.
・ Integrated value of heat (J · h / g) = (T1s × t1s +... + Tns × tns) × 1.507
T1s: first region flameproofing temperature (K)
t1s: First region furnace residence time (h)
Tns: nth region flameproofing temperature (K)
tns: n-th region residence time (h).

Further, when the temperature in the furnace is not constant, the following equation is obtained.
・ Integrated value of heat (J · h / g) = Σ (Tnk × Δtnk) × 1.507
Tnk: Flame resistance temperature (K) at the position k of the nth furnace
Δtnk: Furnace residence time (h) at position k of the nth furnace.

  When the structure of the flameproofing furnace is only one furnace, the integrated value of the amount of heat given when the polyacrylonitrile fiber is heat-treated in air is preferably 100 J · h / g or more and 700 J · h / g or less. More preferably, it is 120 J · h / g or more and 650 J · h / g or less, and further preferably 150 J · h / g or more and 600 J · h / g or less.

  In addition, when the flameproofing furnace is a plurality of furnaces, or when the flameproofing furnace is composed of a plurality of regions having different temperatures, the integrated value of heat is preferably 100 J · h / g or more and 500 J · h / g or less. It is preferably 120 J · h / g or more and 450 J · h / g or less, and more preferably 150 J · h / g or more and 400 J · h / g or less.

  In the present invention, the reason why the integrated value of the amount of heat given to the polyacrylonitrile fiber in the flameproofing process affects the strength of the carbon fiber is not clear, but by giving only the amount of heat below a certain level, the generation of volatile matter is generated. It is considered that generation of voids and defects that are suppressed and affect the strength of the carbon fiber can be suppressed.

  In the present invention, the lower limit of the flameproofing temperature is 220 ° C. or higher, preferably 225 ° C. or higher, and more preferably 230 ° C. or higher. When the flameproofing temperature is less than 220 ° C., the reaction rate is very slow, not only the productivity is impaired, but also the amount of heat given to the polyacrylonitrile fiber is substantially excessive.

  Further, the upper limit of the flameproofing temperature is preferably a temperature that is 20 ° C. or more lower than the weight loss starting temperature obtained from the thermogravimetric analysis measurement of the precursor fiber heat-treated in the flameproofing furnace, more preferably 25 ° C. or less, More preferably, it is 30 degrees C or less. When the flameproofing temperature is less than 20 ° C, high-strength carbon fibers cannot be obtained due to the generation of voids due to the generation of a large amount of volatile components and the generation of defects due to yarn pain, and in some cases, yarn breakage due to intense heat storage May occur. The term “precursor fiber” as used herein refers to a raw material polyacrylonitrile fiber when the number of flameproofing furnaces is one, and the flameproofing furnace is composed of a plurality of furnaces or a plurality of regions having different temperatures. Is a flame-resistant intermediate yarn fiber that has been flame-resistant in the furnace or area immediately before it. Regions with different temperatures are when the temperature of each step in the furnace divided horizontally with respect to the direction of travel of the yarn bundle is different, or in the direction perpendicular to the direction of travel of the yarn bundle. The case where heaters with different temperatures are arranged and each heater section has a different temperature is shown.

  When the flameproofing furnace used for flameproofing is composed of a plurality of furnaces or a plurality of regions (stages or sections) having different temperatures, the heat treatment time in each stage is the same as the heat treatment time in the previous stage or Short is preferred. If the heat treatment time in the subsequent stage is longer than the heat treatment time in the previous stage, and if the amount of heat given by flameproofing is to be within the above-mentioned range, the yarn breakage may occur during the heat treatment in the subsequent stage due to insufficient heat treatment in the previous stage. In addition, when the heat treatment in the former stage is performed to such an extent that the heat treatment in the latter stage can be performed, the amount of heat given by the flame resistance is increased, and the effect of improving the tensile strength of the obtained carbon fiber is reduced.

  The specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace or the first temperature range is preferably 1.32 g / cc or less, more preferably 1.31 g / cc or less, and still more preferably 1 .30 g / cc or less. If this specific gravity exceeds 1.31 g / cc, when performing flame resistance in a temperature range where yarn breakage does not occur due to the heat of flame resistance, the time required for the flame resistance process will be prolonged and the amount of heat given by flame resistance. Increases, and the effect of improving the tensile strength of the resulting carbon fiber decreases.

  In the present invention, the polyacrylonitrile-based polymer for producing carbon fibers is preferably composed of at least 95 mol% of acrylonitrile. For the purpose of improving yarn-making property and promoting flame resistance, the copolymer component is not more than 5 mol%. It may be polymerized.

  The amount of copolymerization is preferably 3 mol% or less, more preferably 1 mol% or less, and still more preferably 0.5 mol% or less. For the purpose of promptly promoting the flameproofing reaction, it is preferable to copolymerize at least 0.1 mol% or more of the flameproofing promoting component as a copolymerization component.

  Specific examples of the flame retardant promoting component which is the above copolymer component include, for example, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid, mesaconic acid, acrylamide and methacrylamide. It is done. For the purpose of preventing the melting point Tm from decreasing under wet heat, it is preferable to use a small amount of a monomer having a high flame resistance promoting effect, and a flame resistance promoting component having a carboxyl group rather than an amide group is preferably used.

  In addition, the number of amide groups and carboxyl groups contained in the flameproofing promoting component is preferably two or more than one, and from that point of view, the flameproofing promoting component that is a copolymerization component is acrylic. Acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid and mesaconic acid are preferred, itaconic acid, maleic acid and mesaconic acid are more preferred, and itaconic acid is most preferably used.

  The polyacrylonitrile-based polymer for producing carbon fibers used in the present invention can be obtained by a known polymerization method such as solution polymerization, suspension polymerization and emulsion polymerization, but for the purpose of increasing the stability during yarn drawing and stretching, It is preferred to use solution polymerization. In the solution polymerization, it is not necessary to isolate polyacrylonitrile from the start to the end of polymerization, or until the stage of spinning to be used for spinning, and the entangled state of polyacrylonitrile molecular chains in the solvent in the state of polyacrylonitrile solution is uniform. Therefore, it is preferable compared to other polymerization methods.

  The polyacrylonitrile for producing carbon fibers in the present invention can be obtained by a known polymerization method such as radical polymerization or anion polymerization, but radical polymerization is preferably used from an industrial viewpoint.

As a requirement of the polyacrylonitrile-based polymer used in the present invention, it is preferable to satisfy either of the following (a) and (b).
(A) Polyacrylonitrile polymer A: intrinsic viscosity is 1.0 to 10.0
(B) Polyacrylonitrile-based polymer B: the intrinsic viscosity satisfies 1.0 to 5.0, and the polydispersity Mz / Mw indicated by the ratio of the Z average molecular weight Mz to the weight average molecular weight Mw is 2.7. -10.0, preferably 1.5-6.0.

  When said intrinsic viscosity is less than 1.0, the tensile elasticity modulus and tensile strength of the carbon fiber obtained may not increase. When the intrinsic viscosity is more than 10.0, gelation when used as a spinning dope becomes remarkable, and spinning may be difficult.

  In the present invention, the above-mentioned intrinsic viscosity can be controlled by changing the amounts of monomers, initiators, chain transfer agents and the like during polymerization.

  In the above (b) polyacrylonitrile-based polymer B, the intrinsic viscosity satisfies 1.0 to 5.0, and the polydispersity Mz / Mw indicated by the ratio of the Z average molecular weight Mz and the weight average molecular weight Mw. A polyacrylonitrile polymer having a 2.7 to 10.0 can be used. (B) The more preferable range of the intrinsic viscosity of the polyacrylonitrile-based polymer B is 1.2 to 4.5, and more preferably 1.5 to 3.5. The polydispersity Mz / Mw indicates that the larger the value, the broader the molecular weight distribution centering on the high molecular weight side. From these characteristics, the polyacrylonitrile-based polymer B has a stable yarn-forming property even at a low intrinsic viscosity as compared with the polyacrylonitrile-based polymer A, but the intrinsic viscosity of the polyacrylonitrile-based polymer B is 1. If it is less than 0, as in the case of the polyacrylonitrile polymer A, the tensile elastic modulus and tensile strength of the obtained carbon fiber do not increase. Further, when the intrinsic viscosity of the polyacrylonitrile polymer B exceeds 5.0, gelation when used as a spinning dope becomes remarkable, and spinning may be difficult. The polydispersity Mz / Mw is preferably 2.7 to 8.0, and more preferably 3.0 to 6.0. If the polydispersity Mz / Mw is less than 2.7, the discharge from the die is not stable, and if it exceeds 10.0, spinning may be difficult. As a method of satisfying the polydispersity Mz / Mw within the above range, a method using a plurality of polymerization initiators having different decomposition temperatures, a method of sequentially adding initiators, and two or more types of polyacrylonitrile polymers having different molecular weights are used. The method of mixing is mentioned.

  In the polyacrylonitrile-based polymer solution used in the carbon fiber production method of the present invention, the polymer concentration in the polyacrylonitrile-based polymer A is preferably 10 to 18% by weight, more preferably 11 to 17% by weight. More preferably, it is 12 to 16% by weight. In the polyacrylonitrile-based polymer B, the polymer concentration is preferably 15 to 25% by weight, more preferably 17 to 23% by weight, and further preferably 18 to 22% by weight.

  The higher the polymer concentration, the higher the equipment efficiency in polymerization and spinning, which is preferable. However, when the polyacrylonitrile-based polymer A exceeds 18% by weight, the polyacrylonitrile-based polymer B exceeds 25% by weight. The gelation of the stock solution becomes remarkable, and stable spinning may be difficult. Further, when the polymer concentration is less than 10% by weight for the polyacrylonitrile polymer A and less than 15% by weight for the polyacrylonitrile polymer B, the compressive strength and tensile strength of the obtained carbon fiber are not increased. Sometimes. Moreover, this polymer concentration can be adjusted by the ratio of the solvent with respect to the polyacrylonitrile-type polymer.

  The solvent used in the polyacrylonitrile-based polymer solution for producing carbon fibers used in the present invention is a solvent capable of dissolving polyacrylonitrile. For example, polyacrylonitrile such as dimethyl sulfoxide, dimethylformamide and dimethylacetamide is preferably exemplified. Can do. Of these, dimethyl sulfoxide is preferably used from the viewpoint of solubility. When solution polymerization is used for the polymerization of the polyacrylonitrile polymer, if the solvent used for the polymerization and the solvent used for spinning are made the same solvent, the step of separating and re-dissolving the obtained polyacrylonitrile polymer will be performed. It becomes unnecessary.

  Next, the manufacturing method of the polyacrylonitrile-type fiber (carbon fiber precursor) for carbon fibers of this invention is demonstrated concretely.

  In the present invention, as described above, the polyacrylonitrile fiber for carbon fiber is spun by discharging the polyacrylonitrile polymer solution (spinning stock solution) used in the present invention from a spinneret by a wet spinning method or a dry-wet spinning method. Then, the fiber obtained in the spinning process can be dried and heat-treated, and then the fiber obtained in the drying and heat-treating process can be steam-stretched.

  In the present invention, in order to obtain a high-strength carbon fiber, it is preferable to pass through a filter having an opening of 1 μm or less before the spinning solution is spun to remove polymerization raw materials and impurities mixed in each step.

  The spinning solution is discharged from a spinneret by a wet spinning method or a dry and wet spinning method, introduced into a coagulation bath, and solidified to form a polyacrylonitrile fiber for carbon fibers (carbon fiber precursor). For the purpose of improving the denseness of the obtained polyacrylonitrile fiber for carbon fiber and enhancing the mechanical properties of the obtained carbon fiber, the spinning dope is temporarily used rather than the wet spinning method in which the spinning dope is directly discharged into a coagulation bath. It is preferable to use a dry-wet spinning method that is discharged into the air and then introduced into the coagulation bath.

  In the present invention, the coagulation bath used in the spinning step preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide and dimethylacetamide used as a solvent for the spinning dope and a so-called coagulation promoting component. As the coagulation accelerating component, a component that does not dissolve the polyacrylonitrile-based polymer and is compatible with the solvent used in the spinning dope can be used. Specifically, it is preferable to use water as a coagulation promoting component.

  Next, after a fiber bundle consisting of a large number of filaments spun from a spinneret is introduced into a coagulation bath to coagulate each filament, a water washing step, a bath drawing step, an oil agent application step, a drying heat treatment step, and steam A polyacrylonitrile fiber for carbon fiber is obtained through a drawing process.

  In the present invention, the fiber bundle derived from the coagulation bath may be directly introduced into the in-bath drawing step by omitting the water washing step, or may be introduced into the in-bath drawing step after removing the solvent in the water washing step. Also good. The stretching in the bath can be usually performed in a single or a plurality of stretching baths maintained at a temperature of 30 to 98 ° C. The draw ratio is preferably 1 to 5 times, more preferably 2 to 4 times.

  For the purpose of preventing adhesion between single fibers after the stretching step in the bath, it is preferable to apply an oil agent made of silicone or the like to the fiber bundle. As the silicone oil, it is preferable to use a modified silicone. As such a silicone oil agent, an oil agent containing amino-modified silicone having high heat resistance can be used.

  The temperature of the drying heat treatment is preferably 160 to 200 ° C, more preferably 165 to 198 ° C, and further preferably 175 to 195 ° C. When the temperature of the drying heat treatment is lower than 160 ° C., the resulting polyacrylonitrile fiber for carbon fiber is insufficiently dense, and the effects of the present invention may not be obtained. On the other hand, when the temperature of the drying heat treatment exceeds 200 ° C., the fusion between the single fibers becomes remarkable, and the tensile strength of the obtained carbon fibers may be lowered.

  The drying heat treatment may be performed by bringing the fiber bundle into contact with a heated roller or by running in a heated atmosphere. From the viewpoint of drying efficiency, the drying heat treatment is performed by contacting with a heated roller. It is preferable.

  The steam stretching is performed by stretching the fiber bundle preferably 3 times or more, more preferably 4 times or more, and still more preferably 5 times or more in the pressurized steam. It is preferable that the draw ratio (total draw ratio) throughout the washing step, the in-bath drawing step and the steam drawing step is 8 to 15 times for the purpose of enhancing the mechanical properties of the obtained carbon fiber. The total draw ratio is more preferably 10 to 14.5 times, still more preferably 11 to 14 times. When the total draw ratio is less than 8, the degree of orientation of the resulting polyacrylonitrile fiber for carbon fiber is lowered, and in the subsequent firing step for producing the carbon fiber, high drawability may not be obtained. When the total draw ratio exceeds 15 times, filament breakage during drawing becomes remarkable, and the quality of the obtained polyacrylonitrile fiber for carbon fiber and carbon fiber tends to be lowered.

  In the present invention, the single fiber fineness of the polyacrylonitrile fiber for carbon fiber is preferably 0.3 to 1.3 dtex. When the single fiber fineness is less than 0.3 dtex, the operability may be reduced due to a decrease in spinnability in the spinning process, the productivity per number of discharge holes may be reduced, and the cost increase may be significant. On the other hand, when the single fiber fineness exceeds 1.3 dtex, the difference between the inner and outer structures of each filament forming the obtained flame-resistant fiber bundle becomes remarkable, and the tensile strength and strand tensile elastic modulus of the obtained carbon fiber decrease. There is.

  The number of filaments forming the fiber bundle of the polyacrylonitrile fiber for carbon fiber is preferably 1,000 to 3,000,000, more preferably 6,000 to 3,000,000, The number is preferably 12,000 to 2,500,000, and most preferably 24,000 to 2,000,000. The number of filaments is preferably greater than 1,000 for the purpose of improving productivity. However, when the number exceeds 3,000,000, the inside of the polyacrylonitrile fiber bundle for carbon fibers cannot be uniformly flameproofed. Sometimes.

  Next, the manufacturing method of the carbon fiber of this invention is demonstrated. Regarding the temperature and time in flame resistance, it is necessary to satisfy the integrated value of the amount of heat given when the above-mentioned polyacrylonitrile fiber for carbon fiber is heat-treated in air, and after flame resistance, preferably 300 to 800 ° C. Carbon fibers can be produced by pre-carbonizing in an inert atmosphere at a temperature, and preferably carbonizing in an inert atmosphere at a temperature of 1,000 to 2,000 ° C.

  The stretch ratio when making flame resistant is preferably 0.80 to 1.30, more preferably 0.90 to 1.20, and still more preferably 0.85 to 1.10. When the draw ratio is less than 0.80, the degree of orientation of the obtained flame-resistant fiber becomes insufficient, and the strand tensile modulus of the obtained carbon fiber may be lowered. On the other hand, when the draw ratio exceeds 1.30, processability may be deteriorated due to generation of fluff and yarn breakage. Moreover, it is preferable to set the specific gravity of the obtained flame-resistant fiber to be in the range of 1.3 to 1.45 for the purpose of improving the mechanical properties of the pre-carbonization and the obtained carbon fiber following the flame resistance. .

  The preliminary carbonization and carbonization are performed in an inert atmosphere. As the inert gas used, for example, nitrogen, argon, xenon, or the like is used, and nitrogen is preferably used from an economical viewpoint.

  The temperature of preliminary carbonization is preferably 300 to 800 ° C., and the rate of temperature increase in preliminary carbonization is preferably set to 500 ° C./min or less.

  The stretch ratio at the time of preliminary carbonization is preferably 1.00 to 1.30, more preferably 1.10 to 1.30, and even more preferably 1.10 to 1.20. When the draw ratio is less than 1.00, the degree of orientation of the obtained preliminary carbonized fiber may be insufficient, and the strand tensile elastic modulus of the carbon fiber may decrease. On the other hand, when the draw ratio exceeds 1.30, the processability may be deteriorated due to the occurrence of fluff or yarn breakage.

  The temperature of carbonization is preferably 1,000 to 2,000 ° C, more preferably 1,200 to 1950 ° C, and still more preferably 1,300 to 1,850 ° C. The higher the maximum carbonization temperature, the higher the tensile modulus of the strand, but the graphitization proceeds and the crystal size increases. As a result, the tensile strength may decrease. Set the temperature.

  The tension during carbonization is preferably 4.0 to 35.0 mN / dTex, more preferably 7.0 to 33.0, and even more preferably 7.5 to 31.0. When the tension is less than 4.0 mN / dTex, the degree of orientation and denseness of the obtained carbon fiber may be insufficient, and the strand tensile modulus may be lowered. On the other hand, when the tension exceeds 35.0 mN / dTex, processability may be deteriorated due to generation of fluff or yarn breakage. Here, the tension in the carbonization step is represented by a value obtained by dividing the tension (mN) measured by the roll on the outlet side of the carbonization furnace by the fineness (dTex) of the polyacrylonitrile-based fiber for carbon fibers when dry.

  The obtained carbon fiber can be subjected to electrolytic treatment in order to modify its surface. Examples of the electrolytic solution used for the electrolytic treatment include acidic solutions such as sulfuric acid, nitric acid and hydrochloric acid, and aqueous solutions of alkalis or salts thereof such as sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, ammonium carbonate and ammonium bicarbonate. Can be used. The amount of electricity required for the electrolytic treatment can be appropriately selected according to the carbonization degree of the applied carbon fiber.

  Such electrolytic treatment can optimize the adhesion between the carbon fiber and the matrix resin in the production of the composite material, resulting in a brittle breakage of the composite material due to excessive adhesion and a tensile strength in the fiber direction. Although the problem and the problem that the tensile strength in the fiber direction is high but the adhesiveness with the resin is inferior and the strength property in the non-fiber direction is not expressed are solved, in the resulting composite material, the fiber direction and the non-fiber direction Strength characteristics balanced in both directions will be expressed.

  After such an electrolytic treatment, a sizing treatment can be performed in order to impart convergence to the obtained carbon fiber. As the sizing agent, a sizing agent having good compatibility with the matrix resin can be appropriately selected according to the type of the matrix resin used in the composite material.

  The carbon fiber obtained by the present invention has high tensile strength and high strand tensile modulus. Therefore, the carbon fiber obtained by the present invention can be applied to various molding methods such as an autoclave molding method using a prepreg, a resin transfer molding method using a preform such as a woven fabric, and a filament winding molding method. It is suitably used for molding sports members such as aircraft members, pressure vessel members, automobile members, fishing rods, and golf shafts using these molding methods.

  The measuring method of various physical property values used in the present invention is as follows.

<Weight reduction start temperature by TG measurement>
The weight reduction start temperature was measured using a TG2010 manufactured by Bruker AXS Co., at a temperature rising rate of 50 ° C./min and 25-600 ° C. under conditions of 100 ml / min of nitrogen gas. Moreover, 10.0 mg of flameproof fiber was used for the measurement sample. The obtained chart decreased with a low gradient from slightly before the weight reduction start temperature, gradually increased the gradient, and then the gradient gradually decreased. In this chart, a tangent line was drawn to each of the flat part before the weight reduction started and the inflection point seen in the sudden weight reduction part, and the weight reduction start temperature was obtained from the intersection of these tangent lines.

<Intrinsic viscosity>
At a temperature of 60 ° C., 150 mg of a polyacrylonitrile-based polymer that has been heat-treated at 120 ° C. for 2 hours and dried is dissolved in 50 ml of dimethylformamide containing 0.1 mol / liter of sodium thiocyanate. About the obtained solution, the fall time between marked lines is measured with the accuracy of 1/100 second using an Ostwald viscometer in the temperature of 25 degreeC. Let the measured fall time be t (seconds). Similarly, measurement is also performed on dimethylformamide added with 0.1 mol / liter of sodium thiocyanate in which the polyacrylonitrile-based polymer is not dissolved, and the dropping time is defined as t ₀ (seconds). The intrinsic viscosity [η] is calculated using the following formula.
[η] = {(1 + 1.32 × ηsp) ^(1/2) −1} /0.198
ηsp = (t / t ₀ ) −1
The above measurement is performed three times, and the arithmetic average is defined as the intrinsic viscosity [η] of the polyacrylonitrile-based polymer. In Examples and Comparative Examples described later, Wako Pure Chemicals special grades were used for the sodium thiocyanate and dimethylformamide.

<Average molecular weight: Z average molecular weight Mz, weight average molecular weight Mw>
The polyacrylonitrile polymer to be measured is dissolved in dimethylformamide (0.01N-lithium bromide added) so that its concentration is 0.1% by weight to obtain a sample solution. About the obtained sample solution, the molecular weight distribution curve was calculated | required from the GPC curve measured on the following conditions using the GPC apparatus, and the Z average molecular weight Mz and the weight average molecular weight Mw were computed. The measurement was performed 3 times and the arithmetic average value was used.
-Column: Column for polar organic solvent GPC-Flow rate: 0.5 ml / min
・ Temperature: 70 ℃
・ Sample filtration: Membrane filter (0.45μm cut)
・ Injection volume: 200 μl
-Detector: A differential refractive index detector.

  For the average molecular weight, use at least 6 types of monodispersed polystyrenes with different molecular weights and known molecular weights to create an elution time-molecular weight calibration curve, and read the molecular weight in terms of polystyrene corresponding to the corresponding elution time on the calibration curve. Was determined by In Examples, CLASS-LC2010 manufactured by Shimadzu Corporation as a GPC apparatus, TSK-GEL-α-M (× 2) manufactured by Tosoh Corporation as a column, Wako Pure Chemical Industries, Ltd. as dimethylformamide and lithium bromide ( Made by Millipore Corporation as a membrane filter, RID-10AV made by Shimadzu Corporation as a differential refractive index detector, and TSK standard polystyrene F as monodisperse polystyrene for creating a calibration curve -20 (Mw: 184000), F-40 (Mw: 427000), F-80 (Mw: 791000), F-128 (Mw: 1300000), F-288 (Mw: 2110000) and F-450 (Mw: 4240000) were used respectively.

<Strand tensile modulus and tensile strength of carbon fiber>
The strand tensile modulus and tensile strength of the carbon fiber are determined according to JIS R7601 (1986) “Resin-impregnated strand test method”. The test piece was prepared by impregnating carbon fiber with the following resin composition and curing conditions of heat treatment at a temperature of 130 ° C. for 35 minutes.

[Resin composition]
3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate (100 parts by weight)
・ Boron trifluoride monoethylamine (3 parts by weight)
Acetone (4 parts by weight)
In addition, the number of strands to be measured is 6, and the arithmetic average value of each measurement result is the strand tensile modulus and tensile strength of the carbon fiber. In Examples and Comparative Examples described below, as the above 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate, “BAKELITE” (registered trademark) ERL- manufactured by Union Carbide Co., Ltd. 4221 was used.

<Specific gravity measurement>
1.0 to 3.0 g of fiber is collected and dried at 120 ° C. for 2 hours. Next, after measuring the absolute dry mass A (g), it was impregnated with methanol and sufficiently degassed, and then the fiber mass B (g) in the solvent bath was measured, and the fiber specific gravity = (A × ρ) / (A The fiber specific gravity was determined by -B).

Example 1
A 2,2′-azobisisobutyronitrile (hereinafter referred to as AIBN) is polymerized from a polyacrylonitrile copolymer consisting of 99.5 mol% of acrylonitrile and 0.5 mol% of itaconic acid using dimethyl sulfoxide as a solvent. Using as an initiator, polymerization was carried out by a solution polymerization method in a nitrogen atmosphere to obtain a polyacrylonitrile-based polymer solution (spinning solution) having an intrinsic viscosity of 2.68.

  The obtained spinning dope was discharged into air once at a temperature of 40 ° C. using a spinneret having a diameter of 0.12 mm and a hole number of 3,000, and passed through a space of about 4 mm, and then a temperature of 3 ° C. A coagulated yarn was obtained by a dry-wet spinning method introduced into a coagulation bath consisting of an aqueous solution of 30% dimethyl sulfoxide controlled to 1%. After the coagulated yarn thus obtained is washed with water by a conventional method, the water bath stretching process is performed independently using two hot water tanks, and after each hot water tank, the maximum temperature for preliminary stretching, after preliminary stretching. Was set to 2.4 times to obtain a water bath drawn yarn. An amino-modified silicone-based silicone oil agent was further added to the obtained water bath drawn yarn, and then the water bath drawn yarn was subjected to dry densification treatment using a heating roller at a temperature of 160 ° C. The obtained dried densified yarn is stretched 5 times in pressurized steam to obtain a total yarn drawing ratio of 12 times to obtain a polyacrylonitrile fiber having a single fiber fineness of 0.7 dtex and a single fiber number of 3,000. It was. The weight reduction start temperature of the obtained polyacrylonitrile fiber by TG measurement in nitrogen was 315 ° C.

  Next, four polyacrylonitrile fibers obtained are combined, 12,000 single fibers are used, a flameproofing furnace constituted by two furnaces is used, the first furnace is set to a temperature of 250 ° C., and the second The furnace was made 290 ° C., and the heat treatment time in the second furnace was made shorter than the heat treatment time in the first furnace, thereby making flame resistance. At that time, the specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.27 g / cc, and the integrated value of the amount of heat given to the polyacrylonitrile fiber was 330 J · h / g. Immediately after leaving the first furnace, the flame reduction starting temperature of the flameproofing intermediate yarn was 333 ° C. The obtained flame-resistant fiber bundle was subjected to preliminary carbonization while being stretched at a stretch tension of 1.4 mN / dTex in a nitrogen atmosphere at a temperature of 300 ° C. to 800 ° C. to obtain a preliminary carbonized fiber bundle. The obtained pre-carbonized fiber bundle was carbonized with a tension of 14 mN / dTex in a nitrogen atmosphere at a maximum temperature of 1,500 ° C. to obtain a carbon fiber bundle. When the strand physical property of the obtained carbon fiber was measured, the tensile strength was 7.3 GPa and the tensile modulus was 350 GPa. The results are shown in Table 1.

(Example 2)
Flame resistance was achieved in the same manner as in Example 1 except that heat resistance was performed in the first furnace until the specific gravity of the flame resistant yarn became 1.33 g / cc. At this time, the integrated value of the amount of heat given to the polyacrylonitrile fiber was 490 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers were 6.5 GPa in tensile strength and 350 GPa in tensile modulus. The results are shown in Table 1.

(Example 3)
Using the same polyacrylonitrile fiber as in Example 1, the first furnace is set to a temperature of 250 ° C., the second furnace is set to a temperature of 270 ° C., the third furnace is set to 285 ° C., and the heat treatment time in the second furnace is set to The flameproofing treatment was performed for shorter than the heat treatment time in one furnace. At that time, the specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.27 g / cc, and the integrated value of the amount of heat given to the polyacrylonitrile fiber was 190 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers were 6.9 GPa in tensile strength and 350 GPa in tensile modulus. The results are shown in Table 1.

Example 4
A carbon fiber was produced in the same manner as in Example 1 except that the carbonization temperature of Example 1 was 1800 ° C. Regarding the strand properties of the obtained carbon fiber, the tensile strength was 6.7 GPa and the tensile modulus was 390 GPa. The results are shown in Table 1.

(Example 5)
Using the same polyacrylonitrile fiber as in Example 1, a flameproofing treatment was performed at a temperature of 260 ° C. using a flameproofing furnace composed of only one furnace. The integrated value of the amount of heat given to the polyacrylonitrile fiber in this flameproofing process was 450 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers were 6.9 GPa in tensile strength and 350 GPa in tensile modulus. The results are shown in Table 1.

(Example 6)
Using the same polyacrylonitrile fiber as in Example 1, a flameproofing treatment was performed at a temperature of 250 ° C. using a flameproofing furnace composed of only one furnace. The integrated value of the amount of heat given to the polyacrylonitrile fiber in this flameproofing process was 690 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers were a tensile strength of 6.6 GPa and a tensile elastic modulus of 350 GPa. The results are shown in Table 1.

(Example 7)
100 parts by weight of acrylonitrile, 1 part by weight of itaconic acid and 130 parts by weight of dimethyl sulfoxide are mixed, and 0.001 part by weight of 2,2′-azobisisobutyronitrile (hereinafter referred to as AIBN) is added as a weight initiator. Then, polymerization was carried out at a temperature of 70 ° C. for 4 hours under a nitrogen atmosphere to obtain a polymer solution. Next, 240 parts by weight of dimethyl sulfoxide, 0.4 parts by weight of AIBN, and 0.1 parts by weight of octyl mercaptan as a chain transfer agent were added to the obtained polymer solution, and held at a temperature of 60 ° C. for 4 hours. And kept at 80 ° C. for 6 hours to obtain a polyacrylonitrile-based polymer solution (spinning solution). The resulting polyacrylonitrile-based polymer solution (spinning stock solution) had an intrinsic viscosity of 2.0, and the polydispersity Mz / Mw was 3.0.

  Using the obtained spinning dope, yarn production was performed in the same manner as in Example 1 to obtain polyacrylonitrile fiber. The weight reduction start temperature of the obtained polyacrylonitrile fiber by TG measurement in nitrogen was 310 ° C.

  Next, four polyacrylonitrile fibers obtained were combined, 12,000 single fibers were used, a flameproofing furnace constituted by two furnaces was used, the temperature of the first furnace was set to 250 ° C., and the second The temperature of the furnace was set to 270 ° C., and the heat treatment time in the second furnace was made shorter than the heat treatment time in the first furnace to perform the flameproofing treatment. At that time, the specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.27 g / cc, and the integrated value of the amount of heat given to the polyacrylonitrile fiber was 470 J · h / g. Moreover, the weight reduction start temperature of the flameproofing yarn immediately after leaving the first furnace was 330 ° C. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers were 7.1 GPa in tensile strength and 350 GPa in tensile modulus. The results are shown in Table 1.

(Example 8)
Polymerization of 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN) was started with a polyacrylonitrile copolymer consisting of 99.5 mol% of acrylonitrile and 0.5 mol% of itaconic acid using dimethyl sulfoxide as a solvent. As an agent, polymerization was performed by a solution polymerization method in a nitrogen atmosphere to obtain a polyacrylonitrile-based polymer solution (spinning solution) having an intrinsic viscosity of 1.8. Using the obtained spinning dope, yarn was produced in the same manner as in Example 1 to obtain polyacrylonitrile fiber. The weight reduction start temperature of the obtained polyacrylonitrile fiber by TG measurement in nitrogen was 310 ° C. After the flameproofing step, carbon fibers were produced in the same manner as in Example 6. As a result, the specific gravity of the polyacrylonitrile fibers heat-treated in the first furnace was 1.29 g / cc, and the strand properties of the obtained carbon fibers Had a tensile strength of 7.4 GPa and a tensile modulus of 350 GPa. The results are shown in Table 1.

Example 9
Using the same polyacrylonitrile-based polymer solution (spinning stock solution) as in Example 8, carbon fibers were produced in the same manner as in Example 7 except that the single fiber fineness was 0.4 dtex. The specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.29 g / cc, and the strand properties of the obtained carbon fiber were a tensile strength of 8.3 GPa and a tensile elastic modulus of 360 GPa. The results are shown in Table 1.

(Example 10)
Carbon fibers were produced in the same manner as in Example 7 except that the same polyacrylonitrile polymer solution (spinning stock solution) as in Example 8 was used and the single fiber fineness was 1.1 dtex. The specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.28 g / cc, and the strand properties of the obtained carbon fiber were a tensile strength of 6.4 GPa and a tensile elastic modulus of 320 GPa. The results are shown in Table 1.

(Example 11)
A carbon fiber was produced in the same manner as in Example 8 except that the carbonization temperature of Example 8 was 1800 ° C. Regarding the strand properties of the obtained carbon fiber, the tensile strength was 6.3 GPa and the tensile modulus was 390 GPa. The results are shown in Table 1.

(Comparative Example 1)
Using the same polyacrylonitrile fiber as in Example 1, the integrated value of the amount of heat given to the polyacrylonitrile fiber in the flameproofing process is 3900 J at a temperature of 215 ° C. using a flameproofing furnace composed of only one furnace. A flameproofing treatment was performed under h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand properties of the obtained carbon fibers had a tensile strength as low as 5.5 GPa although the tensile modulus was 350 GPa. Met. The results are shown in Table 1.

(Comparative Example 2)
In the flameproofing step of Comparative Example 1, when the integrated value of the amount of heat given to the polyacrylonitrile fiber was performed under 500 J · h / g, the yarn was broken in the preliminary carbonization step, and carbon fibers could be obtained. There wasn't.

(Comparative Example 3)
The flameproofing treatment of Example 1 was performed by setting the temperature of the first furnace to 240 ° C., the temperature of the second furnace to 250 ° C., and the heat treatment time in the second furnace shorter than the heat treatment time in the first furnace. At that time, the specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.29 g / cc, and the integrated value of the amount of heat given to the polyacrylonitrile fiber was 920 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand physical properties of the obtained carbon fibers were as low as 5.9 GPa although the tensile modulus was 350 GPa. Met. The results are shown in Table 1.

(Comparative Example 4)
In the flameproofing treatment of Example 1, the heat treatment time in the second furnace was shorter than the heat treatment time in the first furnace. When the temperature in the second furnace was 320 ° C., the polyacrylonitrile heat-treated in the first furnace was used. The specific gravity of the fiber was 1.27 g / cc, and the integrated value of the amount of heat given to the polyacrylonitrile fiber was 290 J · h / g. did. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As a result, the strand physical properties of the obtained carbon fibers were as low as 6.3 GPa, although the tensile modulus was 350 GPa. Met. The results are shown in Table 1.

(Comparative Example 5)
In the flameproofing treatment of Example 1, flameproofing was performed by increasing the residence time in the furnace while making the heat treatment time in the second furnace shorter than the heat treatment time in the first furnace. At that time, the specific gravity of the polyacrylonitrile fiber heat-treated in the first furnace was 1.33 g / cc, and the flame resistance treatment was performed by setting the integrated value of the amount of heat given to the polyacrylonitrile fiber to 960 J · h / g. After the preliminary carbonization step, carbon fibers were produced in the same manner as in Example 1. As for the strand properties of the obtained carbon fiber, although the tensile modulus was 350 GPa, the tensile strength was a low value of 6.1 GPa. The results are shown in Table 1.

(Comparative Example 6)
When flame resistance of Example 5 was performed at a temperature of 300 ° C., yarn breakage frequently occurred during flame resistance, and a flame resistant yarn could not be stably obtained. The results are shown in Table 1.

Claims (9)

  The carbon fiber polyacrylonitrile fiber is heat-treated in air at a temperature of 220 ° C. or higher, and the integrated value of the amount of heat applied at that time is 100 J · h / g to 800 J · h / g. Production method.   The process of heat-treating the polyacrylonitrile fiber for carbon fiber in air is continuously performed in a plurality of stages at a temperature of 220 ° C. or higher, and the heat treatment temperature in each stage is based on the weight loss starting temperature of the polyacrylonitrile fiber heat-treated in the previous stage. Is a temperature lower than 20 ° C., and the integrated value of the amount of heat given to the polyacrylonitrile fiber is 100 J · h / g to 500 J · h / g.   The step of heat-treating the polyacrylonitrile fiber for carbon fiber in air is continuously performed at a temperature of 220 ° C. or higher, and the heat treatment temperature at that time is 20 ° C. or more lower than the weight loss starting temperature of the polyacrylonitrile fiber. And the integrated value of the amount of heat given to the polyacrylonitrile fiber is 100 J · h / g to 700 J · h / g.   The heat treatment time in each stage is the same as or shorter than the heat treatment time in the subsequent stage, and the specific gravity of the polyacrylonitrile fiber heat-treated in the first heat treatment step is 1.32 g / cc or less. The manufacturing method of the carbon fiber of Claim 2.   The method for producing carbon fiber according to any one of claims 1 to 4, wherein the polyacrylonitrile fiber for carbon fiber comprises a polyacrylonitrile polymer having an intrinsic viscosity of 1.0 to 10.   The polyacrylonitrile fiber for carbon fibers is in the range of intrinsic viscosity of 1.0 to 5.0, and the polydispersity Mz / Mw indicated by the ratio of the Z average molecular weight Mz to the weight average molecular weight Mw is 2.7 to 10 The carbon fiber production method according to claim 1, comprising a polyacrylonitrile-based polymer that is 0.0.   The polyacrylonitrile fiber for carbon fiber is a spinning process in which a polyacrylonitrile polymer is spun by spinning from a spinneret by a wet spinning method or a dry-wet spinning method, and a dry heat treatment in which the fiber obtained in the spinning step is subjected to a dry heat treatment. The method for producing a carbon fiber according to claim 5 or 6, wherein the fiber is produced through a steam drawing step of steam drawing the fiber obtained in the step and the drying heat treatment step.   A flameproofing step in which the polyacrylonitrile fiber for carbon fiber according to claim 7 is flameproofed in the air, and the fiber obtained in the flameproofing step is pre-carbonized in an inert atmosphere at a temperature of 300 to 800 ° C. The carbonization process which carbonizes the fiber obtained by this preliminary carbonization process and the fiber obtained by this preliminary carbonization process in the inert atmosphere of the temperature of 1,000-2,000 degreeC, The any one of the Claims 1-4 A method for producing the carbon fiber according to claim 1.   The carbon fiber manufacturing method according to claim 8, wherein the carbonization is carried out at a carbonization step of 4.0 mN / dTex to 35.0 mN / dTex.