阅读说明：本技术 碳纤维束及其制造方法 (Carbon fiber bundle and method for producing same ) 是由 堀之内绫信 渡边史宜 冲嶋勇纪 于 2020-02-26 设计创作，主要内容包括：本发明的课题在于提供能够对工艺油剂向纤维表层的侵入进行抑制,并且能够对纤维间粘接及表层的孔隙进行抑制的碳纤维束的制造方法、碳纤维束,作为解决手段而提供如下碳纤维束：通过广角X射线衍射法所得的微晶尺寸(Lc)为3.0nm以下,在自纤维表面起深度为0～10nm的区域中,存在通过SIMS(二次离子质谱法)算出的Si/C比为10以上的点,自纤维表面起深度为10nm处的通过SIMS算出的Si/C比为1.0以下。(The present invention addresses the problem of providing a method for producing a carbon fiber bundle and a carbon fiber bundle, which can suppress the penetration of a process oil into the fiber surface layer and can suppress the adhesion between fibers and the formation of voids in the surface layer, and which provide the following carbon fiber bundle as a solution: the crystallite size (Lc) obtained by a wide-angle X-ray diffraction method is 3.0nm or less, points with an Si/C ratio of 10 or more calculated by SIMS (secondary ion mass spectrometry) exist in a region with a depth of 0 to 10nm from the fiber surface, and the Si/C ratio calculated by SIMS at a depth of 10nm from the fiber surface is 1.0 or less.)

1. A method for producing a carbon fiber bundle, wherein a polyacrylonitrile-based polymer solution is extruded from a spinneret into air, immersed in a coagulation bath stored in the coagulation bath, and drawn from the coagulation bath into air to obtain a coagulated fiber bundle, and then at least a water washing step, a drawing step, an oil agent application step, and a drying step are performed to obtain a carbon fiber precursor fiber bundle, and then a flame-resistant step of subjecting the carbon fiber precursor fiber bundle to a flame-resistant treatment in an oxidizing atmosphere at a temperature of 200 to 300 ℃, a pre-carbonization step of subjecting the carbon fiber precursor fiber bundle to a pre-carbonization treatment in an inert atmosphere at a maximum temperature of 500 to 1200 ℃, and a carbonization step of subjecting the carbon fiber precursor fiber bundle to a carbonization treatment in an inert atmosphere at a maximum temperature of 1200 to 2000 ℃,

in the manufacturing method, the coagulation bath liquid contains 70-85% of at least 1 organic solvent selected from the group consisting of dimethyl sulfoxide, dimethylformamide and dimethylacetamide, and the temperature of the coagulation bath liquid is-20 ℃,

the immersion time of the polyacrylonitrile-based polymer solution in the coagulation bath is 0.1 to 4 seconds,

after the coagulated fiber bundle is pulled out from the coagulation bath into the air and before the water washing step, an air retention step of retaining the coagulated fiber bundle in the air is performed for 10 seconds or more.

2. The method for producing a carbon fiber bundle according to claim 1, wherein the concentration of the organic solvent in the liquid existing around the coagulated fiber bundle immediately before being introduced into the water-washing bath after staying in the air is higher by 2% or more than the concentration of the organic solvent in the coagulation bath.

3. A carbon fiber bundle in which the crystallite size (Lc) obtained by a wide-angle X-ray diffraction method is 1.0 to 3.0nm or less,

in a region having a depth of 0 to 10nm from the fiber surface, there is a point having an Si/C ratio of 10 or more as calculated by SIMS (secondary ion mass spectrometry),

the Si/C ratio calculated by SIMS at a depth of 10nm from the fiber surface is 1.0 or less.

4. The carbon fiber bundle according to claim 3, wherein the Si/C ratio calculated by SIMS at a depth of 50nm from the fiber surface is 0.5 or less.

5. The carbon fiber bundle according to claim 3 or 4, wherein 50 or less pores having a major diameter of 3nm or more are present in a region from the fiber surface to a depth of 50nm in a cross section of the single fiber, and the average width of the pores is 3 to 15 nm.

6. The carbon fiber bundle according to any one of claims 3 to 5, which has a strand tensile elastic modulus of 200 to 450 GPa.

Technical Field

The present invention relates to a carbon fiber bundle suitable for use in sports applications such as aircraft parts, automobile parts, and ship parts, golf clubs, fishing rods, and other general industrial applications.

Background

Since carbon fibers have higher specific strength and specific modulus than other fibers, they are widely used as reinforcing fibers for composite materials not only in conventional sports applications, aviation and aerospace applications, but also in general industrial applications such as automobiles, civil engineering and construction, pressure vessels, and wind turbine blades, and there is a strong demand for further improvement in performance (particularly improvement in tensile strength of a wire harness).

Among carbon fibers, polyacrylonitrile (hereinafter abbreviated as PAN) carbon fibers, which is the most widely used carbon fibers, are industrially produced by spinning a spinning solution containing a PAN polymer as a precursor thereof by a wet spinning method or a dry-wet spinning method to obtain a carbon fiber precursor fiber, then heating the carbon fiber precursor fiber in an oxidizing atmosphere at a temperature of 200 to 300 ℃ to convert the carbon fiber into a flame-resistant fiber, and then heating and carbonizing the carbon fiber in an inert atmosphere at a temperature of at least 1200 ℃.

Carbon fibers are brittle materials, and therefore, it is necessary to thoroughly suppress defects for the improvement of tensile strength of their strands. In particular, the breakage of carbon fibers often occurs from the surface thereof, and at present, the quality is improved by optimizing the process, and most of the breakage occurs from defects in the vicinity of the outermost surface within 10nm from the surface of the fiber. Defects on the surface of carbon fibers can be classified into three types, mainly, in addition to flaws and depressions generated during the process: defects due to bonding between fibers that occurs during flame-resistant treatment; defects due to pore-like defects (void defects) present in the surface layer of the fibers; defects due to chemical modification of the fiber surface layer, and the like, which are closely related to the process oil applied when spinning the carbon fiber precursor fiber bundle

Generally, a silicone process oil is applied to a carbon fiber precursor fiber for the purpose of suppressing the adhesion between fibers by heating in a flame-retardant step. In this way, the inter-fiber adhesion can be greatly suppressed, and the tensile strength of the wire harness can be improved, but not only is the poor suppression of the inter-fiber adhesion caused by uneven adhesion to the fibers, but also the process oil penetrates into the interior of the precursor fiber, and the process oil stays in the microstructure of the precursor fiber to induce a pore defect (void defect) of about several nm to several tens of nm in a region having a depth of 50nm or less from the fiber surface, and even if no pore is formed, the fiber surface contains an Si element to form an atomic defect, and therefore, even in the case where the pore defect can be suppressed, only a certain strength-improving effect is obtained.

Several proposals have been made so far for the purpose of improving the uniform adhesion of the process oil to the precursor fiber and suppressing the penetration of the process oil into the precursor fiber. Patent document 1 proposes a technique for improving the uniform adhesion of a process finish to a fiber by controlling the denseness and tension of a precursor fiber in a finish-applying step. Patent document 2 proposes a method in which a precursor fiber is stretched up to 8 times or more before an oil agent is applied, thereby increasing the denseness of the precursor fiber and suppressing the penetration of the oil agent. Patent document 3 proposes a method of applying a coagulation bath having a low coagulation rate and appropriately drawing the fibers in a state of containing an organic solvent to improve the denseness of the precursor fibers and suppress void defects. Patent document 4 proposes a technique of reducing the concentration of silicone that permeates into fibers and suppressing the amount of silicone that permeates into the fibers by using a mixed oil of a silicone oil and a non-silicone oil as a process oil. Patent document 5 proposes a method in which the silicone oil is applied in 2 stages to improve the uniform adhesion of the oil to the fiber bundle and to suppress the penetration of the oil into the fiber.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-160312

Patent document 2: japanese patent No. 6359860

Patent document 3: japanese patent No. 4945684

Patent document 4: japanese patent laid-open publication No. 2011-202336

Patent document 5: japanese laid-open patent publication No. 11-124744

Disclosure of Invention

Problems to be solved by the invention

However, in the technique of patent document 1, although the uniform adhesion of the finish can be improved, the penetration of the process finish into the fiber in the vicinity of the outermost surface of the fiber (from the fiber surface to a depth of about 10 nm) which is most important in the tensile strength of the strand cannot be sufficiently suppressed. In the technique of patent document 2, inhibition of oil penetration of the process oil into the interior of the precursor fiber is observed, but there are the following problems: the effect of suppressing penetration in the vicinity of the outermost surface of the fiber is insufficient, and the drawing rate in the process of applying the process finish is increased due to an excessively high draw ratio, so that the uniform adhesion of the process finish is deteriorated. In the technique of patent document 3, although the effect of suppressing the void defect is confirmed, there are the following problems: the effect of suppressing the penetration of the finish oil into the vicinity of the outermost surface of the fiber is insufficient, and the fiber is stretched in a state containing an organic solvent to induce the adhesion between the fibers. Patent document 4 is suspected to be able to suppress the amount of penetration of the silicone oil agent into the fiber, but the effect of suppressing the inter-fiber adhesion is not sufficient as compared with a silicone oil agent containing no non-silicone component, and even if the silicone oil agent is a non-silicone component, the silicone oil agent becomes an atomic defect when penetrating into the fiber, and therefore there is a limit in showing high beam tensile strength. Patent document 5 can suppress the penetration of an oil agent having a depth of 50 to 100nm from the fiber surface, but has a problem that it is difficult to suppress the penetration near the outermost surface of the fiber, and that it is a multi-stage process. That is, the conventional techniques do not relate to a technique capable of suppressing penetration of a process finish, particularly, a process finish into the vicinity of the outermost surface of a fiber (the depth from the fiber surface is about 10 nm), and suppressing inter-fiber adhesion and void defects.

Accordingly, an object of the present invention is to provide a method for producing a carbon fiber bundle, which can suppress the penetration of a process oil agent into a fiber surface layer and can suppress the adhesion between fibers and the surface layer voids, and a carbon fiber bundle.

Means for solving the problems

To achieve the above object, the present invention includes the following configurations.

That is, the method for producing a carbon fiber bundle of the present invention is characterized in that a polyacrylonitrile-based polymer solution is extruded from a spinneret into air, dipped in a coagulation bath stored in a coagulation bath, and drawn from the coagulation bath into air to obtain a coagulated fiber bundle, and then at least a water washing step, a drawing step, an oil agent application step, and a drying step are performed to obtain a carbon fiber precursor fiber bundle, and then a flame-resistant step of subjecting the carbon fiber precursor fiber bundle to a flame-resistant treatment in an oxidizing atmosphere at a temperature of 200 to 300 ℃, a pre-carbonization step of subjecting the carbon fiber precursor fiber bundle to a pre-carbonization treatment in an inert atmosphere at a maximum temperature of 500 to 1200 ℃, and a carbonization step of subjecting the carbon fiber precursor fiber bundle to a carbonization treatment in an inert atmosphere at a maximum temperature of 1200 to 2000 ℃; in the manufacturing method, the coagulation bath liquid contains 70-85% of at least 1 organic solvent selected from the group consisting of dimethyl sulfoxide, dimethylformamide and dimethylacetamide, and the temperature of the coagulation bath liquid is-20 ℃; the immersion time of the polyacrylonitrile polymer solution in the coagulating bath liquid is 0.1 to 4 seconds; after the coagulated fiber bundle is pulled out from the coagulation bath into the air and before the water washing step, an air retention step of retaining the coagulated fiber bundle in the air is performed for 10 seconds or more.

The carbon fiber bundle of the present invention is characterized in that the crystallite size (Lc) obtained by a wide-angle X-ray diffraction method is 3.0nm or less, the single fiber has a point where the Si/C ratio calculated by SIMS (secondary ion mass spectrometry) is 10 or more in a region of 0 to 10nm in depth from the fiber surface, and the Si/C ratio calculated by SIMS at 10nm in depth from the fiber surface is 1.0 or less.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to produce a carbon fiber bundle having excellent bundle tensile strength by suppressing the intrusion of a process oil into the fiber surface layer and suppressing the adhesion between fibers and the voids in the surface layer.

Detailed Description

[ method for producing carbon fiber bundle ]

(spinning method)

As a spinning method for producing the coagulated fiber bundle of the present invention, a dry-wet spinning method is used. The dry-wet spinning method is a spinning method in which a polyacrylonitrile-based (PAN-based) polymer solution, which is a spinning solution, is extruded from a spinneret into the air, immersed in a coagulation bath stored in the coagulation bath, and then drawn from the coagulation bath into the air to obtain a coagulated fiber bundle. In the wet spinning method, stripe-like irregularities of several tens of nm or more are formed in the fiber axial direction on the fiber surface, and the irregularities become defects and break, so it is difficult to improve the tensile strength of the yarn bundle using the present invention.

(PAN-based polymer solution)

The polymer used in the PAN-based polymer solution in the present invention is a PAN-based polymer (polyacrylonitrile or a copolymer containing polyacrylonitrile as a main component, or a mixture containing polyacrylonitrile as a main component). The polyacrylonitrile is used as a main component, and in the copolymer using the polyacrylonitrile as a main component, the acrylonitrile accounts for 85-100 mol% of a polymer skeleton; in the mixture mainly composed of polyacrylonitrile, the copolymer mainly composed of polyacrylonitrile accounts for 85 to 100 mass% of the mixture. The solvent of the PAN-based polymer solution is at least 1 organic solvent selected from the group consisting of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide. The temperature of the PAN-based polymer solution discharged from the spinneret is not particularly limited, and may be appropriately determined from the viewpoint of discharge stability.

(coagulation bath)

In the coagulation bath solution of the present invention, a mixture of an organic solvent used as a solvent in a PAN-based polymer solution, which is at least 1 selected from the group consisting of dimethyl sulfoxide, dimethylformamide and dimethylacetamide, and a so-called coagulation promoting component is used. As the coagulation promoting component, water is preferably used. The organic solvent concentration of the coagulation bath is a very important factor in the present invention. The present invention is characterized in that solidification is performed slowly in the air by passing the liquid through the liquid in a semi-solidified state without completing solidification in the liquid. Therefore, the coagulation bath used needs to be a material that slows the coagulation rate. The concentration of the organic solvent is desirably 70 to 85 mass%, preferably 75 to 82 mass%. If the organic solvent concentration in the coagulation bath is too low, the coagulation rate is high and the solution is difficult to pass through the coagulation bath in a semi-coagulated state; in addition, when the organic solvent concentration in the coagulation bath is high, the coagulation rate is too slow and the formation of fibers is difficult, and the pores in the surface layer increase during the formation of fibers. The temperature of the coagulating bath liquid in the invention needs to be-20 ℃, and preferably-10 ℃. The lower the temperature of the coagulation bath, the slower the coagulation rate, and the easier it is to pass through the coagulation bath in a semi-coagulated state; the higher the coagulation rate, the more difficult it is to pass through the coagulation bath in a semi-coagulated state. The surface porosity is easily suppressed at lower temperatures in the coagulation bath.

(solidification step)

The dipping time of the spinning solution in the coagulation bath liquid is required to be 0.1 to 4 seconds, preferably 0.1 to 2 seconds, and more preferably 0.1 to 1 second. If the dipping time in the coagulation bath is too short, the fibers are difficult to form; if it is too long, it becomes difficult to pass through the coagulation bath in a semi-coagulated state. The dipping time in the coagulation bath can be controlled by varying the dipping length in the coagulation bath, varying the draw speed of the spinning solution.

The semi-coagulated state means a state in which solvent exchange between the spinning solution in the coagulation bath and the coagulation promoting component in the coagulation bath is not completed. The solvent exchange means that the organic solvent (solvent) in the spinning solution and the coagulation promoting component outside the spinning solution are diffused into each other so as to have uniform concentrations, and the concentrations of the organic solvent and the coagulation promoting agent in the spinning solution and the concentrations of the solvent and the coagulation promoting agent outside the spinning solution are the same. Therefore, when the solvent exchange in the coagulation bath liquid is completed, that is, when the coagulation in the coagulation bath liquid is completed, the organic solvent concentration and the coagulation accelerator concentration in the spinning solution become the same as those in the coagulation bath liquid. On the other hand, when the solvent exchange in the coagulation bath is not completed, that is, when the coagulation is in a semi-coagulated state in which the coagulation is not completed at the time point when the coagulation bath is drawn into the air, the organic solvent concentration of the liquid existing around the coagulated fiber bundle after passing through the coagulation bath becomes higher with time than the organic solvent concentration of the coagulation bath. This is because the organic solvent in the spinning solution in a semi-coagulated state is exchanged with the coagulation promoting component of the liquid existing around the organic solvent after passing through the coagulation bath. The solvent exchange after passing through the coagulation bath solution is performed in the air retention step described below, but the solvent exchange after passing through the coagulation bath solution is performed very slowly compared to the solvent exchange in the coagulation bath solution.

(air residence step)

In the present invention, after the spinning solution is passed through the coagulation bath in a semi-coagulated state, the air-entrapping step of entrapping the spinning solution in the air is performed for 10 seconds or more. The air-entrapping step is required to be performed immediately after passing through the coagulation bath and before being introduced into the water-washing bath. As a result, the coagulated fiber bundle passing through the coagulation bath in a semi-coagulated state is gradually coagulated in the air, and the denseness of the fiber bundle, particularly the surface layer, is remarkably improved in this step. This slow solvent exchange is not possible in the coagulation bath, and is only first achieved by coagulation in air. The air residence time is preferably 10 seconds or more, more preferably 30 seconds or more, and still more preferably 100 seconds or more. If the air residence time is too short, the fiber bundle is introduced into the water bath in a state in which the air coagulation is not completed, and the compactness of the fiber bundle is lowered. The setting time in air is completed within 300 seconds even if it is long, and therefore it is not effective if it is longer. The effect of the present invention can be obtained even if the temperature in the air during the air stagnation is not controlled, but the temperature of 5 to 50 ℃ is preferable because the solidification unevenness can be further reduced. In the present invention, it is preferable that the organic solvent concentration of the liquid existing around the coagulated fiber bundle after the retention in the air, immediately before the introduction into the water-washing bath, is higher by 2% or more than the organic solvent concentration in the coagulation bath. Here, the coagulated fiber bundle after staying in the air and immediately before being introduced into the water washing bath means a coagulated fiber bundle at a position of 0.3 seconds immediately before being introduced into the water washing bath. When the organic solvent concentration of the liquid present around the coagulated fiber bundle is higher than the organic solvent concentration in the coagulation bath, the denseness of the fiber bundle is likely to be improved, and is more preferably higher by 3% or more, and still more preferably higher by 5% or more than the organic solvent concentration in the coagulation bath. The organic solvent concentration of the liquid present around the coagulated fiber bundle can be controlled by the organic solvent concentration in the coagulation bath, the temperature, the immersion time in the coagulation bath, and the residence time in the air. The organic solvent concentration of the liquid present around the coagulated fiber bundle can be measured by sampling the liquid present around the coagulated fiber bundle at a position 0.3 seconds before the liquid travels in the air and is introduced into the water washing bath, and using a refractometer or a gas chromatograph.

(Water washing step, drawing step, oil application step, drying step)

In the present invention, a PAN polymer solution is introduced into a coagulation bath to be semi-coagulated and left in the air, and then subjected to a water washing step, a drawing step, an oil agent application step, and a drying step to obtain a carbon fiber precursor fiber bundle.

In the water washing step, the coagulated fiber bundle having undergone the air-entrapping step is introduced into a water washing bath for the purpose of further removing the organic solvent from the coagulated fiber bundle. In order to improve the fiber advancing performance in the water washing step, the fiber may be stretched 1 to 1.5 times in the water washing step.

The stretching step may be carried out in a single or multiple stretching baths adjusted to a temperature of 30 to 98 ℃. The drawing in the bath in the drawing step is referred to as in-bath drawing, and the ratio thereof is referred to as in-bath draw ratio. The draw ratio in the bath is preferably set to 2 to 2.8 times. If the total draw ratio before the finish-application step exceeds 3 times, the denseness of the surface layer is reduced, and the finish easily penetrates into the fiber. The total draw ratio before the finish-application step is the product of the draw ratio in the water-washing step and the draw ratio in the bath.

The finish-applying step is a step of applying a finish for the purpose of preventing adhesion of fibers after the in-bath stretching step. As the oil agent used in this step, an oil agent containing silicone as a main component is preferably used. If the finish does not contain silicone, adhesion between fibers in the flame-retardant process cannot be suppressed, and the tensile strength of the wire harness is reduced. In addition, it is preferable to use an oil containing a modified silicone such as an amino-modified silicone having high heat resistance. Examples of the other silicone oil agent include silicones modified by epoxy modification or alkylene oxide modification. The method of applying the silicone oil is not particularly limited, but it is necessary to apply the silicone oil so that a point having an atomic ratio of Si to C, i.e., an Si/C ratio of 10 or more, exists in a portion having a depth of 0 to 10nm from the surface of the carbon fiber, which is determined by SIMS (secondary ion mass spectrometry). When the Si/C ratio is 10 or less, the effect of suppressing the inter-fiber adhesion is insufficient, and the tensile strength of the strand is lowered.

The drying step may be carried out by a known method. In addition, from the viewpoint of improving productivity and improving the degree of crystal orientation, it is preferable to perform stretching in a heating heat medium after the drying step. As the heating heat medium, for example, pressurized steam or superheated steam is suitably used from the viewpoint of operation stability and cost.

After the drying step, a dry heat stretching step and a steam stretching step may be added.

(firing Process)

Next, a method for producing a carbon fiber bundle of the present invention will be described. The method for producing a carbon fiber bundle of the present invention comprises a flame-resistant step of subjecting a carbon fiber precursor fiber bundle produced by the above method to a flame-resistant treatment in an oxidizing atmosphere at a temperature of 200 to 300 ℃, a pre-carbonization step of subjecting the carbon fiber precursor fiber bundle to a pre-carbonization treatment in an inert atmosphere at a maximum temperature of 500 to 1200 ℃, and a carbonization step of subjecting the carbon fiber precursor fiber bundle to a carbonization treatment in an inert atmosphere at a maximum temperature of 1200 to 2000 ℃.

Air is preferably used as the oxidizing atmosphere in the flame resistance treatment. In the present invention, the pre-carbonization and carbonization are performed in an inert atmosphere. Examples of the gas used in the inert atmosphere include nitrogen, argon, xenon, and the like, and from the viewpoint of economy, nitrogen is preferably used.

(surface modification step)

The obtained carbon fiber bundle may be subjected to electrolytic treatment for surface modification thereof. This is because the adhesion to the carbon fiber matrix can be optimized in the obtained fiber-reinforced composite material by the electrolytic treatment. After the electrolytic treatment, sizing treatment may be performed to impart bundling property to the carbon fiber bundle. The sizing agent is suitably selected depending on the kind of the resin used, and has good compatibility with the matrix resin.

(carbon fiber bundle)

The carbon fiber bundle obtained by the present invention is characterized in that the single fibers have a point in which the atomic ratio Si/C of Si to C calculated by SIMS (secondary ion mass spectrometry) is 10 or more in a region having a depth of 0 to 10nm from the fiber surface, and the single fibers have a Si/C ratio of 1.0 or less calculated by SIMS at a depth of 10nm from the fiber surface. When the Si/C ratio is less than 10 in the entire region of 0 to 10nm in depth, the effect of suppressing the bonding between fibers is insufficient, and the tensile strength of the wire harness is lowered. In addition, when the Si/C ratio at a depth of 10nm from the fiber surface is greater than 1.0, the oil agent penetrates into the fiber surface layer portion, induces a void defect in the surface layer, and the tensile strength of the strand decreases due to the Si element contained in the fiber surface layer portion. Further, it is preferable that the Si/C ratio at a depth of 50nm from the fiber surface is 0.5 or less because the penetration of the oil agent into not only the fiber surface layer but also the inner layer is suppressed, and thus a high tensile strength of the wire harness is exhibited. In the SIMS measurement, carbon fiber bundles are aligned, and primary ions are irradiated from the fiber surface by the following measurement apparatus and measurement conditions to measure generated secondary ions. In the case where the sizing agent adheres to the carbon fiber bundle to be measured, evaluation was performed after the sizing agent was removed by soxhlet extraction using an organic solvent in which the sizing agent was dissolved.

The device comprises the following steps: SIMS4550 manufactured by FEI

Primary ion species: o is₂ ⁺

Primary ion energy: 3keV

Detection of secondary ion polarity: positive ion

Charging compensation: electron gun

Primary ion incident angle: 0 degree

In the carbon fiber bundle of the present invention, it is preferable that the number of pores having a major diameter of 3nm or more existing in a region from the fiber surface to a depth of 50nm in a cross section of a single fiber is 50 or less, and the average width of the pores is 3 to 15 nm. When the number of voids existing in the region from the fiber surface to the depth of 50nm is small, a high tensile strength of the strand is exhibited, and therefore, the number of voids is more preferably 30 or less, and still more preferably 10 or less. In addition, when the average width of the pores is small, a high tensile strength of the wire harness is exhibited, and therefore, it is preferably 3 to 10nm, and more preferably 3 to 5 nm. Here, the average width of the pores means an arithmetic average of major axes of the pores by the following calculation method. The number and average width of pores in the cross section of the carbon fiber bundle were determined as follows. First, a sheet having a thickness of 100nm was prepared by a Focused Ion Beam (FIB) in a direction perpendicular to the fiber axis of a carbon fiber bundle, and the cross section of the carbon fiber was observed by a Transmission Electron Microscope (TEM) at 1 ten thousand times. In the observed image, the length from the end of the void to the longest portion among the white voids present in the region from the fiber surface to the depth of 50nm was defined as the major axis. The number of pores is the total number of pores having a major axis of 3nm or more counted in 1 cross section. The average width of the pores is an arithmetic average of the major axes of all pores having a major axis of 3nm or more in the observed image obtained as described above.

The tensile strength and tensile modulus of the carbon fiber bundle in the present invention were determined by the following procedures in accordance with the resin-impregnated bundle strength test method of JIS-R-7608 (2004). As a resin formulation, "Celloxide (registered trademark)" 2021P/3 boron fluoride monoethylamine/acetone 100/3/4 (parts by mass), and as curing conditions, normal pressure, temperature 125 ℃, time 30 minutes were used. The average value of the tensile strength and tensile elastic modulus of the bundle was determined for 10 carbon fiber bundles. If the tensile elastic modulus of the wire harness is too low, the tensile strength of the wire harness is reduced; if too high, the tensile strength of the wire harness is lowered, and therefore, the tensile elastic modulus of the wire harness is preferably set to 200 to 450GPa, more preferably 250 to 400GPa, and still more preferably 270 to 400 GPa.

The carbon fiber bundle of the present invention has a crystallite size (Lc) of 1.0 to 3.0nm obtained by a wide-angle X-ray diffraction method. If the crystallite size is too small, the tensile strength of the wire harness is reduced; if too high, the tensile strength of the wire harness decreases, and therefore, it is preferably 1.5 to 2.8nm, more preferably 2.0 to 2.8 nm. Carbon fibers are substantially polycrystalline bodies composed of numerous graphite crystallites, and increasing the maximum temperature of carbonization increases the crystallite size and at the same time the crystal orientation progresses, which leads to an increase in the tensile modulus of elasticity of the strands of carbon fibers. If the crystallite size is 1.0nm or more, the tensile elastic modulus of the carbon fiber strand can be increased; when the crystallite size is larger than 3.0nm, the tensile elastic modulus of the strand increases, but the tensile strength of the strand decreases. The crystallite size was measured under the following conditions.

X-ray source: CuK alpha ray (tube voltage 40kV, tube current 30mA)

The detector: goniometer, monochromator and scintillation counter

Scan range: 2 theta is 10 to 40 DEG

Scan mode: step scanning with 0.01 degree of step unit and 1 degree/min scanning speed

In the obtained diffraction pattern, the full width at half maximum was obtained for the peak appearing in the vicinity of 2 θ of 25 to 26 °, and the crystallite size was calculated from this value by the following equation.

Crystallite size (nm) ═ K λ/β₀cosθ_B

Wherein the content of the first and second substances,

k: 1.0, λ: 0.15418nm (wavelength of X-ray)

β₀：(βE²-β₁ ²)^1/2

β_E: apparent full width at half maximum (measured value) rad, beta₁：1.046×10-²rad

θ_B: diffraction angle of Bragg.

Examples

(example 1)

Polyacrylonitrile-based polymer including copolymer of acrylonitrile and itaconic acid was dissolved in dimethyl sulfoxide to prepare spinning solution. The obtained spinning solution was once extruded from the spinneret into the air, introduced into a coagulation bath solution in which dimethyl sulfoxide was mixed in a ratio of 80 mass% and water as a coagulation accelerator was 20 mass%, and the temperature was controlled at 5 ℃, and pulled so that the immersion time in the coagulation bath solution was 0.2s, thereby obtaining a coagulated fiber bundle. Hereinafter, the step of obtaining the coagulated fiber bundle is simply referred to as "coagulation step".

Then, in the air retention step, 120 seconds of air retention was carried out. It was confirmed that the liquid present around the coagulated fiber bundle at a position 0.3 seconds before introduction into the water-washing bath had an organic solvent concentration of 87% and was higher in concentration than the organic solvent concentration in the coagulation bath, and thus the liquid passed through the coagulation bath in a semi-coagulated state and coagulated in the air.

Then, in the water washing step, the coagulated yarn was introduced into a water bath and washed with water, and in the drawing step, bath drawing was performed in warm water at 90 ℃. At this time, the total draw ratio was 2.3 times. Next, in the finish-applying step, an amino-modified silicone-based silicone finish is applied to the fiber bundle. Then, the resultant was dried by using a heated roll at 180 ℃ and subjected to 5-fold drawing in pressurized steam to thereby obtain a polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 1.0dtex, with the total draw ratio of the yarn being 11.5-fold.

Next, the polyacrylonitrile-based precursor fiber bundle obtained is treated in the following firing step to produce a carbon fiber bundle.

In the flame-retardant treatment step, the obtained polyacrylonitrile-based precursor fiber bundle is subjected to flame-retardant treatment in air at a temperature of 200 to 300 ℃ to obtain a flame-retardant fiber bundle.

In the pre-carbonization step, the flame-retardant fiber bundle obtained in the flame-retardant step is subjected to pre-carbonization treatment in a nitrogen atmosphere with the highest temperature of 800 ℃ to obtain a pre-carbonized fiber bundle.

In the carbonization step, the pre-carbonized fiber bundle obtained in the pre-carbonization step was carbonized in a nitrogen atmosphere at a maximum temperature of 1500 ℃.

Next, electrolytic surface treatment, water washing, and drying are performed using an aqueous sulfuric acid solution as an electrolyte solution, and then a sizing agent is applied to the resultant, thereby obtaining a carbon fiber bundle. Spinning conditions and physical properties of the obtained carbon fibers are summarized in table 1, and examples and comparative examples described below are also summarized in tables 1 to 4. The tensile strength of the wire harness was 6.3 GPa.

(example 2)

The same operation as in example 1 was performed except that the dipping time in the coagulation bath in the coagulation step was set to 3.7 seconds. The organic solvent concentration of the liquid present around the coagulated fiber bundle at a position 0.3 seconds before introduction into the water washing bath in the air retention step was 82%, which was lower than that in example 1, and the coagulation was performed to some extent in the coagulation bath. The Si/C ratio at a depth of 10nm from the surface layer of the fiber and the Si/C ratio at a depth of 50nm from the surface layer of the fiber were higher than those of example 1, and the number of pores having a major axis of 3nm or more and the average width of the pores present in the region from the surface layer of the fiber to a depth of 50nm were increased, and the tensile strength of the strand was 5.8GPa and lower than that of example 1. Hereinafter, "the number of pores having a major axis of 3nm or more existing in a region from the surface of the fiber to a depth of 50 nm" is simply referred to as "the number of pores in the surface layer".

(example 3)

The same operation as in example 1 was carried out except that the immersion time in the coagulation bath in the coagulation step was 0.8s, and the residence time in air in the air-residence step was 12 s. The tensile strength of the wire harness was 6.2 GPa.

(example 4)

The same operation as in example 3 was performed except that the residence time in air in the air residence step was changed to 35 s. The tensile strength of the wire harness was 6.4GPa, which was 0.2GPa higher than that of example 3.

(example 5)

The same operation as in example 3 was performed except that the residence time in air in the air residence step was changed to 120 s. The tensile strength of the wire harness was 6.5GPa, which was 0.1GPa higher than that of example 4.

(example 6)

The same operation as in example 3 was performed except that the residence time in air in the air residence step was set to 200 s. The strand tensile strength was 6.5GPa, and it was found that densification by solidification in air was completed in about 120s in the same manner as in example 5.

(example 7)

The same operation as in example 1 was performed except that the dipping time in the coagulation bath in the coagulation step was set to 1.5 seconds. The tensile strength of the wire harness was 5.8 GPa.

(example 8)

The same operation as in example 7 was carried out except that the temperature of the coagulation bath in the coagulation step was set to 15 ℃. The temperature of the coagulation bath was high, and therefore the surface layer had an increased porosity as compared with example 7, and the tensile strength of the strand was 5.6 GPa.

(example 9)

The same operation as in example 7 was carried out except that the temperature of the coagulation bath in the coagulation step was changed to-5 ℃. Since the temperature of the coagulation bath was low, the Si/C ratio at a depth of 10nm from the surface layer of the fiber was lower than that of example 7, the porosity of the surface layer was lower than that of example 8, and the tensile strength of the strand was 6.1 GPa.

(example 10)

The same operation as in example 7 was carried out except that the temperature of the coagulation bath in the coagulation step was changed to-20 ℃. Since the temperature of the coagulation bath was low, the Si/C ratio at a depth of 10nm from the surface layer of the fiber was further decreased as compared with example 9, and the number of pores in the surface layer was decreased to give a tensile strength of 6.4 GPa.

(example 11)

The same operation as in example 7 was carried out except that the organic solvent concentration in the coagulation bath in the coagulation step was 85%. The Si/C ratio at a depth of 10nm from the surface layer of the fiber was smaller than that of example 7, but the organic solvent concentration was higher, so that the surface layer pores were larger than that of example 7, and the tensile strength of the strand was 5.8GPa, which was the same as that of example 7.

(example 12)

The same operation as in example 7 was carried out except that the organic solvent concentration in the coagulation bath in the coagulation step was 83%. The surface layer pores were reduced as compared with example 11, and the tensile strength of the strand was 5.9GPa, which was 0.1GPa higher than that of example 7.

(example 13)

The same operation as in example 7 was carried out except that the organic solvent concentration in the coagulation bath in the coagulation step was 75%. The Si/C ratio and surface voids at a depth of 10nm from the surface layer of the fiber were the same as in example 7, and the tensile strength of the strand was also 5.8GPa, which was the same as in example 7.

(example 14)

The same operation as in example 7 was carried out except that dimethylacetamide was used as the organic solvent of the polyacrylonitrile-based polymer solution as the spinning solution, and dimethylacetamide was used as the organic solvent of the coagulation bath in the coagulation step. The Si/C ratio at a depth of 10nm from the surface layer of the fiber, the Si/C ratio at a depth of 50nm from the surface layer of the fiber, and the number of pores and the average width of pores in the surface layer were not much different from those of example 7, and the tensile strength of the strand was 5.7GPa and also not much different from example 7.

(example 15)

The same operation as in example 7 was performed except that the organic solvent of the polyacrylonitrile-based polymer solution as the spinning solution was dimethylformamide and the organic solvent of the coagulation bath was dimethylformamide. The Si/C ratio at a depth of 10nm from the surface layer of the fiber, the Si/C ratio at a depth of 50nm from the surface layer of the fiber, and the number of pores and the average width of pores in the surface layer were not much different from those of example 7, and the tensile strength of the strand was also 5.8GPa, which was the same as that of example 7.

Comparative example 1

The same operation as in example 1 was carried out except that the immersion time in the coagulation bath in the coagulation step was 10.0 seconds, and the residence time in air in the air residence step was 10 seconds. The organic solvent concentration of the liquid existing around the coagulated fiber bundle at a position 0.3 seconds before the introduction into the water washing bath in the air retention step was 80%, which was the same as the organic solvent concentration in the coagulation bath in the coagulation step, and thus the coagulation in the coagulation bath was completed. The Si/C ratio at a depth of 10nm from the surface layer of the fiber and the Si/C ratio at a depth of 50nm from the surface layer of the fiber were higher than those of example 1, and the number of pores in the surface layer and the average width of pores were increased, and therefore the tensile strength of the strand was 5.1GPa and was 1.2GPa lower.

Comparative example 2

The same operation as in example 7 was carried out except that the dipping time in the coagulation bath in the coagulation step was set to 7.0 s. The concentration of the organic solvent in the liquid present around the coagulated fiber bundle at a position 0.3 seconds before the introduction into the water washing bath in the air retention step was 81%, and it was considered that the coarse coagulation in the coagulation bath was completed because the concentration of the organic solvent was increased by only 1% from the concentration of the organic solvent in the coagulation bath in the coagulation step. The Si/C ratio at a depth of 10nm from the surface layer of the fiber and the number of pores and the average width of pores in the surface layer were also increased as compared with example 7, and therefore the tensile strength of the strand was 5.2GPa and 0.6GPa lower.

Comparative example 3

The same operation as in example 7 was carried out except that the dipping time in the coagulation bath in the coagulation step was set to 5.0 s. The tensile strength of the wire harness was 5.2GPa, which is not much different from that of comparative example 2.

Comparative example 4

The same operation as in example 7 was carried out except that the immersion time in the coagulation bath in the coagulation step was set to 1.5s and the air retention time in the air retention step was set to 1 s. The organic solvent concentration of the liquid present around the coagulated fiber bundle at a position 0.3 seconds before introduction into the water washing bath in the air retention step was 80%, and the coagulation conditions were the same as in example 7, but the organic solvent concentration was the same as in the coagulation bath. Although passing through the coagulation bath in a semi-coagulated state, it is considered that the water bath is introduced before coagulation is sufficiently performed in the air. The tensile strength of the wire harness was 5.1GPa, which was 0.7GPa lower than that of example 7.

Comparative example 5

The same operation as in comparative example 4 was performed except that the air retention time in the air retention step was set to 3 s. The tensile strength of the wire harness was 5.2GPa, which was 0.6GPa lower than that of example 7.

Comparative example 6

The same operation as in comparative example 5 was performed except that the air retention time in the air retention step was set to 7 s. The tensile strength of the strand was 5.2GPa, which was the same as that of comparative example 5.

Comparative example 7

The same operation as in example 2 was carried out except that the organic solvent concentration in the coagulation bath in the coagulation step was changed to 25%. The organic solvent concentration of the liquid present around the coagulated fiber bundle at a position 0.3 seconds before introduction into the water washing bath in the air retention step was 25%, which was the same as the organic solvent concentration in the coagulation bath in the coagulation step. The coagulation bath is considered to be completed in that the coagulation speed is high because the organic solvent concentration in the coagulation bath is low. The tensile strength of the wire harness was 5.1GPa, which was 0.7GPa lower than that of example 2.

Comparative example 8

The same operation as in example 2 was carried out except that the organic solvent concentration in the coagulation bath in the coagulation step was changed to 65%. The tensile strength of the wire harness was 5.0GPa, which was 0.8GPa lower than that of example 2.

Comparative example 9

The same operation as in example 7 was carried out except that the temperature of the coagulation bath in the coagulation step was set to 30 ℃. The tensile strength of the wire harness was 4.8GPa, which was 1.2GPa lower than that of example 7.

Comparative example 10

The same operation as in example 7 was carried out except that the temperature of the coagulation bath in the coagulation step was-30 ℃. The tensile strength of the wire harness was 4.6GPa, which was 1.4GPa lower than that of example 7. In addition, many fuzz was observed.

Comparative example 11

The same operation as in example 14 was carried out except that the dipping time in the coagulation bath in the coagulation step was set to 10.0 seconds. The tensile strength of the wire harness was 5.2GPa, which was 0.5GPa lower than that of example 14.

Comparative example 12

The same operation as in example 15 was carried out except that the dipping time in the coagulation bath in the coagulation step was changed to 10.0 seconds. The tensile strength of the wire harness was 5.2GPa, which was 0.6GPa lower than that of example 15.

Comparative example 13

The same operation as in comparative example 1 was performed except that the amount of the amino-modified silicone added in the oil agent addition step was set to be smaller than that in comparative example 1. The Si/C ratio at a depth of 10nm from the fiber surface layer, the Si/C ratio at a depth of 50nm from the fiber surface layer, and the number of pores and the average width of pores in the surface layer were lower than those in comparative example 1, but the Si/C ratio in the region at a depth of 0 to 10nm from the fiber surface was low, and the fibers were bonded to each other, so that the tensile strength of the strand was 4.9GPa and 0.2GPa lower than that in comparative example 1.

Comparative example 14

The same operation as in comparative example 13 was performed except that the amount of the amino-modified silicone added in the oil agent addition step was set to be smaller than that in comparative example 13. The Si/C ratio at a depth of 10nm from the fiber surface layer and the Si/C ratio at a depth of 50nm from the fiber surface layer were lower than those of comparative example 13, but the Si/C ratio in the region at a depth of 0 to 10nm from the fiber surface layer was lower and the fibers were bonded to each other, so that the tensile strength of the strand was 4.5GPa and 0.4GPa lower than that of comparative example 13.

Comparative example 15

The same operation as in comparative example 1 was performed except that the total draw ratio before the finish oil application step was 3.0 times. The Si/C ratio at a depth of 50nm from the fiber surface layer and the number of pores and the average width of pores in the surface layer were lower than those in comparative example 1, but the Si/C ratio at a depth of 10nm from the fiber surface layer was increased, the tensile strength of the strand was 5.3GPa, and the tensile strength was improved by only 0.2GPa as compared with that in comparative example 1.

Comparative example 16

The same operation as in comparative example 1 was performed except that the total draw ratio before the finish oil application step was set to 4.0 times. The Si/C ratio at a depth of 50nm from the fiber surface layer and the number of pores and the average width of pores in the surface layer were lower than those in comparative example 1, but the Si/C ratio at a depth of 10nm from the fiber surface layer was greatly increased, the tensile strength of the strand was 5.0GPa, and was lower than that in comparative example 1 by 0.1 GPa.

[ Table 1]

[ Table 2]

[ Table 3]

[ Table 4]