JP2020128615A - Carbon fiber bundle and method for producing the same - Google Patents

Abstract

To provide a twisted carbon fiber bundle that is suitable for a carbon fiber reinforced composite material requiring a load capacity per single fiber, has an excellent scratch resistance and process passability, has high productivity, and, furthermore, has excellent handleability and high-level processability.SOLUTION: There is provided a carbon fiber bundle having a single fiber fineness of 1.0 to 4.0 dtex, having a strand modulus E of 250 to 420 GPa, and having a crystallite size Levaluated by bulk measurement of the entire fiber bundle of 1.7 to 6.0 nm. The relation between the strand elastic modulus E and the crystallite size Lsatisfies equation (1). When setting one end as a fixed end and the other as a free end, twist of 2 turns/m or more remains, and, of the double cross-section structure observed on the center side and the circumferential side of the single fiber cross-section, an outer layer ratio Ac (%), which is the ratio of the area occupied by the circumferential side in the cross-sectional area of the single fiber is 85% or more. E≥38 L+190 (1).SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a twisted carbon fiber bundle having a high load resistance per single fiber, excellent processability, high productivity, and excellent handleability and high-order processability, and a method for producing the same.

Composite materials using carbon fibers are used for aviation and space applications, sports applications such as bicycles and golf clubs, etc., and have recently been expanded to industrial applications such as automobile parts and pressure vessels. .. In industrial applications, the high mechanical properties of carbon fibers are required, and the economical efficiency equivalent to that of current materials such as metal materials and glass fiber reinforced composite materials is required. In order to meet the needs, not only cost reduction of expensive carbon fiber, but also weight reduction of structural members (reduction of material usage) by further improvement of mechanical properties of carbon fiber, It is desired to reduce the amount of fibers used. Thus, the demand for composite materials using carbon fibers is increasing, and it is required to improve the productivity of carbon fibers.

The carbon fiber includes a flameproofing step of oxidizing a precursor fiber such as polyacrylonitrile containing a copolymerization component in air at 200 to 300°C, a preliminary carbonization step of heating in an inert atmosphere at 500 to 1200°C, 1200- It is manufactured by undergoing a carbonization step of heating in an inert atmosphere at 3000°C. In order to increase the productivity of carbon fiber, it is possible to improve the productivity by increasing the mass per single fiber, that is, the single fiber fineness, but for that purpose, increase the single fiber fineness of the carbon fiber precursor fiber. It is most effective to do. However, in obtaining a fiber bundle with a large single fiber fineness, the occurrence of structural unevenness in the flameproofing process is an obstacle. In order to reduce the amount of carbon fiber used as described above, it is most effective to improve the strand tensile elastic modulus (hereinafter abbreviated as “strand elastic modulus”) of the carbon fiber that governs the rigidity of the carbon fiber reinforced composite material. Yes. From such a background, it is important that the passability in the carbonization process, the handleability of the carbon fiber bundle and the strand elastic modulus of the carbon fiber are compatible.

In Patent Document 1, by applying a polymer obtained by copolymerizing a bulky, oxygen-permeable unsaturated carboxylic acid alkyl ester to polyacrylonitrile to a carbon fiber precursor fiber bundle, oxygen concentration distribution inside the fiber in a flameproofing process It is proposed that the technology will be uniform, shortening the flameproofing time and improving the physical properties of the carbon fiber. In Patent Document 2, by using a vinyl-based monomer having oxygen permeability and a boron compound having a flame retarding delaying effect, it is possible to suppress a double cross-section structure of a fiber cross section generated at the time of flame resistance and to provide a carbon fiber having excellent tensile properties. The manufacturing method is shown. Patent Document 3 proposes a technique for improving the oxygen permeability and stretchability of a precursor fiber bundle by using polyacrylonitrile obtained by copolymerizing a vinyl-based monomer to improve the productivity and strength of the carbon fiber bundle. .. Patent Documents 4 and 5 disclose that a polymer obtained by copolymerizing a vinyl compound having a hydroxyalkyl group having bulkiness, oxygen permeability, and flameproofing promoting effect with polyacrylonitrile is applied to a carbon fiber precursor fiber bundle. , Has proposed a method for efficiently producing carbon fibers having a large single fiber fineness. Patent Document 6 discloses a technique capable of achieving flame resistance even when the single fiber fineness is further large, and a method for producing carbon fiber having excellent resin impregnation property. Further, Patent Document 7 proposes a technique of adding entanglement or twist to the fiber bundle after the pre-carbonization treatment for the purpose of suppressing the generation of fluff under high tension.

JP 9-31758 A JP, 11-12856, A JP, 2006-257580, A International Publication No. 2012/050171 International Publication No. 2013/157612 JP, 2018-145541, A JP, 2014-141761, A

However, the background art has the following problems.

In Patent Document 1, a component having excellent oxygen permeability is used as a copolymerization component of the carbon fiber precursor fiber, but the single fiber fineness of the carbon fiber is not sufficiently large and the load resistance of the single fiber is insufficient. However, there was a problem of concern about deterioration of operability. In Patent Document 2, since it is a method of suppressing the progress of flame resistance on the surface by boron, there is a problem that the single fiber fineness cannot be increased because the flame resistance unevenness in the fiber cross-sectional direction is deteriorated as the fiber diameter increases. In Patent Document 3, since the carbon fiber precursor fiber bundle has a small amount of a copolymer component having excellent oxygen permeability and the single fiber fineness is small, the production efficiency of the carbon fiber bundle cannot be improved due to a decrease in production amount and deterioration of operability. There was a problem. In Patent Document 4, since the outer layer ratio is low, there is a concern that the quality of monofilaments may be reduced due to a decrease in load resistance of the single fibers. In Patent Document 5, a polyacrylonitrile-based copolymer containing hydroxyalkyl (meth)acrylate, which is slightly inferior in oxygen permeability, is used. Since the outer layer ratio is low, the quality is inferior and the operability may be deteriorated. In Patent Document 6, although the single fiber fineness of the carbon fiber precursor fiber bundle is large, the oxygen permeability of the copolymerization component is poor and the copolymerization amount is insufficient, so that the outer layer ratio is low and the operability is low. There was a problem that a sufficient single fiber load capacity for improvement could not be obtained. Further, according to the example disclosed in Patent Document 7, although it is presumed that twisting remains in the obtained carbon fiber bundle, there is no suggestion or mention regarding the influence of such twisting on the convergence of the carbon fiber bundle. Further, since the single fiber fineness of the precursor fiber used is as thin as 0.7 dtex, the single fiber diameter of the obtained carbon fiber bundle is also small, and there is a problem that fluffs are likely to occur at the time of contact with a guide or a roller. By setting the single fiber fineness of the carbon fiber bundle to less than 1.0 dtex as in Patent Document 7, it is possible to obtain the effect of increasing the outer layer ratio disclosed in Patent Documents 3 to 6, but the single fiber breakage When the area is small, sufficient monofilaments cannot be obtained for the load during handling that is expected to be greater than the effect of improving the outer layer ratio, and fluff is likely to occur during the process. In particular, when the tension in the carbonization process must be applied above a certain level, if the single fiber fineness is reduced and the outer layer ratio is improved, the carbonization process when the tension is applied above a certain level in the carbonization process However, there was a problem that sufficient filaments could not be obtained under the load, and fluff was generated during the process.

In the present invention, it is suitable for carbon fiber reinforced composite materials that require a load bearing capacity per single fiber, has high productivity because it has excellent processability, and has excellent twistability and higher workability. An object is to provide a carbon fiber bundle and a method for producing the same.

In order to achieve such an object, the present invention has the following configurations.

That is, the carbon fiber bundle of the present invention has a single fiber fineness of 1.0 to 4.0 dtex or more, a strand elastic modulus E of 250 to 420 GPa, and a crystallite size L evaluated by bulk measurement of the entire fiber bundle. _c is 1.7 to 6.0 nm, the relationship between the strand elastic modulus E and the crystallite size L _c satisfies the expression (1), and when one end is a fixed end and the other is a free end, 2 turns/ Outer layer ratio Ac, which is the ratio of the area on the circumference side to the single fiber cross-sectional area of the double structure of the cross section observed on the center side and the circumference side of the single fiber cross section, in which the twist of m or more remains, The carbon fiber bundle is 85% or more.
E≧38L _c +190 (1).

The method of producing a carbon fiber bundle of the present invention, acrylonitrile units 90.0 to 97.0 wt% and the structural formula _{CH 2 = CHCOOC n H 2n +} 1 ( in the structural formula is n = 2 to 4, the alkyl chain A single-fiber fineness is 2.0 to 6.0 dtex using a polyacrylonitrile polymer containing 3.0 to 10.0% by mass of an acrylate monomer (X) unit represented by a straight chain. After obtaining the carbon fiber precursor fiber bundle, it is observed on the center side and the circumferential side of the single fiber cross section by the flameproofing treatment in an oxidizing atmosphere under the condition that the flameproofing temperature is 200°C to 300°C. After obtaining a flameproof fiber bundle having an outer layer ratio As of 85% or more, which is the ratio of the area on the circumferential side to the single fiber cross-sectional area in the double-layered cross-section structure, the flame-resistant fiber obtained by the flameproofing treatment. A pre-carbonization treatment for pre-carbonizing the bundle in an inert atmosphere at a maximum temperature of 500 to 1000° C., and a pre-carbonized fiber bundle obtained by the pre-carbonization treatment to carbon in an inert atmosphere at 1000 to 3000° C. A method for producing a carbon fiber bundle, which comprises sequentially performing carbonization treatment for carbonization, wherein the number of twists of the fiber bundle during the carbonization treatment is 2 turns/m or more and the tension is 1.5 mN/dtex or more. Is the way.

According to the present invention, the load resistance per single fiber is high, the productivity is high because of excellent process passability, and since the carbon fiber bundle has a twist, the handleability and high-order processability as a fiber bundle are improved. An excellent carbon fiber bundle is obtained.

In order to increase the withstand load per single fiber, it is important to balance the carbon fiber outer layer ratio and the single fiber fineness, and in order to improve processability and handleability and higher-order processability, carbon The present invention has been reached by clarifying that the twist remains in the fiber bundle.

The single fiber fineness of the carbon fiber bundle of the present invention is 1.0 to 4.0 dtex, preferably 1.2 to 3.0 dtex, and more preferably 1.4 to 2.5 dtex. The single fiber fineness is a mass per unit length of a single fiber, and 1 dtex is a fiber whose mass per 10,000 m of a single fiber is 1 g, and therefore is related to a single fiber diameter. If the monofilament fineness is large, the load resistance per monofilament becomes large, and thus the monofilament breakage (fuzz formation) is less likely to occur due to an external force that increases the load during handling of the carbon fiber such as rubbing. If the monofilament fineness of the carbon fiber bundle is 1.0 dtex or more, the monofilament cross-sectional area is large, and sufficient monofilaments can be obtained for the load during the assumed handling. The processability of the carbonization process/carbonization process is improved. When the monofilament fineness of the carbon fiber bundle is 4.0 dtex or less, the outer layer ratio of the cross-section double structure described later can be suppressed to be small, and fluff is less likely to occur. The single fiber fineness can be calculated from the basis weight of the carbon fiber bundle and the number of filaments. In order to control the single fiber fineness, it is important to control the carbonization yield in the carbonization step from the discharge amount/drawing ratio of the spinning solution and flame resistance in the carbon fiber precursor fiber bundle spinning step. This is mainly achieved by controlling the discharge rate of the spinning solution.

The strand modulus E of the carbon fiber bundle of the present invention is 250 to 420 GPa, preferably 255 to 380 GPa, more preferably 260 to 340 GPa, more preferably 265 to 340 GPa, and most preferably 265 to 265. It is 330 GPa. The higher the strand elastic modulus E, the greater the reinforcing effect of the carbon fibers in the carbon fiber reinforced composite material, and the more rigid the carbon fiber reinforced composite material can be obtained. If no tension is applied in the carbonization step, the carbon fiber bundle may shrink locally to obtain a carbon fiber bundle having a shape similar to the twisting habit, but the carbon fiber thus obtained The bundle tends to have a low strand elastic modulus E and may not be industrially useful. When the strand elastic modulus E is 250 GPa or more, the rigidity of the carbon fiber reinforced composite material can be easily increased, and it is possible to meet the needs in industrial applications where future growth is expected. When the strand elastic modulus E is 420 GPa or less, it is possible to obtain a carbon fiber bundle having good passability in the carbonization step and good handleability of the carbon fiber bundle. The strand elastic modulus E can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608 (2004). In the case where the carbon fiber bundle has a twist, the same number of twists as the number of twists is applied in the opposite direction, and the untwisted product is used for evaluation. The strand elastic modulus E can be controlled by a known method such as tension or maximum temperature when tension is applied in the carbonization treatment.

The crystallite size L _c evaluated by bulk measurement of the entire fiber bundle in the present invention is 1.7 to 6.0 nm, preferably 1.7 to 5.0 nm, and more preferably 2.0 to 4. It is 0 nm, and most preferably 2.3 to 3.8 nm. The crystallite size L _c is an index representing the thickness of the crystallites existing in the carbon fiber in the c-axis direction and the orientation angle with respect to the fiber axis of the crystallites, and is evaluated by wide-angle X-ray diffraction. The detailed evaluation method will be described later. When the crystallite size L _c is large, the stress load inside the carbon fiber is effectively carried out, so that the strand elastic modulus E is easily increased. However, when the crystallite size L _c is too large, it causes stress concentration, and the carbonization process Since the passability may be deteriorated and the handleability of the carbon fiber bundle may be deteriorated, it may be determined depending on the required strand elastic modulus E and the balance between the passability in the carbonization step and the handleability of the carbon fiber bundle. The crystallite size L _c can be controlled mainly by the treatment time after the carbonization treatment and the maximum temperature.

In the carbon fiber bundle of the present invention, the relationship between the strand elastic modulus E and the crystallite size L _c is
E≧38L _c +190 (1)
Is satisfied, preferably the intercept in the above formula is 191 and more preferably the intercept is 192. Since the carbon fiber bundle satisfying the formula (1) has a sufficiently high strand elastic modulus E with respect to the crystallite size L _c , the strand elastic modulus can be maintained without impairing the passability in the carbonization step and the handleability of the carbon fiber bundle. E can be effectively increased, the rigidity of the carbon fiber reinforced composite material can be easily increased, and it is possible to meet the needs in industrial applications where future growth is expected. The carbon fiber bundle satisfying the formula (1) is obtained by a preferred method for producing a carbon fiber bundle of the present invention described later.

The carbon fiber bundle of the present invention has a twist of 2 turns/m or more when one end is a fixed end and the other end is a free end. In the present invention, the fixed end is an arbitrary portion on the fiber bundle that is fixed so as not to rotate about the longitudinal direction of the fiber bundle, and restrains the rotation of the fiber bundle using an adhesive tape or the like. Can be made by The free end refers to the end that appears when a continuous fiber bundle is cut in a cross section perpendicular to its longitudinal direction, is not fixed to anything, and rotates around the longitudinal direction of the fiber bundle. It is the end that can be. When one end is a fixed end and the other end is a free end, the twist remaining means that the carbon fiber bundle has a twist. As a result of studies by the present inventors, it has been found that when the carbon fiber bundle has a twist, the fiber bundle naturally converges without being loosened, so that it has an effect of improving handleability as a fiber bundle. In addition, since the carbon fiber bundle has a twist, even when breakage at the single fiber level, so-called fluff occurs, when the carbon fiber bundle is subjected to high-order processing, it does not easily grow into long fluff and high-order workability is enhanced. I also understood that. This is because when the fluff is going to travel in the longitudinal direction of the fiber bundle, the root of the fluff is included in the twist, which hinders its progress. In addition, when twist is forcibly applied to a general carbon fiber bundle that does not have twist, if the fiber bundle is not always under tension, the carbon fiber bundles that have been forcibly twisted will have higher While the next twist may be formed and the rope may be folded like knitting, when the carbon fiber bundle has a twist, it does not form a higher twist regardless of the presence or absence of tension. The carbon fiber bundle is flexible and easy to handle. When one end is a fixed end and the other end is a free end, the twisting will not be unraveled, and as a result, a twist of 2 turns/m or more will remain, and the effect of improving handleability and high-order workability will be particularly large. I understood. The larger the number of remaining twists, the higher the convergence, which is preferable, but the upper limit is about 500 turns/m due to the constraint of the manufacturing process for twisting. The number of remaining twists is preferably 30 to 120 turns/m, more preferably 46 to 100 turns/m. A carbon fiber bundle in which a twist of 2 turns/m or more remains when one end is a fixed end and the other end is a free end can be produced according to the method for producing a carbon fiber bundle of the present invention described below. The number of twists remaining can be controlled by adjusting the number of twists of the fiber bundle in the carbonization step. Although a detailed evaluation method of the number of remaining twists will be described later, after fixing any point on the fiber bundle firmly with tape etc. to make a fixed end, cut the fiber bundle at a position apart from the fixed end and free end. The fiber bundle is suspended so that the fixed end is at the top and left for a while, then the free end is grasped and untwisted to increase the number of twists required to completely untwist. The number of twists remaining in the present invention is standardized per 1 m.

The carbon fiber bundle of the present invention is the ratio of the area on the circumferential side to the cross-sectional area of the single fiber in the structural difference between the circumferential side and the central side of the single fiber cross section (referred to as the cross-section double structure). The outer layer ratio Ac of the defined carbon fibers is 85% or more, preferably 91% or more, and more preferably 95 to 98%. When the outer layer ratio Ac of the cross-section double structure is 85% or more, the load resistance of the single fiber is increased and the single fiber is less likely to be broken, and the process passability is excellent. The higher the outer layer ratio Ac is, the more desirable. However, when the outer layer ratio Ac is 98% or less, the single fiber strength does not decrease, and the withstand load can be increased. By setting the single fiber fineness of the carbon fiber bundle to less than 1.0 dtex, it is possible to increase the outer layer ratio Ac of the cross-section double structure, but when the single fiber cross-sectional area is small, the effect of improving the outer layer ratio Ac As a result, sufficient monofilaments cannot be obtained under the load assumed during handling, and fluffing is likely to occur during the process. In particular, like the carbon fiber bundle of the present invention, in order to effectively increase the strand elastic modulus E without impairing the passability of the carbonization process and the handleability of the carbon fiber bundle, as described below, in the carbonization process, Tension must be applied above a certain level. Therefore, when the single fiber fineness is reduced and the outer layer ratio Ac is improved, sufficient single fibers cannot be obtained for the load of the carbonization process when tension is applied to a certain level or more in the carbonization process, Since the fluff is generated, the carbon fiber bundle of the present invention cannot be obtained. That is, in the present invention, even when the single fiber fineness is large, the outer layer ratio Ac is high, and thus the strand elastic modulus E can be effectively increased without impairing the passability in the carbonization process and the handleability of the carbon fiber bundle. , Reached to increase the rigidity of carbon fiber reinforced composite materials. The outer layer ratio Ac of the cross-section double structure is such that the carbon fiber bundle is embedded in the resin and the surface perpendicular to the fiber axis is wet-polished to observe the exposed cross section with an optical microscope. It can be obtained by calculating the area of the structure on the circumferential side of the structure. In order to control the outer layer ratio Ac of the double structure in cross section, it can be controlled by changing the treatment time and the treatment temperature of the flameproofing step or the copolymerization component of the polyacrylonitrile polymer.

Next, a method for producing a carbon fiber bundle which is preferable for obtaining the carbon fiber bundle of the present invention will be described.

According to the method for producing a carbon fiber of the present invention, regarding the composition of the raw material used for producing the carbon fiber precursor fiber bundle, the acrylonitrile unit is 90.0 to 97.0 mass% and the acrylate-based monomer (X) unit is 3.0. To 10.0% by mass, preferably 90.0 to 96.0% by mass of acrylonitrile units and 4.0 to 10.0% by mass of acrylate-based monomer (X) units, and more preferably acrylonitrile units. A polyacrylonitrile-based polymer having 90.0 to 95.0 mass% and an acrylate monomer (X) unit of 5.0 to 10.0 mass% is used. The acrylate-based monomer (X) is an acrylic acid ester-based monomer represented by a structural formula CH ₂ ═CHCOOC _n H _2n+1 , n=2 to 4, and an alkyl group having a straight chain. When the acrylate-based monomer unit is 3.0% by mass or more, the outer layer ratio As in the flameproofing process of the carbon fiber precursor fiber bundle becomes high, and when it is 10.0% by mass or less, the strength of the obtained carbon fiber single fiber is high. And, since the withstand load is increased, the process passability is excellent. The ratio of the acrylonitrile unit of the polyacrylonitrile copolymer and the acrylate monomer (X) unit can be controlled by adjusting the composition ratio of each monomer during polymerization. Examples of the acrylate-based monomer (X) include ethyl acrylate, propyl acrylate, and normal butyl acrylate, and ethyl acrylate is particularly preferable from the viewpoint of achieving both quality improvement by improving stretchability and oxygen permeability. As the other copolymerization component, methacrylic acid, acrylic acid, itaconic acid, acrylamide or the like can be used for the purpose of promoting the flame resistance reaction.

In producing the carbon fiber precursor fiber bundle, the fiber is formed by using either a dry/wet spinning method or a wet spinning method. The spinning process is generally a coagulation process in which a spinning solution is discharged from a spinneret into a coagulation bath for spinning, a water washing process in which the fibers obtained in the coagulation process are washed in a water bath, and the fibers obtained in the water washing process are A water bath drawing step of drawing in a water bath, an oil agent step of applying a step oil agent to the fibers obtained in the water bath drawing step, and a dry heat treatment step of drying and heat treating the fibers obtained in the oil agent step. And a steam drawing step of steam drawing the fiber obtained in the dry heat treatment step. Note that the order of the steps can be changed as appropriate. The spinning solution is a solution of the above polyacrylonitrile copolymer in an organic solvent such as dimethylsulfoxide/dimethylformamide/dimethylacetamide or an aqueous solution of nitric acid/zinc chloride/rodan soda in which the polyacrylonitrile copolymer is soluble. It was done.

The coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide and dimethylacetamide used as a solvent for the spinning solution, and a coagulation promoting component. As the coagulation promoting component, a component which does not dissolve the polyacrylonitrile copolymer and which is compatible with the solvent used in the spinning solution can be used. Specifically, it is preferable to use water as the coagulation promoting component. It is preferable to increase the concentration of the organic solvent and set the temperature of the coagulation bath low in a range in which the cross section of the single fiber is a perfect circle and the side surface of the fiber is smooth. For example, when dimethyl sulfoxide is used as the solvent, it is desirable that the concentration of the dimethyl sulfoxide aqueous solution is 5 to 30% by mass, or 70 to 80% by mass, and the coagulation bath temperature is -10 to 30°C.

As the washing bath in the washing step, it is preferable to use a washing bath having a plurality of stages at a temperature of 30 to 98°C. Further, the stretching ratio in the water bath stretching step is preferably 1 to 6 times from the viewpoint of maintaining a cross-sectional shape with high circularity.

After the water bath drawing step, it is preferable to add an oil agent made of silicone or the like to the fiber bundle for the purpose of preventing fusion of the single fibers. The silicone oil agent is preferably a modified silicone, and more preferably one containing an amino-modified silicone having high heat resistance.

A known method can be used for the dry heat treatment step. For example, the drying temperature is, for example, 100 to 200°C.

A carbon fiber precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention is obtained by performing steam stretching, if necessary, after the above-mentioned water washing step, water bath drawing step, oil agent applying step, and dry heat treatment step. To be In the steam drawing, the draw ratio is preferably 2 to 6 times in pressurized steam.

According to the method for producing carbon fiber of the present invention, the single fiber fineness of the carbon fiber precursor fiber bundle is 2.0 to 6.0 dtex, preferably 2.1 to 4.2 dtex, and more preferably 2.2 to 3 dtex. It is 0.5 dtex. When the single fiber fineness of the carbon fiber precursor fiber bundle is 2.0 dtex or more, a single fiber having a high load resistance can be obtained, and fluff is less likely to be generated and the process passability is improved. When the single fiber fineness exceeds 6.0 dtex, flame resistance, pre-carbonization, and scratch resistance in carbonization decrease, and process passability deteriorates. The single fiber fineness is calculated from the mass per unit length of the carbon fiber precursor fiber bundle and the number of filaments. In order to control the single fiber fineness, it is important to control the discharge amount/stretching ratio of the spinning solution in the spinning step of the carbon fiber precursor fiber bundle and the carbonization yield in the firing step, and mainly This is achieved by controlling the discharge rate of the spinning solution.

The carbon fiber bundle of the present invention can be obtained by subjecting the above-mentioned carbon fiber precursor fiber bundle to flameproofing treatment, and then sequentially performing preliminary carbonization treatment and carbonization treatment.

Further, according to the method for producing a carbon fiber bundle of the present invention, the heat treatment temperature for the flameproofing treatment is 200 to 300°C, preferably 220 to 280°C, more preferably 230°C to 270°C. Although the outer layer ratio Ac of the carbon fiber bundle can be improved by lowering the flameproofing temperature, it is necessary to prolong the heat treatment time of the flameproofing treatment, which deteriorates the productivity, and the preliminary carbon The passability of carbonization treatment and carbonization treatment may deteriorate. Therefore, if the heat treatment temperature of the flameproofing treatment is 200° C. or higher, the flameproofing treatment can be efficiently performed at a high temperature and in a short time according to the oxygen permeability of the carbon fiber precursor fiber bundles, and thus the preliminary carbon is not deteriorated The passability of the chemical treatment and the carbonization treatment is improved. When the heat treatment temperature of the flameproofing treatment is 300° C. or lower, thermal runaway of the carbon fiber precursor fiber bundle is suppressed and the workability is excellent.

According to the method for producing a carbon fiber bundle of the present invention, the outer layer ratio As of the flameproof fiber bundle is 85%, preferably 91% or more, and more preferably 95 to 98%. The larger the outer layer ratio As, the smaller the double structure in cross section, and therefore the strength of the single fiber of the carbon fiber bundle after the carbonization treatment of the flameproof fiber bundle is excellent, and the generation of fluff in the process can be suppressed. The outer layer ratio As of the flameproof fiber bundle is determined by observing a cross section perpendicular to the fiber axis, which appears by polishing the surface of the flameproof fiber bundle with a resin, with an optical microscope, and showing the color tone of the fiber cross section in a different region. , It is obtained by calculating the area occupied by the entire cross-sectional area of the outer region. In order to set the outer layer ratio As to 85% or more, the flameproof fiber bundle is sampled in the flameproofing step to check the outer layer ratio As, and the flameproofing temperature is adjusted, or oxygen is used as a raw material of the carbon fiber precursor fiber. This can be achieved by using a copolymer component having excellent permeability.

Further, according to the method for producing a carbon fiber bundle of the present invention, regarding the flameproofing treatment condition, the flameproofing initial temperature Ti (° C.) and the mass composition ratio Za() of the acrylate-based monomer (X) unit of the polyacrylonitrile-based copolymer are %) preferably satisfies the following relationship, more preferably the right side is 1100, and even more preferably the right side is 1200. When the formula is satisfied, the flame resistance treatment can be efficiently performed at a high temperature for a short time depending on the oxygen permeability of the carbon fiber precursor fiber bundle, and thus the productivity may be excellent. This can be achieved by using an appropriate amount of a copolymer component having excellent oxygen permeability as a raw material for the carbon fiber precursor fiber bundle.
Ti×Za≧1000.

In the pre-carbonization treatment for pre-carbonizing the flame-resistant fiber bundle obtained by the flame-resistant treatment, the obtained flame-resistant fiber bundle has a specific gravity of 1. at a maximum temperature of 500 to 1200° C. in an inert atmosphere. It heat-processes until it becomes 5-1.8.

Further, carbonization treatment is performed subsequent to the preliminary carbonization treatment. In the carbonization treatment, the obtained pre-carbonized fiber bundle is heat-treated in an inert atmosphere, preferably at a maximum temperature of 1200 to 3000°C. From the viewpoint of increasing the strand elastic modulus E of the obtained carbon fiber bundle, the maximum temperature in the carbonization treatment is preferably high, but if it is too high, the carbonization treatment passability and the handleability of the carbon fiber bundle may be impaired. Therefore, it is better to set in consideration of trade-off. For the above reason, the maximum temperature in the carbonization treatment is preferably 1400 to 2500°C, more preferably 1700 to 2000°C.

Further, in the present invention, the tension in the carbonization treatment is 1.5 mN/dtex or more, preferably 3 to 18 mN/dtex, and more preferably 5 to 18 mN/dtex. The tension of carbonization treatment is the total fineness (dtex) which is the product of the average fineness (dtex) of the single fibers of the carbon fiber precursor fiber bundle used and the tension (mN) measured at the carbonization furnace outlet side. ). By controlling the tension, the strand elastic modulus E can be controlled without significantly affecting the crystallite size L _c of the obtained carbon fiber bundle, and a carbon fiber bundle satisfying the above-mentioned formula (1) can be obtained. can get. From the viewpoint of increasing the strand elastic modulus E of the carbon fiber bundle, it is preferable that the tension is high, but if it is too high, the process passability and the quality of the obtained carbon fiber may be deteriorated. Good to do. If the tension in the carbonization step is increased without imparting twist, single fiber breakage occurs, the number of fluffs increases, the passability of the carbonization treatment decreases, or the entire fiber bundle breaks, which is necessary. Although tension may not be maintained in some cases, in the carbonization treatment, if twist is imparted to the fiber bundle, fluff is suppressed, and thus high tension can be imparted.

In the method for producing a carbon fiber bundle of the present invention, the number of twists of the fiber bundle during the carbonization treatment is 2 turns/m or more. The twist number is preferably 30 to 120 turns/m, and more preferably 46 to 100 turns/m. By controlling the number of twists within the above range, it is possible to impart a specific twisting habit to the obtained carbon fiber bundle, which is excellent in convergence and becomes a carbon fiber bundle with high handleability and high-order processability as a carbon fiber bundle. .. The upper limit of the number of twists is not particularly limited, but in order to avoid complicating the twisting process, it is preferable to set the upper limit to about 500 turns/m. The number of twists is such that the precursor fiber bundle, the flame-resistant fiber bundle, or the pre-carbonized fiber bundle is once wound around the bobbin, and when the fiber bundle is unwound, the bobbin is swung in a plane orthogonal to the unwinding direction. It can be controlled by a method of imparting a twist by contacting a rotating roller or belt to a running fiber bundle without winding the bobbin.

In the method for producing a carbon fiber bundle of the present invention, there is no particular limitation on the step of introducing twist for controlling the number of twists of the fiber bundle during carbonization treatment, but preferably before carbonization treatment, and more preferably Is before the flameproofing treatment, and more preferably before the precarbonization treatment. Such twisting involves winding the precursor fiber bundle, the flameproof fiber bundle, or the pre-carbonized fiber bundle once on the bobbin, and then, when unwinding the fiber bundle, swivels the bobbin in a plane orthogonal to the unwinding direction. It can be applied by a method or by bringing a rotating roller or belt into contact with a running fiber bundle without winding it on a bobbin.

In the present invention, as the inert gas used in the inert atmosphere, for example, nitrogen, argon and xenon are preferably exemplified, and nitrogen is preferably used from the economical viewpoint.

The carbon fiber bundle obtained by the above-mentioned production method may be subjected to additional carbonization treatment in an inert atmosphere at a maximum temperature of 3000° C., and the strand elastic modulus E may be appropriately adjusted according to the application.

The carbon fiber bundle obtained as described above is preferably subjected to an oxidation treatment to introduce an oxygen-containing functional group. For the electrolytic surface treatment of the present invention, gas phase oxidation, liquid phase oxidation and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. In the present invention, the liquid-phase electrolytic oxidation method is not particularly limited, and a known method may be used.

After such an electrolytic treatment, a sizing treatment can be performed in order to provide the obtained carbon fiber bundle with a focusing property. 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 methods for measuring various physical properties described in the present specification are as follows.

<Number of twists remaining when one end is a fixed end and the other is a free end>
A guide bar was installed at a height of 60 cm from the horizontal plane, and the carbon fiber bundle was fixed at any position of the carbon fiber bundle with tape to the guide bar. Then, the carbon fiber bundle was placed 50 cm away from the fixed end. Cut and form the free end. The free end is enclosed so that it is sandwiched between tapes, and processed so that it does not become loose in single fiber units. The free end is rotated while counting the number of times, and the number of times n (turn) of the rotation until it is completely untwisted is recorded. The number of remaining twists is calculated by the following formula. The average of the above three measurements is taken as the number of twists remaining in the present invention.
Number of remaining twists (turn/m)=n (turn)/0.5 (m).

<Strand elastic modulus E of carbon fiber bundle>
The strand elastic modulus E of the carbon fiber bundle is determined according to the resin impregnated strand test method of JIS R7608 (2004) according to the following procedure. However, in the case where the carbon fiber bundle has twists, evaluation is performed after untwisting by imparting the same number of twists in the reverse rotation as the number of twists. As the resin formulation, "Celoxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industries, Ltd.)/3 boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.)/acetone=100/3/4 (parts by mass) was used. As the curing conditions, normal pressure, a temperature of 125° C., and a time of 30 minutes are used. Ten strands of the carbon fiber bundle are measured, and the average value is taken as the strand elastic modulus E. The strain range for calculating the strand elastic modulus E is 0.1 to 0.6%.

<Crystallite size L _c of carbon fiber bundle>
A carbon fiber bundle to be used for measurement is aligned and solidified using a collodion alcohol solution to prepare a square column measurement sample having a length of 4 cm and a side length of 1 mm. The prepared measurement sample is measured under the following conditions using a wide-angle X-ray diffractometer.

1. Measurement of crystallite size L _c・X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)
・Detector: Goniometer + Monochromator + Scintillation counter ・Scanning range: 2θ = 10-40°
-Scan mode: step scan, step unit 0.02°, counting time 2 seconds.

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

Crystallite size (nm)=Kλ/β ₀ cos θ _B
However,
K: 1.0, λ: 0.15418 nm (wavelength of X-ray)
^{_{β 0: (β E 2 -β}} 1 2) 1/2
β _E : Apparent half width (measured value) rad, β ₁ : 1.046×10 ⁻² rad
θ _B : Bragg diffraction angle.

2. Measurement of Crystal Orientation Degree π _{002 In} the Examples and Comparative Examples described later, Shimadzu XRD-6100 is used as the wide-angle X-ray diffractometer.

<Handling of carbon fiber bundles>
The positions of 30 cm apart in the fiber axis direction of the carbon fiber bundle to be evaluated are separately held by the right hand and the left hand. The right hand and the left hand are brought close to each other until they come into contact with each other, and when a part of the fiber bundle forms a high-order twist like a rope, the handleability is judged to be poor (x) and the evaluation is terminated. When the handling property is not poor in the above operation, the distance between the right hand and the left hand is brought close to a distance of 20 cm, and then both hands are vertically moved a plurality of times vertically while visually observing the state of the fiber bundle. In order to keep the vertical heights of the right and left hand grips always the same, the vertical movements of both hands are performed at the same timing. The distance to be moved up and down is 10 cm, and it is repeated 20 times at a speed of reciprocating once per second. At this time, when the fiber bundle spreads into single fiber units, the handling property is judged to be poor (x) due to insufficient convergence and the evaluation is terminated. Since it is a sensory evaluation, it is difficult to draw a rigorous line, but if any part of the fiber bundle extends 5 cm or more in the direction perpendicular to the fiber axis, it is considered that the fiber bundle has spread to a single fiber unit. Even if the fiber bundle does not extend 5 cm or more in the direction perpendicular to the fiber axis, if some part of the fiber bundle is divided into smaller fiber bundles, the handling property is considered to be somewhat poor (Δ) and the evaluation is terminated. If the handling up to this point does not result in poor or slightly poor handling, the handling is rated good (◯). However, when the diameter of the monofilament is a certain value or more, the resistance to bending of the monofilament, that is, the so-called elasticity tends to be strong, and during the above-mentioned experimental operation, the entire fiber bundle has an appropriate elasticity and tends to be easily handled. there were. Therefore, it is particularly good (⊚) when it is sensual but has a proper elasticity in an actual experimental operation and is easy to handle. When the carbon fiber bundle to be evaluated is subjected to sizing treatment, the sizing agent is burnt off in an oven or removed by washing in a solvent before evaluation. The evaluation is performed in a room with minimal wind, and the central part of the fiber bundle is suspended by gravity.

<Measurement of outer layer ratio (As, Ac) of flame resistant fiber and carbon fiber>
A flame-resistant fiber or carbon fiber cut to a length of 2 cm is embedded in an epoxy resin, a cross section perpendicular to the fiber axis is wet-polished, and then observed with a microscope to take a photograph. The photographs taken were analyzed using image processing software. Depending on the conditions, a gradation may be formed in a certain range between the outer layer and the inner layer, or an intermediate layer may be formed at the boundary between the outer layer and the inner layer and may be observed in a ring shape. The outer edge of the gradation to be formed is defined as the boundary of the double structure. Image analysis was performed on 30 single fibers. The outer layer ratio is calculated according to the following formula after obtaining the average cross-sectional area a ₀ of the flameproof fiber or carbon fiber and the average area a ₁ of the inner layer portion.
Outer layer ratio (%)=(1-a ₁ )÷a ₀ ×100

Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to these. Each measuring method in this example is as described above.

(Example 1)
Polymerization of a monomer mixture obtained by mixing acrylonitrile, itaconic acid, and ethyl acrylate as an acrylate-based monomer in a mass ratio of 93.5:1.0:5.5 by a solution polymerization method using dimethyl sulfoxide (DMSO) as a solvent. Then, a polyacrylonitrile-based copolymer solution was obtained and used as a spinning solution. The obtained spinning solution was once discharged into the air using a spinneret having a hole number of 3,000, allowed to pass through a space, and then coagulated by a dry-wet spinning method in which it was introduced into a coagulation bath made of an aqueous solution of DMSO to obtain a coagulated thread. The obtained coagulated yarn was washed with water, then drawn twice in a warm water bath, a silicone-based oil agent was applied thereto, and heat treatment was performed with a hot drum having a surface temperature of 180°C. Then, it was stretched 4 times in pressurized steam to obtain a carbon fiber precursor fiber bundle having a single fiber fineness of 2.2 dtex. This carbon fiber precursor fiber bundle was heat-treated in air at 250° C. using a hot air circulation oven to obtain a flameproof fiber bundle. The flame-resistant fiber bundle thus obtained is embedded in an epoxy resin (main agent: EPO-KWICK RESIN manufactured by BUEHLER, curing agent: EPO-KWICK HARDENER), and the surface perpendicular to the fiber axis direction of the flame-resistant fiber bundle is wet-polished. Then, the cross section was observed with an optical microscope (Leica Microsystems industrial upright microscope DM2700M), and the outer layer ratio As was calculated using image processing software (Image J). The flame-proofing initial temperature Ti was set to 230° C., and heat treatment was performed in an oven in an air atmosphere of 230 to 280° C. at a draw ratio of 1 to convert to a flame-resistant fiber bundle. The obtained flame-resistant fiber bundle is twisted to give a twist of 22 turns/m, and pre-carbonized at a draw ratio of 0.97 in a nitrogen atmosphere at a temperature of 300 to 800° C. for 22 turns. A pre-carbonized fiber bundle with a twist of /m left was obtained. Next, the pre-carbonized fiber bundle was subjected to carbonization treatment under the conditions shown in Table 1, and then a sizing agent was added to the carbon fiber bundle so that the adhered amount was 1.0% by mass to obtain a carbon fiber bundle. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Example 2)
A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 1 except that the twist number was 87 turns/m and the tension during carbonization was 8.0 mN/dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Example 3)
Flame-resistant fiber bundles and carbon fiber bundles were obtained in the same manner as in Example 1 except that the maximum temperature of carbonization treatment was set to 1900°C. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Example 4)
A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 1 except that the number of twists was 87 turns/m, the tension during carbonization was 8.0 mN/dtex, and the maximum temperature of carbonization treatment was 1900°C. It was The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Example 5)
Except that the raw material of the polyacrylonitrile-based copolymer in Example 1 was the acrylonitrile, itaconic acid, and the monomer composition in which the mass ratio of ethyl acrylate was 96.0:1.0:3.0 as the acrylate-based monomer. A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 4. The carbonization process passability was within an allowable range in the process. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Example 6)
The raw material of the polyacrylonitrile-based copolymer in Example 1 was a monomer composition in which the mass ratio of acrylonitrile, itaconic acid, and normal butyl acrylate as an acrylate-based monomer was 92.0:1.0:7.0. A flameproof fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 4 except for the above. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Comparative Example 1)
A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 4 except that the discharge amount of the spinning solution in Example 1 was changed to obtain a carbon fiber precursor fiber bundle having a single fiber fineness of 1.0 dtex. The carbonization process passability was within a permissible range in the process, and the quality of the obtained carbon fiber bundle was somewhat poor. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Comparative example 2)
A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 1 except that the number of twists was 0 turn/m and the tension during carbonization was 2.0 mN/dtex. In the carbonization process, fluff winding around the roller occurred, and the quality of the obtained carbon fiber bundle was poor. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Comparative example 3)
A flame resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 3 except that the number of twists was 0 turn/m and the tension during carbonization was 2.1 mN/dtex. In the carbonization process, fluff winding around the roller occurred, and the quality of the obtained carbon fiber bundle was poor. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Comparative example 4)
Except that the raw material of the polyacrylonitrile-based copolymer in Example 1 was the acrylonitrile, itaconic acid, and the monomer composition in which the mass ratio of ethyl acrylate was 97.1:1.0:1.9 as the acrylate-based monomer. A flame-resistant fiber bundle and a carbon fiber bundle were obtained in the same manner as in Example 4. In the carbonization process, fluff winding around the roller occurred, and the quality of the obtained carbon fiber bundle was poor. Table 1 shows the evaluation results of the obtained carbon fiber bundles.

(Comparative example 5)
Example 1 was repeated except that the raw material of the polyacrylonitrile-based copolymer was acrylonitrile, itaconic acid, and a monomer composition in which the mass ratio of ethyl acrylate was 99.0:1.0:0 as the acrylate-based monomer. A flameproof fiber bundle was obtained in the same manner as in Example 4. In the carbonization process, fluffs were wrapped around the roller, and the yarn was broken during the carbonization process, so a carbon fiber bundle could not be obtained.

(Comparative example 6)
A flameproof fiber bundle was obtained in the same manner as in Example 5 except that the flameproofing initial temperature Ti was set to 190°C. In the carbonization process, fluffs were wrapped around the roller, and the yarn was broken during the carbonization process, so a carbon fiber bundle could not be obtained.

(Comparative Example 7)
The flameproofing treatment was performed in the same manner as in Example 1 except that the flameproofing initial temperature Ti was set to 310° C. However, the yarn was broken in the flameproofing process, and the flameproof fiber bundle could not be obtained.

Claims (7)

The single fiber fineness is 1.0 to 4.0 dtex, the strand elastic modulus E is 250 to 420 GPa, and the crystallite size L _c evaluated by bulk measurement of the entire fiber bundle is 1.7 to 6.0 nm. , The relationship between the strand elastic modulus E and the crystallite size L _c satisfies the expression (1), and when one end is a fixed end and the other is a free end, a twist of 2 turns/m or more remains, A carbon fiber bundle having an outer layer ratio Ac of 85% or more, which is the ratio of the area on the circumference side to the single fiber cross-sectional area of the double-section structure observed on the center side and the circumference side of the fiber cross section.
E≧38L _c +190 (1) The carbon fiber bundle according to claim 1, wherein a twist of 30 to 120 turns/m remains when one end is a fixed end and the other end is a free end. The carbon fiber bundle according to claim 1, wherein the outer layer ratio Ac is 91% or more. Acrylonitrile units 90.0 to 97.0 wt% and the structural formula _{CH 2 = CHCOOC n H 2n +} 1 ( in the structural formula, a n = 2 to 4, the alkyl chain is linear.) Acrylate monomer represented by (X) Flame resistance after obtaining a carbon fiber precursor fiber bundle having a single fiber fineness of 2.0 to 6.0 dtex using a polyacrylonitrile-based polymer containing 3.0 to 10.0% by mass of unit Under the condition that the temperature is 200° C. to 300° C., of the cross-sectional double structure observed on the center side and the circumferential side of the single fiber cross section by the flameproofing treatment in an oxidizing atmosphere, the area of the circumferential side is After obtaining a flame-resistant fiber bundle having an outer layer ratio As of 85% or more, which is a ratio of the single fiber cross-sectional area, the flame-resistant fiber bundle obtained by the flame-proof treatment is placed in an inert atmosphere at a maximum temperature of 500 to 1200°C. In order to carry out preliminary carbonization treatment for pre-carbonization and carbonization treatment for pre-carbonization fiber bundle obtained by the preliminary carbonization treatment in an inert atmosphere of 1200 to 3000° C. to produce a carbon fiber bundle. A method for producing a carbon fiber bundle, wherein the number of twists of the fiber bundle during the carbonization treatment is 2 turns/m or more and the tension is 1.5 mN/dtex or more. The method for producing a carbon fiber bundle according to claim 4, wherein the flameproofing initial temperature Ti (° C.) and the mass composition ratio Za (%) of the acrylate-based monomer (X) unit satisfy the relationship of Ti×Za≧1000. The method for producing a carbon fiber bundle according to claim 4 or 5, wherein the twist number of the fiber bundle during the carbonization treatment is 30 to 120 turns/m. The method for producing a carbon fiber bundle according to claim 5, wherein the outer layer ratio As is 91% or more.