JP2005314830A - Polyacrylonitrile-based carbon fiber and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyacrylonitrile (PAN)-based carbon fiber having improved wettability to resins and high composite performance such as interlaminar shear strength (ILSS). <P>SOLUTION: The PAN-based carbon fiber has a strand strength of 4,600-5,800 MPa, an elastic modulus of 220-250 GPa and a density of ≥1.74 g/cm<SP>3</SP>and <1.80 g/cm<SP>3</SP>and contains silicon. The silicon in the fiber has an SiOx structure (1≤x≤3) and its content is 110-500 ppm in terms of silicon element based on the mass of the carbon fiber. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polyacrylonitrile (PAN) -based carbon fiber having excellent interface characteristics and a method for producing the same, which is used when a composite is prepared by combining a resin and a carbon fiber.

  As a method for producing carbon fiber, a method is widely known in which a PAN-based precursor fiber (precursor) is used as a raw material fiber, and a carbon fiber is obtained through a flame resistance treatment and a carbonization treatment. The carbon fibers thus obtained have good characteristics such as high specific strength and specific elastic modulus.

  In recent years, industrial uses of composite materials using carbon fibers have been spreading to multiple purposes. Especially in the sports / leisure field, aerospace field, and automobile field, (1) higher performance (higher strength, higher elasticity), (2) lighter (fiber weight reduction and fiber content reduction), ( 3) There is an increasing demand for improving the appearance of higher composite properties when combined (improvement of carbon fiber surface / interface properties).

  In order to pursue high performance in the composite of carbon fiber and resin, the characteristics of the resin are also important, but it is essential to improve the surface characteristics of the carbon fiber itself. In other words, carbon fiber with improved wettability to the resin on the carbon fiber surface is combined with the resin, and the resin and the carbon fiber are more uniformly dispersed, so that the composite material has higher performance (high strength, High elasticity).

  In addition, if high composite physical properties can be improved, the blending ratio of the carbon fibers in the composite can be reduced as compared with the conventional one. Thereby, since the price of the carbon fiber is higher than the price of the resin in the price of the raw material, it is possible to achieve weight reduction and cost reduction as a composite.

  In particular, recently, not only high performance of carbon fibers and composite materials but also cost reduction has been strongly demanded by users, and it is desired to produce carbon fibers and composite materials having high performance efficiently at low cost. Yes.

  In order to improve the strength and quality of the carbon fiber, which is one of the user's requirements, it is desirable to reduce the surface defects of the carbon fiber. As a method for removing the surface defects generated in the carbon fiber manufacturing process after the generation, a method for removing the surface defects by performing a surface treatment such as an etching process on the carbon fiber surface has been proposed (for example, see Patent Document 1). .

  By such a method for removing surface defects, the strand strength of the carbon fiber is improved. However, the removal of surface defects after the generation of the surface defects makes the post-processing equipment complicated and cumbersome, which is contrary to the cost reduction that is the other user's requirement, and is considered difficult to put into practical use.

  As a technique for preventing the occurrence of surface defects rather than the removal after the occurrence of surface defects, there is a technique using an aminosilicone-based oil as the spinning oil for the precursor. As a conventional technique using this oil agent, for example, as disclosed in Patent Document 2, after drying and densifying the precursor, a silicone-based oil agent is applied, and the oil agent is present only in the vicinity of the surface, so that the oil agent is made into a fiber. There is a technology to obtain high-strength fibers without penetrating deep inside.

  However, in the case of fibers formed by applying an oil agent only to the vicinity of the surface, the carbon fiber finally obtained by air oxidation, thermal decomposition, thermal decomposition in an inert gas atmosphere, and thermal reaction in the firing process In many cases, the surface structure originates from the silicon nitride derivative structure. When this carbon fiber is combined with a resin in the future, the wettability with the resin is lowered, and the strength as a composite is lowered.

In addition, the surface on which the silicon nitride derivative structure is formed has a brittle surface structure compared to the surface on which it is not formed so much that defects are likely to occur, which will adversely affect the strength and quality of the carbon fiber (fuzz suppression) in the future. There is a concern that
JP 61-225330 A (Claims) JP 2000-136485 A (Claims)

  While one of the inventors has studied variously in order to solve the above problem, in the precursor production process, an aminosilicone oil is applied before drying and densification, and after drying and densification, a silicone-based oil is applied. Low density suitable for raw materials of lightweight composites with the same performance (strength and elastic modulus) as conventional general-purpose carbon fibers It was learned that a PAN-based carbon fiber was obtained, and an earlier application was filed (Japanese Patent Application No. 2003-24730).

  However, the performance of this PAN-based carbon fiber, for example, the performance of interlaminar shear strength (ILSS) when made into a composite is not so high. Therefore, the inventors of the present invention applied an amino silicone oil agent before drying densification, and reduced the density without applying a silicone oil agent after drying densification, while studying for further performance improvement. It was found that the PAN-based carbon fiber produced without the oil was difficult to be oxidized by air and thermally decomposed during the flameproof firing process.

  Moreover, it is difficult to form a silicon nitride derivative structure on the surface due to thermal decomposition and thermal reaction under an inert gas atmosphere in the carbonization firing process, and instead, a SiOx structure is formed. Furthermore, PAN-based carbon fibers in which silanol groups (Si—OH) and the like are formed are obtained by surface treatment after carbonization firing. The present inventors know that, when this PAN-based carbon fiber with improved surface properties is combined with a resin, the wettability with the resin is improved and the performance as a composite such as interlaminar shear strength (ILSS) can be improved. As a result, the present invention has been completed.

  Accordingly, an object of the present invention is to provide a PAN-based carbon fiber having improved surface characteristics and a method for producing the same, which solves the above problems.

  The present invention for achieving the above object is as follows.

[1] Polyacrylonitrile-based carbon fiber having a strand strength of 4600 to 5800 MPa, an elastic modulus of 220 to 250 GPa, a density of 1.74 g / cm ³ or more and less than 1.80 g / cm ³ , and containing silicon. Polyacrylonitrile-based carbon fiber having a SiOx (1 ≦ x ≦ 3) structure and a content of 110 to 500 ppm of the carbon fiber mass as elemental silicon.

[2] The carbon fiber strand has a single fiber average diameter of 6 to 8 μm, the carbon fiber strand has an electric resistance of 29 to 32 Ω · g / m ² , and a surface silicon amount measured by X-ray photoelectron spectroscopy ( The polyacrylonitrile-based carbon fiber according to [1], wherein Si / C indicating Si) is 0.10 to 0.15, and O / C indicating surface oxygen amount (O) is 0.15 to 0.30.

  [3] The polyacrylonitrile-based carbon according to [1] or [2], wherein the surface has a structure containing silicon having a Si binding energy peak measured by X-ray photoelectron spectroscopy in the range of 102 to 104 eV. fiber.

  [4] A yarn obtained by spinning a copolymer obtained by polymerizing a monomer containing 94% by mass or more of acrylonitrile, an emulsion aqueous solution containing an amino-modified silicone and a dialkylsulfosuccinate as an oil agent in a dry mass of 0. After adhering 1 to 0.3%, after drying and densifying with a dryer at 70 to 150 ° C., wet heat drawing treatment is performed under conditions of a temperature of 100 to 130 ° C. and a draw ratio of 4.0 to 7.0. 0.96 to 1.2d precursor fiber for carbon fiber is obtained, and the obtained precursor fiber is heat-treated as it is in heated air at 230 to 290 ° C. and a draw ratio of 1.02 to 1.08 to provide flame resistant fiber. The flameproofed fiber obtained was heated in an inert gas atmosphere, pre-carbonized at a maximum temperature range of 550 to 750 ° C. and a draw ratio of 1.01 to 1.07, and further in an inert gas atmosphere The temperature rises at 11 and is 11 Production of polyacrylonitrile-based carbon fiber characterized by carbonizing at 0 to 1320 ° C. and a draw ratio of 0.90 to 1.00, and surface-treating in an aqueous solution by electrolytic oxidation with an electric charge of 15 coulomb or more per gram of carbon fiber Method.

  [5] The method for producing a polyacrylonitrile-based carbon fiber according to [4], wherein the moisture content of the carbon fiber precursor fiber is 20 to 60% by mass.

  According to the present invention, when combined with a resin, the wettability with the resin is improved, and the performance as a composite such as interlaminar shear strength (ILSS) can be improved, and the PAN-based carbon fiber with improved surface characteristics and its production A method can be provided.

  The following describes the invention in more detail.

[PAN-based carbon fiber with improved surface properties]
The PAN-based carbon fiber with improved surface characteristics according to the present invention (hereinafter sometimes abbreviated as “carbon fiber”) has a strand strength of 4600-5800 MPa, an elastic modulus of 220-250 GPa, and a density of 1.74 g / ˜. 1.80 g / cm ³ , and PAN-based carbon fiber containing silicon, the contained silicon having a SiOx (1 ≦ x ≦ 3) structure, and the content as silicon is the mass of the carbon fiber It is characterized by being 110-500 ppm.

  The carbon fiber of the present invention has the same properties of strand strength, elastic modulus, elongation and density as compared to conventional general-purpose carbon fibers, and the process of producing carbon fiber precursor fibers in the vicinity of the surface. Silicon is contained in an amount of 110 to 500 ppm of the mass of the carbon fiber due to the influence of the silicone-based oil applied in (1).

The carbon fiber strand has an average single fiber diameter of 6 to 8 μm, an electrical resistance value of the carbon fiber strand of 29 to 32 Ω · g / m ² , and surface silicon measured by X-ray photoelectron spectroscopy. It is preferable that Si / C which shows quantity (Si) is 0.10-0.15. Further, the O / C indicating the surface oxygen amount (O) measured by X-ray photoelectron spectroscopy is preferably 0.15 to 0.30, and particularly preferably 0.21 to 0.30.

  In producing the carbon fiber, it is carbonized at 1500 ° C. or less under an inert gas atmosphere, particularly a nitrogen atmosphere. The firing temperature at that time has a great influence on the internal structure of the carbon fiber, particularly the graphitization growth, and at the same time, it decomposes as a gas, for example, decomposes as ammonia, and the silicon content remaining in the vicinity of the surface or the structure containing silicon. It will be different.

  As a method for simply evaluating the graphite structure of the carbon fiber surface layer portion, the value of the electric resistance value per meter can be substituted. Since carbon fiber has a graphite structure inside the fiber, it exhibits electrical conductivity. As the internal graphite structure develops, the electrical conductivity increases. In particular, since electrons easily flow closer to the surface of the carbon fiber, an electrical resistance value is sometimes used as an index indicating the growth of the graphite structure near the surface of the carbon fiber. This electrical resistance value has a relationship that shows a lower electrical resistance value because the graphite structure near the surface grows and the electrons easily move as the carbon fiber fired at a higher temperature.

  When this electrical resistance value is low, that is, when carbonized at a higher temperature, the silicon compound derived from the oil used in the carbon fiber precursor fiber remaining on the fiber surface is decomposed by heat, and a gas generated from the inside of the fiber, particularly A carbon fiber that does not have the desired silicon structure and SiOx (1 ≦ x ≦ 3) structure, but instead has a silicon nitride derivative structure, and whose surface characteristics cannot be improved, is caused by a reaction caused by ammonia or a similar compound. can get.

  When the electrical resistance value is high, that is, when carbonized at a lower temperature, the performance of the carbon fiber obtained, for example, the strand strength, elastic modulus, density, etc., due to the fact that the structure inside the carbon fiber, especially the graphite structure, does not grow so much This is not preferable. Further, a large amount of contained silicon structure and SiOx (1 ≦ x ≦ 3) structure are formed (residual) thickly on the carbon fiber surface. When a thick silicon-containing layer having this SiOx (1 ≦ x ≦ 3) structure is combined with a carbon fiber and a resin obtained in the future, the physical properties of the composite are deteriorated, which is not preferable.

When viewed from the higher-order structure inside the carbon fiber, if the silicon-containing layer is thick, that is, if there are many impure structures on the surface, the deviation from the structure inside the carbon fiber will increase and the surface will be prone to defects. It is not preferable. In addition, the density of the carbon fibers increases, which is not preferable. Therefore, the electric resistance value is particularly preferably in the range of 29 to 32 Ω · g / m ² .

  Thus, the degree of firing represented by the electrical resistance value has a great influence on the silicon structure contained on the carbon fiber surface and the SiOx (1 ≦ x ≦ 3) structure.

  In addition, in the process of manufacturing the carbon fiber precursor fiber, various oils are applied for the purpose of suppressing process stability and process stability in the subsequent firing, particularly in the flameproofing process, and the occurrence of sticking, and coating the fiber surface. To form. The formation of a film by the application of this oil agent affects the oxygen permeability to the inside of the fiber at the time of flame resistance. It also affects the release of cracked gas generated from the inside of the fiber. Therefore, since it greatly affects the structure of the flame resistant yarn, it is necessary to control the type (structure) of the oil agent to be applied and the adhesion amount to the conditions described later.

  In a general carbon fiber production method, when heat treatment is performed at a higher temperature in the air at the time of flame resistance, the intramolecular cyclization progresses and the oxygen addition rate increases as the specific gravity of the flame resistant yarn increases. However, in the present invention, a specific oil agent is applied before drying and densification in the production process of the carbon fiber precursor fiber, and is penetrated into the vicinity of the surface to make it flame resistant, and the initial carbonization in the first carbonization furnace, By appropriately controlling the temperature at the time of carbonization in the carbonization furnace, it is possible to obtain carbon fibers having the fiber characteristics of conventional general-purpose carbon fibers and having excellent surface characteristics.

  As a precursor fiber for carbon fiber of PAN-based carbon fiber which is a raw material of the carbon fiber of the present invention, a copolymer of acrylonitrile and a comonomer having an olefin structure copolymerizable with this acrylonitrile can be used.

  The acrylonitrile content in this copolymer is preferably 94% by mass or more, and more preferably 95% by mass or more. The comonomer content in the copolymer is preferably 6% by mass or less, and more preferably 5% by mass or less.

  Examples of comonomers include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid, and ammonium salts and alkyl esters thereof, acrylamide, methacrylamide, and derivatives thereof, and combinations of two or more thereof. You can also.

  In particular, in order to reduce the cost, it is preferable to use an unsaturated carboxylic acid as a comonomer in terms of promoting the flame resistance reaction. The content of the unsaturated carboxylic acid in the copolymer is preferably 0.1 to 3% by mass, and more preferably 0.5 to 2% by mass.

  Examples of unsaturated carboxylic acids include acrylic acid, crotonic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid and the like.

  In order to obtain high-strength carbon fibers, it is necessary to increase the molecular orientation of the precursor fibers for carbon fibers. Therefore, it is preferable to copolymerize an unsaturated carboxylic acid ester for the purpose of increasing the degree of molecular freedom in the carbon fiber precursor fiber in order to facilitate high stretching in the carbon fiber precursor fiber manufacturing process. 0.1-6 mass% is preferable and, as for content in the copolymer of unsaturated carboxylic acid ester, 2-5 mass% is still more preferable.

  Examples of unsaturated carboxylic acid esters include alkyl acrylates and alkyl methacrylates. The preferred alkyl group length is 1 to 4 carbon atoms (C), and the particularly preferred alkyl group length is C 1 to 2.

  As a polymerization method of the above monomer and comonomer, solution polymerization, suspension polymerization, emulsion polymerization and the like can be used, but solution polymerization is most preferable because it can be spun as it is.

  The spinning solution (spinning stock solution) was prepared by polymerizing the above monomers and comonomers using organic solvents such as dimethylformamide, dimethyl sulfoxide, dimethylacetamide, and inorganic solvents such as nitric acid, zinc chloride aqueous solution, and rhodan salt aqueous solution as solvents. The polymer solution thus prepared is preferably used as a spinning dope. Among these, it is more preferable to use a zinc chloride aqueous solution having an advantage for dissolving a high molecular weight polymer as a solvent.

  Since the concentration of the spinning dope affects the specific gravity of the carbon fiber precursor fiber, it is preferably 5% by mass or more and 10% by mass or less when a zinc chloride aqueous solution is used as a solvent. More preferably, 7 mass% or more and 9 mass% or less are still more preferable. When the concentration of the spinning dope is too low, the specific gravity of the obtained carbon fiber precursor fiber is low, and carbon fibers having a low specific gravity cannot be obtained. On the other hand, if the concentration is too high, there is a limit to the solubility of the polymer in the solvent, which is not preferable because the spinning dope becomes a non-uniform solution.

  The spinning is preferably wet spinning in which spinning is carried out directly in a coagulation bath containing a coagulation liquid cooled to a low temperature (solvent-water mixture at the time of spinning). Alternatively, a dry-wet spinning method may be used in which a space of about 3 to 5 mm is first discharged into the air and then charged into a coagulation bath and coagulated.

  The spun yarn is gradually coagulated in a coagulation bath having a concentration gradient, and at the same time, while removing the solvent, washed with water and directly stretched in the bath. In stretching in the bath, it is preferable to stretch the spun yarn in several water washing to hot water baths at a draw ratio of 2.0 to 6.0, particularly 4.0 to 6.0.

  Regarding the conditions for stretching in the bath, the temperature gradient between the coagulation bath temperature and the washing temperature or hot water bath temperature is preferably 98 ° C. at the maximum.

  Then, prior to drying and densification, for the purpose of improving heat resistance and spinning stability, it is possible to apply a carbon fiber precursor fiber oil agent that combines a permeable oil agent having a hydrophilic group and a silicone oil agent. In the case of reducing the weight, it is preferable from the viewpoint of obtaining this carbon fiber with high quality.

  The blending ratio of the osmotic oil agent and the silicone oil agent is preferably 65:35 to 75:25 (mass basis).

  The osmotic oil agent preferably has sulfinic acid, sulfonic acid, phosphoric acid, carboxylic acid, its alkali metal salt, ammonium salt or its derivative as a functional group. Among these penetrating oils, it is particularly preferable to use dialkyl sulfosuccinate or a derivative thereof which can easily penetrate.

  The silicone oil may be either unmodified or modified, and among them, epoxy-modified silicone, ethylene oxide-modified silicone, polysiloxane and amino-modified silicone are preferable, and amino-modified silicone is particularly preferable.

  The ratio (attachment amount) applied to the acrylic fiber of the oil containing the permeable oil and the silicone-based oil is 0.1 to 0.3% by mass based on the acrylic fiber dry mass.

  When the amount of the oil agent is less than 0.1% by mass, the convergence property of the acrylic fiber in the flameproofing process is inferior, and the process passability after this flameproofing process is remarkably impaired.

  On the other hand, when the amount of oil agent adhering exceeds 0.3% by mass, the desired single fiber strength cannot be obtained, which is not preferable.

  In the drying and densification, it is preferable to dry with a hot air drier having several layers of rooms with a temperature gradient. About drying temperature, it is preferable to adjust suitably at 70-150 degreeC so that a compactness may improve more, and it is still more preferable to adjust suitably at 80-140 degreeC. The drying time is preferably 1 to 10 minutes.

  Moreover, the fineness and molecular orientation of the produced carbon fiber precursor fiber can be adjusted by stretching at a high temperature. In particular, heat stretching in pressurized steam is effective, and wet heat stretching treatment is particularly preferable under conditions of a temperature of 100 to 130 ° C. and a stretching ratio of 4.0 to 6.0. This hot drawing condition has a great influence on the density of the precursor fiber for carbon fiber.

  As a means for evaluating the denseness, there is an evaluation of apparent specific gravity by Archimedes method.

  About 2 g of carbon fiber precursor fibers are collected and collected into a circle having a diameter of 3 cm or less so that the shape does not collapse. The measurement solvent is preferably water or a hydrophilic solvent. In some cases, it is difficult to remove bubbles at the time of defoaming due to the influence of the oil agent applied to the precursor fiber for carbon fiber. In this case, it is most preferable to use ethanol or acetone.

  Next, the circular sample is immersed in a solvent and degassed under reduced pressure. Under normal temperature, the mass in the solvent is measured, the sample is further dried by heating to obtain the dry mass, and the apparent specific gravity of the precursor fiber for carbon fiber is obtained. The specific gravity is lower than the specific gravity of 1.18 of PAN, preferably 1.160 to 1.175, more preferably 1.163 to 1.174, and even more preferably 1.165 to 1.173. Change the drying densification and heat stretching conditions.

  In the present invention, the single fiber fineness of the precursor fiber for carbon fiber is preferably narrow so that oxidation spots (unevenness) in the flame resistance process are unlikely to occur from the viewpoint of improving the strength. Specifically, 1.2 d or less is preferable, 0.9 to 1.2 d is more preferable, and 0.96 to 1.18 d is still more preferable.

  The obtained precursor fiber for carbon fiber needs to prevent drying of the yarn (precursor fiber) so that the molecular orientation is not easily relaxed. Therefore, the moisture content of the precursor fiber is preferably 20 to 60% by mass, particularly preferably 30 to 50% by mass. If the moisture content of the carbon fiber precursor fiber is too low, the handleability is deteriorated due to a decrease in convergence, and if the moisture content is too high, the surface tension of water wraps around the roller during the flameproofing process. It becomes easy and causes trouble.

  The precursor fiber for carbon fiber produced as described above and having an appropriately adjusted moisture content can be temporarily stored in a sealed container. As the storage container, a cylindrical container is preferable, and a plastic bag is also preferable. However, when storing, it must be able to retain the internal moisture.

  In addition, since the precursor fiber for carbon fiber used in the present invention is not subjected to a heat treatment such as a dry heat roller and uses a yarn after wet heat drawing, if it is stored as it is, the orientation of the fiber is relaxed. This will cause a decrease in strength of the carbon fiber.

  As a method of preventing orientation relaxation of the carbon fiber precursor fiber, the following conventional techniques can be applied.

  That is, as a method for improving the molecular orientation inside the fiber in the post-process (flame-proofing process, carbonization process) after the production of the precursor fiber for carbon fiber, the yarn of the precursor fiber is produced by wet heat drawing. After that, a method of storing the yarn in a wet state with pure water or the like in a storage container can be used.

  According to the method of storing the yarn in the storage container while wet, it is possible to prevent orientation relaxation, oxidation by air, addition of foreign matter in the air, etc. caused by drying of the fiber, and to produce high-strength carbon fiber. it can.

  Next, the precursor fiber for carbon fiber produced in the previous step is subjected to flame resistance treatment in the flame resistance step. This flameproofing treatment is, for example, a horizontal furnace divided into two or more chambers in heated air, through a multi-stage roller group, a temperature of 230 to 290 ° C, preferably 230 to 270 ° C, a draw ratio of 1.02 to 1.08, Preferably, the heat treatment can be performed at 1.03 to 1.07.

  If the stretch ratio for flame resistance is low, the molecular orientation is relaxed, which is not preferable. Further, since the fiber becomes brittle as the flame resistance is usually increased, if the draw ratio is too high, fluff due to single yarn breakage is generated, and the quality of the carbon fiber obtained later is remarkably lowered.

  Therefore, it is preferable to heat-process at 1.02-1.08 about the stretch ratio at the time of flame resistance, and it is still more preferable to heat-process at 1.03-1.07.

  As for the flameproofing reaction, the reaction is initially initiated by oxidation to a nitrile group to cause a cyclization reaction, and further, by adding oxygen to the ring, a flameproofing structure is obtained. Therefore, by defining the degree of cyclization and the degree of oxidation, it is possible to produce a preferable flameproof yarn.

  The specific gravity of the flame resistant fiber is preferably 1.360 to 1.385, more preferably 1.363 to 1.383, and still more preferably 1.365 to 1.380.

  Near the surface of the flame-resistant yarn, various oxide structures are formed by Si oxides due to the effect of the oil applied to the precursor fiber for carbon fibers, amide formation by oxidation of polyacrylonitrile, oxidation to cyclized products, etc. However, a preferable structure is obtained when the ratio of oxygen to silicon is 1 or more as the element ratio in the vicinity of the surface.

  The flame-resistant fiber is gradually heated in a firing furnace (first carbonization furnace) divided into three or more chambers at 300 to 750 ° C. in an inert gas atmosphere such as nitrogen to control the tension of the flame-resistant fiber. The first stage of carbonization (preliminary carbonization) under tension.

  As for the tension condition, the contraction ratio (length after tension / length before tension) is preferably in the range of 1.01 to 1.07, more preferably in the range of 1.02 to 1.05.

  About baking temperature, a temperature gradient is applied in a 1st carbonization furnace, Preferably it is 550-750 degreeC, More preferably, it is 600-700 degreeC in the highest temperature range.

  In order to further promote carbonization and graphitization (high crystallization of carbon), the temperature is raised in an inert gas atmosphere such as nitrogen and gradually in a firing furnace (second carbonization furnace) divided into two or more chambers. A temperature gradient is applied to the yarn, and the tension of the yarn (preliminary carbonized fiber) is controlled and fired under a relaxed condition.

  Regarding relaxation conditions, the shrinkage ratio (length after relaxation / length before relaxation) is preferably in the range of 0.9 to 1.0, more preferably in the range of 0.92 to 0.99, and still more preferably 0. A range of .95 to 0.98 is preferable.

  About a calcination temperature, a temperature gradient is applied in a 2nd carbonization furnace, Preferably it is 1180-1320 degreeC in a highest temperature area | region, More preferably, it is good to set it as 1200-1300 degreeC.

  The temperature gradient is preferably 400 ° C./min or higher, more preferably 400 to 1000 ° C./min, and still more preferably 500 to 900 ° C./min. It is not preferable that the furnace length is too long in terms of productivity and cost, and if the residence time in the high temperature part in the furnace becomes long, graphitization proceeds too much and brittle carbon fibers are obtained. It is not preferable.

  Further, if the temperature gradient is gentle and the residence time is long, densification is advanced in the structure inside the carbon fiber, which is not preferable because the density of the carbon fiber becomes too high. By setting the residence time in the temperature gradient and the maximum temperature range within the above range, the structure inside the carbon fiber is optimized, and the conventional general-purpose carbon fiber can have the fiber characteristics.

  The obtained carbon fiber is surface-treated by electrolytic oxidation treatment in an electrolytic layer using an acid or alkaline aqueous solution. When carbon fiber is combined with resin and used as a material, it is necessary to improve the affinity and adhesion between the carbon fiber and the matrix resin.

  As an electrolytic solution for electrolytic treatment, an acidic or alkaline solution can be used. Examples of acidic substances include nitric acid, sulfuric acid, hydrochloric acid, acetic acid, their ammonium salts, and ammonium hydrogen sulfate.

  Among these electrolytic solutions, ammonium salts such as ammonium sulfate and ammonium hydrogen sulfate that exhibit weak acidity are preferable. Examples of alkaline substances include potassium hydroxide, sodium hydroxide, and ammonia.

  The amount of electricity at the time of electrolytic oxidation needs to be adjusted according to the degree of graphitization of the carbon fiber outer layer portion. In consideration of the compounding with the resin, it is preferably 15 coulombs or more, and particularly preferably 18 coulombs or more per 1 g of the carbon fiber that improves the affinity. If the amount of electricity is too large, the surface may be oxidized more than removing small-scale defects on the carbon fiber surface, and new defects may be generated, and at most 30 coulombs are preferred.

  In addition, after the surface treatment by electrolytic oxidation, the electrolytic solution and its by-products are attached to the carbon fiber, so it is necessary to wash with water and dry it. Further, sizing treatment is performed for the purpose of facilitating the post-processing of the carbon fiber and improving the handleability. The sizing method can be carried out by a conventionally known method, and the sizing agent is used by changing the composition as appropriate according to the use, and after uniformly adhering, is dried. The amount of adhesion is preferably 0.1 to 2.0% by mass, more preferably 0.5 to 1.5% by mass.

  Polyamide-coated carbon fibers were produced under the conditions described in the following examples and comparative examples. In addition, the various physical-property values of each polyamide covering carbon fiber were measured by the above-mentioned method or the following methods.

[Orientation degree in X-ray diffraction measurement]
In addition, about the orientation degree measurement by X-ray diffraction measurement, it can obtain | require as follows.

  The carbon fiber precursor fiber is composed of about 24,000 single fibers (for example, two carbon fiber bundles of 12,000 single fibers), converged with acetone, and aligned in the fiber axis direction.

  A carbon fiber precursor fiber bundle having a length of 3 cm, in which the fibers are aligned, is attached to a mount with a hole having a diameter of 1 cm so that the hole portion is in the center of the fiber. The carbon fiber precursor fiber bundle affixed to the mount is fixed to the sample adjustment jig in a tensioned state so that the fiber axis and the jig axis are parallel.

  Furthermore, this jig is fixed to a wide-angle X-ray diffraction measurement sample stage by a transmission method. When Cu Kα rays are used as the X-ray source and the sample is irradiated, a diffraction pattern appears when 2θ is around 17 degrees.

At the position of the peak (2θ) of the diffraction pattern obtained by the above measurement, the half width is obtained from the two peaks obtained by rotating the measurement sample stage 0 to 360 degrees and scanning in the circumferential direction. The full width at half maximum is H, and the following degree of orientation (%) = [(180−H) / 180] × 100
Can be obtained.

[Carbon fiber surface oxygen and silicon concentrations O / C, Si / C, and Si binding energy]
The surface oxygen and silicon concentrations O / C and Si / C on the carbon fiber surface were determined by XPS (ESCA) according to the following procedure.

After cutting the carbon fibers and arranging them on a stainless steel sample support table, the photoelectron escape angle was set to 90 degrees, MgKα was used as the X-ray source, and the inside of the sample chamber was vacuumed at 1 × 10 ⁻⁶ Pa. Keep it up. As a correction of the peak accompanying charging at the time of measurement, first, the binding energy value B. of the main peak of C ^1s . E. Is adjusted to 284.6 eV. The Si ^1s peak area is obtained by drawing a straight base line in the range of 92 to 116 eV, the O ^1s peak area is obtained by drawing a straight base line in the range of 528 to 540 eV, and the C ^1s peak area is It was determined by drawing a straight baseline in the range of 282 to 296 eV. The surface oxygen concentration O / C on the surface of the carbon fiber was determined by calculating the ratio of the O ^1s peak area to the C ^1s peak area. The surface silicon concentration Si / C on the surface of the carbon fiber was determined by calculating the ratio of the Si ^1s peak area to the C ^1s peak area.

  In addition, the peak of Si binding energy measured by X-ray photoelectron spectroscopy was expanded, and the position of the maximum point was obtained.

[Electric resistance value]
With respect to the measurement of the electric resistance value, it can be performed with reference to the test method A of the strand having a volume resistivity specified in JIS-R-7601. However, in JIS-R-7601, the volume resistivity obtained by multiplying the electrical resistance value by the specific gravity of the carbon fiber is obtained. To obtain the electrical resistance value [X (Ω · g / m ² )], Formula X = Rb × t / L
Rb: electrical resistance (Ω) when the specimen length is L, t: fineness of the specimen (tex), L: specimen length when measuring the resistance (m)
It was performed using.

  In addition, about the test piece length at the time of resistance measurement, it is preferable to measure at about 1 m.

[Carbon fiber strand strength, elastic modulus]
It was measured by the method defined in JIS R7601.

[Interlaminar shear strength of composite (ILSS)]
A curing agent and an accelerator were added to an epoxy resin (trade name: Epicoat manufactured by Yuka Shell Epoxy Co., Ltd.) to prepare an epoxy resin composition for carbon fiber impregnation.

This resin composition was applied onto release paper with a film coater to obtain a resin film. After aligning carbon fibers on this resin film at equal intervals and heating, carbon fiber is impregnated by heating to produce a carbon fiber reinforced epoxy resin composition having a basis weight of 350 g / m ² and a resin impregnation ratio of 35% by mass. did.

The carbon fiber reinforced epoxy resin composition prepared above was laminated so that the thickness after molding was 2.8 mm, placed in a mold, and 0.49 MPa-Gauge (5.0 kgf) at 130 ° C. for 1.5 hours. / Cm ² -Gauge) to form a carbon fiber reinforced molded plate (CFRP plate: composite) in which fibers are arranged in one direction. The ILSS of this CFRP plate was measured at room temperature according to ASTM-D-2344.

[Example 1]
By a solution polymerization method using an aqueous solution of zinc chloride as a solvent, a polymer stock solution having a polymerization degree of 1.6 and a polymer concentration of 7.5% by mass consisting of 95% by mass of acrylonitrile, 4% by mass of methyl acrylate, and 1% by mass of itaconic acid is obtained. Obtained.

  This polymer stock solution was discharged into a 25 mass% zinc chloride aqueous solution at 5 ° C. through a base for 12000 filaments and coagulated to obtain a coagulated yarn.

  This coagulated yarn is washed with water and hot-drawn at 90 ° C., and contains 7 g / l of an amino-modified silicone oil agent (BS-379 manufactured by Aminosilicone Takemoto Yushi Co., Ltd.) and 3 g / l (70:30) of dioctylsulfosuccinate. After being immersed in the emulsion solution, 0.2% by mass was deposited, and it was dried and densified at 70 to 140 ° C. using a hot air dryer, and wet-heat stretched at a stretch ratio of 4.6 at 110 to 120 ° C. to obtain a moisture content of 40. The carbon fiber precursor fiber having a single fiber fineness of 1.17d was obtained by adjusting the mass%. The fiber specific gravity was 1.165, and the degree of orientation by X-ray diffraction was 89.6%.

  The obtained precursor fiber for carbon fiber was flameproofed at a draw ratio of 1.05 in an atmosphere having a temperature distribution of 250 ° C. to 270 ° C. in air. The specific gravity of the flameproof yarn was 1.36.

  The flameproofed yarn is carbonized at a draw ratio of 1.03 in a first carbonization furnace having a temperature distribution of 300 to 650 ° C. in an inert atmosphere, and the maximum temperature is 1250 ° C. in an inert atmosphere. Carbonization was performed in a second carbonization furnace set so as to be (temperature distribution in the atmosphere: 300 to 1250 ° C.).

  Next, an electrolytic oxidation treatment of 20 coulomb per gram of carbon fiber was performed using a 10% by mass ammonium sulfate aqueous solution as an electrolytic solution, followed by washing with water and further sizing treatment to attach a sizing agent-water emulsion solution (concentration 3% by mass). This was dried at 150 ° C. The adhesion amount of the sizing agent was 1.3% by mass. The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Example 2]
After carbonization, the same procedure as in Example 1 was carried out except that 10% by mass ammonium sulfate aqueous solution was used as the electrolytic solution, and after conducting an electrolytic oxidation treatment of 25 coulombs per gram of carbon fiber, it was washed with water. The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Example 3]
The same procedure as in Example 1 was performed except that carbonization was performed in a second carbonization furnace set so that the maximum temperature was 1300 ° C. in an inert atmosphere (temperature distribution in the atmosphere: 300 to 1300 ° C.). The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Example 4]
The same procedure as in Example 1 was performed except that carbonization was performed in a second carbonization furnace set so that the maximum temperature was 1200 ° C. in an inert atmosphere (temperature distribution in the atmosphere: 300 to 1200 ° C.). The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 1]
The oil agent was applied to the carbon fiber precursor fiber after dry densification, not before dry densification, and further dried at 120 ° C. to obtain a carbon fiber precursor fiber. The subsequent firing step was performed in the same manner as in Example 1. The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 2]
Instead of attaching 0.2% by mass of a mixture of amino-modified silicone oil and dioctyl sulfosuccinate to carbon fiber precursor fiber oil, the same amount of osmotic oil having an ammonium phosphate derivative was attached in an amount of 0.1% by mass. The procedure was the same as in Example 1 except that. The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 3]
Instead of adhering 0.2% by mass of the mixture of amino-modified silicone oil and dioctylsulfosuccinate to the oil agent of the carbon fiber precursor fiber, the same amount of the osmotic oil agent having the amino-modified silicone oil and the ammonium phosphate derivative in an amount of 0. The same procedure as in Example 1 was performed except that 1% by mass was adhered. The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 4]
The same procedure as in Example 1 was performed except that carbonization was performed in a second carbonization furnace set so that the maximum temperature was 1400 ° C. in an inert atmosphere (temperature distribution in the atmosphere: 300 to 1400 ° C.). The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 5]
The same procedure as in Example 1 was performed except that carbonization was performed in a second carbonization furnace set so that the maximum temperature was 1150 ° C. in an inert atmosphere (temperature distribution in the atmosphere: 300 to 1150 ° C.). The characteristics of the carbon fiber thus obtained are shown in Table 1.

[Comparative Example 6]
The same procedure as in Example 1 was performed except that the amount of electricity per 1 g of carbon fiber in the electrolytic oxidation treatment was 12 coulomb. The characteristics of the carbon fiber thus obtained are shown in Table 1.

Claims (5)

Polyacrylonitrile-based carbon fiber having a strand strength of 4600-5800 MPa, an elastic modulus of 220-250 GPa, a density of 1.74 g / cm ³ or more and less than 1.80 g / cm ³ , and containing silicon, the silicon contained Has a SiOx (1 ≦ x ≦ 3) structure, and its content is elemental silicon, and is a polyacrylonitrile-based carbon fiber having a carbon fiber mass of 110 to 500 ppm. The average single fiber diameter of the carbon fiber strand is 6 to 8 μm, the electrical resistance value of the carbon fiber strand is 29 to 32 Ω · g / m ² , and the amount of surface silicon (Si) measured by X-ray photoelectron spectroscopy is The polyacrylonitrile-based carbon fiber according to claim 1, wherein Si / C shown is 0.10 to 0.15, and O / C showing the amount of surface oxygen (O) is 0.15 to 0.30. 3. The polyacrylonitrile-based carbon fiber according to claim 1, wherein the surface has a structure containing silicon having a Si binding energy peak measured by X-ray photoelectron spectroscopy in a range of 102 to 104 eV. A yarn obtained by spinning a copolymer obtained by polymerizing a monomer containing 94% by mass or more of acrylonitrile was prepared by using an emulsion aqueous solution containing amino-modified silicone and dialkylsulfosuccinate as an oil agent in a dry mass of 0.1 to 0. After 3% adhesion, drying and densification with a dryer at 70 to 150 ° C., and wet-heat drawing under conditions of a temperature of 100 to 130 ° C. and a draw ratio of 4.0 to 7.0, a single fiber fineness of 0.96 ~ 1.2d precursor fiber for carbon fiber is obtained, and the obtained precursor fiber is directly heat-treated in heated air at 230 to 290 ° C and a draw ratio of 1.02 to 1.08 to obtain a flame-resistant fiber, The obtained flame-resistant fiber is heated in an inert gas atmosphere, pre-carbonized at a maximum temperature range of 550 to 750 ° C. and a draw ratio of 1.01 to 1.07, and further heated in an inert gas atmosphere. 1180-1 in the maximum temperature range 20 ° C., and carbonized at a draw ratio of 0.90 to 1.00, the production method of the polyacrylonitrile-based carbon fiber characterized in that the surface treatment by carbon fiber 1g per electrical quantity 15 coulomb or more electrolytic oxidation in an aqueous solution. The manufacturing method of the polyacrylonitrile-type carbon fiber of Claim 4 whose moisture content of the precursor fiber for carbon fibers is 20-60 mass%.