JP2007002392A - Carbon fiber bundle and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide carbon fiber bundles excellent in quality and a method for producing these carbon fiber bundles and bring about quality stabilization of higher-order processed articled by reducing both of the coefficient of variation in the longitudinal direction of interlaminar shear strength (ILSS) of carbon fiber bundles and the coefficient of variation between carbon fiber bundles. <P>SOLUTION: The present invention provides polyacrylonitrile-based carbon fiber bundles in which coefficient of variation in the longitudinal direction of interlaminar shear strength (ILSS) is ≤2.5% and the method for producing the carbon fiber bundles. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a polyacrylonitrile-based carbon fiber bundle having a small interlayer shear strength (ILSS) variation and a method for producing the same.

  Carbon fiber has extremely excellent performance as a reinforcing fiber of a composite material (composite) due to mechanical properties such that the specific strength and specific elastic modulus are larger than those of other reinforcing fibers. This composite material using carbon fiber as a reinforced fiber has been widely used as a structural material for aerospace, space use, or a demand for weight reduction or fuel consumption reduction in transportation equipment such as automobiles and ships. . However, in order to reflect the excellent mechanical performance of the carbon fiber in the composite material, the base material resin of the composite material and the carbon fiber need to be sufficiently bonded and integrated.

In general, evaluation of adhesion between carbon fiber and base material resin uses interlaminar shear strength (ILSS) in which carbon fiber is impregnated with resin. For this purpose, carbon fibers are usually subjected to oxidation treatment such as electrolytic oxidation, chemical solution oxidation, and gas phase oxidation after firing, and oxygen-containing functional groups are introduced on the surface of carbon fibers to improve wettability with the base resin. . Regarding the surface characteristics of carbon fibers obtained by such an oxidation treatment, Patent Document 1 discloses improvement of adhesive strength with a base resin by specifying a functional group on the outermost surface of carbon fiber, and Patent Document 2 discloses surface treatment. Later, it is disclosed that the adhesion strength is improved by applying a fatty acid compound having a plurality of epoxy groups as a sizing agent.

  However, the carbon fiber surface is not only covered with the graphite crystal plane, but also has many impurities adhering to it, causing a decrease in resin adhesion due to the intervention of impurities between the carbon fiber bundle and the resin. These impurities are derived from a sintered product in which tar mist composed of a metal compound having a large molecular weight among the gas components generated in the firing process is heated and mineralized in the subsequent process, and a silicone-based oil used as a precursor oil. Examples thereof include a sulfur compound derived from a silica compound or a sulfur-containing oil agent.

  In Patent Document 3, as a means for cleaning impurities, the surface-treated carbon fiber is washed in an alkaline aqueous solution (PH> 8), then in an acidic aqueous solution (PH <6), washed until neutralized, and then washed with alkali. A method of washing with water until post-acidification washing and neutralization is disclosed. These impurities are discharged as exhaust gas together with the thermal decomposition of the flame resistant fiber in the temperature range of 400 ° C. to 600 ° C. where the thermal decomposition of the flame resistant fiber proceeds rapidly and mostly. As a result of the examination, it is known that the heat treatment at 1000 ° C. to 2000 ° C. causes the carbon fiber to adhere closely to the carbon fiber and makes it difficult to remove it in the surface treatment process. As a method for preventing tar mist contamination and re-adhesion generated during thermal decomposition, for example, in Patent Document 4 and Patent Document 5, the exhaust gas outlet is adjusted to the thermal decomposition reaction temperature range, and in Patent Document 6, the heating furnace is sequentially installed. It is disclosed to partition into a plurality of heating zones set to be high temperature. However, there is no problem if the temperature in the heating furnace is always kept uniform, but the gas generated by the flame-resistant fiber pyrolysis, the supply gas for keeping the heating furnace in an inert gas, etc. It is very difficult to maintain a set temperature in a plurality of heating zone sections set by convection. As a result of this study, it was found that temperature fluctuations in the heating furnace over time cause variations in the adhesion of impurities and ultimately leads to variations in the resin adhesion of the carbon fiber bundles. Even if it is performed, the variation in impurity adhesion is not sufficiently solved, and as a result, the variation in the longitudinal direction of the carbon fiber bundle and between a plurality of the same calcined carbon fiber bundles cannot be solved.

In view of such a current situation, the present inventors have found that carbon fiber surface impurities are removed in order to improve the variation rate in the longitudinal direction and the machine width direction of the resin adhesive strength in the carbon fiber bundle, and complete the present invention. It came to.
JP-A-4-361619 JP-A-7-279040 JP 61-152873 A JP 59-112029 A Japanese Examined Patent Publication No. 62-0466647 JP-A-62-33822

  An object of the present invention is to provide a carbon fiber excellent in quality stability having a small variation in interlaminar shear strength (ILSS), that is, a fluctuation rate, and a method for producing the same.

  In order to achieve the above-described object, the carbon fiber bundle of the present invention has one of the following configurations. That is, a polyacrylonitrile-based carbon fiber bundle or a plurality of precursor fibers for carbon fibers are characterized in that the longitudinal shear rate of interlaminar shear strength (ILSS) is 2.5% or less in the same firing facility. The polyacrylonitrile-based carbon fiber bundle is characterized in that the variation rate in the machine width direction of the interlaminar shear strength (ILSS) obtained by firing in parallel is 2.5% or less.

  In order to achieve the above-described object, the method for producing a carbon fiber bundle of the present invention has one of the following configurations. That is, in a method for producing a carbon fiber bundle by pre-carbonizing and carbonizing a flame resistant yarn obtained by heating a polyacrylonitrile-based precursor yarn in an oxidizing atmosphere, A method for producing a carbon fiber bundle, or a polyacrylonitrile-based precursor, characterized by controlling a temporal temperature fluctuation rate in a heating furnace maintained in an active atmosphere and a temperature fluctuation rate in a machine width direction to 2.5% or less In a method for producing a carbon fiber bundle by pre-carbonizing and carbonizing a flame-resistant yarn obtained by heating a yarn in an oxidizing atmosphere, 300 to 800 in a heating furnace maintained in an inert atmosphere. In the pre-carbonization treatment in the ° C region, an inert gas for heating the yarn is heated to 300 ° C or higher and then supplied to a heating furnace.

  According to the present invention, as described below, the fluctuation rate of the interlaminar shear strength (ILSS) in the longitudinal direction and the machine width direction of the carbon fiber bundle is reduced to improve its quality stability, and as a result, a high-order processed product. Quality stabilization can be achieved.

As the fiber yarn form of the carbon fiber bundle of the present invention, the twisted yarn obtained by twisting and firing the precursor fiber, the untwisted yarn obtained by untwisting the twisted yarn, and the precursor fiber substantially Although untwisted yarns that are heat-treated without twisting can be used, non-twisted yarns or untwisted yarns are preferable in consideration of higher-order processability, and non-twisted yarns are more preferable from the viewpoint of spreadability during higher-order processing.
Furthermore, in the carbon fiber bundle of the present invention, the rate of change in the longitudinal interlaminar shear strength (ILSS) of the carbon fiber bundle needs to be 2.5% or less, more preferably 2.0% or less. More preferably, it is 5% or less.

  If the variation rate of the longitudinal interlaminar shear strength (ILSS) of the carbon fiber bundle exceeds 2.5%, the resin adhesion stability between the carbon fiber bundle and the base material resin is increased in higher processing such as prepreg molding and filament winding molding. It will decline. For this reason, the shear strength of the molded product is reduced, leading to a reduction in product yield. In prepregs using multiple yarns, the longitudinal variation in interlaminar shear strength is somewhat covered by adjacent yarns, but in particular, filament winding molding that uses several carbon fiber bundles as raw materials. The longitudinal stability of the interlaminar shear strength (ILSS) of the carbon fiber bundle is very important. Since several yarns are used, the variation in the longitudinal direction directly affects the quality of the molded product and directly leads to the product yield. As a result of the study, it was found that the decrease in shear strength due to insufficient resin adhesion had little effect when the interlaminar shear strength (ILSS) of the carbon fiber bundle was 2.5% or less.

  Here, in the present invention, the interlaminar shear strength (ILSS) of the carbon fiber bundle and the longitudinal variation rate thereof are obtained as follows.

  A test piece is prepared by impregnating a carbon fiber bundle with bisphenol A type epoxy resin added with boron trifluoride monoethylamine as a curing agent in a weight ratio of 6: 4 and curing in a 170 ° C. oven for 1 hour. . The prepared test piece is measured by a three-point bending method based on ASTM-D-2344 to determine the interlaminar shear strength. Interlaminar shear strength is determined by the following equation.

Interlaminar shear strength (ζ: Pa) = (3 × P) / (4 × b × t)
Here, P is the maximum load (kg), b is the width (mm) of the test piece, and t is the thickness (mm) of the test piece.

  The variation rate is obtained based on the following formula for the measured interlaminar shear strength of the carbon fiber bundle.

Longitudinal interlayer shear strength fluctuation rate (%) = (σ1 / A1) × 100
Σ1 used in the equation is a standard deviation of all measured interlayer shear strength data, and A1 is an average value of all measured interlayer shear strength data. Here, the total data refers to interlaminar shear strength data for 10 carbon fiber bundles collected every 100 m in the longitudinal direction from each bobbin.

Moreover, since the variation in the machine width direction is also considered as a factor of the interlaminar shear strength (ILSS) variation of the carbon fiber bundle, it is important to reduce this. Usually, in order to produce efficiently, multiple fiber bundles made of polyacrylonitrile, etc., which are precursor fiber bundles of carbon fiber bundles, are placed in parallel and fired simultaneously to obtain multiple carbon fiber bundles simultaneously. However, the fluctuation rate of the interlaminar shear strength (ILSS) in the machine width direction between a plurality of carbon fiber bundles fired at the same time is obtained as follows.

Machine width direction interlaminar shear strength fluctuation rate (%) = (σ2 / A2) × 100
Σ2 used in the equation is a standard deviation of all measured interlayer shear strength data, and A2 is an average value of all measured interlayer shear strength data. Here, the total data refers to the carbon fiber bundle specimens taken from the outermost layer of each of the 28 bobbins sampled from the co-fired lots according to the sampling method specified in MIL-STD-414 (1957). Refers to interlaminar shear strength data.

  Next, the manufacturing method of the carbon fiber bundle of this invention is demonstrated.

  The carbon fiber bundle according to the present invention can be produced as follows. First, a polyacrylonitrile fiber bundle having a constitution in which acrylonitrile is 90% by weight or more and a monomer copolymerizable with acrylonitrile is less than 10% by weight is used as a carbon fiber precursor. The above copolymerizable monomers include acrylic acid, methacrylic acid, itaconic acid or their methyl ester, propyl ester, butyl ester, alkali metal salt, ammonium salt, allyl sulfonic acid, methallyl sulfonic acid, styrene sulfonic acid and It is possible to use at least one selected from the group consisting of these alkali metal salts. This polyacrylonitrile-based precursor fiber bundle is heated and flame-resistant in an oxidizing atmosphere such as air in a temperature range of 200 ° C. to 300 ° C. to produce flame-resistant fibers, and then inert such as nitrogen before carbonization treatment. A pre-carbonization treatment is performed in a temperature range of 300 ° C. to 800 ° C. in an atmosphere. Thus, carbon fiber can be manufactured by carbonizing in the temperature range whose maximum temperature is 1000 degreeC to 2500 degreeC in inert atmosphere, such as nitrogen, after performing a pre-carbonization process. As a surface treatment to be applied after the carbonization treatment, there is an electrolytic oxidation surface treatment for the purpose of generating a functional group on the surface of the carbon fiber and enhancing the adhesion to the resin. The method includes liquid phase oxidation using a chemical solution, electrolytic oxidation in which carbon fiber is treated as an anode in an electrolytic solution, and vapor phase oxidation by plasma treatment in a phase state. As the surface treatment method, an electrolytic oxidation treatment method that is relatively easy to handle and advantageous in terms of manufacturing cost is preferably employed. As the electrolytic solution, either an acidic aqueous solution or an alkaline aqueous solution can be used. As the acidic aqueous solution, sulfuric acid or nitric acid exhibiting strong acidity is preferable, and as the alkaline aqueous solution, an aqueous solution of an inorganic alkali such as ammonium carbonate or ammonium hydrogen carbonate is preferable. Is preferably used.

After the electrolytic oxidation treatment, a sizing agent can be applied to the carbon fiber bundle and used for a molded product. The kind of the sizing agent here is not particularly limited, but a bisphenol A type epoxy resin mainly composed of an epoxy resin or an aliphatic compound having two or more epoxy groups at both ends having a linear structure is preferable. Used. The epoxy group is preferably a highly reactive glycidyl group. Specific examples of the aliphatic compound having an epoxy group in the present invention include glycerin polyglycidyl ethers for glycidyl ether compounds and polyethylene glycol diglycidyl ethers for diglycidyl ether compounds.

As a result of earnest investigation, the inventors of the present invention have found that the carbon fiber bundle surface adhering impurities causing adhesion variation with the base resin during the composite molding of the carbon fiber bundle produced by the above-described method is the temperature distribution in the pre-carbonization furnace process. It was found to be due to. There are two main types of pre-carbonization furnace structures. One is a horizontal furnace in which the yarn runs horizontally and heat treatment is performed.
Although the convection due to the supplied inert gas is relatively small and the temperature fluctuation in the furnace is also relatively small, in order to suppress the suspension due to the carbon fiber's own weight, it is not possible to make the machine length long, and several heat treatments are required, which improves the processability. Not good. The other is a vertical furnace where the yarns run vertically and heat treatment is performed, and since there is no suspension due to the weight of the carbon fiber bundle, the length of the machine can be increased and the processability is excellent, so it is used as the mainstream. Yes. However, there is a drawback that convection due to the generated gas during the heat treatment and the supplied inert gas is likely to occur, and it is difficult to keep the temperature in the furnace stable. Due to convection due to gas generated by flameproof fiber pyrolysis, supply gas, etc., not only the partial or total actual temperature in the compartment divided for each set temperature in the heating furnace varies up and down with respect to the set temperature. , Also varies over time. Due to this temperature fluctuation over time, the gas flows toward a lower temperature range than a higher temperature range with respect to the set temperature. Therefore, when viewed at a certain time, impurities attached to the yarn running in the lower temperature range Increased for yarn running in high temperature range. That is, the temperature fluctuation of the pre-carbonization furnace and the amount of impurities adhering to the yarn are closely related.

When a room temperature gas is supplied into a furnace heated to 300 ° C. to 800 ° C., the supply gas temperature rapidly rises as soon as it enters the pre-carbonization furnace, resulting in local variations in gas temperature. The temperature fluctuation is amplified as the temperature in the pre-carbonization furnace is increased, and this is one of the causes of convection. In that case, when the actual temperature in the previous carbonization furnace was measured every 30 minutes for 24 hours by the measurement method described later, the temperature fluctuation rate in the furnace was 3 to 10%, and the interlaminar shear strength (ILSS) of the obtained carbon fiber bundle Longitudinal direction fluctuation rate and machine width direction fluctuation rate both exceed 2.5%. This is because, as described above, the gas generated by the pyrolysis of the flame-resistant fiber flows from a high temperature to a low temperature, so that impurities attached to the yarn running in the low temperature region when viewed at a certain time. This is because it becomes larger for the yarn traveling in the high temperature region. As a result of the examination, impurities adhering to the carbon fiber bundle are closely adhered to the carbon fiber by being subjected to a heat treatment at 1000 ° C. to 2500 ° C. in the carbonization treatment process, making it difficult to remove in the surface treatment process. Yes. In particular, there is a great influence due to heat treatment in a temperature range of 1000 ° C. to 2000 ° C. However, the design of a carbon fiber bundle having a strand strength of 5000 MPa or more and a strand elastic modulus of 250 GPa or more required by the Japanese Industrial Standard (JIS) -R-7601 (1986) “resin impregnated strand test method” is 1000 ° C. to 2000 ° C. Carbonization in the temperature range of ° C is indispensable. As a means for suppressing temperature fluctuation in the pre-carbonization furnace, it is essential to heat the supply gas to at least 300 ° C., which is a low temperature region in the pre-carbonization furnace. By heating the supply gas to 300 ° C or higher in advance, the temperature fluctuation rate in the furnace can be suppressed to 2.5% or less, and the interlaminar shear strength (ILSS) of the obtained carbon fiber bundle in the longitudinal direction / machine Both the fluctuation rates in the width direction are 2.5% or less, and a carbon fiber bundle excellent in high-order workability is obtained. This effect is particularly noticeable in the above vertical furnace. Further, heating to 500 ° C. or higher at which the thermal reaction is activated by the pre-carbonization treatment is more preferable in terms of stabilizing the furnace temperature. Although there is no restriction | limiting in particular in the temperature upper limit of supply gas, it can be said that it is preferable to set it as 800 degreeC which is the high temperature area | region of a pre-carbonization furnace. In consideration of the sealing property in the furnace, a method of supplying the supply gas from both ends in the pre-carbonization furnace, that is, from both the high temperature section and the low temperature section is preferable.

  The temperature fluctuation in the pre-carbonization furnace is obtained as follows. For the machine width direction, thermocouples are installed every 20 cm from both ends in the heat treatment chamber. About a height direction, a thermocouple is installed in the center part of the part divided for every heating temperature area | region. For each, the furnace temperature is measured every 30 minutes for 8 hours, and the fluctuation is determined.

  According to the present invention, the rate of fluctuation of the interlaminar shear strength (ILSS) of the carbon fiber bundle in the longitudinal direction and the machine width direction is reduced to improve its quality stability, and as a result, produced by the unidirectional prepreg method or the filament winding method. Stabilization of the quality of high-order processed products such as prepregs can be achieved.

Hereinafter, the present invention will be specifically described by way of examples.
Example 1
An acrylic polymer solution composed of 99 mol% acrylonitrile and 1 mol% itaconic acid was prepared by polymerization, and an acrylonitrile precursor fiber having a single fiber fineness of 1 dtex and a filament number of 24,000 was obtained by a dry and wet spinning method. This was heated in air at a temperature of 200 to 270 ° C. to form a flame-resistant fiber bundle, and then pre-carbonized in a vertical pre-carbonization furnace in a temperature range of 300 to 800 ° C. in a nitrogen atmosphere. Nitrogen supplied into the pre-carbonization furnace was heated to 300 ° C. As a result of measuring the temperature in the pre-carbonization furnace divided for each heating temperature region by thermocouple every 30 minutes for 8 hours and every 20 cm in the machine width direction in the furnace, 2% temperature fluctuation was confirmed. Subsequently, carbonization was performed in a temperature range of 1000 to 1800 ° C. to obtain a carbon fiber bundle. Thereafter, the surface treatment is carried out with an aqueous sulfuric acid solution as an electrolytic solution at an electric quantity of 10 coulombs per gram of carbon fiber, followed by washing with water, and then the sizing agent mainly composed of bisphenol A type epoxy resin is 1% by weight with respect to the carbon fiber. Thus, a carbon fiber bundle was obtained.
The obtained carbon fiber bundle had a strand strength of 5200 MPa and a strand elastic modulus of 270 GPa as measured by Japanese Industrial Standard (JIS) -R-7601 (1986) “Resin-impregnated strand test method”. The resulting carbon fiber bundle was impregnated with Epicoat 828 epoxy resin with a curing agent added at a weight ratio of 6: 4, and cured in an oven heated to 170 ° C. for 1 hour to prepare a test piece. The interlaminar shear strength (ILSS) of the prepared test piece was determined by a three-point bending method based on ASTM-D-2344.

As a result, it was possible to obtain a carbon fiber bundle excellent in quality with little variation, in which both the longitudinal shear rate and the machine width direction variation rate of ILSS were 2.0%.
(Comparative Example 1)
When the temperature of the pre-carbonization furnace was measured in the same manner as in Example 1 except that the temperature of the nitrogen supplied to the pre-carbonization furnace was changed to room temperature, a temperature fluctuation of 5% was confirmed. The variation rate of the interlaminar shear strength (ILSS) of the obtained carbon fiber bundle increased to 4% in both the longitudinal direction and the machine width direction, and a carbon fiber bundle having a low quality potential was obtained.
(Example 2)
A solution of an acrylic polymer composed of 99 mol% acrylonitrile and 1 mol% itaconic acid was prepared by polymerization, and an acrylic nitrile precursor fiber composed of a single fiber fineness of 0.5 dtex and a filament number of 24,000 was obtained by a dry and wet spinning method. Using this, the temperature of the pre-carbonization furnace was measured in the same manner as in Example 1 except that the temperature of the nitrogen supplied to the pre-carbonization furnace was changed to 500 ° C., and 1% temperature fluctuation was confirmed. The obtained carbon fiber bundle had a strand strength of 5700 MPa and a strand elastic modulus of 290 GPa as measured by Japanese Industrial Standard (JIS) -R-7601 (1986) “Resin-impregnated strand test method”. The obtained carbon fiber bundle had a longitudinal interlaminar shear strength (ILSS) fluctuation rate of 1.0% and a machine width direction fluctuation rate of 1.5%, and a carbon fiber bundle excellent in quality with little variation was obtained. .

  The carbon fiber of the present invention has low interlaminar shear strength (ILSS) and can be expected to be stable in quality with high-order processability. Therefore, the carbon fiber of the present invention can be applied to space use, automobile use, sports and general industrial use. Is not something The carbon fiber bundle of the present invention is widely used industrially as a reinforcing fiber for composite materials, and is industrially useful.

Claims (5)

A polyacrylonitrile-based carbon fiber bundle having a longitudinal variation rate of interlaminar shear strength (ILSS) of 2.5% or less. The variation rate in the machine width direction of interlaminar shear strength (ILSS) obtained by firing a plurality of precursor fibers for carbon fiber in parallel in the same firing facility is 2.5% or less. A polyacrylonitrile-based carbon fiber bundle. The carbon fiber bundle according to claim 1 or 2, wherein the strand strength is 5000 MPa or more and the strand elastic modulus is 250 GPa or more. In the method of producing carbon fiber bundles by pre-carbonizing and carbonizing a flame-resistant yarn obtained by heating a polyacrylonitrile-based precursor yarn in an oxidizing atmosphere, an inert atmosphere during the pre-carbonizing treatment A method for producing a carbon fiber bundle, characterized by controlling a temperature fluctuation rate with time in a heating furnace maintained at a temperature and a temperature fluctuation rate in a machine width direction to 2.5% or less. A heating furnace maintained in an inert atmosphere in a method for producing a carbon fiber bundle by pre-carbonizing and carbonizing a flame-resistant yarn obtained by heating a polyacrylonitrile-based precursor yarn in an oxidizing atmosphere In the pre-carbonization treatment in the region of 300 to 800 ° C., an inert gas for heat treatment of the yarn is heated to 300 ° C. or higher and then supplied to the heating furnace. .