JP2006233360A - Carbon fiber and method for producing the carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a carbon fiber having especially a high carbonization temperature and having a high strength even while having high elasticity, as stress remaining in the carbon fiber during carbonization is relaxed. <P>SOLUTION: The method for producing the carbon fiber is to produce the carbon fiber by heating the carbon fiber at ≥1400°C, subsequently cool the carbon fiber in an average descending speed of temperature of 30°C/sec to the room temperature. The carbon fiber produced by the method has ≥750 kg/mm<SP>2</SP>tensile strength of single fiber having 5 mm test length and ≥600 kg/mm<SP>2</SP>tensile strength of single fiber having 25 mm test length. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention particularly relates to a carbon fiber having a high elastic modulus and excellent strength development and a method for producing the same.

The following technologies 1) to 5) related to the enhancement of carbon fiber strength are known.
1) Technology to enhance filtration of monomer and polymer stock solutions (Patent Documents 1 and 2)
2) Technology for optimizing coagulation bath conditions and densifying precursor fibers (Patent Document 3)
3) Technology to reduce the single fiber diameter of the precursor fiber and mix flame retarding elements (Patent Document 4)
4) Technology for applying an oil agent excellent in releasability and smoothness to suppress adhesion between single fibers of precursor fibers (Patent Documents 5 and 6)
5) Technology for removing etching by electrolytic treatment of surface defects generated in the firing process (Patent Document 7)

  However, in any technique, the carbon fibers exiting the carbonization furnace are rapidly cooled at an average of 60 ° C./second or more, and at a maximum of 200 ° C./second, and the fiber axis directions having different coefficients of thermal expansion and the respective directions perpendicular thereto. In accordance with the thermal expansion coefficient, rapid volume shrinkage occurred, and the stress generated there remained in the carbon fiber without relaxation.

JP 59-8924 A JP-A-4-12882 Japanese Patent Publication No. 6-15722 Japanese Patent Laid-Open No. 11-241230 Japanese Patent Publication No. 60-18334 Japanese Patent Publication No. 60-99011 Japanese Patent Publication No. 5-4463

  The subject of this invention is providing the carbon fiber which expresses a high intensity | strength physical property, having a high elasticity modulus.

The first gist of the present invention is a carbon fiber having a single fiber tensile strength of 750 kg / mm ² or more at a test length of 5 mm and a single fiber tensile strength of 600 kg / mm ² or more at a test length of 25 mm. The gist lies in a method for producing carbon fiber in which the carbon fiber is heated to 1400 ° C. or higher and then cooled to room temperature at an average temperature drop rate of 30 ° C./second.

  According to the present invention, since the stress remaining on the carbon fiber during carbonization can be relaxed, it is possible to obtain a high-strength carbon fiber even when the carbonization temperature is high and the elastic modulus is high.

First, the manufacturing method of the carbon fiber of this invention is demonstrated in detail.
"Carbon fiber"
The carbon fibers used in the present invention are not limited to carbon fibers having acrylic fibers as precursor fibers, but may be pitch-based carbon fibers.
The preferable manufacturing method of the carbon fiber used by this invention is obtained using the following raw materials and methods.

"Acrylonitrile polymer"
As the acrylonitrile-based polymer constituting the acrylic fiber serving as the precursor fiber, an acrylonitrile homopolymer or a copolymer with a monomer copolymerizable with acrylonitrile can be used. In the latter case, the proportion of acrylonitrile units in the copolymer is preferably 90 mol% or more for the purpose of good carbonization.

  The monomer copolymerizable with acrylonitrile is not particularly limited. For example, acrylic acid esters represented by methyl acrylate and ethyl acrylate, and methacrylic acid esters represented by methyl methacrylate and ethyl methacrylate. , Unsaturated monomers represented by acrylic acid, methacrylic acid, maleic acid, itaconic acid, acrylamide, styrene, vinyltoluene, methallylsulfonic acid, allylsulfonic acid, styrenesulfonic acid, and alkali metal salts thereof. It is done.

  As a monomer copolymerizable with acrylonitrile, a monomer having a carboxylic acid group or an acrylamide monomer is preferably used for the purpose of promoting the cyclization reaction in the carbonization step. As the monomer having such a carboxylic acid group, methacrylic acid and itaconic acid are preferable. As the acrylamide monomer, acrylamide is preferable.

"Polymerization method"
Any known polymerization method such as solution polymerization or suspension polymerization can be employed as the polymerization method for the acrylonitrile-based polymer. It is preferable to perform a treatment for removing unreacted monomers, polymerization catalyst residues, and other impurities as much as possible from the acrylonitrile-based polymer.

  Further, from the standpoints of stretchability in precursor fiber spinning and carbon fiber performance, etc., the degree of polymerization of the copolymer is preferably 1 or more, and particularly preferably 1.4 or more. However, the intrinsic viscosity [η] is usually in a range not exceeding 2.

"Spinning method"
The acrylonitrile-based polymer is dissolved in a solvent to become a spinning dope. As the solvent, organic solvents such as dimethylacetamide, dimethylsulfoxide, dimethylformamide, and aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate can be used. An organic solvent is preferable in that no metal is contained in the acrylic fiber and the process is simplified. Among them, it is more preferable to use dimethylacetamide as a solvent in terms of high density of the coagulated yarn.

  In order to obtain a dense coagulated yarn when spinning, the polymer concentration in the spinning dope is preferably 17% by mass or more, more preferably 19% by mass or more. Although it depends on the molecular weight of the acrylonitrile-based polymer, the polymer concentration is preferably within a range not exceeding 25% by mass in order to have appropriate viscosity and fluidity.

  The spinning method of the acrylic fiber is preferably a wet spinning method or a dry wet spinning method, but may be a dry spinning method. The wet spinning method and the dry and wet spinning method are properly used depending on the application. Usually, it is preferable to select wet-spinning when wet spinning is desired when higher productivity is desired, and when dry-wet spinning is desired when obtaining carbon fibers having higher strand strength.

  The spinning dope is discharged into the coagulation bath from the nozzle hole and becomes a coagulated yarn. It is necessary to set the coagulation bath to a condition with sufficient margin for taking up the coagulated yarn. Then, the concentration and temperature of the solvent contained in the coagulation bath are set so that the cross-sectional shape of the coagulated yarn is circular.

  For the coagulation bath, an aqueous solution of the solvent used in the spinning dope is preferably used. The concentration of the solvent contained is adjusted so that the spinning dope discharged from the nozzle hole becomes a coagulated yarn having a desired fiber diameter. Although depending on the type of solvent used, for example, when dimethylacetamide is used, the concentration is selected to be 50 to 80% by mass.

  The temperature of the coagulation bath is preferably lower from the viewpoint of the density of the coagulated yarn. However, in the case of wet spinning, considering that the temperature of the coagulation bath is lowered too much, the take-up speed of the coagulated yarn is lowered and the overall productivity is lowered, and is usually 50 ° C. or lower, more preferably 20 to 40 ° C. Select the following range.

  In general, it is known that when the wet heat draw ratio is increased, the internal structure of the fiber is broken, and this breakage becomes a source of defects in the carbon fiber, resulting in a decrease in performance. This can be avoided by lowering the wet heat draw ratio, but if the productivity is not changed, it is necessary to increase the draw ratio after dry densification compared to normal, so the spinning process passability becomes worse, On the contrary, productivity may decrease. For these reasons, the wet heat draw ratio is preferably 4 times or less, and the draw ratio after dry densification is preferably 5 times or less.

Furthermore, since the upper limit of the wet heat draw ratio at which fiber breakage occurs differs for each product type, it is necessary to consider the wet heat draw ratio for each production condition.
Prior to wet heat stretching, the solvent may be washed in warm water. Naturally, the draw ratio in such wet heat drawing is not less than 1.

  Furthermore, it is effective that the drawing bath temperature used for the wet heat drawing is as high as possible within a range where the single yarns are not fused. From this viewpoint, it is preferable that the temperature of the stretching bath is a high temperature of 70 ° C. or higher. In the case of multistage stretching, the final bath is preferably heated to a high temperature of 90 ° C or higher.

  After wet heat drawing and washing, the fiber surface is treated with an oil agent by a known method. Although the kind of oil agent is not specifically limited, Aminosilicon type surfactant is used suitably. After this oil agent treatment, dry densification is performed. The drying densification temperature is selected to be above the glass transition temperature of the fiber. The glass transition temperature may differ depending on the state of the fiber itself changing from a water-containing state to a dry state, and a method using a heating roller having a temperature of about 100 to 200 ° C. is preferred.

  For stretching after dry densification, there are various methods such as dry heat stretching using a high-temperature heating roller, a hot platen, or steam stretching using pressurized steam, but in the production method of the present invention, water is plasticized. Steam stretching is preferable because the molecular chain can be made more movable by utilizing the effect. Also, if the steam pressure is too low, the plasticizing effect of water cannot be fully utilized, and if it is too high, the properties of the steam will become unstable, so that it is 170 to 250 kPa for smooth stretching. Desirably, it is more preferably 190 to 220 kPa. Furthermore, the temperature of the tow immediately before entering the steam drawing machine is preferably 80 to 120 ° C, and more preferably 90 to 100 ° C. By setting the temperature to 80 ° C. or higher, it is possible to prevent the spinning from becoming unstable due to the drawing before the temperature of the fiber is increased in the steam drawing machine. On the other hand, when the temperature is set to 120 ° C. or lower, the diffusion of moisture for plasticizing the fibers in the steam drawing machine is good.

  If the total draw ratio including the pre-draw ratio and the post-draw ratio is too low, the fiber orientation is insufficient and the performance of the carbon fiber is lowered, and if it is too high, thread breakage occurs, which is not preferable for production. In this respect, the total draw ratio is preferably 7 to 20 times, and more preferably 10 to 15 times.

"Fireproofing"
As the conditions for making the acrylic fiber flame resistant, the density of the flame resistant fiber is preferably 1.25 g / cm ³ or more, more preferably 1.32 g / cm in an oxidizing atmosphere at 200 to 300 ° C. under tension or stretching conditions. ^It is good to heat until it becomes ³ or more. If the flame resistance is insufficient, adhesion between single yarns is likely to occur during pre-carbonization. As the atmosphere, a known oxidizing atmosphere such as air, oxygen, and nitrogen dioxide can be adopted, but air is preferable from the viewpoint of economy.

"Pre-carbonization"
The flame resistant fiber is preferably pre-carbonized prior to carbonization. The condition is that the carbon dioxide is tensed in an inert atmosphere having a maximum temperature of 550 to 800 ° C., and is pre-carbonized at a heating rate of 500 ° C./min or less, preferably 300 ° C./min or less in a temperature range of 300 to 500 ° C. It is effective to improve the mechanical properties of the carbon fiber. As the atmosphere, a known inert atmosphere such as nitrogen, argon or helium can be adopted, but nitrogen is desirable from the viewpoint of economy.

"Carbonization"
As carbonization conditions, it is carbon to perform carbonization treatment at a temperature rising rate of 500 ° C./min or less, preferably 300 ° C./min or less in a temperature range of 1000 to 1200 ° C. in an inert atmosphere of 1200 to 1800 ° C. It is effective for improving the mechanical properties of the fiber. As the atmosphere, a known inert atmosphere such as nitrogen, argon or helium can be adopted, but nitrogen is desirable from the viewpoint of economy.

"Stress relaxation"
The stress remaining in the carbon fiber obtained as described above is relaxed by heating the carbon fiber to 1400 ° C. or higher and then cooling it to room temperature at an average temperature decrease rate of 30 ° C./second.

Tension applied to the carbon fiber during stress relief is preferably 5kg at heavy / mm ² or less, and more preferably less tension 1kg weight / mm ^2. By setting the tension to 5 kgf / mm ² or less, the stress concentration of the carbon fiber can be successfully eliminated.

  The treatment temperature needs to be 1400 ° C. or higher, more preferably 1500 ° C. or higher, and still more preferably 1600 ° C. or higher. The time for maintaining the temperature is preferably 10 minutes or more, more preferably 20 minutes or more, and further preferably 30 minutes or more. In the case of (B) described later, there is no particular problem with increasing the processing time, but in the case of (A), there is a limit to the reduction in the rotational speed of the roller for transferring the carbon fibers.

  The carbon fiber may be slowly cooled by heating in an inert atmosphere and an average cooling rate of 30 ° C./second to room temperature. (A) Even if a carbonization furnace for producing carbon fiber is used (B ) Once carbon fiber is obtained, the carbon fiber may be placed in a batch furnace. As the inert atmosphere, a known inert atmosphere such as nitrogen, argon, or helium can be employed. Nitrogen is desirable from the economical aspect.

“(A) Method of using a carbonization furnace when producing carbon fiber”
The temperature in the carbonization furnace is set to 1400 ° C. or more, preferably 1500 ° C. or more, and the heat treatment time is 10 minutes or more by adjusting the rotation speed of the roller for transferring the fibers in the carbonization furnace, and the tension is 5 kgf / mm ² or less, and the average cooling rate is 30 ° C./second or less. The average cooling rate is preferably 10 ° C./second or less, more preferably 3 ° C./second or less. By setting the average to 30 ° C./second or less, the stress concentration is eliminated and no new stress concentration occurs. Decreasing the average temperature lowering rate does not cause a new adverse effect on the physical properties of the carbon fiber, but the productivity of the carbon fiber is significantly reduced.

  If the average temperature drop rate does not fall below 30 ° C / s simply by slowing down the roller rotation speed as described above, a new heat retention furnace is provided on the exit side of the carbonization furnace, and the average temperature drop is increased by extending the furnace length. The speed is set to 30 ° C./second or less. There is no particular restriction on the material of the heat insulating material of this heat insulating furnace.

“(B) Once carbon fiber is obtained, use a batch furnace for carbon fiber”
The internal temperature of the batch type furnace containing the carbon fiber is heated to 1400 ° C. or higher, and the carbon fiber is placed in the furnace when the temperature reaches a predetermined temperature. At this time, it is preferable that the carbon fiber has a tension of 5 kgf / mm ² or less. Thereafter, the temperature of the furnace is cooled to an average temperature drop rate of 30 ° C./second or less. More preferably, the carbon fibers are heat-treated side by side in a graphite container. If the tension of the carbon fiber is 5 kgf / mm ² or less, the carbon fiber may be wound around something and subjected to heat treatment.

"Processing after stress relaxation"
The stress-relaxed carbon fiber is subjected to electrolytic oxidation treatment in a conventionally known electrolytic solution, or is subjected to oxidation treatment in a gas phase or a liquid phase, whereby the affinity between the carbon fiber and the matrix resin in the composite material It is preferable to improve the adhesion.

  Furthermore, a sizing agent can be applied by a conventionally known method as necessary.

"Carbon fiber"
As described above, the carbon fiber obtained by the carbon fiber production method described above has a single fiber tensile strength average of 750 kg / mm ² or more at a test length of 5 mm and a single fiber tensile strength average of 600 kg / mm ² or more at a test length of 25 mm. It becomes. Here, the single fiber tensile test is a value measured according to a test method described in JIS R 7606 (an average of 100 tests).

Conventionally, in order to obtain a carbon fiber having high strength and high elastic modulus, it was considered that a single fiber diameter of 4 μm or less was necessary. However, in the present invention, even if the average single fiber diameter is 4.8 μm or more, It is also possible to obtain carbon fibers having a resin-impregnated strand tensile strength of 600 kg / mm ² or more and a tensile elastic modulus of 32 ton / mm ² or more. The smaller the single fiber diameter, the worse the stretchability and processability in the spinning process, so it is advantageous to realize a carbon fiber with a large diameter and high strength and high elastic modulus. is there.

Hereinafter, the present invention will be described more specifically with reference to examples.
Evaluation of physical properties and structural parameters of carbon fibers used in describing the present invention, specifically, “resin impregnated strand characteristics”, “single fiber tensile test”, “crystal size”, and “degree of orientation” The method will be described in advance. In the examples, unless otherwise specified, various physical property values and indices described represent values measured and evaluated by the methods described herein. Usually, a plurality of samples are evaluated and the average value is adopted.

(B) “Resin-impregnated strand characteristics”
The strand strength and strand elastic modulus of the carbon fiber were measured according to the test method described in JIS R7601. The measurement was performed at n = 10.

(B) "Single fiber tensile test"
The single fiber tensile strength and tensile modulus of carbon fiber were measured according to the test method described in JIS R 7606. The measurement was performed at n = 100.

(C) “Crystal size”
The carbon fiber was cut into a length of 50 mm, and 30 mg of this was precisely collected and aligned so that the sample fiber axes were exactly parallel, and then a thickness of 1 mm was uniform using a sample adjusting jig. A fiber sample bundle was arranged. The fiber sample bundle was impregnated with a vinyl acetate / methanol solution and fixed so as not to collapse, and then fixed to a wide-angle X-ray diffraction sample stage. As an X-ray source, a CuKα ray (using Ni filter) X-ray generator manufactured by Rigaku Corporation is used, and the vicinity of 2θ = 25 ° corresponding to the surface index (002) of graphite by a transmission method using a Goniometer manufactured by Rigaku Corporation. Were detected by a scintillation counter. The output was measured at 40 kV-100 mA. The crystal region size Lc was determined from the half width at the diffraction peak using the following formula (1).
Lc = Kλ / (β ₀ cos θ) (1)
(Where K is the Scherrer constant of 0.9, λ is the wavelength of the X-ray used (here, the CuKα ray is used, 1.5418 mm), θ is the Bragg diffraction angle, and β ₀ is the true half-width. , Β ₀ = β _E −β ₁ (β _E is the apparent half-value width, β ₁ is an apparatus constant, here 1.05 × 10 ⁻² rad).
The measurement was performed at n = 5.

(D) “Orientation degree”
Hundreds of carbon fibers are hardened with a vinyl acetate / methanol solution, a sample with a width of about 0.5 mm is prepared, the sample is rotated 360 ° on a plane perpendicular to the X-ray, and the maximum intensity in (002) reflection The degree of orientation was calculated using the formula (2) from the half width of the profile in the meridian direction including. As the X-ray source, a CuKα ray (using Ni filter) X-ray generator manufactured by Rigaku Corporation was used, and the output was 40 kV-100 mA.
Orientation degree: π = (180−B) / 180 × 100 (2)
B: Half width The measurement was performed at n = 5.
Further, wide-angle X-ray diffraction measurement was performed using Ru-200B (manufactured by Rigaku, a rotating counter-cathode X-ray generator).
Example 1

  An acrylonitrile-based polymer composed of 96% by mass of acrylonitrile units, 1% by mass of methacrylic acid units and 3% by mass of acrylamide units was dissolved in dimethylacetamide to prepare a spinning stock solution (polymer concentration 21% by mass, stock solution temperature 60 ° C.). This spinning dope was discharged into a dimethylacetamide aqueous solution having a temperature of 35 ° C. and a concentration of 66% using a die having a diameter of 0.065 mm and a pore number of 12,000 to obtain a coagulated yarn. The coagulated yarn was subjected to cold drawing and hot water drawing, and then immersed in a 1% aqueous solution of an aminosilicon oil and dried and densified with a 180 ° C. heating roller. The wet heat draw ratio so far is 3.9 times. Then, it extended | stretched so that a draw ratio might become 12.5 times in total in pressurized steam, and the acrylic fiber was obtained.

The obtained acrylic fiber is heated to tension in air at 230 to 260 ° C. to convert to a flame-resistant fiber having a density of 1.36 g / cm ³ , and further pre-carbonized by tension in nitrogen at 700 ° C. Carbonized fiber before application.

The obtained pre-carbonized fiber was fired at 1350 ° C. to obtain a carbon fiber before stress relaxation. The carbon fiber before stress relaxation was passed through a stress relaxation furnace (1600 ° C. under a nitrogen stream) at a tension of 1 kgf / mm ² and a transfer rate of 2.5 m / hour. As a result, the temperature was maintained at 1600 ° C. for 20 minutes or more in nitrogen, and the average temperature decreasing rate was about 3 ° C./second and the temperature was gradually cooled to room temperature. The evaluation results are shown in Table 1.
(Example 2)

A carbon fiber was obtained in the same manner as in the example except that the transfer rate of the carbon fiber in the stress relaxation furnace was 5.4 m / hour. As a result, the temperature was maintained at 1600 ° C. for 10 minutes or more in nitrogen, and the temperature was gradually cooled to room temperature at a rate of about 7 ° C./second. The evaluation results are shown in Table 1.
(Example 3)

The pre-stress relaxation carbon fiber obtained in Example 1 was placed in a batch furnace, held at 1600 ° C. in nitrogen for 1 hour, and then gradually cooled to room temperature over about 1 hour. As a result, the temperature lowering rate is about 0.5 ° C./second on average and the temperature is gradually cooled to room temperature. The evaluation results are shown in Table 1.
(Comparative Example 1)

The carbon fiber before stress relaxation obtained in Example 1 was evaluated. The evaluation results are shown in Table 1.
(Comparative Example 2)

  The precarbonized fiber obtained in Example 1 was fired at 1600 ° C. to obtain a carbon fiber before stress relaxation. The evaluation results are shown in Table 1.

Claims (2)

Carbon fiber having a single fiber tensile strength of 750 kg / mm ² or more at a test length of 5 mm and a single fiber tensile strength of 600 kg / mm ² or more at a test length of 25 mm.   A method for producing carbon fiber, in which the carbon fiber is heated to 1400 ° C. or higher and then cooled to room temperature at an average temperature decrease rate of 30 ° C./second.