JP2010111972A - Carbon fiber and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing carbon fibers having a high crystal orientation even if having a small micro-crystal size. <P>SOLUTION: The method for producing the carbon fibers having (the degree of crystal orientation)(%)/(the micro-crystal size, Lc)(nm)≥50 and 235≤ elastic modulus (GPa)≤295 by heat-treating flame retardance-treated fibers at the final temperature of 500 to 700°C in an inert atmosphere to obtain a first carbonization-treated fibers, heat-treating the first carbonization-treated fibers until the final temperature of 750 to 900°C at 100 to 400°C/min temperature elevation rate and at 1.0 to 1.2 fold stretching magnitude to obtain a second carbonization-treated fibers, and heat-treating the second carbonization-treated fibers until the final temperature of 1,000 to 1,300°C at 100 to 400°C/min temperature elevation rate and at 0.95 to 1.1 fold stretching magnitude, wherein the stretching magnitude in a second carbonization treatment is larger than that in the third carbonization treatment. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a carbon fiber having a high degree of crystal orientation and a high single fiber compressive strength despite a low crystal growth, and a method for producing the same.

  Conventionally, a precursor fiber for producing carbon fiber is used as a raw material, and flameproofing treatment is performed on this to obtain a flameproofing fiber. Further, carbonization treatment is applied to this flameproofing fiber to obtain a high-performance carbon fiber. Widely known. This method is also practiced industrially.

  In recent years, industrial applications of composite materials using carbon fibers [for example, carbon fiber reinforced plastic (CFRP) and the like] have been greatly expanded. Recently, it has been recognized that not only the tensile strength in the fiber axis direction but also the compressive strength in the fiber cross-sectional direction is important as a mechanical property that is required to improve the carbon fiber used as a composite material. Various proposals have been made (see, for example, Patent Documents 1 to 4).

In order to obtain the necessary elastic modulus (rigidity) of the fiber, it is effective to grow a graphite crystal structure (increase the crystallite size). However, when the graphite crystal structure is grown, there is a problem that the tensile and compressive strength is lowered due to embrittlement. In particular, the compressive strength in the fiber cross-sectional direction is easily affected by crystal growth and is significantly reduced at a firing temperature of 1200 ° C. or higher. In order to suppress this decrease in tensile / compressive strength, it is one effective measure to lower the carbonization temperature so that graphite crystals do not grow. However, when the carbonization temperature is lowered, there is a problem that a necessary elastic modulus cannot be obtained because the elastic modulus is lowered.
JP 2000-96354 A (Claims) JP 2006-307407 A (Claims) JP-A-2005-344254 (Claims) JP-A-2005-179794 (Claims)

  While the inventor is studying to solve the above problem, the temperature when switching from the primary treatment condition of the second carbonization treatment described in Patent Document 4 to the secondary treatment condition is considerably higher than 950 ° C. It turns out that it is expensive. Therefore, it has been found that the graphite crystallite size of the fiber being processed is considerably increased. In addition, since the secondary treatment at a higher temperature continues after the switching of the treatment conditions, it has been found that the crystallite size is further increased, the embrittlement is advanced, and the tensile and compressive strength is lowered.

  As a result of further studies, it has been found that the strand tensile modulus of carbon fiber depends not only on the crystallite size but also on the degree of crystal orientation. It has been found that the degree of crystal orientation can be increased by increasing the draw ratio at the second carbonization temperature as low as 750 to 900 ° C. Since the second carbonization treatment temperature is low, the strand tensile elastic modulus of the second carbonization treatment fiber is low.

  Therefore, the second carbonization-treated fiber was subjected to a third carbonization treatment at a relatively low temperature of 1300 ° C. or lower with a low draw ratio. As a result, the carbon fiber obtained by the above carbonization treatment had a strand tensile elastic modulus of 235 to 295 GPa, and had an elastic modulus with no practical problem.

  Furthermore, since this carbon fiber has a high ratio of crystal orientation to crystallite size, it has been found that the compressive strength in the single fiber cross-sectional direction is high.

  The carbon fiber having a high crystal orientation degree despite the small crystallite size is obtained by heat-treating the flame-resistant fiber at a temperature of 700 ° C. or lower in an inert atmosphere to obtain the first carbonized fiber. The carbonized fiber is drawn at an end temperature of 900 ° C. or less to obtain a second carbonized fiber, and the second carbonized fiber is drawn at an end temperature of 1300 ° C. or less from the draw ratio during the second carbonization treatment. Has been found to be obtained by heat treatment at a low draw ratio to form a third carbon, and the present invention has been completed.

  Accordingly, an object of the present invention is to provide a carbon fiber and a method for producing the same, which have solved the above problems.

  The present invention for achieving the above object is described below.

[1] Degree of crystal orientation (%) / crystallite size Lc (nm) ≧ 50,
Carbon fiber of 235 ≦ elastic modulus (GPa) ≦ 295.

  [2] The carbon fiber according to claim 1, wherein the fiber cross-sectional direction compressive strength (MPa) / tensile elastic modulus (GPa) ≧ 5.5.

  [3] The carbon fiber according to [1], wherein the crystallite size Lc is 1.3 to 1.8 nm.

  [4] Heat-treating the flame-resistant fiber in an inert atmosphere at an end temperature of 500 to 700 ° C. to obtain a first carbonized fiber, and increasing the temperature of the first carbonized fiber to an end temperature of 750 to 900 ° C. A second carbonized fiber is obtained by heat treatment at 100 to 400 ° C./min and a draw ratio of 1.0 to 1.2, and the second carbonized fiber is heated to a finishing temperature of 1000 to 1300 ° C. at a rate of temperature increase of 100 to 100 ° C. In the method for producing carbon fiber which is heat treated at 400 ° C./min and a draw ratio of 0.95 to 1.1, the draw ratio at the second carbonization treatment is the draw ratio at the third carbonization treatment. It is larger than this, The manufacturing method of the carbon fiber as described in [1] characterized by the above-mentioned.

  The carbon fiber of the present invention is a carbon fiber having a high degree of crystal orientation although the crystal growth is kept low. In addition, since it is possible to achieve both high strand tensile modulus and strength in the fiber axis direction and high single fiber cross-sectional direction compressive strength, the fiber axis direction has excellent rigidity, and the fiber cross-sectional direction. High strength can be shown even against stress from. Therefore, it is useful as a carbon fiber for composite materials.

  According to the production method of the present invention, in the carbon fiber production method in which the flame resistant fiber is subjected to the first carbonization treatment, the second carbonization treatment, and then the third carbonization treatment in an inert atmosphere, the second carbonization end temperature is obtained. Is set to a low temperature of 750 to 900 ° C., the stretching ratio at the time of the second carbonization treatment is made larger than before, the end temperature of the third carbonization is set to a temperature lower than 1300 ° C. Is larger than the draw ratio at the time of the third carbonization treatment, so that growth of crystallite size can be suppressed, and carbon fibers having a high degree of crystal orientation and high compressive strength in the single fiber cross-sectional direction can be easily obtained. it can.

  Hereinafter, the present invention will be described in detail.

  The carbon fiber of the present invention has a ratio of crystal orientation to crystallite size [crystal orientation (%) / crystallite size Lc (nm)] of 50 or more, preferably 50 to 100, and strand tensile elastic modulus of 235 to It is a carbon fiber of 295 GPa. A ratio of the degree of crystal orientation to the crystallite size of less than 50 is not preferable because the compressive strength in the fiber cross-sectional direction decreases.

  When the strand tensile elastic modulus is less than 235 GPa, the strand tensile strength decreases, which is not preferable. When the strand tensile modulus exceeds 295 GPa, it is not preferable because the compressive strength in the fiber cross-sectional direction decreases.

  The ratio of the fiber cross-sectional direction compressive strength to the strand tensile elastic modulus of the carbon fiber of the present invention [fiber cross-sectional direction compressive strength (MPa) / strand tensile elastic modulus (GPa)] is preferably 5.5 or more, and 5.7 to 10 Is more preferable. When the ratio of the compressive strength to the tensile elastic modulus is less than 5.5, it is not preferable as a carbon fiber for composite materials because it is brittle.

  The crystallite size Lc of the carbon fiber of the present invention is preferably 1.3 to 1.8 nm, more preferably 1.3 to 1.6. When the crystallite size Lc is less than 1.3 nm, the strand tensile elastic modulus and the strand tensile strength decrease, which is not preferable. When the crystallite size Lc exceeds 1.8 nm, the compressive strength in the fiber cross-sectional direction decreases, which is not preferable.

  Although the manufacturing method of the carbon fiber of this invention is not specifically limited, For example, it can manufacture with the following method.

<Flame resistant fiber>
The flame-resistant fiber used in the carbon fiber production method of the present invention is obtained by subjecting a precursor fiber such as polyacrylonitrile (PAN), pitch-type, phenol-type, rayon-type, etc. to a flame-proof treatment in which it is oxidized in an oxidizing atmosphere. Conventionally known ones can be used without any limitation. Among these flame-resistant fibers, the use of PAN-based flame resistant fibers is particularly preferable because the highest strength carbon fibers can be obtained.

  When the flame resistant fiber for producing carbon fiber is a PAN flame resistant fiber, it can be obtained by the following method. The PAN-based precursor fiber to be used is obtained by spinning a spinning solution obtained by polymerizing a monomer containing acrylonitrile at 90% by mass or more, preferably 95% by mass or more in a wet or dry wet spinning method, and then washing, drying and stretching. can get. Examples of the monomer to be copolymerized include alkyl acrylate, alkyl methacrylate, acrylic acid, acrylamide, itaconic acid, maleic acid and the like.

  The obtained precursor fiber is subsequently flameproofed at 200 to 280 ° C. in heated air. The treatment at this time is generally a PAN-based flameproof fiber having a fiber specific gravity of 1.3 to 1.5, which is treated in a range of draw ratio of 0.85 to 1.30. The tension (stretch distribution) is not particularly limited.

<Carbonization treatment>
The flame-resistant fiber is heat-treated at an end temperature of 500 to 700 ° C. in an inert atmosphere to obtain a first carbonized fiber.

  The first carbonized fiber is heated to an end temperature of 750 to 900 ° C., preferably 750 to 850 ° C. in an inert atmosphere, and the heating rate is 100 to 400 ° C./min, preferably 150 to 300 ° C./min. The second carbonized fiber is obtained by heat treatment at 1.0 to 1.2 times, preferably 1.05 to 1.15 times.

  When the stretching ratio during the second carbonization treatment is less than 1.0, the stretching effect is small, and the degree of crystal orientation is not sufficiently improved. If the draw ratio during the second carbonization treatment exceeds 1.2 times, yarn breakage tends to occur, which is not preferable.

  When the second carbonization end temperature is less than 750 ° C., it is not preferable because the effect of improving the degree of crystal orientation cannot be sufficiently obtained because it is out of the firing temperature region where the fiber structure suitable for stretching can be obtained. Even when the second carbonization end temperature exceeds 900 ° C., it is not preferable because the effect of improving the degree of crystal orientation cannot be sufficiently obtained because it is out of the firing temperature region where the fiber structure suitable for stretching can be obtained.

  The second carbonized fiber is heat-treated in an inert atmosphere at a temperature rising rate of 100 to 400 ° C./min, preferably 150 to 300 ° C./min, to an end temperature of 1000 to 1300 ° C., preferably 1000 to 1200 ° C. To obtain a third carbonized fiber. In the third carbonization, the draw ratio is controlled to 0.95 to 1.1 times, and the draw ratio during the second carbonization treatment is larger than the draw ratio during the third carbonization treatment. Preferably, it is controlled to 0.95 to 0.98 times. Thereby, the carbon fiber of this invention by which the third carbonization process was carried out is obtained.

  If the draw ratio during the third carbonization treatment exceeds 1.1 times, yarn breakage tends to occur, which is not preferable. When the draw ratio at the time of the third carbonization treatment is less than 0.95 times, the effect of improving the degree of crystal orientation due to the improvement of the draw ratio at the time of the second carbonization treatment cannot be sufficiently reflected in the structure of the carbon fiber. .

  Moreover, when the draw ratio at the time of the third carbonization treatment is equal to or higher than the draw ratio at the time of the second carbonization treatment, yarn breakage occurs, and the quality is deteriorated and the process becomes unstable.

  When the third carbonization end temperature is less than 1000 ° C., the strand tensile elastic modulus and the strand tensile strength decrease, which is not preferable. When the third carbonization end temperature exceeds 1300 ° C., the crystallite size Lc is increased, and the compressive strength in the fiber cross-sectional direction is decreased, which is not preferable.

  When the rate of temperature increase is less than 100 ° C./min, the crystallite size Lc is increased, and the compressive strength in the fiber cross-section direction is decreased, which is not preferable. When the rate of temperature rise exceeds 400 ° C./min, the strand tensile modulus and the strand tensile strength decrease, which is not preferable.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Moreover, the evaluation method about the processing conditions in each Example and a comparative example, and the physical property of a flame-resistant fiber and carbon fiber was implemented with the following method.

<Specific gravity>
Measured by Archimedes method. The sample fiber was degassed in acetone and measured.

<Crystallite size, crystal orientation>
X-ray diffractometer: RINT2000 manufactured by Rigaku Corporation is used, and from the half-value width β of the diffraction peak of the plane index (002) by the transmission method, the following formula (1)
Crystallite size Lc (nm) = 0.9λ / βcosθ (1)
The crystallite size Lc was calculated using λ: X-ray wavelength, β: half width, and θ: diffraction angle. In addition, the half-value widths H _1/2 and H ′ _{1/2 of the} two peaks obtained by scanning the diffraction peak angle in the circumferential direction (from the intensity distribution)
Degree of crystal orientation (%) = 100 × [360− (H _1/2 −H ′ _1/2 )] / 360 (2)
H _1/2 and H ′ _1/2 : The degree of crystal orientation was calculated using the half width.

<Single fiber cross-sectional direction compressive strength>
The single fiber cross-sectional direction compressive strength means the compressive strength when stress is applied in a direction perpendicular to the axial direction of the single fiber. In the measurement, a sample in which a single fiber of carbon fiber was fixed on a slide glass was prepared. Using a micro compression tester MCTM-200 manufactured by Shimadzu Corporation, a flat 50 μm indenter of the compression tester was pressed against the surface of the single fiber of the sample at a load speed of 0.071 mN / sec (7.25 mgf / sec), and the surface of the single fiber The load at the time of breaking was measured (measured at n = 10), and the single fiber cross-sectional direction compressive strength was determined from this load.

<Strand strength, elastic modulus>
The strand strength and elastic modulus of the carbon fiber were measured by the method specified in JIS R 7608.

Example 1
A precursor fiber having a fiber diameter of 10.4 μm is prepared by wet or dry-wet spinning of a copolymer spinning solution of 95% by mass of acrylonitrile / 4% by mass of methyl acrylate / 1% by mass of itaconic acid, followed by washing with water, drying, drawing and oiling. Got. This fiber is flameproofed in heated air in a hot air circulation type flameproofing furnace having an inlet temperature (minimum temperature) of 200 ° C and an outlet temperature (maximum temperature) of 260 ° C to obtain a PAN-based flameproofing fiber having a fiber specific gravity of 1.35. It was.

  Next, the flame-resistant fiber was heat-treated in an inert atmosphere in a first carbonization furnace having an inlet temperature of 300 ° C. and an outlet temperature (first carbonization end temperature) of 600 ° C. to obtain a first carbonized fiber. The first carbonization-treated fiber is subjected to a second carbonization treatment draw ratio (second carbonization draw ratio) and a second carbonization end temperature condition shown in Table 1 in an inert atmosphere in a second carbonization furnace. To obtain a second carbonized fiber. The second carbonization-treated fiber is subjected to a third carbonization treatment draw ratio (third carbonization draw ratio) and a third carbonization end temperature condition shown in Table 1 in an inert atmosphere in a second carbonization furnace. And heat treated. The temperature rising rate during the second carbonization treatment was 200 ° C./min. The heating rate during the third carbonization treatment was 200 ° C./min. As a result, carbon fibers having a fiber diameter of 6.5 μm and various physical properties shown in Table 1 were obtained.

Example 2
The first carbonized fiber obtained in Example 1 was measured under the conditions of the second carbonization stretch ratio, the second carbonization end temperature, the third carbonization stretch ratio, and the third carbonization end temperature shown in Table 1. The carbon fibers having various physical properties shown in Table 1 were obtained in the same manner as in Example 1 except that the two carbonization treatment and the third carbonization treatment were performed.

Comparative Examples 1-3
The carbonization furnace for high-magnification drawing as in the second carbonization furnace in Examples 1 and 2 was not provided, and was obtained in Example 1 in an inert atmosphere only in the low-magnification second carbonization furnace. The first carbonized fiber was treated in the same manner as in Example 1 except that it was heat-treated under the conditions of the second carbonization stretch ratio and the second carbonization end temperature shown in Table 1, and various physical properties shown in Table 1 were obtained. Carbon fiber was obtained.

Comparative Example 4
Carbonization treatment was performed in the same manner as in Example 1 except that the third carbonization end temperature was 1900 ° C., and carbon fibers having physical properties shown in Table 1 were obtained.

Comparative Example 5
Carbonization treatment was performed in the same manner as in Example 1 except that the third carbonization stretch ratio was 1.20, which was larger than the second carbonization stretch ratio. However, the process became unstable due to yarn breakage, and the desired carbon fiber could not be obtained.

Claims (4)

Carbon fiber of crystal orientation degree (%) / crystallite size Lc (nm) ≧ 50 and 235 ≦ elastic modulus (GPa) ≦ 295. The carbon fiber according to claim 1, wherein the fiber cross-sectional direction compressive strength (MPa) / tensile modulus (GPa) ≧ 5.5. The carbon fiber according to claim 1, wherein the crystallite size Lc is 1.3 to 1.8 nm. The flame-resistant fiber is heat-treated at an end temperature of 500 to 700 ° C. in an inert atmosphere to obtain a first carbonized fiber, and the temperature increase rate of the first carbonized fiber is 100 to 400 up to an end temperature of 750 to 900 ° C. A second carbonized fiber is obtained by heat treatment at a rate of 1.0 to 1.2 times at a rate of ° C / minute, and the temperature increase rate is 100 to 400 ° C / second until the second carbonized fiber is finished at 1000 to 1300 ° C. In the method for producing carbon fiber to be third carbonized by heat treatment at a draw ratio of 0.95 to 1.1 times, the draw ratio during the second carbonization treatment is larger than the draw ratio during the third carbonization treatment The method for producing a carbon fiber according to claim 1.