JPH04316614A - Production of carbon fiber - Google Patents

Abstract

PURPOSE:To produce the subject pitch-based carbon fiber excellent in tensile strength and breaking elongation with industrial advantage. CONSTITUTION:In production of a carbon fiber by using, as the raw material, a mesophase pitch obtained by polymerization of a condensed polycyclic hydrocarbon or a material containing it in the presence of hydrogen fluoride and boron trifluoride, melt spinning it and then carrying out infusibilization and carbonization, the melt spinning is carried out in a draft ratio (Ve/Vo) within a specified range (shadowed portion in fig. 1) and the carbonization is carried out at a temperature within a range of 800-1400 deg.C.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates to a method for producing high performance carbon fibers. More specifically, the present invention relates to a method for producing pitch-based carbon fibers having excellent tensile strength and elongation at break.

0002

[Prior Art] High-performance carbon fibers are generally manufactured industrially using P.
Manufactured using AN (polyacrylonitrile) as a raw material. However, PAN is expensive and its carbonization yield is low. In recent years, it has been discovered that even when pitch is used as a raw material, it is possible to produce high-performance carbon fibers with properties equal to or better than those of PAN-based fibers, and this has attracted attention.

[0003] As a method for preparing spinning pitch, which is a raw material for the pitch-based high-performance carbon fiber, there is a method disclosed in Japanese Patent Publication No. 59-3019
No. 2, JP-A-54-160427, JP-A-57-11
Methods such as those described in No. 9984 and Japanese Patent Application Laid-open No. 58-18421 are known, but since each method involves a complicated process, the pitch obtained is not only extremely expensive but also of unstable quality. There is a problem.

[0004] A method of preparing carbon fiber raw material pitch by polymerizing naphthalene in the presence of aluminum chloride is also known (JP-A-61-83317, JP-A-61-83318, JP-A-61-61). No. 83319). However, these methods using aluminum chloride catalysts require a special operation to remove the aluminum chloride catalyst from the pitch obtained by polymerization, and a trace amount of aluminum chloride or its derivatives remains in the carbon fiber. However, there is a problem in that physical properties such as strength of carbon fibers are significantly deteriorated.

In order to solve these problems, naphthalene, anthracene, phenanthrene, acenaphthene,
When fused polycyclic hydrocarbons such as acenaphthylene and pyrene or substances containing them are polymerized in the presence of hydrogen fluoride and boron trifluoride, ■ high mesophase content, ■ high thermal stability during spinning, and ■ It has been reported that mesophase pitch, which is suitable as a raw material for high-performance carbon fiber and is characterized by high infusibility and high carbonization yield, can be stably prepared at low cost (Japanese Patent Laid-Open No. 146920/1983). ,
JP-A-1-139621, JP-A-1-254796
issue).

[0006] The pitch obtained by any of the above methods is a pitch called mesophase pitch, latent mesophase pitch, pre-mesophase pitch, etc., and aromatic planar molecules with developed pitch are formed during melt spinning. Arrange in a direction parallel to the fiber axis. This oriented structure is maintained even during the subsequent infusibility treatment in which the surface is oxidized by gradually increasing the temperature under an oxidizing atmosphere, and is further maintained at the maximum treatment temperature during the carbonization treatment in which the surface is heated in an inert atmosphere. It is thought that carbon fibers with a high degree of orientation, high tensile strength, and high tensile modulus can be obtained because the carbon fibers develop according to the However, if the orientation of pitch molecules in the fiber axis direction becomes excessive, cracks will occur along the fiber axis during the carbon fiber manufacturing stage, and even if no cracks occur, microvoids that exist at the grain boundaries within the fiber will occur. Due to their large size, carbon fibers become extremely brittle, making it difficult to obtain high tensile strength and high tensile modulus that exceed those of PAN-based carbon fibers.

In response, attempts have been made to improve the physical properties of carbon fibers by first controlling the orientation structure of pitch molecules within the cross section of the fibers. That is, based on the idea that the above-mentioned occurrence of cracks and embrittlement of fibers are caused by the macrostructure of the fiber cross section being a so-called radial structure, various proposals have been made to avoid the radial structure. For example, JP-A No. 59-76925, JP-A No. 5
9-168124, JP-A No. 61-167019, etc., the spinning conditions, i.e., the viscosity of the pitch in the nozzle mouth hole, the shear stress or shear rate that the pitch receives, etc., are optimized, and JP-A-59-168127 No., Japanese Patent Application Publication No. 1986-
By specifying the shape of the spinning nozzle, as in No. 104528, attempts are made to obtain carbon fibers with a random structure or an onion structure. Also, JP-A-61-12919, JP-A-61-201021, JP-A-61-258024
JP-A-61-167022 and JP-A-62-177 disclose a method in which a filler layer is provided immediately before the spinning nozzle.
No. 222, JP-A-63-75119, JP-A-63-3
No. 03119, JP-A No. 2-6628, etc., carbon fibers having a random structure or the like are obtained by installing a static or dynamic stirring device on a spinning nozzle. However, carbon fibers with a non-radial structure obtained by these methods do not necessarily have higher tensile strength than those with a radial structure, and the pitch viscosity during spinning is extremely low, resulting in unstable spinning and fiber breakage. This method has problems in that it is disadvantageous in industrial production because it is easy to cause problems and the structure of the nozzle is complicated, making it difficult to manufacture and maintain the nozzle.

Attempts have also been made to increase the strength of carbon fibers by controlling the degree of orientation of pitch molecules in the direction parallel to the fiber axis to prevent cracking and embrittlement of the fibers. For example, JP-A-59-53717, JP-A-1-28231
In No. 7, etc., the orientation structure is controlled by optimizing the draft ratio and the viscosity of the pitch in the nozzle orifice, and in JP-A-62-104926, the orientation structure is controlled by optimizing the draft ratio and the viscosity of the pitch in the nozzle orifice. The alignment structure is controlled by optimizing the viscosity of the pitch. Also, JP-A-60-27
No. 12 controls the orientation structure by optimizing the viscosity of the pitch after it leaves the nozzle mouthpiece. However, none of these methods are fully satisfactory; although it may be possible to suppress crack generation by structural control, the tensile strength and tensile modulus of the resulting carbon fibers are not necessarily satisfactory. The problem is that it does not show a high value.

[0009]

Although various improvements have been made to pitch-based carbon fibers as described above, their performance is still not sufficient. Moreover, no matter which of the above-mentioned known methods is used, high strength is only achieved when the graphitization treatment is performed at a high temperature of about 1500° C. or higher. This not only results in high costs, but also the resulting graphitized fibers have a high tensile modulus but do not have sufficient elongation at break. Fibers cannot be manufactured cheaply. The purpose of the present invention is to solve the above-mentioned problems of the conventional pitch-based carbon fiber, and to
The object of the present invention is to provide a method for manufacturing carbon fiber at low cost, which has properties such as high tensile strength and high elongation at break comparable to general-purpose products.

0010

[Means for Solving the Problems] As a result of intensive studies to achieve the above object, the present inventors have found that a method for producing carbon fibers by melt spinning pitch, infusibility treatment, and carbonization treatment. We have demonstrated that carbon fibers with excellent tensile strength and elongation at break can be manufactured at low cost by using a specific mesophase pitch obtained from condensed polycyclic hydrocarbons as the raw material pitch and by performing melt spinning and carbonization under specific conditions. This heading led to the completion of the present invention.

That is, the present invention uses mesophase pitch obtained by polymerizing a condensed polycyclic hydrocarbon or a substance containing the same in the presence of hydrogen fluoride or boron trifluoride as a raw material, melt-spun it, and then produces a nonwoven fabric. In the method of manufacturing carbon fibers by melting treatment and carbonization treatment, melt spinning is carried out at a draft ratio (Ve/
This is a method for producing carbon fibers, characterized in that the carbonization treatment is carried out at a temperature range of 800 to 1400°C.

28(τD/L)−0.65≦Ve/Vo≦
115 (τD/L) - 0.98 (1) However, τ: Shear stress that the molten pitch receives from the wall of the spinning nozzle capillary (kgf/cm2) D: Diameter of the spinning nozzle capillary (mm) L: Length of the spinning nozzle capillary Size (mm) Ve: Winding speed (
m/min) Vo: Average velocity of the molten pitch in the capillary (
m/min). Hereinafter, the content of the present invention will be explained in detail.

The mesophase pitch used as a raw material in the method of the present invention is a pitch obtained by polymerizing a condensed polycyclic hydrocarbon or a substance containing the same. In other words, this mesophase pitch is disclosed in Japanese Patent Application Laid-open No. 146920/1983.
JP-A-1-139621, JP-A-1-25479
As shown in No. 6, pitch is synthesized from naphthalene, anthracene, phenanthrene, acenaphthene, acenaphthylene, pyrene, etc., condensed polycyclic hydrocarbons having these skeletons, mixtures thereof, or substances containing these.

The mesophase pitch is a pitch obtained by polymerizing a condensed polycyclic hydrocarbon or a substance containing it in the presence of hydrogen fluoride or boron trifluoride. In this polymerization, 0.1 to 20 mol of hydrogen fluoride and 0.0 mol of boron trifluoride are used as polymerization catalysts for 1 mol of pitch raw material.
Use 5-1.0 mol, 180-400°C, 5-10
The reaction is carried out under conditions of 0 atmospheric pressure for 5 to 300 minutes.

According to this synthesis method, by appropriately combining polymerization conditions, a pitch having an arbitrary optically anisotropic phase content of 0 to 100% and an arbitrary softening point of 180 to 400°C can be produced. However, it is preferable that the pitch used in the present invention has an optically anisotropic phase content of 90% or more, particularly substantially 100%, and a softening point of 205%.
It is preferable that it is -255 degreeC. If the content of the optically anisotropic phase is low, the anisotropic phase and isotropic phase will separate in the molten state, which will not only hinder the spinning operation but also cause the resulting carbon fiber to have low tensile strength, low elastic modulus, etc. Become. In addition, if the softening point is low, the low molecular weight components in the pitch will volatilize during spinning, or if the softening point is high, spinning will be required at high temperatures, making pitch thermal decomposition and thermal condensation reactions more likely to occur, resulting in stable spinning. It is difficult to continue for a long time.

[0016] In this specification, "optically anisotropic phase"
When a cross section of a pitch lump solidified near room temperature is polished and observed under crossed nicols using a reflection optical microscope, brightness is observed when the sample or crossed nicols are rotated, that is, optical anisotropy. "Optically anisotropic phase content" means the area fraction of this optically anisotropic phase when observed with a microscope. "Mesophase pitch" refers to a pitch containing such an optically anisotropic phase. Moreover, "softening point of pitch" refers to the solid-liquid transition temperature of pitch measured by a Koka type flow tester.

The mesophase pitch thus obtained has an atomic ratio of hydrogen to carbon of 0.5 to 1.0. Since the atomic ratio of hydrogen to carbon is thus high, the reactivity with oxygen during infusibility is high, and the infusibility treatment can be completed in a short time. Therefore, by using this pitch, it is possible to significantly shorten the infusibility process, which has been considered to be a factor in increasing costs in the carbon fiber manufacturing process.

The inventors investigated a method for manufacturing carbon fiber by melt spinning using mesophase pitch having such characteristics.
) Draft ratio (capillary internal speed Vo
After spinning at a ratio of winding speed Ve to
By carbonizing at 0 to 1400°C, it is possible to obtain carbon fibers that far exceed conventional pitch-based carbon fibers and exhibit high tensile strength and high elongation at break that are comparable to PAN-based high-strength carbon fibers. I found it.

The draft ratio according to equation (1) is expressed by τD on the horizontal axis.
By taking the logarithm of /L and the logarithm of the draft ratio on the vertical axis, it is shown in the range sandwiched between the two straight lines shown in FIG. If the draft ratio is smaller than this range, although the tensile modulus is high, the elongation at break becomes too small and high tensile strength cannot be obtained. Conversely, if the draft ratio is greater than this range, the elongation at break is high but the tensile modulus is low and high tensile strength cannot be obtained.

Here, τD/L is defined by the following equation (2), and can be changed by the combination of the shape of the nozzle used, pitch viscosity, and pitch discharge amount. τD/L=(PD2)/(4L2)
=5.44×10-4(Qη)/(
πD2L) =1.36×10
−8(Voη)/L =8.1
6×10-10 η/t
(2) However, P: Molten pitch discharge pressure (kgf/cm2) D: Diameter of spinning nozzle capillary (mm) L: Length of spinning nozzle capillary (mm) Vo: Average velocity of molten pitch within the capillary (m /min) Q: Melt pitch discharge rate (ml/min) η: Melt pitch viscosity (poise) t: Melt pitch average residence time in the capillary (sec
).

[0021] The melt spinning in the method of the present invention is
Any of extrusion spinning, blow spinning, centrifugal spinning, etc. may be used. Further, the shape of the spinning nozzle to be used may be the same as the nozzle used in general melt spinning and is not particularly limited.

The pitch fibers spun under these conditions are infusible by a conventional method, and then heated at 800 to 1400°C.
carbonized under an inert atmosphere. If the carbonization temperature is lower than 800° C., the growth of graphite crystals is insufficient, so although the elongation at break is large, the tensile modulus is too low and high tensile strength cannot be obtained. On the other hand, the carbonization temperature is 1400℃
If it is higher, the growth of graphite crystals due to graphitization may progress and the tensile strength may be improved, but carbon fibers with a high elongation at break cannot be obtained.

Although the details of the mechanism by which the carbon fibers prepared according to the present invention exhibit extremely high tensile strength and elongation at break are not clear, it is inferred as follows. First, by using mesophase pitch having a special molecular structure and molecular weight distribution as the raw material pitch and performing melt spinning under the above conditions, pitch fibers in which pitch molecules are suitably oriented in the fiber axis direction can be obtained. In this pitch fiber, pitch molecules are suitably oriented and laminated even within the cross section of the fiber. After making such pitch fibers infusible, the excessive growth of graphite crystals is suppressed by carbonizing them at a specific temperature range, resulting in carbon in which microvoids present in the grain boundaries within the fibers are finely dispersed. Fiber is obtained. In addition, this carbon fiber has a sufficiently high degree of orientation of graphite crystals in the fiber axis direction, so
It is thought that as a result, carbon fibers having high tensile strength and high elongation at break can be obtained.

[0024]

EXAMPLES Next, the present invention will be explained in more detail with reference to Examples. Of course, the present invention is not limited to these examples.

Example 1 1 mole of naphthalene, 0.5 mole of hydrogen fluoride, and 0.2 mole of boron trifluoride were charged into a 500 ml acid-resistant autoclave, and the temperature was raised to 260° C. while maintaining the reaction pressure at 25 kgf/cm2. , reacted for 2 hours. Thereafter, the discharge valve of the autoclave was opened, and substantially all of the hydrogen fluoride and boron trifluoride were recovered in gaseous form at normal pressure, and then nitrogen was blown into the autoclave to obtain pitch from which low-boiling components had been removed. The yield of the pitch obtained was 76% by weight based on the raw material naphthalene. The optically anisotropic phase content of this pitch was 100%, the softening point was 231° C., and the H/C atomic ratio was 0.67.

[0026] This mesophase pitch is set to D=0.15.
mmφ, L/D=3, placed in a melt spinning machine with a single-hole nozzle, heated to 310°C, extruded with a pressure of 14 kgf/cm2, and wound at 240 m/min to obtain a fiber with a diameter d=12×10− A pitch fiber with a diameter of 3 mm was obtained. The shear stress at this time is τ = 1.16 kgf/cm2, and the draft ratio is Ve/Vo = 156, so these spinning conditions are within the above-mentioned preferred range. The pitch fiber thus obtained was heated to 28 °C in air at a heating rate of 5 °C/min.
After making it infusible by raising the temperature to 0°C, carbon fibers were obtained by raising the temperature to 1000°C at a rate of 20°C/min in an argon atmosphere. The obtained carbon fiber had a tensile strength of 340 kgf/mm2 and a tensile modulus of 20 tf.
/mm2, and the elongation at break was 1.7%, indicating high strength and high elongation at break.

Comparative Example 1 The same raw material pitch, spinning nozzle, and
Using a spinning machine, after raising the temperature to 320℃, 2kgf/cm2
Pitch fibers having a diameter d=12×10 −3 mmφ were obtained by extruding the fibers under a pressure of 100 m/min and winding them at a speed of 70 m/min. The draft ratio at this time is Ve/Vo=156, which is the same as in Example 1, but the shear stress is τ=0.17 kg.
Since it is as small as f/cm2, it is out of the above-mentioned preferred range. Carbon fibers were obtained from this pitch fiber by the same method as in Example 1. The tensile strength of the obtained carbon fiber is 230k
gf/mm2, tensile modulus was 22tf/mm2, and elongation at break was 1.1%.

Comparative Example 2 The same raw material pitch, spinning nozzle, and
Using a spinning machine, after raising the temperature to 300℃, 50kgf/cm
Pitch fibers having a diameter d=12×10 −3 mmφ were obtained by extruding by applying a pressure of 2 and winding at 270 m/min. The draft ratio at this time is Ve/Vo=156, which is the same as in Example 1, but the shear stress is τ=4.17.
Since it is as large as kgf/cm2, it is out of the above-mentioned preferred range. Carbon fibers were obtained from this pitch fiber by the same method as in Example 1. The tensile strength of the obtained carbon fiber was 26
The tensile modulus was 0 kgf/mm2, the tensile modulus was 26 tf/mm2, and the elongation at break was 1.6%.

Example 2 The same raw material pitch used in Example 1 was used at D=0.3.
It was placed in a spinning machine with a single-hole nozzle of 5 mmφ and L/D=3, heated to 320°C, extruded by applying a pressure of 1 kgf/cm2, and wound at 240 m/min to obtain a yarn with a diameter d=12×10−. A pitch fiber with a diameter of 3 mm was obtained. The shear stress at this time is τ = 0.083 kgf/cm2, and the draft ratio is Ve/Vo = 851, so these spinning conditions are within the above-mentioned preferred range. The pitch fiber thus obtained was heated to 28 °C in air at a heating rate of 5 °C/min.
After making it infusible by raising the temperature to 0°C, carbon fibers were obtained by raising the temperature to 1200°C at a rate of 20°C/min in a nitrogen atmosphere. The obtained carbon fiber had a tensile strength of 420 kgf/mm2 and a tensile modulus of 26 tf/m.
m2 and elongation at break were 1.6%, indicating high strength and high elongation at break.

Examples 3 to 6, Comparative Examples 3 to 7 Naphthalene 6
0 mol, hydrogen fluoride 30 mol, and boron trifluoride 9 mol were charged into a 43 liter acid-resistant autoclave, and the temperature was raised to 260°C while maintaining the reaction pressure at 25 kgf/cm2.
Time reacted. Then open the autoclave discharge valve and
After substantially all hydrogen fluoride and boron trifluoride were recovered in gaseous form at normal pressure, nitrogen was blown into the reactor to obtain pitch from which low-boiling components had been removed. The yield of the pitch obtained was 70% by weight based on the raw material naphthalene. The optically anisotropic phase content of this pitch was 100%, the softening point was 237° C., and the H/C atomic ratio was 0.66.

This mesophase pitch was put into a spinning machine having a single-hole nozzle and melt-spun under the spinning conditions shown in Table 1 to obtain pitch fibers with a diameter d=12×10 −3 mmφ. The obtained pitch fibers are made infusible by heating up to 280°C at a heating rate of 5°C/min in air, and then heated to 1000°C at a heating rate of 20°C/min in a nitrogen atmosphere. Carbon fibers were obtained. Table 1 shows the physical properties of the obtained carbon fiber. In Examples 3 to 6 in Table 1, the spinning conditions were within the above-mentioned preferred range, and the obtained carbon fibers exhibited high tensile strength and high elongation at break. However, in the case of Comparative Examples 3 to 7, the spinning conditions were The physical properties of the carbon fiber are poor because it is outside the preferred range.

[0032]

[Table 1]

[0033]

Effects of the Invention The carbon fiber manufacturing method of the present invention can use pitches synthesized from various raw materials containing naphthalene, anthracene, phenanthrene, etc., and condensed polycyclic hydrocarbons thereof. Moreover, since the mesophase pitch used in the present invention has high reactivity with oxygen, the infusibility process can be significantly shortened. Furthermore, by spinning at a draft ratio within a specific range, high-performance carbon fibers with significantly improved tensile strength and elongation at break can be obtained even during low-temperature carbonization of 800 to 1400°C. As described above, the method of the present invention is an economically superior method that can easily produce high-performance carbon fibers in a short time compared to pitches synthesized from various raw materials.

[0034]

[Brief explanation of drawings]

Figure 1 shows the draft ratio (Ve/
The preferred range of Vo ) is indicated by diagonal lines. τ: Shear stress that the molten pitch receives from the wall surface of the spinning nozzle capillary (kgf/cm2) D: Diameter of the spinning nozzle capillary (mm) L: Length of the spinning nozzle capillary (mm) Ve: Winding speed (
m/min) Vo: Average velocity of the molten pitch in the capillary (
m/min)

Claims (1)

[Claims] 1. Melt-spinning mesophase pitch obtained by polymerizing a condensed polycyclic hydrocarbon or a substance containing it in the presence of hydrogen fluoride or boron trifluoride as a raw material, followed by infusibility treatment; In a method for producing carbon fibers by carbonization treatment, melt spinning is performed at a draft ratio (Ve/Vo) that satisfies the following conditions, and the carbonization treatment is carried out at a draft ratio (Ve/Vo) of 8.
Carbon fiber manufacturing method characterized by carrying out in a temperature range of 00 to 1400°C 28 (τD/L)-0.65 ≦Ve/V
o≦ 115 (τD/L) −0.98 However, τ: Shear stress (kgf/cm2) that the melt pitch receives from the wall surface of the spinning nozzle capillary D: Diameter of the spinning nozzle capillary (mm) L: Length of the spinning nozzle capillary (mm) Ve: Winding speed (
m/min) Vo: Average velocity of the molten pitch in the capillary (
m/min).