JP3039080B2 - Method for determining spinning conditions for carbon fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining spinning conditions for pitch-based carbon fibers. According to this method, for example, the orientation of pitch fibers can be controlled so as to exhibit an improved elastic modulus.

[0002]

2. Description of the Related Art Carbon fiber is a material having a high specific strength and a specific elastic modulus, and is most attracting attention as a filler fiber of a high-performance composite material. Among them, pitch-based carbon fibers have various advantages as compared with polyacrylonitrile-based carbon fibers, such as abundant raw materials, a high yield in the carbonization process, and a high elastic modulus of the fibers.

[0003] As is well known, when a carbon material such as heavy oil, tar, pitch or the like is heated to 350 to 500 ° C, small spheres having an optical anisotropy under polarized light having a diameter of several µm are included in those materials. Is generated. Upon further heating, these small spheres coalesce and grow, eventually reaching a state in which the whole exhibits optical anisotropy.
This anisotropic structure is obtained by stacking and orienting aromatic hydrocarbons, which are planar macromolecules generated by a thermal polycondensation reaction of a carbonaceous raw material, in a layered manner, and is regarded as a precursor of a graphite crystal structure.

A heat-treated product containing such an anisotropic structure is
It is generally called a mesophase pitch. A pitch fiber can be obtained by using such a mesophase pitch as a spinning pitch and melt-spinning through a spinning nozzle.

[0005]

As described above, a carbon fiber having a high elastic modulus can be produced using mesophase pitch which is a spinning raw material having good orientation. Generally, in order to produce carbon fibers having a higher elastic modulus, a higher firing temperature is required in a firing process in which pitch fibers are made infusible, subsequently carbonized and graphitized. This is a disadvantageous condition in production, and a method of solving the problem by changing the spinning conditions to improve the orientation of pitch fibers has been adopted. However, the operating factors of spinning vary widely, and cooling conditions such as the nozzle diameter, the pitch fiber diameter, the temperature and speed of the discharged pitch, the winding speed, the temperature of the atmosphere directly below the nozzle, etc., must be individually considered. In addition, there is a problem that it is difficult to select optimum conditions for producing a desired pitch fiber. The present invention solves this problem.

[0006]

The inventors of the present invention have paid careful attention to such a point, and as a result of intensive studies, have found that the spinning conditions are determined by using the Deborah number, which is the ratio between the relaxation time of the spinning pitch and the representative time in the spinning process. Determined that pitch fibers having a predetermined orientation angle can be easily determined.

[0007] The heated and melted pitch is spun to obtain pitch fibers, then subjected to infusibilization treatment, and further carbonized and / or graphitized to produce carbon fibers. In the melt spinning to be performed, the representative time t of the process in the vicinity where the deformation speed of the molten pitch is maximum and the relaxation time τ of the pitch in the vicinity where the deformation speed of the molten pitch is maximum are determined according to the following formula (I). calculating a Deborah number N _D, using the Deborah number N _D, it is possible to determine the spinning conditions for controlling the orientation of the desired carbon fibers.

[0008]

[Number 2] _{N D = τ / t (I} ) below, in describing the present invention, the spinning pitch used in the present invention, oriented it is likely molecular species forming an optically anisotropic carbon fiber There is no particular limitation as long as it gives. Examples of the carbonaceous raw material for obtaining these spinning pitches include coal-based coal tar, coal-tar pitch, coal liquefaction, petroleum heavy oil, tar, pitch, and the like. These carbonaceous raw materials usually contain impurities such as free carbon, undissolved coal, and ash, and these impurities are well-known in the art such as filtration, centrifugation, or stationary sedimentation using a solvent. It is desirable to remove in advance by the above method. Further, as a carbonaceous raw material, for example, after a heat treatment, a preliminary treatment is performed by a method of extracting a soluble component with a specific solvent, or a method of hydrogenating in the presence of a hydrogen-donating solvent or hydrogen gas. You may leave.

The carbonaceous raw material or the carbonaceous raw material which has been subjected to the pretreatment is then treated at 350 to 500 ° C., preferably 380 to 450 ° C., for 2 minutes to 50 hours, preferably 5 minutes to 5 hours. Conditions are appropriately selected, and an optically anisotropic structure of 40% or more, preferably 70% or more obtained by heat treatment in an atmosphere of an inert gas such as nitrogen or argon, or under blowing. Can be suitably used.

The optical anisotropy ratio of the mesophase pitch referred to in the present invention is the area ratio of the portion showing the optical anisotropy ratio obtained by observation with a polarizing microscope at normal temperature. More specifically, after embedding a large number of mesophase pitch grains having a diameter of several mm in an embedding resin and polishing the surface, the entire pitch polished surface is observed with a polarizing microscope (100 times) throughout, and the optical anisotropy with respect to the entire area is measured. It is determined by measuring the area ratio of the sex part.

The pitch as described above is spun to obtain pitch fibers, then subjected to infusibilization treatment, and further carbonized and / or graphitized to produce carbon fibers. In the melt spinning to be formed, the representative time t of the process in the vicinity where the deformation speed dV (x) / dx of the molten pitch is maximum, and the pitch in the vicinity where the deformation speed dV (x) / dx of the molten pitch is maximum. seeking the relaxation time tau, calculates the Deborah number N _D according to the following formula (I), the using a Deborah number N _D, determining the spinning conditions for controlling the orientation of the desired carbon fibers. Here, x is the distance in the traveling yarn direction with the nozzle discharge portion as the origin, and V (x) is the velocity of the traveling yarn at the position x.

[0012]

N _D = τ / t (I) The value of the deformation speed (dV (x) / dx) here is a speed gradient at the position x from the nozzle, but has a meaning of the deformation speed. . Now, suppose that the speed increases by dV when passing through the section of dx at the time of dt. dV · dt /
Since dx is the elongation strain received during dt,

[0013]

## EQU4 ## dV ・ dt / dx × 1 / dt = dV / dx Therefore, the value of (dV (x) / dx) is the rate of elongation strain. The definition of the neighborhood is shown below. As shown in FIG. 1, the region of both ends of the x ₁ and x ₂ and the vicinity of the x ₂ when taking two points x _1, x ₃ across the position x ₂ of the maximum value of the deformation speed . x ₁ and x ₃ are preferably at positions where the deformation speed is 70% or more of the maximum deformation speed, preferably at positions where the deformation speed is 80% or more.

The definition of the representative time of the spinning process is shown below. Typical spinning process time is fast spinning (short time)
Or a slow (long) time. Although various methods for determining the representative time are conceivable, in the present invention, as the time that most affects the physical properties of the pitch fiber and the carbon fiber, the time that elapses at or near the position where the deformation speed is the maximum is adopted as the representative time. .

The reciprocal of the maximum value of the deformation speed is the elapsed time per unit strain when the running yarn is slenderly deformed at the maximum speed at this position. The reciprocal of the average deformation speed near the maximum value of the deformation speed is also the elapsed time per unit strain in this vicinity. The time passing through the defined predetermined neighborhood may be used as the representative time. Next, how to determine the representative time of the spinning process will be described.

The representative time of the spinning process can also be determined by measuring the change in diameter when the molten pitch discharged from the nozzle is reduced in diameter and solidified into pitch fibers. The linear velocity V _{0 at the} time of discharge from the nozzle, the nozzle diameter D _0, and the fiber diameter D at a distance x from the nozzle
Assuming (x), the linear velocity V at the position x is

[0017]

## EQU5 ## It can be calculated as V (x) = V ₀ (D ₀ / D (x)) ² . FIG. 2 qualitatively shows the results by dotted lines. By differentiating V (x), the value of dV (x) / dx can be obtained as shown by the solid line in FIG. Simultaneously with the measurement of the diameter, as will be described later, it is necessary to obtain the relaxation time on the running yarn. Therefore, as shown by the broken line in FIG.

As a method of actual measurement, it is possible to measure a change in the yarn diameter with a camera equipped with a microscope or the like, and to measure a temperature change with an infrared thermometer. However, this method requires a great deal of time, and it is difficult to perform accurate measurement. Kase, Matsuo et al. Have proposed a method of obtaining the contents shown in Fig. 2 by simulation (J. Polymer Sci. Part A, 3. 2541 (19
65)). According to this, the temperature dependence of viscosity and density is given as the physical property value of the pitch, and the nozzle diameter, the temperature of the discharge pitch, the linear speed, the winding speed, the temperature of the atmosphere under the nozzle and the airflow speed are given as the spinning conditions. 2, the deformation speed dV (x) / dx and the temperature T with respect to the distance x below the nozzle as shown in FIG.
The value of (x) can be known. From this result, a representative time t of the spinning process having the following contents can be obtained and used.

The maximum value of the deformation speed (dV (x) / dx) ma
Reciprocal value of x. As shown in FIG. 1, (dV (x) / dx) m
across the position x2 indicating the ax, the elapsed time between x ₃ from the vicinity x _1. Inverse value of the average value of dV (x) / dx from x ₁ to _{x 3 (dV (x) /} dx) ave.

Further, a method of obtaining the relaxation time will be described.
The relaxation time can be determined by measuring the viscoelasticity of the molten pitch by a vibration method using a viscoelasticity measuring device. That is, as shown in FIG. 3, when the dynamic loss G ″ is measured near the glass transition temperature of the pitch and the angular frequency is changed under a constant temperature condition, the angular frequency dependency of G ″ is upwardly convex. Become. The value of the reciprocal of the angular frequency when G ″ indicates the maximum value G ″ max is the relaxation time τ at the measured temperature. By changing the measurement temperature, the temperature dependence of the relaxation time can be determined. When the maximum value cannot be obtained at the measurement temperature due to the high temperature, as shown by the arrow in FIG. 3, the curve of the angular frequency dependence on the low temperature side having the maximum value is calculated based on the time-temperature conversion rule. By shifting along the angular frequency axis, the relaxation time τ at the temperature where the maximum value could not be measured can be obtained. In the present invention, as a viscoelasticity measuring device, a measuring device of the vibration method Rheometrics RDS-II is used.
Was used. As is known, the temperature dependence of the relaxation time τ can be obtained by a stress relaxation method that applies a step-like stress in addition to the vibration method. The relaxation time τ is a physical property value of the pitch, and can be uniquely obtained as a function of the temperature T as described above. Specifically, as the relaxation time τ, the relaxation time τ (T ₂ ) of the temperature T _{2 at} the position of the maximum value of the deformation speed (dV (x) / dx) max shown in FIG. 1, as shown in FIG. the (dV (x) / dx) position near to sandwich the position x2 indicating the max, mean value of T ₃ from temperature T ₁ of from x ₁ to x ₃ T
The relaxation time τ (Tave) at ave can be used. It is undesirable to use relaxation times at temperatures other than T ₂ and Tave.

The Deborah number N _D can be obtained as the ratio τ / t between the relaxation time τ and the representative time t of the spinning process. This is equivalent to comparing the inherent time scale of the pitch material with the deformation time scale of the spinning process and the length of the two, and if the relaxation time τ is relatively short, , Means that the elastic force generated when the molten pitch is elongated in the spinning process is relaxed and does not remain as residual stress in the pitch fiber,
If the relaxation time τ is relatively long, it means that the elastic force is frozen in the pitch fibers without being relaxed. This content governs the orientation of the molecules of the pitch fiber and thus the orientation of the graphite network of the carbon fiber.

[0022] While the value of the Deborah number N _D can be determined at all positions on the traveling yarn, the result of the study, as Deborah number N _D of determining the orientation of pitch fibers directly, (dV (x)
It has been found that it is preferable to use the value of the position at or near / dx) max. The use of the Deborah number N _D at other positions are not preferred. FIG. 4 shows the relationship between the orientation angle of the pitch fiber obtained by spinning and the Deborah number as the relationship between the Deborah number and the orientation of the pitch fiber. Here, as the Deborah number, the reciprocal value of the maximum value of the deformation speed (dV (x) / dx) max is used as the representative time t, and the maximum value of the deformation speed (dV (x) / dx) is used as the relaxation time τ.
It was determined using the value of the relaxation time corresponding to the temperature at the position of max.

The elapsed time from x ₁ to x ₃ shown in FIG. 1 may be used as the representative time t. Further, as a representative time t from the x ₁ to _{x 3 (dV (x) /} dx) av
The reciprocal of e may be used. As the relaxation time τ, x ₁ in FIG.
Or by using a relaxation time in the average temperature Tave to x ₃ from. Most preferably, it is better to determine the Deborah number N _D at the position of the maximum value of the deformation speed.

Here, the orientation angle ψ is measured by using a bundle of pitch fibers that have been bundled and aligned as a sample and scanning the peak position of 002 diffraction in the longitudinal direction using a fiber X-ray diffractometer to obtain the obtained diffraction intensity. Was used as the orientation angle ψ. When the orientation angle of the pitch fiber is small, the orientation angle of the fired carbon fiber is also small, and the elastic modulus is easily improved. Therefore, it is preferable to reduce the orientation angle at the stage of the pitch fiber.

The conditions for reducing the orientation angle are that the viscosity of the discharged pitch is low, the discharging speed is low, the take-up speed of the pitch fiber is low, the ambient temperature around the discharging yarn is high, and the cooling of the discharging yarn is performed. Although a plurality of individual conditions such as a low wind speed have been empirically determined, the concept of the Deborah number enables comprehensive determination of conditions for obtaining pitch fibers having a desired orientation angle. Orientation angle of pitch fibers as can be seen from Figure 4 the following values have Deborah number, minimum and a constant value of (Deborah number = 1 × 10 ^-1 or less in the embodiment of FIG.)
Thus, advantageous conditions for producing pitch fibers having a small orientation angle can be set within this range. When the combination of τ and t used is changed, the value of the Deborah number moves in parallel to the left and right in FIG. 4, so that the combination to be used must be fixed.

When pitch fibers having a small orientation angle are not particularly required, spinning conditions having a large Deborah number can be selected. Although pitch fibers spun under these conditions are unlikely to have an improved elastic modulus, the fibers do not fuse together during the firing step, and a pliable carbon fiber bundle with good fibrillation properties can be produced. In this way, which one to select may be comprehensively determined from the Deborah number. If the Deborah number is too large, stress cannot be relieved during spinning, and spinning tension increases due to elastic force, so that it cannot be adopted as a preventive condition.

[0027]

FIG. 4 and Table 1 show examples and comparative examples.
The numbers in FIG. 4 correspond to the numbers in Table 1. In the spinning experiment of the example, a mesophase pitch sample having anisotropy of 97% was used, and a nozzle having a diameter of 0.1 mm was used.

[0028] Representative time t of the spinning process used for the Deborah number N _D calculation, the maximum deformation rate was determined by a simulation (dV (x) / dx) is the inverse of the max,
The relaxation time τ is a value corresponding to the temperature at the maximum deformation speed position. When using combined with other t and τ is moved in parallel along the values of Deborah number N _D is the horizontal axis in FIG. 4, it should be noted that the value changes. Since the orientation of the carbon fibers obtained after firing is also improved, the quality of the orientation of the carbon fibers can be determined based on the orientation angle of the pitch fibers.

Even if the spinning operation conditions are changed, the Deborah number is on one curve, so that pitch fibers and carbon fibers having a desired orientation angle can be obtained from this value. That is, in order to obtain a pitch fiber having improved orientation, spinning conditions such as a discharge temperature and a winding speed can be determined so as to obtain the Deborah numbers of Examples 1 to 5. When high fibrillation is not required and fibrillation is regarded as important, Examples 6 to 7
The spinning conditions can be determined so as to satisfy the Deborah number of the above condition. Under the conditions of Examples 8 to 10, the Deborah number is too large, and the tension applied to the traveling yarn in the molten state during the spinning is too large to be easily broken, which is not desirable as the spinning condition.

[0030]

[Table 1]

[0031]

According to the present invention, it is necessary to spin pitch fibers having a small orientation angle in order to produce carbon fibers having improved elasticity even when the firing temperature is lowered. As the spinning conditions for reducing the orientation angle, a plurality of individual conditions can be considered, including the fact that the temperature of the ejected pitch is high. However, if the concept of Deborah number is used, the conditions for obtaining pitch fibers having a desired orientation angle are comprehensive. Can be determined. That is, after determining the nozzle diameter, yarn diameter, etc., which must be preferentially selected from among various individual operating conditions in which the Deborah number is a certain value or less,
To match the desired value of Deborah number N _D, it can be selected to easily operate the remaining operating conditions.

If viscoelasticity measurement and simulation are performed in parallel with the operation, spinning conditions for a desired orientation angle can be determined at any time.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a representative time t of a process.

FIG. 2 is an explanatory diagram of a relaxation time τ.

FIG. 3 is an explanatory diagram of how to find a relaxation time τ.

FIG. 4 is an explanatory view showing the relationship between the orientation angle and Deborah number N _D of pitch fibers.

Claims (6)

(57) [Claims] 1. A pitch melted by heating is spun to obtain a pitch fiber, then subjected to an infusibilization treatment, and further carbonized and / or graphitized to produce a carbon fiber. In melt spinning to make diameter,
A representative time t of the process in the vicinity of the deformation rate is maximum of the melting pitch, seeking and pitch relaxation time τ in the vicinity of the deformation rate is maximized molten pitch, the Deborah number N _D according to the following formula (I) calculated, use the Deborah number N _D
And determining the spinning conditions for controlling the desired orientation of the carbon fibers. N _D = τ / t (I) 2. The method according to claim 1, wherein a reciprocal of the maximum value of the deformation speed of the molten pitch is used as the representative time t of the process. 3. The method for determining spinning conditions for carbon fibers according to claim 1, wherein the relaxation time τ is a relaxation time at the position of the maximum value of the deformation speed of the molten pitch. 4. The method for determining spinning conditions for a carbon fiber according to claim 1, wherein an elapsed time in a predetermined vicinity including the position of the maximum deformation speed of the molten pitch is used as the representative time t of the process. 5. The method for determining spinning conditions for carbon fibers according to claim 1, wherein a reciprocal of an average value of the deformation speed in a predetermined vicinity including the position of the maximum deformation speed of the molten pitch is used as the representative time t of the process. 6. The method according to claim 1, wherein an average relaxation time in a predetermined vicinity including a position of a maximum deformation speed of the molten pitch is used as the relaxation time τ.