JPH0791697B2 - Carbon fiber manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing carbon fiber having high performance from pitch. More specifically, the present invention relates to a method for producing a pitch-based carbon fiber having a high-strength and high elastic modulus in which the orientation structure of carbon molecules exhibits a composite structure of surface layer onion orientation core random in the fiber cross section, without surface cleavage.

(B) Prior art Optically anisotropic pitch is an easily carbonized and graphitized material as described in JP-A-49-19127, and is an excellent raw material for carbon fiber having high strength and high elastic modulus. It shows the property.

Spinning with an optically anisotropic pitch is a fiber formation of a molecule having three-dimensionally extreme anisotropy, and therefore exhibits an orientation behavior that is not observed in ordinary melt spinning of a polymer. JBBarr
Et al. Applied Polymer Symposia 29 P.161-173 (1976)
Have reported that a layered structure corresponding to such orientation behavior exists in pitch-based carbon fibers, and the orientation types are classified into radial type, onion skin type, and random type.

Advances in research on pitch spinning have revealed that the radial type is generally easier to adopt as the orientation type, and the radial type is more susceptible to repeated mechanical deformation than the other types because it tends to cause cleavage cracks on the surface. I've been

JP-A-57-154416 discloses, in order to solve such a problem, a method in which the orientation type is changed to a random type or an onion skin type by cooling with a high-temperature air stream during centrifugal spinning. is doing. Yamada et al., At the 10th Carbon Society of Japan in 1984, the spinning temperature was random type and onion skin type from the bending point appearing in the relationship between the logarithm of pitch viscosity and the reciprocal of absolute temperature. It says that a pattern will appear. These suggest that if the spinning condition is brought to the slow cooling side, it becomes a random type or onion skin type.However, considering the spinnability of the pitch, this spinning condition reduces the spinnability of the pitch. It can be seen that the tendency is to impede the uniformity of yarn and the stability of spinning. Even if the pitch has a large molecular weight such as an optically anisotropic pitch, the molecular weight is smaller than that of a general polymer material, and its spinnability is generally expressed in a glassy supercooled liquid unlike that of a polymer. Considered to be the same. This is because the surface tension of the liquid is small relative to the viscosity, so that the liquid has a cylindrical shape and is stable, and is less likely to be divided into spherical shapes. In the case of pitch spinning, when cooling is transferred to the slow cooling side, the rate of increase in the viscosity of the liquid decreases, and the time in which the columnar shape is unstable becomes longer, causing constriction and breakage in the liquid column. This is not preferable because it becomes easy and spinning becomes unstable.

In order to solve this problem in Japanese Patent Laid-Open No. 57-100186, Otani et al. Reduce the optical anisotropic pitch to make it isotropic, and the degree is made anisotropic by a slight external force. It is disclosed that the spinning can be performed under the condition that the spinnability is good. Yamada et al. Also described in JP-A-58-18421, just before the optical anisotropy, it was possible to perform spinning under conditions where the spinnability was good with a pitch that did not cause the optical anisotropy with a slight external force, and the carbonization characteristics were the optical anisotropy. It discloses that a product that is not much different from the sex pitch can be made. These methods are certainly effective methods for randomizing the orientation of carbon molecules, but since pitch is a substance that is essentially difficult to analyze, it is difficult to grasp the pitch properties and it is difficult to stabilize the quality. Therefore, there are many problems in large-scale industrial implementation.

In order to solve this problem, the applicant of the present invention has disclosed Japanese Patent Laid-Open No. 59-1634.
Filed No. 24. In this method, the optically anisotropic pitch is melt-spun from a spinning nozzle having an irregular cross section, and the liquid column is deformed into a shape close to a circle due to surface tension until the solidification, and the orientation of carbon molecules is Since it is randomized, it has the effect of increasing the strength and elastic modulus after carbonization. This method is certainly an excellent method, but the degree of irregularity of the spinning nozzle is low, and when the cross-sectional shape of the obtained fiber is substantially a perfect circle, the randomization of the orientation of the carbon molecules is insufficient, and the orientation In order to sufficiently randomize, it is necessary to increase the degree of irregularity of the spinning nozzle. In this case, there is a drawback that wear deformation of the spinning nozzle is likely to occur, resulting in a large cost increase due to exhaustion of the nozzle.

As another method for solving this problem, the present applicant has disclosed that
Filed No. 59-163422. This method melt-spins an optically anisotropic pitch from a spinneret with a nozzle outlet cross-sectional area larger than the narrowest cross-sectional area inside the nozzle. The radial orientation is randomized due to the enlargement of the spinning holes and the large expansion ratio after discharging from the spinning holes, and further shifts to the onion skin orientation. This method is certainly an excellent method, but it has a drawback that the unevenness in the thickness of the fiber is large due to the large elongation ratio after discharging from the spinning hole. Although this drawback can be alleviated by reducing the diameter of the spinning hole after expansion, there is a drawback that the processing cost of the spinning hole becomes very high because the deep portion of the spinning hole must be finely cut. To remedy this drawback,
As disclosed in JP-A-59-168127, a method of enlarging the spinning hole and then further reducing it has been proposed, but it is more difficult to process the spinning hole, and a process of bonding two spinnerets is required. It has the disadvantage of being expensive.

Another way to remedy this drawback is US Pat. No. 4,376,74
No. 7 discloses a spinning hole filled with a filter material. Processing of this base is relatively easy,
This method is effective for random orientation of carbon molecules and has good spinnability, but has a drawback that washing of the spinneret is extremely difficult.

As another method for improving this drawback, JP-A-60-
No. 259609 proposes a method of inserting an insert into the introduction portion of a spinning hole. This method has an advantage that the die processing is easy and the washing is easy, but it has a drawback that the random orientation of carbon molecules is not always sufficient.

(C) Problems to be Solved by the Invention In the present invention, when melt-spinning an optically anisotropic pitch or a pitch having a high softening point having carbonization characteristics similar thereto, pitch molecules are oriented in the radial direction with respect to the fiber cross section. Hard to do,
The purpose of the present invention is to make the pitch fibers carbonized or further graphitized by randomly orienting the carbon molecules after carbonization on the surface to give high strength and high elastic modulus with no surface cleavage.

Generally, in a large molecule, in a shear field, the orientation is increased while causing rotational motion in a plane including the shear field direction, but in the case of a disk-shaped molecule such as a pitch, radial orientation is easily caused by the flow in the spinning hole. As shown. When the pitch is such that liquid crystals are formed, the state in which the disks are overlapped is a stable state, so randomization due to thermal diffusion does not occur and a radial structure is likely to occur. Since the pitch exiting the spinning hole is a liquid column and is stretched at a high magnification, the orientation of the molecules is disturbed by surface expansion in this process, and further, the molecular arrangement parallel to the surface develops and becomes an onion skin orientation. Apparently, this mechanism has not been clarified.

(D) Means for Solving the Problems The present invention is a method for producing carbon fibers by melt spinning a pitch having a high softening point, followed by infusibilization treatment and carbonization or further graphitization, in the melt spinning, It is a method for producing carbon fibers, characterized in that an insert that reaches to a position after a discharge port of the spinning hole is inserted into the spinning hole from an introduction hole side of the spinning hole and a pitch flow is spun out. In the present invention, the pitch having a high softening point is a graphitizable pitch such as an optically anisotropic pitch. The graphitizable pitch produces needle coke during calcination, and also carbon fiber having a high elastic modulus even when the pitch fiber is carbonized without stress. The graphitizable pitch includes, in addition to the optically anisotropic pitch, dormant mesophase pitch and premesophase carbonaceous material that exhibit graphitization properties similar to this.

The tip of the insert to be inserted into the spinning hole needs to reach at least up to the outlet of the spinning hole. A specific example of this is shown in FIG. The insert may be concentric with the shape of the spinning hole, but it is preferable that the shape can be fixed by locally contacting the inner surface of the spinning hole.

In this way, one of the effects of inserting the insert that reaches after the discharge hole into the spinning hole is that the pitch molecules are oriented in the shearing direction in the spinning hole, the radial orientation structure freezes and the carbon molecules are converted to the radial direction. It seems to prevent that. The molten pitch flow squeezed between the spinning hole and the insert seems to be once radialized, but is it because of the expansion after the discharge hole and the increase in the amount of the pitch flow retained at the exit from the spinning hole? There is a tendency that the conditions for onion skin orientation become wider, and in particular, only the vicinity of the surface often shows orientation parallel to the surface, and it becomes easy to obtain a composite structure of the surface layer onion orientation core random. As a result, the carbon fiber produced by the present invention is presumed to have excellent strength and durability.

In the present invention, the cross-sectional area of the insert is 0.
It is 03 times to 0.8 times, preferably 0.1 times to 0.6 times the cross-sectional area of the discharge hole. Further, the length of the insert protruding from the discharge hole is preferably 30 times or less the diameter of the discharge hole.

Examples 1 and 2 Thermal catalytic cracking (FCC) heat treatment was carried out at 420 ° C for 2 hours while feeding methane gas to the first distillate at 404 ° C and the final distillate at 560 ° C (at atmospheric pressure), and at 320 ° C. A pitch containing 44.8% of mesophase was prepared by heating for 16 hours, and this was aged to separate the mesophase by sedimentation due to the difference in specific gravity. This pitch contained almost 100% of optically anisotropic component, 47% of quinoline-insoluble matter and 82% of toluene-insoluble matter.

This pitch can be adjusted to
As shown in the figure, an insert reaching up to and after the discharge hole was inserted and spun under the conditions shown in Table 1.

The results of investigating the orientation and physical properties of carbon molecules after carbonization are also shown in Table 1.

In all cases, carbon fibers having a surface onion core random orientation and excellent strength were obtained.

Comparative Examples 1 and 2 As shown in Table 1 except that the insert was not inserted, the pitch was spun with the same die and the same conditions as in Examples 1 and 2.

The orientation of the carbon molecules after carbonization was a radial orientation as shown in Table 1, and the strength also showed a low value.

Comparative Example 3 Instead of the insert of Example 1, a filter made of a sintered alloy (stainless steel thickness 5 mm was used at the introduction portion of a normal spinneret).
Pitch spinning was carried out by pressing open eyes (100-200 mesh).

Randomization of orientation of carbon molecules occurred, but it was extremely difficult to clean the die.

(E) Effect of the Invention The pitch fiber spun according to the present invention is a carbon fiber having a composite structure in which the surface of the carbon molecule has a random onion oriented core portion, and a carbon fiber which is resistant to cracking and has excellent strength and durability.

[Brief description of drawings]

FIG. 1 is a schematic elevational view showing an embodiment of a spinneret used in the present invention, which is an example in which a tip of an insert is protruded from a discharge hole. Explanation of drawing symbols 1: Spinning hole introduction hole 2: Spinning hole contraction part 3: Spinning hole discharge hole 4: Insert

Claims (1)

[Claims] 1. A method for producing a carbon fiber by melt spinning a pitch having a high softening point and then subjecting it to infusibilization treatment and carbonization or further graphitization. When melt spinning is performed, the spinning hole is formed in the spinning hole. The method for producing a carbon fiber characterized in that the pitch flow is spun out by inserting an insert that reaches after the discharge port from the introduction hole side of the spinning hole.