JPS63120112A - Pitch type carbon yarn having high modulus of elasticity and production thereof - Google Patents

Abstract

PURPOSE:To obtain carbon yarn having extremely high modulus of elasticity and high crystallizability at the interior of the yarn than that at the outer surface layer part at a relatively low temperature and economically, by spinning a carbonaceous pitch consisting essentially of an optically anisotropic component to give yarn, making only the outer surface layer part infusible and calcining. CONSTITUTION:A carbonaceous pitch consisting essentially of an optically anisotropic component is spun to give yarn having 9-14mum diameter. Then the yarn, for example, is treated in an oxidizing atmosphere under conditions of 150-200 deg.C infusibility starting temperature, 1-2 deg.C rate of heating and 250-350 deg.C final temperature. After the final temperature is reached, the yarn is immediately cooled to make only the outer surface layer part (e.g. 1-3mu) infusible. Successively, the infusible yarn is calcined in an inert atmosphere under conditions of 20-500 deg.C/min rate of heating, 2,000-3,000 deg.C final temperature and 4-150min time to give the aimed carbon yarn having higher crystallizability at the interior of the yarn than that at the outer surface layer part of the yarn (usually >=10% larger in size of crystallite) in the cross section of the yarn.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a pitch-based carbon fiber and a method for producing the same, and more specifically, relates to a pitch-based carbon fiber that is produced at a low firing temperature and achieves a high modulus of elasticity. .

High modulus carbon fiber is used as a composite material with plastics, metals, carbon, ceramics, etc. in lightweight structural materials (aircraft, spacecraft, automobiles, buildings, etc.) and high-temperature materials (brake discs, rockets, etc.). Used for reinforcing metals and ceramics.

[Conventional technology]

PAN-based carbon fibers with high strength and medium elastic modulus are manufactured using polyacrylonitrile as a raw material, and some fibers fired at 2000° C. or higher exhibit an elastic modulus of about 400 GPa at maximum. However, PAN-based carbon fibers have the disadvantage of high raw material cost, and are difficult to graphitize, so there is a limit to improving the degree of crystallinity (graphitization degree).
Inherently, it is difficult to achieve ultra-high modulus.

Pitch-based carbon fibers are not only economically efficient as their raw materials are inexpensive, but also those manufactured from petroleum-based liquid crystal pitch and fired at around 3000°C are called graphite fibers and exhibit an ultra-high modulus of elasticity of around 700 GPa. (Special Publication No. 59-3567)
.

In pitch-based carbon fibers, in order to improve properties such as strength and elastic modulus, in the cross section of the fiber, crystals are arranged in the circumferential direction on the surface layer of the fiber, and in a radial or mosaic pattern in the center. Carbon fibers with an aligned structure (Japanese Patent Application Laid-Open No. 59-53717), and carbon fibers in which the outer peripheral surface layer of the fiber has a radial orientation structure and the inner core has an onion-ra 1' orientation structure in order to particularly increase the surface strength. Carbon fibers (Japanese Patent Application Laid-Open No. 60-239520) have been proposed.

[Problem that the invention seeks to solve]

As mentioned above, it is possible to manufacture carbon fibers with ultra-high modulus using liquid crystal pitch, and several methods have been proposed to improve the properties of fibers, but none of the methods In order to achieve this ratio, firing at a high temperature around 3000°C is necessary.

A high firing temperature is an obstacle to reducing equipment costs and manufacturing costs, and increasing the firing temperature also has the disadvantage that the tensile strength of the fibers decreases.

[Means and effects for solving the problem] In the process of intensive research and development to obtain carbon fibers having a super-elastic modulus by firing at low temperatures, the present inventors discovered that carbon fibers were removed from the outer surface layer of the fibers and from the inside. They discovered that this was possible by increasing the crystallinity of the fibers, and completed the present invention.

That is, the present invention resides in a pitch-based carbon fiber characterized by having substantially higher crystallinity in the interior of the fiber than in the outer surface layer within the cross section of the fiber. Furthermore, the present invention provides a method for producing such pitch-based carbon fibers by spinning a carbonaceous pinch whose main component is an optically anisotropic portion, and spinning the resulting carbonaceous pitch fiber within the cross section of the fiber. The present invention also relates to a method for producing pitch-based carbon fibers, which is characterized in that only the outer surface layer portion is selectively infusible, and then the fiber with only the outer surface layer portion infusible is fired.

It is known that when the crystallinity of carbon fibers improves, the elastic modulus improves. Conventional carbon fibers require firing at a high temperature of around 3000° C. in order to increase crystallinity to the point where they exhibit an ultra-high modulus of elasticity of about 700 GPa.

On the other hand, according to the present invention, the temperature is about 500°C lower than before.
It is possible to obtain carbon fibers having an elastic modulus substantially equivalent to that of conventional carbon fibers at low firing temperatures.

This is because in the conventional graphitized carbon fiber production method, the crystallinity of the spun liquid crystal pitch fibers was disturbed and decreased during the infusibility process, but in the present invention, the pitch fibers were reduced during the infusibility process. By selectively infusifying only the inner and outer surface layers of the cross section, we achieve the minimum level of infusibility that prevents the fibers from fusing during firing, while maintaining high crystallinity within the cross section of the pinched fibers. This is because by keeping the carbon fibers as they are, it is possible to produce carbon fibers with a high elastic modulus equal to or higher than conventional ones even at a lower firing temperature than conventional ones.

Research on the infusibility reaction of pitch fibers produced from liquid crystal pitch is very limited, and it is thought that infusibility is achieved by progressing polymerization through crosslinking reaction due to oxidation. Furthermore, little research has been conducted on changes in crystal structure during the infusibility process.

As a result of a detailed study of crystal structure changes during the infusibility process using X-ray diffraction, the present inventors found that in the case of pitch fibers with good crystallinity produced from liquid crystal pitch, the crystallinity is disturbed and deteriorated during the infusibility process. I found something to do. This reduction in crystallinity during the infusibility process also reduces the crystal structure of the carbon fiber after carbonization, so it is important to minimize it to the necessary minimum in order to obtain carbon fiber with better performance. Also,
In order to achieve both infusibility to prevent fusion during the carbonization process and to minimize deterioration of the crystal structure, it is necessary to selectively infusible the outer surface layer of the fiber during the infusibility process. I found something good. Since the outer surface layer of the fibers made infusible in this way has been made infusible, the surface will not melt during the subsequent carbonization process and the fibers will not fuse together, and the internal crystal structure will not be disturbed as a whole. Deterioration of the crystal structure is minimized.

Carbon fibers obtained by carbonizing fibers in which only the outer surface layer is selectively infusible in the cross section of the pinch fiber are generally found in the inner part of the fiber than in the outer surface layer in the cross section of the fiber. The crystallinity is terrible. The outer surface layer of carbon fiber with low crystallinity is the part that becomes infusible to prevent the fibers from fusing during carbonization firing, so it is sufficient to have a minimum thickness, but it is necessary to have a larger thickness than that. However, the effects of the present invention can be achieved as long as a highly crystalline portion, that is, a portion that is not infusible, remains inside the fiber.

Furthermore, the change in crystallinity between the outer surface layer and the inside of the fiber does not need to be steep, and it goes without saying that the change may be gradual. In addition, since the thickness of the outer surface layer iβ to be infusible does not increase depending on the fiber diameter, generally speaking, the larger the fiber diameter, the more the proportion of the highly crystalline part inside the fiber can be increased. The elastic modulus of the fibers can also be improved.

The difference in crystallinity between the outer surface layer and the interior of carbon fibers depends on the type and quality of the spinning pitch, the conditions and degree of infusibility, the carbonization conditions, etc., but in the present invention, the crystallinity of the interior is higher than that of the outer surface layer. The crystallite size is 10% or more larger. The size of the crystallite is
Diffraction patterns measured using selected area electron diffraction are measured using a microdensitometer and compared using the reciprocal of the half width of the diffraction intensity. If this difference is less than 10V6, not much effect can be expected.

Next, the method for producing pitch-based carbon fiber as described above according to the present invention will be explained. As the carbonaceous pitch to be spun, a highly crystalline one whose main component is an optically anisotropic portion is used, and a preferable one is a particularly Publication No. 57-88016, 58
It is a carbonaceous pitch with a softening point of 230 to 320°C and an optically anisotropic portion of 90 to 100%, as described in Publication No. 45277, Publication No. 58-37084, etc., but is not limited thereto. . Although spinning can be done by conventional methods, the above preferred carbonaceous pitch is 280-370°C.
It is preferable to perform spinning at a constant temperature within the range of .

According to the present invention, only the outer surface layer of the spun pinch fiber having crystallinity is selectively infusible in the cross section of the fiber. For this purpose, the pitch fibers are preferably infusible for a short time within a certain range, which is shorter than usual. For example, the 5μ obtained by the above preferred embodiment
When pitch fibers having a diameter of m to 20 μm, preferably 9 μm to 14 μm, are infusible in air, the infusibility start temperature is 150°C to 200°C, and the heating rate is 1°C/min or more, preferably 1°C/min. Final temperature 250℃~350min at ~2℃/min
The temperature is raised to ℃ and immediately cooled to room temperature.

If the heating rate is slower than 1° C./min, it will take time to reach the final temperature, and infusibility will progress to the inside of the fiber. Moreover, if the speed is faster than 2° C./min, the fibers will fuse together during the infusibility process. The heating rate is 1°C within the above temperature range.
/min to 2℃/min, the final temperature can be reached in a relatively short time without fusion, and as a result, infusibility occurs only in the outer surface layer, and the crystallinity inside the fiber is disturbed. Therefore, in order to achieve the above-mentioned infusibility, it is necessary to determine the infusibility temperature in accordance with the infusibility temperature. In addition to air, the infusible atmosphere may be oxygen, ozone, nitrogen dioxide, etc. If a strongly oxidizing gas is used, the heating rate should be kept within a correspondingly high range, and the final temperature should be lowered. can.

The minimum thickness of the outer surface layer that must be infusible to prevent the pitch fibers from fusing during carbonization and firing depends on the type of pitch fiber, the degree of infusibility, etc., but is, for example, 1 μm to 3 μm.
It has been found that this thickness is not significantly dependent on the fiber diameter.

The pitch fibers in which only the outer surface layer of the fibers has been selectively infusible can be fired and carbonized using a conventional method.

In this carbonization firing, the interior of the fiber, which has not been made infusible, is fired with high crystallinity, so the crystallinity is higher than that of the outer surface layer of the fiber. Firing conditions include, for example, a temperature increase rate of 20°C/min to 500°C/min, and a final temperature of 2000"C to
The firing time at 3000° C. is 4 minutes to 150 minutes. According to the method of the present invention, 2
Although ultra-high modulus carbon fibers with the same modulus of <7QOGPa are obtained at a firing temperature of 500° C., the firing temperature of the present invention is not limited thereto.

The carbon fiber according to the present invention can achieve an ultra-high modulus of elasticity by low-temperature firing, and also has high tensile strength. Furthermore, the carbon fiber according to the present invention has a unique structure in which the inner part of the fiber has better crystallinity than the outer surface layer within the fiber cross section, and can exhibit unprecedented characteristics.

In addition, the carbon fibers according to the present invention can be modified to a certain extent to a certain extent by arbitrarily selecting the fiber diameter and selective infusibility ratio in addition to the starting pitch raw material, spinning conditions, carbonization firing conditions, etc. It has the advantage of being possible.

〔Example〕

In the examples, the following parameters or measurement methods were used to measure the characteristics of carbon fibers.

X-ray structural parameters orientation angle (φ), lamination thickness Lco. 2), layer spacing (dO
02) is a parameter representing the fine structure of carbon fiber determined by wide-angle X-ray diffraction. The orientation angle (φ) indicates the degree of selective orientation of the crystal with respect to the fiber axis direction, and the smaller this angle, the better the orientation. The lamination thickness (L cooz) represents the apparent lamination thickness of the (002) plane in the carbon microcrystal, and the interlayer spacing (doot) represents the interlayer spacing of the (002) plane of the microcrystal. Generally the lamination thickness is
It is considered that the larger the L coot and the smaller the interlayer spacing (d ooz), the better the crystallinity.

To measure the orientation angle (φ), use a fiber sample stage, scan the counter with the fiber bundle perpendicular to the scanning plane of the counter, and find the time at which the intensity of the diffraction band reaches its maximum (002). 2θ(
approximately 26°). Next, with the counter held in this position, the fiber sample stage was rotated 360° to measure the intensity distribution of the (002) diffraction ring, and the
The half width at the point /2 is defined as the orientation angle (φ).

Lamination thickness Lcooz), F1 layer spacing d002)
The fiber was powdered in a mortar and measured and analyzed in accordance with the Gakushin method "Lattice constant and crystallite size measurement method of artificial graphite", and was determined from the following formula.

do. 2 = □ 2 sin θ K = 1.0, λ = 1.5418 θ: β obtained from the (002) diffraction angle 2θ: Half-power width of the (002) diffraction band obtained by correction EM) Observation method and electron diffraction measurement method Carbon fiber samples are aligned in the fiber axis direction, embedded in heat-curable epoxy resin, and cured. After trimming to expose, an ultra-thin section with a thickness of 1000 angstroms or less is prepared using an ultramicrotome equipped with a diamond knife. This ultra-thin section is placed on a grid treated with engraving. Then, use an electron microscope to take bright-field and dark-field images.A bright-field image is a normal transmission electron microscope (TEM) photograph, and a dark-field image is a photograph of the diffraction from a specific diffraction surface. By capturing an electron beam and forming an image, the collective state of the diffraction surface is observed.The (002) dark-field image in the example was obtained using an objective aperture with a diameter of approximately 10 μm in the same field of view as the bright-field image. hand,
The aggregated state of the (002) diffraction surface was observed by capturing and imaging the diffracted electron beam from the (002) diffraction surface. In the photograph, the (002) diffraction plane is observed to shine white. Therefore, the areas where the white glowing parts are thick are considered to be areas where the (002) crystal plane is well developed and the crystallinity is good.

In order to investigate internal and external differences in crystallinity within the fiber, electron beam diffraction images from specific areas are measured using a controlled field electron diffraction method. Measurement conditions are acceleration voltage 200KV, diameter approximately 1.7μ
Electron diffraction photographs are taken continuously from edge to edge in the direction perpendicular to the fiber axis of the ultrathin section using a selected area aperture of m. The obtained diffraction pattern was analyzed using a microdensitometer to obtain electron beam diffraction images of (002),
Scanning profiles of diffraction intensities in two directions, ie, the equator and the meridian, are measured for the (004), (100), and (110) diffraction lines. The half width (ΔS) of the diffraction intensity of each of the scanning profiles thus obtained is measured. The crystallite size L is determined from the Scherrer equation L=/ΔS.

is a constant and takes a different value for each diffraction line. As is clear from this equation, the crystallite size is inversely proportional to the half-width for the same diffraction line, so the reciprocal of the half-width measured at each measurement point is calculated and the crystallite sizes are compared. can do.

Example 1 Carbonaceous pitch containing about 50% of optically anisotropic phase (AP) was used as a precursor pitch, and it was heated in a cylindrical continuous centrifugal fractionator WF' with an effective volume of 2001 in the rotor. A with a centrifugal force of 10.0OOG while controlling the rotor temperature to 360℃
A pinch of optically anisotropic phase was extracted from the P outlet. The optically anisotropic pinch obtained has an optically anisotropic phase of 9
The content was 9% or more, and the softening point was 271″C.

Next, the obtained optical anisotropy pitch is defined as the nozzle diameter Q, 3
The fibers were spun at 315°C on a mm melt spinning machine.

The obtained pitch fibers were heated at a starting temperature of 180°C in an air atmosphere.
Infusibility was achieved at a final temperature of 290°C and a heating rate of 2°C/min.

After the infusibility treatment is completed, the temperature increase rate is increased to 1 in an argon atmosphere.
Carbonization was carried out at 00°C/min and a final temperature of 2500°C to obtain carbon fibers with a diameter of about 13 μm.

As shown in Table 1, this carbon fiber has an orientation angle (φ) of 6.
8°, lamination thickness (L COO2) is 210 layers, layer spacing (dooz) is 3.395 layers, and elastic modulus is 736G.
It was Pa. Moreover, the tensile strength was 2.77 GPa.

FIG. 1 is a scanning electron micrograph showing a cross section of the obtained carbon fiber, and a difference in the cross-sectional orientation structure is recognized between the inner and outer surface layers. Figure 2 (a) is a (002) dark field image of the longitudinal section of the carbon fiber obtained using a transmission electron microscope. This is thought to be because the (002) layer thickness inside is larger and the crystallinity is better. FIG. 2(a) is a bright field image (ordinary TEM photograph) of the same longitudinal section taken with a transmission electron microscope, and this also shows that the inside of the fiber has better crystallinity than the outer surface layer. In fact, (
002) When the half-width of the diffraction line was measured and calculated from the reciprocal of the half-width as described above, the proportion of crystallites larger in the inside of the fiber than in the outer surface layer was 21%.

The optically anisotropic pitch obtained in W roughening Example 1 was processed using the same melt spinning machine at 315°C and the bi-notch discharge rate was approximately 1/2 that of Example 1.
It was spun with

The obtained pitch fibers were infusible and carbonized under the same conditions as in Example 1 to obtain carbon fibers with a diameter of about 9 μm.

As shown in Table 1, this carbon fiber has an orientation angle (ψ) of 8
．． 9°, laminate thickness reduced. . 2) is 160 people, and the layer interval (
dooz) is 3.401 people, and the elastic modulus is 573G
Pa, and the tensile strength was 2.74 GPa.

In the scanning electron micrograph of the cross-section of the fiber (Fig. 3), no difference in cross-sectional orientation structure is observed between the inner and outer surface layers. Dark-field images (Figure 4 (a)) and bright-field images (Figure 4 (a)) of the longitudinal cross-section of the fiber using a transmission electron microscope show that there is no difference in crystallinity between the inside of the fiber and the outer surface layer. is recognized. In fact, based on the measurement of the half width of (002) diffraction cotton in the electron beam diffraction pattern, the percentage of crystallites thicker inside the fiber than in the outer surface layer is 0.3%, and there is no difference between the inside and outside. It is regarded.

Kasakasa flame 1 The same pitch fiber as in Example 1 was placed in an air atmosphere at a starting temperature of 1.
It was made infusible at 80° C., a final temperature of 290° C., and an A rate of 0.3° C./min.

After the infusibility treatment was completed, carbonization was performed under the same conditions as in Example 1 to obtain carbon fibers with a diameter of about 13 μm. This fiber did not fuse during carbonization.

As shown in Table 1, this carbon fiber has an orientation angle (φ) of 7.
．． 0°, lamination thickness -6°. 2) is 190 people, the layer spacing (aO02) is 3.399 people, and the elastic modulus is 685G.
Pa, and the tensile strength was 2.37 GPa.

A scanning electron micrograph of a cross section of the fiber (FIG. 5) shows no difference in the cross-sectional orientation structure between the inner and outer surface layers. In addition, a dark-field image (
There is no difference in crystallinity between the interior and outer surface layer of the fiber in the bright field image (Figure 6 (a)) and the bright field image (Figure 6 (a)). fact,
As determined by measuring the half width of the (002) diffraction line in the electron beam diffraction pattern, the ratio of crystallites inside the fiber being larger than the outer surface layer is -0.2%, and it is considered that there is no difference between inside and outside.

Comparison 1 This is a commercially available pinch type ultra-high modulus carbon fiber (Union Carbide product U CC-P 100).

A transmission electron micrograph of a cross section of this fiber (FIG. 7) shows that there is no clear difference in cross-sectional orientation structure between the inner and outer surface layers. Furthermore, it is observed that there is no difference between the inner and outer surface layers of the fiber in the dark field image (Figure 8 (a)) and the bright field image (Figure 8 (see)) of the longitudinal cross section of the fiber using a transmission electron microscope. According to the measurement of the half width of the (002) diffraction line in the electron beam diffraction pattern, the proportion of crystallites in the interior of the fiber is larger than that in the outer surface layer is 15%; tends to be small.

The following margins are χ and 5 l.The fiber diameters are 9.6 μm, 11.5 μm, and 12.5 μm・1
Carbon fibers were produced by repeating the same operations as in Example 1 except that the thickness was 4 μm.

Orientation angle (φ) of these carbon fibers, lamination thickness Lc,

Z), the elastic modulus is expressed in relation to the fiber diameter as shown in the graph of FIG. As the fiber diameter increases, the orientation angle (φ) decreases and the lamination thickness decreases. . 2) and the elastic modulus is increasing. This is because the part of the outer surface layer of the fiber that is infusible is almost independent of the fiber diameter, so as the fiber diameter increases, the proportion of the high-crystalline part inside the fiber increases, and as a result, the fiber as a whole This shows that the crystallinity has improved.

〔Effect of the invention〕

According to the present invention, ultra-high modulus fibers of 700 GPa or more can be produced at a lower firing temperature than conventional ones, reducing production equipment costs.
Manufacturing costs can be significantly reduced. Further, a super-elastic fiber is provided which has a unique structure and physical properties in which the inside of the fiber is more uniform in crystallinity than the outer surface layer.

Other benefits include improved tensile strength, and because the product has a larger fiber diameter than conventional products, it improves production efficiency and is easier to handle.

[Brief explanation of the drawing]

Figure 1 is a perspective photograph taken by a scanning electron microscope including a cross section of the carbon fiber obtained in Example 1, and Figures 2 (A) and (B).
3 is a dark-field image and a bright-field image (nine TEM photographs) of the longitudinal section of the carbon fiber obtained in Example 1, respectively, taken by a transmission electron microscope. FIG. A perspective photograph taken by a scanning electron microscope, including a cross section of carbon fiber!
Figures 4 (A) and (B) are dark-field and bright-field images (nine TEM photographs) of the longitudinal section of the carbon fiber obtained in Comparative Example 1, respectively, taken with a transmission electron microscope. 6(A) and (B) are transmission electron micrographs of longitudinal sections of the carbon fibers obtained in Comparative Example 2. A dark-field image and a bright-field image (nine TEN photographs) taken with a microscope, Figure 7 is a perspective photograph taken with a scanning electron microscope 2 containing a cross section of the carbon fiber of Comparative Example 3, and Figure 8 (A). , (A) are a dark-field image and a bright-field image (ordinary TEM photograph) of a longitudinal section of the carbon fiber of Comparative Example 3 taken by a transmission electron microscope, respectively. FIG. 9 is a graph showing the fiber diameter dependence of the characteristics of the carbon fiber of Example 2.

Claims (1)

[Scope of Claims] 1. A pitch-based carbon fiber characterized by having substantially higher crystallinity in the interior of the fiber than in the outer surface layer within the cross section of the fiber. 2. Spinning carbonaceous pitch whose main component is an optically anisotropic part, selectively infusible only the outer surface layer in the cross section of the obtained carbonaceous pitch fiber, and then A method for producing pitch-based carbon fiber, which is characterized by firing fibers in which only the surface layer is infusible.