KR960012991B1 - High modulus pitch-based carbon-fiber and method for preparing the same - Google Patents

Abstract

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Description

High modulus pitch carbon fiber and its manufacturing method

1 is a perspective photograph by a scanning electron microscope including a cross section of the carbon fiber obtained in Example 1,

2 (a) and 2 (b) show dark and bright field images (typical TEM images), respectively, by a transmission electron microscope of the longitudinal section of the carbon fiber obtained in Example 1,

3 is a perspective photograph by a scanning electron microscope including a cross section of the carbon fiber obtained in Comparative Example 1,

4 (a) and (b) are dark field images and bright field images (usually TEM photographs), respectively, by a transmission electron microscope of the longitudinal section of the carbon fiber obtained in Comparative Example 1,

5 is a perspective photograph by a scanning electron microscope including a cross section of the carbon fiber obtained in Comparative Example 2,

6 (a) and (b) are dark field images and bright field images (usually TEM photographs), respectively, by a transmission electron microscope of the longitudinal section of the carbon fiber obtained in Comparative Example 2,

7 is a perspective photograph by a scanning electron microscope including a cross section of the carbon fiber obtained in Comparative Example 3,

8 (a) and (b) are dark field images and bright field images (usually TEM photographs), respectively, by a transmission electron microscope of the longitudinal section of the carbon fiber obtained in Comparative Example 3.

9 is a graph showing the fiber diameter dependence of the properties of the carbon fiber of Example 2. FIG.

TECHNICAL FIELD The present invention relates to a high modulus pitch carbon fiber and a method for manufacturing the same, and more particularly, to a pitch carbon fiber produced at a low carbonization temperature and achieving a high modulus. High modulus carbon fiber is a composite material such as plastic, metal, carbon, ceramic, etc. It is used for lightweight structural materials (aircraft, spacecraft, automobiles, buildings, etc.), high temperature materials (brake discs, rockets, etc.), and reinforces metals and ceramics. Used.

Polyacrylonitrile is used as a raw material, and a PAN-based carbon fiber having high tensile strength and a modulus of elasticity is manufactured. Fibers fired at 2000 ° C. or higher may have an elastic modulus of up to about 400 GPa.

However, the PAN-based carbon fiber has a drawback that the raw material cost is high. However, since it is non-graphitizable, there is a limit in improving the degree of crystallinity (graphitization), and it is difficult to attain ultra high modulus.

Pitch-based carbon fiber is not only cheaper in raw materials and more economical, but also manufactured by petroleum-based liquid crystal pitch and fired around 3000 ° C, called graphite fiber, and exhibits ultra-high modulus of about 700 GPa (Special Publication No. 59-3567) ).

In the pitch-based carbon fiber, in order to improve properties such as tensile strength and elastic modulus, in the cross section of the fiber, crystals are arranged in the circumferential direction at the surface layer portion of the fiber, and are arranged radially or mosaically at the center portion. Branched carbon fibers (Japanese Patent Application Laid-Open No. 59-53717), or carbon fibers (particularly Japanese Patent Application Laid-Open No. 60-239520), in which the outer circumferential layer of the fiber forms a radial alignment structure and the inner core forms an onion-oriented structure in order to increase the surface strength. ) Are proposed.

As described above, it is possible to manufacture carbon fibers having an ultra high modulus using liquid crystal pitch, and some methods for improving the properties of the fiber have been proposed. However, in order to achieve ultra high modulus in any of these methods, it is around 3000 ° C. Firing at high temperature is required.

The high temperature firing temperature impedes the reduction of equipment cost and manufacturing cost, and there is an unreasonable point that the tensile strength of the fiber decreases when the firing temperature is increased.

MEANS TO SOLVE THE PROBLEM In the process of earnest research and development in order to obtain the carbon fiber which has super high elastic modulus by baking at low temperature, the present inventors discovered that it is possible by making the crystallinity of carbon fiber higher inside than the outer surface layer part of fiber, and completed this invention. It was.

That is, the present invention is a pitch-based carbon fiber, characterized in that the fiber is composed of the inner and outer surface layer portion, the inside of the fiber has a substantially higher crystallinity than the outer surface layer portion.

In addition, the present invention is a method for producing such pitch-based carbon fibers, in which the carbonaceous pitch fiber having the optically anisotropic portion as a main component is spun off, and the carbonaceous pitch fiber obtained is selectively stabilized or not in the outer cross section of the fiber. The present invention also relates to a method for producing a pitch-based carbon fiber characterized by melting and then carbonizing a fiber in which only its outer surface portion is infusible.

It is known that the elastic modulus is improved when the crystallinity of the carbon fiber is improved. In order to improve crystallinity until the development of ultra high modulus of about 700 GPa, the conventional carbon fiber was required to be fired at a high temperature around 3000 ° C.

In contrast, according to the present invention, it is possible to obtain carbon fibers having an elastic modulus substantially equivalent to that obtained at a firing temperature of around 3000 ° C. by a conventional method at a firing temperature about 500 ° C. lower than that of the prior art.

This is because in the conventional method for producing graphitized carbon fibers, the crystallinity of the spun fibers has been reduced due to its crystallinity being confused when it is infusible. However, in the present invention, the outer surface layer of the pitch fibers is selectively infusified. Thus, while maintaining the minimum incompatibility to prevent the fusion of the fibers during firing, while maintaining the crystallinity of the inside of the pitch fibers at a high level without substantially disturbing, the firing temperature in the conventional method is equivalent to or even lower than in the conventional method. It is because it becomes possible to manufacture the carbon fiber with high elastic modulus mentioned above.

The research on the stabilization and incompatibility of pitch fibers produced by liquid crystal pitch is very limited, and it is considered that the polymerization proceeds by crosslinking reaction by oxidation once, thereby achieving stabilization and incompatibility.

In addition, little research has been conducted on changes in crystal structure during stabilization and incompatibility.

The present inventors have studied the crystal structure change in the stabilization and incompatibility process in detail by X-ray diffraction, and as a result, in the case of pitch fibers having good crystallinity produced at the liquid crystal pitch, they are disturbed during the stabilization and incompatibility process. I found that.

Since the lowering of the crystallinity in the stabilization and incompatibility process also lowers the crystal structure of the carbon fibers after carbonization, it is important to suppress the lowering of the crystallinity to the minimum necessary to obtain a better performance carbon fiber.

The inventors of the present invention also selectively select the outer surface layer of the fiber during the stabilization and infusification process in order to attain both stabilization and incompatibility to prevent the fusion of the fibers in the carbonization process and to minimize the deterioration of the crystal structure. It was found that it should be stable and incompatible.

Since the outer layer part is stable and incompatible, the surface layer dissolves and the fibers are not fused to each other in the subsequent carbonization process, and the fibers are crystallized as a whole because there is little clutter of the crystal structure. The degradation of the structure is minimized.

In this way, carbonaceousness obtained by carbonizing a fiber which is selectively stabilized and infused only in the outer surface layer in the cross section of the pitch fiber is generally higher in crystallinity in the fiber than in the outer surface layer of the fiber in the cross section of the fiber.

The outer surface layer of low crystallinity of carbon fiber is a part which is incompatible to prevent the fiber from fusion during carbonization, so the minimum thickness is good, but even if the thickness is thicker than that, The effect of the present invention is achieved even if it is thicker than the minimum thickness as long as the unimmobilized portion remains.

In addition, the change in the crystallinity of the outer surface layer portion and the inside of the fiber does not need to be large, but may be a gradual change.

Since the required thickness of the outer surface layer to be stabilized and incompatible does not increase depending on the fiber diameter, in general, as the fiber diameter is increased, the ratio of the high crystallinity portion inside the fiber can be increased, and the elastic modulus of the other material is also increased. It can be improved.

The difference in the crystallinity between the outer surface layer and the inside of the carbon fiber depends on the type and quality of the pitch to be emitted, the conditions and degree of incompatibility, the conditions of carbonization, and the like. Bigger

The size of the crystallites is measured by a microdensitometer and the diffraction pattern measured by the limited field electron diffraction method is compared with the reciprocal of the half width of the diffraction intensity. If the difference in crystallinity between the outer surface layer portion and the inside is 10% or less, no effect can be expected.

Next, a description will be given of the above-described method for producing pitch-based carbon fibers according to the present invention. As for the carbonaceous pitch to be emitted, one having a high crystallinity in which an optically anisotropic portion forms a main component is preferable. Japanese Patent Application Laid-Open No. 57-88016 As described in Japanese Patent Application Laid-Open No. 58-45277, 58-37084, etc., the softening point is 230 to 320 캜, the optically anisotropic portion is 90 to 100%, more preferably 97 to 100%, most preferably 99 Although it is carbonaceous pitch of 100 to 100%, it is not limited to this.

Spinning may be by conventional water method, but the above preferred carbonaceous pitch is preferably spun at a constant temperature within the range of 280 to 370 ° C.

Thus, the pitch fiber which is spun and has crystallinity selectively stabilizes and insolubilizes only the outer surface layer in the cross section of the fiber according to the present invention.

For this purpose, the pitch fiber may be infused for a short time within a specific range shorter than usual.

For example, when the pitch fibers having a diameter of 5 µm to 20 µm, preferably 9 µm to 14 µm, obtained by the above-described preferred shape are stabilized and immobilized in air, the temperature at which the stabilization and incompatibility is set to 150 ° C to 200 ° C The temperature is raised to a final temperature of 250 ° C. to 350 ° C. at 1 ° C./min or more, preferably 1 ° C./min to 2 ° C./min, and immediately cooled to room temperature after the temperature increase.

If the temperature increase rate is later than 1 DEG C / min, it takes time to reach the final temperature, and stabilization and incompatibility proceed to the inside of the fiber. Moreover, if it is faster than 2 degree-C / min, a fiber will fuse | melt in the process of incompatibility.

When the temperature increase rate is set at 1 ° C./min to 2 ° C./min within the above temperature range, it is possible to reach the final temperature in a relatively short time without causing fusion, and as a result, stabilization and incompatibility are performed only in the outer surface layer, and the fiber Internal determinism ends without disturbing.

In addition to air, oxygen, ozone, nitrogen dioxide, and the like may be used as a stable and incompatible atmosphere. When a gas having a high oxidative property is used, the temperature increase rate may be performed within such a fast range and the final temperature may be lowered.

The minimum thickness of the outer surface layer to be stabilized and incompatible in order to prevent the pitch fibers from being fused at the time of carbonization depends on the kind of the pitch fibers, the degree of stabilization and incompatibility, etc., but it is considered to be about 1 μm to 3 μm, It was also found that this thickness was not very dependent on the diameter of the fibers.

Thus, the pitch fiber which selectively stabilized and melt | dissolved only the outer surface layer part of a fiber can be baked and carbonized according to a normal method.

In this carbonization, the fiber inside, which has not been stabilized or incompatible, is baked with high crystallinity, so that the crystallinity is higher than that of the outer surface layer of the fiber.

The firing conditions are, for example, a temperature increase rate of 20 ° C./min to 500 ° C./min, a final temperature of 2000 ° C. to 3000 ° C., and a firing time of 4 minutes to 150 minutes.

According to the method of the present invention, an ultra high modulus carbon having an elastic modulus of 700 GPa at a firing temperature of 2600 DEG C or lower, for example, at a firing temperature of 2500 DEG C lower than 3000 DEG C, which is required to achieve an elastic modulus of 700 GPa in a conventional method. Although a fiber is obtained, the firing temperature of the present invention is not limited thereto.

The carbon fiber according to the present invention can achieve ultra high modulus in low temperature carbonization, and also has high tensile strength.

In addition, the carbon fiber according to the present invention has a unique structure in which the inside is better in crystallinity than the outer surface layer portion in the fiber cross section, and thus the characteristics of the carbon fiber can be obtained.

In addition, the carbon fiber according to the present invention can arbitrarily change the characteristics of the carbon fiber obtained by selecting the starting pitch raw material, spinning condition, carbonization condition, and the like, in particular, the fiber diameter and the ratio of the stable / immobilized portion to the whole fiber. There is an advantage.

According to the present invention, an ultra high modulus carbon fiber having an elastic modulus of 700 GPa or more can be produced at a lower carbonization temperature than in the conventional method, so that manufacturing cost and manufacturing cost can be greatly reduced. In addition, the efficiency and handling of carbon fiber production having a larger diameter is also improved compared to the conventional method.

In the Examples, the characteristics of the carbon fiber were adopted as the following parameters and measurement methods.

X-ray Diffraction Parameters

Orientation angle (ø), the laminated thickness (L _C002), interlayer spacing (d ₂₎ is a parameter representing the microstructure of the carbon fiber obtained in the receiver of X-ray diffraction.

The orientation angle ø indicates the degree of selective orientation of the crystal in the fiber axis direction, which means that the smaller the angle, the better the orientation.

The lamination thickness L _C002 represents the thickness of the lamination of the appearance of the (002) plane in the carbon microcrystalline, and the interlayer d ₂ represents the interlayer spacing of the (002) plane of the microcrystalline.

In general, the larger the lamination thickness (L _C002 ) and the smaller the interlayer thickness (d ₂ ), the better the crystallinity is.

The measurement of the orientation angle ø is performed by scanning the counter tube with the fiber bundle perpendicular to the scanning plane of the counter tube using a fiber sample zone. About 26 °).

Next, the strength distribution of the (002) rotation was measured by rotating the fiber sample table 360 ° with the counter tube held at this position, and the half width at the half point of the maximum strength was set at the orientation angle (ø). do.

The lamination thickness (L _C002 ) and the interlayer thickness (d ₂ ) were made into powder in the form of fibers, and the measurement and analysis were carried out in accordance with the Hakjin method "Measuring method of lattice constant and crystallite size of artificial graphite". Obtained from the equation.

K = 1.0

λ = 1.5418 Å

θ = (002) Diffraction angle 2θ

β = half width of the (002) diffraction band obtained by correction

Transmission electron microscope (TEM) observation and electron diffraction

The carbon fiber sample is oriented in the direction of the fiber axis and immersed in a heat-curable epoxy resin to cure.

After curing the resin and trimming the cured carbon fiber embedding block to expose the fiber, ultra-thin section of 1000 angstroms or less using ultra-mirotome equipped with a diamond knife To produce.

The ultra-thin slices are placed on a pressure-sensitive adhesive grid and photographed in the bright field and dark field using an electron microscope.

A bright field image is a normal transmission electron microscope (TEM) photograph, and a dark field image is integrating and imaging the diffraction electron beam in a specific diffraction surface, and observing the aggregation state of the diffraction surface.

The dark field image of Example (002) was the same as the bright field image, and the aggregate state of the (002) diffraction surface was observed by incorporating the diffraction electron beam on the (002) diffraction surface using an objective diaphragm having a diameter of about 10 μm. will be.

In the photograph, the (002) diffraction surface is observed to shine white. Therefore, the thick white part is considered to be a good crystallinity where the (22) crystal plane is well developed.

In order to investigate the inside and outside of the crystallinity in the fiber, the electron beam diffraction image in a specific portion is measured using the limited field electron beam diffraction method.

The measurement conditions were accelerated voltage 200 KV and a limited field aperture of about 1.7 μm in diameter, and electron beam diffraction photographs were taken continuously from edge to edge in a direction perpendicular to the fiber axis of the ultrathin slice.

The obtained diffraction pattern was measured using a microdensimeter to measure the scanning profile of the diffraction intensity in two directions of the equator and the ultraviolet ray with respect to the (002) diffraction line of the electron diffraction image.

The half width (ΔS) of each diffraction intensity of the scanning profile thus obtained is measured. The size L of the determinant is obtained from Scherrer's equation L = K / ΔS. K is a constant.

As apparent from this equation, since the crystallite size is inversely related to the half width on the same diffraction line, the inverse of the half width measured at each measurement contact can be calculated and the size of the crystallites can be compared.

Example 1

A carbonaceous pitch containing about 50% of optically anisotropic phase (AP) is used as the precursor pitch, and this is a cylindrical continuous centrifugal separator with an effective volume of 200 ml in the rotor. In the optically anisotropic phase rich emission pitch.

The obtained optically anisotropic pitch contained 99% or more of optically anisotropic phases and the softening point was 271 degreeC.

Next, the optically anisotropic pitch obtained was spun at 315 DEG C with a melt spinning machine having a nozzle diameter of 0.3 mm.

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

After completion of the incompatibility treatment, the temperature was raised to 100 ° C./min in an argon atmosphere at a final temperature of 2500 ° C. to obtain carbon fiber having a diameter of about 13 μm.

As shown in Table 1, this carbon fiber had an orientation angle (ø) of 6.8 °, a lamination thickness (L _C002 ) of 210 kPa, a layer spacing (d ₂ ) of 3.395 kPa, and an elastic modulus of 736 GPa. The tensile strength was 2.77 GPa.

FIG. 1 is a scanning electron microscope photograph showing the cross section of the obtained carbon fiber, but the difference is recognized in the inner and outer surface layers in the cross-sectional orientation structure.

Fig. 2 (A) shows the (002) dark field image of the longitudinal cross section of the obtained carbon fiber, and the part where the inner side shines more thickly than the outer surface layer part appears thick, but the inner side has a large (002) lamination thickness and crystallization. It is thought that sex is good.

FIG. 2B is a bright field image (typical TEM photograph) by a transmission electron microscope of the same longitudinal section, and this also shows that the inside of the fiber has better crystallinity than the outer surface layer portion.

In fact, the half width of the (220) diffraction line in the electron beam diffraction pattern was measured and obtained from the reciprocal of the half width as described above. The ratio of the crystallite size of the inside of the fiber was larger than that of the outer surface layer was 21%.

Comparative Example 1

The optically anisotropic pitch obtained in Example 1 was sputtered with a melt spinning machine as in Example 1, and the pitch ejection amount was emitted to about 1/2 of Example 1 at 315 ° C.

The obtained pitch fiber was stabilized, insolubilized, and carbonized under the same conditions as in Example 1 to obtain a carbon fiber having a diameter of 9 µm.

As shown in Table 1, the carbon fiber has an orientation angle (ø) of 8.9 °, a lamination thickness (L _Coo2 ) of _{160 kPa} , a layer spacing (d ₂ ) of 3.401 kPa, an elastic modulus of 573 GPa, and a tensile strength of 2.74 GPa. It was.

In scanning electron micrographs (FIG. 3) of the fiber cross section, the difference in the inner and outer surface layers was not recognized in the cross-sectional orientation structure.

In the dark field image (FIG. 4 (a)) and the bright field image (FIG. 4) of the longitudinal cross section of the fiber by a transmission electron microscope, it was recognized that there was no difference in crystallinity in the fiber interior and the outer surface layer.

In fact, when determined from the measurement of the half width of the (002) diffraction line in the electron diffraction pattern, the ratio of crystallites larger than the outer layer part in the fiber is 0.3%, and it is considered that there is no inner and outer difference.

Comparative Example 2

The same pitch fiber as in Example 1 was stabilized and insolubilized in an air atmosphere at a starting temperature of 180 ° C, a final temperature of 290 ° C, and a heating rate of 0.3 ° C / min.

After the completion of the stabilization and insolubilization treatment, carbonization was carried out under the same conditions as in Example 1 to obtain a carbon fiber having a diameter of about 13 m.

As shown in Table 1, this carbon fiber has an orientation angle (ø) of 7.0 °, a lamination thickness (L _C002 ) of 190Å, a layer spacing (d ₂ ) of 3.399Å, an elastic modulus of 685GPa, and a tensile strength of 2.37GPa. It was.

In the scanning electron micrograph (FIG. 5) of the cross section of a fiber, the difference of an inner part and an outer surface part in a cross-sectional orientation structure is not recognized.

In addition, even in the dark field image (Fig. 6 (a)) and the clear shape (Fig. 6 (b)) of the longitudinal section of the fiber by the transmission electron microscope, the difference in crystallinity between the inside and the outer surface layer of the fiber is not recognized.

In fact, from the measurement of the half width of the (002) diffraction line in the electron beam diffraction pattern, it is considered that the ratio of the crystallite size of the inside of the fiber is larger than that of the outer surface layer is 0.2% and there is no internal and external difference.

Comparative Example 3

This is a commercially available pitch-based ultra-high elastic carbon fiber (commercial carbide carbide product UCC-P 100).

Scanning electron microscopy (FIG. 7) of the cross section of this fiber indicates that there is no clear difference between the inner and outer layers in the cross-sectional orientation structure.

In addition, it is recognized that there is no difference in the inner and outer surface portions of the fiber even in the dark and dark image (Fig. 8 (a)) and the bright field image (Fig. 8 (b)) of the longitudinal cross section of the fiber by the transmission electron microscope.

In the measurement of the half width of the (002) diffraction line in the electron beam diffraction pattern, the proportion of crystallites in which the inside of the fiber is larger than the outer surface layer is 5%, and the inside tends to be slightly smaller in the crystallite size.

TABLE 1

Example 2

Except having made the fiber diameter 9.6 micrometers, 11.5 micrometers, 12.5 micrometers, and 14 micrometers, operation similar to Example 1 was repeated and carbon fiber was produced.

The orientation angles (?), Lamination thickness (L _C002 ), and modulus of elasticity of these carbon fibers are shown in the graph of FIG. As the fiber diameter increases, the orientation angle? Decreases, and the lamination thickness L _C002 and the elastic modulus increase.

This is because the portion of the elastic modulus outer layer stabilized and insolubilized hardly depends on the fiber diameter, so that the proportion of highly crystalline parts inside the fiber increases, such as an increase in the fiber diameter, and as a result, the crystallinity as a whole fiber is increased. It is showing that it is improved.

Claims (2)

The fiber is made of carbonaceous pitch composed of at least 90% of optically anisotropic components, and consists of an inner and outer layer part, the fiber having an average size of crystallites in which the inner part is at least 10% larger than the outer layer part, and the outer layer part of the fiber A pitch-based carbon fiber having a thickness in the range of 1 to 3 µm and having an elastic modulus of at least 700 GPa. In order to obtain carbonaceous pitch fibers, the carbonaceous pitch composed of 90% or more optically anisotropic components is spun off, and the carbonaceous pitch fibers are placed in an oxidizing atmosphere to oxidize only the outer layer portion of the carbonaceous pitch fibers, but not the inside thereof. The average size of the crystallites in the outer surface layer of the fiber is selectively stabilized and insolubilized in the outer surface layer of the carbonaceous pitch fiber having a thickness in the range of 1 to 3 μm, followed by carbonization of the selectively stabilized and insoluble carbonaceous pitch fiber. A process for producing pitch-based carbon fibers, characterized in that it produces a carbon fiber having an average size of crystallites at least 10% higher inside.

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