JPH06146120A - High-strength, high-modulus pitch-based carbon fiber and method for producing the same - Google Patents

Abstract

(57) [Abstract] [Purpose] To obtain a pitch-based carbon fiber having high tensile strength and tensile elastic modulus and little variation in the strength and elastic modulus. [Structure] The pitch fiber is obtained by pretreating the pitch fiber in an oxidizing gas atmosphere at a temperature of 130 to 220 ° C. for an extremely short time and then infusibilizing it in a oxidizing gas atmosphere at a temperature rising rate of 2 to 40 ° C./min. The maximum temperature of the infusible fiber in an inert gas atmosphere is 400 ~
Up to 500 ° C, the temperature rising rate was changed in the range of 10 to 400 ° C / minute to raise the temperature, the maximum temperature was maintained for 1 to 60 minutes to carry out preliminary carbonization, and then carbonization. [Effect] The pitch-based carbon fiber after carbonization by the preliminary carbonization under the above conditions has a roundness of 0.70 to 0.90.
In addition to exhibiting a reduced elliptical cross-sectional shape, the variation in circularity is as small as 5% or less in the coefficient of variation, and high tensile strength and high tensile elastic modulus are developed, and variations in the strength and elastic modulus are also exhibited. Few.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to pitch-based carbon fibers, particularly high-strength, high-modulus pitch-based carbon fibers having an elliptical cross-sectional shape and a method for producing the same. In this specification,
Unless otherwise specified, "carbon fiber" includes not only carbon fiber but also graphite fiber.

[0002]

2. Description of the Related Art Pitch-based carbon fibers produced from carbonaceous pitch such as petroleum-based pitch and coal-based pitch are carbonized in comparison with rayon-based and PAN-based carbon fibers which are currently produced in the largest amount. It has been attracting attention in recent years because it has a high yield, excellent physical properties such as elastic modulus, and has an advantage that it can be manufactured at low cost.

At present, pitch-based carbon fibers are obtained by (1) preparing meso-face pitch suitable for carbon fibers from petroleum-based pitch, coal-based pitch, etc., heating and melting the pitch, and spinning with a spinning machine. After the obtained pitch fibers are converged to form a fiber bundle, (2) the pitch fibers are heated to 250 to 350 ° C. in an infusible furnace under an oxidizing atmosphere to be infusibilized,
(3) The obtained infusible fiber is pre-carbonized by heating it to 400 to 1300 ° C. in an inert gas atmosphere in a pre-carbonization furnace, and (4) the pre-carbonized pre-carbonized fiber is then placed in a carbonization furnace. It is manufactured by heating to 3000 ° C. or lower in an inert gas atmosphere to carbonize or graphitize.

[0004]

According to the pitch-based carbon fiber produced as described above, a fairly high level of tensile strength and tensile elastic modulus is obtained, but higher strength,
The advent of high modulus carbon fibers is desired.

The present inventors have conducted extensive studies to obtain pitch-based carbon fibers having higher tensile strength and tensile modulus.
As a result, we found the following.

That is, the cross-sectional shape of the pitch-based carbon fiber is elliptical (roundness 0.70 to 0.90), and the cross-sectional shape has a uniform cross-sectional shape with less variation. It was found that high tensile strength and tensile modulus can be expressed.

These pitch-based carbon fibers are extruded and spun by a melt spinning machine having a circular porous nozzle,
After pre-treating the spun pitch fiber for an extremely short time, the pitch fiber is infusibilized in an oxidizing gas atmosphere at a temperature of 130 to 220 ° C. for an extremely short time, and the pitch fiber is 2 to 40 in an oxidizing gas atmosphere. Further infusibilized at a temperature rising rate of ° C / min, and the resulting infusibilized fiber has a maximum temperature of 40
The temperature rising rate from 0 to 500 ° C is changed in the range of 10 to 400 ° C / minute to raise the temperature, the maximum temperature is maintained for 1 to 60 minutes to perform pre-carbonization, and subsequently the temperature rising rate is 200 ° C / minute. At a maximum temperature of 600 to 1300 ° C to further pre-carbonize,
It is then obtained by carbonizing.

Usually, the nozzle of the spinneret used for melt-spinning of the pitch is often a nozzle having a circular nozzle hole because of the ease of nozzle processing. Up to the carbon fiber to be obtained, its cross-sectional shape is circular, but by undergoing preliminary carbonization under the above conditions, the obtained carbon fiber becomes an elliptical cross-sectional shape,
In addition, the variation in the cross-sectional shape becomes a uniform elliptical shape with a variation coefficient value of 5% or less, and by doing so, the tensile strength and the tensile elastic modulus are significantly increased. We have found the above.

The present invention has been made based on the above findings,
It is an object of the present invention to provide a pitch-based carbon fiber having high strength and high elastic modulus and less variation in the strength and elastic modulus, and a method for producing the same.

[0010]

The above object can be achieved by the high-strength, high-modulus pitch-based carbon fiber and the method for producing the same according to the present invention. In summary, the present invention is characterized by having a uniform elliptical cross section in which the circularity of the cross section is 0.70 to 0.90 and the variation in the circularity is 5% or less in the variation coefficient value. It is a high strength, high elastic modulus pitch-based carbon fiber.

According to the present invention, the spun pitch fiber is pretreated in an oxidizing gas atmosphere at a temperature of 130 to 220 ° C. for a very short time, and then the pitch fiber is subjected to
The infusible fiber obtained is infusibilized at a temperature rising rate of up to 40 ° C / min, and the temperature rising rate up to 400-500 ° C in the inert gas atmosphere is changed within the range of 10-400 ° C / min. To raise the temperature, hold at the maximum temperature for 1 to 60 minutes, and pre-carbonize,
Continuously, the maximum temperature is 600 at a heating rate of 200 ° C / min or more.
It is a method for producing a high-strength, high-modulus pitch-based carbon fiber, which comprises heating up to ˜1300 ° C., further pre-carbonizing, and then carbonizing.

The present invention will be described in detail below.

The pitch-based carbon fiber of the present invention has an elliptical cross section and has a roundness of 0.70 to 0.90.

In the present invention, the circularity of the fiber cross section means
It is defined as the ratio of the length of the minor axis to the length of the major axis of the ellipse fitted to the fiber cross section, ie, roundness = minor axis length / minor axis length.

The method for measuring the roundness of the fiber cross section is as follows. After embedding carbon fiber in polyester resin, exposing and polishing the cross section of the carbon fiber, take a 800 times cross-section photograph of the fiber with a scanning electron microscope (SEM),
It is determined by measuring the major axis and minor axis of the fiber cross section on the photograph.

In the present invention, 20 fibers were randomly selected and the roundness was measured, and the average of the roundness and the variation in the roundness (variation coefficient value) were obtained for the 20 fibers.

The variation coefficient value (CV value) is obtained from the equation CV value (%) = S / x ₀ × 100, where S is the standard deviation of the circularity and x _{0 is} the average value of the circularity.

The pitch-based carbon fiber of the present invention exhibits an elliptical cross section having a roundness of 0.70 to 0.90, and a variation coefficient value (CV value) of the roundness of 5% or less, which causes little variation. Therefore, a tensile strength of 350 kg / mm ² or more and a tensile elastic modulus of 50 ton / mm ² or more can be easily obtained. As a result, variations in strength and elastic modulus are small, and variations in strength and elastic modulus of the pitch-based carbon fiber of the present invention are 3% or less and 1% or less in CV value, respectively.

When the roundness of the fiber cross section is less than 0.70, the variation coefficient value of the variation of the cross sectional shape cannot be 5% or less, and when it exceeds 0.90, the strength and elastic modulus are not expressed. Since the cross-sectional shape cannot be sufficiently elliptical, desired high tensile strength and tensile modulus cannot be obtained.

Such carbon fibers are produced as follows, and of these, the conditions for pre-carbonizing the infusible fibers are particularly important.

First, as pitch, various heavy oils of petroleum type, pyrolysis tar, catalytic cracking tar, heavy oil obtained by dry distillation of coal, tar and the like are used as raw materials, and meso is obtained by pyrolysis polycondensation. A face pitch (optically anisotropic pitch) can be used, but it is preferable to use catalytic cracking tar as a raw material.

More preferable pitch is hydrogen fluoride,
A mesophase pitch obtained by polymerizing aromatic hydrocarbons (condensed polycyclic hydrogen or a pitch raw material containing these) in the presence of a boron trifluoride catalyst can be used.

Further, the mesophase pitch is obtained by polymerizing mesophase pitch obtained by the above-mentioned catalytic decomposition polycondensation and aromatic hydrocarbons as raw materials in the presence of hydrogen fluoride or boron trifluoride catalyst. It is possible to use a meso-face pitch mixed with the meso-face pitch provided.

The mesoface pitch content of the pitch used as the mesoface pitch is preferably 70 to 100%, more preferably substantially 100% mesoface pitch.

The softening point of mesophase pitch is 180 to
Although those having a temperature of 400 ° C. can be used, those having a softening point of 250 ° C. or higher, and more preferably 270 ° C. or higher are preferable in the present invention.

Such a pitch is spun into pitch fibers by a known melt spinning method. For example, diameter 0.1-0.5m
From a spinneret having about 100 to 2000 m nozzle holes, molten pitch held at a predetermined temperature of 280 to 370 ° C. is extruded into an inert gas to spin pitch fibers. The spun pitch fibers are directly converged into a pitch fiber bundle.

Prior to infusibilization, the pitch fiber thus obtained is preheated for a very short time by rapidly raising the temperature in an oxidizing gas atmosphere at a temperature of 130 to 220 ° C.

Oxidizing gases include ozone, NOx, S
Air containing halogen gas such as Ox and chlorine is used. A mixed gas of air and oxygen containing ozone, NOx, SOx may be used, or an oxygen gas containing ozone, NOx, SOx may be used.

When ozone is used, the ozone content in the oxidizing gas atmosphere containing ozone must be 0.1 wt% or more in order to obtain a sufficient reaction effect, but if it exceeds 10 wt%, excessive reaction will occur. Since it is apt to occur and problems such as the preliminary infusibilization purpose being not achieved occur, 0.1 to 10
The range of wt% is good, and preferably 0.3 to 5 wt%.

The atmosphere for the pretreatment has an oxygen concentration of 20 to
You may use 100% of oxidizing gas. As this example, air, a mixed gas of air and oxygen, or oxygen gas is used.

The treatment temperature in the infusibilizing pretreatment is 130 to 220 ° C. as described above. If the reaction temperature is lower than 130 ° C., the reaction rate is slow, and it is difficult to selectively oxidize only the surface of the pitch fiber. On the contrary, if the reaction temperature is higher than 220 ° C., the pitch fiber causes fusion, which is not preferable.

The treatment time is 7 minutes or less, preferably 0.5 to 5 minutes, since it is necessary to react only the skin layer.

Next, the above-mentioned pretreated pitch fiber is 2 to 40 ° C./minute, preferably 4 to 20 in an oxidizing gas atmosphere.
The temperature is raised to 250 to 350 ° C. at a temperature rising rate of ° C./min to infusibilize.

An oxygen concentration of 20 is used as the oxidizing gas atmosphere.
~ 100% oxidizing gas is used.

The infusibilizing treatment temperature is a maximum temperature of 250 to 3
It is 50 ° C. If the maximum temperature is less than 250 ° C, the pitch fibers cannot be effectively made infusible, while if it exceeds 350 ° C, the inside of the fibers is oxidized and the surface of the fibers is not infusibilized.

The rate of temperature increase up to the maximum temperature is required to be 2 ° C./minute or more in order to make infusible rapidly, but 40
If the temperature exceeds ° C / minute, the oxidation of the fiber does not proceed, so it is set to this value or less.

The infusibilizing time varies depending on the gas atmosphere used, but 20 minutes or less is usually sufficient.

The infusible fiber thus obtained has a circular cross-sectional shape, which is the same as the cross-sectional shape of the pitch fiber obtained by melt spinning.

The infusible fiber obtained as described above is
Next, the maximum temperature is 400-500 ℃ in an inert gas atmosphere.
Up to 10 to 400 ° C./min in heating rate, especially 430
The temperature is raised by changing the temperature rising rate up to 0 ° C., and the maximum temperature is maintained for 1 to 60 minutes, and then the maximum temperature is increased to 600 to 1300 ° C. at a temperature rising rate of 200 ° C./minute or more. This pre-carbonizes the infusible fiber.

In the present invention, by pre-carbonizing the infusible fiber under such conditions, the skin layer of the outer shell is deformed into an elliptical shape, and the variation in cross-sectional shape is reduced to 5% or less in terms of the coefficient of variation. It is what makes me.

In order to control the cross-sectional shape by the above preliminary carbonization, the temperature rising rate up to the maximum temperature of 400 to 500 ° C.,
In particular, the rate of temperature increase up to 430 ° C. is extremely important. At a maximum temperature of 400 to 500 ° C., particularly at a temperature rising rate of up to 430 ° C. of less than 10 ° C./minute, not only does the pre-carbonization treatment take time, but the circularity of the cross-sectional shape exceeds 0.90 and the tensile strength and It is not preferable because the development of the tensile elastic modulus is deteriorated.
On the other hand, if the heating rate up to the maximum temperature exceeds 400 ° C / min, the circularity of the cross-sectional shape will be less than 0.70, the deformation will be too large and the variations will increase, and the tensile strength and tensile modulus will decrease. Similarly, it is not preferable because it is connected.

The maximum temperature is 400 to 500 ° C., especially 430
The holding time at a temperature of up to ° C is extremely important in eliminating variations in cross-sectional shape. If the holding time is short, the variation in the circularity of the cross-sectional shape will be 5% or more in the coefficient of variation,
It is not preferable because the tensile strength and the tensile modulus decrease.
Further, if the holding time is long, the pre-carbonization treatment time becomes long, which is not efficient. Therefore, the maximum temperature is 400-5
Hold at temperatures up to 00 ° C, especially 430 ° C, is 1-60
It is carried out for a time of minutes, preferably 5 to 20 minutes.

The starting temperature of pre-carbonization is not particularly limited,
Preferably, the maximum temperature for infusibilization is the same or lower.

The fibers pre-carbonized as described above are continuously heated at a temperature rising rate of 200 ° C./min or more to 600 to 1200 ° C.
The temperature is raised up to further pre-carbonization.

The pre-carbonized fiber obtained by the above pre-carbonization has a maximum temperature of 3000 ° C. in an inert gas atmosphere.
The carbon fiber is carbonized to obtain a carbon fiber and, if necessary, further graphitized to obtain a graphite fiber.

The carbon fiber of the present invention is manufactured as described above, and has a structure in which the fiber cross section is formed in a long wave shape by undergoing the pre-carbonization process under the above-mentioned unique conditions. The carbon fiber is manufactured as described above, and by undergoing the pre-carbonization process under the above-mentioned unique conditions, it exhibits an elliptical cross section with a circularity of 0.70 to 0.90,
In addition, the variation of the roundness is 5% or less in the variation coefficient value, and it is possible to easily obtain the improved physical property values of the tensile strength of 350 kg / mm ² and the tensile elastic modulus of 50 ton / mm ² or more. As a result, Dispersion of tensile strength and tensile modulus,
The variation coefficient values can be reduced to, for example, 3% or less and 1% or less.

The present invention will be further described below with reference to examples.

[0048]

【Example】

Example 1 Obtained by thermal decomposition polycondensation using catalytic cracking tar as a raw material,
The mesophase pitch for carbon fiber having a softening point of 267 ° C. and consisting of 97% of optically anisotropic phase was passed through a melt spinning machine having a spinneret provided with 500 nozzle holes (circular, diameter 0.3 mm), and the temperature was changed. Pitch fibers were melt spun by extrusion at 350 ° C. Extruded pitch fibers are combined into 3000
It was a pitch fiber bundle of filaments.

Prior to infusibilization, the pitch fiber was
It was introduced into an air atmosphere at 90 ° C. and pretreated for 3 minutes.

Then, the pretreated pitch fiber was introduced into an oxygen-rich atmosphere of oxygen / nitrogen = 60/40 and heated at a temperature rising rate of 8 ° C./minute from 190 ° C. to 300 ° C. to infusibilize it. . The infusibilization time was 14 minutes. The cross section of the infusible fiber thus obtained had a circularity of 0.99.

Next, the infusible fiber was introduced into a nitrogen-containing atmosphere, the temperature was raised from 300 ° C. to 425 ° C. at 200 ° C./min, and then the temperature was kept at 400 ° C. for 10 minutes to carry out a preliminary carbonization treatment.

Then, 2000 at a temperature rising rate of 200 ° C./min.
The temperature was raised to ° C to obtain carbon fiber.

The roundness of the cross section of the carbon fiber thus obtained was 0.80, and the CV value of the variation in roundness was 4.0%.

The tensile strength according to JIS R7601 is 36.
The tensile modulus was as high as 5 kg / mm ² and 58 tons / mm ^2, and the variation in tensile strength was 1.2% (CV value) and the variation in tensile modulus was 0.8% (CV value), which were small.

Example 2 Infusible fibers were introduced into a nitrogen atmosphere, and the temperature was raised from 300 ° C to 42 ° C.
The temperature is raised to 5 ° C at 400 ° C / min and then at 400 ° C for 10
The same process as in Example 1 was carried out except that it was held for a minute.

The roundness of the cross section of the carbon fiber thus obtained was 0.72, and the variation in roundness was 4.5%.
(CV value).

The tensile strength according to JIS R7601 is 38.
The tensile elastic modulus was as high as 0 kg / mm ² and 60 ton / mm ^2, and the variation in the tensile strength was 2.4% (CV value) and the variation in the tensile elastic modulus was 0.9% (CV value), which were small.

Example 3 Infusible fibers were introduced into a nitrogen atmosphere, and the temperature was raised from 300 ° C to 42 ° C.
The same treatment as in Example 1 was performed except that the temperature was raised to 5 ° C at 50 ° C / min, and then the temperature was kept at 400 ° C for 10 minutes.

The roundness of the cross section of the carbon fiber thus obtained was 0.92, and the variation in the roundness was 3.1%.
(CV value).

The tensile strength according to JIS R7601 is 35.
The tensile elastic modulus was as high as 1 kg / mm ² and 56 tons / mm ^2, and the variation in the tensile strength was 2.1% (CV value) and the variation in the tensile elastic modulus was 0.4% (CV value), which were small.

Comparative Example 1 The infusible fiber was heated from 300 ° C. to 425 ° C. at a rate of 200 ° C./min.
No hold for 0 minutes was performed. Other than that was processed like Example 1.

In this case, the roundness of the cross section of the obtained carbon fiber was 0.81, but the variation in the roundness was 21%.
(CV value), which was extremely variable.

The tensile strength based on JISR7601 is 33.
The tensile elastic modulus was as low as 0 kg / mm ² and 48 ton / mm ^2, and the variation in tensile strength was 10% (CV value) and the variation in tensile elastic modulus was as high as 6.1% (CV value). It was

[0064]

As described above, according to the present invention, pre-carbonization of the infusible fiber is carried out under specific conditions, whereby the pitch-based carbon fiber obtained after carbonization exhibits an elliptical fiber cross section and a true carbon fiber. The variation in circularity is 5 in the coefficient of variation
%, The tensile strength and tensile elastic modulus are high, and the variations in strength and elastic modulus are small.

Claims (2)

[Claims] 1. A high feature characterized by having a uniform elliptical cross section in which the circularity of the cross section is 0.70 to 0.90 and the variation in the circularity is 5% or less in the variation coefficient value. Strength,
High modulus pitch-based carbon fiber. 2. A spun pitch fiber at a temperature of 130-2.
After pretreatment for a very short time in an oxidizing gas atmosphere at 20 ° C., the pitch fibers are heated at 2 to 40 ° C. in an oxidizing gas atmosphere.
The temperature of the infusible fiber obtained is infusibilized at a heating rate of 10 / min, and the temperature of the obtained infusible fiber is increased in the inert gas atmosphere by changing the heating rate from the maximum temperature of 400 to 500 ° C within the range of 10 to 400 ° C / min. Then, the maximum temperature is maintained for 1 to 60 minutes to carry out preliminary carbonization, and then the maximum temperature is 600 to 130 at a heating rate of 200 ° C./minute or more.
A method for producing a high-strength, high-modulus pitch-based carbon fiber, which comprises heating to 0 ° C., further pre-carbonizing, and then carbonizing.