JP2960290B2 - Method for producing pitch-based ultrafine carbon fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultrafine carbon fibers from pitches. More specifically, the present invention relates to a method for infusibilizing ultrafine pitch fibers, which converts ultrafine pitch fibers into infusible fibers.

[0002]

2. Description of the Related Art In recent years, carbon fiber has been used for its light weight, high strength,
Due to its properties such as heat resistance, corrosion resistance, and good conductivity, its importance is increasing in various industrial fields such as automobiles and aircraft. In particular, pitch-based carbon fibers differ from PAN-based carbon fibers in that the raw material pitch and spinning method can be freely selected, so that carbon fibers with new characteristics can be manufactured, so future development is expected. Have been. For example, JP-A-4-185720 and JP-A-5-106119 disclose pitch viscosities of 0.1-6.
A melt pitch of 0 poise is discharged from the nozzle, and the flow rate from the periphery is 0.1 to 5 km / sec. By blowing out a high-speed gas that reduces the pitch, the fiber diameter becomes 5μ.
m (less than 2 μm in the example) has been proposed. Since it is impossible to produce such ultra-fine fibers with conventional carbon fibers, application to fields where the fiber diameter is too large to be used can be expected.

[0003] Usually, pitch-based ultrafine carbon fibers are produced by the following steps. First, an optically isotropic pitch and an anisotropic pitch prepared from a petroleum-based or coal-based or other crude material are ejected from a nozzle with a viscosity of 0.1 to 60 poise, and a flow rate of 0 from the surroundings. A fine pitch fiber is spun by blowing out a high-speed gas of 0.1 to 5 km / sec to reduce the pitch. The pitch fibers spun in this manner are collected in a sheet or mat shape on a can or a conveyor belt, and then converted into carbon fibers through infusibilization and carbonization steps. The infusibilization is carried out by adding an oxidizing gas to the pitch fiber and subjecting the pitch molecules to cross-linking to form a polymer. Most commonly, an oxidation reaction using air is used. Next, carbon fibers are produced by subjecting the infusibilized fibers to a carbonization treatment at about 600 to 3000 ° C. in an inert gas. By this treatment, volatile components are decomposed and volatilized in the infusibilized fiber, a six-membered ring structure consisting essentially of carbon atoms is developed, and a high-strength carbon fiber is obtained.

[0004]

In order to produce pitch-based ultrafine carbon fibers, all of the above steps are important. In particular, the infusibilization step requires a long processing time and a large quality of the product. This is particularly important as it is prone to affecting troubles. Therefore, in order to economically produce pitch-based ultrafine carbon fibers, it is necessary to efficiently perform the infusibilizing step, and for that purpose, it is necessary to improve at least the following two points. (1) The purpose of the infusibilizing treatment is to change the thermoplastic pitch fibers to thermosetting, thereby preventing the fibers from being deformed in the subsequent carbonization step. Normally, the temperature is gradually increased in an oxidizing gas such as air while heating to cause the oxidation reaction to proceed, thereby cross-linking the pitch molecules to polymerize them, thereby raising the softening point of the pitch fibers, and finally This is performed by imparting thermosetting properties to the fibers. However, since the reaction rate of this oxidation reaction is extremely slow, a phenomenon often called "fusion" occurs during the temperature rise. This fusion occurs when the rate of temperature rise exceeds the rise in the softening point of the pitch fibers due to the oxidation reaction, and refers to a phenomenon in which the fibers are softened and melted by heat and the adjacent fibers are fixed to each other. Even if the fused fiber is carbonized after carbonization, the fibers remain fixed to each other, so that the fiber lacks flexibility and significantly impairs the value as a product. In order to prevent this, it was necessary to perform the treatment at a sufficiently slow heating rate so that the reaction of the pitch fibers with the oxidizing gas was always performed below the softening point of the pitch fibers. For this reason, the time required for infusibilization becomes longer, which is economically disadvantageous. (2) On the other hand, since this oxidation reaction is an exothermic reaction, it is necessary to proceed with the reaction while quickly removing the reaction heat generated in the reaction. If the heat of reaction is not sufficiently removed, the heat of reaction accumulates and the temperature of the fiber rises, eventually causing a runaway reaction and burning the fiber itself. Usually, the heat of reaction is removed by passing an oxidizing gas and its accompanying gas through the fibers. However, since the diameter of the ultrafine carbon fiber is small, the air permeability into the sheet or mat is poor, and when the thickness of the sheet or mat exceeds a certain limit, it is extremely difficult to remove the reaction heat by the ventilation gas. Therefore, in order to infusibilize a sheet or mat made of very fine pitch fibers so as not to cause a runaway reaction, it is necessary to perform gas ventilation with a thin state as much as possible, that is, with a low basis weight. For this reason, the amount of fiber to be treated per unit area of the infusibilizing furnace is smaller than that of the conventional carbon fiber, and the efficiency of the infusibilizing step has to be reduced.

Here, various methods using an oxidizing agent other than air have been conventionally proposed as a method for accelerating the infusibilizing reaction of the pitch fibers in the item (1). For example, a method using NO ₂ gas (for example, JP-B-48-42696 and JP-B-4-2445), a method using SO ₂ gas (for example, JP-A-60-185819), and a method using chlorine gas (For example, JP-B-48-42696, JP-A-49-75828), a method using hydrogen peroxide (for example, JP-A-2-91223), and a method using nitric acid (for example, JP-A-1-201522). Gazette,
JP-A-1-37488) and a method using cobalt bromide (for example, JP-A-60-81319). Among these methods, the method using NO ₂ gas is as follows.
The effect of promoting the infusibilization reaction is large, and the effect of improving the pliability of the fiber after carbonization is also recognized. However, even if this method is used, in the case of ultrafine carbon fiber having poor air permeability, the above (2) Cannot effectively prevent the runaway reaction of
There was a limit on the basis weight in the infusibilization step. Further, when the infusibilizing treatment is performed with NO ₂ gas, a new problem that the strength of the carbon fiber is reduced has occurred.

As described above, the present invention improves the efficiency of the infusibilizing treatment step, which is a problem in the production process of pitch-based ultrafine carbon fibers, and more specifically, the prevention and runaway of runaway reaction in infusibilizing using NO ₂ gas. It is intended to develop a new infusibilization treatment method that achieves the prevention of reduction.

[0007]

SUMMARY OF THE INVENTION In the present invention, in the step of infusibilizing ultrafine carbon fibers using NO ₂ gas, the efficiency of the infusibilization reaction is improved by preventing runaway reaction and preventing a decrease in strength. It is intended to achieve both productization and improvement in product properties.

That is, in the method for producing pitch-based ultrafine carbon fibers according to the present invention, after the pitches are melted, the molten pitch is discharged from a nozzle, and a high-speed gas flow is blown out from around the nozzle to make the pitch into a fibrous form. A method of obtaining pitch-based ultrafine carbon fibers by thinning and spinning, further infusing the pitch fibers, and further carbonizing, wherein the infusibilization is performed according to the following steps (a) and (b). It is characterized by the following.

(A) The spun pitch fibers are treated with a gas containing NO _2, and (B) the pitch fibers thus treated are further heat-treated in an inert gas atmosphere. N remaining in
O ₂ is substantially removed.

Hereinafter, the present invention will be described in more detail.

As the spinning pitch used in the present invention, any of an optically isotropic pitch and an anisotropic pitch prepared from a petroleum-based or coal-based or other crude material can be used. It is preferable that the material has good spinnability, is thermally stable at the spinning temperature, exhibits uniform fluidity, and does not contain solid particles or gas that hinder spinning.

The pitch fiber which can be used in the present invention is obtained by discharging the pitch at a viscosity of 0.1 to 60 poise from a nozzle, and flowing from the periphery at a flow rate of 0.1 to 5 km / sec.
c. High-speed gas is blown out to narrow the pitch.

According to the invention, the pitch fibers are then replaced with NO ₂
Treated with a gas containing The NO ₂ concentration at this time is desirably in the range of 0.1 to 10% by volume, preferably 1 to 5% by volume. If the concentration is lower than this, the reaction with NO ₂ tends to be insufficient, which is not preferable. on the other hand,
If the concentration is too high, the infusibilization efficiency and the physical properties of the product are not affected, and only the processing cost is increased. Further, the treatment at this time can be performed at a temperature of usually 200 ° C. or less, more preferably 100 ° C. or less. If the treatment temperature is too high, a runaway reaction is likely to occur, and the fibers are burned due to abnormal heat generation, which is not preferable. The processing time may be 1 minute to 20 minutes, preferably 3 minutes to 10 minutes. If the treatment time is short, the reaction with NO ₂ does not sufficiently proceed, which is not preferable. Further, a treatment time longer than this has no effect on the infusibilization efficiency or on the improvement of the physical properties of the product, and only increases the treatment cost.

The pitch fiber thus treated with the gas containing NO ₂ is subjected to a heat treatment. This heat treatment is preferably performed in an inert gas atmosphere under a temperature condition sufficient to remove NO ₂ remaining in the pitch fibers and to perform infusibilization, specifically, 450 ° C. or lower, preferably Is preferably 400 ° C. or less, and usually 300 ° C.
It is desirable to perform a heat treatment at a temperature in the range of 400C to 1400C. If the heating temperature at this time is lower than 300 ° C., the infusibilizing treatment becomes insufficient, so that not only fusion occurs in the carbonization treatment of the next step, but also the strength of the carbon fiber is reduced, which is not preferable. If the temperature exceeds 450 ° C., a runaway reaction may occur during the heat treatment, which is not preferable.

When the above-mentioned infusibilization treatment is performed on ultrafine pitch fibers, sufficient efficiency of the infusibilization reaction is recognized. For example, when infusibilizing is performed using air, the weight of the fiber in which the oxidation reaction proceeds without causing a runaway reaction is 80 g / m ² , and the time required for the infusibilizing is about 12 g / m ^2.
In contrast to the case of 0 minutes, the present invention uses 1000 g.
Even with a basis weight of / m ² or more, the treatment can be performed without causing a runaway reaction, and the treatment can be performed in a short time of 20 minutes or less for infusibility. In an inert gas atmosphere,
By performing the heat treatment at 300 to 400 ° C., it is possible to completely prevent a decrease in the strength of the carbon fiber generated when this treatment is not performed. The infusibilized fiber thus obtained is then placed in an inert gas atmosphere at 600 to 300
By heating at 0 ° C., carbon fibers can be obtained.

[0016]

The first feature of the present invention is that ultrafine pitch fibers are treated with N.
It is to perform treatment at a relatively low temperature with a gas containing O ₂ . N
Although the reaction between O ₂ and pitch fiber is extremely complicated, according to the knowledge of the present inventors, when the chemical structure of the NO ₂ -treated pitch fiber is observed by an infrared absorption spectrum, it is present in the fiber before the treatment. No nitro group and carbonyl group were observed, indicating that the nitration reaction and the oxidation reaction were proceeding simultaneously. The oxidation reaction here is based on active oxygen generated by decomposition of NO ₂ . When the reaction between the pitch fiber and NO ₂ was performed at various temperatures, the intensity ratio of the infrared absorption of the nitro group and the carbonyl group was measured. As shown in FIG. Groups tend to increase. This is because the higher the temperature, the more NO ₂ is decomposed and a large amount of active oxygen is generated. At 200 ° C. or higher, the oxidation reaction becomes the main reaction rather than the nitration reaction. From this, NO ₂
It can be seen that when the reaction with is carried out at a high temperature, a runaway reaction occurs due to the heat of the oxidation reaction. Therefore, in order to prevent a runaway reaction in the NO ₂ treatment, it is important to react at a low temperature and to perform a nitration reaction as a main reaction. Of course, the nitration reaction is also an exothermic reaction, but when the treatment temperature is low, the heat of the reaction does not cause the temperature of the fiber to rise to a temperature at which a runaway reaction is caused.

A second feature of the present invention is that ultrafine fibers treated with NO _{2 are} treated in an inert gas atmosphere,
By performing the heat treatment at 0 to 400 ° C., it is to prevent the strength of the carbon fiber from decreasing. Nitro groups have been introduced into the NO ₂ -treated fiber, and most of the nitro groups are eliminated again in the next carbonization step as NO ₂ . Then, the desorbed NO ₂ is decomposed into NO and O at a high temperature to generate active oxygen, so that oxygen is included in the carbonization atmosphere gas. Since the amount of oxygen is very small, in the case of a fiber with a large fiber diameter as in the past, even if it is subjected to an oxidation reaction due to this oxygen, it remains only on the surface of the fiber, so it does not directly affect the strength of the fiber. Absent.
However, since the diameter of the ultrafine fiber is small, the strength is reduced even if only the surface is oxidized. Therefore, in the case of ultrafine carbon fibers, it is important to prevent an oxidation reaction due to desorbed NO ₂ in the carbonization process. According to the findings of the present inventors, NO ₂ treated when measuring the release of NO ₂ to 120 ° C. As shown in FIG. 2 and 350 ° C. in carbonization process of the fiber
It has been found that there are two peaks. That is, most of the NO ₂ is released at 450 ° C. or lower.
At this temperature, the decomposition of NO ₂ into NO and O does not proceed very much, and even if a small amount of oxygen is generated, the oxidation reaction hardly occurs because the temperature is low. Therefore, once heat treatment is performed at a temperature of 450 ° C. or less, NO ₂ that causes a decrease in fiber strength is substantially removed in advance, thereby effectively preventing a decrease in strength due to a subsequent carbonization treatment. I was able to do it.

[0018]

EXAMPLES Hereinafter, the present invention will be described in more detail by way of examples, but the scope of the present invention is not limited to these examples. (Example 1) Optically isotropic spinning having a softening point of 200 ° C from a pitch discharge hole using a nozzle having a discharge hole having a diameter of 0.25 mm and a gas blowing hole having a diameter of 0.65 mm around the discharge hole. A fine pitch fiber having an average fiber diameter of 1.0 μm is spun by discharging a pitch at 320 ° C. at 500 mg / min and blowing out heated air at 400 ° C. at a rate of 14 l / min from a gas blowing hole. It was collected on a 150-mesh wire net placed below.

The basis weight thus obtained is 1000 g.
/ M ² of ultra-fine pitch fiber mat and NO ₂ of 5 Vo
l. % In nitrogen gas at 30 ° C. for 10 minutes, and then the mat was heated in a nitrogen gas atmosphere at 350 ° C. for 10 minutes to perform infusibilization. No fiber fusion or runaway reaction occurred during this treatment. The infusibilized mat was heated in a nitrogen gas atmosphere to 1000 ° C. at a rate of temperature increase of 20 ° C./min to obtain ultrafine carbon fibers. When the obtained carbon fibers were opened in alcohol and the fiber separation properties were confirmed, it was confirmed that the fibers were not adhered to each other and dispersed into individual fibers, and the tensile strength was 80%.
The strength was as high as kg / mm ² . (Comparative Example 1) A basis weight of 1000 g / m obtained in Example 1
² of ultra-fine pitch fibers of the mat, 5Vol the NO _2. %
In a nitrogen gas containing, at a heating rate of 10 ° C./min, 27
Heat treatment was performed to 0 ° C. As a result, the processing temperature is 200 ° C.
The temperature of the fiber abnormally increased from about the time when the temperature exceeded the limit. Comparative Example 2 An ultrafine carbon fiber was obtained by treating the mat treated with the gas containing NO ₂ in the same manner as in Example 1 except that the heat treatment was not performed at 350 ° C. in a nitrogen gas atmosphere. Although the obtained ultrafine carbon fibers had good fiber separation properties, the tensile strength of the single fibers was as low as 45 kg / mm ² .

[0020]

As described above, according to the method of infusibilizing pitch-based ultrafine carbon fibers of the present invention, it is possible to infusibilize ultra-fine pitch fibers having a high weight per unit area in a short time without causing a runaway reaction. No decrease in tensile strength of
Industrially inexpensive production of ultra-fine carbon fibers becomes possible.

[Brief description of the drawings]

FIG. 1. Nitro groups (1527 cm ⁻¹ ) introduced into pitch fibers observed in infrared absorption spectrum when ultrafine pitch fibers are treated with nitrogen gas containing 5% by volume of NO ₂ at various temperatures. And a carbonyl group (1720c
5 is a graph showing the change in the amount of m ^-1 ) with reference to the absorption at 1600 cm ^-1 .

FIG. 2 After treating ultrafine pitch fibers with a nitrogen gas containing 5% by volume of NO ₂ , this is treated in a nitrogen gas at 20 ° C. /
When the heat treatment at a heating rate of min graph showing the amount of NO ₂ desorbed from the fiber.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshima Riki 3-2-39, Wagi-machi, Kuga-gun, Yamaguchi Prefecture (72) Inventor Haruki Yamazaki 26-26 Suzukawa, Isehara-shi, Kanagawa Pref. (72) Inventor Susumu Shimizu 2-6-6 Nihonbashi Kayabacho, Chuo-ku, Tokyo Inside Tanaka Kikinzoku Kogyo Co., Ltd. (56) References JP-A-5-171519 (JP, A) JP-A Sho 62-191515 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) D01F 9/14

Claims (3)

(57) [Claims] After the pitches are melted, the melted pitch is discharged from a nozzle, a high-speed gas flow is blown out from the periphery of the nozzle to make the pitch finer and spun, and the pitch fiber is made infusible. And a method for obtaining pitch-based ultrafine carbon fibers by further carbonizing, wherein the infusibilization is performed according to the following steps (a) and (b). . (A) The spun pitch fibers are treated with a gas containing NO _2, and (B) the pitch fibers thus treated are further heated in an inert gas atmosphere to remain in the pitch fibers. NO ₂ which is substantially removed. 2. A heat treatment step in the infusibilization (b).
At a temperature of 450 ° C. or lower, preferably 300 to 400 ° C.
The method according to claim 1, which is performed under the following temperature conditions. 3. A process of pitch fibers by a gas containing the NO ₂ is, NO ₂ concentration of 1 to 10 volume%, treatment temperature 200 ° C.
The method according to claim 1, wherein the method is performed under the conditions of a processing time of 1 to 10 minutes.