JPH07292526A - Production of acrylic carbon fiber - Google Patents

Abstract

PURPOSE:To provide a method for mass-producing carbon fibers from acrylic fibers as a base in a short time with good productivity while suppressing the deterioration in strength or elastic modulus in carbonization. CONSTITUTION:This method for producing carbon fibers is to heat-treat acrylic fibers comprising >=85wt.% acrylonitrile and <=15wt.% monomers copolymerizable with the acrylonitrile at 220-300 deg.C in an atmosphere of an inert gas at 0.01-3vol.% oxygen concentration, then heat-treat the resultant fibers at 220-320 deg.C in an atmosphere of an inert gas at >=8vol.% oxygen concentration and subsequently carry out the carbonization treatment in nitrogen at 1400 deg.C.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing carbon fibers from acrylic fibers. More specifically, it relates to a method for producing a large amount of carbon fibers in a short time.

[0002]

2. Description of the Related Art Carbon fibers obtained from conventional acrylic fibers are excellent in mechanical properties such as specific strength and specific elastic modulus.
It is widely industrially manufactured because it can be manufactured with uniform and stable quality and performance. Generally, a process for producing carbon fibers from acrylic fibers is performed by a flame-proofing step in which acrylic fibers are heated in advance in an oxidizing atmosphere such as air, and by further heating in a high-temperature inert gas atmosphere, the flame-resistant fibers are carbonized. It consists of a carbonization process to convert to fibers or a graphitization process. Among the above-mentioned carbon fiber industrial manufacturing processes, the flameproofing process is considered to be an extremely important process that affects productivity. That is, since the flameproofing reaction is an exothermic reaction, if a large amount of acrylic fiber is supplied to the flameproofing process at one time in order to increase the heating temperature in order to accelerate the flameproofing reaction rate or to increase the productivity, it takes a unit time. When the amount of heat generated per unit becomes excessively large and heat removal cannot catch up, a heat storage phenomenon occurs, causing a problem of reaction runaway and uncontrollability. In addition, if the heating temperature is too high, even if the runaway does not occur, the single fibers are fused to each other, which causes the mechanical properties of the finally obtained carbon fibers to deteriorate. In other words, conventionally, in order to control heat generation so that runaway or single fiber fusion does not occur, it is necessary to slowly proceed the reaction,
Therefore, there is a problem that it takes a long time for the flameproofing process and it is difficult to improve the productivity of the carbon fiber.

The flame-resistant reaction of acrylic fibers burns even when exposed to the flame of a match or a gas burner by oxidizing the polymer chains constituting the fibers and cyclizing the nitrile groups bonded to the polymer chains. Instead, it is converted into fibers having a thermally stable structure to the extent that it can pass through the subsequent carbonization step. In the heat treatment in a pure inert atmosphere, mainly the cyclization reaction selectively progresses, the reaction heat generated is small, and there is a possibility that a thick yarn or a large number of yarns can be heat treated at one time. The cyclization reaction rate is slower than in the air, the strength of the flame-resistant yarn decreases, and the mechanical properties such as the strength and elastic modulus of the obtained carbon fiber are increased even if only such a treatment is performed as the oxidization step. However, there are problems such as inferiority of carbon and low carbonization yield. Therefore, in order to shorten the flame-proofing process without deteriorating the physical properties of carbon fiber, Japanese Patent Publication No. 53-22576 and Japanese Patent Publication No. 58-2
14535, Japanese Patent Application Laid-Open No. 58-174630 discloses that a flame-resistant fiber is produced by performing a flame-proofing reaction in the air halfway and then heat-treating for a short time in an inert atmosphere in which the oxygen concentration is adjusted. Was proposed. However, even in such a method, as the first step, since the flame resistance reaction is carried out in the air, the initial generated heat remains large, and a large number of thick yarns or a large number of yarns are processed in a short time. The problem was that it was difficult.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to make an acrylic fiber flame resistant in a short time in a large amount, and to improve physical properties as compared with carbon fibers obtained by the prior art. It is an object of the present invention to provide a method for producing carbon fiber having a carbonization yield comparable to that of the carbon fiber.

[0005]

In order to solve the above problems, the present invention has the following constitution. That is, it is a method for producing an acrylic carbon fiber, characterized in that the acrylic fiber is heat-treated in an atmosphere having an oxygen concentration of 0.01 to 3% by volume, then heat-treated in an active atmosphere, and then carbonized.

The acrylic fiber in the present invention may be a fiber made of polyacrylonitrile. The composition is preferably 85% by weight or more of acrylonitrile and 15% by weight or less of a monomer copolymerizable with acrylonitrile. As the polymerizable monomer, acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, ammonium salts and alkyl esters, acrylamide, methacrylamide and their derivatives, allyl sulfonic acid, methallyl sulfonic acid and those Examples thereof include salts or alkyl esters. Further, it is preferable to copolymerize a polymerizable unsaturated monomer such as unsaturated carboxylic acid which promotes the flameproofing reaction. Specific examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid,
Mesaconic acid etc. can be mentioned. As a method for polymerizing the polyacrylonitrile, conventionally known methods such as suspension polymerization, solution polymerization and emulsion polymerization can be adopted.

As the solvent of polyacrylonitrile used for spinning, known organic and inorganic solvents can be used. Preferred spinning methods include a wet spinning method of spinning directly into a coagulation bath, a dry-wet spinning method of spinning once into air and then solidifying in a bath, or a dry spinning method, and further a melt spinning method. . When using a spinning method using a solvent and a plasticizer, the spun yarn may be directly drawn in a bath,
The film may be stretched in the bath after washing with water to remove the solvent and plasticizer. Dry densification is achieved by drying the yarn after drawing in the bath with a hot drum or the like. The drying temperature, time, etc. can be appropriately selected. If necessary, the dried and densified yarn may be stretched at a higher temperature (for example, in pressure steam). Further, prior to the drying and densification, it is preferable to add a silicone oil agent having high heat resistance in order to prevent adhesion between single fibers during firing. In the production of carbon fibers, it is preferable that the carbon fibers are thin so as not to cause burning spots in the flameproofing step, and the single fiber fineness is preferably 1.5 d or less, more preferably 1.0 d or less, and further preferably 0.8 d or less Is desirable.

In the present invention, the acrylic fiber is first heat treated in an inert atmosphere as a flameproofing step.
At that time, the concentration is 0.01 to 3% by volume, preferably 0.1.
Oxygen of 01 to 1% by volume is mixed. Examples of the inert atmosphere include argon, helium, nitrogen and the like, and a mixture thereof may be used, and is not particularly limited as long as the oxygen concentration can be controlled to the above-mentioned concentration. In the heat treatment in an inert atmosphere having an oxygen concentration of less than 0.01% by volume, the cyclization reaction selectively proceeds, but the reaction rate is low, the treatment time becomes long, and the flame resistant yarn strength decreases. On the other hand, when the oxygen concentration exceeds 3% by volume, the heat of reaction generated is rapidly increased, and a runaway reaction is likely to occur, so that the processing temperature cannot be increased to improve the reaction rate. In particular, the upper limit of the oxygen concentration in this case must be strictly controlled in order to suppress the runaway reaction.

The temperature of the heat treatment in an inert atmosphere with a slight oxygen concentration varies depending on the number of yarns and the fineness of the acrylic fiber, but is 220 to 300 ° C., preferably 230 to 27.
It is preferably 0 ° C. If it is less than 220 ° C, the treatment time becomes long, and if it is 300 ° C or more, the yarn may become brittle. The treatment time varies depending on the treatment temperature, the oxygen concentration, the number of yarns, etc., but it is desirable that the treatment is performed until the specific gravity of the yarn becomes 1.25 or more, more preferably 1.30 or more. It should be noted that applying tension in the treatment here is effective for increasing the strength and elastic modulus of the carbon fiber.

The acrylic fiber that has been heat-treated in an inert atmosphere containing a trace amount of oxygen is then heat-treated in an active atmosphere containing an oxidizing gas. Since the acrylic fiber cyclized in the inert atmosphere is in a state of being easily oxidized, the required oxidation can be achieved by performing the heat treatment for a short time in the active atmosphere. Examples of the oxidizing gas include nitrogen dioxide, sulfur dioxide, and oxygen. These may be used alone or in combination, or may be used as a mixed gas of these gases and an inert gas. From the viewpoint of gas toxicity, it is preferable to use a mixed gas of oxygen and an inert gas. From the viewpoint of cost, a mixed gas of oxygen and nitrogen, particularly air, is most preferably used. If the concentration of the oxidizing gas is too low, the treatment time may be long. Therefore, the concentration is preferably 8% by volume or more, more preferably 15% by volume or more. On the other hand, if the concentration of the oxidizing gas is too high, the runaway temperature becomes low and the process may be in a dangerous state. Therefore, the concentration of the oxidizing gas is preferably 25% by volume or less.

Regarding the heat treatment temperature in the active atmosphere,
Although it depends on the number of yarns, the fineness, the concentration of oxidizing gas, etc., if it is too low, the reaction time becomes long, and if it is too high, a runaway reaction may occur easily.
The temperature range is preferably in the range of ℃, more preferably 230 to 280 ℃. In this heat treatment, it is preferable to apply tension in order to increase the strength and elastic modulus of the carbon fiber. The treatment time varies depending on the treatment temperature, the concentration of oxidizing gas, etc.
When the treatment is performed at 280 ° C., it is sufficient to perform the treatment for 3 to 10 minutes.

In the present invention, the above-mentioned flame resistant yarn is carbonized to obtain a carbon fiber. 400 if necessary before carbonization
It is preferable to perform heat treatment in an inert atmosphere furnace having a temperature gradient of up to 700 ° C. The carbonization method is not particularly limited, and various known methods, for example, 1000 to 2000 ° C. in an inert atmosphere of nitrogen, helium, argon or the like.
There is a method such as heating with. At this time, in order to increase the strength and elastic modulus of the carbon fiber, it is preferable to stretch the carbon fiber to the extent that fluff is not generated.

The carbon fiber thus obtained can be subjected to surface treatment, sizing, etc., if necessary, by a conventionally known method.

That is, the cyclization reaction of acrylic fibers in a short time and in a large amount can be performed by heat treatment in an inert atmosphere having a specific oxygen concentration in the first step, and by the heat treatment in an active atmosphere in the second step. Since a large amount of oxidation reaction can be carried out in a short time, it is possible to obtain a carbon fiber by suppressing a decrease in strength and elastic modulus of the carbon fiber obtained by subsequent carbonization and by suppressing a decrease in carbonization yield. . Such efficiency improvement in the flameproofing process and maintenance of the physical properties and carbonization yield of the obtained carbon fiber cannot be achieved by the conventional technology.

[0015]

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

In this example, the tensile strength and elastic modulus of carbon fiber were measured according to the resin-impregnated strand test method defined in JIS-R-7601. As the resin, Bakelite (registered trademark) ERL4221 (manufactured by Union Carbide Co., Ltd.) / Boron trifluoride monoethylamine (BF3)
MEA) / acetone = 100/3/4 parts was used, and the resin curing condition was 130 ° C. for 30 minutes.

The carbonization yield represents the percentage of the weight of the obtained carbon fiber with respect to the weight of the flame-resistant yarn supplied to the carbonization step.

[Example 1] 1.0d, 48000 consisting of 95% by weight of acrylonitrile and 5% by weight of itaconic acid
The acrylic fiber of the filament is heat-treated for 30 minutes at a tension of 1.2 to 1.5 kg in a nitrogen atmosphere having an oxygen concentration of 0.5% by volume at 260 ° C. (first step), and further at 260 ° C.
In the above air, a tension of 2.5 to 2.8 kg was applied and heat treatment was performed for 5 minutes (second stage). The time required for the flameproofing process was 35 minutes. Next, carbonization was performed in nitrogen at 1400 ° C. to obtain carbon fibers.

Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 1] The acrylic fiber obtained in Example 1 was tried to be flame resistant only in air at 240 ° C for 120 minutes, but a runaway reaction occurred during the treatment, and carbon fiber could not be obtained. It was [Comparative Example 2] In Comparative Example 1, the treatment temperature was lowered to 220 ° C, and flame-resistant treatment was performed for 400 minutes to obtain a carbon fiber. Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 3] The number of filaments was 12000.
An acrylic fiber was obtained in the same manner as in Example 1 except that A carbon fiber was obtained in the same manner as in Example 1 except that this was heat-treated in air only at 240 ° C. for 120 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 4] A carbon fiber was obtained in the same manner as in Example 1 except that the first-stage heat treatment was performed at an oxygen concentration of 0.005% by volume. The time required for the flameproofing process was 35 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 5] A carbon fiber was obtained in the same manner as in Example 1 except that the first-stage heat treatment was carried out at an oxygen concentration of 0.005% by volume for 60 minutes. The time required for the flameproofing process is 65
It was a minute. Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 6] Oxygen concentration of 4 in the first heat treatment
An attempt was made to carry out in the same manner as in Example 1 except that the carbon fiber was not used, but a carbon fiber could not be obtained due to a runaway reaction during the first heat treatment. [Comparative Example 7] The same procedure as in Example 1 was carried out except that the second heat treatment was not performed in air, but a yarn breakage occurred in the carbonization step, and carbon fibers could not be obtained.

Example 2 A carbon fiber was obtained in the same manner as in Example 1 except that the second heat treatment in air was changed to 1 minute. The time required for the flameproofing process was 31 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

Example 3 A carbon fiber was obtained in the same manner as in Example 1 except that the second heat treatment in air was changed to 15 minutes. The time required for the flameproofing process was 45 minutes.
Table 1 shows the physical properties of the obtained carbon fiber.

[Embodiment 4] The second step of heat treatment is performed with an oxygen concentration of 5
A carbon fiber was obtained in the same manner as in Example 1 except that the carbon fiber was used in a volume% of nitrogen. The time required for the flameproofing process was 35 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

[Embodiment 5] The second step of heat treatment is performed with an oxygen concentration of 1
A carbon fiber was obtained in the same manner as in Example 1 except that the carbon fiber was used in 0% by volume of nitrogen. The time required for the flameproofing process was 35 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

[Comparative Example 8] The acrylic fiber obtained in Example 1 was applied with a tension of 1.2 to 1.5 kg in air to give 220
Heat treatment at 60 ° C for 60 minutes, then 2.5 to 2.8k in nitrogen
A carbon fiber was obtained by applying a tension of g and heat-treating at 280 ° C. for 12 minutes. The time required for the flameproofing process was 72 minutes. Table 1 shows the physical properties of the obtained carbon fiber.

[0030]

[Table 1]

[0031]

According to the present invention, an acrylic carbon fiber can be obtained in a short time and in a large amount, that is, with good productivity. That is, the cyclization reaction of acrylic fibers can be carried out in a short time and in a large amount by heat treatment in an inert atmosphere of a specific oxygen concentration, and the subsequent heat treatment in an active atmosphere can oxidize a large amount in a short time. Since the reaction can be carried out, it is possible to obtain a carbon fiber by suppressing a decrease in the strength and elastic modulus of the carbon fiber obtained by the subsequent carbonization and by suppressing a decrease in the carbonization yield.

Claims (4)

[Claims] 1. A method for producing an acrylic carbon fiber, characterized in that an acrylic fiber is heat-treated in an inert atmosphere having an oxygen concentration of 0.01 to 3% by volume, then heat-treated in an active atmosphere, and then carbonized. . 2. The method for producing an acrylic carbon fiber according to claim 1, wherein the active atmosphere is a gas atmosphere having an oxygen concentration of 8% by volume or more. 3. The heat treatment temperature in an inert atmosphere is 220 to.
The method for producing an acrylic carbon fiber according to claim 1, wherein the temperature is in the range of 300 ° C. 4. The heat treatment temperature in an active atmosphere is 220 to 3
It is 20 degreeC, The manufacturing method of the acrylic carbon fiber of Claim 1 characterized by the above-mentioned.