JP2000119341A - Acrylonitrile-based polymer, and precursor fiber for carbon fiber using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an acrylonitrile-based polymer capable of giving a precursor fiber, which can be fired at a high speed and reduce cross-sectional double structure after being treated to improve fire resistance, in order to simultaneously secure high carbon fiber productivity and properties, e.g. modulus of elastcity. SOLUTION: This acrylonitrile-based polymer serves as a stock for precursor fibers for carbon fibers, and has a V (min-1) value of 0.01 or more but below 0.3, where the V value is reciprocal of time before an exothermic peak appears in an isothermal exothermic curve, measured by differential scanning colorimetry effected at 230 deg.C in a flow of air.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an acrylonitrile-based polymer used as a raw material for producing carbon fibers,
More specifically, the present invention relates to an acrylonitrile-based polymer which is excellent in both productivity and performance of carbon fiber and is used as a raw material for producing carbon fiber.

[0002]

2. Description of the Related Art Conventionally, carbon fibers and graphite fibers (hereinafter collectively referred to as carbon fibers) having an acrylic fiber as a precursor have been used for aerospace applications, sports, It is used in a wide range as a reinforcing fiber material for high-performance composite materials for leisure use and the like. Furthermore, in order to improve the performance of these composite materials, the quality and performance of carbon fibers are required to be improved, and furthermore, the reduction in production costs is expected to spread to industrial materials.

[0003] Acrylonitrile-based fibers as precursors of carbon fibers are different from acrylonitrile fibers for clothing and are merely intermediate products for producing carbon fibers as final products. Therefore, what is required is to provide a carbon fiber having excellent quality and performance, and at the same time, it is extremely important that the carbon fiber can be provided at a high productivity and at a low cost in a firing step for forming the carbon fiber.

[0004] The process of producing carbon fibers comprises a spinning process of precursor fibers, a flame-proofing process, a carbonizing process, and a graphitizing process. In this process, the acrylonitrile-based precursor fiber flame-retarding step is a step in which a complex reaction mainly composed of a nitrile group cyclization reaction, an oxidation reaction, and a dehydrogenation reaction occurs. Is known to have a significant effect on In addition, since the flame-proofing process generally requires a long time treatment, shortening the flame-proofing treatment time greatly contributes to the improvement of productivity. Therefore, a precursor fiber having a high oxidation resistance is advantageous for shortening the oxidation time. However, when the oxidation resistance is high as described above, as the oxidation of the fiber surface layer progresses rapidly, the oxygen diffusivity into the interior decreases. A heavy structure occurs. It is known that the properties of the finally obtained carbon fiber are low when the flame resistant fiber having a remarkable double cross section structure is used. As described above, the shortening of the flame resistance time and the reduction of the cross-sectional double structure are contradictory, and a precursor polymer for acrylonitrile-based carbon fiber that achieves these in a well-balanced manner has been desired.

[0005] To shorten the flame-proofing time, for example, by limiting the polymer composition, such as by using a carboxylic acid group-containing vinyl monomer, an attempt is made to shorten the firing time while taking into account the stability in the polymer production and spinning process. Taro (Tokubosho 51-
No. 7209) or a method of adding amines or peroxides to a raw material polymer (JP-B-51-7209,
JP-A-48-87120) and the like have been proposed.
Japanese Patent Application Laid-Open No. 7-292526 proposes a method in which heat treatment is performed in an inert atmosphere and then heat treatment is performed in an active atmosphere. However, although it is thought that the promotion of the reaction itself in flame resistance enables high-speed sintering, the performance of the obtained carbon fiber tends to be rather impaired due to the formation of a double cross-sectional structure of the flame resistant yarn. However, improvement in both productivity and performance of carbon fiber has not been achieved.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of such conventional problems, and has been developed in order to achieve both high productivity and high elastic modulus of carbon fiber. It is an object of the present invention to provide an acrylonitrile-based polymer which can provide a precursor fiber capable of reducing the cross-sectional double structure after the oxidation treatment.

[0007]

DISCLOSURE OF THE INVENTION The present invention relates to an acrylonitrile-based polymer used for producing a precursor fiber for carbon fiber, which is obtained by using a differential scanning calorimeter in an air stream.
When the reciprocal of the appearance time of the exothermic peak in the isothermal exothermic curve measured at 30 ° C. is defined as V (min ⁻¹ ), V is 0.0
It relates to an acrylonitrile-based polymer characterized by being at least 1 and less than 0.3.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION V is a value which is a measure of the oxidization resistance reaction rate. If V is too small, the oxidation-resistant reaction rate is low,
V is 0.01 or more, preferably 0.05 or more, because the flame-resistant reaction does not proceed quickly. On the other hand, if V is too large, the oxidization reaction rate is too high. If the precursor fiber using this polymer is oxidized and converted to oxidized fiber, only the surface will be oxidized, and the interior will be burnt. V is preferably less than 0.3, particularly preferably 0.25 or less.

In the acrylonitrile-based polymer of the present invention, a carboxylic acid group-containing vinyl monomer unit which has an effect of accelerating a flame-resistant reaction in the polymer as a copolymer component is generally added to the acrylonitrile unit in order to obtain V. Is preferable.

In particular, in order to obtain the acrylonitrile polymer of the present invention, when the copolymerization amount of the carboxylic acid group-containing vinyl monomer unit in the polymer is M (mol%), M is (a) 0.5 ± 0.5%. 0.2, (b) 1.5 ± 0.
2, and (c) M of three kinds of polymers in which the copolymerization ratio of the carboxylic acid group-containing vinyl monomer was changed so as to fall within three kinds of composition ranges of 2.5 ± 0.2, and V measured for each polymer. It is preferable to carry out the polymerization under such polymerization conditions that the slope (dV / dM) of the straight line determined by the least squares method from the value of is not more than 0.1.

Here, the polymerization for obtaining the three kinds of polymers in the ranges (a), (b) and (c) needs to be carried out under the same polymerization conditions, and the carboxylic acid group-containing vinyl monomer is used in the same manner. The solution polymerization or the suspension polymerization is carried out, and the solvent or suspension at that time, the type of the initiator, the temperature and the time are the same, and only the charged ratio of the monomers is changed.

The polymerization conditions are such that dV / dM is 0.1.
If it is found that it becomes 1 or less, polymerization conditions for obtaining the acrylonitrile-based polymer of the present invention are determined, and under the same conditions, a carboxylic acid group-containing vinyl monomer such that V becomes 0.01 or more and less than 0.3. The acrylonitrile-based polymer of the present invention can be obtained under the conditions of the charge ratio.

When the dV / dM exceeds 0.1, oxidized spots due to composition unevenness in the polymer tend to occur, and the performance of the final carbon fiber tends to be impaired.

[0014] The acrylonitrile unit in the polymer is preferably at least 97 mol%. When the acrylonitrile content in the copolymer is less than 97 mol%, when the precursor fiber is made into carbon fiber, a comonomer other than acrylonitrile becomes a defect point, and the quality and performance of the carbon fiber are easily impaired.

[0015] The content of the carboxylic acid group-containing vinyl monomer unit is 0.2 to 3 mol%, preferably 0.5 to 2.5 mol%.

Specific examples of the carboxylic acid group-containing vinyl monomer include vinyl monomers having a carboxyl group such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, and salts thereof (for example, ammonium salts). Etc.). Of these, methacrylic acid and itaconic acid are preferred.

In the case of itaconic acid, the polymerization method is more preferably a redox aqueous suspension polymerization. Thus, not only the type of carboxylic acid group-containing vinyl monomer,
The polymerization conditions (polymerization method, initiator) are also important in controlling the flame-resistant reaction.

The acrylonitrile-based polymer of the present invention may be made of acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, or the like, in addition to acrylonitrile and the above-mentioned carboxylic acid group-containing vinyl monomer, as long as the requirements of the present invention are satisfied. It may contain a small amount of monomers such as vinyl group-containing carboxylic acid esters, vinyl acetate, vinyl propionate, acrylamide, methacrylamide, diacetone acrylamide, maleic anhydride, methacrylonitrile, styrene and α-methylstyrene. Of these, acrylamide is particularly preferred as the copolymer component. This is because the oxidization reaction rate is rapidly controlled by the coexistence of a small amount of acrylamide, although the carboxylic acid group content is a dominant factor.

The polymerization method for obtaining the acrylonitrile polymer of the present invention is not limited to any known methods such as solution polymerization and suspension polymerization as long as the requirements of the present invention are satisfied. Inspection and other impurities are preferably eliminated as much as possible. In view of the drawability of the precursor fiber in spinning and the performance development of the carbon fiber, the polymerization degree of the polymer is preferably such that the intrinsic viscosity [η] is 0.8 or more, particularly 1.4 or more. Further, the intrinsic viscosity [η] is 2.
Those having a value of 0 or less are usually used.

The precursor fiber of the present invention is prepared by dissolving such an acrylonitrile-based polymer in a known solvent to prepare a spinning solution, and then spinning, coagulating, and coagulating by a dry method, a dry-wet method, and a wet spinning method according to a known method. Stretching (drawing in a bath or drawing in air and in a bath) and dry densification can be performed to form precursor fibers.

The drawing in the bath may be performed directly on the coagulated fiber, or may be performed after the coagulated fiber is drawn in advance in the air. Stretching in the bath is usually performed in a stretching bath at 50 to 98 ° C. by dividing into one or two or more stages.
Water washing may be performed before, after, or in the middle.

By these operations, the coagulated fiber is preferably drawn about 6 times or more by the time the drawing is completed in the bath.

The fiber after drawing and washing in a bath is subjected to an oil treatment by a known method, and then dried and densified. Drying and densification can be performed by any known method, but a method using a heating roller at about 100 to 200 ° C. is preferable in consideration of the drying speed, simplicity of equipment, and the effect of densifying the fiber. Further, if necessary, before or after dry densification, the fiber may be further stretched by a high-temperature heating roller or pressure steam.

The precursor fiber thus obtained is
By performing heat treatment (flame-resistance treatment) in an oxidizing atmosphere at 00 ° C to 400 ° C, it can be converted into flame-resistant fiber. Furthermore, according to a known method, 1000 ° C to 1500 ° C.
Carbon fibers can be obtained by carbonization in a moderate inert atmosphere.

When the precursor fiber of the present invention is used, high-speed sintering can be performed at the time of the flame-resistant treatment, and the performance of the obtained carbon fiber is excellent.

[0026]

The present invention will be described more specifically with reference to the following examples. The copolymer compositions, the intrinsic viscosities [η] of the copolymers, and the isothermal DSC exothermic curves in Examples and Comparative Examples were measured by the following methods.

(A) "Copolymer composition" ¹ H-NMR method (JEOL GSX-400 type superconducting FT)
-NMR).

(B) "Intrinsic viscosity of copolymer [η]" Measured with a dimethylformamide solution at 25 ° C.

(C) "Isothermal DSC exothermic curve" Using DSC220C manufactured by Seiko Denshi Kogyo, 4.0 mg of an acrylonitrile-based polymer was weighed, placed in an aluminum sample container, and pressed with a stainless steel mesh cover in a dry air stream. Measured at 230 ° C. Exothermic peak time t when the obtained isothermal exothermic curve takes a maximum value
The reciprocal of (min) was V (min ^-1 ).

In the examples, "AN" represents acrylonitrile, "IA" represents itaconic acid, "MAA" represents methacrylic acid, and "AAm" represents acrylamide.

Example 1 Distilled water sufficiently purged with nitrogen was placed in a separable flask and kept at 55 ° C. Thereto, ammonium persulfate, ammonium bisulfite and sulfuric acid, which are redox polymerization initiators, were added.
N, IA aqueous solution was dropped at a constant rate. Thereafter, stirring was continued while the internal temperature was maintained at 55 ° C., and after 2 hours from the completion of the dropwise addition, the polymerization was stopped, and the acrylonitrile-based polymer was obtained by filtering, washing and drying from the polymerized slurry that had precipitated. In the same manner, a polymer having a different composition
Type polymerized. From the NMR analysis of the obtained polymer, the composition was AN / IA = 99.5 / 0.5, 98.
4 / 1.6 and 97.5 / 2.5 (mol%).
The intrinsic viscosity of the polymer was 1.8.

The isothermal heat generation curve of this polymer was measured by DSC.
Measured. As a representative example, AN / IA = 99.5 /
The isothermal exotherm of the 0.5 composition polymer is shown in FIG.
Was. From this heat generation curve, the heat generation peak appearance time is 9.2 minutes.
And V was 0.11. To the other two polymers
Table 1 summarizes the results of similar measurements.
Copolymerization amount M (mol%) of three kinds of polymers and heat generation peak
Reciprocal V of the clock time (min ^-1) Value by the method of least squares
The value of dV / dM was determined to be 2.7 × 10^-2So
(See FIG. 2).

Example 2 A polymer having the composition shown in Table 1 and an intrinsic viscosity of 1.8 was polymerized in the same manner as in Example 1. Table 1 summarizes the results of the isothermal DSC measurement for each polymer. The value of dV / dM was determined by the least squares method from the values of M and V of the three types of polymers and found to be 4.4 × 10 ⁻² .

Example 3 Dimethyl sulfoxide (hereinafter abbreviated as DMSO) was placed in a separable flask and kept at 60 ° C. There, the azo initiator 2,2 = | azodi-
(2,4-Dimethyl-4-methoxyvaleronitrile) was added, and then AN and MAA were added dropwise at a constant rate.
Thereafter, stirring was continued while maintaining the internal temperature at 60 ° C., and polymerization was stopped when 2 hours had passed after the completion of the dropwise addition. The obtained solution was poured little by little into a large amount of water, and the deposited precipitate was filtered, washed and dried to obtain an acrylonitrile-based polymer. In the same manner, a polymer having a different composition
Type polymerized. From the NMR analysis of the obtained polymer, the composition was AN / MAA = 99.4 / 0.6, 9 respectively.
8.5 / 1.5 and 97.3 / 2.7 (mol%). The intrinsic viscosity of the polymer was 1.8.

Table 1 shows the results of the isothermal DSC measurement for each polymer. M of three kinds of polymers,
When the value of dV / dM was calculated from the value of V by the least square method, it was 5.0 × 10 ⁻³ .

[Comparative Example 1] Distilled water / dimethylacetamide (DMAc) which had been sufficiently purged with nitrogen in a separable flask.
= 6/1 and maintained at 60 ° C. There, azobisisobutyronitrile was put, and then AN, MA
A was dropped at a constant rate. Thereafter, stirring was continued while maintaining the internal temperature at 60 ° C., and polymerization was stopped when 2 hours had passed after the completion of the dropwise addition. The precipitated precipitate is filtered, washed,
After drying, an acrylonitrile-based polymer was obtained. In the same manner, two types of polymers having different compositions were further polymerized. From the NMR analysis of the obtained polymer, the composition was A
N / MAA = 99.3 / 0.7, 98.4 / 1.6, 9
It was 7.4 / 2.6 (mol%). In addition, the intrinsic viscosity of the copolymer was 1.8.

The results of the isothermal DSC measurement for each polymer are summarized in Table 1. M of three kinds of polymers,
When the value of dV / dM was determined from the value of V by the least square method, it was 4.2 × 10 ⁻² .

Comparative Example 2 A polymer having the composition shown in Table 1 and an intrinsic viscosity of 1.8 was polymerized in the same manner as in Example 3. Table 1 summarizes the results of the isothermal DSC measurement for each polymer. When the value of dV / dM was determined by the least square method from the values of M and V of the three polymers, it was 2.0 × 10 ⁻¹ .

Comparative Example 3 A polymer having the composition shown in Table 1 and an intrinsic viscosity of 1.8 was polymerized in the same manner as in Example 3. When the isothermal exothermic curve of this polymer was measured in the same manner by DSC, V was obtained.
It was 4.

[0040]

[Table 1] Example 4 Among the polymers obtained in Example 2, AN
A polymer having a composition of /MAA=99.5/0.5 was dissolved in dimethylformamide (hereinafter abbreviated as DMF) to a concentration of 23% by weight to prepare a spinning dope. This spinning stock solution was discharged into the air from a die having a diameter of 0.15 mm, and was coagulated in a mixed solvent of DMF / water. Then, further in air 1.
After being washed and desolvated while stretching 5 times and further 3.4 times in warm water, it was immersed in a silicone oil solution and dried and densified with a 140 ° C. heating roller. Followed by 180
Stretched 1.5 times on a hot plate at ℃, take-up speed 77m / min
Thus, a 1.2 dtex precursor fiber was obtained.

This fiber is placed in an atmosphere of 230 ° C. in air for 60 hours.
The mixture was left standing for a minute to be subjected to an oxidization-resistant treatment, and converted into oxidized-resistant fibers.
After embedding the obtained oxidized fiber in a resin and cutting it to obtain a cross section and polishing it, observing the state of the cross section with a reflection microscope, the fiber cross section is uniformly colored black, and the cross section double structure is Not observed.

Comparative Example 3 From the polymer obtained in Comparative Example 1, a precursor fiber was obtained in the same manner as in Example 5 from a polymer having a composition of AN / MAA = 99.3 / 0.7. This fiber was left in an atmosphere of 230 ° C. in the air for 60 minutes to be subjected to a flameproofing treatment, thereby being converted to a flameproofed fiber. After embedding the obtained oxidized fiber in a resin and cutting it to obtain a cross section and polishing it, the state of the cross section was observed with a reflection microscope. Only the fiber surface was colored black and the inside was not colored. Heavy structures were observed.

[0043]

When the acrylonitrile-based polymer of the present invention is used to produce a precursor fiber for carbon fiber, it can be fired at a high speed during the oxidization treatment, and has a double sectional structure of the oxidized fiber obtained. Can be reduced. Therefore, the productivity of the carbon fiber manufacturing process is improved, and carbon fibers having excellent quality and performance such as high elastic modulus can be obtained, so that both productivity and quality and performance can be achieved.

[Brief description of the drawings]

FIG. 1 is an example of an isothermal heat generation curve of an acrylonitrile-based polymer measured by a differential scanning calorimeter (DSC).

FIG. 2 is a graph showing the relationship between the copolymerization amount M of a carboxylic acid group-containing vinyl monomer and the reciprocal V of an exothermic peak in Examples and Comparative Examples.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiko Hosako 20-1 Miyukicho, Otake City, Hiroshima Prefecture Inside Mitsubishi Rayon Co., Ltd. Central Research Laboratory (72) Inventor Teruyuki Yamada 20-1 Miyukicho, Otake City, Hiroshima Prefecture No. F term in Central Research Laboratory of Mitsubishi Rayon Co., Ltd. (reference) 4J100 AJ02Q AJ08Q AJ09Q AM02P CA04 DA25 FA08 FA21 JA11 4L035 BB06 BB11 BB17 BB71 BB85 BB91 FF01 GG02 GG04 HH10 MB03 MB04 4L037 CS02 CS03 PA55 PA61 PA65

Claims (7)

[Claims] 1. An acrylonitrile-based polymer used for producing a precursor fiber for carbon fiber, the reciprocal of the appearance time of an exothermic peak in an isothermal exothermic curve measured at 230 ° C. in an air stream using a differential scanning calorimeter. To V (mi
An acrylonitrile-based polymer, wherein V is 0.01 or more and less than 0.3 when n ^-1 ). 2. When the amount of a carboxylic acid group-containing vinyl monomer unit in a polymer is represented by M (mol%),
Carboxylic acid group-containing vinyl such that M falls within three types of composition ranges of (a) 0.5 ± 0.2, (b) 1.5 ± 0.2, and (c) 2.5 ± 0.2. Polymerization conditions such that the slope (dV / dM) of a straight line obtained by the least squares method from the M of three types of polymers having different copolymerization rates of the monomers and the value of V measured for each polymer is 0.1 or less. The acrylonitrile-based polymer according to claim 1, which is obtained by polymerizing with acrylonitrile. 3. The acrylonitrile-based polymer according to claim 1, wherein the content of the acrylonitrile unit in the polymer is 97 mol% or more, and the content of the carboxylic acid group-containing vinyl monomer unit is 0.2 to 3 mol%. . 4. The acrylonitrile-based polymer according to claim 3, wherein the carboxylic acid group-containing vinyl monomer as the copolymer component is methacrylic acid. 5. The acrylonitrile-based polymer according to claim 3, wherein the carboxylic acid group-containing vinyl monomer as the copolymerization component is itaconic acid, and the polymerization method is obtained by aqueous suspension polymerization using a redox-based initiator. . 6. A precursor fiber for carbon fiber, comprising the acrylonitrile-based polymer according to claim 1. 7. A carbon fiber obtained by subjecting the precursor fiber for carbon fiber according to claim 6 to flame resistance and carbonization.