JPS63275714A - Production of high-strength carbon fiber - Google Patents

Abstract

PURPOSE:To obtain the titled carbon fiber having high strand strength by such processes that a spinning stock solution containing a specific acrylonitrile-based polymer is spun, being coagulated followed by drawing and calcination. CONSTITUTION:First, an acrylonitrile-based polymer with a weight-average molecular weight >=500,000 containing >=95wt.% of acrylonitrile is dissolved in an organic solvent such as dimethylformamide to prepare a spinning stock solution with a viscosity of 500-1,500 poises at 45 deg.C. Thence, this solution is subjected to dry-wet spinning process, being delivered, through nozzles, into a coagulating bath to form a coagulated yarn with a swelling index of <=200%. This coagulated yarn is then brought to drawing, being treated with lubricant followed by drying at 100-150 deg.C to effect densification. The resulting acrylic precursor is calcined thus obtaining the objective carbon fiber suitable as a material for sports, leisure or the aerospace industry.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to the production of high-strength carbon fibers, and in particular to an acrylonitrile precursor prepared by spinning an acrylonitrile polymer having a weight average molecular weight of 500,000 or more. The present invention relates to a method of producing carbon fiber by firing.

[Conventional technology]

Carbon fibers produced by firing fibers obtained by spinning acrylonitrile polymers as precursors are extremely useful, and can be used as materials for aerospace, sports, leisure, gears, connecting rods, and X-ray photography. Demand is expected to grow in a wide range of areas as an industrial material such as boards. As the advanced use of carbon fiber progresses, the performance requirements for carbon fiber are also becoming stricter, and carbon fiber with high strength and high modulus of elasticity has emerged as an aerospace material that particularly requires reliability. is awaited.

By the way, although the carbon fiber manufacturing process includes many complex and long steps such as polymerization, spinning, and firing, conventional methods for improving the physical properties of carbon fibers include optimizing the firing conditions or using acrylonitrile-based Most of the work involves cleaning polymers, precursors, etc. However, in order to further improve the physical properties of carbon fiber in the future,
There is a limit to the improvement in physical properties if optimization is done simply as an extension of these conventional techniques, and it is desired to improve the physical properties of carbon fiber by going back to the molecular design of the acrylonitrile polymer.

[Problems to be Solved by the Invention] Therefore, the present inventors conducted a fundamental study on the fiber structure of acrylonitrile-based precursors, and found that the 5o By using an acrylonitrile polymer with a weight average molecular weight of 10,000, it is possible to produce a precursor that is highly oriented and has good passability through the firing process.
By firing the precursor, 600 kli/m
``We have discovered that carbon fibers can be made with strand strength exceeding the above, and have completed the present invention.

[Means for solving problems]

The gist of the present invention is to dissolve an acrylonitrile polymer containing 95 wt% or more of acrylonitrile and having a weight average molecular weight of 500,000 or more in an organic solvent, and to prepare a spinning dope having a viscosity of 500 to 1500 poise at 45°C. The coagulated fibers are then discharged from a nozzle using a dry-wet spinning method to obtain coagulated fibers with a degree of swelling of 200% or less, and the resulting coagulated fibers are stretched and the obtained acrylic and aluminum precursors are fired to produce carbon fibers. It's on the tree.

The acrylonitrile polymer used in the present invention needs to have a weight average molecular weight of 500,000 or more, preferably 70,000 or more. In order to produce the high-strength carbon fibers of the present invention, it is necessary to sinter a highly oriented acrylonitrile precursor, but when an acrylonitrile polymer having a weight average molecular weight of less than 5 million is used, It is not possible to obtain a precursor that is large and has good passing through the firing process.

In general, there are two ways to improve the degree of X-ray orientation of an acrylonitrile precursor: ・Improve the molecular weight of the acrylonitrile polymer used ・Improve the stretching ratio; simply improving the degree of X-ray orientation If so, it is also possible to use an acrylonitrile polymer having a weight average molecular weight of less than 50,000, apply a high stretching ratio of 10 times or more, and shape the precursor into a precursor. However, when considering the production of highly oriented precursors using such a method, the draw ratio is set very high, which tends to cause fuzz and yarn breakage during the spinning process. leading to a decline in quality. On the other hand, the carbon fiber firing process can be broadly divided into three steps: flame resistance, pre-carbonization, and carbonization. It is necessary to bake while applying. However, it is impossible to stretch a precursor that has been highly stretched by 10 times or more during the flameproofing and pre-carbonization processes.
Therefore, high-performance carbon fiber cannot be obtained, and in extreme cases, even firing becomes impossible.

However, when an acrylonitrile polymer with a weight average molecular weight of 500,000 or more is used, a highly oriented acrylonitrile precursor with an X-ray orientation degree of 92% or more can be obtained by simply performing a 5 to 10 times stretching operation in the spinning process. Is possible. Moreover, when an acrylonitrile polymer with a weight average molecular weight of 500,000 or more is used, the maximum draw ratio reaches 15 times or more, so when spinning an acrylonitrile precursor, it is necessary to stretch at a draw ratio with sufficient margin. By performing this operation, the stability during the spinning process is greatly improved. Furthermore, since the acrylonitrile precursor has a low draw ratio, it has very good flame resistance and extensibility in the pre-carbonization process, and can be said to be an acrylonitrile precursor suitable for producing high-strength carbon fibers.

The acrylonitrile polymer used in the present invention can be produced by a conventional suspension polymerization method, emulsion polymerization method, or solution polymerization method.
The method described in Publication No. 0, that is, acrylonitrile 10-
7Qwt%, organic solvent 15-60wt%, water 15-5Q
A method of polymerizing a mixture of wt% with a radical initiator and then adding 1 to 10 parts by weight of water and/or an organic solvent to 1 part by weight of the monomer is a method that produces high molecular weight polymers with less branching. This is preferable in that stable coalescence can be obtained. The organic solvent used here is dimethylformamide (D
MF), dimethylacetamide (DMAe), r-butyrolactone, dimethyl sulfoxide (DMSO), and the like. Furthermore, it is true that the performance of carbon fibers greatly depends on the flame-retardant process, but in order to smoothly perform this flame-retardant process, it is necessary to add 0.1% of a polymerizable unsaturated carboxylic acid to the acrylonitrile polymer. It is preferable to copolymerize up to 5 wt%. The copolymerization ratio is 0.1wt%
If the temperature is lower than that, the flame-retardant reaction will not proceed easily, so it is necessary to perform the flame-retardant treatment at a higher temperature.As a result, single fiber fusion tends to occur in the flame-retardant process, making it impossible to produce high-strength carbon fibers. . On the other hand, the copolymerization ratio is 5 wt%
If it exceeds this amount, tar-like substances are likely to be generated during flameproofing, and it is also unfavorable from the viewpoint of carbonization yield of carbon fibers.

Representative examples of such unsaturated carboxylic acids include acrylic acid, methacrylic acid, crotonic acid, and itaconic acid. Other copolymerizable unsaturated monomers include, for example, methyl acrylate, ethyl acrylate, or methacrylate), n+, iso-, or t-
Butyl acrylate or methacrylate, 2-ethylhexyl acrylate or methacrylate, acrylic acid,
Unsaturated monomers such as methacrylic acid, itaconic acid, α-chloroacrylonitrile, 2-hydroxyethyl acrylate, hydroxyalkyl acrylate or methacrylate, acrylamide, diacetone acrylamide, methacrylamide, vinyl chloride, vinylidene chloride, vinyl bromide, vinyl acetate, etc. One example is the body. These polymerizable unsaturated monomers can be used in combination with the above-mentioned polymerizable unsaturated carboxylic acid to copolymerize with acrylonitrile.

Next, in order to produce the high-strength carbon fiber of the present invention, the high molecular weight acrylonitrile is converted into D MF , D
A spinning stock solution is prepared by dissolving MAc, r-butyrolactone, and an organic solvent such as DMSO. In order to obtain high-strength fibers, it is necessary to bring the entire molecular chains that make up the fibers close to the so-called unstretched chain state, which extends in the fiber axis direction.In the spinning and drawing stages, the polymer molecular chains are aligned. It is important to prepare a polymer solution (spinning stock solution) in which the molecular chains are sufficiently loosened in order to make the spinning solution easy to use. at 45℃, 500-150
It is necessary to set it within the range of 0 poise. When performing °C spinning using a spinning dope with a viscosity exceeding 1500 poise, extremely high pressure is applied to the spinning equipment, including the spinning nozzle and dope filtration machine, reducing the durability of the spinning machine. On the other hand, it is also possible to reduce the viscosity by heating the spinning dope to a high temperature, but in this case, problems arise such as the stability of the solvent and the dope being reduced. On the other hand, when a 500 poise unsalted spinning dope is used, the spinnability deteriorates and stable spinning cannot be achieved by the dry-wet spinning method.

Next, the spinning stock solution is discharged from a nozzle by a dry-wet spinning method and coagulated in a coagulation bath to obtain a coagulated thread. In this case, the coagulation bath conditions must be set so that the degree of swelling of the resulting coagulated thread is 200% or less. It is preferable to increase the concentration of the organic solvent in the bath and to set the coagulation bath temperature to be low. When using an acrylonitrile polymer with a weight average molecular weight of 500,000 or more, it is necessary to lower the concentration of the stock solution, which is 15 wt% or less, but even if such a low concentration spinning stock solution is used, the coagulation bath conditions cannot be adjusted. Solvent concentration g 3
By setting the temperature above wt% and below -20℃, it is possible to obtain coagulated threads with a swelling degree of 200% or less and a transparent and dense structure, and macrovoids are a matter of thermal theory in such coagulated threads. There are also no microvoids, which are the main cause of strength reduction in carbon fibers.

However, when the spinning dope is coagulated in a coagulation bath with a low organic solvent concentration and high temperature, only a coagulated thread that is opaque and contains microvoids can be obtained. Even if it is fired into fibers, it cannot be made into high-strength carbon fibers.

Next, the coagulated thread obtained in this way is drawn while washing the organic solvent contained in the coagulated thread with warm water that has a temperature gradient so that the temperature becomes higher in the later steps, and then It is necessary to carry out the stretching at a temperature of ℃ or higher. For such stretching at a temperature of 100° C. or higher, steam stretching or a stretching method in a moist heat atmosphere using a high boiling point solvent as a heating medium is preferable from the viewpoint of stretchability. Note that water-soluble polyhydric alcohols such as ethylene glycol, diethylene glycol, triethylene glycol,
Examples include chrycerin. If necessary, the drawn yarn thus obtained is washed again at 100°C and treated with oil.
Drying and densification are carried out at a temperature of ~150°C.

Subsequently, the acrylonitrile precursor thus obtained was heated to 200 to 300% in an oxidizing atmosphere such as air.
The present invention is made by heat-treating while stretching at a temperature of 50°C to obtain a flame-resistant fiber, followed by pre-carbonization treatment at a temperature of 300 to 800°C, and further carbonization treatment at a temperature of 1000°C or higher. It can be made of high strength carbon fiber.

The carbon fiber of the present invention thus obtained has physical properties of a strand strength of 6001 Qil/m' or more,
It can be used not only as a material for sports and leisure, but also as a material for aerospace, which requires extremely high reliability.

〔Example〕

The present invention will be specifically explained below using Examples.

(13 Weight average molecular weight (Mw) was calculated by measuring the intrinsic viscosity [η] of the polymer at 25° C. using dimethylformamide and using the following formula.

[η] ” 3,35 x 10 (Mri 0°7! (
21 Single fiber fineness is measured using a denier computer made by Toyo Baldwin.

(3) The degree of orientation π is determined by the following formula from the azimuthal diffraction profile obtained from the reflection of the acrylonitrile fiber near the scattering angle 2θ = 17° in the equatorial direction, subtracting the base twin from this, and the half-width of the peak H7 degrees. I asked for it.

(4) Physical properties of carbon fiber were measured according to JIS-R7601.

(5) To determine the degree of swelling of the coagulated threads, first centrifugally dehydrate the coagulated threads to remove the adhering liquid, measure the weight, and determine the weight of the wet threads. After washing with a solvent and drying in a dryer, the weight was measured and determined as an absolutely dry weight (W').It was calculated from W and W' using the following formula.

Example 1 Acrylonitrile polymers polymerized by suspension polymerization and having weight average molecular weights of 560,000 and 710,000 and containing 2 wt % of methacrylic acid were dissolved in DMF to obtain a spinning stock solution. This spinning solution was transferred from a spin tank kept at 50°C with a pore size of 200 μm.
Using a nozzle with 500 holes, it was spun into a coagulation bath of DMF and water using a wet-dry spinning method. Note that the distance between the nozzle surface and the coagulation bath was 5 mtx. The coagulated thread thus obtained was heated twice in warm water at 70°C, twice in purified water, and further heated to 180°C.
After stretching the film twice in glycerin at 140°C, it was treated with an oil agent and dried at 140°C.

The obtained precursor was continuously treated in air at 220 to 250°C for 60 minutes while being elongated by 5% to obtain a flame-resistant yarn. Carbon fibers were obtained by processing for 2 minutes in a heated atmosphere at ~600°C and further processing at 1600°C for 2 minutes. Table 1 shows the strand strength and elastic modulus of the obtained carbon fibers.

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Claims (1)

[Claims] 1. An acrylonitrile polymer containing 95 wt% or more of acrylonitrile and having a weight average molecular weight of 500,000 or more is dissolved in an organic solvent, and the viscosity thereof is 500 to 1,500 at 45°C.
It is characterized by obtaining a spinning stock solution of Poise, discharging it from a nozzle using a dry-wet spinning method to obtain a coagulated thread with a degree of swelling of 200% or less, and firing the acrylic precursor obtained by stretching the obtained coagulated thread. A manufacturing method for high-strength carbon fiber. 2. The method according to claim 1, characterized in that an acrylonitrile polymer produced by carrying out suspension polymerization using a mixed solvent of water/organic solvent as a polymerization medium and an azo initiator is used. Manufacturing method. 3. The manufacturing method according to claim 1, characterized in that an acrylonitrile polymer containing 0.1 to 5 wt% of a polymerizable unsaturated carboxylic acid is used. 4. The manufacturing method according to claim 1, characterized in that a coagulation bath having an organic solvent concentration of 83 wt% or more and a temperature of -20°C or less is used. 5. The manufacturing method according to claim 1, characterized in that a coagulation bath having an organic solvent concentration of 85 wt % or more and a temperature of -30° C. or less is used.