JPS6156328B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength carbon fibers with excellent uniformity (in this specification, "carbon fibers" include "graphite fibers"). More specifically, as a precursor for high-strength carbon fibers, acrylic fibers produced by drawing coagulated fibers obtained in a wet spinning process in a solvent-containing drawing bath and then performing secondary drawing are used, and this is then sintered. The present invention relates to a method for producing high-strength carbon fibers with excellent uniformity. The production of carbon fibers from acrylic fibers is well known and widely practiced. However,
There is still considerable variation in the physical properties of carbon fibers, and therefore it cannot be said that the original capabilities of carbon fibers are fully demonstrated. Furthermore, among the processes for manufacturing carbon fibers, the current situation is that many threads are cut in the so-called flameproofing process in which the fibers are heat-treated in an oxidizing atmosphere, resulting in a decrease in productivity. In order to solve these problems, several improvements in the strength, elastic modulus, degree of orientation, crystallinity, etc. of precursors for carbon fibers have been proposed.
In addition, cut threads in the flameproofing process are thought to be caused by contact, fusion, etc. during the flameproofing process of the precursor, but in order to prevent adhesion and fusion, it is necessary to optimize the flameproofing conditions. The development of oil agents has been proposed. Despite such proposals, the strength of carbon fibers has only reached a few ten percent of the theoretically achievable estimated values, and the elastic modulus has only reached about 50%. Furthermore, with regard to problems such as cut threads during flame resistance, fully satisfactory results have not been obtained. An object of the present invention is to obtain carbon fibers that have high strength and have small variations in strength and other physical properties. Still another object is to provide a precursor that reduces cutting threads during flameproofing and enables stable operation. The method for producing carbon fibers according to the present invention involves dissolving an acrylonitrile-based polymer in a solvent to create a spinning dope, extruding the spinning dope from a spinneret into a coagulation bath and coagulating it without stretching. After drawing the coagulated thread still containing the solvent in a drawing bath containing a solvent at a concentration higher than the coagulation solution concentration in the coagulation bath,
Alternatively, the method is characterized in that the acrylic fiber created by performing secondary stretching after further washing with water is fired. As mentioned above, after sufficiently coagulating the prepared spinning dope in a coagulation bath,
Stretching in a solvent-containing stretching bath results in a homogeneous structure. That is, in a solvent-containing stretching bath, the polymer aggregates formed by coagulation are in a state where they are easy to move, so that more uniform stretching is possible, a uniform structure can be formed, and the degree of orientation is also uniform. Moreover, by pre-stretching in a state containing a solvent, secondary stretchability is good, it becomes possible to achieve a higher stretching ratio, and the physical properties of the obtained fibers are improved. When the acrylic fiber thus obtained is used as a precursor and fired, a uniform carbon fiber with high strength and other physical properties and small variations in these properties can be obtained. Furthermore, the acrylic fiber precursor obtained by the method of the present invention has small variations in strength and other physical properties, and has few structural defects, so the number of cut threads during flame resistance is extremely reduced, and the operating rate during flame resistance is improved. . In the method of the present invention, by fully coagulating in a coagulation bath and then stretching in a stretching bath containing a solvent, it is possible to achieve more uniform stretching, increase orientation, and reduce the degree of swelling. This will advance the process of refinement. Thus, in order to reduce the swelling degree and promote densification, it is necessary to make the solvent concentration in the solvent-containing drawing bath higher than the coagulation solution concentration in the coagulation bath, and the difference in concentration between the two must be 3% by weight or more. It is desirable that Further, the temperature of the solvent-containing drawing bath is preferably 5° C. or more higher than the temperature of the coagulation bath. This is because the degree of swelling decreases and densification progresses more quickly. The stretching ratio in a solvent-containing stretching bath is 3 to 8.
It is desirable to double the amount. As mentioned above, fibers uniformly drawn in a solvent-containing drawing bath have a uniform degree of orientation and extremely excellent secondary drawability. Therefore, by performing secondary stretching, the total stretching ratio can be increased, and a precursor having an even higher degree of orientation and crystallinity can be obtained. In particular, when a nitric acid aqueous solution is used as a solvent for preparing the spinning stock solution, a coagulating liquid, and a drawing bath liquid, it is difficult to increase the drawing ratio when drawing under the same concentration conditions as the coagulating bath. As in the method of the present invention, the effect of the present invention is very significant when the material is sufficiently coagulated in a coagulation bath and then stretched in an aqueous nitric acid solution having a concentration higher than that of the nitric acid in the coagulation solution. Furthermore, the conditions of the preferable solvent-containing drawing bath are as follows:
The concentration is 3% by weight or more higher than that of the coagulation bath, and the temperature is higher than that of the coagulation bath (particularly 5° C. or more). Stretching under these conditions can sufficiently increase the stretching ratio and enable uniform stretching. At the same time, the secondary stretching properties are improved, the total stretching ratio is increased, the degree of orientation is improved, and the stretching ratio is 1.5 denier or less. This makes it easier to spin fine-grained fibers. In the method of the present invention, a solvent for preparing a spinning stock solution,
The coagulating liquid in the coagulating bath and the solvent in the solvent-containing stretching bath can be selected from those known as solvents for acrylic polymers. Examples of such solvents include organic solvents such as dimethylformamide, dimethylacetamide, and dimethyl sulfoxide, and inorganic solvents such as rhodan salt, zinc chloride, nitric acid, and sulfuric acid. Among these, inorganic acid solvents such as nitric acid, which are coagulated at low temperatures to avoid denaturation during spinning, are highly effective. Further, although the solvent in the solvent-containing drawing bath and the coagulating liquid in the coagulating bath may be different, it is preferable that they be the same substance in consideration of recovery and the like. "Acrylic polymer" used in the method of the present invention
refers to polyacrylonitrile and copolymers of acrylonitrile and other unsaturated monomers containing at least 90% by weight of acrylonitrile. Other unsaturated monomers to be copolymerized include ethylenically unsaturated compounds such as acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, vinyl acetate, and itaconic acid. Acrylic polymers can be obtained by commonly used polymerization methods. The spinning itself in the method of the present invention may be carried out in accordance with the generally known wet spinning technique.
The fibers after stretching are washed with water and dried, and any washing method may be used as long as the solvent contained therein can be removed, and the drying method is not particularly limited as long as it can remove water. After stretching in a solvent-containing stretching bath or after further washing with water, secondary stretching is performed. The stretching method in the secondary stretching is conventionally known hot water stretching method, steam stretching method,
Any stretching method such as a pressurized saturated steam stretching method, a heated steam stretching method, a dry heat stretching method, a hot pin stretching method, etc. may be used, or a combination of two or more of these stretching methods may be used. The stretching temperature is not particularly limited either, and may be the same as conventionally used conditions as long as the fibers are sufficiently heated and the desired uniform stretching is achieved. The stretching ratio applied in the secondary stretching is preferably 1.3 to 3 times the fiber length after stretching in a solvent-containing stretching bath. The total stretching ratio in the stretching in the solvent-containing stretching bath and the secondary stretching is preferably 5 to 15 times. In producing carbon fibers from the acrylic fibers thus obtained, any conventional firing method can be employed. Generally, it consists of a flameproofing process in which cyclization is achieved by heating to 150 to 400°C in an oxidizing atmosphere, and then a carbonization process in which carbonization or graphitization is achieved by firing at a high temperature in a non-oxidizing atmosphere or under reduced pressure. The following firing method is suitable. Air is suitable as the atmosphere for the flameproofing process, and nitrogen, helium, argon, etc. are suitable as the atmosphere for carbonization or graphitization. The carbon fiber obtained by the method of the present invention has excellent strength and uniform quality, and
It has the best performance as a composite forming material. Furthermore, there are very few cut threads during the flameproofing process,
Carbon fibers can be manufactured with high productivity. Typical examples of the method of the present invention are shown below. In the examples, percentages are expressed on a weight basis unless otherwise specified. Example 1 An acrylonitrile-based copolymer consisting of 97% acrylonitrile and 3% acrylic acid was dissolved in 70% nitric acid to prepare a spinning stock solution with a polymer concentration of 16.3%. It was extruded into a nitric acid aqueous solution at 10°C to sufficiently solidify it, and stretched at a stretching ratio of 5 times by varying the conditions of the nitric acid-containing stretching bath (the conditions are shown in Table 1 below). moreover,
Secondary stretching was performed in hot water or saturated steam at a stretching ratio of 3 times. After that, wash with normal water and heat to 150℃.
A precursor having a single yarn fineness of 1.5 denier and a filament number of 6000 f was prepared. For comparison, the same spinning stock solution as above was used at 35.3%.
After being extruded into a nitric acid aqueous solution at -3°C and sufficiently solidified, it was washed with water without being stretched in a solvent-containing stretching bath, and then stretched 5 times in hot water at 150°C.
A precursor having a single yarn fineness of 1.5 denier and a filament number of 6000 f was prepared. The precursor obtained as described above was continuously treated for 45 minutes while being stretched 1.5 times at 240°C in an air atmosphere using an electric furnace to obtain a flame-resistant yarn. Carbon fibers were obtained by gradually raising the temperature of the synthetic yarn from 300°C to 1200°C for 3 minutes under a nitrogen atmosphere. The physical properties etc. of the obtained carbon fiber were measured. The results were as shown in Table 1 below.

【table】

Table 1 shows that according to the method of the present invention, carbon fibers with high strength and small variations in strength can be obtained. In addition, adhesion is reduced and the number of cut threads is extremely small. Example 2 A drawn yarn obtained by drawing in a solvent-containing drawing bath in the same manner as in Example 1 was washed with water in a conventional manner to remove the solvent, and then dried using a cylinder at 150°C, and then heated at 200°C. A dry plate was used to perform secondary stretching at a draw ratio of 3 times to produce a precursor having a single filament fineness of 1.5 denier and 6000 filaments. The precursor thus obtained was treated under the same conditions as in Example 1, and the strength and variation thereof of carbon fibers obtained were compared. In addition, the presence or absence of cut threads and adhesion during flame resistance was also determined. The result is below
It was as shown in the table.

[Table] The results in Table 2 also show that high strength carbon fibers with excellent uniformity can be obtained by the method of the present invention. Example 3 Acrylonitrile 98%, methyl methacrylate
A spinning stock solution with a polymer concentration of 16% was prepared by dissolving an acrylonitrile copolymer consisting of 1.0% methacrylic acid and 1.0% methacrylic acid in a 55% rhodan soda aqueous solution. Te-3
After being extruded into a 15.5% aqueous Rodan soda solution at 15.3°C and sufficiently coagulated, the conditions of the drawing bath containing Rodan soda were varied (conditions are shown in Table 3 below), and stretching was carried out at a draw ratio of 5.5 times. . Thereafter, the film was thoroughly washed with water and then subjected to secondary stretching at a stretching ratio of 3 times in heated steam at 120°C. Dry at 150℃, single yarn fineness 1.5 denier, number of filaments
A 6000f precursor was created. The precursor obtained as described above was subjected to flameproofing treatment and carbonization treatment in the same manner as in Example 1 to obtain carbon fibers. The strength, elastic modulus, and variation thereof of the fibers were compared and studied. The results were as shown in Table 3 below.

[Table] Samples Nos. 19 to 22 had very good cut threads in the flame-retardant process, whereas sample No. 16 had many cut threads and had extremely poor operability. Example 4 Using the coagulated thread prepared in Example 1, stretching was carried out at a stretching ratio of 6 times in an aqueous zinc chloride solution (concentration 42%, temperature 15°C). After washing with water, secondary stretching was performed in hot water at 100° C. at a stretching ratio of 2.5 times, followed by drying to produce a precursor having a single yarn fineness of 1.2 denier and 6000 filaments. This fiber was subjected to flameproofing treatment and carbonization treatment in the same manner as in Example 1 to obtain carbon fiber. The physical properties of this carbon fiber were measured.
The tensile strength was 435 Kg/mm ² and the variation σ was 2.2 Kg/mm ² , indicating very high strength and small variation. The elastic modulus is also the same,
At 25.5Ton/mm ² , σ was excellent at 0.4Ton/mm ² . In addition, there were very few cut threads during the flame-retardant process, and the plant was in good operating condition.

Claims (1)

[Claims] 1. When producing carbon fibers from acrylic fibers, an acrylic polymer containing 90% by weight or more of acrylonitrile is dissolved in a solvent to create a spinning stock solution, and the spinning stock solution is passed through a spinneret into a coagulation bath. After coagulating without stretching, the resulting coagulated thread containing the solvent is stretched in a drawing bath containing a solvent at a concentration higher than the coagulation solution concentration in the coagulation bath.
Alternatively, a method for producing high-strength carbon fibers with excellent uniformity, which is characterized by firing acrylic fibers created by subjecting them to secondary stretching after further washing with water. 2. The method for producing carbon fibers according to claim 1, wherein there is a difference of 3% by weight or more between the coagulation solution concentration in the coagulation bath and the solvent concentration in the solvent-containing drawing bath. 3. The method for producing carbon fibers according to claim 1 or 2, wherein the temperature of the coagulating liquid in the coagulating bath is 5° C. or more lower than the temperature of the solvent in the solvent-containing drawing bath. 4. The method for producing carbon fibers according to any one of claims 1 to 3, wherein an aqueous nitric acid solution is used as a solvent for preparing a spinning dope, a coagulating liquid, and a solvent in a solvent-containing drawing bath.