JPH0157165B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field to which the invention pertains] The present invention relates to a method for spinning acrylic fibers for carbon fibers, and more specifically, the present invention relates to a method for spinning acrylic fibers for carbon fibers, and more specifically, the present invention relates to a method for spinning acrylic fibers for carbon fibers, and more specifically, high-strength carbon fibers are produced by improving spinning conditions such as an acrylic polymer, a spinning nozzle, and a coagulation bath. The present invention relates to a spinning method for producing acrylic fibers useful as fiber precursors. [Prior art and its problems] Currently, high-strength continuous carbon fibers can only be obtained from acrylic fibers in practice. High-strength carbon fibers made from acrylic fibers, or highly elastic graphite fibers made from graphitized carbon fibers, can be used as reinforcing materials to obtain lightweight plastic molded products that are stronger or more elastic than metals. It is rapidly expanding and is also being considered for metal reinforcement. Furthermore, in order to improve the quality of molded products and lower the fiber content to reduce costs, there is a desire for higher strength fibers. In the first place, inferring from the crystal structure of carbon fiber,
Although the theoretical strength is thought to be 1800Kg/ ^mm2 , the strength of carbon fibers currently available on the market is at most approximately
It is 350Kg/ ^mm2 . One of the reasons why carbon fibers with lower strength than the theoretical strength can be obtained is as follows. In other words, the method for producing carbon fiber from acrylic fibers generally involves a flame-retardant process in which the filament is heated at 200-300°C in air, and a carbonization process in which the flame-resistant filament thus obtained is heated at 800-1500°C. It is known that in this flameproofing step, a cyclization reaction and an oxidation reaction of polyacrylonitrile occur and heat is generated. However, due to insufficient diffusion of oxygen to the inner layer of the filament or insufficient dissipation of heat, the temperature in the inner layer of the filament may become too high and the fiber structure may be destroyed. It is said that unevenness in the balance between the chemical reaction and the oxidation reaction is part of the reason why there is a limit to the actual strength compared to the theoretical strength and only low strength can be obtained. At the initial stage of arriving at the present invention, the present inventors conducted various studies with the aim of reducing the diameter of each filament of an acrylic fiber in order to obtain a high-strength carbon fiber based on the above-mentioned theory. That is, in general, acrylic fibers are extruded from a nozzle into a coagulation bath and then stretched in at least two of the following stretching processes, such as cold stretching, hot water stretching, dry heat stretching, and steam stretching, to form acrylic polymer molecules. is oriented, increasing its strength and becoming thinner. Therefore, the greater the stretching ratio in these stretching steps, the thinner the filament becomes. Although the limit can be increased somewhat by dividing the stretching process into multiple stages and stretching the film little by little, there is naturally a limit to the stretching ratio. If stretched beyond the limit, the occurrence of fuzz will increase, and sometimes the fibers will be cut, making stable operation impossible. Generally, the final filament speed is determined by the filament speed when leaving the coagulation bath (in the coagulation bath, or between the baths if there are two or more stages of coagulation baths, or the surface speed of this roller if there is an actively rotating roller). The divided value is called the total draft. In other words, the total draft means the value obtained by dividing the final shaking-off or winding yarn speed by the yarn speed on the first roll (roll surface speed). In the initial research carried out by the authors, the limit was about 25 times. It was found that near this limit, the diameter of the filament becomes smaller and the strength of the acrylic fiber increases compared to when the total draft is 20 times, but the strength of the carbon fiber obtained from it becomes weaker. Here,
The inventors' initial research goals mentioned above ran into one paradoxical obstacle. Therefore, in response to the above-mentioned contradictory results, the present inventors proposed that the acrylic polymer solution immediately after the nozzle be stretched before it solidifies, based on the assumption that excessive orientation does not match the structure of the carbon fiber. As a result of extensive experimental research into a method for stretching without orienting molecules, we discovered that the stretchability is greatly influenced by the spinnability of the acrylic polymer solution as well as the structure of the nozzle holes. . In other words, by increasing the nozzle hole length/hole diameter ratio, the upper limit of the nozzle draft (this definition will be explained later) can be raised, and even if the nozzle draft is increased, it will not have much effect on the subsequent stretching process. The present invention was completed by discovering that filament fibers made thinner by increasing the nozzle draft can provide carbon fibers with significantly improved strength. OBJECTS OF THE INVENTION It is therefore a general object of the present invention to obtain high strength carbon fibers to provide useful reinforcing materials for plastics and metals. The main object of the present invention is to provide an acrylic fiber with a small filament diameter that can provide high-strength carbon fiber. Another object of the present invention is to make it possible to increase the drawing ratio of filaments in a coagulation bath, thereby providing carbon fibers with high strength. Another object of the present invention is to provide an industrially advantageous method for producing high-strength carbon fibers by improving the operability of spinning acrylic fibers for carbon fibers. [Summary of the Invention] According to the present invention, when spinning acrylic fibers for carbon fibers, an acrylic polymer solution is extruded into a coagulation bath through a nozzle having a hole length/hole diameter ratio of 2 or more. Draft is 0.5 or more,
The total draft was set to 14 to 20, and the solvent and coagulation bath for the acrylic polymer solution were both a zinc chloride aqueous solution.
The concentration of the acrylic polymer solution is 50 to 150 g/,
There is provided a method for spinning acrylic fibers for carbon fibers, characterized in that the viscosity is 60 poise or more, and the acrylic polymer solution is a solution of a polymer of acrylonitrile and less than 5% of a comonomer. Here, the nozzle draft is defined as the surface velocity of the roller with the driving source that the filament contacts first after leaving the nozzle (hereinafter referred to as the first roller) divided by the linear velocity of the polymer solution in the nozzle hole. refers to value. The polymer solution exits the nozzle hole, comes into contact with the coagulation solution, and gradually solidifies into a filament. At this time, the filament is stretched by the first roller, but the uncoagulated polymer solution stretches more easily than the filament. Therefore, the nozzle draft indicates the magnification by which the polymer solution is stretched before it solidifies. In the four types of stretching described above (i.e., cold stretching, hot water stretching, dry heat stretching, and steam stretching), increasing the stretching ratio in the previous step lowers the upper limit of stretching in the subsequent step, and the limit of the total draft is determined by the solvent. Although the polymer solution, such as the viscosity and solution viscosity, is mostly determined by the polymer solution, the nozzle draft alone hardly changes the maximum stretching ratio in the subsequent process. However, there is a limit to the nozzle draft, beyond which filament breakage increases in the vicinity of the nozzle. The acrylic polymer used in the present invention is obtained by polymerizing monomers containing 85% or more, preferably 90% or more of acrylonitrile. It is not necessary to use a comonomer, but if a small amount is used, the spinning operation will be stabilized and fuzz will be reduced. The comonomers used are not particularly limited, but include vinyl compounds with ester groups, such as methyl acrylate, methyl methacrylate, and vinyl acetate, and vinyl compounds with carboxyl groups, such as acrylic acid and itaconic acid, as well as sulfonic acid groups. It is possible to use a vinyl compound such as para-styrene sulfonic acid. The content of monomers that produce water-soluble polymers such as acrylic acid is preferably 5% or less, and the content of acrylamide is particularly preferably 1% or less. The solvent is not particularly limited either, and dimethylformamide, dimethyl sulfoxide, nitric acid, aqueous rhodan salt solution, aqueous zinc chloride solution, and the like can be used as a solvent, and a diluted solution of these with water can be used in the coagulation bath. For carbon fibers, an inorganic salt aqueous solution, particularly a zinc chloride aqueous solution, is suitable because the cross section of the filament can be made into a uniform circular shape. Furthermore, since a zinc chloride aqueous solution has good spinnability even at a low polymer concentration, it is suitable from this point of view as well, since the filament diameter can be reduced without increasing the draft. It is preferable that the concentration of the acrylic polymer solution is lower because it allows the diameter of the filament to be made smaller without increasing the nozzle draft, but if it is too low, the spinnability decreases. 50-150 depending on solvent
g/ is preferred. When an aqueous zinc chloride solution is used as a solvent, the viscosity becomes a little high, so good results can be obtained at 132 g/or less. Although the lower limit depends on the molecular weight of the polymer, it is preferably 4% or more. It is best to choose a combination of concentration and molecular weight so that the viscosity is 60 poise or higher.
The molecular weight is preferably 60,000 to 250,000. The concentrations of the solvent and coagulation bath may be within commonly used ranges. For zinc chloride, 56-70% as solvent and 20% as coagulation bath
~35% is preferred. Furthermore, when using zinc chloride, it is preferable to add hydrogen chloride in an amount equivalent to or more than the basic salt contained as an impurity in order to prevent filament sticking in the flameproofing step. The nozzle used has a hole shape with a hole length/hole diameter ratio of 2 or more. Here, the pore diameter refers to the minimum diameter of the nozzle hole through which the polymer solution is discharged, and the pore length refers to the length of the minimum diameter portion. In general, nozzles are made by slightly enlarging the introduction part into the nozzle hole due to the desire to increase the plate thickness from the viewpoint of pressure resistance, and the desire to reduce the resistance to the flow of the polymer solution and suppress the increase in nozzle contact pressure. It is normal that there is
In this case, the definition of the hole length (L) and hole diameter (D) is, as shown in FIGS. 1 and 2, L and D that constitute a substantial part of the nozzle, or the minimum diameter D mentioned above, The length L of the minimum diameter D portion is shown. Pore diameter is 50~200μ,
Particularly preferred is one of 80 to 150μ. If the hole diameter is small, the upper limit of the nozzle draft becomes low for unknown reasons, and the filament diameter cannot necessarily be made small, resulting in poor operability. There is no particular limit to the number of holes per nozzle. The polymer solution is forced through the nozzle hole into the coagulation bath and coagulates to form filaments. In addition, shrinkage is suppressed or stretched by the flow of the coagulation bath or the tension by the rolls. In the present invention, the nozzle draft is set to 0.5 or more, preferably 1 or more. This can be easily achieved by setting L/D to 2 or more. In one example,
When L/D was 1, the maximum nozzle draft was 0.37, when L/D was 2, it was 0.8, and when L/D was 3, it was 1.5. In the coagulation bath the polymer solution forms filaments,
Thereafter, the usual steps include water washing, cold stretching, drying, and hot stretching. In order to prevent filaments from sticking together during the drying process, it is preferable to perform hot water treatment in the latter stage of washing. Although the total draft varies depending on conditions such as the solvent and coagulation bath, it can be determined almost unaffected by the nozzle draft. Since the filament is thinner than before due to the nozzle draft, there is no need to forcefully increase the total draft. Forcibly increasing the total draft may weaken the strength of the final carbon fiber, even if the strength of the acrylic fiber for carbon fiber increases. It is preferable to search for The flameproofing process and carbonization process are also carried out without any particular change from conventional methods. The optimal residence time varies depending on the temperature conditions for flame resistance, the atmosphere, the copolymerization components of the polymer, etc., but the smaller the diameter of the filaments that make up the fiber, the easier it is to achieve flame resistance, so lower the temperature or shorten the residence time than before. It is preferable to do so. It is preferable to select conditions such that the increase in oxygen in the filament, which is generally an indicator of flame resistance, is 4 to 12%. According to the present invention, an acrylic fiber having a filament diameter of 10 μm or less and useful as a raw material for high-strength carbon fiber can be easily obtained. [Examples of the Invention] The present invention will be specifically described below with reference to Examples. Example 60% of acrylonitrile containing 2% methyl acrylate and 0.5% itaconic acid as comonomers
% zinc chloride aqueous solution by a conventional method to obtain a solution with a polymer concentration of 5.2% (86 g/). It had a viscosity of 260 poise (45°C) and a molecular weight of 160,000. Using a nozzle with a pore diameter of 100μ and a number of holes of 3000, apply this solution at 5℃, 27%
The fibers were spun into an aqueous solution of zinc chloride, washed with water (cold stretching at the beginning of washing), stretched with hot water, dried, and stretched with steam (steam pressure 2 Kg/mm ² gauge) to obtain acrylic fiber for carbon fiber. After the obtained fibers were subjected to a moist heat relaxation treatment at 90°C, they were heated in a flameproofing furnace at 240°C in the first half and 270°C in the latter half with a tension of 400 g for about 30 minutes (in detail, the yarn speed was adjusted so that the increase in oxygen in the fibers was 6%). regulated) and then carbonized at 1300° C. in high purity nitrogen.
The strand strength of JIS R7601 was used for the strength of carbon fiber. The spinning conditions and results are shown in Table 1.

〔Effect of the invention〕

According to the present invention, acrylic fibers for carbon fibers with small filament diameters can be easily obtained without excessive molecular orientation, and carbon fibers with extremely high strength can be obtained industrially. The method for spinning acrylic fibers for carbon fibers according to the present invention has been described above with reference to preferred embodiments, but it goes without saying that various changes can be made without departing from the spirit of the present invention.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of the nozzle for explaining the definition of the L/D ratio of the nozzle in the present invention, and Figure 2 is the cross-sectional view of the nozzle.
3 is a cross-sectional view of another form of nozzle for the same purpose as in the figure; FIG.

Claims (1)

[Claims] 1. When spinning acrylic fibers for carbon fibers, the acrylic polymer solution is extruded into a coagulation bath through a nozzle with a hole length/hole diameter ratio of 2 or more, with a nozzle draft of 0.5 or more and a total draft of 14 ~20, the solvent and coagulation bath for the acrylic polymer solution are both a zinc chloride aqueous solution, the concentration of the acrylic polymer solution is 50 to 150 g/, the viscosity is 60 poise or more, and the acrylic polymer solution is mixed with acrylonitrile. A method for spinning acrylic fibers for carbon fibers, the method comprising forming a solution of a polymer with less than 5% of a comonomer.