JPS623248B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a process for producing flame-resistant fibers or carbon fibers from acrylic fibers, in which the precursor acrylic fibers are heated in an oxidizing atmosphere, and the evaporated products of oils and the like contained in the fibers, as well as these oils and fibers, are used. The present invention relates to a production method that prevents troubles caused by thermal decomposition products accompanying the oxidation reaction of carbon fibers and efficiently produces high-grade, high-quality flame-resistant fibers or carbon fibers. Conventionally, flame-resistant or carbon fibers have been made using various precursors (hereinafter referred to as
(referred to as a precursor) in an oxidizing atmosphere to thermally stabilize it into a flame-resistant fiber, or further heat the flame-resistant fiber in an inert atmosphere at a high temperature to carbonize it,
Manufactured by converting to carbon fiber. In such flame-retardant or carbon fiber manufacturing methods, when fibers, such as acrylic fibers, are treated with an oil agent etc. in the spinning process, they are heated in the heating process in the oxidizing atmosphere, that is, in the oxidation process. Since the oil agent and its thermal decomposition products are released into the oxidizing atmosphere together with the thermal decomposition products due to the oxidation reaction of the fibers, it is necessary to treat these oil agents and thermal decomposition products to make them harmless, and the oxidation process In addition to the high-temperature heating of the oxidizing atmosphere, energy consumption for this exhaust gas treatment is enormous, and energy conservation is extremely important industrially. In addition, oils evaporated and decomposed from the precursor contained in the oxidation atmosphere, especially silicone oils and thermal decomposition products of the precursor, especially tar-like substances, can have a long lifespan in the oxidation catalyst treatment of the exhaust gas discharged from the oxidation process. There are problems in that the flame resistance may be suddenly reduced, or the single filaments of the flame resistant fibers may be fused together, causing uneven flame resistance. As a means of saving energy in the former oxidation process, Japanese Patent Application Laid-Open No. 57-25417 discloses a method in which exhaust gas from the oxidation process is treated with a catalyst and then recycled for use in the oxidation process. It is difficult to use the latter method for oxidizing the silicone-based oil treatment precursor due to the lifetime of the catalyst. The present inventors solved such problems. In particular, the present invention was discovered through extensive research into methods that can be applied to the treatment of exhaust gases that contain substances that have a strong poisoning effect on oxidation catalysts, such as silicone oils. That is, in the present invention, when heating the precursor in an oxidizing atmosphere to convert it into oxidized fibers, the oxidation process is divided into at least two stages, and each of the divided oxidation stages is independently oxidized. The atmosphere is supplied, and the divided first oxidation process is supplied with at least the exhaust gas discharged from the oxidation process after the second stage or a mixed gas of the exhaust gas and fresh oxidation gas. . In the present invention, in the first oxidation step of the oxidation step divided into at least two stages, that is, the first step in which the precursor is first heated in an oxidizing atmosphere, the precursor is discharged from the second and subsequent oxidation steps. The exhaust gas or a mixture of the exhaust gas with a fresh oxidizing gas, usually fresh air, is supplied. In other words, in such a multi-stage oxidation process, it is necessary to increase the oxidizing atmosphere temperature from the first stage to the second stage, so the temperature of the exhaust gas supplied to the first oxidation process does not need to be heated, or is replaced by fresh air, etc. Also when mixing, the thermal energy required for heating can be significantly reduced. Importantly, as will be described later, in the present invention, the conditions for the first oxidation step are such that the moisture content of the fibers (in an incompletely oxidized state) after the oxidation step is approximately 2 to 4% by weight. By controlling the temperature so as to achieve this, it is possible to substantially completely remove tar-like substances originating from the oily substance generated from the precursor in the oxidation step. Therefore, in the oxidation process of the present invention, in the second and subsequent oxidation processes, the amount of thermal decomposition products generated due to the oxidation of the precursor, or more precisely, the incompletely or partially oxidized fibers, is extremely small, and moreover, the amount of thermal decomposition products generated by the oxidation of the incompletely or partially oxidized fibers is extremely small. The content of silicone oils, their thermal decomposition products, and tar-like substances, which impede the catalytic treatment of the oils, is also significantly reduced. Therefore, the life of the oxidation catalyst treatment of the exhaust gas discharged from the second and subsequent oxidation processes is greatly extended, and the occurrence of string breakage in the oxidation process due to tar-like substances in the second and subsequent oxidation processes. can be prevented. In the present invention, the number of stages in the oxidation step to be divided is not particularly limited, but usually 2 to 4 stages is preferred, and exceeding 4 stages is industrially disadvantageous in terms of equipment design. Here, as the divided first oxidation process conditions, the moisture content of the precursor that has undergone this process is approximately 2 to 4.
% is based on the following facts. That is, FIG. 1 shows the moisture content of the acrylic fiber (which is a measure of the degree of oxidation of the fiber) when the acrylic fiber to which the precursor silicone oil has been applied is heated in an oxidizing atmosphere. This is a diagram showing the relationship between the silicone oil generated from the fiber and the amount of thermal decomposition products of the oil and fiber, where S is the amount of silicone oil generated that evaporates and thermally decomposes in the oxidation process, and T is the amount of silicone oil generated in the oxidation process. shows the amount of tar-like substances generated. From the figure, it can be seen that the amount of these products generated in the oxidation step is generated at the beginning of the oxidation step, and the amount tends to decrease over time. In particular, silicone oils and their thermal decomposition products, low molecular weight silicones, as well as the thermal decomposition products of the fibers themselves, undergo substantial oxidation at the initial stage of oxidation, especially when the moisture content of acrylic fibers reaches about 2-4%. This makes it possible to remove most of it. That is, in the present invention, when the conditions for the first divided oxidation stage are set such that the moisture content of the precursor is about 2 to 4%, most of the thermal decomposition products generated from the precursor are removed at this stage. If the fibers can be removed and have a moisture content within this range, problems such as inter-filament fusion of the precursor and uneven oxidation due to the thermal decomposition products will not be a particular problem in the first oxidation stage. First, even if such fusion or non-uniform oxidation occurs, it can be substantially eliminated by oxidation in the second and subsequent stages, and there will be no defects in the flame resistance and the quality of the resulting carbon fiber. It is. Next, in the present invention, the oxidizing atmosphere is supplied to each of the divided oxidation steps independently. By independently supplying this oxidizing atmosphere to each divided oxidation process, it becomes possible to treat the exhaust gas discharged from each process independently, and the exhaust gas discharged from the latter oxidation process is transferred to the former stage. It can be used as an oxidizing atmosphere in the oxidation process, making it extremely effective in terms of energy conservation. In particular, in the divided first oxidation step of the present invention, the exhaust gas discharged from the second and subsequent oxidation steps can be used as is as an oxidation atmosphere,
There is almost no need to heat the oxidizing atmosphere supplied to this first oxidation step, making temperature control extremely easy and providing great process advantages. In this respect, the exhaust gas discharged from the third oxidation step may be supplied to the second oxidation step, and in such a case, only the oxidizing atmosphere supplied to the final oxidation step is heated. It will also be possible to get it done. The exhaust gas discharged from the divided first oxidation process can be directly combusted as is, and the silicone oil and tar-like substances contained in the exhaust gas discharged from the second and subsequent oxidation processes can be combusted as is. Since the content of substances is small, the life of the catalyst is less likely to be shortened even if it is treated with a catalyst, so it is advantageous in terms of process and energy for exhaust gas treatment. Hereinafter, specific embodiments of the present invention will be explained with reference to the drawings. FIG. 2 is a flowchart showing one example of the oxidation step of the present invention. In the figure, 0 is a precursor, 1 and 2 are the first stage (first) and second stage oxidation furnaces, respectively, 3 is a fresh outside air supply line, 4 and 4' are supply air adjustment valves,
5, 5' are heaters, 6, 6' and 9 are blowers,
7, 7' are circulating gas supply lines, 8, 8' are circulating gas extraction lines, 10 are exhaust gas control valves, 11
1 is an exhaust line, 12 is an exhaust gas treatment facility, and 13 is an atmospheric release line. As shown in the figure, precursor 0 is oxidized in oxidation furnaces 1 and 2 divided into two stages. The mixed gas of the circulating gas extraction line 8' of the furnace 2 and fresh air from the outside air supply line 3 whose supply air amount is adjusted by the supply air adjustment valve 4' is heated to a predetermined temperature by the heater 5'. The exhaust gas is divided into two parts by a blower 6' via a circulating gas supply line 7', and one of the exhaust gases is supplied to the oxidation furnace 2. The remaining exhaust gas is mixed with the circulating gas of the furnace 1 and fed to the furnace 1. On the other hand, the gas that has passed through the circulating gas extraction line 8 of the furnace 1 is divided into two parts, most of which are mixed with the gas itself or fresh air from the outside air supply line 3 and heated to a predetermined temperature by the heater 5. The circulating gas is supplied into the furnace 1 by the blower 6 through the circulating gas supply line 7, and as described above, the divided exhaust gases from the furnace 2 are mixed in the supply line 7. A portion of the exhaust gas from the furnace 1 is normally subjected to direct combustion treatment in an exhaust gas treatment facility via an exhaust line 11 with the exhaust amount adjusted by an exhaust gas control valve 10, and then exhausted via an atmosphere discharge line 13. Next, FIGS. 3 and 4 are flowcharts showing another example of the oxidation process of the present invention. Figure 3 shows the case where the volume of furnace 1 is smaller than that of furnace 2, and the volume ratio of furnace 1/furnace 2 is, for example, 1/2.
-1/5 is mentioned, but is not particularly limited. When such a furnace shape is applied to the oxidation process, the effect of reducing temperature irregularities in the furnace 1 increases, and continuous operation makes it easier to remove and clean tar-like substances attached to the yarn entrance and exit part of the furnace 1. It has the following merits. Figure 4 shows that the oxidation process is carried out in three stages, and the internal volume of the furnace is gradually increased, so that one of the divided exhaust gases from the oxidation process of furnace 2 and furnace 3 is mixed with the circulating gas of furnace 1. This is an example of how it is used. In this case, the volume ratio of each of these furnaces is Furnace 1/Furnace 2/Furnace 14 = 0.2/0.8/1~0.3/
The ratio is generally 0.6/1, but is not particularly limited. FIG. 5 is a flowchart showing a conventional oxidation process. The effects of the present invention will be specifically explained below using examples. The tar in the oxidizing atmosphere, the fluff of the flame-resistant yarn, and the moisture content of the flame-resistant yarn were measured by the following methods. (1) Tar-like substances The oxidizing atmosphere gas is guided through a conduit kept at 200°C, and the tar-like substances are adsorbed onto activated carbon, and the tar-like substances are determined by the weight increase of the activated carbon before and after adsorption. Tar-like substance concentration (g/Nm ³ ) = Increase in weight of activated carbon (g) / Activated carbon layer gas passing amount (
(Nm ³ ) (2) Fluff of flame-resistant yarn A flame-resistant yarn made of 6000 denier 6000 filament is placed on white paper, and the number of fluffs is counted over 1 m. (3) Moisture content Place the flame-retardant thread in a desiccator containing an aqueous ammonium sulfate solution (81% constant humidity at 25°C), and after allowing it to absorb moisture for 16 hours, determine the adsorbed moisture content. Example 1, Comparative Example 1 Acrylic fibers to which 3.0% by weight of hydrocarbon oil was added were continuously supplied at a rate of 100 kg/hour and oxidized according to the flows shown in Figures 2, 3, 4, and 5, respectively. . Table 1 shows the results of investigations regarding the oxidation conditions, fluff of the obtained flame-resistant yarn, workability, energy consumption, etc. In the exhaust gas treatment, kerosene was mixed with the exhaust gas as a combustion improver, and direct combustion treatment was performed. As shown in Table 1, test No. to which the present invention was applied.
1, 2, and 3 all consume less energy. In addition, in Test No. 2 and Test No. 3, in which the processing time of the first stage oxidation process is short, tar-like substances are less likely to accumulate at the entrance and exit part of the first stage oxidation process, and therefore, it is difficult for tar-like substances to accumulate in the yarn during firing. There was little adhesion and the quality of the flame-resistant yarn was good. Furthermore, the effect of reducing kerosene consumption in exhaust gas treatment to which the present invention is applied was recognized.

【table】

[Table] Example 2, Comparative Example 2 A blended oil consisting of 20 parts by weight of dimethylaminosiloxane as a silicone oil and 80 parts by weight of a hydrocarbon oil was used on the same acrylic fibers as in Example 1 and Comparative Example 1. Firing was carried out under the same oxidation conditions as in the corresponding flowcharts of Example 1 and Comparative Example 1, except that 2.5% by weight of was added. In the exhaust gas treatment, kerosene was mixed with the exhaust gas as a combustion aid and direct combustion treatment was performed. Table 2 shows the results regarding the fluff, workability, energy consumption, etc. of the flame-resistant yarn obtained. As shown in Table 2, test No. to which the present invention was applied.
4, 5, and 6 consume less energy. Also the first
Test Nos. 5 and 6 with short processing time for stage oxidation process
The fluff and workability of the flame-resistant yarn were good.

【table】 [Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the moisture content of acrylic fibers and the amount of silicone oil generated from the fibers and the thermal decomposition products of the oils and fibers when the acrylic fibers are heated in an oxidizing atmosphere. . FIG. 2 is a flowchart showing an example of the oxidation step of the present invention, and FIGS. 3 and 4 are flowcharts showing other examples of the oxidation step of the present invention. FIG. 5 is a flowchart showing a conventional oxidation process. 0...precursor, 1...first stage oxidation furnace, 2...
...Second stage oxidation furnace, 3...Fresh outside air supply line,
4, 4', 4''...Air supply control valve, 5, 5',
5″……Heater, 6,6′,6″……Blower,
7, 7', 7''...Circulating gas supply line, 8,
8', 8''...Circulating gas extraction line, 9,9'...
Blower, 10, 10'...exhaust gas control valve,
11...exhaust line, 12...exhaust gas treatment equipment,
13...Atmospheric discharge line, 14...Third stage oxidation furnace, S...Amount of silicone oil generated that evaporates and thermally decomposes in the oxidation process, T...Amount of tar-like material generated in the oxidation process.

Claims (1)

[Claims] 1. Using acrylic fiber as a precursor, at 200 to 300°C
When producing carbon fiber by heating the obtained oxidized fiber in an oxidizing atmosphere at a higher temperature, the oxidation step of heating the precursor in an oxidizing atmosphere is performed in at least two stages. In addition to independently supplying the oxidizing gas to each divided oxidation process, the first stage oxidation process is supplied with at least the exhaust gas discharged from the second and subsequent oxidation processes. A method for producing flame-resistant fibers or carbon fibers, characterized by supplying a mixed gas of exhaust gas and fresh oxidizing gas. 2. The method for producing flame-resistant fibers or carbon fibers according to claim 1, wherein the acrylic fibers are fibers treated with a silicone oil agent. 3 In claims 1 and 2, the moisture content of the fiber after passing through the first oxidation step is about 2
A method for producing flame-resistant fibers or carbon fibers having a content of ~4% by weight.