Flame-resistant fiber bundle and method for producing carbon fiber bundle

Abstract

To provide a flame-resistant furnace, a method for producing a flame-resistant fiber bundle and a method for producing a carbon fiber bundle using the flame-resistant furnace, which solve problems of reduction in sealing property and reduction in energy efficiency of a flame-resistant furnace having a folding roller outside the furnace, and a problem of reduction in working environment due to blowing out of heated gas.SOLUTION: A method for producing a flame-resistant fiber bundle by flame-resistant treatment by passing the fiber bundle through a flame-resistant treatment chamber a plurality of times by using a flame-resistant furnace in which a plurality of polyacrylonitrile fiber bundles are run in parallel in a horizontal direction, the furnace having a flame-resistant treatment chamber to which hot air at a constant temperature is supplied and a circulation unit for circulating the hot air, the furnace having a plurality of slits and turning rollers at both ends for allowing the polyacrylonitrile fiber bundles to pass in and out, and an enclosing unit is provided outside the turning rollers above the flame-resistant treatment chamber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a flame-resistant furnace for producing a flame-resistant fiber bundle, and a method for producing a flame-resistant fiber bundle and a carbon fiber bundle using the same. [Background technology]

[0002] Generally, a method for producing a carbon fiber bundle using polyacrylonitrile fibers as a raw material involves subjecting a polyacrylonitrile fiber bundle, which is a bundle of 1,000 to 80,000 single fibers of a polyacrylonitrile polymer, to a flame-retardant treatment at a temperature of 200 to 300°C to obtain a flame-retardant fiber bundle, and then carbonizing the flame-retardant fiber bundle at a temperature of 300 to 2,000°C.

[0003] A typical flame-retardant treatment method for polyacrylonitrile fiber bundles, which are the raw material for carbon fiber bundles, involves arranging multiple guide rollers on both sides of the entrance and exit of a flame-retardant treatment chamber in which an oxidizing gas at 200 to 300°C circulates, and running the polyacrylonitrile fiber bundle through the flame-retardant treatment chamber, where heated and aerated air is supplied multiple times, while the bundle is folded back by the folding back rollers.

[0004] The flame-proofing furnace with a turn-back roller outside the furnace has the advantage that even if a yarn breakage occurs and part of the fiber bundle gets wrapped around the turn-back roller, the treatment can be easily carried out without stopping the drive.

[0005] However, because the flame-resistant treatment chamber has multiple openings on both sides for the entrance and exit of fiber bundles, the heating gas inside the furnace leaks out of the furnace and outside air leaks into the furnace. This leakage and infiltration is particularly pronounced in large flame-resistant treatment furnaces, where the vertical pressure difference within the furnace is large. Leakage and infiltration cause temperature variations within the flame-resistant treatment chamber, reducing product quality. Furthermore, the inflow of low-temperature outside air into the furnace also lowers the temperature of the heating gas inside the chamber, increasing the power consumption of the heater used to heat the circulating heating gas and reducing energy efficiency. Furthermore, during the flame-resistant treatment of polyacrylonitrile fiber bundles, decomposition gases such as cyanide are generated inside the flame-resistant treatment furnace. Therefore, from a work environment perspective, it is essential to prevent the gas inside the furnace from leaking into the atmosphere and to take measures to separately combust the gas before discharging it outside.

[0006] To solve the above problem, it is necessary to provide sealing chambers outside the openings on both sides of the flame-proofing chamber, which requires extra space. Many techniques have been devised to improve the sealing performance of the sealing chamber, such as circulating gas within the sealing chamber or creating multiple sealing chambers. However, since there are multiple openings on both sides of the flame-proofing furnace, it is extremely difficult to completely prevent the inflow of outside air and the leakage of furnace gas.

[0007] In the flame-proofing furnaces having rollers inside the furnace as in Patent Documents 1, 2 and 3, there is no inflow of outside air or leakage of furnace gas as described above, and therefore they have the advantages of high energy efficiency, no environmental issues, and space saving because a sealing chamber is not required. However, they have the disadvantage of making it difficult to deal with problems such as thread breakage when they occur. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-153987 [Patent Document 2] Japanese Patent Application Publication No. 2018-111891 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-138325 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to solve the problems of the prior art described above, and to provide a flame-resistant furnace for flame-resistant polyacrylonitrile fiber bundles, a method for producing a flame-resistant fiber bundle, and a method for producing a carbon fiber bundle, which can solve the problems of reduced sealing performance and reduced energy efficiency in a flame-resistant furnace having a folding roller outside the furnace, and the problem of a worsening working environment due to the blowing out of heated gas, and to provide a flame-resistant furnace, a method for producing a flame-resistant fiber bundle using the flame-resistant furnace, and a method for producing a carbon fiber bundle. [Means for solving the problem]

[0010] In order to achieve the above object, the method for producing a flame-resistant fiber bundle of the present invention has the following configuration: That is, it is a method for producing a flame-resistant fiber bundle, in which a flame-resistant treatment is performed by passing the polyacrylonitrile fiber bundle through the flame-resistant treatment chamber multiple times using a flame-resistant furnace which has a plurality of polyacrylonitrile fiber bundles running in parallel in the horizontal direction, a flame-resistant treatment chamber to which hot air of a constant temperature is supplied and a circulation section for circulating the hot air, a plurality of slits and turning rollers for allowing the polyacrylonitrile fiber bundle to pass in and out at both ends of the flame-resistant treatment chamber, and an upper flame-resistant treatment enclosure section provided outside the turning rollers and above the flame-resistant treatment section.

[0011] The method for producing a flame-resistant fiber bundle of the present invention has the following configuration: That is, in the method for producing a flame-resistant fiber bundle, the surface temperature of the turning roller is 60°C to 200°C.

[0012] The method for producing a flame-resistant fiber bundle of the present invention has the following configuration: That is, in the method for producing a flame-resistant fiber bundle, the flame-resistant upper enclosure comprises a sheet-like heat-resistant structure having an openable and closable curtain structure, and the heat-resistant structure has a heat-resistant temperature of 300°C or higher.

[0013] The method for producing a flame-resistant fiber bundle of the present invention has the following configuration: That is, in the method for producing a flame-resistant fiber bundle, circulating hot air for flame-resistant treatment is supplied from the top to the bottom of a flame-resistant furnace to a polyacrylonitrile fiber bundle traveling in a flame-resistant treatment chamber.

[0014] The method for producing a carbon fiber bundle of the present invention has the following configuration: the method for producing a carbon fiber bundle is characterized in that the flame-resistant fiber bundle obtained by the method for producing a flame-resistant fiber bundle is pre-carbonized at 300°C to 1000°C, and then carbonized at 1000°C to 2500°C. [Effects of the Invention]

[0015] According to the method for producing a flame-resistant fiber bundle of the present invention, the sealing performance of the flame-resistant furnace is improved and the outflow of heated gas from the flame-resistant treatment chamber is prevented, thereby reducing the amount of energy used to heat the circulating gas and thereby reducing the amount of power consumed by the heater. Furthermore, temperature variations within the flame-resistant treatment chamber are eliminated, making it possible to achieve uniform quality and ensure process stability. Furthermore, by preventing the heated gas in the flame-resistant treatment chamber from leaking out of the flame-resistant treatment furnace, the diffusion of decomposition gas into the working environment can be suppressed. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic configuration diagram of a flame-proofing furnace according to an embodiment of the present invention. FIG. [Figure 2] FIG. 1 is a schematic diagram of a conventional flame-proofing furnace having a folding roller outside the furnace. [Figure 3]FIG. 10 is a schematic diagram of a flame-proofing furnace in which some of the folding rollers become part of the furnace wall of the sealing chamber. [Figure 4] This is a schematic diagram of a type of flame-resistant furnace that has all of the folding rollers inside the furnace. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention is a method for producing a flame-resistant fiber by flame-resistant treatment of a polyacrylonitrile fiber bundle to obtain a flame-resistant fiber bundle, and the method uses a flame-resistant furnace in which a plurality of polyacrylonitrile fiber bundles are run in parallel in the horizontal direction, the furnace having a flame-resistant treatment chamber to which hot air at a constant temperature is supplied and a circulation section to circulate the hot air, and a plurality of slits and turning rollers are provided at both ends of the flame-resistant treatment chamber to allow the polyacrylonitrile fiber bundle to pass in and out, and an upper flame-resistant treatment enclosure provided outside the turning rollers above the flame-resistant treatment chamber, to obtain a flame-resistant fiber bundle by passing the fiber bundle through the flame-resistant treatment chamber multiple times.

[0018] The polyacrylonitrile fiber bundle used as the heat-treated fiber bundle in the method for producing a flame-resistant fiber bundle of the present invention is preferably made of an acrylic fiber containing 100 mol % of acrylonitrile or an acrylic copolymer fiber containing 90 mol % or more of acrylonitrile. The copolymerization components in the acrylic copolymer fiber are preferably acrylic acid, methacrylic acid, itaconic acid, and alkali metal salts thereof, ammonium metal salts thereof, acrylamide, methyl acrylate, etc., but the chemical properties, physical properties, dimensions, etc. of the acrylic fiber bundle are not particularly limited.

[0019] The flame-proofing treatment of the present invention is a method for flame-proofing the polyacrylonitrile fiber bundle in an oxidizing atmosphere, and is carried out in a flame-proofing furnace through which an oxidizing gas flows. The oxidizing gas is not particularly limited, but air is preferred from an economical viewpoint. As shown in FIG. 1 , the flame-proofing furnace 1 comprises a flame-proofing chamber 2 having a plurality of slit-shaped openings 3 in two opposing side walls 7 through which the fiber group A enters and exits, and a sealing chamber 6 located outside the side wall 7 of the flame-proofing chamber 2 and sharing the slit-shaped openings 3 with the flame-proofing chamber 2. The sealing chamber 6 has a plurality of slit-shaped openings 4 through which the fiber group A enters and exits in a sealing chamber side wall (outer side wall) 8 opposite the side wall 7, and is composed of a plurality of folding rollers 5 located outside the sealing chamber side wall 8. The fiber group A to be treated is folded back by rollers 5 arranged in multiple stages, travels horizontally through the flame-resistant treatment chamber 2 and the sealing chamber 6, and is subjected to heat treatment by being blown with heated outside air in the flame-resistant treatment chamber 2, thereby being flame-resistant, and becomes a flame-resistant fiber bundle. The position of the folding back rollers in a flame-resistant furnace having such a configuration and the position of the work area, which is the work environment, will be referred to hereinafter as outside the flame-resistant furnace.

[0020] The structure of the flame-proofing furnace 1 for performing the flame-proofing treatment of the present invention comprises a flame-proofing treatment chamber 2 in which hot air at a constant temperature is supplied to the fiber group A to be treated, a circulation unit (not shown) that circulates the hot air as circulating hot air, and a plurality of slit-shaped openings 3 and 4 that allow the fiber group A to enter and exit the flame-proofing treatment chamber 2 at both ends in the running direction of the fiber group A to be treated, and a plurality of turning rollers 5. The fiber group A to be treated enters and exits the flame-proofing treatment chamber 2 through the slit-shaped openings 3, is turned back by the turning rollers 5, and passes through the flame-proofing treatment chamber 2 in a zigzag pattern.

[0021] During the flame-proofing treatment, the fibers A traveling through the flame-proofing treatment chamber 2 undergo an exothermic reaction, so it is necessary to suppress and control excessive heat generation in the traveling fibers A. A common means for suppressing heat generation is to blow temperature-controlled hot air from outside the flame-proofing treatment chamber through a circulation section onto the traveling yarn to remove heat. There are two possible directions for supplying the hot air: a cross flow, in which the hot air is supplied in a direction perpendicular to the direction in which the fibers A travel through the flame-proofing treatment chamber 2, and a parallel flow, in which the hot air is supplied in a direction parallel to the fibers A. In the present invention, the cross flow, in which the circulating hot air for flame-proofing is directed from the top to the bottom of the heat treatment furnace, is supplied to the polyacrylonitrile fiber bundle traveling through the flame-proofing treatment chamber, and this cross flow allows for temperature control of the entire bundle in the hot air circulation section, and is therefore a preferred form because it is more cost-effective than a parallel flow.

[0022] The flame-resistant furnace 1 has multiple openings on both sides of the flame-resistant treatment chamber for the entrance and exit of the treated fiber group A. The openings allow the heated gas inside the furnace to leak into the atmosphere, degrading the working environment. The heated gas is blown out due to the pressure difference between the top and bottom of the flame-resistant furnace. The upper 60% of the height of the fiber group running through the furnace, from the bottom to the top, is the blowing area, with the upper 50% having more blowing and the additional 30% having particularly more blowing. The upper flame-resistant treatment enclosure 11 is installed outside the return rollers at the top of the flame-resistant furnace, where the heated gas is blown out through the slit-shaped openings 4. By installing the upper flame-resistant treatment enclosure outside the return rollers, the enclosure forms part of the furnace wall, thereby suppressing the leakage of heated gas from the flame-resistant furnace into the atmosphere.

[0023] If the temperature of the treated fiber group A traveling on the turn-back roller exceeds 200°C, the flame-proofing reaction in the treated fiber group A continues, and the generated decomposition products of the polyacrylonitrile fiber bundle and decomposition gas of the oil agent may condense and be transferred to the turn-back roller. The condensed gas is sticky. For this reason, the surface temperature of the turn-back roller is preferably 60 to 200°C because the fuzz of the single yarns of the treated fiber group A is caught by the turn-back roller, causing the fuzz to grow and lead to yarn breakage. Furthermore, the upper limit of the surface temperature of the turn-back roller is preferably 200°C or less, and more preferably 180°C or less. The lower limit of the turn-back roller temperature is preferably 60°C or more because reheating is required when the treated fiber group A is re-introduced into the flame-proofing furnace. If the temperature is less than 60°C, excessive energy is required to reheat the treated fiber group A, which may increase power consumption.

[0024] In the present invention, the upper part of the flame-resistant treatment refers to the upper 30% of the blow-out area in the height direction from the lowest to the highest position in the flame-resistant furnace through which the treated fiber group A runs. Note that, hereinafter, the blow-out area in the height direction from the lowest to the highest position in the flame-resistant furnace through which the treated fiber group A runs may also be simply referred to as the blow-out area.

[0025] The flame-resistant treated upper enclosure in the present invention refers to an enclosure that covers the flame-resistant treated upper portion, and is located outside the turn-back roller and covers the flame-resistant treated upper portion.

[0026] In the present invention, by providing an upper flame-resistant treatment enclosure provided outside the turn-back roller and above the flame-resistant treatment, it is possible to prevent the leakage of heated gas from inside the flame-resistant treatment furnace above the flame-resistant treatment to the outside of the flame-resistant treatment furnace.

[0027] In the present invention, it is sufficient that at least the upper part of the flame-resistant treatment (the upper 30% area which is the blow-out area) is covered by the flame-resistant treatment upper enclosure, it is preferable that the upper 40% area which is the blow-out area is covered by the flame-resistant treatment upper enclosure, it is more preferable that the upper 50% area which is the blow-out area is covered by the flame-resistant treatment upper enclosure, and it is even more preferable that the upper 60% area which is the blow-out area is covered by the flame-resistant treatment upper enclosure.

[0028] In this way, the upper area of ​​the blowing area is covered by the upper enclosure for flame-proofing treatment, which improves the heat retention in the flame-proofing treatment chamber and improves thermal efficiency. As a result, the power consumption of the heater can be significantly reduced compared to conventional flame-proofing furnaces with folding rollers outside the furnace.

[0029] It is preferable that the flame-resistant treated upper enclosure has excellent heat resistance so that it will not deteriorate even when exposed to heated gas for a long period of time in a flame-resistant furnace. Any material and shape that has such properties can be used, but examples of materials include silica cloth and glass cloth, and examples of shapes include curtain type and roll type.

[0030] Furthermore, in one preferred embodiment, the upper enclosure for flame-resistant treatment in the present invention is a sheet-like heat-resistant structure with an openable curtain structure. While this heat-resistant structure is installed, the structure is similar to a flame-resistant furnace with a folding roller inside. This reduces gas leakage from the flame-resistant furnace to near zero, and prevents the treated fiber group A and the folding roller from being cooled by outside air, thereby improving energy efficiency. Furthermore, because the curtain structure is openable, the curtain can be opened during trouble during steady-state operation, start-up preparation, or maintenance, converting the furnace into a conventional flame-resistant furnace with a folding roller outside the furnace, allowing easy access to the folding roller. Furthermore, an accordion-type curtain structure is preferred because it can be folded up compactly when opened, securing work space and preventing interference during work.

[0031] Furthermore, it is preferable that the installed sheet-like heat-resistant structure having a curtain structure can be continuously used at the heat treatment temperature of the flame-resistant furnace, and the heat-resistant temperature is preferably 300°C or higher, which is higher than the heat treatment temperature of the flame-resistant furnace, and more preferably 400°C or higher for long-term durability.

[0032] The flame-resistant furnace 1 is characterized by having a heated gas outlet 9 for blowing heated gas downward and a heated gas inlet 10 for drawing in the heated gas. The heated gas drawn in through the heated gas inlet 10 is recirculated and blown out through the upper heated gas outlet 9, and then supplied to the polyacrylonitrile fiber bundle traveling within the flame-resistant treatment chamber. The heated gas blown out through the heated gas outlet 9 is blown in a direction intersecting the traveling direction of the fiber group A to be treated, preferably perpendicular to the traveling direction. This method is considered superior for flame-resistant treatment at high temperatures and in a short time because it can more efficiently remove heat generated by the oxidation reaction than a method in which the heated gas is blown in a direction parallel to the traveling direction of the fiber group A to be treated. Note that the heated gas outlet 9 and heated gas inlet 10 are located above and below the flame-resistant treatment chamber, respectively, but these positions may be reversed.

[0033] Using the flame-resistant furnace 1 described above, the total number of filaments constituting the polyacrylonitrile fiber bundle, which is the raw material for the treated fiber group A, is 1,000 to 80,000. This polyacrylonitrile fiber bundle is passed through the furnace multiple times for flame-resistant treatment in an oxidizing heated gas such as air, preferably at a temperature of 200 to 300°C, thereby producing a flame-resistant fiber bundle. This flame-resistant treatment oxidizes the polyacrylonitrile fiber bundle, resulting in crosslinking of the linear polymer and structural stabilization through a ring-closing reaction.

[0034] The method for producing a carbon fiber bundle of the present invention includes the steps of obtaining a flame-resistant fiber bundle by the method for producing a flame-resistant fiber bundle of the present invention and carbonizing the flame-resistant fiber bundle in an inert atmosphere at 1000 to 2500°C. Specifically, the flame-resistant fiber bundle obtained by the method for producing a flame-resistant fiber bundle of the present invention described above is pre-carbonized at a temperature of 300 to 1000°C in an inert atmosphere such as nitrogen, and then carbonized at a temperature of 1000 to 2500°C in an inert atmosphere such as nitrogen, to obtain a carbonized fiber bundle. This pre-carbonization and carbonization remove impurities from the flame-resistant fibers and strengthen carbon bonds, thereby resulting in the development of excellent structural properties of the carbon fiber bundle.

[0035] The flame-resistant fiber bundles and carbon fiber bundles thus obtained have reduced temperature variations in the flame-resistant treatment chamber compared to when a conventional flame-resistant furnace is used, and therefore, uniform quality and process stability can be ensured. [Example]

[0036] The present invention will be further illustrated by the following examples.

[0037] The fiber treatment group A, which was made from a polyacrylonitrile fiber bundle containing 12,000 filaments with a single fiber fineness of 1.1 dtex, was subjected to a flame retardant treatment. To confirm the effect of the flame retardant furnace of the present invention, five evaluation items were measured: temperature variation within the flame retardant treatment chamber, heater power consumption, cyanogen gas concentration in the working environment outside the flame retardant furnace, surface temperature of the turn-back roller, frequency of winding on the turn-back roller, and workability during operational troubles.

[0038] The temperature variation in the flame-resistant treatment chamber was calculated by measuring the temperature at each of six measurement points using thermometers installed in the flame-resistant furnace and calculating the difference between the maximum and minimum temperatures. The measurement points were six points in total, equally spaced apart: two points in the height direction from the top to the bottom of the traveling treated fiber group A, and three points in the length direction from a position 1.5 m away from the side wall of the flame-resistant treatment chamber containing the inlet for treated fiber A to a position 1.5 m away from the side wall of the flame-resistant treatment chamber containing the outlet for treated fiber A on the opposite side.

[0039] In order to eliminate fluctuations in the power consumption of the heater in the flame-proofing furnace due to factors such as daily temperature differences, the integrated power consumption values ​​were measured at the same time over a week, and the power consumption per hour was calculated.

[0040] The cyanide gas concentration in the working environment outside the flame-resistant furnace was measured using a suction-type gas detector tube as a representative substance of decomposition gases that require combustion treatment. Measurement points were 0.3 m away from the wall outside the flame-resistant furnace, and measurements were taken at 18 equally spaced locations: three points in the height direction from the top to the bottom of the treated fiber group A and three points in the width direction of the treated fiber group A on the fiber group inlet and fiber group outlet sides.

[0041] The surface temperature of the return roller was measured at the entrance and exit of the treated fiber group A, at three points in the height direction from the top to the bottom of the fiber group and in the width direction of the treated fiber group A.

[0042] The frequency of winding around the turn-back roller is the number of times that a fiber bundle from the fiber group A to be treated, which is running horizontally in the flame-proofing furnace, is wound around the turn-back roller after 500 hours of continuous operation. That is, the frequency of winding around the turn-back roller is defined as small if the number of times that a fiber bundle from the fiber group A to be treated is wound around the turn-back roller after 500 hours of continuous operation is 3 times or less, medium if it is 4 to 9 times, and large if it is 10 times or more.

[0043] As for the workability during operational trouble, the frequency of wrapping around the return roller during operation and the time required to complete the wrapping treatment were measured.

[0044] Example 1 Using the flame-resistant furnace shown in Figure 1, the return rollers placed on both sides of the furnace were arranged in multiple stages, and the upper 60% of the return rollers in the height direction, which is the heated gas blowing area, were covered with a curtain-like heat-resistant structure as an upper flame-resistant treatment enclosure. This heat-resistant structure was designed to be openable and closable.

[0045] The flame-proofing treatment was carried out by providing a heated gas outlet on the top surface of the flame-proofing treatment chamber and a heated gas inlet on the bottom surface, flowing heated gas from the top to the bottom of the flame-proofing treatment chamber, and blowing the heated gas from above onto the treated fiber group A. The heated gas sucked in through the heated gas inlet was returned to the heated gas outlet and recycled. The temperature of the heated gas at the heated gas outlet was controlled to 260°C by an electric heater for heating the flame-proofing furnace, which was provided between the heated gas inlet and the heated gas outlet.

[0046] A sealed chamber equipped with an exhaust mechanism was provided outside the flameproofing treatment chamber.

[0047] The treated fiber group A was made to repeatedly run through the flameproofing treatment chamber while its running direction was reversed by turn-back rollers installed on both sides of the flameproofing treatment chamber.

[0048] Table 1 shows the results of the five measurements mentioned above: temperature variation within the flame-resistant treatment chamber, heater power consumption, cyanide gas concentration in the working environment outside the flame-resistant furnace, surface temperature of the turn-back roller, and workability when operational problems occurred.

[0049] The difference between the maximum and minimum temperatures measured at six locations within the flame-resistant treatment chamber was 40°C, 5°C lower than in Comparative Example 1, which used the conventional flame-resistant treatment furnace shown in Figure 2, in which the folding rollers were exposed to the outside of the furnace. This shows that suppressing leakage outside the flame-resistant treatment furnace improved the heat retention within the flame-resistant treatment chamber and improved thermal efficiency. However, because the folding rollers are not completely inside the furnace, it is not possible to prevent outside air from entering the furnace at the bottom, resulting in temperature variations.

[0050] The power consumption of the heater in the flame-resistant furnace was 80%, a 20% reduction, compared to the power consumption in Comparative Example 1, which used a conventional flame-resistant furnace with a folding roller outside the furnace shown in Figure 2. This is because heat loss was prevented by enclosing the top of the flame-resistant furnace, where the heated gas is blown out, with a curtain. The surface temperature of the curtain rose to 120°C due to the heat of the blown heated gas, but because this was below the heat-resistant temperature of the curtain, there was no deterioration even after long-term production.

[0051] Furthermore, no cyanide gas was detected in the working environment outside the flame-resistant furnace, revealing that the curtain-like heat-resistant structure was able to prevent gas from leaking from inside the furnace.

[0052] By installing a curtain above the flame-resistant furnace, which is the outlet for the heated gas, it had the same effect as having a roller inside the furnace, making it difficult for the turn-back roller to be cooled by the outside air, and the turn-back roller surface temperature was 190°C, which was 10°C higher than in Comparative Example 1, which used a conventional flame-resistant furnace with a turn-back roller outside the furnace as shown in Figure 2. Because production was possible at a turn-back roller surface temperature of 200°C or less, decomposition gases from the oil agent were not transferred to the turn-back roller during 500 hours of continuous operation, the frequency of winding was low, and there was no significant deterioration in operation.

[0053] During steady-state operation, a problem occurred in which the yarn of fiber group A to be treated broke and wrapped around the turn-back roller, but the roller wrapping process was completed in just three minutes, and the treatment was carried out smoothly. The curtain installed on the outside of the turn-back roller is an accordion type that can be easily opened and closed, so the treatment could be carried out immediately by opening the curtain. The curtain is also equipped with a rail, so it can be opened and closed smoothly, and after the problem was treated, production could be continued without any problems by returning the curtain.

[0054] <Example 2> Using the flame-resistant furnace shown in Figure 1, flame-resistant treatment was carried out under the same conditions as in Example 1, except that a curtain-shaped heat-resistant structure was used as the upper enclosure for flame-resistant treatment to cover the outside of the upper 50% of the turn-back rollers in the height direction, which was the heated gas blowing area.

[0055] As in Example 1, the measurement results for five evaluation items, namely, the temperature variation in the flame-proofing treatment chamber, the power consumption of the heater, the cyanide gas concentration in the working environment outside the flame-proofing furnace, the surface temperature of the turn-back roller, the frequency of winding on the turn-back roller, and the workability when an operational problem occurred, are shown in Table 1.

[0056] The temperature variation, which is the difference between the maximum and minimum temperatures at six measurement points in the flame-resistant treatment chamber, was 40°C, the same as in Example 1. However, because the turn-back rollers were not designed to be completely inside the flame-resistant furnace, it was not possible to prevent outside air from entering the flame-resistant furnace at the bottom, and temperature variation occurred.

[0057] The power consumption of the heater in the flame-resistant furnace was 85, a 15% reduction, when the power consumption in Comparative Example 1, which used a conventional flame-resistant furnace with a folding roller outside the furnace shown in Figure 2, was taken as 100. This was because, as in Example 1, heat loss was prevented by enclosing the upper part, which is the heated gas outlet, with a curtain.

[0058] Furthermore, no cyanide gas was detected in the working environment outside the flame-resistant furnace, revealing that the curtain-like heat-resistant structure was able to prevent gas from leaking from inside the furnace.

[0059] By installing a curtain at the heated gas outlet point, the oven was partially turned inside the oven, preventing the turn-back rollers from being cooled by the outside air. As in Example 1, the turn-back roller surface temperature was 190°C, which was 10°C higher than in Comparative Example 1, which used a conventional oven with the turn-back rollers outside the oven (see Figure 2). Because production was possible with the turn-back roller surface temperature at 200°C or below, decomposition gases from the oil agent were not transferred to the rollers during 500 hours of continuous operation, and the frequency of winding was low. There was no significant deterioration in operation.

[0060] During steady-state operation, a problem occurred in which the yarn of the treated fiber group A broke and wound around the turn-back roller, but as in Example 1, the winding process around the turn-back roller was completed in just 3 minutes, and the treatment was carried out smoothly.

[0061] Example 3 Using the flame-resistant furnace shown in Figure 1, flame-resistant treatment was carried out under the same conditions as in Example 1, except that the upper 30% of the turning rollers in the height direction, which was the heated gas blowing area, was covered with a curtain-shaped heat-resistant structure as the upper enclosure for flame-resistant treatment.

[0062] As in Example 1, the measurement results for five evaluation items, namely, the temperature variation in the flame-proofing treatment chamber, the power consumption of the heater, the cyanide gas concentration in the working environment outside the flame-proofing furnace, the surface temperature of the turn-back roller, the frequency of winding on the turn-back roller, and the workability when an operational problem occurred, are shown in Table 1.

[0063] As in Examples 1 and 2, the difference between the maximum and minimum temperatures at the six measurement points inside the flame-resistant treatment chamber was smaller than in the flame-resistant furnace shown in Figure 2. However, as in Examples 1 and 2, the turn-back rollers were not designed to be completely inside the flame-resistant furnace, and therefore it was not possible to prevent outside air from flowing into the flame-resistant furnace at the bottom, resulting in temperature variations.

[0064] The power consumption of the heater in the flame-proofing furnace was 90, a 10% reduction, when the power consumption in Comparative Example 1, which used a conventional flame-proofing furnace with a folding roller outside the furnace shown in Figure 2, was taken as 100. As in Examples 1 and 2, heat loss was prevented by enclosing the upper part, which is the heated gas outlet, with a curtain, but the enclosed area by the upper enclosure above the flame-proofing treatment was small, so the energy-saving effect was reduced.

[0065] Furthermore, no cyanide gas was detected in the working environment outside the flame-resistant furnace, revealing that the curtain-like heat-resistant structure was able to prevent gas from leaking from inside the furnace.

[0066] By installing a curtain at the heated gas outlet point, the furnace became a flame-resistant furnace with a turn-back roller inside the furnace, preventing the turn-back roller from being cooled by outside air. As in Examples 1 and 2, the turn-back roller surface temperature was 190°C, which was 10°C higher than Comparative Example 1, which used a conventional flame-resistant furnace with a turn-back roller outside the furnace (see Figure 2). Because production was possible at a turn-back roller surface temperature of 200°C or less, decomposition gas from the oil agent was not transferred to the turn-back roller during 500 hours of continuous operation, and the frequency of winding was low. There was no significant deterioration in operation.

[0067] During steady-state operation, a problem occurred in which the yarn of the treated fiber group broke and wound around the turn-back roller, but as in Examples 1 and 2, the winding process around the turn-back roller was completed in just 3 minutes, and the treatment was carried out smoothly.

[0068] <Comparative Example 1> Using the conventional flame-resistant furnace shown in FIG. 2, flame-resistant treatment was carried out under the same conditions as in Example 1, except that a folding roller was provided outside the flame-resistant furnace and an upper enclosure for flame-resistant treatment was not provided.

[0069] As in Example 1, the temperature of the heated gas at the outlet was controlled to be 260° C. However, unlike Example 1, the flame-proofing furnace with a turn-back roller outside the furnace has a large amount of outside air leaking in, so it was necessary to introduce heated air into the sealing chamber as well to improve the heat treatment efficiency.

[0070] As in Example 1, the measurement results for five evaluation items, namely, the temperature variation in the flame-proofing treatment chamber, the power consumption of the heater, the cyanide gas concentration in the working environment outside the flame-proofing furnace, the surface temperature of the turn-back roller, the frequency of winding on the turn-back roller, and the workability when an operational problem occurred, are shown in Table 1.

[0071] The temperature variation, which is the difference between the maximum and minimum temperatures at six measurement points within the flame-resistant treatment chamber, was 45°C. It was found that the temperature at the bottom of the flame-resistant treatment chamber was particularly low, and that this was due to outside air being drawn in at the bottom where the heated gas inlet was located. The power consumption of the heater in Comparative Example 1 was set to 100. The power consumption was higher than in Examples 1 to 3, which is due to the above-mentioned leakage of outside air and the decrease in thermal efficiency caused by cooling of treated fiber group A outside the flame-resistant furnace. In addition, the cyanide gas concentration in the working environment outside the flame-resistant furnace was detected at 3 ppm at the top of the furnace, indicating that there was a large leakage of gas from the flame-resistant furnace at the top, where the heated gas outlet was located.

[0072] The surface temperature of the turn-back roller was 180°C. As the turn-back roller was located outside the flame-proofing furnace, it was cooled by the outside air, and production was possible with the turn-back roller surface temperature kept below 200°C. As a result, decomposition gases from the oil agent were not transferred to the roller during 500 hours of continuous operation, and the frequency of winding was low. There was no significant deterioration in operation.

[0073] During normal operation, a thread of treated fiber group A broke and wound around the turn-back roller, but because the turn-back roller was located outside the flame-resistant furnace, the winding process was completed in just three minutes. The thread wound around the turn-back roller during the winding process had no effect on the adjacent threads, and no induced breakage occurred.

[0074] <Comparative Example 2> As shown in FIG. 3, a flame-proofing furnace was used in which the outside of the turn-back roller was covered with a heat-insulating material made of rock wool, and the flame-proofing treatment was carried out under the same conditions as in Example 1, except that 50% of the volume of the turn-back roller was present inside the flame-proofing furnace.

[0075] As in Example 1, the temperature of the heated gas at the outlet was controlled to be 260°C.

[0076] As in Example 1, the measurement results for five evaluation items, namely, the temperature variation in the flame-proofing treatment chamber, the power consumption of the heater, the cyanide gas concentration in the working environment outside the flame-proofing furnace, the surface temperature of the turn-back roller, the frequency of winding on the turn-back roller, and the workability when an operational problem occurred, are shown in Table 1.

[0077] Among the measurement points in the flame-resistant treatment chamber, the temperature variation, which is the difference between the highest and lowest temperatures, was 20°C. Because the turn-back rollers were essentially inside the flame-resistant furnace, the temperature variation inside the flame-resistant furnace was small. The power consumption of the heater in the flame-resistant furnace was reduced to 60, assuming the power consumption in Comparative Example 1, which used a conventional flame-resistant furnace with turn-back rollers outside the furnace shown in Figure 2, as 100. This was because the inflow of outside air at the bottom was prevented, resulting in low power consumption.

[0078] Cyanogen gas was not detected in the working environment outside the flame-resistant furnace, as in Example 1. As for the surface temperature of the turn-back roller, the turn-back roller was not cooled by outside air, and the heat-insulating material was located in close proximity to the turn-back roller, as in Example 1, so the surface temperature of the turn-back roller rose to 230°C. During 500 hours of continuous operation, the rise in the surface temperature of the turn-back roller caused increased contamination of the turn-back roller and damage to the treated fiber group A, and the frequency of wrapping around the turn-back roller increased.

[0079] During steady-state operation, a yarn from fiber group A to be treated broke and wound around the turn-back roller, so a winding treatment was carried out. The winding treatment required time to remove the heat-insulating material installed on the outside of the turn-back roller and to adjust the slit spacing, and it took 30 minutes to complete the winding treatment around the turn-back roller. Similar treatment was required each time the number of winding locations increased, which took even more time.

[0080] <Comparative Example 3> The flame-proofing treatment was carried out under the same conditions as in Example 1, except that the turn-back roller was located entirely within the flame-proofing furnace as shown in Figure 4. The flame-proofing furnace with the turn-back roller inside does not require a sealing chamber, which is different from Examples 1 to 3 and Comparative Examples 1 and 2.

[0081] As in Example 1, the temperature of the heated gas at the outlet was controlled to be 260°C.

[0082] As in Example 1, the measurement results for five evaluation items, namely, the temperature variation in the flame-proofing treatment chamber, the power consumption of the heater, the cyanide gas concentration in the working environment outside the flame-proofing furnace, the surface temperature of the turn-back roller, the frequency of winding on the turn-back roller, and the workability when an operational problem occurred, are shown in Table 1.

[0083] Among the measurement points within the flame-resistant treatment chamber, the temperature variation (the difference between the highest and lowest temperatures) was only 15°C. This is because the turn-back roller was completely inside the flame-resistant treatment chamber and completely unaffected by external air. The power consumption of the heater in the flame-resistant treatment chamber was reduced to 60°C, the same as in Comparative Example 2, assuming the power consumption of Comparative Example 1, which used a conventional flame-resistant treatment chamber with the turn-back roller outside the furnace (see Figure 2), as 100. Furthermore, no cyanogen gas was detected in the working environment outside the flame-resistant treatment chamber. The surface temperature of the turn-back roller was 230°C. This is because the turn-back roller was not cooled by external air and was completely inside the furnace. During 500 hours of continuous operation, the frequency of wrapping around the turn-back roller increased due to the rise in the turn-back roller surface temperature, which caused contamination of the turn-back roller and damage to the treated fiber group A.

[0084] During steady-state operation, a yarn from fiber group A to be treated broke and wound around a turn-back roller, so a winding treatment was carried out. However, because the turn-back roller was located inside the flame-proofing furnace, direct contact with the roller was not possible, and the winding treatment could not be carried out. Since heat generation and heat accumulation from the yarn wound around the turn-back roller could lead to a fire, the drive was quickly stopped and the temperature was lowered, and the machine was shut down. The shutdown resulted in a large production loss.

[0085] [Table 1] [Industrial Applicability]

[0086] The flame-resistant furnace and flame-resistant method according to the present invention are effective in applications requiring flame-resistant treatment, particularly in the production of carbon fiber bundles. [Explanation of symbols]

[0087] 1:Flameproofing furnace 2: Flameproofing treatment room 3: Slit-shaped opening in the flame-resistant treatment chamber 4: Slit-shaped opening in the sealing chamber 5: Folding roller 6:Seal chamber 7: Side wall of flameproofing treatment room 8: Side wall of the seal chamber (outer side wall) 9: Heated gas outlet 10: Heated gas intake port 11: Flame-resistant upper enclosure 12: Heat-resistant structure attached rail 13: Flame-resistant treated upper part (upper 30% of the blowing area) 14: Height direction from the lowest to the highest position in the flame-resistant treatment furnace

Claims

1. A method for producing a flame-resistant fiber bundle, in which a plurality of polyacrylonitrile fiber bundles are run in parallel in the horizontal direction, the method comprising: a flame-resistant treatment chamber to which hot air of a constant temperature is supplied and a circulation section for circulating the hot air; a plurality of slits and turning rollers are provided at both ends of the flame-resistant treatment chamber to allow the polyacrylonitrile fiber bundle to pass in and out; and an upper flame-resistant treatment enclosure provided outside the turning rollers above the flame-resistant treatment chamber, the method comprising: making the polyacrylonitrile fiber bundle pass through the flame-resistant treatment chamber a plurality of times to obtain a flame-resistant fiber bundle.

2. 2. The method for producing an oxidized fiber bundle according to claim 1, wherein the surface temperature of the turn-back roller is 60 to 200°C.

3. 2. The method for producing a flame-resistant fiber bundle according to claim 1, wherein the flame-resistant treated upper enclosure is made of a sheet-like heat-resistant structure having an openable and closable curtain structure, and the heat-resistant structure has a heat-resistant temperature of 300°C or higher.

4. 2. The method for producing a flame-resistant fiber bundle according to claim 1, wherein the circulating hot air for flame-resistant treatment according to claim 1 is supplied from the top to the bottom of a flame-resistant furnace to a polyacrylonitrile-based fiber bundle traveling in a flame-resistant treatment chamber.

5. A method for producing a carbon fiber bundle, comprising pre-carbonizing the flame-resistant fiber bundle obtained by the method according to any one of claims 1 to 4 at 300 to 1000°C, and then carbonizing the same at 1000 to 2500°C.

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