Heat treatment furnace of carbon fiber sheet and manufacturing method of carbon fiber sheet using the same

Abstract

To provide a heat treatment furnace and a manufacturing method of a carbon fiber sheet, capable of stable and continuous production of a high-quality carbon fiber sheet with few poor appearances such as a through-hole or a stain, while suppressing deformation of the carbon fiber sheet.SOLUTION: The heat treatment furnace for manufacturing the carbon fiber sheet by continuously conveying a carbon fiber sheet precursor while heating includes at least one gas discharge channel for discharging exhaust gas generated while heating the carbon fiber sheet precursor from inside the furnace to the outside. The heat treatment furnace also includes a shielding plate that shields a top part of a conveyance face of the carbon fiber sheet precursor in the vicinity of an opening inside the furnace in the gas discharge channel, provided in a position not contacting the carbon fiber sheet precursor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heat treatment furnace for a carbon fiber sheet and a method for producing a carbon fiber sheet using the same. [Background technology]

[0002] Carbon fiber sheets are used for a variety of purposes, including the molding of carbon fiber reinforced plastics (CFRP) and electrodes for fuel cells. One method for continuous production of carbon fiber sheets is to graphitize the resin by continuously heat-treating a carbon fiber sheet precursor made by impregnating a mat made by papermaking short carbon fibers with resin and curing it in a heat treatment furnace.

[0003] In this method, in the step of graphitizing the resin of the carbon fiber sheet precursor, decomposition gas generated during the thermal decomposition of the resin accumulates in the heat treatment furnace, and the solidified gas aggregates come into contact with the edge of the carbon fiber sheet precursor, causing cracks and chips at the edge of the carbon fiber sheet, which causes the carbon fiber sheet to break. In addition, the decomposition gas aggregates of the resin accumulated in the form of icicles on the inner wall surface of the ceiling of the heat treatment furnace tend to fall onto the carbon fiber sheet precursor being transported in the heat treatment furnace, which can cause appearance defects such as stains and through holes in the carbon fiber sheet.

[0004] In order to prevent breakage or poor appearance of the carbon fiber sheet precursor due to such gas aggregates, a heat treatment method has been proposed in which a heat-resistant sheet wider than the carbon fiber sheet precursor is placed on top of the carbon fiber sheet precursor that is continuously transported inside a heat treatment furnace so as to cover the carbon fiber sheet precursor (Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-191406 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the heat treatment method of Patent Document 1, when a heat-resistant sheet is arranged so as to be in contact with the carbon fiber sheet precursor, meandering and deformation of the carbon fiber sheet precursor become significant, which may result in cracks or breaks causing the carbon fiber sheet precursor to break, adversely affecting the continuous conveyance of the carbon fiber sheet precursor. Furthermore, even if the heat-resistant sheet is arranged in a manner that does not contact the carbon fiber sheet precursor, the heat-resistant sheet of Patent Document 1 has micropores, and gas condensates that have fallen onto the heat-resistant sheet may pass through the micropores and adhere to the carbon fiber sheet precursor, causing tiny stains or through-holes.

[0007] An object of the present invention is to provide a heat treatment furnace and a manufacturing method for carbon fiber sheets that can stably and continuously produce high-quality carbon fiber sheets with few appearance defects such as through holes and stains while suppressing deformation of the carbon fiber sheets. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention provides a heat treatment furnace and a manufacturing method for a carbon fiber sheet, which can stably continue to transport a carbon fiber sheet precursor while preventing gas aggregates from adhering to the carbon fiber sheet precursor by the following steps. (1) A heat treatment furnace for producing a carbon fiber sheet by heating a carbon fiber sheet precursor while continuously transporting the same, the heat treatment furnace having at least one gas exhaust path for exhausting exhaust gas generated during heating of the carbon fiber sheet precursor from inside the furnace to outside the furnace, and a shielding plate provided in a position near an opening of the gas exhaust path in the furnace to shield an upper part of a transport surface of the carbon fiber sheet precursor and not in contact with the carbon fiber sheet precursor. (2) The heat treatment furnace according to (1), wherein the shielding plate is provided at a position that shields a straight line that connects the center of the furnace opening of the gas exhaust path and the conveying surface in the shortest distance. (3) The heat treatment furnace according to (1) or (2), wherein the heat treatment furnace is capable of heating the fiber sheet precursor 1 to 2000°C or higher, and the shielding plate is installed in a region of 500 to 1500°C within the heat treatment furnace. (4) The heat treatment furnace according to any one of (1) to (3), wherein the shielding plate is installed within a range of 50% or less of the distance from the furnace entrance to the exit. (5) The heat treatment furnace according to any one of (1) to (4), wherein the shielding plate is wider than the carbon fiber sheet precursor. (6) The heat treatment furnace according to any one of (1) to (5), wherein the shielding plate is made of a carbon material or a carbon fiber reinforced composite material. (7) The heat treatment furnace according to any one of (1) to (6), wherein the shielding plate is provided detachably. (8) The heat treatment furnace according to (7), wherein the shielding plate is placed on spacers erected on both sides of the conveying surface. (9) A method for producing a carbon fiber sheet, comprising a heating step of heating a carbon fiber sheet precursor using the heat treatment furnace according to any one of (1) to (8). (10) A method for producing a carbon fiber sheet according to (9), comprising a low-temperature heating step of heating a carbon fiber sheet precursor to less than 1000°C and a high-temperature heating step of heating the carbon fiber sheet precursor to 2000°C or higher in this order, wherein the high-temperature heating step involves heat treatment using a heat treatment furnace according to any one of (1) to (8). [Effects of the Invention]

[0009] According to the present invention, it is possible to stably and continuously produce high-quality carbon fiber sheets with little deformation, in which deformation of the carbon fiber sheet is suppressed and appearance defects such as stains and through holes are suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional schematic view of a heat treatment furnace according to an embodiment of the present invention, viewed from the side in a direction perpendicular to the conveying direction. [Figure 2] 1 is a cross-sectional schematic view of a heat treatment furnace according to an embodiment of the present invention, viewed from the conveying direction. [Figure 3]FIG. 10 is a cross-sectional view of a heat treatment furnace according to another embodiment of the present invention, viewed from the conveyance direction. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will now be described in detail with reference to the drawings.

[0012] Fig. 1 is a cross-sectional schematic diagram of a heat treatment furnace 100 according to one embodiment of the present invention, viewed from the side in a direction perpendicular to the conveying direction of a carbon fiber sheet precursor 1 (hereinafter simply referred to as the "conveying direction"); Fig. 2 is a cross-sectional schematic diagram of the heat treatment furnace 100 viewed from the conveying direction. Fig. 3 is a cross-sectional schematic diagram of a heat treatment furnace 100 according to another embodiment, viewed from the conveying direction. In Fig. 1, the carbon fiber sheet precursor 1 is conveyed from the left side to the right side of the figure.

[0013] The carbon fiber sheet precursor 1 may be a flame-resistant fiber nonwoven fabric using flame-resistant acrylic fiber or the like, a flame-resistant woven yarn fabric, or a sheet in which short carbon fibers are bound with an organic substance.

[0014] The flame-resistant fiber nonwoven fabric may be produced by either a dry method in which staple fibers such as acrylic flame-resistant fibers are carded, or a wet method in which staple fibers of flame-resistant fibers are dispersed in water and then paper-made. To increase the strength of the nonwoven fabric, the nonwoven fabric may be subjected to a hydroentanglement treatment or a needle punching treatment to form entangled fibers. For the same purpose, the nonwoven fabric may be subjected to a treatment to add a binder that bonds the fibers together. Similarly, in the case of the dry method, the fiber entanglement treatment may be performed by hydroentanglement or needle punching, or a binder that bonds the fibers together may be added. As another method for obtaining a flame-resistant fiber nonwoven fabric, the nonwoven fabric may be subjected to a flame retardant treatment.

[0015] Examples of flame-resistant woven fabrics include those made by weaving long carbon fibers or those made by spinning flame-resistant acrylic yarns or the like into spun yarns and then woven into fabrics.

[0016] In the case of a sheet in which short carbon fibers are bound with an organic substance, any of polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, and phenol-based carbon fibers can be used as the carbon fibers. However, PAN-based carbon fibers or pitch-based carbon fibers are preferred, as they can increase the bending strength and tensile strength of the resulting carbon fiber sheet, and PAN-based carbon fibers are even more preferred. The length of the short carbon fibers is preferably within the range of 3 to 12 mm. A length of 6 to 9 mm is preferred because it allows for good dispersibility during papermaking of the short carbon fibers and results in a carbon fiber sheet with high tensile strength and tear resistance. The organic substance can be a thermoplastic resin such as polyvinyl alcohol (PVA) or a thermosetting resin such as a phenolic resin, for bonding the carbon fibers contained in the carbon fiber sheet precursor 1; a thermosetting resin such as a phenolic resin or pitch can be used to bond the carbon fibers in the carbon fiber sheet 2 with a charcoal formed by heat treatment of the organic substance; or starch or PVA can be used to improve the processability of obtaining the carbon fiber sheet precursor 1.

[0017] The carbon fiber sheet precursor 1 can contain carbon powder, metal powder, inorganic powder, metal fiber, inorganic fiber, etc. depending on the performance required of the carbon fiber sheet. When used as a fuel cell electrode substrate, it is preferable to contain carbon powder to improve conductivity and reduce impurities. When using carbon powder, the ratio of carbon powder is preferably 10 to 50 mass % of the carbon fiber sheet to improve conductivity.

[0018] 1 heat-treats a carbon fiber sheet precursor 1 while it is continuously traveling, and converts the carbon fiber sheet precursor 1 into a carbon fiber sheet 2 by carbonizing the flame-resistant yarns in the carbon fiber sheet precursor 1 or the organic matter binding the short carbon fibers. The heat-treatment furnace 100 is provided with an internal furnace space 101 that passes through it and allows the carbon fiber sheet precursor 1 to pass through. The carbon fiber sheet precursor 1 is inserted from the inlet of the heat-treatment furnace (hereinafter referred to as the furnace inlet) and moves continuously toward the outlet of the heat-treatment furnace (hereinafter referred to as the furnace outlet) in a floating state without touching the hearth 102 within the internal furnace space 101, which is maintained at a high temperature, or moves continuously while sliding on the hearth 102.

[0019] A gas exhaust path 103 is provided in the ceiling of the heat treatment furnace 100 to exhaust gas generated during heating of the carbon fiber sheet precursor 1 from the inside of the furnace to the outside of the furnace. Such a gas exhaust path may be provided on one or both wall surfaces of the conveying surface of the carbon fiber sheet precursor 1.

[0020] Because the heat treatment furnace 100 is an open-type heat treatment furnace with an open furnace entrance and exit, the temperature near the furnace entrance and exit is about room temperature, and a temperature distribution occurs within the furnace. As a result of extensive research by the inventors into the deposition of carbides on the hearth, they have found that the main location where gas aggregates that cause poor appearance of the carbon fiber sheet 2 occur is in a temperature range of 500 to 1500°C, although this varies depending on the structure and temperature distribution of the heat treatment furnace 100, and that this temperature range is the region in the furnace where the concentration of exhaust gas is highest, near the furnace opening of a gas exhaust path 103 that exhausts exhaust gas generated in the heat treatment furnace 100 during transportation from the furnace to the outside.

[0021] Therefore, in the heat treatment furnace 100, a shielding plate 50 is provided near the furnace opening of the gas discharge path 103. When the gas discharge path 103 is provided on the upper wall surface of the heat treatment furnace 100, the shielding plate 50 is preferably provided at a position that blocks a straight line connecting the center of the furnace opening of the gas discharge path 103 and the transport surface of the carbon fiber sheet precursor 1 at the shortest distance. As described above, the gas discharge path 103 may be provided on one or both wall surfaces of the transport surface of the carbon fiber sheet precursor 1. However, even in these cases, the shielding plate 50 must be provided on the inner ceiling surface of the heat treatment furnace 100 near the gas discharge path 103 because a considerable amount of the gas aggregates accumulate. When the gas discharge path 103 is provided on the lower wall surface of the heat treatment furnace 100, the shielding plate 50 is preferably provided so as to cover the upper surface of the carbon fiber sheet precursor 1, which is an extension of the straight line connecting the center of the furnace opening of the gas discharge path 103 and the transport surface of the carbon fiber sheet precursor 1 at the shortest distance. Furthermore, when the gas exhaust path 103 is provided on the side wall surface of the heat treatment furnace 100, it is preferable that the shielding plate 50 is provided so as to cover the upper surface of the carbon fiber sheet precursor 1 at the position of the straight line connecting the center of the furnace opening of the gas exhaust path 103 and the end of the carbon fiber sheet precursor 1 by the shortest distance.

[0022] The shielding plate 50 is provided at a position in the furnace space 101 where it does not come into contact with the carbon fiber sheet precursor 1 during transportation. By providing the shielding plate at a position where it does not come into contact with the carbon fiber precursor sheet 1, it is possible to prevent resistance such as friction from being applied to the carbon fiber sheet precursor 1, suppress meandering and deformation of the carbon fiber sheet precursor 1, and prevent breakage of the carbon fiber sheet precursor 1 and the carbon fiber sheet 2 due to cracks or breakage caused by these.

[0023] Furthermore, in the case of a heat treatment furnace capable of heating the carbon fiber sheet precursor 1 to 2000°C or higher, by placing a shielding plate 50 in the region of the heat treatment furnace 100 where the temperature reaches 500 to 1500°C, as shown in Figure 1, it is possible to prevent gas condensates that cause appearance defects such as stains and through holes from adhering to the carbon fiber sheet precursor 1.

[0024] The shielding plate 50 may be a solid (hole-free, including micropores) plate- or sheet-like material such as a nonwoven fabric or woven fabric, but is preferably made of a material that does not undergo thermal decomposition or react with the furnace atmosphere gas in the inert gas atmosphere within the heat treatment furnace 100 at temperatures ranging from 500 to 1500°C, and is preferably made of a carbon material or carbon fiber reinforced composite material that has excellent heat resistance. In particular, from the viewpoint of durability, the shielding plate 50 is more preferably made of a carbon fiber reinforced carbon composite material. A carbon fiber reinforced carbon composite material not only ensures that the shielding plate 50 is free of micropores, but also provides sufficient rigidity, thereby reliably solving the problems of the present invention.

[0025] The shielding plate 50 may be installed over the entire length inside the heat treatment furnace 100, but in that case, exhaust gas generated in the high-temperature region inside the heat treatment furnace 100 may stagnate on the carbon fiber sheet precursor transport surface side of the shielding plate 50, and the above-mentioned gas aggregates may adhere to the carbon fiber sheet precursor transport surface side of the shielding plate 50, increasing the risk of appearance defects in the carbon fiber sheet 2. Therefore, the shielding plate 50 is preferably installed within a range of 50% or less of the distance from the furnace entrance to the furnace exit, including the straight line connecting the center of the furnace opening of the gas exhaust path 103 and the transport surface by the shortest distance.

[0026] The shielding plate 50 may be permanently installed in the heat treatment furnace 100, but is preferably detachable from the heat treatment furnace 100 for easy maintenance. Furthermore, the shielding plate 50 is preferably placed on spacers 104 erected on both sides of the conveying surface of the carbon fiber sheet precursor 1 so that it can be easily removed from the furnace entrance or exit of the heat treatment furnace 100. The spacers 104 may be rod-shaped and extend in the conveying direction of the carbon fiber sheet precursor 1 installed on the hearth 102 as shown in FIG. 2(A), or may be protrusions installed on both sides of the conveying surface of the carbon fiber sheet precursor 1 in the heat treatment furnace 100 as shown in FIG. 2(B). The width of the spacer 104 is preferably about 5% of the width of the shielding plate 50 to ensure a conveying area for the carbon fiber sheet precursor 1.

[0027] To prevent adhesion of gas condensates to the carbon fiber sheet precursor 1 across the entire width of the carbon fiber sheet precursor 1, the shielding plate 50 is preferably wider than the carbon fiber sheet precursor 1, more specifically, is preferably 1.01 to 2.00 times the width of the carbon fiber sheet precursor 1. If the width of the shielding plate 50 is smaller than 1.01 times the width of the carbon fiber sheet precursor 1, for example, if the width of the carbon fiber sheet precursor is 1 meter, the carbon fiber sheet precursor 1 will protrude beyond the area covered by the shielding plate 50 even if it meanders just 5 mm on one side in the width direction, making it difficult to cover the entire width of the carbon fiber sheet precursor 1, or the spacer 104 on which the shielding plate 50 is placed will come into contact with the carbon fiber sheet precursor 1, causing cracks or chips in the carbon fiber sheet precursor 1 and the carbon fiber sheet 2, resulting in breakage. On the other hand, if the width of the shielding plate 50 is greater than 2.00 times the width of the carbon fiber sheet precursor 1, the width of the heat treatment furnace 100 must be designed to be larger in order to install the shielding plate 50, which will increase the installation costs of the heat treatment furnace 100.

[0028] Another aspect of the present invention is a method for producing a carbon fiber sheet, which includes a heating step of heating a carbon fiber sheet precursor using the above-mentioned heat treatment furnace. Generally, the production of a carbon fiber sheet includes a low-temperature heating step of heating the carbon fiber sheet precursor to less than 1000°C and a high-temperature heating step of heating the carbon fiber sheet precursor to 2000°C or higher in this order, and the heat treatment furnace 100 of the present invention is particularly suitable for use in the high-temperature heating step. [Example]

[0029] [Example] In a process of heat-treating a carbon fiber sheet precursor 1 in a heat treatment furnace with a maximum temperature of 2000°C or higher, a shielding plate 50 made of a carbon fiber reinforced composite material was inserted into the heat treatment furnace from the furnace inlet side and installed in a range that included directly below a gas exhaust path 103 provided in the ceiling of the heat treatment furnace and corresponded to 30% of the distance from the furnace inlet to the furnace outlet. The atmospheric temperature at the end of the shielding plate 50 on the furnace outlet side was 1500°C. The carbon fiber sheet precursor 1 was heat-treated while being dragged between the shielding plate 50 and the hearth 102. The carbon fiber sheet precursor 1 had a width of 400 mm and a length of 500 mm, and the width of the shielding plate 50 was 600 mm.

[0030] When the production status of carbon fiber sheet 2 was observed for about three months using the above method, the number of through holes with a diameter of more than 0.1 mm was found to be an average of 0.4 per carbon fiber sheet 2. During this period, no cracks or breaks occurred in carbon fiber sheet precursor 1 or carbon fiber sheet 2 due to meandering or deformation.

[0031] [Comparative Example] The same procedure as in Example 1 was carried out except that a carbon fiber fabric was placed in contact with the carbon fiber sheet precursor 1 instead of the shielding plate 50. When the production status of the carbon fiber sheet 2 was observed for about three months, the average number of through holes was 5.1 per sheet. During this time, cracks and breaks occurred in the carbon fiber sheet precursor 1 and the carbon fiber sheet 2 due to meandering or deformation. [Explanation of symbols]

[0032] 1: Carbon fiber sheet precursor 2: Carbon fiber sheet 50: Shielding plate 100: Heat treatment furnace 101: Furnace space 102: Hearth 103: Gas exhaust route 104: Spacer

Claims

1. A heat treatment furnace for producing a carbon fiber sheet by heating a carbon fiber sheet precursor while continuously transporting it, the heat treatment furnace having at least one gas exhaust path for exhausting exhaust gas generated during heating of the carbon fiber sheet precursor from inside the furnace to outside the furnace, and a shielding plate is provided in a position that shields an upper part of the transport surface of the carbon fiber sheet precursor near an opening of the gas exhaust path in the furnace and does not come into contact with the carbon fiber sheet precursor.

2. 2. The heat treatment furnace according to claim 1, wherein the shielding plate is provided at a position that shields a straight line that connects the center of an opening in the furnace of the gas exhaust path and the transfer surface at the shortest distance.

3. The heat treatment furnace according to claim 1 or 2, wherein the heat treatment furnace is capable of heating the carbon fiber sheet precursor to 2000°C or higher, and the shielding plate is installed in a region of 500 to 1500°C in the heat treatment furnace.

4. 4. The heat treatment furnace according to claim 1, wherein the shielding plate is installed within a range of 50% or less of the distance from the furnace inlet to the furnace outlet.

5. The heat treatment furnace according to any one of claims 1 to 4, wherein the shielding plate is wider than the carbon fiber sheet precursor.

6. 6. The heat treatment furnace according to claim 1, wherein the shielding plate is made of a carbon material or a carbon fiber reinforced composite material.

7. 7. The heat treatment furnace according to claim 1, wherein the shielding plate is provided in a detachable manner.

8. 8. The heat treatment furnace according to claim 7, wherein the shielding plate is placed on spacers provided upright on both sides of the conveying surface.

9. A method for producing a carbon fiber sheet, comprising a heating step of heating a carbon fiber sheet precursor using the heat treatment furnace according to any one of claims 1 to 8.

10. The method for producing a carbon fiber sheet according to claim 9, further comprising a low-temperature heating step of heating a carbon fiber sheet precursor to less than 1000° C. and a high-temperature heating step of heating the carbon fiber sheet precursor to 2000° C. or higher in this order, and a heat treatment is performed in the high-temperature heating step using a heat treatment furnace according to any one of claims 1 to 8.

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