Method for manufacturing carbon fiber sheets and plate-shaped jigs used therein

Abstract

To provide a manufacturing method of a carbon fiber sheet having excellent workability by using a light-weight firing jig having excellent handling ability.SOLUTION: A manufacturing method of a carbon fiber sheet includes a firing step of firing a carbon fiber sheet precursor comprising a carbon fiber or a carbon fiber precursor, wherein the plurality of carbon fiber sheet precursors are laminated in the firing step, and firing is conducted in a lamination state that a tabular jig comprising a carbon short fiber bonded with a resin carbide is interposed between the laminated carbon fiber sheet precursors.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing carbon fiber sheets used as electrode substrates and electrolytic electrodes for solid polymer fuel cells, and to a plate-shaped jig used when firing a carbon fiber sheet precursor in the said manufacturing method. [Background technology]

[0002] Electrode substrates used in polymer electrolyte fuel cells require properties such as conductivity, mechanical strength, and gas permeability. Carbon fiber sheets, in which carbon fibers are bound together with carbides, are suitable as electrode materials possessing these properties, and their manufacturing process is mainly divided into papermaking, resin impregnation, pressing, and firing processes.

[0003] To explain each step, in the papermaking process, carbon short fibers are dispersed to make carbon fiber paper, and then in the resin impregnation process, the carbon fiber paper is impregnated with a resin such as phenolic resin. The sheet-like intermediate product after impregnation is called a prepreg. This prepreg is pressure-molded in the pressing process to become a carbon fiber sheet precursor. By firing this carbon fiber sheet precursor at a high temperature of 2,000°C or higher in the firing process, the resin inside the carbon fiber sheet precursor is replaced by carbides while releasing decomposition gases, and a carbon fiber sheet can be obtained in which the carbon fibers are bound together by the carbides.

[0004] In the firing process, multiple carbon fiber sheet precursors are sometimes stacked and fired. However, the shrinkage of the resin contained in the carbon fiber sheet precursors during firing often caused shape abnormalities such as warping and wrinkling in the carbon fiber sheets. When shape abnormalities occurred, the contact between the electrode substrate and each component deteriorated when manufacturing fuel cells by stacking electrode substrates, catalysts, and electrolyte membranes using carbon fiber sheets. This resulted in misalignment or partial delamination of the stacked and bonded materials, rendering them unusable.

[0005] As a method to suppress such shape abnormalities, Patent Document 1 discloses a method in which, when laminating carbon fiber sheet precursors, graphite plates with high thermal conductivity and high rigidity are inserted at regular intervals and fired. This allows for uniform heating of the carbon fiber sheets through the thermal conductivity of the graphite plates, thereby uniformizing the shrinkage of the resin during firing, and suppresses the occurrence of warping and wrinkling due to the rigidity of the graphite plates, thereby suppressing the occurrence of shape abnormalities. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-48182 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, graphite sheets like those described in Patent Document 1 are dense, heavy, brittle, and easily broken, which poses a significant burden during lamination. The present invention aims to provide a method for manufacturing carbon fiber sheets with improved workability by using a lightweight and easy-to-handle firing jig. [Means for solving the problem]

[0008] The present invention, which aims to achieve the above objective, is as follows: (1) A method for manufacturing a carbon fiber sheet, comprising a firing step of firing a carbon fiber sheet precursor consisting of carbon fibers or carbon fiber precursors, wherein in the firing step, a plurality of carbon fiber sheet precursors are laminated, and a plate-shaped jig made of carbon short fibers bound with resin carbide is laminated so as to be interposed between the laminated carbon fiber sheet precursors and fired. (2) The method for manufacturing a carbon fiber sheet according to (1), wherein the plate-shaped jig is a structure for papermaking of carbon short fibers. (3) The method for manufacturing a carbon fiber sheet according to (1) or (2), wherein the plate-shaped jig is laminated with 5 to 60 carbon fiber sheet precursors interposed between them. (4) The method for manufacturing a carbon fiber sheet according to (1) to (3), wherein the dimensions of the plate-shaped jig are 0.5 cm or more larger in both width and length than the dimensions of the carbon fiber sheet precursor. (5) The method for manufacturing a carbon fiber sheet according to any one of claims (1) to (4), wherein the maximum temperature of the firing step is 600 to 1000°C or 2000 to 3000°C. Furthermore, the present invention is characterized by providing the following plate-shaped jig. (6) A plate-shaped jig used in the method for manufacturing carbon fiber sheets described in (1), wherein the thickness of the plate-shaped jig is 0.5 mm to 3.0 mm. (7) Density is 0.3 g / cm 3 ~1.0g / cm 3 The plate-shaped jig described in (6). (8) 14 cc / cm² in the thickness direction 2 A plate-shaped jig as described in (6) or (7), wherein the gas permeability resistance, calculated by dividing the differential pressure when air is permeated at a rate of / sec by the thickness, is 50 mmAq / mm or less. 。 (9) A plate-shaped jig according to any one of (6) to (8), wherein the ratio of bending strength to elastic modulus is 0.005 or more. (10) A plate-shaped jig according to any of (6) to (9), wherein the surface roughness Rmax is 20 to 300 μm. (11) A plate-shaped jig according to any of (7) to (10), wherein the in-plane thermal conductivity is 10 W / (m·K) or more. (12) A plate-shaped jig according to any of (6) to (11), wherein the Fe and V content is 20 ppm or less. [Effects of the Invention]

[0009] The present invention makes it possible to improve the workability of the firing process in the production of carbon fiber sheets by using a lightweight, easy-to-handle plate-shaped jig while preventing deterioration of the gas permeability of the carbon fiber sheet precursor.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, a plate-shaped jig used in the present invention (hereinafter sometimes referred to as "the plate-shaped jig of the present invention" or simply "plate-shaped jig"), a method for manufacturing the same, and a method for manufacturing a carbon fiber sheet using the plate-shaped jig will be described in detail.

[0011] The plate-shaped jig of the present invention is a plate-shaped body formed by binding carbon short fibers with a resin carbide. As the structure of the plate-shaped jig, a laminated structure of carbon fiber paper is preferable. By forming a laminated structure of carbon fiber paper, a plate-shaped jig having a surface shape and high gas permeability, and excellent handling properties in terms of light weight and high strength can be obtained.

[0012] Such a laminated structure of carbon fiber paper can typically be produced by thermally pressing a carbon fiber paper body impregnated with a thermosetting resin, and more specifically, it can be produced by a manufacturing method including a papermaking process, a resin impregnation process, a pressing process, and a firing process.

[0013] In the papermaking process, a long carbon fiber paper wound in a roll shape is obtained by binding carbon short fibers dispersed randomly in a substantially two-dimensional plane with an organic binder. By using carbon fiber paper obtained by papermaking, the plate-shaped jig becomes a papermaking structure of carbon short fibers, and it is possible to prevent the surface shape of the jig from being transferred and the uneven shape from changing during the firing of the carbon fiber sheet. As a method for dispersing carbon short fibers randomly, there are mainly a wet method in which carbon short fibers are dispersed in a liquid and then papermade, and a dry method in which carbon short fibers are dispersed in the air and then deposited. The wet method is preferable because the dispersibility of carbon short fibers and the orientation in the two-dimensional plane direction are good, so the surface roughness of the plate-shaped jig is small, and there are few abnormal dispersion defects and local convex portions due to carbon fiber protrusion.

[0014] In the resin impregnation process, a carbon fiber paper is impregnated with a thermosetting resin and cut into a certain length to obtain a prepreg, which is a composite of the carbon fiber paper and the thermosetting resin. As the thermosetting resin, phenolic resin, epoxy resin, furan resin, melamine resin, etc. can be used, and a mixed resin containing at least one of these may also be used. The thermosetting resin needs to remain as a resin carbide that binds between the carbon short fibers after the firing process. Among them, phenolic resin is particularly preferable because it has a high residual carbon rate, good binding property with carbon fibers, and is easy to handle in the resin impregnation and pressing processes. Also, in the resin impregnation process, it is preferable to mix and impregnate carbon particles such as graphite powder and carbon black into the resin in order to increase the thermal conductivity.

[0015] In the pressing process, one or more prepregs are stacked and heated and pressurized to cure the thermosetting resin and bond the prepregs to obtain a molded product. At this time, the thickness of the molded product can be controlled by the number of stacked prepregs. When multiple sheets are stacked, the finally obtained plate-like jig will be a laminated structure of carbon fiber paper making. The heating temperature is preferably 100 - 250°C. More preferably, it is 120 - 200°C, and even more preferably, it is 140 - 180°C. If it is less than 100°C, it is necessary to extend the molding time. Also, when the molding time is short and the curing of the thermosetting resin is insufficient, the shrinkage during firing becomes large, resulting in warping and large thickness variations. Also, when it exceeds 250°C, the resin cures before the thickness is made uniform by pressurization, resulting in variations in the thickness of the molded product. The pressurization pressure is preferably 0.1 - 2 MPa. More preferably, it is 0.3 - 1 MPa. If it is less than 0.1 MPa, the adhesion between the carbon fibers and the resin, and thus the adhesion between the carbon fibers and the resin carbide after firing, is weak and non-uniform, so the mechanical strength such as the bending strength of the plate-like jig decreases. If it exceeds 2 MPa, the plate-like jig becomes highly dense, and the gas permeability may become too low. Furthermore, the carbon short fibers may break, the bending strength of the jig may decrease, or the generation of dust may increase.

[0016] In the firing process, the molded product is heated in a batch furnace in an inert atmosphere to carbonize the thermosetting resin and obtain a plate-like jig in which the carbon short fibers are bound with the resin carbide. The maximum firing temperature in the firing process is 2,000 - 3,000Set it to °C. If the maximum firing temperature is too low, a large amount of metal impurities will remain in the plate-shaped jig, and will be transferred from the plate-shaped jig to the carbon fiber sheet during the firing of the carbon fiber sheet. Also, when the plate-shaped jigs are stacked with no load at the ends and fired at a high temperature exceeding the processing history of the jigs, warping and wrinkles are likely to occur at the ends. Conversely, if the maximum firing temperature is too high, not only will the operating cost of the firing furnace increase, but the consumption of the firing furnace will also become severe, the maintenance cost will increase, and ultimately the production cost will increase. Incidentally, the firing process may be divided into two stages: firing at a maximum temperature of 1,000 °C or lower and firing at a maximum temperature of 2,000 °C or higher.

[0017] Through the above steps, a plate-shaped jig formed by binding carbon short fibers with a resin carbide can be manufactured. .child Such a plate-shaped jig has a porous structure with low density and good gas permeability, while having small surface irregularities.

[0018] The thickness of the plate-shaped jig is preferably 0.5 mm to 3.0 mm. If it is thinner than 0.5 mm, the breaking load will decrease and it will be easily broken during handling. If it is thicker than 3.0 mm, the occupancy rate of the furnace height during firing will be high and the firing amount will decrease. Incidentally, the thickness shall be measured using a micrometer under a pressure of 0.15 MPa.

[0019] The density of the plate-shaped jig can be adjusted by the material ratio in the resin impregnation process and the pressing pressure in the pressing process, and is preferably 0.3 g / cm 3 ~1.0 g / cm 3 More preferably, the density of the plate-shaped jig is 0.4 g / cm 3 ~0.7 g / cm 3 When the density is less than 0.3 g / cm 3 the strength of the plate-shaped jig tends to decrease and it becomes easily broken. When the density is greater than 1.0 g / cm 3 when the plate-shaped jig has an end, it tends to break due to deflection caused by its own weight or the gas permeation resistance becomes too high.

[0020] The bending strength and bending modulus of a plate-shaped jig increase with higher density, but the ratio of bending strength to modulus tends to decrease, and the ratio also decreases similarly with improved adhesion between carbon fibers and resin carbides. It is preferable that the ratio of bending strength to bending modulus be 0.005 or higher. More preferably, it is 0.007 or higher, and even more preferably 0.01 or higher. If it is less than 0.005, the plate-shaped jig is prone to cracking due to deflection during handling. The ratio of bending strength to modulus is calculated by dividing the bending strength by the modulus when each unit is expressed in MPa. The bending test is conducted in accordance with JIS K7074 (3-point bending test).

[0021] The thermal conductivity of a plate-shaped jig can be improved by increasing its density. Since the plate-shaped jig plays a role in uniformly transferring heat during the firing process of the carbon fiber sheet precursor, a high thermal conductivity is preferable. However, excessive density increases the weight, leading to a deterioration in the handling of the firing jig itself. In this invention, it is preferable to set the in-plane thermal conductivity of the plate-shaped jig to 10 to 100 W / (m·K). If it is lower than 10 W / (m·K), heat will not be transferred uniformly to the carbon fiber sheet precursor, and if the density increases to the point where it is higher than 100 W / (m·K), it tends to lead to a deterioration in the main physical properties mentioned above.

[0022] The plate-shaped jig of the present invention has a density of 14 cc / cm² in the thickness direction of the plate-shaped jig. 2 It is preferable that the gas permeability resistance, calculated by dividing the differential pressure when air is passed through at a rate of / sec by the thickness, be 50 mmAq / mm or less. Here, mmAq refers to millimeters of water column. Furthermore, the thickness of the carbon fiber sheet used when calculating the gas permeability resistance is the thickness value measured using a micrometer while applying a pressure of 0.15 MPa in the thickness direction. If the gas permeability resistance exceeds 50 mmAq / mm, as mentioned above, it hinders the escape of decomposition gases generated from the carbon fiber sheet precursor during the firing process of the carbon fiber sheet precursor, leading to an increase in the gas permeability resistance of the carbon fiber sheet fired in contact with the plate-shaped jig.

[0023] Furthermore, the plate-shaped jig of the present invention has a surface roughness R measured in accordance with JIS B 0601. max It is preferable that the thickness is 20-300 μm. max When the value exceeds 300, the transfer of the surface irregularities of the plate-shaped jig to the carbon fiber sheet becomes significant during the firing of the carbon fiber sheet precursor, as described later. When the carbon fiber sheet is used as a substrate for solid polymer fuel cell electrodes, poor bonding with the catalyst layer and localized pressurization of the electrolyte membrane occur, reducing the lifespan of the fuel cell.

[0024] Next, the method for manufacturing the carbon fiber sheet of the present invention will be described.

[0025] The present invention provides a method for manufacturing carbon fiber sheets, characterized in that, in a firing step of firing carbon fiber sheet precursors consisting of carbon fibers or carbon fiber precursors, multiple carbon fiber sheet precursors are stacked, and the plate-shaped jig of the present invention described above is stacked so as to be interposed between the stacked carbon fiber sheet precursors during firing.

[0026] When laminating, it is preferable to interpose a plate-shaped jig every 5 to 60 carbon fiber sheet precursors, that is, to laminate them so that 5 to 60 carbon fiber sheet precursors are sandwiched between adjacent plate-shaped jigs. If the number of carbon fiber sheet precursors interposed between adjacent plate-shaped jigs is less than 5, the number of plate-shaped jigs used in one firing becomes excessive, reducing productivity by compressing the volume inside the firing furnace. Conversely, if the number of carbon fiber sheet precursors interposed between plate-shaped jigs is more than 60, the effect of the rigidity of the plate-shaped jigs in suppressing warping and wrinkling of the carbon fiber sheets is weakened, increasing the risk of shape abnormalities. While it is preferable from the viewpoint of workability and productivity to have one plate-shaped jig interposed every certain number of carbon fiber sheet precursors, this does not preclude the interposition of two or more plate-shaped jigs in a continuous laminated state within the carbon fiber precursor laminate.

[0027] The plate-shaped jig used for firing the carbon fiber sheet precursor preferably has dimensions on each side that are at least 0.5 cm larger in plan view than the dimensions on each side of the carbon fiber sheet precursor in plan view. If the difference in dimensions between the plate-shaped jig and the carbon fiber sheet precursor is less than 0.5 cm, sufficient pressure will not be applied to the edges of the carbon fiber sheet precursor, and wrinkles tend to form at the edges of the carbon fiber sheet.

[0028] The firing of the carbon fiber sheet precursor after lamination is preferably carried out in two stages: a firing process in the low-temperature range with a maximum temperature of 600 to 1000°C, and a firing process in the high-temperature range with a maximum temperature of 2000 to 3000°C. The method of the present invention can be applied to either of these firing processes. In this case, the first firing stage mainly removes decomposition gases generated from the resin. If the maximum temperature is lower than 600°C, the decomposition gases cannot be sufficiently removed, and conversely, if the maximum temperature is higher than 1000°C, not only will the operating cost of the firing furnace increase, but the firing furnace will also wear out more rapidly, increasing its maintenance costs and thus increasing production costs. The second firing stage mainly graphitizes the carbon fibers and resin carbides. If the maximum temperature is lower than 2000°C, the graphitization of the carbon fiber sheet will not proceed sufficiently, and the physical properties of the carbon fiber sheet (thermal conductivity and electrical resistance) will deteriorate, resulting in a decrease in battery performance when used as an electrode substrate for a fuel cell. Conversely, if the maximum temperature exceeds 3000°C, not only will the operating cost of the firing furnace increase, but the furnace will also wear out more rapidly, raising maintenance costs and ultimately increasing production costs.

[0029] The plate-shaped jig used during the firing of carbon fiber sheet precursors preferably has an Fe and V content of 20 ppm or less. If the content exceeds 20 ppm, Fe and V will be transferred to the carbon fiber sheet precursor in contact with the plate-shaped jig, leading to a decrease in power generation performance when the carbon fiber sheet is used as an electrode substrate for a fuel cell. The Fe and V content was measured by fluorescent X-ray analysis after cutting the plate-shaped jig into 30 mm squares and setting the measurement surface to 30 mmΦ. [Examples]

[0030] (Example 1) Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca®" T300-6K (average fiber diameter: 7 μm, number of single fibers: 6,000) is cut to a length of 12 mm, dispersed in water, and then immersed in polyvinyl alcohol and dried to produce a basis weight of approximately 30 g / m². 2 A carbon fiber sheet with a width of 1,150 mm was obtained and wound into a roll.

[0031] Next, the carbon fiber sheet was continuously impregnated with a 6% methanol solution of a resin obtained by mixing resol-type phenolic resin and novolac-type phenolic resin in a 1:1 weight ratio. After heating and drying at 90°C for 3 minutes, it was cut to a length of 1,250 mm to obtain a sheet-like prepreg (1,150 mm × 1,250 mm).

[0032] Next, 20 sheets of the obtained prepreg were stacked, and the resol-type phenolic resin was cured by applying a pressure of 0.60 MPa at a temperature of 155°C for 30 minutes to obtain a molded product. After that, the four edges of the molded product were trimmed to obtain a molded product with dimensions of 1,050 mm x 1,050 mm.

[0033] Next, the resulting molded product was fired in a nitrogen atmosphere at a maximum temperature of 2,550°C to carbonize the resin and obtain a plate-shaped jig with a thickness of 2 mm.

[0034] Carbon fiber sheet precursors (500 mm x 500 mm) with a thickness of 550 μm were stacked in a horizontal arrangement of 2 rows x 2 columns, with one plate-shaped jig obtained in Example 1 interposed every 8 sheets. The stacked carbon fiber sheet precursors were fired in a nitrogen atmosphere at a maximum temperature of 2,550 °C to obtain carbon fiber sheets.

[0035] (Comparative Example 1) A carbon fiber sheet was obtained in the same manner as in Example 1, except that a graphite plate (Toyo Tanso IG-56, 1,050 mm x 1,050 mm x 5.4 mm thick) was used instead of a plate-shaped jig.

[0036] (Example 2) A carbon fiber sheet was obtained in the same manner as in Example 1, except that the plate-shaped jig was interposed every 15 carbon fiber sheet precursors (1,000 mm x 1,000 mm) with a thickness of 0.245 mm.

[0037] (Comparative Example 2) A carbon fiber sheet was obtained in the same manner as in Example 2, except that a graphite plate (Toyo Tanso IG-56, 1,050 mm x 1,050 mm x 5.4 mm thick) was used instead of a plate-shaped jig.

[0038] Table 1 shows the thickness, density, gas permeability resistance, bending strength, and weight of the plate-shaped jig obtained in Example 1 and the graphite plates used in Comparative Examples 1 and 2 when pressurized at 0.15 MPa. Furthermore, the jig and graphite plate were lifted 50 times, with both ends held 50 cm from the front, to a height of 1 m above the floor. The number of times the jig and graphite plate broke was measured. The evaluation methods for thickness and gas permeability are as follows. Thickness: Measured using a micrometer while applying a surface pressure of 0.15 MPa. Gas permeability resistance: 14 cc / cm² in the thickness direction 2 This value is obtained by measuring the differential pressure between the upstream and downstream sides when air is passed through at a rate of / sec, and dividing that value by the thickness mentioned above.

[0039] [Table 1]

[0040] Table 1 shows that the plate-shaped jig used in Example 1 of the present invention achieves weight reduction, improved gas permeability, and an improved ratio of bending strength to elastic modulus, which is an indicator of resistance to cracking, compared to the graphite plates used in Comparative Examples 1 and 2.

[0041] Next, the gas permeability resistance of the carbon fiber sheets laminated in contact with the plate-shaped jigs obtained in Example 1 and Comparative Example 1 was measured using the method described above, and the results are shown in Table 2.

[0042] [Table 2]

[0043] Table 2 shows that the carbon fiber sheet obtained in Example 2 of the present invention exhibits improved gas permeability, which is the objective of the present invention, compared to the carbon fiber sheet obtained in Comparative Example 1.

[0044] Next, the stacking height and stacking weight of the carbon fiber sheet precursors stacked in Example 2 and Comparative Example 2 were measured, and the stacking height, stacking weight, and firing efficiency are shown in Table 3. Note that the stacking height and stacking weight refer to the total height and total weight of 15 carbon fiber sheet precursors and one plate-shaped jig or graphite plate stacked in Example 2 and Comparative Example 2, respectively, and the firing efficiency is the value obtained by dividing the number of stacked carbon fiber sheet precursors by the stacking height.

[0045] [Table 3]

[0046] As shown in Table 3, Example 2, compared to Comparative Example 2, reduced the stacking height by 3.4 mm per layer, and increased the firing capacity from 1.7 sheets / mm to 2.7 sheets / mm. This indicates that by using the plate-shaped jig of the present invention, the number of sheets fired can be increased compared to Comparative Example 2, thereby improving productivity.

Claims

1. A method for manufacturing a carbon fiber sheet, comprising a firing step of firing a carbon fiber sheet precursor consisting of carbon fibers or carbon fiber precursors, wherein in the firing step, a plurality of carbon fiber sheet precursors are stacked, and a plate-shaped jig made of carbon short fibers bound with resin carbide is stacked between the stacked carbon fiber sheet precursors and fired in that state, A method for manufacturing a carbon fiber sheet, wherein the plate-shaped jig has a thickness of 0.5 mm to 3.0 mm, a density of 0.3 g / cm³ to 1.0 g / cm³, a gas permeation resistance of 50 mmAq / mm or less when 14 cc / cm² / sec of air is permeated in the thickness direction, and a ratio of bending strength to bending modulus of elasticity of 0.005 or more.

2. The method for manufacturing a carbon fiber sheet according to claim 1, wherein the plate-shaped jig is a laminated structure of carbon fiber papermaking.

3. A method for manufacturing a carbon fiber sheet according to claim 1 or 2, wherein the plate-shaped jig is interposed between every 5 to 60 carbon fiber sheet precursors during lamination.

4. A method for manufacturing a carbon fiber sheet according to any one of claims 1 to 3, wherein the dimensions of the plate-shaped jig in plan view are 0.5 cm or more larger than the dimensions of the carbon fiber sheet precursor in plan view.

5. A method for manufacturing a carbon fiber sheet according to any one of claims 1 to 4, wherein the maximum temperature of the firing step is 600 to 1000°C or 2000 to 3000°C.

6. A method for manufacturing a carbon fiber sheet according to Claim 1, wherein carbon short fibers are bonded with a resin carbide. A plate-shaped jig having a surface roughness Rmax of 20 to 300 μm.

7. The plate-shaped jig according to claim 6, wherein the in-plane thermal conductivity is 10 to 100 W / (m·K).

8. The plate-shaped jig according to claim 6 or 7, wherein the Fe and V content is 20 ppm or less.

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