Manufacturing method of carbon fiber sheet and tabular jig used therefor

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 producing a carbon fiber sheet used for electrode substrates for polymer electrolyte fuel cells, electrodes for electrolysis, etc., and to a plate-like jig used when firing a carbon fiber sheet precursor in the production method. [Background technology]

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

[0003] To explain each process, in the papermaking process, short carbon 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 a pressing process to become a carbon fiber sheet precursor. In the firing process, this carbon fiber sheet precursor is fired at a high temperature of 2,000°C or higher, whereby the resin inside the carbon fiber sheet precursor is converted into charcoal while emitting decomposition gases, resulting in a carbon fiber sheet in which the carbon fibers are bonded together by the charcoal.

[0004] In the firing process, multiple carbon fiber sheet precursors are sometimes stacked and fired. During this process, shrinkage of the resin contained in the carbon fiber sheet precursor during firing often causes problems with the carbon fiber sheets becoming warped, wrinkled, or otherwise irregular in shape. When this occurs, when a fuel cell is manufactured by stacking an electrode substrate, catalyst, and electrolyte membrane using carbon fiber sheets, the contact between the electrode substrate and each component deteriorates, causing misalignment or partial peeling of the stacked and bonded materials, making the material unusable.

[0005] As a method for suppressing such shape abnormalities, Patent Document 1 discloses that when stacking carbon fiber sheet precursors, a graphite plate having high thermal conductivity and high rigidity is inserted every certain number of sheets and then fired. This makes it possible to uniformly heat the carbon fiber sheets by the thermal conduction of the graphite plate, thereby making the firing shrinkage of the resin uniform, and also to suppress the occurrence of warping and wrinkles by the rigidity of the graphite plate, thereby suppressing the occurrence of shape abnormalities. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-48182 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the graphite plates described in Patent Document 1 are high density, heavy, brittle, and easily broken, which causes a problem of a heavy burden during the lamination work. An object of the present invention is to provide a method for producing a carbon fiber sheet with better workability by using a firing jig that is lightweight and easy to handle. [Means for solving the problem]

[0008] The present invention to achieve the above object is as follows. (1) A method for producing a carbon fiber sheet, comprising a firing step of firing a carbon fiber sheet precursor made of carbon fiber or a carbon fiber precursor, wherein in the firing step, a plurality of the carbon fiber sheet precursors are stacked, and a plate-like jig formed by binding short carbon fibers with a resin carbide is interposed between the stacked carbon fiber sheet precursors, and the stacked state is fired. (2) The method for producing a carbon fiber sheet according to (1), wherein the plate-like jig is a papermaking structure of short carbon fibers. (3) The method for producing a carbon fiber sheet according to (1) or (2), wherein the plate-like jig is placed between every 5 to 60 sheets of the carbon fiber sheet precursors. (4) The method for producing a carbon fiber sheet according to any one of (1) to (3), wherein the dimensions of the plate-like jig are larger than the dimensions of the carbon fiber sheet precursor by 0.5 cm or more in both width and length. (5) The method for producing a carbon fiber sheet according to any one of claims (1) to (4), wherein the maximum temperature in the firing step is 600 to 1000°C or 2000 to 3000°C. The present invention is also characterized by providing the following plate-shaped jig. (6) A plate-like jig used in the method for producing a carbon fiber sheet according to (1), the plate-like jig having a thickness of 0.5 mm to 3.0 mm. (7) The plate-like jig according to (6), which has a density of 0.3 g / cm 3 to 1.0 g / cm 3 . (8) 14cc / cm in thickness direction 2 The plate-like jig according to (6) or (7), wherein the gas permeation resistance calculated by dividing the pressure difference when air is passed through at 1 / sec by the thickness is 50 mmAq / mm or less. (9) A plate-like jig according to any one of (6) to (8), having a ratio of bending strength to modulus of elasticity of 0.005 or more. (10) Surface roughness R max The plate-like jig according to any one of (6) to (9), wherein the thickness is 20 to 300 μm. (11) The plate-like jig according to any one of (7) to (10), having a thermal conductivity in an in-plane direction of 10 W / (m·K) or more. (12) The plate-like jig according to any one of (6) to (11), wherein the contents of Fe and V are 20 ppm or less. [Effects of the Invention]

[0009] According to the present invention, in the production of a carbon fiber sheet, the workability of the firing step can be improved by using a plate-like jig that is lightweight and easy to handle while preventing deterioration of the gas permeability of the carbon fiber sheet precursor. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the plate-shaped jig used in the present invention (hereinafter sometimes referred to as the "plate-shaped jig of the present invention" or simply as the "plate-shaped jig"), its manufacturing method, 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 made of short carbon fibers bound with a resin carbide. The structure of the plate-shaped jig is preferably a laminated structure of carbon fiber paper. By using a laminated structure of carbon fiber paper, the plate-shaped jig can have a surface shape and high gas permeability, and is lightweight, high-strength, and easy to handle.

[0012] Such a carbon fiber paper laminate structure can typically be produced by hot pressing a carbon fiber paper body impregnated with a thermosetting resin, and more specifically, can be produced by a manufacturing method comprising a papermaking step, a resin impregnation step, a pressing step, and a firing step.

[0013] In the papermaking process, carbon short fibers are dispersed randomly in a roughly two-dimensional plane and bound together with an organic binder to produce long carbon fiber paper wound into a roll. By using the carbon fiber paper produced by papermaking, the plate-shaped jig becomes a paper structure of carbon short fibers, preventing the surface shape of the jig from being transferred to the carbon fiber sheet during firing, thereby preventing changes in the uneven shape. Methods for randomly dispersing carbon short fibers include the wet method, in which carbon short fibers are dispersed in a liquid and then made into a paper, and the dry method, in which carbon short fibers are dispersed in air and allowed to settle. The wet method is preferred because it provides good dispersibility and orientation of the carbon short fibers in a two-dimensional plane, resulting in low surface roughness of the plate-shaped jig and fewer localized convexities due to poor dispersion or protruding carbon fibers.

[0014] In the resin impregnation process, the carbon fiber paper is impregnated with a thermosetting resin and cut to a certain length to obtain a prepreg, which is a composite of the carbon fiber paper and the thermosetting resin. Examples of thermosetting resins that can be used include phenolic resin, epoxy resin, furan resin, and melamine resin, and a mixed resin containing at least one of these may also be used. The thermosetting resin must remain as a resin char that binds the short carbon fibers after the firing process. Among these, phenolic resins are particularly preferred because they have a high residual carbon content, have good binding properties with the carbon fibers, and are easy to handle in the resin impregnation and pressing processes. In addition, in the resin impregnation process, it is also preferable to mix carbon particles such as graphite powder or carbon black into the resin and impregnate it to increase 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 together to obtain a molded product. The thickness of the molded product can be controlled by the number of prepregs stacked. When multiple prepregs are stacked, the final plate-shaped jig is a laminated structure of carbon fiber paper. The heating temperature is preferably 100 to 250°C, more preferably 120 to 200°C, and even more preferably 140 to 180°C. A temperature below 100°C requires a longer molding time. Furthermore, if the molding time is short and the thermosetting resin is not cured sufficiently, shrinkage during baking increases, resulting in warping and thickness variations. Furthermore, if the temperature exceeds 250°C, the resin cures before the thickness is uniformed by pressing, resulting in thickness variations in the molded product. The pressing pressure is preferably 0.1 to 2 MPa, more preferably 0.3 to 1 MPa. If the pressure is less than 0.1 MPa, the adhesion between the carbon fiber and the resin, and even between the carbon fiber and the resin carbide after firing, will be weak and uneven, resulting in a decrease in the mechanical strength of the plate-shaped jig, such as its bending strength. If the pressure exceeds 2 MPa, the plate-shaped jig will become too dense and its gas permeability may become too low. Furthermore, the short carbon fibers may break, reducing the bending strength of the jig and increasing dust generation.

[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-shaped jig in which the short carbon fibers are bound with resin carbide. The maximum firing temperature in the firing process is 2,000 to 2,700°C. If the maximum firing temperature is too low, many metal impurities remain in the plate-shaped jig and are transferred from the plate-shaped jig to the carbon fiber sheet during firing. Furthermore, if the plate-shaped jig is stacked without load on the edges and fired at a high temperature exceeding the jig's processing history, warping and wrinkles are likely to occur at the edges. Conversely, if the maximum firing temperature is too high, not only will the operating costs of the firing furnace increase, but the furnace will also suffer from severe wear and tear, increasing maintenance costs and ultimately production costs. The firing process may be performed in two stages: firing at a maximum temperature of 1,000°C or less and firing at a maximum temperature of 2,000°C or more.

[0017] The above steps allow the production of a plate-shaped jig made of short carbon fibers bound with a resin carbide. Such a plate-shaped jig has a porous structure with low density and good gas permeability, while also having minimal 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 more likely to break when handled, while if it is thicker than 3.0 mm, it will occupy a larger height in the furnace during firing, reducing the amount of fired material. The thickness is measured using a micrometer at 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 up to 0.3 g / cm 3 ~1.0g / cm 3 The density of the plate-shaped jig is preferably 0.4 g / cm 3 ~0.7g / cm 3 The density is 0.3 g / cm 3 If it is smaller than 1.0g / cm, the strength of the plate-shaped jig will decrease and it will tend to be easily broken. 3 If it is larger, there is a tendency that when the end of the plate-shaped jig is held, it may be broken due to bending caused by its own weight, or that the gas permeation resistance may become too high.

[0020] The bending strength and bending modulus of the plate-shaped jig increase as the plate-shaped jig is densified, but the ratio of bending strength to modulus tends to decrease, and this ratio also decreases as the adhesion between the carbon fiber and the resin carbide improves. The ratio of bending strength to modulus is preferably 0.005 or more, more preferably 0.007 or more, and even more preferably 0.01 or more. If it is less than 0.005, the plate-shaped jig is prone to cracking due to bending during handling. The ratio of bending strength to modulus is the value obtained by dividing the bending strength value by the modulus value, with each unit being MPa. The bending test is performed in accordance with JIS K7074 (three-point bending test).

[0021] The thermal conductivity of the 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, which leads to poor handling of the firing jig itself. In the present invention, the thermal conductivity of the plate-shaped jig in the in-plane direction is preferably 10 to 100 W / (m·K). If it is lower than 10 W / (m·K), it becomes difficult to transfer heat uniformly to the carbon fiber sheet precursor. However, if the density is increased to more than 100 W / (m·K), it tends to deteriorate the main physical properties mentioned above.

[0022] The plate-shaped jig of the present invention has a flow rate of 14 cc / cm in the thickness direction of the plate-shaped jig. 2 It is preferable that the gas permeation resistance, calculated by dividing the pressure difference when air permeates at 1 / sec by the thickness, is 50 mmAq / mm or less. Here, mmAq refers to millimeters of water column. Furthermore, the thickness of the carbon fiber sheet used in calculating the gas permeation resistance is the thickness measured with a micrometer while a pressure of 0.15 MPa is applied in the thickness direction. If the gas permeation resistance exceeds 50 mmAq / mm, as described above, the escape of decomposition gases generated from the carbon fiber sheet precursor during the firing process of the carbon fiber sheet precursor is hindered, resulting in an increase in the gas permeation resistance of the carbon fiber sheet fired in contact with a plate-shaped jig.

[0023] In addition, 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 R is 20 to 300 μm. max If the surface roughness exceeds 300, the transfer of the surface irregularities of the plate-like jig to the carbon fiber sheet becomes significant during the firing of the carbon fiber sheet precursor described below, and when the carbon fiber sheet is used as an electrode substrate for a polymer electrolyte fuel cell, poor bonding with the catalyst layer and localized pressure on the electrolyte membrane occur, resulting in a shortened life of the fuel cell.

[0024] Next, a method for producing a carbon fiber sheet according to the present invention will be described.

[0025] The method for producing a carbon fiber sheet of the present invention is characterized in that, in a firing step of firing a carbon fiber sheet precursor made of carbon fiber or a carbon fiber precursor, a plurality of carbon fiber sheet precursors are stacked, and the plate-like jig of the present invention described above is interposed between the stacked carbon fiber sheet precursors and fired in that state.

[0026] During stacking, it is preferable to interpose a plate-shaped jig every 5 to 60 sheets of carbon fiber sheet precursor, i.e., stack the sheets so that 5 to 60 sheets of carbon fiber sheet precursor 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 will be excessive, which will constrict the volume inside the firing furnace and reduce productivity. Conversely, if the number of carbon fiber sheet precursors interposed between plate-shaped jigs is more than 60, the rigidity of the plate-shaped jigs will weaken the effect of suppressing the occurrence of warping and wrinkles in the carbon fiber sheets, increasing the risk of shape abnormalities. Note that, from the standpoints of workability and productivity, it is preferable to interpose one plate-shaped jig between a certain number of carbon fiber sheet precursors. However, this does not exclude the possibility of interposing two or more plate-shaped jigs in a continuous stack in the carbon fiber precursor stack.

[0027] The plate-like jig used for firing the carbon fiber sheet precursor preferably has a side dimension in plan view that is at least 0.5 cm larger than the side dimension in plan view of the carbon fiber sheet precursor. If the difference in size between the plate-like jig and the carbon fiber sheet precursor is less than 0.5 cm, sufficient pressure is not applied to the edge of the carbon fiber sheet precursor, and wrinkles tend to occur at the edge of the carbon fiber sheet.

[0028] The calcination of the laminated carbon fiber sheet precursor is preferably carried out in two stages: a calcination step at a low temperature range with a maximum temperature of 600 to 1000°C, and a calcination step at a 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 calcination steps. In this case, the first calcination stage is primarily intended to remove decomposition gases generated from the resin. If the maximum temperature is lower than 600°C, the decomposition gases cannot be sufficiently removed. Conversely, if the maximum temperature is higher than 1000°C, not only does the operating cost of the calcination furnace increase, but the calcination furnace also becomes subject to severe wear, increasing its maintenance cost and production cost. The second calcination stage is primarily intended to graphitize the carbon fiber and resin charcoal. If the maximum temperature is lower than 2000°C, the graphitization of the carbon fiber sheet does not proceed sufficiently, and the physical properties (thermal conductivity and electrical resistance) of the carbon fiber sheet deteriorate, resulting in reduced cell performance when used as an electrode substrate for a fuel cell. Conversely, if the maximum temperature is higher than 3000°C, not only will the operating costs of the kiln increase, but the kiln will also be subject to increased wear and tear, increasing maintenance costs and production costs.

[0029] The plate-like jig used when firing the carbon fiber sheet precursor preferably has an Fe and V content of 20 ppm or less. If the content exceeds 20 ppm, Fe and V are transferred to the carbon fiber sheet precursor in contact with the plate-like jig, resulting in 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 contents are values ​​measured by fluorescent X-ray analysis after cutting the plate-like jig into 30 mm squares and setting the measurement surface to 30 mmΦ. [Example]

[0030] Example 1 Toray Industries, Inc.'s polyacrylonitrile carbon fiber "TORAYCA (registered trademark)" T300-6K (average fiber diameter: 7 μm, number of single fibers: 6,000) was cut to a fiber length of 12 mm, dispersed in water, and made into a paper sheet. This was then immersed in polyvinyl alcohol and dried to produce a paper sheet with a basis weight of approximately 30 g / m. 2 A carbon fiber sheet having 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 wt% methanol solution of a resin made by mixing resol-type phenolic resin and novolac-type phenolic resin in a 1:1 weight ratio, heated and dried at 90°C for 3 minutes, and then cut to a length of 1,250 mm to obtain a sheet-like prepreg (1,150 mm x 1,250 mm).

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

[0033] The resulting molded product was then 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 × 500 mm) with a thickness of 550 μm were stacked horizontally in a 2-row × 2-column arrangement, with one plate-like jig obtained in Example 1 sandwiched between every eight sheets. The stacked carbon fiber sheet precursors were fired at a maximum temperature of 2,550°C in a nitrogen atmosphere 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 the plate-shaped jig.

[0036] Example 2 Carbon fiber sheets were obtained in the same manner as in Example 1, except that the plate-like jig was placed between every 15 carbon fiber sheet precursors (1,000 mm x 1,000 mm) having 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 the plate-shaped jig.

[0038] The thickness, density, gas permeation 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 are shown in Table 1. In addition, both ends of the jig and the graphite plate were held at a position 50 cm from the front, and the jig and the graphite plate were lifted to a height of 1 m from the floor 50 times, and the number of times the jig and the graphite plate were broken was counted. The thickness and gas permeability were evaluated as follows. Thickness: Measured using a micrometer with a surface pressure of 0.15 MPa applied. Gas permeation resistance: 14cc / cm in thickness direction 2 The differential pressure between the upstream and downstream sides is measured when air is passed through at a rate of 1 / sec, and this value is divided by the thickness mentioned above.

[0039] [Table 1]

[0040] From Table 1, it can be seen that the plate-shaped jig used in Example 1 of the present invention has achieved weight reduction, improved gas permeability, and an improved ratio of bending strength to elastic modulus, which is an index of crack resistance, compared to the graphite plates used in Comparative Examples 1 and 2.

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

[0042] [Table 2]

[0043] From Table 2, it was found that the carbon fiber sheet obtained in Example 2 of the present invention had improved gas permeability, which is the object 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. The stacking height and stacking weight are the total height and total weight of 15 carbon fiber sheet precursors and one plate-like jig or graphite plate stacked in Example 2 and Comparative Example 2, and the firing efficiency is a value obtained by dividing the number of stacked carbon fiber sheet precursors by the stacking height.

[0045] [Table 3]

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

Claims

1. A method for producing a carbon fiber sheet, comprising a firing step of firing a carbon fiber sheet precursor made of carbon fibers or a carbon fiber precursor, wherein in the firing step, a plurality of the carbon fiber sheet precursors are stacked, and a plate-like jig formed by binding short carbon fibers with a resin carbide is interposed between the stacked carbon fiber sheet precursors, and the stacked state is fired.

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

3. The method for producing a carbon fiber sheet according to claim 1 or 2, wherein the plate-shaped jig is interposed between every 5 to 60 sheets of the carbon fiber sheet precursors.

4. The method for producing a carbon fiber sheet according to claim 1 , wherein a dimension of the plate-shaped jig in a plan view is 0.5 cm or more larger than a dimension of the carbon fiber sheet precursor in a plan view.

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

6. 2. A plate-shaped jig used in the method for producing a carbon fiber sheet according to claim 1, which is made by binding short carbon fibers with a resin carbide and has a thickness of 0.5 mm to 3.0 mm.

7. Density is 0.3 g / cm ３ ~1.0g / cm ３ The plate-shaped jig according to claim 6 ,

8. 14cc / cm in thickness direction ２ 8. The plate-like jig according to claim 6 or 7, wherein the gas permeation resistance when air is allowed to pass through the plate-like jig at 1000 sq. m. / sec is 50 mmAq / mm or less.

9. 9. The plate-like jig according to claim 6, wherein the ratio of bending strength to bending modulus is 0.005 or more.

10. The plate-shaped jig according to any one of claims 6 to 9, having a surface roughness Rmax of 20 to 300 µm.

11. The plate-shaped jig according to any one of claims 7 to 10, having a thermal conductivity in an in-plane direction of 10 to 100 W / (m · K).

12. The plate-shaped jig according to claim 6 , wherein the Fe and V contents are 20 ppm or less.