JP2004308098A - Method for producing carbon fiber sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously producing a dense carbon fiber sheet, capable of preventing the sheet from being deteriorated in compression characteristics and decreased in resistance to permeation of gases. <P>SOLUTION: This method for producing the carbon fiber sheet comprises continuously baking a precursor fiber sheet in a heating furnace, while conveying the sheet therein, so as to produce the sheet having a thickness of 0.1-0.25 mm and a bulk density of 0.3-0.7 g/cm<SP>3</SP>, wherein the precursor sheet prior to baking is continuously heated and pressurized by hot plates or molds arranged parallel to each other, while intermittently conveying the sheet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a sheet of carbon fiber woven fabric, nonwoven fabric, paper and the like.

  Sheets such as carbon fiber woven fabric, non-woven fabric, and paper are used for a wide variety of applications such as molding CFRP (carbon fiber reinforced plastic), repairing and reinforcing concrete structures, radio wave absorbers, and fuel cell electrodes. ing.

  By the way, as is well known, for example, a raw material fiber (precursor) such as an acrylic fiber is fired in a relatively low-temperature oxidizing atmosphere of about 200 to 400 ° C. to be flame-resistant (flame-resistance). After the fibers are formed), they are fired and carbonized in an inert atmosphere at a high temperature of 1,000 ° C. or more. The carbon fiber sheet may be woven or made into a non-woven fabric using the carbon fiber thus produced as a woven yarn. However, since the carbon fiber is brittle and easily fluffed, the weaving and non-woven fabric operation is very difficult. In addition, since it is inefficient to fire the raw material fibers and the oxidized fibers one by one, the raw fibers and the oxidized fibers are made into a woven fabric or a nonwoven fabric in advance, and then baked into a carbon fiber sheet. (For example, see Patent Document 1).

However, when the raw fiber is subjected to the oxidization treatment in the form of a woven or nonwoven fabric having a high yarn bulk density, heat is generated in the woven or nonwoven fabric because the oxidization treatment is a treatment involving an exothermic reaction, and stable temperature control is achieved. The resulting carbon fiber sheet is extremely difficult and tends to have uneven quality. When carbonizing oxidized woven fabric or non-woven fabric, these oxidized fiber fabrics tend to be porous because of shrinkage due to carbonization, and the performance may not be sufficiently exhibited depending on the application. For example, when it is used as an electrode substrate (hereinafter referred to as a gas diffusion layer) of a polymer electrolyte fuel cell, cell characteristics are greatly increased due to deterioration of compression characteristics such as compression ratio and compression residual strain, or reduction of gas permeation resistance. Lower it. Therefore, it is said that it is preferable to densify the flame-resistant fiber cloth which is the precursor fiber sheet before firing (for example, see Patent Document 2), but a specific means for efficiently densifying the continuous sheet is proposed. However, only the compression treatment by hot press or calender roll is shown. Further, in the method of Patent Document 2, it is considered that the thickness of the carbon fiber sheet is preferably 0.15 to 10 mm, and a pressure of 10 to 100 (more preferably 15 to 90) MPa is required in the case of hot pressing. . Therefore, if a compression process of 1 m ² is performed by one press, a pressing force of 10 ⁷ to 10 ⁸ N (about 10 ³ to 10 ⁴ tf) is required, and a large-scale press system is used or the production efficiency is reduced. And it is necessary to reduce the processing area per operation. Further, the compression treatment using a calender roll has a problem that a single fiber is broken or flattened due to a local compression force, and the carbon fiber sheet after firing tends to be brittle.
Japanese Patent Publication No. 61-11323 JP-A-2002-194650

  An object of the present invention is to solve the above-mentioned problems of the conventional technology and to provide a method for producing a dense carbon fiber sheet capable of suppressing deterioration of compression characteristics and reduction of gas permeation resistance.

In order to achieve the above object, the present invention employs the following configuration. That is,
(1) A carbon fiber sheet having a thickness of 0.1 to 0.25 mm and a bulk density of 0.3 to 0.7 g / cm ³ by firing the precursor fiber sheet while continuously transporting it in a heating furnace. Wherein the precursor fiber sheet before firing is continuously heated and pressed by hot plates parallel to each other while being intermittently conveyed.

(2) When the effective pressing length in the transport direction of the hot plate parallel to each other is L _P and the feed amount of the precursor fiber sheet during intermittent transport is L _F , L _F / L _P is 0.04. The method for producing a carbon fiber sheet according to (1), wherein the number is from 1.5 to 1.5.

(3) When the effective pressing length in the conveying direction of the hot plates parallel to each other is L _P and the feed amount of the precursor fiber sheet during intermittent conveyance is L _F , L _F / L _P is 0.1. (1) The method for producing a carbon fiber sheet according to (1), wherein

  (4) The method for producing a carbon fiber sheet according to any one of (1) to (3), wherein the temperature of the hot plate parallel to each other is 140 to 300 ° C. and the pressure is 0.1 to 40 MPa. .

  (5) The carbon fiber sheet according to any one of (1) to (4), wherein preheating and pressing is performed by at least one set of calender rolls before continuous heating and pressing with the hot plates parallel to each other. Production method.

(6) The precursor fiber sheet is fired while being continuously conveyed in a heating furnace, and has a thickness of 0.1 to 0.25 mm and a bulk density of 0.3 to 0.7 g / cm ^3. Wherein the precursor fiber sheet before firing is continuously heated and pressed by a mold while intermittently being conveyed.

(7) Assuming that the effective press length in the conveying direction of the mold is L _P and the feed amount of the precursor fiber sheet during intermittent conveyance is L _F , L _F / L _P is 0.04 to 1. 5. The method for producing a carbon fiber sheet according to (6), wherein

  (8) The method for producing a carbon fiber sheet according to (6) or (7), wherein the temperature of the mold is 140 to 300 ° C and the pressure is 0.1 to 40 MPa.

  (9) The method for producing a carbon fiber sheet according to any one of (6) to (8), wherein preheating and pressing is performed by at least one set of calender rolls before continuous heating and pressing with a mold.

  (10) The method for producing a carbon fiber sheet according to any one of (1) to (9), wherein an oxidized fiber cloth is used as the precursor fiber sheet.

  (11) The method for producing a carbon fiber sheet according to (10), wherein a woven fabric, a nonwoven fabric, or paper is used as the flame-resistant fiber fabric.

  (12) The method for producing a carbon fiber sheet according to any one of (1) to (9), wherein a paper obtained by binding carbon fibers with a binder is used as the precursor fiber sheet.

  (13) The method for producing a carbon fiber sheet according to (12), wherein a carbon fiber having an average fiber diameter in a range of 5 to 20 μm is used.

  (14) The method for producing a carbon fiber sheet according to (12) or (13), wherein a carbon fiber having an average fiber length in a range of 3 to 20 mm is used.

  (15) The method for producing a carbon fiber sheet according to any one of (10) to (14), wherein a precursor fiber sheet impregnated with a resin is used.

  (16) The method for producing a carbon fiber sheet according to (15), wherein a thermosetting resin is used as the resin impregnated in the precursor fiber sheet.

  (17) The method for producing a porous carbon substrate according to (16), wherein a phenol resin is used as the thermosetting resin.

  (18) The method for producing a carbon fiber sheet according to any one of (15) to (17), wherein a phenol resin that does not use a metal catalyst or an alkali catalyst during synthesis is used as the resin to be impregnated.

  (19) The method for producing a carbon fiber sheet according to any one of (1) to (18), wherein the precursor fiber sheet contains carbonaceous powder.

  (20) The method for producing a carbon fiber sheet according to (19), wherein a particle diameter of the carbonaceous powder is in a range of 0.01 to 10 μm.

  (21) The method for producing a carbon fiber sheet according to (19) or (20), wherein graphite or carbon black is used as the carbonaceous powder.

  (22) A carbon fiber sheet produced by the method according to any one of (1) to (21).

  (23) A gas diffusion layer for a polymer electrolyte fuel cell, manufactured by the method according to any one of (1) to (21).

  The present invention relates to a method of manufacturing a carbon fiber sheet by firing a precursor fiber sheet while continuously transporting the same in a heating furnace, wherein the precursor fiber sheet before firing is intermittently transported while being intermittently transported. Since it is continuously heated and pressed by a plate or a mold, as is clear from the comparison between the example and the comparative example, a dense carbon fiber sheet capable of suppressing the deterioration of the compression characteristics and the gas permeation resistance is continuously obtained. be able to.

As described above, in the method for producing a carbon fiber sheet of the present invention, the precursor fiber sheet is fired while being continuously conveyed in a heating furnace to have a thickness of 0.1 to 0.25 mm and a bulk density of 0. In the method for producing a carbon fiber sheet of 0.3 to 0.7 g / cm ³ , the precursor fiber sheet before firing is continuously heated and pressed by a hot plate or a mold parallel to each other while being intermittently conveyed. It is a feature.

  The heating and pressurizing treatment is performed intermittently while the precursor fiber sheet is conveyed, that is, while alternately repeating the pressurizing and feeding of the precursor fiber sheet, the long precursor fiber which is a continuous body in the conveying direction. This is because the sheet is continuously densified without being made into a sheet shape.

  "Continuous heating and pressurization" in the present invention refers to continuous densification of the continuous precursor fiber sheet, which is a continuous body, without forming a single sheet, and the long precursor fiber sheet. May include a part that is not heated and pressurized. For example, when a sheet surface described later is patterned into a three-dimensional shape, a three-dimensional pattern is lined up like a film frame on a continuous precursor fiber sheet that is long. , Ie, between the patterns, it is not necessary to apply heat and pressure.

When the precursor fiber sheet is intermittently conveyed and the continuous heating and pressurization is performed by a hot plate or a mold parallel to each other, the effective press length in the conveyance direction is L _P , and the precursor fiber sheet when intermittently conveyed when the feed amount and L _F, L _F / L _P is preferably from 0.04 to 1.5, 0.1 to 0.98 is more preferable. In the case of a substantially two-dimensional plane, it is most preferably 0.04 to 0.45. When L _F / L _P is less than 0.04, the densification effect by heating and pressing can be more averaged, so that the flatness can be improved, that is, the thickness variation in the transport direction can be reduced. In addition, the time ratio required for opening and closing the press and feeding the precursor fiber sheet increases, and the production efficiency deteriorates. On the other hand, when L _F / L _P exceeds 1.5, the ratio of the undensified portion becomes larger than the densified portion, so that the portion where the material is lost increases and the production efficiency deteriorates. In this case, if the range of L _F / L _P is reduced to less than 0.1, in addition to the above-mentioned effects, the press equipment is required to have a face plate size of 1 to 1.5 m square, ie, L _P of about 1 to 1.5 m. When using, the balance between the feed amount L _F , the heating and pressurizing time, etc. is improved, and if it exceeds 0.98, the uniform densification effect in the case of a substantially two-dimensional plane intended by the present invention is obtained. In addition, the effect of patterning the sheet surface into a three-dimensional shape is added. That is, the surface of the sheet can be patterned into a three-dimensional shape by using a mold having an uneven pattern and setting L _F / L _P to 1 to 1.5. This is effective, for example, when it is desired to change the gas permeability in the gas flow path in the gas diffusion layer for a polymer electrolyte fuel cell manufactured by the method according to any of the present invention.

  Here, the effective pressurized length refers to the length of a portion where the precursor fiber sheet is in contact with a hot plate or a mold and is heated and pressed. The term “feed amount” refers to the amount of the precursor fiber sheet that is sent (or taken off) in the transfer direction when the press is opened per transfer.

  The heating and pressing conditions in a hot plate or a mold parallel to each other vary depending on the composition of the precursor fiber sheet to be used. For example, when an acrylic flame-resistant fiber cloth is used as the precursor fiber sheet, the temperature is 140 to 300 ° C. The heating and pressurization may be performed at a surface pressure of 0.1 to 40 MPa for 0.2 to 15 minutes.

  The hot plates parallel to each other were arranged at the center of each of the 25 sections, the surface in contact with the precursor fiber sheet of the hot plate divided into five equal parts in the transport direction and five equal parts in the direction perpendicular to the transport direction. When the lead piece is deformed under heat and pressure, the difference between the maximum value and the minimum value of the thickness of the deformed lead piece in at least 13 sections is 1 mm or less. Further, the material of the hot plate is preferably a metal such as a general steel material such as carbon steel, but a heat-resistant plastic, rubber or the like can also be used. The material of both hot plates may be the same, but different ones may be used. For example, one hot plate may be made of stainless steel, and the other hot plate may be made of silicon rubber. In this case, the parallelism is calculated by measuring a steel plate having a known thickness in advance as a hot plate and substituting the thickness with a separately measured hot plate made of stainless steel or silicon rubber.

  In addition, the mold can be provided with necessary pattern irregularities and inclined portions. By doing so, the thickness of the precursor fiber sheet after heating and pressurizing can be changed for each part, for example, for a polymer electrolyte fuel cell manufactured by the method according to any one of the present invention. In the gas diffusion layer, the gas permeability can be changed in the gas flow path.

  Next, the heating and pressing conditions in the case of using a flame-resistant fiber fabric as the precursor fiber sheet will be described.

  A more preferable treatment temperature in the heating and pressurization with a hot plate or a mold parallel to each other is 160 to 300 ° C, more preferably 170 to 230 ° C. If the temperature is too low, the effect of densification of the precursor fiber sheet by heating and pressing is insufficient, and if the temperature is less than 140 ° C., the effect is small. If the temperature is too high, the oxidation of the acrylic fiber oxidized fiber fabric, which is the precursor fiber sheet, proceeds in air, causing problems such as a decrease in strength. Furthermore, equipment maintenance and process management become difficult due to the high temperature.

The surface pressure is preferably 2 to 25 MPa, more preferably 3 to 15 MPa, and still more preferably 4 to 8 MPa. If the pressure is low, the effect of densifying the precursor fiber sheet is insufficient. If the pressure is high, when the precursor fiber sheet is bent, a linear pattern likely to be caused by buckling of fibers or separation between fibers occurs, and the gas permeability of the fired carbon fiber sheet decreases, and Good characteristics cannot be exhibited as a gas diffusion layer of a battery. In addition, problems such as adhesion to a press surface, which is a pressing surface, and release paper occur. Further, the press equipment also requires a pressing force of 2550 tf in order to press 1 m ² at 25 MPa, and it is necessary to use a large-scale press system or to reduce the production efficiency and reduce the processing area per operation.

  The heating and pressurizing time is preferably 1.5 to 10 minutes, more preferably 3.5 to 6 minutes. If the heating and pressurizing time is short, the effect of densification by heating and pressurizing cannot be sufficiently obtained. Further, even if the heating and pressurization is performed for more than 6 minutes, a further increase in the densification effect cannot be expected much.

  In addition, if a preliminary compression process is performed by a calender roll before heating and pressing with a hot plate or a mold, the same densification effect can be obtained even if the pressing force with the hot plate or the mold is reduced. This is preferable because the press equipment can be simplified or the processing area per operation can be increased.

  In this case, the treatment temperature is the same as described above for the pressurization with a calender roll, a hot plate, or a mold, but the pressing force on the calender roll is preferably 5 to 200 kN / m, more preferably 10 to 100 kN. / M, more preferably 20 to 70 kN / m. If the pressure is low, the effect of densification of the precursor fiber sheet by pressurization is insufficient, and if the pressure is high, the local compression force acts on the calender roll, which causes breakage or flattening of the single fiber, and after firing. The carbon fiber sheet becomes brittle. The pressing force by a hot plate or a mold can be reduced by using a calendar roll together, and the pressure by the hot plate or the mold when used together is preferably 0.2 to 8 MPa, more preferably 0.1 to 8 MPa. 3 to 2 MPa. If the pressure is too low, the effect of densifying the precursor fiber sheet is insufficient. If the pressure is too high, the gas permeability of the fired carbon fiber sheet will decrease, and sufficient characteristics as a gas diffusion layer of the fuel cell will not be exhibited. Further, it is necessary to use a large-scale press system as the pressure increases, or to reduce the production efficiency and reduce the processing area per operation.

  As described above, the precursor fiber sheet before firing is continuously heated and pressed by a hot plate parallel to each other or a mold while intermittently being conveyed, but it has been considered preferable until now, but there is no specific means. Continuous densification before firing can be enabled.

  As the precursor fiber, the above-mentioned flame-resistant fiber, for example, in addition to acrylic flame-resistant fiber obtained by flame-resistant acrylic fiber, rayon fiber, phenol fiber, various synthetic fibers such as infusible pitch fiber are used alone or in combination. However, it is particularly preferable to use carbon fiber excellent in various properties such as strength and elastic modulus, and acrylic oxidized fiber from which a carbon fiber sheet can be obtained.

  As a sheet made of such precursor fibers, an oxidized fiber fabric such as a woven fabric, a nonwoven fabric, and a paper can be used.

  Further, as the precursor fiber sheet, paper formed by binding chopped carbon fiber (short fibers) or the like with a binder such as a phenol resin or a PVA resin can also be used.

  When a paper made of carbon fibers bound with a binder is used as the precursor fiber sheet, the carbon fiber preferably has a length of 3 to 20 mm, and more preferably 5 to 15 mm. Is more preferable in order to improve the dispersibility of carbon fibers when dispersing and making paper to obtain a precursor fiber sheet. The carbon fiber preferably has a fiber diameter of 4 to 20 μm, more preferably 4 to 13 μm, and more preferably 4 to 10 μm, in order to obtain a suitable pore diameter.

  The average fiber diameter of the carbon fibers is determined as a simple average value by selecting any 10 carbon fibers from a cross-sectional photograph of the fibers by an electron microscope at a magnification of 5,000 and measuring the fiber diameter. If the cross-sectional shape is not circular, for example, an elliptical diameter, the average value of the major axis and the minor axis is defined as the fiber diameter.

  The average fiber length of the carbon fibers is determined by heating the precursor fiber sheet at 600 ° C. in the air and burning off any other binder or the like while leaving the carbon fibers. , A 5 × optical microscope photograph is taken, the length of each carbon fiber is measured from the photograph, and the result is determined as a simple average value.

  These precursor fiber sheets include thermosetting resins such as epoxy resins, unsaturated polyester resins, phenolic resins, polyimide resins, and melamine resins, and thermoplastic resins such as acrylic resins, polyvinylidene chloride resins, and polytetrafluoroethylene resins. The resin may be impregnated or contained as a fibrous material. In particular, when using a paper made of carbon fibers bound with a binder as a precursor fiber sheet, a resin having a large amount of resin carbide after the carbonization is used to bind the carbon fibers even after carbonization. It is preferable to impregnate the fiber sheet.

  When a thermosetting resin is used, it is more preferable to use a phenol resin having a high bending strength and a high conductivity in the thickness direction because the amount of the resin carbide after carbonization is large.

  It is preferable to use a phenol resin that does not use a metal catalyst or an alkali catalyst during the synthesis. Examples of the phenol resin include a novolak type phenol resin using an acid catalyst in the synthesis, an alkali resol type phenol resin using an alkali catalyst, and an ammonia resol type phenol resin using an ammonia catalyst. When ions such as sodium and calcium are present in the phenolic resin, these metal ions cause a decrease in proton conductivity of the polymer electrolyte membrane, which causes a problem that battery performance is reduced. Therefore, as the phenol resin, an ammonia resol-type phenol resin R or a novolak-type phenol resin N can be used, and it is preferable to use a mixture of both to improve bending strength.

  The mixing ratio is such that if R is too large, the bending strength will be low and the electrical resistance in the thickness direction will be high, and if N is too large, the mixed resin will not be sufficiently hardened in the subsequent heating step and will be handled. R: N = 2: 1 to 1: 3 is more preferable, and R: N = 3: 2 to 1 is more preferable, because it is difficult to reduce the carbon content during the carbonization of the resin. : 2.

  The carbonaceous powder described below is preferably 300 parts by weight or less, more preferably 200 parts by weight or less, and even more preferably 150 parts by weight or less, based on 100 parts by weight of the phenol resin. If the amount of the carbonaceous powder is too large with respect to the resin, the resin carbide cannot sufficiently bind the carbon fiber and the carbonaceous powder, and problems such as falling off of the carbonaceous powder occur.

  When impregnating the precursor fiber sheet with the resin, the resin may be in an uncured or unsolidified state, but in the case of uncured or unsolidified, the compression treatment of the present invention, that is, curing or solidifying at the same time as heating and pressing. It is preferable to keep it. When the resin impregnated or contained as a fibrous material is uncured or unsolidified, the surface pressure in heating and pressing with a hot plate or a mold parallel to each other according to the present invention is caused by resin outflow during heating and pressing. In order to prevent this, the pressure is preferably 0.1 to 3 MPa, more preferably 0.2 to 2.0 MPa. The temperature of the parallel hot plate or the mold and the time of heating and pressurization by these may be appropriately selected depending on the curing characteristics (thermal properties) of the resin used.

  Further, it is preferable to use a precursor fiber sheet containing carbonaceous powder. By including the carbonaceous powder, the conductivity of the base material itself is improved, and a decrease in conductivity due to a water-repellent treatment can be suppressed in order to reduce cracking of the resin carbide. As the carbonaceous powder, carbon black, graphite powder, expanded graphite, carbonaceous milled fiber, and the like are preferable, and carbon black and graphite powder are more preferable. Most preferred is graphite powder.

  This carbonaceous powder preferably has a weight fraction in the range of 1 to 60%, more preferably in the range of 10 to 55%, and still more preferably in the range of 20 to 50%. The most preferred range is 15-35%. If the amount of the carbonaceous powder is less than 1%, the conductivity will be low. If it exceeds 60%, the density becomes high, and a suitable pore size cannot be obtained, and the battery characteristics become low. By including the carbonaceous powder, the conductivity in the thickness direction can be improved. If the rate of temperature rise is high during carbonization of the resin, cracks may occur in the resin part, causing a decrease in conductivity in the thickness direction and a decrease in bending strength. When the speed is high, cracking of the resin can be prevented. To obtain such an effect, the particle size of the carbonaceous powder is preferably about 0.01 to 10 μm, more preferably 0.01 to 7 μm, and more preferably 0.01 to 5 μm. It is further preferable to improve the bending strength of the steel and obtain a suitable pore diameter.

  The particle size of the carbonaceous powder is determined as the number average particle size of the obtained particle size distribution by performing dynamic light scattering measurement of the carbonaceous powder added when manufacturing the carbon fiber sheet.

  From these, when using paper formed by binding carbon fibers with a binder as a precursor fiber sheet, 20 to 300 parts by weight of thermosetting resin and 100 to 200 parts by weight of carbonaceous powder are used for 100 parts by weight of carbon fibers. Preferably in the range of 30 to 250 parts by weight of the thermosetting resin, more preferably in the range of 10 to 160 parts by weight of the carbonaceous powder, and 40 to 200 parts by weight of the thermosetting resin. More preferably, the content is within the range of 20 to 120 parts by weight. If the amount of the thermosetting resin is too small, the carbon fiber sheet after firing becomes too thick, and the conductivity in the thickness direction decreases, which is not preferable. If the amount of the thermosetting resin is too large, the density of the porous carbon plate base material is too high, the pore size is too small, and the drainage of water when used as a gas diffuser for a fuel cell is poor, and the cell performance is poor. It is not preferable because it lowers. If the amount of the carbonaceous powder is too small, the effect of improving the conductivity cannot be obtained, which is not preferable. If the amount is too large, the density becomes high and the pore diameter becomes too small as in the case of the thermosetting resin, which is not preferable. Also, it is not preferable to add a large amount of carbonaceous powder from the viewpoint of cost.

  Hereinafter, the present invention will be described in more detail based on embodiments shown in the drawings.

  1 to 4 show an example of implementing the method of the present invention, in which a pre-compression step using calender rolls (FIG. 1), a compression step using hot plates parallel to each other (FIG. 2), a pre-carbonization furnace, It consists of a carbonization step (FIGS. 3 and 4) using a carbonization furnace. Note that the arrow in the drawing indicates the traveling direction of the sheet.

(Preliminary compression process)
The precursor fiber sheet 1 unwound from the unwinder 2 is pre-compressed by the calender roll 3, then wound up into a required length roll by the winder 4, and then sent to the next compression step. Can be

(Compression process)
The pre-compressed precursor fiber sheet 5 unwound from the unwinder 6 is introduced into a hot press 8 having hot plates 7 parallel to each other. The hot plate 7 is kept at 140 to 300 ° C., and the pre-compressed precursor fiber sheet 5 is compressed (densified) while being pressed by the hot press 8. After being compressed for a predetermined time, the hot press 8 is opened, and in the meantime, the precursor fiber sheet compressed by the winding machine 9 is wound, and at the same time, a new pre-compressed precursor is unwound from the unwinding machine 6. The body fiber sheet 5 is unwound. After the compression process (densification) and the release (winding and unwinding) are alternately performed, the winding length of the winding machine 9 reaches the required length, and then the film is sent to the next process, the carbonization process.

(Carburizing process)
After being unwound from the unwinding machine 11, the precursor fiber sheet 10 subjected to the compression treatment is conveyed by the transport rolls 12 and introduced into the heating furnace 14 by the endless conveyor belt 13. The inside of the heating furnace 14 is kept under an inert gas atmosphere of about 300 to 1,200 ° C., and the compressed precursor fiber sheet 10 is transported under tensionless by the endless conveyor belt 13. Is pre-carbonized.

  The pre-carbonized precursor fiber sheet is then introduced into the next heating furnace 17 by the transport roll 15 and the endless conveyor belt 16. The heating furnace 17 is also configured in the same manner as the heating furnace 14, but the atmosphere is maintained under an inert gas atmosphere of about 1,200 to 3,000 ° C., and the pre-carbonized precursor fiber sheet 10 The carbon fiber sheet 18 is carbonized while being conveyed under tension by the endless conveyor belt 16. The carbon fiber sheet 18 is transported by a transport roll 19 to, for example, a winder 20.

  Here, the precursor fiber sheet in the heating furnace can be dragged on the hearth 21 as shown in FIG. In this case, the hearth 21 can be made of a material that can withstand the ambient temperature in the heating furnace, such as graphite or ceramics.

  In the above, the pre-compression step with a calender roll, the compression step with a hot plate parallel to each other, the pre-carbonization furnace, the carbonization step in the carbonization furnace are performed independently, but this makes each step independent. This is because the processing can be performed at a processing speed suitable for each step, and the facility scale can be easily optimized. However, the present invention is not limited to this, and may be performed continuously without winding in each step. Further, in the case of a precursor fiber sheet such as a woven fabric, a nonwoven fabric, or paper, the preliminary compression process using a calender roll may be performed in a weaving process, a nonwoven fabric process, and a papermaking process.

The present invention is particularly suitable for obtaining a relatively thin carbon fiber sheet having a thickness of about 0.1 to 0.25 mm and a bulk density of about 0.3 to 0.7 g / cm ³ . The carbon fiber sheet obtained according to the present invention can be used for various applications as described above. However, compression properties such as compressibility and compression set and gas permeability have a large effect on battery characteristics. It is particularly suitable as a gas diffusion layer for a molecular fuel cell.

  The thickness of the carbon fiber sheet is measured by placing the carbon fiber sheet on a smooth table, using a micrometer having a circular flat indenter having a diameter of 5 mm, and applying a surface pressure of 0.15 MPa. The number of measurements is 5, and the average value is the thickness.

The bulk density is calculated by dividing the weight of the carbon fiber sheet cut to 0.01 m ² (100 cm ² ) by the cut area and the thickness measured by the above method. At that time, the weight of the carbon fiber sheet is measured according to the “standard state” described in JIS L 0105. The number of measurements is 5, and the average value is the bulk density.

  The method of collecting a test piece in the measurement of the thickness and the bulk density was measured based on “Sampling and preparation of sample and test piece” described in JIS L 0105.

  Hereinafter, Examples and Comparative Examples will be described. The data described in Examples and Comparative Examples were measured using the following method.

(Thickness change due to compression)
Place the carbon fiber sheet on a smooth table, measure the thickness at the time of applying a surface pressure of 0.15 MPa using a micrometer having a circular flat indenter having a diameter of 5 mm, further increase the load, and apply the surface pressure of 1.0 MPa. The thickness was measured, and the difference between the thickness at a surface pressure of 0.15 MPa and the thickness at a surface pressure of 1.0 MPa was defined as a change in thickness due to compression. The change in thickness due to compression is an index indicating the compression characteristics of the gas diffusion layer for a polymer electrolyte fuel cell, and the smaller the better, the better, preferably 0.15 mm or less, more preferably 0.10 mm or less, Preferably it is 0.08 mm or less. If the change in thickness due to compression is large, it becomes difficult to apply the carbon layer to the surface of the nonwoven fabric. Furthermore, when used for the electrode diffusion layer of the fuel cell, the gas flow path of the separator is buried, and the thickness of the fuel cell is hardly made constant, and the tightening pressure of the fuel cell is reduced with time. .

(Tensile strength)
A carbon fiber sheet having a width of 15 mm is gripped with a tensile test jig attached to an Autograph AG-10TA (manufactured by Shimadzu Corporation) at a span length of 30 mm, and pulled at a speed of 2 to 4 mm / min until it breaks. Divided by. Tensile strength is an index that indicates the mechanical properties of the gas diffusion layer for polymer electrolyte fuel cells.If the tensile strength is low, the tensile strength of the carbon fiber sheet, such as the application of the carbon layer or the bonding with the catalyst layer or electrolyte membrane, is high. It is preferably 0.7 kN / m or more, more preferably 1.0 kN / m or more, and still more preferably 1.2 kN / m or more, since it is broken by the tension at the time of performing the next processing. The tensile strength only needs to satisfy the above value in at least one direction, and in the case of a continuous long sheet, the tensile strength in the longitudinal direction should satisfy the above value.

(Differential pressure during air permeation)
When the air of 14 cm ³ / (cm ² · sec) = 14 cm / sec is transmitted in the thickness direction of the carbon fiber sheet, the pressure difference between the upstream side and the downstream side of the air across the carbon fiber sheet is defined as a differential pressure. It was measured. The differential pressure at the time of air permeation is an index indicating the gas permeability of the gas diffusion layer for a polymer electrolyte fuel cell, and is preferably 10 to 100 Pa, more preferably 20 to 80 Pa, and still more preferably 30 to 70 Pa. It is. If the differential pressure is too large, that is, if the gas permeation resistance is too large, the permeability of air, hydrogen, and water is low, and water clogging is likely to occur, resulting in a low battery voltage. When the differential pressure is too small, that is, when the gas permeation resistance is too small, the water is easily discharged or dried, so that the membrane is dried, the electric resistance is increased, and the battery voltage is decreased.

(Fuel cell voltage)
The carbon fiber sheet is immersed in the aqueous PTFE dispersion, then pulled up and dried, 20% of PTFE is adhered, a mixture of carbon black and PTFE is applied on the carbon fiber sheet, and the mixture is heat-treated at 380 ° C. Created a sheet. The ratio of the carbon black to the PTFE mixture is 8: 2, and the amount of adhesion is about 2 mg / cm 2. On the other hand, a membrane-catalyst sheet was prepared in which a mixture of platinum-supporting carbon as a catalyst and Nafion was attached to both surfaces of Nafion 112 (manufactured by EI du Pont de Nemours and Company). The supported amount of platinum as a catalyst is about 0.5 mg / cm ² . The membrane-catalyst sheet was sandwiched between two nonwoven fabrics with a carbon layer with the carbon layer facing inward, and heated and pressed at 130 ° C. and 3 MPa to be integrated to obtain a membrane-electrode assembly (MEA). This MEA was sandwiched between grooved separators, and battery characteristics were measured by a conventional method. The battery temperature was 70 ° C, the hydrogen gas humidification temperature was 80 ° C, the air gas humidification temperature was 60 ° C, and the gas pressure was atmospheric pressure. At 0.7 A / cm 2, the hydrogen utilization rate is 70%, and the air utilization rate is 40%. The higher the voltage, the better.

(Examples 1 and 2)
As a precursor fiber sheet, an acrylic flame-resistant fiber having a specific gravity of 1.41 g / cm ³ was cut into 51 mm after crimping, processed into a card, and subjected to hydroentanglement to obtain an acrylic flame-resistant fiber nonwoven fabric having a width of 600 mm and a length of 150 m. Was. The basis weight of the acrylic flame-resistant fiber nonwoven fabric was 130 g / m ² .

This acrylic flame-resistant nonwoven fabric was pre-compressed with a calender roll 3 at a temperature of 200 ° C., a linear pressure of 49 kN / m and a speed of 2 m / min. Further, a hot plate 7 was set on a 100 t press 8 manufactured by Kawajiri Co., Ltd. so as to be parallel to each other, and the precursor was repeatedly opened and closed at two levels of a hot plate temperature of 230 ° C., a surface pressure of 0.39 and 4.9 MPa. While the fiber sheet was intermittently conveyed, a compression treatment was performed so that the same portion was heated and pressed for a total of 5 minutes. In this case, the effective pressure圧長L _P is 300mm of the hot plate, the feed amount L _F of the precursor fiber sheet at the time of transporting intermittently and 60 mm, and a L _F / L _P = 0.2. That is, the compression treatment was performed by repeating heating and pressing for 1 minute, opening the mold, and feeding the precursor fiber sheet (60 mm).

  A heating furnace 14 and a heating furnace 17 are arranged in series on the compressed precursor fiber sheet thus obtained, as shown in FIG. 3, and subjected to a pre-carbonization treatment at a maximum temperature of 650 ° C. in a nitrogen gas atmosphere. A carbonization treatment at a maximum temperature of 1,950 ° C. was performed (two-stage firing) to obtain a carbon fiber nonwoven fabric as a carbon fiber sheet. The lengths of the portions of the heating furnace 14 and the heating furnace 17 where the inert (nitrogen gas) atmosphere is maintained are 6.8 m and 6.7 m, respectively, and are the precursor fiber sheets in the pre-carbonization treatment in the heating furnace 14. The endless conveyor belt 13 for the transfer of the precursor fiber sheet in the carbonization processing in the heating furnace 17 is used for the endless conveyor belt 13 for the transfer of the SUS310S made of air-permeable compound balanced belt, A carbon fiber woven belt having air permeability in the thickness direction was used. The transport speed is 0.2 m / min, and the transport roll 12, the transport roll 15, and the transport roll 19 are controlled to place the precursor fiber sheet on the endless conveyor belts 13, 16 when the precursor fiber sheet is placed on the endless conveyor belts 13, 16. No slippage occurred with the conveyor belt.

  Table 1 shows the thickness, bulk density, thickness change due to compression, tensile strength, differential pressure during air permeation, and the fuel cell voltage of Example 1 of the obtained carbon fiber sheet.

  The above results show that, according to the present invention, it is possible to obtain a dense carbon fiber sheet that can suppress the deterioration of compression characteristics and the decrease of gas permeation resistance, which cause the deterioration of cell characteristics of a polymer electrolyte fuel cell. Is shown.

(Examples 3 and 4)
A carbon fiber sheet was obtained in the same manner as in Example 1 except that the pre-compression by a calender roll was not performed, and the pressure applied by a hot plate parallel to each other was set to 4.9 and 9.8 MPa. Table 1 shows the thickness, bulk density, change in thickness due to compression, tensile strength, differential pressure during air permeation, and fuel cell voltage of the obtained carbon fiber sheet.

(Example 5)
As a precursor fiber sheet, a polyacrylonitrile-based carbon fiber “Torayca” T300 (average fiber diameter: 7 μm) manufactured by Toray Industries, Inc. is cut to a length of 12 mm, dispersed in water, and formed on a wire mesh. It was immersed in an aqueous solution of polyvinyl alcohol, pulled up and dried to obtain a carbon fiber paper having about 30% by weight of polyvinyl alcohol as a binder adhered to 100 parts by weight of carbon single fiber.

  Next, 75 parts by weight of flake graphite (average particle size: 5 μm) is uniformly dispersed in 100 parts by weight of a resin in a 6% by weight methanol solution of a mixed resin containing the same amount of novolak type phenol resin as the resole type phenol resin. The carbon fiber paper was immersed in the liquid thus obtained, pulled up, and 100 parts by weight of the mixed resin and 25 parts by weight of flake graphite were adhered to 100 parts by weight of the carbon fiber.

Further, the precursor fiber sheet containing the phenolic resin and flaky graphite is intermittently conveyed while repeating opening and closing of the press, and the same portion is heated and pressed for a total of 6 minutes by hot plates parallel to each other. Compressed. At this time, hot plate temperature 170 ° C., surface pressure 0.74 MPa, the effective pressurizing圧長L _P 1200 mm of the hot plate, and the precursor fiber sheet feed amount L _F 100 mm of the time of transporting intermittently, L _F / L _{P was set to} 0.08. That is, compression processing was performed by repeating heating and pressing for 30 seconds, opening of the mold, and feeding (100 mm) of the precursor fiber sheet, followed by winding into a roll.

  Further, as shown in FIG. 3, the heating furnace 14 and the heating furnace 17 are arranged in series, and a pre-carbonization treatment at a maximum temperature of 650 ° C. and a carbonization treatment at a maximum temperature of 1,950 ° C. are performed in a nitrogen gas atmosphere. (Two-stage firing) to obtain a carbon fiber sheet. The conveying speed of the precursor fiber sheet is 0.5 m / min. The endless conveyor belt 13 (SUS310S compound balanced belt) moving at 0.2 m / min in the heating furnace 14 and the endless conveyor in the heating furnace 17. Firing was performed while dragging on the hearth (made of graphite) from which the belt 16 was removed.

  Table 1 shows the thickness, bulk density, change in thickness due to compression, tensile strength, differential pressure during air permeation, and fuel cell voltage of the obtained carbon fiber sheet.

(Comparative Example 1)
A carbon fiber sheet was obtained in the same manner as in Example 1 except that preliminary compression by a calender roll and compression by a hot plate parallel to each other were not performed. Table 1 shows the thickness, bulk density, change in thickness due to compression, tensile strength, differential pressure during air permeation, and fuel cell voltage of the obtained carbon fiber sheet.

(Comparative Example 2)
A carbon fiber sheet was obtained in the same manner as in Example 1 except that the pre-compression by a calender roll was not performed and the pressure applied by a hot plate parallel to each other was set to 19.6 MPa. At this time, in order to secure the pressure, the effective pressure圧長L _P of the hot plate and 80 mm, a feed amount L _F of the precursor fiber sheet at the time of intermittently conveyed and 16 mm, L _F / L _P = 0.2. Table 1 shows the thickness, bulk density, change in thickness due to compression, tensile strength, differential pressure during air permeation, and fuel cell voltage of the obtained carbon fiber sheet.

(Comparative Example 3)
The pre-compression using a calender roll was performed at a temperature of 200 ° C., a linear pressure of 392 kN / m, and a speed of 2 m / min. Hereinafter, a carbon fiber sheet was obtained in the same manner as in Example 1. Table 1 shows the thickness, bulk density, change in thickness due to compression, tensile strength, differential pressure during air permeation, and fuel cell voltage of the obtained carbon fiber sheet.

  From Table 1 above, in Example 1, the sheet was densified to the most preferable areas, with the change in thickness due to compression being 0.03 mm and the differential pressure during air permeation being 44.1 Pa. The sheet is densified to the most preferable region where the change is 0.02 mm and the differential pressure during air transmission is 50 Pa. On the other hand, in Comparative Example 1, the change in thickness due to compression was 0.19 mm, and the sheet was insufficiently densified. Therefore, the carbon fiber sheet of the present invention exhibited poor compression characteristics and low gas permeation resistance. It can be seen that the method is excellent in providing a method for producing a dense carbon fiber sheet capable of suppressing the decrease.

  The present invention can be applied not only to the electrode material of the polymer electrolyte fuel cell but also to various electrodes and radio wave absorbers, but the application range is not limited to these.

FIG. 2 is a schematic longitudinal sectional view showing a pre-compression step in one embodiment of a production process of a carbon fiber sheet used for carrying out the present invention. FIG. 2 is a schematic longitudinal sectional view showing a compression step in one embodiment of a production process of a carbon fiber sheet used for carrying out the present invention. FIG. 3 is a schematic longitudinal sectional view showing a firing step in one embodiment of a manufacturing process of a carbon fiber sheet used for carrying out the present invention. FIG. 3 is a schematic longitudinal sectional view showing a firing step in one embodiment of a manufacturing process of a carbon fiber sheet used for carrying out the present invention.

Explanation of reference numerals

1: Precursor fiber sheet 2: Unwinder 3: Calender roll 4: Rewinder 5: Precompressed precursor fiber sheet 6: Unwinder 7: Hot plate 8: Hot press 9: Rewinder 10 : Precursor fiber sheet subjected to compression treatment 11: Unwinder 12: Conveying roll 13: Endless conveyor belt 14: Heating furnace for pre-carbonization treatment 15: Conveyance roll 16: Endless conveyor belt 17: Heating furnace for carbonization treatment 18: Carbon Fiber sheet 19: Conveyance roll 20: Winding machine 21: Hearth

Claims (23)

The precursor fiber sheet is fired while being continuously conveyed in a heating furnace to produce a carbon fiber sheet having a thickness of 0.1 to 0.25 mm and a bulk density of 0.3 to 0.7 g / cm ^3. A method for producing a carbon fiber sheet, wherein a precursor fiber sheet before firing is continuously heated and pressed by hot plates parallel to each other while being intermittently conveyed. Assuming that the effective pressing length in the conveying direction of the hot plates parallel to each other is L _P and the feed amount of the precursor fiber sheet during intermittent conveyance is L _F , L _F / L _P is 0.04 to 1.0. The method for producing a carbon fiber sheet according to claim 1, wherein When the effective pressing length in the conveying direction of the hot plates parallel to each other is L _P and the feed amount of the precursor fiber sheet during intermittent conveyance is L _F , L _F / L _P is 0.1 to 0.1. The method for producing a carbon fiber sheet according to claim 1, wherein the number is 98. The method for producing a carbon fiber sheet according to any one of claims 1 to 3, wherein the temperature of the hot plate parallel to each other is 140 to 300 ° C, and the pressing force is 0.1 to 40 MPa. The method for producing a carbon fiber sheet according to any one of claims 1 to 4, wherein preheating and pressurizing is performed by at least one or more sets of calender rolls before continuous heating and pressurizing with a hot plate parallel to each other. The precursor fiber sheet is fired while being continuously conveyed in a heating furnace to produce a carbon fiber sheet having a thickness of 0.1 to 0.25 mm and a bulk density of 0.3 to 0.7 g / cm ^3. A method for producing a carbon fiber sheet, wherein a precursor fiber sheet before firing is continuously heated and pressed by a mold while intermittently being conveyed. When the effective pressing length in the conveying direction of the mold is L _P and the feed amount of the precursor fiber sheet during intermittent conveyance is L _F , L _F / L _P is 0.04 to 1.5. The method for producing a carbon fiber sheet according to claim 6, wherein: The method for producing a carbon fiber sheet according to claim 6, wherein the temperature of the mold is 140 to 300 ° C. and the pressure is 0.1 to 40 MPa. The method for producing a carbon fiber sheet according to any one of claims 6 to 8, wherein preheating and pressing is performed by at least one set of calender rolls before continuous heating and pressing with a mold. The method for producing a carbon fiber sheet according to any one of claims 1 to 9, wherein an oxidized fiber cloth is used as the precursor fiber sheet. The method for producing a carbon fiber sheet according to claim 10, wherein a woven fabric, a nonwoven fabric, or paper is used as the flame-resistant fiber fabric. The method for producing a carbon fiber sheet according to any one of claims 1 to 9, wherein a paper obtained by binding carbon fibers with a binder is used as the precursor fiber sheet. The method for producing a carbon fiber sheet according to claim 12, wherein a carbon fiber having an average fiber diameter in a range of 5 to 20 m is used. 14. The method for producing a carbon fiber sheet according to claim 12, wherein an average fiber length of the carbon fibers is in a range of 3 to 20 mm. The method for producing a carbon fiber sheet according to any one of claims 10 to 14, wherein a precursor fiber sheet impregnated with a resin is used. The method for producing a carbon fiber sheet according to claim 15, wherein a thermosetting resin is used as the resin impregnated in the precursor fiber sheet. The method for producing a porous carbon substrate according to claim 16, wherein a phenol resin is used as the thermosetting resin. The method for producing a carbon fiber sheet according to any one of claims 15 to 17, wherein a phenol resin that does not use a metal catalyst or an alkali catalyst during synthesis is used as the resin to be impregnated. The method for producing a carbon fiber sheet according to any one of claims 1 to 18, wherein the precursor fiber sheet contains carbonaceous powder. The method for producing a carbon fiber sheet according to claim 19, wherein a particle diameter of the carbonaceous powder is in a range of 0.01 to 10 m. 21. The method for producing a carbon fiber sheet according to claim 19, wherein graphite or carbon black is used as the carbonaceous powder. A carbon fiber sheet produced by the method according to claim 1. A gas diffusion layer for a polymer electrolyte fuel cell, manufactured by the method according to claim 1.

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