JP2022146056A - Manufacturing method of fiber sheet - Google Patents

Abstract

To provide a fiber sheet including less fiber-fiber binding and having homogeneous dispersion of fibers.SOLUTION: A manufacturing method of a fiber sheet includes: a fibrillating step of fibrillating fiber bundles in a slurry including at least fiber bundles and a dispersion liquid by using an agitation device composed of at least an agitator and an agitation tank; a dispersion stabilizing step of agitating the slurry after the fibrillating step by using the agitation device; and a sheet making step of making a fiber sheet from the slurry after the dispersion stabilizing step. In the fibrillating step, agitation is run with an eccentricity rate of the agitator set in 5% or over and 30% or under, and in the dispersion stabilizing step, agitation is run with an eccentricity rate of the agitator set in 0% or over and under 5%.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for producing a fiber sheet for a fuel cell electrode substrate, which is used as an electrode of a polymer electrolyte fuel cell used as a power source for a fuel cell vehicle or the like.

A fuel cell is a device that electrically extracts the energy generated when water is produced by reacting hydrogen and oxygen.It is expected to be a clean energy source because it is highly energy efficient and emits only water.

Electrodes used in polymer electrolyte fuel cells consist of a catalyst layer formed on the surface of the polymer electrolyte membrane and a gas diffusion layer formed outside the catalyst layer on both sides of the polymer electrolyte membrane. have a structure. Gas diffusion electrodes are in circulation as separate members for forming gas diffusion layers in the electrodes. As the gas diffusion electrode, a dense layer called a microporous layer (MPL) is generally used on a conductive porous substrate. As the conductive porous substrate of the gas diffusion electrode, a substrate containing carbon fibers is generally used because of its chemical stability.

Carbon fibers include PAN (polyacrylonitrile)-based carbon fibers and pitch-based carbon fibers, and PAN-based carbon fibers account for 90% or more of the amount of carbon fibers used in the world. PAN-based carbon fibers are divided into regular tow type (fiber bundle consisting of 1,000 to 24,000 single fibers) and large tow type (40,000 or more single fibers) depending on the number of single fibers that make up the fiber bundle (tow). fiber bundle) consisting of fibers.

A carbon fiber sheet is generally used for the conductive porous substrate. It is difficult to keep the time, and if the fibers agglomerate, a sheet with a uniform basis weight cannot be obtained. In addition, if aggregates (hereafter referred to as "bundles") are present, in short-circuit measurement, which is one of the reliability evaluations of fuel cells, the fibers in the bundles break during compression and break through the polymer electrolyte membrane, resulting in short circuits. There are cases where In order to suppress the occurrence of this bundling, as shown in Patent Document 1 and the like, as a method for producing a slurry containing fibers, a technique is known in which a circulation line is provided in a stirring tank to prevent the fibers from entangling with each other.

JP 2017-6857 A

The technique described in Patent Document 1 has a certain effect in realizing uniform dispersion of fibers, but when applied to carbon fibers that are difficult to disperse, it can be stably maintained for a long time without aggregating carbon fibers. Not effective enough to keep. In addition, it is necessary to use long fibers with a relatively long fiber length in order to maintain the mechanical strength of the electrode base material for fuel cells. means have not yet been provided.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a fibrous sheet in which the fibers are uniformly dispersed with less bundling.

As a result of various investigations by the present inventors, it is effective for the dispersibility of fibers to divide the agitation method into a "fibrillation process" in which the fiber bundle is loosened and a "dispersion stabilization process" in which the fibrillated single fibers are kept without aggregating. I got the knowledge that it is.

The present invention provides a defibration step of fibrillating fiber bundles in a slurry containing at least fiber bundles and a dispersion using a stirring device comprising a stirrer including at least a stirring blade and a stirring shaft and a stirring tank, and the defibrating step. It has a dispersion stabilization step of stirring the slurry after the fiberization step using the stirring device, and a papermaking step of making a fiber sheet from the slurry after the dispersion stabilization step, and in the defibration step, the eccentricity of the stirrer is 5%. A method for producing a fiber sheet, characterized by stirring at a rate of 30% or more, and stirring at an eccentricity of the stirrer at 0% or more and less than 5% in the dispersion stabilizing step.

According to the present invention, there is provided a fiber sheet in which the fibers are less bound together and the fibers are uniformly dispersed.

The schematic diagram of the stirring apparatus used for the fibrillation process in the Example of this invention. Schematic diagram of a stirring device used in a dispersion stabilization step in an example of the present invention.

As the fiber sheet in the present invention, a paper sheet is preferably used because of the gas diffusibility required for the fuel cell electrode base material.

As a papermaking body, a wet body obtained by dispersing fibers in a dispersion liquid and making paper is fixed with a binder (binder resin) and dried to form a sheet (wet papermaking), which is generally used. The dispersion in which the fibers are dispersed preferably contains an antifoaming agent, a surfactant, and a thickener. The antifoaming agent suppresses foaming during stirring and is not particularly limited, but urethane-based, polyoxyalkylene-based, and silicone-based antifoaming agents are preferred. Surfactants are effective in defibrating fiber bundles and are not particularly limited, but polyethylene glycol-based and polyethylene oxide-based surfactants are preferred. The thickener thickens the solution and is effective in suppressing physical contact between the fibers, and is not particularly limited, but is preferably polyethylene oxide or polyacrylic acid.

The method for producing a fiber sheet of the present invention includes a defibration step of agitating slurry with an agitator having an eccentricity of 5% or more and 30% or less; It has a dispersion stabilization step of stirring as less than 5%. Here, the slurry refers to the fiber bundles thrown into the dispersion liquid, and the state of the slurry after the defibration step means that 80 parts by mass or more of the 100 parts by weight of the fiber bundles put into the dispersion liquid are unraveled. It refers to the state where it becomes a single fiber. As for the stirring vessel 20, separate stirring vessels may be used in each step, or one stirring vessel may be used with the position of the stirrer changed. What is important is to stir the fiber bundle with an eccentricity of 0% or more and less than 5% after defibration of the fiber bundle. If eccentricity is not used during fibrillation, swirling flow becomes dominant, and the agitation effect is small, making it easier for binding to occur. A baffle plate can be provided so as to generate an upstream and downstream flow on the inner wall of the stirring tank 20 without eccentricity, but in that case, retention tends to occur in the vicinity of the baffle plate, and the flow velocity also slows down, which also tends to cause bundling. Therefore, it is preferable to eccentrically stir during defibration.

On the other hand, if eccentric stirring is continued after defibration, the probability of contact between fibers increases upstream and downstream, and binding increases over time. As a result, the contact between the fibers is suppressed, and the dispersed state can be stably maintained.

As for the flow state during stirring, the apparent number of circulations per stage of the stirring blade is preferably 2.0 times/minute or more and 15.0 times/minute or less in both the fibrillation process and the dispersion stabilization process. More preferably, it is 4.0 times/minute or more and 8.0 times/minute or less. The apparent number of circulations is calculated by the following formula, and is an index representing the fluidity of the liquid in the stirring vessel. The discharge coefficient is a numerical value calculated from the shape of the stirring blade.
Apparent number of circulations (times/min) = (discharge coefficient x number of revolutions (rpm) x (diameter of stirring blade (m)) ³ )/liquid volume (m ³ ).

If the apparent number of circulations is 2.0 times/minute or more, fluidity is improved, and aggregation of fibers can be further suppressed. If it is 15.0 times/min or less, foaming of the dispersion can be suppressed, and aggregation can be further suppressed.

As for the type of stirring impeller, it is preferable to use a propeller impeller or a paddle impeller in both the fibrillation step and the dispersion stabilization step, from the balance between the shearing ability and the discharge ability of the impeller. Turbine blades are excellent in shearing capacity, but inferior in discharge capacity, which may result in retention and poor defibration properties.

As for the size of the stirring blades, the diameter of the stirring blades/the inner diameter of the stirring tank (d/D) is preferably 0.20 or more and 0.50 or less in both the fibrillation step and the dispersion stabilization step. Here, the diameter of the impeller is the diameter when the surface of the impeller parallel to the liquid surface is circular, and the diameter of the circumscribed circle of the trajectory drawn by the impeller during rotation when it is not circular. If it is less than 0.20, the blade may be small and the discharge capacity may be inferior. If it exceeds 0.50, the motor power needs to be increased. be.

The number of stages of stirring blades is preferably two or more in both the fibrillation process and the dispersion stabilization process. In one stage, the top and bottom parts in the stirring tank cannot be uniformly mixed, and retention may occur.

The height of the first stage 33 of the stirring impeller (height 35 from the bottom of the stirring vessel to the bottom of the first stage of stirring impeller) is equal to the height of the stirring vessel 25 (the height of the stirring vessel 30% or less of the height from the bottom to the top cover 21) is preferable. If it exceeds 30%, the fluidity of the bottom portion of the stirring tank 20 is deteriorated, which may cause bundling.

The eccentricity of the stirrer 30 in the fibrillation step is 5% or more and 30% or less, more preferably 15% or more and 25% or less. The eccentricity represents how far the stirring shaft 31 of the stirrer 30 is away from the central axis of the stirring vessel 20, and is calculated by the following formula.
Eccentricity of the stirrer = (distance R from the central axis of the stirring tank to the stirring axis/inner diameter D of the stirring tank) x 100.

If the eccentricity is less than 5%, the eccentricity is insufficient and upstream and downstream are difficult to occur.

The eccentricity of the stirrer 30 in the dispersion stabilization step is 0% or more and less than 5%. By making it less than 5%, it is possible to suppress upstream and downstream generation and suppress aggregation of fibers.

The viscosity of the dispersion liquid is preferably 5 mPa·s or more and 50 mPa·s or less, more preferably 8 mPa·s or more and 20 mPa·s or less, in consideration of fiber dispersibility and dehydration (filterability) during papermaking.

The average flow velocity of the slurry during stirring is preferably 1 m/s or more in the fibrillation step and/or the dispersion stabilization step. By setting the average flow velocity to 1 m/s or more, the fiber bundle can be easily fibrillated and aggregation can be suppressed. The average flow velocity can be obtained by calculating the volume average flow velocity using numerical fluid analysis software STAR-CCM+ (manufactured by SIEMENS) or the like. As a calculation method, there is a method of inputting the density and viscosity of the dispersion liquid, executing the fluid analysis until a steady state is reached, and calculating the volume average velocity in the fluid region from the steady state analysis result.

The average shear rate of the slurry during stirring is preferably 6/s or more in the fibrillation step and/or the dispersion stabilization step. By setting the average shear rate to 6/s or higher, the fiber bundle can be easily defibrated and aggregation can be suppressed. The average shear rate can be obtained by calculating the volume average shear rate using numerical fluid analysis software STAR-CCM+ (manufactured by SIEMENS) or the like. As a calculation method, there is a method in which the density and viscosity of the dispersion are input, the fluid analysis is performed until a steady state is reached, and the volume average shear rate in the fluid region is calculated from the steady state analysis results.

The type of fibers used in the present invention is not particularly limited, and uniform dispersion can be achieved even when carbon fibers are used. Since carbon fibers are inorganic fibers and have almost no functional groups on their surfaces, they are difficult to chemically bond with components such as surfactants and are difficult to be dispersed and stabilized. There are also fibers with a sizing agent (bundle) added to the surface of the carbon fiber. It becomes easy to aggregate. By using the production method of the present invention, it is possible to achieve both defibration properties and subsequent dispersion stability even when carbon fibers having the above problems are used.

The fibers used in the present invention preferably have an average single fiber length in the range of 3 to 20 mm, more preferably in the range of 5 to 15 mm. When the average length of the single fibers is 3 mm or more, the fiber sheet has excellent mechanical strength. On the other hand, when the average length of the single fibers is 20 mm or less, the fibers are excellent in dispersibility and a homogeneous fiber sheet can be obtained. The longer the average length of the single fibers, the greater the contact area between the fibers and the easier it is to bind them together.

The shape of the stirring vessel 20 used in the present invention is not particularly limited, but it is preferably cylindrical in view of slurry fluidity, and the bottom wall 23 is not horizontal but inclined toward the discharge port 24 in terms of slurry dischargeability. preferable.

As a method for producing the fiber sheet of the present invention, the slurry after the above dispersion stabilization step is generally paper-made to obtain a wet body, a binder (binder resin) is applied, dried, and wound on a roll. used for purposes. From the viewpoint of suppressing bundling, it is particularly preferable to send the slurry to a paper machine while stirring under the same conditions after the dispersion stabilization step to make paper. As a method for obtaining a wet body, a wet papermaking method is used in which the fibers and the dispersion liquid in the slurry are separated by a fourdrinier paper machine, a cylinder paper machine, or an inclined paper machine, and the fibers are formed into a sheet. A tilted paper machine is preferred because it allows for a thin fiber concentration in the slurry.

Although the binder resin to be applied to a wet body is not particularly limited, an aqueous solution of polyvinyl alcohol is preferable from the viewpoint of adhesion to fibers. Various commercially available applicators can be used to apply the binder resin. As a coating method, a method such as a spray coater, a curtain coater, or a die coater can be used.

After coating the wet body with the binder resin, it is preferable to dry the wet body coated with the binder resin at a temperature of 100 to 180°C. As a drying method, commonly used drying methods such as multi-cylinder dryer, Yankee dryer, and hot air drying can be used. is preferred.

When producing carbon paper that is suitably used as a conductive porous substrate from the viewpoint of conductivity and gas diffusion, carbon fibers are used as the above fibers to produce a carbon fiber sheet, which is then coated with a resin carbide. It's good to tie up. The method for producing carbon paper from a carbon fiber sheet is not particularly limited, but the carbon fiber sheet is impregnated with a resin composition that serves as a binder, and then heated and pressed to adjust to the desired thickness and density. A method of firing under an inert atmosphere is preferably used to carbonize the resin composition.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to the following descriptions.

[Measurement of viscosity of dispersion liquid]
95 ml of the dispersion liquid was placed in a 110 ml screw tube, and the rotor No. The viscosity was measured three times at 20 (M1) and 60 rpm, and the average value was taken as the viscosity (mPa·s) of the dispersion.

[Method for checking defibration state]
1 L of the slurry was diluted with the dispersion liquid to a concentration of 0.05 part by mass of carbon fibers, and non-single fibers (non-fibrillated fibers and aggregated fibers) and single fibers were each dried at a temperature of 140° C. for 1 hour. . After that, each weight was measured, and the defibration process was completed when the amount of single fibers reached 80 parts by mass or more with respect to 100 parts by mass of dried fibers.

[Measurement of number of bundles of fiber sheets]
Ten samples were prepared by cutting the fiber sheet in the longitudinal direction by 1 m, and the number of bundles (fiber aggregates with a width of 1 mm or more or a length of 30 mm or more) was counted and divided by the area of the fiber sheet. The above calculation was performed for 10 samples, and the average value was taken as the number of bundles (pieces/m ² ).

(Example 1)
To 100 parts by mass of ion-exchanged water with a liquid volume of 6.5 m ³ , add 0.01 parts by mass of an antifoaming agent (KM-73; manufactured by Shin-Etsu Chemical Co., Ltd.) and 0.01 parts by mass of a thickening agent ("Noptex ( Registered trademark) "E-R060; manufactured by San Nopco Co., Ltd.), 0.01 parts by mass of a surfactant ("Alcox (registered trademark)"CP-B1; manufactured by Meisei Chemical Industry Co., Ltd.) was added, and was stirred for 10 minutes with the stirring device 10 of . The stirring device 10 in FIG. 1 uses two stages of propeller blades having a diameter of 0.77 m and a discharge coefficient of 0.415 as the stirring blades, and the diameter of the stirring blades/inner diameter of the stirring vessel (d/D) is 0.35, The height of the first stage 33 of the stirring blade was 20% of the height 25 of the stirring tank, the distance R from the center axis of the stirring tank to the stirring axis was 0.50 m, and the eccentricity was 23%. The rotation speed of the stirring blades during stirring was 140 rpm, and the apparent number of circulations per stage of the stirring blades was 4.1 times/minute.

Next, 0.15 part by mass of Toray Industries, Inc.'s PAN-based carbon fiber "Torayca (registered trademark)" T300 (average single fiber diameter: 7 μm) cut to a length of 6 mm was put into the stirring device 10 of FIG. , under the above stirring conditions, for 5 minutes to defibrate the fiber bundle (fibrillation step). The disentanglement state was confirmed in advance according to the method described in [Confirmation method of disentanglement state] above, and the stirring time in the disentanglement step was determined. After that, the liquid was sent to the stirring device 10 of FIG. The stirrer 10 of FIG. 2 was similar to the stirrer 10 of FIG. 1 except that the eccentricity was 0%. Stirring was started at the same rotational speed as in the defibration step (dispersion stabilization step). While stirring the above, the liquid is sent to a paper machine to make paper, a binder solution (10% by mass aqueous solution of polyvinyl alcohol) is applied, dried and wound into a roll, and carbon fibers with a basis weight of 40 g / m ² A long fiber sheet (carbon fiber paper body) consisting of was obtained. The amount of the binder solution applied was adjusted so that 15 parts by mass of polyvinyl alcohol was applied to 100 parts by mass of the carbon fiber paper. Stirring under the conditions of the dispersion stabilization step was continued for a total of 40 minutes until the liquid transfer (papermaking) was completed. The average flow velocity and average shear rate of the slurry during stirring in each step were obtained by calculating the volume average flow velocity and volume average shear rate, respectively, using numerical fluid analysis software STAR-CCM+ (manufactured by SIEMENS). . Regarding the obtained fiber sheet, the number of bundles measured according to the method described in [Measurement of number of bundles of fiber sheet] was 0/m ² .

(Examples 2-5)
A fiber sheet was produced in the same manner as in Example 1, except that the blending amount of the thickening agent in the dispersion was varied to set the viscosity of the dispersion to 3 to 60 mPa·s.

(Examples 6-9)
A fiber sheet is produced in the same manner as in Example 1, except that in the fibrillation process and the dispersion stabilization process, the number of rotations of the stirrer 30 is changed to set the apparent number of circulations to 1.0 to 17.0 times/minute. did.

(Example 10)
The same as in Example 1 except that the stirring blades in the stirrer 30 in FIGS. to produce a fiber sheet.

(Example 11)
The same as Example 1 except that the stirring blades in the stirrer 30 in FIGS. to produce a fiber sheet.

(Examples 12-15)
The same procedure as in Example 1 was carried out except that the diameter of the stirring blades in the stirrer 30 in FIGS. to produce a fiber sheet.

(Example 16)
A fiber sheet was produced in the same manner as in Example 1, except that the number of stages of stirring blades in the stirrer 30 shown in FIGS. 1 and 2 was set to one.

(Example 17)
A fiber sheet was produced in the same manner as in Example 1 except that the height of the first stage 33 of the stirring blades in the stirring device 10 of FIGS. 1 and 2 was 35% of the height of the stirring tank 20.

(Examples 18-20)
A fiber sheet was produced in the same manner as in Example 1, except that the eccentricity of the stirring device 10 in FIG. 1 was set to 13 to 30%.

(Example 21)
A fiber sheet was produced in the same manner as in Example 1, except that the eccentricity of the stirring device 10 in FIG. 2 was changed to 4%.

(Comparative example 1)
A fiber sheet was produced in the same manner as in Example 1, except that the stirring device 10 of FIG. 1 having an eccentricity of 23% was used in the dispersion stabilization step.

(Comparative example 2)
A fiber sheet was produced in the same manner as in Example 1, except that the stirring device 10 of FIG. 2 with an eccentricity of 0% was used in the defibration step.

(Comparative Example 3)
A fiber sheet was produced in the same manner as in Example 1, except that the eccentricity of the stirring device 10 in FIG. 1 was changed to 3%.

(Comparative Example 4)
A fiber sheet was produced in the same manner as in Example 1, except that the eccentricity of the stirring device 10 in FIG. 1 was changed to 32%.

(Comparative Example 5)
A fiber sheet was produced in the same manner as in Example 1, except that the eccentricity of the stirring device 10 in FIG. 2 was changed to 10%.

10 Stirrer 20 Stirring tank 21 Upper lid 22 Side wall 23 Bottom wall 24 Outlet 25 Stirring tank height D Stirring tank inner diameter 30 Stirrer 31 Stirring shaft 32 Drive motor 33 Stirring blade 1st stage 34 Stirring blade 2nd stage 35 Height R from the bottom of the stirring tank to the bottom of the first stage of stirring blades Distance d from the central axis of the stirring tank to the stirring shaft Diameter of stirring blades 40 Dispersion or slurry

Claims (11)

A defibration step of fibrillating the fiber bundles in the slurry containing at least the fiber bundles and the dispersion liquid using a stirring device comprising a stirrer including at least a stirring blade and a stirring shaft and a stirring tank, and after the fibrillation step. It has a dispersion stabilizing step of stirring the slurry using the stirring device, and a papermaking step of making a fiber sheet from the slurry after the dispersion stabilizing step, and in the defibrating step, the eccentricity of the stirrer is 5% to 30%. , and in the dispersion stabilizing step, the eccentricity of the stirrer is set to 0% or more and less than 5%. 2. The method for producing a fiber sheet according to claim 1, wherein the apparent number of circulations per stage of the stirring blade in the fibrillation step and the dispersion stabilization step is 2.0 times/minute or more and 15.0 times/minute or less. The method for producing a fiber sheet according to claim 1 or 2, wherein the stirring blade is either a propeller blade or a paddle blade. The method for producing a fiber sheet according to any one of claims 1 to 3, wherein in the stirring device, the ratio (d/D) of the diameter of the stirring blade to the inner diameter of the stirring vessel is 0.20 or more and 0.50 or less. The method for producing a fiber sheet according to any one of claims 1 to 4, wherein the number of stages of the stirring blades is two or more. The method for producing a fiber sheet according to any one of claims 1 to 5, wherein the height of the first stage of the stirring blade in the stirring device is 30% or less of the height of the stirring vessel. The method for producing a fiber sheet according to any one of claims 1 to 6, wherein the eccentricity of the stirrer in the fibrillation step is 15% or more and 25% or less. The method for producing a fiber sheet according to any one of claims 1 to 7, wherein the dispersion has a viscosity of 5 mPa·s or more and 50 mPa·s or less. The method for producing a fiber sheet according to any one of claims 1 to 8, wherein in the fibrillation step and/or the dispersion stabilizing step, the slurry is stirred so that the average flow velocity is 1 m/s or more. . 10. Production of the fiber sheet according to any one of claims 1 to 9, characterized in that in the fibrillation step and/or the dispersion stabilizing step, the slurry is stirred so that the average shear rate is 6/s or more. Method. The method for producing a fiber sheet according to any one of claims 1 to 10, wherein the fiber bundle is a carbon fiber bundle.

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