JP2004330126A - Electrode for electro-osmotic dehydrator, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for an electro-osmotic dehydrator small in depletion and long in service life, and to provide a production method therefor. <P>SOLUTION: The electrode is formed of carbon short fibers dispersed in random directions in a two-dimensional plane with an average fiber length in the range of 5-25 mm, and carbon short fibers with an average fiber length in the range of 0.01-1 mm both bound with carbon, having a bulk density of 1-1.4 g/cm<SP>3</SP>. The production method for the electrode comprises processes of: obtaining a composite sheet containing 75-90 wt.% of carbonizable organic matter by impregnating a mixture of the carbon short fibers with the average fiber length of 0.01-1 mm and a carbonizable organic matter; obtaining molded goods by laminating a plurality of composite sheets and molding at a pressure of 0.3-3 Mpa; and carbonizing the organic matter by baking the molded goods in an inert atmosphere at 650-1,000°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrode made of a carbon fiber / carbon composite material and used for an electroosmotic dehydrator, and a method for manufacturing the electrode.
[0002]
[Prior art]
Electrodes of the electroosmotic dehydrator have not only high bending strength to withstand the force of pressing (during dehydration), but also less contact with filter cloth and sludge and less wear due to abrasion. It is required to have high corrosion resistance to acidic and alkaline environments due to electrochemical oxidation and energization by electricity and to withstand long-term use.
[0003]
As such an electrode for an electroosmotic dehydrator, conventionally, a composite sheet obtained by impregnating a sheet made of a short carbon fiber having an average fiber length in a range of 2 to 20 mm with a carbonizable organic substance such as a phenol resin. Is baked at a temperature of 1,200 ° C. or less to carbonize organic matter, and short carbon fibers having an average fiber length in the range of 2 to 20 mm dispersed in random directions in a two-dimensional plane are interlinked with carbon. Is known (for example, see Patent Document 1). It is also known that, after firing, an organic substance is impregnated again and fired again in order to increase the bulk density and extend the life (for example, see Patent Document 2). However, in each case, the organic material is impregnated into the voids of the short carbon fibers dispersed in random directions in the two-dimensional plane and carbonized. Contains a mass of organic carbonized material having a size of several tens of μm to 100 μm that does not include short carbon fibers, and the mass of the carbonized material breaks and falls off during use, causing a problem of remarkable depletion. There is. In addition, the latter including the re-impregnation step and the re-baking step is expensive in addition to the production cost, and the carbonized material of the organic substance due to the re-impregnation has a small void at the time of the re-impregnation, so that a large physical adhesive force cannot be obtained. It is easy to fall off, and the service life has not been extended to the extent that the bulk density increases.
[0004]
[Patent Document 1]
JP-A-3-135494
[Patent Document 2]
JP-A-3-221108
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide an electrode for an electroosmotic dehydrator having a small life and a long life, and a method of manufacturing the same.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a short carbon fiber having an average fiber length in a range of 5 to 25 mm dispersed in random directions in a two-dimensional plane, and an average fiber length of 0.01 to Provided is an electrode for an electroosmotic dehydrator in which short carbon fibers within a range of １1 mm are bound to each other by carbon, and the bulk density is within a range of 1 to 1.4 g / cm ^3. . Here, the electrode preferably has a half-width of the (002) peak of carbon determined by wide-angle X-rays in the range of 5.5 to 7.5 °. Further, it is preferable that the weight loss rate at 500 ° C. by a thermobalance is 10% or less and that at 700 ° C. is at least 90%.
[0008]
The electrode for an electroosmotic dehydrator described above is a sheet made of carbon short fibers dispersed in random directions in a two-dimensional plane and having an average fiber length in the range of 5 to 25 mm. Obtaining a composite sheet containing a carbonizable organic substance in a range of 75 to 90% by weight by impregnating with a mixture containing carbon short fibers and a carbonizable organic substance in a range of 0.01 to 1 mm; A step of laminating a plurality of sheets and molding them under a pressure in the range of 0.3 to 3 MPa to obtain a molded article; and a step of subjecting the molded article to a temperature in the range of 650 to 1,000 ° C. in an inert atmosphere. And a step of carbonizing the organic matter by firing. Instead of using short carbon fibers, precursor short fibers of short carbon fibers may be used, and the precursor short fibers may be carbonized at the time of firing to form short carbon fibers.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
The electrode for an electro-osmotic dehydrator according to the present invention (hereinafter, simply referred to as an electrode) will be described together with a method for producing the electrode. In the present invention, the average fiber length dispersed in random directions in a two-dimensional plane is described. Sheets made of short carbon fibers in the range of 5 to 25 mm can be carbonized by impregnating with a mixture containing short carbon fibers having an average fiber length in the range of 0.01 to 1 mm and a carbonizable organic substance. After obtaining a composite sheet containing an organic substance in the range of 75 to 90% by weight, a plurality of the composite sheets are laminated and molded under a pressure in the range of 0.3 to 3 MPa to obtain a molded product. Then, the molded article is fired in an inert atmosphere at a temperature in the range of 650 to 1,000 ° C. to carbonize the organic substance, and the short carbon fibers are bound with the carbonized organic substance. An electrode formed by binding short fibers with carbon. Therefore, the obtained electrode has a relatively short carbon fiber having an average fiber length in the range of 5 to 25 mm and a relatively short carbon fiber having an average fiber length in the range of 0.01 to 1 mm. Will be included. Instead of using short carbon fibers, short fibers of the precursor fiber may be used and carbonized at the time of firing to obtain short carbon fibers.
[0010]
As the carbon fibers constituting the short carbon fibers, those using polyacrylonitrile (PAN) -based fibers, pitch-based fibers, rayon-based fibers, phenol-based fibers, or the like as precursor fibers can be used. Above all, it is preferable to use a PAN-based carbon fiber having a PAN-based fiber as a precursor fiber because an electrode having excellent mechanical properties such as bending strength can be obtained. If the carbon fiber used is too thin, it is expensive and the mechanical strength of the electrode is reduced even if it is slightly consumed. It is preferable to select and use one having an average fiber diameter in the range of 3 to 20 μm because the mass of the particles becomes large and the mechanical strength of the electrode also decreases.
[0011]
The relatively long short carbon fibers having an average fiber length in the range of 5 to 25 mm are dispersed in random directions in a two-dimensional plane. In other words, although lying in a random direction in a two-dimensional plane, it is likely that an excessive portion of bound carbon is formed between short carbon fibers or large voids are formed between the carbon fibers by being oriented in a random direction. As a result, peeling and falling off of the short carbon fiber and generation of cracks in the electrode can be prevented. In addition, the lying state improves the bending strength of the electrode. On the other hand, relatively short carbon short fibers having an average fiber length in the range of 0.01 to 1 mm can penetrate between relatively long carbon short fibers to form a lump consisting of only binding carbon between the short carbon fibers. To prevent wear of the electrode and falling off of short carbon fibers, and increase the bulk density of the electrode. This is because the carbonizable organic substance described later shrinks and loses weight by firing and carbonizing, whereas the weight of the short carbon fiber hardly changes by firing.
[0012]
The relatively short carbon fibers have an average fiber length in the range of 5 to 25 mm as described above. When the average fiber length is less than 5 mm, the bending strength required for an electrode cannot be obtained. Further, if the thickness exceeds 25 mm, it is difficult to form a sheet, so that the uniformity of the electrodes is reduced and the life is greatly reduced. The preferred average fiber length range of the relatively short carbon fiber is 9 to 18 mm. On the other hand, relatively short carbon fibers have an average fiber length in the range of 0.01 to 1 mm. When the average fiber length is less than 0.01 mm, it easily falls off together with the binding carbon, and the life of the electrode is greatly reduced. In addition, when the length exceeds 1 mm, it is difficult to penetrate between relatively short carbon fibers, so that the dispersibility is deteriorated. Therefore, the uniformity of the electrode is reduced, and the life is also greatly reduced. A preferable range of the average fiber length of the relatively short carbon short fibers is 0.01 to 0.2 mm, and a more preferable range is 0.02 to 0.05 mm. The average fiber length of the short carbon fibers is an average fiber length by weight average.
[0013]
The short carbon fibers having an average fiber length in the range of 0.01 to 1 mm are in a weight ratio of 0.05 to 0.5 with respect to the short carbon fibers 1 in which the average fiber length is in the range of 5 to 25 mm. It is preferable to be within the range. When the ratio is extremely low, the effect of preventing the electrode from being worn out and the short carbon fibers from falling is reduced, and when the ratio is extremely high, the bending strength of the electrode is reduced and it becomes difficult to impregnate the sheet during production. Sometimes.
[0014]
Also, it is preferable that the two types of short and long carbon fibers be contained in the electrode in the range of 20 to 35% by weight. When the content of the short carbon fiber is extremely low, the bending strength of the electrode is reduced. On the other hand, when the height is extremely high, the number of voids in the electrode increases, the bulk density of the electrode decreases, and the amount of binding carbon decreases, thereby shortening the life of the electrode.
[0015]
As the organic substance that is carbonized by firing, a phenol resin, a condensed polycyclic aromatic resin (COPNA resin), a furan resin, a pitch, or the like can be used. Among them, it is preferable to use a phenol resin or a COPNA resin which is easy to handle and has a high carbonization yield.
[0016]
There are various methods for obtaining a composite sheet of short carbon fibers and a carbonizable organic substance, but the most preferable method is to form a relatively long short carbon fiber by a wet or dry method, and to apply a relatively small amount to the obtained sheet. This is a method of impregnating a mixture of short carbon short fibers and an organic substance. This method is simple, can disperse relatively short carbon fibers uniformly, has high dimensional and shape stability during firing, and has high bending strength of the resulting electrode. As another method, there is a method in which a relatively long carbon short fiber and a relatively short carbon short fiber are mixed, paper is formed by a wet or dry method, and the obtained paper sheet is impregnated with an organic substance. In these methods, instead of using short carbon fibers, it is also possible to use short fibers of precursor fibers which are carbonized at the time of firing (at the time of carbonization of an organic substance) and become short carbon fibers. As a relatively long fiber having an average fiber length in the range of 5 to 25 mm, it is preferable to use short carbon fibers in order to suppress deformation of the composite sheet during firing and carbonization.
[0017]
The carbonizable organic matter is contained in the composite sheet in the range of 75 to 90% by weight. If the amount of the organic substance that can be carbonized is less than 75% by weight, the number of voids increases, the density of the electrode decreases, and the life decreases. On the other hand, if the content is more than 90% by weight, the short carbon fibers are reduced and the bending strength of the electrode is reduced, or the organic substance flows out due to the pressurization at the time of molding and molding cannot be performed.
[0018]
The composite sheet is usually heated and pressed before firing to form a molded product. Molding makes it easier to control the thickness and bulk density. At this time, a plurality of composite sheets are laminated and heated and pressed so that the obtained electrode has a desired thickness. The heating temperature is a temperature at which the organic material hardens or solidifies. The pressing force is in the range of 0.3 to 3 MPa. If the pressure is lower than 0.3 MPa, the density of the electrode cannot be increased. On the other hand, when the pressure is higher than 3 MPa, the density becomes too high, so that a decomposition gas when organic matter is carbonized becomes difficult to escape, and cracks occur in the obtained electrode.
[0019]
The firing of the molded article is performed in an inert atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere. As a result, the organic substance is carbonized, and the short carbon fibers are bound to each other by the carbonized organic substance to form an electrode. The firing temperature is in the range of 650 to 1,000 ° C. When the firing temperature is lower than 650 ° C., the conductivity of the organic carbonized material is lowered and the electric resistance of the electrode is increased. When the firing temperature is higher than 1,000 ° C., the consumption of the organic material during firing is increased. A more preferred range is 700 to 850 ° C.
[0020]
Thus, the short carbon fibers having an average fiber length in the range of 5 to 25 mm and the short carbon fibers having the average fiber length in the range of 0.01 to 1 mm dispersed in random directions in the two-dimensional plane. And an electrode having a bending strength of 20 MPa or more, which is made of a carbon fiber / carbon composite material and is bonded to each other by carbon. This electrode has a bulk density in the range of 1 to 1.4 g / cm ³ . When the bulk density is lower than 1 g / cm ³ , the life is greatly reduced because the amount of the binding carbon is small. Further, in order to obtain a bulk density higher than 1.4 g / cm ³ , an extremely high molding pressure is required. However, when a molded body molded at such an extremely high pressure is fired, as described above, Decomposition gas when organic matter is carbonized becomes difficult to escape, and the resulting electrode has many cracks and lowers bending strength. In the present invention, the bulk density is increased and the life is extended by using a relatively short carbon fiber having an average fiber length of 0.01 to 1 mm in combination with an organic substance capable of being carbonized. A preferable range of the bulk density is 1.1 to 1.3 g / cm ³ . The bulk density is determined by dividing the weight of the electrode by the volume calculated from the outer diameter.
[0021]
In the electrode of the present invention, it is preferable that the half width of the (002) peak of carbon determined by wide-angle X-ray is in the range of 5.5 to 7.5 °. The half width varies depending on the firing temperature, that is, the carbonization temperature of the organic substance, and becomes smaller as the carbonization proceeds. However, when it is less than 5.5 °, the electrode tends to be consumed more. On the other hand, if it exceeds 8 °, the conductivity tends to decrease. Such an electrode can be obtained by setting the firing temperature lower than the carbonizing temperature of the short carbon fiber used. A more preferable range of the half width of the (002) peak of carbon is 6 to 7 °.
[0022]
The electrode of the present invention preferably has a weight loss rate at 500 ° C. of 10% or less by a thermobalance and a weight loss rate at 700 ° C. of 90% or more. The weight loss rate depends on the firing temperature, that is, the carbonization temperature of the organic substance. As the carbonization of organic matter proceeds, the weight loss rates at 500 ° C. and 700 ° C. decrease. When the weight loss rate at 500 ° C. exceeds 10%, the carbonization of the organic substance is insufficient, and the electrical resistance of the electrode increases due to oxidation during use. When the weight loss rate at 700 ° C. is less than 90%, the carbonization of the organic matter is excessively advanced, and the consumption of the electrode during use increases.
[0023]
[Examples and Comparative Examples]
Example:
PAN-based carbon fiber “Treca” T300-3K manufactured by Toray Industries, Inc. (average single fiber diameter: 7 μm, number of single fibers: 3,000, tensile strength: 3,530 MPa, tensile modulus: 230 GPa), average fiber length: 12 mm , And wet-laid using water as a papermaking medium, and polyvinyl alcohol was further adhered as a binder to obtain a short carbon fiber sheet.
[0024]
Next, the obtained short carbon fiber sheet is impregnated with a mixture of the above-mentioned milled fiber (average fiber length: 0.03 mm) of the carbon fiber and a 60% by weight methanol solution of a resol-type phenol resin, dried, and dried. Got. The content of short carbon fibers having an average fiber length of 12 mm in the composite sheet is 10.5% by weight, the content of milled fibers having an average fiber length of 0.03 mm is 2.6% by weight, and the content of phenol resin is 84.6. % By weight, and the content of the polyvinyl alcohol binder was 2.3% by weight. This composition is a composition in which two types of short and long carbon short fibers are considered to be contained at 22% by weight in the electrode.
[0025]
Next, 30 composite sheets were stacked, heated and press-molded at 150 ° C. and 0.5 MPa to cure the phenol resin to obtain a molded plate. The thickness of the formed plate was 12 mm.
[0026]
Next, the formed plate was fired in a nitrogen atmosphere at a maximum temperature of 800 ° C. for 10 minutes to carbonize the phenol resin to obtain an electrode.
[0027]
The bulk density of the obtained electrode was 1.14 g / cm ³ . The bending strength according to the three-point bending test method was 56 MPa. Further, the half width of the (002) peak of carbon was 6.6 °. The weight loss rate at 500 ° C. by a thermobalance was 5%, and that at 700 ° C. was 99%. Further, when the weight loss rate was determined as an index of the life, it was 5%. The weight reduction rate was determined by immersing the two electrodes in a 4 g / l aqueous solution of CaCl ₂ with the electrodes facing each other, and conducting electricity for 500 hours while switching the polarity at a current density of 20 mA / cm ² at intervals of 20 minutes. Is the ratio of weight loss to the initial weight.
Comparative example:
In Examples, the content of the short carbon fiber having an average fiber length of 12 mm in the composite sheet was 16.4% by weight, and the content of the phenol resin was 81.3% by weight without using the milled fiber.
[0028]
The bulk density of the obtained electrode was 1.09 g / cm ³ . The bending strength measured in the same manner as in Example 1 was 69 MPa. Further, the half width of the (002) peak of carbon was 6.6 °. The weight loss rate at 500 ° C. by a thermobalance was 4%, and that at 700 ° C. was 99%. Further, the weight reduction rate measured in the same manner as in the example was 8%.
[0029]
【The invention's effect】
The electroosmotic dehydrator electrode of the present invention is a carbon short fiber having an average fiber length in the range of 5 to 25 mm dispersed in random directions in a two-dimensional plane, and an average fiber length of 0.01. And short carbon fibers in the range of 1 to 1 mm are mutually bound by carbon and the bulk density is in the range of 1 to 1.4 g / cm ^3. As is clear from the comparison with the above, the wear is small and the life is long.

Claims (5)

Short carbon fibers having an average fiber length in the range of 5 to 25 mm, and carbon short fibers having an average fiber length in the range of 0.01 to 1 mm, dispersed in random directions in a two-dimensional plane. They are bound to each other by a carbon, and the bulk density is in the range of 1~1.4g / cm ³ electroosmotic type dehydrator electrode. The electrode for an electroosmotic dehydrator according to claim 1, wherein the half width of the (002) peak of carbon determined by wide-angle X-ray is in the range of 5.5 to 7.5 °. The electrode for an electroosmotic dehydrator according to claim 1 or 2, wherein a weight loss rate at 500 ° C by a thermobalance is 10% or less and that at 700 ° C is at least 90%. In a sheet made of short carbon fibers having an average fiber length in a range of 5 to 25 mm dispersed in random directions in a two-dimensional plane, a carbon short fiber having an average fiber length in a range of 0.01 to 1 mm is provided. A step of impregnating a mixture containing fibers and a carbonizable organic substance to obtain a composite sheet containing the carbonizable organic substance in a range of 75 to 90% by weight; Molding under a pressure in the range of ３3 MPa to obtain a molded article; and firing the molded article in an inert atmosphere at a temperature in the range of 650 to 1,000 ° C. to carbonize organic substances. The method for producing an electrode for an electroosmotic dehydrator according to claim 1, comprising: In a sheet composed of short carbon fibers having an average fiber length in a range of 5 to 25 mm dispersed in random directions in a two-dimensional plane, a carbon having an average fiber length in a range of 0.01 to 1 mm A step of impregnating a mixture containing a short fiber precursor short fiber and a carbonizable organic substance to obtain a composite sheet containing the carbonizable organic substance in the range of 75 to 90% by weight, and a plurality of composite sheets Are laminated and molded under a pressure in the range of 0.3 to 3 MPa to obtain a molded product, and the molded product is fired at a temperature in the range of 650 to 1,000 ° C. in an inert atmosphere. The method for producing an electrode for an electroosmotic dehydrator according to any one of claims 1 to 3, further comprising a step of carbonizing the precursor short fiber and an organic substance.