Carbon material precursor and method for producing carbon material using same

Abstract

The present invention addresses the problem of providing a carbon material precursor that contains, as a main component, cellulosic fibers which are sustainable biomass resources, and exhibits less shrinkage caused by a carbonization treatment. The present invention also addresses the problem of providing a method for producing a carbon material capable of suppressing shrinkage caused by a carbonization treatment even when a precursor containing cellulosic fibers as a main component is used.　The present invention involves preparing a carbon material precursor which contains an unfired fiber mixture of cellulosic fibers and synthetic polymer fibers, and in which the content of the cellulosic fibers in the unfired fiber mixture is not less than 50 wt%, and the content of the synthetic polymer fibers in the unfired fiber mixture is not less than 5 wt% but less than 50 wt%. As the synthetic polymer fibers, fibers which have a thermal decomposition temperature higher than the thermal decomposition temperature of the cellulosic fibers and which do not melt at a temperature below the thermal decomposition temperature of the cellulosic fibers are used. By firing such a carbon material precursor, a carbon material is obtained.

Description

Carbon material precursor and method for producing carbon material using the same

[0001] The present disclosure relates to a carbon material precursor and a method for producing a carbon material using the same.

[0002] Carbon fiber materials, such as carbon fiber sheets, are used in a wide range of applications, including reinforcing materials and gas diffusion electrodes. Generally, the precursors of carbon fiber materials are primarily synthetic polymer fibers. However, synthetic polymer fibers are petrochemical products that rely on fossil resources, and improvements are needed in light of the recent growing global awareness of sustainable development.

[0003] In contrast to synthetic polymer fibers that rely on fossil resources, natural fibers, such as cellulosic fibers typified by natural pulp, are fiber materials derived from sustainable biomass resources. Attempts have been made to use natural pulp as one component of a precursor for a carbon material. For example, it has been proposed to produce a porous carbon sheet by carbonizing a precursor fiber sheet containing short carbon fibers, a resin, and pulp (see, for example, Patent Document 1). Furthermore, with the aim of reducing the appearance defects that occur when using commonly used natural pulp, a precursor fiber sheet for a carbon material (porous carbon sheet) has been proposed (see, for example, Patent Document 2). The precursor fiber sheet contains short carbon fibers with an average length of 3 to 10 mm, natural pulp with an ash content of 0.15% by mass or less, and a resin that carbonizes upon heating.

[0004] JP 2011-195374 A Japanese Patent No. 5988009 A

[0005] Although there have been several proposals to use cellulosic fibers such as natural pulp as precursors for carbon fiber materials, they are often used when the fibers are mixed with already carbonized carbon fibers and then calcined again, so that the cellulosic fibers are added only as an auxiliary agent. Therefore, the present inventors attempted to obtain a carbon material by calcining a precursor sheet mainly composed of cellulosic fibers, but encountered the problem that when the precursor sheet prepared from the cellulosic fibers was carbonized, the carbonization treatment caused significant shrinkage and resulted in a lack of shape stability.

[0006] In view of the above circumstances, one of the problems to be solved is to provide a carbon material precursor that is mainly composed of cellulosic fibers, which are a sustainable biomass resource, and that exhibits little shrinkage upon carbonization. Another problem to be solved is to provide a method for producing a carbon material that can suppress shrinkage upon carbonization even when a precursor mainly composed of cellulosic fibers is used.

[0007] As a result of extensive research, the inventors have discovered a temperature range in which cellulosic fibers shrink significantly during the burning process. Based on this finding, the inventors have come up with the idea of ​​adding another component suitable for suppressing the shrinkage of cellulosic fibers in that temperature range.

[0008] The invention presented in this disclosure can be understood in multiple aspects and may include, for example, the following embodied aspects as means for solving problems. Note that in this disclosure, the invention presented in this disclosure is also simply referred to as "the present invention" either as a comprehensive concept or according to each individual aspect.

[0009] [1] A carbon material precursor comprising an unburned fiber mixture of cellulosic fibers and synthetic polymer fibers, wherein the content of the cellulosic fibers in the unburned fiber mixture is 50% by weight or more, and the content of the synthetic polymer fibers in the unburned fiber mixture is 5% by weight or more but less than 50% by weight, and the synthetic polymer fibers have a thermal decomposition temperature higher than the thermal decomposition temperature of the cellulosic fibers and do not melt at or below the thermal decomposition temperature of the cellulosic fibers. [2] The carbon material precursor according to [1] above, wherein the cellulosic fibers include either one or both of chemical pulp and mechanical pulp. [3] The carbon material precursor according to [1] above, wherein the synthetic polymer fibers have a thermal decomposition temperature of 350°C or higher under conditions of an inert gas atmosphere and a heating rate of 10°C / min. [4] The carbon material precursor according to [1] above, wherein the synthetic polymer fibers have a melting point of 350°C or higher or are not observed. [5] The carbon material precursor according to any one of [1] to [4] above, wherein the synthetic polymer fiber is one or more fibers selected from the group consisting of aramid fiber, polyether ether ketone fiber, polyimide fiber, and polyphenylene sulfide fiber. [6] A method for producing a carbon material, comprising a step of calcining a carbon material precursor, wherein the carbon material precursor comprises an uncalcined fiber mixture of cellulosic fiber and synthetic polymer fiber, the uncalcined fiber mixture containing the cellulosic fiber being 50% by weight or more and the synthetic polymer fiber being 5% or more and less than 50%, and the synthetic polymer fiber having a thermal decomposition temperature higher than the thermal decomposition temperature of the cellulosic fiber but not melting at or below the thermal decomposition temperature of the cellulosic fiber. [7] The method for producing a carbon material according to [6] above, wherein the carbon material precursor is in a sheet form, and the carbon material is a carbon fiber sheet. [8] The method for producing a carbon material according to [6] above, wherein the firing temperature is 700 to 2500° C. [9] The method for producing a carbon material according to any one of [6] to [9] above, wherein in the firing step, the heating rate until the maximum temperature is reached is 45° C. / min or less.

[0010] In one or more aspects of the invention presented in the present disclosure, it is possible to provide a precursor that can suppress the degree of shrinkage due to carbonization even when the carbon material precursor is mainly composed of cellulosic fibers, which are a sustainable biomass resource. In one or more aspects of the invention presented in the present disclosure, it is possible to provide a method for producing a carbon material that can suppress shrinkage due to carbonization even when a precursor mainly composed of cellulosic fibers is used.

[0011] This disclosure has been filed as an international application under the Patent Cooperation Treaty, and the original language of the application is Japanese. It is intended that this disclosure will be translated into the languages ​​required by each designated and elected state upon entry into those states. In this disclosure, unless otherwise specified, Japanese nouns may be singular or plural, depending on the context or the full text of this disclosure. Furthermore, when translated into a language such as English that distinguishes between countable and uncountable nouns, and between singular and plural countable nouns, unless otherwise specified, the singular designation includes the plural, and the plural designation includes the singular, depending on the context or the full text of this disclosure.

[0012] Hereinafter, embodiments of the present invention will be described. In this disclosure, the term "one embodiment" with respect to the present invention refers to any one embodiment for describing the present invention in detail, unless otherwise specified, and does not deny or limit the existence of other or multiple embodiments. As will be described below, the present invention may include multiple embodiments within its scope. Furthermore, multiple embodiments may also be provided as modified forms, for example, by various combinations of the components (or technical features) shown in this disclosure. Furthermore, in this disclosure, when the term "embodiment" is simply used, it includes one or multiple embodiments unless otherwise specified.

[0013] In this disclosure, unless otherwise specified, the expression "AA to BB" in relation to a numerical range means "AA or more and BB or less" (where "AA" and "BB" represent arbitrary numerical values). Furthermore, unless otherwise specified, the units of the lower and upper limits are the same as the units immediately following the latter (i.e., "BB" here). Furthermore, in this disclosure, the combination of the lower and upper limits of a numerical range can be arbitrarily selected from the group of lower or upper limit values ​​exemplified as preferred numerical values, etc. Furthermore, the expression "X and / or Y" means both X and Y, or either one of them.

[0014] 1. Carbon Material Precursor One embodiment of the present invention provides a carbon material precursor that is primarily composed of cellulosic fibers but that can suppress the degree of shrinkage after carbonization. In this disclosure, the term "carbon material precursor" refers to a precursor containing a material prior to carbonization by calcination. Furthermore, in this disclosure, the term "cellulosic fibers" simply refers to uncalcined cellulosic fibers.

[0015] A carbon material precursor according to one embodiment of the present invention includes cellulosic fibers and other unburned materials capable of suppressing shrinkage of the cellulosic fibers. The thermal decomposition temperature of cellulose is 250 to 400°C, and within this temperature range, the degree of shrinkage of the cellulosic fibers is large. The other unburned materials are materials that are ultimately burned and carbonized, and that shrink less than the cellulosic fibers within the thermal decomposition temperature range of cellulose. Examples of such other unburned materials include synthetic polymer fibers having specific thermal properties. In this disclosure, the term "synthetic polymer fibers" simply refers to unburned synthetic polymer fibers. In this disclosure, a fiber mixture containing cellulosic fibers and synthetic polymer fibers is also referred to as an unburned fiber mixture.

[0016] In the present disclosure, "containing cellulosic fibers as a main component" means that the content of cellulosic fibers is at least higher than that of other components, and a higher content of cellulosic fibers is preferable from the viewpoint of sustainability, etc. Examples of lower limits of the content of cellulosic fibers in the total unburned material (or in the unburned fiber mixture) are preferably 50, 51, 52, 53, 54, or 55% by weight or more, more preferably 60, 65, or 68% by weight or more, and even more preferably 70, 75, or 78% by weight or more.

[0017] On the other hand, the amount of cellulosic fiber may be relatively reduced from the viewpoint of suppressing shrinkage due to carbonization. For example, when an unburned fiber mixture containing cellulosic fiber and synthetic polymer fiber is used, the content of cellulosic fiber in the unburned fiber mixture may be 95, 90, 85, 83, 82, 81, or 80 wt. % or less, depending on the type of synthetic polymer fiber. Furthermore, the content of cellulosic fiber may be reduced to less than 80 wt. % to further suppress shrinkage or to modify the properties of the carbonized material obtained by burning.

[0018] When a specific synthetic polymer fiber is used as another unburned material to suppress shrinkage of the cellulosic fiber, the content of the synthetic polymer fiber in the unburned fiber mixture may preferably be 5% by weight or more and less than 50% by weight. More specifically, this is as follows. The lower limit of the content of the synthetic polymer fiber in the unburned fiber mixture depends on the type of synthetic polymer fiber, but from the viewpoint of suppressing shrinkage of the cellulosic fiber, it may be preferably 5% by weight or more, more preferably 10% by weight or more, and even more preferably 15% by weight or more. The content of the synthetic polymer fiber may be higher than the above lower limit from the viewpoint of further suppressing shrinkage due to the carbonization treatment, but in light of the lower limit of the content of the cellulosic fiber, it may preferably be less than 50% by weight. The content of the cellulosic fiber may be relatively increased, and the content of the synthetic polymer fiber in the unburned fiber mixture may be 45, 40, 30, or 20% by weight or less.

[0019] As the synthetic polymer fiber, a synthetic polymer fiber having a predetermined thermal property can be used as a material that contributes to suppressing shrinkage of the cellulosic fiber due to the carbonization treatment. Examples of the thermal property indicator of the synthetic polymer fiber include the thermal decomposition temperature and the melting point. In one embodiment of the present invention, one or a combination of two or more of these thermal property indicators can be used.

[0020] A preferred embodiment of the carbon fiber precursor may be exemplified by a precursor that satisfies the following conditions (a) to (e): (a) It contains an unburned fiber mixture of cellulosic fibers and synthetic polymer fibers. (b) The content of the cellulosic fibers in the unburned fiber mixture is 50% by weight or more. (c) The content of the synthetic polymer fibers in the unburned fiber mixture is 5% by weight or more and less than 50% by weight. (d) The thermal decomposition temperature of the synthetic polymer fibers is higher than the thermal decomposition temperature of the cellulosic fibers. (e) The synthetic polymer fibers do not melt at temperatures equal to or lower than the thermal decomposition temperature of the cellulosic fibers.

[0021] In one embodiment of the present invention, a carbon material precursor uses cellulosic fibers as a material contained in the precursor. In this disclosure, "cellulosic fibers" refers to fibers primarily composed of cellulose and its derivatives. As described below, the cellulosic fibers may be modified. Cellulose and its derivatives are carbohydrates (polysaccharides) that are the main components of plant cell walls and plant fibers. They account for one-third of natural plant matter and are the most abundant biological resource (biomass) on Earth. In other words, this disclosure provides various means for effectively utilizing cellulosic fibers, an abundant and sustainable biomass resource, and provides technologies that can contribute to sustainable development and reduce the production costs of carbon materials.

[0022] Cellulose-based fibers that can be used in one embodiment of the present invention include cellulose fibers and cellulose derivative fibers. Cellulose derivatives are included in the category of cellulose derivatives as long as they maintain a structure in which D-glucopyranose units are linked by β1,4 bonds (cellulose skeleton) within the molecule. Cellulose is usually linked in a linear chain, but cellulose derivatives may have a substituent on the cellulose, a side chain, or may be branched. In the following, in this disclosure, cellulose and its derivatives will also be simply referred to as cellulose unless a clear distinction is required.

[0023] Examples of materials that can be used as cellulosic fibers include natural cellulosic fibers such as papermaking chemical pulp, mechanical pulp, cotton, and herbaceous plant-derived fibers, and regenerated cellulosic fibers such as viscose rayon. In a preferred embodiment of the present disclosure, the cellulosic fibers are, for example, natural fibers, and more specifically, examples include chemical pulp, mechanical pulp, cotton, and herbaceous plant-derived fibers. Pulps such as chemical pulp and mechanical pulp can be mass-produced in the papermaking industry as raw materials for various types of paper, and may be preferable from the perspective of reducing raw material costs. One type of cellulosic fiber may be used, or two or more types may be used in combination.

[0024] In a preferred embodiment of the present invention, chemical pulp may be used alone as the cellulosic fiber. Chemical pulp is prepared by chemically treating woody biomass resources, and therefore generally contains fewer impurities such as inorganic substances and lignin than mechanical pulp, or is a more refined pulp. Chemical pulp contains less ash derived from inorganic substances, and when chemical pulp is burned, a carbon material with relatively high carbon purity and high chemical stability can be obtained. Furthermore, chemical pulp production processes have long involved chemical recycling and energy recovery, and chemical pulp can be obtained using a method with low environmental impact.

[0025] In another preferred embodiment of the present invention, mechanical pulp can be used alone as the cellulosic fiber. Mechanical pulp has a higher density of carbon material precursor than chemical pulp, and therefore the density remains relatively high even after calcination, which is expected to improve the strength of carbon materials, particularly carbon fiber sheets. Furthermore, since mechanical pulp generates a lot of fluff and short fibers during the manufacturing process, the precursor sheet obtained by papermaking has a relatively high smoothness, and the smoothness of the carbon fiber sheet after calcination can also be expected. Furthermore, mechanical pulp can be prepared from woody biomass resources by physical processes such as pulverization and cutting, and has a high yield, making it suitable from a cost perspective. Furthermore, since lignin and other materials originally present in the woody tissue remain, an improvement in yield after calcination can also be expected.

[0026] Furthermore, in another preferred embodiment of the present invention, a mixture of chemical pulp and mechanical pulp can be used as the cellulosic fiber. Chemical pulp and mechanical pulp each have the properties described above, and the mixing ratio of these pulps can be adjusted depending on the application of the carbon material, the desired performance required of the carbon material, etc.

[0027] In a preferred embodiment, the carbon material precursor of the present disclosure contains another unburned material that can suppress shrinkage of the cellulosic fiber during carbonization. Suitable examples of such another unburned material include a material that is ultimately burned and carbonized, and that shrinks less than the cellulosic fiber in the thermal decomposition temperature range of cellulose. More specifically, preferred examples of the other unburned material include synthetic polymer fibers with predetermined thermal properties. Examples of the predetermined thermal properties that can serve as indicators include the thermal decomposition temperature or melting point.

[0028] As a preferred embodiment of the other unburned materials described above, synthetic polymer fibers having a thermal decomposition temperature higher than that of the cellulosic fiber used together and not melting at temperatures equal to or lower than the thermal decomposition temperature of the cellulosic fiber may be used. By mixing such synthetic polymer fibers with the cellulosic fiber, it is believed that the synthetic polymer fibers can act as a support that suppresses the shrinkage of the cellulosic fiber in the temperature range where the cellulosic fiber is likely to shrink.

[0029] As mentioned above, cellulosic fibers shrink to a greater extent in their thermal decomposition temperature range. The thermal decomposition temperature of cellulosic fibers is approximately 250 to 400°C. In the present disclosure, "higher than the thermal decomposition temperature of the cellulosic fiber" does not necessarily mean 400°C or higher. It is sufficient that the temperature is higher than the thermal decomposition temperature of the specific cellulosic fiber used together. For example, when using a cellulosic fiber with a thermal decomposition temperature of around 300°C, the thermal decomposition temperature of the companion synthetic polymer fiber should be higher than that temperature of around 300°C. In practice, it may be preferable to allow for some leeway and use a fiber with a thermal decomposition temperature that is about 10 to 50°C higher than the thermal decomposition temperature of the cellulosic fiber used together.

[0030] To give a specific example of the thermal decomposition temperature of synthetic polymer fibers using numerical values, under conditions of an inert gas atmosphere and a temperature increase rate of 10°C / min, the thermal decomposition temperature is preferably 350°C or higher, more preferably 380°C or higher, and even more preferably 400°C or higher. Using synthetic polymer fibers having a thermal decomposition temperature higher than the above lower limit is suitable for further suppressing shrinkage of cellulosic fibers during carbonization treatment.

[0031] It should be noted that for some polymeric materials, such as cellulosic fibers and synthetic polymer fibers, it is difficult to pinpoint the thermal decomposition temperature even for a single product, and the thermal decomposition temperature may be specified as a certain numerical range in light of common technical knowledge in the relevant technical field. When one or both of the thermal decomposition temperatures of the cellulosic fibers and the synthetic polymer fibers are expressed as numerical ranges, comparisons using the thermal decomposition temperature as an index are made using both numerical ranges or only one numerical range. In such cases, a preferred embodiment of the present invention is to use the maximum value (or upper limit) of the numerical range for the thermal decomposition temperature of the cellulosic fibers and the minimum value (or lower limit) of the numerical range for the thermal decomposition temperature of the synthetic polymer fibers.

[0032] In one embodiment of the present invention, as described above, synthetic polymer fibers that do not melt under temperature conditions below the thermal decomposition temperature of the cellulosic fiber used therewith may be used. From this perspective, the melting point (Tm) may be used as an indicator of other thermal properties of the synthetic polymer fiber. The melting point of the synthetic polymer fiber may preferably be 350°C or higher. More specifically, the lower limit of the melting point of the synthetic polymer fiber may preferably be 350°C or higher, more preferably 375°C or higher, and even more preferably 400°C or higher. Having the lower limit of the melting point of the synthetic polymer fiber equal to or higher than the above lower limit is advantageous in terms of suppressing shrinkage of the cellulosic fiber during carbonization. The upper limit of the melting point of the synthetic polymer fiber does not need to be particularly specified from the perspective of suppressing shrinkage of the cellulosic fiber during carbonization.

[0033] It should be noted that synthetic polymer fibers may have substantially no observable melting point. This is because some synthetic polymer fibers may undergo thermal decomposition or thermal hardening without melting or almost simultaneously with melting when heated. In an embodiment of the present invention, synthetic polymer fibers having substantially no observable melting point may be used as long as they do not melt at or below the thermal decomposition temperature of the cellulose-based fiber used together.

[0034] Furthermore, even for a single product, it is difficult to specify the melting point of a polymer material such as a synthetic polymer fiber; rather, it may be specified within a certain range of values ​​in light of common general knowledge in the relevant technical field. Therefore, when one or both of the thermal decomposition temperature of the cellulosic fiber and the melting point of the synthetic polymer fiber are expressed as a range of values, comparisons using the thermal decomposition temperature or the melting point as an indicator are made using both ranges of values, or just one of the ranges of values. In such cases, a preferred embodiment of the present invention uses the maximum value (or upper limit) of the range of values ​​for the thermal decomposition temperature of the cellulosic fiber for comparison, and the minimum value (or lower limit) of the range of values ​​for the melting point of the synthetic polymer fiber for comparison.

[0035] Preferred types of synthetic polymer fibers include, for example, aramid fibers, polyether ether ketone fibers, polyimide fibers, and polyphenylene sulfide fibers, and more preferably, aramid fibers, polyimide fibers, and polyphenylene sulfide fibers. One type of synthetic polymer fiber may be used, or two types may be used in combination. These synthetic polymer fibers can be obtained with thermal properties that contribute to further suppressing the shrinkage of cellulosic fibers during carbonization.

[0036] The shape of the carbon material precursor is not particularly limited. Examples of the carbon material precursor include a sheet-shaped carbon material precursor and a roll-shaped carbon material precursor obtained by winding a sheet-shaped carbon material precursor. The carbon material precursor is easy to handle and transport, and can also be easily transferred to the subsequent manufacturing process.

[0037] A precursor containing cellulosic fibers as a main component may be susceptible to significant shrinkage due to carbonization when in a sheet form. The carbon material precursor according to one embodiment of the present invention is suppressed from shrinking due to carbonization, even though it contains cellulosic fibers as a main component, and therefore a sheet form may be one of the preferred application forms of the present invention.

[0038] The carbon material precursor may be composed solely of the unburned fiber mixture, or may further contain other optional components depending on the intended use of the resulting carbon material and the performance required of the carbon material. For example, a catalyst may be added to promote the progress of carbonization by burning. Furthermore, a binder may be added to improve the shape stability of the carbon material precursor or the carbon material obtained by carbonizing the carbon material precursor. Examples of other optional components include a conductive additive, a metal catalyst, and a surfactant.

[0039] 2. Method for Producing a Carbon Material Precursor A carbon material precursor according to one embodiment of the present invention includes a cellulosic fiber and another unsintered material capable of suppressing shrinkage of the cellulosic fiber. As described above, the other unsintered material may be, for example, a synthetic polymer fiber having predetermined thermal properties.

[0040] In one embodiment of the present invention, the carbon material precursor can be obtained by mixing cellulosic fibers and synthetic polymer fibers to prepare an unburned fiber mixture. From the viewpoint of suppressing shrinkage of the cellulosic fibers during carbonization, it is preferable that the cellulosic fibers and synthetic polymer fibers are dispersed in the unburned fiber mixture while being entangled with each other. By entangling the two fibers with each other, it is expected that the synthetic polymer fibers will act as a support to prevent shrinkage of the cellulosic fibers when the temperature reaches a range where the cellulosic fibers are likely to shrink during the carbonization process by burning.

[0041] In one embodiment, to thoroughly mix the cellulosic fibers and synthetic polymer fibers, an unburned fiber mixture can be prepared, for example, as follows: Cellulosic fibers and synthetic polymer fibers are each added to a dispersion medium such as water or an aqueous solution to prepare a defibrated slurry for each fiber. To thoroughly defibrate and disperse the fibers, a retention aid, surfactant, thickener, etc. may be added. The respective defibrated slurries are then mixed and stirred to prepare a mixed slurry in which both fibers are thoroughly mixed. Furthermore, other optional components may be added to each defibrated slurry or the mixed slurry as needed, and then stirred and mixed. The resulting mixed slurry is placed in a desired mold or the like, dried, and the dispersion medium is removed, thereby preparing an unburned fiber mixture of the desired shape.

[0042] 3. Method for Producing a Carbon Material A carbon material can be obtained by calcining and carbonizing a carbon material precursor. In one embodiment of the present invention, the carbon material precursor is calcined by being subjected to a heat treatment at a predetermined temperature or higher in an inert gas atmosphere.

[0043] In the present disclosure, the inert gas atmosphere refers to an oxygen-free or low-oxygen atmosphere in which carbon is less likely to undergo a combustion reaction and is carbonized. In one embodiment of the present invention, preferred examples of the inert gas include helium gas, argon, and nitrogen. Among these, nitrogen may be preferable from the viewpoint of production costs.

[0044] In one embodiment of the present invention, the heating temperature (maximum temperature) for calcining the carbon material precursor may preferably be 700 to 2500°C. More specifically, this is as follows. The lower limit of the heating temperature (maximum temperature) for calcining the carbon material precursor may preferably be 700°C or higher, more preferably 750°C or higher, and even more preferably 800°C or higher. Heating to such a temperature or higher allows the carbon material precursor to be sufficiently carbonized. From the viewpoint of calcining the carbon material precursor, there is no particular upper limit to the maximum temperature that can be achieved by heating, but from the viewpoint of production costs, a temperature of approximately 2500, 2300, 2000, 1800, or 1500°C or lower may be preferable.

[0045] In one embodiment of the present invention, the heating rate in the step of calcining the carbon material precursor may preferably be 3 to 45°C / min or less. More specifically, this is as follows. The lower limit of the heating rate in the calcination step may preferably be 3°C / min or more, more preferably 5 or 10°C / min or more, and even more preferably 15°C / min or more. By setting the lower limit of the heating rate in this manner, the calcination step can be carried out without taking an unnecessarily long time while suppressing shrinkage of the precursor mainly composed of cellulose-based fibers. The upper limit of the heating rate in the calcination step may preferably be 45°C / min or less, more preferably 40 or 35°C / min or less, and even more preferably 30°C / min or less. Setting the upper limit of the heating rate in this manner can avoid an overly rapid temperature rise, contributing to suppression of shrinkage of the precursor mainly composed of cellulose-based fibers.

[0046] In a method for producing a carbon material according to one embodiment of the present invention, a final carbon material can be produced from a precursor containing cellulosic fibers as a main component by a single calcination process, rather than mixing a carbon material obtained by prior carbonization with cellulosic fibers and then calcining the resulting mixture, i.e., two or more times. Furthermore, shrinkage of the precursor can be suppressed during the calcination process. Therefore, according to one or more embodiments of the present invention, it is possible to reduce production costs and the number of production steps.

[0047] The present invention will be described in more detail below with reference to examples, but the technical scope (or technical reach) of the invention presented in this disclosure is not limited to the following examples. The following measurement methods, calculation methods, evaluation methods, and other methods can be used to determine numerical values ​​when the invention presented in this disclosure is specified using numerical values.

[0048] The "thermal decomposition temperature" and "melting point" in the present disclosure can be determined by the following method.

[0049] <Identification of Thermal Decomposition Temperature> When the thermal decomposition temperature is described in the reference specifications of the product, that value is used. When not described, the thermal decomposition temperature is the temperature range at which a significant weight loss occurs when heated at 10°C / min using a TGA (thermogravimetric analyzer) in a nitrogen atmosphere. Here, the "temperature range at which a significant weight loss occurs" can be specified as the temperature range from the start point of a significant weight loss to the end point of the significant weight loss.

[0050] <Determination of Melting Point (Tm)> When the melting point is described in the reference specifications of the product, the value is used. When not described, the melting point is determined as the melting peak top temperature when the temperature is increased at 10°C / min in a nitrogen atmosphere using a DSC (differential scanning calorimeter).

[0051] Example 1 A first slurry was prepared by dispersing 16 g of bone-dry weight of softwood-derived chemical pulp (NUKP (softwood unbleached kraft pulp)) and 4 g of bone-dry weight of 1 mm cut aramid fiber (Kevlar 49, manufactured by DuPont-Toray Co., Ltd., pyrolysis temperature 539°C, "Kevlar" is a registered trademark) in water, and adding water to make the total amount approximately 2 kg. The first slurry was disintegrated in a TAPPI disintegrator to prepare a second slurry. Half of the second slurry was used to produce a 25 cm square handsheet in a standard handsheet. The obtained sheet was dried in a cylinder dryer set at 100°C and then conditioned for 24 hours in an environment of 23°C and 50% humidity to obtain a carbon material precursor sheet.

[0052] Comparative Example 1 A carbon material precursor sheet was obtained in the same procedure as in Example 1, except that a first slurry was prepared by dispersing 20 g of softwood-derived chemical pulp (NUKP (softwood unbleached kraft pulp)) in an amount equivalent to bone dry weight of 20 g in water, and adding water to make the total amount about 2 kg.

[0053] Comparative Example 2 A carbon material precursor sheet was obtained in the same procedure as in Example 1, except that a first slurry was prepared by dispersing 16 g (in bone-dry weight) of softwood-derived chemical pulp (NUKP (softwood unbleached kraft pulp)) and 4 g (in bone-dry weight) of PE pulp (average fiber length 1.2 mm, SWP manufactured by Mitsui Chemicals, Inc., melting point 125 to 137°C, "PE" is an abbreviation for polyethylene, and "SWP" is a registered trademark) in water, and adding water to make the total amount about 2 kg.

[0054] Comparative Example 3 A carbon material precursor sheet was obtained in the same procedure as in Example 1, except that a first slurry was prepared by dispersing 8 g (in bone-dry weight) of softwood-derived chemical pulp (NUKP (softwood unbleached kraft pulp)), 8 g (in bone-dry weight) of mechanical pulp (collected from a pulp preparation process at a newsprint manufacturing facility of Nippon Paper Industries Co., Ltd.), and 4 g (in bone-dry weight) of PE pulp (average fiber length 1.2 mm, SWP manufactured by Mitsui Chemicals, Inc.) in water, and adding water to make the total amount about 2 kg.

[0055] <Shrinkage Test> Two 10 x 10 cm test sheets were cut out from each of the sheets of Example 1 and Comparative Examples 1 to 3. Each cut sheet was placed in a firing furnace, heated to 900°C at a heating rate of 15°C / min, and fired at a temperature of around 900°C for 60 minutes. The length of one side of each sheet after firing was measured, and the average value was calculated for each Example and Comparative Example. The results are shown in Table 1.

[0056]

Claims

1. A carbon material precursor comprising an unburned fiber mixture of cellulosic fibers and synthetic polymer fibers, wherein the content of the cellulosic fibers in the unburned fiber mixture is 50% by weight or more, and the content of the synthetic polymer fibers in the unburned fiber mixture is 5% by weight or more and less than 50% by weight, and the synthetic polymer fibers have a thermal decomposition temperature higher than the thermal decomposition temperature of the cellulosic fibers and do not melt at or below the thermal decomposition temperature of the cellulosic fibers.

2. The carbon material precursor according to claim 1, wherein the cellulose-based fibers contain either or both of chemical pulp and mechanical pulp.

3. The carbon material precursor according to claim 1, wherein the synthetic polymer fiber has a thermal decomposition temperature of 350°C or higher under conditions of an inert gas atmosphere and a heating rate of 10°C / min.

4. The carbon material precursor according to claim 1, wherein the synthetic polymer fiber has a melting point of 350°C or higher or none is observed.

5. The carbon material precursor according to any one of claims 1 to 4, wherein the synthetic polymer fiber is one or more fibers selected from the group consisting of aramid fiber, polyether ether ketone fiber, polyimide fiber, and polyphenylene sulfide fiber.

6. A method for producing a carbon material, comprising a step of calcining a carbon material precursor, wherein the carbon material precursor comprises an uncalcined fiber mixture of cellulosic fibers and synthetic polymer fibers, the content of the cellulosic fibers in the uncalcined fiber mixture is 50% by weight or more, and the content of the synthetic polymer fibers in the uncalcined fiber mixture is 5% or more and less than 50%, and the synthetic polymer fibers have a thermal decomposition temperature higher than the thermal decomposition temperature of the cellulosic fibers and do not melt at or below the thermal decomposition temperature of the cellulosic fibers.

7. The method for producing a carbon material according to claim 6, wherein the carbon material precursor is in the form of a sheet, and the carbon material is a carbon fiber sheet.

8. The method for producing a carbon material according to claim 6, wherein the sintering temperature is 700 to 2500°C.

9. The method for producing a carbon material according to any one of claims 6 to 8, wherein in the calcination step, the rate of temperature increase until the maximum temperature is reached is 45°C / min or less.

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