Composition containing a sinterable inorganic material, and method for producing a sinterable inorganic material-containing composition

Abstract

The present invention provides a sinterable inorganic material-containing composition that offers excellent handling during slip casting, as well as superior moldability and shape retention of the wet molded body obtained by injecting the slurry into a mold, and enables the production of sintered ceramic products with high strength. [Solution] The sinterable inorganic material-containing composition of the present invention contains cellulose microfibers satisfying the following conditions (a) to (d), and a sinterable inorganic material, wherein the amount of cellulose microfibers added per 100 parts by mass of the sinterable inorganic material is less than 2.0 parts by mass. (a) Number-average fiber diameter is between 2 nm and 500 nm. (b) Average aspect ratio is between 30 and 300 (c) Having 70-100% cellulose type II crystal structure (d) Having anionic functional groups

Description

[Technical Field]

[0001] The present invention relates to a sinterable inorganic material-containing composition and a method for producing the sinterable inorganic material-containing composition. [Background Art]

[0002] As a method for producing ceramic products, a technique called slip casting is known, in which ceramic powder as a sinterable inorganic material is dispersed in a volatile liquid such as water to form a slurry, which is then poured into a mold formed of a porous body having pores that allow water to pass through, and then dried by allowing water to pass through each pore of the mold to obtain a molded product. The obtained wet molded body is subjected to processes such as drying and firing to obtain a ceramic product as a sintered body. In order to improve the strength of the molded body before firing, an organic binder is used in ceramic slurry; after firing, the organic binder is thermally decomposed, so that the finally obtained ceramic product does not contain the organic binder.

[0003] In the above-mentioned slip casting, a method using a polyvinyl alcohol-based polymer as an organic binder (binding agent) is known (Patent Document 1).

[0004] It has also been proposed to use an inorganic material-containing composition containing an inorganic material, cellulose fibers containing anion-modified cellulose, water, and a dispersant for slip casting (Patent Document 2). [Prior Art Literature] [Patent Literature]

[0005] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 09-136916 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2018-90454 [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] In Patent Document 1, the polyvinyl alcohol-based polymer used as a binder had poor viscosity-enhancing effects on the slurry, making it prone to aggregation and precipitation of inorganic materials, which could lead to cracks and unevenness in the resulting wet molded articles. Furthermore, it was necessary to increase the amount of binder added to achieve a viscosity suitable for casting. Moreover, the strength of the inorganic wet molded articles obtained before firing using the polyvinyl alcohol-based polymer as a binder was insufficient.

[0007] On the other hand, the cellulose fibers used in Patent Document 2 tend to thicken easily when preparing slurries and are difficult to handle. Furthermore, the cellulose fibers in Patent Document 2 are characterized by having a cellulose type I crystal structure. Cellulose type I is known as the cellulose crystal structure that constitutes the cell walls of higher plants, but it has relatively low thermal stability and is known to be converted to cellulose type II. Therefore, in industrial applications where high chemical stability is required, the change in crystallinity can affect physical properties, and there have been many cases where it has been difficult to use.

[0008] Furthermore, in type I cellulose, all hydrogen atoms directly bonded to glucopyranose are located in the axial direction, and all hydroxyl groups are located in the equatorial direction. This means that it exhibits hydrophobicity in the axial direction and hydrophilicity in the equatorial direction. Therefore, cellulose nanofibers mainly composed of type I cellulose may sometimes exhibit insufficient hydrophilicity. For example, when using cellulose nanofibers in ceramic processing, it is necessary to uniformly disperse the cellulose nanofibers in a small amount of water within a high-solids content sinterable inorganic material slurry. However, with cellulose nanofibers mainly composed of type I cellulose, this dispersion is often insufficient, making their use difficult in some cases.

[0009] On the other hand, type II cellulose has a crystalline structure characterized by an antiparallel chain structure and is known to have high thermal stability. Therefore, it is not converted to type I cellulose and is suitable for industrial applications where chemical stability is required, although there are few examples of its use.

[0010] Furthermore, it is known that the (1-10) crystal plane is exposed in the antiparallel chain structure of cellulose type II. This crystal plane has high surface energy and the best hydrophilicity. Therefore, it is considered suitable for ceramic processing in sinterable inorganic material slurries with low water content and high solid content. However, until now, there have been no known cases of using cellulose nanofibers mainly composed of cellulose type II.

[0011] Therefore, the present invention aims to provide a sinterable inorganic material-containing composition that offers excellent handling during casting, as well as superior moldability and shape retention of the wet molded body obtained by injecting the slurry into a mold, and that enables the production of sintered ceramic products with high strength. [Means for solving the problem]

[0012] The inventors of this invention conducted thorough research to achieve this objective and found that using specific cellulose microfibers is extremely effective, thus completing the present invention.

[0013] The present invention provides the following: (1) A sinterable inorganic material-containing composition comprising cellulose fine fibers satisfying the following conditions (a) to (d), wherein the amount of cellulose fine fibers added per 100 parts by mass of the sinterable inorganic material is less than 2.0 parts by mass. (a) Number-average fiber diameter is between 2 nm and 500 nm. (b) Average aspect ratio is between 30 and 300 (c) Having 70-100% cellulose type II crystal structure (d) Having anionic functional groups (2) The sinterable inorganic material-containing composition according to (1), wherein the cellulose fine fibers, when dispersed in a 1% by mass aqueous solution, have a viscosity (60 rpm) of 1 mPa·s or more and 50 mPa·s or less at 25°C. (3) The cellulose fine fibers have a degree of polymerization of 30 or more and 300 or less, the sinterable inorganic material-containing composition according to (1). (4) A method for producing a sinterable inorganic material-containing composition, comprising: a powder mixing step of mixing a powder containing cellulose microfibers satisfying the following conditions (a) to (d) with a powder of a sinterable inorganic material; and a slurry preparation step of adding water to the mixed powder obtained in the powder mixing step and mixing the resulting mixture to obtain a slurry. (a) the number-average fiber diameter is 2 nm or more and 500 nm or less (b) the average aspect ratio is 30 or more and 300 or less (c) the cellulose II crystal structure accounts for 70 to 100% thereof (d) it has an anionic functional group (5) The method for producing a sinterable inorganic material-containing composition according to (4), wherein the powder containing the cellulose microfibers has a moisture content of 20 mass% or less, and an average particle diameter of less than 45 µm. Effects of the Invention

[0014] According to the present invention, there can be provided a sinterable inorganic material-containing composition which is excellent in handleability during slip casting, as well as in moldability and shape retention of a wet molded body obtained by injecting a slurry into a mold, and which enables obtaining a sintered ceramic product having high strength. Brief Description of the Drawings

[0015] [Figure 1] It is a photograph showing an example of a wet molded body in which no void was confirmed at the center in the evaluation of moldability during slip casting (judgment: ○). [Figure 2] It is a photograph showing an example of a wet molded body in which a void was confirmed at the center in the evaluation of moldability during slip casting (judgment: ×). Mode for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described in detail. In the present invention, "to" includes the endpoint values. That is, "X to Y" includes the values X and Y at both ends thereof.

[0017] The sinterable inorganic material-containing composition of the present invention contains cellulose microfibers satisfying the following conditions (a) to (d), and a sinterable inorganic material, wherein the amount of cellulose microfibers added per 100 parts by mass of the sinterable inorganic material is less than 2.0 parts by mass. (a) Number-average fiber diameter is between 2 nm and 500 nm. (b) Average aspect ratio is between 30 and 300 (c) Having 70-100% cellulose type II crystal structure (d) Having anionic functional groups

[0018] (Cellulose microfibers) The cellulose microfibers used in this invention satisfy the following conditions (a) to (d). (a) The number-average fiber diameter is 2 nm or more and 500 nm or less; (b) The average aspect ratio is 30 or more and 300 or less; (c) It has a cellulose type II crystalline structure of 70 to 100%; and (d) It has anionic functional groups.

[0019] (Number-average fiber diameter, number-average fiber length) The cellulose microfibers used in this invention have a number-average fiber diameter of 2 nm to 500 nm. The number-average fiber length is preferably 60 nm to 600 nm, and more preferably 100 nm to 400 nm. The number-average fiber diameter and number-average fiber length of the cellulose microfibers can be determined by diluting a dispersion of cellulose microfibers, analyzing 200 randomly selected fibers using an atomic force microscope (AFM), and taking the average.

[0020] (Average aspect ratio) The cellulose microfibers used in this invention have an average aspect ratio of 30 to 300. Preferably, the average aspect ratio is 40 to 250, and more preferably 50 to 200. The average aspect ratio can be calculated using the following formula. Average aspect ratio = Number average fiber length / Number average fiber diameter

[0021] Such cellulose microfibers can be obtained, for example, by alkaline treatment of a cellulose raw material to obtain cellulose having a cellulose type II crystalline structure of 70-100%, introducing anionic functional groups to the obtained cellulose, and then subjecting the anionically modified cellulose to a defibrillation treatment.

[0022] (Cellulose raw material) Cellulose raw materials include wood-derived kraft pulp or sulfite pulp, powdered cellulose obtained by grinding these materials with a high-pressure homogenizer or mill, or microcrystalline cellulose powder obtained by refining these materials through chemical treatment such as acid hydrolysis. In addition, plant-derived cellulose raw materials such as kenaf, hemp, rice, bagasse, and bamboo can also be used. However, if a large amount of lignin remains in the cellulose raw material, it may inhibit chemical reactions such as oxidation reactions of the raw material. Therefore, in the present invention, cellulose raw materials obtained by a method for producing chemical pulp are preferred. To further remove lignin, it is even more preferable to subject the cellulose raw material obtained in this way to a known bleaching treatment.

[0023] (Cellulose type II crystal structure) In the present invention, for example, cellulose having 70-100%, preferably 90-100%, of a cellulose type II crystalline structure can be obtained by treating the above-mentioned cellulose raw material in an alkaline solution. Hereinafter, this treatment will also be simply referred to as "alkaline treatment." Alkaline treatment can be carried out by dispersing the cellulose raw material in water, adding alkali to the aqueous dispersion to adjust the hydroxide ion concentration in the water to a range of 10%-20%, and stirring the reaction system. Alternatively, alkaline treatment can be carried out by dispersing the cellulose raw material in water that has been previously adjusted to the hydroxide ion concentration range.

[0024] When the cellulose type II crystal structure is in the range of 70-100%, it is possible to obtain cellulose microfibers that mainly consist of antiparallel chain structures where adjacent molecular chains are oriented in opposite directions. These microfibers have high chemical stability and good hydrophilicity, and exhibit good dispersion and reinforcing properties even in high-concentration, low-water ceramic dispersions (slurries). Compositions containing these cellulose microfibers and sinterable inorganic materials have excellent handling properties during casting, where high temperature resistance and chemical stability are required. If the cellulose type II crystal structure is less than 70%, the chemical stability and hydrophilicity will be insufficient, and there is a risk that the cellulose microfibers will not be sufficiently dispersed in the ceramic dispersion (slurry).

[0025] You can also use commercially available cellulose that has a cellulose type II crystal structure of 70-100%.

[0026] Here, the content of cellulose type II crystal structure can be calculated by the area ratio of the peaks at diffraction angle 2θ in the graph obtained by measurement using wide-angle X-ray diffraction. The procedure is as follows. First, cellulose is frozen with liquid nitrogen, compressed, and then used to create tablet pellets. These samples are then measured using an X-ray diffraction analyzer (LabX XRD-6000, Shimadzu Corporation). The resulting graphs are then analyzed using the graph analysis software PeakFit (Hulinks) to separate the peaks, and the ratio of Type I to Type II crystals is calculated from the area ratio. For peak separation, Type I and Type II crystals are distinguished based on the diffraction angles shown below, and the ratio is calculated using the following formula. Crystal type I: 2θ=14.7°, 16.5°, 22.5° Crystal type II: 2θ=12.3°, 20.2°, 21.9° Cellulose type II ratio = (Crystal type II area) / (Crystal type I area + Crystal type II area) × 100

[0027] (Anionic functional group) Next, anionic functional groups are introduced into the cellulose having 70-100% of the obtained cellulose type II crystal structure to obtain anionically modified cellulose. Examples of anionic functional groups include carboxyl groups, carboxymethyl groups, and phosphate groups. Methods for introducing anionic functional groups into cellulose include introducing anionic functional groups into the pyranose ring of cellulose by carboxylation (oxidation) or substitution reaction. In this specification, carboxylation (oxidation) reaction refers to a reaction in which the hydroxyl group of the pyranose ring is directly oxidized to a carboxyl group. In this specification, substitution reaction refers to a reaction in which anionic functional groups are introduced into the pyranose ring by a substitution reaction other than carboxylation (oxidation).

[0028] (carboxylation) Carboxylated cellulose (also called oxidized cellulose) can be obtained by carboxylating (oxidizing) cellulose using known methods. Although not particularly limited, it is preferable to adjust the amount of carboxyl groups to 0.6 to 2.5 mmol / g relative to the oven-dry mass of anionically modified cellulose microfibers during carboxylation, and more preferably to 1.0 mmol / g to 2.3 mmol / g.

[0029] One example of a carboxylation (oxidation) method is to oxidize cellulose in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromide, iodide, or a mixture thereof. This oxidation reaction selectively oxidizes the primary hydroxyl group at the C6 position of the glucopyranose ring on the surface of the cellulose, resulting in the formation of an aldehyde group and a carboxyl group (-COOH) or carboxylate group (-COOH) on the surface. - Cellulose fibers having the following characteristics can be obtained. The concentration of cellulose during the reaction is not particularly limited, but 5% by mass or less is preferred.

[0030] An N-oxyl compound is a compound that can generate a nitroxyl radical. Any compound that promotes the desired oxidation reaction can be used as an N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO).

[0031] The amount of N-oxyl compound used is not particularly limited, as long as it is a catalytic amount that can oxidize the raw material cellulose. For example, 0.01 to 10 mmol is preferred, 0.01 to 1 mmol is more preferred, and 0.05 to 0.5 mmol is even more preferred, per 1 g of oven-dried cellulose. Also, about 0.1 to 4 mmol / L relative to the reaction system is preferable.

[0032] Bromides are compounds containing bromine, and examples include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, and examples include alkali metal iodides. The amount of bromide or iodide used can be selected within a range that promotes the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol per 1 g of oven-dried cellulose.

[0033] As an oxidizing agent, known substances can be used, such as halogens, hypohalous acids, halogenous acids, perhalous acids or their salts, halogen oxides, and peroxides. Among these, sodium hypochlorite is preferred because it is inexpensive and has a low environmental impact. The appropriate amount of oxidizing agent to use is, for example, 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and most preferably 3 to 10 mmol per 1 g of oven-dried cellulose. Also, for example, 1 to 40 mol per 1 mol of N-oxyl compound is preferred.

[0034] The cellulose oxidation process can proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, and can also be room temperature of about 15 to 30°C. As the reaction progresses, carboxyl groups are formed in the cellulose, causing a decrease in the pH of the reaction solution. To ensure the oxidation reaction proceeds efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at 8 to 12, preferably 10 to 11. Water is preferred as the reaction medium due to its ease of handling and the low likelihood of side reactions.

[0035] The reaction time in an oxidation reaction can be set appropriately according to the degree of oxidation, and is usually 0.5 to 6 hours, for example, 0.5 to 4 hours.

[0036] Furthermore, the oxidation reaction may be carried out in two stages. For example, by filtering out the oxidized cellulose after the first stage of the reaction and then oxidizing it again under the same or different reaction conditions, the oxidation can be carried out efficiently without being inhibited by the salt produced as a by-product in the first stage of the reaction.

[0037] Another example of a carboxylation (oxidation) method involves contacting cellulose with an ozone-containing gas. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring, and also causes decomposition of the cellulose chain. The ozone concentration in the ozone-containing gas is 50-250 g / m³. 3 Preferably, it is 50-220 g / m² 3It is more preferable that the following conditions are met. The amount of ozone added to cellulose is preferably 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass, when the solid content of cellulose is 100 parts by mass. The ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, and is preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, it is possible to prevent excessive oxidation and decomposition of cellulose, and the yield of oxidized cellulose is good. After ozone treatment, a follow-up oxidation treatment may be performed using an oxidizing agent. The oxidizing agent used for the follow-up oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, as well as oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, these oxidizing agents can be dissolved in water or a polar organic solvent such as alcohol to create an oxidizing agent solution, and the follow-up oxidation treatment can be performed by immersing cellulose in the solution.

[0038] The amount of carboxyl groups in oxidized cellulose can be adjusted by controlling the reaction conditions, such as the amount of oxidizing agent added and the reaction time.

[0039] (carboxymethylation) Carboxymethylated cellulose can be obtained by carboxymethylating cellulose using a known method. Alternatively, commercially available carboxymethylated cellulose may be used. In either case, it is preferable that the degree of carboxymethyl group substitution per anhydrous glucose unit of cellulose is 0.01 to 0.50. An example of a method for producing such carboxymethylated cellulose is as follows: Cellulose is used as the starting material, and 3 to 20 times the mass of water and / or lower alcohols, specifically water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol, etc., are used as the solvent, either individually or as a mixture of two or more. When lower alcohols are mixed, the mixing ratio of the lower alcohols is 60 to 95% by mass. As a mercing agent, 0.5 to 20 times the molar amount of alkali metal hydroxide per anhydrous glucose residue of the starting material is used, specifically sodium hydroxide and potassium hydroxide. The starting material, solvent, and mercerizing agent are mixed, and the mercerizing treatment is carried out at a reaction temperature of 0 to 70°C, preferably 10 to 60°C, and for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. After that, a carboxymethylating agent is added at a rate of 0.05 to 10.0 molars per glucose residue, and the etherification reaction is carried out at a reaction temperature of 30 to 90°C, preferably 40 to 80°C, and for a reaction time of 30 minutes to 10 hours, preferably 1 hour to 4 hours.

[0040] In this specification, "carboxymethylated cellulose," a type of anionically modified cellulose used in the preparation of cellulose microfibers having anionic functional groups, refers to a material in which at least a portion of its fibrous shape is maintained even when dispersed in water. Therefore, it is distinguished from carboxymethylcellulose, a type of water-soluble polymer described later. When an aqueous dispersion of "carboxymethylated cellulose" is observed with an electron microscope, fibrous material can be observed. On the other hand, when an aqueous dispersion of carboxymethylcellulose, a type of water-soluble polymer, is observed, no fibrous material is observed. Furthermore, when "carboxymethylated cellulose" is measured by X-ray diffraction, peaks of cellulose type I crystals or cellulose type II crystals can be observed, but cellulose type I and cellulose type II crystals are not observed in carboxymethylcellulose, a water-soluble polymer.

[0041] (Esterification) Esterified cellulose can be obtained by mixing cellulose with powder or aqueous solution of phosphate compound A, or by adding an aqueous solution of phosphate compound A to a cellulose slurry.

[0042] Examples of phosphate compounds A include phosphoric acid, polyphosphate, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, or esters thereof. These may also be in salt form. Among these, compounds having a phosphate group are preferred because they are low-cost, easy to handle, and can improve defibration efficiency by introducing a phosphate group into the cellulose of pulp fibers. Examples of compounds having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, and ammonium metaphosphate. These can be used individually or in combination of two or more. Of these, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are more preferred from the viewpoint of high efficiency in introducing a phosphate group, ease of defibration in the defibration process described below, and ease of industrial application. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Furthermore, it is preferable to use the phosphate compound A as an aqueous solution because this increases the uniformity of the reaction and the efficiency of phosphate group introduction. The pH of the aqueous solution of phosphate compound A is preferably 7 or less because it increases the efficiency of phosphate group introduction, but a pH of 3 to 7 is preferable from the viewpoint of suppressing hydrolysis of pulp fibers.

[0043] The following method is an example of a method for producing phosphate-esterified cellulose. A phosphate compound A is added to a dispersion of cellulose with a solid content concentration of 0.1 to 10% by mass while stirring to introduce phosphate groups into the cellulose. When the amount of cellulose is 100 parts by mass, the amount of phosphate compound A added is preferably 0.2 to 500 parts by mass, and more preferably 1 to 400 parts by mass, in terms of phosphorus element content. If the proportion of phosphate compound A is above the lower limit, the yield of fine cellulose fibers can be further improved. However, if it exceeds the upper limit, the effect of improving the yield plateaus, which is undesirable from a cost perspective.

[0044] In this process, in addition to cellulose and phosphate compound A, powder or aqueous solution of compound B other than A may be mixed. Compound B is not particularly limited, but a nitrogen-containing compound exhibiting basicity is preferred. Here, "basicity" is defined as the aqueous solution exhibiting a pink to red color in the presence of phenolphthalein indicator, or the pH of the aqueous solution being greater than 7. The nitrogen-containing compound exhibiting basicity used in this invention is not particularly limited as long as it achieves the effects of the present invention, but a compound having an amino group is preferred. Examples include, but are not particularly limited, urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, urea is preferred because it is low-cost and easy to handle. The amount of compound B added is preferably 2 to 1000 parts by mass, and more preferably 100 to 700 parts by mass, per 100 parts by mass of solid content of cellulose. The reaction temperature is preferably 0 to 95°C, and more preferably 30 to 90°C. The reaction time is not particularly limited, but is approximately 1 to 600 minutes, with 30 to 480 minutes being more preferable. When the esterification reaction conditions are within this range, it is possible to prevent the cellulose from being excessively esterified and becoming easily soluble, resulting in a good yield of phosphate-esterified cellulose. After dehydrating the obtained phosphate-esterified cellulose suspension, it is preferable to heat-treat it at 100 to 170°C from the viewpoint of suppressing hydrolysis of cellulose. Furthermore, it is preferable to heat it at 130°C or lower, preferably 110°C or lower, while water is present during the heat treatment, and then heat-treat it at 100 to 170°C after removing the water.

[0045] The degree of phosphate group substitution per glucose unit in phosphate-esterified cellulose is preferably 0.001 to 0.40. Introducing phosphate substituents to cellulose causes electrical repulsion between the cellulose molecules. Therefore, cellulose with introduced phosphate groups can be easily defibrillated. If the degree of phosphate group substitution per glucose unit is less than 0.001, sufficient defibrillation is not possible. On the other hand, if the degree of phosphate group substitution per glucose unit is greater than 0.40, swelling or dissolution may occur, making it impossible to obtain fine cellulose fibers. To efficiently defibrillate, it is preferable to boil the phosphate-esterified cellulose obtained above and then wash it with cold water. These modifications due to esterification are modifications due to substitution reactions. The degree of substitution in esterified cellulose and the degree of substitution when the same esterified cellulose is pulverized are usually the same.

[0046] Phosphate-esterified cellulose can also be described as cellulose into which an anionic group, such as a phosphorus oxoacid group or a substituent derived from a phosphorus oxoacid group (sometimes simply referred to as "phosphorus oxoacid group"), has been introduced. Here, the phosphorus oxoacid group is a divalent functional group, for example, obtained by removing a hydroxyl group from phosphoric acid, and is specifically represented as -PO3H2. Substituents derived from the phosphorus oxoacid group include substituents such as salts of the phosphorus oxoacid group and phosphorus oxoacid ester groups. Note that substituents derived from the phosphorus oxoacid group may also be contained in cellulose nanofibers as groups formed by the condensation of a phosphoric acid group (e.g., a pyrophosphate group). Furthermore, the phosphorus oxoacid group may be, for example, a phosphonic acid group, and substituents derived from the phosphorus oxoacid group may be salts of the phosphonic acid group, etc.

[0047] The amount of phosphorus oxoacid groups, such as phosphate groups or phosphite groups, introduced into the phosphated cellulose is not particularly limited. When phosphated esterification is performed, it is preferable to adjust the amount of phosphorus oxoacid groups to 0.5 to 5 mmol / g relative to the oven-dry mass of the anionically modified cellulose microfibers, and it is even more preferable to adjust it to 0.5 to 2.5 mmol / g.

[0048] (Fibreation) The apparatus used for defibration of anionically modified cellulose is not particularly limited, but high-speed rotary, colloidal mill, high-pressure, roll mill, and ultrasonic devices can be used. It is preferable to apply a strong shear force to the aqueous dispersion of anionically modified cellulose during defibration. In particular, to efficiently defibrate, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying a pressure of 50 MPa or more to the aqueous dispersion and applying a strong shear force. The pressure is more preferably 100 MPa or higher, and even more preferably 140 MPa or higher. Furthermore, prior to defibration and dispersion in the high-pressure homogenizer, the aqueous dispersion may be pre-treated using known mixing, stirring, emulsifying, and dispersion devices such as a high-speed shear mixer, if necessary.

[0049] In the present invention, a sinterable inorganic material-containing composition slurry (hereinafter also simply referred to as "slurry") may be produced using an aqueous dispersion of anionically modified cellulose microfibers (hereinafter also simply referred to as "cellulose microfibers") obtained by defibrating anionically modified cellulose, without going through a drying process. Alternatively, a sinterable inorganic material-containing composition slurry may be produced using a redispersion obtained by drying and pulverizing this aqueous dispersion to obtain a powder of cellulose microfibers, which is then redispersed in water.

[0050] (viscosity) From the viewpoint of handling, the cellulose microfibers used in the present invention preferably have a viscosity (at 60 rpm) of 1 mPa·s to 50 mPa·s at 25°C when dispersed in a 1% by mass aqueous solution, and more preferably 3 mPa·s to 30 mPa·s. If the viscosity is too high, mixing with the sinterable inorganic material may be insufficient, and if the viscosity is too low, the shape retention may decrease.

[0051] (Degree of polymerization) The cellulose microfibers used in this invention preferably have a degree of polymerization of 30 to 300, and more preferably 50 to 200, from the viewpoint of shape retention and stability. The degree of polymerization can be adjusted by changing the raw materials and manufacturing conditions of the cellulose microfibers.

[0052] (Dispersant) When using cellulose microfibers that have undergone processes such as drying and pulverization in the present invention, they may contain a dispersant from the viewpoint of improving redispersibility. Examples of dispersants include water-soluble polymers and surfactants. It is preferable to use water-soluble polymers because they cover the areas with low charge density on the surface of the cellulose microfibers, suppressing the formation of hydrogen bonds and preventing aggregation of cellulose microfibers during drying.

[0053] (Water-soluble polymer) In the present invention, water-soluble polymers include, for example, cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginate, pullulan, starch, potato starch, kudzu starch, modified starch (cationized starch, phosphorylated starch, phosphate cross-linked starch, phosphate monoesterified phosphate cross-linked starch, hydroxypropyl starch, hydroxypropylated phosphate cross-linked starch, acetylated adipate cross-linked starch, acetylated phosphate cross-linked starch, acetylated oxidized starch, sodium octenyl succinate starch, acetic acid starch, oxidized starch), corn starch, gum arabic, and gellan. Examples include gum, polydextrose, pectin, chitin, water-soluble chitin, chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol, polyacrylamide, sodium polyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acids, polylactic acid, polymalic acid, polyglycerin, latex, rosin-based sizing agents, petroleum resin-based sizing agents, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide / polyamine resin, polyethyleneimine, polyamine, plant gum, polyethylene oxide, hydrophilic crosslinked polymer, polyacrylate salt, starch-polyacrylic acid copolymer, tamarind gum, guar gum, and colloidal silica, as well as mixtures of one or more thereof. Among these, cellulose derivatives are preferred in terms of compatibility with cellulose microfibers, and carboxymethylcellulose and its salts are particularly preferred. Water-soluble polymers such as carboxymethylcellulose and its salts are thought to improve redispersibility by penetrating between cellulose microfibers and increasing the distance between them.

[0054] When using carboxymethylcellulose or its salts as the water-soluble polymer, it is preferable to use carboxymethylcellulose with a carboxymethyl group substitution degree of 0.55 to 1.6 per anhydrous glucose unit, more preferably 0.55 to 1.1, and even more preferably 0.65 to 1.1. Furthermore, longer molecules (higher viscosity) are preferable because they have a greater effect in widening the distance between fine fibers. The B-type viscosity of a 1% by mass aqueous solution of carboxymethylcellulose at 25°C and 600 rpm is preferably 3 to 14000 mPa·s, more preferably 7 to 14000 mPa·s, and even more preferably 1000 to 8000 mPa·s. Note that "carboxymethylcellulose or its salts" as the water-soluble polymer referred to here is completely soluble in water and is therefore distinguished from the carboxymethylated cellulose whose fiber shape can be observed in water as described above.

[0055] In the present invention, when a dispersant is included in the cellulose microfiber powder, the mixing ratio of cellulose microfibers (absolutely dry solids) to the dispersant is preferably 2:8 to 8:2, and more preferably 5:5 to 7:3, from the viewpoint of obtaining an effect of improving redispersibility. If the mixing ratio of the dispersant is too high compared to the above upper limit, problems such as a decrease in viscosity characteristics such as thixotropy, which is a characteristic of cellulose microfibers, and a decrease in dispersion stability may occur. If the mixing ratio of the dispersant is too low compared to the above lower limit, sufficient redispersibility cannot be obtained.

[0056] (Method for producing powder) The method for producing the cellulose microfiber-containing powder that can be used in the present invention is not particularly limited. For example, one method involves dehydrating and drying an aqueous suspension containing cellulose microfibers and a dispersant as needed to obtain a dry solid, then pulverizing it with a pulverizer or the like, classifying the resulting pulverized material as needed, and recovering it to obtain the powder. Another method involves spray-drying the aqueous suspension to simultaneously dehydrate, dry, and pulverize it, classifying it as needed, and recovering it to obtain the powder. Examples of spraying methods include atomizers, single-fluid nozzles, and two-fluid nozzles.

[0057] Furthermore, from the viewpoint of ensuring good redispersibility, it is preferable to adjust the pH of the aqueous suspension containing cellulose microfibers to 7-12, more preferably 7.5-11, using an alkali such as sodium hydroxide, sodium bicarbonate, or potassium hydroxide before dehydrating and drying it.

[0058] While it is difficult to generalize about spray drying conditions as they vary depending on the spraying method, one approach is to adjust the treatment conditions such as the concentration and viscosity of the liquid used for drying, spray pressure, temperature, amount of drying air, and liquid flow rate, create a calibration curve showing the relationship between treatment conditions and particle size, and then determine the appropriate treatment conditions based on the calibration curve to achieve the desired average particle size.

[0059] Any conventionally known method can be used to dehydrate and dry an aqueous suspension containing cellulose microfibers, such as spray drying, freeze-drying, pressing, air drying, hot air drying, and vacuum drying.

[0060] The method for obtaining a pulverized product by pulverizing a dried solid containing cellulose microfibers is not particularly limited. For example, the pulverized product can be obtained by pulverizing the dried solid containing cellulose microfibers with a pulverizer.

[0061] The pulverized material containing cellulose fine fibers may be classified as needed. If classification is performed, it can be done using a classification device that is attached to or mounted on the pulverizer.

[0062] The type of equipment used to recover the pulverized material containing cellulose fine fibers and the granulated material obtained by spray drying is not particularly limited. Examples of recovery equipment include cyclones and bag filters.

[0063] The powder containing cellulose microfibers (hereinafter also referred to as "cellulose microfiber-containing powder") may be classified after recovery.

[0064] (moisture content) The cellulose fine fiber-containing powder that can be used in the present invention has a moisture content of 20% by mass or less, preferably 0 to 15% by mass, more preferably 0 to 10% by mass, and even more preferably 0 to 9.5% by mass, from the viewpoint of transportability and storage. If the moisture content is too high above the above upper limit, there is a problem that the possibility of spoilage increases. It may also be dried to a moisture content of 0% (absolutely dry).

[0065] (Average particle size) The cellulose fine fiber-containing powder that can be used in the present invention has an average particle diameter of less than 45 μm, preferably 5 μm or more and less than 20 μm, from the viewpoint of fluidity and uniformity of the sinterable inorganic material-containing composition slurry. If the average particle diameter is too large compared to the above upper limit, there is a problem that it is difficult to mix with the inorganic material and it is difficult to achieve uniformity. Here, the average particle diameter (D50) is the particle diameter that accounts for 50% of the particles when accumulated from the minimum value in the volume-based particle diameter distribution. The particle diameter distribution can be measured using a laser diffraction / scattering particle size analyzer.

[0066] (Cumulative 10% diameter) The cellulose fine fiber-containing powder that can be used in the present invention preferably has a cumulative 10% diameter (D10) of 10 μm or less, more preferably 8 μm or less, and even more preferably 5 μm or less, from the viewpoint of water absorption and dispersibility of the cellulose fine fiber-containing powder in a sinterable inorganic material-containing composition slurry. If D10 is too large compared to the above upper limit, there is a problem that the water absorption of the cellulose fine powder in the sinterable inorganic material-containing composition slurry will be low and the dispersibility will deteriorate. Furthermore, it is preferable that it be 0.1 μm or more. Here, the cumulative 10% diameter (D10) is the particle size that accounts for 10% of the volume-based particle size distribution when accumulated from the minimum value. The particle size distribution can be measured using a laser diffraction / scattering particle size analyzer.

[0067] (Sinterable inorganic materials) Sinterable inorganic materials are materials that constitute sintered bodies, and ceramics are an example of such materials. Specific examples of ceramics include oxides such as Al2O3, BeO, and ZrO2; carbides such as TiC, ZrC, B4C, tungsten carbide (WC), and SiC; borides such as CrB and ZrB2; and nitrides such as titanium nitride, zirconium nitride, boron nitride, and aluminum nitride. These materials may be used individually or in combination of two or more.

[0068] From the viewpoint of handling, the sinterable inorganic material is preferably in powder form. When the sinterable inorganic material is in powder form, the average particle size is not particularly limited and may be, for example, 0.05 to 30 μm or 0.1 to 10 μm. Alternatively, the average particle size may be 0.05 to 3 μm, 0.1 to 2 μm, or 0.1 to 1 μm. Furthermore, the average particle size may be 0.5 to 10 μm, 1.0 to 9 μm, or 1.0 to 8 μm.

[0069] In this specification, when the sinterable inorganic material is a powder, the average particle size refers to the particle size (D50) that accounts for 50% of the volume-based particle size distribution obtained using a laser diffraction-scattering particle size analyzer, calculated by integrating the minimum value.

[0070] The sinterable inorganic material-containing composition of the present invention may further contain water, and is usually used in casting as a slurry composition containing water.

[0071] (Composition containing sinterable inorganic materials) The sinterable inorganic material-containing composition of the present invention may contain, in addition to cellulose microfibers, a sinterable inorganic material, and water as needed, that satisfy the conditions (a) to (d) described above, other auxiliary agents to the extent that they do not impair the effects of the present invention. Examples of other auxiliary agents include dispersants used in ceramic molding, plasticizers, organic solvents, preservatives, defoamers, thickeners, lubricants, and inorganic salts.

[0072] In the sinterable inorganic material-containing composition of the present invention, the amount of cellulose microfibers added per 100 parts by mass of sinterable inorganic material is less than 2.0 parts by mass in terms of solid content, preferably 0.05 to 0.9 parts by mass, more preferably 0.07 to 0.8 parts by mass, and even more preferably 0.1 to 0.7 parts by mass, from the viewpoint of processability. If the amount of cellulose microfibers added is too high, the slurry becomes excessively thick and difficult to handle, and the wet molded article obtained using such a slurry may have poor moldability. If the amount of cellulose microfibers added is too low, the dispersion state of the inorganic material in the slurry deteriorates, and problems such as cracking may occur when the wet molded article is dried.

[0073] From the viewpoint of fluidity, the viscosity of the sinterable inorganic material-containing composition of the present invention is preferably 5 to 50 Pa·s, and more preferably 10 to 40 Pa·s, at 25°C and a shear rate of 1.12 (1 / s). If the viscosity is lower than the lower limit, the fluidity of the sinterable inorganic material-containing composition slurry may be too high, making it impossible to obtain a molded body. If the viscosity is higher than the upper limit, the fluidity of the sinterable inorganic material-containing composition slurry may deteriorate significantly, making processing impossible.

[0074] Dispersants used in ceramic molding include well-known surfactants and polymeric dispersants. Examples of such dispersants include known anionic dispersants such as polycarboxylic acid-based, naphthalene sulfonic acid formalin condensate-based, and alkyl sulfonic acid-based dispersants; known nonionic dispersants such as polyethylene glycol and higher alcohol alkylene oxide-based dispersants; known cationic dispersants such as quaternary ammonium-based dispersants; and polymeric dispersants such as modified polyamide-based, modified polyurethane-based, modified polyester-based, and polyvinylpyrrolidone-based dispersants. The surfactants are preferably those that can dissolve and disperse in water. Examples include sulfonic acid-based surfactants such as sodium alkyl sulfosuccinate, sodium alkyl sulfonate, and alkyl sulfate ester salts; phosphate ester-based surfactants such as polyoxyethylene alkyl phosphate esters; nonionic surfactants such as alkylene oxide adducts of higher alcohols and alkylarylphenol alkylene oxide adducts; anionic surfactants; cationic surfactants; amphoteric surfactants; and polymeric surfactants. These may be used individually or in combination of two or more.

[0075] Furthermore, dispersants used in the aforementioned ceramic molding include natural, semi-synthetic, and synthetic dispersants. Examples of natural dispersants include pectin, carrageenan, locust bean gum, xanthan gum, gellan gum, tamarind gum, gelatin, pullulan, agar, glucomannan, gum arabic, starch, and hyaluronic acid. Examples of semi-synthetic dispersants include carboxymethyl starch, pregelatinized starch, cyclodextrin, dextrin, and hydroxypropyl starch. Examples of synthetic dispersants include polyvinyl alcohol, polyvinylpyrrolidone, carboxyvinyl polymer, sodium polyacrylate, polyethylene glycol, polyoxyethylene, and polypropylene glycol.

[0076] When the sinterable inorganic material-containing composition of the present invention contains a dispersant, the solid content of the dispersant is preferably in the range of 0.1 to 2% by mass of the total composition, and more preferably in the range of 0.3 to 1% by mass. If the dispersant content is less than the lower limit, the dispersion of the sinterable inorganic material may be insufficient, and if it is more than the upper limit, impurities may remain after firing.

[0077] When the sinterable inorganic material-containing composition of the present invention contains water, the water content is preferably in the range of 10 to 40% by mass of the total composition, and more preferably in the range of 15 to 30% by mass, from the viewpoint of processability. If the water content is less than the lower limit, the dispersion of each material in the sinterable inorganic material-containing composition slurry will be insufficient, and the fluidity of the sinterable inorganic material-containing composition slurry will deteriorate significantly, potentially making processing impossible. If the water content is more than the upper limit, the drying of the sinterable inorganic material-containing composition before firing will be insufficient, potentially leading to defects in the product after firing.

[0078] (Method for producing a sintered inorganic material-containing composition) The sinterable inorganic material-containing composition of the present invention (hereinafter sometimes simply referred to as "the composition") can be obtained by blending cellulose fine fibers and a sinterable inorganic material that satisfy the conditions (a) to (d) described above, and further blending water and other auxiliary agents as necessary, and then mixing and stirring these together.

[0079] When using cellulose microfibers in the form of an aqueous dispersion, for example, a composition can be obtained by adding and mixing water, the aqueous dispersion of cellulose microfibers, and any necessary auxiliary agents such as dispersants to a sinterable inorganic material in this order.

[0080] When using cellulose microfibers as described above, if the resulting powder, obtained by dehydrating and drying an aqueous suspension containing cellulose microfibers, is used directly without redispersing it in water, the composition can be obtained by performing a powder mixing step of mixing the cellulose microfiber powder with a sinterable inorganic material powder, and a slurry preparation step of adding water and, if necessary, a dispersant or other auxiliary agent to the mixed powder obtained in the powder mixing step to obtain a slurry. The powder mixing of cellulose microfiber powder and sinterable inorganic material powder is a highly efficient and industrially effective method. At this time, if the average particle size of the cellulose microfiber powder is too large, uniformity will be impaired, so an average particle size of less than 45 μm is preferable. The ceramic mixed slurry prepared in the slurry preparation step has better workability if it has less moisture. The cellulose microfiber powder absorbs and disperses the moisture, thereby exhibiting the functionality of the cellulose microfibers. At this time, by using type II cellulose, which has high hydrophilicity, it is possible to obtain a wet molded article with excellent moldability and shape retention, as it is uniformly dispersed even under low moisture conditions. Furthermore, the wet-molded bodies produced in this manner have a uniform composition, and excellent ceramic molded bodies (ceramic products) can be obtained after firing.

[0081] The sinterable inorganic material-containing composition of the present invention contains specific cellulose microfibers, which prevents it from becoming excessively viscous and provides excellent handling during casting. Furthermore, the wet-molded articles obtained by injecting the slurry composition into a mold exhibit excellent moldability. Moreover, by using specific cellulose microfibers instead of polyvinyl alcohol, which has been conventionally used as a binder in the ceramics field, the sinterable inorganic material-containing composition of the present invention allows for the creation of high-strength ceramic wet-molded articles with only a small amount of additive.

[0082] Furthermore, the method for producing the sinterable inorganic material-containing composition of the present invention includes a mixing step of mixing a powder containing specific cellulose fine fibers with a powder of a sinterable inorganic material, thereby enabling the production of a wet-molded article that is uniformly dispersed and has excellent moldability and shape retention.

[0083] The sinterable inorganic material-containing composition of the present invention is suitably used in slip casting. By performing slip casting using the composition of the present invention, a molded body can be obtained, and by drying and firing it, a sintered ceramic product can be obtained. Examples of ceramic products include sanitary ware and industrial structural members. [Examples]

[0084] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to these examples. Unless otherwise specified, the measurement / calculation methods for each numerical value in each example are as described in the specification. In the following description, "parts" refers to parts by mass unless otherwise specified, and "%" refers to mass percent unless otherwise specified.

[0085] (Carboxylate group amount) The amount of carboxyl groups was calculated using the following formula from the amount of sodium hydroxide consumed during the neutralization stage of the weak acid, where the change in electrical conductivity was gradual (a). This was done by preparing 60 mL of a 0.5% by mass slurry (aqueous dispersion) of carboxylated cellulose, adding 0.1 M hydrochloric acid aqueous solution to adjust the pH to 2.5, and then adding 0.05 N sodium hydroxide aqueous solution dropwise until the pH became 11, and measuring the electrical conductivity. Carboxylate group content [mmol / g carboxylated cellulose] = a [mL] × 0.05 / mass of carboxylated cellulose [g].

[0086] (Degree of polymerization) The degree of polymerization of the cellulose microfibers used in the examples and comparative examples was determined by dissolving 0.15 g of cellulose microfibers, obtained by freeze-drying an aqueous dispersion of cellulose microfibers, in 30 mL of 0.5 M copper ethylenediamine solution, and measuring the viscosity using a Canon-Fenske viscometer. Taking the viscosity of this cellulose microfiber copper ethylenediamine solution as η and the viscosity of the 0.5 M copper ethylenediamine solution as η0, the intrinsic viscosity [η] of the cellulose microfibers in the copper ethylenediamine solution and the degree of polymerization DP of the cellulose microfibers were calculated using the following formulas. Intrinsic viscosity [η] = (η / η0) / {c(1 + A × η / η0)} (where c is the cellulose microfiber concentration (g / dL), and A = 0.28) Degree of polymerization DP = Intrinsic viscosity [η] / (K × a) (where K = 5.7 × 10) -3 a=1)

[0087] (Viscosity of CNF aqueous dispersion) The viscosity of the cellulose microfibers used in the examples and comparative examples was measured using a B-type viscometer (manufactured by Eiko Seiki Co., Ltd.) at 25°C and 60 rpm after preparing a 1% by mass aqueous dispersion.

[0088] (Moisture content of CNF powder) The moisture content of the CNF powder used in the examples and comparative examples was calculated as the difference between the mass of the obtained powder after drying at 105°C for 3 hours or more (oven-dry mass) and the mass before drying.

[0089] (Particle size) The average particle size (D50) of the CNF powder used in the examples and comparative examples is the particle size that accounts for 50% of the total particle size distribution when calculated from the minimum value, and the cumulative 10% diameter (D10) is the particle size that accounts for 10% of the total particle size distribution when calculated from the minimum value.

[0090] Particle size was measured using a laser diffraction / scattering particle size analyzer (Mastersizer 3000, manufactured by Spectris Co., Ltd.) under conditions of a particle refractive index of 1.5. For the measurement, a dry powder sample was directly fed into the instrument.

[0091] (Manufacturing Example 1) (Manufacturing of cellulose microfibers) 500g (absolutely dry) of bleached coniferous wood-derived kraft pulp (Leonia DKP, cellulose type II crystalline structure ratio: 100%) was added to 50L of an aqueous solution containing 7.8g (0.05mol) of TEMPO (Sigma Aldrich) and 75.5g (0.74mol) of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. After adding 1.6L of 2M sodium hypochlorite aqueous solution to the reaction system, the oxidation reaction was started (oxidation treatment). During the reaction, the pH in the system decreased, but 3M sodium hydroxide aqueous solution was added sequentially to adjust the pH to 10. After reacting for 2 hours, the mixture was filtered through a glass filter and thoroughly washed with water to obtain carboxylated cellulose. A slurry of carboxylated cellulose was prepared by diluting this slurry with water to a concentration of 5.0% (w / v), and hydrogen peroxide was added to this slurry at a concentration of 2% (w / w) relative to the carboxylated cellulose. The pH was then adjusted to 11.3 with 3M sodium hydroxide. This slurry was left at 80°C for 2 hours to undergo hydrolysis. This was adjusted to 5.0% (w / v) with water and treated five times in an ultra-high pressure homogenizer (20°C, 140 MPa) to obtain a dispersion of carboxylated cellulose nanofibers. This was designated as cellulose fine fiber 1. The pH of the obtained dispersion (solid content concentration 1%) was 7.5. The carboxyl group content was 1.7 mmol / g, the average fiber diameter was 3 nm, the aspect ratio was 117, the proportion of cellulose type II crystal structure was 100%, the degree of polymerization was 120, and the viscosity when it was dissolved in 1% by mass water was 10 mPa·s.

[0092] (Manufacturing of dried product) As cellulose microfibers, a 5.0% by mass aqueous suspension (pH 7.5) of cellulose microfiber 1 (carboxyl group amount: 1.7 mmol / g, average fiber diameter: 3 nm, aspect ratio: 117, proportion of cellulose type II crystal structure: 100%) having an anionic functional group carboxyl group obtained in Production Example 1 was prepared. Drying was performed using a two-fluid nozzle type spray dryer (manufactured by Okawara Chemical Machinery Co., Ltd., L-8i) to obtain a powdered CNF dried product.

[0093] The obtained dried material was classified and recovered using a cyclone to obtain cellulose fine fiber 1 powder (carboxylated CNF1 powder). The moisture content of the obtained powder was 4.4% by mass, the average particle size was 8.0 μm, and the cumulative 10% diameter (D10) in the volume-based particle size distribution was 3.6 μm.

[0094] (Manufacturing example 2) (Manufacturing of cellulose microfibers) Cellulose microfibers 2 were produced by the same procedure as in Production Example 1, except that the amount of hydrogen peroxide added during the hydrolysis of carboxylated cellulose obtained by the same reaction as in Production Example 1 was 1% (w / w), and the subsequent slurry temperature was 50°C. The pH of the obtained dispersion (solid content concentration 1%) was 7.7. The carboxyl group content was 1.7 mmol / g, the average fiber diameter was 3 nm, the aspect ratio was 180, the proportion of cellulose type II crystal structure was 100%, the degree of polymerization was 272, and the viscosity when it was dispersed in 1% by mass water was 30 mPa·s.

[0095] (Manufacturing of dried product) A powder of cellulose microfiber 2 (carboxylated CNF2 powder) was obtained in the same manner as in Production Example 1, except that cellulose microfiber 2 having a carboxyl group, which is an anionic functional group obtained in Production Example 2, was used as the cellulose microfiber. The moisture content of the obtained powder was 9.5% by mass, the average particle size was 10 μm, and the cumulative 10% diameter (D10) in the volume-based particle size distribution was 5.0 μm.

[0096] (Example 1) (Preparation of a redispersion solution of cellulose microfibers) Water was added to the carboxylated CNF1 powder obtained in Production Example 1, and the mixture was stirred for 30 minutes at 3000 rpm using a homodisperser to prepare CNF redispersion solution 1 with a CNF solid content of 5% by mass.

[0097] (Preparation of casting slurry) A casting slurry with a total solid content of 77% by mass was prepared by adding and mixing 100 parts by mass of aluminum oxide powder (product name: AES-11, manufactured by Sumitomo Chemical Co., Ltd.) as a sinterable inorganic material, water, 0.5 parts by mass of a dispersant (main component: ammonium polycarboxylate salt, non-volatile content: 40%, product name: Cerna D-305, manufactured by Chukyo Oil & Fat Co., Ltd.), and 0.5 parts by mass of the CNF redispersion solution 1 prepared as described above, in this order, and stirring for 60 minutes using a ball mill (manufactured by Lechner).

[0098] (Example 2) A casting slurry was prepared in the same manner as in Example 1, except that the amount of CNF redispersion solution 1 added was changed to 0.3 parts by mass in terms of solid content.

[0099] (Example 3) A casting slurry was prepared in the same manner as in Example 1, except that the amount of CNF redispersion solution 1 added was changed to 0.1 parts by mass in terms of solid content.

[0100] (Example 4) (Preparation of casting slurry) 100 parts by mass of aluminum oxide powder (product name: AES-11, manufactured by Sumitomo Chemical Co., Ltd.) as a sinterable inorganic material and 0.5 parts by mass of carboxylated CNF1 powder obtained in Production Example 1 were pre-mixed to obtain a mixed powder. Water and 0.5 parts by mass of a dispersant (main component: ammonium polycarboxylate salt, non-volatile content: 40%, product name: Celna D-305, manufactured by Chukyo Oil & Fat Co., Ltd.) were added and mixed in that order, and the mixture was stirred for 60 minutes using a ball mill (manufactured by Lecce Co., Ltd.) to prepare a casting slurry with a total solid content of 77% by mass.

[0101] (Example 5) A casting slurry was prepared in the same manner as in Example 4, except that 0.3 parts by mass of carboxylated CNF2 powder obtained in Production Example 2 was used instead of 0.5 parts by mass of carboxylated CNF1 powder obtained in Production Example 1.

[0102] (Comparative Example 1) In preparing the casting slurry, the casting slurry was prepared in the same manner as in Example 1, except that instead of adding 0.5 parts by mass of CNF redispersion solution 1 in terms of solid content, 1 part by mass of polyvinyl alcohol (product name: Celna WF-804, manufactured by Chukyo Oil & Fat Co., Ltd.) was added.

[0103] (Comparative Example 2) The casting slurry was prepared in the same manner as in Example 1, except that the amount of CNF redispersion solution 1 added was changed to 2.0 parts by mass in terms of solid content.

[0104] (evaluation) The casting slurry obtained in each example and comparative example was poured into a plaster mold and allowed to stand for 2 hours to dry, thereby obtaining five cylindrical wet molded bodies. The obtained wet molded bodies were removed from the mold and further dried at 25°C for 3 days to obtain additionally dried wet molded bodies (also called "dried molded bodies"). The obtained dried molded bodies were fired in an electric furnace at 1500°C to obtain sintered bodies as cast molded products. The following evaluations were performed on the casting slurry, dried molded body, and sintered body obtained in each example and comparative example. The moldability was evaluated by visual inspection of the molded body. Furthermore, the casting slurry in Comparative Example 2 was found to have excessively high viscosity, resulting in poor molding; therefore, the experiment was terminated without evaluating the dried molded body. The results are shown in Table 1.

[0105] (Evaluation of casting slurry) (viscosity) The viscosity of the slurry was measured using a rheometer (model number: MCR301, manufactured by Anton Paar) at 25°C and a shear rate of 1.12 (1 / s).

[0106] (Liquidity) ○: Excellent fluidity. ×: Poor liquidity.

[0107] (Formability during casting) ○: No void was observed in the center of the cylindrical wet-molded body obtained after removing it from the plaster mold. ×: A void was observed in the center of the cylindrical wet-molded body obtained after removing it from the plaster mold. For reference, Figure 1 shows examples of what would be considered a "○" (correct) evaluation based on the above criteria, and Figure 2 shows examples of what would be considered a "×" (incorrect) evaluation, both illustrated with photographs.

[0108] (Evaluation of dried molded products) (Dry formability) ○: None of the dried molded parts showed any cracks. △: Some of the dried molded parts had cracks (1-2 cracks were observed). ×: More than half of the dried molded products had cracks (three or more products had cracks).

[0109] (Three-point bending strength test) The maximum point stress and bending strength were measured using a three-point bending strength test (JIS R1601) on dried molded and sintered products.

[0110] (Diameter shrinkage rate during firing) The diameter of the dry molded body and the diameter of the sintered body were measured, and the diameter shrinkage rate of the cylindrical molded body was determined by calculating it according to the following formula. Diameter shrinkage rate (%) = (Diameter of dry molded body - Diameter of fired body) / (Diameter of dry molded body) × 100

[0111] (Shortness rate during firing) The length of the dry molded body and the length of the sintered body were measured, and the length shrinkage rate of the cylindrical molded body was determined by calculating it according to the following formula. Length shrinkage rate (%) = (Length of dry molded body - Length of fired body) / (Length of dry molded body) × 100

[0112] (Measurement of bulk density and apparent density) The calcined body was dried in a dryer at 110°C for 24 hours, and then its mass was measured after cooling to room temperature in a desiccator. This value was defined as the dry mass. Next, the calcined body was immersed in water for 24 hours to reach a saturated state. The calcined body was then suspended in the water by wire, and its mass was measured. This value was defined as the water mass. Finally, the calcined body was removed from the water, the surface moisture was quickly wiped off with a cloth, and its mass was measured. This value was defined as the saturated mass. The bulk density and apparent density were calculated using the following formulas. Bulk density (g / cm³) 3 )=Dry mass / (saturated water mass - mass in water) Apparent density (g / cm³) 3 )=Dry mass / (Dry mass - Mass in water)

[0113] [Table 1]

[0114] Table 1 shows that using cellulose microfibers 1 and 2 allows for the production of high-strength pre-fired wet molded articles (dry molded articles) with a smaller amount of additive compared to Comparative Example 1, which uses the conventional binder polyvinyl alcohol. The articles exhibit good strength, excellent shape retention, and moldability. Furthermore, they result in ceramic molded articles with low diameter shrinkage after firing and minimal deformation during firing. Additionally, they produce ceramic molded articles with high bulk density, apparent density, and a dense structure. Compared to Comparative Example 2, which uses a large amount of cellulose microfibers, adjusting the amount of additive to create a ceramic slurry with appropriate viscosity improves moldability.

Claims

1. A sinterable inorganic material-containing composition comprising cellulose microfibers satisfying the following conditions (a) to (d), and a sinterable inorganic material, wherein the amount of cellulose microfibers added to 100 parts by mass of the sinterable inorganic material is less than 2.0 parts by mass. (a) Number-average fiber diameter of 2 nm or more and 500 nm or less (b) Average aspect ratio is between 30 and 300 (c) Having 70-100% cellulose type II crystalline structure (d) Having anionic functional groups

2. The sinterable inorganic material-containing composition according to claim 1, wherein the cellulose fine fibers, when dispersed in a 1% by mass aqueous dispersion, have a viscosity (60 rpm) at 25°C of 1 mPa·s or more and 50 mPa·s or less.

3. The sinterable inorganic material-containing composition according to claim 1, wherein the cellulose fine fibers have a degree of polymerization of 30 or more and 300 or less.

4. A powder mixing step in which a powder containing cellulose fine fibers that satisfy the following conditions (a) to (d) is mixed with a powder of a sinterable inorganic material, A method for producing a sinterable inorganic material-containing composition, comprising: a slurry preparation step of adding water to the mixed powder obtained in the powder mixing step and mixing to obtain a slurry. (a) Number-average fiber diameter of 2 nm or more and 500 nm or less (b) Average aspect ratio is between 30 and 300 (c) Having 70-100% cellulose type II crystalline structure (d) Having anionic functional groups

5. The method for producing a sinterable inorganic material-containing composition according to claim 4, wherein the powder containing the cellulose fine fibers has a moisture content of 20% by mass or less and an average particle diameter of less than 45 μm.

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