Aqueous suspension containing nitride and cellulose nanofiber, and mold wash

Abstract

To provide an aqueous suspension of nitride that is excellent in dispersion stability.SOLUTION: Provided is an aqueous suspension containing a nitride and a cellulose nanofiber.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aqueous suspension containing a nitride and cellulose nanofibers, and a mold wash containing this aqueous suspension. [Background technology]

[0002] Boron nitride, a type of nitride, is a ceramic material that has characteristics different from metals and carbonaceous materials, namely, that it is thermally conductive yet insulating, and also has lubricity, corrosion resistance, and thermal stability superior to that of graphite. Because of these characteristics, boron nitride has attracted attention as a material used in heat dissipation materials, insulating materials, mold release agents, lubricants, heat-resistant coatings, and the like.

[0003] For such boron nitride, attempts have been made to uniformly treat the surface of the boron nitride with other substances and to isolate and disperse boron nitride in a medium in order to increase its affinity with other materials such as resins without impairing its thermal conductivity and other properties as much as possible. This method typically employs a substance (dispersant) that aids in the affinity of boron nitride in the medium. Furthermore, from the standpoints of cost, toxicity, and environmental considerations, aqueous media such as water are preferred for this method.

[0004] However, boron nitride has poor wettability and is therefore very difficult to disperse in water, so when dispersing boron nitride in water, a dispersant with a large molecular weight is used to disperse boron nitride in water (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-127793 Summary of the Invention [Problem to be solved by the invention]

[0006] The boron nitride slurry obtained using the dispersant of Patent Document 1 has the problem of insufficient dispersibility. Furthermore, the insufficient dispersibility hinders the properties of boron nitride (thermal conductivity, corrosion resistance, thermal stability, etc.).

[0007] An object of the present invention is to provide an aqueous suspension of a nitride having excellent dispersion stability, and a mold wash containing the aqueous suspension.

[0008] As a result of extensive research into solving the above problems, the present inventors discovered that the above object could be achieved by using cellulose nanofibers, and thus completed the present invention. [Means for solving the problem]

[0009] That is, the present invention provides the following. (1) An aqueous suspension containing nitride and cellulose nanofibers. (2) The aqueous suspension according to (1), wherein the cellulose nanofibers are anion-modified cellulose nanofibers. (3) The aqueous suspension according to (2), wherein the anion-modified cellulose nanofibers are cellulose nanofibers having a carboxyl group or cellulose nanofibers having a carboxyalkyl group. (4) The aqueous suspension according to (3), wherein the anion-modified cellulose nanofibers have carboxymethyl groups as the carboxyalkyl groups, and are carboxymethylated cellulose nanofibers having a degree of carboxymethyl substitution of 0.01 to 0.50. (5) The aqueous suspension according to any one of (1) to (4), further comprising carboxymethyl cellulose. (6) The aqueous suspension according to any one of (1) to (5), wherein the nitride is boron nitride. (7) The aqueous suspension according to any one of (1) to (5), wherein the nitride is aluminum nitride. (8) A mold wash comprising the aqueous suspension according to any one of (1) to (7). [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an aqueous suspension of a nitride having excellent dispersion stability, and also to provide a mold wash containing this aqueous suspension. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below. In the present invention, "to" includes the extreme values. That is, "X to Y" includes the values ​​X and Y at both ends.

[0012] The aqueous suspension of the present invention comprises a nitride and cellulose nanofibers.

[0013] (nitrides) Examples of nitrides that can be used in the present invention include boron nitride, silicon nitride, titanium nitride, and aluminum nitride, and from the viewpoint of use as a mold wash, it is preferable to use boron nitride and aluminum nitride.

[0014] (boron nitride) The crystal structure of boron nitride is not particularly limited, and either hexagonal boron nitride or cubic boron nitride can be used, but the latter is produced under ultra-high pressure and high temperature conditions, so the former, hexagonal boron nitride, is preferred.

[0015] (aluminum nitride) The crystal structure of aluminum nitride is not particularly limited, and either hexagonal aluminum nitride or cubic aluminum nitride can be used, but the former, hexagonal aluminum nitride, is preferred because it is energetically more stable.

[0016] The boron nitride and aluminum nitride are not particularly limited, and any of powder, particle, fiber, tubular, and block forms can be used. The average particle size of such boron nitride and aluminum nitride is usually 5 nm to 100 μm, preferably 10 nm to 50 μm. When smoothness is required, a smaller average particle size is preferable, whereas when a continuous phase is required for insulation, heat dissipation, or the like, a larger average particle size is preferable. The average particle size of the boron nitride and aluminum nitride used can be appropriately selected depending on the application. For example, when used as a mold wash, the average particle size is preferably 0.1 to 50 μm, more preferably 0.3 to 30 μm. The average particle size of boron nitride and aluminum nitride can be measured by electron microscope observation (SEM or TEM).

[0017] (Cellulose nanofiber) In the present invention, cellulose nanofibers (CNFs) are fine fibers with a fiber diameter of approximately 3 to 500 nm, which are obtained by pulp or other cellulose raw materials being refined to the nanometer level. The average fiber diameter and average fiber length of cellulose nanofibers can be obtained by averaging the fiber diameters and fiber lengths obtained from the observation of each fiber using an atomic force microscope (AFM) or a transmission electron microscope (TEM). Cellulose nanofibers can be obtained by applying mechanical force to pulp to refine it, or by defibrating modified cellulose obtained by chemical modification, such as carboxylated cellulose (also called oxidized cellulose), carboxymethylated cellulose, cellulose with a phosphate ester group introduced, or cationized cellulose. The average fiber length and average fiber diameter of fine fibers can be adjusted by chemical modification treatment or defibration treatment.

[0018] The average aspect ratio of the cellulose nanofibers used in the present invention is usually 50 or more. There is no particular upper limit, but it is usually 1000 or less. The average aspect ratio can be calculated using the following formula: Aspect ratio = average fiber length / average fiber diameter

[0019] (cellulose raw material) Known cellulose raw materials are derived from plants (for example, wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (for example, ascidians), algae, microorganisms (for example, acetic acid bacteria (Acetobacter)), microbial products, etc.), and any of these can be used in the present invention. Cellulose fibers derived from plants or microorganisms are preferred, and plant-derived cellulose fibers are more preferred.

[0020] (anion-modified) The cellulose nanofibers used in the present invention are preferably anionically modified cellulose nanofibers, which can be obtained by defibrating anionically modified cellulose raw material. Anion modification refers to the introduction of anionic groups into cellulose, specifically the introduction of anionic groups into the pyranose ring by oxidation or substitution reaction. In the present invention, the oxidation reaction refers to a reaction in which the hydroxyl group of the pyranose ring is directly oxidized to a carboxyl group. Furthermore, in the present invention, substitution reaction refers to a reaction in which anionic groups are introduced into the pyranose ring by a substitution reaction other than the oxidation.

[0021] The anion-modified cellulose used as the raw material for anion-modified cellulose nanofibers is one that maintains at least a portion of its fibrous shape even when dispersed in water or a water-soluble organic solvent. If one that does not maintain its fibrous shape (i.e., one that dissolves in a dispersion medium) is used, nanofibers cannot be obtained. "Maintaining at least a portion of the fibrous shape when dispersed" means that a fibrous substance can be observed when a dispersion of the anion-modified cellulose is observed under an electron microscope. Furthermore, anion-modified cellulose that allows observation of a cellulose type I crystal peak when measured by X-ray diffraction is preferred. The crystallinity of the cellulose in the raw material anion-modified cellulose is preferably 50% or more, more preferably 60% or more, for crystalline type I. By adjusting the crystallinity within the above range, it is possible to obtain sufficient crystalline cellulose fibers that do not dissolve even after the fibers are finely divided by defibration. The crystallinity of cellulose type I in the anion-modified cellulose nanofiber is preferably 50 to 90%, more preferably 60 to 80%, and particularly preferably 65 to 75%. If the crystallinity is less than 50%, the dispersion effect will be reduced. The crystallinity of cellulose can be controlled by the crystallinity of the raw cellulose and the degree of anion modification. The crystallinity of anion-modified cellulose and anion-modified CNF can be measured as follows: The sample was placed in a glass cell and measured using an X-ray diffraction measurement device (LabX XRD-6000, manufactured by Shimadzu Corporation). The degree of crystallinity was calculated using the method of Segal et al., where the diffraction intensity at 2θ = 10° to 30° in the X-ray diffraction pattern was used as the baseline, and the degree of crystallinity was calculated using the following formula from the diffraction intensity of the 002 plane at 2θ = 22.6° and the diffraction intensity of the amorphous part at 2θ = 18.5°. Xc=(I002c-Ia) / I002c×100 Xc: Crystallinity of cellulose type I (%) I002c: 2θ=22.6°, diffraction intensity of the 002 plane Ia: 2θ=18.5°, diffraction intensity of the amorphous part.

[0022] (carboxylation) Carboxylated (oxidized) cellulose can be used as anion-modified cellulose. In the present invention, the carboxyl group refers to -COOH (acid type) or -COOM (salt type). Here, M is a metal ion, such as sodium or potassium. Carboxylated cellulose (also called "oxidized cellulose") can be obtained by carboxylating (oxidizing) the above-mentioned cellulose raw material using a known method. Although not particularly limited, the amount of carboxyl groups is preferably 0.6 to 3.0 mmol / g, more preferably 1.0 to 2.0 mmol / g, based on the bone-dry mass of the anion-modified cellulose nanofiber. One example of the carboxylation (oxidation) method is a method in which the cellulose raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, and mixtures thereof. This oxidation reaction selectively oxidizes the primary hydroxyl groups at the C6 position of the glucopyranose ring on the cellulose surface, leaving aldehyde groups and carboxyl groups (-COOH) or carboxylate groups (-COO) on the surface. - The cellulose concentration during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0023] An N-oxyl compound refers to a compound capable of generating a nitroxy radical. Any compound that promotes the target oxidation reaction can be used as the N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO). The amount of the N-oxyl compound used is not particularly limited, as long as it is a catalytic amount capable of oxidizing the cellulose raw material. For example, 0.01 to 10 mmol is preferred, 0.01 to 1 mmol is more preferred, and 0.01 to 0.5 mmol is even more preferred, per 1 g of bone-dry cellulose raw material. The concentration of the N-oxyl compound in the reaction system is preferably about 0.1 to 4 mmol / L.

[0024] Bromides are compounds containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, examples of which include alkali metal iodides. The amount of bromide or iodide used can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is, for example, 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 bone-dry cellulose raw material. The modification is a modification due to an oxidation reaction.

[0025] Known oxidizing agents can be used, such as halogens, hypohalous acids, halous acids, perhalogen acids or their salts, halogen oxides, and peroxides. Among these, sodium hypochlorite is preferred because it is inexpensive and environmentally friendly. The appropriate amount of oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, and even more preferably 2.5 to 25 mmol, per 1 g of bone-dry cellulose raw material. Furthermore, for example, 1 to 40 mol is preferred per 1 mol of the N-oxyl compound.

[0026] The oxidation process of cellulose raw materials can proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, or may be room temperature, about 15 to 30°C. As the reaction proceeds, carboxyl groups are generated in the cellulose, causing the pH of the reaction solution to decrease. To efficiently proceed with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to the reaction system as needed to maintain the pH of the reaction solution at about 9 to 12, preferably about 10 to 11. Water is preferred as the reaction medium because it is easy to handle and does not easily cause side reactions. The reaction time in the oxidation reaction can be appropriately set depending on the degree of oxidation progress and is usually 0.5 to 6 hours, for example, about 0.5 to 4 hours.

[0027] Alternatively, the oxidation reaction may be carried out in two stages. For example, the oxidized cellulose obtained by filtration after the first stage of the reaction can be oxidized again under the same or different reaction conditions, thereby efficiently introducing carboxyl groups into the cellulose raw material without reaction inhibition by salts produced as by-products in the first stage of the reaction.

[0028] The amount of carboxyl groups in the oxidized cellulose can be adjusted by controlling the reaction conditions, such as the amount of oxidizing agent added, reaction time, etc. It is preferable that the amount of carboxyl groups in the anion-modified cellulose nanofibers is the same as the amount of carboxyl groups when the cellulose nanofibers are formed.

[0029] In the present invention, in the oxidized cellulose obtained by the above process, the carboxyl groups introduced into the cellulose raw material are usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the defibration step, the alkali metal salt of the oxidized cellulose may be substituted with another cationic salt such as a phosphonium salt, an imidazolinium salt, an ammonium salt, or a sulfonium salt. The substitution can be carried out by a known method.

[0030] (carboxyalkylation) Preferred anionic groups include carboxyalkyl groups such as carboxymethyl groups. In the present invention, the carboxyalkyl group refers to -RCOOH (acid type) or -RCOOM (salt type). Here, R is an alkylene group such as a methylene group or an ethylene group, and M is a metal ion. Carboxyalkylated cellulose may be obtained by a known method, or a commercially available product may be used. The degree of carboxyalkyl substitution per anhydroglucose unit of cellulose is preferably 0.50 or less. Furthermore, when the anionic group is a carboxymethyl group, the degree of carboxymethyl substitution is preferably 0.50 or less. If the degree of substitution is greater than 0.50, crystallinity decreases and the proportion of soluble components increases, resulting in loss of nanofiber functionality. Furthermore, the lower limit of the degree of carboxyalkyl substitution is preferably 0.01 or more. Considering operability, the degree of substitution is particularly preferably 0.02 to 0.50, and more preferably 0.10 to 0.30. An example of a method for producing such carboxyalkylated cellulose includes the following steps: The modification is a substitution reaction. The following describes carboxymethylated cellulose as an example. i) mixing the starting material with a solvent and a mercerizing agent, and subjecting the mixture to mercerization at a reaction temperature of 0 to 70°C, preferably 10 to 60°C, for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours; ii) Subsequently, a step of adding a carboxymethylating agent in an amount of 0.05 to 10.0 times the moles per glucose residue, and carrying out an etherification reaction at a reaction temperature of 30 to 90°C, preferably 40 to 80°C, for a reaction time of 30 minutes to 10 hours, preferably 1 hour to 4 hours.

[0031] The above-mentioned cellulose raw material can be used as the starting material. As the solvent, 3 to 20 times by mass of water or a lower alcohol, specifically water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol, etc., can be used alone or in combination. When a lower alcohol is mixed, the mixing ratio is 60 to 95% by mass. As the mercerizing agent, 0.5 to 20 times by mole of an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, can be used per anhydrous glucose residue of the starting material.

[0032] As mentioned above, the degree of carboxymethyl substitution per glucose unit of cellulose is less than 0.04, and preferably 0.01 or more and less than 0.60. Introducing carboxymethyl substituents into cellulose causes electrical repulsion between cellulose molecules. Therefore, cellulose with carboxymethyl substituents introduced can be easily nanofibrillated. Note that if the carboxymethyl substituents per glucose unit are less than 0.02, nanofibrillation may be insufficient. The degree of substitution in anion-modified cellulose nanofibers is usually the same as the degree of substitution when the cellulose nanofibers are formed.

[0033] In the present invention, in the carboxyalkylated cellulose obtained by the above process, the carboxyalkyl group introduced into the cellulose raw material is usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the defibration process, the alkali metal salt of the carboxyalkylated cellulose may be substituted with another cation salt such as a phosphonium salt, an imidazolinium salt, an ammonium salt, or a sulfonium salt. The substitution can be carried out by a known method.

[0034] In this specification, "carboxymethylated cellulose," a type of anion-modified cellulose used in preparing cellulose nanofibers, refers to cellulose that maintains at least a portion of its fibrous shape when dispersed in water. Therefore, it is distinguished from carboxymethyl cellulose, a type of water-soluble polymer. When an aqueous dispersion of "carboxymethylated cellulose" is observed under an electron microscope, a fibrous substance can be observed. On the other hand, when an aqueous dispersion of carboxymethyl cellulose, a type of water-soluble polymer, is observed, no fibrous substance can be observed. Furthermore, when "carboxymethylated cellulose" is measured by X-ray diffraction, a peak corresponding to cellulose type I crystals can be observed, whereas cellulose type I crystals are not observed in the water-soluble polymer carboxymethyl cellulose.

[0035] (esterification) Esterified cellulose can also be used as anion-modified cellulose. Examples of methods include mixing a powder or aqueous solution of phosphoric acid compound A with a cellulose raw material, or adding an aqueous solution of phosphoric acid compound A to a slurry of the cellulose raw material. Examples of phosphoric acid compound A include phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, or esters of these. These may be in the form of salts. Among the above, compounds containing phosphoric acid groups are preferred because of their low cost, ease of handling, and the ability to introduce phosphoric acid groups into the cellulose of pulp fibers to improve defibration efficiency. Examples of compounds containing phosphoric acid groups include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, 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. Phosphate groups can be introduced using one or more of these compounds. Among these, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are preferred from the viewpoints of high efficiency of phosphate group introduction, ease of defibration in the defibration step described below, and ease of industrial application. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Furthermore, it is desirable to use the phosphoric acid compound A as an aqueous solution, as this allows the reaction to proceed uniformly and increases the efficiency of phosphate group introduction. The pH of the aqueous solution of phosphoric acid compound A is preferably 7 or less, as this increases the efficiency of phosphate group introduction, but a pH of 3 to 7 is preferred from the viewpoint of suppressing hydrolysis of pulp fibers.

[0036] The following method can be mentioned as an example of a method for producing phosphated cellulose. A phosphoric acid compound A is added to a suspension of a cellulose raw material having a solids concentration of 0.1 to 10% by mass while stirring, to introduce phosphate groups into the cellulose. When the cellulose raw material is taken as 100 parts by mass, the amount of phosphoric acid compound A added is preferably 0.2 to 500 parts by mass, more preferably 1 to 400 parts by mass, in terms of elemental phosphorus. When the proportion of phosphoric acid compound A is equal to or greater than the lower limit, the yield of fine fibrous cellulose can be further improved. However, when the proportion exceeds the upper limit, the yield improvement effect plateaus, which is undesirable from a cost perspective.

[0037] In addition to the phosphoric acid compound A, a powder or aqueous solution of compound B may be mixed. Compound B is not particularly limited, but is preferably a nitrogen-containing compound exhibiting basicity. "Basicity" here is defined as an aqueous solution exhibiting a pink to red color in the presence of a phenolphthalein indicator, or a pH of greater than 7. The nitrogen-containing compound exhibiting basicity used in the present invention is not particularly limited as long as it exhibits the effects of the present invention, but is preferably a compound having an amino group. Examples include urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, urea is preferred because of its low cost and ease of handling. The amount of compound B added is preferably 2 to 1,000 parts by mass, more preferably 100 to 700 parts by mass, per 100 parts by mass of the solid content of the cellulose raw material. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. The reaction time is not particularly limited, but is approximately 1 to 600 minutes, more preferably 30 to 480 minutes. When the esterification reaction conditions are within these ranges, it is possible to prevent the cellulose from being excessively esterified and becoming more soluble, resulting in a good yield of phosphated cellulose. After dehydrating the resulting phosphated cellulose suspension, it is preferable to heat-treat it at 100 to 170°C in order to suppress hydrolysis of the cellulose. Furthermore, it is preferable to heat the suspension at 130°C or lower, preferably 110°C or lower, while it contains water, and then, after removing the water, heat-treat it at 100 to 170°C.

[0038] The degree of phosphate substitution per glucose unit of the phosphate-esterified cellulose is preferably 0.001 or more and less than 0.40. Introducing phosphate group substituents into cellulose causes electrical repulsion between cellulose units. Therefore, cellulose with phosphate groups introduced can be easily nano-fibrillated. If the degree of phosphate substitution per glucose unit is less than 0.001, nano-fibrillation is not sufficient. On the other hand, if the degree of phosphate substitution per glucose unit is greater than 0.40, the cellulose may swell or dissolve, making it impossible to obtain nanofibers. To achieve efficient fibrillation, the phosphate-esterified cellulose raw material obtained above is preferably boiled and then washed with cold water. The modification by esterification is a modification by substitution reaction. The degree of substitution in anion-modified cellulose nanofibers is preferably the same as the degree of substitution when the cellulose nanofibers are obtained.

[0039] In the present invention, in the cellulose phosphate obtained by the above process, the phosphate group introduced into the cellulose raw material is usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the defibration step, the alkali metal salt of the cellulose phosphate may be substituted with another cationic salt such as a phosphonium salt, an imidazolinium salt, an ammonium salt, or a sulfonium salt. The substitution can be carried out by a known method.

[0040] (defibration) In the present invention, the device for defibrating anionically modified cellulose is not particularly limited. However, it is preferable to apply a strong shear force to the aqueous dispersion of anionically modified cellulose using a device such as a high-speed rotary device, colloid mill device, high-pressure device, roll mill device, or ultrasonic device. In particular, for efficient defibration, it is preferable to apply a pressure of 50 MPa or more to the aqueous dispersion and to use a wet high-pressure or ultra-high-pressure homogenizer capable of applying a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. Furthermore, prior to defibration and dispersion treatment with a high-pressure homogenizer, the CNF can be pretreated, if necessary, using a known mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer. The number of treatments (passes) in the defibration device may be one or two or more times, with two or more being preferred.

[0041] In the dispersion treatment, anionically modified cellulose is usually dispersed in a solvent. The solvent is not particularly limited as long as it can disperse anionically modified cellulose, but examples include water, organic solvents (e.g., hydrophilic organic solvents such as methanol), and mixtures thereof. Since the cellulose raw material is hydrophilic, the solvent is preferably water.

[0042] The solids concentration of the anion-modified cellulose in the dispersion is usually 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. This ensures an appropriate amount of liquid relative to the amount of cellulose fiber raw material, which is efficient. The upper limit is usually 10% by mass or less, preferably 6% by mass or less. This allows fluidity to be maintained.

[0043] Prior to the defibration treatment or dispersion treatment, a pretreatment may be carried out as necessary. The pretreatment may be carried out using a mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer.

[0044] When the anion-modified cellulose nanofibers obtained through the defibration treatment are in the salt form, they may be used as they are, or may be converted into the acid form by acid treatment using a mineral acid, a method using a cation exchange resin, etc. Furthermore, they may be made hydrophobic by a method using a cationic additive.

[0045] The cellulose nanofibers used in the present invention may be an aqueous dispersion of anion-modified cellulose nanofibers obtained through the above-mentioned defibration treatment, or may be a powder obtained by drying and pulverizing the cellulose nanofibers, or may be redispersed in an aqueous solvent such as water.

[0046] From the viewpoint of imparting shape retention, the cellulose nanofibers used in the present invention preferably have a Brookfield viscosity of 1000 to 8400 mPa·s, more preferably 2000 to 7000 mPa·s, at a solids concentration of 1%, at 60 rpm, and at 25°C.

[0047] The carboxymethylated cellulose nanofibers used in the present invention preferably have a viscosity of 6.0 or more when dispersed in water at 1% (w / v) solids at 25°C and 6 rpm, divided by the viscosity of 1% (w / v) solids at 25°C and 60 rpm (also simply referred to as "viscosity at 6 rpm divided by viscosity at 60 rpm"). A higher viscosity indicates a greater change in viscosity due to differences in shear stress, indicating higher thixotropy. Cellulose nanofibers with high thixotropy are suitable for use as shape-retention agents and viscosity modifiers. There is no upper limit to the viscosity at 6 rpm divided by the viscosity at 60 rpm, but in practice, the upper limit is thought to be around 15.0. The viscosity of carboxymethylated cellulose nanofibers at 6 rpm (aqueous dispersion with 1% (w / v) solids content, 25°C) is preferably 15,000 mPa·s or higher, and more preferably 20,000 mPa·s or higher. The higher the viscosity at a low shear rate (6 rpm), the greater the likelihood of thixotropy. There is no particular upper limit to the viscosity at 6 rpm, but in reality it is thought to be around 50,000 mPa·s. The viscosity of the carboxymethylated cellulose nanofiber at 60 rpm (aqueous dispersion with a solid content of 1% (w / v), 25°C) is preferably about 1000 to 8400 mPa·s, more preferably about 2000 to 7000 mPa·s, even more preferably about 2500 to 7000 mPa·s, and even more preferably about 3000 to 7000 mPa·s.

[0048] (aqueous suspension) The aqueous suspension of the present invention is obtained by dispersing nitrides and cellulose nanofibers as essential components in an aqueous solvent. The aqueous solvent used in the present invention is preferably water, a water-soluble organic solvent, or a mixture thereof. In consideration of the dispersibility of chemically modified pulp and CNF, the aqueous solvent is preferably water or a mixture of water and a water-soluble organic solvent. Furthermore, from the viewpoints of cost, environmental friendliness, and safety, it is more preferable to use water as the aqueous solvent.

[0049] The water-soluble organic solvent is an organic solvent that dissolves in water. Examples include methanol, ethanol, 2-propanol, butanol, glycerin, acetone, methyl ethyl ketone, 1,4-dioxane, N-methyl-2-pyrrolidone, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetonitrile, and combinations thereof. Among these, lower alcohols having 1 to 4 carbon atoms, such as methanol, ethanol, and 2-propanol, are preferred. From the viewpoints of safety and availability, methanol and ethanol are more preferred, and ethanol is even more preferred. The amount of the water-soluble organic solvent in the mixed solvent is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. There is no upper limit to this amount, but it is preferably 95% by mass or less, more preferably 90% by mass or less. Furthermore, the aqueous solvent may contain a non-water-soluble organic solvent to the extent that the effects of the invention are not impaired.

[0050] In the aqueous suspension of the present invention, the blending ratio of each component is not particularly limited, but the content of nitride in the aqueous suspension is preferably 1 to 50 mass%, more preferably 2 to 30 mass%, and even more preferably 3 to 20 mass%, from the viewpoint of releasability between the metal and the mold. Furthermore, the solids concentration of cellulose nanofibers in the aqueous suspension is preferably 0.01 to 3 mass%, more preferably 0.02 to 2 mass%, and even more preferably 0.05 to 1 mass%, from the viewpoint of dispersibility.

[0051] The aqueous suspension of the present invention may contain a dispersant other than CNF, as long as the effects of the present invention are not impaired. Examples of dispersants other than CNF include water-soluble polymers, acrylic dispersants, polycarboxylic acid dispersants, polyoxyethylene-type nonionic surfactants, silicone materials, and silane coupling agents.

[0052] Water-soluble polymers include cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, ethylcellulose), xanthan gum, xyloglucan, dextrin, dextran, carrageenan, locust bean gum, alginic acid, alginate, pullulan, starch, potato starch, arrowroot flour, cationic starch, phosphorylated starch, corn starch, gum arabic, gellan gum, polydextrose, pectin, chitin, water-soluble chitin, chitosan, casein, albumin, soy protein lysate, peptone, polyvinyl alcohol, poly Examples of suitable polymers include acrylamide, sodium polyacrylate, polyvinylpyrrolidone, polyvinyl acetate, polyamino acids, polylactic acid, polymalic acid, polyglycerin, latex, rosin-based sizing agents, petroleum resin-based sizing agents, urea resins, melamine resins, epoxy resins, polyamide resins, polyamide-polyamine resins, polyethyleneimine, polyamines, vegetable gums, polyethylene oxide, hydrophilic crosslinked polymers, polyacrylates, starch-polyacrylic acid copolymers, tamarind gum, guar gum, and colloidal silica, as well as mixtures of one or more thereof. Among these, carboxymethyl cellulose is preferred from the viewpoint of improving dispersibility.

[0053] (Method of producing aqueous suspension) The method for producing the aqueous suspension of the present invention is not particularly limited, and any method may be used as long as it can disperse the nitride and cellulose nanofibers in an aqueous solvent. Examples include a method in which an aqueous dispersion of cellulose nanofibers is added to the nitride while stirring it in water and further stirring, a method in which the nitride is mixed with an aqueous dispersion of cellulose nanofibers and stirred, or a method in which a dispersion of the nitride in an aqueous solvent is gradually added to an aqueous dispersion of cellulose nanofibers and stirred. The aqueous suspension of the present invention can be obtained by stirring and mixing using conventional equipment and means, and the temperature conditions, etc., are not particularly limited.

[0054] The aqueous suspension of the present invention contains cellulose nanofibers, so that nitrides such as boron nitride and aluminum nitride are stably dispersed and sedimentation is unlikely to occur. Therefore, this aqueous suspension is suitable for use in paints, coating agents, additives, lubricants, mold release agents, mold washes, cosmetics, and solid resins. The aqueous suspension of the present invention can be used in a wide range of applications, without being limited to the above applications.

[0055] (coating agent) The aqueous suspension of the present invention can be used as a mold wash. A mold wash is a coating agent that is applied to the surface of a mold used in casting to protect it from the heat of molten aluminum or other metals and to prevent seizure. In addition to the aqueous suspension, the mold wash of the present invention can contain other additives such as binders (organic binders such as phenolic resins, polyester resins, polyether resins, urethane resins, acrylic resins, vinyl acetate resins, silicone resins, butadiene-based synthetic rubbers, epoxy resins and emulsions thereof, fluororesins, rosin, rosin-modified resins, maleic acid resins, dammar resins, mastic resins, copal, shellac, starch, and dextrin), antifoaming agents, etc. Furthermore, if necessary, mold release agents such as graphite, graphite fluoride, molybdenum disulfide, tungsten disulfide, kaolin, bentonite, mica, talc, silica, quartz, calcium carbonate, magnesium carbonate, silicon carbide, and titanium oxide can be used in combination.

[0056] (Method of manufacturing a mold wash) The method for producing the mold wash of the present invention is not particularly limited as long as it is possible to add other additives to an aqueous suspension and mix them, and examples of the method include a method using a stirrer such as a disper or homomixer, and a method using a mixer such as a ball mill. [Example]

[0057] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0058] (Production Example 1) (Production of carboxymethylated cellulose nanofibers) 200 g of softwood bleached kraft pulp (NBKP, Nippon Paper Industries Co., Ltd.) (dry mass) and 111 g of sodium hydroxide (dry mass) were added to a mixer capable of mixing pulp, and water was added to adjust the pulp solids content to 20% by mass. After 30 minutes of stirring at 30°C, 216 g of sodium monochloroacetate (active ingredient equivalent) was added. After 30 minutes of stirring, the temperature was raised to 70°C and stirred for 1 hour. The reaction mixture was then removed, neutralized, and washed to obtain carboxymethylated pulp with a carboxymethyl substitution degree of 0.25 per glucose unit. The carboxymethylated pulp was then adjusted to 1% solids with water and defibrated by processing five times in a high-pressure homogenizer at 20°C and 150 MPa to obtain an aqueous dispersion of carboxymethylated cellulose nanofibers. The B-type viscosity of this aqueous dispersion at 60 rpm and 25°C was 1500 mPa·s. The obtained carboxymethylated cellulose nanofibers had an average fiber diameter of 50 nm and an aspect ratio of 120.

[0059] The resulting carboxymethylated cellulose nanofibers were dispersed in water to a solids content of 0.7% by mass. Carboxymethyl cellulose (product name: F350HC-4, viscosity (1% by mass, 25°C, 60 rpm) approximately 3000 mPa·s, degree of carboxymethyl substitution approximately 0.90) (hereinafter sometimes referred to as "CMC") was added at 40% by mass relative to the carboxymethyl cellulose nanofibers (i.e., 40 parts by mass of carboxymethyl cellulose solids per 100 parts by mass of carboxymethyl cellulose nanofiber solids). The mixture was stirred for 60 minutes using a TK homomixer (12,000 rpm). The resulting aqueous dispersion of CMC-containing carboxymethylated cellulose nanofibers was dehydrated and dried in a drum dryer to obtain a dry solid, which was then pulverized. The pulverized product was then classified using a 30-mesh mesh to obtain a powder of CMC-containing carboxymethylated cellulose nanofibers.

[0060] (Method for measuring the degree of carboxymethyl substitution per glucose unit) Approximately 2.0 g of carboxymethylated cellulose fiber (bone dry) was weighed and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of a solution prepared by adding 10 mL of concentrated nitric acid to 90 mL of methanol was added and the mixture was shaken for 3 hours to convert the carboxymethylated cellulose salt (carboxymethylated cellulose) into hydrogenated carboxymethylated cellulose. 1.5 to 2.0 g of hydrogenated carboxymethylated cellulose (bone dry) was weighed and placed in a 300 mL Erlenmeyer flask with a stopper. The hydrogenated carboxymethylated cellulose was moistened with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH was added, and the mixture was shaken at room temperature for 3 hours. Excess NaOH was back-titrated with 0.1 N H2SO4 using phenolphthalein as an indicator. The degree of carboxymethyl substitution (DS) was calculated using the following formula: A = [(100 × F' - (0.1N H2SO4) (mL) × F) × 0.1] / (bone-dry mass of hydrogenated carboxymethyl cellulose (g)) DS=0.162×A / (1-0.058×A) A: Amount of 1N NaOH (mL) required to neutralize 1 g of hydrogenated carboxymethyl cellulose F: Factor of 0.1N H2SO4 F': Factor of 0.1N NaOH

[0061] (Production Example 2) (Production of carboxylated cellulose nanofibers) 500 g (bone-dry) of bleached, unbeaten kraft pulp (85% brightness) derived from softwood was added to 500 mL of an aqueous solution containing 780 mg of TEMPO (Sigma-Aldrich) and 75.5 g of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 6.0 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by the addition of 3 M aqueous sodium hydroxide. The reaction was terminated when the sodium hypochlorite was consumed and the pH no longer changed. The reaction mixture was filtered through a glass filter to separate the pulp, which was then thoroughly washed with water to obtain carboxylated pulp (carboxylated cellulose). The carboxyl group content of this carboxylated cellulose was 1.42 mmol / g. The solids concentration of the carboxylated cellulose was adjusted to 5% by mass with water, and 10% hydrochloric acid was added to convert the sodium salt-type carboxyl groups (-COONa) in the carboxylated cellulose to the acid-type (-COOH). The resulting solution was then suction filtered using a glass filter and dehydrated. The solids concentration of the carboxylated cellulose was again adjusted to 5% by mass with water, and then dehydrated. This process was repeated three times to obtain a dispersion of acid-type carboxylated cellulose with a solids concentration of 25% by mass. This aqueous dispersion of carboxylated pulp was diluted with water to a solids concentration of 3% by mass, and then defibrated three times using an ultra-high pressure homogenizer at 150 MPa to obtain carboxylated cellulose nanofibers with an average fiber diameter of 40 nm and an aspect ratio of 150.

[0062] The resulting aqueous dispersion of carboxylated cellulose nanofibers was dehydrated and dried in a drum dryer to obtain a dry solid, which was then pulverized. The pulverized product was then classified using a 30 mesh to obtain a powder of carboxylated cellulose nanofibers.

[0063] (Measurement of Carboxyl Group Amount) 60 mL of a 0.5% by mass slurry (aqueous dispersion) of a carboxylated cellulose sample is prepared, and a 0.1 M aqueous hydrochloric acid solution is added to adjust the pH to 2.5. Then, a 0.05 N aqueous sodium hydroxide solution is added dropwise, and the electrical conductivity is measured until the pH reaches 11. The electrical conductivity is calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of a weak acid, where the change in electrical conductivity is gradual: Amount of carboxyl groups [mmol / g carboxylated cellulose] = a [mL] x 0.05 / mass of carboxylated cellulose [g].

[0064] Example 1 (Production of aqueous suspension) 10 g of boron nitride powder (Showa Denko K.K., scaly boron nitride powder "SHOBN (registered trademark) UHP-S1", average particle size: 0.5 μm (D50)) was added to 100 g of water at 100°C, and while stirring with a propeller, 1 g of the powder of CMC-containing carboxymethylated cellulose nanofibers obtained in Production Example 1 above was added. The resulting mixture was stirred at 2000 rpm for 10 minutes with a homodisper to obtain a uniformly dispersed suspension. This suspension remained dispersed even after 24 hours.

[0065] Example 2 10 g of boron nitride powder (Showa Denko K.K., scaly boron nitride powder "SHOBN (registered trademark) UHP-S1", average particle size: 0.5 μm (D50)) was added to 100 g of water at 100°C, and while stirring with a propeller, 1 g of the carboxylated cellulose nanofiber powder obtained in Production Example 2 above was added. The resulting mixture was stirred at 2000 rpm for 10 minutes with a homodisper, yielding a uniformly dispersed suspension. This suspension remained dispersed even after 24 hours.

[0066] Example 3 10 g of aluminum nitride powder (Tokuyama Corporation, aluminum nitride (AlN) powder / granules, E grade, average particle size: 1.0 μm (D50)) was added to 100 g of water at 100°C, and while stirring with a propeller, 1 g of the CMC-containing carboxymethylated cellulose nanofiber powder obtained in Production Example 1 above was added. The resulting mixture was stirred at 2000 rpm for 10 minutes with a homodisper to obtain a uniformly dispersed suspension. This suspension remained dispersed even after 24 hours.

[0067] (Comparative Example 1) 10 g of boron nitride powder (Showa Denko K.K., scaly boron nitride powder "SHOBN (registered trademark) UHP-S1", average particle size: 0.5 μm (D50)) was added to 100 g of water at 100°C and stirred at 2000 rpm for 10 minutes with a Homo Disper, but the boron nitride did not mix well with the water and did not disperse well. Furthermore, it had completely precipitated after 24 hours.

[0068] (Comparative Example 2) Ten grams of aluminum nitride powder (Tokuyama Corporation, aluminum nitride (AlN) powder / granules, E grade, average particle size: 1.0 μm (D50)) was added to 100 g of water at 100°C and stirred at 2000 rpm for 10 minutes with a homodisper, but the aluminum nitride did not mix well with the water and did not disperse well. Furthermore, it had completely settled after 24 hours.

Claims

1. nitride and carboxylated cellulose nanofibers, the amount of carboxyl groups in the carboxylated cellulose nanofiber is 0.6 to 3.0 mmol / g relative to the bone dry mass of the cellulose nanofiber; The carboxylated cellulose nanofiber has a degree of crystallinity of cellulose type I of 50 to 90%, The content of the nitride is 1 to 50 mass %, The aqueous suspension has an average particle size of the nitride of 0.5 to 30 μm.

2. The aqueous suspension of claim 1 further comprising carboxymethyl cellulose.

3. 3. The aqueous suspension according to claim 1, wherein the nitride is boron nitride.

4. 3. The aqueous suspension according to claim 1, wherein the nitride is aluminum nitride.

5. A mold wash comprising the aqueous suspension according to any one of claims 1 to 4.