Fine particle dispersant and flocculant

Abstract

To enable both dispersion and aggregation of fine particles irrespective of their kind by using cellulose nanofibers (CNF), and to facilitate use as a dispersant or a flocculant, while allowing transition between a dispersed state and an aggregated state.SOLUTION: A mixed solution containing fine particles and unmodified CNF is formed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a dispersant and a flocculant using cellulose nanofibers. [Background technology]

[0002] In mixtures containing fine particles in a liquid, there are cases where the need to maintain a stable dispersion state of the fine particles and cases where the need to aggregate the fine particles is desired. Maintaining a dispersion state of fine particles is required for applications such as preventing pigment sedimentation, while aggregation is primarily required in water treatment, and technologies for dispersion and aggregation to meet each need are being investigated. Fine fibers made of cellulose and cellulose derivatives known as cellulose nanofibers (hereinafter abbreviated as "CNF") are known to function as dispersants and aggregates for fine particles by utilizing their properties. There is also a demand for maintaining a dispersion state until use, while agglomerating the fine particles after use so that they can be removed by filtration or other methods.

[0003] Non-Patent Document 1 investigates carboxy-modified CNF, known as TEMPO-oxidized CNF. It has been reported that carboxy-modified CNF forms a loose physical network structure in water through hydrogen bonds, thereby suppressing the settling of fine particles and maintaining a stable dispersion state.

[0004] Patent Document 1 describes a method for producing a dispersion in which a mixture containing biomass nanofibers such as mechanically defibrated CNF and inorganic particles is used as a pressurized fluid to disperse the inorganic particles. It explains that the biomass nanofibers form a three-dimensional network in water, and the inorganic particles become entangled in the three-dimensional network, thereby stabilizing the dispersion (Patent Document 1

[0014]

[0018] , etc.).

[0005] Patent Document 2 proposes a method for obtaining a pigment dispersion that can maintain a stable dispersion state by supplying a pigment to a CNF dispersion using an anion-modified CNF such as carboxymethylated CNF or phosphorylated CNF as a dispersion stabilizer.

[0006] Patent Document 3 proposes that suitable battery separators can be obtained by dispersing inorganic fine particles using phosphorylated CNF to create a battery separator coating liquid that has excellent dispersibility and is resistant to viscosity reduction.

[0007] Patent Document 4 proposes a papermaking method in which calcium-containing inorganic particles are agglomerated by adding a dispersion containing TEMPO-oxidized CNF, thereby improving retention in the papermaking process. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2022-47967 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-218493 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-117695 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-74528 [Non-patent literature]

[0009] [Non-Patent Document 1] Yosuke Ushiroi: "Use of TEMPO-oxidized cellulose nanofiber as a thickener", Forest Science, Vol. 81 p. 27-30 (2017) Summary of the Invention [Problem to be solved by the invention]

[0010] However, the mechanically defibrated CNF used in Patent Document 1 had a fiber width that was too large, so although it was able to temporarily form a three-dimensional network, it was not able to maintain a stable dispersed state for a long period of time.

[0011] Furthermore, anion-modified CNFs such as TEMPO-oxidized CNF and phosphorylated CNF used in Non-Patent Document 1 and Patent Documents 2 and 3 have a small difference between the pH and isoelectric point of the water that serves as the dispersion medium for the microparticles, making them effective for dispersing microparticles with a small surface charge. However, because they are anion-modified, there is a large difference between the pH and isoelectric point of the water in which the microparticles are dispersed, making it impossible to maintain a dispersed state for microparticles with a large positive surface charge, and the types of microparticles that can be stabilized in a dispersed state are limited.

[0012] On the other hand, when chemically modified CNF is used as the flocculant as in Patent Document 4, flocs are easily formed by promoting aggregation of the fine particles through ionic bonds via calcium ions in the CNF. In this case, too, the surface charge of the fine particles to be aggregated is used, so the types of fine particles that can be aggregated are limited.

[0013] Therefore, the object of this invention is to use CNF to enable dispersion and aggregation regardless of the type of fine particles, making it easier to use as a dispersant or aggregator. [Means for solving the problem]

[0014] This invention solves the above problem by a first solution, which is a mixed liquid containing microparticles and unmodified CNF.

[0015] Unmodified CNF can maintain a dispersed state even for microparticles with a large positive surface charge, which anion-modified CNF cannot handle. This eliminates the need to check the type of microparticle each time it is used, and it can be used universally regardless of the microparticle's surface charge. Furthermore, this mixture can be dispersed by increasing the content of unmodified CNF, or aggregated by decreasing the content, and the state can be freely changed by changing the content.

[0016] By utilizing this property, a second solution can be adopted to further limit the first solution, in which the liquid height of the dispersion layer in which the unmodified CNF is used as a dispersant to disperse the microparticles is made into a suspension that is 90% or more of the liquid height of the entire mixed liquid after being left to stand for 7 days.

[0017] Conversely, a third solution can be adopted as a further limitation of the first solution, in which the flocculation liquid has a supernatant layer with a light transmittance of 95% or more. In this flocculation liquid, the unmodified CNF acts as a flocculant to flocculate the fine particles, resulting in a clear supernatant layer.

[0018] Furthermore, as a fourth solution, a preparation method can be adopted in which the dispersion or aggregation state of the microparticles is transitioned by changing the content of the unmodified CNF in the mixed liquid. [Effects of the Invention]

[0019] The mixed solution of this invention allows dispersion and aggregation of particles regardless of their surface charge. This allows for general-purpose use without examining the attributes of the target particles. Furthermore, because the dispersion and aggregation states can be transitioned simply by changing the concentration of unmodified CNF, it can be freely adjusted to the state required at any given time, such as for storage or removal. [Brief explanation of the drawings]

[0020] [Figure 1] (a) 50,000x FE-SEM image of RCNF in the example, (b) 50,000x FE-SEM image of mechanically defibrated CNF in the example [Figure 2] (a) Micrograph of Comparative Example 11, which contains only fine particles with no RCNF added; (b) Micrograph of Example 15, which contains a small amount of RCNF and is in an aggregated state; (c) FE-SEM image of Example 2, which contains a large amount of RCNF and is in a dispersed state. [Figure 3] Comparative photographs of the mixed liquids in the examples showing the dispersed and aggregated states DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention is described in detail below. The present invention is a mixture of unmodified CNF and fine particles, and the unmodified CNF can be used as both a dispersant and a flocculant.

[0022] CNF is a fine fiber made by processing a cellulose material. Hereinafter, the term CNF refers to both unmodified and modified CNF. In this invention, "unmodified CNF" refers to CNF that has not been modified by functional groups in the cellulose molecular structure, or that can be considered unmodified. Direct mechanical defibration of cellulose material without changing the cellulose molecular structure leaves the cellulose unmodified, but it is difficult to reduce the fiber width to the preferred range described below. Cellulose that has been chemically modified in part and then defibrated to regenerate cellulose is more likely to achieve the preferred fiber width described below. However, it is preferable that the chemically modified material be regenerated to a level where it can be considered unmodified by removing the introduced functional groups.

[0023] Examples of such modified cellulose that can be regenerated and then restored to unmodified cellulose include phosphorylated cellulose, which is obtained by phosphorylating cellulose, and cellulose that has been treated with alkali and carbon disulfide to form xanthate groups (-OCSS). - M + Examples include xanthate cellulose, which has been introduced with functional groups. When cellulose materials made from pulp, etc. are chemically modified, the ionic repulsion between the introduced functional groups can be utilized, making it easier to create nanofibers than if the cellulose material were simply defibrated. CNF that has been chemically modified and defibrated in this way is called "modified CNF." CNF obtained by a regeneration process that removes functional groups from the molecular structure of the chemically modified cellulose of modified CNF and returns it to cellulose is called "regenerated CNF." Hereinafter, when simply referring to CNF, this includes not only unmodified CNF (including regenerated CNF) but also modified CNF.

[0024] Among the regenerated CNFs, not only those in which the hydroxyl groups have been temporarily substituted with other functional groups such as xanthate groups and then all of the hydroxyl groups have been returned to their original hydroxyl groups, but also those in which a very small amount of other functional groups remain. Specifically, the modified cellulose units of the total cellulose units are preferably 0.1% or less, more preferably 0.01% or less, and particularly preferably below the detection limit and considered to be 0. Furthermore, these regenerated CNFs may be purified before use if necessary to remove or reduce impurities.

[0025] The number-average fiber width of the unmodified CNF used in this invention must be 3 nm or more. The present invention can be implemented with a fiber width smaller than this, but achieving a fiber width of less than 3 nm requires a great deal of energy and is not very practical in terms of work efficiency. On the other hand, the number-average fiber width of the unmodified CNF must be 10 nm or less. If the fiber width exceeds 10 nm, it becomes difficult to form a dense network, and its effectiveness as a dispersant becomes insufficient, making it difficult to maintain a stable dispersed state for a long period of time.

[0026] When CNF, including unmodified CNF, is mixed with fine particles in water, it adsorbs to the surface of the fine particles and causes them to aggregate. However, with unmodified CNF, this effect is exerted regardless of the positive or negative or large or small surface charge of the fine particles.

[0027] On the other hand, CNF, including unmodified CNF, forms a network structure when CNFs interact with each other in water at a certain concentration or above. This network structure captures fine particles, allowing them to be stably dispersed for a long period of time. When capturing fine particles with this network structure, unmodified CNF is versatile enough to accommodate either fine particles with a strong positive or negative surface charge.

[0028] The fine particles to be dispersed or aggregated in this invention can be any of inorganic particles, organic particles, and organic-inorganic composite particles. However, particles that are easily soluble in water become solutes and are not the target of dispersion or aggregation in the first place, so it is easier to use particles that are relatively insoluble. The shape of the fine particles is not particularly limited, but examples include powder, granules, flakes, spheres, and scales.

[0029] Examples of inorganic particles include metals, metal oxides, hydroxides, carbides, carbonates, sulfates, silicates, nitrides, titanic acid compounds, carbons, various magnetic powders, and the like. Examples of the metal include iron, aluminum, chromium, nickel, zinc, lead, tungsten, tin, titanium, copper, silver, gold, and alloys thereof. Examples of the metal oxide include silica, aluminum oxide (alumina), zirconium oxide (zirconia), titanium oxide, magnesium oxide, indium tin oxide, zinc oxide, cesium oxide, tungsten oxide, tin oxide, cerium oxide, copper oxide, iron oxide, calcium oxide, and barium oxide. Examples of the hydroxide include calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and iron hydroxide. Examples of the carbonates include calcium carbonate, magnesium carbonate, zinc carbonate, barium carbonate, dawsonite, and hydrotalcite. Examples of the sulfate include calcium sulfate and barium sulfate. Examples of the silicates include calcium silicate, wollastonite, xonotlite, kaolin, talc, clay, mica, montmorillonite, bentonite, dolomite, aluminum silicate, magnesium silicate, zirconium silicate, activated clay, sepiolite, imogolite, sericite, glass fiber, glass beads, and zeolite. Examples of the nitride include aluminum nitride, boron nitride, and silicon nitride. Examples of the titanate compound include barium titanate, calcium titanate, and strontium titanate. Examples of the carbons include carbon black, graphite, carbon fiber, activated carbon, bamboo charcoal, wood charcoal, carbon nanotubes, and fullerenes. Examples of the various magnetic powders include ferrite, magnetite, chromium oxide, barium ferrite, and cobalt-containing iron oxide.

[0030] Examples of organic particles include organic resins, organic pigments, and organic fluorescent dyes. Examples of organic resins include thermoplastic resins, thermosetting resins, natural resins such as collagen, and natural polymers such as cellulose. Examples of organic pigments include azo-based, phthalocyanine-based, indigo-based, quinacridone-based, anthraquinone-based, perylene-based, and pyrrolopyrrole-based pigments. Examples of organic fluorescent dyes include rhodamine dyes, squarylium dyes, cyanine dyes, aromatic hydrocarbon dyes, oxazine dyes, carbopyronine dyes, and pyrromethene dyes.

[0031] Organic-inorganic composite particles include those formed by combining either organic or inorganic particles as a core material with the other particle coated or modified on its surface, and those formed by mixing organic and inorganic materials. Examples include polymer particles whose surfaces are coated with iron oxide, metallic copper, titanium oxide, etc., and particles formed by combining vinyl polymers and silicones at the molecular level.

[0032] These particles can be changed for various purposes, such as storage, use, and fixation, by transitioning between a dispersed state and an aggregated state at the required timing.

[0033] The number-average particle diameter of the microparticles dispersed or aggregated in this invention is preferably 0.01 μm or more, and more preferably 0.1 μm or more. If the diameter is less than 0.01 μm, the surface area per mass becomes too large, and the amount of unmodified CNF required to surround the microparticles becomes too large, making dispersion or aggregation difficult. On the other hand, the number-average particle diameter of the microparticles is preferably 100 μm or less, and more preferably 50 μm or less. If the diameter exceeds 100 μm, it becomes difficult to disperse the microparticles in water and they tend to remain aggregated, making it difficult to achieve the effects of the present invention.

[0034] The isoelectric point of the microparticles, which indicates whether the surface charge is positive or negative, is preferably 4 to 11, and more preferably 5 to 10. The isoelectric point is the pH at which the surface charge is 0; at pH levels below the isoelectric point, the surface charge becomes positive, and above the isoelectric point, the surface charge becomes negative. Unmodified CNF can accommodate a surface charge that is either positive or negative to some extent, so the isoelectric point of applicable microparticles ranges around 7, allowing for highly versatile use. However, if the surface charge of the microparticles is extremely positive or negative, the electrostatic repulsion between the microparticles becomes strong, and even with unmodified CNF, the amount required for aggregation, in particular, may vary significantly.

[0035] In this invention, water is preferably used as the dispersion medium for the mixture of microparticles and unmodified CNF. However, water-soluble substances other than water may be added as long as the effects of the invention are not impaired. Examples of water-soluble substances include, but are not limited to, alcohols, diols, glycerin, sugars, salts, pH adjusters, and preservatives.

[0036] In this invention, the mixed liquid can be in either a dispersed state, where microparticles are uniformly dispersed throughout almost the entire liquid to form a dispersion layer, or an aggregated state, where a supernatant layer with a low microparticle content is located above the liquid and a dispersion layer with a high microparticle content is located below the liquid. In the dispersed state, the liquid height at the interface between the dispersion layer and the supernatant layer (hereinafter referred to as the "liquid height of the dispersion layer") is preferably 90% or more, and more preferably 95% or more, of the total liquid height of the mixed liquid. Furthermore, it is more preferable to maintain a liquid height of 90% or more in the dispersion layer for 7 days or more. In the aggregated state, it is preferable that the liquid height of the dispersion layer is 60% or less of the total liquid height of the mixed liquid and that the light transmittance of the supernatant layer is 95% or more.

[0037] In this invention, the content of the particles that can be aggregated depends on the true specific gravity of the particles, but if the aggregation is insufficient, the efficiency of separating the dispersion layer and the supernatant layer may decrease.

[0038] In this invention, it is assumed that a CNF suspension is added to a microparticle suspension to bring the mixed solution into an aggregated state. The unmodified CNF content in the mixed solution excluding the microparticles is preferably 0.03% by mass or more, more preferably 0.05% by mass or more. If it is less than 0.03% by mass, the amount adsorbed to the microparticles is insufficient, resulting in insufficient aggregation. On the other hand, it is preferably 0.2% by mass or less, more preferably 0.15% by mass or less. If it exceeds 0.2% by mass, not only will the unmodified CNF be adsorbed to the microparticles, but it will also be more likely to form a network structure within the mixed solution, increasing the liquid height of the dispersion layer and potentially reducing the efficiency of separating the dispersion layer from the supernatant layer.

[0039] In this invention, it is assumed that fine particles are added to the CNF suspension to bring the mixture into a dispersed state. The content of unmodified CNF in the mixture excluding the fine particles is preferably 0.6% by mass or more, and more preferably 0.7% by mass or more, relative to the amount of the mixture excluding the fine particles. If the content is less than 0.6% by mass, the network structure formed by the unmodified CNF in the liquid is not formed or is insufficient, making it difficult to stabilize and maintain the fine particles in a dispersed state. On the other hand, if there is an excess of unmodified CNF, a sufficient network structure is formed and there is no problem in maintaining the dispersed state. However, if there is too much unmodified CNF, not only will it be wasted, but a large amount of dispersion medium must be added to return the mixture to an aggregated state. Therefore, a realistic and preferable content is 1.5% by mass or less.

[0040] In this invention, by adjusting the content of unmodified CNF in the mixed solution, it is also possible to freely transition between a dispersed state and an aggregated state. That is, it is possible to freely increase the content of unmodified CNF in an aggregated solution that has once become aggregated, thereby changing it to a dispersed state, or conversely, it is possible to freely decrease the content of unmodified CNF in a suspension that has become dispersed, thereby changing it to an aggregated state. To increase the content of unmodified CNF, it is recommended to add unmodified CNF. To decrease the content of unmodified CNF, it is recommended to add a dispersion medium. [Example]

[0041] Hereinafter, specific examples of the present invention will be described. First, as a preliminary step to obtaining unmodified CNF, xanthated CNF is produced, and unmodified CNF, which is regenerated CNF, is obtained as regenerated cellulose by removing the xanthate groups from the xanthated CNF.

[0042] (Sample 1: Preparation of unmodified CNF) The following materials were used to manufacture xanthated CNF: Kraft pulp (manufactured by Nippon Paper Industries Co., Ltd.: NBKP, α-cellulose content: 90% by mass) will be referred to as "NBKP" below.

[0043] <Alkali treatment> NBKP was weighed out so that the pulp solids (pulp solids are the amount excluding water in the pulp; the same applies below) was 100 g. To this was added 2,500 g of an 8.5 mass% aqueous sodium hydroxide solution, and the mixture was stirred at room temperature for 3 hours to perform an alkali treatment. The alkali-treated pulp was subjected to solid-liquid separation using a centrifugal dehydrator (Kokusan Co., Ltd., H-110A, 400 mesh filter cloth) to obtain a dehydrated alkali cellulose. The sodium hydroxide content of this dehydrated alkali cellulose was 7.5 mass% and the pulp solids content was 27.4 mass%.

[0044] <Xanthate treatment> The dehydrated alkali cellulose prepared above was weighed to a pulp solids content of 100 g, and 35 g of carbon disulfide (35% by mass relative to the pulp solids content) was added thereto, and the sulfurization reaction was carried out at room temperature for 4.5 hours to perform a xanthation treatment, thereby obtaining xanthated cellulose.

[0045] <Defibrillation processing> The xanthated cellulose prepared by the above xanthation treatment was weighed to a pulp solids content of 100 g, and distilled water was added to disperse the xanthated cellulose to a pulp solids concentration of 5% by mass. While centrifuging using the centrifugal dehydrator, the xanthated cellulose was thoroughly washed with distilled water to remove impurities, alkali, unreacted carbon disulfide, etc. After washing, all of the xanthated cellulose was collected, and distilled water was added to make a 20 kg aqueous suspension with a cellulose concentration of 0.5% by mass (hereinafter referred to as "cellulose concentration"). This aqueous suspension was defibrated using a high-pressure homogenizer (H20 model, manufactured by Sanwa Engineering Co., Ltd.) at a flow rate of 2.5 L / min and a pressure of 40 MPa for a total of five passes to obtain xanthated CNF.

[0046] <Recycling and redispersion processing> To 16.4 kg of an aqueous suspension of zanted CNF (cellulose concentration: 0.5% by mass) obtained by the above procedure, 360 ml of a 1 M sulfuric acid aqueous solution was added, and the mixture was stirred with an agitator for 1 hour for regeneration treatment. After the treatment, it was neutralized with a 1 M sodium hydroxide aqueous solution to obtain a regenerated CNF aqueous suspension. When the average degree of zantate substitution of the regenerated CNF (hereinafter referred to as "RCNF") was measured, it was less than 0.001, which is the lower limit of measurement. Thus, it was confirmed that the zantate groups were almost completely eliminated by the acid treatment and returned to hydroxyl groups.

[0047] <Measurement of Degree of Zantate Substitution> The average degree of zantate substitution of RCNF was measured by the Bredee method. The degree of zantate substitution is a value for the degree to which zantate groups are introduced per glucose unit of cellulose. The procedure of the Bredee method was as follows. 40 g of the RCNF aqueous suspension was weighed, 50 mL of a 0.5 M sodium hydroxide aqueous solution (5°C) was added, and the mixture was stirred. After standing for 15 minutes, it was neutralized with a 1.5 M acetic acid aqueous solution. 100 mL of distilled water was added thereto and stirred well, and then 10 mL of a 1.5 M acetic acid aqueous solution and 10 mL of a 0.05 mol / L iodine aqueous solution were added. This solution was titrated with a 0.05 mol / L sodium thiosulfate aqueous solution, and the degree of zantate substitution was calculated from the following formula (1) based on the titration amount of the sodium thiosulfate aqueous solution and the cellulose content in the sample

[0048] Degree of zantate substitution = (0.05 × 10 × 2 - 0.05 × titration amount of sodium thiosulfate (mL)) ÷ 1000 ÷ (cellulose content in sample (g) / 162.1)......(1)

[0049] While centrifugally dehydrating the RCNF aqueous suspension obtained above using the centrifuge, distilled water was added and it was washed thoroughly. All of the washed RCNF was recovered, and distilled water was added to make an 8 kg aqueous suspension with a CNF concentration of 1.0% by mass. This aqueous suspension was redispersed by passing it through the high-pressure homogenizer three times at a flow rate of 2.5 L / min and a pressure of 40 MPa.

[0050] <Measurement of Fiber Width of CNF> An aqueous suspension of RCNF diluted with distilled water to a CNF concentration of 0.01% by mass was dyed and dried on a support membrane to prepare a dried specimen. This specimen was observed using a field emission scanning electron microscope (FE-SEM, Hitachi High-Tech Corporation, S-4800) at an accelerating voltage of 25 kV. Measurements of 50 randomly selected nanofibers from the 130,000x magnification image revealed that the fiber widths ranged from 3.9 nm to 9.7 nm, with a number-average fiber width of 6.4 nm. The physical properties of the specimens used are summarized in Table 1.

[0051] [Table 1]

[0052] (Sample 2: mechanically defibrated CNF) As the CNF obtained by mechanical fiberization, BiNFi-s manufactured by Sugino Machine Co., Ltd. was used. When measured in the same manner as Sample 1, the fiber width was in the range of 24.1 to 53.2 nm, and the number average fiber width was 31.2 nm.

[0053] (Sample 3: Phosphorylated CNF) The modified CNF used was phosphorylated CNF, which was produced by the following procedure. A phosphorylation reagent was obtained by dissolving 18 g of ammonium dihydrogen phosphate and 48 g of urea in 60 g of distilled water. 10 g of bone-dry NBKP was placed in a tray and uniformly impregnated with 31.5 g of the phosphorylation reagent. The tray was placed in a constant-temperature incubator (Yamato Scientific Co., Ltd.: DKN602) at 165°C and dried for 30 minutes to obtain phosphorylated pulp. After thorough washing, this phosphorylated pulp was added with distilled water to prepare an aqueous suspension with a solids content of 1% by mass. A 1M sodium hydroxide solution was added to this aqueous suspension to obtain an aqueous suspension containing cellulose fibers with a pH of 12.5. This was dehydrated and washed three times with distilled water, and distilled water was added to the cellulose fibers obtained after dehydration to obtain 1 kg of an aqueous suspension with a solids content of 0.75% by mass. This aqueous suspension was defibrated by passing it through a high-pressure homogenizer (ECONIZER LABO-02, manufactured by Sanmaru Machinery Industry Co., Ltd.) a total of seven times at a pressure of 60 MPa to obtain an aqueous suspension of phosphorylated CNF. 26.7 g of the resulting aqueous suspension of phosphorylated CNF was diluted with distilled water to obtain 100 g of an aqueous suspension with a solids content of 0.2% by mass, and 0.1M hydrochloric acid solution was added to this to adjust the pH to 2.54. The pH and electrical conductivity were measured while adding 0.1 M sodium hydroxide solution dropwise, and the electrical conductivity at each added amount was plotted until the pH reached approximately 10. The amount of phosphate groups introduced was measured and found to be 1.55 mmol / g. This confirmed that the modified cellulose substituted with phosphate groups had been defibrated.

[0054] The obtained phosphorylated CNF was measured in the same manner as in Sample 1, and the fiber width was found to be in the range of 3.8 to 9.9 nm, and the number average fiber width was 5.6 nm.

[0055] (Sample 4: TEMPO oxidized CNF) TEMPO-oxidized CNF was prepared as modified CNF according to the Reference Example of Japanese Patent No. 6668558, and an aqueous suspension of TEMPO-oxidized CNF with an average oxidation degree of 2 mmol / g was obtained. This CNF was defibrated to form modified cellulose substituted with carboxyl groups. The resulting TEMPO-oxidized CNF was measured in the same manner as Sample 1, and the fiber width was found to be in the range of 3.7 to 7.9 nm, with a number-average fiber width of 6.3 nm.

[0056] (Sample 5: Carboxymethylcellulose) Carboxymethyl cellulose (manufactured by Nacalai Tesque, Inc.: 07326-95, first-class EP) that is not a fine fiber and is used as a thickener was used.

[0057] <Fiber width evaluation of CNF> FE-SEM photographs at 50,000 times magnification taken for each of the test samples 1 (RCNF) and test sample 2 (mechanically defibrated CNF) are shown in Figs. 1(a) and (b), respectively. It was confirmed that the mechanically defibrated CNF had a significantly larger fiber width and was in a significantly different state compared to RCNF.

[0058] <Preparation (dispersion) of fine particle mixture> The RCNF aqueous suspension of test sample 1 was diluted with distilled water, and each of the fine particles shown in Table 2 was added to adjust the total amount to 40 g. The mixture was subjected to dispersion treatment at 8000 rpm for 5 minutes using a homogenizer (manufactured by Nippon Seiki Co., Ltd.: Excel Auto Homogenizer ED4) to obtain a mixture containing each fine particle and RCNF. Thereafter, this mixture was transferred to a test tube (manufactured by Marumoto Co., Ltd.: A-21) with a diameter of 21 mm. Note that the data shown within the range in Table 2 are the data published by the provider.

[0059] <Preparation (aggregation) of fine particle mixture> Distilled water was added to each of the fine particles (dry state) shown in Table 2, and the mixture was subjected to dispersion treatment at 8000 rpm for 5 minutes using the homogenizer to prepare an aqueous suspension of each fine particle. An RCNF aqueous suspension with a CNF concentration of 1.5 mass% was added to these fine particle aqueous suspensions to adjust the total amount to 40 g, and the mixture was again subjected to dispersion treatment at 8000 rpm for 5 minutes using the homogenizer to obtain a mixture containing each fine particle and RCNF. Thereafter, this mixture was transferred to the test tube.

[0060]

Table 2

[0061] <Measurement of light transmittance of supernatant layer of fine particle mixture> After the dispersion treatment in the above preparation, the sample was left to stand for 30 minutes, and a portion 2 cm above the interface between the supernatant layer and the dispersion layer in the test tube was gently sampled with a dropper. The light transmittance of this sampled supernatant layer at a wavelength of 660 nm was measured using a UV-1800 (10 mm cell) manufactured by Shimadzu Corporation.

[0062] <Evaluation of dispersion and aggregation state> The particle mixture in the test tube was allowed to stand for 30 minutes after dispersion, 7 days at 25°C, and 1 month, and the proportion of the dispersed layer in each mixture was calculated using the following formula (2) and evaluated according to the following criteria: The dispersed layer is the layer in which the particles are dispersed, and it may occupy the entire mixture or the lower layer below the supernatant layer. Indicator of dispersion: If the proportion of the dispersion layer, i.e., the liquid height occupied by the dispersion layer from the bottom of the test tube (liquid height of the dispersion layer) is 90% or more of the total liquid height of the mixture, can be maintained from 30 minutes after dispersion processing until 1 month after being left to stand, it is judged to be in a good dispersion state. If the proportion of the liquid height of the dispersion layer is less than 90% after 7 days of standing after dispersion processing, it is judged that the dispersion state has not been maintained sufficiently. Indicator of aggregation: After leaving the mixture in a test tube for 30 minutes, if the height of the dispersion layer from the bottom of the test tube (liquid height of the dispersion layer) is 60% or less of the total liquid height of the mixture, and the light transmittance of the supernatant layer at 660 nm is 95% or more, the mixture is judged to be in a good state of aggregation.

[0063] Dispersion layer liquid height (mm) / total mixed liquid height (mm) × 100 (%) = dispersion layer ratio (%) … (2)

[0064] <Viscosity measurement of mixed liquid> The prepared microparticle mixture was left to stand at 25°C for 7 days, and then gently shaken to uniformly disperse the microparticles in the microparticle mixture. The viscosity of the microparticle mixture was measured using a B-type viscometer (B8M) manufactured by Tokimec Inc. with rotor 3 at 3 rpm.

[0065] <Examples and corresponding comparative examples in a dispersed state> For each combination of microparticles and CNF shown in Table 3, a mixed solution was prepared by adjusting the type and amount of each to the values ​​shown in the table, and the dispersion state was evaluated. The results are shown in Table 3. Note that the mixing ratios of the mixed solution, microparticles, and CNF in the table are all in mass %.

[0066] [Table 3]

[0067] It was confirmed that Example 1, in which a sufficient amount of RCNF was added to the microparticle mixture, was able to maintain the microparticles in a dispersed state for a long period of time compared to Comparative Example 1, in which no RCNF was added. It was also confirmed that Example 4, in which the amount of RCNF was also increased, was able to adequately maintain the dispersed state even when the amount of microparticles was increased. However, in Reference Examples 1 and 2, in which the amount of RCNF added was small, the liquid height occupied by the layer in which the microparticles were dispersed (dispersion layer) decreased depending on the amount of RCNF. For this reason, it was found that in order to uniformly disperse the microparticles, RCNF needs to be at least 0.6% by mass relative to the amount of the mixture excluding the microparticles.

[0068] Furthermore, it was confirmed that in Examples 6 to 10, in which the type of fine particles was changed, the dispersed state could be maintained in the same manner when RCNF was used instead of CNF.

[0069] On the other hand, in Comparative Example 3, which used mechanically defibrated CNF, even though it contained the same amount of CNF as in Example 2, the liquid height of the dispersion layer began to decrease after 30 minutes of standing, and after 7 days of standing, the liquid height of the dispersion layer decreased significantly, and after 1 month of standing, it decreased even more significantly. It was confirmed that mechanically defibrated CNF, which has a large fiber width, cannot form a dense network structure and cannot maintain a dispersed state, and that CNF with a small fiber width, such as RCNF, is necessary.

[0070] In Comparative Examples 4 and 5, in which the same amount of CNF as in Example 2 was used but the type of CNF was changed to TEMPO-oxidized CNF and phosphorylated CNF, a dispersion state similar to that of Example 2 was maintained. However, when the microparticles were changed from calcium carbonate to aluminum oxide or zinc oxide, the dispersion state could not be maintained in Comparative Examples 6 and 7, in which TEMPO-oxidized CNF and phosphorylated CNF were used, unlike Examples 9 and 10, in which RCNF was used. The reason for this is that aluminum oxide and zinc oxide have a positive surface charge in neutral aqueous suspensions, and therefore the microparticles in chemically modified CNF interact with these functional groups, disrupting the network structure in water and reducing the dispersibility of the microparticles. This is supported by the significantly lower viscosity of Comparative Example 7 compared to Comparative Example 5. Furthermore, in Comparative Examples 6 and 7, the supernatant layer had low light transmittance, was turbid, and was confirmed to be insufficiently aggregated.

[0071] In Comparative Example 8, we investigated an example in which carboxymethyl cellulose (CMC), a commonly used thickener, was used instead of the fine fiber CNF. Even when 3.9% by mass of RCNF was added, which was significantly higher than the content of RCNF relative to the amount of the mixture excluding the fine particles in the Examples, the viscosity of the mixture in Comparative Example 8 remained at the same level as in Example 2, where the content of RCNF relative to the amount of the mixture excluding the fine particles was 0.78% by mass, and the dispersibility of the fine particles was also reduced. This suggests that the use of RCNF not only exerts a thickening effect, but also that the network structure formed by the RCNF contributes to maintaining the dispersed state.

[0072] <Examples showing an aggregated state and corresponding comparative examples> For each combination of fine particles and CNF shown in Table 4, a mixed solution was prepared by adjusting the type and amount of each to the values ​​shown in the table, and the state of aggregation was evaluated. The results are shown in Table 4.

[0073] [Table 4]

[0074] In Examples 11 to 13, in which the amount of RCNF in the mixed liquid was significantly less than that of the Examples in Table 3 above, the liquid height of the dispersion layer decreased, and the fine particles aggregated, resulting in an increased light transmittance and a clear supernatant layer. In contrast, in Comparative Examples 9 and 10, in which no RCNF was added, the light transmittance of the supernatant layer was low and it was cloudy. These comparisons confirmed that RCNF functions satisfactorily as a flocculant.

[0075] However, even when RCNF is added, if the RCNF content of the mixture excluding the fine particles is too low, as in Reference Example 3, the supernatant layer becomes cloudy, and it was confirmed that the flocculation effect is naturally insufficient. From this, it is thought that if RCNF is to be used as a flocculant, approximately 0.05 to 0.2 mass% of RCNF is required relative to the mixture excluding the fine particles. On the other hand, in Example 14, where the RCNF content of the mixture excluding the fine particles is similar to that of Example 13, and the amount of fine particles added is large, the light transmittance of the supernatant layer is sufficiently high, but the liquid height of the dispersion layer is somewhat high. From this, it was confirmed that even if there is a sufficient amount of RCNF, if there are too many fine particles, it may become impossible to flocculate.

[0076] In Examples 15 to 17, in which the type of microparticles was changed, the light transmittance of the supernatant layer was sufficiently high compared to Comparative Examples 11 to 13, which did not contain RCNF. This confirmed that RCNF exerts a sufficient flocculation effect regardless of the type of microparticles.

[0077] In Comparative Example 14, which used mechanically defibrated CNF, the supernatant layer was cloudy compared to Example 12, which used the same amount of RCNF, confirming that RCNF exhibits higher aggregation ability.

[0078] In Comparative Examples 15 and 16, the CNF used in Example 12 was changed from RCNF to chemically modified CNF. In Comparative Examples 15 and 16, the light transmittance of the supernatant layer was lower than in Example 12, and the results were almost the same as in Comparative Example 10, which did not contain CNF, and no flocculation effect was obtained. From this, it is thought that RCNF does not have highly polar functional groups compared to chemically modified CNF, and therefore, even when adsorbed to the surface of fine particles, the repulsion between the fine particles is small, and it functions stably as a flocculant.

[0079] <Check each condition with enlarged photos> For Comparative Example 11, which contained only calcium carbonate (2) without added RCNF, and Example 15, which contained a small amount of RCNF and in which the calcium carbonate was in an aggregated state, the mixed solution in the dispersion layer was sampled, the RCNF was fluorescently stained, and photographs were taken at 100x magnification using a fluorescence microscope (Keyence Corporation: BZ-X100). The respective photographs are shown in Figures 2(a) and (b). While Figure 2(b) is difficult to distinguish due to the black-and-white processing, the colored portions of the particles from the upper left to the lower right are fluorescently stained, and it can be confirmed that RCNF is adsorbed all around each microparticle.

[0080] For Example 2, which contained a large amount of RCNF and dispersed calcium carbonate (1), the mixture from the dispersion layer was collected, freeze-dried, and photographed with an FE-SEM. The photograph is shown in Figure 2(c). It was confirmed that the RCNF formed a network structure and that the fine particles were incorporated into the network.

[0081] <Comparison of dispersed and aggregated states> Figure 3 shows photographs of the conditions after 7 days in test tubes, where calcium carbonate was used as the microparticles and the RCNF content was different. The corresponding tables are also shown.

[0082] In the photographs, (1) and (6) represent Comparative Example 1, which does not contain RCNF. In the photographs, (2) to (5) represent Example 11, Reference Example 1, Example 2, and Example 1, each containing 0.11 mass% of RCNF relative to the amount of the mixed liquid excluding the fine particles, respectively. In Example 11, which contains a small amount of RCNF, the fine particles are sufficiently aggregated. As the amount of RCNF increases, the liquid height of the dispersion layer gradually rises, despite aggregation, and in Examples 1 and 2, where the amount is 0.70 mass% or more, the dispersion state is such that almost the entire surface becomes a dispersion layer.

[0083] In the photograph, (6) is Comparative Example 1, which, like (1), does not contain RCNF. In the photograph, (7) is Reference Example 3, in which the RCNF is 0.01 mass% relative to the amount of the mixed liquid excluding the fine particles, and in the photograph, (8) is Example 12, in which the RCNF is 0.06 mass%. In Reference Example 3, the fine particles still float in the supernatant layer, making it turbid and indicating insufficient aggregation. In Example 12, the white parts that appear in the photograph are the light source reflected on the surface of the test tube, and the supernatant layer is sufficiently transparent.

[0084] <Study on the order of addition> Example 18: Dispersion state 21.7 g of distilled water was added to 5 g of calcium carbonate (1) and used with the homogenizer (8,000 rpm for 5 minutes) to prepare a microparticle aqueous suspension. 23.3 g of an RCNF aqueous suspension with a CNF concentration of 1.5% by mass was added to this microparticle aqueous suspension, and the suspension was dispersed under the same conditions as in Example 2. The suspension was then collected in the test tube. The height of the microparticle dispersion layer was measured after 30 minutes, 7 days, and 1 month of standing, and compared with that of Example 2. The results are shown in Table 5. In Example 2, in which microparticles were added to the RCNF aqueous suspension, and Example 18, in which the order was reversed and RCNF was added to the microparticle aqueous suspension, the RCNF content relative to the amount of calcium carbonate (1) in the mixed solution was the same at 0.78% by mass. Although there was a slight difference in the maintenance of the dispersed state, it was confirmed that the dispersed state could be maintained sufficiently even when the order was reversed.

[0085] [Table 5]

[0086] Example 19: Aggregation state The RCNF aqueous suspension of Sample 1 was diluted with distilled water to obtain an RCNF aqueous suspension with a CNF concentration of 0.78% by mass. 5 g of calcium carbonate (1) was added to 45 g of this RCNF aqueous suspension to adjust the total volume to 50 g. Dispersion treatment was performed under the same conditions as in Example 12, and the suspension was then collected in the test tube. After leaving the suspension to stand for 30 minutes, the liquid height of the dispersion layer and the light transmittance of the supernatant layer were measured and compared with those of Example 12. The results are shown in Table 6. The liquid height of the dispersion layer and the light transmittance of the supernatant layer were nearly identical in Example 12, in which the RCNF aqueous suspension was added to the microparticle aqueous suspension, and Example 19, in which the microparticles were added to the RCNF aqueous suspension in the reversed order. This confirmed that both procedures, even when performed in reverse, provided sufficient flocculation effects.

[0087] [Table 6]

[0088] <State transition from dispersed state to aggregated state> Example 20 4 g of the dispersed microparticle mixture of Example 4 (RCNF: 1.0 mass%, calcium carbonate (I): 20 mass%) was collected, 36 g of distilled water was added thereto, and the mixture was dispersed using the homogenizer, and then collected in the test tube. After leaving the mixture to stand for 30 minutes, the liquid height of the dispersed layer and the light transmittance of the supernatant layer were measured. The results are shown in Table 7. The dispersed mixture was able to transition to an aggregated state, with a supernatant layer having a sufficiently high light transmittance.

[0089] [Table 7]

[0090] <State transition from aggregated state to dispersed state> Example 21 20 g of the aggregated particle mixture from Example 11 (RCNF: 0.1% by mass, calcium carbonate (I): 10% by mass) was collected while stirring to make it uniform, and 20 g of an RCNF aqueous suspension with a CNF concentration of 1.3% by mass was added to it. After dispersion treatment using a homogenizer, the mixture was collected in the test tube and the change in the liquid height of the dispersion layer over time was observed. The results are shown in Table 8. The mixture, which was in an aggregated state, was able to transition so that it could maintain a dispersed state for a long period of time.

[0091] [Table 8]

[0092] <State transition summary> From Examples 20 and 21, it was confirmed that the mixed solution of RCNF and microparticles can transition from a dispersed state to an aggregated state or from an aggregated state to a dispersed state by adjusting the RCNF content, and that RCNF acts as a dispersant and aggregate.

Claims

1. A mixture containing microparticles and unmodified CNF.

2. The mixture according to claim 1, wherein the liquid height of the dispersion layer in which the microparticles are dispersed using the unmodified CNF as a dispersant is 90% or more of the liquid height of the entire mixture after being left standing for 7 days.

3. 2. The mixture of claim 1, having a supernatant layer with a light transmittance of 95%.

4. A preparation method for transitioning the dispersion or aggregation state of the microparticles by changing the content of the unmodified CNF in the mixed solution described in any one of claims 1 to 3.

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