Cellulose nanofiber powder dispersant and production method therefor

Abstract

Provided is a cellulose nanofiber powder dispersant capable of dispersing fillers (individual particles) in a solvent in water or oil. This cellulose nanofiber powder dispersant comprises 75-90 wt% of cellulose nanofibers, 7-12 wt% of ethylene glycol, 0.2-0.5 wt% of an acrylic acid copolymer, 0.4-0.8 wt% of water-soluble sorbitan, 0.6-1.0 wt% of a higher fatty acid, and 0.4-0.8 wt% of an alkylammonium salt.

Description

Cellulose nanofiber powder dispersant and method for producing same

[0001] The present invention relates to a cellulose nanofiber powder dispersant and a method for producing the same. In particular, the present invention relates to a cellulose nanofiber powder dispersant that has excellent dispersing effects for various fillers such as micro-sized plant fibers, synthetic fibers, inorganic and organic pigments, carbon, calcium carbonate, and glass fibers, whether in water or oil, and a method for producing such a cellulose nanofiber powder dispersant.

[0002] Cellulose nanofiber (also known as "Cellulose Nano Fiber"; hereafter, simply referred to as "CNF") is a next-generation plant-derived material that is said to be five times stronger than steel at one-fifth the weight. Utilizing cellulose nanofiber in automobiles, home appliances, etc. is expected to reduce weight, improve energy efficiency, and make a significant contribution to combating global warming. Toward the social implementation of CNF, the Ministry of Economy, Trade and Industry and the Ministry of Agriculture, Forestry and Fisheries are collaborating to implement model projects in various fields, including automobiles, home appliances, and housing / building materials, to evaluate and verify the CO2 reduction effects and to demonstrate solutions to related issues.

[0003] As one application example of cellulose nanofibers, Patent Document 1 (JP 2022-044861 A) discloses a hot melt adhesive containing cellulose nanofiber fibers, which comprises a resin that melts in the range of 70°C to 160°C and a powder of cellulose nanofiber fibers. It is disclosed that such a hot melt adhesive can instantly solidify the bonded area, has excellent adhesive properties even at low temperatures, and can firmly maintain the bonded surface.

[0004] Furthermore, in recent years, cellulose nanofibers have begun to be used widely in water-based paints, and are also beginning to be used in cosmetics and foods that incorporate their moisturizing properties.In addition, cellulose nanofibers have a lower specific gravity than glass, metal, and carbon, and their nano-sized fiber length is expected to improve functionality such as increasing the strength of resins, making them an ideal material to add to resin materials as a reinforcing material.

[0005] Cellulose nanofibers are generally sold in the form of a dispersion (slurry, sol, etc.) in which they are dispersed in a dispersing medium such as water at a solids content of approximately 1 to 10% by weight, and the cellulose nanofiber dispersion at a predetermined concentration after production is usually used for various purposes as an industrial material or an additive material for foods and cosmetics. However, compounding with hydrophobic materials such as resins and rubbers requires the removal of water. Currently, there are several methods for removing water from a dispersion in which cellulose nanofibers are dispersed in water, including precipitation, centrifugation, filtration, spray drying, and freeze drying.

[0006] For example, Patent Document 2 (JP 2022-028316 A) discloses a container for freeze-drying cellulose nanofibers, which includes a main body that stores a brine solution inside and a jacket that covers the outer periphery of the main body and is supplied with a heat medium, and the main body is capable of cooling the brine solution by supplying the heat medium to the jacket. According to Patent Document 2, it is possible to provide a container that can freeze a cellulose nanofiber dispersion while suppressing aggregation of the cellulose nanofibers.

[0007] JP 2022-044861 A JP 2022-028316 A

[0008] However, there has been little research into the use of cellulose nanofiber powder, which is a dried form of cellulose nanofiber, as a dispersant. According to the inventors' findings, cellulose nanofiber powder with an appropriate composition can be used in both water and oil, and in some cases can achieve dispersion performance that exceeds the critical micelle concentration of existing surfactants.

[0009] The present invention was completed based on the above findings, and in one embodiment, it aims to provide a cellulose nanofiber powder dispersant that can disperse fillers (solid particles) in water or oil solvents. In another embodiment, it aims to provide a method for producing such a cellulose nanofiber powder dispersant.

[0010] After extensive research, the present inventors discovered that the above-mentioned problems can be solved by a method different from that of conventional techniques. Specifically, by preparing a cellulose nanofiber powder dispersant containing specific components, the cellulose nanofiber powder dispersant can effectively disperse a filler in a solvent, whether in water or oil. The present invention was completed based on this finding, and is exemplified below.

[0011] [1] A cellulose nanofiber powder dispersant comprising 75 to 90% by weight of cellulose nanofibers, 7 to 12% by weight of ethylene glycol, 0.2 to 0.5% by weight of an acrylic acid copolymer, 0.4 to 0.8% by weight of water-soluble sorbitan, 0.6 to 1.0% by weight of a higher fatty acid, and 0.4 to 0.8% by weight of an alkylammonium salt. [2] The cellulose nanofiber powder dispersant according to [1], wherein the cellulose nanofiber powder has an average fiber length in the range of 0.1 μm to 3.0 μm. [3] The cellulose nanofiber powder dispersant according to [1] or [2], wherein the cellulose nanofiber powder has an average fiber diameter in the range of 0.5 nm to 10 nm. [4] A method for producing a cellulose nanofiber powder dispersion, comprising: Step A of preparing a cellulose nanofiber dispersion; and Step B of supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill to dry the cellulose nanofiber dispersion and obtain a cellulose nanofiber powder, wherein the cellulose nanofiber dispersion is prepared to contain, in terms of solid content, 75 to 90 wt % cellulose nanofibers, 7 to 12 wt % ethylene glycol, 0.2 to 0.5 wt % acrylic acid copolymer, 0.4 to 0.8 wt % water-soluble sorbitan, 0.6 to 1.0 wt % higher fatty acid, and 0.4 to 0.8 wt % alkylammonium salt. [5] The method according to [4], wherein Step B comprises heating the surface temperature of the two-roll mill to 95°C to 130°C.

[0012] According to one embodiment of the present invention, a cellulose nanofiber powder dispersant capable of dispersing a filler (solid particles) in a solvent such as water or oil can be provided. According to another embodiment of the present invention, a method for producing such a cellulose nanofiber powder dispersant can be provided.

[0013] Fig. 1 is a schematic diagram illustrating the structure and operating principle of a two-roll mill in one embodiment of the present invention. Fig. 2 is a photograph showing the state of a cellulose nanofiber dispersion dried by a two-roll mill in an example of the present invention. Fig. 3 is a photograph showing a cellulose nanofiber powder dispersion obtained by a two-roll mill in an example of the present invention.

[0014] Next, embodiments of the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes and improvements may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

[0015] (1. Raw materials for cellulose nanofibers) Cellulose nanofibers are made by finely grinding cellulose, the main component of plant fibers, to nano-size particles, and their main raw material is wood pulp (the raw material for paper). They are used primarily for the purposes of reinforcing resin materials and preventing resin shrinkage at low temperatures. In the present invention, there are no particular limitations on the raw materials for cellulose nanofibers.

[0016] Because cellulose nanofibers are typically plant-derived materials, they are in the form of a slurry dispersed in water when extracted from the plant. The solids content is typically 1 to 10% by weight. For example, when producing a powder dispersant from a cellulose nanofiber dispersion, it is first necessary to remove only the water from the cellulose nanofiber aqueous dispersion dispersed in water to extract the cellulose nanofibers alone. It is also conceivable that the cellulose nanofibers alone may be redispersed in water or a dispersant other than water, and then extracted again from the redispersion. Therefore, in the present invention, the dispersant in the cellulose nanofiber dispersion is not limited to water. However, the dispersant in the cellulose nanofiber dispersion is preferably water.

[0017] As mentioned above, the solids content of a cellulose nanofiber dispersion is usually 1 to 10% by weight, but it may also be further diluted to less than 1% by weight. Therefore, the cellulose nanofiber dispersion can also be pre-dried. There are no particular restrictions on the pre-drying method, and any conventional method can be used. The cellulose nanofiber dispersion can be pre-dried, for example, to a maximum solids content of 12% by weight, and then the drying method of the present invention described below can be carried out.

[0018] As described below, cellulose nanofiber powder is obtained by drying a cellulose nanofiber dispersion under specific conditions. The average fiber length of the cellulose nanofiber powder is preferably 0.1 μm or more. This is expected to improve dispersion stability. From this perspective, the average fiber length of the cellulose nanofiber powder is more preferably 0.2 μm or more, even more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. If the average fiber length of the cellulose nanofiber powder is 0.1 μm or more, it has a high aspect ratio relative to the average fiber diameter.

[0019] Furthermore, it is preferable that the average fiber length of the cellulose nanofiber powder is 3.0 μm or less. This prevents the cellulose nanofiber powder from becoming spherical during processing. From this perspective, it is more preferable that the average fiber length of the cellulose nanofiber powder is 2.5 μm or less, even more preferably 1.5 μm or less, and even more preferably 1.0 μm or less.

[0020] The average fiber length of the cellulose nanofiber powder refers to the D50 (median diameter) of the fibers of the cellulose nanofiber powder measured according to the laser diffraction / scattering method of JIS Z8825:2022.

[0021] The average fiber diameter of the cellulose nanofiber powder is preferably 0.5 nm or more. This is expected to improve dispersion stability. From this perspective, the average fiber diameter of the cellulose nanofiber powder is more preferably 0.7 nm or more, even more preferably 1 nm or more, and even more preferably 3 nm or more.

[0022] Furthermore, it is preferable that the average fiber diameter of the cellulose nanofiber powder is 10 nm or less. If the cellulose nanofiber powder becomes too thick, the aspect ratio decreases, and steric hindrance may occur, making it impossible to maintain interparticle distance. From this perspective, it is more preferable that the average fiber diameter of the cellulose nanofiber powder is 8 nm or less, even more preferably 7 nm or less, and even more preferably 5 nm or less.

[0023] The average fiber diameter of the cellulose nanofiber powder refers to the average particle diameter of the fibers of the cellulose nanofiber powder measured according to the dynamic light scattering method of JIS Z8828:2019.

[0024] (2. Cellulose Nanofiber Powder) Hereinafter, the utility and advantages of the cellulose nanofiber powder of the present invention as a dispersant will be described.

[0025] Traditionally, surfactants suitable for hydrophilic and hydrophobic solvents are used to disperse fillers (solid particles) in water or oil solvents. When preventing separation and stabilizing the particles are desired rather than simply dispersing them, amphoteric surfactants are used as effective filler dispersants in mechanically emulsified water and oil mixtures. At low concentrations, amphoteric surfactants dissolved in water tend to gather and align at the interface (surface) rather than exist as individual molecules. Increasing the concentration of amphoteric surfactant in water results in the water surface being covered with surfactant molecules, and numerous surfactant molecules gather together in the water, forming micelles with the hydrophilic groups facing toward the water. If micelles are formed at the critical micelle concentration, incorporating hydrophobic fillers into the micelles enables dispersion in water.

[0026] However, inorganic pigments that have been surface-treated with aluminum or silica, or carbon intended for use at high concentrations, have strong intermolecular attractive forces, so large aggregates form before they can be incorporated into micelles in water. The same thing happens in oil. For this reason, surfactants alone have limitations on dispersibility, and mechanical dispersion is currently required.

[0027] On the other hand, cellulose nanofiber powder has a more flexible fiber shape with a larger aspect ratio than carbon nanotubes, nanocarbon particles, or nanosilica, and when it diffuses into the mesh structure, it acts as a steric hindrance to other fillers, resulting in high retention stability, making it a potential candidate for use as a dispersant.

[0028] Here, dispersions such as 1-8 wt% cellulose nanofiber aqueous solutions can also diffuse into water and disperse other fillers. Therefore, in recent years, cellulose nanofiber aqueous solutions have been used as filler dispersants in paints and coatings. They are also used in the cosmetics industry, where they have been shown to improve the uniformity of particle dispersion in water-soluble moisturizing materials. As such, several methods for using cellulose nanofiber aqueous solutions as dispersants have been established. This is because cellulose nanofibers are extremely long, nanoscale fibers that are structurally hydrophilic. This hydrophilicity and nanoscale diffusion ability can be utilized to utilize the strong interfacial entropy of cellulose nanofiber aqueous solutions as a dispersing ability. Therefore, cellulose nanofiber powder is expected to have better dispersibility than surfactants as a dispersant.

[0029] However, with conventional technology, this dispersion was only possible with aqueous cellulose nanofiber solutions, and hydrophobic substances could only be dispersed in water, limiting the scope of application. Therefore, drying the cellulose nanofiber solution and using it as a powder was considered, but simply drying the cellulose nanofiber dispersion made it difficult to return to its original dispersion state, even in water, let alone in oil, and thus it could not function as a dispersant for other fillers.

[0030] Therefore, the present invention successfully produced a cellulose nanofiber powder that exhibits excellent dispersion stability and dispersion retention after diffusion in a solvent, even among nanomaterials, regardless of whether the solvent is water or oil, by dispersing the cellulose nanofiber itself in either water or oil and obtaining a dried product. This cellulose nanofiber powder dispersant can be used as a dispersant for a variety of fillers, including micro-sized plant fibers, synthetic fibers, inorganic and organic pigments, carbon, calcium carbonate, and glass fiber. In some preferred embodiments of the present invention, the cellulose nanofiber powder dispersant is a micelle that disperses fillers, and has the advantage of being usable in areas beyond the limitations of conventional technology.

[0031] In one embodiment of the present invention, the cellulose nanofiber powder dispersant comprises 75 to 90% by weight of cellulose nanofibers, 7 to 12% by weight of ethylene glycol, 0.2 to 0.5% by weight of an acrylic acid copolymer, 0.4 to 0.8% by weight of a water-soluble sorbitan, 0.6 to 1.0% by weight of a higher fatty acid, and 0.4 to 0.8% by weight of an alkylammonium salt.

[0032] If the amount of cellulose nanofibers is less than 75% by weight, the dispersing effect of the dispersant having cellulose nanofibers as its core will be weakened. From this perspective, the amount of cellulose nanofibers is 75% by weight or more, preferably 78% by weight or more, more preferably 80% by weight or more, even more preferably 82% by weight or more, and even more preferably 85% by weight or more. On the other hand, if the amount of cellulose nanofibers exceeds 90% by weight, the effects of other components will be weakened, and the dispersing effect of the dispersant will decrease. From this perspective, the amount of cellulose nanofibers is 90% by weight or less, preferably 89% by weight or less, and more preferably 88% by weight or less.

[0033] Ethylene glycol functions as a water-soluble solvent, has a strong bonding power with other chemicals, and is also used as a heat stabilizer. Although cellulose nanofibers do not chemically bond, ethylene glycol has a high adsorption property to plant fibers and has a wetting effect on cellulose nanofibers.

[0034] The amount of ethylene glycol added is 7 to 12 wt % relative to the total weight of the cellulose nanofiber powder dispersant. If the amount of ethylene glycol added is less than 7 wt %, the effect cannot be fully exerted. From this perspective, the amount of ethylene glycol added is preferably 8 wt % or more, more preferably 9 wt % or more. On the other hand, if the amount of ethylene glycol added exceeds 12 wt %, the product becomes sticky and processability deteriorates. From this perspective, the amount of ethylene glycol added is preferably 11 wt % or less, more preferably 10 wt % or less. Ethylene glycol is usually added in the form of an aqueous solution, but it may also be added as a single substance.

[0035] Acrylic acid copolymers have the function of accelerating the evaporation time of water, and, in the case of fibrous fillers in particular, they have the effect of suppressing the unraveling of fiber bundles and the formation of fuzzball-like aggregates. Specific examples of acrylic acid copolymers that can be used preferably include acrylic acid-styrene copolymer, acrylic acid-methacrylic acid copolymer, acrylic acid-vinyl acetate copolymer, and acrylic acid-ethylene copolymer. These acrylic acid copolymers all have the above-mentioned effects, so they can be used interchangeably.

[0036] The amount of acrylic acid copolymer added is 0.2 to 0.5 wt % relative to the total weight of the cellulose nanofiber powder dispersant. If the amount of acrylic acid copolymer added is less than 0.2 wt %, the effect cannot be fully exerted. From this perspective, the amount of acrylic acid copolymer added is preferably 0.3 wt % or more. On the other hand, if the amount of acrylic acid copolymer added exceeds 0.5 wt %, the effect will plateau. From this perspective, the amount of acrylic acid copolymer added is preferably 0.4 wt % or less. The acrylic acid copolymer is usually added in the form of an aqueous solution, but it may also be added as a single solution.

[0037] Water-soluble sorbitan is excellent at stabilizing emulsions, has a high activation effect on the filler surface, and exhibits little change in viscosity with temperature even at low temperatures. It also has the effect of suppressing the swelling of pulp and cellulose nanofibers, and at the same time, even if the filler dries, it remains easily soluble in water as long as the water-soluble sorbitan is attached.

[0038] The amount of water-soluble sorbitan added is 0.4 to 0.8 wt % relative to the total weight of the cellulose nanofiber powder dispersant. If the amount of water-soluble sorbitan added is less than 0.4 wt %, the effect cannot be fully exerted. From this perspective, the amount of water-soluble sorbitan added is preferably 0.5 wt % or more. On the other hand, if the amount of water-soluble sorbitan added exceeds 0.8 wt %, the effect will plateau. From this perspective, the amount of water-soluble sorbitan added is preferably 0.7 wt % or less. Note that water-soluble sorbitan is usually added in the form of an aqueous solution, but it may also be added as a single substance.

[0039] The higher fatty acid melts during the treatment with the two-roll mill described below, creating a wax-like effect. That is, the dried cellulose nanofibers have improved releasability from the plated or metal roll surface and peel off naturally. In this specification, the higher fatty acid refers to a fatty acid having 10 to 25 carbon atoms, which may be saturated or unsaturated, and may be linear, branched, or of another shape.

[0040] The amount of higher fatty acid added is 0.6 to 1.0 wt % relative to the total weight of the cellulose nanofiber powder dispersant. If the amount of higher fatty acid added is less than 0.6 wt %, the effect cannot be fully exerted. From this perspective, the amount of higher fatty acid added is preferably 0.7 wt % or more. On the other hand, if the amount of higher fatty acid added exceeds 1.0 wt %, the effect will plateau. From this perspective, the amount of higher fatty acid added is preferably 0.9 wt % or less. Note that higher fatty acids are usually added in the form of an aqueous solution, but they may also be added as a single solution.

[0041] Alkylammonium salts have the effect of preventing the aggregation of cellulose nanofibers as the water content decreases during drying.

[0042] Examples of alkylammonium salts include, but are not limited to, distearyldimethylammonium chloride, behenyltrimethylammonium chloride, stearyltrimethylammonium chloride, stearyltrimethylammonium chloride, cetyltrimethylammonium chloride, lauryltrimethylammonium chloride, and benzalkonium chloride. These are usually added in the form of an aqueous solution. Alkylammonium salts have the effect of preventing the aggregation of cellulose nanofibers as the water content decreases.

[0043] The amount of alkylammonium salt added is 0.4 to 0.8 wt % relative to the total weight of the cellulose nanofiber powder dispersant. If the amount of alkylammonium salt added is less than 0.4 wt %, the effect cannot be fully exerted. From this perspective, the amount of alkylammonium salt added is preferably 0.5 wt % or more, more preferably 0.6 wt % or more. On the other hand, if the amount of alkylammonium salt added exceeds 0.8 wt %, the effect will plateau. From this perspective, the amount of alkylammonium salt added is preferably 0.7 wt % or less. Note that alkylammonium salts are usually added in the form of an aqueous solution, but they may also be added as a single substance.

[0044] In a preferred embodiment of the present invention, the higher fatty acid and alkylammonium salt are added as a mixture with isopropyl alcohol (IPA) during the preparation of the cellulose nanofiber powder dispersant. For example, among higher fatty acids, the general chemical formula for carboxylic acid is R-COOH (R represents a carboxylic acid substituent), and the chemical formula for isopropyl alcohol is C3H8O. The reaction formula for a mixture of the two is R-COOH + C3H8O → R-COOC3H7 + HO. That is, the higher fatty acid reacts with isopropyl alcohol to produce a fatty acid ester (R-COOC3H7) and water (HO). The fatty acid ester enhances the heat resistance and wetting properties of the fibers, while the isopropyl alcohol absorbs the resulting water, lowering the boiling point of the water in the slurry medium and accelerating its evaporation time. Accelerated water removal enhances the adsorption of other chemicals to the cellulose nanofibers.

[0045] When the cellulose nanofiber dispersion is dried, the isopropyl alcohol disappears due to evaporation or the like, and therefore, in some embodiments of the present invention, the cellulose nanofiber powder dispersant does not contain isopropyl alcohol.

[0046] In one embodiment of the present invention, the cellulose nanofiber powder dispersant of the present invention can be obtained by adding each of the above additives to a cellulose nanofiber dispersion and drying it. Stirring is preferred for mixing. During stirring, it is desirable for the cellulose nanofibers to be kneaded in a spiral pattern while maintaining their orientation in the liquid or sol. While there are no particular limitations on the stirring method or device, suitable devices include a Super Mixer (manufactured by Kawata Corporation), a Henschel Mixer (manufactured by Nippon Coke and Engineering Co., Ltd.), and a High-Speed ​​Mixer (manufactured by Earth Technica Corporation).

[0047] (3. Method for Producing Cellulose Nanofiber Powder Dispersion) The method for producing a cellulose nanofiber powder dispersion of the present invention comprises step A of preparing a cellulose nanofiber dispersion, and step B of supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill to dry the cellulose nanofiber dispersion and obtain cellulose nanofiber powder, the cellulose nanofiber dispersion being prepared to contain, in terms of solids content, 75 to 90% by weight of cellulose nanofibers, 7 to 12% by weight of ethylene glycol, 0.2 to 0.5% by weight of acrylic acid copolymer, 0.4 to 0.8% by weight of water-soluble sorbitan, 0.6 to 1.0% by weight of higher fatty acid, and 0.4 to 0.8% by weight of alkylammonium salt.

[0048] In step A of preparing a cellulose nanofiber dispersion, the types and amounts of each component of the cellulose nanofiber dispersion are as described above, and therefore will not be described here.

[0049] After step A, the cellulose nanofiber dispersion is supplied to a two-roll mill, and the two-roll mill is rotated to remove the cellulose nanofiber dispersion and obtain a thick, dried cellulose nanofiber powder (step B).

[0050] By supplying the cellulose nanofiber dispersion to the middle of the two-roll mill, the cellulose nanofiber dispersion is absorbed into the roll gap (nip) (Figure 1). A characteristic of the two-roll mill is that before entering the nip, the input raw material rotates in the same direction as the rolls in the roll bank above the roll gap. At this time, the cellulose nanofibers maintain their orientation, and when they enter the nip, the fibers are aligned in the same direction. This causes shear shear in the nip while maintaining the orientation of the cellulose nanofibers. The raw material, whose boiling point is lowered by self-heating due to shear shear and the heating temperature of the two-roll mill, easily vaporizes, shortening the drying time. The distance between the rolls (clearance) is not particularly limited, but in this embodiment, it is preferably 0.3 to 1.5 mm.

[0051] The greater the rotation ratio (i.e., the difference in rotation speed) between the front roll and the rear roll in a two-roll mill, the faster the material will be cut into the nip. On the other hand, if the rotation ratio between the front roll and the rear roll is too high, the shear generated by the rotation ratio will be small and the time it takes for water to evaporate will be short, so it is preferable to set the rotation ratio to 1 to 3. Furthermore, if there is no rotation ratio, the cellulose nanofiber dispersion will not be able to cut into the nip and will tend to stagnate on the roll bank.

[0052] Self-heating in the nip and heat transfer from the heated two-roll mill cause the intermolecular forces of water and thermal energy to attract each other while in liquid form, causing interactions with nearby molecules. Under the sudden high temperature, the water molecules gain enough energy to overcome the intermolecular forces with the fibers, and the water and alcohol instantly transition from liquid to gas. As mentioned above, the cellulose nanofibers enter the nip while maintaining their orientation, so they undergo vaporization in a steam explosion in the same direction, with little random stress on the cellulose nanofibers, allowing them to dry in a state close to their original form (with the double helix structure still largely intact).

[0053] Here, kneading machines other than two-roll mills, such as pressure kneaders, Banbury kneaders, and extruders, are considered. However, because these machines knead randomly, even if the water evaporates instantly, the cellulose nanofibers tend to entangle with each other and form lumps. Furthermore, conventional techniques such as freeze drying, drying ovens, and spray drying do not provide a means for imparting orientation to the cellulose nanofibers, and the drying process takes time, which tends to loosen the double helix structure and form lumps. Therefore, two-roll processing offers advantages not available with conventional techniques. The two-roll mill may be any device capable of kneading the raw materials between two rolls, and may also have a third or subsequent roll.

[0054] Furthermore, if the surface temperature of the two-roll mill is too high, there is a risk that the cellulose nanofibers will be altered, and the surface will harden during powdering, making it difficult to produce fine particles, so the temperature is preferably 130°C or less, and more preferably 120°C or less. Therefore, in one embodiment of the present invention, the surface temperature of the two-roll mill is 95°C to 130°C.

[0055] As a result of processing using the two-roll mill, cellulose nanofiber powder is obtained. When cellulose nanofiber powder is obtained, the following appearance and properties are typically obtained. - Shape: Plate-like flakes with a size of 1 mm to 5 mm and a thickness of 0.5 mm or less. - Color: Milky white and transparent. - Although hard, it can be easily broken into a fine powder with your fingertips. No special crushing process is required. - There is no burning or partial discoloration. - There is almost no scattering into the air even when weighed and packaged.

[0056] Furthermore, because fatty acids and amphoteric surfactants adhere to the surface of the dehydrated cellulose nanofiber powder, it is less likely to burn. The adhesion of higher fatty acids or higher fatty acid amides in particular provides high heat resistance. Furthermore, the two-roll mill offers the advantage of high productivity, as it allows for continuous supply of raw materials and recovery of the cellulose nanofiber powder dispersant as a dried product.

[0057] Examples of the present invention will be described below together with comparative examples. These examples are provided for a better understanding of the present invention and its advantages, and are not intended to limit the invention.

[0058] (Cellulose nanofiber powder dispersant production process) As the "hydrophilic dispersant" listed in Table 1 below, ethylene glycol, acrylic acid copolymer, and water-soluble sorbitan were placed in one container and mixed with a dissolver mixer at room temperature and 700 rpm for 10 minutes. Furthermore, as the "IPA diluent fatty acid" listed in Table 1, IPA (isopropyl alcohol), a higher fatty acid, and an alkyl ammonium salt were placed in one container and mixed with a dissolver mixer at room temperature and 700 rpm for 10 minutes. The mixtures of "hydrophilic dispersant" and "IPA diluent fatty acid" were mixed and stirred at 1500 rpm for 5 minutes in a Henschel mixer (high-speed type) manufactured by Nippon Coke & Engineering Co., Ltd., and mechanically emulsified. This mixture was liquid, and an aqueous cellulose nanofiber solution with a solids content of 8% by weight was added to it and mixed with a dissolver mixer at room temperature and 500 rpm for 5 minutes. The amounts of each component added were as shown in Table 1.

[0059]

[0060] The "acrylic acid copolymer" is BYK-199BF (main component: acrylic acid-methacrylic acid copolymer) manufactured by BYK Corporation, the "water-soluble sorbitan" is Rheodol TW-L120 (content composition: polyoxyethylene sorbitan monolaurate) manufactured by Kao Corporation, and the "higher fatty acid" is a 1:1 mixture of BYK-109 (high molecular weight alkylol aminoamide) and BYK-9077 (polyglycol polyester modified polyalkyleneimine) manufactured by BYK Corporation (main component: chemical formula C 10 H 20 O2) was used, and the "alkyl ammonium salt" was BYK-9076 (a 50 to 60% by mass solution of alkyl ammonium salt of high molecular weight copolymer) manufactured by BYK.

[0061] Regarding "hydrophilic dispersants," a mixture of acrylic acid copolymer, methacrylic acid copolymer, and ethylene glycol is acrylic acid copolymer (C3H4O2) n , methacrylic acid copolymer (C5H8O2) n However, when ethylene glycol C2H6O2 is dissolved and mixed in a solvent, (C3H4O2) m +(C5H8O2) n +C2H6O2→(C3H4O2) m (C5H8O2) n (C2H6O2) mixture, which supports the diffusion of fillers in water.

[0062] Regarding "IPA diluent fatty acid", (C3H4O2) m (C5H8O2) n (C2H6O2) + C3H8O (IPA) → (C3H4O2) m (C5H8O2) n By the reaction of (C2H6O2) (C3H8O), isopropyl alcohol participates in the mixing reaction and combines with the copolymer. 10 H 20 O2 and alkylammonium salt NR4 + is dissolved, R4N + C3H7O - +C 10 H 20O2→R4N+C3H7OC 10 H 19 O2 + H is produced, which is a mixture of alkyl moieties substituted with ester groups, which supports filler wetting in oil.

[0063] Since both the "hydrophilic dispersant" and the "IPA diluent fatty acid" adhere to and wet the cellulose nanofibers, if they are emulsified by high-speed stirring before being added to the cellulose nanofiber aqueous solution, the mixed additives will remain stable and dispersed in the aqueous solution without causing the cellulose nanofibers to aggregate. The final mixture obtained is a highly viscous paste.

[0064] The above mixture was loaded into a two-roll mill manufactured by Yasuda Seiki Seisakusho Co., Ltd. (roll diameter: 8 inches; surface treatment: gloss plating; nip clearance: 0.5 mm). The loading amount was 300 g. The rolls were heated to a surface temperature of 120°C. Next, the two-roll mill was operated so that the rotation speed of the front roll was 7 rpm and the rotation speed of the rear roll was 6 rpm, and a sample of the dried product was taken after three rotations of the front roll. The time required for drying was within 30 seconds. Figure 2A is a photograph showing the state of the cellulose nanofiber dispersion dried using the two-roll mill. Figure 2B is a photograph showing the cellulose nanofiber powder dispersion obtained using the two-roll mill.

[0065] For the dried cellulose nanofiber powder dispersant, the amount of each component was confirmed by gas chromatograph mass for the entire composition using a GC-MS series manufactured by Shimadzu Corporation. The temperature was gradually increased using a ceramic crucible combustion system manufactured by Buhler Co., Ltd. to remove charcoal, and the peak elements of the residue were confirmed using an IRA1S series FTIR spectrometer manufactured by Shimadzu Corporation. The water-soluble portion was subjected to mass analysis using a Nexera series high-performance liquid chromatograph manufactured by Shimadzu Corporation. The mass of each component was calculated from the results of these analyses. The results are shown in Table 1.

[0066] (Test 1: Verification of carbon black dispersibility) The following test was conducted to investigate the effect of the cellulose nanofiber powder dispersant on the dispersion of carbon black pigment in water and oil. Mitsubishi Carbon Black #30 was used as the carbon black pigment.

[0067] Specifically, for the oil mixture, assuming a hydrophobic paint, phthalate ester (DOP) and vinyl chloride resin (degree of polymerization 800) were added simultaneously with carbon black pigment and the above dried product (cellulose nanofiber powder dispersant), and the mixture was stirred for 5 minutes with a Three-One Motor Stirrer, and then dispersed once with a three-roll mill (6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Tests were conducted by changing the amount of cellulose nanofiber powder dispersant added as shown in Table 2.

[0068] For the underwater test, assuming a water-based paint, carbon black pigment, water-soluble acrylic resin (Nikasol manufactured by Nippon Carbide Industries Co., Ltd.), and the above dried product were simultaneously added to water, stirred for 5 minutes with a three-one motor mixer, and dispersed by one pass with a three-roll mill (6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Tests were conducted by changing the amount of cellulose nanofiber powder dispersant added as shown in Table 3.

[0069] For the dispersion liquid after dispersion treatment, pigment particle residues (number of particles with a particle diameter (maximum dimension) of 5 μm or more) were counted using a double-groove grind meter (0-100 μm gauge) manufactured by Taiyu Kizai Co., Ltd., and further, the values ​​for the viscosity reduction during dispersion and the thixotropic thickening due to re-aggregation were read using a B-type viscometer BM type manufactured by Toki Sangyo Co., Ltd. (sample temperature 25°C, No. 4 rotor, 6 rpm), and dispersibility was compared. The results are shown in Tables 2 and 3.

[0070]

[0071]

[0072] As can be seen from Tables 2 and 3, in oil, the amount of cellulose nanofiber powder dispersant added relative to the weight of the carbon black pigment is preferably 2 to 8%, with 3 to 5% being the most effective. In water, the amount of cellulose nanofiber powder dispersant added relative to the weight of the carbon black pigment is preferably 2 to 8%, with 3 to 5% being the most effective. Therefore, for carbon black pigment, the dispersion effect is observed with approximately the same amount of cellulose nanofiber powder dispersant added in both water-based and oil-based solutions.

[0073] (Test 2: Verification of dispersibility of titanium dioxide inorganic pigment) The following test was carried out to investigate the effect of the cellulose nanofiber powder dispersant on the dispersion of titanium dioxide inorganic pigment in water and oil. CR-60 titanium dioxide manufactured by Ishihara Sangyo Kaisha, Ltd. was used as the titanium dioxide inorganic pigment.

[0074] Specifically, for the oil mixture, assuming a hydrophobic paint, phthalate ester (DOP) and vinyl chloride resin (degree of polymerization 800) were added simultaneously to titanium dioxide inorganic pigment and the above dried product (cellulose nanofiber powder dispersant), and the mixture was stirred for 5 minutes with a Three-One Motor Stirrer, and then dispersed once with a three-roll mill (6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Tests were conducted by changing the amount of cellulose nanofiber powder dispersant added as shown in Table 4.

[0075] For the underwater test, assuming a water-based paint, the titanium dioxide inorganic pigment and the above dried product were added simultaneously to water, stirred for 5 minutes with a Three-One Motor Stirrer, and dispersed once with a three-roll mill (a 6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Tests were conducted by changing the amount of cellulose nanofiber powder dispersant added as shown in Table 5.

[0076] For the dispersion liquid after dispersion treatment, pigment particle residues (number of particles with a particle diameter (maximum dimension) of 5 μm or more) were counted using a double-groove grind meter (0-100 μm gauge) manufactured by Taiyu Kizai Co., Ltd., and further, the values ​​of the viscosity reduction during dispersion and the thixotropic thickening due to re-aggregation were read using a B-type viscometer BM type manufactured by Toki Sangyo Co., Ltd. (sample temperature 25°C, No. 4 rotor, 6 rpm), and dispersibility was compared. The results are shown in Tables 4 and 5.

[0077]

[0078]

[0079] As can be seen from Tables 4 and 5, in oil, the amount of cellulose nanofiber powder dispersant added relative to the weight of the titanium dioxide inorganic pigment is preferably 2 to 9%, with 3 to 6% being most effective. In water, the amount of cellulose nanofiber powder dispersant added relative to the weight of the titanium dioxide inorganic pigment is preferably 2 to 9%, with 4 to 6% being most effective. Therefore, for titanium dioxide inorganic pigment, the dispersion effect is observed with approximately the same amount of cellulose nanofiber powder dispersant added in both water-based and oil-based solutions.

[0080] (Test 3: Verification of dispersibility of mixed pigments) The following test was conducted to investigate the effect of the cellulose nanofiber powder dispersant on the dispersion of carbon black pigment and titanium dioxide inorganic pigment in water and oil. Mitsubishi Carbon Black #30 was used as the carbon black pigment, and CR-60 titanium dioxide manufactured by Ishihara Sangyo Kaisha, Ltd. was used as the titanium dioxide inorganic pigment.

[0081] Specifically, for the oil mixture, assuming a hydrophobic paint, phthalate ester (DOP) and vinyl chloride resin (degree of polymerization 800) were added simultaneously to carbon black pigment, titanium dioxide inorganic pigment, and cellulose nanofiber powder dispersant, and the mixture was stirred for 5 minutes with a Three-One Motor Stirrer and dispersed once through a three-roll mill (6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Note that the composition of the cellulose nanofiber powder dispersant was changed as shown in Table 6, and a dried product was produced using the above process.

[0082] For the underwater case, assuming a water-based paint, carbon black pigment, titanium dioxide inorganic pigment, and the above dried product were simultaneously added to water, stirred for 5 minutes with a Three-One Motor Stirrer, and dispersed by one pass through a three-roll mill (a 6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). Note that the composition of the cellulose nanofiber powder dispersant was changed as shown in Table 7, and the dried product was manufactured using the above process.

[0083] For the dispersion liquid after dispersion treatment, pigment particle residues (number of particles with a particle diameter (maximum dimension) of 5 μm or more) were counted using a double-groove grind meter (0-100 μm gauge) manufactured by Taiyu Kizai Co., Ltd., and further, the values ​​of the viscosity reduction during dispersion and the thixotropic thickening due to re-aggregation were read using a B-type viscometer BM type manufactured by Toki Sangyo Co., Ltd. (sample temperature 25°C, No. 4 rotor, 6 rpm), and dispersibility was compared. The results are shown in Tables 6 and 7.

[0084]

[0085]

[0086] As can be seen from Table 6, in oil, reducing the higher fatty acid and alkylammonium salt of the cellulose nanofiber powder dispersant proportionally worsens dispersibility. Furthermore, as can be seen from Table 7, in water, reducing the acrylic acid copolymer and water-soluble sorbitan of the cellulose nanofiber powder dispersant proportionally worsens dispersibility. Therefore, when comparing aqueous and oil-based solvents, reducing the amount of the solvent-specific components reduces the dispersion effect in each solvent. From this, it can be understood that in order to make it applicable to both aqueous and oil-based solvents, it is necessary to use cellulose nanofiber as a skeleton and to form a matrix of components suitable for both aqueous and oil-based solvents.

[0087] (Test 4: Comparison of dispersibility between the present invention and commercially available surfactants) Next, the difference in dispersibility between the cellulose nanofiber powder dispersant of the present invention and a commercially available surfactant was compared.

[0088] First, a nonionic phosphate ester was selected as the surfactant. Phosphate esters are compounds in which a hydrocarbon chain and a phosphate group are bonded. They have surfactant properties and are used to disperse fillers through interactions at the interface of oily liquids and solids. Maleic anhydride, a carboxylic acid, was selected as the dispersant. These components were stirred for five minutes using a Three-One motor mixer, and then passed through a three-roll mill (a 6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.) once to perform a dispersion process. Tests were conducted by changing the amount of each component added as shown in Table 8.

[0089] For the dispersion liquid after the dispersion treatment, the pigment particle residues (the number of particles with a particle diameter (maximum dimension) of 5 μm or more) were counted using a double-groove grind meter (0-100 μm gauge) manufactured by Taiyu Kizai Co., Ltd., and further, the viscosity reduction during dispersion and the thixotropic thickening due to re-aggregation were read using a B-type viscometer BM type manufactured by Toki Sangyo Co., Ltd. (sample temperature 25°C, No. 4 rotor, 6 rpm), and dispersibility was compared. The results are shown in Table 8.

[0090]

[0091] As can be seen from Table 8, when the cellulose nanofiber powder dispersant of the present invention (CNF powder dispersant) was used, there were no problems with dispersibility or viscosity stability, and it was good. On the other hand, while the phosphate ester dispersed, the initial viscosity was very high, resulting in a large torque (ampere) when mixing the pigment. Maleic anhydride was sufficient for use with commercially available paints in terms of dispersion and viscosity, but after one hour, some separation of carbon and titanium dioxide was observed, causing color separation.

[0092] Next, a polyoxyethylene alkyl ether, which is a nonionic surfactant and has the highest stability in aqueous solvents, was selected as the surfactant, and a polymer-based polyvinyl alcohol, which is often used in aqueous gravure printing, was selected as the dispersant, and a comparative test was conducted. These components were stirred for 5 minutes using a Three-One motor mixer, and then dispersed once using a three-roll mill (a 6-inch roll mill manufactured by Inoue Seisakusho Co., Ltd.). The amount of each component added was varied as shown in Table 9, and the test was conducted.

[0093] For the dispersion liquid after the dispersion treatment, the pigment particle residues (the number of particles with a particle diameter (maximum dimension) of 5 μm or more) were counted using a double-groove grind meter (0-100 μm gauge) manufactured by Taiyu Kizai Co., Ltd., and further, the viscosity reduction during dispersion and the thixotropic thickening due to re-aggregation were read using a B-type viscometer BM type manufactured by Toki Sangyo Co., Ltd. (sample temperature 25°C, No. 4 rotor, 6 rpm), and dispersibility was compared. The results are shown in Table 9.

[0094]

[0095] As can be seen from Table 9, when the cellulose nanofiber powder dispersant of the present invention (CNF powder dispersant) was used, there were no problems with dispersibility or viscosity stability, and it was good. On the other hand, polyoxyethylene alkyl ether had good dispersion, but its viscosity was only slightly higher than that of the cellulose nanofiber powder dispersant. With regard to polyvinyl alcohol, the amount added was not enough to produce an effect, and it was not possible to disperse it.

Claims

1. A cellulose nanofiber powder dispersion comprising 75-90% by weight of cellulose nanofiber, 7-12% by weight of ethylene glycol, 0.2-0.5% by weight of an acrylic acid copolymer, 0.4-0.8% by weight of a water-soluble sorbitan, 0.6-1.0% by weight of a higher fatty acid, and 0.4-0.8% by weight of an alkyl ammonium salt.

2. The cellulose nanofiber powder dispersant according to claim 1, wherein the average fiber length of the cellulose nanofiber powder is within the range of 0.1 μm to 3.0 μm.

3. The cellulose nanofiber powder dispersant according to claim 1 or 2, wherein the average fiber diameter of the cellulose nanofiber powder is within the range of 0.5 nm to 10 nm.

4. A method for producing a cellulose nanofiber powder dispersion, comprising: step A of preparing a cellulose nanofiber dispersion; and step B of supplying the cellulose nanofiber dispersion to a two-roll mill and rotating the two-roll mill to dry the cellulose nanofiber dispersion and obtain a cellulose nanofiber powder, wherein the cellulose nanofiber dispersion is prepared to contain, in terms of solid content, 75 to 90% by weight of cellulose nanofibers, 7 to 12% by weight of ethylene glycol, 0.2 to 0.5% by weight of acrylic acid copolymer, 0.4 to 0.8% by weight of water-soluble sorbitan, 0.6 to 1.0% by weight of higher fatty acid, and 0.4 to 0.8% by weight of alkylammonium salt.

5. The method according to claim 4, wherein in step B, the surface temperature of the two-roll mill is heated to 95°C to 130°C.

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