Method for producing modified cellulose nanofibers

Abstract

To provide a method for producing a modified cellulose nanofiber that can produce a modified cellulose nanofiber, in a simple production process, which allows the application range of cellulose to be easily widened, is imparted with excellent hydrophobicity, and has excellent affinity with hydrophobic substance.SOLUTION: A method for producing a modified cellulose nanofiber includes a cellulose nanofiber dispersion preparation step for preparing a cellulose nanofiber dispersion having cellulose nanofiber dispersed in water, a silane coupling agent-containing dispersion preparation step for adding a silane coupling agent having epoxy groups to the cellulose nanofiber dispersion to obtain a silane coupling agent- containing dispersion, and a drying step for drying the silane coupling agent-containing dispersion to modify the cellulose nanofiber with the silane coupling agent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing modified nanocellulose modified with a silane coupling agent, and in particular to a method for producing modified cellulose nanofibers that also exhibit affinity for hydrophobic substances. [Background technology]

[0002] Nanocellulose is a cellulose fiber with a thickness on the order of nanometers that is obtained by defibrating plant fibers such as pulp in an aqueous medium. Nanocellulose is classified into cellulose nanofibers, cellulose crystals, cellulose whiskers, bacterial nanocellulose, etc., depending on the structure of the cellulose. Because cellulose nanofibers are lightweight and strong, they have attracted attention for use as a reinforcing material for thermoplastic resins, for example (see, for example, Patent Document 1).

[0003] On the other hand, because cellulose nanofibers are hydrophilic, it is difficult to uniformly disperse them in organic solvents or mix them uniformly with hydrophobic materials such as resins, limiting the scope of their application. To address this issue, for example, a method for producing hydrophobic cellulose nanofibers has been proposed in the past, in which hydrophobic properties are imparted to cellulose nanofibers by reacting hydrated cellulose nanofibers with a vinyl organic acid and then recovering the product (see, for example, Patent Document 2). Hereinafter, cellulose nanofibers obtained by imparting desired properties such as hydrophobicity to naturally derived cellulose nanofibers obtained from plant fibers, etc., may be referred to as modified cellulose nanofibers.

[0004] However, the method for producing modified cellulose nanofibers shown in Patent Document 2 requires strict reaction conditions between cellulose nanofibers and organic vinyl acid, and the production process is complicated. Furthermore, in the method for producing modified cellulose nanofibers modified with organic vinyl acid, the molecular structure of the cellulose nanofiber itself changes depending on the degree of modification of the cellulose nanofiber, leaving room for improvement in terms of improving the hydrophobicity of the cellulose nanofibers.

[0005] In addition, a method for producing modified cellulose nanofibers has been proposed to impart hydrophobic properties to cellulose nanofibers. This method involves attaching two types of modifying groups to the surface of cellulose nanofibers: one selected for wettability (e.g., aromatic groups), and the other selected for steric repulsion and hydrophobicity (e.g., polyalkylene glycol groups). This method converts cellulose nanofibers to hydrophobic properties with a small mass of modifying groups. However, the production process for modified cellulose nanofibers using two types of modifying groups (e.g., aromatic groups and polyalkylene glycol groups) is complex, leaving room for improvement in productivity. Furthermore, modified cellulose nanofibers using two types of modifying groups (e.g., aromatic groups and polyalkylene glycol groups) lack highly reactive functional groups, making it difficult to expand their application fields. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-11392 [Patent Document 2] International Publication No. 2016 / 010016 Summary of the Invention [Problem to be solved by the invention]

[0007] In view of the above circumstances, the present invention aims to provide a method for producing modified cellulose nanofibers that can easily expand the range of applications of cellulose and that can produce modified cellulose nanofibers that have excellent hydrophobicity and therefore excellent affinity with hydrophobic substances through a simple production process. [Means for solving the problem]

[0008] The gist of the configuration of the present invention is as follows. [1] A cellulose nanofiber dispersion preparation step of preparing a cellulose nanofiber dispersion in which cellulose nanofibers are dispersed in water; a silane coupling agent-containing dispersion preparation step of adding a silane coupling agent having an epoxy group to the cellulose nanofiber dispersion to obtain a silane coupling agent-containing dispersion; a drying step in which the silane coupling agent-containing dispersion is subjected to a drying treatment to modify the cellulose nanofibers with the silane coupling agent. Method for producing modified cellulose nanofibers. [2] The method for producing modified cellulose nanofibers according to [1], wherein the drying treatment is a drying treatment using a freeze-drying method. [3] The method for producing modified cellulose nanofibers according to [1] or [2], further comprising a dialysis step of dialyzing the silane coupling agent-containing dispersion using a dialysis membrane between the silane coupling agent-containing dispersion preparation step and the drying step. [4] The method for producing modified cellulose nanofibers according to any one of [1] to [3], wherein the silane coupling agent has at least an epoxy group and an alkoxy group. [5] The method for producing modified cellulose nanofibers according to any one of [1] to [4], wherein the silane coupling agent has an epoxy group, an alkoxy group, and an alkyl group. [6] The silane coupling agent is a compound represented by the following general formula (1): [ka] (In the general formula (1), X represents an organic functional group containing an epoxy group, and R 1 , R 2 , R 3 and each independently represent chloride, an alkoxy group having 1 to 5 carbon atoms, or an alkyl group having 1 to 5 carbon atoms. [7] R 1 , the R 2 , the R 3 The method for producing modified cellulose nanofibers according to [6], wherein at least one of the groups is an alkoxy group having 1 to 5 carbon atoms and at least one of the groups is an alkyl group having 1 to 5 carbon atoms. [8] The method for producing modified cellulose nanofibers according to any one of [1] to [7], wherein the silane coupling agent contains 3-glycidoxypropyl(dimethoxy)methylsilane. [9] A method for producing modified cellulose nanofibers according to any one of [1] to [8], wherein in the silane coupling agent-containing dispersion preparation step, the silane coupling agent is added in an amount of 0.001 mol to 6.0 mol per 1.0 mol of β-glucose unit of the cellulose nanofiber.

[10] A method for producing modified cellulose nanofibers according to any one of [1] to [9], wherein the modified cellulose nanofibers have an average fiber length of 20 μm or more and 200 μm or less, and an average fiber diameter of 4 nm or more and 100 nm or less.

[0009] In the method for producing modified cellulose nanofibers described in [1], silanol groups of a silane coupling agent are chemically bonded to the hydroxyl groups of cellulose to produce modified cellulose nanofibers in which cellulose is modified with a silane coupling agent. In other words, the modified cellulose nanofibers have a structure in which the silane coupling agent is chemically bonded to the hydroxyl groups of cellulose. Note that "cellulose nanofiber" refers to cellulose fibers with an average fiber diameter of less than 1.0 μm, i.e., an average fiber diameter on the nanometer order. [Effects of the Invention]

[0010] According to the method for producing modified cellulose nanofibers of the present invention, a silane coupling agent having an epoxy group is added to a cellulose nanofiber dispersion and then dried, thereby producing modified cellulose nanofibers in which at least a portion of the hydroxyl groups of the cellulose are modified with the silane coupling agent having an epoxy group. Therefore, modified cellulose nanofibers can be produced using a simple production process, resulting in excellent production efficiency. Furthermore, according to the method for producing modified cellulose nanofibers of the present invention, modified cellulose nanofibers that are imparted with excellent hydrophobicity and have excellent affinity with hydrophobic substances can be produced. Therefore, the modified cellulose nanofibers obtained by the production method of the present invention are uniformly dispersed in hydrophobic substances such as organic solvents and resins, thereby expanding the range of applications of cellulose nanofibers and reliably imparting the properties of cellulose nanofibers to the target applications.

[0011] Furthermore, the method for producing modified cellulose nanofibers of the present invention can produce modified cellulose nanofibers having highly reactive epoxy groups as functional groups, which allows the obtained modified cellulose nanofibers to be further modified via the epoxy groups. Therefore, the modified cellulose nanofibers obtained by the production method of the present invention can easily be used with a wider range of hydrophobic substances.

[0012] According to the method for producing modified cellulose nanofibers of the present invention, the drying process is carried out using freeze-drying, which makes it possible to facilitate the drying process.

[0013] According to the method for producing modified cellulose nanofibers of the present invention, a dialysis step of dialyzing the silane coupling agent-containing dispersion is further included between the silane coupling agent-containing dispersion preparation step and the drying step. This makes it possible to remove the silane coupling agent remaining in the silane coupling agent-containing dispersion, thereby improving the purity of the modified cellulose nanofibers.

[0014] According to the method for producing modified cellulose nanofibers of the present invention, the silane coupling agent contains at least an epoxy group and an alkoxy group, thereby reliably modifying cellulose and producing modified cellulose nanofibers that are reliably imparted with excellent hydrophobicity.

[0015] According to the method for producing modified cellulose nanofibers of the present invention, the silane coupling agent contains an epoxy group, an alkoxy group, and an alkyl group, which reliably modifies cellulose to impart excellent hydrophobicity while reliably preventing aggregation of the modified cellulose nanofibers, thereby producing modified cellulose nanofibers that can be more reliably dispersed uniformly in hydrophobic substances such as organic solvents and resins.

[0016] According to the method for producing modified cellulose nanofibers of the present invention, the silane coupling agent is a compound represented by the following general formula (1): [ka] (In the general formula (1), X represents an organic functional group containing an epoxy group, and R 1 , R 2 , R 3 and each independently represent chloride, an alkoxy group having 1 to 5 carbon atoms, or an alkyl group having 1 to 5 carbon atoms. By using a compound represented by the formula (I), it is possible to produce modified cellulose nanofibers with reliably improved hydrophobicity.

[0017] According to the method for producing modified cellulose nanofibers of the present invention, the silane coupling agent contains 3-glycidoxypropyl(dimethoxy)methylsilane, making it possible to produce modified cellulose nanofibers that are reliably imparted with superior hydrophobicity while more reliably preventing the modified cellulose nanofibers from aggregating together.

[0018] According to the method for producing modified cellulose nanofibers of the present invention, by adding 0.001 mol to 6.0 mol of a silane coupling agent per 1.0 mol of β-glucose units of the cellulose nanofiber in the silane coupling agent-containing dispersion preparation step, it is possible to reliably impart excellent hydrophobicity to the modified cellulose nanofiber while preventing the silane coupling agent from remaining in the silane coupling agent-containing dispersion. [Brief explanation of the drawings]

[0019] [Figure 1] Figure (a) is a photograph showing the dispersion state of a dispersion sample of Example 1, immediately after preparation, using tetrahydrofuran (THF) as the dispersion solvent; Figure (b) is a photograph showing the dispersion state of a dispersion sample of Example 2, immediately after preparation, using tetrahydrofuran as the dispersion solvent; and Figure (c) is a photograph showing the dispersion state of a dispersion sample of Comparative Example 1, immediately after preparation, using tetrahydrofuran as the dispersion solvent. DETAILED DESCRIPTION OF THE INVENTION

[0020] The method for producing modified cellulose nanofibers of the present invention can produce modified cellulose nanofibers in which at least a portion of the hydroxyl groups of cellulose are modified with a silane coupling agent having an epoxy group. That is, the modified cellulose nanofibers obtained by the production method of the present invention are cellulose nanofibers modified with a silane coupling agent.

[0021] <Method of manufacturing modified cellulose nanofiber> The method for producing modified cellulose nanofibers of the present invention is described in detail below. The method for producing modified cellulose nanofibers of the present invention comprises: (1) a cellulose nanofiber dispersion preparation step of preparing a cellulose nanofiber dispersion by dispersing cellulose nanofibers in water; (2) a silane coupling agent-containing dispersion preparation step of adding a silane coupling agent having an epoxy group to the cellulose nanofiber dispersion to obtain a silane coupling agent-containing dispersion; and (3) a drying step of subjecting the silane coupling agent-containing dispersion to a drying treatment to modify the cellulose nanofibers with the silane coupling agent.

[0022] According to the method for producing modified cellulose nanofibers of the present invention, by adding a silane coupling agent having an epoxy group to a cellulose nanofiber dispersion and then drying the mixture, modified cellulose nanofibers in which at least a portion of the hydroxyl groups of the cellulose are modified with the silane coupling agent having an epoxy group can be produced, thereby enabling the production of modified cellulose nanofibers through a simple production process. Therefore, the method for producing modified cellulose nanofibers of the present invention has excellent production efficiency for modified cellulose nanofibers. Furthermore, the method for producing modified cellulose nanofibers of the present invention can produce modified cellulose nanofibers that are imparted with excellent hydrophobicity and have excellent affinity with hydrophobic substances. Therefore, the modified cellulose nanofibers obtained by the production method of the present invention are uniformly dispersed in hydrophobic substances such as organic solvents and resins, thereby expanding the range of applications of cellulose nanofibers and reliably imparting the properties of cellulose nanofibers to the target applications.

[0023] Furthermore, the method for producing modified cellulose nanofibers of the present invention can produce modified cellulose nanofibers having highly reactive epoxy groups as functional groups, which allows the obtained modified cellulose nanofibers to be further modified via the epoxy groups. Therefore, the modified cellulose nanofibers obtained by the production method of the present invention can easily be used with a wider range of hydrophobic substances depending on the properties of the hydrophobic substances.

[0024] (1) Cellulose nanofiber dispersion preparation process The cellulose nanofiber dispersion preparation step is a step of preparing a cellulose nanofiber aqueous dispersion in which cellulose nanofibers used as the raw material for modified cellulose nanofibers (hereinafter sometimes referred to as "raw cellulose nanofibers") are dispersed in water. The raw cellulose nanofibers form the skeleton of the modified cellulose nanofibers.

[0025] Raw cellulose nanofibers may not have been subjected to a specific hydrophobic treatment, and are generally hydrophilic. Examples of methods for preparing raw cellulose nanofibers include a method in which a cellulose precursor derived from a natural product is defibrated to convert the cellulose precursor into nanofibers, thereby obtaining cellulose nanofibers. Examples of cellulose precursors that are the raw material for cellulose nanofibers include plant fibers such as pulp. The method for defibrating the cellulose precursor is not particularly limited, and examples include defibration treatments using a mixer, high-speed homomixer, ultrasonic homogenizer, low-pressure homogenizer, high-pressure homogenizer, high-speed rotary mixer, grinder, freeze grinding, media mill, ball mill, etc.

[0026] The size of the raw cellulose nanofibers is not particularly limited as long as the average fiber diameter is less than 1.0 μm. However, from the viewpoints of uniformity of the cellulose modification sites with the silane coupling agent and expanding the range of applications of modified cellulose nanofibers, the average fiber diameter is preferably 4 nm to 100 nm, and particularly preferably 10 nm to 50 nm. The average fiber length of the raw cellulose nanofibers is also not particularly limited. From the viewpoints of uniformity of the cellulose modification sites with the silane coupling agent and expanding the range of applications of modified cellulose nanofibers, the average fiber diameter is preferably 20 μm to 200 μm, and particularly preferably 50 μm to 150 μm. The average fiber diameter and average fiber length of the raw cellulose nanofibers can be measured, for example, by scanning probe microscopy or nitrogen adsorption method, and by electron microscopes (transmission electron microscope (TEM) and scanning electron microscope (SEM)) or scanning probe microscope.

[0027] As described above, raw cellulose nanofibers dispersed in water in advance can be prepared as a cellulose nanofiber dispersion. The content of raw cellulose nanofibers in the cellulose nanofiber dispersion is not particularly limited, but the lower limit is preferably 0.05 parts by mass, and particularly preferably 0.1 parts by mass, per 100 parts by mass of water, from the viewpoints of ensuring uniform dispersion of the raw cellulose nanofibers in water while improving the production efficiency of modified cellulose nanofibers, as the raw cellulose nanofibers are appropriately diluted with water until they can be sufficiently stirred using a stirrer such as a magnetic stirrer, and the appropriate dispersion state is determined particularly depending on the stirrer, stirring vessel, and amount of raw cellulose nanofibers. On the other hand, the upper limit of the cellulose nanofiber content in the cellulose nanofiber dispersion is preferably 1.0 part by mass, and particularly preferably 0.8 part by mass, from the viewpoints of ensuring uniform dispersion of the raw cellulose nanofibers in water while ensuring uniform modification of the raw cellulose nanofibers with the silane coupling agent in the drying process described below, while the cellulose nanofibers are diluted with water as appropriate using a stirrer such as a magnetic stirrer until they can be sufficiently stirred. In particular, when determining the appropriate dispersion state based on the stirrer, stirring vessel, and amount of raw cellulose nanofibers,

[0028] One method for dispersing the raw cellulose nanofibers prepared as described above in water is to add the raw cellulose nanofibers to water and stir under specified stirring conditions. Stirring conditions can be selected appropriately depending on the size of the raw cellulose nanofibers and the amount added to water, and examples include a stirring temperature of 10°C to 60°C, a stirring time of 1 minute to 120 minutes, and a stirring speed of 600 rpm to 1350 rpm. The stirring means is not particularly limited, and examples include a stirrer, stirring blades, etc.

[0029] (2) Silane coupling agent-containing dispersion preparation process The silane coupling agent-containing dispersion preparation step is a step of adding a silane coupling agent having an epoxy group to the cellulose nanofiber aqueous dispersion prepared in the (1) cellulose nanofiber dispersion preparation step to obtain a dispersion containing raw cellulose nanofibers and a silane coupling agent. In the method for producing modified cellulose nanofibers of the present invention, a cellulose nanofiber dispersion in which hydrophilic raw cellulose nanofibers are dispersed in water can be used as the reaction system, eliminating the need for pretreatment of the raw cellulose nanofibers. In addition, the silane coupling agent can be used as a modifier for hydrophobizing the raw cellulose nanofibers. Therefore, in the method for producing modified cellulose nanofibers of the present invention, the raw cellulose nanofibers can be hydrophobized by a simple procedure.

[0030] The silane coupling agent is a compound having a functional group and a hydrolyzable silyl group in one molecule, and upon reaction with water, the hydrolyzable silyl group becomes a silanol group. The silane coupling agent has a substituent for modifying the hydroxyl groups of the cellulose that constitutes the cellulose nanofiber. In the method for producing modified cellulose nanofibers of the present invention, the silane coupling agent must have an epoxy group as a functional group. The silanol groups of the silane coupling agent chemically bond with the hydroxyl groups of the cellulose that constitutes the cellulose nanofiber, thereby modifying the hydroxyl groups of the cellulose. The silanol groups generated by hydrolysis of the hydrolyzable silyl groups of the silane coupling agent chemically bond to the hydroxyl groups of the cellulose, thereby modifying the cellulose with the silane coupling agent.

[0031] The hydrolyzable silyl group of the silane coupling agent is preferably an alkoxy group, since this allows for reliable modification of cellulose and production of modified cellulose nanofibers that are reliably imparted with excellent hydrophobicity. In other words, the silane coupling agent is preferably an organosilicon compound having at least an epoxy group and an alkoxy group.

[0032] Furthermore, the hydrolyzable silyl group of the silane coupling agent preferably has an alkoxy group and an alkyl group, since this allows for the production of modified cellulose nanofibers that reliably modify cellulose to impart excellent hydrophobicity while reliably preventing aggregation of the modified cellulose nanofibers. In other words, organosilicon compounds having at least an epoxy group, an alkoxy group, and an alkyl group are more preferred as silane coupling agents. Reliable prevention of aggregation of the modified cellulose nanofibers allows for more uniform dispersion in hydrophobic substances such as organic solvents and resins.

[0033] As the silane coupling agent, for example, a silane coupling agent represented by the following general formula (1) can be used, from the viewpoint of being able to produce modified cellulose nanofibers with reliably improved hydrophobicity: [ka] (In the general formula (1), X represents an organic functional group containing an epoxy group, and R 1 , R 2 , R 3 are each independently a chloride, an alkoxy group having 1 to 5 carbon atoms, or an alkyl group having 1 to 5 carbon atoms. In the compound of general formula (1), R 1 , R 2 , R 3 The chemical bond between and Si forms a hydrolyzable silyl group.

[0034] R in the compound of general formula (1) 1 , R 2 , R 3 As for R in the compound of general formula (1), from the viewpoint of being able to more reliably modify cellulose, it is more preferable that they are each independently an alkoxy group having 1 to 3 carbon atoms or an alkyl group having 1 to 3 carbon atoms, and particularly preferable that they are an alkoxy group having 1 to 2 carbon atoms or an alkyl group having 1 to 2 carbon atoms. Furthermore, from the viewpoint of being able to more reliably modify cellulose and reliably impart excellent hydrophobicity, while being able to produce modified cellulose nanofibers that can more reliably suppress aggregation of modified cellulose nanofibers, it is preferable that R1 , R 2 , R 3 Among these, it is preferred that at least one is an alkoxy group having 1 to 5 carbon atoms and at least one is an alkyl group having 1 to 5 carbon atoms.

[0035] X in the compound of general formula (1) is not particularly limited as long as it is an organic functional group containing an epoxy group. However, from the viewpoint of being able to reliably produce modified cellulose nanofibers to which excellent hydrophobicity has been imparted, X is preferably a compound represented by the following general formula (2): ER 4 -OR 5 - (2) (In the general formula (2), E represents an epoxy group, R 4 , R 5 and each independently represent an aliphatic hydrocarbon group having 1 to 5 carbon atoms, preferably an aliphatic hydrocarbon group having 1 to 3 carbon atoms. An organic functional group represented by the following formula is preferred.

[0036] Specific examples of silane coupling agents include 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyl(dimethoxy)methylsilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, etc. Among these, 3-glycidoxypropyl(dimethoxy)methylsilane is preferred, since it allows for the production of modified cellulose nanofibers that are reliably imparted with superior hydrophobicity and can more reliably prevent aggregation of the modified cellulose nanofibers.

[0037] The amount of silane coupling agent added in the silane coupling agent-containing dispersion preparation step is not particularly limited, but the lower limit is preferably 0.001 mol, more preferably 0.005 mol, and particularly preferably 0.020 mol per 1.0 mol of β-glucose unit of cellulose, in order to ensure excellent hydrophobicity of the modified cellulose nanofibers. On the other hand, the upper limit of the amount of silane coupling agent added in the silane coupling agent-containing dispersion preparation step is preferably 6.0 mol, and particularly preferably 3.0 mol, per 1.0 mol of β-glucose unit of cellulose, in order to prevent the silane coupling agent from remaining in the silane coupling agent-containing dispersion. The above-mentioned "β-glucose unit" refers to a structural unit of cellulose.

[0038] One example of a method for dispersing a silane coupling agent having an epoxy group in a cellulose nanofiber dispersion is to add the silane coupling agent having an epoxy group to the cellulose nanofiber dispersion and stir the mixture under specified stirring conditions. Stirring conditions can be appropriately selected depending on the size and amount of the raw cellulose nanofibers, the amount of silane coupling agent added, the size of the stirring vessel, the type of stirrer, and other factors. Examples of suitable stirring conditions include a stirring temperature of 10°C to 60°C, a stirring time of 1 minute to 120 minutes, and a stirring speed of 600 rpm to 1350 rpm. The stirring time and stirring speed can be determined under appropriate conditions depending on the type of stirrer, the size of the stirring vessel, the size and amount of the raw cellulose nanofibers, and other factors. The stirring means is not particularly limited and may include, for example, a stirrer, stirring blades, etc.

[0039] (3) Drying process The drying step is a step in which the raw cellulose nanofibers are modified with a silane coupling agent by subjecting the silane coupling agent-containing dispersion obtained in (2) the silane coupling agent-containing dispersion preparation step to a drying treatment. By modifying the raw cellulose nanofibers with the silane coupling agent, modified cellulose nanofibers can be produced.

[0040] The drying step is a step of removing water from the silane coupling agent-containing dispersion obtained in the silane coupling agent-containing dispersion preparation step to obtain the target modified cellulose nanofibers. The modified cellulose nanofibers obtained as described above are modified cellulose nanofibers in which the silanol groups of the silane coupling agent having epoxy groups are chemically bonded to the hydroxyl groups of the cellulose constituting the raw cellulose nanofibers by dehydration condensation, resulting in cellulose nanofibers modified with the silane coupling agent.

[0041] The drying method for the silane coupling agent-containing dispersion is not particularly limited, but freeze-drying is preferred, for example, from the perspective of ease of drying. By suppressing the ring-opening reaction of the epoxy groups of the silane coupling agent, the resulting modified cellulose nanofibers can be further modified via the epoxy groups, making it easy to expand the range of hydrophobic substances to which they can be applied depending on the properties of the hydrophobic substances. Furthermore, freeze-drying more reliably produces modified cellulose nanofibers that have excellent affinity with hydrophobic substances.

[0042] The freeze-drying conditions are not particularly limited, but examples include a freezing temperature of −45° C. or lower, a reduced pressure condition (vacuum degree) of 30 Pa or lower, and a freeze-drying time of 72 hours to 120 hours.

[0043] Optional Process In the method for producing modified cellulose nanofibers of the present invention, in addition to the above-mentioned (1) cellulose nanofiber dispersion preparation step, (2) silane coupling agent-containing dispersion preparation step, and (3) drying step, other steps can be added as needed.

[0044] An example of another step that can be added as needed is a dialysis step in which the silane coupling agent-containing dispersion is dialyzed using a dialysis membrane between (2) the silane coupling agent-containing dispersion preparation step and (3) the drying step. By further including the dialysis step, the silane coupling agent remaining in the silane coupling agent-containing dispersion can be removed, thereby improving the purity of the modified cellulose nanofibers.

[0045] The dialysis time in the dialysis step is not particularly limited, but is preferably 48 hours or more in order to more reliably remove the silane coupling agent remaining in the silane coupling agent-containing dispersion, and is particularly preferably 48 hours or more and 120 hours or less in order to more reliably remove the silane coupling agent while reliably obtaining the production efficiency of modified cellulose nanofibers.

[0046] Preferred dialysis membranes used in the dialysis step include regenerated cellulose (RC), polystyrene (PS), polymethyl methacrylate (PMMA), cellulose triacetate (CTA), polyethersulfone (PES), polyester polymer alloy (PEPA), acrylonitrile sodium methacrylate sulfonate polymer (AN69), and ethylene vinyl alcohol polymer (EVAL), because they are highly hydrophilic and strong, allowing dialysis in water.

[0047] <Modified cellulose nanofiber> The modified cellulose nanofibers obtained by the production method of the present invention are cellulose nanofibers modified with a silane coupling agent having an epoxy group. More specifically, the modified cellulose nanofibers obtained by the production method of the present invention are modified cellulose nanofibers in which silanol groups of the silane coupling agent having an epoxy group are chemically bonded to hydroxyl groups of cellulose by dehydration condensation, thereby modifying the cellulose nanofibers with the silane coupling agent.

[0048] The proportion of silane coupling agent in the modified cellulose nanofibers obtained by the production method of the present invention is not particularly limited, but the lower limit is preferably 0.001 mol of silane coupling agent per 1.0 mol of β-glucose unit of cellulose, from the viewpoint of reliably imparting excellent hydrophobicity to the cellulose, more preferably 0.005 mol, even more preferably 0.020 mol, and particularly preferably 0.20 mol. On the other hand, the upper limit of the proportion of silane coupling agent in the modified cellulose nanofibers is preferably 3.0 mol per 1.0 mol of β-glucose unit of cellulose, from the viewpoint of maintaining the crystalline structure of the cellulose nanofibers to prevent a loss of high strength, and particularly preferably 1.0 mol per 1.0 mol of β-glucose unit of cellulose, from the viewpoint of more reliably maintaining the crystalline structure of the cellulose nanofibers. The above-mentioned ratio of the silane coupling agent to be charged means the ratio of the number of silane coupling agents per β-glucose unit, which is a structural unit of cellulose.

[0049] The size of the modified cellulose nanofibers obtained by the production method of the present invention is not particularly limited, but examples include an average fiber diameter of less than 1.0 μm, corresponding to the average fiber diameter of the raw cellulose nanofibers being less than 1.0 μm. The size of the modified cellulose nanofibers corresponds to the average fiber diameter of the raw cellulose nanofibers described above, and from the viewpoints of uniformity of the cellulose modification sites with the silane coupling agent and expanding the range of applications of modified cellulose nanofibers, the average fiber diameter is preferably 4 nm to 100 nm, and particularly preferably 10 nm to 50 nm. Furthermore, the average fiber length of the modified cellulose nanofibers corresponds to the average fiber length of the raw cellulose nanofibers described above, and from the viewpoints of uniformity of the cellulose modification sites with the silane coupling agent and expanding the range of applications of modified cellulose nanofibers, the average fiber length is preferably 20 μm to 200 μm, and particularly preferably 50 μm to 150 μm.

[0050] The success of producing modified cellulose nanofibers by modifying raw cellulose nanofibers with a silane coupling agent can be confirmed by visually observing the dispersion state of the modified cellulose nanofibers in the organic solvent immediately after dispersing the resulting modified cellulose nanofibers in the organic solvent, and by visually observing the time required for the modified cellulose nanofibers to aggregate and settle in the organic solvent after dispersing the resulting modified cellulose nanofibers in the organic solvent. If the time required for the modified cellulose nanofibers to aggregate and settle in the organic solvent is longer than the specified time, it is proven that the modified cellulose nanofibers have excellent dispersibility in the organic solvent and have been modified with a silane coupling agent.

[0051] Examples of organic solvents that can be used to confirm that modified cellulose nanofibers modified with a silane coupling agent have been produced include alcohols having 1 to 5 carbon atoms, such as tetrahydrofuran, acetone, ethanol, and propanol.

[0052] Furthermore, the production of modified cellulose nanofibers in which the hydroxyl groups of raw cellulose nanofibers have been modified with a silane coupling agent can be confirmed by Fourier transform infrared spectroscopy (FT-IR). That is, by comparing the infrared absorption spectrum of the raw cellulose nanofibers not modified with a silane coupling agent with the infrared absorption spectrum of the resulting modified cellulose nanofibers, it can be confirmed that at least some of the hydroxyl groups of the cellulose have been modified with a silane coupling agent having an epoxy group.

[0053] Specifically, in the case of modified cellulose nanofibers modified with a silane coupling agent, when the silane coupling agent has an alkyl group, the 1260 cm -1 , 798cm -1 , 763cm -1 In contrast, the raw cellulose nanofiber exhibits an absorption peak at 1260 cm due to the Si-C bond. -1 , 798cm -1 , 763cm -1 Therefore, no absorption peak appears in the region of 1260 cm in the infrared absorption spectrum. -1 , 798cm -1 , 763cm -1 By checking whether or not there is an absorption peak in this region, it can be confirmed whether or not modified cellulose nanofibers modified with a silane coupling agent have been produced.

[0054] In addition, in the case of modified cellulose nanofibers modified with a silane coupling agent having an epoxy group, even if the silane coupling agent does not have an alkyl group, the 900 cm -1 The absorption peak due to epoxy groups appears in the region around 900 cm of the infrared absorption spectrum of the raw cellulose nanofiber. -1 In the modified cellulose nanofibers, an absorption peak appears in the region around 900 cm -1In the region around 900 cm, the absorption peak derived from the epoxy group and the absorption peak derived from the raw cellulose nanofiber overlap, resulting in a stronger peak intensity. -1 By checking the intensity of the peaks in the surrounding areas, it is possible to confirm whether modified cellulose nanofibers modified with a silane coupling agent have been produced. [Example]

[0055] Next, examples of the present invention will be described, but the present invention is not limited to these examples as long as they do not depart from the spirit of the present invention.

[0056] <Production of modified cellulose nanofibers> Example 1 A cellulose nanofiber dispersion was prepared by adding 0.2 g of cellulose nanofiber aqueous dispersion (average fiber length 100 μm, average fiber diameter 30–40 nm, Mori Machinery Corporation's "CellFim C-100" (water content approximately 95% by mass)) to 95 g of water while stirring with a stirrer (cellulose nanofiber dispersion preparation step). The prepared cellulose nanofiber dispersion was stirred with a stirrer at a stirring speed of 1350 rpm at room temperature. 3-glycidoxypropyl(dimethoxy)methylsilane (hereinafter referred to as "GOPDMS"; a silane coupling agent with an epoxy group) was added in an amount of 3.0 mol per 1.0 mol of β-glucose units of cellulose. GOPDMS was added, and the mixture was stirred for 60 minutes after the addition of GOPDMS to prepare a silane coupling agent-containing dispersion in which cellulose nanofibers and silane coupling agent were dispersed in water (silane coupling agent-containing dispersion preparation step). Next, the resulting silane coupling agent-containing dispersion was dialyzed in pure water using a dialysis membrane. Dialysis using the dialysis membrane was carried out for 24 hours. From the addition of GOPDMS to the end of dialysis using the dialysis membrane, the process was carried out at room temperature of 25°C (dialysis step). The dialyzed silane coupling agent-containing dispersion was then dried in a freeze dryer under conditions of a freezing temperature of -45°C, a reduced pressure (vacuum degree) of 10 Pa, and a freeze-drying time of 72 hours, thereby sublimating the water in the frozen silane coupling agent-containing dispersion (drying step). In this way, modified cellulose nanofibers of Example 1 were produced in which cellulose nanofibers were modified with GOPDMS. Note that in Example 1, the blending ratio of cellulose nanofibers and GOPDMS was 3.0 mol of GOPDMS per 1.0 mol of β-glucose unit of cellulose, based on the blending ratio of cellulose nanofibers and GOPDMS.

[0057] The compound obtained in Example 1 was analyzed by FT-IR (PerkinElmer Co., Ltd.'s "Spectrum One (A)"), and the infrared absorption spectrum at 1260 cm was obtained. This confirmed that modified cellulose nanofibers obtained by modifying cellulose nanofibers with GOPDMS were produced. -1 , 798cm -1, 763cm -1 In addition, an absorption peak due to the Si-C bond appears in the region of 900 cm in the infrared absorption spectrum compared to the raw cellulose nanofiber. -1 This was confirmed by the strong peak intensity in the surrounding area.

[0058] Example 2 The modified cellulose nanofibers of Example 2 were produced in the same manner as in Example 1, except that instead of using GOPDMS, the amount of 3-glycidoxypropyltrimethoxysilane (hereinafter sometimes referred to as "GOPTS", equivalent to a silane coupling agent having an epoxy group) added was adjusted to 3.0 mol per 1.0 mol of β-glucose unit of cellulose. In Example 2, the blending ratio of cellulose nanofibers and GOPTS meant that the charge ratio of GOPTS per 1.0 mol of β-glucose unit of cellulose was 3.0 mol.

[0059] The compound obtained in Example 2 was analyzed by FT-IR in the same manner as in Example 1, and the infrared absorption spectrum at 900 cm -1 The peak intensity in the region around 900 cm of the raw cellulose nanofiber -1 This was confirmed by the fact that the peak intensity was stronger than that in the surrounding area.

[0060] Comparative Example 1 The raw cellulose nanofibers used in Examples 1 and 2, i.e., cellulose nanofibers not modified with a silane coupling agent, were used.

[0061] <Evaluation items> (1) Dispersibility (hydrophobicity) 0.02 g of the modified cellulose nanofiber samples of Examples 1 and 2 and the raw cellulose nanofiber sample of Comparative Example 1 was weighed out and added to 20 ml of tetrahydrofuran in a container, and stirred with a stirrer at a stirring speed of 1350 rpm for 24 hours to prepare a first dispersion. Furthermore, 1.0 ml of the prepared first dispersion was taken and placed in a separate container, and 10 ml of tetrahydrofuran was added to prepare a second dispersion, which was used as a dispersion sample.

[0062] The dispersion state of the cellulose nanofiber sample in tetrahydrofuran immediately after the preparation of the second dispersion (5 seconds after preparation) was visually observed and evaluated as follows. ◎: The cellulose nanofiber sample is in an excellent dispersion state, and the cellulose nanofiber sample exhibits excellent hydrophobicity. ○: Although some aggregation of the cellulose nanofiber sample was observed, the cellulose nanofiber sample was well dispersed and hydrophobicity was observed. ×: Most of the cellulose nanofiber sample was in an aggregated state, and no hydrophobicity was observed in the cellulose nanofiber sample.

[0063] (2) Settling time For the dispersion samples obtained as described above, the time from immediately after the preparation of the second dispersion until the amount of sedimentation of the cellulose nanofiber sample stabilized was measured as the sedimentation time. The sedimentation time was measured for Examples 1 and 2 and Comparative Example 1, with n=3 for each sample, and evaluated as follows from the average value. ◎: Settling time 100 seconds or more ○: Settling time: 40 seconds or more and less than 100 seconds ×: Settling time less than 40 seconds

[0064] The evaluation results are shown in the following Table 1. Regarding the evaluation of dispersibility, Fig. 1(a) shows a photograph of the dispersed state immediately after preparation of a dispersion sample of Example 1 using tetrahydrofuran as the dispersion solvent, Fig. 1(b) shows a photograph of the dispersed state immediately after preparation of a dispersion sample of Example 2 using tetrahydrofuran as the dispersion solvent, and Fig. 1(c) shows a photograph of the dispersed state immediately after preparation of a dispersion sample of Comparative Example 1 using tetrahydrofuran as the dispersion solvent.

[0065] [Table 1]

[0066] As can be seen from Table 1 above, the modified cellulose nanofibers of Examples 1 and 2, which were produced using a cellulose nanofiber dispersion preparation process, a silane coupling agent-containing dispersion preparation process, and a drying process in which the silane coupling agent-containing dispersion was subjected to a drying treatment, achieved a dispersibility rating of ∘ or better and a settling time rating of ∘ or better, indicating that modified cellulose nanofibers exhibited excellent dispersion properties in organic solvents, which are hydrophobic substances. In particular, Example 1, in which modified cellulose nanofibers were produced using 3-glycidoxypropyl(dimethoxy)methylsilane, exhibited even better dispersibility and settling time than Example 2, in which modified cellulose nanofibers were produced using 3-glycidoxypropyltrimethoxysilane. This is thought to be due in part to the difference in the number of silanol groups generated by hydrolysis of the hydrolyzable silyl groups of the silane coupling agent.

[0067] On the other hand, in Comparative Example 1, which was raw cellulose nanofiber that was not modified with a silane coupling agent having an epoxy group, dispersibility in hydrophobic substances was not obtained, the settling time was short, and dispersion properties were not obtained. [Industrial Applicability]

[0068] The method for producing modified cellulose nanofibers of the present invention can produce modified cellulose nanofibers that have excellent hydrophobic properties and excellent affinity with hydrophobic substances using a simple production process, and therefore can be used in a wide range of fields, and is highly useful, for example, in the field of resin compositions that are used as materials for molded products.

Claims

1. a cellulose nanofiber dispersion preparation step of preparing a cellulose nanofiber dispersion in which cellulose nanofibers are dispersed in water; a silane coupling agent-containing dispersion preparation step of adding a silane coupling agent having an epoxy group to the cellulose nanofiber dispersion to obtain a silane coupling agent-containing dispersion; a drying step of subjecting the silane coupling agent-containing dispersion to a drying treatment to modify the cellulose nanofibers with the silane coupling agent, In the silane coupling agent-containing dispersion preparation step, the silane coupling agent is added in an amount of 0.001 mol or more and 6.0 mol or less per 1.0 mol of β-glucose units of the cellulose nanofibers, In the modified cellulose nanofibers, a hydroxyl group of cellulose is chemically bonded to a silanol group generated by hydrolysis of a hydrolyzable silyl group of the silane coupling agent, The silane coupling agent is represented by the following general formula (1): 【Chemical 1】 (in general formula (1), X represents an organic functional group containing an epoxy group, and R 1 , R 2 , and R 3 each independently represent chloride, an alkoxy group having 1 to 5 carbon atoms, or an alkyl group having 1 to 5 carbon atoms, and at least one of R 1 , R 2 , and R 3 is an alkoxy group having 1 to 5 carbon atoms, and at least one is an alkyl group having 1 to 5 carbon atoms).

2. The method for producing modified cellulose nanofibers according to claim 1, wherein the drying treatment is a drying treatment using a freeze-drying method.

3. 3. The method for producing modified cellulose nanofibers according to claim 1, further comprising a dialysis step of dialyzing the silane coupling agent-containing dispersion using a dialysis membrane between the silane coupling agent-containing dispersion preparation step and the drying step.

4. The method for producing modified cellulose nanofibers according to any one of claims 1 to 3, wherein the silane coupling agent contains 3-glycidoxypropyl(dimethoxy)methylsilane.

5. The method for producing modified cellulose nanofibers according to any one of claims 1 to 4, wherein the modified cellulose nanofibers have an average fiber length of 20 µm or more and 200 µm or less, and an average fiber diameter of 4 nm or more and 100 nm or less.

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