Nanocellulose and dispersion liquid thereof and methods of production thereof

Abstract

To provide nanocellulose excellent in viscosity stability.SOLUTION: In a slurry, the nanocellulose has an average fiber length of 100 nm or more and 500 nm or less, and an average fiber width of 2.0 nm or more and 5.0 nm or less, and further satisfies at least one of the following conditions. A condition A: the standard deviation of fiber length is 600 nm or less. A condition B: the kurtosis of fiber length is 11 or more. A condition C: the skewness of fiber length is 3.0 or more. A condition D: the range of fiber length is 4000 nm or less. A condition E: the standard deviation of fiber width is 1.5 nm or less. A condition F: the kurtosis of fiber width is 0.3 or more. A condition G: the skewness of fiber width is 0.5 or more. A condition H: the range of fiber width is 6.8 nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to nanocellulose, a dispersion thereof, and a method for producing the same. More specifically, the present invention relates to nanocellulose having an average fiber length and an average fiber width within a specific range, and having a fiber length distribution or a fiber width distribution within a specific range, a nanocellulose dispersion containing the same, and a method for producing nanocellulose.

Background Art

[0002] Nanocellulose such as cellulose nanofibers (hereinafter also referred to as "CNF") is produced by mechanically defibrating cellulose or oxidized cellulose in water, and obtained as a viscous nanocellulose aqueous dispersion. Utilizing its viscosity, the use of nanocellulose as a thickener, dispersant, binder, etc. has been studied. However, it is known that the viscosity of the nanocellulose aqueous dispersion is related to the fiber shape such as the fiber length and fiber width of nanocellulose. Further, when a slurry is prepared by mixing nanocellulose and inorganic particles for the above applications, if the viscosity stability of the slurry is poor, there is a risk of change in the quality of the product.

[0003]

[0004] For example, Japanese Patent Application Laid-Open No. 2018-162549 (Patent Document 1) describes CNF having an acid (H) type carboxylated CNF that has a high viscosity in a low shear region and an extremely short fiber length, specifying the average fiber length and average fiber width of the CNF, and also specifying the viscosity of the CNF aqueous dispersion.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, Patent Document 1 relates to acid-type carboxylated CNF that is highly viscous in the low-sag region, and describes CNF with an average fiber length of 50 to 500 nm, a proportion of fiber lengths of 300 nm or less being 50% or more, and a proportion of fiber lengths of 600 nm or more being less than 20%. However, there is a problem with the viscosity stability of the slurry containing this CNF. In addition, Patent Document 2 only describes CNF with a fiber length in micrometers and does not describe technologies related to microcrystalline cellulose with a fiber length distribution in nanometers.

[0007] In view of the above situation, an object of the present invention is to provide nanocellulose having excellent viscosity stability of a slurry containing nanocellulose.

Means for Solving the Problems

[0008] As a result of intensive studies to solve the above problems, the present inventor has found that by setting the standard deviation, kurtosis, skewness, or range, which are indicators showing the fiber length distribution or fiber width distribution of nanocellulose, within a specific range, the viscosity stability of a slurry containing nanocellulose is excellent, and the present invention has been completed.

[0009] That is, the first invention of the present invention is nanocellulose having an average fiber length of 100 nm or more and 500 nm or less and an average fiber width of 2.0 nm or more and 5.0 nm or less, and satisfying at least one of the following conditions A to H. Condition A: The standard deviation of the fiber length is 600 nm or less Condition B: The kurtosis of the fiber length is 11 or more. Condition C: The skewness of the fiber length is 3.0 or more. Condition D: The range of the fiber length is 4000 nm or less. Condition E: The standard deviation of the fiber width is 1.5 nm or less. Condition F: The kurtosis of the fiber width is 0.3 or more. Condition G: The skewness of the fiber width is 0.5 or more. Condition H: The range of the fiber width is 6.8 nm or less.

[0010] The second invention of the present invention is the nanocellulose according to the first invention, wherein the standard deviation of the fiber length is 10 nm or more and 500 nm or less.

[0011] The third invention of the present invention is the nanocellulose according to the first invention or the second invention, wherein the kurtosis of the fiber length is 12 or more and 30 or less.

[0012] The fourth invention of the present invention is the nanocellulose according to any one of the first to third inventions, wherein the skewness of the fiber length is 3.0 or more and 6.0 or less.

[0013] The fifth invention of the present invention is the nanocellulose according to any one of the first to fourth inventions, wherein the range of the fiber length is 450 nm or more and 4000 nm or less.

[0014] The sixth invention of the present invention is the nanocellulose according to any one of the first to fifth inventions, wherein the standard deviation of the fiber width is 0.5 nm or more and 1.5 nm or less.

[0015] The seventh invention of the present invention is the nanocellulose according to any one of the first to sixth inventions, wherein the kurtosis of the fiber width is 0.3 or more and 2.5 or less.

[0016] The eighth invention of the present invention is the nanocellulose according to any one of the first to seventh inventions, wherein the skewness of the fiber width is 0.5 or more and 1.5 or less.

[0017] The ninth invention of the present invention is the nanocellulose according to any one of the first to eighth inventions, wherein the fiber width ranges from 3.0 nm to 6.8 nm.

[0018] The tenth invention of the present invention is the nanocellulose according to any one of the first to ninth inventions, which contains carboxylated nanocellulose.

[0019] The eleventh invention of the present invention is the nanocellulose according to any one of the first to tenth inventions, which substantially does not contain an N-oxyl compound.

[0020] The twelfth invention of the present invention is the nanocellulose according to any one of the first to eleventh inventions, which is produced by defibrating oxidized cellulose obtained by oxidizing a cellulose-based raw material with hypochlorous acid or a salt thereof.

[0021] The thirteenth invention of the present invention is a nanocellulose dispersion in which the nanocellulose according to any one of the first to twelfth inventions is dispersed in a dispersion medium.

[0022] Further, the fourteenth invention of the present invention is a method for producing the nanocellulose according to any one of the first to twelfth inventions, which includes a step of producing by defibrating oxidized cellulose obtained by oxidizing a cellulose-based raw material with hypochlorous acid or a salt thereof.

Advantages of the Invention

[0023] According to the present invention, by setting the numerical values of the standard deviation, kurtosis, skewness, or range of the fiber length of the nanocellulose, or the standard deviation, kurtosis, skewness, or range of the fiber width of the nanocellulose within a specific range, the viscosity stability of the slurry containing the nanocellulose can be enhanced. Since the slurry containing the nanocellulose has excellent viscosity stability, the nanocellulose of the present invention is useful, for example, as a thickener, a dispersant, a binder, and the like.

Embodiments for Carrying Out the Invention

[0024] When a cellulose-based raw material is treated in the coexistence of 2,2,6,6-tetramethyl-1-piperidine-N-oxyl radical (hereinafter also referred to as "TEMPO"), sodium bromide, and sodium hypochlorite which is an inexpensive oxidizing agent, carboxy groups can be efficiently introduced onto the surface of the cellulose. By subjecting the oxidized cellulose with the introduced carboxy groups to fibrillation treatment using a mixer or the like, fine nanocellulose with a fiber length in the nanometer unit can be produced. Further, in the absence of TEMPO, when the cellulose-based raw material is treated with hypochlorous acid or its salt, oxidized cellulose is obtained, and by subjecting this to fibrillation treatment, fine nanocellulose is obtained.

[0025] Note that when using TEMPO, there is a risk that harmful N-oxyl compounds may remain in the nanocellulose. Therefore, nanocellulose substantially free of N-oxyl compounds obtained by a production method that does not use TEMPO is preferred. In this specification, "substantially free of N-oxyl compounds" means that the residual nitrogen component derived from the N-oxyl compounds contained in the nanocellulose is 2.0 ppm or less as an increase from the raw material pulp. The residual nitrogen component derived from the N-oxyl compounds in the nanocellulose of this specification is preferably 1.0 ppm or less as an increase from the raw material pulp. Also, when the content of the N-oxyl compound is preferably 2.0 mass ppm or less, more preferably 1.0 mass ppm or less as an increase from the cellulose-based raw material, it also means "substantially free of N-oxyl compounds". By substantially not containing N-oxyl compounds, it is possible to suppress the residue of N-oxyl compounds, which are a concern for the environment and the human body, in the nanocellulose. The content of the N-oxyl compound can be measured by known means. Examples of known means include the method using a trace total nitrogen analyzer. Specifically, the nitrogen component derived from the N-oxyl compounds in the nanocellulose can be measured as the amount of nitrogen using a trace total nitrogen analyzer (for example, manufactured by Mitsubishi Chemical Analytech Co., Ltd., device name: TN-2100H, etc.).

[0026] When cellulose is oxidized, carboxyl groups are generated in at least some of the structural units that make up the cellulose molecular chain. Due to the electrostatic repulsion and osmotic pressure effects caused by the carboxyl groups, fibrillation can be achieved with relatively weak energy, contributing to the reduction of production costs. The nanocellulose obtained by this oxidation method is called carboxylated nanocellulose. And because the dispersibility in water and the like is improved by the carboxyl groups, the viscosity stability of the slurry containing nanocellulose is also enhanced. Therefore, as the nanocellulose of the present invention, it is preferable to include carboxylated nanocellulose.

[0027] The nanocellulose of the present invention is an aggregate of nanocellulose fibers. When the nanocellulose of the present invention contains carboxylated nanocellulose, it only needs to contain at least one carboxylated nanocellulose fiber, and it is preferable that the carboxylated nanocellulose is the main component. Here, the carboxylated nanocellulose fiber being the main component means that the proportion of carboxylated nanocellulose in the total amount of nanocellulose exceeds 50% by mass, preferably exceeds 70% by mass, more preferably exceeds 80% by mass. The upper limit of the above proportion is 100% by mass, but it may be 98% by mass or 95% by mass.

[0028] The nanocellulose of the present invention has an average fiber length of 100 nm or more and 500 nm or less, and an average fiber width of 2.0 nm or more and 5.0 nm or less. The average fiber length is preferably in the range of 100 nm or more and 450 nm or less, and more preferably in the range of 100 nm or more and 400 nm or less. When the average fiber length exceeds 500 nm, the slurry thickens sharply and handling becomes difficult. Also, when the average fiber length is less than 100 nm, it becomes difficult to exhibit the viscosity, which is a characteristic of nanocellulose.

[0029] The average fiber width is preferably in the range of 2.0 nm or more and 4.5 nm or less, more preferably in the range of 2.5 nm or more and 4.0 nm or less. When the average fiber width is less than 2.0 nm, it becomes difficult to exhibit the strength improvement when nanocellulose is added to the resin. Further, when the average fiber width is greater than 5.0 nm, it similarly becomes difficult to exhibit the strength improvement due to stress concentration.

[0030] Note that the average fiber length and the average fiber width are calculated by mixing nanocellulose and water so that the concentration of nanocellulose is approximately 1 to 10 ppm, naturally drying the sufficiently diluted nanocellulose aqueous dispersion on a mica substrate, observing the shape of nanocellulose using a scanning probe microscope, randomly selecting an arbitrary number of fibers from the obtained image, setting the cross-sectional height of the shape image = fiber width, and the perimeter length ÷ 2 = fiber length. For calculating such average fiber width and average fiber length, image processing software can be used. At this time, the conditions for image processing are arbitrary, but even for the same image, there may be a difference in the calculated values depending on the conditions for image processing. The range of the difference in values due to the conditions for image processing is preferably within ±100 nm for the average fiber length. The range of the difference in values due to the conditions is preferably within ±10 nm for the average fiber width. A more detailed measurement method follows the method described in the examples below.

[0031] Regarding the nanocellulose of the present invention, the standard deviation of the fiber length is preferably 600 nm or less, more preferably 500 nm or less, and even more preferably in the range of 10 nm or more and 500 nm or less. When the standard deviation of the fiber length exceeds 600 nm, the slurry using such nanocellulose is likely to have a non-uniform portion in the nanocellulose concentration, and the state of the slurry, particularly the viscosity stability, deteriorates. Therefore, the smaller the standard deviation, the more preferable. However, if the standard deviation is to be less than 10 nm, it is necessary to significantly increase the number of defibrillation times, which is not economically preferable.

[0032] Regarding the nanocellulose of the present invention, the kurtosis of the fiber length is preferably 11 or more, more preferably 12 or more, and even more preferably in the range of 12 or more and 30 or less. The kurtosis is a numerical value indicating the concentration degree of the fiber length distribution. For a slurry using nanocellulose with a kurtosis of less than 11, uneven parts in the nanocellulose concentration are likely to occur, and the state of the slurry, especially the viscosity stability, deteriorates. Therefore, the larger the kurtosis, the more preferable it is. However, in order to make the kurtosis exceed 30, it is necessary to significantly reduce the number of defibrillation times, which is not preferable because the nanocellulose formation becomes insufficient.

[0033] Regarding the nanocellulose of the present invention, the skewness (distribution shape) of the fiber length is preferably 3.0 or more, more preferably in the range of 3.0 or more and 6.0 or less, and even more preferably in the range of 3.0 or more and 4.0 or less. When the skewness of the fiber length is within a certain range, although the mechanism is unclear, the slurry using the nanocellulose has high viscosity stability in particular in the state of the slurry. When the skewness of the fiber length is less than 3.0, the viscosity stability decreases. When the skewness of the fiber length exceeds 6.0, it is necessary to significantly reduce the number of defibrillation times, which is not preferable because the nanocellulose formation becomes insufficient. Note that the higher the skewness of the fiber length, the more it indicates that the fiber length distribution is biased toward the side with a smaller width.

[0034] Also, regarding the nanocellulose of the present invention, the range of the fiber length (the difference between the maximum value and the minimum value) is preferably 4000 nm or less, more preferably 450 nm or more and 4000 nm or less, even more preferably in the range of 500 nm or more and 4000 nm or less, still more preferably in the range of 550 nm or more and 4000 nm or less, and even more preferably in the range of 700 nm or more and 4000 nm or less. For a slurry using nanocellulose with a fiber length range exceeding 4000 nm, uneven parts in the nanocellulose concentration are likely to occur, and the state of the slurry, especially the viscosity stability, tends to deteriorate. Therefore, the smaller the fiber length range, the more preferable it is. However, in order to make the lower limit of the fiber length range less than 500 nm, it is necessary to significantly increase the number of defibrillation times, which is not economically preferable.

[0035] Regarding the nanocellulose of the present invention, the standard deviation of the fiber width is 1.5 nm or less, preferably in the range of 0.5 nm or more and 1.5 nm or less, more preferably in the range of 1.0 nm or more and 1.5 nm or less. When the standard deviation of the fiber width exceeds 1.5 nm, the slurry using nanocellulose is likely to have non-uniform portions in the nanocellulose concentration in the slurry, and the state of the slurry, particularly the viscosity stability, tends to decrease. Therefore, the smaller the standard deviation, the more preferable. However, in order to make the standard deviation less than 1.0 nm, it is necessary to significantly increase the number of defibrillation times, which is not economically preferable.

[0036] Regarding the nanocellulose of the present invention, the kurtosis of the fiber width is preferably 0.3 or more, more preferably in the range of 0.3 or more and 2.5 or less, and even more preferably in the range of 0.7 or more and 2.5 or less. The lower limit of the kurtosis of the fiber width is more preferably 0.35 or more, further preferably 0.4 or more, even more preferably 0.5 or more, and still more preferably 0.6 or more. A slurry using nanocellulose with a small kurtosis of the fiber width is likely to have non-uniform portions in the nanocellulose concentration, and the state of the slurry, particularly the viscosity stability, tends to decrease. Therefore, the larger the kurtosis, the more preferable. However, if the kurtosis of the fiber width is to exceed 2.5, it is necessary to significantly reduce the number of defibrillation times, resulting in insufficient nanocellulose formation, which is not preferable.

[0037] Regarding the nanocellulose of the present invention, the skewness (distribution shape) of the fiber width is preferably 0.5 or more, more preferably 0.6 or more, further preferably 0.7 or more, and even more preferably 0.8 or more. The range of the skewness of the fiber width is preferably in the range of 0.5 or more and 1.5 or less, more preferably in the range of 0.6 or more and 1.5 or less, further preferably in the range of 0.7 or more and 1.5 or less, even more preferably in the range of 0.8 or more and 1.5 or less, and still more preferably in the range of 0.85 or more and 1.5 or less. When the skewness of the fiber width is 0.5 or more, although the mechanism is unclear, the slurry using the nanocellulose has high viscosity stability in particular in the state of the slurry. When the skewness of the fiber width is less than 0.5, the viscosity stability decreases. When the skewness of the fiber width exceeds 1.5, it is necessary to significantly reduce the number of defibration times, which is not preferable because the nanocellulose formation becomes insufficient. Note that the higher the skewness of the fiber width, the more the fiber width distribution is biased toward the smaller side.

[0038] Furthermore, regarding the nanocellulose of the present invention, the range of the fiber width (the difference between the maximum value and the minimum value) is preferably 6.8 nm or less, more preferably in the range of 3.0 nm or more and 6.8 nm or less, even more preferably in the range of 4.0 nm or more and 6.8 nm or less, still more preferably in the range of 5.0 nm or more and 6.8 nm or less, and even more preferably in the range of 5.2 nm or more and 6.7 nm or less. A slurry using nanocellulose with a large range of fiber width is likely to have a non-uniform portion in the nanocellulose concentration, and the state of the slurry, particularly the viscosity stability, tends to decrease. Therefore, the smaller the range of the fiber width, the more preferable. However, in order to make the lower limit of the range of the fiber width less than 3.0 nm, it is necessary to significantly increase the number of defibration times, which is not economically preferable.

[0039] The average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range in the nanocellulose of the present invention, as well as the standard deviation, kurtosis, skewness, and range of the fiber width can be controlled within a predetermined range, respectively, for example, by selecting the type of cellulose-based raw material, controlling the degree of carboxylation of nanocellulose, specifically, controlling the time of the oxidation reaction; adjusting the stirring of the oxidation reaction; controlling the method of the defibration treatment; adjusting the mesh size of the filter used when solid-liquid separating the oxidized cellulose after the oxidation reaction; and so on. The control methods for the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range in the nanocellulose of the present invention, as well as the standard deviation, kurtosis, skewness, and range of the fiber width are not limited to these, and two or more of these methods may be combined. In the method of adjusting the aperture of a filter, by increasing the aperture, fine fibers are removed, and it becomes easier to control the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range, as well as the standard deviation, kurtosis, skewness, and range of fiber width within a predetermined range.

[0040] Note that the standard deviation of fiber length and the standard deviation of fiber width in the present invention respectively represent how widely the statistical target values are distributed from their average. The standard deviation can be obtained from the following formula (1) where the number of data is n and each data is x.

Equation

[0041]

Equation

[0042] Note that for the kurtosis of fiber length and the kurtosis of fiber width respectively, compared with the normal distribution, if the kurtosis is large, the distribution has a sharp peak and a long and thick tail, and if the kurtosis is small, the distribution has a more rounded peak and a short and thin tail. The kurtosis of fiber length and the kurtosis of fiber width can be obtained from the following formula (2) where the number of data is n, each data is xi, and the standard deviation is s.

Equation

[0043]

Equation

[0044] The skewness of fiber length and the skewness of fiber width respectively represent the degree of asymmetry on both sides of the distribution around the average value. Positive skewness indicates a distribution with an asymmetric tail that spreads towards more positive values, and negative skewness indicates a distribution with an asymmetric tail that extends towards more negative values. The skewness of fiber length and the skewness of fiber width can be obtained from the following formula (3), where the number of data is n, each data is xi, and the standard deviation is s.

Number

[0045]

Number

[0046] The average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range in the nanocellulose of the present invention, and the standard deviation, kurtosis, skewness, and range of fiber width may be obtained using commercially available spreadsheet software. For example, for calculating the standard deviation, the STDEV function of Microsoft Excel can be used, for calculating kurtosis, the KURT function can be used, and for calculating skewness, the SKEW function can be used.

[0047] When the average fiber length is relatively short, the fiber length distribution or fiber width distribution is narrow (small standard deviation, small range), and / or the fiber length distribution or fiber width distribution is sharp (large kurtosis), and / or the fiber length distribution or fiber width distribution is skewed towards the smaller side (large skewness), it is considered that non-uniform parts of the nanocellulose concentration in the slurry are less likely to occur, and the state of the slurry, particularly the viscosity stability, is higher. Similarly, for the fiber width or fiber width distribution, when the distribution is narrow (small standard deviation, small range), and / or the distribution is sharp (large kurtosis), and / or the fiber width distribution or fiber width distribution is skewed towards the smaller side (large skewness), it is considered that non-uniform parts of the nanocellulose concentration in the slurry are less likely to occur, and the state of the slurry, particularly the viscosity stability, is higher.

[0048] Hereinafter, the manufacturing method of the nanocellulose of the present invention will be exemplified and described. Note that the nanocellulose of the present invention is not limited to these manufacturing methods. The nanocellulose of the present invention can be produced, for example, by reacting a cellulose-based raw material with sodium hypochlorite as an oxidizing agent to produce oxidized cellulose, and further defibrating the oxidized cellulose. Since "nanocellulose" is fibrous cellulose obtained by refining oxidized fibrous cellulose, it is also referred to as "microfine cellulose fiber" or "CNF". In addition, nanocellulose represents a general term for nanosized cellulose, and includes cellulose nanofibers, cellulose nanocrystals, and the like.

[0049] The cellulose-based raw material in the present invention is not particularly limited as long as it is a material mainly composed of cellulose, and examples thereof include pulp, natural cellulose, regenerated cellulose, and microfine cellulose obtained by depolymerizing a cellulose raw material by mechanical treatment. In addition, as the cellulose-based raw material, commercially available products such as crystalline cellulose using pulp as a raw material can be used as they are. The cellulose-based raw material may be subjected to chemical treatment such as alkali treatment in order to facilitate penetration of the oxidizing agent used in the method described later.

[0050] <Step of producing oxidized cellulose> The concentration of the cellulose-based raw material during the reaction is not particularly limited, but is preferably 10% by mass or less. Generally, the reaction is carried out in a state where the cellulose raw material is added to the liquid containing the oxidizing agent. In addition, the available chlorine concentration of sodium hypochlorite in the reaction system is not particularly limited, but is preferably 6% by mass or more and 43% by mass or less, more preferably 7% by mass or more and 43% by mass or less, still more preferably 10% by mass or more and 43% by mass or less, and even more preferably 14% by mass or more and 43% by mass or less. The higher the available chlorine concentration in the reaction system, the more smoothly the reaction proceeds. On the other hand, sodium hypochlorite having an available chlorine concentration exceeding 43% by mass tends to be unstable.

[0051] The available chlorine concentration in hypochlorous acid or its salts such as sodium hypochlorite is a well-known concept and is defined as follows. Hypochlorous acid is a weak acid that exists only as an aqueous solution, and hypochlorite is a compound in which the hydrogen of hypochlorous acid is replaced by other cations. For example, since sodium hypochlorite, which is a hypochlorite, exists only in solution, the amount of available chlorine in the solution is measured instead of the concentration of sodium hypochlorite. The available chlorine of sodium hypochlorite means that the oxidizing power of the divalent oxygen atoms generated by the decomposition of sodium hypochlorite corresponds to two atomic equivalents of monovalent chlorine. Therefore, the bound chlorine atoms of sodium hypochlorite (NaClO) have the same oxidizing power as two atoms of unbound chlorine (Cl2), and available chlorine = 2×(chlorine in NaClO). For the specific measurement of the available chlorine concentration, the sample is precisely weighed, water, potassium iodide, and acetic acid are added and left standing, and the liberated iodine is titrated with a sodium thiosulfate solution using an aqueous starch solution as an indicator for measurement.

[0052] As for the adjustment of the available chlorine concentration of an aqueous sodium hypochlorite solution, there are a method of concentrating an aqueous sodium hypochlorite solution with a low available chlorine concentration, and a method of adjusting by using sodium hypochlorite pentahydrate crystals with an available chlorine concentration of about 43% by mass as they are or diluted with water. Examples of the method of adjusting the available chlorine concentration to a range of 6% by mass or more and 43% by mass or less, which is a preferable range, include the above-mentioned methods. Among these, adjusting the available chlorine concentration using sodium hypochlorite pentahydrate as an oxidizing agent is preferable because self-decomposition is less, that is, the decrease in the available chlorine concentration is less, and the adjustment is simple.

[0053] The amount of the aqueous sodium hypochlorite solution used as an oxidizing agent can be selected within a range that promotes the oxidation reaction. The mixing method of the cellulose-based raw material and the aqueous sodium hypochlorite solution is not particularly limited, but from the viewpoint of ease of operation, it is preferable to add the cellulose-based raw material to the aqueous sodium hypochlorite solution and mix them.

[0054] The reaction temperature in the oxidation reaction is preferably 15°C or higher and 100°C or lower, more preferably 20°C or higher and 90°C or lower. In order to efficiently proceed with the oxidation reaction, it is preferable to maintain the pH of the reaction system at 5 or higher and 14 or lower, and more preferably at 7 or higher and 14 or lower. An alkaline agent such as sodium hydroxide or an acid such as hydrochloric acid can be added to adjust the pH. The oxidation reaction time can be set according to the degree of progress of oxidation. For example, it is preferably reacted for about 15 minutes or more and 50 hours or less. From the viewpoint of adjusting the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range, standard deviation of fiber width, kurtosis, skewness, and range, etc. within the scope of the present invention, the oxidation reaction time is preferably 20 minutes or more, more preferably more than 20 minutes, and even more preferably 25 minutes or more.

[0055] In the oxidation reaction, the primary hydroxyl group of cellulose contained in the cellulose-based raw material is oxidized to a carboxyl group to produce oxidized cellulose. The oxidized cellulose in the present invention can also be said to be an oxide of the cellulose-based raw material. Further, when the cellulose-based raw material is oxidized with hypochlorous acid or its salt to obtain oxidized cellulose, the oxidized cellulose can also be said to be an oxide of the cellulose-based raw material by hypochlorous acid or its salt. The amount of carboxyl groups in the oxidized cellulose is not particularly limited. However, when the oxidized cellulose is defibrated and nano-sized in the next step to produce nanocellulose, the amount of carboxyl groups per 1 g of oxidized cellulose is preferably 0.2 mmol / g or more and 3.0 mmol / g or less, more preferably 0.35 mmol / g or more and 3.0 mmol / g or less, even more preferably 0.4 mmol / g or more and 3.0 mmol / g or less, still more preferably 0.5 mmol / g or more and 3.0 mmol / g or less, and even more preferably 0.55 mmol / g or more and 2.0 mmol / g or less. Further, the oxidation reaction may be carried out in two or more stages.

[0056] In addition, in order to enhance the viscosity stability of the slurry containing nanocellulose, the higher the amount of carboxy groups in the nanocellulose, the more preferable it is, and it is preferably 0.4 mmol / g or more. Since the cost in the oxidation reaction increases, it is preferably 0.8 mmol / g or less. Incidentally, the amount of carboxy groups in the oxidized cellulose or nanocellulose can be measured by the following method.

[0057] Pure water is added to a 0.5 mass% slurry of oxidized cellulose or nanocellulose to adjust the volume to 60 ml. After adding 0.1 M hydrochloric acid aqueous solution to adjust the pH to 2.5, a 0.05 N sodium hydroxide aqueous solution is added dropwise, and the electric conductivity is measured until the pH reaches 11. From the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid where the change in electric conductivity is gentle, it is calculated using the following formula. Amount of carboxy groups (mmol / g of oxidized cellulose or nanocellulose) = a (ml) × 0.05 / mass of oxidized cellulose or mass of nanocellulose (g)

[0058] Note that the cellulose-based raw material may contain protein components, but the mixed protein components can be removed by performing filtration and washing after the oxidation reaction is completed. As the methods of filtration and washing, known methods can be applied. In addition, by comparing the conductivity values of the washing water and the washing wastewater, etc., it can be used as a criterion for reaching the end point of washing.

[0059] Before the isolation treatment such as the above filtration, from the viewpoint of improving the filterability and yield of the isolation treatment, an acid is added to the solution containing oxidized cellulose or nanocellulose, for example, the pH is set to 4.0 or less, and at least a part of the carboxy groups generated by oxidation in the salt form (-COO - X + :X + refers to cations such as sodium and lithium) can be converted to the proton form (-COO - H + ). Incidentally, in the infrared absorption spectrum, the proton form is around 1720 cm -1 and the salt form is around 1600 cm-1 Since peaks are observed nearby, they can be distinguished.

[0060] In a solution containing oxidized cellulose or nanocellulose, when the pH is set to 4.0 or less for isolation treatment, in order to improve the handleability when used thereafter, for example, a base is added to make the pH 6.0 or more, and at least a part of the carboxy group is in the salt form (-COO - X + :X + refers to cations such as sodium and lithium). Further, the solution containing oxidized cellulose or nanocellulose may be made into a composition containing oxidized cellulose or nanocellulose by substituting the solvent or the like. Also in the composition containing oxidized cellulose or nanocellulose, for example, the pH is set to an alkaline condition of 10 or more, and at least a part of the carboxy group is in the salt form (-COO - X + :X + refers to cations such as sodium and lithium).

[0061] The method for producing oxidized cellulose or nanocellulose may further include a step of mixing the obtained oxidized cellulose or nanocellulose with a compound having a modifying group in order to control the physical properties of the oxidized cellulose. The compound having a modifying group is not particularly limited as long as it can form an ionic bond or a covalent bond with the carboxy group or hydroxyl group that oxidized cellulose or nanocellulose has. Examples of the compound having a modifying group capable of forming an ionic bond include primary amines, secondary amines, tertiary amines, quaternary ammonium compounds, and phosphonium compounds. Examples of the compound having a modifying group capable of forming a covalent bond include alcohols, isocyanate compounds, and epoxy compounds. As described above, oxidized cellulose or nanocellulose includes salt form, proton form, and modified form by a modifying group.

[0062] From the perspective of adjusting the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range, standard deviation of fiber width, kurtosis, skewness, and range, etc. within the scope of the present invention, it is preferable to use a propeller-type stirring blade or the like that can uniformly mix the reaction system during stirring. Also, the rotation speed of the propeller-type stirring blade is preferably in the range of 50 rpm or more and 500 rpm or less, and more preferably in the range of 80 rpm or more and 200 rpm or less.

[0063] <Process for producing nanocellulose by defibrating oxidized cellulose> The oxidized cellulose obtained above can be used to produce nanocellulose by defibrating and nanosizing it. The nanocellulose of the present invention includes those obtained by nanosizing cellulose such as cellulose nanocrystals. In the method for defibrating the oxidized cellulose, shortening of the defibrating time can be achieved by performing weak stirring such as using a stirrer in a solvent or mechanical defibrating. However, if the mechanical defibrating is too strong, the nanocellulose may break or be cut.

[0064] The method of the mechanical defibrating is not particularly limited. For example, after sufficiently washing the oxidized cellulose with a solvent, it can be appropriately selected according to the purpose. Examples include known mixing and stirring devices such as a screw-type mixer, paddle mixer, disperser-type mixer, turbine-type mixer, homomixer under high-speed rotation, high-pressure homogenizer, ultra-high pressure homogenizer, double-cylindrical homogenizer, ultrasonic homogenizer, water flow counter-collision type disperser, beater, disk-type refiner, conical refiner, double-disk type refiner, grinder, single-axis or multi-axis kneader, etc. By treating these alone or in combination of two or more kinds in a solvent, the oxidized cellulose can be nanosized to produce nanocellulose. From the perspective of adjusting the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range, standard deviation of fiber width, kurtosis, skewness, and range, etc. within the scope of the present invention, it is preferable to use an ultra-high pressure homogenizer. When defibrillation is performed using an ultra-high pressure homogenizer, the pressure during the defibrillation treatment is preferably 100 MPa or more, more preferably 120 MPa or more, and still more preferably 150 MPa or more. The number of defibrillation treatment times is not particularly limited, but from the viewpoint of sufficiently advancing defibrillation, it is preferably 2 times or more, more preferably 3 times or more. In addition, by adjusting defibrillation conditions such as the number of defibrillation treatment times during mechanical defibrillation and the concentration of oxidized cellulose in the oxidized cellulose dispersion, the average fiber length, average fiber width, standard deviation of fiber length, kurtosis, skewness, and range, standard deviation of fiber width, kurtosis, skewness, and range, etc. can be within the scope of the present invention.

[0065] The solvent used for the defibrillation treatment is not particularly limited and can be appropriately selected according to the purpose. Examples include water, alcohols, ethers, ketones, carbonic esters, acetonitrile, N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, etc. These can be used alone or in combination of two or more.

[0066] Examples of the alcohols include methanol, ethanol, isopropanol, isobutanol, sec-butyl alcohol, tert-butyl alcohol, methyl cellosolve, ethylene glycol, and glycerin. Examples of the ethers include ethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran. Examples of the ketones include acetone and methyl ethyl ketone. Examples of the carbonic esters include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, diphenyl carbonate, and ethyl methyl carbonate.

[0067] By selecting an organic solvent as the solvent, it becomes easier to isolate the oxidized cellulose obtained in the above process and the nanocellulose obtained by defibrating it. Further, since nanocellulose dispersed in the organic solvent is obtained, it becomes easier to mix with a resin that dissolves in the organic solvent, its resin raw material monomer, etc.

Example

[0068] Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[0069] <Evaluation method for viscosity stability> Regarding an aqueous slurry (50 g) containing titanium oxide R-820 (5% by mass) manufactured by Ishihara Sangyo Co., Ltd. and various nanocelluloses, the addition amount of nanocellulose was changed so that the initial viscosity of the slurry became 300 mPa·s when the viscosity was measured by the method described below. For the mixing to prepare the slurry, a mixer "Awatori Renkitaro ARE-310" manufactured by Shinki Co., Ltd. (mixing mode, revolution: 2000 rpm, rotation: 800 rpm, 20 minutes) was used. Then, the viscosity immediately after preparation (initial viscosity) and the viscosity after standing for 1 week were measured, the viscosity change rate was calculated from the following formula, and the viscosity stability was determined according to the following criteria. Viscosity change rate (%) = (viscosity of the slurry after standing for 1 week) / (viscosity of the slurry immediately after preparation) × 100 Criteria for judging viscosity stability (absolute value of viscosity change rate) A: Less than 105% B: 105% or more and less than 110% C: 110% or more and less than 115% D: 115% or more Note that the standing was carried out indoors (23 ± 2°C).

[0070] <Viscosity measurement method> The initial viscosity of the slurry was stirred with a spatula at a speed such that no bubbles entered, and then measured with an E-type viscometer (TV-22) manufactured by Toki Sangyo Co., Ltd. at 25°C and 100 rpm (shear rate 200 s -1) were measured under the conditions. The viscosity after standing for one week was also measured under the same conditions using the above equipment.

[0071] <Method for Measuring Fiber Length and Fiber Width of Nanocellulose> The obtained nanocellulose dispersion was diluted 1000 - 1000000 times with pure water, and then naturally dried on a mica substrate. The shape of the nanocellulose was observed in AC mode using a scanning probe microscope "MFP-3D infinity" manufactured by Oxford Asylum. Regarding the fiber length, the obtained image was binarized and analyzed using the image processing software "ImageJ". For 100 or more fibers, the number average fiber length was determined as fiber length = "perimeter length" ÷ 2. Regarding the fiber width, using the software attached to "MFP-3D infinity", for 50 or more fibers, the number average fiber width was determined as the cross-sectional height of the shape image = fiber width.

[0072] <Example 1> As a cellulose-based raw material, softwood pulp (SIGMA-ALDRICH NIST RM 8495, bleached kraft pulp) was cut into 5 mm squares with scissors and mechanically defibrated into a cotton-like shape by processing at 25,000 rpm for 1 minute using a "Wander Blender WB-1" manufactured by Osaka Chemical Co., Ltd. 350 g of sodium hypochlorite pentahydrate crystals with an available chlorine concentration of 42% by mass were placed in a beaker, and pure water was added and stirred to adjust the available chlorine concentration to 21% by mass. Then, 35% hydrochloric acid was added and stirred to obtain an aqueous solution with a pH of 11. The sodium hypochlorite aqueous solution was heated to 30 °C in a constant temperature water bath while stirring at 100 rpm using a propeller-type stirring blade of a stirrer (Three One Motor, BL600) manufactured by Shinto Kagaku Co., Ltd., and then 50 g of the cellulose-based raw material was added. After supplying the cellulose-based raw material, while maintaining the temperature at 30 °C in the same constant temperature water bath, 48% by mass sodium hydroxide was added while adjusting the pH during the reaction to 11, and stirring was carried out under the same conditions with a stirrer for 30 minutes. After the reaction was completed, the product was separated into solid and liquid by suction filtration using a PTFE mesh filter with an aperture of 20 μm, and the obtained oxidized cellulose was washed with pure water. Pure water was added to the oxidized cellulose to prepare a 5% dispersion, which was then treated 10 times at 200 MPa using a high-pressure homogenizer "Starburst Lab HJP-25005" manufactured by Sugino Machine, Ltd. to obtain a nano-cellulose aqueous dispersion. In the high-pressure homogenizer, the oxidized cellulose aqueous dispersion is circulated through the built-in high-pressure defibrillation unit to promote defibrillation. One pass through the defibrillation unit is called one pass. The residual nitrogen component derived from the N-oxyl compound in the nano-cellulose was measured as the nitrogen content using a trace total nitrogen analyzer (manufactured by Mitsubishi Chemical Analytech Co., Ltd., model name: TN-2100H), and as a result of calculating the increase from the raw material pulp, it was 1 ppm or less.

[0073] <Example 2> The same conditions as in Example 1 were used except that the number of passes in the high-pressure homogenizer was changed to 15 passes.

[0074] <Example 3> The same conditions as in Example 1 were used except that powdered cellulose (KC Flock W-100GK) manufactured by Nippon Paper Industries Co., Ltd. was used as the cellulose-based raw material.

[0075] <Example 4> The same conditions as in Example 1 were used except that powdered cellulose (VP-1) from TDI was used as the cellulose-based raw material.

[0076] <Example 5> The same conditions as in Example 1 were used except that the oxidized cellulose concentration of the oxidized cellulose dispersion during defibrillation was set to 2% by mass.

[0077] <Example 6> The same conditions as in Example 3 were used except that the oxidized cellulose concentration of the oxidized cellulose dispersion during defibrillation was set to 2% by mass.

[0078] <Example 7> The conditions were the same as in Example 3, except that the oxidized cellulose concentration in the oxidized cellulose dispersion during fibrillation was set to 2% by mass and the number of passes through the ultra-high pressure homogenizer was set to 8 passes.

[0079] <Example 8> The conditions were the same as in Example 3, except that the oxidized cellulose concentration in the oxidized cellulose dispersion during fibrillation was set to 3% by mass.

[0080] <Comparative Example 1> The conditions were the same as in Example 1, except that stirring during the oxidation reaction was carried out with a stirrer, pure water was added to the oxidized cellulose to prepare a 1% dispersion, and fibrillation was carried out for 10 minutes under the conditions of CYCLE 0.5 and AMPLIYUDE 50 using an ultrasonic homogenizer "UP-400S" manufactured by Hielscher. In the ultrasonic homogenizer, the ultrasonic oscillation part was immersed in the oxidized cellulose aqueous dispersion placed in the container, and fibrillation was advanced by the oscillated ultrasonic waves.

[0081] <Comparative Example 2> The conditions were the same as in Example 4, except that the oxidation reaction time was set to 20 minutes.

[0082] <Comparative Example 3 (TEMPO Oxidation)> Into a beaker, 0.8 g of 2,2,6,6-tetramethyl-1-piperidine-N-oxyl radical (TEMPO, Sigma-Aldrich) and 5 g of sodium bromide (FUJIFILM Wako Pure Chemical Corporation) were placed, 5000 mL of pure water was added, and stirring was carried out at 200 rpm for dissolution using a propeller-type stirring blade with a stirrer (Three One Motor, BL600) manufactured by Shinto Kagaku Co., Ltd. After heating to 25°C in a constant temperature water bath, 50 g of mechanically defibrated softwood pulp (SIGMA-ALDRICH NIST RM 8495, bleached kraft pulp) in a cotton-like form was added as a cellulose-based raw material, and 0.1 M sodium hydroxide solution was added to make an aqueous solution with a pH of 10. Then, 131.5 g of an aqueous sodium hypochlorite solution (industrial grade) was added to initiate the reaction. After supplying the aqueous sodium hypochlorite solution, while maintaining the temperature at 25°C in the same constant temperature water bath, 0.1 M sodium hydroxide was added to adjust the pH during the reaction to 10, and stirring was carried out under the same conditions with a stirrer for 120 minutes. After the reaction was completed, the product was separated by solid-liquid separation by suction filtration using a PTFE mesh filter with a pore size of 20 μm, and the obtained oxidized cellulose was washed with pure water. Pure water was added to the oxidized cellulose to prepare a 0.5% dispersion, and it was treated with a high-pressure homogenizer "Starburst Labo" manufactured by Sugino Machine at 200 MPa for 3 passes to obtain a nano-cellulose aqueous dispersion. The residual nitrogen component derived from N-oxyl compounds in the nano-cellulose was measured as the nitrogen content under the same conditions as in Example 1, and as a result of calculating the increase from the raw material pulp, it was 5 ppm.

[0083] The results of the nano-cellulose obtained in Examples 1 to 8 and Comparative Examples 1 to 3 are summarized in Table 1 below. The cellulose-based raw materials in Table 1 are as follows. · Softwood: Mechanically defibrated softwood pulp (SIGMA-ALDRICH NIST RM 8495, bleached kraft pulp) in a cotton-like form · KC: Powdered cellulose (KC Flock W-100GK) manufactured by Nippon Paper Industries Co., Ltd. · VP: Powdered cellulose (VP-1) of TDI Co., Ltd.

[0084]

Table 1

[0085]

Table 2

Claims

1. Nanocellulose having an average fiber length of 100 nm or more and 500 nm or less and an average fiber width of 2.0 nm or more and 5.0 nm or less, which satisfies at least one of the following conditions A to H. Condition A: The standard deviation of fiber length is 600 nm or less. Condition B: The kurtosis of the fiber length is 11 or more. Condition C: The fiber length skewness is 3.0 or more. Condition D: Fiber length range is 4000 nm or less Condition E: Standard deviation of fiber width is 1.5 nm or less Condition F: The kurtosis of the fiber width is 0.3 or more. Condition G: The fiber width skewness is 0.5 or more. Condition H: The fiber width range is 6.8 nm or less.

2. 2. The nanocellulose of claim 1, wherein the standard deviation of the fiber length is 10 nm or more and 500 nm or less.

3. The nanocellulose according to claim 1 or 2, wherein the kurtosis of the fiber length is 12 or more and 30 or less.

4. The nanocellulose according to any one of claims 1 to 3, wherein the fiber length skewness is 3.0 or more and 6.0 or less.

5. The nanocellulose according to any one of claims 1 to 4, having a fiber length range of 450 nm or more and 4000 nm or less.

6. The nanocellulose according to any one of claims 1 to 5, wherein the standard deviation of the fiber width is 0.5 nm or more and 1.5 nm or less.

7. The nanocellulose according to any one of claims 1 to 6, wherein the kurtosis of the fiber width is 0.3 or more and 2.5 or less.

8. The nanocellulose according to any one of claims 1 to 7, wherein the fiber width skewness is 0.5 or more and 1.5 or less.

9. The nanocellulose according to any one of claims 1 to 8, wherein the fiber width is in the range of 3.0 nm to 6.8 nm.

10. The nanocellulose of any one of claims 1 to 9, comprising carboxylated nanocellulose.

11. The nanocellulose of any one of claims 1 to 10, which is substantially free of N-oxyl compounds.

12. The nanocellulose according to any one of claims 1 to 11, which is produced by defibrating oxidized cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof.

13. A nanocellulose dispersion in which the nanocellulose according to any one of claims 1 to 12 is dispersed in a dispersion medium.

14. The method for producing nanocellulose according to any one of claims 1 to 12, comprising a step of defibrating oxidized cellulose obtained by oxidizing a cellulosic raw material with hypochlorous acid or a salt thereof.

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