Cellulose nanocrystal dispersion

Abstract

To provide a cellulose nanocrystal dispersion that is excellent in dispersibility of cellulose nanocrystal, can express excellent barrier properties and handleability, and also has excellent productivity and economical efficiency.SOLUTION: A cellulose nanocrystal dispersion contains a sulfate group and / or a sulfo group derived from sulfuric acid treatment, and an anionic functional group derived from hydrophilicization treatment. In a water dispersion with the cellulose nanocrystal of 2 mass% in solid content, a visible light transmittance at 600 nm is 45%T or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a cellulose nanocrystal dispersion, and more particularly, to a cellulose nanocrystal dispersion that is excellent in dispersibility and coatability and can form a coating film excellent in transparency and gas barrier properties.

Background Art

[0002] Nanocellulose has been proposed for use in various applications as a highly functional biomass raw material, such as functional additives and film composite materials. In particular, materials such as films made of cellulose nanofibers (CNF) and laminates containing cellulose nanofibers are known to be excellent in gas barrier properties such as oxygen barrier properties because they can suppress the dissolution and diffusion of gases due to strong hydrogen bonds and cross-linking interactions between cellulose fibers, and barrier materials using cellulose nanofibers have been proposed. For the refinement of cellulose fibers, chemical treatment for introducing hydrophilic functional groups such as carboxyl groups and phosphate groups into the hydroxyl groups of cellulose is performed together with mechanical treatment, whereby the energy required for the refinement treatment can be reduced, and the barrier properties and dispersibility in aqueous solvents are improved.

[0003] Such a dispersion using cellulose nanofibers is also known. For example, in Patent Document 1 below, a dispersion in which cellulose nanofibers having a crystallinity of 70% or more, a degree of polymerization of 160 or less by a viscosity method using a copper ethylenediamine solution, and a fiber diameter of 50 nm or less are dispersed in a dispersion medium is proposed. Also, in Patent Document 2 below, a dispersion in which cellulose nanofibers having a number average fiber length of 250 nm or less and a number average fiber diameter of 2 to 5 nm are dispersed in a dispersion medium is proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] However, in the cellulose nanofiber dispersion described in Patent Document 1 above, as a process for preparing cellulose nanofibers to be dispersed in a dispersion medium, a process for reducing the viscosity of cellulose nanofibers chemically treated using a TEMPO catalyst or a micronization process for nanofibrillation is required, which is not sufficiently satisfactory in terms of productivity and economy. In addition, since cellulose nanofibers have a long fiber length, they are not satisfactory in terms of gas barrier properties. Although it is possible to obtain high gas barrier properties by shortening the fiber length, further treatment is required for this purpose, resulting in poor economy. In Patent Document 2 above, cellulose nanofibers with a short fiber length that can be reduced in viscosity to improve the dispersibility of the dispersion are prepared. However, it is necessary to use special raw materials to prepare cellulose nanofibers with a short fiber length, which is also inferior in terms of productivity and economy.

[0006] As nanocellulose with a shorter fiber length than cellulose nanofibers, cellulose nanocrystals (CNC) formed by hydrolyzing cellulose fibers with a strong acid are known. Since cellulose nanocrystals have a short fiber length, they are excellent in handleability during coating and also in the smoothness of the coated surface. However, generally, cellulose nanocrystals are inferior in gas barrier properties compared to cellulose nanofibers into which carboxyl groups and the like have been introduced as described above. The present inventors have found that cellulose nanocrystals can exhibit excellent gas barrier properties due to their high content of anionic functional groups (Japanese Patent Application No. 2018-177610). However, such cellulose nanocrystals are generally solidified in powder form from the viewpoints of storage stability and transportability. To obtain a dispersion, it is necessary to disperse the powdered cellulose nanocrystals in a dispersion medium while suppressing aggregation, and this is still not satisfactory in terms of productivity and economy. Further, according to the research by the present inventors, it has been found that a dispersion obtained by dispersing such solidified (powder-form) cellulose nanocrystals reduces the gas barrier properties of the cellulose nanocrystals containing anionic functional groups.

[0007] Therefore, an object of the present invention is to provide a cellulose nanocrystal dispersion excellent in the dispersibility of cellulose nanocrystals, capable of exhibiting excellent barrier properties and handleability, and also excellent in productivity and economy.

Means for Solving the Problems

[0008] According to the present invention, there is provided a cellulose nanocrystal dispersion containing a sulfate group and / or a sulfo group derived from sulfuric acid treatment and an anionic functional group derived from hydrophilic treatment, The total amount of the sulfate group and / or sulfonic group and anionic functional groups is more than 0.17 mmol / g and 4.0 mmol / g or less, wherein the average particle diameter of the cellulose nanocrystals in the cellulose nanocrystal dispersion measured by a laser diffraction particle size distribution measuring device is 0.35 to 10.1 μm, the median diameter is 0.40 to 10.6 μm, and the mode diameter is 0.40 to 10.9 μm, and the visible light transmittance at 600 nm in an aqueous dispersion having a solid content of 2% by mass of the cellulose nanocrystals is 85.1%T or more.

[0009] In the cellulose nanocrystal dispersion of the present invention, 1 .Before the crystallinity of the cellulose nanocrystals is 60% or more, the fiber width is 50 nm or less, and the aspect ratio is 5 to 50, 2 . the anionic functional group is at least one of a sulfate group, a sulfo group, a phosphate group, and a carboxyl group, 3The hydrophilic treatment is a combination of a never-dry treatment or a never-dry treatment and a treatment using any one of carbodiimide, sulfuric acid, sulfur trioxide-pyridine complex, phosphoric acid-urea, TEMPO catalyst, and oxidizing agent. is preferred.

Advantages of the Invention

[0010] In the cellulose nanocrystal dispersion of the present invention, cellulose nanocrystals having a short fiber length and a small fiber diameter are uniformly dispersed without aggregation. Therefore, it has excellent coatability and handleability, and also excellent transparency. The visible light transmittance in the aqueous dispersion with a solid content of 2% by mass of cellulose nanocrystals is 45%T or more, showing excellent transparency. In the present invention, cellulose nanocrystals have sulfate groups and / or sulfonic groups derived from sulfuric acid treatment and anionic functional groups derived from hydrophilic treatment. A dense self-assembled structure is formed due to the charge repulsion between cellulose nanocrystals, so that excellent gas barrier properties can be exhibited. Moreover, it is composed of cellulose nanocrystals with a short fiber length, and since these are uniformly dispersed in the dispersion, the gas barrier properties are further improved in combination with the self-assembled structure described above. As is clear from the results of the examples described later, the cellulose nanocrystal dispersion of the present invention is superior in transparency (visible light transmittance) and gas barrier properties compared to a dispersion in which solidified cellulose nanocrystals are dispersed at the same solid content. In addition, there is no need to redisperse the solidified cellulose nanocrystals to prepare a dispersion, and it is also excellent in terms of economy and handleability. Moreover, since the cellulose nanocrystals are treated by never-dry treatment without undergoing dry solidification, a dispersion in which the cellulose nanocrystals are in a fine and homogeneous dispersed state can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0012] The cellulose nanocrystal dispersion of the present invention is a dispersion in which cellulose nanocrystals containing sulfate groups and / or sulfonic groups derived from sulfuric acid treatment and anionic functional groups derived from hydrophilic treatment are uniformly dispersed without aggregating in a dispersion medium. Therefore, the visible light transmittance of the aqueous dispersion with a solid content of cellulose nanocrystals of 2% by mass is 45%T or more, and it has excellent transparency. In addition, the gas barrier property by cellulose nanocrystals is exhibited when the self-assembled structure formed by the charge repulsion between cellulose nanocrystals becomes a barrier for the permeation path of the permeating gas. In the cellulose nanocrystal dispersion of the present invention, due to the presence of anionic functional groups such as sulfate groups and / or sulfonic groups and carboxyl groups on the surface of cellulose nanocrystals, the self-assembled structure can be efficiently formed by the charges (anions) of these anionic functional groups, and excellent gas barrier property can be exhibited in combination with the above-described uniform dispersibility.

[0013] Furthermore, in the cellulose nanocrystal dispersion of the present invention, it is preferable that the average particle diameter of the cellulose nanocrystals is 10.1 μm or less, the median diameter is 10.6 μm or less, and the mode diameter is 10.9 μm or less as measured by a laser diffraction particle size distribution analyzer. The particle size distribution is an index indicating what sizes (particle diameters) of particles are contained in what ratios (relative particle amounts with the whole being 100%) in the sample particle group of the dispersion to be measured. The particle diameter is specified from the light intensity distribution data pattern by the laser diffraction and scattering method using a laser diffraction particle size distribution apparatus, and the volume-based particle size distribution is obtained by calculation. Also, the number-based particle size distribution can be calculated based on the volume-based particle size distribution. When a laser beam is irradiated onto the particle group to be measured by a laser diffraction particle size distribution analyzer, a spatial light intensity distribution pattern of diffracted and scattered light is generated. Among these, the light intensity distribution pattern of the forward scattered light is condensed by a lens and forms a ring-shaped diffracted and scattered image on the detection surface located at the focal distance. This is detected by a ring sensor in which detection elements are arranged concentrically. Also, the side scattered light and the backward scattered light are detected by a side scattered light sensor and a backward scattered light sensor, respectively. In this way, the light intensity distribution pattern is detected using various detection elements to obtain light intensity distribution data. In a state where the cellulose nanocrystals in the dispersion are present at a low concentration, the average particle diameter, median diameter, and mode diameter of the cellulose nanocrystals can be determined by analyzing the measurement results of a laser diffraction particle size distribution apparatus. In a dispersion in which the average particle diameter, median diameter, and mode diameter of the cellulose nanocrystals in the dispersion are within the above ranges, there are advantages such as good dispersibility, excellent gas barrier properties and handling properties during coating, and excellent smoothness of the coated surface.

[0014] The particle size distribution is usually expressed as the amount of particles (cumulative or frequency) with respect to the particle size scale. Also, in the laser diffraction / scattering method, which is the measurement principle of a laser diffraction type particle size distribution measuring device, the particle size distribution is determined based on the light intensity distribution pattern of the diffracted / scattered light calculated assuming that the particles are spherical. Therefore, in particle size distribution measurement, the measurement is always carried out based on the premise or assumption that the particles are spherical. In the case of fibrous particles such as cellulose nanocrystals, a relatively broad distribution is obtained where the minor axis of the fibrous particles approximately corresponds to the lower limit of the distribution and the major axis corresponds to the upper limit of the distribution. This is because it is considered that the directions of the fibrous particles are random inside the cell, and even if the directions of the particles are somewhat constant to a certain extent, since the shape of the sensor for detecting the diffracted / scattered light is a quarter circle, as a result, diffracted / scattered light in only a certain direction cannot be detected. When fibrous particles of various shapes and sizes are included, the distribution range of the measurement results spreads from the smallest minor axis to the largest major axis among them. Also, in the case of fibrous particles, the greater the variation in the shape and size of the particles included in the particle group to be measured, the wider the distribution range, and the smaller the variation, the narrower the distribution range. As the shape and size increase, the particle size distribution as a whole shifts to a larger value, and as the shape and size decrease, the particle size distribution as a whole shifts to a smaller value. If the fibrous particles are aggregated, the particle size distribution as a whole shifts to a larger value, and if they are dispersed, the particle size distribution as a whole shifts to a smaller value. Thus, the fibrous properties of cellulose nanocrystals can be quantitatively evaluated by a laser diffraction type particle size distribution measuring device.

[0015] The average particle size is calculated based on a logarithmic scale. Multiply the value of each particle size by the relative amount of particles (difference %), and divide by the total relative amount of particles (100%). Specifically, first divide the particle size range to be measured (maximum particle size: x1, minimum particle size: xn+1) into n parts, and each particle size interval is set as [xj, xj+1] (j = 1, 2, ··· n). The division in this case is an equal division on the logarithmic scale. Also, based on the logarithmic scale, the representative particle size in each particle size interval is given by the following formula (1)

[0016] [Number] is calculated by. Since this representative particle size is represented by a logarithm, it no longer becomes the unit of particle size at this point. Further, let qj (j = 1, 2, ··· n) be the relative particle amount (difference %) corresponding to the particle size interval [xj, xj+1], and assuming the total of all intervals is 100%, the average value μ on the logarithmic scale is given by the following formula (2)

[0017] [Number] can be calculated by. This μ is a numerical value on the logarithmic scale and has no unit as a particle size. Therefore, to convert it back to the unit of particle size, 10 μ That is, 10 to the power of μ is calculated. This 10 μ becomes the average particle size.

[0018] The particle size distribution data is expressed as the integrated % or frequency % with respect to the particle size scale, but it can also be expressed as the particle size with respect to the scale of the integrated %. For example, the particle size at the point where the integrated % distribution curve intersects the horizontal axis at 10% can be called the 10% diameter, the particle size at the point where it intersects the horizontal axis at 50% can be called the 50% diameter, and the particle size at the point where it intersects the horizontal axis at 90% can be called the 90% diameter. Any arbitrary integrated % can be used as needed. The calculation method for the arbitrary % particle size is shown in Fig. 1 and the following formulas (3) and (4).

[0019] [Number] Integrated % Q a The arbitrary % particle size x a at is obtained, and for this x a the two particle sizes x j+1 and x j on both sides of, and the integrated % Q j+1 and Q j are known, when the arbitrary % particle size x a at the integrated % Q aIt can be calculated using the above formulas (3) and (4). The 50% particle size is the median diameter, and Q a is set to 50% to calculate the median diameter. Also, the mode diameter can be obtained from the particle size with the largest ratio of the appearance frequency (%).

[0020] In the cellulose nanocrystal dispersion of the present invention, it is preferable that the cellulose nanocrystal has undergone a never-dry treatment. However, regarding whether the cellulose nanocrystal in the cellulose nanocrystal dispersion has undergone a never-dry treatment or a drying and solidification treatment, the difference in molecular mobility and dispersibility of the cellulose nanocrystal dispersion can be evaluated by relaxation time measurement analysis using pulsed NMR and cryo-SEM observation. That is, for the cellulose nanocrystal dispersion, using Bruker's TD-NMR THE MINISPEC MQ20, the free induction decay (M(t)) measured by the solid echo method of pulsed NMR is fitted by analysis software (TDNMR-A) using the following formula (5), and can be analyzed by approximating it to one component (α) (component (α): the component with the lowest molecular mobility and the most restricted molecular motion). Furthermore, by using the following formulas (6) and (7), it is possible to examine whether it can be analyzed in two or more components (the above component (α), component (β): the component with higher molecular mobility than component (α) and unrestricted molecular motion, component (γ): the component with higher mobility than component (β)). When an error is returned on the analysis software, it is suggested that there are no two or more components.

[0021] M(t)=αexp{-(1 / Wa)×(t / Tα)×Wa}···(5) M(t)=αexp{-(1 / Wa)×(t / Tα)×Wa} +βexp{-(1 / Wa)×(t / Tβ)×Wa}···(6) M(t)=αexp{-(1 / Wa)×(t / Tα)×Wa} +βexp{-(1 / Wa)×(t / Tβ)×Wa} +γexp{-(1 / Wa)×(t / Tγ)×Wa} ··· (7)

[0022] The symbols in the above (5) to (7) are as follows. α: Proton ratio (%) of component (α) Tα: T2 relaxation time (μsec) of component (α) β: Proton ratio (%) of component (β) Tβ: T2 relaxation time (μsec) of component (β) γ: Proton ratio (%) of component (γ) Tγ: T2 relaxation time (μsec) of component (γ) t: Observation time (μsec) Wa: Shape coefficient (1 < Wa < 2)

[0023] The dispersion obtained by concentrating cellulose nanocrystals by the Never-dry treatment (corresponding to Example 1 described later) becomes a dispersion with higher viscosity and transparency than the dispersion (corresponding to Comparative Example 1 described later) that has been redispersed using a disperser after drying such as spray drying. That is, as is clear from the results of the examples described later, in Example 1 which is a cellulose nanocrystal dispersion by the Never-dry treatment, as a result of analysis according to the above formulas (5) to (7), the analysis result of the numerical value of only one component (α) with a short relaxation time is obtained. This indicates that only cellulose nanocrystals by the Never-dry treatment are present in the solvent and have a uniform mobility of one component, showing the good homogeneity of cellulose nanocrystals in the dispersion. Also, as is clear from FIG. 2 showing the cryo-SEM observation image of the cellulose nanocrystal dispersion of Example 1, it is observed that the cellulose nanocrystals by the Never-dry treatment are finely and uniformly dispersed in the dispersion. This result is suggested to be correlated with the analysis result of pulsed NMR.

[0024] On the other hand, in Comparative Example 1, which is a cellulose nanocrystal dispersion that has been solidified by a drying process, as a result of analysis according to the above formulas (5) to (7), numerical analysis results in two components, namely, a component (α) with a short relaxation time and a component (β) with a longer relaxation time, have been obtained. This indicates that two components, namely, cellulose nanocrystals that have undergone drying and solidification and cellulose nanocrystals that have been aggregated with limited redispersion ability, are present in the solvent and have non-uniform motility, showing the inhomogeneity of cellulose nanocrystals in the dispersion. Also, as is clear from FIG. 3 showing a cryo-SEM observation image of the cellulose nanocrystal dispersion of Comparative Example 1, aggregated cellulose nanocrystals are present and a non-uniform dispersion state is observed, suggesting that this result correlates with the analysis result of pulsed NMR.

[0025] In the dispersion of the present invention, it is preferable that the cellulose nanocrystals contain an anionic functional group such as a sulfate group and / or a sulfonic group, and a carboxyl group in a total amount of more than 0.17 mmol / g and 4.0 mmol / g or less. When the total amount of anionic functional groups is less than the above range, a sufficient self-assembled structure may not be formed compared to the case within the above range, and there is a possibility that the desired gas barrier property cannot be obtained. On the other hand, when the total amount of anionic functional groups is more than the above range, the crystal structure of cellulose nanocrystals may not be maintained, and conversely, the gas barrier property may be impaired. In addition, in this specification, the sulfate group is a concept including a sulfuric acid ester group.

[0026] In the present invention, it is particularly preferable that the cellulose nanocrystal is a cellulose nanocrystal hydrolyzed by sulfuric acid treatment, since it already contains sulfate groups and / or sulfonic groups that contribute to the formation of a self-assembled structure. That is, there are cellulose nanocrystals obtained by acid hydrolysis of cellulose fibers by sulfuric acid treatment or hydrochloric acid treatment. However, since cellulose nanocrystals obtained by hydrochloric acid treatment do not have sulfate groups and / or sulfonic groups, they cannot improve the barrier properties as compared with cellulose nanocrystals obtained by sulfuric acid treatment, which have sulfate groups and / or sulfonic groups that contribute to the formation of a self-assembled structure. The anionic functional groups possessed by the cellulose nanocrystals of the present invention are determined by the method for hydrophilizing the cellulose nanocrystals described later, and it is particularly preferable that they are carboxyl groups, phosphate groups, sulfate groups and / or sulfonic groups. Thereby, the self-assembled structure described above is efficiently formed, and the gas barrier property is improved.

[0027] In the present invention, it is desirable that the total amount of sulfate groups and / or sulfonic groups and anionic functional groups in the cellulose nanocrystal is in the above-described range, and the crystallinity is in the range of 60% or more. Also, it is preferable that the cellulose nanocrystal is a cellulose nanocrystal having a fiber diameter of 50 nm or less and an aspect ratio of 5 to 50.

[0028] (Method for producing a cellulose nanocrystal dispersion) The cellulose nanocrystal dispersion of the present invention having the above-described characteristics is importantly a cellulose nanocrystal dispersion that has not undergone solidification such as powder (never-dry treatment) in the preparation of cellulose nanocrystals. As described above, when preparing a dispersion using solidified cellulose nanocrystals, not only does the manufacturing process become complicated due to the solidification treatment and the dispersion treatment of solid cellulose nanocrystals, but there is also a problem that the gas barrier property deteriorates. That is, cellulose nanocrystals that have been subjected to a drying treatment and solidified are difficult to align the fibers in the dispersion and are difficult to form a dense self-organized structure, and the gas barrier property is inferior to that of the dispersion of the present invention. Therefore, the cellulose nanocrystal dispersion of the present invention is a dispersion obtained by subjecting cellulose nanocrystals containing sulfate groups and / or sulfonic acid groups, obtained by subjecting a cellulose raw material to sulfuric acid treatment, to a hydrophilization treatment, and then subjecting it to a centrifugation step and a filtration separation step after the hydrophilization treatment step, and does not undergo a spray drying step or the like for solidification.

[0029] [Cellulose nanocrystal] Cellulose nanocrystals are rod-shaped cellulose crystal fibers obtained by subjecting cellulose fibers such as pulp to acid hydrolysis treatment with sulfuric acid or hydrochloric acid. In the present invention, however, cellulose nanocrystals obtained by sulfuric acid treatment having sulfate groups and / or sulfonic acid groups that can contribute to the formation of a self-organized structure are used. It is preferable that the cellulose nanocrystals contain sulfate groups and / or sulfonic acid groups in an amount of 0.18 to 4.0 mmol / g, particularly 0.20 to 2.0 mmol / g. Also, as described above, the cellulose nanocrystals preferably have a fiber diameter in the range of 50 nm or less, particularly 2 to 50 nm, a fiber length in the range of 100 to 500 nm, an aspect ratio in the range of 5 to 50, and a crystallinity of 60% or more, particularly 70% or more, and can be preferably used.

[0030] [Hydrophilization treatment] In the present invention, by performing a hydrophilization treatment on the cellulose nanocrystal having a sulfate group and / or a sulfo group described above, the amount of the sulfate group and / or the sulfo group is adjusted, or an anionic functional group such as a carboxyl group or a phosphate group is introduced into the hydroxyl group at the 6-position of cellulose, and the total amount of anionic functional groups such as a sulfate group, a sulfo group, a carboxyl group, and a phosphate group is more than 0.17 mmol / g and 4.0 mmol / g or less, particularly in the range of 0.20 to 2.0 mmol / g. A cellulose nanocrystal is prepared. As the hydrophilization treatment, it is carried out in combination with a never-dry treatment or a never-dry treatment and a treatment using any one of a water-soluble carbodiimide, sulfuric acid, a sulfur trioxide-pyridine complex, phosphoric acid-urea, a TEMPO catalyst, and an oxidizing agent. By the treatment using any one of a carbodiimide, sulfuric acid, and a sulfur trioxide-pyridine complex, the amount of the sulfate group and / or the sulfo group of the cellulose nanocrystal is adjusted, and further, the nanocellulose is further short-fibrillated. In addition, by the treatment using any one of phosphoric acid-urea or a TEMPO catalyst and an oxidizing agent, an anionic functional group such as a phosphate group or a carboxyl group is introduced, and the total amount of anionic functional groups of the cellulose nanocrystal is adjusted to the above range. In addition, as long as the total amount of anionic functional groups is within the above range, any one treatment may be performed for the hydrophilization treatment, but the same treatment may be performed multiple times or in combination with other treatments multiple times.

[0031] <Hydrophilization treatment by never-dry treatment> Cellulose nanocrystals are solidified into powders or the like by performing a drying treatment such as spray drying, heating, or reduced pressure. However, when solidified by the drying treatment, a part of the anionic functional groups contained in the cellulose nanocrystals is desorbed and the hydrophilicity is reduced. That is, the never-dry treatment that does not undergo solidification such as powder for cellulose nanocrystals containing anionic functional groups can be cited as a hydrophilization treatment. Examples of the anionic functional group include a sulfate group and / or a sulfo group, a phosphate group, and a carboxyl group.

[0032] <Hydrophilization treatment using carbodiimide> In the treatment using carbodiimide, cellulose nanocrystal and carbodiimide are stirred in a solvent such as dimethylformamide, sulfuric acid is added thereto, and then reacted at a temperature of 0 to 80 ° C for 5 to 300 minutes to form a sulfate ester. Carbodiimide and sulfuric acid are preferably used in amounts of 5 to 30 mmol and 5 to 30 mmol, respectively, per 1 g (solid content) of cellulose nanocrystal. Next, it is preferable to add an alkaline compound such as sodium hydroxide to convert the sulfo group introduced into the cellulose nanocrystal from the H type to the Na type in order to improve the yield. Thereafter, a filtration treatment using a dialysis membrane or the like is performed to remove impurities and the like, thereby preparing a sulfate group and / or sulfo group-modified cellulose nanocrystal. Examples of the carbodiimide include 1-ethyl-3- (3-dimethylaminopropyl) carbodiimide, which is a water-soluble compound having a carbodiimide group (-N = C = N-) in the molecule. Dicyclohexylcarbodiimide or the like that dissolves in an organic solvent can also be used.

[0033] <Hydrophilic treatment using sulfuric acid> The cellulose nanocrystal used in the present invention is obtained by hydrolyzing cellulose fibers with sulfuric acid, and this cellulose nanocrystal is further hydrophilized using sulfuric acid. Sulfuric acid is preferably used in an amount of 40 to 60% by mass based on 1 g (solid content) of cellulose nanocrystal. The reaction is carried out at a temperature of 40 to 60 ° C for 5 to 300 minutes, and then a filtration treatment using a dialysis membrane or the like is performed to remove impurities and the like, thereby preparing a sulfate group and / or sulfo group-modified cellulose nanocrystal.

[0034] <Hydrophilic treatment using sulfur trioxide-pyridine complex> In the treatment using a sulfur trioxide-pyridine complex, a sulfur trioxide-pyridine complex and cellulose nanocrystal are reacted in dimethyl sulfoxide at a temperature of 0 to 60 ° C for 5 to 240 minutes to introduce a sulfate group and / or a sulfo group into the hydroxyl group at the 6-position of the cellulose glucose unit. The sulfur trioxide-pyridine complex is preferably blended in an amount of 0.5 to 4 g by mass per 1 g (solid content) of cellulose nanocrystals. After the reaction, it is preferable to add an alkaline compound such as sodium hydroxide to convert the sulfate group and / or sulfonate group introduced into the cellulose nanocrystals from the H form to the Na form in order to improve the yield. Thereafter, dimethylformamide or isopropyl alcohol is added, washed by centrifugation or the like, and then impurities and the like are removed by filtration using a dialysis membrane or the like. The obtained concentrated solution is dispersed in water to prepare sulfate group and / or sulfonate group-modified cellulose nanocrystals.

[0035] <Hydrophilic treatment using phosphoric acid-urea> The hydrophilic treatment using phosphoric acid-urea can be carried out in the same manner as the conventionally known treatment for introducing a phosphate group using phosphoric acid-urea. Specifically, in the presence of a urea-containing compound, a cellulose nanocrystal and a phosphate group-containing compound are reacted at a temperature of 135 to 180°C for 5 to 120 minutes to introduce a phosphate group into the hydroxyl group of the cellulose glucose unit. Examples of the phosphate group-containing compound include phosphoric acid, lithium salt of phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, ammonium salt of phosphoric acid, etc. Among them, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, etc. can be preferably used alone or in combination. The phosphate group-containing compound is preferably added in an amount of 10 to 100 mmol per 10 g (solid content) of cellulose nanocrystals. Examples of the urea-containing compound include urea, thiourea, biuret, phenylurea, benzylurea, dimethylurea, etc. Among them, urea can be preferably used. The urea-containing compound is preferably used in an amount of 150 to 200 mmol per 10 g (solid content) of cellulose nanocrystals.

[0036] <Hydrophilic treatment using TEMPO catalyst> The hydrophilization treatment using a TEMPO catalyst (2,2,6,6 - tetramethylpiperidine - 1 - oxyl) can be carried out in the same manner as the conventionally known oxidation method using a TEMPO catalyst. Specifically, cellulose nanocrystals having a sulfate group and / or a sulfo group are subjected to a hydrophilization reaction that oxidizes the hydroxyl group at the 6 - position of the cellulose glucose unit to a carboxyl group under the conditions of an aqueous system, normal temperature, and normal pressure via a TEMPO catalyst (2,2,6,6 - tetramethylpiperidine 1 - oxyl). As the TEMPO catalyst, in addition to the above - mentioned 2,2,6,6 - tetramethylpiperidine 1 - oxyl, derivatives of TEMPO such as 4 - acetamido - TEMPO, 4 - carboxy - TEMPO, and 4 - phosphonoxy - TEMPO can also be used. The amount of the TEMPO catalyst used is in the range of 0.01 - 100 mmol, preferably 0.01 - 5 mmol, per 1 g of cellulose nanocrystals (solid content).

[0037] During the hydrophilization oxidation treatment, it is preferable to use a co - oxidant such as an oxidizing agent, bromide, or iodide alone or in combination with the TEMPO catalyst. Examples of the oxidizing agent include known oxidizing agents such as halogens, hypohalous acids, halous acids, perhalogenic acids or their salts, halogen oxides, and peroxides. In particular, sodium hypochlorite and sodium hypobromite can be preferably used. The amount of the oxidizing agent is in the range of 0.5 - 500 mmol, preferably 5 - 50 mmol, per 1 g of cellulose nanocrystals (solid content). After adding the oxidizing agent and allowing a certain time to elapse, a post - oxidation treatment can also be carried out by adding the oxidizing agent again. As the co - oxidant, alkali metal bromides such as sodium bromide and alkali metal iodides such as sodium iodide can be preferably used. The amount of the co - oxidant is in the range of 0.1 - 100 mmol, preferably 0.5 - 5 mmol, per 1 g of cellulose nanocrystals (solid content). Also, the reaction solution preferably uses water or an alcohol solvent as the reaction medium.

[0038] The reaction temperature for the hydrophilization treatment is in the range of 1 to 50 °C, particularly 10 to 50 °C, and it may be at room temperature. Also, the reaction time is preferably 1 to 360 minutes, particularly 60 to 240 minutes. As the reaction progresses, carboxyl groups are generated in the cellulose, resulting in a decrease in the pH of the slurry. However, in order to efficiently progress the oxidation reaction, it is desirable to maintain the pH in the range of 9 to 12 using a pH adjuster such as sodium hydroxide.

[0039] [Washing and fibrillation treatment] Before or after the hydrophilization treatment, the cellulose nanocrystal is washed with water or by centrifugation while adding water to wash the acid, catalyst, etc. used in the hydrophilization treatment. Next, it is preferable to perform a fibrillation treatment. However, since the cellulose nanocrystal has a short fiber length, it is not always necessary to perform a fibrillation treatment. In addition, as a micronization device, an ultra-high pressure homogenizer, a mixer, a grinder, etc. can be used, and a fibrillation treatment can be performed using water or the like as a dispersion medium, and a dispersion liquid can be prepared simultaneously with the fibrillation.

[0040] [Dispersion treatment] The cellulose nanocrystal subjected to the hydrophilization treatment or, if necessary, the fibrillation treatment is subjected to a dispersion treatment without being solidified (spray-dried to be in powder form). Since it is not solidified, there is no need for redispersion, and it is excellent in productivity and economy. Also, since it is not solidified, it becomes possible to form the above-described dense self-assembled structure and to exhibit excellent gas barrier properties. For the dispersion treatment, a disperser such as an ultrasonic disperser, a homogenizer, or a mixer can be preferably used, and a stirring method using a stirring rod, a stirring stone, etc. may also be used. The dispersion medium of the dispersion liquid may be only water, but may also be an alcohol such as methanol, ethanol, or isopropanol, a ketone such as 2-butanone or acetone, or a mixed solvent of an aromatic solvent such as toluene and water. The cellulose nanocrystal dispersion preferably contains cellulose nanocrystals (solid content) in the range of 0.1 to 90% by mass. In an aqueous dispersion with a solid content of 2% by mass, the viscosity is 5.5 to 40 mPa·s (rotational viscometer, temperature 30°C, spindle rotation speed 100 rpm), the zeta potential is in the range of -50 to -55 mV, and it has excellent handleability and coatability. Also, in an aqueous dispersion with a solid content of 2% by mass, the visible light transmittance is 45%T or more, showing excellent transparency.

Examples

[0041] Examples of the present invention will be described below. The following examples are examples of the present invention, and the present invention is not limited to these examples. The measurement methods for each item are as follows (Incidentally, Example 1 and Example 3 are reference examples) 。

[0042] <Amount of anionic functional groups> Weighed the nanocellulose-containing dispersion, added ion-exchanged water to prepare 100 ml of a 0.05 to 0.3% by mass nanocellulose-containing dispersion. Added 0.1 g of a cation-exchange resin and stirred. Then, filtration was performed to separate the cation-exchange resin and the nanocellulose dispersion. Using a potentiometric automatic titrator (manufactured by Kyoto Electronics) for the dispersion after cation-exchange, a 0.05 M sodium hydroxide solution was dropped, and the change in the electrical conductivity shown by the nanocellulose-containing dispersion was measured. From the obtained conductivity curve, the titration amount of sodium hydroxide consumed for the neutralization of anionic functional groups was determined, and the amount of anionic functional groups (mmol / g) was calculated using the following formula. Amount of anionic functional groups (mmol / g) = Titration amount of sodium hydroxide consumed for the neutralization of anionic functional groups (ml) × Concentration of the sodium hydroxide (mmol / ml) ÷ Solid mass of nanocellulose (g)

[0043] <Visible light transmittance> Using a spectrophotometer (UV-3100PC, Shimadzu Corporation), the visible light transmittance (%T) at 600 nm was determined when the cellulose nanocrystals were made into an aqueous dispersion with a solid content of 2% by mass.

[0044] <Particle size distribution> Using a laser diffraction particle size distribution analyzer (SALD-3100, Shimadzu Corporation), the average particle size, median diameter, and mode diameter of cellulose nanocrystals were determined using an aqueous dispersion with a solids content of 2% by mass. The light source of the apparatus is a semiconductor laser with a wavelength of 690 nm. The measurement results with an absorbance of 0.015 or less and a refractive index of 1.55 were used for the analysis.

[0045] <Example 1> Cellulose nanocrystals were prepared by decomposing pulp with 64% by mass sulfuric acid. The cellulose nanocrystals were prepared by concentrating and washing with an ultracentrifuge without drying and solidifying, and finally added to ion-exchanged water so that the cellulose nanocrystals had a solids content of 2% by mass, and treated with an ultrasonic disperser for 10 minutes to prepare a cellulose nanocrystal dispersion.

[0046] <Example 2> Cellulose nanocrystals prepared by the never-dry treatment were prepared in the same manner as in Example 1, and finally added to ion-exchanged water so that the cellulose nanocrystals had a solids content of 2% by mass, and treated with an ultra-high pressure homogenizer for 10 minutes to prepare a cellulose nanocrystal dispersion.

[0047] <Example 3> Cellulose nanocrystals prepared by a never-dry process were prepared in the same manner as in Example 1. To a 10 g (solid content) aqueous dispersion of cellulose nanocrystals, 0.8 mmol of TEMPO catalyst (manufactured by Sigma Aldrich) and 12.1 mmol of sodium bromide were added, ion-exchanged water was added to make up to 1 L, and the mixture was stirred until uniformly dispersed. Then, 5 mmol of sodium hypochlorite was added to initiate the oxidation reaction. During the reaction, the pH in the system was maintained at 10.0 to 10.5 with a 0.5 N aqueous sodium hydroxide solution, and the hydrophilization treatment was carried out with stirring at 30 °C for 4 hours. The hydrophilized cellulose nanocrystals were washed with ion-exchanged water while adding ion-exchanged water using an ultracentrifuge (50,000 rpm, 10 minutes) until the pH reached 8. Then, it was placed inside a dialysis membrane (manufactured by Spectrum, fractionation molecular weight 3500 - 5000 D) and left standing in ion-exchanged water to remove impurities and the like, and cellulose nanocrystals were prepared. Ion-exchanged water was added to the washed cellulose nanocrystal dispersion described above, and the mixture was dispersed using an ultrasonic disperser to prepare a cellulose nanocrystal dispersion with a solid content of 2% by mass of cellulose nanocrystals oxidized by a TEMPO catalyst without solidifying the cellulose nanocrystals in the process.

[0048] <Comparative Example 1> The pulp was decomposed with 64% by mass sulfuric acid, washed, and dried and solidified to prepare cellulose nanocrystals that were dried and solidified. 1 g (solid content) of the cellulose nanocrystals was added to ion-exchanged water, and the mixture was dispersed using an ultrasonic disperser for 10 minutes to obtain a cellulose nanocrystal dispersion with a solid content of 2% by mass of cellulose nanocrystals.

[0049] <Comparative Example 2> Cellulose nanocrystals dried and solidified in the same manner as in Comparative Example 1 were prepared. The cellulose nanocrystals were oxidized with a TEMPO catalyst in the same manner as in Example 3 to prepare a cellulose nanocrystal dispersion with a solid content of 2% by mass of cellulose nanocrystals.

[0050]

Table 1

[0051] <Pulsed NMR> The cellulose nanocrystal dispersion liquids obtained in Example 1 and Comparative Example 1 described above were concentrated to a solid content of 5% by mass, and the free induction decay (M(t)) of the cellulose nanofiber dispersion liquid at 30 °C measured by the solid echo method of pulsed NMR was measured under the following conditions. Measuring device: TD-NMR the minispec mq20 manufactured by Bruker Sample amount: Approximately 200 mg Observed nucleus: 1H Measurement: T2 Measurement method: Solid echo method 90° pulse width: 2.2 μsec Number of integrations: 32 times Measurement temperature: 30 °C (The sample internal temperature was measured 15 minutes after the device temperature reached the set temperature. The device temperature was adjusted so that the sample internal temperature would reach the measurement temperature, and the measurement was started) Repetition time: 8 sec

[0052] Also, using the above-described formula (5), fitting was performed by analysis software (TDNMR-A), and analysis was performed approximating the above-described component (α). Also, using the above-described formulas (6) and (7), analysis was performed with two or more of the above-described components (α), component (β), and component (γ). When an error was returned on the analysis software, it was confirmed that two or more components did not exist.

[0053] In the cellulose nanocrystal dispersion liquid of Example 1, the proton ratio of component (α) was 100 (%) and the T2 relaxation time of component (α) was 1579 (μsec), and an analysis result of numerical values with one component was obtained. This indicates that only cellulose nanocrystals by the never-dry treatment are dispersed in the solvent and have a uniform mobility of one component, and it can be seen that the cellulose nanocrystals have excellent homogeneity in the dispersion liquid. In the cellulose nanocrystal dispersion of Comparative Example 1, the proton ratio of Component (α) was 86.7 (%), the T2 relaxation time of Component (α) was 1519 (μsec), the proton ratio of Component (β) was 13.6 (%), and the T2 relaxation time of Component (β) was 7723 (μsec). As a result, the analysis results of the numerical values of the two components of Component (α) and Component (β) were obtained. From this result, it can be seen that in the solvent, there are two components: cellulose nanocrystals that have undergone dry solidification and cellulose nanocrystals that are aggregated and dispersed. They have non-uniform motility in the two components, indicating the inhomogeneity of cellulose nanocrystals in the dispersion.

[0054] <Cryo-SEM> The cellulose nanocrystal dispersions obtained in Example 1 and Comparative Example 1 were concentrated to a solid content of 5% by mass, separated, cryo-fractured at -160 °C, sublimated water at -80 °C, and cross-sectional adjustment by FIB and FE-SEM observation were performed on the fracture surface while maintaining the cooling state. SEM observation photos are shown in Figures 2 and 3. Equipment: FEI, Helios NanoLab 600, Helios G4 UX Acceleration voltage: FIB 30 kV, SEM 1 - 2 kV Observation image: Backscattered electron image Processing set temperature: -160 °C Observation set temperature: -80 °C

[0055] As is clear from Figure 2, in the cellulose nanocrystal dispersion of Example 1, it was observed that the cellulose nanocrystals without the never-dry treatment were finely and homogeneously dispersed. Also, as is clear from Figure 3, in the cellulose nanocrystal dispersion of Comparative Example 1, it was observed that there were aggregated cellulose nanocrystals and the dispersion was not homogeneous.

Industrial Applicability

[0056] The cellulose nanocrystal dispersion of the present invention is excellent in coatability and handleability, can form a coating film excellent in gas barrier properties and transparency, and is used as a coating agent capable of imparting gas barrier performance. Further, it is superior in dispersibility to a dispersion in which solidified cellulose nanocrystals are dispersed, and by forming a molded body composed of a mixture with a polyvalent cation resin or a crosslinking agent, as a gas barrier film or as an interface peel strength with a hydrophobic substrate made of a thermoplastic resin or the like is also improved, and thus it is suitably used as a gas barrier laminate.

Claims

1. A cellulose nanocrystal dispersion containing sulfate groups and / or sulfonic groups derived from sulfuric acid treatment and anionic functional groups derived from hydrophilic treatment, wherein the total amount of the sulfate groups and / or sulfonic groups and the anionic functional groups is more than 0.17 mmol / g and 4.0 mmol / g or less, and the average particle size of the cellulose nanocrystals in the cellulose nanocrystal dispersion measured by a laser diffraction particle size distribution measuring device is 0.35 to 10.1 μm, the median diameter is 0.40 to 10.6 μm, and the mode diameter is 0.40 to 10.9 μm, and the visible light transmittance at 600 nm in an aqueous dispersion with a solid content of 2% by mass of the cellulose nanocrystals is 85.1% T or more. A cellulose nanocrystal dispersion characterized by the above.

2. The cellulose nanocrystal dispersion according to claim 1, wherein the crystallinity of the cellulose nanocrystals is 60% or more, the fiber width is 50 nm or less, and the aspect ratio is 5 to 50.

3. The cellulose nanocrystal dispersion according to claim 1 or 2, wherein the anionic functional group is at least one of a sulfate group, a sulfonic group, a phosphate group, and a carboxyl group.

4. The cellulose nanocrystal dispersion according to any one of claims 1 to 3, wherein the hydrophilic treatment is a combination of a never-dry treatment or a never-dry treatment and a treatment using any one of carbodiimide, sulfuric acid, sulfur trioxide-pyridine complex, phosphoric acid-urea, TEMPO catalyst, and oxidizing agent.

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