Cellulose nanofibers and aqueous dispersion composition containing same

Abstract

The present invention addresses the problem of providing cellulose nanofibers which can suppress the appearance of viscosity and which can achieve excellent physical properties in film-forming applications such as coatings and films. The present invention provides: cellulose nanofibers satisfying the following conditions: (A) the carboxyl group is in the range of 0.8-1.10 mmol / g with respect to the absolute dry weight of the cellulose nanofibers, and (B) the carboxyl group is in the range of 0.8-1.10 mmol / g with respect to the absolute dry weight of the cellulose nanofibers; and (B) an aspect ratio of 20 or more but less than 50.

Description

Technical Field

[0001] The present invention relates to cellulose nanofibers and an aqueous dispersion composition containing the cellulose nanofibers. Background Art

[0002] Natural fibers or synthetic fibers with a diameter of about 1 to 100 nm are sometimes generally referred to as nanofibers. Cellulose nanofibers, one type of nanofiber, are expected to be used in a variety of applications, such as food, cosmetics, coatings, and reinforcing materials for composite materials.

[0003] As a method for obtaining cellulose nanofibers, there is known a method of oxidizing cellulose fibers in water in the presence of an N-oxyl compound or the like, removing impurities, and applying a dispersing force (Patent Document 1).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2008-001728. Summary of the invention

[0007] Problems to be solved by the invention

[0008] On the other hand, as cellulose nanofibers are used in various applications, it is desirable to develop cellulose nanofibers with various characteristics. For example, cellulose nanofibers with a small aspect ratio calculated from an average fiber length and an average fiber diameter are also one of them. By having a small aspect ratio, cellulose nanofibers with a short fiber length relative to the fiber diameter are obtained. Therefore, as an example of the use of such cellulose nanofibers, it is considered to apply a cellulose nanofiber dispersion to a substrate to form a film on the substrate, or to mix the cellulose nanofiber dispersion in a coating containing a pigment or an adhesive, etc.

[0009] The following are problems in the case of applying a cellulose nanofiber dispersion to a substrate to form a film on the substrate. First, if the viscosity of the dispersion is too high, it is difficult to apply uniformly. On the other hand, if the cellulose nanofiber dispersion is diluted for uniform application, it is necessary to repeat the application and drying several times until the desired film thickness is reached, which is inefficient. In addition, when the cellulose nanofiber dispersion is mixed with a coating containing a pigment and a binder, if the viscosity of the dispersion is too high, it cannot be mixed uniformly in the coating.

[0010] In view of the above circumstances, the present invention aims to provide cellulose nanofibers which can suppress the development of viscosity and can obtain excellent physical properties in film-forming applications such as coatings and films.

[0011] Means for solving problems

[0012] That is, the present invention is the following (1) to (4).

[0013] (1) Cellulose nanofibers satisfying the following conditions (A) to (B):

[0014] (A) the cellulose nanofibers have carboxyl groups, and the amount of the carboxyl groups is in the range of 0.8 to 1.10 mmol / g relative to the absolute dry mass of the cellulose nanofibers;

[0015] (B) The aspect ratio is 20 or more and less than 50.

[0016] (2) The cellulose nanofibers described in (1), further satisfying the following condition (C):

[0017] (C) The average fiber diameter is 500 nm or less.

[0018] (3) An aqueous dispersion composition comprising the cellulose nanofibers according to (1) or (2).

[0019] (4) The composition described in (3), which is used for a film.

[0020] Effects of the Invention

[0021] The present invention can provide cellulose nanofibers which can suppress the development of viscosity and can exhibit excellent physical properties such as coating properties, transparency, and wettability in film-forming applications such as coating materials and films. DETAILED DESCRIPTION

[0022] [1. Cellulose nanofibers]

[0023] In this specification, cellulose nanofibers are sometimes also referred to as cellulose nanofibrils, fibrillated cellulose, or nanocellulose crystals.

[0024] The cellulose nanofibers (hereinafter sometimes referred to as CNF) of the present invention are as follows: (A) having carboxyl groups, wherein the amount of the carboxyl groups is 0.80 to 1.10 mmol / g relative to the absolute dry weight of the cellulose nanofibers; and (B) having an aspect ratio of 20 or more and less than 50.

[0025] (Carboxyl group amount (condition (A)))

[0026] Cellulose nanofibers are cellulose nanofibers containing carboxyl groups, so-called oxidized cellulose nanofibers (hereinafter, sometimes referred to as oxidized CNF). The amount of carboxyl groups relative to the absolute dry weight of oxidized cellulose is 0.80 or more, preferably 0.85 or more, and more preferably 0.90 or more. As a result, the energy required for defibration can be suppressed to a low level, and a transparent film can be formed when applied to a substrate. The upper limit is 1.10 or less. As a result, adverse effects on the physical properties of film-forming materials such as films can be suppressed. Therefore, the amount of carboxyl groups in oxidized cellulose is 0.80 to 1.10 mmol / g, more preferably 0.85 to 1.10 mmol / g, and further preferably about 0.90 to 1.10 mmol / g. It should be noted that in this specification, when indicating the degree of modification, the amount of carboxyl groups represents the amount of carboxyl groups (-COOH) and carboxylate groups (-COO - The amount of carboxyl groups in the oxidized cellulose nanofibers is an indicator of the degree of modification of the oxidized cellulose. The amount of carboxyl groups can be adjusted by the reaction conditions such as the amount of the oxidant added during the production, the reaction time, etc., which will be described in the following section.

[0027] The amount of carboxyl groups can be measured using oxidized pulp (oxidized cellulose) before defibration under the following conditions. That is, 60 ml of a 0.5 mass % slurry of oxidized pulp is prepared, a 0.1 M hydrochloric acid aqueous solution is added to adjust the pH to 2.5, and then a 0.05 N sodium hydroxide aqueous solution is added dropwise. The conductivity is measured until the pH reaches 11, and the amount of sodium hydroxide (a) consumed in the weak acid neutralization stage where the conductivity changes slowly can be calculated using the following formula.

[0028] Carboxyl group content [mmol / g pulp] = a [ml] × 0.05 / oxidized pulp mass [g].

[0029] It should be noted that, in principle, the degree of modification does not change during the fibrillation treatment for preparing cellulose nanofibers, and therefore the degree of modification of oxidized cellulose (unfibrillated) can be regarded as the degree of modification of defibrated cellulose nanofibers.

[0030] (Average fiber diameter (condition (C)))

[0031] The average fiber diameter of the cellulose nanofibers of the present invention is preferably 500 nm or less, more preferably 50 nm or less. There is no particular restriction on the lower limit, for example, it is 3 nm or more. Therefore, the average fiber diameter is preferably 3 nm or more or 500 nm or less, more preferably 3 nm or more or 50 nm or less, and further preferably 3 nm or more or 20 nm or less. The average fiber diameter and the average fiber length can be determined, for example, by modulating a 0.001 mass % aqueous dispersion of cellulose nanofibers, thinly spreading the diluted dispersion on a mica sample stand, heating and drying at 50°C to prepare a sample for observation, measuring the cross-sectional height of the shape image observed with an atomic force microscope (AFM), and calculating it as the number average fiber diameter or fiber length.

[0032] (Aspect ratio (Condition (B)))

[0033] The aspect ratio of the oxidized CNF is 20 or more, preferably 25 or more, and more preferably 30 or more. Thus, the decrease in viscosity can be suppressed, the generation of crawling (wrinkling of the paint film) can be suppressed when applied to the substrate, and a uniform film can be formed. The upper limit is less than 50, preferably 45 or less. Thus, the operability during coating becomes good, and a uniform film can be formed. In addition, degassing during coating can be easily performed. Therefore, the aspect ratio is 20 or more and less than 50, preferably 25 to 45, and more preferably in the range of 30 to 45. By having the aspect ratio in this range, the viscosity can be adjusted to an appropriate range.

[0034] The aspect ratio can be calculated using the following formula:

[0035] Aspect ratio = average fiber length / average fiber diameter

[0036] [2. Method for producing cellulose nanofibers]

[0037] The cellulose nanofibers of the present invention can be obtained by defibrillating oxidized cellulose obtained by introducing carboxyl groups into a cellulose raw material.

[0038] (Cellulose raw materials)

[0039] Examples of the cellulose raw material include plant materials (e.g., wood, bamboo, hemp, jute, kenaf, farm waste, cloth, pulp (coniferous unbleached kraft pulp (NUKP), coniferous bleached kraft pulp (NBKP), broadleaf unbleached kraft pulp (LUKP), broadleaf bleached kraft pulp (LBKP), coniferous unbleached sulfite pulp (NUSP), coniferous bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animal materials (e.g., ascidian), algae, microorganisms (e.g., acetic acid bacteria (acetobacter)), and materials derived from microbial products. Preferably, the cellulose raw material is derived from a plant or a microorganism, and more preferably, the cellulose raw material is derived from a plant.

[0040] (Introduction of carboxyl groups (oxidation))

[0041] Carboxyl groups can be introduced into the cellulose raw material by oxidizing (carboxylating) the above-mentioned cellulose raw material according to a known method.

[0042] -TEMPO oxidation-

[0043] As an example of oxidation, there is the following method: oxidizing the cellulose raw material in water using an oxidant in the presence of an N-oxyl compound and bromide, iodide or a mixture thereof. Through this oxidation reaction, the primary hydroxyl group at the C6 position of the pyranose ring on the surface of the cellulose is selectively oxidized. As a result, a cellulose having an aldehyde group and a carboxyl group (-COOH) or a carboxylate group (-COO - The concentration of cellulose during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0044] N-oxyl compounds refer to compounds that can generate nitroxide free radicals. As N-oxyl compounds, any compound can be used as long as it promotes the target oxidation reaction. For example, 2,2,6,6-tetramethylpiperidin-1-oxyl free radical (TEMPO) and its derivatives (for example, 4-hydroxy TEMPO) can be listed.

[0045] The amount of the N-oxyl compound used is not particularly limited as long as it is a catalytic amount that can oxidize the cellulose as a raw material. It is preferably 0.01 mmol to 10 mmol, more preferably 0.01 mmol to 1 mmol, further preferably 0.02 mmol to 0.5 mmol, 0.05 mmol to 0.5 mmol, relative to the absolute dry weight of 1 g of cellulose. In addition, its concentration is preferably about 0.1 mmol / L to 4 mmol / L relative to the reaction system.

[0046] Bromide refers to a compound containing bromine, including alkali metal bromide that can be dissociated in water to be ionized. In addition, iodide refers to a compound containing iodine, including alkali metal iodide.

[0047] The amount of bromide or iodide used can be selected within the range that can promote the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 mmol to 100 mmol, more preferably 0.1 mmol to 10 mmol, and further preferably 0.5 mmol to 5 mmol per absolute dry weight of 1 g of cellulose.

[0048] As the oxidizing agent, a known oxidizing agent can be used, for example, halogen, hypohalous acid, halogenous acid, perhalogen acid or salts thereof, halogen oxides, peroxides, etc. Among them, sodium hypochlorite is preferred because it is inexpensive and has little environmental impact.

[0049] The amount of the oxidant used is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, and further preferably 1 to 25 mmol, relative to 1 gram of the absolute dry weight of cellulose, and preferably 1 to 40 mol, for example, relative to 1 mol of the N-oxyl compound.

[0050] The oxidation step of cellulose will allow the reaction to proceed efficiently even under relatively mild conditions. For this reason, the reaction temperature is preferably 4°C to 40°C, and can be room temperature of about 15°C to 30°C. As the reaction proceeds, carboxyl groups are generated in cellulose, so the pH of the reaction solution decreases. In order to make the oxidation reaction proceed efficiently, it is preferred to add an alkaline solution such as an aqueous sodium hydroxide solution during the reaction to maintain the pH of the reaction solution at about 8 to 12, preferably about 9 to 12. More preferably, it is 10 to 12. For reasons such as ease of operation and the difficulty in causing side reactions, the reaction medium is preferably water.

[0051] The reaction time of the oxidation reaction can be appropriately set depending on the progress of the oxidation, and is usually 0.5 to 6 hours.

[0052] In addition, the oxidation reaction can be carried out in two stages. For example, after the first stage reaction is completed, the cellulose is filtered and the obtained carboxylated cellulose is oxidized again under the same or different reaction conditions, so that the oxidation can be carried out efficiently without being hindered by the by-product salt in the first stage reaction.

[0053] -Ozone oxidation-

[0054] Another example of oxidation is a method of bringing a cellulose raw material into contact with an ozone-containing gas. In this oxidation reaction, at least the hydroxyl groups at the 2nd and 6th positions of the pyranose ring are oxidized, and the cellulose chain is decomposed at the same time.

[0055] The ozone concentration in the ozone-containing gas is preferably 50 g / m3 ~250g / m 3 , more preferably 50g / m 3 ~220g / m 3 The amount of ozone added to the cellulose raw material is preferably 0.1 to 30 parts by mass, more preferably 5 to 30 parts by mass, based on 100 parts by mass of the solid content of the cellulose raw material.

[0056] The ozone treatment temperature is preferably 0° C. to 50° C., more preferably 20° C. to 50° C. The ozone treatment time is not particularly limited, but is about 1 minute to 360 minutes, preferably about 30 minutes to 360 minutes. When the ozone treatment conditions are within the above range, excessive oxidation and decomposition of cellulose can be prevented, and the yield of oxidized cellulose becomes good.

[0057] After the ozone treatment, an additional oxidation treatment may be performed using an oxidant. The oxidant used for the additional oxidation treatment is not particularly limited, and examples thereof include chlorine compounds such as chlorine dioxide and sodium chlorite, or oxygen, hydrogen peroxide, persulfate, and peracetic acid. For example, the additional oxidation treatment may be performed by dissolving these oxidants in a polar organic solvent such as water or alcohol to prepare an oxidant solution, and immersing the cellulose raw material in the solution.

[0058] (Short fiber processing)

[0059] After oxidation and before defibration, it is preferred to perform a short-fiber treatment. By performing the short-fiber treatment, the size of the cellulose nanofibers such as the aspect ratio and the average fiber diameter can be adjusted, and the increase in the viscosity of the dispersed composition containing the cellulose nanofibers can be suppressed.

[0060] As a short-fiberization treatment, for example, there can be mentioned: a treatment of hydrolyzing oxidized cellulose under alkaline or acidic conditions (preferably alkaline conditions). The reaction pH is preferably 8 or more, more preferably 9 or more, and further preferably 10 or more. In this way, short-fiberization based on hydrolysis can be carried out moderately. The upper limit is preferably 14 or less, more preferably 13 or less, and further preferably 12 or less. In this way, coloration after hydrolysis can be suppressed, and transparent oxidized cellulose can be obtained. Therefore, the pH is preferably 8 to 14, more preferably 9 to 13, and further preferably 10 to 12. The alkali used to adjust the pH value only needs to be water-soluble, and from the viewpoint of manufacturing cost, sodium hydroxide is preferred.

[0061] During the short-fiber treatment, it is preferred to use auxiliary agents such as oxidants and reducing agents. In this way, the formation of double bonds during β separation during hydrolysis (including hydrolysis under alkaline conditions) and the coloring of oxidized cellulose caused thereby can be suppressed. As the oxidant, for example, one or a combination of two or more selected from oxygen, ozone, hydrogen peroxide, and hypochlorite can be cited. From the viewpoint of not easily generating free radicals, oxygen, hydrogen peroxide, and hypochlorite are preferred, and hydrogen peroxide is more preferred. As the reducing agent, for example, one or a combination of two or more selected from sodium borohydride, bisulfite, and sulfite can be cited. From the viewpoint of reaction efficiency, the amount of the auxiliary agent used is preferably 0.1 to 10% by mass, more preferably 0.3 to 5% by mass, and further preferably 0.5 to 3% by mass, relative to the absolute dry weight of cellulose.

[0062] The reaction temperature of the short-fiberization treatment is preferably 40°C or more, more preferably 50°C or more, and further preferably 60°C or more. Thus, hydrolysis can be carried out moderately and short-fiberization can be fully carried out. The upper limit is preferably 120°C or less, more preferably 100°C or less, and further preferably 90°C or less. Thus, the coloring of the oxidized cellulose after hydrolysis can be suppressed. Therefore, from the viewpoint of reaction efficiency, 40 to 120°C, more preferably 50 to 100°C, and further preferably 60 to 90°C are preferred. The reaction time of the hydrolysis is preferably 0.5 hours to 24 hours, more preferably 1 hour to 10 hours, and further preferably 2 hours to 6 hours. The short-fiberization treatment can be carried out, for example, by adding the oxidized cellulose to an alkaline solution (preferably an alkaline aqueous solution). The concentration of the oxidized cellulose in the alkaline solution is preferably 1 to 20% by mass, more preferably 3 to 15% by mass, and further preferably 4% to 10% by mass.

[0063] (Fiber removal)

[0064] There is no particular limitation on the device used for defibration, and examples thereof include: high-speed rotary, colloid mill, high pressure, roller mill, ultrasonic and other types of devices, preferably high pressure or ultra-high pressure homogenizers, more preferably wet high pressure or ultra-high pressure homogenizers. The device is preferably capable of applying a strong shear force to the cellulose raw material or oxidized cellulose (usually a dispersion). The pressure that the device can apply is preferably 50 MPa or more, more preferably 100 MPa or more, and further preferably 140 MPa or more. The device is preferably a wet high pressure or ultra-high pressure homogenizer that can apply the above-mentioned pressure to the cellulose raw material or oxidized cellulose (usually a dispersion) and can apply a strong shear force. Thus, defibration can be effectively performed. The number of treatments (passes) in the defibration device can be 1 time, or it can be 2 times or more, preferably 2 times or more.

[0065] [3. Dispersion composition]

[0066] The cellulose nanofibers described above can be used in the form of a dispersion composition. The dispersion composition is a composition containing at least cellulose nanofibers and a solvent, wherein the cellulose nanofibers are dispersed in a dispersion medium.

[0067] The dispersion composition can be produced by a dispersion treatment. In the dispersion treatment, a component containing at least oxidized cellulose is usually dispersed in a solvent. The solvent is not particularly limited as long as it is a solvent in which the oxidized cellulose can be dispersed, and examples thereof include water, an organic solvent (for example, a hydrophilic organic solvent such as methanol), and a mixed solvent thereof. Since the cellulose raw material is hydrophilic, the solvent is preferably water. In this specification, a dispersion composition containing water as a solvent is referred to as an aqueous dispersion composition.

[0068] The solid content concentration of the oxidized cellulose in the dispersion is usually 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.3% by mass or more. Thus, the amount of liquid is appropriate relative to the amount of the cellulose fiber raw material, and the efficiency is high. The upper limit is usually 10% by mass or less, preferably 6% by mass or less. Thus, the fluidity can be maintained.

[0069] The order of the dispersion treatment during the fiberization treatment and the dispersion composition modulation in the manufacture of cellulose nanofibers is not particularly limited, and any one of them can be performed first, or it can be performed simultaneously, preferably after the dispersion treatment, the fiberization treatment is performed. The combination of each treatment can be repeated more than 2 times as long as it is performed at least once. In addition, the solid content concentration of the dispersion composition can be adjusted after the dispersion treatment and the fiberization treatment.

[0070] Before the defibration treatment or the dispersion treatment, a pretreatment may be performed as needed. The pretreatment may be performed using a mixing, stirring, emulsifying, or dispersing device such as a high-speed shear mixer.

[0071] The dispersion composition may contain other components as necessary. Examples of other components include any known components such as preservatives.

[0072] [3. Uses of cellulose nanofibers and aqueous dispersion compositions]

[0073] The above-mentioned cellulose nanofibers and aqueous dispersion compositions can be used in various industrial fields such as food and cosmetics. As applications, for example, they can be used as reinforcing materials for resins, rubbers, etc., and for film-making applications, and are also suitable for film-making applications such as films. The material of the substrate is not particularly limited, and can be used as various resins (for example, polyethylene terephthalate, polypropylene, polyethylene), and metals (for example, aluminum).

[0074] Example

[0075] Hereinafter, manufacturing examples, embodiments and comparative examples are listed to more specifically illustrate the present invention. It should be noted that these manufacturing examples, embodiments and comparative examples are only examples and are not intended to limit the scope of the present invention. In manufacturing examples, embodiments and comparative examples, unless otherwise specified, "parts" and "%" are based on mass standards.

[0076] (Example 1)

[0077] 5 g (absolute dry weight) of bleached kraft pulp (DKP manufactured by Buckeye) from coniferous trees was added to 500 ml of an aqueous solution containing 19.5 mg (0.025 mmol relative to the absolute dry weight of 1 g of cellulose) of TEMPO (SigmaAldrich) and 514 mg (1.0 mmol relative to the absolute dry weight of 1 g of cellulose) of sodium bromide, and stirred until the pulp was evenly dispersed. After adding 8 ml of a 2M aqueous sodium hypochlorite solution to the reaction system, the pH was adjusted to 10.3 with a 0.5 N aqueous hydrochloric acid solution to start the oxidation reaction. During the reaction, the pH in the system decreased, and 3.0 N aqueous sodium hydroxide solution was gradually added to adjust the pH to 10. After reacting for 2 hours, it was filtered with a glass filter and washed thoroughly with water to obtain carboxylated cellulose. The amount of carboxyl groups in the obtained carboxylated cellulose was 0.94 mmol / g. Then, 2% (w / v) hydrogen peroxide was added to the 5% (w / v) slurry of oxidized cellulose, and the pH was adjusted to 12 with 1M sodium hydroxide. The slurry was hydrolyzed at 80°C for 2 hours. The slurry was adjusted to 4.0% (w / v) with water and treated 5 times with an ultra-high pressure homogenizer (20°C, 140 MPa) to obtain an aqueous dispersion containing cellulose nanofibers A. The carboxyl group content of cellulose nanofibers A was 0.94 mmol / g, the average fiber diameter was 6.9 nm, the average fiber length was 235 nm, and the aspect ratio was 34.

[0078] (Example 2)

[0079] The same operation as in Example 1 was performed except that the 2M sodium hypochlorite aqueous solution added to the reaction system was changed to 7.5 ml and the hydrolyzed oxidized cellulose fibers were treated 10 times with an ultrahigh pressure homogenizer (20° C., 140 MPa), thereby obtaining an aqueous dispersion containing cellulose nanofibers B. The cellulose nanofibers B had a carboxyl group content of 0.87 mmol / g, an average fiber diameter of 7.3 nm, an average fiber length of 208 nm, and an aspect ratio of 28.

[0080] (Example 3)

[0081] The same operation as in Example 1 was carried out except that the 2M sodium hypochlorite aqueous solution added to the reaction system was changed to 8.5 ml, thereby obtaining an aqueous dispersion containing cellulose nanofibers C. The cellulose nanofibers C had a carboxyl group content of 1.06 mmol / g, an average fiber diameter of 7.2 nm, an average fiber length of 352 nm, and an aspect ratio of 49.

[0082] (Comparative Example 1)

[0083] The same operation as in Example 1 was carried out except that the 2M sodium hypochlorite aqueous solution added to the reaction system was changed to 14 ml, thereby obtaining an aqueous dispersion containing cellulose nanofibers D. The cellulose nanofibers D had a carboxyl group content of 1.87 mmol / g, an average fiber diameter of 5.7 nm, an average fiber length of 230 nm, and an aspect ratio of 40.

[0084] (Comparative Example 2)

[0085] 5 g (absolute dry weight) of bleached coniferous kraft pulp (whiteness 85%) was added to 500 mL of an aqueous solution containing 19.5 mg (0.025 mmol relative to the absolute dry weight of 1 g of cellulose) of TEMPO (SigmaAldrich) and 514 mg (1.0 mmol relative to the absolute dry weight of 1 g of cellulose) of sodium bromide, and the mixture was stirred until the pulp was evenly dispersed. After adding 11 ml of a 2M aqueous sodium hypochlorite solution to the reaction system, the pH was adjusted to 10.3 with a 0.5N aqueous hydrochloric acid solution to start the oxidation reaction. A 0.5N aqueous sodium hydroxide solution was gradually added to adjust the pH to 10. After reacting for 2 hours, the mixture was filtered with a glass filter and washed thoroughly with water to obtain oxidized cellulose. The amount of carboxyl groups in the obtained oxidized cellulose was 1.6 mmol / g. The mixture was adjusted to 1.0% (w / v) with water, and treated three times with an ultra-high pressure homogenizer (20°C, 140 MPa) to obtain an aqueous dispersion containing cellulose nanofibers E. The cellulose nanofibers E had a carboxyl group content of 1.6 mmol / g, an average fiber diameter of 3.0 nm, an average fiber length of 753 nm, and an aspect ratio of 251.

[0086] (Comparative Example 3)

[0087] The same operation as in Example 1 was carried out except that the amount of hydrogen peroxide added to the 5% (W / V) slurry of oxidized cellulose was changed to 10% (W / V) relative to the oxidized cellulose, thereby obtaining an aqueous dispersion containing cellulose nanofibers F. The cellulose nanofibers F had a carboxyl group content of 0.92 mmol / g, an average fiber diameter of 6.5 nm, an average fiber length of 126 nm, and an aspect ratio of 19.

[0088] (Comparative Example 4)

[0089] The same operation as in Example 1 was carried out, except that the amount of hydrogen peroxide added to the 5% (W / V) slurry of oxidized cellulose was changed to 0.5% (W / V) relative to the oxidized cellulose, and the hydrolyzed oxidized cellulose fibers were treated four times with an ultrahigh pressure homogenizer (20° C., 140 MPa), to obtain an aqueous dispersion containing cellulose nanofibers G. The cellulose nanofibers G had a carboxyl group content of 0.95 mmol / g, an average fiber diameter of 7.2 nm, an average fiber length of 370 nm, and an aspect ratio of 51.

[0090] (Comparative Example 5)

[0091] The same operation as in Example 1 was carried out except that the 2M sodium hypochlorite aqueous solution added to the reaction system was changed to 6.5 ml, thereby obtaining an aqueous dispersion containing cellulose nanofibers H. The cellulose nanofibers H had a carboxyl group content of 0.71 mmol / g, an average fiber diameter of 8.2 nm, an average fiber length of 268 nm, and an aspect ratio of 33.

[0092] (Comparative Example 6)

[0093] The same operation as in Example 1 was carried out except that the 2M sodium hypochlorite aqueous solution added to the reaction system was changed to 10 ml, thereby obtaining an aqueous dispersion containing cellulose nanofibers I. The cellulose nanofibers I had a carboxyl group content of 1.18 mmol / g, an average fiber diameter of 5.3 nm, an average fiber length of 241 nm, and an aspect ratio of 45.

[0094] (evaluate)

[0095] The concentration of the aqueous dispersion containing cellulose nanofibers A to G obtained in each example was adjusted to 3%. On the other hand, a PET film as a substrate was placed on a horizontal platform, and the above 3% aqueous dispersion was applied to the surface thereof using a coating bar to a concentration of 780 g / m 2 As the coating rod, a glass rod with tape wrapped around both ends and adjusted to a height of 0.7 mm was used. The coating film was formed by drying at 35° C. for 1 day, and the following evaluation was performed.

[0096] -Coating properties-

[0097] The operability and the surface state of the coating film when applied with a coating rod are evaluated visually. The uniform state of the coating film surface after coating is evaluated as good coating property, the uneven state is confirmed as poor coating property, and the situation where no coating film is formed and depressions are generated is evaluated as poor coating property. It should be noted that when the aspect ratio is large, the viscosity is high, the operability is likely to deteriorate, bubbles enter the coating film, and it is possible to form an uneven coating film surface. In addition, when the aspect ratio is small, sometimes no coating film is formed or depressions are generated.

[0098] -Wettability-

[0099] 5 μl of water was dripped onto the coating surface, and the contact angle of water was measured 1 second after dripping at a temperature of 23°C and a humidity (relative humidity) of 50% using a contact angle meter (trade name: DAT1122, manufacturer: Fibro Corporation). The contact angle of 28° or less was evaluated as good wettability, less than 30° was evaluated as slightly poor, and more than 30° was evaluated as poor.

[0100] - Transparency -

[0101] The transmittance of light at a wavelength of 660 nm was measured using a UV-VIS spectrophotometer UV-1800 (manufactured by Shimadzu Corporation) using a square cuvette with an optical path length (optical path length) of 10 mm (blank: PET film). Transparency was judged to be good when the transmittance was 96% or more, slightly poor when it was 90% or more, and poor when it was less than 90%.

[0102] [Table 1]

[0103] Table 1

[0104]

[0105] Since the aspect ratio and the amount of carboxyl groups of the cellulose nanofibers of the present invention are adjusted to an appropriate range, they are suitable for various uses such as food or cosmetics, film-making materials or reinforcement materials for composite materials. In particular, they can show good physical properties in film-forming applications such as films and coatings, and are more suitable for such applications.

Claims

1. Cellulose nanofibers satisfying the following conditions (A) to (B): (A) the cellulose nanofibers have carboxyl groups, and the amount of the carboxyl groups is in the range of 0.8 to 1.10 mmol / g relative to the absolute dry weight of the cellulose nanofibers; (B) The aspect ratio is 20 or more and less than 50.

2. The cellulose nanofiber according to claim 1, further satisfying the following condition (C): (C) The average fiber diameter is 500 nm or less. 3 . An aqueous dispersion composition comprising the cellulose nanofibers according to claim 1 . The composition according to claim 3, which is used for a film.

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