JP5178931B2 - Method for producing cellulose nanofiber - Google Patents

Description

  The present invention relates to a method for producing cellulose nanofibers with low energy.

  When the cellulosic material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as TEMPO) and sodium hypochlorite which is an inexpensive oxidant, Carboxyl groups can be efficiently introduced onto the surface of cellulose microfibrils. It is known that when this cellulose-based raw material into which carboxyl groups have been introduced is treated with a mixer or the like in water, a highly viscous and transparent cellulose nanofiber aqueous dispersion can be obtained (Non-Patent Document 1, Patent Documents 1 and 2). ).

  Cellulose nanofiber is a new biodegradable water-dispersed material. Moreover, since the carboxyl group is introduce | transduced by the oxidation reaction on the surface of the cellulose nanofiber, the cellulose nanofiber can be freely modified with the carboxyl group as a base point. Furthermore, since the cellulose nanofibers obtained by the above method are in the form of a dispersion, they can be further modified by blending with various water-soluble polymers or by combining with organic and / or inorganic pigments. Furthermore, cellulose nanofibers can be made into sheets or fibers. Taking advantage of these characteristics of cellulose nanofibers, the development of high-performance products that are applied to high-performance packaging materials, transparent organic substrate members, high-performance fibers, separation membranes, regenerative medical materials, etc. is expected as environmentally-circulating materials. .

JP 2008-001728 A JP 2010-235679 A

Saito, T .; , Et al. Cellulose Commun. , 14 (2), 62 (2007)

  In a conventional method for obtaining cellulose nanofibers, a dispersion of a cellulose-based raw material oxidized using TEMPO is treated using a mixer, and the oxidized cellulose-based raw material is defibrated. However, there is a problem in that the viscosity of the dispersion becomes extremely high during the treatment and it is difficult to perform an efficient defibration treatment. In particular, when the viscosity of the dispersion is too high, the dispersion proceeds only around the stirring blades of the mixer, which causes a problem of non-uniform dispersion. For example, even when the concentration of the oxidized cellulose raw material in the dispersion is as low as 0.3 to 0.5% (w / v), the B-type viscosity (60 rpm, 20 ° C.) is 800 to 4000 mPa · s. It was sometimes high. For this reason, the present inventors tried defibrating using a homogenizer having higher defibrating and dispersing power than a mixer. However, it was still observed that the cellulosic raw material was remarkably thickened at the initial stage of dispersion to deteriorate the fluidity, and the power consumption required for the dispersion treatment was increased. Furthermore, since the cellulose nanofiber dispersion liquid adheres to the inside of the apparatus, sufficient dispersion cannot be performed, and operations such as taking out the dispersion liquid from the apparatus become difficult, and the yield of the dispersion liquid may be reduced.

  Usually, cellulose nanofiber is applied to various uses in the state of dispersion. In particular, when the dispersion is used as a coating liquid, the concentration of cellulose nanofibers in the cellulose nanofiber dispersion is preferably high from the viewpoint of increasing the amount of cellulose nanofibers in the coating film. For example, when a cellulose nanofiber dispersion is applied to a base material to form a film on the base material, if the cellulose nanofiber concentration is low, the number of times of coating must be increased, resulting in a reduction in work efficiency. For this reason, cellulose nanofibers that give a high-concentration dispersion are desired, but as described above, thickening is observed when trying to obtain a high-concentration dispersion. A lot of energy is required.

  In view of the above, an object of the present invention is to provide a method for producing a cellulose nanofiber dispersion excellent in fluidity with low energy.

As a result of intensive studies, the present inventors have found that the above-mentioned problems can be solved by hydrolyzing in an alkaline solution having a pH of 8 to 14 before the oxidized cellulose-based raw material is defibrated, thereby completing the present invention. It was. That is, the said subject is solved by the following this invention.
(A) Cellulose-based raw material is oxidized with (a3) an oxidizing agent in the presence of (a1) an N-oxyl compound and (a2) a compound selected from the group consisting of bromide, iodide or a mixture thereof. The process of
(B) A step of hydrolyzing the oxidized cellulose-based material obtained in the step A in an alkaline solution having a pH of 8 to 14, and (C) a dispersion containing the hydrolyzed oxidized cellulose-based material obtained in the step B And a step of fibrillating the hydrolyzed oxidized cellulose-based raw material in a dispersion medium to form nanofibers.

  According to the present invention, a cellulose nanofiber dispersion excellent in fluidity can be produced with low energy.

Hereinafter, the present invention will be described in detail. In the present specification, “to” used for expressing a numerical range includes values at both ends.
1. Production method of cellulose nanofiber The production method of the present invention comprises (A) a cellulose-based raw material comprising (a1) an N-oxyl compound and (a2) a compound selected from the group consisting of bromide, iodide or a mixture thereof. (A3) a step of oxidizing using an oxidizing agent in the presence, (B) a step of hydrolyzing the oxidized cellulose-based raw material obtained in step A in an alkaline solution having a pH of 8 to 14, and (C) the step Preparing a dispersion containing the hydrolyzed oxidized cellulose-based raw material obtained in B, and fibrillating the hydrolyzed oxidized cellulose-based raw material in a dispersion medium to form nanofibers.

1-1. Process A
In step A, the cellulosic raw material is used in the presence of a compound selected from the group consisting of (a1) an N-oxyl compound and (a2) bromide, iodide or a mixture thereof, using (a3) an oxidizing agent. Oxidize.

(1) N-oxyl compound (a1)
The N-oxyl compound refers to a compound that can generate a nitroxy radical. As the N-oxyl compound used in the present invention, any compound can be used as long as it promotes the target oxidation reaction. For example, the N-oxyl compound used in the present invention includes a compound represented by the following general formula (Formula 1).

In Formula 1, R < ₁ > -R < ₄ > is the same or different and shows a C1-C4 alkyl group.
Of the substances represented by Formula 1, 2,2,6,6-tetramethyl-1-piperidine-oxy radical (hereinafter referred to as TEMPO) is preferable. Further, the N-oxyl compound represented by any one of the following formulas 2 to 4, that is, the hydroxyl group of 4-hydroxy TEMPO was etherified with alcohol or esterified with carboxylic acid or sulfonic acid to impart moderate hydrophobicity. 4-Acetamide TEMPO, which is obtained by acetylating the amino group of 4-hydroxy TEMPO derivative or 4-amino TEMPO represented by the following formula 5 and imparting appropriate hydrophobicity, is inexpensive and provides uniform oxidized cellulose. Is particularly preferable.

In formulas 2 to 4, R is a linear or branched carbon chain having 4 or less carbon atoms.
Furthermore, an N-oxyl compound represented by the following formula 6, that is, an azaadamantane type nitroxy radical, is preferable because it can efficiently oxidize a cellulose-based raw material in a short time and can produce a cellulose nanofiber having a high degree of polymerization.

In Formula 6, R ₅ and R ₆ are the same or different and each represents hydrogen or a C _{1 to} C ₆ linear or branched alkyl group.
The amount of the N-oxyl compound used is not particularly limited as long as it is a catalyst amount that can promote the oxidation reaction to such an extent that the cellulose-based raw material can be converted into nanofibers. For example, 0.01 to 10 mmol is preferable, 0.01 to 1 mmol is more preferable, and 0.05 to 0.5 mmol is more preferable with respect to 1 g of the absolutely dry cellulosic material.

(2) Bromide or iodide (a2)
Bromide refers to a compound containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Further, iodide refers to a compound containing iodine, and examples thereof include alkali metal iodide. The amount of bromide or iodide used can be selected as long as the oxidation reaction can be promoted. The total amount of bromide and iodide is, for example, preferably from 0.1 to 100 mmol, more preferably from 0.1 to 10 mmol, and even more preferably from 0.5 to 5 mmol, based on 1 g of cellulosic raw material.

(3) Oxidizing agent (a3)
As the oxidizing agent used in oxidizing the cellulosic raw material, known ones can be used, for example, halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide, etc. Can be used. Among these, from the viewpoint of cost, sodium hypochlorite, which is the most widely used in industrial processes and has a low environmental load, is particularly suitable. The amount of the oxidizing agent used can be selected within a range that can promote the oxidation reaction. For example, 0.5 to 500 mmol, preferably 0.5 to 50 mmol, and more preferably about 2.5 to 25 mmol can be used for 1 g of cellulosic raw material.

(4) Cellulose raw material The “cellulosic raw material” used in the oxidation step (step A) of the present invention is not particularly limited, and is a pulverized pulp or sulfite pulp derived from various woods, and a powder obtained by pulverizing them with a high-pressure homogenizer or a mill. Cellulose or microcrystalline cellulose powder purified by chemical treatment such as acid hydrolysis can be used. In addition, plant-derived cellulosic materials such as kenaf, hemp, rice, bacus, and bamboo can also be used. From the viewpoint of mass production and cost, it is preferable to use powdered cellulose, microcrystalline cellulose powder, or bleached kraft pulp or bleached sulfite pulp having a whiteness (ISO 2470) of 80% or more. In particular, it is preferable to use powdered cellulose or microcrystalline cellulose powder because cellulose nanofibers can be produced which give a dispersion having a lower viscosity even at a high concentration. In addition, a cellulose-based raw material derived from hardwood is also preferable because it can produce cellulose nanofibers that give a low-viscosity dispersion with low power consumption.

  Powdered cellulose is a rod-like particle made of microcrystalline cellulose obtained by removing a non-crystalline part of wood pulp by acid hydrolysis and then grinding and sieving. The degree of polymerization of cellulose in the powdered cellulose is about 100 to 500, the degree of crystallinity of the powdered cellulose by X-ray diffraction method is 70 to 90%, and the volume average particle diameter by a laser diffraction type particle size distribution analyzer is preferably 100 μm. Or less, more preferably 50 μm or less. When the volume average particle diameter is 100 μm or less, a cellulose nanofiber dispersion excellent in fluidity can be obtained. As the powdered cellulose used in the present invention, for example, an undegraded residue obtained after acid hydrolysis of a selected pulp is dried, crushed and sieved, and a rod shaft having a certain particle size distribution is obtained. The crystalline cellulose powder can be used. Alternatively, commercially available products such as KC Flock (registered trademark) (manufactured by Nippon Paper Chemical Co., Ltd.), Theolas (trademark) (manufactured by Asahi Kasei Chemicals Corporation), and Avicel (registered trademark) (manufactured by FMC) may be used.

(5) Conditions for oxidation reaction of cellulosic raw material In the oxidation step (step A), it is preferable to use water as a reaction medium in order to suppress side reactions. In Step A, the reaction can be efficiently advanced even under relatively mild conditions. Therefore, the reaction temperature may be a room temperature of about 15 to 30 ° C. In addition, since a carboxyl group produces | generates in a cellulose with progress of reaction, the fall of pH of a reaction liquid is recognized. In order to advance the oxidation reaction efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at about 9 to 12, preferably about 10 to 11. The reaction time in the oxidation reaction can be appropriately set, and is preferably about 0.5 to 4 hours, for example.

From the viewpoint of avoiding the side reaction in the next step B, the oxidized cellulose-based material obtained in step A is preferably washed before being subjected to step B.
1-2. Process B
In step B, the oxidized cellulose-based material obtained in step A (hereinafter also simply referred to as “oxidized cellulose-based material”) is hydrolyzed in an alkaline solution having a pH of 8-14.
In step B, it is preferable to use water as a reaction medium in order to suppress side reactions.
In step B, it is preferable to use an oxidizing agent or a reducing agent as an auxiliary agent. As the oxidizing agent or reducing agent, those having activity in an alkaline region of pH 8-14 can be used. Examples of the oxidizing agent include oxygen, ozone, hydrogen peroxide, and hypochlorite, and two or more of these may be used in combination. However, when an oxidizing agent that generates radicals such as ozone is used, there is a problem that the oxidized cellulose-based raw material after hydrolysis is colored by the generated radicals. Therefore, as the oxidizing agent used in the present invention, oxygen, hydrogen peroxide, hypochlorite and the like that do not easily generate radicals are preferable, and hydrogen peroxide is particularly preferable from the viewpoint of preventing coloring. These are more preferably not used in combination with an oxidizing agent that generates radicals such as ozone, and hydrogen peroxide is more preferably used alone. Examples of the reducing agent used in the present invention include sodium borohydride, hydrosulfite, and sulfite, and two or more of these may be used in combination. From the viewpoint of reaction efficiency, the additive is preferably added in an amount of 0.1 to 10% by weight, more preferably 0.3 to 5% by weight, and 0.5 to 2% by weight with respect to the absolutely dry oxidized cellulose raw material. Is more preferable.

  8-14 are preferable, as for pH of the reaction liquid in a hydrolysis reaction, 9-13 are more preferable, and 10-12 are more preferable. If the pH is less than 8, sufficient hydrolysis does not occur and the energy required for Step C may not be reduced. Moreover, when pH exceeds 14, although a hydrolysis will progress, the problem that the oxidized cellulose raw material after a hydrolysis may color arises. The alkali used for adjusting the pH may be water-soluble, but sodium hydroxide is optimal from the viewpoint of production cost. Moreover, from a viewpoint of reaction efficiency, 40-120 degreeC is preferable, 50-100 degreeC is more preferable, and 60-90 degreeC is more preferable. If the temperature is low, sufficient hydrolysis does not occur, and the energy required for Step C may not be reduced. On the other hand, when the temperature is high, the hydrolysis proceeds, but there may be a problem that the oxidized cellulose-based raw material after the hydrolysis is colored. The reaction time for hydrolysis is preferably 0.5 to 24 hours, more preferably 1 to 10 hours, and further preferably 2 to 6 hours. From the viewpoint of reaction efficiency, the concentration of the oxidized cellulose-based raw material in the reaction solution is preferably 1 to 20% by mass, more preferably 3 to 15% by mass, and further preferably 5 to 10% by mass.

  By hydrolyzing in an alkaline solution having a pH of 8 to 14, the energy required for defibration in Step C can be reduced. The reason is presumed as follows. In the amorphous region of the cellulosic raw material oxidized with the N-oxyl compound, carboxyl groups are scattered, and the hydrogen at the C6 position where the carboxyl groups are present is attracted by the carboxyl groups. As a result, the electric charge is deficient. Therefore, the hydrogen is easily extracted with hydroxide ions under alkaline conditions of pH 8-14. Then, the cleavage reaction of the glucoside bond by β elimination proceeds, and the oxidized cellulose raw material is shortened. Thus, by shortening the fiber length of the oxidized cellulosic raw material, the viscosity of the dispersion containing the raw material can be lowered. As a result, the energy required for defibration in step C is reduced. However, if the hydrolysis is simply carried out under alkaline conditions, the cellulosic material tends to be colored yellow. This is thought to be because a double bond is generated during β elimination. Therefore, in the hydrolysis under alkaline conditions of pH 8 to 14, when an oxidizing agent or a reducing agent is used, this double bond can be removed by oxidation or reduction, so that coloring can be suppressed. In particular, when hydrogen peroxide or the like that hardly generates radicals is used as an oxidizing agent, coloring is unlikely to occur.

1-3. Process C
In Step C, a dispersion in which the hydrolyzed oxidized cellulose raw material obtained in Step B (hereinafter also referred to as “the cellulose raw material obtained in Step B”) is dispersed is prepared, and the hydrolyzed oxidation The cellulose raw material is fibrillated and dispersed into nanofibers while being dispersed in a dispersion medium. “To make nanofiber” means processing a cellulose-based raw material into cellulose nanofiber which is a single microfibril of cellulose having a width of 2 to 5 nm and a length of about 1000 to 5000 nm, preferably about 1 to 5 μm. Means. In view of ease of handling, the dispersion medium is preferably water. In order to defibrate while dispersing the cellulosic raw material obtained in step B in a dispersion medium, the dispersion liquid is strongly resisted using a high-speed rotating type, colloid mill type, high pressure type, roll mill type, ultrasonic type device, etc. It is preferable to apply an appropriate shear force. In particular, it is preferable to use a wet high-pressure or ultrahigh-pressure homogenizer that can apply a pressure of 50 MPa or more to the dispersion and can apply a strong shearing force. The pressure is more preferably 100 MPa or more, and further preferably 140 MPa or more. By this treatment, the cellulose-based raw material obtained in Step B is defibrated to form cellulose nanofibers, and the cellulose nanofibers are dispersed in the dispersion medium.

  The concentration of the cellulosic raw material obtained in Step B in the dispersion used for defibration is preferably 1 to 50% (w / v), more preferably 2 to 10% (w / v). In the present invention, since the hydrolysis treatment is performed in the step B, the viscosity of the system does not increase during the defibrating treatment even if the concentration of the cellulosic raw material obtained in the step B is increased in this way.

1-4. Low viscosity treatment In order to further reduce the energy required for defibration, it was obtained in the oxidized cellulose-based raw material obtained in step A or in step B between step A and step B or between step B and step C. The hydrolyzed oxidized cellulose raw material (hereinafter collectively referred to as “the cellulose raw material obtained in Step A or B”) may be subjected to a viscosity reduction treatment by a method different from the method of Step B. The viscosity reduction treatment is to further appropriately cut the cellulose chain of the cellulose-based raw material obtained in step A or B (shorten the cellulose chain). Since the raw material thus treated has a low viscosity when used as a dispersion, the viscosity reduction treatment can be said to be a treatment for obtaining a cellulosic raw material that gives a low-viscosity dispersion. The viscosity-reducing treatment may be any treatment that reduces the viscosity of the cellulosic material obtained in step A or B. For example, the treatment of irradiating the cellulosic material obtained in step A or B with ultraviolet rays, the same material Treatment with hydrogen peroxide and ozone, treatment with hydrolyzing the same raw material with acid, and combinations thereof.

(1) Ultraviolet irradiation When the viscosity reduction treatment is performed by irradiating the cellulose-based raw material obtained in step A or B with ultraviolet rays, the wavelength of the ultraviolet rays is preferably 100 to 400 nm, more preferably 100 to 300 nm. . Among these, ultraviolet rays having a wavelength of 135 to 260 nm are particularly preferable because they directly act on cellulose and hemicellulose to cause a reduction in molecular weight, and the cellulose raw material obtained in the step A or B can be shortened.

  What is necessary is just to use what can irradiate the light of the wavelength range of 100-400 nm as a light source which irradiates an ultraviolet-ray. Specific examples thereof include a xenon short arc lamp, an ultra-high pressure mercury lamp, a high-pressure mercury lamp, a low-pressure mercury lamp, a deuterium lamp, a metal halide lamp, and the like, and one or more of these can be used in any combination. . In particular, when a plurality of light sources having different wavelength characteristics are used in combination, ultraviolet rays having different wavelengths can be simultaneously irradiated to increase the number of cellulose chains and hemicellulose chains to be cut, thereby facilitating shortening of the fibers.

  As a container for containing the cellulosic raw material obtained in step A or B, for example, when using ultraviolet light having a wavelength longer than 300 nm, a hard glass container can be used, but when using ultraviolet light having a shorter wavelength than that, In this case, it is preferable to use a quartz glass container that transmits UV light more. As the material of the portion not participating in the reaction by the ultraviolet rays in the container, a material having little deterioration with respect to the wavelength of the ultraviolet rays may be appropriately selected.

  In order to carry out the reaction efficiently, it is preferable to disperse the cellulose-based raw material obtained in step A or B in a dispersion medium to form a dispersion, and to irradiate the dispersion with ultraviolet rays. The dispersion medium is preferably water from the viewpoint of suppressing side reactions. From the viewpoint of increasing energy efficiency, the concentration of the oxidized cellulose-based raw material in the dispersion is preferably 0.1% by mass or more. Moreover, in order to maintain the fluidity | liquidity of the cellulose raw material obtained by the process A or B in an ultraviolet irradiation device, and to improve reaction efficiency, the said density | concentration is preferable 12 mass% or less. Therefore, the concentration of the oxidized cellulose-based raw material in the dispersion is preferably 0.1 to 12% by mass, more preferably 0.5 to 5% by mass, and further preferably 1 to 3% by mass.

  From the viewpoint of reaction efficiency, the reaction temperature is preferably 20 ° C. or higher. On the other hand, if the temperature is too high, the cellulosic raw material obtained in step A or B may be deteriorated and the pressure in the reaction apparatus may exceed atmospheric pressure. Therefore, the reaction temperature is preferably 95 ° C. or lower. Therefore, the reaction temperature is preferably 20 to 95 ° C, more preferably 20 to 80 ° C, and further preferably 20 to 50 ° C. Further, when the reaction temperature is within this range, there is an advantage that it is not necessary to design an apparatus in consideration of pressure resistance. The pH of the system in the reaction is not limited, but in view of simplification of the process, a neutral region, for example, about pH 6.0 to 8.0 is preferable.

  The degree of ultraviolet irradiation can be arbitrarily set by adjusting the residence time of the cellulosic raw material obtained in step A or B in the irradiation reaction apparatus or the energy amount of the irradiation light source. For example, the concentration of the cellulosic raw material dispersion obtained in step A or B in the irradiation apparatus is diluted with water or the like, or obtained by blowing with an inert gas such as air or nitrogen to obtain it in step A or B. It is possible to adjust the amount of ultraviolet radiation received by the cellulosic material. These conditions are appropriately selected in order to obtain the desired quality of the raw material after treatment (fiber length, cellulose polymerization degree, etc.).

  When the ultraviolet irradiation treatment is carried out in the presence of an auxiliary agent such as oxygen, ozone, or peroxide (hydrogen peroxide, peracetic acid, sodium percarbonate, sodium perborate, etc.), the efficiency of the photooxidation reaction is further improved. Since it can raise, it is preferable. In particular, when ultraviolet rays having a wavelength region of 135 to 242 nm are irradiated, ozone is generated from oxygen in the air normally present in the gas phase around the light source. It is preferable to use this ozone as an auxiliary agent. In the present invention, the ozone generated by continuously supplying air to the periphery of the light source is continuously extracted, and the extracted ozone is injected into the cellulosic raw material obtained in the step A or B. Ozone can be used as an auxiliary for the photooxidation reaction without supplying ozone from the outside. Furthermore, a larger amount of ozone can be generated in the system by supplying oxygen to the gas phase around the light source. As described above, in the present invention, ozone that is secondarily generated in the ultraviolet irradiation reactor can be used.

  The ultraviolet irradiation treatment can be repeated a plurality of times. The number of repetitions can be appropriately set according to the quality of the raw material after the treatment and the relationship with the post-treatment such as bleaching. For example, ultraviolet rays of 100 to 400 nm, preferably 135 to 260 nm can be irradiated 1 to 10 times, preferably about 2 to 5 times. At this time, the irradiation time per time is preferably 0.5 to 10 hours, and more preferably 0.5 to 3 hours.

(2) Oxidative decomposition with hydrogen peroxide and ozone Ozone used in the treatment can be generated by a known method using an ozone generator using air or oxygen as a raw material. As described above, in order to efficiently perform the oxidation reaction, it is preferable to use a dispersion obtained by dispersing the cellulose-based raw material obtained in Step A or B in a dispersion medium such as water. The amount (mass) of ozone used in the present invention is preferably 0.1 to 3 times the absolute dry mass of the cellulosic material obtained in step A or B. If the amount of ozone used is at least 0.1 times the absolute dry mass of the cellulosic raw material obtained in step A or B, the amorphous part of cellulose can be sufficiently decomposed, and defibration and dispersion in step C The energy required for processing can be greatly reduced. On the other hand, if the amount of ozone used is excessively large, excessive decomposition of cellulose may occur, but if the amount used is 3 times or less the absolute dry mass of the cellulosic raw material obtained in step A or B, excessive decomposition will occur. Can be suppressed. Therefore, the amount of ozone used is more preferably 0.3 to 2.5 times, more preferably 0.5 to 1.5 times the absolute dry mass of the cellulosic material obtained in Step A or B.

  The amount (mass) of hydrogen peroxide used is preferably 0.001 to 1.5 times the absolute dry mass of the cellulosic material obtained in step A or B. When hydrogen peroxide is used in an amount of 0.001 times or more of the cellulosic raw material obtained in step A or B, a synergistic action between ozone and hydrogen peroxide occurs, and an efficient reaction is possible. On the other hand, for the decomposition of the cellulosic material obtained in step A or B, it is sufficient to use hydrogen peroxide in an amount of about 1.5 times or less of the cellulosic material obtained in step A or B. A large amount of use leads to an increase in cost. Therefore, the amount of hydrogen peroxide used is more preferably 0.1 to 1.0 times the absolute dry mass of the cellulosic material obtained in Step A or B.

  From the viewpoint of reaction efficiency, the pH of the system in the oxidative decomposition treatment with ozone and hydrogen peroxide is preferably 2 to 12, more preferably 4 to 10, and still more preferably 6 to 8. The temperature is preferably 10 to 90 ° C, more preferably 20 to 70 ° C, and further preferably 30 to 50 ° C. The treatment time is preferably 1 to 20 hours, more preferably 2 to 10 hours, and further preferably 3 to 6 hours.

  As a device for performing the treatment with ozone and hydrogen peroxide, a commonly used device can be used. Examples include a conventional reactor equipped with a reaction chamber, a stirrer, a chemical injector, a heater, and a pH electrode.

  When the treatment with ozone and hydrogen peroxide is carried out immediately before Step C, the ozone and hydrogen peroxide remaining in the aqueous solution also work effectively in the defibrating treatment in the next Step C, thus giving a lower viscosity dispersion. A cellulose nanofiber can be manufactured and is preferable.

  The reason why the cellulose-based raw material obtained in Step A or B can be efficiently reduced in viscosity with hydrogen peroxide and ozone is presumed as follows. Carboxyl groups are localized on the surface of the cellulose-based raw material oxidized using the N-oxyl compound, and a hydrated layer is formed. For this reason, since the raw materials are close to each other and form a network, the viscosity of the dispersion containing the raw materials is high. However, there is a microscopic gap between the raw materials that is not seen in ordinary pulp due to the charge repulsive force between the carboxyl groups. When the raw material is treated with ozone and hydrogen peroxide, ozone and excess Hydroxyl radicals excellent in oxidizing power are generated from hydrogen oxide, penetrate into the microscopic gaps, efficiently oxidatively decompose cellulose chains in the raw material, and shorten the raw material.

(3) Hydrolysis with acid In this treatment, an acid is added to the cellulose raw material obtained in step A or B to hydrolyze the cellulose chain (acid hydrolysis treatment). As the acid, it is preferable to use a mineral acid such as sulfuric acid, hydrochloric acid, nitric acid, or phosphoric acid. As described above, in order to perform the reaction efficiently, it is preferable to use a dispersion liquid in which the cellulose-based raw material obtained in Step A or B is dispersed in a dispersion medium such as water. The conditions for the acid hydrolysis treatment may be any conditions that allow the acid to act on the amorphous part of the cellulose. For example, the addition amount of the acid is preferably 0.01 to 0.5% by mass, more preferably 0.1 to 0.5% by mass with respect to the absolutely dry mass of the cellulose-based material obtained in the step A or B. . It is preferable that the amount of acid added is 0.01% by mass or more because hydrolysis of cellulose proceeds and the treatment efficiency in Step C is improved. Moreover, the excessive hydrolysis of a cellulose can be prevented as the said addition amount is 0.5 mass% or less, and the fall of the yield of a cellulose nanofiber can be prevented. The pH of the system during acid hydrolysis is preferably from 2.0 to 4.0, more preferably from 2.0 to less than 3.0. However, when the alkali used in Step B remains in the cellulose-based raw material dispersion medium obtained in Step B, it is preferable to adjust the pH of the system to the above range by appropriately increasing the amount of acid added. From the viewpoint of reaction efficiency, the reaction is preferably performed at a temperature of 70 to 120 ° C. for 1 to 10 hours.

In order to efficiently carry out the treatment of the next step B or C, it is preferable to neutralize by adding an alkali such as sodium hydroxide after the acid hydrolysis treatment.
The reason why the viscosity-reducing treatment of the cellulosic raw material obtained in Step A or B can be efficiently carried out by the acid hydrolysis treatment is presumed as follows. As described above, the carboxyl group is localized on the surface of the cellulosic raw material oxidized using the N-oxyl compound, a hydrated layer is formed, and the raw materials are close to each other to form a network. ing. When an acid is added to the raw material and hydrolysis is performed, the balance of charges in the network is lost, a strong network of cellulose molecules is lost, the specific surface area of the raw material is increased, and shortening of the raw material is promoted. Cellulosic raw materials are reduced in viscosity.

2. Cellulose nanofibers produced by the present invention Cellulose nanofibers produced by the present invention are single microfibrils of cellulose having a width of 2 to 5 nm and a length of about 100 to 5000 nm, preferably about 1 to 5 μm. The cellulose nanofibers obtained by the present invention have a B-type viscosity (60 rpm, 20 ° C.) in an aqueous dispersion having a concentration of 1.0% (w / v) of 500 mPa · s or less, preferably 300 mPa · s or less, more preferably Is 100 mPa · s or less. The B-type viscosity in an aqueous dispersion having a concentration of 2% (w / v) is preferably 2000 mPa · s or less, and more preferably 1000 mPa · s or less. When the B-type viscosity in an aqueous dispersion with a concentration of 2% (w / v) is 2000 mPa · s or less, it has excellent miscibility with various pigments and binder resins, and when it is 1000 mPa · s or less, it is above a certain level. It is possible to efficiently obtain a coating layer having an excellent film thickness and excellent surface properties. The lower limit of the B-type viscosity is not particularly limited, but is usually about 1 mPa · s or more, or about 5 mPa · s or more. The B-type viscosity can be measured using a normal B-type viscometer. For example, it can be measured using a TV-10 viscometer manufactured by Toki Sangyo Co., Ltd. under the conditions of 20 ° C. and 60 rpm.

  According to the production method of the present invention, since cellulose nanofibers can be well dispersed in a dispersion medium, light diffusion hardly occurs, and the transparency of the resulting dispersion of cellulose nanofibers is high. The cellulose nanofibers obtained by the present invention preferably have a light transmittance (wavelength of 660 nm) in a 0.1% (w / v) aqueous dispersion of 90% or more, more preferably 95% or more. Preferably, it is 99% or more. When the transparency is 95% or more, the cellulose nanofibers can be used without problems for film applications, and when it is 99% or more, the film uses such as a display and a touch panel that require high optical properties (transparency). Can be used without problems. Specifically, it is obtained by measuring the amount of light transmitted through a test body in which a 0.1% dispersion liquid is put in a quartz cell (optical path: 10 mm) using an ultraviolet / visible spectrophotometer.

  As a cellulose nanofiber manufactured by this invention, a thing with little coloring is preferable. Colored cellulose nanofibers may have low strength. In addition, for example, when a coating containing cellulose nanofibers with little coloring is coated on a transparent film and dried, it is difficult to discolor (color) due to heat during drying, and thus a transparent film with few appearance defects can be obtained. There are advantages. Such cellulose nanofibers can be obtained by using an oxidizing agent or a reducing agent in Step B, in particular, by using hydrogen peroxide, which hardly generates radicals, as the oxidizing agent.

  Cellulose nanofibers produced according to the present invention are excellent in fluidity and transparency, and also excellent in barrier properties and heat resistance, so that they can be used for various applications such as packaging materials in addition to the above. .

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.
[Example 1]
Step A: Bleached unbeaten kraft pulp derived from coniferous tree (manufactured by Nippon Paper Industries Co., Ltd., whiteness 84%) 5 g (absolutely dried) 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 755 mg (7 mmol) of sodium bromide Was added to 500 ml of the dissolved aqueous solution and stirred until the pulp was uniformly dispersed. After adding 18 ml of aqueous sodium hypochlorite solution (effective chlorine 5%) to the reaction solution, the pH was adjusted to 10.3 with 0.5N aqueous hydrochloric acid solution to initiate the oxidation reaction. During the reaction, the pH of the reaction solution was lowered, but a 0.5N aqueous sodium hydroxide solution was successively added to adjust the pH to 10. After making it react for 2 hours, it filtered with the glass filter and obtained the cellulose-type raw material oxidized by fully washing with water.

  Step B: A 5% (w / v) aqueous dispersion of an oxidized cellulose-based material is prepared, and 1% (w / v) hydrogen peroxide is added to the oxidized cellulose-based material in the dispersion. And the pH was adjusted to 12 with 1M sodium hydroxide. This aqueous dispersion was heated at 80 ° C. for 2 hours to hydrolyze the oxidized cellulose-based raw material, and then filtered through a glass filter and sufficiently washed with water.

The coloration of the obtained dispersion of oxidized cellulose raw material was visually evaluated according to the following criteria:
3 Not colored at all 2 Not colored at all 1 Colored.

  Step C: An aqueous dispersion having a cellulose-based raw material concentration of 2% (w / v) obtained in Step B is prepared, and treated with an ultra-high pressure homogenizer (treatment pressure 140 MPa) 10 times to obtain a transparent gel dispersion. Obtained.

  The B-type viscosity (60 rpm, 20 ° C.) of the obtained 2% (w / v) cellulose nanofiber aqueous dispersion was measured using a TV-10 viscometer (manufactured by Toki Sangyo Co., Ltd.). Further, the obtained 2% (w / v) cellulose nanofiber dispersion was diluted with water to prepare a 0.1% (w / v) cellulose nanofiber aqueous dispersion, and a UV-VIS spectrophotometer The transmittance of 660 nm light was measured using UV-265FS (Shimadzu Corporation). Furthermore, the power consumption required for defibration and dispersion treatment was determined by (power during processing) × (processing time) / (amount of processed sample). The results are shown in Table 1.

[Example 2]
In Step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that 1% (w / v) sodium hypochlorite was added to the oxidized cellulose-based raw material instead of hydrogen peroxide. . The results are shown in Table 1.

[Example 3]
In step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that the treatment was performed under an oxygen pressure condition of 0.6 MPa. The results are shown in Table 1.

[Example 4]
In Step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that 2% (w / v) ozone was further added to the oxidized cellulose-based raw material. The results are shown in Table 1.

[Example 5]
In Step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that hydrogen peroxide was not added. The results are shown in Table 1.

[Examples 6 to 8]
In Step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that when adding hydrogen peroxide, the pH was adjusted to 8, 10, and 14 with 1 M sodium hydroxide, respectively. The results are shown in Table 1.

[Examples 9 to 12]
In step B, nanofiber dispersions were obtained and evaluated in the same manner as in Example 1 except that the temperatures were 50 ° C., 60 ° C., 90 ° C., and 100 ° C., respectively. The results are shown in Table 1.

[Example 13]
Before Step C, an aqueous dispersion containing 2% (w / v) of the cellulosic material obtained in Step B is prepared, and 254 nm ultraviolet rays are emitted using a 20 W low-pressure ultraviolet lamp while flowing the aqueous dispersion. A nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that the step of irradiation for 6 hours was performed. The results are shown in Table 1.

[Example 14]
A nanofiber dispersion was prepared in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from hardwood was used instead of bleached unbeaten kraft pulp derived from conifers (whiteness 85%, manufactured by Nippon Paper Industries Co., Ltd.). Obtained and evaluated. The results are shown in Table 2.

[Example 15]
In step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 14 except that 1% (w / v) sodium hypochlorite was added to the oxidized pulp instead of hydrogen peroxide. The results are shown in Table 2.

[Example 16]
In step B, a nanofiber dispersion was obtained and evaluated in the same manner as in Example 14 except that the treatment was performed under an oxygen pressure condition of 0.6 MPa. The results are shown in Table 2.

[Example 17]
A nanofiber dispersion was obtained and evaluated in the same manner as in Example 1 except that bleached unbeaten sulfite pulp (whiteness of 86%) derived from hardwood was used instead of bleached unbeaten kraft pulp derived from softwood. . The results are shown in Table 2.

[Comparative Example 1]
An aqueous dispersion having a concentration of 2% (w / v) was prepared using the oxidized cellulosic raw material obtained in Step A of Example 1. Without carrying out Step B, a viscosity reduction treatment was performed by irradiating ultraviolet rays at 254 nm for 6 hours using a 20 W low-pressure ultraviolet lamp while flowing the aqueous dispersion. Using the aqueous dispersion obtained by this treatment, Step C of Example 1 was performed to obtain and evaluate a nanofiber dispersion. The results are shown in Table 1.

[Comparative Example 2]
An aqueous dispersion having a concentration of 2% (w / v) was prepared using the oxidized cellulosic raw material obtained in Step A of Example 1. Without carrying out Step B, 2 mass% of commercially available cellulase (Novozyme 476, manufactured by Novozymes Japan) was added to the aqueous dispersion and held at 50 ° C. Using the aqueous dispersion obtained by this treatment, Step C of Example 1 was performed to obtain and evaluate a nanofiber dispersion. The results are shown in Table 1.

[Comparative Example 3]
An aqueous dispersion having a concentration of 2% (w / v) was prepared using the oxidized cellulosic raw material obtained in Step A of Example 1. Without carrying out Step B, ozone and hydrogen peroxide were added to the aqueous dispersion, and the mixture was stirred at room temperature for 6 hours to perform a viscosity reduction treatment. The amounts of ozone and hydrogen peroxide used are respectively an ozone concentration of 6 g / L (corresponding to 0.6 times the absolute dry mass of the oxidized cellulose raw material), and a hydrogen peroxide concentration of 3 g / L (oxidized cellulose system). Equivalent to 0.3 times the absolute dry mass of the raw material). Using the aqueous dispersion obtained by this treatment, Step C of Example 1 was performed to obtain and evaluate a nanofiber dispersion. The results are shown in Table 1.

[Comparative Example 4]
A 0.1% aqueous hydrochloric acid solution was added to the oxidized cellulose raw material obtained in Step A of Example 1 to prepare a 5% (w / v) aqueous dispersion having a pH of 2.8. Without performing Step B, the aqueous dispersion was stirred at 90 ° C. for 2 hours for acid hydrolysis treatment. The amount of hydrochloric acid added was 0.1% by mass with respect to the oxidized cellulose raw material. Using the aqueous dispersion obtained by this treatment, Step C of Example 1 was performed to obtain and evaluate a nanofiber dispersion. The results are shown in Table 1.

  From the results of Tables 1 and 2, in Examples 1 to 17 in which Step B for hydrolyzing an oxidized cellulose-based raw material in an alkaline solution was performed, the viscosity was lower than in Comparative Examples 1 to 4 in which Step B was not performed. It is clear that cellulose nanofibers that give an aqueous dispersion with high transparency can be produced with low power consumption.

Claims (5)

(A) A cellulosic raw material selected from kraft pulp or sulfite pulp is used in the presence of a compound selected from the group consisting of (a1) an N-oxyl compound and (a2) bromide, iodide or a mixture thereof. (A3) a step of oxidizing using an oxidizing agent,
(B) a step of hydrolyzing the oxidized cellulose-based material obtained in the step A in an alkaline solution having a pH of 8 to 14, and (C) a dispersion containing the hydrolyzed oxidized cellulose-based material obtained in the step B Preparing a nanofiber by fibrillating while dispersing the hydrolyzed oxidized cellulose raw material in a dispersion medium,
The manufacturing method of the cellulose nanofiber containing this.   The method according to claim 1, wherein an oxidizing agent or a reducing agent is added to the alkaline solution in the step B.   The method according to claim 1 or 2, wherein the hydrolysis in the step B is performed at a temperature of 40 to 120 ° C for 0.5 to 24 hours. Wherein a said concentration of the oxidizing cellulosic material in the dispersion used in step B is 1~50% (w / v), The method according to any one of claims 1 to 3. The cellulosic material is a cellulosic material derived from hardwood, the method according to any one of claims 1 to 4.