JP5731253B2 - Method for producing cellulose nanofiber - Google Patents

Description

  The present invention relates to a method for producing cellulose nanofibers. In more detail, this invention relates to the manufacturing method of the cellulose nanofiber which gives the cellulose nanofiber dispersion liquid of high concentration and high fluidity.

  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 introduced with a carboxyl group is treated in water with a mixer or the like, 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 combining with organic / inorganic pigments. Furthermore, cellulose nanofibers can be made into sheets or fibers. Utilizing these characteristics, cellulose nanofibers can be applied to high-performance packaging materials, transparent organic substrate materials, high-performance fibers, separation membranes, regenerative medical materials, etc. It is expected to develop functional products.

  However, the dispersion of cellulose nanofibers obtained by oxidizing a cellulose-based raw material derived from wood (hardwood pulp, softwood pulp, etc.) using TEMPO and defibrating with a mixer is 0.3 to 0.5%. Even at a concentration as low as (w / v), the B-type viscosity (60 rpm, 20 ° C.) is 800 to 4000 mPa · s, which is very high. For this reason, the conventional cellulose nanofiber was not easy to handle, and it was not easy to apply it to the above-mentioned uses.

JP 2008-001728 A JP 2010-235679 A

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

As a result of preliminary studies on conventional wood-derived cellulose nanofibers, the inventors have obtained the following knowledge.
First, it tried to apply | coat the dispersion liquid of the cellulose nanofiber derived from wood to a base material, and to form a film on a base material. However, since the viscosity of the dispersion is too high to perform uniform coating, the concentration of cellulose nanofibers in the dispersion is very low, about 0.05 to 0.4% (w / v), and the dispersion B It was necessary to adjust the mold viscosity (60 rpm, 20 ° C.) to about 10 to 3000 mPa · s. When such Ru with a low concentration of the dispersion, also may need to repeat the coating and drying multiple times in order to obtain a film of desired thickness, the efficiency is poor.

  Next, an attempt was made to mix a cellulose nanofiber dispersion derived from wood with a paint containing a pigment and a binder and apply it to paper or the like. However, in this case as well, the viscosity of the dispersion is too high to uniformly mix with other components, and the concentration of cellulose nanofibers in the dispersion has to be lowered. A coating using such a low-concentration dispersion has a problem that it is difficult to apply because it does not have the viscosity required for application, and the drying load is large. In addition, the coating film penetrates into the base paper and the effective coating film becomes thin, and there is also a problem that desired functions expected for the coating film, such as improvement in glossiness and surface strength, and suppression of printing unevenness, are not exhibited.

  In these studies, the viscosity of the cellulosic material derived from oxidized wood leaf tree is significantly increased when dispersed in water, and the dispersion proceeds only around the stirring blades, resulting in a non-uniform dispersion. There was also. Therefore, an attempt was made to defibrate the oxidized cellulosic material derived from wood leaves using a homogenizer with higher defibration / dispersion power than the mixer. Thus, there is a problem that the fluidity is deteriorated and the power consumption required for the dispersion process is greatly increased. Furthermore, there is a problem that the cellulose nanofiber dispersion liquid adheres to the inside of the apparatus and the dispersion is not sufficiently performed, or the operation of taking out the dispersion liquid from the apparatus becomes difficult and the yield of the dispersion liquid is lowered.

  As a result of the above studies, the inventors have found that conventional wood-derived cellulose nanofibers are difficult to be applied to various fields because the viscosity is too high when a high-concentration dispersion is used. .

  In view of the above circumstances, an object of the present invention is to provide a cellulose nanofiber that gives a cellulose nanofiber dispersion that has a high concentration and a low viscosity.

As a result of intensive studies, the present inventors have found that the above problem can be solved by setting the hemicellulose content contained in the hardwood-derived cellulosic material to less than a certain amount. That is, the said subject is solved by the following this invention.
(A) Presence of a compound selected from the group consisting of (a1) N-oxyl compounds and (a2) bromides, iodides or mixtures thereof, from a broad-leaved tree-derived raw material having a hemicellulose content of less than 17% by mass (A3) a step of oxidizing using an oxidizing agent, and (B) a dispersion liquid in which the concentration of the oxidized cellulose raw material obtained in the step A is 1% (w / v) or more is prepared, and the oxidized cellulose A method for producing cellulose nanofibers, comprising: a step of defibrating a system raw material and dispersing in a dispersion medium to form nanofibers.

  According to the present invention, it is possible to provide a cellulose nanofiber that gives a cellulose nanofiber dispersion liquid having a high concentration and a low viscosity.

Hereinafter, the present invention will be described in detail. In the present specification, “to” includes values at both ends.
1. Production method of cellulose nanofiber The production method of the present invention comprises (A) a hardwood-derived cellulosic material having a hemicellulose content of less than 17% by mass, (a1) an N-oxyl compound, and (a2) bromide and iodide. Alternatively, in the presence of a compound selected from the group consisting of these mixtures, (a3) a step of oxidizing using an oxidizing agent, and (B) the concentration of the oxidized cellulose-based raw material obtained in step A is 1% (w / V) A step of preparing the above dispersion, defibrating the oxidized cellulose-based raw material, dispersing it in a dispersion medium, and forming nanofibers.

1-1. Process A
In step A, a hardwood-derived cellulosic material having a hemicellulose content adjusted to less than 17% by mass is oxidized under specific conditions.

(1) Cellulose-based raw material The hardwood-derived cellulose-based raw material used in the present invention is required to have a hemicellulose content of less than 17% by mass, and preferably less than 13% by mass. Hemicellulose is a polysaccharide that constitutes the cell wall of plants, and is a polysaccharide in which an acetyl group or a sugar residue is bonded to the main chain of xylan or glucomannan. In the present invention, a cellulose-based raw material derived from hardwood having a hemicellulose content of less than 17% by mass is used. Therefore, a cellulose nanofiber that gives a cellulose nanofiber dispersion having a low viscosity and excellent fluidity even at a high concentration. Can be manufactured. This mechanism will be described later.

  The broad-leaved cellulosic raw material includes hardwood-derived pulp, powdered cellulose obtained by pulverizing them with a high-pressure homogenizer, a mill, or the like, or microcrystalline cellulose powder obtained by purifying them by chemical treatment such as acid hydrolysis.

  Examples of methods for producing pulp from hardwood (pulping treatment) include methods for producing mechanical pulp such as ground pulp and thermomechanical pulp, methods for producing chemical pulp such as kraft pulp, sulfite pulp, and organosolv pulp. . However, if a large amount of hardwood-derived lignin remains in the cellulosic raw material, there is a risk of inhibiting the oxidation reaction of the raw material. Therefore, in the present invention, the cellulosic raw material obtained by the method for producing chemical pulp is preferred. . In order to further remove lignin, it is more preferable to subject the cellulosic raw material thus obtained to a known bleaching treatment. Bleaching methods include chlorination (C), chlorine dioxide bleach (D), alkali extraction (E), hypochlorite bleach (H), hydrogen peroxide bleach (P), alkaline hydrogen peroxide treatment stage (Ep ), Alkaline hydrogen peroxide / oxygen treatment stage (Eop), ozone treatment (Z), chelate treatment (Q), and the like. For example, the bleaching treatment can be performed by C / D-E-H-D, Z-E-D-P, Z / D-Ep-D, Z / D-Ep-D-P, D-Ep-D, D- It can be carried out in a sequence such as Ep-DP, D-Ep-PD, Z-Eop-DD, Z / D-Eop-D, Z / D-Eop-D-ED. “/” In the sequence means that the processes before and after “/” are continuously performed without cleaning. The whiteness (ISO 2470) of the cellulosic raw material (pulp) thus obtained is desirably 80% or more.

  The method for setting the hemicellulose content of the hardwood-derived cellulosic raw material to less than 17% by mass is not particularly limited, but each condition in the pulping treatment and bleaching treatment (chemical treatment concentration, treatment time, treatment temperature, etc.) is appropriately adjusted. Thereby, hemicellulose content can be made less than 17 mass%. Usually, since the hemicellulose content in bleached hardwood pulp (LBKP) generally used in the papermaking field is 17% by mass or more and less than 20% by mass, the hemicellulose content is higher than the cellulosic material used in the present invention. Many.

  The broad-leaved trees are, for example, the genus Acacia, the genus Eucalyptus, the genus Robinia, and the genus Ulmus. As Acacia genus Acacia (hereinafter abbreviated as A.), A. mangiumu, A. et al. auriculaeformis, A. et al. dealbata, A.M. Mearnsii and the like can be mentioned. Eucalyptus (hereinafter abbreviated as E.) grandis, E .; euphilia, E .; eulograndis and the like. As the genus Robinia (hereinafter abbreviated as R.), R. pseudoacacia, R.P. hispida, R.H. pseudoacacia f. inermis and the like. As the genus Ulmus (hereinafter abbreviated as U.), U. americana, U.S.A. parvifolia, U.S.A. and the like.

  The hemicellulose content of the cellulosic material can be measured as follows. After reacting 300 mg of lyophilized pulp in 3 mL of 72% sulfuric acid at room temperature for 2 hours, dilute the sulfuric acid concentration to 2.5% by mass and heat at 105 ° C. for 1 hour. A monosaccharide solution is obtained. The solution is diluted, and monosaccharides are quantified by ion chromatography (DXion 500, manufactured by Dionex, column: AS-7, eluent: water, flow rate 1.1 mL / min). From the amount of xylose and mannose contained in the acid hydrolysis solution, hemicellulose can be obtained by the following formula.

  Hemicellulose content (% by mass) = (xylose amount (mg) × 0.88 + mannose amount (mg) × 0.9) / pulp amount (mg) × 100 (%)

(2) N-oxyl compound (a1)
An N-oxyl compound is a compound capable of generating 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. The 4-hydroxy TEMPO derivative is particularly preferable because it is inexpensive and can provide uniform oxidized cellulose.

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 5, that is, an azaadamantane type nitroxy radical, is preferable because cellulose nanofibers having a high degree of polymerization can be produced in a short time.

In Formula 5, 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 capable of converting the cellulose-based raw material 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.

(3) Bromide or iodide (a2)
Bromide is a compound containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Further, an iodide is 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 the cellulosic raw material.

(4) Oxidizing agent (a3)
As the oxidizing agent used in the oxidation of 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. In general, broad-leaved cellulosic raw materials are less likely to introduce carboxyl groups (that is, less susceptible to oxidation) than coniferous cellulosic raw materials, so the amount of oxidizing agent used is adjusted to an appropriate range for oxidation. It is preferable to promote the progress of. The appropriate amount of the oxidizing agent used varies depending on the hardwood species to be used, but is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, for example, with respect to 1 g of cellulosic raw material. 5 to 25 mmol is more preferable, and 5 to 20 mmol is most preferable.

(5) Oxidation reaction conditions This step allows the reaction to proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature may be a room temperature of about 15 to 30 ° C. As the reaction proceeds, a carboxyl group is generated in the cellulose, so that the pH of the reaction solution is reduced. 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 medium is preferably water because it is easy to handle and hardly causes side reactions.

  The reaction time in the oxidation reaction can be appropriately set according to the progress of oxidation, and is not particularly limited. For example, the reaction time is 0.5 to 6 hours, preferably 2 to 6 hours, and more preferably about 4 to 6 hours. is there. The oxidation reaction may be performed in two stages. For example, oxidized cellulose obtained by filtration after the completion of the first stage reaction is oxidized again under the same or different reaction conditions, so that the cellulose is not subject to reaction inhibition by the salt produced as a by-product in the first stage reaction. The carboxyl group can be efficiently introduced into the system material, and the oxidation of the cellulosic material can be promoted.

  In this step, it is preferable to set conditions so that the amount of carboxyl groups of the oxidized cellulose raw material is 1.0 mmol / g or more with respect to the absolute dry mass of the cellulose raw material. In this case, the carboxyl group amount is more preferably 1.0 mmol / g to 3.0 mmol / g, further preferably 1.4 mmol / g to 3.0 mmol / g, and particularly preferably 1.5 mmol / g to 2.5 mmol. / G. The amount of the carboxyl group can be adjusted by adjusting the oxidation reaction time, adjusting the oxidation reaction temperature, adjusting the pH during the oxidation reaction, adjusting the addition amount of the N-oxyl compound, bromide, iodide, or oxidizing agent.

  In order to enhance the dispersibility in the next step B, the oxidized cellulose-based raw material obtained in step A is preferably washed.

1-2. Process B
In this step, (B) a dispersion liquid in which the concentration of the oxidized cellulose raw material obtained in the step A is 1% (w / v) or more is prepared, and the oxidized cellulose raw material is defibrated to be contained in the dispersion medium. Disperse into nanofibers. “To make nanofiber” means to process a cellulosic raw material into cellulose nanofiber which is a single microfibril of cellulose having a width of about 2 to 5 nm and a length of about 1 to 5 μm. A dispersion is a liquid in which the oxidized cellulose raw material is dispersed in a dispersion medium. From the viewpoint of ease of handling, the dispersion medium is preferably water. As this dispersion, the reaction mixture obtained in Step A may be used as it is, or may be diluted as necessary.

  In order to disentangle the oxidized cellulose raw material and disperse it in the dispersion medium, a strong shear is applied to the dispersion using an apparatus such as a high-speed rotation type, a colloid mill type, a high pressure type, a roll mill type, or an ultrasonic type. It is preferable to apply a force. In particular, in order to efficiently obtain cellulose nanofibers, 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 oxidized cellulose-based raw material is defibrated to form cellulose nanofibers, and the cellulose nanofibers are dispersed in the dispersion medium.

  The concentration of the oxidized cellulose raw material in the dispersion used for the treatment is 1% (w / v) or more, preferably 1 to 5% (w / v), more preferably 2 to 5% (w / v). It is. In the present invention, a cellulose-based raw material derived from hardwood with less than a specific amount of hemicellulose is used. Therefore, even if the oxidized cellulose-based raw material concentration is increased in this way, the viscosity of the system does not increase during the defibrating process and the cellulose Fiber can be obtained.

1-3. In the present invention, in order to obtain a cellulose nanofiber that gives a cellulose nanofiber dispersion having a lower viscosity, before the step B, the oxidized cellulose-based raw material (hereinafter simply referred to as “cellulose”) obtained in the step A is obtained. It is preferable to reduce the viscosity of the raw material). The viscosity reduction treatment is to appropriately cut the cellulose chain of the oxidized cellulose raw material (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 reduction treatment may be any treatment that lowers the viscosity of the cellulosic raw material. For example, the treatment of irradiating the oxidized cellulosic raw material with ultraviolet rays, the oxidized cellulosic raw material with hydrogen peroxide and ozone Examples thereof include a process of oxidizing and decomposing with an acid, a process of hydrolyzing an oxidized cellulosic raw material with an acid, and a combination thereof.

(1) Ultraviolet irradiation When the oxidized cellulose-based raw material is irradiated with ultraviolet rays to reduce the viscosity, 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 can directly act on cellulose and hemicellulose to cause low molecular weight and shorten the cellulose-based raw material.

  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 oxidized cellulosic raw material when performing ultraviolet irradiation, for example, when using ultraviolet light having a wavelength longer than 300 nm, a hard glass container can be used, but ultraviolet light having a shorter wavelength than that can be used. When used, it is preferable to use a quartz glass container which allows more ultraviolet rays to pass through. 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 oxidized cellulose-based raw material in a dispersion medium to form a dispersion, and 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 cellulose nanofibers in the dispersion is preferably 0.1% by mass or more. Further, in order to improve the flowability of the cellulose-based raw material in the ultraviolet irradiation device and increase the reaction efficiency, the concentration is preferably 12% by mass or less. Therefore, the concentration is preferably 0.1 to 12% by mass, more preferably 0.5 to 5% by mass, and further preferably 1 to 3% by mass.

  In view 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 may be deteriorated or 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 in the irradiation reaction apparatus or the energy amount of the irradiation light source. Further, for example, by diluting the concentration of the cellulosic raw material dispersion in the irradiation apparatus with water or the like, or by blowing and diluting with an inert gas such as air or nitrogen, the irradiation amount of ultraviolet rays received by the cellulosic material can be adjusted. . These conditions are appropriately selected so that the quality (fiber length, cellulose polymerization degree, etc.) of the cellulosic raw material after the treatment is set to a desired value.

  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 by air that normally exists in the gas phase portion around the light source, and this ozone is preferably used 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 ozone extracted from outside the system is injected into the oxidized cellulosic raw material. Without supply, ozone can be used as an auxiliary for the photo-oxidation reaction. 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 relationship with the quality of the cellulosic raw material after the treatment and 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 the cellulose-based raw material as a dispersion by dispersing it 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. If the amount of ozone used is at least 0.1 times the absolute dry mass of the cellulosic raw material, the amorphous part of the cellulose can be sufficiently decomposed, and the energy required for the defibration / dispersion treatment in the next step B It can be greatly reduced. On the other hand, if the amount of ozone used is excessively large, excessive decomposition of cellulose can occur, but if the amount used is three times or less the absolute dry mass of the cellulose-based raw material, excessive decomposition can be suppressed. Therefore, the amount of ozone used is more preferably 0.3 to 2.5 times the absolute dry mass of the cellulosic raw material, and even more preferably 0.5 to 1.5 times.

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

  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 with a reaction chamber, a stirrer, a chemical injector, a heater, and a pH electrode.

  After the treatment with ozone and hydrogen peroxide, ozone and hydrogen peroxide remaining in the aqueous solution work effectively in the defibration / dispersion treatment in the next step B, so manufacturing cellulose nanofibers that give a dispersion with a lower viscosity. it can.

  The reason why the low-viscosity treatment of the oxidized cellulosic raw material can be efficiently performed 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 cannot be 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 microscopic gaps, and efficiently oxidatively decompose cellulose chains in the raw material to shorten the cellulose-based raw material.

(3) Hydrolysis with acid In this treatment, an acid is added to the oxidized cellulose raw material 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 an oxidized cellulose-based raw material 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 amount of acid added is preferably 0.01 to 0.5% by mass, more preferably 0.1 to 0.5% by mass, based on the absolute dry mass of the cellulosic material. It is preferable for the amount of acid added to be 0.01% by mass or more because hydrolysis of cellulose proceeds and the processing efficiency in the next step B 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. From the viewpoint of acid hydrolysis 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 in the next step B, it is preferable to neutralize by adding an alkali such as sodium hydroxide after the acid hydrolysis treatment.
The reason why the low-viscosity treatment of the oxidized cellulose raw material can be efficiently performed 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 for hydrolysis, the balance of charges in the network is lost, the strong network of cellulose molecules is lost, the specific surface area of the raw material is increased, and the cellulose-based raw material is shortened. This promotes the viscosity of the cellulosic material.

2. Cellulose nanofibers obtained by the production method of the present invention The cellulose nanofibers produced by the present invention are cellulose single microfibrils having a width of about 2 to 5 nm and a length of about 1 to 5 μm. The cellulose nanofibers obtained according to the present invention have a B-type viscosity (60 rpm, 20 ° C.) of an aqueous dispersion having a concentration of 1.0% (w / v), preferably 800 mPa · s or less, more preferably 500 mPa · s or less, more preferably 100 mPa · s or less. 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 of the cellulose nanofiber aqueous dispersion can be measured using a normal B-type viscometer. For example, the B type viscosity can be measured using a TV-10 viscometer manufactured by Toki Sangyo Co., Ltd. under the conditions of 20 ° C. and 60 rpm.

  The cellulose nanofibers produced according to the present invention have excellent fluidity even in such a relatively high concentration aqueous dispersion. For this reason, it is suitable for the use as a coating agent by applying on a base material. In particular, since the coating layer made of cellulose nanofibers obtained by the present invention is excellent in barrier properties, it can be applied to various uses such as packaging materials. In particular, when applied to a coating agent, the B-type viscosity (60 rpm, 20 ° C.) of a 1% (w / v) cellulose nanofiber dispersion is preferably 500 mPa · s or less from the viewpoint of coating properties, and 100 mPa -It is more preferable that it is s or less, and it is further more preferable that it is 50 mPa * s or less. Since such a cellulose nanofiber dispersion liquid has a high viscosity and a low viscosity, a coating layer having a certain thickness or more and an excellent surface property can be efficiently formed.

  The reason why cellulose nanofibers having excellent fluidity in such an aqueous dispersion having a relatively high concentration can be obtained by setting the hemicellulose content in the cellulose-based raw material derived from hardwood to less than 17% by mass is limited. Although not, it is guessed as follows.

  In general, hemicellulose is present in cellulosic materials, and cellulose microfibrils are restrained to some extent from being strongly bonded by hydrogen bonds. Therefore, a certain amount of gaps exist between cellulose microfibrils, and the amount of primary hydroxyl groups present on the microfibril surface is relatively large. Therefore, the primary hydroxyl group is oxidized during the oxidation reaction to become a carboxyl group. On the other hand, in the part where hemicellulose does not exist in the cellulosic raw material, cellulose microfibrils are strongly bonded by hydrogen bonding, so that a plurality of microfibrils are closely adhered to each other and the surface area is reduced. As a result, the amount of primary hydroxyl groups present on the surface of the microfibril is reduced. Therefore, during the oxidation reaction, the secondary hydroxyl group having low reactivity is also oxidized to produce a carbonyl group. Under alkaline conditions, a beta elimination reaction takes place starting from a carbonyl group, and the degree of polymerization of cellulose decreases. As a result, shortening of the cellulose nanofiber occurs, and the viscosity of the dispersion liquid decreases.

  By the way, cellulose-based raw materials derived from hardwoods are superior as raw materials for cellulose nanofibers because they are finer and shorter than cellulose-based raw materials derived from coniferous trees, but there are some parts with a lot of hemicellulose and some parts are non-uniform doing. Therefore, in order to cause the shortening of the fiber, it is necessary to make the hemicellulose in the cellulose-based raw material derived from the hardwood less than a certain amount. Therefore, in the present invention, by setting the hemicellulose content in the hardwood-derived cellulosic material to less than 17% by mass, cellulose nanofibers having excellent fluidity even in such a relatively high concentration aqueous dispersion can be obtained. It is thought that it is obtained.

  The amount of carboxyl groups in the cellulose nanofibers obtained by the present invention is preferably 1.0 mmol / g or more. The amount of carboxyl groups in cellulose nanofibers was adjusted to pH 2.5 by preparing 60 ml of a 0.5% by weight slurry (= 0.5 (w / v) aqueous dispersion) of cellulose nanofibers and adding 0.1 M hydrochloric acid aqueous solution. Then, 0.05N sodium hydroxide aqueous solution was added dropwise to measure the electrical conductivity until the pH reached 11, and the amount of sodium hydroxide consumed in the neutralization step of the weak acid where the change in electrical conductivity was slow ( From a), it can be calculated using the following equation.

  Carboxyl group amount [mmol / g pulp] = a [ml] × 0.05 / oxidized pulp mass [g]

Hereinafter, the present invention will be described in more detail with reference to examples.
[Example 1]
Both are hardwoods. grandis and E.M. Bleached unbeaten kraft pulp (hemicellulose content: 14.9% by weight, whiteness: 85%) derived from a mixture of eulograndis (mixing ratio 30:70) 5 g (absolutely dry), TEMPO (Sigma Aldrich) 78 mg (0 0.5 mmol) and 754 mg (7.4 mmol) of sodium bromide were dissolved in 500 ml of an aqueous solution and stirred until the pulp was uniformly dispersed.

  After adding 16 ml of 2M aqueous sodium hypochlorite solution to the reaction system, the pH was adjusted to 10.3 with 0.5N aqueous hydrochloric acid solution to initiate the oxidation reaction. During the reaction, the pH in the system 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 oxidized pulp by fully washing with water.

  An aqueous dispersion 2L containing 1% (w / v) (= 1% by mass) of oxidized pulp obtained by the above reaction was prepared, and irradiated with ultraviolet rays for 6 hours with a 20 W low-pressure ultraviolet lamp while the aqueous solution was flowing. Thereafter, the aqueous solution was treated 10 times with an ultrahigh pressure homogenizer (20 ° C., 140 MPa) to obtain a transparent gel dispersion.

  The B-type viscosity (60 rpm, 20 ° C.) of the obtained 1% (w / v) cellulose nanofiber dispersion was measured using a TV-10 viscometer (Toki Sangyo Co., Ltd.). In addition, the transparency (660 nm light transmittance) of a 0.1% (w / v) cellulose nanofiber dispersion was measured using a UV-VIS spectrophotometer UV-265FS (manufactured by Shimadzu Corporation). The results are shown in Table 1.

[Example 2]
R. as a hardwood. A nanofiber dispersion was obtained in the same manner as in Example 1 except that pseudoacacia (hemicellulose content: 16.7% by mass, whiteness: 85%) was used. The results are shown in Table 1.

[Example 3]
As hardwood A. A nanofiber dispersion was obtained in the same manner as in Example 1 except that mangium (having a hemicellulose content of 12.3% by mass and a whiteness of 86%) was used. The results are shown in Table 1.

[Example 4]
U. as hardwood. A nanofiber dispersion was obtained in the same manner as in Example 1 except that Americana (hemicellulose content 15.1% by mass, whiteness 85%) was used. The results are shown in Table 1.

[Example 5]
As E. hardwood. grandis and E.M. A nanofiber dispersion was obtained in the same manner as in Example 1, except that bleached sulfite pulp (hemicellulose content: 3.3% by mass, whiteness: 86%) derived from the mixture of camaldulensis was used. The results are shown in Table 1.

[Comparative Example 1]
As E. hardwood. g lobulus and E.I. A nanofiber dispersion was obtained in the same manner as in Example 1 except that an obliqua mixed material (blending ratio 30:70) (hemicellulose content: 17.3% by mass, whiteness: 85%) was used. The results are shown in Table 1.

[Comparative Example 2]
As broad-leaved trees A nanofiber dispersion was obtained in the same manner as in Example 1 except that tremuloides (haemicellulose content: 18.5% by mass, whiteness: 85%) was used. The results are shown in Table 1.

[Comparative Example 3]
A nanofiber dispersion was obtained in the same manner as in Example 1 except that a domestic hardwood mixed material (hemicellulose content 22.1% by mass, whiteness 84%) was used as the hardwood. The results are shown in Table 1.

In Example 1 using a hardwood-derived cellulosic material having a hemicellulose content of less than 17%, it is lower than Comparative Example 1 using a hardwood-derived cellulosic material having a hemicellulose content of 17% by mass or more. It turns out that the cellulose nanofiber which gives a viscous dispersion liquid is obtained. Therefore, according to the present invention, it is possible to obtain a cellulose nanofiber dispersion provide high concentration and high fluidity of the minute dispersion liquid.

  Since the dispersion liquid of cellulose nanofibers obtained by the present invention has a high concentration and high fluidity, for example, when a coating material containing the dispersion liquid is applied to a substrate to form a film, the coating material is applied once. There is an advantage that a film having a thickness of about 5 to 30 μm can be formed simply by doing so. In order to prepare a paint having a viscosity of about 10 to 3000 mPa · s (B-type viscosity, 60 rpm, 20 ° C.) when using a dispersion of cellulose nanofibers obtained from a conventional cellulose-derived raw material derived from wood, cellulose The concentration of nanofibers had to be reduced to about 0.05 to 0.4% (w / v). Therefore, in order to produce a film having a thickness of about 5 to 30 μm, it has been necessary to repeat coating and drying many times.

Claims (6)

(A) A hardwood-derived pulp having a hemicellulose content of less than 17% by weight in the presence of a compound selected from the group consisting of (a1) N-oxyl compounds and (a2) bromides, iodides or mixtures thereof. (A3) a step of oxidizing using an oxidizing agent, and (B) a dispersion having a concentration of oxidized pulp obtained in step A of 1% (w / v) or more prepared, and the oxidized pulp A process of defibrating and dispersing in a dispersion medium to form nanofibers,
The manufacturing method of the cellulose nanofiber containing this. The manufacturing method of Claim 1 whose density | concentration of the oxidized pulp in the dispersion liquid in the said process B is 1-5% (w / v).   The manufacturing method of Claim 1 or 2 whose B-type viscosity (60 rpm, 20 degreeC) of the aqueous dispersion in the density | concentration 1% (w / v) of the obtained cellulose nanofiber is 10-3000 mPa * s. Carboxyl group amount of oxidized pulp obtained in step (A) is in the 1.0 mmol / g or more relative to the absolute dry weight of oxidized pulp in any one of claims 1 to 3 The manufacturing method as described. The step (B) performing defibration and dispersion treatment under a pressure of more than 50MPa in process according to any one of claims 1 to 4. The manufacturing method according to any one of claims 1 to 5, wherein the hardwood-derived pulp in the step (A) is a hardwood-derived kraft pulp.