WO2012132663A1

WO2012132663A1 - Method for producing cellulose nanofibers

Info

Publication number: WO2012132663A1
Application number: PCT/JP2012/054361
Authority: WO
Inventors: 志穂辻; 宮脇　正一; 知章小柳
Original assignee: 日本製紙株式会社
Priority date: 2011-03-30
Filing date: 2012-02-23
Publication date: 2012-10-04
Also published as: JPWO2012132663A1

Abstract

Cellulose nanofibers are produced by means of a method involving a step (A) in which a cellulose-based starting material derived from hardwood having a hemicellulose content of 17 mass% or more is oxidized using an oxidizing agent (a3) in the presence of an N-oxyl compound (a1) and a compound (a2) selected from among bromide, iodide and a mixture thereof, and a step (B) in which a dispersion liquid in which the concentration of the oxidized cellulose-based starting material obtained from step (A) is 0.3% (w/v) or more is prepared and the oxidized cellulose-based starting material is formed into nanofibers by fibrillating the oxidized cellulose-based starting material while dispersing same in a dispersion medium.

Description

Method for producing cellulose nanofiber

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 aqueous dispersion of high transparency.

When a cellulosic raw material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as TEMPO) and an inexpensive oxidizing agent sodium hypochlorite, Carboxyl groups can be efficiently introduced onto the surface of cellulose microfibrils (oxidation of cellulosic materials). 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 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. .

However, cellulose nanofibers obtained by oxidizing cellulose based raw materials (hardwood pulp) derived from hardwood using TEMPO and then defibrating with an ultra-high pressure homogenizer have low transparency when used as a dispersion. There was a problem that pulp fibers that did not become remained. For this reason, the conventional cellulose nanofiber has not been easily applied to the above-described uses.

JP 2008-001728 A JP 2010-235679 A

An object of the present invention is to provide a cellulose nanofiber that gives a highly transparent cellulose nanofiber dispersion.

As a result of intensive studies, the present inventors have used the above-mentioned cellulose-based raw material derived from hardwood having a hemicellulose content of at least mass% as a cellulose-based raw material to be subjected to an oxidation step using an N-oxyl compound such as TEMPO. I found that the problem could be solved. That is, the said subject is solved by the following this invention.
(A) The 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 17% by mass or more (A3) a step of oxidizing using an oxidizing agent, and (B) preparing a dispersion having a concentration of the oxidized cellulose-based raw material obtained in the step A of 0.3% (w / v) or more, A method for producing cellulose nanofiber, comprising a step of fibrillating and forming nanofibers while dispersing an oxidized cellulose raw material in a dispersion medium.

According to the present invention, it is possible to provide a cellulose nanofiber that gives a highly transparent cellulose nanofiber dispersion.

Hereinafter, the present invention will be described in detail. In the present specification, “to” used in representing 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 derived from hardwood having a hemicellulose content of 17% by mass or more, (a1) an N-oxyl compound, and (a2) bromide, iodide. Alternatively, in the presence of a compound selected from the group consisting of these mixtures, (a3) the step of oxidizing using an oxidizing agent, and (B) the concentration of the oxidized cellulose-based raw material obtained in step A is 0.3% (W / v) A step of preparing a dispersion liquid as described above and defibrating and dispersing into nanofibers while dispersing the oxidized cellulose raw material in a dispersion medium.

1-1. Process A
In step A, a cellulosic material derived from hardwood having a hemicellulose content of 17% by mass or more is oxidized under specific conditions.

(1) Cellulose-based raw material The “cellulosic raw material” used in the oxidation step (step A) performed using an N-oxyl compound or the like in the present invention is derived from a broad-leaved tree (hereinafter referred to as a “hard-leaved-derived cellulose-based raw material”) And the hemicellulose content is 17% by mass or more. 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 this invention, the cellulose nanofiber which is excellent in transparency can be manufactured when it is set as a dispersion liquid by using the cellulose raw material derived from hardwood with hemicellulose content of 17 mass% or more. Further, when a hardwood-derived cellulose material having a hemicellulose content of 23% by mass or more, more preferably 25% by mass or more, and further preferably 28% by mass or more is used, a cellulose nanofiber dispersion having excellent transparency can be obtained. It is preferable because not only the amount of carboxyl groups introduced into the cellulose by oxidation increases, but the amount of energy required for defibration (step B) decreases. This mechanism will be described later. Although the upper limit of hemicellulose content of a cellulosic raw material is not limited, Usually, what is necessary is just 30 mass% or less.

The hardwood-derived cellulose-based material refers to hardwood-derived pulp, powdered cellulose obtained by pulverizing hardwood-derived pulp 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, etc. .

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. . Among them, in the present invention, 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, so that the lignin content is reduced by the chemical pulp manufacturing method It is preferable to use it as a cellulosic material. 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 may be performed by C / D-EHD, ZEDP, Z / D-Ep-D, Z / D-Ep-DP, 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-DED. “/” 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, and more desirably 85% or more. The whiteness of the cellulosic material (pulp) correlates with the amount of lignin remaining in the pulp. Cellulose-based raw materials with a small amount of lignin, that is, a high whiteness, facilitate the introduction of carboxyl groups into the cellulose chain because the oxidation reaction proceeds well in step A, and are easy to disperse in the next step B. This is preferable because cellulose nanofibers that give a high dispersion can be provided.

The method for obtaining a broad-leaved cellulosic material having a hemicellulose content of 17% by mass or more is not particularly limited. Generally, the ratio of hemicellulose in a raw material falls by the process at the time of manufacturing a chemical pulp, and said bleaching process. In addition, the proportion of hemicellulose originally contained varies depending on the species of the broad-leaved tree, and further, the degree of decrease due to the above treatment also varies depending on the species. Therefore, for example, depending on the type of tree used, by appropriately adjusting each condition in the pulping treatment and bleaching treatment (chemical treatment concentration, treatment time, treatment temperature, etc.), it is derived from a broad-leaved tree having a hemicellulose content of 17% by mass or more. The cellulosic material can be obtained.

“Cellulose-based raw materials derived from broad-leaved trees” that can be preferably used in the present invention include, for example, Eucalyptus, Birchula, Begus, Fagus, Acer, Populus , Genus Fraxinus, or hardwood such as Carpinus. Eucalyptus (hereinafter abbreviated as E.) calophylla, E .; citriodora, E .; diversiccolor, E.I. globulus, E.I. grandis, E .; gummifera, E .; marginata, E.M. nesophila, E.I. nitens, E.I. amygdalina, E .; camaldulensis, E .; delegenasis, E.I. gigantea, E .; muelleriana, E.M. obliqua, E .; regnans, E .; Sieberiana, E.I. viminalis, E .; camaldulensis, E .; marginata and the like. As birch genus Betala (hereinafter abbreviated as B.), ermanii, B.M. pendula, B.M. populinifolia, B.I. carpinifolia, B.I. mandhurica, B.M. verrucosa, B.M. papyrifera, B. et al. alleghaniensis and the like. As the beech genus Fagus (hereinafter abbreviated as F.), grandifolia, F.M. orientalis, F.M. sylvatica, F.M. Crenata can be mentioned. As a maple genus Acer (hereinafter abbreviated as A.), A. camprestre, A.M. dasycarpum, A.M. ginnal, A.M. platanoides, A.M. mono, A.M. spicatum, A. apicatum, A.M. saccharinum, A.M. rubrum, A.R. pseudoplatanus and the like. As the genus Populus (hereinafter abbreviated as P.), P. maximowiczi, P.M. alba, P.I. sieboldii, P.A. coreana, P.M. deltoides, P.A. grandidentata, P.M. tacamachaca, P.M. tremuloids, P.M. and trichocarpa. As the genus Fraxinus (hereinafter abbreviated as F.), americana, F.A. excelsior, F.M. mandhurica, F.M. pnesylvanica and the like. Examples of the genus Carpinus (hereinafter abbreviated as C.) include C.I. belulus, C.I. cordadata, C.I. japonica, C.I. laxiflora, C.I. tschonoskii, C.I. turczaninovii and the like.

When these various broad-leaved trees are subjected to pulping treatment and bleaching treatment under the same conditions, the content of hemicellulose contained in the obtained cellulose raw material varies depending on the tree species. In particular, the birch genus (Betula) is preferably used in the present invention because it tends to maintain a relatively high hemicellulose content even after pulping or bleaching to obtain a cellulose material with high whiteness. Can do. A cellulose-based raw material having a high whiteness and a high hemicellulose content has an advantage that a cellulose nanofiber dispersion having a high transparency can be produced as described above. Most preferred is a cellulosic material having a whiteness of 85% or more and a hemicellulose content of 23% by mass or more.

The hemicellulose content of the cellulosic material can be measured as follows. After reacting 300 mg of lyophilized cellulosic raw material in 3 mL of 72 mass% sulfuric acid at room temperature for 2 hours, diluted to a sulfuric acid concentration of 2.5 mass%, and further heated at 105 ° C. for 1 hour, A monosaccharide solution is obtained by an acid hydrolysis reaction. This acid hydrolyzed solution is diluted, and monosaccharides are quantified using ion chromatography (DX-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 (mass%) = (xylose amount (mg) × 0.88 + mannose amount (mg) × 0.9) / cellulose raw material amount (mg) × 100 (%)

(2) N-oxyl compound (a1)
An N-oxyl compound refers to 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 to R ₄ are the same or different and each represents an alkyl group having about 1 to 4 carbon atoms.

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 the formulas 2 to 4, R is a straight 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 cellulosic raw material in a short time and 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 -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 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 even more preferable with respect to 1 g of cellulosic raw material.

(3) 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.

(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 to be used varies depending on the hardwood species to be used. For example, 0.5 to 500 mmol is preferable, 0.5 to 50 mmol is more preferable, and 0.5 to 50 mmol is more preferable with respect to 1 g of completely dry cellulosic material. 5 to 25 mmol is more preferable, and 5 to 20 mmol is most preferable.

(5) Conditions for oxidation reaction of cellulosic raw material In this step, the reaction can 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 make the oxidation reaction proceed efficiently, an alkaline solution such as an aqueous sodium hydroxide solution is added 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, 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-based material is 1.0 mmol / g or more with respect to the absolutely dry mass of the oxidized cellulose-based 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, 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.
The carboxyl group amount of the oxidized cellulose-based raw material was adjusted to pH 2 by preparing 60 ml of a 0.5 mass% slurry (= 0.5% (w / v) aqueous dispersion) of the oxidized cellulose-based raw material and adding 0.1 M hydrochloric acid aqueous solution. Then, 0.05N sodium hydroxide aqueous solution was added dropwise to measure the electric conductivity until the pH reached 11, and the hydroxylation consumed in the neutralization step of the weak acid where the change in electric conductivity was slow. It can calculate from the amount of sodium (a) using the following formula.
Amount of carboxyl group [mmol / g oxidized cellulose material] = a [ml] × 0.05 / oxidized cellulose material mass [g]

In order to efficiently perform defibration in the next step B, it is preferable to wash the oxidized cellulosic material obtained in step A.

1-2. Process B
In Step B, using the oxidized cellulose-based material obtained in Step A, a dispersion having a concentration of the oxidized cellulose-based material of 0.3% (w / v) or higher is prepared, and the oxidized cellulose-based material is used as a dispersion medium. While being dispersed, it is defibrated into nanofibers. “To make nanofiber” means to process a cellulose-based 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. The dispersion of oxidized cellulose material is a liquid in which the oxidized cellulose material is dispersed in a dispersion medium. From the viewpoint of ease of handling, the dispersion medium is preferably water.

In order to defibrate while dispersing the oxidized cellulose raw material in a dispersion medium, a strong shearing force is applied to the dispersion using a high-speed rotating type, colloid mill type, high pressure type, roll mill type, ultrasonic type device, etc. Is preferably applied. 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-based material in the dispersion to be subjected to the treatment is 0.3% (w / v) or more, preferably 1 to 2% (w / v), more preferably 3 to 5% (w / V).

1-3. In the present invention, in order to reduce the viscosity of the cellulose nanofiber dispersion and improve the handleability, the oxidized cellulose raw material obtained in step A (hereinafter simply referred to as “ It is preferable to reduce the viscosity of the “oxidized cellulose 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 treated in this way has a low viscosity when used as a dispersion, it can be said that the viscosity reduction treatment is a treatment for obtaining an oxidized cellulose-based raw material that gives a low-viscosity dispersion. The viscosity-reducing treatment may be any treatment that lowers the viscosity of the oxidized cellulose-based material. For example, the treatment of irradiating the oxidized cellulose-based material with ultraviolet rays, the oxidized cellulose-based material with hydrogen peroxide and Examples include a treatment for oxidizing and decomposing with ozone, a treatment for hydrolyzing an oxidized cellulose 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 perform the viscosity reduction treatment, the wavelength of the ultraviolet rays is preferably 100 to 400 nm, more preferably 100 to 300 nm. Of these, ultraviolet rays having a wavelength of 135 to 260 nm are particularly preferred because they directly act on cellulose and hemicellulose to cause low molecular weight and shorten the oxidized cellulose raw material.

As the light source for irradiating ultraviolet rays, a light source capable of irradiating light in a wavelength region of 100 to 400 nm may be used. 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 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 cellulose nanofibers in the dispersion is preferably 0.1% by mass or more. The concentration is preferably 12% by mass or less in order to improve the fluidity of the oxidized cellulose-based raw material in the ultraviolet irradiation device and increase the reaction efficiency. Accordingly, 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.

Also, 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 oxidized cellulose-based raw material may be deteriorated and the pressure in the reaction apparatus may exceed atmospheric pressure. Therefore, the reaction temperature is preferably 95 ° C. or lower. Accordingly, 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 oxidized cellulose raw material in the irradiation reaction apparatus or the energy amount of the irradiation light source. Also, for example, by diluting the concentration of the oxidized cellulose 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 amount of ultraviolet radiation received by the oxidized cellulose raw material can be reduced. Can be adjusted. These conditions are appropriately selected so that the quality (fiber length, degree of cellulose polymerization, etc.) of the oxidized cellulose-based raw material after 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 irradiating ultraviolet rays in the wavelength region of 135 to 242 nm, ozone is generated from oxygen in the air normally present in the gas phase 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 multiple times. The number of repetitions can be appropriately set according to the relationship with the quality of the oxidized cellulose-based 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 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 an oxidized cellulose-based material in a dispersion medium such as water for the reaction. The amount (mass) of ozone used in the present invention is preferably 0.1 to 3 times the absolute dry mass of the oxidized cellulose raw material. If the amount of ozone used is at least 0.1 times the absolute dry mass of the oxidized cellulose raw material, the amorphous part of cellulose can be sufficiently decomposed, and the energy required for the defibration / dispersion treatment in the next step B Can be greatly reduced. On the other hand, when the amount of ozone used is excessively large, excessive decomposition of cellulose can occur, but when the amount used is three times or less the absolute dry mass of the oxidized cellulose-based raw material, excessive decomposition can be suppressed. Therefore, the amount of ozone used is more preferably 0.3 to 2.5 times, and even more preferably 0.5 to 1.5 times the absolute dry mass of the oxidized cellulose raw material.

The amount (mass) of hydrogen peroxide used is preferably 0.001 to 1.5 times the absolute dry mass of the oxidized cellulose material. When hydrogen peroxide is used in an amount 0.001 times or more that of the oxidized cellulose 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 oxidized cellulose-based material, it is sufficient to use hydrogen peroxide in an amount about 1.5 times or less that of the cellulose-based material, and a larger amount used 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 oxidized cellulose raw 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 pH 4 to 10, and further preferably pH 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 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 effectively work even in the defibration treatment of the next step B, so that cellulose nanofibers that give a dispersion with a lower viscosity can be produced.

The reason why the low-viscosity treatment of the oxidized cellulose raw material can be efficiently performed with hydrogen peroxide and ozone is presumed as follows. A carboxyl group is localized on the surface of the cellulosic raw material oxidized with 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 the microscopic gaps, and efficiently oxidatively decompose cellulose chains in the raw material to shorten the oxidized cellulose raw material into short fibers.

(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 absolutely dry mass of the oxidized cellulose raw 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 carried out at a temperature of 70 to 120 ° C. for 1 to 10 hours.

In order to efficiently perform the treatment of 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 reduced viscosity of the oxidized cellulose raw material 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 with the N-oxyl compound, a hydration 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, the strong network of cellulose molecules is lost, the specific surface area of the raw material is increased, and the oxidized cellulose raw material is shortened. Is promoted, and the viscosity of the cellulosic material is reduced.

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 2 to 5 nm and a length of about 1 to 5 μm. The dispersion of cellulose nanofibers obtained by the present invention has excellent transparency. The dispersion medium in the dispersion is preferably water. In the present invention, the transparency is evaluated by the transmittance of light having a wavelength of 660 nm. Specifically, using a UV / visible spectrophotometer, a 0.1% (v / w) dispersion in a quartz cell (optical path 10 mm). It is calculated | required by measuring the quantity of the light which permeate | transmits the test body which put in.

The aqueous dispersion of cellulose nanofibers obtained by the present invention has a light transmittance of 660 nm at a concentration of 0.1% (w / v), preferably 94% or more, more preferably 95% or more. Preferably it is 97% or more, particularly preferably 99% or more. When the transparency is 95% or more, it can be used without problems for general film applications, and when the transparency is 99% or more, high optical properties (transparency) such as a display and a touch panel are required. Can be used without problems for film applications.

In the present invention, by setting the hemicellulose content in the cellulose-based raw material derived from hardwood to 17% by mass or more, cellulose nanofibers that give such a highly transparent aqueous dispersion can be obtained. Although this reason is not limited, it is guessed as follows.

The cellulose-based raw material derived from hardwood is considered to be superior as a raw material for cellulose nanofiber because the fibers are thinner and shorter than the cellulose-based raw material derived from conifer. In general, cellulosic materials have hemicellulose between cellulose microfibrils. If hemicellulose is present between cellulose microfibrils, it is considered that voids are formed between cellulose microfibrils because hemicellulose is eluted during an oxidation reaction under alkaline conditions. Thereby, it is estimated that the catalyst easily reaches the surface of the cellulose microfibril, and the primary hydroxyl group on the surface of the cellulose microfibril is selectively oxidized. As a result, it is considered that carboxyl groups are uniformly introduced on the surface of the cellulose microfibril and nano-dispersion is facilitated by charge repulsion. As in the present invention, a cellulose-based raw material derived from hardwood having a high hemicellulose content of 17% by mass or more, preferably 23% by mass or more, more preferably 25% by mass or more, and further preferably 28% by mass or more, as in the present invention. It is presumed that the nano-dispersion of cellulose microfibrils is further promoted by using.

Further, the amount of carboxyl groups of the cellulose nanofibers produced according to the present invention is preferably 1.2 mmol / g or more, more preferably 1.7 mmol / g or more, and further preferably 1.8 mmol / g or more. The amount of carboxyl groups in cellulose nanofibers was prepared by preparing 60 ml of a 0.5% by weight slurry (= 0.5% (w / v) aqueous dispersion) of cellulose nanofibers, adding 0.1 M hydrochloric acid aqueous solution to pH 2.5. 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 cellulose nanofiber] = a [ml] × 0.05 / cellulose nanofiber mass [g]

Such a cellulose nanofiber having a carboxyl group can form a sheet excellent in barrier properties and heat resistance. Therefore, the cellulose nanofiber manufactured by this invention can be used for various uses, such as a packaging material.

Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
Both are hardwoods. ermanii and B.M. 5 g (absolutely dry) of bleached unbeaten kraft pulp (hemicellulose content 28.4 mass%, whiteness 85%) derived from a mixture of pendula (mixing ratio 50:50), 78 mg of TEMPO (Sigma Aldrich) 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.

2 L of an aqueous dispersion containing 1% (w / v) (= 1% by mass) of oxidized pulp obtained by the above reaction was prepared, and irradiated with ultraviolet rays with a 20 W low-pressure ultraviolet lamp for 6 hours while flowing the aqueous solution. Then, it processed 10 times with the ultrahigh pressure homogenizer (20 degreeC, 140 Mpa), and obtained the transparent gel-like dispersion liquid.

Water is added to the dispersion thus obtained to prepare a cellulose nanofiber dispersion having a concentration of 0.1% (w / v), and the transparency (transmittance of 660 nm light) is measured with a UV-VIS spectrophotometer UV-VIS. Measurement was performed using 265FS (Shimadzu Corporation). The results are shown in Table 1.

[Example 2]
P. is a hardwood. A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from alba (having a hemicellulose content of 17.9% by mass and a whiteness of 86%) was used.

[Example 3]
Both are hardwoods. Crenata and C.I. Cellulose nanofibers in the same manner as in Example 1 except that bleached unbeaten kraft pulp (hemicellulose content 22.1% by mass, whiteness 84%) derived from a mixture with belulus (mixing ratio 50:50) was used. An aqueous dispersion was produced and evaluated.

[Example 4]
B. a hardwood. A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from pendula (hemicellulose content: 23.3 mass%, whiteness: 86%) was used.

[Example 5]
B. a hardwood. A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from verrucosa (having a hemicellulose content of 25.6% by mass and a whiteness of 86%) was used.

[Comparative Example 1]
R. hardwood. A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from pseudoacacia (having a hemicellulose content of 16.7% by mass and a whiteness of 85%) was used.

[Comparative Example 2]
A. Hardwood A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from Mangiumu (hemicellulose content 12.3 mass%, whiteness 86%) was used.

[Comparative Example 3]
U. is a hardwood. A cellulose nanofiber aqueous dispersion was produced and evaluated in the same manner as in Example 1 except that bleached unbeaten kraft pulp derived from Americana (hemicellulose content 15.1% by mass, whiteness 85%) was used.

From this result, the Example using the cellulosic raw material derived from hardwood having a hemicellulose content of 17% by mass or more is more transparent than the comparative example using the cellulosic raw material having a hemicellulose content of less than 17% by mass. It can be seen that cellulose nanofibers that give a high aqueous dispersion can be produced.

Claims

(A) The 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 17% by mass or more (A3) a step of oxidizing using an oxidizing agent, and (B) preparing a dispersion having a concentration of the oxidized cellulose raw material obtained in the step A of 0.3% (w / v) or more, A process of defibration and nanofiber formation while dispersing an oxidized cellulose-based raw material in a dispersion medium,
The manufacturing method of the cellulose nanofiber containing this.
The manufacturing method of the cellulose nanofiber of Claim 1 using the cellulose raw material derived from a hardwood whose hemicellulose content is 23 mass% or more.
The manufacturing method of the cellulose nanofiber of Claim 2 whose whiteness (ISO 2470) of the cellulose raw material derived from hardwood is 85% or more.
The cellulose nanofiber according to any one of claims 1 to 3, wherein the transmittance of light at a wavelength of 660 nm of the aqueous dispersion at a concentration of 0.1% (w / v) of the obtained cellulose nanofiber is 94% or more. Fiber manufacturing method.
The amount of carboxyl groups of the oxidized cellulose raw material obtained in the step (A) is 1.2 mmol / g or more with respect to the absolute dry mass of the oxidized cellulose raw material. The manufacturing method of the cellulose nanofiber of crab.
The production method according to any one of claims 1 to 5, wherein defibration is performed in the step (B) under a pressure of 50 MPa or more.