WO2023188554A1 - Method for producing cellulose nanofiber and cellulose nanofiber - Google Patents

Abstract

[Problem] To provide: a method for producing a highly transparent cellulose nanofiber; and a cellulose nanofiber. [Solution] Provided is a method for producing cellulose nanofibers, the method being characterized by adding a dilute alkali to a carbamated cellulose fiber to make the same finer. Also provided is a cellulose nanofiber obtained by said method.

Description

Method for producing cellulose nanofibers and cellulose nanofibers

The present invention relates to a method for producing cellulose nanofibers and cellulose nanofibers.

In recent years, the use of fine fibers such as cellulose nanofibers and microfiber cellulose (microfibrillated cellulose) as reinforcing materials for resins has been in the spotlight. However, while fine fibers are hydrophilic, resins are hydrophobic, and therefore, when fine fibers are used as reinforcing materials for resins, there is a problem with the dispersibility of the fine fibers. Therefore, the present inventors proposed replacing the hydroxy groups of the fine fibers with carbamate groups (see Patent Document 1). According to this proposal, the dispersibility of fine fibers is improved, thereby improving the reinforcing effect of the resin. However, this proposal is intended to improve the handling properties of the fine fibers and the reinforcing properties of the resin, and is not intended to improve the transparency of the fine fibers.

JP 2019-1876 Publication

The problem to be solved by the present invention is to provide a method for producing highly transparent cellulose nanofibers and cellulose nanofibers.

Carbamate cellulose fibers are soluble in dilute alkali, and coagulation regenerates cellulose fibers. Utilizing the mechanism by which cellulose fibers are soluble in dilute alkali, we have come up with the following means for solving the above problems.

That is, the method for producing cellulose nanofibers is characterized by adding dilute alkali to carbamate cellulose fibers to make them fine.

It is also a cellulose nanofiber obtained by this method.

According to the present invention, a highly transparent cellulose nanofiber manufacturing method and cellulose nanofiber are obtained.

Next, a mode for carrying out the invention will be described. Note that this embodiment is an example of the present invention. The scope of the present invention is not limited to the scope of this embodiment.

The method for producing cellulose nanofibers of this embodiment mainly includes the following steps.
(1) Carbamate the raw material pulp (2) Addition of dilute alkali, preferably dilute alkali and additives (3) Refinement

More preferably, the raw material pulp is microfibrillated into microfiber cellulose (MFC) prior to the addition of the dilute alkali and additives in (2) above. Below, they will be explained in order.

(Raw material pulp)
Raw material pulp includes, for example, wood pulp made from hardwoods, coniferous trees, etc., non-wood pulp made from straw, bagasse, cotton, linen, bark fiber, etc., and waste paper pulp made from recycled waste paper, waste paper, etc. (DIP) and the like, one or more types can be selected and used. Note that the various raw materials mentioned above may be in the form of a pulverized material (powdered material) called, for example, cellulose powder.

However, in order to avoid contamination with impurities as much as possible, it is preferable to use wood pulp as the raw material pulp. As the wood pulp, one or more types can be selected and used from, for example, chemical pulps such as hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), mechanical pulp (TMP), and the like.

The hardwood kraft pulp may be a bleached hardwood kraft pulp, an unbleached hardwood kraft pulp, or a semi-bleached hardwood kraft pulp. Similarly, the softwood kraft pulp may be a bleached softwood kraft pulp, an unbleached softwood kraft pulp, or a semi-bleached softwood kraft pulp.

Mechanical pulps include, for example, stone ground pulp (SGP), pressurized stone ground pulp (PGW), refiner ground pulp (RGP), chemical ground pulp (CGP), thermoground pulp (TGP), ground pulp (GP), One or more types can be selected and used from thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), refiner mechanical pulp (RMP), bleached thermomechanical pulp (BTMP), and the like.

(Carbamate formation)
The raw material pulp is carbamated to obtain carbamate cellulose fibers (carbamate cellulose fibers).

Note that carbamate formation refers to a state in which a carbamate group (carbamic acid ester) is introduced into cellulose fibers. The carbamate group is a group represented by -O-CO-NH-, for example, a group represented by -O-CO-NH ₂ , -O-CONHR, -O-CO-NR ₂ and the like. That is, the carbamate group can be represented by the following structural formula (1).

Here, R is each independently a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, At least one of an aromatic group and a group derived therefrom.

Examples of the saturated linear hydrocarbon group include linear alkyl groups having 1 to 10 carbon atoms such as methyl group, ethyl group, and propyl group.

Examples of the saturated branched hydrocarbon group include branched alkyl groups having 3 to 10 carbon atoms such as isopropyl group, sec-butyl group, isobutyl group, and tert-butyl group.

Examples of the saturated cyclic hydrocarbon group include cycloalkyl groups such as a cyclopentyl group, a cyclohexyl group, and a norbornyl group.

Examples of the unsaturated linear hydrocarbon group include linear alkenyl groups having 2 to 10 carbon atoms such as ethenyl group, propen-1-yl group, propen-3-yl group, ethynyl group, propyn-1 Examples include straight-chain alkynyl groups having 2 to 10 carbon atoms such as -yl group and propyn-3-yl group.

Examples of the unsaturated branched hydrocarbon group include branched alkenyl groups having 3 to 10 carbon atoms such as propen-2-yl group, buten-2-yl group, buten-3-yl group, butyn-3 Examples include branched alkynyl groups having 4 to 10 carbon atoms such as -yl group.

Examples of the aromatic group include phenyl group, tolyl group, xylyl group, and naphthyl group.

Examples of the derivative group include the above-mentioned saturated linear hydrocarbon groups, saturated branched hydrocarbon groups, saturated cyclic hydrocarbon groups, unsaturated linear hydrocarbon groups, unsaturated branched hydrocarbon groups, and aromatic groups. Examples include groups in which one or more hydrogen atoms of the group are substituted with a substituent (eg, a hydroxy group, a carboxy group, a halogen atom, etc.).

In carbamate cellulose fibers, some or all of the highly polar hydroxy groups are substituted with relatively less polar carbamate groups. Therefore, carbamate cellulose fine fibers obtained by refining carbamate cellulose fibers have low hydrophilicity, have low viscosity, and have good handling properties.

The substitution ratio of carbamate groups to hydroxy groups in cellulose fibers is preferably 0.5 to 5.0 mmol/g, more preferably 1.0 to 3.0 mmol/g, particularly preferably 1.5 to 2.0 mmol/g. It is. Considering operability, it is preferable to set the substitution rate to 0.5 mmol/g or more because the energy required for microfibrillation is reduced. On the other hand, if the substitution rate exceeds 5.0 mmol/g, the cellulose fibers will have difficulty maintaining their fiber shape, and when dilute alkali and additives are added, they will dissolve and cellulose nanofibers will not be obtained.

In this embodiment, the substitution rate of carbamate groups (mmol/g) refers to the amount of carbamate groups contained per 1 g of cellulose raw material having carbamate groups. The substitution rate of carbamate groups is determined by measuring the N atoms present in the carbamate-formed pulp by the Kjeldahl method, and calculating the carbamate conversion rate per unit weight. Further, cellulose is a polymer having anhydroglucose as a structural unit, and has three hydroxy groups per structural unit.

The process of carbamateing raw material pulp can be mainly divided into, for example, mixing treatment, removal treatment, and heat treatment. Note that the mixing treatment and the removal treatment can also be collectively referred to as a preparation treatment for preparing a mixture to be subjected to heat treatment.

Incidentally, as a method for carbamate formation, for example, there is a method in which raw material pulp is formed into a sheet, and this sheet-shaped raw material pulp is coated with urea or the like and heat treated, that is, a method that is not a mixing treatment. In this embodiment, this method of forming into a sheet is not denied, and below, as an example, a mode in which raw material pulp, urea, etc. are mixed and processed will be explained in detail.

In the mixing process, cellulose fibers (raw pulp) and urea or a derivative of urea (hereinafter also simply referred to as "urea etc.") are mixed in a dispersion medium.

Examples of urea and urea derivatives include urea, thiourea, biuret, phenylurea, benzylurea, dimethylurea, diethylurea, tetramethylurea, and compounds in which the hydrogen atom of urea is replaced with an alkyl group. can. These urea or urea derivatives can be used alone or in combination.

The lower limit of the mixing mass ratio of urea etc. to cellulose fibers (urea etc./cellulose fibers) is preferably 1/100, more preferably 10/100. On the other hand, the upper limit is preferably 300/100, more preferably 200/100. By setting the mixing mass ratio to 1/100 or more, the efficiency of carbamate formation is improved. On the other hand, even if the mixing mass ratio exceeds 300/100, carbamate formation reaches a ceiling.

The dispersion medium is usually water. However, other dispersion media such as alcohol and ether, or a mixture of water and other dispersion media may also be used.

In the mixing treatment, for example, cellulose fibers and urea may be added to water, cellulose fibers may be added to an aqueous solution of urea, or urea may be added to a slurry containing cellulose fibers. Further, in order to mix uniformly, it may be stirred after addition. Furthermore, the dispersion containing cellulose fibers, urea, etc. may contain other components.

In the removal treatment, the dispersion medium is removed from the dispersion containing cellulose fibers, urea, etc. obtained in the mixing treatment. By removing the dispersion medium, urea and the like can be efficiently reacted in the subsequent heat treatment.

The removal of the dispersion medium is preferably performed by volatilizing the dispersion medium by heating. According to this method, only the dispersion medium can be efficiently removed while leaving components such as urea.

The lower limit of the heating temperature in the removal treatment is preferably 50°C, more preferably 70°C, particularly preferably 90°C when the dispersion medium is water. By setting the heating temperature to 50° C. or higher, the dispersion medium can be efficiently volatilized (removed). On the other hand, the upper limit of the heating temperature is preferably 120°C, more preferably 100°C. If the heating temperature exceeds 120° C., the dispersion medium and urea may react, and urea may decompose alone.

The heating time in the removal treatment can be adjusted as appropriate depending on the solid content concentration of the dispersion. Specifically, it is, for example, 1 to 24 hours.

In the heat treatment following the removal treatment, the mixture of cellulose fibers and urea etc. is heat treated. In this heat treatment, some or all of the hydroxyl groups of the cellulose fibers react with urea or the like and are replaced with carbamate groups. More specifically, when urea or the like is heated, it is decomposed into isocyanic acid and ammonia as shown in reaction formula (1) below. Isocyanic acid has very high reactivity, and for example, carbamate groups are formed on the hydroxyl groups of cellulose as shown in reaction formula (2) below.

NH ₂ -CO-NH ₂ → H-N=C=O + NH ₃ ...(1)

Cell-OH + H-N=C=O → Cell-O-CO-NH ₂ ...(2)

The lower limit of the heating temperature in the heat treatment is preferably 120°C, more preferably 130°C, particularly preferably at least the melting point of urea (about 134°C), even more preferably 140°C, and most preferably 150°C. By setting the heating temperature to 120° C. or higher, carbamate formation is efficiently performed. The upper limit of the heating temperature is preferably 280°C, more preferably 260°C, particularly preferably 240°C. If the heating temperature exceeds 280°C, the cellulose fibers may be decomposed and cellulose nanofibers may not be obtained.

The lower limit of the heating time in the heat treatment is preferably 10 seconds, more preferably 20 seconds, and particularly preferably 1 minute. By setting the heating time to 10 seconds or more, the carbamate reaction can be carried out reliably. On the other hand, the upper limit of the heating time is preferably 15 hours, more preferably 10 hours. If the heating time exceeds 15 hours, it is not economical, and 15 hours is enough to carry out carbamate formation.

However, prolonged heating time causes deterioration of the cellulose fibers. Therefore, the pH conditions in the heat treatment become important. The pH is preferably pH 9 or higher, more preferably pH 9 to 13, particularly preferably pH 10 to 12, which is an alkaline condition. Further, as a second best measure, acidic conditions or neutral conditions with a pH of 7 or less, preferably a pH of 3 to 7, particularly preferably a pH of 4 to 7 are preferred. Under neutral conditions of pH 7 to 8, the efficiency of the carbamate formation reaction is poor, and the heating time may be prolonged and the amount of chemicals may be required, which is not economical. On the other hand, under alkaline conditions with a pH of 9 or higher, the cellulose fibers swell and the reaction to urea etc. is promoted, resulting in an efficient carbamate reaction, making it possible to ensure a sufficient average fiber length of the cellulose fibers. . On the other hand, under acidic conditions with a pH of 7 or less, the reaction of decomposing urea etc. into isocyanic acid and ammonia proceeds, promoting the reaction to cellulose fibers and efficiently carrying out the carbamate reaction. However, if possible, it is preferable to perform the heat treatment under alkaline conditions. If the condition is acidic, acid hydrolysis of cellulose may proceed and the average fiber length may become short.

The pH can be adjusted by adding an acidic compound (for example, acetic acid, citric acid, etc.) or an alkaline compound (for example, sodium hydroxide, calcium hydroxide, etc.) to the mixture.

As a device for heating in the heat treatment, for example, a hot air dryer, a paper machine, a dry pulp machine, etc. can be used.

The mixture after heat treatment may be washed. This cleaning may be performed with water or the like. By this washing, unreacted residual urea and the like can be removed.

(microfibrillation)
Immediately after the cellulose fibers are carbamated, dilute alkali and additives can be added to make them fine, but in this embodiment, the carbamate cellulose fibers are first microfibrillated to form microfiber cellulose. Hereinafter, first, this microfibrillation will be explained.

Note that the carbamate-formed cellulose fibers can be pretreated by a chemical method prior to microfibrillation. Examples of pretreatments using chemical methods include hydrolysis of polysaccharides with acids (acid treatment), hydrolysis of polysaccharides with enzymes (enzyme treatment), swelling of polysaccharides with alkalis (alkali treatment), and oxidation of polysaccharides with oxidizing agents ( Examples include oxidation treatment), reduction of polysaccharide with a reducing agent (reduction treatment), and the like. However, as a pretreatment using a chemical method, it is preferable to perform enzyme treatment, and in addition, it is more preferable to perform one or more treatments selected from acid treatment, alkali treatment, and oxidation treatment. The enzyme treatment will be explained in detail below.

As the enzyme used for the enzyme treatment, it is preferable to use at least one of a cellulase enzyme and a hemicellulase enzyme, and it is more preferable to use both in combination. The use of these enzymes makes it easier to defibrate cellulose fibers. Note that cellulase enzymes cause the decomposition of cellulose in the presence of water. Furthermore, hemicellulase enzymes cause the decomposition of hemicellulose in the presence of water.

Examples of cellulase enzymes include Trichoderma, Acremonium, Aspergillus, Phanerochaete, and Trametes. It is produced by the genus Humicola, the genus Bacillus, the genus Schizophyllum, the genus Streptomyces, and the genus Pseudomonas. Enzymes can be used. These cellulase enzymes can be purchased as reagents or commercial products. Commercially available products include, for example, Cellleucine T2 (manufactured by HPI Co., Ltd.), Meicelase (manufactured by Meiji Seika Co., Ltd.), Novozyme 188 (manufactured by Novozyme Co., Ltd.), Multifect CX10L (manufactured by Genencor Co., Ltd.), and Cellulase-based enzyme GC220 (manufactured by Genencor Co., Ltd.). For example,

Further, as the cellulase enzyme, either EG (endoglucanase) or CBH (cellobiohydrolase) can be used. EG and CBH may be used alone or in combination. It may also be used in combination with hemicellulase enzymes.

Examples of hemicellulase enzymes that can be used include xylanase, an enzyme that decomposes xylan, mannase, an enzyme that decomposes mannan, and arabanase, an enzyme that decomposes alaban. can. Pectinase, which is an enzyme that degrades pectin, can also be used.

Hemicellulose is a polysaccharide excluding pectin, which is present between cellulose microfibrils in plant cell walls. Hemicellulose is diverse and varies depending on the type of wood and the wall layers of the cell wall. In the secondary wall of softwood, glucomannan is the main component, and in the secondary wall of hardwood, 4-O-methylglucuronoxylan is the main component. Therefore, when obtaining fine fibers from softwood bleached kraft pulp (NBKP), it is preferable to use mannase. Moreover, when obtaining fine fibers from hardwood bleached kraft pulp (LBKP), it is preferable to use xylanase.

The amount of enzyme added to cellulose fibers is determined by, for example, the type of enzyme, the type of wood used as a raw material (softwood or hardwood), the type of mechanical pulp, etc. However, the amount of enzyme added to the cellulose fibers is preferably 0.1 to 3% by mass, more preferably 0.3 to 2.5% by mass, particularly preferably 0.5 to 2% by mass. If the amount of the enzyme added is less than 0.1% by mass, there is a risk that the effect of the addition of the enzyme may not be sufficiently obtained. On the other hand, if the amount of enzyme added exceeds 3% by mass, cellulose may be saccharified and the yield of fine fibers may decrease. Another problem is that it is not possible to recognize an improvement in the effect commensurate with the increase in the amount added.

When using a cellulase enzyme as the enzyme, the pH during enzyme treatment is preferably in the weakly acidic region (pH = 3.0 to 6.9) from the viewpoint of reactivity of the enzyme reaction. On the other hand, when a hemicellulase enzyme is used as the enzyme, the pH during enzyme treatment is preferably in a weakly alkaline region (pH = 7.1 to 10.0).

The temperature during the enzyme treatment is preferably 30 to 70°C, more preferably 35 to 65°C, particularly preferably 40 to 60°C, regardless of whether a cellulase enzyme or a hemicellulase enzyme is used as the enzyme. . If the temperature during the enzyme treatment is 30° C. or higher, the enzyme activity will be less likely to decrease, and the treatment time can be prevented from becoming longer. On the other hand, if the temperature during enzyme treatment is 70° C. or lower, deactivation of the enzyme can be prevented.

The time for enzyme treatment is determined by, for example, the type of enzyme, the temperature of enzyme treatment, the pH at the time of enzyme treatment, etc. However, the general enzyme treatment time is 0.5 to 24 hours.

After the enzyme treatment, it is preferable to deactivate the enzyme. Examples of methods for inactivating enzymes include adding an alkaline aqueous solution (preferably pH 10 or higher, more preferably pH 11 or higher), adding hot water at 80 to 100°C, and the like.

Next, the method of alkali treatment will be explained.

When treated with alkali prior to fibrillation, some of the hydroxyl groups of hemicellulose and cellulose in the pulp dissociate, and the molecules become anions, weakening intramolecular and intermolecular hydrogen bonds, promoting the dispersion of cellulose fibers during fibrillation. Ru.

Examples of the alkali used in the alkali treatment include sodium hydroxide, lithium hydroxide, potassium hydroxide, ammonia aqueous solution, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, etc. Organic alkalis and the like can be used. However, from the viewpoint of manufacturing cost, it is preferable to use sodium hydroxide.

If enzyme treatment, acid treatment, or oxidation treatment is performed prior to fibrillation, the microfiber cellulose obtained by fibrillation can have lower water retention, higher crystallinity, and higher homogeneity. . In this regard, when the water retention degree of microfiber cellulose is low, it becomes easy to dehydrate, and the dehydration property of the cellulose fiber slurry improves.

When raw pulp is subjected to enzyme treatment, acid treatment, or oxidation treatment, the amorphous regions of hemicellulose and cellulose contained in the pulp are decomposed. As a result, the energy for fibrillation can be reduced, and the uniformity and dispersibility of cellulose fibers can be improved. However, it is preferable to avoid excessive pretreatment because it may reduce the aspect ratio of microfiber cellulose and dissolve when dilute alkali and additives are added, making it impossible to obtain cellulose nanofibers.

Fibrillation of cellulose fibers can be carried out using, for example, a beater, a high-pressure homogenizer, a homogenizer such as a high-pressure homogenizer, a grinder, a millstone friction machine such as a grinder, a single-shaft kneader, a multi-shaft kneader, a kneader refiner, a jet mill, etc. This can be done by beating cellulose fibers using However, it is preferable to use a refiner or a jet mill, and more preferably to use a single disc refiner (SDR).

The average fiber diameter (average fiber width; average diameter of single fibers) of the microfiber cellulose obtained by fibrillation is preferably 0.1 to 15 μm, more preferably 0.2 to 10 μm, particularly preferably 0.5 to 15 μm. It is 10 μm. If the average fiber diameter of microfiber cellulose is less than 0.1 μm, it may dissolve when dilute alkali and additives are added, and cellulose nanofibers may not be obtained. On the other hand, if the average fiber diameter of the cellulose microfibers exceeds 15 μm, the cellulose microfibers will not be different from pulp, and the light transmittance of the cellulose nanofibers may decrease.

The average fiber diameter of microfiber cellulose can be adjusted, for example, by the degree of fibrillation, pretreatment, etc.

The method for measuring the average fiber diameter of microfiber cellulose is as follows.
First, 100 ml of an aqueous dispersion of fine fibers (microfiber cellulose) with a solid content concentration of 0.01 to 0.1% by mass was filtered through a Teflon (registered trademark) membrane filter, filtered once with 100 ml of ethanol, and once with 20 ml of t-butanol. Replace the solvent three times with Next, it is freeze-dried, coated with osmium, and used as a sample. This sample is observed using an electron microscope SEM image at a magnification of 3,000 times to 30,000 times depending on the width of the constituent fibers. Specifically, two diagonal lines are drawn on the observed image, and three straight lines passing through the intersections of the diagonals are arbitrarily drawn. Furthermore, the widths of a total of 100 fibers that intersect with these three straight lines are visually measured. Then, the median diameter of the measured value is taken as the average fiber diameter.

The average fiber length (average length of single fibers) of microfiber cellulose is preferably 0.10 to 2.0 mm, more preferably 0.2 to 1.5 mm, particularly preferably 0.3 to 1.2 mm. be. If the average fiber length is less than 0.10 mm, there is a possibility that cellulose nanofibers will not be obtained due to dissolution during addition of dilute alkali and additives. On the other hand, if the average fiber length exceeds 2.0 mm, the light transmittance of the obtained cellulose nanofibers may decrease.

The average fiber length of microfiber cellulose can be arbitrarily adjusted, for example, by selecting the raw material pulp, pretreatment, fibrillation, etc.

The fine ratio A of microfiber cellulose is preferably 10 to 100%, more preferably 20 to 100%, and particularly preferably 25 to 100%. When the fine ratio A is 10% or more, the energy required for the process of refining after addition of dilute alkali and additives is reduced, and economical efficiency becomes advantageous.

On the other hand, the fine ratio B of the microfiber cellulose is preferably 1 to 50%, more preferably 2 to 40%, and particularly preferably 3 to 35%. If the Fine ratio B is less than 1%, the light transmittance of the obtained cellulose nanofibers may decrease because there are many fibers with short fiber length or many fibers with large fiber width. On the other hand, if the Fine ratio B exceeds 50%, the number of thin and long fibers increases, and the fibers may become entangled with each other and aggregate.

The fine ratios A and B can be adjusted by pretreatment such as enzyme treatment. However, especially in the case of enzyme treatment, if too much enzyme is added to the treatment, the fibers may be broken down to monosaccharides, which may reduce the yield of cellulose nanofibers. Therefore, from this point of view, the amount of enzyme added is preferably 2% by mass or less, more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less. Furthermore, one option is not to perform enzyme treatment (addition amount: 0% by mass).

The fine ratios A and B can be adjusted, for example, by a method of mixing two or more types of microfiber cellulose with different fine ratios. However, manufacturing efficiency is better if one cellulose raw material is simply refined to adjust the fine ratio. Therefore, for example, it is preferable to use a mixture of a plurality of pulp raw materials as the cellulose raw material.

Specifically, for example, it is preferable to use a pulp raw material in which NKP (softwood kraft pulp) and LKP (hardwood kraft pulp) are mixed, and 5 to 95% by mass of NKP (preferably NBKP) and LKP (preferably NBKP) are used. It is more preferable to use a pulp raw material consisting of 5 to 95 mass % of LBKP.), and it is particularly preferable to use a pulp raw material of 25 to 75 mass % of NKP and 25 to 75 mass % of LKP. NKP is characterized by having many long, hard (thick) fibers, and LKP is characterized by having many short, soft (thin) fibers, so according to the above blending ratio, Fine ratios A and B can be easily achieved. Can be adjusted.

In this embodiment, "Fine ratio A" refers to the mass-based ratio of cellulose fibers having a fiber length of 0.2 mm or less and a fiber width of 75 μm or less. Moreover, "Fine ratio B" refers to the mass-based ratio of cellulose fibers whose fiber length exceeds 0.2 mm and whose fiber width is 10 μm or less.

The aspect ratio of the microfiber cellulose is preferably 2 to 15,000, more preferably 10 to 10,000. When the aspect ratio is less than 2, the shape is not fibrous and becomes close to cellulose nanocrystals. On the other hand, when the aspect ratio exceeds 15,000, the microfiber cellulose becomes entangled with each other, and there is a possibility that the dispersion becomes insufficient.

Aspect ratio is the value obtained by dividing the average fiber length by the average fiber width. As the aspect ratio increases, the number of places where snags occur increases, so when cellulose nanofibers are used as an additive, the reinforcing effect increases, but on the other hand, the fibers become more entangled, which may worsen handling properties.

The fiber length, fine rate, etc. of microfiber cellulose are values measured using a fiber analyzer "FS5" manufactured by Valmet.

The fibrillation rate of microfiber cellulose is preferably 1.0 to 30.0%, more preferably 1.5 to 20.0%, particularly preferably 2.0 to 15.0%. When the fibrillation rate exceeds 30.0%, the contact area with water becomes too large, which may make dehydration difficult. On the other hand, if the fibrillation rate is less than 1.0%, the energy required for the process of refining after addition of dilute alkali and additives may increase, which may adversely affect economic efficiency.

In this embodiment, the fibrillation rate refers to cellulose fibers being disintegrated in accordance with JIS-P-8220:2012 "Pulp - Disintegration Method", and the obtained disintegrated pulp being processed by FiberLab. (Kajaani).

The crystallinity of the microfiber cellulose is preferably 50% or more, more preferably 55% or more, particularly preferably 60% or more. When the degree of crystallinity is less than 50%, there is a possibility that the cellulose nanofibers will be dissolved in dilute alkali and additives, making it difficult to obtain cellulose nanofibers. On the other hand, the upper limit of the crystallinity of microfiber cellulose is not limited, but it is thought that about 90% is the upper limit because the crystallinity decreases during beating and microfibrillation.

The degree of crystallinity of microfiber cellulose can be arbitrarily adjusted, for example, by selecting the raw material pulp, pretreatment, and refining treatment.

The crystallinity of microfiber cellulose is a value measured in accordance with JIS K 0131 (1996).

The pulp viscosity of the microfiber cellulose is preferably 2 cps or more, more preferably 4 cps or more. If the pulp viscosity of the microfiber cellulose is less than 2 cps, it may dissolve when dilute alkali and additives are added, and cellulose nanofibers may not be obtained.

The pulp viscosity of microfiber cellulose is a value measured in accordance with TAPPI T230.

The freeness of the microfiber cellulose is preferably 500 ml or less, more preferably 300 ml or less, particularly preferably 100 ml or less. When the freeness of the microfiber cellulose exceeds 500 ml, the beating from the pulp has not progressed, so the energy required for micronization after addition of dilute alkali and additives increases, and the light transmittance of the resulting cellulose nanofibers decreases. there is a possibility.

The freeness of microfiber cellulose is a value measured in accordance with JIS P8121-2 (2012).

The zeta potential of microfiber cellulose is preferably -150 to 20 mV, more preferably -100 to 0 mV, particularly preferably -80 to -10 mV. If the zeta potential is less than -150 mV, the amount of carbamate groups introduced may be low, and the light transmittance of cellulose nanofibers obtained by micronization after addition of dilute alkali and additives may decrease. On the other hand, when the zeta potential exceeds 20 mV, the amount of carbamate groups introduced is high, and there is a possibility that cellulose nanofibers will not be obtained due to dissolution when dilute alkali and additives are added.

The water retention degree of microfiber cellulose is preferably 30 to 400%, more preferably 90 to 350%, particularly preferably 100 to 300%. If the water retention is less than 30%, the light transmittance of cellulose nanofibers obtained by micronization after addition of dilute alkali and additives may decrease because it is the same as the raw material pulp. On the other hand, if the degree of water retention exceeds 400%, dehydration properties tend to be poor, and cellulose nanofibers may not be obtained due to dissolution in dilute alkali and additives. In this regard, the water retention of microfiber cellulose can be lowered by substituting the hydroxy groups of the fibers with carbamate groups, and the dehydration and drying properties can be improved.

The water retention degree of microfiber cellulose can be arbitrarily adjusted, for example, by selecting the raw material pulp, pretreatment, fibrillation, etc.

The water retention level of microfiber cellulose is JAPAN TAPPI No. 26 (2000).

(Addition of dilute alkali and additives)
Next, the addition of a dilute alkali and additives to microfiber cellulose obtained by fibrillating carbamate cellulose fibers will be explained.

First, if necessary, the carbamate microfiber cellulose is dispersed in an aqueous medium to form a dispersion (slurry). It is particularly preferable that the aqueous medium consists entirely of water, but an aqueous medium that is partially composed of other liquids that are compatible with water can also be used. As other liquids, lower alcohols having 3 or less carbon atoms can be used.

The solid content concentration of the slurry is preferably 0.1 to 20% by mass, more preferably 0.5 to 5% by mass. If the solid content concentration is within the above range, defibration can be performed efficiently.

Examples of dilute alkalis added to carbamate microfiber cellulose include sodium hydroxide, potassium hydroxide, lithium hydroxide, magnesium hydroxide, calcium hydroxide, manganese hydroxide, iron(II) hydroxide, and iron(II) hydroxide. III), copper(II) hydroxide, zinc hydroxide, lanthanum hydroxide, aluminum hydroxide, etc. However, from the viewpoint of economy, it is preferable to use sodium hydroxide.

Furthermore, as the additive, it is preferable to use at least one of urea and urea derivatives, and it is more preferable to use urea.

As described above, according to the method in which micronization is performed with additives such as dilute alkali and urea added, the dilute alkali inhibits the hydrogen bonds in the hydrophilic region of cellulose, and furthermore, the additives such as urea are Because cellulose fibers gather in hydrophobic regions due to Waals force and work to prevent re-aggregation of cellulose fibers, the stability of dispersion of cellulose fibers in a solvent can be improved, and the transparency of the resulting cellulose nanofibers can be improved. ) will be improved.

In addition to the above, it is preferable to further add zinc oxide (ZnO) for refining. According to the method where zinc oxide (ZnO) is added and refined, zinc oxide forms a hydrogen bond with cellulose, selectively shearing the hydrogen bonds between cellulose molecules, and at the same time, the cellulose becomes charged, causing electrostatic repulsion. generation, the cellulose fibers become easier to disperse in the solvent, and the transparency (transparency) of the obtained cellulose nanofibers improves.

When the dilute alkali is sodium hydroxide, the amount of sodium hydroxide added is preferably 0.1 to 100 kg, particularly preferably 1 to 90 kg, per 1 kg of cellulose fiber. If the amount added is less than 0.1 kg, the light transmittance of the obtained cellulose nanofibers may decrease. On the other hand, if the amount added exceeds 100 kg, cellulose may dissolve and cellulose nanofibers may not be obtained.

Furthermore, the amount of additives such as urea added is preferably 0.1 to 100 kg, more preferably 2.5 to 90 kg, particularly preferably 5 to 80 kg, per 1 kg of cellulose. If the amount added is less than 0.1 kg, the light transmittance of the obtained cellulose nanofibers may decrease. On the other hand, even if the amount added exceeds 100 kg, the increase in light transmittance reaches a ceiling and no advantage is obtained in terms of economy and cost.

Furthermore, the amount of zinc oxide added to the cellulose dispersion solution is preferably 0.1 to 20 g, more preferably 0.1 to 10 g, per 1 kg of cellulose fiber. If the amount added exceeds 20 g, the solution may become cloudy.

The dilute alkali and additives are added to an aqueous dispersion of carbamate cellulose fibers and microfiber cellulose. The addition can be carried out either before microfibrillation or before microfibrillation. Further, stirring is not particularly limited as long as it is a method that can uniformly disperse the dilute alkali and additives in the aqueous dispersion.

(miniaturization)
After adding dilute alkali, urea, etc. to the slurry of carbamate microfiber cellulose as described above, the carbamate microfiber cellulose is refined (fibrillated) in this state. Since this refinement is similar to the fibrillation of the carbamate cellulose fibers described above, the following will focus on the differences from the fibrillation.

In this embodiment, the carbamate microfiber cellulose is refined so that the cellulose fibers become cellulose nanofibers. Cellulose nanofibers are fine fibers similar to microfiber cellulose, but they have higher light transmittance and are transparent.

Cellulose nanofibers differ in the degree of refinement from microfiber cellulose, for example, the average fiber diameter is less than 0.1 μm. Specifically, the average fiber diameter (average fiber width; average diameter of single fibers) of cellulose nanofibers is preferably 1 to 20 nm, more preferably 1 to 15 nm, particularly preferably 1 to 10 nm. When the average fiber diameter of the cellulose nanofibers is less than 1 nm, the surface area of the fibers becomes large, resulting in poor dehydration properties, which may increase manufacturing costs for products such as sheets. On the other hand, when the average fiber diameter of cellulose nanofibers exceeds 20 nm, transparency (transparency) may be poor.

The average fiber diameter of the cellulose nanofibers can be adjusted by, for example, the degree of fibrillation of the carbamate microfiber cellulose, pretreatment, fibrillation, etc.

Note that the measurement method for the physical properties of cellulose nanofibers is the same as that for microfiber cellulose unless otherwise specified.

The average fiber length (length of a single fiber) of cellulose nanofibers is preferably 0.1 to 1,000 μm, more preferably 0.5 to 500 μm. When the average fiber length of the cellulose nanofibers is less than 0.1 μm, the shape is not fibrous but close to that of cellulose nanocrystals. On the other hand, if the average fiber length of the cellulose nanofibers exceeds 1,000 μm, the fibers tend to become entangled with each other, and there is a possibility that the dispersibility may not be sufficiently improved.

The average fiber length of cellulose nanofibers can be adjusted, for example, by pretreatment, fibrillation, etc.

The cellulose nanofiber crystallinity is preferably 50% or more, more preferably 60% or more. If the crystallinity of cellulose nanofibers is within the above range, the mechanical properties (particularly strength and dimensional stability) of cellulose can be improved.

The degree of crystallinity can be arbitrarily adjusted by, for example, the degree of fibrillation of the carbamate microfiber cellulose, pretreatment, fibrillation, etc.

If necessary, the cellulose nanofibers obtained by fibrillation can be dispersed in an aqueous medium to form a dispersion before being mixed with other cellulose fibers. It is particularly preferable that the entire amount of the dispersion medium is water (aqueous solution). However, the dispersion medium may be another liquid that is partially compatible with water. As other liquids, for example, lower alcohols having 3 or less carbon atoms can be used.

The type B viscosity of the cellulose nanofiber dispersion (concentration 1%) is preferably 10 to 2,000 cp, more preferably 30 to 1,500 cp. When the B-type viscosity of the dispersion liquid is within the above range, mixing with other cellulose fibers becomes easy, and the dewaterability of the cellulose fiber slurry improves.

The B-type viscosity (solid content concentration 1%) of the dispersion is a value measured in accordance with JIS-Z8803 (2011) "Liquid viscosity measurement method". Type B viscosity is the resistance torque when stirring a dispersion liquid, and means that the higher the viscosity, the more energy is required for stirring.

The solid content concentration of the slurry is preferably 0.1 to 10.0% by mass, more preferably 0.5 to 5.0% by mass. When the solid content concentration is less than 0.1% by mass, excessive energy may be required during dehydration and drying. On the other hand, if the solid content concentration exceeds 10.0% by mass, the fluidity of the slurry itself may decrease, and the dispersant may not be mixed uniformly.

(others)
The cellulose nanofiber of this embodiment can be used, for example, in the form of a transparent sheet by drying a slurry.

Next, examples of the present invention will be described.
First, softwood kraft pulp and an aqueous urea solution having a solid content concentration of 10% were mixed at a solid content conversion ratio of 2:1. A small amount of 20% citric acid solution was also added (0.0008 g/g-urea). This mixture was dried at 105°C to obtain a dry product. This dried product was heat-treated at 160° C. for 1 hour to obtain carbamate-modified pulp. The carbamate-modified pulp thus obtained was diluted with distilled water and stirred, and dehydrated and washed twice. The washed carbamate-modified pulp was beaten in a refiner to obtain carbamate-modified microfiber cellulose (cellulose fiber). This carbamate-modified microfiber cellulose had a carbamate group introduction amount of 1.4 mmol/g and fine A of 77%.

Next, Reagent A was prepared by mixing 80.0 g of 50% sodium hydroxide aqueous solution, 50.0 g of urea, and 130.8 g of water. The prepared reagent A and carbamate-modified microfiber cellulose (NBKP: moisture 97.9% by mass) dry weight 5g were mixed and subjected to defibration (refining) treatment using a high-pressure homogenizer, resulting in a concentration of 1.0% by mass. An aqueous dispersion of carbamate-modified cellulose nanofibers was obtained. In some test examples, zinc oxide was added. In other test examples, reagent A was prepared according to the blending ratios shown in Table 1, and then similar operations were performed.

The total light transmittance of each of the obtained aqueous dispersions of carbamate-modified cellulose nanofibers was examined as follows. The results are shown in Table 1.

(Total light transmittance)
After degassing the resulting aqueous dispersion of carbamate-modified cellulose nanofibers, total light transmittance was measured using an absorbance meter in accordance with JIS K 7361. Zero point correction was performed using ion-exchanged water placed in the same glass cell, and the total light transmittance was calculated from the average value of transmittances measured at wavelengths of 350 to 880 nm.

The present invention can be used as a method for producing cellulose nanofibers and cellulose nanofibers.

Claims (7)

1. Add dilute alkali to carbamate cellulose fibers to make them fine.
   A method for producing cellulose nanofibers, characterized by:
2. Adding a dilute alkali and at least one of urea and urea derivative additives to the carbamate cellulose fibers to make them fine.
   A method for producing cellulose nanofibers, characterized by:
3. The dilute alkali is sodium hydroxide, the amount of the sodium hydroxide added is 0.1 to 100 kg, and the amount of the additive is 0.1 to 100 kg with respect to 1 kg of the cellulose fiber. ,
   The method for producing cellulose nanofibers according to claim 2.
4. The carbamate-formed cellulose fibers have an average fiber diameter of 1 to 16 μm, and the refinement is carried out so that the average fiber diameter is 1 to 20 nm.
   The method for producing cellulose nanofibers according to claim 1 or 2.
5. Carbamate the carbamate under alkaline conditions of pH 9 or higher or acidic conditions of pH 7 or lower so that the average fiber length of the carbamate-formed cellulose fibers is 0.10 to 2.0 mm.
   The method for producing cellulose nanofibers according to claim 1 or 2.
6. The crystallinity of the carbamate cellulose fiber is 50% or more,
   The method for producing cellulose nanofibers according to claim 1 or 2.
7. Obtained by the method according to claim 1 or claim 2,
   Cellulose nanofibers are characterized by: