JP2011046793A - Method for producing cellulose nanofiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a cellulose nanofiber that has a high molecular weight and also avoids discoloration when molded into a molding. <P>SOLUTION: The method for producing a cellulose nanofiber includes: a first oxidation step in which cellulose is oxidized in a first reaction solution including an oxidizing agent including a hypohalous acid, and an N-oxyl compound; and a second oxidation step in which the oxidized cellulose obtained in the first oxidation step is oxidized in a second reaction solution including an oxidizing agent which oxidizes an aldehyde group. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

  The present inventors oxidize natural fiber materials such as cellulose in the presence of a TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) catalyst and perform a mechanical fibrillation treatment. Thus, a method for producing highly crystalline ultrafine fibers (nanofibers) having a diameter of several nanometers has already been proposed (see Patent Document 1). By this production method, each cellulose nanofiber was separated in water, and a cellulose nanofiber dispersion, which was a novel material that could be applied to various uses, could be obtained.

JP 2008-001728 A

  However, the method described in Patent Document 1 has a problem that the degree of polymerization of the obtained nanofibers is low. Further, when a nanofiber molded body such as a film is produced by removing a liquid component from the nanofiber dispersion, the nanofiber molded body is colored when heated.

  The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a method for producing cellulose nanofibers that have a high molecular weight and are prevented from being colored during heating.

In order to solve the above problems, the method for producing cellulose nanofibers of the present invention is a first method in which cellulose is oxidized in a first reaction solution containing an N-oxyl compound and a reoxidant for the N-oxyl compound. And a second oxidation step of oxidizing the oxidized cellulose obtained in the first oxidation step in a second reaction solution containing an oxidizing agent that oxidizes aldehyde groups. To do.
According to this production method, the hydroxyl group at the cellulose C6 position is oxidized in the first oxidation step to introduce aldehyde groups and carboxyl groups into the cellulose, and the aldehyde generated in the first oxidation step in the second oxidation step. Since the group is oxidized to a carboxyl group, the oxidation treatment necessary for the characteristics of the cellulose nanofiber can be quickly performed in the first oxidation step, and the aldehyde group that causes a reduction in molecular weight or coloring is converted into the second oxidation step. Can be substituted with a carboxyl group. Thereby, a method for producing cellulose nanofibers having a high molecular weight and also prevented from being colored during heating is realized.

After the first oxidation step is completed, the oxidizing agent that oxidizes aldehyde groups is added to the first reaction solution, the pH is adjusted, and the second oxidation step is continuously performed. it can.
According to this manufacturing method, after the first oxidation step is performed, the second oxidation step is continuously performed without isolating the oxidized cellulose. Cellulose nanofibers can be produced.

In the method for producing cellulose nanofibers of the present invention, the pH of the first reaction solution is preferably 8 or more and 12 or less, and the pH of the second reaction solution is preferably 3 or more and 7 or less.
According to this production method, in the first oxidation step, the reaction for oxidizing the hydroxyl group at the cellulose C6 position efficiently proceeds, and in the second oxidation step, the reaction for oxidizing the aldehyde group to a carboxyl group efficiently proceeds. be able to. Thereby, it is possible to efficiently produce cellulose nanofibers having a high molecular weight and preventing coloring during heating.

In the first oxidation step, it is preferable to introduce a carboxyl group of 0.3 mmol / g or more and 1 mmol / g or less into the cellulose.
According to this production method, it is possible to produce cellulose nanofibers that can be easily separated and dispersed by a light fibrillation treatment.
More preferably, the first and second oxidation steps are performed so that the amount of carboxyl groups after the end of the second oxidation step is 0.7 mmol / g or more. This makes it possible to produce cellulose nanofibers with a high yield.

It is preferable that the processing time of the first oxidation process is 20 minutes or less, and the processing time of the second oxidation process is 180 minutes or less.
According to this production method, the cellulose nanofiber has a high molecular weight and is prevented from being colored during heating without significantly reducing the production time as compared with a conventionally known method for producing cellulose nanofiber. Can be manufactured.

The total processing time of the first and second oxidation steps is preferably 120 minutes or less.
According to this production method, the production time can be reliably shortened as compared with the conventionally known methods for producing cellulose nanofibers.

The figure which shows the manufacturing method of the cellulose nanofiber of this invention. The photograph of the dispersion liquid produced in the Example. The photograph of the dispersion liquid produced in the Example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In the method for producing cellulose nanofibers according to the present embodiment, as shown in FIG. 1, the cellulose is oxidized in a first reaction solution containing an N-oxyl compound and a reoxidant of the N-oxyl compound. Oxidation step ST11 and the second oxidation step ST12 in which the oxidized cellulose obtained in the first oxidation step is oxidized in a second reaction solution containing an oxidizing agent that oxidizes an aldehyde group and an N-oxyl compound. And a dispersion treatment step ST13 for dispersing the oxidized cellulose obtained in the second oxidation step ST12 in a medium.

<First oxidation step>
First, the first oxidation step ST11 will be described.
In the first oxidation step ST11, first, a dispersion in which natural cellulose is dispersed in water is prepared. Natural cellulose is purified cellulose isolated from cellulose biosynthetic systems such as plants, animals, and bacteria-producing gels. Specifically, it is isolated from softwood pulp, hardwood pulp, cotton pulp such as cotton linter and cotton lint, non-wood pulp such as straw pulp and bagasse pulp, bacterial cellulose, cellulose isolated from sea squirt, seaweed A cellulose etc. can be illustrated.

  Moreover, you may give the process which expands surface areas, such as beating, to the natural cellulose isolated and refine | purified. Thereby, reaction efficiency can be improved and productivity can be improved. In addition, it is preferable to use natural cellulose that has been stored in an undried state after isolation and purification. By storing in an undried state, it is possible to keep the microfibril bundle in an easily swellable state, so that the reaction efficiency is improved and cellulose nanofibers with a small fiber diameter are easily obtained.

  Typically, water is used as a dispersion medium for natural cellulose in the first reaction solution used in the first oxidation step ST11. The natural cellulose concentration in the first reaction solution is not particularly limited as long as the reagent (oxidant, catalyst, etc.) can be sufficiently dissolved. Usually, the concentration is preferably about 5% or less with respect to the weight of the reaction solution.

An N-oxyl compound is used as the catalyst added to the first reaction solution. As the N-oxyl compound, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and a TEMPO derivative having various functional groups at the C4 position can be used. Examples of TEMPO derivatives include 4-acetamido TEMPO, 4-carboxy TEMPO, 4-phosphonooxy TEMPO, 4-amino-TEMPO, 4- (2-bromoacetamido) -TEMPO, 4-hydroxy TEMPO, 4-oxy TEMPO, and the like. Can be mentioned. In particular, TEMPO and 4-acetamido TEMPO have obtained favorable results in the reaction rate.
A catalytic amount is sufficient for the addition of the N-oxyl compound. Specifically, it may be added in a range of 0.1 to 4 mmol / L with respect to the reaction solution. Preferably, it is an addition amount range of 0.1 to 2 mmol / L.

As the oxidizing agent in the first oxidation step ST11, hypohalous acid or a salt thereof (hypochlorous acid or a salt thereof, hypobromous acid or a salt thereof, hypoiodous acid or a salt thereof, or the like) is used. The content of the oxidizing agent is preferably in the range of 1 to 50 mmol / L.
As hypohalite, in the case of hypochlorous acid, alkali metal salts such as lithium hypochlorite, potassium hypochlorite, sodium hypochlorite, calcium hypochlorite, hypochlorous acid, etc. Examples include magnesium, alkaline earth metal salts such as strontium hypochlorite, and ammonium hypochlorite. Moreover, hypobromite and hypoiodite corresponding to these can also be used.
Among the above, a preferable oxidizing agent in the first oxidation step ST11 is an alkali metal hypohalite, and a more preferable oxidizing agent is an alkali metal hypochlorite (such as sodium hypochlorite).

  Furthermore, in the first oxidation step ST11, a catalyst component in which a bromide or iodide is combined with an N-oxyl compound may be used. For example, ammonium salt (ammonium bromide, ammonium iodide), bromide or alkali metal iodide (bromide such as lithium bromide, potassium bromide, sodium bromide, lithium iodide, potassium iodide, sodium iodide, etc. Iodide), bromide or alkaline earth metal iodides (calcium bromide, magnesium bromide, strontium bromide, calcium iodide, magnesium iodide, strontium iodide, etc.) can be used. These bromides and iodides can be used alone or in combination of two or more.

In the first oxidation step ST11, the pH of the first reaction solution is maintained in the range of pH 8 to 12 suitable for the action of the hypohalite that is the oxidizing agent. Furthermore, it is preferable to maintain in the range of pH 10-11.
The pH of the reaction solution is a basic substance (ammonia, potassium hydroxide, sodium hydroxide, etc.) or an acidic substance (acetic acid, oxalic acid, succinic acid, glycolic acid, malic acid, citric acid, benzoic acid and other organic acids, or It can be adjusted by appropriately adding an inorganic acid such as nitric acid, hydrochloric acid, sulfuric acid or phosphoric acid.

[Specific example of first oxidation step ST11]
In the first oxidation step ST11, specifically, a cellulose raw material suspended in water, an N-oxyl compound (such as TEMPO) and an alkali metal bromide (or alkali metal iodide), and the next as an oxidizing agent. A first reaction solution to which sodium chlorite (hypochlorite) is added is prepared, and the oxidation reaction is performed with stirring as necessary under a temperature condition of 0 ° C. to room temperature (10 ° C. to 30 ° C.). Make it progress.

After completion of the reaction, the oxidized fibrous TEMPO catalyst is oxidized by performing a treatment to decompose the oxidizing agent (such as sodium hypochlorite) as necessary, and then repeatedly filtering the first reaction solution and washing with water. Obtain cellulose.
As will be described in detail later, a second reaction solution is prepared using the first reaction solution after use without extracting oxidized cellulose from the first reaction solution, and the second oxidation step. ST12 can also be executed continuously.

In the first oxidation step ST11, since the carboxyl group is generated as the reaction proceeds, the pH of the reaction solution decreases. Therefore, in order to sufficiently proceed the oxidation reaction, an aqueous solution containing an alkali metal component such as an aqueous sodium hydroxide solution is added to the reaction solution, and the reaction system is in the alkaline region (pH 8 to 12, preferably pH 10 to 11). Is preferably maintained.
Further, in the first oxidation step ST11, the pH of the reaction solution decreases with the progress of the oxidation reaction, so that the point in time when the progress of the pH decrease is not recognized can be set as the reaction end point.

  In addition, the reaction temperature in 1st oxidation process ST11 can also be made higher than room temperature, and reaction efficiency can be raised by making it react at high temperature. On the other hand, since chlorine gas is easily generated from sodium hypochlorite, it is preferable to prepare a chlorine gas treatment system when the reaction is performed at a high temperature.

<Second oxidation step>
Next, the second oxidation step ST12 will be described.
The raw material used for the second oxidation step ST12 is oxidized cellulose obtained in the first oxidation step ST11. That is, it is oxidized cellulose that is oxidized in a first reaction solution that uses natural cellulose as a raw material and contains an N-oxyl compound and a reoxidant (hypohalous acid or a salt thereof).

  The oxidizing agent used in the second oxidation step ST12 is an oxidizing agent that can oxidize aldehyde groups and convert them into carboxyl groups. Specifically, halous acid or a salt thereof (chlorous acid or a salt thereof, bromous acid or a salt thereof, iodic acid or a salt thereof), peracid (hydrogen peroxide, peracetic acid, persulfuric acid, perbenzoic acid) Acid). These oxidizing agents can be used alone or in combination of two or more. Further, it may be used in combination with an oxidase such as laccase. The content of the oxidizing agent is preferably in the range of 1 to 50 mmol / L.

Examples of halous acid salts include, in the case of chlorous acid, alkali metal salts such as lithium chlorite, potassium chlorite, and sodium chlorite, calcium chlorite, magnesium chlorite, and strontium chlorite. Examples thereof include alkaline earth metal salts and ammonium chlorite. In addition, bromite and iodate corresponding to these can also be used.
A preferred oxidizing agent in the second oxidation step ST12 is an alkali metal halous acid salt, and more preferably an alkali metal chlorite.

In the second oxidation step, no catalyst is required, but the same N-oxyl compound and other catalysts as in the first oxidation step may be included. Moreover, the bromide and iodide used with an N-oxyl compound may be contained.
For example, after the oxidation treatment in the first oxidation step ST11, if the second oxidation step is performed without washing and isolation, the N-oxyl compound is brought together with the oxidized cellulose from the first oxidation step ST11. . Even if the N-oxyl compound is contained in the second reaction solution, the reaction in the second oxidation step is hardly affected.

  In the second oxidation step ST12, the aldehyde group generated at the C6 position of cellulose in the first oxidation step ST11 is converted into a carboxyl group by using an oxidizing agent capable of oxidizing the aldehyde group to a carboxyl group. . Thereby, the beta elimination reaction caused by the aldehyde group can be prevented, and high molecular weight cellulose nanofibers can be obtained.

It is also preferable to add a buffer solution to the second reaction solution. As the buffer solution, various buffer solutions such as a phosphate buffer solution, an acetate buffer solution, a citrate buffer solution, a borate buffer solution, a tartaric acid buffer solution, and a Tris buffer solution can be used.
By suppressing the pH change during the reaction using a buffer solution, it is not necessary to continuously add acid or alkali to maintain the pH, and it is not necessary to install a pH meter. And since addition of an acid or an alkali is unnecessary, a reaction container can be sealed.

  The second reaction solution used in the second oxidation step ST12 may contain hypohalous acid or a salt thereof, but the content thereof is reduced as much as possible (for example, 1 mmol / L or less). Is preferable). These oxidizing agents oxidize TEMPO to generate N-oxoammonium cations and oxidize the primary alcohol at the cellulose C6 position. Therefore, when hypohalous acid or a salt thereof is included, introduction of an aldehyde group and a carboxyl group at the cellulose C6 position proceeds also in the second oxidation step ST12. Therefore, in order to avoid fluctuations in the amount of carboxyl groups and residual aldehyde groups on the oxidized cellulose surface, it is preferable to reduce the content of hypohalous acid or a salt thereof as much as possible.

In the present invention, the first oxidation step ST11 and the second oxidation step ST12 may be performed continuously. In this case, after the completion of the first oxidation step ST11, a step of adjusting the pH of the first reaction solution after use, an oxidant that oxidizes aldehyde groups is added to the first reaction solution, and a second step is performed. And a step of preparing a second reaction solution used in the oxidation step, and preparing a second reaction solution from the first reaction solution.
Thereby, the catalyst (N-oxyl compound) and the sub-oxidant (sodium hypochlorite) used in the second oxidation step ST12 can be used as they are in the first oxidation step ST11. Since it is not necessary to extract oxidized cellulose (washing and isolation) after one oxidation step ST11, it becomes possible to produce cellulose nanofibers very efficiently.

[Specific example of second oxidation step ST12]
Specifically, in the second oxidation step ST12, sodium chlorite (chlorite) as an oxidizing agent is placed in the suspension of the oxidized cellulose obtained in the first oxidation step ST11 in water. A second reaction solution added quantitatively is prepared, and the oxidation reaction is allowed to proceed with stirring as necessary under a temperature condition of about room temperature to 100 ° C. The process of extracting TEMPO oxidized cellulose after completion of the oxidation reaction is the same as in the first oxidation step ST11 described above.

  In the second oxidation step ST12, the pH of the reaction solution is maintained in a neutral to acidic range. More specifically, a pH range of 3 to 7 is preferable. In particular, care should be taken that the pH of the reaction solution does not exceed 8. By setting it to such a pH range, the aldehyde group can be oxidized to a carboxyl group while preventing the beta elimination reaction due to the aldehyde group at the C6 position of the cellulose produced in the first oxidation step ST11, High molecular weight cellulose nanofibers can be produced.

[Dispersion process]
Next, in the dispersion step ST13, the oxidized cellulose obtained in the oxidation treatment step or the oxidized cellulose that has undergone the purification step is dispersed in the medium.
As a medium (dispersion medium) used for dispersion, an aqueous solvent is used. The aqueous solvent in the present embodiment is a solvent that is only water except for components that are inevitably mixed, or a mixed solvent of water and an organic solvent such as an alcohol compatible with less than 20% by weight of water. Typically, water is used as the dispersion medium.

  Various devices can be used as a dispersion device (defibration device) used in the dispersion step ST13. For example, a household mixer, an ultrasonic homogenizer, a high-pressure homogenizer, a biaxial kneading device, a defibrating device such as a stone mill can be used. In addition to these, a dispersion of cellulose nanofibers can be easily obtained with a defibrating apparatus generally used for household use or industrial production. In addition, if a defibrating apparatus having a strong and beating ability such as various homogenizers and various refiners is used, cellulose nanofibers having a small fiber diameter can be obtained more efficiently.

  Although the density | concentration of a cellulose nanofiber aqueous dispersion is not specifically limited, It is preferable to set it as the range of 0.05 weight% or more and 2 weight% or less. More preferably, it is 0.1 weight% or more and 0.5 weight% or less. By setting it as this range, a cellulose nanofiber can be uniformly disperse | distributed in water, without gelatinizing.

  By the dispersion step ST13, a cellulose nanofiber dispersion liquid in which cellulose nanofibers are dispersed in a medium is obtained. In this cellulose nanofiber aqueous dispersion, cellulose nanofibers in which the primary hydroxyl group at the C6 position in a part of cellulose is oxidized to carboxylic acid sodium salt (sodium salt of carboxyl group) is an aqueous solvent due to the charge repulsive force of the carboxyl group. It is uniformly dispersed therein.

  In the manufacturing method of the present embodiment described above, first, in the first oxidation step ST11, hypohalous acid or a salt thereof as a TEMPO reoxidant is used in the first reaction solution. Since the reaction proceeds in an environment of pH 8 to 11 where the agent acts efficiently, the oxidation treatment of cellulose can proceed efficiently. In the first oxidation step ST11 in the present invention, although depending on the amount of oxidizing agent used and the amount of cellulose processed, the treatment can be completed in about several minutes to 20 minutes.

  On the other hand, in the first oxidation step ST11, oxidized cellulose containing an aldehyde group is generated. TEMPO oxidized by the above oxidizing agent oxidizes the primary hydroxyl group of cellulose C6 to an aldehyde group, and a part of this aldehyde group is oxidized to a carboxyl group, but all aldehyde groups are not oxidized. , It will always remain. If the aldehyde group remains, a beta elimination reaction caused by the aldehyde group occurs in the alkaline first reaction solution, and the molecular chain of the cellulose is cut to reduce the molecular weight of the oxidized cellulose (cellulose nanofiber). End up. Further, when cellulose nanofibers containing aldehyde groups are dried and solidified to form a molded product, coloring occurs during heating.

  Therefore, in the present invention, in the second oxidation step ST12, the aldehyde group of the oxidized cellulose obtained in the first oxidation step ST11 is oxidized. By this second oxidation step ST12, an oxidized cellulose substantially free of aldehyde groups can be obtained, and the molecular weight of oxidized cellulose resulting from the beta elimination reaction of the aldehyde groups and the coloring during heating caused by the aldehyde groups are reduced. Can be prevented. Furthermore, in the present invention, since the second reaction solution is adjusted to pH 3 to 7, it is possible to prevent the beta elimination reaction of the aldehyde group from occurring during the treatment of the second oxidation step ST12.

  Thus, according to the manufacturing method of this embodiment, the dispersion liquid of the cellulose nanofiber by which each one was isolate | separated can be manufactured efficiently in a short time. Moreover, the cellulose nanofiber obtained by the manufacturing process of this embodiment is a high molecular weight, and coloring when it is set as a molded object will also be prevented.

  In particular, a significant feature of the present embodiment is that the time required for producing cellulose nanofibers can be significantly shortened. Specifically, the effect of shortening the time is obtained for both the case where the cellulose nanofiber is produced only by the first oxidation step ST11 and the case where the cellulose nanofiber is produced only by the second oxidation step ST12. It is done.

  First, when manufacturing cellulose nanofiber only by 2nd oxidation process ST12, oxidizing agents, such as halohalic acid or its salt, are used for the 2nd reaction solution, and pH is made into the range of 3-7. Therefore, the oxidation reaction of TEMPO hardly proceeds and the oxidation reaction of cellulose itself becomes very slow. For example, in order to obtain a uniformly dispersed cellulose nanofiber aqueous dispersion (concentration 0.1% (g / mL)), a long-time oxidation treatment of about 24 hours is required.

  Moreover, also when manufacturing a cellulose nanofiber only by 1st oxidation process ST11, the oxidation process for about 2 hours is required. This is because cellulose nanofibers containing an aldehyde group at the C6 position of cellulose are produced only in the first oxidation step ST11, and this aldehyde group has an action of aggregating the cellulose nanofibers. This is because it is necessary to introduce many carboxyl groups into the cellulose surface.

  On the other hand, in the method for producing cellulose nanofibers of the present embodiment, the first oxidation step ST11 can be completed in about several minutes to 20 minutes, and the subsequent second oxidation step ST12 is also performed for 60 to 180 minutes. Can be completed to a degree. Therefore, cellulose nanofibers can be produced by oxidation treatment for about 1 to 3 hours, and when producing cellulose nanofibers only in the second oxidation step ST12, depending on conditions, the first oxidation step The manufacturing time can be shortened even when cellulose nanofibers are manufactured using only ST11.

  In this embodiment, since the aldehyde group is oxidized to a carboxyl group in the second oxidation step ST12, there is no need to excessively oxidize the cellulose in the first oxidation step ST11. The processing time of the first oxidation step ST11 can be shortened, and in the second oxidation step ST12, it is not necessary to oxidize the hydroxyl group at the cellulose C6 position, and only the aldehyde group generated in the first oxidation step ST11 is oxidized. This is because the processing time of the second oxidation step ST12 can be greatly shortened.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

  In this example, cellulose nanofiber aqueous dispersions having various conditions shown in Table 1 were prepared by sequentially performing the first oxidation step, the second oxidation step, and the dispersion step.

"First oxidation process"
Conical bleached kraft pulp (1 g by dry weight), TEMPO (0.016 g; 0.1 mmol), sodium bromide (0.1 g; 1 mmol) that has been stored in a wet state after the bleaching treatment is placed in an Erlenmeyer flask that can be sealed. And deionized water (100 mL) was added and stirred. Thereafter, a predetermined amount (0.63 to 1.25 mL; 1.3 to 2.5 mmol) of an aqueous sodium hypochlorite solution (effective chlorine concentration 2 M) was added to prepare a first reaction solution, and the reaction was started. . During the reaction, pH decreased with the formation of carboxyl groups, so 0.5 M sodium hydroxide was added dropwise to keep the pH at 10. The reaction was considered to be complete (10 minutes, 15 minutes, 120 minutes from the start of the reaction) when the pH no longer dropped.
Thereafter, the oxidized pulp not treated with sodium chlorite (oxidized cellulose 1, 3, and 5 shown in Table 1) was filtered through a glass filter and sufficiently washed with deionized water. After washing, the pulp sample was dehydrated until it had a solid content of 10 to 15%, and then stored refrigerated.

"Second oxidation process"
When TEMPO catalytic oxidation was completed, 1M acetic acid was added dropwise to lower the pH to 3.5 once, and then the pH was raised to 5 with 0.5M sodium hydroxide (to provide a buffering effect). To this was added sodium chlorite (effective chlorine amount 80%, 1.13 g; 10 mmol) to prepare a second reaction solution, which was sealed.
After heating to 50 ° C. in a water bath and stirring for 3 hours, the mixture was cooled to room temperature and 1M sodium thiosulfate was added until it became colorless, to reduce the remaining sodium chlorite. Then, it filtered with the glass filter and fully wash | cleaned with deionized water. After washing, the oxidized pulp (oxidized cellulose 2, 4 shown in Table 1) was dehydrated and stored refrigerated until the solid content became 10-15%.
Moreover, the oxidation which processed the softwood bleached kraft pulp (dry weight 1g) only by the 2nd oxidation process (the oxidation treatment time was made into 24 hours) without passing through the 1st oxidation process by the process similar to the above. Pulp (oxidized cellulose 6 shown in Table 1) was prepared, dehydrated and stored refrigerated.

"Dispersion process"
Water was added to kraft pulp (natural cellulose shown in Table 1) and oxidized pulp (oxidized cellulose 1 to 6 shown in Table 1) that had been stored in a wet state to a concentration of 0.1% (w / v) to 20 mL. Thereafter, the pH was adjusted to 8 with 0.05 M sodium hydroxide. The pulp slurry was treated with a mechanical homogenizer (Excel Auto Homogenizer, Nippon Seiki Seisakusho) for 5 minutes and then treated with an ultrasonic homogenizer (US-300T, Nippon Seiki Seisakusho) for 3 minutes for a total of 2 times. .

"Measurement of carboxyl and aldehyde groups"
The amount of carboxyl groups and the amount of aldehyde groups were measured for the cellulose nanofibers prepared by the above procedure. Table 1 shows the measured carboxyl group amount and aldehyde group amount.
First, the amount of carboxyl groups can be measured by the following method.
60 ml of a 0.5 to 1 wt% dispersion was prepared from a pulp sample whose dry weight was precisely weighed, adjusted to about 2.5 with 0.1 M hydrochloric acid aqueous solution, and then 0.05 M sodium hydroxide aqueous solution was added. Drop and measure electrical conductivity. The measurement is continued until the pH is 11. Then, the amount of functional groups is determined from the amount of sodium hydroxide (V) consumed in the neutralization step of the weak acid where the change in electrical conductivity is gradual, using the following equation. This amount of functional groups is the amount of carboxyl groups.
Functional group amount (mmol / g) = V (ml) × 0.05 / mass of cellulose (g)

  The amount of aldehyde groups can also be measured from the same formula as above. The pulp sample subjected to the above-mentioned measurement of the amount of carboxyl groups is further oxidized at room temperature for 48 hours in a 2% aqueous sodium chlorite solution adjusted to pH 4 to 5 with acetic acid, and the amount of functional groups is measured again by the above method. An amount obtained by subtracting the amount of the carboxyl group from the measured amount of the functional group is the amount of the aldehyde group.

"Degree of polymerization"
In this example, the degree of polymerization of cellulose was measured for each pulp sample. Table 1 shows the measurement results of the degree of polymerization.
The degree of polymerization is “the number of average glucose components contained in one cellulose molecule”. If the degree of polymerization is 162, the molecular weight is obtained. In this example, each oxidized cellulose sample was dissolved in a 0.5 M copper ethylenediamine solution in advance with sodium borohydride and the remaining aldehyde groups were reduced to alcohol, and the degree of polymerization was determined by the viscosity method. . Since the copper ethylenediamine solution is alkaline, if an aldehyde group remains in the oxidized cellulose, a beta elimination reaction may occur in the dissolution process and the molecular weight may decrease. Thus, the aldehyde group was converted to an alcoholic hydroxyl group. The following literature was referred to for the formula for determining the degree of polymerization of cellulose from the viscosity of cellulose dissolved in a 0.5 M copper ethylenediamine solution.

(Reference) Isogai, A., Mutoh, N., Onabe, F., Usuda, M., “Viscosity measurements of cellulose / SO ₂ -amine-dimethylsulfoxide solution”, Sen'i Gakkaishi, 45, 299-306 (1989) ).

2 and 3 are photographs of each dispersion prepared in this example. The dispersions (a) to (e) shown in FIG. 2 correspond to natural cellulose and oxidized cellulose 1 to 4 shown in Table 1, respectively. The dispersions (f) and (g) shown in FIG. 3 correspond to oxidized celluloses 6 and 7, respectively.
As shown in FIG. 2, it can be seen that as the amount of carboxyl groups increases, the cohesiveness decreases and a uniform dispersion can be obtained. In addition, the comparison between the oxidized cellulose 1 in (b) and the oxidized cellulose 2 in (c) and the comparison between the oxidized cellulose 3 in (d) and the oxidized cellulose 4 in (e) showed that the time was shortened. It can be seen that the dispersibility of the cellulose nanofibers tends to be insufficient only by the oxidation step, but the dispersibility of the cellulose nanofibers is remarkably improved by additionally performing the second oxidation step.

  In addition, the oxidized cellulose 4 and the oxidized cellulose 5 which changed only the processing time of the 2nd oxidation process became a substantially equivalent result. From this, in the method for producing cellulose nanofibers according to the present invention, the treatment time of the second oxidation step is sufficient to be 60 minutes or less, and the properties of the cellulose nanofibers obtained even when the treatment time is longer than that are almost sufficient. It was confirmed that there was no effect.

  Further, from the comparison between FIG. 2 (e) and FIGS. 3 (f) and 3 (g), the oxidized cellulose 4 which has been subjected to the first oxidation step for 15 minutes and the second oxidation step for 60 minutes is A transparent dispersion equivalent to oxidized cellulose 6 obtained by performing only the process for 60 minutes is obtained, and it is confirmed that the dispersibility of cellulose nanofibers can be improved more than oxidized cellulose 7 obtained by performing only the second oxidation process for 24 hours. It was.

  Also, from the test results of this example, it is confirmed that the amount of carboxyl groups introduced by the first and second oxidation steps needs to be 0.7 mmol / g or more in order to efficiently produce cellulose nanofibers. It was done. More specifically, when the amount of introduced carboxyl groups is less than 0.7 mmol / g, the yield of cellulose nanofibers is drastically lowered because oxidized cellulose cannot be sufficiently defibrated, 5% or less.

  ST11 First oxidation step, ST12 Second oxidation step, ST13 Dispersion step

Claims (6)

A first oxidation step in which cellulose is oxidized in a first reaction solution containing an N-oxyl compound and a reoxidant of the N-oxyl compound;
A second oxidation step of oxidizing the oxidized cellulose obtained in the first oxidation step in a second reaction solution containing an oxidizing agent that oxidizes aldehyde groups;
The manufacturing method of the cellulose nanofiber characterized by having.   After the first oxidation step is completed, the oxidizing agent that oxidizes aldehyde groups is added to the first reaction solution, the pH is adjusted, and the second oxidation step is continuously performed. The manufacturing method of the cellulose nanofiber of Claim 1 characterized by the above-mentioned.   The method for producing cellulose nanofiber according to claim 1 or 2, wherein the pH of the first reaction solution is 8 or more and 12 or less, and the pH of the second reaction solution is 3 or more and 7 or less. .   The cellulose nanofiber according to any one of claims 1 to 3, wherein a carboxyl group of 0.3 mmol / g or more and 1 mmol / g or less is introduced into the cellulose in the first oxidation step. Method.   5. The process according to claim 1, wherein a treatment time of the first oxidation process is 20 minutes or less and a treatment time of the second oxidation process is 180 minutes or less. A method for producing cellulose nanofibers.   6. The method for producing cellulose nanofiber according to claim 5, wherein the total treatment time of the first and second oxidation steps is 120 minutes or less.