JP2009209217A - Method for modifying cellulose, modified cellulose, cello-uronic acid and cellulose crystallite - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for modifying cellulose while inhibiting the reduction of molecular weight. <P>SOLUTION: This method for modifying the cellulose is characterized by comprising a process of oxidizing an alkali-treated cellulose or a regenerated cellulose in a neutral or an acid reaction solution containing an N-oxyl compound and an oxidant for oxidizing an aldehyde group. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for modifying cellulose, modified cellulose, cellouronic acid, and cellulose microcrystals.

  Cellulose, which is biomass that exists naturally in large quantities, is used in various fields, taking advantage of its safety and strength. In addition, various modified celluloses added with new functions by chemical modification are used. As a typical example, a carboxyl group is chemically introduced using a hydroxyl group of cellulose as a contact point.

For example, the treatment methods described in Patent Documents 1 and 2 oxidize the primary hydroxyl group of β-glucose to a carboxyl group using sodium hypochlorite as the main oxidizing agent. According to this treatment method, no toxic or deleterious substances are used, such as partial carboxymethylation using alkali and monochloroacetic acid or carboxylation by adding N ₂ O ₄ in chloroform. Can be introduced.
JP-A-10-251302 JP 2001-49591 A

  In the conventional treatment method, an aqueous solution of alkali-treated cellulose or regenerated cellulose containing a catalytic amount of NaBr and TEMPO is added with an aqueous solution of sodium hypochlorite (NaClO) as a main oxidant (a TEMPO catalytic oxidation reaction). ). In this treatment method, the pH drops due to the formation of carboxyl groups during the reaction, so a dilute aqueous sodium hydroxide solution (usually about 0.5M NaOH) is always added to maintain the pH of the reaction system at 8-11. .

  9 and 10 show the mechanism of oxidizing primary hydroxyl group of cellulose to carboxyl group via aldehyde group by adding catalytic amount of sodium bromide (NaBr) and TEMPO using sodium hypochlorite as main oxidizing agent. . When the above method is applied to regenerated cellulose or mercerized cellulose (cellulose swelled with 5% or more NaOH aqueous solution and then washed with water), all the primary hydroxyl groups at the C6 position of the cellulose are all selectively aldehydes. By oxidizing to a carboxyl group via a group, water-soluble cellouronic acid is obtained. In addition, when applied to natural cellulose, only the primary hydroxyl group at the C6 position located on the surface of the cellulose can be selectively oxidized to a carboxyl group or an aldehyde group while maintaining the crystal structure of cellulose. A modified cellulose in which the hydroxyl group at the C6 position is substituted with a carboxyl group or a sodium salt of a carboxyl group can be obtained.

  However, as the inventors further researched, problems with the conventional treatment method and the modified cellulose obtained thereby have also been clarified. Hereinafter, this problem will be described in detail.

(1) First, it was found that the degree of polymerization (molecular weight) of the modified cellulose obtained by the conventional treatment method is greatly reduced than before the treatment. For example, when regenerated cellulose is treated, the degree of polymerization, which is 350 or more when untreated, is reduced to about 40.
Accordingly, an object of the present invention is to provide a method for modifying cellulose that can suppress a decrease in molecular weight.

(2) Moreover, in the conventional processing method, it is necessary to always make the pH of the reaction solution constant during the TEMPO catalytic oxidation reaction. For this purpose, a pH meter must be installed in the reaction solution, and an open-type reaction system in which a dilute aqueous NaOH solution is continuously added must be constructed. Since the reaction vessel cannot be sealed, treatment of gas generated by the reaction and reaction efficiency This is also disadvantageous.
Accordingly, an object of the present invention is to provide a method for modifying cellulose that facilitates pH control by improving the reaction system and enables the reaction vessel to be sealed.

  In order to solve the above-mentioned problems, the cellulose modification method of the present invention comprises an alkali-treated cellulose or regenerated cellulose in a neutral or acidic reaction solution containing an N-oxyl compound and an oxidizing agent that oxidizes an aldehyde group. It has the process of oxidizing.

  In the modification method of the present invention, an alkali-treated cellulose or regenerated cellulose is oxidized using an oxidizing agent that oxidizes an aldehyde group in the presence of an N-oxyl compound. It can be oxidized and can prevent the formation of an aldehyde group at the C6 position.

Here, in the conventional treatment method, TEMPO catalytic oxidation is performed under weakly alkaline conditions of pH 8 to 11, so that an aldehyde group (CHO group) is generated as an intermediate at the C6 position as shown in the center of FIG. This aldehyde group undergoes a beta elimination reaction very easily under conditions of pH 8-11. As a result, as shown on the right side of FIG. 11, it is considered that the molecular chain of the cellulose is cut and the molecular weight of the resulting modified cellulose is remarkably reduced.
On the other hand, the modification method of the present invention can prevent the formation of aldehyde groups as described above, and the pH of the reaction solution is neutral or acidic even if aldehyde groups exist for a short time. Therefore, the beta elimination reaction that occurs in weak alkali to strong alkalinity does not occur. Therefore, according to the present invention, the breakage of the cellulose molecular chain due to the reaction of the aldehyde group can be prevented, and a modified cellulose having a high degree of polymerization can be obtained.

  In addition, since the aldehyde group is rapidly oxidized, only a negatively charged carboxyl group is generated on the surface of the cellulose. Therefore, when the water-insoluble modified cellulose is dispersed in water or the like, the carboxyl group The modified cellulose can be uniformly dispersed by the charge repulsion effect of.

It is preferable to add a buffer to the reaction solution. By setting it as such a method, it becomes unnecessary to add an acid and an alkali for pH maintenance, and a pH meter becomes unnecessary. Therefore, in the reforming method according to the present invention, the reaction vessel can be sealed.
And if a reaction container is sealed, heating and pressurization with respect to a reaction system are possible. Further, since the gas generated from the reaction solution is not released out of the system, the reforming method is excellent in terms of safety. Further, since the gas generated by the decomposition of the oxidant is not released to the atmosphere, there is an advantage that the amount of the oxidant used can be reduced.

As the oxidizing agent, halous acid or a salt thereof can be used. Further, as the oxidizing agent, a mixture of hydrogen peroxide and oxidase or peracid can be used.
By using these oxidizing agents, the primary hydroxyl group can be oxidized to a carboxyl group, and the production of an aldehyde group at the C6 position can be effectively prevented.

  The pH of the reaction solution is preferably 4 or more and 7 or less. By setting it as such a range, an oxidizing agent can be made to act on a cellulose efficiently and the modification | denaturation of a cellulose can be implemented efficiently in a short time.

  It is preferable to add hypohalous acid or a salt thereof to the reaction solution. By using such a reforming method, the reaction rate can be remarkably improved, and the processing efficiency can be greatly improved.

  The N-oxyl compound is preferably 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO). Moreover, in the modification | reformation method which concerns on this invention, processing efficiency can be improved by using 4-acetamido TEMPO as an N-oxyl compound.

Next, the modified cellulose of the present invention is a water-soluble cellouronic acid characterized in that all or 50% or more of the C6-hydroxyl group of cellulose is substituted only with a carboxyl group or a metal salt of a carboxyl group. . Alternatively, it is a water-insoluble modified cellulose characterized in that at least a part of the C6 hydroxyl group of cellulose located on the surface is substituted only with a carboxyl group or a metal salt of a carboxyl group.
The modified cellulose according to the present invention is a modified cellulose obtained by the above-described modification method of the present invention, and can take various forms depending on the modification object.
For example, when regenerated cellulose is used, it is possible to obtain celouronic acid having a higher degree of polymerization than when a conventional treatment method is used. In addition, when alkali-treated cellulose is used, water-insoluble cellulose microcrystals can be obtained. The cellulose microcrystals can be uniformly dispersed in a dispersion medium such as water by the charge repulsion action of the carboxyl group introduced on the surface thereof.

  As a metal salt, a typical salt obtained by the above modification method is a sodium salt, but other metal may be used, and it becomes water-insoluble by substitution of the metal or imparts functionality. Modified cellulose is also included in the scope of the present invention. Examples of substitutable metals include various metals such as potassium, lithium, and other alkali metals, as well as calcium, magnesium, aluminum, silica, titanium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, silver, and barium. It is done. These metals can be freely selected depending on applications and necessary physical properties.

According to the cellulose modification method of the present invention, the primary hydroxyl group of cellulose is substituted only with a carboxyl group or a metal salt of a carboxyl group, and a modified cellulose having a higher degree of polymerization than conventional can be obtained. Such a modification method can be applied to the preparation of celouronic acid and the functionalization of cellulose microcrystals.
Since the modified cellulose of the present invention has a high degree of polymerization and a low molecular weight during the modification treatment, it has a high molecular weight, so it becomes a high-molecular-weight celouronic acid. Moreover, it becomes a functionalized cellulose microcrystal.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Method for modifying cellulose)
The method for modifying cellulose according to the present invention comprises using an N-oxyl compound as an oxidation catalyst on an alkali-treated cellulose or regenerated cellulose, which is a modification object, under a condition that the reaction solution is neutral or acidic. The cellulose is oxidized by the action of an oxidizing agent that oxidizes the group.

  As the cellulose to which the modification method according to the present invention can be applied, regenerated cellulose (viscose rayon, copper ammonia rayon (obtained by dissolving natural cellulose obtained by removing impurities such as lignin from plant resources and refining it in a solvent) ( (Cupra), tencel, polynosic, lyocell, etc.) and cellulose (natural cellulose, regenerated cellulose) treated with an alkali.

  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.

The alkali treatment of cellulose can be performed by, for example, a method of spraying or moistening an alkali solution on cellulose, or a method of immersing or suspending cellulose in an aqueous alkali solution.
As the alkali, alkali metal components such as alkali metal hydroxides (sodium hydroxide, potassium hydroxide, lithium hydroxide, etc.) and alkali metal carbonate aqueous solutions (sodium bicarbonate, potassium bicarbonate, etc.) can be used. These alkali metal compounds may be used alone or in admixture of two or more. Among these, it is preferable to use an alkali metal hydroxide, particularly sodium hydroxide. The concentration of the alkali used is a concentration necessary to swell the cellulose to convert the cellulose I type crystal structure into an amorphous or cellulose II type, and is preferably from 6% to 30%. The alkali treatment time is preferably from 1 minute to 1 day. After the alkali treatment, it is washed with water by filtration or the like to remove the alkali and subjected to the next oxidation reaction in an undried state.

  In the step of oxidizing cellulose, water is typically used as the cellulose dispersion medium in the reaction solution. The cellulose concentration in the 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.

  Moreover, when performing the modification process of fibrous cellulose, you may perform the process which expands surface areas, such as beating. Thereby, reaction efficiency can be improved and productivity can be improved. In addition, it is preferable to use what was preserve | saved in the never-dry state after isolation and refinement | purification, or an alkali treatment. By storing in a never dry state, the microfibril focusing body can be maintained in a state in which it easily swells, so that the reaction efficiency can be increased.

An N-oxyl compound is used as a catalyst added to the 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. 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 the addition amount range of 0.1-2 mmol / l.

  As the oxidizing agent, an oxidizing agent capable of oxidizing an aldehyde group generated by oxidation of a hydroxyl group is used. Examples of such an oxidizing agent include halous acid or a salt thereof (such as sodium chlorite), a mixture of hydrogen peroxide and oxidase (laccase), peracid, and the like. In addition, as peracid, various things, such as persulfuric acid (potassium hydrogen persulfate etc.), peracetic acid, perbenzoic acid, can be used. The content of the oxidizing agent is preferably in the range of 1 to 50 mmol / l.

By using an oxidizing agent that can oxidize an aldehyde group to a carboxyl group in this way, generation of an aldehyde group at the C6 position can be prevented.
FIG. 1 is a diagram showing a carboxyl group generation mechanism in the present invention. As shown in FIG. 1, in the oxidation reaction using an N-oxyl compound as a catalyst, the primary hydroxyl group of the glucose component may be selectively oxidized to produce an intermediate containing an aldehyde group. However, in the present invention, since an oxidizing agent that oxidizes an aldehyde group is included, the aldehyde group of this intermediate is quickly oxidized and converted to a carboxyl group. Thereby, the modified cellulose which does not contain an aldehyde group can be obtained.

In addition, it is preferable to add hypohalous acid or a salt thereof on the premise that the above-described oxidizing agent is used as the main oxidizing agent. For example, the reaction rate can be greatly improved by adding a small amount of sodium hypochlorite.
FIG. 2 is a diagram showing a reaction mechanism in the case of containing sodium hypochlorite. As shown in FIG. 2, sodium hypochlorite added to the reaction solution functions as an oxidizing agent for TEMPO, and the oxidized TEMPO oxidizes the primary hydroxyl group at the C6 position of cellulose and aldehyde group at the C6 position. Is generated. And the produced | generated aldehyde group is rapidly oxidized to a carboxyl group by the sodium chlorite which is a main oxidizing agent. Further, during the oxidation of the aldehyde group, sodium chlorite changes to sodium hypochlorite. Furthermore, the produced sodium hypochlorite is supplemented as an oxidizing agent for TEMPO.
Thus, by adding sodium hypochlorite to the reaction solution, the oxidation reaction of TEMPO can be promoted and the reaction rate can be increased.

  If the amount of hypohalite added is too large, these function as the main oxidizing agent, so that the molecular weight of the cellulose may be reduced and the desired modified cellulose may not be obtained. Therefore, the amount of hypohalite added is preferably about 1 mmol / l or less.

  In the present invention, the pH of the reaction solution is maintained in a neutral to acidic range. More specifically, a pH range of 4 to 7 is preferable. In particular, care should be taken that the pH of the reaction solution does not exceed 8. This is to prevent a beta elimination reaction due to an aldehyde group temporarily generated at the C6 position of cellulose.

Furthermore, it is preferable to add a buffer to the 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.

Here, Fig.3 (a) is a figure which shows an example of the apparatus for implementing the modification | reformation method of this invention. FIG.3 (b) is a figure which shows the apparatus for implementing the conventional processing method.
As shown in FIG. 3 (a), in the production method of the present invention, the reaction vessel 100 contains a reaction solution 110 containing an object to be modified (cellulose), a catalyst, an oxidizing agent, a buffer solution, etc. The reaction vessel 100 is sealed by 101. Moreover, the reaction vessel 100 can be heated using a heating device such as the hot tub 120, and the reaction temperature can be increased. In some cases, the reaction vessel 100 may be provided with a pressurizing device that pressurizes the inside.

  On the other hand, in the conventional processing method shown in FIG. 3B, the upper portion of the reaction vessel 200 containing the reaction solution 210 is opened, and the pH electrode 251 of the pH adjusting device 250 provided side by side through this opening. In addition, a nozzle 252 for supplying a diluted NaOH solution for pH adjustment is installed in the reaction solution 210. As described above, in the conventional processing method, the reaction vessel 210 has to be made into an oven type, so that a part of chlorine gas generated by the decomposition of sodium hypochlorite as a co-oxidant is released into the atmosphere. If it does so, the apparatus which processes the emitted chlorine gas will be needed, and the loss of an oxidizing agent will have to add sodium hypochlorite more than necessary.

  Thus, in the reforming method according to the present invention, the reaction vessel 100 can be sealed, so that the reaction efficiency can be increased by raising the temperature of the reaction solution 110. Therefore, according to the present invention, cellulose can be efficiently modified in a short time. On the other hand, it is possible to raise the temperature of the reaction solution 210 by the conventional treatment method, but the amount of chlorine gas released increases, which is not preferable in terms of exhaust gas treatment and the amount of oxidant used.

  In the cellulose modification method of the present invention, modified cellulose in different forms can be obtained depending on the mode of cellulose that is the modification target. It should be noted that the conditions for the reforming treatment described above should be changed as appropriate according to the reforming target shown below.

  First, when regenerated cellulose is used as an object to be modified, high molecular weight celouronic acid can be obtained. This cellulonic acid is used in various applications such as paper additives, glues, adhesives, emulsifiers and protective colloids, suspending agents, synthetic detergent builders, viscosity stabilizers (emulsifiers, creams, jams and other stabilizers), It can be used as a binder, a thickener and the like.

Furthermore, when alkali-treated cellulose is used as the modification target, water-insoluble cellulose microcrystals are obtained. These cellulose microcrystals are 10-500 nm size microcrystals (nanocrystals) having a cellulose II type crystal structure, and are novel substances that have not been obtained in the past.
Cellulose crystallites obtained from this alkali-treated cellulose are themselves water-insoluble, but can be dispersed in a medium by a slight dispersion treatment. That is, a cellulose microcrystal dispersion liquid in which cellulose microcrystals are dispersed in a medium can be obtained.

  As a medium (dispersion medium) used for dispersion, water is usually preferable, but other than water, a hydrophilic organic solvent can be used according to the purpose. Such hydrophilic organic solvents include water-soluble alcohols (methanol, ethanol, isopropanol, isobutanol, sec-butanol, tert-butanol, methyl cellosolve, ethyl cellosolve, ethylene glycol, glycerin, etc.), ethers (Ethylene glycol dimethyl ether, 1,4-dioxane, tetrahydrofuran, etc.), ketones (acetone, methyl ethyl ketone), N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide and the like can be exemplified. Further, a mixture of plural kinds of hydrophilic organic solvents may be used.

  As a dispersion apparatus used in the dispersion step, a general bath-type ultrasonic irradiation apparatus or a home mixer is sufficient, but an apparatus such as an ultrasonic homogenizer, a high-pressure homogenizer, or a biaxial kneading apparatus may be used. In addition to these, a cellulose crystallite dispersion can be easily obtained with a dispersion apparatus generally used for household or industrial production.

(Modified cellulose)
Next, in the modified cellulose obtained by the above-described modification method of the present invention, at least a part of the C6 hydroxyl group of the cellulose located on the surface is substituted only with a carboxyl group or a metal salt of a carboxyl group. It can be specified as modified cellulose. Moreover, the celouronic acid of the present invention can be specified as celouronic acid in which all or 50% or more of the C6 hydroxyl group of cellulose is substituted only with a carboxyl group or a metal salt of a carboxyl group.
Alternatively, at least a part of the C6 hydroxyl group of the cellulose located on the surface is substituted with a metal salt of a carboxyl group and specified as a modified cellulose or ceurouronic acid having an aldehyde group content of less than 0.05 mmol / g Can do.

That is, it can be considered that there is no or no aldehyde group at the C6-position on the surface of the modified cellulose or on the cellulonic acid. In addition, the case where it can be regarded that there is no aldehyde group corresponds to the content of the aldehyde group being less than 0.05 mmol / g.
By setting it as such a range, the fall of the polymerization degree resulting from an aldehyde group is suppressed, and a desired modification | reformation effect can be acquired. The amount of the aldehyde group is more preferably 0.01 mmol / g or less, and still more preferably 0.001 mmol / g or less.
In addition, since the detection limit of aldehyde groups in currently known measurement methods is about 0.001 mmol / g, a desirable mode is modified cellulose in which aldehyde groups are not detected even if measurement is performed.
In the conventional treatment method, both a carboxyl group and an aldehyde group are always generated in the TEMPO catalytic oxidation. Therefore, the modified cellulose of the present invention can be specified as clearly different from the modified cellulose obtained by the conventional treatment method due to the above characteristics.

  Moreover, the modified cellulose according to the present invention does not contain an aldehyde group at the C6 position, so that it is difficult to cause modification by heating. That is, when cellulose is treated by a conventional treatment method, it becomes a modified cellulose that produces coloration when heated to a high temperature, but such coloration does not occur in the modified cellulose according to the present invention. Therefore, according to this invention, the modified cellulose which can be used suitably also for the use heated is obtained.

  In addition, it is thought that coloring arises in the modified cellulose obtained by the conventional processing method for the following reasons. Aldehyde groups generated on the surface of the modified cellulose obtained by the conventional treatment method are 0.5 mmol / g or less (usually 0.3 mmol / g or less), which is a small amount compared to carboxyl groups. It also remains on the surface of cellulose. Therefore, it is considered that coloring occurs due to a reaction similar to caramelization in a reducing sugar having an aldehyde group.

In the modified cellulose according to the present invention, the carboxyl group content is preferably 0.05 mmol / g to 5 mmol / g, and more preferably 0.05 mmol / g to 1 mmol / g. Furthermore, it is desirable that it is 0.05 mmol / g-1 mmol / g.
Examples of the metal forming the metal salt include sodium, potassium, lithium, calcium, magnesium, aluminum, silica, titanium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, silver, and barium. . These metals can be appropriately selected according to required physical properties and applications.

  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.

Example 1
In this example, a method for producing cellouronic acid using the method for modifying cellulose according to the present invention will be described.

1 g of regenerated cellulose (Benryse manufactured by Asahi Kasei Co., Ltd .: polymerization degree 680) was dispersed in a 0.1 M aqueous acetic acid solution adjusted to pH 4.8.
To a dispersion in an Erlenmeyer flask, 0.096 g (0.45 mmol) of 4-acetamido-TEMPO, 0.83 ml (1 mmol) of a 9% solution of sodium hypochlorite, and commercially available 80% sodium chlorite were added. 1.70 g (15 mmol) was added to prepare a reaction solution. Thereafter, the Erlenmeyer flask was sealed and stirred with a magnetic stirrer while maintaining the temperature at 60 ° C.

  Next, the reaction solution was poured into a solution of ethanol: water = 3: 1, and the solid content in the reaction solution was filtered or centrifuged and washed with a washing solution of ethanol: water = 3: 1. Subsequently, the obtained solid content was dried to obtain cellulonic acid obtained by TEMPO oxidation of cellulose.

And the obtained cellouronic acid was dissolved in water and the ¹³ C-NMR spectrum was measured. The measurement results are shown in FIG. As shown in FIG. 4, since no C6 peak in the vicinity of 60 ppm was observed, it was confirmed that the cellulouric acid having a uniform chemical structure was obtained by the above production procedure.

In addition, the present inventors prepared cerouronic acid by changing the types of the catalyst and the buffer and the reaction time from the above production procedure.
Specifically, four types of reaction solutions each combining a type of catalyst (TEMPO, 4-acetamido-TEMPO) and a type of buffer solution (pH 4.8 acetate buffer solution, pH 6.8 phosphate buffer solution) were prepared. Using this, the reaction time (8 hours, 24 hours, 72 hours) was changed to prepare celouronic acid.
And about the obtained cellulonic acid, the measurement of the quantity of a carboxyl group and the measurement of the polymerization degree were performed.

The amount of the carboxyl group can be measured by the following method.
First, 60 ml of a 0.5 to 1% by weight slurry was prepared from a cellulonic acid sample that was precisely weighed in dry weight, adjusted to a pH of about 2.5 with a 0.1 M hydrochloric acid aqueous solution, and then 0.05 M sodium hydroxide. The aqueous solution is dropped and the electrical conductivity is measured. 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)

FIG. 5 is a graph showing the measurement results of the amount of carboxyl groups for each sample.
From the results shown in FIG. 5, when 4-acetamide-TEMPO is used, the efficiency of the acetate buffer at pH 4.8 is higher in the initial stage (conditions where the reaction time is short), but acetic acid is used under conditions of 24 hours or more. It was confirmed that there was no difference between the buffer solution and the phosphate buffer solution. On the other hand, when TEMPO was used, the reaction efficiency was higher when the phosphate buffer having pH 6.8 was used.

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

  Next, the measurement result of the degree of polymerization will be described. In the present specification, the degree of polymerization means “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) ).

  FIG. 6 is a graph showing the measurement results of the degree of polymerization. As shown in FIG. 6, under any condition, the degree of polymerization of the cellulonic acid is lower than that of the cellulose not subjected to the modification treatment (degree of polymerization 680), and is about 130 to 220. However, since the degree of polymerization of celouronic acid obtained by the conventional treatment method is about 40, the celouronic acid obtained by the modification method according to the present invention has a sufficiently high degree of polymerization compared to the conventional product. .

(Example 2)
In this example, functionalization of cellulose microcrystals using the cellulose modification method according to the present invention will be described.

In this example, cellulose subjected to alkali treatment by the following procedure was used as the modification target.
Softwood bleached kraft pulp and hardwood kraft pulp were each immersed in a 20% aqueous sodium hydroxide solution and washed with water to obtain alkali-treated cellulose (mercelized cellulose). Then, the alkali-treated cellulose was modified with the reaction solution used in Example 1 above. The obtained modified cellulose was different from the cellulonic acid prepared in Example 1 and was not soluble in water even after purification.

Moreover, about the obtained modified cellulose, the measurement of the amount of carboxyl groups and the measurement of the X-ray diffraction pattern were performed.
FIG. 7 is a graph showing the measurement results of the amount of carboxyl groups. As shown in FIG. 7, when alkali-treated cellulose was used, it was confirmed that the amount of carboxyl groups introduced was smaller than when regenerated cellulose was used. Moreover, from the measurement result of the X-ray diffraction pattern, it was confirmed that the modified cellulose of this example had a cellulose II type crystal form and maintained its crystallinity after the modification treatment. .

Next, when the obtained modified cellulose was dispersed in water and subjected to ultrasonic irradiation treatment, a transparent dispersion liquid was obtained.
FIG. 8 is an electron micrograph of the dried product of the aqueous dispersion. As shown in FIG. 8, it was found that the modified cellulose obtained in this example was a cellulose crystallite (nanocrystal) having a size of about 10 to 500 nm.
Thus, by applying the cellulose modification method according to the present invention to alkali-treated cellulose, a cellulose microcrystal having a cellulose II type crystal form and a carboxyl group introduced on the surface and a dispersion thereof are prepared. can do.

The figure which shows the production | generation mechanism of the carboxyl group in the modification | reformation method which concerns on this invention The figure which shows the oxidation mechanism of the cellulose in the modification | reformation method which concerns on this invention The figure which shows the apparatus used with the modification | reformation method based on this invention, and the conventional processing method The graph which shows the NMR spectrum measurement result which concerns on Example 1 Results of measurement of carboxyl group content of cellouronic acid according to Example 1 Measurement result of degree of polymerization of cellouronic acid according to Example 1 Result of measurement of carboxyl group content of modified cellulose according to Example 2 Electron micrograph of modified cellulose according to Example 2 The figure which shows the oxidation mechanism of the cellulose in the conventional processing method The figure which shows the oxidation mechanism of the cellulose in the conventional processing method Diagram explaining molecular chain scission by beta elimination reaction

Explanation of symbols

  100 reaction vessel, 101 cap, 110 reaction solution, 120 heating device

Claims (7)

  A method for modifying cellulose, comprising a step of oxidizing alkali-treated cellulose or regenerated cellulose in a neutral or acidic reaction solution containing an N-oxyl compound and an oxidizing agent that oxidizes an aldehyde group.   The method for modifying cellulose according to claim 1, wherein a buffer solution is added to the reaction solution.   The method for modifying cellulose according to claim 1 or 2, wherein a halogenous acid or a salt thereof is used as the oxidizing agent.   The N-oxyl compound is 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) or 4-acetamido-TEMPO. 2. The method for modifying cellulose according to item 1.   A modified cellulose characterized in that at least a part of the C6 hydroxyl group of cellulose located on the surface is substituted only with a carboxyl group or a metal salt of a carboxyl group.   Cellulonic acid, wherein all or 50% or more of the C6 hydroxyl group of cellulose is substituted only with a carboxyl group or a metal salt of a carboxyl group.   A water-insoluble cellulose microcrystal, wherein at least a part of a C6-position hydroxyl group of cellulose located on a surface is substituted only with a carboxyl group or a metal salt of a carboxyl group.

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