Method for producing cellulose gel and method for producing porous material

Abstract

The present invention addresses the problem of providing, in a simple manner, a gel and a porous material which have little environmental burden or impact on health and which have high strength.　This method for producing a gel is characterized by having: a first step for mixing hydroxyl group-containing fibrous cellulose with an alkaline reagent to prepare a cellulose solution; a second step for freezing the cellulose solution and reacting the cellulose with the alkaline reagent for a prescribed period of time so as to modify a cellulose I type crystal structure into a cellulose II type crystal structure; and a third step for defrosting the frozen material.

Description

Method for producing cellulose gel and method for producing porous body

[0001] The present invention relates to a method for producing a cellulose gel by freeze aggregation and a method for producing a porous body.

[0002] Cellulose is the main component of plant cell walls and is a natural polymer that has been used since ancient times. Cellulose is a linear polymer formed by β-glucose bonds in a β-(1-4) structure, and these linear chains are regularly arranged through hydrogen bonds, forming a crystalline structure known as cellulose I in nature. The surface of natural cellulose is composed only of hydroxyl groups, which have low reactivity, and the molecules form a bundled aggregate structure. Therefore, in order to utilize it artificially, technologies have been developed that involve dissolving cellulose, regenerating it, spinning it, and processing it, or derivatizing cellulose through chemical reactions, spinning it, and processing it.

[0003] The cellulose gel produced by this technique and the porous body obtained by drying the gel are expected to be applied in a variety of fields, including as an adsorbent and a material for regenerative medicine.

[0004] On the other hand, the above-mentioned cellulose dissolution-regeneration spinning-processing or cellulose derivatization-spinning-processing technologies through chemical reactions may not be easy to use and may not place a low burden on the environment or the human body, depending on the reagents used during dissolution, derivatization, or processing. Therefore, there is a demand for more environmentally friendly cellulose processing technologies.

[0005] Although environmental friendliness is taken into consideration, some cellulose gels and porous bodies are not strong enough to be used as the above-mentioned adsorbents or regenerative medical materials, and there is a need to develop technology to create stronger cellulose gels and porous bodies.

[0006] Patent Document 1 discloses a method for producing porous cellulose particles, in which cellulose adjusted to an average degree of polymerization of 450 by hydrolyzing pulp is dissolved in an 8 wt % aqueous sodium hydroxide solution at −6°C, and the solution is poured into hexane at −16°C in the form of atomized fine particles using a spray nozzle to obtain a frozen body, which is then poured into a 50% aqueous sulfuric acid solution at −20°C, and the frozen body is kept at −20°C to obtain cellulose particles.

[0007] Japanese Unexamined Patent Publication No. 64-43530

[0008] The method for producing porous cellulose particles described in Patent Document 1 uses hexane as a freezing solvent, and therefore it is difficult to say that it has a low environmental impact or a low burden on the human body.

[0009] In addition, in Patent Document 1, the temperature of the freezing solvent is set to -16°C, but because the solvent in which cellulose is dissolved is sprayed in the form of fine particles, the cellulose solution is introduced into hexane in the form of fine particles with a small heat capacity, which causes the cellulose solution to freeze very rapidly. Furthermore, because the frozen body is immediately washed with an aqueous sulfuric acid solution, it is not considered that the cellulose I crystalline structure be sufficiently transformed into the cellulose II crystalline structure, as in the cellulose gel of the present invention disclosed by the authors of the present invention, and therefore sufficient strength cannot be obtained.

[0010] The present invention has been made in view of the above problems, and an object of the present invention is to provide a gel and a porous body that are easy to use and have high strength.

[0011] To solve the above problems, the method for producing a gel of the present invention comprises the following steps: a first step of mixing hydroxyl-containing fibrous cellulose with an alkaline reagent to prepare a cellulose solution; a second step of freezing the cellulose solution to form a frozen mass, and reacting the cellulose with the alkaline reagent for a predetermined time to denature the cellulose I crystal structure into cellulose II; and a third step of melting the frozen mass. This specification incorporates the disclosure of Japanese Patent Application No. 2023-200223, from which the present application claims priority.

[0012] By carrying out the present invention, it becomes possible to easily provide a gel and a porous body that are high in strength.

[0013] It should be noted that new problems and effects not described in the above problems and effects will become apparent from the description of the embodiments of the invention.

[0014] 1 is a diagram showing the flow of a method for synthesizing a cellulose gel according to a first embodiment; FIG. 2 is a diagram schematically showing the synthesis of a cellulose gel according to the first embodiment; FIG. 3 is a diagram schematically showing the flow of a method for synthesizing a cellulose gel according to a second embodiment; FIG. 4 is a diagram schematically showing the synthesis of a cellulose gel according to the second embodiment; FIG. 5 is a diagram schematically showing the flow of a method for synthesizing a cellulose gel according to a third embodiment; FIG. 6 is a diagram schematically showing the synthesis of a cellulose gel according to the third embodiment; FIG. 7 is a diagram showing photographs illustrating the external shapes of cellulose gels according to the present invention, respectively (a) a photograph showing the external shape of a cellulose gel according to Experimental Example 1, (b) a photograph showing the external shape of a cellulose gel according to Experimental Example 2, (c) a photograph showing the external shape of a cellulose gel according to Experimental Example 5, and (d) a photograph showing the external shape of a cellulose gel according to Comparative Example 1; FIG. 8 is a diagram showing the self-supporting ability and restoring ability of cellulose gels according to Experimental Examples 1 and 2 and Comparative Examples 1 to 4 according to the present invention; FIG. 9 is a diagram showing the results of a compressive strength test according to the present invention, respectively (a) the results of the cellulose gel according to Experimental Example 1, (b) the results of the cellulose gel according to Experimental Example 2, (c) the results of the cellulose gel according to Comparative Example 2, and (d) the results of the cellulose gel according to Comparative Example 3.

[0023] Figures showing the results of PXRD measurements according to the present invention, where (a) is a diagram showing the relationship between diffraction intensity and Q value in each experimental example, and (b) is a diagram showing the crystallinity and crystal size in each experimental example. Figures showing the results of SEM measurements according to the present invention, where (a) is the result for the cellulose gel according to Experimental Example 1, (b) is the result for the cellulose gel according to Experimental Example 2, and (c) is the result for the cellulose gel according to Comparative Example 1. Figures showing the results of FT-IR measurements in each experimental example according to the present invention. Figures showing the results of an adsorption test in each experimental example according to the present invention. Figures showing the physical properties, etc. in Experimental Example 5 according to the present invention, where (a) is a photograph showing the external shape of the cellulose gel according to Experimental Example 5, (b) is the result of a compressive strength test of the cellulose gel according to Experimental Example 5, and (c) is the result of SEM measurement of the cellulose gel according to Experimental Example 5. Figures showing (a) the structural formula of a cellulose nanofiber according to the present invention, and (b) the structural formula of citric acid.

[0015] Hereinafter, embodiments of the gel and porous body according to the present disclosure will be described with reference to the drawings.

[0016] First Embodiment A first embodiment will be described with reference to Figures 1 and 2. Figure 1 is a flow chart showing a method for synthesizing a cellulose gel according to the first embodiment.

[0017] In the method for synthesizing a cellulose gel according to the present invention, as shown in FIG. 1 , fibrous cellulose having hydroxyl groups is prepared in step S11. Then, in step S12, the fibrous cellulose is dissolved in water to prepare a solution. Then, in step S13, an alkaline reagent is added to the solution, and the alkaline reagent is adjusted to a predetermined concentration to prepare a mixed solution. Note that an adsorbent, photocatalyst, etc., as described in detail below, may be added when preparing the mixed solution. Next, in step S14, the mixed solution is poured into a desired container and frozen to obtain a frozen body. Finally, in step S15, the frozen body is left to stand for a predetermined time to allow a gelation reaction, after which it is dissolved and washed for a predetermined time to obtain a cellulose gel. The cellulose gel obtained in this manner, as described below, differs from cellulose gel obtained without freezing and contains a large amount of cellulose II crystalline structure, which is more water-absorbent and has high strength.

[0018] In the present invention, "high strength" means a strength sufficient to allow the molded cellulose gel to stand on its own without losing its shape.

[0019] <Fibrous Cellulose> The hydroxyl group-containing fibrous cellulose used in the present invention is, for example, a cellulose polymer, and specific examples include cellulose nanofiber, nanocellulose, cellulose derivatives, etc. Furthermore, "containing a hydroxyl group" means that the cellulose polymer may have at least one or more hydrophilic groups other than hydroxyl groups, such as carboxyl groups, sulfonic groups, other anionic groups, amino groups, and amide groups, but it is preferable that the hydroxyl group is the main hydrophilic group.

[0020] The specific substitution amount of hydroxyl groups in the cellulose nanofibers is preferably 50% or more, and more preferably 80% or more, of all functional groups in the cellulose nanofibers.

[0021] <Alkaline Reagent> The alkaline reagent used in the present invention may be, for example, at least one of monovalent bases such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; divalent bases such as magnesium hydroxide and calcium hydroxide; and trivalent bases such as aluminum hydroxide, iron hydroxide, and ammonia.

[0022] On the other hand, from the viewpoint of environmental load, cost, etc., it is preferable to use sodium hydroxide.

[0023] <Freezing rate> The solution in which fibrous cellulose having hydroxyl groups is dissolved is frozen, and there are no particular restrictions on the freezing rate or freezing temperature. However, by adjusting the freezing rate, it is possible to control the ice particle size to some extent. More specifically, rapid freezing results in smaller ice particle size, while slow freezing results in larger ice particle size. Therefore, if you want to adjust the surface area of ​​the porous body obtained by removing water from the cellulose gel, you can adjust the freezing rate.

[0024] Details will be explained in the Principles and Experimental Examples, but if the freezing rate is too fast, sufficient strength may not be obtained, but it is thought that a certain level of freezing rate will improve strength.

[0025] <Melting rate> After the frozen body is reacted for a predetermined time, the frozen body is melted to obtain a cellulose gel. At this time, there is no particular limitation on the rate at which the frozen body is melted, but it is considered preferable to melt the frozen body slowly rather than rapidly.

[0026] <Principle> The principle of the present invention will be explained using Figure 2. Note that the diagram used in Figure 2 is for the purpose of facilitating understanding and does not accurately represent specific dimensions, etc. Hereinafter, for the purpose of facilitating understanding of the principle of the present invention, the explanation will be given assuming that cellulose nanofiber 1 is used as the fibrous polymer having hydroxyl groups and sodium hydroxide is used as the alkaline reagent. First, the top diagram in Figure 2 corresponds to steps S11 and S12 in Figure 1 and shows the state in which cellulose nanofiber 1 has dissolved in water. In this state, cellulose nanofiber 1 has not been sufficiently defibrated and takes the form of cellulose I with a crystalline structure 1a.

[0027] Part A in Figure 2 is a diagram showing the crystalline structure of cellulose nanofiber 1, showing the details of the crystalline structure 1a of cellulose I. The dotted lines in the crystalline structure 1a of cellulose I indicate hydrogen bonds of cellulose, which form hydrogen bonds within the cellulose molecule. This structure is weaker than the crystalline structure 10a of cellulose II shown later.

[0028] Next, the diagram in the upper middle section of Figure 2 corresponds to step S13 in Figure 1, and shows the state in which sodium hydroxide has been added to a predetermined concentration in an aqueous solution of cellulose nanofibers in water. As shown in the figure, sodium ions 2 (Na ＋ ) enters the cellulose nanofiber 1, and the hydrogen of the hydroxyl group (-OH) is replaced by a sodium ion, -ONa ＋ As a result, the cellulose nanofibers 1 swell and are partially dissolved.

[0029] The diagram in the lower middle section of Figure 2 corresponds to step S14 in Figure 1 and shows the frozen state of a mixed solution of cellulose nanofibers 1 and sodium hydroxide. When the mixed solution is frozen, layer separation occurs between the ice crystals and the nanocellulose and bound water, with some of the hydroxyl groups replaced with sodium ions, resulting in a separation into ice crystals 3 and a freeze-concentrated layer 4 containing the cellulose nanofibers and bound water. In particular, the cellulose nanofibers 1 are pushed out by the ice crystals 3 and are present in high density in the freeze-concentrated layer 4 together with the sodium ions 2 and hydroxide ions that are not incorporated into the ice crystals 3. As a result, more of the hydrogen in the hydroxyl groups (-OH) of the cellulose nanofibers 1 is replaced with sodium ions.

[0030] In this state, the effects of salting out and freeze-concentration cause a rearrangement of the hydrogen bonds in the cellulose nanofiber 1, and even at a lower concentration of sodium hydroxide than that required for conventional mercerization, the crystalline structure 1a of cellulose I can be changed to the crystalline structure 10a of cellulose II.

[0031] The diagram at the bottom of Fig. 2 corresponds to step S15 in Fig. 1 and shows the structure of the cellulose gel 100 after the frozen mixed solution is thawed over a predetermined period of time and washed with water. The cellulose gel 100 produced by the above-described method is a gel having a sheet structure 10 rather than a fibrous structure of cellulose nanofibers.

[0032] Part B in Figure 2 shows the crystalline structure of the sheet structure 10, and is a detailed view of the crystalline structure 10a of cellulose II. The dotted lines in the crystalline structure 10a of cellulose II indicate hydrogen bonds in cellulose, which form not only within cellulose molecules but also between cellulose molecules. This structure makes it stronger than the crystalline structure 1a of cellulose I.

[0033] It is believed that the cellulose gel according to the present invention has high strength due to the sheet structure 10 and the crystalline structure 10a of cellulose II.

[0034] Second Embodiment Next, a second embodiment will be described with reference to FIGS.

[0035] 3 is a flowchart showing a method for synthesizing a gel according to the second embodiment. Note that the same reference numerals are used for components similar to those in the first embodiment, and descriptions of parts that overlap with those in the first embodiment will be omitted.

[0036] The difference between the gel synthesis method according to the first embodiment and this embodiment is that step S21 of adding a cross-linking agent to the frozen body is added between step S14 of freezing the mixed solution and step S15 of melting the frozen body.

[0037] <Crosslinking Agent> The crosslinking agent used in the present invention is selected from agents capable of crosslinking fibrous cellulose having hydroxyl groups. In particular, in the present invention, a crosslinking agent that forms a hydrogen bond or an ionic bond with fibrous cellulose is preferred. For example, when crosslinking is performed using an acid, either an organic acid or an inorganic acid may be used. The organic acid may be at least one of formic acid, acetic acid, lactic acid, malic acid, succinic acid, maleic acid, oxalic acid, citric acid, ascorbic acid, etc., and the inorganic acid may be at least one of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, polyphosphoric acid, etc.

[0038] On the other hand, when a basic crosslinking agent is used, for example, ammonium carbonate, ammonium citrate, or the like can be used.

[0039] On the other hand, when a metallic crosslinking agent is used, for example, polyvalent metal salts such as calcium chloride, calcium carbonate, aluminum chloride, aluminum sulfate, aluminum lactate, and zirconium (particularly zirconium chloride oxide) can be used.

[0040] It is believed that the valence of the crosslinking agent affects the strength of the gel when synthesized; for example, it is presumed that the greater the valence, the stronger the synthesized gel will be.

[0041] <Principle> The principle of the second embodiment of the present invention will be described with reference to Fig. 4. Note that the same components as those in the first embodiment are designated by the same drawing numbers.

[0042] As described above, the difference between the gel synthesis method according to the first embodiment and this embodiment is that step S21 of adding a cross-linking agent to the frozen body is added between step S14 of freezing the mixed solution and step S15 of thawing the frozen body. Therefore, the explanation of the top diagram and the upper middle diagram of Fig. 4, which overlap with the first embodiment, will be omitted.

[0043] The diagram in the middle bottom of Figure 4 corresponds to steps S14 and S21 in Figure 3, and shows the state after a mixed solution of cellulose nanofibers 1 and sodium hydroxide is frozen and a crosslinking agent is added. The crosslinking agent penetrates into the freeze-concentrated layer 4 where cellulose nanofibers 1 and sodium ions 2 are densely packed, and reacts to form crosslinked structures 5 between the cellulose nanofibers 1.

[0044] The diagram at the bottom of Figure 4 corresponds to step S25 in Figure 3 and shows the structure of a cellulose gel 200 after the frozen mixed solution is thawed over a predetermined time period and washed with water. The cellulose gel 200 produced by the above-described method is a gel having a sheet structure 20 rather than a fibrous structure of cellulose nanofibers. The gel according to the present invention is thought to have a higher strength than the first embodiment due to the sheet structure 20, the crystalline structure 10a of cellulose II, and the cross-linked structure 5.

[0045] Part C in Fig. 4 is an enlarged view of the cross-linked structure 5 between cellulose fibers. Note that X shown in the figure varies depending on the type of cross-linking agent, and for example, one derived from citric acid or the like is incorporated.

[0046] Third Embodiment Next, a third embodiment will be described with reference to FIGS. 5 and 6. FIG.

[0047] 5 is a flowchart showing a method for synthesizing a gel according to the third embodiment. Note that the same reference numerals are used for the same components as those in the first embodiment, and descriptions of parts that overlap with those in the first embodiment will be omitted.

[0048] The difference between the gel synthesis method of the first embodiment and this embodiment is that a step S31 has been added in which a gas is flowed through the cellulose gel obtained in step S15 to cause it to adsorb.

[0049] <Gas> The gas used in the present invention is selected from gases that are adsorbed by the cellulose gel, and is preferably selected from gases that can crosslink the hydroxyl-containing fibrous cellulose, such as at least one of carbon dioxide, ammonia, hydrogen sulfide, nitrous acid, nitric oxide, and nitrogen dioxide.

[0050] As a method for adsorbing a gas into a gel, various methods can be used depending on the gas to be adsorbed, such as flowing the gas directly through the gel or dissolving the gas in a solution such as water and then adsorbing it.

[0051] When using a method in which the gas is first dissolved in an aqueous solution or the like and then adsorbed, a known bubble generator can be used, and in order to increase the efficiency of dissolution in a solution such as water, a fine bubble generator that generates bubbles with a diameter of 100 μm or less can be used as appropriate.

[0052] <Principle> The principle of the third embodiment of the present invention will be described with reference to Fig. 6. Fig. 6 is a diagram for explaining step S31 shown in Fig. 5.

[0053] The top diagram in Figure 6 shows the cellulose gel 100 obtained through step S15. Part D in Figure 6 is an enlarged view of a portion of the cellulose gel 100, and as explained in the first embodiment, the cellulose gel 100 has a structure in which sheet structures 10 in which cellulose nanofibers are freeze-crosslinked are folded over one another.

[0054] 6 shows an example of reacting cellulose gel 100 with gas 33. In this example, cellulose gel 100 is placed in a container 32, water is added, and then gas 33 is caused to flow into the solution for a predetermined time using a bubble generator 31, causing the gas 33 to be adsorbed into the cellulose gel 100.

[0055] For example, CO ２ When using CO ２ dissolves in water and becomes bicarbonate ions and carbonate ions, which are adsorbed onto the cellulose gel 100. At this time, the bicarbonate ions and carbonate ions react with functional groups in the cellulose gel 100 to form a crosslinked structure as shown in the second embodiment.

[0056] The lower diagram in FIG. 6 shows the cellulose gel 300 after it has reacted with the gas 33 .

[0057] Part E in Figure 6 is an enlarged view of a portion of the cellulose gel 300, and even after the gas 33 has been adsorbed, the sheet structure 30 is maintained. On the other hand, what differs from the cellulose gel 100 is that a crosslinked structure 6 is provided in the sheet structure 30. Part F in Figure 6 is an enlarged view of the sheet structure 30. Note that G shown in the figure changes depending on the type of gas to be adsorbed, and for example, a functional group derived from carbon dioxide or the like is incorporated.

[0058] <Additives> Common to all embodiments, it is also possible to impart specific properties to the resulting cellulose gel by adding various substances in step S12 or step S13.

[0059] For example, it is possible to impart higher adsorption performance to the gel by adding clay minerals or activated carbon powder in step S12 or step S13. Although clay minerals and activated carbon powder are given as examples of adsorbents, any material that has adsorption performance and disperses in a solvent can be added.

[0060] Furthermore, by adding iodine or silver ions in step S12 or step S13, it is possible to impart bactericidal properties, antiviral properties, etc. to the gel. Although iodine and silver ions are given as examples of materials that impart bactericidal properties, any material that has bactericidal properties and disperses in a solvent can be added.

[0061] Furthermore, catalytic activity can be imparted to the cellulose gel by adding titanic acid or the like in step S12 or step S13. Although titanic acid is given as an example of a material that imparts photocatalytic activity, any material that has photocatalytic activity and disperses in a solvent can be added.

[0062] The porous body obtained by drying the cellulose gel can be controlled to have a thickness that ranges from a filter to a sponge. Details will be explained in the experimental examples below.

[0063] Furthermore, by adding hydrophobic molecules such as cholesterol or fatty acids to the gel, the gel can be made hydrophobic and used as an adsorbent for hydrophobic molecules. Although cholesterol and fatty acids are given as examples of materials that impart hydrophobicity, any material that is hydrophobic and can form bonds with the functional groups of the polymer can be added.

[0064] Regarding the method for drying the cellulose gel, various drying methods such as freeze drying, ethanol substitution drying, heat drying, etc. can be used.

[0065] In the above, for ease of understanding, an example using water and fibrous cellulose having hydroxyl groups was given as an example, but if an organic solvent that can be solidified by cooling is used, it is possible to obtain a gel with high strength, similar to that obtained when water is used.

[0066] On the other hand, using water alone instead of an organic solvent offers various advantages, such as lower procurement costs for water, reduced cooling costs due to its freezing temperature of 0°C, shorter gel preparation times, and easier production of larger cellulose gels and porous bodies. Furthermore, since no organic solvent components remain in the cellulose gel or porous body, gels and porous bodies that are gentle on the human body can be provided. It goes without saying that water alone may contain some organic components as impurities.

[0067] Next, each experimental example according to the present invention will be described.

[0068] Experimental Example 1 (NC-Na0.2-F) A commercially available cellulose nanofiber (biomass nanofiber BiNFi-s (WTFo-10002), purity 99.5% or higher, manufactured by Sugino Machine Co., Ltd.) was used to prepare a cellulose nanofiber solution at a concentration of 2 weight percent in water.

[0069] The cellulose nanofibers used were unchemically modified, had an average width of approximately 10 to 50 nm, and a length of several micrometers, and were made from hardwood pulp, which was physically defibrated by water jet treatment (the structural formula of the cellulose nanofibers used in this example is shown in Figure 15(a)). The cellulose nanofibers were composed of approximately 70 to 80% cellulose and approximately 20 to 30% hemicellulose.

[0070] A sodium hydroxide solution (1.0 mol / L, 190-02171, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the obtained cellulose nanofiber solution so that the sodium hydroxide concentration of the cellulose nanofiber solution was 0.2 mol / L to prepare a mixed solution, which was then transferred to a plastic container and left to stand at -20°C for 12 hours to obtain a frozen body.

[0071] The resulting frozen body was then left to thaw at 4°C for 12 hours and washed with water to obtain a cellulose gel 100 as shown in Fig. 7(a). The cellulose gel obtained in Experimental Example 1 will be referred to as NC-Na0.2-F (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.2 mol / L, and F indicates that it was produced by freezing).

[0072] The obtained cellulose gel was subjected to measurements of water content, a compressive strength test, observation with a scanning electron microscope (hereinafter simply referred to as "SEM"), Fourier transform infrared spectroscopy (hereinafter simply referred to as "FT-IR"), powder X-ray diffraction (hereinafter simply referred to as "PXRD"), and an adsorption test. Note that the SEM measurement, FT-IR measurement, and PXRD measurement were performed using freeze-dried samples of the cellulose gel.

[0073] <Measurement of Water Content> The water content is calculated by the following equation (1), where the weight of the obtained gel is Ws (g) and the weight of the freeze-dried gel is Wd (g).

[0074]

[0075] The cellulose gel was freeze-dried using a freeze-dryer (FDU-1100) manufactured by EYELA.

[0076] <Compression Strength Test> The compression strength test of the cellulose gel was performed by repeating the compression test three times using a texture analyzer (TA.XAPlus) manufactured by Stable Micro Systems, Inc. The measurement was performed using a cylindrical probe (diameter 20 mm) that was pressed at a speed of 1 mm / sec until 80% compression was reached, and then raised at 1 mm / sec, with the zero position being the point at which a load of 2 gf was detected.

[0077] Based on the results of the compressive strength test, the compressive modulus of elasticity was calculated from the slope of the stress-strain curve in the linear region (0.01 to 0.05 ST), and the Young's modulus was calculated from the average value of these measurement results.

[0078] <SEM Measurement> SEM measurement was carried out using a scanning electron microscope (TM4000PlusII) manufactured by Hitachi High-Technologies Corporation.

[0079] <FT-IR Measurement> FT-IR measurement was carried out at room temperature using a Fourier transform infrared spectrophotometer (FT / IR-6600) and a diffuse reflectance measuring device (DR-81) manufactured by JASCO Corporation.

[0080] <PXRD Measurement> PXRD measurement was performed using an X-ray diffractometer (MiniFlex 600) manufactured by Rigaku Corporation, with a Cu—K radiation source. α Measurements were performed at room temperature using a diffraction angle of 0.15418 nm (λ = 0.15418 nm). Diffraction data was collected at intervals of 0.02 degrees. The scattering vector Q was calculated using the following equation (2).

[0081]

[0082] The degree of crystallinity and crystal size were estimated by analyzing the PXRD profile.

[0083] <Adsorption test> In the adsorption test, 0.1 mol / L of PbCl ２ Aqueous solution, 0.1 mol / L CuCl ２ aqueous solution, and 0.1 mol / L ZnCl ２ Aqueous solutions were prepared, and 35 mg of cellulose gel sample was added to 7 ml of each solution, followed by stirring at room temperature for 24 hours. The pH was not adjusted during this process.

[0084] In the adsorption measurement, the supernatant of the aqueous solution obtained by the above method was taken out and measured using an IPC optical emission spectrometer (700 series) manufactured by Agilent Technologies, Inc. The removal efficiency Reff was calculated by the following (Equation 3).

[0085]

[0086] Here, Ci and Ct are the Pb in the aqueous solution, respectively. ２＋ , Cu ２＋ or Zn ２＋where Ci is the concentration of the aqueous solution at the start of the adsorption test, and Ct is the concentration of the aqueous solution after t hours have elapsed since the start of the adsorption test.

[0087] <Experimental Example 2 (NC-Na0.2-FC)> Next, Experimental Example 2 will be described, focusing on the differences from Experimental Example 1. The difference from Experimental Example 1 is that citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 030-05525) was added to the obtained frozen body; specifically, in this experiment, 2 mL of a 1.0 mol / L citric acid solution was added to the frozen body (the structural formula of citric acid used in this example is shown in Figure 15(b)). Note that other points are the same as those in Experimental Example 1, and therefore description thereof will be omitted.

[0088] This method resulted in the production of a cellulose gel 200 as shown in Fig. 7(b). The cellulose gel obtained in Experimental Example 2 will be referred to as NC-Na0.2-FC (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.2 mol / L, F indicates that the gel was produced by freezing, and C indicates that citric acid was added).

[0089] <Experimental Example 3 (NC-Na0.1-F)> Next, Experimental Example 3 will be explained, focusing on the differences from Experimental Example 1. The difference from Experimental Example 1 is the amount of sodium hydroxide added to the cellulose nanofiber solution; in this experiment, sodium hydroxide solution was added so that the sodium hydroxide concentration in the cellulose nanofiber solution was 0.1 mol / L. Note that explanations of other aspects that are similar to Experimental Example 1 will be omitted.

[0090] In this way, the cellulose gel of Experimental Example 3 was obtained. The cellulose gel obtained in Experimental Example 3 will be referred to as NC-Na0.1-F (NC indicates cellulose nanofiber, Na0.1 indicates that the sodium hydroxide concentration was 0.1 mol / L, and F indicates that it was prepared by freezing).

[0091] Experimental Example 4 (NC-Na0.2-BFC (0.5:1)) Next, Experimental Example 4 will be described. This Experimental Example is based on Experimental Example 2 and differs from Experimental Example 2 in that bentonite powder (manufactured by Volkray Japan, bentonite from Wyoming, USA) was added to a cellulose nanofiber solution prepared to a concentration of 2 weight percent with respect to water. In this Experimental Example, the weight of bentonite was added so that the weight of the dried cellulose nanofiber was 0.5, assuming the weight of the dried cellulose nanofiber was 1. More specifically, 16 mg of bentonite powder was added, and then a sodium hydroxide solution (1.0 mol / L, 190-02171, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added so that the sodium hydroxide concentration of the cellulose nanofiber solution was 0.2 mol / L to prepare a mixed solution. The mixed solution was then transferred to a plastic container and allowed to stand at -20°C for 12 hours to obtain a frozen body. Note that other aspects are the same as those of Experimental Example 2, and therefore further description will be omitted.

[0092] In this way, the cellulose gel of Experimental Example 4 was obtained. The cellulose gel obtained in Experimental Example 4 will be referred to as NC-Na0.2-BFC (0.5:1) (NC indicates cellulose nanofibers, Na0.2 indicates a sodium hydroxide concentration of 0.2 mol / L, B indicates the addition of bentonite, F indicates that it was prepared by freezing, C indicates the addition of citric acid, and (0.5:1) indicates the amount of bentonite added relative to the weight of the cellulose nanofibers).

[0093] <Experimental Example 5 (NC-Na0.2-BFC (2:1))> Next, Experimental Example 5 will be described, focusing on the differences from Experimental Example 4. The difference from Experimental Example 1 is the amount of bentonite added; in this Experimental Example, when the weight of dried cellulose nanofibers is taken as 1, the weight of bentonite was added so that it was 2; more specifically, 64 mg of bentonite powder was added. Note that other points are the same as those in Experimental Example 4, and therefore a description thereof will be omitted.

[0094] In this manner, a cellulose gel 700 as shown in Fig. 7(c) was obtained. The cellulose gel obtained in Experimental Example 5 is referred to as NC-Na0.2-BFC (2:1).

[0095] <Experimental Example 6 (NC-Na0.2-BFC (3:1))> Next, Experimental Example 6 will be described, focusing on the differences from Experimental Example 4. The difference from Experimental Example 1 is the amount of bentonite added; in this Experimental Example, when the weight of dried cellulose nanofibers is taken as 1, the weight of bentonite was added so that it was 3; more specifically, 96 mg of bentonite powder was added. Note that other points are the same as those in Experimental Example 4, and therefore a description thereof will be omitted.

[0096] In this manner, the cellulose gel of Experimental Example 6 was obtained. The cellulose gel obtained in Experimental Example 6 is referred to as NC-Na0.2-BFC (3:1).

[0097] Comparative Example 1 (NC-Na0.2-RC) Next, a description will be given of Comparative Example 1. A commercially available cellulose nanofiber (biomass nanofiber BiNFi-s (WTFo-10002), purity 99.5% or higher, manufactured by Sugino Machine Co., Ltd.) was used to prepare a cellulose nanofiber solution at a concentration of 2 weight percent relative to water.

[0098] As in Experimental Example 1, the cellulose nanofibers used were unchemically modified, had an average width of approximately 10-50 nm, and a length of several micrometers, were made from hardwood pulp, and were physically defibrated by water jet treatment. The cellulose nanofibers were composed of approximately 70-80% cellulose and approximately 20-30% hemicellulose.

[0099] A sodium hydroxide solution (1.0 mol / L, 190-02171, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the obtained cellulose nanofiber solution so that the sodium hydroxide concentration of the cellulose nanofiber solution was 0.2 mol / L to prepare a mixed solution, and then 2 mL of a 1.0 mol / L citric acid solution was added to the mixed solution. The mixed solution was then transferred to a plastic container and allowed to stand at room temperature for 12 hours to obtain a cellulose gel.

[0100] The resulting cellulose gel was washed with water to obtain cellulose gel 210 as shown in Fig. 7(d). The cellulose gel obtained in Comparative Example 1 will be referred to as NC-Na0.2-RC (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.2 mol / L, R indicates that the gel was prepared at room temperature without freezing, and C indicates that citric acid was added).

[0101] Comparative Example 2 (NC-P-F) Next, a description will be given of Comparative Example 2. A commercially available cellulose nanofiber (biomass nanofiber BiNFi-s (WTFo-10002), purity 99.5% or higher, manufactured by Sugino Machine Ltd.) was used to prepare a cellulose nanofiber solution at a concentration of 2 weight percent relative to water.

[0102] As in Experimental Example 1, the cellulose nanofibers used were unchemically modified, had an average width of approximately 10-50 nm, and a length of several micrometers, were made from hardwood pulp, and were physically defibrated by water jet treatment. The cellulose nanofibers were composed of approximately 70-80% cellulose and approximately 20-30% hemicellulose.

[0103] The resulting cellulose nanofiber solution was transferred to a plastic container and left to stand at −20° C. for 12 hours to obtain a frozen body.

[0104] The resulting frozen body was then allowed to stand at 4°C for 12 hours to thaw, and washed with water to obtain the cellulose gel of Comparative Example 2. The cellulose gel obtained in Comparative Example 2 will be referred to as NC-P-F (NC indicates cellulose nanofiber, P indicates that no sodium hydroxide was added, and F indicates that it was prepared by freezing).

[0105] Comparative Example 3 (NC-P-FC) Next, Comparative Example 3 will be described. The difference from Comparative Example 2 is that citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 030-05525) was added to the obtained frozen body; specifically, in this experimental example, 2 mL of a 1.0 mol / L citric acid solution was added to the frozen body. Note that other points are the same as in Comparative Example 2, and therefore description thereof will be omitted.

[0106] In this way, the cellulose gel of Comparative Example 3 was obtained. The cellulose gel obtained in Comparative Example 3 is referred to as NC-P-FC (NC indicates cellulose nanofiber, P indicates that sodium hydroxide was not added, F indicates that it was prepared by freezing, and C indicates that citric acid was added).

[0107] <Comparative Example 4 (NC-P-RC)> Next, we will explain Comparative Example 4. The difference from Comparative Example 1 is that a cellulose gel was created from a cellulose nanofiber solution adjusted to 2 weight percent with respect to water without adding a sodium hydroxide solution. Note that other points are the same as those of Comparative Example 1, and therefore explanations will be omitted.

[0108] In this way, the cellulose gel of Comparative Example 4 was obtained. The cellulose gel obtained in Comparative Example 4 is referred to as NC-P-RC (NC indicates cellulose nanofiber, P indicates that sodium hydroxide was not added, R indicates that it was prepared at room temperature without freezing, and C indicates that citric acid was added).

[0109] <Comparative Example 5 (NC-Na0.05-F)> Next, Comparative Example 5 will be described. This comparative example is based on Experimental Example 1, but the amount of sodium hydroxide added was changed. Specifically, it differs from Experimental Example 1 in that the amount of sodium hydroxide added to the cellulose nanofiber solution is different; in this experimental example, sodium hydroxide solution is added so that the sodium hydroxide concentration in the cellulose nanofiber solution is 0.05 mol / L. Note that other points are the same as in Experimental Example 1, so explanation will be omitted.

[0110] In this way, the cellulose gel of Comparative Example 5 was obtained. The cellulose gel obtained in Comparative Example 5 will be referred to as NC-Na0.05-F (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.05 mol / L, and F indicates that it was prepared by freezing).

[0111] <Comparative Example 6 (NC-Na0.03-F)> Next, Comparative Example 6 will be described. This comparative example is based on Experimental Example 1, but the amount of sodium hydroxide added was changed. Specifically, it differs from Experimental Example 1 in that the amount of sodium hydroxide added to the cellulose nanofiber solution is different; in this experimental example, sodium hydroxide solution is added so that the sodium hydroxide concentration in the cellulose nanofiber solution is 0.03 mol / L. Note that other points are the same as in Experimental Example 1, so explanation will be omitted.

[0112] In this way, the cellulose gel of Comparative Example 6 was obtained. The cellulose gel obtained in Comparative Example 6 will be referred to as NC-Na0.03-F (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.03 mol / L, and F indicates that it was prepared by freezing).

[0113] <Comparative Example 7 (NC-Na0.01-F)> Next, Comparative Example 7 will be described. This comparative example is based on Experimental Example 1, but the amount of sodium hydroxide added was changed. Specifically, it differs from Experimental Example 1 in that the amount of sodium hydroxide added to the cellulose nanofiber solution is different; in this experimental example, sodium hydroxide solution is added so that the sodium hydroxide concentration in the cellulose nanofiber solution is 0.01 mol / L. Note that other points are the same as in Experimental Example 1, so explanation will be omitted.

[0114] In this way, the cellulose gel of Comparative Example 7 was obtained. The cellulose gel obtained in Comparative Example 7 will be referred to as NC-Na0.01-F (NC indicates cellulose nanofiber, Na0.2 indicates that the sodium hydroxide concentration was 0.01 mol / L, and F indicates that it was prepared by freezing).

[0115] <Results of moisture content measurement> In measuring the moisture content, in order to confirm the difference in moisture content depending on whether or not a crosslinking agent was used, the differences were mainly compared between Experimental Example 1 and Experimental Example 2. As a result, the moisture content was 96% in Experimental Example 1 and 97% in Experimental Example 2, and there was no significant difference in moisture content depending on whether or not a crosslinking agent was used.

[0116] <Results of Compression Strength Test> In the compression strength test, measurements were carried out on the cellulose gels of Experimental Examples 1 and 2 and Comparative Examples 1 to 4 to confirm the difference in effect depending on whether or not sodium hydroxide was present. Figure 8 shows the results of confirming the self-supporting ability and recovery ability in Experimental Examples 1 and 2 and Comparative Examples 1 to 4. Regarding self-supporting ability, when the cellulose gel was molded and allowed to stand still without the application of external force, a "good" was recorded if the gel was able to stand on its own while maintaining its shape, and an "unsatisfactory" was recorded if the gel could not stand on its own while maintaining its shape.

[0117] Regarding resilience, cellulose gels that did not break when compressed with a finger were marked with "〇", cellulose gels that restored their shape when compressed with a finger were marked with "◎", cellulose gels that broke when compressed with a finger were marked with "×", and cellulose gels that could not be molded into the desired shape were marked with "-".

[0118] As shown in Figure 8, in Experimental Examples 1 and 2, in which the sodium hydroxide concentration was adjusted to 0.2 mol / L and then frozen, cellulose gels were obtained that were strong enough to withstand compression with a finger. In Experimental Example 2, in which citric acid was added to the frozen body, a cellulose gel was obtained that was strong enough to restore its shape when compression with a finger was applied to the cellulose gel.

[0119] 9 shows the results of the compressive strength test, and since no free-standing cellulose gel was obtained for Comparative Example 1, the results of the compressive strength test for Experimental Example 1 are shown in Fig. 9(a), Experimental Example 2 in Fig. 9(b), Comparative Example 2 in Fig. 9(c), and Comparative Example 3 in Fig. 9(d). Note that good overlap of data in each cycle, i.e., substantially identical data in each cycle, indicates high restorability.

[0120] As shown in Figure 9(a), in Experimental Example 1, in which the sodium hydroxide concentration was adjusted to 0.2 mol / L and frozen, the cellulose gel did not break even after three cycles of compressive strength testing, and although there was a slight difference from the first to the third cycle, the data were almost the same for each cycle.

[0121] Furthermore, as shown in Figure 9(b), in Experimental Example 2, in which the concentration of sodium hydroxide was adjusted to 0.2 mol / L, frozen, and cross-linked with citric acid, the cellulose gel was not destroyed even after three cycles of compressive strength testing, and the data were similar from the first cycle to the third cycle.

[0122] On the other hand, the cellulose gel of Comparative Example 2, which was frozen without adding sodium hydroxide, and the cellulose gel of Comparative Example 3, which was frozen without adding sodium hydroxide and then added with citric acid, were destroyed in the first cycle of the compressive strength test, as shown in Figures 9(c) and 9(d), respectively.

[0123] From the above results, it is clear that the addition of sodium hydroxide is necessary to give the cellulose gel high restorability.

[0124] Furthermore, in order to give the cellulose gel created greater restorability, it is preferable to crosslink the frozen body, which has been frozen with the addition of sodium hydroxide, using a crosslinking agent (citric acid).

[0125] <Results of PXRD Measurement> In order to confirm the effect of sodium hydroxide concentration on the obtained cellulose gel, the cellulose gel obtained in Experimental Example 1, Experimental Example 3, Comparative Example 2, and Comparative Examples 5 to 7 was freeze-dried and powdered, and the measurement was carried out. Figure 10(a) shows the results of the PXRD measurement for the cellulose gels obtained in Experimental Example 1, Experimental Example 3, Comparative Example 2, and Comparative Examples 5 to 7. －１ 1 shows the results of X-ray diffraction patterns for each of the experimental examples and comparative examples in the range of 1000 to 15000.

[0126] As can be seen from FIG. 10A, the Q value of NC-PF (corresponding to Comparative Example 2) and NC-Na0.05-F to NC-Na0.01-F (corresponding to Comparative Examples 5 to 7) was 10.3 nm. －1 , 11.9 nm －1 , and 16 nm －1 It can be seen that there are characteristic peaks (shown by circles in Figure 10(a)) at the diffraction pattern of the crystalline structure of cellulose I shown in Figure 10(a) . These peaks correspond to the (1-10), (110), and (200) planes in the diffraction pattern of the crystalline structure of cellulose I. This indicates that the crystalline structure of cellulose I is dominant in Comparative Examples 2 and 5 to 7.

[0127] On the other hand, two diffraction patterns of the crystalline structure of cellulose II are described, and for NC-Na0.1-F (corresponding to Experimental Example 3) and NC-Na0.2-F (corresponding to Experimental Example 1), the Q value is 14.1 nm －1 , and 15.6 nm －1 For NC-Na0.2-F, a peak appears at 8.6 nm －1 A peak appears at 8.6 nm, and the peak derived from cellulose II crystals disappears. －1 , 14.1 nm －1 , and 15.6 nm －1These peaks correspond to the (1-10), (110), and (020) planes in the diffraction pattern of the crystalline structure of cellulose II shown in Figure 10(a), and indicate that NC-Na0.1-F is transforming into crystalline cellulose II.

[0128] FIG. 10(b) shows the crystallinity values ​​calculated from the results of PXRD measurements for Experimental Example 1, Comparative Example 2, and Comparative Examples 5 to 7. Note that the crystallinity values ​​shown are for cellulose I in Comparative Example 2 and Comparative Examples 5 to 7, and for Experimental Example 1, the crystallinity values ​​are for cellulose II. The crystallinity values ​​of cellulose I in NC-P-F (corresponding to Comparative Example 2) were 52%, cellulose I in NC-Na0.01-F (corresponding to Comparative Example 7) were 53%, and cellulose I in NC-Na0.05-F (corresponding to Comparative Example 5) were approximately 45%, indicating that the crystallinity of cellulose I gradually decreased with increasing sodium hydroxide concentration. Meanwhile, the crystallinity of cellulose II in NC-Na0.2-F (corresponding to Experimental Example 1) was 17%. 10(a) and 10(b) show that cellulose II crystals appear and the degree of crystallinity of cellulose II increases when the sodium hydroxide concentration is increased to 0.05 mol / L. This indicates that when the sodium hydroxide concentration is increased above 0.05 mol / L, cellulose II crystals are formed and the strength of the cellulose gel increases.

[0129] Furthermore, Figure 10(b) also plots the crystal size for each experimental example and comparative example. The crystal size of NC-P-F (corresponding to Comparative Example 2) was 4.8 nm, the crystal size of NC-Na0.01-F (corresponding to Comparative Example 7) was 4.2 nm, and the crystal size of NC-Na0.2-F (corresponding to Experimental Example 1) was 2.6 nm, indicating that the crystal size decreased with increasing sodium hydroxide concentration. This means that the cellulose nanofibers were dissolved by sodium hydroxide, and that the dissolution rate of the cellulose nanofibers increased with increasing sodium hydroxide concentration.

[0130] In other words, as explained in the principle above, the formation of the freeze-concentrated layer 4 increases the sodium hydroxide concentration in the layer, which causes the cellulose nanofibers to dissolve more in the freeze-concentrated layer 4, creating a state in which the cellulose nanofibers are dissolved at a high density.

[0131] Conventionally, a sodium hydroxide concentration of approximately 3.0 mol / L was required to convert cellulose I into cellulose II, so this method increases the effect of sodium hydroxide by approximately 15 to 30 times.

[0132] <Regarding SEM Measurement> Fig. 11 shows the results of SEM measurement of Experimental Example 1, Experimental Example 2, and Comparative Example 1. Fig. 11(a) shows an SEM image of NC-Na0.2-F (corresponding to Experimental Example 1), Fig. 11(b) shows an SEM image of NC-Na0.2-FC (corresponding to Experimental Example 2), and Fig. 11(c) shows an SEM image of NC-Na0.2-RC (corresponding to Comparative Example 1).

[0133] 11(a) and (b), it can be seen that the fibers derived from the raw material cellulose nanofibers have disappeared and a sheet structure has been formed in the cellulose gels of Experimental Examples 1 and 2. On the other hand, as can be seen in Fig. 11(c), it can be seen that the fibers derived from the cellulose nanofibers remain in the cellulose gel of Comparative Example 1.

[0134] From the above results, it can be seen that simply increasing the sodium hydroxide concentration to 0.2 mol / L without freezing the cellulose gel will not result in a sheet-like microstructure, and a high-strength cellulose gel will not be obtained.

[0135] Furthermore, upon closer observation of Figure 11(b), it was found that the sheet had pores of approximately 70 μm in size, had a more uniform sheet structure than the structure shown in Figure 11(a), and had minute particles attached to the surface of the sheet.

[0136] Analysis of these particles revealed that they contained only carbon and oxygen, and therefore were likely composed of cellulose nanofibers and / or citric acid. This suggests that amorphous cellulose may have been extruded when the crystalline structure changed from cellulose I to cellulose II. It is speculated that the cellulose gel of Experimental Example 2 has higher strength and recovery than Experimental Example 1 because the sheet structure was formed by extruding amorphous cellulose.

[0137] <FT-IR Measurement> FIG. 12 shows the results of measurements using freeze-dried powders of the cellulose gels obtained in Experimental Example 1, Experimental Example 3, Comparative Example 2, and Comparative Examples 5 to 7. －１ ~4000cm －１ FT-IR measurement results are shown in the range of

[0138] The characteristic feature in Figure 12 is the O-H stretching mode at 3050 cm －１ ~3600cm －１ Specifically, as the concentration of sodium hydroxide increases, the O-H band broadens and its relative intensity decreases. This means that the hydrogen bonds within the cellulose gel also become more diverse as the concentration of sodium hydroxide increases. This result suggests that cellulose I is formed by intramolecular hydrogen bonds, while cellulose II is formed by both intramolecular and intermolecular hydrogen bonds.

[0139] <Adsorption Test> FIG. 13 shows the results of the adsorption test of the cellulose gels of Experimental Examples 2 and 4 to 6. ２＋ , Cu ２＋ , Zn ２＋ The graph shows the removal efficiency Reff of each of the above.

[0140] In the case of NC-Na0.2-FC (corresponding to Experimental Example 2) to which bentonite was not added, Pb ２＋ , Cu ２＋ , and Zn ２＋ The Reff values ​​for the three compounds were 17%, 64%, and 13%, respectively, and no specific adsorption was observed.

[0141] On the other hand, for NC-Na0.2-BFC (0.5:1) to NC-Na0.2-BFC (3:1) (corresponding to Experimental Examples 4 to 6, respectively), the adsorption performance tended to improve as the amount of bentonite increased. ２＋ , Cu ２＋ , Zn ２＋ The removal efficiencies Reff of the respective compounds were 79%, 74%, and 76% for NC-Na0.2-BFC (0.5:1), 94%, 91%, and 93% for NC-Na0.2-BFC (2:1), and 98%, 100%, and 98% for NC-Na0.2-BFC (3:1).

[0142] In particular, when compared with bentonite powder itself, Experimental Examples 5 and 6 achieved a Reff of 90% or more for each metal ion, and Experimental Example 6 achieved results almost equivalent to those of bentonite powder.

[0143] When adsorption is performed using bentonite powder, the problem is that it is difficult to recover the bentonite powder after adsorption because it is a powder. On the other hand, when bentonite is supported on cellulose gel as in the present invention, the bentonite does not peel off and it is possible to simply recover the cellulose gel, which is advantageous in terms of ease of use.

[0144] From the above viewpoint, it is preferable to add an amount of bentonite powder that is not peeled off from the cellulose gel. If an amount greater than that supported in Experimental Example 6 is added, some bentonite will peel off. Therefore, if the weight of the cellulose nanofiber is taken as 1, it is preferable that the weight of the bentonite be 3.0 or less.

[0145] Figure 14 shows the physical properties of NC-Na0.2-BFC (2:1) (corresponding to Experimental Example 5), where Figure 14(a) shows photographs of the cellulose gel before and after applying compressive force with a finger, Figure 14(b) shows the results of a compressive strength test, and Figure 14(c) shows the results of SEM observation.

[0146] As shown in Figure 14(a), even when the cellulose gel was compressed with a finger, the bentonite did not peel off. The SEM observation results in Figure 14(c) show that the bentonite particles are supported by a sheet structure, so they do not easily peel off even when pressure is applied.

[0147] Furthermore, in the compressive strength test shown in Figure 14(b), similar to the NC-Na0.2-FC (corresponding to Experimental Example 2) shown in Figure 9(b), the data were almost the same from the first to third cycles, indicating that there was no decrease in strength even when bentonite was supported on cellulose gel.

[0148] As described above, by using the present invention, it is possible to provide a gel and a porous body that are easy to use, have low environmental impact and low burden on the human body, and have high strength.

[0149] The present invention will be briefly summarized below.

[0150] The method for producing cellulose gel described in the present invention includes a first step of mixing fibrous cellulose having hydroxyl groups with an alkaline reagent to prepare a cellulose solution, a second step of freezing the cellulose solution to form a frozen body, and reacting the cellulose with the alkaline reagent for a predetermined time to denature the cellulose I crystal structure into cellulose II, and a third step of melting the frozen body.

[0151] By using the above method, when the cellulose solution is frozen, the alkaline reagent and fibrous cellulose having hydroxyl groups are forced into the nanospaces between the ice crystals, and the fibrous cellulose having hydroxyl groups can be denatured from cellulose I to cellulose II crystal structure while being confined at a high density, making it possible to easily provide a high-strength cellulose gel.

[0152] Furthermore, in the method for producing cellulose gel described in the present invention, as shown in Figures 10(a) and 10(b), a cellulose material in which peaks due to the crystalline structure of cellulose II are not visible is transformed into a state in which peaks due to the crystalline structure of cellulose II are visible (in the present invention, this is expressed as expressing the crystalline structure of cellulose II).

[0153] By using the above method, it is possible to provide a strong cellulose gel that has a crystalline structure of cellulose II in the cellulose gel.

[0154] In order to obtain a cellulose gel with higher strength, it is preferable that the crystallinity of cellulose II in the above reaction is 10% or more, more preferably 15% or more.

[0155] Furthermore, in the method for producing cellulose gel described in the present invention, sodium hydroxide is used as the alkaline reagent.

[0156] By using the above method, it is possible to obtain cellulose gel using reagents that have a low environmental impact.

[0157] In the method for producing a cellulose gel according to the present invention, the lower limit of the concentration of sodium hydroxide in the cellulose solution is set to 0.1 mol / L or more.

[0158] By using the above method and setting the above sodium hydroxide concentration, the sodium hydroxide concentration in the frozen concentrated layer can be increased, thereby promoting the mercerization of fibrous cellulose having hydroxyl groups.

[0159] In the method for producing a cellulose gel according to the present invention, the lower limit of the concentration of sodium hydroxide in the cellulose solution is set to 0.2 mol / L or more.

[0160] By using the above method and achieving the above sodium hydroxide concentration, the sodium hydroxide concentration in the frozen concentrated layer can be made higher, making it possible to more efficiently promote mercerization of fibrous cellulose having hydroxyl groups.

[0161] In addition, in the method for producing a cellulose gel described in the present invention, the upper limit of the sodium hydroxide concentration in the cellulose solution is set to less than 3.0 mol / L.

[0162] By using the above method, the sodium hydroxide concentration becomes lower than 3.0 mol / L, which is the concentration required for mercerization at room temperature, and therefore it is possible to reduce the environmental load.

[0163] The upper limit of the concentration of sodium hydroxide in the cellulose solution is set to less than 1.0 mol / L.

[0164] By using the above method, the concentration of cellulose is lower than the level at which it must be treated as a toxic substance, making it possible to produce cellulose gel simply and safely.

[0165] Furthermore, in the method for producing cellulose gel described in the present invention, in the second step, at least one crosslinking agent selected from the group consisting of formic acid, acetic acid, lactic acid, malic acid, succinic acid, maleic acid, oxalic acid, citric acid, ascorbic acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and polyphosphoric acid is added.

[0166] Since the crosslinking agents used in the above-mentioned methods are all chemicals with low environmental impact, it is possible to provide cellulose gel in an environmentally friendly manner.

[0167] As shown in this experimental example, by adding citric acid, it is possible to provide a cellulose gel with higher strength.

[0168] In addition, in the method for producing cellulose gel described in the present invention, at least one additive selected from the group consisting of clay mineral powder, activated carbon powder, iodine, silver ions, titanic acid, and zirconium is added in the first step.

[0169] By adding additives as in the above method, it is possible to impart any desired function to the resulting cellulose gel.

[0170] In the method for producing cellulose gel according to the present invention, bentonite is used as the clay mineral powder, and the weight of this bentonite is set to be three times or less the weight of the dried fibrous cellulose.

[0171] By limiting the amount of bentonite added to three times or less as in the above method, it is possible to obtain a cellulose gel in which the bentonite is less likely to peel off during use.

[0172] In the method for producing cellulose gel according to the present invention, bentonite is used as the clay mineral powder, and the weight of this bentonite is at least twice the weight of the dried fibrous cellulose.

[0173] By doubling or more the amount of bentonite added as in the above method, it is possible to ensure performance of about 80% or more compared to ordinary bentonite powder.

[0174] Furthermore, in the method for producing a cellulose gel according to the present invention, a gas is adsorbed onto the obtained cellulose gel.

[0175] In particular, when carbon dioxide is adsorbed into cellulose gel, it is incorporated between cellulose molecules to form a cross-linked structure, which can improve the strength of the cellulose gel itself that is produced.

[0176] The gas may be adsorbed simultaneously with the thawing of the frozen body, or after the frozen body has been completely thawed.

[0177] As for the adsorption method, an appropriate method can be used depending on the gas to be adsorbed, such as a method in which the gas is dissolved in a liquid and adsorbed onto a cellulose gel in the liquid, or a method in which the gas is directly flowed onto a cellulose gel or a porous body made by drying the gel and adsorbed.

[0178] Although the embodiments of the present invention have been described above using the drawings, the specific configuration is not limited to this embodiment, and even if there are design changes and the like within the scope that does not deviate from the gist of the present invention, they are also included in the present invention.

[0179] 1: Cellulose nanofibers 2: Sodium ions 3: Ice crystals 4: Freeze-concentrated layer 10: Sheet structure All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.

Claims

1. A method for producing a cellulose gel, comprising: a first step of mixing fibrous cellulose having hydroxyl groups with an alkaline reagent to prepare a cellulose solution; a second step of freezing the cellulose solution to form a frozen body, and reacting the cellulose with the alkaline reagent for a predetermined period of time to denature the cellulose I crystal structure into cellulose II; and a third step of melting the frozen body.

2. A method for producing a cellulose gel according to claim 1, characterized in that the crystallinity of the cellulose II is 10% or more.

3. The method for producing a cellulose gel according to claim 2, wherein the alkaline reagent is sodium hydroxide.

4. A method for producing a cellulose gel as described in claim 3, characterized in that the concentration of sodium hydroxide in the cellulose solution is 0.1 mol / L or more.

5. A method for producing a cellulose gel as described in claim 4, characterized in that the concentration of sodium hydroxide in the cellulose solution is less than 3.0 mol / L.

6. A method for producing a cellulose gel as described in claim 5, characterized in that the concentration of sodium hydroxide in the cellulose solution is less than 1.0 mol / L.

7. A method for producing a cellulose gel as described in claim 6, characterized in that the concentration of sodium hydroxide in the cellulose solution is 0.2 mol / L or more.

8. The method for producing a cellulose gel according to claim 2, characterized in that in the second step, at least one crosslinking agent selected from the group consisting of formic acid, acetic acid, lactic acid, malic acid, succinic acid, maleic acid, oxalic acid, citric acid, ascorbic acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and polyphosphoric acid is added.

9. The method for producing a cellulose gel according to claim 8, wherein the crosslinking agent is citric acid.

10. A method for producing a cellulose gel as described in claim 2, characterized in that in the first step, at least one additive selected from the group consisting of clay mineral powder, activated carbon powder, iodine, silver ions, titanic acid, and zirconium is added.

11. A method for producing a cellulose gel as described in claim 10, wherein the clay mineral powder is bentonite, and the bentonite is added in an amount three times or less the weight of the dried fibrous cellulose.

12. The method for producing a cellulose gel according to claim 11, wherein the bentonite is added in an amount at least twice the weight of the dried fibrous cellulose.

13. The method for producing a cellulose gel according to claim 2, further comprising adsorbing carbon dioxide into the cellulose gel after or simultaneously with the third step.

14. A method for producing a cellulose gel according to any one of claims 1 to 13, further comprising drying the resulting cellulose gel to produce a porous body.

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