Cellulose nanofiber slurry and its powder

Abstract

To provide a method for dispersing cellulose nanofibers without causing re-aggregation, even when the cellulose nanofiber slurry is further diluted with water or when the powder obtained from the slurry is diluted with an organic solvent. [Solution] PVA, a nonionic dispersant with only hydroxyl groups, is added to a cellulose nanofiber slurry prepared by kneading cellulose nanofibers with water, and after mixing and stirring, PVP, a nonionic dispersant with hydroxyl groups and lipophilic groups, is added to produce a cellulose nanofiber slurry. To produce a powder, isopropyl alcohol is added to the mixture, and the mixture is stirred and mixed to remove water to produce flakes, which are then pulverized in a grinder or similar to produce a powder.

Description

[Technical Field]

[0001] The present invention relates to a slurry of cellulose nanofibers and a powder obtained by drying and pulverizing the slurry, and more specifically to a slurry of cellulose nanofibers and a powder thereof that are dispersed without causing re-agglomeration even when the slurry is diluted with a solvent such as water. [Background technology]

[0002] Water is generally used as a solvent to disperse cellulose nanofibers. Water is compatible with cellulose nanofibers that contain hydroxyl groups, and adding a small amount of surfactant or dispersant to water enables more effective diffusion and dispersion (Patent Document 1).

[0003] However, if you try to dilute a cellulose nanofiber slurry that has already been dispersed in water (for example, a slurry with a weight ratio of 1 to 8% cellulose nanofiber to water) by adding more water, the cellulose nanofibers will aggregate in the dilution water, which acts as a solvent, as shown in Figure 7.

[0004] Therefore, in order to break up the agglomerates, it is necessary to break up the agglomerates mechanically or by adding a dispersant.

[0005] Generally, when mechanically deflocculating the agglomerates, a machine such as a three-roll mixer or a planetary mixer is used to forcibly deflocculate the agglomerates.

[0006] On the other hand, when dispersing the particles with a dispersant, the dispersant is changed depending on the type of solvent to dissolve the particles.

[0007] Specifically, when the solvent is water, surfactants used include anionic surfactants such as sodium lauryl sulfate and sulfonic acid surfactants, cationic surfactants such as silica tetramethylammonium salts and alkyltrimethylammonium salts, zoitial anionic surfactants such as sodium laurylbenzenesulfonate, and nonionic surfactants such as polyoxyethylene alkyl ethers and polyvinyl alcohol (PVA), and these surfactants are used to disperse cellulose nanofibers in water.

[0008] On the other hand, when the solvent is an organic solvent, surfactants used include anionic surfactants such as sodasteroyl monoglutamic acid and sodasteroyl glutamic acid, cationic surfactants such as silica tetramethylammonium salt, zoitial anionic surfactants such as sodium laurylbenzenesulfonate, and nonionic surfactants such as polyvinylpyrrolidone (PVP).The slurry is dispersed in the solvent using these surfactants, and the concentration of the cellulose nanofiber slurry is adjusted, after which it is used as a liquid slurry, or the slurry is dried and added as a powder to resins, etc. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication WO2018 / 143149 Summary of the Invention [Problem to be solved by the invention]

[0010] However, when attempting to disperse cellulose nanofiber slurry using such surfactants, the following problems arise.

[0011] That is, when a dispersant is added to a solvent such as water or an organic solvent to dilute the mixture, the dispersant must be changed depending on the type of solvent, and the mixture gradually separates after thickening, making it difficult to completely deflocculate the particles. For this reason, it is often necessary to forcibly deflocculate the particles using a machine such as a mixer.

[0012] Furthermore, even if the cellulose nanofibers are dried, powdered, and then added to the same or a different solvent, they undergo strong fiber aggregation during drying. Even if forced agitation using a mixer or other device is attempted, the fibers exhibit poor diffusibility and cannot be dispersed beyond a controlled slurry. Therefore, in practice, a liquid slurry with a controlled concentration is often used instead of a powder, limiting the development of products containing cellulose nanofibers.

[0013] Therefore, the present invention has been made with a focus on the above-mentioned problems, and aims to make it possible to maintain dispersion without causing re-agglomeration, even when the cellulose nanofiber slurry is further diluted with water, or when the powder obtained from the slurry is diluted with an organic solvent. [Means for solving the problem]

[0014] That is, in order to solve the above-mentioned problems, the present invention provides a method for producing a cellulose nanofiber slurry by kneading cellulose nanofibers with water, comprising the steps of: adding a nonionic dispersant having only hydroxyl groups to the cellulose nanofiber slurry; mixing and stirring the cellulose nanofiber slurry to which the nonionic dispersant having only hydroxyl groups has been added; adding a nonionic dispersant having hydroxyl groups and lipophilic groups to the cellulose nanofiber slurry that has been mixed and stirred; and stirring the cellulose nanofiber slurry to which the nonionic dispersant having hydroxyl groups and lipophilic groups has been added; a step of adding a sorbitan ester to a slurry of cellulose nanofibers that has been mixed and stirred with the nonionic dispersant having only a hydroxyl group and the nonionic dispersant having both a hydroxyl group and a lipophilic group, and stirring the mixture; The device is equipped with a cellulose nanofiber slurry producing device.

[0015] In this way, even if a slurry of cellulose nanofibers that has been dispersed once is diluted again with water, it is possible to maintain the dispersion without causing reagglomeration.Furthermore, even if this cellulose nanofiber slurry is dried to form a powder and then diluted with water or an organic solvent, it is possible to prevent reagglomeration and maintain the dispersion.

[0016] In this invention, the nonionic dispersant having only a hydroxyl group is at least one of polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polyvinyl amine, and cellulose ether.

[0017] The nonionic dispersant having a hydroxyl group and a lipophilic group is at least one of polyethylene glycol, polyvinyl porlidone, methyl cellulose, hydroxypropyl methyl cellulose, and polyacrylamide.

[0018] Further, the method includes a step of adding a nonionic dispersant having only hydroxyl groups to a cellulose nanofiber slurry obtained by kneading cellulose nanofibers with water, a step of mixing and stirring the cellulose nanofiber slurry to which the nonionic dispersant having only hydroxyl groups has been added, a step of adding a nonionic dispersant having hydroxyl groups and lipophilic groups to the cellulose nanofiber slurry that has been mixed and stirred, and a step of stirring the cellulose nanofiber slurry to which the nonionic dispersant having hydroxyl groups and lipophilic groups has been added. a step of adding a sorbitan ester to a slurry of cellulose nanofibers that has been mixed and stirred with the nonionic dispersant having only a hydroxyl group and the nonionic dispersant having a hydroxyl group and a lipophilic group, and stirring the resulting slurry; and a step of adding a sorbitan ester to the slurry of cellulose nanofibers that has been mixed and stirred. The method for producing cellulose nanofiber powder includes the steps of adding isopropyl alcohol and mixing and stirring, adding isopropyl alcohol and mixing and stirring, then removing water to produce flakes, and grinding the produced flakes into a fine powder using a grinder or the like.

[0019] In this way, even when the powdered cellulose nanofibers are diluted with water or an organic solvent, they can be dispersed without re-aggregation. [Effects of the Invention]

[0020] According to the present invention, even when a slurry of cellulose nanofibers that has been dispersed once is diluted again with water, the cellulose nanofibers can be kept dispersed without causing reagglomeration.Furthermore, even when the cellulose nanofiber slurry is dried to form a powder and then diluted with water or an organic solvent, the cellulose nanofibers can be kept dispersed without causing reagglomeration. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 shows a process for producing cellulose nanofiber slurry or powder in one embodiment of the present invention. [Figure 2] A comparative example showing the state of a cellulose nanofiber slurry immediately after mixing and stirring PVA and PVP in the same form. [Figure 3] A comparative example showing the state of a cellulose nanofiber slurry prepared by mixing and stirring PVA and PVP in the same form after 48 hours of mixing and stirring. [Figure 4] A comparative example showing the state of cellulose nanofiber slurry powder obtained by mixing and stirring PVA and PVP in the same form and diluting it with water. [Figure 5] A comparative example showing the state in which the cellulose nanofiber slurry powder obtained by mixing and stirring PVA and PVP in the same form is diluted with toluene. [Figure 6] A photograph showing a slurry of cellulose nanofibers made by mixing and stirring PVA, PVP, and sorbitan in the same form. [Figure 7] A photograph showing the state of a conventional cellulose nanofiber slurry diluted with water only. DETAILED DESCRIPTION OF THE INVENTION

[0022] An embodiment of the present invention will now be described.

[0023] The cellulose nanofiber slurry in this embodiment is a cellulose nanofiber slurry that has been previously dispersed in water and that is then diluted with water or an organic solvent to prevent re-aggregation. The process involves adding a non-ionic dispersant having only hydroxyl groups to a cellulose nanofiber slurry prepared by kneading cellulose nanofibers with water, mixing and stirring the cellulose nanofiber slurry to which the non-ionic dispersant having only hydroxyl groups has been added, adding a non-ionic dispersant having hydroxyl groups and lipophilic groups to the cellulose nanofiber slurry that has been mixed and stirred, and stirring the cellulose nanofiber slurry to which the non-ionic dispersant having hydroxyl groups and lipophilic groups has been added. a step of adding a sorbitan ester to a slurry of cellulose nanofibers that has been mixed and stirred with the nonionic dispersant having only a hydroxyl group and the nonionic dispersant having both a hydroxyl group and a lipophilic group, and stirring the mixture; The method for producing a slurry of cellulose nanofibers in this embodiment will be described in detail below.

[0024] The cellulose nanofiber slurry used initially contains 1 to 8 weight percent cellulose nanofibers (average length 10 μm). To produce this initial slurry, water, pure water, distilled water, hypochlorous acid, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), and, if necessary, enzymes such as cellulase are added to a cellulose raw material such as wood pulp. A coarse dispersion is then achieved using a stirring mixer. Next, to perform the nano-dispersion process, the mixture is kneaded for an extended period using a high-pressure homogenizer, ultrasonic homogenizer, or planetary mixer, pretreated in a three-roll mill, and refined to an average fiber diameter of 5 nm using a bead mill. The pH is then adjusted as necessary.

[0025] The resulting cellulose nanofiber slurry is then commercially available in a forcibly dispersed state in water.

[0026] If a commercially available cellulose nanofiber slurry is purchased and then diluted with water, the cellulose nanofibers will aggregate, necessitating forced mechanical dispersion again. Therefore, in this embodiment, the following method is used to ensure that the cellulose nanofibers remain dispersed even after dilution without forced mechanical dispersion.

[0027] 1. First process (step S1)

[0028] In the first step, commercially available cellulose nanofiber slurry is placed in a mixer, heated to 65°C, and stirred with a mixer. After this heating and stirring is complete, a nonionic dispersant containing only hydroxyl groups is added while the material temperature is still at 65°C, and the mixture is stirred with a mixer. Examples of nonionic dispersants containing only hydroxyl groups include polyvinyl alcohol (PVA), polyvinyl acetal (PVB), polyvinyl butyral (PVB), polyvinylamine (PVAm), and cellulose ethers such as methylcellulose and hydroxypropylmethylcellulose (HPMC). These nonionic dispersants containing only hydroxyl groups have the following effects:

[0029] <Improved dispersibility>

[0030] Nonionic dispersants that only have hydroxyl groups are highly hydrophilic and highly soluble in water, meaning that they can provide good dispersibility in aqueous media that are added later.

[0031] <Providing stability>

[0032] Since nonionic dispersants having only hydroxyl groups can be adsorbed onto the surface of cellulose nanofibers, they can be dispersed in water with steric hindrance between the cellulose nanofibers, improving the stability of the slurry.

[0033] At this time, the nonionic dispersant having only hydroxyl groups is added at a weight ratio of 1.0 to 1.5 percent to the cellulose nanofiber slurry (weight ratio of CNF to water: 1 to 8 percent by weight), and this is stirred in a mixer to completely dissolve the granules of the nonionic dispersant having only hydroxyl groups.

[0034] 2. Second process (step S2)

[0035] Next, after stirring is complete, a nonionic dispersant having both lipophilic and hydroxyl groups is added while the material temperature is kept at 80°C, and the mixture is stirred using a mixer under the same conditions as the previous stirring. When adding this nonionic dispersant, it is added at a weight ratio of 0.4 to 0.7 percent relative to the cellulose nanofiber slurry (CNF to water ratio of 1 to 8 weight percent). Examples of such nonionic dispersants having lipophilic and hydroxyl groups include at least one of polyethylene glycol, polyvinyl porlidone, methylcellulose, hydroxypropyl methylcellulose, and polyacrylamide. These nonionic dispersants function as follows:

[0036] <Surfactant action>

[0037] Because it has both hydrophilic and lipophilic groups, it acts as a surfactant, making it easier to disperse cellulose nanofibers in water or organic solvents. Furthermore, this nonionic dispersant with both hydrophilic and lipophilic groups retains its lipophilic groups even when dried and powdered and then placed in an organic solvent medium, so dispersibility is not impaired.

[0038] <Improved stability>

[0039] By adding such nonionic dispersants with both hydrophilic and lipophilic groups, some of the nonionic dispersants with both hydrophilic and lipophilic groups bond to the nonionic dispersants with only hydrophilic groups adsorbed on the surface of cellulose nanofibers. Both of these nonionic dispersants are highly hydrophilic polymers, each containing numerous hydroxyl (-OH) and carbonyl (-C=O) groups. These functional groups readily form hydrogen bonds, allowing them to interact through these hydrogen bonds when mixed. Furthermore, these nonionic dispersants are both water-soluble and readily dissolve in the same aqueous solvent, facilitating the formation of homogeneous mixtures and enabling physical mixing. The nonionic dispersants with only hydrophilic groups adsorb to the surface of cellulose nanofibers through hydrogen bonding. The later-added nonionic dispersants with both hydrophilic and lipophilic groups then further increase steric hindrance through the compatibilization of the carboxyl groups of the cellulose nanofibers, thereby increasing the intermolecular distance between the cellulose nanofiber fibers and stabilizing them.

[0040] <Electrostatic Interaction>

[0041] The dispersion of cellulose nanofibers can be stabilized by electrostatic interactions due to the same polarity between two types of nonionic dispersants: a nonionic dispersant with only hydroxyl groups and a nonionic dispersant with both hydroxyl groups and lipophilic groups.

[0042] 3. Third process (step S3)

[0043] After stirring and mixing with these two types of nonionic dispersants, the stirring was stopped and the material temperature was maintained at 80°C. Sorbitan ester was then added (0.1 to 0.2% by weight of the total slurry) and the mixture was stirred in a mixer at 80°C under the same conditions. Stirring and mixing with these two types of nonionic dispersants further improved stability and prevented aggregation and precipitation, and adding a small amount of sorbitan ester further promoted hydrogen bond stabilization. This made the three-dimensional nanofiber structure in the slurry less likely to collapse, resulting in a cellulose nanofiber slurry that was less likely to aggregate even when left in a container for long periods of time. Even when diluted with water, the three-dimensional structure dispersed, preventing fiber aggregation.

[0044] 4. Fourth step (step S4)

[0045] After the stirring in the third step is completed, when the temperature of the whole mixture has dropped below 50°C, the mixture is discharged from the mixer and the cellulose nanofiber slurry is transferred to a container. At this time, the mixture is kept at 25°C and checked for any discoloration, bubbles, or loose particles, and the final cellulose nanofiber slurry is taken out.

[0046] The treated cellulose nanofiber slurry thus extracted can be used in a dispersed state without re-agglomeration, even if it is subsequently diluted by adding water.

[0047] 5. Fifth step (step S5)

[0048] The cellulose nanofiber slurry from the fourth step can be used in its liquid form or powdered. To produce a powder, isopropyl alcohol (IPA) is added to the slurry obtained in the fourth step and mixed. The isopropyl alcohol is added at a weight ratio of 3.0 to 5.0% of the total slurry. The slurry is then heated and kneaded using a heated two-roll mill to remove moisture and form flakes. The heating temperature of the two-roll mill is preferably around 105°C. The amount of material fed is preferably such that the material rotates automatically in the roll bank. The speed ratio of the two-roll mill is set to 4:3 for the front roll and 3:3 for the rear roll. The rolls are then rotated two to three times to remove 60 to 70% of the moisture, forming flakes that peel off from the roll surface. The isopropyl alcohol (IPA) is added to lower the boiling point of water by approximately 10°C; at this point, the isopropyl alcohol (IPA) has completely evaporated. The flakes are then collected and pulverized in an airflow mill to completely remove the moisture. The moisture content of the powder is preferably 1000 ppm or less, and the average particle size of the powder is D50 = 5 μm.

[0049] The cellulose nanofibers powdered in this way can be dispersed without agglomeration even when placed in water, and even when placed in an organic solvent, the lipophilic groups allow them to be dispersed without agglomeration. [Example]

[0050] Next, an example of this embodiment will be described.

[0051] In the examples, polyvinyl alcohol (PVA) is used as a nonionic dispersant containing only hydroxyl groups. Water-soluble polyvinyl alcohol dissolves in water most quickly at low temperatures, has strong hydroxyl group bonds with cellulose nanofibers, and exhibits good surface adsorption. PVP is used as a nonionic dispersant containing both hydroxyl groups and lipophilic groups. This PVP is water-soluble and has a similar polarity to PVA, making it easy to achieve affinity. Furthermore, because PVP contains lipophilic groups, it can be easily dispersed in a solvent-based medium other than aqueous media after removing water and drying, followed by powderization. Polyethylene glycol can also be powdered in a similar manner and then dispersed in a solvent-based medium, but PVP is best combined with PVA to prevent the aggregation of cellulose nanofibers. Therefore, these combinations were tested.

[0052] The state of dispersion in a slurry of cellulose nanofibers in which PVA and PVP were added to water is shown in Figures 2 and 3. Note that Figure 2 shows the state immediately after adding PVA and PVP, and Figure 3 shows the state after 48 hours.

[0053] As shown in Figure 2, when only PVA was added to water (Test 1) and when only PVP was added to water (Test 2), a small amount of aggregation remained, but when both PVA and PVP were added (Tests 3 and 4), no reagglomeration was observed immediately after dilution. Furthermore, when sorbitan ester was added (Test 4), it was confirmed that the viscosity decreased.

[0054] The state after a certain time has passed since dilution is shown in Figure 3. As shown in Figure 3, when both PVA and PVP were added (Test 3), separation into two layers was observed 48 hours after dilution, but when sorbitan ester was added (Test 4), as shown in Figure 6, almost no change in the dispersed state was observed compared to immediately after dilution.

[0055] Next, Figure 4 shows the state of cellulose nanofibers that have been pulverized into a powder and diluted with water, and Figure 5 shows the state of cellulose nanofibers that have been diluted with an organic solvent.

[0056] As shown in Figure 4, when only PVA was added to water (Test 1) and when only PVP was added (Test 2), aggregation remained, but when both PVA and PVP were added (Test 3), many small aggregates were observed immediately after dilution. Furthermore, when sorbitan ester was added (Test 4), no reagglomeration was observed, and it was confirmed that the viscosity increased. Furthermore, after 5 days, only when sorbitan ester was added (Test 4) no reagglomeration was observed, and it was confirmed that the mixed state was maintained.

[0057] Figure 5 shows the results of adding 5 weight percent cellulose nanofiber powder and 95 weight percent toluene, an organic solvent, and stirring and mixing for 5 minutes at 25°C and 500 rpm using a three-one motor mixer (3 blades).

[0058] When only PVA was added to toluene (Test 1) and when only PVP was added (Test 2), aggregation occurred, but when both PVA and PVP were added (Test 3), only small aggregates were observed immediately after dilution. In contrast, when sorbitan ester was added (Test 4), no reagglomeration was observed, and an increase in viscosity was confirmed. However, after 5 days, only when sorbitan ester was added (Test 4), no reagglomeration was observed, and it was confirmed that the mixed state was maintained.

[0059] This confirmed that the dispersion of slurries containing cellulose nanofibers with PVA and PVP added, and the powders obtained by drying these slurries, improved when diluted with water or organic solvents. Furthermore, the addition of sorbitan ester further improved and maintained the dispersion state.

Claims

1. A step of adding a nonionic dispersant having only hydroxyl groups to a cellulose nanofiber slurry obtained by kneading cellulose nanofibers with water; a step of mixing and stirring a slurry of cellulose nanofibers to which the nonionic dispersant having only hydroxyl groups has been added; adding a nonionic dispersant having a hydroxyl group and a lipophilic group to the mixed and stirred cellulose nanofiber slurry; a step of stirring the slurry of cellulose nanofibers to which the nonionic dispersant having a hydroxyl group and a lipophilic group has been added; A method for producing a cellulose nanofiber slurry comprising:

2. 2. The method for producing a cellulose nanofiber slurry according to claim 1, wherein the nonionic dispersant having only hydroxyl groups is at least one nonionic dispersant selected from the group consisting of polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polyvinyl amine, and cellulose ether.

3. 2. A method for producing a cellulose nanofiber slurry according to claim 1, wherein the nonionic dispersant having a hydroxyl group and a lipophilic group is at least one nonionic dispersant selected from the group consisting of polyethylene glycol, polyvinyl porlidone, methyl cellulose, hydroxypropyl methyl cellulose, and polyacrylamide.

4. A step of adding a nonionic dispersant having only hydroxyl groups to a cellulose nanofiber slurry obtained by kneading cellulose nanofibers with water; a step of mixing and stirring a slurry of cellulose nanofibers to which the nonionic dispersant having only hydroxyl groups has been added; adding a nonionic dispersant having a hydroxyl group and a lipophilic group to the mixed and stirred cellulose nanofiber slurry; a step of stirring the slurry of cellulose nanofibers to which the nonionic dispersant having a hydroxyl group and a lipophilic group has been added, and then adding isopropyl alcohol and stirring the mixture; adding isopropyl alcohol, mixing and stirring, and then removing water to form flakes; A step of pulverizing the produced flakes into fine powder using a grinder or the like; A method for producing cellulose nanofiber powder comprising:

5. A cellulose nanofiber powder produced by the production method of claim 4.

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