JP7367264B2 - Method for producing cellulose nanofiber and/or chitin nanofiber coating - Google Patents

Description

The present invention relates to a method for producing a coating of cellulose nanofibers and/or chitin nanofibers. More specifically, the present invention relates to a method of depositing anionic cellulose nanofibers or cationic cellulose nanofibers and/or chin nanofibers on a conductive substrate to form a coating using the principle of electrophoresis.

Cellulose nanofibers obtained by introducing anionic or cationic groups into cellulose and defibrating them using the charge repulsion of these introduced groups have a very fine fiber diameter and are generally It has been widely studied because of its characteristics such as high homogeneity, various functionalities based on the introduced groups, and high strength. For example, anionic cellulose nanofibers obtained by introducing anionic groups into cellulose and defibrating them can be obtained by oxidizing some of the hydroxyl groups of cellulose to carboxyl groups using a surface oxidation reaction of cellulose with an N-oxyl compound. Oxidized cellulose nanofibers obtained by fibrillation and carboxymethylated cellulose nanofibers with a degree of carboxymethyl substitution of 0.01 to 0.30 and an average fiber diameter of 3 to 500 nm have been reported (patent patent). References 1 and 2).

It has also been reported that chitin nanofibers are produced by a simple method from shells and shells of crustaceans such as shrimp and crabs, which are normally discarded, and applied to paint compositions and the like (Patent Document 3).

Japanese Patent Application Publication No. 2008-1728 International Publication No. 2014/088072 International Publication No. 2010/073758

These nanofibers have a long fiber length relative to their small fiber diameter, generally have high elasticity, and exhibit high viscosity when made into a dispersion. Methods for forming a nanofiber coating on a substrate using these nanofiber dispersions include coating the nanofiber dispersion on the substrate with a bar coater, and coating the nanofiber dispersion on the substrate using a nanofiber dispersion. One possible method is to spray it onto the material. In these methods, shearing force is applied to the nanofibers in the dispersion, thereby deforming the nanofibers and applying them onto the substrate. However, since nanofibers have high elasticity, a force is exerted to return them to their original shape after application, which tends to cause unevenness in the final nanofiber coating. In addition, the nanofiber dispersion has pseudoplasticity, and its viscosity decreases while shearing force is applied, but the viscosity recovers when it is released from the shearing force after being applied on the substrate. Leveling after application cannot be expected. Furthermore, the above-mentioned nanofiber dispersion has low affinity with metals and hydrophobic materials, and has poor wettability with these substrates, so it is extremely difficult to form a coating on these substrates. Cheap. Therefore, it has been difficult to uniformly coat the above-mentioned nanofibers on a substrate. An object of the present invention is to provide a method capable of forming a uniform nanofiber coating with few irregularities on a base material such as metal.

As a result of intensive studies for the above purpose, the present inventors have found that by using the principle of electrophoresis, uniform cellulose nanofibers and/or chitin nanofibers with few irregularities can be grown on conductive substrates such as metals. It has been found that a coating can be formed. Specifically, a dispersion of anionic or cationic cellulose nanofibers having anionic or cationic groups, a dispersion of chitin nanofibers, or a dispersion containing cationic cellulose nanofibers and chitin nanofibers is used. prepare. A conductive base material to be coated (substrate to be coated) is prepared and immersed in the dispersion liquid. When using anionic cellulose nanofibers, electricity is applied using the conductive base material as an anode, and when using cationic cellulose nanofibers and/or chitin nanofibers, electricity is applied using the conductive base material as a cathode. By doing so, these nanofibers are transferred and deposited onto the conductive substrate by the principle of electrophoresis, thereby forming a coating of these nanofibers on the conductive substrate. It has been found that this makes it possible to form a uniform nanofiber coating with few irregularities on the conductive substrate without being affected by the high elasticity of the nanofibers or the pseudoplasticity of the nanofiber dispersion. They also discovered that because the nanofibers are tightly adsorbed onto the conductive substrate by electricity, they can be coated uniformly without being repelled. The present invention includes the following.
[1] Putting a dispersion of anionic cellulose nanofibers into an electrolytic bath,
immersing a conductive base material and an electrode in the dispersion liquid in the electrolytic bath; and applying electricity using the conductive base material as an anode and the electrode as a cathode to transfer the anionic cellulose nanofibers to the conductive base material. A method for producing a coating of anionic cellulose nanofibers, comprising coating the conductive substrate with the anionic cellulose nanofibers by electrophoresis.
[2] Putting a dispersion of cationic cellulose nanofibers and/or chitin nanofibers into an electrolytic cell,
immersing a conductive base material and an electrode in the dispersion liquid in the electrolytic bath; and applying current to the conductive base material as a cathode and the electrode as an anode to the cationic cellulose nanofibers and/or chitin nanofibers; cationic cellulose nanofibers and/or chitin nanofibers, comprising coating the conductive substrate with the cationic cellulose nanofibers and/or chitin nanofibers by electrophoresing them onto the conductive substrate. method of manufacturing the coating.
[3] The method according to [1], wherein the anionic cellulose nanofiber is an oxidized cellulose nanofiber having a carboxyl group and/or a carboxylate group.
[4] The method according to [1], wherein the anionic cellulose nanofiber is a carboxyalkylated cellulose nanofiber.
[5] The method according to [1], wherein the anionic cellulose nanofiber is a phosphated cellulose nanofiber.
[6] The method according to [1], wherein the anionic cellulose nanofiber is a sulfated cellulose nanofiber.
[7] The method according to any one of [1] to [6], wherein the conductive base material is metal or carbon.

According to the present invention, anionic or cationic cellulose nanofibers or chitin nanofibers that are highly elastic and form a dispersion with high pseudoplasticity are used to coat a conductive substrate with uniformity and less peeling. will be able to form. These nanofibers have high elasticity, so when bar coating or spray coating is used, there is a force that causes them to return to their original shape after application, which tends to cause unevenness in the final nanofiber coating. In addition, the above nanofiber dispersion has pseudoplasticity, and its viscosity decreases while shearing force is applied, but the viscosity recovers when it is applied to the substrate and released from the shearing force. It was difficult to level (smooth) afterwards, and the resulting unevenness was likely to remain. Furthermore, the above-mentioned nanofiber dispersion has low affinity with metals and hydrophobic materials, and has poor wettability with these substrates, so it is extremely difficult to form a coating on these substrates. The problem was that it was easy. On the other hand, in the present invention, nanofibers are moved to a conductive substrate using the principle of electrophoresis and are electrically deposited/adhered to the conductive substrate, so that the nanofibers are not affected by the high elasticity and pseudoplasticity of the nanofibers, and the nanofibers are transferred to the conductive substrate. A coating can be formed thereon. This results in a uniform coating with fewer irregularities and wrinkles. Furthermore, since the nanofibers are tightly adsorbed onto the substrate by electricity, the nanofibers can be coated uniformly on the substrate without being repelled, and the problem of peeling of the coating is less likely to occur.

The present invention is a method for producing a coating of cellulose nanofibers (hereinafter referred to as "CNF") or chitin nanofibers, and more specifically, using the principle of electrophoresis to coat a conductive substrate. The method involves depositing anionic or cationic CNF or chitin nanofibers thereon to form a coating. In this specification, anionic CNFs, cationic CNFs, and chitin nanofibers may be collectively referred to simply as "nanofibers" or "NFs." In the method of the present invention, first, the above dispersion of NF is placed in an electrolytic cell. Next, a conductive base material and an electrode are immersed in this dispersion, and when using anionic CNF, the conductive base material is used as an anode, and when using cationic CNF or chitin NF, the conductive base material is used as an anode. Electricity is applied using the material as a cathode. When energized, the NF moves electrophoretically toward the conductive substrate and is deposited on the conductive substrate, and a gel-like coating of NF is generated on the conductive substrate. This gel-like coating may be dried if desired. Drying may be performed using conventional methods such as hot air drying after the conductive substrate with the gel-like coating is removed from the electrolytic cell.

(Nanofiber)
In the present invention, nanofibers (NF) refer to anionic CNF, cationic CNF, or chitin NF having an average fiber diameter of less than 1 μm. Preferably, the average fiber diameter is about 3 nm to 500 nm, more preferably about 3 nm to 150 nm, even more preferably about 3 nm to 20 nm. The aspect ratio is 30 or more, preferably 50 or more, and more preferably 100 or more. The upper limit of the aspect ratio is not limited, but is about 500 or less. The average fiber diameter and average fiber length of NF were randomly selected using atomic force microscopy (AFM) if the diameter was less than 20 nm, and field emission scanning electron microscopy (FE-SEM) if the diameter was 20 nm or more. It can be measured by analyzing 200 fibers and calculating the average. Additionally, the aspect ratio can be calculated using the following formula:
Aspect ratio = average fiber length/average fiber diameter.

(Anionic CNF)
Anionic CNF is NF in which an anion group is introduced into the molecular chain of cellulose. Anionic CNF can be obtained by defibrating anionic cellulose obtained by introducing an anion group into the pyranose ring of cellulose to have an average fiber diameter of less than 1 μm.

The type of cellulose used as a raw material for anionic cellulose is not particularly limited. For example, bleached or unbleached mechanical pulp (e.g., thermomechanical pulp (TMP), groundwood pulp) and chemical pulp (e.g., sulfite pulp) made from softwood, hardwood, cotton, straw, bamboo, hemp, jute, kenaf, etc. , kraft pulp), dissolving pulp, regenerated cellulose, fine cellulose, microcrystalline cellulose excluding an amorphous region, etc., and any of these can be used as a cellulose raw material.

Anionic cellulose can be produced by introducing anionic groups into such cellulose raw materials. The method of introducing the anionic group is not particularly limited, but examples include methods of directly oxidizing the hydroxyl group of the pyranose ring of cellulose to a carboxyl group, or introducing an anionic group by an esterification reaction at the hydroxyl group of the pyranose ring. Anionic CNF can be obtained by defibrating the obtained anionic cellulose to have an average fiber diameter of less than 1 μm. The defibration method is not particularly limited, and any known defibration device such as a high-speed rotation type, colloid mill type, high pressure type, roll mill type, or ultrasonic type may be used. Among these, it is preferable to use a wet high pressure or ultra high pressure homogenizer.

(oxidized CNF)
An example of anionic CNF is oxidized CNF having a carboxyl group and/or a carboxylate group. In this specification, the carboxyl group refers to -COOH (acid type) and -COOM (metal salt type) (wherein M is a metal ion), and the carboxylate group refers to -COO ^- . Oxidized CNF having a carboxyl group and/or a carboxylate group (herein also simply referred to as "oxidized CNF") is obtained by obtaining oxidized cellulose using a known method of oxidizing the hydroxyl group of the pyranose ring of cellulose to a carboxyl group. and then defibration. As a method for oxidizing cellulose, for example, oxidation is performed in the presence of an N-oxyl compound such as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and bromide and/or iodide. Examples include a method of oxidizing cellulose in water using an oxidizing agent, and a method of oxidizing cellulose by bringing it into contact with a cellulose raw material using a gas containing ozone as an oxidizing agent.

The total amount of carboxyl groups and carboxylate groups in oxidized CNF is preferably 0.4 to 3.0 mmol/g, more preferably 0.6 to 2.0 mmol/g, and 1 .0 to 2.0 mmol/g is more preferable, and 1.1 to 2.0 mmol/g is even more preferable. The amount of carboxyl groups and carboxylate groups in oxidized CNF can be adjusted by controlling reaction conditions such as the amount of oxidizing agent added and reaction time. The amount of carboxyl and carboxylate groups can be measured by the following method:
Prepare 60 ml of 0.5% by mass slurry (aqueous dispersion) of oxidized CNF, add 0.1M hydrochloric acid aqueous solution to adjust the pH to 2.5, and then dropwise add 0.05N sodium hydroxide aqueous solution to bring the pH to 11. The electrical conductivity is measured until the electrical conductivity changes slowly, and the amount of sodium hydroxide (a) consumed during the neutralization stage of the weak acid is calculated using the following formula:
Amount of carboxyl and carboxylate groups [mmol/g oxidized CNF] = a [ml] x 0.05/mass of oxidized CNF [g].

(Carboxyalkylated CNF)
An example of anionic CNF is carboxyalkylated CNF having a carboxyalkyl group. In this specification, the carboxyalkyl group refers to -RCOOH (acid type) and -RCOOM (metal salt type). Here, R is an alkylene group such as a methylene group or an ethylene group, and M is a metal ion. As the carboxyalkylated CNF having a carboxyalkyl group, carboxymethylated CNF having a carboxymethyl group in which R is a methylene group is most preferable (hereinafter, "carboxymethyl" is referred to as "CM"). Carboxyalkylated CNF is obtained by obtaining carboxyalkylated cellulose using a known method of treating a cellulose raw material with a mercerizing agent and then treating it with a carboxyalkylating agent to introduce a carboxyalkyl group, and then defibrating it. Obtainable.

The degree of carboxyalkyl substitution per anhydroglucose unit of the carboxyalkylated CNF is preferably less than 0.40. Further, the lower limit of the degree of carboxyalkyl substitution is preferably 0.01 or more. Considering operability, the degree of substitution is particularly preferably 0.02 or more and 0.35 or less, even more preferably 0.10 or more and 0.35 or less, and 0.15 or more and 0.35 or less. is more preferable, and even more preferably 0.15 or more and 0.30 or less. Note that anhydroglucose units mean individual anhydroglucoses (glucose residues) that constitute cellulose, and carboxyalkyl substitution degree refers to carboxyalkyl substitution among hydroxyl groups (-OH) in glucose residues that constitute cellulose. The proportion (number of carboxyalkyl groups per glucose residue) of those substituted with groups (-ORCOOH or -ORCOOM) is shown. The degree of carboxyalkyl substitution can be adjusted by controlling reaction conditions such as the amount of mercerizing agent and reaction time. The degree of CM substitution per glucose unit can be measured by the following method:
Accurately weigh approximately 2.0 g of CM-modified CNF (absolutely dry) and place it in a 300 mL Erlenmeyer flask with a stopper. Add 100 mL of a solution prepared by adding 100 mL of special grade concentrated nitric acid to 900 mL of methanol, and shake for 3 hours to convert salt-type CM-ized CNF to hydrogen-type CM-ized CNF. Accurately weigh 1.5 g to 2.0 g of hydrogen-type CM-formed CNF (absolutely dry) and place it in a 300 mL Erlenmeyer flask with a stopper. The hydrogenated CM-ized CNF is moistened with 15 mL of 80% by mass methanol, 100 mL of 0.1N NaOH is added, and the mixture is shaken at room temperature for 3 hours. Excess NaOH is back-titrated with 0.1N H ₂ SO ₄ using phenolphthalein as indicator. The degree of CM substitution (DS) is calculated by the following formula:
A=[(100×F'-(0.1N H ₂ SO ₄ )(mL)×F)×0.1]/(absolute dry mass (g) of hydrogen-type CM-modified CNF)
DS=0.162×A/(1-0.058×A)
A: Amount of 1N NaOH required to neutralize 1g of hydrogen-type CM-modified CNF (mL)
F: Factor of 0.1N H ₂ SO ₄ F': Factor of 0.1N NaOH The degree of substitution of carboxyalkyl groups other than CM groups can also be measured by the same method as above.

(Phosphate esterified CNF)
An example of anionic CNF is phosphoric acid esterified CNF. Phosphoric acid esterified CNF is produced by mixing phosphoric acid compound powder or aqueous solution with the cellulose raw material mentioned above, or by adding an aqueous solution of phosphoric acid compound to a slurry of cellulose raw material. It can be obtained by introducing an acidic group into cellulose to obtain phosphoric acid esterified cellulose, which is then defibrated. Examples of phosphoric acid compounds include phosphoric acid, polyphosphoric acid, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, and esters or salts thereof. Specifically, for example, but not limited to, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, the following: Potassium phosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, Examples include triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate, and the like. A phosphoric acid group derived from a phosphoric acid compound can be introduced into cellulose by using one type or a combination of two or more of these. In this specification, phosphoric acid groups derived from phosphoric acid compounds include phosphoric acid groups, phosphorous acid groups, hypophosphorous acid groups, pyrophosphoric acid groups, metaphosphoric acid groups, polyphosphoric acid groups, phosphonic acid groups, and polyphosphonic acid groups. Phosphate-esterified cellulose and phosphate-esterified CNF include those in which one or more of these phosphate groups are introduced into the molecular chain of cellulose. When reacting a cellulose raw material with a phosphoric acid compound, it is preferable to use the phosphoric acid compound as an aqueous solution because the reaction can proceed uniformly and the efficiency of introducing the above groups is increased. The pH is preferably 3 to 7. Further, a nitrogen-containing compound such as urea may be added.

The degree of substitution of a phosphate group per glucose unit in the phosphoric acid esterified CNF (hereinafter simply referred to as "degree of phosphate group substitution") is preferably 0.001 or more and less than 0.40. The degree of phosphate group substitution per glucose unit can be measured by the following method:
A slurry of phosphoric acid esterified CNF having a solid content of 0.2% by mass is prepared. To the slurry, 1/10 of the volume of strongly acidic ion exchange resin (Amber Jet 1024; manufactured by Organo, preconditioned) was added, and after shaking for 1 hour, the resin was poured onto a mesh with an opening of 90 μm. By separating the CNF and the slurry, hydrogen-type phosphated CNF is obtained. Next, 50 μL of a 0.1 N aqueous sodium hydroxide solution is added to the slurry treated with the ion exchange resin once every 30 seconds, and the change in the electrical conductivity value of the slurry is measured. Among the measurement results, by dividing the amount of alkali (mmol) required in the region where the electrical conductivity suddenly decreases by the solid content (g) in the titration target slurry, the amount per gram of hydrogen-type phosphated CNF can be calculated. Calculate the amount of phosphate groups (mmol/g). Furthermore, the degree of phosphoryl substitution (DS) per glucose unit of phosphorylated CNF is calculated by the following formula:
DS=0.162×A/(1-0.079×A)
A: Amount of phosphate groups per 1 g of hydrogen-type phosphate-esterified CNF (mmol/g).

(Sulfate esterified CNF)
An example of anionic CNF is sulfate-esterified CNF. Sulfate-esterified CNF can be obtained by reacting the above-mentioned cellulose raw material with a sulfate-based compound to introduce a sulfate-based group derived from the sulfate-based compound into cellulose to obtain sulfate-esterified cellulose, and then fibrillating this. I can do it. Examples of sulfuric acid compounds include sulfuric acid, sulfamic acid, chlorosulfonic acid, sulfur trioxide, and esters or salts thereof. Among these, it is preferable to use sulfamic acid because the solubility of cellulose is low and the acidity is low.

For example, when sulfamic acid is used as the sulfuric acid compound, the amount of sulfamic acid used can be adjusted as appropriate, taking into consideration the amount of anionic groups introduced into the cellulose chain. For example, it can be used preferably in an amount of 0.01 to 50 mol, more preferably 0.1 to 3.0 mol, per mol of glucose unit in the cellulose molecule.

The amount of sulfate groups per glucose unit (hereinafter simply referred to as "sulfate group amount") in the sulfate-esterified CNF is preferably 0.1 to 3.0 mmol/g. The amount of sulfate groups per glucose unit can be measured by the following method:
The aqueous dispersion of sulfated CNF is subjected to solvent replacement with ethanol and t-butanol in this order, and then freeze-dried. Add 15 ml of ethanol and 5 ml of water to 200 mg of the obtained sample, and stir for 30 minutes. Then, 10 ml of 0.5N aqueous sodium hydroxide solution was added, stirred at 70°C for 30 minutes, and further stirred at 30°C for 24 hours. Next, add phenolphthalein as an indicator, titrate with hydrochloric acid, and calculate using the following formula:
Sulfate group amount [mmol/g sample] = (5-(0.1 x hydrochloric acid titration [ml] x 2))/0.2.

(cationic CNF)
Cationic CNF is cationic CNF in which a cationic group is introduced into the molecular chain of cellulose. Cationic CNF can be obtained by defibrating cationic cellulose obtained by introducing a cationic group into the pyranose ring of cellulose to have an average fiber diameter of less than 1 μm.

The type of cellulose used as a raw material for cationic cellulose is not particularly limited, and is the same as that described in the column of anionic CNF. Further, the method of defibrating cationic cellulose to form cationic CNF is not particularly limited, and is the same as that described in the column of anionic CNF.

Cationic cellulose is produced by adding a cationizing agent such as glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium hydrate, or its halohydrin type to the oxidized cellulose, and alkali metal hydroxide (sodium hydroxide) as a catalyst. , potassium hydroxide, etc.) in the presence of water or an alcohol having 1 to 4 carbon atoms, and by defibrating the obtained cationic cellulose to obtain cationic CNF. I can do it.

The degree of cation substitution per glucose unit in cationic CNF is preferably 0.02 to 0.50. The degree of cation substitution can be adjusted by adjusting the amount of cationizing agent added and the composition ratio of water or alcohol having 1 to 4 carbon atoms. The degree of cation substitution per glucose unit can be measured by the following method:
After drying the cationic CNF, the nitrogen content was measured using a total nitrogen analyzer (TN-10 manufactured by Mitsubishi Chemical Corporation), and the degree of cationic substitution (mole of substituents per mole of anhydroglucose unit) was calculated using the following formula. Calculate the average value of numbers):
Degree of cation substitution = (162×N)/(1-151.6×N)
N: Nitrogen content.

(Chitin NF)
Chitin NF can be mentioned as cationic NF other than cationic CNF. Chitin is an aminopolysaccharide in which N-acetylglucosamine is linked in a chain, and is industrially obtained from the shells and shells of shrimp and crabs. Chitin derived from living things such as shrimp and crabs usually has a matrix around it, such as protein or calcium carbonate, and is treated to remove these matrices. For the deproteinization treatment, known alkali treatment methods, proteolytic enzyme methods, and the like can be used. To remove ash such as calcium carbonate, known methods such as acid treatment and ethylenediaminetetraacetic acid treatment can be used. Chitin NF can be obtained by defibrating biologically derived chitin that has been subjected to deproteinization and demineralization using a device such as a stone grinder or a high-pressure homogenizer. A method for producing chitin NF is described, for example, in International Publication No. 2010/073758.

(NF dispersion)
A dispersion of anionic or cationic CNF or chitin NF as described above is prepared and placed in an electrolytic cell. The dispersion medium of the dispersion liquid is preferably water, but may contain a water-soluble organic solvent as long as it does not interfere with NF electrophoresis. Examples of water-soluble organic solvents include methanol, ethanol, isopropanol, 2-propanol, butanol, glycerin, acetone, methyl ethyl ketone, 1,4-dioxane, N-methyl-2-pyrrolidone, tetrahydrofuran, N,N-dimethylformamide, Included are N,N-dimethylacetamide, dimethylsulfoxide, acetonitrile, ethylene glycol, and combinations thereof. The proportion of water in the dispersion medium is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass (all water).

The solid content of NF in the dispersion liquid is preferably 0.05 to 10.0% by mass, more preferably 0.1 to 7.5% by mass, and 0.05 to 10.0% by mass, more preferably 0.1 to 7.5% by mass, in order to facilitate electrophoresis during subsequent energization. .1 to 5.0% by mass is more preferable, and even more preferably 0.1 to 3.0% by weight.

The dispersion liquid preferably consists of water as a dispersion medium and NF, but may contain substances other than NF as long as they do not interfere with the electrophoresis of NF. Examples of such substances include, when using anionic CNF, anionic polymers such as acrylic resin, carboxymethyl cellulose, and uronic acid. Furthermore, when using cationic CNF and/or chitin NF, examples include alkyl amines, amine compounds, and cationic polymers. In addition, polyethylene glycol, a silane coupling agent, a carbodiimide group-containing polymer, etc. can be used for any of anionic CNF, cationic CNF, and chitin NF. Further, in order to increase the electrical conductivity of the dispersion, an electrolyte such as zinc chloride, aluminum chloride, magnesium chloride, sodium chloride, sodium hydroxide, acetic acid, carbonic acid, etc. may be added. The ratio of the solid content of NF to the total solid content in the dispersion is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, even more preferably 90% or more, and even more preferably 95% or more.

The electrolytic cell in which the dispersion liquid is placed is not particularly limited, and any electrolytic cell may be used as long as it can cause electrophoresis of NF by applying electricity. For example, an electrolytic cell used for electrodeposition coating can be used.

(conductive base material, electrode)
After putting the above-mentioned NF dispersion into an electrolytic cell, a conductive substrate and an electrode are immersed in the dispersion in the electrolytic cell. The conductive substrate is a substrate on which a coating of NF is formed. The conductive base material is not particularly limited as long as it is made of a material having conductivity. Examples include metals such as aluminum, copper, iron, zinc, titanium, nickel, lead, silver, platinum, tungsten, bismuth, stainless steel, brass, and chromium, alloys thereof, and carbon. Further, it may be a material that is imparted with conductivity by containing one or more of these (for example, rubber to which carbon black is added).

The electrode is used as a counter electrode to the conductive base material during energization, and its type is not particularly limited. For example, an electrode used as a counter electrode of the object to be coated in the field of electrodeposition coating can be used.

(energized)
After the conductive base material and the electrode are immersed in the NF dispersion, electricity is applied to electrophores the NFs to form a coating of NFs on the conductive base material. When applying electricity, when anionic CNF is used, the conductive base material is used as an anode and the electrode is used as a cathode. Moreover, when using cationic CNF and/or chitin NF, the conductive base material is used as a cathode, and the electrode is used as an anode. This causes the NF to migrate to the conductive substrate, deposit on the conductive substrate, and coat the conductive substrate.

Conditions for energization are not particularly limited, and may be adjusted as appropriate depending on the desired thickness of the coating. For example, the voltage range is preferably about 0.1 to 1000V, more preferably about 1 to 500V, even more preferably about 5 to 500V, even more preferably about 5 to 100V, even more preferably about 5 to 50V. The range of current density is preferably about 0.1 to 10,000 mA/cm ² , more preferably about 1 to 10,000 mA/cm ² . The energization time is preferably about 1 to 100 minutes, more preferably about 2 to 60 minutes. In addition, if you want to increase the density of NF in the gel-like NF coating on the conductive substrate, you can set the voltage during energization to a higher value within the above range, or change the voltage during energization. You may change it to a higher value. For example, such a high voltage may include, but is not particularly limited to, a voltage of about 50 to 1000 V, more preferably about 100 to 500 V, and more preferably 0.1 to 10 minutes at such voltage. For example, it may be held for a period of about 1 to 5 minutes.

After energizing the NF dispersion, the conductive substrate is placed in an electrolytic bath containing ion-exchanged water, and the NF coating on the conductive substrate is cleaned by placing the conductive substrate in an electrolytic bath containing ion-exchanged water and allowing it to stand still while being energized for a certain period of time. You may go. The voltage range for energization during cleaning is not particularly limited, but is preferably about 0.1 to 1000 V, more preferably about 10 to 500 V, and the energization time is preferably about 1 to 100 minutes. , more preferably about 5 to 60 minutes.

(NF coating)
By the above-mentioned energization, a coating of NF is formed on the conductive base material. The coating obtained immediately after energization is a layer of a transparent to translucent gel-like NF dispersion containing NF and a dispersion medium. The conductive substrate having the gel-like coating of NF may be removed from the electrolytic cell and, if necessary, dried to remove the dispersion in the gel-like coating. The drying method in this case is not particularly limited, and examples thereof include natural drying, warm air/hot air drying, and vacuum drying.

The thickness of the gel-like NF coating on the conductive substrate can be adjusted by adjusting the NF solid content of the NF dispersion, the current application time, etc. coating can be formed. The thickness of the coating obtained by drying the gel-like coating (removal of the dispersion medium) is not particularly limited, but is, for example, about 0.5 to 50 μm. The coating after drying is a dense thin film consisting mainly of NF.

The ratio of the solid content of NF in the total solid content in the gel-like coating and the coating after drying is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, even more preferably 90% or more, More preferably 95% or more.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples.

(Preparation of oxidized CNF)
500 g (absolutely dry) of bleached unbeaten kraft pulp derived from coniferous trees (85% whiteness) was added to 500 ml of an aqueous solution containing 780 mg of TEMPO (Sigma Aldrich) and 75.5 g of sodium bromide, and the pulp was uniformly dispersed. Stir until combined. An aqueous sodium hypochlorite solution was added to the reaction system at a concentration of 6.0 mmol/g to start the oxidation reaction. During the reaction, the pH in the system decreased, but the pH was adjusted to 10 by successively adding a 3M aqueous sodium hydroxide solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH within the system stopped changing. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was sufficiently washed with water to obtain oxidized pulp. The pulp yield at this time was 90%, and the time required for the oxidation reaction was 90 minutes. The oxidized pulp obtained in the above process was adjusted to 0.5% (w/v) with water and defibrated five times using an ultra-high pressure homogenizer (20°C, 150 MPa) to obtain a dispersion of oxidized CNF. Obtained. The amount of carboxyl groups in the obtained oxidized CNF was 1.42 mmol/g.

(Preparation of CNF for CM)
130 parts of water and a solution of 20 parts of sodium hydroxide dissolved in a mixed solvent of 10 parts of water and 90 parts of isopropanol (IPA) were added to a twin-screw kneader whose rotational speed was adjusted to 150 rpm, and hardwood pulp (Nippon Paper Industries, Ltd.) was added. LBKP) manufactured by Co., Ltd.) was charged at a dry weight of 100 parts when dried at 100° C. for 60 minutes. Mercerization treatment was performed by stirring and mixing at 35° C. for 80 minutes. Further, while stirring, a mixed solvent of 23 parts of water and 207 parts of IPA and 40 parts of sodium monochloroacetate were added, and after stirring for 30 minutes, the temperature was raised to 70° C. and etherification treatment was performed for 90 minutes. After the reaction was completed, the mixture was neutralized with acetic acid until the pH reached 7, washed with aqueous methanol, deliquified, dried, and pulverized to obtain a sodium salt of CM pulp. The degree of CM substitution in the obtained CM pulp was 0.17. The CM-modified pulp obtained in the above process was adjusted to 0.5% (w/v) with water and defibrated three times using an ultra-high pressure homogenizer (20°C, 150 MPa) to form a dispersion of CM-modified CNF. I got it.

(Preparation of phosphoric acid esterified CNF)
100 g of hardwood pulp (manufactured by Nippon Paper Industries Co., Ltd., LBKP) was immersed in 400 g of an aqueous solution containing 120 g of urea and 45 g of ammonium dihydrogen phosphate, dried in an oven at 70°C for 24 hours, and then dried at 150°C for 10 minutes. Heated. Thereafter, it was washed five times with ion-exchanged water to obtain a phosphoric acid esterified pulp. When the amount of phosphoric acid groups in the phosphoric acid esterified pulp was measured by the above-mentioned method, it was found to be 0.87 mmol/g. The phosphoric acid esterified pulp obtained in the above process was adjusted to 0.5% (w/v) with water, and defibrated three times using an ultra-high pressure homogenizer (20°C, 150 MPa) to convert it into a phosphoric acid ester. A dispersion of CNF was obtained.

(Preparation of sulfate-esterified CNF)
After drying 100 g of hardwood pulp (manufactured by Nippon Paper Industries, Ltd., LBKP) in an oven at 105°C for 24 hours, 2000 g of a 60% aqueous sulfuric acid solution was added, and the mixture was stirred at 50°C for 1 hour. Thereafter, it was washed five times with ion-exchanged water to obtain a sulfuric acid esterified pulp. The amount of sulfate groups in the sulfuric acid esterified pulp was measured using the method described above and was found to be 0.79 mmol/g. The sulfate-esterified pulp obtained in the above process was adjusted to 0.5% (w/v) with water, and defibrated three times using an ultra-high pressure homogenizer (20°C, 150 MPa) to obtain sulfate-esterified CNF. A dispersion was obtained.

(Preparation of cationic CNF)
Add 200 g of dry weight of pulp (NBKP, manufactured by Nippon Paper Industries, Ltd.) and 24 g of sodium hydroxide (dry weight) to a pulper that can stir the pulp, and add water so that the pulp solid content is 15% by mass. added. After stirring at 30°C for 30 minutes, the temperature was raised to 70°C, and 200g (in terms of active ingredient) of 3-chloro-2-hydroxypropyltrimethylammonium chloride was added as a cationizing agent. After reacting for 1 hour, the reactant was taken out, neutralized, and washed to obtain a cation-modified pulp with a degree of cation substitution per glucose unit of 0.05. The cationic pulp obtained in the above process was adjusted to 0.5% (w/v) with water and defibrated three times using an ultra-high pressure homogenizer (20°C, 150 MPa) to form a dispersion of cationized CNF. I got it.

(Preparation of chitin NF)
500 g of snow crab shells were left standing in 5 L of a 20% aqueous sodium hydroxide solution for 24 hours, and washed three times with 5 L of ion-exchanged water. Then, it was left standing in 5 L of 10% hydrochloric acid for 24 hours, and washed three times with 5 L of ion-exchanged water. Thereafter, it was left standing in 2 L of ethanol and washed three times with 5 L of ion-exchanged water. A suspension of washed crab shells with a concentration of 0.5% (w/v) was adjusted to pH 6 with acetic acid, pulverized with a household mixer for 1 hour, and further pulverized with a homodisper (manufactured by Primix) at 6000 rpm for 5 hours. Preliminary defibration was performed. This suspension was defibrated three times using an ultra-high pressure homogenizer (20° C., 150 MPa) to obtain a chitin nanofiber aqueous dispersion.

(Test 1 Formation of coating by electrophoresis)
An aqueous dispersion of each NF prepared by the method described above was prepared. The solid content of NF in the dispersion liquid is as shown in Table 1. 500 mL of an aqueous dispersion of each NF was placed in an electrolytic cell, and the conductive base material (25 cm ² ) listed in Table 1 and an aluminum plate (25 cm ² ) as a counter electrode were immersed in the dispersion, and a voltage of 18 Electricity was applied for about 1 hour at a voltage of .5V and a current value of 0.1A or less. After energization, each conductive substrate was taken out of the electrolytic bath, put back into the electrolytic bath containing ion-exchanged water prepared separately, and washed by leaving it still while energizing for 1 hour under the same conditions as above. The uniformity of the gel-like coating of NF formed on the conductive substrate was evaluated by the method described below. In addition, the presence or absence of repellency of the coating on the base material was evaluated by the method described below.

After uniformity and repellency evaluation, the conductive substrate with the gel-like coating of NF was dried at 50° C. into a thin film. The resulting dried coating was evaluated for the presence or absence of irregularities, wrinkles, and peeling using the methods described below. The results are shown in Table 1.

(Test 2 Formation of coating by electrophoresis)
Using the NF aqueous dispersion and conductive base material listed in Table 2, the conductive base material was placed in the same manner as in Test 1, except that the voltage was 30 V and the current value was 0.3 A or less, and electricity was applied for 2 minutes. A gel-like coating of NF was formed on the surface. The uniformity of the coating and the presence or absence of repellency on the substrate were evaluated in the same manner as in Test 1 using the methods described below. After these evaluations, the conductive substrates with each coating were dried at 50°C. The coating after drying was evaluated for the presence or absence of irregularities, the presence or absence of wrinkles, and the presence or absence of peeling using the methods described below in the same manner as in Test 1. The results are shown in Table 2.

(Test 3 Formation of coating by electrophoresis)
A NF coating was formed and evaluated in the same manner as Test 2, except that the voltage was changed to 20V. The results are shown in Table 3.

(Test 4 Formation of coating by electrophoresis)
An NF coating was formed and evaluated in the same manner as in Test 2, except that the voltage was changed to 10V. The results are shown in Table 4.

(Test 5 Formation of coating by electrophoresis)
An NF coating was formed and evaluated in the same manner as Test 2, except that the voltage was changed to 5V. The results are shown in Table 5.

(Comparative test bar coat/spray coat)
As a comparative example, a dispersion of each NF having the solid content shown in Table 6 was placed on each conductive substrate. Hand coated using 40 wire bars (bar coat). Further, each NF dispersion having the solid content shown in Table 6 was sprayed onto each conductive substrate using a two-fluid nozzle (spray coating). The uniformity of each of the obtained coatings and the presence or absence of repellency on the substrate were evaluated by the method described below in the same manner as in the Examples. After these evaluations, the conductive substrates with each coating were dried at 50°C. The coating after drying was evaluated for the presence or absence of irregularities, the presence or absence of wrinkles, and the presence or absence of peeling using the methods described below in the same manner as in Test 1. The results are shown in Table 6.

(Evaluation of uniformity)
In the state before drying, if the NF coating formed on the conductive substrate is visually smooth, the uniformity is evaluated as good (good), while if it is visually uneven and the thickness is uneven. Uniformity was evaluated as poor (bad).

(Evaluation of presence or absence of bounce)
In the state before drying, if the NF coating remains without flowing, it is evaluated as having no repulsion (no repulsion).On the other hand, if the coating aggregates locally over time or the surface of the conductive substrate is exposed. If it was, it was rated as having reluctance.

(Evaluation of presence or absence of unevenness)
Regarding the surface of the coating after drying, if it is visually smooth, it is evaluated as having no unevenness (absent), while if it is visually observed that the surface is wavy or the thickness is uneven, it is evaluated as having unevenness. It was evaluated as follows.

(Evaluation of presence or absence of wrinkles)
Regarding the surface of the coating after drying, if it is visually observed to be glossy and uniform, it is evaluated as having no wrinkles (absent), while if it is visually observed to be locally cloudy or locally rough, it is evaluated as having no wrinkles. It was rated as having wrinkles.

(Evaluation of presence or absence of peeling)
Regarding the coating after drying, if the coating does not lift off from the conductive substrate when visually observed, it is evaluated as no peeling (no peeling), and on the other hand, when the coating does not lift off from the conductive substrate when visually observed, it is evaluated as peeling ( Yes).

The results in Table 6 show that when NF is coated on a base material such as metal using conventional bar coat or spray coat, the uniformity is poor, and depending on the type of base material, repulsion may occur, and the coating after drying. It can be seen that unevenness, wrinkles, and peeling occur. On the other hand, in the method of the present invention in which each NF is deposited on a conductive substrate using the principle of electrophoresis to form a coating, the results in Tables 1 to 5 show that the coating is uniform and non-repellent on the conductive substrate. It can be seen that a coating can be formed, and that no unevenness, wrinkles, or peeling occurs in the coating after drying.

Claims (6)

placing a dispersion of anionic cellulose nanofibers in an electrolytic cell;
immersing a conductive substrate and an electrode in the dispersion in the electrolytic cell;
The conductive base material is made of the anionic cellulose nanofibers by electrophoresing the anionic cellulose nanofibers onto the conductive base material by applying electricity using the conductive base material as an anode and the electrode as a cathode. A method for producing a coating of anionic cellulose nanofibers, the method comprising coating the conductive substrate, placing the coated conductive substrate in an electrolytic bath containing ion-exchanged water, and washing by applying electricity. The method according to claim 1, wherein the anionic cellulose nanofibers are oxidized cellulose nanofibers having carboxyl groups and/or carboxylate groups. 2. The method of claim 1, wherein the anionic cellulose nanofibers are carboxyalkylated cellulose nanofibers. The method according to claim 1, wherein the anionic cellulose nanofibers are phosphated cellulose nanofibers. The method according to claim 1, wherein the anionic cellulose nanofibers are sulfated cellulose nanofibers. The method according to claim 1, wherein the conductive substrate is metal or carbon.