Method for producing cellulose nanofiber, alcohol-based CNF dispersion, and method for producing CNF-containing resin molded article

Abstract

To provide a method for producing a cellulose nanofiber, an alcohol-based CNF dispersion, and a method for producing a CNF-containing resin molded article, enabling improved dispersibility of fibers in resin when used as a synthetic resin additive, and allowing suppression of discoloration of the resin molded article upon addition to resin.SOLUTION: The present invention employs a method in which, from a composition comprising a pyridinium-type quaternary ammonium salt, an alcohol, and wet fibers resulting from dissolving and precipitating plant fibers, the wet fibers are separated and collected; and the wet fibers are then washed using pyridine.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing cellulose nanofibers that are mainly used by adding them to resins, an alcohol-based CNF dispersion, and a method for producing a CNF-containing resin molded product. [Background technology]

[0002] CNF (cellulose nanofiber) is a plant fiber that has been defibrated to the nano-order. It has mechanical properties such as light weight, high strength, high elastic modulus, and low linear expansion coefficient, chemical / biological properties such as adsorption, gas barrier properties, water retention, water repellency, and biodegradability, optical properties such as transparency, ultraviolet absorption, dimensional stability (low linear expansion coefficient), high thermal conductivity / electrical insulation, as well as thermal and electrical properties. Therefore, it is attracting attention as an additive that can improve the physical properties of synthetic resins and add functions to them.

[0003] Furthermore, the above-mentioned CNF defibration processes can be broadly divided into two types: mechanical (physical) and chemical. However, mechanical processes have limitations on the quality of the defibrated CNF, while chemical processes have the problem of high processing costs due to the cost of the chemicals used and the length of the reaction time, resulting in expensive defibrated CNF.

[0004] Furthermore, because cellulose fibers are hydrophilic, even if the above-mentioned CNF is kneaded in powder form with synthetic resin, the fibers are difficult to disperse, resulting in unstable quality.Similarly, many CNFs are provided as aqueous CNF dispersions in which they are dispersed in water, but aqueous CNF dispersions have the disadvantage of being poorly compatible with resins, resulting in poor dispersibility of the CNF when kneaded with resin.

[0005] Therefore, the inventors of this invention previously developed a fiber-defibrating method in which plant fibers are dissolved using pyridinium-based quaternary ammonium salts and then precipitated with alcohol (Patent Application No. 2022-206590). However, with this fiber-defibrating method, even when the CNFs were washed with alcohol, a considerable amount of pyridinium-based quaternary ammonium salt remained, and when a CNF dispersion in which CNFs were dispersed in alcohol was added to a resin and molded, the resin would discolor when heated. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-1728 [Patent Document 2] Japanese Patent Application Publication No. 2019-48924 [Patent Document 3] Japanese Patent Publication No. 2020-79420 [Patent Document 4] Patent Publication No. 2021-175799 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0007] The present invention aims to solve the problems of the prior art described above, and in summary, to provide a method for producing cellulose nanofibers, an alcohol-based CNF dispersion, and a method for producing CNF-containing resin molded bodies, which have excellent fiber dispersibility in resins when used as an additive to synthetic resins and can suppress discoloration of resin molded bodies when added to resins. [Means for solving the problem]

[0008] As a means of solving the above problem, the inventors adopted a method of separating and recovering wet fibers from a composition containing a pyridinium-based quaternary ammonium salt, alcohol, and wet fibers precipitated by dissolving plant fibers, and then washing the wet fibers with pyridine (the effects will be described later).

[0009] In this specification, "CNF (cellulose nanofiber)" refers to cellulose fibers, the main component of plant cell walls and fibers, that have been loosened to a nanoscale width of 100 nm or less. Generally, it refers to fibers with a width of 3-100 nm, an aspect ratio of 10 or more, and a length of less than 100 μm. In addition, in this specification, "wet fiber" refers to cellulose nanofibers in an undried state, with a trace amount of pyridinium-based quaternary ammonium salt attached.

[0010] In the present invention, the separated and recovered wet fibers may be washed with alcohol and then washed with pyridine. In this case, it is preferable to wash the wet fibers with alcohol until they turn white, and then wash them with pyridine to reduce the amount of pyridine used and efficiently perform the washing process.

[0011] In the present invention, the wet fibers washed with pyridine can be washed again with alcohol, thereby removing pyridine, which is a harmful component to the human body.

[0012] In the present invention, in an alcohol-based CNF dispersion, cellulose nanofibers containing a trace amount of pyridinium-based quaternary ammonium salt are dispersed in alcohol, and the content of pyridinium-based quaternary ammonium salt in the alcohol is set to less than 2.0 mg / L, thereby suppressing discoloration of the resin molded body when the pyridinium-based quaternary ammonium salt is added to the resin and molded.

[0013] The present invention employs a method for producing CNF-containing resin molded products by adding the above-mentioned alcohol-based CNF dispersion and then heat-molding the product, thereby making it possible to suppress discoloration of the surfaces of resin pellets and resin molded products. [Effects of the Invention]

[0014] In the present invention, by separating and recovering wet fibers defibrated using a pyridinium-based quaternary ammonium salt and then washing the wet fibers with pyridine, the residual rate of pyridinium-based quaternary ammonium salt in the fibers can be reduced, thereby reducing the amount of pyridinium-based quaternary ammonium salt that dissolves into alcohol when the CNF is made into an alcohol-based CNF dispersion.

[0015] Furthermore, when the alcohol-based CNF dispersion is added to a resin material and subjected to extrusion or injection molding, the problem of the resin discoloring to dark brown or black due to heating during molding, which is caused by the adverse effects of the pyridinium-based quaternary ammonium salt, can be suppressed, thereby enabling the production of resin molded products with good appearance. DETAILED DESCRIPTION OF THE INVENTION

[0016] "Method for producing cellulose nanofibers" [1] Plant fiber defibration and CNF precipitation process [1-1] Process description The manufacturing process for cellulose nanofibers of the present invention is described below. First, plant fibers or chemical fibers made from plant fibers are added to a pyridinium-based quaternary ammonium salt, and the plant fibers are dissolved by heating. The fibers are then defibrated by stirring using a stirrer bar or similar. Alcohol is then added to the defibrated solution, causing CNF to precipitate as wet fibers from the quaternary ammonium salt.

[0017] [1-2] Plant fibers or chemical fibers made from plant fibers As the plant fibers, wood pulp or non-wood pulp can be used, and as non-wood raw materials, fibers such as cotton, linen, ramie, hemp, jute, Manila hemp (abaca), sisal (henequen), and bamboo can be used, and as chemical fibers made from the plant fibers, recycled fibers such as rayon made from wood pulp and cupra made from cotton linters can be used.

[0018] [1-3] Pyridinium-based quaternary ammonium salts Amines and halides can be used as raw materials for the pyridinium-based ammonium salt. The amines are preferably compounds represented by the following [Chemical Formula 1] to [Chemical Formula 4], as shown in the embodiments of Japanese Patent No. 6295495 and JP-A-2015-20954. The halides are preferably benzyl halides represented by the following [Chemical Formula 5]. For example, 1-benzylpyridinium chloride, obtained by chemically reacting pyridine and benzyl chloride by heating, is preferably used. Other raw materials for pyridinium-based ammonium salts include those having skeletons represented by the following [Chemical Formula 6] to [Chemical Formula 15], which can be appropriately selected or combined. The ratio of amine to halide in the ammonium salt is not particularly limited, but it is preferable to prepare it in a molar ratio of 1 to 650:1 (preferably 1:1). [ka] (In the formula, R1 independently represents hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] [ka] (In the formula, R2, R3, and R4 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] [ka] (In the formula, X independently represents a halogen.) [ka] (In the formula, R1, R2, R3, R4, and R5 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, R3, and R4 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, and R3 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, R3, R4, and R5 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, R3, and R4 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, R3, R4, R5, and R6 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, R3, R4, and R5 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, and R4 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R1, R2, and R3 independently represent hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.) [ka] (In the formula, R independently represents hydrogen, a hydroxyl group, a halogen, an alkyl group, an alkenyl group, an alkynyl group, a nitro group, a cyano group, an alkoxycarbonyl group, or an aromatic group.)

[0019] As the ammonium salt including the pyridinium-based quaternary ammonium salt, there can be used 1-benzyl-4-(dimethylamino)pyridinium chloride obtained by reacting benzyl chloride with 4-dimethylaminopyridine by heating, N,N,N-triethylbenzeneaminium chloride obtained by reacting benzyl chloride with triethylamine by heating, 1-benzylpyrrolidine hydrochloride obtained by reacting benzyl chloride with pyrrolidine by heating, and 1-benzylimidazolium chloride obtained by reacting benzyl chloride with imidazole by heating.

[0020] [1-4] Precipitation alcohol The alcohol used to precipitate the wet fibers may be any alcohol that dissolves in the pyridinium ammonium salt but does not dissolve the defibrated fibers, and alcohols such as methanol, ethanol, propyl alcohol, and butanol can be used, with methanol or ethanol being preferred.

[0021] [2] Cellulose nanofiber separation and recovery process The wet fibers are recovered from the alcohol diluted solution of the pyridinium-based quaternary ammonium salt by separation means such as centrifugation, filtration, or decantation. The separated solution is separated into the pyridinium-based quaternary ammonium salt and other solvents using liquid-liquid separation means such as vacuum concentration, and then recovered. Even when the washing step described below is repeated, performing this step allows only the washed wet fibers to be separated and recovered.

[0022] [3] CNF cleaning process [3-1] Process description The wet fibers separated and recovered as described above are further washed with alcohol to remove the pyridinium-based quaternary ammonium salt. This washing process is preferably repeated multiple times. Generally, it is repeated until the color of the wet fibers changes from the orange or reddish-purple color derived from the pyridinium-based quaternary ammonium salt to white. After washing the wet fibers with alcohol until the orange or reddish-purple color disappears, the wet fibers are further washed with pyridine to remove any remaining pyridinium-based quaternary ammonium salt in the fibers. Note that washing the wet fibers with alcohol until they turn white and then washing with pyridine can reduce the amount of pyridine used and enable efficient washing. If necessary, the fibers are then substituted and washed again with alcohol to remove any remaining pyridine. This removes pyridine, a component harmful to humans.

[0023] Regarding washing of wet fibers using the above alcohol and pyridine, it is preferable to wash until the content of pyridinium-based quaternary ammonium salts contained in the alcohol separated after washing is less than 2.0 mg / L (preferably 1.0 mg / L, more preferably 0.5 mg / L).

[0024] [3-2] Cleaning alcohol The alcohol used for washing the wet fibers may be methanol, ethanol, propyl alcohol, butanol, etc., and among these, methanol or ethanol is preferred. It is preferable to use the same type of alcohol for washing as the alcohol for precipitation, but a different type of alcohol may also be used.

[0025] [4] Adding CNF to resin [4-1] Process description The wet fibers can be dried to produce powdered CNF, but when added to a resin, they are preferably dispersed in alcohol and added to the resin in the form of an alcohol-based CNF dispersion. Furthermore, due to the characteristics of the manufacturing method, the CNF produced by this method contains trace amounts of pyridinium-based quaternary ammonium salts. However, by washing with pyridine, the content of pyridinium-based quaternary ammonium salts in the alcohol of the alcohol-based CNF dispersion can be reduced to less than 2.0 mg / L (preferably 1.0 mg / L, more preferably 0.5 mg / L), thereby reducing the adverse effects of the pyridinium-based quaternary ammonium salts.

[0026] [4-2] Alcohol for dispersion The alcohol used in the alcohol-based CNF dispersion can be methanol, ethanol, propyl alcohol, butanol, etc., and among these, methanol or ethanol is preferred. It is preferable to use the same type of alcohol for the dispersion as the above-mentioned alcohol for precipitation and washing, but different types of alcohol can also be used.

[0027] [5] Manufacturing of CNF-containing resin moldings [5-1] Process description By adding the alcohol-based CNF dispersion and then heat-molding the resin during molding, a CNF-containing resin molded product can be produced. The CNF in the resin improves the strength of the CNF-containing resin molded product, and the decrease in toughness is suppressed. Furthermore, since the pyridine cleaning prevents the adverse effects of pyridinium-based quaternary ammonium salts, the discoloration to dark brown or black that occurs on the surface of the resin molded product due to heating during molding is suppressed.

[0028] [5-2] Resin materials The resin to which the alcohol-based CNF dispersion can be added includes, for example, thermoplastic resins such as polypropylene resin, polyethylene resin, vinyl chloride resin, polystyrene resin, polyester resin, and acrylic resin, as well as thermosetting resins such as phenolic resin, urethane resin, and epoxy resin. Furthermore, the alcohol-based CNF dispersion is compatible with resins, and when mixed with a thermoplastic or thermosetting resin and molded, the CNF disperses well in the resin.

[0029] [6] Uses of CNF and CNF-containing resin moldings [6-1] Uses of CNF The above-mentioned CNFs can be suitably used in applications that utilize their properties, such as high strength, high elastic modulus, light weight, dimensional stability (low linear expansion coefficient), thixotropy, viscosity control, adsorption, gas barrier properties, heat retention, water retention, moisture retention, water repellency, pore control, biodegradability, increased surface area, low thermal expansion coefficient, network structure, transparency, ultraviolet absorption, and high thermal conductivity / electrical insulation.

[0030] [6-2] Applications of CNF-containing resin moldings The CNF-containing resin molded articles can be used for non-foamed or foamed articles (e.g., packaging materials, storage containers, automobile parts, home appliances, stationery and miscellaneous goods, piping, flooring, wallpaper, films), adhesives, etc. Specifically, they can be used for table tennis rackets and speakers by taking advantage of their high strength, for automobile parts by improving the elastic modulus of resins, and for foamed articles such as shoe soles by taking advantage of their foam-stabilizing properties. [Example]

[0031] [Effectiveness Demonstration Test 1] Next, we will explain the demonstration test of the cleaning effect of pyridine on the above-mentioned wet fabric. In this test, an ultraviolet-visible spectrophotometer (UH3900D, manufactured by Hitachi High-Techno-Science Corporation) was used to measure the absorbance at 200 nm, the absorption wavelength of 1-benzylpyridinium chloride. A calibration curve for the methanol cleaning solution of 1-benzylpyridinium chloride was created, and the amount of 1-benzylpyridinium chloride contained in 1 L of the methanol cleaning solution was quantified.

[0032] "Example 1" In this example, 10 g of wood pulp was added to 170 mL of 1-benzylpyridinium chloride and heated and stirred at 135°C to defibrate plant fibers. After defibration, methanol was added to dilute the mixture, and wet fibers were precipitated. The wet fibers were separated by centrifugation and washed with methanol. The washing and separation were repeated until the color of the 1-benzylpyridinium chloride disappeared. The wet fibers, which had lost their color due to 1-benzylpyridinium chloride, were washed with pyridine and recovered by filtration. The pyridine used for washing was concentrated under reduced pressure, allowing 1-benzylpyridinium chloride to be recovered as a residue. Further, the pyridine adhering to the wet fibers was removed with methanol, and 1 L of the methanol used for pyridine removal was used for analysis. Quantitation was performed using a UV-visible spectrophotometer (UH3900D, Hitachi High-Techno-Science Corporation) based on the absorbance at 200 nm, confirming that the concentration was less than 2.0 mg / L.

[0033] "Comparative Example 1" In this comparative example, the wet fiber was washed with only methanol using the same method as in the example. The wet fiber, from which the color of 1-benzylpyridinium chloride had disappeared, was washed with methanol and filtered. 1 L of the methanol washings was quantified based on the absorbance at 200 nm using a UV-visible spectrophotometer (UH3900D, manufactured by Hitachi High-Techno-Science Corporation). The results showed that 2.0 mg to 3.0 mg of 1-benzylpyridinium chloride continued to elute.

[0034] "Verification results of effectiveness" In the case of washing with only methanol in Comparative Example 1, the amount of pyridinium-based quaternary ammonium salt contained in the wet fiber eluted into methanol was 2.0 to 3.0 mg / L, whereas in Example 1, it was confirmed that when further washing with pyridine after washing with methanol was performed, the remaining 1-benzylpyridinium chloride was removed, and the amount eluted from the wet fiber into methanol was less than 2.0 mg / L.

[0035] [Effectiveness Demonstration Test 2] Next, we will explain the demonstration test of the effect of suppressing discoloration due to heating of resin molded products to which the above-mentioned CNFs have been added. In this test, we prepared resin pellets by adding an alcohol-based CNF dispersion in which wet fibers were dispersed in methanol, and then injection-molded A1 multipurpose test pieces using these resin pellets. The degree of discoloration was confirmed visually.

[0036] Specifically, the resin pellets were produced by molding resin strands using an extrusion molding machine (Parker Corporation, PRMN-25FS), cooling them with an air-cooled net conveyor (KCK Engineering, C-02), and pelletizing them with a small pelletizer (Toyo Seiki Seisaku-sho, MPETC1). The extrusion molding conditions included screw temperature control set to 170-240°C and an extruder speed of 30-50 rpm. For the A1 multipurpose test specimens, the temperature of a small injection molding machine (Imoto Manufacturing Co., Ltd., IMC-070B) was set to 170-200°C, and the resin was melted in the machine for 15-40 minutes. The resin was then injected into a mold heated to 70-120°C. After the mold cooled, the desired test specimens were obtained.

[0037] "Example 2" In this example, an alcohol-based CNF dispersion was prepared by dispersing the pyridine-washed wet fibers from Example 1 in methanol. This was then mixed with polylactic acid resin (Terramac TE-2000, Unitika Co., Ltd.) in an extruder to produce resin pellets for injection molding. The resin pellets were prepared by adding polylactic acid resin and wet fibers (alcohol-based CNF dispersion) to the extruder inlet so that the ratio of wet fiber to the total resin pellet was between 0.1 wt% and 5 wt%. These resin pellets were then heated and molded using a small injection molding machine to produce A1 multipurpose test specimens. Visual inspection of the color of these test specimens revealed a light brown appearance.

[0038] "Comparative Example 2" In this comparative example, an alcohol-based CNF dispersion was used, in which the wet fibers of Comparative Example 1, which had been washed with only methanol, were dispersed in methanol. This was mixed with polylactic acid resin (Terramac TE-2000, Unitika Co., Ltd.) in an extrusion molding machine to produce resin pellets for injection molding. The resin pellets were then heated and molded using a small injection molding machine to produce A1 multipurpose test pieces. Visual inspection of this test piece revealed that its appearance had turned dark brown. Furthermore, inspection of the mechanical properties of the test piece revealed that it was brittle and had reduced strength.

[0039] "Verification results of effectiveness" In the above Comparative Example 2, the resin molded body was significantly discolored and its strength was reduced due to heating during molding, whereas in the above Example 2, it was confirmed that the degree of discoloration due to heating was less than in Comparative Example 2, and the reduction in strength was also less.

Claims

1. A method for producing cellulose nanofibers, comprising separating and recovering wet fibers from a composition containing a pyridinium-based quaternary ammonium salt, alcohol, and wet fibers precipitated by dissolving plant fibers, and then washing the wet fibers with pyridine.

2. The method for producing cellulose nanofibers according to claim 1 , wherein the separated and recovered wet fibers are washed with alcohol and then washed with pyridine.

3. The method for producing cellulose nanofibers according to claim 1 or 2, wherein the wet fibers washed with pyridine are washed again with alcohol.

4. An alcohol-based CNF dispersion in which cellulose nanofibers containing a trace amount of pyridinium-based quaternary ammonium salt are dispersed in alcohol, and the content of pyridinium-based quaternary ammonium salt in the alcohol is less than 2.0 mg / L.

5. A method for producing a CNF-containing resin molded product, comprising adding the alcohol-based CNF dispersion according to claim 4 and then heat-molding the mixture.

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