Porous cellulose object production method, and porous cellulose object

Abstract

The present invention addresses the problem of providing: a porous cellulose object production method which makes it possible to produce a porous cellulose object having a high porosity and a high transmittance; and the porous cellulose object. This porous cellulose object production method is one for obtaining a porous cellulose object from a cellulose nanofiber solution comprising a solvent including water and cellulose nanofibers. The method comprises: a gelation step in which the cellulose nanofiber solution is caused to gel using a gellant solution containing a gellant, thereby obtaining a wet cellulose gel; and a drying step in which the wet cellulose gel is dried to obtain the porous cellulose object. The gelation step is a step in which the cellulose nanofiber solution is caused to gel in the state of containing metal ions and having a pH adjusted to 4-10.

Description

Method for producing porous cellulose body and porous cellulose body

[0001] The present invention relates to a method for producing a porous cellulose body. The present invention also relates to a porous cellulose body.

[0002] Porous materials obtained by drying dispersions of structures with three-dimensional network structures, such as cellulose nanofibers, have high porosity and therefore have insulating properties similar to air, making them promising for a variety of applications.

[0003] For example, Patent Document 1 describes cellulose aerogel as such a porous body, and also describes a method for producing cellulose aerogel by gelling it using 5% sulfuric acid (

[0067] ).

[0004] International Publication No. 2010 / 104061

[0005] The present inventors have investigated the method for producing a cellulose aerogel (porous cellulose body) described in Patent Document 1 and have found that the resulting porous cellulose body may not be able to achieve both high porosity and high permeability.

[0006] Therefore, an object of the present invention is to provide a method for producing a porous cellulose body that can produce a porous cellulose body having high porosity and high permeability. Another object of the present invention is to provide a porous cellulose body.

[0007] As a result of extensive research into the above-mentioned problems, the present inventors discovered that the porosity and permeability of the resulting porous cellulose body can be increased by including a step of gelling a cellulose nanofiber solution containing metal ions and having a pH adjusted to 4 or more and 10 or less, and thus completed the present invention. In other words, the present inventors discovered that the above-mentioned problems can be solved by the following configuration.

[0008] [1] A method for producing a porous cellulose body, comprising obtaining a porous cellulose body from a cellulose nanofiber solution containing a water-containing solvent and cellulose nanofibers, the method comprising: a gelling step of gelling the cellulose nanofiber solution using a gelling agent solution containing a gelling agent to obtain a cellulose wet gel; and a drying step of drying the cellulose wet gel to obtain the porous cellulose body, the gelling step being a step of gelling the cellulose nanofiber solution in a state where the cellulose nanofiber solution contains metal ions and has a pH adjusted to 4 or more and 10 or less. [2] The method for producing a porous cellulose body according to [1], wherein the cellulose wet gel has an elastic modulus of 5 KPa or more, and the transmittance at a wavelength of 550 nm of a 10 mm thick cellulose wet gel is 90% or more. [3] The method for producing a porous cellulose body according to [1] or [2], wherein a pH adjusting solution containing at least one pH adjuster selected from the group consisting of sodium hydroxide, potassium hydroxide, and cesium hydroxide is used in the gelling step before using the gelling agent solution. [4] The method for producing a porous cellulose body according to [3], wherein the concentration of the pH adjuster in the pH-adjusted solution is 0.01 mol / L or more and 0.1 mol / L or less. [5] The method for producing a porous cellulose body according to any one of [1] to [4], wherein the gelling agent is at least one metal salt selected from the group consisting of magnesium chloride, aluminum chloride, calcium chloride, and iron chloride. [6] The method for producing a porous cellulose body according to any one of [1] to [5], wherein the concentration of the gelling agent in the gelling agent solution is 0.01 mol / L or more and 1.0 mol / L or less. [7] The method for producing a porous cellulose body according to any one of [1] to [6], wherein the drying method in the drying step is at least one method selected from the group consisting of superheated steam drying, microwave drying, and freeze-drying. [8] A cellulose porous body having a porosity of 90% or more, wherein a 2 mm thick cellulose porous body has a transmittance of 90% or more at a wavelength of 550 nm, and the cellulose porous body contains 1 mass % or more and 10 mass % or less of a metal relative to the total mass of the cellulose porous body.

[0009] According to the present invention, it is possible to provide a method for producing a porous cellulose body that can produce a porous cellulose body having high porosity and high permeability.Furthermore, according to the present invention, it is possible to provide a porous cellulose body.

[0010] The present invention will be described in detail below. The following description of the components may be based on representative embodiments of the present invention, but the present invention is not limited to such embodiments.

[0011] The following describes the meaning of each description in this specification. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. Furthermore, in this specification, in a numerical range described in stages, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the Examples. Furthermore, in this specification, each component may be a single substance corresponding to the component, or two or more substances may be used in combination. Here, when two or more substances are used in combination for each component, the content of that component refers to the total content of the substances used in combination, unless otherwise specified.

[0012] [Method for producing porous cellulose body] The method for producing a porous cellulose body of the present invention (hereinafter also referred to as "the production method of the present invention") is a method for producing a porous cellulose body by obtaining a porous cellulose body from a cellulose nanofiber solution containing a water-containing solvent and cellulose nanofibers. The production method of the present invention includes a gelling step in which the cellulose nanofiber solution is gelled using a gelling agent solution containing a gelling agent to obtain a cellulose wet gel. The production method of the present invention also includes a drying step in which the cellulose wet gel is dried to obtain a porous cellulose body. Furthermore, in the production method of the present invention, the gelling step is a step in which the cellulose nanofiber solution is gelled in a state in which metal ions are contained and the pH is adjusted to 4 or more and 10 or less.

[0013] According to the manufacturing method of the present invention, a porous cellulose body with high porosity and high permeability can be obtained. The reason why the above effect is exhibited is not necessarily clear in detail, but the present inventors speculate as follows. First, as a method for gelling a cellulose nanofiber solution, an acid (e.g., hydrochloric acid, sulfuric acid, etc.) is added to the solution to remove COO, which is a substituent of the cellulose nanofiber. － is converted to COOH and gelled by hydrogen bonding between O and H (see, for example, Patent Document 1). However, with this method, as shown in Comparative Example 1 below, the porosity and permeability of the resulting porous cellulose body are both low. This is because the addition of an acid causes the charge repulsion of the substituents of the cellulose nanofiber to be lost, i.e., the COO － Therefore, in the present invention, the cellulose nanofiber solution is gelled in a state where it contains metal ions and the pH is adjusted to 4 or more and 10 or less, so that the substituents of the cellulose nanofibers are converted to COO － It is presumed that the high porosity and high transmittance were achieved because crosslinking could be carried out in this state, that is, crosslinking could be carried out in a state in which aggregation of the cellulose nanofibers was suppressed.

[0014] The production method of the present invention will be described in detail below.

[0015] [Cellulose Nanofiber Solution] The cellulose nanofiber solution used in the production method of the present invention contains a water-containing solvent and cellulose nanofibers.

[0016] <Water-Containing Solvent> The water-containing solvent is not particularly limited as long as it contains at least water, and may be water alone or a mixed solvent of water and an organic solvent. Examples of the organic solvent include: ester-based solvents such as ethyl acetate, butyl acetate, isopropyl acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and propylene glycol monoethyl ether acetate; ether-based solvents such as propylene glycol monomethyl ether, ethylene glycol monomethyl ether, ethylene glycol isopropyl ether, ethylene glycol-t-butyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether (butyl cellosolve), and propylene glycol monobutyl ether; alcohol-based solvents such as methanol, ethanol, ethoxypropanol, butanol, methoxybutanol, methyl methoxybutanol, propyl alcohol, and 2-ethylhexanol; ketone-based solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; aliphatic hydrocarbon-based solvents such as Swazol, Shellsol, and mineral spirits; aromatic solvents such as xylene and toluene; and aprotic polar solvents such as acetonitrile and dimethyl sulfoxide. These may be used alone or in combination of two or more.

[0017] (Cellulose nanofibers) Cellulose nanofibers (hereinafter also referred to as "CNF") are not particularly limited as long as they are materials obtained from cellulosic raw materials. The cellulosic raw materials are not particularly limited as long as they are materials mainly composed of cellulose, and examples thereof include pulp, natural cellulose, regenerated cellulose, and fine cellulose obtained by depolymerizing a cellulose raw material through mechanical treatment. Note that commercially available products such as crystalline cellulose made from pulp can be used as the cellulosic raw material as is. The cellulosic raw materials may be subjected to chemical treatment such as alkali treatment to facilitate penetration of an oxidizing agent.

[0018] The fiber length of the cellulose nanofiber is not particularly limited, but is preferably 100 nm to 5000 nm, more preferably 50 nm to 2000 nm, and even more preferably 100 nm to 700 nm. The fiber diameter of the cellulose nanofiber is not particularly limited, but is preferably 1 nm to 100 nm, and more preferably 2 nm to 10 nm.

[0019] There are no particular limitations on the method for obtaining cellulose nanofibers from cellulosic raw materials, and any method known in the technical field of the present invention can be used. For example, cellulose nanofibers can be produced by a method in which the cellulosic raw material is oxidized with sodium hypochlorite as an oxidizing agent in the presence of a compound having a piperidine skeleton, such as 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter abbreviated as "TEMPO"), as a catalyst.

[0020] The content of cellulose nanofibers in the cellulose nanofiber solution is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more, relative to the total mass of the cellulose nanofiber solution. The content of cellulose nanofibers in the cellulose nanofiber solution is preferably less than 10% by mass, more preferably 8% by mass or less, and even more preferably 5% by mass or less, relative to the total mass of the cellulose nanofiber solution.

[0021] The cellulose nanofiber solution may be concentrated to achieve the above-mentioned preferred content. The concentration method is not particularly limited, but preferably includes a step of concentrating under conditions of a temperature of 20 to 80°C and a humidity of 50 to 90%.

[0022] (Other Components) The cellulose nanofiber solution may contain other components in addition to those described above. Examples of other components include antifreeze agents. When freeze-drying is applied in the drying step described below, the cellulose nanofiber solution preferably contains an antifreeze agent. Examples of antifreeze agents include water-soluble polymers, sugars, and antifreeze proteins, with antifreeze proteins being preferred. The content of the antifreeze agent is preferably 1 to 40% by mass relative to the content of cellulose nanofibers in the cellulose nanofiber solution.

[0023] [Gelling Step] The gelling step included in the production method of the present invention is a step of gelling the cellulose nanofiber solution using a gelling agent solution containing a gelling agent to obtain a cellulose wet gel. The gelling step included in the production method of the present invention is a step of gelling the cellulose nanofiber solution while adding metal ions and adjusting the pH to between 4 and 10. Examples of metal ions include magnesium ions, aluminum ions, calcium ions, and iron ions, and these may be used alone or in combination of two or more. Of these, aluminum ions or calcium ions are preferred, with aluminum ions being more preferred.

[0024] <Gelling Agent Solution> The gelling agent contained in the gelling agent solution is not particularly limited as long as it can gel the cellulose nanofiber solution, but from the viewpoint of including the metal ions in the cellulose nanofiber solution, it is preferably a metal salt. Here, the metal salt refers to a compound containing a metal ion and an anion formed by removing a hydrogen atom from an acid. Suitable examples of such metal salts are metal salts made of a metal with a valence of 2 or more from the viewpoint of cross-linking cellulose fibers together, and specific examples include magnesium chloride (MgCl ２ ), aluminum chloride (AlCl ３ ), calcium chloride (CaCl ３ ), and iron chloride (FeCl ３Among these, aluminum chloride or calcium chloride is preferred, and aluminum chloride is more preferred.

[0025] In the present invention, from the viewpoint of crosslinking cellulose fibers without reducing transmittance, the concentration of the gelling agent (particularly the metal salt) in the gelling agent solution is preferably 0.01 mol / L or more and 1.0 mol / L or less, more preferably more than 0.01 mol / L and less than 1.0 mol / L, and even more preferably 0.1 mol / L or more and 0.5 mol / L or less.

[0026] The gelling agent solution preferably contains a solvent, more preferably water, along with the gelling agent. The solvent contained in the gelling agent solution may be the same as the water-containing solvent contained in the cellulose nanofiber solution, and preferred embodiments are also the same.

[0027] <pH Adjusting Solution> In the present invention, before using the gelling agent solution described above in the gelation step, it is preferable to use a pH adjusting solution containing at least one pH adjuster selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH), and cesium hydroxide (CsOH).

[0028] Furthermore, in the present invention, the concentration of the pH adjuster in the pH-adjusted solution is preferably 0.01 mol / L or more and 0.1 mol / L or less, more preferably more than 0.01 mol / L and less than 0.1 mol / L, and even more preferably 0.02 mol / L or more and 0.06 mol / L or less.

[0029] The pH-adjusted solution preferably contains a solvent, more preferably water, along with the pH adjuster. The solvent contained in the pH-adjusted solution may be the same as the water-containing solvent contained in the cellulose nanofiber solution, and preferred embodiments are also the same.

[0030] <Gelling method> In the gelling step of the manufacturing method of the present invention, as described above, the cellulose nanofiber solution is gelled while containing metal ions and the pH is adjusted to 4 or more and 10 or less (preferably 4 or more and 9 or less), thereby obtaining a cellulose wet gel.

[0031] Here, the pH (4 or more and 10 or less) of the cellulose nanofiber solution during gelation is measured using the following method. That is, after dripping the gelling agent solution into the cellulose nanofiber solution, the pH is measured within 1 to 10 minutes by contacting a compact pH meter (AP-20 manufactured by AS ONE) with the surface onto which the gelling agent solution has been dripped (the surface on which gelation is progressing). Furthermore, if a pH adjusting solution is used before using the gelling agent solution, the pH adjusting solution is dripped into the cellulose nanofiber solution, and then the gelling agent solution is dripped and then measured using the method described above.

[0032] Methods for obtaining a cellulose wet gel (gelation method) include, for example, a method of contacting the cellulose nanofiber solution with the gelling agent solution; a method of mixing the cellulose nanofiber solution with the pH-adjusted solution and then contacting the mixture with the gelling agent solution; and a method of placing the cellulose nanofiber solution in a container of a desired shape, supplying the gelling agent solution to the liquid surface of the cellulose nanofiber solution, and allowing the mixture to stand. Specific examples include a method of placing a mixture of the cellulose nanofiber solution and the pH-adjusted solution in a container of a desired shape, supplying the gelling agent solution to the liquid surface of the mixture, and allowing the mixture to stand. To obtain a more uniform cellulose wet gel, the gelling agent solution may be supplied while shaking or stirring the cellulose nanofiber solution. The amount of gelling agent solution supplied is not particularly limited, but may be, for example, 1 to 100% by mass, preferably 5 to 50% by mass, and more preferably 10 to 30% by mass, of the mass of the cellulose nanofiber solution. The amount of pH adjuster supplied is not particularly limited, but for example, the mass of the pH adjuster supplied relative to the mass of the cellulose nanofiber solution can be 1 to 100% by mass, preferably 5 to 50% by mass, and more preferably 10 to 30% by mass. Another method for obtaining a cellulose wet gel includes placing the cellulose nanofiber solution in a container of a desired shape, supplying the pH-adjusted solution and the gelling agent solution to the container in that order, and shaking the container to gel the solution. In this method, the pH-adjusted solution and the gelling agent solution can be supplied in that order while stirring the cellulose nanofiber solution.

[0033] The temperature when carrying out the gelation method is preferably 10 to 40°C, more preferably 15 to 30°C. In the gelation method in which the pH-adjusted solution and the gelling agent solution are allowed to stand after being supplied, the standing time is, for example, 0.1 to 24 hours, preferably 0.5 to 12 hours. In the method in which gelation is carried out while shaking, the shaking time is preferably 6 to 12 hours.

[0034] The shape of the cellulose wet gel obtained by the gelation step is not particularly limited, but a specific three-dimensional shape is preferable because the porosity of the cellulose porous body obtained by the drying step is likely to be high. A cellulose wet gel having a specific three-dimensional shape refers to a three-dimensional shape having at least one flat surface (also referred to as "surface X"), where the surface area of ​​surface X is equal to or greater than the sum of the surface areas of the surfaces directly in contact with surface X. For example, an example of a cellulose wet gel having a specific three-dimensional shape is a rectangular parallelepiped shape in which the surface area of ​​the bottom surface is greater than the sum of the surface areas of the side surfaces. Another example is a cylindrical shape in which the surface area of ​​the bottom surface is greater than the surface area of ​​the side surfaces. The specific three-dimensional shape is often a sheet-like shape. A sheet-like shape refers to a shape having two opposing main surfaces, a thin thickness relative to the main surfaces, and an overall flat shape. The ratio of the thickness of the cellulose wet gel to the maximum in-plane length of the main surfaces of the sheet-like cellulose wet gel is preferably 0.5 or less, more preferably 0.3 or less. There is no particular lower limit to the ratio, but it may be 0.00001 or more, and is often 0.001 or more.

[0035] When superheated steam drying or microwave drying is applied in the drying step described below, a cellulose porous body with a higher porosity is likely to be obtained if the cellulose wet gel has the above-mentioned specific three-dimensional shape. This is because, during drying, much of the water inside the cellulose wet gel moves in a direction perpendicular to the plane X, and the capillary force of water generated at that time tends to be perpendicular to the direction of water movement (i.e., from the side to the inside). Therefore, when the cellulose wet gel has a specific three-dimensional shape (e.g., a rectangular parallelepiped or cylindrical shape) and the top surface corresponds to the plane X, the capillary force of water tends to be parallel to the bottom surface. It is thought that the capillary force in the above direction is easily weakened by the anchoring effect of the cellulose wet gel, which suppresses volumetric shrinkage during drying and results in a cellulose porous body with a higher porosity.

[0036] <Cellulose Wet Gel> A cellulose wet gel is obtained by the gelling step.

[0037] In the present invention, because a porous cellulose body having a higher porosity and a higher transmittance can be obtained, it is preferable that the elastic modulus of the cellulose wet gel obtained by the gelation step be 5 KPa or more, and that the transmittance at a wavelength of 550 nm of a 10 mm thick cellulose wet gel be 90% or more. Here, the elastic modulus of the cellulose wet gel is preferably 7 KPa or more, more preferably 10 KPa or more, and even more preferably 14 KPa or more, in order to facilitate the production of a porous cellulose body with an even higher porosity. The upper limit of the elastic modulus is not particularly limited, but may be, for example, 50 KPa or less, and is often 30 KPa or less, or may be 20 KPa or less. Furthermore, the transmittance at a wavelength of 550 nm of a 10 mm thick cellulose wet gel is preferably 95% or more, and more preferably 96% or more, in order to facilitate the production of a porous cellulose body with an even higher transmittance.

[0038] The modulus of elasticity of the cellulose wet gel is measured by the following method. The compressive modulus of the cellulose wet gel is measured under the following conditions, and the result is regarded as the modulus of elasticity of the cellulose wet gel. Apparatus: Shimadzu Micro Autograph MST Load cell: 2 N Max Indenter: Acrylamide resin indenter with a flat tip having a diameter of 15 mm Sample size: 10 mm square Compression speed: 100% / min (10 mm / min) Number of samples: 3 Compressive modulus: Calculated from strain of 0-1%

[0039] The transmittance of a 10 mm thick cellulose wet gel at a wavelength of 550 nm is measured as follows: A porous cellulose body is processed to a thickness of 10 mm to obtain a measurement sample. The measurement sample is placed in a spectrophotometer (V-670, manufactured by JASCO Corporation), and the transmittance at wavelengths (300 to 800 nm) is calculated to obtain the transmittance at 550 nm.

[0040] [Drying Step] The drying step included in the production method of the present invention is a step of drying the cellulose wet gel to obtain a porous cellulose body. In the drying step, the solvent component contained in the cellulose wet gel is removed.

[0041] In the drying step, the method (drying method) is not particularly limited as long as the solvent component contained in the cellulose wet gel is removed and a porous cellulose body is obtained. Among these, at least one method selected from the group consisting of superheated steam drying, microwave drying, and freeze-drying is preferred as the drying method, since it tends to increase the porosity of the resulting porous cellulose body. Each method will be described below.

[0042] <Superheated Steam Drying> Superheated steam drying refers to a method of drying using vapor of a solvent heated to a temperature equal to or higher than its saturation temperature.

[0043] In superheated steam drying, 10 ２ Pa or more 10 ６ It is also preferable to set the pressure at or below 10 Pa and the temperature at or above the boiling point of the solvent contained in the cellulose wet gel. Furthermore, it is also preferable to set the internal temperature of the cellulose wet gel at or above the external temperature of the cellulose wet gel. The internal temperature of the cellulose wet gel can be directly confirmed by a thermocouple. Furthermore, the external temperature of the cellulose wet gel can be directly confirmed by a thermocouple. In the superheated steam drying, from the viewpoint of obtaining a porous cellulose body with a higher porosity, the pressure in the superheated steam drying is set to 10 ３ Pa or more 10 ５ Pa or less is preferable, ４ Pa or more 10 ５ Pa or less is more preferable.

[0044] The temperature of the superheat drying treatment is not particularly limited as long as it is a temperature equal to or higher than the boiling point of the solvent described above, but is preferably 100 to 200°C, more preferably 100 to 180°C. The internal temperature of the cellulose wet gel is preferably equal to or higher than the external temperature of the cellulose wet gel, more preferably 100 to 200°C, and even more preferably 100 to 180°C. The external temperature of the cellulose wet gel is preferably equal to or lower than the internal temperature of the cellulose wet gel, more preferably 100 to 200°C, and even more preferably 100 to 180°C. Furthermore, because expansion and rupture of the cellulose wet gel can be suppressed, the temperature difference between the internal temperature of the cellulose wet gel and the external temperature of the cellulose wet gel in the superheat drying treatment is preferably within 50°C, more preferably 10 to 50°C, and even more preferably 20 to 40°C.

[0045] When the cellulose wet gel to be subjected to the drying step contains water, a preferred method is to dry it using a superheated steam dryer set at a temperature of 100 to 180° C. and a humidity of 10 to 50%, for example.

[0046] <Microwave drying> Microwave drying refers to a drying method in which the molecules of a solvent contained in a cellulose wet gel are vibrated by microwaves, causing heating and evaporation. When microwave drying is applied as a drying step for a cellulose wet gel, a method in which microwaves with a wavelength of 1 mm to 10 cm are irradiated at an output of 1 to 5 kW is preferred in order to prevent the cellulose wet gel from expanding and bursting. Microwave drying may also be used in combination with the above-mentioned superheated steam drying.

[0047] <Freeze-drying> Freeze-drying refers to a method of freezing the solvent contained in a cellulose wet gel and sublimating the frozen solvent. Freezing of a cellulose wet gel can be carried out by cooling it to a temperature below the freezing point of the solvent component contained therein, and can be carried out by a known method. Among these, freezing by rapid cooling is preferred, since the ice crystals of the solvent that are generated when the solvent component freezes are less likely to become large, and the permeability of the resulting porous cellulose body is higher. Examples of rapid cooling methods include contacting the gel with liquid nitrogen and contacting the gel with a member cooled with liquid nitrogen.

[0048] In freeze-drying, the frozen solvent is sublimated and removed from the cellulose wet gel to obtain a porous cellulose body. The sublimation process is preferably carried out in an environment below the freezing point of the solvent contained in the frozen cellulose wet gel. Furthermore, the sublimation process is preferably carried out in a reduced pressure environment where the pressure is lower than atmospheric pressure, in order to improve the sublimation rate.

[0049] In freeze-drying, the solvent contained in the cellulose wet gel may be replaced with another solvent before the freezing. Examples of the other solvent include a solvent containing an alcohol-based solvent and water, or an alcohol-based solvent. Examples of the alcohol-based solvent include alcohols that are miscible with water in any ratio, and t-butyl alcohol is preferred.

[0050] [Porous cellulose body] The porous cellulose body of the present invention has a porosity of 90% or more. Furthermore, the porous cellulose body of the present invention has a transmittance of 90% or more at a wavelength of 550 nm when the porous cellulose body is 2 mm thick. Furthermore, the porous cellulose body of the present invention contains a metal in an amount of 1% by mass or more and 10% by mass or less, based on the total mass of the porous cellulose body. The porous cellulose body of the present invention can be obtained, for example, by the production method of the present invention.

[0051] Here, the porosity of the porous cellulose body of the present invention refers to a value measured by the following procedure. First, the volume V and mass W of the porous cellulose body are measured. Next, the density D of the cellulose nanofibers constituting the porous cellulose body is measured to be 1.5 g / cm. ３ Next, the porosity is calculated using the following formula: Porosity = 100 × {volume V - (mass W / density D)} / volume V

[0052] The transmittance at 550 nm of the 10 mm thick porous cellulose body is measured in the same manner as the cellulose wet gel described above.

[0053] Furthermore, the metal content contained in the porous cellulose body of the present invention refers to a value measured by the absolute calibration curve method of inductively coupled plasma optical emission spectroscopy (ICP-OES). Here, the metal contained in the porous cellulose body of the present invention at a content of 1% by mass or more and 10% by mass or less is preferably at least one selected from the group consisting of magnesium (Mg), aluminum (Al), calcium (Ca), and iron (Fe). Of these, aluminum or calcium is preferred, and aluminum is more preferred. Furthermore, the metal content in the porous cellulose body of the present invention is preferably 3% by mass or more and 10% by mass or less.

[0054] The porous cellulose body of the present invention preferably has an indentation breaking stress of 0.5 MPa or more, and more preferably 1 MPa to 5000 MPa. The indentation breaking stress is obtained by performing a measurement similar to the compressive modulus measurement using a porous cellulose body as a sample instead of a sample prepared using a cellulose wet gel. Specifically, the measurement is performed on the porous cellulose body using the same equipment and conditions as for the cellulose wet gel, and the stress value at which the stress no longer increases or decreases with applied strain is taken as the indentation breaking stress.

[0055] The porous cellulose material of the present invention may contain components used in the production method of the present invention, in addition to the above-mentioned metals.

[0056] [Uses] The porous cellulose body obtained by the production method of the present invention and the porous cellulose body of the present invention can be used in a variety of applications. For example, the porous cellulose body obtained by the production method of the present invention and the porous cellulose body of the present invention can be suitably used for windows or components thereof. Specifically, they can be suitably used for applications such as interlayer films for laminated glass and interlayer films for double-glazed glass. For laminated glass and double-glazed glass, conventionally known configurations can be used for the number and type of glass sheets, layers other than the glass sheets and interlayer films (e.g., light-shielding layer, heat-shielding layer, flame-retardant layer, etc.), and sealing structures.

[0057] The present invention will be described in more detail below with reference to examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the examples shown below.

[0058] Example 1 Preparation of Cellulose Nanofiber Solution 10 g of softwood kraft pulp was suspended in 1,000 g of pure water containing 0.16 g of TEMPO and 1 g of sodium bromide. 25 g of a 2 M aqueous sodium hypochlorite solution was added to initiate the oxidation reaction. The temperature of the reaction system was maintained at 25°C, and the pH of the system was maintained at 10 by adding a 0.5 mol / L aqueous sodium hydroxide solution during the reaction. After 2 hours, the oxidation reaction was terminated by adding approximately 100 mL of ethanol to the reaction system. The solution was then filtered and washed repeatedly with pure water using a glass filter to obtain oxidized cellulose. The resulting oxidized cellulose was passed through a high-pressure homogenizer three times to prepare a dispersion containing 1% by mass of cellulose nanofibers. The resulting cellulose nanofibers had a carboxyl group content of 1.5 mmol / g, a fiber diameter of 3 nm, and a fiber length of 500 nm.

[0059] The cellulose nanofiber (CNF) aqueous dispersion obtained above was kept in a thermo-hygroscopic dryer at a temperature of 40°C and a humidity of 80% for 8 to 10 days to prepare a concentrated dispersion with a CNF concentration of 4% by mass. The concentrated dispersion had an orientation degree of 78 to 83%, and was confirmed to have structural anisotropy.

[0060] [Gelling step] To 100 parts by mass of the CNF concentrated dispersion obtained by the above procedure, 20 parts by mass of a pH-adjusted solution containing NaOH as a pH adjuster (NaOH concentration: 0.1 mol / L) was added from above the CNF concentrated dispersion using a dropper, and the mixture was allowed to stand at room temperature for 6 hours or more. ３ A gelling agent solution containing AlCl ３A cellulose wet gel was obtained by adding 4.1 parts by mass (= 20 x (17 / 83)) of CNF (concentration: 1.0 mol / L) from the top of the CNF concentrated dispersion using a dropper and leaving it to stand at room temperature for 6 hours or more to gel. The gelation was carried out in a 15 cm diameter Petri dish so that the thickness of the resulting cellulose wet gel was 10 mm. The resulting cellulose wet gel had a CNF content of approximately 0.4 mass% and contained water as a solvent. A hexahedral measurement sample measuring 10 mm long x 10 mm wide x 10 mm thick was cut out from the resulting cellulose wet gel.

[0061] [Drying step] The measurement sample cut into the above shape was placed in a superheated steam dryer set at a temperature of 120°C and a humidity of 30%, and left for 20 minutes to remove the solvent contained in the cellulose wet gel, thereby obtaining a porous cellulose body of Example 1.

[0062] [Examples 2 to 7] The porous cellulose bodies of Examples 2 to 7 were obtained in the same manner as in Example 1, except that various conditions in the gelation process (i.e., the concentrations of the pH adjuster and gelling agent, and the amount of gelling agent solution) were changed as shown in Table 1 below.

[0063] [Example 8] The cellulose porous body of Example 8 was obtained in the same manner as in Example 1, except that the following microwave drying was performed instead of superheated steam drying in the drying step. The microwave drying was performed using a microwave dryer. The microwave output was adjusted so that the internal temperature of the cellulose wet gel was 140°C and the external temperature of the cellulose wet gel was 100°C. The drying time was 5 minutes. Note that when the microwave drying was performed, nitrogen gas was sprayed onto the surface of the cellulose wet gel to maintain the external temperature at 100°C.

[0064] Comparative Example 1 In the gelation step, no pH adjusting solution was used, and AlCl ３ A gelling agent solution containing AlCl ３ Concentration: 1.0 mol / L) instead of sulfuric acid (H ２ SO ４ ) containing a gelling agent solution (H ２ SO４ The porous cellulose bodies of Comparative Example 1 were obtained in the same manner as in Example 1, except that 20 parts by mass of a cellulose syrup containing 20% ​​by mass of cellulose syrup (concentration: 1.0 mol / L) was blended and room temperature drying was used instead of superheated steam drying in the drying step. The drying method (room temperature drying) used in Comparative Example 1 was a method in which the cellulose syrup was left to stand for 2 days under conditions of 25°C and a relative humidity of 40%.

[0065] [Comparative Examples 2 and 3] As shown in Table 1 below, various conditions in the gelation step (i.e., concentrations of pH adjuster and gelling agent, blending amount of gelling agent solution) were changed, and hot air drying was used instead of superheated steam drying in the drying step, but other than that, the porous cellulose bodies of Comparative Examples 2 and 3 were obtained in the same manner as in Example 1. The drying method (hot air drying) used in Comparative Examples 2 and 3 was a method in which drying was carried out in a hot air oven at 80°C for 30 minutes.

[0066] [pH of CNF concentrated dispersion in gelation step] The pH of the CNF concentrated dispersion in the gelation step was measured by the method described above for Examples 1 to 8 and Comparative Examples 1 to 3. The measurement results are shown in Table 1 below.

[0067] [Presence or absence of precipitates during gelation process] For Examples 1 to 8 and Comparative Examples 1 to 3, the measurement samples after the gelation process were held up to a fluorescent lamp and visually checked for the presence or absence of cloudy matter. The results are shown in Table 1 below.

[0068] [Measurement of transmittance of cellulose wet gel] The transmittance of the cellulose wet gel obtained in the gelation step was measured by the method described above for Examples 1 to 8 and Comparative Examples 1 to 3. The measurement results are shown in Table 1 below.

[0069] [Measurement of Elastic Modulus of Cellulose Wet Gel] The elastic modulus of the cellulose wet gel obtained in the gelation step was measured by the method described above for Examples 1 to 8 and Comparative Examples 1 to 3. The measurement results are shown in Table 1 below.

[0070] [Porous cellulose body] The metal (Al) content in the porous cellulose bodies prepared in Examples 1 to 8 and Comparative Examples 1 to 3 was measured by the method described above. Similarly, the porosity and permeability of the prepared porous cellulose bodies were also measured by the method described above. These measurement results and the thickness of the prepared porous cellulose bodies are shown in Table 1 below.

[0071]

[0072] The results shown in Table 1 indicate that when the pH of the cellulose nanofiber solution in the gelation step (during gelation) is outside the range of 4 to 10, the porosity and transmittance of the resulting porous cellulose body are both low, regardless of the presence or absence of metal ions (Comparative Examples 1 to 3). In contrast, when the cellulose nanofiber solution in the gelation step (during gelation) contains metal ions (aluminum) and has a pH of 4 to 10, the porosity and transmittance of the resulting porous cellulose body are both high (Examples 1 to 8). In particular, a comparison between Example 1 and Example 7 indicates that when the transmittance of a 10 mm thick cellulose wet gel at a wavelength of 550 nm is 90% or higher, the porosity and transmittance of the resulting porous cellulose body are both higher.

Claims

1. A method for producing a cellulose porous body, comprising obtaining a cellulose porous body from a cellulose nanofiber solution containing a solvent containing water and cellulose nanofibers, the method including: a gelation step of gelating the cellulose nanofiber solution using a gelling agent solution containing a gelling agent to obtain a cellulose wet gel; and a drying step of drying the cellulose wet gel to obtain a cellulose porous body, wherein the gelation step is a step of gelating the cellulose nanofiber solution in a state where metal ions are contained and the pH is adjusted to 4 or more and 10 or less.

2. The method for producing a cellulose porous body according to claim 1, wherein the elastic modulus of the cellulose wet gel is 5 kPa or more, and the transmittance of the cellulose wet gel having a thickness of 10 mm at a wavelength of 550 nm is 90% or more.

3. The method for producing a cellulose porous body according to claim 1 or 2, wherein in the gelation step, before using the gelling agent solution, a pH adjustment solution containing at least one pH adjuster selected from the group consisting of sodium hydroxide, potassium hydroxide, and cesium hydroxide is used.

4. The method for producing a cellulose porous body according to claim 3, wherein the concentration of the pH adjuster in the pH adjustment solution is 0.01 mol / L or more and 0.1 mol / L or less.

5. The method for producing a cellulose porous body according to claim 1 or 2, wherein the gelling agent is at least one metal salt selected from the group consisting of magnesium chloride, aluminum chloride, calcium chloride, and iron chloride.

6. The method for producing a cellulose porous body according to claim 5, wherein the concentration of the gelling agent in the gelling agent solution is 0.01 mol / L or more and 1.0 mol / L or less.

7. The method for producing a cellulose porous body according to claim 1 or 2, wherein the drying method in the drying step is at least one method selected from the group consisting of superheated steam drying, microwave drying, and freeze drying.

8. A cellulose porous body having a porosity of 90% or more, wherein the transmittance of the cellulose porous body having a thickness of 2 mm at a wavelength of 550 nm is 90% or more, and the cellulose porous body contains 1% by mass or more and 10% by mass or less of metal based on the total mass of the cellulose porous body.

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