Method for producing porous cellulose body, and porous cellulose body

Abstract

The present invention addresses the problem of providing a method for producing a porous cellulose body, with which it is possible to obtain a porous cellulose body that has a high porosity and a high transmittance. A method for producing a porous cellulose body according to the present invention is for obtaining a porous cellulose body from a cellulose nanofiber solution that contains cellulose nanofibers and a solvent including water, and the method includes: a gelation step for making the cellulose nanofiber solution to gel with use of a gelling agent solution so as to obtain a cellulose wet gel; and a drying step for drying the cellulose wet gel so as to obtain a porous cellulose body. The gelling agent solution contains at least one gelling agent that is selected from the group consisting of an acid and a metal salt, and the concentration of the gelling agent is not less than 0.001 mol / L but less than 0.1 mol / L.

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] As a method for producing a porous body, for example, Patent Document 1 describes a method in which a hydrogel containing a porous body having a three-dimensional network structure is subjected to supercritical drying or the like (see [Claim 1], [Claim 3], [Claim 8], [Claim 11], and [Claim 12]).

[0004] Japanese Patent Application Laid-Open No. 2022-148855

[0005] The present inventors have investigated the method for producing a porous material (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] The present inventors have conducted extensive research to solve the above problems and have completed the present invention. That is, they have found that the above problems can be solved by the following configuration.

[0008] [1] A method for producing a porous cellulose body, in which a porous cellulose body is obtained from a cellulose nanofiber solution containing a water-containing solvent and cellulose nanofibers, comprising: a gelling step in which the cellulose nanofiber solution is gelled using a gelling agent solution to obtain a cellulose wet gel; and a drying step in which the cellulose wet gel is dried to obtain the porous cellulose body, wherein the gelling agent solution contains at least one gelling agent selected from the group consisting of acids and metal salts, and the concentration of the gelling agent is 0.001 mol / L or more and less than 0.1 mol / L. [2] 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 with a gelling agent solution to obtain a cellulose wet gel; and a drying step of drying the cellulose wet gel to obtain a porous cellulose body, wherein the cellulose nanofiber content in the cellulose nanofiber solution is 0.1 mass% or more and less than 10 mass% relative to the total mass of the cellulose nanofiber solution, the cellulose wet gel has an elastic modulus of 5 KPa or more, and the cellulose wet gel having a thickness of 10 mm has a transmittance of 90% or more at a wavelength of 550 nm. [3] The method for producing a porous cellulose body according to [1] or [2], wherein the gelling agent comprises at least one selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, aluminum chloride, and calcium chloride. [4] The method for producing a porous cellulose body according to any one of [1] to [3], 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. [5] A porous cellulose body having a porosity of 90% or more, wherein the porous cellulose body having a thickness of 2 mm has a transmittance at a wavelength of 550 nm of 90% or more.

[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] The methods for producing a porous cellulose body of the present invention (first and second embodiments) are described below. Hereinafter, the method for producing a porous cellulose body according to the first embodiment will also be referred to as the "first production method of the present invention," and the method for producing a porous cellulose body according to the second embodiment will also be referred to as the "second production method of the present invention." Hereinafter, when there is no need to distinguish between the first production method of the present invention and the second production method of the present invention, they will also be referred to as the "production method of the present invention."

[0013] [Method for producing porous cellulose body (first embodiment)] The method for producing a porous cellulose body according to the first embodiment (first 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 first production method of the present invention comprises a gelling step in which the cellulose nanofiber solution is gelled using a gelling agent solution to obtain a cellulose wet gel, and a drying step in which the cellulose wet gel is dried to obtain the porous cellulose body. In the first production method of the present invention, the gelling agent solution used in the gelling step contains at least one gelling agent selected from the group consisting of acids and metal salts, and the concentration of the gelling agent is 0.001 mol / L or more and less than 0.1 mol / L.

[0014] According to the first production method of the present invention, a porous cellulose body having 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. In the first production method of the present invention, the cellulose wet gel obtained in the gelling step is dried in the drying step to obtain a porous cellulose body. In the drying step, the solvent contained in the cellulose wet gel is removed, and during drying, the cellulose nanofibers can be aggregated by the capillary force of the solvent.

[0015] Here, in the first production method of the present invention, the concentration of the gelling agent in the gelling agent solution used in the drying and gelling steps is within the above-mentioned range. When the gelling agent concentration is equal to or greater than the lower limit of the above-mentioned range, the network formed by the cellulose nanofibers in the cellulose wet gel is relatively strong, and aggregation is less likely to occur during drying. When aggregation is less likely to occur during drying, the resulting porous cellulose body is likely to have a high porosity. Furthermore, when the gelling agent concentration is less than the upper limit of the above-mentioned range, a cellulose wet gel is likely to be obtained in which the cellulose nanofibers are highly dispersible, maintaining the dispersed state of the cellulose nanofibers in the cellulose nanofiber solution. When the cellulose nanofibers are highly dispersible, the porous cellulose body obtained by drying is likely to have a high transmittance. Furthermore, when such aggregation does not occur during drying, the transmittance of the resulting porous cellulose body is likely to be maintained at a high level. As a result, it is believed that a porous cellulose body obtained by subjecting a cellulose wet gel gelled using a gelling agent solution containing a gelling agent in the above-mentioned concentration range to a drying step will have a high porosity and high transmittance.

[0016] The first production method of the present invention will be described in detail below.

[0017] [Cellulose Nanofiber Solution] The cellulose nanofiber solution used in the first production method of the present invention contains a water-containing solvent and cellulose nanofibers.

[0018] (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.

[0019] (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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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%.

[0024] (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.

[0025] [Gelling step] In the first production method of the present invention, a gelling step is carried out in which the cellulose nanofiber solution is gelled using a gelling agent solution to obtain a cellulose wet gel. The gelling agent solution used in the first production method of the present invention and the method for carrying out the gelling step are described below.

[0026] (Gelling Agent Solution) As described above, the gelling agent solution contains at least one gelling agent selected from the group consisting of acids and metal salts, and the concentration of the gelling agent is 0.001 mol / L or more and less than 0.1 mol / L. The gelling agent solution contains the gelling agent and a solvent.

[0027] The gelling agent is at least one selected from the group consisting of acids and metal salts, and is not particularly limited as long as it can gel the cellulose nanofiber solution. The gelling agent has the function of reducing the electrostatic repulsion between the cellulose nanofibers, and the reduction in electrostatic repulsion forms a network between the cellulose nanofibers, resulting in a cellulose wet gel. Examples of acids include phosphoric acid (H ３ P.O. ４ ), hydrochloric acid (HCl), sulfuric acid (H ２ SO ４ ), acetic acid (CH ３ COOH), citric acid (C(OH)(CH ２ COOH) ２ COOH), and lactic acid (CH ３Examples of suitable acid salts include CH(OH)(COOH), with hydrochloric acid, sulfuric acid, or acetic acid being preferred. The acid refers to a substance contained as a solute in the gelling agent solution. A metal salt refers to a compound containing a metal ion and an anion formed by removing a hydrogen atom from an acid. Preferred examples of metal salts include compounds containing a metal ion and an anion formed by removing a hydrogen atom from the acid. Preferred examples of metal ions include magnesium ions, aluminum ions, calcium ions, and iron ions. Examples of metal salts include magnesium chloride, aluminum chloride, calcium chloride, and iron chloride, with aluminum chloride or calcium chloride being preferred. It is particularly preferred that the gelling agent contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, aluminum chloride, and calcium chloride.

[0028] The gelling agent may be ionized in the gelling agent solution, or may be bonded to other components contained in the gelling agent solution.

[0029] The solvent contained in the gelling agent solution preferably contains water. Examples of the solvent contained in the gelling agent solution include the same solvents as the water-containing solvent contained in the cellulose nanofiber solution, and the preferred embodiments are also the same. It is also preferable that the solvent contained in the gelling agent solution is only water.

[0030] The concentration of the gelling agent in the gelling agent solution is 0.001 mol / L or more, preferably 0.005 mol / L or more, and more preferably 0.01 mol / L or more. The concentration of the gelling agent in the gelling agent solution is less than 0.1 mol / L, preferably 0.08 mol / L or less, and more preferably 0.05 mol / L or less.

[0031] (Gelling Method) In the gelling step, the cellulose nanofiber solution is gelled using the gelling agent solution to obtain a cellulose wet gel, and the method is not particularly limited as long as a cellulose wet gel is obtained.

[0032] Examples of methods for obtaining a cellulose wet gel (gelation method) include contacting a cellulose nanofiber solution with a gelling agent solution. Specifically, one method involves 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 it to stand. To obtain a more uniform cellulose wet gel, the gelling agent solution may be supplied while the cellulose nanofiber solution is being shaken or stirred. While the amount of gelling agent solution supplied is not particularly limited, for example, the mass of the gelling agent solution supplied relative to the mass of the cellulose nanofiber solution may 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 gelling agent solution to the cellulose nanofiber solution, and gelling the solution while shaking. In the above method, the gelling agent solution may be supplied 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 gelling agent solution is 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 process, 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 resulting capillary force of water 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 believed 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 gelation step. It is also preferable that the cellulose wet gel obtained in the gelation step of the first production method of the present invention has the same properties as the cellulose wet gel obtained in the gelation step of the second production method of the present invention described below. The properties satisfied by the cellulose wet gel will be described in detail later.

[0037] [Drying Step] In the first production method of the present invention, a drying step is carried out in which the cellulose wet gel is dried to obtain a porous cellulose body. In the drying step, the solvent component contained in the cellulose wet gel is removed.

[0038] 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 drying method selected from the group consisting of superheated steam drying, microwave drying, and freeze-drying is preferred, as it tends to increase the porosity of the resulting porous cellulose body. Each method will be described below.

[0039] (Superheated Steam Drying) Superheated steam drying refers to a method of drying using vapor of a solvent heated to a saturation temperature or higher.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] (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.

[0044] (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.

[0045] 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.

[0046] 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.

[0047] [Method for producing porous cellulose body (second embodiment)] The method for producing a porous cellulose body according to the second embodiment (second 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 second production method of the present invention includes a gelling step in which the cellulose nanofiber solution is gelled using a gelling agent solution to obtain a cellulose wet gel, and a drying step in which the cellulose wet gel is dried to obtain a porous cellulose body. In the second production method of the present invention, the cellulose nanofiber content in the cellulose nanofiber solution is 0.1 mass% or more and less than 10 mass% relative to the total mass of the cellulose nanofiber solution. Furthermore, the cellulose wet gel has an elastic modulus of 5 KPa or more, and a transmittance of a 10 mm thick cellulose wet gel at a wavelength of 550 nm is 90% or more.

[0048] According to the second production method of the present invention, a porous cellulose body having high porosity and high permeability can be obtained. The reason why the above effect is exhibited is not necessarily clear in detail, but the inventors speculate as follows. In the second production method of the present invention, the cellulose wet gel obtained in the gelling step is dried in the drying step to obtain a porous cellulose body. In the drying step, the solvent contained in the cellulose wet gel is removed, and during drying, the cellulose nanofibers can be aggregated by the capillary force of the solvent.

[0049] Here, in the second manufacturing method of the present invention, the elastic modulus of the cellulose wet gel is 5 KPa or more. When the elastic modulus of the cellulose wet gel is within the above range, the cellulose nanofiber network in the cellulose wet gel is relatively strong and aggregation is unlikely to occur during drying. When aggregation is unlikely to occur during drying, the resulting porous cellulose body is likely to have a high porosity. Furthermore, the above-mentioned transmittance of the cellulose wet gel is a predetermined value or more. When aggregation is unlikely to occur during drying in the cellulose wet gel, a porous cellulose body is likely to be obtained while maintaining the transmittance of the cellulose wet gel. Note that such a cellulose wet gel is likely to be obtained when the cellulose nanofiber content in the cellulose nanofiber solution is within the above-mentioned range. As a result, it is thought that the porous cellulose body obtained by the second manufacturing method of the present invention, which satisfies each of the above requirements, will have a high porosity and a high transmittance.

[0050] The second production method of the present invention will be described in detail below.

[0051] [Cellulose nanofiber solution] The cellulose nanofiber solution used in the second production method of the present invention contains a solvent containing water and cellulose nanofibers. The cellulose nanofiber content in the cellulose nanofiber solution is 0.1 mass% or more and less than 10 mass% relative to the total mass of the cellulose nanofiber solution. The cellulose nanofiber solution used in the second production method of the present invention is the same as the cellulose nanofiber solution used in the first production method of the present invention except that the cellulose nanofiber content is in the above-mentioned range, so further explanation will be omitted.

[0052] [Gelling step] In the second manufacturing method of the present invention, a gelling step is carried out in which the cellulose nanofiber solution is gelled using a gelling agent solution to obtain a cellulose wet gel. In the second manufacturing method of the present invention, the obtained cellulose wet gel satisfies the above-mentioned predetermined requirements. Below, the gelling agent solution used in the second manufacturing method of the present invention and the obtained cellulose wet gel are described. Note that the method for carrying out the gelling step is the same as in the first manufacturing method of the present invention, so description thereof will be omitted.

[0053] (Gelling agent solution) The gelling agent solution preferably contains a gelling agent and a solvent. The gelling agent is at least one selected from the group consisting of acids and metal salts, and is not particularly limited as long as it can gel the cellulose nanofiber solution. Examples and preferred embodiments of the gelling agent are the same as those in the first production method of the present invention, and therefore will not be described here.

[0054] The solvent contained in the gelling agent solution preferably contains water. Examples of the solvent contained in the gelling agent solution include the same solvents as the water-containing solvent contained in the cellulose nanofiber solution, and the preferred embodiments are also the same. It is also preferable that the solvent contained in the gelling agent solution is only water.

[0055] The concentration of the gelling agent in the gelling agent solution is preferably 0.001 mol / L or more, more preferably 0.005 mol / L or more, and even more preferably 0.01 mol / L or more. The concentration of the gelling agent in the gelling agent solution is preferably less than 0.1 mol / L, more preferably 0.08 mol / L or less, and even more preferably 0.05 mol / L or less.

[0056] (Cellulose Wet Gel) The cellulose wet gel obtained in the gelling step in the second production method of the present invention has an elastic modulus of 5 KPa or more, and a 10 mm thick cellulose wet gel has a transmittance of 90% or more at a wavelength of 550 nm.

[0057] The elastic modulus of a cellulose wet gel is measured by the following method. The compressive elastic modulus of the cellulose wet gel is measured under the following conditions, and this is taken as the elastic modulus of the cellulose wet gel. Apparatus: Shimadzu Micro Autograph MST Load cell: 2 N Max Indenter: A flat acrylamide resin indenter with a φ15 mm tip Sample size: 10 mm square Compression speed: 100% / min (10 mm / min) n number: 3 Compressive elastic modulus: Calculated from strain of 0-1% The elastic modulus of the cellulose wet gel is 5 KPa or more, and is preferably 7 KPa or more, more preferably 10 KPa or more, and even more preferably 14 KPa or more, in that a porous cellulose body with a higher porosity can be easily obtained. There is no particular upper limit to the elastic modulus, but it can be, for example, 50 KPa or less, and is often 30 KPa or less, and may be 20 KPa or less.

[0058] The transmittance of a 10 mm thick cellulose wet gel at a wavelength of 550 nm is measured by the following method. 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. The transmittance is 90% or more, and is preferably 95% or more, and more preferably 96% or more, in that a porous cellulose body with a higher transmittance is more likely to be obtained. The transmittance is often 99.9% or less.

[0059] [Drying step] In the second production method of the present invention, a drying step is carried out in which the cellulose wet gel is dried to obtain a porous cellulose body. In the drying step, the solvent component contained in the cellulose wet gel is removed. Examples and preferred aspects of the drying step in the second production method of the present invention are the same as the examples and preferred aspects of the drying step in the first production method of the present invention, and therefore, description thereof will be omitted.

[0060] [Porous cellulose body] The porous cellulose body of the present invention has a porosity of 90% or more, and a 2 mm thick porous cellulose body has a transmittance of 90% or more at a wavelength of 550 nm. The porous cellulose body of the present invention can be obtained, for example, by the production method of the present invention (the first production method of the present invention or the second production method of the present invention).

[0061] The porosity of the porous cellulose body of the present invention is 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

[0062] 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.

[0063] Furthermore, 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.

[0064] The porous cellulose body of the present invention contains cellulose nanofibers and may contain the components used in the production method of the present invention.

[0065] [Uses] The porous cellulose body obtained by the production method of the present invention (the first production method of the present invention and the second 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-insulating glass. For laminated glass and double-insulating 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.

[0066] 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.

[0067] 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.

[0068] 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 0.4% by mass. The concentrated dispersion had an orientation degree of 78 to 83%, and was confirmed to have structural anisotropy.

[0069] [Gelling Process] A 0.05 mol / L aqueous HCl solution (hydrochloric acid) serving as a gelling agent solution was added to the CNF-enriched dispersion obtained by the above procedure using a dropper from above. The mixture was then shaken at room temperature for at least 6 hours using a shaker to gel the mixture, resulting in a cellulose wet gel. Gelation was carried out in a 15 cm diameter dish so that the resulting cellulose wet gel had a thickness of 10 mm. The gelling agent solution was added in an amount equivalent to 20% by mass of the CNF-enriched dispersion. The resulting cellulose wet gel contained approximately 0.4% by mass of CNF and contained water as a solvent. A hexahedral measurement sample measuring 10 mm long, 10 mm wide, and 10 mm thick was cut out from the resulting cellulose wet gel.

[0070] [Drying step] The cellulose wet gel obtained above 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.

[0071] [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. The measurement results are shown in the table below.

[0072] [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. The measurement results are shown in the table below.

[0073] [Examples 2 to 10] Except for changing the type of gelling agent contained in the gelling agent solution and the content of the gelling agent as shown in the table below, the porous cellulose bodies of Examples 2 to 10 were obtained in the same manner as Example 1. The transmittance and elastic modulus of the cellulose wet gels obtained in the gelling step of Examples 2 to 10 were measured in the same manner as Example 1.

[0074] [Example 11] The porous cellulose body of Example 11 was obtained in the same manner as in Example 6, 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. Note that the transmittance and elastic modulus of the cellulose wet gel obtained in the gelation step of Example 11 were measured in the same manner as in Example 1.

[0075] Example 12 A porous cellulose body was obtained in the same manner as in Example 1, except that the solvent contained in the cellulose wet gel was replaced with t-butyl alcohol and the drying method was changed to freeze-drying. Specifically, a cellulose wet gel was first obtained in the same manner as in Example 1. Next, the obtained cellulose wet gel was immersed in t-butyl alcohol and shaken to replace the water contained in the cellulose wet gel with t-butyl alcohol. The cellulose wet gel from which the solvent had been replaced was contacted with liquid nitrogen under atmospheric pressure, and the temperature was lowered to -100°C to obtain a frozen gel. In the frozen gel, the t-butyl alcohol contained in the cellulose wet gel was frozen. Next, the frozen gel was left to stand in a chamber reduced to 200 Pa for 4 hours, and the t-butyl alcohol was sublimated and removed from the frozen gel. After the above procedure, the treated frozen gel was removed from the chamber to obtain a porous cellulose body.

[0076] Example 13 A porous cellulose material was obtained in the same manner as in Example 1, except that the gelation step was changed to the following procedure.

[0077] [Gelling Process] To the CNF-enriched dispersion obtained by the procedure described in Example 1, a gelling agent solution (a 0.05 mol / L HCl aqueous solution (hydrochloric acid)) was added from above the CNF dispersion using a dropper. The mixture was allowed to stand at room temperature for at least 6 hours to gel, resulting in a cellulose wet gel. Gelation was carried out in a 15 cm diameter Petri dish so that the resulting cellulose wet gel had a thickness of 10 mm. The gelling agent solution was added in an amount equivalent to 20% by mass of the CNF-enriched dispersion. The resulting cellulose wet gel had a CNF content of approximately 0.4% by mass and contained water as a solvent. A hexahedral measurement sample measuring 10 mm long, 10 mm wide, and 10 mm thick was cut out from the resulting cellulose wet gel.

[0078] For the cellulose wet gel obtained by the procedure of Example 13, the results obtained were different between the measurement sample cut out from near the center of the petri dish and the measurement sample cut out from near the edge of the petri dish. Furthermore, for the porous cellulose body obtained by the procedure of Example 13, the porosity and transmittance measurement results were different between the sample cut out near the center of the petri dish and the sample cut out near the edge of the petri dish. The results are shown in the table below. Meanwhile, for the cellulose wet gels obtained in Examples 1 to 12, similar results were obtained at all positions. Furthermore, for the porous cellulose bodies obtained in Examples 1 to 12, similar results were obtained at all positions.

[0079] Comparative Example 1 A porous cellulose body of Comparative Example 1 was obtained in the same manner as in Example 1, except that the content of the gelling agent contained in the gelling agent solution in Example 1 was changed to the content shown in the table below, and the drying method in the drying step was changed to the method shown below (oven drying). The oven drying was a method in which drying was performed in a hot air oven at 80°C for 30 minutes. The transmittance and elastic modulus of the cellulose wet gel obtained in the gelation step of Comparative Example 1 were measured in the same manner as in Example 1.

[0080] [Comparative Examples 2 and 3] In Example 1, the content of the gelling agent contained in the gelling agent solution was changed to the content shown in the table below, and the porous cellulose bodies of Comparative Examples 2 and 3 were obtained in the same manner as in Example 1. The transmittance and elastic modulus of the cellulose wet gels obtained in the gelation steps of Comparative Examples 2 and 3 were measured in the same manner as in Example 1.

[0081] [Results] The gelation process method and the above measurement results for each Example and Comparative Example are shown in Table 1. In Table 1, in the row for Example 13, two numerical values ​​are listed, which represent the maximum and minimum values. Specifically, this indicates that there were measurement samples with a transmittance of 95% and 96% for the cellulose wet gel.

[0082]

[0083] From the results shown in Table 1, and comparison between each Example and each Comparative Example, it was confirmed that when the concentration of the gelling agent in the gelling agent solution used in the gelling step is within a predetermined range, a porous cellulose body with high porosity and high transmittance can be obtained. Furthermore, comparison between each Example and each Comparative Example confirmed that when the cellulose nanofiber content in the cellulose nanofiber solution is within a predetermined range, the elastic modulus of the cellulose wet gel is a predetermined value or greater, and the transmittance of the cellulose wet gel at a wavelength of 550 nm is a predetermined value or greater, a porous cellulose body with high porosity and high transmittance can be obtained. Comparison between Example 5 and Examples 2, 8, 9, and 10 confirmed that when the gelling agent contained in the gelling agent solution is aluminum chloride, the porosity and transmittance of the resulting porous cellulose body were superior. Comparison between Example 6 and Example 11 confirmed that when heated steam drying was performed as the drying method, the porosity and transmittance of the resulting porous cellulose body were superior. A comparison between Examples 4 to 6 and Example 7 confirmed that when the gelling agent concentration in the gelling agent solution was 0.01 mol / L or more, the porosity of the resulting porous cellulose body was superior.

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 to obtain a cellulose wet gel, and a drying step of drying the cellulose wet gel to obtain a cellulose porous body, wherein the gelling agent solution contains at least one gelling agent selected from the group consisting of an acid and a metal salt, and the concentration of the gelling agent is 0.001 mol / L or more and less than 0.1 mol / L.

2. 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 to obtain a cellulose wet gel, and a drying step of drying the cellulose wet gel to obtain a cellulose porous body, wherein the content of the cellulose nanofibers in the cellulose nanofiber solution is 0.1% by mass or more and less than 10% by mass based on the total mass of the cellulose nanofiber solution, 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 the gelling agent contains at least one selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, aluminum chloride, and calcium chloride.

4. 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.

5. 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.

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