JP6953156B2 - Silica sheet - Google Patents

Description

The present invention relates to a silica sheet.

Silica (silicon dioxide, SiO ₂ ) is the main component of sand, and it is said that about 60% of the crust is composed of minerals containing silica. As described above, silica is abundant in nature and relatively easy to manufacture, so that it can be manufactured at low cost. Further, silica has a property that it easily adsorbs various substances due to its porous nature and a large surface area, and easily releases the adsorbed substances depending on the surrounding environment. Therefore, silica has high utility value as an industrial material.
For example, silica has the properties of absorbing water and releasing adsorbed water, and is expected to be used as a water absorbing material.

On the other hand, silica is usually a powder, and has a drawback that it is easily scattered and difficult to handle, especially when it is used in a large amount. Therefore, attempts have been made to form silica into a specific shape by using a substance that functions as a binder in combination.
For example, as a sheet containing silica (silica sheet), a sheet containing silica and cellulose fibers in a ratio of 1: 1000 to 1: 1 is disclosed (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2014-176510

However, conventional silica sheets such as those disclosed in Patent Document 1 have a problem that they are liable to cause a large warp in a water-absorbed state and are inferior in practicality.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silica sheet containing silica and suppressing warpage after water absorption.

In order to solve the above problems, the present invention contains silica and cellulose nanofibers, and the ratio of [silica content (parts by mass)] / [cellulose nanofibers content (parts by mass)] is larger than 1. A silica sheet of 12 or less is provided.

According to the present invention, there is provided a silica sheet containing silica and suppressing warpage after water absorption.

It is the image imaging data of the sheet obtained in Example 1 with an electron microscope. 3 is a graph showing the relationship between the silica / CNF mass ratio of the sheet and the water absorption capacity (times) in Examples 1 to 3 and Comparative Examples 1 to 3. It is the imaging data of the dried product (sheet) at the stage where the drying at the time of manufacturing a sheet is completed in Example 1. FIG. 6 is imaging data of a dried product (sheet) at the stage when drying at the time of sheet production is completed in Comparative Example 7. It is the imaging data of the dried product (powder) at the stage where the drying at the time of sheet production is completed in Comparative Example 8. It is the imaging data of the composition (I) and the composition (II) inside the transparent container in Test Example 1. It is data which shows the analysis result of FT-IR in Example 6.

<< Silica sheet and its manufacturing method >>
The silica sheet of the present invention contains silica and cellulose nanofibers (sometimes abbreviated as "CNF" in the present specification), and contains [silica content (parts by mass)] / [cellulose nanofibers]. Amount (parts by mass)] ratio (in this specification, it may be abbreviated as "silica / CNF mass ratio") is greater than 1 and 12 or less.
In this specification, the silica sheet may be simply abbreviated as "sheet".

The silica sheet of the present invention has the ability to adsorb and release various substances including water when the silica / CNF mass ratio is within such a specific range, and suppresses warpage after water absorption. Has been done.
In addition, the silica sheet is lightweight and has excellent handleability.

The silica sheet can be formed by using a sheet-forming composition containing a component for constituting the silica sheet and a solvent. More specifically, the silica sheet is a method in which, for example, the sheet-forming composition is deflated by filtration or the like, and the obtained sheet-like solid is dried; the sheet-forming composition is used as a silica sheet. It can be produced by a method of coating the surface of an object to be formed and drying the obtained sheet-shaped coated product.
The sheet-forming composition may be filtered by a known method, and suction filtration is preferable.
The coating method of the sheet-forming composition is not particularly limited, and various known coating methods of the liquid composition may be used.

The ratio of the contents of the components that do not vaporize at room temperature in the sheet-forming composition is usually the same as the ratio of the contents of the components in the silica sheet. In addition, in this specification, "normal temperature" means a temperature which is not particularly cooled or heated, that is, a normal temperature, and examples thereof include a temperature of 15 to 25 ° C.

The sheet-forming composition may be dried by a known method such as blast drying or heating blast drying. Conditions such as drying temperature and drying time may be appropriately set according to the drying method.
For example, in the case of heating and blowing drying, the drying temperature is preferably 80 to 120 ° C., and the drying time is preferably 10 minutes to 12 hours.

<Sheet forming composition>
Examples of the sheet-forming composition include silica, cellulose nanofibers, a solvent, and, if necessary, other components that do not fall under any of these.

[silica]
The silica is not particularly limited and may be known, but it is preferably amorphous silica and preferably particulate (fine particles).

Examples of the amorphous silica include synthetic amorphous silica such as dry silica and wet silica.
Dry silica can be obtained, for example, by a combustion method in which silicon tetrachloride is burned in an oxygen / hydrogen flame.
Wet silica can be obtained, for example, by a precipitation method or a gel method in which sodium silicate is neutralized with an inorganic acid, a sol-gel method in which alkoxysilane is hydrolyzed, or the like.
The amorphous silica is preferably wet silica.

The specific surface area of the amorphous silica is preferably 50 to 500 m ² / g, more preferably 100 to 400 m ² / g. When the specific surface area of the amorphous silica is at least the above lower limit value, the amount of the object to be adsorbed on the silica sheet becomes larger. Further, when the specific surface area of amorphous silica is not more than the above upper limit value, the effect of suppressing the uptake of non-purpose components other than silica, such as cellulose nanofibers, into the inside of silica becomes higher.
The specific surface area of amorphous silica can be measured by a known method such as a permeation method or a gas adsorption method using gas molecules such as nitrogen.

The average particle size of the amorphous silica is preferably 0.5 to 15 μm, more preferably 2 to 10 μm. When the average particle size is at least the lower limit value, the voids between the amorphous silica particles in the silica sheet are maintained at an appropriate width, and the adsorption amount and adsorption rate of the adsorption target object are further increased. Further, when the average particle size is equal to or less than the upper limit value, the degree of unevenness on the surface of the silica sheet is reduced.
In the present specification, the "average particle diameter" in the present specification was measured by a method using a Coulter counter, it means a median D ₅₀ of the cumulative volume distribution.

The oil absorption of amorphous silica is preferably 100 to 350 mL / 100 g, and more preferably 150 to 300 mL / 100 g. When the oil absorption amount is at least the lower limit value, the adsorption amount of the object to be adsorbed on the silica sheet is further increased. Further, when the oil absorption amount is not more than the upper limit value, the fluidity of the sheet-forming composition described later for producing the silica sheet is further improved, and the silica sheet can be produced more easily.

The pore volume of amorphous silica is preferably 0.5 mL / g or more. When the pore volume is at least the lower limit value, the amount of adsorption of the object to be adsorbed on the silica sheet is further increased. The upper limit of the pore volume is not particularly limited as long as the effect of the present invention is not impaired, and may be, for example, 2.0 mL / g.

The silica contained in the silica sheet and the sheet-forming composition may be only one type, may be two or more types, and when there are two or more types, the combination and ratio thereof can be arbitrarily adjusted.
In the present specification, for example, when silica is amorphous silica, its specific surface area, average particle size, oil absorption amount and pore volume, and one or more other physical properties other than these are mutually exclusive. If they are clearly different, the types of silica shall be different from each other.

[Cellulose Nanofiber (CNF)]
It is presumed that the cellulose nanofibers (CNF) function as a binder in the silica sheet and as a dispersant for silica in the sheet-forming composition.
In particular, unlike other oligomers or polymers, cellulose nanofibers can stably form a large number of networks of fine fibers, and are therefore presumed to be particularly excellent as binders and dispersants.

The cellulose nanofibers are not particularly limited and may be known.
More specifically, examples of cellulose nanofibers include cellulose or derivatives thereof, which are microfibrils or microfibril aggregates having a fiber width of 3 to 200 nm.

Cellulose nanofibers can be produced by a known production method. For example, production having a step of obtaining a cellulose nanofiber dispersion liquid by performing a defibration treatment on a cellulose nanofiber precursor in a dispersion medium such as water. Can be manufactured by the method.

The cellulose nanofiber precursor is cellulose that has not been defibrated, and is composed of, for example, an aggregate of microfibrils.
As the cellulose nanofiber precursor, for example, oxidized cellulose, that is, one in which a carboxy group is introduced into at least a part of the glucopyranose ring in the cellulose molecule by the oxidation treatment of cellulose may be used.

The cellulose raw material for obtaining the cellulose nanofiber precursor is not particularly limited as long as it contains cellulose.
Examples of the cellulose raw material include naturally occurring cellulose having a crystal structure of cellulose I, and more specifically, for example, various wood pulps, non-wood pulps, bacterial celluloses, waste paper pulps, cotton, baronia celluloses, and the like. Examples include squirrel cellulose and the like.
Various commercially available products such as cellulose powder and microcrystalline cellulose powder can also be used as a cellulose raw material.

The method for defibrating the cellulose nanofiber precursor is not particularly limited. As a specific method of defibration treatment, for example, an ultrasonic homogenizer, a low-pressure homogenizer, a high-pressure homogenizer, a counter-collision homogenizer, an ultra-high-pressure homogenizer, a ball mill, a planetary mill, a high-speed rotary mixer, a grinding machine, or the like was used. A mechanical defibration treatment method can be mentioned.
It can be confirmed that the processed product obtained by the defibration treatment is cellulose nanofibers, for example, by observing the fibers of the treated product using an atomic force microscope (AFM).

In the silica sheet and the sheet-forming composition, the ratio (silica / CNF mass ratio) of [silica content (parts by mass)] / [cellulose nanofiber content (parts by mass)] is greater than 1 and 12 or less. It is preferably 1.1 to 12, more preferably 1.3 to 11.5, further preferably 1.5 to 11, and 1.7 to 10.5. Is particularly preferable. When the silica / CNF mass ratio is at least the above lower limit value, the adsorption amount (for example, water absorption amount) of the object to be adsorbed on the silica sheet is increased, and the warp of the silica sheet after water absorption is suppressed. On the other hand, when the silica / CNF mass ratio is not more than the upper limit value, the hardness and strength of the silica sheet are improved, and the stability of the silica sheet is improved.

Among them, when the silica / CNF mass ratio is preferably 3 or more, more preferably 4 or more, further preferably 4.5 or more, and particularly preferably 5 or more, the effect of suppressing warpage of the silica sheet after water absorption is remarkable. It gets higher. In this case, the upper limit of the silica / CNF mass ratio may be any of the above values, and the silica / CNF mass ratio is preferably 12 or less, more preferably 11.5 or less, still more preferably 11 or less. , Especially preferably 10.5 or less. These lower limit value and upper limit value can be arbitrarily combined.

On the other hand, when the silica / CNF mass ratio is preferably 9 or less, more preferably 8 or less, still more preferably 7 or less, and particularly preferably 6 or less, the hardness and strength of the silica sheet are remarkably improved. In this case, the lower limit of the silica / CNF mass ratio may be any of the above values, and the silica / CNF mass ratio is preferably 1.1 or more, more preferably 1.3 or more, still more preferably. It is 1.5 or more, particularly preferably 1.7 or more. These lower limit value and upper limit value can be arbitrarily combined.

[solvent]
The solvent in the sheet-forming composition imparts fluidity to the sheet-forming composition, and dissolves or disperses components other than the solvent in the sheet-forming composition.
In the sheet-forming composition, when silica, cellulose nanofibers, or the other components are blended as a dispersion or a solution and no solvent is used other than the one used as the dispersion medium or solvent at that time. , The solvent means the dispersion medium used for the preparation of the dispersion or the solvent used for the preparation of the solution.
Further, when a solvent is separately blended in addition to the dispersion medium used for preparing the dispersion or the solvent used for preparing the solution, the solvent is the dispersion medium or solvent and this separately blended. Means both solvent and. When silica, cellulose nanofibers, and the other components are all blended as a solid substance, the solvent means a solvent blended separately from these.
Examples of the solvent include water, alcohol and the like.

The solvent contained in the sheet-forming composition may be only one kind, two or more kinds, and when two or more kinds, the combination and ratio thereof can be adjusted arbitrarily.

[Other ingredients]
The other components are not particularly limited as long as they do not impair the effects of the present invention, and may be, for example, an organic compound or an inorganic compound.
Preferred of the other components include, for example, binders other than cellulose nanofibers, antioxidants, preservatives and the like.

The other components contained in the silica sheet and the sheet-forming composition may be only one kind, two or more kinds, and when there are two or more kinds, the combination and ratio thereof can be adjusted arbitrarily.

(Binders other than cellulose nanofibers)
Examples of binders other than cellulose nanofibers include water-soluble polymers.
Examples of the water-soluble polymer include polyvinyl alcohol; modified polyvinyl alcohol such as cation-modified polyvinyl alcohol, anion-modified polyvinyl alcohol, and silanol-modified polyvinyl alcohol; oxidized starch; phosphoric acid esterified starch; locons; casein; carboxymethyl cellulose (CMC). ); Polyurethanes such as ester-based polyurethanes and carbonate-based polyurethanes can be mentioned.
The polyurethane may be, for example, cationic, anionic or nonionic.

The ratio of the content of the other component to the total content of the component other than the solvent in the sheet-forming composition (that is, the content of the other component in the silica sheet) is 10% by mass or less. It is preferably 8% by mass or less, more preferably 6% by mass or less, and may be, for example, 4% by mass or less, 2% by mass or less, 1% by mass or less, or the like.
The lower limit of the content ratio (content of the other component of the silica sheet) is not particularly limited and may be 0% by mass, but for example, if it is 0.1% by mass, the other The effect of using the above components can be obtained more remarkably.

In other words, the ratio of the total content of silica and cellulose nanofibers to the total content of components other than the solvent in the sheet-forming composition (that is, the total content of silica and cellulose nanofibers in the silica sheet) is 90. It is preferably 9% by mass or more, more preferably 92% by mass or more, particularly preferably 94% by mass or more, and for example, 96% by mass or more, 98% by mass or more, 99% by mass or more, etc. It may be.
The upper limit of the content ratio (total content of silica and cellulose nanofibers in the silica sheet) is not particularly limited and may be 100% by mass, but for example, 99.9% by mass. The effect of using the other components is more remarkable.

<Manufacturing method of sheet-forming composition>
The sheet-forming composition can be produced by blending silica, cellulose nanofibers, a solvent, and if necessary, the other ingredients, and stirring these blending components sufficiently. In the sheet-forming composition, undissolved components such as silica and cellulose nanofibers are preferably uniformly dispersed.

The blending order of each component is not particularly limited.
Further, when blending each component, all the components may be added and then mixed, or some components may be sequentially added and mixed, or all the components may be sequentially added and mixed. You may.

The temperature at the time of blending each component is not particularly limited and may be appropriately adjusted according to the type of the blended component and the like, but is usually preferably 15 to 30 ° C.
Stirring may be performed by a known method for preparing a dispersion, such as a method of rotating and mixing a stirrer or a stirring blade; a method of mixing using a mixer. The stirring time may be appropriately adjusted according to the total amount of the sheet-forming composition to be produced, the stirring method, and the like, and is preferably 10 minutes to 24 hours, for example.

Examples of the method for producing the sheet-forming composition include (i) a production method having a step of adding a solution or dispersion of cellulose nanofibers to a dispersion of silica, and (ii) a solution or dispersion of cellulose nanofibers. A production method having a step of adding a dispersion liquid of silica, a production method having a step of adding a solvent after adding solid silica to a solution or dispersion of (iii) cellulose nanofibers, and the like. Be done.

In the production methods (i) to (iii), the silica dispersion is preferably an aqueous dispersion of silica, and the cellulose nanofiber solution or dispersion is an aqueous dispersion of cellulose nanofibers. preferable.

When an aqueous dispersion of cellulose nanofibers is used, since this aqueous dispersion has a high viscosity, stronger stirring conditions are required when adding other components to the aqueous dispersion to obtain a sheet-forming composition. Therefore, when an aqueous dispersion of cellulose nanofibers is used, the production method (i) is preferable among the above production methods.

The thickness of the silica sheet may be appropriately selected depending on the intended use of the silica sheet, and is not particularly limited, but is preferably 0.5 to 3 mm, more preferably 0.8 to 1.5 mm, for example. Can be done.

<< How to use silica sheet >>
As described above, the silica sheet has the ability to adsorb and release various substances such as water.
The substance to be adsorbed and released from the silica sheet may be a single substance of only one kind, or may be a mixture of two or more kinds of substances.

When the silica sheet does not adsorb a substance (object to be adsorbed) at all, or when the amount of adsorbed substance is small, the silica sheet easily adsorbs a sufficient amount of this substance by bringing it into contact with the target substance. .. That is, the silica sheet can be used as an adsorbent for substances.
On the other hand, when a sufficient amount of the substance is adsorbed, the silica sheet easily releases the adsorbed substance by placing it in an environment where the amount of the adsorbable substance is small. That is, the silica sheet can also be used as a substance-releasing material. The silica sheet can release the adsorbed substance, for example, in both a gas and a liquid.

The substance to be adsorbed and released from the silica sheet is not particularly limited, but is preferably a liquid or a gas.
Preferred substances to be adsorbed and released include, for example, antioxidants listed as the other components; preservatives; water; potassium atom (K) -containing compounds, nitrogen atom (N) -containing compounds, and phosphorus atoms (P). ) Compounds for fertilizers such as contained compounds; pesticides; plant seeds; aromatic compounds (fragrances, etc.) and the like.

A silica sheet adsorbing a sufficient amount of a target substance can be used as various functional materials.
For example, a silica sheet adsorbing water, fertilizer compounds, pesticides or plant seeds is a plant cultivation substrate for directly holding and cultivating plants; mixed with the soil in which the plants are cultivated, into the soil. It can be used as a soil conditioner for supplying water, fertilizer compounds, pesticides or plant seeds.
A silica sheet adsorbing a fragrant compound can be used as an fragrance, for example.
The applications listed here are examples, and the applications of the silica sheet are not limited to these.

On the other hand, a silica sheet that does not adsorb substances at all or has a small amount of substances adsorbed can be used, for example, as a raw material for producing the functional material.
Further, a silica sheet that does not adsorb substances at all or has a small amount of adsorbed substances can also be used as various functional materials.
For example, since the silica sheet has excellent water absorption, it can be used as a water absorption sheet, a coaster for a cup, a sweat absorption pad, an insole for footwear, a toilet sheet for pets, a wallpaper, a bath mat, and the like.

For example, the amount of water absorbed by the silica sheet can be preferably 1.3 mass times or more, for example, 1.7 mass times or more, based on the mass of the silica sheet when it is not absorbing water (in other words, when it is dried). It can be any of the above and 2.0 mass times or more. On the other hand, the amount of water absorbed by the silica sheet can be preferably 3 times by mass or less with respect to the mass of the silica sheet when it does not absorb water. The water absorption amount of the silica sheet can be expressed by arbitrarily combining these lower limit values and upper limit values.

Even if the silica sheet is immersed in water, for example, it does not disperse or dissolve in water, and the sheet shape can be stably maintained.

As described above, the silica sheet has the ability to adsorb various substances, but is suitable for adsorbing or releasing water because it is excellent in water absorption, for example. In that case, after adsorbing water (after water absorption). ) Silica sheet is suppressed from warping.
Further, the silica sheet can suppress warpage even after adsorption of a component other than water, depending on the type or amount of the component.
As described above, the silica sheet has high shape stability and is therefore highly practical.

Since the silica sheet can repeatedly adsorb and release the target substance, it can be repeatedly used in the above-mentioned applications and is highly economical.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the examples shown below.

<< Manufacture of silica sheet >>
[Example 1]
Silica (“AY-8A2” manufactured by Tosoh Silica, D ₅₀ : 8 μm, oil absorption: 250 mL / 100 g, specific surface area: 350 m ² / g) (4.8 g) was added to ion-exchanged water (39.6 g). , The aqueous dispersion of silica was obtained by stirring at 250 rpm for 1 hour using a stirring blade.
Next, a cellulose nanofiber aqueous dispersion (“WMa-1002” manufactured by Sugino Machine Limited) (120 g) having a concentration of 2% by mass was added to the obtained aqueous dispersion of silica (120 g), and the mixture was cooled at room temperature (23 ° C.). By stirring at 250 rpm for 1 hour, a sheet-forming composition which is an aqueous dispersion was obtained.
Next, 41.1 g of the obtained sheet-forming composition was weighed, suction-filtered until no water came out, and the separated solid matter was kept in the sheet-like shape and maintained at 105 ° C. using an oven. A silica sheet (thickness 1.12 mm, mass 1.80 g) was obtained by drying in the oven for 1 hour.
Table 1 shows the blending amount (solid content amount) of the blending components other than the solvent (water) at the time of producing the sheet-forming composition.

[Examples 2 to 4, Comparative Examples 1 to 8]
A silica sheet or a comparative sheet is used in the same manner as in Example 1 except that one or both of the types and amounts of the compounding components at the time of producing the sheet-forming composition are as shown in Table 1. Manufactured.
In addition, in Table 1, the description of "-" in the column of the compounding component means that the component is not compounded.

As the polyvinyl alcohol (PVA), "Kuraray Poval PVA117" (solid) manufactured by Kuraray Co., Ltd. was dissolved in water to prepare a 10 mass aqueous solution.
As carboxymethyl cellulose (CMC), "Cerogen 7A" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. was used.

<< Evaluation of silica sheet >>
The following items were evaluated for the silica sheet or the comparison sheet obtained above. The results are shown in Table 2.

[Stability]
The stability of the dried sheet obtained by the above method was evaluated according to the following criteria from the viewpoint of hardness and strength.
(Evaluation criteria)
A: The sheet is sufficiently hard and has sufficient strength.
B: The sheet is hard and has strength that does not cause any problem in practical use.
C: The sheet is easily cracked by a small impact and its strength is insufficient.
D: The sheet is broken or otherwise damaged, or the sheet is not in the shape of a sheet and is in the form of powder.

[Water absorption capacity]
The sheet obtained above was placed horizontally in the opening of a transparent container having a capacity of 100 mL, and water was dropped onto the sheet drop by drop from above, and water leaked into the transparent container from the back surface of the sheet. The mass of the sheet at the time point (the mass of the sheet at the time of water leakage) was measured. Then, a numerical value (multiple) corresponding to the water absorption capacity of the sheet was obtained according to the following formula. The difference between the mass of the sheet at the time of water leakage and the mass of the sheet before water absorption is the amount of water absorbed by the sheet.
[Water absorption capacity of sheet (times)] = ([Mass of sheet at the time of water leakage (g)]-[Mass of sheet before water absorption (g)]) / [Mass of sheet before water absorption (g)]

[Warp after water absorption]
The sheet at the time of water absorption capacity evaluation was dried at 105 ° C. for 1 hour using an oven. Next, the presence or absence of warpage of the sheet is confirmed, and if there is warpage, the convex surface (convex surface) of both sides of the sheet is brought into contact with the horizontal plane and placed on the horizontal plane. Of the corners, the height (cm) from the horizontal plane on which the sheet was placed was the highest, and this height (cm) was measured and used as the warp value (cm).

As is clear from the above results, the sheets of Examples 1 to 4 have a silica / CNF mass ratio of 2.0 to 10.1, high stability, high water absorption capacity, and sufficient warpage after water absorption. It was suppressed. The sheet of Example 4 was used in combination with polyvinyl alcohol as a binder as another component, but the action of the cellulose nanofibers was not inhibited and had good properties.

The image data (magnification 1000 times) of the sheet obtained in Example 1 with an electron microscope is shown in FIG.
From FIG. 1, it was presumed that silica particles were stably held in this sheet by a minute network of cellulose nanofibers.

On the other hand, the sheet of Comparative Example 1 did not use silica (not a silica sheet) and had high stability, but had low water absorption capacity, and nevertheless, the warp after water absorption was extremely large.
Although the sheet of Comparative Example 2 used silica, the amount used was too small (silica / CNF mass ratio was too small), so that the result was similar to that of the sheet of Comparative Example 1.
Although the amount of silica used in the sheet of Comparative Example 3 was larger than that of the sheet of Comparative Example 1, it was still small (the silica / CNF mass ratio was still too small), so that it was similar to the sheets of Comparative Examples 1 and 2. The result was.

Contrary to Comparative Examples 1 to 3, the sheet of Comparative Example 4 had low stability because the amount of silica used was too large (the silica / CNF mass ratio was too large). Since the sheet of Comparative Example 4 was easily cracked by a small impact, the water absorption capacity and the warp after water absorption could not be evaluated.
In Comparative Example 5, since the amount of silica used was too large (silica / CNF mass ratio was too large) as compared with Comparative Example 4, when the suction filtration of the sheet-forming composition was completed, one of the sheets was used. The part was already cracked, and the water absorption capacity and the warp after water absorption could not be evaluated.
In Comparative Example 6, since the cellulose nanofibers were not used and no binder was used at all, the filtered product (silica) was not in the form of a sheet at the stage when the suction filtration of the aqueous dispersion of silica was completed, and the powder was powdered. It was a body. Therefore, the water absorption capacity and the warp after water absorption could not be evaluated.

In Comparative Example 7, since the cellulose nanofibers were not used and polyvinyl alcohol was used as the binder, the stability of the sheet was low. In Comparative Example 7, cracks were already observed in the separated sheet-like solid matter during the suction filtration of the sheet-forming composition, and at the stage when the suction filtration was completed, more than half of the sheets were cracked. It was closed, and the water absorption capacity and warpage after water absorption could not be evaluated.

In Comparative Example 8, since carboxymethyl cellulose was used as a binder instead of using cellulose nanofibers, cracks were already observed in the separated solid matter during suction filtration of the sheet-forming composition, and suction filtration was performed. At the stage when the above was completed, the filtered material was not in the form of a sheet but was in the form of powder. Therefore, the water absorption capacity and the warp after water absorption could not be evaluated.

In Examples 1 to 3 and Comparative Examples 1 to 3, as the silica / CNF mass ratio increases, the stability of the sheet decreases, the water absorption capacity of the sheet improves, and the effect of suppressing warpage of the sheet after water absorption improves. There was a tendency for the sheet to warp (the warpage of the sheet was reduced). That is, the silica / CNF mass ratio was clearly correlated with these evaluation results. In terms of sheet stability and sheet formation potential, Comparative Examples 4 to 6 showed the same tendency, and a similar correlation was observed.

A graph showing the relationship between the silica / CNF mass ratio of the sheet and the water absorption capacity (times) in Examples 1 to 3 and Comparative Examples 1 to 3 is shown in FIG.
In addition, the imaging data of the dried product (sheet or powder) at the stage when the drying at the time of sheet production is completed in Example 1 and Comparative Examples 7 to 8 are shown in FIGS. 3 to 5. FIG. 3 is the imaging data in Example 1, FIG. 4 is the imaging data in Comparative Example 7, and FIG. 5 is the imaging data in Comparative Example 8.

<< Consideration on the function of cellulose nanofibers >>
[Test Example 1]
A composition for forming a sheet, which is an aqueous dispersion containing silica and cellulose nanofibers in the same manner as in Example 1 (in this specification, it may be abbreviated as "composition (I)"). Was prepared.
Further, in the same manner as in the case of this sheet-forming composition (composition (I)) except that cellulose nanofibers were not used, a comparative composition (in the present specification, "composition (composition (composition)"). II) ”may be abbreviated). That is, the composition (II) contains silica in the same content as the composition (I) and does not contain cellulose nanofibers.
Next, the obtained composition (I) and composition (II) were separately placed in a transparent container and sealed, and then the precipitate was uniformly dispersed and stored at room temperature for 1 day.
Then, these transparent containers were turned upside down and allowed to stand, and the composition (I) and the composition (II) inside the transparent container were immediately visually observed. The imaging data of the composition (I) and the composition (II) acquired at this time are shown in FIG.

As is clear from FIG. 6, in the case of the composition (II), a considerable amount of silica remains adhered to the bottom of the inverted transparent container (the area surrounded by the line in FIG. 6) in a precipitated state. It was. That is, in the composition (II), silica was precipitated during the static storage, and the state of the dispersion liquid could not be maintained.
On the other hand, in the case of the composition (I), no precipitate adhered to the bottom of the inverted transparent container, and the state of the dispersion liquid was maintained. That is, the composition (I) did not precipitate and the state of the dispersion liquid was maintained from the time of static storage to the time of this visual observation.
From these results, it was confirmed that the cellulose nanofibers function as a dispersant for the coexisting silica in the aqueous dispersion such as the above-mentioned sheet-forming composition.

<< Confirmation of adsorption and release of substances on silica sheet >>
[Example 5]
Using the sheet obtained in Example 1, a saturated aqueous potassium chloride solution was suction-filtered at room temperature.
The sheet was then dried in the oven at 105 ° C. for 1 hour.
Then, the dried sheet was immersed in pure water at room temperature for 5 minutes. At this time, the entire sheet was immersed in pure water.
Next, the immersed sheet was taken out from the pure water, the pure water after the sheet was taken out was completely dried and removed, and the residue of the solid was recovered, and the solid was subjected to Fourier transform infrared spectroscopy (Fourier transform infrared spectroscopy). FT-IR) was performed.
As a result, it was possible to observe a peak that coincided with the peak indicated by potassium chloride, and it was confirmed that potassium chloride was present in the pure water. This indicated that during suction filtration of the saturated potassium chloride aqueous solution, potassium chloride was adsorbed on the sheet, and then the potassium chloride was released from the sheet into pure water.

[Example 6]
The sheet obtained in Example 1 was immersed in a saturated aqueous potassium chloride solution at room temperature for 5 minutes. At this time, the entire sheet was immersed in the saturated aqueous potassium chloride solution.
Then, the soaked sheet was taken out from the saturated aqueous potassium chloride solution and dried in an oven at 105 ° C. for 1 hour.
Then, the dried sheet was immersed in pure water at room temperature for 5 minutes. At this time, the entire sheet was immersed in pure water.
Next, the immersed sheet was taken out from the pure water, the pure water after the sheet was taken out was completely dried and removed, and the residue of the solid was recovered, and the solid was subjected to Fourier transform infrared spectroscopy (Fourier transform infrared spectroscopy). FT-IR) was performed.
As a result, as in the case of Example 5, a peak corresponding to the peak indicated by potassium chloride could be observed, and it was confirmed that potassium chloride was present in the pure water. The analysis result at this time is shown in FIG. 7 together with the analysis result of potassium chloride. This indicates that after the potassium chloride was adsorbed on the sheet when immersed in the saturated potassium chloride aqueous solution, the potassium chloride was released from the sheet into pure water as in the case of Example 5. rice field.

The present invention can be used as an adsorbent or a release material for various substances such as water.

Claims (2)

A silica sheet containing silica and cellulose nanofibers.
The ratio of [the content of silica (parts by weight)] / [amount of cellulose nanofibers (parts by weight)] is, Ri 2-8 der,
The silica is amorphous silica having an average particle size of 0.5 to 15 μm.
A silica sheet in which the silica sheet is for adsorbing or releasing a substance . The silica sheet according to claim 1, wherein the ratio is 3 to 5.4.

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