JP6286909B2 - Method for producing porous sheet - Google Patents

Description

  The present invention relates to a method for producing a porous sheet containing fine fibers.

A porous sheet containing fine fibers has advantages such as high mechanical strength, and application to various uses is being studied (Patent Document 1).
Patent Document 2 discloses a method for producing a porous sheet containing fine fibers, adding an organic solvent to an aqueous dispersion containing fine cellulose fibers, and then making paper and drying.

JP 2008-24788 A JP 2012-1111849 A

However, in the method for producing a porous sheet described in Patent Document 2, since the organic solvent is used, the working environment may be impaired. Therefore, an exhaust device for improving the working environment may be installed, and an organic solvent recovery device may be installed to prevent the organic solvent from being released into the atmosphere. The installation of these devices has been a factor in increasing the cost of the porous sheet.
The subject of this invention is providing the manufacturing method of the porous sheet which can manufacture a porous sheet, without using an organic solvent.

In the method for producing a porous sheet of the present invention, a water-soluble dispersion containing fine fibers is mixed by dissolving or suspending a soluble substance that dissolves in a cleaning liquid composed of water, an inorganic acid aqueous solution, or an inorganic alkaline aqueous solution. A slurry preparing step of preparing a slurry, a sheet forming step of drying the mixed slurry to obtain a mixed sheet containing fine fibers and particles of the soluble substance, and cleaning the mixed sheet using the cleaning liquid, And a porosification step of drying to make it porous.
In the method for producing a porous sheet of the present invention, the soluble substance is preferably an inorganic salt.
When the soluble substance is an inorganic salt, the cleaning liquid is preferably water.

  According to the method for producing a porous sheet of the present invention, a porous sheet can be produced without using an organic solvent.

<Porous sheet>
The porous sheet obtained by the method for producing a porous sheet of the present invention is a sheet containing fine fibers.

  The fine fiber used in the present invention is a fiber obtained by refining a fiber raw material by chemical, biological, mechanical treatment, or a combination thereof.

  Although it does not specifically limit as a fiber raw material, For example, semi-synthetic fiber and regenerated fiber, such as an inorganic fiber, an organic fiber, a synthetic fiber, are mentioned. Examples of inorganic fibers include, but are not limited to, glass fibers, rock fibers, and metal fibers. Examples of organic fibers include, but are not limited to, fibers derived from natural products such as cellulose, carbon fibers, pulp, chitin, and chitosan. Examples of synthetic fibers include, but are not limited to, nylon, vinylon, vinylidene, polyester, polyolefin (for example, polyethylene, polypropylene, etc.), polyurethane, acrylic, polyvinyl chloride, aramid, and the like. Semi-synthetic fibers include but are not limited to acetate, triacetate, promix and the like. Examples of the regenerated fiber include, but are not limited to, rayon, cupra, polynosic rayon, lyocell, and tencel.

  The fiber raw material used in the present invention is not particularly limited, but preferably contains a hydroxyl group or an amino group because introduction of substituents described later is facilitated.

  Although it does not specifically limit as a fiber raw material, It is preferable to use a pulp from the point of being easy to acquire and cheap. The pulp is selected from wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP). Chemical pulp, semi-chemical pulp (SCP), semi-chemical pulp such as Chemi-groundwood pulp (CGP), mechanical pulp such as groundwood pulp (GP), thermomechanical pulp (TMP, BCTMP), etc. It is not limited. Non-wood pulp includes cotton pulp such as cotton linter and cotton lint, non-wood pulp such as hemp, straw and bagasse, cellulose isolated from sea squirts and seaweed, chitin, chitosan, etc. Not. Examples of the deinking pulp include, but are not limited to, deinking pulp made from waste paper. The pulp of the present invention may be used alone or in combination of two or more. Among the above pulps, wood pulp containing cellulose and deinked pulp are preferable in terms of availability. Among wood pulps, chemical pulp has a large cellulose ratio, so the yield of fine cellulose fibers during fiber refinement (defibration) is high, and the degradation of cellulose in the pulp is small and long fibers with a large axial ratio. Although it is especially preferable at the point from which a fine cellulose fiber is obtained, it is not specifically limited. Of these, kraft pulp and sulfite pulp are most preferably selected, but are not particularly limited. A sheet containing fine cellulose fibers of long fibers having a large axial ratio has high strength.

(Fine cellulose fiber)
The fine cellulose fibers are cellulose fibers or rod-like particles having a type I crystal structure that is much finer and shorter than pulp fibers usually used in papermaking applications.
The degree of crystallinity of fine cellulose fibers determined by the X-ray diffraction method is preferably 60% or more, more preferably 65% or more, and further preferably 70% or more. If the degree of crystallinity is not less than the lower limit, further excellent performance can be expected in terms of heat resistance and low linear thermal expansion.
The degree of crystallinity can be determined by measuring the X-ray diffraction profile and using the pattern by a conventional method.

  The fine cellulose fiber is cellulose having an average fiber width of 2 to 1000 nm determined by observation with an electron microscope. The average fiber width of the fine cellulose fibers is preferably 2 to 100 nm, more preferably 2 to 50 nm, further preferably 2 to 30 nm, and particularly preferably 2 to 15 nm. When the average fiber width of the fine cellulose fibers exceeds the above upper limit, the characteristics as the fine cellulose fibers (high strength, high rigidity, high dimensional stability, high dispersibility when combined with resin, transparency) are obtained. Becomes difficult. If the average fiber width of the fine cellulose fibers is less than the lower limit value, the cellulose molecules are dissolved in the dispersion medium, so that characteristics (high strength, high rigidity, high dimensional stability) as fine cellulose fibers can be obtained. It becomes difficult.

Measurement of the average fiber width by observation with an electron microscope of fine cellulose fibers is performed as follows. A slurry containing fine cellulose fibers is prepared, and the slurry is cast on a carbon film-coated grid that has been subjected to a hydrophilization treatment to obtain a transmission electron microscope (TEM) observation sample. When wide fibers are included, an operation electron microscope (SEM) image of the surface cast on glass may be observed. Observation by an electron microscope image is performed at any magnification of 1000 times, 5000 times, 10000 times, 20000 times, 50000 times or 100,000 times depending on the width of the constituent fibers. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions.
(1) One straight line X is drawn at an arbitrary location in the observation image, and 20 or more fibers intersect the straight line X.
(2) A straight line Y perpendicularly intersecting the straight line X is drawn in the same image, and 20 or more fibers intersect the straight line Y.
For the electron microscope observation image as described above, the width (minor axis of the fiber) of at least 20 fibers (that is, at least 40 in total) is read for each of the fibers intersecting with the straight line X and the fibers intersecting with the straight line Y. . In this way, at least three or more sets of electron microscope images as described above are observed, and the fiber width of at least 40 × 3 sets (that is, at least 120 sets) is read. The fiber widths thus read are averaged to obtain the average fiber width. This average fiber width is equal to the number average fiber diameter.

The fine cellulose fiber may have an anionic group or a cationic group. Examples of the anionic group include a carboxylic acid group, a sulfonic acid group, and a phosphoric acid group. Examples of the cationic group include a group having an onium such as ammonium, phosphonium, and sulfonium. When the fine cellulose fiber has an anion group or a cation group, the content is preferably 0.1 to 2.0 mmol / g, and more preferably 0.1 to 1.5 mmol / g.
If the content of the cationic group or the anionic group is within the above range, the hydration property of the fine cellulose fiber will not be too high, and the viscosity when slurried will be low. When the content of the anionic group or the cationic group exceeds the upper limit, the hydration property becomes too high, and the fine cellulose fiber may be dissolved.
Note that cellulose has a small amount (specifically, less than 0.1 mmol / g) of carboxy groups even without a treatment for introducing carboxy groups.

The content of the anionic group can be quantified according to the method of “Test Method T237 cm-08 (2008): Carboxyl Content of pull” of TAPPI in the United States.
The content of the cationic group can be quantified by measuring the amount of a specific element contained in the fine cellulose fiber. For example, in the case of a cationic group having an ammonium salt structure, the amount of nitrogen can be quantified by measuring with a nitrogen measuring device.

The fine cellulose fiber is obtained by defibrating a fiber raw material (for example, various pulps) containing cellulose to make it fine. From the viewpoint of easily refining cellulose, it is preferable to subject the cellulose to an anion treatment, a cationization treatment or an enzyme treatment before the defibrating treatment.
The anionization treatment is a treatment for introducing an anionic group into cellulose by adding an anionic agent. Examples of the anionizing agent include a carboxylic acid having a plurality of carboxyl groups or an anhydride thereof, or a salt thereof, an oxo acid containing a phosphorus atom or a salt thereof, ozone, 2,2,6,6-tetramethylpiperidinooxy A radical etc. are mentioned.
The cationization treatment is a treatment for introducing a cation group into cellulose by adding a cationizing agent. Examples of the cationizing agent include glycidyltrialkylammonium halides or halohydrin type compounds thereof.
The enzyme treatment is a treatment for decomposing cellulose with cellulase.
The defibrating equipment used in the defibrating process includes a high-speed defibrator, a grinder (stone mortar grinder), a high-pressure homogenizer and an ultra-high pressure homogenizer, a high-pressure collision grinder, a ball mill, a bead mill, a disk refiner, a conical refiner, A wet milling apparatus such as a biaxial kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, a beater, or the like can be used as appropriate.

(Porosity)
The porosity of the porous sheet is preferably 10% by volume or more, and more preferably 20 to 80% by volume. When the porosity of the porous sheet is small, it may be difficult for the resin to enter when the porous sheet contains a resin to obtain a composite material. On the other hand, when the porosity of the porous sheet is high, in the composite material containing the resin, a sufficient reinforcing effect by the fine cellulose fibers cannot be obtained, and the linear thermal expansion coefficient tends to increase.

The porosity here refers to the volume ratio of the voids in the porous sheet, and the porosity can be determined from the area, thickness, and mass of the porous sheet according to the following formula.
Porosity (volume%) = {(1-B / (M × A × t)} × 100
Here, A is the area (cm ² ) of the porous sheet, t (cm) is the thickness, B is the mass (g) of the porous sheet, and M is the density of cellulose. In the present invention, it is assumed that M = 1.5 g / cm ³ . The film thickness of the porous sheet is measured at 10 points at various positions of the porous sheet using a film thickness meter (for example, PDN-20 manufactured by PEACOK), and the average value is adopted.
The porosity can be adjusted by, for example, the amount of the soluble substance added in the porous sheet manufacturing method described later. That is, the porosity increases as the amount of the soluble substance added increases.

  Moreover, when calculating | requiring the porosity of a porous sheet, the porosity can also be calculated | required by carrying out image analysis of spectral analysis and SEM observation of the cross section of a composite material.

(Air permeability)
The air permeability of the porous sheet is preferably 70000 seconds or less, more preferably 500 to 60000 seconds, and still more preferably 1000 to 50000 seconds. The air permeability is as follows: Oken Air Permeability: JAPAN TAPPI Paper Pulp Test Method No. It is the value measured according to 5-2: 2000 (Oken type).
The air permeability can be adjusted by, for example, the amount of the soluble substance added in the method for producing a porous sheet described later. That is, the greater the amount of soluble material added, the lower the air permeability.

<Method for producing porous sheet>
The method for producing a porous sheet of the present invention includes a slurry preparation step, a sheet forming step, and a porous step.

(Slurry preparation process)
The slurry preparation step is a step of preparing a mixed slurry by dissolving or suspending a soluble substance in an aqueous dispersion containing fine fibers.

An aqueous dispersion containing fine fibers is a dispersion in which fine fibers are suspended in water. The aqueous dispersion may contain a small amount of a water-soluble organic solvent such as alcohol or ketone, preferably 30% by mass or less.
The concentration of the fine fibers in the aqueous dispersion is preferably 0.05 to 20% by mass, and more preferably 0.1 to 10% by mass. If the concentration of fine fibers in the aqueous dispersion is equal to or higher than the lower limit, the production efficiency of the porous sheet is excellent, and if the concentration is lower than the upper limit, the dispersion stability of the aqueous dispersion is excellent.

The soluble substance in the present invention is soluble in the cleaning liquid. Specifically, the soluble substance is at least one of a soluble compound that dissolves in the cleaning liquid and a soluble metal that dissolves in the cleaning liquid. Here, it is sufficient that at least a part of the soluble substance is dissolved in the cleaning liquid, and specifically, 1 g or more may be dissolved in 100 g of the cleaning liquid.
Examples of the soluble compound include inorganic salts, inorganic oxides, inorganic hydroxides, and the like, and inorganic salts are preferable in terms of high solubility.
Inorganic salts include sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium sulfate, sodium hydrogen sulfate, potassium sulfate, potassium hydrogen sulfate, magnesium sulfate, sodium nitrate, potassium nitrate, sodium phosphate, potassium phosphate, ammonium phosphate Sodium carbonate, sodium hydrogen carbonate, ammonium carbonate and the like.
The said inorganic salt melt | dissolves in all of water (however, an inorganic acid and an inorganic alkali are not included), inorganic acid aqueous solution, and inorganic alkali aqueous solution.
Examples of the soluble metal include aluminum, zinc, iron and copper. Aluminum and zinc are soluble in inorganic acids and also in inorganic alkalis. Iron and copper are soluble in inorganic acids.

  The preferable addition amount of the soluble substance varies depending on the type. For example, when the soluble substance is sodium chloride, the preferable addition amount is preferably 1 to 10000 parts by mass, more preferably 10 to 3000 parts by mass with respect to 100 parts by mass of the fine fibers. If the amount of sodium chloride added is not less than the lower limit, a porous sheet having a sufficiently high porosity can be produced, and if it is not more than the upper limit, it can be easily dissolved and removed in the porosification step.

The cleaning liquid in the present invention is composed of water (however, it does not include inorganic acid and inorganic alkali), inorganic acid aqueous solution and inorganic alkaline aqueous solution. Examples of inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and the like. Two or more inorganic acids may be used in combination. Examples of the inorganic alkali include sodium hydroxide and potassium hydroxide. Two or more inorganic alkalis may be used in combination.
When the soluble substance is an inorganic salt, the cleaning liquid is preferably water. If the cleaning liquid is water, it becomes easy to treat the waste liquid after cleaning, and the cost is low.

(Sheet making process)
The sheet forming step is a step of drying the mixed slurry to obtain a mixed sheet containing fine fibers and particles of the soluble substance.
In the sheet forming step, the mixed slurry is preferably formed into a sheet or a liquid film before drying. Examples of the method for forming the mixed slurry into a sheet form include a papermaking method, and examples of the method for forming a liquid film include a coating method.

In the paper making method, a paper machine used in normal paper production can be used. Examples of the paper machine include continuous paper machines such as a long net type, a circular net type, and an inclined type.
Examples of the filter medium used for papermaking include a porous membrane or a woven fabric made of a polymer such as polytetrafluoroethylene, polyethylene, or polypropylene. The pore diameter of the filter medium may be selected as long as fine fibers do not pass through, and is specifically selected in the range of 0.1 to 20 μm.
After paper making, water may be squeezed using a roll press.

In the coating method, the mixed slurry is applied onto the process substrate.
As the process substrate, a sheet, a plate, or a cylindrical body can be used. As the material for the process substrate, resin or metal is used, and resin is preferable in that a mixed sheet can be easily manufactured. In addition, the surface of the process substrate may be hydrophobic or hydrophilic.
Examples of the resin include polytetrafluoroethylene, polyethylene, polypropylene, polyethylene terephthalate, polyvinyl chloride, polyvinylidene chloride, polystyrene, and acrylic resin.
Examples of the metal include aluminum, stainless steel, zinc, iron, and brass.

  As a coating machine for coating the mixed slurry, for example, a roll coater, a gravure coater, a die coater, a curtain coater, an air doctor coater, etc. can be used, and the thickness can be easily made uniform. A coater and a spray coater are preferable, and a die coater is more preferable.

As a drying method, a method of bringing a sheet into contact with a heated cylinder dryer, a method of bringing a sheet into contact with a heated Yankee dryer, a method of drying a sheet with hot air, a method of irradiating the sheet with infrared rays, and a method of irradiating the sheet with microwaves And a method of vacuum drying the sheet.
When heating at the time of drying, it is preferable that heating temperature shall be 40-150 degreeC. If the heating temperature is at least the lower limit, drying can be performed quickly. If the heating temperature is at most the upper limit, the cost required for heating and the discoloration due to the heat of cellulose can be suppressed.

By the drying, the soluble substance contained in the mixed slurry is precipitated and formed into particles. The average particle diameter of the soluble substance particles formed by drying is preferably 0.001 to 100 μm, and more preferably 0.005 to 50 μm. The average particle diameter of the soluble substance can be determined from the observation result of an electron microscope. The average particle diameter obtained from the observation result of the electron microscope is based on the number.
The average particle size of the soluble substance is adjusted by, for example, the type of the soluble substance, the precipitation rate, the presence or absence of a nucleating agent, and the precipitation method (such as precipitating particles with a controlled particle diameter using a uniform precipitation method). Can do.

(Porosification process)
The porous step is a step of obtaining a porous sheet by washing the mixed sheet with the washing liquid and drying to make it porous. In the porosification step, particles of the soluble substance contained in the mixed sheet are dissolved to form pores.

Examples of the method for cleaning the mixed sheet include a method of immersing the mixed sheet in a cleaning liquid placed in a container (immersion method) and a method of spraying the cleaning liquid onto the mixed sheet (spraying method). The dipping method is preferable in that the apparatus configuration for cleaning becomes simple.
In the immersing method, it is preferable to repeatedly immerse the mixed sheet in a new cleaning solution in order to sufficiently wash. In the spraying method, it is preferable to spray the cleaning liquid on both surfaces of the mixed sheet. In any of the dipping method and the spraying method, the mixed sheet may be moved during cleaning.
As a drying method after washing, a method similar to the drying method in the sheet forming step can be applied.

(Function and effect)
It is difficult to obtain a porous sheet containing fine fibers by directly drying an aqueous dispersion containing fine fibers. This is because the fibers are aggregated with drying and a porous network structure cannot be formed. This is considered particularly remarkable when the affinity between fine fibers and water is high.
In the present invention, the soluble substance is precipitated as particles between the fine fibers with drying. Since the precipitated particles of the soluble substance suppress aggregation of fine fibers, a fine fiber network structure can be formed while using an aqueous dispersion.
Since the particles of the soluble substance existing between the fine fibers are dissolved in the cleaning liquid, they can be removed by subsequent cleaning with the cleaning liquid.
After washing, it must be dried again, but once formed fine fiber network structure is maintained after drying or does not disappear completely. Remains as a hole.
Therefore, in the method for producing a porous sheet, a porous sheet can be produced without using an organic solvent. Since no organic solvent is used, the working environment for producing the porous sheet can be easily improved.

(Production Example 1)
Reagent A was obtained by dissolving 26.4 g of sodium dihydrogen phosphate dihydrate and 19.7 g of disodium hydrogen phosphate in 53.9 g of water. Further, 44.3 g of urea was dissolved in 55.7 g of water to obtain reagent B.
The dried sheet of bleached softwood bleached kraft pulp was treated with a cutter mill and a pin mill to obtain cotton-like fibers. 10 g of this cotton-like fiber was sampled in an absolutely dry mass, and a chemical solution obtained by mixing 10.5 g of reagent A and 11.3 g of reagent B was sprayed uniformly on the collected cotton-like fiber using a spray bottle. Thereafter, cotton-like fibers sprayed with a chemical solution were rubbed by hand to obtain a chemical solution-impregnated pulp.
The obtained chemical solution-impregnated pulp was dried and heated at 130 ° C. for 1 hour using a blower with a damper to obtain phosphorylated pulp.
300 ml of ion-exchanged water was poured into the resulting phosphorylated pulp, and the mixture was stirred and dispersed uniformly, followed by filtration and dehydration. Next, 300 ml of ion exchange water was poured again, and the mixture was stirred and dispersed uniformly, followed by filtration and dehydration to obtain a dehydrated sheet. The obtained dehydrated sheet was diluted with 300 ml of ion-exchanged water, and a 1N sodium hydroxide aqueous solution was added little by little while stirring to obtain a pulp slurry having a pH of 12 to 13. Next, the pulp slurry was dehydrated to obtain a dehydrated sheet, and then 300 ml of ion-exchanged water was poured, and the mixture was stirred and dispersed uniformly, followed by filtration and dehydration to obtain a dehydrated sheet. Thereafter, 300 ml of ion exchange water was poured again, and the mixture was stirred and dispersed uniformly, followed by filtration and dehydration to obtain a dehydrated sheet of phosphorylated pulp.
After adding to the ion-exchange water to the obtained dehydration sheet | seat of the phosphorylated pulp, it stirred and obtained the slurry whose pulp density | concentration is 0.5 mass%. The slurry was defibrated for 30 minutes under a condition of 21,500 rotations / minute using a defibrating apparatus (Cleamix-2.2S, manufactured by M Technique Co., Ltd.) to obtain a defibrated pulp slurry.
Next, ion-exchanged water is added to the defibrated pulp slurry to adjust the slurry solid content concentration to 0.2% by mass, and using a cooling high-speed centrifuge (Kokusan Co., Ltd., H-2000B) under the condition of 12000G × 10 minutes. Centrifugation was performed, and the resulting supernatant was recovered. The supernatant was used as a fine cellulose fiber aqueous dispersion.

Example 1
1.6 g of sodium chloride was dissolved in 52 g of water to obtain an aqueous sodium chloride solution.
The fine cellulose fiber aqueous dispersion obtained in Production Example 1 was diluted with water so that the cellulose concentration became 0.15% by mass, and 105 g of the diluted slurry was fractionated. While stirring the separated slurry using a magnetic stirrer, an aqueous sodium chloride solution was added little by little using a peristaltic pump. The slurry to which sodium chloride was added in this manner was subjected to reduced pressure filtration on a filter holder KG-90 manufactured by Advantech, using a polypolytetrafluoroethylene membrane filter having a pore diameter of 0.5 μm manufactured by Advantech, and a wet paper was obtained. . The wet paper was dried using a cylinder dryer heated to 120 ° C. to obtain a sheet.
The obtained sheet was placed in a petri dish poured with 100 ml of water and allowed to stand for 10 minutes. After standing, the sheet was taken out from the petri dish, the water in the petri dish was replaced, and then the sheet was put into the petri dish again. This operation was repeated twice to wash the sheet.
The washed sheet was dried using a cylinder dryer heated to 120 ° C. to obtain a fine cellulose fiber sheet.

(Comparative Example 1)
The fine cellulose fiber aqueous dispersion obtained in Production Example 1 was diluted with water so that the cellulose concentration became 0.15% by mass, and 105 g of the diluted slurry was fractionated. The separated slurry was filtered under reduced pressure on a filter holder KG-90 manufactured by Advantech, using a polytetrafluoroethylene membrane filter having a pore diameter of 0.5 μm manufactured by Advantech, to obtain a wet paper. This wet paper was dried using a cylinder dryer heated to 120 ° C. to obtain a fine cellulose fiber sheet.

(Comparative Example 2)
The fine cellulose fiber aqueous dispersion obtained in Production Example 1 was diluted with water so that the cellulose concentration became 0.15% by mass, and 105 g of the diluted slurry was fractionated. The separated slurry was filtered under reduced pressure on a filter holder KG-90 manufactured by Advantech, using a polytetrafluoroethylene membrane filter having a pore diameter of 0.5 μm manufactured by Advantech, to obtain a wet paper. To this wet paper, 3.8 ml of ethylene glycol monotertiary butyl ether (organic solvent) was poured and further filtered under reduced pressure. Subsequently, it dried using the cylinder dryer heated at 120 degreeC, and obtained the fine cellulose fiber sheet.

<Evaluation>
About the fine cellulose fiber sheet obtained by each Example and each comparative example, the air permeability, the total light transmittance after liquid paraffin impregnation, and haze were measured with the following method. The measurement results are shown in Table 1.

[Air permeability]
The air permeability of the fine cellulose fiber sheet was measured using the JAPAN TAPPI paper pulp test method No. It was measured according to 5-2: 2000 (Oken type). The smaller the value of the air permeability, the higher the air permeability. Further, the higher the air permeability, the higher the resin impregnation property.

[Total light transmittance and haze after liquid paraffin impregnation]
After impregnating the obtained fine cellulose fiber sheet with liquid paraffin under reduced pressure, the total light transmittance and haze were measured according to JIS K7136 using a haze meter HM-150 manufactured by Murakami Color Research Laboratory. . The higher the total light transmittance and the lower the haze, the higher the resin impregnation property.
In Comparative Example 1, since the sheet was not impregnated with liquid paraffin, the total light transmittance and haze were not measured.

  Although the fine cellulose fiber sheet obtained by the method of Example 1 was not manufactured using an organic solvent, air permeability was ensured. Moreover, the fine cellulose fiber sheet obtained by the method of Example 1 had the same impregnation property as the fine cellulose fiber sheet of Comparative Example 1 obtained using an organic solvent.

Claims (4)

A slurry preparation step of preparing a mixed slurry by dissolving or suspending a soluble substance that dissolves in a cleaning liquid consisting of water, an inorganic acid aqueous solution, or an inorganic alkaline aqueous solution in an aqueous dispersion containing fine cellulose fibers;
Sheeting step of drying the mixed slurry to obtain a mixed sheet containing fine cellulose fibers and particles of the soluble substance;
A porous sheet manufacturing method comprising: cleaning the mixed sheet with the cleaning liquid, and drying to make the porous sheet porous.   The method for producing a porous sheet according to claim 1, wherein the soluble substance is an inorganic salt.   The method for producing a porous sheet according to claim 2, wherein the cleaning liquid is water. The method for producing a porous sheet according to any one of claims 1 to 3 , wherein an average fiber width obtained by observing the fine cellulose fiber with an electron microscope is 2 to 1000 nm.