WO2016072230A1 - Waterproof cellulose sheet and method for manufacturing waterproof cellulose sheet - Google Patents

Abstract

A waterproof cellulose sheet that comprises cellulose nanofibers carrying functional groups represented by -COOA, wherein: when standardized by regarding the total ionic strength detected by time-of-flight secondary ion mass spectrometry as 1, the alkali metal ionic strength or alkaline earth metal ionic strength is 0.020 or smaller; in -COOA, A represents an alkali metal, an alkaline earth metal or a hydrogen atom; and 20% (inclusive) to 90% (exclusive) of the functional groups represented by -COOA are -COOH groups. Thus, a waterproof cellulose sheet having good waterproofness and high transparency is provided.

Description

Water-resistant cellulose sheet and method for producing water-resistant cellulose sheet

The present invention relates to a water-resistant cellulose sheet comprising cellulose nanofibers and a method for producing the same. Specifically, the present invention relates to a water-resistant cellulose sheet having good water resistance and transparency, and a method for producing the same.

Utilization of a cellulose sheet made of cellulose nanofibers as a substrate for printed electronics, water resistant sheets, agricultural sheets, food packaging films, gas barrier films, gas separation membranes, and the like has been studied.

For example, Patent Document 1 describes a gas separation membrane mainly composed of cellulose nanofibers having a carboxylic acid type group on the surface as a gas separation membrane for mainly separating hydrogen gas from a mixed gas.
In Patent Document 1, as a method for producing this gas separation membrane, a step of preparing a first cellulose nanofiber dispersion by dispersing cellulose nanofibers having carboxylate type groups in a solvent, By adding acid to the cellulose nanofiber dispersion, the carboxylate type group is replaced with the carboxylic acid type group, and the cellulose nanofiber gel is gelled, and the cellulose nanofiber gel is redispersed in the solvent. A process for producing a gas separation membrane having a step of preparing a second cellulose nanofiber dispersion and a step of forming the second cellulose nanofiber dispersion into a membrane is described.

Patent Document 2 discloses a non-woven fabric made of cellulose nanofibers having a number average fiber diameter of 500 nm or less as a water-resistant cellulose sheet made of cellulose nanofibers used for filter media, blood separation membranes, polishing tapes, and the like. A water-resistant cellulose sheet having a weight ratio of 1 to 99% by weight, a porosity of 50% or more, a tensile strength corresponding to a basis weight of 10 g / m ² is 6 N / 15 mm or more, and a dry / wet strength ratio of tensile strength is 50% or more. Are listed.

JP 2014-14791 A JP 2012-46843 A

Such a cellulose sheet made of cellulose nanofibers may be required to have excellent water resistance, low haze, and excellent transparency depending on the application. However, a cellulose sheet excellent in both water resistance and transparency has not been realized.

For example, since the cellulose sheet described in Patent Document 1 is formed into a sheet after gelling cellulose nanofibers by acid treatment, sufficient water resistance is obtained because the surface roughness is large and the surface area is large. I can't. Moreover, since the gelled material formed by the acid treatment remains in the cellulose sheet described in Patent Document 1, the gelled material diffuses light, and has high haze and low transparency.

Moreover, the cellulose sheet described in patent document 2 improves the water resistance of the nonwoven fabric which consists of a cellulose nanofiber using a crosslinking agent. Therefore, a cellulose sheet having a smooth surface and high transparency cannot be crosslinked to the inside, and the effect of improving water resistance is hardly obtained.
Furthermore, even if the transparent cellulose sheet is swollen to the inside in a swollen state, it is difficult to return the swollen cellulose sheet to the original transparent and smooth cellulose sheet.

An object of the present invention is to solve such problems of the prior art, and is a water-resistant cellulose sheet comprising cellulose nanofibers, which not only has excellent water resistance, but also has low haze and transparency. An object is to provide an excellent water-resistant cellulose sheet and a method for producing the same.

In order to solve this problem, the water-resistant cellulose sheet of the present invention is a water-resistant cellulose sheet comprising cellulose nanofibers having a functional group represented by -COOA,
Alkaline metal ion intensity or alkaline earth metal ion intensity when normalized with 1 as the total ion intensity detected by time-of-flight secondary ion mass spectrometry is 0.020 or less,
A in the functional group represented by —COOA represents any one of an alkali metal, an alkaline earth metal, and a hydrogen atom, and 20% or more and less than 90% of the functional group represented by —COOA is a —COOH group. A water-resistant cellulose sheet is provided.

In such a water-resistant cellulose sheet of the present invention, the surface roughness Ra is preferably 0.1 μm or less.
The thickness is preferably 5 to 100 μm.
Furthermore, it is preferable that A of the functional group represented by —COOA is H or Na.

The method for producing a water-resistant cellulose sheet of the present invention is a method for producing the water-resistant cellulose sheet of the present invention,
A process of oxidizing cellulose nanofibers,
A step of forming an oxidized cellulose nanofiber into a sheet, and
There is provided a method for producing a water-resistant cellulose sheet, comprising a step of treating a cellulose nanofiber formed into a sheet shape with an acidic aqueous solution.

In such a method for producing a water-resistant cellulose sheet of the present invention, the pH of the acidic aqueous solution is preferably 4 or less.
Furthermore, it is preferable to oxidize the cellulose nanofibers using an N-oxyl compound as a catalyst.

According to the present invention, it is possible to provide a water-resistant cellulose sheet having not only excellent water resistance but also low haze and excellent transparency, and a method for producing the same.

Hereinafter, the water-resistant cellulose sheet of the present invention and the method for producing the water-resistant cellulose sheet will be described in detail.
In the present specification, a numerical range expressed using “to” includes numerical values described before and after “to” as a lower limit value and an upper limit value.

The water-resistant cellulose sheet of the present invention is composed of cellulose nanofibers having a functional group represented by —COOA, and is normalized when the total ion intensity detected by time-of-flight secondary ion mass spectrometry is set to 1. In which the alkali metal ion strength or alkaline earth metal ion strength is 0.020 or less, and 20% or more and less than 90% of the functional groups represented by —COOA are —COOH groups. In the following description, the water-resistant cellulose sheet is also simply referred to as “cellulose sheet”.
In the functional group represented by —COOA, A represents an alkali metal, an alkaline earth metal, or a hydrogen atom (H).

The cellulose sheet of the present invention has an alkali metal ion strength or alkaline earth metal ion strength of 0.020 or less when normalized with the total ion strength detected by time-of-flight secondary ion mass spectrometry as 1. In addition, when 20% or more and less than 90% of the functional group represented by —COOA is a —COOH group, it exhibits excellent water resistance, low haze, and good transparency.
The reason why both the water resistance and the transparency are good is not clear in detail, but is estimated as follows.
That is, among the —COOA groups possessed by cellulose nanofibers, the —COOA group present in the vicinity of the surface has a higher proportion of —COOH groups than the —COOA groups present in the interior. As a result, it is considered that excellent water resistance was developed.
In addition, since the internal structure of the cellulose nanofiber is less hydrophobic than the structure near the surface, the formation of a gelled product or the like inside the sheet is suppressed during sheet formation, and as a result, excellent transparency is expressed. Conceivable.

In the present invention, “consisting of cellulose nanofibers having a functional group represented by —COOA” preferably means that the content of cellulose nanofibers having a functional group represented by —COOA in the cellulose sheet is 50 It shows that it is at least mass%. In the cellulose sheet of the present invention, the content of cellulose nanofibers having a functional group represented by —COOA is more preferably 70% by mass or more, and particularly preferably 90% by mass or more.

Cellulose nanofibers are cellulose microfibrils constituting the basic skeleton of plant cell walls or the like, or fibers constituting the same. Various known CNFs can be used in the cellulose sheet of the present invention. In the following description, the cellulose nanofiber is also referred to as “CNF”.
Examples of such CNF include plant-derived fibers contained in wood, bamboo, hemp, jute, kenaf, cotton, beet pulp, potato pulp, agricultural waste, cloth, paper and the like. These may be used alone or in combination of two or more.
Examples of the wood include sitka spruce, cedar, cypress, eucalyptus, and acacia.
Examples of the paper include deinked waste paper, corrugated waste paper, magazines, and copy paper.
As the pulp, for example, chemical pulp (kraft pulp (KP), sulfite pulp (SP)), semi-chemical pulp (SCP) obtained by pulping plant raw materials chemically or mechanically or using both in combination. , Semi-ground pulp (CGP), chemimechanical pulp (CMP), groundwood pulp (GP), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), and the like.

In CNF, it is preferable that lignin is removed as much as possible in terms of preventing aggregation.

CNF is usually an aggregate of unit fibers having a fiber diameter of about 4 nm. In the cellulose sheet of the present invention, the fiber diameter of CNF is not particularly limited, but the average fiber diameter is preferably 2 to 40 nm, and more preferably 3 to 25 nm.
By setting the average fiber diameter of CNF to 2 nm or more, it is preferable in terms of improving the strength of the cellulose sheet.
Moreover, it is preferable at points, such as transparency of a cellulose sheet, film forming property (reduction of an aggregate), by making the average fiber diameter of CNF into 40 nm or less.

In the cellulose sheet of the present invention, such CNF has a functional group represented by —COOA in which a part of primary hydroxyl groups of CNF are oxidized.
In the functional group represented by —COOA, A is an alkali metal (Li (lithium), Na (sodium), K (potassium), Rb (rubidium), Cs (cesium), Fr (francium))), alkaline earth metal 1 or more selected from (Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), Ra (radium)), and H (hydrogen atom).
As the alkali metal and alkaline earth metal, Na is preferably exemplified in terms of versatility, price, environmental load, and the like of an oxidizing agent used for the oxidation treatment of CNF described later.

Here, the cellulose sheet of the present invention is an alkali metal ion when normalized with the total ion intensity detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS) being set to 1. The strength or alkaline earth metal ion strength is 0.020 or less.
In the following description, time-of-flight secondary ion mass spectrometry is also referred to as “TOF-SIMS”. Further, the alkali metal ion intensity or alkaline earth metal ion intensity when the total ion intensity detected by TOF-SIMS is normalized to 1 is also referred to as “alkaline ion amount ratio”.

The cellulose sheet of the present invention is preferably produced by the production method of the present invention described in detail later.
In the production method of the present invention, the primary hydroxyl group exposed on the surface of CNF as described above is oxidized to form an alkali metal salt of —COOH group or —COOH group such as —COONa group or —COOCa group. Alkaline earth metal salt. In the following description, these groups are also referred to as “—COONa groups”.
Next, the oxidized CNF is defibrated using an aqueous solvent to prepare a CNF dispersion. Next, this dispersion is applied to a substrate and dried to produce a CNF sheet as a precursor, and then the precursor sheet is acid-treated with an acidic aqueous solution to produce the cellulose sheet of the present invention. By this acid treatment, the hydrophilic —COONa group is converted to a hydrophobic —COOH group, thereby imparting water resistance to the cellulose sheet.
As is well known, TOF-SIMS analyzes the extreme surface of a depth of about 1 nm. That is, an alkali ion content ratio of 0.020 or less indicates that the surface —COONa groups are almost —COOH groups.

When the alkali ion amount ratio exceeds 0.020, there arises a disadvantage that sufficient water resistance cannot be obtained.
That is, the lower the alkali ion amount ratio, the higher the water resistance. The alkali ion content ratio is preferably 0.01 or less, and more preferably 0.005 or less.

In the cellulose sheet of the present invention, CNF has a functional group represented by —COOA, and 20% or more and less than 90% of the functional group represented by —COOA is a —COOH group.
That is, it is considered that most of the —COONa groups are —COOH groups on the surface of the cellulose sheet of the present invention, and more —COONa groups are present in the interior than the surface.

In the present invention, less than 20% to 90% of the functional groups to be -COOH group represented by -COOA, obtained by infrared spectroscopy, a peak value of absorbance of 1733cm ^-1 ± 50cm ^-1 A It shows that the COOH ratio obtained from [COOH] and A [COO] which is the maximum absorbance of 1636 cm ⁻¹ ± 50 cm ⁻¹ is 20% or more and less than 90%.
Specifically, the COOH ratio is “COOH ratio [%] =
(A [COOH] / (A [COOH] + A [COO])) × 100 ”.
As an example, infrared spectroscopic analysis is performed by freeze-pulverizing a cellulose sheet, mixing the obtained powder and KBr powder, placing the sample in a mold and applying pressure, and measuring it. Good.

When the COOH ratio is less than 20%, there are problems such as insufficient water resistance.
When the COOH ratio is 90% or more, the heat resistance is lowered and inconveniences such as yellowing easily occur by heating. The phrase “yellowing is likely to occur due to heating” means that transparency is likely to decrease due to heating.

Here, if the amount of —COOH groups in the functional group represented by —COOA is large, it is disadvantageous in terms of transparency at high temperatures, such as yellowing when exposed to high temperatures. Considering this point, the COOH ratio is preferably 85% or less, more preferably 80% or less, and still more preferably 70% or less. In consideration of water resistance, the COOH ratio is preferably 30% or more, more preferably 40% or more.
Therefore, the COOH ratio is preferably 30 to 80%, more preferably 40 to 70%, because better heat resistance, that is, transparency and better water resistance can be obtained.

The surface roughness Ra of the cellulose sheet of the present invention is not particularly limited, and may be appropriately set according to the use of the cellulose sheet. Here, the surface roughness Ra of the cellulose sheet is preferably 0.1 μm or less, more preferably 0.05 μm or less, and particularly preferably 0.03 μm or less.
By making the surface roughness Ra of the cellulose sheet 0.1 μm or less, the haze can be reduced and the transparency can be improved. The smoothness is improved and the functional layer is applied on the surface of the cellulose sheet. This is preferable in that unevenness hardly occurs.
In addition, what is necessary is just to measure surface roughness Ra (arithmetic mean roughness Ra) based on JISB0601 (1994).

The thickness of the cellulose sheet of the present invention is not particularly limited, and may be appropriately set according to the mechanical strength required for the cellulose sheet, the use of the cellulose sheet, and the like. Here, the thickness of the cellulose sheet is preferably 5 to 100 μm, more preferably 20 to 80 μm, and particularly preferably 40 to 60 μm.
By setting the thickness of the cellulose sheet to 5 μm or more, it is possible to suitably form a film using a CNF dispersion, which will be described later. This is preferable in that generation of water can be suppressed.
By setting the thickness of the cellulose sheet to 100 μm or less, it is preferable in that the film forming time of the cellulose sheet can be shortened, the haze can be lowered to improve the transparency, and the material cost can be reduced.

Such a cellulose sheet of the present invention includes CNF having a functional group represented by —COOA, inevitable impurities such as moisture, a chemical added by oxidation of CNF or acid treatment of a precursor sheet, which will be described later, CNF You may contain the impurities (lignin, hemicellulose, etc.) contained in a raw material, surfactant, resin (latex, water-soluble resin, etc.), etc. which are added as needed.
However, in the cellulose sheet of the present invention, the content of CNF having a functional group represented by —COOA is preferably 50% by mass or more, more preferably 70% by mass or more, and 90% by mass or more. Particularly preferred is as described above.

Hereinafter, the manufacturing method of the cellulose sheet of this invention which manufactures such a cellulose sheet of this invention is demonstrated.
In the method for producing a cellulose sheet of the present invention, first, the CNF as described above is oxidized to oxidize the primary hydroxyl groups of CNF to form —COOH groups or —COONa groups. As described above, the —COONa group is an alkali metal salt of —COOH group or an alkaline earth metal salt of —COOH group.
The oxidation treatment of CNF may be performed by a known method. Therefore, it may be chemically oxidized or physically oxidized.

As a specific example of the oxidation treatment of CNF, an N-oxyl compound such as TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is used as a catalyst in a reaction solution in which CNF is dispersed in an aqueous solvent. And a method of oxidizing CNF by using an oxidizing agent.

The CNF used as a raw material can be variously used for the above-mentioned CNF. Examples of the aqueous solvent include a solvent that contains only water except unavoidable impurities, and a mixed solvent containing 20% by mass or less of alcohol having compatibility with water.
What is necessary is just to set suitably the solid content density | concentration (absolute dryness) of CNF in the reaction liquid of an acidic treatment according to the oxidizing agent and N-oxyl compound to be used. Specifically, in terms of oxidation reaction efficiency, it is preferably 10% by mass or less, more preferably 5% by mass or less, and particularly preferably 0.1 to 4% by mass.

As the N-oxyl compound, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPO derivatives having various functional groups at the C4 position can be used. Examples of the TEMPO derivative include 4-hydroxy TEMPO, 4-acetamido TEMPO, 4-carboxy TEMPO, 4-phosphonooxy TEMPO, and the like.
Further, 4-hydroxy TEMPO derivatives imparted with hydrophobicity by etherification of the hydroxyl group of 4-hydroxy TEMPO with alcohol or esterification with carboxylic acid or sulfonic acid are also preferably used.

A catalytic amount is sufficient for the addition amount of the N-oxyl compound. Specifically, the content of the N-oxyl compound in the reaction solution is preferably 0.01 to 10 parts by mass and more preferably 0.1 to 5 parts by mass with respect to 100 parts by mass of absolutely dry CNF.
The addition amount of the N-oxyl compound is preferably 0.01 parts by mass or more from the viewpoint of shortening the oxidation treatment time and reducing the storage facility. By adding the N-oxyl compound in an amount of 10 parts by mass or less, it is preferable in that the cost for the oxidation treatment can be reduced, and the occurrence of yellowing of CNF during heating, which is considered to be caused by a reaction byproduct, can be prevented. .

As the oxidizing agent, various substances that can promote the target oxidation reaction can be used.
Specifically, hypohalous acid or a salt thereof (such as hypochlorous acid or a salt thereof, hypobromite or a salt thereof, hypoiodous acid or a salt thereof), a halogenous acid or a salt thereof (chlorite or a salt thereof) Salt thereof, bromous acid or salt thereof, iodic acid or salt thereof, perhalogen acid or salt thereof (perchloric acid or salt thereof, periodic acid or salt thereof), halogen (chlorine, bromine, iodine, etc.) ), Halogen oxides (ClO, ClO ₂ , Cl ₂ O ₆ , BrO ₂ , Br ₃ O ₇ etc.), nitrogen oxides (NO, NO ₂ , N ₂ O ₃ etc.), peracids (hydrogen peroxide, excess Acetic acid, persulfuric acid, perbenzoic acid, etc.).

As hypohalite, in the case of hypochlorous acid, alkali metal salts such as lithium hypochlorite, sodium hypochlorite, sodium hypochlorite, calcium hypochlorite, hypochlorous acid, etc. Examples include magnesium, alkaline earth metal salts such as strontium hypochlorite, and ammonium hypochlorite. Moreover, hypobromite and hypoiodite corresponding to these can also be used.
Examples of halous acid salts include, in the case of chlorous acid, alkali metal salts such as lithium chlorite, potassium chlorite, and sodium chlorite, calcium chlorite, magnesium chlorite, and strontium chlorite. Examples thereof include alkaline earth metal salts and ammonium chlorite. In addition, bromite and iodate corresponding to these can also be used.
As perhalogenates, for example, in the case of perchlorates, alkali metal salts such as lithium perchlorate, potassium perchlorate, sodium perchlorate, calcium perchlorate, magnesium perchlorate, strontium perchlorate Examples thereof include alkaline earth metal salts such as ammonium perchlorate. Moreover, perbromate and periodate corresponding to these can also be used.

Of these, alkali metal hypohalous acid salts and alkali metal halous acid salts are preferably used, and among them, alkali metal hypochlorite and alkali metal chlorite salts are preferably used.

These oxidizing agents can be used alone or in combination of two or more. Further, it may be used in combination with an oxidase such as laccase.

The addition amount of the oxidant may be appropriately set according to the oxidant to be used, the CNF concentration in the reaction solution, the target introduction amount of —COOA group, and the like.
Specifically, 0.2 to 500 mmol is preferable and 0.2 to 50 mmol is more preferable per 1 g of absolutely dry CNF. By introducing the oxidizing agent in an amount of 0.2 to 500 mmol per 1 g of absolutely dry CNF, the efficiency of introducing the functional group represented by —COOA into CNF can be improved. Basically, the greater the amount of oxidizing agent used, the greater the amount of functional group introduced by -COOA into CNF.

Depending on the type of oxidizing agent used, a catalyst component in which a bromide or iodide is combined with an N-oxyl compound may be used.
Examples of bromides and iodides include ammonium salts (ammonium bromide and ammonium iodide), bromides and alkali metal iodides (bromides such as lithium bromide, potassium bromide and sodium bromide, lithium iodide and iodide). Potassium iodide, iodide such as sodium iodide), bromide or alkaline earth metal iodide (calcium bromide, magnesium bromide, strontium bromide, calcium iodide, magnesium iodide, strontium iodide, etc.) it can. These bromides and iodides can be used alone or in combination of two or more.
Specifically, for example, when an alkali metal hypochlorite is used as an oxidizing agent, it is preferable to use a catalyst component in which an N-oxyl compound and bromide or iodide are combined. When an alkali metal chlorite is used as the oxidizing agent, it is preferable to use an N-oxyl compound alone as a catalyst component.

What is necessary is just to set suitably the addition amount of a bromide and / or iodide in the range which can accelerate | stimulate an oxidation reaction.
Specifically, the amount is preferably 0.1 to 100 parts by mass, more preferably 1 to 60 parts by mass with respect to 100 parts by mass of absolutely dry CNF.
By making the addition amount of bromide and / or iodide 0.1 parts or more, the oxidation reaction can be advanced efficiently. In addition, by controlling the addition amount of bromide and / or iodide to 100 parts by mass or less, the cost for the oxidation treatment can be reduced, the occurrence of yellowing of CNF during heating, which is considered to be caused by a reaction byproduct, can be prevented, etc. This is preferable.

The oxidation treatment of CNF can proceed smoothly even under mild temperature conditions. Therefore, the oxidation treatment temperature of CNF may be appropriately set from an appropriate temperature. Specifically, 0 to 50 ° C. is preferable, and 10 to 30 ° C. (room temperature) is more preferable.
The oxidation treatment time may be appropriately set according to the content of CNF, the type and content of the oxidizing agent, and the like. Here, if the oxidation treatment time is too long, the introduction amount of the functional group represented by —COOA becomes excessive and the strength of CNF may be lowered. Therefore, the oxidation treatment is terminated without water-solubilizing CNF. Is preferred. Considering this point, the oxidation treatment is preferably 30 minutes to 4 hours, more preferably about 2 hours.

Here, in the oxidation treatment, as the reaction proceeds, a functional group represented by -COOA is generated in CNF, and the pH of the reaction solution is lowered.
Therefore, in order to allow the oxidation reaction to proceed efficiently, it is preferable to maintain the pH of the reaction solution at 9 to 12 during the oxidation treatment, and it is more preferable to maintain the pH of the reaction solution at 10 to 11. The pH of the reaction solution may be adjusted by a known method such as a method of adding an alkali such as an aqueous sodium hydroxide solution to the reaction system.

As described above, in the production method of the present invention, various known methods can be used for the oxidation treatment of CNF besides this.
As an example, International Publication No. 2009/107637, International Publication No. 2009/084566, International Publication No. 2010/116826, Japanese Unexamined Patent Publication No. 2009-228186, Japanese Unexamined Patent Publication No. 2009-173909, Japanese Unexamined Patent Publication No. 2010-209510. The method described in JP2009-243014A and the like is exemplified.

Further, as the oxidation treatment of CNF, a treatment for converting CNF into CMC (carboxymethyl cellulose) can also be used.

In addition to the above-mentioned various CNFs, CNF subjected to the above-described oxidation treatment can be used as the raw material CNF in converting CNF into CMC.

Conversion of CNF to CMC can be achieved by introducing a carboxyalkyl group into the hydroxyl group of CNF. Preferred carboxyalkyl groups include a carboxymethyl group, a carboxyethyl group, a carboxypropyl group, and the like.
CNF can be converted to CMC by reacting CNF (oxidized CNF) with a chloroacetate such as chloroacetic acid or sodium chloroacetate using an alkali such as sodium hydroxide as a catalyst. Here, the amount of carboxymethyl group introduced can be increased by increasing the amount of chloroacetic acid, the amount of chloroacetate and the reaction time, and the amount of the functional group represented by —COOA to CMC-converted CNF can be increased.

When the oxidation treatment of CNF is completed, CNF is defibrated.
By oxidation treatment, a —COONa group in which a primary hydroxyl group is oxidized is generated in CNF. As described above, the —COONa group is an alkali metal salt of —COOH group or an alkaline earth metal salt of —COOH group. Since the —COONa group is ionic, the CNF having this functional group is suitably defibrated, and each CNF is suitably stretched.

The defibration of the oxidized CNF may be performed using the oxidized reaction solution as it is. Alternatively, the oxidized CNF may be separated by filtration or centrifugation, washed with an aqueous solvent, etc., and then dispersed again in the aqueous solvent.
As the defibrating method, various known methods can be used. As an example, there is exemplified a method using various apparatuses commonly used for industrial and household purposes, such as a home mixer, an ultrasonic homogenizer, a high-pressure homogenizer, a twin-screw kneader, a roll mill, and a stone mill.

Next, the dispersion liquid obtained by defibrating CNF is applied (cast) to the substrate to form a coating film. For the application of the CNF dispersion, various known application methods can be used. In the following description, a dispersion obtained by defibrating CNF is also referred to as a “CNF dispersion”.
The CNF dispersion applied to the substrate is preferably adjusted so that the solid content concentration is 0.1 to 2% by mass as necessary.

Moreover, it is preferable to defoam the CNF dispersion before applying the CNF dispersion to the substrate. By defoaming the CNF dispersion, the surface roughness Ra of the cellulose sheet to be produced can be reduced, and a cellulose sheet having lower haze, better transparency, and better water resistance can be obtained.
As the defoaming treatment, various known methods for defoaming from an aqueous solution or a dispersion, such as a method using stirring, a method using stirring under reduced pressure, ultrasonic treatment, heat treatment, and the like can be used.

As the substrate, various types of film and plate-like materials such as a resin plate-like material such as polyethylene terephthalate (PET), a film-like material, a glass plate, and a metal plate can be used.
Further, the thickness of the coating film of the CNF dispersion may be appropriately set according to the thickness of the target cellulose sheet, the solid content concentration of CNF in the CNF dispersion, and the like.

By drying the coating film of the CNF dispersion liquid, a CNF sheet having a —COONa group, which is a precursor of the cellulose sheet of the present invention, is formed on the substrate. Drying may be performed by natural drying or a heating means such as an oven may be used. In the case of natural drying, the coating film can be dried usually in 2 to 3 days. Further, when heated in an oven at 100 ° C., the coating film can be dried usually in 2 to 3 hours.
In the following description, a CNF sheet having a —COONa group, which is a precursor of the cellulose sheet of the present invention, is also referred to as a “precursor sheet”.
In addition, the coating film of the CNF dispersion may be finished on the substrate, or the sheet is peeled off from the substrate with the aqueous solvent remaining before the coating film of the CNF dispersion is dried. Then, the undried sheet may be further dried to obtain a precursor sheet.

When a precursor sheet made of CNF having a —COONa group is produced in this manner, the precursor sheet is acid-treated with an acidic aqueous solution. By this acid treatment, the —COONa group of CNF on the surface of the precursor sheet is oxidized to form a hydrophobic —COOH group.
Thus, the haze is composed of CNF having a functional group represented by —COOA, the alkali ion content ratio is 0.020 or less, and 20% or more and less than 90% of the functional groups represented by —COOA are —COOH groups. The (water-resistant) cellulose sheet of the present invention having low and high transparency and good water resistance is produced. As described above, in —COOA, A is any one of an alkali metal, an alkaline earth metal, and a hydrogen atom.

In addition, the acid treatment of the precursor sheet is basically performed by peeling the precursor sheet from the substrate. However, depending on the use of the cellulose sheet and the like, the precursor sheet may be acid-treated while the precursor sheet is adhered to the substrate.

The acid used in the acidic aqueous solution for acid treatment may be an organic acid or an inorganic acid. Inorganic acids are preferable, and hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid and the like are more preferable. A plurality of these acids may be used in combination.
The acidic aqueous solution may be prepared by dissolving these acids in water, preferably pure water. Here, the acidic aqueous solution preferably has a pH of 4 or less, more preferably a pH of 3 or less, and particularly preferably a pH of 2 or less.
The precursor sheet is preferably acid-treated using an acidic aqueous solution having a pH of 4 or less, so that the —COONa group of CNF on the surface of the precursor sheet can be suitably converted to a —COOH group.

Various methods can be used for the acid treatment of the precursor sheet with an acidic aqueous solution. As an example, a method of immersing the precursor sheet in an acidic aqueous solution, a method of applying an acidic aqueous solution to the surface of the precursor sheet, a method of exposing the precursor sheet to the vapor of the acidic aqueous solution, and the like are exemplified.

What is necessary is just to set the temperature of the acidic aqueous solution which acid-treats a precursor sheet | seat suitably for the temperature which reaction fully advances according to the acid, pH, etc. which are contained in acidic aqueous solution.
Specifically, 0 to 60 ° C. is preferable, 0 to 55 ° C. is more preferable, and 0 to 50 ° C. is particularly preferable.
By setting the temperature of the acidic aqueous solution to be acid-treated to 0 ° C. or higher, it is preferable in that the CNF —COONa group on the precursor sheet surface can be suitably converted to —COOH group.
By setting the temperature of the acidic aqueous solution to 60 ° C. or lower, it is preferable in that an excessive —COOH group is prevented from being generated in CNF and a cellulose sheet having good heat resistance is obtained.

What is necessary is just to set suitably the time which can fully process the acid treatment time of the precursor sheet | seat by acidic aqueous solution according to the acid, pH, etc. which are contained in acidic aqueous solution.
Specifically, the acid treatment time is preferably 0.1 seconds or more, more preferably 1 second to 1 day, and particularly preferably 3 seconds to 30 minutes. By setting the acid treatment time of the precursor sheet with the acidic aqueous solution to 0.1 seconds or longer, it is preferable in that the —COONa group of CNF on the surface of the precursor sheet can be suitably converted to —COOH group.

When the acid treatment with the acidic aqueous solution is completed, the cellulose sheet is washed with a polar solvent such as water or alcohol to wash away the acidic aqueous solution. That is, the acid treatment time of the precursor sheet is the time from when the precursor sheet is brought into contact with the acidic aqueous solution until the start of cleaning.
Washing is preferably performed using alcohol or water, and more preferably using water.

The water-resistant cellulose sheet and the method for producing the water-resistant cellulose sheet of the present invention have been described in detail above. However, the present invention is not limited to the above-described examples, and various improvements can be made without departing from the gist of the present invention. Of course, you may make changes.

Hereinafter, the present invention will be described in more detail with reference to specific examples of the present invention.
[Example 1]
To 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 754 mg (7.4 mmol) of sodium bromide was dissolved, 5 g of CNF was added in an absolutely dry weight, and the mixture was stirred until the pulp was uniformly dispersed. As CNF, bleached unbeaten sulfite pulp (manufactured by Nippon Paper Chemical Co., Ltd.) derived from conifers was used.
After adding 18 ml of aqueous sodium hypochlorite solution (effective chlorine 5%) to the reaction solution, the reaction solution was adjusted to pH 10.3 with 0.5N aqueous hydrochloric acid solution, and oxidation treatment was started. During the reaction, the pH of the reaction solution was measured, and the reaction solution was adjusted to pH 10 by appropriately adding a 0.5N sodium hydroxide solution.
After performing the oxidation treatment for 2 hours, it was filtered through a glass filter and washed thoroughly with water to obtain an oxidized CNF.

CNF dispersion A was prepared by adding the oxidized CNF to pure water so that the solid concentration (absolute dryness) was 1% by mass. Next, this CNF dispersion A was treated with a homogenizer to defibrate the oxidized CNF.
Furthermore, the CNF dispersion A that had been defibrated was subjected to defoaming treatment by stirring for 10 minutes at 1000 rpm using a stirring type defoaming apparatus.

The defoamed CNF dispersion A was applied onto a PET film, dried in an oven at 100 ° C. for 2 hours, and dried. Thus, a precursor sheet A made of oxidized CNF was formed.

Precursor sheet A was peeled from the PET film and immersed in an aqueous hydrochloric acid solution at a temperature of 25 ° C. and a pH of 3.5 for 1 minute to perform acid treatment of precursor sheet A.
The precursor sheet A subjected to acid treatment was sufficiently washed with water and dried to prepare a cellulose sheet. The thickness of the cellulose sheet was 50 μm. The thickness of the cellulose sheet was measured with a stylus type thickness measuring machine.

With respect to the produced cellulose sheet, sodium ion strength was measured when the total ionic strength detected by TOF-SIMS was normalized to 1. In the following description, the sodium ion intensity when normalized with the total ion intensity detected by TOF-SIMS as 1 is also referred to as “Na amount ratio”.
For TOF-SIMS, TOF-SIMS V manufactured by ION-TOF was used. The measurement conditions are as follows.
Primary ion: Bi ₃ ⁺ (current value: 0.2 pA)
Measurement mode: bunching mode (high mass resolution)
Secondary ion polarity: positive
Measurement area: 500 × 500μm
Surface resolution: 256 × 256pixel
Integration: 16 times Charging correction: Yes Sample size was 500 μm square. As the Na amount ratio, an average value measured twice was used.
As a result, the Na amount ratio of the cellulose sheet was 0.009.

Further, the COOH ratio of the produced cellulose sheet was measured with an infrared spectroscopic analyzer (manufactured by JASCO Corporation, FT-IR4000).
As described above, the COOH ratio is obtained by infrared spectroscopy, and A [COOH] is the maximum value of the absorbance of 1733 cm ^-1 ± 50 cm ^-1, the maximum value of the absorbance at 1636 cm ^-1 ± 50 cm ^-1 Is calculated by the following formula COOH ratio [%] = (A [COOH] / (A [COOH] + A [COO])) × 100.
Infrared spectroscopic analysis was performed by preparing a measurement sample by freeze-pulverizing a cellulose sheet, mixing the obtained powder and KBr powder, putting the mixture in a mold, and applying pressure.
As a result, the COOH ratio of the cellulose sheet was 26%.

Furthermore, the surface roughness Ra of the produced cellulose sheet was measured with a surface shape measuring machine (manufactured by Zygo, New View 7100).
The measurement conditions were a measurement area of 0.14 × 0.11 mm, and the magnification of the objective lens was 50 times.
As a result, the surface roughness Ra of the cellulose sheet was 0.02 μm.

[Example 2]
A cellulose sheet was prepared in the same manner as in Example 1 except that the hydrochloric acid aqueous solution for acid treatment of the precursor sheet A was changed to pH 1. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.004, a COOH ratio of 58%, and a surface roughness Ra of 0.02 μm.
[Example 3]
A cellulose sheet was produced in the same manner as in Example 1 except that the aqueous hydrochloric acid solution for acid treatment of the precursor sheet A was adjusted to pH 0.3. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.003, a COOH ratio of 76%, and a surface roughness Ra of 0.03 μm.
[Example 4]
A cellulose sheet was prepared in the same manner as in Example 1 except that the hydrochloric acid aqueous solution for acid treatment of the precursor sheet A was adjusted to pH 0.1 and a temperature of 80 ° C. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.002, a COOH ratio of 88%, and a surface roughness Ra of 0.04 μm.

[Example 5]
A cellulose sheet was prepared in the same manner as in Example 1 except that the solid content concentration of the CNF dispersion A was 0.1% by mass and the aqueous hydrochloric acid solution for acid treatment of the precursor sheet A was adjusted to pH 1. The thickness of the cellulose sheet was 5 μm. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.004, a COOH ratio of 59%, and a surface roughness Ra of 0.01 μm.
[Example 6]
A cellulose sheet was prepared in the same manner as in Example 1 except that the solid content concentration of the CNF dispersion A was 0.6% by mass and the aqueous hydrochloric acid solution for acid treatment of the precursor sheet A was adjusted to pH 1. The thickness of the cellulose sheet was 30 μm. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.004, a COOH ratio of 53%, and a surface roughness Ra of 0.02 μm.
[Example 7]
A cellulose sheet was prepared in the same manner as in Example 1 except that the solid content concentration of the CNF dispersion A was 2% by mass and the aqueous hydrochloric acid solution for acid treatment of the precursor sheet A was adjusted to pH 1. The thickness of the cellulose sheet was 100 μm. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had a Na amount ratio of 0.004, a COOH ratio of 44%, and a surface roughness Ra of 0.04 μm.

[Comparative Example 1]
A CNF dispersion A was prepared in the same manner as in Example 1 and defibrated.
With the defibrated CNF dispersion A kept at 25 ° C., 1M hydrochloric acid was added to adjust the pH to 2, and stirring was continued for 30 minutes to obtain a CNF dispersion B obtained by acid-treating the CNF dispersion A. . This acid treatment gelled CNF.
Thereafter, the CNF dispersion B was centrifuged to recover the gelled CNF.

After the gelled CNF was sufficiently washed with water, it was poured into pure water so that the solid content concentration was 1% by mass to prepare CNF dispersion C. Next, the CNF dispersion C was defoamed in the same manner as in Example 1.
The CNF dispersion C subjected to defoaming treatment was applied onto a PET film and dried at room temperature for 3 days to form a cellulose sheet.
After drying the CNF dispersion C, the cellulose sheet was peeled from the PET film. The thickness of the cellulose sheet was 50 μm.
As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had an Na amount ratio of 0.035, a COOH ratio of 28%, and a surface roughness Ra of 0.34 μm.

[Comparative Example 2]
A cellulose sheet was produced in the same manner as in Comparative Example 1 except that the amount of 1M hydrochloric acid added in the acid treatment of CNF dispersion A was changed to pH 1 of CNF dispersion B. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had an Na amount ratio of 0.026, a COOH ratio of 55%, and a surface roughness Ra of 0.66 μm.
[Comparative Example 3]
A cellulose sheet was prepared in the same manner as in Comparative Example 1 except that the amount of 1M hydrochloric acid added in the acid treatment of CNF dispersion A was changed to pH 0.5 of CNF dispersion B. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had an Na amount ratio of 0.011, a COOH ratio of 90%, and a surface roughness Ra of 0.72 μm.

[Comparative Example 4]
A cellulose sheet was produced in the same manner as in Example 1 except that the aqueous hydrochloric acid solution for acid treatment of the precursor sheet A was adjusted to pH 5. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had an Na amount ratio of 0.022, a COOH ratio of 15%, and a surface roughness Ra of 0.02 μm.

[Comparative Example 5]
CNF used as a raw material in Example 1 was put into pure water so as to have a solid content concentration of 1% by mass without being oxidized, and defibrated and degassed in the same manner as in Example 1. CNF dispersion D was prepared.
Using this CNF dispersion D, a precursor sheet D obtained by drying the CNF dispersion D was produced in the same manner as in Example 1.
This precursor sheet D was immersed in an aqueous hydrochloric acid solution and washed in the same manner as in Example 1 to produce a cellulose sheet.
In this example, since the oxidation treatment of CNF as a raw material is not performed, the produced cellulose sheet does not have Na and COOH groups. Moreover, as a result of measuring similarly to Example 1, surface roughness Ra of the produced cellulose sheet was 0.12 micrometer.

[Comparative Example 6]
In Example 1, the precursor sheet A was directly used as a cellulose sheet without being immersed in an aqueous hydrochloric acid solution. As a result of measurement in the same manner as in Example 1, the produced cellulose sheet had an Na amount ratio of 0.058, a COOH ratio of 0%, and a surface roughness Ra of 0.02 μm.

[Water resistance evaluation]
The produced cellulose sheet was immersed in pure water for 24 hours, and the swelling ratio was calculated from the thickness before and after the immersion, and the water resistance was evaluated.
[Swelling ratio] = [Thickness after immersion] / [Thickness before immersion]

[Haze]
The haze of the produced cellulose sheet was measured using a D65 light source with a turbidimeter (Nippon Denshoku Industries Co., Ltd., NDH2000 measurement method 3).

[Heat-resistant]
The produced cellulose sheet was heated in an oven at 100 ° C. for 1 hour, and then visually evaluated.
Good when no yellowing is observed:
The case where yellowing is observed is evaluated as “No”: The results are shown in the following table.

As shown in Table 1, the cellulose sheet of the present invention satisfying the Na amount ratio of 0.020 or less and the COOH ratio in the range of 20% or more and less than 90%, regardless of the pH and film thickness of the acid treatment, Both show good water resistance and low haze. In Example 4, the COOH ratio is slightly higher, which is inferior in heat resistance, but all others are also excellent in heat resistance.
On the other hand, all of Comparative Examples 1 to 3 in which the acid treatment was performed with the CNF dispersion and then the coating for forming the cellulose sheet was performed, because CNF was in a gel state. The haze is large, and the surface of the cellulose sheet has many —COONa groups and the surface roughness Ra is large, so that the water resistance is low. Furthermore, Comparative Example 3 in which the COOH ratio exceeds the range of the present invention has low heat resistance.
Since Comparative Example 4 has a high Na amount ratio and a low COOH ratio, it is inferior in water resistance. In Comparative Example 5 in which CNF oxidation treatment is not performed, CNF remains having a primary hydroxyl group and does not have a functional group represented by —COOA, and thus has low water resistance and slightly high haze. Further, Comparative Example 6 in which acid treatment is not performed has low water resistance because CNF on the surface of the CNF sheet has a hydrophilic —COONa group.
From the above results, the effects of the present invention are clear.

It can be suitably used for printed electronics substrates, gas barrier films, agricultural sheets, food packaging films, and the like.

Claims (7)

1. A water-resistant cellulose sheet comprising cellulose nanofibers having a functional group represented by -COOA,
   Alkaline metal ion intensity or alkaline earth metal ion intensity when normalized with 1 as the total ion intensity detected by time-of-flight secondary ion mass spectrometry is 0.020 or less,
   A in the functional group represented by —COOA represents any one of an alkali metal, an alkaline earth metal, and a hydrogen atom, and 20% or more and less than 90% of the functional group represented by —COOA is a —COOH group. A water-resistant cellulose sheet characterized by being.
2. The water-resistant cellulose sheet according to claim 1, wherein the surface roughness Ra is 0.1 μm or less.
3. The water-resistant cellulose sheet according to claim 1 or 2, which has a thickness of 5 to 100 µm.
4. The water-resistant cellulose sheet according to any one of claims 1 to 3, wherein A of the functional group represented by -COOA is H or Na.
5. A method for producing a water-resistant cellulose sheet according to any one of claims 1 to 4,
   A process of oxidizing cellulose nanofibers,
   Forming the oxidized cellulose nanofiber into a sheet, and
   A process for producing a water-resistant cellulose sheet, comprising: treating the cellulose nanofibers formed into the sheet shape with an acidic aqueous solution.
6. The method for producing a water-resistant cellulose sheet according to claim 5, wherein the pH of the acidic aqueous solution is 4 or less.
7. The method for producing a water-resistant cellulose sheet according to claim 5 or 6, wherein the oxidation treatment of the cellulose nanofiber is performed using an N-oxyl compound as a catalyst.