JP7655728B2 - Method for producing chemically modified microfibril cellulose fibers - Google Patents

Description

The present invention relates to a method for producing chemically modified microfibril cellulose fibers.

Cellulose nanofibers and microfibril cellulose fibers obtained by micronizing cellulose are extremely fine fibers with widths on the nano to micro order. They are expected to be used in a variety of fields as new materials with properties not found in ordinary pulp, such as high strength, high elasticity, and thixotropy.

For example, in the papermaking industry, paper strength improvers are sometimes used to increase the strength of paper, and the addition of cellulose nanofibers to reinforce this effect is being considered (Patent Document 1, etc.).

JP 2016-166444 A

However, the method described in Patent Document 1 has the problem that cellulose nanofibers are expensive, and the addition of cellulose nanofibers increases the cost of the resulting paper. In addition, because cellulose nanofibers have a very small fiber width, when they are added to papermaking raw materials and used for papermaking, they are discharged together with the water during dehydration in the wire part of the papermaking machine and water squeezing in the press part, which causes the resulting paper to have insufficient strength.

The inventors came up with the idea that by using microfibril cellulose fibers, which have a lower degree of defibration than cellulose nanofibers and have a high BET specific surface area, instead of cellulose nanofibers, it would be possible to produce strong paper at low cost. As a method for producing microfibril cellulose fibers with such characteristics, the use of a single disc refiner has been considered, but there is a problem in that it is difficult to increase the processing volume.

The object of the present invention is to provide a method for producing chemically modified microfibril cellulose fibers with excellent production efficiency, which can produce chemically modified microfibril cellulose fibers with a high BET specific surface area and an average fiber width within a specific range.

The present invention provides the following:
(1) A method for producing chemically modified microfibril cellulose fibers having a BET specific surface area of 50 ^m2 /g or more and an average fiber width of 500 nm or more, comprising: a chemical modification step of chemically modifying a raw pulp to obtain a chemically modified pulp; and a beating step of beating the chemically modified pulp obtained in the chemical modification step under a condition of a solid content concentration of 15% by weight or less using a double disc refiner having two discs DA and DB and a disc DM between them, in which either the DA and the DB or the DM is fixed and the other rotates.
(2) The method for producing a chemically modified microfibrillated cellulose fiber according to (1), wherein the number of passes in the beating treatment step is 30 or less.
(3) The method for producing chemically modified microfibril cellulose fibers according to (1) or (2), wherein the chemical modification is carried out by oxidation using an N-oxyl compound, a compound selected from the group consisting of bromides, iodides and mixtures thereof, and an oxidizing agent.
(4) The method for producing a chemically modified microfibril cellulose fiber according to (1) or (2), wherein the chemical modification is a carboxymethyl modification.
(5) The method for producing a chemically modified microfibrillated cellulose fiber according to any one of (1) to (4), characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and the DB, counted from the raw material inlet side, is larger than the blade width X2 of the second disc.
(6) The method for producing a chemically modified microfibrillated cellulose fiber according to any one of (1) to (4), characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and the DB, counted from the raw material inlet side, is smaller than the blade width X2 of the second disc.
(7) The method for producing a chemically modified microfibrillated cellulose fiber according to any one of (1) to (4), characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and the DB, counted from the raw material inlet side, is the same as the blade width X2 of the second disc.

The present invention provides a method for producing chemically modified microfibril cellulose fibers with excellent production efficiency, which can produce chemically modified microfibril cellulose fibers with a high BET specific surface area and a specific range of average fiber width.

FIG. 2 is a schematic diagram showing an example of a double disc refiner used in the production method of the present invention.

The manufacturing method of the chemically modified microfibril cellulose fiber of the present invention will be described below with reference to the drawings. In the present invention, "~" includes the end values. In other words, "X~Y" includes the values X and Y at both ends.

The method for producing chemically modified microfibril cellulose fibers having a high BET specific surface area and an average fiber width within a specific range of the present invention includes a chemical modification step in which raw pulp is chemically modified to obtain chemically modified pulp, and a beating step in which the chemically modified pulp obtained in the chemical modification step is beated under conditions of a solid content concentration of 15% by weight or less using a double disc refiner having two discs, DA and DB, and a disc DM between them, in which either the DA and DB or the DM is fixed and the other rotates.

(Chemically modified microfibril cellulose fiber)
Microfibrillated cellulose fibers (hereinafter also referred to as "MFC") are fibers having an average fiber width of 500 nm or more obtained by defibrating a cellulose-based raw material such as pulp, and chemically modified microfibrillated cellulose fibers (hereinafter also referred to as "chemically modified MFC") are MFCs obtained by defibrating a chemically modified cellulose-based raw material. In the present invention, the average fiber width is a length-weighted average fiber width, and the fiber width can be measured using a fiber tester manufactured by ABB Corporation or a fractionator manufactured by Valmet. The lower limit of the fiber diameter is preferably 500 nm or more, and the upper limit is not particularly limited, but is about 60 μm or less. MFCs are obtained by relatively weakly defibrating or beating a cellulose-based raw material using a beater or disperser. Therefore, MFCs have a larger fiber width than cellulose nanofibers obtained by strongly defibrating a cellulose-based raw material using a high-pressure homogenizer, and have a shape in which the fiber surface is efficiently fluffed (externally fibrillated) while suppressing the fineness of the fiber itself.

(BET specific surface area)
The chemically modified microfibril cellulose fiber obtained by the production method of the present invention has a BET specific surface area of 50 ^m2 /g or more, preferably 70 ^m2 /g or more. If the BET specific surface area is high, for example, when used as a papermaking additive, it is easier to bond to pulp, which has the advantage of improving the yield and improving the effect of imparting strength to paper. The BET specific surface area can be measured by the following method with reference to the nitrogen gas adsorption method (JIS Z8830):
(1) Approximately 2% slurry of chemically modified microfibril cellulose fibers (dispersion medium: water) was separated and placed in a centrifuge container so that the solid content was approximately 0.1 g, and 100 mL of ethanol was added.
(2) Add a stirrer and stir at 500 rpm for at least 30 minutes.
(3) Remove the stirrer, and sediment the chemically modified microfibril cellulose fibers in a centrifuge at 7,000 G for 30 minutes at 30° C.
(4) The supernatant is removed while trying to avoid removing as much of the chemically modified microfibril cellulose fibers as possible.
(5) Add 100 mL of ethanol, add a stir bar, stir under the conditions of (2), centrifuge under the conditions of (3), and remove the supernatant under the conditions of (4). Repeat this process three times.
(6) The solvent in (5) is changed from ethanol to t-butanol, and the stirring, centrifugation, and removal of the supernatant are repeated three times in the same manner as in (5) at room temperature above the melting point of t-butanol.
(7) After the final solvent removal, add 30 mL of t-butanol, mix gently, then transfer to a recovery flask and freeze using an ice bath.
(8) Cool in the freezer for at least 30 minutes.
(9) Place in a freeze-dryer and freeze-dry for 3 days.
(10) BET measurement is performed using a BET measurement device (Micromeritics) (pretreatment conditions: under nitrogen flow at 105° C. for 2 hours, relative pressure of 0.01 to 0.30, sample amount of about 30 mg).

(viscosity)
The chemically modified microfibril cellulose fiber obtained by the production method of the present invention has a B-type viscosity at 1% by weight, 60 rpm and 25°C of preferably 10 to 6000 mPa·s, more preferably 20 to 5000 mPa·s, and even more preferably 50 to 4000 mPa·s, from the viewpoint of the degree of defibration.

(Chemical modification process)
In the chemical modification step, the raw pulp is chemically modified to obtain a chemically modified pulp.

(Raw pulp)
Examples of raw pulp include unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), unbleached hardwood sulfite pulp (LUSP), bleached hardwood sulfite pulp (LBSP), thermomechanical pulp (TMP), pressurized groundwood pulp (PGW), refiner ground wood pulp (RGP), alkaline hydrogen peroxide mechanical pulp (APMP), alkaline hydrogen peroxide thermomechanical pulp (APTMP), pulp derived from herbs such as linter, jute, hemp, paper mulberry, Mitsumata, and kenaf, pulp derived from bamboo, recycled pulp, and waste paper pulp, but are not limited to these.

(Chemical Modification)
Chemical modification refers to the introduction of functional groups into pulp, and the chemical modification is preferably anionic modification, i.e., the chemically modified pulp preferably has anionic groups. Examples of anionic groups include acid groups such as carboxyl groups, carboxyl group-containing groups, phosphoric acid groups, phosphoric acid group-containing groups, and sulfate ester groups. Examples of carboxyl group-containing groups include -COOH groups, -R-COOH (R is an alkylene group having 1 to 3 carbon atoms), and -O-R-COOH (R is an alkylene group having 1 to 3 carbon atoms). Examples of phosphoric acid group-containing groups include polyphosphoric acid groups, phosphorous acid groups, phosphonic acid groups, and polyphosphonic acid groups. Depending on the reaction conditions, these acid groups may be introduced in the form of a salt (for example, a carboxylate group (-COOM, M is a metal atom)). In the present invention, the chemical modification is preferably oxidation or etherification.

The oxidation can be carried out as known in the art. For example, there is a method in which raw pulp is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a substance selected from the group consisting of bromides, iodides, and mixtures thereof. According to this method, the primary hydroxyl group at the C6 position of the glucopyranose ring on the surface of the cellulose is selectively oxidized to generate a group selected from the group consisting of an aldehyde group, a carboxyl group, and a carboxylate group. Alternatively, there is an ozone oxidation method. According to this oxidation reaction, at least the hydroxyl groups at the 2nd and 6th positions of the glucopyranose ring constituting the cellulose are oxidized, and the cellulose chain is decomposed.

An N-oxyl compound is a compound that can generate a nitroxy radical. Any compound that promotes the desired oxidation reaction can be used as an N-oxyl compound. Examples include 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO).

An example of a method for measuring the amount of carboxyl groups is described below. 60 mL of a 0.5 wt % slurry (aqueous dispersion) of oxidized cellulose is prepared, and a 0.1 M aqueous hydrochloric acid solution is added to adjust the pH to 2.5, after which a 0.05 N aqueous sodium hydroxide solution is added dropwise and the electrical conductivity is measured until the pH reaches 11. The amount of carboxyl groups can be calculated using the following formula from the amount of sodium hydroxide (a) consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual.
Amount of carboxyl groups [mmol/g oxidized cellulose] = a [mL] × 0.05 / weight of oxidized cellulose [g]

The amount of carboxyl groups in oxidized cellulose measured in this manner is preferably 0.1 mmol/g or more, more preferably 0.5 mmol/g or more, and even more preferably 0.8 mmol/g or more, based on the bone dry weight. The upper limit of this amount is preferably 3.0 mmol/g or less, more preferably 2.5 mmol/g or less, and even more preferably 2.0 mmol/g or less. Therefore, this amount is preferably 0.1 mmol/g or more and 3.0 mmol/g or less, more preferably 0.5 mmol/g or more and 2.5 mmol/g or less, and even more preferably 0.8 mmol/g or more and 2.0 mmol/g or less.

Examples of etherification include carboxymethylation (etherification), methylation (etherification), ethylation (etherification), cyanoethylation (etherification), hydroxyethylation (etherification), hydroxypropylation (etherification), ethylhydroxyethylation (etherification), and hydroxypropylmethylation (etherification). Among these, carboxymethylation is preferred. Carboxymethylation can be carried out, for example, by mercerizing the raw pulp as the starting raw material, and then etherifying it.

The degree of carboxymethyl substitution per glucose unit of carboxymethyl cellulose is measured, for example, by the following method. That is, 1) Approximately 2.0 g of carboxymethyl cellulose (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 2) 100 mL of a solution of 1000 mL of nitric acid methanol and 100 mL of special-grade concentrated nitric acid is added, and the mixture is shaken for 3 hours to convert the carboxymethyl cellulose salt (carboxymethyl cellulose) into hydrogen-type carboxymethyl cellulose. 3) Approximately 1.5 g to 2.0 g of hydrogen-type carboxymethyl cellulose (bone dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 4) The hydrogen-type carboxymethyl cellulose is moistened with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH is added, and the mixture is shaken at room temperature for 3 hours. 5) Using phenolphthalein as an indicator, the excess NaOH is back-titrated with 0.1 N H ₂ SO ₄ . 6) The degree of carboxymethyl substitution (DS) is calculated by the following formula:
A = [(100 x F' - (0.1 _{N H2SO4} ) (mL) x F) x 0.1] / (bone dry weight of hydrogen type carboxymethyl cellulose (g))
DS=0.162×A/(1-0.058×A)
A: The amount of 1N NaOH required to neutralize 1 g of hydrogen-type carboxymethyl cellulose (mL)
F: Factor of 0.1N _H2SO4 F': Factor _of 0.1N NaOH

The degree of carboxymethyl substitution per anhydrous glucose unit in carboxymethylated cellulose is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.10 or more. The upper limit of the degree of substitution is preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. Therefore, the degree of carboxymethyl group substitution is preferably 0.01 or more and 0.50 or less, more preferably 0.05 or more and 0.40 or less, and even more preferably 0.10 or more and 0.30 or less.

(Beating process)
In this step, a double disc refiner is used to beat the chemically modified pulp, the solid content of which has been adjusted to 15% by weight or less. Beating the chemically modified pulp leads to finer fibers, which are smaller in length and width, and fibrillation, which is more fuzzy. In the beating step of the present invention, the beating process using the double disc refiner may be a circulation process, or a continuous process in which the beating process is performed continuously using a plurality of double disc refiners. In the present invention, when the beating process using the double disc refiner is a circulation process or a continuous process, the number of passes of the beating process is not particularly limited as long as the desired chemically modified microfibrillated cellulose fiber can be obtained. From the viewpoint of productivity and from the viewpoint of suppressing excessive shortening of the fibers and deterioration due to heat generated during the process, the number of passes is preferably 30 or less, more preferably 20 or less, even more preferably 10 or less, and even more preferably 5 or less.

In the present invention, when the beating process is a circulation process, the number of passes of the beating process in the case of partial circulation is the number of times that the raw material to be treated passes through one double disc refiner and is recovered multiplied by n as follows:
n=1/(1-a)
Here, a = circulation ratio. (When the circulation ratio is 50%, a = 0.5, and when the circulation ratio is 75%, a = 0.75.)

When multiple double disc refiners with partial circulation are used for continuous processing, the number of passes in the beating process should be calculated for each refiner using the method described above and then added together.

When the beating process is a circulation process and batch processing is performed with complete circulation, the number of passes of the beating process is the number of times the raw material passes through the double disc refiner before being recovered.

In addition, in the present invention, when multiple double disc refiners are used to perform continuous beating, the number of passes of the beating process is increased by one each time the raw material passes through one double disc refiner.

(Disc Refiner)
A disc refiner is a device in which disks with beating blades (disc plates (sometimes simply called "disks")) face each other at close range and rotate in one direction or in the opposite direction at a specified rotation speed, providing the slurry passing between them with a pressurized beating effect and a continuous sending-out effect through centrifugal force. Among disc refiners, those with one beating gap formed by the disc plate are called single disc refiners (sometimes abbreviated as "SDR"), and those with two beating gaps formed by the disc plate are called double disc refiners (sometimes abbreviated as "DDR").

(Double disc refiner)
A double disc refiner is equipped with two discs DA and DB and a disc DM between them, with either the DA and DB or the DM being fixed and the other rotating, or with the DA and DB rotating in opposite directions to the DM. Examples of double disc refiners with the two discs DA and DB as fixed discs and the DM as a floating disc that rotates freely between them include the double disc refiner manufactured by Aikawa Iron Works Co., Ltd., the double disc refiner manufactured by Mitsubishi Heavy Industries/Beloit (Jones), the twin hydra disc manufactured by Ishikawajima Industrial Machinery/Black & Clawson, and the twin disc refiner manufactured by Hitachi Zosen (Hitachi Zosen Tomioka Machinery)/Escher Wyss.

In the present invention, a double disc refiner is used because it can increase the processing volume compared to a single disc refiner and has superior production efficiency.

The blade width of the two disks DA and DB is preferably 0.3 to 1.5 mm, and more preferably 0.5 to 1.3 mm. The groove width of DA and DB is preferably 0.5 to 2.0 mm, and more preferably 0.8 to 1.7 mm. The blade width and groove width of DA and DB may be the same or different. There are no particular limitations on the blade angle, but it is preferably 0 to 40°, and more preferably 5 to 20°.

Double disc refiners are broadly classified into two types, monoflow and duoflow, depending on the raw material flow method. In the monoflow type, the raw material flows from the beating gap closer to the raw material inlet side to the other beating gap, while in the duoflow type, the raw material is inserted from the center and flows in parallel through the beating gaps formed on both sides of the DM. In the manufacturing method of the present invention, the monoflow type is preferred as the raw material flow method from the viewpoint of efficient fiberization.

An example of a double disc refiner that can be used in the present invention is described with reference to Figure 1. Figure 1 is a cross-sectional view showing an outline of a double disc refiner with a monoflow type raw material flow system. Note that the double disc refiner that can be used in the present invention is not limited to the one shown in Figure 1.

The double disc refiner 2 shown in Figure 1 has a rotating disc DM8 arranged in a beating chamber 6 which beats the pulp raw material fed through the raw material inlet 4, and blades having a predetermined blade width and groove width are attached to both sides of the rotating disc DM8. Fixed discs DA10 and DB12 are arranged on the inner wall of the beating chamber 6, facing the blades attached to both sides of the rotating disc DM8. Blades having a predetermined blade width and groove width are attached to the fixed discs DA10 and DB12. The rotating disc DM8 is attached to a drive shaft 14, which is connected to a motor 16 and driven to rotate. The beating chamber 6 is provided with a circulation pipe 18 for circulating the beaten raw material in the beating chamber 6, a discharge port 20 for discharging the beaten raw material outside the beating chamber 6, and an outlet pipe 22 for sending the raw material from the discharge port 20 to a recovery tank (not shown). The flow rate of the circulation pipe 18 and the outlet pipe 22 is adjusted by a manual valve 24.

When the double disc refiner 2 is used to beat the pulp raw material, the pulp raw material is input from the raw material inlet 4 of the double disc refiner 2 and sent to the beating chamber 6. The raw material sent to the beating chamber 6 is first beaten in the beating gap formed between the fixed disc DA10 and the rotating disc DM8, then beaten in the beating gap formed between the fixed disc DB12 and the rotating disc DM8, and a part of the raw material is sent again to the beating chamber 6 via the circulation pipe 18. The beaten raw material is discharged from the discharge port 20 and sent to a recovery tank via the outlet pipe 22. The amount of raw material circulated and the amount of raw material discharged from the discharge port are adjusted by the manual valve 24.

In the case of a monoflow type raw material flow system, the relationship between the blade width (X1) of the first disk DA, counting from the raw material inlet 4 side, and the blade width (X2) of the second disk DB, of the two disks DA and DB, has the following advantages:
X1 = X2: The fiber width and fiber length of the obtained MFC are likely to be uniform.
X1>X2: The raw material is beaten and fibrillated on a wide blade, and then beaten by a narrow blade to proceed cutting, so that the raw material is less likely to pass through unfibrillated.
X1<X2: The raw material is beaten with a narrow blade and cut, and then beaten on a wide blade and fibrillation proceeds, so that fine fibers with advanced fibrillation are likely to be obtained.

In addition, when the raw material flow method is a monoflow type, the relationship between the groove width (Y1) of the first disk DA, counting from the raw material inlet 4 side, and the groove width (Y2) of the second disk DB, of the two disks DA and DB, has the following advantages.
Y1 = Y2: The fiber width and fiber length of the obtained MFC are likely to be uniform.
Y1>Y2: Since the raw material passes through a wide beating gap and then a narrow beating gap, excessive load is not applied, and this is suitable for processing highly viscous materials.
Y1<Y2: When the raw material first passes through a narrow beating gap, defibration progresses and the viscosity tends to increase, but the raw material then passes through a wider beating gap, allowing it to pass through without stagnation.

In the manufacturing method of the present invention, when a double disc refiner is used, the operating conditions are such that the clearance between DA and DM and between DB and DM is preferably 0.6 mm or less, more preferably 0.4 mm or less, and even more preferably 0.2 mm or less. There is no particular lower limit, but in order to avoid metal touch, 0.02 mm or more is preferable. The operating temperature is preferably 5 to 120°C. The flow rate, processing time, and other conditions are appropriately adjusted so that chemically modified microfibril cellulose fibers having the desired fiber width are obtained.

In the manufacturing method of the present invention, the solids concentration of the chemically modified pulp to be subjected to the beating process is 15% by weight or less, preferably 0.3 to 10% by weight, and more preferably 0.5 to 6% by weight, from the viewpoint of pulp slurry transportation. If the solids concentration is too low, the pulp will not come into contact with the blades of the plate during beating, resulting in poor beating efficiency. The solids concentration may vary during mechanical processing including beating, but in the present invention, the solids concentration at the start of beating is referred to as the solids concentration in the beating process.

One method for adjusting the solids concentration of chemically modified pulp to 15% by weight or less is dilution.

The pH of the chemically modified pulp to be subjected to the beating process is preferably 6 or higher, more preferably 7 or higher, from the viewpoint of ease of defibration. Methods for adjusting the pH to the above range include adding chemicals such as NaOH, KOH, or sodium bicarbonate, or reducing the amount of acidic chemicals added after chemical modification. The pH of the chemically modified pulp may fluctuate during mechanical processing including beating, but in the present invention, the pH at the start of beating is defined as the pH in the beating process.

The chemically modified pulp to be subjected to the beating process may be one that has been subjected to an alkali treatment. Any alkali, such as NaOH or KOH, can be used for this alkali treatment. When the chemically modified pulp used in the present invention is anionically modified pulp, the terminals of the modified groups become Na-type or the like by adding an alkali, and the repulsion between the fibers increases. Therefore, the electrostatic repulsion of the fibers can be utilized to efficiently carry out mechanical treatments such as beating, defibration, and fiber defibration.

The chemically modified pulp to be subjected to the beating process may be one that has been subjected to an acid treatment. When the chemically modified pulp used in the present invention is anionically modified pulp, the terminal of the modified group becomes H-type by the acid treatment, and the affinity for water is reduced.

The manufacturing method of the present invention may include one or more mechanical treatment steps using a device other than a double disc refiner before or after the beating step using the double disc refiner. Also, the manufacturing method of the present invention may include a mechanical treatment step using a device other than a double disc refiner before or after the beating step, in which a chemically modified pulp having a solids concentration of more than 15% by weight is mechanically treated.

In the present invention, mechanical processing refers to mixing fibers and further refining or fibrillating them, and includes beating, defibrating, dispersing, kneading, etc. Here, mechanical processing under conditions where the solid content concentration is higher than 15% by weight is sometimes called "high concentration mechanical processing", and especially when the mechanical processing is beating, it is also called "high concentration beating". Similarly, mechanical processing under conditions where the solid content concentration is 15% by weight or less is sometimes called "low concentration mechanical processing", and especially when the mechanical processing is beating, it is also called "low concentration beating". In the present invention, as long as the beating processing step of low concentration beating using a double disc refiner is performed at least once, mechanical processing may be performed multiple times.

In the present invention, the mechanical treatment may be a circulation operation (batch treatment), a partial circulation operation, or a continuous treatment in which mechanical treatment is performed continuously using multiple devices. A combination of high-concentration mechanical treatment and low-concentration mechanical treatment may be performed. When combining these mechanical treatments, the order of the treatments is not limited, but it is preferable to perform the high-concentration mechanical treatment first from the viewpoint of ease of concentration. For example, MFC can be obtained by subjecting chemically modified pulp to high-concentration mechanical treatment, diluting the chemically modified pulp obtained by the treatment to 15% by weight or less, and performing a low-concentration beating treatment using a double disc refiner.

Devices that can be used for low-concentration mechanical treatment include, for example, high-speed rotary, colloid mill, high-pressure, roll mill, and ultrasonic types of equipment, including high-pressure or ultra-high-pressure homogenizers, refiners such as conical refiners, beaters, PFI mills, kneaders, dispersers, top finers, seven finers, beat finers, twin beat finers, Henschel mixers, and homomix line mills, which use a metal or blade to act on the pulp fibers around a rotating shaft, or use friction between the pulp fibers, or disperse or defibrate the pulp fibers by cavitation, water flow, or water pressure.

Devices that can be used for high-concentration mechanical processing include, for example, high-speed rotary, colloid mill, high-pressure, roll mill, and ultrasonic types of equipment, and can use high-pressure or ultra-high-pressure homogenizers, refiners, beaters, PFI mills, kneaders, dispersers, top finers, and other devices that use a metal or blade to act on the pulp fibers around a rotating shaft, or that use friction between the pulp fibers themselves.

The present invention provides a method for producing chemically modified microfibril cellulose fibers with excellent production efficiency, which can produce chemically modified microfibril cellulose fibers with a high BET specific surface area and a specific range of average fiber width.

In the manufacturing method of the present invention, chemically modified pulp is used as the raw material, and electrostatic repulsion occurs between the substituents introduced by chemical modification, so MFC can be obtained efficiently by beating using a double disc refiner.

The chemically modified microfibril cellulose fiber obtained by the manufacturing method of the present invention is made from chemically modified pulp and has functional groups on the fiber surface, and therefore has various functionalities derived from the functional groups. Therefore, the chemically modified microfibril cellulose fiber obtained by the manufacturing method of the present invention can be used for various purposes, and can be used as a thickener, gelling agent, paste, food additive, excipient, paint additive, adhesive additive, abrasive, rubber/plastic compounding material, water retention agent, shape retention agent, muddy water regulator, filter aid, overflow prevention agent, admixture, etc. in various fields where additives are generally used. Examples of such fields include food, beverages, cosmetics, medicine, papermaking, various chemical products, paints, sprays, agricultural chemicals, civil engineering, construction, electronic materials, flame retardants, household goods, adhesives, cleaning agents, fragrances, lubricating compositions, etc.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

Example 1
<Preparation of chemically modified pulp 1>
40 kg (bone-dry) of bleached unbeaten kraft pulp (whiteness 85%: manufactured by Nippon Paper Industries Co., Ltd.) derived from coniferous trees was added to 4000 L of an aqueous solution in which 312 g (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (manufactured by Sigma Aldrich) and 4112 g of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. A sodium hypochlorite aqueous solution was added to the reaction system so that the sodium hypochlorite concentration was 5.5 mmol/g, and an oxidation reaction was started at room temperature. The pH of the system decreased during the reaction, but a 3M aqueous sodium hydroxide solution was added successively to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system no longer changed. After adding hydrochloric acid to the reaction mixture to adjust the pH to 2, the pulp was thoroughly washed by repeatedly dehydrating and diluting with water, and finally dehydrated until the pulp solids concentration reached 20% by weight to obtain a chemically modified pulp (TEMPO oxidized pulp). The pulp yield was 90%, and the carboxyl group content was 1.41 mmol/g.

The obtained TEMPO oxidized pulp was dispersed in tap water, sodium hydroxide was added, and the mixture was stirred to obtain an aqueous dispersion of TEMPO oxidized pulp with a pH of 7.7 and a solids concentration of 1.1% by weight.

<Beating>
3,000 kg of the obtained aqueous dispersion of TEMPO oxidized pulp was beaten in a monoflow double-disc refiner (AWN20 manufactured by Aikawa Iron Works Co., Ltd., plate: blade width (X1): 0.8 mm, groove width (Y1): 1.3 mm, blade width (X2): 0.6 mm, groove width (Y2): 1.0 mm) operated for 45 minutes under conditions of clearance: 0.4 mm or less and circulation rate of 75.5%, to obtain TEMPO oxidized pulp as MFC. The number of passes in the beating treatment was 8.1. The MFC was then evaluated by the method described below. The results are shown in Table 1.

Example 2
The TEMPO oxidized pulp having a solids concentration of 20% by weight obtained in Example 1 was dispersed in tap water, and sodium hydroxide was added and stirred to obtain an aqueous dispersion of TEMPO oxidized pulp having a pH of 7.6 and a solids concentration of 1.1% by weight.
3000 kg of the aqueous dispersion of the TEMPO oxidized pulp was beaten in the same manner as in Example 1, except that the plate was changed to one with a blade width (X1): 0.6 mm, groove width (Y1): 1.0 mm, blade width (X2): 0.6 mm, and groove width (Y2): 1.0 mm, to obtain an MFC. The MFC was then evaluated by the method described below. The results are shown in Table 1.

Example 3
The TEMPO oxidized pulp having a solids concentration of 20% by weight obtained in Example 1 was dispersed in tap water, and sodium hydroxide was added and stirred to obtain an aqueous dispersion of TEMPO oxidized pulp having a pH of 7.6 and a solids concentration of 1.1% by weight.
3000 kg of the aqueous dispersion of the TEMPO oxidized pulp was beaten in the same manner as in Example 1, except that the plate was changed to one with a blade width (X1): 0.6 mm, groove width (Y1): 1.0 mm, blade width (X2): 0.8 mm, and groove width (Y2): 1.3 mm, to obtain an MFC. The MFC was then evaluated by the method described below. The results are shown in Table 1.

(Comparative Example 1)
The TEMPO oxidized pulp having a solids concentration of 20% by weight obtained in Example 1 was dispersed in ion-exchanged water, sodium hydroxide was added, and the mixture was stirred to obtain an aqueous dispersion of TEMPO oxidized pulp having a pH of 8.8 and a solids concentration of 2% by weight.
74 kg of this aqueous dispersion of TEMPO oxidized pulp was beaten by circulating it for 10 minutes using a single disc refiner (14-inch lab refiner (RF-14 type) manufactured by Aikawa Iron Works Co., Ltd., plate: blade width: 0.6 mm, groove width: 1.0 mm) with a clearance of 0.23 to 0.25 mm to obtain a TEMPO oxidized MFC. The MFC was then evaluated by the method described below. The results are shown in Table 1.

(Comparative Example 2)
The TEMPO oxidized pulp having a solid content of 20% by weight obtained in Example 1 was dispersed in ion-exchanged water, sodium hydroxide was added, and the mixture was stirred to obtain an aqueous dispersion of TEMPO oxidized pulp having a pH of 8.6 and a solid content of 4% by weight.
58 kg of this aqueous dispersion of TEMPO oxidized pulp was beaten by circulating it for 10 minutes using a single disc refiner (14-inch lab refiner (RF-14 type) manufactured by Aikawa Iron Works Co., Ltd., plate: blade width: 0.8 mm, groove width: 1.5 mm) with a clearance of 0.23 to 0.25 mm to obtain a TEMPO oxidized MFC. The MFC was then evaluated by the method described below. The results are shown in Table 1.

As can be seen from Table 1, a method for producing chemically modified microfibril cellulose fiber, which includes a chemical modification step of chemically modifying raw pulp to obtain chemically modified pulp, and a beating step of beating the chemically modified pulp obtained in the chemical modification step using a double disc refiner having two discs, DA and DB, and a DM between them, in which either the DA and the DB or the DM is fixed and the other rotates, under conditions of a solid content concentration of 15% by weight or less, is more productive and has a smaller power consumption rate than a method using a single disc refiner, i.e., is more efficient in producing chemically modified microfibril cellulose fiber having a BET specific surface area of 50 ^m2 /g or more and an average fiber width of 500 nm or more.

The physical properties and characteristics were evaluated as follows.
<Average fiber length and average fiber width (measurement method A)>
The slurry of chemically modified microfibril cellulose fiber was diluted with water to a solids concentration of 0.25%, and then subjected to a fractionator twice, approximately 250 g each (50 g of which was used for measurement), under conditions of a flow rate of 5.7 L/min, a water temperature of 25±1°C, and a total outflow volume of 22 L. Approximately 2,000 images of the chemically modified microfibril cellulose fiber classified by flow rate were obtained inside the device using a CCD camera attached to the fractionator.
The fiber analysis parameters of the analysis software IMG (Metso) were set as shown in Tables 2 to 4, and approximately 2,000 images were analyzed to obtain data such as average fiber length, average fiber width, and fiber length distribution. The average value of two measurements and analyses was used as the measurement data.

・平均繊維長（Length）：長さ加重平均繊維長
・平均繊維幅（Width）：長さ加重平均繊維幅
・繊維長分布（Fraction percentage of length weighted distribution）：長さ加重繊維長分布（各フラクションの設定は表２に記載した通り。）

Average fiber length (Length): Length-weighted average fiber length Average fiber width (Width): Length-weighted average fiber width Fraction percentage of length weighted distribution: Length-weighted fiber length distribution (The settings for each fraction are as shown in Table 2.)

<Average fiber length and average fiber width (measurement method B)>
The measurement was performed under the same conditions as in Measurement Condition A, except that the fiber analysis parameters of the analysis software IMG (Metso) were set as shown in Tables 5 to 7.

The percentage (%) of fibers with a fiber length of 0 to 0.2 mm in the fiber length distribution (length weighted) was evaluated using measurement method A. In addition, the percentage (%) of fibers with a fiber width of 0 to 5 μm in the fiber width distribution (length weighted) was evaluated using measurement method A.

<Viscosity>
Ion-exchanged water was added to the dispersion after the treatment to prepare a 1 wt % slurry, which was then left to stand at 25° C. for 3 hours and then stirred for 5 minutes using a PRIMIX homodisper (3000 rpm). Immediately after stirring, the viscosity was measured after 1 minute at a rotation speed of 60 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) using an appropriate rotor from No. 1 to 4.

<BET specific surface area>
The BET specific surface area was measured by the following method with reference to the nitrogen gas adsorption method (JIS Z 8830):
(1) Ion-exchanged water was added to the treated dispersion as necessary to prepare an approximately 2% slurry (dispersion medium: water), and this was separated and placed in a centrifuge container so that the solid content was approximately 0.1 g, and 100 mL of ethanol was added.
(2) A stirrer was added and the mixture was stirred at 500 rpm for at least 30 minutes.
(3) The stirrer was removed, and the fibrillated chemically modified cellulose fibers were sedimented in a centrifuge at 7,000 G for 30 minutes at 30° C.
(4) The supernatant was removed while trying to avoid removing as much of the fibrillated chemically modified cellulose fibers as possible.
(5) 100 mL of ethanol was added, a stirrer was added, and the mixture was stirred under the conditions of (2), centrifuged under the conditions of (3), and the supernatant was removed under the conditions of (4). This process was repeated three times.
(6) The solvent in (5) was changed from ethanol to t-butanol, and stirring, centrifugation, and removal of the supernatant were repeated three times in the same manner as in (5) at room temperature above the melting point of t-butanol.
(7) After the final removal of the solvent, 30 mL of t-butanol was added and mixed gently, then transferred to a recovery flask and frozen in an ice bath.
(8) Cool in a freezer for at least 30 minutes.
(9) The mixture was placed in a freeze-dryer and freeze-dried for 3 days.
(10) BET measurement was performed using a BET measurement device (Micromeritics) (pretreatment conditions: under nitrogen flow at 105° C. for 2 hours, relative pressure of 0.01 to 0.30, sample amount of about 30 mg).

<Productivity>
The productivity was calculated according to the following formula:
Productivity (BDkg/h) = Treatment amount (as-reacted kg) × Treatment concentration (%) × 0.01 ÷ Treatment time (h)

<Electricity consumption rate>
The power consumption rate was calculated using the following formula.
Electricity consumption rate (kwh/BDT) = Processing load (kw) / Productivity (BDkg/h) x 1000

2...Double disc refiner, 4...Raw material inlet, 6...Beating chamber, 8...Rotary disc DM, 10...Fixed disc DA, 12...Fixed disc DB, 14...Drive shaft, 16...Motor, 18...Circulation piping, 20...Discharge port, 22...Outlet piping, 24...Manual valve

Claims (6)

A method for producing chemically modified microfibril cellulose fibers having a BET specific surface area of 50 m ² /g or more and an average fiber width of 500 nm or more, comprising the steps of:
a chemical modification step of chemically modifying the raw pulp to obtain a chemically modified pulp;
and a beating process in which the chemically modified pulp obtained in the chemical modification process is beated under a condition of a solid content concentration of 0.3 to 10% by weight using a double disc refiner having two discs, DA and DB, and a DM between them, in which either the DA and DB or the DM is fixed and the other rotates.
The method for producing chemically modified microfibril cellulose fibers, wherein the number of passes in the beating treatment step is less than 10. The method for producing chemically modified microfibril cellulose fibers according to claim 1, wherein the chemical modification is carried out by oxidation using an N-oxyl compound, a compound selected from the group consisting of bromides, iodides and mixtures thereof, and an oxidizing agent. The method for producing chemically modified microfibril cellulose fibers according to claim 1, wherein the chemical modification is carboxymethyl modification. The method for producing chemically modified microfibrillated cellulose fibers according to any one of claims 1 to 3, characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and DB, counted from the raw material inlet side, is larger than the blade width X2 of the second disc. The method for producing chemically modified microfibrillated cellulose fibers according to any one of claims 1 to 3, characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and DB, counted from the raw material inlet side, is smaller than the blade width X2 of the second disc. The method for producing chemically modified microfibrillated cellulose fibers according to any one of claims 1 to 3, characterized in that the double disc refiner has a monoflow type raw material flow system, and the blade width X1 of the first disc of the DA and DB, counted from the raw material inlet side, is the same as the blade width X2 of the second disc.

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