JP7769475B2 - Paper with a clear coating layer containing fibrillated chemically modified cellulose fibers - Google Patents

Description

The present invention relates to paper having a clear coating layer containing fibrillated, chemically modified cellulose fibers.

Cellulose nanofibers (hereinafter also referred to as "CNF") are expected to be a new material, and various studies are being conducted on them. Furthermore, as disclosed in Patent Document 1, fibrillated chemically modified cellulose fibers (hereinafter also referred to as "MFC"), which have a lower degree of defibration than cellulose nanofibers, are also expected to be a new material, and various studies are being conducted on them.

International Publication No. 2019/189588

Paper is required to have high printing surface strength, but Patent Document 1 makes no mention of this characteristic. In light of this situation, the objective of the present invention is to provide paper with high printing surface strength.

The above problems are solved by the present invention described below.
(1) A paper comprising a base paper and a clear coating layer,
the clear coating layer comprises starch, fibrillated chemically modified cellulose fibers, and a metal salt;
The fibrillated chemically modified cellulose fibers have a cellulose I type crystallinity of 50% or more, a degree of anionization of 0.10 to 2.00 meq/g, and an average fiber diameter of more than 500 nm.
(2) The paper according to (1), wherein the starch is oxidized starch.
(3) The paper according to (1) or (2), wherein the metal salt contains a divalent or higher metal element.
(4) The paper according to any one of (1) to (3), further comprising a pigment coating layer.
(5) The paper according to any one of (1) to (4), wherein the fibrillated chemically modified cellulose fibers are anionically modified.
(6) The paper according to any one of (1) to (5), wherein the fibrillated chemically modified cellulose fibers have 0.1 to 3.0 mmol/g of carboxyl groups relative to the bone dry weight of the fibrillated chemically modified cellulose fibers.
(7) The paper according to any one of (1) to (5), wherein the chemically modified cellulose in the fibrillated chemically modified cellulose fibers is a carboxyalkylated cellulose having a carboxyalkyl substitution degree per glucose unit of the chemically modified cellulose of 0.01 to 0.50.

This invention provides paper with high printing surface strength.

The present invention is described in detail below. The paper of the present invention has a clear coating layer containing starch, fibrillated chemically modified cellulose fibers (MFC), and a metal salt on one or both sides of the base paper. Furthermore, in the present invention, "X to Y" includes the end values X and Y.

1. Paper with a clear coating layer containing starch, MFC, and metal salt (1) Fibrillated chemically modified cellulose fibers (MFC)
The MFC of the present invention is obtained by defibrating chemically modified cellulose. Chemically modified cellulose has cellulose chains that constitute the fibers chemically modified. Examples of chemically modified cellulose include, but are not limited to, carboxylated cellulose in which a carboxyl group has been introduced, carboxyalkylated cellulose in which a carboxyalkyl group such as a carboxymethyl group has been ether-bonded, and phosphated cellulose in which a phosphate group has been introduced. The production methods for these are described below.

Chemically modified cellulose may take the form of a salt, and in the present invention, chemically modified cellulose also includes salt-type chemically modified cellulose. Examples of salt-type chemically modified cellulose include those that form metal salts such as sodium salts.

Chemically modified cellulose maintains at least a portion of its fibrous shape when dispersed in water. In other words, when an aqueous dispersion of MFC is observed with an electron microscope, a fibrous substance can be seen, and when measured by X-ray diffraction, the peaks of cellulose type I crystals can be observed.

MFC is obtained by moderately beating or fibrillating (fibrillating) chemically modified cellulose raw materials using a refiner or similar device. Compared to chemically modified cellulose fibers that have not been beaten or fibrillated, MFC has cellulose microfibril fluff on the fiber surface. Furthermore, compared to chemically modified cellulose nanofibers, MFC has a larger fiber diameter and a shape in which the fiber surface is efficiently fluffed (external fibrillation) while minimizing the fiber itself (internal fibrillation).

Compared to fibrillated cellulose fibers that have not been chemically modified, MFC has characteristics such as high water retention and thixotropy due to the chemical modification. Furthermore, compared to MFC obtained by fibrillating chemically modified cellulose raw materials that are chemically modified after beating unmodified cellulose raw materials, the strong hydrogen bonds that exist between the fibers are weakened during fibrillation because the cellulose fibers are chemically modified during fibrillation, making it easier for the fibers to separate during fibrillation and resulting in less damage to the fibers.

The average fiber diameter (average fiber width) of MFC is greater than 500 nm, preferably greater than 1 μm, and more preferably greater than 10 μm. The upper limit of the average fiber diameter is preferably 60 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less, and even more preferably 20 μm or less. By performing appropriate fibrillation to an extent that the average fiber diameter falls within this range, the MFC exhibits higher water retention than undefibrated cellulose fibers, and even in small amounts, it can achieve greater strength and yield improvement effects than finely defibrated cellulose nanofibers.

The average fiber length of MFC is preferably 10 μm or more, more preferably 30 μm or more, even more preferably 50 μm or more, and may be 100 μm or more, 200 μm or more, or 300 μm or more. There is no particular upper limit to the average fiber length, but it is preferably 3000 μm or less, more preferably 1500 μm or less, even more preferably 900 μm or less, and even more preferably 500 μm or less. Because chemically modified cellulose raw materials are used for beating or defibration, fibrillation can be promoted without excessively shortening the fibers. Furthermore, chemical modification improves affinity with water, allowing for high water retention even when the fiber length is long.

The above average fiber diameter and average fiber length can be determined using an image analysis-type fiber analyzer, such as the L&W Fiber Tester Plus manufactured by ABB Corporation or a fractionator manufactured by Valmet Ltd. Specifically, when a fractionator is used, the length-weighted fiber width and length-weighted average fiber length can be determined, respectively.

The aspect ratio of the MFC is preferably 10 or more, more preferably 20 or more, and even more preferably 30 or more. There is no particular upper limit to the aspect ratio, but it is preferably 1000 or less, more preferably 100 or less, and even more preferably 80 or less. The aspect ratio can be calculated by the following formula:
Aspect ratio = average fiber length/average fiber diameter.

The fibrillation rate (Fibrillation %) of MFC measured using a fractionator manufactured by Valmet Co., Ltd. is preferably 1.0% or more, more preferably 2.5% or more, and even more preferably 3.5% or more. The fibrillation rate varies depending on the type of cellulose raw material used, but it is considered that fibrillation has occurred within the above range. In the present invention, it is preferable to perform fibrillation so as to improve the fibrillation rate (f ₀ ) of the chemically modified cellulose raw material before fibrillation. If the fibrillation rate of MFC is f, the difference in fibrillation rates Δf = f - f ₀ should be greater than 0, preferably 0.2% or more, more preferably 1% or more, and even more preferably 2.5% or more.

The amount of carboxyl groups in MFC is preferably 0.1 mmol/g or more, more preferably 0.6 mmol/g or more, and even more preferably 1.0 mmol/g or more, based on the bone dry weight of MFC. 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, the amount is preferably 0.1 to 3.0 mmol/g, more preferably 0.6 to 2.5 mmol/g, and even more preferably 1.0 to 2.0 mmol/g.

The chemically modified cellulose in MFC is preferably a carboxyalkylated cellulose having a carboxyalkyl substitution degree per glucose unit of 0.01 to 0.50. The upper limit of this substitution degree is preferably 0.40 or less. If the carboxyalkyl substitution degree exceeds 0.50, the cellulose becomes more susceptible to dissolution in water, and the fibrous form may not be maintained in water. Furthermore, a certain degree of substitution is necessary to obtain the benefits of carboxyalkylation. For example, if the substitution degree is less than 0.02, the benefits of introducing a carboxyalkyl group may not be obtained depending on the application. Therefore, the carboxyalkyl substitution degree is preferably 0.02 or more, more preferably 0.05 or more, more preferably 0.10 or more, more preferably 0.15 or more, more preferably 0.20 or more, and even more preferably 0.25 or more. Furthermore, the carboxyalkyl substitution degree is preferably the carboxymethyl substitution degree.

<Crystallization degree of cellulose type I>
The crystallinity of cellulose in MFC is 50% or more, preferably 60% or more, for crystalline type I. The crystallinity of cellulose can be controlled by the degree of chemical modification. There is no particular upper limit to the crystallinity of cellulose type I, but in reality, it is thought that the upper limit is about 90%.

The method for measuring the crystallinity of cellulose type I of MFC is as follows:
The sample is placed in a glass cell and measured using an X-ray diffraction measurement device (LabX XRD-6000, manufactured by Shimadzu Corporation). The degree of crystallinity is calculated using the method of Segal et al., where the diffraction intensity at 2θ = 10° to 30° in the X-ray diffraction pattern is used as a baseline, and the degree of crystallinity is calculated using the diffraction intensity of the 002 plane at 2θ = 22.6° and the diffraction intensity of the amorphous portion at 2θ = 18.5° according to the following formula:
Xc=(I002c-Ia)/I002c×100
Xc = Crystallinity of cellulose type I (%)
I002c: 2θ=22.6°, diffraction intensity of the 002 plane Ia: 2θ=18.5°, diffraction intensity of the amorphous portion

<Degree of anionization>
The degree of anionization (also referred to as "anion charge density") of the MFC is 0.10 to 2.00 meq/g. The method for measuring the degree of anionization is as follows:
MFC was dispersed in water to prepare an aqueous dispersion with a solid content of 10 g/L, which was then stirred for 10 minutes or more at 1,000 rpm using a magnetic stirrer. The resulting slurry was diluted to 0.1 g/L, and 10 mL of the solution was sampled and titrated with 1/1,000 normality diallyldimethylammonium chloride (DADMAC) using a streaming current detector (Mutek Particle Charge Detector 03). The amount of DADMAC added until the streaming current became zero was used to calculate the degree of anionization according to the following formula:
q = (V × c) / m
q: degree of anionization (meq/g)
V: Amount of DADMAC added until the streaming current becomes zero (L)
c: DADMAC concentration (meq/L)
m: weight (g) of MFC in the measurement sample.

In the present invention, as can be seen from the above measurement method, the "degree of anionization" corresponds to the equivalent of DADMAC required to neutralize the anionic groups in a unit weight of MFC, and corresponds to the equivalent of anions per unit weight of MFC.

The degree of anionization of MFC is preferably 1.50 meq/g or less, more preferably 1.30 meq/g or less, even more preferably 1.00 meq/g or less, and even more preferably 0.80 meq/g or less. Compared to MFCs with a higher degree of anionization, MFCs with a degree of anionization within this range are thought to be chemically modified uniformly throughout the cellulose rather than locally, and are thought to be able to more stably achieve the effects unique to MFCs, such as imparting water retention.

<Water retention capacity>
The water retention capacity of the MFC measured by the following method is preferably 15 or more. The water retention capacity is measured by the following method:
40 mL of slurry (medium: water) with a solids content of 0.3 wt% MFC is prepared. The weight of the slurry at this stage is designated as A. Next, the entire amount of the slurry is centrifuged at 30°C and 25,000 G for 30 minutes using a high-speed refrigerated centrifuge to separate the aqueous phase from the sediment. The weight of the sediment at this stage is designated as B. The aqueous phase is placed in an aluminum cup and dried overnight at 105°C to remove the water, and the weight of the solids in the aqueous phase is measured. The weight of the solids in this aqueous phase is designated as C. The water retention capacity is calculated using the following formula:
Water retention capacity = (B + C - 0.003 x A) / (0.003 x A - C).

As shown in the formula above, water retention capacity corresponds to the weight of water in the sediment relative to the weight of the solid fiber content in the sediment. The higher the value, the stronger the fiber's ability to retain water. The water retention capacity of the MFC of the present invention is preferably 15 or more, more preferably 20 or more, and even more preferably 30 or more. There is no particular upper limit, but in reality it is thought to be around 200 or less.

The above-mentioned method for measuring water retention capacity applies to fibrillated fibers and is generally not applicable to fibers that have not been fibrillated or defibrated, or to cellulose nanofibers that have been defibrated down to single microfibrils. When attempting to measure the water retention capacity of cellulose fibers that have not been fibrillated or defibrated using the above-mentioned method, a dense sediment cannot be formed under the above-mentioned centrifugation conditions, making it difficult to separate the sediment from the aqueous phase. Furthermore, cellulose nanofibers hardly settle under the above-mentioned centrifugation conditions.

<Viscosity>
The MFC of the present invention preferably exhibits a relatively low viscosity when dispersed in water as a dispersion medium (aqueous dispersion). This allows the material to be easily handled despite being fibrillated. In the present invention, the viscosity is measured by the following method:
The MFC was weighed into a polypropylene container and dispersed in 160 mL of ion-exchanged water to prepare an aqueous dispersion with a solid content of 1% by weight. The temperature of the aqueous dispersion was adjusted to 25°C, and the viscosity after 1 minute was measured at 60 rpm using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.) in accordance with the method of JIS-Z-8803.

The viscosity (25°C, 60 rpm) of the MFC of the present invention is preferably 10,000 mPa·s or less. The lower limit is preferably 10 mPa·s or more, more preferably 20 mPa·s or more, and even more preferably 50 mPa·s or more, and the upper limit is preferably 7,000 mPa·s or less.

<Others>
The MFC of the present invention preferably has an electrical conductivity of 500 mS/m or less when dispersed in water at a solid content concentration of 1.0 wt %. The electrical conductivity is more preferably 300 mS/m or less, even more preferably 200 mS/m or less, even more preferably 100 mS/m or less, and particularly preferably 70 mS/m or less. The lower limit of the electrical conductivity is preferably 5 mS/m or more, more preferably 10 mS/m or more. The electrical conductivity is measured as follows:
200 g of an aqueous dispersion of MFC with a solid content of 1.0 wt % was prepared and thoroughly stirred, after which the electrical conductivity was measured using an electrical conductivity meter (HORIBA ES-71).

The BET specific surface area of the MFC of the present invention is preferably 30 m ² /g or more, more preferably 50 m ² /g or more, and even more preferably 100 m ² /g or more. A high BET specific surface area has advantages such as easier bonding to pulp when used as a papermaking additive, improving yield and increasing 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 MFC (dispersion medium: water) was separated so that the solid content was approximately 0.1 g and placed in a centrifuge container, and 100 mL of ethanol was added.
(2) Add a stirrer and stir at 500 rpm for at least 30 minutes.
(3) Remove the stirring bar and sediment the MFC in a centrifuge at 7000 G for 30 minutes at 30°C.
(4) Remove the supernatant while trying to remove as little MFC as possible.
(5) Add 100 mL of ethanol, add a stir bar, stir under the conditions of (2), remove the stir bar, 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 supernatant removal 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 carried out (pretreatment conditions: under nitrogen flow at 105°C for 2 hours, relative pressure 0.01 to 0.30, sample amount approximately 30 mg).

The Schopper-Riegler freeness of the MFC of the present invention is not particularly limited, but is preferably 1°SR or more. The Schopper-Riegler freeness is measured in accordance with JIS P82121-1:2012, specifically as follows:
MFC was dispersed in water to prepare an aqueous dispersion with a solids content of 10 g/L. A stirrer was added and the mixture was stirred at 1000 rpm for 10 minutes or more. The resulting slurry was diluted to 2 g/L. A 60-mesh screen (wire diameter 0.17 mm) was installed in a Mutec DFR-04, and the amount of liquid passing through the mesh from 1000 ml of test liquid was measured for 60 seconds. The Schopper-Riegler freeness was calculated according to JIS P8121-1:2012.

Schopper-Riegler freeness is an index of the degree of drainage of a fiber suspension, with the lower limit being 0°SR and the upper limit being 100°SR. The closer the Schopper-Riegler freeness is to 100°SR, the less drainage (discharge volume) is indicated.

The lower limit of the Schopper-Riegler freeness of the MFC of the present invention is not particularly limited, but is preferably 1°SR or more, more preferably 10°SR or more, more preferably 25°SR or more, more preferably 40°SR or more, and even more preferably 50°SR or more. The upper limit is not particularly limited, but is 100°SR or less.

When the MFC of the present invention is dispersed in water at a solids concentration of 1.0% by weight, the transparency (transmittance of 660 nm light) is preferably less than 60%, more preferably 40% or less, even more preferably 30% or less, even more preferably 20% or less, and particularly preferably 10% or less. The lower limit is not particularly limited and may be 0% or more. When the transparency is in this range, the degree of fibrillation is appropriate, making it easier to achieve the effects of the present invention. Transparency can be measured by the following method.

An aqueous dispersion of MFC (solids concentration 1.0% (w/v), dispersion medium: water) was prepared, and the transmittance of light with a wavelength of 660 nm was measured using a UV-VIS spectrophotometer UV-1800 (Shimadzu Corporation) in a square cell with an optical path length of 10 mm.

When water is used as a dispersion medium, the MFC of the present invention has a solids concentration of approximately 2% by weight or more and is in the form of a translucent to white gel, cream, or paste. The MFC may be in the form of a dispersion obtained after production, or may be dried or redispersed in water as needed. The drying method is not limited, and known methods such as freeze drying, spray drying, tray drying, drum drying, belt drying, thin spreading on a glass plate or the like and drying, fluidized bed drying, microwave drying, and heated fan-type vacuum drying can be used. After drying, the MFC may be pulverized as needed using a cutter mill, hammer mill, pin mill, jet mill, or the like. The method for redispersing the MFC in water is also not particularly limited, and known dispersion devices can be used.

<Method for manufacturing MFC>
The MFC of the present invention can be produced by first preparing a chemically modified cellulose raw material and then fibrillating it. As mentioned above, types of chemical modification include, but are not limited to, carboxylation, carboxyalkylation, and phosphate esterification of cellulose. The chemically modified cellulose raw material to be fibrillated may be commercially available, or may be produced by chemically modifying cellulose, as described below.

1) Cellulose The cellulose used as a raw material for MFC is not particularly limited, and examples thereof include those derived from plants, animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), and microbial products. Examples of plant-derived cellulose include wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, and pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), softwood dissolving pulp, hardwood dissolving pulp, recycled pulp, and waste paper pulp). Cellulose powder obtained by pulverizing the above-mentioned cellulose may also be used. Any one or a combination of these may be used as the cellulose, but cellulose derived from plants or microorganisms is preferred, cellulose derived from plants is more preferred, and pulp derived from plants is even more preferred.

In the present invention, in order to maintain a crystallinity of cellulose type I of 50% or more in MFC, it is preferable to use cellulose with a high crystallinity of cellulose type I as the raw material. The crystallinity of cellulose type I of the cellulose is preferably 70% or more, and more preferably 80% or more. The method for measuring the crystallinity of cellulose type I is as described above.

2) Chemical Modification Chemical modification refers to the introduction of functional groups into cellulose, and the introduction of anionic groups is preferred. Examples of anionic groups include acid groups such as carboxyl groups, carboxyl group-containing groups, phosphate groups, and phosphate group-containing groups. Examples of carboxyl group-containing groups include -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 phosphate group-containing groups include polyphosphate groups, phosphite 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 (e.g., a carboxylate group (-COOM, M is a metal atom)). Chemical modification is preferably oxidation or etherification. Oxidation or etherification can be carried out according to known methods, such as those described in JP 2019-104833 A. In addition, the amount of carboxyl groups in MFC and the degree of carboxymethyl substitution per glucose unit of chemically modified cellulose can also be measured according to known methods such as those described in JP-A-2019-104833.

3) Fibrillation MFC is obtained by defibrating or beating the chemically modified cellulose raw material. Defibration or beating in the fibrillation is preferably carried out in a wet manner (i.e., in the form of a dispersion in water or the like) using a refiner such as a disk-type, conical-type, or cylindrical-type, a high-speed defibrator, a shear-type mixer, a colloid mill, a high-pressure jet disperser, a beater, a PFI mill, a kneader, a disperser, or the like, but is not particularly limited to these devices, and any device that imparts mechanical defibration force in a wet manner may be used.

In the present invention, it is preferable to use DDR-treated MFC, which is obtained by treating and fibrillating the raw material, chemically modified cellulose, using a double disc refiner (sometimes abbreviated as "DDR"), a type of disc-type refiner (disc refiner). Because DDR-treated MFC is highly fibrillated, paper containing DDR-treated MFC in the clear coating layer exhibits high print surface strength. The treatment performed by this device may be a beating treatment or a defibration treatment, with beating being preferred.

<Disc refiner>
A disc refiner is a device in which two discs (disc plates (sometimes simply called "discs")) with beating blades face each other at close range, with either one or both rotating in opposite directions at a predetermined rotation speed, providing the slurry passing between them with a pressurized beating effect and a continuous feeding effect through centrifugal force. Disc refiners with two beating gaps formed by the disc plates are called double disc refiners (sometimes abbreviated as "DDR").

<Double disc refiner>
A double disc refiner has 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 the DA and DB and the DM rotating in opposite directions. Examples of double disc refiners with fixed discs, DA and DB, and the DM being 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 this embodiment, the beating process using a double disc refiner may be a circulating process, or a continuous process in which the beating process is carried out continuously using multiple double disc refiners. When the beating process using a double disc refiner is a circulating process or a continuous process, the number of passes of the beating process is not limited as long as the desired MFC is obtained. However, from the viewpoint of productivity and from the viewpoint of preventing excessive fiber shortening and deterioration due to heat generated during the process, 30 passes or less is preferable, 20 passes or less is more preferable, 10 passes or less is even more preferable, and 5 passes or less is even more preferable.

When the beating process is a circulation process and partial circulation is used, the number of passes of the beating process is the number of times the raw material to be processed passes through one double disc refiner and is recovered multiplied by n as follows:
n=1/(1−a)
Here, a = circulation rate. (When the circulation rate is 50%, a = 0.5, and when the circulation rate is 75%, a = 0.75.)

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

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

When multiple double disc refiners are used for continuous beating, the number of passes in the beating process is counted as one each time the raw material passes through one double disc refiner.

The blade width of the two discs 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 0 to 40° is preferred, and 5 to 20° is particularly preferred.

Double disc refiners are broadly divided 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 closest 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 perspective of efficient fiberization.

In one embodiment, a double-disc refiner has a raw material inlet, a beating chamber that beats the pulp raw material fed through the inlet, and a rotating disc DM placed within the chamber. Blades with predetermined blade widths and groove widths are attached to both sides of the rotating disc DM. Fixed discs DA and DB are placed on the inner wall of the beating chamber, facing the blades attached to both sides of the rotating disc DM. Blades with predetermined blade widths and groove widths are attached to the fixed discs DA and DB. The rotating disc DM is attached to a drive shaft, which is connected to a motor and driven to rotate. The beating chamber is equipped with a circulation pipe for circulating the beaten raw material back into the beating chamber, a discharge port for discharging the beaten raw material outside the beating chamber, and an outlet pipe for sending the material from the discharge port to a recovery tank. The flow rates in the circulation pipe and outlet pipe are adjusted by manual valves.

When the raw material flow method is a monoflow type, the following advantages are obtained depending on the relationship between the blade width (X1) of the first disk DA, counting from the raw material inlet side, and the blade width (X2) of the second disk DB, of the two disks DA and DB.
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 the wide blade, and then beaten and cut by the narrow blade, making it difficult for the raw material to pass through unfibrillated.
X1<X2: After the raw material is beaten with narrow blades and cutting progresses, it is beaten on wide blades and fibrillation progresses, so that fine fibers with advanced fibrillation are likely to be obtained.

Furthermore, when the raw material flow method is a monoflow method, the relationship between the groove width (Y1) of the first disk DA, counting from the raw material inlet 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: The raw material passes through a wide beating gap and then a narrow beating gap, so that excessive load is not easily 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.

As operating conditions for the double disc refiner, 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 lower limit, but to avoid metal touch, 0.02 mm or more is preferred. The operating temperature is preferably 5 to 120°C. The flow rate, processing time, and other conditions are adjusted as appropriate to obtain chemically modified microfibril cellulose fibers with the desired fiber width.

Chemically modified cellulose is subjected to the beating process as an aqueous dispersion, and from the perspective of transportation, the solids concentration at this time is usually 15% by weight or less, preferably 0.3 to 10% by weight, and more preferably 0.5 to 6% by weight. If the solids concentration is too low, it is difficult for the cellulose to come into contact with the blades of the beating plate during beating, resulting in reduced beating efficiency. The solids concentration may fluctuate 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 during the beating process.

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

Before or after the beating process using a double disc refiner, one or more mechanical treatment steps may be performed using a device other than a double disc refiner. Also, before or after the beating process, a mechanical treatment step may be performed on an aqueous dispersion of chemically modified cellulose having a solids concentration of more than 15% by weight.

The mechanical treatment refers to mixing fibers and further refining or fibrillating them, and includes beating, defibrating, dispersing, kneading, etc. Here, mechanical treatment under conditions where the solids concentration is higher than 15% by weight is sometimes referred to as "high-concentration mechanical treatment," and when the mechanical treatment is beating, it is also referred to as "high-concentration beating." Similarly, mechanical treatment under conditions where the solids concentration is 15% by weight or less is sometimes referred to as "low-concentration mechanical treatment," and when the mechanical treatment is beating, it is also referred to as "low-concentration beating." In the present invention, it is preferable to perform the beating treatment step at a solids concentration of 15% by weight or less at least once using a double-disc refiner, and in this case, mechanical treatment may be performed multiple times.

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

Equipment that can be used for low-concentration mechanical treatment includes, 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 homomic line mills, which use a metal or blade to act on the pulp fibers around a rotating shaft, or by friction between the pulp fibers, or by cavitation, water flow, or water pressure to disperse or defibrate the pulp fibers.

Equipment that can be used for high-concentration mechanical processing includes, for example, high-speed rotary, colloid mill, high-pressure, roll mill, and ultrasonic types of equipment, and can be used to treat pulp fibers with metal or blades around a rotating shaft, or by using friction between pulp fibers themselves, such as high-pressure or ultra-high-pressure homogenizers, refiners, beaters, PFI mills, kneaders, dispersers, and top finers.

The preferred average fiber diameter (average fiber width) of DDR-treated MFC is as described above. The average fiber length of DDR-treated MFC is preferably 10 μm or more, more preferably 30 μm or more, even more preferably 50 μm or more, and may be 100 μm or more, 200 μm or more, or 300 μm or more. There are no particular limitations on the upper limit of the average fiber length, but it is preferably 3000 μm or less, more preferably 1500 μm or less, even more preferably 900 μm or less, and even more preferably 500 μm or less. Because the chemically modified cellulose raw material is processed in a double-disc refiner, fibrillation can be promoted without extremely shortening the fibers. In addition, because the chemical modification improves affinity with water, high water retention can be achieved even with long fiber lengths.

The solids concentration of the raw material in the dispersion of chemically modified cellulose raw material to be subjected to fibrillation is preferably 0.1% by weight or more, more preferably 0.5% by weight or more, even more preferably 1.0% by weight or more, and even more preferably 2.0% by weight or more. The upper limit of the concentration is preferably 40% by weight or less, and even more preferably 30% by weight or less.

Before preparing the dispersion for fibrillation, the chemically modified cellulose raw material obtained by the above method may be dried and pulverized in advance. The dry-pulverized chemically modified cellulose raw material may then be dispersed in a dispersion medium and subjected to (wet) fibrillation. The equipment used for dry-pulverizing the raw material is not particularly limited, and examples include impact mills such as hammer mills and pin mills, media mills such as ball mills and tower mills, and jet mills.

As mentioned above, fibrillation is carried out in a range that maintains an average fiber diameter of more than 500 nm, preferably 1 μm or more, and more preferably 10 μm or more. The upper limit of the average fiber diameter is preferably 60 μm or less, more preferably 40 μm or less, even more preferably 30 μm or less, and even more preferably 20 μm or less. By performing moderate fibrillation to an extent that the average fiber diameter falls within this range, the cellulose exhibits higher water retention than undefibrated cellulose fibers, and even in small amounts, it is possible to obtain higher strength-improving effects and improved yields compared to finely defibrated cellulose nanofibers.

As described above, it is preferable to carry out fibrillation so as to improve the fibrillation rate (f ₀ ) of the chemically modified cellulose raw material before fibrillation. If the fibrillation rate of MFC is f, the difference in fibrillation rates Δf = f - f ₀ should be greater than 0, preferably 0.2% or more, more preferably 1% or more, and even more preferably 2.5% or more. The fibrillation rate (Fibrillation %) can be measured using a fractionator manufactured by Valmet Co., Ltd.

(2) Starch Starch is a polymer of D-glucose, preferably a mixture of amylose and amylopectin. In this embodiment, starch also includes polymeric compounds derived from starch. Examples of such polymers include starch that has been denatured, modified, or processed. In the present invention, oxidized starch in which an anionic group has been introduced by an oxidation reaction is preferred. Examples of the anionic group include a carboxyl group.

Oxidized starch is a white powder or granular substance that does not dissolve in cold water. Its aqueous suspension, except at very low oxidation levels, has a lower gelatinization temperature as the oxidation level increases, forming a low-viscosity, transparent paste. Oxidized starch is typically produced by oxidizing starch in a wet or dry reaction using an oxidizing agent such as hypochlorite (preferably sodium), bleaching powder, hydrogen peroxide, potassium permanganate, or ozone. Depending on the oxidation conditions, oxidized starch typically has carboxyl and carbonyl groups and a structure in which glycosidic bonds have been cleaved.

(3) Metal Salts The metal salts that can be used in this embodiment are not limited, and examples include magnesium salts, calcium salts, aluminum salts, sodium salts, and potassium salts. Among these, metal salts containing divalent or higher metal elements, such as magnesium salts, calcium salts, and aluminum salts, are preferred because the metal elements form chelates with anionic groups present in CNF or starch, improving the strength of the coating film. The counter ions are also not limited, but are preferably other than organic acid ions from the standpoints of availability, water solubility, and the ease with which metal elements ionized in water form the chelates. Examples of counter ions include inorganic acid ions such as sulfate ions, chloride ions, nitrate ions, and carbonate ions, as well as hydroxide ions. Therefore, specific examples of preferred metal salts include aluminum sulfate (aluminum sulfate), magnesium sulfate, calcium chloride, magnesium chloride, and aluminum chloride.

(4) Base Paper Base paper is the base layer of paper and contains pulp as a main component. The pulp raw material for the base paper is not particularly limited, and mechanical pulps such as ground pulp (GP), thermomechanical pulp (TMP), and chemithermomechanical pulp (CTMP), and chemical pulps such as deinked pulp (DIP), softwood kraft pulp (NKP), and softwood kraft pulp (LKP), can be used. Deinked (recycled paper) pulp can be derived from sorted waste paper such as fine paper, medium-quality paper, low-grade paper, newspaper, flyers, and magazines, or unsorted waste paper that is a mixture of these.

While known fillers can be added to base paper, they may not be necessary for applications that do not require high opacity or whiteness, such as paperboard, or when using raw materials with a high ash content, such as recycled paper. If fillers are added, examples include inorganic fillers such as heavy calcium carbonate, light calcium carbonate, clay, silica, light calcium carbonate-silica composite, kaolin, calcined kaolin, delaminated kaolin, magnesium carbonate, barium carbonate, barium sulfate, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide, and amorphous silica produced by neutralizing sodium silicate with mineral acid; and organic fillers such as urea-formaldehyde resin, melamine resin, polystyrene resin, and phenolic resin. These may be used alone or in combination. Among these, calcium carbonate and light calcium carbonate, which are typical fillers used in neutral and alkaline papermaking and provide high opacity, are preferred. The filler content in the base paper is preferably 5 to 25% by weight, and more preferably 6 to 20% by weight, based on the weight of the base paper. In this invention, a decrease in paper strength is suppressed even if the ash content in the paper is high, so the filler content in the base paper is more preferably 10% by weight or more.

Internal additives such as bulking agents, dry strength agents, wet strength agents, drainage aids, dyes, and neutral sizing agents may be used as needed.

The base paper is produced using a known papermaking method. For example, it can be produced using a Fourdrinier paper machine, a gap former paper machine, a hybrid former paper machine, an on-top former paper machine, a cylinder paper machine, etc., but is not limited to these.

The base paper may be single-ply or multi-ply. The base paper may contain the above-mentioned MFC. In the case of multi-layer base paper, some or all of the multiple paper layers may contain MFC. If the base paper contains MFC, its content is preferably 0.0001% by weight or more, more preferably 0.0003% by weight or more, and even more preferably 0.001% by weight or more, based on the pulp weight of the entire base paper.

(5) Clear Coating Layer The starch:MFC (weight ratio) in the clear coating layer is preferably 1000:1 to 20:1, more preferably 350:1 to 30:1, and even more preferably 300:1 to 50:1. The amount of metal salt is preferably 0.05 to 1 part by weight, more preferably 0.1 to 0.5 parts by weight, per 100 parts by weight of starch. Having the weight ratio of each component within this range improves the film-forming properties of the starch-based clear coating layer, and as a result, high print surface strength can be achieved.

The coating weight of the clear coating layer is preferably 0.01 to 3.0 g/ ^m2 , more preferably 0.1 to 2.0 g/ ^m2 , in terms of solids content per side. The clear coating can be formed by applying a clear coating liquid containing starch as a primary component to the base paper using a coater (coating machine) such as a size press, gate roll coater, premetering size press, curtain coater, or spray coater. For example, when coating with a gate roll coater, the clear coating liquid preferably has a B-type viscosity (30°C, 60 rpm) of 5 to 450 mPa·s at a solids concentration of 5 wt%, from the standpoint of coatability, and more preferably 10 to 300 mPa·s. When coating with a gate roll coater, if the B-type viscosity of the clear coating liquid is less than 5 mPa·s, the viscosity is too low, making it difficult to ensure the required coating weight, while if it exceeds 450 mPa·s, boiling may occur, resulting in poor operability. The solids concentration of the clear coating liquid is adjusted so as to achieve the above concentration, and is preferably 2 to 14% by weight.

The amount of MFC derived from the clear coating layer is preferably 1.0×10 ⁻⁵ to 0.1 g/m ² per side, more preferably 1.0×10 ⁻⁴ to 5.0×10 ⁻² g/m ² .

(6) Pigment Coating Layer The paper of this embodiment may have a pigment coating layer. The pigment coating layer is a layer containing a white pigment as a main component. Examples of white pigments include commonly used pigments such as calcium carbonate, kaolin, clay, calcined kaolin, amorphous silica, zinc oxide, aluminum oxide, satin white, aluminum silicate, magnesium silicate, magnesium carbonate, titanium oxide, and plastic pigments. Examples of calcium carbonate include light calcium carbonate and heavy calcium carbonate.

The pigment coating layer contains an adhesive. Examples of such adhesives include proteins such as the starch, casein, soy protein, and synthetic protein mentioned above; polyvinyl alcohol; cellulose derivatives such as carboxymethyl cellulose and methyl cellulose; conjugated diene polymer latexes such as styrene-butadiene copolymer and methyl methacrylate-butadiene copolymer; acrylic polymer latex; and vinyl polymer latexes such as ethylene-vinyl acetate copolymer. These can be used alone or in combination of two or more types, and it is preferable to use a starch-based adhesive in combination with a styrene-butadiene copolymer.

The pigment coating layer may contain various auxiliary agents commonly used in paper manufacturing, such as dispersants, thickeners, antifoaming agents, colorants, antistatic agents, and preservatives, and may also contain MFC. In this case, the amount of MFC is preferably 1 x 10 ^-3 to 1 part by weight per 100 parts by weight of pigment. Within this range, a pigment coating liquid with appropriate water retention properties can be obtained without significantly increasing the viscosity of the coating liquid.

The pigment coating layer can be formed by applying a coating solution to one or both sides of the base paper using a known method. From the viewpoint of coating suitability, the solids concentration in the coating solution is preferably about 30 to 70% by weight. The pigment coating layer may consist of one layer, two layers, or three or more layers. The coating weight of the pigment coating layer may be adjusted appropriately depending on the application, but when used as coated paper for printing, it is 5 g/ ^m2 or more in total per side, and preferably 10 g/ ^m2 or more. The upper limit is preferably 30 g/ ^m2 or less, and more preferably 25 g/ ^m2 or less.

When the paper in this embodiment further comprises a pigment coating layer, it is possible to obtain pigment coated paper that has excellent surface strength and printing gloss in addition to high ink mileage.

(7) Properties: The paper of this embodiment has high print surface strength. The reason for this is presumably, but not limited to, that functional groups such as COOH groups in starch and MFC form crosslinked structures with metal ions, thereby improving the strength of the clear coating layer. Furthermore, the basis weight of the paper of this embodiment, measured in accordance with JIS P 8124, is typically about 20 to 500 g/ ^m² , preferably 30 to 250 g/ ^m² .

(8) Method for Producing Paper of the Present Embodiment The paper of the present embodiment is preferably produced by a process of coating a clear coating liquid containing starch, MFC, and a metal salt onto base paper prepared by a known method. Specifically, the paper of the present embodiment is preferably produced by a process comprising the following steps:
Step 1: A step of preparing a clear coating liquid containing starch, MFC, and a metal salt. Step 2: A step of forming a clear coating layer on a base paper using the clear coating liquid.

The starch, MFC, and metal salt used in step 1 are as described above. The method for preparing the coating liquid and its properties are also as described above. The coating in step 2 can also be carried out as described above. Step 1 preferably includes the following steps:
Step 1A: A step of preparing a mixed solution containing starch and MFC. Step 1B: A step of preparing a clear coating solution containing the mixed solution and a metal salt. The starch in Step 1A is preferably cooked starch. Step 1A may also be prepared by adding starch to an aqueous dispersion of MFC. In this case, the mixed solution can also be prepared by adding starch to an aqueous dispersion of MFC and subjecting the resulting solution to cooking. In the clear coating solution prepared in this manner, the starch and MFC are uniformly dispersed.

The paper of this embodiment may be produced by a method including the following step 3 in addition to the above steps 1 and 2.
Step 3: Forming a pigment coating layer containing a pigment and an adhesive on the clear coating layer containing starch, MFC, and metal salt.Step 3 can be carried out by a known method.

[Example 1]
130 parts by weight of water and a solution of 20 parts by weight of sodium hydroxide dissolved in 100 parts by weight of water were added to a twin-screw kneader whose rotation speed was adjusted to 100 rpm, and 100 parts by weight of hardwood bleached kraft pulp (LBKP, manufactured by Nippon Paper Industries Co., Ltd.) was added in a dry weight amount when dried at 100°C for 60 minutes.

The contents were stirred and mixed at 30°C for 90 minutes to prepare a mercerized cellulose raw material. While stirring, 100 parts by weight of isopropanol (IPA) and 60 parts by weight of sodium monochloroacetate were added, and after stirring for 30 minutes, the temperature was raised to 70°C and the carboxymethylation reaction was carried out for 90 minutes. The concentration of IPA in the reaction medium during the carboxymethylation reaction was 30%.

After the reaction was complete, the mixture was neutralized with acetic acid to a pH of approximately 7, yielding a carboxymethylated cellulose raw material (sodium salt) with a carboxymethyl substitution degree per glucose unit of chemically modified cellulose of 0.21 and a crystallinity degree of cellulose type I of 72%. The method for measuring the crystallinity of cellulose type I was as described above. The carboxymethyl substitution degree per glucose unit of chemically modified cellulose was measured according to the known method described above.

An aqueous dispersion of the resulting carboxymethylated cellulose raw material with a solids concentration of 2% by weight was prepared and treated for 10 minutes using a Top Finer manufactured by Aikawa Iron Works Co., Ltd. to produce fibrillated carboxymethyl cellulose fibers (CM-MFC).

<Clear coating liquid 1>
Oxidized starch (SK20, manufactured by Nippon Cornstarch Co., Ltd.) and aluminum sulfate (aluminum sulfate) were added to the aqueous dispersion of CM-MFC prepared as described above to prepare Clear Coating Liquid 1, which had a weight ratio of starch:CM-MFC:metal salt of 100:0.33:0.33. Table 1 shows the Brookfield viscosity of Clear Coating Liquid 1 at 30°C and 60 rpm when the solids concentration was 5% by weight.

<Pigment coating liquid>
To 100 parts by weight of ground calcium carbonate, 2.0 parts by weight of latex as an adhesive and 6.7 parts by weight of oxidized starch were added to prepare a pigment coating liquid with a solid content of 60% by weight.

<Paper>
To LBKP (manufactured by Nippon Paper Industries Co., Ltd., c.s.f. 360 ml), 0.5 wt. % aluminum sulfate, 0.77 wt. % cationized starch, and 0.05 wt. % paper strength agent were added to prepare a pulp slurry with a solids concentration of 0.7 wt. Base paper was produced using the obtained pulp slurry on a paper machine. The clear coating liquid 1 was applied to both sides of the base paper using a gate roll coater to a solids content of 0.2 g/ ^m2 per side. The pigment coating liquid was then applied to both sides to a solids content of 7.5 g/ ^m2 per side, and the resulting paper was dried by a standard method to obtain coated paper. The coated paper was evaluated using the methods described below.

[Comparative Example 1]
Coated paper was produced and evaluated in the same manner as in Example 1, except that no metal salt was used.

[Example 2]
According to a standard method, bleached softwood kraft pulp (NBKP, manufactured by Nippon Paper Industries Co., Ltd.) was oxidized using TEMPO to obtain a carboxylated cellulose raw material with a carboxyl group content of 1.42 mmol/g. An aqueous dispersion of the obtained carboxylated cellulose raw material with a solids concentration of 2% by weight was prepared, and the dispersion was treated for 10 minutes using a Top Finer manufactured by Aikawa Iron Works Co., Ltd. to prepare fibrillated carboxylated cellulose fibers (TEMPO-MFC).

<Clear coating liquid 2>
Clear coating liquid 2 was prepared in the same manner as clear coating liquid 1, except that TEMPO-MFC was used instead of CM-MFC.
<Paper>
A coated paper was produced and evaluated in the same manner as in Example 1, except that Clear Coating Liquid 2 was used instead of Clear Coating Liquid 1.

[Comparative Example 2]
Coated paper was produced and evaluated in the same manner as in Example 2, except that no metal salt was used.
The evaluation results are shown in Table 1.

It is clear that the coated paper of the present invention has excellent surface printing strength.

<Evaluation method>
1) Basis weight: Based on JIS P8124.
2) Print Glossiness Using a Roland offset sheet-fed printing press (four colors), offset sheet-fed inks (NEX-M manufactured by Toyo Ink Co., Ltd.) were used to print in the order of blue and red (CM) at a printing speed of 8,000 sheets/hr so that the ink density of the solid area was 1.60 blue and 1.50 red. The glossiness of the blue and red (CM) solid printed area of the obtained print was measured in accordance with JIS P-8142.

3) Picking Evaluation Using a Roland offset sheet-fed printing press, a solid blue image was printed at a speed of 8000 sph using LeoEcho Y Indigo Ink manufactured by Toyo Ink Co., Ltd. The number of picks that occurred on the F and W sides during printing of 10 sheets was measured.

Claims (7)

A paper comprising a base paper and a clear coating layer,
the clear coating layer comprises starch, fibrillated chemically modified cellulose fibers, and a metal salt;
the weight ratio of the starch to the fibrillated chemically modified cellulose fibers is 1000:1 to 20:1;
the amount of the metal salt is 0.05 to 1 part by weight based on 100 parts by weight of the starch;
The fibrillated chemically modified cellulose fibers have a cellulose I type crystallinity of 50% or more, a degree of anionization of 0.10 to 2.00 meq/g, and an average fiber diameter of more than 500 nm. The paper according to claim 1, wherein the starch is oxidized starch. Paper according to claim 1 or 2, wherein the metal salt contains a divalent or higher metal element. The paper according to any one of claims 1 to 3, further comprising a pigment coating layer. The paper according to any one of claims 1 to 4, wherein the fibrillated chemically modified cellulose fibers are anionically modified. The paper according to any one of claims 1 to 5, wherein the fibrillated chemically modified cellulose fibers have 0.1 to 3.0 mmol/g of carboxyl groups based on the bone dry weight of the fibrillated chemically modified cellulose fibers. The paper according to any one of claims 1 to 5, wherein the chemically modified cellulose in the fibrillated chemically modified cellulose fibers is a carboxyalkylated cellulose having a degree of carboxyalkyl substitution per glucose unit of the chemically modified cellulose of 0.01 to 0.50.