AU2019244030A1 - Paper comprising microfibrillated cellulose fibers - Google Patents

Abstract

Provided is paper containing microfibril cellulose fiber that has an average fiber diameter of 500 nm or more and is chemically modified by mechanical treatment.

Description

DESCRIPTION PAPER COMPRISING MICROFIBRILLATED CELLULOSE FIBERS TECHNICAL FIELD

[0001] The present invention relates to paper comprising microfibrillated cellulose fibers.

BACKGROUND ART

[0002] Paper is used in various fields, such as printing paper, recording paper, or other

information recording medium uses as well as packaging uses. For any of these uses,

sufficient strength is required during use or processing. For the purpose of improving the

strength or stiffness of paper, Patent Literature (PTL) 1, for example, discloses a paper

substrate added with oxidized pulp and PTL 2 discloses a paper product comprising a co

processed microfibrillated cellulose and inorganic particle composition.

CITATION LIST PATENT LITERATURE

[0003] PTL 1: WO 2014/097929

PTL 2: Japanese Unexamined Patent Application Publication No. 2017-203243

SUMMARY OF INVENTION TECHNICAL PROBLEM

[0004] PTL 1 uses oxidized pulp, but the oxidized pulp exhibits poor dispersibility in a

stock and less-than-satisfactory air resistance and reinforcing effects of paper. PTL 2 uses

microfibrillated cellulose obtained by mechanically treating unmodified pulp together with

inorganic particles. The microfibrillated cellulose also exhibits poor dispersibility in a stock

and less-than-satisfactory air resistance and reinforcing effects of paper. In view of this, an

object of the present invention is to provide paper having excellent strength and air

resistance.

SOLUTION TO PROBLEM

[0005] In chemically modified cellulose, compared with unmodified cellulose, it is possible

to efficiently release cellulose fibers due to electrostatic repulsion of introduced functional groups. For this reason, when chemically modified cellulose is beaten, further fibrillated further shortened microfibrillated cellulose is obtained compared with the case of beating chemically unmodified cellulose. The present inventors thus found that such microfibrillated cellulose exhibits satisfactory dispersibility when added to a stock and that paper obtained from the stock has excellent mechanical strength and high air resistance.

Specifically, the above-mentioned problems are resolved by the following present invention.

(1) Paper comprising mechanically-treated chemically-modified microfibrillated

cellulose fibers having a mean diameter of fibers of 500 nm or more.

(2) The paper according to (1), where the microfibrillated cellulose fibers have a

mean fibril index of 4.0 or more as measured by a fiber analyzer.

(3) The paper according to (1) or (2), where the microfibrillated cellulose fibers have

a cellulose I crystallinity of 50% or more.

(4) The paper according to any one of (1) to (3), where the chemical modification is

anionic modification.

(5) The paper according to any one of (1) to (4), where the paper has a pigment

coating layer.

(6) The paper according to any one of (1) to (5), where the paper has a clear coating

layer.

(7) The paper according to any one of (1) to (6), where the paper when conditioned

in accordance with JIS P 8111 at 23°C/50 2% has a water content of 10% by weight or less.

(8) A production method for the paper according to any one of (1) to (7),

comprising: preparing the microfibrillated cellulose fibers by wet grinding a cellulose raw

material; and preparing a stock containing the microfibrillated cellulose fibers.

(9) The production method according to (8), further comprising chemically

modifying the cellulose raw material before the wet grinding.

ADVANTAGEOUS EFFECTS OF INVENTION

[0006] According to the present invention, it is possible to provide paper having excellent

strength and air resistance.

BRIEF DESCRIPTION OF DRAWINGS

[0007] Fig. 1 shows pulp (COOH amount of 0.3 mmol/g, CSF of 573 mL) solely subjected

to chemical modification.

Fig. 2 shows MFC (COOH amount of 0.3 mmol/g, CSF of 67 mL) obtained through

beating treatment with an SDR after chemical modification.

DESCRIPTION OF EMBODIMENTS

[0008] Hereinafter, the present invention will be described in detail. In the present

invention, the wording "X to Y" includes the lower and the upper limits of X and Y.

[0009] 1. Paper Comprising Microfibrillated Cellulose Fibers

The paper of the present invention includes a base paper layer and may include one

or more coating layers. Such a coating layer may be a pigment coating layer containing an

inorganic pigment and an adhesive or a clear coating layer containing an adhesive as a main

component but not inorganic pigment. The paper of the present invention may contain

microfibrillated cellulose fibers in any of these layers that constitute the paper. For

example, microfibrillated cellulose fibers are contained in a base paper layer, a pigment

coating layer, or a clear coating layer. These layers will be described hereinafter.

[0010] (1) Microfibrillated Cellulose Fibers

Microfibrillated cellulose fibers (hereinafter, also referred to as "MFC") are fibers

having a mean diameter of fibers of 500 nm or more obtained by fibrillating a cellulosic raw

material, such as pulp. Different from cellulose nanofibers refined to a diameter of fiber of

less than 500 nm, the microfibrillated cellulose of the present invention has a mean diameter

of fibers of 500 nm or more sincefibrillation of the fiber surface is efficiently promoted

while maintaining to some extent the shape of pulp fibers. The mean diameter of fibers is

preferably more than 1Ipm, more preferably 2 pm or more, and further preferably 10 Pm or

more. The upper limit is preferably 60 pm or less. The mean diameter of fibers in the

present invention indicates a length-weighted mean diameter of fibers, and such a diameter of

fiber can be measured with a fiber tester from ABB Ltd. MFC is obtained through

relatively mild mechanical treatment, such as refining or beating, of a chemically modified cellulosic raw material with a beater, a disperser, a refiner, or the like. Accordingly, compared with cellulose nanofibers having a mean diameter of fibers of single nano to about less than 500 nm obtained through vigorous refining treatment of a cellulosic raw material,

MFC has a large diameter of fiber and a shape of afiber surface that has been efficiently

fluffed (externally fibrillated) while suppressing refining of individual fibers (internal

fibrillation).

[0011] The mechanically-treated chemically-modified cellulose fibers (hereinafter, also

referred to as "mechanically-treated chemically-modified MFC") used in the present

invention may be obtained by subjecting pulp to mechanical treatment after chemical

modification or by subjecting mechanically treated pulp to chemical modification but is

preferably obtained by the former in view offibrillation efficiency. In other words, since

the mechanically-treated chemically-modified MFC is obtained through relatively mild

mechanical treatment, such as refining or beating, of a chemically modified cellulosic raw

material, strong hydrogen bonding existing between fibers weakens. Accordingly,

compared with MFC that has solely underwent mechanical refining or beating treatment

without chemical modification, fibers are readily released and thus have less damage and a

shape reflecting moderate internal fibrillation and external fibrillation (see Fig. 2).

Moreover, an aqueous dispersion obtained by dispersing the mechanically-treated

chemically-modified MFC in water exhibits high hydrophilicity, water retention, and

viscosity.

[0012] As in the foregoing, MFC is different from a cellulosic raw material in the degree of

fibrillation. In general, it is not easy to quantify the degree of fibrillation. However,inthe

present invention, it was found possible to quantify the degree of fibrillation from changes in

freeness or water retention value before and after mechanical treatment for MFC. MFC of

the present invention is preferably obtained by mechanically refining or beating pulp to the

degree that the freeness (Fo) of pulp before the mechanical treatment is reduced by 10 mL or

more. In other words, a difference in freeness (AF = Fo - F, where F is freeness after

treatment) is preferably 10 mL or more, more preferably 20 mL or more, and further preferably 30 mL or more. Although the freeness of pulp varies depending on the degree of modification, it is possible to determine the degree of fibrillation independently from the degree of chemical modification by such a definition based on the freeness of raw material pulp. As just mentioned, since Fo varies depending on the degree of modification of pulp, it is difficult to set a single upper limit for AF. However, the freeness F after treatment is preferably more than 0 mL. Since vigorous mechanical refining is required to obtain MFC having F of 0 mL, the resulting MFC is likely to have a mean diameter of fibers of less than

500 nm (cellulose nanofibers). Moreover, when a large amount of MFC having a freeness

of 0 mL is added to the papermaking process, there is a risk of worsening the drainage of a

pulp slurry for papermaking. The mean fibril index of the mechanically-treated chemically

modified MFC of the present invention is preferably 4.0 or more, more preferably 4.5 or

more, further preferably 5.0 or more, and most preferably 8.0 or more. The mean fibril

index is a value calculated as an item, such as "mean fibril area," "mean fibril index," or

"fibril area," when fibers are measured by a fiber analyzer and is an indicator of the degree of

fibrillation of main fibers. The mean fibril index can be measured by Fiber Tester or Fiber

Tester Plus from ABB Ltd., for example, and is preferably defined as "meanfibril area"

measured by Fiber Tester Plus in the present application.

[0013] The lower limit of Canadian standard freeness for the mechanically-treated

chemically-modified MFC of the present invention is not limited but is preferably more than

mL. The upper limit of the freeness is preferably 500 mL or less, more preferably 350 mL

or less, still more preferably 150 mL or less, and particularly preferably 100 mL or less.

Cellulose nanofibers refined to a single nano-level typically has a Canadian standard freeness

of 0 mL.

[0014] As mentioned above, the degree offibrillation of the mechanically-treated

chemically-modified MFC of the present invention can also be quantified by an increase in

water retention value (H). The mechanically-treated chemically-modified MFC of the

present invention is preferably obtained by mechanically refining or beating pulp to the

degree that a difference in water retention value (AH = HO - H), which is defined as a difference between a water retention value (HO) of pulp before treatment and a water retention value (H) of the pulp after the treatment, increases by 10% or more and is more preferably obtained by mechanically refining or beating pulp to the degree that AH increases by 50% or more. In chemically modified MFC, the water retention value varies depending on pH as a ratio of the free acid form to the Na form varies. For this reason, a water retention value is preferably measured under the same pH conditions before and after refining. The water retention value of the mechanically-treated chemically-modified MFC of the present invention is preferably 210% or more, more preferably 250% or more, and further preferably 500% or more. When the water retention value falls within these ranges, it is possible to obtain the effects of the present invention at a high level due to sufficient fibrillation of the mechanically-treated chemically-modified MFC.

[0015] In the mechanically-treated chemically-modified MFC of the present invention, the

degree of fibrillation can be assessed by the freeness in the case of a low degree of

fibrillation. However, in the case of vigorous fibrillation, an apparent freeness increases in

some cases since fibers are fibrillated and simultaneously shortened and the resulting fibers

pass through a mesh. In such a case, since it is impossible to accurately assess the degree of

fibrillation by the freeness, assessment by a change in water retention value is preferable.

Specifically, the mechanically-treated chemically-modified MFC of the present invention is

preferably chemically modified cellulose subjected to mechanical treatment to satisfy AF of

mL or more or AH of 10% or more.

[0016] When the mechanically-treated chemically-modified MFC of the present invention

has a high cellulose I crystallinity, the strength of the MFC is high and the strength of paper

containing the MFC is also enhanced. In this view, the cellulose I crystallinity is preferably

% or more and more preferably 50% or more. The upper limit of the cellulose I

crystallinity is not limited but is preferably 90% or less. The cellulose I crystallinity can be

measured by X-ray diffraction. For example, mechanically-treated chemically-modified

MFC is freeze-dried using liquid nitrogen and compressed to prepare a tablet-shaped pellet.

Subsequently, the sample is analyzed with an X-ray diffractometer (X'pert PRO MPD from

PANalytical B.V.) and peaks of the resulting graph are separated using PeakFit graph

analysis software (from Hulinks Inc.) to obtain a crystal type I crystallinity based on the

following diffraction angles.

Crystal type I: 20= 14.8, 16.80, 22.6

Crystal type II: 20= 12.1°, 19.80, 22.0

[0017] 1) Cellulosic Raw Material

Exemplary cellulosic raw materials include, but are not particularly limited to, those

derived from plants, animals (ascidian, for example), algae, microorganisms (Acetobacter,

for example), and materials produced by microorganisms. Examples of those derived from

plants include wood, bamboo, hemp, jute, kenaf, farmland residues and wastes, cloth, and

pulp (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), thermomechanical

pulp (TMP), recycled pulp, wastepaper, and so forth). The cellulose raw material used in

the present invention may be any of these materials or combinations thereof but is preferably

cellulose fibers derived from plants or microorganisms and more preferably cellulose fibers

derived from plants.

[0018] A mean diameter of fibers of cellulose fibers is not particularly limited but, in the

case of common pulp, is about 30 to 60 pm for softwood kraft pulp and about 10 to 30 Pm

for hardwood kraft pulp. In the case of other pulps, a mean diameter of fibers after common

refining is about 50 pm. For example, when a refined raw material of several centimeter

size, such as chips, is used, the mean diameter of fibers is preferably adjusted to about 50 Pm

or less and more preferably to about 30 pm or less through mechanical treatment with a

disintegrator, such as a refiner or a beater. Hereinafter, a production method for the

mechanically-treated chemically-modified MFC will be described.

[0019] 2) Chemical Modification

Chemical modification herein refers to introduction of a functional group into a

cellulosic raw material. In the present invention, an anionic group is preferably introduced.

Exemplary anionic groups include acid groups, such as a carboxy group, a carboxy

containing group, a phosphate group, and a phosphate-containing group. Exemplary

carboxy-containing groups include -COOH group, -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).

Exemplary phosphate-containing groups include a polyphosphate group, a phosphorous acid

group, a phosphonic acid group, and polyphosphonic acid group. These acid groups are

introduced in the form of salts (carboxylate group (-COOM, M is a metal atom), for example)

in some cases depending on reaction conditions. In the present invention, chemical

modification is preferably oxidation or etherification. Hereinafter, oxidation and

etherification will be described in detail.

[0020] [Oxidation]

Oxidized cellulose is obtained by oxidizing a cellulose raw material. The oxidation

method is not particularly limited and examples include a method of oxidizing a cellulose

raw material in water using an oxidant 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 hydroxy group at the C-6 position of glucopyranose

ring on cellulose surface is selectively oxidized to generate a group selected from an

aldehyde group, a carboxy group, and a carboxylate group. The concentration of a cellulose

raw material during the reaction is not particularly limited but is preferably 5% by weight or

less.

[0021] AnN-oxyl compound is a compound that can generate a nitroxyl radical.

Exemplary nitroxyl radicals include (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). Any

N-oxyl compound may be used provided that the compound promotes intended oxidation

reactions. The amount of an N-oxyl compound used is not particularly limited provided that

the amount is a catalytic amount that enables oxidation of raw material cellulose. For

example, the amount is preferably 0.01 mmol or more and more preferably 0.02 mmol or

more based on 1 g of bone-dry cellulose. The upper limit is preferably 10 mmol or less,

more preferably 1 mmol or less, and further preferably 0.5 mmol or less. Accordingly, the amount of an N-oxyl compound used is preferably 0.01 to 10 mmol, more preferably 0.01 to

1 mmol, and further preferably 0.02 to 0.5 mmol based on 1 g of bone-dry cellulose.

[0022] Bromides are compounds containing bromine and examples include alkali metal

bromides that can ionize through dissociation in water, such as sodium bromide. Iodides are

compounds containing iodine and examples include alkali metal iodides. The amount of a

bromide or an iodide used may be selected from the range in which oxidation reactions can

be promoted. The total amount of a bromide and an iodide is preferably 0.1 mmol or more

and more preferably 0.5 mmol or more based on 1 g of bone-dry cellulose. The upper limit

is preferably 100 mmol or less, more preferably 10 mmol or less, and further preferably 5

mmol or less. Accordingly, the total amount of a bromide and an iodide is preferably 0.1 to

100 mmol, more preferably 0.1 to 10 mmol, and further preferably 0.5 to 5 mmol based on 1

g of bone-dry cellulose.

[0023] Exemplary oxidants include, but are not particularly limited to, halogens,

hypohalous acids, halous acids, perhalic acids, salts thereof, halogen oxides, and peroxides.

Among these oxidants, due to low cost and low environmental load, hypohalous acids or salts

thereof are preferable, hypochlorous acid or salts thereof are more preferable, and sodium

hypochlorite is further preferable. The amount of an oxidant used is preferably 0.5 mmol or

more, more preferably 1 mmol or more, and further preferably 3 mmol or more based on 1 g

of bone-dry cellulose. The upper limit is preferably 500 mmol or less, more preferably 50

mmol or less, and further preferably 25 mmol or less. Accordingly, the amount of an

oxidant used is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, further

preferably 1 to 25 mmol, and particularly preferably 3 to 10 mmol based on 1 g of bone-dry

cellulose. When an N-oxyl compound is used, the amount of an oxidant used is preferably 1

mol or more based on 1 mol of the N-oxyl compound, and the upper limit is preferably 40

mol. Accordingly, the amount of an oxidant used is preferably 1 to 40 mol based on 1 mol

of an N-oxyl compound.

[0024] The conditions, such as pH and temperature, during oxidation reactions are not

particularly limited. In general, oxidation reactions efficiently proceed even under relatively mild conditions. The reaction temperature is preferably 4°C or higher and more preferably 15°C or higher. The upper limit of the reaction temperature is preferably 40°C or lower and more preferably 30°C or lower. Accordingly, the reaction temperature is preferably 4°C to 40°C and may be about 15°C to 30°C, in other words, room temperature.

The pH of a reaction solution is preferably 8 or more and more preferably 10 or more. The

upper limit of pH is preferably 12 or less and more preferably 11 or less. Accordingly, the

pH of a reaction solution is preferably 8 to 12 and more preferably about 10 to 11. In

general, since carboxy groups are formed within cellulose as oxidation reactions proceed, the

pH of a reaction solution tends to decrease. For this reason, to allow oxidation reactions to

proceed efficiently, it is preferable to maintain the pH of a reaction solution within the above

mentioned ranges through addition of an alkaline solution, such as an aqueous sodium

hydroxide solution. A reaction medium for oxidation is preferably water due to easy

handling, suppressed side reactions, or the like.

[0025] The reaction time for oxidation may be appropriately set according to the progress

of oxidation and is typically 0.5 hours or more. The upper limit is typically 6 hours or less

and preferably 4 hours or less. Accordingly, the reaction time for oxidation is typically 0.5

to 6 hours and about 0.5 to 4 hours, for example. Oxidation may be carried out as two- or

more-stage reactions. For example, by obtaining oxidized cellulose through filtration after

the end of the first-stage reaction and by oxidizing the resulting oxidized cellulose again

under the same or different reaction conditions, efficient oxidation is possible without

reaction inhibition due to sodium chloride generated as a side product in the first-stage

reaction.

[0026] Another example of the carboxylation (oxidation) method is ozone oxidation.

Through the oxidation reactions, hydroxy groups at least at the C-2 and C-6 positions of

glucopyranose ring that constitutes cellulose are oxidized while decomposing the cellulose

chains. Ozone treatment is typically performed by allowing contact between a gas

containing ozone and a cellulose raw material. The ozone concentration in a gas is

preferably 50 g/m3 or more. The upper limit is preferably 250 g/m3 or less and more preferably 220 g/m3 or less. Accordingly, the ozone concentration in a gas is preferably 50 to 250 g/m3 and more preferably 50 to 220 g/m 3. The amount of ozone added is preferably

0.1% by weight or more and more preferably 5% by weight or more based on 100% by

weight of the solids content of a cellulose raw material. The upper limit of the amount of

ozone added is typically 30% by weight or less. Accordingly, the amount of ozone added is

preferably 0.1 to 30% by weight and more preferably 5 to 30% by weight based on 100% by

weight of the solids content of a cellulose raw material. The ozone treatment temperature is

typically 0°C or higher and preferably 20°C or higher, and the upper limit is typically 50°C

or lower. Accordingly, the ozone treatment temperature is preferably 0°C to 50°C and more

preferably 20°C to 50°C. The ozone treatment time is typically 1 minute or more and

preferably 30 minutes or more, and the upper limit is 360 minutes or less. Accordingly, the

ozone treatment time is typically about 1 to 360 minutes and preferably about 30 to 360

minutes. When ozone treatment conditions fall within the above-mentioned ranges, it is

possible to prevent excessive oxidation and decomposition of cellulose and thus to achieve a

satisfactory yield of oxidized cellulose.

[0027] Ozone-treated cellulose may be further subjected to additional oxidation treatment

using an oxidant. Exemplary oxidants used for additional oxidation treatment include, but

are not particularly limited to, chlorine compounds, such as chlorine dioxide and sodium

chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. Exemplary

methods for additional oxidation treatment include a method of dissolving such an oxidant in

water or a polar organic solvent, such as an alcohol, to prepare an oxidant solution and

immersing a cellulose raw material in the oxidant solution. The amount of carboxy groups,

carboxylate groups, or aldehyde groups of oxidized MFC can be adjusted by controlling

oxidation conditions, such as the amount of an oxidant added and the reaction time.

[0028] An exemplary measurement method for the amount of carboxy groups will be

described hereinafter. A 60 mL of 0.5 weight% slurry (aqueous dispersion) of oxidized

cellulose is prepared and adjusted to pH 2.5 by adding 0.1 M aqueous hydrochloric acid.

Subsequently, the electric conductivity is measured to pH 11 while adding 0.05 N of aqueous sodium hydroxide solution dropwise. The amount of carboxy groups can be calculated using the following formula from the amount (a) of sodium hydroxide consumed in the weakly acidic neutralization stage where changes in electric conductivity are gradual.

Amount of carboxy groups [mmol/g-oxidized cellulose]= a [mL] x 0.05/weight of

oxidized cellulose [g]

[0029] The thus-measured amount of carboxy groups of oxidized cellulose is preferably 0.1

mmol/g or more, more preferably 0.5 mmol/g or more, and further preferably 0.8 mmol/g or

more based on the bone-dry weight of the oxidized cellulose. The upper limit is preferably

3.0 mmol/g or less, more preferably 2.5 mmol/g or less, and further preferably 2.0 mmol/g or

less. Accordingly, the amount of carboxy groups is preferably 0.1 to 3.0 mmol/g, more

preferably 0.5 to 2.5 mmol/g, and further preferably 0.8 to 2.0 mmol/g.

[0030] [Etherification]

Examples of etherification include carboxymethylation (carboxymethyl ether

formation), methylation (methyl ether formation), ethylation (ethyl ether formation),

cyanoethylation (cyanoethyl ether formation), hydroxyethylation (hydroxyethyl ether

formation), hydroxypropylation (hydroxypropyl ether formation),

ethylation/hydroxyethylation (ethyl/hydroxyethyl ether formation), and

hydroxypropylation/methylation (hydroxypropyl/methyl ether formation). Among such

etherification, a method for carboxymethylation will be described hereinafter as an example.

[0031] The degree of carboxymethyl substitution per anhydrous glucose unit of

carboxymethylated cellulose or MFC is preferably 0.01 or more, more preferably 0.05 or

more, and further 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 further preferably 0.35 or less.

Accordingly, the degree of carboxymethyl substitution is preferably 0.01 to 0.50, more

preferably 0.05 to 0.40, and further preferably 0.10 to 0.30.

[0032] Exemplary methods for carboxymethylation include, but are not particularly limited

to, a method of mercerizing a cellulose raw material, followed by etherification. A solvent

is commonly used for such a reaction. Exemplary solvents include water, alcohols (lower alcohols, for example), and mixed solvents thereof. Exemplary lower alcohols include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, isobutanol, and tert butanol. The mixing ratio of a lower alcohol in a mixed solvent is typically 60 weight% or more as the lower limit and 95 weight% or less as the upper limit and thus is preferably 60 to weight%. The amount of solvent is typically 3 times the weight of a cellulose raw material. The upper limit is not particularly limited but is 20 times the weight of a cellulose raw material. Accordingly, the amount of solvent is preferably 3 to 20 times the weight of a cellulose raw material.

[0033] Mercerization is typically conducted by mixing a raw material with a mercerizing

agent. Exemplary mercerizing agents include alkali metal hydroxides, such as sodium

hydroxide and potassium hydroxide. The amount of a mercerizing agent used is preferably

0.5 molar times or more, more preferably 1.0 molar time or more, and further preferably 1.5

molar times or more per anhydrous glucose residue of a raw material. The upper limit is

typically 20 molar times or less, preferably 10 molar times or less, and more preferably 5

molar times or less. Accordingly, the amount of a mercerizing agent used is preferably 0.5

to 20 molar times, more preferably 1.0 to 10 molar times, and further preferably 1.5 to 5

molar times.

[0034] The reaction temperature for mercerization is typically 0°C or higher and preferably

°Corhigher. The upper limit is typically 70°C or lower and preferably 60°C or lower.

Accordingly, the reaction temperature is typically 0°C to 70°C and preferably 10°C to 60°C.

The reaction time is typically 15 minutes or more and preferably 30 minutes or more. The

upper limit is typically 8 hours or less and preferably 7 hours or less. Accordingly, the

reaction time is typically 15 minutes to 8 hours and preferably 30 minutes to 7 hours.

[0035] Etherification reactions are typically performed by adding a carboxymethylating

agent to a reaction system after mercerization. Exemplary carboxymethylating agents

include sodium monochloroacetate. The amount of a carboxymethylating agent added is

generally preferably 0.05 molar times or more, more preferably 0.5 molar times or more, and

further preferably 0.8 molar times or more per glucose residue of a cellulose raw material.

The upper limit is typically 10.0 molar times or less, preferably 5 molar times or less, and

more preferably 3 molar times or less. Accordingly, the amount is preferably 0.05 to 10.0

molar times, more preferably 0.5 to 5 molar times, and further preferably 0.8 to 3 molar

times. The reaction temperature is typically 30°C or higher and preferably 40°C or higher.

The upper limit is typically 90°C or lower and preferably 80°C or lower. Accordingly, the

reaction temperature is typically 30°C to 90°C and preferably 40°C to 80°C. The reaction

time is typically 30 minutes or more and preferably 1 hour or more. The upper limit is

typically 10 hours or less and preferably 4 hours or less. Accordingly, the reaction time is

typically 30 minutes to 10 hours and preferably 1 to 4 hours. During carboxymethylation

reactions, a reaction solution may be stirred as necessary.

[0036] The degree of carboxymethyl substitution per glucose unit of carboxymethylated

cellulose is measured, for example, by the following method. Specifically, 1) about 2.0 g of

carboxymethylated cellulose (bone dry) is accurately weighed and placed in a 300 mL

Erlenmeyer flask with a glass stopper. 2) The carboxymethyl cellulose salt

(carboxymethylated cellulose) is converted to free acid-form carboxymethylated cellulose by

adding 100 mL of a solution prepared by adding 100 mL of special grade concentrated nitric

acid to 1000 mL of-methanol, followed by shaking for 3 hours. 3) Accurately weighed 1.5

to 2.0 g of the free acid-form carboxymethylated cellulose (bone dry) is placed in a 300-mL

Erlenmeyer flask with a glass stopper. 4) The free acid-form carboxymethylated cellulose is

wetted with 15 mL of 80 % methanol, added with 100 mL of 0.1N NaOH, and shaken for 3

hours at room temperature. 5) By using phenolphthalein as an indicator, excessive NaOH is

back-titrated with 0.iN H 2 SO 4 . 6) The degree of carboxymethyl substitution (DS) is

calculated using the following formula:

A = [(100 x F'- (0.1N H 2 SO4 ) (mL) x F) x 0.1] / (bone-dry mass (g) of free acid

form carboxymethylated cellulose)

DS = 0.162 x A/(1 - 0.058 x A)

A: amount of 1 N NaOH (mL) required for neutralizing 1 g of free acid-form

carboxymethylated cellulose

F: factor of 0.1 N H 2 SO4

F': factor of 0.1 N NaOH

[0037] 3) Mechanical Refining or Beating

In this process, chemically modified cellulose is mechanically refined or beaten to a

mean diameter of fibers of 500 nm or more. In the present invention, mechanical refining or

beating is referred to as "mechanical treatment," and mechanical treatment of an aqueous

dispersion of a raw material cellulose is referred to as "wet grinding." Refining or beating

treatment may be performed once or a plurality of times independently or in combination.

When refining or beating is performed a plurality of times, the timing is optional and an

apparatus used may be the same or different.

[0038] Exemplary apparatuses used for refining or beating treatment incudes, but are not

particularly limited to, a high-speed rotary apparatus, a colloid mill, a high-pressure

apparatus, a roll mill, and an ultrasonic apparatus. Apparatuses, such as a high-pressure or

ultrahigh-pressure homogenizer, a refiner, a beater, a PFI mill, a kneader, and a disperser, in

which a metal or blade and pulp fibers are interacted with the rotational axis as the center or

pulp fibers are interacted with each other through friction may be used.

[0039] When an aqueous dispersion of chemically modified pulp is subjected to refining or

beating, in other words, subjected to wet grinding, the concentration (solids concentration) of

chemically modified pulp in the aqueous dispersion is generally preferably 0.1% by weight or

more, more preferably 0.2% by weight or more, and further preferably 0.3% by weight or

more. At such concentrations, the amount of liquid is appropriate relative to the amount of

chemically modified cellulose, and this results in efficiency. The upper limit of the

concentration is not particularly limited provided that refining or beating is possible, and is

generally preferably 90% by weight or less, more preferably 50% by weight of less, and

further preferably 40% by weight of less.

[0040] Through this process, mechanically-treated chemically-modified MFC is obtained.

The mechanically-treated chemically-modified MFC has a mean diameter of fibers, as a

length-weighted mean diameter of fibers, of 500 nm or more, preferably 1Ipm or more, and more preferably 10 pm or more. The upper limit of the mean diameter of fibers is preferably 60 pm or less and more preferably 40 pm or less. The mean length of fibers, as a length-weighted mean length of fibers, is preferably 300 pm or more, more preferably 500 pm or more, and further preferably 800 pm or more. The upper limit of the mean length of fibers is preferably 3,000 pm or less, more preferably 1,500 Pm or less, and further preferably 1,100 pm or less. According to the present invention, since raw material pulp is chemically modified in advance, fibrillation or shorter fiber formation readily progresses during mechanical refining or beating. Moreover, the mechanically-treated chemically modified MFC of the present invention differs from CNF. In CNF, due to vigorous refining treatment, the formation of finer fibers generally progresses while reducing the fiber width.

However, in the mechanically-treated chemically-modified MFC of the present invention,

since treatment for promoting fibrillation, such as relatively mild refining or beating, but not

vigorous refining treatment for CNF is performed, fibers are fibrillated or shortened while

maintaining the fiber width.

[0041] The length-weighted mean diameter of fibers and length-weighted mean length of

fibers are obtained by using a fiber tester from ABB Ltd. or a fractionator from Valmet

Corporation. The average aspect ratio of the mechanically-treated chemically-modified

MFC is preferably 10 or more and more preferably 30 or more. The upper limit is not

particularly limited and is preferably 1,000 or less, more preferably 100 or less, and further

preferably 80 or less. The average aspect ratio is calculated from the following formula.

Average aspect ratio = mean length of fibers/mean diameter of fibers

[0042] The amount of carboxy groups of the mechanically-treated oxidized MFC obtained

in this process is preferably the same as the amount of carboxy groups of the foregoing

oxidized cellulose. Similarly, the substitution degree per glucose unit of the mechanically

treated carboxymethylated MFC obtained in this process is preferably the same as the

substitution degree of the corresponding carboxymethyl cellulose.

[0043] The mechanically-treated chemically-modified MFC of the present invention may

contain metal ions, and the total amount is preferably 0 or more and less than 10 mg/g.

Exemplary metal ions include Ag, Au, Pt, Ni, Mn, Fe, Ti, Al, Zn, and Cu, and di- or higher

valent metal ions are preferable. The content of the metal ions can be determined from

scanning electron microscope images and ICP atomic emission spectroscopy for an extract by

a strong acid. Specifically, the presence of metal ions cannot be observed on scanning

electron microscope images whereas ICP atomic emission spectroscopy can confirm that a

metal is contained. In contrast, when a metal is reduced to be present as metal particles,

metal particles can be observed on scanning electron microscope images. The

mechanically-treated chemically-modified MFC of the present invention is used by mixing

with other paper chemicals in the production processes for paper (papermaking and coating

application, for example). Since many chemicals having cationic or anionic charges are

used as paper chemicals, there is a risk of causing trouble, such as aggregation, when the

charge balance in the system is disrupted. Accordingly, the use of mechanically-treated

chemically-modified MFC having a high metal ion content could disrupt the charge balance

inthesystem. For this reason, the metal ion content is preferably less than 10 mg/g to

reduce trouble in the production processes for paper.

[0044] (2) Paper

The paper of the present invention may contain mechanically-treated chemically

modified MFC in a base paper layer or in a coating layer, as described hereinafter. The

former and the latter are also referred to as "internal addition" and "external addition,"

respectively. The content of mechanically-treated chemically-modified MFC in the paper of

the present invention is preferably less than 20% by weight and more preferably 10% by

weight or less based on the entire pulp (the total of mechanically-treated chemically-modified

MFC and other pulps). When the amount of mechanically-treated chemically-modified

MFC is 20% by weight or more, there is a risk of worsening drainage of pulp and impairing

smooth operation in the case of internal addition and, similarly, there is a risk of worsening

drying efficiency of coating solutions in the case of external addition. The paper of the

present invention can be employed for uses, such as western paper, paperboard, information

paper, industrial paper, and corrugated fiberboard.

[0045] (3) Base Paper Layer

A base paper layer is a base layer for paper and contains pulp as a main component.

In the present invention, base paper may be single-layered or multilayered. A base paper

layer preferably contains mechanically-treated chemically-modified MFC. In the case of a

multilayered base paper layer, at least any one layer or all the layers may contain the MFC.

The content of the MFC is preferably 1 X10-4 to 10 parts by weight and more preferably 3 x

-4 to 1 part by weight based on 100 parts by weight of pulp in each layer.

[0046] Exemplary pulp raw materials for base paper used in the present invention include,

but are not particularly limited to, mechanical pulp, such as ground pulp (GP),

thermomechanical pulp (TMP), or chemi-thermomechanical pulp (CTMP); deinked pulp

(DIP); and chemical pulp, such as softwood kraft pulp (NKP) or hardwood kraft pulp (LKP).

As deinked (wastepaper) pulp, those derived from sorted wastepaper, such as high-grade

paper, medium-grade paper, low-grade paper, newsprint, leaflets, and magazines; and

unsorted wastepaper as mixtures thereof may be used.

[0047] Base paper may be added with a publicly known filler but needs not be added with a

filler for the uses that do not require certain opacity or whiteness, such as paperboard.

When a filler is added, exemplary fillers include inorganic fillers, such as ground calcium

carbonate, precipitated calcium carbonate, clay, silica, precipitated 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 a mineral acid; and organic fillers, such as urea-formaldehyde resin,

melamine resins, polystyrene resin, and phenolic resins. These fillers may be used alone or

in combination. Among these fillers, calcium carbonate and precipitated calcium carbonate,

which are representative fillers in neutral papermaking and alkaline papermaking and which

can attain high opacity, are preferable. The content of a filler in base paper is preferably 5

to 20% by weight based on the weight of the base paper. Since lowering in strength is

suppressed in the present invention even at a high ash content of paper, the content of a filler in base paper is more preferably 10% by weight or more.

[0048] As chemicals for internal addition, bulking agents, dry strengthening agents, wet

strengthening agents, drainage aids, dyes, neutral size, and so forth may be used as necessary.

[0049] The base paper is produced by a publicly known papermaking method using, for

example, a Fourdrinier machine, a gap former, a hybrid former or on-top former, or a

cylinder machine, although not limited thereto.

[0050] When added to base paper, the mechanically-treated chemically-modified MFC of

the present invention may be added in any step in the preparation process for a pulp slurry

but is preferably added in the pulp refiner step or the mixing step to enhance mixing

efficiency of the MFC. When the MFC is added in the mixing step, the MFC mixed in

advance with other aids, such as a filler and a retention aid, may be added to a pulp slurry.

[0051] The density of base paper is preferably 0.2 g/cm3 or more and more preferably 0.4

g/cm3 or more. The paper of the present invention has many fibrils on the surface of the

mechanically-treated chemically-modified MFC. In the paper of the present invention,

strengthening effects are exerted presumably because fibrillated mechanically-treated

chemically-modified MFC is present between pulp fibers that constitute the paper, thereby

increasing binding points between pulp fibers. For this reason, further high strengthening

effects can be exerted in relatively high-density paper in which the distance between pulp

fibers is close. Further, the basis weight of base paper is not particularly limited but is

preferably 20 g/m2 or more, more preferably 30 g/m2 or more, and further preferably more

than 40 g/m2 .

[0052] (4) Pigment Coating Layer

A pigment coating layer is a layer containing a white pigment as a main component.

Exemplary 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.

[0053] The pigment coating layer contains an adhesive. Exemplary adhesives include various starches, such as oxidized starch, cationic starch, urea/phosphoric acid-esterified starch, hydroxyethyl-etherified starch or other etherified starches, and dextrin; proteins, such as casein, soy protein, and synthetic proteins; 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 latexes; and vinyl polymer latexes, such as ethylene-vinyl acetate copolymer. These adhesives may be used alone or in combination, and the use of a starch-based adhesive and styrene-butadiene copolymer in combination is preferable.

[0054] The pigment coating layer may contain various aids used in the common paper

production field, such as dispersants, thickeners, defoamers, colorants, antistatic agents, and

preservatives, and may contain the mechanically-treated chemically-modified MFC of the

present invention. When the pigment coating layer contains the MFC, the amount is

preferably 1 x 10-3 to 1 part by weight based on 100 parts by weight of a pigment. When

the amount falls within this range, it is possible to obtain a pigment coating solution having

appropriate water retention properties without substantially increasing the viscosity of the

coating solution.

[0055] The pigment coating layer can be provided by applying a coating solution to either

or both surfaces of base paper by a publicly known method. The solids concentration of a

coating solution is preferably about 30 to 70% by weight in view of smooth application.

One, two, or three or more pigment coating layers may be provided. When a plurality of

pigment coating layers are present, the MFC may be contained in any of these pigment

coating layers. The amount applied for a pigment coating layer may be appropriately

adjusted depending on the uses but, in the case of coated printing paper, is 5 g/m2 or more

and preferably 10 g/m 2 or more per surface in total. The upper limit is preferably 30 g/m 2 or

less and more preferably 25 g/m2 or less.

[0056] (4) Clear Coating Layer

The paper of the present invention may have a clear (transparent) coating layer on

either or both surfaces of base paper. By providing a clear coating layer on base paper, it is possible to enhance the surface strength and smoothness of the base paper as well as to improve application properties when a pigment coating is provided. The amount applied for a clear coating is preferably 0.1 to 1.0 g/m2 and more preferably 0.2 to 0.8 g/m2 as solids content per surface. In the present invention, a clear coating layer is formed by applying

(size pressing) to base paper a coating solution (surface treatment solution) primarily

containing starch, oxidized starch, or other various starches and a water-soluble polymer,

such as polyacrylamide or polyvinyl alcohol, by using a coater, such as a size press, a gate

roll coater, a premetering size press, a curtain coater, or a spray coater. The mechanically

treated chemically-modified MFC of the present invention may be contained in a clear

coating layer.

[0057] (5) Characteristics

In the paper of the present invention, the paper when conditioned in accordance with

JIS P 8111 at 23°C/50 2% preferably has a water content of 10% by weight or less. Here,

mechanically-treated chemically-modified MFC, which has a relatively high water retention

ratio, makes dewatering or drying difficult in the paper production processes in some cases.

However, it is possible to attain satisfactory dewatering properties or drying properties in the

paper production processes preferably by controlling the water content within the above

mentioned range through adjustment of the amount of the MFC and the amount of modifying

groups. Moreover, paper having the water content of 10% by weight or less exhibits

sufficient strength. Meanwhile, when the water content exceeds 10% by weight, hydrogen

bonding existing between cellulose fibers of pulp is disrupted by water, thereby posing a risk

of lowering in strength, particularly, stiffness of paper. In this view, the lower limit of the

water content is preferably 4% by weight or more, although not limited thereto.

[0058] The paper of the present invention, which contains mechanically-treated chemically

modified MFC having a relatively large diameter of fiber and a high degree of fibrillation,

exhibits excellent strength as well as excellent air resistance.

[0059] 2. Production Method for Paper

The paper of the present invention may contain mechanically-treated chemically- modified MFC in any one layer of base paper, a clear coating layer, and a pigment coating layer that constitute the paper. To obtain high strength-enhancing effects, the paper is preferably produced via a step of preparing a stock containing mechanically-treated chemically-modified MFC. Meanwhile, to obtain high air resistance-enhancing effects, the paper is preferably produced via a step of preparing a coating solution containing mechanically-treated chemically-modified MFC. As in the foregoing, mechanically-treated chemically-modified MFC can be prepared by mechanically refining or beating a chemically modified cellulose raw material. A stock can be prepared in accordance with a publicly known method, for example, by adding the MFC, a filler, and additives as necessary to a slurry obtained by disintegrating pulp. Further, a coating solution can be prepared in accordance with a publicly known method, for example, by adding the MFC and additives as necessary to a binder, such as starch, or by further adding a white pigment to prepare a pigment coating solution.

[0060] It is possible to produce paper through papermaking of the thus-obtained stock by a

publicly known method or through application of the thus-obtained coating solution to base

paper. As in the foregoing, a clear coating layer or a pigment coating layer may be provided

on the surface of paper.

EXAMPLES

[0061] Hereinafter, the present invention will be described by means of Examples.

(1) Evaluation

Each item was measured in accordance with the following.

Basis weight: JIS P 8223: 2006

Thickness and density: JIS P 8118: 2014

Ash content: JIS P 8251: 2003

Canadian standard freeness (CSF: mL): JIS P 8121-2: 2012

Air resistance: JIS P 8117: 2009 with an Oken smoothness/air permeability tester

Breaking length: JIS P 8113: 1998

Tensile stiffness: ISO/DIS 1924-3

ISO stiffness: ISO 2493

MFC characteristics: the characteristics of MFC in Examples 1 and 2 were measured

with Fiber Tester Plus from ABB Ltd. The measurement conditions are as follows.

[0062] Measurement conditions for Fiber Tester Plus: MFC dispersed in water at 0.05%

was measured in accordance with the specified method. Measurement items were set to

mean length of fibers (Mean Length), mean diameter of fibers (Mean width), mean fibril

index (Mean fibril area), and mean fines proportion (Mean fines).

[0063] (2) Preparation of MFC

[Production Example 1] Preparation 1 of Mechanically-treated Chemically-modified

MFC from NBKP

NBKP (from Nippon Paper Industries Co., Ltd.) was subjected to TEMPO oxidation

treatment by a common method to produce TEMPO-oxidized pulp A to C shown in Table 1.

Each obtained TEMPO-oxidized pulp was dispersed in water to prepare a 3 weight%

dispersion and treated with a refiner. Each mechanically-treated chemically-modified MFC

was obtained by changing treatment conditions, such as the clearance of a refiner and the

number of treatments, according to target CSF and mean fibril index. The physical

properties are shown in Table 1, and the characteristics of the obtained MFC are shown in

Tables 2 and 3.

[0064] [Production Example 2] Preparation 2 of Mechanically-treated Chemically-modified

MFC from NBKP

NBKP (from Nippon Paper Industries Co., Ltd.) was subjected to TEMPO oxidation

treatment by a common method to produce TEMPO-oxidized pulp D (CSF of 554 mL at pH

7.2) having the amount of COOH groups of 1.37mmol/g. Each obtained TEMPO-oxidized

pulp was dispersed in water to prepare a 4 weight% dispersion and treated with a refiner.

Each mechanically-treated chemically-modified MFC-D was obtained by changing the

number of treatments with the refiner. The characteristics of the MFC are shown in Table 4.

It was impossible to measure proper CSF for mechanically-treated chemically-modified

MFC-D due to excessively progressed formation of finer fibers.

[0065] [Table 1]

Table 1 COOH CSF AF = Fo -F amount mmol/g mL mL TEMPO-oxidized pulp A 0.3 Fo 537 Mechanically-treated chemically- 0.3 F 67 470 modified MFC-A TEMPO-oxidized pulp B 0.58 Fo 364 Mechanically-treated chemically- 0.58 F 54 310 modified MFC-B TEMPO-oxidized pulp C 1.53 Fo 65 Mechanically-treated unmodified 0 Fo 400 pulp (NBKP)

[0066] [Example 1-1]

A mixed pulp was prepared from 96% by weight of LBKP (from Nippon Paper

Industries Co., Ltd., CSF of 400 mL) and 4% by weight of mechanically-treated chemically

modified MFC-A (CSF of 67 mL, COOH group amount of 0.30 mmol/g). A pulp slurry of

0.35 weight% solids concentration was prepared by adding to the mixed pulp 1.5% by weight

of aluminum sulfate, 0.025% by weight of polyethylenimine, 0.6% by weight of

polyacrylamide, and 0.2% by weight of a size based on the total amount of the mixed pulp.

A hand-formed sheet with a basis weight of 50 g/m2 was produced using the obtained pulp

slurry and evaluated. Hand forming was performed in accordance with JIS P 8222.

[0067] [Example 1-2]

A hand-formed sheet was produced and evaluated in the same manner as Example

1-1 except for using mechanically-treated chemically-modified MFC-B (CSF of 54 mL,

COOH group amount of 0.58 mmol/g) in place of mechanically-treated chemically-modified

MFC-A.

[0068] [Comparative Examples 1 and 2]

Each hand-formed sheet was produced and evaluated in the same manner as

Example 1-1 except for using mechanically-untreated TEMPO-oxidized pulp A (COOH

group amount of 0.30 mmol/g, CSF of 573 mL) or TEMPO-oxidized pulp B (COOH group

amount of 0.58 mmol/g, CSF of 364 mL) in place of mechanically-treated chemically

modified MFC.

[0069] [Comparative Example 3]

A hand-formed sheet was produced and evaluated in the same manner as

Comparative Example 1 except for using unmodified MFC (NBKP, COOH group amount of

mmol/g, CSF of 450 mL) in place of TEMPO-oxidized pulp A.

[0070] [Comparative Example 4]

A pulp slurry of 0.35 weight% solids concentration was prepared by adding 1.5% by

weight of aluminum sulfate, 0.025% by weight of polyethylenimine, 0.6% by weight of

polyacrylamide, and 0.2% by weight of a size to 100% by weight of LBKP (CSF of 400 mL).

A hand-formed sheet with a basis weight of 50 g/m2 was produced using the obtained pulp

slurry and evaluated. The results are shown in Table 2.

[0071] [Examples 2-1 to 2-3]

Each hand-formed sheet was produced and evaluated in the same manner as

Example 1-1 except for using magazine wastepaper-derived undeinked pulp (from Nippon

Paper Industries Co., Ltd., CSF of 350 mL) in place of LBKP and changing the amount of

mechanically-treated chemically-modified MFC-A added to 0.01 weight%, 1.0 weight%, or

weight%.

[0072] [Comparative Example 5]

A hand-formed sheet was produced and evaluated in the same manner as

Comparative Example 4 except for changing the basis weight to 43.6 g/m2 . The results are

shown in Table 3.

[0073] [Table 2]

Table 2

Chemically Mechanically- Chemically Mechanically LBKP NBKP modified treated modified treated alone 4% MFC chemically- MFC chemically modified MFC modified MFC

Ex. Comp. Ex. 1 Ex. 1-1 Comp. Ex. 2 Ex. 1-2 o.Ex 100 96 96 96 96 96 Amount of LBKP (CSF 400 mL) % - 4 - - - pulpadded NBKP (chemically unmodified) MFC (chemically modified) - - 4 4 4 4 mmol/g - 0 0.3 0.58 MFC COOH amount 0 x O x 0 characteristics Mechanical treatment (beating) - - CSF mL - 450 573 67 364 54 Basis weight g/m 2 49.7 49.6 49.7 49.7 49.7 49.9 Thickness pm 81.4 81.0 83.0 80.9 84.3 82.8 Density g/cm 3 0.61 0.61 0.60 0.61 0.59 0.60 Paper physical Ash content % 1.1 1.2 1.1 1.2 1.1 1.2 properties Tensile strength kN/m 3.52 3.41 3.39 3.56 3.44 3.52 Breaking length km 4.7 4.6 4.5 4.8 4.6 4.7 ISO stiffness pN-m 2/m 72.1 67.0 69.1 70.8 66.2 72.1 Thickness-corrected stiffness 13.4 12.6 12.1 13.4 11.0 12.7

Chemically Mechanically- Chemically Mechanically Type of pulp LBKP NBKP modified cheally- modified chtrealy modified MFC modified MFC CSF mL 400 450 573 67 364 54 Characteristics Mean length of fibers (Mean length) am 762 769 1885 866 1557 1002 Mean diameter of fibers (Mean pm 19.7 19.9 28.6 29.4 29.8 31.2 width) Mean fibril index (Mean fibril area) % 3.5 3.6 1.2 5.0 1.7 4.8 Mean fines proportion (Mean fines) % 20.9 21.0 16.9 50.4 18.0 50.8

[0074] [Table 3]

Table 3 Comp. Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex. 5 Pulp Undeinked pulp COOH group amount mmol/g - 0.3 CSF mL - 67 MFC Mean length of fibers ptm - 866 characteristics Mean diameter of fibers pm - 29.4 Mean fibril index % - 5.0 5.0 5.0 Mean fines proportion % - 50.4 MFC content in entire pulp 0% 0.01% 1.0% 10% Basis weight g/m 2 43.6 43.5 42.9 41.3 Thickness pm 78.5 76.0 73.9 71.8 3 0.57 0.58 0.58 Paper quality Density g/cm 0.56 Ash content % 12.8 12.7 12.6 13.0 Air resistance sec 5.0 6.5 7.0 9.0 Breaking length km 2.15 2.25 2.37 2.35

[0075] From Tables 2 and 3, it is clear that the paper of the present invention has excellent

strength and air resistance.

[0076] [Examples 3-1 to 3-2, Comparative Example 7]

A mixed pulp was prepared from 96% by weight of LBKP (from Nippon Paper

Industries Co., Ltd., CSF of 400 mL) and 4% by weight of mechanically-treated chemically

modified MFC-D (COOH group amount of 1.37 mmol/g), each of which is different in the

number of refiner passes. A pulp slurry of 0.35 weight% solids concentration was prepared

by adding to the mixed pulp 1.5% by weight of aluminum sulfate, 0.025% by weight of

polyethylenimine, 0.6% by weight of polyacrylamide, and 0.2% by weight of a size based on

the total amount of the mixed pulp. Each hand-formed sheet with a target basis weight of

g/m 2 was produced using the obtained pulp slurry and evaluated. Hand forming was

performed in accordance with JIS P 8222. The MFC used in Comparative Example 7 is not

mechanically-treated chemically-modified MFC. The results are shown in Table 4.

[0077] [Table 4]

Table 4 Comp. Comp. Ex. 3-1 Ex. 3-2 Ex. 6 Ex. 7 Pulp Undeinked pulp Beating concentration % - 4 Number of refiner passes times - 0 3 10 COOH group amount mmol/g - 1.37 MFC CSF mL - 123 10 10 characteristics Mean length of fibers pm - 2126 1052 755 Mean diameter of fibers pm - 30.4 31.7 30.6 Mean fibril index % - 3.9 12.5 17 Mean fines proportion % - 5.1 30.1 41.1 MFC content in entire pulp 0% 4% Basis weight g/m 2 51.5 50.7 51.4 51.5 Thickness ptm 93 92 90 90 g/cm 0.56 0.57 0.57 Paper quality Density 0.55 Ash content % 2.4 2.1 2.2 2.6 Air resistance sec 10 15 28 38 Breaking length km 2.88 3.02 3.19 3.27

[0078] Table 4 reveals that the paper containing mechanically-treated chemically-modified

MFC has a large breaking length and a high air resistance and that the paper has a larger

breaking length and a higher air resistance as the degree of fibrillation of the MFC increases.

[0079] [Comparative Example 6]

A hand-formed sheet was produced and evaluated in the same manner as Example

3-1 except for not using mechanically-treated chemically-modified MFC.

Claims (9)

1. Paper comprising mechanically-treated chemically-modified microfibrillated

cellulose fibers having a mean diameter of fibers of 500 nm or more.

2. The paper according to Claim 1, wherein the microfibrillated cellulose fibers have a

mean fibril index of 4.0 or more as measured by a fiber analyzer.

3. The paper according to Claim 1 or 2, wherein the microfibrillated cellulose fibers

have a cellulose I crystallinity of 50% or more.

4. The paper according to any one of Claims I to 3, wherein the chemical modification

is anionic modification.

5. The paper according to any one of Claims I to 4, wherein the paper has a pigment

coating layer.

6. The paper according to any one of Claims I to 5, wherein the paper has a clear

coating layer.

7. The paper according to any one of Claims 1 to 6, wherein the paper when

conditioned in accordance with JIS P 8111 at 23°C/50 2% has a water content of 10% by

weight or less.

8. A production method for the paper according to any one of Claims I to 7,

comprising:

preparing the microfibrillated cellulose fibers by wet grinding a cellulose raw

material; and

preparing a stock containing the microfibrillated cellulose fibers.

9. The production method according to Claim 8, further comprising chemically

modifying the cellulose raw material before the wet grinding.