AU2019390849A1

AU2019390849A1 - Paper comprising cellulose-nanofiber-containing coating layer

Info

Publication number: AU2019390849A1
Application number: AU2019390849A
Authority: AU
Inventors: Kiyoshi Hatakeyama; Shiori KAKINOKI; Ryo SOTOOKA; Takehiro Yoshimatsu
Original assignee: Nippon Paper Industries Co Ltd
Current assignee: Nippon Paper Industries Co Ltd
Priority date: 2018-11-26
Filing date: 2019-11-26
Publication date: 2021-07-01
Also published as: JPWO2020111025A1; CN113039325A; TW202028567A; WO2020111025A1; JP7425743B2

Abstract

Provided is a paper comprising a base paper and a coating layer, wherein the coating layer includes starch and cellulose nanofibers. The starch is preferably thermochemically modified starch, or more preferably ammonium-persulfate-modified starch or urea•acid modified starch.

Description

DESCRIPTION PAPER COMPRISING CELLULOSE-NANOFIBER-CONTAINING COATING LAYER TECHNICAL FIELD

[0001] The present invention relates to a paper comprising a cellulose nanofiber-containing

coating layer.

BACKGROUND ART

[0002] Cellulose nanofibers are expected as a new material and have been studied from

various viewpoints. For example, PTL 1 discloses a printing paper prepared by coating or

impregnating a paper with a papermaking additive composed of a cellulose nanofiber. The

paper disclosed in PTL 1 has excellent air permeation resistance, inking properties, and

strike-through resistance.

CITATION LIST PATENT LITERATURE

[0003] PTL1: Japanese Unexamined Patent Application Publication No. JP 2009-263850

SUMMARY OF INVENTION TECHNICAL PROBLEM

[0004] Papers are required to feature print gloss and surface strength. However, PTL 1 is

not descriptive of such a characteristic and workability. In view of these circumstances, an

object of the present invention is to provide a paper having high print gloss and surface

strength.

SOLUTION TO PROBLEM

[0005] The aforementioned object is achieved by the following invention.

(1) A paper comprising a base paper and a coating layer, wherein the coating layer

comprises a starch and a cellulose nanofiber.

(2) The paper as set forth in (1), wherein the starch is a thermochemically modified starch.

(3) The paper as set forth in (1) or (2), wherein the coating layer is a clear coating layer,

wherein the weight ratio of the thermochemically modified starch the cellulose nanofiber is in the range of from 350:1 to 67:1.

(4) The paper as set forth in (3), further comprising a pigment coating layer formed on the

clear coating layer.

(5) The paper as set forth in (1) or (2), wherein the coating layer is a pigment coating layer.

(6) The paper as set forth in any of (2) to (5), wherein the thermochemically modified

starch is selected from the group consisting of an ammonium persulfate-modified starch, a

urea- and acid-modified starch, and a combination thereof.

(7) The paper as set forth in any of (1) to (6), wherein the cellulose nanofiber is an

anionically modified cellulose nanofiber.

(8) The paper as set forth in any of (1) to (7), wherein the cellulose nanofiber has a

carboxyl group content of from 0.1 to 3.0 mmol/g.

(9) The paper as set forth in any of (1) to (8), wherein the cellulose nanofiber has a degree

of carboxymethyl substitution per glucose unit of from 0.01 to 0.50.

(10) The paper asset forth in any of (1) to (9), wherein the cellulose nanofiber has a type B

viscosity (60 rpm, 20°C) of from 500 to 7000 mPa-s as measured as a 1% (w/v) water

dispersion.

ADVANTAGEOUS EFFECTS OF INVENTION

[0006] The present invention can provide a paper having high print gloss and surface

strength.

DESCRIPTION OF EMBODIMENTS

[0007] Hereunder, the present invention will be described in detail. The paper of this

invention has a coating layer comprising a starch and a CNF, the coating layer which is

formed on one side or both sides of a base paper. In one embodiment, the paper of this

invention comprises a clear base paper comprising a starch and a CNF, and in another

embodiment, the paper of this invention comprises a pigment coating layer comprising a

starch and a CNF. In this invention, ranges "from X to Y" include both endpoints X and Y.

[0008] 1. Paper having a clear base paper comprising a starch and a CNF (first

embodiment)

(1) Cellulose nanofiber

The "cellulose nanofiber" (hereinafter also referred to as "CNF") is a single cellulose microfibril obtained by defibrating a cellulose-based raw material and having an

average fiber diameter of less than 500 nm.

[0009] The CNF is preferably chemically modified. Chemically modified CNFs can be

prepared by chemically modifying a cellulose-based raw material to prepare a chemically

modified cellulose and by mechanically defibrating the chemically modified cellulose.

1) Cellulose-based raw material

Examples of cellulose-based raw materials include, but are not particularly limited

to, cellulose-based raw materials derived from plants, animals (e.g., sea squirt), algae,

microorganisms (e.g., Acetobacter), and microorganism products. Examples of cellulose

based raw materials derived from plants include wood, bamboo, cotton, jute, kenaf, farm

waste products, cloth, and pulps (e.g., softwood (nadelholz) unbleached kraft pulp (NUKP),

softwood bleached kraft pulp (NBKP), hardwood (laubholz) unbleached kraft pulp (LUKP),

hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood

bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper).

The cellulose-based raw material can be any or a combination of the aforementioned

materials, but is preferably a cellulose fiber derived from a plant or microorganism, more

preferably a cellulose fiber derived from a plant.

[0010] 2) Chemical modification

The "chemical modification (chemically modified)" refers to the introduction of a

functional group, preferably an anionic group, into a cellulose-based raw material.

Examples of anionic groups include a carboxyl group, carboxyl group-containing groups, and

acid groups such as a phosphoric group and phosphoric group-containing groups. Examples

of carboxyl group-containing groups include -R-COOH (wherein R is an alkylene group

having 1 to 3 carbon atoms), and -O-R-COOH (wherein R is an alkylene group having 1 to 3

carbon atoms). Examples of phosphoric group-containing groups include, but are not

limited to, a polyphosphoric group, a phosphite group, a phosphonic group, and a polyphosphonic group. Depending on the reaction conditions, such acid groups may be introduced in the form of a salt (e.g., carboxylate group (-COOM, wherein M is a metal atom)). The chemical modification is preferably oxidization or etherification. The following paragraphs provide detailed descriptions of these chemical modifications.

[0011] [Oxidization]

By oxidizing a cellulose raw material, an oxidized cellulose is obtained. The

method of oxidizing a cellulose raw material is not particularly limited. One exemplary

method is to oxidize a cellulose raw material in water using an oxidizing agent in the

presence of an N-oxyl compound and a substance selected from the group consisting of

bromides, iodides and mixtures thereof. According to this method, primary hydroxyl

groups at C6 position of glucopyranose rings found on the surface of cellulose fibers are

selectively oxidized to produce groups selected from the groups consisting of aldehyde

groups, carboxyl groups, and carboxylate groups. The concentration of a cellulose raw

material during the oxidization reaction is not particularly limited, but is preferably not more

than 5% by weight.

[0012] The N-oxyl compound refers to a compound capable of producing nitroxyl radicals.

Examples of nitroxyl radicals include 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Any

N-oxyl compound can be used as long as it is a compound capable of promoting a desired

oxidization reaction. The amount of an N-oxyl compound used is not particularly limited

as long as it is a catalyst amount that allows oxidization of cellulose as a raw material. For

example, the amount of an N-oxyl compound used is preferably not less than 0.01 mmol,

more preferably not less than 0.02 mmol, per 1 g of absolute dry cellulose. The upper limit

of this amount is preferably not more than 10 mmol, more preferably not more than 1 mmol,

still more preferably not more than 0.5 mmol. Therefore, the amount of an N-oxyl

compound used is in the range of preferably from 0.01 to 10 mmol, more preferably from

0.01 to 1 mmol, still more preferably from 0.02 to 0.5 mmol, per 1 g of absolute dry cellulose.

[0013] Bromides refer to compounds containing bromine, and examples thereof include

alkali metal bromides which are able to dissociate and ionize in water, such as sodium bromide. Iodides refer to compounds containing iodine, and examples thereof include alkali metal iodides. The amount of a bromide or an iodide used can be selected within a range that can promote an oxidization reaction. The total amount of a bromide and an iodide is preferably not less than 0.1 mmol, more preferably not less than 0.5 mmol, per 1 g of absolute dry cellulose. The upper limit of this amount is preferably not more than 100 mmol, more preferably not more than 10 mmol, still more preferably not more than 5 mmol.

Therefore, the total amount of a bromide and an iodide is in the range of preferably from 0.1

to 100 mmol, more preferably from 0.1 to 10 mmol, still more preferably from 0.5 to 5 mmol,

per 1 g of absolute dry cellulose.

[0014] Examples of an oxidizing agent include, but are not particularly limited to, halogens, hypohalous acids, halous acids, perhalic acids, salts thereof, halogen oxides, and peroxides.

Inter alia, preferred because of low cost and low environmental impact are hypohalous acids

or salts thereof, more preferably hypochlorous acid or a salt thereof, still more preferably

sodium hypochlorite. The amount of an oxidizing agent used is preferably not less than 0.5

mmol, more preferably not less than 1 mmol, still more preferably not less than 3 mmol, per

1 g of absolute dry cellulose. The upper limit of this amount is preferably not more than

500 mmol, more preferably not more than 50 mmol, still more preferably not more than 25

mmol. Therefore, the amount of an oxidizing agent used is in the range of preferably from

0.5 to 500 mmol, more preferably from 0.5 to 50 mmol, still more preferably from 1 to 25

mmol, particularly preferably from 3 to 10 mmol, per 1 g of absolute dry cellulose. When

an N-oxyl compound is used, the amount of an oxidizing agent used is preferably not less

than 1 mol per 1 mol of the N-oxyl compound, and the upper limit of this amount is

preferably 40 mol. Therefore, the amount of an oxidizing agent used is preferably in the

range of from 1 to 40 mol per1 mol of the N-oxyl compound.

[0015] The conditions for an oxidization reaction, such as pH and temperature, are not

particularly limited, and the oxidization reaction generally progresses efficiently even under

relatively mild conditions. The reaction temperature is preferably not less than 4°C, more

preferably not less than 15°C. The upper limit of the temperature is preferably not more than 40°C, more preferably not more than 30°C. Therefore, the reaction temperature is in the range of preferably from 4 to 40°C, or approximately from 15 to 30°C, or namely may be room temperatures. The pH of a reaction solution is preferably not less than 8, more preferably not less than 10. The upper limit of the pH is preferably not more than 12, more preferably not more than 11. Therefore, the pH of a reaction solution is in the range of preferably from 8 to 12, more preferably approximately from 10 to 11. Since carboxyl groups are generally produced in cellulose during the progress of an oxidization reaction, the pH of a reaction solution tends to drop. Therefore, in order to allow an oxidization reaction to progress efficiently, it is preferred to add an alkaline solution such as aqueous sodium hydroxide solution to maintain the pH of a reaction solution within the aforementioned range.

The reaction medium used for oxidization is preferably water because of its workability and

unlikeliness to cause a side reaction.

[0016] The reaction time for oxidization can be determined as appropriate according to the

state of progress of the oxidization reaction, and is generally not less than 0.5 hour. The

upper limit of this reaction time is generally not more than 6 hours, preferably not more than

4 hours. Therefore, the reaction time for oxidization is generally in the range of from 0.5 to

6 hours, for example approximately from 0.5 to 4 hours. The oxidization reaction may be

performed in two or more divided stages. For example, when an oxidized cellulose

obtained by filtration after the completion of the first reaction stage is oxidized again under

the same or different reaction conditions, the cellulose can be oxidized efficiently without

being interfered with by a salt produced as a byproduct during the first reaction stage.

[0017] Another example of the carboxylation (oxidization) method is ozone oxidization.

According to this oxidization reaction method, not only hydroxyl groups located at least at

C2 and C6 positions of glucopyranose rings constituting cellulose are oxidized, but also

degradation of cellulose chains take place. The ozone treatment is generally performed by

bringing a cellulose raw material into contact with an ozone-containing air. The ozone

concentration in the air is preferably not less than 50 g/m 3 . The upper limit of the ozone

concentration is preferably not more than 250 g/m3 , more preferably not more than 220 g/m3 .

Therefore, the ozone concentration in the air is in the range of preferably from 50 to 250

g/m , more preferably from 50 to 220 g/m. The amount of ozone added is preferably not

less than 0.1% by weight, more preferably not less than 5% by weight, based on 100% by

solid weight of a cellulose raw material. The upper limit of this amount is generally not

more than 30% by weight. Therefore, the amount of ozone added is in the range of

preferably from 0.1 to 30% by weight, more preferably from 5 to 30% by weight, based on

100% by solid weight of a cellulose raw material. The ozone treatment temperature is

generally not less than 0°C, preferably not less than 20°C, and the upper limit of this

temperature is generally not more than 50°C. Therefore, the ozone treatment temperature is

in the range of preferably from 0 to 50°C, more preferably from 20 to 50°C. The ozone

treatment time is generally not less than 1 minute, preferably not less than 30 minutes, and

the upper limit of the ozone treatment time is generally not more than 360 minutes.

Therefore, the ozone treatment time is generally in the range of approximately from 1 to 360

minutes, preferably approximately from 30 to 360 minutes. When the conditions for ozone

treatment fall within the aforementioned ranges, cellulose can be prevented from being

excessively oxidized and degraded, resulting in a satisfactory yield of an oxidized cellulose.

[0018] Ozone-treated cellulose maybe further subjected to additional oxidization treatment

with an oxidizing agent. The oxidizing agent used for additional oxidization treatment is

not particularly limited, and examples thereof include chlorine-based compounds such as

chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and

peracetic acid. An exemplary method of performing additional oxidization treatment is a

method in which such an oxidizing agent is dissolved in water or a polar organic solvent such

as alcohol to prepare a solution of the oxidizing agent and a cellulose raw material is

immersed in the oxidizing agent solution. The contents of carboxyl groups, carboxylate

groups and aldehyde groups in an oxidized cellulose nanofiber can be adjusted by controlling

the oxidization conditions, such as the amount of an oxidizing agent added and reaction time.

[0019] An exemplary method of determining carboxyl group content is described below.

mL of a 0.5% by weight slurry (water dispersion) of an oxidized cellulose is prepared, adjusted to pH 2.5 by adding a 0.1 M aqueous hydrochloric acid solution, and then measured for electrical conductivity while a 0.05 N aqueous sodium hydroxide solution is added dropwise until the pH reaches 11. The carboxyl group content can be calculated according to the following equation, based on the amount (a) of sodium hydroxide consumed during the stage of neutralization with weak acid where the electrical conductivity changes slowly.

Carboxyl group content [mmol/g oxidized cellulose]

= a [mL] x 0.05 / mass of oxidized cellulose [g]

[0020] The carboxyl group content in an oxidized cellulose, as determined in such a

manner, is preferably not less than 0.1 mmol/g, more preferably not less than 0.5 mmol/g,

still more preferably not less than 0.8 mmol/g, based on absolute dry weight. The upper

limit of this content is preferably not more than 3.0 mmol/g, more preferably less than 2.5

mmol/g, still more preferably not more than 2.0 mmol/g. Therefore, said carboxyl group

content is in the range of preferably from 0.1 to 3.0 mmol/g, more preferably from 0.5 to 2.5

mmol/g, still more preferably from 0.8 to 2.0 mmol/g.

[0021] [Etherification]

Examples of etherification treatments include, but are not limited to, carboxylation

(carboxymethyl etherification), methylation (methyl etherification), ethylation (ethyl

etherification), cyanoethylation (cyanoethyl etherification), hydroxyethylation (hydroxyethyl

etherification), hydroxypropylation (hydroxypropyl etherification), ethylhydroxyethylation

(ethylhydroxyethyl etherification), and hydroxypropylmethylation (hydroxypropylmethyl

etherification). Among them, the carboxymethylation method will be described below as an

example.

[0022] The degree of carboxymethyl substitution per anhydrous glucose unit in a

carboxymethylated cellulose or CNF obtained by carboxymethylation is preferably not less

than 0.01, more preferably not less than 0.05, still more preferably not less than 0.10. The

upper limit of this degree is preferably not more than 0.50, more preferably not more than

0.40, still more preferably not more than 0.35. Therefore, the degree of carboxymethyl

substitution is in the range of preferably from 0.01 to 0.50, more preferably from 0.05 to 0.40, still more preferably from 0.10 to 0.30.

[0023] The method of carboxymethylation is not particularly limited, and examples thereof

include such a method as mentioned above, in which a cellulose raw material used as a

starting material is subjected to mercerization followed by etherification. For the

carboxymethylation reaction, a solvent is generally used. Examples of a solvent include

water, alcohols (e.g., lower alcohols) and mixed solvents thereof. Examples of lower

alcohols include methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol,

isobutanol, and tertiary butanol. As for the mixing proportion of a lower alcohol in a mixed

solvent, the lower limit is generally not less than 60% by weight, and the upper limit is not

more than 95% by weight -- thus, said mixing proportion is preferably in the range of from

to 95% by weight. The amount of a solvent is generally 3 times by weight that of the

cellulose raw material. The upper limit of this amount is not particularly limited, but is

preferably 20 times by weight. Therefore, the amount of a solvent is preferably in the range

of from 3 to 20 times by weight.

[0024] Mercerization is generally performed by mixing a starting material with a

mercerizing agent. Examples of a mercerizing agent include alkali metal hydroxides such

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

preferably not less than 0.5 times moles, more preferably not less than 1.0 times mole, still

more preferably not less than 1.5 times moles, per anhydrous glucose residues in a starting

material. The upper limit of this amount is generally not more than 20 times moles,

preferably not more than 10 times moles, more preferably not more than 5 times moles.

Therefore, the amount of a mercerizing agent used is in the range of preferably from 0.5 to 20

times moles, more preferably from 1.0 to 10 times moles, still more preferably from 1.5 to 5

times mole.

[0025] The reaction temperature for mercerization is generally not less than 0°C, preferably

not less than 10°C, and the upper limit of this reaction temperature is generally not more than

°C, preferably not more than 60°C. Therefore, this reaction temperature is generally in

the range of from 0 to 70°C, preferably from 10 to 60°C. The reaction time for mercerization is generally not less than 15 minutes, preferably not less than 30 minutes.

The upper limit of this reaction time is generally not more than 8 hours, preferably not more

than 7 hours. Therefore, this reaction time is generally in the range of from 15 minutes to 8

hours, preferably from 30 minutes to 7 hours.

[0026] The etherification reaction is generally performed by adding a carboxymethylating

agent to the reaction system after mercerization. Examples of a carboxymethylating agent

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

generally preferably not less than 0.05 times moles, more preferably not less than 0.5 times

moles, still more preferably not less than 0.8 times moles, per glucose residues in a cellulose

rawmaterial. The upper limit of this amount is generally not more than 10.0 times moles,

preferably not more than 5 times moles, more preferably not more than 3 times moles.

Therefore, this amount is in the range of preferably from 0.05 to 10.0 times moles, more

preferably from 0.5 to 5 times moles, still more preferably from 0.8 to 3 times moles. The

reaction temperature for etherification is generally not less than 30°C, preferably not less than

°C, and the upper limit of this reaction temperature is generally not more than 90°C,

preferably not more than 80°C. Therefore, this reaction temperature is generally in the

range of from 30 to 90°C, preferably from 40 to 80°C. The reaction time for etherification

is generally not less than 30 minutes, preferably not less than 1 hour, and the upper limit of

this reaction time is generally not more than 10 hours, preferably not more than 4 hours.

Therefore, this reaction time is generally in the range of from 30 minutes to 10 hours,

preferably from 1 to 4 hours. During the carboxymethylation reaction, a reaction solution

may be stirred depending on the need.

[0027] The degree of carboxymethyl substitution per glucose unit in a carboxymethylated

cellulose is determined according to, for example, the following procedure: 1) approximately

2.0 g (absolute dry) of a carboxymethylated cellulose is precisely weighted out and placed

into a 300 mL stoppered conical flask; 2) 100 mL of a mixed solution of 1000 mL of nitric

acid-methanol and 100 mL of premium grade concentrated nitric acid is added, and shaking

is continued for 3 hours to convert a carboxymethycellulose salt (carboxymethylated cellulose) to a H-type carboxymethylated cellulose; 3) 1.5-2.0 g of the H-type carboxymethylated cellulose (absolute dry) is precisely weighted out and placed into a 300 mL stoppered conical flask; 4) the H-type carboxymethylated cellulose is wetted with 15 mL of 80% methanol, 100 mL of 0.1 N NaOH is added, and shaking is continued at room temperature for 3 hours; 5) excess NaOH is back titrated with 0.1 N H2 SO4 using phenolphthalein as an indicator; and 6) the degree of carboxymethyl substitution (DS) is calculated according to the following equation.

A = [(100 x F' - (0.1 N H 2 SO 4 ) (mL) x F) x 0.1] / (absolute dry mass (g) of H-type

carboxymethylated cellulose)

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

A: Amount (mL) of 1 N NaOH required for neutralization of 1 g of H-type

carboxymethylated cellulose

F: Factor for 0.1 N H 2 SO4

F': Factor for 0.1 N NaOH

[0028] 3) Mechanical defibration

A chemically modified cellulose is mechanically defibrated to obtain a CNF.

Defibration treatment may be performed once or may be repeated two or more times. It is

preferred to defibrate a mixture comprising a chemically modified cellulose and a dispersion

medium. The dispersion medium is preferably water. The apparatus used for defibration

is not particularly limited, and examples thereof include different types of apparatus, such as

high-speed rotating type, colloid mill type, high pressure type, roll mill type, and ultrasonic

type, with a high pressure or ultrahigh pressure homogenizer being preferred, and a wet-type

high pressure or ultrahigh pressure homogenizer being more preferred. The apparatus is

preferably capable of applying a strong shear force to a chemically modified cellulose. The

pressure that can be applied by the apparatus is preferably not less than 50 MPa, more

preferably not less than 100 MPa, still more preferably not less than 140 MPa. The

apparatus is preferably a wet-type high pressure or ultrahigh pressure homogenizer. By

using such a homogenizer, defibration can be carried out effectively.

[0029] In the case of defibrating a liquid dispersion of a chemically modified pulp, the

solids concentration of the modified cellulose in the liquid dispersion is generally not less

than 0.1% by weight, more preferably not less than 0.2% by weight, still more preferably not

less than 0.3% by weight. At such a solids concentration, the relative amount of a

dispersion medium to the amount of the modified cellulose becomes appropriate, leading to

greater efficiency. The upper limit of this concentration is generally preferably not more

than 20% by weight, more preferably not more than 15% by weight, still more preferably not

more than 10% by weight. By controlling the solids concentration to lie within such an

upper limit, the flowability of the liquid dispersion can be maintained.

[0030] 4) Characteristics

The average fiber diameter of a CNF is generally approximately not less than 2 nm

but less than 500 nm, preferably in the range of from 2 to 100 nm, in terms of length

weighted average fiber diameter. The upper limit of this average fiber diameter is more

preferably not more than 50 nm. The average fiber length of a CNF is preferably in the

range of from 50 to 2000 nm, in terms of length-weighted average fiber length. The length

weighted average fiber diameter and length-weighted average fiber length (hereinafter also

simply referred to as "average fiber diameter" and "average fiber length", respectively) are

determined by observing a fiber using an atomic force microscope (AFM) or a transmission

electron microscope (TEM). The average aspect ratio of a CNF is generally not less than 10.

The upper limit of this average aspect ratio is not particularly limited, and is generally not

morethan1000. The average aspect ratio can be calculated according to the following

equation.

Average aspect ratio = average fiber length / average fiber diameter

[0031] It is preferable that the carboxyl group content and the degree of carboxymethyl

substitution per glucose unit in a CNF should be the same as those in a chemically modified

cellulose.

[0032] In this embodiment, it is preferred to use a CNF that gives a type B viscosity (60

rpm, 20°C) of from 500 to 7000 mPa-s as measured as a 1% (w/v) water dispersion (i.e., a water dispersion in which 1 g (dry weight) of a CNF is contained in 100 mL of water). The type B viscosity is an index for identifying the characteristics of a CNF, such as functional group content, average fiber length and average fiber diameter, and is adjusted as appropriate depending on the intended use.

[0033] The type B viscosity of a water dispersion of a CNF can be measured by a known

technique. For example, the type B viscosity can be measured using the Viscometer TV-10

produced by Toki Sangyo Co., Ltd. During the measurement, the temperature is set to 20°C

and the rotation speed of a rotor is set to 60 rpm. The water dispersion of a CNF of the

present invention has a thixotropic property, characterized in that the viscosity of the water

dispersion drops when stirring is started and a shear force is applied whereas the water

dispersion increases in viscosity and turns into a gel when it is allowed to stand still.

Therefore, it is preferred to measure the type B viscosity of the water dispersion under

adequate stirring.

[0034] (2) Starch

Starch is a polymer of D-glucose, preferably a mixture of amylose and amylopectin.

In this embodiment, the "starch" also includes starch-derived polymeric compounds.

Examples of such starch-derived polymeric compounds include those obtained by denaturing,

modifying or processing a starch, with thermochemically modified starches being particularly

preferred. Examples of thermochemically modified starches includes starches gelatinized

and oxidized instantaneously by heating in the presence of an oxidizing agent. Such

thermochemically modified starches are characterized by having low functional group

content. Among them, preferred is an ammonium persulfate-modified starch which is

prepared using ammonium persulfate as an oxidizing agent. Other examples of

thermochemically modified starches include a urea- and acid-modified starch which is

prepared by modifying a starch with urea and an acid. A urea- and acid-modified starch is

prepared by following, for example, the method described in Japanese Unexamined Patent

Application Publication No. JP 2004-238523. In this embodiment, an ammonium

persulfate-modified starch and a urea- and acid-modified starch may be used in combination.

[0035] (3) Base paper

The base paper is a layer serving as a base material for paper, and comprises a pulp

as a main component. The pulp material used to make a base paper is not particularly

limited, and examples thereof that can be used include: mechanical pulps such as ground pulp

(GP), thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP); deinked

pulps (DIP); and chemical pulps such as nadelholz (softwood) kraft pulp (NKP) and laubholz

(hardwood) kraft pulp (LKP). As deinked (waste paper) pulps, use can be made of those

pulps derived from sorted waste papers such as high-quality paper, medium-quality paper,

low-quality paper, newspaper waste paper, leaflet waste paper, and magazine waste paper, or

those pulps derived from unsorted waste papers comprising a mixture of different waste

papers.

[0036] It is acceptable that a known filler may be added to the base paper. However, when

a base paper is used to make papers that do not require whiteness or opacity, such as paper

board, or when a high ash content pulp material such as waste paper is used to make a base

paper, such a filler may not be added to the base paper. In the case of adding a filler,

examples of fillers used include: inorganic fillers such as heavy calcium carbonate, light

calcium carbonate, clay, silica, light calcium carbonate-silica composite, kaolin, fired 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-formalin resin, melamine resin, polystyrene resin and phenol

resin. Such fillers may be used alone or in combination. Among them, preferred is heavy

calcium carbonate or light calcium carbonate, which are representative fillers used to make

neutral and alkaline papers and can give papers high opacity. The content of a filler in the

base paper is in the range of preferably from 5 to 20% by weight, more preferably from 6 to

% by weight, based on the weight of the base paper. In the present invention, it is more

preferred that the content of a filler in the base paper should be not less than 10% by weight,

since the decline in paper strength can be reduced even when the paper has high ash content.

[0037] Depending on the need, various wet end additives may be used, such as bulking

agent, dry paper strengthening agent, wet paper strengthening agent, freeness improver, dye,

and/or neutral sizing agent.

[0038] The base paper is made by following a known papermaking method. Papermaking

can be carried out using, for example, but not limited to, a fourdrinier paper machine, a gap

former-type paper machine, a hybrid former-type paper machine, an on-top former-type

paper machine, or a cylinder paper machine.

[0039] The base paper may be composed of a single layer or of multiple layers. The base

paper may contain a CNF described above. In the case of a multi-layer base paper, some of

multiple paper layers may contain a CNF, or all of multiple paper layers may contain a CNF.

When the base paper contains a CNF, the content of the CNF is preferably not less than

0.0001% by weight, more preferably not less than 0.0003% by weight, still more preferably

not less than 0.001% by weight, based on the total pulp weight of the base paper.

[0040] (4) Clear coating layer

The starch:CNF ratio (weight ratio) in a clear coating layer is in the range of

preferably from 1000:1 to 20:1, more preferably from 350:1 to 67:1, still more preferably

from 300:1 to 67:1. When this weight ratio falls within the aforementioned range, the film

forming capability of the starch-based clear coating layer is enhanced, so that high ink

mileage, high print gloss, and high surface strength can be achieved.

[0041] The coating amount of the clear coating layer is in the range of preferably from 0.01

to 3.0 g/m2 , morepreferably from 0.1 to 2.0 g/m2 , interms of solids per one side. Aclear

coating can be formed by, for example, coating a base paper with a starch-based clear coating

liquid using a coater such as size press, gate roll coater, premetered size press, curtain coater,

or spray coater. In one instance, in the case of coating a clear coating liquid using a gate

roll coater, it is preferred from the viewpoint of coating suitability that the clear coating

liquid should have a type B viscosity (30°C, 60 rpm) of from 5 to 450 mPa-s, more

preferably from 10 to 300 mPa-s, at a solids concentration of 5% by weight. Inthecaseof

coating a clear coating liquid using a gate roll coater, if the clear coating liquid has a type B viscosity of less than 5 mPa-s, it may be difficult, because of too low viscosity, to coat an adequate amount of the clear coating liquid. If the clear coating liquid has a type B viscosity of more than 450 mPa-s, boiling may occur, leading to deterioration of workability.

The solids concentration of the clear coating liquid is adjusted so as to ensure that the

aforementioned range of viscosity can be achieved, but is preferably in the range of from 2 to

14% by weight.

[0042] The content of a CNF originating from the clear coating layer is in the range of

preferably from 1.0x10-5 to 0.1 g/m 2 , more preferably from 1.Ox1O-4 to 5.0x10-2 gm 2, per

one side.

[0043] (4) Pigment coating layer

In this embodiment, the paper may have a pigment coating layer. The pigment

coating layer is a layer comprising a white pigment as a main component. Examples of a

white pigment include commonly used pigments such as calcium carbonate, kaolin, clay,

fired kaolin, amorphous silica, zinc oxide, aluminum oxide, satin white, aluminum silicate,

magnesium silicate, magnesium carbonate, titanium oxide, and plastic pigments. Example

of calcium carbonate include light calcium carbonate and heavy calcium carbonate.

[0044] The pigment coating layer contains a binder. Examples of a binder include, but are

not limited to: different types of starches as mentioned above; different types of proteins,

such as casein, soybean protein, and synthetic protein; 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.

Such binders may be used alone, or two or more thereof may be used in combination. It is

preferable to use a starch-based binder and a styrene-butadiene copolymer in combination.

[0045] The pigment coating layer may contain different auxiliary agents commonly used in

the field of paper production, such as dispersant, thickener, antifoamer, colorant, antistatic

agent, and/or antiseptic agent. The pigment coating layer may contain a CNF. When the

base paper contains a CNF, the content of the CNF is preferably in the range of from1x10-3 to 1 part by weight based on 100 parts by weight of the pigment. When this content falls within the aforementioned range, there can be obtained a pigment coating liquid having moderate water retention ability without showing a significant increase in viscosity. Further, the pigment coating layer of this embodiment may be a pigment coating layer described later in relation to the second embodiment.

[0046] The pigment coating layer can be formed by coating one or both sides of a base

paper with a pigment coating liquid by a known method. From the viewpoint of coating

suitability, the solids concentration of the pigment coating liquid is preferably in the range of

approximately from 30 to 70% by weight. One, two or three or more pigment coating layers

may be formed. The coating amount of a pigment coating layer is adjusted as appropriate

depending on the intended use, but in the case of production of a coated paper for printing,

said coating amount is not less than 5 g/m2 , preferably not less than 10 g/m2 , in total per one

side. The upper limit of this coating amount is preferably not more than 30 g/m 2 , more

preferably not more than 25 g/m2 .

[0047] When the paper of this embodiment further comprises a pigment coating layer, a

pigment coated paper having not only high ink mileage but also excellent surface strength

and print gloss can be obtained.

[0048] (5) Characteristics

The paper of this embodiment is characterized by having high ink mileage and

excellent print gloss and surface strength, and also by being easy to produce. The paper of

this embodiment generally has a basis weight of approximately from 20 to 500 g/m2 ,

preferably from 30 to 250 g/m2 , as measured according to JIS P 8124.

[0049] (6) Method of producing a paper having a clear coating layer comprising a starch

and a CNF

The paper of this embodiment is preferably produced through coating a base paper

prepared by a known method with a clear coating liquid comprising a CNF. To be specific,

the paper of this embodiment is preferably produced by a method comprising the following

steps.

Step 1: Preparing a clear coating liquid comprising a starch and a CNF;

Step 2: Forming a clear coating layer on the base paper using the prepared clear

coating liquid.

[0050] The starch and CNF used at step 1 are as described in preceding paragraphs. The

preparation method and characteristics of the coating liquid are also as described in preceding

paragraphs. The coating at step 2 can also be carried out as described in preceding

paragraphs.

[0051] The paper of this embodiment maybe produced by a method comprising not only

steps 1 and 2 as mentioned above but also step 3 as mentioned below.

Step 3: Forming a pigment coating layer comprising a pigment and a binder on the

clear coating layer comprising a starch and a CNF.

The pigment coating liquid used at step 3 may be a pigment coating liquid

comprising a starch and a CNF as used in the second embodiment.

[0052] 2. Paper having a pigment coating layer comprising a starch and a CNF (second

embodiment)

(1) CNF, starch, base paper

In this embodiment, the CNF, starch, and base paper as described in relation to the

first embodiment can be used.

[0053] (2) Pigment coating layer

The paper of this embodiment has a pigment coating layer comprising a starch and a

CNF, the pigment coating layer which is formed on one side or both sides of a base paper.

The pigment coating layer is a layer comprising a white pigment as a main component.

Examples of the white pigment that can be used include those described in relation to the first

embodiment. The starch:CNF weight ratio in the pigment coating layer is not limited, but is

preferably in the range of from 300:1 to 2:1. When this weight ratio falls within the

aforementioned range, the film-forming capability of the pigment coating layer is enhanced,

so that high print gloss and high surface strength can be achieved. From this viewpoint, this

weight ratio is more preferably in the range of from 200:1 to 5:1.

[0054] The pigment coating layer may contain a binder in addition to a starch. Examples

of the binder are as described in relation to the first embodiment.

[0055] The pigment coating layer may contain different auxiliary agents commonly used in

agent, and/or antiseptic agent.

[0056] The pigment coating layer can be formed by coating one or both sides of a base

paper with a coating liquid by a known method. From the viewpoint of coating suitability,

the solids concentration of the pigment coating liquid is preferably in the range of

said coating amount is not less than 1 g/m2 , preferably not less than 5 g/m 2, in total per one

side. The upper limit of this coating amount is preferably not more than 30 g/m 2, more

preferably not more than 20 g/m 2 . The content of a CNF originating from the pigment

coating layer is in the range of preferably from 1.0x10-5 to 0.1 g/m 2, more preferably 1.OxI04

to 5.Ox10-2 g/m 2 , per one side.

[0057] (3) Clear coating layer

The paper of this embodiment may have a clear coating layer formed on one side or

both sides of a base paper. By forming a clear coating, the surface strength and smoothness

of the base paper can be enhanced. The clear coating layer is formed from a clear coating

liquid comprising, as a main component, a water-soluble polymer such as starch,

polyacrylamide or polyvinyl alcohol. The clear coating layer may be a clear coating layer

comprising a starch and a CNF as described above in relation to the first embodiment.

Since the clear coating layer has high film-forming capability, high ink mileage, high print

gloss, and high surface strength can be achieved.

[0058] In this embodiment, the coating amount of a clear coating layer is in the range of

preferably from 0.01 to 3.0 g/m 2 , more preferably from 0.1 to 2.0 g/m2 , in terms of solids per

one side. A clear coating can be formed by, for example, coating a base paper with a clear coating liquid using a coater such as size press, gate roll coater, premetered size press, curtain coater, or spray coater. In one instance, in the case of coating a clear coating liquid using a gate roll coater, it is preferred from the viewpoint of coating suitability that the clear coating liquid should have a type B viscosity (30°C, 60 rpm) of from 5 to 450 mPa-s, more preferably from 10 to 300 mPa-s, at a solids concentration of 5% by weight. Inthecaseof coating a clear coating liquid using a gate roll coater, if the clear coating liquid has a type B viscosity of less than 5 mPa-s, it may be difficult, because of too low viscosity, to coat an adequate amount of the clear coating liquid. If the clear coating liquid has a type B viscosity of more than 450 mPa-s, boiling may occur, leading to deterioration of workability.

14% by weight. Further, the pigment coating layer of this embodiment may be a pigment

coating layer as described above in relation to the first embodiment.

[0059] (4) Characteristics

The paper of this embodiment generally has a basis weight of approximately from

to 500 g/m2 , preferably from 30 to 300 g/m2 , as measured according to JIS P 8124.

[0060] (5) Method of producing a paper having a pigment coating layer comprising a

starch and a CNF

prepared by a known method with a pigment coating liquid comprising a CNF. To be

specific, the paper of this embodiment is preferably produced by a method comprising the

following steps.

Step 1: Preparing a pigment coating liquid comprising a pigment, a starch and a

CNF;

Step 2: Forming a pigment coating layer on the base paper using the prepared

pigment coating liquid.

[0061] The starch and CNF used at step 1 are as described in preceding paragraphs. The

preparation method and characteristics of the coating liquid are also as described in preceding paragraphs. The coating at step 2 can also be carried out as described in preceding paragraphs.

[0062] The paper of this embodiment may be produced by a method comprising not only

steps 1 and 2 as mentioned above but also step 3 as mentioned below.

Step 3: Forming a clear coating layer on the base paper prior to step 2.

The clear coating liquid used at step 3 may be a clear coating liquid comprising a

starch and a CNF as used in the first embodiment.

EXAMPLES

[0063] [Example Al]

<CNF>

First, 5.00 g (absolute dry) of a bleached, unbeaten kraft pulp (whiteness: 85%;

produced by Nippon Paper Industries Co., Ltd.) derived from softwood was added to 500 mL

of an aqueous solution of 39 mg (0.05 mmol per g of absolute dry cellulose) of TEMPO

(produced by Sigma Aldrich) and 514 mg (1.0 mmol per g of absolute dry cellulose) of

sodium bromide, and stirring was continued until the pulp was uniformly dispersed. To the

reaction system, an aqueous solution of sodium hypochlorite was added to a sodium

hypochlorite concentration of 5.5 mmol/g to initiate oxidization reaction at room temperature.

In order to avoid a decrease in the pH of the system during the reaction, a 3 M aqueous

sodium hydroxide solution was sequentially added to adjust pH to 10. The reaction was

terminated once sodium hypochlorite had been consumed so that the pH of the system no

longer changed. The mixture after the reaction was filtered through a glass filter to separate

a pulp, which was fully washed with water to give an oxidized pulp (carboxylated cellulose).

The pulp yield was 90%, the time required for the oxidization reaction was 90 minutes, and

the carboxyl group content was 1.5 mmol/g. The oxidized pulp was adjusted with water to a

concentration of 1% (w/v) and treated three times using an ultrahigh-pressure homogenizer

(20°C, 150 MPa) to obtain a water dispersion of CNF. The resulting CNF had an average

fiber diameter of 3 nm and an aspect ratio of 250.

[0064] <Clear coating liquid 1>

An oxidized starch (SK20 produced by Japan Corn Starch Co., Ltd.) was added to

the water dispersion of CNF prepared as mentioned above, to thereby prepare a clear coating

liquid 1 having a starch:CNF weight ratio of 30:1. The typeB viscosityat30°C and60 rpm

of this clear coating liquid 1 at a solids concentration of 5% by weight is shown in Table 1.

[0065] <Paper>

0.5% by weight of aluminum sulfate, 0.77% by weight of a cationized starch, and

0.05% by weight of a paper strengthening agent were added to LBKP (produced by Nippon

Paper Industries Co., Ltd.; 360 mL c.s.f.) to thereby prepare a pulp slurry having a solids

concentration of 0.7% by weight. With the use of the obtained pulp slurry, a base paper was

produced by a paper machine. On both sides of the produced base paper, the clear coating

liquid 1 was coated in a coating amount of 1.2 g/m2 in terms of solids per one side using a

gate roll coater, followed by drying by a conventional procedure, to thereby obtain a clear

coated paper. The obtained paper was evaluated by procedures described later. The

results are shown in Table 1.

[0066] [Comparative Example Al]

A clear coated paper was produced by the same procedures as in Example Al except

that no CNF was used.

[0067] [Example A2]

0. 7 % by weight of aluminum sulfate, 0.30% by weight of a cationized starch, and

0.06% by weight of a paper strengthening agent were added to LBKP (produced by Nippon

Paper Industries Co., Ltd.; 420 mL c.s.f.) to thereby prepare a pulp slurry having a solids

concentration of 0.7% by weight. With the use of the obtained pulp slurry, a base paper

with a basis weight of 34.5 g/m2 was produced by a paper machine.

[0068] <Pigment coating liquid 1>

2.0 parts by weight of a latex as a binder and 6.7 parts by weight of an oxidized

starch were added to 100 parts by weight of heavy calcium carbonate to thereby prepare a

pigment coating liquid having a solids concentration of 60% by weight.

[0069] <Pigment coated paper>

A clear coating liquid 1 was prepared by the same procedure as in Example Al

except that the starch:CNF weight ratio in the clear coating liquid was changed to 60:1. The

clear coating liquid 1 was coated on both sides of the base paper prepared above in a coating

amount of 0.2 g/m2 in terms of solids per one side using a gate roll coater, followed by drying

by a conventional procedure, to thereby form clear coating layers. Further, the pigment

coating liquid 1 prepared above was coated on both sides of the prepared clear coated paper,

followed by drying by a conventional procedure. The thus-obtained pigment coated paper

was evaluated by procedures described later. The results are shown in Table 1.

[0070] [Example A3]

A pigment coated paper was produced by the same procedures as in Example A2

except that the starch:CNF weight ratio in a clear coating liquid 1 was changed to a value

shown in Table 1.

[0071] [Comparative Example A2]

A pigment coated paper was produced by the same procedures as in Example A2

except that no CNF was used.

[0072] [Example A4]

A pigment coated paper was produced by the same procedures as in Example A2

shown in Table 1.

[0073] [Example A5]

0.1% by weight of ammonium persulfate as an oxidizing agent was added to a raw

(unmodified) starch to thereby prepare a starch slurry having a solids concentration of 25%

by weight. The prepared starch slurry was steamed and thermochemically modified at

150°C by a jet cooker, cooled, and then adjusted to pH 7 with an aqueous solution of sodium

hydroxide. Further, water was added to obtain an aqueous solution of ammonium

persulfate-modified starch having a solids concentration of 12% by weight.

[0074] <Clear coating liquid 2>

The thus-prepared aqueous solution of ammonium persulfate-modified starch was

added to the water dispersion of CNF prepared as mentioned above, to thereby prepare a

clear coating liquid 2 having a starch:CNF weight ratio of 67:1. The type B viscosity at

°C and 60 rpm of this clear coating liquid 2 at a solids concentration of 5% by weight is

shown in Table 1.

A pigment coated paper was produced by the same procedures as in Example A2

except that the clear coating liquid 1 was replaced with the clear coating liquid 2.

[0075] [Examples A6, A7]

Pigment coated papers were produced by the same procedures as in Example A5

except that the starch:CNF weight ratio in a clear coating liquid 2 was changed to values

shown in Table 1.

[0076] [Examples A8 to A12]

Pigment coated papers were produced by the same procedures as in Example A2

except that the starch:CNF weight ratio in a clear coating liquid 1 was changed to values

shown in Table 1.

[0077] [Comparative Example A3]

A pigment coated paper was produced by the same procedures as in Example A5

except that no CNF was used.

[Comparative Example A4]

A pigment coated paper was produced by the same procedures as in Example A8

except that no CNF was used.

U" <C' ojC

- ~ ~ Co ~0 1 rUO4 - C

o in-00 Lo

m-10 L C 'r 0 .OC r -- Cd - - 0

< 10 U5 0 Lo g co< d

0 00 0 0 r v -n

<OH Q 6 C, -1W *

0. CD x IfOC.J

< U C',0 :C

C')U' 0 0lC )

U~0 ~ ( 0 fl m CD) !2 CD0i 2"n - 0R::

0 10 C'',~ ' III oi r- L o to m

w~b E LO 22 U7b

rp a. i

ot

0 0i *0 CO CCbA 0 0

C0 0 (

0 CD 0C C OL CC. 0 00

-0 0. 0 0 to w

0 E to~ C.

0 4 .

L bd bO~c CL 1C .

0 0.C 0 0 C . 0 MC Q o 2

bo- 0 CC C

-~~ 0CCC 0.C -0 0 ;

00mC E '

[Table 1-2] Table 1 Comparative Example Al A2 A3 A4 Themochemically modified starch Parts by - - 100 Compositional Oxidized starch weight 100 100 - 100 profile CNF 0 0 0 0 Weight ratio Starch/CNF - - - Clear coating liquid Coating amount per one side g/m 2 1.2 0.2 0.2 0.2 Physical properties Concentration % 5 5 5 5 of coating materialI Type B viscosity (30°C, 60rpm) mPa-s 6.8 6.8 6.7 8.8 Suitability for gate roll coater coating A A A A Heavy calcium carbonate Parts by - 100 100 100 Compositional Pigment coating profile Latex weight - 2 2 2 liquid | Oxidized starch - 6.7 6.7 6.7 Coating amount per one side g/m 2 - 7.6 7.5 7.5 White paper quality Basis weight I g/m 2 100.5 50.1 51.2 51.1 Print gloss CM % - 53.9 54.5 55.1 Print quality Picking evaluation LeoEcoo Y Cyan n* - 54 54 49 Ink mileage Density: 1.3 After 24h 1.43 0.98 0.97 0.99 * Large in number: bad; small in number: good

[0079] The clear coated papers of the present invention had high print gloss and high ink

mileage. In particular, the papers of Example Al to A3, A5 to A12, which were made with

a CNF providing a liquid dispersion having a specified viscosity, were excellent in

workability during production. Further, the pigment coated papers, which were prepared by

further forming pigment coating layers on the clear coated papers of this invention, had high

ink mileage, and excellent print gloss and surface strength.

[0080] <Evaluation procedures>

1) Basis weight

The basis weight was measured according to JIS P8124.

2) Print gloss

Solids areas were printed in the order of cyan and magenta (CM) by a sheet-fed

offset printer (4-color) produced by Roland DG with sheet-fed offset inks (NEX-M produced

by Toyo Ink Co., Ltd.) at a printing speed of 8000 sheets/hr so as to ensure that the inking

densities on solid areas were 1.60 for cyan and 1.50 for magenta. The gloss of the cyan and

magenta (CM) solid printed areas of a printed sheet was determined according to JIS P 8142.

[0081] 3) Picking evaluation

A solid cyan area was printed by a sheet-fed offset printer produced by Roland DG using the sheet-fed offset ink, LeoEco Y Cyan, produced by Toyo Ink Co., Ltd. at a printing speedof8000sph. The number of pickings generated on the F and W sides of every 10 printed sheets were counted.

[0082] 4) Ink mileage

The "ink mileage" refers to the number of prints that can be printed with a unit

amount of ink. The amount of ink on paper per unit area required to obtain the same print

density was regarded as color developability and used as a simple and convenient index for

evaluating ink mileage.

Excellent ink mileage means excellent color developability with a small amount of

ink on paper. To be specific, solid printing was done by a Prfbau print tester (IGT), and

after standing overnight after printing on the assumption of sheet-fed printing, the print

density of a printed sheet was determined by a spectrophotometric colorimeter to take

readings for all densities. Also, the difference in the weight of a removable printing disc

before and after printing was regarded as the amount of ink on paper. By varying the

amount of ink coated on the printing disc, the relation between the amount of ink on paper

and print density was determined, and the amount of ink on paper required to obtain a

specified density was calculated based on the determined relational expression. During the

measurement, the printing pressure was set to 700 N and the printing speed was set to 2.0 m/s.

[0083] 5) Suitability for gate roll coater coating (workability during production)

The occurrence of boiling during coating of clear coating liquids on a base paper

using a gate roll coater was visually rated on the four-grade scale detailed below. The

ratings of "A" and "B" were considered favorable.

A: No boiling occurs, and coating suitability (workability during production) is

excellent

B: Boiling occurs a little, but coating suitability (workability during production) is

generally excellent

C: Since boiling occurs, coating suitability (workability during production) is

somewhat impaired

D: Since boiling occurs frequently, coating suitability (workability during

production) is considerably impaired

[0084] [Example BI]

<CNF>

fiber diameter of 3 nm and an aspect ratio of 250.

[0085] <Clear coating liquid 3>

A clear coating liquid 3 containing 30% by weight of an oxidized starch (SK20

produced by Japan Corn Starch Co., Ltd.) was prepared.

[0086] <Pigment coating liquid 2>

2.0 parts by weight of a latex as a binder, 6.7 parts by weight of an oxidized starch,

and 0.2 parts by weight of the CNF prepared as mentioned above were added to 100 parts by

weight of heavy calcium carbonate to thereby prepare a pigment coating liquid 2 having a solids concentration of 60% by weight.

[0087] <Base paper>

0.7% by weight of aluminum sulfate, 0.30% by weight of a cationized starch, and

with a basis weight of 34.5 g/m2 was produced by a paper machine. On both sides of the

produced base paper, the clear coating liquid 3 was coated in a coating amount of 0.2 g/m2 in

terms of solids per one side, followed by drying by a conventional procedure, to thereby form

clear coating layers. Further, the pigment coating liquid 2 prepared above was coated on

both sides of the prepared clear coated paper, followed by drying by a conventional

procedure, to thereby obtain a pigment coated paper. The obtained paper was evaluated by

the procedures described above. The results are shown in Table 2.

[0088] [Example B2]

An oxidized starch (SK20 produced by Japan Corn Starch Co., Ltd.) was added to

liquid 4 having a starch:CNF weight ratio of 30:1. The typeB viscosityat30°C and60 rpm

of this clear coating liquid 4 at a solids concentration of 5% by weight was 130 mPa-s. A

pigment coated paper was produced by the same procedures as in Example B1 except for

using the clear coating liquid 4.

[0089] [Comparative Example BI]

A pigment coated paper was produced by the same procedures as in Example B1

except that no CNF was used.

[0090] [Table 2] Table 2 Com. Ex. Ex. B1 Ex. B2 Compositional Oxidized starch Parts by 100 100 100 Clear coating profile CNF weight 0 0 3.3 liquid Weight ratio Starch/CNF - - 30 Coating amount per one side g/m 2 0.2 0.2 0.2 Heavy calcium carbonate 100.0 100.0 100.0 Compositional Latex Parts by 2.0 2.0 2.0 Pigment oating profile Oxidized starch weight 6.7 6.7 6.7 liquid ICNF 0.0 0.2 0.2 Coating amount per one side g/m 2 7.7 7.5 7.7 White paper quality Basis weight g/m 2 50.1 49.4 50.8 Print gloss CM % 53.9 56.1 59.5 Print quality Picking evaluation LeoEcoo Y Cyan n* 54 26 23 Ink mileage Density: 1.3 After 24h 0.98 0.95 0.93 * Large in number: bad; small in number: good

[0091] The pigment coated papers of the present invention had high ink mileage, and

excellent print gloss and surface strength.

Claims

1. A paper comprising a base paper and a coating layer, wherein the coating layer

comprises a starch and a cellulose nanofiber.

2. The paper according to claim 1, wherein the starch is a thermochemically modified

starch.

3. The paper according to claim 1 or 2, wherein the coating layer is a clear coating layer,

wherein the weight ratio of the thermochemically modified starch the cellulose

nanofiber is in the range of from 350:1 to 67:1.

4. The paper according to claim 3, further comprising a pigment coating layer formed on

the clear coating layer.

5. The paper according to claim 1 or 2, wherein the coating layer is a pigment coating layer.

6. The paper according to any of claims 2 to 5, wherein the thermochemically modified

urea- and acid-modified starch, and a combination thereof.

7. The paper according to any of claims 1 to 6, wherein the cellulose nanofiber is an

anionically modified cellulose nanofiber.

8. The paper according to any of claims 1 to 7, wherein the cellulose nanofiber has a

carboxyl group content of from 0.1 to 3.0 mmol/g.

9. The paper according to any of claims 1 to 8, wherein the cellulose nanofiber has a degree of carboxymethyl substitution per glucose unit of from 0.01 to 0.50.

10. The paper according to any of claims I to 9, wherein the cellulose nanofiber has a type

B viscosity (60 rpm, 20°C) of from 500 to 7000 mPa-s as measured as a 1% (w/v) water

dispersion.