JP7425743B2 - Paper with a coating layer containing cellulose nanofibers - Google Patents

Description

The present invention relates to paper provided with a coating layer containing cellulose nanofibers.

Cellulose nanofibers are expected to be a new material and various studies are being conducted. For example, Patent Document 1 discloses printing paper in which paper is coated or impregnated with a papermaking additive made of cellulose nanofibers. The paper of Patent Document 1 has excellent air permeability resistance, ink receptivity, and print-through prevention properties.

JP2009-263850A

Paper is required to have properties such as printing gloss and surface strength, but Patent Document 1 does not include any description of these properties and workability. In view of these circumstances, an object of the present invention is to provide paper with high printing gloss and surface strength.

The above problem is solved by the present invention as described below.
(1) A paper comprising a base paper and a coating layer,
Paper in which the coating layer contains starch and cellulose nanofibers.
(2) The paper according to (1), wherein the starch is thermochemically modified starch.
(3) the coating layer is a clear coating layer,
The paper according to (1) or (2), wherein the weight ratio of the thermochemically modified starch to cellulose nanofibers is 350:1 to 67:1.
(4) The paper according to (3), further comprising a pigment coating layer on the clear coating layer.
(5) The paper according to (1) or (2), wherein the coating layer is a pigment coating layer.
(6) The paper according to any one of (2) to (5), wherein the thermochemically modified starch is selected from the group consisting of ammonium persulfate modified starch, urea/acid modified starch, and combinations thereof.
(7) The paper according to any one of (1) to (6), wherein the cellulose nanofibers are anion-modified cellulose nanofibers.
(8) The paper according to any one of (1) to (7), wherein the cellulose nanofiber has 0.1 to 3.0 mmol/g of carboxyl groups.
(9) The paper according to any one of (1) to (8), wherein the cellulose nanofibers have a degree of carboxymethyl substitution of glucose units of 0.01 to 0.50.
(10) The cellulose nanofibers have a type B viscosity (60 rpm, 20°C) of 500 to 7000 mPa·s when made into an aqueous dispersion with a concentration of 1% (w/v), (1) to (9) Paper listed in any of the above.

The present invention can provide paper with high printing gloss and surface strength.

The present invention will be explained in detail below. The paper of the present invention includes a coating layer containing starch and CNF on one or both sides of the base paper. In one embodiment, the paper of the present invention comprises a clear coat layer containing starch and CNF, and in another embodiment the paper of the present invention comprises a pigment coat layer containing starch and CNF. Furthermore, in the present invention, "X~Y" includes the end values of X and Y.

1. Paper with a clear coating layer containing starch and CNF (first embodiment)
(1) Cellulose Nanofiber Cellulose nanofiber (also referred to as "CNF") is a single microfibril of cellulose obtained by defibrating a cellulose-based raw material, and has an average fiber diameter of less than 500 nm.

Preferably, the CNF is chemically modified. Chemically modified CNF can be produced by chemically modifying a cellulose-based raw material to prepare chemically modified cellulose, and mechanically defibrating this.
1) Cellulose-based raw materials Cellulose-based raw materials are not particularly limited, and include, for example, those derived from plants, animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria), and microbial products. Plant-derived materials include, for example, wood, bamboo, hemp, jute, kenaf, agricultural residue, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp ( LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.). The cellulose raw material may be any one or a combination of these, but preferably cellulose fibers derived from plants or microorganisms, more preferably cellulose fibers derived from plants.

2) Chemical Modification Chemical modification refers to the introduction of functional groups into cellulosic raw materials, and it is preferable to introduce anionic groups. Examples of the anionic group include acid groups such as a carboxyl group, a carboxyl group-containing group, a phosphoric acid group, and a phosphoric acid group-containing group. Examples of the carboxyl group-containing group include -R-COOH (R is an alkylene group having 1 to 3 carbon atoms) and -OR-COOH (R is an alkylene group having 1 to 3 carbon atoms). Examples of the phosphoric acid group-containing group include a polyphosphoric acid group, a phosphorous acid group, a phosphonic acid group, a polyphosphonic acid group, and the like. Depending on the reaction conditions, these acid groups may be introduced in the form of a salt (for example, a carboxylate group (-COOM, M is a metal atom)). The chemical modification is preferably oxidation or etherification. These will be explained in detail below.

[Oxidation]
Oxidized cellulose is obtained by oxidizing cellulose raw materials. The oxidation method is not particularly limited, but as an example, cellulose raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a substance selected from the group consisting of bromides, iodides, and mixtures thereof. One method is to do so. According to this method, the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface is selectively oxidized to produce a group selected from the group consisting of aldehyde groups, carboxyl groups, and carboxylate groups. The concentration of the cellulose raw material during the reaction is not particularly limited, but is preferably 5% by weight or less.

An N-oxyl compound is a compound that can generate nitroxy radicals. Examples of nitroxyl radicals include 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). As the N-oxyl compound, any compound can be used as long as it promotes the desired oxidation reaction. The amount of the N-oxyl compound used is not particularly limited as long as it is a catalytic amount that can oxidize cellulose as a raw material. For example, it is preferably 0.01 mmol or more, more preferably 0.02 mmol or more, per 1 g of bone dry cellulose. The upper limit is preferably 10 mmol or less, more preferably 1 mmol or less, and even more preferably 0.5 mmol or less. Therefore, the amount of the N-oxyl compound to be used is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and even more preferably 0.02 to 0.5 mmol, per 1 g of bone-dry cellulose.

Bromide is a compound containing bromine, and includes, for example, an alkali metal bromide that can be dissociated and ionized in water, such as sodium bromide. Moreover, iodide is a compound containing iodine, and includes, for example, alkali metal iodide. The amount of bromide or iodide to be used can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is preferably 0.1 mmol or more, more preferably 0.5 mmol or more, per 1 g of bone-dry cellulose. The upper limit of the amount is preferably 100 mmol or less, more preferably 10 mmol or less, and even more preferably 5 mmol or less. Therefore, the total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol, per 1 g of bone-dry cellulose.

Examples of the oxidizing agent include, but are not limited to, halogen, hypohalous acid, halous acid, perhalogenic acid, salts thereof, halogen oxides, peroxides, and the like. Among these, hypohalous acid or its salt is preferred, hypochlorous acid or its salt is more preferred, and sodium hypochlorite is even more preferred because it is inexpensive and has little environmental impact. The amount of the oxidizing agent used is preferably 0.5 mmol or more, more preferably 1 mmol or more, and even more preferably 3 mmol or more, per 1 g of bone-dry cellulose. The upper limit of the amount is preferably 500 mmol or less, more preferably 50 mmol or less, and even more preferably 25 mmol or less. Therefore, the amount of the oxidizing agent used is preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and particularly preferably 3 to 10 mmol, per 1 g of bone dry cellulose. When using an N-oxyl compound, the amount of the oxidizing agent used is preferably 1 mol or more per 1 mol of the N-oxyl compound, and the upper limit is preferably 40 mol. Therefore, the amount of the oxidizing agent used is preferably 1 to 40 mol per 1 mol of the N-oxyl compound.

Conditions such as pH and temperature during the oxidation reaction are not particularly limited, and the oxidation reaction generally proceeds efficiently even under relatively mild conditions. The reaction temperature is preferably 4°C or higher, more preferably 15°C or higher. The upper limit of the temperature is preferably 40°C or less, more preferably 30°C or less. Therefore, the reaction temperature is preferably 4 to 40°C, and may be about 15 to 30°C, that is, room temperature. The pH of the reaction solution is preferably 8 or higher, more preferably 10 or higher. The upper limit of pH is preferably 12 or less, more preferably 11 or less. Therefore, the pH of the reaction solution is preferably about 8 to 12, more preferably about 10 to 11. Usually, as carboxyl groups are generated in cellulose as the oxidation reaction progresses, the pH of the reaction solution tends to decrease. Therefore, in order to allow the oxidation reaction to proceed efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution within the above range. Water is preferable as the reaction medium for oxidation because it is easy to handle and does not easily cause side reactions.

The reaction time for oxidation can be appropriately set according to the degree of progress of oxidation, and is usually 0.5 hours or more, and the upper limit is usually 6 hours or less, preferably 4 hours or less. Therefore, the reaction time for oxidation is usually about 0.5 to 6 hours, for example about 0.5 to 4 hours. The oxidation may be carried out in two or more reaction steps. For example, by oxidizing the oxidized cellulose obtained by filtration after the completion of the first-stage reaction again under the same or different reaction conditions, the efficiency can be improved without being inhibited by the salt produced as a by-product in the first-stage reaction. Can be oxidized well.

Another example of a carboxylation (oxidation) method is ozone oxidation. This oxidation reaction oxidizes at least the hydroxyl groups at the 2- and 6-positions of the glucopyranose rings constituting cellulose, and causes decomposition of the cellulose chain. Ozone treatment is usually performed by bringing a gas containing ozone into contact with the cellulose raw material. The ozone concentration in the gas is preferably 50 g/m ³ or more. The upper limit is preferably 250 g/m ³ or less, more preferably 220 g/m ³ or less. Therefore, the ozone concentration in the gas is preferably 50 to 250 g/m ³ , more preferably 50 to 220 g/m ³ . The amount of ozone added is preferably 0.1% by weight or more, more preferably 5% by weight or more, based on 100% by weight of the solid content of the cellulose raw material. The upper limit of the amount of ozone added is usually 30% by weight or less. Therefore, the amount of ozone added is preferably 0.1 to 30% by weight, more preferably 5 to 30% by weight, based on 100% by weight of the solid content of the cellulose raw material. The ozone treatment temperature is usually 0°C or higher, preferably 20°C or higher, and the upper limit is usually 50°C or lower. Therefore, the ozone treatment temperature is preferably 0 to 50°C, more preferably 20 to 50°C. The ozone treatment time is usually 1 minute or more, preferably 30 minutes or more, and the upper limit is usually 360 minutes or less. Therefore, the ozone treatment time is usually about 1 to 360 minutes, preferably about 30 to 360 minutes. When the ozone treatment conditions are within the above range, excessive oxidation and decomposition of cellulose can be prevented, and the yield of oxidized cellulose can be improved.

The ozone-treated cellulose may be further subjected to additional oxidation treatment using an oxidizing agent. The oxidizing agent used in the additional oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. Examples of methods for the additional oxidation treatment include a method in which an oxidizing agent solution is prepared by dissolving these oxidizing agents in a polar organic solvent such as water or alcohol, and the cellulose raw material is immersed in the oxidizing agent solution. The amount of carboxyl groups, carboxylate groups, and aldehyde groups contained in oxidized cellulose nanofibers can be adjusted by controlling oxidation conditions such as the amount of oxidizing agent added and reaction time.

An example of a method for measuring the amount of carboxyl groups will be explained below. Prepare 60 mL of 0.5% by weight slurry (aqueous dispersion) of oxidized cellulose, add 0.1M hydrochloric acid aqueous solution to adjust the pH to 2.5, and then dropwise add 0.05N sodium hydroxide aqueous solution to bring the pH to 11. Measure the electrical conductivity until the It can be calculated using the following formula from the amount (a) of sodium hydroxide consumed in the neutralization stage of the weak acid where the change in electrical conductivity is gradual.
Amount of carboxyl group [mmol/g oxidized cellulose] = a [mL] x 0.05/weight of oxidized cellulose [g]

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

[etherification]
Etherification includes carboxymethyl (ether), methyl (ether), ethyl (ether), cyanoethyl (ether), hydroxyethyl (ether), hydroxypropyl (ether), and ethylhydroxyethyl (ether). and hydroxypropylmethyl (ether). As an example of these methods, the method of carboxymethylation will be described below.

The degree of carboxymethyl substitution per anhydroglucose unit in carboxymethylated cellulose or CNF obtained by carboxymethylation is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.10 or more. The upper limit of the degree of substitution is preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. Therefore, the degree of carboxymethyl group substitution is preferably from 0.01 to 0.50, more preferably from 0.05 to 0.40, even more preferably from 0.10 to 0.30.

Although the carboxymethylation method is not particularly limited, for example, a method of mercerizing a cellulose raw material as a bottom forming raw material and then etherifying it can be mentioned. A solvent is usually used in the reaction. Examples of the solvent include water, alcohol (eg, lower alcohol), and mixed solvents thereof. Examples of the lower alcohol include methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, and tertiary butanol. The lower limit of the mixing ratio of lower alcohol in the mixed solvent is usually 60% by weight or more, and the upper limit is 95% by weight or less, preferably 60 to 95% by weight. The amount of solvent is usually 3 times the weight of the cellulose raw material. The upper limit of the amount is not particularly limited, but is 20 times the weight. Therefore, the amount of solvent is preferably 3 to 20 times the weight.

Mercerization is usually performed by mixing the bottom forming raw material and a mercerization agent. Examples of the mercerizing agent include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. The amount of the mercerizing agent to be used is preferably 0.5 times or more, more preferably 1.0 times or more, and even more preferably 1.5 times or more by mole per anhydroglucose residue in the bottom raw material. The upper limit of the amount is usually 20 times the mole or less, preferably 10 times the mole or less, and more preferably 5 times the mole or less. Therefore, the amount of the mercerizing agent used is preferably 0.5 to 20 times by mole, more preferably 1.0 to 10 times by mole, and even more preferably 1.5 to 5 times by mole.

The reaction temperature for mercerization is usually 0°C or higher, preferably 10°C or higher, and the upper limit is usually 70°C or lower, preferably 60°C or lower. Therefore, the reaction temperature is usually 0 to 70°C, preferably 10 to 60°C. The reaction time is usually 15 minutes or more, preferably 30 minutes or more. The upper limit of the time is usually 8 hours or less, preferably 7 hours or less. Therefore, the reaction time is usually 15 minutes to 8 hours, preferably 30 minutes to 7 hours.

The etherification reaction is usually carried out by adding a carboxymethylating agent to the reaction system after mercerization. Examples of carboxymethylating agents include sodium monochloroacetate. The amount of the carboxymethylating agent added is usually preferably 0.05 times or more, more preferably 0.5 times or more, and still more preferably 0.8 times or more, per glucose residue in the cellulose raw material. The upper limit of the amount is usually 10.0 times the mole or less, preferably 5 times the mole or less, and more preferably 3 times the mole or less. Therefore, the amount is preferably 0.05 to 10.0 times the mole, and more. The amount is preferably 0.5 to 5, more preferably 0.8 to 3 times the mole. The reaction temperature is usually 30°C or higher, preferably 40°C or higher, and the upper limit is usually 90°C or lower, preferably 80°C or lower. Therefore, the reaction temperature is usually 30 to 90°C, preferably 40 to 80°C. The reaction time is usually 30 minutes or more, preferably 1 hour or more, and the upper limit is usually 10 hours or less, preferably 4 hours or less. Therefore, the reaction time is usually 30 minutes to 10 hours, preferably 1 hour to 4 hours. The reaction solution may be stirred as necessary during the carboxymethylation reaction.

The degree of carboxymethyl substitution per glucose unit of carboxymethylated cellulose can be measured, for example, by the following method. That is, 1) Accurately weigh about 2.0 g of carboxymethylated cellulose (absolutely dried) and place it in a 300 mL Erlenmeyer flask with a stopper. 2) Add 100 mL of a solution prepared by adding 100 mL of special grade concentrated nitric acid to 1000 mL of methanol nitric acid, and shake for 3 hours to convert carboxymethylcellulose salt (carboxymethylated cellulose) into hydrogen-type carboxymethylated cellulose. 3) Accurately weigh 1.5 to 2.0 g of hydrogen-type carboxymethylated cellulose (absolutely dried) and place it in a 300 mL Erlenmeyer flask with a stopper. 4) Wet hydrogen carboxymethylated cellulose with 15 mL of 80% methanol, add 100 mL of 0.1N NaOH, and shake at room temperature for 3 hours. 5) Back titrate excess NaOH with 0.1N H ₂ SO ₄ using phenolphthalein as indicator. 6) Calculate the degree of carboxymethyl substitution (DS) by the following formula:
A=[(100×F'-(0.1N H ₂ SO ₄ )(mL)×F)×0.1]/(absolute dry mass (g) of hydrogen-type carboxymethylated cellulose)
DS=0.162×A/(1-0.058×A)
A: Amount of 1N NaOH (mL) required to neutralize 1g of hydrogen-type carboxymethylated cellulose
F: Factor of 0.1N H ₂ SO ₄ F': Factor of 0.1N NaOH

3) Mechanical defibration Chemically modified cellulose is mechanically defibrated to obtain CNF. The defibration treatment may be performed once or multiple times. It is preferable to subject a mixture containing chemically modified cellulose and a dispersion medium to defibration treatment. Water is preferred as the dispersion medium. The device used for defibration is not particularly limited, but examples include high-speed rotation type, colloid mill type, high pressure type, roll mill type, and ultrasonic type devices. High pressure or ultra-high pressure homogenizers are preferred, and wet high pressure homogenizers are preferred. Or an ultra-high pressure homogenizer is more preferable. Preferably, the device is capable of applying strong shear force to the chemically modified cellulose. The pressure that can be applied by the device is preferably 50 MPa or more, more preferably 100 MPa or more, and still more preferably 140 MPa or more. The device is preferably a wet high pressure or ultra high pressure homogenizer. Thereby, defibration can be performed efficiently.

When defibrating a dispersion of chemically modified pulp, the solid content concentration of modified cellulose in the dispersion is usually preferably 0.1% by weight or more, more preferably 0.2% by weight or more, and 0.1% by weight or more, more preferably 0.2% by weight or more. More preferably, the amount is .3% by weight or more. This makes the amount of liquid appropriate for the amount of modified cellulose, making it efficient. The upper limit of the concentration is usually preferably 20% by weight or less, more preferably 15% by weight or less, and even more preferably 10% by weight or less. This allows fluidity to be maintained.

4) Characteristics The average fiber diameter of CNF is generally about 2 nm or more and less than 500 nm in terms of length weighted average fiber diameter, but preferably 2 to 100 nm. The upper limit is more preferably 50 nm or less. The average fiber length is preferably 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" or "average fiber length") can be determined using an atomic force microscope (AFM) or a transmission electron microscope (TEM). It is determined by observing each fiber. The average aspect ratio of nanofibers is usually 10 or more. The upper limit is not particularly limited, but is usually 1000 or less. The average aspect ratio can be calculated using the following formula.
Average aspect ratio = average fiber length / average fiber diameter

The amount of carboxyl groups and the degree of substitution per glucose unit in CNF are preferably the same as those in chemically modified cellulose.

In this embodiment, when an aqueous dispersion with a concentration of 1% (w/v) (that is, an aqueous dispersion containing 1 g of CNF (dry weight) in 100 mL of water) has a B-type viscosity of 500 to 7000 mPa·s. (60 rpm, 20°C) is preferably used. It is an index that specifies the characteristics of the B-type viscosity CNF, such as the amount of functional groups, average fiber length, and average fiber diameter, and is adjusted as appropriate depending on the application.

The B-type viscosity of an aqueous CNF dispersion can be measured by a known method. For example, it can be measured using a VISCOMETER TV-10 viscometer manufactured by Toki Sangyo Co., Ltd. The temperature at the time of measurement was 20° C., and the rotation speed of the rotor was 60 rpm. The aqueous dispersion of CNF of the present invention has thixotropic properties, and its viscosity decreases when stirred and subjected to shear stress, and when left standing, the viscosity increases and gels. It is preferable to measure the type B viscosity under the conditions.

(2) Starch Starch is a polymer of D-glucose, preferably a mixture of amylose and amylopectin. In this embodiment, starch also includes starch-derived polymer compounds. Examples of the polymer include those obtained by modifying, modifying, and processing starch, and among these, thermochemically modified starch is preferred. Thermochemically modified starch includes starch that is instantaneously gelatinized and oxidized by heating in the presence of an oxidizing agent. Such thermochemically modified starch is characterized by a small amount of functional groups. Among these, ammonium persulfate-modified starch using ammonium persulfate as the oxidizing agent is preferred. In addition, examples of thermochemically modified starch include urea/acid modified starch modified with urea and acid. Urea/acid-modified starch is produced, for example, by the method described in JP-A No. 2004-238523. In this embodiment, ammonium persulfate modified starch and urea/acid modified starch may be used in combination.

(3) Base paper Base paper is the base layer of paper and contains pulp as its main component. The pulp raw material for the base paper is not particularly limited, and includes mechanical pulp such as ground pulp (GP), thermomechanical pulp (TMP), and chemi-thermomechanical pulp (CTMP), deinked pulp (DIP), softwood kraft pulp (NKP), and softwood. Chemical pulp such as kraft pulp (LKP) can be used. As deinked (waste paper) pulp, those derived from sorted waste paper such as high-quality paper, medium-quality paper, low-grade paper, newspaper, leaflets, magazines, etc., and unsorted waste paper that is a mixture of these can be used.

Known fillers can be added to the base paper, but it is not necessary to add fillers for applications where opacity and whiteness are not required, such as paperboard, or when using raw materials with a high ash content, such as waste paper. When adding fillers, fillers include heavy calcium carbonate, light calcium carbonate, clay, silica, light calcium carbonate-silica composite, kaolin, calcined kaolin, delamic kaolin, magnesium carbonate, barium carbonate, barium sulfate, hydroxide. Inorganic fillers such as amorphous silica produced by neutralizing aluminum, calcium hydroxide, magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide, and sodium silicate with mineral acids, urea-formalin resin, and melamine-based fillers. Examples include organic fillers such as resins, polystyrene resins, and phenolic resins. These may be used alone or in combination. Among these, calcium carbonate and light calcium carbonate are preferred because they are typical fillers for neutral papermaking and alkaline papermaking and provide high opacity. The filler content in the base paper is preferably 5 to 25% by weight, more preferably 6 to 20% by weight, based on the weight of the base paper. In the present invention, the content of filler in the base paper is more preferably 10% by weight or more, since a decrease in paper strength is suppressed even if the ash content in the paper is high.

As internal chemicals, bulking agents, dry paper strength improvers, wet paper strength improvers, freeness improvers, dyes, neutral sizing agents, etc. may be used as necessary.

The base paper is manufactured by a known papermaking method. For example, it can be carried out using a Fourdrinier paper machine, a gap former paper machine, a hybrid former paper machine, an on-top former paper machine, a circular wire paper machine, etc., but is not limited thereto.

The base paper may be single layer or multilayer. The base paper may contain the CNF. In the case of multilayer base paper, some of the paper layers may contain CNF, or all the layers may contain CNF. When the base paper contains CNF, its content is preferably 0.0001% by weight or more, more preferably 0.0003% by weight or more, and even more preferably 0.001% by weight or more, based on the pulp weight of the entire base paper.

(4) Clear coating layer The starch:CNF (weight ratio) in the clear coating layer is preferably 1000:1 to 20:1, more preferably 350:1 to 67:1, and even more preferably 300:1 to 20:1. :1 to 67:1. When the weight ratio is within this range, the film formability of the clear coating layer mainly composed of starch is improved, and as a result, high ink mileage, printing gloss, and surface strength can be achieved.

The coating amount of the clear coating layer is preferably 0.01 to 3.0 g/m ² in terms of solid content per side, more preferably 0.1 to 2.0 g/m ² . For clear coating, use a coater (coating machine) such as a size press, gate roll coater, premetering size press, curtain coater, or spray coater to apply a clear coating liquid containing starch as the main component to the base paper. It can be formed by coating on top. For example, when coating with a gate roll coater, the clear coating liquid should have a type B viscosity (30°C, 60 rpm) of 5 to 450 mPa・s when the solid content concentration is 5% by weight from the viewpoint of coating suitability. is preferable, and more preferably 10 to 300 mPa·s. When coating with a gate roll coater, if the B type viscosity of the clear coating liquid is less than 5 mPa・s, the viscosity is too low and it is difficult to secure a coating amount, and if it exceeds 450 mPa・s, boiling may occur. Operability may deteriorate. The solid content concentration of the clear coating liquid is adjusted so as to achieve the above concentration, and is preferably 2 to 14% by weight.

The amount of CNF derived from the clear coating layer is preferably 1.0×10 ⁻⁵ to 0.1 g/m ² , more preferably 1.0×10 ⁻⁴ to 5.0×10 ⁻² g per one side. / ^m2 .

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

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

The pigment coating layer may contain various auxiliary agents such as dispersants, thickeners, antifoaming agents, colorants, antistatic agents, and preservatives used in the general paper manufacturing field, and may contain CNF. You may. In this case, the amount of CNF is preferably 1×10 ⁻³ to 1 part by weight per 100 parts by weight of the pigment. In the case of the above range, a pigment coating liquid with appropriate water retention can be obtained without significantly increasing the viscosity of the coating liquid. Further, the pigment coating layer in this embodiment may be the pigment coating layer described in the second embodiment.

The pigment coating layer can be provided by applying a coating liquid to one or both sides of the base paper using a known method. The solid content concentration in the coating liquid is preferably about 30 to 70% by weight from the viewpoint of coating suitability. The pigment coating layer may be one layer, two layers, or three or more layers. The coating amount of the pigment coating layer may be adjusted as appropriate depending on the application, but in the case of coated paper for printing, the total amount per side is 5 g/m ² or more, preferably 10 g/m ² or more. . The upper limit is preferably 30 g/m ² or less, more preferably 25 g/m ² or less.

When the paper in this embodiment further has a pigment coating layer, it is possible to obtain a pigment-coated paper that has not only high ink mileage but also excellent surface strength and printing gloss.

(5) Characteristics The paper of this embodiment has the characteristics of high ink mileage, printing gloss, and surface strength, and is easy to manufacture. The basis weight of the paper of this embodiment measured according to JIS P 8124 is usually about 20 to 500 g/m ² , preferably 30 to 250 g/m ² .

(6) Method for producing paper with a clear coating layer containing starch and CNF The paper of this embodiment is produced by applying a clear coating liquid containing CNF on base paper prepared by a known method. Preferably, it is manufactured. Specifically, the paper of this embodiment is preferably manufactured by a method including the following steps.
Step 1: Step of preparing a clear coating solution containing starch and CNF Step 2: Step of forming a clear coating layer on base paper using the clear coating solution

The starch and CNF used in step 1 are as described above. The method for preparing the coating liquid and its properties are also as described above. Coating in step 2 can also be performed as described above.

The paper of this embodiment may be manufactured by a method that includes the following step 3 in addition to the steps 1 and 2 described above.
Step 3: Step of forming a pigment coating layer containing a pigment and an adhesive on the clear coating layer containing starch and CNF In Step 3, the starch and CNF used in the second embodiment as a pigment coating liquid. You may use the pigment coating liquid which has.

2. Paper with a pigment coating layer containing starch and CNF (second embodiment)
(1) CNF, starch, and base paper In this embodiment, the CNF, starch, and base paper described in the first embodiment can be used.

(2) Pigment Coating Layer The paper of this embodiment has a pigment coating layer containing starch and CNF on one or both sides of the base paper. The pigment coating layer is a layer containing a white pigment as a main component. As the white pigment, those explained in the first aspect can be used. The weight ratio of starch to CNF in the pigment coating layer is not limited, but is preferably 300:1 to 2:1. When the weight ratio is within this range, the film formability of the pigment coating layer is improved, and as a result, high printing gloss and surface strength can be achieved. From this point of view, the weight ratio is more preferably 200:1 to 5:1.

The pigment coating layer may contain an adhesive other than starch. The adhesive is as described in the first aspect.

The pigment coating layer may contain various auxiliary agents such as dispersants, thickeners, antifoaming agents, colorants, antistatic agents, and preservatives used in the general paper manufacturing field.

The pigment coating layer can be provided by applying a coating liquid to one or both sides of the base paper using a known method. The solid content concentration in the coating liquid is preferably about 30 to 70% by weight from the viewpoint of coating suitability. The pigment coating layer may be one layer, two layers, or three or more layers. The coating amount of the pigment coating layer may be adjusted as appropriate depending on the application, but in the case of coated paper for printing, the total amount per side is 1 g/m ² or more, preferably 5 g/m ² or more. . The upper limit is preferably 30 g/m ² or less, and preferably 20 g/m ² or less. The amount of CNF derived from the pigment coating layer is preferably 1.0×10 ⁻⁵ to 0.1 g/m ² , more preferably 1.0×10 ⁻⁴ to 5.0×10 ⁻² g per one side. / ^m2 .

(3) Clear coating layer The paper of this embodiment may have a clear coating layer on one or both sides of the base paper. By applying a clear coating, the surface strength and smoothness of the base paper can be improved. The clear coating layer is formed from a clear coating liquid containing various starches, water-soluble polymers such as polyacrylamide, polyvinyl alcohol, etc. as main components. The clear coating layer may be a clear coating layer containing starch and CNF as described in the first aspect. Since the clear coating layer has high film-forming properties, high ink mileage, printing gloss, and surface strength can be achieved.

The coating amount of the clear coating layer in this embodiment is preferably 0.01 to 3.0 g/m ² in terms of solid content per side, more preferably 0.1 to 2.0 g/m ² . Clear coating involves applying a clear coating liquid onto base paper using a coater (coating machine) such as a size press, gate roll coater, premetering size press, curtain coater, or spray coater. It can be formed by For example, when coating with a gate roll coater, the clear coating liquid should have a type B viscosity (30°C, 60 rpm) of 5 to 450 mPa・s when the solid content concentration is 5% by weight from the viewpoint of coating suitability. is preferable, and more preferably 10 to 300 mPa·s. When coating with a gate roll coater, if the B type viscosity of the clear coating liquid is less than 5 mPa・s, the viscosity is too low and it is difficult to secure a coating amount, and if it exceeds 450 mPa・s, boiling may occur. Operability may deteriorate. The solid content concentration of the clear coating liquid is adjusted so as to achieve the above concentration, and is preferably 2 to 14% by weight. Further, the clear coating layer in this embodiment may be the clear coating layer described in the first embodiment.

(4) Properties The basis weight of the paper of this embodiment measured according to JIS P 8124 is usually about 10 to 500 g/m ² , preferably 30 to 300 g/m ² .

(5) Method for producing paper with a pigment coating layer containing starch and CNF The paper of this embodiment is produced by applying a pigment coating liquid containing CNF on base paper prepared by a known method. Preferably, it is manufactured. Specifically, the paper of this embodiment is preferably manufactured by a method including the following steps.
Step 1: A step of preparing a pigment coating liquid containing a pigment, starch, and CNF. Step 2: A step of forming a pigment coating layer on base paper using the pigment coating liquid.

The starch and CNF used in step 1 are as described above. The method for preparing the coating liquid and its properties are also as described above. Coating in step 2 can also be performed as described above.

The paper of this embodiment may be manufactured by a method that includes the following step 3 in addition to the steps 1 and 2 described above.
Step 3: Step of forming a clear coating layer on the base paper before Step 2 In Step 3, the clear coating solution containing starch and CNF used in the first embodiment may be used as the clear coating solution. .

[Example A1]
<CNF>
5.00 g (absolutely dry) of bleached unbeaten kraft pulp derived from coniferous trees (85% whiteness: manufactured by Nippon Paper Industries Co., Ltd.) was mixed with 39 mg (0.05 mmol per 1 g of absolutely dry cellulose) of TEMPO (manufactured by Sigma Aldrich). 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) was added to 500 mL of an aqueous solution and stirred until the pulp was uniformly dispersed. An aqueous sodium hypochlorite solution was added to the reaction system so that the sodium hypochlorite concentration was 5.5 mmol/g, and the oxidation reaction was started at room temperature. During the reaction, the pH in the system decreased, but the pH was adjusted to 10 by successively adding a 3M aqueous sodium hydroxide solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH within the system stopped changing. The reaction mixture was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain oxidized pulp (carboxylated cellulose). The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.5 mmol/g. This was adjusted to 1% (w/v) with water and treated with an ultra-high pressure homogenizer (20° C., 150 MPa) three times to obtain an aqueous CNF dispersion. The average fiber diameter of the CNF was 3 nm, and the aspect ratio was 250.

<Clear coating liquid 1>
Oxidized starch (manufactured by Nippon Cornstarch Co., Ltd., SK20) was added to the aqueous dispersion of CNF produced as described above to produce clear coating liquid 1 in which the weight ratio of starch to CNF was 30:1. Table 1 shows the type B viscosity at 30° C. and 60 rpm when the solid content concentration of the clear coating liquid 1 is 5% by weight.

<Paper>
To LBKP (manufactured by Nippon Paper Industries Co., Ltd., c.s.f. 360ml), 0.5% by weight of sulfuric acid, 0.77% by weight of cationized starch, and 0.05% by weight of paper strength agent were added. A pulp slurry having a solid content concentration of 0.7% by weight was prepared. Using the obtained pulp slurry, base paper was manufactured using a paper machine. On top of the base paper, the clear coating liquid 1 was applied to both sides of the base paper using a gate roll coater so that the solid content per side was 1.2 g/m ² , and dried by a standard method to form clear coated paper. Obtained. The paper was evaluated by the method described below. The results are shown in Table 1.

[Comparative example A1]
Clear coated paper was produced in the same manner as Example A1 except that CNF was not used.

[Example A2]
<Base paper>
To LBKP (manufactured by Nippon Paper Industries Co., Ltd., c.s.f. 420ml), 0.7% by weight of sulfuric acid, 0.30% by weight of cationized starch, and 0.06% by weight of paper strength agent were added. A pulp slurry having a solid content concentration of 0.7% by weight was prepared. Using the obtained pulp slurry, base paper with a basis weight of 34.5 g/m ² was manufactured using a paper machine.

<Pigment coating liquid 1>
To 100 parts by weight of heavy calcium carbonate, 2.0 parts by weight of latex and 6.7 parts by weight of oxidized starch as adhesives were added to prepare a pigment coating liquid with a solid content of 60% by weight.

<Pigment coated paper>
The same clear coating solution 1 as in Example A1 was applied to the base paper using a gate roll coater to give a solid content of 0.2 g/ ^m2 per side, except that the starch:CNF weight ratio was 60:1. was coated on both sides and dried according to a standard method to form a clear coating layer.Furthermore, the pigment coating solution 1 was coated on both sides and dried according to a standard method. The pigment coated paper was evaluated by the method described below. The results are shown in Table 1.

[Example A3]
Pigment coated paper was produced in the same manner as Example A2, except that the weight ratio of starch to CNF in clear coating liquid 1 was changed as shown in Table 1.

[Comparative example A2]
Pigment coated paper was produced in the same manner as Example A2 except that CNF was not used.

[Example A4]
Pigment coated paper was produced in the same manner as Example A2, except that the weight ratio of starch to CNF in clear coating liquid 1 was changed as shown in Table 1.

[Example A5]
A starch slurry having a solid content concentration of 25% by weight was prepared by adding 0.1% by weight of ammonium persulfate as an oxidizing agent to raw starch (unmodified starch). This starch slurry was steamed at 150°C using a jet cooker and subjected to thermochemical denaturation treatment, and after cooling, an aqueous sodium hydroxide solution was added to adjust the pH to 7, and water was further added to obtain a solid content concentration of 12% by weight. An aqueous solution of ammonium persulfate-modified starch was obtained.

<Clear coating liquid 2>
The ammonium persulfate modified starch aqueous solution produced as described above was added to the aqueous dispersion of CNF produced as described above to produce clear coating liquid 2 in which the weight ratio of starch to CNF was 67:1. Table 1 shows the type B viscosity at 30° C. and 60 rpm when the solid content concentration of the clear coating liquid 2 is 5% by weight.
Pigment coated paper was produced in the same manner as in Example A2, except that clear coating liquid 2 was used in place of clear coating liquid 1.

[Example A6, A7]
Pigment-coated papers were produced in the same manner as in Example A5, except that the weight ratio of starch to CNF in clear coating liquid 2 was changed as shown in Table 1.

[Examples A8 to A12]
Pigment-coated papers were produced in the same manner as in Example A2, except that the weight ratio of starch to CNF in clear coating liquid 1 was changed as shown in Table 1.

[Comparative example A3]
Pigment coated paper was produced in the same manner as Example A5 except that CNF was not used.
[Comparative example A4]
Pigment coated paper was produced in the same manner as Example A8 except that CNF was not used.

The clear coated paper of the present invention had high printing gloss as well as high ink mileage. In particular, in Examples A1 to A3 and A5 to A12, in which CNF was used to provide a dispersion having a specific viscosity, workability during production was also good. In addition, the pigment-coated paper of the present invention, in which a pigment coating layer was further provided on the clear-coated paper, had good printing gloss and surface strength in addition to high ink mileage.

<Evaluation method>
1) Basis weight According to JIS P8124.
2) Printing glossiness: Ink adhesion on solid areas using offset sheet-fed ink (NEX-M manufactured by Toyo Ink Co., Ltd.) on a Roland offset sheet-fed printing press (4 colors) at a printing speed of 8000 sheets/hr. Indigo and red (CM) were printed in this order so that the meat density was 1.60 for blue and 1.50 for red. The glossiness of the blue-red (CM) solid printed area of the obtained printed matter was measured based on JIS P-8142.

3) Picking Evaluation Using an offset sheet-fed printing machine manufactured by Roland, an indigo solid was printed at a speed of 8000 sph using Leo Echo Y indigo manufactured by Toyo Ink Co., Ltd. as the ink. The number of pickings on the F side and W side that occurred while printing 10 sheets was measured.

4) Ink mileage Ink mileage is the number of copies that can be printed per unit amount of ink. The amount of ink on the paper surface per unit area required to obtain the same printing density was defined as color development, and this was evaluated as a simple index of ink mileage.
Good ink mileage means good color development with a small amount of ink on paper. Specifically, solid printing was performed using a Plüfbau test printer (IGT), and assuming sheet-fed printing, the print density of the printed material was measured with a spectrophotometer overnight after printing, and the total density was determined. I read it. In addition, the difference in weight of the removable print disc before and after printing was taken as the amount of ink on the paper surface. The amount of ink applied to the print disk was changed to determine the relationship between the amount of ink on the paper surface and the print density, and the amount of ink on the paper surface necessary to obtain a predetermined density was calculated from the relational expression. The printing pressure at the time of measurement was 700 N, and the printing speed was 2.0 m/s.

5) Suitability for coating with gate roll coater (workability during manufacturing)
The occurrence of boiling when the clear coating liquid was applied to the base paper using a gate roll coater was visually evaluated according to the following four criteria. It is preferable that the following evaluation is "A" or "B".
A: No boiling occurred, and coating suitability (workability during manufacturing) was good. B: Boiling occurred slightly, but coating suitability (workability during manufacturing) was generally good. C: Boiling occurred, Coating suitability (workability during manufacturing) is slightly reduced D: Boiling occurs frequently, and coating suitability (workability during manufacturing) is significantly reduced

[Example B1]
<CNF>
5.00 g (absolutely dry) of bleached unbeaten kraft pulp derived from coniferous trees (85% whiteness: manufactured by Nippon Paper Industries Co., Ltd.) was mixed with 39 mg (0.05 mmol per 1 g of absolutely dry cellulose) of TEMPO (manufactured by Sigma Aldrich). 514 mg of sodium bromide (1.0 mmol per 1 g of bone-dry cellulose) was added to 500 mL of an aqueous solution and stirred until the pulp was uniformly dispersed. An aqueous sodium hypochlorite solution was added to the reaction system so that the sodium hypochlorite concentration was 5.5 mmol/g, and the oxidation reaction was started at room temperature. During the reaction, the pH in the system decreased, but the pH was adjusted to 10 by successively adding a 3M aqueous sodium hydroxide solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH within the system stopped changing. The reaction mixture was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain oxidized pulp (carboxylated cellulose). The pulp yield was 90%, the time required for the oxidation reaction was 90 minutes, and the amount of carboxyl groups was 1.5 mmol/g. This was adjusted to 1% (w/v) with water and treated with an ultra-high pressure homogenizer (20° C., 150 MPa) three times to obtain an aqueous CNF dispersion. The average fiber diameter of the CNF was 3 nm, and the aspect ratio was 250.

<Clear coating liquid 3>
Clear coating liquid 3 containing 30% by weight of oxidized starch (SK20, manufactured by Nippon Cornstarch Co., Ltd.) was produced.

<Pigment coating liquid 2>
To 100.0 parts by weight of heavy calcium carbonate, 2.0 parts by weight of latex as an adhesive, 6.7 parts by weight of oxidized starch, and 0.2 parts by weight of CNF produced as described above were added to give a solid content of 60% by weight. Pigment coating liquid 2 was prepared.

<Base paper>
To LBKP (manufactured by Nippon Paper Industries Co., Ltd., c.s.f. 420ml), 0.7% by weight of sulfuric acid, 0.30% by weight of cationized starch, and 0.06% by weight of paper strength agent were added. A pulp slurry having a solid content concentration of 0.7% by weight was prepared. Using the obtained pulp slurry, base paper with a basis weight of 34.5 g/m ² was manufactured using a paper machine. On top of the base paper, the clear coating liquid 3 is applied to both sides of the base paper so that the solid content per side is 0.2 g/m ² , dried by a standard method to form a clear coating layer, and further, Pigment coating liquid 2 was applied to both sides and dried by a standard method to obtain pigment coated paper. The paper was evaluated using the method described above. The results are shown in Table 2.

[Example B2]
Oxidized starch (manufactured by Nippon Cornstarch Co., Ltd., SK20) was added to the aqueous dispersion of CNF produced as described above to produce clear coating liquid 4 in which the weight ratio of starch to CNF was 30:1. When the solid content concentration of the clear coating liquid 4 was 5% by weight, the type B viscosity at 30° C. and 60 rpm was 130 mPa·s. Pigment coated paper was obtained in the same manner as in Example B1 except that the clear coating liquid 4 was used.

[Comparative example B1]
Pigment coated paper was produced in the same manner as Example B1 except that CNF was not used.

The pigment-coated paper of the present invention had good printing gloss and surface strength in addition to high ink mileage.

Claims (8)

A paper comprising a base paper, a clear coating layer, and a pigment coating layer on the clear coating layer,
Paper in which the clear coating layer contains starch and anion-modified cellulose nanofibers. Paper according to claim 1, wherein the starch is a thermochemically modified starch. In the clear coating layer,
The paper according to claim 2 , wherein the weight ratio of the thermochemically modified starch to the cellulose nanofibers is 350:1 to 67:1. 4. The paper of claim 2 or 3, wherein the thermochemically modified starch is selected from the group consisting of ammonium persulfate modified starch, urea/acid modified starch, and combinations thereof. The paper according to any one of claims 1 to 4, wherein the cellulose nanofiber has 0.1 to 3.0 mmol/g of carboxyl groups. Paper according to any of claims 1 to 4, wherein the cellulose nanofibers have a degree of carboxymethyl substitution of glucose units of 0.01 to 0.50. The cellulose nanofibers have a type B viscosity (60 rpm, 20° C.) of 500 to 7000 mPa·s when made into an aqueous dispersion with a concentration of 1% (w/v), according to any one of claims 1 to 6. paper. The paper according to claim 5, wherein the cellulose nanofibers have carboxyl groups of 0.5 to 3.0 mmol/g.