JP7769474B2 - Paper with a clear coating layer containing cellulose nanofibers - Google Patents

Description

The present invention relates to paper having a cellulose nanofiber-containing clear coating layer.

Cellulose nanofibers are expected to be a new material, and various studies are being conducted on them. For example, Patent Document 1 discloses printing paper in which a papermaking additive made from cellulose nanofibers is coated or impregnated into the paper. The paper in Patent Document 1 has excellent air resistance, ink receptivity, and resistance to strike-through.

JP 2009-263850 A

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

The above problems are solved by the present invention described below.
(1) A paper comprising a base paper and a clear coating layer,
The paper, wherein the clear coating layer contains starch, cellulose nanofibers, and a metal salt.
(2) The paper according to (1), wherein the starch is oxidized starch.
(3) The paper according to (1) or (2), wherein the metal salt contains a divalent or higher metal element.
(4) The paper according to any one of (1) to (3), further comprising a pigment coating layer.
(5) The paper according to any one of (1) to (4), wherein the cellulose nanofibers are anion-modified cellulose nanofibers.
(6) The paper according to any one of (1) to (5), wherein the cellulose nanofibers have 0.1 to 3.0 mmol/g of carboxyl groups relative to the bone dry weight of the cellulose nanofibers.
(7) The paper according to any one of (1) to (5), wherein the cellulose nanofibers have a degree of carboxyalkyl substitution per anhydroglucose unit of 0.01 to 0.50.

This invention provides paper with high printing surface strength.

The present invention is described in detail below. The paper of the present invention has a clear coating layer containing starch, cellulose nanofibers (hereinafter also referred to as "CNF"), and a metal salt on one or both sides of the base paper. Furthermore, in the present invention, "X to Y" includes the end values X and Y.

1. Paper with a clear coating layer containing starch, CNF, and metal salt (1) Cellulose nanofiber (CNF)
CNF is a single microfibril of cellulose obtained by defibrating a cellulosic raw material, and has an average fiber diameter of less than 500 nm.

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

2) Chemical Modification Chemical modification refers to the introduction of functional groups into cellulosic raw materials, and the introduction of anionic groups is preferred. Examples of anionic groups include acid groups such as carboxyl groups, carboxyl group-containing groups, phosphate groups, and phosphate group-containing groups. Examples of carboxyl group-containing groups include -R-COOH (R is an alkylene group having 1 to 3 carbon atoms) and -O-R-COOH (R is an alkylene group having 1 to 3 carbon atoms). Examples of phosphate group-containing groups include polyphosphate groups, phosphite groups, phosphonic acid groups, and polyphosphonic acid groups. Depending on the reaction conditions, these acid groups may be introduced in the form of a salt (e.g., a carboxylate group (-COOM, M is a metal atom)). Chemical modification is preferably oxidation or etherification. Oxidation or etherification can be carried out according to known methods, such as those described in JP 2019-104833 A.

3) Mechanical defibration: CNF is obtained by mechanically defibrating chemically modified cellulose. The defibration treatment may be performed once or multiple times. A mixture containing chemically modified cellulose and a dispersion medium is preferably subjected to the defibration treatment. Water is preferably used as the dispersion medium. The device used for defibration is not particularly limited, but examples include high-speed rotary, colloid mill, high-pressure, roll mill, and ultrasonic devices. A high-pressure or ultra-high-pressure homogenizer is preferred, and a wet high-pressure or ultra-high-pressure homogenizer is more preferred. The device is preferably capable of applying a strong shear force to the chemically modified cellulose. The pressure applied by the device is preferably 50 MPa or more, more preferably 100 MPa or more, and even more preferably 140 MPa or more. A wet high-pressure or ultra-high-pressure homogenizer is preferred. This allows for efficient defibration.

When defibration is performed on a dispersion of chemically modified pulp, the solids concentration of the modified cellulose in the dispersion is typically preferably 0.1% by weight or more, more preferably 0.2% by weight or more, and even more preferably 0.3% by weight or more. This ensures that the amount of liquid relative to the amount of modified cellulose is appropriate and efficient. The upper limit of this concentration is typically 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 typically 2 nm or more and less than 500 nm, preferably 2 to 100 nm, in terms of length-weighted average fiber diameter. Its 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 simply referred to as "average fiber diameter" and "average fiber length") are determined by observing each fiber using an atomic force microscope (AFM) or a transmission electron microscope (TEM). The average aspect ratio of nanofibers is typically 10 or more. There is no particular upper limit, but it is typically 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 in CNF is preferably 0.1 mmol/g or more, more preferably 0.5 mmol/g or more, and even more preferably 0.8 mmol/g or more, based on the bone dry weight of CNF. The upper limit of this amount is preferably 3.0 mmol/g or less, more preferably 2.5 mmol/g or less, and even more preferably 2.0 mmol/g or less. Therefore, this amount is preferably 0.1 to 3.0 mmol/g, more preferably 0.5 to 2.5 mmol/g, and even more preferably 0.8 to 2.0 mmol/g.

The degree of carboxyalkyl substitution per anhydroglucose unit in CNF is preferably 0.01 or more, more preferably 0.05 or more, and even more preferably 0.10 or more. The upper limit of this substitution degree is preferably 0.50 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. Therefore, the degree of carboxyalkyl substitution is preferably 0.01 to 0.50, more preferably 0.05 to 0.40, and even more preferably 0.10 to 0.30. The degree of carboxyalkyl substitution is preferably the degree of carboxymethyl substitution.

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

In this embodiment, it is preferable to use CNF that gives a Brookfield viscosity (60 rpm, 20°C) of 500 to 7000 mPa·s when made into a 1% (w/v) aqueous dispersion (i.e., an aqueous dispersion containing 1 g of CNF (dry weight) in 100 mL of water). The Brookfield viscosity is an index that specifies the properties of the CNF, such as the amount of functional groups, average fiber length, and average fiber diameter, and is adjusted appropriately depending on the application.

The Brookfield viscosity of an aqueous dispersion of CNF can be measured by known methods. For example, it can be measured using a VISCOMETER TV-10 viscometer manufactured by Toki Sangyo Co., Ltd. The temperature during measurement is 20°C, and the rotor rotation speed is 60 rpm. The aqueous dispersion of CNF of the present invention is thixotropic, and its viscosity decreases when stirred and shear stress is applied, but when left to stand, its viscosity increases and it gels. Therefore, it is preferable to measure the Brookfield viscosity after thorough stirring.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[Example 1]
<CNF>
5.00 g (bone-dry) of bleached, unbeaten kraft pulp derived from coniferous trees (brightness 85%: manufactured by Nippon Paper Industries Co., Ltd.) was added to 500 mL of an aqueous solution containing 39 mg (0.05 mmol per 1 g of bone-dry cellulose) of TEMPO (manufactured by Sigma-Aldrich) and 514 mg (1.0 mmol per 1 g of bone-dry cellulose) of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. A sodium hypochlorite aqueous solution was added to the reaction system so that the sodium hypochlorite concentration was 5.5 mmol/g, and the oxidation reaction was initiated at room temperature. During the reaction, the pH of the system decreased, but 3 M sodium hydroxide aqueous solution was gradually added to adjust the pH to 10. The reaction was terminated when the sodium hypochlorite was consumed and the pH of the system no longer changed. The reaction mixture was filtered through a glass filter to separate the pulp, which was then thoroughly washed with water to obtain oxidized pulp (carboxylated cellulose). The pulp yield was 90%, the oxidation reaction time was 90 minutes, and the carboxyl group content was 1.5 mmol/g. Water was added to the oxidized pulp to prepare a 1% (w/v) mixture, which was then processed three times in an ultra-high-pressure homogenizer (20°C, 150 MPa) to obtain an aqueous CNF dispersion. The CNF had an average fiber diameter of 3 nm and an aspect ratio of 250.

<Clear coating liquid 1>
Oxidized starch (SK20, manufactured by Nippon Cornstarch Co., Ltd.) and aluminum sulfate (aluminum sulfate) were added to the aqueous dispersion of CNF prepared as described above to prepare Clear Coating Solution 1, which had a weight ratio of starch:CNF:metal salt of 100:1.5:0.33. Table 1 shows the Brookfield viscosity of Clear Coating Solution 1 at 30°C and 60 rpm when the solids concentration was 5% by weight.

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

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

[Comparative Example 1]
Coated paper was produced and evaluated in the same manner as in Example 1, except that CNF and metal salt were not used.
[Comparative Example 2]
Coated paper was produced and evaluated in the same manner as in Example 1, except that no metal salt was used.

[Example 2]
<Clear coating liquid 2>
Clear coating liquid 2 was prepared in the same manner as clear coating liquid 1, except that the amounts of CNF and metal salt were changed to those shown in Table 1.
<Paper>
A coated paper was produced and evaluated in the same manner as in Example 1, except that Clear Coating Liquid 2 was used instead of Clear Coating Liquid 1.

[Comparative Example 3]
Coated paper was produced and evaluated in the same manner as in Example 2, except that no metal salt was used.

[Example 3]
<Clear coating liquid 3>
Clear Coating Liquid 3 was prepared in the same manner as Clear Coating Liquid 1, except that the amount of CNF was changed to the amount shown in Table 1.
<Paper>
A coated paper was produced and evaluated in the same manner as in Example 1, except that Clear Coating Liquid 3 was used instead of Clear Coating Liquid 1.

[Examples 4 and 5]
Clear Coating Liquids 4 and 5 were prepared in the same manner as Clear Coating Liquid 3, except that magnesium sulfate and calcium chloride, respectively, were used instead of aluminum sulfate. Coated papers were produced and evaluated in the same manner as in Example 3, except that these coating liquids were used.

[Comparative Example 4]
Except for not using the metal salt, coated paper was produced and evaluated in the same manner as in Example 3. The evaluation results are shown in Table 1.

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

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

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

Claims (7)

A paper comprising a base paper and a clear coating layer,
the clear coating layer contains starch, cellulose nanofibers, and a metal salt;
the weight ratio of the starch to the cellulose nanofibers is 1000:1 to 20:1;
The amount of the metal salt is 0.05 to 1 part by weight based on 100 parts by weight of the starch.
paper. The paper according to claim 1, wherein the starch is oxidized starch. Paper according to claim 1 or 2, wherein the metal salt contains a divalent or higher metal element. The paper according to any one of claims 1 to 3, further comprising a pigment coating layer. The paper according to any one of claims 1 to 4, wherein the cellulose nanofibers are anionically modified cellulose nanofibers. The paper according to any one of claims 1 to 5, wherein the cellulose nanofibers have 0.1 to 3.0 mmol/g of carboxyl groups relative to the bone dry weight of the cellulose nanofibers. The paper according to any one of claims 1 to 5, wherein the cellulose nanofibers have a degree of carboxyalkyl substitution per anhydroglucose unit of 0.01 to 0.50.