JP2021161597A - Paper comprising cellulose nanofiber-containing clear coating layer - Google Patents

Abstract

To provide paper having high print surface strength.SOLUTION: Paper comprises base paper and a clear coating layer, the clear coating layer including starch, cellulose nanofiber, and metal salt. The starch is preferably oxidized starch.SELECTED DRAWING: None

Description

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

Cellulose nanofibers are expected as a new material, and various studies have been conducted. For example, Patent Document 1 discloses a printing paper coated or impregnated with a papermaking additive made of cellulose nanofibers. The paper of Patent Document 1 has excellent air permeation resistance, ink penetration property, and strike-through prevention property.

Japanese Unexamined Patent Publication No. 2009-263850

Paper is required to have high print surface strength, but Patent Document 1 does not describe this characteristic. In view of such circumstances, it is an object of the present invention to provide a paper having high printing surface strength.

The problem is solved by the following invention.
(1) Paper having a base paper and a clear coating layer.
Paper in which 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 metal element having a divalent value or higher.
(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 a carboxyl group of 0.1 to 3.0 mmol / g with respect to the absolute dry weight of the cellulose nanofibers.
(7) The paper according to any one of (1) to (5), wherein the cellulose nanofiber has a carboxyalkyl substitution degree per anhydrous glucose unit of 0.01 to 0.50.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a paper having a high printing surface strength.

Hereinafter, the present invention will be described in detail. The paper of the present invention includes 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. Further, in the present invention, "X to Y" includes X and Y which are the fractional values thereof.

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

The CNF is preferably chemically modified. The chemically modified CNF can be produced by chemically modifying a cellulosic raw material to prepare chemically modified cellulose and mechanically defibrating it.
1) Cellulose-based raw materials Cellulose-based raw materials are not particularly limited, and examples thereof include those derived from plants, animals (for example, sea squirts), algae, microorganisms (for example, acetobacter), and microbial products. Plant-derived materials include, for example, wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (conifer unbleached kraft pulp (NUKP), conifer bleached kraft pulp (NBKP), broadleaf unbleached kraft pulp (NBKP). LUKP), broadleaf bleached kraft pulp (LBKP), coniferous unbleached sulphite pulp (NUSP), coniferous bleached sulphite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, used paper, etc.). The cellulose raw material may be any one or a combination of these, but is preferably a cellulosic 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 a functional group into a cellulosic raw material, and it is preferable to introduce an anionic group. Examples of the anionic group include an acid group 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, and a polyphosphonic acid group. Depending on the reaction conditions, these acid groups may be introduced in the form of salts (for example, carboxylate groups (-COOM, M is a metal atom)). The chemical denaturation is preferably oxidation or etherification. Oxidation or etherification can be carried out according to a known method such as that described in JP-A-2019-104833 and the like.

3) Mechanical defibration Chemically modified cellulose is mechanically defibrated to obtain CNF. The defibration treatment may be performed once or a plurality of times. It is preferable that the mixture containing the chemically modified cellulose and the dispersion medium is subjected to the defibration treatment. Water is preferable as the dispersion medium. The device used for defibration is not particularly limited, and examples thereof include high-speed rotary type, colloid mill type, high pressure type, roll mill type, ultrasonic type and the like, and a high pressure or ultrahigh pressure homogenizer is preferable, and a wet high pressure type is preferable. Alternatively, an ultrahigh pressure homogenizer is more preferable. The device preferably applies a strong shear force to the chemically modified cellulose. The pressure applied to the device is preferably 50 MPa or more, more preferably 100 MPa or more, still more preferably 140 MPa or more. The device is preferably a wet high pressure or ultra high pressure homogenizer. As a result, defibration can be performed efficiently.

When defibration is carried out on a dispersion of chemically modified pulp, the solid content concentration of the modified cellulose in the dispersion is usually preferably 0.1% by weight or more, more preferably 0.2% by weight or more, and 0. .3% by weight or more is more preferable. As a result, the amount of liquid becomes appropriate with respect to the amount of modified cellulose, which makes it efficient. The upper limit of the concentration is usually preferably 20% by weight or less, more preferably 15% by weight or less, still more preferably 10% by weight or less. This makes it possible to maintain liquidity.

4) Characteristics The average fiber diameter of CNF is usually about 2 nm or more and less than 500 nm in terms of length-weighted average fiber diameter, but is 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. Atomic force microscope (AFM) or transmission electron microscope (TEM) is used for the length-weighted average fiber diameter and the length-weighted average fiber length (hereinafter, also simply referred to as "average fiber diameter" and "average fiber length"). It is obtained 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 by 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, still more preferably 0.8 mmol / g or more, based on the absolute dry weight of CNF. 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, and even more preferably 0.8 to 2.0 mmol / g.

The degree of carboxyalkyl substitution per anhydrous glucose 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 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 substitution of the carboxyalkyl group is preferably 0.01 to 0.50, more preferably 0.05 to 0.40, and even more preferably 0.10 to 0.30. The carboxyalkyl substitution is preferably a 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, a B-type viscosity of 500 to 7000 mPa · s when an aqueous dispersion having a concentration of 1% (w / v) (that is, an aqueous dispersion containing 1 g of CNF (dry weight) in 100 mL of water) is used. It is preferable to use a CNF that provides (60 rpm, 20 ° C.). It is an index for specifying characteristics such as the amount of functional groups, the average fiber length, and the average fiber diameter of the B-type viscosity CNF, and is appropriately adjusted according to the application.

The B-type viscosity of the aqueous dispersion of CNF 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 is 20 ° C., and the rotation speed of the rotor is 60 rpm. The aqueous dispersion of CNF of the present invention has thixotropy property, and has the property that the viscosity decreases by stirring and applying shear stress, and the viscosity increases and gels in the stationary state, so that the mixture is sufficiently stirred. It is preferable to measure the B-type viscosity in the state.

(2) Starch Starch is a polymer of D-glucose, preferably a mixture of amylose and amylopectin. In this embodiment, the starch also includes a polymer compound derived from starch. Examples of the polymer include those obtained by modifying, modifying, or processing starch. In the present invention, oxidized starch in which an anionic group is introduced by an oxidation reaction is preferable. Examples of the anionic group include a carboxyl group.

Oxidized starch is a white powder or granules that are insoluble in cold water. Unless the oxidation level is very low, the aqueous suspension lowers the gelatinization start temperature as the oxidation level increases, forming a low-viscosity transparent paste liquid. Oxidized starch is usually a wet or dry reaction by oxidizing the starch with an oxidizing agent such as hypochlorite (preferably Na salt), bleaching powder, hydrogen peroxide, potassium permanganate, or ozone. Manufactured. Although it depends on the oxidation conditions, the oxidized starch usually has a carboxy group and a carbonyl group, and has a structure in which the glycosidic bond is cleaved.

(3) Metal salt The metal salt that can be used in the present invention is not limited, and examples thereof include magnesium salt, calcium salt, aluminum salt, sodium salt, and potassium salt. Among them, metal salts containing divalent or higher metal elements such as magnesium salt, calcium salt, and aluminum salt form a chelate with an anionic group present in CNF or starch, and the strength of the coating film is improved. Therefore, it is preferable. Further, the counter ion is also not limited, but from the viewpoint of easy availability, water solubility, and the metal element ionized in water easily forms the chelate, a sulfate ion other than the organic acid ion is preferable. Examples thereof include inorganic acid ions such as ions, chloride ions, nitrate ions and carbonate ions, and hydroxide ions. Therefore, specific examples of preferable metal salts include aluminum sulfate (sulfate band), magnesium sulfate, calcium chloride, magnesium chloride, aluminum chloride and the like.

(4) Base paper The base paper is a base layer of paper and contains pulp as a main component. The pulp raw material of the base paper is not particularly limited, and mechanical pulp such as ground pulp (GP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), deinked pulp (DIP), coniferous kraft pulp (NKP), and coniferous tree. Chemical pulp such as kraft pulp (LKP) can be used. As the deinked (waste paper) pulp, selected waste paper such as high-quality paper, medium-quality paper, low-grade paper, newspaper, leaflets, and magazines, and unsorted waste paper obtained by mixing these can be used.

A known filler can be added to the base paper, but it is not necessary to add the filler when using paperboard or other applications where opacity or whiteness is not required, or when using raw materials with a large amount of brought-in ash such as used paper. When a filler is added, the filler includes heavy calcium carbonate, light calcium carbonate, clay, silica, light calcium carbonate-silica complex, kaolin, calcined kaolin, deramikaolin, 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 acid, urea-formalin resin, and melamine. 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, which are typical fillers for neutral papermaking and alkaline papermaking and can obtain high opacity, are preferable. The content of the filler 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, even if the ash content in the paper is high, the decrease in paper strength is suppressed, so that the content of the filler in the base paper is more preferably 10% by weight or more.

As the internal chemicals, bulking agents, dry paper strength improvers, wet paper strength improvers, drainage improvers, dyes, neutral sizing agents and the like may be used as necessary.

The base paper is produced by a known papermaking method. For example, it can be performed using a long net paper machine, a gap former type paper machine, a hybrid former type paper machine, an on-top former type paper machine, a round net paper machine, and the like, but the present invention is not limited thereto.

The base paper may be single-layer or multi-layer. The base paper may contain the CNF. In the case of the multilayer base paper, some of the plurality of paper layers may contain CNF, or all layers may contain CNF. When the base paper contains CNF, the content thereof is preferably 0.0001% by weight or more, more preferably 0.0003% by weight or more, still more preferably 0.001% by weight or more, based on the total pulp weight of the 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 the metal salt is preferably 0.05 to 1 part by weight, more preferably 0.1 to 0.5 parts by weight, based on 100 parts by weight of starch. When the weight ratio of each component is in this range, the film-forming property of the clear coating layer mainly composed of starch is improved, and as a result, high print surface strength can be achieved.

The coating amount of the clear coating layer is preferably 0.01~3.0g / ^{m 2} in terms of solid per side content, and more preferably 0.1 to 2.0 g / ^{m 2.} For clear coating, for example, a coater (coating machine) such as a size press, a gate roll coater, a pre-metering size press, a curtain coater, or a spray coater is used, and a clear coating liquid containing starch as a main component is used as a base paper. It can be formed by coating on top. As an example, when coating with a gate roll coater, the clear coating liquid has a B-type 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 10 to 300 mPa · s is more preferable. 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 the coating amount, and if it exceeds 450 mPa · s, boiling occurs. Operability may deteriorate. The solid content concentration of the clear coating liquid is adjusted so as to achieve the above concentration, but 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 in this embodiment may include a pigment coating layer. The pigment coating layer is a layer containing a white pigment as a main component. 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 the adhesive include proteins such as starch, casein, soybean protein, and synthetic protein, polyvinyl alcohol, cellulose derivatives such as carboxymethyl cellulose and methyl cellulose, styrene-butadiene copolymer, and conjugated diene of methyl methacrylate-butadiene copolymer. Examples thereof include a system polymer latex, an acrylic polymer latex, and a vinyl polymer latex such as an ethylene-vinyl acetate copolymer. These can be used alone or in combination of two or more, and it is preferable to use a starch-based adhesive and a styrene-butadiene copolymer in combination.

The pigment coating layer may contain various auxiliaries such as dispersants, thickeners, defoamers, colorants, antistatic agents, and preservatives used in the general paper manufacturing field, and contains CNF. You may. In this case, the amount of CNF is preferably ^{1 × 10 -3} to 1 part by weight with respect to 100 parts by weight of the pigment. In the above range, a pigment coating liquid having an appropriate water retention property can be obtained without significantly increasing the viscosity of the coating liquid.

The pigment coating layer can be provided by applying a coating liquid to one side or both sides of the base paper by 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 amount of the pigment coating layer applied may be appropriately adjusted depending on the intended use, 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, and preferably 25 g / m ² or less.

When the paper in this embodiment further has a pigment-coated layer, it is possible to obtain a pigment-coated paper having excellent surface strength and print glossiness in addition to high ink mileage.

(7) Characteristics The paper of this embodiment has high printing surface strength. The reason for this is not limited, but it is presumed that functional groups such as COOH groups in starch and CNF form a crosslinked structure with metal ions to improve the strength of the clear coating layer. 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 .

(8) Method for producing paper of this embodiment The paper of this embodiment is produced through a step of applying a clear coating liquid containing starch, CNF and a metal salt on a base paper prepared by a known method. Is preferable. Specifically, the paper of this embodiment is preferably produced by a method including 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 method for preparing the coating liquid and its characteristics 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: Step of preparing a mixed solution containing starch and CNF Step 1B: Step of preparing a clear coating solution containing the mixed solution and a metal salt The starch in step 1A is preferably steamed starch. Further, step 1A may be prepared by adding starch to the aqueous dispersion of CNF. In this case, starch can be added to the aqueous dispersion of CNF, and the solution can be steamed to prepare a mixed solution. In the clear coating liquid prepared in this way, starch and CNF are uniformly dispersed.

The paper of this embodiment may be produced by a method including the following steps 3 in addition to the above-mentioned steps 1 and 2.
Step 3: A step of forming a pigment coating layer containing a pigment and an adhesive on a 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 (absolutely dry) of bleached unbeaten kraft pulp derived from coniferous tree (whiteness 85%: manufactured by Nippon Paper Industries, Ltd.) with 39 mg (0.05 mmol per 1 g of absolutely dried cellulose) of TEMPO (manufactured by Sigma Aldrich) 514 mg of sodium bromide (1.0 mmol with respect to 1 g of dry cellulose) was added to 500 mL of an aqueous solution, and the mixture was 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. Although the pH in the system decreased during the reaction, a 3M aqueous sodium hydroxide solution was sequentially added to adjust the pH to 10. The reaction was terminated when sodium hypochlorite was consumed and the pH in the system did not change. 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. Water was added to the oxidized pulp to prepare a 1% (w / v) mixture, which was treated three times with an ultrahigh pressure homogenizer (20 ° C., 150 MPa) to obtain an aqueous dispersion of CNF. The average fiber diameter of CNF was 3 nm and the aspect ratio was 250.

<Clear coating liquid 1>
Oxidized starch (manufactured by Japan Corn Starch, SK20) and aluminum sulfate (aluminum sulfate band) are added to the aqueous dispersion of CNF produced as described above, and the weight ratio of starch: CNF: metal salt is 100: 1.5: A clear coating liquid 1 of 0.33 was produced. Table 1 shows the B-type viscosities at 30 ° C. and 60 rpm when the solid content concentration of the clear coating liquid 1 was 5% by weight.

<Pigment coating liquid>
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 were added as adhesives to prepare a pigment coating liquid having a solid content of 60% by weight.

<Paper>
To LBKP (manufactured by Nippon Paper Industries, Ltd., csf 360 ml), 0.5% by weight of sulfuric acid band, 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, a base paper was produced by a paper machine. On the base paper, the clear coating liquid 1 was applied to both sides of the base paper with a gate roll coater so that the ^{solid content per side was 0.2 g / m 2.} Next, the pigment coating liquid was applied to both sides so that the solid content per side was 7.5 g / m ^2, and dried by a conventional method to obtain a coated paper. The coated paper was evaluated by the method described later.

[Comparative Example 1]
A coated paper was produced and evaluated in the same manner as in Example 1 except that CNF and a metal salt were not used.
[Comparative Example 2]
A 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>
The clear coating liquid 2 was prepared by the same method as the clear coating liquid 1 except that the amounts of CNF and the metal salt were set to the amounts shown in Table 1.
<Paper>
A coated paper was produced and evaluated by the same method as in Example 1 except that the clear coating liquid 2 was used instead of the clear coating liquid 1.

[Comparative Example 3]
A 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>
The clear coating liquid 3 was prepared in the same manner as the 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 the clear coating liquid 3 was used instead of the clear coating liquid 1.

[Examples 4 and 5]
Clear coating liquids 4 and 5 were prepared in the same manner as the clear coating liquid 3 except that magnesium sulfate and calcium chloride were used instead of the sulfuric acid band, respectively. A coated paper was produced and evaluated by the same method as in Example 3 except that these were used as the coating liquid.

[Comparative Example 4]
A coated paper was produced and evaluated in the same manner as in Example 3 except that no metal salt was used. The results of these evaluations 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 According to JIS P8124.
2) Printing gloss Using an offset sheet-fed printing press (4 colors) manufactured by Roland Co., Ltd., using offset sheet-fed ink (NEX-M manufactured by Toyo Ink Co., Ltd.), printing on solid areas at a printing speed of 8000 sheets / hr. Printing was performed in the order of indigo (CM) so that the meat density was indigo 1.60 and red 1.50. The glossiness of the indigo red (CM) solid printed portion of the obtained printed matter was measured based on JIS P-8142.

3) Picking evaluation Using an offset sheet-fed printing press manufactured by Roland Corporation and Leo Echo Y indigo manufactured by Toyo Ink Co., Ltd. as ink, solid indigo was printed at a speed of 8000 sph. The number of picking on the F side and the W side generated during printing 10 sheets was measured.

Claims (7)

Paper with a base paper and a clear coating layer
Paper in which the clear coating layer contains starch, cellulose nanofibers, and a metal salt. The paper according to claim 1, wherein the starch is oxidized starch. The paper according to claim 1 or 2, wherein the metal salt contains a metal element having a divalent value or higher. 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 anion-modified cellulose nanofibers. The paper according to any one of claims 1 to 5, wherein the cellulose nanofibers have a carboxyl group of 0.1 to 3.0 mmol / g with respect to the absolute dry weight of the cellulose nanofibers. The paper according to any one of claims 1 to 5, wherein the cellulose nanofibers have a carboxyalkyl substitution degree per anhydrous glucose unit of 0.01 to 0.50.