Coated paper, method for producing same, and coating agent for paper

Abstract

The purpose of the present invention is to provide coated paper having exceptional transparency and drying properties during production. The present invention provides a coated paper in which at least one surface of a paper substrate is coated with a coating agent, wherein: the coating agent contains a polyester resin X and a polyester resin Y; the polyester resin X is a reaction product of at least one polyol (D1) and at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), the melt viscosity of the polyester resin X at 50°C being 200-180,000 mPa·s; and the polyester resin Y is a reaction product of at least one polyol (D2) and at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), the melt viscosity of the polyester resin Y at 110°C being 700 mPa·s or greater.

Description

Coated paper, its manufacturing method, and paper coating agent

[0001] This invention relates to coated paper, a method for producing the same, and a coating agent for paper.

[0002] Common methods for producing highly transparent paper include using pulp fibers with a high degree of beating during the papermaking process, and impregnating the spaces between the fibers of the paper with resin or other substances. The method using pulp fibers with a high degree of beating during the papermaking process is used in the production of glassine paper, tracing paper, etc. The method of impregnating the spaces between the fibers of the paper with resin or other substances is used in the production of oil paper, wax paper, etc.

[0003] In recent years, as a shift towards sustainable and recyclable materials, the replacement of plastic film with paper has been explored. For example, the replacement of plastic film with paper is progressing in the field of packaging materials. If the transparency of plastic film can be imparted to paper, its applications are expected to expand even further.

[0004] Patent Document 1 proposes applying a rosin-based aqueous clearing agent to a portion of the base paper and heating it to obtain a windowed packaging bag. Patent Document 2 proposes printing and impregnating a clearing agent mainly composed of vegetable oil onto a specific portion of opaque paper, forming an oil layer protective film on both sides to make the opaque paper transparent.

[0005] Japanese Patent Publication No. 2011-63286 Japanese Patent Publication No. Sho 62-15395

[0006] The technologies described in Patent Documents 1 and 2 involve impregnating the voids between the cellulose fibers of paper with a clearing agent, thereby increasing the transparency of the semi-transparent areas impregnated with the clearing agent. However, the rosin-based aqueous clearing agent used in Patent Document 1 does not provide sufficient transparency. Patent Document 2 requires an oil layer protection layer to prevent the impregnated vegetable oil from leaching out. If an oil layer protection layer is not provided, the vegetable oil will leach out or dry out over time, causing the paper to become cloudy.

[0007] The inventors, after extensive research, discovered that a polyester resin incorporating rosins and aliphatic carboxylic acids is useful as a clearing agent. Rosins and aliphatic carboxylic acids each exhibit refractive indices close to those of cellulose pulp, but using them together brings the refractive index closer to that of cellulose pulp than the refractive indices of each substance alone. However, as with Patent Document 2, the aliphatic carboxylic acids leach out or dry, causing the paper to become cloudy. Resinizing these substances can suppress the clouding of the paper. Further research toward practical application revealed that it is difficult to achieve both transparency and good drying properties during manufacturing with the above-described polyester resin. Poor drying properties result in stickiness remaining after drying in high-speed coating, preventing the next process from being carried out and reducing productivity.

[0008] The present invention aims to provide coated paper with excellent transparency and drying properties during manufacturing, a method for manufacturing the same, and a paper coating agent useful for manufacturing the coated paper.

[0009] The present invention has the following embodiments: [1] Coated paper in which a coating agent is applied to at least one surface of a paper substrate, wherein the coating agent comprises a polyester resin Y, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2) and aliphatic monocarboxylic acids (C2) and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C. [2] Coated paper having a coating agent applied to at least one surface of a paper substrate, wherein the coating agent comprises polyester resin X and polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1) and aliphatic monocarboxylic acids (C1) and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2) and aliphatic monocarboxylic acids (C2) and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C. [3] Coated paper in which a first coating agent and a second coating agent are applied in this order to at least one surface of a paper substrate, wherein the first coating agent comprises a polyester resin X, and the second coating agent comprises a polyester resin Y, wherein the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.[4] The coated paper according to [2], wherein the ratio of polyester resin Y to the total mass of polyester resin X and polyester resin Y is 20 to 80% by mass. [5] The coated paper according to [3], wherein the ratio of polyester resin Y to the total mass of polyester resin X and polyester resin Y is 10 to 60% by mass. [6] The coated paper according to any one of [2] to [5], wherein the refractive index of polyester resin X is 1.4 or more and 1.6 or less. [7] The coated paper according to any one of [1] to [6], wherein the refractive index of polyester resin Y is 1.4 or more and 1.6 or less. [8] A method for producing coated paper, comprising coating a coating agent onto at least one surface of a paper substrate, wherein the coating agent comprises a polyester resin Y, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C. [9] A method for producing coated paper, comprising coating a coating agent on at least one surface of a paper substrate, wherein the coating agent comprises polyester resin X and polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

[10] A method for manufacturing coated paper, comprising coating a first coating agent and a second coating agent in this order on at least one surface of a paper substrate, wherein the first coating agent comprises a polyester resin X, the second coating agent comprises a polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

[11] The method for manufacturing coated paper according to [9], wherein the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 20 to 80% by mass.

[12] The method for manufacturing coated paper according to

[10] , wherein the amount of the second coating agent applied is such that the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 10 to 60% by mass.

[13] The method for manufacturing coated paper according to any one of [9] to

[12] , wherein the refractive index of the polyester resin X is 1.4 or more and 1.6 or less.

[14] The method for manufacturing coated paper according to any one of [8] to

[13] , wherein the refractive index of the polyester resin Y is 1.4 or more and 1.6 or less.

[15] A coating agent for paper comprising a polyester resin Y, wherein the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

[16] A coating agent for paper comprising polyester resin X and polyester resin Y, wherein the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1) and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2) and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

[17] The coating agent for paper according to

[16] , wherein the content of polyester resin Y is 20 to 80% by mass relative to the total mass of polyester resin X and polyester resin Y.

[0010] According to the present invention, it is possible to provide coated paper with excellent transparency and drying properties during manufacturing, a method for manufacturing the same, and a paper coating agent useful for manufacturing the coated paper.

[0011] This is a schematic cross-sectional view showing an example of coated paper according to the first embodiment. This is a schematic cross-sectional view showing an example of coated paper according to the second embodiment.

[0012] In this specification, the "~" indicating a numerical range means that the values ​​described before and after it are included as the lower and upper limits. The lower and upper limits of the numerical ranges disclosed herein can be arbitrarily combined to form new numerical ranges. "Polycarboxylic acid" means a compound having two or more carboxyl groups and its anhydride. "Melt viscosity" is measured using a B-type viscometer. "Refractive index" is measured in accordance with JIS K 7142 (2014). "Iodine value" is measured in accordance with JIS K 0070 (1992).

[0013] <<First Embodiment>> The coated paper of the first embodiment is coated paper in which a coating agent is applied to at least one surface of a paper substrate. The coating agent includes at least polyester resin Y, which is one of polyester resin X and polyester resin Y. The coating agent may contain only polyester resin Y, or it may contain both polyester resin X and polyester resin Y. The coating agent will be described in detail later.

[0014] When a coating agent is applied to at least one surface of a paper substrate, at least a portion of the coating agent impregnates the interior of the paper substrate, making it transparent. The transparent area (transparent region) may be the entire surface or only a portion of the paper substrate when viewed from above. At least a portion of the coating agent may be located on at least one surface of the paper substrate. In this case, diffuse reflection of light from the surface of the paper substrate can be suppressed, further enhancing transparency. The resin contained in the coating agent may also have heat-sealing properties. If a resin with heat-sealing properties is used, the coating agent located on at least one surface of the paper substrate can also be used as a sealant layer.

[0015] Figure 1 is a schematic cross-sectional view showing an example of coated paper according to the first embodiment. In this example, the coated paper 1 has a coating agent 12 applied to one side of a paper substrate 11. In Figure 1, the upper side is the coated surface 1a. For the sake of explanation, the dimensions in Figure 1 are different from those of the actual product. Here, an example is shown in which the coating agent 12 is applied to one side of the paper substrate 11, but the coating agent 12 may also be applied to both the one side of the paper substrate 11 and the other side opposite it.

[0016] In coated paper, the visible light transmittance in the transparent region is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. The visible light transmittance is measured using a transmission meter. Further details are described in the examples below.

[0017] The amount of coating agent to be applied cannot be stated definitively as it depends on the type of paper substrate (void amount, etc.), but the total mass of polyester resin X and polyester resin Y per unit area of ​​the transparent region (or the mass of polyester resin Y only if polyester resin X is not included) is 10 to 80 g / m². ２ Preferably, 20 to 70 g / m ２ More preferably, 30-60 g / m ２ This is even more preferable. If the amount of coating agent applied is above the lower limit of the range, the transparency of the transparent region is more easily enhanced. If the amount of coating agent applied is below the upper limit of the range, the recyclability can be improved. The masses of polyester resin X and polyester resin Y are calculated on a solid content basis, and the same applies below. Solid content refers to non-volatile components, which are components excluding the liquid medium described later.

[0018] <Coating Agent> In the first embodiment, the coating agent, as described above, includes at least polyester resin Y, one of polyester resin X and polyester resin Y. Polyester resin X (hereinafter also referred to as "resin X") is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity at 50°C (hereinafter also referred to as "melt viscosity (50°C)") of 200 to 180,000 mPa·s. Polyester resin Y (hereinafter also referred to as "resin Y") is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity at 110°C (hereinafter also referred to as "melt viscosity (110°C)") of 700 mPa·s or more.

[0019] [Rosins (A1), (A2)] Rosins (A1) and (A2) are plant-derived components. Therefore, using rosins (A1) and (A2) as raw materials for resin X can contribute to carbon neutrality. More specifically, rosins (A1) are compounds derived from pine. The type of pine is not particularly limited, and examples include Merkus pine, slash pine, and horsehair pine. These can be used individually or in combination of two or more types.

[0020] The rosin (A1) is not particularly limited and includes known unmodified rosin and its derivatives. Examples of unmodified rosin include crude rosin and refined rosin. Examples of crude rosin include gum rosin, tall rosin, and wood rosin. Examples of refined rosin include refined products of crude rosin. Examples of rosin derivatives include hydrogenated rosin, disproportionated rosin, and polymerized rosin. These can be used individually or in combination of two or more. The origin of the rosin is not particularly limited and includes, for example, China, Vietnam, Indonesia, and Brazil. These can be used individually or in combination of two or more. From the viewpoint of film-forming properties and water resistance, unmodified rosin is preferred as the rosin (A1), and gum rosin is more preferred.

[0021] Examples of rosin (A2) include those the same as rosin (A1), and the preferred embodiments are also the same. Rosin (A2) may be the same as or different from rosin (A1).

[0022] [Polycarboxylic acids (B1), (B2)] Polycarboxylic acids (B1) and (B2) are components that adjust the molecular weight of resin X. Examples of polycarboxylic acids (B1) include oxalic acid, malonic acid, succinic acid, fumaric acid, maleic acid, itaconic acid, citraconic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, dodecanediic acid, tetradecanediic acid, hexadecanedioic acid, phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, oxaloacetate, methylmalonic acid, dimethylmalonic acid, methylsuccinic acid, 2,2-dimethylsuccinic acid, methylglutaric acid, dimethylglutaric acid, diglycolic acid, 1 Examples include 3-acetonedicarboxylic acid, ketoglutaric acid, cyclopropane-1,1-dicarboxylic acid, cyclobutane-1,1-dicarboxylic acid, cyclohexane-1,1-dicarboxylic acid, 2-oxoadipic acid, 4-oxoheptanedioic acid, 5-oxoazelaic acid, phenylenedioxydiacetic acid, indan-2,2-dicarboxylic acid, naphthalene-2,6-dicarboxylic acid, naphthalene-2,7-dicarboxylic acid, cyclohexanedicarboxylic acid, ethylenediaminetetraacetic acid, and their anhydrides. These can be used individually or in combination of two or more. As the polycarboxylic acid (B1), dicarboxylic acids are preferred from the viewpoint of adjusting the molecular weight.

[0023] The polycarboxylic acid (B1) preferably contains an α,β-unsaturated dicarboxylic acid. Since the α,β-unsaturated dicarboxylic acid reacts with rosins (A1), the presence of an α,β-unsaturated dicarboxylic acid in the polycarboxylic acid (B1) improves its transparency properties. Examples of α,β-unsaturated dicarboxylic acids include fumaric acid, maleic acid, itaconic acid, citraconic acid, and their anhydrides. Examples of anhydrides include maleic anhydride, itaconic anhydride, and citraconic anhydride. These can be used individually or in combination of two or more. In addition, an α,β-unsaturated dicarboxylic acid can be used in combination with other dicarboxylic acids.

[0024] From the viewpoint of adjusting the molecular weight, the polycarboxylic acid (B1) preferably contains at least one selected from the group consisting of succinic acid, fumaric acid, maleic anhydride, and adipic acid, and more preferably contains fumaric acid.

[0025] As the polyvalent carboxylic acid (B2), the same ones as the polyvalent carboxylic acid (B1) can be mentioned, and the preferred embodiments are also the same. The polyvalent carboxylic acid (B2) may be the same as or different from the polyvalent carboxylic acid (B1).

[0026] [Aliphatic monocarboxylic acids (C1), (C2)] As the aliphatic monocarboxylic acid (C1), for example, formic acid, acetic acid, trifluoroacetic acid, propionic acid, acrylic acid, methacrylic acid, valeric acid, mercaptoacetic acid, sorbic acid, etc. can be mentioned. As the aliphatic monocarboxylic acid (C1), further, fatty acids derived from oils and fats can be exemplified. More specifically, linseed oil fatty acid, yuzu oil fatty acid, pistachio oil fatty acid, rice oil fatty acid, safflower oil fatty acid, apricot oil fatty acid, cottonseed oil fatty acid, sesame oil fatty acid, corn oil fatty acid, watermelon oil fatty acid, soybean oil fatty acid, poppy oil fatty acid, apple oil fatty acid, sunflower oil fatty acid, cactus oil fatty acid, tall oil fatty acid, walnut oil fatty acid, tung oil fatty acid, clove oil fatty acid, and castor oil fatty acid can be mentioned. These can be used alone or in combination of two or more.

[0027] The refractive index of the aliphatic monocarboxylic acid (C1) is, for example, 1.35 to 1.65, preferably 1.40 to 1.60.

[0028] As the aliphatic monocarboxylic acid (C1), from the viewpoint of transparency suitability, those having a predetermined iodine value are preferable. The iodine value of the aliphatic monocarboxylic acid (C1) is, from the viewpoint of transparency suitability, for example, 0 mg / 100 mg or more, preferably 70 mg / 100 mg or more, more preferably 100 mg / 100 mg or more.

[0029] From the viewpoint of transparency suitability, the aliphatic monocarboxylic acid (C1) preferably contains as a main component an aliphatic monocarboxylic acid with an iodine value of 20 g / 100 g or more. The main component refers to 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more. Therefore, the content ratio of the aliphatic monocarboxylic acid with an iodine value of 20 g / 100 g or more relative to the total amount of the aliphatic monocarboxylic acid (C1) is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. It is particularly preferable that the aliphatic monocarboxylic acid (C1) consists of an aliphatic monocarboxylic acid with an iodine value of 20 g / 100 g or more.

[0030] Examples of aliphatic monocarboxylic acids with an iodine value of 20 g / 100 g or more include linseed oil fatty acids, yuzu oil fatty acids, pistachio oil fatty acids, rice bran oil fatty acids, safflower oil fatty acids, apricot kernel oil fatty acids, cottonseed oil fatty acids, sesame oil fatty acids, corn oil fatty acids, watermelon oil fatty acids, soybean oil fatty acids, poppy seed oil fatty acids, apple oil fatty acids, sunflower oil fatty acids, cactus oil fatty acids, tall oil fatty acids, walnut oil fatty acids, tung oil fatty acids, clove oil fatty acids, and castor oil fatty acids.

[0031] Examples of aliphatic monocarboxylic acid (C2) include those the same as those of aliphatic monocarboxylic acid (C1), and the preferred embodiments are also the same. The aliphatic monocarboxylic acid (C2) may be the same as or different from the aliphatic monocarboxylic acid (C1).

[0032] [Polyols (D1) and (D2)] As the polyol (D1), for example, dihydric alcohols, trihydric alcohols, and alcohols with four or more hydroxyl groups can be mentioned. As the dihydric alcohol, for example, linear alkyldiols, branched alkyldiols, and ether diols can be mentioned. Examples of the linear alkyldiol include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, etc. Examples of the branched alkyldiol include propylene glycol, 1,3-butanediol, 1,2-butanediol, 3-methyl-1,5-pentanediol, 2,6-dimethyl-1-octene-3,8-diol, etc. Examples of the ether diol include diethylene glycol, triethylene glycol, dipropylene glycol, etc. Also, examples of the dihydric alcohol include 1,4-dihydroxy-2-butene, isosorbide, cyclohexanedimethanol, cyclohexanediol, tricyclodecane dimethylol, bisphenol A, hydrogenated bisphenol A, hydrogenated bisphenol F, hydrogenated bisphenol S, hydrogenated catechol, hydrogenated resorcinol, hydrogenated hydroquinone, and dicyclopentadiene diallyl alcohol copolymer, etc. As the trihydric alcohol, for example, glycerin, trimethylolethane, trimethylolpropane, trimethylolhexane, trimethyloctane, etc. can be mentioned. As the alcohol with four or more hydroxyl groups, for example, alcohols with 4 to 8 hydroxyl groups can be mentioned. Examples of the alcohol with 4 to 8 hydroxyl groups include pentaerythritol, diglycerin, ditrimethylolpropane, sorbitan, sorbitol, dipentaerythritol, inositol, and tripentaerythritol, etc. These can be used alone or in combination of two or more.

[0033] From the viewpoint of adjusting the molecular weight of resin X, the polyol (D1) preferably contains at least one selected from the group consisting of trihydric alcohols and tetrahydric or higher alcohols, more preferably contains a trihydric alcohol, even more preferably contains at least one selected from the group consisting of trimethylolpropane, glycerin, and pentaerythritol, and particularly preferably contains glycerin.

[0034] The number of carbon atoms in the polyol (D1) is, for example, 2 or more, preferably 3 or more, and also, for example, 30 or less, preferably 20 or less, more preferably 10 or less, and even more preferably 8 or less.

[0035] Examples of polyol (D2) include those similar to polyol (D1), and preferred embodiments are also similar. Polyol (D2) may be the same as polyol (D1) or different.

[0036] [Polyester Resin X] Resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1). From the viewpoint of keeping the refractive index of resin X within the preferred range described above, the carboxylic acid preferably includes rosins (A1) and aliphatic monocarboxylic acids (C1), and from the viewpoint of transparency, it is particularly preferable that it consists of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1). Therefore, it is particularly preferable that resin X is a reaction product of rosins (A1), polycarboxylic acids (B1), aliphatic monocarboxylic acids (C1), and polyol (D1).

[0037] The melt viscosity (50°C) of resin X is 200 to 180,000 mPa·s. When the melt viscosity (50°C) of resin X is 180,000 mPa·s or less, resin X penetrates easily into the interior of the paper substrate, resulting in coated paper with fewer internal voids and excellent transparency. When the melt viscosity (50°C) of resin X is 200 mPa·s or more, sufficient drying properties can be ensured in combination with resin Y, and bleed-through of the coating liquid during application can be suppressed. The melt viscosity (50°C) of resin X is preferably 100,000 mPa·s or less, more preferably 50,000 mPa·s or less, even more preferably 30,000 mPa·s or less, preferably 300 mPa·s or more, more preferably 500 mPa·s or more, and even more preferably 1,000 mPa·s or more.

[0038] The refractive index of resin X is preferably 1.4 to 1.6, more preferably 1.45 to 1.58, even more preferably 1.47 to 1.57, and particularly preferably 1.48 to 1.56. When the refractive index of resin X is within the above range, the transparency of the coated paper is better. This is because the refractive index of cellulose fibers is generally in the range of 1.4 to 1.6. By setting the refractive index of resin X to a value close to that of cellulose fibers, the refraction of light at the interface between the coating agent and the pulp fibers in the paper substrate can be reduced.

[0039] The iodine value of resin X is preferably 10 g / 100 g or more, more preferably 20 g / 100 g or more, particularly preferably 30 g / 100 g or more, and also preferably 140 g / 100 g or less, more preferably 120 g / 100 g or less, and particularly preferably 100 g / 100 g or less. When the iodine value of resin X is above the lower limit, the transparency is better, and when it is below the upper limit, the drying properties are better. Note that the lower the iodine value, the higher the melt viscosity (at 50°C).

[0040] [Method for producing polyester resin X] Resin X can be produced by reacting at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1) with at least one polyol (D1) so that the melt viscosity (50°C) is 200 to 180,000 mPa·s. To adjust the melt viscosity (50°C) of resin X to the above range, for example, the content ratio of raw material components, reaction temperature, and reaction time are adjusted. To adjust the refractive index of resin X to the above range, for example, the content ratio of rosins (A1) and aliphatic monocarboxylic acid (C1) is adjusted. To adjust the iodine value of resin X to the above range, for example, the iodine value and content ratio of aliphatic monocarboxylic acid (C1) are adjusted.

[0041] When reacting the raw materials, they may be reacted all at once or in multiple stages. In a single reaction, resin X can be produced by a dehydration condensation reaction between a known acid and an alcohol. Suitable conditions for the dehydration condensation reaction are approximately 150 to 300°C and 2 to 20 hours.

[0042] When the polycarboxylic acid (B1) includes an α,β-unsaturated dicarboxylic acid, a multi-step reaction is preferred. Specifically, first, a rosin (A1) is reacted with an α,β-unsaturated dicarboxylic acid to obtain a first product. Then, the first product is reacted with an aliphatic monocarboxylic acid (C1) and a polyol (D1) to obtain resin X.

[0043] In the step of obtaining the first product, an addition reaction is carried out between the unsaturated bond of the rosin (A1) and the unsaturated bond of the α,β-unsaturated dicarboxylic acid. The reaction temperature in the step of obtaining the first product is, for example, 150°C or higher, preferably 170°C or higher, more preferably 180°C or higher, and also, for example, 230°C or lower, preferably 220°C or lower, more preferably 200°C or lower. The reaction time in the step of obtaining the first product is, for example, 0.1 hours or more, preferably 0.5 hours or more, and also, for example, 5 hours or less, preferably 3 hours or less. In the step of obtaining the first component, a known reaction catalyst may be added in an appropriate proportion as needed. Furthermore, in the step of obtaining the first product, the raw material components may be reacted without a solvent, or in the presence of a known solvent.

[0044] In the step of reacting the first product with an aliphatic monocarboxylic acid (C1) and a polyol (D1), the carboxyl group of the first product or the aliphatic monocarboxylic acid (C1) is esterified with the hydroxyl group of the polyol (D1). The first product, the aliphatic monocarboxylic acid (C1), and the polyol (D1) may be reacted together or in a multi-step reaction. The reaction temperature in the step of reacting the first product with the aliphatic monocarboxylic acid (C1) and the polyol (D1) is, for example, 150°C or higher, preferably 160°C or higher, more preferably 170°C or higher, even more preferably 180°C or higher, and also, for example, 230°C or lower, preferably 220°C or lower, more preferably 210°C or lower, and even more preferably 200°C or lower. The reaction time in the step of reacting the first product with the aliphatic monocarboxylic acid (C1) and the polyol (D1) is, for example, 1 hour or more, preferably 3 hours or more, and also, for example, 48 hours or less, preferably 24 hours or less. In this reaction, if necessary, the condensed water produced by the esterification reaction can be removed by known distillation method. In the step of reacting the first product with the aliphatic monocarboxylic acid (C1) and the polyol (D1), if necessary, a known reaction catalyst can be added in an appropriate proportion. In the step of reacting the first product with the aliphatic monocarboxylic acid (C1) and the polyol (D1), the starting material components may be reacted without a solvent, or in the presence of a known solvent.

[0045] If the aliphatic monocarboxylic acid (C1) contains fatty acids derived from oils and fats, the oil in which the aliphatic monocarboxylic acid (C1) is a constituent fatty acid may be reacted with a polyol (D1) to obtain a second product containing a modified product in which the polyol (D1) has been modified with the oil, and the resin X may be obtained by reacting this second product with the first product. When the oil and fat are reacted with the polyol (D1), a modified product containing an ester (fatty acid ester) of the polyol (D1) and fatty acids derived from the oil is obtained by transesterification. This fatty acid ester produces polyol (D1) and fatty acids by hydrolysis. Therefore, the modified product can be used in place of the aliphatic monocarboxylic acid (C1) and polyol (D1). Specifically, as described above, the rosin (A1) is reacted with an α,β-unsaturated dicarboxylic acid to obtain the first product. Separately, the oil in which the aliphatic monocarboxylic acid (C1) is a constituent fatty acid is reacted with the polyol (D1) to obtain the second product. Next, the first product and the second product are reacted to obtain resin X.

[0046] Examples of oils and fats whose constituent fatty acids are aliphatic monocarboxylic acids (C1) include linseed oil, yuzu oil, pistachio oil, rice bran oil, safflower oil, apricot oil, cottonseed oil, sesame oil, corn oil, watermelon oil, soybean oil, poppy oil, apple oil, sunflower oil, cactus oil, tall oil, walnut oil, tung oil, clove oil, and castor oil.

[0047] The blending ratio in the step of obtaining the second product is set as appropriate, but for example, the amount of hydroxyl groups of polyol (D1) per mole of oil is, for example, 3 moles or more, preferably 10 moles or more, and for example, 50 moles or less, preferably 40 moles or less.

[0048] The reaction temperature in the step to obtain the second product is, for example, 230°C or higher, preferably 240°C or higher, more preferably 250°C or higher, and also, for example, 300°C or lower, preferably 280°C or lower, more preferably 270°C or lower. If the reaction temperature is excessively low, the reaction in the step to obtain the modified product may not proceed, and the aliphatic monocarboxylic acid (C1) may not be incorporated into the resin X. If the reaction temperature is excessively high, a decomposition reaction may occur, which may lead to an increase in low molecular weight components. The reaction time in the step to obtain the second product is, for example, 0.5 hours or more, preferably 1 hour or more, and also, for example, 20 hours or less, preferably 10 hours or less. In the step to obtain the second product, a known reaction catalyst may be added in an appropriate proportion as needed. In addition, in the step to obtain the second product, the raw material components may be reacted without a solvent, or in the presence of a known solvent.

[0049] The reactant obtained in this manner contains resin X. In addition to resin X, the reactant may also contain unreacted raw material components. Unreacted raw material components are removed from the reactant as needed.

[0050] [Polyester Resin Y] Resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2). From the viewpoint of keeping the refractive index of resin Y within the preferred range described above, the carboxylic acid preferably includes rosins (A2) and aliphatic monocarboxylic acids (C2), and from the viewpoint of drying properties, it is particularly preferable that it consists of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2). Therefore, it is particularly preferable that resin Y is a reaction product of rosins (A2), polycarboxylic acids (B2), aliphatic monocarboxylic acids (C2), and polyol (D2).

[0051] The melt viscosity (110°C) of resin Y is 700 mPa·s or higher. A melt viscosity (110°C) of 700 mPa·s or higher of resin Y provides excellent drying properties. The melt viscosity (110°C) of resin Y is preferably 1000 mPa·s or higher, more preferably 5000 mPa·s or higher, and even more preferably 10000 mPa·s or higher. There is no particular upper limit to the melt viscosity (110°C) of resin Y, but it is preferably 400000 mPa·s or lower, which is the measurement limit for melt viscosity at 110°C when measured using a B-type viscometer. Resin Y may also be one whose melt viscosity (110°C) cannot be measured. Note that the melt viscosity (50°C) of resin Y usually exceeds 180000 mPa·s.

[0052] The refractive index of resin Y is preferably 1.4 to 1.6, more preferably 1.45 to 1.58, even more preferably 1.47 to 1.57, and particularly preferably 1.48 to 1.56, similar to resin X.

[0053] The iodine value of resin Y is preferably 20 g / 100 g or less. The lower limit of the iodine value of resin Y is not particularly limited. When the iodine value of resin Y is below the upper limit, the drying properties are better. Note that the lower the iodine value, the higher the melt viscosity (110°C) tends to be.

[0054] [Method for producing resin Y] Resin Y can be produced by reacting at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2) with at least one polyol (D2) such that the melt viscosity (110°C) is 700 mPa·s or higher. Resin Y can be produced in the same manner as resin X, except that the melt viscosity (110°C) is 700 mPa·s or higher. To adjust the melt viscosity (110°C) of resin Y to the above range, for example, the content ratio of raw material components, reaction temperature, and reaction time can be adjusted.

[0055] [Content of resin X and resin Y] The ratio of the total mass of resin X and resin Y to the total solid content of the coating agent (or the ratio of resin Y only if resin X is not included) is preferably 20% by mass or more, more preferably 40% by mass or more, even more preferably 80% by mass or more, and may be 100% by mass.

[0056] When the coating agent contains resin X and resin Y, the ratio of resin Y to the total mass of resin X and resin Y is preferably 20 to 80% by mass, more preferably 20 to 70% by mass, and even more preferably 20 to 60% by mass. When the ratio of resin Y is above the lower limit of the above range, the drying properties are better, and when it is below the upper limit of the above range, the transparency is better.

[0057] [Other components] The coating agent may further contain other components besides resins X and Y, as needed.

[0058] The coating agent may contain, for example, a liquid medium. The liquid medium in the coating agent is removed by drying during the manufacturing of the coated paper. Examples of liquid media include organic solvents, water, and mixtures thereof. The liquid medium is preferably capable of dissolving or dispersing resins X and Y. Organic solvents are preferred as the liquid medium because they offer excellent penetration into the paper substrate, do not cause the bumps that occur when water is used, and have excellent drying properties.

[0059] Examples of organic solvents include alcohols, ethers, esters, and nonpolar solvents. Examples of alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, n-pentanol, n-hexanol, and other C1-C6 alcohols.

[0060] Examples of ethers include ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monopropyl ether, diethylene glycol monopropyl ether, triethylene glycol monopropyl ether, tetraethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, tetraethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monoisobutyl ether, triethylene glycol Polyethylene glycol monoisobutyl ether, tetraethylene glycol monoisobutyl ether, ethylene glycol monotertiary butyl ether, diethylene glycol monotertiary butyl ether, triethylene glycol monotertiary butyl ether, tetraethylene glycol monotertiary butyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol monoethyl ether, tetrapropylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monopropyl ether, tripropylene glycol monopropyl ether, propylene glycol monoisopropyl ether, dipropylene glycol monoisopropyl ether, tripropylene glycol monoisopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, propylene glycol monoisobutyl ether,Examples include glycol ethers such as dipropylene glycol monoisobutyl ether, tripropylene glycol monoisobutyl ether, propylene glycol monotertiary butyl ether, dipropylene glycol monotertiary butyl ether, and tripropylene glycol monotertiary butyl ether.

[0061] Examples of esters include ethyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, diethylene glycol monoethyl ether acetate, and diethylene glycol monobutyl ether acetate. Examples of nonpolar solvents include paraffinic hydrocarbons such as pentane, hexane, heptane, octane, nonane, decane, and dodecane; isoparaffinic hydrocarbons such as isohexane, isooctane, and isododecane; alkylnaphthenic hydrocarbons such as liquid paraffin; aromatic hydrocarbons such as benzene, toluene, xylene, alkylbenzene, and solvent naphtha; and silicone oils.

[0062] The content of the liquid medium is set appropriately according to the purpose and application. For example, the liquid medium may be, for example, 5% by mass or more, preferably 10% by mass or more, relative to the total amount of the liquid medium, resin X, and resin Y. Alternatively, the liquid medium may be, for example, 80% by mass or less, preferably 70% by mass or less, relative to the total amount of the liquid medium, resin X, and resin Y. Or, the total amount of resin X and resin Y may be, for example, 20% by mass or more, preferably 30% by mass or more, relative to the total amount of the liquid medium, resin X, and resin Y. Alternatively, the total amount of resin X and resin Y may be, for example, 95% by mass or less, preferably 90% by mass or less, relative to the total amount of the liquid medium, resin X, and resin Y. If the liquid medium or the total amount of resin X and resin Y is within the above range relative to the total amount of the liquid medium, resin X, and resin Y, a rapid increase in viscosity can be suppressed, and the productivity, coatability, and drying properties of the coating agent can be improved.

[0063] The coating agent may contain, for example, resins other than resins X and Y. Examples of other resins include acrylic resins, styrene-modified acrylic resins, silicone acrylic resins, modified silicone acrylic resins, rosin phenol resins, rosin ester resins, terpene phenol resins, coumarone indene resins, petroleum resins, epoxy resins, modified epoxy resins, polyester resins, vinyl acetate resins, ethylene-vinyl acetate resins, urethane resins, urea resins, melamine resins, and cellulose resins. These can be used individually or in combination of two or more. In the coating agent, the content ratio of other resins is set appropriately according to the purpose and application. For example, with respect to 100 parts by mass of the total of resins X and Y, the other resin may be 0 parts by mass or more, or 20 parts by mass or more, or 80 parts by mass or less, or 60 parts by mass or less.

[0064] The coating agent may contain, for example, wax. The inclusion of wax can suppress blocking when used on paper substrates. Examples of waxes include fatty acid amide wax, carnauba wax, rice wax, polyolefin wax, paraffin wax, Fischer-Tropsch wax, beeswax, microcrystalline wax, polyethylene oxide wax, and amide wax. These can be used individually or in combination of two or more. Preferably, the waxes are fatty acid amide wax, carnauba wax, polyolefin wax, paraffin wax, and microcrystalline wax, and more preferably, carnauba wax, polyolefin wax, and paraffin wax.

[0065] More specifically, examples of fatty acid amide waxes include pelargonic acid amide, capric acid amide, undecyl acid amide, lauric acid amide, tridecyl acid amide, myristic acid amide, pentadecyl acid amide, palmitic acid amide, heptadecyl acid amide, stearic acid amide, nonadecanoic acid amide, arachidic acid amide, behenic acid amide, lignoceric acid amide, oleic acid amide, cetoleic acid amide, linoleic acid amide, linolenic acid amide, and mixtures thereof. Fatty acid amides from animal and vegetable oils are also examples of fatty acid amide waxes. These can be used individually or in combination of two or more types.

[0066] More specifically, examples of carnauba waxes include MICROKLEAR 418 (manufactured by Micro Powders, Inc.) and refined carnauba wax No. 1 powder (Nippon Wax Co., Ltd.). These can be used alone or in combination of two or more types. More specifically, examples of olefin waxes include polyethylene wax, polypropylene wax, MPP-635VF (Micro Powders, Inc.), and MP-620VF XF (Micro Powders, Inc.). These can be used alone or in combination of two or more types. More specifically, examples of paraffin waxes include MP-28C, MP-22XF, and MP-28C (all from Micro Powders, Inc.). These can be used alone or in combination of two or more types.

[0067] From the standpoint of heat resistance, the melting point of wax is, for example, 60°C or higher and, for example, 130°C or lower.

[0068] In coating agents, the proportion of wax is set appropriately according to the purpose and application. For example, with respect to 100 parts by mass of resin X and resin Y combined, the amount of wax is, for example, 0 parts by mass or more, preferably 1 part by mass or more, more preferably 3 parts by mass or more, or, for example, 50 parts by mass or less, preferably 30 parts by mass or less.

[0069] The coating agent may contain additives in appropriate proportions. Examples of additives include fillers, thickeners, foaming agents, antioxidants, light stabilizers, heat stabilizers, flame retardants, color adjusters, and drying accelerators such as cobalt octylate and cobalt naphthenate. These can be used individually or in combination of two or more types.

[0070] The coating agent can be manufactured by mixing resin X and resin Y, and other components as needed.

[0071] <Paper Substrate> The paper substrate includes pulp. Examples of pulp constituting the paper substrate include chemical pulp, mechanical pulp, recycled paper pulp, and non-wood pulp. These pulps may be used individually or in combination of two or more. Chemical pulp is preferred among these.

[0072] Chemical pulps include softwood chemical pulp and hardwood chemical pulp, but it is preferable to include both softwood and hardwood chemical pulp in the paper substrate. Hardwood chemical pulp has a shorter and finer fiber structure than softwood chemical pulp. When hardwood chemical pulp is used, the short and fine fiber structure results in superior paper substrate structure. If the paper substrate structure is poor, even if the coating agent is impregnated, the heterogeneity caused by the structure may result in uneven penetration of the coating agent, impairing transparency. To obtain coated paper with excellent transparency, it is preferable to have a paper substrate structure with excellent structure. However, while hardwood chemical pulp with its fine and short fiber structure has good structure, the gaps and voids become smaller, making it easy for areas where the coating agent does not penetrate easily to occur. Also, because there are more interfaces, transmitted light tends to be refracted and attenuated, reducing transparency. Furthermore, because it is easily coated with the coating agent, its defibration properties when recycled as waste paper tend to be inferior to those of softwood chemical pulp. Therefore, by using both hardwood chemical pulp and softwood chemical pulp as the paper base material, it is possible to achieve both transparency and recyclability in coated paper.

[0073] Examples of softwood chemical pulps include unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), semi-bleached softwood kraft pulp (NSBKP), and softwood sulfite pulp (NSP). Among these, bleached softwood kraft pulp (NBKP) and softwood sulfite pulp (NSP) are preferred from the viewpoint of achieving both transparency and strength in coated paper.

[0074] The Canadian standard filtration efficiency (CSF) of softwood chemical pulp is preferably 300 to 700 mL, more preferably 400 to 680 mL, even more preferably 420 to 650 mL, and particularly preferably 450 to 600 mL. When the CSF of the softwood chemical pulp is above the lower limit of the above range, the voids in the paper substrate can be retained, resulting in excellent impregnation of the coating agent. When the CSF of the softwood chemical pulp is below the upper limit of the above range, the form of the paper substrate can be improved, making it easier to obtain coated paper with a transparent region that has excellent transparency and visibility.

[0075] Examples of hardwood chemical pulps include unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), semi-bleached hardwood kraft pulp (LSBKP), and hardwood sulfite pulp (LSP). Among these, bleached hardwood kraft pulp (LBKP) and hardwood sulfite pulp (LSP) are preferred from the viewpoint of achieving both transparency and strength in coated paper.

[0076] The Canadian Standard CSF (Chemical Saturation Factor) of hardwood chemical pulp is preferably 250 to 680 mL, more preferably 350 to 650 mL, even more preferably 370 to 630 mL, and particularly preferably 400 to 600 mL. If the CSF of the hardwood chemical pulp is above the lower limit of the above range, the strength of the paper substrate can be increased. If the CSF of the hardwood chemical pulp is below the upper limit of the above range, the form of the paper substrate can be improved, and coated paper with a transparent area that is excellent in transparency and visibility can be easily obtained.

[0077] The mass ratio of coniferous chemical pulp to hardwood chemical pulp (coniferous chemical pulp: hardwood chemical pulp) is preferably 80:20 to 20:80, more preferably 75:25 to 25:75, and even more preferably 70:30 to 30:70.

[0078] When the paper substrate contains coniferous chemical pulp and hardwood chemical pulp, the paper substrate may also contain other pulps in addition to these chemical pulps, such as mechanical pulp, recycled paper pulp, and non-wood pulp. Examples of mechanical pulps include stone ground pulp (SGP), pressure stone ground pulp (PGW), refiner ground pulp (RGP), thermo ground pulp (TGP), chemiground pulp (CGP), crushed wood pulp (GP), and thermomechanical pulp (TMP). Examples of recycled paper pulp include disintegrated recycled paper pulp, disintegrated and deinked recycled paper pulp, and disintegrated, deinked and bleached recycled paper pulp. Examples of recycled paper used as raw material for recycled paper pulp include brown recycled paper, kraft envelope recycled paper, magazine recycled paper, newspaper recycled paper, flyer recycled paper, office recycled paper, corrugated cardboard recycled paper, white recycled paper, Kent recycled paper, imitation recycled paper, and land deed recycled paper. Examples of non-wood pulps include various types of pulps, such as those chemically or mechanically produced from non-wood fibers like kenaf, cotton, hemp, and reeds. These pulps may be used individually or in combination of two or more types.

[0079] When recycled paper pulp is used as the pulp, the recycled paper pulp content is preferably 10% by mass or less, and more preferably 5% by mass or less, relative to the total mass of pulp constituting the paper base material. When the recycled paper pulp content is below the above upper limit, the coated paper can be suitably applied as packaging material for food and beverages. The lower limit for the recycled paper pulp content is 0% by mass.

[0080] The visible light transmittance of paper substrates is, for example, 10-20%.

[0081] The basis weight of the paper substrate is 35 to 150 g / m². ２ Preferably, 40-100 g / m ２ More preferably, 43-80 g / m ２ This is even more preferable. If the basis weight is above the lower limit of the above range, the paper will have sufficient strength and will be a coated paper suitable for use as packaging paper, printing paper, etc. If the basis weight is below the upper limit of the above range, the transparency of the coated paper can be increased. The basis weight is measured in accordance with JIS P8124.

[0082] The air permeability of the paper substrate is preferably 10 to 40 seconds, more preferably 12 to 35 seconds, and even more preferably 15 to 33 seconds. If the air permeability is above the lower limit of the above range, the paper strength is obtained, resulting in coated paper suitable for applications such as packaging paper and printing paper. If the air permeability is below the upper limit of the above numerical range, the penetration of the coating agent is excellent, and the transparency of the coated paper can be increased. The air permeability is measured using the Wang Ren method air permeability, which conforms to J. TAPPI-5-2:2000.

[0083] The density of the paper substrate is 0.5 to 0.90 g / cm³. ３ Preferably, 0.6 to 0.8 g / cm³ ３ This is more preferable. If the density is above the lower limit of the range, the paper strength is obtained, resulting in coated paper suitable for applications such as packaging paper and printing paper. If the density is below the upper limit of the range, the penetration of the coating agent is excellent, and the transparency of the coated paper can be increased. The density is measured in accordance with JIS P8118.

[0084] The porosity of the paper substrate is preferably 30 to 80%, more preferably 40 to 70%, and even more preferably 50 to 70%. If the porosity of the paper substrate is above the lower limit of the above range, the transparency of the transparent area is easily enhanced. If the porosity of the paper substrate is below the upper limit of the above range, the physical strength of the sheet is less likely to decrease. The porosity of the paper substrate is the value obtained by dividing the density measured in accordance with JIS P8118 by the true density of cellulose, which is 1.50.

[0085] The ParkerPrintSurf smoothness of at least one surface of the paper substrate is preferably 7 μm or less, and more preferably 5 μm or less. There is no particular lower limit. The smaller the value, the smoother the surface. ParkerPrintSurf smoothness can evaluate the smoothness of fine details, and the smaller this value, the less light scattering on the paper surface can be reduced, thereby improving visibility through the transparent parts. ParkerPrintSurf smoothness is determined in accordance with ISO 8791-4:1992 (soft backing / clamping pressure 500 kPa).

[0086] In addition to pulp, the paper substrate may contain known papermaking aids such as paper strength enhancers, sizing agents, fillers, and colorants. The inclusion of fillers such as talc and calcium carbonate improves the smoothness and whiteness of the paper substrate, but it also increases the opacity of the paper, making it difficult to impart transparency. Therefore, it is preferable to keep the filler content in the paper substrate within a range that does not impair transparency and visibility, and it is even more preferable that the paper substrate does not contain any fillers.

[0087] The method for manufacturing paper substrates is not particularly limited. For example, a method may include a step of beating pulp which will be the raw material for the paper substrate, a step of papermaking from a pulp slurry containing the beated pulp, and a step of drying the wet sheet obtained by papermaking.

[0088] In the beating process, it is preferable to beating the raw pulp so that it reaches the Canadian standard filtration degree. The beating machine is not particularly limited. For example, known beating machines such as double disc refiners can be used. The paper machine used for papermaking is not particularly limited. For example, long screen paper machines, short screen paper machines, cylinder screen paper machines, etc. The drying process is also not particularly limited. For example, a dryer attached to the paper machine can be used.

[0089] The paper substrate may be subjected to a smoothing treatment. By applying a smoothing treatment, light scattering on the paper surface can be reduced, thereby improving visibility through the transparent parts. Examples of smoothing treatments include tightness press, machine calender, gloss calender, soft nip calender, and supercalender. However, these devices increase the density of the paper, so care must be taken to reduce the linear pressure to prevent the density from becoming too high. On the other hand, a transfer method in which the paper is attached to a smooth surface while it is still wet and then the smooth surface is transferred by drying is preferable because it does not increase the density of the paper. For example, technologies such as Yankee cylinders, cast drums, and film transfer can be used. Among these, a Yankee dryer using a Yankee cylinder is preferable because it is attached to the paper machine and offers excellent productivity.

[0090] As a paper substrate, commercially available papers such as kraft paper, glossy kraft paper, fine paper, electrophotographic paper, inkjet recording paper, thermal recording paper, laser thermal paper, thermal transfer recording paper, art paper, coated paper, cast coated paper, white cardboard, colored cardboard, corrugated cardboard liner, glassine paper, rice paper, India paper, and Japanese paper can also be used. Among these, fine paper, electrophotographic paper, kraft paper, and glossy kraft paper, which have a low pigment content, are preferred because they provide excellent visibility of the transparent areas due to the coating agent.

[0091] <Method for manufacturing coated paper> The coated paper of this embodiment can be manufactured by coating at least one surface of a paper substrate with the coating agent described above.

[0092] The coating agent can be applied by known methods. Examples include spray coaters, roll coaters, gravure coaters, nip coaters, bar coaters, blade coaters, lip coaters, fountain coaters, curtain coaters, flow coaters, comma coaters, dip coaters, brush coating, immersion methods, flexographic printing, inkjet printing, gravure printing, offset printing, gravure offset printing, and silkscreen printing. Among these, methods that apply, impregnate, and print using rolls and roll nibs are preferred because they efficiently push the coating agent into the paper substrate, resulting in high transparency. Examples of such methods include roll coaters, gravure coaters, nip coaters, bar coaters, blade coaters, etc., which combine rolls such as backup rolls and backing rolls with applicator rolls, and gravure printing, gravure offset printing, and offset printing, which combine an impression cylinder with a plate cylinder or blanket cylinder.

[0093] The coating agent may be applied to the entire surface of the paper substrate or to a portion of it. Furthermore, during the application of the coating agent, there may be areas in the cross-section of the paper substrate where the coating agent does not reach a portion of the surface opposite the coated surface. The coating agent may be applied to only one surface of the paper substrate, or to one surface and the other surface separately. The coating agent may be applied in a single step or in multiple steps. If applied in multiple steps, the components and composition of the coating agent used in each step may be the same or different.

[0094] After applying the coating agent, drying is performed as needed to remove the liquid medium. Drying can be carried out by known methods. Drying conditions may be natural drying at room temperature or heated drying. Heated drying is preferred. The drying temperature in heated drying is, for example, 40°C or higher, preferably 50°C or higher. The drying temperature is, for example, 150°C or lower, preferably 130°C or lower. The drying time is, for example, 1 second or more, preferably 5 seconds or more. The drying time is, for example, 600 seconds or less, preferably 500 seconds or less. After drying, humidity control may be performed to adjust the moisture content. Humidity control conditions are, for example, a temperature of 23°C and a relative humidity of 50%.

[0095] <Applications> The coated paper of this embodiment can be used as packaging paper with a transparent area for applications such as boxes, bags, envelopes, and clear files. In addition to packaging paper, it can be used for various applications such as printing paper, book paper, copy paper, information paper, label paper, and household paper, where an image (characters, symbols, pictures, objects, etc.) on the opposite side of the paper can be seen through the paper substrate.

[0096] ≪Second Embodiment≫ The coated paper of the second embodiment is coated paper in which a first coating agent and a second coating agent are applied in this order to at least one surface of a paper substrate. The first coating agent contains resin X. The second coating agent contains resin Y. Therefore, the coated paper of the second embodiment contains resin X and resin Y.

[0097] When the first and second coating agents are applied in this order to at least one surface of a paper substrate, at least a portion of the first and second coating agents impregnates the interior of the paper substrate, making it transparent. The transparent area (transparent region) may be the entire surface or only a portion of the paper substrate in a plan view. A portion of the first and second coating agents may be located on at least one surface of the paper substrate. In this case, diffuse reflection of light by the surface of the paper substrate can be suppressed, and transparency can be further enhanced. Furthermore, since the resin contained in each coating agent has heat-sealing properties, the coating agent located on at least one surface of the paper substrate can also be used as a sealant layer.

[0098] Figure 2 is a schematic cross-sectional view showing an example of coated paper according to the second embodiment. In this example, the coated paper 2 has a first coating agent 22 and a second coating agent 23 applied in that order to one surface of a paper substrate 21. In Figure 2, the upper surface is the coated surface 2a. For the sake of explanation, the dimensions in Figure 2 are different from those of the actual product. In the coated paper 2, at least a portion of the first coating agent 22 is impregnated into the interior of the paper substrate 21. A portion of the first coating agent 22 may be located on one surface of the paper substrate 21. At least a portion of the second coating agent 23 is located closer to the coated surface 2a than the first coating agent 22. A portion of the second coating agent 23 may be mixed with the first coating agent 22. Here, an example is shown in which the first coating agent 22 and the second coating agent 23 are applied in this order to one side of the paper substrate 21. However, the first coating agent 22 and the second coating agent 23 may also be applied in this order to one side of the paper substrate 21 and the other side opposite it.

[0099] In coated paper, the visible light transmittance in the transparent region is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more.

[0100] The coating amounts of the first coating agent and the second coating agent are preferably such that the ratio of resin Y to the total mass of resin X and resin Y is 10 to 60% by mass. The ratio of resin Y to the total mass of resin X and resin Y is more preferably 20 to 60% by mass, and even more preferably 30 to 60% by mass. When the ratio of resin Y is at least the lower limit of the above range, the drying property is more excellent, and when it is at most the upper limit of the above range, the transparency is more excellent.

[0101] The total coating amount of the first coating agent and the second coating agent cannot be generally determined depending on the type of the paper base material (such as the void amount, etc.). However, as the total mass of resin X and resin Y per unit area of the transparency region, 10 to 80 g / m ２ is preferable, 20 to 70 g / m ２ is more preferable, and 30 to 60 g / m ２ is even more preferable. When the total coating amount is at least the lower limit of the above range, it is easy to enhance the transparency of the transparency region. When the total coating amount is at most the upper limit of the above range, the recyclability can be enhanced.

[0102] <First coating agent> The first coating agent contains resin X. As resin X, the same ones as those listed in the first embodiment can be mentioned, and the preferable embodiments are also the same.

[0103] The mass ratio of resin X to the total solid content of the first coating agent is preferably 40% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and may be 100% by mass.

[0104] The first coating agent can further contain other components other than resin X and resin Y, if necessary. As the other components, the same ones as those listed in the first embodiment can be mentioned, and the preferable embodiments are also the same.

[0105] <Second coating agent> The second coating agent contains resin Y. As resin Y, the same ones as those listed in the first embodiment can be mentioned, and the preferable embodiments are also the same.

[0106] The ratio of the mass of resin Y to the total solid content of the second coating agent is preferably 40% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, and may also be 100% by mass.

[0107] The second coating agent may further contain other components besides resins X and Y, as needed. Examples of other components are the same as those listed in the first embodiment, and the preferred embodiments are also the same.

[0108] <Paper Substrate> Examples of paper substrates include those similar to the paper substrate in the first embodiment, and the preferred embodiments are also similar.

[0109] <Method for manufacturing coated paper> The coated paper of this embodiment can be manufactured by applying the first coating agent and the second coating agent described above to at least one surface of a paper substrate in that order.

[0110] The application of the first coating agent and the application of the second coating agent can each be carried out in the same manner as the application of the coating agent in the first embodiment.

[0111] After applying the first coating agent and then the second coating agent, drying is performed as necessary to remove the liquid medium. Drying can be carried out in the same manner as drying after coating the coating agent in the first embodiment. After drying, humidity control may be performed to adjust the moisture content. For example, the humidity control conditions are a temperature of 23°C and a relative humidity of 50%.

[0112] <Applications> The coated paper of this embodiment can be used as packaging paper with a transparent area for applications such as boxes, bags, envelopes, and clear files. In addition to packaging paper, it can be used for various applications such as printing paper, book paper, copy paper, information paper, label paper, and household paper, where an image (characters, symbols, pictures, objects, etc.) on the opposite side of the paper can be seen through the paper substrate.

[0113] Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to the following examples. Unless otherwise specified, "parts" and "%" are based on mass. In the following description, specific numerical values ​​such as blending ratios (content), physical properties, and parameters may be replaced with the corresponding upper limits (numerical values ​​defined as "less than or equal to" or "less than") or lower limits (numerical values ​​defined as "greater than or equal to" or "greater than") of the blending ratios (content), physical properties, and parameters described in the "Modes for Carrying Out the Invention" above.

[0114] <Measurement Method> (1) Refractive Index The refractive index was measured using a multi-wavelength Abbe refractometer DR-M4 (manufactured by Atago Corporation) in accordance with JIS K 7142 (2014).

[0115] (2) The viscosity of the resin at 50°C and 110°C was measured using a Type B viscometer (TVB-15, manufactured by Toki Sangyo Co., Ltd.) equipped with a melt viscosity constant temperature bath.

[0116] (3) Iodine value The iodine value was measured in accordance with JIS K 0070 (1992).

[0117] <Production of Polyester Resin> (Production Example 1) 45.3 parts of gum rosin were added to a four-necked flask equipped with a stirrer, a reflux condenser with a water separator, and a thermometer, while blowing nitrogen gas into it, and the temperature was raised to 210°C. Next, 6.4 parts of fumaric acid were added, and the mixture was stirred at 210°C for about 90 minutes. This yielded the first product. 36.0 parts of linseed oil and 12.4 parts of glycerin were added to another four-necked flask equipped with a reflux condenser with a water separator and a thermometer, and the mixture was stirred at 250°C for about 90 minutes. This yielded the second product. The first product was mixed into the four-necked flask containing the second product, and after mixing, the mixture was cooled to 190°C. Subsequently, a dehydration condensation reaction was carried out at 190°C for 8 hours to obtain rosin-modified polyester resin (resin 1). The refractive index of resin 1 was 1.51, its melt viscosity at 50°C was 10860 mPa·s, and its melt viscosity at 110°C was 183 mPa·s. The refractive index of linseed oil fatty acid is 1.49.

[0118] (Manufacturing Example 2) 86.3 parts of gum rosin were added to a four-necked flask equipped with a stirrer, a reflux condenser with a water separator, and a thermometer, while nitrogen gas was blown in, and the temperature was raised to 210°C. Next, 1.6 parts of fumaric acid were added, and the mixture was stirred at 210°C for about 90 minutes. After the reaction, the mixture was cooled to 190°C, 9 parts of linseed oil and 3.1 parts of glycerin were added, and a dehydration condensation reaction was carried out at 250°C for 5 hours to obtain a rosin-modified polyester resin (resin 2). The refractive index of resin 2 was 1.52, the melt viscosity at 50°C was 2,330,000 mPa·s, and the melt viscosity at 110°C was 740 mPa·s.

[0119] (Manufacturing Examples 3 and 4) Rosin-modified polyester resins (resins 3 and 4) were obtained in the same manner as in Manufacturing Example 1, except that the ratio of raw material components was changed to the ratios shown in Table 1. The physical properties of each resin are shown in Table 1.

[0120]

[0121] <Preparation of Resin Solutions> 70 parts of resin 1 and 30 parts of isopropyl alcohol were placed in an Erlenmeyer flask equipped with a reflux condenser and stirred at 50°C for approximately 1 hour. This yielded resin solution 1 with a solid content of 70%. Resins 3 and 4 were prepared in the same manner as resin 1 to obtain resin solutions 3 and 4 with a solid content of 70%. For resin 2, 70 parts of resin 2 and 30 parts of ethyl acetate were placed in an Erlenmeyer flask equipped with a reflux condenser and stirred at 50°C for approximately 1 hour. This yielded resin solution 2 with a solid content of 70%. Furthermore, 50 parts of resin solution 2 and 50 parts of isopropyl alcohol were mixed to obtain resin solution 5 with a solid content of 35%.

[0122] <Preparation of coated paper by mixed coating> (Example 1) Paper base material: basis weight 50 g / m² ２ We prepared a single-sided glossy kraft paper. The pulp that makes up this single-sided glossy kraft paper is 40% bleached hardwood kraft pulp and 60% bleached softwood kraft pulp. The visible light transmittance of this single-sided glossy kraft paper is 15%, and the density is 0.72 g / cm³. ３The thickness was 70 μm. A coating solution was obtained by mixing 50 parts of resin solution 1 and 50 parts of resin solution 2. The coating solution, adjusted to 25°C, was applied to the surface of the paper substrate using a wire-equipped bar coater No. 36, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0123] (Comparative Example 1) A coating solution was obtained by mixing 50 parts of resin solution 3 and 50 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0124] (Comparative Example 2) A coating solution was obtained by mixing 50 parts of resin solution 4 and 50 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0125] (Comparative Example 3) A coating solution was obtained by mixing 50 parts of resin solution 1 and 50 parts of resin solution 4. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0126] (Example 2) A coating solution was obtained by mixing 90 parts of resin solution 1 and 10 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0127] (Example 3) A coating solution was obtained by mixing 80 parts of resin solution 1 and 20 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0128] (Example 4) A coating solution was obtained by mixing 20 parts of resin solution 1 and 80 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0129] (Example 5) A coating solution was obtained by mixing 10 parts of resin solution 1 and 90 parts of resin solution 2. Coated paper was obtained in the same manner as in Example 1, except that this coating solution was used.

[0130] (Example 6) Paper substrate with a basis weight of 33 g / m² ２Glassine paper was prepared. The pulp that makes up the glassine paper is sulfite pulp, and the visible light transmittance of this glassine paper is approximately 58%, with a density of 1.12 g / cm³. ３ The thickness was 30 μm. The above glassine paper was used instead of glossy kraft paper, and the amount of solid resin adhering was 10.0 g / m². ２ Coated paper was obtained in the same manner as in Example 1, except for the difference described above.

[0131] (Example 7) The coating solution was applied to both the front and back surfaces of the glassine paper, and the amount of solid resin adhering was 13.3 g / m². ２ Coated paper was obtained in the same manner as in Example 6, except for the difference described above.

[0132] <Preparation of coated paper by overcoating> (Example 8) As the paper substrate, the same single-sided gloss kraft paper used in Example 1 was prepared. Resin solution 1, adjusted to 25°C, was applied to the surface of the paper substrate using a wire-equipped bar coater No. 32 and dried at 110°C for 1 minute. The amount of solid resin adhering at this time was 29 g / m² ２ Subsequently, a resin solution 5 adjusted to 25°C was applied to the same coated surface using a wire-wound bar coater No. 4, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 1 g / m². ２ The total amount of resin adhering to the surface is 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0133] (Example 9) As the paper substrate, the same single-sided gloss kraft paper used in Example 1 was prepared. Resin solution 1, adjusted to 25°C, was applied to the surface of the paper substrate using a wire-type bar coater No. 32 and dried at 110°C for 1 minute. The amount of solid resin adhering at this time was 27 g / m². ２ Subsequently, resin solution 2, adjusted to 25°C, was applied to the same coated surface using a wire-equipped bar coater No. 4, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 3 g / m². ２ The total amount of solid resin adhering to the surface is 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0134] (Example 10) As the paper substrate, the same single-sided gloss kraft paper used in Example 1 was prepared. Resin solution 1, adjusted to 25°C, was applied to the surface of the paper substrate using a bar coater No. 14 with a winding wire, and dried at 110°C for 1 minute. The amount of solid resin adhering at this time was 12 g / m². ２ Subsequently, resin solution 2, adjusted to 25°C, was applied to the same coated surface using a wire-wound bar coater No. 18, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 18 g / m². ２ The total amount of resin adhering to the surface is 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0135] (Example 11) As the paper substrate, the same single-sided gloss kraft paper used in Example 1 was prepared. Resin solution 1, adjusted to 25°C, was applied to the surface of the paper substrate using a bar coater No. 12 with a winding. The amount of solid resin adhering at this time was 9 g / m². ２ Subsequently, resin solution 2, adjusted to 25°C, was applied to the same coated surface using a wire-wound bar coater No. 24, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 21 g / m². ２ The total amount of resin adhering to the surface is 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0136] (Example 12) As the paper substrate, the same type of glossy kraft paper used in Example 1 was prepared. Resin solution 2, adjusted to 25°C, was applied to the surface of the paper substrate using a wire-equipped bar coater No. 36, and dried at 50°C for 3 minutes to obtain coated paper. The amount of solid resin adhering at this time was 30 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0137] (Comparative Example 4) As the paper substrate, the same type of glossy kraft paper used in Example 1 was prepared. Resin solution 1, adjusted to 25°C, was applied to the surface of the paper substrate using a winding bar coater No. 36, and dried at 110°C for 1 minute to obtain coated paper. The amount of solid resin adhering at this time was 29 g / m². ２ The resulting coated paper was then conditioned for 12 hours in a constant temperature and humidity environment (22°C, 50% relative humidity).

[0138] <Evaluation> The following evaluation was performed on each case. The results are shown in Table 2.

[0139] (Drying properties) After drying at 110°C for 1 minute, the coated surface was touched with a finger to check for stickiness. ○ indicated no stickiness, △ indicated slight stickiness but no practical problems, and × indicated stickiness where the coated surface adhered to the finger. In the case of multiple coats, the stickiness of the coated surface after the second coat was evaluated.

[0140] (Bleed-through) During the application of the coating solution, the presence or absence of bleeding of the coating solution to the opposite side (back side) was visually checked. ○ was used when no bleeding was observed, △ when slight bleeding of spots was observed, and × when more significant bleeding was observed.

[0141] (Transparency) The visible light transmittance of coated paper immediately after humidity adjustment was measured. Visible light transmittance was measured using a transmission meter LS160 (manufactured by ZHELLY) with visible light at a wavelength of 530 nm.

[0142]

[0143] As shown in the results above, the coated papers of Examples 1 to 12 exhibited excellent drying properties and transparency, and no bleed-through of the coating liquid was observed. On the other hand, Comparative Example 1, in which the melt viscosity (50°C) of resin X was less than 300 mPa·s, showed poor drying properties and bleed-through of the coating liquid was observed. Comparative Example 2, in which the melt viscosity (50°C) of resin X exceeded 180,000 mPa·s, showed poor transparency. Comparative Example 3, in which the melt viscosity (110°C) of resin Y was less than 700 mPa·s, showed poor drying properties. Comparative Example 4, in which only resin X was coated, showed poor drying properties.

[0144] 1 Coated paper 1a Coated surface 11 Paper substrate 12 Coating agent 2 Coated paper 2a Coated surface 21 Paper substrate 22 First coating agent 23 Second coating agent

Claims

1. Coated paper having a coating agent applied to at least one surface of a paper substrate, wherein the coating agent comprises a polyester resin Y, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

2. Coated paper having a coating agent applied to at least one surface of a paper substrate, wherein the coating agent comprises polyester resin X and polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

3. Coated paper in which a first coating agent and a second coating agent are applied in this order to at least one surface of a paper substrate, wherein the first coating agent comprises a polyester resin X, and the second coating agent comprises a polyester resin Y, wherein the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

4. The coated paper according to claim 2, wherein the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 20 to 80% by mass.

5. The coated paper according to claim 3, wherein the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 10 to 60% by mass.

6. The coated paper according to any one of claims 2 to 5, wherein the refractive index of the polyester resin X is 1.4 or more and 1.6 or less.

7. The coated paper according to any one of claims 1 to 5, wherein the refractive index of the polyester resin Y is 1.4 or more and 1.6 or less.

8. A method for producing coated paper, comprising applying a coating agent to at least one surface of a paper substrate, wherein the coating agent comprises a polyester resin Y, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

9. A method for producing coated paper, comprising applying a coating agent to at least one surface of a paper substrate, wherein the coating agent comprises polyester resin X and polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

10. A method for manufacturing coated paper, comprising coating a first coating agent and a second coating agent in this order onto at least one surface of a paper substrate, wherein the first coating agent comprises a polyester resin X, the second coating agent comprises a polyester resin Y, the polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1), and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

11. The method for manufacturing coated paper according to claim 9, wherein the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 20 to 80% by mass.

12. The method for manufacturing coated paper according to claim 10, wherein the amount of the second coating agent applied is such that the ratio of the polyester resin Y to the total mass of the polyester resin X and the polyester resin Y is 10 to 60% by mass.

13. The method for manufacturing coated paper according to any one of claims 9 to 12, wherein the refractive index of the polyester resin X is 1.4 or more and 1.6 or less.

14. The method for manufacturing coated paper according to any one of claims 8 to 12, wherein the refractive index of the polyester resin Y is 1.4 or more and 1.6 or less.

15. A paper coating agent comprising a polyester resin Y, wherein the polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2), and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

16. A coating agent for paper comprising polyester resin X and polyester resin Y, wherein polyester resin X is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A1), polycarboxylic acids (B1), and aliphatic monocarboxylic acids (C1) and at least one polyol (D1), and has a melt viscosity of 200 to 180,000 mPa·s at 50°C, and polyester resin Y is a reaction product of at least one carboxylic acid selected from the group consisting of rosins (A2), polycarboxylic acids (B2), and aliphatic monocarboxylic acids (C2) and at least one polyol (D2), and has a melt viscosity of 700 mPa·s or more at 110°C.

17. The paper coating agent according to claim 16, wherein the content of the polyester resin Y is 20 to 80% by mass relative to the total mass of the polyester resin X and the polyester resin Y.

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