Pigment coated paper

Abstract

The present invention addresses the problem of providing pigment coated paper excellent in smoothness, in which the basis weight of the base paper is high. As a solution, provided is pigment coated paper having, on at least one surface of base paper, two pigment coated layers, which are an undercoat layer and a topcoat layer, wherein: the basis weight of the base paper is 150 g / m2 to 400 g / m2; the content ratio (RKU) of kaolin in the undercoat layer is 10 mass% or more with respect to the total pigment amount; the content ratio (RKT) of kaolin in the topcoat layer is RKU or more with respect to the total pigment amount; and the coating amount (CWU) of the undercoat layer and the coating amount (CWT) of the topcoat layer satisfy CWU > CWT.

Description

pigment coated paper

[0001] The present invention relates to pigment coated paper.

[0002] The smoothness of printing paper is extremely important for obtaining high-quality printed materials. During the printing process, paper with good smoothness allows ink to spread evenly, resulting in finely detailed printed materials. In the printing industry, the required levels of gloss and smoothness are increasing year by year, and there is a demand for coated paper that produces high-quality printed materials while minimizing cost increases and allowing for high productivity.

[0003] Double-coated paper produced by a multi-layer coating method has been widely used because it has superior gloss and smoothness compared to single-layer coated paper, and because high-quality, low-cost printing paper can be obtained by using an inexpensive paint for the undercoat coating. Patent Document 1 proposes a coated printing paper with two pigment coating layers, each composed primarily of a pigment and an adhesive, on both sides of a base paper, the base paper being dehydrated in a gap-type twin-wire former and then two or more shoe presses, and then dried to form a paper, in which the undercoat coating layer adjacent to the base paper contains 15 to 50 parts by mass of a water-soluble adhesive per 100 parts by mass of pigment, and the topcoat coating layer contains 2 to 30 parts by mass of calcium trisulfoaluminate. This coated printing paper has excellent surface smoothness and printability.

[0004] Patent Document 2 describes a coated printing paper having two pigment coating layers per side, each layer consisting mainly of a pigment and an adhesive, on a base paper that has been made by dewatering using a gap-type twin wire former and then drying using two or more shoe presses, in which the undercoat coating layer adjacent to the base paper contains 15 to 50 parts by mass of a water-soluble adhesive per 100 parts by mass of pigment, and the average particle diameter of the pigment is in the range of 1.0 to 3.5 μm, and the particle diameter (D ７５ ) and the particle diameter corresponding to 25 mass% (D ２５ ) ratio, D ７５ / D ２５Coated printing paper with excellent surface smoothness and printability has been proposed, which contains 50 parts by mass or more of precipitated calcium carbonate having a molecular weight of 1.5 to 4.0. Patent Documents 1 and 2 describe that, from the viewpoint of productivity, it is preferable to produce the paper on a machine in which the papermaking to coating steps are carried out continuously at a coating speed of 1,400 m / min or more (since the process is on a machine, the papermaking speed is also the same speed) (see, for example, claim 7 of Patent Document 1 and claim 9 of Patent Document 2).

[0005] Here, coated paper with a high basis weight is used for paper boxes and the like, but there are cases where high printing quality is required for applications such as outer boxes for products. The coated printing paper described in Patent Documents 1 and 2 has a basis weight of 30 to 70 g / m ２ However, it is described that it is preferable to use a base paper of high basis weight (for example, a basis weight of 150 g / m) at a papermaking speed of 1,400 m / min or more as described in Patent Documents 1 and 2. ２ It is extremely difficult to make high-basis weight paper (or higher). This is because high-basis weight paper takes a long time to dehydrate, and as the papermaking speed increases, the dewatering section is unable to sufficiently dehydrate the paper. Furthermore, thick coating is easier the faster the coating speed (line speed), but it has been difficult to make high-basis weight paper and to coat a thick film with excellent smoothness online.

[0006] JP 2008-274464 A JP 2009-221613 A

[0007] An object of the present invention is to provide a pigment coated paper having a high basis weight and excellent smoothness.

[0008] The means for solving the problems of the present invention are as follows: 1. A base paper has two pigment coating layers, an undercoating layer and a top coating layer, on at least one side thereof, and the basis weight of the base paper is 150 g / m ２ More than 400g / m ２ the content of kaolin relative to the total pigment in the undercoating layer (R ＫＵ ) is 10% by mass or more, and the content of kaolin relative to the total pigment in the top coating layer (R ＫＴ ) is R ＫＵ or more, and the coating weight (CW Ｕ) and the coating weight of the top coating layer (CW Ｔ ) and CW Ｕ >CW Ｔ 2. PPS smoothness of the top coating layer (PPS Ｔ 3. The pigment coated paper according to 1., characterized in that the PPS smoothness (PPS Ｂ 4. The pigment coated paper according to 1. or 2., characterized in that the difference in coating weight between the undercoating layer and the top coating layer (CW) is 5.5 μm or more. Ｕ -CW Ｔ ) is 1 g / m ２ 5. The pigment coated paper according to any one of 1. to 3., characterized in that the PPS smoothness (PPS Ｂ ) and the PPS smoothness of the top coating layer (PPS Ｔ ) and the smoothness improvement rate S calculated from the following formula 1 Ｂ－Ｔ 5. The pigment coated paper according to any one of 1. to 4., wherein (Formula 1) S is 50% or more. Ｂ－Ｔ = (PPS Ｂ -PPS Ｔ ) / PPS Ｂ × 100 (%) 6. PPS smoothness of the base paper (PPS Ｂ ) and the PPS smoothness of the undercoating layer (PPS Ｕ ) and the smoothness improvement rate S calculated from the following formula 2 Ｂ－Ｕ 6. The pigment coated paper according to any one of 1. to 5., wherein S (Formula 2) is 15% or more. Ｂ－Ｕ = (PPS Ｂ -PPS Ｕ ) / PPS Ｂ × 100 (%) 7. Density is 0.90 g / cm ３ 8. The pigment coated paper according to any one of 1. to 7., characterized in that the GM value is 6.0 or more. 9. The pigment coated paper according to any one of 1. to 7., characterized in that the GM value is 6.0 or more. ２ More than 400g / m ２ a step of making the following base paper; ＫＵan under-coating layer forming step of forming an under-coating layer in which the content of kaolin relative to the total pigment (R ＫＴ ) is R ＫＵ a top coating layer forming step of forming a top coating layer having a coating weight CW of at least 55% by mass, Ｕ and the coating weight CW of the top coating layer Ｔ But, CW Ｕ >CW Ｔ and the papermaking speed is 100 m / min or more and 450 m / min or less.

[0009] The pigment coated paper of the present invention has excellent smoothness and is capable of high-quality printing, even though the base paper has a high basis weight. The pigment coated paper of the present invention is capable of rigid, high-quality printing, and is therefore particularly suitable for packaging applications such as paper boxes.

[0010] The pigment coated paper of the present invention has two pigment coating layers, an undercoating layer and a top coating layer, on at least one side of the base paper, and the basis weight of the base paper is 150 g / m ２ More than 400g / m ２ The content of kaolin relative to the total pigment in the undercoating layer (R ＫＵ ) is 10% by mass or more, and the content of kaolin relative to the total pigment in the top coating layer (R ＫＴ ) is R ＫＵ The coating weight of the undercoating layer (CW Ｕ ) and the coating weight of the top coating layer (CW Ｔ ) and CW Ｕ >CW Ｔ In this specification, the expression "A to B (A and B are numbers)" means a numerical range including the values ​​of A and B, that is, A or more and B or less.

[0011] "Base Paper" Base paper is a sheet made of papermaking fibers, fillers, various auxiliaries, etc. Wood pulp is preferably used as the papermaking fibers. Examples of wood pulp include chemical pulps such as softwood kraft pulp, hardwood kraft pulp, and sulfite pulp; mechanical pulps such as thermomechanical pulp, stone-grind pulp, and refiner-grind pulp; and recycled paper pulp obtained from newspaper, coated paper, and fine paper. One or more of these wood pulps can be used in combination. If necessary, one or more fiber materials other than cellulose fibers, such as non-wood pulp from kenaf, hemp, or bamboo, glass fiber, or polyethylene fiber, can be blended. From the viewpoint of the strength of pigment-coated paper, it is preferable that the total papermaking fibers contain wood pulp at 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, even more preferably 98% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass or more. On the other hand, recycled paper pulp may be added to reduce costs and promote recycling, but from the viewpoint of strength, the recycled paper pulp content is preferably 40% by mass or less. Furthermore, the ISO brightness of the pulp used is preferably 70% or more, more preferably 75% or more, and even more preferably 80% or more. If pulp with an ISO brightness of less than 70% is used for the base paper, the color of the base paper may be visible through the coating layer, making the coating layer look dull and making uneven coating of the coating layer more noticeable.

[0012] The base paper may contain a filler. Examples of fillers that can be used include known fillers such as white carbon, talc, kaolin, clay, heavy calcium carbonate, light calcium carbonate, titanium oxide, zeolite, and synthetic resin fillers. In the present invention, the filler is an optional component, and the base paper may not contain a filler. Examples of auxiliary agents that can be used as needed include aluminum sulfate and various anionic, cationic, nonionic, or amphoteric retention aids, drainage aids, paper strength agents, and internal sizing agents. Furthermore, dyes, fluorescent whitening agents, pH adjusters, antifoaming agents, pitch control agents, slime control agents, and the like may also be added as needed.

[0013] The basis weight of the base paper of the present invention is 150 g / m ２ More than 400g / m ２ The basis weight of the base paper is 150 g / m ２ If the base paper has a basis weight of less than 400 g / m, the strength may be insufficient for packaging applications such as paper boxes. ２ If the weight exceeds 170 g / m, the workability such as bending will be reduced, and the pigment coated paper will become heavy, resulting in high costs for transportation. ２ More than 190 g / m ２ More preferably, 210 g / m or more ２ More preferably, 350 g / m ２ Preferably, 300 g / m or less ２ More preferably, 250 g / m or less ２ The following is even more preferred:

[0014] The base paper of the present invention has a density of 0.90 g / cm ３ From the viewpoint of weight reduction, it is preferable that the density of the base paper is 0.85 g / cm or less. ３ The lower limit of the density of the base paper is not particularly limited, but from the viewpoint of strength, it is preferably 0.65 g / cm ３ More than 0.70 g / cm ３ The above is more preferable.

[0015] The method for producing the base paper (papermaking) is not particularly limited, and the base paper can be produced by acidic or neutral papermaking using a known Fourdrinier former, on-top hybrid former, gap former machine, or the like. The base paper may be composed of one layer or two or more layers. The papermaking speed is not particularly limited as long as it can produce a base paper of the desired basis weight, but is, for example, about 100 m / min or more and 450 m / min or less. The papermaking speed is preferably 150 m / min or more, more preferably 200 m / min or more, and even more preferably 250 m / min or more.

[0016] The surface of the base paper can be treated with various chemicals (external additives, clear coating). Examples of chemicals that can be used include known external additives such as polyacrylamide, polyvinyl alcohol, surface sizing agents, starch, latex, water-resistant agents, water retention agents, thickeners, and lubricants, and these can be used alone or in combination of two or more. The method for surface treatment of the base paper is not particularly limited, but known coating devices such as rod metering size presses, pond-type size presses, gate roll coaters, spray coaters, blade coaters, and curtain coaters can be used. The amount of external additive coating is not particularly limited, but should be 0.1 to 3.0 g / m2 in solids per side. ２ is preferably 0.2 to 2.0 g / m ２ is more preferred.

[0017] From the viewpoint of preventing penetration of the paint for the pigment coating layer into the base paper, the base paper preferably has a Stoeckigt sizing degree (JIS P 8122 Paper and paperboard - Sizing degree test method - Stoeckigt method) of 30 seconds or more, more preferably 40 seconds or more, and even more preferably 50 seconds or more. By making the base paper have a Stoeckigt sizing degree of 30 seconds or more, penetration of the binder components and colorants in the coating solution into the base paper is suppressed, and a pigment coated paper with excellent coating layer strength and excellent color development of the colorants can be obtained.

[0018] According to the present invention, paper without color unevenness can be obtained even when the base paper has an uneven surface. However, when a smooth base paper is used, paper with even less unevenness can be obtained. Therefore, the base paper of the present invention may be subjected to a calendering treatment. There are no particular restrictions on the calendering method, and a smoothing treatment device such as a normal super calender, gloss calender, soft nip calender, thermal calender, or shoe calender can be used. These may also be combined. The base paper preferably has an Oken smoothness of 50 seconds or less, more preferably 40 seconds or less, even more preferably 30 seconds or less, and even more preferably 20 seconds or less.

[0019] "Pigment Coating Layer" A pigment coating layer is a layer formed from a coating liquid containing a pigment and an adhesive. In addition to the pigment and adhesive, the pigment coating layer may contain various known additives that are blended into pigment coating layers, such as dispersants, thickeners, water retention agents, defoamers, water-resistant agents, and colorants, as needed. The pigment coated paper of the present invention has two pigment coating layers: an undercoating layer and a top coating layer. The top coating layer is provided on the undercoating layer, and the undercoating layer and the top coating layer are in contact with each other.

[0020] Pigment In the present invention, both of the two pigment coating layers, the undercoating layer and the top coating layer, contain kaolin as the pigment. The pigment coating layer may also contain pigments other than kaolin. The pigment other than kaolin is optional and may not be present. Pigments other than kaolin that are known in the papermaking field can be used without particular limitation. For example, one or more of inorganic pigments such as calcium carbonate (heavy / light), clay, delaminated clay, talc, titanium dioxide, aluminum hydroxide, mica, illite, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, and satin white, as well as solid, hollow, or core-shell organic pigments, can be used. As a pigment other than kaolin, calcium carbonate is preferred because it is inexpensive and has excellent whiteness and flowability. In particular, to obtain pigment-coated paper with high whiteness, it is preferable that the top coating layer contain calcium carbonate.

[0021] The undercoating layer has a kaolin content relative to the total pigment (R ＫＵ ) is 10 mass % or more. The mechanism is unclear, but R ＫＵ A coating solution having a content of 10% by mass or more can be applied in a thick film even at a slow coating speed, and the papermaking of a high basis weight base paper at a slow papermaking speed and the application of the coating layer can be carried out continuously on-machine. In the case of an off-machine process in which papermaking and coating are carried out separately, the process that determines the production speed is the slowest line speed, which in the present invention is the papermaking speed. In offline coating, from the viewpoint of the stability of the coating solution, it is preferable that the time when no coating is carried out is as short as possible, and it is preferable that coating be carried out at approximately the same speed as the papermaking speed. Therefore, R ＫＵ A coating solution having a content of 10% by mass or more is also suitable for offline coating. ＫＵ is preferably 14% by mass or more, more preferably 18% by mass or more. ＫＵ Although there is no particular upper limit, since kaolin is a relatively expensive pigment, it is preferably 80% by mass or less, more preferably 60% by mass or less, and even more preferably 40% by mass or less.

[0022] The top coating layer has a kaolin content relative to the total pigment (R ＫＴ ) is the content of kaolin relative to the total pigment in the undercoating layer, R ＫＵ That's all. ＫＴ is R ＫＵ By satisfying the above, the top coating layer can be applied in a thick film, and a coating surface with excellent smoothness can be formed. ＫＵ R is equal to or greater than ＫＴ is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and even more preferably 30% by mass or more. ＫＵ The upper limit of R is not particularly limited, but is preferably 85% by mass or less, more preferably 70% by mass or less, and even more preferably 55% by mass or less. ＫＴ and R ＫＵ The difference is preferably 1% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more.

[0023] In the present invention, the kaolin preferably has a volume 50% average particle size (D50, hereinafter also referred to as average particle size) of 0.2 μm or more and 10 μm or less, more preferably 0.5 to 6 μm, and even more preferably 1 to 3 μm, as measured by a laser diffraction / scattering method, from the viewpoint of hiding power. Examples of measuring devices for the laser diffraction / scattering method include the particle size distribution measuring device "Partica" manufactured by Horiba, Ltd., and the particle size distribution measuring device "MASTER SIZER S" manufactured by Malvern Instruments. Furthermore, from the viewpoint of thick-film coating properties, the aspect ratio of the kaolin is preferably 1.2 to 100, more preferably 2 to 80, and even more preferably 5 to 50. The aspect ratio was calculated using the measured value of the "MASTER SIZER S" according to the calculation method described in "Basic Research on the Relationship between Coating Pigment Properties and Coated Paper Quality," Journal of the Japan Paper and Pulp Technical Association, Vol. 65, No. 12.

[0024] When the top coating layer contains a pigment other than kaolin, the average particle size (D50) of the pigment other than kaolin is preferably 0.2 μm or more and 3.0 μm or less, more preferably 0.2 μm or more and 2.0 μm or less. As such a pigment, calcium carbonate, which has excellent whiteness and fluidity as described above, is preferably used, and heavy calcium carbonate is more preferably used. Furthermore, the pigment other than kaolin contained in the top coating layer is preferably a pigment whose ratio (D75 / D25) of the particle size at 75 cumulative mass% to the particle size at 25 cumulative mass% in the particle size distribution curve measured by the sedimentation method is 3.0 or more and 5.0 or less. D75 / D25 is a value indicating the sharpness of the particle size distribution, with a smaller value indicating a sharper distribution. A sharper particle size distribution tends to result in higher smoothness, gloss, etc.

[0025] Adhesives: The adhesive (binder) used in the pigment coating layer can be any adhesive known in the papermaking field, without any particular limitations. For example, synthetic adhesives such as various copolymers such as styrene-butadiene, styrene-acrylic, ethylene-vinyl acetate, butadiene-methyl methacrylate, and vinyl acetate-butyl acrylate, polyvinyl alcohol, maleic anhydride copolymers, and acrylic acid-methyl methacrylate copolymers can be used alone or in combination with other adhesives. Proteins such as casein, soy protein, and synthetic protein can be used; starches such as oxidized starch, cationic starch, urea phosphate esterified starch, and etherified starch such as hydroxyethyl etherified starch, and dextrin can be used; and cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose can be used alone or in combination. In the present invention, the undercoating layer preferably uses a combination of a synthetic adhesive and a starch from the viewpoint of workability. The top coating layer can also use a combination of a synthetic adhesive and a starch, but when used in combination, it is preferable to use a synthetic adhesive in a larger amount than the starch from the viewpoint of smoothness and gloss.

[0026] The undercoating layer preferably contains 8 to 35 parts by mass of adhesive per 100 parts by mass of pigment. When coating is performed by blade coating, the adhesive preferably contains 8 to 20 parts by mass of adhesive per 100 parts by mass of pigment. Furthermore, among synthetic adhesives, styrene-butadiene copolymers or styrene-acrylic copolymers are preferred, with styrene-acrylic copolymers being more preferred. In the undercoating layer, it is preferred to use a synthetic adhesive and a starch-based adhesive in combination, with the ratio of synthetic adhesive:starch-based adhesive being preferably 3:7 to 7:3, and more preferably 4:6 to 6:4.

[0027] The top coating layer preferably contains 8 to 20 parts by mass of adhesive per 100 parts by mass of pigment. Furthermore, among synthetic adhesives, styrene-butadiene copolymers or styrene-acrylic copolymers are preferred, with styrene-acrylic copolymers being more preferred. In the top coating layer, synthetic adhesives and starches can be used in combination, but from the viewpoint of smoothness and gloss, it is preferable for the synthetic adhesive to be contained in greater amounts than the starches. Since this makes it easier to obtain a smoother surface, the top coating layer preferably contains a higher kaolin content relative to the total pigment than the under-coating layer.

[0028] The coating method for the pigment coating layer is not particularly limited, and coating can be performed using known coating equipment and coating systems. Examples of coating equipment include blade coaters, bar coaters, roll coaters, air knife coaters, reverse roll coaters, curtain coaters, gravure coaters, spray coaters, size press coaters, and gate roll coaters. Of these, a roll coater or blade coater is preferred for the undercoating layer, and a blade coater is preferred for the top coating layer. A water-based coating system using water as the main medium is preferred. In the present invention, either an on-machine coater or an off-machine coater, in which the papermaking machine and the coating equipment are integrated, may be used, but coating using an on-machine coater is more preferred. When coating is performed using an off-machine coater, which is a separate machine from the coating device, the base paper is made on the paper machine, then wound up onto a reel, and then the reel is put into the off-machine coater for coating.However, when coating is performed using an on-machine coater, the steps of winding up the base paper and putting it into the coater can be omitted, resulting in better production efficiency.

[0029] According to the present invention, it is possible to obtain pigment coated paper with a smooth surface. This is because the unevenness of the base paper is filled in the pigment coating layer. In the present invention, since the unevenness of the base paper is filled in, it is possible to avoid carrying out a smoothing treatment such as a calendar treatment on the base paper before forming the pigment coating layer, and the PPS smoothness (PPS Ｂ) can be, for example, 5.5 μm or more, 6.0 μm or more, 6.5 μm or more, 7.0 μm or more, etc. Smoothing treatment of the base paper is optional, and if a higher degree of smoothness is required, smoothing treatment can be performed.

[0030] The pigment coated paper of the present invention has a coating weight (CW) of the undercoating layer. Ｕ ) and the coating weight of the top coating layer (CW Ｔ ) and CW Ｕ >CW Ｔ In the pigment coated paper of the present invention, the undercoating layer fills in the unevenness of the base paper, and the top coating layer is formed on top of this undercoating layer, so that a smooth surface can be formed even with a small coating weight of the top coating layer. Ｕ -CW Ｔ ) is 1 g / m ２ More than 1.5 g / m is preferable. ２ More preferably, 2 g / m or more ２ If the coating weight of the under-coating layer is too large compared to the coating weight of the top coating layer, the drying load of the under-coating layer increases, which may result in a decrease in runnability, and if the coating weight of the top coating layer is too small, the surface feel of the coated paper may deteriorate. Ｕ -CW Ｔ ) is 10 g / m ２ Preferably, 9 g / m or less ２ More preferably, 8 g / m or less ２ The following is even more preferred:

[0031] Smoothness of undercoating layer (PPS Ｕ ) is preferably 5.5 μm or less. Ｕ When the thickness is 5.5 μm or less, it becomes easy to form a smooth top coating layer thereon. Ｕ The thickness is more preferably 5.2 μm or less, even more preferably 4.9 μm or less, and even more preferably 4.6 μm or less. The pigment coated paper of the present invention has a smooth surface due to the top coating layer provided on the under coating layer, so the under coating layer does not need to have a high degree of smoothness. ＵThe coating weight of the undercoating layer (CW) is about 4.0 μm or more. Ｕ ) is 6 g / m2 in solid content per side to fill in the unevenness of the base paper. ２ More than 7 g / m is preferable. ２ More preferably, 8 g / m or more ２ More preferably, 9 g / m ２ More preferably, 10 g / m ２ More preferably, 11 g / m ２ The above is even more preferable. Ｕ There is no particular upper limit to the CW Ｕ is 20 g / m ２ The following are preferred: CW Ｕ is 20 g / m ２ Even if the thickness exceeds CW, the smoothness of the undercoating layer saturates and there is almost no further change, and the costs for raw materials, drying, etc. increase. Ｕ is 18 g / m ２ More preferably, 16 g / m or less ２ The following is even more preferred:

[0032] Smoothness of top coating layer (PPS Ｔ ) is preferably 3.0 μm or less from the viewpoint of printability. Ｔ The coating weight (CW) of the top coating layer is more preferably 2.7 μm or less, even more preferably 2.4 μm or less, even more preferably 2.1 μm or less, and even more preferably 1.8 μm or less. Ｔ ) is 5 g / m2 in solid content per side in order to obtain a smooth surface. ２ More than 6 g / m is preferable. ２ More preferably, 7 g / m or more ２ More preferably, 8 g / m ２ More preferably, 9 g / m ２ More preferably, 10 g / m ２ The above is even more preferable. In addition, from the viewpoint of saturation of the smoothness of the top coating layer, deterioration of drying efficiency, and high cost, it is preferable to use a CW Ｔ is 18 g / m ２ Preferably, 16 g / m or less ２ The following is more preferred:

[0033] PPS smoothness of base paper (PPS Ｂ ) and the PPS smoothness of the top coating layer (PPS Ｔ ) and the smoothness improvement rate S calculated from the following formula 1 Ｂ－Ｔ is preferably 50% or more. (Formula 1) S Ｂ－Ｔ = (PPS Ｂ -PPS Ｔ ) / PPS Ｂ ×100(%) Smoothness improvement rate S Ｂ－Ｔ If the smoothness improvement rate S is 50% or more, it can be determined that a very smooth surface has been obtained compared to the state of the base paper. Ｂ－Ｔ is preferably higher, more preferably 60% or more, even more preferably 65% ​​or more, and even more preferably 70% or more. Ｂ－Ｔ is preferably 90% or less, and may be 85% or less, or 80% or less.

[0034] PPS smoothness of base paper (PPS Ｂ ) and the PPS smoothness of the undercoating layer (PPS Ｕ ) and the smoothness improvement rate S calculated from the following formula 2 Ｂ－Ｕ is preferably 15% or more. Ｂ－Ｕ = (PPS Ｂ -PPS Ｕ ) / PPS Ｂ ×100(%) Smoothness improvement rate S Ｂ－Ｕ When the smoothness improvement rate S is 15% or more, the unevenness of the base paper is filled to a certain extent, and it becomes easy to obtain a smooth top coating layer. Ｂ－Ｕ In the pigment coated paper of the present invention, the undercoating layer does not need to completely fill in the unevenness of the base paper, and the smoothness improvement rate S Ｂ－Ｕ It is sufficient if the ratio is 50% or less, and it may be 45% or less, or 40% or less.

[0035] The method for drying the pigment coating layer is not particularly limited, and for example, a conventional method such as a steam heater, gas heater, infrared heater, electric heater, hot air heater, microwave, or cylinder dryer can be used.

[0036] Paper provided with a pigment coating layer can be subjected to calendering, such as supercalendering or high-temperature soft nip calendering, as needed. Calendering can further improve the smoothness and gloss of pigment-coated paper. In the present invention, soft nip calendering is preferred. Soft nip calendering improves both whiteness and opacity. Whiteness unevenness is a difference in whiteness intensity in a small area caused by unevenness in the coating layer due to the unevenness of the base paper. In particular, calendering can cause unevenness in the area that comes into contact with the calender, resulting in uneven density in the pigment coating layer, which can result in whiteness unevenness. Because the pigment-coated paper of the present invention has an excellent smoothness of the top coating layer, even after calendering, whiteness unevenness can be suppressed to a level comparable to that of an untreated paper.

[0037] "Pigment Coated Paper" The pigment coated paper of the present invention preferably has a GM value of 6.0 or more. The GM value is the bending stiffness B ＭＤ and bending stiffness B in the CD direction ＣＤ and the geometric mean ((B ＭＤ ×B ＣＤ )^(1 / 2)), and the larger the GM value, the better the stiffness. The bending stiffness is calculated from the bending (15°) resistance measured in accordance with JIS P8125-1 using the following formula (3). (Formula 3) B ＭＤ (B ＣＤ ) = bending (15°) resistance × 3.352 × 0.025 The GM value is more preferably 6.5 or more, even more preferably 7.0 or more, even more preferably 7.5 or more, and even more preferably 8.0 or more. There is no particular upper limit for the GM value, but for paper of the basis weight of the present invention, it is about 10.

[0038] The pigment coated paper of the present invention has a bending stiffness B in the machine direction measured in accordance with JIS P8125-1. ＭＤ is 8.0 mN m ２ / m or more, and bending stiffness B in the CD direction ＣＤ is 4.0 mN·m ２ / m or more is preferable from the viewpoint of stiffness. ＭＤ is 9.0 mN·m ２ / m or more is more preferable, and 10.0 mN m ２ / m or more is more preferable, and 11.0 mN m ２ / m or more is even more preferable. Bending stiffness in CD direction B ＣＤ is 4.5 mN·m ２ / m or more is more preferable, and 5.0 mN m ２ / m or more is more preferable, and 5.5 mN m ２ / m or more is even more preferable. ＭＤ and bending stiffness B in the CD direction ＣＤ There is no particular upper limit to the value of B. ＭＤ is 15 mN·m ２ / m, B ＣＤ is 7.5 mN·m ２ / m.

[0039] The present invention will be specifically illustrated by the following examples, but the present invention is not limited to the descriptions in the examples. Furthermore, parts by mass in the examples refer to parts by mass on an bone-dry basis unless otherwise specified.

[0040] "Example 1" Base paper containing 100% chemical pulp and weighing 190 g / m ２ Base paper with a Stockigt sizing degree of 380 seconds was made at a papermaking speed of 240 m / min. 73.5 parts by mass of heavy calcium carbonate B (75% of particles with a particle size of 2 μm or less by precipitation method, D50 = 1.61 μm) and 26.5 parts by mass of first-class kaolin (average particle size 2 μm, aspect ratio 10) were mixed as pigments, and 18 parts by mass of acrylate-styrene copolymer latex was blended as an adhesive. Water was then added to obtain under-coating liquid A with a solids concentration of 71.1% by mass. Under-coating liquid A was coated onto the base paper using a blade coater at a coating speed of 400 m / min per side. The dry coating amount of under-coating liquid A per side was 11.2 g / m ２ It was.

[0041] Comparative Example 1 Coating was carried out in the same manner as in Example 1, except that the pigment in the undercoating liquid A was 100 parts by mass of heavy calcium carbonate B, and the solid content concentration was 68.6% by mass. The dry coating amount of the undercoating liquid per side was 5.8 g / m２ It was.

[0042] It was confirmed that adding kaolin to the coating solution makes it possible to coat a thick film even at a slow coating speed.

[0043] "Example 2" Base paper containing 100% chemical pulp and weighing 190 g / m ２ Base paper with a Stockigt sizing degree of 380 seconds was made at a papermaking speed of 245 m / min. 20 parts by weight of heavy calcium carbonate A (90% particles with a particle diameter of 2 μm or less by sedimentation method, D50 = 1.23 μm) as a pigment, 60 parts by weight of heavy calcium carbonate B, and 20 parts by weight of first-grade kaolin were mixed together, and 9 parts by weight of acrylate ester-styrene copolymer latex and 9 parts by weight of starch were blended as adhesives. Water was then added to obtain an undercoating liquid with a solids concentration of 64.9% by weight. 40 parts by weight of heavy calcium carbonate A and 20 parts by weight of heavy calcium carbonate B as pigments, and 40 parts by weight of first-grade kaolin were mixed together, and 15 parts by weight of acrylate ester-styrene copolymer latex and 0.3 parts by weight of a synthetic water retention agent were blended as adhesives. Water was then added to obtain a topcoating liquid with a solids concentration of 65.8% by weight.

[0044] The undercoating liquid was applied to the base paper using a blade coater to a dry coating amount of 14.2 g / m per side. ２ The coated layer was then dried at a coating speed of 300 m / min. A top coating solution was then applied thereon using a blade coater until the dry coating amount per side was 11.9 g / m. ２ The coating was carried out at a coating speed of 300 m / min so as to obtain coated paper.

[0045] "Example 3" Base paper containing 100% chemical pulp and weighing 199 g / m ２ The base paper having a Stockigt sizing degree of 440 seconds was made at a papermaking speed of 240 m / min. The dry coating amount of the undercoating liquid per side was 14.3 g / m ２ , the dry coating amount of the top coating liquid per side is 8.8 g / m ２ A coated paper was obtained in the same manner as in Example 2, except that the dry coating amount of the under-coating liquid per side was 7.5 g / m. ２, the dry coating amount of the top coating liquid per side is 14.7 g / m ２ Coated paper was obtained in the same manner as in Example 3, except that the base paper was smoothed (high-temperature soft nip calender, 140 m / min, 50°C, 40 kN / m), and the base paper (Stöckigt sizing degree 440 seconds) was coated with an under-coating liquid in a dry coating amount of 13.6 g / m2 on one side. ２ , the dry coating amount of the top coating liquid per side is 10.3 g / m ２ A coated paper was obtained in the same manner as in Example 3, except that the above conditions were met.

[0046] "Example 5" Base paper containing 100% chemical pulp and weighing 184.3 g / m ２ The base paper having a Stockigt sizing degree of 380 seconds was made at a papermaking speed of 270 m / min. The dry coating amount of the undercoating liquid per side was 12.5 g / m ２ , the dry coating amount of the top coating liquid per side was 11.3 g / m ２ A coated paper was obtained in the same manner as in Example 2, except that the top coating layer was smoothed (high-temperature soft nip calender, 250 m / min, 30°C, 100 kN / m).

[0047] "Example 7" Dry coating amount of undercoating liquid per side: 14.8 g / m ２ , the dry coating amount of the top coating liquid per side is 9.2 g / m ２ A coated paper was obtained in the same manner as in Example 5. Example 8 A coated paper was obtained in the same manner as in Example 7, except that the top coating layer was smoothed (high-temperature soft nip calender, 250 m / min, 30°C, 100 kN / m).

[0048] "Comparative Example 3" Dry coating amount of undercoating liquid per side: 10.1 g / m ２ , the dry coating amount of the top coating liquid per side was 14.1 g / m ２A coated paper was obtained in the same manner as in Example 5. Comparative Example 4 A coated paper was obtained in the same manner as in Comparative Example 3, except that the top coating layer was smoothed (high-temperature soft nip calender, 250 m / min, 30°C, 100 kN / m).

[0049] Example 9 30 parts by mass of heavy calcium carbonate A, 40 parts by mass of heavy calcium carbonate B, and 30 parts by mass of first-grade kaolin were mixed as pigments, and then 9 parts by mass of acrylate ester-styrene copolymer latex and 9 parts by mass of starch were added as adhesives, and water was further added to obtain under-coating liquid B with a solids concentration of 63.0% by mass. 50 parts by mass of heavy calcium carbonate A and 50 parts by mass of first-grade kaolin were mixed as pigments, and then 15 parts by mass of acrylate ester-styrene copolymer latex and 0.01 parts by mass of a synthetic water retention agent were added as adhesives, and then water was further added to obtain top-coating liquid B with a solids concentration of 62.1% by mass. The dry coating amount of under-coating liquid B on one side of the base paper was 14.2 g / m. ２ , the dry coating amount of top coating liquid B per side was 11.4 g / m ２ A coated paper was obtained in the same manner as in Example 2, except that the above conditions were met.

[0050] Example 10 30 parts by mass of heavy calcium carbonate A, 50 parts by mass of heavy calcium carbonate B, and 20 parts by mass of first-grade kaolin were mixed as pigments, and then 9 parts by mass of acrylate-styrene copolymer latex and 9 parts by mass of starch were added as adhesives, and water was further added to obtain an under-coating liquid C with a solids concentration of 63.0% by mass. 60 parts by mass of heavy calcium carbonate A and 40 parts by mass of first-grade kaolin were mixed as pigments, and then 15 parts by mass of acrylate-styrene copolymer latex and 0.01 parts by mass of a synthetic water retention agent were added as adhesives, and then water was further added to obtain a top-coating liquid C with a solids concentration of 62.1% by mass. The dry coating amount of under-coating liquid C per side of the base paper was 14.1 g / m. ２ , the dry coating amount of top coating liquid C per side was 10.2 g / m ２ A coated paper was obtained in the same manner as in Example 2, except that the above conditions were met.

[0051] The following measurements were carried out. The results are shown in Tables 1 to 3. Basis weight: Measured in accordance with JIS P8124. Paper thickness: Measured in accordance with JIS P8118. Density: Calculated from basis weight and paper thickness.

[0052] PPS smoothness, smoothness improvement rate: In accordance with JIS P8151 "Paper and paperboard - Surface roughness and smoothness test method (air leak method) - Printsurf tester method," smoothness was measured using a soft backing at a clamp pressure of 1 MPa. The smoothness improvement rate was calculated from the smoothness value. ISO bending stiffness, GM value: In accordance with JIS P8125-1, bending stiffness B in the MD direction was measured. ＭＤ , bending stiffness in CD direction B ＣＤ Also, B ＭＤ and B ＣＤ From the value of , the GM value (B ＭＤ and B ＣＤ The geometric mean of the

[0053]

[0054]

[0055]

[0056] The pigment-coated paper obtained in the Examples of the present invention had a high base paper basis weight and excellent stiffness, and a thick coating layer with excellent smoothness. Therefore, the pigment-coated paper of the present invention is suitable for product packaging, such as outer boxes. The results of Example 3 and Comparative Example 2, as well as Examples 5-8 and Comparative Examples 3 and 4, confirmed that a smooth surface can be obtained by increasing the undercoat weight relative to the topcoat weight, even when using the same base paper and the same coating material. The results of Examples 3 and 4 confirmed that the present invention can produce pigment-coated paper with equivalent smoothness, regardless of whether the base paper was smoothed or not. The results of Examples 5 and 6, and 7 and 8 confirmed that smoothing treatment of the top coating layer further improved smoothness. It was also confirmed that a smooth surface can be obtained by increasing the undercoat weight relative to the topcoat weight, even when using the same base paper and the same coating material.

Claims

1. At least one side of the base paper has two pigment coating layers, an undercoating layer and a top coating layer, and the basis weight of the base paper is 150 g / m ２ More than 400g / m ２ The content of kaolin relative to the total pigment in the undercoating layer (R ＫＵ ) is 10% by mass or more, and the content of kaolin relative to the total pigment in the top coating layer (R ＫＴ ) is R ＫＵ or more, and the coating weight (CW Ｕ ) and the coating weight of the top coating layer (CW Ｔ ) and CW Ｕ >C.W. Ｔ A pigment coated paper characterized by satisfying the above.

2. PPS smoothness of top coating layer (PPS Ｔ 2. The pigment coated paper according to claim 1, characterized in that the particle size is 3.0 μm or less.

3. PPS smoothness of the base paper (PPS Ｂ 3. The pigment coated paper according to claim 1, wherein the particle size is 5.5 μm or more.

4. Difference in coating weight between undercoat layer and top coat layer (CW Ｕ -C.W. Ｔ ) is 1 g / m ２ 3. The pigment coated paper according to claim 1, wherein said pigment coated paper is 5. PPS smoothness of the base paper (PPS Ｂ ) and the PPS smoothness of the top coating layer (PPS Ｔ ) and the smoothness improvement rate S calculated from the following formula 1 Ｂ－Ｔ The pigment coated paper according to claim 1 or 2, characterized in that S is 50% or more. Ｂ－Ｔ = (PPS Ｂ - PPS Ｔ ) / PPS Ｂ ×100(%) 6. PPS smoothness of the base paper (PPS Ｂ ) and the PPS smoothness of the undercoating layer (PPS Ｕ ) and the smoothness improvement rate S calculated from the following formula 2 Ｂ－Ｕ The pigment coated paper according to claim 1 or 2, characterized in that S is 15% or more. Ｂ－Ｕ = (PPS Ｂ - PPS Ｕ ) / PPS Ｂ ×100(%) 7. Density is 0.90 g / cm ３ 3. The pigment coated paper according to claim 1, wherein:

8. Pigment coated paper according to claim 1 or 2, characterized in that the GM value is 6.0 or more.

9. Basis weight 150g / m ２ More than 400g / m ２ A step of making the following base paper: ＫＵ forming an under-coating layer having a kaolin content (R ＫＴ ) is R ＫＵ and forming a top coating layer having a coating weight CW of 55% by mass or less. Ｕ and the coating weight CW of the top coating layer Ｔ But, CW Ｕ >C.W. Ｔ and a papermaking speed is 100 m / min or more and 450 m / min or less.

Images