JPH0241494A - Production of cast coated paper - Google Patents

Abstract

PURPOSE:To obtain the title coated paper of low gloss giving refined image with repose having a delustered back surface layer by coating the back surface of a base paper with a pigment of specified granular size and by making a hot calendering with a smoothing elastic roll with its surface roughness specified. CONSTITUTION:The back surface of a base paper is coated with a composition comprising (A) 100 pts.wt. of a pigment containing >=50wt.% of a component 2-50mum in mean granular size and (B) pref. 5-50 pts.wt. of an adhesive (e.g., casein) followed by coating the normal surface with (C) a coating composition comprising a conventional inorganic pigment and an adhesive. The resulting product is then put to cast finish followed by making a calendering with both of a metallic roll and an elastic roll with its surface roughness satisfying the formula heated to >=60 deg.C to carry out smoothing treatment, thus obtaining the objective coated paper.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for producing cast-coated paper, particularly so-called matte-matte paper in which the pigment coating layer on the back surface has low gloss and exhibits low ink gloss. The present invention relates to a method for efficiently producing cast coated paper having a matte back coating layer of the type.

``Prior Art'' Cast coated paper has particularly excellent gloss and smoothness among coated papers for printing, and is widely used as covers for magazines, catalogs, pamphlets, annual reports, etc.

Conventional methods for manufacturing cast-coated paper include wet casting (USP 1719166), rewetting casting (ISP 2759847), and gelling casting (USP 2919205), all of which use minerals. A method that is common in principle is adopted in that the coating layer, which is mainly composed of pigments and adhesives, is pressed against a heated finished surface and dried while it is in a plastic state and contains water. Most of the cast-coated paper produced according to the manufacturing method is so-called single-sided cast paper, which has a highly glossy cast finish on one side and a high-quality paper-like surface on the other side.

On the other hand, matte coated paper gives an elegant and calm image compared to high-gloss coated paper, so in recent years it has become an important printing paper for everything from single-color prints such as text paper to various multi-color prints. It is commonly used as one of the grades.

As mentioned above, the basic cast finishing method itself will not be changed, but a pigment coating layer with good ink receptivity, retention and plate reproducibility will be provided on the back side, and this will be calendered as necessary. A method has been proposed in which a pigment coating layer provided on the surface of the base paper is dried and finished by pressing it against a heated surface while it is in a plastic state due to water content. However, although this method improves the printability of the back side, it is considerably inferior in smoothness compared to general coated printing paper because it is not calendered. It has also been proposed to perform heat calendering treatment after finishing the cast, but this causes scratches on the surface of the cast coated paper, significantly impairing the appearance of the cast surface, and as the main pigment of the pigment coating layer on the back side. Calcium carbonate with an average particle size of about 0.4 to 2 μm is usually used, but the microporosity (micropores) on the surface of the coating layer is reduced by heat calendering, so smoothness improves but the ink shines. comes out, and as a result, it turns into a Mazda gloss type. Particularly in solid areas of offset multicolor printing, problems arise in that the ink gloss becomes mottled, resulting in very ugly printed matter.

``Problems to be Solved by the Invention'' In light of the above-mentioned circumstances, the inventors of the present invention have conducted intensive research into methods for improving the above-mentioned drawbacks that occur when heat calendering is applied after casting finish. By using a pigment with a particle size different from that of the conventional pigment coating layer and by specifying the surface roughness of the elastic roll of the heating calender, we have found a method that can solve the conventional difficulties and finally completed the present invention.

"Means for Solving the Problems" The present invention involves forming a pigment coating layer containing pigments and an adhesive as main components on the back side of base paper, and then coating the surface of the base paper with a pigment coating layer containing pigments and an adhesive as main components. Manufacture of cast-coated paper by forming a layer, casting-finishing the coating layer on the surface, and passing the paper through a heated calender consisting of a metal roll and an elastic roll heated to 60°C or higher to give a strong gloss finish. In the method, the pigment coating layer on the back side contains a pigment having an average particle diameter of 2 to 15 μm at 50% by weight or more of the total pigment, and the surface roughness Rmax CJTS B0651 satisfies the following formula.
This is a method for producing cast coated paper whose back surface is a matte coated layer surface, which is characterized by carrying out a smoothing treatment using a heated calender consisting of an elastic roll having an elastic roll (as defined above).

(Note) Rmax≦-0,1D-0,01T+35 (Note) Rmax i Surface roughness of elastic roll (μm) D; Shore D hardness of elastic roll T; Surface temperature of metal roll during operation (°C ) "Operation" The feature of the present invention is that when trying to obtain a matte coating layer with a matte-matte type on the back side, the relationship between the smoothing of the coating layer and the ink gloss, that is, the ink gloss appears when the coating layer is smoothed. In order to reduce the gloss of the ink, the smoothness must be reduced.To solve this problem, as a pigment for coating compositions,
The average particle size used in conventional matte coated paper is 0.
Even coarser than 4-2.0 μm, with an average particle size of 2.0-2.0 μm
It is essential that coarse pigments of 15 μm, more preferably 3.0 to 10 μm, account for 50% by weight or more of the total pigment. Incidentally, if it is less than 2.0 μm, the ink gloss becomes too high, and if it exceeds 15 μm, the micro-smoothness of the surface of the coating layer decreases, resulting in poor printability.

In addition, coarse pigments of 2.0 to 15 μm account for 50% by weight of the total pigments.
Even if it is less than that, the ink gloss becomes too high and the desired effect of the present invention cannot be expected. In particular, in the method of the present invention, it is preferable to use coarse-grained heavy calcium carbonate as the main pigment, which has an amorphous particle shape and is not oriented in parallel in principle even during the coating process or heating calender treatment. .

Although various types of pigments are used in the present invention, it is difficult to directly measure the average particle diameter of mixed pigments because the specific gravity differs depending on the type of pigment. particle size distribution analyzer (SA-CF
2), after measuring the individual average particle diameter of each pigment used in advance, depending on the formulation of each pigment,
The average particle size of all pigments was calculated by calculation.

Pigments used in the present invention include common pigments for coated paper such as tare, kaolin, aluminum hydroxide, calcium carbonate, titanium dioxide, barium sulfate, zinc oxide, satin white, calcium sulfate, talc, plastic pigments, etc. One or more of these may be appropriately selected and used.

Examples of adhesives include proteins such as casein, soy protein, and synthetic proteins; conjugated diene polymers such as styrene/butadiene copolymers, methyl methacrylate/butadiene copolymers, acrylic esters, and/or methacrylates; Acrylic polymer latices such as polymers or copolymers of acid esters, vinyl polymer latices such as ethylene/vinyl acetate copolymers, or various polymer latexes of these can be combined with monomers containing functional groups such as carboxyl groups. Alkali-soluble or alkali-insoluble polymer latex modified with; synthetic resin adhesives such as polyvinyl alcohol, olefin/maleic anhydride resin, and melamine resin; starches such as positive starch and oxidized starch; carboxymethyl cellulose, hydroxyl One or more conventional adhesives for coated paper, such as cellulose derivatives such as ethyl cellulose, are appropriately selected and used.

In addition, these adhesives are used in an amount of 5 to 100 parts by weight of the pigment.
It is used in an amount of about 50 M parts, more preferably about 10 to 30 parts by weight. In addition, various auxiliary agents such as antifoaming agents, colorants, mold release agents, flow modifiers, etc. are appropriately blended into the coating liquid as necessary. Amine, amide, polyacrylamine, etc., or a polyvalent metal salt such as zinc, aluminum, magnesium, calcium, barium, etc. may be added in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the pigment.

The coating composition thus prepared can be used in general coated paper production, such as blade coaters, air knife coaters, roll coaters, reverse roll coaters, bar coaters, curtain coaters, die slot coaters, etc.
The base paper is coated in one layer or in multiple layers using an on-machine or off-machine coater equipped with a coating device such as a gravure coater, a Champlex coater, or a size press coater. The solid content concentration of the coating liquid at that time is generally about 40 to 75% by weight, but considering operability, it is about 4% by weight.
A range of 5 to 70% by weight is preferred. The base paper is 30 to 400 grams per square meter, which is used for general coated printing paper.
Paper-based or board-based base paper is used, but the method of making the base paper is not particularly limited, and may be either acid paper-making or alkaline paper-making. Base paper can also be used.Also,
Base paper pre-coated with a size press, bill blade, etc. can also be used.

The amount of coating liquid applied to the base paper is generally about 10 to 50 g/rd per side in terms of dry weight, but when considering the white paper quality and printability of the resulting coated paper, it is 15 to 30 g/rd.
It is desirable to adjust within a range of approximately lri. In addition, various methods for drying the wet coating layer include conventionally known methods such as steam heating, hot air heating, gas heater heating, electric heater heating, infrared heater heating, high frequency heating, laser heating, and electron beam heating. will be adopted as appropriate.

The opposite side (front side) of the base paper coated on the back side as described above contains various inorganic pigments and plastic pigments such as clay, kaolin, calcium carbonate, aluminum hydroxide, and titanium oxide, as well as casein, starch, synthetic resin latex, etc. A coating composition mainly composed of natural and synthetic adhesives is applied, and while the coating layer contains water and is in a plastic state, it is pressed against a heated finished surface and dried, so that the moisture in the coating layer is removed. After substantial evaporation, the finished surface is released from the mold to yield a cast coated paper. Various conventionally known methods can be applied to the cast finishing method itself. For example, a paint mainly composed of the inorganic pigment is applied to the surface of a base paper that has a coating layer on the back side, and the paint flows. There is a method in which the coated layer is immediately pressed onto a heated finished surface to dry it while it is still wet (wet casting method), or after applying a paint mainly composed of inorganic pigments, the coated layer is passed through a coagulation bath while still wet. The coating layer is gelled and immediately dried by pressing it against the heated finished surface (gelling casting method), or after coating a coating mainly containing inorganic pigments, it is dried first and then coated again with a wetting liquid. The main method is the so-called cast finishing method, in which the layer is moistened, softened, pressed against a heated finished surface and dried (rewet casting method), in other words, the layer is moistened and in a plastic state and then pressed against a heated finished surface and dried. It is applicable to the invention.

In the manufacturing method of the present invention, the cast coated paper thus obtained is passed through a calender heated to 60°C or higher to give a surface finish.
For example, Super Calendar Gross Calender (Japanese Patent Application Laid-Open No. 40-132305, Publication Patent Publication No. 63-5001)
88), Soft Compact Calender (Gi Pulp Technology Times, August 1962 issue).

pp. 31-36, PPI, January 1987 issue, 45
~47 pages, WFP, 1985, 22.873-87
Various types of calendars, such as (page 7), are used in on-machine and off-machine formats.

Note that the surface of the metal roll may be mirror-finished by hard chrome plating or the like.

In the method of the present invention, an elastic roll having a surface roughness that satisfies the following formula is selectively used as the elastic roll of the various calenders as described above.

Rmax≦-0,1D-0,01T+35 (Note) Rmax; Surface roughness of elastic roll (μm) D; Shore D hardness of elastic roll) T; Surface temperature of metal roll during operation ('C) Above relationship As is clear from the equation, the harder the Shore D height of the roll used in the method of the present invention is, the higher the surface temperature of the metal roll, the smaller the surface roughness Rmax. There is a need to.

Conventionally, no particular attention has been paid to the surface roughness of the elastic rolls constituting the calender, and as in the present invention, the surface roughness of the elastic rolls is determined based on the relationship between the Shore D hardness of the elastic rolls and the surface temperature of the metal rolls. There is no known technical idea that by regulating the surface roughness, the roughness of the resulting cast coated paper can be completely eliminated.

According to experiments conducted by the present inventors, surface roughness Rv
Even with an elastic roll with aax of 5 μm or less, the length of 200 m
Pressure treatment of cast coated paper under conditions of increasing the surface temperature of the metal roll to 60"C or higher and applying a linear pressure of 100 kg/cm or higher at a high speed of 100 kg/cm or higher will result in uneven formation and coating of the coated base paper. It has become clear that surface roughness rapidly decreases due to unevenness caused by unevenness.

In particular, thick paper has uneven texture and poor smoothness, so the surface roughness decreases significantly, and this tendency becomes even more pronounced when using elastic rolls with low hardness or elastic rolls made of materials with low resilience. It was found that this was promoted.

Therefore, in the method of the present invention, it is necessary to specify the surface roughness of the elastic roll in the state in which it is actually used.
If the surface roughness of the elastic roll deviates from the specifications of the present invention during operation, it is necessary to stop the calender and replace it with a roll that satisfies the conditions defined by the present invention.

As the material for the elastic roll, it is also possible to use materials that have traditionally been used in calenders for coated paper, such as cotton wool, fill-mat cotton, white cotton paper, woolen paper, and asbestos. Elastic rolls, which are the main material, have poor high-temperature durability and the surface roughness decreases quickly, so they are made of one or more layers of resins such as urethane, polyamide, epoxy, isocyanate, silicone, vinylidene fluoride, and phenol. (Japanese Unexamined Patent Publication No. 62-282093) made of synthetic fibers such as nylon, tetron, aramid, etc.
An elastic roll or the like molded from a material containing 0% or more (hereinafter collectively referred to as a resin roll) is preferable, and a resin roll made of aramid, epoxy, polyamide, etc., which has strong heat resistance, is particularly preferably used.

Regarding the hardness of the elastic roll, it is desirable to use a roll with Shore D hardness of 75" or more, considering durability under high temperature and high pressure and roughening resistance under paper passing. Also, the higher the surface temperature of the elastic roll, the more difficult it will be. Since roughening of the roll surface progresses rapidly, it is preferable to cool the elastic roll with cooling liquid or cooling air from inside or outside the roll.

In addition, in the method of the present invention, the surface roughness R of the elastic roll
max is measured based on the method defined in JIS B0651, but since the width of a normal calender roll of cast coated paper is about 1500 to 3800 mm, the measured value varies considerably in the width direction of the roll.

Therefore, for example, 5urft manufactured by Mitoyo Seisakusho is used as a measuring instrument.
Using es t201, set the measurement length (L) to the maximum measurement length 8
mm, over the elastic roll hit at least 10
If the roll width is wide, measure more times), and use the average value of the obtained measurement values to determine the surface roughness Rm.
It is necessary to specify ax.

The processing conditions when surface-treating cast-coated paper using a heated calender depend on the type of cast-coated paper intended, base paper conditions, properties of the coating layer, coating amount, paper moisture, finishing speed, etc. Adjustments will be made as appropriate.

Calender roll pressure conditions are 100 to 500 linear pressure.
kg/cm is preferred, and the number of pressure nips per calender is usually from 2 to 6 nib per drum or 10-liter for soft compact calenders. Incidentally, in the case of a super calender, about 3 to 13 nips are common.

Therefore, in terms of surface finish, cast coated paper is 2 to 12
It comes into pressure contact with an elastic roll about the size of a book. The degree of reduction in the surface roughness of the elastic roll depends on the formation of the cast-coated paper passing through it, uneven coating, basis weight, paper moisture, material of the elastic roll,
It takes time to constantly measure and control the hardness, which varies depending on many conditions such as hardness, roll temperature, pressure, and speed. Since the second half elastic roll is used, the method of the present invention requires operation using a roll having a surface roughness Rmax that satisfies the relationship of the above general formula as at least one of the second half elastic rolls. Of course, it is most preferable that all elastic rolls satisfy the above general formula.

The heating calender finishing conditions of the present invention are 8 cycles so that the white paper gloss on the back side is 25% or less, more preferably 20% or less.

The moisture content of the cast-coated paper before entering the calender nip is preferably about 3 to 10%, and the finishing speed of the calender varies depending on the paper weight, paper type, etc. A range of about minutes is preferable.Also,
In order to control and humidify the cast coated paper after surface treatment, we have installed a water coating device using a roll, an electrostatic heating device, a steam humidifying device, etc., and various types of equipment that are conventionally known in the field of coated paper manufacturing. Of course, it is also possible to use appropriate combinations of techniques.

"Example" The present invention will be described in more detail with reference to Examples below, but the present invention is of course not limited to these.

In addition, "parts" and "%" in the examples indicate "parts by weight" and "% by weight", respectively, unless otherwise specified.

Examples 1 to 3 Heavy calcium carbonate 10 with a total pigment average particle size of 14 μm
A pigment slurry having a solid content concentration of 65% was prepared by dispersing with a Coles disperser using 0.2 parts of sodium polyacrylate as a dispersant. To this pigment slurry, 3 parts of phosphoric acid-modified starch (solid content) and 12 parts of styrene-butadiene copolymer latex (trade name: JSRO696, manufactured by Japan Synthetic Rubber Co., Ltd.) (solid content) were added, and water was added to adjust the solid content. A coating liquid with a concentration of 60% was prepared.

Apply 75g/n of this coating liquid to the back of the base paper with a dry coating amount of 20g/n.
Coat with a blade coater so that it is 120 g/%.
It was dried in a dryer at 0.degree. C. to obtain a single-sided coated paper with a moisture content of 6%.

Separately, an aqueous coating liquid for casting with a solid content concentration of 53% was obtained, which consisted of 80 parts of kaolin, 20 parts of calcium carbonate, 10 parts of casein (solid content), and 8 parts of styrene-butadiene latex (solid content). This coating liquid was applied to the surface of the base paper to be coated on the back side, and finished by casting according to the wet-on-wet method as disclosed in Japanese Patent Publication No. 38-25160. That is, the surface of the base paper to be coated is coated with a casting coating liquid using a first coating device so that the dry weight becomes 11 g/rrf, and then, while this coating layer is in a wet state, a second coating liquid is immediately applied. The dry weight of the same casting coating liquid is 9.
g/I7? The paper was coated so as to have a surface temperature of 90° C., and immediately pressed against a chrome-plated cast drum having a surface temperature of 90° C., and after drying, the mold was released to obtain a cast coated paper.

This cast coated paper was coated with a metal roll with a surface temperature of 75°C and a surface roughness Rmax of 5 μm [Example IL 16 μm [Example 2], 20 μm [Example 3] and a Shore hardness of 86
Two soft compact calenders with 4 nips on one side are made of polyurethane resin extensible rolls (trade name: ERAGLAS, manufactured by Kinryūsha Co., Ltd.). The surface of the second roll was in contact with the metal roll, and the speed was set at 200.
A cast coated paper was obtained by performing surface pressure finishing under conditions of a linear pressure of 150 kg/cm and a linear pressure of 150 kg/cm.

The obtained cast coated paper was evaluated according to the evaluation items, and the results are shown in Table 1.

Comparative Examples 1 to 4 Comparative Example 1 is Example 1 in which the calendering treatment is not performed after casting, and Comparative Examples 2 to 4 are the case in which the resin elastic roll Rmax used in Example 1 is 37 μm (Comparative Example 2) and 42 μm, respectively. [Comparative example 3L50μm [Comparative example 4
] Cast coated paper was produced in the same manner as in Example 1, except that the results were evaluated, and the evaluation results are shown in Table 1.

Example 4 Instead of heavy calcium carbonate, which was used in Example 1 and had an average particle diameter of 14 μm, all pigments had an average particle diameter of 8 μm.
Cast coated paper was produced in the same manner as in Example 1 except that 80 parts of heavy calcium carbonate and 20 parts of talc were used as pigments, and the evaluation results are shown in Table 1.

Comparative Example 5 Cast coated paper was produced in the same manner as in Example 4, except that 100 parts of heavy calcium carbonate with a total pigment average particle diameter of 1.5 μm was used as the pigment, and the evaluation results are presented. -1.

Examples 5 to 7 Heavy calcium carbonate 6 with a total pigment average particle diameter of 3 μm
0 parts and 40 parts of light calcium carbonate as pigments,
A coating liquid having a solid content concentration of 45% was prepared by using 3 parts of oxidized starch and 13 parts of styrene-butadiene latex as an adhesive. This coating liquid was applied to the back side of high-quality base paper with a weight of 80 g/rrr using an air knife coater until the dry weight was 20 g/rrr.
The coating was applied and dried to give a coating weight of g/rrr, and a back-coated base paper for casting was prepared.

Separately, 60 parts of kaolin, 40 parts of light calcium carbonate, and 0.5 parts of sodium polyacrylate were dispersed in water using a Coles disperser to prepare a pigment slurry with a solid content concentration of 60%. This is supplemented with 0 tributyl phosphate as an antifoaming agent.
．． 5 parts, ammonium stearate 1.0 as mold release agent
10 parts of a 15% aqueous casein solution (solid content) dissolved using ammonia as an adhesive and acrylic acid/butadiene/methyl methacrylate (ratio; 2/33/65
) Add 16 parts of copolymer latex (solid content) and further add Z
A coating liquid having a solid content concentration of 45% was prepared by adding 3,043 parts of N and water.

The obtained coating liquid was cast and finished according to the wet casting method described in Japanese Patent Application Laid-Open No. 59-216996.

This cast coated paper was bonded to a metal roll with a surface temperature of 105°C and a surface roughness Rmax of 7 μm [Example 5], 1) μm
[Example 6 L 19 μm [Example 7] Using a soft compact calender with 4 nips on one side made of elastic rolls made of epoxy resin with Shore D hardness of 90D, the calender was heated at a speed of 300 m7 minutes and a linear pressure of 200 kg/a.
The paper was passed twice under conditions of m. The cast-coated paper was passed so that the back side of the paper was in contact with the metal roll in the first pass, and the paper was passed through the paper so that the front face was in contact with the metal roll in the second pass, and the surface was finished under pressure to obtain a cast-coated paper. Table 1 shows the evaluation results for each coated paper.

Comparative Example 6 RIIIax of the elastic roll used in Example 5 was 42μ
Cast coated paper was produced in the same manner as in Example 5, except that Akebono was used, and the evaluation results are shown in Table 1.

Comparative Example 7 Cast coated paper was produced in the same manner as in Example 5 except that 45 parts of heavy calcium carbonate and 55 parts of kaolin with a total pigment average particle size of 1.2 μm were used in Example 5, and the evaluation results were are shown in Table-1.

Table-1 [Remarks]; Evaluation method in Table-1 *Cast surface roughness The cast coated paper surface was visually evaluated.

◎－Extremely good 〇-Good △－inferior × - Extremely poor *Cast coated paper back side glossy Measurement was performed at an angle of 75° according to the JIS P8142 method.

The higher the number, the better the gloss.

*Smoothness of the back side of cast coated paper Evaluation was made visually.

◎=Extremely Good〇-Good△=Poor
Four colors (black, magenta, cyan, yellow) were printed at a printing speed of 0.00 sheets/hour, and the ink gloss of the four-color overlapping area was measured at an angle of 60 @ using Gross Meter GM-3D (manufactured by Murakami Color Co., Ltd.). The higher the value, the better the gloss.

The average particle diameter of all pigments was calculated by measuring the average particle diameter of each pigment in advance and based on the blending ratio thereof. (Measuring instrument: Particle size distribution measuring device 5A-CF2 manufactured by Shimadzu Corporation), and the surface roughness Rmax of the elastic roll
The measurement was performed using 5urftest 201 manufactured by Sanyo Seisakusho with a measurement length (L) of 8+nm.

"Effects" As is clear from the results in Table 1, all of the cast coated papers obtained in the examples of the present invention had significantly improved surface roughness, and the matte coating layer on the back surface was also matte. It was a cast-coated paper with an extremely elegant and calm matte surface.

When these were used as covers, there was almost no difference in quality from the main text paper. In particular, by simultaneously using a double-sided matte coated paper finished with the same pigment coating layer as the back side pigment coating layer shown in the example of the present invention as the text paper, there is no sense of incongruity between the back side of the cast cover and the text paper. The result was a magazine with a classy feel to it.

Claims (3)

[Claims] (1) After forming a pigment coating layer mainly composed of pigment and adhesive on the back side of the base paper, a coating layer mainly composed of pigment and adhesive is formed on the surface of the base paper, and the coating layer on the surface is coated. In a method for producing cast-coated paper, in which the layer is cast-finished and the paper is passed through a heated calender consisting of a metal roll and an elastic roll heated to 60°C or higher to obtain a strong gloss finish, the pigment-coated layer on the back side of the paper is cast-finished. A surface roughness Rmax (defined in JISB0651) that contains pigments with an average particle diameter of 2 to 15 μm at 50% by weight or more of the total pigments and satisfies the following formula:
1. A method for producing cast coated paper having a matte coated layer on the back surface, which comprises smoothing the paper using a heated calender made of an elastic roll having the following properties. (Note) Rmax≦-0.1D-0.01T+35 (Note) Rmax: Surface roughness of elastic roll (μm) D: Shore D hardness of elastic roll (°) T: Surface temperature of metal roll during operation (°C) ) (2) The method for producing cast coated paper according to claim (1), wherein the pigment in the back pigment coating layer is heavy calcium carbonate. (3) The method for producing cast coated paper according to claim (1) or (2), wherein the white paper gloss of the back coating layer is 25% or less as measured in accordance with JISP8142 method.