JP2013108195A - Coated paper - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide coated paper that has high opacity and superior flexibility and printability even though being bulky.SOLUTION: The coated paper comprises base paper and coating layers stacked on both faces of the base paper. The coating layer contains pigment and an adhesive as a main component. The base paper contains particles of a silica-heavy calcium carbonate composite and a bulking agent. The coated paper has an interlaminar strength of 500 kPa or more and an internal bond strength of 102 J/mor less per unit area.

Description

  The present invention relates to coated paper.

  As a means for increasing the opacity of the coated paper, a method of internally adding various particles to the coated paper is generally used. As the above particles, kaolin, talc, titanium dioxide, hydrated silicic acid (white carbon), urea-formalin polymer fine particles, etc. are used, and attempts have been made to combine the particles in order to increase the functionality of each particle. . As a coated paper using the composite particles, a coated paper using a light calcium carbonate-silica composite has been proposed (see Japanese Patent Application Laid-Open No. 2007-270407). However, the coated paper cannot be said to be sufficiently satisfied with the recent demand for high opacity of coated paper, and a coated paper having higher opacity is demanded.

  In addition, the coated paper may be required to have excellent printability and excellent flexibility even when it is bulky as the weight is reduced. Usually, flexibility decreases as the paper thickness increases, and thus it is difficult to achieve both bulkiness and flexibility. Therefore, as a paper which is bulky and excellent in flexibility, a coated paper for printing in which the product of the basis weight, density, Young's modulus in the paper making direction, and tear length in the paper making direction is limited to a predetermined range has been proposed. (See JP 2002-138389 A). However, even if the above four products are limited to a predetermined range, for example, when one value is extremely large, either bulkiness or flexibility is lowered. Therefore, it is difficult to say that the coated paper for printing is a paper in which both are sufficiently compatible.

JP 2007-270407 A JP 2002-138389 A

  The present invention has been made based on the above circumstances, and an object of the present invention is to provide a coated paper that is excellent in flexibility and printability even when the opacity is high and bulky.

The invention made to solve the above problems is
Having a base paper and a coating layer laminated on both sides of the base paper,
The coated layer is a coated paper containing a pigment and an adhesive as main components,
The base paper contains silica composite heavy calcium carbonate particles and a bulking agent,
The interlayer strength is 500 kPa or more, and the internal bond strength per unit area is 102 J / m ² or less.

  Since the coated paper contains silica composite heavy calcium carbonate particles internally, it has high opacity. The reason why the opacity is increased by using such particles is not clear, but this aggregate (silica composite heavy calcium carbonate) can be obtained by aggregating heavy calcium carbonate having a non-uniform particle size and shape with silica. Particles) It is considered that the internal scattering property is improved and the opacity is increased.

Further, since the coated paper has an interlayer strength of 500 kPa or more and an internal bond strength per unit area of 102 J / m ² or less, the coated paper is excellent in flexibility and printability even when it is bulky. The reason for this is not clear, but the following assumptions are presumed. The internal bond strength is the energy required for the paper to undergo shear deformation when the shear force is applied in the thickness direction of the paper and to be torn in the thickness direction. The interlayer strength is a force per unit area necessary for tearing the paper in the thickness direction. Therefore, it is considered that by reducing the internal bond strength per unit area and increasing the interlayer strength, the force required for the paper to shear and deform in the thickness direction is reduced and the flexibility is increased. Moreover, according to the coated paper, since it has a predetermined interlayer strength, it can exhibit sufficient printability.

  The adhesive contains a latex adhesive, the latex adhesive contains a polymer obtained from a monomer containing acrylonitrile, and the proportion of acrylonitrile in the monomer is preferably 21% by mass or more. . By using such an adhesive for the coating layer, the strength of the coating layer can be further softened, so that the flexibility and printability of the coated paper can be further increased.

It is preferable that the average cross-sectional area of the pulp fiber obtained by separating the coated paper is 200 μm ² or more and 500 μm ² or less. By setting the average cross-sectional area of the pulp fiber in the above range, the flexibility and printability of the coated paper can be further enhanced.

The density of the coated paper is preferably 0.65 g / cm ³ or more and 0.85 g / cm ³ or less, and the paper thickness is preferably 66.7 μm or more and 200 μm or less. By setting the density and thickness of the coated paper in the above ranges, the bulkiness and flexibility can be achieved at a higher level.

The flexibility (S) represented by the following formula (1) in the coated paper is preferably 60 × 10 ⁶ / m ² or more and 110 × 10 ⁶ / m ² or less.
S (1 / m ² )
= Interlayer strength (Pa) / {internal bond strength per unit area (J / m ² ) × paper thickness (m)}
... (1)

As described above, the internal bond strength is energy necessary for shearing deformation of the paper when the shearing force is applied in the thickness direction of the paper and tearing the sheared paper in the thickness direction. The interlayer strength is a force per unit area necessary for tearing the paper in the thickness direction. Therefore, the value obtained by dividing the internal bond strength per unit area by the interlayer strength is considered to be a value representing the force necessary for shear deformation of the paper in the thickness direction. Further, even if this value is the same, if the paper thickness is thick, the flexibility is lowered. Therefore, a value obtained by multiplying the above value by the paper thickness is an index relating to rigidity against shear deformation. Therefore, by taking the reciprocal of this index (internal bond strength per unit area × paper thickness / interlayer strength) and setting this as the flexibility (S: unit 1 / m ² ), this flexibility can be applied to the coated paper. This is considered to be an index representing the flexibility per unit area (indicating that the larger the value, the higher the flexibility). In addition, if this value is too large, the flexibility itself is increased, but the strength of the paper is lowered, so that it is considered that the printability and the ease of turning are lowered.

  Here, the internal bond strength per unit area is a value in the flow direction measured in accordance with JAPAN TAPPI 18-2: 2000 “Internal Bond Tester Method”. The interlaminar strength is a value measured according to TAPPI T 541 pm-83 “internal bond strength of paperboard (tensile strength in Z direction)”. The average cross-sectional area of the pulp fiber is the fiber lab. Of the pulp fiber obtained by disaggregating the coated paper by JIS P 8220: 1998 “pulp-disaggregation method”. It is the value measured using (Kajaani). The density and paper thickness are values measured in accordance with JIS-P8118 (1998) “Paper and paperboard—Test method for thickness and density”.

  As described above, according to the coated paper, even when the opacity is high and bulky, the flexibility and the printability are excellent.

  Hereinafter, embodiments of the coated paper of the present invention will be described in detail.

  The coated paper of the present invention has a base paper and a coating layer laminated on both sides of the base paper.

(Base paper)
The base paper is usually obtained by papermaking a pulp slurry containing pulp as a main component. The base paper contains silica composite heavy calcium carbonate particles and a bulking agent.

  As said pulp, a well-known thing can be used, A virgin pulp, a used paper pulp, or these combined things can be used suitably.

  Examples of virgin pulp include hardwood bleached kraft pulp (LBKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), softwood unbleached kraft pulp (NUKP), hardwood semi-bleached kraft pulp (LSBKP), Chemical pulp such as softwood semi-bleached kraft pulp (NSBKP), hardwood sulfite pulp, softwood sulfite pulp, etc .; Stone Grand Pulp (SGP), Pressurized Stone Grand Pulp (TGP), Chemi Grand Pulp (CGP), Ground Pulp (GP), Various well-known pulps such as mechanical pulps such as thermomechanical pulp (TMP); pulps chemically or mechanically produced from non-wood fibers such as kenaf, hemp, and straw can be used.

  Waste paper pulp includes, for example, tea waste paper, craft envelope waste paper, magazine waste paper, newspaper waste paper, flyer waste paper, office waste paper, corrugated waste paper, Kamihiro waste paper, Kent waste paper, imitation waste paper, and old paper waste paper. Examples of such a paper include disaggregation / deinked waste paper pulp (DIP) and disaggregation / deinking / bleached waste paper pulp.

  Among these pulps, it is preferable to use a combination of hardwood bleached kraft pulp (LBKP) and softwood bleached kraft pulp (NBKP). By using these two kinds of pulps, it is possible to easily adjust the flexibility and the like. In this case, as content of LBKP, 50 mass% or more is preferable, 70 mass% or more is more preferable, 75 mass% or more and 90 mass% or less are more preferable. LBKP has a short fiber length and reduces the interfiber spacing of the resulting base paper. Therefore, by increasing the content of LBKP in this way, the adhesive forming the coating layer enters between the pulp fibers of the base paper, and the adhesive and the pulp fibers are closely bonded. As a result, the anchor effect of the coating layer is enhanced, cracking of the coating layer can be suppressed, and the occurrence of mottling during printing can be reduced. Moreover, by setting it as such a pulp ratio, a softness | flexibility can be made into a suitable range and a softness | flexibility can be improved.

The average cross-sectional area of the pulp fibers obtained by macerating the coated paper is preferably 200 [mu] m ² or more 500 [mu] m ² or less, more preferably 250 [mu] m ² or more 450 [mu] m ² or less. By setting the average cross-sectional area of the pulp fiber in the above range, the flexibility and printability of the coated paper can be further enhanced. If this average cross-sectional area exceeds the above upper limit, mottling is likely to occur because the fibers are thick. Moreover, since there are few bonds between fibers, internal bond strength and interlayer intensity | strength are easy to fall, and adjustment of a softness | flexibility may become difficult. On the other hand, when the average cross-sectional area is less than the above lower limit, the density is increased because the fibers are thin, and as a result, the flexibility may decrease.

  As a means for adjusting the average cross-sectional area of the pulp fiber, it can be performed by setting the kind of pulp to be used. In conifers, fir, spruce, and cypress have a small average cross-sectional area, todomatsu, black pine, Japanese pine, radiata pine, tsutsuga, cedar, and hiba are medium, and larch and red pine are large. In broad-leaved trees, the average cross-sectional areas of red oak, zelkova, and sharpener are small, dronoki, beech, mizunara, eucalyptus, cynomolgus, wig, linden and yachidamo are medium, and makanba and cricket are large. That is, it is preferable to select a pulp type having the above-mentioned average average cross-sectional area as NBKP and LBKP. For example, when using Radiata pine as a conifer and eucalyptus as a broad-leaved tree, the average cross-section is within the scope of the present invention. This is preferable because it is easy to adjust the flexibility to a suitable range.

  Furthermore, by setting the ratio of LBKP and NBKP in the pulp type in the above range and setting the average cross-sectional area of the pulp fiber in the above range, the interfiber gap, interfiber bond, density, etc. in the base paper are in a suitable state with a good balance It is possible to improve both flexibility and printability.

(Silica composite heavy calcium carbonate particles)
As described above, the base paper contains silica composite heavy calcium carbonate particles obtained by combining heavy calcium carbonate with silica as a filler, and is preferable because coated paper with higher opacity can be obtained. The silica composite heavy calcium carbonate particles are those obtained by combining heavy calcium carbonate with silica (hereinafter also referred to as composite particles). In other words, it is a composite particle in which at least a part of the surface of the heavy calcium carbonate particle is coated with silica. The silica composite heavy calcium carbonate particles are usually a mixture of heavy calcium carbonate particles that are coated with silica and heavy calcium carbonate particles that are only coated with silica and do not aggregate. It is an aggregate of existing particles.

  Both heavy calcium carbonate and light calcium carbonate have a refractive index of 1.56, and the effect of improving opacity is comparable. For this reason, although it was thought that the opacity improvement effect of silica composite heavy calcium carbonate particles and silica composite light calcium carbonate was the same, as a result of inventors' diligent investigation, silica composite heavy calcium carbonate was It was found to have a higher opacity improving effect than silica composite light calcium carbonate. This is thought to be due to the following factors.

  In the calcium carbonate compounded with silica, several calcium carbonate particles are bonded with silica, and the effect of improving the opacity of the composite particles varies greatly depending on the particle size distribution of the calcium carbonate particles. In other words, when light calcium carbonate having a narrow particle size distribution (nearly uniform particle size) is compounded with silica, the scattering inside the composite particle is reduced, whereas a wide particle size distribution (non-uniform particle size) When the heavy calcium carbonate having bismuth is combined with silica, the scattering properties inside the composite particles are easily improved, and the opacity is easily improved.

  In addition, since heavy calcium carbonate is harder and stronger than light calcium carbonate, the calcium carbonate particles are not broken by impact and the composite is not dissolved, and the effect of improving opacity can be maintained. When a coating layer is provided on a base paper by blade coating, the amount of coating tends to increase due to an increase in coating speed in recent years, and the blade pressure increases to scrape off excess paint. Tends to be overloaded. In particular, when the coating speed is 1,300 m / min or more, there is a possibility that decomposition of the composite particles due to blade pressing, which has not been a problem in the past, may occur. And if a composite is dissolved by decomposition, it becomes difficult to obtain sufficient scattering properties, and the effect of improving opacity tends to decrease. That is, as in the present invention, when silica composite heavy calcium carbonate is used as a filler, the effect of improving opacity is higher when performing high-speed blade coating, compared to silica composite light calcium carbonate. Excellent coated paper can be obtained.

  On the other hand, silica composite heavy calcium carbonate tends to have a higher density than silica composite light calcium carbonate, and in order to obtain the same level of opacity improvement effect as silica composite light calcium carbonate, the amount added is increased. It is necessary and it is said that it is hard to get bulky. Therefore, in the present invention, by further increasing the bulkiness by using a bulking agent together, it is possible to obtain the advantage of heavy calcium carbonate excellent in opacity improvement effect, and even in the case of bulkiness, the opacity is excellent. Coated paper can be obtained.

  Moreover, according to the coated paper, by adding silica composite heavy calcium carbonate particles internally, it is possible to weaken the internal bond strength while relatively suppressing a decrease in interlayer strength. The reason for this is not clear, but because heavy calcium carbonate is manufactured by grinding, the corners are sharp but the overall shape is nearly spherical, and the silica-composite heavy calcium carbonate particles are relatively nearly spherical. As a result, the presence of the filler is unlikely to be resistant to shear deformation in the thickness direction of the base paper, and the internal bond strength is considered to decrease. On the other hand, light calcium carbonate has a spindle shape, a column shape, a cubic shape, etc. and is far from spherical, and even if it is compounded with silica, it is not nearly spherical and is considered to be a relatively complicated shape. In calcium particles, it is considered that the presence of a filler provides resistance to shear deformation in the thickness direction of the base paper, resulting in an increase in internal bond strength and a decrease in flexibility.

  Although it does not specifically limit as content of the said silica composite heavy calcium carbonate, 1 mass% or more and 30 mass% or less are preferable with respect to a pulp (absolute dry weight) in a pulp slurry, and 5 mass% or more and 20 mass% or less. Is more preferable. When the content of the silica composite heavy calcium carbonate is less than the above lower limit, the opacity improvement effect may not be sufficiently obtained. Conversely, when the content of the silica composite heavy calcium carbonate exceeds the above upper limit, the strength between the fibers may be weakened by the silica composite heavy calcium carbonate, and the internal bond strength and the interlayer strength may be reduced.

  The average particle size of the composite particles is preferably 2 μm or more and 15 μm or less, more preferably 3 μm or more and 10 μm or less, and further preferably 3.5 μm or more and 6 μm or less. By setting the average particle diameter of the composite particles in such a range, opacity when used as a filler can be efficiently increased. When the average particle diameter of the composite particles is less than the lower limit, the yield may not be sufficiently improved when used as a filler, and the opacity improving ability is not sufficient. On the other hand, when this average particle diameter exceeds the above upper limit, when used as a filler, as a result of reducing the strength between the pulp fibers, the paper strength may decrease or the wire wear degree may increase, and the particle size is large. Thereby, the uniform dispersibility in a slurry falls and there exists a possibility that opacity and opacity after printing may fall.

  In addition, in this invention, an average particle diameter says a 50% volume average particle diameter, and uses the Nikkiso Co., Ltd. micro track particle size distribution measuring apparatus (model number: MT-3300), and the number of times of measurement: Avg / 2 (average of two measurements) ), Measurement time: 10 seconds, distribution display: volume, particle size classification: standard, calculation mode MT-3300II, measurement upper limit 2000 μm, measurement lower limit 0.021 μm.

  The frequency ratio of the particles per division occupying the mode value in the particle size distribution of the composite particles measured by logarithmic division in the range of 0.021 μm to 2,000 μm is preferably 3% or more and 12% or less. The frequency ratio of the particles occupying this mode of the composite particles is preferably 4.5% or more and 9.5% or less, and more preferably 5% or more and 9% or less. Thus, by making this frequency ratio relatively high, the particle size distribution becomes narrow as described above, and the above performance can be effectively achieved. In addition, it is preferable that it is larger than the frequency ratio of the particle which occupies the mode value in the same particle size distribution of the said heavy calcium carbonate particle.

  When the mode value after silica coating is less than 3%, sufficient primary particle silica coating cannot be performed, and in particular, the effect of rounding many knife edges peculiar to heavy calcium carbonate cannot be expressed, exceeding 12%. In the silica coating, the inventors have found that a small particle size has a specific effect that aggregation of a plurality of particles proceeds together with the silica coating, whereas a large particle size has only a silica coating and has almost no aggregation. Problems that are not expressed can arise.

  In addition, the particle size distribution in the present invention uses a Microtrack particle size distribution measuring device (model number: MT-3300) manufactured by Nikkiso Co., Ltd., the number of measurements: Avg / 2, the measurement time: 10 seconds, the distribution display: volume, particle size classification: It can be measured under conditions of standard, calculation mode MT-3300II, measurement upper limit 2000 μm, measurement lower limit 0.021 μm.

  The composite particles are coated with silica so that the frequency ratio of the particles occupying the mode value is higher than the frequency ratio of the particles occupying the mode value of the primary particles of the calcium carbonate particles. By increasing the size as follows, the dispersion of the particle diameter is reduced by the silica coating based on the heavy calcium carbonate particles having a large dispersion of the particle diameter. According to the composite particle which is an aggregate of such particles, since there are few particles having a small particle size, it is more preferable to adjust the silica content to 2% by mass or more and 30% by mass or less to reduce the particle size. Is selectively agglomerated by silica and easily stays between fibers during papermaking, improving the yield and contributing to bulkiness due to the flexible properties. Further, according to the composite particle, the surface of the particle having a large particle diameter originally covered with silica can reduce the wire wear degree. Moreover, according to the composite particles, the primary particle diameter has a wide particle size distribution, that is, it is composed of an aggregate of calcium carbonate particles having various particle diameters, so that the light scattering property is high and the opacity can be increased. Furthermore, the composite particles have a porous shape due to the coating by precipitation on the surface of silica, have a high oil absorption, and can increase the opacity after printing.

  When silica coating is performed using primary particles having a mode value higher than 12%, there is a problem that a composite particle having a mode value lower than the mode value of the primary particles is generated due to agglomeration due to the silica coating. Cause opacity problems.

  Difference in frequency ratio occupying the mode value between primary particles of calcium carbonate coated with silica and composite particles after composite (frequency of occupying mode value in primary particles of calcium carbonate from frequency ratio occupying mode value in composite particles) When the ratio (%) is at least 2% or more, more preferably 2.5% or more, composite particles having high yield and opacity and low wire wear can be obtained. preferable.

<Heavy calcium carbonate particles>
By obtaining composite particles using heavy calcium carbonate particles, composite particles having a narrow particle size distribution and a high yield can be obtained. This is probably because the heavy calcium carbonate particles are indefinite and have a wide particle size distribution. When silica is deposited on such heavy calcium carbonate particles, a small particle size is agglomerated by a plurality of particles together with the silica coating, whereas a large particle size is mostly agglomerated only by the silica coating. Thus, composite particles having a narrow particle size distribution width can be obtained from heavy calcium carbonate particles having a wide particle size distribution. In addition, since the particle size distribution of the heavy calcium carbonate particles forming the composite particles is wide in this way, the obtained composite particles have different light scattering properties in each particle, and it is considered that high opacity can be exhibited as a whole. . When light calcium carbonate particles with a narrow particle size distribution width and high shape uniformity are used, silica coating is performed almost uniformly on each light calcium carbonate particle, so the particle size distribution of the resulting composite particles Is usually difficult to narrow, and it is difficult to exhibit the above-described effects using heavy calcium carbonate particles.

  The heavy calcium carbonate particles preferably have an average particle size (primary particle size) of 0.5 μm to 3 μm, and more preferably 0.8 μm to 2.5 μm. By setting the average particle size of the heavy calcium carbonate particles in such a range, the yield and opacity when the composite particles are used as a filler in papermaking can be increased in combination with the aggregation by silica described later.

  Further, as the heavy calcium carbonate particles, those in which the frequency ratio of the primary particles occupying the mode value in the particle size distribution is smaller than the frequency ratio in the particle size distribution of the obtained composite particles are preferably used. It is done. By using heavy calcium carbonate particles with a low frequency ratio, a small particle size is agglomerated with a plurality of particles together with the silica coating, whereas a large particle size is almost agglomerated only with the silica coating. Thus, the heavy calcium carbonate particles having a wide particle size distribution can be made into composite particles having a narrow particle size distribution width. As a result, the yield of the composite particles can be improved, and the oil absorption and opacity can be increased.

  In addition, the calcium carbonate particle which has the above wide particle size distribution can be easily obtained by using heavy calcium carbonate particle. This heavy calcium carbonate can be prepared by a method of pulverizing and classifying natural limestone, and commercially available heavy calcium carbonate available as a powder can be pulverized and classified as necessary. . Examples of the pulverization here include pulverization using a dry pulverizer such as a roll mill, jet mill, dry ball mill, and impact pulverizer, wet pulverization such as a wet ball mill, a vibration mill, a stirring tank mill, a flow tube mill, and a coball mill. The pulverization by a mill can be mentioned, and these pulverizers can be used in appropriate combination.

  Examples of the classification method include, for example, sieving such as resonant vibration sieve, low head screen, electromagnetic screen, etc., dry classification such as micron separator, cyclone, etc., wet classification such as decanter type centrifuge, liquid cyclone, drug classifier, etc. These classifiers can be used in appropriate combination.

<Silica>
The silica that coats the heavy calcium carbonate particles is not particularly limited, and known silica can be used, and the calcium carbonate particles are efficiently coated by depositing and coating silica in an aqueous solution as described later. be able to.

  The silica content is preferably 2% by mass or more and 30% by mass or less, and more preferably 3% by mass or more and 25% by mass or less. By setting the silica content in such a range, for heavy calcium carbonate particles having a small particle size, the amount of silica deposited on the surface is sufficient to allow a plurality of particles to be flexibly aggregated. For large heavy calcium carbonate particles, the coating stays on the surface, particularly the tip portion such as a knife edge that is likely to precipitate, and does not have a coating that causes agglomeration with other particles. Therefore, according to the composite particles, by covering the heavy calcium carbonate particles having a wide particle size distribution with silica having such a mass ratio, the composite particles can be easily controlled to an aggregate state having a narrow particle size distribution. Yield improvement, oil absorption and opacity can be increased, and wire wear can be reduced.

  When the silica content is less than the above lower limit, the heavy calcium carbonate particles cannot be sufficiently agglomerated, and the particle size distribution of the resulting composite particles is difficult to narrow, resulting in improved yield when used as a filler. There is a risk of not. On the other hand, when the silica content exceeds the above upper limit, the aggregation of heavy calcium carbonate particles having a relatively large particle size can easily proceed. As a result, in the resulting composite particles, the number of particles having a large particle size increases, similarly, the particle size distribution is not easily narrowed, the ability to improve opacity is not sufficient, and the number of particles having a large particle size is large. There is a risk that paper dust is likely to be generated.

  The oil absorption of the composite particles is preferably 30 ml / 100 g or more and 100 ml / 100 g or less, more preferably 40 ml / 100 g or more and 80 ml / 100 g or less. When composite particles having such an oil absorption are used as an internal filler, the composite particles absorb ink vehicle or organic solvent impregnated in the paper layer in the paper layer, and the printing opacity of the paper is reduced. By suppressing the reduction and absorbing the ink vehicle, the organic solvent, and the like, the ink drying property and the effect of preventing blurring can be remarkably exhibited. On the other hand, when the oil absorption is less than 30 ml / 100 g, the above effect is not sufficient, and the composite particles may tend to inhibit the ink absorption and drying properties. On the other hand, if the oil absorption exceeds 100 ml / 100 g, the ink absorbability is high, so that the ink sinks, and so-called color developability may be inferior.

<Manufacturing method>
The method for producing the composite particles is not particularly limited. For example, (1) a suspension step in which heavy calcium carbonate particles are suspended in an aqueous alkali silicate solution to obtain a suspension;
(2) A mineral acid is added to this suspension, silica is deposited on the surface of the heavy calcium carbonate particles, and the resulting suspension is coated to agglomerate the calcium carbonate particles.
By producing in this way, in the agglomeration step, among the heavy calcium carbonate particles, those having a small particle size proceed with agglomeration of a plurality of particles together with the silica coating, while those having a large particle size are coated with silica. Thus, almost no aggregation occurs, and heavy calcium carbonate particles with a wide particle size distribution can be made into composite particles with a narrow particle size distribution. Therefore, according to the said manufacturing method, the composite particle with a high yield, opacity, etc. and a low degree of wire wear can be obtained.

(1) Suspension step In this step, preferably, wet-pulverized heavy calcium carbonate is mixed in an aqueous alkali silicate solution. The aqueous alkali silicate solution is not particularly limited, but a sodium silicate solution (No. 3 water glass) is preferable because it is easily available.

The concentration of the alkali silicate solution is preferably 3 to 10% by mass in terms of the silicic acid content in the aqueous solution (in terms of SiO ₂ ). If the amount exceeds 10% by mass, the formed composite particles are coated with the white carbon to be produced, and the amorphous form and optical characteristics of the calcium carbonate in the core may not be exhibited. On the other hand, if the amount is less than 3% by mass, the silica component in the composite particles is lowered, so that composite particles coated with silica are hardly formed.

  Moreover, as solid content concentration of this suspension, 3-35 mass% is preferable. By adjusting this concentration within the above range, it becomes easy to control the particle size, particle size distribution, silica content and the like of the obtained composite particles to a desired range.

(2) Aggregation step In this step, mineral acid is added to the suspension, and silica is deposited and coated on the surface of the heavy calcium carbonate particles, thereby aggregating at least a part of the heavy calcium carbonate particles.

  Examples of the mineral acid include diluted solutions of mineral acids such as dilute sulfuric acid, dilute hydrochloric acid, and dilute nitric acid, but dilute sulfuric acid is preferred in terms of price and handling. Further, the concentration when adding dilute sulfuric acid is preferably 0.2 to 4.0 mol%. Heavy calcium carbonate suitably used in the above production method is derived from minerals, and further contains impurities such as calcium and aluminum as constituent elements within a predetermined range. Alteration may occur in the composite particles.

  Moreover, since silica precipitates in a short time, so that there is much mineral acid addition amount, it is preferable to adjust an addition rate according to those conditions. In addition, the addition within 5 minutes makes the structure of the uniform reaction system insufficient.

  The reaction temperature in this aggregation step is preferably 60 ° C. or higher and 100 ° C. or lower. From the results of the present inventors' extensive studies, the reaction temperature with the heavy calcium carbonate used in the present invention affects the formation of silica, the crystal growth rate, and the mechanical strength of the formed heavy calcium carbonate-silica composite particles. Effect. When the reaction temperature is less than 60 ° C., the silica formation / growth rate is slow, the coverage of the formed composite particles is inferior, the coating is easily peeled off, and the coating is easily broken by the shearing force applied when the filler-added paper is made. On the other hand, if the temperature exceeds 100 ° C., the reaction process becomes complicated because an autoclave must be used because it is an aqueous reaction. The optimum reaction temperature is 65 to 95 ° C.

In this agglomeration step, silica sol is produced by adding a mineral acid as described above, and composite particles are obtained by adjusting the suspension to neutral to weakly alkaline, preferably pH in the range of 8-11. Can do. At this time, the mineral acid may be added in such a range that the temperature of the suspension is 60 ° C. or more and 100 ° C. or less and the silica content in the composite particles is 2% by mass or more and 30% by mass or less. By controlling to such a temperature and the amount of mineral acid added, more preferably, the particle size can be maintained by maintaining the retention time of the silica aggregation step with respect to heavy calcium carbonate for 60 to 120 minutes, more preferably 70 to 100 minutes. In the case of small particles, agglomeration of a plurality of particles is progressing together with the silica coating. On the other hand, for heavy calcium carbonate particles having a large particle size, the silica is deposited on the surface, particularly on the tip portion such as a knife edge that tends to precipitate. The above-mentioned composite particles having a narrow particle size distribution can be efficiently obtained because the coating does not occur so as to remain and agglomerate with other particles. If the retention time is less than 60 minutes, the aggregation of heavy calcium carbonate with a small particle size becomes insufficient, and if it exceeds 120 minutes, excessive silica coating may occur, resulting in composite particles having an excessive particle size. .
(Bulky agent)
The base paper contains a bulking agent. Although it does not specifically limit as said bulking agent and it can use a well-known thing, A fatty acid ester type | system | group, a fatty acid amide type | system | group, or a polyamine type bulking agent is preferable, and a fatty acid ester type | system | group is more preferable. By using such a bulking agent, the bulk can be increased while suppressing a decrease in internal bond strength and interlayer strength due to addition of silica composite heavy calcium carbonate or the like.

  Although it does not specifically limit as content of the said bulking agent, 0.2 mass% or more and 20 mass% or less are preferable with respect to a pulp (absolute dry weight) in a pulp slurry, 0.5 mass% or more and 15 mass% or less. Is more preferable. When the content of the bulking agent is less than the above lower limit, the bulk may not be sufficiently increased. On the other hand, when the content of the bulking agent exceeds the above upper limit, the strength between fibers may be weakened by the bulking agent, and the internal bond strength and interlayer strength may be reduced.

  Examples of the base paper include other paper strength enhancers such as starches, polyacrylamide and epichlorohydrin, sizing agents such as rosin, alkyl ketene dimer, ASA (alkenyl succinic anhydride) and neutral rosin, and sulfate bands. , Coagulants such as polyethyleneimine, and coagulants such as polyacrylamide and copolymers thereof.

  It is preferable that a coating liquid containing a surface sizing agent and a paper strength improver is applied to both surfaces of the base paper as an undercoat coating. By performing such undercoating, the flexibility and printability of the coated paper can be enhanced.

  The surface sizing agent is not particularly limited and includes, for example, starch, styrene sizing agent, acrylate sizing agent, alkyl ketene dimer, etc. Among them, starch, styrene sizing agent and acrylate sizing agent can be mentioned. Sizing agents are preferred, and starch is particularly preferred.

  The paper strength improver is not particularly limited. For example, polyacrylamide (PAM), polyvinyl alcohol (PVA), acrylic resin, polyamide / polyamine resin, urea / formalin resin, melamine / formalin resin, polyethylene Among them, PAM, acrylic resin and PVA are preferable, and PAM is more preferable.

  Furthermore, it is more preferable to use starch as the surface sizing agent and PAM as the paper strength improver. By using a coating liquid for undercoating of such a combination, even with a small coating amount, it is possible to form a flexible and highly adhesive film between the base paper and the coating layer (top coating layer). As a result, the printability and flexibility of the coated paper can be further enhanced. Furthermore, when using starch as a surface sizing agent and PAM as a paper strength improver on a base paper having an average cross-sectional area within a predetermined range as described above, both printability and flexibility can be improved. .

  In addition, when using starch and PAM combining, as these mass ratio, 9: 1 to 1: 1 is preferable and 5: 1 to 2: 1 is more preferable. By setting it as such mass ratio, the said effect can be heightened more.

The coating amount of the coating solution in the undercoat coating is not particularly limited, but is preferably 0.1 g / m ² or more and 1 g / m ² or less per side in terms of solid content, and 0.3 g / m ² or more and 0. 0.7 g / m ² or less is more preferable. When this coating amount is less than the above lower limit, the effect of the above-described undercoat coating is not sufficiently exhibited, and for example, printability may be reduced. On the other hand, when the coating amount exceeds the above upper limit, the coating may increase the surface strength and decrease the flexibility.

(Coating layer)
The coating layer contains a pigment and an adhesive as main components, and is laminated on both surfaces of the base paper by applying a coating liquid (top coating). That is, the coating liquid contains the pigment and the adhesive as main components.

  The pigment is not particularly limited. For example, kaolin, delaminated kaolin, talc, clay, calcium carbonate (light calcium carbonate, heavy calcium carbonate), satin white, calcium sulfite, gypsum, barium sulfate, white carbon, Inorganic pigments such as calcined kaolin, structured kaolin, diatomaceous earth, magnesium carbonate, titanium dioxide, aluminum hydroxide, calcium hydroxide, silica, magnesium hydroxide, zinc hydroxide, zinc oxide, magnesium oxide, bentonite, sericite, and polystyrene resins Examples thereof include fine particles, urea formalin resin fine particles, fine hollow particles, and porous fine particles.

  Among these, it is preferable to use calcium carbonate from the viewpoint of increasing whiteness. Further, the oil absorption amount of this calcium carbonate is preferably less than 40 ml / 100 g, more preferably 10 ml / 100 g or more and 30 ml / 100 g or less. By using calcium carbonate having a low oil absorption in this way, it is possible to suppress the printing ink from being absorbed and to reduce the occurrence of mottling. In addition, the absorption of dampening water is also suppressed, and the elongation of the coating layer is suppressed, so that the printability can be improved.

  As content of the said calcium carbonate with respect to the said pigment, 45 mass% or more is preferable, and 50 mass% or more and 100 mass% or less are more preferable. Thus, by increasing the content of calcium carbonate, the whiteness of the coated paper can be increased.

  The pigment used together with calcium carbonate in the pigment is not particularly limited, but clay having excellent ink acceptability is preferable.

  The adhesive is not particularly limited and known ones can be used. Examples thereof include proteins such as casein and soybean protein; styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex, styrene-methyl. Conjugated diene latex such as methacrylate-butadiene copolymer latex; acrylic latex such as polymer latex or copolymer latex of acrylate ester and / or methacrylate ester; vinyl latex such as ethylene-vinyl acetate polymer latex Latexes such as alkali partially soluble or insoluble latex obtained by modifying these various copolymer latexes with a functional group-containing monomer such as a carboxyl group; olefin-maleic anhydride resin, melamine resin, urea resin, Urethane resin Synthetic resin-based adhesive; can be mentioned carboxymethyl cellulose, cellulose derivatives such as hydroxyethyl cellulose; oxidized starch, positive starch, esterified starch, starches such as dextrin.

  Among these, a latex adhesive is preferable, and the latex adhesive preferably contains a polymer obtained from a monomer containing acrylonitrile. When acrylonitrile is included as a monomer, the symmetry of the resulting polymer is increased, so that the melting point is increased. Accordingly, when such a coating layer is used, the flexibility of the coating layer is increased, so that the flexibility of the coated paper can be increased. Moreover, printability can also be improved by using a polymer containing acrylonitrile as a monomer.

  The proportion of acrylonitrile in the monomer is preferably 21% by mass or more, and preferably 22% by mass or more and 30% by mass or less. Thus, by relatively increasing the ratio of acrylonitrile, the flexibility and printability of the coated paper can be increased. Furthermore, on the base paper having an average cross-sectional area within a predetermined range as described above, starch is used as a surface sizing agent and PAM is used in combination as a paper strength improver, and the proportion of acrylonitrile is 21% by mass or more. When latex is used in combination, flexibility can be particularly enhanced.

  Furthermore, the polymer obtained from the monomer containing acrylonitrile preferably contains styrene as the monomer. The polymer obtained by copolymerizing the monomer containing acrylonitrile and styrene in this way is not clear for reasons, but has high adhesion to calcium carbonate and can firmly fix the pigment even in a small amount. . Therefore, by using such a polymer, the amount of the adhesive can be reduced and the flexibility of the coated paper can be increased. In this case, the proportion of styrene in all the monomers is preferably 20% by mass or more and 40% by mass or less in order to suitably exhibit the above function.

  As content of the adhesive agent in the said coating layer (solid content in a coating liquid), less than 10 mass% is preferable, and 3 mass% or more and 8 mass% or less are more preferable. Thus, by reducing the content of the adhesive, a flexible coating layer can be obtained, and the flexibility of the coated paper can be increased. In addition, by using the above-mentioned oil absorption amount of calcium carbonate as a pigment and using the above-mentioned acrylonitrile and styrene as monomers as an adhesive, even if the content of the adhesive is reduced in this way, The fixing property is high and the printability can be improved.

  For the coating layer (coating liquid), in addition to pigments and adhesives, for example, fluorescent whitening agents, dyed substances of fluorescent whitening agents, antifoaming agents, release agents, colorants, water retention Various commonly used auxiliaries such as an agent can be contained.

Examples of the coating amount of the coating layer, preferably per side 5 g / ^{m 2} or more 18 g / ^{m 2} or less, more preferably 7 g / ^{m 2} or more 12 g / ^{m 2} or less. When the coating amount is less than the lower limit, it is difficult to exhibit sufficient printability. On the contrary, when the coating amount exceeds the upper limit, the surface of the coated paper is cured by the coating layer, so that the flexibility is likely to be lowered.

(Interlayer strength and internal bond strength per unit area)
The interlayer strength of the coated paper is 500 kPa or more, and the internal bond strength per unit area is 102 J / m ² or less. Thus, by reducing the internal bond strength per unit area and increasing the interlayer strength, it is considered that the force required for the paper to shear and deform in the thickness direction is reduced and the flexibility is increased. . Moreover, according to the coated paper, since it has a predetermined interlayer strength, it can exhibit sufficient printability.

  The interlayer strength is preferably 500 kPa to 1,000 kPa, and more preferably 550 kPa to 900 kPa. If the interlaminar strength exceeds the above upper limit, shear deformation is likely to occur, and therefore, printability may be deteriorated such that mottling is likely to occur. Conversely, when the interlayer strength is less than the above upper limit, flexibility and printability may be reduced.

The internal bond strength per the unit area is preferably 40 J / ^{m 2} or more 80 J / ^{m 2} or less, 52J / ^{m 2} or more 68j / ^{m 2} or less is more preferable. When the internal bond strength exceeds the above upper limit, the flexibility may decrease. On the other hand, when the internal bond strength is less than the above lower limit, the printability may be lowered, for example, the paper may be pulled in both directions when passing through the blanket during printing.

  The interlaminar strength and internal bond strength are, for example, beating, basis weight, coating amount of coating liquid when forming a coating layer and its components, coating method, pulp type forming base paper, flexible It can be adjusted by the type and amount of the internal additive such as an agent, the linear pressure in the calendar process, and the like. For example, when the beating is performed by lowering the beating concentration in the beating of the pulp, the fibers are cut more and the external and internal fibrillation hardly occurs, so that shear deformation easily occurs and the internal bond strength is easily reduced. Cutting beating includes methods such as increasing the beating temperature and adjusting the beating blade (narrowing the blade width, increasing the blade length, and reducing the contact angle). Also, as a coating method for clear coating liquids such as starch and sizing agents, the interlayer strength can be improved by using a two-roll size press that immerses the paper in pounds where the coating liquid can easily penetrate into the paper. Can do.

  Among these, as described above, by using silica composite heavy calcium carbonate particles as a filler, among the interlaminar strength and the internal bond strength, the internal bond strength can be relatively greatly reduced, and as a result, The flexibility of the resulting coated paper can be increased.

(Flexibility (S))
In the coated paper, the flexibility (S) represented by the following formula (1) is 60 × 10 ⁶ / m ² or more and 110 × 10 ⁶ / m ² or less, and 65 × 10 ⁶ / m ² or more and 90 It is preferable that it is x10 < ⁶ > / m < ² > or less.
S (1 / m ² )
= Interlayer strength (Pa) / {internal bond strength per unit area (J / m ² ) × paper thickness (m)}
... (1)

  This flexibility is a value related to the force required to shear and deform the paper in the thickness direction as described above, and the greater the value, the more unnecessary the force, that is, the higher the flexibility. When the flexibility parameter is less than the above lower limit, the flexibility is lowered, and for example, when turning the page, the page is likely to stand up, and it is difficult to turn. On the other hand, when the flexibility parameter exceeds the upper limit, the strength of the paper is lowered, printability is deteriorated (for example, mottling occurs), and turning is difficult.

  The flexibility can be adjusted by changing the internal bond strength per unit area and the interlayer strength in addition to the paper thickness. The internal bond strength and interlayer strength per unit area are, for example, basis weight. The amount of coating liquid applied to form the coating layer and its components, the type and amount of internal additives such as pulp types and softeners that form the base paper, and the linear pressure in calendering, etc. Can do.

(Other quality, etc.)
The paper thickness of the coated paper is not particularly limited, but is preferably 66.7 μm or more and 200 μm or less. According to the coated paper, even in such a bulky state, the flexibility is adjusted, so that both flexibility and printability can be achieved.

The The density of the coated paper is not particularly limited, but is preferably 0.65 g / cm ³ or more 0.85 g / cm ³ or less, 0.7 g / cm ³ or more 0.8 g / cm ³ or less is more preferable. By setting it as such a density, the softness | flexibility in the said coated paper and the printability by the anchor effect of a coating layer can be exhibited with sufficient balance. When the density is less than the above lower limit, the printability may deteriorate due to the anchor effect of the coating layer being weakened. Conversely, when the density exceeds the above upper limit, the flexibility may decrease. Furthermore, on the base paper having an average cross-sectional area within a predetermined range as described above, starch is used as a surface sizing agent and PAM is used in combination as a paper strength improver, and the proportion of acrylonitrile is 21% by mass or more. When latex is used in combination and the density is within the above range, both flexibility and printability can be enhanced.

The basis weight of the coated paper is not particularly limited, but is preferably 50 g / m ² or more and 150 g / m ² or less, and more preferably 70 g / m ² or more and 120 g / m ² or less. By setting the basis weight within the above range, the flexibility and printability of the coated paper can be exhibited in a well-balanced manner such that the flexibility parameter can be easily adjusted.

  Although it does not specifically limit as whiteness of the said coated paper, 80% or more is preferable and 83% or more is preferable. According to the coated paper, such high whiteness can be provided, for example, preferably by adjusting the content of the calcium carbonate. The upper limit of the whiteness is not particularly limited, but is 95%, for example. In addition, whiteness means the value measured based on "the measuring method of paper, paperboard, and pulp-ISO whiteness (diffuse blue light reflectance)" of JIS-P8148.

(Production method)
It does not specifically limit as a manufacturing method of the said coated paper, It can obtain with the manufacturing method of a well-known coated paper. Specifically, for example, the raw pulp slurry is made into paper, used for the press part and the pre-dryer part to produce the base paper, then the undercoat is applied in the undercoater part, and after drying and flattening treatment, the coating is performed. It can be obtained by forming a coating layer by applying a liquid (top coating liquid) and applying a calendar treatment.

  There are no particular limitations on the coating apparatus used for coating, and for example, a size press, a blade metering size press, a rod metalling size press, a blade coater, a bar coater, a gate roll coater, a rod coater, an air knife coater or the like may be used. it can.

  In addition, as a calendar device for calendar processing, for example, various calenders such as a super calender, a gloss calender, a soft compact calender, or a combination of a drum and an elastic roll can be appropriately used in on-machine or off-machine specifications.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

  In addition, the measuring method and evaluation standard of each measured value in a present Example are as describing below.

[Measurement of average particle diameter, frequency ratio A and frequency ratio B]
Using a laser diffraction particle size distribution measuring apparatus [Microtrack / Nikkiso Co., Ltd.] (model number: MT-3300), 50% volume average particle diameter (μm), frequency ratio A (%) and frequency ratio B (%) were measured. . In the preparation of the measurement sample, particles were added to a 0.1% sodium hexametaphosphate aqueous solution and dispersed with an ultrasonic wave for 1 minute.

[Silica content]
Component analysis was performed with a fluorescent X-ray analyzer (RIGAKU SYSTEM 3080E2), and the silica content (mass%) was calculated from the oxide conversion ratio of calcium and the oxide conversion ratio of silicon.

[Oil absorption of composite particles]
It measured according to the kneading method described in JIS-K5101. That is, 2 g to 5 g of a sample dried at 105 ° C. to 110 ° C. for 2 hours is taken on a glass plate, and refined linseed oil (having an acid value of 4 or less) is dropped from the burette to the center of the sample little by little and kneaded with a spatula each time. The kneading operation was repeated, and the dripping amount of the refined linseed oil was determined with the end point when the whole was first assembled into one rod shape, and the oil absorption was calculated by the following formula (1).
Oil absorption amount = [linseed oil amount (mL) × 100] / paper (g) (1)

[Wire wear degree (mg)]
Using a wear degree test apparatus (manufactured by Nippon Filcon Co., Ltd.) and pumping a filler dispersion with a solid content concentration of 5%, the test conditions (weight = 650 g, wire = plastic wire / SS-40 ... Nippon Filcon Abrasion degree test was carried out using a product manufactured by the company, test time = 3 hours), and the weight (mg) of the reduced wire was used as the wire wear degree. It shows that wire abrasion property is so large that a numerical value is large.

[Basis weight (unit: g / m ² )]
Measured according to JIS-P8124 (1998) “Paper and paperboard—basis weight measurement method”.

[Paper thickness (unit: μm)]
Measured according to JIS-P8118 (1998) “Paper and paperboard—Test methods for thickness and density”.

[Density (unit: g / cm ³ )]
Measured according to JIS-P8118 (1998) “Paper and paperboard—Test methods for thickness and density”.

[Internal bond strength per unit area (unit: J / m ² )]
The strength in the flow direction was measured according to JAPAN TAPPI 18-2: 2000 “Internal Bond Tester Method”.

[Interlayer strength (unit: kPa)]
Measured according to TAPPI T 541 pm-83 “Internal bond strength of paperboard (tensile strength in Z direction)”.

[Flexibility (unit: 1 / m ² )
Calculation was performed according to the above formula (1).

[Whiteness (Unit:%)]
It was measured according to “Measuring method of paper, paperboard and pulp-ISO whiteness (diffuse blue light reflectance)” described in JIS-P8148.

[Opacity (%)]
It measured based on the method of JIS-P8149.

[Mottling]
Printing was performed on the coated paper under the following conditions to produce a print test specimen.
・ Printing machine: RI-3 type, manufactured by Meisei Seisakusho Co., Ltd. ・ Ink: WebRexNouverHIMARK process, manufactured by Dainichi Seika Co., Ltd. ・ Ink amount: 0.3 ml for the upper roll and 0.2 ml for the lower roll
Test method: The ink was kneaded for 3 minutes each with the upper and lower rolls (kneaded for 2 minutes, then the roll was inverted and further kneaded for 1 minute), and two-color simultaneous printing was performed at a rotation speed of 30 rpm.
About this printing test body, the Mott Clicking was visually evaluated as follows.
A: No mottling is observed.
○: Slight mottling is observed.
Δ: Some mottling is observed, but there is no practical problem.
×: Strong mottling occurs and cannot be used.

[Average cross-sectional area of pulp fiber (unit: μm ² )]
The pulp fiber obtained by disaggregating the coated paper according to JIS-P8220: 1998 “pulp-disaggregation method” is referred to as FiberLab. (Kajaani) was used for measurement.

[Flexibility (Ease of turning)]
100 sheets of the coated paper obtained were cut into A5 size and sandwiched between clips to create a booklet model. As flexibility, the ease of turning when turning the page and the difficulty of standing the page were evaluated by 10 monitors based on the following criteria.
A: Excellent (very easy to turn over and hard to stand)
○: Excellent (easy to turn and hard to stand)
△: Slightly problematic (There are pages that are difficult to turn and pages that stand)
×: There is a problem (it is difficult to turn over and is easy to stand)

The chemicals used in each example are as follows.
(Bulky agent)
・ Fatty acid ester system: KB-210, manufactured by Kao Corporation ・ Fatty acid amide system: Peremin TS-218, manufactured by Miyoshi Yushi Co., Ltd. ・ Polyamine system: DKS Taflon NT-350J, manufactured by Daiichi Kogyo Seiyaku (external sizing agent)
Starch: Mermaid M-205, manufactured by Shikishima Starch Co., Ltd. Styrene: SS2710, manufactured by Seiko PMC Co., Ltd. Acrylate: SE2066, manufactured by Seiko PMC Co., Ltd. (paper strength improver)
-PAM: Haricoat G51, manufactured by Harima Kasei Co., Ltd.-Acrylic: Polymer Set 500, manufactured by Arakawa Chemical Industries, Ltd.-PVA: Gohsenal T-330, manufactured by Nippon Synthetic Chemical Industries (pigment)
・ Clay (Kaolin clay): KCS, manufactured by Huber ・ Calcium carbonate Light calcium carbonate: TP-121, manufactured by Okutama Kogyo Co., Ltd., oil absorption 42 ml / 100 g
Light calcium carbonate: Albafil, Pfizer MSP, Oil absorption 40ml / 100g
Light calcium carbonate: Argonite, manufactured by Shiroishi Kogyo Co., Ltd., oil absorption 34 ml / 100 g
Heavy calcium carbonate: NS600, manufactured by Nitto Fuka Kogyo Co., Ltd., oil absorption 30 ml / 100 g
Heavy calcium carbonate: FAX82, manufactured by Nissei Kyobensha, oil absorption 25ml / 100g
(latex)
PA8200, manufactured by Japan A & L (acrylonitrile 22% by mass, styrene 25% by mass)
However, Examples 30 and 31 were prepared by changing the monomer ratio as shown in Table 2.

(Manufacture of silica composite heavy calcium carbonate particles)
<Production of composite particles 1>
Heavy calcium carbonate (average particle size of primary particles 1.6 μm, frequency ratio of particles occupying the mode in the particle size distribution measured by dividing 132 logarithm in the range of 0.021 μm to 2,000 μm (frequency ratio A) 4 .8%) slurry (concentration: 10%) in 200 g, 60 g of sodium silicate aqueous solution was added, and dispersion treatment was carried out for 20 minutes at 3,000 rpm using a homomixer to prepare heavy calcium carbonate and sodium silicate aqueous solution. A dispersion slurry was prepared. Next, this slurry was put into a 1 L four-necked flask equipped with a stirrer, a temperature sensor, and a reflux condenser, and heated to 88.5 ° C. in an oil bath while stirring. Next, while maintaining the slurry in the container at 88.5 ° C. (± 2 ° C.), 150 mL of 1N sulfuric acid was added dropwise over 100 minutes at a dropping rate of 2.5 mL / min using a metering pump. Quality calcium carbonate particles were obtained. The pH of the reaction solution at the start of the reaction was 11.5. Furthermore, no. A wet cake of silica composite heavy calcium carbonate particles was obtained by filtering, washing with water and filtering again using two filter papers.

<Manufacture of composite particles 2-11>
Using heavy calcium carbonate having the average particle size (μm) and frequency ratio A (%) of the primary particles described in Table 1, the amount of sulfuric acid added and the reaction conditions were adjusted so that the silica content described in Table 1 was obtained. Except having adjusted, the same operation | work as manufacture of the composite particle 1 was performed, and the composite particles 2-11 were obtained. The reaction initiation pH and reaction temperature varied depending on the particle type, the amount of mineral acid added, and the like.

<Particles i and ii>
Particle i is a commercially available light calcium carbonate, and particle ii is a commercially available heavy calcium carbonate.

(Production of silica composite light calcium carbonate particles)
<Production of composite particles iii>
The silica composite was prepared in the same manner as the composite particle 1 except that light calcium carbonate (average particle size of primary particles 2.2 μm, frequency ratio A 7.5%) was used instead of heavy calcium carbonate. Light calcium carbonate particles were obtained.

(Production of aggregated light calcium carbonate particles)
<Production of composite particles iv>
Light calcium carbonate (particles i) was aggregated with a flocculant (polydiallyldimethylammonium chloride) to obtain aggregated light calcium carbonate particles.

  The average particle size (μm) of the composite particles obtained in Table 1, the frequency ratio of particles occupying the mode in the particle size distribution measured by dividing 132 logarithm in the range of 0.021 μm to 2,000 μm (frequency ratio B :%), Oil absorption (mL / 100 g) and wire wear (mg).

[Example 1]
As raw material pulp, softwood bleached kraft pulp (NBKP tree species: Radiata pine) and hardwood bleached kraft pulp (LBKP tree species: Eucalyptus) are blended at a mass ratio of 20:80, and each solid (solid dry weight) is solid. Minutes, 10% by mass of silica composite heavy calcium carbonate particles (composite particle 1) as filler, 8.0% by mass of fatty acid ester bulking agent, cationic polyacrylamide (product number: Percoll 47, manufactured by Ciba Japan) , Anionic polyacrylamide (product number: Teriofoam M100, manufactured by Ciba Japan Co., Ltd.), internal sizing agent (product number: AK-720H, manufactured by Harima Kasei Co., Ltd.) 0.02% by mass and cationized starch (product number: Amilofax T) -2600, manufactured by Abebe Japan Co., Ltd.) 1.0% by mass was added to obtain a pulp slurry.

  Next, a winder part is used using a papermaking system including a wire part, a press part, a pre-dryer part, an undercoater part, an after-dryer part, a pre-calender part, a top coater part, a scuff dryer part, a calendar part, and a reel part. Coated paper was obtained.

Specifically, the pulp slurry was first made with a wire part, and then subjected to a press part and a predryer part to produce a base paper having a basis weight of 78.0 g / m ² . Next, in the undercoater part, both sides of the undercoat coating liquid containing starch as an external sizing agent and PAM as a paper strength improver at a mass ratio of 3: 1 are adjusted to 0.5 g / m ² per side. The undercoat was applied and dried in an after dryer part. Thereafter, it was flattened with a pre-calender (linear pressure 20 kN / m), and coated with a top coater part (containing calcium carbonate and clay having an oil absorption of 25 ml / 100 g as a pigment in a mass ratio of 50:50, A latex containing a polymer obtained by polymerizing a monomer containing 22% by mass of acrylonitrile and 25% by mass of styrene (the coating liquid containing 48% butadiene and 5% carboxylic acid) containing 8% of polymer on each side. Both sides were overcoated so as to be 0.0 g / m ² . Next, the calender part was subjected to a flattening treatment at a linear pressure of 200 kN / m and a speed of 1,500 m / min, and provided to a winder part to obtain a coated paper having a basis weight of 95.0 g / m ² . In addition, paper was made using a gap former in the wire part, a rod metering size press coater was used in the undercoater part, and a blade coater was used in the top coater part. In the calendar part, a multi-nip calendar was used. All the above parts used an on-machine papermaking system.

[Examples 2-39 and Comparative Examples 1-9]
In the manufacture of the base paper, the tree species of the pulp used and the blended mass ratio, the type and content of the filler, the type and content of the bulking agent and the basis weight, and the external sizing agent and the paper strength improver in the primer coating Tables 2 and 3 show the types and mass ratios, coating amounts, types and contents of pigments in top coating, monomer ratios constituting the polymer contained in the latex of the adhesive, and coating amounts. Coated papers of Examples 2 to 39 and Comparative Examples 1 to 9 were obtained in the same manner as Example 1 except that the change was made as described above.

[Evaluation]
For each coated paper obtained, basis weight, paper thickness, density, internal bond strength per unit area, interlayer strength, flexibility, whiteness, opacity, mottling, average fiber cross-sectional area by the above method. And the flexibility was measured or evaluated. The evaluation results are shown in Table 4.

  As shown in Table 4, each coated paper of the examples has high opacity and sufficient flexibility (easy to turn) and printability (occurrence of mottling is suppressed). I understand.

  Moreover, when Example 1 and Comparative Examples 1-4 are compared, Example 1 has especially small internal coupling strength compared with Comparative Examples 1-4, As a result, a softness | flexibility (S) becomes large and is flexible. The nature is increasing. This is because the silica composite heavy calcium carbonate particles are agglomerated in a relatively spherical state. As a result, the presence of the filler is hardly resistant to deformation in the shear direction of the base paper, and the internal bond strength is low. It is thought that it falls. On the other hand, in the non-composite particles (Comparative Examples 1 and 2) having a small particle diameter and the particles considered to be aggregated in a relatively complicated shape (Comparative Examples 3 and 4), the presence of filler is the base paper. As a result of resistance to deformation in the shear direction, it is considered that the internal bond strength is increased and the flexibility is lowered.

  Furthermore, Example 1 has higher opacity than Comparative Examples 1 to 4. This is considered to be due to the high scattering property and the like inside the silica composite heavy calcium carbonate particles.

  As described above, the coated paper of the present invention is excellent in flexibility and printability even when it is bulky, and can be suitably used as a printing paper for magazines, for example.

Claims (5)

Having a base paper and a coating layer laminated on both sides of the base paper,
The coated layer is a coated paper containing a pigment and an adhesive as main components,
The base paper contains silica composite heavy calcium carbonate particles and a bulking agent,
A coated paper having an interlayer strength of 500 kPa or more and an internal bond strength per unit area of 102 J / m ² or less. The adhesive includes a latex adhesive,
This latex adhesive contains a polymer obtained from a monomer containing acrylonitrile,
The coated paper according to claim 1, wherein a ratio of acrylonitrile in the monomer is 21% by mass or more. The coated paper according to claim 1 or 2, wherein an average cross-sectional area of pulp fibers obtained by disaggregation is 200 µm ² or more and 500 µm ² or less. The coated paper according to claim 1, wherein the density is 0.65 g / cm ³ or more and 0.85 g / cm ³ or less, and the paper thickness is 66.7 μm or more and 200 μm or less. The softness (S) represented by the following formula (1) is 60 × 10 ⁶ / m ² or more and 110 × 10 ⁶ / m ² or less, The coating according to any one of claims 1 to 4. paper.
S (1 / m ² )
= Interlayer strength (Pa) / {internal bond strength per unit area (J / m ² ) × paper thickness (m)}
... (1)