Printable sheet and method for manufacturing a printable sheet

Abstract

The present invention provides a printing sheet having a coating layer for receiving ink on at least one surface of a substrate, wherein the printing suitability is excellent, the antistatic property is good without easily causing paper jam and the like during printing, and the properties such as water resistance, weather resistance and the like are excellent, and particularly, a printing sheet having excellent adhesion / bonding of the substrate and the coating layer and a manufacturing method thereof. The printing sheet is a printing sheet having a coating layer on one or both surfaces of a substrate, wherein the coating layer is formed by incorporating clay in a proportion of 35 mass% or more and 65 mass% or less and heavy calcium carbonate in a proportion of 5 mass% or more and 30 mass% or less in a continuous phase composed of an acrylic polymer, and the volume average particle diameter of the heavy calcium carbonate is 0.05 μm or more and 2.00 μm or less.

Description

Technical Field

[0001] This invention relates to printing sheets and methods for manufacturing printing sheets. More specifically, this invention relates to techniques for improving the adhesion / bonding properties, printability, water resistance, weather resistance, and antistatic properties of printing sheets having a coating on the surface of a substrate. Background Technology

[0002] Historically, printing sheets have been known to use paper or plastic sheets made of polyester, polypropylene, etc., as their base material. In particular, for applications requiring properties such as water resistance and tear strength, such as posters or outdoor printed materials, synthetic paper, which is obtained by adding inorganic fillers and a small amount of additives to plastic sheets or thermoplastic resins and molding them into sheets, is mainly used.

[0003] Furthermore, with environmental protection becoming an increasingly international issue, reducing the consumption of thermoplastics and paper materials is being widely discussed. From this perspective, it is advocated to incorporate inorganic powder into thermoplastics to form thermoplastic compositions, and these compositions are also being practically applied to printing sheets as described above (for example, see Patent Document 1, etc.).

[0004] Incidentally, especially when considering the use of sheets made of the aforementioned materials for printing applications, the surface of the plastic sheet is often modified to improve the adhesion or color development of the ink on the printing surface to the plastic sheet. Specifically, a coating, also known as an ink receiving layer or printing receiving layer, is formed by applying a coating liquid made of clay / latex or acrylic polymer (see, for example, Patent Documents 2-4, etc.).

[0005] However, when coating with the aforementioned clay / latex, the whiteness of the printed sheet is low due to the color of the clay components, and the color is sometimes less than ideal as a paper substitute. Furthermore, because the latex used contains polystyrene, it is highly susceptible to degradation by sunlight, especially ultraviolet light, and there is a problem of yellowing over time.

[0006] On the other hand, when coating conventional acrylic polymers as described in references 2 and 3, the hydrophilic acrylic polymer material added to achieve the antistatic effect is highly environmentally dependent, especially in low humidity conditions such as winter, where the antistatic effect decreases and static electricity is easily generated. If static electricity occurs, printing may experience jamming or the sheets may stick together, making this method undesirable. Furthermore, when coating acrylic polymers onto olefin-based plastic sheets such as polyethylene or polypropylene, the low polarity of the substrate plastic sheet can sometimes weaken the adhesion between the substrate plastic sheet and the coating liquid, depending on the manufacturing conditions. Even if coating is performed, peeling from the sheet to the coating interface may occur. In addition, if only acrylic polymers are coated, printability, especially in laser beam printers (LBP), suffers from low toner fixing properties, requiring high-temperature fixing. Consequently, the sheet melts due to heat, leading to frequent paper jams within the printing apparatus.

[0007] To illustrate, Patent Document 4 discloses the following: using a mixture of at least two acrylic polymer latexes having an acid value of 20-60 mg KOH / g and a Tg of less than 35 degrees Celsius, and at least one acrylic polymer latex having a Tg of greater than 90 degrees Celsius, as the acrylic polymer to improve adaptability to high-speed printing at low temperatures; incorporating hollow polymer particles into the coating liquid composition to improve the thermal insulation properties of the coating; and further incorporating silica particles with a primary particle size of less than 100 nm into the coating liquid composition to improve the smoothness of the coating. However, when these incorporations are implemented into the coating liquid composition, there is concern that the adhesion between the plastic sheet as the substrate and the coating liquid may be further reduced, and from the viewpoint of the aforementioned antistatic effect, there is no particular improvement.

[0008] Furthermore, Patent Document 5 discloses the following: Tetrapotassium pyrophosphate is used as a dispersant in a polymer adhesive composed of vinyl acetate, ethylene-acrylic acid, styrene-acrylic acid, or styrene-butadiene-acrylic acid polymers. A specified amount of titanium dioxide particles are added along with clay and calcium carbonate, and the mixture is then coated onto a substrate to obtain a sheet with high whiteness. While the addition of a specified amount of titanium dioxide particles does indeed improve whiteness, it does not significantly improve the aforementioned problems of substrate-coating adhesion, poor water resistance, or poor weather resistance of the coating.

[0009] Furthermore, Patent Document 6 proposes the following: A printing sheet with high smoothness is obtained by coating a composition onto a smooth substrate surface composed of fibers having a weighted average fiber length greater than 0.9 mm. The composition is made by incorporating pigments such as calcium carbonate precipitated aragonite, clay, and hollow spherical polyethylene pigments into an acrylic adhesive at a binder-pigment ratio of 100:15 to 100:40. However, while improved sheet smoothness can be expected when using such a composition, improvements in sheet whiteness, printability, substrate-coating adhesion, and antistatic properties are not anticipated.

[0010] Furthermore, Patent Document 7 discloses the following: As a coating composition applied to a substrate using a Direct Fountain Coater (DF) method to obtain printing sheets, a coating composition is shown containing clay with a high aspect ratio in a proportion of 5 to 30 parts by weight and calcium carbonate containing more than 90% particles smaller than 2 μm in a proportion of 70 to 95 parts by weight, thereby economically obtaining sheets with high whiteness. According to the technology shown in this document, the high proportion of calcium carbonate can be expected to improve the whiteness of the sheet, but on the other hand, there is concern that the high proportion of calcium carbonate may alter the texture of the sheet (increase the gloss of the printed matter). It should be noted that in Patent Document 7, SBR resin is used as the binder component of this coating composition, which suffers from the aforementioned problems of poor weather resistance or poor water resistance due to the presence of polystyrene components.

[0011] Patent document 8 discloses a printing sheet in which a material with the following composition is formed as an ink receiving layer on a substrate. This material is a material incorporating hard polymer particles, such as polymethyl methacrylate (PMMA), into a film-forming adhesive, or a material further incorporating particles such as kaolin, calcined clay, structured clay, heavy calcium carbonate, precipitated calcium carbonate, titanium dioxide, aluminum trihydride, satin white, silicon dioxide, zinc oxide, and barium sulfate. While incorporating PMMA particles can improve the adhesion between the substrate and the coating or enhance water resistance, it may also increase the matte finish of the sheet. Furthermore, when PMMA particles are primarily used as the particles incorporated into the receiving layer, for example, during LBP printing, the fixing properties with toner are insufficient, requiring fixing at high temperatures. Consequently, similar to the above, the sheet may melt due to heat, causing paper jams. Moreover, from an economic point of view, PMMA particles are also disadvantageous, leading to increased sheet prices.

[0012] Furthermore, Patent Document 9 discloses the following: For creating holograms, a sheet is used as the receiving layer, formed on a substrate, wherein the material is, for example, a polymeric pigment such as styrene-acrylic copolymer particles incorporated into an acrylic polymer, styrene-acrylic copolymer, etc., and further incorporated with precipitated calcium carbonate (PCC), heavy natural calcium carbonate, or kaolin or other clay. However, similar to the technology in Patent Document 8, using polymeric particles such as styrene-acrylic copolymer particles may increase the matte finish of the sheet, and furthermore, the adhesion of toner in LBP printing may be insufficient or may cause paper jams inside the printer. Moreover, from an economic point of view, polymeric particles are also disadvantageous, leading to an increase in the price of the sheet. Furthermore, due to the presence of styrene, there are also problems with moisture resistance.

[0013] Patent Document 10 discloses a structure for inkjet recording sheets comprising an ink receiving layer with amorphous silica and an adhesive as main components on one side of a substrate, and a back layer on the side of the substrate opposite to the side with the ink receiving layer. The back layer contains pigments selected from the group consisting of delaminated clay, kaolin, talc, calcium carbonate, and combinations of delaminated clay and talc, and a latex as an adhesive, such as a styrene-acrylic acid copolymer, a styrene-butadiene copolymer, or an ethylene-vinyl acetate copolymer. It should be noted that in this recording sheet, the back layer is formed by incorporating clay and calcium carbonate into the latex of the adhesive on the side opposite to the receiving layer. However, this back layer is provided to improve the sheet's processability and is not a configuration that solves the problems of the receiving layer sought to be addressed in this invention application. Furthermore, the presence of styrene as an adhesive component leads to moisture resistance issues.

[0014] Patent Document 11 discloses a structure in which a surface sizing layer composed of oxidized starch, a sizing agent, and an acrylic polymer is formed on the outer surface of a paper backing forming the surface of a gypsum board, and a coating is then applied to that surface. The coating is composed of at least one inorganic filler selected from calcium carbonate, clay, aluminum silicate, calcined clay, titanium dioxide, and mixtures thereof, and an adhesive such as a styrene-acrylic copolymer. Although it is described that decorative patterns can be printed on the coating in this document, the use of highly hydrophilic materials as adhesives, due to the requirement for water permeability when forming gypsum board from this paper backing, fails to solve the technical problems of poor weather resistance or poor water resistance, even when such a coating is applied to printing sheets like those of this invention.

[0015] Patent Document 12 discloses a coating film having an ink-absorbing layer, wherein the ink-absorbing layer is formed by containing at least three inorganic pigments, such as light calcium carbonate, clay, and silica, in a binder resin at a ratio of 2:1:2 to 2:3:2. Furthermore, Patent Document 13 discloses a coating composition with precipitated calcium carbonate as the main component and containing clay and a binder resin. However, printing sheets using coatings described in these patent documents, which contain more other inorganic pigments (especially light calcium carbonate) than clay, often fail to achieve sufficient gloss and exhibit poor printability.

[0016] Previously, in order to solve the aforementioned technical problems, the applicant explored the composition of single-sided or double-sided coatings on the substrate. The results showed that by incorporating clay and light calcium carbonate in a well-balanced, specified ratio into a coating composed of acrylic polymers, problems such as poor adhesion between the substrate and the coating, poor water resistance of the coating, yellowing due to poor weather resistance, and paper jams during the printing process caused by static electricity could be solved. This resulted in printing sheets with improved adaptability to LBP printing (fixing and transfer properties) or offset printing. This technology has been filed as a patent application and is published in Patent Document 14.

[0017] Existing technical documents

[0018] Patent documents

[0019] Patent Document 1: International Publication No. WO2014 / 109267

[0020] Patent Document 2: Japanese Patent Publication No. 7-20739

[0021] Patent Document 3: Japanese Patent Application Publication No. 2015-6793

[0022] Patent Document 4: International Publication No. WO2006 / 051092

[0023] Patent Document 5: Japanese Patent Application Publication No. 2014-189941

[0024] Patent Document 6: Japanese Patent Application Publication No. 2007-520642

[0025] Patent Document 7: International Publication No. WO2011 / 114456

[0026] Patent Document 8: Japanese Patent No. 2005-520065

[0027] Patent Document 9: Japanese Patent No. 2004-533922

[0028] Patent Document 10: Japanese Patent No. 4403131

[0029] Patent Document 11: Japanese Patent No. 2002-513873

[0030] Patent Document 12: Japanese Patent Application Publication No. 2001-225422

[0031] Patent Document 13: Japanese Patent Application Publication No. 2018-508607

[0032] Patent Document 14: Japanese Patent Application Publication No. 2021-011080 Summary of the Invention

[0033] The problem the invention aims to solve

[0034] Patent Document 14 describes a printing sheet with excellent printability, good adhesion between the substrate and coating, good antistatic properties that prevent paper jams during printing, and good water resistance and weather resistance, demonstrating excellent printability. However, in recent years, with the diversification of printing methods and the expansion of applications for printed sheets, there is a demand for further improvements in various physical properties, especially the adhesion / bonding between the substrate and coating.

[0035] This invention was made in view of the above-mentioned actual situation, and the problem it aims to solve is to provide a further improved printing sheet and a method for manufacturing the same. Furthermore, this invention aims to solve the problem of providing a printing sheet having a coating for receiving ink on at least one surface of a substrate, wherein it exhibits excellent printability, good antistatic properties to prevent paper jams during printing, and excellent water resistance, weather resistance, and other properties; particularly, it provides a printing sheet and a method for manufacturing the same with extremely good adhesion / bonding between the substrate and the coating.

[0036] Solution to the problem

[0037] The inventors further investigated the composition of coatings made of acrylic polymers and found that by using heavy calcium carbonate with a specific average particle size instead of light calcium carbonate and combining it with talc in a specified amount, the aforementioned problems could be solved, resulting in printing sheets with particularly excellent adhesion / bonding between the substrate and the coating.

[0038] That is, the present invention, which solves the above problems, is a printing sheet having a coating on one or both sides of a substrate. The coating is formed by adding clay in a continuous phase composed of an acrylic polymer at a ratio of 35% to 65% by mass and heavy calcium carbonate in a ratio of 5% to 30% by mass. In the printing sheet, the volume average particle size of the heavy calcium carbonate is 0.05 μm or more and 2.00 μm or less.

[0039] In one embodiment of the printing sheet of the present invention, a printing sheet is shown, wherein the substrate is a substrate containing polyolefin resin and inorganic powder in a mass ratio of 50:50 to 10:90.

[0040] In one embodiment of the printing sheet of the present invention, a printing sheet is shown, wherein the inorganic powder is calcium carbonate powder.

[0041] In one embodiment of the printing sheet of the present invention, a printing sheet is shown, wherein the volume average particle size of the clay is 0.05 μm or more and 2.00 μm or less.

[0042] In one embodiment of the printing sheet of the present invention, a printing sheet is shown, wherein the acrylic polymer is an alkyl (meth)acrylate.

[0043] In one embodiment of the printing sheet of the present invention, a printing sheet is shown, wherein the alkyl methacrylate is selected from one or more of the following groups: methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, nonyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, and 2-hydroxypropyl methacrylate.

[0044] In one embodiment of the printing sheet of the present invention, a printing sheet is shown that has a coating on one or both sides of a substrate. The coating is formed by adding clay in a continuous phase composed of an acrylic polymer at a ratio of 35% to 65% by mass and heavy calcium carbonate in a ratio of 5% to 30% by mass (excluding coatings where, when the total mass of the coating is set to 100% by mass, zinc pyrithione and / or zinc oxide with a volume average particle size of 0.05 μm to 2.00 μm is added in the continuous phase at a ratio of 0.40% to 2.00% by mass). In the printing sheet,

[0045] The volume average particle size of the heavy calcium carbonate is greater than 0.05 μm and less than 2.00 μm.

[0046] In one embodiment of the printing sheet of the present invention, a printing sheet is also shown, wherein the inorganic powder in the coating is substantially composed only of the clay and the heavy calcium carbonate.

[0047] The present invention, which solves the above problems, is a method for manufacturing a printing sheet, wherein an aqueous emulsion of an acrylic polymer is coated on one or both sides of a substrate. The aqueous emulsion of the acrylic polymer is formed by adding clay in a proportion of 35% to 65% by dry weight and heavy calcium carbonate with a volume average particle size of 0.05 μm to 2.00 μm in a proportion of 5% to 30% by dry weight to a continuous phase composed of an acrylic polymer.

[0048] In one embodiment of the method for manufacturing printing sheet of the present invention, a method for manufacturing printing sheet is shown, wherein a substrate containing polyolefin resin and inorganic powder in a mass ratio of 50:50 to 10:90 is extruded into a sheet, and the acrylic polymer aqueous emulsion is coated on one or both sides of the substrate sheet by a stretching process.

[0049] Invention Effects

[0050] According to the present invention, a printing sheet having a coating for receiving ink on at least one surface of a substrate is provided, which exhibits excellent printability, good antistatic properties to prevent paper jams during printing, and excellent water resistance, weather resistance, and other properties. Furthermore, according to the present invention, the printing sheet also exhibits good whiteness or texture (appropriate gloss), good ink drying speed for oil-based offset printing, UV offset printing, etc., and excellent offset printing adaptability. In addition, because the substrate and coating of the printing sheet of the present invention have excellent adhesion, coating peeling is less likely to occur during printing, subsequent bending or other processing, or during use. Therefore, it can be used in various printing / processing procedures, not only for general printed materials such as menus or posters, but also for food applications such as milk cartons, medical / hygiene applications such as collapsible cups, and containers for storing medicines. Detailed Implementation

[0051] The present invention will now be described in detail based on its embodiments.

[0052] <<Printing Sheets>>

[0053] The printing sheet of the present invention comprises a sheet-like substrate and a coating formed on at least one surface of the substrate. The coating comprises a coating formed by adding clay in a continuous phase composed of an acrylic polymer at a ratio of 35% to 65% by mass and heavy calcium carbonate with a volume average particle size of 0.05 μm to 2.00 μm at a ratio of 5% to 30% by mass.

[0054] It should be noted that for the printing sheet in this invention, as long as the printing sheet has the above-described coating on at least one surface of the substrate, there are no particular limitations on other components. For example, the following layers can be arbitrarily provided: an intermediate layer with a certain function between the substrate and the coating, such as a sealant layer for good adhesion between the substrate and the coating, an internal printing layer for imparting color and patterns on the printing sheet, a shielding layer, or a protective layer, an adhesive layer, etc., on the surface of the substrate without a coating, and a protective layer on the surface of the coating.

[0055] (1) Substrate

[0056] The material of the substrate in the printing sheet of the present invention is not particularly limited. It can be made of a plastic sheet with resin-based materials as the main component, a paper-based material, or synthetic paper. Furthermore, from the viewpoint of environmental protection and from the viewpoint of improving mechanical strength, heat resistance, and other properties, it is preferable to use a sheet made of a thermoplastic plastic containing inorganic powder, which is formed by filling a thermoplastic plastic with an inorganic powder in a high proportion. In particular, it is a sheet made of a thermoplastic plastic containing inorganic powder in a mass ratio of 50:50 to 10:90 containing polyolefin resin and inorganic powder.

[0057] (Resin composition)

[0058] There is no particular limitation on the resin constituting the plastic sheet or the thermoplastic plastic sheet incorporating inorganic powder. Various resins can be used depending on the purpose and function of the printing sheet. Examples include: polyethylene resins, polypropylene resins, polymethyl-1-pentene, ethylene-cyclic olefin copolymers, and other polyolefin resins; functionalized polyolefin resins such as ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, metal salts (ionomers) of ethylene-methacrylic acid copolymers, ethylene-alkyl acrylate copolymers, ethylene-alkyl methacrylate copolymers, maleic acid-modified polyethylene, and maleic acid-modified polypropylene; polyamide resins such as nylon-6, nylon-6,6, nylon-6,10, and nylon-6,12; and polyterephthalic acid (PTA). This includes aromatic polyester resins such as polyethylene glycol esters and their copolymers, polyethylene naphthalate, and polybutylene terephthalate; thermoplastic polyester resins such as polybutylene succinate and polylactic acid; polycarbonate resins such as aromatic polycarbonate and aliphatic polycarbonate; polystyrene resins such as atactic polystyrene, syndiotactic polystyrene, acrylonitrile-styrene (AS) copolymer, and acrylonitrile-butadiene-styrene (ABS) copolymer; polyvinyl chloride resins such as polyvinyl chloride and polyvinylidene chloride; polyphenylene sulfide; and polyether resins such as polyethersulfone, polyetherketone, and polyetheretherketone. These can be used alone or in combination of two or more.

[0059] Among these thermoplastic resins, polyolefin resins, aromatic polyester resins, and aliphatic polyester resins are preferred in terms of ease of molding, performance, and economy.

[0060] Here, polyolefin resin refers to a polyolefin resin in which olefin components are the main components. Specific examples include polypropylene resins and polyethylene resins as described above, as well as polymethyl-1-pentene, ethylene-cyclic olefin copolymers, and mixtures of two or more of these. It should be noted that "as the main component" means that the polyolefin resin contains 50% or more of olefin components by mass, preferably 75% or more by mass, more preferably 85% or more by mass, and even more preferably 90% or more by mass. It should be noted that the manufacturing method of the polyolefin resin used in this invention is not particularly limited, and it can be any method, such as using a Ziegler-Natta catalyst, a metallocene catalyst, oxygen, a peroxide, or other free radical initiator, to obtain the polyolefin resin.

[0061] As the polypropylene-based resin, resins in which the propylene component unit is 50% by mass or more can be listed, such as propylene homopolymers or copolymers of propylene with other α-olefins that can be copolymerized. Examples of other α-olefins that can be copolymerized with propylene include ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene, and 3-methyl-1-hexene, which have 4 to 10 carbon atoms. Propylene homopolymers include any one of linear or branched polypropylene exhibiting isotactic, syndiotactic, atactic, semi-isotactic, and various stereoregularities. Furthermore, the above copolymers can be random copolymers or block copolymers, and can be not only binary copolymers but also ternary copolymers. Specifically, examples include ethylene-propylene random copolymers, butene-1-propylene random copolymers, ethylene-butene-1-propylene random ternary copolymers, and ethylene-propylene block copolymers.

[0062] Furthermore, as the polyethylene-based resin, resins with an ethylene component of 50% or more by mass can be listed, such as: high-density polyethylene (HDPE), low-density polyethylene (LDPE), medium-density polyethylene, linear low-density polyethylene (LLDPE), ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-propylene-butene copolymer, ethylene-butene copolymer, ethylene-hexene copolymer, ethylene-4-methylpentene copolymer, ethylene-octene copolymer, etc., and mixtures of two or more thereof.

[0063] Among the polyolefin resins mentioned, polypropylene resins are preferred due to their particularly excellent balance between mechanical strength and heat resistance.

[0064] (Inorganic powder)

[0065] As mentioned above, when the substrate is a thermoplastic sheet incorporating inorganic powder, there are no particular limitations on the inorganic powder that can be incorporated into the sheet. Examples include: carbonates, sulfates, silicates, phosphates, borates, oxides, or hydrates of calcium, magnesium, aluminum, titanium, iron, zinc, etc. Specifically, examples include: calcium carbonate, magnesium carbonate, zinc oxide, titanium oxide, silicon dioxide, aluminum oxide, clay, talc, kaolin, aluminum hydroxide, magnesium hydroxide, aluminum silicate, magnesium silicate, calcium silicate, aluminum sulfate, magnesium sulfate, calcium sulfate, magnesium phosphate, barium sulfate, silica sand, carbon black, zeolite, molybdenum, diatomaceous earth, sericite, volcanic ash, calcium sulfite, sodium sulfate, potassium titanate, bentonite, graphite, etc. These can be synthetic or derived from natural minerals, and can be used alone or in combination of two or more.

[0066] Furthermore, there are no particular limitations on the shape of inorganic powder; it can be any shape, such as particle, flake, granule, or fibrous. In addition, as a particle, it can be spherical particles obtained by general synthesis methods, or it can be particles of irregular shape obtained by crushing natural minerals.

[0067] As for these inorganic powders, calcium carbonate, magnesium carbonate, zinc oxide, titanium oxide, silicon dioxide, aluminum oxide, clay, talc, kaolin, aluminum hydroxide, magnesium hydroxide, etc. are preferred, with calcium carbonate powder being particularly preferred. Furthermore, as calcium carbonate, it can be either so-called light calcium carbonate prepared by synthesis or so-called heavy calcium carbonate obtained by mechanically crushing and classifying natural raw materials such as limestone with CaCO3 as the main component, or a combination of them, but from an economic point of view, heavy calcium carbonate is preferred.

[0068] Here, heavy calcium carbonate refers to calcium carbonate obtained by mechanically crushing / processing natural limestone, etc., and is clearly distinguished from synthetic calcium carbonate manufactured through chemical precipitation reactions, etc. It should be noted that there are dry and wet crushing methods, but from an economic point of view, the dry method is preferred.

[0069] Furthermore, to improve the dispersibility or reactivity of inorganic powders, the surface of the inorganic powders can be pre-modified using conventional methods. Examples of surface modification methods include physical methods such as plasma treatment, and chemical surface treatment methods using coupling agents or surfactants. Examples of coupling agents include silane coupling agents or titanium coupling agents. Surfactants can be any type of surfactant, including anionic, cationic, nonionic, and amphoteric surfactants, such as higher fatty acids, higher fatty acid esters, higher fatty acid amides, and higher fatty acid salts.

[0070] The inorganic powder is preferably composed of particles, with an average particle size preferably between 0.1 μm and 50.0 μm, more preferably between 1.0 μm and 10.0 μm, and even more preferably between 1.0 μm and 5.0 μm. It should be noted that the average particle size of the inorganic powder described in this specification refers to a value calculated from the specific surface area measured according to the air permeation method of JIS M-8511. As the measuring instrument, for example, the specific surface area measuring device SS-100 manufactured by Shimadzu Corporation is preferably used. It is particularly preferable that the particle size distribution does not contain particles larger than 50.0 μm. On the other hand, if the particles become too fine, the viscosity will increase significantly when mixed with the aforementioned thermoplastic resin, potentially making the manufacture of the molded article difficult. Therefore, the average particle size is preferably set to 0.5 μm or more.

[0071] Inorganic powders can be fibrous, powdery, flake-like, or granular.

[0072] The average fiber length of the fibrous inorganic powder is preferably 3.0 μm or more and 20.0 μm or less. The average fiber diameter is preferably 0.2 μm or more and 1.5 μm or less. Furthermore, the aspect ratio is typically 10 or more and 30 or less. It should be noted that the average fiber length and average fiber diameter of the fibrous inorganic powder are measured using an electron microscope, and the aspect ratio is the ratio of the average fiber length to the average fiber diameter (average fiber length / average fiber diameter).

[0073] When the substrate is a thermoplastic sheet containing inorganic powder as described above, the ratio (by mass%) of the thermoplastic resin to the inorganic powder contained therein is preferably 50:50 to 10:90, more preferably 40:60 to 20:80, and even more preferably 40:60 to 25:75. This is because if the proportion of inorganic powder in the thermoplastic resin to inorganic powder ratio is less than 50% by mass, the desired texture, impact resistance, and other physical properties of the thermoplastic resin composition containing inorganic powder cannot be obtained, while if it is greater than 90% by mass, molding processes such as extrusion molding and vacuum forming become difficult.

[0074] (Other additives)

[0075] Furthermore, when the substrate is the aforementioned plastic sheet or the aforementioned thermoplastic sheet incorporating inorganic powder, other additives may be added to its composition as needed as auxiliary agents. Examples of such additives include: plasticizers, colorants, lubricants, coupling agents, flow improvers, dispersants, antioxidants, UV absorbers, flame retardants, stabilizers, antistatic agents, and foaming agents. These additives may be used individually or in combination of two or more.

[0076] (Paper-based materials)

[0077] Specific examples of paper-based materials include glassine paper, coated paper, premium paper, dust-free printing paper, impregnated paper, and laminated paper on which thermoplastic resins such as polyethylene are laminated.

[0078] (Base material composition)

[0079] The substrate can be composed of a single sheet made of the aforementioned materials, or it can be composed of multiple layers. Furthermore, when the substrate is the plastic sheet or the thermoplastic sheet incorporating inorganic powder, the sheet can be an unstretched sheet, or a sheet stretched along a uniaxial or biaxial direction, such as longitudinal or transverse.

[0080] There is no particular limitation on the thickness of the substrate, but it is usually 10 μm or more and 300 μm or less, preferably 25 μm or more and 200 μm or less.

[0081] Furthermore, when using a substrate made of plastic sheet or thermoplastic sheet incorporating inorganic powder, surface treatment can be performed on one or both sides as needed, such as by oxidation or texturing, to improve adhesion to the coating applied to its surface. Examples of oxidation methods include corona discharge treatment, flame treatment, plasma treatment, glow discharge treatment, wet chromic acid treatment, hot air treatment, and ozone / ultraviolet irradiation treatment. Examples of texturing methods include sandblasting and solvent treatment. Additionally, a primer treatment may also be applied.

[0082] (2) Coating

[0083] The coating of the printing sheet of the present invention can be provided on only one side of the substrate or on both sides. There is no particular limitation on the thickness of the coating; for example, it is preferably 1 μm or more and 10 μm or less, more preferably 2 μm or more and 8 μm or less, and particularly preferably 3 μm or more and 5 μm or less. If the thickness is within this range, the coating functions fully as an ink receiving layer, exhibiting good ink receiving characteristics such as colorability and color development. Furthermore, the water resistance, surface antistatic properties, and ink adhesion of the printing sheet also become good.

[0084] Therefore, in this invention, the coating is formed by adding clay and heavy calcium carbonate to the continuous phase of an acrylic polymer that forms the matrix in the following proportions: clay is added in a proportion of 35% by mass or more and 65% by mass or less, more preferably 38% by mass or more and 60% by mass or less, and even more preferably 40% by mass or more and 55% by mass or less; and heavy calcium carbonate with a volume average particle size of 0.05 μm or more and 2.00 μm or less is added in a proportion of 5% by mass or more and 30% by mass or less, more preferably 7% by mass or more and 27% by mass or less, and even more preferably 9% by mass or more and 25% by mass or less.

[0085] By ensuring that clay and heavy calcium carbonate of a specified particle size are present in a balanced manner in the continuous phase composed of acrylic polymers in the coating, the water resistance and whiteness of the printing sheet are maintained at the desired levels. Simultaneously, the sheet surface does not lose its smoothness, forming fine irregularities, thus achieving improved weather resistance due to the scattering of incident light. Furthermore, the presence of clay in the continuous phase of acrylic polymers reduces resistivity, particularly surface resistivity, enabling toner transfer even without applying high voltage during LBP printing. Moreover, the presence of heavy calcium carbonate in the continuous phase of acrylic polymers mitigates yellowing caused by clay, maintaining whiteness. Additionally, oil resistance is improved, ink drying speed is enhanced, and adaptability to oil-based offset printing is improved. Furthermore, in the printing sheet of the present invention, since the combination of heavy calcium carbonate and clay of the specified particle size is well-balanced in the coating, the adhesion / bonding between the substrate and the coating is extremely good.

[0086] Although this invention is not limited to any particular theory, the effectiveness of this invention is believed to stem from the anchoring effect resulting from the specified particle size of heavy calcium carbonate described above. As previously mentioned, since heavy calcium carbonate is obtained by mechanically crushing / processing natural limestone, it generally consists of irregularly shaped particles with more irregularities compared to light calcium carbonate of the same particle size. It is possible that by incorporating heavy calcium carbonate with a specific volume average particle size into such a coating along with clay, physical irregularities are formed on the coating surface, thereby improving adhesion at the bonding interface with the substrate through an anchoring effect.

[0087] Furthermore, it is believed that: the presence of clay (and heavy calcium carbonate) on the coating surface creates fine irregularities on the sheet surface, which improves weather resistance due to the scattering of incident light; the presence of heavy calcium carbonate in addition to clay on the coating surface ensures adequate water resistance; and the presence of clay provides good antistatic properties. It is further speculated that: by incorporating clay and heavy calcium carbonate in a well-balanced, prescribed ratio into the coating, it is possible to improve the printability (fixing and transfer properties) and offset printing suitability of the resulting printing sheet for LBP.

[0088] As explained in the comparative examples described later and in the aforementioned Patent Document 14, if heavy calcium carbonate with a volume average particle size greater than 2.00 μm is used, the adhesion between the substrate and the coating decreases compared to using the same amount of light calcium carbonate. This is why light calcium carbonate was chosen along with talc in the invention of Patent Document 14. However, upon further investigation, it was found that if heavy calcium carbonate with a volume average particle size of 0.05 μm or more but less than 2.00 μm is used, high adhesion is observed even compared to using light calcium carbonate with the same particle size. When using heavy calcium carbonate with a large particle size greater than 2.00 μm, the effect of the unevenness of the particle surface is difficult to achieve, and a decrease in adhesion occurs due to particle shedding. In contrast, when using heavy calcium carbonate with the aforementioned volume average particle size, the particles do not shed, and the effect of the small size and amorphous shape is significantly apparent. This unexpected effect may be due to this.

[0089] If the amount of clay added to the coating is less than the above-mentioned range, the reduction in surface resistivity of the sheet due to the added clay will be insufficient, and the resistivity will increase. For example, when used for LBP printing, the adhesion of the toner may decrease, the transferability may also decrease, and the adhesion to the substrate may also decrease. On the other hand, if the amount of clay added exceeds the above-mentioned range, although the transferability when used for LBP printing will be good, the water resistance of the coating will decrease, which will have an adverse effect on the appearance. It is also possible that the coating cannot be formed into a continuous layer with sufficient strength.

[0090] Furthermore, by keeping the amount of heavy calcium carbonate within the aforementioned range, the whiteness of the sheet can be maintained at the desired level. That is, although the coating of the printing sheet of the present invention contains clay at a proportion of 30% by mass or more as described above, the yellowish tint of the sheet is reduced (b...). *While the whiteness may increase, as described above, incorporating heavy calcium carbonate at a proportion of 5% or more by mass effectively improves whiteness. Furthermore, by keeping the amount of heavy calcium carbonate within the aforementioned range, appropriate oil resistance is achieved, improving the quick-drying properties of oil-based inks and enhancing the adaptability to oil-resistant printing plates. Furthermore, incorporating heavy calcium carbonate at a proportion of 30% or less by mass can smooth the surface of the coating and improve gloss. It should be noted that if the proportion exceeds 30% by mass, gloss decreases. From the viewpoint of improving gloss, a proportion of 25% or less by mass is particularly preferred, and 20% or less by mass is even more preferred.

[0091] To explain, the coating of the printing sheet of the present invention is formed by mixing clay and heavy calcium carbonate in a specified ratio as described above. Therefore, by appropriately adjusting the amount and particle size of the clay and heavy calcium carbonate and the thickness of the formed coating within a specified range, the surface of the coating can be made into a matte surface with a certain degree of roughness or a glossy surface with improved gloss.

[0092] The components that form the coating of the present invention will now be described in detail.

[0093] (Acrylic polymers that form a continuous phase)

[0094] Acrylic polymers that serve as coating matrices include polymers obtained with (meth)acrylic acid, (meth)acrylates, (meth)acrylamides, and (meth)acrylonitrile as the main monomer components. It should be noted that the term "(meth)acrylic acid" as used in this specification is used to include both "acrylic acid" and "methacrylic acid".

[0095] More specifically, there is no particular limitation on the monomer components constituting the acrylic polymer, and one or more of the following components may be selected:

[0096] Acrylic acid, methacrylic acid;

[0097] For example, alkyl acrylates with 1 to 18 carbon atoms, such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, hexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, nonyl acrylate, lauryl acrylate, stearyl acrylate, palmityl acrylate, or cyclohexyl acrylate.

[0098] For example, alkyl methacrylates with 1 to 18 carbon atoms, such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, hexyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate, nonyl methacrylate, lauryl methacrylate, stearyl methacrylate, palmityl methacrylate, and cyclohexyl methacrylate;

[0099] For example, alkyl esters with hydroxyl groups on the side chain of (meth)acrylic acid, such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, and monohydroxyethyl acrylate of phthalate;

[0100] For example, polyethylene glycol (n preferably 3 or more and 20 or less) diacrylates having ethylene glycol units in the molecule, trimethylolpropane EO-modified (n preferably 3 or more and 20 or less) triacrylates, and phenol EO-modified (n preferably 3 or more and 20 or less) acrylates.

[0101] For example, allyloxyethyl acrylate or allyloxyethyl methacrylate, etc., are alkenyloxyalkyl esters of (meth)acrylic acid.

[0102] For example, methoxybutyl acrylate, methoxybutyl methacrylate, methoxyethyl acrylate, methoxyethyl methacrylate, ethoxybutyl acrylate, ethoxybutyl methacrylate, etc. are alkyl esters with alkoxy groups on the side chain of (meth)acrylic acid.

[0103] For example, alkenyl esters of (meth)acrylic acid such as allyl acrylate or allyl methacrylate;

[0104] For example, glycidyl acrylate, glycidyl methacrylate, and methyl glycidyl acrylate or methyl glycidyl methacrylate are alkyl esters with an epoxy group on the side chain of acrylic acid.

[0105] For example, mono- or dialkylaminoalkyl esters of (meth)acrylic acid such as diethylaminoethyl acrylate, diethylaminoethyl methacrylate, methylaminoethyl acrylate, and methylaminoethyl methacrylate.

[0106] For example, organosilicon-modified (meth)acrylates with silyl groups, alkoxysilyl groups, or hydrolyzable alkoxysilyl groups as side chains;

[0107] For example, acrylamide, methacrylamide;

[0108] For example, (meth)acrylamides containing hydroxymethyl groups, such as N-hydroxymethylacrylamide and N-hydroxymethylmethacrylamide;

[0109] For example, (meth)acrylamides having alkoxyhydroxymethyl groups, such as N-alkoxyhydroxymethylacrylamide (e.g., N-isobutoxyhydroxymethylacrylamide, etc.) and N-alkoxyhydroxymethylmethacrylamide (e.g., N-isobutoxyhydroxymethylmethacrylamide, etc.);

[0110] For example, (meth)acrylamides having an alkoxyalkyl group, such as N-butoxymethylacrylamide or N-butoxymethylacrylamide; and

[0111] Acrylonitrile, methacrylonitrile, and other acrylic monomers can also be listed as monomeric components constituting the acrylic resin.

[0112] Furthermore, when introducing cross-linking structures into acrylic polymers through photocuring reactions or other methods to improve the film strength of the coating, bifunctional or multifunctional acrylic monomers can also be incorporated. Specifically, examples include 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, the aforementioned polyethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate with hydroxypentanoic acid, dicyclopentyl di(meth)acrylate, caprolactone-modified dicyclopentenyl di(meth)acrylate, ethylene oxide-modified di(meth)acrylate, and allylated di(meth)acrylate. Hexyl ester, di(meth)acrylate of isocyanurate, trimethylolpropane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, propionic acid modified dipentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propylene oxide modified trimethylolpropane tri(meth)acrylate, tri(acryloyloxyethyl)isocyanurate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, propionic acid modified dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, caprolactone modified dipentaerythritol hexa(meth)acrylate and other multifunctional (meth)acrylates.

[0113] These monomeric components can be used alone or in combination.

[0114] That is, in this invention, the acrylic polymer constituting the continuous phase of the coating can be a homopolymer composed only of any one of the various monomer components in the above examples, or it can be a copolymer composed of a combination of various monomer components in the above examples.

[0115] Furthermore, in one embodiment of the present invention, a copolymer containing other monomer components besides the aforementioned monomer components can be used as an acrylic polymer.

[0116] For monomer components other than those listed above, any monomer that forms a copolymer with the aforementioned monomer components is acceptable and is not particularly limited. Examples include: vinyl acetate, vinyl chloride, vinylidene chloride, vinyl lactate, vinyl butyrate, vinyl tert-carbonate, vinyl benzoate, and other ethylene-based monomers, ethylene, butadiene, styrene, etc. However, from the viewpoint of the weather resistance of the resulting sheet, it is preferable to have no styrene.

[0117] It should be noted that the method for forming a coating in the printing sheet of the present invention is not particularly limited, but from the viewpoint of coatability when forming such a coating, it is generally preferred to use it in a form dispersed in water / or dissolved in an organic solvent, and particularly preferred to use it in a form dispersed in water, that is, in the form of an aqueous emulsion of an acrylic polymer. Therefore, as the above-mentioned acrylic polymer, it is preferable to have an aqueous emulsion form in the stage of forming the coating.

[0118] Emulsification polymerization in the manufacture of aqueous emulsions of acrylic polymers is well known to those skilled in the art. Two or more anionic, cationic, amphoteric, and nonionic surfactants can be used alone or in combination as surfactants in the emulsification polymerization. Among these, nonionic and cationic surfactants are preferred. For nonionic surfactants, there are no particular limitations; examples include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, dehydrated sorbitan alkyl esters, and polyoxyethylene dehydrated sorbitan alkyl esters. For cationic surfactants, there are no particular limitations; examples include dodecyltrimethylammonium chloride, stearyltrimethylammonium chloride, and N-2-ethylhexylpyridine chloride, but nonionic surfactants are most preferred. Furthermore, polyoxyethylene alkylphenyl ethers are particularly preferred among these. The amount of surfactant is not particularly limited, but it is generally preferred to use 1 to 5% by mass of the total monomer.

[0119] Furthermore, as a protective colloid agent, it can be combined with water-soluble polymers such as gelatin and polyvinyl alcohol.

[0120] In addition, examples of free radical polymerization initiators used in emulsion polymerization include: persulfates such as potassium persulfate and ammonium persulfate, water-soluble initiators such as hydrogen peroxide water, tert-butyl hydroperoxide, and hydrochloride of azobis(amidine)propane, and oil-soluble initiators such as benzoyl peroxide, diisopropyl peroxide dicarbonate, cumyl peroxynedecanoate, cumyl peroxyoctanoate, and azobis(isobutyronitrile) ...

[0121] The polymerization reaction is not particularly limited, but it is usually carried out at a temperature of 35–90°C under stirring, and the reaction time is usually 3–40 hours. Furthermore, by adding an alkaline substance at the beginning or end of the emulsion polymerization to adjust the pH, the storage stability, freezing stability, and chemical stability of the emulsion can be improved. In this case, the pH of the resulting emulsion is preferably adjusted to 5–9, and alkaline substances such as ammonia, ethylamine, diethylamine, triethylamine, ethanolamine, triethanolamine, dimethylethanolamine, sodium hydroxide, and potassium hydroxide can be used for this purpose.

[0122] Although not particularly limited, alkyl (meth)acrylates can be preferably used as an example of an acrylic polymer that serves as the coating matrix.

[0123] (clay)

[0124] Furthermore, in this invention, in the continuous phase of the acrylic polymer that forms the matrix, heavy calcium carbonate (described later) is combined with clay in a predetermined ratio.

[0125] The clay used in this invention is not particularly limited, and any known clay may be used appropriately. It should be noted that, in this specification, "clay" includes not only clay minerals with a layered structure, but also clay minerals without a layered structure, such as illite or diaspore. Examples of clay minerals with a layered structure include: smectite, vermiculite, montmorillonite, bentonite, illite, lithium montmorillonite, halloysite, soapstone, bedesite, smectite, chlorite, mica, brittle mica, sericite, glauconite, hydrotalcite, and other expansive minerals; and kaolinite minerals (kaolinite, kaolinite), serpentine, pyrophyllite, talc, chlorite, zeolite, and other non-expanding minerals. Furthermore, examples of such clay include natural clay, synthetic clay, and organic clay.

[0126] It should be noted that the term "organized clay" is not particularly limited and can include any known organized clay, but preferably organic clay formed by organicating clay with an organicating agent. The clay prior to such organication is not particularly limited, as long as it is a so-called clay mineral, and can be any clay as in the examples above. Furthermore, such clay can be natural or synthetic.

[0127] Furthermore, there are no particular limitations on the organicating agent, and known organicating agents capable of organicating clay can be appropriately used. For example, hexylammonium ions, octylammonium ions, 2-ethylhexylammonium ions, dodecylammonium ions, laurylammonium ions, octadecylammonium ions, dioctyldimethylammonium ions, trioctylammonium ions, di(octadecyl)dimethylammonium ions, tri(octadecyl)ammonium ions, etc., can be used.

[0128] There are no particular limitations on the clay used in this invention, but kaolin clay is particularly preferred from the viewpoint of uniform dispersion in the coating.

[0129] Furthermore, there is no particular limitation on the particle size of the clay, although it is somewhat influenced by the thickness of the coating to be formed. However, the volume average particle size is preferably 0.05 μm or more and 2.00 μm or less, more preferably 0.1 μm or more and 1.7 μm or less, particularly preferably 0.2 μm or more and 1.5 μm or less, and even more preferably 1.0 μm or less. If the clay has such a particle size range, it can be well dispersed in the formed coating, and the desired effects, such as improved substrate adhesion, improved printability, and antistatic properties, can be better achieved.

[0130] It should be noted that the shape of the clay is not particularly limited and can be any shape, such as spherical, ellipsoidal, flat, or irregular. However, from the viewpoint of uniform dispersion in the coating, a shape close to spherical is preferred. From this viewpoint, the aspect ratio of the clay is preferably 5 or less, more preferably 3 or less, and even more preferably 2 or less. The above aspect ratio refers to the major axis / minor axis.

[0131] Furthermore, for clay, from the viewpoint of achieving uniform in-plane properties of the coating, uniform particle size is desirable.

[0132] Furthermore, there is no particular limitation on the specific gravity of the clay, but from the viewpoint that it can be evenly dispersed in the overall coating, it is preferably 1.5 to 3.0, and more preferably 2.0 to 2.8.

[0133] (Heavy calcium carbonate)

[0134] In this invention, the coating, together with the aforementioned clay, contains heavy calcium carbonate with a specified particle size. As mentioned earlier, "heavy calcium carbonate" refers to calcium carbonate manufactured by mechanically crushing and classifying natural raw materials such as limestone, which have CaCO3 as the main component, and is clearly distinguished from light calcium carbonate (synthetic calcium carbonate) manufactured through chemical precipitation reactions, etc.

[0135] As described above, the heavy calcium carbonate used in this invention has a volume average particle size of 0.05 μm or more and 2.00 μm or less. The volume average particle size is preferably 0.07 μm or more and 1.50 μm or less, more preferably 0.10 μm or more and 1.00 μm or less, and particularly preferably 0.20 μm or more and 0.50 μm or less. Using heavy calcium carbonate with such a particle size range not only improves the desired whiteness of the sheet for printing, but also enhances the smoothness of the coating and the gloss of the sheet.

[0136] (Other additives)

[0137] In this invention, the coating may also contain other components such as additives not mentioned above.

[0138] Specific examples of additives include: crosslinking agents, pH adjusters, thickeners, flow improvers, defoamers, antifoaming agents, surfactants, release agents, penetrants, coloring pigments, coloring dyes, fluorescent whitening agents, ultraviolet absorbers, antioxidants, preservatives, mildew inhibitors, antibacterial agents such as zinc pyrithione, water-resistant agents, ink fixers, curing agents, and weather-resistant materials. It should be noted that when the overall mass of the coating is set as 100% by mass, the individual amounts of these additives, primarily the antibacterial agent, are preferably less than 0.40% by mass, more preferably less than 0.30% by mass, and particularly preferably less than 0.20% by mass. Regarding antibacterial agents such as zinc pyrithione, it is also possible to completely exclude these antibacterial agents from the coating.

[0139] Examples of crosslinking agents include: aldehyde compounds, melamine compounds, isocyanate compounds, zirconium compounds, titanium compounds, amide compounds, aluminum compounds, boric acid, borates, carbodiimide compounds, and oxazoline compounds.

[0140] Furthermore, as an ink fixer, a cationic resin other than an acrylic resin or a polyvalent metal salt is preferred. Examples of cationic resins include: polyethyleneimine resins, polyamine resins, polyamide resins, polyamide-epimeric alcohol resins, polyamine-epimeric alcohol resins, polyamide-polyamine-epimeric alcohol resins, polydiallylamine resins, and dicyandiamide condensates. Examples of polyvalent metal salts include: calcium compounds, magnesium compounds, zirconium compounds, titanium compounds, and aluminum compounds. Among these, calcium compounds are preferred, and calcium nitrate tetrahydrate is more preferred.

[0141] There are no particular limitations on defoamers; for example, mineral oil-based, polyether-based, and silicone-based defoamers can be used, with mineral oil-based defoamers being preferred. For hydrophobic silica-type mineral oil-based defoamers, although not limited to the following, examples include: NOPCO 8034, NOPCO 8034-L, SN-DEFOAMER AP, SN-DEFOAMER H-2, SN-DEFOAMER TP-33, SN-DEFOAMER VL, SN-DEFOAMER 113, SN-DEFOAMER 154, SN-DEFOAMER 154S, SN-DEFOAMER 313, SN-DEFOAMER 314, SN-DEFOAMER 316, SN-DEFOAMER 317, SN-DEFOAMER 318, SN-DEFOAMER 319, SN-DEFOAMER 321, SN-DEFOAMER 323, SN-DEFOAMER 364, SN-DEFOAMER 414, SN-DEFOAMER 456, SN-DEFOAMER 474, SN-DEFOAMER 476-L, SN-DEFOAMER 480, SN-DEFOAMER 777, SN-DEFOAMER 1341, SN-DEFOAMER 1361 (manufactured by S.N.C.), BYK-1740 (manufactured by BYK Chemical Company), etc.; For mineral oil-based defoamers of metallic soaps, although not limited to the following, examples include: NOPCO DF-122, NOPCO DF-122-NS, NOPCO NDW, NOPCO NXZ, SN-DEFOAMER 122-SV, SN-DEFOAMER 269, SN-DEFOAMER 1010 (manufactured by S.N.C.), etc.; For amide wax-type mineral oil-based defoamers, although not limited to the following, examples include: NOPCO 267-A, NOPCO DF-124-L, SN-DEFOAMER, etc. TP-39, SN-DEFOAMER 477T, SN-DEFOAMER 477-NS, SN-DEFOAMER 479, SN-DEFOAMER 1044, SN-DEFOAMER 1320, SN-DEFOAMER 1340, SN-DEFOAMER 1360, and SN-DEFOAMER 5100 (manufactured by Sanopco Co., Ltd.). These can be used alone or in combination of two or more. It should be noted that there is no particular limitation on the amount of defoamer used, but it is preferably 0.01 to 0.03% by mass relative to the total amount of coating liquid forming the coating.

[0142] It should be noted that the coating of the present invention may also contain microparticles other than the clay and heavy calcium carbonate of the specified particle size, as long as it does not impair the excellent effects described above resulting from the combined use of these clays and heavy calcium carbonate within the specified amount range. However, it is preferable that it is substantially free of other microparticles, such as zinc oxide. In particular, it is preferable that it is substantially free of polymer microparticles such as (meth)acrylate resin particles, represented by polymethyl methacrylate particles, or zinc oxide with a volume average particle size of 0.05 μm or more and 2.00 μm or less. Even if it is contained, it is preferable that it is less than 0.10% by mass in the coating. If the coating is substantially composed of only clay and heavy calcium carbonate of the specified particle size as microparticles (powder) and free of microparticles other than unavoidable impurities, the adhesion to the substrate or printability becomes particularly good.

[0143] Furthermore, for example, when using an acrylic polymer aqueous emulsion as the acrylic polymer matrix, the coating solution for forming the coating can be prepared as follows: The clay or its dispersion in water, and the heavy calcium carbonate or its dispersion in water, are added to the dispersion medium of the acrylic polymer aqueous emulsion, i.e., water. Dispersion is typically carried out for 1 to 5 minutes using a suitable mixer or disperser, such as a wet colloid mill, a bladed turbine, or a paddle mill, at a rotation speed of 500 to 3000 rpm. It should be noted that if the clay and heavy calcium carbonate are directly added to the acrylic polymer aqueous emulsion, aggregation may occur. Therefore, it is preferable to add the acrylic polymer aqueous emulsion to a pre-mixed dispersion of clay and / or heavy calcium carbonate in a medium such as water, as needed. It should be noted that since clay and heavy calcium carbonate are substances with essentially the same specific gravity, the clay and heavy calcium carbonate will not exist unevenly in the dispersion; they can be dispersed in a uniformly distributed state.

[0144] <Method for Manufacturing Printing Sheets>

[0145] As a method for manufacturing the printing sheet of the present invention, a known method for forming a coating on the surface of a substrate can be used. For example, it can be carried out by applying a coating liquid composed of an acrylic polymer aqueous emulsion to one or both sides of the substrate by a suitable method such as roller coating, blade coating, rod coating, brush coating, spraying, or dipping, and then drying and curing the coating. The acrylic polymer aqueous emulsion is prepared by adding clay in a proportion of 35% to 65% by dry weight and heavy calcium carbonate with a volume average particle size of 0.05 μm to 2.00 μm in a proportion of 5% to 30% by dry weight. There are no particular limitations on the temperature conditions for drying or curing the coating; for example, it can be carried out at a temperature of 90 to 120°C.

[0146] To illustrate, in embodiments using sheets made of thermoplastic plastic containing inorganic powder as the substrate, the following steps can be taken: The substrate containing polyolefin resin and inorganic powder in a mass ratio of 50:50 to 10:90 is extruded into a sheet, and through a stretching process, a coating liquid composed of an acrylic polymer aqueous emulsion is applied to one or both sides of the substrate sheet using an appropriate method. The coating is then dried and cured. The acrylic polymer aqueous emulsion is prepared by incorporating clay at a ratio of 35% to 65% by dry mass and heavy calcium carbonate with a volume average particle size of 0.05 μm to 2.00 μm at a ratio of 5% to 30% by mass. When mixing inorganic powder with polyolefin resin during the molding of sheets made of thermoplastic plastic incorporating inorganic powder, the polyolefin resin and inorganic powder can be melted and mixed before being fed from the hopper to the molding machine, or they can be melted and mixed during molding in the molding machine. The same applies to other additives besides inorganic powder. Furthermore, melt mixing is preferably performed while applying high shear stress to the polyolefin resin to uniformly disperse the inorganic powder within it; for example, a biaxial mixer is preferred. When the aforementioned inorganic powder is incorporated into the polyolefin resin, higher temperatures increase the likelihood of odor generation. Therefore, an embodiment where the treatment is carried out at a temperature below +55°C is preferred, more preferably above and below +55°C, and even more preferably above +10°C and below +45°C.

[0147] It should be noted that, for extrusion molding into sheet form, it is preferable to mold at the same temperature.

[0148] Furthermore, there are no particular limitations on the stretching process when the sheet is formed. Stretching can be performed during or after forming along a uniaxial or biaxial direction, or along multiple directions (such as using a tubular method). For biaxial stretching, it can be performed sequentially or simultaneously.

[0149] If the formed sheet is stretched (e.g., longitudinally and / or laterally), the density of the sheet decreases. This decrease in density results in better whiteness of the sheet.

[0150] Example

[0151] The invention will now be described in more detail with reference to embodiments. It should be noted that these embodiments are described only for the purpose of illustrating specific forms and implementations, in order to more readily understand the concept and scope of the invention disclosed in this specification and set forth in the appended claims, and the invention is not limited to these embodiments in any way.

[0152] (Evaluation Method)

[0153] The physical property values ​​in the following examples and comparative examples were evaluated using the following methods.

[0154] (Adhesion)

[0155] To examine the adhesion of the coating to the substrate, a peel test based on cellophane tape was conducted.

[0156] • Measurement tape

[0157] Cellophane tape based on JIS Z1522:2009 (width: 24mm)

[0158] • Measurement Steps - 1

[0159] (1) Take out the tape to a length of about 75mm.

[0160] (2) Apply the tape to the sheet to be tested and rub it with your fingers until it is visible from the back. Note that you should press down with your fingertips, not your nails.

[0161] (3) Within 5 minutes of applying the tape, lift the end of the tape so that the tearing direction is at an angle of about 60° to the coating, and tear it reliably within 0.5 to 1.0 seconds. Observe the peeled surface and visually inspect whether the coating layer is adhered. Evaluate the adhesion between the coating and the substrate based on the following evaluation criteria.

[0162] Evaluation Criteria

[0163] ○ No coating peeling at all.

[0164] △ Coating peeling is less than 20%.

[0165] × Coating peeling is over 20%.

[0166] • Measurement Steps - 2

[0167] For samples with an evaluation result of 0, repeat the above operations (1) to (3) 5 times (6 times for pasting / peeling each). Samples with no coating peeling are evaluated as "◎".

[0168] (Surface resistivity)

[0169] The determination was conducted according to JIS K 6911:2006. The determination was performed using a 100 mm square sheet as the sample, under the following conditions.

[0170] Temperature 23℃, humidity 50%

[0171] (Whiteness)

[0172] Immediately after the printing sheet is manufactured, visually inspect the condition of the coating surface and compare it with L. * a * b * Comparison of color space chromaticity diagrams to determine whiteness (b) * ).

[0173] (LBP Printing Adaptability)

[0174] To examine the LBP printing adaptability of printing sheets, color and black-and-white test patterns were printed on each sheet using a laser printer (product name: Versant80Press, manufactured by Fuji Xerox Co., Ltd.), and the fixing properties of the toner were visually observed. The printing adaptability was evaluated based on the following evaluation criteria.

[0175] Evaluation Criteria

[0176] ○ The test pattern was printed clearly, with no toner peeling at all.

[0177] △The test pattern was printed well, but slight toner peeling occurred.

[0178] × The test pattern was not printed smoothly, and significant toner peeling occurred.

[0179] (Adaptability to offset printing)

[0180] To examine the offset printing suitability of printing sheets, test patterns were printed on each sheet using oil-based offset inks on an offset printing press (product name: RMGT920, manufactured by Ryobi Shinmyo Printing Machine Co., Ltd.). The printing bleeding, misalignment, and fixing properties were observed visually. The offset printing suitability (ink drying speed) was evaluated based on the following evaluation criteria.

[0181] Evaluation Criteria

[0182] ○ The test pattern was printed clearly, without any ink bleeding or misalignment.

[0183] △The test pattern was printed well, but slight bleeding and misalignment occurred during printing.

[0184] × The test pattern was not printed successfully, with significant ink bleeding and misalignment.

[0185] (Water resistance)

[0186] Wipe the surface of the coating with a damp cloth (KimWipes) (a product name manufactured by CRECIA Co., Ltd. of Japan) for 10 seconds and visually inspect for any watermarks. Evaluate the water resistance based on the following evaluation criteria.

[0187] Evaluation Criteria

[0188] ○ There are absolutely no watermarks left on the surface of the coating.

[0189] Although there were very few watermarks on the surface of the coating, such watermarks had seeped in.

[0190] The surface of the × coating showed significant watermarks that had seeped in.

[0191] (Weather resistance)

[0192] In the metal halide weathering test, the blackboard temperature was 63℃ (±2℃), the humidity was 50% (±5%), and the illuminance was 1200W / m². 2 The experiment was conducted for 24 hours, and the condition of the coating surface before and after the experiment was visually observed, comparing it with L. * a * b * Color space chromaticity diagrams are compared, colors are measured, and evaluations are conducted based on the following evaluation criteria.

[0193] Evaluation Criteria

[0194] ○ Before and after testing on the coated surface, the lightness L * and chromaticity a * b * There was essentially no change; no yellowing actually occurred.

[0195] △ Before and after the test on the coated surface, the lightness L * and chromaticity a * The change is small, but the chromaticity b * As the value increases, a slight yellowing occurs.

[0196] × Before and after testing on the coated surface, the brightness L * and chromaticity a * b * There are changes, especially in chroma b. * The value increases significantly, causing yellowing.

[0197] (Material)

[0198] The ingredients used in the following examples and comparative examples are as follows.

[0199] Substrate

[0200] S1: 36.0 parts by mass of polypropylene homopolymer (melting point 160℃), 60.0 parts by mass of heavy calcium carbonate powder with an average particle size of 2.2 μm (measured by air permeation method based on JIS M-1511) as inorganic powder, and 2.0 parts by mass of sodium alkyl sulfonate (average number of carbon atoms in alkyl group = 12) as lubricant are fed into an extruder equipped with a twin-screw extruder (T-die extrusion molding device). In a mixture with L / D = 25, the raw materials are mixed at a temperature below 220°C. The mixed materials are then formed into sheets using a T-die at a molding temperature of 220°C. The sheets are then wound and stretched simultaneously using a winding machine to produce a sheet of thermoplastic plastic containing inorganic powder as the base material. It should be noted that the thickness of the sheet obtained in this way is 200 μm.

[0201] • Aqueous emulsions of acrylic polymers

[0202] M1: An aqueous emulsion of (meth)acrylate alkyl ester copolymer containing n-butyl acrylate, methyl methacrylate, 2-ethylhexyl acrylate, methacrylic acid, vinyl acetate, and rosin derivatives in a mass ratio of 26:16:44:8:6:3 (solids:water = 50:50 (mass ratio)).

[0203] Ma: An aqueous emulsion of a styrene-acrylate copolymer containing styrene, benzyl acrylate, butyl acrylate, 1,6-hexanediol dimethacrylate, and 2-hydroxyethyl methacrylate in a mass ratio of 84.0:26.0:32.0:0.1:0.9 (solids:water = 20:80 (mass ratio)).

[0204] ·clay

[0205] C1: Kaolin clay (volume average particle size 0.29 μm, specific gravity 2.5)

[0206] C2: Kaolin clay (volume average particle size 1.50 μm, specific gravity 2.6)

[0207] Calcium carbonate powder

[0208] H1: Heavy calcium carbonate (volume average particle size 0.25 μm, specific gravity 2.6)

[0209] H2: Heavy calcium carbonate (volume average particle size 0.34 μm, specific gravity 2.6)

[0210] H3: Heavy calcium carbonate (volume average particle size 3.00 μm, specific gravity 2.6)

[0211] L1: Light calcium carbonate (volume average particle size 0.05 μm, specific gravity 2.6)

[0212] L2: Light calcium carbonate (volume average particle size 1.00 μm, specific gravity 2.6)

[0213] [Examples 1-6, Comparative Examples 1-8, and Reference Example 1]

[0214] The acrylic polymer aqueous emulsion and clay, as well as the type and amount of calcium carbonate, were prepared according to Table 1. The mixture was stirred at 3000 rpm for 3 minutes using a bladed turbine to prepare the coating solution. Note that the amounts of clay and calcium carbonate shown in Table 1 are converted from the dried mass of the acrylic polymer aqueous emulsion. It should be noted that water was used as the dispersion medium in all coating solutions, and the solids concentration was set at 46% by mass. Furthermore, 0.1% by mass of surfactant as a dispersant and 0.1% by mass of hydrophobic silica-based mineral oil defoamer as a defoamer were added to all coating solutions. However, these dispersants and defoamers are not essential components, and the coating solution can be prepared even without their addition. The prepared coating solution was applied to the surface of the substrate at the specified film thickness shown in Table 1 using microgravure printing and dried at 110°C to produce a printing sheet. The adhesion, surface resistivity, whiteness, LBP printing suitability, offset printing suitability, water resistance, and weather resistance of the obtained printing sheets were measured under the above conditions. The results are shown in Table 2.

[0215] [Table 1]

[0216] [Table 1: Substrate / Coating Composition for Each Sample]

[0217]

[0218] 1) Types of aqueous emulsions 2) Unit: % by mass

[0219] [Table 2]

[0220] [Table 2 Evaluation results for each sample]

[0221]

[0222] According to the present invention, the printing sheet containing talc and heavy calcium carbonate of a specific particle size in a specified amount in the coating exhibits excellent adhesion / adhesion to the substrate, and displays sufficiently good properties in terms of whiteness, LBP printing adaptability, oil-based offset printing adaptability, surface resistivity, water resistance, and weather resistance.

[0223] On the other hand, the printing sheets of Comparative Examples 1, 3, and 4, which contain light calcium carbonate along with clay in the coating, exhibit excellent printability and good initial adhesion evaluation results, but their evaluation results after multiple adhesion / peeling cycles are worse than those of the printing sheets according to the embodiments of the present invention. Furthermore, the printing sheet of Comparative Example 2, which contains heavy calcium carbonate H3 with a volume average particle size of 3.00 μm along with talc in the coating, has poor printability and low coating adhesion. This demonstrates the importance of using heavy calcium carbonate with a volume average particle size of 0.05 μm or more and 2.00 μm or less as the calcium carbonate powder.

[0224] Furthermore, in Comparative Example 5, which is a conventional composition consisting only of an acrylic coating, adhesion is poor, and there are problems with LBP printing suitability, offset printing suitability, surface resistivity, water resistance, and weather resistance, resulting in yellowing. In Comparative Example 6, which consists only of clay without the addition of heavy calcium carbonate, there are slight deficiencies in offset printing suitability, water resistance, and weather resistance.

[0225] As described above, this invention provides a printing sheet having a coating for receiving ink on at least one surface of a substrate, which exhibits excellent printability, good antistatic properties to prevent paper jams during printing, and excellent water resistance, weather resistance, and other properties. In particular, it provides a printing sheet with extremely good adhesion between the substrate and the coating, and a method for manufacturing the same.

Claims

1. A printing sheet having a coating on one or both sides of a substrate, wherein the coating is formed by adding clay in a proportion of 35% to 65% by mass and heavy calcium carbonate in a proportion of 5% to 30% by mass to a continuous phase composed of an acrylic polymer, but the coating does not include a coating in which zinc pyrithione is added in a proportion of 0.40% to 2.00% by mass and / or zinc oxide with a volume average particle size of 0.05 μm to 2.00 μm in a proportion of 0.10% to 1.00% by mass when the total mass of the coating is set to 100% by mass. In the printing sheet, The volume average particle size of the heavy calcium carbonate is greater than 0.05 μm and less than 2.00 μm.

2. The printing sheet according to claim 1, wherein, The substrate is a substrate containing polyolefin resin and inorganic powder in a mass ratio of 50:50 to 10:

90.

3. The printing sheet according to claim 2, wherein, The inorganic powder is calcium carbonate powder.

4. The printing sheet according to claim 1, wherein, The clay has a volume average particle size of ≥0.05μm and ≤2.00μm.

5. The printing sheet according to any one of claims 1 to 4, wherein, The acrylic polymer is an alkyl (meth)acrylate.

6. The printing sheet according to claim 5, wherein, The alkyl methacrylate is selected from one or more of the following groups: methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, nonyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, and 2-hydroxypropyl methacrylate.

7. A method for manufacturing a printing sheet, wherein, An aqueous emulsion of an acrylic polymer is coated on one or both sides of a substrate. The aqueous emulsion is formed by adding clay at a ratio of 35% to 65% by dry weight and heavy calcium carbonate with a volume average particle size of 0.05 μm to 2.00 μm at a ratio of 5% to 30% by dry weight to a continuous phase composed of an acrylic polymer. However, the aqueous emulsion does not include an aqueous emulsion of an acrylic polymer containing zinc pyrithione at a ratio of 0.40% to 2.00% by dry weight and / or zinc oxide with a volume average particle size of 0.05 μm to 2.00 μm at a ratio of 0.10% to 1.00% by dry weight to a ratio of 100% by dry weight.

8. The method for manufacturing printing sheet according to claim 7, wherein, A substrate containing polyolefin resin and inorganic powder in a mass ratio of 50:50 to 10:90 is extruded into a sheet. The acrylic polymer aqueous emulsion is then coated on one or both sides of the substrate sheet through a stretching process.

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