AU2010263666B2

AU2010263666B2 - Printing papers

Info

Publication number: AU2010263666B2
Application number: AU2010263666A
Authority: AU
Inventors: Yuji Abe; Mitsuhiro Akaike; Kiyomi Hanaya; Yusuke Kondo; Yoshikazu Ninomiya; Fuminari Nonomura; Yoshitake Yamada; Masato Yamaguchi
Original assignee: Nippon Paper Industries Co Ltd
Current assignee: Nippon Paper Industries Co Ltd
Priority date: 2009-06-26
Filing date: 2010-06-22
Publication date: 2013-12-19
Anticipated expiration: 2030-06-22
Also published as: JP2016065355A; JP6046326B2; JP4889797B2; JP5843835B2; JP2011026753A; JP2011196010A; JP2011026757A; JP2011026756A; TWI493088B; JP2014012924A; JP5686984B2; AU2010263666A1; JP5743721B2; TW201120281A; JP4889798B2; JP2011026754A

Abstract

Disclosed is printing paper which has high whiteness degree and high opacity at the same time. Specifically disclosed is printing paper which contains one or more of colorants selected from the group consisting of purple pigments, purple dyes, blue pigments and blue dyes, and has a opacity of not less than 85%. The printing paper has an a* value of 0 or more but less than 7 and a b* value of -15 or more but less than -3 as determined by a measurement in accordance with JIS P 8150 including ultraviolet light.

Description

- 1 DESCRIPTION PRINTING PAPERS TECHNICAL FIELD [0001] The present invention relates to printing papers having both high brightness and high opacity. BACKGROUND ART [0002] Recently, there has been a growing tendency of seeking after visually impressive whiteness in printing papers, especially coated printing papers used for printing catalogs in mail order business and the like. Coated papers include various grades such as woodfree coated papers (Al coated paper, A2 coated paper, A3 coated paper, lightweight coated paper), wood-containing coated papers, ultra-lightweight coated papers, etc., and are commercially available from various manufacturers under various brand names varying in basis weight, brightness, gloss, smoothness, tone, etc. to meet different purposes and requirements. [0003] Printing papers made from woodfree materials use a stock mainly derived from bleached kraft pulp so that they contain little colored foreign matter and provide high brightness but low opacity. In contrast, printing papers made from wood-containing materials contain much recycled pulp or mechanical pulp in addition to bleached kraft pulp. For this reason, the printing papers made from wood-containing materials have relatively higher opacity than woodfree printing papers, but disadvantageously lower brightness than woodfree printing papers. Thus, there is a demand for increasing both brightness and opacity in printing papers. [0004] Generally, high-basis weight printing papers may have combined relatively high brightness and opacity, but are required to have visually more impressive whiteness. Printing papers having higher opacity at the same weight basis are also required. Thus, there is a demand for increasing both brightness and opacity even in high-basis weight printing papers. On the other hand, a demand for low-basis weight printing papers is growing to meet requirements for increasing the number of pages of each book while -2 reducing the costs of paper, shipping and mailing. Low-basis weight printing papers have difficulty in combining high brightness and opacity, and suffer from the problem of print through especially during printing on thin papers. [0005] Methods for achieving a balance between brightness and opacity have been disclosed by internally or externally adding titanium oxide known for its high refractive index, forming pores of appropriate sizes in coating layers by incorporating a hollow plastic pigment into the coating layers, or combining both techniques (patent document 1, patent document 2). However, materials used in these methods are all expensive, resulting in a cost increase. CITATION LIST PATENT DOCUMENTS [0006] Patent document 1: JPA 2000-336593 Patent document 2: JPB SH052-118116. SUMMARY OF INVENTION TECHNICAL PROBLEMS [0007] Under these circumstances, the present invention intends to provide printing papers having high brightness and high opacity. SOLUTION TO PROBLEMS [0008] As a result of careful studies to solve the above problems, we attained the present invention on the basis of the finding that printing papers having high brightness and high opacity can be prepared by including any one or more colorants having a purple or blue color in components constituting the printing papers. [0009] Accordingly, the present invention includes: (1) a printing paper which has a basis weight of 70 g/m2 or less containing one or more colorants selected from the group consisting of purple pigments, purple dyes, blue pigments and blue dyes and having an opacity of 85 % or more, said printing paper having an a* value of 0 or more and less than 7 and a b* value of -15 or more and less than -3 as determined according to the method of JIS P 8150 (using a light source including a UV component); (2) the printing paper as defined in (1) wherein the colorant exists in any one of layers constituting the printing paper; -3 (3) the printing paper as defined in (1) which has more than one layer and a basis weight of 70 g/m 2 or less wherein the colorant exists in multiple layers; (4) the printing paper as defined in (1) which has a basis weight of more than 70 g/m 2 (5) the printing paper as defined in (1) wherein the colorant is a purple pigment or a blue pigment. ADVANTAGEOUS EFFECTS OF THE INVENTION [0010] According to the present invention, printing papers having both high brightness and high opacity can be obtained. Particularly, printing papers of the present invention show intense apparent whiteness and appear distinctly whiter than the brightness determined by instrumental analysis. Especially, printing papers of the present invention show remarkably high opacity at low basis weight. Moreover, printing papers of the present invention have good print surface appearance and print gloss, high smoothness, good ink adhesion and good printability. Additionally, the present invention allows printing papers to be kept at high opacity even if a lot of low opacity chemical pulp is used in the stock. Especially when the colorant exists in more than one layer, color unevenness of printing papers can be reduced. BRIEF DESCRIPTION OF DRAWING [0011] Figure 1 shows changes in hue in the L*a*b* color system after colorants are added. DESCRIPTION OF EMBODIMENTS [0012] Printing papers of the present invention may be coated printing papers having a coating layer based on a pigment and an adhesive or printing papers without such a coating layer. The present invention is more advantageous in coated printing papers. The term "coated printing paper" refers to a paper comprising a paper substrate (hereinafter sometimes referred to as "base paper") and at least one coating layer on one or both side(s) of the paper substrate wherein the coating layer is based on a pigment and an adhesive. As used herein, printing papers having a pigment coating layer containing a white pigment are referred to as -4 coated printing papers or simply coated papers, while printing papers without such a pigment coating layer are referred to as uncoated printing papers or simply uncoated papers. [0013] 1. Colorants As used herein, the term "colorant" refers to a pigment or dye of a color other than white. In the present invention, printing papers contain one or more colorants selected from the group consisting of purple pigments, purple dyes, blue pigments and blue dyes. The term "pigment" refers to a white or colored powder insoluble or less soluble or present in the dispersed state in water or oils or organic solvents, and encompasses inorganic and organic pigments. In the present invention, either inorganic or organic pigment may be used as the colorant. The term "dye" refers to an organic dye that selectively absorbs or reflects visible light to provide a specific color and that stains fibers or pigments or the like by appropriate dyeing techniques. Dyes are soluble in solvents (water or organic solvents, etc.). In the present invention, dyes may also be used, but preferably a purple pigment and/or blue pigment is used to improve light fastness and prevent discoloration/coloration of paper with time. As used herein, the expression "a purple pigment and/or blue pigment is used" means that both blue and purple pigments are used or either blue or purple pigment is used. As used herein, the term "purple pigment and/or blue pigment" is sometimes referred to as "blue purple pigment". [0014] Commercially available blue pigments include, for example, EMT-blue DS-18 from Toyo Ink Mfg. Co., Ltd., etc., and commercially available purple pigments include, for example, SA violet C12896 from Mikuni Color Ltd. A blue or purple pigment may be used alone or both may be used in combination. In order to improve opacity, a purple pigment is preferably used. In the present invention, colorants other than blue and purple such as black, red and yellow may be added as appropriate. [0015] As described above, both inorganic and organic pigments can be used as blue-purple pigments. Specific examples of blue pigments include, for example, ultramarine, azurite, Prussian blue (dark blue), ultramarine blue, smalt, cobalt blue (cobalt aluminate), cerulean blue (cobalt stannate), cobalt chrome blue, cobalt-aluminum-silicon oxide, cobalt-zinc-silicon -5 oxide, manganese blue, and phthalocyanine. Specific examples of purple pigments include, for example, inorganic pigments such as cobalt violet (cobalt arsenate, cobalt phosphate, cobalt-lithium phosphorus oxide, ammonium cobalt phosphate hydrate, cobalt borate, etc.), ultramarine violet, violet iron oxide, manganese violet, mineral violet; and organic pigments such as indigoid, quinacridone, oxazine, anthraquinone, carbonium and xanthene pigments. [0016] In the present invention, the surface color of printing papers can be made bluish white to enhance apparent whiteness while improving opacity to prevent print through during printing by including a predetermined amount of a blue-purple pigment in the printing papers to control the hue in the range described below. [00171 As used herein, the term "blue and purple pigments" refers to colorants that confer the respective colors on printing papers when they are contained in the printing papers. When each colorant is added, the hue of paper can be changed into the direction shown in Figure 1. Figure 1 shows changes in hue of paper on the basis of the L'a'b* color system before and after it contains colorants of the present invention. When a' values indicate 0* in the positive direction and 1800 in the negative direction and b' values indicate 900 in the positive direction and 2700 in the negative direction and the origin corresponds to the color of paper before colorants are added, the hue of paper is shown to change to the sector of 2100 or more and less than 2800 labeled as "bluish" in Figure 1 after a blue colorant is added or to the sector of 2800 or more and less than 3350 labeled as "purplish" in Figure 1 after a purple colorant is added. [0018] Colorant content The content of colorants used in the present invention is not specifically limited, but preferably 0.7 mg or more and 3.5 mg or less, more preferably 0.9 mg or more and 3.0 mg or less in total per m2 of printing paper. [0019] As described above, a purple pigment and/or blue pigment is preferably used in the present invention, and the total content of these pigments is also preferably as defined above. If the total content of purple and blue pigments is less than 0.7 mg, the hiding power contributing to opacity will be unpreferably insufficient because the pigments absorb little -6 light. If the total content is more than 3.5 mg, the pigments absorb much light to greatly contribute to improvement of opacity, but the hue will be unpreferably far from the origin so that it is not perceived as white. The content of the pigments can be appropriately controlled within the range shown above depending on the brightness of the stock or base paper or the like. [0020] The total amount of the colorants is the total of the contents in a base paper layer, a size press layer and a pigment coating layer if a printing paper comprises them and calculated by the equation below, assuming that a blue pigment and a purple pigment are used as colorants. [0021] Total amount (mg/m 2 ) = {Basis weight of base paper (g/m 2 ) x Pulp content in base paper x Contents of blue pigment and purple pigment in base paper (mg/g pulp) + Coating mass of size press solution (both sides) (g/m 2 ) x [Total of blue pigment and purple pigment in size press solution in parts by weight / Total solids content in size press solution in parts by weight] + Coating mass of pigment coating solution (both sides) (g/m 2 ) x [Total of blue pigment and purple pigment in pigment coating solution in parts by weight / Total solids content in pigment coating solution in parts by weight]} x 1000 [0022] Layers containing colorants In the present invention, colorants may be contained in any layer of printing papers. In the present invention, a colorant may exist in any one of layers constituting a printing paper, for example. In this case, the printing paper can be advantageously prepared relatively easily. Especially, a blue-purple pigment should preferably be contained in a coating layer. When identical layers exist on both sides of a base paper (e.g., coating layers are provided on both sides), the expression "a blue-purple pigment exists in one layer" means that the coating layers on both sides contain a blue-purple pigment or one of the coating layers on both sides contains a blue-purple pigment. In the present invention, a blue-purple -7 pigment is preferably used as a colorant as described above, and therefore, the following description relates to the case where a blue-purple pigment is used as a colorant unless otherwise specified. [0023] The blue-purple pigment may be contained in a base paper or a size press solution or a coating layer if it is included in printing papers. In the case of printing papers comprising more than one base paper layer and/or coating layer, it may be contained in any one or more layers or all of the layers. For ease of preparation, the blue-purple pigment is preferably contained in a coating layer. [0024] Generally, printing papers are classified into coated papers having a pigment coating layer containing a conventional pigment and uncoated papers without such a pigment coating layer. The conventional pigment encompasses kaolin, clay or the like as described below, and a coating layer containing this pigment is herein simply referred to as "pigment coating layer". Coated and uncoated papers optionally have a clear (transparent) coating layer formed by applying an aqueous solution of a water-soluble polymer binder such as starch, polyacrylamide or polyvinyl alcohol (size press solution) on a base paper. Thus, printing papers of the present invention comprise any one or both of a clear coating layer and a pigment coating layer on one or both sides of a base paper. [0025] Accordingly, printing papers of the present invention include the following embodiments. 1) A coated paper comprising a base paper layer and a pigment coating layer. In this coated paper, the purple-blue pigment exists in both or one of the base paper layer and the pigment coating layer. 2) A coated paper comprising a base paper layer, a clear coating layer and a pigment coating layer. In this coated paper, the purple-blue pigment exists in one or more layers selected from the base paper layer, the clear coating layer and the pigment coating layer. 3) An uncoated paper comprising a base paper layer. In this uncoated paper, the purple-blue pigment exists in the base paper layer. 4) An uncoated paper comprising a base paper layer and a clear coating layer. In this -8 uncoated paper, the purple-blue pigment exists in both or one of the base paper layer and the clear coating layer. [0026] Especially when a blue-purple pigment exists in multiple layers, apparent whiteness and opacity of printing papers can be stably improved and color unevenness of the printing papers can be reduced. As used herein, the expression "a blue-purple pigment exists in multiple layers" means that a blue-purple pigment exists in multiple different layers (e.g., a base paper layer and a coating layer). When identical layers exist on both sides of a base paper (e.g., coating layers are provided on both sides) and these two coating layers contain a blue-purple pigment, it is not considered that a blue-purple pigment exists in two layers. Embodiments in which a blue-purple pigment exists in more than one layer tend to be advantageous especially in low-basis weight printing papers liable to problems such as print through, i.e., printing papers having a basis weight of 70 g/m 2 or less. [0027] When a blue-purple pigment of the present invention exists in multiple layers, the proportions of the blue-purple pigment existing in the layers are not specifically limited, but advantages of the present invention can be efficiently attained with small amounts of the pigment by increasing the proportion of the pigment existing in the coating layer corresponding to the outermost layer. For example, the amount of the blue-purple pigment in outer layers is preferably higher than the amount of the blue-purple pigment in inner layers. [0028] 2. Preparation of printing papers Printing papers of the present invention can be prepared by known techniques. For example, coated printing papers of the present invention can be prepared by passing a paper stock as described below through the wire part, then the press part and the predryer part to prepare a base paper, which can be coated with a coating solution as described below in the coater part, then passed through the after-dryer part, calender part, reel part, winder part, etc. Uncoated printing papers can be prepared by passing a paper stock through the wire part, then the press part and the predryer part to prepare a base paper, which can be clear-coated with a water-soluble polymer (binder). [0029] Base paper -9 Printing papers of the present invention comprise at least a base paper layer. The base paper used in the present invention may consist of a single layer or multiple layers. When the base paper of the present invention has a multilayer structure, a blue-purple pigment may be contained in any one or more of multiple layers constituting the base paper. To ensure that a blue-purple pigment exists in the base paper layer, the base paper may be made from a paper stock containing the blue-purple pigment. The technique for preparing the base paper used in the present invention is not specifically limited, and it can be prepared by known techniques using known materials. [0030] Raw material pulp Chemical pulp can be used as a stock used for the base paper of the present invention. Various pulps other than chemical pulp can also be used depending on the purpose. Examples of such pulps include deinked pulp (DIP), groundwood pulp (GP), refiner groundwood pulp (RGP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemigroundwood pulp (CGP), semichemical pulp (SCP), etc. Deinked pulp may be derived from sorted waste papers such as woodfree paper, wood-containing paper, mechanical paper, news, advertising leaflets and magazines or unsorted waste papers including mixtures of them. When deinked pulp is used in the present invention, high brightness pulp derived from sorted waste paper mainly consisting of woodfree paper is preferably included. [0031] In a preferred embodiment of the present invention, the base paper of the present invention is made from a stock including chemical pulp. In the present invention, chemical pulp prepared by both of the kraft process and the sulfite process can be used, but chemical pulp prepared by the kraft process (hereinafter sometimes simply referred to as kraft pulp) is preferred in terms of production cost. Generally, chemical pulp has high brightness because wood-derived lignin is removed during the production process, but the resulting paper tends to have low opacity. Thus, an improvement in opacity was a major issue especially in low basis weight printing papers using chemical pulp, and can be achieved according to the present invention.

-10 [0032] In a preferred embodiment of the present invention, the chemical pulp content in the stock is 60 % by weight or more, more preferably 80 % by weight or more, still more preferably 90 % by weight or more, most preferably 100 % by weight in total pulp. If the chemical pulp content is 100 % by weight, the stock contains no lignin and a minimum amount of elements staining the printing paper. In addition, high brightness printing paper can be obtained because the stock containing no lignin reduces UV absorption so that the effect of fluorescent dyes is not inhibited. Lower chemical pulp contents are disadvantageous in terms of brightness. [0033] Fillers In the present invention, any known fillers can be used as fillers for the base paper. For example, inorganic fillers such as ground calcium carbonate, precipitated calcium carbonate, clay, silica, precipitated calcium carbonate-silica complexes, kaolin, calcined kaolin, delaminated kaolin, white carbon, talc, magnesium carbonate, barium carbonate, barium sulfate, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide and amorphous silica prepared by neutralization of sodium silicate with mineral acids, and organic fillers such as urea-formalin resins, melamine resins, polystyrene resins and phenol resins can be used alone or in combination. Among them, ground calcium carbonate and precipitated calcium carbonate, which are typical fillers in the preparation of neutral and alkaline papers, are preferably used to improve opacity as well. The filler content in paper is not specifically limited, but preferably 1-40 % solids by weight, more preferably 10-35 % solids by weight. [0034] In the present invention, known papermaking additives can be used. For example, internal papermaking additives such as aluminum sulfate, various anionic, cationic, nonionic or amphoteric retention aids, freeness improvers, various paper strength enhancers and internal sizing agents can be used as appropriate. Dry paper strength enhancers include polyacrylamide, cationized starch, etc., while wet paper strength enhancers include polyamidoamine-epichlorohydrin, etc. These chemicals are added so far as they may not affect formation and runnability. Neutral sizing agents include alkyl ketene dimers, alkenyl - 11 succinic anhydride, neutral rosin sizing agents, etc. In addition, dyes, fluorescent whitening agents, pH modifiers, antifoaming agents, pitch control agents, slime control agents and the like can also be added as appropriate. [0035] Papermaking process/paper machine The process for preparing the base paper in the present invention is not specifically limited, and can be performed using a Fourdrinier machine with a top wire or the like, an on top former, a gap former, a cylinder machine, a board machine combining a Fourdrinier machine and a cylinder machine, a Yankee dryer machine, etc. The pH during papermaking may be acidic or neutral or alkaline, but preferably neutral or alkaline. The machine speed is not specifically limited. [0036] Basis weight of the base paper The basis weight of the base paper of the present invention depends on the basis weight of the end product printing paper. In the case of printing papers having a basis weight of 70 g/m 2 or less (hereinafter also referred to as "low-basis weight printing papers"), the basis weight of the base paper is preferably 60 g/m 2 or less, more preferably 20 g/m 2 _ 60 g/m2, still more preferably 20 g/m2-40 g/m2. In contrast to conventional low-basis weight papers having low opacity due to their small thickness, high opacity can be achieved at low basis weight according to the present invention. [0037] When a coated paper is to be prepared by the present invention, the base paper may be preliminarily smoothed through an on-line soft calender, on-line chilled calender or the like before the coating step. [0038] In the case of printing papers having a basis weight of more than 70 g/m 2 (hereinafter also referred to as "high-basis weight printing papers"), the basis weight of the base paper is preferably 30 g/m 2 -200 g/m 2 , more preferably 40 g/m 2 -130 g/m 2 . [0039] Clear coating Printing papers of the present invention may comprise a clear (transparent) coating layer on one or both side(s) of the base paper described above. The surface strength or smoothness of the base paper can be improved and coatability during pigment coating can be - 12 improved by clear coating on the base paper. In the present invention, a blue-purple pigment can be contained in the clear coating layer, in which case a clear coating solution containing the blue-purple pigment may be applied on the base paper. The clear coating mass is preferably 0.1-1.0 g/m 2 , more preferably 0.2-0.8 g/m 2 expressed as solids per side. [0040] As used herein, the term "clear coating" refers to applying (size press coating) a coating solution (surface-treating solution) based on a water-soluble polymer such as starch, various starch derivatives such as oxidized starch, polyacrylamide or polyvinyl alcohol using a coater (coating machine) such as a size press coater, gate roll coater, premetering size press, curtain coater or spray coater, for example. [0041] Pigment coating Printing papers of the present invention may also be coated papers having a pigment coating layer provided by pigment coating. Printing papers of the present invention may have a single or multiple pigment coating layer(s). To obtain low-basis weight printing papers, a single layer is preferred. To obtain high-basis weight printing papers, however, multiple pigment coating layers are preferred. When the pigment coating layer has a multilayer structure, a blue-purple pigment may be contained in any one or more of multiple layers constituting the pigment coating layer. To ensure that a blue-purple pigment exists in the pigment coating layer, pigment coating can be performed by using a coating containing the pigment. In the present invention, the coating technique is not specifically limited, and known materials can be used by known techniques. [0042] Precalendering In the present invention, the base paper is preferably preliminarily smoothed by precalendering the base paper before coating through an on-line soft calender, on-line chilled calender or the like to provide a uniform coating layer after coating. In this case, the linear pressure of the treatment is preferably 30-100 kN/m, more preferably 50-100 kN/m. The moisture content in the base paper during precalendering is also important, and preferably 3 5 %. [0043] Coating step - 13 Coated printing papers of the present invention can comprise a coating layer formed by applying a coating solution based on a pigment and an adhesive on the base paper obtained as described above. Coating can be applied on the surface of one or both side(s) of the base paper, but preferably on both sides to ensure that the base paper is not curled and has the same properties on both sides. [0044] Pigments contained in the coating layer Pigments other than the blue-purple pigment used in the coating layer of the present invention (hereinafter also referred to as "white pigments") may be of any type conventionally used for coated papers. For example, inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, ground calcium carbonate, precipitated calcium carbonate, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica and satin white; and organic pigments such as solid, hollow or core shell pigments can be used alone or in combination, as appropriate. [0045] Low-basis weight printing papers preferably contain ground calcium carbonate and precipitated calcium carbonate to provide high brightness, and especially precipitated calcium carbonate having uniform particle size and morphology to improve opacity as well. When calcium carbonate is contained in a coating solution, the content of precipitated calcium carbonate or ground calcium carbonate or the total of both is preferably 50 parts by weight or more, more preferably 70 parts by weight or more, still more preferably 80 parts by weight or more per 100 parts by weight of pigments. To form a uniform coating layer on the base paper, the mean particle size determined by a laser diffraction particle size analyzer such as Mastersizer S from Malvern is preferably 0.2-5 ptm, more preferably 0.2-3 pim. [0046] On the other hand, high-basis weight printing papers normally contain a high proportion of clays such as kaolin, clay, engineered kaolin, delaminated clay. At such a proportion, sheet gloss is improved, but brightness decreases. In the present invention, however, high quality can be kept without compromising brightness even at a high proportion of clays by adding a blue-purple pigment. Pigments that can be used to improve brightness are ground calcium carbonate and precipitated calcium carbonate as described above, - 14 especially precipitated calcium carbonate having uniform particle size and morphology to improve opacity as well. [0047] Thus, high-basis weight printing papers preferably contain a combination of one or more pigments selected from the group consisting of kaolin, clay, engineered kaolin and delaminated clay (also referred to as "first pigment") and one or more pigments selected from the group consisting of ground calcium carbonate and precipitated calcium carbonate (also referred to as "second pigment") as white pigments in the pigment coating layer. Especially, kaolin is preferably used as a first pigment and precipitated calcium carbonate is used as a second pigment. The proportion (weight ratio) of the total of first pigments and the total of second pigments is preferably 40/60 - 70/30, more preferably 46/54 - 60/40, still more preferably 50/50 - 60/40. If the proportion of the first pigment is less than 40, gloss tends to decrease. If the proportion of the first pigment is more than 70, however, brightness of the printing paper tends to decrease. As described above, the blue-purple pigment preferably exists in the pigment coating layer, in which case the blue-purple pigment is preferably present at approximately 0.001-0.05 parts by weight per 100 parts by weight of the total of white pigments. [0048] As described above, high-basis weight printing papers preferably comprise multiple pigment coating layers. In this case, the first and second pigments may be used as white pigments in the outermost layer in the proportions shown above, while any white pigments may be used as white pigments in inner layers other than the outermost layer (layers on the side of the base paper). However, white pigments in inner layers preferably include precipitated calcium carbonate or ground calcium carbonate to achieve a balance between brightness and gloss. The total content of precipitated calcium carbonate or ground calcium carbonate in inner layers is preferably 50 parts by weight or more, more preferably 70 parts by weight or more, still more preferably 80 parts by weight or more per 100 parts by weight of the total of white pigments in the inner layers. When the pigment coating layers has a multilayer structure, the blue-purple pigment may exist in one or both of the inner layers and the outermost layer, but preferably exist predominantly in the outermost layer, as described - 15 above. In this case, the blue-purple pigment preferably exists in an amount of 0.001-0.05 parts by weight per 100 parts by weight of the total of white pigments in the outermost layer and 0.001-0.01 parts by weight per 100 parts by weight of the total of white pigments in the inner layers. [0049] Adhesives Adhesives (binders) used in the present invention are not specifically limited, and any conventional adhesives for coated papers can be used. For example, preferred adhesives include synthetic adhesives such as various copolymers including styrene butadiene copolymers, styrene-acrylic copolymers, ethylene-vinyl acetate copolymers, butadiene-methyl methacrylate copolymers and vinyl acetate-butyl acrylate copolymers, or polyvinyl alcohols, maleic anhydride copolymers and acrylic-methyl methacrylate copolymers; proteins such as casein, soybean protein and synthetic proteins; starches such as oxidized starches, cationic starches, urea phosphate-esterified starches, etherified starches such as hydroxyethyl etherified starches, and dextrin; and cellulose derivatives such as carboxyethyl cellulose, hydroxyethyl cellulose and hydroxymethyl cellulose; and one or more of these conventional adhesives for coated papers can be appropriately selected and used. [0050] The amount of adhesives used in the present invention is preferably 20-40 parts by weight, more preferably 25-35 parts by weight per 100 parts by weight of pigments in terms of printability and coatability. This range is preferred because if the total amount of adhesives exceeds 40 parts by weight, the viscosity of the coating increases to cause operation problems such as boiling during film transfer coating, but if the amount is less than 20 parts by weight, it is difficult to obtain sufficient surface strength during film transfer coating on the base paper. [0051] In addition to the pigments and adhesives, the coating solution of the present invention may contain various additives used in conventional pigments for coated papers such as dispersants, thickeners, water retention agents, antifoamers, waterproofing agents, etc., as appropriate.

- 16 [0052] Preparation of the coating solution In the present invention, the method for preparing the coating solution is not specifically limited, and can be appropriately tailored to the type of the coater. When blade coaters are used, the coating solution preferably has a solids content of 40-70 % by weight, more preferably 60-70 % by weight. The coating solution preferably has a Brookfield viscosity in the rage of 500-1000 mPa.s measured at 60 rpm. [0053] Coating techniques/coaters In the present invention, any conventional coaters can be used. On-machine or off machine coaters may be used, and on-machine coaters include roll coaters such as size press coaters and gate roll coaters; Billblade coaters, blade metering size press coaters, etc. The coating speed is not specifically limited, but preferably 500-1800 m/min using blade coaters or 500-3000 m/min using size press coaters under the current technology. [0054] Coating mass For low-basis weight printing papers, the coating mass of the coating solution is preferably 2-15 g/m2, more preferably 5-12 g/m2, still more preferably 5-10 g/m 2 expressed as solids per side. The present invention is more advantageous in improving opacity when the coating mass is low. [0055] For high-basis weight printing papers, the coating mass of the coating solution is appropriately controlled to ensure that the resulting printing papers have a basis weight exceeding 70 g/m2 , but the coating mass is preferably 15-40 g/m 2, more preferably 18 37 g/m 2 , still more preferably 21-34 g/m 2 expressed as solids per side. In the present invention, printing papers with high opacity can be obtained even if the coating mass is relatively low. [0056] Drying step In the present invention, the technique for drying the wet coating layer is not specifically limited. For example, various techniques can be used alone or in combination including a steam-heated cylinder, hot air dryer, gas heater dryer, electric heater dryer, infrared heater dryer or the like.

- 17 [0057] Surface treatment In the present invention, the paper prepared as described above is surface-treated, if desired. In a preferred embodiment, a printing paper of the present invention may be surface-treated using a calender such as a supercalender or hot soft nip calender. Smoothness and gloss of the printing paper can be improved by surface treatment. In the present invention, soft nip calendering is preferred. Both brightness and opacity are improved by soft nip calendering. In the case of soft nip calendering using metallic rolls having a surface temperature of 20 *C-60 *C, the linear pressure is 30-60 kN/m, more preferably 40-60 kN/m. In the case of hot soft nip calendering using metallic rolls having a surface temperature of 40 *C-250 C, the linear pressure is 60-400 kN/m, preferably 150 300 kN/m, more preferably 100-350 kN/m. As the temperature increases, gloss and smoothness of the surface of the coated paper is improved. [0058] 3. Printing papers Basis weight Printing papers of the present invention may have a basis weight of more than 70 g/m2 or may have a basis weight of 70 g/m2 or more. Generally, high-basis weight papers have sufficient thickness and high opacity, but printing papers of the present invention have higher opacity and higher brightness. [0059] Low-basis weight papers have low opacity because of their small thickness, but high opacity can be achieved at low basis weight according to the present invention. The present invention tends to be advantageous in a low basis weight range, and more advantageous at a basis weight of 60 g/m2 or less. [0060] Hue Printing papers of the present invention have a hue expressed as an a value of 0 or more and less than 7 and a b' value of -15 or more and less than -3 as determined according to the method defined in JIS P 8150 using a light source including a UV component. More preferably, the a value is 0 or more and less than 5 and the b value is -10 or more and less than -5, and still more preferably, the a' value is 0 or more and less than 5 and the b' value is - 18 -9 or more and less than -5. Such relatively low b values help to enhance apparent whiteness, improve opacity and prevent print through during printing of printing papers. (0061] Ash content The ash content in printing papers of the present invention is preferably 10 % by weight or more in the case of uncoated papers or preferably 30 % by weight or more in the case of coated papers. If the ash content in the printing papers is less than 10 % by weight, opacity is not sufficiently improved. [0062] Intensity of fluorescent whitening effect Printing papers of the present invention preferably have an intensity of fluorescent whitening effect of 4.0 or more to improve brightness, though sufficient brightness can be obtained even if the intensity of fluorescent whitening effect is 5.5 or less. [0063] Opacity Printing papers of the present invention have an opacity of 85 % or more. EXAMPLES [0064] The following examples and comparative examples further illustrate the present invention without, however, limiting the invention exclusively to the examples as a matter of course. In the examples below, parts and % refer to parts by weight and % by weight, respectively. [0065] {Quality evaluation methods] The quality of printing papers of the present invention was evaluated by the quality evaluation methods below. (1) Hue (a*, b'): determined by the colorimeter CMS-35SPX from Murakami Color Research Laboratory Co., Ltd. according to JIS P8150 using a light source including a UV component. (2) ISO brightness: determined by the colorimeter CMS-35SPX from Murakami Color Research Laboratory Co., Ltd. according to JIS P8148 using a light source including a UV component. (3) Opacity: determined by the colorimeter CMS-35SPX from Murakami Color Research Laboratory Co., Ltd. according to JIS P8149.

-19 (4) Ash content: determined according to JIS P8251. (5) Intensity of fluorescent whitening effect: defined as the difference obtained by subtracting the ISO brightness value determined by the colorimeter CMS-35SPX from Murakami Color Research Laboratory Co., Ltd. using a light source including no UV component from the brightness value determined using a light source including a UV component. (6) Tint: Tint on the surface of the printing paper was visually evaluated under interior fluorescent lighting. (7) Apparent whiteness: Whiteness on the surface of the printing paper was visually evaluated under interior fluorescent lighting. The reason why whiteness was evaluated is that brightness does not always correlate to the whiteness viewed by human eyes. Visual evaluation was based on the 4-class scale below: @: very white, o: white, A: looks slightly dark and dull or slightly yellowish, x: looks dark and dull or yellowish. (8) Print through during printing: Solid printing was performed in black on one side of the printing paper using a rotary offset press and print through was visually evaluated by observing the printed area from the reverse side. Visual evaluation was based on the 4-class scale below: @: the printed area can be hardly identified from the reverse side, 0: the printed area is invisible from the reverse side, A: the printed area is slightly visible, x: the printed area is visible so that the image quality or appearance on the reverse side is deteriorated. (9) Color unevenness: Color unevenness of the printing paper was visually evaluated according to the 4-class scale below: @: no color unevenness can be identified, o: no visible color unevenness, A: some color unevenness, x: visible color unevenness. [0066] Other quality parameters of printing papers of the present invention other than those described above were also assessed. [0067] [Materials] The following materials were contained in size press solutions and coating solutions. 1. Oxidized starch (SK-20 from Japan Cornstarch Corporation) 2. Pigments - Fine-grained precipitated calcium carbonate - 20 - Fine-grained kaolin (Hydragloss from KaMin LLC) 3. Colorants - Blue pigment (EMT-Blue DS-18 from Toyo Ink Mfg. Co., Ltd.) - Purple pigment (SA Violet C12896 from Mikuni Color Ltd.) - Black pigment (SA Black A035 from Mikuni Color Ltd.) 4. Fluorescent dye (BLANKOPHOR NCC LIQUID from Kemira Japan KK) 5. Adhesive - Styrene-butadiene synthetic polymer latex 6. Water-soluble polymer - Urea phosphate-esterified starch (Starcoat 16 from Nihon Shokuhin Kako Co., Ltd.) [0068] Examples A1-A6 [Preparation of stocks] A pulp slurry containing 65 % hardwood kraft pulp (LBKP) and 35 % softwood kraft pulp (NBKP) was prepared, and precipitated calcium carbonate was added as a filler in the proportions shown in Table 1 and a cationic paper strength enhancer was added as an internal paper strength enhancer in an amount of 0.5 % vs. pulp to prepare stocks. In Examples A5 and A6, a pigment was added to the stocks. [0069] [Preparation of size press solutions (surface-treating solutions)] Various materials in the proportions shown in Table 1 were mixed/stirred at ambient temperature to give size press solutions having a solids content of 8 % by weight. Note that the amounts of the colorants are values per 100 parts by weight of oxidized starch. The coating mass of the size press solutions was 0.5 g/m 2 in all Examples and Comparative examples. [0070] [Preparation of pigment coating solutions] As white pigments, 85 parts of fine-grained precipitated calcium carbonate and 15 parts of fine-grained kaolin Hydragloss (from KaMin LLC) were added, and 7 parts of a synthetic polymer latex mainly composed of styrene and butadiene monomers, 4 parts of a water-soluble polymer consisting of phosphate-esterified starch, and 4 parts of the fluorescent - 21 dye were added based on the total of the white pigments and colorants were further added in the proportions shown in Table 1, and all of the materials were mixed/stirred at ambient temperature to prepare coating solutions having a solids content of 67 % by weight. Note that the amounts of the colorants, adhesive and water-soluble polymer are values per 100 parts by weight of the white pigments. [0071] [Preparation of printing papers] The above stocks were used in a twin wire paper machine at a machine speed of 1100 m/min to make papers, which were then coated on both sides with the above size press solutions (surface-treating solutions) at a coating mass of 0.5 g/m 2 per side using a gate roll coater and dried to give base papers having a basis weight of 41.8 g/m 2 . [0072] Subsequently, the above coating solutions were applied on both sides of the base papers at the coating masses shown in Table 1 per side using a blade coater at a coating speed of 1100 m/min and dried. [0073] The papermaking and coating steps were continuously performed on-line, so that the coating speed and calendering speed were also 1100 m/min. The resulting coated papers were further surface-treated through a hot soft nip calender with 4 nips under conditions of the maximum treatment temperature of 200 'C and the maximum treatment linear pressure of 250 kN/m. [0074] In this manner, coated printing papers having a basis weight of 70 g/m2 or less and containing colorants in multiple layers were obtained. [0075] Comparative examples 1-2 Printing papers were prepared in the same manner as in Example Al except that the blue-purple pigment was added to the coating solutions only. The printing papers obtained in these Comparative examples had a basis weight of 70 g/m 2 or less and opacity and hue (a', b') outside the ranges defined herein. [0076] The formulations of Examples A and the Comparative examples are shown in Table 1 and the performances of the printing papers are shown in Table 2. [0077] - 22 0 0 0 00 coo 0 00 C C) C)- -C0 00 CO C 00 (u cz M) C)CD CD 0 0 D 0 aC) 0 0 .- _ - - - - - - - - - - - - COn C)C R o6 CD 0 0 0 0 C 4. 00 E! M 00 - x~ l;; x L E' -6b cc U - I- -~~~~~. 0 -0 ,~ - 0 ' - 0 0 0 ~ 6~ 0. .o60 0 0 E ~ a E EL U _ C' Cl. E. E E r~~. 0. W = a ) 0 . C .

0 o , 4. 0 ) U OD W A ~2 0. 0 E 0 COc 2 a) U, C . .0 .2 -uo-. ~ u u. u a)~ n +, +O C ~ . C - 23 - 0 odr O N N ~ 0 r 00C o 0 o _ N L- -oo -' 0N - - - n - - - -3 0 o ~ 0 ~- ~-~00 u~ 00 'IT I I' - e a0 CN m V : . N . S - - 0 ~ ~ 00 r u tn C0 000 oo o6 .N- N 00 0 . ------------ -. a o E~ V© o D 00 0 0 o 0 oo -I. o OON O 0 0-- - - - - - 06 v o0 i N 00 o - - -- - - - -- U, o o ------------------------ _2 0 o00 r~ o6 t * t 00 r xr od o6 o D og (D tf xr. N 00 N - N N E~ - -c-"00, M0 0 e O 'o o 'IT 00 00 o L -E N N N 0 m e o .,.. ~~00 S .- . -- o. D o -4 .. 00 <, a) C-4, M V ) 0 m 00 0. 0 00 m 00 C It N') 00 c' ' E 0 0 00 00m0 C' CO N 'I -4 0 000 00~o~ q , E~a n in00 c0 CD z N 0 C, vi ~ mO0 f N -i4 0= *400 00 000 e LE tn N0 M O 0 0 E U) 0 o 0. > -0 E b=0 N >, j 0 u 0 0 .u .Z; +- ' C> a ~ c w ~ -) 0.~~ -M CL - 24 [0079] As shown in Table 2, the printing papers obtained in the present examples are excellent coated printing papers characterized by combined high brightness and high opacity, invisible print through during printing, especially during offset printing and a wide range of color reproduction. [0080] In contrast, Comparative examples 1 and 2 having opacity and hue (a', b) outside the range defined herein were inferior to the products embodying the present invention in apparent whiteness, print through during printing, color unevenness, etc. [0081] Examples B1-B8 Printing papers were prepared in the same manner as in Examples A1-A6 except that the blue-purple pigment was added to the coating solutions only. The printing papers obtained in Examples B were coated printing papers having a basis weight of 70 g/m2 or less and containing colorants in only one layer. [0082] The formulations of Examples B and the performances of the resulting printing papers are shown in Tables 3 and 4. The results of the Comparative examples are also shown in Tables 3 and 4. [0083] - 25 en -" 0)~ ~ ~~C Q j 0~~ C)- C 2 N 0 'IT o u -l 0 C 0 00 C) CD 0u 00) 00 C) 0 C)C 0 0 c!:: 2 N6 C )N 0 0 - ICut C)- 0 0 -: C: CD 0 c i 0 0 0Dm') 0- 0 0 0i N~~ 00C C N~~ 000 0 CD = - - c 0 0 0 CD Cu 0 0! 0) m 0 0M. 0 tn 0D 0 - cc N) tn a)) N 0 CJ> 000 NCu < -4 0 0 0 CD o6E 1) 0. CD 0 )0 a CuN 00 m C:) 0 ) 0 0 c 0 N ~~~0 E(0 O 0~ co 00 0) 0 -U N ~ . 0L .9 n '~ O ' 0 0 -E 0t Cu u 0 0 0 0 0"- -z uu c' CA*6 C ' ~ ~ ~ C 2u 0 0 o0 0o Cu u Z m 00 E .Le 0)0.~ 0 0 '0 0) u 0) 00 ~.) 0)00.

-26 a' 00 c - 0 - -1 -I C oo E 0 E .0 0 x "T ex C -oo -- -M o6I U Q i ~ ~ o N cc el C, n IC 4 e N rN N I- In - -- 0 0 00 - 00~~0 m~~0 o L~ -an '*~ o o oo 1 - - In - ' 000<]0 qt N5 0 E - - 2 0 < S00 *N *N 0 )C It -n 00 n -~ 0 0 -o N-* a' toro 0- 1 - z D In n E - - - - - - -- - o 3 o n - -N - N om 2 - - 3 o-:oR n oi © o0 o I N - i) 00 'na \, LIrO m 00 eo -n N - , In 00 ee . e 0 a' In NL,, I 00 In o O -c : -- 2 c . " -- ) o) " o 91N - - -0) -J 0 o0 * - o o~aI co - o eoee -2 . N - - -n 0 I.n . o-c on - m 0 << C7, - I i o k! 0 a'I R 1C tn I N W I 00 I tn N 00- In N6 0 0 I a ~~O m~~~ ~ N - ' ~ 000I kNf ) 00 In; ~ N - ' ) 00 In (2) -0 m0 I 0I Nn- ) m - 00 ,r o6 0 tn N z ~ I 00 In E 00 o N 0, ~ ~ I- aN0 z~ 00 tn 0o6 ~ o6- :E*0 ( ( " 0n _0 m__ - 27 [0085] As shown in Table 3, the printing papers obtained in the present examples are excellent coated printing papers characterized by combined high brightness and high opacity, invisible print through during printing, especially during offset printing and a wide range of color reproduction. [0086] In contrast, Comparative examples 1 and 2 having opacity and hue (a*, b) outside the range defined herein were inferior to the products embodying the present invention in apparent whiteness, print through during printing, color unevenness, etc. [0087] Moreover, a comparison between Examples A and B revealed that the printing papers of Examples A containing pigments in more than one layer formed on both sides of the base paper especially show less color unevenness as compared with the printing papers of Examples B containing pigments in only one layer. [0088] Example C1 [Preparation of a stock] A pulp slurry containing 90 % hardwood kraft pulp (LBKP) and 10 % softwood kraft pulp (NBKP) was prepared, and 15 % precipitated calcium carbonate was added as a filler and a cationic paper strength enhancer was added as an internal paper strength enhancer in an amount of 0.5 % vs. pulp to prepare a stock. [0089] [Preparation of a pigment coating (undercoating) solution] An undercoating solution having a solids content of 60 % by weight was prepared by mixing/stirring 100 parts of fine-grained precipitated calcium carbonate, 21 parts of oxidized starch, 3.5 parts of a styrene-butadiene synthetic polymer latex, and 0.004 parts of the purple pigment at ambient temperature. [0090] [Preparation of a pigment coating (top coating) solution] As pigments, 54 parts of fine-grained precipitated calcium carbonate and 46 parts of fine-grained kaolin Hydragloss (from KaMin LLC) were added, and 10 parts of a synthetic polymer latex mainly composed of styrene and butadiene monomers, 2.3 parts of a water soluble polymer consisting of phosphate-esterified starch, 4 parts of the fluorescent dye, 0.01 part of the purple pigment and 0.003 parts of the blue pigment were added based on the total - 28 of the pigments, and all of the materials were mixed/stirred at ambient temperature to prepare a coating solution having a solids content of 67 % by weight. Note that the amounts of the colorants, adhesive and water-soluble polymer here are values per 100 parts by weight of the white pigments. [0091] [Preparation of a printing paper] The above stock was used in a twin wire paper machine at a machine speed of 1000 m/min to make a paper, which was then coated on both sides with the above undercoating solution (surface-treating solution) at a coating mass of 4.0 g/m2 per side using a gate roll coater and dried to give a base paper having a basis weight of 52.1 g/m 2 . [0092] Subsequently, the above coating solution was applied on both sides of the base paper at a coating mass of 9.5 g/m 2 per side using a blade coater at a coating speed of 1000 m/min and dried. [0093] The papermaking and coating steps were continuously performed on-line, so that the coating speed and calendering speed were also 1000 m/min. The resulting coated paper was further surface-treated through a hot soft nip calender with 4 nips under conditions of the maximum treatment temperature of 200 *C and the maximum treatment linear pressure of 250 kN/m. In this manner, a coated printing paper having a basis weight of more than 70 g/m 2 was obtained. [0094] Example C2 The above stock was used in a twin wire paper machine at a machine speed of 1000 m/min to make a paper, which was then coated on both sides with the above undercoating solution (surface-treating solution) at a coating mass of 4.5 g/m 2 per side using a gate roll coater and dried to give a base paper having a basis weight of 54.6 g/m 2 . [0095] Subsequently, the above coating solution was applied on both sides of the base paper at a coating mass of 10.0 g/m2 per side using a blade coater at a coating speed of 1000 m/min and dried. The resulting coated paper was surface-treated through a hot soft nip calender in the same manner as in Example C1 to give a coated printing paper. [0096] Example C3 - 29 A pulp slurry containing 85 % hardwood kraft pulp (LBKP) and 15 % softwood kraft pulp (NBKP) was used as a stock and used in a twin wire paper machine at a machine speed of 1000 m/min to make a paper, which was then coated on both sides with the above undercoating solution (surface-treating solution) at a coating mass of 4.0 g/m 2 per side using a gate roll coater and dried to give a base paper having a basis weight of 56.6 g/m 2 . [0097] As pigments, 47 parts of fine-grained precipitated calcium carbonate and 53 parts of fine-grained kaolin Hydragloss (from KaMin LLC) were added, and 10.5 parts of a synthetic polymer latex mainly composed of styrene and butadiene monomers, 2.3 parts of a water soluble polymer consisting of phosphate-esterified starch, 4 parts of the fluorescent dye, 0.009 parts of the purple pigment and 0.002 parts of the blue pigment were added based on the total of the pigments, and all of the materials were mixed/stirred at ambient temperature to give a pigment coating solution having a solids content of 67 % by weight, which was applied on both sides of the base paper at a coating mass of 10.5 g/m 2 per side using a blade coaster at a coating speed of 1000 m/min and dried. The resulting coated paper was surface treated through a hot soft nip calender in the same manner as in Example C1 to give a coated printing paper. [0098] Example C4 A coated printing paper was obtained in the same manner as in Example C1 except that a pigment coating (top coating) solution contained 30 parts of fine-grained precipitated calcium carbonate and 70 parts of fine-grained kaolin Hydragloss (from KaMin LLC) in place of 54 parts of fine-grained precipitated calcium carbonate and 46 parts of Hydragloss (from KaMin LLC) as pigments. [0099] Comparative example 3 A coated printing paper was obtained in the same manner as in Example C1 except that the pigment coating (undercoating) solution and the pigment coating (top coating) solution did not contain the purple and blue pigments. [0100] Comparative example 4 A coated printing paper was obtained in the same manner as in Example C3 except - 30 that the pigment coating (undercoating) solution and the pigment coating (top coating) solution did not contain the purple and blue pigments. [0101] The formulations of Examples C and Comparative examples 3-4 and the performances of the resulting printings papers are shown in Tables 5 and 6. [0102] E -31

C)

m- 0 0 U 0 -D 0 S t 00 C:.) 0 0 C CD o) 0. 00 .50 5 C.0 r41 0 00 co 0~~ 0 0r. 0 (A 0 C Cl o 0. W 0 0 z 00 ~ 0 0 u tin i- 6.-' 00 -L I ,c. -5 u.

-32 E m Nn 6q 0 0r U> ~r~oV)f o6.~< 0? 0 E "DN N1 C, VE -n XIO X 000 W N 0 "D 002 o M "~ E C\ M NN kr))O q in OV - o w E C\ mO~ 0 N 0 Ur *-* 00 \0 r- Um((D ( E~ CN 0 w- p u 0 0 a 4. Q - 33 [0104] As shown in Table 6, the printing papers obtained in the present examples are excellent coated printing papers characterized by combined high brightness and high opacity, invisible print through during printing, especially during offset printing and a wide range of color reproduction. In contrast, the Comparative examples containing no blue-purple pigment were inferior to the products embodying the present invention in apparent whiteness, print through during printing, color unevenness, etc. [0105] It is understood that any acknowledgment of any prior art in this specification is not to be taken as an admission that this acknowledged prior art forms part of the common general knowledge in Australia or elsewhere. [0106] Where the terms "comprise , comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Claims

1. A coated paper for offset printing which has a basis weight of 70 g/m2 or less containing one or more colorants selected from the group consisting of purple pigments, purple dyes, blue pigments and blue dyes and having an opacity of 85 % or more, said printing paper having an a* value of 0 or more and less than 7 and a b* value of -15 or more and less than -3 as determined according to the method of JIS P 8150 using a light source comprising a UV component.

2. The coated paper for offset printing of claim 1 wherein the colorant exists in any one of layers constituting the coated paper for offset printing.

3. The coated paper for offset printing of claim 1 or 2 which is a coated paper comprising a base paper layer and a pigment coating layer wherein the colorant exists in the pigment coating layer.

4. The coated paper for offset printing of claim 1 or 2 which is an uncoated paper wherein the colorant exists in a base paper layer.

5. The coated paper for offset printing of claim 1 which has more than one layer and a basis weight of 70 g/m 2 or less wherein the colorant exists in multiple layers.

6. The coated paper for offset printing of claim 5 which is a coated paper comprising a base paper layer and a pigment coating layer wherein the colorant exists in the base paper layer and the pigment coating layer.

7. The coated paper for offset printing of claim 5 or 6 which is a coated paper comprising a base paper layer, a clear coating layer and a pigment coating layer wherein the colorant exists in more than one layer selected from the base paper layer, the clear coating layer and the pigment coating layer.

8. The coated paper for offset printing of claim 5 which is an uncoated paper comprising a base paper layer and a clear coating layer wherein the pigment exists in the base paper layer and the clear coating layer.

9. The coated paper for offset printing of any one of claims 1-8 wherein the colorant is a purple pigment or a blue pigment. - 35

10. The coated paper for offset printing of claim 9 wherein the total content of the purple and blue pigments is 0.7-3.5 mg/m2

11. The coated paper for offset printing of any one of claims 1-10 wherein 60 % by weight or more of chemical pulp is contained in the stock.

12. A coated paper for offset printing which has a basis weight of 70 g/m 2 or less substantially herein described with reference to the examples.