CA2058023A1

CA2058023A1 - Coating composition for newsprint

Info

Publication number: CA2058023A1
Application number: CA002058023A
Authority: CA
Inventors: Takahiro Shimazoe; Kenzo Miyamoto
Original assignee: Individual
Current assignee: Zeon Corp
Priority date: 1990-12-19
Filing date: 1991-12-18
Publication date: 1992-06-20
Also published as: FI915965A; KR920012659A; JPH0544197A; DE4141860A1; FI915965A0; FR2670810A1

Abstract

Abstract of the Disclosure There is provided a coating composition for newsprint excellent in offset printabilities at the time of cold set type high speed web offset printing such as surface strength, print gloss and ink receptivity. The composition comprises 100 parts by weight of a pigment, preferably 50 weight % or more of which is clay, and 3 to 50 parts by weight, preferably 5 to 40 parts by weight in terms of solid of an amphoteric copolymer latex obtained by copolymerizing an ethylenic unsaturated amine with butadiene or the like. The composition may be one containing 15 parts by weight or less, preferably 10 parts by weight or less of a water soluble polymer binder such as a phosphoric acid-esterified starch, in addition to the amphoteric copolymer latex, in such a range that the total amount of the pigment binders becomes 3 to 50 parts by weight in terms of solid per 100 parts by weight of the pigment.

Description

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COAT I NG COMP OS I T I ON FOR NEWS PR I NT

This invention relates to a coating composition for paper for printing of newspaper thereinafter, referred to as "a coating composition for newsprint") suitable for mul~i-color offset printing. More speci-fically, this invention relates to a coating compositionfor newsprint excellent in offset printabilities at the time of cold set type high speed web offset printing such as surface strength, print gloss and ink receptivity.
Recently, multi-color printing is conspicuous in daily newspapers, particularly, sports papers, and multi-color printing of the advertisement pages of daily papers is also rapidly increasing. However, these multi-color printing papers are poor-looking compared with inserted leaflets whose material is often coated paper, and thus short of ability appealing to readers.
On the other hand, due to the social mission of newspapers, it is required to print them in large quan-tities in a short time. Consequently, printing speed is made faster and faster, and improvement of offset print-abilities such as picking and an ink set is desired.
Heretofore, as a countermeasure for multi-color printing and offset printing, a method has been adopted whereby surface strength is enhanced using starch, polyvinyl alcohol or the like. ~owever, by this method/
suficient offset printability such as print gloss could not be obtained. Recently, in order to improve these characteristics, pigment coating is proposed wherein the amount of the oil to be absorbed in the coated layer is enhanced by selection of pigment (Japanese Laid-Open Patent Publication Nos. 174697/1989 and 169768/1990).
Further, a method for improvement is known wherein a large amount of a pigment binder or an organic pigment is used (Japanese haid-Open Patent Publication Nos.

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19595/1990 and 74698/1990). However, these methods cannot satisfy all of the requirements for offset print-abilities in a cold set web offset press using a cold set type ink such as an ink set, surface strength, print gloss, and water dissipation by use of fountain solution.
Moreover, these method could not provide newsprint having brightness, opacity, etc.~ in addition to the above offset printabilities.
The present inventors studied for solving the disadvantages of the prior art, and considered, as one of methods for solution, to use a usual coating composition for slightly coated paper as it is, but merely with a reduced coating amount thereof. However, in this method, in the case of multi-color offset printing using a cold set type ink, the coated paper has a poor oil absorption property and a poor water absorp~ion property and thus is poor in an ink set and ink receptivity, and consequently, newsprint excellent in offset printabilities and thus suitable for color printing could not be obtained.
Therefore, the present inventors further continued to study intensively. As a result, they found that, by using an amphoteric copolymer latex as a pigment binder and setting the amount of the pigment binder in an appropriate range, but without changing the pigment composition having hitherto generally been used for coated paper and slightly coated paper, there can be obtained newsprint which has remarkably improved offset printabilities such as an ink set, water dissipation, surface strength (particularly wet pick resistance) and print gloss, and is also good in brightness, opacity, etc., and completed this invention based on this finding.
Thus according to this invention there is provided a coating composition for newspaper-printing paper which comprises 100 parts by weight of a pigment and as a pigment binder 3 to 50 parts by weight in terms of solid of an amphoteric copolymer latex.
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, - . , :, , , ,: , : , ': ,.. . . .. ' According to this invention is further provided a coating composition for newsprint which comprises as a pigment binder 15 parts by weight or less of a water soluble polymer compound in addition to the amphoteric copolymer latex, in such a range that the total amount of the pigment binders becomes 3 to 50 parts by weight in terms of solid per 100 parts by weight of the pigment.
The amphoteric copolymer latex to be used as a pigment binder in this invention may be a copolymer latex which gelates at a certain pH, and can be obtained by introducing a specific monomer unit into a copolymer or polymerizing monomers using an amphoteric surfactant, or by another hitherto known method.
Although monomers to be used for synthesis of the amphoteric copolymer latex to be used in this inven-tion and their compositions are not particularly limited, a monomer mixture is usually used comprising 20 to 50 weight % of an aliphatic conjugated diene monomer, 10 to 74 weight ~ of an aromatic vinyl monomer, 0.5 to 20 weight % of an ethylenic unsaturated acid monomer and 0 to 40 weight % of a monomer copolymerizable with these monomers.
The aliphatic conjug~ted diene monomer imparts flexibility to the polymer. When the amount is less than 20 weight %, the copolymer becomes too hard, and when it is above 50 weight %, water resistance is lowered.
Specific examples of the aliphatic conjugated diene monomer are 1,3-butadiene, 2-methyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., and 1,3-butadiene is particularly preferred.
By use of the aromatic vinyl monom~r, effects of imparting appropriate hardness and water resis~ance to the copolymer are obtained. In an amount below 10 weight % sufficient effects cannot be obtained, and in amount above 74 weight ~ its film formation property is lowered and its adhesive property is lowered. ~xamples of the .

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aromatic vinyl monomer are styrene, ~-methylstyrene, monochlorostyrene, vinyltoluene, etc., and styrene is prèferred among them.
The ethylenic unsaturated acid monomer is suitably used for enhancing the adhesive strength of the copolymer and improving the colloidal stability of the copolymer latex. When the amount is below 0.5 weight ~, the above effects cannot be obtained, and when it is used in an amount above 20 weight %, a problem takes place in polymerization stability or the like. Further, when an ethylenic unsaturated amine monomer is used, the ethylenic unsaturated acid monomer is also effective for adjus-tment of the gelation point. In this case, when the amount of the ethylenic unsaturated acid monomer is too large, the gelation point of the resulting copolymer latex becomes out of the object of the invention.
Specific examples of the ethylenic unsaturated acid monomer are unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid and butene-tricarboxylic acid; unsaturated polycarboxylic acid alkyl esters having at least one carboxyl group such as itaconic acid monoethyl ester, fumaric acid monobutyl ester and maleic acid monobutyl ester; unsaturated sulfonic acids or their salts such as acrylic acid sulfoethyl sodium salt, methacrylic acid sul~opropyl sodium salt and acrylamidopropanesul~onic acid; etc.
In synthesis of an amphoteric copolymer latex to be used in this invention, a monomer copolymerizable with the above monomers can be copolymerized. Such a monomer, if necessary, is used preferably in the range of 40 weight % or less. Specific examples of such monomers are unsaturated carboxylic acid alkyl esters such as methyl acrylate, methyl methacrylate, ethyl acrylate and butyl acrylate; ethylenic unsaturated nitrile compounds such as acrylonitrile and methacrylunitile; unsaturated :

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carboxylic acid hydroxyalkyl es-ters such as ~-hydroxy-ethyl acrylate, ~-hydroxypropyl acrylate and ~-hydroxy-ethyl methacrylate; unsaturated carboxylic acid amides and their derivatives such as acrylamide, methacrylamide, N-methylolacrylamide and diacetoneacrylamide; unsaturated carboxylic acid glycidyl esters such as glycidyl acryla~e and glycidyl methacrylate; other vinyl compounds such as acrolein and allyl alcohol, etc. The unsaturated car-boxylic acid ester imparts moderate hardness to the copolymer, and, at the same time, is effective to enhance air permeability, and ~he amount is preferably 5 to 30 weight %. In an amount above 40 weight %, it has an desirable influence on water resistance, etc. The ethylenic unsaturated nitrile is effective to enhance the print gloss of the coated paper, and its amount is pref-erably S to 30 weight %. In an amount above ~0 weight %, the ink receptivity and ink set of the coated paper is 10~7ered. The unsaturated carboxylic acid hydroxyalkyl ester is effective for improvement of the mechanical stability of the copolymer latex. By use of the unsaturated carboxylic acid amide or its derivative, it is possible to improve the chemical stability, mechanical stability and water resistance of the copolymer latex.
When a comonomer is used to introduce a gelation site into the copolymer as a method of obtaining an amphoteric copolymer latex, its amount is 0.5 to 20 weight %, preferably 1 to 10 weight % based on all the monomers. In an amount below 0.5 weight %, it is difficult to make the latex amphoteric, whereas in an amount above 20 weight ~, it becomes difficult to impart a gelation point suitable for the composition of the invention, and, besides, there arise problems~ for example, that a large amount of coagula is formed at the time of preparation of a copolymer latex.
As examples of a monomer to introduce a gelation site into the copolymer, there can be mentioned : ,: - -.

ethylenic unsaturated amines represented by the general formula Rl R3 CH2=C-A-R2-N

, wherein Rl represents H or a methyl group, R2 represents an alkylene group having 2 to lO carbon atoms, R3 and R~ each independently represents H or an alkyl group having l to 12 carbon atoms, and A represents -C(=O~-, -C(=O)NH- or -O-.
Specific examples of the ethylenic unsaturated amine are ethylenic unsaturated carboxylic acid aminoalkyl esters such as methylaminoethyl (meth)acryl-ate, t-butylaminoethyl (meth)acrylate, N,N-dimethylamino-ethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)-acrylate, diethylaminoethyl (meth)acrylate and dibutyl- i 15 aminoethyl (meth)acrylate; ethylenic unsaturated ~:
carboxylic acid aminoalkylamide such as methylaminoethyl (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylamide ;
and N,N-dimethylaminopropyl (meth)acrylamide; aminoalkyl vinyl ethers such as aminoethyi vinyl ether, methylamino-ethyl vinyl ether and N,N-dimethylaminoethyl vinyl ether;
vinylpyridines such as 2-vinylpyridine, 4-vinylpyridine, 2-methyl-5-vinylpyridine and 2,4-diethyl-5-vinylpyridine;
etc.
An amphoteric copolymer latex to be used in `
2~ this invention can also be obtained by using a suitable kind of surfactant. Such surfactants include cationic surfactants and amphoteric surfactants. In emulsion polymerization of the above monomer mixture, such a surfactant may either be used together with a nonionic surfactant or the like or added after polymerization using a nonionic surfactant or the like. The amount of -- : ~ :

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such a cationic or amphoteric surfactant is 0.1 to 5 weight %, preferably 0.2 to 2 weight ~ based on the weight of a copolymer.
As cationic surfactants to be used for the above purpose, those of quaternary ammonium salt type are suitable. Specific examples thereof are, for example, lauryltrimethylammonium chloride, cetyltrimethylammonium chloride, lauryltrimethylammonium bromide, beef tallow trimethylammonium chloride, N-(dodecylcolaminoformyl-methyl)pyridinium chloride, cetylethylmorphonium etho-sulfate, etc. As specific examples of the amphoteric surfactant, alkylimidazoline type amphoteric surfactants, etc. can be mentioned.
The gelation point of the amphoteric copolymer latex can be adjusted by changing the kind and amount of monomers to be used in synthesis of the copolymer such as an ethylenic unsaturated acid monomer, an ethylenic unsaturated amine monomer and a hydrophilic monomer. The gelation point can also be adjusted by changing the kind or amount of an emulsifier or the kind or amount of an initiator. It is preferred to adjust the gelation point of the copolymer latex to be used in this invention in the range of pH 3.5 to 8.5. The gelation point can be measured by the method disclosed in Japanese Patent Publication No. 49360/1984.
An amphoteric copolymer latex to be used in this lnvention can be prepared by a known emulsion poly-merization method. There is no particular limitation on the polymerization temperature and the pressure, the way of addition oE the monomers, etc., and subsidiary mat~rials to be used for polymerization (a polymeriza~ion initiator, a molecular weigbt modifier~ a pH adjusting-agent and the like~.
Pigments to be used in this invention are not particularly limited, and there can be mentioned as specifîc examples mineral pigments such as clay, calcium ..
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The coating composition for newsprint of this invention comprises a pigment and an amphoteric copolymer latex as an indispensable pigment binder.
The amount of the amphoteric copolymer latex is 3 to 50 parts by weight in terms of solid per 100 parts by weight of the pigment, but preferably 5 to 40 parts by weight, more preferably 10 to 40 parts by weight. When this amoun~ is below 3 parts by weight, it becomes difficult to suppress binder migration in the drying process at the time of preparation of coated newsprint, and consequently, ink receptivity becomes unsatisfactory and offset printabilities such as surface strength and print gloss become poor. On the other hand, in an amount above 40 parts by weight blocking resistance lowers due to tackiness, etc. of the latex and runnability becomes bad. Particularly, in an amount above 50 parts by weight~ this tendency becomes impractically terrible, and at the same time the brightness, opacity, water dissipa~
tion, ink set and the like of the resulting coated news-print becomes bad.
In this invention, it is possible to use a water soluble polymer binder together as a pigment binder. Although use of the water soluble binder is not indispensable, by using it together, i~ becomes easier to adjust the viscosity of a coating color and there arise effects in imparting opacity and adhesive strength to the newsprint. Specific examples of the water soluble polymer binder are starches such as phosphoric acid-esterified starFh and enzyme-modified atarch; casein;
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polyvinyl alcohol; cellulose derivatives such as methyl-cellulose and carboxylmethylcellulose; etc., but other water soluble polymer binders which are usually used in this technical fields may also be used.
The amount of the water soluble polymer binder is ~5 parts by weight or less, preferably 10 parts by weight or less in terms of solid per 100 parts by weight of the pigment. When the amount of the water soluble polymer binder is above 15 parts by weight, increase of the viscosity of the coating color and decrease of fluidity take place and thus coating runnability becomes bad. The amount of the water soluble polymer binder must be an amount in such a range that the total amount of it and the amphoteric copolymer latex as an indispensable pigment binder is 3 to 50 parts by weight per 100 parts by weight of the pigment.
Further, so far as the effect of this invention is not impaired, it is also possible to use together as a pigment binder a conventional styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex or the like in the coating composition of the invention.
Further, it is possible to add, if necessary, to the coating composition of this invention, besides water, the pigment and the pigment binder, a pigment-dispersing agent, a viscosity-modifier, water-retention agent, insolubilizerl a dye, a fluorescent dye, a lubricant, a pH-adjusting agent, an antifoaming agent, a surfactant, an antiseptic or the like. In this inven-tion, it is necessary to maintain the pH of the coating composition to a pH equal to or higher than the gelation point of the amphoteric copolymer latex. When the pH is below the gelation point, the improvement effect of ink receptivity lowers. Although the pH-adjusting agent is not particularly limited, ammonia water is pre~erably USed.
Although the reason why the coating composition . . .

of this invention has excellent offset printabilities is not certain, the following is considered as one possible reason. Namely it is generally considered that poor ink receptivity, etc. is caused by ununiformity of the distribution of the binder at the drying process in ooating paper with a coating compositio~, and that the ununiformity is caused by so-called migration. On the other hand, when the amphoteric copolymer latex is used as a pigment binder, the p~ of the coating composLtion is lowered at the drying process and the amphoteric copolymer latex gelates at the gelation point, and thus migration does not occur~ Thus, binder distribution becomes uniform and excellent printabilities are obtained.
This invention is more specifically illustrated by the following examples. Unless otherwise indicated, parts and ~ in the examples are based on weight.
Further, the weight of latexes is in terms of solid.
In examples, various characteristics of coating compositions and coated papers were evaluated by the following methods.
Coating color viscosity A BL type viscometer is used. Color viscosity is expressed by the value (unit: cp~ measured after 1 minute at 60 rpm using the No. 4 rotor.
Color fluidity A Hercules type high shear viscometer is used.
Color fluidity is expressed by the apparent viscosity ~cp) at a revolution number of 4,000 rpm with G bob (clearance 0.02 cm).
Ink set By use of an RI tester (produced by Akira Seisaku-sho Co., Ltd.), solid printing was carried out with a newspaper ink and a coated paper is superposed thereon. Then, the degree of transfer of the ink is judged by visual observation.~ Ink set is evaluated by a ; . . ~ .: ...., : ~ : .
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2 ~ 7~ i five point method. 5 is best and 1 is worst.
Water dissipation After application of fountain solution by a rubber roller, a coated newsprint is subjected to solid printing using an RI tester and a newspaper ink, and the degree of transfer of the ink (the concentration of the ink) is judged by visual observation~ Water dissipation is evaluated by a five point method. 5 is best and 1 is worst.
Dry pick resistance The degree of picking when printing was carried out using an RI tester and an ink for a picking test (its tack value is 24) is judged by visual observation. Dry pick resistance is evaluated by a five point method. 5 is best and 1 is worst.
Wet pick resistance After application of water by a Molton roller, a coated newsprint is subjected to solid printing using an RI tester and an ink for a picking test (its tack value is 16) and the degree of picking is judged by visual observation. Wet pick resistance is evaluated by a five point method, and 5 is best and 1 is worst.
Print gloss After solid printing is carried out using an RI
tester and a newspaper ink, print gloss is measured by a glossmeter (produced by Murakami Shikisai Co., ~td.
GM-26D).

Brightness is measured using a Hunter reflecto-meter tproduced by Murakami Shikisai Co., Ltd. CM-8P) and expressed by %. The larger the value is, the larger brightness is.
Opacity Opacity is measured using a Hunter reflecto-meter (Produced by Murakami Shikisai Co., Ltd. CM~8P) and expressed by %. The larger the value is, the larger --~

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opacity is.
slockin~ resistance Blocking resistance is expressed by the degree of adhesion of a coated newsprint to a metal-made calender roll when the paper was twice passed through a supercalender at 70C and 150 kg/cm.
O : No adhesion : A little adhesion X : Considerable adhesion [Preparation of amphoteric copolymer latexes]
Into an autoclave equipped with a stirrer were charged 32 parts of water, 1.1 parts of tetrasodium ethylenediaminetetraacetate, 1.1 parts of sodium lauryl sulfate and 0.2 part of potassium persulfate, and then a 10 ~ portion of a monomer emulsion separately prepared in a tank equipped with a stirrer and having a composition shown in Table 1 was added. The reaction system was heated to 80C to carry out reaction for 1 hour. Then, 0.8 part of potassium persulfate was added together with 20 parts of water, and then the remaining 90 % portion of the monomer emulsion was continuously supplied into the autoclave over a period of 4 hours. During the time, the autoclave was maintained at 80C. Thereafter, the reaction system was maintained at 80C for further one hour, and then ammonia water was added to adjust the pH
of the latex to 9Ø Then, 4.0 parts of water, 2.0 parts of N,N-dimethylaminoethyl methacrylate and 0.1 part of ammonium persulfate were added and reaction was carried out at 80C for 2 hours. Thus, amphoteric copolymer latexes A, B and C were obtained. All the gelation points of these latexes were 7.D.

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Into an autoclave equipped with a stirrer were charged 32 parts of water, 1.1 parts of tetrasodium ethylenediaminetetraacetate, 1.1 parts of sodium lauryl sulfate and 0.2 part of potassium persulfate, and a 10 portion of a monomer emulsion separately prepared in a tank equipped with a stirrer and having a composition shown in Table 2 was added. The reaction system was heated to 80C to be allowed to react for 1 hour. 0.8 part of potassium persulfate was added together with 20 parts of water, and the remaining 90 % portions of the monomer emulsion was continuously supplied into the autoclave over a period of 4 hours. During the time~ the autoclave was maintained at 80c. The reaction was continued at 80C for further 2 hours. Consequently, latexes D, E and F were obtained. None of these latexes exhibited any gelation point.

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[Coating compositions for newsprint]
Coating compositions for newsprint 1 to 6 each having a solid concentration of 50 -~ and a pH of 9.5 were prepared by a coating color formulation shown in Table 3 using the latexes A to F.

Table 3 . . ~ . . __.
Clay 60 parts Ground calcium carbonate 40 Dispersant (1) 0.2 Sodium hydroxide 0.2 Phosphoric acid-esterified starch (2) 6 Latex 20 (1) Sodium polyacrylate produced by Toagosei Chemical Co., Ltd., Aron T-40 (2) produced by Nihon Shokuhinkako Co., Ltd., ~Example 1]
Sheets of base paper for newsprint were coated with each of the thus obtained coating compositions under 10 conditions shown in Table 4~ Resulting coated newsprint was kept in an air-conditioned room of 20C and a relative humidity of 65+5 % for a whole day and night, and then subjected to various evaluativn tests. The results are shown in Table 5.

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Table 4 Method : Laboratory blade coater Drying : Hot~air oven, 130C x 20 seconds Calender : Laboratory supercalender 40C, 55 kg/cm, 2 nips Base paper : Newsprint (46 g~m ) Coating weight: 5 g/m2/side _ :` : :

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From the results of Table 5, it is seen that in the case where the coating compositions l to 3 of this invention co~prising an amphoteric latex were used, coated newsprint excellent in an ink set, water dissipa-tion, surface strength, print gloss, brightness andopacity could be obtained, whereas in the case of the coating compos.itions 4 to 6 wherein usual latexes were used, only coated papers poor in these characteristics could be obtained.
[EXample 2]
Coated newsprint were prepared in the same manner as in Example l except that coating compositions 7 to 12 were used obtained by coating color formulations shown in Table 6 using the latex A or D, and subjected to the same various evaluation tests as in Example 1. The results are shown in Table 7.

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From the results of Tabl~ 7, it is seen that in the case where the coating compositions 7 to 9 of the invention were used, as the amount of the pigment binder was increased, surface strength and print gloss were enhanced and at the same time coated newsprint excellent in an ink set, water dissipation, brightness and opacity were obtained, whereas in the case of coating composi-tions 10 to 12 wherein usual latexes were used, although print gloss and surface strength were enhanced in accordance with increase of the amount of the pigment binder, an ink set and water dissipation were strikingly lowered and only coated newspaper could be obtained which did not withstand practical use as newsprint.
[Example 3]
Coated newsprint were prepared in the same manner as in Example 1 except that coating compositions 13 to 20 obtained by coating color formulations shown in Table 8 using the latex A were used, and subjected to the same various evaluation test as in Example 1.
The results are shown in Table 9.

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It is seen from the results of Table 9 that in the case where the coating compositions of the invention were used, coated newsprint were obtained excellent in an ink set~ water dissipation, surface strength, print gloss, brightness and opacity, whereas in the case where the amount of the pigment binder was too large (composi-tion 18), a coated newsprint strikingly poor in an ink set and water dissipation was obtained, in the case where the amount of tbe amphoteric copolymer latex was too small (composition 19), a coated newsprint having strikingly low surface strength was obtained, and in the case where the amount of the water soluble polymer binder was too large (compositions 19 and 20), coating color viscosity became too high and coated papers poor in various characteristics were obtained.
Thus according to this invention, it is possible to obtain a coated newsprint more excellent in an ink set, water dissipation, surface strength (parti-cularly, wet picking resistance and print gloss as cold set type of~set printabilities and comparable brightness and opacity, compared with the prior art.

, - :

Claims

1. A coating composition for newsprint which comprises 100 parts by weight of a pigment and as a pigment binder 3 to 50 parts by weight in terms of solid of an amphoteric copolymer latex.

2. The coating composition for newsprint of claim 1 which comprises as a pigment binder 15 parts by weight or less of a water soluble polymer in addition to the amphoteric copolymer latex, in such a range that the total amount of the pigment binders becomes 3 to 50 parts by weight in terms of solid per 100 parts by weight of the pigment.

3. The coating composition for newsprint of claim 1 which comprises 100 parts by weight of the pigment and as a pigment binder 5 to 40 parts by weight in terms of solid of the amphoteric copolymer latex.

4. The coating composition for newsprint of claim 2 which comprises as a pigment binder 10 parts by weight or less of the water soluble polymer in addition to the amphoteric copolymer latex, in a range such that the total amount of the pigment binders becomes 3 to 50 parts by weight in terms of solid per l00 parts by weight of the pigment.

5. The coating composition for newsprint of claim 3 which comprises as a pigment binder 10 parts by weight or less of the water soluble polymer in addition to the amphoteric copolymer latex, in a range such that the total amount of the pigment binders becomes 5 to 40 parts by weight in terms of solid per 100 parts by weight of the pigment.

6. The coating composition for newsprint of any of claims 1 to 5 wherein the amphoteric copolymer latex is one obtained by copolymerizing a monomer mixture comprising 20 to 50 weight % of an aliphatic conjugated diene monomer, 10 to 74 weight % of an aromatic vinyl monomer, 0.5 to 20 weight % of an ethylenic unsaturated acid monomer, 0.5 to 20 weight % of a monomer to intro-duce a site for making the copolymer gelate into the copolymer, and 40 weight % or less of a monomer copoly-merizable with them.

7. The coating composition for newsprint of any of claims 1 to 5 wherein the amphoteric copolymer latex is one obtained by copolymerizing a monomer mixture comprising 20 to 50 weight % of an aliphatic conjugated diene monomer, 10 to 74 weight % of an aromatic vinyl monomer, 0.5 to 20 weight % of an ethylenic unsaturated acid monomer, 0.5 to 20 weight % of an ethylenic unsaturated amine as a monomer to introduce a site for making the copolymer gelate into the copolymer, and 40 weight % or less of a monomer copolymerizable with them.

8. The coating composition for newsprint of any of claims 1 to 5 where in the amphoteric copolymer latex is one obtained by copolymerizing, using an amphoteric surfactant, a monomer mixture comprising 20 to 50 weight % of an aliphatic conjugated diene monomer, 10 to 74 weight % of an aromatic vinyl monomer, 0.5 to 20 weight of an ethylenic unsaturated acid monomer and 40 weight %
or less of a monomer copolymerizable with them.