SHEET SURFACE TREATING AGENT AND INK-JET PRINTING PAPER

Abstract: There is provided a treating agent for a sheet surface comprising a graft copolymer composed of a back bone polymer and branch polymer, either of which is a polymer having vinyl alcohol units, and the other of which is a polymer having cationic groups, the treating agent is applied onto a pulp sheet or plastic sheet, and the coated sheet is used in ink jet printing. Cationic groups are introduced by graft copolymerization using di(meth)allylamine-based and/or (meth)acryl-based monomer. Alternatively, the graft copolymer of the present invention is obtained by grafting N-vinylcarboxylic acid amide monomer followed by hydrolysis thereof. A formula weight ratio of vinylalcohol units and the introduced cationic groups of 1:20 to 2:1 is preferable in terms of balanced performance.

Description: Technical Field The present invention relates to a treating agent for a sheet surface and a paper for ink jet printing which has been treated with the agent. Background Art Ink jet printing is a non-contact printing method that offers numerous advantages including high-speed printing, printing at low noise levels, ease of performing color printing, or the like, and consequently has been rapidly popularized for use in printers and plotters. Ink jet printing allows printing on ordinary printing paper, coated paper, PPC paper (paper for plain paper copy), medium-quality paper and even plastic film. As a paper for use in ink jet printing, there is a coated paper for obtaining a high image quality, which has a coating layer containing synthetic silica or the like. In contrast, as low-priced common-use paper, one obtained by penetrate-treating paper with starch or the like by means of a size press is used. Common-use paper has the major problem of the occurrence of so-called feathering when it is printed with an aqueous ink which is used in the ink jet printing. Since the ink used for the ink jet printing is both aqueous and anionic, a cationic waterproofing agent is applied to the surface of the paper. Although it is effective to coat or penetrate-treat paper with a cationic polymer to improve water resistance, this results in the occurrence of the problem of decreased color density during printing. Various methods have been proposed to inhibit this decrease in color density, examples of which include a method wherein nonionic resin fine particles are combined with nonionic, anionic or cationic water-soluble polymer and coated (Japanese Patent Laid-Open Publication No. 9-1925), a method wherein emulsion particles are coated which are synthesized by copolymerization of acrylonitrile and acrylic esters to increase printing density (Japanese Patent Laid-Open Publication No. 8-50366), a method wherein cationic emulsion particles of acrylic esters are coated (Japanese Patent Laid-Open Publication No. 9-99632), and a method wherein colloidal silica and a water-soluble polymer are coated for the purpose of improving dye color development and printing density (Japanese Patent Laid-Open Publication No. 9-109544). However, since all of these methods use water-insoluble fine particles, they bond weakly with the ink dye for ink jet printing, and the water-soluble cationic polymer that is used in combination with them does not demonstrate adequate water resistance. In addition, the use of amixture of polymer having vinyl alcohol units such as polyvinyl alcohol and a cationic polymer for ink jet printing paper is known. Polyvinyl alcohol has a good film-forming ability, and has the effect of inhibiting decreases in color density. On the other hand, cationic polymers have the effect of increasing water resistance. However, polyvinyl alcohol and cationic polymer have poor miscibility, and it is difficult to apply their mixture uniformly in the microscopic state. Consequently, these substances have shortcomings that include large decreases in color density, thereby requiring further improvement. The use of a copolymer comprising a polymer moiety having vinyl alcohol units and a polymer moiety having cationic monomer units for ink jet printing paper as being useful in the present invention has heretofore been unknown. Moreover, due to the considerable increase in viscosity that occurs when a conventional cationic polymer or the like is mixed into a coating color, it is necessary to dilute with water at the time of application. Thus, the polymer concentration is unable to be increased, and the coated amount of the polymer ends up being low, which had previously presented a problem. A surface treating agent that is able to effectively solve the problems of decreased quality, namely decreased color density, feathering and insufficient water resistance as mentioned above has yet to be developed. In addition, the light resistance of printed ink images or characters is also insufficient. Disclosure of the Invention An object of the present invention is to provide a treating agent for a sheet surface that improves water resistance and light resistance without decreasing color density during printing when printing with an aqueous ink, for example, one used in ink jet printing. Another obj ect of the present invention is to provide a treating agent for a sheet surface that prevents the problem of feathering that particularly occurs with common-use paper for ink jet printing. A further object of the present invention is to provide a treating agent for a sheet surface that prevents the occurrence of the considerable increase in viscosity when mixed with coating colors and can be applied in a sufficient polymer concentration. A still further object of the present invention is to provide a coated paper for ink jet printing that improves water resistance and light resistance without decreasing color density during printing. A still further object of the present invention is to provide a common-use paper for ink jet printing that prevents feathering without decreasing color density during printing, and improves water resistance and light resistance. As a result of earnest studies to solve the above-mentioned problems, the inventors of the present invention found that miscibility between polyvinyl alcohol and cationic polymers, which had presented a problem in the prior art, is improved by using a water soluble graft copolymer comprising a polymer moiety having vinyl alcohol units and a polymer moiety having cationic groups, especially a graft copolymer containing vinyl alcohol units in either the back bone polymer or the branch polymer while the other polymer has cationic groups. It was also found that by treating a sheet surface such as paper, an excellent printing is possible without decreasing color density as well as better water resistance and light resistance than current commercially available papers, thereby leading to completion of the present invention. Furthermore, the occurrence of feathering can be prevented thereby. The copolymer comprises polymer moieties which are a back bone polymer and a branch polymer, and is a graft copolymer composed of the back bone polymer and branch polymer, either of which is a polymer having vinyl alcohol units, and the other of which is a polymer having cationic groups. Furthermore, the present invention is to provide the above described treating agent for a sheet surface, in which the back bone polymer of the graft copolymer is composed of water-soluble or water-dispersible polymer having vinyl alcohol units, and the branch polymer is composed of at least one repeating unit selected from the group consisting of a repeating unit represented by the following formula (1), a repeating unit represented by the following formula (2), a repeating unit represented by the following formula (3) and a repeating unit represented by the following formula (4) : (wherein, R 1 and R 2 represent H or CH 3 , R 3 and R 4 represent hydrogen, alkyl groups having 1∼4 carbon atoms or benzyl groups, and X - represents a counter ion); (wherein, A represents O or NH, B represents C 2 H 4 , C 3 H 6 or C 3 H 5 OH, R 5 represents H or CH 3 , R 6 and R 7 represent alkyl groups having 1 ∼4 carbon atoms, R 8 represents hydrogen, an alkyl group having 1 ∼4 carbon atoms or a benzyl group, and X - represents a counter ion) ; (wherein, R 9 represents H or CH 3 , and X - represents a counter ion) ; and (wherein, R 10 and R 11 represent H or CH 3 , and X - represents a counter ion). In the preferred embodiments, the branch polymer comprises at least one repeating unit selected from the group consisting of the repeating unit represented by the formula (1) and the repeating unit represented by the formula (2). Furthermore, in the preferred embodiments the branch polymer comprises the repeating unit represented by the formula (1) and the repeating unit represented by the formula (3) and/or the repeating unit represented by the formula (4). In addition, in the preferred embodiments, the graft copolymer can be obtained by radical polymerization of a monomer composition generating at least one repeating unit selected from the group consisting of the repeating unit represented by the formula (1), the repeating unit represented by the formula (2), the repeating unit represented by the formula (3), and the repeating unit represented by the formula (4) in the presence of the water-soluble or water-dispersible polymer having vinyl alcohol units. According to the invention, the formula weight ratio of vinyl alcohol units of the water-soluble or water-dispersible polymer having vinyl alcohol units and the cationic groups is from 1:20 to 2:1. Still further, the proportion of vinyl alcohol units contained in the water-soluble or water-dispersible polymer having vinyl alcohol units is preferably from 70 mol% to 100 mol%. In the radical polymerization reaction, the pH of the reaction system may be preferably from 1.0 to 6.0. Furthermore, the degree of polymerization of the water-soluble or water-dispersible polymer having vinyl alcohol units may be preferably from 100 to 2500. Further, in the preferred embodiments, the water-soluble or water-dispersible polymer having vinyl alcohol units may be grafted by 40% or more by radical polymerization. Preferably, a copolymer is used having the following solubility: when 10 times weight of methanol is added to the polymer aqueous solution obtained after the grafting reaction in which the concentration of the graft copolymer mixture is 20 wt%, to form a precipitate, the amount of dry matter of the formed precipitate is 60 wt% or less of the water-soluble or water-dispersible polymer having vinyl alcohol units used as raw material. Further, the intrinsic viscosity of the polymer mixture after grafting reaction in 2% aqueous ammonium sulfate solution at 25°C may preferably be from 0.1 to 2.0 dl/g. Still further, the monomer generating the repeating unit represented by formula (1) maypreferablybe a salt of diallylamine, a salt of diallylmonomethylamine, or a salt of diallyldimethylamine. In addition, the monomer generating the repeating unit represented by formula (2) may preferably be a salt or quaternary compound of a dialkylaminoethyl(meth)acrylate or a salt or quaternary compound of a dialkylaminopropyl(meth)acrylamide. In addition, in the preferred embodiments the monomer generating the repeating unit represented by formula (3) is N-vinylformamide or N-vinylacetamide. Furthermore, the monomer generating the repeating unit representedby formula (4) may preferably be a monomer composition of N-vinylformamide and acrylonitrile. Still further, the present invention is to provide the above treating agent for a sheet surface, wherein said graft copolymer is a graft copolymer in which a vinyl ester of a carboxylic acid is graft copolymerized with a polymer of monomers comprising an N-vinylcarboxylic acid amide, or a hydrolysis product of the polymer as the back bone polymer raw material, to form a branch copolymer, and the branch copolymer is made to contain vinyl alcohol units by hydrolyzing the resulting graft copolymer. Furthermore, the present invention is to provide a paper for ink jet printing prepared by coating a coating color comprising the above described treating agent, a filler, and a binder onto a sheet surface. Further, the present invention is to provide a paper for ink jet printing prepared by making a treating solution comprising the above mentioned treating agent penetrate into a sheet surface. Furthermore, the paper preferably comprises the treating agent in an amount of from 0.02 to 5 g/m 2 . Best Mode for Carrying out the Invention The present invention is described in detail hereinafter. The treating agent according to the present invention comprises a graft copolymer comprising a polymer moiety having vinyl alcohol units and a polymer moiety having cationic groups. The graft copolymer comprises polymer moieties which are a back bone polymer and branch polymers respectively, either the back bone polymer or branch polymer being a polymer having vinyl alcohol units, and the other being a polymer having cationic groups. Particularly, a graft copolymer having vinyl alcohol units in its back bone polymer may be preferably used. The graft copolymer of the present invention can be obtained by grafting monomers capable of generating cationic repeating units on a water-soluble or water-dispersible polymer having vinyl alcohol units or units capable of forming vinyl alcohol groups upon subsequent hydrolysis. The water-soluble or water-dispersible polymer containing vinyl alcohol units that serves as the raw material of the present invention may be preferably obtained by alkaline hydrolysis of a homopolymer of a vinyl ester of a carboxylic acid, or a copolymer of a vinyl ester of a carboxylic acid with copolymerizable monomers. Since polyvinyl alcohol, a hydrolysis product of polyvinyl acetate, is widely used in practical terms, polyvinyl alcohol that is a hydrolysis product of polyvinyl acetate may be most preferably used in the present invention as well. Examples of vinyl esters of carboxylic acids include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate. In addition, typical examples of the above-mentioned copolymerizable monomers include nonionic, cationic or anionic monomers such as ethylene, styrene, salts or quaternary compounds of dimethylaminopropyl(meth)acrylamide, salts or quaternary compounds of diallylamine, N-vinylformamide, N-vinylacetamide, vinylsulfonate, acrylamido-2-methyl-propanesulfonate, (meth)acrylic acid, itaconic acid, dimethylacrylamide and N-isopropylacrylamide. Vinyl alcohol units are typically formed within the polymer by alkaline hydrolysis of a homopolymer or copolymer of these monomers. In the present invention, the vinyl alcohol units in the water-soluble or water-dispersible polymer having vinyl alcohol units represent preferably 70∼100 mol%, more preferably 85∼100 mol%, and are considered to become the back bone polymer of the graft copolymer. In the case the vinyl alcohol units are outside this range, the grafting rate decreases, thereby preventing the object of the present invention from being achieved. The degree of polymerization of the water-soluble or water-dispersible polymer having vinyl alcohol units such as polyvinyl alcohol is preferably 100 to 2500, and more preferably 300 to 2000. It is preferable that the branchpolymers of the graft copolymer contain cationic repeating units represented by the above-mentioned formula (1), formula (2), formula (3), or formula (4). These repeating units may be present in the branch polymers either alone or a plurality of kinds thereof simultaneously. In the present invention, a graft copolymer composed of a polymer moiety having vinyl alcohol units and a polymer moiety having cationic groups can be realized by polymerizing a monomer composition generating the cationic repeating units represented by the above-mentioned formula (1) and/or formula (2) in the presence of a water-soluble or water-dispersible polymer having vinyl alcohol units that serves as the back bone polymer. In addition, a monomer composition comprising a monomer generating the repeating unit represented by the above mentioned formula (1) and a monomer generating the repeating unit represented by the above mentioned formula (3) and/or a monomer generating the repeating unit represented by the above mentioned formula (4) can be used as in the present invention. By forming the branch polymer from these monomers followed by alkali or acid hydrolysis, a graft copolymer of the present invention can be obtained, which comprises the repeating units represented by the above mentioned formulas (1) and (3) and/or (4). A graft copolymer having in its branches cationic repeating units represented by the above-mentioned formulas (3) or (4) can be obtained by graft copolymerizing an N-vinylcarboxylic acid amide monomer or a monomer composition of an N-vinylcarboxylic acid amide monomer and acrylonitrile instead of the repeating units represented by the formula (1) or formula (2) onto the above-mentioned water-soluble or water-dispersible polymer having vinyl alcohol units, followed by hydrolysis thereof. In this system, however, three types of polymers are present in the form of a mixture, namely a polymer of cationic monomers that have not been graft polymerized, the above-mentioned ungrafted water-soluble or water-dispersible polymers having vinyl alcohol units, and the formed graft copolymer. The present invention effectively demonstrates prevention of feathering and improvement of water resistance even when this mixture is used as a treating agent for a sheet surface. The graft copolymer demonstrates good effects as a treating agent for a sheet surface even if it is not isolated, but rather in the state of a mixture with the other polymers. Thus, in this case, a complex isolation process is not required thereby further enhancing the practical value of the present inventionwith respect to decreased production cost. The treating agent for a sheet surface of the present invention can be prepared by radical polymerizing di(meth)allylamine-based monomers generating the repeating unit represented by the above-mentioned formula (1) and/or (meth)acrylic cationic monomers generating the repeating unit represented by the above-mentioned formula (2) after dissolving or dispersing the water-soluble or water-dispersible polymer having vinyl alcohol units in an aqueous medium. Examples of di(meth)allylamine-based monomers generating the repeating unit represented by the above-mentioned formula (1) referred to here in include salts of di(meth)allylamine, salts or quaternary compounds of di(meth)allylmonoalkylamines, and salts or quaternary compounds of di (meth) allylbenzylamines. Examples of salts referred to here in include hydrochlorides, sulfates and acetates. Examples of quaternary amine compounds include quaternary compounds with methylhalides or benzylhalides. Preferred examples include hydrochlorides, sulfates, acetates and quaternary compounds from diallylamine, diallylmonomethylamine, or diallylbenzylamine with methylhalides. Specific examples of these compounds include monomethylbenzyldiallylammonium chloride, dimethyldiallylammonium chloride, diallylamine hydrochloride, diallylamine sulfate and diallylamine acetate, while dimethyldiallylammonium chloride is particularly preferable. In addition, specific examples of (meth)acrylic cationic monomers generating the repeating unit represented by the above-mentioned formula (2) referred to herein include salts or quaternary compounds of dialkylaminoethyl(meth)acrylate and salts or quaternary compounds of dialkylaminopropyl(meth)acrylamides. Examples of salts referred to herein include hydrochlorides, sulfates and acetates. Examples of quaternary amine compounds include quaternary compounds with methylhalides and benzylhalides. Specific examples of these compounds that are particularly preferable include quaternary ammonium salts such as (meth)acryloyloxyethyldimethylbenzylammonium chloride, (meth)acryloyloxyethyltrimethylammonium chloride, (meth)acryloylaminopropyldimethylbenzylammonium chloride and (meth)acryloylaminopropyltrimethylammonium chloride, while dimethylaminoethyl(meth)acrylate salts, such as hydrochlorides, sulfates and acetates, as well as dimethylaminopropyl (meth)acrylamide salts, such as hydrochlorides, sulfates and acetates, can be used preferably. Furthermore,the repeating unitsrepresented by these formulas (1) and/or (2) may be introduced from single monomers or a mixture of a plurality of monomers. Together with the above-mentioned cationic monomers, nonionic (meth)acrylic monomers, anionic (meth) acrylic monomers or various types of vinyl monomers can also be copolymerized. In addition, cationic repeating units represented by the above-mentioned formula (3) can be introduced by graft copolymerizing an N-vinylcarboxylic acid amide onto the water-soluble or water-dispersible polymer having vinyl alcohol units using a similar preparative method followed by hydrolysis thereof. On the other hand, cationic repeating units represented by the above-mentioned formula (4) can also be introduced by graft copolymerizing an N-vinylcarboxylic acid amide and acrylonitrile onto the water-soluble or water-dispersible polymer having vinyl alcohol units using a similar preparative method followed by hydrolysis thereof. N-vinylacetamide or N-vinylformamide can be used as the N-vinylcarboxylic acid amide. Further, in the above graft copolymerization, a monomer generating a repeating unit represented by the above mentioned formula (1) can be graft copolymerized because of its hydrolysis resistance. The grafting rate of graft copolymers can be analyzedbynuclear magnetic resonance or infrared spectroscopy. In addition, the weight ratio of methanol insoluble matter to the charged amount of raw material polymer can be used as an indicator of the grafting rate of the back bone polymer by utilizing the increased solubility in methanol of the back bone polymer as a result of grafting. Namely, as grafting proceeds, the raw material polymer that has become highly cationic becomes soluble in methanol. The weakly cationic grafted polymer becomes insoluble in methanol and is measured. Thus, the difference between the charged amount of raw material (polymer and graft monomers) and the amount of insoluble matter is the amount of graftedpolymer that has become highly cationic, and is considered to indicate the minimum value of the grafting rate of the raw material polymer. More specifically, 10 times weight of methanol is added to the aqueous polymer solution after performing the graft polymerization procedure of the present invention in which the concentration of polymer mixture (concentration calculated from the sum of the amount of polymer such as polyvinyl alcohol charged as raw material and the amount of charged monomers for graft copolymerization) is 20 wt% . The formed precipitate is filtered with No. 5B filter paper and dried to a constant weight at 105°C to determine the amount of insoluble matter. The weight ratio is then determined between this amount of insoluble matter and the amount of charged polymer (such as polyvinyl alcohol). In the present invention, it is preferable that the amount of insoluble matter be 60 wt% or less. Namely, it is preferable that at least 40% of back bone polymer charged as raw material be grafted. Since homopolymers of cationic monomers are also soluble in methanol, this assay method is only valid as an indicator of grafting rate of the back bone polymer. If a method were developed for determining the grafting rate of the cationic monomers as well, it would be possible to more specifically stipulate the desirable composition of graftcopolymer, the findings of the inventors of the present invention only apply to the grafting ratio of the back bone polymer. The formula weight ratio of the vinyl alcohol units of water-soluble or water-dispersible polymer having vinyl alcohol units serving as raw material in the present invention and the above-mentioned cationic groups is selected from the range of 1:20 to 2:1. This range is preferably 1:20 to 1:1, and most preferably 1:20 to 1:2. Graft copolymer produced within this range is particularly preferable. If the amount of water-soluble or water-dispersible polymer having vinyl alcohol units is less than 4.7 formula wt%, the effect on color density is too large, making it unsuitable for use as a treating agent for a sheet surface. If the amount of the above-mentioned cationic groups is less than 33 formula wt%, it is not practical as a result of low water resistance. Graft copolymerization may be carried out in a monomer solution in the presence of water-soluble or water-dispersible polymer having vinyl alcohol units in an aqueous medium. Although water alone is preferably used as the aqueous medium, organic solvents which mix uniformly with water, such as dimethylsulfoxide, ethanol and N-methylpyrrolidone, may also be mixed with water. For the polymerization method, the entire amount of monomer may be charged into a polymerization vessel in the presence of the water-soluble or water-dispersible polymer having vinyl alcohol units followed by initiation of polymerization, or a portion of the monomer is charged into the polymerization vessel, and remaining monomer is then charged according to the progress of polymerization after initiating polymerization. Either method may be suitably employed. The water-soluble or water-dispersible polymer having vinyl alcohol units to serve as the back bone polymer is present in the polymerization system in the dissolved or dispersed state at a concentration of 2 to 25 wt%, while monomer to be grafted is present in the polymerization system at a monomer concentration of 5 to 60 wt%. Cationic monomer is then graft copolymerized to the back bone polymer by solution polymerization, reverse phase emulsion polymerization, reverse phase suspension polymerization and so forth, and particularly preferably by aqueous solution polymerization. In addition, a graft copolymer can be obtained and used in a similar application, where the graft copolymer has a polymer structure having cationic groups for the back bone polymer, and a polymer structure having vinyl alcohol units for the branch polymers, by graft copolymerizing a vinyl ester of carboxylic acid such as vinylacetate with a (co)polymer containing N-vinylcarboxylic acid amide and/or hydrolysis product of the (co)polymer as back bone polymer, followed by hydrolysis thereof. The graft copolymerization can be performed by polymerizing under the above-mentioned conditions using a conventional radical generator. Examples of conventional radical generators that may be used preferably include azo-based initiators such as 2-2'-azobis-2-amidinopropane · 2 hydrochloride, sodium 4,4'-azobis-4-cyanovalerate and 2,2'-azobis-N,N'-dimethyleneisobutylamidine·2 hydrochloride. In addition, in place of the use of these azo-based initiators, oxidizing initiators such as tetravalent cerium compounds or redox-type initiators such as a combination of ammonium persulfate and sodium hydrogen sulfite can be either used in combination or alone. The amount of polymerization initiator used is normally about 100 to 10,000 ppm relative to the amount of monomers. If it is necessary to adjust the molecular weight of the polymer, a conventional chain transfer agent may be used, e.g. alcohols such as methanol, ethanol or isopropanol, or sulfur-containing compounds such as 2-mercaptoethanol. The amount used thereof preferably is 0.1 to 200 wt% relative to the amount of monomers in the case of alcohols, and 0.01 to 10 wt% relative to the amount of monomers in the case of 2-mercaptoethanol. The polymerization reaction is typically carried out at a temperature of 10°C to 100°C, and preferably 30°C to 80°C, and within a pH range of 1.0 to 6.0, and particularly preferably pH 2.0 to 5. 0 after removing oxygen gas from the system. If the polymerization reaction is carried out at a pH outside the above range, the grafting reaction does not proceed thereby preventing the obj ect of the present invention from being achieved. A polymer mixture obtained by the above graft copolymerization having an intrinsic viscosity at 25°C in 2 % ammonium sulfate aqueous solution of 0.1 to 2.0 dl/g is preferable as a treating agent for a sheet surface, while that having an intrinsic viscosity of 0.2 to 0.5 dl/g is particularly preferable. If the viscosity is outside the above range, it does not fall within the suitable viscosity range during treating, thereby preventing the obtaining of good treating. Examples of sheets to be treated with the treating agent of the present invention include pulp sheets such as high-quality paper, medium-quality paper, paper board, synthetic paper and plastic sheets. It is also applicable to a composite sheet comprising paper and synthetic paper. The treating agent according to the present invention can be applied in printing methods other than ink jet printing which use a similar aqueous ink. In the case of treating a paper surface with the treating agent of the present invention the amount should be chosen by taking into consideration the required properties. In general, the applied polymer content of the treating agent is within the range of 0.02 to 5 g/m 2 . In the case of coated paper, a paper for ink jet printing according to the present invention can be produced by preparing a coating color composed of fillers, binders and the treating agent of the present invention and coating a sheet surface with the coating color. In the case of common-use paper, a paper for ink jet printing according to the present invention can be produced by penetrate treating a sheet surface with the treating agent of the present invention alone or in combination with oxidized starch, polyvinyl alcohol or surface sizing agent. The treating agent according to the present invention can be used in combination with other coating agents, examples of which include ink charge neutralizing substances such as cationic surface active agents, polycondensed aluminum ions and polycondensed cationic polymers, water-soluble polymers and latex such as oxidized starch, cationic starch, other modified starch and polyvinyl alcohol, and coating pigments or fillers for ink jet printing such as fine synthetic silica, alumina, talc, kaolin clay and calcium carbonate. The paper for ink jet printing according to the present invention can be obtainedby treating a sheet surface with the treating agent according to the present invention alone or with a mixture of the treating agent of the present invention with other coating agents by use of a size press, a gate roll coater or a blade coater. The treating agent of the present invention comprises a graft copolymer composed of a back bone polymer (main chain) and branch polymers (side chains) , with one having a polymer structure containing vinyl alcohol units, while the other has a polymer structure containing cationic groups. Generally polyvinyl alcohol and cationic polymer have a low level of miscibility with each other. On the other hand, the roles of both differ when used as a treating agent for an ink jet printing paper. The cationic polymer increases water resistance, while the polyvinyl alcohol prevents a decrease in color density. Both components cannot be coated onto a paper surface as a uniform phase when in the form of a simple mixture; however, in the presence of the graft copolymer of the present invention (which itself is a uniform phase), especially in a case where a water-soluble or water-dispersible binder like polyvinyl alcohol is used, miscibility of the binder and cationic polymer is improved, thereby enabling various types of polymers to be applied to a paper surface in a uniform state. Moreover, the treating agent of the present invention prevents increases in viscosity of a coating color during mixing, thereby contributing to performance by allowing a large amount of polymer to be coated onto the paper. The effect of preventing increases in viscosity of a coating color is also surmised to be the result of improved miscibility between the polyvinyl alcohol and cationic polymer components. In this manner, as a result of having the characteristic of allowing uniform coating of a large amount of polymer, the paper for ink jet printing coated with the present graft copolymer can be given desirable properties. EMBODIMENTS Although the following provides a detailed explanation of specific embodiments of the present invention, the present invention is not limited to the following embodiments. [Synthesis Example-1] 52.2 g of a 23% aqueous solution of polyvinyl alcohol (abbreviated as PVA) (trade name: PVA205 (saponification index: 88%, degree of polymerization: 500) manufactured by Kurare Co.), 166 g of dimethyldiallylammonium chloride (DADMAC, concentration 65%, manufactured by Daiso Co.) (abbreviated as DDMC) and 44.6 g of deionized water were added to a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH of the raw material mixed liquid was adjusted to 3.5 with 2.4 g of 10% aqueous sulfuric acid, and the monomer concentration was adjusted to 40.8%. Nitrogen replacement was performed for 30 minutes while stirring the raw material mixture and maintaining the temperature at 60°C. After that, 5.4 g (0.5wt% per monomer) of a 10% aqueous solution of polymerization initiator V-50 (2,2'-azobis-amidinopropane dihydrochloride: manufactured by Wako Pure Chemical Industries Co.) were added to start polymerization. After maintaining the temperature at 60°C for 4 hours, 0.2 wt% per monomer of polymerization initiator V-50 was further added, and the reaction continued for 8 hours after its initiation. After cooling, 130 g of deionized water were added to bring the total amount to 400 g and obtain a uniform reaction product. The polymer concentration as determined from the total of PVA and DDMC was 30%. This polymer mixture was designated as Sample-1. The cation equivalent value of the Sample-1 (pure polymer content) was measured by colloidal titration. At this time, the formula weight ratio of hydroxyl groups as determined from the saponification index of PVA and cationic groups as determined by colloidal titration was 26:74. In addition, the intrinsic viscosity of the Sample-1 at 25°C in 2% ammonium sulfate aqueous solution was measured. To 3. 6 g of a 20% solution of Sample-1, 36.0 g of methanol were added to form a precipitate. The insoluble precipitate that formed at this time was filtered with No. 5B filter paper. After drying it for 1 hour at 105°C, the dried precipitate was weighed to calculate the weight ratio (% insoluble matter) of methanol-insoluble matter relative to the polyvinyl alcohol charged as raw material. Polymer properties are shown in Table 1. [Synthesis Examples-2] The same polymerization procedure as in Synthesis Example-1 was performed except for using the charged amounts of PVA205 and DDMC shown in Table 1 to obtain Sample-2. The properties of the polymer are shown in Table 1. [Synthesis Examples-3, 4] The same polymerization procedures as in Synthesis Example-1 were performed except for using PVA105 (saponification index: 98%, degree of polymerization: 500) manufactured by Kurare Co. as polyvinyl alcohol and using the charged amounts of PVA105 and DDMC shown in Table 1 to obtain Samples-3 and 4. The properties of these polymers are shown in Table 1. [Comparative Synthesis Example-1] Polymerization of DDMC was performed according to the same procedure as in Synthesis Example-1 without adding PVA to obtain polydimethyldiallylammonium chloride (P-DDMC), and a polymer mixture obtained by mixing an equal amount of PVA as that used in the Synthesis Example-1 with the P-DDMC was designated as Comparative Sample-1. The Comparative Sample-1 separated into 2 phases, and again separated into 2 phases within 1 day even after mixed with a homogenizer. The cation equivalent value, the intrinsic viscosity, and the weight ratio (% insoluble matter) of methanol-insoluble matter of Comparative Sample-1 were measured in accordance with the same procedure as Synthesis Example-1. Polymer properties are shown in Table 1. Furthermore, a polymer mixture obtained by mixing an equal amount of Sample-1 with Comparative Sample-1 did not separate into 2 phases and was designated as Mixed Sample-1 according to the present invention. [Comparative Synthesis Examples-2∼4] The same polymerization procedures as in Comparative Synthesis Example-1 were performed using the charged amounts of PVA and P-DDMC shown in Table 1 to obtain Comparative Samples- 2 ∼4 . The properties of these polymers are shown in Table 1. Name of Sample PVA Kind of Monomer Formula weight ratio VA:Cationic Group Cationic Equivalent Value (meq/g) Intrinsic Viscosity (g/dl) Insoluble Matter (%) Sample-1 PVA205 DDMC 26:74 5.57 0.39 25 Sample-2 PVA205 DDMC 33:67 5.37 0.20 40 Sample-3 PVA105 DDMC 21:79 5.70 0.30 10 Sample-4 PVA105 DDMC 50:50 4.71 0.51 50 Comparative Sample-1 PVA205 DDMC 26:74 5.57 0.40 100 Comparative Sample-2 PVA205 DDMC 33:67 5.37 0.21 100 Comparative Sample-3 PVA105 DDMC 21:79 5.70 0.30 100 Comparative Sample-4 PVA105 DDMC 50:50 4.71 0.50 100 DDMC: dimethyldiallylammonium chloride VA: vinylalcohol units [Examples 1∼5, Comparative Examples 1∼5] (Application of the treating agent) Synthetic powdered silica (Nipseal HD-2, manufactured by Nippon Silica Kogyo Co.), polyvinylalcohol (PVA105 (saponification index: 98%, degree of polymerization: 500) manufactured by Kurare Co.), and the treating agents (Samples-1∼4 or Comparative Samples-1∼4) were mixed in a weight ratio of 50:45:5 to prepare coating colors having a concentration of 25%. After coating 8.0 g/m 2 (Sample and Comparative Sample polymer amount of 0.4 g/m 2 ) of these coating colors onto commercially available PPC papers (Stoeckigt sizing degree: approx. 20 seconds) using a wire bar (PDS04, manufactured by Wavestar Co.), the coated papers were dried for 2 minutes at 105°C and then used in printing and later testing. (Printing of Test Paper and Measurement of Water and Light Resistance) Cyan, magenta, yellow and black patterns and characters were printed on the coated test papers using the BJC-600J ink jet printer manufactured by Canon Co. Color densities after printing were measured with an NR-3000 colorimeter (manufactured by Nihon Denshoku Co.) indicating as L*, a* and b*. Cyan was evaluated with the value of -b*, magenta with the value of a*, yellow with the value of b* and black with the value of L*. Changes in color densities before and after water resistance and light resistance tests were measured using an RD-918 Macbeth reflection densitometer. The water resistance test was performed by measuring color density before and after immersing a solid-printed test piece in deionized water (flowing water) moving at 300ml/min for 10 minutes, and then calculating the rate A, B and C of residual color. A: not changed, B: slightly blurred, C: significantly blurred The light resistance test was conducted by illuminating a solid-printed test piece for 40 hours at an illumination intensity of 500 W/m 2 , wavelength of 300∼800 nm and temperature of 50°C using a light resistance tester (manufactured by Shimadzu Co., XS-180CPS), measuring the color density before and after illumination, and calculating the rate A, B and C of residual color. A: not changed, B: slightly faded, C: faded. The results of each measurement are shown in Table 2. Name of Sample Water Resistance Light Resistance Color Density C M Y B C M Y B C (-b ) M (a ) Y (b ) B (L ) Example 1 Mixed Sample-1 A B A A A B A A 43.1 64.1 75.0 33.5 Example 2 Sample-1 A B A A A B A A 43.4 64.0 75.1 33.0 Example 3 Sample-2 A B A A A B A A 43.2 64.4 75.8 33.5 Example 4 Sample-3 A B A A A B A A 43.1 64.0 75.9 33.0 Example 5 Sample-4 A B A A A B A A 44.1 63.8 75.5 33.2 Comparative Example 1 Comparative Sample-1 B C A B A C A A 42.3 63.0 73.2 35.4 Comparative Example 2 Comparative Sample-2 B C A B A C A A 42.6 63.7 73.0 35.0 Comparative Example 3 Comparative Sample-3 B C A B A C A A 39.5 62.1 72.0 35.5 Comparative Example 4 Comparative Sample-4 B C A B A C A A 44.2 64.0 73.5 34.6 Comparative Example 5 Comparative Sample-1 A B A A A B A A 33.8 49.0 52.3 27.5 C: cyan, M: magenta, Y: yellow, B: black [Synthesis Examples-5∼8] A 23% aqueous solution of polyvinyl alcohol (abbreviated as PVA) (trade name: PVA117 (saponification index: 98%, degree of polymerization: 1700) manufactured by Kurare Co.) and monomers having the compositions described in Table 3 were charged into a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH was adjusted to 3.5 and the monomer concentration was adjusted to 20% by addition of deionized water. Nitrogen replacement within the system was performed for 30 minutes while stirring the raw material mixture andmaintaining the temperature at 60°C. Next, 0.3 wt% (per monomer) of ammoniumperoxodisulfate and 0.01 wt% (per monomer) of sodium hydrogen sulfite were added to initiate polymerization. The reaction was allowed to continue for 4 hours while maintaining the reaction system at 60°C and then cooled to obtain a uniform product. These polymer mixtures were designated as Samples-5∼8. The cation equivalent values, the intrinsic viscosities, and the weight ratio (% insoluble matter) of methanol-insoluble matter of the Samples-5∼8 were measured by the same procedures as in Synthesis Example-1. Polymer properties are shown in Table 3. [Comparative Synthesis Examples-5∼8] With the exception of polymerizing at the pH values described in Table 3, monomers were polymerized using the same procedure as in Synthesis Examples-5∼8 to obtain Comparative Samples-5∼8. The pH values thereof were adjusted with hydrochloric acid and sodium hydroxide. The Comparative Samples-5∼8 separated into 2 phases, and again separated into 2 phases within 1 day even when mixed with a homogenizer. The cation equivalent values, the intrinsic viscosities, and the weight ratio (% insoluble matter) of methanol-insoluble matter of Comparative Samples-5∼8 were measured by the same procedures as in Synthesis Example-1. Polymer properties are shown in Table 3. Name of Sample Formula weight ratio VA:Cationic Group Kind of Monomer Polymerization pH Cationic Equivalent Value (meq/g) Intrinsic Viscosity (g/dl) Insoluble Matter (%) Sample-5 25:75 DAMC 2.5 4.70 0.39 35 Sample-6 25:75 DPMC 3.5 4.50 0.39 20 Sample-7 25:75 DABC 3.5 3.50 0.41 10 Sample-8 25:75 DPBC 4.5 3.30 0.42 44 Comparative Sample-5 25:75 DAMC 0.0 4.70 0.40 99 Comparative Sample-6 25:75 DPMC 7.0 4.50 0.41 99 Comparative Sample-7 25:75 DABC 0.5 3.50 0.42 98 Comparative Sample-8 25:75 DPBC 6.5 3.30 0.39 98 DAMC: acryloyloxyethyltrimethylammonium chloride DPMC: acryloylaminopropyltrimethylammonium chloride DABC: acryloyloxyethylbenzyldimethylammonium chloride DPBC: acryloylaminopropylbenzyldimethylammonium chloride VA: vinylalcohol units [Example-6∼9, Comparative Example-6∼9] (Evaluation Test for the Treating Agent) Polyvinyl alcohol (trade name: PVA105 (saponification index: 98%, degree of polymerization: 500) manufactured by Kurare Co.), oxidized starch (Ace C, manufactured by Oji Cornstarch Co.) and treating agents (Samples-5∼8, Comparative Samples-5∼8) were mixed in a weight ratio of 0.3:2.7:1 and coated and immersed in the amount of 4.0 g/m 2 as the amount of solid content (Sample and Comparative Sample polymer amount of 1. 0 g/m 2 ) in the same manner as in Examples-1 ∼5 followed by drying, printing and testing after printing. The results of water resistance, light resistance and evaluating color densities as described above are shown in Table 4. In addition, the degrees of featheringwere evaluatedbyvisual inspection using a magnifying glass. o ○: feathering is not recognized, Δ: feathering is recognized, ×: feathering is significantly recognized Name of Sample Water Resistance Light Resistance Color Density Feathering C M Y B C M Y B C (-b*) M (a*) Y (b*) B (L*) Example 6 Sample-5 A B A A A B A A 45.6 64.0 74.1 33.0 o ○ Example 7 Sample-6 A B A A A B A A 46.1 64.4 74.8 33.5 o ○ Example 8 Sample-7 A B A A A B A A 46.7 64.0 74.9 33.0 o ○ Example 9 Sample-8 A B A A A B A A 46.8 63.8 74.5 33.2 o ○ Comparative Example 6 Comparative Sample-5 B C A B A C A A 43.8 63.2 73.2 35.4 × Comparative Example 7 Comparative Sample-6 B C A B A C A A 44.1 63.4 73.1 35.0 Δ Comparative Example 8 Comparative Sample-7 B C A B A C A A 44.0 63.1 72.8 35.5 × Comparative Example 9 Comparative Sample-8 B C A B A C A A 44.2 64.0 73.5 33.6 Δ C: cyan, N: magenta, Y: yellow, B: black [Synthesis Examples-9∼10] A 20% aqueous solution of polyvinyl alcohol (abbreviated as PVA) (trade name: PVA105 (saponification index: 98%, degree of polymerization: 500) manufactured by Kurare Co.) and N-vinylcarboxylic acid amide monomers having the compositions described in Table 5 were charged into a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH was adjusted to 5.5 and the total concentration of PVA105 and monomers was adjusted to 20% by addition of deionized water. Nitrogen replacement within the system was performed for 30 minutes while stirring the raw material mixture and maintaining the temperature at 60 °C. Next, 0.5 wt% (per monomer) of polymerization initiator V-50 was added to initiate polymerization. After maintaining at 60°C for 4 hours, 0.2 wt% (per monomer) of polymerization initiator V-50 was added, and the reaction was allowed to continue for 8 hours from the time of initiation to obtain a uniform product. This polymer mixture was alkaline hydrolyzed to hydrolyze 95% of the N- vinylcarboxylic acid amide units therein to obtain vinylamino units. The mixtures of hydrolyzed polymers were designated as Samples-9 and -10. The cation equivalent values, the intrinsic viscosities, and the weight ratio (% insoluble matter) of methanol-insoluble matter of Samples-9∼10 were measured by the same procedures as in Synthesis Example-1. Polymer properties are shown in Table 5. [Comparative Synthesis Examples-9∼10] Polymerization of N-vinylcarboxylic acid amide monomer was performed according to the same procedure as in Synthesis Examples-9 ∼10 without adding PVA to obtain poly-N-vinylcarboxylic acid amide, and a polymer mixture was obtained by mixing an equal amount of PVA as that used in the Synthesis Examples-9 ∼ 10 with the poly-N-vinylcarboxylic acid amide. This polymer mixture was alkaline hydrolyzed to hydrolyze 95% of the N-vinylcarboxylic acid amide units therein to obtain a mixed polymer of cationic polymer having vinylamino units and PVA. These polymer mixtures were designated as Comparative Samples-9∼10. The Comparative Samples-9 and -10 separated into 2 phases, and again separated into 2 phases within 1 day even after mixed with a homogenizer. The cation equivalent value, the intrinsic viscosities, the weight ratio (% insoluble matter) of methanol-insoluble matter of the Comparative Samples-9∼10 were measured by the same procedures as in Synthesis Example-1. Polymer properties are shown in Table 5. Name of Sample Formula weight ratio VA:Cationic Group Kind of Monomer Cationic Equivalent Value (meq/g) Intrinsic Viscosity (g/dl) Insoluble Matter (%) Sample-9 25:75 NVF 10.00 0.38 29 Sample-10 33:67 NVA 9.40 0.41 31 Comparative Sample-9 25:75 NVF 10.00 0.40 100 Comparative Sample-10 33:67 NVA 9.40 0.40 100 NVF: N-vinylformamide NVA: N-vinylacetamide VA: vinylalcohol units [Synthesis Examples 11∼12] A23% aqueous solution of polyvinylalcohol (trade name: PVA105 (saponification index: 98%, degree of polymerization: 500) manufactured by Kurare Co.) and monomer compositions described in Table 6 were charged into a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH was adjusted to 5.0 and the total concentration of PVA105 and monomers was adjusted to 30% by addition of deionized water. Nitrogen replacement within the system was performed for 30 minutes while stirring the raw material mixture and maintaining the temperature at 60°C. Next, 0.3 wt% (permonomer) of hydroxylamine hydrochloride as a crosslinking preventor, 0.5 wt% (per monomer) of 2-mercaptoethanol as a chain transfer agent, and 0.5 wt% (per monomer) of polymerization initiator V-50 were added to initiate polymerization. After maintaining at 60°C for 6 hours, an equivalent amount of hydrochloric acid as that of N-vinylformamide used was added, and the reaction was allowed to continue for 5 hours at 90°C. The obtained polymers were designated as Samples-11 and 12. The cation equivalent values, the intrinsic viscosities, and the weight ratio (% insoluble matter) of methanol-insoluble matter of the Samples-11∼12 were measured by the same procedure as in Synthesis Example-1. In addition, amidine proportions of the products were measured by NMR method. Polymer properties are shown in Table 6. [Comparative Synthesis Examples-11∼12] Polymerization of monomer compositions described in Table 6 was performed according to the same procedure as in Synthesis Examples-11∼12 without adding PVA, and a polymer mixture was obtained by mixing an equal amount of PVA as that used in the Synthesis Examples-11∼12 with the obtained products followed by reaction with hydrochloric acid. These polymer mixtures were designated as Comparative Samples-11∼12. Polymer properties are shown in Table 6. Name of Sample Formula weight ratio VA:Cationic Group NVF/AN mol ratio Polymerization pH Cationic Equivalent Value (meq/g Intrinsic Viscosity (g/dl) Insoluble Matter (%) Polymer composition P/Q/R % Sample-11 25:75 60/40 5.0 7.20 0.35 25 44/39/17 Sample-12 33:67 60/40 5.0 6.71 0.42 34 49/28/23 Comparative Sample-11 25:75 60/40 5.0 7.15 0.32 98 45/40/15 Comparative Sample-12 33:67 60/40 5.0 6.76 0.39 97 48/30/22 NVF: N-vinylformamide AN: acrylonitrile VA: vinylalcohol units P: mol% of primary amino groups Q: mol% of amidine groups R: mol% of cyano groups [Examples 10∼13, Comparative Example 10∼13] (Application of Treating Agent) The same procedures as in Examples-1∼5 were repeated except for using the treating agents of Samples-9 ∼12 and Comparative Samples-9∼12 to obtain coating colors. The coated papers were dried and then used in printing and later testing. (Printing of Test Paper and Measurement of Water and Light Resistance) Evaluation testing was performed in the same manner as in Examples-1∼5, and the color densities, water resistance and light resistance of each sample were evaluated. The results of each measurement are shown in Table 7. Name of Sample Water Resistance Light Resistance Color Density C M Y B C M Y B C (-b*) M (a*) Y (b*) B (L*) Example 10 Sample-9 A B A A A B A A 44.4 63.9 73.3 33.2 Example 11 Sample-10 A B A A A B A A 45.2 64.5 73.8 33.5 Example 12 Sample-11 A A A A A A A A 43.3 65.0 72.8 33.7 Example 13 Sample-12 A A A A A A A A 45.0 65.0 73.1 33.0 Comparative Example 10 Comparative Sample-9 B C A B A C A A 42.3 63.0 73.2 35.4 Comparative Example 11 Comparative Sample-10 B C A B A C A A 42.6 63.7 73.0 35.0 Comparative Example 12 Comparative Sample-11 B C A B A C A A 42.1 63.3 73.0 35.3 Comparative Example 13 Comparative Sample-12 B C A B A C A A 42.0 63.6 73.1 35.2 C:cyan, M:magenta, Y:yellow, B:black [Synthesis Examples 13∼14] A 20% aqueous solution of N-vinylformamide was charged into a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH was adjusted to 5.5. Nitrogen replacement within the system was performed for 30 minutes while stirring the raw material and maintaining the temperature at 60°C. Next, 0.3 wt% (per monomer) of hydroxylamine hydrochloride as a crosslinking preventor, 0.5 wt% (per monomer) of 2-mercaptoethanol as a chain transfer agent, and 0.5 wt% (per monomer) of V-50 as a polymerization initiator were added to initiate polymerization. After maintaining at 60°C for 6 hours, an amount of hydrochloric acid equivalent to that of N-vinylformamide used was added, and the reaction was allowed to continue for 5 hours at 90°C. The reaction solution was added with a large amount of acetone. After removal of water, a powdered polyvinylamine hydrochloride was obtained. After that, the obtained polyvinylamine hydrochloride and the respective vinylacetate monomer composition described in Table 8 were charged into a 500 ml four-neck flask equipped with a thermometer, stirrer, nitrogen feed tube and condenser, after which the pH was adjusted to 3.5 and the total concentration of polyvinylamine hydrochloride and vinylacetate monomer was adj usted to 25 % by addition of deioni zed water. Nitrogen replacement within the system was performed for 30 minutes while stirring the raw material and maintaining the temperature at 60 °C. Next, 0.5 wt% (per monomer) of ammoniumperoxodisulfate as a polymerization initiator was added to initiate polymerization. After maintaining at 60°C for 4 hours, 0.2 wt% (per monomer) of polymerization initiator V-50 was added, and the reaction was allowed to continue for 8 hours. The obtained polymer was alkaline hydrolyzed followed by neutralization with hydrochloric acid to obtain a graft copolymer comprising a back bone polymer having 95 mol% of vinylamino units (in the form of hydrochloride) and branch polymers comprising polyvinylalcohol (95 mol% hydrolysis product of polyvinylacetate). The hydrolysis products of the polymer mixture were designated as Samples-13 and 14. The cation equivalent values, the intrinsic viscosities, and the weight ratio (% insoluble matter) of methanol-insoluble matter of the Samples-13∼14 were measured by the same procedure as the Synthesis Example-1. Polymer properties are shown in Table 8. Name of Sample Formula weight ratio VA:Cationic Group Cationic Equivalent Value (meq/g) Intrinsic Viscosity (g/dl) Insoluble Matter (%) Sample-13 25:75 9.88 0.35 36 Sample-14 33:67 9.25 0.44 33 VA: vinylalcohol units [Examples 14∼15] (Application of Treating Agent) The same procedures as in Examples-1∼5 were repeated except for using the treating agents of Samples-13∼14 to obtain coating colors. The coated papers were dried and then used in printing and later testing. (Printing of Test Paper and Measurement of Water and Light Resistance) Evaluation testing was performed in the same manner as in Examples-1∼5, and the color densities, water resistance and light resistance of each sample were evaluated. The results of each measurement are shown in Table 9. Name of Sample Water Resistance Light Resistance Color Density C M Y B C M Y B C (-b*) M (a*) Y (b*) B (L*) Example 14 Sample-13 A B A A A B A A 45.4 64.7 73.5 33.1 Example 15 Sample-14 A B A A A B A A 45.3 64.5 73.8 33.6 C:cyan, M:magenta, Y:yellow, B:black Industrial Applicability The treating agent of the present invention preferably comprises a copolymer obtained by polymerizing di(meth)allylamine-based monomers and/or (meth)acryl-based cationic monomers in the presence of a water-soluble or water-dispersible polymer containing vinyl alcohol units by radical polymerization. According to the present invention, feathering, a particular problem of common-use paper, is prevented, water resistance and light resistance are improved without decreasing color density during printing, and there is strong bonding with the ink dye, thereby allowing the product of the present invention to be used as a treating agent for an ink jet printing paper. The present invention therefore has a significant applicability in the industrial field.

Claims: A treating agent for a sheet surface, comprising a water soluble copolymer comprising a polymer moiety having vinyl alcohol units and a polymer moiety having cationic groups, said copolymer being a graft copolymer in which said respective polymer moieties are a back bone polymer and a branch polymer, either of which has the vinyl alcohol units, and the other of which has the cationic groups and wherein the formula weight ratio of the vinyl alcohol units of said water-soluble polymer and the cationic groups is from 1:20 to 2:1. A treating agent according to claim 1, wherein said back bone polymer of said graft copolymer is derived from a water-soluble or water-dispersible polymer having vinyl alcohol units, and said branch polymer comprises of at least one repeating unit selected from the group consisting of a repeating unit represented by the following formula (1), a repeating unit represented by the following formula (2), a repeating unit represented by the following formula (3) and a repeating unit represented by the following formula (4): (wherein, R 1 and R 2 represent H or CH 3 , R 3 and R 4 represent hydrogen, alkyl groups having 1∼4 carbon atoms or benzyl groups, and X - represents a counter ion); (wherein, A represents O or NH, B represents C 2 H 4 , C 3 H 6 or C 3 H 5 OH, R 5 represents H or CH 3 , R 6 and R 7 represent alkyl groups having 1 ∼4 carbon atoms, R 8 represents hydrogen, an alkyl group having 1 ∼4 carbon atoms or a benzyl group, and X - represents a counter ion) ; (wherein, R 9 represents H or CH 3 , and X - represents a counter ion) ; and (wherein, R 10 and R 11 represent H or CH 3 , and X - represents a counter ion). A treating agent according to claim 2, wherein said branch polymer comprises at least one repeating unit selected from the group consisting of the repeating unit represented by said formula (1) and the repeating unit represented by said formula (2). A treating agent for a sheet surface according to claim 2, wherein said branch polymer comprises the repeating unit represented by said formula (1) and the repeating unit represented by said formula (3) and/or the repeating unit represented by said formula (4). A treating agent according to claim 2, wherein said graft copolymer is obtainable by radical polymerization of a monomer composition generating at least one repeating unit selected from the group consisting of the repeating unit represented by said formula (1), the repeating unit represented by said formula (2), the repeating unit represented by said formula (3), and the repeating unit represented by said formula (4) in the presence of the water-soluble or water-dispersible polymer having vinyl alcohol units. A treating agent according to any one of claims 2 to 5, wherein the proportion of vinyl alcohol units contained in said water-soluble or water-dispersible polymer having vinyl alcohol units is from 70 mol% to 100 mol%. A treating agent according to any one of claims 2 to 6, wherein the degree of polymerization of said water-soluble or water-dispersible polymer having vinyl alcohol units is from 100 to 2500. A treating agent according to any one of claims 2 to 7, wherein said water-soluble or water-dispersible polymer having vinyl alcohol units is grafted by 40% or more. A treating agent according to any one of claims 2 to 8, which comprises a polymer composition having the following solubility property: if 10 times the weight of methanol is added to a 20 wt% aqueous solution of the polymer composition the graft polymer is obtained by precipitating the amount of dry matter of the formed precipitate is 60 wt% or less of the water-soluble polymer. A treating agent according to any one of claims 2 to 9, wherein the intrinsic viscosity of said graft copolymer in 2% ammonium sulfate aqueous solution at 25°C is from 0.1 to 2.0 dl/g. A treating agent according to any one of claims 2 to 10, wherein the monomer generating the repeating unit represented by said formula (1) is a salt of diallylamine, a salt of diallylmonomethylamine, or a salt of diallyldimethylamine. A treating agent according to any one of claims 2 to 10, wherein the monomer generating the repeating unit represented by said formula (2) is a salt or quaternary compound of a dialkylaminoethyl(meth)acrylate or a salt or quaternary compound of a dialkylaminopropyl(meth)acrylamide. A treating agent according to any one of claims 2 or 4 to 10, wherein the monomer generating the repeating unit represented by said formula (3) is N-vinylformamide or N-vinylacetamide. A treating agent according to any one of claims 2 or 4 to 10, wherein the monomer generating the repeating unit represented by said formula (4) is a monomer composition of N- vinylformamide and acrylonitrile. A treating agent according to claim 1, wherein said graft copolymer is obtainable by graft copolymerization of a vinyl ester of a carboxylic acid with a polymer derived from a monomer composition containing an N-vinylcarboxylic acid amide, or a hydrolysis product of such polymer as the back bone polymer raw material, to form the branch polymer, followed by hydrolysing the branch polymer to form vinyl alcohol units. A paper for ink jet printing prepared by coating a composition comprising the treating agent according to any one of claims 1 to 15, a filler, and a binder onto a sheet surface. A paper for ink jet printing prepared by making a treating solution comprising the treating agent according to any one of claims 1 to 15 penetrate into a sheet surface. A paper for ink jet printing according to claim 16 or claim 17, wherein said paper comprises said treating agent in an amount of from 0.02 to 5 g/m 2 .

Inventors: KAMADA OSAMU, ONO MOTOSUKE, SUGIYAMA TOSHIAKI, KAMADA, OSAMU, ONO, MOTOSUKE, SUGIYAMA, TOSHIAKI, KAMADA, Osamu, Hyomo Corporation, ONO, Motosuke, Hymo Corporation, SUGIYAMA, Toshiaki, Hymo Corporation
Applicants: HYMO CORP, HYMO CORPORATION
Application year: 1998
Main classification: D21H27
IPC codes: B41M5, C08F2, C08F261, D21H17, D21H19