COATINGS AND COATED PAPERS FOR GRAVURE PRINTING

Abstract: Paper having improved printability in rotogravure print­ ing is provided by coating a paper substrate with an aqueous coating formulation containing a pigment such as kaolin clay, a binder such as a styrene/butadiene/itaconic acid terpolymer and a polyamidoamine such as the reaction product derived from ethylene diamine and methylacrylate.

Description: This invention relates to coated papers suitable for gravure printing and to coating compositions used to prepare such coated papers. The invention further relates to a process for preparing such a coated paper. The use of rotogravure printing processes in printing high volume, high quality publications such as magazines, catalogues, and product brochures is well-known. In such processes, a gravure master is employed which has a working surface that is generally smooth except for a regular array of tiny, discrete intaglio cell cavities excavated to a depth below the working surface. This array of cavities corresponds to the images to be printed on the paper. Ink is applied to the working surface such that the cavities retain a significant amount. The master is then pressed against a paper web and the ink transfers from the cavities to the paper thereby producing a printed reproduction of the desired image on the paper. Rotogravure printing, unlike offset printing and letter press printing, is very sensitive to poor ink transfer. Missing gravure dots and defective reproduction dots are often noticed by the reader. Thus a smooth and ink receptive paper is required. The paper is normally a coated paper comprising a wood pulp web as the substrate and a coating containing a pigment and a binder, e.g., a natural binder such as starch, a synthetic binder such as a styrene/butadiene latex or mixture thereof. Unfortunately, most of the coated papers used in rotogravure printing are either very expensive and/or are not sufficiently smooth and ink receptive to permit the high speed, high quality, printing that is desired in rotogravure printing processes. Therefore, it is highly desirable to provide a coated paper that is less expensive and has improved printability in rotogravure processes. In one aspect, the present invention is such a coated paper comprising a paper substrate and a coating resulting from an aqueous coating formulation containing a pigment and a binder, characterized in that said coating formulation further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages, branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation sufficient to cause the pigment to agglomerate or flocculate, after the paper is coated, and said pigment is present in an opacifying amount. In another aspect, the present invention is an aqueous coating formulation comprising a pigment and a binder characterized in that it further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines, and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation sufficient to cause the pigment to agglomerate or flocculate after the paper is coated, and said pigment is present in an opacifying amount. In a further aspect the invention relates to a process for preparing a coated paper suitable for rotogravure printing by treating a paper substrate with an aqueous coating formulation containing a pigment and a binder characterized in that said coating formulation further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation sufficient to cause the pigment to agglomerate or flocculate after the paper is coated and said pigment is present in an opacifying amount. The coated papers of this invention are also useful in offset printing. Moreover, the coating formulations are suitable as paper board coatings, for example, cylinder board coatings, bleached and unbleached Fourdrinier board coating and cast coating. Of the prior art JP-A-58 89391 and US-A-4,054,717 may be mentioned. JP-A-5 889 391 teaches an aqueous paper coating containing a pigment (talc), a binder (gelatin, hydroxyethyl cellulose) and epichlorhydrin-modified polyamide polyamine. It is Applicants understanding that 5 parts of epichlorhydrin-modified polyamide polyamine per 100 parts talc or pigment as used in this reference will stabilize talc or pigment particles rather than flocculate or agglomerate these particles in contrast to the present invention. In fact, flocculation of the pigment particles will be detrimental to the desired reduction of ink spreading. US-A-4,054,717 teaches the use of water-soluble amine-containing polymer in paper coating compositions to increase the wet rub resistance of such coating compositions. In contrast, the polyamidoamine used in paper coating compositions of the present invention results in decrease in the wet rub resistance of the claimed coating compositions. This is believed to be attributed to the formation of open, porous coatings by polyamidoamines as flocculants. Surprisingly, the practice of the present invention enables the rotogravure printing of relatively inexpensive paper without significant loss of printing quality or speed of printing. The coating formulations of this invention exhibit excellent fluidity, thus permitting them to be applied to paper substrates at very high speeds without the coatweight control problems often experienced with conventional formulations. Any paper which can be conventionally employed in rotogravure printing can be suitably employed in the practice of this invention. Examples of such papers include those prepared from high yield pulps such as unbleached, semi-bleached or bleached pulps as well as mixtures of two or more of such pulps. Such pulps are similar to those used in newsprint paper, paper board, medium-grade paper, medium-grade coated paper, ground wood paper and others such as described in US-A-4,298,652. Suitable pigments for purposes of this invention are any of those conventionally employed in rotogravure coating formulations, including such pigments as titanium dioxide, kaolin clay, barium sulfate, precipitated or ground calcium carbonate, aluminum hydroxide, satin white, calcium sulfite, zinc sulfite, and plastic pigments. Of these, kaolin clay is preferred. Binders preferably employed include aqueous dispersions of copolymers of styrene, butadiene, and/or acrylonitrile with ethylenically unsaturated carboxylic acids as illustrated in US-A-3,409,569; aqueous dispersions of copolymers of conjugated diolefins and unsaturated carboxylic acids; aqueous dispersions of copolymers of lower alkyl acrylate esters with acrylic acid or methacrylic acid such as shown in US-A-3,365,410; latexes of copolymers of vinyl esters of saturated carboxylic acids and ethylenically unsaturated carboxylic acids such as vinyl acetate/acrylic acid copolymers and vinyl acetate/butyl acrylate/acrylic acid copolymers; and various other alkali sensitive synthetic resin emulsions as well as mixtures thereof. Of the foregoing synthetic resin binders, the latexes of styrene/butadiene/ethylenically unsaturated carboxylic acid terpolymers are most preferred. Also suitable, although not as preferred as the foregoing binders are latexes of styrene and butadiene binary copolymers, methacrylate/butadiene binary copolymers and other polymers of acrylic and methacrylic esters; latexes of hydroxyalkyl polymers including alkyl acrylate/hydroxyethyl acrylate copolymers latexes of vinyl acetate polymers and copolymers such as ethylene/vinyl acetate copolymer and the like. Also suitable, but less preferred are natural binders such as oxidized or esterified starch and proteins such as casein, soybean protein and petroleum protein. Polyamidoamines which are suitable employed as pigment agglomerating agents in the practice of this invention are characterized as having a backbone containing both amide and amine linkages. In addition, the polyamidoamine backbone may bear one or more pendant amine, amide, amidoamine or ammonium moieties. Examples of such amidoamine polymers are linear polyamidoamines as described in U.S. Patent No. 4,435,548; ammonium polyamidoamines as described in U.S. Patent No. 4,416,729; and curable ammonium polyamidoamines as described in U.S. Patent No. 4,402,748. Of the foregoing polyamidoamines, the linear polyamidoamines and branched polyamidoamines are preferred. The linear polyamidoamines are conveniently prepared as described in U.S. Patent No. 3,305,493 by contacting an alkylene diamine or a polyalkylene polyamine with an ethylenically unsaturated compound such as an alkyl acrylate such as methyl acrylate, acrylamide, acrylic acid or methacrylic acid under reactive conditions. Preferably, the foregoing reactants are employed in stoichiometric ratio to form a linear polyamidoamine represented by the formula: wherein each R is independently hydrogen or lower alkyl, e.g., methyl or ethyl; Y is a terminal group characteristic of polyamidoamines; V is hydrogen or the residue of polymerization to form a polyamidoamine; each m is independently a whole number from 2 to 6, preferably 2; each n is independently a whole number from 1 to 3, more preferably 1 or 2, most preferably 1; and p is a whole number sufficient to provide the polyamidoamine with a weight average molecular weight (M w ) of 500 to 20,000. More preferably, the linear polyamidoamine has a M w of 1,000 to 20,000, especially 2,000 to 15,000, and most preferably 4,000 to 10,000. The branched polyamidoamine is prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of the linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound. Of such carboxylic compounds, methacrylate is most preferred. The reaction of the unsaturated compound with the linear polyamidoamine should occur at a temperature at which substantial cross-linking of the polymer is avoided. Generally, such temperature is in the range from about 0°C to 200°C, with temperatures from 20°C to 100°C being preferred. While the foregoing reaction can be conducted neat, it is generally desirable to employ a diluent which is a solvent for the polyamidoamine but which is substantially inert to the reactants. Water and lower alkanols having from 1 to 4 carbons are generally preferred diluents, with methanol being the most preferred. The ratio of carboxylic compound to equivalents of amino hydrogens in the linear polyamidoamine can be varied to produce the desired substitution of pendant carboxylic groups on the polyamidoamine. If complete substitution is desired, a stoichiometric amount or an excess of the unsaturated carboxylic compound is employed. However, if a lesser degree of substitution is desired, the reactants can be combined in the desired ratio and essentially completely reacted. In preparing such a branched polyamidoamine, it is generally desirable that at least 50 mole percent of the total amino hydrogens of the linear polyamidoamine be reacted with the unsaturated carboxylic compound, preferably at least 75 mole percent of such amino hydrogens are reacted. The resulting polyamidoamine bearing the pendant carboxyl moieties are then reacted with amines via an amidation reaction to produce the desired branched polyamidoamine. Such desired branched polyamidoamine are represented by the formula: wherein Y, V, R, m, n and p are as defined above and D at each occurrence is independently hydrogen or wherein R¹ at each occurrence is independently H, a C₁-C₄ alkyl or a C₁-C₄ hydroxyalkyl and B at each occurrence is independently or wherein x is an integer from 2 to 10 and y is an interger from 1 to 6 provided that at least 10 mole percent, preferably at least 20 percent, of D groups is The coating formulation used in the present invention contains sufficient pigment to provide the desired degree of opacity to the resulting coated paper. Preferably, such amounts of pigment is in the range from 85 to 96, most preferably from 90 to 94 parts by weight based on dry weight of the coating formulation. The total binder employed in the coating formulation is that which is sufficient to bond the pigment to the paper. Preferably, such an amount is in the range from 4 to 15, most preferably from 6 to 10 dry parts based on the dry weight of the coating formulation. The amount of polyamidoamine employed is that which is sufficient to cause the pigment interaction or agglomeration after the paper is coated. For the purposes of this invention, the ability of the polyamidoamine to cause the pigment to interact or agglomerate is measured by the improvement in rotogravure printability attained when using the polyamidoamine. A polyamidoamine is said to provide a suitable capability of pigment interaction or agglomeration if, when used in a paper coating formulation, it improves the rotogravure printability over a formulation without the polyamidoamine. Preferably, the amount of polyamidoamine is from 0.06 to 0.10 dry weight parts per 100 dry weight parts of the coating formulation. In addition to the foregoing essential ingredients, the coating formulation may also contain optional additives such as viscosity modifiers, dispersants, antifoaming agents, lubricants, and water retention agents to the extent that they do not affect the ability of the polyamidoamine to flocculate the pigment. In preparing the coated paper of the present example, a paper web or sheet as conventionally employed in rotogravure printing is employed. For example, such papers include those derived from bleached and unbleached sulfate pulps, bleached and unbleached sulfite pulps, bleached and unbleached soda pulps, neutral sulfite pulps, semichemical and chemical ground wood pulps, ground wood pulps and any combination of such pulps. The base paper or paper substrate is single coated or multiple coated on either surface or both surfaces thereof with the foregoing coating formulation by means of an on-machine or off-machine coater so that the coating weight on one surface is above 4.5 grams per square meter, preferably above 6 grams per square meter on a dry weight basis. The makeup of the coating formulation on each surface and that of the coating formulation forming each layer in a multiple coating may be changed as needed, coating that may be done by any process and by means of any conventional coating machines, for example, an air knife coater, a roll coater, a puddle type or inverted blade coater with beveled or bent blade, a bill blade coater, a twin blade coater, or a short dwell coater. Among these coating machines, the puddle, inverted blade and short dwell coaters are preferably employed. Upon application of the coating, the coating is subjected to conventional procedures for drying and curing, for example, by passing through calender or supercalender rolls, or air drying ovens. The following examples are given for purposes of illustrating the invention. Unless otherwise stated, all parts and percentages are by weight. Example 1 A coating formulation is prepared consisting of 2 parts of a 48 percent solids latex of a styrene/butadiene/itaconic (43.3/55.0/1.7) terpolymer, 100 parts of kaolin clay and 0.14 part of an aqueous solution of 61.5 percent of a linear polyamidoamine having an M w of about 6000 which is prepared by reacting equal molar amounts of ethylenediamine with methylacrylate. Using this formulation, rotogravure base stock paper (35.5 grams per meter square) is coated on one side using a lab coater having a puddle blade and operating at a coating speed of 12.2 meters per minute. The coated paper is calendered between nip rolls operating at 2 to 3 nips and 65.6°C (150°F) at 210 to 265 kN/m. The resulting coated paper is printed using a rotogravure print method as described in detail hereinafter and then tested for printability, gloss and brightness. The results of these tests are reported in Table I. For purposes of comparison, the foregoing procedure is repeated using different amounts of branched polyamidoamines in substitution for the linear polyamidoamine used in the foregoing procedure. The resulting papers are similarly printed and tested and the results are reported in Table I. Also for comparison, the foregoing procedure is repeated except that no polyamidoamine is employed in combination with the latex and pigment. The resulting coated paper is similarly printed and tested and the results are reported in Table I. As is evidenced by the data shown in Table I, all papers coated with formulations containing polyamidoamines, e.g., Sample Nos. 1-6, exhibit improved printability as compared to the paper coated formulation containing no polyamidoamine (Sample No. C). Brightness and gloss are also improved by the presence of the polyamidoamine. Example 2 Following Example 1, several coating formulations are prepared except that starch is substituted for the latex used in Example 1. Coated papers are similarly prepared and tested and the results are reported in Table II. As evidenced by the data of Table II, the rotogravure formulations of this invention exhibit improved printability even when a starch binder is employed.

Claims: A coated paper comprising a paper substrate and a coating resulting from an aqueous coating formulation containing a pigment and a binder, characterized in that said coating formulation further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages, branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of the said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation sufficient to cause the pigment to agglomerate or flocculate, after the paper is coated, and said pigment is present in an opacifying amount. The paper of Claim 1 wherein the polyamidoamine is a branched or linear polyamidoamine prepared from an alkylene diamine and an ethylenically unsaturated acrylate or methacrylate ester or acrylamide or methacrylamide, the binder is a copolymer of styrene, butadiene and an ethylenically unsaturated carboxylic acid and the pigment is kaolin clay. The paper of Claim 1 or 2 wherein the aqueous coating formulation comprises on a dry basis about 85 to about 96 weight parts of pigment, from about 4 to about 15 weight parts of binder and from about 0.01 to about 0.5 weight part of the polyamidoamine. An aqueous coating formulation comprising a pigment and a binder characterized in that it further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages, branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of the said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation sufficient to cause the pigment to agglomerate or flocculate after the paper is coated and said pigment is present in an opacifying amount. The formulation of Claim 4 wherein the polyamidoamine is a branched or linear polyamidoamine prepared from an alkylene diamine and an ethylenically unsaturated acrylate or methacrylate ester acrylamide or methacrylamide, the binder is a copolymer of styrene, butadiene and an ethylenically unsaturated carboxylic acid and the pigment is kaolin clay. The formulation of Claim 4 or 5 wherein the aqueous coating formulation comprises on a dry basis from about 85 to about 96 weight parts of pigment, from about 4 to about 15 weight parts of binder and from about 0.01 to about 0.5 weight part of the polyamidoamine. A process for preparing a coated paper suitable for rotogravure printing by treating a paper substrate with an aqueous coating formulation containing a pigment and a binder characterized in that said coating formulation further contains a polyamidoamine selected from the group consisting of linear polyamidoamines having a backbone containing both amide and amine linkages, branched polyamidoamines prepared by contacting the aforementioned linear polyamidoamine with an ethylenically unsaturated carboxylic compound under conditions sufficient to produce a Michaels addition reaction between the amine moiety of the said linear polyamidoamine and the ethylenically unsaturated moiety of the carboxylic compound and ammonium polyamidoamines and being present in an amount from about 0.01 to about 0.5 weight part per 100 dry weight parts of the coating formulation, sufficient to cause the pigment to agglomerate or flocculate after the paper is coated and said pigment is present in an opacifying amount.

Inventors: IK LEE, DO, WERKEMA, DONALD A., WILLENCY, RICHARD A.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1984
Main classification: D21H19
IPC codes: D21H19, C08L7, C08L77, B41M5, C08L21