Water resistant paper compositions containing carboxyl group-containing latex and aziridine derivatives

Abstract

 Paper coating compositions containing a carboxyl group-containing latex such as a styrene/butadiene/acrylic acid latex, a poly-functional aziridine compound such as diphenylmethane-bis-4,4'-N,N'-ethyleneurea and a pigment such as clay are disclosed. Papers coated with these compositions have excellent sheet gloss, ink gloss, wet pick resistance, dry pick resistance and ink receptivity.

Description

[0001] This invention relates to paper coating compositions containing a pigment, a latex binder and an aziridine compound. This invention also relates to papers coated with such paper coating compositions.

[0002] In an effort to improve quality of various papers, especially papers intended for high quality printing, papers have been coated with paper coating compositions. Paper coating compositions are generally combinations of a binder (latex and/or "natural" binder) and a pigment. Paper coating compositions are applied to the surface of paper in order to give the paper greater weight, stiffness, opacity, whiteness, brightness, gloss, smoothness and ink receptivity.

[0003] Recent improvements in printing techniques have stimulated demand for higher quality papers. Characteristics such as improved whiteness, gloss, printability and water resistance are in particular high demand by the market.

[0004] Heretofore, many methods have been proposed for improving the water resistance of paper. For example, it is well known to treat paper sheets with a urea-formaldehyde resin, melamine-formaldehyde resin, or similar thermosetting amino resins. However, these resins undergo a viscosity increase during application, and can emit hygenically undesirable formaldehyde gas during drying processes. Sodium hydroxide has also been added to such a liquid resin as a stabilizer, but has an adverse effect on the water resistance of the paper if excess amounts are used. This method is impractical due to the limited range of treatment conditions. Although zirconium ammonium carbonate is known to be highly effective in imparting water resistance to paper, it has highly unsatisfactory rheology. Specifically, it has a high viscosity when formulated into paper coating compositions. Casein, which is predominantly used as a natural binder for paperboard coatings, imparts good water resistance to paper. However, if added in sufficient concentrations to impart desired water resistance, the resulting coating compositions have excessive.viscosity. This is especially true for paper coating compositions having a total solids of 55 weight percent or greater. Reactive starch, obtained by denaturing starch which is ordinarily used as a natural binder for paper coating compositions, has also been proposed for use as a water resistant agent in paper coating compositions. However, such attempts fail because in order to impart sufficient water resistance to the paper, the viscosity of the resulting coating compositions becomes excessive.

[0005] Thus, it would be desirable to provide a paper coating composition having improved rheology, binding strength and water resistance. It would also be desirable to have such a paper coating composition which has excellent sheet gloss and print gloss, without reducing the whiteness of the paper. It would further be desirable to have such a paper coating composition having excellent ink receptivity.

Summary of the Invention

[0006] In one aspect, the invention is a paper coating composition containing a pigment, a carboxyl group-containing copolymer latex and a poly-functional aziridine compound. In another aspect, the invention comprises a paper coated with such a paper coating composition.

[0007] Papers coated with the coating compositions of the present invention are well suited to offset printing and show excellent resistance to fountain solutions (damping water) used in such printing processes, while performing well at a wide pH range of such water. Surprisingly, papers coated according to the invention exhibit excellent sheet gloss, ink gloss, wet pick resistance, dry pick resistance, and ink receptivity.

Detailed Description of the Invention

[0008] The paper coating compositions of the invention require the use of a poly-functional aziridine compound. These compounds may be represented by the formula:

wherein n represents an integer of 2 or more, Z represents an organic or inorganic group of atoms having a valence of n and each R¹ independently represents hydrogen or a methyl group.

[0009] More desirably, Z is an organic group having an amide group adjacent to each aziridine ring, the entire formula being shown by the following structure:

in which R² is an organic radical having a valence of n and R¹ and n are as defined hereinbefore. Preferably, these compounds are represented by the formula:

wherein R¹ is as defined hereinbefore. Most preferably, both of _R¹ are hydrogen. The compound of Formula III wherein both of R¹ are hydrogen (diphenylmethane-bis--4,4'-N,N'-ethyleneurea) is produced and commercially sold by Meisei Chemical Works Company, Japan, under the trade name SU 125F.

[0010] The invention also requires a carboxyl group-containing latex. By "carboxyl group," is meant that the latex has carboxylic acid functionalities or a salt thereof. Such latexes are typically prepared by emulsion polymerizing one or more ethylenically unsaturated monomers through an addition polymerization. The latex must contain sufficient carboxylation to enable the latex to react with the aziridine compound so as to impart water resistance to the paper coating composition. Carboxyl content is typically added to the latex by including as a comonomer thereof one or more ethylenically unsaturated carboxylic acids. Preferably, these latexes will contain from about 0.5 to about 11, more preferably from about 1 to about 6, and most preferably from about 2 to about 4, weight percent of an ethylenically unsaturated carboxylic acid. Typical examples of suitable ethylenically unsaturated carboxylic acids include itaconic acid, acrylic acid, methacrylic acid, fumaric acid, maleic acid, vinylbenzoic acid and isopropenylbenzoic acid. Preferred species thereof include acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid. If insufficient acid (e.g., typically less than 0.5 weight percent) is used, the papers coated with the paper coating compositions of the invention will not have sufficient water resistance to be commercially important. However, excess acid content is also undesirable and will cause excessive viscosity increase.

[0011] As comonomers of the latex, other than the ethylenically unsaturated carboxylic acids, a wide variety of suitable comonomers may be used. Typical -examples of suitable comonomers include aliphatic conjugated dienes such as 1,3-butadiene; monovinylidene aromatic monomers such as styrene; alkyl esters of ethylenically unsaturated carboxylic acids such as methylmethacrylate; vinyl esters of unsaturated carboxylic acids such as vinyl acetate; and so forth. Mixtures of two or more of these monomers may also be used. Examples of suitable copolymer latexes include latexes obtained by emulsion copolymerization of a monomer mixture of an aliphatic conjugated diene, an ethylenically unsaturated carboxylic acid and a monovinylidene aromatic compound; latexes of an aliphatic conjugated diene, an ethylenically unsaturated carboxylic acid and a vinyl unsaturated acid ester; and latexes of an acrylate, vinyl acetate and a monoethylenically unsaturated carboxylic acid.

[0012] These latexes may also contain other monomers such as ethylene, propylene, acrylonitrile, vinylidene chloride, and the like.

[0013] The latexes of the invention may be easily produced by any well-known emulsion polymerization using a free radical catalyst in an aqueous medium. Examples of such processes include batch reactions, continuous addition batch reactions and continuous reactions. The polymerization process may include the addition of various additives to the polymerization system. For instance, one may add antioxidants, UV stabilizers, chain transfer agents and so forth. Typically, the latexes will be made at about 30 to 65 percent solids by weight.

[0014] A third component of the paper coating compositions of the invention is a pigment. Any pigment generally suitable for paper coating applications will be suitable for practice in the instant invention. While calcium carbonate is easily formulated to extremely high solids, the spherical morphology of calcium carbonate particles causes an open coating structure which fails to get a smooth, impermeable surface on the coated paper. Typical paper coating clays have a platelet morphology, and are desired for use in the invention. More recently, "plastic pigments," which are synthetic polymers, have been used in paper coating compositions. These materials are also suitable for use in the invention.

[0015] The aziridine compound, latex and pigment are blended together, typically with additional water, to form a paper coating composition. The latex must be present in an amount sufficient to bind the pigment particles to a paper substrate. If insufficient latex is used, the resulting coating composition will not have sufficient binding strength. If excess latex is used, the resulting compositions will exhibit great decreases in ink receptivity and sheet gloss. In another respect, the latex must be present in a quantity such that the polyaziridine compound may cross-link the latexes to an extent that the paper coating composition has increased water resistance. In this respect, the quantity of latex necessary for a given coating composition will vary depending upon the extent of carboxylation of the latex. The greater the extent of carboxylation, the less latex will be required for purposes of the cross-linking reaction. Typically, the latex will be present at about 1 to about 30, preferably 3 to about 25, and most preferably 5 to about 20 parts by weight (solids) based upon 100 parts by weight of pigment.

[0016] The polyaziridine compound must be present in an amount sufficient to impart water resistance to the paper coating composition. If excess aziridine is used the coated papers will have very low ink gloss and ink receptivity. If insufficient aziridine compound is used, the coated papers will have low binding strength. Typically, the polyaziridine compound will be present in the paper coating composition at about 0.01 to about 10, preferably about 0.05 to about 5, most preferably about 0.1 to about 2 parts by weight based upon 100 parts by weight of the pigment.

[0017] If desired, the paper coating compositions may contain additional ingredients. Water-soluble natural binders such as casein, denatured starch and protein; synthetic pastes such as polyvinylalcohol; anti-foaming agents; lubricants and other water resistant agents may be added.

[0018] The paper coating compositions are adjusted to a total solids content convenient for the particular coating application, paper and latex being employed. Typically, the total solids will be from about 35 to about 65 weight percent.

[0019] Coating compositions made according to the invention may be applied to paper in any convenient manner. Several modes of application are well known to those skilled in the art. Conventional means such as a letter-press roll coater, off-set roll coater, size press, air knife, or blade coater may be used.

[0020] After application, the coating is dried by any convenient method. Generally, drying is accomplished by causing a current of heated air to impinge upon the surface of the coated sheet. The temperature of the air may vary up to about 160°C, and the duration of the contact is controlled such that the coating is heated to a temperature sufficient to cause at least limited deformation of the polymeric binder particles so as to achieve the desired adhesion of the paper coating pigment particles among themselves and to the paper substrate. Typically, a temperature of about 100°C or less in the coating is sufficient for such . purpose and, accordingly, the aforementioned duration of contact between the coating and the heated air is generally limited so as to prevent the coating temperature from exceeding 100°C.

[0021] After drying, the coated paper products can be finished pursuant to processes conventionally employed in the art such as calendering, super calendering, and the like.

[0022] Further aspects of the invention will be apparent from the following examples. In the examples all parts and percentages are by weight unless otherwise specified. In the examples all measurements are made according to the following methods.

Average Particle. Size

[0023] The average particle size of the carboxyl group-containing latexes is measured by light scattering.

Viscosity

[0024] The viscosity of the paper coating compositions is measured with a Brookfield (BL) viscometer, using Spindle No. 3 at 60 rpm and 25°C.

Dry Pick Resistance

[0025] The dry pick resistance (dry binding strength) of paper coating compositions coated onto paper substrates is measured using an RI Printing Tester. Several runs are made for each composition with Tack No. 13 (as measured by an Inkometer) ink, and the state of picking on the paper surface is evaluated with the naked eye. Smaller values mean better results.

Wet Pick Resistance

[0026] The wet pick resistance (wet binding strength) of coated papers is also measured on an RI Printing Tester. The coated paper specimens are dampened with water supplied by a Molton roll. Immediately after the water is applied, the sample is tested with Tack No. 13 ink, and the printed surface evaluated with the naked eye. Smaller values mean better results.

Sheet Gloss

[0027] Sheet gloss (pre-printing gloss) of the unprinted, coated papers is measured with a gloss meter at an incidence angle of 75°. The results are expressed in percent, larger values meaning better results.

Ink Gloss

[0028] The ink gloss (post-printing gloss) is measured by placing the coated paper specimens on an RI Printing Tester and printing over their entire surface with a web off-set ink. The gloss of the printed surface is measured with a gloss meter at an angle of incidence of 75°. The results are expressed in percent, higher values meaning better results.

Ink Receptivity

[0029] Coated paper specimens are placed on an RI Printing Tester and dampened with water supplied by a Molton roll. Immediately thereafter, the specimen is printed with Tack No. 6 ink. The printed surface is visually checked for the ink adhesion. Smaller values mean better ink receptivity.

Example 1

[0030] A reaction vessel is charged with 80 parts water, 3 parts itaconic acid, 0.5 part sodium alkyl- benzenesulfonate and 0.01 part of a chelating agent. The reaction vessel is heated to 80°C, and 40 parts butadiene, 57 parts styrene, 0.5 part tert-dodecyl mercaptan, a solution of 0.3 part sodium hydroxide and 1.2 parts ammonium persulfate in 20 parts water, are separately added thereto at a constant rate for 5 hours with stirring. To further the degree of conversion and stability of latex, agitation is continued for an additional 3 hours. The resulting latex has an average particle size of 1,800 Angstroms (180 nm), a degree of conversion of 98 percent, and is labeled "Latex A."

[0031] In a similar manner, Latexes B-E are prepared using modifications shown in Table I.

[0032] Using the copolymer Latexes A-E, paper coating compositions are prepared by blending 100 parts clay (UW-90, produced by Engelhard Corp. (Philbro), Edison, NJ, USA), 0.01 part dispersing agent (Dispex F40 produced by Toagosei Chemical Industry Co., Ltd., Japan), 0.5 part denatured starch (Oji Ace B produced by Oji Corn Starch Co., Ltd., Japan), 15 parts of one of copolymer latexes A-E, and as the poly-functional aziridine compound, two parts diphenylmethane-bis-4,4'-N,N'-ethyleneurea (SU 125F produced by Meisei Chemical Works, Ltd., Japan).

[0033] After adjusting the total solids to 60 percent, each of the paper coating compositions is applied onto one side of a high quality paper substrate (weighing 80 g/m²) with a manual blade coater at a coating weight of 15 g/m² (solids). The samples are dried at 150°C for 30 seconds and super-calendered. The treated specimens are evaluated, and the results shown in Table II.

Comparative Example 1

[0034] Paper coated compositions are prepared following the procedure of Example 1, except that diphenylmethane-bis-4,4'-N,N'-ethyleneurea is not used. The resulting paper coating compositions are coated onto paper specimens and evaluated as in Example 1. The results are shown in Table II.

Example 2

[0035] Using copolymer Latexes A-E prepared in Example 1, coating compositions were prepared by blending 90 parts clay (UW-.90, produced by Engelhard Corp., Edison, NJ, USA), 10 parts precipitated calcium carbonate (PZ produced by Shiraishi Calcium Co., Ltd., Japan), 0.2 part dispersing agent (Dispex F40 produced by Toagosei Chemical Industry Co., Ltd., Japan), 7 parts casein, 12 parts of one of the copolymer Latexes A-E, and 3 parts diphenylmethane-bis-4,4'-N,N'-ethyleneurea (SU 125F produced by Meisei Chemical Works, Ltd., Japan).

[0036] After adjusting the total solids to 40 percent, each of the paper coating compositions is applied onto one side of a white cardboard substrate (weight 270 g/m²) with a wire bar coater at a coating rate of 18 g/m² (solids). After drying at 105°C for 60 seconds, each coated specimen is super-calendered. The thus treated specimens are subjected to evaluations in the aforementioned manner, the results of which are given in Table III.

Comparative Example 2

[0037] Paper coating compositions are prepared by repeating the procedure of Example 2 except that diphenylmethane-bis-4,4'-N,N'-ethyleneurea is not used. Using the resultant compositions, coated paper specimens are prepared and subjected to evaluation in the same manner as Example 2. These results are also shown in Table III.

Example 3

[0038] Using copolymer Latex A, prepared in Example 1, a coating composition is prepared by blending 100 parts clay (UW-90, produced by Engelhard Corp., Edison, N.J., U.S.A.), 0.01 part dispersing agent (Dispex F40 produced by Toagosei Chemical Industry Co., Ltd., Japan), 0.5 part denatured starch (Oji Ace B produced by Oji Corn Starch Co., Ltd., Japan), 15 parts of copolymer Latex A, and two parts diphenylmethane-bis-4,4'-N,N'-ethyleneurea (SU 125F produced by Meisei Chemical Works, Ltd., Japan).

[0039] Additional paper coating compositions are prepared in the same manner except that the quantity of copolymer Latex A and diphenylmethane-bis-4,4'-N,N'--ethyleneurea are varied as shown in Table IV. The coating compositions are put on paper substrates and the coated paper specimens are subjected to evaluation in the same manner as in Example 1. The results are shown in Table IV.

[0040] The data shows that the paper coating compositions of the invention provide improved wet pick resistance, ink gloss and ink receptivity, without sacrificing viscosity, dry pick resistance or sheet gloss.

Claims

1. A paper coating composition comprising

a. a pigment;

b. a carboxyl group-containing latex in sufficient quantity to bind the pigment particles to a substrate; and

c. a poly-functional aziridine compound in sufficient quantity to impart water resistance to the coating composition.

2. The composition of Claim 1 wherein the copolymer of the latex contains 0.5 to 10 weight percent of an ethylenically unsaturated carboxylic acid.

3. The composition of Claim 2 wherein the latex solids are present at 1 to 30 weight parts, based on 100 weight parts of the pigment.

4. The composition of Claim 3 wherein the aziridine compound is present at 0.05 to 5 weight parts, based on 100 weight parts of the pigment.

5. The composition of Claim 1 wherein the aziridine compound has the formula:

wherein n represents an integer of 2 or more, Z represents an organic or inorganic group of atoms having a valence of n and each R¹ independently represents hydrogen or a methyl group.

6. The composition of Claim 1 wherein the aziridine compound has the formula:

in which R² is an organic radical having a valence of n and _R¹ and n are as defined hereinbefore.

7. The composition of Claim 1 wherein the aziridine compound has the formula:

wherein R¹ is as defined hereinbefore.

8. The composition of Claim 1 wherein the aziridine compound is diphenylmethane-bis-4,4'-N,N'- ethvleneurea.

9. The composition of Claim 2 wherein the aziridine compound is diphenylmethane-bis-4,4'-N,N'- ethyleneurea.

10. The composition of Claim 3 wherein the aziridine compound is diphenylmethane-bis-4,4'-N,N'- ethyleneurea.

11. The composition of Claim 4 wherein the aziridine compound is diphenylmethane-bis-4,4'-N,N'- ethyleneurea.

12. The composition of Claim .11 wherein the latex is a copolymer of a monovinylidene aromatic monomer, a conjugated diene and an ethylenically unsatu= rated carboxylic acid.

13. The composition of Claim 12 wherein the acid is acrylic acid, methacrylic acid, fumaric acid or maleic acid.

14. A coated article comprising a substrate coated on at least one surface with the coating composition of Claim 1, 4, 5, 11 or 13.