WO2004046259A1

WO2004046259A1 - Improvement of paper or paperboard

Info

Publication number: WO2004046259A1
Application number: PCT/FI2003/000878
Authority: WO
Inventors: Jonas Hannus; Marjo KÄPYNEN; Päivi MIETTINEN; Anna-Liisa Tammi
Original assignee: Raisio Chemicals Ltd
Priority date: 2002-11-18
Filing date: 2003-11-18
Publication date: 2004-06-03
Also published as: FI20022046A0; AU2003279424A1

Abstract

The present invention relates to an improvement in a printing process, especially to a pigment which has been modified using a salt of styrene maleic acid copolymer as a dispersing agent, prior to incorporation in a paper coating composition for preparing a coated paper or paperboard for use in a printing process. Also disclosed is a paper coating composition for preparing a coated paper or paperboard and a printing process, wherein said coated paper or paperboard is employed.

Description

Improvement of paper or paperboard

Field of the invention

The present invention relates to an improvement in a printing process, especially to a pigment which has been modified using a salt of styrene maleic acid copolymer as a dispersing agent prior to incorporation in a paper coating composition for preparing a coated paper or paperboard for use in a printing process, especially a rotogravure printing process. Also disclosed are a paper coating composition for preparing a coated paper or paperboard and a printing process, wherein said coated paper or paperboard is employed.

Description of the background art

The rotogravure printing process is generally most suited to those applications in which a very large number of copies is required to be printed, because the process by which a rotogravure printing cylinder is prepared is expensive relative to the cost of preparing printing plates for use in other methods of printing. A rotogravure printing cylinder has on its surface a matrix of cells or depressions which vary in depth according to the quantity of ink which is required to be transferred to the paper from each individual cell. The surface of the cylinder is initially smooth and highly polished, and the matrix of cells on the surface is prepared by an expensive photographic etching process. Because of the large number of copies to be printed, it is generally advantageous to run the printing press at high speed, and this necessitates the use of a printing ink which dries quickly on the paper to avoid "offset", or the transfer of undried printing ink from one sheet of paper to the next. It is therefore desirable to use a printing ink which has a solvent which is more volatile than water. Such volatile solvents are almost invariably organic and oleophilic, and it has been found that a gravure printing ink based on such a solvent is not always well absorbed into a conventional paper coating formed by applying to the surface of the base paper a composition containing a pigment with a hydrophilic surface. Conse- quently, there is a need for a coating composition which provides the coated paper with better printability properties especially in the rotogravure printing process.

Conventional dispersing agents used in coating compositions are for example inorganic dispersing agents, such as the water soluble salts of polyphosphoric acid or polysilicic acid, or polycarboxylate dispersing agents such as the water soluble salts of poly(acrylic acid) or of poly(methacrylic acid). The most commonly used pigment dispersants are polyacrylate salts, e.g. sodium polyacrylate. Mixtures of different dispersing agents have also been operable.

Conventional dispersing agents confer the paper coating pigment and the surface coating containing said pigment with a hydrophilic character. However, especially in a rotogravure printing process hydrophilicity of the surface to be printed is not desirable as it may lead to an unsatisfactory printing result (mottling, waving). Consequently, the coating pigments have been modified with various surface treatment agents to confer hydrophobic or enhanced hydrophobic character on the pigment surfaces (see for example US 5,879,512; EP 565 961).

US 5,879,512 describes a method wherein conventional dispersing agents are employed together with surface treatment agents comprising a fatty acid, a fatty alcohol or an anionic surfactant having a long chain hydrophobic group and a polar group. In said method, an aqueous emulsion or solution of the surface treatment agent is added to the dispersed aqueous suspension of the pigment material.

EP 565 691 discloses a method whereby a pigment material is treated with a chemical surface treatment agent, e.g. a saturated or unsaturated fatty acid or an amine, prior to the preparation of the paper coating composition. However, in use of such a method, the treated pigment material has to be subsequently dispersed with a dispersing agent in an aqueous suspension. Such a process is difficult to carry out satisfactorily because the treated pigment material is not easily dispersed with conventional dispersing agents. In the abstract of JP 62258732, the use of a copolymer of styrene and maleic acid and/or maleic anhydride as a dispersing agent in water solutions of calcium carbonate is mentioned. However, as the salt of the copolymer, an alkali metal salt, preferably a sodium salt, is used, and no reference to a paper coating composition or to a rotogravure printing process is made.

US 2,576,914 discloses paper coating compositions wherein as a pigment binder is used a copolymeric material obtained by copolymerizing an alkyl ester of a suitable acid or anhydride, such as maleic acid or maleic anhydride, with a polymerizable vinyl compound, e.g. styrene. The coating composition further contains long chain unsaturated fatty acids, mineral or pigment, and water. The copolymeric material is used in an amount of from 5 to 30% by weight, based on the mineral or pigment. However, as a result of the esterification of the anhydride or acid moiety, the binding to pigment is decreased.

Summary of the invention

According to the present invention there is provided a pigment for use in a paper coating composition for preparing a coated paper or paperboard especially for use in a rotogravure printing process, wherein at least part of the pigment has been modified using a salt of styrene maleic acid copolymer as a dispersing agent prior to incorporation in the paper coating composition. Further aspects of the invention are a paper coating composition containing the pigment according to the invention, as well as a printing process wherein the paper or paperboard employed has been coated with a paper coating composition according to the invention. Still further aspects of the invention are the use of a salt of styrene maleic acid copolymer for dispersing paper coating pigments, as well as the use of a pigment modified using a salt of styrene maleic acid copolymer as a dispersing agent in a rotogravure paper coating composition. Within this disclosure, by a "paper" is meant paper or paperboard or any material, preferably cellulosic material, which is or can be made suitable for a printing process, especially for a rotogravure printing process.

As used herein, by "modification of a pigment" is meant treatment of the pigment particle surface with a salt of styrene maleic acid copolymer at any stage of the preparation of an aqueous pigment slurry. Preferably the pigment is treated as early as possible during the preparation process when the pigment particle surface is preferably still intact.

Within this application, "a salt of styrene maleic acid copolymer" means a salt of the hydrolysis product of styrene maleic anhydride copolymer.

The term "coating process" refers herein to a process, wherein at least one uniform coating layer of for example 1-20 g/m² of a coating composition is applied onto at least one side of a base paper, mainly in order to improve the printability properties of the paper.

Detailed description of the invention

The pigment according to the invention is modified by using a dispersing agent, wherein the dispersing agent comprises of a salt of styrene maleic acid copolymer. The salt is advantageously an ammonium, sodium, potassium or calcium salt, or alternatively a combination of said ion forms. A particularly advantageous salt is the ammonium salt of styrene maleic acid copolymer. The copolymer may also contain other copolymerised monomers, such as different acrylates, butadiene, acryla- mides or acrylonitriles. The salt of styrene maleic acid copolymer may be partly in an amide and/or imide form. Preferably the acid or anhydride is not esterified.

The styrene share of the salt of styrene maleic acid copolymer used in the dispersing agent for dispersing the pigment according to the invention can vary between 25 and 90%, preferably between 60 and 80%. A large amount of styrene is advantageous regarding this invention, because the hydrophobic properties of the dispersing agent are particularly due to the styrene.

When the compositions of coating mixtures are given, the proportions of the components are usually calculated so that the total amount of pigments is marked as 100 and the amounts of the other components are given as relative to the amount of the total pigment. The proportions of all components are given as dry parts. In this disclosure the amounts of the components of the coating composition according to the invention are given as described above. The term pph (parts per hundred) is also used in connection with said components.

Typically, the amount of the dispersing agents used will be at least 0.1 pph by weight, based on the weight of dry pigment, and will preferably be less than 5 pph by weight, based on the weight of dry pigment. More preferably the amount of the dispersing agents is from 0.3 to 3.0 pph by weight, even more preferably from 0.5 to 1.5 pph by weight, based on the weight of dry pigment.

Mixtures with different conventional dispersing agents are also operable. In mixtures with conventional dispersing agents, the share of the salt of styrene maleic acid copolymer should be at least 5%, preferably at least 20%, in order to confer the pigment according to the invention, the corresponding paper coating composition and the coated paper with the desired properties.

The dry solids content of the pigment dispersion is usually 50 to 80%, preferably 55 to 70%.

The pigment material for use in the present invention may be any conventional pigment used in paper coating compositions. However, inorganic pigments are preferred. Especially preferred are calcium-containing pigments, such as ground or precipitated calcium carbonate, calcium sulphate, calcined kaolin or a blend of any of these. One of the advantages of the present invention is that part or all of the pigments conventionally used in the rotogravure printing process, such as talc and kaolin, can be replaced by calcium-containing pigments, thus achieving e.g the desired brightness.

The pigment material modified according to the present invention may be mixed with conventionally dispersed or untreated pigments for the preparation of paper coating compositions and printing papers. Preferably, in any such admixture, the pigment according to the present invention should be present in an effective amount, preferably in an amount of at least 20% by weight, based on the total weight of the pigment. In preferable embodiments of the present invention, the pigment according to the present invention will be the sole pigment used. The paper coating composition according to the invention further comprises binder(s) and, if desired, conventional paper coating composition additives.

The amount of binder solids present in the coating composition will vary according to the type of binder(s) used and the type of process by which the coated paper is to be printed, but will normally be in the range of from 3 pph to 30 pph, preferably 20 pph or less, by weight based on the dry weight of the pigment. Preferably, the binder is a latex in which case it is preferred that the quantity of the latex used is such that the amount of latex solids is in the range of 3 pph to 10 pph by weight, based on the dry weight of the pigment. The latex may be, for example, a styrene- butadiene latex or an acrylic latex, which may or may not be of the alkali-swelling type. The latex to be employed will normally be one which is in the form of an aqueous emulsion containing about 30% to 60% by weight of latex solids, preferably about 40 to 50% by weight of latex solids.

The paper coating composition according to the invention may also contain small amounts of conventional additives, such as lubricants, thickeners, optical bright- eners, preservatives, hardeners, defoamers and foaming inhibition agents. The pigments according to the invention can be incorporated into coating compositions for use in preparing coated paper or paperboard which has especially advantageous properties when used in a rotogravure printing process. The print quality on printing by the rotogravure process is superior, as compared with a paper which has been coated with a paper coating composition containing only conventionally dispersed pigment material. In the printing process the use of ESA (electrostatic assist) can be reduced or, alternatively, the printing speed can be higher with the same effect of ESA.

A further aspect of the invention is a printing process in which an image is printed onto a paper web, which has been coated with a paper coating composition containing a pigment which has been modified using a salt of styrene maleic acid copolymer as a dispersing agent. The printing process is preferably a rotogravure printing process.

The invention also relates to a paper or paperboard coated with a paper coating composition in accordance with the invention, and to the use of a salt of styrene maleic acid copolymer for dispersing paper coating pigments.

The invention will now be illustrated with the following example.

Example

Three different paper coating compositions (B and C being in accordance with the invention) were prepared according to the recipes given in table 1 below:

Table 1. Coating compositions A B

a.i = according to the invention s.c. = solid content

Composition A was prepared using calcium carbonate dispersed with a conventional dispersing agent.

Composition B has been prepared from a mixture of calcium carbonate dispersed with conventional dispersing agents and calcium carbonate dispersed with a dispersing agent according to the invention.

In composition C the calcium carbonate has been dispersed using solely a dispersing agent according to the invention.

In all the compositions the total amount of dispersing agents was 0.8 pph, based on the weight of dry pigment.

The compositions A, B, and C were used to coat rotogravure base papers of 56 g/m², the papers were dried to a moisture content of 6%, and calendered. After calendering, the papers with a moisture content of 4-5% were tested for a rotogravure printing process.

The printability of the coated papers in a rotogravure printing process has been evaluated using a Heliotest and by measuring the dispersive, polar and total surface energies of the surfaces. Missing dots are inevitable at 5% halftone but disastrous when occurring at 20% and 30%) halftones. The number of missing dots is a traditional measure of rotogravure printability of a paper, h laboratory printing, the so-called Heliotest indicates the number of missing dots. The Heliotest fitting developed by Centre Technique du Papier is used with an IGT AC2 laboratory printer. The test figure is 110 mm long with an 8 mm wide raster where the tone changes from dark to light. The size of the dots is constant over the entire printing area. The number of missing dots is defined as a distance from the dark end of the printed image within which 20 missing dots can be detected. The longer the distance the better the rotogravure printability of the paper. A dot where more than half of the dot area is missing is considered as a missing dot.

The results from the tests are presented in table 2.

Table 2.

According to the results an increase of the part of pigment dispersed in accordance with the invention leads to a better result in regard to the Heliotest. The increase in the dispersive part of the surface energy is also clearly visible, which tells that the paper surface now has a hydrophobic character.

Claims

1. A pigment for use in a paper coating composition, characterized in that at least part of the pigment has been modified using a salt of styrene maleic acid copolymer as a dispersing agent prior to incorporation of the pigment in the paper coating composition.

2. The pigment according to claim 1 for preparing a coated paper or paperboard for use in a rotogravure printing process.

3. The pigment according to claim 1 or 2, characterized in that the quantity of the dispersing agent employed is from 0.1 to below 5.0 pph by weight, based on the weight of dry pigment, preferably from 0.3 to 3.0 pph by weight, and most preferably from 0.5 to 1.5 pph by weight, based on the weight of dry pigment.

4. The pigment according to claim 3, characterized in that the salt is the ammonium salt of styrene maleic acid copolymer.

5. The pigment according to any one of claims 1 to 4, characterized in that the pigments employed are calcium-containing pigments.

6. A paper coating composition for preparing a coated paper or paperboard which paper coating composition comprises binder(s), pigment(s) and, if desired, conventional paper coating composition additives, characterized in that prior to incorporation in the paper coating composition at least part of the pigment(s) has been modified using a salt of styrene maleic acid copolymer as a dispersing agent.

7. The paper coating composition according to claim 6 for preparing a paper or paperboard for use in a rotogravure printing process.

8. The paper coating composition according to claim 6 or 7, characterized in that the salt is the ammonium salt of styrene maleic acid copolymer.

9. The paper coating composition according to any one of claims 6 to 8, characterized in that the pigments employed are calcium-containing pigments.

10. The paper coating composition according to any one of claims 6 to 9, characterized in that the quantity of the dispersing agent employed is from 0.1 to below 5.0 pph by weight, based on the weight of dry pigment, more preferably from 0.3 to 3 pph by weight, and most preferably from 0.5 to 1.5 pph by weight, based on the weight of dry pigment.

11. The paper coating composition according to any one of claims 7 to 10, characterized in that at least 20% of the pigment(s) employed in the composition has been modified using a salt of styrene maleic acid copolymer as a dispersing agent.

12. The paper coating composition according to any one of claims 7 to 10, characterized in that the pigments employed have been modified with dispersing agents comprising at least 5%, preferably at least 20%, of a salt of styrene maleic acid copolymer.

13. A printing process, especially a rotogravure printing process, in which an image is printed onto a paper web, characterized in that the paper employed has been coated with a paper coating composition containing a pigment at least part of which has been modified using a salt of styrene maleic acid copolymer as a dispersing agent.

14. The printing process according to claim 13, characterized in that the quantity of the dispersing agent employed is from 0.1 to below 5.0 pph by weight, based on the weight of dry pigment, more preferably 0.3 to 3 pph by weight, and most preferably 0.5 to 1.5 pph by weight, based on the weight of dry pigment.

15. The printing process according to claim 13 or 14, characterized in that the salt is the ammonium salt of styrene maleic acid copolymer.

16. A paper or paperboard coated with a paper coating composition according to any one of claims 6 to 12.

17. Use of a salt of styrene maleic acid copolymer for dispersing paper coating pigments.

18. The use of a pigment, which has been modified using a salt of styrene maleic acid copolymer as a dispersing agent, in a rotogravure paper coating composition.

19. The use according to claim 17 or 18, wherein the quantity of the dispersing agent employed is from 0.1 to below 5.0 pph by weight, based on the weight of dry pigment, more preferably from 0.3 to 3 pph by weight, and most preferably from 0.5 to 1.5 pph by weight, based on the weight of dry pigment.

20. The use according to claim 19, characterized in that the salt is the ammonium salt of styrene maleic acid copolymer.