The invention relates to a method of decreasing the dusting tendency of printing paper.

Background

The predominant part of all paper and paperboard that is to be used in laser printers, inkjet printers and other different printing methods such as offset printing and flexoprinting, is surface treated with a surface sizing composition in order to improve printability as well as other properties of the paper or paperboard. Surface sizing compositions mainly consists of modified starches or other hydrocolloids or surface sizing agents.

Inkjet printing puts high demands on a printing paper, since the ink must be quickly dried on the substrate and yet provide a high print quality with, e.g. a high optical print density, minimized feathering and bleeding and low strike-through. In recent years it has been found that when multivalent salts, such as calcium chloride, are added to the surface size, the applied ink will precipitate fast on the surface of the paper and give rise to a significant improvement in print quality. This is especially

advantageous in inkjet printing. US6207258 discloses a composition useful for surface treating a sheet substrate for inkjet printing, the composition comprising a salt of a divalent metal. One problem connected to printing of uncoated paper is dusting, i.e. the tendency of a paper surface to shed loose and weakly bonded particles, which may accumulate on the printing machinery. Said weakly bonded particles mostly comprise different forms of calcium carbonate originating from fillers, such as precipitated calcium carbonate (PCC) , chalk and ground calcium carbonate (GCC) , present in the base-paper .

In the art, attempts to reduce the dusting tendency of paper have been done by increasing the bonding within the paper. However, in recent years, tougher demands on keeping down production costs have increased the use of fillers in printing paper manufacturing. At higher filler contents, the bonding within the paper is weakened whereby the dusting tendency increases.

Description of the invention

One object of the present invention is to decrease the dusting tendency of printing paper.

This object, as well as other advantages, can be achieved by the method of the invention. The invention relates to a method of reducing the dusting tendency of printing paper, preferably of uncoated printing paper. The method of the invention comprises the steps of; providing a base paper comprising filler in an amount of at least 20 weight%, which filler comprises calcium carbonate and applying a sizing composition to the surface of said base paper, wherein the pH of said sizing composition has been adjusted to between 4 and 6. The filler in said base paper may comprise calcium carbonate in the form of e.g. precipitated calcium carbonate (PCC) , ground calcium carbonate (GCC) and/or chalk.

It has been shown that by adjusting the pH of the surface sizing composition to between 4 and 6, a calcium salt is formed from the filler located on the surface of the paper and the amount of calcium carbonate at the surface is reduced, whereby problems with dusting are avoided.

Moreover, the calcium salt formed at the surface of the paper enhances the printing properties of the produced paper. A pH adjusted to between 4 and 6 gives rise to an effective reduction of calcium carbonate at the surface of the paper and in the surface size circulation. Since it is only the calcium carbonate located at the surface that is to be reduced, the pH should not be lowered to below 4. Moreover, at pH below 4, also problems with corrosion may occur .

The pH of the sizing composition may be adjusted by the addition of an acidic media. The acidic media may be selected from the group consisting of mineral acid and/or organic acids, and/or metal salts. The acidic media may, e.g. comprise hydrochloride acid, a metal salt, citric acid, sulfomic acid, alum, sulfuric acid, acetic acid, phosphoric acid, sodium dihydrogen phosphate, malic acid, tartaric acid, adipic acid and/or succinic acid. In one mode, the acidic media comprises a salt of a multivalent metal, such as a divalent or trivalent metal, e.g.

magnesium acetate. When an acidic salt comprising

multivalent ions is added to the surface sizing

composition, not only a reduced dusting tendency is achieved, but at the same time the print quality is improved .

The sizing composition may comprise starch. Preferably, the sizing composition comprises starch to an amount of 8 - 15 % dry weight. In one preferred mode, the sizing

composition comprises cationic starch. A decrease of pH of a sizing composition comprising cationic starch, renders the starch, especially tertiary amino type starch, even more cationic active whereby the ability of the starch to bind to the fibers improves. In this way, release of starch is avoided at the recycling of broke, whereby slime formation is reduced.

The method of the invention may be used to reduce dusting of all kinds of printing papers, e.g. paper suitable for printing by use of laser, inkjet, offset and or gravure . Preferably, the printing paper is suitable for inkjet or rotogravure printing.

Detailed description of a preferred embodiment

The invention is further described below by way of an example, wherein the dusting tendency of a paper according to the invention is compared with a paper surface sized with a conventional starch solution.

A conventional cationic surface size starch, based on corn with the trade name Meribond 164 supplied by Tate & Lyle with a degree of substitution of 0,014, was cooked with a jet cooker. The starch solution, with a pH of 7,5 was diluted to a concentration of 12 %. To this solution, 12 g per litre/solution sulphanilic agent (OBA) was added.

A first part of said starch solution was used as reference (hereinafter: Ref. sizing composition), which Ref. sizing composition had a pH of 7,5. The pH of a second part of said solution (hereinafter sizing composition 1) was decreased to 4,5 by the addition of 7 ml citronic acid with a concentration of 10%, per litre of the starch/OBA

mixture. The pH of a third part of said solution

(hereinafter sizing composition 2) was decreased to 4,5 by the addition of 10 ml sulfomic acid with a concentration of 10%, per litre of the starch/OBA mixture.

The base-paper used in the test is made from chemical pulp and the ratio of softwood pulp to hardwood pulp is 50/50. It comprises 25 % of precipitated calcium carbonate with a scalenohedral crystal form. Furthermore, 1 % cationic starch, 0,5 % OBA, 0,15 % alkyl succinic anhydride, is added. The retention system used is Compozil® from

AkzoNobel Corporate.

One sample (Ref. Sample) of said base paper was surface sized with the Ref. sizing composition, while two samples (Sample 1 and Sample 2) were surface sized with sizing composition 1 and 2 respectively. All samples were

surface-sized in a Sym Sizer® device supplied by Metso Corperation on both the bottom and the top side of the paper with the surface sizing compositions to an amount of 1 , 3 g/m ² .

The surface sized samples (Ref Sample, Sample 1 and Sample 2) were thereafter printed in an inkjet printer. Visual comparisons showed that the printing of the Ref Sampled gave rise to problems with dusting particles accumulating on the printing blanket. The printing of Samples 1 and 2 did not give rise to any accumulation on the blanket. Thus, the treatment of a printing paper in accordance with the invention remarkably reduces the dusting tendency when the paper is printed by use of e.g. inkjet printing techniques.

While specific embodiments and examples of the products and methods of the invention have been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.