A METHOD FOR MAKING PRINTING PAPER AND A MIXTURE COMPOSITION

FIELD OF THE INVENTION The invention relates to a method for making printing paper as defined in the preamble of claim 1 and to a mixture composition for the surface-sizing of printing paper as defined in the preamble of claim 6.

BACKGROUND OF THE INVENTION

Known from the prior art are various methods for making different kinds of printing papers. Also known from the prior art are different kinds of papers for use in printing, e.g. different types of inkjet- based printing.

As to the inkjet-based printing techniques, it is important to ensure sufficient printing ink absorption in the high speed inkjet papers to provide a sufficiently good colour print. The important crite- rion is that the printing inks of different colours do not blend together. In uncoated papers, sufficient absorption can be ensured by low hydrophobic sizing level of the base paper, which has been disclosed e.g. in patent application WO 2006067273. It is known that anionic aqueous printing ink must be bound to the paper in order for the print to be waterfast. Waterfastness is essential for example in bank balance statements and other similar prints. It is known that waterfastness can be improved by add- ing cationic polymers to the surface-size or coating on the surface of the paper. The most commonly used polymer is polyDADMAC. Many of the fixatives used at the wet end of the paper machine are suitable for this purpose. Patent application WO 2006067273 discloses the surface treatment of printing paper with a cati-

onic polymer together with an internal sizing treatment of the paper. The suggested optimal amount is 2g/m ² , in which case about half of the amount of the surface-size should be replaced with the cationic polymer. The amount of the cationic polymer as presented above increases the production costs of the printing paper.

OBJECTIVE OF THE INVENTION The objective of the invention is to eliminate the drawbacks referred to above. One specific objective of the invention is to disclose a new method and surface-size mixture composition for making printing paper. In particular, the objective of the inven- tion is to disclose a new way of improving printabil- ity of printing paper.

SUMMARY OF THE INVENTION

The method and the mixture composition in ac- cordance with the invention are characterized by what has been presented in the claims .

The invention is based on a method for making printing paper, which method comprises forming an un- coated paper. In accordance with the invention, a sur- face-size mixture comprising starch and polyvinyl pyr- rolidone is arranged on the surface of the uncoated paper for improving the printability of the paper.

Printability means print quality, waterfast- ness and/or degree and rate of absorption. Print qual- ity means print sharpness and/or evenness.

The invention is specifically based on improving inkjet printability in uncoated papers by a new surface-size composition.

In this context, paper means any fibre-based paper, board or fibre product or the like. The paper may have been made from chemical pulp, mechanical

pulp, chemimechanical pulp and/or the like. The paper may be in the form of a wet web, dried web or sheet, or in other form which is suitable for the purpose. The paper may comprise suitable fillers and additives. In one embodiment of the invention, uncoated fine paper which mainly does not comprise mechanical pulp, e.g. WFU or UWF, is used in the method. Alternatively, it is possible to use uncoated paper which does comprise mechanical pulp, e.g. SC paper or news- print.

One embodiment of the invention comprises producing high speed inkjet paper with good printability.

In one embodiment of the invention, the surface-size mixture is in the form of a solution, dis- persion or paste. The mixture can be arranged or applied on the surface of the paper by any manner known per se in the art .

In one embodiment, the surface-size mixture is arranged on the surface of the formed paper web in con- junction with the web formation, e.g. in conjunction with the drying. In one embodiment, the mixture is arranged on the surface of the formed web in a separate step after the formation of the web.

In one embodiment of the method, a base paper web is formed in a manner known per se and cut to the correct width. The web is coated with the surface-size mixture. If desired, the web is cut into sheets or the like, and the web or the paper products cut off from it are provided with the desired print in the desired man- ner. In one embodiment, the print can be provided by a digital printing method, e.g. inkjet-based technique.

Furthermore, the invention is based on a mixture composition to be used for making printing paper. In accordance with the invention, the mixture composi- tion comprises starch and polyvinyl pyrrolidone, and

the mixture composition can be arranged on the surface of an uncoated paper as surface-size.

In one embodiment of the invention, the mixture composition comprises less than 5% cross-linker. The cross-linker may be e.g. an epoxy-based, a glyco- sal-based or a zirconium-based cross-linker.

In one embodiment of the invention, the mixture composition comprises a cationic polymer. In one embodiment, the mixture composition comprises less than 15% cationic polymer.

In one embodiment of the invention, the mixture composition comprises 45 - 99% starch.

In one embodiment of the invention, the mixture composition comprises 1 - 50% polyvinyl pyrroli- done. The polyvinyl pyrrolidone may be in any state and be present as a pure agent or mixture.

The method and the surface-size mixture in accordance with the invention provide considerable advantages compared to the prior art . The surface-sizing composition in accordance with the invention provides a printing paper of excellent print quality and improved printability characteristics. Furthermore, the surface-sizing composition in accordance with the invention provides quicker col- our absorption / setting rate in the printing.

The invention provides an industrially applicable, easy and fast way of making printing paper. The method in accordance with the invention is easy, simple and affordable to be executed as a production process.

The method in accordance with the invention is suitable to be used for making different kinds of printing papers, specifically for making inkjet-type, such as high speed inkjet, printing papers.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

The tests of this example involved examining different kinds of surface-sizing compositions to be used in WFU paper for high speed inkjet printing. The tests were conducted in a laboratory scale.

The tests involved examining and testing the effect of PVP (polyvinyl pyrrolidone) as a surface- sizing agent in non-pigmented compositions for surface-sizing of paper. The obtained results were compared to the results obtained with a reference composition. The reference composition used comprised mainly a starch-containing surface-size mixture with- out any PVP content.

The base paper used in the tests was a low internal-sized uncoated fine paper with the basis weight of about 82g/m ² and Cobb values of about 60 - 70g/m ² . The paper was surface-sized on one side with an RK laboratory coater and a rod at about 3g/m ² . The surface-size mixture compositions are presented in Tables 1 and 2. The pH value of the surface-size mixture was 7.5.

The surface-sized papers were printed by two inkjet printers; Epson Stylus C46 with Versamark ink and Canon Pixma iP3000.

The printed papers were rated as to the printing ink absorbency, line spreading and roughness, waterfastness (K, M) , wet rub and densities. The visual properties of the printed papers were examined by three persons .

Waterfastness means blackness after wetting / blackness before wetting x 100%. A higher value indicates better waterfastness; the value may exceed 100%. Wet rub means blackness of the white area after rubbing / blackness of the black area before rubbing. A

lower value indicates better wet rub x 100%; the value may be zero.

Table 1 presents the agents used in the test- s. Table 2 presents the surface-sizing agent compositions used in the individual tests. The solids content of the compositions was about 12%. Table 3 presents the results analysed from the printed paper.

Table 1

In the tests, the N-vinyl-2-pyrrolidone ho- mopolymer was used as the PVP. It dissolves in water and many organic solvents, such as alcohols, amines, acids, chlorinated hydrocarbons and amides. In a solution, it exhibits excellent wetting properties and good film-forming capability. It is more hydrophilic than starch and a better OBA activator, but it is more difficultly cross-linked.

Table 2

Table 3

The tests showed that the use of PVP together with starch in the surface-sizing provided good water- fastness and wet rub to the printed paper, substantially without reducing the density compared to other compositions. PVP was found to remain on the surface of the paper, i.e. it exhibits lower migration into paper than starch. Faster ink absorption, lower ink spreading and better waterfastness were therefore obtained with the PVP. Fast ink absorption rate is useful because it reduces the spreading of the ink. The tests showed that PVP together with starch improves considerably the waterfastness of the print and the black and white print quality, such as print sharpness and evenness, compared to the paper treated with the reference mixture (Ref) . In summary of the tests one can conclude that

PVP together with starch provides a very effective composition for surface-sizing of printing paper.

Example 2

The tests of this example involved examining different surface-sizing compositions to be used in WFU paper for high speed inkjet printing. The tests were conducted in a pilot scale. The tests involved examining and testing the effect of PVP (polyvinyl pyrrolidone) as a surface- sizing agent in non-pigmented compositions for surface-sizing of paper. The obtained results were compared to the results obtained with a reference compo- sition. The reference composition used comprised mainly a starch-containing surface-size mixture without any PVP content .

The base paper according to Example 1 was used in the tests, and it was coated accordingly with a surface-size mixture at 2.5g/m ² . The surface-size mixture compositions are presented in Table 4. The solids content of the compositions was 12%.

The papers were printed by inkjet printers; Epson Stylus C48 with Versamark ink, Canon Pixma iP3000 and HP Deskjet 6540.

Table 4

In the tests, it was found that the Digat absorption time was reduced when using PVP in the surface-size composition, compared to the results obtained with the reference surface-size mixture. In the

tests, the absorption time for black ink when using PVP (compositions 5-8 and 11) was in the range of 100 - 200ms, and when using only starch (Ref) it was more than 300ms. Waterfastness was found to be good in all compositions, preferably more than 100%. Also wet rub was good in all compositions, preferably less than 12%.

Also colour density was found to be at a good level when using PVP, irrespective of the printing device, relative to the results obtained with the reference composition.

Example 3

This test involved examining printing of papers that were surface-sized with the compositions in accordance with Test 2 by digital high speed inkjet printing machines . Waterfastness, wet rub, waterbleed, colour density, print evenness and print-through were found to be good with all compositions containing PVP relative to the reference composition.

Waterbleed means blackness of the white area after wetting / blackness of the black area before wetting x 100%. A lower value indicates better result. The goal is a result of less than 10%; the value may be zero.

Furthermore, it was found in the tests that the setting time of Digat ink was shorter when using a

PVP-containing surface-size composition compared to the reference composition. The setting time with the

PVP-containing composition was mainly less than 150ms.

Runnability and printability of the paper which contains PVP in the surface-sizing composition were found to be good in the printing tests.

In the printing tests of Examples 2 and 3, composition 8 with 10 parts PVP and 90 parts starch seemed to emerge as the preferred surface-sizing mixture composition. Results from the pilot tests (Example 2) and printing tests (Example 3) supported the results obtained in the laboratory scale. A PVP-containing surface-size composition provides an excellent printing paper with good printability and print quality. The method in accordance with the invention is suitable in different embodiments to be used for making the most different kinds of printing papers.

The invention is not limited merely to the examples referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims.