Strength agent, its use and method for increasing strength properties of paper

Abstract

The invention relates to a strength agent for paper, board or the like. The strength agent comprises a first component, which is refined cellulosic fibres having a refining level of >70° SR, and a second component, which is a synthetic cationic polymer having a charge density of 0.1-2.5 meq/g, determined at pH 2.7, and an average molecular weight of >300 000 g/mol. The invention relates also to a use of the strength agent and to a method for increasing strength properties of paper, board or the like.

Claims

1. A strength agent for paper, board or the like, which agent comprises a first component, which is mechanically refined cellulosic fibres having a refining level of >70° SR, a second component, which is a synthetic cationic polymer having a charge density of 0.1-2.5 meq/g, determined at pH 2.7, and an average molecular weight of >300000 g/mol.

2. The strength agent according to claim 1, wherein the cellulosic fibres have a refining level of 70-98° SR, preferably 75-90° SR, more preferably 77-87° SR.

3. The strength agent according to claim 1, wherein the first component consists of cellulosic fibres, which are obtained by kraft pulping and which have been subjected solely to mechanical refining.

4. The strength agent according to claim 1, wherein the cellulosic fibres are bleached softwood fibres obtained by kraft pulping.

5. The strength agent according to claim 1, wherein the synthetic cationic polymer has a charge density of 0.2-2.5 meq/g, preferably 0.3-1.9 meq/g, more preferably 0.4-1.35 meq/g.

6. The strength agent according to claim 1, wherein the synthetic cationic polymer has an average molecular weight of 300000-6000000 g/mol, preferably 400000-4000000 g/mol, more preferably 500000-1900000 g/mol.

7. The strength agent according to claim 1, wherein the synthetic cationic polymer is a copolymer of methacrylamide or acrylamide and at least one cationic monomer.

8. The strength agent according to claim 7, wherein the cationic monomer is selected from a group consisting of methacryloyloxyethyltrimethyl ammonium chloride, acryloyloxyethyl-trimethyl ammonium chloride, 3-(methacrylamido) propyltrimethyl ammonium chloride, 3-(acryloylamido) propyltrimethyl ammonium chloride, diallyldimethyl ammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropylacrylamide and dimethylamino-propylmethacrylamide.

9. The strength agent according to claim 1, wherein it comprises cationic or amphoteric starch with a substitution degree in the range of 0.01-0.5, preferably 0.04-0.3, more preferably 0.05-0.2.

10. The strength agent according to claim 1, wherein it comprises 70-99.8 weight-%, preferably 90-99 weight-% of refined cellulosic fibres and 0.5-10 weight %, preferably 1-5 weight-%, of synthetic cationic polymer.

11. The strength agent according to claim 1, wherein the strength agent comprises refined cellulosic fibres and synthetic cationic polymer in ratio of 100:1-5:1, preferably 70:1-20:1.

12. Use of the strength agent according to claim 1 for increasing strength properties of paper, board or the like.

13. Use according to claim 11, wherein the strength agent is added to the pulp in such amount that the dose of the first component is in the range of 0.1-10 weight-%, preferably 0.5-8 weight-%, more preferably 1.5-6 weight-%, and the dose of the second component is in the range of 0.02-0.5 weight-%, preferably 0.07-0.4 weight-%, more preferably 0.12-0.25 weight-%, calculated per dry fibre stock.

14. A method for increasing strength properties of paper, board or the like, comprising obtaining a fibre stock, adding to the fibre stock a strength agent comprising a first component and a second component according to claim 1.

15. The method according to claim 14, wherein the fibre stock comprises mineral filler.

16. The method according to claim 14, wherein the first component of the strength agent is added to the stock, and thereafter the second component of the strength agent.

17. The method according to claim 14, wherein the second component of the strength agent is added to the stock, and thereafter the first component of the strength agent.

18. The method according to claim 14, wherein the strength agent or any of its components is added to thick fibre stock, which has a consistency of at least 20 g/l, preferably more than 25 g/l, more preferably more than 30 g/l.

Description

EXAMPLE 1

[0046] Hand sheets were formed as described above. Sheet basis weight was 80 g/m.sup.2.

[0047] The fibre suspension comprised 50 weight-% of long fibre fraction, which was pine kraft pulp, SR 18, and 50 weight-% short fibre fraction, which was eucalyptus pulp, SR18.

[0048] The strength agent comprised:

[0049] 1) a first component, which was pine kraft pulp with refining level of SR 90. The refining of the pine kraft pulp was performed with Valley-beater, 1.64 weight-%, calculated as dry fibre, and

[0050] 2) a second component which was cationic polyacrylamide, average molecular weight 800 000 g/mol, charge density 1.3 meg/g.

[0051] The results of Example 1 are given in Table 2. All the dosages are given as kg/pulp ton and as active component.

TABLE-US-00002 TABLE 2 Results of Example 1 1.sup.st 2.sup.nd Tensile Scott Zeta Test component component index Bond, potential, Point dose dose [Nm/g] [J/m2] [mV] 1 — — 38.1 150 −91 2 50 — 42.1 171 −87 3 — 2 44.1 228 −30 4 50 1 44.3 228 −58 5 50 2 49.2 260 −33 6 50 4 48.1 258 6

[0052] From Table 2 it can be seen that the strength agent according to the invention comprising both refined cellulosic fibres and synthetic cationic polymer improves the tensile index and Scott Bond values of the obtained paper. It is also seen that when strength agent is used, lower amounts of synthetic cationic polymer yield similar results than higher amount of synthetic cationic polymer alone. This may indicate that by using the present invention, lower amount of synthetic cationic polymers can be used, which have positive effect on overall process economy, as usually the synthetic polymers are the expensive components in manufacture of paper or board.

[0053] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.