Paper strength improving composition, manufacture thereof and use in paper making

1. A method of making a paper comprising the steps of:

a) providing a cationic wet strength resin comprising a polyamidoamine epihalohydrin, a condensation copolymer of epihalohydrin and amine, or a combination thereof,

b) providing an anionic polymer,

c) co-mixing the cationic wet strength resin and the anionic polymer to provide a composition comprising polyelectrolyte complexes,

d) providing an aqueous pulp slurry, draining the aqueous pulp slurry to form a wet fiber web, and drying the wet fiber web to obtain the paper,

wherein said co-mixed composition is introduced to the aqueous pulp slurry or on the formed wet fiber web,

and further wherein the anionic polymer has a standard viscosity of about 1.1 to 6 cP, measured at 0.1 weight-% polymer concentration in 1 M NaCl, at 25° C. and pH 8.0—8.5, using Brookfield DVII T viscometer.

2. The method according to claim 1 , wherein the anionic polymer comprises an anionic synthetic polymer, an anionic polysaccharide, or any combination thereof.

3. The method according to claim 2 , wherein the anionic synthetic polymer comprises a copolymer of non-ionic monomers and anionic monomers, a homopolymer of anionic monomers, a partially or completely hydrolysed poly(meth)acrylamide, an anionic glyoxalated polyacrylamide, or any combination thereof.

4. The method according to claim 3 , wherein the copolymer of non-ionic monomer and anionic monomer has a molar ratio of anionic monomer to non-ionic monomer in the range of 5:95—95:5.

5. The method according to claim 2 , wherein the anionic polysaccharide comprises anionic cellulose, anionic starch, anionic vegetable gum, anionic microfibrillar cellulose, or any combination thereof.

6. The method according to claim 1 , wherein the anionic polymer has a charge density of about −0.1 to −10 meq/g (dry), as measured at pH 7.

7. The method according to claim 1 , wherein the cationic wet strength resin has a weight average molecular weight of about 150 000 to 1 000 000 Dalton.

8. The method according to claim 1 , wherein the cationic wet strength resin has a charge density of about 1.5 to 6.0 meq/g, as measured at pH 4.

9. The method according to claim 1 , wherein the cationic wet strength resin comprises a polyamidoamine epihalohydrin resin.

10. The method according to claim 9 , wherein the polyamidoamine epihalohydrin has an epihalohydrin:amine molar ratio of at least 0.80.

11. The method according to claim 1 , wherein the weight ratio of the anionic polymer to the cationic wet strength resin is about 5:95 to 50:50.

12. The method according to claim 1 , wherein the solids content of the cationic wet strength resin is at least 15 weight-%, before co-mixing with the anionic polymer.

13. The method according to claim 1 , wherein the co-mixed composition of step c) is diluted with water before introducing to the aqueous pulp slurry or on the formed wet fiber web; preferably the co-mixed composition of step c) is diluted to a solids content of at most 5 weight-%.

14. The method according to claim 1 , wherein the co-mixed composition has a net cationic charge, as measured at pH 4; preferably the co-mixed composition has a charge density of at least 0.01 meq/g, as measured at pH 4.

15. The method according to claim 1 , wherein the co-mixed composition is introduced to the aqueous pulp slurry or on the formed wet fiber web at most 2 hours after initiation of co-mixing.

16. The method according to claim 1 , wherein the paper is selected from tissue, towel, carrier board, linerboard, fluting, liquid packaging board, folding box board, solid bleached sulfate board, solid unbleached sulfate board, and white lined chipboard.