Sizing composition, its use and a method for producing paper, board or the like
The invention relates to a composition for sizing of a surface of paper, board or the like and its use for increasing strength properties of paper, board or the like. The composition has a solids content of 3-30%, and it comprises degraded non-ionic starch, and at least 0.5 weight-% of anionic polyacrylamide, which has a molecular weight, MW, >500 000 g/mol and <2 500 000 g/mol and an anionicity in the range of 4-35 mol-%. The invention further relates also to a method for producing paper, board or the like, which comprises addition of a first strength composition, which comprises a cationic agent, to a fiber stock, formation of a fibrous web from the fiber stock, drying of the fibrous web to dryness of at least 60%, and application of a second strength composition, which comprises an anionic hydrophilic polymer and a starch component, on the surface of the fibrous web.
1. A sizing composition for sizing of a surface of paper, or board, the sizing composition having a solids content of 3-30% and comprising:
degraded non-ionic starch, and
at least 0.5-10 weight-% of anionic polyacrylamide, which has an average molecular weight, MW, in a range of 540,000 g/mol to 1,400,000 g/mol, and an anionicity in a range of 4-35 mol-%.
2. The composition according to claim 1 , wherein the anionic polyacrylamide has the average molecular weight in a range of 650,000-1,400,000 g/mol.
3. The composition according to claim 1 , wherein the anionic polyacrylamide has the anionicity in a range of 4-24 mol-%, 4-17 mol-% or 5-17 mol-%.
4. The composition according to claim 1 , wherein the anionic polyacrylamide has the anionicity in a range of 7-15 mol-% or 9-13 mol-%.
5. The composition according to claim 1 , wherein the anionic polyacrylamide is a copolymer of acrylamide and unsaturated carboxylic acid monomers, being (meth)acrylic acid, maleic acid, crotonic acid, itaconic acid or their mixture.
6. The composition according to claim 1 , wherein the composition comprises 0.75-5 weight-% or 1-2.5 weight-%, of anionic polyacrylamide.
7. The composition according to claim 1 , wherein the starch is enzyme treated or thermally degraded starch.
8. The composition according to claim 1 , wherein the starch, prior to its degradation, has an amylase content of 15-30%, 20-30% or 24-30%.
9. The composition according to claim 1 , wherein the composition is free from inorganic mineral fillers or pigments.
10. A method for producing paper, board or the like, which method comprises:
adding a first strength composition, which comprises a cationic agent, to a fibre stock,
forming a fibrous web from the fibre stock,
drying the fibrous web to dryness of at least 60%, and
applying on the surface of the fibrous web a second strength composition having a solids content of 3-30%, which comprises at least 0.5-10 weight-% of an anionic hydrophilic polymer which is a anionic polyacrylamide, which has an average molecular weight, MW, in a range of 540,000 g/mol to 1,400,000 g/mol, and an anionicity in a range of 4-35 mol-%, and a starch component, which is degraded non-ionic starch.
11. The method according to claim 10 , wherein the cationic agent in the first strength composition comprises cationic starch or at least one cationic synthetic polymer or a mixture of cationic starch and cationic synthetic polymer(s).
12. The method according to claim 11 , wherein the cationic synthetic polymer is selected from a group comprising copolymers of (meth)acrylamide and cationic monomers; glyoxylated polyacrylamide; polyvinylamine; N-vinyl formamide; copolymer of acrylamide and diallyldimethylammonium chloride (DADMAC); polyamidoamine epihalohydrin and any of their mixtures.
13. The method according to claim 11 wherein the cationic synthetic copolymer is a copolymer originating from >20 mol-% of non-ionic monomers and 3-30 mol-%, 5-20 mol-% or 6-10 mol-%, of cationic monomers.
14. The method according to claim 11 wherein the cationic synthetic polymer has an average molecular weight of 600,000-950,000 g/mol.
15. The method according to claim 10 , wherein the cationic agent has charge density of 0.05-5 meq/g, 0.1-3 meq/g, 0.3-2 meq/g or 0.5-1.4 meq/g, at a pH of 7.
16. The method according to claim 10 , wherein the first strength composition is added to the fibre stock in amount of 0.2-15 kg/ton, 0.4-9 kg/ton produced paper or 1-5 kg/ton produced paper, calculated as dry product.
17. The method according to claim 10 , wherein the second strength composition comprises 0.1-20 weight-%, 0.5-10 weight-% or 0.7-4 weight-% of anionic hydrophilic polymer, and 80-99.9 weight-%, 90-99 weight-% or 96-99 weight-% of starch.
18. The method according to claim 10 , wherein the anionic monomer is selected from unsaturated mono- or dicarboxylic acids.
19. The method according to claim 10 , wherein the anionic hydrophilic polymer of the second strength agent has an average molecular weight of 350 000-950 000 g/mol.
20. The method according to claim 10 , wherein the anionic hydrophilic polymer of the second strength agent originates from >20 mol-% of non-ionic monomers and 4-17 mol-%, of anionic monomers.
21. The method according to claim 10 , wherein the fibre stock comprises at least 10-30% or 11-19% of inorganic mineral filler, measured by ash content at 525° C.
22. The method according to claim 10 , wherein the fibre stock comprises at least 20 weight-% or at least 50 weight-%, of fibres originating from recycled paper or board.
23. The method according to claim 10 , wherein the second strength composition is applied on the fibre web in such amount that the anionic hydrophilic polymer is applied on the web in an amount of 0.1-5 kg/t, 0.2-3 kg/t or 0.5-2 kg/t.