Crumple-resistant security sheet, a method of manufacturing such a sheet, and a security document including such a sheet
The present invention relates to a crumple-resistant security sheet comprising fibers; an anionic polymer in a proportion lying in the range 5% to 45% by dry weight relative to the total dry weight of the fibers, and presenting a glass transition temperature greater than 40° C.; and a main cationic flocculation agent in a quantity lying in the range 1% to 5% by dry weight relative to the total dry weight of the fibers.
1. A method of manufacturing a crumple-resistant security sheet, the method comprising the steps of forming the sheet by a wet-process technique from an aqueous suspension containing:
fibers;
a stabilized aqueous dispersion of an anionic polymer in a proportion lying in a range of 5% to 45% by dry weight relative to a total dry weight of the fibers, and having a glass transition temperature greater than −40° C.; and
a cationic flocculation agent in a quantity lying in a range of 1% to 5% by dry weight relative to the total dry weight of the fibers;
wherein the anionic polymer is configured to precipitate onto the fibers in the presence of the cationic flocculation agent;
then drying the sheet, and
applying a coating layer to at least one face of the security sheet.
2. The manufacturing method according to claim 1 , wherein the anionic polymer has a glass transition temperature lying in the range −30° C. to 10° C.
3. The manufacturing method according to claim 1 , wherein the anionic polymer is structured not to be crosslinkable.
4. The manufacturing method according to claim 1 , wherein the anionic polymer comprises a polymer having carboxyl functions.
5. The manufacturing method according to claim 1 , wherein the anionic polymer comprises a carboxylated styrene butadiene copolymer.
6. The manufacturing method according to claim 1 , wherein the cationic flocculation agent is a cationic resin.
7. The manufacturing method according to claim 6 , wherein the cationic resin is a polyamide-amine-epichlorohydrin (PAAE) resin.
8. The manufacturing method according to claim 1 , wherein the cationic flocculation agent is selected from the group consisting of polyacrylamides, polyethyleneimines, polyvinylamines, and mixtures thereof.
9. The manufacturing method according to claim 1 , wherein the fibers comprise cellulose fibers.
10. The manufacturing method according to claim 9 , wherein the cellulose fibers represent at least 70% by dry weight of a total quantity of the fibers.
11. The manufacturing method according to claim 1 , wherein the fibers comprise synthetic fibers.
12. The manufacturing method according to claim 11 , wherein the synthetic fibers are selected from the group consisting of polyamide fibers, polyester fibers, and mixtures thereof.
13. The manufacturing method according to claim 1 , wherein the sheet further comprises a secondary cationic flocculation agent in a quantity in the range 0.001% to 0.006% by dry weight relative to the total dry weight of the fibers.
14. The manufacturing method according to claim 13 , wherein the secondary cationic flocculation agent is selected from the group consisting of polyacrylamides, polyethyleneimines, polyvinylamines, and mixtures thereof.
15. The manufacturing method according to claim 1 , wherein the sheet includes at least one security element.
16. The manufacturing method according to claim 1 , wherein the security sheet has a thickness of in a range of 95-142 μm.
17. The manufacturing method according to claim 1 , wherein the sheet has tear strength greater than 1300 mN.
18. The manufacturing method according to claim 1 , wherein the sheet has a double-folding endurance in the range of 3908 folds to 8807 folds.
19. The manufacturing method according to claim 1 , wherein the sheet has a wet strength in the range of 57.7% to 63.9%.
20. A security document including a security sheet obtained by the manufacturing method according to claim 1 .
21. The security document according to claim 20 , wherein the document is a banknote.