Crumple-resistant security sheet, a method of manufacturing such a sheet, and a security document including such a sheet

Abstract

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.

Claims

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 m.

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.

Description

(1) The invention is described in more detail below by means of the following non-limiting examples and comparative examples.

(2) The Applicant performed three series of tests: Series 1 and 2 were conducted on non-coated sheets, and Series 3 were conducted on sheets each coated with a coating layer, as generally applies to sheets included in security documents such as banknotes.

(3) Measurements of pre-crumpling porosity and of post-crumpling porosity, of folding endurance (i.e. resistance to double-folding), and of tear strength were taken on the resulting sheets.

(4) Series 1

COMPARATIVE EXAMPLE 1

(5) A security sheet was made whose composition corresponded to the basic composition of a large number of banknotes currently in circulation.

(6) For this purpose, said sheet was formed by a wet-process technique on a cylinder-mold paper-making machine, from an aqueous suspension containing only cotton fibers and a wet strength agent (a PAAE resin in this example) in a proportion of 2.1% by dry weight relative to the weight of the fibers.

(7) The resulting sheet presented a weight expressed in grams per square meter of 85.2 g/m.sup.2, and thickness of 142 micrometers (m).

EXAMPLE 2

(8) On a cylinder-mold paper-making machine, a sheet of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 25 C. in a proportion of 11% by dry weight relative to the weight of the fibers, and a main flocculation agent in the form of a PAAE resin in a proportion of 2.3% by dry weight relative to the total weight of the fibers. The PAAE resin also acted as a wet strength agent, as in Comparative Example 1.

(9) The resulting sheet presented a weight of 87.6 g/m.sup.2, and a thickness of 124 m.

EXAMPLE 3

(10) A sheet of paper of the invention was made by using the composition of Example 2 and by adding thereto a polyacrylamide as a secondary flocculation agent in a proportion of 0.001% relative to the total weight of the fibers.

(11) The resulting sheet presented a weight of 86.9 g/m.sup.2 and a thickness of 125 m.

EXAMPLE 4

(12) A sheet of paper of the invention was made that comprised the same ingredients as in Example 3, the anionic polymer being present in a proportion of 25% by dry weight relative to the weight of the fibers, the main flocculation agent being present in a proportion of 2.6% by dry weight relative to the total weight of the fibers, and the secondary cationic flocculation agent being present in a proportion of 0.004% by dry weight relative to the total weight of the fibers.

(13) The resulting sheet presented a weight of 86.5 g/m.sup.2 and a thickness of 121 m.

(14) Series 2

COMPARATIVE EXAMPLE 5

(15) A security sheet was made whose composition corresponded to the basic composition of a large number of banknotes currently in circulation.

(16) For this purpose, said sheet was formed by a wet-process technique on a laboratory handsheet former, from an aqueous suspension containing only cotton fibers and a wet strength agent (a PAAE resin in this example) in a proportion of 2.5% by dry weight relative to the total weight of the fibers.

(17) The resulting sheet presented a weight of 80.5 g/m.sup.2, and thickness of 137 m.

EXAMPLE 6

(18) On a laboratory handsheet former, a sheet of paper of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 5 C. in a proportion of 25% by dry weight relative to the total weight of the fibers, a PAAE resin as a main flocculation agent (also acting as a wet strength agent) in a proportion of 3.1% by dry weight relative to the total weight of the fibers, and a polyacrylamide as a secondary flocculation agent in a proportion of 0.003% by dry weight relative to the total weight of the fibers.

(19) The resulting sheet presented a weight of 82.7 g/m.sup.2, and a thickness of 132 m.

EXAMPLE 7

(20) On a laboratory handsheet former, a sheet of paper of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 5 C. in a proportion of 11% by dry weight relative to the total weight of the fibers, a PAAE resin as a main flocculation agent (also acting as a wet strength agent) in a proportion of 2.8% by dry weight relative to the total weight of the fibers, and a polyacrylamide as a secondary flocculation agent in a proportion of 0.002% by dry weight relative to the total weight of the fibers.

(21) The resulting sheet presented a weight of 83.4 g/m.sup.2, and a thickness of 136 m.

(22) Series 3

COMPARATIVE EXAMPLE 8

(23) A sheet was formed by a wet process technique on a cylinder-mold paper-making machine, from an aqueous suspension of only cotton fibers that also contained a wet strength agent (PAAE resin) in a proportion of 2.1% by dry weight relative to the total weight of the fibers. After being formed, the resulting sheet of paper was coated with a coating layer designed to improve the durability of the sheet, and comprising a polyurethane binder and a colloidal silica, as described in Application EP 1 319 104.

(24) The resulting sheet presented a weight of 85.8 g/m.sup.2, and a thickness of 97 m.

COMPARATIVE EXAMPLE 9

(25) A security sheet was made that comprised the same ingredients as in Comparative Example 8, but in which a fraction of the cotton fibers was replaced with polyamide fibers so that the proportion of cotton fibers was 85% by dry weight and the proportion of polyamide fibers was 15% by dry weight relative to the total dry weight of the fibers.

EXAMPLE 10

(26) On a cylinder-mold paper-making machine, a sheet of paper of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 26 C. in a proportion of 11% by dry weight relative to the total dry weight of the fibers, and a PAAE resin as a main flocculation agent (also acting as a wet strength agent) in a proportion of 2.3% by dry weight relative to the total dry weight of the fibers.

(27) The resulting sheet presented a weight of 92.8 g/m.sup.2, and a thickness of 103 m.

EXAMPLE 11

(28) On a cylinder-mold paper-making machine, a sheet of paper of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 26 C. in a quantity of 11% by dry weight relative to the total weight of the fibers, a PAAE resin as a main flocculation agent in a proportion of 2.1% by dry weight relative to the total weight of the fibers, and a polyacrylamide as a secondary flocculation agent in a proportion of 0.001% by dry weight relative to the total dry weight of the fibers.

(29) The resulting sheet presented a weight of 86.9 g/m.sup.2, and a thickness of 100 m.

EXAMPLE 12

(30) On a cylinder-mold paper-making machine, a sheet of paper of the invention was made that comprised only cotton fibers, a carboxylated styrene butadiene copolymer having a glass transition temperature of 26 C. in a quantity of 25% by dry weight relative to the total dry weight of the fibers, a PAAE resin as a main flocculation agent (also acting as a wet strength agent) in a quantity of 2.6% by dry weight relative to the total dry weight of the fibers, and a polyacrylamide as a secondary flocculation agent in a proportion of 0.004% by dry weight relative to the total dry weight of the fibers.

(31) The resulting sheet presented a weight of 82.9 g/m.sup.2, and a thickness of 95 m.

EXAMPLE 13

(32) On a cylinder-mold paper-making machine, a sheet of paper of the invention was made by using the composition of Example 12, but by replacing a fraction of the cotton fibers with polyamide fibers so that the proportion of polyamide fibers was 15% by weight relative to the total dry weight of the fibers.

(33) The resulting sheet presented a weight of 85.4 g/m.sup.2, and a thickness of 108 m.

(34) Tests and Results

(35) The measurements of porosity before and after crumpling (sheet crumpled eight times for each test) were taken in compliance with French Standard NF Q03-076. The crumpling was performed by an NBS Crumpling Device of the IGT brand.

(36) The folding endurance measurements were performed in compliance with International Standard ISO 5626.

(37) The tear strength measures were performed in compliance with European Standard EN 21974.

(38) In order to evaluate wet resistance, bursting strength was measured in compliance with French Standard NF Q03-053, on wet and dry sheets. The wet strength value was then obtained using the following formula:

(39) $\mathrm{Wet}\mathrm{Strength}=\frac{\mathrm{Wet}\mathrm{Bursting}\mathrm{Strength}}{\mathrm{Dry}\mathrm{Bursting}\mathrm{Strength}}100$

(40) TABLE-US-00001 TABLE 1 Comparative Test Example Example 2 Example 3 Example 4 Pre-crumpling 22 26 24 27 porosity (cm.sup.3/min) Post-crumpling 206 147 145 89 porosity (cm.sup.3/min) Improvement % reference 28.7% 29.7% 56.84% Wet strength (%) 48.6 50.3 52 52.5 Double-folding 2620 3061 3304 4012 endurance (number of folds) Improvement % reference +16.8% +26.1% +53.1%

(41) TABLE-US-00002 TABLE 2 Comparative Test Example 5 Example 6 Example 7 Pre-crumpling porosity 131 101 117 (cm.sup.3/min) Post-crumpling 1043 545 855 porosity (cm.sup.3/min) Improvement reference 47.8% 18.1% Double-folding 666 1479 1248 endurance (number of folds) Improvement reference +122.1% +87.4%

(42) TABLE-US-00003 TABLE 3 Comparative Comparative Example Example Example Example Test Example 8 Example 9 10 11 12 13 Pre-crumpling 0 0 0 0 0 0 porosity (cm.sup.3/min) Post- 103 41 24 15 12 crumpling porosity (cm.sup.3/min) Improvement reference 61.2% 77% 85% 88% Wet strength 54.5 57.7 60.0 61.2 63.9 (%) Double- 3074 4655 4331 3908 5579 8807 folding endurance (number of folds) Improvement % reference +41% +27% +81% +186% Tear strength 760 870 820 760 660 1380 (mN) Improvement % 13% reference 6% 13% 24% +59%

(43) Series 1

(44) A shown in Table 1 which gives the results of Series 1, the security sheets of Examples 2 to 4 present post-crumpling porosities that are considerably improved relative to Comparative Example 1 which is taken as the reference (reduction in post-crumpling porosity by in the range 28% to 56%).

(45) In the same way, for the sheets of the invention, the double-folding endurance is considerably increased relative to the sheet of Comparative Example 1 (increase lying in the range 16% to 53%).

(46) Finally, it should be noted that the sheets of Examples 2 to 4 of the invention present wet strength values that are very close to and even slightly greater than the wet strength value of Comparative Example 1, thereby showing that the flocculation agent used (PAAE resin) continues to act as effectively as a wet strength agent.

(47) Series 2

(48) As shown in Table 2 which gives the results of Series 2, the sheets of Examples 6 and 7 of the invention present post-crumpling porosities that are considerably improved relative to Comparative Example 5 which is taken as the reference (reduction of in the range 17% to 48% of the post-crumpling porosity).

(49) In the same manner, for the sheets of the invention, the double-folding endurance is considerably increased relative to the sheet of Comparative Example 5 which is taken as the reference (increase lying in the range 87% to 122%).

(50) Series 3

(51) As shown in Table 3 which is recapitulative of the results of Series 3, the security sheets of Comparative Examples 8 and 9 and of the various examples 10 to 13 present pre-crumpling porosities that are substantially zero, unlike the sheets of Series 1 and 2. This can be explained by the presence of a coating layer that blocks off the pores at the surface of the sheets.

(52) After crumpling, all of the sheets of the examples present porosities less than the porosity of Comparative Example 8. The rate of improvement relative to Comparative Example 8 which is taken as the reference varies in the range 77% to 88%. The post-crumpling porosities of the sheets of the invention are very close to the pre-crumpling porosity of the comparative example 8.

(53) As regards double-folding endurance, the sheets of the invention of Examples 10 to 13 present improvements lying in the range 27% to 186% relative to the sheet not including anionic polymer of Comparative Example 8 which is taken as the reference.

(54) As regards tear strength, Examples 10 to 13 were compared with Comparative Example 9 in order to determine the synergy between the presence of synthetic fibers and the presence of an anionic polymer.

(55) The sheet of Comparative Example 9 does not contain any anionic polymer but it does contain polyamide fibers in a proportion of 15%. The tear strength of the sheet of Comparative Example 8 is 13% less than the tear strength of the sheet of Comparative Example 2, which confirms the effect of the synthetic fibers.

(56) Examples 10 to 12 present tear strength values that are less than or equal to those of Comparative Example 8, and less than those of Comparative Example 9, i.e. the presence of an anionic polymer alone has no beneficial influence on tear strength.

(57) Example 13 presents a tear strength value greater than that of Comparative Example 8 but also significantly greater (+59%) than that of Comparative Example 9. Therefore, the combination of the presence of synthetic fibers and of the presence of an anionic polymer in the composition of the security sheet has a synergistic effect on the tear strength of said sheet.

(58) Finally, it should be noted that the sheets of Examples 10 to 13 of the invention present wet strength values very close to and even slightly greater than the wet strength value of Comparative Example 8, which shows that the flocculation agent used (PAAE resin) continues to act as effectively as a wet strength agent.