PROCESS FOR THE PRODUCTION OF STRONGLY ADHERENT LIQUID CRYSTAL FILMS ON FLEXIBLE SUBSTRATES

Abstract

A process for producing strongly adherent liquid crystal films on flexible substrates, security elements, obtainable by the process and security documents, including the security elements, may involve coating a flexible substrate first with a primer layer and then with a liquid crystal film the adherence of the liquid crystal film to the substrate is improved while keeping the optical performance of the liquid crystal film in an acceptable range.

Claims

1. A process for producing an adherent liquid crystal films on a flexible substrate, the process comprising (a) optionally, exposing the flexible substrate to a corona discharge or a plasma discharge treatment; (b) applying on the substrate a primer composition comprising (b1) a polyurethane (A) comprising, in reacted form, (b1a) an organic polyisocyanate having a functionality of at least 2, (b1b) a compound comprising an isocyanate-reactive group and a radically polymerizable unsaturated group, (b1c) a photoinitiator comprising an isocyanate-reactive group, (b2) a polyfunctional polymerizable compound (B), and (b3) a solvent; (c) evaporating the solvent with IR-radiation and/or thermal drying, to obtain an uncured primer layer; (d) curing the uncured primer layer with UV/VIS radiation or an electron beam, to obtain a primered substrate; (e) optionally, exposing the primered substrate to a corona discharge or a plasma discharge treatment; (f) applying a liquid crystal composition onto a primer coating of the primered substrate; the primer coating, (g) evaporating the solvent by applying IR-radiation and/or thermal drying, to obtain a liquid crystal film; and (h) curing the liquid crystal film with UV/VIS radiation or electron beam.

2. The process of claim 1, wherein the flexible substrate is a biaxially oriented polyethylene terephthalate film or a biaxially oriented polypropylene film.

3. The process of claim 1, wherein the primer composition is applied by slot die-coating, knive-coating, reverse roll-coating, metering rod coating, gravure-printing, flexo-printing, screen-printing, or ink jet printing.

4. The process of claim 1, wherein the liquid crystal composition is applied by slot die-coating, knive-coating, reverse roll-coating, metering rod coating, gravure-printing, flexo-printing, screen-printing, or ink jet printing.

5. The process of claim 1, wherein the liquid crystal film has a peel_force >20 N/m.

6. The process of claim 1, wherein the liquid crystal composition comprises: an achiral nematic polymerizable monomer; and a chiral polymerizable monomer; a substance comprising a photoinitiator; a reactive diluent comprising a photopolymerizable group; solvent, defoamer, deaerator, lubricant, leveling agent, thermally curing auxiliary, radiatively curing auxiliary, substrate wetting aid, wetting aid, dispersing aid, hydrophobizing agent, in-can stabilizer, and/or a scratch resistance auxiliary; and a component comprising a dye and/or pigment.

7. The process of claim 6, wherein the achiral nematic polymerizable monomer comprises a compound of formula (Ib) or (Ia) ##STR00035## wherein Z.sup.1 and Z.sup.2 are independently ##STR00036## A.sup.1 and A.sup.2 are independently spacers comprising 4 to 8 carbon atoms, Y.sup.1 and Y.sup.4 are independently a single chemical bond, oxygen, —CO—, —O—CO—, or —CO—O—, R.sup.b is hydrogen, C.sub.1-C.sub.6-alkyl, or CO—O—C.sub.1-C.sub.6-alkyl; wherein the chiral polymerizable monomer composes ##STR00037## wherein the substance comprises the photoinitiator, solvent leveling agent, and in-can stabilizer.

8. The process of claim 1, wherein the polyurethane (A) has a formula ##STR00038## wherein n is a positive number which is on average 1 up to 5, and R.sup.11 is a group of formula ##STR00039## wherein R.sup.13, R.sup.14, and R.sup.15 are independently H, an alkyl group comprising 1 to 4 carbon atoms, or an alkyloxy group comprising 1 to 4 carbon atoms, p is 0 or an integer in a range of from 1 to 10, and Y.sub.i, for i being 1 to p, is independently —CH.sub.2—CH.sub.2—O—, —CH.sub.2—CH(CH.sub.3)—O—, —CH(CH.sub.3)—CH.sub.2—O—, —CH.sub.2—C(CH.sub.3).sub.2—O—, —C(CH.sub.3).sub.2—CH.sub.2—O—, —CH.sub.2—CHVin-O—, —CHVin-CH.sub.2—O—, —CH.sub.2—CHPh-O—, or —CHPh-CH.sub.2—O—, Ph being phenyl and Yin being vinyl.

9. The process of claim 8, wherein the polymerizable compound (B) is 1,2-propanediol diacrylate, 1,3-propanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, trimethylolpropane triacrylate, ditrimethylol tetracrylate, dipentaerythritol hexaacrylate, triacrylate of singly to vigintuply alkoxylated glycerol, or singly to vigintuply alkoxylated pentaerythritol.

10. The process of claim 1, wherein the solvent (b3)) comprises a C.sub.2-C.sub.6alcohol, ether, and/or ether-C.sub.2-C.sub.6alcohol, and optionally water.

11. A security element, comprising: a flexible substrate; a primer layer; and a liquid crystal film; obtained by the process of claim 1.

12. The element of claim 11, further comprising: a black layer; a white layer; a metallic layer; a plasmonic layer; an embossed layer comprising a diffractive grating; a micromirror; a lens; a magnetic layer; fluorescent layer; an interference layer; a colored layer; an IR-absorbing layer; an IR-transparent layer; and/or a conductive layer.

13. The element of claim 11, wherein the layers might be fully, or partially printed on the security element.

14. A security document, comprising; the security element of claim 11 configured as a laminate onto the document, or embedded as an optionally windowed thread into the document, or as a window on the document.

15. The document of claim 14, which is a banknote, tax stamp, ID-card, voucher, entrance ticket, or label.

16. A primer composition, comprising: (b1) a polyurethane (A) comprising, in reacted form, (b1a) an organic polyisocyanate having a functionality of at least 2, (b1b) a compound comprising an isocyanate-reactive group and a radically polymerizable unsaturated group, (b1c) a photoinitiator comprising an isocyanate-reactive group; (b2) a polyfunctional polymerizable compound (B); (b3) a solvent, wherein a proportion of the solvent is 90.0 to 99.5 wt. %, a proportion of the compound (B) and the polyurethane (A) is 10.0 to 0.5 wt. %, and wherein the proportions of solvent, compound (B), and polyurethane (A) add up to 100 wt.

17. The composition of claim 16, wherein the polyurethane (A) has a formula ##STR00040## wherein n is a positive number which is on average 1 up to 5, and R.sup.11 is a group of formula ##STR00041## wherein R.sup.13, R.sup.14, and R.sup.15 are independently H, an alkyl group comprising 1 to 4 carbon atoms, or an alkyloxy group comprising 1 to 4 carbon atoms, p is 0 or an integer in a range of from 1 to 10, and Y.sub.i, for i being 1 to p, is independently —CH.sub.2—CH.sub.2—O—, —CH.sub.2—CH(CH.sub.3)—O—, —CH(CH.sub.3)—CH.sub.2—O—, —CH.sub.2-C(CH.sub.3).sub.2—O—, —C(CH.sub.3).sub.2—CH.sub.2—O—, —CH.sub.2—CHVin-O—, —CHVin-CH.sub.2—O—, —CH.sub.2—CHPh-O—, or —CHPh-CH.sub.2—O—, in which Ph being phenyl and Vin being vinyl.

18. The composition of claim 16, wherein the polymerizable compound (B) is selected from is 1,2-propanediol diacrylate, 1,3-propanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, trimethylolpropane triacrylate, ditrimethylol tetracrylate, dipentaerythritol hexaacrylate, triacrylate of singly to vigintuply alkoxylated trimethylolpropane, singly to vigintuply alkoxylated glycerol, or singly to vigintuply alkoxylated pentaerythritol.

19. The composition of claim 16, wherein the solvent comprises a C.sub.1-C.sub.6alcohol and/or, ether-C.sub.1-C.sub.6alcohol.

Description

EXAMPLES

[0336] Primer Formulation:

TABLE-US-00001 Compound Weight (g) Photoinitiator.sup.1) 6.07 Sartomer ® 344 (polyethylene glycol (400) 3.45 diacrylate) Isopropanol 495.24 1,2-dimethoxypropanol 495.24 .sup.1)Compound of formula (V), wherein n is 1, and R.sup.11 is a group of formula [00034]in which R.sup.13 and R.sup.14 each independently of one another are an alkyl group containing 1 to 4 carbon atoms, Y.sub.i for is a group of —CH.sub.2—CH.sub.2—O—, —CH.sub.2—CH(CH.sub.3)—O—, and —CH(CH.sub.3)—CH.sub.2—O—, preferably —CH.sub.2—CH.sub.2—O—.

[0337] LC 1: compound I.a

[0338] LC 2: compound I.b

[0339] Dopant: compound IV.a

[0340] Liquid crystal ink for examples 1-4

TABLE-US-00002 Mixture A Mixture B Mixture C Mix D LC 1 49.4420 g 0.0000 24.7210 g 83.631 g LC 2 0.0000 48.8365 g 24.4183 g 250.893 g Dopant 2.3081 g 2.9135 g 2.6108 g 18.689 g Irgacure 907 2.5875 g 0.0000 1.2938 g 0.000 Irgacure TPO 0.0000 2.5875 g 1.2938 g 17.507 g BYK 361N 0.0259 g 0.0259 g 0.0259 g 0.175 g Cyclopentanone 93.45 g 93.45 g 93.45 g 632.28 g

Comparative Examples 1-3

[0341] The liquid crystal based ink is applied with a 12 μm spiral applicator on Lumirror® 40.01 film, dried in a convection oven and UV cured. The liquid crystal film shows an excellent color flop from red to green. Adhesion test is done with a 90° peel test.

TABLE-US-00003 Comparative Liquid Drying UV Example crystal ink temperature curing 1 Mix A  90° C. Mercury lamp 2 Mix B 105° C. Mercury lamp 3 Mix C 130° C. Mercury lamp

Examples 1α-d to 3α-d

[0342] The primer formulation is applied with a spiral applicator on PET film Lumirror® 40.01, dried 2 minutes at 80° C. in a convection oven, followed by UV curing.

[0343] The spiral applicator used for the primer formulation is indicated in the table.

[0344] The liquid crystal based ink is applied subsequently using a 12 μm spiral applicator, followed by drying in a convection oven and UV curing. The liquid crystal film shows a good color flop from red to green. Adhesion test is done with a 90° peel test.

TABLE-US-00004 Spiral applicator Drying for primer Liquid temperature Example formulation crystal ink of LC ink UV curing 1a 4 μm Mix A  90° C. Mercury lamp 1b 6 μm Mix A  90° C. Mercury lamp 1c 12 μm  Mix A  90° C. Mercury lamp 1d 24 μm  Mix A  90° C. Mercury lamp 2a 4 μm Mix B 105° C. Mercury lamp 2b 6 μm Mix B 105° C. Mercury lamp 2c 12 μm  Mix B 105° C. Mercury lamp 2d 24 μm  Mix B 105° C. Mercury lamp 3a 4 μm Mix C 130° C. Mercury lamp 3b 6 μm Mix C 130° C. Mercury lamp 3c 12 μm  Mix C 130° C. Mercury lamp 3d 24 μm  Mix C 130° C. Mercury lamp

[0345] The alignment of the liquid crystals is slightly affected when printed on the top of the primer, resulting in an increased number of microdomains as can be seen under a microscope. The adhesion performance is good.

[0346] The adhesion performance of the liquid crystal film on the PET film with and without primer is also tested with a 90° peel test.

[0347] Peel tester: Zwicki BZ1.0/TH1S

[0348] Test conditions:

TABLE-US-00005 Peel angle 90° Tape width 15 mm Temperature/Humidity: 23° C./50% humidity Force sensor 50N Test speed 50 mm/min Peelarm Scotch over a length of 15 cm

[0349] Results:

TABLE-US-00006 Sample of Primer Peelforce [N/m] Comparative No 6.4 Example 1 Example 1a yes 24 Example 1b yes 23 Example 1c yes 23 Example 1d yes 23 Comparative No 4.2 Example 2 Example 2a yes 24 Example 2b yes 22 Example 2c yes 24 Example 2d yes 20 Comparative No 2.7 Example 3 Example 3a yes 19 Example 3b yes 20 Example 3c yes 23 Example 3d yes 24

Comparative Example 4

[0350] The liquid crystal based ink Mix D is gravure printed on Hostaphan® RNK 23 micron film using a 701/cm gravure cylinder, dried at 100° C. and UV-cured with a Ga doped Hg lamp. Adhesion is tested by tape test.

Example 4

[0351] Hostaphan® RNK, a biaxially oriented polyethylene terephthalate (BOPET) film, is subjected to a corona treatment at an output of 350 W min/m.sup.2 (SOFTAL, Type CLNE015-130-1KB3) The primer formulation is printed first by gravure using a 1001/cm ceramic cylinder on the BOPET film, drying temperature of the printing press: 70° C., UV curing: Mercury lamp, printing speed: 10 m/min. The liquid crystal based ink Mix D is printed subsequently using a 701/cm cylinder.

[0352] Adhesion is Assessed by Tape Test

TABLE-US-00007 Adhesion Surface tension as Optical Sample of promoter delivered (mN/m) Effect Comparative No 41 Excellent color flop from Example 4 green to blue, no adhesion Example 4 yes 50 Good color flop from green to blue, Good adhesion

[0353] The alignment of the liquid crystals is slightly affected when printed on the top of the primer, resulting in an increased number of microdomains as can be seen under a microscope. The adhesion performance is good.

[0354] The adhesion performance of the liquid crystal film on the PET film with and without primer is also tested with a 90° peel test.

[0355] Peel tester: Zwicki BZ1.0/TH1S

[0356] Test Conditions:

TABLE-US-00008 Peel angle 90° Tape width 15 mm Temperature/Humidity: 23° C./50% humidity Force sensor 50N Test speed 50 mm/min Peelarm Scotch over a length of 15 cm

[0357] Results:

TABLE-US-00009 Sample of Primer Peelforce [N/m] Comparative No 2.1 Example 4 Example 4 yes 51

[0358] When measuring the transmission spectrum of the liquid crystal film on the PET film with primer (Example 4) and without primer (Comparative Example 4) the difference of the transmission minimum of the liquid crystal film on the PET film with primer and without primer is less than 20 nm. The transmission minimum represents the wavelength at the halfwidth of the transmission band at 80% transmission. That means, the adherence of the liquid crystal film to the substrate is improved in Example 4 while simultaneously keeping the optical performance of the liquid crystal film in an acceptable range.