Printing ink, use of micro-wires as antennas in security documents, method for producing a security document and methods for authentication of security documents

1. A printing ink comprising at least one film forming binder—and a plurality of at least one type of electrically conductive micro-wires wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles and wherein the mean length of said type of micro-wires is in a range of between 3 μm to 250 μm.

2. Printing ink according to claim 1 wherein the mean length of the micro-wires is in a range of between 5 μm to 100 μm.

3. Printing ink according to claim 1 wherein the mean length of the micro-wires is in a range of between 10 μm to 50 μm.

4. Printing ink according to claim 1 wherein the length of each micro-wire within at least said one type of micro-wires is in a range of +10% to −10% of the mean length.

5. Printing ink according to claim 4 wherein the length of each micro-wire within at least said one type of micro-wires is in a range of between +5% to −5% of the mean length.

6. Printing ink according to claim 1 wherein the mean diameter of at least said one type of micro-wires is equal or less than 50 μm.

7. Printing ink according to claim 6 wherein the mean diameter of at least said one type of micro-wires is equal or less than 20 μm.

8. Printing ink according to claim 6 wherein the mean diameter of at least one type of micro-wires is equal or less than 10 μm.

9. Printing ink according to claim 1 wherein the material of at least said one type of micro-wires is selected from the group consisting of metals, metal-alloys and non conductive materials comprising at least one layer of an electrically conductive material, the layer having a thickness in the range of 3 μm to 250 μm.

10. Printing ink according to claim 9 wherein the metal or metal-alloy are selected from the group consisting of copper, copper-alloys, aluminum, aluminum-alloys and stainless steels.

11. Printing ink according to claim 1 wherein the material of said one type of micro-wires is selected so as to have additional detectable properties, particularly magnetic and/or luminescence properties.

12. A method of use of at least one type of micro-wire having a mean length of between 3 μm to 250 μm as antennas in markings which have been printed on security documents wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles and the length of each individual micro-wire within said one type of micro-wire preferably is in a range of +10% to −10% of the mean length.

13. Document, in particular a security document, with a first and a second surface having a printed marking on at least one of its surfaces, said marking comprising a plurality of at least one type of thin micro-wires fixed in a randomly spatial arrangement within said marking, wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles, the mean length of said one type of micro-wires is in a range of between 3 μm to 250 μm and the length of each micro-wire within said one type of micro-wire is preferably in a range of +10% to −10% of the mean length of said one type so as to generate an amplified resonant antenna signal when said marking is exposed to electromagnetic radiation having frequencies corresponding to the micro-wires length.

14. A method for producing a security document having a first and a second surface with at least one marking applied on at least one of said surfaces said marking comprises a plurality of at least one type of micro-wires fixed in a randomly spatial arrangement, said method comprising steps of

a) subjecting a printing ink comprising at least one film forming binder and a plurality of at least one type of electrically conductive micro-wires, wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles and wherein the mean length of said type of micro-wires is in a range of between 3 μm to 250 μm, to a grinding process in a milling machine and in particular in a three-roll-mill;

b) printing said printing ink obtained after step a) in a printing process selected from intaglio-, offset-, letter-press or silk screen printing processes; and

c) optionally drying said marking;

wherein the material of said one type of micro-wires is selected such that the physical structure of the micro-wires is not substantially changed during step a) and step b).

15. A method for authentication of a security document with at least a first and a second surface having a printed marking on at least one of its surfaces, said marking comprising a plurality of at least one type of thin micro-wires fixed in a randomly spatial arrangement within said marking, wherein said micro-wires are separated from each other, the mean length of said one type of micro-wires is in a range of between 3 μm to 250 μm and the length of each micro-wire within said one type of micro-wire is preferably in a range of +10% to −10% of the mean length of said one type, said method comprising the steps of

a) exposing said marking to a narrowband radiation, in particular produced by a tunable backward-wave tube, and

b) detecting the spectrum coming from the marking by a broadband detecting element.

16. A method for authentication of a security document with at least a first and a second surface, said security document having a printed marking on at least one of its surfaces, said marking comprising a plurality of at least one type of thin micro-wires fixed in a randomly spatial arrangement within said marking, wherein said micro-wires are separated from each other, the mean length of said one type of micro-wires is in a range of between 3 μm to 250 μm and the length of each micro-wire within said one type of micro-wire is preferably in a range of +10% to −10% of the mean length of said one type, said method comprising the steps of

a) exposing said marking to a broadband radiation, in particular produced by a thermal light source, and

b) detecting the spectrum coming from the marking by a narrowband detecting element or by a scanning interferometer.

17. A method for authentication of a security document having a first and a second surface with a printed marking on at least one of said surfaces, said marking comprising a plurality of at least one type of thin micro-wires fixed in a randomly spatial arrangement within said marking, wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles, the mean length of said one type of micro-wires is in a range of between 3 μm to 250 μm and the length of each micro-wire within said one type of micro-wire is preferably in a range of +10% to −10% of the mean length of said one type so as to generate an amplified resonant antenna signal when said marking is exposed to electromagnetic radiation having frequencies corresponding to the micro-wires length, wherein the physical and/or chemical parameters are detected by a scanning electron microscope (SEM/EDX).

18. A method for authentication of a security document with at least a first and a second surface having a printed marking on at least one of its surfaces, said marking comprising a plurality of at least one type of thin micro-wires fixed in a randomly spatial arrangement within said marking, wherein said micro-wires are separated from each other such that they do not touch each other and thereby act as individual electric dipoles, the mean length of said one type of micro-wires is in a range of between 3 μm to 250 μm and the length of each micro-wire within said one type of micro-wire is preferably in a range of +10% to −10% of the mean length of said one type, wherein the natural thermal emission of said micro-wires is detected at their resonance frequency by passive detection methods.