Security printing

Abstract

A composition comprising a first infrared absorbing material which is a doped metal oxide comprising indium and/or tin; and a second infrared absorbing material which is a compound selected from: or a salt or polymer thereof, wherein —M is a metal selected from a group 3-10 (Group 1MB-VIII) element or a lanthanide; —R.sub.1 is selected from hydrogen, phosphonate, sulphonate, nitro, halo, cyano, thiocyano, thioalkyl, thioaryl, alkyl, alkoxy, aryl, aryloxy, amines, substituted amines and substituted aryl; —one of R.sub.2 and R.sub.3 is oxygen and the other of R.sub.2 and R.sub.3 is NO; —n is a number corresponding to half the co-ordination number of the metal M; —each L and L′ is independently a ligand complexed to the metal M; and —y is a number corresponding to the co-ordination number of the metal M. ##STR00001##

Claims

1. A composition comprising a first infrared absorbing material which is a doped metal oxide comprising indium and/or tin; and a second infrared absorbing material which is a compound selected from: ##STR00013## or a salt or polymer of said compound, wherein M is a metal selected from a group 3-10 element or a lanthanide; R.sub.1 is selected from hydrogen, phosphonate, sulphonate, nitro, halo, cyano, thiocyano, thioalkyl, thioaryl, alkyl, alkoxy, aryl, aryloxy, amines, substituted amines and substituted aryl; one of R.sub.2 and R.sub.3 is oxygen and the other of R.sub.2 and R.sub.3 is NO; n is a number corresponding to half a co-ordination number of the metal M; each L and L′ is independently a ligand complexed to the metal M; and y is a number corresponding to the co-ordination number of the metal M.

2. A composition according to claim 1 wherein the first infrared absorbing material is a doped tin oxide or a doped indium oxide.

3. A composition according to claim 1 wherein the first infrared absorbing material is a doped tin oxide comprising from 2 to 20 mol % of one or more dopant elements selected from indium, antimony, tungsten, phosphorus, copper, niobium, manganese, fluorine and nickel.

4. A composition according to claim 3 wherein the first infrared absorbing material is an antinomy doped tin oxide.

5. A composition according to claim 1 wherein the first infrared absorbing material is an indium oxide doped with 1 to 20 mol % tin.

6. A composition according to claim 1 wherein the first infrared absorbing material is selected from antinomy doped tin oxide, tin doped indium oxide and reduced tin doped indium oxide.

7. A composition according to claim 1 wherein the second infrared absorbed material is selected from salts having one of the following formulae: ##STR00014##

8. A composition according to claim 1 wherein the second infrared absorbing material has the formula ##STR00015##

9. A composition according to claim 1 which is an ink composition.

10. A composition according to claim 1 comprising 0.1 to 50 wt % of the first infrared absorbing material and 0.01 to 50 wt % of the second infrared absorbing material.

11. A method of providing a covert security image or a hidden coating on a banknote, the method comprising applying a first infrared absorbing material and a second infrared absorbing material according to claim 1 to said banknote.

12. An article comprising a substrate which carries a first infrared absorbing material which is a doped metal oxide comprising indium and/or tin and a second infrared absorbing material which is a compound selected from: ##STR00016## or a salt or polymer of said compound, wherein M is a metal selected from a group 3-10 element or a lanthanide; R.sub.1 is selected from hydrogen, phosphonate, sulphonate, nitro, halo, cyano, thiocyano, thioalkyl, thioaryl, alkyl, alkoxy, aryl, aryloxy, amines, substituted amines and substituted aryl; one of R.sub.2 and R.sub.3 is oxygen and the other of R.sub.2 and R.sub.3 is NO; n is a number corresponding to half a co-ordination number of the metal M; each L and L′ is independently a ligand complexed to the metal M; and y is a number corresponding to the co-ordination number of the metal M.

13. An article according to claim 12 which is a banknote.

14. An article according to claim 12 wherein the first infared absorbing material and the second infared absorbing material are provided at a locus of the article and wherein the locus of the article which carries the first and second infrared absorbing materials has an absorbance of at least 50% of at least one wavelength in the range 800-900 nm and an absorbance of at least 50% of at least one wavelength in the range 1000-1500 nm.

15. An article according to any of claim 12 wherein the first and second infrared absorbing materials are included within or on a surface of the substrate and do not substantially alter a colour of the substrate in a visible range.

16. A method of manufacturing an article of claim 12, the method comprising incorporating the first infrared absorbing material and the second infrared absorbing material into or onto the substrate.

17. A method of detecting a genuine article as claimed in claim 12, the method comprising measuring at a locus on the article an absorption A.sub.1 of radiation at a first wavelength λ.sub.1, and an absorption A.sub.2 of radiation at a second wavelength λ.sub.2, wherein λ.sub.1 and λ.sub.2 are in a range of 700 to 2000 nm.

18. A method of assessing a quality of an article as claimed in claim 12, the method comprising measuring at a locus of the article which carries the first and second infrared absorbing materials, an absorption A.sub.1 of at least one wavelength of λ.sub.1 of radiation in a range of 700 to 2000 nm.

Description

EXAMPLE 1

(1) 12 wt % of pigment A and 5 wt % of pigment B were dosed into a lime green offset printing ink. Pigment A is a reduced indium tin oxide comprising 10 mol % tin in an indium oxide matrix.

(2) Pigment B is the compound:

(3) ##STR00012##

(4) A print of the original offset ink at the thickness of 1 to 2 microns was made and a print of the ink comprising pigment A and pigment B was also made at the same thickness. The infrared spectra of the prints were recorded and are shown in FIG. 1.

(5) To the naked eye the prints look identical and as FIG. 1 shows the spectra are very similar in the visible range but quite different in the infrared region of the electromagnetic spectrum.

EXAMPLE 2

(6) Two blue ink compositions were prepared. Composition 1 contained 20 wt % of pigment C, an antimony doped tin oxide containing 10 wt % antimony. Composition 2 contained 20 wt % of pigment C and 0.3 wt % pigment B.

(7) Compositions 1 and 2 were printed at a thickness of approximately 8 microns. Infrared spectra of the prints were recorded and are shown in FIG. 2. The spectra were very similar in the visible region and could not be distinguished by the naked eye. The spectra are closely similar at infrared wavelengths above 950 nm. However there is a significant difference in the region 800 to 900 nm. Thus specific apparatus which detect radiation at these wavelengths would be needed to distinguish these prints.