Laser marking
An ink formulation comprises a marking component, e.g. ammonium octamolybdate, and a metal salt that absorbs laser irradiation at 780-2500 nm, e.g. reduced indium tin oxide, and thereby causes the marking component to change colour.
1. An ink formulation comprising a marking component and a non-stoichiometric metal salt, wherein the marking component in the absence of the metal salt undergoes a colour change in response to laser irradiation at a wavelength above 2500 nm but not between 780-2500 nm, and the metal salt absorbs laser radiation at 780-2500 nm, thereby causing the marking component to change colour.
2. The formulation according to claim 1 , wherein the metal salt has excess metal.
3. The formulation according to claim 1 , wherein the metal salt has a metal deficiency.
4. The formulation according to claim 2 , wherein the metal salt compound is an oxide, mixed oxide, nitride or boride.
5. The formulation according to claim 4 , wherein the compound is antimony tin oxide, indium tin oxide, aluminum zinc oxide or indium zinc oxide.
6. The formulation according to claim 5 , wherein the non-stoichiometric compound mixed metal oxide is in a reduced form.
7. The formulation according to claim 6 , wherein the compound is reduced indium tin oxide.
8. The formulation according to claim 1 , which additionally comprises a colour-forming compound.
9. The formulation according to claim 1 , wherein the non-stoichiometric compound is present in the form of particles with an D3,2 average particle size in the range from 10 nm to 10 μm.
10. The formulation according to claim 9 , wherein the size is less than 1 μm.
11. The formulation according to claim 10 , wherein the size is less than 100 nm.
12. The formulation according to claim 1 , wherein the marking component is a compound including an oxymetal anion.
13. The formulation according to claim 12 , wherein the marking component is a molybdate.
14. The formulation according to claim 12 , wherein the marking component is an octamolybdate.
15. The formulation according to claim 12 , wherein the marking component is ammonium octamolybdate.
16. The formulation according to claim 1 , which additionally comprises a binder.
17. The formulation according to claim 1 , which is water-based.
18. The formulation according to claim 1 , which comprises an organic solvent.
19. A substrate coated with a formulation according to claim 1 .
20. The substrate according to claim 19 , including a marking component and coated with a non-stoichiometric compound, wherein the marking component undergoes a colour change in response to laser irradiation at a wavelength above 2500 nm but not between 780-2500 nm, and the compound absorbs laser radiation at 780-2500 nm, thereby causing the marking component to change colour.
21. The substrate according to claim 19 , which is paper, cardboard, plastic, textile, wood, metal, glass, leather, a foodstuff or a pharmaceutical composition.
22. A method of applying an inorganic non-stoichiometric compound on to a substrate that comprises a marking component, wherein the marking component undergoes a colour change in response to laser irradiation at a wavelength above 2500 nm but not between 780-2500 nm, and the compound absorbs laser radiation at 780-2500 nm, thereby causing the marking component to change colour; and wherein said method comprises using a sputtering process.
23. A method for forming an image on a substrate, which comprises irradiating with a laser substrate coated with a formulation according to claim 1 .
24. The method according to claim 23 , wherein the laser is a fibre, diode, diode array or CO 2 laser.
25. The method according to claim 24 , wherein the laser has an operating wavelength in the range 780 to 2500 nm.