INK FOR LASER IMAGING

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In a first aspect, the invention provides an ink formulation comprising : (a) a near infrared absorbing system comprising a tungsten bronze of the formula M2xWyOz, wherein M2 is one or more elements selected from the group consisting of H, He, alkali metals, alkaline earth metals, rare-earth elements, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Hf, Os, Bi and I, or ammonium, wherein x, y and z satisfy the relationships 0.001≤x/y≤2 and 2.2≤z/y≤3, and the tungsten bronze has an absorptivity ratio of α/β "a to β" of <2; (b) a colour change agent; and (c) a binder. The invention also provides alternative ink formulations for laser marking, a method of forming an image using the ink formulations and a substrate coated with the ink formulation of the invention. Ink formulations according to the invention have been found to have good laser imaging efficacy, and an essentially negligible impact on the background colour of the coating.