Security inks and machine readable security features

A security ink for printing a machine readable security feature, said having a viscosity between 50 and 3000 mPa s at 25° C., a pH between about 7.0 and about 9.0, and comprising:

• a) water in an amount of at least about 45 wt-%,
• b) a binder comprising one or more acrylic resins, said binder being present in an amount from about 10 wt-% to about 40 wt-%,
• c) one or more IR absorbing materials in a total amount from about 5 wt-% to about 25 wt-%,
• said one or more IR absorbing materials comprising copper (Cu) and one or more anions selected from the group consisting of phosphates (PO43−), hydrogenophosphates (HPO42−), pyrophosphates (P2O74−), metaphosphates (P3O93−), fluorides, chlorides, sulfates (SO42−) and hydroxides (OH−) and
• d) one or more zinc aluminum phosphate compounds in an amount from about 0.125 wt-% to about 5.0 wt-%,
• wherein a ratio (R) between the amount of the one or more zinc aluminum phosphate compounds versus the sum of the amount of the one or more zinc aluminum phosphate compounds and the one or more IR absorbing materials is between about 2.0 and about 20,
• e) optionally one or more additives selected from fillers, waxes, surfactants, anti-foaming agents, thickening agents and mixtures thereof,
• the weight percents being based on the total weight of the security ink.