Method for treating the surface of metal foils with UV-cured protective varnish

A method for treating the surface of metal foils with UV-cured protective varnish, the method comprising the steps of:

• applying atmospheric plasma on at least a portion of a surface to be treated, of a metal foil, increasing a surface free energy of the metal foil, making the surface free energy compatible with a surface tension of the varnish to be applied to the metal foil, wherein the surface free energy of the metal foil after applying the atmospheric plasma is from 29 mN/m to 72 mN/m, the metal foil is coated with tin or chrome;
• applying, on the surface of the metal foil, submitted to the atmospheric plasma, a UV-cured varnish layer, selected from compounds of oligomers, monomers and photo-initiators, with a viscosity from 40 to 60 Sec CF4/25° C., specific gravity from 1.01 to 1.05/25° C. and with a solids content from 99% to 100%;
• and
• curing the varnish layer, applied on the surface of the metal foil, by polarization of the varnish compounds, by interaction of the photo-initiator with UV radiation in a curing unit, forming a varnish coating adhered to the surface of the metal foil,
• wherein the atmospheric plasma is released, on the surface to be treated of the metal foil, from at least one plasma nozzle, the atmospheric plasma being produced by a controlled discharge of electrical energy made by a plasma generator, inside the plasma nozzle, the atmospheric plasma being fed by compressed air or nitrogen or oxygen from a compressed gas generator, and
• wherein the plasma generator generates 300 to 400 Volts, with a pulse frequency from 20 to 30 kHz and a power from 900 to 1000 W, with the gas to be compressed being selected from oxygen or nitrogen and in an amount from 50 to 60 liters per minute, with a pressure from 240 to 250 mbar.