Security document, e.g. a banknote, is of an opaque paper where zones of an increased transparency are formed by an ultrasonic sonotrode as a security which can be recognized by the public but cannot be photocopied

摘要

To produce a security document, the paper has an opaque fiber screen to be treated with a local exposure to vibration from a sonotrode at supersonic frequencies, to increase the transparency in selected zones. To produce a security document, the paper is a screen of cellulose. The local transparent zones are impregnated with a varnish, where the coefficient of optical refraction is equal to that of the document paper. The varnish contains a solid additive, with an optical coefficient of refraction which equals that of the paper, using talc or kaolin or carbonate of lime. The impregnation is by printing through silk-screen, flexography or heliogravure. The varnish has a component which can be polymerized by ultra violet light, used on the varnish after impregnation. The varnish can also contain a thermofusable component, and the treated zone is heated before impregnation and cooled afterwards, to evaporate the water content of the varnish. The sonotrode operates at a frequency of 30-40 KHz, with an amplitude of 8-16 microns. The document has a number of zones with an increased transparency, forming a repetitive pattern. The zones of increased transparency can also be used to take an additional security feature as a filament, fibers or special pigments.