Uv-vis radiation curable security inks for producing dichroic security features

A UV-Vis radiation curable security ink for producing a security feature exhibiting a blue color upon viewing in transmitted light and a metallic yellow color upon viewing in incident light, wherein said ink comprises:

• a) from about 7.5 wt-% to about 20 wt-% of silver nanoplatelets having a mean diameter in the range of 50 to 150 nm with a standard deviation of less than 60%, a mean thickness in the range of 5 to 30 nm with a standard deviation of less than 50%, and a mean aspect ratio higher than 2.0, wherein the mean diameter is determined by transmission electron microscopy and the mean thickness is determined by transmission electron microscopy, and
• wherein the silver nanoplatelets bear a surface stabilizing agent of general formula (I)
• wherein
• the residue RA is a C2-C4alkyl group substituted with a hydroxy group;
• the residue RB is selected from a C1-C4 alkyl group, and a C2-C4alkyl group substituted with a hydroxy group;
• and
• Cat+ is a cation selected from the group consisting of Na+, K+, Cs+ and Rb+;
• b) a perfluoropolyether surfactant functionalized with at least a hydroxy group;
• c) from about 3 wt-% to about 12 wt-% of a polyvinyl chloride copolymer containing at least
• 69 wt-% of vinyl chloride;
• d) d1) from about 25 wt-% to about 55 wt-% of a cycloaliphatic epoxide, and from about 1 wt-% to about 10 wt-% of a cationic photoinitiator;
• or
• d2) from about 30 wt-% to about 65 wt-% of a mixture of a cycloaliphatic epoxide and a radically curable compound, from about 1 wt-% to about 6 wt-% of a cationic photoinitiator, and from about 1 wt-% to about 6 wt-% of a free radical photoinitiator;
• and optionally
• e) a cationically curable compound selected from the group consisting of:
• e1) a vinyl ether having two vinyloxy residues in an amount lower than 50% of the weight percent (wt-%) of the cycloaliphatic epoxide of d);
• e2) a vinyl ether having one vinyloxy residue in an amount lower than about 5 wt-%;
• e3) an epoxide other than a cycloaliphatic epoxide in an amount lower than about 10 wt-%;
• e4) an oxetane having two oxetanyl residues in an amount lower than about 20 wt-%;
• e5) an oxetane having one oxetanyl residue in an amount lower than about 3.5 wt-%;
• and
• e6) a mixture of e1) and/or e2) and/or e3) and/or e4) and/or e5);
• the weight percents being based on the total weight of the UV-Vis radiation curable security ink.