Security element

Security element, comprising

• (a) a substrate, which optionally contains indicia or other visible features in or on its surface, and
• (b) on at least part of the substrate surface a metal layer having a thickness of from 5 to 1000 nm,
• (c) optionally on at least part of the metal layer a dielectric layer having a thickness of 1 to 20 nm,
• (d) on at least part of the metal layer, or the dielectric layer, a layer obtained by overcoating the metal layer, or the dielectric layer with a composition, comprising
• (i) silver nanoparticles having an average diameter of from 5 to 500 nm in an amount of 0.1 to 20% by weight,
• (ii) a solvent in an amount of 50 to 99.8% by weight,
• (iii) surface stabilizing agent(s) in an amount of 0.0001 to 5% by weight, and
• (iv) optionally a binder in an amount of from 0 to 20% by weight, wherein the amounts of components (i), (ii), (iii), and (iv) are based on the total weight of the composition, and
• (e) a protective layer on top of layer (d), wherein the maximum absorption wavelength of the silver nanoparticles in layer (d) is controlled by the amount of (surface) stabilizing agent(s) and optionally binder relative to the amount of silver nanoparticles, to be in the range of 700 to 1600 nm;
• wherein the surface stabilizing agent(s) comprises a surface stabilizing agent of formula
• on their surface, wherein indicates the bond to the silver,
• R1 is H, C1-C18alkyl, phenyl, C1-C8alkylphenyl, or CH2COOH;
• R2, R3, R4, R5, R6 and R7 are independently of each other H, C1-C8alkyl, or phenyl;
• Y is O, or NR8;
• R8 is H, or C1-C8alkyl;
• k1 is an integer in the range of from 1 to 500,
• k2 and k3 are independently of each other 0, or integers in the range of from 1 to 250;
• k4 is 0, or 1, and optionally a compound of formula
• wherein
• R21a is a hydrogen atom, a halogen atom, a C1-C8alkoxy group, or a C1-C8alkyl group,
• R21b is a hydrogen atom, or a group of formula —CHR24—N(R22)(R23),
• R22 and R23 are independently of each other a C1-C8alkyl, a hydroxyC1-C8alkyl group, or a group of formula —[(CH2CH2)—O]n1—CH2CH2—OH, wherein n1 is 1 to 5, R24 is H or C1-C8alkyl;
• and optionally a compound of formula
• wherein R25 can be the same, or different in each occurrence and is a hydrogen atom, a halogen atom, a C1-C18alkyl group, a C1-C18alkoxy group, or a group —C(═O)—R26,
• R26 is a hydrogen atom, a hydroxy group, a C1-C18alkyl group, unsubstituted or substituted amino group, unsubstituted or substituted phenyl group, or a C1-C18alkoxy group, and
• n3 is a number of 1 to 4,
• m3 is a number of 2 to 4, and
• the sum of m3 and n3 is 6.