Printing diffraction gratings on paper and board

A method for forming a surface relief microstructure on a paper substrate, the method comprising:

• A) applying a curable composition to at least a portion of a frontside of a paper substrate;
• B) contacting at least a portion of the curable composition with a surface relief microstructure;
• C) curing the curable composition with a UV lamp, which is arranged on a backside of the paper substrate, to form a cured composition;
• and
• D) depositing a metallic layer, and optionally a layer of a transparent high refractive index material, on at least a portion of the cured composition,
• wherein:
• the UV lamp has an emission peak in a UV-A range of 320 nm to 400 nm and additionally in a near VIS range of 400 nm to 450 nm;
• the curable composition comprises a photoinitiator which absorbs in the UV-A range and also in the near VIS range;
• the photoinitiator is a mixture comprising a compound formula (XII):
• R50 is unsubstituted cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl;
• or is cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl substituted by one or more halogen, C1-C12alkyl, C1-C12alkoxy, C1-C12alkylthio or by NR53R54;
• or R50 is unsubstituted C1-C20alkyl or is C1-C20alkyl which is substituted by one or more halogen, C1-C12alkoxy, C1-C12alkylthio, NR53R54 or by —(CO)—O—C1-C24alkyl;
• R51 is —(CO)R′52;
• or R51 is C1-C12alkyl which is unsubstituted or substituted by one or more halogen, C1-C12alkoxy, C1-C12alkylthio, or by NR53R54;
• R52 and R′52 independently of each other are unsubstituted cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl, or are cyclohexyl, cyclopentyl, phenyl, naphthyl or biphenylyl substituted by one or more halogen, C1-C4alkyl or C1-C4alkoxy;
• or R52 is a 5- or 6-membered heterocyclic ring comprising an S atom or N atom;
• R53 and R54 independently of one another are hydrogen, unsubstituted C1-C12alkyl or C1-C12alkyl substituted by one or more OH or SH wherein the alkyl chain optionally is interrupted by one to four oxygen atoms;
• or R53 and R54 independently of one another are C2-C12-alkenyl, cyclopentyl, cyclohexyl, benzyl or phenyl;
• the mixture further comprises at least one selected from the group consisting of a monoacylphosphine oxide compound, a benzophenone compound, an alpha-amino ketone compound, an alpha-hydroxyketone compound, an alpha-alkoxyketone compound, a phenyl
• glyoxylate compound, an oxime ester compound, and an onium salt compound;
• the metallic layer is deposited by applying a metal-based ink on the cured composition;
• the metal-based ink comprises metal pigment particles produced by physical vapor deposition, a binder, and optionally a solvent;
• the metal pigment particles have a thickness in the range of 5 to 50 nm;
• and the curing of the curable composition with the UV lamp, arranged on the backside of the paper substrate, fully cures the curable composition.