Low energy curing offset and letterpress printing inks and printing process

1. A low energy radically curable offset or letterpress printing ink having a viscosity in the range of about 2.5 to about 25 Pa s at 40° C. and 1000 s −1 for printing a security feature on a substrate or security document, said low energy radically curable offset or letterpress printing ink comprising:

i) from about 10 wt. % to about 80 wt. % of radically curable (meth)acrylate compounds;

ii) from about 1 wt. % to about 20 wt. % of one or more photoinitiators of formula (I):

wherein

R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , R 11 , R 12 , R 13 , R 14 are identical or different from each other and are selected from the group consisting of hydrogen, C 1 -C 4 -alkyls;

iii) from about 1 wt. % to about 60 wt. % of one or more machine readable materials selected from the group consisting of luminescent materials, magnetic materials, IR absorbing materials and mixtures thereof; and

iv) from about 0.5 wt. % to about 20 wt. % of one or more fillers and/or extenders, the weight percents being based on the total weight of the low energy radically curable offset or letterpress printing ink.

2. The low energy radically curable offset or letterpress printing ink according to claim 1 , wherein at least one of the one or more photoinitiators is of formula (II):

3. The low energy radically curable offset or letterpress printing ink according to claim 1 , wherein the one or more fillers and/or extenders are selected from the group consisting of carbonates, silicas, talcs, clays and mixtures thereof.

4. The low energy radically curable offset or letterpress printing ink according to claim 1 , wherein the radically curable (meth)acrylate compounds consist of one or more radically curable (meth)acrylate oligomers and one or more radically curable (meth)acrylate monomers.

5. The low energy radically curable offset or letterpress printing ink according to claim 1 , further comprising a) one or more dyes and/or b) inorganic pigments, organic pigments or mixtures thereof.

6. The low energy radically curable offset or letterpress printing ink according to claim 1 , further comprising one or more waxes selected form the group consisting of paraffin waxes, polyethylene waxes, fluorocarbon waxes, polytetrafluoroethylene waxes, carnauba waxes and mixtures thereof.

7. A process for printing a security feature on a substrate by an offset or letterpress printing process comprising the steps of:

a) applying the low energy radically curable offset or letterpress printing ink recited in claim 1 by offset printing or letterpress printing so as to form a coating or layer, and

b) curing the coating or layer with an ultraviolet lamp having a wavelength of 280 to 400 nm at a dose of at least 50 mJ/cm 2 .

8. A security feature comprising a coating or a layer made of the low energy radically curable offset or letterpress printing ink recited in claim 1 .

9. A security document comprising one or more security features recited in claim 8 .

10. The low energy radically curable offset or letterpress printing ink according to claim 1 , wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , R 11 , R 12 , R 13 , R 14 are selected from the group consisting of hydrogen, methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, fluorine, chlorine and bromine.

11. The process for printing a security feature on a substrate by an offset or letterpress printing process according to claim 7 , wherein in step b) curing the coating or layer with the ultraviolet lamp is performed at a dose of at least 100 mJ/cm 2 .