Elena Perez-Barrado1 2 Richard Darton1 Anna Dobrowolska2 Dieter Guhl2

1, Keele University, Stafordshire, , United Kingdom
2, Keeling & Walker Ltd., Stoke-on-Trent, Staffordshire, United Kingdom

Materials that either absorb or reflect in the near infrared (NIR) region have numerous applications. In particular, compounds that absorb in the NIR can be found in the market of smart windows, security inks, agricultural films, laser marking, laser welding and functional coatings, amongst others [1-2]. In this work we focus our attention in pigments for security inks and laser marking technology. There is a current interest in the industry to find non-toxic and non-expensive security inks for documents such as passports, visas, certificates, etc, to avoid counterfeiting and/or identity fraud. Moreover, these pigments can also be diluted in polymer compositions and used in the laser marking of plastics. It is an important industry considering the number of plastic objects that surround us that need marking [3].
Pigments used in this work are copper hydroxyphosphates with formula Cu2PO4OH. As a mineral it can be found in nature with the name libethenite, which belongs to adamite-type compounds and has space group Pnnm [4]. Several Cu2PO4OH samples were synthesized using different conditions and later characterized by XRD, FTIR, Raman, TGA, SEM and TEM. The optical properties were checked by UV-VIS-NIR spectroscopy. In order to assess the applicability of the materials as laser marking additives and as pigments in security inks, low density polyethylene testing plates and inks were tested.
The libethenite phase was detected in all samples after using different synthesis conditions. Interestingly, a sample was synthesized by using a low-cost synthetic procedure and was easily upscaled to 400 gr. After monitoring the temperature of the coated films, it was possible to observe a significant temperature decrease when compared to a resin-only coated film. The behaviour of the coated inks allows us to suggest that they could be used as security inks and be detected by a Raman detector.
In addition, several laser codes were successfully marked on testing plates. Because of the positive outcome of the laser-marking test we could suggest the use of libethenite for the marking of plastics.

[1] US Patent 20130264529 A1.
[2] US Patent 8778494 B2.
[3] R. Wissemborski, R. Klein, Welding and marking of plastics with lasers, LTJ 7 (2010) 19-22.
[4] I-S. Cho, D. W. Kim, S. Lee, C. H. Kwak, S-T. Bae, J. H. Noh, S. H. Yoon, H. S. Jung, D-W. Kim, K. S. Hong, Synthesis of Cu2PO4OH hierarchical superstructures with photocatalytic activity in visible light, Adv. Func. Mater. 18 (2008) 2154-2162.