Novel inorganic-organic hybrid photochromic and fluorescent ink for anticounterfeiting documents was developed using a pigment/resin ink formula enclosing a long-lived luminescent inorganic pigment with good thermal photostability. The produced ink exhibited an optimal excitation wavelength at 360 nm with an absorption colour and fluorescence changes in the printed document. To develop a transparent printed film from pigment/resin ink, the phosphorescent pigment has to be well dispersed physically without agglomeration. The pigment/resin hybrid was applied effectively onto commercial cellulose paper sheets using screen-printing technology. An homogeneous photochromic layer was deposited on cellulose paper document surface to afford a considerable greenish-yellow colour as demonstrated by CIE coloration measurements under a UV lamp, even at a pigment concentration as low as 0.1 wt% of the ink formulation. The printed paper sheets exhibited three excitation bands at 235, 274 and 378 nm and three emission bands at 416, 418 and 436 nm. Fluorescence optical microscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and wavelength-dispersive X-ray fluorescence spectrometry of the printed paper sheet were explored. The screen-printed paper sheets displayed a reversible and fast photochromism during ultraviolet irradiation without fatigue. The rheological properties, stability, and printability of the ink were studied.
Keywords: fluorescence; photochromism; screen print; security ink; strontium aluminate.
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