Photostability and Colorimetric Properties of Azobenzene‐Based Photochromic Coatings: Performance Enhancement by TiO 2 Nanoparticles and UV Protective Varnish

ABSTRACT Azobenzene‐based photochromic pigments are widely studied for their tunable photochemical behavior and adaptability as functional printed coatings. This study evaluated the colorimetric, chemical, and functional properties of printed coatings containing blue, violet, and orange photochromic pigments modified with titanium dioxide (TiO 2 ) nanoparticles and protected with an overprint varnish. The effects of these modifications on UV‐induced aging, lightfastness, pigment activation, and photofading of printed layers on paperboard were systematically examined. Incorporation of TiO 2 nanoparticles produced higher lightness values of coatings compared to unmodified samples, while varnish application resulted in a paler, less saturated appearance. FTIR‐ATR and 2D correlation spectroscopy revealed chemical changes in the printed coatings due to UV exposure, particularly with peaks corresponding to carbonyl and aromatic vibrations showing altered intensity in aged samples. UV‐induced aging caused the most pronounced degradation and color shifts in unmodified coatings, whereas samples containing nano‐TiO 2 showed smaller color shifts throughout aging cycles; the least change was observed in varnished surfaces. Fluorescence spectroscopy indicated that the detected emission originated predominantly from the paperboard substrate, with negligible fluorescence contribution from the photochromic pigments under applied conditions. Monitoring pigment activation and photofading revealed visible differences between modified and unmodified coatings, with the greatest decrease in photochromic effect occurring immediately after removal of the UV source. Nano‐TiO 2 addition partially slowed fading, and varnish provided the highest color stability. Overall, the proposed modifications demonstrated potential for improving lightfastness and delaying photofading, emphasizing interactions among ink components and offering opportunities for controlled application of photochromic functional coatings.