UV-Induced Colour Changes in Bio-Based Coating Formulations Containing Turmeric and Madder Studied by EPR and FTIR

The study investigated the mechanism of colour change in coating formulations containing natural colorants—madder (Rubia tinctorum) and turmeric (Curcuma longa)—under UV irradiation using electron paramagnetic resonance (EPR) and Fourier-transform infrared spectroscopy (FTIR). Samples based on Epidian® 5 epoxy resin and Polimal® 109 unsaturated polyester resin containing radical and cationic photoinitiators were examined. EPR analysis revealed the involvement of transition metal ions (Fe3+, Cu2+) and organic radicals in the photo-oxidation processes. Characteristic signals at g ≈ 4.3, g = 2.2–2.3 and g = 2.001 were attributed to Fe3+, Cu2+ and aromatic radicals, respectively. UV irradiation caused the formation of oxygen- and phenoxyl-type radicals in turmeric-containing epoxy coatings and phenoxyl and semiquinone radicals in madder-containing coatings. The colour changes correlated with structural transformations identified by FTIR spectroscopy, while polyester coatings showed slower colour-change dynamics. The results confirm the usefulness of EPR and FTIR spectroscopy for analyzing UV-induced processes in polymer coating formulations containing plant-derived colorants and provide insight into the photochemical stability of bio-based components considered for more environmentally friendly coating materials.