Stability and ageing behaviour of a polysiloxane-based titania protective coating on terracotta samples

This study investigates the performance of a protective coating composed of a commercial polysiloxane mixed with titanium dioxide nanoparticles (P25), applied onto terracotta samples via airbrushing. The coating was characterized through colorimetric analysis, static contact angle (SCA) measurements, and infrared spectroscopy to evaluate its aesthetic impact, wettability properties, and chemical stability under UV exposure. Results indicate that both polysiloxane alone and the polysiloxane-titania formulation altered the appearance of terracotta samples, with a ΔE* value of approximately 3. Polysiloxane alone induced a yellowing effect, while the addition of titanium dioxide led to surface whitening. Accelerated aging did not cause significant aesthetic degradation, suggesting good visual durability. As expected, the strong artificial aging process induced the loose of superhydrophobic coating and its water repellency from 136° to 16° SCA values, changing to a hydrophilic state due to increased surface energy. ATR infrared spectroscopy revealed UV-induced polymer aging (segment scission or crosslinking) which may have contributed to these changes. These characteristics make the polymer more suitable for indoor surfaces, characterized by low-irradiance environments, or as a temporary protective treatment for artifacts requiring subsequent restoration intervention. These limitations observed with the commercial product have guided ongoing research in our group toward an eco-friendly formulation with significantly improved UV resistance, tailored for long-term heritage conservation needs.