Polyhydroxyalkanoate/TiO2 nanocomposite as sustainable coating for protection of marble, basalt, and travertine in the conservation of cultural heritage assets

In this work, a nanocomposite based on nano-TiO2 and the biodegradable polymer polyhydroxyalkanoate (PHA) was developed and tested as protective coating for the treatment of lithotypes commonly employed in cultural heritage assets, specifically marble, basalt, and travertine. TiO2 nanoparticles were synthetized through hydrothermal synthesis and their structural and morphological features as well as photocatalytic activity were compared with the commercial TiO2-P25 nanoparticles. TiO2-P25 commercial product consisted mainly of anatase nanograins, while the synthesized material displayed a distinctive cypress-cone morphology with anatase and rutile in equal proportion. Despite these structural and phase differences, both nanoparticle types demonstrated similar photocatalytic efficiency in degrading methylene blue, indicating that morphology and phase ratio did not significantly affect performance. Pure PHA and PHA/TiO2 nanocomposite coatings were prepared with the aim to obtain a sustainable protective layer with enhanced hydrophobic and self-cleaning properties. The coatings were characterized using colorimetric analysis, water absorption tests, and static contact angle measurements. These evaluations were performed on untreated samples (t0), after coating application (t1), and following UV-accelerated aging (t2). The results showed that both PHA-based treatments did not induce notable color variations, though some variability was observed in basalt and travertine, materials that tend to be less uniform. In terms of hydrophobicity and water resistance, the pure PHA coating produced moderate improvements, with stable performance even after UV exposure, suggesting good durability. In contrast, the nanocomposite coating initially provided superior enhancement, making the treated stone surfaces significantly more water-repellent and hydrophobic. However, this effect decreased after UV aging: the surfaces became more hydrophilic, indicating that the TiO2 addition, while beneficial in the short term, compromised long-term stability under UV stress. These findings underline the trade-off between short-term performance and long-term durability when coupling photocatalytic nanomaterials with biopolymers.