Nano-TiO₂ coatings for cultural heritage protection: The role of the binder on hydrophobic and self-cleaning efficacy

Nano-sized titanium dioxide has demonstrated its efficiency in many application fields thanks to its photocatalytic features that provide self-cleaning properties to the materials with simple and non-expensive procedures. For this reason, it has been successfully used also for the practice of restoration of stone built heritage. However, some aspects are still unresolved and need to be further investigated, such as the method for binding these particles to stone surfaces. In this work, nano-TiO₂ was combined with three different binders and applied on two stone substrates, namely the Carrara marble and the Noto calcarenite, two lithotypes extensively used in built heritage. The performance of all tested coatings was evaluated by scanning electron microscopy (SEM), roughness measurements, capillary water absorption test, static contact angle calculation, colorimetric measurements, UV aging and self-cleaning test. Results suggested the key role of interaction between coating and stone surface in terms of penetration of the product, hydrophobicity, variations of surface roughness and durability, which define the performance of the coatings. Specifically, among the three tested products, the best behaviour in terms of hydrophobicity, durability and self-cleaning properties was shown by both the acrylic (Fosbuild) and fluorinated (Akeograd P) suspensions. Conversely, the Paraloid id B72 - TiO₂ mixture led to an intense superficial alteration of both stones and showed scarce water-repellent and photo-degrading effect.